Recent Developments in Sulfur Production from Hydrogen Sulfide

3 Recent Developments in Sulfur Production from Hydrogen Sulfide-Containing Gases J. Β. H Y N E

Downloaded by UNIV OF PITTSBURGH on June 26, 2014 | http://pubs.acs.org Publication Date: March 29, 1982 | doi: 10.1021/bk-1982-0183.ch003

Alberta Sulphur Research Ltd. and University of Calgary, Department of Chemistry, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4

Since the 1950's there has been a remarkable growth in sulfur production from the hydrogen sul fide of n a t u r a l and refinery gases. Once a minor source of world brimstone, sour gas now makes a very significant c o n t r i b u t i o n t o world sulfur pro duction. The e a r l y p i o n e e r i n g work on sour gas prod u c t i o n and conversion t o sulfur has been f o l l o w e d in the last ten years by some remarkable develop ments in both recovery and p r o c e s s i n g . Deep, h o t , high pressure sour gas d e p o s i t s , p r e v i o u s l y avoided as t e c h n o l o g i c a l l y unworkable, can now be produced using modern m e t a l l u r g y and a n t i c o r r o s i o n t e c h niques. Improved methods f o r s e p a r a t i n g the a c i d gases (H S, CO ) from the hydrocarbon component using kinetic r a t h e r than e q u i l i b r i u m c o n t r o l o f sweetening s o l u t i o n l o a d i n g have been developed. The efficiency of the Claus Process, long the means of conversion of H S t o sulfur, has been i n c r e a s e d through improvements in r e a c t i o n furnace, c a t a l y s t bed and computerized feed composition c o n t r o l l e a d i n g t o recovery efficiencies in excess o f 98%. Recent development of a Claus Process under pressure may yield f u r t h e r important improvements. Add on tail gas clean-up processes have f u r t h e r reduced p l a n t e f f l u e n t in response t o environmental pro t e c t i o n requirements. The sulfur product itself has a l s o been the subject o f recent research and development w i t h improvements in h a n d l i n g , storage and t r a n s p o r t a t i o n o f both liquid and new types of formed, solid sulfur. 2

2

2

0097-6156/82/0183-0037$05.00 / 0 © 1982 American Chemical Society

In Sulfur: New Sources and Uses; Raymont, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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SULFUR: NEW SOURCES A N D USES

Recovered sulfur has grown s t e a d i l y in importance as a world source of brimstone s i n c e the mid 1950's. Between 1965 and 1977 recovered sulfur's share of world supply grew from 18% t o 30% (1) and continues to grow in r e l a t i o n to Frasch mined and other forms of n a t i v e elemental sulfur. A l l o f t h i s has meant a r a p i d growth in the number o f sulfur recovery f a c i l i t i e s and new developments in the v a r i o u s techniques and processes a s s o c i a t e d w i t h the industry. I t is the purpose of t h i s paper t o review some o f these developments r e l a t i n g to the p r o d u c t i o n o f sulfur from hydrogen s u l f i d e c o n t a i n i n g gases. These gases can be n a t u r a l or man made; the former being found as sour n a t u r a l gas d e p o s i t s around the w o r l d , the l a t t e r the r e s u l t of h y d r o d e s u l f u r i z a t i o n o f sulfur-containing crude oil in modern r e f i n e r y p r a c t i c e . Although these h i g h e r sulfur c o n t a i n i n g crudes (>0.5%) have i n c r e a s e d in volume in recent years (from 36% to 46% o f U.S. supply between 1973 and 1978 (2)) the t o t a l tonnage o f sulfur recovered from oil is l e s s than t h a t from sour n a t u r a l gas. Canada alone provided some 45% of w o r l d production of recovered sulfur in 1976 v i r t u a l l y a l l from sour n a t u r a l gas. Thus the emphasis in t h i s review w i l l be on recovery of sulfur from sour n a t u r a l gas sources. Apart from the i n i t i a l d i s c u s s i o n of development o f these sources, however, the balance of the review a p p l i e s e q u a l l y t o sulfur recovered from man made gases. The sour n a t u r a l gas sulfur recovery i n d u s t r y covers v i r t u a l l y the e n t i r e gamut o f chemistry. From the sour gas r e s e r v o i r to the Claus p l a n t end product problems are encountered in t h e r modynamics, k i n e t i c s , c o r r o s i o n , c a t a l y s i s , redox, rheology and the environment - p l u s a l l the r e s t ! I n reviewing recent d e v e l opments in such a wide ranging f i e l d it is only p o s s i b l e t o s e l e c t examples. I tishoped, however, that these h i g h l i g h t s w i l l serve t o i l l u s t r a t e the dynamism of the i n d u s t r y in recent years and the progress it has made in developing a new source o f one o f the world's most b a s i c and e s s e n t i a l elements in an environmentally acceptable manner. The f o u r major s e c t i o n s o f the review cover developments in new sour gas Sources, improvements in P r o d u c t i o n techniques a s s o c i a t e d w i t h the s e p a r a t i o n and conversion of H2S to elemental sulfur, aspects of the Environmental impact and i t s m i n i m i z a t i o n and, f i n a l l y , new developments in the forming and h a n d l i n g o f the sulfur Product i t s e l f . Sources The f i r s t major source o f recovered sulfur from H 2 S c o n t a i n ing n a t u r a l gas was the Lacq f i e l d in southern France developed by SNPA in the mid 1950's. This 15% H 2 S c o n t a i n i n g gas stream was the forerunner of many subsequent sour gas developments around the w o r l d . I n the e a r l y days o f sour gas p r o d u c t i o n the major product sought a f t e r was the methane; hydrogen s u l f i d e and


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the sulfur d e r i v e d from it was very much a by product, n o t i n f r e q u e n t l y r e f e r r e d t o as a waste product! The i n t e r v e n i n g years t o 1981 have seen s e v e r a l world market sulfur demand c y c l e s which have e f f e c t e d the r a t e o f acceptance of these new sulfur sources but s l o w l y the recovered sulfur product has taken i t s p l a c e alongside other w o r l d sulfur sources such as Frasch mined and p y r i t e s . I n very recent times there have even been moves t o re-open very sour gas w e l l s as sulfur w e l l s as world demand f o r t h i s key commodity has grown and p r i c e s have crossed $100/tonne FOB the p l a n t gate. H2S Rich Deposits. Some examples of H 2 S r i c h n a t u r a l gas w e l l s around the world are l i s t e d in Table 1. While some of these sources are r e l a t i v e l y w e l l e s t a b l i s h e d (e.g. France, Canada, West Germany) many new h i g h H 2 S gas d e p o s i t s a r e now being developed. I n the USSR the Orenburg sour gas f i e l d s have been in production f o r some years but the new c e n t r a l A s i a n deposits a t Astrakhan a r e only now being brought on stream. Deep d r i l l i n g i n t o the sediments o f the M i s s i s s i p p i b a s i n in the southern USA is uncovering hot, h i g h p r e s s u r e , h i g h H 2 S containing gases that present some new challenges in m e t a l l u r g y and phase behaviour. Most r e c e n t l y of a l l the Chinese have discovered and are now c o n s i d e r i n g the development o f a r e l a t i v e l y shallow (7,000') and very c o o l (25°C) sour gas r e s e r v o i r a t Zhaolanzhuang south of Peking which may c o n t a i n l e v e l s o f H 2 S o f up t o 90%. Because o f the low r e s e r v o i r temperature and pressure changes during p r o d u c t i o n some i n t e r e s t i n g phase behaviour problems may be encountered. Table I Some H2S R i c h Sour Gas F i e l d s Location

% H?S

France

(Lacq)

16.0

W. Germany (Varnhorn)

22.4

Canada (Harmattan)

53.5

Canada (Bearberry)

90.0

(Panther R i v e r )

70 - 80

U.S.A. ( M i s s i s s i p p i )

25 - 45

(Smackover) USSR (Astrakhan)

22.5

China (Zhaolanzhuang)

60 - 90

These new developments of h i g h H S n a t u r a l gas come a t a time when the world sulfur market is p a r t i c u l a r l y bouyant, s u f f i c i e n t l y 2




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so to encourage re-examination of some o l d e r high H2S deposits p r e v i o u s l y not considered economic because of depressed sulfur p r i c e s . I n t h i s category are some Western Canadian f i e l d s such as Bearberry, Panther R i v e r and Hunter V a l l e y a l l in deeply b u r i e d f o o t h i l l s formations w i t h r e l a t i v e l y high temperatures and pressures. Some of the t e c h n i c a l problems a s s o c i a t e d w i t h the development of such h i g h H 2 S c o n t a i n i n g r e s e r v o i r s w i l l be discussed below. Recovered sulfur sources in Middle East c o u n t r i e s have a l s o been developed r e c e n t l y . Most important o f these is the Saudi Arabian gas recovery program at B e r r i , Shedgum and Uthmaniyah which w i l l i n i t i a l l y add some 4,000 tons/d t o recovered sulfur production. This a d d i t i o n a l 1.5 m i l l i o n tons/annum of recovered sulfur w i l l f u r t h e r enhance the growing dominance o f recovered sulfur in the t o t a l world market p i c t u r e . S u l f u r D e p o s i t i o n . The development of high H S c o n t a i n i n g n a t u r a l gas resources has not been without i t s problems and a few of these w i l l be touched on here. High H 2 S c o n t a i n i n g gases under pressure are an e x c e l l e n t " s o l v e n t " f o r elemental sulfur which is o f t e n found in a s s o c i a t i o n w i t h sour n a t u r a l gas r e s e r v o i r s . The so c a l l e d s o l u b i l i t y o f sulfur in such f l u i d s has been the s u b j e c t o f s t u d i e s by authors such as Kennedy and Wieland (_3) and Brunner and W o l l (4) . F i e l d s t u d i e s (.5,6. ,Z) have shown t h a t as the pressure and temperature o f the sour gas f l u i d changes during the t r i p from bottom hole to w e l l head the sulfur c a r r y i n g power of the f l u i d decreases and sulfur deposit i o n can occur in the t u b u l a r s . Although p h y s i c a l s o l u t i o n of the sulfur in the high pressure f l u i d phase c o n t r i b u t e s t o the s o l v e n t c a r r y i n g c a p a b i l i t y it is now g e n e r a l l y accepted that hydrogen p o l y s u l f i d e formation and decomposition p l a y s a major r o l e in the d e p o s i t i o n phenomenon ( 8 ) . Thus as pressure and temperature change the p o s i t i o n of the p o l y s u l f i d e e q u i l i b r i u m moves 2

HS + S ^ ± H S sour deposited p o l y s u l f i d e gas f l u i d sulfur carried sulfur 2

x

2

x + i

e i t h e r p i c k i n g up o r d e p o s i t i n g the elemental sulfur. I f temperatures are above the m e l t i n g p o i n t of the sulfur, plugging w i l l not occur but below 120°C s o l i d sulfur may w e l l be deposited. While the most l i k e l y l o c a t i o n o f such d e p o s i t i o nisinthe p r o d u c t i o n t u b u l a r s c o n d i t i o n s of pressure and temperature can be encountered where the sulfur is deposited in the formation around bottom h o l e . When t h i s occurs formation damage can r e s u l t and p r o d u c i b i l i t y can be e f f e c t e d . Another f e a t u r e of the p o l y s u l f i d e e q u i l i b r i u m is that the forward and reverse r e a c t i o n s are not instantaneous. Thus the f l o w i n g system can be in meta-stable e q u i l i b r i u m because o f the


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41


i n a b i l i t y of the chemical e q u i l i b r i u m to t r a c k the pressure and temperature changes o c c u r r i n g in the f l o w i n g sour gas f l u i d . P r e d i c t i o n of l o c a t i o n of sulfur d e p o s i t i o n thus becomes a complex matter i n v o l v i n g H 2 S content, sulfur content, p r e s s u r e , temperature and flow r a t e s . Solvent Systems. The problem of sulfur d e p o s i t i o n in sour gas producing w e l l s has r e s u l t e d in the development of new s o l v e n t systems f o r t r e a t i n g plugged w e l l s o r f o r continuous c i r c u l a t i o n to prevent d e p o s i t i o n . The hazards of u s i n g carbon d i s u l f i d e as a sulfur s o l v e n t have r e s u l t e d in the development o f techniques based on e i t h e r d i s u l f i d e l i q u i d s (e.g. Merox) o r amines both of which are capable of combining c h e m i c a l l y w i t h the deposited sulfur and c a r r y i n g it t o the s u r f a c e ( 8 ) . RSSR

RNH

2

+

+ HS 2

S

^ RSS SR catalyst

x

+

X

S τ x

* RNH3 H S

X + 1

The mechanisms f o r these chemical s o l v e n t processes are now q u i t e w e l l understood. C o r r o s i o n problems downhole have been encoun tered in the i o n i c amine system (9) but the d i s u l f i d e oil s o l v e n t appears to be without such c o m p l i c a t i o n s and can c a r r y up to 60% w/w of sulfur. Handling the m a t e r i a l r e q u i r e s some i n s e n s i t i v i t y to malorodous f l u i d s ! Corrosion. The development of deep, h o t , h i g h pressure sour gas r e s e r v o i r s has brought w i t h it a s u i t e of new problemsinthe m a t e r i a l s area. S u l f i d e s t r e s s c r a c k i n g , hydrogen embrittlement and general c o r r o s i o n problems have been w e l l recognizedinthe sour gas i n d u s t r y f o r many years (10), but these and r e l a t e d m a t e r i a l f a i l u r e problems are s i g n i f i c a n t l y a m p l i f i e d when temperatures r i s e to near 200°C pressures to 16,000+ p s i and H 2 S content to 30% or h i g h e r . The s i t u a t i o n can be f u r t h e r compli cated i f connate b r i n e and carbon d i o x i d e are a l s o presentinthe production f l u i d (11). The c h l o r i d e i o n is b e l i e v e d to be i n v o l v e d in p i t t i n g c o r r o s i o n which can r e s u l t in f a i l u r e o f t u b u l a r s by puncture of the pipe w a l l (12, 13). Much recent work has been c a r r i e d out on the development o f s p e c i a l a l l o y m a t e r i a l s capable of w i t h s t a n d i n g the h o s t i l e environment found in deep, hot h i g h pressure sour gas w e l l s . Asphahani has r e c e n t l y surveyed (14) progress to date in the f i e l d w i t h p a r t i c u l a r reference to the C O 2 / H 2 S / C I environment. The inadequacy of t y p i c a l 410 type s t a i n l e s s s t e e l s in t h i s environment has l e d to a search f o r more r e s i s t a n t a l l o y m a t e r i a l s and high i r o n , n i c k e l base a l l o y s such as I n c o l o y 825 (30 Fe, 42 N i , 22 Cr, 3 Mo, 2 Cu) and H a s t e l l o y G (20 Fe, 49.5 N i , 22 Cr, 7 Mo, 1.5 Cu) appear to o f f e r best performances. They show a



42


general r e s i s t a n c e t o c o r r o s i v e a t t a c k and t o c h l o r i d e / s u l f i d e s t r e s s c r a c k i n g a t elevated temperatures. The presence o f CO2 can a l s o be t o l e r a t e d . The H a s t e l l o y G appears t o r e s i s t l o c a l i z e d p i t t i n g c o r r o s i o n b e t t e r than the Incoloy 825. Although the use o f i n h i b i t o r s can m i t i g a t e c o r r o s i o n t o some extent in such h o s t i l e environments (15) designing systems w i t h appropriate phase behaviour under bottom hole c o n d i t i o n s is d i f f i c u l t . To date oil based i n h i b i t o r c a r r i e r systems have been employed but these r e q u i r e l a r g e volumes o f f l u i d c i r c u l a t i o n and it is o f t e n d i f f i c u l t t o maintain a continuous oil wet surface on the t u b u l a r s . A w a t e r - i n h i b i t o r system would overcome many o f these problems s i n c e the sour gas f l u i d is already saturated w i t h water in the r e s e r v o i r . Such a system, however, remains t o be designed. U n t i l such an i n h i b i t o r system is developed the use o f the so c a l l e d " e x o t i c " a l l o y s f o r downhole tubulars in the new deep sour gas plays w i l l be r e q u i r e d . Exxon's 21,000' D.B. McDonald Smackover w i l d c a t in M i s s i s s i p p i (16) w i t h expected bottom hole c o n d i t i o n s of 400°F and 23,000 p s i and H S between 25% and 45% w i l l have an i n t e r n a l tubing s t r i n g of H a s t e l l o y C-276 w i t h a y i e l d s t r e n g t h o f over 150,000 p s i . This s t r i n g w i l l c a r r y cont i n u o u s l y c i r c u l a t i n g c o r r o s i o n i n h i b i t o r t o p r o t e c t the other t u b u l a r s . A t an estimated $42 m i l l i o n t o t a l d r i l l i n g , completion and t e s t i n g costs f o r such developments it is j u s t as w e l l that the gas w i l l b r i n g $3+ per MCF and the recovered sulfur $100+ per ton! 2

Production S e l e c t i v e Absorption in the Sweetening Process. I n recent years removal o f the a c i d gas (H2S, CO2) components from a gas stream has i n c r e a s i n g l y been by absorption in a s o l v e n t system c o n t a i n i n g amines. While non-reactive solvent sweetening processes are in use, the a b i l i t y of the b a s i c amine t o r e a c t chemica l l y w i t h the a c i d gas t o y i e l d water s o l u b l e s a l t s has favored the chemical sweetening system. Thus R NH

+

HS

* R NH

2

R NH

+

C0

R NH

2

2

2

2

2

2

2

+

+ HS~

+

+ HCO3

H0 2

base

acid

salt

The e q u i l i b r i a are r e a d i l y reversed by heating and the s a l t loaded aqueous amine s o l u t i o n s can be made t o r e l e a s e the c a r r i e d H2S and CO2 a f t e r s e p a r a t i o n from the hydrocarbon gas being sweetened. Normal p r a c t i c e has been t o allow the amine sweetening s o l u t i o n t o approach e q u i l i b r i u m l o a d i n g l e v e l s in the gas/ s o l u t i o n contactor. While v a r i o u s amine s o l u t i o n s show d i f f e r e n t


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43

e q u i l i b r i u m a f f i n i t i e s f o r CO2 and H 2 S the e q u i l i b r i u m s e l e c t i v i t y is not g r e a t . Recent work (17, 18) however, has shown t h a t i f the d i f f e r e n t i a l r a t e o f the H 2 S and CO2 r e a c t i o n s w i t h amines are utilized much greater s e l e c t i v i t y of a b s o r p t i o n can be achieved. Thus h i g h loadings and s h o r t contact times between gas and solvent favor the s e l e c t i v e n o n - e q u i l i b r i u m chemisorption of H 2 S over C O 2 . I t has been shown that s e l e c t i v i t y as h i g h as 27 t o 1 f o r H2S over CO2 can be obtained in methyldiethanolamine s o l u t i o n s (MDEA). This n o n - e q u i l i b r i u m l o a d i n g of the sweetening s o l u t i o n can be coupled w i t h new i n s t r u m e n t a t i o n f o r continuous monitoring o f the H S l o a d i n g l e v e l in the amine u s i n g s u l f i d e i o n s e l e c t i v e e l e c t r o d e s . This can s i g n i f i c a n t l y improve the e f f i c i e n c y o f both the a b s o r p t i o n and r e g e n e r a t i o n steps in the sweetening process and reduce the amount of CO2 in the a c i d gas feed to the f r o n t end furnace o f the sulfur p l a n t . As w i l l be seen l a t e r this improves the combustion c h a r a c t e r i s t i c s in the furnace and reduces the amount o f COS appearing in the Claus p l a n t t a i l gas stream.


2

The Front End Reaction Furnace. An e s s e n t i a l component o f any sulfur p l a n t based on the chemistry of the H 2 S / S O 2 redox r e a c t i o n is the r e a c t i o n furnace. Here a p o r t i o n of the feed H2S is o x i d i z e d by combustion w i t h a i r to y i e l d the SO2 which w i l l act as oxidant in the downstream c a t a l y s t u n i t s . While the general o v e r a l l thermodynamics of the combustion process in the f r o n t end furnace have long been appreciated there have been some important recent developments in our understanding of some of the more s o p h i s t i c a t e d aspects o f the combustion chemistry. These, in t u r n , have l e d t o h i g h e r sulfur conversions in t h i s i n i t i a l h i g h temperature r e g i o n of the sulfur p l a n t and lower l o a d i n g of the downstream c a t a l y s t beds. O v e r a l l conversions have thus been improved and the amount of unrecovered sulfur going to atmosphere in the i n c i n e r a t e d t a i l gas has been f u r t h e r reduced. These and other improvements have r e s u l t e d l a r g e l y from a b e t t e r understanding of the combustion process. The homogeneity of the gas mixture being burned has been improved by b e t t e r design of a c i d gas ( H 2 S , CO2) i n j e c t i o n n o z z l e s and, in some i n s t a n c e s the i n t r o d u c t i o n of i n t e r n a l furnace s t r u c t u r e s (checkered w a l l s ) to a s s i s t in mixing of the a i r and a c i d gas feeds. While the o v e r a l l chemistry of the combustion process was w e l l understood v i z . , sulfur

6H S 2

+

30

2

» 6H 0 2

+ 6S

production S0 production 2

X

— 2H S 2

+

30

2

> 2H 0 2

+

2S0

2

the r a t e c o n t r o l l i n g steps i n v o l v i n g a v a r i e t y of i n t e r m e d i a t e species were not known a t the h i g h furnace temperatures employed (19) . Rate data were a v a i l a b l e f o r many of these processes a t




44

25°C but e x t r a p o l a t i o n t o 1,000°C o r higher was not p o s s i b l e . This is an i n t e r e s t i n g i l l u s t r a t i o n o f the gap between the condi t i o n s chosen f o r the b a s i c research s t u d i e s and those of the p l a n t o p e r a t i o n . Recent research (20) has helped b r i d g e t h i s gap by f u r t h e r i n v e s t i g a t i o n o f r e a c t i o n r a t e s a t e l e v a t e d tempe r a t u r e s . The work o f SNPA (SNEA) in p a r t i c u l a r (21) has r e s u l ted in a much b e t t e r understanding o f the r e l a t i o n s h i p between residence time, temperature and extent o f r e a c t i o n w i t h i n the furnace. The p r o d u c t i o n o f COS in the f r o n t end r e a c t i o n furnace presents s p e c i a l problems s i n c e sulfur in t h i s form may be d i f f i c u l t t o remove in the downstream c a t a l y t i c beds under conditions that are optimal f o r the Claus redox r e a c t i o n between H2S and SO^ COS (and C S 2 ) were known t o be generated from hydrocarbon impuri t i e s c a r r i e d over in the a c i d gas feed thus the e f f i c i e n c y o f the up-stream sweetening process became an important f a c t o r . The r e a c t i o n of C O 2 , a common c o n s t i t u e n t o f the a c i d gas feed, w i t h H2S and/or sulfur under furnace temperature c o n d i t i o n s has a l s o been shown t o be an important source of COS. HS 2

3/2 S

+ C0 +

2

2C0

•> COS +

2

•> 2C0S

2

H 0 2

+ S0

2

Thus m i n i m i z i n g the CO2 content o f the a c i d gas feed, as f o r example by the s e l e c t i v e a b s o r p t i o n o f CO2 and H 2 S discussed p r e v i o u s l y , takes on added s i g n i f i c a n c e . Furnace temperatures have a l s o been shown to be important in c o n t r o l l i n g the formation of COS. While COS has l i t t l e e f f e c t on the downstream Claus c a t a l y s t e f f i c i e n c y i t s presence in the gas stream leads t o h i g h e r l o a d i n g of the r e d u c t i v e t a i l gas clean-up processes (e.g. SCOT, BSR, see environment) o r t o higher SO2 emissions in the stack gas. The recent developments regarding the c o n t r o l o f i t s formation in the f r o n t end furnace are thus a s i g n i f i c a n t c o n t r i b u t i o n t o the improvement o f e n v i r o n mental q u a l i t y c o n t r o l . Adequate c o n t r o l of the chemistry in the f r o n t end furnace can s i g n i f i c a n t l y e f f e c t the l i f e t i m e and e f f i c i e n c y o f the downstream c a t a l y s t beds in a sulfur p l a n t . Inadequate removal of Ce+ hydrocarbons from the a c i d gas feed can r e s u l t in c a t a l y s t f o u l i n g by polymeric m a t e r i a l s formed under furnace c o n d i t i o n s . Toluenes, ethylbenzenes and xylenes have been shown t o be p a r t i c u l a r l y troublesome in t h i s regard. Oxygen breakthrough i n t o the c a t a l y s t beds can a l s o shorten the e f f e c t i v e l i f e t i m e of the Alumina c a t a l y s t by s u l f a t i o n i . e . 3S0

2

+ 1.50

A 1

2

A1 0 2

2

( S 0 0

3

3

Thus the c o n t r o l o f the r a t i o o f a i r t o a c i d gas being fed t o the


3.

HYNE


45


f r o n t end furnace becomes a c r u c i a l parameter. S i g n i f i c a n t advances have been made in f r o n t end furnace feed r a t i o c o n t r o l through the use o f o n - l i n e gas chromatography w i t h automatic feed back to the r a t i o c o n t r o l l e r (22). Further recent improvements i n v o l v e the use o f o n - l i n e spectrophotometric d e t e c t o r s (UV, IR) which reduce maintenance and speed up response times (23, 24). In t h i s manner the p r e c i s e s t o i c h i o m e t r y r e q u i r e d f o r o p t i m i z i n g sulfur recovery in the Claus converters can be maintained. Recent improvements in the o p e r a t i o n o f the f r o n t end r e a c t i o n furnace have not only i n c r e a s e d sulfur recovery at t h i s stage in the p l a n t but have reduced the p r o d u c t i o n of u n d e s i r a b l e by-products which, u l t i m a t e l y , leads to a r e d u c t i o n in the e n v i r onmental impact of the processes. The Claus C a t a l y t i c Qmverters. These u n i t s represent the heart o f any sulfur recovery p l a n t . While the b u l k of the sulfur y i e l d may be obtained in the f r o n t end r e a c t i o n furnace the h i g h o v e r a l l recovery l e v e l s demanded by environmental r e g u l a t i o n s are l a r g e l y dependent on the e f f i c i e n c y of the Claus c a t a l y t i c converters. Recent developments in Claus converter e f f i c i e n c y can be d i v i d e d i n t o two c a t e g o r i e s ; improvements in process o p e r a t i n g technique and improvements in c a t a l y s t management. P a s k a l l (25) has r e c e n t l y reviewed the v a r i o u s m o d i f i c a t i o n s to the Claus process that r e s u l t in optimum sulfur recovery e f f i c i e n c y . O v e r a l l p l a n t conversion e f f i c i e n c i e s in the range o f 97% were considered to be the upper l i m i t a t the beginning of the 1970 s (26). While t h i s is a very r e s p e c t a b l e conversion e f f i c iency f o r an i n d u s t r i a l process the unrecovered 3% in a 2,000 tonne/d sulfur p l a n t represents 60 tonnes/d of sulfur l o s t , mainly to atmosphere as 120 tonnes/d of SO2. M o d i f i c a t i o n s t o the f o u r stage Claus converter t r a i n however, can r a i s e o v e r a l l conversions to over 98.5% thus h a l v i n g the sulfur l o s s t o the p l a n t t a i l gas. This e i t h e r reduces environmental impact or the load on t a i l gas d e s u l f u r i z a t i o n u n i t s that w i l l be discussed l a t e r . Among the most e f f e c t i v e o f the m o d i f i c a t i o n s to Claus opera t i n g procedure is accurate temperature c o n t r o l of the c a t a l y s t beds. Gamson and E l k i n s (27) in the e a r l y 1950's showed that e q u i l i b r i u m sulfur conversion e f f i c i e n c i e s in the c a t a l y t i c redox r e a c t i o n r i s e d r a m a t i c a l l y as operating temperatures are lowered toward the dewpoint of sulfur. While some h i g h l y e f f i c i e n t subdewpoint Claus type processes are now in use the b u l k of sulfur production from H2S s t i l l r e q u i r e s that the converters be operated above the dewpoint. C a r e f u l c o n t r o l o f converter bed temperature has, however, c o n t r i b u t e d to improved e f f i c i e n c i e s . This has in l a r g e p a r t r e s u l t e d from b e t t e r i n s t r u m e n t a t i o n o f the Claus t r a i n and e f f e c t i v e i n f o r m a t i o n feed back systems. A major component o f the 3% sulfur l o s s in e a r l i e r m u l t i stage Claus p l a n t s was e n t r a i n e d sulfur mist in the process gas stream. P r e c i s e c o n t r o l o f i n t e r s t a g e sulfur condenser temperaf




46

tures to ensure that the product l i q u i d sulfur temperature is as low as p o s s i b l e without f r e e z i n g has reduced sulfur vapor pressure and the amount of e n t r a i n e d sulfur m i s t . While it is o f t e n d i f f i c u l t to persuade operators to work w i t h l i q u i d sulfur a t temperatures as l i t t l e as 5 - 10 C above the f r e e z i n g p o i n t , especi a l l y under quick c h i l l w i n t e r c o n d i t i o n s , the b e n e f i t s in terms of reduced sulfur m i s t entrainment are q u i t e n o t i c e a b l e in f u r t h e r reducing the already s m a l l sulfur l o s s . I n t e r e s t i n g l y , the c o n t r o l o f l i q u i d sulfur product temperature near the f r e e z i n g p o i n t has become an important f a c t o r in maximizing the q u a l i t y o f the s p h e r i c a l l y formed sulfur products now being favored around the world (see Product s e c t i o n ) . Elsewhere in t h i s review we have commented on the problem o f COS p r o d u c t i o n as a r e s u l t of h i g h temperature r e a c t i o n s occurring in the f r o n t end furnace. S u l f u r in t h i s form is not subject t o conversion to elemental sulfur in the c a t a l y t i c redox Claus r e a c t i o n and thus appears as COS in the t a i l gas where it is i n c i n e r a t e d to S 0 thus adding to l o s s e s t o the environment. The COS and any C S can be hydrolyzed to H S which can then be conv e r t e d by the redox Claus r e a c t i o n . 2

2

2

COS 2H S 2

+

H 0 . ,—: 5 C 0 + H S * hydrolysis * + S0 > H 0 + 3/8 S 2

2

2

r e d Q x

2

2

8

The h y d r o l y s i s r e a c t i o n however, r e q u i r e s h i g h e r temperatures (650°F) than the optimum f o r the Claus redox. Nonetheless it has been c l e a r l y demonstrated in p l a n t s t u d i e s during the 1970 s that running the f i r s t converter bed at t h i s h i g h e r COS h y d r o l y s i s temperature and s a c r i f i c i n g some Claus redox e f f i c i e n c y that can be recovered in l a t e r beds does r e s u l t in an improved o v e r a l l sulfur recovery e f f i c i e n c y e s p e c i a l l y in four stage Claus converter systems. During the 1970's there was considerable research a c t i v i t y in the f u r t h e r i n v e s t i g a t i o n and improvement o f the Claus catalyst^ i t s use and regeneration (21, 28-31). A b e t t e r understanding o f the nature o f the a c t i v e s i t e in the alumina c a t a l y s t and i t s i n t e r a c t i o n w i t h S 0 and H S has r e s u l t e d in recommendations f o r improved operating procedures and c a t a l y s t regeneration techniques. Although there s t i l l remains some " a r t " in the p r o d u c t i o n of h i g h a c t i v i t y c a t a l y s t s , surface a r e a , pore s i z e and other f a c t o r s r e l e v a n t t o the a c c e s s i b i l i t y o f the reactant gases t o the c a t a l y t i c s i t e s are c l e a r l y of primary importance. Thus any deposit i o n o f product or by-product w i t h i n the c a t a l y s t pore s t r u c t u r e is u n d e s i r a b l e . I n recent years the r e l a t i o n s h i p between impure Claus feed c o n t a i n i n g hydrocarbons and c a t a l y s t l i f e t i m e has been w e l l demonstrated (32). Carbon of hydrocarbon polymer d e p o s i t i o n on the c a t a l y s t u s u a l l y r e s u l t s in b l o c k i n g access of the r e a c t ant gases to the i n t e r n a l c a t a l y t i c s i t e s . Product sulfur depos1

2

2



3.

HYNE

47


i t i o n w i t h i n the pore s t r u c t u r e can a l s o occur even a t temper atures s i g n i f i c a n t l y above the dewpoint due, in p a r t , t o the c a p i l l a r y f o r c e s i n v o l v e d . Past p r a c t i c e has been t o remove these d e a c t i v a t i n g d e p o s i t s by so c a l l e d " r e g e n e r a t i v e b u r n - o f f " where temperatures in excess of 600°C can be reached in the c a t a l y s t . I t has been c l e a r l y demonstrated that e x t e n s i v e and i r r e v e r s i b l e damage t o the c a t a l y s t can occur d u r i n g such procedures and they have been l a r g e l y r e p l a c e d by the "heat soak" technique. I n t h i s procedure the i n l e t temperature t o the c a t a l y s t bed is h e l d a t some 15 - 20 C above normal f o r s e v e r a l hours and it is g e n e r a l l y e f f e c t i v e in removing i n t r a p o r e r e t a i n e d sulfur. A major d e a c t i v a t i n g mechanism has been shown t o be the s u l f a t i o n o f the a c t i v e h y d r o x y l o r oxide i o n s i t e s by chemical r e a c t i o n w i t h the chemisorbed S 0 (29, 30). 2

A1 0 2

2A1 0 (S0 ) 2

3

+

2

3

+ S0

4S0

+

2

> A1 0 (S0 )

2

2

30

2

3

2

>2Α1 (80^) 2

3

I t should be noted that the s u l f a t i o n r e a c t i o n r e q u i r e s the presence of oxygen. This serves t o h i g h l i g h t the importance of a i r / a c i d gas r a t i o c o n t r o l in the f r o n t end furnace feed and the prevention of oxygen breakthrough i n t o the c a t a l y s t beds. This can be o f p a r t i c u l a r importance when oxygen enriched a i r feed is used in the f r o n t end r e a c t i o n furnace (31). Regeneration of the s u l f a t e d c a t a l y s t can be accomplished under reducing c o n d i t i o n s as opposed t o the p r e v i o u s l y p r a c t i c e d "regenerative b u r n - o f f " which is an o x i d a t i v e process and probably generates s u l f a t e on the c a t a l y s t . The reducing c o n d i t i o n s r e q u i r e d can be generated by going o f f s t o i c h i o m e t r i c r a t i o of the H S/S0 being fed t o the c a t a l y s t and r a i s i n g the temperature above normal Claus c o n d i t i o n s (33). This generates a reducing, excess H S atmosphere which reverses the s u l f a t i o n r e a c t i o n . 2

2

2

2Α1 (80^) 2

3

+

18H S 2

> 2A1 0 2

3

+

18H 0 2

+ 3S

8

Although the Claus c a t a l y t i c conversion is a h i g h l y e f f i c i e n t process as p r e s e n t l y employed in sulfur recovery p l a n t s the c o n t i n u i n g e f f o r t s t o reduce sulfur emissions t o atmosphere demand that the l a s t p o s s i b l e ounce of e f f i c i e n c y be squeezed from the process. Whether f u r t h e r s m a l l but c r i t i c a l improvementsinthe already h i g h sulfur recovery e f f i c i e n c y can be achieved by more f i n e tuning of the converters and t h e i r c a t a l y s t charge remains t o be seen. What cannot be accomplished in the c a t a l y t i c converters w i l l be achieved in the t a i l gas d e s u l f u r i z a t i o n processes. The Richard Process - A Claus A l t e r n a t i v e ? The Claus r e a c t i o n process is by f a r the most common f o r r e c o v e r i n g elemental sulfur from hydrogen s u l f i d e . This heterogeneous gas phase c a t a l y s i s over alumina o f the redox r e a c t i o n between H S and S 0 , 2

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 In Sulfur: New Sources and Uses; Raymont, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

2



48

however, has a number of problems, some of which have been noted above. Although s o l u t i o n s t o these problems are being found research e f f o r t has a l s o been d i r e c t e d toward developing q u i t e d i f f e r e n t r e a c t i o n c o n d i t i o n s f o r the redox r e a c t i o n . One such development is the so c a l l e d Richard Process (34) announced in 1980. In t h i s new process the H 2 S / S O 2 r e a c t i o n is c a r r i e d out in l i q u i d sulfur a t pressures in excess o f f i v e atmospheres. Typical Claus c a t a l y s t s are s t i l l employed but temperatures are lower (below the dewpoint of sulfur) and thus the redox r e a c t i o n occurs in the l i q u i d sulfur phase a t the s u r f a c e of the c a t a l y s t . Vapor l o s s e s due t o sulfur m i s t entrainment are reduced and i n t e r s t a g e condensers in the t r a d i t i o n Claus t r a i n are not r e q u i r e d thus a v o i d i n g w a s t e f u l heat t r a n s f e r problems. The authors c l a i m that o v e r a l l sulfur r e c o v e r i e s in excess of 99% are p o s s i b l e without the use o f t a i l gas c l e a n up u n i t s . We b e l i e v e that the key f e a t u r e in t h i s new process may be the presence of the l i q u i d sulfur phase i t s e l f . Wiewiorowski (35) in 1969 showed that the redox r e a c t i o n of H2S w i t h SO2 proceeded smoothly in l i q u i d sulfur in the presence of a base c a t a l y s t . One of the major thermodynamic b a r r i e r s t o the p r o d u c t i o n of sulfur ( S 8 ) a t Claus r e a c t i o n temperatures from molecules o f H2S and SO2 is that the product molecule contains e i g h t sulfur atoms and reagents only one each. Thus there is a c o n s i d e r a b l e entropy b a r r i e r in b u i l d i n g an SQ molecule from the monothio-reagents. There is some evidence t o suggest that the p o l y t h i o Se chain is b u i l t up g r a d u a l l y on the s u r f a c e o f the c a t a l y s t v i a the i n t e r mediacy of hydrogen p o l y s u l f i d e s ( H 2 S ) . When χ > 8 the S r i n g can break out from the H 2 S . Thus there is no need f o r the e n t r o p i c a l l y unfavorable simultaneous aggregation o f a l l r e a c t a n t species in a p a r t i c u l a r c o n f i g u r a t i o n as would be r e q u i r e d by the minimum s t o i c h i o m e t r i c a l l y balanced equation. X

8

X

16H S 2

+

8S0

2

^

3S

8

+

16H 0 2

When the r e a c t i o n medium is l i q u i d sulfur, as in the Richard Process, the H 2 S r e a c t a n t can r e a d i l y form hydrogen p o l y s u l f i d e e.g. H S + S =F=* H S +i 2

x

2

x

which can then undergo c a t a l y z e d redox r e a c t i o n w i t h SO2 t o form water and a new, l a r g e r sulfur molecule. s 2H 0 + S catalyst The presence o f the l i q u i d sulfur phase in the R i c h a r d process may f a v o r the formation o f the product l i q u i d sulfur by a i d i n g the formation of the i n t e r m e d i a t e hydrogen p o l y s u l f i d e b u i l d i n g b l o c k . The success o f a 5LTD demonstration p l a n t may w e l l determine whether f u t u r e sulfur recovery from H 2 S feed continues to be by 2H S 2

X + 1

+

S0

2

2

y


3.

HYNE


49


heterogeneous gas phase redox r e a c t i o n o r l i q u i d phase in sulfur under pressure. D i r e c t Thermal Decomposition of H 2 S . S u l f u r recovered from the hydrogen s u l f i d e removed from e i t h e r n a t u r a l o r r e f i n e r y gas streams is a u s e f u l and now v a l u a b l e by-product. The hydrogen of the H 2 S , however, has t r a d i t i o n a l l y been converted to water y i e l d i n g u s e f u l energy and an environmentally acceptable end product - water. The demand f o r hydrogen by the petrochemical and petroleum p r o c e s s i n g i n d u s t r i e s , however, continues t o i n c r e a s e e s p e c i a l l y as h y d r o d e s u l f u r i z a t i o n of h e a v i e r crudes becomes more commonplace. Recovery of both hydrogen and sulfur from H 2 S has thus been the s u b j e c t of renewed i n v e s t i g a t i o n in recent y e a r s . Work on the thermal decomposition of H2S has been reported by s e v e r a l workers (36, 37, 38). The key to an e f f e c t i v e economic process would appear t o l i e in f i n d i n g an e f f e c t i v e c a t a l y s t f o r the decomposition a t r e a d i l y a c c e s s i b l e temperatures and a means f o r s e p a r a t i n g the products from each other a t r e a c t i o n tempera ture. xH S 2

catalyst heat

xH

2

+

S

x

separation

Kotera e t a l (37) have patented a technique that r e q u i r e s conden s i n g out the product sulfur a f t e r each decomposition c y c l e . This i n v o l v e s s i g n i f i c a n t temperature swings in the process. Raymont (36) has shown that hydrogen ροlysulfides may be important i n t e r mediates in the mechanism j u s t as they appear t o be in the redox r e a c t i o n of the Claus process. Chivers e t a l (38) have e x p l o r e d the e f f i c a c y o f a number o f t r a n s i t i o n metal s u l f i d e s as c a t a l y s t s and the prospects f o r f i n e tuning t h i s a l l important c a t a l y t i c r o l e look promising. While these recent developments have not as y e t been f u l l y commercialized the p o s s i b i l i t i e s f o r producing two v a l u a b l e chemical commodities r a t h e r than one from H2S e x t r a c t e d from gas streams are encouraging. Environment The f a c t t h a t sulfur recovery processes do not achieve 100% e f f i c i e n c y means t h a t there is the p o t e n t i a l f o r l o s s t o the environment in the v i c i n i t y of the p l a n t . Over the l a s t decade there have been very s i g n i f i c a n t improvements in the sulfur recovery e f f i c i e n c y of t y p i c a l Claus processes but these have o f t e n been matched by i n c r e a s i n g l y r e s t r i c t i v e r e g u l a t i o n s r e g a r ding maximum p e r m i s s i b l e emission of sulfur t o the atmosphere. This has been e s p e c i a l l y t r u e in densely populated urban areas w i t h unusual m e t e o r o l o g i c a l c o n d i t i o n s (e.g. the Los Angeles Basin) but even in r u r a l s e t t i n g s concern regarding the long term


SULFUR: NEW


50

SOURCES AND

USES

e f f e c t s of a c i d r a i n have f u r t h e r heightened s e n s i t i v i t y to i n d u s t r i a l sulfur emissions. S u l f u r values emitted to the atmosphere from a sulfur recovery p l a n t are p r i m a r i l y in the form of sulfur d i o x i d e . This r e s u l t s from the combustion of a l l p l a n t t a i l gases in an i n c i n e r a t o r before v e n t i n g to atmosphere. Thus unconverted H 2 S , C S 2 , COS and unrecovered sulfur vapor in the t a i l gas are l a r g e l y o x i d i z e d to the common sulfur e f f l u e n t S O 2 . S u l f u r d i o x i d e is a l s o the form in which the sulfur content of many hydrocarbon f u e l s f i n a l l y reach atmosphere a f t e r combustion. Technologies f o r the removal of SO2 from f l u e gases have t h e r e f o r e r e c e i v e d a great d e a l of a t t e n t i o n during recent years (39) but more in regard to f u e l combustion e f f l u e n t d e s u l f u r i z a t i o n than in conne c t i o n w i t h sulfur recovery p l a n t s . Recent developments in d e s u l f u r i z i n g the e f f l u e n t s from sulfur p l a n t s have concentrated on the p l a n t t a i l gas before i n c i n e r a t i o n . This avoids the f u r t h e r d i l u t i o n of the e f f l u e n t stream w i t h n i t r o g e n from a i r used as the oxidant and many of the sulfur values are s t i l l in a reduced or at l e a s t unoxidized state. High E f f i c i e n c y Claus Reactions. One of the e a r l i e s t comme r c i a l processes f o r t a i l gas d e s u l f u r i z a t i o n was the S u l f r e e n Process developed by L u r g i and SNPA and f i r s t introduced in 1970 (40) as a r e t r o f i t to SNPA s two stage Claus u n i t in Lacq, France. The f i r s t North American u n i t was i n s t a l l e d at A q u i t a i n e Canada's Ram R i v e r p l a n t in A l b e r t a (41). The S u l f r e e n u n i t s r a i s e d the o v e r a l l sulfur recovery in these two-stage Claus u n i t s to over 98% comparable to the w e l l tuned 4-stage Claus converter t r a i n s discussed p r e v i o u s l y . The p r i n c i p l e upon which the S u l f r e e n u n i t s are based is the h i g h e q u i l i b r i u m conversion p o s s i b l e in the Claus redox r e a c t i o n at temperatures below the dewpoint of sulfur. C a t a l y s t used is now alumina although e a r l i e r v e r s i o n s of the process employed a c t i v a t e d carbon. Because of o p e r a t i o n below the sulfur dewpoint the c a t a l y s t becomes loaded w i t h sulfur from the redox r e a c t i o n between the low c o n c e n t r a t i o n H S and SO2 in the t a i l gas and r e a c t o r s are s e q u e n t i a l l y taken out of s e r v i c e f o r heat soak regeneration to remove sulfur. S u l f u r c o n c e n t r a t i o n l e v e l s of 2,000 ppm (0.2%) at the stack mouth are p o s s i b l e u s i n g the S u l f r e e n process. Using e s s e n t i a l l y the same sub-dewpoint Claus r e a c t i o n p r i n c i p l e the Cold Bed Absorption (CBA) process of Amoco (42) achieves the same l e v e l of t a i l gas d e s u l f u r i z a t i o n . The low temperature h i g h e f f i c i e n c y swing converters can be in l i n e r a t h e r than as a t a i l gas clean up add on u n i t . These developments and s i m i l a r t a i l gas d e s u l f u r i z a t i o n processes based on the Claus such as the Bumines C i t r a t e (43) IFP (44) and Clean A i r (45) processes are simply methods f o r f o r c i n g the Claus redox r e a c t i o n of the upstream converter u n i t s f u r t h e r to completion. Thus they i n c r e a s e sulfur recovery from H 2 S and 1

2


3.

HYNE


51

SO2 components o f the process gas stream. None, however, d i r e c t l y a t t a c k the problem of removal of other sulfur values in the gas such as C S 2 , COS and e n t r a i n e d sulfur m i s t . These sulfur components can e a s i l y exceed 1% of the t o t a l sulfur feed and f a i l u r e t o remove them from the process stream can f r u s t r a t e any attempt t o achieve 99%+ recovery even when the Claus r e a c t a n t s , H 2 S and S O 2 , have been reduced to low ppm l e v e l s . Reductive T a i l Gas Treatments. I t was l a r g e l y as a r e s u l t of the e f f o r t to achieve b e t t e r than 99% recovery that the reduct i v e t a i l gas d e s u l f u r i z a t i o n processes (46) were developed in the 1970 s. The two main methods are the Beavon S u l f u r Removal (BSR) (47) and the S h e l l Claus Off-Gas Treatment (SCOT) (48) processes. Both of these processes are now w i d e l y used as t a i l gas d e s u l f u r i z a t i o n u n i t s on sulfur recovery p l a n t s and can r e a d i l y achieve p o i n t source emission l e v e l s below 250 ppm and below 100 ppm i f necessary to meet r e g u l a t o r y standards. The f i r s t step in these h i g h l y e f f i c i e n t r e d u c t i v e t a i l gas processes is the r e d u c t i o n or h y d r o l y s i s o f a l l sulfur values t o H S i.e. = H S + 2H 0 + 3H S0


f

2

2

S

X

COS

cs

2

+ xH

2

2

2

2

=

xH S 2

+ H0

= C0

2

+ HS

+ 2H 0

= C0

2

+ 2H S

2

2

2

2

The r e a c t i o n s are c a t a l y z e d by cobalt-molybdate and u t i l i z e e x i s t i n g H in the process gas stream that is formed by thermal c r a c k i n g in the f r o n t end furnace, or make up hydrogen from water s h i f t , methane reforming o r the l i k e . The conversion of a l l e n t r a i n e d sulfur values to H2S is h i g h l y e f f i c i e n t . I n the BSR process the hydrogen s u l f i d e is then t r e a t e d by the S t r e t f o r d Process (49) which i n v o l v e s o x i d a t i o n t o elemental sulfur using aerobic oxygen in a s e r i e s of complex steps i n v o l v i n g a n t h r o q u i none d i s u l f o n i c a c i d and vanadate i o n redox couples. Although the s o l u t i o n o x i d a t i o n process is a complex one there is v i r t u a l l y no l i m i t t o the degree of sulfur removal a t t a i n a b l e . I n the SCOT process the hydrogen s u l f i d e from the r e d u c t i o n step is absorbed and concentrated in a t r a d i t i o n a l alkanolamine scrubbing process, regenerated by heat and returned to the Claus a c i d gas feed stream. More r e c e n t l y the BSR process has been m o d i f i e d by c o u p l i n g to the Selectox step (50). This r e p l a c e s the complex S t r e t f o r d by d i r e c t l y o x i d i z i n g some o f the produced H2S to SO2 a t low temperature over the new p r o p r i e t r y Selectox-32 c a t a l y s t . The SQ2 and H 2 S then r e a c t to form sulfur in the t r a d i t i o n a l Claus redox r e a c t i o n . While o v e r a l l conversions by the BSR/Selectox are somewhat l e s s than that p o s s i b l e w i t h the BSR ( S t r e t f o r d ) , values of 99.8% are claimed. 2



52


The recovered sulfur i n d u s t r y e x i s t s p r i m a r i l y as a r e s u l t of the n e c e s s i t y of removing sulfur values from hydrocarbon f u e l s before combustion so that sulfur emissions to atmosphere are reduced. I n the case o f sour gas, the p r i n c i p a l source o f r e c o vered sulfur, the product that r e s u l t s from recovery o f the sulfur is clean-burning, n o n - p o l l u t i n g methane. I n the case o f r e f i n e r i e s h a n d l i n g h i g h sulfur crude the product is low sulfur gasol i n e and o i l s . Thus every ton of sulfur recovered is a ton that is not added t o the atmosphere. The recovery process i t s e l f however, is a l s o the s u b j e c t o f o p t i m i z a t i o n and recent developments in recovery e f f i c i e n c y have f u r t h e r ensured that the e n v i r onmental impact in the immediate v i c i n i t y of these d e s u l f u r i z a t i o n f a c i l i t i e s w i l l be minimized. Product Recovered elemental sulfur is one o f the purest b u l k commodi t i e s a v a i l a b l e on the market today. T y p i c a l analyses quoted f o r b u l k shipments are 99.95% p u r i t y w i t h ash, carbon and other contaminant l e v e l s in the 10 - 100 ppm range. This is a remarkably pure b u l k commodity f o r a 5ç/lb p r i c e . What them remains to be developed by way o f improvement of product q u a l i t y ? While a s i g n i f i c a n t p o r t i o n of the elemental sulfur moving in world trade is handled, shipped and s t o r e d in the l i q u i d form, the b u l k of the export shipments, e s p e c i a l l y from Canada, Poland and soon from Saudi A r a b i a , move in the s o l i d i f i e d s t a t e . The l a s t decade has seen some important changes in the form of the s o l i d i f i e d sulfur p a r t l y t o improve h a n d l i n g c h a r a c t e r i s t i c but, more i m p o r t a n t l y , to minimize environmental impact. 1

S l a t e d S u l f u r . U n t i l the l a t e I 9 6 0 s s o l i d elemental sulfur was recovered from s t o c k p i l e b l o c k s by e x c a v a t i o n ( f r o n t end l o a d e r s , b u l l dozers, etc.) and shipped in the r e s u l t i n g "crushed b u l k " form. I n open r a i l r o a d gondola cars and exposed s t o c k p i l e s at t r a n s f e r p o i n t s , the dust a s s o c i a t e d w i t h the crushed b u l k form was g e n e r a l l y regarded as environmentally u n d e s i r a b l e . By the l a t e I 9 6 0 s o r e a r l y 1970 s, the s l a t e d form of s o l i d sulfur had been w i d e l y adopted throughout the w o r l d f o r the storage and t r a n s p o r t a t i o n o f the element and r e s u l t e d in s i g n i f i c a n t improvements in environmental impact. Formed by c o o l i n g and s o l i d i f y i n g a t h i n sheet o f l i q u i d on a water cooled moving b e l t the f r e s h s l a t e d i d indeed have a r e s i s t a n c e to breakdown that r e s u l t e d in reduced dust formation, but a f t e r ageing the e s s e n t i a l l y two dimensional s o l i d form was s t i l l q u i t e a f r i a b l e m a t e r i a l . 1

f

New Forms. Even a t the time of i n t r o d u c t i o n of s l a t e it was recognized that the best geometric form f o r f r i a b l e s o l i d sulfur was a s p h e r i c a l p a r t i c l e e n c l o s i n g maximum volume in minimum surface area and w i t h l e a s t o p p o r t u n i t y f o r fulcrum e f f e c t s t o increase rupture f o r c e s . A number of methods f o r forming l i q u i d


3.

HYNE

Sulfur Production from Hydrogen Sulflde-Containing Gases

53

sulfur i n t o s o l i d s p h e r i c a l or pseudo s p h e r i c a l p a r t i c l e s were devised and d u r i n g the 1970's e x t e n s i v e t e s t i n g and e v a l u a t i o n o f these forms was c a r r i e d out (51). The Canadian i n d u s t r y a l s o e s t a b l i s h e d "premium product" s p e c i f i c a t i o n s which attempted t o i d e n t i f y minimum values f o r c e r t a i n c r i t i c a l p r o p e r t i e s o f the formed m a t e r i a l . These i n c l u d e d f r i a b i l i t y as measured by s t a n dardized tumbling t e s t s , angle of repose (storage f a c t o r ) , f l o w a b i l i t y , u n i f o r m i t y , water r e t e n t i o n and the l i k e . By the l a t e 1970 s the many e n t r i e s in the sulfur forming f i e l d had been reduced t o a few prime candidates. Some o f these together w i t h the general f e a t u r e s of the process by which they are made are noted in Table I I . Downloaded by UNIV OF PITTSBURGH on June 26, 2014 | http://pubs.acs.org Publication Date: March 29, 1982 | doi: 10.1021/bk-1982-0183.ch003

f

Table I I MECHANISMS OF SULFUR FORMING

ι DRY PROCESSES

WET PROCESSES

LIQUID DROPLET

FLETCHER SUPEL CAMBRIAN

SPRAY INJECTION

POPCORN NUGGET

LIQUID DROPLET

POLISH AIR PRILLS STEARNS-ROGERS PRILLS

BUILD-UP FROM NUCLEUS PEC PROCOR GX S Τ AMI CARBON

COMMERCIAL FORMS

The new sulfur forming processes can be c l a s s i f i e d i n t o two primary c a t e g o r i e s - wet processes u s i n g water as the c o o l i n g medium f o r the l i q u i d sulfur and dry processes in which a i r is the heat s i n k i n t o which the l i q u i d sulfur heat flows d u r i n g s o l i d i f i c a t i o n . Although the wet processes are i n h e r e n t l y simpler and l e s s c a p i t a l i n t e n s i v e the product r e t a i n s c o n s i d e r a b l e amounts o f water which makes it l e s s a t t r a c t i v e t o customers and introduces some c o r r o s i o n problems. The wet product can, o f course, be d r i e d but the a d d i t i o n a l c o s t s i n v o l v e d make l a r g e scale drying a less a t t r a c t i v e proposition. f

Premium Products. By the l a t e 1970 s two dry process forms, P o l i s h A i r P r i l l s and Procor GX granules were emerging as the f r o n t runners in the premium product s t a k e s . Although a i risthe common heat s i n k in both o f these processes the mechanism o f



54

formation is very d i f f e r e n t . The P o l i s h P r i l l (52) developed in the l a t e I960 s a t the P o l i s h sulfur mining center o f Tarnobrzeg, is a m o d i f i e d a i r p r i l l i n g technique s i m i l a r t o those developed for urea and other f e r t i l i z e r m a t e r i a l s . A spray o f l i q u i d d r o p l e t s flows downward counter c u r r e n t t o an upward flow o f c o o l i n g a i r s o l i d f y i n g the d r o p l e t s i n t o s p h e r i c a l p a r t i c l e s . Because the s o l i d i f i c a t i o n occurs from the o u t s i d e inward and because s o l i d sulfur has a s m a l l e r specific volume than l i q u i d the s o l i d i f y i n g sphere tends t o c o l l a p s e inward on i t s e l f and a s m a l l h o l e o f t e n develops l e a d i n g i n t o a more c r y s t a l l i n e (slow inner c o o l i n g ) hollow c e n t r e . Provided the outer s h e l l is w e l l formed and amorphous the s m a l l (1.5 - 3.5 mm diameter) p r i l l has e x c e l l e n t r e s i s t a n c e t o r u p t u r e . The key f e a t u r e o f the p r i l l i n g process is the avoidance o f s u p e r c o o l i n g o f the l i q u i d d r o p l e t during i t s decent through the p r i l l i n g tower. This is achieved by ensuring that the upward f l o w i n g a i r contains n u c l e a t i n g sulfur dust which impacts on the l i q u i d sulfur d r o p l e t s and initiates solidification. The Procor g r a n u l a t i n g process (53) operates on the p r i n c i p l e of s e q u e n t i a l b u i l d up o f l a y e r s of sulfur on t o a seed nucleus. A r o t a t i n g g r a n u l a t i n g drum w i t h the a x i s a t a s l i g h t angle c a r r i e s the growing p a r t i c l e s s e v e r a l times through a spray o f l i q u i d sulfur as the p a r t i c l e s move down the i n c l i n e d a x i s o f the drum. Each a p p l i c a t i o n o f l i q u i d sulfur i n c r e a s e s the s i z e o f the granule u n t i l the d e s i r e d diameter is achieved. C r u c i a l t o the q u a l i t y of the r e s u l t i n g granule is the bonding between successive l a y e r s o f the sulfur. This r e q u i r e s accurate c o n t r o l of temperature and time i n t e r v a l between a p p l i c a t i o n s t o ensure that each subsequent l i q u i d sulfur a p p l i c a t i o n has s u f f i c i e n t heat content t o fuse e f f e c t i v e l y on t o the p r e c u r s o r s u r f a c e . I f t h i s is not achieved the product has onion s k i n p r o p e r t i e s and f r a c t ures by s p a l l i n g . Both processes are now in use in Canada as w e l l as elsewhere in the w o r l d . European and South A f r i c a n s a l e s have been negoti a t e d f o r Procor GX and the P o l i s h A i r P r i l l has been s e l e c t e d for i n s t a l l a t i o n in Saudi A r a b i a . The products represent a very s i g n i f i c a n t improvement in dust s u p r e s s i o n in the h a n d l i n g , storage and t r a n s p o r t a t i o n o f s o l i d elemental sulfur and w h i l e the processes r e q u i r e c a r e f u l f i n e tuning and o p e r a t i o n a l monitor i n g they are c l e a r l y another c o n t r i b u t i o n to the improvement o f the environmental impact o f the recovered sulfur i n d u s t r y .


1

Conclusion The recovered sulfur i n d u s t r y has come o f age in the l a s t two decades and is now a major world source o f t h i s e s s e n t i a l element* Much progress has been made in the improvement o f v a r i o u s process s t e p s , p a r t i c u l a r l y those r e l a t e d t o o v e r a l l recovery e f f i c i e n c y . These improvements continue t o reduce u n d e s i r a b l e process l o s s e s to atmosphere and the environmental impact of the i n d u s t r y . As


3.

HYNE


55

s o c i e t y ' s continued demand f o r hydrocarbon f u e l s f o r c e s g r e a t e r u t i l i z a t i o n of h i g h e r sulfur c o n t a i n i n g sources the sulfur recovery i n d u s t r y w i l l grow in both s i z e and importance but the i n d u s t r y w i l l continue t o t r y t o meet the d u a l goals of energy s u f f i c i e n c y and environmental p r o t e c t i o n .


Literature Cited 1. Newton, B.F., Hydrocarbon Processing, 1978, Jan., 1981-184. 2. Oil & Gas J. Newsletter, 1978, Mar. 3. Kennedy, H.T., Wieland, D.R., Petrol. Trans. AIME, 1960, 219, 166. 4. Brunner, E., Woll, W., BASF.AG. WAA D-6700, 1977, Ludwigshafen, W. Germany. 5. Hyne, J.B., Oil & Gas J., 1968, Nov., 107-133. 6. Kur, CH., J. Pet. Tech., 1972, 1142. 7. Lubben, Η., Proc. World Pet. Congress, 1975, 389-399. 8. Hyne, J.B., Derdall, G.D., World Oil, 1980, Oct., 111-120. 9. Lyle, F.F., Lechler, S., Brandt, J., Blount, F.E., Snavely, S.E., Materials Performance, 1978, 17 (1), 24. 10. Adams, N., Oil & Gas J., 1980, Mar. 3, 55-62. 11. Mehdizakeh, P., McGlasson, R.L., Landers. J.E., Corrosion, 1966, 22, 325-334. 12. Bernhardsson, S.O., Oredsson, J., Tynell, Μ., Corrosion/80 (NACE), 1980, Mar. 3-7, Chicago, Paper #14. 13. Herbsleb, G., Corrosion/80 (NACE), Mar. 3-7, Chicago, Paper #13. 14. Asphahani, A.I., Corrosion/80 (NACE), Mar. 3-7, Chicago, Paper #12. 15. Place, M., Soc. Pet. Eng., Sept. 1979, 54th Annual Tech. Conf., Las Vegas SPE 8310. 16. Leonard, J., World Oil, Dec. 1980, 109-112. 17. Vidaurri, F.C., Kahre, L.C., Hydrocarbon Processing, Nov. 1977, 333-337. 18. Goar, B.G., Oil & Gas J., May 5 1980, 240-242. 19. Hyne, J.B. Gas Proc./Can., Mar. April. 1972, 12-16. 20. Kerr, R.K., Berlie, E.M., Energy Proc./Can., 1977, 69 (5), 42-46. 21. Grancher, P., Hydrocarbon Processing, Sept. 1978, 258-262. 22. Hamilton, T., Mathes, Α., Little, Μ., Strong, C., Oilweek, Jan. 1976, 39-40. 23. Beamish, Μ., Oilweek, 1978, 28, 28-40. 24. Saltzman, R.S., Hune, E.B., ISA Transactions, 1973, 12, 103-107. 25. Paskall, H.B., Capability of Modified Claus Process, Dept. of Energy and Nat. Resources, Alberta Report, published by Western Research and Development, March 1979. 26. Chalmers, W.W., Becker, H.W., Rankine, R.P., Pacific N.W. Air Pollution Assoc, Mtg., Calgary, Nov. 1971, Paper 71-AP-8


SULFUR: NEW SOURCES AND USES

56

27. 28. 29. 30. 31. 32.


33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.

Gamson, B.W., E l k i n s , R.H., Chem. Eng. Progress, 1953, 49, 213-215. Pearson, M.J., Hydrocarbon P r o c e s s i n g , Feb. 1973, 81-85. D a l l a Lana, K.G., Energy Proc./Can., Mar.Apr. 1978, 34-38. G r a u l i e r , M., Papee, D., Energy Proc. Can., Mar.-Apr. 1974. Gray, M.R., Svrcek, W.Y., Gas C o n d i t i o n i n g Conference, U n i v e r s i t y of Oklahoma, Norman, Oklahoma, March 2-4, 1981. Norman, W.S., Gas C o n d i t i o n i n g Conference, March 1976, Norman, Oklahoma. Goar, B.G., Hydrocarbon Proc. J u l y 1974, 129-132. J a g o d z i n s k i , R.F., K e r r , R.K., paper Can. Soc. Chem. Eng. Conf., Sept. 1980, Edmonton, A l b e r t a . Wiewiorowski, R.K., U.S. Patent #3,447,903, 1969. Raymont, M.E.D., Hydrocarbon Proc. 54, 1975, 139 and Ph.D. T h e s i s , Univ. of Calgary, 1974. K o t e r a , Y., I n t . J. Hydrogen Energy, 1976, 1, 219 and U.S. Patent 3,962,409, 1976. C h i v e r s , T., Hyne, J.B., Lau, C., Int. J. Hydrogen Energy, 1980, 5, 499-506. Hyne, J.B., Oil & Gas J., Aug. 1972, 64-67. Guyot, G., M a r t i n , J.E., Paper Can. Nat. Gas Proc. Assoc. Mtg., June 1971, Edmonton, A l b e r t a . Morin, M.M., P h i l a r d e a u , Y.M., Energy Proc./Can., 1977 69, ( 5 ) , 30-40. Goddin, C.S., Hunt, E.B., Palm, J.W., Hydrocarbon Proc., 1974, 53, ( 1 0 ) , 122-124. Madenberg, R.S., Seesee, T.A., Chem. Eng., J u l y 14, 1980 and Chem. Eng. News, June 14, 1971. U.S. Patent, 3,598,529, Aug. 10, 1971. Thomas, G.L., Plum, E., Paper 71st N a t i o n a l AlChE Mtg., Feb. 1972 20-23. Semrau, K., Advances in Chem., S e r i e s #139, 1975, 1-21. Beavon, D.K., F l e c k , Advances in Chem., S e r i e s #139, 1975, 93-99. Swaim, D.C., Advances in Chem. S e r i e s #139, 1975, 111-119. Beavon, D.K., Ellwood, P., Chem. Eng., July 1964, 128. Beavon, D.K., Hass, R.H., Muke, B., Oil & Gas J., Mar. 12, 1979, 76-80. Hyne, J.B., Hydrocarbon Proc., Apr. 1977, 125-128. Leszczynska, H., Sulphur #132, Mar.Apr., 1976, 44-46. Procor Sulphur S e r v i c e s D i v i s i o n , Procor GX Process D e s c r i p t i o n , 5622 B u r l e i g h Cres., S.E., Calgary, A l b e r t a .

RECEIVED October 5,

1981.


Recent Developments in Sulfur Production from Hydrogen Sulfide

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