Polybed Pressure-Swing Adsorption Hydrogen Processing

13 1

Polybed Pressure-Swing Adsorption Hydrogen Processing R. T. CASSIDY Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch013

Union Carbide Corporation, Linde Division/EP&P, Tarrytown, NY 10591

A process has been commercialized which represents a breakthrough in the production of high purity hydrogen in the quantities needed for large hydrogen plants serving the refining, chemicals production, and f e r t i l i z e r manufacturing industries. The patented process employs multiple adsorbent beds in a pressure-swing cycle to separate hydrogen from a variety of hydrogen-bearing feed streams. The process is characterized by simple plant design, high product purities and recoveries, and low energy requirements. Commercial systems have proven stable and reliable in operation and are non-polluting. Hydrogen is important for a multitude of indust r i a l processes including hydrogenation, hydrocracking, ammonia and methanol syntheses, etc. Traditionally hydrogen has been available at purities up to about 97% in large plants. Although pressure-swing adsorption technology is capable of providing ultra-pure hydrogen, i t had previously been limited to relatively low recoveries and capacities. U n i o n C a r b i d e e n g i n e e r s d e v e l o p e d a new c o n c e p t in p r e s s u r e - s w i n g a d s o r p t i o n t e c h n o l o g y which s o l v e s many o f t h e problems i n h e r e n t i n c o n v e n t i o n a l hydrogen p r o c e s s i n g and extends t h e c a p a c i t y range c o n s i d e r a b l y beyond t h a t o f p r e v i o u s p r e s s u r e - s w i n g p r o c e s s e s . The c o n c e p t u t i l i z e s s e v e r a l a d s o r b e r s i n an i n t e g r a t e d c y c l e t h a t improves a d s o r b e n t p r o d u c t i v i t y and p r o d u c t recovery. The t e c h n i q u e was a p p l i e d t o a steam r e f o r m i n g p l a n t t o r e p l a c e t h e c o n v e n t i o n a l hydrogen p u r i f i c a t i o n s steps (1). Optimization o f the pressure-swing system r e s u l t e d i n t h e POLYBED PSA system.

1

POLYBED is a trademark o f Union C a r b i d e

Corporation.

0-8412-0582-5/80/47-135-247$05.00/0 ©1980 American Chemical Social

In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

SYNTHETIC ZEOLITES

248 Background

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch013

Pressure-Swing

Adsorption

A d i a b a t i c p r e s s u r e - s w i n g a d s o r p t i o n (PSA) p r o c e s s es u s i n g m o l e c u l a r s i e v e s , a c t i v a t e d c a r b o n and s i l i c a g e l have become w i d e l y a c c e p t e d as v i t a l p r o c e s s i n g t o o l s i n the p e t r o c h e m i c a l , c h e m i c a l , p e t r o l e u m r e f i n i n g and atmospheric gas s e p a r a t i o n i n d u s t r i e s d u r i n g the past ten y e a r s . A p p l i c a t i o n s i n c l u d e p u r i f i c a t i o n o f hydrogen, h e l i u m , and methane; s e p a r a t i o n o f i s o and n o r m a l - p a r a f f i n s ; and a i r s e p a r a t i o n . The b a s i c process i s c y c l i c i n nature c o n s i s t i n g of s e v e r a l s t e p s by which a gaseous p r o d u c t i s s e p a r a t e d from a m i x t u r e a t moderate t o h i g h p r e s s u r e . The a d s o r b e n t r e g e n e r a t i o n i s c a r r i e d out a t low p r e s s u r e . In t h e s i m p l e s t type o f PSA system, an a d s o r b e r s e p a r a t e s the d e s i r e d p r o d u c t from the f e e d m i x t u r e a t f e e d p r e s s u r e and ambient t e m p e r a t u res. When t h e ads o r b e n t bed has been s a t u r a t e d w i t h i m p u r i t i e s , i t i s d e p r e s s u r i z e d t o a lower p r e s s u r e and i n a d i r e c t i o n c o u n t e r c u r r e n t t o the f e e d f l o w d i r e c t i o n . The d e p r e s s u r i z a t i o n (blowdown) c a u s e s d e s o r p t i o n and removal o f much o f t h e i m p u r i t i e s from t h e a d s o r b e n t . The bed i s then purged, a l s o c o u n t e r c u r r e n t l y t o t h e f e e d f l o w d i r e c t i o n , w i t h p r o d u c t gas t o complete t h e removal o f i m p u r i t i e s t o a low r e s i d u a l l e v e l . Alternately, t h e bed may be evacuated t o a sub-atmospheric p r e s s u r e t o remove i m p u r i t i e s . The a d s o r b e r i s then p r e s s u r i z e d w i t h p r o d u c t gas and i t i s put back on l i n e t o b e g i n the a d s o r p t i o n s t e p once a g a i n . The e a r l i e r PSA u n i t s t y p i c a l l y u t i l i z e d 2,3, o r 4 a d s o r b e r beds i n a c y c l i c sequence i n which one o f the beds was always on an a d s o r p t i o n s t e p w h i l e t h e o t h e r bed o r beds were b e i n g r e g e n e r a t e d . However, t h e s e systems were i n h e r e n t l y i n e f f i c i e n t f o r two reasons. F i r s t , the blowdown from a d s o r p t i o n p r e s s u r e t o the low waste p r e s s u r e caused l a r g e l o s s e s o f t h e d e s i r e d p r o d u c t gas which was s t o r e d i n t h e bed a t high pressure. Second, some pure p r o d u c t gas was c y c l e d t o waste s i n c e i t was used t o purge a r e g e n e r a t i n g bed. Improved c y c l e s were d e v e l o p e d t o i n c r e a s e t h e e f f i c i e n c i e s o f PSA systems. The i n n o v a t i o n c o n s i s t e d o f w i t h d r a w i n g p r o d u c t - q u a l i t y gas from an a d s o r b e r t h a t has completed i t s a d s o r p t i o n s t e p by d e p r e s s u r i z i n g t h e bed c o c u r r e n t l y , i n t h e d i r e c t i o n o f f e e d and p r o d u c t f l o w , t o some i n t e r m e d i a t e p r e s s u r e . By c o n t r o l of the mass-transfer f r o n t s of the i m p u r i t i e s , the p u r i t y o f t h i s l o w e r - p r e s s u r e gas i s m a i n t a i n e d a t



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PSA Hydrogen Processing

a h i g h l e v e l , and i t i s used t o p e r f o r m some o f t h e p r e s s u r i z a t i o n and a l l o f t h e p u r g i n g o f o t h e r a d s o r b e r s i n t h e c y c l e . Thus, t h e p r o d u c t gas p r e v i o u s l y l o s t i n t h e s e f u n c t i o n s c a n be r e c o v e r e d as p r o d u c t at the plant battery l i m i t s . A l s o , s i n c e blowdown s t a r t s a t much lower p r e s s u r e , l o s s e s o f p r o d u c t stored w i t h i n t h e bed a r e r e d u c e d . A c y c l e sequence c h a r t f o r a PSA system i n c o r p o r a t i n g t h e s e i n n o v a t i o n s i s shown i n F i g u r e 1. The gas for p r e s s u r i z a t i o n o f adsorbers i s t r a n s f e r r e d v i a e q u a l i z a t i o n s i n p r e s s u r e between t h e bed s u p p l y i n g the gas and t h e bed r e c e i v i n g i t . The system i s a r ranged so t h a t one a d s o r b e r i s always " o n - l i n e " sup p l y i n g product gas. The advantages o f PSA systems o v e r o t h e r p r o c e s s i n g schemes a r e : ο

U l t r a - h i g h p u r i t y - p r e f e r r e d f o r hydrogen, h e l i u m and methane p u r i f i c a t i o n s w i t h v i r t u a l l y t o t a l removal o f i m p u r i t i e s .

ο

No e x t e r n a l h e a t i n p u t r e q u i r e d tion.

ο

Single step processing - several constituents i n a gas stream c a n be removed i n one s t e p , e.g., CO, CHit and C 0 from h y d r o g e n - b e a r i n g streams.

f o r regenera

2

ο

Long a d s o r b e n t l i f e - o r i g i n a l a d s o r b e n t charge g e n e r a l l y l a s t s t h e l i f e o f t h e p l a n t .

T r a d i t i o n a l l y PSA p r o c e s s e s have found p a r t i c u l a r l y w i d e s p r e a d a p p l i c a t i o n i n hydrogen p r o d u c t i o n systems w h e r e i n t h e PSA u n i t i s used t o produce u l t r a h i g h p u r i t y hydrogen (99.999%+) from steam-reformed h y d r o c a r b o n s and o t h e r hydrogen s o u r c e s . However, s e v e r a l l i m i t a t i o n s have e x i s t e d w i t h t h e s e p r o c e s s e s . The r e c o v e r y o f hydrogen, even i n the more e f f i c i e n t Union C a r b i d e f o u r - b e d system, i s t y p i c a l l y 70-75%. The s i z e o f t h e a d s o r b e n t beds i s l i m i t e d by f a c t o r s such as t h e p h y s i c a l s t r e n g t h o f t h e a d s o r b e n t mater i a l s , v e s s e l t r a n s p o r t a t i o n , e f f i c i e n c y o f flow d i s t r i b u t i o n and o t h e r p r a c t i c a l c o n s i d e r a t i o n s . As a r e s u l t o f r e c o v e r y and bed s i z e l i m i t a t i o n s , t h e p r o d u c t i o n r a t e o f 2-, 3-, o r 4-bed systems g e n e r a l l y has an upper l i m i t o f 12-13 MMSCFD. D u r i n g t h e 1970's t h e t r e n d i n t h e p e t r o c h e m i c a l i n d u s t r y s h i f t e d t o s y n t h e s i s g a s a s a b a s i c raw mater i a l , and t h e need f o r h i g h e r - o c t a n e g a s o l i n e c r e a t e d major new demands f o r hydrogen i n p e t r o l e u m r e f i n i n g . I t was r e c o g n i z e d t h a t c o n v e n t i o n a l PSA u n i t s were not b r o a d l y a p p l i c a b l e i n t h e s e a r e a s because o f t h e i r



250

SYNTHETIC ZEOLITES

VESSEL NUMBER ADSORPTION

1 2

C D t

3

EJ1

4

EQl t

EQl

PURGE EQ2 EQl t t t

C D

EQ2 EQl t t

ADSORPTION

R *

E^2 C D PURGE t t R t

EQ2 C D PURGE t t

R •

EQ2 EQl t t

ADSORPTION

EQl

C D

EQl 4

R t C D *

E^2

ADSORPTION E^2 C D PURGE EQ2 t t

EQ — EQUALIZATION

•

C D+- COCURRENT DEPRESSURIZATION

f - COUNTERCURRENT FLOW

COCURRENT FLOW

C D|-COUNTERCURRENT DEPRESSURIZATION R-

REPRESSURIZATION Figure 1.

Generalized PSA cycle sequence chart



13.

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l i m i t e d p r o d u c t i o n c a p a c i t y . Furthermore, t h e concept of s i m p l y u s i n g s e v e r a l o f t h e s e u n i t s i n p a r a l l e l was not t r u l y v i a b l e . T h e i r r e l a t i v e l y low p r o d u c t r e c o v ery (70-75%) i n l a r g e u n i t s would r e s u l t i n l a r g e q u a n t i t i e s o f low-value f u e l - g a s o r waste g a s . I t was e v i d e n t t h a t a new-generation, high-capa c i t y , h i g h - r e c o v e r y system was needed. The problem became one o f making more e f f i c i e n t u s e o f t h e a d s o r b e n t s used i n t h e s e systems and, c o n c u r r e n t l y , making a major i n c r e a s e i n u n i t c a p a c i t y . The POLYBED PSA System S i n c e t h e p r e v i o u s l y s t a t e d l i m i t a t i o n s on v e s s e l s i z e were s t i l l i n e f f e c t , i t was d e c i d e d t o p u r sue a roate u s i n g m u l t i p l e a d s o r b e r beds i n a h i g h l y i n t e g r a t e d system. I t was n e c e s s a r y t o d e f i n e i n t e g r a t e d h i g h - c a p a c i t y systems w i t h a t l e a s t 80% p r o d u c t r e c o v e r y and t o d e v i s e c o n t r o l schemes f o r such s y s tems. While o p t i m i z i n g p u r i t y and r e c o v e r y f o r t h e l a r g e - s c a l e system, some o f t h e c o n c e r n s were: ο

D e t e r m i n a t i o n o f t h e minimum number (N) of hydrogen r e c o v e r y s t a g e s , and number o f a d s o r b e r s f o r a wide range o f o p e r a t i n g parameters.

ο

F o r each N, a d e f i n i t i o n o f optimum f e e d - t o purge p r e s s u r e r a t i o s .

ο

D e f i n i t i o n o f t h e minimum f i n a l purge c o n d i t i o n which s t i l l p r o v i d e s n e c e s s a r y a d s o r b ent cleanup.

ο

D e t e r m i n a t i o n o f m a s s - t r a n s f e r f r o n t s , shapes and movement d u r i n g each s t e p .

ο

Avoidance o f b u i l d u p o f "heavy" i m p u r i t i e s which c o u l d cause i r r e v e r s i b l e a d s o r b e n t deactivation.

ο

Design o f a r e l i a b l e process c o n t r o l

system.

In d e a l i n g w i t h t h e problems mentioned above, c o n s i d e r a t i o n was g i v e n t o those o c c u r r e n c e s i n t h e p r o c e s s which were l a r g e l y u n s t e a d y - s t a t e phenomena. In each s t e p i n t h e p r o c e s s t h e r e i s a change i n t h e a d s o r b a t e ( i m p u r i t y ) l o a d i n g on t h e a d s o r b e n t bed and a change i n t h e t o t a l gas s t o r a g e w i t h i n t h e a d s o r b e r vessel. These changes a r e caused by: ο

P r e s s u r e change such as d e p r e s s u r i z a t i o n and r e p r e s s u r i z a t i o n .



252

SYNTHETIC ZEOLITES

o

L o a d i n g changes which a r e t h e r e s u l t o f changing p a r t i a l pressures o f the i m p u r i t i e s w i t h i n t h e system.

ο

Temperature changes w i t h i n t h e a d s o r b e r s caused by e x p a n s i o n o r c o n t r a c t i o n o f gas d u r i n g p r e s s u r e changes, and t h e heat o f a d s o r p t i o n and d e s o r p t i o n .

ο

Movement o f t h e m a s s - t r a n s f e r f r o n t s i n each a d s o r b e r as a r e s u l t o f changes i n p r e s s u r e , l o a d i n g ( i . e . , movement a l o n g t h e i s o t h e r m s ) , temperature, f l o w d i r e c t i o n and v o l u m e t r i c flow.

ο

Changes i n c o m p o s i t i o n o f t h e gas e n t e r i n g and l e a v i n g each a d s o r b e r , caused by items mentioned above.

When d e a l i n g w i t h seven t o t e n , o r more, a d s o r b ers and a d d i t i o n a l i n t e r m e d i a t e p r e s s u r e l e v e l s , as w e l l as t h e dynamic b e h a v i o r o f t h e a d s o r b e n t system i t s e l f , t h e p r o c e s s must a l s o be s u b j e c t e d t o more s o p h i s t i c a t e d c o n t r o l than when d e a l i n g w i t h f o u r o r fewer a d s o r b e r beds and o n l y one o r two i n t e r m e d i a t e pressure l e v e l s . Process

Description

The POLYBED PSA hydrogen system uses f i v e o r more a d s o r b e r s i n a s t a g g e r e d sequence. Several adsorbers a r e on a d s o r p t i o n s t e p s a t any time w h i l e t h e o t h e r s a r e i n v a r i o u s s t a g e s o f r e g e n e r a t i o n . The d i f f e r e n t c y c l e s t e p s which an a d s o r b e r e x p e r i e n c e s d u r i n g one complete c y c l e a r e summarized below and i n T a b l e I . Adsorption The f e e d gas e n t e r s t h e a d s o r b e r a t r e l a t i v e l y h i g h p r e s s u r e ( t y p i c a l l y 250 t o 600 p s i g ) . A l l impur i t i e s a r e s e l e c t i v e l y adsorbed and p u r i f i e d hydrogen p r o d u c t i s d e l i v e r e d a t about 5 p s i l e s s than f e e d pressure. The a d s o r p t i o n s t e p i s t e r m i n a t e d when t h e mass t r a n s f e r f r o n t r e a c h e s a p r e d e t e r m i n e d p o i n t i n t h e bed. Hydrogen Recovery

Steps

The a d s o r b e r i s d e p r e s s u r i z e d i n s t a g e s t o r e c o v er hydrogen from t h e v o i d spaces i n t h e bed. P u r i t y of t h e hydrogen t a k e n from t h e bed d u r i n g t h e s e s t e p s i s maintained. T h i s i s a c h i e v e d by c o n t r o l l i n g t h e



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TABLE I CYCLE STEPS

Steps

Functions

Adsorption

Hydrogen Recovery

Trap

Stages

impurities

Recover H s t o r e d i n t h e a d s o r b e n t bed t o p r e s s u r i z e and purge o t h e r a d s o r b e r s . 2

Blowdown

Desorb i m p u r i t y .

Regeneration

Purge i m p u r i t i e s from adsorbent and r e s t o r e p r e s s u r e i n vessel prior to adsorption step.


SYNTHETIC ZEOLITES

254

d e p r e s s u r i z a t i o n s t o keep the i m p u r i t y f r o n t w i t h i n the bed a t a l l t i m e s . The c o n t r o l system r o u t e s the gas t o o t h e r a d s o r b e r s f o r r e p r e s s u r i z a t i o n and p u r g i n g . Blowdown


A f i n a l c o n t r o l l e d d e p r e s s u r i z a t i o n occurs i n w h i c h t h e v e s s e l i s brought t o i t s l o w e s t p r e s s u r e . I m p u r i t i e s a r e desorbed and removed t o some e x t e n t . Regeneration The a d s o r b e r i s purged a t t a i l gas p r e s s u r e w i t h hydrogen r e c o v e r e d from o t h e r a d s o r b e r s . The gas des o r b s i m p u r i t i e s from t h e a d s o r b e n t and f l o w s out the t a i l gas l i n e . The i m p u r i t y f r o n t i s r e t u r n e d t o t h e f e e d end o f the a d s o r b e n t bed. The t a i l gas can be used as f u e l o r f o r subsequent p r o c e s s i n g . Recovered hydrogen from o t h e r a d s o r b e r s i s used t o r e p r e s s u r i z e the v e s s e l i n s e v e r a l s t a g e s t o i n t e r mediate l e v e l s . Some p r o d u c t gas i s i n t r o d u c e d t o complete t h e r e p r e s s u r i z a t i o n . T h i s sequence o f s t e p s completes one c y c l e on an adsorber. A f t e r t h e r e p r e s s u r i z a t i o n i s completed, t h e a d s o r b e r i s a g a i n s w i t c h e d t o the a d s o r p t i o n s t e p and the sequence i s r e p e a t e d . I n t e g r a t i o n of Process

Steps

As was mentioned b e f o r e , s e v e r a l o f t h e a d s o r b e r s i n a POLYBED system a r e on a d s o r p t i o n s t e p s a t any time. However, the sequencing i s s t a g g e r e d so t h a t the a d s o r b e r s a r e i n d i f f e r e n t phases o f t h e i r a d s o r p t i o n steps. F o r example, as a bed b e g i n s i t s a d s o r p t i o n s t e p , one o f the o t h e r s c o u l d be o n e - q u a r t e r t o oneh a l f o f t h e way through i t s a d s o r p t i o n s t e p . Still a n o t h e r a d s o r b e r c o u l d be o n e - h a l f t o t h r e e - q u a r t e r s through a d s o r p t i o n . The r e m a i n i n g v e s s e l s a r e i n v a r i o u s s t a g e s o f hydrogen r e c o v e r y o r r e g e n e r a t i o n . The i n t e g r a t e d sequencing p r o v i d e s the o p p o r t u n i t i e s f o r more e f f i c i e n t hydrogen r e c o v e r y through e x t r a s t e p s compared t o t r a d i t i o n a l PSA c y c l e s . Improvements o f b e t t e r t h a n t e n p e r c e n t a g e p o i n t s have been r e a l i z e d (2). A d s o r b e n t u t i l i z a t i o n i s improved so t h a t p r o d u c t i v i t y , i . e . , hydrogen produced per mass u n i t a d s o r b e n t , i n c r e a s e d by 10 t o 20%. The s o p h i s t i c a t e d sequencing and the r e q u i r e m e n t s f o r c o n t r o l o f the t r a n s f e r o f gas streams among t h e a d s o r b e r s p r e s e n t e d a c h a l l e n g e i n c o n t r o l system design. Through use o f s o l i d s t a t e programmable hardware


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and modern p r e s s u r e c o n t r o l t e c h n i q u e s , h i g h l y e f f i c i ent and r e l i a b l e c o n t r o l systems were implemented (2,3).


Conventional Hydrogen Production Using Liquid Absorption Purification A r e v i e w o f c o n v e n t i o n a l hydrogen p r o d u c t i o n v i a steam r e f o r m i n g i s u s e f u l t o a p p r e c i a t e t h e advantages of t h e POLYBED PSA system. The c o n v e n t i o n a l system c o n s i s t s o f a feed d e s u l f u r i z e r , reforming furnace, h i g h - t e m p e r a t u r e and low-temperature s h i f t c o n v e r t e r s , C 0 removal system and a methanator (see F i g u r e 2 ) . The o p e r a t i o n o f t h e r e f o r m i n g s e c t i o n i s dependent on the degree o f CO c o n v e r s i o n i n t h e s h i f t c o n v e r t e r s and t h e e x t e n t o f C 0 removal from t h e hydrogen stream. L e s s CO c o n v e r s i o n and C 0 removal r e s u l t i n g r e a t e r hydrogen l o s s e s i n t h e methanator. In t u r n , a g r e a t e r steam-to-carbon r a t i o i s r e q u i r e d i n t h e r e f o r m e r t o m a i n t a i n a p r o d u c t p u r i t y o f 97% hydrogen. Generally, the hydrogen p u r i t y a t t a i n a b l e by c o n v e n t i o n a l systems i s 97-98%. A l t h o u g h t h e r e f o r m e r c a t a l y s t tends t o be rugged and r e l i a b l e , t h e c a t a l y s t i n t h e low-temperature s h i f t c o n v e r t e r and t h e methanator tends t o be l e s s s o . I n a d d i t i o n , t h e p o t e n t i a l e x i s t s f o r temperature e x c u r s i o n s i n t h e methanator i n t h e e v e n t o f C 0 breakthrough from t h e wash system. The C 0 removal system r e q u i r e s a s i g n i f i c a n t h e a t i n p u t and a c o r r o s i v e s o l v e n t , which can p r e s e n t d i f f i c u l t i e s . "Conventional" technology i s therefore character i z e d by: 2

2

2

2

2

ο

Hydrogen r e c o v e r i e s o f 90-95%.

ο

H

ο

Complex f l o w scheme f o r i m p u r i t y r e m o v a l .

ο

Wet p r o c e s s i n g , u s i n g c o r r o s i v e s o l v e n t which may present h a n d l i n g and o p e r a t i o n a l problems.

2

p u r i t y l i m i t e d t o 97-98%.

I n t e g r a t i o n i n t o Hydrogen P l a n t The POLYBED PSA system r e p l a c e s s e v e r a l o f t h e u n i t s a s s o c i a t e d w i t h hydrogen p u r i f i c a t i o n i n a c o n v e n t i o n a l p l a n t — s p e c i f i c a l l y , t h e low temperature s h i f t c o n v e r t e r , t h e C 0 removal system and t h e methan a t o r (see F i g u r e 2 ) . The t a i l gas from t h e PSA system i s used as f u e l t o t h e r e f o r m e r and i t i s supplemented by an e x t e r n a l s u p p l y . However, t h e amount o f f u e l consumed i s g r e a t l y r e d u c e d . Because some hydrogen 2



Reformer

Λ

Heat recovery

Stack

Fuel

TJ

Reformer

a"

Heat recovery

Stack

POLYBED PSA HYDROGEN PLANT

Feed

Dtsulfurizer

H e a t

f

w

c e o l i n

*

t-0U

PSA system PSA purge gas drum

Fuel

Conventional and PSA steam reformer hydrogen plants

*» V

L0U

L.T. shift

Heat recovery and cooling

H.T. shift

Process steam

λ

HT. shift

Process steam

Figure 2.

CONVENTIONAL HYDROGEN PLANT

TU

Dtsulfurizer


Hydrocarbon Processing

Hydrogen product

Hydrogen product


13.

CASSIDY


257

i s l o s t t o f u e l through t h e t a i l gas o f t h e POLYBED PSA u n i t , more hydrogen must be produced i n t h e r e f o r m i n g section. T h i s i n c r e a s e s t h e f e e d r e q u i r e m e n t and t h e size o f the furnace. O f f - s e t t i n g t h i s disadvantage i s a more e f f i c i e n t h e a t r e c o v e r y w i t h t h e n e t e f f e c t t h a t t h e o v e r a l l hydrogen p r o d u c t i o n c o s t s , b o t h o p e r a t i n g and c a p i t a l , a r e l e s s than t h o s e o f a c o n v e n t i o n a l p l a n t (1). One o f t h e major t e c h n i c a l problems t h a t had t o be overcome t o i n t e g r a t e t h e POLYBED system w i t h t h e steam r e f o r m e r was t h e v a r i a t i o n i n t a i l gas f l o w and composition. Because o f t h e c y c l i c n a t u r e o f t h e p r o c e s s , t a i l gas i s r e j e c t e d by t h e POLYBED u n i t d u r i n g blowdown and purge w i t h s i g n i f i c a n t f l o w and composition variations. The f l u c t u a t i o n s would have made i t i m p o s s i b l e t o use t h e t a i l gas f o r f u e l and a s o p h i s t i c a t e d system was d e v e l o p e d t o b a l a n c e t a i l gas h e a t i n g v a l u e . T h i s b u f f e r / m i x i n g tank system has proven t o be v e r y r e l i a b l e i n h o l d i n g heat i n p u t v a r i a t i o n t o ± 1% (2) . Commercial O p e r a t i o n The f i r s t commercial u n i t went on stream a t Wint e r s h a l l AG's R e f i n e r y i n L i n g e n , West Germany i n August 1977. I t produces 41 MMSCFD o f 99.999% p u r i t y hydrogen a t a r e c o v e r y o f 86%. T h i s i s 3% above d e s i g n and lowers t h e c o s t s f o r f e e d and u t i l i t i e s by 2% compared t o p r e v i o u s l y p u b l i s h e d comparisons. Both f l e x i b i l i t y and r e l i a b i l i t y a r e enhanced by an a d d i t i o n a l f e a t u r e o f t h e system: t h e two s e c t i o n s of f i v e a d s o r b e r s c a n be d e c o u p l e d t o p e r m i t o p e r a t i o n on a s i n g l e t r a i n o f f i v e beds. T h i s a l l o w s hydrogen p r o d u c t i o n t o c o n t i n u e a t some reduced l e v e l i f a p r o blem o c c u r s w i t h p a r t o f t h e u n i t . When o p e r a t i n g i n t h i s mode, t h e W i n t e r s h a l l p l a n t has produced between 50-60% o f t h e f u l l d e s i g n p r o d u c t f l o w f o r ten-bed o p e r a t i o n . V e r y good r e c o v e r y has been r e a l i z e d i n t h i s mode as w e l l (2) . The f i r s t domestic hydrogen p l a n t u t i l i z i n g POLYBED PSA was r e c e n t l y brought onstream a t A s h t a b u l a , Ohio f o r U n i o n C a r b i d e ' s L i n d e D i v i s i o n . The system i s d e s i g n e d t o produce 9 MMSCFD o f hydrogen. The puri t y i s 99.9999% s i n c e o n l y 1 ppm t o t a l i m p u r i t i e s c a n be t o l e r a t e d i n a l i q u e f a c t i o n system downstream o f t h e POLYBED u n i t . S e v e r a l o t h e r hydrogen p l a n t s which i n t e g r a t e steam r e f o r m i n g w i t h POLYBED PSA a r e i n v a r i o u s s t a g e s o f d e s i g n and c o n s t r u c t i o n (3).


SYNTHETIC ZEOLITES

258

Other A p p l i c a t i o n s o f t h e POLYBED PSA Concept


The a p p l i c a b i l i t y o f POLYBED i s n o t r e s t r i c t e d t o steam r e f o r m i n g hydrogen p l a n t s . The advantages o f h i g h c a p a c i t y , h i g h r e c o v e r y and h i g h p u r i t y make the c o n c e p t s u i t a b l e f o r economic p u r i f i c a t i o n o f hy drogen from a v a r i e t y o f p r o c e s s streams. Other a p p l i cations include: ο

R e f i n e r y H Upgrading - s u b s t i t u t e f o r c r y o g e n i c p r o c e s s i n g (an e n e r g y - i n t e n s i v e process).

ο

Ammonia Loop V e n t Streams - hydrogen o r a h y d r o g e n / n i t r o g e n m i x t u r e can be r e c o v e r e d while r e j e c t i n g i n e r t s .

ο

E t h y l e n e P l a n t O f f - G a s - H c a n be r e c o v e r e d from streams c o n t a i n i n g C s and o t h e r h y d r o carbons .

2

2

1

2

ο

Coal G a s i f i c a t i o n / L i q u e f a c t i o n - the sizes of even d e m o n s t r a t i o n p l a n t s r e q u i r e l a r g e - s c a l e ^ e f f i c i e n t hydrogen r e c o v e r y / p u r i f i c a t i o n .

ο

P a r t i a l oxidation

ο

Methanol

ο

Coke Oven Gases.

systems.

Purge.

Environmental Considerations The POLYBED PSA system i s c l e a n and n o n - p o l l u t i n g . The o n l y m a t e r i a l s l e a v i n g t h e u n i t a r e p r o d u c t and a t a i l gas which i s used f o r f u e l o r subsequent p r o c e s s ing. The system r e q u i r e s no water, combustion a i r , o r f u e l , and d i s c h a r g e s n o t h i n g t o t h e atmosphere. Exist i n g commercial u n i t s a r e d i s t i n g u i s h e d by t h e i r c l e a n appearance and freedom from odor and c o r r o s i o n problems. Conclusion The development and c o m m e r c i a l i z a t i o n o f t h e POLYBED PSA system opens t h e door t o a new g e n e r a t i o n o f a p p l i c a t i o n s i n gas p r o c e s s i n g . High volume, h i g h r e c o v e r y and h i g h p u r i t y c a n a l l be p r o v i d e d w i t h a single unit. The development e f f o r t r e q u i r e d an i n d e p t h u n d e r s t a n d i n g o f t h e t h e o r y and p r a c t i c a l o p e r a t i n g c o n s t r a i n t s o f a d s o r p t i o n systems as a b a s i s f o r innovative design. The s u c c e s s f u l c o m m e r c i a l i z a t i o n r e s u l t e d from t h e a p p l i c a t i o n o f b a s i c c h e m i c a l engineer i n g c o n c e p t s and i m p l e m e n t a t i o n o f modern c o n t r o l t e c h nology. I n r e c o g n i t i o n o f t h e development o f POLYBED PSA, U n i o n C a r b i d e r e c e i v e d an honors award from t h e


13.

CASSIDY

259


K i r k p a t r i c k Award Committee and C h e m i c a l Magazine i n December 1979 ( 4 ) .

Engineering


Literature Cited (1)

Heck, J . L., and Johansen, T . , "Process Improves Large Scale Hydrogen Production", Hydrocarbon Processing, January 1978, pp. 175-179.

(2)

Corr, F., Dropp, F . , and Rudelstorfer, E . , "PSA Produces Low-Cost High-Purity H ", Hydrocarbon Processing, March 1979, pp. 119-122. 2

(3)

(4)

Heck, J . L., "A Report on the Performance of the First 'POLYBED' PSA Unit Operating in the United States", Oil and Gas Journal, February 11, 1980, pp. 122-130. Chemical Engineering, December 3, 1979, pp. 90-91.

RECEIVED

April 24, 1980.


Polybed Pressure-Swing Adsorption Hydrogen Processing

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