_ ,. ±. „„„„«,, Inventor: Krishnamurthy, Ramachandran, §) Date of publication of application : 30.09.87 A-1 1 Devonsnire Drivej Cranbury NJ 0851 2 (US)
Bulletin 87/40 Inventor: Lerner, Steven L., 41 Holly Glen Lane South, Berkeley Heights NJ 07922 (US)
@ Representative : Wickham, Michael et al, c/o Patent and Trademark Department The BOC Group pic Chertsey
@ Designated Contracting States : BE DE FR GB IT NL Road, Windlesham Surrey GU20 6HJ (GB)
Argon recovery from hydrogen depleted ammonia plant purge gas.
U) CO CM
O) CO CM
§7) A gas stream comprising hydrogen, nitrogen, methane and argon is subjected to the following steps: (i) separation of methane and most of the nitrogen in the
gas stream in a pressure swing adsorption system 2 using molecular sieve or activated carbon material.
(ii) separation of most of the hydrogen in a membrane se- parator 5. The separated hydrogen may be used as purge gas for regeneration of the pressure swing adsorption system 2 of step (i).
(iii) separation of the nitrogen and residual hydrogen in a cryogenic distillation unit 8 to obtain essentially pure liquid argon product. In a Step (ii) above may be eliminated and separation of
hydrogen is then accomplished by cryogenic separation in combination with step (iii). In a further alternative, separation of most of the hydrogen is accomplished by metal hydride adsorption in place of membrane separation. The present process is applicable to the recovery of argon from ammonia synthesis plant purge gas stream, which is pre-heated typically in a membrane to reduce its hydrogen content. The pre-treat- ment may also include an ammonia removal step.
raft UNIT
COOLER
SEPARATOR'"' DISTILLATION UNIT
i i i
AUIUHUM Afci
_ i _ q <u o y o o
Argon Recovery from Hydrogen Depleted Ammonia
Plant Purge Gas
?his i nven t ion r e l a t e s to argon recovery from hydrogen d e p l e t e d
immonia p lan t purge g a s .
Jhe economic p roduc t i on of argon via air s e p a r a t i o n p l an t s is l i n k e d
;o the p roduc t ion of e q u i v a l e n t q u a n t i t i e s of n i t r o g e n or oxygen o r
joth. In recent years , the demand for argon has been growing at a
(lore rapid rate than the c o r r e s p o n d i n g growth rate of e i t h e r
l i t r ogen or oxygen. A l t e r n a t i v e sources for argon has thus become
/ery a t t r a c t i v e . One such a l t e r n a t i v e source is the purge gas from
in ammonia s y n t h e s i s p l a n t .
[n an ammonia s y n t h e s i s p l a n t , i t becomes necessa ry to purge a
f r a c t i o n of the gas stream in order to main ta in the i n e r t
c o n c e n t r a t i o n below a s p e c i f i e d l eve l . Undes i r ab ly high i n e r t
levels reduce the p a r t i a l p r e s su re of the r e a c t a n t s and cause an
unfavorable s h i f t of the ammonia s y n t h e s i s r e a c t i o n e q u i l i b r i u m ,
yiethane and argon are the i n e r t gases of concern. A t y p i c a l
composi t ion of the ammonia purge gas, a v a i l a b l e at a p p r o x i m a t e l y
1900 psig, is as fo l lows: 60.5% H2, 20% N2, 4.5% Ar, 13% CH4
and 2% NH .
Since the ammonia s y n t h e s i s process is energy i n t e n s i v e , economics
have favored recovery of the hydrogen in the purge gas for r e c y c l e
to the ammonia s y n t h e s i s loop. C u r r e n t l y , three kinds of p r o c e s s e s
are employed for th is purpose; in order of most to l e a s t p r e v a l e n t ,
these are membrane s e p a r a t i o n , c ryogenic s e p a r a t i o n and p r e s s u r e
swing a d s o r p t i o n s e p a r a t i o n . In f ac t , the use of a membrane
s e p a r a t o r has become very popular for hydrogen recovery and r e c y c l e ,
and a number of ammonia p l an t s in the United S ta tes and abroad have
hydrogen membrane s e p a r a t o r un i t s i n s t a l l e d .
MW/NJP/8627
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P resen t t echno logy for argon recovery from ammonia s y n t h e s i s p l a n t
purge gas does not o p t i m a l l y i n t e g r a t e th i s hydrogen membrane
s e p a r a t o r , but r a t h e r employs a c ryogenic process tha t c o n s i s t s of a
p r e - t r e a t m e n t s e c t i o n for ammonia removal and three s u b s e q u e n t
s e p a r a t o r y columns. In such a c o n v e n t i o n a l des ign, the f i r s t two
columns are for s t r i p p i n g hydrogen and n i t r o g e n in the feed gas and
the f i n a l column is for s e p a r a t i n g argon and methane to ob ta in p u r e
l i q u i d argon product and also pure methane for use as f u e l .
The pr imary o b j e c t of the i n v e n t i o n is to provide an improved
process s u i t a b l e for the recovery of argon from h y d r o g e n - d e p l e t e d
ammonia p l a n t purge gas. The process employs an a d v a n t a g e o u s
combinat ion «of non -c ryogen ic and c ryogenic s e p a r a t o r y s teps which
makes p o s s i b l e post membrane recovery of argon from the n o n - p e r m e a t e
gas s tream. The p r e s e n t i n v e n t i o n employs a PSA system t o
accompl ish removal of methane and most of the n i t r o g e n from t h e
purge g a s .
In the fo l lowing d e s c r i p t i o n of the i n v e n t i o n , the term " p r e s s u r e
swing a d s o r p t i o n " or i t s acronym "PSA" is used in r e f e r e n c e to a
type of p rocess and appa ra tus tha t is now well known and widely u s e d
with r e s p e c t to s e p a r a t i n g the components of a gaseous mixture . A
PSA system b a s i c a l l y comprises pass ing a feed gas mixture t h r o u g h
one or more a d s o r p t i o n beds c o n t a i n i n g a s ieve m a t e r i a l which has a
g r e a t e r s e l e c t i v i t y for a more s t r o n g l y adsorbed component than a
more weakly adsorbed component of the gas mix ture . In the o p e r a t i o n
of a t y p i c a l 2-bed PSA system, the connec t ing condu i t s , v a l v e s ,
t imer s , and the l ike are c o o r d i n a t e d and a r ranged so tha t when
a d s o r p t i o n is o c c u r r i n g in a f i r s t bed, r e g e n e r a t i o n is o ccu r r i ng i n
a second bed. In the usual cyc le , s e q u e n t i a l s teps with r e spec t t o
each bed inc lude p r e s s u r i z a t i o n , p roduct r e l e a s e and ven t ing . B a s i c
PSA systems are d e s c r i b e d in U.S. Pa ten t 2 ,944 ,627 , U.S. Pa ten t No.
3 ,801 ,513 , and U.S. Pa ten t No. 3 , 9 6 0 , 5 2 2 .
MW/NJP/8627
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ar ious m o d i f i c a t i o n s and improvements to the Dasic rt>A process ana
ipparatus have been de sc r ibed in the l i t e r a t u r e , for example, i n
\.S. Pa tent No. 4 ,415,340, i ssued on November 15, 1983 and U.S.
' a tent No. 4 ,340,398 i ssued on July 20, 1982.
:he p r e s e n t i nven t ion is not l i m i t e d to the use of any p a r t i c u l a r
>SA process or appara tus des ign. A s p e c i f i c design tha t leads t o
ligh argon y ie ld is, however, d e t a i l e d below as an example .
k. new and improved process has been developed for r e c o v e r i n g a r g o n
from the purge gas stream o r i g i n a t i n g from an ammonia s y n t h e s i s
>lant. The process is set out in claim 1 be low.
Ammonia s y n t h e s i s p lan t purge gas comprises hydrogen, n i t r o g e n ,
irgon, methane, and ammonia. This purge gas is c o n v e n t i o n a l l y
subjec ted to ammonia a b s o r p t i o n and a f i r s t membrane s e p a r a t i o n o f
lydrogen for recyc le to the ammonia p l a n t . The hydrogen d e p l e t e d
non-permeate gas stream from a f i r s t membrane s e p a r a t o r , c o m p r i s i n g
the a f o r e s a i d four components, and any r e s i d u a l mois ture from t h e
ammonia a b s o r p t i o n , is s u b j e c t e d , accord ing to a f i r s t embodiment o f
the p r e sen t i n v e n t i o n , to the fo l lowing s t e p s :
s e p a r a t i o n of methane and r e s i d u a l mois tu re ana mos>t
Df the n i t r o g e n in the gas stream in a p r e s s u r e swing
adso rp t i on system using molecular s ieve or a c t i v a t e d
carbon m a t e r i a l .
( i i ) s e p a r a t i o n of most of the hydrogen in a s e c o n a
membrane s e p a r a t o r . The s e p a r a t e d hydrogen may be
used as purge gas for r e g e n e r a t i o n of the p r e s s u r e
swing a d s o r p t i o n systems of step ( i ) .
MW/NJP/8627
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( i i i ) s e p a r a t i o n of the n i t r o g e n and r e s i d u a l hydrogen by
cryogenic d i s t i l l a t i o n to ob ta in e s s e n t i a l l y p u r e
l i q u i d argon p r o d u c t .
A second embodiment of the p r e s e n t i n v e n t i o n comprises the f o l l o w i n g
s teps :
(i) the post membrane purge gas is passed to a PSA sys t em
where a l l of the methane and r e s i d u a l mois ture in t h e
feed gas and most of the n i t r o g e n are removed.
( i i ) the product gas from the PSA system is passed to a
c ryogenic d i s t i l l a t i o n uni t a f t e r p r e - c o o l i n g w i t h
cold waste gas from the d i s t i l l a t i o n un i t and o p t i o n a l
expans ion to a d e s i r e d lower p r e s s u r e . In a s i n g l e
column, pure l i q u i d argon is produced as a bo t tom
produc t , whereas the d i s t i l l a t e or top product is a
hydrogen n i t r o g e n mix ture . The d i s t i l l a t e may be u s e d
as a purge gas for PSA r e g e n e r a t i o n a f t e r h e a t
exchange with the PSA p r o d u c t .
In a t h i r d embodiment, hydrogen in the non-permeate stream i s
s e p a r a t e d by using metal hydr ides employing a p r e s s u r e swing c y c l e .
The hydr ides used for hydrogen s e p a r a t i o n and the molecu la r s i e v e
used for methane and n i t r o g e n s e p a r a t i o n can be combined in a s i n g l e
PSA p r o c e s s .
I t w i l l be a p p r e c i a t e d by those s k i l l e d in the a r t t ha t a l though t h e
p rocess d e s c r i b e d by each of the three embodiments above is w i t h
r e f e r e n c e to a post membrane purge gas, a PSA or c ryogenic uni t may
be used i n s t e a d of the f i r s t membrane s e p a r a t o r to recover hyd rogen
for r e c y c l e to the ammonia s y n t h e s i s p l an t . In th i s case, two minor
p rocess m o d i f i c a t i o n s should be made. F i r s t , the feed gas i s
MW/NJP/8627
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Dmpressed to a de s i r ed PSA o p e r a t i n g p r e s su re ana, se touu, x ,
ranonia is p resen t in the feed, the PSA process for methane
e p a r a t i o n is s u i t a b l y modif ied to s i m u l t a n e o u s l y remove ammonia.
he p resen t i nven t ion has s eve ra l impor tan t advantages over t h e
r io r a r t ' s three stage c ryogenic recovery of argon. A c o n s i d e r a b l e
educ t ion in c a p i t a l cost and o p e r a t i n g expense is achieved t h r o u g h
he use of a gas phase methane s e p a r a t i o n . In f ac t , the h i g h
r e s su re of the purge gas e x i t i n g from the ammonia s y n t h e s i s p l a n t
an be used to provide most or al l of the energy r equ i r emen t s in t h e
.on-cryogenic s e p a r a t i o n . Fur thermore , i t is p o s s i b l e , as a f u r t h e r
nergy saving measure, to pass the high p r e s s u r e purge gas s t r e a m
:hrough a t u rb ine in order to provide cool ing needed for t h e
myogenic s e p a r a t i o n . In a d d i t i o n , the compact un i t s employed i n
:he p re sen t process are more p o r t a b l e and, as a r e s u l t , the p u r g e
fas a v a i l a b l e at numerous ammonia p lan t s i t e s over a w i d e
reograph ica l range can be more e x p e d i t i o u s l y tapped to meet t h e
jrowing demand for argon. F i n a l l y , the PSA uni t for s e p a r a t i n g
lethane in the p r e sen t i n v e n t i o n also performs the f u n c t i o n o f
removing t race l eve l s of water and ammonia, whereas p r i o r a r t
myogenic methods for argon recovery t y p i c a l l y r e q u i r e a s e p a r a t e
adsorp t ion uni t to perform th i s f u n c t i o n . .
rhe process accord ing to the i n v e n t i o n wi l l now be d e s c r i b e d by way
Df examples with r e f e r e n c e to the accompanying d r a w i n g s .
rhe i nven t ion wi l l be more c l e a r l y unders tood by r e f e r e n c e to t h e
fo l lowing d e s c r i p t i o n of exemplary embodiments t h e r e o f i n
con junc t ion with the fo l lowing drawings in which :
FIG. 1 is a box diagram i l l u s t r a t i n g a f i r s t embodiment of t h e
p resen t i n v e n t i o n in which argon is recovered from a m u l t i c o m p o n e n t
gas s t r e a m .
MW/NJP/8627
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i*IG. 2 is a schemat ic p rocess flow diagram i l l u s t r a t i n g a n o t h e r
imbodiment of the p r e s e n t i n v e n t i o n for t r e a t i n g hydrogen PSA w a s t e
jas .
?IG. 3 is a schemat ic of one p o s s i b l e c o n f i g u r a t i o n of the PSA u n i t .
I l l u s t r a t i n g valve p o s i t i o n s and a u x i l i a r y e q u i p m e n t .
?IG. 4 is a t iming diagram i l l u s t r a t i n g a f u l l cycle sequence of PSA
a p e r a t i o n c o r r e s p o n d i n g to the c o n f i g u r a t i o n shown in FIG. 3.
An ammonia absorber and hydrogen membrane s e p a r a t o r c o n v e n t i o n a l l y
t r e a t s the purge gas e x i t i n g from a ammonia s y n t h e s i s p l a n t . As
e x e m p l i f i e d by Monsanto 's PRISM membrane s e p a r a t o r s y s t e m ,
hydrogen membrane s e p a r a t o r s are well known in the ar t for t r e a t i n g
ammonia s y n t h e s i s p l an t purge gas in order to ob ta in r e c y c l a b l e
hydrogen. R e f e r r i n g to FIG. 1, i l l u s t r a t i n g a f i r s t embodiment o f
the p r e s e n t i n v e n t i o n , the non-permeate gas stream 1, from a
hydrogen membrane s e p a r a t o r comprises argon, hydrogen, methane and
n i t r o g e n .
The non-permeate gas t y p i c a l l y has a compos i t ion , by volume, in t h e
fo l lowing range: 7-20% hydrogen, 6-12% argon, 45-54% n i t r o g e n and ,
25-33% methane. It wi l l be a p p r e c i a t e d by those s k i l l e d in the a r t
t ha t the p r e s e n t i n v e n t i o n app l i e s as well to feed streams tha t l i e
ou t s ide th i s compos i t ion r a n g e .
According to the p r e s e n t i n v e n t i o n , the non-permeate feed gas s t r e a m
1 is passed to a PSA un i t 2, which s e p a r a t e s out a stream 3 ,
compr i s ing most of the n i t r o g e n and e s s e n t i a l l y a l l of the me thane .
This methane c o n t a i n i n g stream 3 may be used as fuel for the p r i m a r y
reformer of the ammonia s y n t h e s i s p l a n t . The PSA product gas s t r e a m
4 is then passed through a membrane s e p a r a t o r 5 to remove a ma jo r
p o r t i o n of the hydrogen. The s e p a r a t e d hydrogen in stream 6 can be
MW/NJP/8627
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sed as purge gas for r e g e n e r a t i o n or tne ft>A unic c, at. i u e i , u i
p t i o n a l l y recyc led to the ammonia s y n t h e s i s loop. The product ga s
tream 7a, compris ing argon and remaining q u a n t i t i e s of hydrogen and
d t rogen , is p r e f e r a b l y p r e - c o o l e d with cold waste gas stream 9 and
i p t i o n a l l y expanded in a t u rb ine for f u r t h e r coo l ing . The c o o l e d
;tream 7b, is then t r e a t e d in a c ryogenic d i s t i l l a t i o n uni t 8 t o
iroduce e s s e n t i a l l y pure l i q u i d argon, which ex i t s as product s t r e a m
.0. The remaining q u a n t i t i e s of hydrogen and n i t r o g e n ex i t from t h e
:ryogenic d i s t i l l a t i o n uni t 8 as d i s t i l l a t e stream 9.
?he d i s t i l l a t e stream 9 can be used for p r e - c o o l i n g the feed ga s
stream 1 and subsequen t ly can be used as purge gas for r e g e n e r a t i o n
>f the PSA uni t 2. The re f lux for the d i s t i l l a t i o n uni t 8 i s
su i tab ly provided by l i q u i d n i t r o g e n in a r e c i r c u l a t i n g
r e f r i g e r a t i o n loop with the column r e b o i l e r ac t ing as the heat s i n k
:or a heat pump. O p t i o n a l l y , l i q u i d n i t r o g e n can be s to red in t a n k s
ind c i r c u l a t e d in metered amounts to provide the r e f l u x . The
l i t r o g e n vapor can be sent to the ammonia p l an t air compressor . The
small amount of cool ing provided by n i t r o g e n vapor wil l r e s u l t in a
narginal i n c r e a s e in air in take and favor improved c o m p r e s s o r
o p e r a t i o n .
In a second embodiment, the product stream from the PSA uni t may be
d i r e c t l y t r e a t e d in the c ryogenic d i s t i l l a t i o n un i t . In a s i n g l e
column, pure argon may be produced as a bottom product and a
hydrogen plus n i t r o g e n mixture may be produced as a d i s t i l l a t e
stream. As mentioned before , th is d i s t i l l a t e stream can be used t o
cool the feed to the column and can be s u b s e q u e n t l y used as p u r g e
gas for PSA methane r e g e n e r a t i o n .
Re fe r r ing now to FIG. 2, a t h i r d embodiment of the p r e s e n t i n v e n t i o n
comprises , in place of a hydrogen membrane s e p a r a t o r fo l lowing t h e
MW/NJP/8627
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PSA u n i t , the i n c o r p o r a t i o n of metal hydr ide m a t e r i a l into t h e
molecular s ieve bed of the PSA un i t , in order to accomplish t h e
removal of hydrogen s i m u l t a n e o u s l y with the removal of methane and
n i t r o g e n .
In th i s embodiment, an ammonia s y n t h e s i s p l an t purge gas stream 11
e n t e r s a hydrogen PSA recovery un i t 12, r a t he r than a h y d r o g e n
membrane s e p a r a t o r . The stream 13 comprises a h igh ly c o n c e n t r a t e d
hydrogen gas for r ecyc le to the ammonia s y n t h e s i s loop. The
hydrogen d e p l e t e d stream 14, compr is ing n i t r o g e n , argon, me thane ,
hydrogen, and ammonia, en t e r s via a compressor 15 a PSA uni t 16,
where i t is t y p i c a l l y s tepped up to a p r e s s u r e in the range of 50 t o
100 ps ia . The a d s o r p t i o n beds of the PSA uni t comprise b o t h
molecu la r s ieve and metal hydr ide m a t e r i a l . The molecular s i e v e
adsorbs e s s e n t i a l l y a l l of the methane, most of the n i t r o g e n , and
a l l of the ammonia. The metal hydr ides adsorb most of the r e m a i n i n g r
hydrogen. The vent gas stream 17 thus comprises a mixture o f
ammonia, methane, n i t r o g e n , and hydrogen. The argon e n r i c h e d
produc t stream 18a from the PSA uni t 16 is then cooled and expanded
in expans ion means 19, such as an expansion valve or t u r b i n e , and i s
i n t r o d u c e d into a c ryogenic d i s t i l l a t i o n column 20, which produces a
d i s t i l l a t e stream 21 c o n s i s t i n g p r i m a r i l y of n i t r o g e n , with t r a c e s
of hydrogen, and a pure l i q u i d argon f i na l p roduct stream 22. The
d i s t i l l a t e stream 21 can be used to purge the PSA uni t 16, i n
a d d i t i o n to the purge i n h e r e n t l y due to the hydrogen desorb ing from
the metal h y d r i d e s .
As the metal hydr ide m a t e r i a l , a commercia l ly a v a i l a b l e p r o d u c t ,
Hystor a l l oy 207 (La Ni4 ? A1Q 3> from Ergenics in Wycoff, New
J e r s e y , may be employed. A number of o ther commercia l ly a v a i l a b l e
hydr ide a l loys may also be used. Within the PSA un i t , the m e t a l
hydr ide m a t e r i a l may be a r ranged in a two l aye red bed, w i t h
molecu la r s ieve adso rben t m a t e r i a l . P lac ing the metal h y d r i d e
MW/NJP/8627
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n a t e r i a l on top of the molecular sieve adsorben t m a t e r i a l i s
advantageous for two reasons . F i r s t , the c a p a c i t y of the m e t a l
nydride m a t e r i a l is b e t t e r u t i l i z e d , s ince the hydrogen p a r t i a l
pressure during a d s o r p t i o n is h ighes t at the top of the bed and,
secondly, because desorbed hydrogen would act as purge gas f o r
Resorpt ion of methane from the molecular sieve adso rben t m a t e r i a l .
rhe metal hydride m a t e r i a l is s u i t a b l y crushed to a f ine powder f o r
use in the beds of the PSA un i t . The size of the crushed hydr ide i s
t y p i c a l l y about 10 microns. The amount of metal hydr ide can be
var ied, as wi l l be a p p r e c i a t e d by those s k i l l e d in the a r t , in o r d e r
to ensure tha t hydrogen in the product is below the a l lowable l i m i t
for the l a t e r cryogenic s e p a r a t i o n at the o p e r a t i n g p r e s s u r e
necessary to produce pure l i q u i d a r g o n .
The hydr ides may be placed in a s epa ra t e bed on top of each z e o l i t e
bed to provide ease of r ep lacement . A s i n t e r e d metal disc f i l t e r i s
s u i t a b l y employed at the top of the beds and may have a pore size o f
5 microns or smal ler to p reven t p a r t i c u l a t e carry over. However,
the p a r t i c u l a t e carry over problem may also be add res sed in a
d i f f e r e n t manner. The beds may be f i l l e d with th ree l a y e r s : f i r s t ,
a layer of pure z e o l i t e , second, a layer of a hydr ide plus z e o l i t e
mixture and, f i n a l l y , a layer of pure z e o l i t e molecular s ieve a g a i n .
In every case, the molecular s ieve or a c t i v a t e d carbon m a t e r i a l
con ta ined in the PSA uni t must have a g r e a t e r s e l e c t i v i t y f o r
methane than argon. Both calcium and sodium a l u m i n o s i l i c a t e
z e o l i t e s may be employed. Carbon molecular s ieves and s i l i c a
molecular s ieves are also f u n c t i o n a l . S u i t a b l e z e o l i t e s i n c l u d e ,
but are not l im i t ed to the fo l lowing : 5A, 10X, 13X, and
morden i t e s . P r e f e r r e d s ieves are the 5A medical grade z e o l i t e ,
commercia l ly a v a i l a b l e from Union Carbide, the 5A HC s i e v e
MW/NJP/8627
- 10 - 3 2 3 9 2 3 5
commercial ly a v a i l a b l e from Laporte I n d u s t r i e s , or molecular s i e v e s
tfith comparable pore s ize and molecu la r a t t r a c t i o n . The 5A m e d i c a l
jrade z e o l i t e p rov ides e x c e l l e n t a rgon/methane s e l e c t i v i t y and
e x h i b i t s the a b i l i t y to remove v i r t u a l l y a l l of the methane, so t h a t
the PSA produc t gas can con ta in as low as f r a c t i o n a l par t p e r
mi l l i on (ppm) l eve l s of methane. Removal of the methane to low
l eve l s is an impor tan t c r i t e r i o n ; any methane in the product gas
c o n c e n t r a t e s in the pure argon product in the c ryogenic d i s t i l l a t i o n
uni t . Hence, expens ive downstream p u r i f i c a t i o n s teps would be
r e q u i r e d if the PSA produc t gas were to con ta in u n d e s i r a b l e l e v e l s
of methane. A methane l eve l equal to or less than 20 ppm i s
t y p i c a l l y r e q u i r e d in the p roduc t , a methane level of 1 ppm i s
d e s i r e d , and a methane level of 0.5 ppm or below is p r e f e r r e d .
A s u i t a b l e o p e r a t i n g p r e s s u r e for the PSA un i t is in the range of 25
psig to 1000 ps ig . A range of 100 to 400 ps ig is p r e f e r r e d . By
vary ing the p roduc t to feed ratio1, e i t h e r by changing the p r o d u c t
flow or cycle time, the argon y i e ld at va r ious o p e r a t i n g p r e s s u r e s
c o r r e s p o n d i n g to zero methane c o n c e n t r a t i o n in the product can be
de te rmined by thermal c o n d u c t i v i t y a n a l y s i s of the PSA p r o d u c t
stream using a gas chromatograph. There is a moderate r e d u c t i o n i n
argon y i e l d with i n c r e a s i n g p r e s s u r e . The v a r i a t i o n of argon y i e l d
with p r e s s u r e i n d i c a t e s tha t the s e p a r a t i o n of methane i s
r e g e n e r a t i o n c o n t r o l l e d ; the h igher the amount of methane removed
dur ing PSA bed r e g e n e r a t i o n , the b e t t e r wi l l be the a r g o n / m e t h a n e
s e l e c t i v i t y .
The PSA uni t must be r e g e n e r a t e d p e r i o d i c a l l y . S u i t a b l e modes o f
r e g e n e r a t i o n inc lude (i) r e g e n e r a t i o n at a tmospher ic p r e s s u r e
coupled with product purge, ( i i ) r e g e n e r a t i o n at or below 25 p s i a
coupled with purge using hydrogen or a h y d r o g e n - n i t r o g e n mix ture . a t
low p r e s s u r e s (about 15 p s i g ) , and ( i i i ) vacuum r e g e n e r a t i o n .
MW/NJP/8627
- 11 - 0 2 3 9 2 3 5
When using product purge, it may be advantageous to r e s t r i c t t h e
purge to d i f f e r e n t p o r t i o n s of the half cycle . T y p i c a l l y , t h e
product r e l e a s e d immediately a f t e r p r e s s u r i z a t i o n of the bed
conta ins mostly hydrogen. The re fo re , i t is advantageous to r e s t r i c t
purge to two time pe r iods ; the f i r s t per iod is immediate ly f o l l o w i n g
p r e s s u r i z a t i o n of the adsorb ing bed, and the second per iod i s
•towards the end of the hal f cycle when product p u r i t y is i n
dec l i ne . By s u i t a b l e choice of time for the two purge s t eps , t h e
maximum argon y ie ld using th is mode of r e g e n e r a t i o n can be
de termined t oge the r with the minimum product hyd rogen
c o n c e n t r a t i o n . Advantages of product purge are tha t r e l a t i v e l y
lower energy is r equ i red for r e g e n e r a t i o n , and the hyd rogen
c o n c e n t r a t i o n in the product is c o m p a r a t i v e l y low.
A d i s advan t age of product purge is r e l a t i v e l y low argon y ie ld as a
r e s u l t of the loss of the product purge gas i t s e l f . For s e p a r a t i n g
a l l of the methane in the feed gas, the purge gas r e q u i r e m e n t
t y p i c a l l y accounts for g r e a t e r than 30 pe rcen t of the argon l o s t .
As an a l t e r n a t i v e to product purge, the hydrogen r ich stream 6 shown
in FIG. 1 is idea l for use as purge. In the f i r s t embodiment of t h e
p re sen t i n v e n t i o n , the hydrogen, r ich stream 6, s e p a r a t e d by t h e
membrane 5, wil l be at low p r e s su re (about 15 psig) and i s
consequen t ly not of much value for r e c y c l i n g to the ammonia
s y n t h e s i s loop. It can t h e r e f o r e be p r o f i t a b l y used as purge g a s .
An a l t e r n a t e mode of r e g e n e r a t i o n in the p r e s e n t i n v e n t i o n is vacuum
r e g e n e r a t i o n . The y i e ld ob ta ined using vacuum r e g e n e r a t i o n i s
g e n e r a l l y s u p e r i o r to the y i e ld using hydrogen purge or p r o d u c t
purge. Vacuum r e g e n e r a t i o n , however, i n c r e a s e s the c a p i t a l
inves tment for the process s l i g h t l y and the energy r e q u i r e m e n t
a p p r e c i a b l y . Since the vent stream is used as a fue l , r e c o m p r e s s i o n
to about 25 psia is also neces sa ry unless s p e c i a l lower p r e s s u r e
MW/NJP/8627
Li -
turners are used. In d e t e r m i n i n g the bes t r e g e n e r a t i o n p r o c e d u r e ,
:he i n c r e a s e in argon y i e l d t ha t r e s u l t s with vacuum r e g e n e r a t i o n
mst be weighed a g a i n s t the i nc r emen ta l c a p i t a l cost and e n e r g y
:hanges .
Che va r ious embodiments d e s c r i b e d above for argon recovery from t h e
nembrane non-permeate gas in NH3 p l an t s are also e q u a l l y
app l i cab l e to argon recovery in NH p l a n t s which have i n s t a l l e d on
r.hem PSA or c ryogenic E. recovery u n i t s . The argon r ich s t r e a m s
from these un i t s are at low p r e s s u r e as opposed to the n o n - p e r m e a t e
stream which is t y p i c a l l y at 1900 ps ig . In c o n t r a s t , a f t e r h y d r o g e n
PSA, the argon r ich stream is t y p i c a l l y at 8 ps ia to 25 ps ia and ,
a f t e r c ryogen ic hydrogen recovery , the argon r ich stream i s
t y p i c a l l y at 25 ps ia to 100 ps ia . The feed stream from H2~PSA
uni ts wi l l r equ i r e r ecompress ion to 50-100 ps ig , whereas t h e
c ryogenic p l an t feed may only o p t i o n a l l y r equ i r e r e c o m p r e s s i o n .
Fol lowing r ecompress ion , the embodiments d i s c u s s e d above f o r
post-membrane t r e a t m e n t are a p p l i c a b l e . Any ammonia in the feed i s
s e p a r a t e d along with methane in the z e o l i t e PSA.
The fo l lowing working examples i l l u s t r a t e a des ign for each of t h e
th ree embodiments, based on exper iments or, where a p p r o p r i a t e ,
t h e o r e t i c a l c a l c u l a t i o n s assuming well mixed s t r e a m s .
EXAMPLE 1
This example i l l u s t r a t e s a des ign based on the second embodiment o f
the p r e s e n t i n v e n t i o n . An ammonia s y n t h e s i s p l an t purge s t r e a m
compr i s ing app rox ima te ly 60.5% 20% N2, 4.5% Ar, 13% CK^,
and 2% NH e n t e r s a p r e - t r e a t m e n t s e c t i o n c o n s i s t i n g of a w a t e r
MW/NJP/8627
0 2 3 9 2 3 5
scrubber to remove ammonia. P r e - t r e a t e d gas then en te r s a PRISM
membrane system, producing two s t reams: a f i r s t hydrogen r i c h
permeate stream for r ecyc l e , compr is ing 85.4% H^, 5.3% N2, 8%
CH and 1.3% Ar and a second argon en r i ched non-permeate s t r e a m
compris ing 9% Yi , 54% N2 12% Ar and 25% CH4> At a p r e s su re o f
150 psig, the non-permeate stream is fed to a PSA un i t , at a m b i e n t
t empera tu re , compris ing beds c o n t a i n i n g 5A medical g r a d e
a l u m i n o s i l i c a t e z e o l i t e . A t y p i c a l PSA c o n f i g u r a t i o n d e p i c t i n g t h e
var ious valves is shown in FIG. 3. R e f e r r i n g to FIG. 3, the PSA
unit comprises a d s o r p t i o n bed A, a d s o r p t i o n bed B, e q u a l i z a t i o n t a n k
C, b a c k f i l l tank D, product r e s e r v o i r E, b a c k p r e s s u r e r e g u l a t o r 16
and valves 1 through 15. The PSA uni t is ope ra ted in a c c o r d a n c e
with the fu l l cycle sequence shown in Table 1. FIG. 4 shows a
t iming diagram for the fu l l cycle s e q u e n c e .
MW/NJP/8627
- 14 -
TABLE I
0 2 3 9 2 3 !
Step No Bed A Bed B Valves Open
1 Bed ba lance Bed ba lance 3, 4, 9,
10, 13
2 Feed P r e s s u r i z a t i o n E q u a l i z a t i o n with tank 1, 8, 13
3 Feed P r e s s u r i z a t i o n Vent to atmosphere 1, 6, 14,
13
4 Cons tan t feed & Vacuum r e g e n e r a t i o n 1, 6, 15,
p roduc t r e l e a s e 13
5 Cons tan t feed & E q u a l i z a t i o n with tank 1, 8, 13
produc t r e l e a s e
6 Cons tan t feed & Product b a c k f i l l 1, 12
p roduc t r e l e a s e
7 Bed ba lance Bed ba lance 3, 4, 9,
10, 13
8 E q u a l i z a t i o n with Feed p r e s s u r i z a t i o n 2, 7, 13
t a n k
9 Vent to a tmosphere Feed p r e s s u r i z a t i o n 2, 5, 14,
13
10 Vacuum r e g e n e r a t i o n Constant feed and 2, 5, 15,
product r e l e a s e 13
11 E q u a l i z a t i o n with Constant feed and 2, 7, 13
tank product r e l e a s e
12 Product b a c k f i l l Constant feed and 2, 11
product r e l e a s e
E s s e n t i a l l y a l l of the CH and g r e a t e r than 80% of t h e
N are removed in the vent stream of th i s PSA uni t by 2 using 1.5 ps ia (100 Torr) abso lu t e vacuum. The PSA
produc t gas is c r y o g e n i c a l l y d i s t i l l e d to produce p u r e
l i q u i d argon as a column bottom p roduc t . The t e m p e r a t u r e
and p r e s s u r e c o n d i t i o n s , flow r a t e s and composi t ion o f
va r ious s t reams are summarized in Table I I .
MW/NJP/8627
- lb -
'ABLE I I
Flow (Volume
irrpsn, T«mp. Pressure Rate H2 Composit ion P e r c e n t )
in (K) (ps ia) (Uni t s / Ar N2 CH4
FIG.l) M i n . )
1 298 150 100.0 9.0 12.0 54.0 25 .0
Min 25
3 293 Min 1.5 74.3 0.6 4.0 61.7 33 .7
Max 25
4 303 150 25.7 33.3 35.1 31 .6
7b 116 45 25.7 33.3 35.1 31 .6
9 83 42 16.9 50.5 1.6 4 7 . 9
10 98 40 8.8 100 .0
EXAMPLE I
rhis example i l l u s t r a t e s a design based on the f i r s t
embodiment of the p re sen t i n v e n t i o n . The n o n - p e r m e a t e
Eeed stream is t r e a t e d in a 2-bed PSA at 400 psig. The
PSA c o n f i g u r a t i o n is s i m i l a r to the one shown for Example
L. Purge gas is used for PSA r e g e n e r a t i o n and hence
a p p l i c a t i o n of vacuum is e i t h e r re laxed or e l i m i n a t e d . I n
this p a r t i c u l a r example, the vent p r e s su re i s
a tmospher ic . The PSA product gas is t r e a t e d in a membrane
to give two p roduc t s : the hydrogen r ich permeate at 30
psia is used as purge gas for PSA r e g e n e r a t i o n , while t h e
argon r ich non-permeate is c r y o g e n i c a l l y d i s t i l l e d as i n
Example 1 to produce pure l i q u i d argon as a bo t t om
product . Temperature , p r e s s u r e , flow rate and c o m p o s i t i o n
of var ious streams are summarized in Table I I I .
MW/NJP/8627
0 2 3 9 2 3 1
CABLE I I I
Flow (Volume
Stream Temp. P res su re Rate Composit ion P e r c e n t )
( in (K) (ps ia) (Uni t s / Ar N2 CH4
FIG.l) Min . )
1 298 400 100.0 9.0 12.0 54.0 25 .0
Min 200
3 293 Min 1.5 81.9 9.1 4.3 56.1 3 0 . 5
Max 25
4 303 400 27.5 36.0 33.1 30 .9
6 303 Min 20 9.4 88.4 7.5 4 . 1
Max 35
7a 303 400 18.1 8.8 46.5 4 4 . 7
7b 116 40 18.1 8.8 46.5 4 4 . 7
9 83 35 10.0 16.0 2.5 81 .5 4 4
10. 96 33 8.1 100.0 1x10- 1x10-
EXAMPLE 3
The example i l l u s t r a t e s a design based on the t h i r d
embodiment of the p r e s e n t i n v e n t i o n . The n o n - p e r m e a t e
feed stream at 400 psig p r e s s u r e is t r e a t e d in a 2 -bed PSA
system. The beds con t a in metal hydride (Hystor a l l o y
207) and z e o l i t e in a two layered arrangement and in t h e
approximate weight r a t i o 1:6. The metal hydr ide i s
l oca t ed at the product end of the bed. All of the CH^,
70% of the H2 and g r e a t e r than 80% of the N2 a r e
removed in the vent stream with r e g e n e r a t i o n at vacuum
(1.5 p s i a ) . The argon r ich p roduc t gas from the PSA i s
c r y o g e n i c a l l y d i s t i l l e d to produce pure l i q u i d argon a s
bottom p roduc t . Tempera ture , p r e s s u r e , flow ra te and
compos i t ion of va r ious streams are summarized in Table IV.
MW/NJP/8627
- J. / -
VBLE IV
LOW
in (K) (ps ia) (Uni t s / Ar N2 CH4
E-IG.l) Min . )
1 298 400 100.0 9.0 12.0 54.0 2 5 . 0
3 293 Min 1.5 78.1 8.1 4.6 55.3 3 2 . 0
Max 2 5
4 303 400 21.9 12.3 38.4 49 .3
7o H6 40 21.9 12.3 38.4 49 .3
g 83 35 13.8 19.6 1.80 78 .6
10 98 33 8.1 100
fodif i c a t i o n s to the descr iDea emooainiein_s> , wj.i-n.ni
cope and s p i r i t of the p r e sen t i n v e n t i o n , wi l l be e v i d e n t
:o those s k i l l e d in the a r t . For example, vacuum
e g e n e r a t i o n could be r ep laced with purge gas
•egenera t ion . The purge gas may be heated before p a s s a g e
:hrough the PSA for r e g e n e r a t i o n . In a d d i t i o n , feed gas
nay be cooled to give a f avo rab le t empera tu re swing .
MW/NJP/8627
CLAIMS
A process for the recovery of argon from a gas stream c o m p r i s i n g
hydrogen, n i t r o g e n , methane, and argon compris ing the f o l l o w i n g
s t e p s :
(i) pass ing said gas stream through a p r e s s u r e swing
a d s o r p t i o n means i n c l u d i n g at l e a s t one adsorben t bed
for s e p a r a t i n g e s s e n t i a l l y a l l of the methane and
most of the n i t r o g e n , thereby producing a p r o d u c t
stream compris ing an enhanced or p r e d o m i n a n t
p r o p o r t i o n of argon; and
( i i ) pas s ing said product stream to a means for s e p a r a t i n g
out hydrogen; and
( i i i ) pass ing the hydrogen dep le t ed product stream to a
c ryogenic d i s t i l l a t i o n means for producing an
e s s e n t i a l l y pure argon f i na l p r o d u c t .
2. A process for the recovery of argon from a gas stream c o m p r i s i n g
hydrogen, n i t r o g e n , methane and argon compris ing pass ing s a i d
gas stream through a p r e s s u r e swing a d s o r p t i o n means i n c l u d i n g
at l e a s t one adso rben t bed for s e p a r a t i n g e s s e n t i a l l y a l l of t h e
methane and most of the n i t r o g e n thereby producing a p r o d u c t
stream compr is ing an enhanced or predominant p r o p o r t i o n o f
argon, and pass ing said product stream to a c r y o g e n i c
d i s t i l l a t i o n column for s e p a r a t i n g out hydrogen and n i t r o g e n and
producing an e s s e n t i a l l y pure argon p r o d u c t .
3. A process as claimed in claim 1, wherein said means f o r
s e p a r a t i n g out hydrogen comprises a membrane s e p a r a t o r or one o r
more beds compr is ing metal h y d r i d e s .
MW/NJP/8627
±y -
. A process as claimed in claim 3, wherein tne metai n y a r i a e s <ne
i n c o r p o r a t e d into said at l e a s t one a d s o r p t i o n bed .
. A process as claimed in claim 1, 3 or 4, in which p e r i o d i c a l l y
said a d s o r p t i o n bed is r e g e n e r a t e d with a purge gas compr is ing a
h y d r o g e n - r i c h stream e x i t i n g from said means for s e p a r a t i n g o u t
h y d r o g e n .
i. A process as claimed in any one of claims 1 to 4, w h e r e i n
p e r i o d i c a l l y said a d s o r p t i o n bed is r e g e n e r a t e d by being s u b j e c t
to a vacuum or with a purge gas compris ing r ecyc led product ga s
or a gaseous mixture of hydrogen and n i t r o g e n e x i t i n g from t h e
top of said cryogenic d i s t i l l a t i o n means.
r. A process as claimed in any one of the p reced ing c la ims, i n
which the methane c o n c e n t r a t i o n in said product argon is e q u a l
to or less than 20 ppm.
}. A process as claimed in any one of the p reced ing c la ims, i n
which the methane c o n c e n t r a t i o n in said product argon is l e s s
than or equal to 1 ppm.
3. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
the feed gas to said p r e s su re swing a d s o r p t i o n is eooled by
expansion from a high p r e s s u r e to the o p e r a t i n g p r e s s u r e o f s a i d
p re s su re swing a d s o r p t i o n means.
10. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
r e f r i g e r a t i o n for said c ryogenic d i s t i l l a t i o n means is o b t a i n e d
by r e c i r c u l a t i n g a r e f r i g e r a n t in a r e f r i g e r a t o r heat pump c y c l e
and using the column r e b o i l e r of said c ryogenic d i s t i l l a t i o n
means as a heat sink for the c y c l e .
MW/NJP/8627
- 2 0 - u z j y ^ J o
1. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
r e f r i g e r a t i o n for said c ryogenic d i s t i l l a t i o n means is o b t a i n e d
by v a p o r i s i n g l i q u i d n i t r ogen from s to rage tanks and pass ing t h e
n i t r o g e n vapor to an ammonia s y n t h e s i s compressor air i n t a k e
s e c t i o n .
2. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
the product stream from said p r e s s u r e swing a d s o r p t i o n means i s
cooled by heat exchange with waste gas l eav ing said c r y o g e n i c
d i s t i l l a t i o n means and by expansion to the p r e s s u r e s a i d
c ryogenic d i s t i l l a t i o n means before e n t e r i n g the same.
.3. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
any ammonia or r e s i d u a l water p r e s e n t in the said gas s t r e a m
e n t e r i n g the p r e s s u r e swing a d s o r p t i o n means is d i s cha rged in a
vent gas of said p r e s s u r e swing a d s o r p t i o n means.
L4. A process as claimed in any one of the p reced ing c la ims, w h e r e i n
said gas stream compr is ing hydrogen, n i t r o g e n , methane and a r g o n
is formed by reducing the c o n c e n t r a t i o n of hydrogen in an
ammonia p l an t purge g a s .
L5. A process as claimed in claim 14, in which the ammonia p l a n t
purge gas is also s u b j e c t to t r e a t m e n t to s e p a r a t e ammonia
t h e r e f r o m .
l£. A process as claimed in claim 14 or claim 15, wherein t h e
c o n c e n t r a t i o n of hydrogen in the ammonia p l an t purge gas i s
reduced in a membrane s e p a r a t o r .
17. A process as claimed in any one of the p reced ing c la ims, i n
which the p r e s s u r e energy of a high p r e s su re non-permeate ga s
s tream, through the use of a t u r b i n e , is used to dr ive a
compressor for c i r c u l a t i n g r e f r i g e r a n t in a r e f r i g e r a t o r h e a t
pump cycle employing a column r e b o i l e r of sa id c r y o g e n i c
d i s t i l l a t i o n means as a heat sink for the c y c l e .
MW/NJP/8627
1 I 3 ~ 1
1 — ' 1 — 1
. , — 1
NT"
cr >• ex
/ V / cu 1
en
CO I
opean raiem ice
legory SlIUIl Ul UWVUlllW i» - - • ■ of relevant passages claim 'PLICATION (IM, 01.*)
>, 2 3 r d S e p t e m b e r 1974 , p a g e 50, no. 6 5 6 6 9 s , C o l u m b u s , O h i o ,
3; K.YU.BAICHTOK e t a l . : D i f f u s i o n m e t h o d f o r t h e s p a r a t i o n of h y d r o g e n in a / s t e m f o r c o m p l e x s e p a r a t i o n o f
a rge g a s e s f rom t h e s y n t h e s i s £ a m m o n i a " & MEMBRAN. SKHNOL . -N0V0E NAPRAVLENIE NAUKE EKH. 1973 , 1 8 8 - 9 0
HEMICALi A B b l K ^ u i o , v u x . 4, 3 r d O c t o b e r 1983 , p a g e 1 6 6 , o. 107381W, C o l u m b u s , O h i o , US; . V . S 0 L 0 V E I e t a l . : " H y d r o g e n u r i f i c a t i o n u s i n g i n t e r m e t a l l i c ompound h y d r i d e s " & PROBL. ASHINOSTR. 1982 , 17, 1 0 3 - 6
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