Synthetic Membranes - American Chemical Society

16 Engineering Aspects of the Continuous Membrane Column

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 27, 1981 | doi: 10.1021/bk-1981-0154.ch016

JOHN M. THORMAN

1

and SUN-TAK HWANG

Chemical and Materials Engineering, The University of Iowa, Iowa City, IA 52242

The partial enrichment of gas mixtures via membranes has long been recognized as a n o v e l separation technique. P r i o r to 1950 only a l i m i t e d amount of research had been conducted i n t h i s f i e l d . E a r l y a p p l i c a t i o n s were unique and included hydrogen p u r i f i c a t i o n through s i l v e r - p a l l a d i u m a l l o y s , helium recovery through silica g l a s s , and uranium isotope s e p a r a t i o n . However, in recent years there has been an explosion of activity d i r e c t e d toward the imminent and widespread commercialization of gas permeation technology, Gas permeation separations are becoming l e s s novel and more practical. In p a r t i c u l a r , s u b s t a n t i a l progress has been made during the past decade. Plug-flow separation models of c a p i l l a r y permeators have been confirmed experimentally by s e v e r a l i n v e s t i g a t o r s (1,2, 3,4) . S p e c i f i c studies of c a p i l l a r y membranes and permeators have a l s o been made concerning a x i a l pressure l o s s (2,5,6,7) , c a p i l l a r y deformation (4,5,8), process v a r i a b l e s and broken f i b e r s ( 7 ) , flow patterns and purge streams (9), the pressure dependency of permeability c o e f f i c i e n t s (10,11,12,13), and two-membrance permeators (15,16). Work with axisymmetric membranes has been i n i t i a t e d (17). Cascade separations have been advanced by other researchers (18,19,20,21,22). Commerical u n i t s , such as the Du Pont Permasep (23) and Monsanto Prism Separator (24) , have been developed f o r e n r i c h i n g hydrogen, carbon monoxide, ammonia, and other i n d u s t r i a l gases. Extensive reviews of these and other recent advances i n the area of gas permeation can be found i n books by Hwang and Kammermeyer (25) and Meares (26). In a d d i t i o n to the above c o n t r i b u t i o n s another i d e a , r e f e r r e d to as "the continuous membrance column," was developed i n the l a t e 1970's (27,28,29). In essence t h i s concept s t a t e s that the gas permeation cell, t r a d i t i o n a l l y regarded as a s i n g l e - s t a g e separation u n i t , i s a c t u a l l y a s e l f - c o n t a i n e d continuous cascade. The

1

Current address: Monsanto Chemical Intermediates Company, Texas City, T X 77590.

0097-6156/81/0154-0259$05.25/0 © 1981 American Chemical Society

In Synthetic Membranes: Volume II; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


260

SYNTHETIC M E M B R A N E S : HF AND U F

USES

more permeable gas i s s t r i p p e d from the high-pressure stream along the membrane; however, when the c e l l i s operated i n the countercurrent, plug-flow mode, and the amount of permeation i s maximized r e l a t i v e t o product stream f l o w r a t e s , a strong i n t e r n a l r e f l u x a c t i o n i s c r e a t e d . I n p r i n c i p l e , the components of a b i n a r y mixture can be enriched i n d e f i n i t e l y , provided that the membrane employed e x h i b i t s some f i n i t e s e l e c t i v i t y . Although the s e p a r a t i o n mechanisms f o r membrane and e q u i l i brium processes d i f f e r , o p e r a t i o n of a membrance column i s analogous to that of packed d i s t i l l a t i o n and e x t r a c t i o n columns. As shown i n F i g u r e 1, a feed stream i s c e n t r a l l y introduced, and product streams a r e withdrawn from the ends of the column. The column can be d i v i d e d i n t o s t r i p p i n g and e n r i c h i n g s e c t i o n s . Note that gas on the low-pressure s i d e of the membrane i s r e c y c l e d to the high-pressure s i d e v i a a compressor. I n t h i s manner the more permeable gas i s c o n t i n u a l l y c a r r i e d toward and c o l l e c t e d near the compressor, w h i l e the l e s s permeable gas i s s t e a d i l y t r a n s f e r r e d toward and concentrated a t the opposite end of the column. The absence of backmixing i s very important, s i n c e any a x i a l mixing w i l l tend to e q u a l i z e compositions. The degree of s e p a r a t i o n achieved depends on product f l o w r a t e s , membrane s e l e c t i v i t y , amount of l o c a l permeation, and column l e n g t h . E a r l i e r papers on the continuous membrane column (28,29) have discussed the s e p a r a t i o n of C O 2 - N 2 , C O 2 - O 2 and O 2 - N 2 ( a i r ) mixtures i n s t r i p p e r , e n r i c h e r and t o t a l column u n i t s composed of 35 s i l i c o n e rubber c a p i l l a r i e s . A c h a r a c t e r i z a t i o n of the membrane column using a membrane u n i t concept (analogous to t r a n s f e r u n i t concept — HTU, NTU) has a l s o been presented. The purpose of t h i s paper i s to present some new data and d i s c u s s i o n s on the extended study of continuous membrane column. S p e c i f i c a l l y , the t o p i c s of multicomponent s e p a r a t i o n s , inherent s i m u l a t i o n d i f f i c u l t i e s , composition minima i n the e n r i c h i n g s e c t i o n , v a r i a t i o n of experimental parameters, and l o c a l HMU v a r i a t i o n along the column w i l l be covered. Multicomponents Systems Thus f a r , only b i n a r y mixtures have been separated i n the t o t a l membrane column. R e s u l t s of t h i s work have been discussed elsewhere (28,29). A sample s h e l l - s i d e composition p r o f i l e from a t o t a l column experiment w i t h a C O 2 - O 2 mixture i s shown i n F i g u r e 2. Table I summarizes the t o t a l column data obtained to date. One of the next steps i n developing the continuous membrane column w i l l be to o b t a i n extensive data on multicomponent systems. Some p r e l i m i n a r y experiments w i t h a C^-CH^-^ mixture using a s t r i p p e r have a l r e a d y been conducted. The r e s u l t s of two such experiments are presented i n F i g u r e s 3 and 4. The agreement between experiment and model i s e x c e l l e n t .


16.

THORMAN

261

Continuous Membrane Column

AND HWANG

MOST PERMEABLE GAS

- COMPRESSOR

LOW PRESSURE


PRESSURIZED FEED ( GAS MIXTURE).

HIGH PRESSURE „ MEMBRANE

LEAST PERMEABLE GAS

Figure 1.

Schematic of the total column

100

co -o 2

2

o

90 80

^ ^ ^ ^ ^

70 -

s

EXPT CALC'D

FEED* 57.2 %

C0

2

6 0

o £

50 40 ENRICHING SECTION

30 20

I

0.0

I

I

I

I

I

2.0 3.0 LENGTH,m

1.0

10

Figure 2.

STRIPPING SECTION I

I

4.0

I

I

5.0

Shell-side composition profile of the total membrane column for the C0 —0 mixture 2

2



5.12

(87.7)

Compressor Load (ymol/s)

T o t a l Column Length (m)

10.60

5.77

Bottom Product Flow Rate (ymol/s)

Feed Flow Rate (ymol/s)

4.83

17.8

Bottom Product Composition (% 1st Gas)

Top Product Flow Rate (ymbl/s)

94.5 (94.4)

Top Product Composition (% 1st Gas)

2

54.8

2

C0 -N

Feed Composition (% 1st Gas)

Gas System N

2 2

5.12

(91.7)

4.96

2.59

2.37

8.2

94.6 (95.2)

52.6

C 0

2°2

5.12

(84.0)

14.64

6.74

7.90

20.5

87.3 (86.3)

57.2

C0

57.0

2°2

6.13 (12.66) 4.24

(88.69) 5.12

4.34

1.79

15.1

36.8 (36.8)

21.0

N

°2- 2

14.56

7.10

7.46

22.6

88.3 (88.7)

C0

TABLE I . PERFORMANCE OF THE CONTINUOUS MEMBRANE COLUMN


2

4.24

(13.13)

13.853

12.92

0.993

19.6

41.7 (41.7)

20.9

2

0 -N


2.04 (1.80) 0.71 (0.80) 2.28 (1.96) 99.27

2.04 (1.80) 99.07

8.06 (6.92) 0.80 (0.80) 98.78

7.62 (5.98) 1.24 (1.16) 99.27

Enriching Section Pressure Loss (kPa)

Stripping Section Pressure Loss (kPa)

S h e l l - S i d e Pressure (kPa)

Stripping Section Temperature (K)

Enriching Section Temperature (K) 298.7

298.3

1.03 (1.03) 0.83 (0.88)

7.81 (6.82)

7.35 (6.03)

223.86 (222.73)

226.62 (224.89)

297.1

297.1

296.3

296.3

302.7

302.7

297.8

299.5

298.5

299.4

98.59

227.34 (227.11) 230.14 (230.28)

223.89 (222.56)

224.85 (223.31)

101.22

2.11

2.11

2.01

2.01

2.01

2.01

Stripping Section Length (m)

Pressure a t Compressor (kPa)

2.13

2.13

3.11

3.11

3.11

3.11

(cont'd.)

Enriching Section Length (m)

TABLE I .


ON

to

2

s

K

R

S'

o

> o a

>

o


264

SYNTHETIC

MEMBRANES:

HF AND U F

USES

Figure 3.

Shell-side composition profiles for the C0 -CH -N per at total reflux

Figure 4.

-N mixture in a stripShell-side composition profiles for the C0, -CH per with bottom product

2

fy

2t

2

2

mixture in a strip-

h


16.

THORMAN


AND HWANG

265

In modeling multicomponent systems i t i s necessary to modify the a x i a l pressure l o s s , gas permeation and c o n c e n t r a t i o n g r a d i ents p r e v i o u s l y d e s c r i b e d (28). The governing equations, r e q u i r e d to execute the numerical s i m u l a t i o n over i n f i n i t e s i m a l segments along the membrane column as shown i n F i g u r e 5 a r e : dP dz

>

4Re z _ 8yqRT w Re r . Re /(3P) - rrN(r.) P/(2yqRT) rrN(r.) P Z 1; z x 1

K yqRT 4

rrN(r_ ) P

3

L

4


(i) 27rN

, /, r . .) E [Q.(x.P ^ J j

dz

X

n(r

(2)

- y.P ) ] Jo

and dx.

2TTNQ.(X.P - y.P )

dz

£n(r / r . ) o 1

(3)

j dz for j

l,2,...,n-l

where 2

i ( l + 0.75Re - 0.0407Re + 0.0125Re w w w K

-1 4- 0.056Re

0

w

3

- 0.0153Re w

(A)

(5)

The f o l l o w i n g o v e r a l l and component balances are a l s o r e q u i r e d : q - q = G -G B

x.q

(6)

B

y

J

G

B. B

for

j = 1,2,••»,n-l

(7)

Of course,

E j

x =i

(8)

J

A l l procedures f o r executing the numerical s i m u l a t i o n of a multicomponent s e p a r a t i o n are s i m i l a r to those described f o r a b i n a r y system, except f o r e v a l u a t i n g the i n i t i a l permeate composit i o n at the r e s i d u e end of the s t r i p p e r . A g a i n , the i n i t i a l permeate composition w i l l be that of the mixture which permeates through the endmost increment of the membrane. The a p p r o p r i a t e relations are:


266

SYNTHETIC

J

o

i

J

\

j

J

for

j =

MEMBRANES:

HF

AND

UF

USES

V

1,2,.

and


E

y

B

= i

do)

A t r i a l - a n d - e r r o r s o l u t i o n i s necessary. By e s t i m a t i n g the v a l u e of dq/dz each of the component equations (9) and (10) can be solved f o r y . I f y > 1, y < 0 or ^ y f 1, then dq/dz B

B >

B
0

F i g u r e 6 i l l u s t r a t e s the change i n HMU and NMU over 1 0 0 mm sect i o n s along the column. The curves are f o r the C O 2 - O 2 system, but q u a l i t a t i v e l y represent both the e n r i c h i n g and s t r i p p i n g s e c t i o n s f o r a l l of the C O 2 - N 2 , C O 2 - O 2 and O 2 - N 2 systems i n v e s t i gated. The HMU trend shows t h a t e f f i c i e n c y goes up (HMU goes down) s i g n i f i c a n t l y as the a x i a l flow r a t e decreases w i t h i n the column. In some i n s t a n c e s , l o c a l HMU values change almost an order of magnitude. Since HMU and NMU vary i n v e r s e l y over a given i n t e r v a l , the number of membrane u n i t s achieved i n each s e c t i o n i n creases d r a m a t i c a l l y w i t h decreasing flow r a t e s . Note that i t i s the m o d i f i c a t i o n of column o p e r a t i n g v a r i a b l e s contained i n the HMU expression that u l t i m a t e l y determine the v a l u e HMU when the column l e n g t h i s f i x e d . Composition Minima U s u a l l y i n an e n r i c h e r o r the e n r i c h i n g s e c t i o n of the membrane column, the more permeable component i s s t e a d i l y concent r a t e d from the feed i n l e t to the compressor. However, some of the r e s u l t s show that the s h e l l - s i d e and even the tube-side comp o s i t i o n p r o f i l e s can pass through a minimum. Note the e x p e r i mental data i n Figures 7 and 8. I n these cases the feed f l o w i s r e l a t i v e l y slow and r e f l u x a c t i o n , r a t h e r than b u l k f l o w , i s predominant. F i g u r e 8 i l l u s t r a t e s t h a t a composition minimum can a l s o occur during o p e r a t i o n of the t o t a l column when the r e s i d u e flow r a t e from the e n r i c h i n g s e c t i o n i s too slow. Figures 7 and 8 i n c o r p o r a t e c a l c u l a t e d tube- and s h e l l - s i d e c o n c e n t r a t i o n p r o f i l e s f o r the unique C 0 - N 2 and O 2 - N 2 ( a i r ) data, and a l s o i l l u s t r a t e l o c a l permeate composition v a r i a t i o n . Each f i g u r e shows that the c o n c e n t r a t i o n of more permeable gas s t e a d i l y decreases i n the d i r e c t i o n of flow along the high-pressure (tube) 2


268

SYNTHETIC

MEMBRANES:

HF AND U F

PT

PO

qT

I

•

G

x t

1

L

y r

P+dP q +dq 1 x + dx

t

1 1


USES

T

Po G+dG y+dy

dz P q X

1 t •

1

Po G y

z

Figure 5. Modeling of the membrane column

Figure 6.

Local variation of NMU and HMU within a section of the membrane column


THORMAN


AND HWANG

^.

C0 -N 2


LOCAL PERMEATE COMPOSITION SHELL-SIDE COMPOSITION TUBE-SIDE COMPOSITION

2

\

o

—

\ "

\

-

O

\

269

EXPERIMENTAL

\

•s FEED = 44.1 % C 0 8.31 /x.mol/s

^ N .

2

—

0.0

Figure 7.

Figure 8.

TOP

X.

i

i

0.5

1.0 LENGTH, m

FEED-

1.5

Composition minima for the C0 -N product 2

system in an enricher with top

2

Composition minimum for the 0 -N top product 2

• —

2.0

2

(air) system in an enricher with



270

SYNTHETIC

MEMBRANES:

HF

AND

UF

USES

s i d e of the e n r i c h e r . This r e s u l t s i n a lower percentage of more permeable gas passing through the membrane. E v e n t u a l l y , because of strong r e f l u x a c t i o n , the l o c a l permeate composition matches the s h e l l - s i d e (low-pressure s i d e ) composition. At that p o i n t enrichment ceases. Continuing toward the bottom of the e n r i c h e r , the s h e l l - s i d e c o n c e n t r a t i o n of more permeable gas d e c l i n e s u n t i l the r e l a t i v e amount of permeate i s i n s u f f i c i e n t to f u r t h e r lower the c o n c e n t r a t i o n of the incoming low-pressure stream. The prof i l e then increases toward the feed i n l e t . Hence a s h e l l - s i d e c o n c e n t r a t i o n minimum i s formed. F i g u r e 7 shows that a c o n c e n t r a t i o n minimum can a l s o occur on the high-pressure s i d e proceeding again toward the bottom of the e n r i c h e r , the c o n c e n t r a t i o n of more permeable gas passing through the membrane g r a d u a l l y becomes l e s s than that on the tube (high pressure) s i d e . Thus, the c o n c e n t r a t i o n of the more permeable gas increases toward the bottom product o u t l e t . There i s no tube-side composition minimum i n F i g u r e 8, s i n c e the l o c a l permeate composition does not f a l l below the tube-side composition profile. I t should be pointed out that o p e r a t i o n of an e n r i c h e r and t o t a l column (Figure 9) i s l e s s e f f i c i e n t when a c o n c e n t r a t i o n minimum occurs. I n other words, the same enrichment can be accomplished w i t h a s h o r t e r column. This c o n d i t i o n can be e a s i l y remedied by i n c r e a s i n g the f l o w r a t e on the high-pressure s i d e of the membrane. Simulation D i f f i c u l t i e s The numerical s i m u l a t i o n of the t o t a l membrane column works w e l l , but does c o n t a i n some inherent problems. These problems r e l a t e to r e s t r i c t i o n s i n the d i r e c t i o n that i n t e g r a t i o n s a r e executed, and to the i n f l u e n c e of propagated e r r o r s i n the f i n a l results. A l l c a l c u l a t i o n s , except those i n v o l v i n g a t o t a l r e f l u x s t r i p p e r , should be i n i t i a t e d a t the bottom of a column s e c t i o n for two reasons. F i r s t , numerical i n s t a b i l i t y i s observed i n a s t r i p p i n g s e c t i o n w i t h a bottom product when i n t e g r a t i o n i s d i r e c t e d from the feed p o i n t toward the bottom o u t l e t . I n t e g r a t i o n i s s t a b l e , however, from the bottom o u t l e t to the feed p o i n t , except i n some instances of wide-open f l o w . Secondly, a t the top of the column i t i s d i f f i c u l t to a c c u r a t e l y measure flow r a t e s of streams e n t e r i n g and e x i t i n g the e n r i c h i n g s e c t i o n near the comp r e s s o r . Were values of these flow r a t e s provided, along w i t h gas composition and pressure a t the compressor, i n t e g r a t i o n could be executed from the top of the e n r i c h i n g s e c t i o n to the feed point. The propagation of measurement and c a l c u l a t i o n e r r o r s can a l s o be a problem. I n g e n e r a l , experimental e r r o r s i n boundary c o n d i t i o n s a t the bottom of the column and c a l c u l a t i o n e r r o r s i n the model are propagated along the s t r i p p i n g s e c t i o n , introduced



16.

THORMAN

AND HWANG


271

to and f u r t h e r compounded i n the m a t e r i a l balance around the feed p o i n t , and a m p l i f i e d again i n the e n r i c h i n g s e c t i o n . I f a t any step i n the s i m u l a t i o n the absolute values of composition or flow r a t e s become comparable w i t h t h e i r r e s p e c t i v e e r r o r s , there i s a strong l i k e l i h o o d that the ensuing c a l c u l a t e d p r o f i l e s w i l l d e v i a t e s i g n i f i c a n t l y from the experimental p r o f i l e s . The most s u s c e p t i b l e q u a n t i t i e s i n the s i m u l a t i o n a r e the boundary condit i o n s a t the residue o u t l e t s of the s t r i p p i n g and e n r i c h i n g sections . The e f f e c t of p e r t u r b i n g a boundary c o n d i t i o n a t the b o t tom of a s t r i p p i n g s e c t i o n i s i l l u s t r a t e d i n Figure 10. The experimental composition of the bottom product f o r the C O 2 - N 2 system i s only 1.7% C O 2 . The simulated s h e l l - s i d e composition p r o f i l e based on t h i s v a l u e f o l l o w s the experimental t r e n d , but i s c o n s i s t e n t l y higher. A f t e r s l i g h t l y a l t e r i n g the bottom comp o s i t i o n to 1.3% C O 2 , the c a l c u l a t e d p r o f i l e and c o n d i t i o n s a t the opposite end of the s t r i p p e r agree c l o s e l y w i t h the e x p e r i mental data. I n t h i s example only one i n i t i a l c o n d i t i o n was v a r i e d . I t should be remembered that both the composition and flow r a t e of the bottom product may c o n t a i n measurement e r r o r s comparable to t h e i r absolute values. I n such cases i t i s recommended that measurements be taken c a r e f u l l y and that i n i t i a l cond i t i o n s i n the s i m u l a t i o n be perturbed w i t h i n the range of e x p e r i mental e r r o r i n order to appreciate the p o s s i b l e range of c a l c u l a ted r e s u l t s . The inherent s i m u l a t i o n d i f f i c u l t y accompanying a low residue flow r a t e from the e n r i c h i n g s e c t i o n of a t o t a l column i s shown i n F i g u r e 11. R e f e r r i n g to F i g u r e 1, the m a t e r i a l balance around the feed p o i n t i n v o l v e s an e x t e r n a l feed and the residue stream from the e n r i c h i n g s e c t i o n , which combine to form an i n t e r n a l feed to the s t r i p p i n g s e c t i o n . I n the column s i m u l a t i o n the composition and flow r a t e of the residue stream are determined using experimental values f o r the e x t e r n a l feed, and c a l c u l a t e d values f o r the i n t e r n a l feed to the s t r i p p i n g s e c t i o n . I n some cases the c a l c u l a t e d and experimental e r r o r s may negate one another. However, when the actual residue flow r a t e i s s m a l l r e l a t i v e to the e x t e r n a l feed, i t i s more l i k e l y that e r r o r s i n the c a l c u l a t e d flow r a t e and composition of i n t e r n a l feed to the s t r i p p i n g s e c t i o n w i l l be a m p l i f i e d i n determining the calculated values of the residue stream from the e n r i c h i n g s e c t i o n . Depending on how s e n s i t i v e the e n r i c h i n g s e c t i o n s i m u l a t i o n i s to propagated e r r o r s i n the boundary c o n d i t i o n s , the c a l c u l a t e d p r o f i l e s for the e n r i c h i n g s e c t i o n can d i f f e r markedly from experimental profiles. Should the c a l c u l a t e d p r o f i l e s i n the e n r i c h i n g s e c t i o n be erroneous due to u n c e r t a i n t i e s i n the boundary conditions the c o r r e c t p r o f i l e s can be found by employing a shooting technique. The input data f o r the s i m u l a t i o n are again based on measurements subj e c t to experimental e r r o r . By p e r t u r b i n g one or more values w i t h i n the bounds of experimental e r r o r , the e n r i c h i n g s e c t i o n


272

SYNTHETIC

C 0

2

- N

MEMBRANES:

HF

AND

UF

USES

2

D O

3


O O o

o

°

FEED = 54.2 %

C0

2

STRIPPING SECTION

SECTION ENRICHING i 0.0

Figure 9.

i 1.0

i

0

i

i

i

2.0 3.0 LENGTH,m

i

i 4.0

i

i 5.0

Shell-side composition profile minimum for the C0 -N total membrane column 2

2

mixture in the

90

LENGTH, m

Figure 10.

Sensitivity of the stripper shell-side composition profile to the bottom product composition


THORMAN


AND HWANG

0 -N (AIR)

40

2

2

273

FEED COMPOSITION

(% o ) 2

20.9 21.1 21.3 21.9


35

CM

o

X °

30

25

STRIPPING SECTION

ENRICHING SECTION

20 I

I

0.0

Figure 11.

EXPT

I

I

1.0

I

1

I

2.0 3.0 LENGTH, m

1

1

_ L

4.0

5.0

Sensitivity of the shell-side composition profile to the feed composition in the total column simulation

0 -N (AIR) 2

0

1

FEED

2

2

3

4

5

LENGTH,m

Figure 12.

Effect of column length on enrichment in a total reflux enricher


274

SYNTHETIC

MEMBRANES:

HF AND U F

USES

boundary c o n d i t i o n s can be r e c a l c u l a t e d . This procedure i s one of t r i a l - a n d - e r r o r and can be e x e r c i s e d when an experimental prof i l e , such as the s h e l l - s i d e composition i s a v a i l a b l e f o r f i t t i n g . Figure 11 i l l u s t r a t e s the a p p l i c a t i o n of t h i s shooting technique by p e r t u r b i n g only the e x t e r n a l feed composition.


Parameter V a r i a t i o n The e f f e c t of column l e n g t h on the degree of enrichment i n a t o t a l r e f l u x e n r i c h e r i s shown i n F i g u r e 12. Feed flow r a t e s were p r a c t i c a l l y the same t o t o t a l r e f l u x e n r i c h e r s 3, 4 or 5m i n l e n g t h . The output pressure of the compressor was comparable i n each i n s t a n c e , ranging from 229-231 kPa. The data i n d i c a t e that e x t r a membrane surface area r e s u l t s i n a f u r t h e r accumulation of the more permeable gas i n the e n r i c h e r . The amount of permeation becomes g r e a t e r r e l a t i v e t o the feed flow r a t e ; hence g r e a t e r enrichment of mixture occurs along the column. For t h i s s p e c i f i c case, an 0 -N2 ( a i r ) m i x t u r e , the l e v e l of oxygen a t the compressor increased approximately 4% w i t h each a d d i t i o n a l meter of tube bundle. F i g u r e 13 shows that e s s e n t i a l l y i d e n t i c a l p r o f i l e s a r e obtained when s t r i p p e r s of 3, 4 and 5m are operated a t the same "cut ( r a t i o of permeate t o f e e d ) . However, u n l i k e the e x p e r i mental work l e a d i n g t o F i g u r e 12, feed flow r a t e s were v a r i e d w i t h column l e n g t h i n order to maintain the v a l u e of " c u t " constant. Once a g a i n , the experimental and c a l c u l a t e d composition p r o f i l e s are i n e x c e l l e n t agreement. The e f f e c t of feed f l o w r a t e on the performance of a t o t a l r e f l u x e n r i c h e r of f i x e d length was discussed i n an e a r l i e r paper (29). I n g e n e r a l , the degree of enrichment increased as the amount of permeation became greater r e l a t i v e t o the feed f l o w rate. 2

11

Conclusions 1.

According t o membrane u n i t a n a l y s i s , the e f f i c i e n c y of a permeation c e l l increases s i g n i f i c a n t l y as a x i a l f l o w r a t e decreases.

2.

A composition minimum can occur i n an e n r i c h e r or i n the e n r i c h i n g s e c t i o n o f a t o t a l column on the low- and h i g h pressure s i d e s of the membrane. Such o p e r a t i o n i s i n e f f i c i e n t , and can be remedied by i n c r e a s i n g the a x i a l f l o w r a t e on the high-pressure s i d e of the column.

3.

The s e p a r a t i o n model, which was p r e v i o u s l y a p p l i e d only t o b i n a r y systems, has been s u c c e s s f u l l y extended to d e s c r i b e the s e p a r a t i o n of a multicomponent gas mixture.



THORMAN

Figure 13.

AND

HWANG


275

Shell-side composition profile variation with a comparable cut and a variable column length


276

SYNTHETIC

MEMBRANES:

HF

AND

UF

USES

4.

Inherent d i f f i c u l t i e s accompany some s i m u l a t i o n s of the continuous membrane column. P e r t u r b a t i o n of boundary cond i t i o n s w i t h i n the range of experimental e r r o r may be necessary i n c e r t a i n instances to achieve a proper f i t of experimental data.

5.

I n c r e a s i n g the amount of permeation r e l a t i v e to the feed flow r a t e i n a t o t a l r e f l u x e n r i c h e r enhances the degree of enrichment of the more permeable gas.


Acknowledgment This m a t e r i a l i s based upon work supported by the N a t i o n a l Science Foundation under Grant No. ENG78-10850. Nomenclature G

=

s h e l l - s i d e f l o w r a t e , umole/s

HMU =

height of a membrane u n i t as d e f i n e d by Equation (12), n

K

f u n c t i o n of Re

n

=

JL

K

as d e f i n e d by Equation (4)

W

=

f u n c t i o n of Re

n

=

t o t a l number of components i n mixture

N

=

number of c a p i l l a r i e s

0

z

w

as d e f i n e d by Equation (5)

NMU =

number of membrane u n i t s as d e f i n e d by Equation (11)

P

=

absolute l o c a l tube-side pressure, kPa

=

atmospheric pressure, kPa

=

P /P o

q

=

tube-side f l o w r a t e , ymol/s

Q

=

2 p e r m e a b i l i t y c o e f f i c i e n t mol-m/s-m -Pa

P P

Q

r

r

capillary radius, n

R

gas

Re w Re z

Reynolds number a t w a l l defined as r,v

constant p/u

a x i a l Reynolds number



16.

THORMAN


AND HWANG

T

absolute temperature, K

v

r a d i a l v e l o c i t y a t w a l l , m/s

rw

277

x

= mole f r a c t i o n of more permeable component on tube s i d e

y

= mole f r a c t i o n of more permeable component on s h e l l s i d e

z

= a x i a l coordinate measured from bottom of column, m

Z

=

t o t a l column h e i g h t , m

Greek L e t t e r u

= v i s c o s i t y of gas m i x t u r e , Pa-s

TT

=

3.14159 •••

3 p

=

gas d e n s i t y , kg/m

Subscripts 1

= more-permeable component

2

=

B

= at bottom of column

i

=

o

= outside

r

=

ratio

T

=

a t top o f column

w

= at c a p i l l a r y w a l l

less-permeable component

inside

z = axial Abstract

direction

Engineering aspects of "the continuous membrane column," an i n n o v a t i o n in membrane s e p a r a t i o n technology are d i s c u s s e d . The gaseous permeation cell is no longer regarded as a s i n g l e stage, but r a t h e r as a continuous cascade. The membrane column e x p l o i t s the countercurrent p l u g - f l o w o p e r a t i o n of modern gas permeators. By maximizing the amount of permeation relative to product stream


278

SYNTHETIC MEMBRANES:

USES

flow r a t e s , a strong internal r e f l u x a c t i o n is created. Thus, a b i n a r y feed mixture can be introduced to the membrane column, and n e a r l y complete s e p a r a t i o n can be achieved on a continuous b a s i s . Experiments were conducted w i t h a permeator composed of 35 silicone rubber capillaries (pressurized internally). Results are presented f o r the b i n a r y systems O2-N2 (air), CO2-N2, CO2-O2, and the multicomponent system CO2-CH4-N2. Particular attention is given t o s e p a r a t i o n of the CO -CH -N2 mixture in a s t r i p p e r , c o n d i t i o n s f o r observing composition minima in the e n r i c h i n g s e c t i o n , inherent s i m u l a t i o n difficulties in modeling the membrane column, variation of experimental parameters, and local HMU variation along the column. 2


HF AND U F

4

Literature Cited 1.

Blaisdell, 1249.

2.

Thorman, J.M.; Rhim, H.; Hwang, S.T. 30, 751.

3.

Ohno, M.; Morisue, T.; Ozaki, O.; H e k i , H.; Miyauchi, T. Radiochem. Radioanal. L e t t . , 1976, 27, 299.

4.

S t e r n , S.A.; Onorato, F.J.; Libove, C. 567.

5.

Thorman, J.M.; Hwang, S.T.

6.

Pan, C.-Y.; Habgood, H.W. 210.

7.

Antonson, C.R.; Gardner, R.J.; King, C.F.; Ko, D.Y. Ind. Eng. Chem. Process Des. Develop, 1977, 16, 463.

8.

Blaidell,

C.T.; Kammermeyer, K.

Chem. Eng. Sci., 1973, 28,

Chem. Eng.Sci.,1975,

AIChE J., 1977, 23,

Chem. Eng. Sci., 1978, 33, 15. Can. J. Chem. Eng., 1978b, 56,

C.T.; Kammermeyer, K.

AIChE J., 1972, 18, 1015.

9.

Pan, C.-Y.; Habgood, H.W. 323.

Ind. Eng. Chem. Fundam., 1974, 13

10.

S t e r n , S.A.; Mullhaupt, J.T.; G a r e i s , P.J. 15, 64.

11.

S t e r n , S.A.; Fang, S.-M.; Jobbins, R.M. Phys., 1971, B 5 ( 1 ) , 41.

12.

S t e r n , S.A.; Fang,S.-M.; 1972, Part A-2, 201.

13.

Fang,S.-M.; S t e r n , S.A.; F r i s c h , H.L: 30, 77.

Frisch,

AIChE J., 1969,

J . Macromol. Sci.-

H.L.; J. Polymer. Sci.,

Chem. Eng.Sci.,1975,



16.

THORMAN

AND HWANG


279

14.

Ohno, M.; Morisue, T.; Ozaki, O.; M i y a u c h i , T. J . Nucl. Sci. Technol., 1978a, 15, 411.

15.

Ohno, M.; Morisue, T.; Ozaki, O.; Miyauchi, T. J . N u c l . Sci. Technol., 1978b, 15, 376.

16.

Ohno, M.; Ozaki, O.; S a i t o , H.; Kimura, S.; M i y a u c h i , T. J . Nucl. Sci. Technol., 1977, 14, 589.

17.

S o u r i r a j a n , S.; Agrawal, J.P. "Reverse Osmosis in S y n t h e t i c Membranes," S. S o u r i r a j a n (Ed.), N a t l . Res. C o u n c i l Can., 1977, Chapter 26.

18.

H i g a s h i , K.; D o i , H.; S a i t o , T.; Energ. N u c l . , 1970, 17, 98.

19.

Rainey, R.H.; C a r t e r , W.L.; Blumkin, S. Report ORNL-4522, Oak Ridge N a t i o n a l Laboratory, Oak Ridge, Tenn., April 1971.

20.

Yamamoto, I . ; Kanagawa, A.; J. N u c l . Sci. Technol., 1975, 12, 120.

21.

H i g a s h i , K.; Miyamoto, Y. J . N u c l . Sci. Technol., 1976, 13, 30.

22.

Pan. C.-Y.; Habgood, H.W. 197.

23.

Gardner, R.J.; Crane, R.A.; Hannan, J.F. Chem. Eng. Prog., 1977, 73(10), 76.

24.

Knieriem, M. Jr. Hydrocarbon P r o c e s s i n g , 1980, 59(7), 65.

25.

Hwang, S.-T.; Kammermeyer, K. "Membranes in Separations," W i l e y - I n t e r s c i e n c e , New York, 1975.

26.

Meares, P. (Ed.) "Membrane Separation Processes," E l s e v i e r , New York, 1976.

27.

Thorman, J.M. "Engineering Aspects of C a p i l l a r y Gas Permea t o r s and the Continuous Membrane Column," Ph.D. T h e s i s , U n i v e r s i t y of Iowa, Iowa City, Iowa, 1979.

28.

Hwang, S.-T.; Thorman, J.M.; AIChE J., 1980, 26, 558.

29.

Hwang, S.-T.; Thorman, J.M.; Yuen, K.M. Sep.Sci.Technology, 1980, 15(4), 1069.

30.

B e c k e t t , R.; Hurt, J . "Numerical C a l c u l a t i o n s and A l g o r i t h m s , " McGraw-Hill, New York, 1967.

RECEIVED

Can.J.Chem. Eng., 1978a, 56,

December 4, 1980.


Synthetic Membranes - American Chemical Society

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