9 The Shell Claus Offgas Treating (SCOT) Process C. DONALD SWAIM, JR.
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Ford, Bacon, and Davis Texas, Inc., Dallas, Tex. 75238
The Shell Claus Offgas Treating (SCOT) acceptance by the oil refining industry
Process won instant when it was an-
nounced in September 1972, and today it is the preferred method of meeting the most stringent emission regulations. Its functions are familiar to refinery operators, economical carbon steel is used throughout, and there are no waste discharges except the vent gas containing less than 500
ppm
hydrogen sulfide (250 ppm sulfur dioxide after incineration) and a clean water condensate.
This paper describes the
SCOT Process and discusses the operating experience of the first commercial
plants placed on stream. Two small skid-
-mounted units were designed and placed in operation
in
California within 8½ mos of contract award.
Specified recovery efficiency of sulfur recovery units ( S R U ) ^
used to
be based on economic considerations. Any increase in S R U efficiency
which added to its cost had to increase profits based upon the sales value of the additional sulfur recovered. Sulfur recovery units were based on the classic Claus process which was, and still is, the cheapest way to recover over 90%
of the sulfur in hydrogen sulfide-bearing
streams.
Most were very simple one or two catalytic stage plants. As the problem of reducing sulfur emissions has become more urgent, the complexity and cost of SRU's has risen, partially because of such routine sophisticated modifications as closed loop control of acid gas/air ratio, three or more catalytic reactor stages, high pressure steam reheat, ammonia and hydrogen cyanide handling capability, etc.
Such plants are approaching the
theoretical limits of sulfur recovery for the Claus process. Following the National Environmental Policy Act of 1969, the drive of federal, state, and local regulations toward zero sulfur emissions has 111
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
112
SULFUR
REMOVAL
AND RECOVERY
c a u s e d m a n u f a c t u r i n g a n d e n g i n e e r i n g - c o n s t r u c t i o n i n d u s t r i e s to d e v e l o p n u m e r o u s processes to c a p t u r e the r e s i d u a l s u l f u r i n C l a u s t a i l gas.
As
a p p l i e d specifically to S R U offgas, a f e w of these processes h a v e b e e n successfully c o m m e r c i a l i z e d , n a m e l y the W e l l m a n - L o r d , I F P , a n d P a r sons—Beavon processes.
T h e most recent to j o i n this p a r a d e of successful
c o m m e r c i a l processes is the S h e l l C l a u s T a i l G a s Offgas T r e a t i n g ( S C O T ) Process.
I t is l i c e n s e d i n the U . S . b y S h e l l D e v e l o p m e n t C o . , b y S h e l l
N i h o n G i j u t s u i n J a p a n a n d the F a r E a s t , a n d i n a l l other countries b y Shell Internationale Research Maatschappij.
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Process Description
and
Chemistry
T h e f o l l o w i n g process d e s c r i p t i o n is f r o m or d e r i v e d f r o m a p a p e r p r e s e n t e d b y N a b e r , W e s s l i n g h a n d G r o e n e n d a a l of S I R M (1,2,3).
The
S C O T process m a y b e d i v i d e d i n t o t w o s e c t i o n s — r e d u c t i o n - q u e n c h a n d a m i n e . T h e r e d u c t i o n step converts essentially a l l s u l f u r values i n C l a u s offgas to h y d r o g e n sulfide. T h e e l e m e n t a l s u l f u r a n d s u l f u r d i o x i d e are h y d r o g e n a t e d , a n d the c a r b o n y l sulfide a n d c a r b o n d i s u l f i d e are h y d r o l y z e d to h y d r o g e n sulfide a c c o r d i n g to the f o l l o w i n g m a i n reactions : S +
H
2
=
H S
+
3H
2
=
H S +
2H 0 2
(2)
COS +
H 0
=
H S +
C0
2
(3)
2H 0
=
2H S +
S0
CS
2
2
+
2
2
(1)
2
2
2
2
C0
(4)
2
N o r m a l l y , C l a u s S R U t a i l gas contains m o r e t h a n e n o u g h h y d r o g e n a n d c a r b o n m o n o x i d e to r e d u c e the s u l f u r a n d s u l f u r d i o x i d e , b u t a n o u t side source of h y d r o g e n or h y d r o g e n - r i c h gas m u s t be p r o v i d e d i n case of a n upset i n the S R U w h i c h w o u l d cause the s u l f u r d i o x i d e content to rise a b o v e n o r m a l . C a r b o n m o n o x i d e is as g o o d as h y d r o g e n for r e d u c t i o n b y the f o l l o w i n g shift r e a c t i o n : CO +
H 0 2
=
C0
2
+
H
(5)
2
T h e a m i n e section absorbs most of the h y d r o g e n sulfide f r o m the gas w h i l e c o a b s o r b i n g as l i t t l e c a r b o n d i o x i d e as possible.
These acid
gases are r e c y c l e d to the i n l e t of the C l a u s u n i t a n d b e c o m e p a r t of its feed.
B e c a u s e t h e solvent selects h y d r o g e n sulfide a n d rejects most
the c a r b o n d i o x i d e , the size of the C l a u s S R U is i n c r e a s e d b y o n l y
of
5-6%
because of r e c y c l i n g inert c a r b o n d i o x i d e . A s s h o w n o n t h e process flow sheet, F i g u r e 1, r e d u c i n g gas is a d d e d to t h e S R U offgas, a n d the t e m p e r a t u r e is r a i s e d to the r e q u i r e d r e a c t o r i n l e t t e m p e r a t u r e i n a fired heater. A l t e r n a t i v e l y , i f a source of r e d u c i n g
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
swAiM
SCOT
113
Process
SRU Tail Gas
Vent Gas To Incinerator
Reducing Gas Air Feed Heater
Absorber
Quench Tower
Reactor
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Lean Cooler/-
Boiler Feed Water
Quench Cooler
Waste Heat
A
1
Condensate
Figure 1.
SCOT process flow diagram
gas is n o t a v a i l a b l e , a r i c h gas b u r n e r m a y b e u s e d w h i c h b u r n s n a t u r a l gas w i t h s u b s t o i c h i o m e t r i c a i r t o y i e l d b o t h the n e e d e d h y d r o g e n a n d c a r b o n m o n o x i d e as w e l l as reactor preheat. P r e h e a t e d gas enters the reactor c o n t a i n i n g a b e d o f c o b a l t - m o l y b d e n u m catalyst w h e r e s u l f u r a n d its c o m p o u n d s are c o n v e r t e d t o h y d r o g e n sulfide a t a b o u t 300 ° C .
H e a t is r e c o v e r e d f r o m the h o t reactor effluent
b y g e n e r a t i n g steam i n a w a s t e heat b o i l e r w h i c h p r o v i d e s a b o u t o n e t h i r d o f the steam r e q u i r e d f o r t h e s u b s e q u e n t
S C O T stripper w h i l e
p a r t i a l l y c o o l i n g t h e reactants. T h e gas is c o o l e d to near a m b i e n t t e m p e r a t u r e b y d i r e c t contact w i t h water i n a packed quench tower.
T h e circulating quench water may b e
c o o l e d b y c o o l i n g w a t e r or b y a n a i r cooler w i t h c o o l i n g w a t e r t r i m . T h e substantial q u a n t i t y o f w a t e r v a p o r c o n t a i n e d i n n o r m a l s u l f u r r e c o v e r y u n i t t a i l gas is l a r g e l y c o n d e n s e d i n the q u e n c h t o w e r , a n d the condensate is c o n t i n u o u s l y w i t h d r a w n t o m a i n t a i n a constant l e v e l i n the q u e n c h tower bottom.
T h i s condensate is i n contact w i t h the h y d r o g e n sulfide
i n the gas stream a n d c o n s e q u e n t l y m u s t b e s t r i p p e d b e f o r e d i s c a r d i n g to t h e sewer. I f t h e m a i n p l a n t has a sour w a t e r s t r i p p e r , this w a t e r m a y be p i p e d to that t o w e r , o r i f not, a s m a l l sour w a t e r s t r i p p e r m a y b e a d d e d t o this stream as a n i n t e g r a l p a r t o f the S C O T p l a n t itself. H y d r o gen sulfide is r e t u r n e d either t o the S C O T system o r t o the sour w a t e r strippers so that o n l y c l e a n w a t e r is d i s c h a r g e d f r o m the p l a n t . T h e c o o l e d gas f r o m the t o p o f the q u e n c h t o w e r enters the S C O T absorber
where
the hydrogen
sulfide is a b s o r b e d
selectively b y a n
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
114
SULFUR REMOVAL
A N D RECOVERY
a l k a n o l - a m i n e s o l u t i o n . T h e system is d e s i g n e d so t h a t a l l b u t the s m a l l a m o u n t of h y d r o g e n sulfide a l l o w e d b y a n t i p o l l u t i o n regulations is r e m o v e d f r o m the gas s t r e a m w h i l e o n l y a b o u t 2 0 - 3 0 % d i o x i d e is c o - a b s o r b e d
of the
carbon
b y the a m i n e s o l u t i o n . T h e o v e r h e a d gas f r o m
the absorber c o n t a i n i n g t h e d e s i g n e d a m o u n t of h y d r o g e n sulfide, u s u a l l y a b o u t 2 0 0 - 5 0 0 p p m , is sent to a n i n c i n e r a t o r w h e r e r e s i d u a l s u l f u r c o m p o u n d s are o x i d i z e d to s u l f u r d i o x i d e b e f o r e d i s c h a r g e t h r o u g h a stack to the atmosphere. T h e r i c h a m i n e f r o m the b o t t o m of the S C O T a b s o r b e r is p u m p e d t h r o u g h a l e a n - r i c h exchanger to be h e a t e d w h i l e c o o l i n g the l e a n a m i n e s o l u t i o n a n d is f e d to the S C O T s t r i p p e r . H e a t i n p u t to the
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S C O T s t r i p p e r t h r o u g h its r e b o i l e r generates w a t e r v a p o r to s t r i p out the c a r b o n d i o x i d e a n d h y d r o g e n sulfide w h i c h t h e n go o v e r h e a d w i t h the w a t e r v a p o r , p a s s i n g t h r o u g h a condenser w h e r e the w a t e r is c o n d e n s e d for reflux to the s t r i p p e r . T h e u n c o n d e n s e d gases c o n t a i n i n g n e a r l y a l l of the h y d r o g e n sulfide a n d 2 0 - 3 0 %
of the c a r b o n d i o x i d e are t h e n
r e t u r n e d to t h e C l a u s u n i t w h e r e t h e y j o i n the m a i n a c i d gas feed.
The
hot, r e g e n e r a t e d a m i n e s o l u t i o n is p u m p e d f r o m the b o t t o m of t h e s t r i p p e r t h r o u g h the lean—rich e x c h a n g e r a n d a w a t e r - c o o l e d l e a n a m i n e cooler to t h e top of the absorber.
Table I.
S C O T Operating Requirements for 100 L T / D S R U
Electric power ( K W ) S t e a m (50 psig) ( l b s / h r ) B o i l e r feed w a t e r ( G P M ) F u e l gas ( m i l l i o n B t u / h r ) Cooling water ( G P M ) C a t a l y s t , based 3-yr life ( $ / y r ) Alkanolamine (J/yr) C a p i t a l costs
34 6,400 6.4 2.9 1,200 10,000 2,000 $1,400,000
T h e a m i n e section appears c o n v e n t i o n a l b u t w h e r e the u s u a l m o n o e t h a n o l a m i n e ( M E A ) a n d d i e t h a n o l a m i n e ( D E A ) s w e e t e n i n g processes a p p r o a c h the e q u i l i b r i u m s o l u b i l i t y of c a r b o n d i o x i d e a n d h y d r o g e n s u l fide,
the s e l e c t i v i t y for a b s o r b i n g h y d r o g e n sulfide a n d r e j e c t i n g c a r b o n
d i o x i d e is a t t a i n e d b y the difference i n r e a c t i o n rates of the gases w i t h the a m i n e ( u s u a l l y d i i s o p r o p a n o l a m i n e , D I P A ) .
T h e a i m is to
absorb
n e a r l y a l l the h y d r o g e n sulfide before the c a r b o n d i o x i d e has h a d t i m e to react w i t h the a m i n e . T h e a b s o r p t i o n takes p l a c e at near a t m o s p h e r i c pressure. T h i s differs f r o m t h e c o n v e n t i o n a l a m i n e p l a n t w h i c h u s u a l l y operates at a c o n s i d e r a b l y h i g h e r pressure. U n l i k e the s u l f u r oxide p r o c esses, this process is g e n e r a l l y n o n - c o r r o s i v e a n d c a r b o n steel is u s e d t h r o u g h o u t except i n the f e w cases w h e r e a l l o y is r e q u i r e d because of conditions.
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
SCOT
swAiM
115
Process
T h e S C O T Process is c o m p e t i t i v e w i t h other processes f o r r e d u c i n g sulfur d i o x i d e levels to the 2 0 0 - 5 0 0 p p m r a n g e .
T a b l e I shows c a p i t a l
a n d o p e r a t i n g costs for a t y p i c a l u n i t to serve a 100 l o n g
ton/day
( L T / D ) S R U o p e r a t i n g at 9 4 % r e c o v e r y efficiency p e r pass. SCOT
Process Development
T h e S C O T process was first m a d e p u b l i c i n S e p t e m b e r 1972, at a technical meeting i n Japan b y Shell Internationale Research Maatschappij (SIRM).
S h e l l h a d p r o v e d t h e effectiveness a n d l i f e of the catalyst i n
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t h e r e d u c t i o n step i n b e n c h - s c a l e w o r k at t h e i r A m s t e r d a m l a b o r a t o r y a n d a s e m i - c o m m e r c i a l d e m o n s t r a t i o n o n C l a u s S R U t a i l gas at Shell's G o r dorf, G e r m a n y refinery. C o n f i d e n c e i n the effectiveness
a n d selectivity
of t h e a m i n e a b s o r p t i o n step w a s b a s e d o n S h e l l s extensive use of the A D I P process i n w o r l d w i d e a p p l i c a t i o n s b o l s t e r e d b y l a b o r a t o r y b e n c h scale testing. Commercial
Plants
O n the s t r e n g t h of the S I R M w o r k , D o u g l a s O i l C o . ( a s u b s i d i a r y of C o n t i n e n t a l O i l C o . ) a n d C h a m p l i n P e t r o l e u m C o . b o u g h t the S C O T process for t h e i r refineries i n C a l i f o r n i a to m e e t the v e r y strict L o s A n g e l e s A P C D C o d e (4, 5 ) .
B o t h of these p l a n t s w e r e a s s e m b l e d , c o m -
plete w i t h p i p i n g , instrumentation, insulation, a n d electrical w i r i n g , on skids i n the D a l l a s , Texas shops of F o r d , B a c o n a n d D a v i s a n d s h i p p e d b y t r u c k to t h e p l a n t sites. T o w e r s s h i p p e d d i r e c t l y to the jobsite b y t h e i r v e n d o r s w e r e p l a c e d , a l o n g w i t h the skids, o n p r e p a r e d f o u n d a t i o n s i n 1 day.
B o t h units w e r e started u p the last w e e k of J u n e 1973, less t h a n
9 m o n t h s f r o m contract a w a r d . F i g u r e 2 shows t h e C h a m p l i n P e t r o l e u m SCOT
Unit.
T h e s e t w o plants represent n o t o n l y t h e first c o m m e r c i a l a p p l i c a t i o n of the process a n y w h e r e , b u t also the first t i m e the h y d r o g e n a t i o n - q u e n c h section a n d the a m i n e section h a d b e e n o p e r a t e d as a n i n t e r g r a t e d w h o l e . E v e n Shell's p i l o t p l a n t a n d d e m o n s t r a t i o n u n i t h a d not b r o u g h t the separate sections together. W h i l e these C a l i f o r n i a plants w e r e s m a l l u n i t s a d d e d onto e x i s t i n g 9 a n d 15 L T / D S R U ' s r e s p e c t i v e l y , t h e y d i d p r o v i d e the o p p o r t u n i t y to d i s c o v e r a n d r e m e d y the i n e v i t a b l e p r o b l e m s i n a n e w process.
M u c h w a s l e a r n e d w h i c h i n c r e a s e d t h e confidence i n the d e s i g n
of m u c h l a r g e r plants w h i c h w e r e f o l l o w i n g o n . A t a n e a r l y date, S h e l l C a n a d a L t d . d e c i d e d to i n s t a l l the
SCOT
process at t h e i r W a t e r t o n , A l b e r t a gas p r o c e s s i n g p l a n t . T h i s p l a n t , d e s i g n e d i n the N e t h e r l a n d s , w i l l treat the t a i l gas f r o m a S R U c a p a c i t y of 2,100 L T / D . B e c a u s e the t o t a l s u l f u r e m i s s i o n a l l o w a b l e f r o m a single
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
116
SULFUR
REMOVAL
AND
RECOVERY
f a c i l i t y i n A l b e r t a is w e l l a b o v e the c a p a b i l i t y of the S C O T process, this u n i t w i l l treat o n l y a b o u t t w o t h i r d s of the t o t a l S R U c a p a c i t y of Waterton plant.
T h e r e m a i n i n g one t h i r d of the S R U t a i l gas m a y
i n c i n e r a t e d a l o n g w i t h the S C O T t a i l gas a n d d i s c h a r g e d
the be
to t h e air
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w i t h o u t e x c e e d i n g the a l l o w a b l e e m i s s i o n rate.
Figure
2.
SCOT unit in 15 LT/D SRU of Petroleum Co., Wilmington, Calif.
Champlin
I n q u i c k succession, a n u m b e r of S C O T units w e r e o r d e r e d i n areas w h e r e the a n t i p o l l u t i o n c o d e requires v e r y l o w emission levels.
These
w e r e o n n e w S R U ' s specifically d e s i g n e d to integrate the C l a u s S R U ' s w i t h S C O T plants.
T h e y w e r e B P O i l at M a r c u s H o o k , P a . ; M a r a t h o n
O i l at D e t r o i t , M i c h . ; S t a n d a r d O i l C o . ( O h i o ) at L i m a , O h i o ;
South-
w e s t e r n O i l a n d R e f i n i n g C o . at C o r p u s C h r i s t i , T e x . ; a n d T e x a c o Inc. at P o r t A r t h u r , T e x . A t this w r i t i n g , t h i r t e e n S C O T units are u n d e r c o n tract i n the U . S . a n d C a n a d a , as d e t a i l e d i n T a b l e I I . F i g u r e 3 is a m o d e l of a 160 L T / D S C O T u n i t u n d e r
construction.
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
swAiM
Table I I .
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Process
117
SCOT Process
User
Champlin Petroleum Douglas O i l Murphy Oil U . S . Steel Β Ρ Oil Sun O i l Marathon Oil Shell O i l Shell C a n a d a Southwestern O i l & Refining Texaco Inc. Shell O i l
Figure 3.
SCOT
U n i t s — U . S . and
Location Wilmington, Calif. Paramount, Calif. Meraux, L a . Clairton, P a . Marcus Hook, P a . Duncan, Okla. Detroit, M i c h . Houston, Tex. Waterton, Alta. Corpus Christi, Tex. Port Arthur, Tex. Norco, L a .
Canada
SRU
Startup
(LT/D) 15 8.8 40 130 160 28 80 325 2100 125 235 40
J u n e 1973 J u n e 1973 m i d 1974 late 1974 late 1974 e a r l y 1975 e a r l y 1975 e a r l y 1975 e a r l y 1975 e a r l y 1975 late 1975 late 1975
Model of SCOT unit under construction in 160 LT/D
SRU
I n J a p a n , v i r t u a l l y a l l S R U ' s are i n h i g h p o p u l a t i o n d e n s i t y C o n s e q u e n t l y , the S C O T process has b e e n q u i c k l y a d o p t e d there. teen S C O T units are slated to be b u i l t i n J a p a n at this t i m e .
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
areas. Seven
118
SULFUR
Commercial Plants Startup
and Operation
REMOVAL
AND RECOVERY
Problems
W h i l e there w e r e f e w m e c h a n i c a l p r o b l e m s d u r i n g startup of the t w o C a l i f o r n i a plants because of t h e c o n v e n t i o n a l e q u i p m e n t , some process problems delayed full compliance w i t h L o s Angeles A P C D Regulations. Understated Sulfur Load Design.
T h i s p r o b l e m is m e n t i o n e d first
because it h a d an important bearing on meeting guaranteed performance, even i f there h a d b e e n no other p r o b l e m s .
T h i s u n i t m u s t consistently
r e m o v e t h e last b i t of s u l f u r f r o m a stream that is subject to r a t h e r w i d e fluctuations
c a u s e d b y r o u t i n e changes or upsets i n p r e c e d i n g processes.
T h i s is e s p e c i a l l y t r u e i n refineries w h e r e m u l t i p l e a m i n e units a n d v a r i a -
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tions i n refinery feedstocks c a n cause w i d e s w i n g s i n b o t h rate a n d c o m p o s i t i o n of the a c i d gas f e e d i n g the S R U . U p s e t s i n a m i n e units can cause upsets i n the S R U o p e r a t i o n , a n d t h e S R U m a y be subject
to
upsets of its o w n i f a d e q u a t e i n s t r u m e n t a t i o n is not p r o v i d e d . B o t h the C a l i f o r n i a plants w e r e a d d e d to e x i s t i n g S R U ' s . T h e s u l f u r content of the off-gases w e r e at times w e l l a b o v e that stated b y
the
owners for o r i g i n a l d e s i g n of the S C O T units. T h e s o l u t i o n to this p r o b l e m was s i m p l y to a d d i m p r o v e d S R U controls i n o l d e x i s t i n g p l a n t s a n d d e s i g n for o p t i m u m c o n t r o l i n n e w plants a n d / o r p r o v i d e e n o u g h excess c a p a c i t y i n the S C O T u n i t to h a n d l e m a x i m u m a n t i c i p a t e d s u l f u r content of the t a i l gas. Solvent Stripper Design.
I n o p e r a t i o n at g r e a t e r - t h a n - d e s i g n s u l f u r
loads, i t was f o u n d that the solvent c i r c u l a t i o n rate h a d to be i n c r e a s e d to m e e t the specifications for r e s i d u a l h y d r o g e n sulfide i n the absorber offgas. T o a c c o m p l i s h this, the s t r i p p e r w a s r e p l a c e d w i t h one of l a r g e r d i a m e t e r to a c c o m m o d a t e the i n c r e a s e d l i q u i d a n d v a p o r traffic i n t h e tower.
I n c i d e n t a l to the u p g r a d i n g of s t r i p p e r c a p a c i t y w e r e i n c r e a s e d
steam to r e b o i l e r , i n c r e a s e d reflux condenser, a n d i n c r e a s e d p u m p c a p a c i t y . T h e s e changes h a v e b e e n a c c o m p l i s h e d at the C h a m p l i n P e t r o leum S C O T
u n i t , a n d i t has b e e n
operating smoothly w i t h
minimal
operator a t t e n t i o n a n d b e t t e r i n g its g u a r a n t e e d p e r f o r m a n c e b y a w i d e m a r g i n since O c t o b e r 1973. Sulfur Dioxide Breakthrough.
N o r m a l l y , e n o u g h s u l f u r d i o x i d e is
c o n v e r t e d to h y d r o g e n sulfide i n the S C O T reactor so t h a t s u l f u r d i o x i d e is not detected i n reactor effluent. I n one of the p l a n t s , t h r o u g h mistakes i n o p e r a t i o n d u r i n g s t a r t u p , the s u l f u r d i o x i d e content was a l l o w e d to rise to the p o i n t w h e r e t h e q u e n c h w a t e r b e c a m e a c i d i c , a n d s u l f u r d i o x i d e was c a r r i e d over i n t o the S C O T absorber w h e r e i t f o r m e d
a
c o m p o u n d w i t h D I P A that was not r e g e n e r a b l e at s t r i p p e r c o n d i t i o n s . A s a result, the c a r b o n steel q u e n c h w a t e r p u m p a n d the c a r b o n steel quench water pipes where turbulence was h i g h were r a p i d l y corroded a n d the h y d r o g e n sulfide a b s o r b i n g c a p a c i t y of the a m i n e s o l u t i o n w a s lost.
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
9.
SCOT Process
swAiM
119
While the consequences of sulfur dioxide breakthrough are serious, it is easy to prevent using the following procedures and precautions: 1. If initial catalyst sulfiding is to be done with sulfur dioxidecontaining S R U tail gas, the reactor effluent must be isolated, bypassing the quench system.
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2. There must always be an excess of hydrogen to guard against surges in sulfur dioxide content of S R U tail gas. 3. Protection against contamination of the amine by small amounts of sulfur dioxide is provided by the reaction of sulfur dioxide with hydrogen sulfide to form sulfur in low temperature Claus reaction in the water phase. The quench water itself provides very sensitive early warn ing of potential trouble from sulfur dioxide breakthrough, and proper instrumentation can sound an alarm and/or divert the S R U tail gas to the incinerator until the breakthrough is corrected. These indicators are color, p H , and turbidity, in order of increasing situation severity. W i t h proper instrumentation the amine should never be contaminated. #
The possibility of sulfur dioxide contamination of the D I P A has been considered so remote and the rate of D I P A degradation is so low that none of the plants now being designed and constructed have D I P A reclaiming facilities, as is common practice in M E A and D E A amine units. Since November 1973, none of the startup problems have recurred in the Champlin Unit, and the plant has operated well below the sulfur emission levels guaranteed by Shell and required by A P C D regulations. Performance
Testing and
Compliance
The performance of the Champlin S C O T unit has been tested by the Los Angeles A P C D emission source test team.
They found that the
emission level was considerably below the statutory limits of 500 p p m sulfur dioxide.
The plant has also been subject to a lengthy test by
the mobile laboratory of the E P A with similar
findings.
Performance
tests made by Shell Development Co. proved that the Champlin Plant met and exceeded its guarantee level of 500 ppm hydrogen sulfide in absorber offgas and that the selectivity of the solvent for hydrogen sulfide exceeded
expectations.
Literature
Cited
1. Naber, J. E . , Wesselingh, Μ. Α., Groenendaal, W., Chem. Eng. Progr. (Dec. 1973) 6 9 , 29. 2. N G / L N G / S N G Handbook, Hydrocarbon Process. (April 1973) 52, 114. 3. Naber, J. E., Wesselingh, Μ. Α., Groenendaal, W., Energy Process. Canada (Sept.-Oct. 1973) 32. 4. The British Sulphur Corp., Ltd., Sulphur (Nov.-Dec. 1972) 103, 53. 5. Ibid., (May-June 1973) 106, 61. R E C E I V E D June 6,
1974
In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.