8/10/2019 Industrial & Engineering Chemistry Volume 49 Issue 3 1957 Leva, Max.; Wen, Chin-Yung. -- Absorption and Humidi

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C H E M I C A L E N Q l N E E R l N G R E V I E W S

I

UNIT

I

Absorption and Humidification

P E R A T I O N S

R E V I E W

THEREas modest activity in the de-

velopment of packings and general test-

ing durin g the past year. Most efforts

were directed toward obtaining addi-

tional dat a for certain types of Sted man

packing.

I n

the field of capacity data

and processes a few interesting contribu-

tions app eare d; in particular, additional

d a t a on the hot-carbonate carbon di-

oxide-removal process were given. I n

the study of fundamenta ls several articles

treated absorption theories, penetration,

and surface renewal. In one, the correla-

tion of diffusion coefficients was con sid-

ered, and new data in wetted-wall

columns were given.

Column Studies

In order to correlate pressure drop for

two-phase flow thro ugh towers, carrying

Stedman packing, Morton and K ing

(27) modif ied the Carman equa t ion for

f low through porous media . Th e pro-

posed correlation involves a term that

pertains to voidage of the packing.

Liquid rate and flow characteristics

of

the discontinuous (liquid) phase un-

doubted ly affect voidage. Regardless

of

reflux rate, a constant thickness of liquid

film around the apertures was assumed.

This in effect postulates th at voidage re-

mained c onsta nt for the reflux rates that

were operative. A compar ison of the

proposed correlation with experimental

data shows satisfactory agreement for

low irrigation rates. Deviations become

more severe at elevated liquid rates.

This was a t t r ibuted to the advent of

loading of the packing. Th e experi-

ments also supported the now generally

accepted belief that loading and flood-

ing a re phenomena th a t deve lop gradu-

ally and not suddenly at a definite flow

point, as has been postulated earlier.

Morton, Cerigo , and King

(20)

also re-

ported holdup and flooding data for re-

duced pressure operation with Stedman

packing 6 inches in diameter. For tri-

angular pyramid-type Stedman packing

holdup below the loading point was ex-

pressed by

where H i s column holdup in ga l lons per

cubic foot of tower and R is the liquid

irrigation rate in gallons per hour per

square foot of tower cross section. T h e

limiting vapor velocity at the flooding

point, in feet per second, was given by

H = 0.095R0.6S

MAX LEVA, a consulting chemical engineer has worked in

fluidization research and in the development of tower

packings gas absorption and gas drying installations.

A native of Ludwigshafen Germany Leva holds a

B.S.

from the University of Cincinnati and

M.S.

from Carnegie

Institute of Technology. He is a regis tered professional

engineer and member of the ACS and

AIChE.

CHIN-YUNG

WEN

is teaching chemical engineering at

West Virg inia University. He holds

q

B.S. degree from

the National Taiwan University and a M.S. and Ph.D. from

West Virgin ia University. Wen has done research on gas

absorption fluidization and solid-gas

low.

He

s

a member

of Sigma Xi and

Phi

Lambda Upsilon and several technical

societies.

where G a nd

L

are liquid and gas rates,

a nd

pa

a nd p are gas and liquid densi-

ties.

A new industrial disti l lation and ab-

sorption packing, Spraypak, investi-

gated by McWilliam s and others

(79),

consists of

a

single layer of expanded

metal. Grap hical correlations were given

for pressure dro p, l iquid holdup , and en-

trainment. Spra ypak was claimed to

have a lower pressure drop than bubble

cap plates, handling the same through-

put , an d H E T P va lues were a lso re -

ported lower.

Liquid-liquid holdup was investigated

by Wicks and Beckmann 27). Toluene

was the dispersed and water the con-

tinuous phase. Rin gs were used as pack-

ing and sizes ranged from inch

throu gh all intermediate sizes to

1

inch.

Column diameters were 3,

4,

a nd 6

inches. T he da ta were corre la ted em-

pirically by an equatio n based on dimen-

sional analysis, Phenom enonwise three

different types of holdup were observed.

The dispersed-phase drops that rise

freely to the interface were described as

free holdup. Ope rationa l holdup

includes free holdup, in additio n to drop-

lets that are freed from the interstices

by pulsations. Und er total holdup

they defined the entire amount

of

dis-

persed phase in the effective packed

volume at any time. T h e possible effect

of packing arrangement in liquid-liquid

flooding was examined statistically by

Johnson and Beeckmans

(73).

The i r

studies disclosed that the phenomenon

is definitely influenced by arra nge me nt of

pieces. T he results were presented in

form of an equation.

Results of wetted area studies in a

column

5

inches in diameter were re-

ported by Hikita and Kataoka (70).

Packings investigated were rings

15, 25,

a nd 35 mm . in diameter. Liquids used

were aqueous solutions of methanol and

glycerol. As found earlier by others. in

the operating range below the loading

zone gas rate had no effect on wetted

area , but extent of wetted area was liquid-

ra te dependent . Th e da ta were corre -

lated by

m

5 = 0.0464 LI3 g)

t

where

m

=

-1.42d-0.

a, = wetted a rea

VOL. 49 NO.

3

PART II MARCH 1957 457

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The hot-carbonate process for removal of CO offers considerable steam saving, over

existing processes, during regeneration.

G = total packing area

L = l iquid rate: kg. per sq.

meter

per

= surface

tens ion ,

d y n e s per cni.

hour

0 = packing diameter. cm.

\:'erted area \vas virtually independent

of

packed height.

Effective \vetted surface area i n Ras-

chig ring

col i imns \vas

also the s u b j r c t of

a revieiv by St'hitt (25),1vho summarized

past developments, based

on

direct

niras-

i irements an d absorption behavior.

\

nomograph permitting solution

of

problems that pertain to gas-liquid

packed toiver opera tion \vas presented

by

Jacobs (72). T h e nomograph

is

based

on a

flooding correlation prrsented ear-

lier? and the data extend to Kaschis

rings and Intalox saddles.

A brief study in distillation.

descr ibed

ti>. Kirschbauin 77). einpliasiztd the

importance of ~ v a l l effect in

packed

Raschig ring coluinns. One-inch

Kas-

chig

rings \vere examined in t \ vo

400-

mm.-diameter columns.

One column

had

a smooth inside \vall: whereas the

other

c o lu mn

carried transverse ribs

a t

close

pitch. I n the latter the ring a r -

r a n g e m e n t adjacent to the

\\.all

was t ho r -

oughly irregular. Height effect studirs

disclosed

a

better performance in t h i s

t \ p of colurnn, \ \-hich vas atiribured to

tlie absence or rniniiriizcd intensit)- of

\\.all

efrect.

Capacity Datu

\Vhyness

26)

reported 0 1 1

~ h cbsorp-

tion of silicon tetrafluoride by ivatcr. .I

s -stem

of

descending ivater d r o p s

as

\vel1

as a \\-etted-w all colurnn

\vas

used.

For

absorption by drops.

the

concentration of

fluosilicic acid in the effluent increascd

\vith partial pressure of silicon tetra-

fluoride in the gas until the latter had

reached a value of about

80

mm.

of

mer-

cu ry .

A further partial pressure increase

had no effect on e R u e n t concentration.

Microscopic investigation indicated that

format ion of a silica film near the sur-

face of the drops might have hindered

fu r ther absorption . This should be kept

i n min d i n th e specification of height of a

spray absorber .

)Vetted-wall absorption data

Icere

satisfactorily correlated by a t\vo-filni

a p p r o a c h .

'I-'hr valmr

p r e s s ~ i r ~ :f siiicoii

tetrafluoride o \ ~ r luosilicic acid

\ \ a s

found negligible ; hence, in gas

a l ) s o r p -

t ion there was

no

back pressure f r u n i the

liquid phase. Th e chief rrsistancr \.as

fo~ind o be in tlie gas film, as e v i d r n c c d

by the virtually nonexistent intrrcc1)t

ivhen

1 I i f ;a

\\'as

plotted against

Q . 3 ,

N e i v absorption data for tlie s).steiii

sulfur dioxide--watcr ~vcrrobtained 11

Parkison

( 22).

The

I-inch

Raschi?

rinq

to\ver \vas 8 inches n diameter, packrd t i l )

to 2 fecr. Concrntrarions of stilfiur di-

oxide

ivere 0 . 5

to

1.5:

and

5 1 l o r ; ,

~Tempera turcs~vcre 0

F.

:I f'c\\, r u n s

ivere

also

niadc a t 5(lc and 90' F. T c n i -

peraturc dependence \vas

i n

agrernieiit

with

t h a t

alread . established

b

SVIiitiic -

and L-ivian . 'The capac i ty da t a ~ve re v -

pendent o n both liquid

a n d

gas

raws.

. I b s o r p t i o n of carbo n dioxide in T ~ L I I ~ ~

\vas invrstiqarcd b Fiijita a n d

Ha -n-

ka\va ( ( j ) in a packed to\vt:r. iis \vel1 as

under conditions of so-called "rod-like ir-

rigation." T h e toiver bringing abour

this condition

is iiierel . d i i

rm pty shcl l.

rlirough

ivhich rod-like

strrarns

of

\\-;iter

pass

do\vnivard

from

copl ier

tubes.

T h e r e is no splashing. For some s1)ccific

opvrating conditions the rodlikc t o \ \ ' e r

apprar cd

superior to thc. Kaschiq- r inq

a n d

Berl sadd l c - i ) ac i cd t o~ve r . Capic-

ir . da ta ~ v v r c orrelii trd for tlie packrd

tu \ \ - r rby

I ) , , = O , 0 2 5 ( 4 L ' q L ? { x

\ \ - here I ) , , = liquid phase diffiisivit)

I ,

= liquid mass

velocit .

ii

=

effective contact area per

p =

dr ns i r y and

\.iscc,sil\.

o I

Z

= toivci- heiTlii

unit tc11vrr volurne

liquid

Henson, Field, and

Ha)-nr.s

I )

dc-

scribed further their process

for

absoi.t)-

ing carbon dioxide from inert atmos-

pheres? by using ho t carbo nate solut ions .

T h e

studies

were

made in pilot installa-

tions

Lvhere

the towers were 6 a n d

8

inches in d iameter and up

LO 30

feet

i n

height. One of

the

advantages of the

new

process over older existing proc-

esses

is considerable steam saving durin g

regene ration. Com parisons ivith the

monoethanolamine process are given.

Fu r the r da t a for the system amm oi~ ia-

\ \ a [ c l \ \ -vr.rr.

and

h [ u l l e r

5).

T h r i r

c o l u m n

\vas 100

mni . i n diameter, packed

a b o u ~

2000

min .

h i q h

\vith Kaschig r inqs 10

i n r n .

i n

d ia n ie t r r . I n vsscncc , t h e y rneasurcd thc

rate of

an i n i on i a

roncrntration increase

i n tlir

dcsccndirig liquid. F rm i

a

con-

sideration

of

material

balancc: iind tlic.

definition of

K,;a

i n ternis of qas i'ati's

;inti

terininal coiiceiitra~iOiis. h e y arrivcd at

an cxpr(mion

11

hich rclatrd concentra-

tion incrcmc

of

ammonia in the liquid

di r ect ly

to parl icd hci5ht. T h u s :

\vherc ,r

is

the conccniration

of

atrirrioriia

i i i thc

l iquid.

c ' is

i in intryration

u i n -

stant.

cs i s r l i c c onc e ~ i t r i i t i~nn

and is t h e coltirrin height. .Is

was

tci

c.sp(:cwd, ~ i l o i

f ii

i's. log

x produced

Ibr tlie niqjor portiun t h r o u g h t h e co l un i n

a lincar rrlation, but tlierc secrried

to

Le

a deviation from linearity in the 1)asc o f

the coluinr~.

.I'liis

could have

I)cc:n

caused ti . nialdis tri bu t ion.

Cooling roiver pcrforniance \vas

dc-

scribrd b y Itiazurni 77). H e a t a nd

inass t ransfrr

data \vcrc

obtaiiicd f o i ,

constru ction \vitli rcd\vood and Xlasonitc

slats.

'T'lic

c:ffei'c.t of ronstruction

\vas

in-

dicated and

ii

gt.ncralizing equation w a s

cs tcd .

(;as absorption calculations

ciutiined by T,llis f ) , I'or two-

( : o t n p c ~ n e n ~ ;

nd inu1ticornt)oiieni systems

the graph i ca l

nirthod

of evalualion of the

n u n i h e r o f

equilibrium stagrs

\vas

givrn

i v i t l i emphasis o n ii varying ratio

of

l iquid

to gas rate as pertains to high conccntr;i-

lions of

s o l u t e

in the inlct strcams.

Pack-

ing cal)acit)- data for / 4-inch Raschig

ring-s

in

2

small

dis t i l lat ion-es t rac.t ivc

dis t i l lat i r in unit.

ope r a t i ny \ \ i r h nirtIi\ 1-

c .cloliesane-loiitciic xvith an d vithuul

furfural, \yere g i v m by

C h r n r r ,

E l l i s 2

and

(;ranville ( 7 ) . I n borh cases ( ) } ) t i -

inuin operation \ v a s found near t l i r

loacl-

i n g point . Kirschbauni and o thers

( I , )

have continued their earlier distillatioii

studies, \ \ -he re in

[ h e

effect

of

pressurr o n

rectification \\ as onsidered.

Fundamentals

.I rvic\c

of

the film

r l ieory

and

pcnc -

tration theory

\vas

given b y 1)ar ick-

458 INDUSTRIAL A ND ENGINEERING CHEMISTRY

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ABSORPTION AND HUMIDIFICATION

werts

3) .

The effects of chemical reac-

tion based on Higbies and Danckwerts

mo del of surface renewal wer e discussed.

Neither the film model nor the simple

penetration models seem to be valid for

correlation of observed absorption rates.

The invalidity of the simple penetration

theory is a t t r ibuted to the uncer ta in ty

of

the surface-age distribution function,

greater rates of surface renewal caused by

relatively fast flow of liquid in some re-

gions, and slower rates of renewal caused

by relatively stagnant l iquid in other

locations of the packings. Fo r this reason

over-all effects are not readily calculated,

unless the distribution of k L values

throughout the packing is known. T he

only case where the penetration theory

might be applicable is in prediction of the

effect

of

chemical absorption in industrial

equipment , and only then when smal l

scale data are available from small scale

equip men t that operates essentially simi-

larly from a hydr odyn amic poin t of view.

Surface rejuvenation was preferred

to surface renewal because a com-

plete renewal between two smooth sur-

faces of packing does not occur.

Based essentially o n the H igbie-Dan ck-

werts f i lm model Hanr a t ty (9) derived a

ra te equa t ion an d a concentra tion pro-

file equation for mass transfer between a

turbu lent fluid an d a solid surface. T he

probability function + e,) has been as-

sumed to have the form

where

A

and are constants,

0

is total

contact t ime between a fluid and the

wall, and n

=

I 2, 3. , . The t ransfer

equation was solved for the assumed

surface-age distribution function. T he

measured concentration profile lies be-

tween +( ) = Ae-@clr a nd

+( )

= =

constant. Althpugh these results d o not

necessarily prove that the discontinuous

film model is a description of actuality,

the agreement between predicted veloc-

ity profile by th e model an d t ha t of meas-

ured values suggests the possibility of a

closer approximation of the mass trans-

fer mechanism,

if

proper age-probability

functions a re selected.

In order to show tha t the Danckwerts

surface renewal concept m ay be appli-

cable to the mass transfer of solid-liquid

systems, Johnson and H uan g

(74)

tudied

the r ates of dissolution of several organic

solids from smoothed flat surfaces into

turbulent l iquids in an agitated vessel.

Their experimental correlation indi-

cated a n expone nt of

0.5

for the Schmidt

number and thus agreed wi th

k L c q / D

in Danckwerts rate equation, based on

surface renewal. Th e autho rs concluded

that this provides new and significant

evidence for the applicabili ty of t he

Danckwerts theory of surface renewal,

and tha t the i r type of appara tus can be

used for further confirmation of the

theory , especially for the case where mass

transfer is accompanied by a chemical

reaction.

A graphical correlation of binary gas

diffusion coefficients was developed by

Fair and Lerner 5) , ased on the Hirsch-

felder-Bird-Spotz diffusion equation and

the theorem of corresponding states. De-

spite the fact that the crit ical diffusion

coefficient and reduced diffusion co-

efficient which they defined do not pos-

sess great physical significance, the au-

thors were able to show the barrier gas

ratio (the ratio of the critical diffusion

coefficient for various gases through a

single barrier gas and the crit ical diffu-

sion coefficient for these gases through

air) to be independent of the properties

of the diffusing gas. This enables rapid

calculation. T he accuracy of the method

a t high pressure (near the crit ical value)

may n ot be too good.

Cairns and Roper

(2)

continued their

studies of wetted-wall columns. Fr om

the ir ad iaba t ic dehumidif ica t ion da ta

they concluded that the prediction of

Colb urn and Drew applies. Correlation

of the increased heat transfer rates re-

quires, however, consideration of P,,/P

a nd a (where PB, is the log-mean partial

pressure of the nondiffusing component

in th e gas film,

P

s total pressure, and a

is the ratio of the sensible heat carried

by the diffusing vapors and the heat

transferred in absence of mass transfer)

a nd no t a alone.

Mass transfer studies in a wetted-wall

tower carryi ng a falling film of wate r an d

carbon d ioxide were made by K amei an d

others

75).

The experimental values

of height of transfer unit per unit test

section were smaller than the theoreti-

cal values derived by Pigford 23) based

on true molecular diffusion in a perfect

lami nar l iquid layer. T he deviation is

attribu ted to ripples on th e surface of the

lami nar film. For carbon dioxide absorp-

tion without gas flow

where

HL

R e L = Reynolds number , per ta in ing

PL, PL = viscosity, density of liquid

D L

=

molecular diffusivity

=

height of a transfer unit , feet

=

height

of

wetted-wall colum n

to falling liquid film

Results of another mass transfer study

i n a wetted-wall tower were reported by

Schwarz and Hoelscher

24 ) ,

w ho in -

vestigated concentration profiles of water

vapor a t var ious e leva tions. T he de-

scending films were essentially free of

ripples and the air stream had developed

to complete turbulence. M an y of the

discrepancies in the li terature, concerning

mass transfer da ta, were believed du e to

lack of consideration of entrance ef-

fects. T h e mass transfer rates observed

did not reach constancy until the down-

stream distance ha d exceeded six column

diameters.

Results of a n adiab atic humidification

study in a perforated plate tower were re-

ported by Kamei , Takamatsu , and

Nakazaki

(76).

Another study of

Yo-

shida and Hyodo (28) involved the

vaporization of organic solvents from

their wet-bulb surface into water-wet a ir.

Essentially satu ration relationships were

presented for the cases where nonhygro-

scopic and hyg roscopic-type solvents were

involved.

l i terature Cited

Benson, H. E.,

Field,

J.

H. , Haynes,

W.

P.,

Chem. Eng. Progr. 52, 433,

10 (1956).

Cairns, R. C . , Roper, G. H., Chem.

Eng. Sci. 4,

221-8 (1955).

Danckwerts. P. V.,

A.Z.Ch.E. Journal

1.456-6311955):

Ellis,

S. R.

M., Petroleum ReJner 35,

NO. 2,127-31 (1956).

Fair, J. R., Lerner, B. J., A.Z.Ch.E.

Journal

2, 1, 13 (1956).

Fujita, S., Hayakawa, T.,

Chem.

Eng . (Japun)

20, 113-117 (1956).

Garner. F.

H..

Ellis. S R .

M..

Gran-

ville,W. H.. J .

Inst.

Petroleum 42,

148-54 (1956).

Guyer, A. Guyer, A . Jr., Muller, F.,

Helv.

Chim. Acta

38, 1545-53

(1955); 50, No. 4.

Hanratty,

T.

J., A.Z.Ch.E. Journal 2,

Hikita, H., Kataoka,

T.,

Chem. Eng.

Inazumi. H..

Zbid..

19. 579-85 (1955).

359-62 (1956).

( J u p a n ) 20, 528-33 (1956).

Jacobs,

J.

K.

Petroieum Refiner

35,

Johnson, A . I., Beeckmans,

J. M. L.,

Can. J . Technol. 33, 434-44 (19 55) .

Johnson, A. I., Huang, C. J.,

A . I .

Ch.E. Journal 2,412-19 (1956).

Kamei. S Oishi.

J..

Iii ima. H..

NO. 6, 187-8 (1956).

. ,

Itoi,M.,Kamada,

6f. hem.

Eng.

( J a p a n )20, 65-70 (1956).

Kamei, S.,

Takamatsu, T.,

Nakazaki,

S.,

Zbid.,

20, 23-7 (1 956 ).

Kirschbaum,

E.,

Chem. Zng.

Tech.

28,

639 (1956).

Kirschbaum,

E.,

Busch,

W.,

Billet,

R.,

Zbid.,

28,475-80 (1956).

McWilliams, J. A Pra t t , H.

R.

C.,

Dell, F. R., Jones, D. A . Trans.

Znst. Chem. Engrs.

(London) 34, 17-

43 11956).

-

Morton, Frank, Cerigo,

D.

G., King,

Morton, Frank, Kine,

P. J., Zbid.,

P.

J.,Zbid.,

34, 146 (1956).

34,155 (1956). -

(Julv 1956).

Parkison, R. V.,

Tappi

39, 522-7

Pigford,R. L. h.D. thesis, Univers ity

of Illinois, 1941.

Schwarz, W. H., Hoelscher, H. E.,

A.Z.Ch.E. Journal 2,101-6 (1956).

Whitt, F. R., Brit. Chem. Eng. 1956,

439.

Whyness,

A.

L., Trans. Znst. Chem.

Engrs. (London)

34, 117-26 (1956).

Wicks, C. E., Beckmann, R. B.,

A.I.CI1.E. Journal

1, No. 4, 426-33

(1955).

( J u p n n ) 20, 528-40 (19k6).

Yoshida, T., Hyodo, T

Chem. Eng.

VOL.

49 NO.

3

PART II MARCH

1957

59