Beavon Sulfur Removal Process for Claus Plant ... - ACS Publications

7 Beavon Sulfur Removal Process for Claus Plant Tail Gas DAVID K. BEAVON

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The Ralph M. Parsons Co., Pasadena, Calif. 91124 RAY N. FLECK Union Oil Co. of California, Brea, Calif. 92621

The Beavon sulfur removal process for the cleanup of Claus plant tail gas is a two-step process in which the sulfur contaminants are first catalytically

hydrolyzed and/or

hydro-

genated to hydrogen sulfide and the hydrogen sulfide is then converted to elemental sulfur and recovered in a Stretford process unit. Commercial

plants reduce the

concentration

of sulfur compounds as hydrogen sulfide in the tail gas from 1-3

vol % to less than 100 ppm.

less than 1 ppm hydrogen sulfide.

The treated gas contains The chemistry, design

criteria, operating experience, and economics of the process are discussed.

/ ^ l a u s plants are used in petroleum refineries and elsewhere to partially ^

oxidize hydrogen sulfide to elemental sulfur using air as the oxidant.

The efficiency of such plants is limited, and a nitrogen-rich tail gas is produced which contains water, carbon dioxide, and smaller of other substances including sulfur compounds. Typically, about

amounts 3-10%

of the entering sulfur is produced with the tail gas as carbonyl sulfide, methyl mercaptan, carbon disulfide, hydrogen sulfide, sulfur dioxide, and elemental sulfur as vapor or entrained droplets.

Disposal of the incin-

erated tail gas has in the past been an air pollution problem because it contains about 10,000-20,000 ppm sulfur dioxide. A number of commercial plants are now using the Beavon sulfur removal process to convert the sulfur content of Claus tail gas first to hydrogen sulfide and finally to elemental sulfur. These plants reduce the sulfur content of the tail gas from about 1-3%

to less than 100 ppm of

which less than 1 ppm is present as hydrogen sulfide. The foregoing con93

In Sulfur Removal and Recovery; Pfeiffer, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

94

SULFUR

REMOVAL

AND RECOVERY

centrations are c a l c u l a t e d as the s u l f u r d i o x i d e e q u i v a l e n t o n a v o l u m e basis. T h e p r o c e s s e d t a i l gas c a n b e d i r e c t l y d i s c h a r g e d t o t h e atmosphere without environmental problems. Chemistry of the Claris Process I n a C l a u s p l a n t a b o u t o n e - t h i r d of the h y d r o g e n sulfide is c o m b u s t e d to s u l f u r d i o x i d e , a n d the b a l a n c e reacts a c c o r d i n g to R e a c t i o n 1; t h e r m a l l y at h i g h temperatures a n d c a t a l y t i c a l l y at l o w e r temperatures (i,

2): 2 H S + S0


2

?± 3 S + 2 H 0

2

(1)

2

T h e c a t a l y t i c r e a c t i o n p r o c e e d s stagewise w i t h interstage r e m o v a l of the s u l f u r to shift t h e e q u i l i b r i u m .

Interstage r e m o v a l of w a t e r to shift

t h e e q u i l i b r i u m e v e n f u r t h e r is i m p r a c t i c a l because of p l u g g i n g p r o b l e m s (3) w i t h solid sulfur. D u r i n g the c o m b u s t i o n of the h y d r o g e n sulfide some of t h e s u l f u r reacts w i t h h y d r o c a r b o n s n o r m a l l y present to f o r m c a r b o n d i s u l f i d e a n d m e t h y l m e r c a p t a n . C a r b o n y l sulfide is also f o r m e d e i t h e r b y the p a r t i a l h y d r o l y s i s of c a r b o n d i s u l f i d e a n d / o r b y the r e a c t i o n 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 sulfide.

Some hydrogen and carbon monoxide

are also

f o r m e d i n the C l a u s c o m b u s t i o n step. Chemistry

of the Catalytic

Reactor

T h e heart of t h e B e a v o n process is a c a t a l y t i c reactor w h i c h converts essentially a l l of t h e s u l f u r i n t h e t a i l gas to h y d r o g e n sulfide either b y hydrogénation or h y d r o l y s i s . H y d r o l y s i s reactions are t y p i f i e d b y R e a c tions 2 a n d 3: CS

+ 2 H

2

COS +

0 ^ 2 H S + C0

2

2

H 0 ï± H S + C 0 2

2

2

(2) (3)

2

R e s i d u a l 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 t y p i c a l l y t o t a l a b o u t 25 p p m c a l c u l a t e d as s u l f u r d i o x i d e . Hydrogénation reactions are t y p i f i e d b y R e a c t i o n s 4, 5, a n d 6: S + H S0

+ 3 H

2

CH SH + 3

2

2

—> H S

(4)

2

-> H S + 2 H 0

H

2

2

(5)

2

-> H S + C H 2

4


(6)

7.

BEAVON AND FLECK

Beavon Sulfur Removal

95

Process

W h i l e c a r b o n d i s u l f i d e a n d c a r b o n y l sulfide c a n also b e c o n v e r t e d

to

h y d r o g e n sulfide b y hydrogénation, b o t h are p r e d o m i n a n t l y c o n v e r t e d

by

hydrolysis.

The

s u p p l y of h y d r o g e n

is s u p p l e m e n t e d

b y the

carbon

m o n o x i d e content of the reactor f e e d gas w h i c h undergoes R e a c t i o n 7, the w a t e r - g a s shift r e a c t i o n : CO +

H 0 H 2

2

+

C0

(7)

2

Chemistry of the Stretford Process T h e h y d r o g e n s u l f i d e - c o n t a i n i n g stream f r o m the c a t a l y t i c step c a n Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: April 1, 1975 | doi: 10.1021/ba-1975-0139.ch007

be p r o c e s s e d several w a y s to r e c o v e r the s u l f u r content. mercial development

uses the S t r e t f o r d process (4-10)

T h e initial comto convert

the

h y d r o g e n sulfide to e l e m e n t a l s u l f u r b y a w e t c h e m i s t r y process d e v e l o p e d b y T . N i c k l i n a n d others at N o r t h W e s t G a s of S t r e t f o r d , E n g l a n d . T h e S t r e t f o r d process has b e e n a c c e p t e d w i d e l y for t r e a t i n g coke o v e n gas a n d r e c e i v e d the Queen's A w a r d for i n d u s t r i a l

developments.

I n this process the gas is first c o n t a c t e d i n a n absorber w h e r e h y d r o g e n sulfide is a b s o r b e d i n t o a n a l k a l i n e s o l u t i o n a n d e v e n t u a l l y c o n v e r t e d to s u l f u r b y R e a c t i o n 8 w h e r e V 2 V+ The reduced V

+ 4

5

+

+ 5

is present as s o d i u m m e t a v a n a d a t e .

H S - » 2 V+ 2

4

+

2H+ + S

(8)

is l a t e r o x i d i z e d b y a i r b l o w i n g i n t h e presence of the

d i s o d i u m salt of a n t h r a q u i n o n e , 2,7-disulfonic a c i d ( A D A ) . T h e o v e r a l l o x i d a t i o n is r e p r e s e n t e d b y R e a c t i o n 9: 2V Sulfur formed

+ 4

+ 2H++

1/2 0

2

-> 2 V +

i n R e a c t i o n 8 is r e c o v e r e d

5

+

H 0 2

(9)

b y flotation d u r i n g the a i r

b l o w i n g i n R e a c t i o n 9. S o m e of the h y d r o g e n sulfide f e e d u n d e r g o e s side reactions a n d is c o n v e r t e d to s o d i u m t h i o s u l f a t e a n d s o d i u m sulfate. m o r e of the i n c o m i n g h y d r o g e n

However, 9 8 %

or

sulfide f e e d is n o r m a l l y c o n v e r t e d

to

e l e m e n t a l sulfur. Plant

Design

Pilot Plant.

T h e B e a v o n s u l f u r r e m o v a l process was s t u d i e d i n a

p i l o t p l a n t w h e r e the scale-up f a c t o r w a s 100 a n d 200 for t h e first t w o c o m m e r c i a l units.

G e n e r a l process o p e r a b i l i t y w a s e s t a b l i s h e d at this

stage. Commercial Design.

T h e d e s i g n for a c o m m e r c i a l p l a n t is s h o w n

s c h e m a t i c a l l y i n F i g u r e 1. R e d u c i n g gas is generated b y p a r t i a l o x i d a t i o n


96

SULFUR

REMOVAL

AND RECOVERY CLEAN OFF G A S

CLAUS TAIL GAS

cru

FUEL GAS

ABSORBER


REDUCING GAS GENERATOR

CATALYST BED

Figure 1.

OXIDIZER

WASTE HEAT BOILER

SULFUR

FROTH FILTER & WASH

SULFUR MELTER

Flow diagram of the Beavon sulfur removal process

of a f u e l gas a n d t h e n m i x e d w i t h the f e e d t a i l gas as a source of h y d r o g e n and carbon monoxide.

T h e hot r e d u c i n g gas also preheats the t a i l gas.

T h e m i x e d gases flow to the c a t a l y t i c step after w h i c h the b u l k of the w a t e r m a y b e r e m o v e d , a n d t h e gas stream is passed to the S t r e t f o r d absorber.

I n the S t r e t f o r d process, e l e m e n t a l s u l f u r r e c o v e r e d as a f r o t h

i n the o x i d i z e r is filtered, w a s h e d , a n d passed to a m e l t e r to separate m o l t e n s u l f u r a n d the e n t r a i n e d w a t e r . Special Design Considerations.

T h e i n i t i a l plants consisted of

two

C l a u s / B e a v o n strings so that w h e n one s t r i n g w a s shut d o w n the other c o u l d process the h y d r o g e n sulfide l o a d f r o m the e n t i r e refinery w i t h o u t p o l l u t i o n . S o m e p l a n t s n o w u n d e r c o n s i d e r a t i o n w i l l h a v e o n l y a single s t r i n g w i t h s p a r i n g of some items of e q u i p m e n t , e.g., p u m p s , to p r o v i d e sufficient r e l i a b i l i t y . H y d r o l y s i s , hydrogénation, a n d the shift r e a c t i o n take p l a c e c o n c u r r e n t l y at m o d e r a t e temperatures a n d a t m o s p h e r i c pressure over a n ext r u d e d c o b a l t m o l y b d a t e catalyst w h i c h is sulfided. Space velocities are a b o u t 2000 c u ft of t a i l gas p l u s r e d u c i n g g a s / h r / c u ft of catalyst. B e cause of the excessive heat released w h e n sufficient a i r contacts

these

catalysts, extraneous a i r m u s t b e e x c l u d e d f r o m the catalyst at a l l times, especially d u r i n g start-up a n d shutdown. F o r m a x i m u m c o n v e r s i o n of s u l f u r c o m p o u n d s i t is d e s i r a b l e to o p erate as close to c h e m i c a l e q u i l i b r i u m as possible.

B o t h the p i l o t p l a n t

a n d c o m m e r c i a l plants a t t a i n e d a f a i r l y close a p p r o a c h to e q u i l i b r i u m w i t h respect to the c o n v e r s i o n of s u l f u r c o m p o u n d s . reactions

t h r o u g h o u t the reactor

together

with

The many competing a close a p p r o a c h

to

e q u i l i b r i u m c o m p l i c a t e s process analysis. S e v e r a l things w e r e d o n e at the p i l o t p l a n t stage to m i n i m i z e reactor scale-up problems.

T h e catalyst p a r t i c l e size i n the p i l o t p l a n t w a s the

same as that n o w u s e d i n c o m m e r c i a l p l a n t s . C a t a l y s t b e d d e p t h s i n the p i l o t a n d c o m m e r c i a l u n i t s w e r e s i m i l a r i n order to h a v e s i m i l a r mass


7.

BEAVON AND FLECK

Beavon

velocities t h r o u g h b o t h beds.

Sulfur Removal

97

Process

B e c a u s e of the large gas v o l u m e s a n d l o w

pressure d r o p r e q u i r e m e n t s , the catalyst b e d is r e l a t i v e l y s h a l l o w . d e s i g n r e q u i r e s p r o p e r gas d i s t r i b u t i o n at the reactor i n l e t to

This

prevent

channeling. T h e pilot plant operated exclusively on a commercial Claus p l a n t t a i l gas i n o r d e r to i n c l u d e the effects of a n y a n d a l l u n k n o w n t a i l gas constituents i n the process e v a l u a t i o n . F i n a l l y , the p i l o t p l a n t w a s o p e r a t e d for over 3 months to m a k e c e r t a i n that the catalyst d i d not deter i o r a t e r a p i d l y w i t h use. A s of m i d - J a n u a r y 1974, U n i o n ' s t w o c o m m e r c i a l units h a d each o p e r a t e d for 6 m o n t h s w i t h o u t o b s e r v a b l e catalyst d e a c t i v a t i o n . T h e u l t i m a t e catalyst l i f e is e x p e c t e d to b e m o r e t h a n a year.


U n f o r t u n a t e l y , upsets i n a n o i l refinery C l a u s u n i t are r o u t i n e because of f r e q u e n t s u d d e n changes i n f e e d c o m p o s i t i o n a n d

flow.

The Beavon

u n i t p r o v i d e s excess h y d r o g e n i n the c a t a l y t i c section to

hydrogenate

s u l f u r d i o x i d e surges.

T h e r e s u l t i n g h y d r o g e n sulfide surges as w e l l as

those f r o m the C l a u s u n i t are h a n d l e d i n the S t r e t f o r d u n i t b y p r o v i d i n g a s u i t a b l e excess of c h e m i c a l s i n the S t r e t f o r d s o l u t i o n .

Because

the

hydrogénation reactions are e x o t h e r m i c , s u l f u r d i o x i d e surges, for a m p l e , w i l l increase the t e m p e r a t u r e rise across the catalyst b e d .

exThe

d e s i g n tolerates surges w i t h o u t d a m a g i n g the catalyst. A waste-heat b o i l e r m a y be i n s t a l l e d o n the reactor outlet gas l i n e i f the i n s t a l l a t i o n is large e n o u g h to generate steam e c o n o m i c a l l y . S t e a m m a y b e g e n e r a t e d at a n y pressure u p to a b o u t 200 p s i g . W a t e r b u i l d u p m u s t be p r e v e n t e d i n the S t r e t f o r d l i q u o r .

Water

enters the S t r e t f o r d u n i t w i t h the feed gas a n d is also a r e a c t i o n p r o d u c t i n the S t r e t f o r d process.

A d d i t i o n a l w a t e r enters w i t h the filter w a s h . A n

e v a p o r a t i v e cooler evaporates excess w a t e r a n d controls t e m p e r a t u r e . T h e absorber is a s i m p l e splash-deck tower.

B e c a u s e the b a c k pres-

sure of h y d r o g e n sulfide over S t r e t f o r d s o l u t i o n is n e g l i g i b l e , the a b s o r b e r c a n b e s i z e d to r e d u c e the i n l e t h y d r o g e n sulfide c o n c e n t r a t i o n b y a f a c t o r of 100,000.

C o m m e r c i a l absorbers have met this d e s i g n c r i t e r i o n .

I n p r a c t i c e the o x i d a t i o n of S t r e t f o r d s o l u t i o n b y R e a c t i o n 9 is k i n e t i c a l l y l i m i t e d so that a s u b s t a n t i a l i m p r o v e m e n t i n p e r f o r m a n c e be o b t a i n e d b y s t a g i n g the process.

can

It is e s t i m a t e d that the o v e r a l l r e s i -

d e n c e t i m e i n the o x i d i z e r c a n be decreased b y as m u c h as 5 0 %

when

three o x i d i z e r stages are u s e d i n p l a c e of a single stage.

Commercially,

a

stirrers c h u r n

three-stage

o x i d i z e r is u s e d

air i n t o s m a l l b u b b l e s .

in which power-driven

A n analysis of c o m m e r c i a l o x i d i z e r

i n d i c a t e s that mass transfer is not a significant factor.

performance

H e n c e the b u b b l e

size is s u i t a b l y s m a l l . Operating

Experience

A t the present t i m e seven units are o p e r a t i n g a n d a b o u t

fifteen

more

are i n v a r i o u s stages of e n g i n e e r i n g a n d c o n s t r u c t i o n . B y the t i m e of this


98

SULFUR

REMOVAL

AND RECOVERY

p u b l i c a t i o n , t h e t o t a l o p e r a t i n g t i m e l o g g e d w i l l b e a b o u t 36 u n i t - m o n t h s . T o d a t e t h e h y d r o g e n sulfide content of t h e treated t a i l gas has e x c e e d e d 1 p p m o n l y d u r i n g severe upsets. T h e aggregate t i m e of these upsets is p r o b a b l y less t h a n h a l f a d a y . T h e treated t a i l gas p u r i t y has at a l l times e x c e e d e d t h e most r i g i d regulations b y a w i d e m a r g i n . Operating Problems. D u r i n g t h e startups of t h e first t w o p l a n t s there w a s some difficulty, w i t h s o l u t i o n o v e r l o a d i n g , m a i n l y because of upsets i n t h e C l a u s u n i t s a n d i n spite of the fact t h a t these plants are d e s i g n e d w i t h generous

over-capacity.

T h e frequency

of these upsets q u i c k l y

d e c r e a s e d as p e r s o n n e l l e a r n e d to m a i n t a i n p r o p e r o p e r a t i n g ratios i n t h e C l a u s units.

F o r example, a 5 %

deficiency

i n t h e a m o u n t of a i r i n


the C l a u s unit c a n double the load o n the B e a v o n plant. T h e f r o t h filter w a s t h e most t r o u b l e s o m e single p i e c e of m e c h a n i c a l e q u i p m e n t . T r o u b l e s d i m i n i s h e d g r e a t l y as t h e operators b e c a m e f a m i l i a r w i t h this n e w t y p e of e q u i p m e n t .

M e t h o d s to e l i m i n a t e t h e filter h a v e

n o w b e e n d e v e l o p e d a n d w i l l b e tested o n a c o m m e r c i a l scale. Construction Materials. C a r b o n steel is u s e d for most of t h e p l a n t s ; i n s o m e areas i t is p r o t e c t e d against r u s t i n g b y a c o a l t a r e p o x y c o a t i n g . T h e w a t e r o u t l e t l i n e of t h e s u l f u r m e l t e r is stainless steel. N o n o t a b l e c o r r o s i o n has o c c u r r e d . Table I.

Beavon Sulfur Removal Process Operating Costs"

Commodity Power F u e l gas Soft w a t e r Catalyst replacement Chemicals

Unit Cost

6

T o t a l costs L e s s c r e d i t for 50 p s i g s t e a m L e s s c r e d i t for incinerator fuel N e t cost a 6

0.725*f/kw-hr S1.00/MM Btu 20φ/Μ g a l — —

Consumption Rate

Daily Cost ($)

300 k w 125 M M B t u 12 M g a l / d a y — —

52 125 2 8 140 327

S 1 . 0 0 / M lb $1.00/MM Btu

2,500 l b / h r

60

250 M M B t u

250 17

Basis: 100 long-ton/day Claus Unit. Assumed catalyst life is 3 yrs. Economics. T h e c a p i t a l i n v e s t m e n t f o r a B e a v o n p l a n t to process t a i l

gas f r o m a 1 0 0 - l o n g - t o n / d a y C l a u s u n i t is a b o u t $1,250,000.

Operating

costs f o r a u n i t of this size a r e g i v e n i n T a b l e I. T h e f u e l a n d steam r e q u i r e d b y t h e process

is less t h a n o n e - f o u r t h of that r e q u i r e d f o r

s i m p l e i n c i n e r a t i o n . S h o u l d f u e l gas cost increase to $ 2 / M M B t u as has b e e n forecast, t h e energy savings w o u l d m a k e t h e o p e r a t i o n profitable.


7.

BEAVON AND F L E C K

Beavon Sulfur Removal Process

99

Conclusions The Beavon sulfur removal process is now a reliable, established method for cleaning up Claus plant tail gas well beyond any proposed regulations. As with any new process, work is currently directed toward reducing capital and operating costs. T h e capital investment has already been reduced by about 2 0 % over the original design.

A further cost

reduction now seems possible, thereby increasing the application possibilities of this process.


Literature

Cited

1. Gamson, B. W., Elkins, R. H . , Chem. Eng. Progr. (1953) 49, 203. 2. Palm, J. W., Hydrocarbon Process. (1972) 51 (3), 105. 3. Bailleul, M . R., Berthier, P., Guyot, G., Proc. Ann. Conv., Nat. Gas Process. Ass., 49th (March 17-19, 1970) 89. 4. Nicklin, T., Brunner, E . , "Hydrogen Sulfide Removal by the Stretford Liquid Purification Process," Inst. Gas Eng. Meetg. (May 16-19, 1961), Publication No. 593. 5. Nicklin, T., Brunner, E . , Hydrocarbon Process. Petrol. Refiner (1961) 40(12), 141. 6. Ellwood, P., Chem. Eng. (1964) 71(15), 128. 7. Oil Gas J. (1971) 69(41), 68. 8. Nicklin, T., et al., U.S. Patent #2,997,439 (Aug. 22, 1961). 9. Ibid., #3,035,889 (May 22, 1962). 10. Ibid., #3,097,926 (July 16, 1963). RECEIVED April 4, 1974. The process is patented in the U.S. (#3,752,877, D. K. Beavon, Aug. 14, 1973) and Canada (#916,898, D. K. Beavon, Dec. 19, 1972), and patents are being sought in other countries. The process is licensed by the Union Oil Co. of California and is designed by the Ralph M . Parsons Co.


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