14 Separation of Hydrogen Sulfide-Hydrogen Mixtures by Heatless Adsorption 1
M. D. WHITLEY and C. E. HAMRIN, JR.
Downloaded by SUNY STONY BROOK on October 17, 2014 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch014
Department of Chemical Engineering and Institute for Mining and Minerals Research, University of Kentucky, Lexington, KY 40506
Many processes have been developed f o r the removal o f hydrogen s u l f i d e from gas streams. They can be c l a s s i f i e d as l i q u i d a b s o r p t i o n , l i q u i d o x i d a t i o n , dry o x i d a t i o n , and a d s o r p t i o n . One o f these processes i s u s u a l l y i n c l u d e d i n a c o a l g a s i f i c a t i o n o r l i q u e f a c t i o n flowsheet s i n c e the coal s u l f u r i s converted to H2S and f i n a l l y elemental s u l f u r . The S t r e t f o r d and Townsend d i r e c t HpS to S processes and the R e c t i s o l process f o l l o w e d by a Claus p l a n t are f r e q u e n t l y i n c l u d e d on c o a l c o n v e r s i o n flowsheets (V). Kohl and R i e s e n f e l d (2) present p e r t i n e n t d e t a i l s f o r many comm e r c i a l processes. C y c l i c a d s o r p t i o n processes such as parametric pumping, c y c l i n g zone a d s o r p t i o n and h e a t l e s s a d s o r p t i o n have r e c e i v e d much a t t e n t i o n both t h e o r e t i c a l l y and e x p e r i m e n t a l l y i n the past s e v e r a l y e a r s ( 3 ) . Skarstrom (4) has reviewed a p p l i c a t i o n s o f h e a t l e s s a d s o r p t i o n (synonymous with pressure swing a d s o r p t i o n ) to a i r d r y i n g , hydrogen p u r i f i c a t i o n and a i r f r a c t i o n a t i o n . Stewart and Hack (5) have presented o p e r a t i n g c h a r a c t e r i s t i c s o f pressure swing a d s o r p t i o n systems f o r reducing i m p u r i t i e s i n a hydrogen stream from 40 vol percent to 1 ppm. I m p u r i t i e s i n c l u d ed ammonia, water, methane, carbon monoxide, carbon d i o x i d e , n i t r o g e n , and s e v e r a l hydrocarbons. In t h i s study h e a t l e s s ads o r p t i o n i s used to separate hydrogen s u l f i d e - h y d r o g e n mixtures and the experimental r e s u l t s are compared with t h e o r e t i c a l models. Process D e s c r i p t i o n H e a t l e s s a d s o r p t i o n i s a c y c l i c a l process f o r the p u r i f i c a t i o n o f gaseous mixtures by the s e p a r a t i o n o f the gas i n t o two streams. One stream which c o n t a i n s more o f the more s t r o n g l y adsorbable m a t e r i a l s i s c a l l e d the purge. The o t h e r process stream i s c a l l e d the product stream and c o n t a i n s l e s s o f the moreadsorbable components. Pressure r e d u c t i o n d u r i n g the purging c y c l e coupled with the use o f product purge gas and s h o r t c y c l e s allows the process to operate without the use o f heat f o r bed 1
C u r r e n t address:
S h e l l O i l C o . , New O r l e a n s , LA
70161
0-8412-0582-5/80/47-135-261$05.00/0 © 1980 American Chemical Society
In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
SYNTHETIC ZEOLITES
262
r e g e n e r a t i o n and without an e x c e s s i v e demand f o r purge volume. When operated a t the proper purge gas f r a c t i o n with r e l a t i v e l y s h o r t c y c l e s , the process can reduce the content o f more s t r o n g l y held components i n the product to exceedingly low l e v e l s . A duo-bed h e a t l e s s a d s o r p t i o n c y c l e has been d e s c r i b e d by Shendalman and M i t c h e l l (6_). The c y c l e c o n s i s t s o f f o u r d i s t i n c t steps ( F i g u r e 1 ) .
Downloaded by SUNY STONY BROOK on October 17, 2014 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch014
Step 1: Feed gas mixture flows i n t o column 2 w h i l e product flows out. A p o r t i o n o f t h i s product i s passed through column 1 a t a lower pressure a c t i n g as a purge. Step 2: Column 2 o r i g i n a l l y a t feed pressure i s reduced t o a lower pressure (blowdown) w h i l e column 1 i s i n c r e a s e d t o feed pressure ( r e p r e s s u r i z a t i o n ) . Step 3: Feed flows i n t o column 1 a t high pressure with a p o r t i o n o f the product used t o purge column 2 a t reduced p r e s s u r e . Step 4: Column 1 undergoes blowdown w h i l e column 2 i s repressurized. Diagrams showing the p r e s s u r e s , f e e d , product and purge flow r a t e s f o r each column d u r i n g a c y c l e a r e presented by Weaver and Hamrin ( 7 ) . T h i s system has c e r t a i n c h a r a c t e r i s t i c s which are n o t present i n conventional a d s o r p t i o n processes. A l o s s i n recovery o f m a t e r i a l i s experienced d u r i n g both blowdown and purging. The volume o f gas l o s t during blowdown i s dependent on the pressure d i f f e r e n t i a l ; the higher the pressure d i f f e r e n t i a l the more gas l o s t . The l e n g t h o f the c y c l e time a l s o a f f e c t s the amount l o s t during blowdown. The purge l o s s e s depend only on the chosen purge r a t e . Theory Shendalman and M i t c h e l l (6) have developed an e q u i l i b r i u m model which assumes t h a t the a d s o r p t i o n isotherm i s l i n e a r . The model p r e d i c t s the l i m i t i n g product composition f o r v a r y i n g opera t i n g c o n d i t i o n s as w e l l as the product composition as a f u n c t i o n o f c y c l e number f o r the i n i t i a l t r a n s i e n t p e r i o d . The equation f o r the product c o n c e n t r a t i o n , Y [ J , o f the more s t r o n g l y adsorbed component i s r
P»
F
/ i \ 2nk(l-e)/[e+k(l-e)] P
where y £ i s t h e feed c o n c e n t r a t i o n o f the more s t r o n g l y adsorbed
In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by SUNY STONY BROOK on October 17, 2014 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch014
14.
WHITLEY AND
HAMRiN
Separation of
H S-H* 2
263
component and the s u b s c r i p t s L and H r e f e r t o low and high pres sure r e s p e c t i v e l y , η i s the h a l f c y c l e number, k i s the p a r t i t i o n c o e f f i c i e n t , Ρ i s the pressure i n the column, and ε i s the bed void fraction. The mass balance model o f Weaver and Hamrin (7) was m o d i f i e d to d e s c r i b e the o p e r a t i o n o f t h i s system. A s e r i e s o f mass b a l ances were w r i t t e n around columns 1 and 2 d e s c r i b i n g the v a r y i n g steps i n the process. The balances were w r i t t e n with r e s p e c t to the more s t r o n g l y adsorbed component, hydrogen s u l f i d e . Due to the f a c t t h a t hydrogen does not adsorb on the molecular s i e v e , t h i s makes the e x p r e s s i o n l e s s complicated and l e s s d i f f i c u l t to s o l v e than the o r i g i n a l model. D e t a i l s are presented elsewhere (8).
Experimental Apparatus and Procedure The experimental system used i n t h i s study was modeled a f t e r the h e a t l e s s adsorber used by Shendalman and M i t c h e l l (
