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AN EXPERIMENTAL STUDY

OF THE EFFECT OF IMPELLER DESIGNS

ON THE PERFORMANCE OF

GAS INDUCING CONTACTORS

BY

Khaled R. Moftah

A thesis submitted in confomi ty w i t h the requirezztents

for the degree of Master of Applied Science

Department of Chemical Engineering

and Applied C2zentistry

Universi t y of Toronto

O Copyright by Khaled R. Moftah 1997

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Vin Experimental Study of the Effect of Impeller ~esigns on the

Performance of Gas Inducing Contactorsw

Khaled R. Moftah

M. A. Sc. Thesis, 1997

Department of Chemical Engineering and Applied Chemistry

University of Toronto

Five impeller designs were tested in a laboratory scale gas

inducing contactor (GIC). Testing included induced static head,

rate of gas induction, mixing tirne and mass transfer

coefficient.

The Reference design for cornparisons was the straight blade

impeller with shroud.

For the designs studied the following results were obtained.

The induced static head, rate of gas induction, mixing tirne and

mass transfer coefficient of the down-pumping impeller were

enhanced over the reference design.

The mass transfer coefficient of the up-pumping impeller was

also enhanced. Unlike the other designs this impeller showed

instability in the power consumption in the range 1400-1750 RPM.

T h e induced s ta t ic head and the rate of gas induction of the

double row impeller were enhanced, but the mass transfer

coefficient was only marginally improved.

The straight blade impeller with cut-off w a s inferior in al1

respects.

iii

It was a great chance and good luck for me to do this

work at the University of Toronto. 1 thank al1 those who

helped me during the course of the work.

My chief debt is to my supervisor; Professor James srnith

who provided me with valuable help and guidance

throughout this work. 1 appreciate the support and

encouragement which he provided to me.

Thanks to Apollo Environmental Systems Corp., and to its

President, Dr. Peter Walton for the financial support to

the work. Thanks are due also to the University of

Toronto and to the Natural Science and Engineering

Research Council (NSERC) for their financial support.

It was to my honour that professor Donald Kirk chaired

the discussion committee, who gave me a great help and

valuable comments during the reviewing process.

1 would like to pass my gratitude to Silvano Meffe,

Apollo Engineer, whose discussions and suggestions were

very valuable to the completion of the study.

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENT

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

1. INTRODUCTION

1.1 General

1.2 Gas ~nducing Contactors

1.3 Mass Transf er Coefficient

1.4 Experimental procedures

1.4.1 ~ o w e r

1.4.2 Induced Static Head

1.4.3 Rate of Gas Induction

1.4.4 Mixing Time

1.4.5 Mass Transf er coefficient

1.4.5.1 The Dynarnic Method

1.4.5.2 The Steady State method

1.4.5.3 The Gas balance Method

1.4.5.4 The oxygen electrode

1.5 Other Parameters

1.5.1 Gas Hold-up

1.5.2 Bubble Size

2. THEORETICAL BACKGROUND

2.1 Impeller Design

2.2 Mass Transfer Coefficient

2.2.1 The Film Mode1

2.2.2 Still Surface Mode1

2.2.3 Surface Rej uvenation Mode1

2.2.4 Surface Renewal Mode1

v

viii

ix

1

1

2

2-2.5 Definite-Thickness Surface Renewal Mode1

2.2.6 Other Models

2.3 Oxygen-Sodium Sulfite System

2-3-1 General

2.3-2 Continuous Feed System

3. EXPERIMENTAL

3.1 Equipment Setup

3.2 The Studied Impellers

3.3 Testing Methods

3.3.1 Power

3.3.2 Induced static head

3.3-3 Rate of Gas Induction

3.3.4 Mixing Time

3.3.5 Mass Transfer Coefficient

3.3.6 Dissolved Oxygen Concentration

3.4 Data Acquisition

3.5 Materials

4. RESULTS and DISCUSSION

4.1 General

4.2 Power Consumption Under Free Induction

4.3 Power Consumption Under Forced Gassing

4.4 Induced tat tic Head

4.5 Rate of Gas Induction

4.6 Mixing T i m e

4.7 Mass Transf er Coefficient

4.7.1 General

4 . 7 . 2 Systems with Fast Chemical Reactions

4 . 7 . 3 Effect of the Shroud

5. CONCLUSIONS

6 RECOMMENDATIONS

7. REFERENCES

8 - NOMENCLATURE

9 . APPENDICES

Appendix A l :

Appendix A2:

Appendix A3:

Appendix A4 :

Appendix A5 :

Appendix A6:

Appendix A7:

Appendix A8:

Power vs. RPM under Free Induction

Power vs. RPM under Forced Gassing

Induced Static Head vs. RPM

Rate of Gas Induction vs . RPM

Mixing Time vs. RPM

Mass transf e r Coefficient vs . RPM Correlations

Miscellaneous

vii

LIST OF FIGURES

Figure 1: Experimental Setup

Figure 2: Schematic of Impeller Designs and the Shroud

Figure 3: Specific Power vs. RPM under Free Induction

Low RPM Range

Figure 4: specific Power vs. RPM under Free Induction

High RPM Range

Figure 5: ~pecific Power vs. RPM under Free Induction

Shroud In and Out of Place

Figure 6: Specific Power vs. RPM under Free Induction for

Impeller 11%

Shroud In and Out of Place

Figure 7: Specific ~ydraulic Power vs. Superf ic ia l Air Velocity

Figure 8: Induced Static Head vs. RPM

Figure 9: Rate of Gas Induction vs. Hydraulic Power

Figure 10: Mixing Time vs. Specific Power

Figure 11: Mass Transfer Coefficient vs. Specific Hydraulic Power

Figure 12: Schematic of the Shroud around Impeller 1

viii

LIST OF TABLES

Table 1: Ratios of Parameter Values of D i f f erent Impellers

Table 2 : Performance of Impellers Studied in G I C

1. INTRODUCTION

Liqu ids are mechan ica l ly agitated t o enhance mixing, b l end ing ,

d i s s o l u t i o n , s o l i d suspens ion , e m u l s i f i c a t i o n sed imen ta t ion ,

c r y s t a l l i s a t i o n , c o a g u l a t i o n , f l o t a t i o n , h e a t and mass t r a n s f e r .

Each p roces s has s p e c i f i c requirements . Achieving the r e q u i r e d g o a l

a t a minimum expense of energy is a primary t a s k i n t h e d e s i g n o f

the a g i t a t i o n u n i t .

Mechanical a g i t a t i o n of l i q u i d s induces c u r r e n t s i n t h e l i q u i d which

s i m u l t a n e o u s l y enhance more than one parameter, e . g . mixing,

d i s s o l u t i o n , and mass t r a n s f e r . Energy i s dissipated i n i nduc ing

h y d r a u l i c c u r r e n t s and overcoming t h e d r a g a s s o c i a t e d wi th t h e

relative motion between t h e s o l i d p a r t s and t h e l iquid. The la t ter

are largely t h e b l a d e s b u t i n c l u d e t h e hub, t h e s h a f t , and o t h e r

moving components. B a f f l e s and the shroud a round t h e i m p e l l e r

i n c r e a s e t h e power consumption as they i n c r e a s e t h e t u r b u l e n c e i n

t h e l i q u i d mix ture . The mode of d i s s i p a t i o n o f the mechanical energy

i n the liquid o r l i q u i d mixture determines the f i n a l performance o f

the u n i t w i t h r ega rd to t h e r e q u i r e d p r o c e s s o r combinat ion o f

processes.

Al1 ene rgy i n p u t w i l l be d i s s i p a t e d i n t h e l iquid mix tu re i n any

a g i t a t e d system, b u t some des igns are b e t t e r t h a n o t h e r s . They

consume less energy t o a c h i e v e the same t a s k . This is because t h e y

i n d u c e the right combinat ion of h y d r a u l i c c u r r e n t s i n terms o f

q u a n t i ty , d i r e c t i o n , a n d head .

Des igns c a n d i f f e r i n t h e geomet r ies o f any of t h e vessel, t h e

baffles, the impe l l e r , the s tand-pipe , the shroud , t h e sparger , and

t h e combinat ions t h e r e o f . B e t t e r c l e a r a n c e , submergence, b a f f l e

s h a p e and wid th , r a t i o o f i m p e l l e r diameter t o ves se1 d i ame te r ,

i m p e l l e r shape , type, width and d i a m e t e r p l u s o t h e r r e l e v a n t

dimensions are sough t i n the process of o p t i m i s a t i o n o f the d e s i g n .

Gas Inducing contactors

Gas-Inducing-Type Mechanica l ly A g i t a t e d C o n t a c t o r s (GIMACr s) o r

simply Gas Inducing c o n t a c t o r s (GIC's) are mechanical arrangements

where al1 o r a p a r t o f the i m p e l l e r is placed i n s i d e a s t and -p ipe

( s t a t o r ) connected t o t h e gas i n l e t , the gas b e i n g mixed and reacted

w i t h the l i q u i d o r compounds suspended o r d i s s o l v e d i n t h e l iquid.

These u n i t s create s u c t i o n head i n t h e s tand-p ipe s u f f i c i e n t t o move

the gas i n t o the l iquid by induc ing a v o r t e x . Thus t h e irnpeller i n

t h e s e u n i t s performs m u l t i p l e f u n c t i o n s i n c l u d i n g suck ing the g a s

from t h e head space i n s i d e t h e s tand-p ipe ( s t a t o r ) , d i s p e r s i o n o f

the sucked gas i n t o the bulk the l i q u i d as w e l l as a g i t a t i o n o f t h e

bu lk l i q u i d .

I n the l a s t 10 years se l f - induced f l o t a t i o n cells have been used i n

g a s - l i q u i d c o n t a c t i n g o p e r a t i o n s , p a r t i c u l a r l y f o r sc rubbing

hydrogen s u l f i d e f r o m waste gases and f o r s t r i p p i n g hydrogen s u l f i d e

f rom l i q u i d s u l f u r . S p e c i a l d e s i g n s based on modif ied f l o t a t i o n

cells have been p a t e n t e d by Smith, et al., 1990 and 1992. The

feas ib i l i ty o f u s i n g t h e gas - induc ing c o n t a c t o r ( G I C ) f o r oxygen

t r a n s f e r i n aqueous systems is a t t r a c t i v e , s i n c e many i n d u s t r i a l and

municipal o p e r a t i o n s r e q u i r e oxygenation t o effect var ious ox ida t ion

r e a c t i o n s . Examples i n c l u d e go ld cyanida t ion , w a s t e w a t e r t r e a t m e n t

and a e r o b i c d i g e s t i o n . The

economics and p r a c t i c a l i t y of us ing modified f l o t a t i o n ce11 as a G I C

w i l l depend l a r g e l y on mass t r a n s f e r performance.

These u n i t s can be used where it is desirable t o have complete

u t i l i z a t i o n o f t h e s o l u t e gas and /o r where the convers ion p e r p a s s

i s low. T h i s is done by recyc l ing gas f r o m the head s p a c e back i n t o

t h e s t and-p ipe and the i m p e l l e r zone- This is achieved w i t h o u t t h e

need fo r a n e x t e r n a l l o o p and cornpressor. They are p a r t i c u l a r l y

u s e f u l where t h e s o l u t e gas i s available at low p r e s s u r e .

These c o n t a c t o r s are u s e f u l f o r u n i t p r o c e s s e s such as a l k y l a t i o n ,

o z o n a t i o n , o x i d a t i o n , hydrogena t ion , s u l f o n a t i o n , s u f o x i d a t i o n ,

h y d r o c h l o r i n a t i o n , ammonolysis and f e r m e n t a t i o n . GICr s are a l s o

use fu l i n f l u e gas d e s u l f u r i z a t i o n , oxyhydro lys i s i n o r e l e a c h i n g ,

and o x i d a t i v e leaching of m e t a l s . GiC' s are w i d e l y u t i l i z e d i n f r o t h

f l o t a t i o n f o r b e n i f i c a t i o n of o r e s as w e l l as i n w a s t e water

t r e a t m e n t .

Many GIC d e s i g n s have been d e s c r i b e d i n t h e l i t e r a t u r e . The p i p e

i m p e l l e r , c o n s i s t i n g of a hol low (p ipe ) t u b u l a r shaf t and t u b u l a r

i m p e l l e r w a s d i s c u s s e d by Z loka rn ik , 1966, Z lokarn ik , and J o d a t ,

1967, J o s h i and Sharma, 1977 and J o s h i , 1980. The f l a t t e n e d

c y l i n d r i c a l impeller was i n v e s t i g a t e d by M a r t i n , 1972 a n d J o s h i , and Shanna, 1977. Zundelevich, 1979 p resen ted t h e t u r b o a e r a t o r - The

shrouded-disc-curved-blade t u r b i n e , and t h e p i t c h e d - b l a d e t u r b i n e

w e r e i n v e s t i g a t e d by Raidoo, e t a l . , 1987 and Mundale, a n d J o s h i , 1995. T h e p i t c h e d blade i m p e l l e r w a s found t o have s u p e r i o r gas

i n d u c t i o n capaci ty .

The W e m c o F l o t a t i o n ce11 consists o f a cage-type o u t e r s t a t o r and

an i n n e r i m p e l l e r made up o f round p o s t s . Sawant et a l , 1980 have

shown that t h e wemco f l o t a t i o n ce11 prov ides h i g h e r rates o f gas

i nduc t ion compared to t h e p i p e and f l a t t e n e d c y l i n d r i c a l i m p e l l e r s

a t the sarne power consumption. I n another d e s i g n d i scussed by Sawant

et a l . , 1981; the Denver and Wemco type o f gas induc ing i m p e l l e r s ,

t h e i m p e l l e r w a s composed of straight blades.

I n a l 1 these des igns radial f l o w i s formed and a l o w p r e s s u r e

reg ion is formed i n the v i c i n i t y of t h e impe l l e r . When the pres su re

at t h e i m p e l l e r becomes l o w e r t han t h e p r e s s u r e i n t h e s t and-p ipe

gas i s induced i n t o the l i q u i d . Then t h e g a s i s s h e e r e d and

dispersed i n t o t h e b u l k o f t h e l i q u i d by t h e r o t a t i o n o f t h e

inrpeller. Sheer ing occurs a t the i m p e l l e r region where a g i t a t i o n i s

h i g h w h i l e d i s p e r s i o n i s effected by t h e mixing c u r r e n t s .

The rate of gas i n d u c t i o n and t h e gas hold-up d e c r e a s e w i t h an

i n c r e a s e i n t h e i m p e l l e r submergence. That i s w h y t h e s e i m p e l l e r s

are p l a c e d n e a r t h e t o p o f t h e l i q u i d . On t h e o t h e r hand gas

d i s p e r s i o n d e c r e a s e s as t h e c l e a r a n c e form t h e bottom i n c r e a s e s , t h a t i s why double i m p e l l e r d e s i g n s w e r e i n v e s t i g a t e d and

recommended b y S a r a ~ n a n a n d J o s h i , 1995 and 1996. They u s e d t h e

p i t c h e d b l a d e dom-pumping i m p e l l e r which i s t h e most e f f i c i e n t

i m p e l l e r f o r gas induc t ion and used i t a t the t o p row whi le s t u d i e d

d i f f e r e n t i m p e l l e r shapes i n t h e bottom r o w . They found t h a t up-flow

impellers i n t h e bottom row w e r e s u p e r i o r f o r gas i n d u c t i o n

compared t o the s t r a i g h t blade and the dom-f low d e s i g n s .

When d i s c u s s i n g t h e performance o f d i f f e r e n t i m p e l l e r s

d i f f e r e n t i a t i o n i s made between mixing and a g i t a t i o n . While

a g i t a t i o n is a n y motion i m p a r t e d t o t h e f l u i d , mix ing is r e s e r v e d

f o r t h e homogenisation process which i s effected through specific

h y d r a u l i c c u r r e n t s c a u s i n g random dis t r i b u t i o n th roughout t h e

c o n t a i n e r . These c u r r e n t s s p r e a d o u t t h e mix ture i n t h e t a n k t h u s

r e d u c i n g any g r a d i e n t e . g . of c o n c e n t r a t i o n , t empera ture o r phase

s e p a r a t i o n . A g i t a t i o n i n a c i r c u l a t o r y manner is impor t an t f o r t h e

development of the cavities behind the impe l l e r . These c a v i t i e s are

r e s p o n s i b l e f o r most o f t h e i n t e r a c t i o n , and hence rnost mass

t r a n s f e r between t h e g a s and t h e l i q u i d where mos t of t h e mass

t r a n s f e r occurs around the i m p e l l e r . T h e s e c a v i t i e s a l s o reduce t h e

power consumption. I n a d d i t i o n a g i t a t i o n induces t u r b u l e n c e which

r e d u c e s t h e t h i c k n e s s o f t h e gas l iquid i n t e r f a c i a l f i l m . T h i s i n

t u r n i n c r e a s e s t h e mass t r a n s f er c o e f f i c i e n t .

If the l i q u i d i s saturatêd w i t h i n and n e a r the zone o f i n t e n s e

a g i t a t i o n around the impeller, more a g i t a t i o n (and more power) w i l l

have no b e n e f i c i a l effect e x c e p t t o t h e e x t e n t that mixing is

enhanced. The re fo re , f o r t h e hydrogen s u l f ide sc rubb ing system,

mixing can be much less impor tan t i n b r i n g i n g f r e s h reagent t o t h e

r e a c t i o n zone t h a n fo r oxygen transfer, w h e r e s a t u r a t i o n of t h e

e n t i r e l i q u i d body i s d e s i r e d , and mixing assumes g r e a t e r

importance, I n b i o l o g i c a l r e a c t o r s high a g i t a t i o n may be de t r imen ta l

t o s h e a r s e n s i t i v e b r o t h s a n d t h i s s h o u l d be t aken i n t o

c o n s i d e r a t i o n i n the d e s i g n o f t h e i m p e l l e r .

The primary objective of t h e p r e s e n t s t u d y , then, was t o e v a l u a t e

the performance of a r ange of impellers i n gas induc ing c o n t a c t o r s ,

f o r a i r w a t e r systern, which w i l l affect a g i t a t i o n and mixing i n the

c o n t a c t i n g vesse1 i n d i f f e r e n t ways.

I t was shown by Klas sen and Mokrosov, 1963 t h a t d e c r e a s i n g t h e

i m p e l l e r width decreased its a e r a t i o n ra te a n d t h i s w a s exp la ined

by Degner and Treweek, 1976 as due t o the decrease of t h e i m p e l l e r

pumpi ng capaci ty .

I n the p r e s e n t stuciy w i d e impel le rs w e r e used as well as one double

row i m p e l l e r s . These wide i m p e l l e r s may be seen as composed of two

merged rows o f b l a d e s . The f i n a l g o a l of l o w submergence, low

c l e a r a n c e and high pumping capacity i s thus ach ieved ,

1.3 Mass Transfer Coefficient

The mass t r a n s f e r rate per u n i t volume i n g a s l i q u i d c o n t a c t i n g i s

def i n e d f r o m the e q u a t i o n :

The mass t r a n s f e r rate is enhanced through t w o processes. T h e f i r s t

is the i n c r e a s e i n the vo lume t r i c mass t r a n s f e r c o e f f i c i e n t k,a. The

v o l u m e t r i c mass t r a n s f e r c o e f f i c i e n t i n t u r n i n c l u d e s two t e m . The

f i rs t i s the specific i n t e r f a c i a l area a ( I n t e r f a c i a l area per u n i t

volume, m-Il . For the same gas bubble s i z e t h e h i g h e r the gas hold-up

the h ighe r t h e i n t e r f a c i a l area. The G a s hold-up i s the ratio of the

volume o f t h e gas p h a s e t o t h e t o t a l volume of t h e g a s l i q u i d

mix tu r e . I n vessels o f c o n s t a n t cross s e c t i o n th roughout t h e h e i g h t ,

gas hold-up can be c a l c u l a t e d from the ratio o f the i n c r e a s e i n the

h i g h t due t o the i n t r o d u c t i o n o f the g a s t o the to ta l h e i g h t of the

g a s / l i q u i d mixture.

T h e i n t e r f a c i a l area depends a l s o on t h e bubble size. The smaller

the average bubble size t h e h ighe r t h e i n t e r f a c i a l area. The bubb le

s i z e is affected by the type o f l iquid and the p r e s e n c e o f c e r t a i n

o r g a n i c o r i n o r g a n i c compounds i n t h e s o l u t i o n . High i o n i c

c o n c e n t r a t i o n produces a s m a l l bubble s i z e w i t h larger i n t e r f acial

area and l e s s e n s bubble coa lescence . Air/water sys t em is cons ide r ed

a c o a l e s c i n g system. A d d i t i o n o f s u r f a c e active materials promotes

t h e f i n e bubb le non-coalesc ing system, b u t f o m a t h i n layer i n the

gas l i q u i d i n t e r f a c e which i n c r e a s e s t h e resis t a n c e to mass

t r a n s f e r .

The o t h e r mechanism o f enhanc ing t h e v o l u m e t r i c mass t r a n s f e r

c o e f f i c i e n t i s t h e t h i n n i n g of the t h i c k n e s s o f t h e l i q u i d boundary

layer ( f i l m ) a round t h e gas. T h i s w i l l d i r e c t l y decrease t h e

r e s i s t a n c e t o mass transfer t h e r e b y i n c r e a s i n g t h e mass t r a n s f e r

c o e f f i c i e n t proper k,. I n many cases the gas boundary f i l m is

c o n s i d e r e d to produce far less r e s i s t a n c e t o mass t r a n s f e r and i s

thus n e g l e c t e d . The overall mass t r a n s f e r c o e f f i c i e n t i s t h u s

approx imate ly equated t o the l i q u i d f i l m mass t r a n s f er c o e f f i c i e n t . Thinning of the l i q u i d f i l m is enhanced by i n c r e a s e o f the a g i t a t i o n

a n d t u r b u l e n c e i n the m i x t u r e .

The second factor which can enhance t h e mass transfer rate i s t h e

d r i v i n g f o r c e o r the d i f f e r e n c e i n c o n c e n t r a t i o n between t h e

s a t u r a t e d l i q u i d f i l m i n c o n t a c t w i t h t h e g a s and t h e bu lk o f t h e

l i q u i d (c' - CL). T h i s is maximised t o maximise t h e mass t r a n s f e r

rate. There i s a maximum for t h e equilibrium c o n c e n t r a t i o n c'. A t

a c e r t a i n temperature, p r e s s u r e and c o n c e n t r a t i o n of the gas i n t h e

g a s m i x t u r e , t h e e q u i l i b r i u m c o n c e n t r a t i o n i s es t a b l i s h e d by i ts

Henry's l a w c o n s t a n t f o r s p a r i n g l y soluble gasses . T h i s a l s o depends

on the presence of other materials, especially i o n i c species i n t h e

s o l u t i o n .

On the o t h e r hand C, is c o n t r o l l e d by process requ i r emen t s , L e . i n

f e r m e n t e r s where a minimum c o n c e n t r a t i o n of t h e dissolved oxygen

c o n c e n t r a t i o n i n the bulk o f t h e l iquid i s r e q u i r e d . CL can be

lowered t o a l m o s t zero f o r fas t chemica l r e a c t i o n s through good

mixing .

Mixing i s r e q u i r e d f o r good mass t r a n s f e r t o carry the s a t u r a t e d

bubbles from nea r the impeller or from the gass ing zone, and replace

them w i t h fresh bubbles and/or b r i n g f r e s h l iquid to t h e r e a c t i o n

zone. Mass t r a n s f e r occurs between the en t r apped g a s i n s i d e t h e

bubble and t h e l iquid around it. F u r t h e m o r e g a s bubbles have

c e r t a i n l i f e t i m e t o reach the s u r f a c e of t h e mixture and b u r s t there

o r c o a l e s c e w i t h o t h e r bubb le s w i t h i n the b u l k of t h e l i q u i d . T h e

more t h e r e c i r c u l a t i o n of t h e gas bubbles i n t h e l i q u i d the m o r e the

usage of the gas contained i n it- Thus, quick escape o f gas bubbles

t o t h e s u r f a c e i s d e t r i m e n t a l t o mass t r a n s f e r .

1.4 Experirnental procedures

I n t h e fo l lowing s e c t i o n measurements of power, gas hold-up,

i nduced static head, rate of gas i n d u c t i o n , rnixing t i m e and mass

t r a n s f e r c o e f f i c i e n t w i l l be d i s c u s s e d . Each is impor t an t i n

e v a l u a t i n g t h e overall performance o f the d i f f e r e n t i m p e l l e r s .

1.4.1 Power

Power i s a primary parameter . O the r performance parameters o f

d i f f e r e n t d e s i g n s are compared at t h e same speed o r power

consurnption, Power may be c o r r e l a t e d with the impeller speed, whi le

o t h e r parameters, e . g . mass t r a n s f er c o e f f i c i e n t , mixing t i m e , g a s

hold-up etc. are a l s o c o r r e l a t e d wi th t h e i m p e l l e r speed. Any

p r o p e r t y can then be c o r r e l a t e d t o power from mathematical

c a l c u l a t i o n s . Ai t e r n a t i v e l y any p r o p e r t y can be directly c o r r e l a t e d

w i t h power £rom s imul taneous measurement o f i t s v a l u e and t h e

agi t a t o r power consurnption.

Power may be rneasured by measuring the torque impar t ed on t h e s h a f t

o f t h e mixer u t i l i z i n g a t o r q u e s e n s o r . Shiue and Wong, 1984,

Greaves and Lob, 1984 and Greaves and K. Kobbacy, 1981 averaged t h e

simultaneous s i g n a l s of t h e t o r q u e around t h e s h a f t and speed ove r

a p e r i o d of time u t i l i z i n g d a t a a c q u i s i t i o n system i n o r d e r t o

r e d u c e t h e n o i s e . Here c o r r e c t i o n f o r f r i c t i o n a l power i s made by

s u b t r a c t i n g t h e f r i c t i o n a l (no h y d r a u l i c load) p o w e r from t h e t o t a l

p o w e r consumption. Another method i s by measur ing t h e t o r q u e

i m p a r t e d on t h e liquid by h o l d i n g t h e ves se1 on a t o r q u e table.

Rewatkar , 1990, Rewatkar , and J o s h i , 1993, Warmoeskerken e t a l . , 1981 and J o s h i , and Sharma, 1977 a p p l i e d t h i s t echn ique .

A l t e r n a t i v e l y power can be measured around t h e motor f o r a c t u a l

p o w e r consumption, a n d s u b t r a c t i n g the f r i c t i o n a l power when t h e

m i x t u r e runs i n a i r to c a l c u l a t e t h e h y d r a u l i c power. U l t ima te ly

p o w e r around t h e m o t o r i s what i s p r a c t i c a l l y consumed.

1.4.2 Induced Static Head

T h e induced static head is a measure o f the i m p e l l e r d r i v i n g f o r c e .

It i s measured f r o m t h e r ead ing of a manometer connected t o t h e

s tand-p ipe when the impeller runs with no access of a i r (gas) t o t h e

stand-pipe, i .e. the suction line is closed as performed by Raidoo

et al., 1987.


The rate of gas induction is a measure of the self gassing capacity

of an impeller in the gas inducing contactor ( G I C ) . The Gas

induction rate is measured from a reading of a rotameter connected

to the suction port. The disadvantage of this method is the decrease

in the flow rate and suction head due to pressure drop in the

rotameter. Wide rotameters with small friction may be used. A

capillarimeter having very low pressure drop was used for the same

purpose by Raidoo, et al. , 1987.

Another method of measuring the gas induction rate, which was

followed in this study, is to connect the suction port to a forced

air main and connect a manometer open to the atmosphere to a hole

near to the suction port. The air flow rate is varied and the

reading of the rotameter connected to the suction line is taken when

the reading in the manometer is zero, which indicates that the

pressure around the impeller, inside the stand-pipe is equal to the

ambient pressure.

1.4.4 Mixing Time

Mixing time is a reversed measure of the homogenization capacity of

an impeller. Mixing time is frequently measured as the time elapsed

to reach a certain degree of homogeneity after introduction of a

tracer as a pulse signal. Homogeneity is clearly important in the

air water system as discussed earlier. Homogeneity is measured by

monitoring the difference in concentration, pH, color, or any other

suitable parameter throughout the agitated volume, or by monitoring

changes of the values of the studied parameter in a certain place,

along the course of time. The tracer used could be a concentrated

salt s o l u t i o n and t he measured p r o p e r t y is t h e c o n d u c t i v i t y . T h i s

t echnique was followed by Kramers e t a l , , 1953, Prochaza and Landau,

1961, Biggs, 1963, Holmes et a l . , 1964, Kafarov e t al,, 1971, Raghav

Rao a n d J o s h i , 1988 and Kramers e t a l , 1953.

The d i s a d v a n t a g e of t h i s method i s t h e i n c r e a s i n g level o f back

n o i s e and l o s s o f s e n s i t i v i t y , u n l e s s Eresh s o l u t i o n i s used f o r

every test ,

A c i d s and b a s e s are used where t h e mon i to red v a r i a b l e is t h e pH,

Norwood and Metzner, 1960 used a photographic method a n d methyl red

i n d i c a t o r based on acid base r e a c t i o n . Bernan and Lahre r , 1976 used

m e t h y l red i n & c a t o r t o de t e rmine t h e mix ing t i m e . I n t h e i r s t u d y ,

as w e l l as i n p rev ious s t u d i e s , t h e r e w a s a n o t i c e d t i m e delay

encountered with the u s e of phenolphtha le in . T h i s w a s a t t r i b u t e d t o

the i n t e r f e r e n c e o f carbon d i o x i d e f rom ambient a i r .

I n t h e pH-indicator method t h e test s o l u t i o n can be used r e p e a t e d l y

a f te r a d j u s t i n g t h e p H t o t h e required s t a r t i n g va lue . Its

d isadvantage is the n o n - l i n e a r i t y o f the response curve, Pand i t and

J u s h i , 1989 used both conductivity and p H e l e c t r o d e s for

measurements o f t h e mixing t i m e . The l i m i t a t i o n o f t h e s e methods

is t h e i n t e r f e r e n c e due t o t h e presence o f the measur ing e l e c t r o d e

and that the measusernent i s done locally.

A thermal method measures t h e t e m p e r a t u r e d i f f e r e n c e between t w o

d i f f e r e n t points i n t he vesse1 after i n t r o d u c t i o n o f a h o t tracer.

This was described by Shiue and Havas, et a l . , 1978, Ahmad, 1985 and

Wong, 1984 who used two thermistors w e r e c apab le o f detecting 0 . 0 1

OC d i f f e r e n c e . The homogenization (mixing) t i m e w a s selected i n this

work as t h e t i m e span frorn t h e s t a r t of t h e a d d i t i o n o f t h e tracer

t o r e a c h a tempera ture d i f f e r e n c e o f less than 5% o f t h e t o t a l

t empera tu re d i f f e r e n c e .

Colorimetry and Photographie method

s o l u t i o n due t o acid, a l k a l i o r

presence of a suitable i r i d i c a t o r .

ana lyses t h e c o l o r change i n the

o t h e r tracer a d d i t i o n i n the

The n e u t r a l i z a t i o n d e c o l o r a t i o n

w a s used by Brennan a n d Lahre r , 1976. Ca r reau et a l . , 1976 u s e d

iodine-sodium t h i o s u l f a t e system as a d e c o l o r a t i o n method f o r

measurements o f t h e mixing t i m e .

Hoogendoorn and D e n Hartog , 1967 u t i l i s e d b o t h t h e d e c o l o r a t i o n

method aided by a m o v i e f i l m a n d t h e the rma l response method t o

measure mixing t i m e i n v i scous f l u i d s f o r dif f e r e n t impellers . T h e y

found t h a t i n most cases bo th methods y i e l d e d comparable r e s u l t s .

The photographic method permits a n a l y s i s o f d i f f e r e n t r eg ions i n t h e

s o l u t i o n a n d a r r i v i n g a t an o v e r a l l mixing t i m e v a l u e . A v ideo o r

cinema camera is used and frames are backplayed s e q u e n t i a l l y f o r

s tudy and d e t e r m i n a t i o n o f t h e e n d p o i n t .

O p t i c a l measurement of t h e refractive index change upon mixing o f

two l i q u i d layers o f d i f f e r e n t d e n s i t i e s was used t o measure t h e

mixing t i m e by Van de Vusse, 1955.

F luorescence t e c h n i q u e was described by Lee and Brodkey, 1963.

Other o p t i c a l methods such as absorbante and chemiluminescence have

been used t o monitor mixing t i m e by Harnby et a l . , 1985 and Walker,

1987.

Hool, 1992 used a scheme based on the e l ec t rochemica l g e n e r a t i o n o f

a chromophore (tri-iodide i o n ) f rom r e a g e n t s p r e s e n t i n t h e t a n k

(po tass ium i o d i d e ) a n d used a c o l o r i m e t e r t o measure o p t i c a l

t ransmiss ion through a s e c t i o n of t h e tank as a c r i t e r i o n f o r mixing

t i m e . The s o l u t i o n is reusab le upon the a d d i t i o n of a redox r e a g e n t

( a s c o r b i c acid) t o c o n v e r t t h e chemical species to t h e s t a r t i n g

c o n d i t i o n .

Shenoy and Toor, 1990 used fiber o p t i c s t o measure the time-averaged

l o c a l c o l o r i n t e n s i t y o f bromothymol b l u e as a measure of

micromixing i n get systems .

The f low-fo l lower method measures t h e c i r c u l a t i o n t i m e which is a

measure o f t h e pumping capacity of t h e i m p e l l e r . C i r c u l a t i o n t i m e

i s r e l a t e d t o mixing t i m e i n such a way t h a t after a number o f

c i r c u l a t i o n s a c e r t a i n degree of homogeneity o r mixing i s ach ieved .

Normally 5 c i r c u l a t i o n s are s u f f i c i e n t f o r e s s e n t i a l l y complete

mix ing , and the t o t a l t i m e o f t h e s e 5 c i r c u l a t i o n s i s c a l c u l a t e d

as t h e mixing t i m e .

I n e r t small plast ic chips w e r e used as f low-fo l lowers by Sykes ,

1965. This method i s limited f o r clear s o l u t i o n s . The c h i p s shou ld

be equibuouyant . Fo l lower s i n c o r p o r a t i n g radio i s o t o p e s have been

described by Aiba, 1958. Th i s method is n o t l i m i t e d by t h e c la r i ty

o f t h e s o l u t i o n .

The Vlow-fo l lowervr method i s good a l s o i n v i s u a l i z i n g the f low

p a t t e r n s i n t h e system. Nienow and Kuobi, 1990 used a T . V . camera

with a r e c o r d e r t o p l ayback and f o l l o w t h e movement o f a s i n g l e

p a r t i c l e f low-f o l l o w e r i n v i scous f l u i d s .

Mixing t i m e v a l u e s depend upon t h e t echn ique u s e d , t h e d imens ions

of system and the p o s i t i o n o f the monitor ing e l e c t r o d e . Care shou ld

be exercised when comparing r e s u l t s based on d i f f e r e n t methods as

t h e i r r e s u l t s may n o t be similar. Aiso, t h e d e f i n i t i o n o f the degree

o f mixedness i s a r b i t r a r y . S ince t h e approach t o f i n a l complete

mix ing at the rnolecular level i s asyrnptot ic , normal ly a c u t - o f f

value is used at Say 90% of t h e f i n a l v a l u e . H o w e v e r t h e r e is no

precise d e f i n i t i o n o f t h i s v a l u e , n o r i s t h e r e g e n e r a l agreement

o f i t s v a l u e . Detailed d i s c u s s i o n o f t h e s e t o p i c s i s p r e s e n t e d by

Bryan t , 1977.


The oxygen mass t r a n s f e r r a t e is of c o n s i d e r a b l e interest

i n d u s t r i a l l y a n d fo r t h i s and o t h e r reasons it was selected as a

measure of the mass t r a n s f e r c a p a b i l i t y of d i f f e r e n t con tac to r s , The

mass t r a n s f e r c o e f f i c i e n t i s measured e i t h e r by a dynamic method or

t h e steady state method. There are many v a r i a n t s of t h e dynamic

gassing-in o r gass ing-out method. D i f f e r e n t step changes were used

as w e l l as d i f f e r e n t mathematical models f o r c a l c u l a t i o n s of the

volumetr ic mass t r a n s f e r c o e f f i c i e n t KLa. The vo lumet r i c mass

t r a n s f e r coef f î c i e n t d e f i n i t i o n is c o n s i s t e n t w i t h a volumetr ic

i n t e r f a c i a l area having a dimension o f m? For s i m p l i c i t y , i n gas

l iquid c o n t a c t i n g , t h e volurnetr ic mass t r a n s f e r c o e f f i c i e n t i s

f r e q u e n t l y ref erred t o as t h e mass t r a n s f e r c o e f f i c i e n t . Fur the r ,

o f t e n the g a s f i l m r e s i s t a n c e i s n e g l i g i b l y small and usually

i g n o r e d . Thus, t h e liquid f i l m mass t r a n s f e r c o e f f i c i e n t k,a i s

loosely equated w i th t h e o v e r a l l mass t r a n s f e r c o e f f i c i e n t KLa .

1.4.5.1 The Dynamic method

I n t h e gass ing-out method, t h e 1 is deoxygenated and t h e

response of a polarographic oxygen e l e c t r o d e ( t h e Clark cell) i s

monitored f o l l o w i n g a s t a r t of t h e a i r o r oxygen E l o w . This

procedure w a s fol lowed by Wise, 1951.

Bandyopadhyay e t al., 1967 f i rs t proposed t h e method based on

c a l c u l a t i n g k,a f r o m t h e s l o p e of a p l o t o f the l oga r i thm o f the

e l e c t r o d e r ead ing versus t i m e . This method does not make allowance

f o r e l e c t r o d e response o r t h e gas d p a m i c s .

Van de Sande, 1974 and Shioya and Dunn, 1979 c a l c u l a t e d t h e f irst

moment of t h e impulse response curve from the area above t h e s t e p

response cu rve . Thus, a c o r r e c t i o n f o r c a l c u l a t i n g kLa can be

de termined .

Other a u t h o r s applied more complicated models for d i f f u s i o n .

Reference is made to Heineken, 1970 and 1971, Linek, 1972, and Linek

e t a l . , 1973.

Dunn and E inse l e , 1975 and Dang e t a l . , 1977 worked o u t c o r r e c t i o n

graphs a n d models to account f o r t h e gas phase r e s i d e n c e t i m e .

Heineken, 1971 a l t e r n a t e l y sparged a i r and n i t r o g e n i n water w h i l e

m a i n t a i n i n g c o n s t a n t gas f l o w rate and de te rmined k,a form the

r e s p o n s e curve o b t a i n e d f rom a step change o f t h e oxygen

c o n c e n t r a t i o n . The c a l i b r a t i o n curve was o b t a i n e d by an a l t e r n a t e

s t e p change from zero oxygen t o s a t u r a t i o n c o n d i t i o n .

Linek and Vacek, 1976 used an enzyme catalyst f o r g l u c o s e o x i d a t i o n

and monitored the e x p o n e n t i a l change of t h e d i s s o l v e d oxygen u s i n g

a p o l a r o g r a p h i c oxygen e l e c t r o d e .

Dang et a l . , 1977 a p p l i e d a l i n e a r mode1 f o r a n a l y s i s o f the

e l e c t r o d e r e sponse , thus o b t a i n e d t h e k,a .

Linek and Benes, 1977 d i scussed a sideway lateral d i f f u s i o n over the

e l e c t r o d e f i l m and proposed a l i n e a r combinat ion o f two d i f f u s i o n

r eg ions .

Linek, 1977 d i s c u s s e d t h e error i n t r o d u c e d due t o n e g l e c t i n g t h e

s t a r t - u p pe r iod as w e l l as t h e i n t e r f e r e n c e o f gas bubbles wi th t h e

e l e c t r o d e r ead ings .

Chapman, et al., 1982 a p p l i e d a s t e p change i n t h e i n l e t gas

c o n c e n t r a t i o n f o r t h e i r s t u d y on k,a. Both l i q u i d and g a s response

measurements were t aken . The method is t h u s free of t h e error

encountered w i t h the gas r e s i d e n c e tirne d i s t r i b u t i o n .

Mignone and E r t o l a , 1984 and Mignone, 1990 applied a sudden change

i n t h e a g i t a t i o n speed and c a l c u l a t e d kLa u s i n g a first moment

m e thod .

Yoshida e t al., 1996 a p p l i e d t h e dynarnic g a s s i n g o u t method t o

determine kLa u s i n g t h e e q u a t i o n :

T h i s i s W i s e ' s , 1951 e q u a t i o n which does n o t correct f o r t h e

electrode r e sponse .

Values of kLa o b t a i n e d were c o r r e c t e d f o r t h e t i m e l a g o f t h e

oxygen e l e c t r o d e a c c o r d i n g t o t h e f i r s t o r d e r response delay by

Rucht i et al. , 1981 a c c o r d i n g t o equat ion :

F u r t h e r t h e v o l u m e t r i c m a s s t r a n s f e r c o e f f i c i e n t based on t h e

d i s p e r s i o n volume was c a l c u l a t i n g accord ing to:

I n t h e s e calculations mass t r a n s f e r from t h e head space w a s

neglec ted . I n a c t u a l s i t u a t i o n s t h e r e are o t h e r r e s i s t a n c e s t o t h e

mass t r a n s f e r , e - g . t h e r e s i s t a n c e of the l iquid f i l m cove r ing t h e

p l a s t i c membrane. Aiso, t h e r e are i n t e r f e r e n c e s due t o t h e h i t t i n g

o f t h e g a s b u b b l e s w i t h t h e membrane l e a d i n g t o an electrode

response somewhere between t h e s a t u r a t i o n c o n c e n t r a t i o n at t h e

boundary o f t h e gas bubb le and t h e bulk d i s s o l v e d c o n c e n t r a t i o n .

Account should be made f o r t h e e l e c t r o d e response i f the rneasurement

is dynamic. These issues

a l , , 1987, Votroba e t a l

w e r e d iscussed by Heineken, 1970, Linek e t

, , 1977 and 1978, Vacek, 1978 and o t h e r s .

Linek e t al., 1987 showed t h a t t h e on ly c o r r e c t v a r i a n t o f t h e

dynamic method is the when pure oxygen is introduced as a step i n p u t

t o t h e l i q u i d b a t c h freed Erom any d i s s o l v e d gas.

1.4.5.2 The Steady State Method

This method relies on t he absorp t ion of oxygen from t h e gas phase

and simultaneously and e q u i v a l e n t l y removing it from t h e l i q u i d

phase through a chemical o r enzymatic r e a c t i o n .

In the steady state semibatch chernical method an oxid izable chemical

i s in t roduced a t t h e s ta r t , then oxygen o r air i s cont inuous ly

a g i t a t e d i n t o the l i q u i d where t h e decrease i n the concen t ra t ion o f

t h e chemical is monitored with t i m e , hence k,a is ca lcu la ted . Sodium

s u l f i t e may be used as the o x i d i z a b l e chemical. The r e a c t i o n i s

complex. However , t h e o v e r a l l r e a c t i o n proceeds according to t h e

equa t ion :

I n t h e sodium s u l f i t e method test starts w i t h a def i n i t e

c o n c e n t r a t i o n o f sodium s u l f i t e and t h e d e p l e t i o n i n i t s

c o n c e n t r a t i o n i s monitored with t i m e under a g i t a t i o n and a e r a t i o n

c o n d i t i o n s . The method w a s f i rst proposed by Cooper e t a l . , 1 9 4 4

and followed by Scholtz and Gaden, 1956, Yoshida e t a l . , 1960, Yagi

and Inoue, 1962, Barron and O'Hern, 1966, D e W a a l and Okeson, 1966,

Linek and Tvrdik , 1971, Chen and Barron, 1972, and Bengtsson, and

B j e r l e , 1975.

External ti t r a t i o n of aliquots is required a t i n t e r v a l s t o e s t a b l i s h

the rate of oxygen consumption. k,a is c a l c u l a t e d according to t h e

e q u a t i o n :

kLa = (Moles of ~ 0 , ~ - at tl - Moles o f ~ 0 , ~ - at t2) / (2VL(C* - CL) * a t )

The method c l a k an o v e r a l l mass t r a n s f e r c o e f f i c i e n t , neve r the l e s s

a l i q u o t s for t i t r a t i o n are taken practically from smal l r e g i o n s i n

t h e vesse1 which, p r e c i s e l y produces r e g i o n a l o r l o c a l kLa.

Mehta and Sharma, 1971 u t i l i s e d a system b a s e d on CO2 a b s o r p t i o n i n

d i f f e r e n t a lka lonamine mediums to measure k,a.

Sodium s u l f i t e system and o t h e r chemical methods based on carbon

d i o x i d e a n d i n o r g a n i c a l k a l i , o r amine as w e l l as esters and

a l k a l i n e s o l u t i o n s w e r e i n v e s t i g a t e d and surranarized by Sharma and

Danckwerts, 1970 who cited 67 r e f e r e n c e s ,

Matsumura and Masunaga, 1979 deve loped a method based on s u l f i t e

o x i d a t i o n where t h e change i n t h e p r e s s u r e i n t h e head space o f a

closed r e a c t o r was used t o c a l c u l a t e k,a.

S t r i c t c o n d i t i o n s should be observed t o ensure t h a t t h e r e a c t i o n is

fas t enough so t h a t the c o n t r o l l i n g step is t h e phys i ca l absorp t ion .

On t h e o t h e r hand it s h o u l d not be so f a s t as t o cause a r e a c t i o n

a t t h e w a t e r a i r i n t e r f a c e , thus exc ludes the p h y s i c a l a b s o r p t i o n

f r o m c o n t r o l l i n g the rate of its consumption. t i n e k and Vacek, 1981

reviewed t h e s e and o t h e r requi rements and cited 1 4 1 r e f e r e n c e s .

S i c k e t a l . , 1983 s t u d i e d t h e o x i d a t i o n o f hyd raz ine as a system

t o measure k,a.

Ogut and Hatch, 1988 used t h e s u l f i t e o x i d a t i o n method t o de te rmine

k,a i n b o t h Newtonian and non-Newtonian f l u i d s .

Sharma, 1993 p r e s e n t e d a n e x c e l l e n t d i s c u s s i o n and cited 1 4 3

r e f e r e n c e s on many mul t iphase r e a c t i o n s and t h e i r u t i l i s a t i o n f o r

t h e d e t e r m i n a t i o n of k,a,

R e s u l t s o f t h e chemical method are g e n e r a l l y h i g h e r t h a n t h e

phys i ca l method. This is because of the chemical enhancement, which

i s d u e t o t h e o c c u r r e n c e o f some r e a c t i o n at t h e s u r f a c e . This

should be kept minimum. The o t h e r reason i s the change i n t h e w a t e r

system c o n t a i n i n g h i g h c o n c e n t r a t i o n of i o n s from t h e l a r g e g a s

bubble s i z e t o small bubble s i z e .

The steady state cont inuous f e e d chernical method (SCFCM) alleviates

t h e drawbacks o f the chemical steady state semibatch method. The

c o n c e n t r a t i o n of the i o n s i n s o l u t i o n i s k e p t very small and t h e

effect on t h e l i q u i d - g a s system i s t h u s n e g l i g i b l y small. This

method was u t i l i s e d by Imai and Takei , 1987 and Linek e t a l . , 1990.

Reference i s made a l s o t o Alves and Vasconcelos , 1993 .

The method i s simple and there is no need f o r e x t e r n a l t i t r a t i o n o f

samples . Chemical consumption is low compared t o t h e semibatch

method and t h e r e i s no need t o treat the water b e f o r e dumping it t o

t h e sewer as it c o n t a i n s low concen t r a t ions o f sodium s u l f a t e (less

than 5000 ppm). Th i s method w a s adop ted for t h e p r e s e n t s t u d y .

A l t e r n a t i v e l y t h e i n t e r f a c i a l area a and t h e mass t r a n s f e r

c o e f f i c i e n t k, can be determineci s e p a r a t e l y and t h e v o l u m e t r i c m a s s

t r a n s f e r c o e f f i c i e n t i s t h u s c a l c u l a t e d by m u l t i p l y i n g t h e s e two

pa rame te r s . Westerterp e t a l . , 1963 i n v e s t i g a t e d CO, and s u l f i t e

systems a s chemical methods f o r t h e determinat ion of the i n t e r f a c i a l

area.

The effect o f the chemical abso rp t ion rate on t h e i n t e r f a c i a l area

was i n v e s t i g a t e d by Linek and Mayrhoferova, 1969.

Robinson and Wilke, 1974 studied the concurren t chemical a b s o r p t i o n

of CO, and d e s o r p t i o n of oxygen i n the sea rch f o r d e t e r m i n a t i o n o f

k, a n d i n t e r f a c i a l area. However t h e i r conc lus ion o f t h e d e c r e a s e

of k, with the i n c r e a s e of power w a s i n c o r r e c t as shown by P r a s h e r ,

1975.

Matheron and Sandall , 1979 i n v e s t i g a t e d and compared t h e effective

area for p h y s i c a l and chernical a b s o r p t i o n .

S r i d h a r and P o t t e r , 1978 used t h e l i g h t t r a n s m i s s i o n t echn ique t o

de te rmine the i n t e r f a c i a l area. Aï T a w e e l e t a l . 1 9 8 4 , expanded t h e

a p p l i c a b i l i t y of t h e method.

1.4.5.3 The Gas Balance Method

The g a s (oxygen) b a l a n c e method has t h e u n i v e r s a l advantage o f

measu r ing an o v e r a l l ( b u l k ) mass t r a n s f er c o e f f i c i e n t compared t o

a n y method u t i l i z i n g t h e oxygen c o n c e n t r a t i o n probe o r ti t r a t i o n

a l i q u o t s which w i l l measure the l o c a l oxygen c o n c e n t r a t i o n o r l o c a l

sodium s u l f i t e c o n c e n t r a t i o n and consequen t ly the l o c a l mass

t r a n s f e r c o e f f i c i e n t .

The p r e s e n c e o f t h e oxygen e l e c t r o d e a l t e r s t h e e f f i c i e n c y o f

mix ing and i n t e r f e r s w i t h t h e hydrodynamics o f t h e system. The

e l e c t r o d e a c t s as a b a f f l e and may increase t h e power consumption

and t h e m a s s t r a n s f e r c o e f f i c i e n t i n s m a l l bench scale tests- On

s c a l e - u p t h e i n t e r f e r e n c e due t o t h e 02 e l e c t r o d e w i 1 1 n o t b e

apprec i ab le and the a c t u a l mass t r a n s f e r c o e f f i c i e n t might be less

than what was expected. I n the gas ba lance method t h i s i n t e r f e r e n c e

is e l i rn ina t ed .

The gas b a l a n c e method i s faster than t h e conven t iona l t i t r a t i o n

method. The method i s i n s e n s i t i v e t o t h e mode o f gas phase f l o w

( w e l l mixed o r p lug f low or i n be tween) . The l a s t assumption is

v a l i d when t h e dif f e r e n c e between t h e oxygen c o n c e n t r a t i o n i n the

i n l e t and out le t flow i s small which is encountered i n many

a p p l i c a t i o n s ,

The d i f f i c u l t y with the gas balance method is that it requires very

s e n s i t i v e measurement o f t h e c o n c e n t r a t i o n of oxygen i n t h e i n l e t

as w e l l as i n the o u t l e t gas. This needs p r e c i s e measurements o f the

gas flow rate, temperature and pressure. Also, it i s n e c e s s a r y t o

p r o v i d e a well-mixed head space above the l i q u i d . C o r r e c t i o n

c a l c u l a t i o n s may be required t o account f o r t h e l a g i n t h i s r e g i o n .

These are n e c e s s a r y t o a c c u r a t e l y c a l c u l a t e t h e rate of oxygen

consumption.

The gas ba lance method was u t i l i s e d i n s u l f i t e ox ida t ion system with

gas a n a l y s i s as described by Siege l1 and Gaden, 1962, Dussab e t al.,

1985, Ogut and Hatch, 1988 and Denis e t a l . , 1990.

The governing e q u a t i o n i s :

The same p r i n c i p l e s apply t o what i s called t h e oxygen t r a n s f e r

rate method. Mukhopadhyay and Ghosw, 1976 and Cor r i eu e t al., 1976

r e p o r t e d some details a b o u t t h i s rnethod.

V a n ' t Riet, 1979 reviewed and compared d i f f e r e n t methods f o r k,a

measurements and cited 76 r e f e r e n c e s . Linek et a l . , 1982 p u t t h e

comparison between t h e dynamic and steady state methods on a

t h e o r e t i c a l as w e l l as exper imental bases and discussed the reasons

f o r d i s c repancy .

1.4.5.4 T h e Oxygen Electrode

I t i s a normal practice t o use a Clark p o l a r o g r a p h i c ce11 o r a

ga lvan ic ce11 t o measure t h e d i s s o l v e d oxygen c o n c e n t r a t i o n e i t h e r

i n i ndus t ry o r i n l a b o r a t o r y experiments. The Clark ce11 i s composed

of a nob le m e t a l e l e c t r o d e which i s made n e g a t i v e with respect t o

a s u i t a b l e r e f e r e n c e e l e c t r o d e (calomel o r ~ g / A g ~ l ) i n a n e u t r a l

potassium c h l o r i d e so lu t ion by applying b i a s v o l t a g e . The d i s s o l v e d

oxygen i s reduced at the su r face of t h e ca thode . I n t h e p l a t e a u

region of the polarogram (current versus b i a s v o l t a g e ) r e d u c t i o n of

oxygen i s so fast t h a t t h e rate o f r e a c t i o n i s limited by t h e

d i f f u s i o n o f oxygen t o the cathode su r face . The cathode, anode and

the e l e c t r o l y t e are separated from the measuring medium by a p l a s t i c

membrane which is permeable t o oxygen gas b u t i s o l a t e s o t h e r i o n s ,

In t h e p l a t e a u region the r e s i s t ance t o mass t r a n s f e r i s mainly due

t o d i f f u s i o n i n t h e p l a s t i c membrane t h e p l a t e a u c u r r e n t is thus

p r o p o r t i o n a l t o t he a c t i v i t y o r p a r t i a l p r e s s u r e ( t e n s i o n ) of

oxygen .

The g a l v a n i c ce11 e l e c t r o d e does n o t r e q u i r e an e x t e r n a l vo l t age

source t o reduce oxygen at the cathode. When a r e l a t i v e l y b a s i c

m e t a l such as z i n c is used as the anode and a r e l a t i v e l y noble metal

such as g o l d i s used as t h e cathode, t h e v o l t a g e genera ted by t h e

e l ec t rode p a i r i s suf f i c i e n t t o spontaneously reduce oxygen a t t h e

cathode surface. I n t h i s ce11 t h e anode s u r f a c e i s g r a d u a l l y

oxid ized and t h e r e is a l i m i t e d l i f e of t h e probe.

Details o f theory as w e l l a s t h e c o n s t r u c t i o n and o p e r a t i o n of

d i s s o l v e d oxygen e l e c t r o d e s are found i n t h e art icle of Lee and

Tsao, 1979.

1.5 O t h e r Parameters

Gas Hold-up

Measuring methods of gas hold-up i n c l u d e s i m p l e he igh t measurement

( v i s u a l obse rva t ion) which can be a i d e d by t h e use of a float and

by measurement at d i f f e r e n t l o c a t i o n s and t a k i n g t h e average .

Other methods rely on instruments , mainly conduct iv i ty cells, as was

p e r f ormed by Faust, 1 9 4 4 . I n t h e improved v e r s i o n of t h i s method

a number of conduct iv i ty e l e c t r o d e s are i n s e r t e d i n t h e mixture and

connected t o a p rocess ing u n i t (computer) t o g e t an average o f t h e

s i g n a l va lues over t i m e .

K a w e c k i et a l . , 1967 and Loiseau e t al-, 1977 used an overflow

method . I n the overf low method desc r ibed by Warmoeskerkin, e t a l ,

1981 t h e l e v e l of the l i q u i d s u r f a c e i n the a g i t a t e d v e s s e l w a s

ma in ta ined by connect ing a small q u i e s c e n t tank t o t h e main

c o n t a i n e r and a r e c i r c u l a t i o n pump. They c l a i m e d s e n s i t i v e and

a c c u r a t e measurements a s s o c i a t e d w i t h t h i s technique .

O t h e r methods used p r e s s u r e tapping a t two p o i n t s by Yoshida and

Miura, 1963, t r a c e r techniques by Matsumura et a l . , 1978, u l t r a s o n i c

by Buja lsk i e t al., 1988 and Machon e t a l . , 1991. A surranary of these

methods is presented by Rewatkar, e t a l . , 1993 who j u s t followed the

v i s u a l obse rva t ion method .

Bubble S i z e

The i n t e r f a c i a l area can be ca lcu la ted from determinat ion of t h e gas

hold-up and t h e average bubble s i z e - T o w e l l e t a l . , 1965 u t i l i s e d

a photographic technique t o determine bubble s i z e . This was followed

by Koets ier and Thoenes, 1972. Kaweck e t a l . , 1967 connected a small

v e s s e l at the i m p e l l e r level and photographed t h e bubbles whi le

r i s i n g i n the small v e s s e l . D i r e c t s u c t i o n and measurement i n a

c a p i l l a r y have been performed by Reith and Beek, 1970, Todtenhaupt,

1971 and Van de Sande, 1974. Ryan, E . J . , 1972 used culminated l i g h t

t o photograph the g a s bubbles i n bubble columns.

2 . THEORETICAL BACKGROUND

2.1 Impeller Design

I m p e l l e r des ign i s a combination of science and a r t . Many shapes

c o u l d be tested i n s e a r c h f o r t h e optimum performance i n t h e

s p e c i f i c s i t u a t i o n . When i m p e l l e r s are made o u t of composite

material, a n a l m o s t i n f i n i t e s e l e c t i o n of shapes i s poss ib le . T h e

Rushton disc s t r a i g h t blade t u r b i n e impeller was t h e s t andard shape

f o r long tirne. Th i s i m p e l l e r s u i t s gas l iqu id d i s p e r s i o n where t h e

gas is introduced from t h e bottom of t h e vesse1 . However, t h e disc

may i n h i b i t gas induct ion i n G I C f s where the gas i s in t roduced from

t h e top.

I n Our case, where a G I C was u t i l i z e d t o s tudy the air-water system,

new i m p e l l e r des igns were i n t roduced i n search f o r better

performance.

The w i d e s t r a i g h t blade i m p e l l e r surrounded by a shroud (pe r fo ra ted

b a f f l e d c y l i n d e r ) w a s known t o be very effective i n s t r i p p i n g

hydrogen s u l f i d e from contaminated gas by Smith, e t a l . 1994 and

1995. The same impe l l e r was tested i n t h e p r e s e n t s tudy t o

i n v e s t i g a t e its performance i n t h e a i r - w a t e r system and compare i t

to o t h e r des igns .

One o f t h e d e s i g n s i s a modi f i ca t ion t o t h e s t r a i g h t blade wide

i m p e l l e r where t h e upper end i s curved to perform dom-pumping i n

a concave shape. Down-pumping was stated by Raghav Rao and J u s h i ,

1988, t o be more effective i n mixing i n comparison with the s t r a i g h t

b lade impeller. The down-pumping impel le r was a l s o reported to have

h i g h e r rate of gas i nduc t ion (Raidow et a l . , 1987 and Mundale and

Jushi , 1995) .

Another modi f i ca t ion s tud ied , w a s t h e opposi te t o t h e p rev ious

m o d i f i c a t i o n . The bottom end of t h e s t r a i g h t blade i m p e l l e r was

curved t o induce up-pumping c u r r e n t s . This f e a t u r e resembles a

s t r a i g h t b l a d e upper row and a p i t ched Made up-pumping lower row

of blades. The pitched blade shape w a s r e p o r t e d t o have better

rnixing and gas dispersion t han t h e s t r a i g h t b l a d e impeller (Raghav

Rao and J o s h i , 1988 and Raghav Rao et a l , 1987).

A t h i r d main v a r i a t i o n was the double b lade i m p e l l e r . T h e upper row

was a p i t c h e d s t r a i g h t blade while t h e bottom r o w was a concave

hol low type. Th i s combination was in t roduced i n the p r e s e n t s t u d y

i n sea rch f o r a combination between t h e effects o f b o t h t h e p i t c h e d

b lade and t h e hollow type. Dif fe ren t mul t ip l e i m p e l l e r systems w e r e

reported t o have greater gas induct ion rate compared t o s i n g l e b lade

systems (Saravanan and J u s h i , 1995) . Warrnoeskerken and Smith, 1989

repor ted t h a t the hollow blade impel ler had b e t t e r gas handl ing and

d i s p e r s i o n compared t o t h e straight b l a d e i m p e l l e r .

The f o u r t h m o d i f i c a t i o n was t h e s t r a i g h t b l a d e wi th a c u t - o f f . It

i s known t h a t t h e power consumption o f an i m p e l l e r i s f u n c t i o n o f

its diameter and width. This impe l l e r was s t u d i e d mainly i n s e a r c h

f o r lower power consumption .

Mass Transfer Coefficient

D i f f e r e n t models have been proposed t o expla in mass t r a n s f er between

gas and l i q u i d i n a g i t a t e d v e s s e l s . T h e main models are:

2.2.1 The Film Mode1

A s f irst proposed by Whitman, 1923, t h e mode1 p i c t u r e s a s t a g n a n t

d i f f u s i o n f i l m of thickness 6 where mass t r a n s f e r occurs o n l y by

molecular d i f f u s i o n , i . e. t h e r e i s no convect ion - The bulk of the

l i q u i d is k e p t uniform i n composit ion by a g i t a t i o n . G a s

concentration i n the f i l m fal ls f rom C' ( e q u i l i b r i u m concen t ra t ion )

at i t s surface subjected t o the gas t o CL at the i n n e r edge. Then;

and

The hydrodynamic c h a r a c t e r i s t i c s o f the system are accounted f o r by

t h e parameter 6, which depends on the geometry, l i q u i d a g i t a t i o n and

physical p r o p e r t i e s . This model is simple b u t is n o t very r e a l i s t i c .

The idea o f the sharp discontinuity near the s u r f a c e i s

inconceivable .

S t i l l Surface Model

T h i s model assumes p r o g r e s s i v e t r a n s i t i o n £rom p u r e l y molecular

d i f fus ion to predominantly convective t r a n s p o r t as t h e d i s t ance from

the surface i n c r e a s e s . According t o King's s t i l l s u r f a c e model

(King, 1966) the t r a n s p o r t i s a sum of t h e molecular d i f f u s i o n and

eddy h f f u s i o n which is propor t iona l t o some power n of the d i s t ance

from the s u r f a c e . This model p rov ides an e x p r e s s i o n f o r k, which

con ta ins two parameters r e l a t i n g t o t h e hydrodynamic cond i t ions .

2.2.3 Surface rejuvenation Model

The s u r f a c e r e j u v e n a t i o n model is a s t i l l s u r f a c e model suggested

by Dankwerts, 1955, and Andrew, 1961. According t o t h i s model

d i f f u s i o n t a k e s p l a c e f o r a pe r iod at a d imin i sh ing rate i n t o a

s tagnant l iquid sur face . A convective d i s tu rbance then rep laces t h e

liquid up t o a c e r t a i n depth beyond t h e s u r f a c e by l i q u i d of t h e

bulk concentrat ion. Quiescent d i f f u s i o n recommences w i t h a s teeper

concen t ra t ion g r a d i e n t resenibling the c o n d i t i o n a t t h e start.

T r a n s p o r t p roceeds at a d imin i sh ing rate u n t i l a new d i s t u r b a n c e

o c c u r s , and s o on. I t c o n t a i n s two pa rame te r s t o describe t h e

hydrodynamic features o f t r a n s f e r corresponding t o t h e t h i c k n e s s o f

t h e u n d i s t u r b e d layer and t h e r a t e of r e j u v e n a t i o n .

2 . 2 . 4 Surface Renewal Models

F e a t u r e s o f t h e s e models are l i k e t h e s u r f a c e r e j u v e n a t i o n . The

d i f f e r e n c e is t h a t t h e y c o n s i d e r t h e a b s o r p t i o n layer as though it

w e r e q u i e s c e n t and i n f i n i te ly deep , t h u s r e d u c i n g t h e hydrodynamic

p a r a m e t e r s t o one which i s t h e frequency of renewal o f t h i s

h y p o t h e t i c a l l y i n f i n i t e l y deep layer. The form o f t h e s u r f a c e -

renewal model o r i g i n a l l y proposed by Higbie, 1935 assuxnss t h a t every

element of t h e s u r f a c e is exposed t o the gas f o r t h e same l e n g t h o f

t i m e 0 , b e f o r e b e i n g r e p l a c e d by l i q u i d o f t h e bu lk compos i t ion .

Mathematical d e r i v a t i o n y i e l d s :

Where 0 i s the l e n g t h of t i m e the element of t h e s u r f a c e i s exposed

t o t h e gas and it i s the o n l y parameter a c c o u n t i n g for system

hydrodynamics ,

T h i s seems u n r e a l i s t i c and Dankwerts, 1951 supposed a n o t h e r model

where t h e chance o f an element of surface be ing replaced w i t h f r e s h

l i q u i d i s independent o f t h e l e n g t h o f t i m e fo r which i t h a s been

exposed. Mathematical d e r i v a t i o n y i e l d s an average rate R equal to:

Where s is the s i n g l e parameter accounting f o r t h e hydrodynamics o f

the system and h a s t h e dimension o f a r e c i p r o c a l t i m e .

The i n f i n i t e l y deep layer is a g a i n inconceivable . Many modi f i ca t ions

t o t h i s system have been bas& on a f i n i t e t h i c k n e s s of t h e s u r f a c e

layer as f o l l o w s ;

2 - 2 3 Definite-Thickness Surface Renewal Models

Dobbins, 1956 and Toor and Marche l lo , 1958 have proposed a f i l m

renewal model i n which a s t a g n a n t f i l m of d e f i n i t e t h i c k n e s s ( l i k e

t h a t i n t h e f i l m model) ex is t s at t h e s u r f a c e , b u t is replaced

piocewise frorn time t o t i m e by l i q u i d having t h e bu lk compos i t ion .

Marchello and Toor, 1963 have proposed a m o d e l i n which t h e l i q u i d

i n a f i l m of d e f i n i t e t h i c k n e s s i s mixed t o uniform c o n c e n t r a t i o n

a t i n t e r v a l s . I n b o t h models k, i s determined by two parameters

c h a r a c t e r i s t i c o f t h e hydrodynamics .

2.2.6 Other Models

Kish inevsk i i e t a l . , 1949 , 1 9 5 5 and 1956 have developed a model i n

which t u r b u l e n c e i s supposed t o ex tend t o t h e i n t e r f a c e , t h e rate

of a b s o r p t i o n b e i n g de te rmined by a combination of molecu la r a n d

eddy d i f f u s i o n ,

For tescue and Pearson, 1967 have s o l v e d t h e e q u a t i o n f o r d i f f u s i o n

i n t o a s u r f a c e comprising a r e g u l a r system of e d d i e s , These r e g u l a r

eddies can , s o m e t i m e s , be c h a r a c t e r i z e d f r o m t h e macroscopic

f e a t u r e s of the system. The mass t r a n s f e r c o e f f i c i e n t k, is related

t o t h e s e eddies and consequen t ly can be c a l c u l a t e d .

The two q u e s t i o n s here are: which model more closely r e p r e s e n t s t h e

a c t u a l case? And, which model predicts realist ic values ? One migh t

assume that t h e model d e s c r i b i n g real i ty more c l o s e l y is t h e one

which g i v e s more accurate p r e d i c t i o n s . There i s no w a y , f o r now, t o

i n v e s t i g a t e and d e f i n e t h e r e a l s i t u a t i o n . T h i s i s because o f its

extreme complexity and f o r the p r o b a b i l i s t i c n a t u r e of the phenomena

on t h e mic rosca le . Thus, t h e o n l y c r i t e r i o n w i l l be based upon

conf ormi ty wi th exper imenta l r e s u l ts .

If w e knew f o r s u r e what is happening, then an a c c e p t a b l e approach

might be possible by c o r r e l a t i n g the r e l evan t parameters. Yet models

are requi red t o put th ings i n a s imple forrn which may n o t r e p r e s e n t

the a c t u a l phys ica l system b u t are easy to s o l v e with t h e r e q u i r e d

accuracy. The best m o d e l would be the one which satisfies t h e need

f o r s i m p l i c i t y and accuracy over a wide range o f a p p l i c a t i o n s o r

which makes the b e s t compromise o u t of t h e s e .

Some models might s u i t s p e c i f i c c o n d i t i o n s better than o t h e r s , and

one model might succeed i n a s p e c i f i c des ign . I n some o t h e r cases

dif f e r e n t models based on two dif f e r e n t concepts might e q u a l l y w e l l

p red ic t gas- l iquid mass t r a n s f e r . Actual ly p r e d i c t i o n s based on t h e

Higbie and Davidson and For tescue models are c l o s e l y similar i n

most cases.

I t t u r n s o u t t o be a mat te r o f convenience when s e l e c t i n g t h e

governing model for c a l c u l a t i o n s as shown by Dankwerts, 1970.

Difference i n p r e d i c t i o n becomes s i g n i f i c a n t when t r e a t i n g v iscous

so lu t ions where d i f f u s i v i t y i s low. D i f f e r e n t models differ i n t h e

mass t r a n s f e r c o e f f i c i e n t f u n c t i o n with regard t o d i f f u s i v i t y . I n

the f i l m model the mass t r a n s f e r c o e f f i c i e n t i s p r o p o r t i o n a l t o t h e

d i f f u s i v i t y while i n t h e s u r f a c e renewal model t h e p r o p o r t i o n a l i t y

is t o i t s square r o o t .

2 . 3 Oxygen-Sodium Sul f i t e Sys t e m

2.3.1 General

The r e a c t i o n between oxygen and sodium s u l f i t e proceeds according

t o t h e equa t ion :

The r e a c t i o n is catalyzed by Cu (2+) o r Co (2+) t o be f a s t enough t o

be limited by p h y s i c a l absorp t ion , thus keeping t h e bu lk

c o n c e n t r a t i o n of oxygen a t zero . On t h e o t h e r hand care should be

observed n o t t o reach a very high rate where t h e r e a c t i o n can

proceed i n t h e d i f f u s i o n a l f i l m .

The r e a c t i o n has been used as a t o o l t o s tudy gas l i q u i d a b s o r p t i o n

and t o determine kLa, which t r a n s l a t e s t o how effective is t h e

d e s i g n of t h e gas-liquid r e a c t o r . According t o t h e p r e v i o u s l y

mentioned assumptions, and with negl igence o f the gas side

r e s i s t a n c e the r a t e o f oxygen uptake p e r u n i t volume o f the s o l u t i o n

i s k,a&*, where a i s t h e i n t e r f a c i a l a r e a per u n i t volume o f

d i s p e r s i o n .

The r e a c t i o n has very cornplex k i n e t i c s and t h e o r d e r d i f f e r s

according t o concen t ra t ion . Aiso , t h e rate of t h e r e a c t i o n depends

h e a v i l y on t h e p H of the s o l u t i o n . I n t h e semibatch system the

experiment starts with a c e r t a i n concentrat ion of sodium s u l f i t e i n

the s o l u t i o n which should n o t fa11 below a c e r t a i n v a l u e a t t h e end

of the test t o keep the k i n e t i c s within the r e q u i r e d v a l u e s . The p H

should be kep t c o n s t a n t du r ing t h e course of t h e r e a c t i o n t o

main ta in c o n s t a n t rate.

2.3.2 Continuous Feed System

The p r e s e n t system opera te s a t s t e a d y state w h e r e t h e r e i s a

continuous feed o f sodium s u l f i t e s o l u t i o n and an equal cont inuous

d ra in o r overflow. This f e a t u r e permits keeping d i s s o l v e d oxygen i n

the bulk o f the l i q u i d a t a c e r t a i n v a l u e CL. From t h e known sodium

s u l f i t e feed rate and t h e measured CL t h e mass transfer c o e f f i c i e n t

kLa i s c a l c u l a t e d .

I n t h i s method there i s no need t o cons ider the response l a g of the

02 electrode, o r t h e response lag due to g a s phase mixing. E'urther

i n t h i s system t h e i o n i c c o n c e n t r a t i o n is small and i t s effect on

the hydrodynarnics could be neglec ted . Also , details on the k i n e t i c s

of the r e a c t i o n are no t required.

The chernical enhancement f a c t o r E is a f u n c t i o n of the dimensionless

term y. y is c a l c u l a t e d from the equa t ion :

I t i s requ i red t o keep y less than 0 . 1 , so t h a t E approximates 1.

Under t h e s e c o n d i t i o n s the chemical r e a c t i o n i n t h e gas f i l m i s

n e g l i g i b l e and t h e r e a c t i o n occurs o n l y i n t h e bu lk o f the l i q u i d .

From the equat ion , and the o r d e r and rate of the o x i d a t i o n r e a c t i o n

a maximum concen t ra t ion of t h e s u l f i t e i o n i n s o l u t i o n i s

c a l c u l a t e d t o be 5.5 mol/ m3. I m a i e t a l . , 1987 d i s c u s s e d details

and o f t h e m e t h o d .

The residual s u l f i t e concen t ra t ion i n t h e s o l u t i o n i s very small

compared t o t h a t i n the feed s o l u t i o n and thus cou ld be n e g l e c t i n g .

Also oxygen concen t ra t ion i n t h e feed s o l u t i o n is a lmos t zero ,

y i e l d i n g the equa t ion :

3.1 Equipment setup:

The s tudy of the mass t r a n s f e r c o e f f i c i e n t was performed i n a 20x20

cm square base plexi g l a s s vessel. The water volume w a s 5 1 and the

c l e a r a n c e between the bottom of t h e i m p e l l e r s and t h e base o f t h e

v e s s e l w a s 4 cm ( t h i s value ranged from 3 .7 t o 4 cm) .

The s tudy of the mass t r a n s f e r c o e f f i c i e n t was always performed a t

250C by h e a t i n g t h e c o n t e n t s of t h e r e a c t o r to t h e d e s i r e d

temperature b e f o r e the start of the test and keeping a small cool ing

water flow during t h e test. Temperature a t t h e end of a test was

t0.5 OC from t h e s t a r t i n g value.

The mixer u s e d f o r t h e s t u d y w a s a bench scale Wemco gas f l o t a t i o n

c e l l . Figure 1 shows a genera l schematic o f the experimental set-up.

3.2 The s tudied Impellers

Tt was the o b j e c t i v e of this study t o compare the performance of the

s t r a i g h t blade i m p e l l e r ve r sus down-pumping and up-pumping

impel le rs . Many v a r i e t i e s of double impel le rs w e r e d e s c r i b e d i n t h e

l i t e r a t u r e . One double b l a d e impe l l e r was i nc luded the comparison.

The s t r a i g h t blade i m p e l l e r with eu t -o f f w a s thought worthy of

i n v e s t i g a t i o n as a way t o reduce power consumption.

The s t u d i e d impellers were:

Impe l l e r 1:

This w a s a s t r a i g h t blade t u r b i n e wi th t h e fo l lowing dimensions:

Hub diameter: 1 . 9 cm, b l a d e length: 1.5 cm, i m p e l l e r width: 5 cm,

number of blades: 6.

Ficure - 1: Eypei-inlental Setup

Impellers II and IfC:

Both inipellers were dom-pumping. Impeller 11 was a turbine inipeller

w l t h blades having a straight p o r t i o n and a curved end. Dimensions

were: width of the s t r a i g h t p o r t i o n of the b lade : 2.5 cm, width of

the upper curved down-pumping end : 2.5 cm, curve radius : 5 cm. Other

dimensions were l i k e impeller 1 e x c e p t t h a t the hub extends below

t h e blade by 1.3 cm.

Impeller IIC had the same dimensions of i rnpel le r II e x c e p t t h a t i t

had a t r i angu l a r eut-off a t the bottom straight part. The t r i a n g u l a r

cu t -o f f had a 1.5 cm base at t h e hub.

I m p e l l e r I I B :

T h i s impeller f e a t u r e d i m p e l l e r II t u r n e d ups ide down and running

at o p p o s i t e d i r e c t i o n t o be up-pumping-

I m p e l l e r III:

This was a rather complex design. The i m p e l l e r c o n t a i n e d t w o s h o r t

blade rows. T h e upper row s t r a i g h t b l a d e s w e r e i n c l i n e d at 30" t o

the v e r t i c a l axis to effect down-pumping. The wid th o f the b l a d e s

w a s 2 m. The lower row hol low blades had 2 cm width and 2 .5 cm

radius of curvature . Other dimensions were l i k e impeller 1.

T h e Shroud:

The shroud was made of rubber material t o sur round the i m p e l l e r by

a t t a c h i n g t o the stand pipe ( s t a t o r ) as an e x t e n s i o n t o it. The

shroud had a m u l t i t u d e of holes i n it . Dimensions w e r e :

C y l i n d r i c a l o u t s i d e d iameter : 1 0 . 7 cm, t h i c k n e s s : 2.5 rm, h e i g h t :

5 .5 cm. The shroud had t w o rows of 16 h o l e s having 1.2 cm diameter .

One row of 16 l o n g i t u d i n a l i n s i d e baffles h a v i n g w i d t h 7 mm,

th ickness 7 mm and l ength 3.7 cm w e r e attached t o the i n n e r w a l l of

the shroud. After i n s t a l l a t i o n the extended length of the shroud

below t h e opening of t h e stand p i p e w a s 4 cm.

The shroud, after f i x i n g around the impe l l e r , acted as an ex tens ion

of the stand-pipe ( s t a t o r ) thereby conf in ing t h e r e a c t i o n zone. The

mul t i tude of the ho les i n the shroud permi ts i n t e r a c t i o n between the

i n s i d e and o u t s i d e zones-

F igure 2 i s a schemat ic of i m p e l l e r d e s i g n s and t h e shroud.

3.3 Testing Methods

3.3.1 Power and impeller speed

Power was c a l c u l a t e d from t h e readings of a vo l t amete r and anuneter

i n t h e motor c i r c u i t . N e t (Hydraul ic) power w a s c a l c u l a t e d by

d e d u c t i n g t h e f r i c t i o n a l (no-load power, i m p e l l e r running i n t h e

a i r ) from the t o t a l power as read from t h e ammeter and vo l t amete r .

Impe l l e r speed was measured by an o p t i c a l tachometer.

Induced Suction Head

The induced static head was measured, i n conjunct ion wi th t h e rate

of gas induc t ion , by t ak ing t h e reading o f a manometer connected t o

a h o l e n e a r t h e s u c t i o n hole when t h e s u c t i o n l i n e i s c l o s e d .


The rate of gas induct ion was rneasured by monitoring t h e r ead ing of

a rotameter connected t o the suct ion l i n e while keeping the pressure

d r o p around t h e i m p e l l e r at zero. The p r e s s u r e drop around t h e

impel le r was measured by a manometer connected to t h e s u c t i o n l i n e

i n t h e s tand-pipe and open t o atmosphere. The p r e s s u r e d r o p around

t h e i m p e l l e r was k e p t a t z e r o by changing t h e f o r c e d f low r a t e o f

t h e a i r i n t o t h e s u c t i o n U n e .

Figure 2: Lchematic oi Impeller Desions and the Siiroud

SIDE ELEV SIDE ELEV VIE W

Ç!DE ELEV VIE Ti\i

Impeller 1 Impeller IIB Impeller II

4

ELEV SIDE ELEV VIE W VIE W VIE W

Impelle r IV Impeller III Impeller IIC

No tes: Rotation of al1 impellers is from right to left. I

One blade o u t of

3.3.4 Mixing Time

Mixing t i m e was measured by the c o l o r a t i o n method. I n i t i a l l y a dye

tracer made up of 5% eriochrome black T and 5% indigo carmine w a s

used, but t h i s tracer gave bad r e s o l u t i o n on the f rames studied.

Then i o d i n e solution was used as a t r a c e r where a drop w a s

i n t r o d u c e d at the t o p t o an a c i d i f i e d water s o l u t i o n t o which a

starch i n d i c a t o r was added. The c o l o r a t i o n p rogress was s t u d i e d by

v i sua l observa t ion o f t h e backplayed frames taken by a high speed

PHOTRON WC-11B (186 f r a m e s / s ) black and white video camera. The

mixing tirne was thus determined by the period elapsed from the start

o f the i n t roduc t ion o f the tracer t o f i n a l even dark grey i n t e n s i t y .


Mas s transfer c o e f f i c i e n t was determined by t h e s teady s t a t e

continuous feed chernical method. Modif ica t ions to this method were

the i n s e r t i o n of the oxygen probe f r o m the side n e a r t h e bottom. In

this p o s i t i o n the effect of gas bubble h i t t i n g the e l e c t r o d e and

s u p e r f i c i a l l y i n c r e a s i n g t h e mass t r a n s f er c o e f f i c i e n t is minimal.

Mass t r a n s f e r c o e f f i c i e n t values were obtained at dif ferent f eed

rates and an average was c a l c u l a t e d . Concent ra t ion of the sodium

s u l p h i t e s o l u t i o n was 1 M o r 0 .75 M as mentioned i n t h e r e l e v a n t

tables. Also the o u t l e t stream was c o n t r o l l e d by a ro tameter and

adjusted exactly t o be equal t o t h e feed rate rather than a n

overflow pipe.

The sodium sulphite feed rate was c o n t r o l l e d using a Masterflex

m e t e r i n g pump of Cole-Parmer Instrument Company o r by a d j u s t i n g a

r o t a m e t e r i n s t a l l e d i n t h e feed l i n e to the d e s i r e d value. T h e

rotameter was calibrateci by weighing the dispenseà feed over periods

of t i m e . In t h i s case the density o f the sodium s u l p h i t e s o l u t i o n

was calculated f r o m weighing 2 1 s o l u t i o n i n a volumetric flask. The

flow rate was c a l c u l a t e d from weighing the flow passed in a b o u t 1

minute at a s p e c i f i c rotameter reading using a s t o p watch and an

a n a l y t i c a l b a l a n c e ,

3.3.6 Dissolved Oxygen

The d i s so lved oxygen concentrat ion was measured using a 5730 mode1

YS1 (YelIow Spr ings Instrument Co) oxygen m e t e r f i t t e d with

standard 0.001 i n . E'EP T e f l o n membrane. Oxygen meter readings were

t aken from r e c o r d s of the data a c q u i s i t i o n system a t t a c h e d t o it,

as w e l l as direct readings of t h e dia1 d e f l e c t i o n .

The oxygen m e t e r reading w a s taken when i t reached a steady value

a t c e r t a i n sodium su lph ide feed rate. This was normally attained

after 1 minute. Then the f e e d rate was changed and the corresponding

steady state va lue of t h e bu lk concentva t ion of oxygen was taken.

The tes t was ended after t a k i n g Eew readings at a c e r t a i n RPM. A t

the end of a test about 50 m i of sodium s u l p h i t e w e r e normally

dispensed to the r eac to r bringing its sodium sulphate concen t ra t ion

t o 0 . 0 1 mol/l. A fresh s o l u t i o n was then prepared f o r the next t e s t .

No attempt w a s made t o c o n t r o l the p H of the s o l u t i o n . Dur ing t h e

test p H v a r i e d dur ing the test i n t h e range of 8 .6 t o 8.9.

3 . 4 Data Acquisition

The oxygen m e t e r , p H m e t e r and t h e thermocouple were connected t o

Labtech Notebook data a c q u i s i t i o n sof tware ve r s ion 5 . 0 i n

conjunction with an IBM compatible persona1 computer and a Metrabyte

DAS-16G1 data a c q u i s i t i o n i n t e r f a c e device.

3 . 5 Materials

Distilled water (conductivity = 40 pS) was used for the mass

transf er coefficient test unless otherwise stated. Tap w a t e r was

used for other tests. Anachemia anhydrous sodium sulfite (assay

>98%) was used for the mass transfer coefficient tests.

4 . RESULTS and DISCUSSION

4.1 General

Five impeller designs were tested in self inducing gas-liquid

contactor. Details of irnpeller designs are found in the

introduction. They included straight blade turbine (impeller I),

straight blade turbine with a curved down-pumping end (impeller II

and IIC) , straight blade turbine with a curved up-pumping end (impeller IIB), double blade impeller; the upper rcw pitched down-

pumping straight blades and the lower row concave c u ~ e d hollow type

blades (impeller III) and a straight blade with cut-off impeller

(irnpeller IV) . Thus, the main variants of impeller designs were represented. The number of blades in any row was 6.

Testing included power consumption under free induction as well as

forced gassing condition, self induced static head, rate of gas

induction, mixing time and mass trans£er coefficient . Impeller 1 with the shroud in its place around the irnpeller was considered the

reference case for comparison. This was because of the previously

known high performance of this design in specific processes,

especially in stripping of hydrogen sulfide from bioreactor off-gas.

Comparison of the new designs to this irnpeller was t h u s desirable.

The new designs were introduced for a search to a better performance

especially in oxygen-water system.

These new impellers included the pitched down-pumping blade (the

upper row in impeller III) and the two variants straight blade with

curved down-pumping end, impeller II and up-pumping end, impeller

IIB. The effect of the pitched blade was thus achieved in a smoother

fashion through the curved end.

The vessel w a s an unbaf f l ed square base c o n t a i n e r . H i s t o r i c a l

reasons w e r e behind t h i s s e l e c t i o n . The square base vessel has t h e

advantage o f reducing t h e s w i r l i n g t o some e x t e n t wi thou t t h e need

t o install b a f f l e s . A t a la ter s t a g e one baffle w a s i n t roduced i n

t h e v e s s e l i n some mass t r a n s f e r c o e f f i c i e n t tests.

The a h o f t h e test was t o p r imar i ly i n v e s t i g a t e the performance o f

d i f f e r e n t impellers with regard t o mixing t i m e and mass t r a n s f e r

c o e f f i c i e n t . T h e i d e a was t o improve t h e mixing t i m e t o g e t h e r w i t h

the mass t r a n s f e r c o e f f i c i e n t . Mixing p l a y s a c r u c i a l r o l e i n mass

t r a n s f e r where the l i q u i d i s q u i c k l y s a t u r a t e d wi th t h e s o l u t e gas

as i n t h e oxygen-water system. This w a s d i s c u s s e d i n t h e

i n t r o d u c t i o n and r e f e r e n c e is made t o t h a t s e c t i o n .

During t h e t e s t i n g i t became apparent t h a t o t h e r parameters p l a y

important r o l e s i n the mass t r a n s f e r c a p a b i l i t y . Induced static head

and t h e rate of gas i nduc t ion c o n t r i b u t e t o t h e mass t r a n s f e r

c a p a b i l i t y . Thus, t h e s e pparameters w e r e tested fo r an understandmg

of t h e o v e r a l l performance o f t h e i rnpel le rs s t u d i e d .

Power was first measured and c o r r e l a t e d with RPM. Mixing tirne, mass

t r a n s f er c o e f f i c i e n t and o the r parameters w e r e c o r r e l a t e d wi t h R P M ,

f r o m which c o r r e l a t i o n with power was d e r i v e d .

A genera l idea of t h e r e p r o d u c i b i l i t y of t h e tests cou ld be judged

from the r a w da ta and c o r r e l a t i o n c o e f f i c i e n t s . I t should be no ted

t h a t t h e r epor t ed power consumption values a r e averages . Power

consumption under free induc t ion normally f l u c t u a t e d by f5% around

t h e average . Some tests have been repea ted and t h o s e are shown i n

the raw data tables. Good agreement was observed. Power consumption

under restricted a i r f low ( f o r c e d gass ing) or a t h i g h e r RPM values

fluctuated i n a severer manner reaching &IO%.

I n the mixing t i m e test, averages w e r e ob ta ined from r e p e a t e d

testing under the same cond i t ion . The scatter of t h e p o i n t s around

the average is an ind ica t ion of i t s r e p e a t a b i l i t y . The d i f f e r e n c e

between the maximum o r a minimum and t h e average, i n m o s t c a s e s ,

was less than 10% - Induced static head r e s u l t s f luc tua ted around t h e

r e p o r t e d averagê by about +5%. Rate o f gas induct ion f l u c t u a t e d

around t h e average w i t h i n +IO%.

4.2 Power Consumption U n d e r F r e e Induction

Power consumption ve r sus RPM w a s d i v i d e d i n t o two regions ; t h e low

RPM r e g i o n from 300 RPM t o 600 RPM and t h e high RPM region above 600

RPM. There was no phys ica l s i g n i f i c a n c e f o r t h i s d i v i s i o n . The

reason f o r t h e d i v i s i o n i s t o g e t as smooth c o r r e l a t i o n a s p o s s i b l e ,

e s p e c i a l l y f o r t h e lower RPM range. This c o r r e l a t i o n i s used f o r

mixing t i m e c a l c u l a t i o n s i n t h e low range . In t h i s low range no

a t tempt w a s made t o c a l c u l a t e t h e hydraulic power consumption.

Tables 1-Al through 6-A1 p r e s e n t raw data of t h e power consumption

under free induc t ion . Table 1-Al p resen t r a w data of t h e motor power

consumption i n t h e low RPM range (300 - 600) under free i n d u c t i o n

f o r 4 a n c l e a r a n c e value. T a b l e s 2-A1 and 3-AI show t h e same data

f o r 2.5 cm c lea rance value. I n Table 2-A1 t h e shroud is o u t o f p l a c e

and i n Table 3-A1 t h e shroud i s i n p l a c e . C o r r e l a t i o n s of s p e c i f i c

motor power consumption and RPM based on t h e s e data a r e shown i n

t a b l e s 1-A7 and 2-A7 f o r 4 cm and 2 .5 cm c lea rance values

r e s p e c t i v e l y .

Tables 4-Al and 5-A1 present raw data of the rnotor power

consumption i n t h e high RPM range (over 600 RPM) f o r 4 cm c lea rance .

Table 6-Al presents s i rn i la r d a t a f o r 2 .5 cm c lea rance va lue . T a b l e

3-A7 shows c o r r e l a t i o n s of t h e s p e c i f i c power and RPM based o n the

data f o r 4 cm c lea rance while T a b l e 4-A7 shows t h e same c o r r e l a t i o n s

f o r 2 .5 cm c l e a r a n c e i n t h e h i g h RPM range.

S u b t r a c t i n g t h e f r i c t i o n a l power consumption (no h y d r a u l i c l o a d )

from t h e motor p o w e r consumption magnifies t h e error i n t h e

c a l c u l a t i o n s and c o r r e l a t i o n s . The f a c t t h a t t h e f r i c t i o n a l power

has a n i n t e r c e p t , i n d i c a t i n g power consumption a t z e r o RPM is

impor tan t i n t h i s respect.

Motor power consumption i s c o r r e l a t e d t o RPM u s i n g a p o w e r model.

The exponent produced by r e g r e s s i o n analysis i s s l i g h t l y over 1. I n

fact l i n e a r c o r r e l a t i o n s can be as good as p o w e r c o r r e l a t i o n i n t h e

low RPM r e g i o n .

The c o r r e l a t i o n of t h e motor power consumption w i t h RPM us ing t h e

power model seems art i f icial s i n c e t h e r e is always a n i n t e r c e p t f o r

t h e real case. On t h e o t h e r hand a l i n e a r c o r r e l a t i o n n e g l e c t s t h e

fact that the h y d r a u l i c power p o r t i o n is a power f u n c t i o n of the RPM

o r i m p e l l e r speed. A good c o r r e l a t i o n c o e f f i c i e n t of the power m o d e l

i n the range s t u d i e d permi ts comparison of mixing t i m e data i n t h i s

range.

The g e n e r a l t r e n d of t h e motor power consumption is t o i n c r e a s e

when changing t h e c l e a r a n c e f rom 4 t o 2 . 5 cm. This is p r e s e n t e d on

Tables 1-A7 and 2-A7. Figures 3 and 4 show t h i s t r e n d f o r selected

i m p e l l e r d e s i g n s .

Among d i f f e r e n t i m p e l l e r s i m p e l l e r IV h a s t h e lowes t power

consumption. Other impe l l e r s have such similar power consumption so

t h a t no d e f i n i t e trend cou ld be drawn. In t h i s low RPM range, the

effect o f t h e sh roud on the power consumption i s weak.

I n t h e h i g h RPM ( i m p e l l e r speed) range power c o r r e l a t i o n s are

calculated between t h e motor power as w e l l as t h e h y d r a u l i c power

and the FSM. This i s shown i n Table 4-A7. The effect of i n s e r t i n g

t h e sh roud a round the i m p e l l e r i s clear i n t h i s r a n g e where it

i n c r e a s e s t h e power consumption. F igure 5 depicts t h i s effect f o r

i m p e l l e r s 1 and I I .

-08s

-09s

- OPS

-0ZS

-00s

-08P

-09P

-0PP

- OZP

-0OP

-08s

-098

- OPS

Figure 4: Power vs. RPM under Free Induction High RPM range

Square base vessel, 5 1 water, No baffles, free induction

Specific power, kW/m ̂ 3

RPM

The exponents o f the h y d r a u l i c power consumption are higher than

those for the motor power consumption. This is q u i t e understanàable.

The l i n e a r f r i c t i o n a l power has a moderating effect when added t o

the h y d r a u l i c power t o form t h e motor power.

Impel le r IIS (up-purnping) shows i n s t a b i l i t y i n the power

consumption and p u l s a t i o n i n t h e 1400 t o 1750 RPM range. The power

i n s t a b i l i t y i s qui t s h a r p when t h e shroud i s i n s e r t e d around t h e

i m p e l l e r . Power consumption of t h i s impe l l e r , wh i l e t h e shroud i s

i n place, i s thus divided i n t o two d i f f e r e n t r e g i o n s . The exponent

of t h e RPM of i m p e l l e r I I B wi th t h e shroud i s h i g h producing a

s t e e p e r curve. Th i s behaviour i s dep ic ted i n T a b l e 5-A1 and i n

figure 6.

The i n s t a b i l i t y of impel le r 11% i n the 1400 t o 1750 RPM range might

be exp la ined based upon t h e observed d e l a y of developing i t s

cavities. The up-pumping c h a r a c t e r seems t o be s t r o n g enough t o

hinder the gas i n d u c t i o n . In t h i s range i t seems t h a t t h e impe l l e r

s t rugg les heav i ly . S t r o n g h y d r a u l i c c u r r e n t s are induced near t h e

i m p e l l e r while t h e sucked gas i s no t enough t o go through the

l iquid. This behaviour i s magnified when t h e shroud i s i n place

around t h e i m p e l l e r . The shroud confines t h e volume around t h e

i m p e l l e r thereby i n t e n s i f i e s t h e hydrau l i c c u r r e n t s i n t h i s zone.

A t h igher RPM va lues gas induct ion occurs and t h e power consumption

was stable and h i g h -

Nienow, 1993 mentioned i n s t a b i l i ty of torque f o r A310 hydrofo i l

i m p e l l e r (down-pumping i m p e l l e r , manufactured by the Mixing

Equipment Co. (see a l s o Oldshu, 1989) a t c e r t a i n a i r f low r a t e f o r

normal con tac to r s with a i r introduced f r o m t h e bottom. A t a c e r t a i n

RPM range a c e r t a i n a i r flow is induced by i m p e l l e r I I B i n t h e G I C

and hence t h e sarne phenornenon w a s observed. By comparison impe l l e r

II (down-pumping) has l i t t l e power f l u c t u a t i o n s . Impeller III

(double blade) shows t h e most stable power consumption.

Figure 6: Power vs. RPM under Free Induction for lmpeller IIB . Square Vessel, 5 1 Water, No baffles, 4 cm Clearance

-

Specific Power, kW/m A 3

RPM

The i n s t a b i l i t y of t h e up-pumping i m p e l l e r i n GIC where a i r i s

int roduced from top i n the s tand-pipe is q u i t e t h e same encountered

by a dom-pmping impeller where a i r is in t roduced from t h e bottom.

I n both cases s t r o n g l i q u i d c u r r e n t s oppose the i n t r o d u c t i o n o f t h e

g a s .

4 . 3 Power Under Forced Gassing Condition

T a b l e s 1-A2 through 6-A2 i n Appendix A2 p r e s e n t t h e r a w data o f

t h i s s t u d y . Table 1-A2 depicts t h e r a w data o f t h e m o t o r power

consmpt ion i n t h e high range of E?PM f o r 4 cm c l e a r a n c e a t 0 . 3 l / s

(18 l/min) a i r , whi le T a b l e 2-A2 depicts the same d a t a a t 0 . 7 l / s

(42 l /min) a i r . C o r r e l a t i o n s between power consumption and RPM

based on t h e s e d a t a a r e p r e s e n t e d on T a b l e 5-A7.

From these &ta it is s e e n t h a t t h e power consumptions o f i m p e l l e r s

1 and II are h i g h e r a t 0.3 l /s (18 l /min) a i r than a t free

i n d u c t i o n . I t i s concluded t h a t 0 - 3 l /s a i r i s less than t h e free

induc t ion capacity of t h e s e i m p e l l e r s . A t 0 . 7 1 / s (42 l /min) and up

t o 1600 RPM the power is n e a r l y the same as the free induc t ion which

i n d i c a t e s self a e r a t i o n c a p a c i t y of n e a r l y 0.7 l / s (42 I /min) a t

t h i s RPM.

Under restricted gass ing i n c r e a s i n g RPM i n c r e a s e s power consumption

i n a higher rate compared t o t h e free induc t ion case. This i n d i c a t e s

h i g h e r i n d u c t i o n c a p a c i t y a t h i g h e r RPM v a l u e s as t h e d i f f e r e n c e

r ep re sen t s the reduc t ion i n power due t o the i n t r o d u c t i o n o f the gas

i n t o t h e l iquid.

Tables 3-A2 through 6-A2 i n Appendix A2 show t h e power consumption

a t variable a e r a t i o n rates and s e l e c t e d RPM v a l u e s for 4 cm

c l e a r a n c e . C o r r e l a t i o n s o f t h e h y d r a u l i c power consumption based

on t h e s e d a t a are p r e s e n t e d on table 6-A7.

The c o r r e l a t i o n o f the p o w e r with t h e s u p e r f i c i a l air v e l o c i t y g ives

a n i n s i g h t t o how t h e s e i m p e l l e r s differ i n t h e i r behaviour under

f o r c e d a e r a t i o n . I t is clear t h a t i m p e l l e r II h a s a f l a t t e r curve

i n d i c a t e d by the low exponent produced by r e g r e s s i o n a n a l y s i s , while

impeller 1 h a s a s t r o n g f u n c t i o n o f i t s power consumption with t h e

s u p e r f i c i a l a e r a t i o n rate. These c o r r e l a t i o n s i n d i c a t e t h a t t h e

power consumption inc reases as t h e a i r (gas) f l o w rate to the stand-

p i p e d e c r e a s e s than t h e self a e r a t i o n c a p a c i t y o f i m p e l l e r s 1 and

I I . T h i s i s much more severe f o r i m p e l l e r 1 t h a n i m p e l l e r I I , and

even severer when t h e shroud i s i n i t s p l a c e a round t h e i m p e l l e r .

Reference f o r t h i s behaviour is made t o table 6-A7 and Figure 7 .

T h i s d i f f e r e n c e i n behaviour between Impel ler 1 a n d II reveals more

flexibility i n o p e r a t i o n o f i m p e l l e r II . I n a c c i d e n t a l s h u t o f t h e

gas the i n c r e a s e i n power consumption i s no t as h i g h as i m p e l l e r 1.

I n fact this should be taken i n t o c o n s i d e r a t i o n i n t h e des ign phase.

Huge ove r d e s i g n of t h e motor f o r i m p e l l e r 1 i s r e q u i r e d though

seldorn used .

4 . 4 Induced Static Head

The induced s tat ic head g i v e s an i n d i c a t i o n o f t h e i m p e l l e r induced

d r i v i n g f o r c e . The induced s t a t i c head of f o u r i m p e l l e r d e s i g n s w a s

t e s t e d f o r 4 and 5 1 l i q u i d volumes. Tests w e r e performed to see the

r e l a t i v e performance o f d i f f e r e n t i m p e l l e r s .

T a b l e s 1-A3 through 4-A3 i n Appendix A3 show t h e raw d a t a o f the

induced s ta t ic head, measured i n cm w a t e r , at 4 cm c l e a r a n c e and

l i q u i d volumes of 4 1 a n d 5 1. Table 7-A7 and F i g u r e 8 show the

c o r r e l a t i o n s of t h e induced s t a t i c head versus RPM based on t h e s e

data. The a i r i n l e t is blocked t o measure t h e induced s t a t i c head.

Power consumption under t h i s c o n d i t i o n does n o t r e p r e s e n t a c t u a l

case. That is why induced s ta t ic head i s o n l y c o r r e l a t e d w i t h =M.

The test f o r t h e 4 1 l i q u i d volume is n o t a s c o n c l u s i v e because the

liquid level is only high enough t o cover f e w m i l l i m e t r e s of the t i p

o f t he s t a n d p i p e under s t a t i c c o n d i t i o n s .

Figure 7: Power vs. Superficial air velocity -

Square base vessel, no baffles, 5 1 water, 4 cm clearance

Specific Hydraul ic Power, kW/m A 3 30

O 1 1 I 1 I I I I I I

27 32 37 42 47 52 57 62 67 72 77

Superficial Air Velocity, m/h

When t h e i m p e l l e r r o t a t e s it forms a v o r t e x around t h e s t a n d p i p e

which i f n o t w e l l covered by l i q u i d w i l l b reak t h e i nduced head

i n s i d e i t , T e s t i n g w i t h sh roud i n p l a c e showed t h a t i m p e l l e r s II

and IV w e r e i n f e r i o r t o i m p e l l e r 1 w i t h shroud w h i l e i m p e l l e r III

produced comparable r e s u l t s .

I m p e l l e r I V w i t h shroud produced about 60% t o 70% o f t h e induced

static head o f i m p e l l e r 1 wi th shroud, and produced about 70 t o 90%

o f t h e s ta t ic head o f i m p e l l e r 1 wi th shroud , when t h e sh roud was

o u t o f p l a c e . This i s s e e n i n T a b l e 7-A7-

Performance o f i m p e l l e r II w i t h shroud i s about 70 t o 78 % o f t h e

r e f e r e n c e case w i t h r e g a r d t o i t s self induced s t a t i c head . This

i n d i c a t e s d e t e r i o r a t i n g ef fect o f t h e shroud on i m p e l l e r II .

I m p e l l e r III w i t h sh roud produced a l m o s t t h e same induced s ta t ic

head as i m p e l l e r I w i t h sh roud .

I m p e l l e r s 1 and II w i t h o u t shroud produced a b o u t 20% t o 30% more

static head compared t o the r e f e r e n c e case of i m p e l l e r 1 with

shroud. Impeller III wi thout shroud produced more o r less t h e same

amount of i m p e l l e r 1 w i t h sh roud a t low RPM. A t h i g h e r RPM (1200)

i t produced about 40% more static head w i t h comparison t o i m p e l l e r

1 w i t h shroud . A i l t h e s e r e s u l t s are shown i n T a b l e 7-A7.

Figure 8 reflects the induced s tat ic head r e s u l t s d a t a for 5 1

w a t e r i n a g r a p h i c a l p r e s e n t a t i o n .

4 . 5 Rate of Gas Induction

Table 1-A4 shows the raw data o f t h e rate o f gas i n d u c t i o n as w e l l

as c o r r e l a t i o n w i t h RPM and T a b l e 8-A7 shows c o r r e l a t i o n s wi th

power based on the r a w data. I t can be s e e n t h a t a t h i g h h y d r a u l i c

power i m p e l l e r s 1, II and III, a l1 w i t h o u t shroud have d e f i n i t e

advan tage o v e r i m p e l l e r 1 w i t h shroud i n p l a c e , p roduc ing 10% t o 50% more induced g a s .

Figure 8: Induced Static Head vs. RPM Square base vessel, no baffles, 5 1 water, 4 cm clearance

lnduced static head, cm water 35

(

RPM

These r e s u l t s t e n d t o match, i n g e n e r a l , wi th t h e induced s ta t ic

head r e s u l t s , H o w e v e r t h e h i g h gas i n d u c t i o n rate of i m p e l l e r III

compared t o i m p e l l e r 1 w i t h shroud ( abou t 40% i n c r e a s e ) i s n o t

matched by i ts moderate improvement of t h e induced stat ic head.

C o r r e l a t i o n s of t h e rate o f gas i n d u c t i o n a r e p r e s e n t e d i n T a b l e

8-A7 i n a l i n e a r model. The f irst l i n e a r p o r t i o n of t h e curves are

t a k e n i n t h i s c o r r e l a t i o n . From t h i s c o r r e l a t i o n a minimum

(critical) i m p e l l e r speed f o r gas i n d u c t i o n w a s c a l c u l a t e d d e f i n e d

as t h e RPM v a l u e a t z e r o rate of gas i n d u c t i o n . Th i s i s a

mathematical d e f i n i t i o n compared t o the normal p h y s i c a l obse rva t ion

which, accord ing t o Mundale and J o s h i , 1995, w a s d e f i n e d as t h e

smallest r o t a t i o n a l speed a t which gas bubbles are n o t i c e d on t h e

free l i q u i d s u r f a c e , The v a l u e s o b t a i n e d from t h e two d e f i n i t i o n s

may n o t c o i n c i d e .

Raidoo e t a l , , 1987 c o l l e c t e d d a t a of gas i n d u c i n g i m p e l l e r s which

showed d i f f e r e n t models of t h e c o r r e l a t i o n o f t h e rate of g a s

i n d u c t i o n w i t h power. Some w e r e more o r less l i n e a r wh i l e o t h e r s

had d i s t i n c t i v e power form. Success o f t h e l i n e a r c o r r e l a t i o n i n

e s t a b l i s h i n g t h e minimum rate o f gas induc t ion w i l l depend upon t h e

f l a t n e s s of t h e curve .

According t o t h e mathematical d e f i n i t i o n , i m p e l l e r 1 w i t h shroud

had a d i s t i n c t i v e low critical speed £or gas i n d u c t i o n ( abou t 300

RPM) b u t it had t h e l o w e s t nominator i n t h e l i n e a r e q u a t i o n which

lowered i t s rate o f gas i n d u c t i o n .

I t w a s n o t i c e d h e r e t h a t t h e low cri t ical speed f o r g a s i n d u c t i o n

o f i m p e l l e r 1 w i t h shroud w a s a lso reflected i n t o l o w crit ical

power f o r gas induct ion . But t h e low power c o e f f i c i e n t slowed down

i t s rate o f gas i n d u c t i o n a t h i g h power compared t o o t h e r

impellers. T h i s behaviour i s d e p i c t e d i n Tables 1-A4 and 8-A7.

When comparing t h e s e r e s u l t s with p o w e r under forced g a s s i n g one

s h o u l d b e c a r e f u l . Power consumption f l a t t e n s at higher g a s s i n g

cond i t ion and p o w e r v e r s u s one specific a e r a t i o n rate i s difficult

t o be d e f i n e d . Another i m p o r t a n t o b s e r v a t i o n i s t h a t t h e

experimental r e s u l t s of the rate of gas i nduc t ion a c c o r d i n g t o this

p r o c e d u r e are h i g h . This is because of b r i n g i n g the d i f f e r e n c e i n

p r e s s u r e around the impeller t o zero , whereas f o r power consumption

u n d e r f o r c e d g a s s i n g t h e p r e s s u r e d r o p i n t h e l i n e and f i t t i n g s

reduces the rate of gas induction as read Erom t h e r o t m e t e r i n t h e

f o r c e d g a s s i n g l i n e .

As shown by the a n a l y s i s o f Mundale and J o s h i , 1995 , who refered t o

o t h e r a u t h o r s , a forced v o r t e x that runs a l o n g t h e s t and-p ipe

( s t a t o r ) and t a k e s a t y p i c a l p a r a b o l o i d shape appea r s i n t h e

i m p e l l e r zone before and with the beg inn ing of gas i n d u c t i o n . The

v o r t e x would become deeper at i n c r e a s i n g i m p e l l e r speeds

e v e n t u a l l y reach the impel le r at the beginning o f the gas

p r o c e s s ,

and would

i n d u c t i o n

The s t a t o r and i t s vanes (which are comparable to t h e holes i n the

shroud) allow a predominant ly r a d i a l l y outward flow, which carries

a w a y t h e gas bubbles formed i n t h e i m p e l l e r zone. Any gas bubbles

t h a t cannot corne o u t of t h e s t a t o r a l o n g with t h e r a d i a l l y outward

f l o w would travel upward i n r e sponse to t h e buoyancy forces, and

would thus g e t r e c y c l e d to t h e stand-pipe t he reby r educ ing t h e n e t

gas i n d u c t i o n .

Following t h e prev ious a n a l y s i s by Mundale and Joshi, the t a n g e n t i a l

v e l o c i t y field wi th in the s t a t o r , the axial downward velocity field,

and t h e radial outward velocity field are i m p o r t a n t i n t h e

performance of the gas inducing c o n t a c t o r s and the right combination

between these fields i s the t a r g e t .

F i g u r e 9 shows t h e rate o f gas i n d u c t i o n va lues vs. power.

4 . 6 Mixing Time

R a w data of mixing tirne f o r the low range o f RPM are c i t e d i n T a b l e

1-A5 i n Appendix AS. This test i s performed while the shroud is i n

p l a c e f o r a l1 i m p e l l e r s . These data are summarized i n Table 9-A7.

C o r r e l a t i o n s o f t h e mixing t i m e wi th r e c i p r o c a l RPM are a l s o

presented i n t h e same Table.

The i n t e r c e p t s o f t h e s e c o r r e l a t i o n s are forced t o ze ro . This is t h e

s imples t mode1 fol lowed by Voncken, 1996. Some a u t h o r s r e p o r t a

s l i g h t i n c r e a s e o f t h e product of t h e rnixing t i m e and RPM with

i n c r e a s i n g Reyno ld t s number. I n p r a c t i c e t h e constancy o f t h e

product i s a good assumption a t f u l l t u rbu lence (Reynoldts number

> 10 0 0 0 ) . The v a l i d i t y o f t h e assumption i s shown from t h e h igh

cor re la t ion c o e f f i c i e n t v a l u e s obtained. I n t h e low RPM range t h e

self induced g a s i s n e g l i g i b l e and its effect on t h e mixing t i m e

could be ignored .

Raw d a t a of mixing tirne a t t h e high RPM range are presen ted on

t a b l e s 2-A5 through 5-A5 i n Appendix 5. Average v a l u e s a r e

summarised on T a b l e 10-A7. I n t h i s range gas induc t ion i s

not iceable . D a t a on t h e effect of the induced gas on the mixing t i m e

are scarce.

Forced gass ing dec reases power consumption and dec reases t h e

pumping c a p a c i t y o f t h e i m p e l l e r due t o the cavity format ion .

Experiments perfonned by E i n s e l e and Finn, 1980 and Middleton, 1979

showed an i n c r e a s e (worsening) of the mixing t i m e by a f a c t o r o f 1 - 2

t o 2 due t o a e r a t i o n .

I n t h e low RPM r ange , a t 2 .5 cm c lea rance , mixing t i m e of i m p e l l e r

II with t h e shroud is far better than i m p e l l e r I w i t h t h e shroud,

being about 55% of the i m p e l l e r 1 values. I m p e l l e r IV i s worse than

i m p e l l e r 1 b e i n g h i g h e r by abou t 35%. Impel le r III shows s l i g h t l y

higher mixing t i m e va lues compareci t o impel le r 1, b u t no conclus ion

can be made as t h e s e v a l u e s are c l o s e t o each o t h e r . The s u p e r i o r

mixing time o f i m p e l l e r II might b e expla ined based on i ts s t r o n g

dom-pumping axial c u r r e n t s which improves mixing tirne i n g e n e r a l .

Figure 10 show mixing time va lues vs . power f o r d i f f e r e n t i m p e l l e r s

i n t h e l o w range .

I n t h e h i g h RPM range while t h e shroud i s i n place around t h e

impellers the same t r e n d is observed. Mixing tirne values of impe l l e r

II are a b o u t 75% of i m p e l l e r 1 v a l u e s . I m p e l l e r IV mixing t i m e

v a l u e s are 33% t o 50% h i g h e r than i m p e l l e r 1 which showed lower

mixing abi l i ty . The cu t -o f f of i m p e l l e r IV lowers i ts power

consumption b u t severely d e t e r i o r a t e s i ts mixing c a p a b i l i t y .

I m p e l l e r III shows a small improvement i n t h i s range compared t o

impel le r 1 which meant a reversal of t h e t rend a t lower RPM v a l u e s .

Table 10-A5 shows a summary of mixing t i m e v a l u e s for d i f f e r e n t

i m p e l l e r s .

Raidoo e t al., 1987 repor ted an improvement (decrease) of the mixing

t i m e o f one p i t c h e d b l a d e down-pumping des ign compared t o t h e

shrouded disc t u r b i n e and a t t r i b u t e d t h e enhancement t o t h e h igher

l i q u i d c i r c u l a t i o n v e l o c i t i e s which would a l s o favour t h e rate of

gas i n d u c t i o n .

I t i s i m p o r t a n t to n o t i c e t h a t d i f f e r e n c e s i n mixing time between

d i f f e r e n t impellers d imin i sh as t h e i m p e l l e r speed i n c r e a s e s . This

i s because o f t h e normal l e v e l l i n g - o f f o f t h e mixing t i m e curve

with i n c r e a s e d i m p e l l e r speed.

Present r e s u l t s did n o t show clear d i f f e r e n c e i n mixing t i m e between

t h e cases where t h e shroud w a s i n p l a c e and o u t o f p lace .

4 . 7 Mass Transfer Coefficient

T a b l e s P A 6 through 7-A6 i n Appendix A6 d e p i c t t h e mass t r a n s f e r

c o e f f i c i e n t r e s u l t s under free induc t ion . C o r r e l a t i o n s o f t h e mass

t r a n s f e r c o e f f i c i e n t t o RPM are p re sen ted i n T a b l e 11-A7. Table 12-

A7 reflects t h e d a t a on table 10-A7 i n a c o r r e l a t i o n between t h e

mass t r a n s f e r c o e f f i c i e n t and t h e m o t o r power consumption. Table 13-

A7 shows c o r r e l a t i o n s between the mass t r a n s f e r c o e f f i c i e n t and t h e

h y d r a u l i c power consumption .

The non-enhancinq c o n c e n t r a t i o n of t h e c a t a l y s t w a s es t a b l i s h e d as

seen f r o m Table 1-A6. Less than 1 m o l / m 3 (5 m l of 1 mol/m3 c a t a l y s t

s o l u t i o n i n t o 5 1 w a t e r ) of c o b a l t s u l f a t e i n t h e r e a c t i o n mix ture

t h e r e w a s no enhanc ing effect. Some tests had been performed wi th

tap w a t e r and compared f o r dist i l led w a t e r . I t i s concluded t h a t

t h e r e is no enhanc ing effect of u s i n g t a p wate r with t h e c a t a l y s t

at t h e s p e c i f i e d c o n c e n t r a t i o n , i e t h e added effect o f t h e

catalytic i m p u r i t i e s i n t a p water and t h e catalyst a t 1 mrnol/m3 is

w i t h i n t h e range o f non-enhancement- On t h e o t h e r hand c a t a l y t i c

i m p u r i t i e s i n t a p water are e f f i c i e n t i n b r i n g i n g t h e r e a c t i o n i n t o

t h e fas t regime .

D a t a i n tables 7-A6 differ from t h o s e i n Tables 3-A6 and 4-A6 i n

t h a t t h e oxygen e l e c t r o d e w a s i n t r o d u c e d a t a h i g h level o p p o s i t e

t o t h e shroud w h i l e i n o t h e r T a b l e s i t was in t roduced a t a l o w

level . The d i f f e r e n c e is t h a t i m p e l l e r 1 with t h e shroud showed

h i g h e r l o c a l mass t r a n s f e r c o e f f i c i e n t when t h e e l e c t r o d e was

i n t r o d u c e d o p p o s i t e t o t h e shroud compared t o t h e low side e n t r y .

O t h e r i m p e l l e r s did n o t show t h a t d i f f e r e n c e . The I n t r o d u c t i o n of

t h e sh roud might have l e s s e n e d mix ing c u r r e n t s far away from t h e

i r n p e l l e r , t he reby l o w e r i n g t h e l o c a l mass t r a n s f e r c o e f f i c i e n t a t

remote a r e a s . For t h e s a k e o f comparison wi th o t h e r impellers t h e

high mass t r a n s f e r c o e f f i c i e n t obtained a t the high e l e c t r o d e level

f o r i m p e l l e r I with t h e shroud i n p l a c e w a s taken. The observed

t r ends i n cornparison of t h e mass t r a n s f e r c o e f f i c i e n t s of d i f f e r e n t

impel le r des igns are magnif ied i f t h e v a l u e s of t h e l o w side e n t r y

o f the 02 electrode w e r e used f o r comparison.

As s e e n f rom tables 12-A7 and 13-A7, i m p e l l e r s II (dom-pumping) , IIB (up-pumping) , I I C (dom-purnping) wi thou t t h e shroud al1 have a

def i n i te improvement o f t h e mass t r a n s f e r c o e f f i c i e n t compared t o

the impel le r I wi th t h e shroud f o r the same motor power consumption

o r hydraul ic power conswnption. The mass t r a n s f e r c o e f f i c i e n t w a s

i n c r e a s e d by a b o u t 20% t o 50% with r e f e r e n c e t o t h e same s p e c i f i c

power consumption. This effect is e s p e c i a l l y pronounced a t h i g h e r

RPM v a l u e s o r h igh power i n p u t . A t l o w s p e c i f i c power v a l u e s

Iqeiler I I B (up-pumping) does no t produce enhanced k,a values with

comparison t o i m p e l l e r 1 wi th t h e shroud. When c o n s i d e r i n g t h e

u n s t a b l e power consumption o f impel le r I I B i n t h e 1550 - 1700 RPM

values it might n o t be recomended, but i f h igher REM values are t h e

normal o p e r a t i o n , t h i s i m p e l l e r i s q u i t e stable and improves t h e

mass t r a n s f e r c o e f f i c i e n t q u i t e s i g n i f i c a n t l y . Thus i n t h e h igh RPM

v a l u e s (above 1800) i m p e l l e r I I B i s advantageous. I t absorbs more

energy and makes use of i t as r e f l e c t e d i n i t s mass t r a n s f e r

c a p a b i l i ty .

I m p e l l e r I V showed v e r y low mass t r a n s f e r c o e f f i c i e n t , be ing on ly

4 0 % o f i m p e l l e r 1 wi th t h e shroud. I m p e l l e r III (shroud o u t o f

p l a c e ) shows a s l i g h t improvement over i m p e l l e r 1 w i t h t h e shroud

i n place. This needs f u r t h e r t e s t i n g to confirm t h e d i f f e r e n c e . I t

was thought t h a t i m p e l l e r 1 should perform b e t t e r than t h e

ob ta ined r e s u l t s .

The i n s e r t i o n o f one b a f f l e i n the square base tank had an

improvement effect on t h e mass t r a n s f e r c o e f f i c i e n t as shown from

T a b l e s 4 -A6 through 6-A6. I t a l s o l e v e l l e d out t h e power

consumption and produced more stable c u r r e n t s i n t h e c o n t a c t o r . I t

increased the power consumption by about 15% but t h e improvement i n

t h e mass t r a n s f er c o e f f i c i e n t outweighed t h e sacrifice .

The hydrau l i c power was obtained by s u b t r a c t i n g the f r i c t i o n a l power

from t h e motor power. Th i s process magnif ies t h e relative e r r o r .

A f t e r al1 t h e motor power consumption is what w e pay f o r . Figure Il

shows comparison between d i f f e r e n t i m p e l l e r s w i th regard t o t h e i r

mass t r a n s f er c a p a b i l i t y based on h y d r a u l i c power consumption . Reference i s made t o Table 12-A7 f o r comparison based on t h e motor

power consumption .

4 - 7 . 2 Systems w i t h fast chemical reactions

The p r e s e n t s t u d y of t h e a i r / w a t e r system showed a d e f i n i t e

advantage o f up-pumping and dom-pumping i m p e l l e r s where t h e shroud

w a s o u t o f p l a c e compared t o i m p e l l e r I with t h e shroud i n place.

I t a l s o showed t h a t t h e r e i s no b i g advantage of in t roduc ing t h e

sh roud t o i m p e l l e r 1 i tself . The r e f e r e n c e f o r comparison h e r e i s

the mass t r a n s f e r c o e f f i c i e n t based on l o c a l measurement of oxygen

absorp t ion i n t o sodium s u l f i t e s o l u t i o n .

C a r e should be taken when apply ing t h i s comparison t o one o f t h e

a c t u a l u t i l i z a t i o n s where the chemistry i s d i f f e r e n t , e.g. s t r i p p i n g

of hydrogen s u l f i d e . The f irst important d i f f e r e n c e is that hydrogen

s u l f i d e s t r i p p i n g r e a c t i o n r a t e i s very h i g h , Exact va lue t h e

r e a c t i o n rate and t h e c o r r e c t regime f o r mass t r a n s f e r under t h e

cond i t ion s t u d i e d should be known t o ob ta in r e l i a b l e r e s u l t s .

Secondly a t t e n t i o n should be p a i d t o evidence o f the f ine-bubble

non-coalescing regime. This is impor tant because under t h e s e

condi t ions the gas phase i s no more i d e a l l y mixed i n t h e c o n t a c t o r

and a mode1 f o r i t s mixing behaviour should be fol lowed i n t h e

c a l c u l a t i o n of t h e mass t r a n s f e r capability.

What i s emphasized, i s that results with chemical enhancement can

n o t be cornpared t o pure p h y s i c a l a b s o r p t i o n o r t o cases where the

chemical enhancement i s n e g l i g i b l e .

Harnby, E d w a r d s and Nienow, 1992, identified five main regimes

depending on t h e ratio o f r e a c t i o n t i m e t o d i f f u s i o n tirne. I n the

first regime mass transfer i s c o n t r o l l e d by k i n e t i c s f o r the slow

r e a c t i o n s . Fo r t h e modera t e ly f a s t r e a c t i o n s i t i s c o n t r o l l e d by

d i f f u s i o n , i .e. k,. For fas t r e a c t i o n s , r e a c t i o n occurs i n t h e f i l m

and mass t r a n s f e r i s independent o f k,. I n t h e very fas t and

i n s t a n t a n e o u s r e a c t i o n s mass t r a n s f e r rate depends on k,. I n the

latter case the r e l a t i o n is direct p r o p o r t i o n a l i t y . Th i s is because

r e a c t i o n o c c u r s a t the i n t e r f a c e and i s c o n t r o l l e d by the slow

d i f f u s i o n of t h e r e a c t a n t from the b u l k of t h e l iquid to the

i n t e r f a c e .

Under some c i rcumstances , where t h e c h d c a l enhancement i s high t h e

gas f i l m resistance becomes impor t an t and i s i n c o r p o r a t e d i n t o the

o v e r a l l nwss t r a n s f e r c o e f f i c i e n t . The equat ion for this , acco rd ing

t o t h e f i l m concep t 1s:

Where E, is the e q u i l i b r i u m c o n s t a n t .

Dankwerts p r e s e n t e d a n example of H,S a b s o r p t i o n into

monoethanolamine i n a packed column. I n the mentioned example t h e

gas side mass t r a n s f e r w a s f a r less than t h e l i q u i d side and t h e

o v e r a l l mass t r a n s f e r was thus c o n t r o l l e d by t h e gas side

resistance. A g i t a t i o n d o e s n o t affect t h e gas side mass t r a n s f e r

c o e f f i c i e n t i n the s e n s e of thinning its f i l m thickness . High power

i n p u t w i l l i n c r e a s e the i n t e r f a c i a l a r e a and cornparison between

d i f f e r e n t i m p e l l e r s c o u l d be based upon t h i s c r i t e r i o n .

A l t e r n a t i v e l y the gas hold-up and the average bubb le s i z e c o u l d be

de te rmined and the i n t e r f a c i a l area i s c a l c u l a t e d from bo th

parameters . In G a s Inducing C o n t a c t o r s (GIC) t h e self induced ra te

of g a s i n d u c t i o n i s a l s o a n i m p o r t a n t c r i t e r i o n w i t h t h i s regard.

High rate o f gas induc t ion i n c r e a s e s the gas hold-up p r o p o r t i o n a l l y .

The f i n a l judgement on t h e performance of d i f f e r e n t i m p e l l e r des igns

s h o u l d be based upon a c t u a l mass t r a n s f e r rate performance o f

d i f f e r e n t arrangements and t h i s comparison s h o u l d be based on

power consumption.

4 . 7 . 2 E f f e c t of the Shroud

F i g u r e 12 shows t h e shroud s e c u r e d around i m p e l l e r 1.

T h e shroud makes an ex tens ion t o t h e s tand-pipe ( s t a t o r ) a round t h e

i m p e l l e r . This i s b e n e f i c i a l i f t h e l iquid level i s low and t h e

v o r t e x created d u r i n g t h e r o t a t i o n o f t h e i m p e l l e r l o w e r s it than

t h e brim o f t h e s t and-p ipe . V i s u a l o b s e r v a t i o n i n d i c a t e d t h a t t h e

uppe r row of t h e h o l e does n o t pe rmi t easy currents across t h e

shroud. Many des igns i n G I C i nc luded d i f f u s e r s o r shrouds around t h e

impeller b u t t h e s e extended for a very s h o r t l ength below the stand-

p i p e . Also, openings between d i f f u s e r blades were quite w i d e .

G i l e s , 1988, s t u d i e d and compared t h e mod i f i ed Wemco #28 c o n t a c t o r

models i n the removal e f f i c i e n c y of hydrogen s u l f i d e from a

contaminated gas . H e r epo r t ed 99.9% and h ighe r removal e f f i c i e n c y .

However t h e f e a t u r e s which w e r e s t u d i e d inc luded d i £ f e r e n t shroud

d e s i g n s . N o comparison w a s made t o t h e case where t h e sh roud was

removed .

L e e , 1990 used t h e r e a c t i o n between carbon d i o x i d e and c a u s t i c

s o l u t i o n t o s tudy t h e new designs. H e reported i n t e r f a c i a l area o f

7x103 nil , gas side mass t r a n s f e r c o e f f i c i e n t equal t o5 9x10

mol/ (m3spa) , and l i q u i d side mass t r a n s f e r c o e f f i c i e n t e q u a l t o 7

s-= .

Figure - 1 The Chroucl Arouncl Impeller I in the Square B;ise \:essel

I t i s i m p o r t a n t t o n o t i c e here that t h e r e i s a d i f f e r e n c e between

the area fo r chernical r e a c t i o n and the area f o r p h y s i c a l absorp t ion ,

The high rate o f t h e chemical r e a c t i o n i n t e r f e r e s w i t h t h e

de te rmina t ion of t h e mass t r a n s f e r c o e f f i c i e n t and the i n t e r f a c i a l

area. The mass t r a n s f e r c o e f f i c i e n t and i n t e r f acial area r e s u l t s

under chemical enhancement might be taken as apparent or pseudo.

This is t o d i f f e r e n t i a t e it from p h y s i c a l absorption. It i s not

generally a p p r o p r i a t e to dei ine a t r a n s f e r c o e f f i c i e n t as R/ (c' - C) except fo r phys i ca l absorp t ion . Comparison between t h e two cases

is mis lead ing .

A rnarked enhancement o v e r phys i ca l mass t r a n s f er c o e f f i c i e n t was

encountered i n abso rp t ion of carbon &oxide i n t o alkaline s o l u t i o n s .

Reference is made h e r e t o Linek, and Mayrhoferov, 1969 and Sharma,

1993.

Adamson, 1994 i n h i s report t o Apol lo Environmental S e r v i c e s Co.

(not published) on t h e experimental s t u d y on t h e n e w shrouded des ign

concluded that t h e Apollo c o n t a c t o r i s capable of t r a n s f e r r i n g 0 . 4

kg/kWh oxygen from a i r into the l i q u i d . H e obse rved t h a t when t h e

sh roud was removed and i n t r o d u c i n g a i r b e n e a t h t h e irnpeller

e f f i c i e n c i e s were comparable t o the normal operation mode (wi th t h e

shroud), When a i r was i n t roduced above the i m p e l l e r , the mass

t r a n s f e r e f f i c i e n c y w a s approximately 20% less t h a n where t h e shroud

i s i n p l a c e . T h i s might be a t t r i b u t e d t o t h e very low submergence

and escaping of s o m e a i r around t h e impeller t a k i n g a s h o r t u p r i g h t

p a t h .

C e r t a i n l y t h e shroud would have an improvement effect i n t h e s e

circumçtances as it f o m a physical barrier around the impe l l e r and

an e x t e n s i o n t o the s tand-p ipe . This effect would e x t e n t t h e t i m e

o f contact between the gas bubbles and t h e l iquid. These r e s u l t s

match, i n g e n e r a l , wi th the r e s u l t s o f t h e p r e s e n t s t u d y f o r

air/water sys tem.

5 . CONCLUSIONS

Table 1 shows r a t i o s o f v a r i o u s parameters w i t h r e f e r e n c e t o

i m p e l l e r 1 w i t h the shroud i n p l a c e ( t h e r e f e r e n c e case). T a b l e 2

i s a sunniary o f t h e o v e r a l l performance o f t h e i m p e l l e r d e s i g n s

s t u d i e d i n t h e gas i nduc ing c o n t a c t o r (GIC). G e n e r a l l y t h e down-

pumping impeller had a better performance compared to t h e s t r a i g h t

blade. This i s vaiid f o r mix ing t i m e , induced s ta t ic head, rate of

gas induc t ion and mass t r a n s f e r c o e f f i c i e n t . The up-pumping impe l l e r

h a d a l s o a better performance with r e g a r d t o t h e mass t r a n s f e r

c o e f f i c i e n t b u t showed power i n s t a b i l i t y i n t h e range 1400 t o 1750

R P M ,

The double blade impeller i s m a r g i n a l l y b e t t e r t h a n t h e r e f e r e n c e

case with r e g a r d t o t h e mass t r a n s f e r c o e f f i c i e n t . The s t r a i g h t

blade impe l l e r wi th a cu t -of f consumed less energy a t t h e same RPM,

but was i n f e r i o r i n performance with r e f e r e n c e to the s t r a i g h t b l a d e

i m p e l l e r wi th the shroud i n place. The i n f e r i o r performance was n o t

o f f s e t by the lower power consumption and t h e n e t r e s u l t was n o t i n

i ts f a v o r .

The s t u d i e d a i r / w a t e r system (with n e g l i g i b l e chemical enhancement)

d i f fers f rom t h e i n s t a n t a n e o u s i r r e v e r s i b l e hydrogen s u l f i d e

a b s o r p t i o n system. R e s u l t s of t e s t i n g of one system are n o t

t r a n s f e r a b l e t o the other. T h i s is concluded form cornparison o f t h e

present r e s u l t s o f a i r / w a t e r system and t h e hydrogen s u l f i d e

a b s o r p t i o n r e s u l t s o b t a i n e d by o t h e r s .

I n s e r t i o n o f one b a f f l e i n t h e square base vesse1 m a r g i n a l l y

i n c r e a s e d power consumption . 1 t s i g n i f i c a n t l y improved t h e mass

t r a n s f e r c o e f f i c i e n t . I t also reduced s w i r l i n g and s t a b i l i z e d power

consumption . The o v e r a l l r e s u l t i s benef i c ia l .

Table 1: R a t i o s of Parameter Values of I m p e l l e r s S t u d i e d

M l ra t ios are with reference t o i m p e l l e r 1 w i t h s h r o u d i n place under the same c o n d i t i o n s

RE'M 1 II III IV 1 II III IV

I n I n I n I n O O O O

R a t i o s II III IV 1 II III IV

RPM I n I n I n O O O O

R a t e o f gas i n d u c t i o n R a t i o s 1200 - 2200 R P M

SFP 1 1 II III I n O O O

1 II III O O O

100/~xing t i m e (200 - 600 RPM)

SFP 1 II III I V

I n I n I n I n

R a t i o s

II III IV

Mass transfer c o e f f i c i e n t I Ratios I

SFP 1 1 II I I B T I C III IV 1 I II I I B I I C III IV In O O O O O 0 1 0 O O O O O

I 12 0.06 0.05 0.07 0.06 0.08 0.06 0.02 1 0.84 1.25 1.05 1.33 1 .O5 0.40 16 0.08 0.07 0.11 0.1 0.11 0.09 0.03 10.83 1.35 1.21 1.34 1.08 0.41 20 0.11 0.09 0.16 0.15 0.15 0.12 0.05 1 0.82 1.43 1.36 1.35 1.11 0.42 24 0.13 0.11 0.2 0.2 0.18 0.15 0.06 10.81 1-51 1.48 1.35 1.13 0.43

Table 2: Performance of Impellers Studied in G I C

Al1 cornpariosons are with regard to impeller 1 with shroud in place Al1 cornparisons were for impellers without shroud , except for mixing time w h e r e the shroud was in place

impeller IIB XII

design straight straight straight double blade straight blade blade blade the upper blade turbine with curved with curved row pitched turibine

down-pumping up-pumping down-pumping with end end the lower cut-of f

hollow type

power steep at f la t ter at s h o w s f la t ter a t least no gassing no gassing instability no gassing power

shows some at 1400 - stable consumption power 1750 RPM power stable fluctuation stable above consumption at high RPM 1800 RPM

shroud insertion of the shroud increased the power consumption of al1 impellers with regard t o the same without shroud

induced higher higher not tested comparable- less static without higher at head shroud high RPM

rate of higher higher not tested higher not tested gas w i t h o u t induction shroud

mixing t ime w i t h shroud

much not tested slightly worse better worse

mass slightly higher- higher comparable- much tarnsfer lower I I C is also at higher slightly less coef f . without higher power higher

shroud

6 . RECOMMENDATIONS

For the sake of completeness t h e gas hold-up may be tested f o r t h e

i m p e l l e r s s t u d i e d . This w i l l f i l 1 a gap i n t h e f u l l unders tanding

o f the designs s tud ied . Mass t r a n s f er c o e f f i c i e n t depends s t r o n g l y

on t h i s proper ty and this t e s t i n g w i l l confirm the presen t f ind ings .

To e s t a b l i s h t h e r e l a t i o n s h i p between t h e mass t r a n s f e r and t h e

hydraul ic power consumption accura te measurements of t h e la t ter are

required. This can be achieved only by u t i l i z i n g a to rque table. I t

i s thus recornmended t o perform the mass t r a n s f e r test us ing a torque

table t o o b t a i n more a c c u r a t e c o r r e l a t i o n s .

If the vessel dimensions change from t h e s q u a r e shape t o t h e

c y l i n d r i c a l one f o r f u l l scale p l a n t s , t e s t i n g w i l l be r e q u i r e d i n

t h i s type o f vessel. Tes t ing of the mass t r a n s f e r c o e f f i c i e n t on the

f u l l scale o r a p i l o t scale v e s s e l would be h e l p f u l .

The down-pumping impeller ( impel ler II) and t h e up-pumping i m p e l l e r

( i m p e l l e r I I B ) have s u p e r i o r performances compared t o i m p e l l e r 1.

D i f f e r e n t modif icat ions t o these des igns could be tested for optimum

performance, e s p e c i a l l y wi th regard t o t h e degree of c u r v a t u r e of

t h e lower end of impel le r I I B . Lowering the degree o f c u r v a t u r e may

decrease i t s i n s t a b i l i t y i n t h e middle R P M range . I n c r e a s e of t h e

impeller diameter , e s p e c i a l l y at the upper gas inducing end i s a l s o

worth of s t u d y as it may improve t h e gas i n d u c t i o n rate and thus

d e c r e a s e t h e i n s t a b i l i t y o f power consumption. Tes t ing of t h e new

impe l l e r designs with t h e shroud i n p l a c e w i l l shed l i g h t on i t s

effect on t h e i r performance.

Preliminary tests i n d i c a t e t h a t t h e mass t r a n s f e r c o e f f i c i e n t is a

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131 Wise, W. S., Gen. Microbiol., 5, 167 (1951)

132 Yagi, S., Inoue, H., InThe absorption of oxygen into sodium

sulphite solution^^^, Chernical Engineering Science, vol. 17,

p.411 (1962)

133 Yoshida, F m and Y. Miura, "Gas absorption in agitated Gas-

liquid contactors", Ind. Eng. Chem. Proc. Des. and Dev. , 2, 263-268 (1963)

Yoshida, M. ; Kitamura, A. ; Yamagiwa, K. and A. Ohkawa, %as

hold-up and volumetric oxygen transfer coefficient in an

agitated vesse1 without baffles having forward-reverse

rotating impellersI1, Can J. Chem. Eng., 74, 31-39 (1996)

Yoshida, F; Ikeda, A.; Imakawa, S. and Y. Miura, llOxygen

absorption rates in stirred gas-liquid contactorsw, Industrial

and Engineering Chemistry, vol. 52, No. 5 (May 1960)

Zlokarnik, M.; Judat, H o , "The tube and disk stirrers - Two efficient stirrers for the gassing of liquidsl', Chem. Ing.

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Zlokarnik, M., Chemic. Ing. Technil., 38, 717 (1966)

Zundelevich, V., I1Power consumption and gas capacity of self

inducing turbo agitators", AIChE J., 25, 763-773 (1979)

8 . NOMENCLATURE

c hernical enhancement factor, dimensionless

thickness of difision film, rn

gas hold-up based on gas liquid dispersion volume, dimensionless

time duration for surface renewal. s

time constant for dissolved OZ electrode, s

volumetric (specific) interfacial area based on liquid volume. m-I

volumetnc interfacial area based on dispersion volume. m"

concentration of the gas at the liquid interface at equilibrium with gas, gmol/m3

oxygen concentration in inlet gas, gmol/m3

oxygen concentration in outlet sas, gmol/m3

concentration of reactant in the bulk of the liquid (=Cd. gmol/m3

concentration of reactant (sodium sulfite) in the feed Stream. çmol/m3

oxygen concentration in the gas phase, gmoVm3

concentration of the gas in the bulk of liquid, gmol/m3

concentration of the gas in the bulk of the liquid

at initial condition. gmoVm3

oxygen concentration in the bulk of the liquid at steady state. gmol/mJ

difisivity of dissolved gas A., m2/s

Chemical enhancement factor. dimensionless

ratio of reaction rate to difision rate. dimensionless

absorption rate, gmole/s

gas film mass transfer coefficient

liquid film mass transfer coefficient, mis

overall volumetric (specific) mass transfer coefficient

liquid film volumetnc mass transfer coefficient, s-'

liquid film volumetric mass transfer coefficient

based on dispersion volume, s"

irreversible reaction rate constant of rn.n order

order of reaction with reference to gas component

order of reaction with reference to liquid reactant

volumetric flow rate of gas induction. m3/s

Oas inlet flow rate. m3/s C

gas outlet flow rate, m3/s

average rate of absorption into liquid per unit interfacial area. gmoV(m2s)

fiactional rate of surface renewal, s-'

time at start of test

time at end of test

time duration of test, s

average gas absorption rate per unit volume. gmol/(rn3s)

volume of jas liquid dispersion. m3

volume of liquid, m3

volume of gas. m3

Notation:

AEt CRPM e F FP FR GP HSFP

HSGP

1 In ISH KLA MT NF NLP O or Out RGI RPM S SAV SEP SGP

III, IV are impeller designations as shown in Figure 2

air f low rate, l/min critical rate (RPM) for gas induction equilibrium condition f ree induction motor power under free induction, W sodium sulfite feed rate, ml/min motor power under forced gassing condition, W Hydraulic specific power under free induction, kW/m63 hydraulic specific power under gassing condition, kW/mA3 ampere shroud in place around the impeller induced static head, cm water mass transfer coefficient, sA-1 mixing time, s number of frames counted on the video frictional power (no hydraulic load), shroud out of place (No shroud around rate of gas induction, l/min revolution per minute steady state condition superficial air velocity, m/h

camera W the impeller)

specific motor power under free induction, kW/mA3 specific motor power under forced gassing condition, kW/mA3 no gas power (gas inlet blocked) , W volt difference between oxygen concentration at equilibrium and at steady state, mg/ 1 oxygen concentration, mog/ 1

Appendir A l Power vs. RPM under Free Induction - -- -- - - - - - -- -- -

Vesse1 Square base vesse1

Liq. V o l . 5 1 water

Baffles No b a f f l e s

Air Free induction

T a b l e 1-Al Low RPM range 4 c m clearance Shroud In & Out

T a b l e 2-A1 Low RPM range 2 . 5 c m c learance Shroud Out

Table 3-Al Law RPM range 2 . 5 c m clearance Shroud In

Table 4-A1 High EiPM Range 4 cm clearance Shroud In & Out

T a b l e 5-A1 High RPM range 4 c m clearance Impeller IIB

Table 6-A1 High RPM range 2.5 cm clearance Shroud In & Out

Table 1-Al : Power vs. R P M

Vesse1 Square base L i q . Vol. 5 1 B a f f l e s No baff les Clearance 4 cm A i r Free induction RPM Low range

1,In * RPM 1 V FP SFP

IV, In RPM SFP

4 . 5 6.1 9.3 11.8 14.5 17.5 21.0 24.3 27.4 2 9 . 4

1 4 RPM 1 V FP SFP

III,O RPM 1 SFP

5.6 6.4 7.6

10.1 11.9 12.2 13.5 15.2 17.8

IV,O RPM 1 V FP SFP

*: Full range of RPM was included, qood correlation was obtaine

Table 2 - A l : Power vs. RPM

V e s s e 1 Square base Liq. Vol. 5 1 B a f f l e s N o baffles Clearance 2.5 cm Air F r e e induction R P M Low range Shroud Out

Impeller 1

RPM 1 V FP SFP

Impeller 1, repeated t e s t

RPM 1 V FP SFP

Impeller IV

RPM 1 V FP SFP

Irnpeller I L

RPM 1 V FP SFP

5.8 7.9 7.3 8.6 10.6 12.3 13.8 15.1 16.9

Impeller III

RPM I V FP SFP

Impeller I V , repeated test

RPM 1 V FP SFP

Note: Maximum deviation of the repeated tests is about 12%. These tests were not used in the correlation.

Table 3-Al: Power vs. RPM

Vesse1 Square base L i q . Vol. 5 1 Baffles N o baffles Clearance 2 . 5 cm Air Free i n d u c t i o n RPM Low range Shroud In

Impeller 1

RPM 1 V SFP

3 . 5 6 . 9 7 . 2 9 . 5

1 0 . 5 1 2 . 8 1 3 . 7 1 5 . 5 1 7 . 7 1 8 . 4

Irnpeller III RPM 1 V FP SFP

Impeller II

RPM I V FP

.- -

SFP

3 . 6 5.2 6.4 8 . 3

1 0 . 1 1 2 . 4 1 7 . 0

Impeller IV

RPM I V SFP

2 . 6 3 . 7 5.0 6 . 1 7 . 3 8 . 3

1 0 . 1 1 1 . 1 1 3 . 3 1 4 . 6 1 7 . 0 1 7 . 3 19.9 2 2 . 3 2 4 . 4

Table 4 - A 1 : Power vs. RPM

Vesse1 Square Base L i q u i a Volume 5 1 Clearance 4 cm Baffles No Baffles Air Free induction RPM High range

- -

1,In RPM V 1 FP SFP

1, In, Repeat RPM V 1 FP SFP

I , O RPM V 1 FP SFP

IV, O RPM V 1 FP SFP

RPM

1220 1330 1550 1720 1825 1980 1500 1800 2100 2400 2700

1 FP SFP

II, O RPM V 1 FP SFP

II1,O RPM V 1 FP SFP

Table 5-Al : Power vs. RPM

Vesse1 Square base ~ i q . Vol. 5 1 Clearance 4 cm Baffles No baffles Air Free induction RPM High range Impeller I I B

IIB, In

RPM V I FP SFP HSFP RPM V 1 FP SFPHSFP

*: Unstable power consurnption and high pulsation, much severe when the shroud is in p l a c e around the impeller.

Table 6-A1: Power vs. RPM

Vesse1 Square base Liq. Vol. 5 1 Clearance 2.5 cm Baffles No baffles A i r F r e e induction RPM H i ~ h range

11,o RPM 1 V FP SFP

L O R P M 1 V FP SFP

III, O RPM 1 V FP SFP

1,In RPM 1 V FP SFP

II, In RPM 1 V FP SFP

III, In RPM 1 V FP SFP

RPM 1 V FP SFP RPM I v FP SFP

Note: The 750 RPM reading was not taken in t h e c o r r e l a t i o n while the shroud was out of place

Appendix A2 Power va. RPM under Forced Gassing


Liquid volume 5 1 water

Baffles No baffles

Clearance 4 cm

Table 1-A2 Air: 18 l/min

Table 2-A2 Air: 42 l/min

Impellers 1, II, III, IV

Impellers 1, II

Power vs. superficial air velocity


Liquid volume 5 1 water

Baffles No baffles

Clearance 4 cm

Table 3-A2 Impeller 1 Shroud In

Table 4-A2 Impeller 1 Shroud Out

Table 5-A2 Impeller II Shroud In

Table 6-A2 Impelller II Shroud In

Table 1-A2: Power vs . RPM under F o r c e d Gassing

Vesse1 Square base L i q . Vol. 5 1 Clearance 4 cm Baffles No baffles Air 18 l /min RPM H i g h range

I,O RPM 1 V GP SGP

I,In RPM 1 V GP SGP

II1,O RPM V 1 GP SGP

II, O RPM 1 V GP SGP

II,In RPM I V GP S G P

IV,O RPM

1320 1480 1740 1960 2100 2260 2450

V I GP SGP

Table 2-A2 : Specific Power vs. RPM under Forced Gassing

Vesse1 Liq. Vol. Clearance Baffles A i r

Square base 5 1 4 cm No baffles 4 2 l /min

If0 RPM V 1 GP SGP

I , O , RPM

1300 1400 1500 1600 1800 1980

repeated test V 1 GP SGP

1,In RPM V 1 GP SGP

II, In RPM V 1 GP SGP

II, O RPM V 1 GP SGP

T a b l e 3-A2: Power vs. Superficial Air Velocity

V e s s e 1 Square base Liq. Vol. 5 1 Clearance 4 cm B a f f l e s No baffles Impeller 1, shroud In place

***** NLP = 3.45 + 0.0334 * RPM

1,In RPM = 1500

AR SAV V 1 SGP HSGP

12.2 6.2 4.5 3.6 3.1 2.7

1, In RPM = 1900 AR SAV V 1 SGP HSGP

I,In RPM = 1700

AR SAV V 1 S G P HSGP

1,In RPM = 2000 AR SAV V 1 SGP HSGP

Table 4-A2: Power vs. superficial Air Velocity

Vesse1 Square base Liq. Vol. 5 1 Clearance 4 cm Bai f les No baffles Impeller 1, shroud Out of place

L O RPM = 1500

AR SAV V 1 SGP HSGP

L O RPM = 2000

AR SAV V 1 SGP HSGP

1,O RPM = 2000, Repeated t e s t

AR SAV V I SGP HSGP

Table 5-A2: Power vs. Superficial A i r Velocity

Vesse1 Square base, side length = 20 cm Liq. Vol. 5 1 Clearance 4 cm Baffles No baffles Impeller II, shroud In place

NLP - 3 . 4 5 + 0.0334 * RPM

I1,In R P M = 1500 AR SAV V I SGP HSGP

11,1n RPM = 1700 AR SAV V 1 SGP HSGP

I1,In RPM = 1900 AR SAV V I SGP HSGP

II,In RPM = 2000 AR SAV V 1 SGP HSGP

Table 6-A2: Power vs. Superficial Air Velocity

Vesse1 Square base, side length = 20 cm L i q . Vol. 5 1 C l e a r a n c e 4 cm B a f f l e s No baffles Impeller II, shroud Out of place

NLP = 3 . 4 5 + 0 .O334 * RPM --p.- . - .- - . . -

II,O R P M = 1500

AR SAV V 1 SGP HSGP

AR SAV V 1 SGP NLP HSGP

AR SAV V 1 SGP NLP HSGP

Appendix A3 Induced Static Head vs. RPM


Baffles No baffles

Clearance 4 cm

Air N o a ir , a i r i n l e t connected to water manometer

Table 1-A3 Impeller I L i q u i d volume: 4 & 5 1

Table 2-A3 Impeller II L i q u i d volume: 4 & 5 1

Table 3-A3 Impeller III L i q u i d volume: 5 1

Table 4-A3 Impeller IV Liquid volume: 5 1

Table 1-A3: Induced Static Head vs. RPM

Vesse1 Square base Liq. Vol. 4 & 5 L Clearance 4 cm Baffles No baffles Air No air, Stand pipe connected to manometer Impeller 1

Method: measure the head difference (ISH) in a water manometer connected to the suction hole in the stand pipe.

I,In, 4 1 RPM V 1 UP ISH

I,In, 5 1 R P M V I UP ISH

I,O, 4 1 RPM V 1 UP ISH

I,O, 5 1 RPM V 1 UP ISH

Table 2-A3 : Induced Static Head vs. RPM

Vesse1 Square base L i q . V o l . 4 & 5 L Clearance 4 c m Baffles N o b a f f l e s Air No a i r , Stand pipe connected to manometer ImpeIler II

Method: measure the head difference ( I S H ) i n a water manometer connected to the suc t ion h o l e i n t h e stand p ipe .

II,Inf 4 1 RPM V 1 UP ISH

IIfInf 5 1 RPM V I UP ISH

ISB

4 . 3 7.8

13.25 17.2

II,O, 5 1 RPM V I ISH

3 . 7 6 . 8

10.5 15

2 0 . 7 5 2 5 . 7 5 31.5

3 7 - 75


Vesse1 Square base L i q . Vol. 5 1 Clearance 4 cm Baffles No b a f f l e s A i r No a ir , Stand pipe connected to manometer Impeller III

Method: measure the head difierence (ISH) in a water manometer, connected to the suct ion hole i n the stand pipe.

ISH

5.8 9.6

13.4 16.85 20.5

25-05 30.15 35.2

ISH

2.2 5 4 8.8

13 1 17.6

23.65 29.65 35.75


Vesse1 Square base Liq. Vol. 5 1 Clearance 4 cm Baffles No baffles Air No air, Stand pipe connected to manometer ImpeIler IV

Method: measure the head difference (ISH) in a water manometer connected to the suction hole in the stand pipe.

IV,In, 5 1 RPM V ISH

cm

2.05 4.15 6.4 8.9 11.6 14.2 17.5 21.3 25.5

28 33.3

IV,O, 5 1 RPM V 1 ISH

cm

Appendix A4 Rate of Gas Induction vs. RPM


L i q u i d volume 5 1

Baffles No baffles

Clearance 4 cm

Table 1-A4 Rate of gas induction vs. RPM

Table 1-A4 : Rate of Gas Induction vs. RPM

Vesse1 Base side Liquid Volume Clearance Baffles

Square base 2 0 c m 5 l 4 cm No baffles

Method: Observe reading of rotameter connected to the suction hole when DP around the impeller is zero. (between a hole near the suction hole and ambient air)

Calibration of Rotameter Rate of Gas Induction (RGI) = A + B * Rotameter Reading (RR) A = 4.55 B = 7.77

1 In

R P M RR

1 In

RGI

28 34 4 3 47 5 2 56

1 O RGI

27 38 47 55 60

II III O O

RGI RR

III O

RGI

22 40 55 66 78 83

Linear correlation of R G I with RPM

1, In : RGI = -9.1 + 0.0313 * RPM I f 0 : RGI = -29.7 + 0.0418 * RPM f1,O : RGI = -37.3 + 0.0552 * RPM II1,O : RGI= -47.4 + 0.0614 * RPM

Calculated RGI : RPM 1200 1400 1600 1800 2000

1, In : RGI = Ir0 : RGI = II,O : RGI = II1,O : RGI=

CRPM

Appendix A5 Mixing T h e vs. RPM

V e s s e 1 Square base vesse1

Liquid volume 5 1

B a f f l e s N o baffles

Clearance 2.5 cm

A i r F r e e induction

Table 1-A5 T r a c e r II 200 - 600 RPM T a b l e 2-A5 T r a c e r 1 1000 - 1600 R P M

Table 3-A5 Tracer II 1000 R P M

Table 4-A5 Tracer II: 1250 RPM

Table 5-A5 Tracer II 1500 RPM

Table 1-A5 : Mixing T i m e vs. RPM

Vesse1 Square Base Liq. Vol. 5 1 Clearance 2.5 cm Baffles No baffles Air Free induction

Method Coloration tirne by visual observation and stop watch Solution 10 ml starch indicator, 10 ml (1/1) sulfuric acid Tracer one drop iodine solution ( .1 N) at the top, Addition in the middle of the surface area using a dropper

AVE 29 17 33 38 1 19 10 22 24

RPM 1 II III IV In In In In M T M T M T M T

AVE 15 8 17 22 1 11 6 12 17

RPM 1 II III IV In In In In MT MT MT MT

AVE 10 5 10 13

Table 2-A5

Vesse1 Liq. Vol. Clearance Air Baffles Air

Mixing Time vs. RPM

Square Base 5 1 2.5 cm Free induction No baffles Free induction

Method Coloration t i m e using video camera 186 frames per second

Tracer 2 ml water solution of 5% eriochrome black T and 5% indigo carmine; tracer ONE

Addition At the top in the middle of the area, by a beaker

1 II III RPM In In In

NF NF NF

AVE MT 1.7 0.9 1.2

1 II III IV II RPM In In In In In

NF NF NF NF NF

AVE MT 1.3 1.0 1.0

AVE MT 1.8

*: Tracer is 2.5 ml potassium pyrogallate solution

Table 3-A5 : Mixing Time at 1000 R P M

Vesse1 Square Base Liq. Vol. 5 1 Clearance 2.5 cm Baffles No b Air Free induction

Method Coloration time using video camera, 186 frames per second

Solution 10 ml starch indicator, 10 ml (1/1) sulfuric acid Tracer 0.3-0.4 ml iodine solution (0.1 N) , (tracer TWO) Addition At the top in the middle of the area, usinf a dropper

* * * * * * * * RPM 1 1 1 1 II II II 1 1 II II III III

In In In In In I n In O O O O O O NF NF NF NF NF NF NF NF NF NF NF NF NF

RPM

1 1 1 1 II II II 1 1 II II III III In In In In I n I n In O O O O O O

Ave . MT 2.0 2.0 2.0 2.1 1.6 1.6 1.5 1.9 1.7 1.8 1.9 1.9 1.9

1 II I II III In In O O O

Ave MT 2.0 2.8 1.8 1.8 1.9

*: Repeated test

Table 4-A5 : Mixing Time at 1250 RPM

Vesse1 Square Base Liq-Vol. 5 1 Clearance 2.5 c m Baffles No baffles Air F r e e induction

Method c o l o r a t i o n time using video camera 186 frames per second

Solution 10 ml starch indicator, 10 ml (1/1) sulfuric acid 0.3-0.4 ml iodine solution (0.1 N)

Addi t ion A t the top i n the middle of the area, using a drop

RPM 1 II III I V 1 II In In In In O O NF NF NF NF NF NF

R P M

Ave M T 1.8 1.1 1.1 2.4 1.7 1.3

Table 5-A5 Mixing T i m e a t 1500 RPM

V e s s e 1 Liq. Vo l . Clearance Baff les Air

Square Base 5 1 2 . 5 cm No baff 1 Free induction

Solution: 10 ml starch i n d i c a t o r , 10 ml ( 1 / 1 ) sulfuric a c i d Tracer : 0 . 4 - 0 . 5 m l iod ine s o l u t i o n ( 0 . 1 N ) (tracer TWO) Addition: At the rop half way from the corner

RPM

1500

D D D D D 1 1 III11 IV 1 1 In In In NF NF NF NF NF

* * * D D D M P S D

If II II In In In NF NF NF

AVE MT 1 . 2 1 . 3 0 . 8 1 . 1 1 . 8 1 . 9 1 . 1 1 . 2 1 . 0 1 . 1 1 .1

Ave MT

1,In 1 . 3 II, I n 1 . 0 III, In 1.1 IV, I N 1 . 9

D: Dropper 1 MP: Micropipette 1 S:Syringe *: Repeated test

Appendix A6 Mass Transfer Coefficient vs. RPM

V e s s e 1 Square b a s e vesse1

Liquid volume 5 1

Baffles No baffles

Clearance 4 cm --

Temp . 25 OC

Feed Ins ide stand-pipe

Oxygen probe Side entry

Table P A 6 Imp. II & IV Shroud Out

Table 2-A6 Imp. IIB Shroud In and Out

Table 3-A6 Imp. 1 Shroud In

Table 4-A6 Imp. 1 Shroud Out

Table 5-A6 Imp. IIC Shroud Out

Table 6-A6 Imp. III Shroud Out

Table 7-A6 I m p 1 Shroud In and Out H i g h side entry of 02 e l e c t r o d e

Table 1-A6 : Mass Transfer Coefficient vs. RPM

Vesse1 L i q . Vol. Clearance Baffles Air Impellers

Square base 5 1 4 cm No baffles F r e e induction II & IV

Temperature 25 Deg. C Feed Inside stand-pipe Oxygen probe Side entry

Catalyst solution (cobalt sulfate) : 1 m o l / 1 Sodium s u l f i t e feed solution -75 M Rotameter reading = 20 =0.2425 g/s = 0.2195 ml/s (Rho=l. 105) F= 0.2195 *10A-6*16*1000* .75 / (2 *O~O05) = 0.263

File : OXY224 II,O

RPM [O21 [02][D02] K1A e s

1310 8.3 2.5 5.8 0.061 1560 8.3 4.3 4 0.088 1770 8.3 5.3 3 0.117 1920 8.3 5.7 2.6 0,135 2180 8.3 6.3 2 0.176 2300 8.3 6.5 1.8 0.195

Test run with 1 M s u l f i t e sol.

File : OXY235 II, O Study on catalyst, TW

[Oz] [OS] [DO21 KLA C.S. e s ml

File : Oxy255 II,O

RPM [OS] [O21 CD021 K1A e s

1360 8.3 5 3.3 0.080 1490 8.3 5.5 2.8 0.094 1640 8.3 5.8 2.5 0.105 1800 8.3 6.2 2.1 0.125 1950 8.3 6.5 1.8 0.146 2060 8.3 6.7 1.6 0.165 2330 8.3 7 1-3 0.203

File : OXY303 IV,O

RPM [O21 [OZ] [DO21 K1A e s

2420 8.3 3.9 4.4 0.060 2110 8.3 2.6 5.7 0.046 2600 8.3 4.2 4.1 0.064 1970 8.3 1.8 6.5 0.041


Vesse1 tiq. V o l . Clearance Baffles Air Impeller

Square base 5 1 4 cm No baffles Free induction IIB

Temperature 25 Deg. C Feed Inside stand-pipe Oxygen probe Side entry

Catalyst solution (cobalt sulfate) : 1 moï/mA3 Rotameter reading = 20 =0.2425 g/s = 0.2195 ml/s F= 0.2195 *IOA-6*16*1000*. 75/ (2*0.005) = 0.263

File : OXY266 IIB, Out

RPM [OS] [OS] [DO21 K1A e s

File : OXY272 IIB,O

RPM [O2 ] [O21 [DO2 ] K1A e s

OXY273 IIB, In

RPM [O21 [O21 [DOS ] K1A e s D.02 K1A

Table 3-A6 : Mass Transfer Coefficient vs- RPM

Vesse1 Liq. Vol. Clerance Baffles Air Impeller

Square base 5 1 4 cm No baffles Free induction 1, shroud In

Sodium sulfite solution : 1 M KLA = FR / (37.5 * [D02])

1800 RPM Ave . FR [OS] [DO21 K1A K1A

s 12 3.8 4.5 0.071 0.070 18 1.6 6.7 0.072 18 1.5 6.8 0.071 12 3.7 4.6 0.070 6 6 2.3 0.070

--- - - - - - -

2100 RPM Ave . FR [O21 [DO21 K1A K1A

S

12 4.6 3.7 0.086 0.086 18 2.7 5.6 0.086 22 1.6 6.7 0.088 22 1.4 6.9 0.085 18 2.6 5.7 0.084 12 4.6 3.7 0.086 6 6.4 1.9 0.084

2400 RPM Ave . FR [02] [DO21 K1A K1A

S

18 3.55 4.750.101 0,098 24 1.6 6.7 0.096 12 5 3 - 3 0.097 6 6.7 1-6 0.100 18 3.3 5 0.096

2700 RPM Ave . FR [O21 CD021 K1A K1A

S

3000 R P M Ave . FR [OS] [DO21 K1A K1A

s 12 6.4 1.9 0.126 0.125 * 15 5.9 2.4 0.125 18 5.4 2.9 0.124

*: Test run with 0.75 M sodium s u l f i t e solution

Table 4-Ad: Mass Transfer Coefficient vs. RPM

Vesse L i q . Vol. Clearance Baffles A i r Impeller

Square base 5 1 4 cm No baffles Free induction 1, shroud Out

Sodium sulfite solution : 1 M KLA = FR / ( 3 7 . 5 * [ D O S ] )

1 8 0 0 RPM A v e . FR 1023 [DO21 K1A KlA

S

2 1 0 0 RPM Ave , FR [OS] [DOS] K1A K1A

s

2 4 0 0 RPM A v e . FR [ O S ] [DOS] K1A K1A

s

2 7 0 0 RPM A v e . FR [O21 [DO2 ] K1A K1A

S

3 0 0 0 RPM A v e . FR [O21 CD021 K1A KlA

s

3 0 0 0 RPM with I baffle FR [OS] [DO21 K1A

s

- --- - - --

*: Test run w i t h 0 . 7 5 sodium sulfite solution

T a b l e 5-A6 : Mass T r a n s f e r Coefficient vs. RPM

Vesse1 L. Vol. Impeller clearance Air

Square base 5 1 IIC, shroud O u t 4 cm Free induction

Sodiumsulfite solution : 1M KLA = FR / (37.5 * [D02])

1800 RPM A v e . FR [ O 2 1 C D 0 2 1 K 1 A K1A

S

A v e . [DOS] K1A K 1 A

2 7 0 0 RPM FR [ O S ] [ D O S ] K 1 A Ave .

s K 1 A

2400 RPM Ave . FR [ O 2 1 [ D O 2 1 K 1 A K1A

S

3 0 0 0 RPM A v e . FR [O21 [ D O 2 1 K 1 A K 1 A

s

3 0 0 0 RPM, 1 B a f f l e FR [ O 2 1 [ D O 2 1 K1A A v e

s

*: Test result with 0 . 7 5 mol/l sodium s u l f i t e feed solution


Vesse1 Square base Liq. Vol. 5 1 D i s t e l l e d water Impeller III, shroud Out Temp . 25 Deg C Clearance 4 c m Air Free induction

Side low entry of 02 electrode 3 m l Catalyst Solution (Imol/mA3) Sodium S u l f i t e Feed Solution: 1000 moi/mA3 (1 M) KLA=FR/(37.5 * [DOS])


S


S

2700 RPM Ave . FR (021 [DO21 K1A K1A

s

2700 R P M Ave . FR (021 [DO21 K1A K1A

S

3000 RPM Ave .

FR [O21 CD021 K1A KLA s

18 6.9 1.4 0.257 0.248 * 21 6.6 1.7 0.247 24 6 - 3 2 0.240

3000 RPM with one 2.7 cm baffle

FR ~ 0 2 3 1~023 K ~ A s

21 7.05 1.25 0.336

*: Test results with 0.75 mol/l sodium sulfite feed s o l u t i o n

Table 7-A6: Mass Transfer Coefficient vs. RPM

Vesse1 Square base Liq. Vol. 5 1 Clearance 4 c m Baffles N o B a f f l e s I m p e l l e r 1 Air F r e e induction

Temp 25 D e g C Feed Inside stand-pipe Probe entry High s ide entry Catalyst 2 m l

Catalyst solution (cobalt sulfate) : 1 mmol/l Calibration o f f e e d rotameter: Rotameter reading = 20 =0.2425 g/s = 0 . 2 1 9 5 ml/s (Rho = 1 . 1 0 5 ) Sodium sulfite s o l u t i o n = 1 M = 2 N = 1000 mol/mA3 * * F = 0 .351 K1A = F / CD021

Tap water, 2 m l C a t a l y s t solution - 1,In File : OXY147

RPM [O21 [OS] [DO21 K1A e s

1,1n F i l e OXY287

RPM CO21 [O21 [DO21 K1A e s

Appendix A7 Correlations


Liquid volume 5 1

Baffles No baffles

A i r Free induction

Table 1-A7

Table 2-A7

Table 3-A7

Table 4-A7

Table 5-A7

Table 6-A7

Table 7-A7

Table 8-A7

Table 9-A7

Table 10-A7

Table 11-A7

Table 12-A7

Table 13-A7

Power vs. RPM 4 cm clearance Low RPM

Power vs. RPM 2.5 cm clearance Low R P M

Power vs. D M 4 cm clearance H i g h W M

Power vs. RPM 2.5 cm clearance High R P M

Power vs. RPM under Forced Gassing 4 cm Cl.

Power vs. Superficial Air Velocity 4 cm clearance

Induced Static Head vs. RPM 4 cm clearance

Rate of Gas ~nduction vs. Power 4 cm clearance

Mixing Time vs. RPM 2.5 cm clearance Low RPM

Mixing Time vs. RPM 2.5 cm clearance High RPM

( su-ary 1

Mass Transfer Coefficient vs. RPM 4 cm cl.

Mass Transfer Coefficient vs. Power 4 cm cl.

Mass Transfer Coefficient vs. Hydraulic Power - 4 cm clearance

Table 1-A7: Correlations of Power vs. R P M

Vesse1 Liq. Vol. Baffles Clearance Air RPM Raw Data

Square base s 1 No baffles 4 cm F r e e induction Low range Table 1-Al

Rn 2 If In SFP = 4.41E-03 * RPM A 1.116 O. 9973 IVJn SFP = 3.70E-03 * RPM A 1.135 1 0.9984

Ir0 S F P = 3.45E-03 * RPM A 1.129 II1,O SFP = 5.74E-03 * RPM A 1.067 IV,O SFP = 3.03E-03 * RPM A 1.139

Calculated SFP

RPM SFP SFP 1 IV In In

SFP SFP SFP 1 III IV O O O

Table 2-A7: Correlations of Power vs. RPM

Vesse1 L i q . V o l . Baff Les Clearance a r RPM Raw data

Square base 5 1 No baffles 2.5 cm Free i n d u c t i o n Low range Tables 2-A1 6 3-Ai

I J n : SFP= 4 .971 -03 * R P M A 1 . 0 9 4 1 0 .9961 I 1 , I n : S F P = 6 .143-03 * RPM A 1 . 0 7 9 1 0 .9913 III , ln : SFP = 9.06E-03 * RPM A 1 . 0 3 5 1 0 . 9 9 8 3 IV,In : SFP = 4.843-03 * RPM A 1 . 0 7 7 1 0 .9998

I f 0 : S F P = 1.30E-02 * RPM A 0.936 1 I I , O : SFP = 6.80E-03 * RPM A 1 . 0 5 6 1 I I I , O : SFP = 1.30E-02 RPM A 0 . 9 5 1 1 IV,O : S F P = 6 .723 -03 * RPM A 1 . 0 1 4 1

Calculated SFP

RPM 1 II III IV In

1 In

II In

III In

IV O O O O

Table 3-A7 : Correlation of Power vs. RPM

Vesse1 Liquid Volume Clearance Baffles Air RPM Raw data

Square Base 5 1 4 cm No Baffles Free induction High range Tables 4-A1 & 5-A1

NLP = 3.69 + 0.0334 * SFP vs . RPM

I,In : SFP = 1.672E-03 * RPM 1 , O : S F P = 1.710E-03 * RPM II, O : SFP = 9.0713-04 * RPM IIB,O : SFP = 7.093E-04 ir RPM fIC,O : SFP = 4.5893004 * RPM III,O : SFP = 2.933E-04 * RPM IV,O : SFP = 8.794E-04 * RPM

RPM

Calculated SFP : RPM 1

In

** 1 II IIB IIC III IV SNLP O O O O O O

Calculated HSFP

RPM 1 1 II IIB IIC III IV In O O O O O O

**: Fictitious specific no-load power for 5 1 liquid volume

Table 4-A7: Correlations of Power vs. RPM

Vesse1 L i q . Vol. Clearance Baffles A i r RPM Raw data

I,O II,O III, O IV,O

1,In I1,In III,I IV, In

SFP SFP SFP SFP

SFP SFP SFP SFP

Square base 5 1 2.5 cm No baffles F r e e induction High range Table 6-Al

R ^2

Calculated SFP

R P M A 1.121 RPM A 1.395 R P M A 1.229 RPM A 1.231

RPM " 0.917 RPM " 1.087 R P M A 0.991 R P M " 0.833

RPM 1000

0.999 0.998 0. 998 0.980

0.985 0.986 0.996 0.996

L O 9.0 II,O 8.9 III, O 8.9 IV, O 8.1

1,In 11.5 I1,In 11.8 III, In 12. O IV, In 10.5

Table 5-A7 : Correlations of Power vs . RPM under F o r c e d Gassing

Vesse1 Square base Liq. Vol. 5 1 Clearance 4 c m Baffles No Baffles A i r F o r c e d and free induction RPM High range Raw data Tables 1-A2 & 2-A2

F r e e Induction (high RPM range) RA 2

1, In : SFP = 1.67E-03 * RPM A 1.236 11,In : SFP = 1.14E-03 * RPM A 1.308 &O : SFP = 1.71E-03 * RPM A 1,218 II,O : SFP = 9.07E-04 * RPM A 1.313 IIB,O : SFP = 7.093-04 * RPM A 1.338

18 l/min Air

Calculated Specific Power RPM 1200 1400 1600 1800 2000 2200

1,In : SGP = 7.94E-06 * RPM A 2.034 I1,In : SGP = 2.51E-05 * RPM A 1.885 1,O : SGP = 1.33E-03 * RPM A 1.270 II,O : SGP = 6.803-04 * RPM A 1.367 II1,O : SGP = 1.86E-03 * RPM A 1.224 IV,O : SGP = 2.203-03 * RPM A 1.179

42 l/min Air

Free

0. 9999 0. 9998 O. 9973 1. O000 0.9970 O. 9968

I,In : SGP = 1.82E-05 * RPM A 1.854 I1,In : SGP = 7,313-05 * RPM A 1.692 1,O : SGP = 2.973-03 * RPM A 1,145 II,O : SGP = 1,19E-03 * RPM A 1.274

I,In : 10.7 12.9 15.3 17.7 20.1 22.6 I1,In : 12.1 14.9 17.7 20.6 23.7 26.8 If0 : 9.6 1.6 13.7 15.8 17.9 20.1 II, O : 10.0 12.3 14.6 17.1 19.6 22.2 IIB , O : 9.3 11.5 13.7 16.1 18.5 21.0

0.9821 0.9970 O. 9994 0. 9970

I,In : 14.6 19.9 26.1 33.2 41.1 49.9 I1,In : 16.0 21.4 27.5 34.3 41.9 50.1 L O : 10.8 13.2 15.6 18.1 20.7 23.4 II, O : 11.0 13.6 16*3 19.2 22.1 25.2

T a b l e 6-A7: Correlations of Power vs. Superficial A i r Velocity

Vesse1 Square base, side length = 20 cm Liq. Vol, 5 1 Clearance 4 cm Baffles No baffles Raw data Table 3-A2 through 6-A2

RPM

1,In 1500 : HSGP = 1,In 1700 : HSGP = I n 1900 : HSGP =

II,In 1500 : HSGP = I1,In 1900 : HSGP =

II,O 1900 : HSGP = II,O 2000 : HSGP =

1232 * SAV " -1,399 12580 * SAV A -1,871 27726 * SAV A -1,985

18.9 * SAV A -0,317 O. 9790 97.9 * SAV A -0.621 1 0.9894

34.0 * SAV -0,432 0. 9920 56.2 * S A V A -0,555 1 0.9934

Calculated HSGP:

RPM 1500

SAV AR

Table 7-A7 : Correlations of the Induced Static Head vs. RPM

V e s s e 1 Square base L i q . Vol. 5 L Clearance 4 cm Baffles No baffles Air No a i r , a i r i n l e t connected to m a n o m e t e r Raw data Tables 1-A3 through 4-A3

Method: m e a s u r e m e n t of the head difference in a water m a n o m e t e r connected to a hole in the stand p ipe .

1,fn II, In III, In IV, In

Ir0 II,O III, O IV, O

RPM A 2 . 0 9 2 RPM A 2 . 2 6 0 RPM A 2 . 2 4 7 RPM A 2 . 4 1 0

R P M A 2 . 0 7 2 RPM A 2 . 4 7 3 RPM A 3 . 0 7 2 RPM A 2 . 7 9 7

Calculated ISK for 5 1 liquid v o l u m e :

1 II III IV 1 II In In In In O O

III IV

Table 8-A7: C o r r e l a t i o n s of the R a t e of Gas I n d u c t i o n vs P o w e r

Vesse1 Base s ide L i q u i d Volume C l e a r a n c e Baffles

Square base 20 Qn

5 1 4 cm No baffles

SFP VS. RPM, 1 5 0 0 - 3 0 0 0 RPM

1 , I n : S F P = 1 . 6 7 E - 0 3 * R P M A 1.236 I r 0 : S F P = 1 . 7 1 E - 0 3 * = M A 1 . 2 1 8 I I , O : S F P = 9 . 0 7 E - 0 4 * R P M A 1.313 III ,O : SFP = 2 . 9 3 3 - 0 4 * R P M A 1 . 4 5 8

RPM 1 I n

SFP

1 I n

R G I

1 O

S F P

1 O

RGI

II O

S F P

II 0

RGI

III 0

SFP

III 0

RGI

C o r r e l a t i o n of RGI vs. S F P R A

I f I n : RGI = -3.3 + 2 .93 * SFP 1 0 . 9 7 4 I r 0 : R G I = - 1 6 . 4 + 3.91 * SFP 1 0 . 9 8 3 II,O : R G I = - 1 5 . 6 + 4.53 * SFP 1 0 .992 III ,O : RGI = -24.5 + 5 . 5 0 * SFP 1 O . 98

C r i t i c a l p o w e r for gas i nduc t ion

1 , I n : CSFP = 1.1 I f 0 : CSFP = 4.2 I I f O : CSFP = 3.5 I I I , O : C S F P = 4 . 5

Note: The first 4 p o i n t s were used i n the corre la t ion of i m p e l l e r 1 , I n - T h e first 5 p o i n t s w e r e used i n t h e correlation of impeller I I 1 , O

Table 9-A7: Correlations of Mixing Tirne vs. RPM

Vesse1 Square Base L i q . Vol. 5 1 Clearance 2.5 cm Bai i les No ba A i r Free induction RPM Low range Raw data Table 1-A5

Method Coloration t i m e by visual observation and stop watch Solution 10 ml starch indicator, 10 ml (Wl) s u l f u r i c acid Tracer 1 drop (about 0.2 ml) iodine solution (0.1 N) Addition A t the top, half way from the corner, by a dropper

Coorelat ions

1,In : MT = 5790 / RPM I1,In . MT = 3229 / RPM III, In : MT = 6543 / RPM IV, In : MT = 7747 / RPM

Calculated MT

RPM RPM RPM RPM RPM 200 300 400 500 600

1,In 28.9 19.3 14.5 11.6 9.6 II,In 16.1 10.8 8.1 6.5 5.4 III, In 32.7 21.8 16.4 13.1 10.9 IV, In 38.7 25.8 19.4 15.5 12.9

Calculated 100/MT

RPM RPM RPM RPM RPM 200 300 400 500 600

Correlations

1,In . MT = 49.7 / SFP II, I n . . MT = 27.7 / SFP I11,In : MT = 56.2 / SFP IV, In . . MT = 66.5 / SFP

Calculated MT

SFP 2 3 4 5 6

1,In : 24.8 16.6 12.4 9.9 8.3 I I J n : 13.9 9.2 6.9 5.5 4.6 II1,In : 28.1 18.7 14.0 11.2 9.4 IV, In : 33.2 22.2 16.6 13.3 11.1

, Calculated 100/MT

Table 10-A7 :

Vesse1 L i q . Vol. Clearance Baffles Air RPM Raw data

Summary o f fixing T h e vs . RPM

Square Base 5 1 2 . 5 cm No baffles Free induction High range Tables 2-A5 through 5-A5

Method Coloration time using video camera 186 frames per second

Tracers : ONE: water so lut ion o f S%eriochrome black T and 5% indigo carmine TWO: 0 . 3 0 . 5 m l iodine solution ( 0 . 1 N) , water is acidified and starch indicator added to i t

AVE MT (s)

RPM I II III I V 1 I I III In In In In O O O NF NF NF NF NF NF NF

Tracer

1000 1.7 0.9 1.2 1.8 ONE

ONE ONE ONE

TWO

Table 11-A7: Correlations of Mass Transfer Coefficient vs. RPM

V e s s e 1 L. Vol* Shroud Clearance Temp Raw data

Square base 5 1 Distelled eater Out 4 cm 25 Deg C Tables P A 6 through 7-A6

correlations of K L A vs- R P M

1,In KLA = 1.81E-06 * RPMA 1 . 4 4 8 Ir0 KLA = 2.76E-06 * RPMA 1.349 I1,O KLA = 8.233-08 * RPMA 1.897 IIB, O KLA = 5.29E-09 * RPMA 2.238 IIC,O KLA = 4 -353-07 * RPMA 1.676 II1,O KLA = 7.35E-08 * = M A 1.874 IV, O KLA = 9.953-08 * RPMA 1.704

Calculated KLA R f M 1200

1,In . . 0 . 052 I f 0 . . 0.039 II,O . . O. 057 IIB,O : 0.041 IIC, O : 0.063 III, O . 0.043 IV, O . . 0.018

Table 12-A7 : Correlations of Mass Transfer Coefficient vs. Power

Vesse1 L. Vol. Shroud Clearance Temp Raw data

Square base 5 1 Distelled eater Out 4 cm 25 Deg C Tables 1 -A6 through 7-A6

Correlations of KLA vs. RPM

KLA = 1.81 E-06 RPMA 1.448 KLA = 2.76 E-06 RPM A 1.349 KLA = 8.23E-08 * RPMA 1.897 KLA = 5.29E-09 * RPM A 2,238 KLA = 4.35E-07 RPM A 1.676 KLA = 7.35E-08 RPM A 1.874 KLA = 9.95E-08 * RPM A 1.704

Correlations of SFP vs. RPM

I,ln : SFP = 1.67E-03 * I,O : SFP = 1.71 E-03 * Il,O 1 SFP = 9.07E-04 IlB,O : SFP= 7.09E-04 HC,O : SFP= 4.59E-04 * I11,O : SFP= 2.93E-04 IV.0 : SFP = 8.79E-04 k

RPM A 1.236 RPMA 1.218 RPMA 1.313 RPM A 1.338 RPM A 1.407 RPMA 1.458 RPM A 1.311

Correlations of KLA vs. SFP

I,ln : KLA= 3.24E-03 SFP A 1.172 I D : KLA= 3.20E-O3 SFP A 1.108 11,O : KLA= 2.05E-03 * SFP A 1.445 IIB,O : KLA= 9.79E-04 SFP A 1.673 IIC,O : KLA= 4.12E-03 SFP A 1.191 111,O : KLA= 2.56E-03 * SFP A 1.285 lV,O : KLA= 9.33E-04 * SFP A 1.300

Calculated KLA

SFP 10 12 14 16 18 20 22 24

Table 13-A7 : Correlations of Mass Transfer Coefficient vs. Hydraulic Power

Vesse1 L. Vol. Shroud Clearance Temp Raw data

Square base 5 1 Distelled eater Out 4 cm 25 Deg C Tables 1-A6 through 7-A6

Correlations of KLA vs. HSFP

I,In KLA = 3.11E-02 * HSFP A 0.709 I r 0 KLA = 3.94E-02 * HSFP " 0.533 II,O KLA = 4.42E-02 * HSFP " 0.744 IIB,O KLA = 5.05E-O2 * HSFP " 0.683 IK,O KLA = 5.45E-02 * HSFP " 0,576 III, O KLA = 4 . 15E-02 * HSFP " 0.645 IV,O KLA = 1.80E-02 * HSFP A 0.588

Calculated KLA

1,In I , O II, O IIB,O IIC,O 111,o IV, O

HSFP 2 4 6 8 10

Table 1-A8 Equilibrium Concentration of Oxygen in Distilled

water

T a b l e 2-A8 Pressure and Altitude Correction for Oxygen

Concentration in Distilled Water

T a b l e 3-248 ~alibration of Rotameter for Forced Air Flow

T a b l e 4-A8 Frictional (no hydraulic load) Power vs. RPM

Table 2-A8 : Pressure and altitude correction for oxygen concentration in distilled water

Press. A l t . Corr. F I mm Hg f t

Altitude at 200 College St. =109.2 m =358 ft Add =6 m ( = 1 8 ft) for 2nd storey above ground Crrection factor = 0.99 Equilibrium concntration of oxygen at 25 Deg C, at testing site (Walbreg Buliding, U of T) = 8.3

Table 3-A8 : Calibration of the rotameter for forced air flow F i l e AIRCAL

Time Q Flow RR S ftA3 ftA3/s

Regression Output: Constant O. 002679 Std Err of Y Est O . 000119 R Squared 0 . 999945 N o . of Observations 7 Degrees of Freedom 5

X Coefficient (s) 0 .004575 Std Err of C o e f . 0 .000015

Air Flow = 0 . 0 0 2 7 + O . 0046 * RR Air F l o w = 0 . 0 7 5 9 + O . 1296 * RR Air Flow = 4.55 + 7 . 7 7 * RR

R . R . : R o t a m e t e r Reading

Flow RR l/min

Flow l/min

Table 4-A8: Frictional (No hydraulic load) power vs . R P M

F i r s t determination

RPM V 1 NLP

Regression Output: Constant 3.69 Std E r r of Y Est O. 5692 R Squared O . 9990 No. of Observations 6 Degrees of Freedom 4

X Coefficient (s) O. 0334 Std Err of Coef. O. O005

Second determination

RPM V 1 NLP

Regression Output: Constant 3.4456 Std Err of Y Est O. 1400 R Squared 0,9997 No. of Observations 6 Degrees of Freedom 4

X Coefficient (s) O. 0334 Std Err of Coef. 0.000240

. .-

NLP = 3.44 + 0.0334 * RPM

i MAGt LVALUATION TEST TARGET (QA-3)

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