distillatio column design methanol water

Distillation Column Design Methanol-Water separation

Welahetti W.P.K 080533H

Distillation Column Design 2012

Prasanna Welahetti Page 2

Table of Contents 1 Introduction ....................................................................................................................... 4

2 Theory ................................................................................................................................ 4

2.1 Operating Line equation for Top Operating Line (TOP) .............................................. 4

2.2 Operating Line equation for Bottom operating Line (BOP) ........................................ 6

2.3 Equation of q factor .................................................................................................. 7

3 Calculation ......................................................................................................................... 7

3.1 Data ............................................................................................................................. 7

3.2 Calculation of amount of distillate and bottom product ............................................ 7

3.3 Equilibrium data of methanol and water mixture ...................................................... 9

3.4 Calculation of Minimum Reflux Ratio ......................................................................... 9

3.4.1 Drawing q line ...................................................................................................... 9

3.4.2 Drawing Top Operating Line (TOP) at minimum reflux ..................................... 10

3.5 Calculation of Operating Reflux Ratio ....................................................................... 11

3.6 Calculation of Actual number of stages required ..................................................... 11

3.7 Calculation of liquid and vapuor amounts in both sections ..................................... 13

3.8 Liquid and Vapour Density calculation ...................................................................... 14

3.8.1 Rectifying section densities ............................................................................... 15

3.8.2 Stripping section densities ................................................................................. 15

3.9 Calculation of column diameter ................................................................................ 16

3.9.1 Diameter of rectifying section ........................................................................... 16

3.9.2 Calculation of diameter of stripping section ..................................................... 19

3.10 Calculation of hole diameter and area .................................................................. 20

3.11 Number of Holes in a tray at rectifying section ..................................................... 20

3.12 Number of Holes in a tray at stripping section...................................................... 21

3.13 Flow pattern identification and down comer selection ........................................ 21

3.13.1 Flow pattern of rectifying section ...................................................................... 22

3.13.2 Flow pattern of stripping section ....................................................................... 22

3.14 Column efficiency calculation ................................................................................ 23

3.15 Calculation of actual number of stages required .................................................. 25

3.16 Calculation of height of the column ...................................................................... 25



3.17 Calculation of feed tray location ........................................................................... 26

4 References ....................................................................................................................... 27

5 Summary .......................................................................................................................... 28

Table of Figure

2.1 TOP Line ............................................................................................................................... 4

2.2 Bottom Operating Line ........................................................................................................ 6

3.1 Data ...................................................................................................................................... 7

3.2 Mass balance ........................................................................................................................ 8

3.3 Methanol Water Equilibrium Data ...................................................................................... 9

3.4 Temperature Composition Diagram of methanol/water .................................................... 9

3.5 R min Figure ......................................................................................................................... 10

3.6 Vapour Liquid Equilibrium Curve ....................................................................................... 12

3.7 Column 1 ............................................................................................................................ 13

3.8 Feeding Section .................................................................................................................. 14

3.9 Recommended general conditions and dimensions for tray tower .................................. 18

3.10 selection of Liquid-flow arrangement ............................................................................. 21

3.11 Reverse Flow Down Comer .............................................................................................. 22

3.12 Cross flow down comer ................................................................................................... 22

3.13 Van Winkles correlation .................................................................................................. 23

Content of Table

3.1 Density data ....................................................................................................................... 14

3.2 Density of Mixture ............................................................................................................. 15

5.1 Summary ............................................................................................................................ 28



1 Introduction

This is continuous distillation with reflux. There are two sections in fractionation column.

The upper section is called the rectification section and lower section is called stripping

section. Rectifying section enriches with more volatile components and stripping section

enriches with less volatile components.

2 Theory

Here we can use the Mc-Cabe-Thiele method,

Assumptions;

Constant molar overflow

There is no any chemical reaction

There are no any mass generation, mass accumulation and mass generation

2.1 Operating Line equation for Top Operating Line (TOP)

2.1 TOP Line



Material balance around the envelope shown:

Overall Balance;

Material Balance for More Volatile component (MVC);

Thus we have,

Under constant molar overflow assumption:

The subscripts can be dropped. Thus, the equation simplifies to:

Re-arranging in the form y = f(x), and Introducing Reflux Ratio R = L / D,

Equation 2.1

This is the Operating Line Equation for the rectifying section (Top Operating Line) or TOL

in short.

Characteristics: Straight Line Equation

Slope Intercept ,



2.2 Operating Line equation for Bottom operating Line (BOP)

2.2 Bottom Operating Line

Material balance around the envelope shown: Under constant molar overflow assumption:

Overall Balance;

Material Balance for More Volatile component (MVC);

Thus we have,

Equation 2.2



2.3 Equation of q factor

3 Calculation

3.1 Data

3.1 Data

3.2 Calculation of amount of distillate and bottom product

These two values can calculate using simple material equations.

Assumptions

At steady state there are no any material losses, material accumulation and material

generation.



There is no any chemical reaction inside the column

3.2 Mass balance

Overall Material balance around the envelope shown:

Material balance for MVC around the envelope shown:

Solving above two equations;



3.3 Equilibrium data of methanol and water mixture

3.3 Methanol Water Equilibrium Data

3.4 Temperature Composition Diagram of methanol/water

3.4 Calculation of Minimum Reflux Ratio

3.4.1 Drawing q line

Feed is at saturated liquid condition; therefore according to q line equation q value should

be 1.

Therefore,



3.4.2 Drawing Top Operating Line (TOP) at minimum reflux

At the minimum reflux ratio; maximum numbers of theoretical stages are given. Therefore

TOP line should be pass through interception of equilibrium curve and q-line.

3.5 R min Figure

Gradient of TOL



3.5 Calculation of Operating Reflux Ratio

No hard and fast rules can be given for the selection of the design reflux ratio, but for many

systems the optimum will lie between 1.2 to 1.5 times the minimum reflux ratios. Assume

operating reflux ratio is 1.25 of minimum reflux ratio. Therefore operating Reflux ratio (R);

3.6 Calculation of Actual number of stages required

Slop of the TOL

Therefore TOL equation;

BOL can easily draw with starting (Xw,Xw) to ending intersection of q line and TOL.



3.6 Vapour Liquid Equilibrium Curve



According to figure 3.6 vapour- liquid equilibrium curve it can count number of theoretical

plates.

Total number of stages

There are total condenser and partial reboiler. Total condenser is not act as equilibrium

stage, but partial reboiler is act as equilibrium stage. So, reboiler stage need reduce from

total theoretical stages.

Number of theoretical stages required

3.7 Calculation of liquid and vapuor amounts in both sections

3.7 Column 1

Rectifying section

D

LR o

According to constant molar overflow;

Material balance for above envelop



Stripping Section

3.8 Feeding Section

According to given data, feed is at saturated liquid condition therefore all feeding liquids

join with stripping section liquid.

3.8 Liquid and Vapour Density calculation

Inside temperature of the distillation column is varying point to point. Therefore average

temperature was considered for a section.

Position

Compositions of Methanol Liquid

Compositions of Methanol Vapour Temperature

Density of Water(kg/m3)

Density of Methanol(kg/m3)

Liquid Vapour Liquid Vapour

Feed 0.35 0.68 77C 973 0.27 742 2.01

Distillate 0.96 0.99 65C 980 0.165 752 1.12

Residue 0.02 0.04 100C 958 0.59 714 4.33

3.1 Density data



Density of liquid at 77C

=892.15kg/m3

Similarly;

Temperature Density of liquid mixture kg/m3 Density of vapour mixture kg/m3

77C 892.15 1.45

65C 761.12 1.11

100C 953.12 0.74

3.2 Density of Mixture

3.8.1 Rectifying section densities

Rectifying section is operating in between 65C -77C. Therefore at the average temperature

density values can calculate.

Liquid density of rectifying section L =

L =826.6 kg/m3

Vapour density of rectifying section v =

v =1.28 kg/m3

3.8.2 Stripping section densities

Stripping section is operating in between 100C -77C. Therefore at the average temperature

density values can calculate.

Similarly;

Liquid density of rectifying section L =922.64 kg/m3

Vapour density of rectifying section v =1.095 kg/m3



3.9 Calculation of column diameter

3.1 Flooding velocity, sieve plates

3.9.1 Diameter of rectifying section

3.2 Liquid Vapour factor

Assume tray spacing = 0.45 m

According to figure 3.1;

The flooding velocity can be estimated from the correlation given by Fair



v

vLf KU

1

Dc- Column diameter

Ac- Column cross sectional area

Ad-Down comer area

Aa Active area

Ah-Holes area

An- Net area

Ua-Actual vapour velocity required

through the column

Assume Ad=12% of Ac.

Assume 80% flooding condition;

Average molecular weight of the mixture assuming 50% methanol

(When it calculate the average molar weight, feed methanol composition is 35%,

assume inside the distillation column 50% methanol in mixture)

hmVn /)36001/(2518.1693

smVn /175.13



UaAn

Vn

Therefore according to above derived equations;

Therefore the column diameter of rectifying section is 0.999m

3.9.1.1 Rechecking the assume tray space value

Here we have assumed tray spacing for the column. According to the Recommended

general conditions and dimensions for tray tower table, it can check.

3.9 Recommended general conditions and dimensions for tray tower

According to above Recommended general conditions and dimensions for tray tower

table, diameter of the column is correct. Because assume tray space was 0.45m and

Tower diameter Tray spacing

0.15

1 or less 0.5

1 to 3 0.6

3 to 4 0.75

4 to 8 0.9



received column diameter is 0.999m. Therefore assumed column space is correct(It is in

range).

3.9.2 Calculation of diameter of stripping section

This is similar to rectifying section.

Assume tray spacing = 0.45 m

According to figure 3.1;

Assume 80% flooding condition;

There is no vapour amount changes in stripping section with compare to rectifying

section. Therefore,

smVm /175.13

UaAn

Vn



Therefore according to above derived equations;

Therefore stripping section column diameter is 0.837m

3.10 Calculation of hole diameter and area

Hole size less than 6.5 mm. Entrainment may be greater with larger hole sizes. (Page No.

568 Coulson & Richardson 6th volume 4th edition).

Therefore hole diameter is considered as 6mm.

3.11 Number of Holes in a tray at rectifying section

For a plate there is one down comer on the plate and top down comer also affected to

bottom plate. Therefore two down comer areas need to reduce from active area. So,

Substituting to above equation;

Assume holes area is 10% of active area;

Total holes area in a tray

Area of a hole

Number of hole in a plate



3.12 Number of Holes in a tray at stripping section

All equations are similar to rectifying section calculation;

Assume holes area is 10% of active area;

Number of hole in a plate

3.13 Flow pattern identification and down comer selection

The most common flow pattern is cross flow pattern. It can identify using below graph

(figure 3.10 selection of liquid flow pattern).

3.10 selection of Liquid-flow arrangement



3.13.1 Flow pattern of rectifying section

Liquid flow rate of rectifying section

Column Diameter

Therefore the flow pattern is Reverse flow(According to figure 3.10)

3.11 Reverse Flow Down Comer

3.13.2 Flow pattern of stripping section

Liquid flow rate of stripping section

Column Diameter

Therefore the flow pattern is Cross flow. (According to figure 3.10)

3.12 Cross flow down comer



3.14 Column efficiency calculation

Here we can use Van Winkles correlation for calculate to efficiency of column.

3.13 Van Winkles correlation

Maximum and minimum temperatures of inside the column are respectively 100C and

65C. Assume liquid temperature is average temperature of column. Assume properties of

liquid mixture are equal to water properties.

Average temperature of column

Surface tension of liquid at 82.5 is calculated according to their molar basis of feed.

As the ; I have taken maximum Ua value from both sections.



As vapour and liquid densities; average densities were taken,

Weir height for 1 atm column is 40-90mm. Therefore here Im assuming weir height of the

column is 45mm.

Fractional area calculation;

Surface tension number calculation;

Liquid Schmidt number calculation;

Reynolds number calculation;



Therefore;

Therefore efficiency of column

3.15 Calculation of actual number of stages required

I have already calculated required number of theoretical plates.

Number of theoretical stages required

In other way below equation can use for efficiency calculation;

Therefore number of actual stages required =17

3.16 Calculation of height of the column

No. of plates

Tray spacing Hs = 0.45 m

H= 0.5 meter each for liquid hold up and vapor disengagement



H=1 m

Assume thickness of a plate is 5mm.

Total thickness of trays

= 0.085m

Height of column =

Therefore the height of the column is 8.234m.

3.17 Calculation of feed tray location

Assume here plate numbers are count from top to bottom.

According to figure 3.6 vapour-liquid equilibrium curve; it can find the feed tray location.

Feed composition of methanol is 0.35. This value is at 10th plate (theoretical) of the column.

To calculate the actual feed tray location this value need to divide with column efficiency.

Therefore feed tray location



4 References

Separation Process En gineering,2nd Edition, Phillip C.Wankat

Chemical Engineering volume 6, 4th Edition, Coulson & Richardsons

Mass-transfer Operations,3rd Edition, Robert E.Treybal

http://pubs.acs.org/doi/abs/10.1021/je00019a016

http://www.engineeringtoolbox.com/water-dynamic-kinematic-viscosity-d_596.html

http://www.engineeringtoolbox.com/methanol-properties-d_1209.html



5 Summary

Name Sign Result

Distillate flow rate D 87.77 kmol/hr

Waste flow rate W 162.23 kmol/hr

q value q 1

Minimum Reflux ratio 0.742

Operating Reflux ratio R

Number of theoretical plates 13

Liquid flow rate of rectifying section L0,Ln 81.41 kmol/hr

Vapour flow rate of rectifying section

Flooding velocity of rectifying section

Column Diameter of rectifying Section Dc 0.999m

Flooding velocity of Stripping section

Column Diameter of Stripping Section Dc 0.837m

Tray Space of both sections lt 0.45m

Hole diameter Dh 6mm

Number of hole in a plate at rectifying Section 2106

Number of hole in a plate at Stripping Section 1478

Down comer type of rectifying section Reverse flow down comer

Down comer type of Stripping section Cross flow down comer

Column efficiency Emv 79%

Number of actual plates 17

Height of the column

Feed tray location 13th plate

5.1 Summary

distillatio column design methanol water

Documents

calculation of diameter

calculation of liquid

calculation of column

operating line equation

calculation of hole

diameter of rectifying

operating line bop

stripping section densities