Industrial Gas Separations - American Chemical Society

7

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 14, 2016 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch007

Membrane Gas Separations for Chemical Processes and Energy Applications WILLIAM J. SCHELL Separex Corporation, Spectrum Separations Division, Anaheim, CA 92806 C. DOUGLAS HOUSTON Separex Corporation, DGH, Inc. Division, Tyler, TX 75703

Membranes have been used commercially for many years for water desalination by the reverse osmosis pro cess. These membranes consist of a microporous sub structure of cellulose acetate and a thin layer of dense cellulose acetate (active layer) on the upper surface, cast onto a supporting cloth for added mechanical strength. The active layer serves as the separating barrier, and due to its thinness, pro vides very high transport rates. It has been deter mined that these membranes, when dried, are also suitable for gas separation. As certain gases per meate more rapidly than others a gas mixture of two or more gases of varying permeability may be sepa rated into two streams, one enriched in the more permeable components and the other enriched in the less permeable components. The membrane system to be described consists of spiral-wound elements con nected in series and contained within pressure vessels. A rubber U-cup attached to the element serves to seal the element with the inner diameter of the pressure vessel, thereby forcing the feed gas to flow through the element. The pressure vessels usually contain six elements each and are mounted in racks on a skid. Applications that will be discussed include recovery of hydrogen from re finery process streams and tail gases, carbon di oxide removal and sweetening of natural gas, de hydration of natural gas on offshore platforms and production of carbon dioxide for enhanced o i l re covery. The advantages of membrane separation over conventional processes typically include greatly reduced capital costs, lower energy consumption, smaller size and weight, lower installation costs due to its modular design, and simplified opera tion. 0097-6156/83/0223-0125$06.00/0 © 1983 American Chemical Society Whyte et al.; Industrial Gas Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


126

I N D U S T R I A L GAS

SEPARATIONS

I t h a s b e e n r e c o g n i z e d f o r many y e a r s t h a t n o n p o r o u s p o l y m e r f i l m s e x h i b i t a h i g h e r p e r m e a b i l i t y t o w a r d some g a s e s t h a n t o w a r d s o t h e r s . As e a r l y as 1831, i n v e s t i g a t i o n s were r e p o r t e d on t h e phenomenon o f e n r i c h m e n t o f a i r w i t h r u b b e r membranes (Réf. 1 ) ; h o w e v e r , n o t u n t i l 1950 h a d t h e p r a c t i c a l p o s s i b i l i t y o f t h i s a n d o t h e r g a s s e p a r a t i o n s w i t h p e r m s e l e c t i v e membranes b e e n s e r i o u s l y studied. W e l l e r and S t e i n e r i n t h e i r c l a s s i c papers, demonstrated the f e a s i b i l i t y o f s e p a r a t i n g oxygen from a i r and d e s c r i b e d p r a c t i c a l p r o c e s s e s f o r s e p a r a t i o n o f h y d r o g e n a n d h e l i u m f r o m methane ( R e f . 2_,3) . A l t h o u g h t h e i r r e s u l t s were h i g h l y v a l u a b l e i n t h e d e v e l o p m e n t o f t h e s c i e n c e o f membrane s e p a r a t i o n , t h e c a l c u l a t e d membrane a r e a r e q u i r e m e n t s f o r i n d u s t r i a l p r o c e s s e s were e n o r m o u s , due t o t h e v e r y l o w p e r m e a t i o n r a t e s o f t h e g a s e s t h r o u g h t h e s e dense f i l m s . The t e c h n i c a l b r e a k t h r o u g h i n t h e a p p l i c a t i o n o f membranes t o gas s e p a r a t i o n came w i t h t h e d e v e l o p m e n t o f a p r o c e s s f o r p r e p a r i n g c e l l u l o s e acetate i n a s t a t e which r e t a i n s i t s permselect i v e c h a r a c t e r i s t i c s b u t a t g r e a t l y increased permeation rates (Ref. . T h e s e f l a t - s h e e t c e l l u l o s e a c e t a t e membranes, w h i c h were o r i g n a l l y d e v e l o p e d f o r r e v e r s e - o s m o s i s w a t e r d e s a l i n a t i o n ( R e f . 6), a r e p r e p a r e d f r o m a s o l u t i o n o f t h e p o l y m e r w h i c h i s c a s t on a s u p p o r t i n g c l o t h , p a r t i a l l y d r i e d t h e n s e t o r g e l l e d i n a w a t e r b a t h . A t t h i s s t a t e t h e membranes a r e h e a t e d i n w a t e r t o improve t h e i r s e l e c t i v i t y c h a r a c t e r i s t i c s , f o l l o w e d by d r y i n g w i t h a s o l v e n t - e x c h a n g e t e c h n i q u e . T h e s e membranes h a v e v a s t l y i n creased permeation r a t e s , w h i l e r e t a i n i n g t h epermselective chara c t e r i s t i c s o f f i l m s o f c e l l u l o s e a c e t a t e , due t o t h e f o r m a t i o n o f a t h i n , d e n s e l a y e r o n t h e a i r - d r i e d s u r f a c e o f t h e membrane. This s o - c a l l e d " a c t i v e " l a y e r has c h a r a c t e r i s t i c s s i m i l a r t o those of c e l l u l o s e acetate f i l m s but w i t h a t h i c k n e s s o f t h e order o f 0.1 m i c r o m e t e r (urn) o r l e s s , w h e r e a s t h e t o t a l membrane t h i c k n e s s may r a n g e f r o m a p p r o x i m a t e l y 75 t o 125 ym ( s e e F i g u r e 1 ) . The m a j o r p o r t i o n o f t h e membrane i s an o p e n - p o r e s p o n g e - l i k e s u p p o r t s t r u c t u r e t h r o u g h w h i c h t h e g a s e s f l o w w i t h o u t r e s t r i c t i o n . The p e r m e a b i l i t y and s e l e c t i v i t y c h a r a c t e r i s t i c s o f these asymmetric membranes a r e f u n c t i o n s o f c a s t i n g s o l u t i o n c o m p o s i t i o n , f i l m c a s t i n g c o n d i t i o n s and p o s t - t r e a t m e n t , and a r e r e l a t i v e l y i n d e p e n d e n t o f t o t a l membrane t h i c k n e s s . M e t h o d s were d e v e l o p e d l a t e r t o i n c o r p o r a t e t h i s a s y m m e t r i c membrane s t r u c t u r e f o r g a s s e p a r a t i o n i n a h o l l o w f i b e r c o n f i g u r a t i o n r a t h e r than t h e f l a t - s h e e t ( R e f . 7 ) . H o l l o w f i b e r s have a g r e a t e r p a c k i n g d e n s i t y (membrane a r e a p e r p a c k a g i n g volume) t h a n f l a t s h e e t s , b u t t y p i c a l l y have l o w e r p e r m e a t i o n r a t e s . The mecha n i s m f o r g a s s e p a r a t i o n i s i n d e p e n d e n t o f membrane c o n f i g u r a t i o n , h o w e v e r , a n d i s b a s e d on t h e p r i n c i p l e t h a t c e r t a i n g a s e s p e r m e a t e more r a p i d l y t h a n o t h e r s . T h i s i s due t o a c o m b i n a t i o n o f d i f f u s i o n a n d s o l u b i l i t y d i f f e r e n c e s , w h e r e b y a g a s m i x t u r e o f two o r more g a s e s o f v a r y i n g p e r m e a b i l i t y may b e s e p a r a t e d i n t o two s t r e a m s , one e n r i c h e d i n t h e more p e r m e a b l e components a n d t h e o t h e r e n r i c h e d i n t h e l e s s permeable components.

Whyte et al.; Industrial Gas Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

SCHELL

A N D HOUSTON

Membrane

Gas

Separations


7.


127

128

I N D U S T R I A L GAS S E P A R A T I O N S


Membrane E l e m e n t C o n f i g u r a t i o n I n o r d e r f o r membranes t o b e u s e d i n a c o m m e r c i a l s e p a r a t i o n s y s t e m t h e y must b e p a c k a g e d i n a manner t h a t s u p p o r t s t h e memb r a n e a n d f a c i l i t a t e s h a n d l i n g o f t h e two p r o d u c t g a s s t r e a m s . These packages a r e g e n e r a l l y r e f e r r e d t o as elements o r b u n d l e s . The m o s t common t y p e s o f membrane e l e m e n t s i n u s e t o d a y i n c l u d e the s p i r a l - w o u n d , h o l l o w f i b e r , t u b u l a r , and p l a t e and frame configurations. The s y s t e m s c u r r e n t l y b e i n g m a r k e t e d f o r g a s s e p a r a t i o n a r e o f t h e s p i r a l - w o u n d t y p e , s u c h a s t h e SEPAREX and Delsep p r o c e s s e s , and t h e h o l l o w - f i b e r type such as t h e P r i s m s e p a r a t o r a n d t h e C y n a r a Company p r o c e s s . S p i r a l - w o u n d e l e m e n t s , a s shown i n F i g u r e 2, c o n s i s t p r i m a r i l y o f o n e o r more membrane " l e a v e s , " e a c h l e a f c o n t a i n i n g t w o memb r a n e l a y e r s s e p a r a t e d b y a r i g i d , p o r o u s , f l u i d - c o n d u c t i v e mater i a l known a s t h e "permeate c h a n n e l s p a c e r . " The p e r m e a t e c h a n n e l s p a c e r f a c i l i t a t e s t h e f l o w o f t h e " p e r m e a t e " , an e n d p r o d u c t o f t h e s e p a r a t i o n . A n o t h e r c h a n n e l s p a c e r known a s t h e " h i g h p r e s s u r e c h a n n e l s p a c e r " s e p a r a t e s one membrane l e a f f r o m a n o t h e r a n d f a c i l i t a t e s the f l o w o f the h i g h pressure stream through t h e e l e ment. The membrane l e a v e s a r e wound a r o u n d a p e r f o r a t e d h o l l o w t u b e , known a s t h e " p e r m e a t e t u b e " , t h r o u g h w h i c h t h e p e r m e a t e i s removed. The membrane l e a v e s a r e s e a l e d w i t h an a d h e s i v e o n t h r e e s i d e s t o s e p a r a t e t h e f e e d gas from t h e permeate g a s , w h i l e t h e f o u r t h s i d e i s open t o t h e p e r m e a t e t u b e . The o p e r a t i o n o f t h e s p i r a l - w o u n d e l e m e n t c a n b e s t b e e x p l a i n e d b y means o f an e x a m p l e . I n o r d e r t o separate carbon d i o x i d e and/or hydrogen s u l f i d e ( a c i d gases) from a m i x t u r e o f these a c i d gases and hydrocarbon gases, t h e gaseous m i x t u r e e n t e r s t h e p r e s s u r e tube a t h i g h p r e s s u r e and i s i n t r o d u c e d i n t o t h e element v i a the "high pressure channel spacer." The more p e r m e a b l e a c i d g a s e s r a p i d l y p a s s t h r o u g h t h e membrane i n t o t h e "permeate c h a n n e l s p a c e r " where t h e y a r e c o n c e n t r a t e d a s a l o w p r e s s u r e g a s s t r e a m . This low pressure a c i d gas stream f l o w s through t h e element i n t h e "permeate c h a n n e l s p a c e r " a n d i s c o n t i n u o u s l y e n r i c h e d b y a d d i t i o n a l a c i d g a s e n t e r i n g f r o m o t h e r s e c t i o n s o f t h e membrane. When t h e l o w p r e s s u r e a c i d g a s s t r e a m r e a c h e s t h e p e r m e a t e t u b e a t the c e n t e r o f t h e element, t h e a c i d gas "permeate" i s removed. The h i g h p r e s s u r e " r e s i d u a l g a s " m i x t u r e r e m a i n s i n t h e h i g h p r e s s u r e c h a n n e l s p a c e r , l o s i n g more a n d more o f i t s a c i d g a s a n d b e i n g e n r i c h e d i n hydrocarbon gas as i t f l o w s through t h e element, and e x i t s a t t h e o p p o s i t e e n d o f t h e e l e m e n t . The membrane s y s t e m c o n s i s t s o f membrane e l e m e n t s connected i n s e r i e s a n d c o n t a i n e d w i t h i n p r e s s u r e t u b e s a s shown i n F i g u r e 3. A r u b b e r U-cup a t t a c h e d t o t h e e l e m e n t s e r v e s t o s e a l t h e e l e ment w i t h t h e i n n e r d i a m e t e r o f t h e p r e s s u r e t u b e , t h e r e b y f o r c i n g the feed gas t o f l o w through t h e element. The p r e s s u r e t u b e s u s u a l l y c o n t a i n s i x e l e m e n t s e a c h a n d a r e mounted i n r a c k s on a skid. Commercial s i z e elements a r e t y p i c a l l y 8 inches i n diameter by 40 i n c h e s l o n g a n d c o n t a i n f r o m 150 t o 275 s q u a r e f e e t o f membrane a r e a .



7.

SCHELL

A N DHOUSTON

Membrane

Gas

129

Separations

ο

•Η

-Ρ ο U -Ρ

03

β Ο

ϋ -Ρ β

I

il The p r o d u c t k i D i i s t e r m e d t h e p e r m e a b i l i t y c o e f f i c i e n t o f t h e membrane f o r component i . T h i s c o e f f i c i e n t i s i n d e p e n d e n t o f mem b r a n e t h i c k n e s s and p r e s s u r e d i f f e r e n t i a l a n d t h e f r e q u e n t l y u s e d u n i t s a r e , c c ( S T P ) - cm/cm^-sec-cm Hg. A n o t h e r p a r a m e t e r o f


132


SEPARATIONS

i n t e r e s t i s t h e p e r m e a t i o n r a t e , F^, d e f i n e d a s k±O±/Z which i s a m e a s u r e d c h a r a c t e r i s t i c o f a g i v e n membrane, w i t h u n i t s c c ( S T P ) / c m - s e c - c m Hg o r S C F H / f t - 1 0 0 p s i . The t o t a l p r e s s u r e s , P a n d Ρ , a r e g i v e n b y t h e sums, P ^ + P j a n d P ^ + P j , respective ly. The r a t i o P " i / P j i s d e f i n e d a s t h e i d e a l s e p a r a t i o n f a c t o r f o r component i w i t h r e s p e c t t o component j i n t h e membrane a n d i s written α i / j . From t h e p r e v i o u s d i s c u s s i o n i t c a n b e s e e n t h a t , i f compo n e n t i i s t h e more p e r m e a b l e , i n c r e a s i n g P i , e i t h e r b y i n c r e a s i n g t h e t o t a l p r e s s u r e o r t h e c o n c e n t r a t i o n o f component i , w i l l r e s u l t i n a h i g h e r membrane p e r m e a b i l i t y r a t e . I n a d d i t i o n , h i g h e r v a l u e s f o r αi/j r e s u l t i n g r e a t e r e f f i c i e n c y i n gas s e p a r a t i o n . I t c a n b e shown t h a t t h e c o m p o s i t i o n o f t h e p e r m e a t e g a s , P ^ , f o r permeation o f a b i n a r y gas m i x t u r e i s g i v e n by t h e following quadratic equation: f

2

2

1

1 1

1

1

1 1

1

1


1

1 1

Pj (

pll

1 1

_

P i

. II)

αi/j (

p

I. p l l ) _

(Pi (

P

i

1

i.

- Pi P

i

1 1

)

ii)

P a r a m e t e r s A f f e c t i n g The P r o c e s s The s e p a r a t i o n e f f i c i e n c y f o r a g i v e n membrane w i t h a p a r t i c u l a r b i n a r y g a s m i x t u r e w i l l b e d e p e n d e n t m a i n l y upon t h r e e f a c t o r s : gas c o m p o s i t i o n , t h e p r e s s u r e r a t i o between f e e d and p e r meate g a s , a n d t h e s e p r a t i o n f a c t o r f o r t h e two c o m p o n e n t s . A h i g h e r s e p a r a t i o n f a c t o r g i v e s a more s e l e c t i v e membrane, r e s u l t ing i n a g r e a t e r s e p a r a t i o n e f f i c i e n c y . This parameter i s a f u n c t i o n o f t h e membrane m a t e r i a l a n d i s d e t e r m i n e d b y t h e i n d i v i d u a l gas permeation r a t e s . The g a s c o m p o s i t i o n a n d p r e s s u r e r a t i o a r e i n t e r r e l a t e d i n t e r m s o f t h e i r e f f e c t on s e p a r a t i o n e f f i c i e n c y . Their i n t e r a c t i o n i s p r o n o u n c e d when t h e f e e d g a s h a s a l o w c o n c e n t r a t i o n o f t h e more p e r m e a b l e g a s , w i t h t h e r e s u l t t h a t t h e e f f e c t i v e s e p a r a t i o n i s small a t low pressure r a t i o s , i r r e s p e c t i v e o f the a i / j v a l u e o f t h e membrane. T h i s b e h a v i o r i s shown i n F i g u r e 4. This phenomenon i s due t o t h e f a c t t h a t t h e p a r t i a l p r e s s u r e o f t h e more p e r m e a b l e component on t h e p e r m e a t e ( l o w p r e s s u r e ) s i d e c a n not exceed i t s p a r t i a l p r e s s u r e on t h e f e e d ( h i g h p r e s s u r e ) s i d e . For h i g h c o n c e n t r a t i o n gases t h e pressure r a t i o has o n l y a s m a l l i n f l u e n c e , a s shown i n F i g u r e 5. H e n c e , i f c o m p r e s s i o n e n e r g y i s an i m p o r t a n t f a c t o r i n t h e e c o n o m i c s o f a p a r t i c u l a r s e p a r a t i o n , l o w e r p r e s s u r e r a t i o s may b e u s e d w i t h l i t t l e o r no l o s s i n s e p a ration efficiency. By c o n t r a s t ( s e e F i g u r e 4), t h e p r e s s u r e r a t i o i s a very important consideration i n achieving e f f i c i e n t separa t i o n o f l o w c o n c e n t r a t i o n gas m i x t u r e s . A n o t h e r i m p o r t a n t f a c t o r i n membrane g a s s e p a r a t i o n i s t h e pressure d i f f e r e n t i a l , Ρ - P ; the greater t h i s d i f f e r e n c e the l e s s membrane a r e a r e q u i r e d . The membrane a r e a i s e x a c t l y p r o p o r t i o n a l t o the inverse of thepressure d i f f e r e n t i a l , i . e . , A r e a = c o n s t a n t (Ρ - P ) " ! . The e f f e c t i v e s e p a r a t i o n f a c t o r i s 1

1

1 1

1 1


7.

SCHELL

Membrane

A N D HOUSTON

100,

ι

Gas

133

Separations

ι

PR = PRESSURE : RATIO 1 οι = M E M B R A N E S E P A R A T ION

1

F A C T OR

80

-

> Ο LU LU

(/)

A *

10

s**

LU >

-θΓ^0.05

1

5 10 M E M B R A N E SEPARATION

F i g u r e 5. E f f e c t o f p r e s s u r e on e f f e c t i v e s e l e c t i v i t y .

ratio

PR == 2

50

100

FACTOR

a n d membrane s e p a r a t i o n


factor


134


SEPARATIONS

u n a f f e c t e d b y t h e p r e s s u r e d i f f e r e n t i a l a s l o n g as t h e p r e s s u r e r a t i o i s unchanged. O t h e r s y s t e m v a r i a b l e s t h a t w i l l h a v e a n e f f e c t on t h e s e p a r a t i o n p r o c e s s a r e temperature and r e l a t i v e h u m i d i t y o f t h e gas. I n c r e a s i n g t h e t e m p e r a t u r e r a i s e s most p e r m e a b i l i t i e s by a b o u t 10 t o 1 5 % p e r 10°C a n d h a s l i t t l e e f f e c t on s e p a r a t i o n f a c t o r s . The e f f e c t o f r e l a t i v e h u m i d i t y i s v a r i a b l e d e p e n d i n g upon t h e membrane used. High r e l a t i v e h u m i d i t i e s , g r e a t e r than 95%, a r e gene r a l l y d e t r i m e n t a l due t o membrane p l a s t i c i z a t i o n . Contamination w i t h l i q u i d w a t e r h a s b e e n f o u n d t o d r a m a t i c a l l y r e d u c e membrane performance f o r c e l l u l o s e a c e t a t e . The f l o w d y n a m i c s o f t h e p e r m e a t e s t r e a m t h r o u g h t h e p r o d u c t channel spacer m a t e r i a l , or through the f i b e r bore i n the case o f h o l l o w f i b e r s y s t e m s , may a l s o a d v e r s e l y a f f e c t t h e s y s t e m p e r formance. I f t h e p e r m e a t e f l o w i s s u f f i c i e n t l y h i g h , a back p r e s s u r e w i l l be o b s e r v e d i n t h e p e r m e a t e s t r e a m . I t c a n be s e e n f r o m F i g u r e s 4 a n d 5 t h a t t h i s w o u l d r e s u l t i n a l o w e r e n r i c h m e n t due to a l o w e r i n g o f t h e e f f e c t i v e p r e s s u r e r a t i o . I t has been found t h a t i n s p i r a l - w o u n d elements t h e permeate f l o w behaves as i f i t were under l a m i n a r c o n d i t i o n s . Gas v i s c o s i t y i s t h e r e f o r e t h e d e t e r m i n i n g f a c t o r i n comparing t h e r e l a t i v e back p r e s s u r e o f v a r i o u s g a s e s f l o w i n g a t t h e same r a t e s . T h i s e f f e c t may be e s s e n t i a l l y e l i m i n a t e d by p r o p e r c o n s t r u c t i o n o f t h e membrane element. C e l l u l o s e A c e t a t e Membrane D a t a Membranes m a n u f a c t u r e d b y S p e c t r u m S e p a r a t i o n s , I n c . , a s u b s i d i a r y o f SEPAREX CORPORATION, a r e o f t h e c e l l u l o s e a c e t a t e t y p e . They a r e s i m i l a r t o t h o s e made f o r r e v e r s e o s m o s i s e x c e p t t h e y must be d r i e d f o r g a s s e p a r a t i o n u s e . A p r o p r i e t a r y p r o c e s s i s u s e d t o a c c o m p l i s h t h i s s o t h a t t h e membrane d o e s n o t c o l l a p s e and l o s e i t s a s y m m e t r i c c h a r a c t e r upon r e m o v a l o f t h e w a t e r . The p e r m e a t i o n r a t e s a r e m e a s u r e d a t v a r i o u s p r e s s u r e s f o r the g a s e s o f i n t e r e s t i n o r d e r t o c h a r a c t e r i z e t h e membranes. Permation r a t e s f o r water vapor, h e l i u m , hydrogen, hydrogen s u l f i d e a n d c a r b o n d i o x i d e a r e t h e h i g h e s t w i t h N 2 , C H 4 , CO a n d C 2 H 6 b e i n g t h e l o w e s t . The h i g h r a t e f o r H2O, H 2 S a n d CO2 i s due t o t h e h i g h s o l u b i l i t y o f t h e s e g a s e s i n t h e membrane. Gas p e r meation and s e l e c t i v i t y d a t a f o r SEPAREX™ s p i r a l - w o u n d elements are shown i n T a b l e I . I t may be n o t e d t h a t t h e o x y g e n / n i t r o g e n s e l e c t i v i t y i s o f an o r d e r o f magnitude l e s s t h a n t h e o t h e r gas c o u p l e s , due t o t h e v e r y s m a l l d i f f e r e n c e i n m o l e c u l a r s i z e a n d s o l u b i l i t y o f O2 and N 2 . T h i s l o w s e p a r a t i o n f a c t o r makes o x y g e n e n r i c h m e n t t h e most d i f f i c u l t t o a c c o m p l i s h , b o t h t e c h n i c a l l y a n d e c o n o m i c a l l y , o f t h e m a j o r gas s e p a r a t i o n s o f i n t e r e s t . The s e l e c t i v i t y and p e r m e a t i o n r a t e s a c h i e v e d i n f i n i s h e d e l e m e n t s a r e somewhat l o w e r t h a n t h e membrane due t o b o u n d a r y l a y e r p r o b l e m s , permeate back p r e s s u r e , problems w i t h i m p e r f e c t s e a l s , d e f e c t s i n the membrane and c o n t a m i n a n t s f o u n d i n g a s m i x t u r e i n t h e f i e l d .


7.

SCHELL

AND

Membrane

HOUSTON

Table I

ELEMENT Gas C o u p l e H 0 (Vapor)/CH H /CH C0 /CH H S/C H He/CH 0 /N 2

2

4

2

4

2

3

4


2

2

8

4

Gas

Separations

135

PERFORMANCE S e p a r a t i o n Factor,α 200 - 400 45 - 55 20 - 30 75 - 110 60 - 85 4 - 5

P e r m e a t i o n R a t e a t 500 p s i g , SCFH Gas He H C0 0 2

2

2

4-in. Dia. 3,300 2,800 1,400 200

8-in. D i a . 14,000 12,000 6,000 850

Applications T h e r e a r e many c o m m e r c i a l a p p l i c a t i o n s f o r membrane g a s s e p a r a t i o n s , some o f w h i c h a r e c u r r e n t l y b e i n g m a r k e t e d , and o t h e r s being tested. S e v e r a l f i e l d t e s t s t u d i e s have been u n d e r t a k e n u t i l i z i n g t h e SEPAREX process i n a 2 - i n . diameter element s i z e . Due t o t h e m o d u l a r c o n f i g u r a t i o n o f membrane s y s t e m s , a f u l l s i z e s y s t e m c a n be d i r e c t l y d e s i g n e d f r o m t h e t e s t r e s u l t s w i t h a s m a l l p i l o t plant. Although the flow rates f o r a p i l o t unit are considerably l o w e r t h a n might be e n c o u n t e r e d i n a f u l l - s i z e system, a l l p r o c e s s parameters such as p r o d u c t p u r i t i e s , p r e s s u r e d r o p , p r o d u c t r e c o v e r i e s , optimum p r e s s u r e and t e m p e r a t u r e , membrane a r e a r e q u i r e d and s e r i e s / p a r a l l e l a r r a n g e m e n t o f t h e e l e m e n t s c a n be d i r e c t l y determined. A c i d Gas S e p a r a t i o n f r o m M e t h a n e . Removal o f h y d r o g e n s u l f i d e ( H S ) and c a r b o n d i o x i d e ( C 0 ) f r o m n a t u r a l gas i s an i d e a l a p p l i c a t i o n f o r membranes i n t h a t H S a n d C 0 p e r m e a t e t h r o u g h t h e membrane a p p r o x i m a t e l y 30 t i m e s f a s t e r t h a n membrane, e n a b l i n g a h i g h r e c o v e r y o f t h e a c i d gases w i t h o u t s i g n i f i c a n t l o s s o f p r e s s u r e i n t h e methane p i p e l i n e p r o d u c t g a s ( R e f . 5/8) . The membrane s y s t e m w o u l d be c o n s i d e r e d a b u l k r e m o v a l p r o c e s s i n t h i s c a s e a n d has t h e g r e a t e s t e c o n o m i c a d v a n t a g e o v e r c o n v e n t i o n a l t e c h n o l o g y when t h e a c i d g a s c o n t e n t i s o v e r 1 0 % . The most o b v i o u s p l a c e f o r membrane s y s t e m s i n t h i s a p p l i c a t i o n i s i n t h e r e t r o f i t t i n g o f e x i s t i n g sour gas p r o c e s s i n g plants. T h i s w o u l d i n c r e a s e t h e c a p a c i t y and r e d u c e t h e e n e r g y l o a d o f t h e e x i s t i n g system o r e l i m i n a t e t h e need f o r expanding t h e e x i s t i n g p l a n t when w e l l h e a d p r e s s u r e l o s s o r i n c r e a s e d a c i d gas c o n t e n t o c c u r s . 2

2

2

2


136


SEPARATIONS

A n o t h e r a p p l i c a t i o n f o r membrane s y s t e m s r e l a t e d t o s o u r g a s p r o c e s s i n g i s t h e r e c o v e r y o f carbon d i o x i d e f o r use as a m i s c i b l e f l o o d f o r e n c h a n c e d o i l r e c o v e r y (EOR) f r o m d e p l e t e d o i l f i e l d s ( R e f . 9 ) . I t h a s b e e n f o u n d t h a t C 0 a t c o n c e n t r a t i o n s above 9 0 % w i l l s o l u b i l i z e o i l absorbed i n the s u b s t r a t a , a l l o w i n g f o r secondary and t e r t i a r y r e c o v e r y . T h e r e a r e many g a s w e l l s c o n t a i n i n g a b o u t 4 0 % t o 8 0 % C 0 t h a t c o u l d be p r o c e s s e d w i t h a membrane s y s tem, p r o v i d i n g a h i g h p u r i t y C 0 f o r EOR a n d a f u e l g a s b y product. I n EOR p r o d u c t i o n , C 0 i n j e c t e d i n t o t h e f o r m a t i o n e v e n t u a l l y appears w i t h l i g h t hydrocarbon gases a s s o c i a t e d w i t h t h e crude o i l . A membrane s y s t e m c a n a g a i n be u s e d t o r e c o v e r C 0 f o r r e i n j e c t i o n and r e c o v e r l i g h t h y d r o c a r b o n s f o r s a l e as p i p e l i n e gas. F o r e x a m p l e , a t y p i c a l a s s o c i a t e d g a s c o n t a i n s 70-85% C 0 a t 100°F a n d 40 p s i g ( R e f . 1 0 ) . I f such gas i s sent through a twos t a g e SEPAREX p r o c e s s a t 300 p s i g , C 0 w o u l d b e r e c o v e r e d i n t h e permeate stream a t a c o n c e n t r a t i o n o f 95% and a r e c o v e r y o f 98%, a l o n g w i t h a r e s i d u a l h i g h p r e s s u r e l i g h t hydrocarbon p i p e l i n e gas c o n t a i n i n g 3% o r l e s s C 0 . A C 0 - C H methane p r o c e s s g a s s t r e a m , s i m i l a r t o a t y p i c a l h i g h C 0 n a t u r a l g a s h a s b e e n u n d e r t e s t b y SEPAREX f o r C 0 r e moval i n a 2 - i n . d i a m e t e r element p i l o t p l a n t s i n c e September 1981. The f e e d g a s c o n t a i n s 30% C 0 a n d i s d e l i v e r e d t o t h e memb r a n e t e s t u n i t a t 250-450 p s i g u n d e r a m b i e n t t e m p e r a t u r e c o n d i tions. The o b j e c t i v e o f t h e s y s t e m i s t o r e d u c e t h e C 0 l e v e l o f t h e methane t o l e s s t h a n 3.5%. The membrane s y s t e m c o n s i s t s o f 5 p r e s s u r e tubes i n s e r i e s , each tube c o n t a i n i n g three 4 0 - i n . long e l e m e n t s . The g a s i s c o n d i t i o n e d t o m a i n t a i n i t a t a minimum o f 20°F above t h e d e w p o i n t . The s y s t e m was o p e r a t e d a t a v a r i e t y o f f l o w r a t e s , p r e s s u r e s , r e c o v e r i e s and temperatures. Selected data a r e p r e s e n t e d i n F i g u r e s 6 t h r o u g h 8. A p l o t o f t h e feed and r e s i d u a l gas f l o w r a t e s as a f u n c t i o n o f s t a g e c u t i s shown i n F i g u r e 6. S t a g e c u t i s d e f i n e d a s t h e f r a c t i o n o f f e e d g a s t h a t p e r m e a t e s t h r o u g h t h e membrane f o r a g i v e n number o f e l e m e n t s ( s i n g l e s t a g e ) . I n t h i s c a s e a s t a g e c o n s i s t s o f f i f t e e n elements i n s e r i e s . I t can be seen t h a t a t a f e e d f l o w r a t e o f a b o u t 180 SCFH a l l o f t h e g a s w o u l d p e r m e a t e t h e membrane, r e s u l t i n g i n no e n r i c h m e n t . T h i s i s f u r t h e r demons t r a t e d i n F i g u r e 7, where t h e p e r m e a t e g a s c o m p o s i t i o n a t z e r o r e s i d u a l gas f l o w i s equal t o the feed c o m p o s i t i o n . By c o m b i n i n g F i g u r e s 6 a n d 7 i t i s p o s s i b l e t o r e l a t e s t a g e c u t t o t h e r e s i d u a l and permeate gas s t r e a m c o m p o s i t i o n s . These c a l c u l a t i o n s a r e u s e f u l i n d e t e r m i n i n g r e c o v e r i e s and i n d e s i g n i n g s e r i e s / p a r a l l e l f l o w a r r a n g e m e n t s i n l a r g e s y s t e m s . The d a s h e d portions o f the curves i n the low flow r a t e region o f Figure 7 r e p r e s e n t s p r e d i c t e d C 0 compositions under u n i f o r m f l o w c o n d i t i o n s , whereas t h e s o l i d l i n e s r e p r e s e n t a c t u a l o b t a i n e d d a t a . The d e v i a t i o n b e t w e e n p r e d i c t e d a n d a c t u a l p e r f o r m a n c e i s most l i k e l y due t o f e e d g a s c h a n n e l i n g i n t h e e l e m e n t s o r t h e d e v e l opment o f s t a g n a n t f l o w r e g i o n s a s a r e s u l t o f t h e l o w p r e s s u r e d i f f e r e n c e across the h i g h pressure s i d e (feed t o r e s i d u a l ) o f the elements. 2

2

2


2

2

2

2

2

2

4

2

2

2

2

2



7.

SCHELL

A N D HOUSTON

Membrane

Gas

137

Separations

STAGE CUT

F i g u r e 6.

S t a g e c u t as f u n c t i o n o f f l o w

rate.


138

INDUSTRIAL

GAS

SEPARATIONS

C0 IN PERMEATE 2

FEED CONDITIONS 30% C0 365 PSIA 70° F


2

4000

1000 2000 RESIDUAL FLOW RATE, SCFH F i g u r e 7.

E f f e c t o f f l o w r a t e on e n r i c h m e n t .

FEED CONDITIONS 30% C0 365 PSIA CORRECTED TO 70° F 2

10.

LU

< cc _ ζ

Industrial Gas Separations - American Chemical Society

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