8 Gas-Adsorption Processes: State of the Art GEORGE E. KELLER, II Union Carbide Corporation, Ethylene Oxide and Glycol Division, Technical Center, South Charleston, WV 25303
Gas-adsorption processes are used for a wide variety of separations throughout the chemical and other industries. But to effect various separations, quite different process embodiments are necessary. These differences are primarily concerned with the method by which the adsorbent is regenerated. This choice is influenced by the percentage of adsorbate in the feed, ease of desorption and degree of separation required. In this paper, we discuss temperature-swing, inert-purge, displacement-purge, pressure-swing, parametric-pumping, chromatographic and other cycles; the separations for which they are best suited; and their limitations. Commercial examples are given. Finally, predictions are made as to future improvements in various technologies as well as to what new separations will be amenable to adsorption separation. G a s - a d s o r p t i o n p r o c e s s e s i n v o l v e the s e l e c t i v e c o n c e n t r a t i o n ( a d s o r p t i o n ) o f one o r more components ( a d s o r b a t e s ) o f a g a s ( o r vapor) at the s u r f a c e o f a microporous s o l i d ( a d s o r b e n t ) . The a t t r a c t i v e f o r c e s c a u s i n g the a d s o r p t i o n a r e g e n e r a l l y weaker t h a n t h o s e o f c h e m i c a l bonds and a r e s u c h t h a t , by i n c r e a s i n g the temperature of the adsorbent o r r e d u c i n g an a d s o r b a t e ' s p a r t i a l p r e s s u r e , t h e a d s o r b a t e c a n be d e s o r b e d . The d e s o r p t i o n s t e p i s q u i t e important i n the o v e r a l l p r o c e s s . First, d e s o r p t i o n a l l o w s recovery of adsorbates i n those separations where t h e y a r e v a l u a b l e , and s e c o n d , i t p e r m i t s r e u s e o f t h e adsorbent for further c y c l e s . T h i s paper i s concerned w i t h a broad range o f g a s - a d s o r p t i o n p r o c e s s e s as p r a c t i c e d i n the o r g a n i c - c h e m i c a l , p e t r o l e u m , n a t u r a l - g a s and a l l i e d i n d u s t r i e s . E m p h a s i s w i l l be on a d e s c r i p t i o n of processes r a t h e r than a r e v i e w of adsorber d e s i g n
0097-6156/83/0223-0145$07.25/0 © 1983 American Chemical Society
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p r o c e d u r e s , w h i c h have b e e n c o v e r e d i n o t h e r p u b l i c a t i o n s ( I 7 6 ) , or a d i s c u s s i o n of t r a d i t i o n a l a d s o r b e n t s , w h i c h have a l s o been c o v e r e d e l s e w h e r e (7-15)« The o b j e c t i v e s o f t h i s p a p e r a r e ( i ) t o r e v i e w t h o s e s i t u a t i o n s i n w h i c h a d s o r p t i o n c a n be an economically v i a b l e process, ( i i ) to provide a d e s c r i p t i o n of p r e s e n t - d a y a d s o r p t i o n t e c h n o l o g y , and ( i i i ) t o i n d i c a t e t h e l i k e l y d i r e c t i o n s t h a t new a d s o r p t i o n t e c h n o l o g y and u s e s w i l l take. Criteria
f o r When t o Use
Gas
Adsorption
D i s t i l l a t i o n and r e l a t e d v a p o r - l i q u i d p r o c e s s e s a r e by f a r t h e most common means o f s e p a r a t i n g homogeneous m i x t u r e s i n t h e i n d u s t r i e s mentioned above. The r e a s o n s f o r t h i s a r e t h e b a s i c p r o c e s s s i m p l i c i t y and s c a l a b i l i t y o f d i s t i l l a t i o n ( w h i c h h e l p s p r o m o t e l o w i n v e s t m e n t p e r a n n u a l u n i t o f f e e d ) and t h e a b i l i t y t o a c h i e v e many t h e o r e t i c a l s t a g e s ( w h i c h e f f e c t s h i g h d e g r e e s of s e p a r a t i o n ) i n many s y s t e m s . On t h e o t h e r h a n d , d i s t i l l a t i o n i s fundamentally a high-energy-usage process. I n most d i s t i l l a t i o n s t h e e n e r g y u s a g e i s a t l e a s t one and f r e q u e n t l y two o r d e r s o f m a g n i t u d e g r e a t e r t h a n t h e minimum t h e r m o d y n a m i c w o r k of s e p a r a t i o n . We can u s e t h e f o l l o w i n g as r o u g h c r i t e r i a t o d e c i d e w h e t h e r gas a d s o r p t i o n m i g h t be a v i a b l e a l t e r n a t i v e f o r d i s t i l l a t i o n f o r a given separation. 1. The r e l a t i v e v o l a t i l i t y b e t w e e n t h e k e y components t o be s e p a r a t e d i s i n t h e o r d e r o f 1.2 t o 1.5 o r l e s s . 2. The b u l k o f t h e f e e d i s a r e l a t i v e l y l o w - v a l u e , m o r e v o l a t i l e p r o d u c t , and t h e p r o d u c t o f i n t e r e s t i s i n r e l a t i v e l y l o w c o n c e n t r a t i o n ( a b o u t 10 w e i g h t p e r c e n t or l e s s ) . I n such s i t u a t i o n s l a r g e r e f l u x r a t i o s can be r e q u i r e d , r e s u l t i n g i n h i g h e n e r g y c o s t s . 3. One s e t o f components whose b o i l i n g r a n g e o v e r l a p s t h a t of a second s e t of c h e m i c a l l y o r g e o m e t r i c a l l y d i s s i m i l a r components must be s e p a r a t e d f r o m t h e s e c o n d s e t . To make s u c h a s e p a r a t i o n , e v e n t h o u g h v a r i o u s r e l a t i v e v o l a t i l i t i e s may be r e a s o n a b l y l a r g e , s e v e r a l d i s t i l l a t i o n c o l u m n s w o u l d be r e q u i r e d . 4. S e p a r a t i o n by d i s t i l l a t i o n r e q u i r e s c r y o g e n i c o p e r a t i o n o r p r e s s u r e s a b o v e a b o u t 20 a t m o s p h e r e s . Gas a d s o r p t i o n s h o u l d be c o n s i d e r e d f o r a g i v e n s e p a r a t i o n when one o r more o f t h e a b o v e c r i t e r i a a p p l y t o d i s t i l l a t i o n and when a s u i t a b l e a d s o r b e n t e x i s t s . In general a suitable a d s o r b e n t i s one w h i c h shows a d e q u a t e s e l e c t i v i t y ( g r e a t e r t h a n two f o r t h e k e y c o m p o n e n t s ) and c a p a c i t y , c a n be r e g e n e r a t e d e a s i l y and c a u s e s no damage t o t h e p r o d u c t s by p r o m o t i n g byproduct-forming reactions. I t i s also usually desirable that the adsorbent s e l e c t i v e l y adsorb the component(s) i n lower c o n c e n t r a t i o n s i n t h e f e e d t o m i n i m i z e t h e amount o f a d s o r b e n t r e q u i r e d per u n i t of feed processed.
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S e p a r a t i o n s and
Processes
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Processes
C o m m e r c i a l g a s - a d s o r p t i o n p r o c e s s e s ( s e e T a b l e I ) c a n be d i v i d e d i n t o b u l k s e p a r a t i o n s , i n w h i c h a b o u t 10 w e i g h t p e r c e n t o r more o f a s t r e a m must be a d s o r b e d , and p u r i f i c a t i o n s , i n which u s u a l l y c o n s i d e r a b l y l e s s than 10 weight p e r c e n t o f a s t r e a m must be a d s o r b e d . Such a d i f f e r e n t i a t i o n i s d e s i r a b l e t o make b e c a u s e i n g e n e r a l d i f f e r e n t p r o c e s s c y c l e s a r e u s e d f o r t h e d i f f e r e n t c a t e g o r i e s , a s w i l l be d i s c u s s e d l a t e r . B e l o w , f o u r b a s i c p r o c e s s c y c l e s and two c o m b i n a t i o n s a r e d e s c r i b e d i n t h e i r s i m p l e s t forms. In other sections, recent u s e s and m o d i f i c a t i o n s o f t h e s e c y c l e s , a s w e l l a s o t h e r new process c y c l e s , are d e s c r i b e d . Temperature-Swing C y c l e . I n t h i s c y c l e , a stream c o n t a i n i n g a s m a l l amount o f a n a d s o r b a t e i s p a s s e d t h r o u g h t h e a d s o r b e n t bed a t a r e l a t i v e l y l o w t e m p e r a t u r e . A f t e r e q u i l i b r i u m between a d s o r b a t e i n t h e f e e d and o n t h e a d s o r b e n t i s r e a c h e d , t h e bed t e m p e r a t u r e i s r a i s e d t o a h i g h e r v a l u e , and more f e e d i s p a s s e d through the bed. D e s o r p t i o n o c c u r s and a new, l o w e r e q u i l i b r i u m loading i s established. The n e t bed r e m o v a l c a p a c i t y , c a l l e d t h e d e l t a l o a d i n g , i s t h e d i f f e r e n c e b e t w e e n t h e s e two l o a d i n g s . ( T h i s v a l u e i s a c t u a l l y an upper l i m i t , s i n c e e q u i l i b r i u m l o a d i n g s i n p r a c t i c a l o p e r a t i o n a r e n o t a t t a i n e d . ) When t h e bed i s s u b s e q u e n t l y c o o l e d and f e e d i s a g a i n p a s s e d t h r o u g h t h e b e d , p u r i f i c a t i o n w i l l o c c u r down t o a c o n c e n t r a t i o n i n e q u i l i b r i u m w i t h the r e s i d u a l l o a d i n g on the a d s o r b e n t . The t i m e r e q u i r e d t o h e a t , d e s o r b and c o o l a bed i s u s u a l l y i n t h e r a n g e o f a few h o u r s t o o v e r a day. Because d u r i n g t h i s l o n g r e g e n e r a t i o n t i m e t h e bed i s n o t p r o d u c t i v e l y s e p a r a t i n g , t e m p e r a t u r e - s w i n g p r o c e s s e s a r e used a l m o s t e x c l u s i v e l y t o remove s m a l l c o n c e n t r a t i o n s o f a d s o r b a t e s f r o m f e e d s . O n l y i n s u c h s i t u a t i o n s c a n t h e o n - s t r e a r a t i m e be a s i g n i f i c a n t f r a c t i o n of t h e t o t a l c y c l e t i m e o f t h e p r o c e s s . I n e r t - P u r g e C y c l e . I n t h i s c y c l e , the a d s o r b a t e , i n s t e a d of b e i n g removed b y t e m p e r a t u r e i n c r e a s e , I s removed b y p a s s i n g a n o n - a d s o r b i n g g a s c o n t a i n i n g no a d s o r b a t e t h r o u g h t h e bed. T h i s has the e f f e c t o f l o w e r i n g the p a r t i a l p r e s s u r e o f the a d s o r b a t e a r o u n d t h e p a r t i c l e s , and d e s o r p t i o n o c c u r s . I f enough p u r g e g a s i s p a s s e d t h r o u g h t h e b e d , t h e a d s o r b a t e w i l l be c o m p l e t e l y removed, and t h e maximum d e l t a l o a d i n g w i l l e q u a l t h e e q u i l i b r i u m l o a d i n g . A s i n t h e t e m p e r a t u r e - s w i n g c y c l e and a l l o t h e r s , a c t u a l d e l t a l o a d i n g s w i l l be l e s s t h a n t h e e q u i l i b r i u m d e l t a l o a d i n g . D u r i n g the subsequent a d s o r p t i o n s t e p , r e m o v a l o f a d s o r b a t e f r o m t h e f e e d w o u l d be e s s e n t i a l l y c o m p l e t e u n t i l t h e a d s o r b e n t becomes n e a r l y f u l l y l o a d e d and breakthrough occurs.
American Chemical Society Library
1455 16th St. N. W. Washington. 0. C. 2003·
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INDUSTRIAL
GAS
SEPARATIONS
TABLE I REPRESENTATIVE COMMERCIAL GAS-ADSORPTION
Separation* I·
Adsorbent
Gas B u l k S e p a r a t i o n s Normal P a r a f f i n s / I s o p a r a f f i n s , Aromatics N /0 0 /N CO, C H , C 0 , N , A, N H / H Acetone/Vent Streams C H4/Vent Streams S e p a r a t i o n o f Perfume Components 2
2
2
2
4
2
2
3
2
2
II.
Gas P u r i f i c a t i o n H 0/01efin-Containing Cracked Gas, N a t u r a l Gas, A i r , S y n t h e s i s Gas, E t c . C0 /C H4, N a t u r a l Gas, E t c . Organics/Vent Streams S u l f u r Compounds/Natural Gas, H y d r o g e n , L i q u i f i e d P e t r o l e u m Gas ( L P G ) , E t c . Solvents/Air Odors/Air NO /N S0 /Vent Streams Hg/Chlor-Alkali C e l l Gas Effluent 2
2
x
2
2
2
*Adsorbates a r e l i s t e d
SEPARATIONS
first.
Zeolite Zeolite Carbon M o l e c u l a r S i e v e Zeolite, Activated C Activated C Activated C
S i l i c a , Alumina,
Zeolite Activated Zeolite
Activated Activated Zeolite Zeolite Zeolite
Zeolite
C, O t h e r s
C C
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Gas-Adsorption
Processes
149
D i s p l a c e m e n t - P u r g e C y c l e * T h i s c y c l e , w h i c h i s somewhat s i m i l a r t o the p r e v i o u s one, d i f f e r s from i t i n t h a t a gas o r v a p o r w h i c h a d s o r b s a b o u t as s t r o n g l y a s t h e a d s o r b a t e i s u s e d t o remove t h e a d s o r b a t e ( s e e F i g u r e 1 ) . Removal i s t h u s f a c i l i t a t e d b o t h by a d s o r b a t e p a r t i a l - p r e s s u r e r e d u c t i o n i n t h e f l u i d a r o u n d t h e p a r t i c l e s and by c o m p e t i t i v e a d s o r p t i o n o f t h e d i s p l a c e m e n t medium. As w i t h t h e i n e r t - p u r g e c y c l e , t h e maximum d e l t a l o a d i n g i s t h e e q u i l i b r i u m l o a d i n g . The use o f t h e two d i f f e r e n t t y p e s o f p u r g e f l u i d s c a u s e s two m a j o r d i f f e r e n c e s i n t h e p r o c e s s e s . F i r s t , s i n c e t h e displacement-purge f l u i d i s a c t u a l l y adsorbed, i t i s present when t h e a d s o r p t i o n p a r t o f t h e c y c l e b e g i n s and t h e r e f o r e contaminates the l e s s - a d s o r b e d p r o d u c t . ( I n the i n e r t - p u r g e c y c l e t h i s c o n t a m i n a t i o n i s u s u a l l y much s m a l l e r . ) I n p r a c t i c a l t e r m s t h i s means t h a t t h e d i s p l a c e m e n t - p u r g e f l u i d must be recovered from both product streams. Second, s i n c e the heat of a d s o r p t i o n of the displacement-purge f l u i d i s a p p r o x i m a t e l y e q u a l t o t h a t o f t h e a d s o r b a t e , a s t h e two e x c h a n g e on t h e a d s o r b e n t , t h e h e a t g e n e r a t e d ( o r consumed) i s v i r t u a l l y z e r o , and t h e a d s o r b e n t ' s t e m p e r a t u r e r e m a i n s u n c h a n g e d . When a n i n e r t - p u r g e gas i s u s e d , a t e m p e r a t u r e r i s e o c c u r s i n t h e r e g i o n w h e r e a d s o r p t i o n o c c u r s , and t h i s t e m p e r a t u r e r i s e c a n s e v e r e l y l i m i t t h e a d s o r p t i o n c a p a c i t y o f t h e bed i n some c a s e s . Pressure-Swing C y c l e . I n a g a s - a d s o r p t i o n c y c l e , the p a r t i a l p r e s s u r e o f an a d s o r b a t e c a n be r e d u c e d by r e d u c i n g t h e t o t a l pressure of the gas. T h i s change c a n be u s e d t o e f f e c t a desorption. The l o w e r t h e t o t a l p r e s s u r e o f t h e d e s o r b i n g s t e p , t h e g r e a t e r t h e maximum d e l t a l o a d i n g w i l l b e . The t i m e r e q u i r e d t o l o a d , d e p r e s s u r i z e , d e s o r b and r e p r e s s u r i z e a bed i s u s u a l l y a few m i n u t e s and c a n i n some c a s e s be o n l y a few s e c o n d s . Thus, even though p r a c t i c a l d e l t a l o a d i n g s a r e s u b s t a n t i a l l y l e s s t h a n t h e maximum d e l t a l o a d i n g s to minimize t h e r m a l - g r a d i e n t problems, the very s h o r t c y c l e t i m e s make a p r e s s u r e - s w i n g c y c l e q u i t e a t t r a c t i v e f o r b u l k - g a s separations. Combined C y c l e s . O f t e n a t e m p e r a t u r e - s w i n g c y c l e i s combined w i t h an i n e r t - p u r g e t o f u r t h e r f a c i l i t a t e d e s o r p t i o n . S e v e r a l p o s s i b i l i t i e s a r e shown i n F i g u r e 2. The i n e r t - p u r g e s t r e a m c a n be a f r a c t i o n o f t h e l e s s - a d s o r b e d p r o d u c t , o r a s e p a r a t e p u r g e s t r e a m can be u s e d . I f the feed i s a t supera t m o s p h e r i c p r e s s u r e , many t i m e s t h e d e s o r p t i o n s t e p w i l l be c a r r i e d out a t atmospheric p r e s s u r e . I n p r e s s u r e - s w i n g p r o c e s s e s , i t i s q u i t e common t o u s e a f r a c t i o n o f t h e l e s s - a d s o r b e d gas p r o d u c t as a l o w - p r e s s u r e p u r g e g a s , a s shown i n F i g u r e 2. O f t e n the purge f l o w i s i n the o p p o s i t e d i r e c t i o n from the feed f l o w . Rules f o r the minimum f r a c t i o n o f l e s s - a d s o r b e d g a s p r o d u c t f o r d i s p l a c i n g
FEED A+B
Τ
A+D
JLJT DISTILLATION
ΓΛ
DESORBING BED WITH A BEING REPLACED BY D
Figure 1. Displacement-purge c y c l e . A , a d s o r b a t e ; B, l e s s - a d s o r b e d component; a n d D, d i s p l a c e m e n t a g e n t .
ADSORBING BED PRELOADED WITH D AND WITH D BEING REPLACED BY A
B+D
^DISTILLATION
Β
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Processes
LESS-ADSORBED PRODUCT PRODUCT PURGE GAS
BED IN LOW-PRESSURE DESORPTION MODE
BED IN HIGH-PRESSURE ADSORPTION MODE
τ
FEED
τ
ADSORBED PRODUCT PLUS LESS-ADSORBED PRODUCT PURGE GAS
F i g u r e 2. Combined p r e s s u r e - s w i n g , cycle (pressure-swing adsorption).
non-adsorbed-product-purge
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t h e a d s o r b a t e h a v e b e e n g i v e n by S k a r s t r o m ( 1 6 ) * T h i s p r o c e s s has b e e n c a l l e d h e a t l e s s f r a c t i o n a t i o n o r , more commonly, pressure-swing adsorption (PSA). The l a t t e r name w i l l be u s e d i n t h i s paper. Gas
Bulk-Separation
Processes
Gas b u l k - s e p a r a t i o n p r o c e s s e s c o n s i s t m o s t l y o f p r e s s u r e s w i n g a d s o r p t i o n (PSA) v a r i a t i o n s a n d d i s p l a c e m e n t - p u r g e a n d inert-purge processes. R e c e n t l y , however, a chromatographic c y c l e h a s been c o m m e r c i a l i z e d . B e l o w , t h e s e c y c l e s w i l l be discussed· PSA. A s m e n t i o n e d e a r l i e r , PSA p r o c e s s e s u s e a f r a c t i o n of t h e less-adsorbed product t o a i d i n purging t h e adsorbate from t h e adsorbent. Thus a n i n e v i t a b l e f e a t u r e o f p r e s e n t - d a y PSA p r o c e s s e s i s t h e l o s s o f p a r t o f t h e l e s s - a d s o r b e d p r o d u c t to t h e purge stream. This has the a d d i t i o n a l i m p l i c a t i o n that t h e p u r g e s t r e a m c a n n o t be o f h i g h p u r i t y , a n d a s a r e s u l t PSA i s l i m i t e d a t p r e s e n t t o t h o s e a p p l i c a t i o n s where o n l y one p u r e product i s d e s i r e d : oxygen and n i t r o g e n (but n o t both s i m u l t a n e o u s l y ) from a i r , hydrogen recovery i n cases i n which complete r e c o v e r y i s n o t r e q u i r e d , and chemical p r o c e s s purge-stream treatment i n which the less-adsorbed product i s removed f r o m t h e p r o c e s s a n d t h e a d s o r b a t e - e n r i c h e d s t r e a m i s recycled t o the process. M o s t o f t e n , PSA p r o c e s s e s u s e t h r e e o r more b e d s i n parallel. The a d v a n t a g e s a r e ( i ) e n e r g y s a v i n g s , w h i c h a c c r u e from u s i n g g a s from a h i g h - p r e s s u r e bed t o p a r t i a l l y r e p r e s s u r i z e a l o w - p r e s s u r e bed f o l l o w i n g d e s o r p t i o n , and ( i i ) h i g h e r product r e c o v e r i e s , which accrue from u s i n g a combination of purging steps i n the c y c l e . R e c e n t l y two w i d e l y d i v e r g e n t p r o c e s s v a r i a t i o n s o f PSA h a v e b e e n c o m m e r c i a l i z e d by U n i o n C a r b i d e C o r p o r a t i o n . The f i r s t o f t h e s e i s c a l l e d POLYBED PSA, w h i c h i s u s e d f o r h y d r o g e n recovery (17-21). P l a n t s w i t h c a p a c i t i e s o f up t o a b o u t 1.2 χ 10^ NM^/day o f h y d r o g e n h a v e b e e n b u i l t . Hydrogen r e c o v e r i e s o f 86 p e r c e n t v s 70-75 p e r c e n t f o r o t h e r PSA p r o c e s s e s h a v e b e e n d e m o n s t r a t e d , a s w e l l a s p u r i t i e s o f 99.999 m o l e p e r c e n t (20). POLYBED PSA i n v o l v e s t h e u s e o f f i v e o r more b e d s , w i t h e x t e n s i v e gas i n t e r c h a n g e s and p r e s s u r e e q u a l i z a t i o n s between t h e b e d s . One p r o c e s s embodiment ( 1 7 ) i s shown i n F i g u r e 3. Thus POLYBED PSA c a n be t h o u g h t o f a s a p r o c e s s w h i c h a i m s f o r maximum r e c o v e r y o f t h e l e s s - a d s o r b e d component i n v e r y h i g h p u r i t y a t t h e expense o f a complex f l o w s h e e t . I t s growing c o m m e r c i a l a c c e p t a n c e s u g g e s t s t h a t i t competes q u i t e s u c c e s s f u l l y w i t h c r y o g e n i c s e p a r a t i o n i n many c a s e s . The s e c o n d p r o c e s s , c a l l e d p r e s s u r e - s w i n g p a r a m e t r i c pumping ( P S P P ) , was d e v e l o p e d t o m i n i m i z e p r o c e s s c o m p l e x i t y
8.
Gas-Adsorption
KELLER
r m ι
r-f
ν-ή
Processes
153
HYDROGEN
r-p
r-p
r-p
pp
•
r-?
[-?
ADSORBED GAS PLUS HYDROGEN F i g u r e 3.
P o l y b e d PSA p r o c e s s .
154
I N D U S T R I A L GAS
SEPARATIONS
and i n v e s t m e n t a t t h e e x p e n s e o f p r o d u c t r e c o v e r y . The name p a r a m e t r i c pumping was c o i n e d by W i l h e l m ( 2 2 ) , who d e s c r i b e d a n adsorption-based separation process i n v o l v i n g r e v e r s i n g f l o w s . When t h e f l o w i s i n one d i r e c t i o n a p a r a m e t e r , s u c h a s t e m p e r a t u r e , w h i c h i n f l u e n c e s a d s o r p t i v i t y , i s a t one v a l u e , w h i l e t h e p a r a m e t e r i s changed t o a n o t h e r v a l u e when t h e f l o w i s i n theopposite d i r e c t i o n . Such a p r o c e s s w i l l c r e a t e a s e p a r a t i o n b e t w e e n components w i t h d i f f e r e n t a d s o r p t i v i t i e s . Chen ( 2 3 ) h a s c o r r e c t l y p o i n t e d o u t t h a t PSA p r o c e s s e s c o n s t i t u t e a subset o f p a r a m e t r i c pumping, i n w h i c h p r e s s u r e i s the parameter used t o i n f l u e n c e a d s o r p t i v i t y . B e c a u s e o f i t s h i g h l y u n u s u a l n a t u r e , PSPP w i l l be d e s c r i b e d i n some d e t a i l . PSPP e x i s t s i n b o t h s i n g l e - b e d ( 2 4 - 2 6 ) and m u l t i p l e - b e d ( 2 7 ) e m b o d i m e n t s . S c h e m a t i c d i a g r a m s o f b o t h a r e shown i n F i g u r e 4. The s i n g l e - b e d p r o c e s s w i l l be described f i r s t . The b e d c a n v a r y f r o m a b o u t 0.3 t o 1.3 o r more m e t e r s i n l e n g t h a n d c o n t a i n s a d s o r b e n t p a r t i c l e s w i t h d i a m e t e r s i n t h e r a n g e o f 177 t o 420 um. Feed g a s u n d e r p r e s s u r e i s s u p p l i e d i n p u l s e s o f up t o a b o u t a s e c o n d i n l e n g t h f r o m a c o m p r e s s o r and a s u r g e t a n k . The p u l s e i s c o n t r o l l e d by a s o l e n o i d v a l v e and a t i m e r . D u r i n g t h i s f e e d p u l s e t h e exhaust s o l e n o i d valve i s c l o s e d . F o l l o w i n g t h e feed pulse b o t h v a l v e s a t t h e f e e d e n d a r e c l o s e d f o r a b o u t 0.5 t o t h r e e seconds; t h i s p e r i o d i s c a l l e d the delay. Next t h e s o l e n o i d v a l v e o n t h e e x h a u s t o r p u r g e l i n e opens f o r a p e r i o d o f a b o u t f i v e t o twenty seconds. Since t h e pressure i n t h i s l i n e i s maintained below that i n t h e feed l i n e , reverse f l o w o f gas from t h e bed o c c u r s . And f i n a l l y a n o t h e r d e l a y p e r i o d w i t h b o t h s o l e n o i d v a l v e s c l o s e d i s u s e d i n some c a s e s . I t s length i s g e n e r a l l y l e s s t h a n one s e c o n d . However, w h i l e p r e s s u r e s a n d f l o w d i r e c t i o n s a r e f l u c t u a t i n g s u b s t a n t i a l l y a t t h e f e e d e n d o f t h e bed, a c o n t i n u o u s f l o w emerges f r o m t h e p r o d u c t e n d a n d t h r o u g h a s m a l l surge tank. This flow i s enriched i n less-adsorbed components; i n t h e case o f a i r s e p a r a t i o n , t h e p r o d u c t c o n s i s t s t y p i c a l l y o f 90 t o 95 p e r c e n t o x y g e n , w i t h t h e b a l a n c e a r g o n and a s m a l l amount o f n i t r o g e n . C o n v e r s e l y t h e e x h a u s t s t r e a m i s somewhat d e p l e t e d i n o x y g e n a n d a r g o n . The n e t e f f e c t o f t h i s p r o c e s s i s t o p r o d u c e , u s i n g a s i n g l e a d s o r b i n g bed a n d two s u r g e t a n k s , a c o n s t a n t f l o w o f l e s s - a d s o r b e d product - p r i m a r i l y oxygen i n t h i s case - from a constant f l o w o f a i r from a compressor. The m u l t i p l e - b e d p r o c e s s u s e s e s s e n t i a l l y t h e same r a n g e o f bed l e n g t h s a n d a d s o r b e n t p a r t i c l e s i z e s . The t i m e c y c l e s o f t h e t h r e e b e d s c a n be s e q u e n c e d i n s u c h a way t h a t c o m p r e s s e d feed i s always f l o w i n g t o t h e process. This reduces the s i z e o f t h e f e e d s u r g e t a n k o r c o m p l e t e l y e l i m i n a t e s t h e need f o r i t . In general the f r a c t i o n o f the o v e r a l l c y c l e devoted t o the f e e d p u l s e i n t h e m u l t i p l e - b e d p r o c e s s i s much g r e a t e r - a b o u t
8.
KELLER
Gas-Adsorption
Processes
155
EXHAUST: — IMPURE ADSORBED PRODUCT
FEED
SURGE TANK
ADSORBER
D—IX}-**- LESSSURGE ADSORBED TANK PRODUCT EXHAUST:
FEED
-•IMPURE ADSORBED PRODUCT
ADSORBERS
τ F i g u r e k.
τ
τ
LESSADSORBED PRODUCT
One-bed a n d t h r e e - b e d ρ r e s s u r e - s w i n g p a r a m e t r i c pumps.
156
INDUSTRIAL
GAS
SEPARATIONS
o n e - t h i r d - than the f r a c t i o n i n the single-bed process - i n the o r d e r o f one-tenth. The m u l t i p l e - b e d p r o c e s s u s u a l l y p r o d u c e s somewhat h i g h e r l e s s - a d s o r b e d p r o d u c t p r e s s u r e s a n d s l i g h t l y h i g h e r l e s s - a d s o r b e d p r o d u c t r e c o v e r i e s compared t o the s i n g l e - b e d process. The most u n u s u a l f e a t u r e o f PSPP i s t h e c o n t i n u o u s l y changing a x i a l pressure p r o f i l e . Whereas a l l o t h e r p r o c e s s e s o p e r a t e w i t h v i r t u a l l y a c o n s t a n t p r e s s u r e through t h e bed a t any g i v e n t i m e , t h e f a s t c y c l e o f PSPP a n d t h e f l o w r e s i s t a n c e c a u s e d by t h e u s e o f v e r y s m a l l a d s o r b e n t p a r t i c l e s c r e a t e s u b s t a n t i a l p r e s s u r e d r o p s i n t h e b e d , a s shown i n F i g u r e 5. These p r e s s u r e p r o f i l e s a r e h i g h l y c r i t i c a l t o t h e performance of t h e process. F o r example, t h e unusual p r o f i l e d u r i n g the e x h a u s t p a r t o f t h e c y c l e , w h i c h shows a maximum p r e s s u r e somewhere i n s i d e t h e b e d , s i m u l t a n e o u s l y p e r m i t s p u r g i n g o f p a r t o f t h e bed w h i l e l e s s - a d s o r b e d p r o d u c t s a r e c o n t i n u a l l y produced a t t h e o p p o s i t e end o f t h e bed. PSPP h a s b e e n c o m m e r c i a l i z e d f o r t h e p r o d u c t i o n o f o x y g e n and f o r t h e r e c o v e r y a n d r e c y c l e o f e t h y l e n e a n d a s m a l l amount of chlorocarbons (the adsorbed stream) t o an e t h y l e n e c h l o r i n a t i o n process w h i l e purging n i t r o g e n (the less-adsorbed component) f r o m t h e p r o c e s s . As m e n t i o n e d b e f o r e , POLYBED PSA and PSPP r e p r e s e n t q u i t e d i f f e r e n t a p p r o a c h e s t o i m p r o v i n g t h e b a s i c PSA p r o c e s s . Table I I shows t h e p l u s e s a n d m i n u s e s o f t h e s e a p p r o a c h e s compared t o a b a s i c PSA p r o c e s s . A new PSA p r o c e s s h a s been d e v e l o p e d by B e r g b a u F o r s c h u n g GmbH f o r t h e p r o d u c t i o n o f n i t r o g e n f r o m a i r ( 2 8 , 2 9 ) . The process uses a unique carbon-based molecular s i e v e as t h e adsorbent. The a d s o r b e n t ' s m i c r o - g e o m e t r y i s s u c h t h a t , e v e n t h o u g h t h e a d s o r p t i o n i s o t h e r m s o f o x y g e n and n i t r o g e n a r e a l m o s t i d e n t i c a l , o x y g e n d i f f u s e s many t i m e s more r a p i d l y i n t o the pores. T h i s c r e a t e s an o x y g e n - d e p l e t e d gas phase u n t i l b o t h g a s e s e q u i l i b r a t e , w h i c h r e q u i r e s o v e r a n h o u r . By o p e r a t i n g the process w i t h a time c y c l e c o n s i d e r a b l y l e s s than t h e e q u i l i b r i u m t i m e , t h e a d s o r b e n t becomes o x y g e n - s e l e c t i v e and p r o d u c e s a h i g h - n i t r o g e n p r o d u c t . N i t r o g e n p u r i t i e s o f up t o 99.9 p e r c e n t c a n be p r o d u c e d . The p r o c e s s , d e p i c t e d I n F i g u r e 6, u s e s a vacuum d e s o r p t i o n s t e p t o f a c i l i t a t e production of high-purity nitrogen. N i t r o g e n p r o d u c t i o n u s i n g carbon molecular s i e v e s i s t h e o n l y known c o m m e r c i a l p r o c e s s u s i n g d i f f e r e n c e s i n i n t r a p a r t i c l e d i f f u s i v i t y , r a t h e r than i n h e r e n t adsorbent s e l e c t i v i t y o r s e l e c t i v e molecular e x c l u s i o n , as the basis f o r theseparation. A n o t h e r method o f u s i n g PSA t o o b t a i n n i t r o g e n f r o m a i r h a s r e c e n t l y been r e v e a l e d b y T o r a y I n d u s t r i e s , I n c . ( 3 0 ) . The s t a n d a r d PSA p r o c e s s h a s b e e n m o d i f i e d t o p r o d u c e a n i m p u r e (33 p e r c e n t oxygen) l e s s - a d s o r b e d g a s and a r e l a t i v e l y pure (up t o 99.9 p e r c e n t ) p u r g e s t r e a m o f n i t r o g e n . A d i a g r a m i s shown
8.
KELLER
Gas-Adsorption
157
Processes TABLE I I
COMPARISON OF THE PERFORMANCES OF POLYBED AND PRESSURE-SWING PARAMETRIC PUMPING WITH BASIC PSA Polybed
PSA
PSA
Pressure-Swing P a r a m e t r i c Pumping
Less
Same o r G r e a t e r
Productivity
A b o u t t h e Same
Much G r e a t e r
Degree o f S e p a r a t i o n
Greater
Same o r L e s s
Process Complexity
Greater
Less
Adaptability to Large Flows
Greater
Less
A b o u t t h e Same (Several Minutes)
Much L e s s (Several
Same
Lower
Compression
Cycle
Cost
Time
Non-Adsorbed Pressure
Seconds)
Product
F i g u r e 5. P r e s s u r e p r o f i l e s i n an a d s o r b e n t b e d p r o d u c i n g 90 mole p e r c e n t o x y g e n . 1, m i d d l e o f f e e d ; 2, m i d d l e o f d e l a y ; 3, e a r l y i n e x h a u s t ; a n d U, l a t e i n e x h a u s t .
I N D U S T R I A L GAS
SEPARATIONS
NITROGEN
- IMPURE OXYGEN F i g u r e 6.
Bergbau Forschung n i t r o g e n - p r o d u c t i o n
process.
8.
KELLER
Gas-Adsorption
Processes
159
i n F i g u r e 7. D r i e d a i r i s passed through the adsorber a t superatmospheric p r e s s u r e ; n i t r o g e n i s p r e f e r e n t i a l l y adsorbedPart of t h e n i t r o g e n p r o d u c t from p r e v i o u s c y c l e s i s t h e n passed i n t o t h e bed t o d e s o r b s m a l l amounts o f o x y g e n , a f t e r w h i c h t h e bed i s r e d u c e d t o a t m o s p h e r i c p r e s s u r e t o d e s o r b n i t r o g e n . F i n a l l y more n i t r o g e n i s r e c o v e r e d by vacuum d e s o r p t i o n . The a d s o r b e n t u s e d i n t h i s p r o c e s s i s a p p a r e n t l y t h e same - z e o l i t e m o l e c u l a r s i e v e s - as t h a t u s e d i n o x y g e n PSA p r o c e s s e s . T h i s p r o c e s s has been demonstrated i n p i l o t - s c a l e . Its c o m m e r c i a l s t a t u s i s unknown. D i s p l a c e m e n t - P u r g e and I n e r t - P u r g e C y c l e s . By f a r t h e most w i d e s p r e a d embodiment o f t h e s e c y c l e s i s t h e s e p a r a t i o n o f n o r m a l and i s o - p a r a f f i n s i n a v a r i e t y o f p e t r o l e u m f r a c t i o n s . T h e s e f r a c t i o n s i n g e n e r a l c o n t a i n s e v e r a l c a r b o n numbers and c a n i n c l u d e m o l e c u l e s f r o m a b o u t C5 t o a b o u t C^g. A l l of these s e p a r a t i o n s u s e 5A m o l e c u l a r s i e v e a s t h e a d s o r b e n t . I t s 0.5 nm p o r e d i a m e t e r i s s u c h t h a t n o r m a l p a r a f f i n s c a n e n t e r but i s o - p a r a f f i n s are e x c l u d e d ; t h i s c o n s t i t u t e s the b a s i s f o r separation. A r e c e n t review (13) l i s t s s i x d i f f e r e n t commercial vaporphase p r o c e s s e s . These p r o c e s s e s use e i t h e r d i s p l a c e m e n t - p u r g e or i n e r t - p u r g e c y c l e s . The f o r m e r a r e u s e d when h i g h e r molecular-weight f r a c t i o n s a r e being processed. I n such cases, t h e a d s o r b a t e s a r e h e l d so t i g h t l y by t h e a d s o r b e n t t h a t t h e use o f an a d s o r b i n g d i s p l a c e m e n t agent i s n e c e s s a r y t o e f f e c t desorption. I n t u r n , h o w e v e r , two d i s t i l l a t i o n columns a r e r e q u i r e d to r e c o v e r the displacement f l u i d from the product streams. When l o w e r - m o l e c u l a r - w e i g h t f r a c t i o n s a r e b e i n g processed, the s i m p l e r i n e r t - p u r g e c y c l e s s u f f i c e . With these c y c l e s , c o o l e r s f o l l o w e d by v a p o r - l i q u i d s e p a r a t o r s c a n be u s e d to r e c o v e r p r o d u c t s from the purge gas. The n o r m a l / i s o - p a r a f f i n s e p a r a t i o n p r o c e s s u s e d most o f t e n i s Union Carbide's I s o S i v process. I s o S i v u n i t s w i t h feed c a p a c i t i e s up t o 3600 m e t r i c t o n s p e r day have b e e n b u i l t . In t h e m i d - 1 9 7 0 s I s o S i v was c o m b i n e d w i t h t h e Hysomer p a r a f f i n i s o m e r i z a t i o n p r o c e s s d e v e l o p e d by S h e l l R e s e a r c h B.V. ( 3 1 , 3 2 ) ; t h i s combination i s c a l l e d the T o t a l I s o m e r i z a t i o n Process (TIP) and c a n p r o d u c e a c o m p l e t e l y i s o - p a r a f f i n p r o d u c t f r o m a n o r m a l o r m i x e d p a r a f f i n f e e d . A s c h e m a t i c d i a g r a m i s shown i n F i g u r e 8. Hydrogen, which i s r e q u i r e d i n the i s o m e r i z a t i o n s t e p , a l s o s e r v e s as the purge gas f o r d e s o r b i n g the normal p a r a f f i n s , w h i c h a r e r e c y c l e d t o t h e Hysomer u n i t . The p r o c e s s i s c a r r i e d o u t i n t h e v a p o r p h a s e , and u t i l i t y r e q u i r e m e n t s a r e c o n s i d e r a b l y b e l o w t h o s e o f d i s t i l l a t i o n ( 3 2 ) . P r o d u c t f r o m a T I P u n i t has a R e s e a r c h O c t a n e Number o f 88 t o 92, compared t o a RON o f 79 t o 82 f o r t h e p r o d u c t f r o m a Hysomer u n i t a l o n e . The c l o s e - c o u p l i n g o f r e a c t i o n and s e p a r a t i o n u n i t s f o r d r i v i n g e q u i l i b r i u m r e a c t i o n s to complete c o n v e r s i o n , though
2
F i g u r e 7·
PRESSURIZED AIR
2
IMPURE OXYGEN + C0 /H 0
NITROGEN
process.
PRESSURIZED REFLUX
IMPURE OXYGEN
SEPARATION SECTION
T o r a y r e v e r s e PSA n i t r o g e n - p r o d u c t i o n
2
C(VH 0 REMOVAL
8.
KELLER
Gas-Adsorption
161
Processes
LIGHT PARAFFINS
1
H + RECYCLE NORMAL PARAFFINS 2
PARAFFIN FEED MAKEUP H Ο
Ζ ω
ce Ο
HIGH-OCTANE ISOMER PRODUCT
CO
û
