The Shell Claus Offgas Treating (SCOT) Process - ACS Publications

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.


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


9.

swAiM

SCOT

113

Process

SRU Tail Gas

Vent Gas To Incinerator

Reducing Gas Air Feed Heater

Absorber

Quench Tower

Reactor


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


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


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.


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


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


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


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.


9.

swAiM

Table I I .


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 .


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 -


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.


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.


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


The Shell Claus Offgas Treating (SCOT) Process - ACS Publications

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