Sulfur: New Sources and Uses - American Chemical Society

2 Sulfur Recovery from New Energy Sources D. K. F L E M I N G

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 2, 2017 | http://pubs.acs.org Publication Date: March 29, 1982 | doi: 10.1021/bk-1982-0183.ch002

Institute of Gas Technology, Chicago, IL 60616

This paper is presented as an overview of the t e c h n i c a l aspects of the sulfur question in s y n f u e l s production. In g e n e r a l , the d i s p o s i t i o n of sulfur species in synf u e l s processes is a complex f u n c t i o n of •

Feedstock type - and the chemical forms o f in the specific feedstock utilized

•

Primary feedstock p r o c e s s i n g

•

Synfuels product upgrading systems employed.

sulfur

S u l f u r recovery t o a u s e f u l by-product can range from very low t o very h i g h f r a c t i o n s o f the total sulfur in the feed, depending upon the combinations of the above f a c t o r s in a specific s y n f u e l s facility. The d i s c u s s i o n examines the d i s p o s i t i o n of sulfur, presents a v a i l a b l e sulfur removal and recovery techniques, and o u t l i n e s p o s s i b l e problems that might be encountered in the t r a n s f e r of proved sulfur technology t o these new energy processes.

T h i s paper is designed as a t e c h n i c a l overview o f the sulfur question in s y n f u e l s production. I tisnot intended as a d e t a i l e d l i t e r a t u r e review; r a t h e r , the d i s c u s s i o n is a c o m p i l a t i o n of eng i n e e r i n g experience in the f i e l d . I n the context of the sulfur symposium, the paper is d i r e c t e d a t those who are i n t e r e s t e d in sulfur i t s e l f and not, f o r example, in the environmental i m p l i c a t i o n s o f sulfur. In g e n e r a l , the feedstocks considered f o r the s y n f u e l s indust r y c o n t a i n s m a l l but s i g n i f i c a n t q u a n t i t i e s of sulfur. Indeed, one of the main d r i v i n g f o r c e s behind the development o f the s y n f u e l s i n d u s t r y is the conversion of sulfur-laden f u e l i n t o one that is low in sulfur content. 0097-6156/82/0183-0021 $05.00/0 © 1982 American Chemical Society

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

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SULFUR: NEW SOURCES A N D USES


The chemical processing techniques considered f o r s y n f u e l s flowsheet f o r the removal and recovery o f sulfur are s i m i l a r t o those employed in other i n d u s t r i e s - notably n a t u r a l gas sweeteni n g , petroleum h y d r o d e s u l f u r i z a t i o n , and coke oven gas treatment but w i t h c e r t a i n s i g n i f i c a n t d i f f e r e n c e s a t t r i b u t a b l e t o the operat i n g c o n d i t i o n s encountered in s y n f u e l s processing. I t is a l s o appropriate in t h i s i n t r o d u c t i o n t o address the question of the q u a n t i t y of sulfur to be expected as a by-product from the s y n f u e l s i n d u s t r y . Although the f r a c t i o n o f sulfur in the feedstock is low, the q u a n t i t i e s of feedstock r e q u i r e d t o support a f u l l s y n f u e l s i n d u s t r y are immense. Although scenarios d i f f e r , as do time frames, the most common s i z e p r o j e c t e d f o r the t o t a l s y n f u e l s i n d u s t r y is two m i l l i o n b a r r e l s / d a y o f f u e l - o i l equival e n t (BPDFOE)l. Such an i n d u s t r y would r e q u i r e about 50 energy r e f i n e r i e s , each producing 250 b i l l i o n Btu/day o f c l e a n energy product. Each of the f a c i l i t i e s is a massive undertaking. A coal r e f i n e r y o f t h i s s i z e w i l l consume the f u l l output o f three o f the l a r g e s t underground c o a l mines. An oil shale p l a n t w i l l r e q u i r e three times the feedstock o f a c o a l r e f i n e r y - even more i f lower grade, Eastern shale is considered. The c a p i t a l requirements are huge (but not when considered r e l a t i v e t o our imported f u e l b i l l ) . The manpower r e q u i r e d to design, c o n s t r u c t , operate, and provide feedstock to these energy r e f i n e r i e s w i l l a l s o be s i g n i f i c a n t . The investment o f these resources w i l l be necessary to provide energy that is in a p r e f e r r e d form and that can be consumed in an environmentally s a t i s f a c t o r y manner. The 2 m i l l i o n BPDFOE i n d u s t r y w i l l produce a by-product o f 5 to 10 m i l l i o n tons/year of sulfur; the exact production w i t h i n t h i s range is dependent upon the eventual mix o f feedstocks and technology that is employed. I f h i g h - s u l f u r Eastern c o a l were the predominant feedstock f o r these r e f i n e r i e s , even more by-product sulfur w i l l be produced, but much l e s s would be r e a l i z e d from oil shale r e f i n e r i e s . A s m a l l , but s t i l l s i g n i f i c a n t i n f l u e n c e upon the t o t a l sulfur production w i l l be the environmental r e g u l a t i o n s that are promulgated. Of course, environmentally, sulfur has a bad name - a name that is p o s s i b l y undeserved. Considering the h i s t o r i c a l odor and S 0 problems that have been a s s o c i a t e d w i t h sulfur and c o a l , however, the f u t u r e environmental r e g u l a t i o n s on the s y n f u e l s i n d u s t r y w i l l probably r e q u i r e a high recovery o f s u l fur. X

A s i n g l e energy r e f i n e r y would r e q u i r e about 20,000 tons/day of h i g h - s u l f u r Eastern c o a l , producing about 600 long tons/day of elemental sulfur by-product. For the purposes o f t h i s paper, t h e f u t u r e p r i c e o f sulfur has been p r o j e c t e d to be about $100/LT, w i t h a f l o o r provided by the energy cost t o produce d i s c r e t i o n a r y sulfur. At t h i s s a l e p r i c e , the sulfur by-product c r e d i t is about $ 0 . 2 5 / m i l l i o n Btu o f energy produced. This c r e d i t , f o r 2 m i l l i o n


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BPDFOE p r o d u c t i o n , is $0.5 t o $1 b i l l i o n annually. Therefore, the sulfur question in s y n f u e l s production has s i g n i f i c a n t economic importance.


S u l f u r in Feedstock - Occurrence In attempting t o assess the d i s p o s i t i o n of sulfur in the overa l l s y n f u e l s system, one must f i r s t examine the c h a r a c t e r i s t i c s of the sulfur in the feedstock. Then, the feedstock is examined f o r sulfur " g e t t e r " m a t e r i a l s . With e v a l u a t i o n of the o p e r a t i n g parameters of t h e primary energy conversion r e a c t i o n system, the f a t e of sulfur in the feedstock can be p r o j e c t e d . The sulfur in the general energy conversion facility feedstock may be c l a s s i f i e d as organic and i n o r g a n i c sulfur. The organic sulfur is bound t o the carbonaceous m a t e r i a l in the feed; the inorganic sulfur is g e n e r a l l y a s s o c i a t e d w i t h the ash f r a c t i o n of the feed. This d i s t i n c t i o n is convenient in assessment of the feedstock because it a f f o r d s a q u a l i t a t i v e measure of the f a t e of s u l fur d u r i n g p r o c e s s i n g . During p r o c e s s i n g , however, sulfur can t r a n s f e r between i n o r g a n i c and o r g a n i c a l l y bound species.2 Each of the major c l a s s i f i c a t i o n s of sulfur types may be f u r t h e r subdivided. The i n o r g a n i c a l l y bound sulfur is c o n v e n t i o n a l l y subdivided among p y r i t e , s u l f a t e , and s u l f i d e types of sulphur^. Again, sulfur can t r a n s f e r among these types, as w e l l as back and f o r t h t o o r g a n i c a l l y bound sulfur or escape from the s o l i d as a gas. The p y r i t e sulfur is g e n e r a l l y considered t o be sulfur associ a t e d w i t h i r o n p y r i t e , FeS2- I n most cases only the second sulfur of the p y r i t e molecule can be considered t o be in t h i s c l a s s . This sulfur can be e x p e l l e d from i r o n p y r i t e w i t h moderate h e a t i n g (approximately 500°C) t o form i r o n s u l f i d e , FeS, and elemental s u l fur condensate in the c o o l e r vapor space above the sample. I r o n s u l f i d e (FeS), on the other hand, is much more d i f f i c u l t to decompose. Even a t 800° to 850°C, FeS is r e l a t i v e l y s t a b l e in reducing atmospheres, depending upon the r e l a t i v e concentrations of H2S, H2O, and H2 in the r e a c t i n g gas^. When one assesses the t h e r modynamics of t h i s system, together w i t h the r e a c t i o n c o n d i t i o n s in most s y n f u e l processes, it w i l l be found that i r o n s u l f i d e is r e l a t i v e l y s t a b l e u n t i l high temperatures are reached. The t h i r d form of i n o r g a n i c sulfur - the s u l f a t e v a r i e t y - is r e a l i t i v e l y unimportant in energy conversion. Fresh c o a l contains very l i t t l e s u l f a t e ; i t s presence is u s u a l l y i n d i c a t i v e of a weathered ( o x i d i z e d ) feedstock. A d d i t i o n a l l y , s u l f a t e may be formed to a minor degree from other types of sulfur by moderate h e a t i n g of the feedstock, by r e a c t i o n w i t h the bound oxygen in t h a t feed. This e f f e c t can even occur under a hydrogen atmosphere .



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The o r g a n i c a l l y bound sulfur can a l s o be g e n e r a l l y d i v i d e d into three types. However, before c o n s i d e r i n g the o r g a n i c a l l y bound sulfur, we must recognize the b a s i c d i f f e r e n c e in energy r e f i n e r y feedstocks. Western oil shales and t a r sands are p r i m a r i l y p a r a f f i n i c in nature, but c o a l is p r i m a r i l y aromatic. The Eastern oil shales appear to have been formed from both animal and vegetable matter, because both aromatic and p a r a f f i n i e species are d e r i v e d from the p r o c e s s i n g . The degree of a r o m a t i c i t y o f the feedstock is important because it appears t o c o n t r o l the extent o f sulfur bonding i n t o the more complex forms o f coke t h a t are d e r i v e d from these m a t e r i a l s . The three general forms o f organic sulfur are c h a r a c t e r i z e d by the temperatures at which they are r e l e a s e d from the feedstock under hydrogen treatment^. A very l o o s e l y bonded form of sulfur is r e l e a s e d a t temperatures that are lower than the range at which the i r o n p y r i t e decomposes. G e n e r a l l y t h i s sulfur form e x i s t s as -SH groups on the organic s t r u c t u r e . This is the predominant form of organic sulfur in p a r a f f i n i c feeds but is not so common in aromatic s t r u c t u r e s (perhaps 25% t o 40% o f the organic sulfur). M

lf

The second form of organic sulfur is r e l e a s e d a t temperatures greater than that o f p y r i t e decomposition, but l e s s than i r o n s u l f i d e decomposition. I n g e n e r a l , t h i s sulfur can be considered as being part o f the " v o l a t i l e matter" of the carbon feedstock that is r e l e a s e d as the " o i l s " are d r i v e n out o f the system. T y p i c a l l y , these m a t e r i a l s are t h i o p h e n i c in nature, but they may be r e l e a s e d as hydrogen s u l f i d e as the aromatic s t r u c t u r e is broken. Most of the organic sulfur probably e x i s t s in t h i s form, although it may convert t o the t h i r d organic type in p r o c e s s i n g o r conventional analysis. The t h i r d form of organic sulfur is more t i g h t l y bound i n t o the carbon l a t t i c e of the c o a l char. Apparently, when c o a l p o l y merizes, o r chars, some of the sulfur is o r g a n i c a l l y trapped w i t h that c o a l char. Tests have i n d i c a t e d t h a t approximately 30% of the o r i g i n a l sulfur content of the c o a l (whether it was o r i g i n a l l y o r ganic o r i n o r g a n i c sulfur) may be trapped in the c h a r r i n g process and not r e l e a s e d by extensive hydrotreatment a t 800°C^. This s u l f u r is, however, g a s i f i e d when the char is reacted at s t i l l higher temperatures. With the above background on the apparent occurrence of sulfur in the carbonaceous f e e d s t o c k s , one can o b t a i n a q u a l i t a t i v e e s t i mate o f i t s f a t e during treatment, depending upon the residence time, temperature, and hydrogen p a r t i a l pressure. Fate o f S u l f u r in Feedstock - M i n e r a l Matter C h a r a c t e r i s t i c s The m i n e r a l matter c o n s t i t u e n t s w i t h i n the feedstock can have


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an important e f f e c t upon the f a t e of sulfur d u r i n g p r o c e s s i n g . The a f f i n i t y of sulfur f o r i r o n has p r e v i o u s l y been d i s c u s s e d . The other primary m a t e r i a l that is both commonly present and has a high a f f i n i t y f o r sulfur is calcium. Both i r o n and calcium may be considered t o be sulfur " g e t t e r s , w i t h a higher a f f i n i t y f o r s u l f u r than c o a l . The s u l f i d e s of calcium are q u i t e s t a b l e in the environments of energy conversion processes. I n f a c t , in an earl i e r program a process was developed f o r the h y d r o t r e a t i n g of c o a l in the presence of i r o n or calcium to d e s u l f u r i z e the c o a l , conv e r t i n g the c o a l - s u l f u r i n t o i r o n or calcium s u l f i d e from which the sulfur could be recovered. Approximately 90% of the sulfur in the c o a l could be removed by t h i s technique^.


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The s t a b i l i t y of the i n o r g a n i c s u l f i d e s appears t o be a major f a c t o r in the d i s p o s i t i o n of t r a c e metals in the feed. As examp l e s , l e a d , t i n , cadmium and n i c k e l can be l a r g e l y recovered as the s u l f i d e s in most energy conversion processes, although many forms of these m a t e r i a l s are v o l a t i l e ^ . Fate of S u l f u r in Energy Conversion - P r o c e s s i n g Conditions The e n t i r e spectrum of energy conversion processes may be cons i d e r e d as part of a g e n e r a l , three-dimensional system of time, temperature, and hydrogen p a r t i a l pressure.* The most common type of system employs a s i n g l e r e a c t i o n stage, c o n t a c t i n g the feed w i t h r e a c t i n g gas t o produce the d e s i r e d product. Several staged processes e x i s t (e.g. COGAS and HYGAS): in these cases, each stage should be evaluated and the impacts of i n t e g r a t i o n should be a s sessed. I n c o u n t e r c u r r e n t , fixed-bed processes, a continuum of v a r y i n g c o n d i t i o n s e x i s t s . These systems can be evaluated s i m i l a r l y t o the staged systems. For e i t h e r the s i n g l e - s t a g e , m u l t i stage, or countercurrent systems, the sum of the t o t a l system is arranged t o y i e l d a high conversion of the feedstock t o the des i r e d product s p e c i e s . For example, s i n g l e - s t a g e l i q u e f a c t i o n processes normally operate a t r e l a t i v e l y low temperatures and high hydrogen p a r t i a l pressures (as sometimes i n f l u e n c e d by hydrogen donor s o l v e n t s or c a t a l y s t s ) w i t h s u f f i c i e n t residence time t o y i e l d the d e s i r e d conv e r s i o n . Extremely-high-temperature (1500°C) g a s i f i c a t i o n processes r e q u i r e very short residence times t o e f f e c t conversion, but more moderate temperatures (1000°C) r e q u i r e longer residence times. P y r o l y s i s processes simply cook the feedstock in an e s s e n t i a l l y i n e r t atmosphere a t r e l a t i v e l y low temperatures (500°to 700°C); h y d r o p y r o l y s i s operates under s i m i l a r c o n d i t i o n s , but in a reduci n g atmosphere. * Although apparently obvious, t h i s concept is not g e n e r a l l y r e c ognized. Rather, the conventional approach is to consider each s y n f u e l process as a separate and d i s t i n c t e n t i t y .



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When one considers a s i n g l e - s t a g e system, the operating temperature is u s u a l l y s u f f i c i e n t that the primary sulfur species present in the o f f - g a s are hydrogen s u l f i d e and c a r b o n y l s u l f i d e ; these w i l l be present in the r a t i o determined by the H2/CO content of the c o n t a c t i n g gas. In m u l t i s t a g e systems and in some l i q u e f a c t i o n processes, one must consider the " d e v o l a t i l i z a t i o n " of the feedstock. U s u a l l y the temperature during d e v o l a t i l i z a t i o n is n o t h i g h , so the r e a c t i o n s t o and COS are k i n e t i c - l i m i t e d . Thus, the o f f - g a s w i l l probably a l s o c o n t a i n carbon d i s u l f i d e , mercaptans, organic s u l f i d e s , and thiophenes. The r e l a t i v e q u a n t i t i e s of these m a t e r i a l s that are present from d e v o l a t i l i z a t i o n , of course, are f u n c t i o n s of the time, temperature, r e a c t i n g gas comp o s i t i o n , and c o n t a c t i n g mode. These species are most apparent in the simplest of c o a l conversion processes - such as coke product i o n ; t h e i r presence decreases in more advanced processes. Fate o f S u l f u r Compounds - Product Upgrading The primary purpose o f the energy conversion facility is the production of l i q u i d o r gaseous f u e l s ; most of the sulfur w i l l be removed from these products. L i q u i d f u e l streams w i l l be hydrod e s u l f u r i z e d to meet combusion standards, w i t h the sulfur t r a n s f e r r e d t o the gas phase.** I n the case o f oil s h a l e , extensive hydrotreatment w i l l be r e q u i r e d t o remove the r e f r a c t o r y n i t r o g e n compounds from the oil. With t h i s degree of treatment, the sulfur w i l l a l s o be removed. I f the product is a low- or medium-Btu gas, produced f o r d i r ect combustion, the stream w i l l probably be d e s u l f u r i z e d f o r environmental reasons. I n t h i s i n s t a n c e , approximately 95% sulfur removal is t y p i c a l of a reasonable l e v e l o f p u r i f i c a t i o n - b e t t e r than d i r e c t combustion of the c o a l by a f a c t o r o f two, yet not r e q u i r i n g an e x c e s s i v e energy p e n a l t y . I f the gaseous product is a s y n t h e s i s gas, as in the p r o d u c t i o n of methane, methanol, o r Fishcher-Tropsch l i q u i d s , extreme d e s u l f u r i z a t i o n is r e q u i r e d t o p r o t e c t the sulfur-sensitive downstream c a t a l y s t s . In a d d i t i o n to the primary product and by-product streams, sulfur w i l l be present in other streams in the facility. For example, water that is condensed from the product w i l l c o n t a i n some d i s s o l v e d sulfur compounds, as w i l l the stack from the b o i l e r house that is r e q u i r e d to p r o v i d e steam f o r the system. D i s s o l v e d sulfur gases w i l l normally be s t r i p p e d from f o u l water t o the gas streams, but some f i x e d sulfur (e.g. thiocyanate) may not be a t tacked u n t i l b i o l o g i c a l o x i d a t i o n or other water treatment. The sulfur compounds present in the b o i l e r house stack might be r e covered t o s a l a b l e products, depending on the f l u e gas d e s u l f u r i ** I n some i n s t a n c e s , where the l i q u i d stream is a s m a l l by-product it may not be d e s u l f u r i z e d o n s i t e . Rather, that m a t e r i a l might be s o l d as feedstock to a petroleum r e f i n e r y .


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z a t i o n system utilized. Note that sulfur and many of i t s common compounds ( u n l i k e n i t r o g e n , a l s o present in energy r e f i n e r i e s ) are r e l a t i v e l y w e l l studied and c h a r a c t e r i z e d chemicals. With thermodynamic data on the many p o s s i b l e sulfur forms ( t h i o s u l f a t e s , t h i o cyanates, p o l y s u l f i d e s , e t c . ) , the f a t e of sulfur can be adequatel y projected.


S u l f u r Removal from Gas Streams As o u t l i n e d above, most of the sulfur released from the feed w i l l e v e n t u a l l y appear in the gas phase. The primary sulfur problem is s y n f u e l s production, t h e r e f o r e , is gas-phase d e s u l f u r i z a tion. This p o r t i o n of the d i s c u s s i o n is l i m i t e d t o a general overview. S i g n i f i c a n t l i t e r a t u r e e x i s t s on t h i s s u b j e c t * , however, most gas p u r i f i c a t i o n processes have not been e x t e n s i v e l y t e s t e d on c o a l - or oil shale-derived gases; the engineering t r a n s f e r of technology from other systems e n t a i l s a measure of r i s k . 7

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The two primary approaches to removal o f s u l f u r o u s compounds from process gas streams are 1) the d i r e c t - o x i d a t i o n processes and 2) the acid-gas removal systems9,10. in the d i r e c t - o x i d a t i o n processes, hydrogen s u l f i d e is s e l e c t i v e l y removed from the process gas, and the sulfur is recovered in i t s elemental form. The a c i d gas removal processes, as a group, remove a c i d i c components (H2S, CO2, HCN, etc.) and regenerate these species i n t o a separate s i d e stream. In g e n e r a l , the d i r e c t - o x i d a t i o n processes employ a redox couple that has s u f f i c i e n t o x i d a t i o n p o t e n t i a l t o convert H2S i n t o elemental sulfur but i n s u f f i c i e n t p o t e n t i a l t o o x i d i z e sulfur t o higher s t a t e s . Examples of m a t e r i a l s that have t h i s redox potent i a l are vanadium compounds, a r s e n i c compounds, i r o n compounds, and c e r t a i n organic s p e c i e s . T y p i c a l l y , the redox m a t e r i a l s , d i s solved in a hot potassium carbonate s o l u t i o n w i t h the species in i t s o x i d i z e d form, contacts the I^S-laden gas and the t^S d i s s o l v e s as the h y d r o s u l f i d e . This sulfur r e a c t s w i t h the redox couple, forming elemental sulfur and the reduced s t a t e of the couple. A i r blowing of the s o l u t i o n r e o x i d i z e s the couple and removes the e l e mental sulfur from s o l u t i o n as a product f r o t h . D i r e c t - o x i d a t i o n processes can conveniently remove H2S from the process gas t o l e v e l s of 100 ppm. At s i g n i f i c a n t l y greater c o s t , l e v e l s of perhaps 10 ppm can be achieved. The systems are c h a r a c t e r i z e d by r e l a t i v e l y low sulfur-carrying c a p a c i t i e s and, t h e r e f o r e , high l i q u o r r e c i r c u l a t i o n r a t e s . They a r e not generall y a p p l i c a b l e t o gas streams w i t h high p a r t i a l pressures of carbon d i o x i d e ; the carbon d i o x i d e a l s o d i s s o l v e s in the s o l u t i o n , causi n g a pinch in the column and a r e d u c t i o n of sulfur p u r i f i c a t i o n capability.



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The d i r e c t - o x i d a t i o n systems a r e specific t o hydrogen s u l f i d e ; other sulfur species are apparently not attacked. S o l u t i o n degradation problems may be caused by t h i o s u l f a t e formation as w e l l as thiocyanate formation ( i f HCN is present in the gas to be t r e a t e d ) . S o l u t i o n r e g e n e r a t i o n techniques have been developed t o attempt t o minimize the impact o f these e f f e c t s . The systems must be operated w i t h c a u t i o n , in some cases, the s o l u t i o n s c o n t a i n species that a r e considered t o x i c o r environmentally hazardous. Some of the commercial processes comprising t h i s group i n c l u d e S t r e t f o r d , Takahex, Giammarco-Vetrocoke, Ferrox and o t h e r s . The acid-gas removal systems remove hydrogen s u l f i d e , as w e l l as other a c i d gases, from the process stream and regenerate them as a separate s i d e stream. I n the a p p l i c a t i o n of acid-gas removal processes, one must consider t h e downstream recovery process. The d i r e c t o x i d a t i o n processes, discussed above, are a p p l i c a b l e f o r t h i s duty; u s u a l l y , however, a Claus p l a n t is a l e s s c o s t l y approach. The c o n v e n t i o n a l Claus r e a c t i o n , however, r e q u i r e s H2S concentrations t h a t are higher than 15%; over 30% is p r e f e r r e d f o r smooth o p e r a t i o n o f the system**-. I n a t y p i c a l energy r e f i n e r y , the q u a n t i t y of sulfur in the t o t a l a c i d gas may be in the range of 1% t o 5%. This range o f H2S c o n c e n t r a t i o n is s a t i s f a c t o r y f o r a d i r e c t - o x i d a t i o n process. U s u a l l y , however, the combination of a Claus sulfur p l a n t w i t h a " s e l e c t i v e " acid-gas removal system is an economically p r e f e r r e d a l t e r n a t i v e 1 2 . A s e l e c t i v e acid-gas system is designed t o remove hydrogen s u l f i d e from the gas a t a g r e a t er r a t e than carbon d i o x i d e . Hence, the sulfur in the gas can be recovered a t a c o n c e n t r a t i o n t h a t is s a t i s f a c t o r y f o r Claus p l a n t operation. The three general c l a s s i f i c a t i o n s o f acid-gas removal processes a r e 1) amines, 2) a c t i v a t e d hot potassium carbonate, and 3) p h y s i c a l s o l v e n t s . These systems each have p r e f e r r e d o p e r a t i n g ranges, as g e n e r a l l y c o n t r o l l e d by the acid-gas p a r t i a l pressure, have d i f f e r e n t energy requirements, and o f f e r d i f f e r e n t degrees o f selectivity. The amine-based acid-gas removal systems have a high a f f i n i t y f o r a c i d i c species and a r e operable when the p a r t i a l pressure o f the a c i d gases is r e l a t i v e l y low. T h i s h i g h chemical a f f i n i t y , however, means t h a t s i g n i f i c a n t energy is r e q u i r e d t o break the amine-acid bond f o r r e g e n e r a t i o n of the a c i d gases. This h i g h energy demand g e n e r a l l y d i c t a t e s the s e l e c t i o n o f an a l t e r n a t i v e acid-gas removal system unless the system must operate a t low p r e s sure. Although common in oil r e f i n e r i e s , t h i s type o f system may not f i n d s i g n i f i c a n t a p p l i c a t i o n in s y n f u e l s f a c i l i t i e s where the a c i d gas p a r t i a l pressure is u s u a l l y h i g h . Amines a r e o c c a s i o n a l l y used in combination w i t h other s y s tems f o r secondary p u r i f i c a t i o n . For example, an a l t e r n a t i v e pro-


2.

FLEMING


29

cess may be used f o r the more economical bulk removal of the a c i d gases, but an amine scrubber is used f o r improved cleanup when the p a r t i a l pressure of the a c i d gases is low. Primary amines do not e x h i b i t s i g n i f i c a n t s e l e c t i v i t y f o r H2S over CO2; moderate s e l e c t i v i t y is o b t a i n a b l e in some secondary and t e r t i a r y amines. Some of the common amine-based processes (abbreviated as per custom) i n c l u d e MEA, DEA, DGA, MDEA, TEA, and DIPA .


8

The a c t i v a t e d hot potassium carbonate systems are g e n e r a l l y a p p l i e d when the p a r t i a l pressure of a c i d gases is above a p p r o x i mately 15 p s i g ! 3 . For these systems, the a c i d i c c o n s t i t u e n t must f i r s t d i s s o l v e in the s o l u t i o n and then react w i t h the b a s i c s o l u t i o n . A moderate degree of s e l e c t i v i t y can be achieved by t a k i n g advantage of the r e l a t i v e l y r a p i d k i n e t i c s (not e q u i l i b r i u m ) of H2S d i s s o l u t i o n compared w i t h CO2 d i s s o l u t i o n . Energy demands f o r r e g e n e r a t i o n of the hot carbonate systems are l e s s than f o r the amine-based processes, but s t i l l h i g h compared w i t h the p h y s i c a l solvent processes. Some of the process names a s s o c i a t e d w i t h hot carbonate systems i n c l u d e B e n f i e l d , Catacarb, and A l k a z i d . The p h y s i c a l solvent-based acid-gas removal systems do not r e l y upon the a c i d i c p r o p e r t i e s of the a c i d gas. Rather, the p o l a r a c i d gases have a much greater s o l u b i l i t y in many o i l s and s o l v e n t s , compared w i t h f u e l gas s p e c i e s . These s o l u b i l i t i e s are f u n c t i o n s of temperature and, of course, pressure. Thus, these systems operate by d i s s o l v i n g the a c i d gases in the s o l v e n t s a t h i g h pressure and r e l a t i v e l y low temperature. The a c i d gases can be regenerated by temperature and/or pressure swing. G e n e r a l l y , a pressure swing is utilized to f u r t h e r minimize the a l r e a d y low thermal energy requirements of the system. However, s i g n i f i c a n t mechanical energy may be r e q u i r ed f o r s o l u t i o n pumping (at high o p e r a t i n g p r e s s u r e s ) , r e f r i g e r a t i o n (at low o p e r a t i n g temperatures), or f l a s h gas recompression ( i f a h i g h degree of s e l e c t i v i t y is r e q u i r e d ) . In g e n e r a l , the p h y s i c a l solvent-based processes appear t o have economic advantages when the p a r t i a l pressure of a c i d gas is greater than approximately 200 p s i g ^ . By making use of the d i f ference in s o l u b i l i t y between hydrogen s u l f i d e and carbon d i o x i d e , a s i g n i f i c a n t degree of s e l e c t i v i t y can be achieved. This s e l e c t i v i t y is u s u a l l y improved a t lower temperatures and w i t h g r e a t e r f l a s h gas r e c y c l e (which changes the r e l a t i v e c o n c e n t r a t i o n of H2S and CO2 in the scrubber feed). Some of the process names a s s o c i a ted w i t h the p h y s i c a l s o l v e n t systems i n c l u d e R e c t i s o l , S e l e x o l , P u r i s o l , and S u l f o l a n e . In many s y n f u e l r e f i n e r y flowsheets, the acid-gas p a r t i a l pressure is b o r d e r l i n e between obvious preferance f o r hot carbonate and p h y s i c a l solvent-based acid-gas removal systems. I n these



30


cases, a c a r e f u l process t r a d e - o f f study, on a p l a n t - s p e c i f i c basis, is r e q u i r e d . For example, low-cost power f a v o r s a p h y s i c a l solvent system, but a s u r p l u s of low-pressure steam from the process f a v o r s a hot carbonate acid-gas removal process. Some acid-gas removal systems i n c l u d e mixed s o l v e n t s - usua l l y an amine in a p h y s i c a l s o l v e n t . This approach modifies the r e l a t i v e c h a r a c t e r i s t i c s of the p h y s i c a l solvent by the amine and provides many of the advantages of each. G e n e r a l l y , only a moderate degree o f s e l e c t i v i t y can be achieved, but the t o t a l system appears t o be improved. T y p i c a l in t h i s area are the S u l f i n o l and Amisol processes. S u l f u r Recovery Systems The d i r e c t - o x i d a t i o n processes f o r recovery o f sulfur from raw gases are a l s o a p p l i c a b l e on acid-gas streams. U s u a l l y , a d i r e c t o x i d a t i o n process would be a p p l i e d when a n o n s e l e c t i v e acid-gas removal system had been employed and the sulfur concent r a t i o n in the a c i d gas is r e l a t i v e l y low. At higher H2S concent r a t i o n s , as achieved through s e l e c t i v e acid-gas removal, the conv e n t i o n a l Claus process appears t o be more economic. The reason that a c o n v e n t i o n a l p l a n t r e q u i r e s higher sulfur l e v e l s in the feed gas may be found in the heat balance of the system. I n a conventional Claus p l a n t , where the e n t i r e gas stream is reacted w i t h s t o i c h i o m e t r i c oxygen t o d r i v e the hydrogen s u l f i d e t o elemental sulfur, the f i r s t r e a c t i o n stage is an I ^ S - a i r flame, and high sulfur c o n c e n t r a t i o n s are r e q u i r e d so that the flame temperatures are high enough to be s t a b l e * * . I n a s p l i t flow process, o n e - t h i r d of the I ^ S - c o n t a i n i n g stream is reacted w i t h s u f f i c i e n t oxygen to r e a c t the H2S t o SO2. The r e s u l t i n g SO2 is reacted w i t h the remaining H2S o f the o r i g i n a l sulfur-plant feed t o form elemental sulfur. With t h i s approach, the H2S concent r a t i o n s may be somewhat lower - down t o 15% - and s t i l l maintain a s t a b l e flame in the combustion s e c t i o n * * . For leaner sulfur streams, s e v e r a l techniques have been suggested t o provide the heat balance. Among them are 1) use o f oxygen r a t h e r than a i r f o r combustion*^, 2) a d d i t i o n of hydrocarbon or other f u e l species t o the sulfur burner s e c t i o n t o provide heat, 3) r e c y c l e of product sulfur t o the burner s e c t i o n t o provide heat**, and 4) preheat of i n l e t a c i d gas and a i r * ^ . Recently, d i s c u s s i o n s have appeared in the t e c h n i c a l l i t e r a t u r e on the s a t i s f a c t o r y o p e r a t i o n of sulfur p l a n t s a t f e e d - c o n c e n t r a t i o n down t o 1% t o 5% sulfur, u t i l i z i n g preheat of the i n l e t area. For t h a t matter, according to other l i c e n s o r s , t h i s approach was p r a c t i c e d 25 years ago. I tisknown t h a t s u f f i c i e n t heat is produced in the 2 / ° 2 r e a c t i o n , i f p r o p e r l y recuperated, t o operate c a t a l y t i c a l l y w i t h t h i s low sulfur c o n c e n t r a t i o n in the a c i d gas. H

s


2.

FLEMING

Sulfur Recovery from New

Energy Sources

31

The Union Oil s e l e c t i v e sulfur o x i d a t i o n c a t a l y s t ^ is the b a s i s f o r many modified sulfur p l a n t designs announced in recent years*7 18 This system may be i d e a l f o r a s y n f u e l s facility because of the low H2S/CO2 r a t i o of s y n f u e l raw-gas streams. I f a p h y s i c a l solvent is employed f o r acid-gas removal, some hydrocarbon w i l l be l o s t to the acid-gas stream. With the s e l e c t i v e sulfur o x i d a t i o n c a t a l y s t , t h i s f u e l is not o x i d i z e d , r a t h e r it is a v a i l a b l e f o r t a i l - g a s r e d u c t i o n over cobalt-molybdenom, p r i o r to f i n a l treatment. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 2, 2017 | http://pubs.acs.org Publication Date: March 29, 1982 | doi: 10.1021/bk-1982-0183.ch002

5

#

The primary problem w i t h the systems that have been developed to u t i l i z e lower feed sulfur c o n c e n t r a t i o n s is the reduced e f f i c i e n c y of sulfur recovery. This e f f e c t is not so apparent in l o s t sulfur revenues as it is in the increased s i z e and cost of the downstream t a i l - g a s t r e a t i n g u n i t r e q u i r e d to minimize the sulfur emissions from the o v e r a l l sulfur-treatment section. Sulfur-Plant Tail-Gas Treating A number of approaches are p o s s i b l e to minimize the sulfur content in the t a i l gas from the sulfur p l a n t , and s e v e r a l processes have been developed f o r each of these approaches^>10. A complete d i s c u s s i o n of t a i l - g a s treatment is beyond the scope of t h i s document. The processes a v a i l a b l e have a wide range of r e covery e f f i c i e n c i e s ; some of them r e s u l t in a n e a r l y sulfur-free t a i l gas. In g e n e r a l , the cost of the p r o c e s s i n g i n c r e a s e s w i t h the degree of p u r i t y achieved, and, more i m p o r t a n t l y , those processes that do achieve higher p u r i t y in the f i n a l t a i l gas genera l l y have e x c e s s i v e energy demands. In most cases, the a d d i t i o n a l sulfur recovered in the t a i l - g a s t r e a t i n g processes is i n s u f f i c i e n t to pay f o r the c a p i t a l requirement of the t a i l - g a s t r e a t i n g process, l e t alone the o p e r a t i n g c o s t s in terms of energy. For that matter, on a t o t a l system b a s i s , some of these processes may not be environmentally c o s t - e f f e c t i v e . Systems A n a l y s i s The o v e r a l l problem, from sulfur in a raw gas through sulfur recovery and t a i l gas t r e a t i n g , has h i s t o r i c a l l y been addressed by a case study approach. In t h i s approach, each p l a n t s e c t i o n is optimized^ based upon predetermined subsystem i n t e r f a c e s . A pref e r r e d approach, r a t h e r , is to evaluate the e n t i r e system a n a l y t i c a l l y , a l l o w i n g the i n t e r f a c e s between the subprocesses to f l o a t , to determine the o v e r a l l c o s t - e f f e c t i v e . a p p r o a c h . In t h i s technique, compromises are taken in each of the u n i t processes so that the t o t a l system operates more e f f e c t i v e l y . A f u r t h e r question a r i s e s on the need f o r the e x c e p t i o n a l l y high p u r i t y t a i l - g a s t r e a t i n g processes. The energy demands of these systems are e x c e s s i v e , compared w i t h l e s s e f f i c i e n t systems.


32


Indeed, more p o l l u t a n t s may be emitted in the production of energy t o r e p l a c e the energy used f o r t a i l - g a s treatment, than are recovered by the more e f f i c i e n t t a i l - g a s t r e a t i n g processes.


P o t e n t i a l Problems One of the primary problems t o be encountered in energy conv e r s i o n f a c i l i t i e s w i l l be the v a r i a b i l i t y o f the feedstock. A c o a l o r oil shale facility w i l l probably see greater v a r i a b i l i t y , day-by-day, than experienced in planned changes in oil r e f i n e r y crude runs. Even w i t h e x t e n s i v e blending in the feed s t o c k p i l e , the v a r i a t i o n in feed c h a r a c t e r i s t i c s w i l l be s i g n i f i c a n t . The feedstock v a r i a t i o n w i l l impact on the c h a r a c t e r i s t i c s of the raw-gas. Added t o the e f f e c t o f the feedstock v a r i a t i o n is the c y c l i c impact o f l o a d i n g of lockhoppers, f u l l of f r e s h feedstock, i n t o the r e a c t o r . Thus, the downstream p u r i f i c a t i o n system can be expected t o be faced w i t h a wide and c o n s t a n t l y changing range of concentrations of minor s p e c i e s . Nowhere is t h i s e f f e c t more important than in the sulfur p l a n t . I n the sulfur recovery s e c t i o n , the a i r flow t o the Claus r e a c t o r must be c a r e f u l l y proportioned t o the sulfur flow in the feed; even s l i g h t v a r i a t i o n s cause s i g n i f i c a n t penalty in the s u l fur p l a n t e f f i c i e n c y and increased load on the t a i l - g a s t r e a t i n g unit15. The sulfur p l a n t of the energy r e f i n e r y , as contrasted t o other sulfur p l a n t a p p l i c a t i o n s , w i l l have a h i g h c o n c e n t r a t i o n of carbon d i o x i d e . This CO2 w i l l have a s i g n i f i c a n t impact, both c h e m i c a l l y and t h e r m a l l y , on the sulfur p l a n t o p e r a t i o n and downstream t a i l - g a s t r e a t i n g . Energy demands and/or sulfur emissions may be adversely a f f e c t e d , r e l a t i v e t o known o p e r a t i o n s . The sulfur p l a n t w i l l a l s o be impacted by minor c o n s t i t u e n t s in the raw gas. Depending upon the primary conversion system utilized, the raw gas may c o n t a i n s u g n i f i c a n t q u a n t i t i e s o f cyanides, ammonia, o i l s , and l i g h t hydrocarbons. S u l f u r may be present in the forms of carbonyl s u l f i d e , carbon d i s u l f i d e , mercaptans, thiophenes, o r other s p e c i e s . Design engineers should be aware o f these species and t h e i r v a r i a b i l i t y , in order t o prov i d e s u f f i c i e n t p r o t e c t i o n in the o v e r a l l system. Excess o i l s and hydrocarbons in t h e sulfur p l a n t feed can r e a d i l y darken the sulfur, m i n i m i z i n g i t s s a l e s v a l u e * . Ammonia can cause f o u l i n g of the Claus c a t a l y s t b e d s . Cyanides have a tendency t o polymerize, causing s i g n i f i c a n t problems w i t h the formation o f P r u s s i a n blue. Organic forms o f sulfur are not so r e a d i l y reacted in low-temperature Claus beds and may cause problems in a c h i e v i n g the d e s i r e d sulfur p l a n t e f f i c i e n c y . A d d i 5

1 5


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FLEMING


Energy Sources

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t i o n a l l y , some of these organic forms of sulfur are not r e a d i l y recoverable in c e r t a i n types of t a i l - g a s t r e a t i n g processes. Another area of concern that has not r e c e i v e d adequate a t t e n t i o n is the p o s s i b l e contamination of the sulfur products. Feedstocks to these r e f i n e r i e s w i l l c o n t a i n a f u l l spectrum of the elements of the p e r i o d i c t a b l e . T h e o r e t i c a l a n a l y s i s i n d i cates that c e r t a i n of these m a t e r i a l s may undergo chemical react i o n s and end up in the sulfur p l a n t feed. T h e o r e t i c a l l y , we can expect a s i g n i f i c a n t c o n t a i m i n a t i o n by a r s e n i c , selenium, t e l l u r i u m , and perhaps mercury^. New

Developments

The major f a c t o r a f f e c t i n g sulfur in the s y n f u e l s i n d u s t r y is the growing tendency to design f o r s u l f u r i c a c i d manufacture, r a t h e r than elemental sulfur production. From a t e c h n i c a l viewp o i n t , many of the problems inherent w i t h the sulfur p l a n t can be overcome by proper design of a s u l f u r i c a c i d p l a n t . On an economic b a s i s , the value of s u l f u r i c a c i d , per u n i t of sulfur, is s i g n i f i c a n t l y higher. Thus, the o v e r a l l p l a n t economics appear f a v o r a b l e w i t h s u l f u r i c - a c i d production. For a p l a n t l o cated in the I l l i n o i s c o a l b a s i n , the market f o r s u l f u r i c a c i d appears s t r o n g . Thus, we can a n t i c i p a t e seeing more process designs that i n c l u d e s u l f u r i c a c i d p r o d u c t i o n , r a t h e r than element a l sulfur manufacture. However, u n l i k e elemental sulfur, sulfuri c a c i d cannot be i n v e n t o r i e d and long d i s t a n c e t r a n s p o r t a t i o n can be economically u n v i a b l e . An area of research that is r e c e i v i n g increased a t t e n t i o n is the removal of sulfur from process gas streams at elevated temperature. This approach has s i g n i f i c a n t e f f i c i e n c y - i m p a c t upon the o v e r a l l energy conversion system, p a r t i c u l a r l y i f the product is a medium-Btu gas that is to be f i r e d o n s i t e ( f o r example, i n t o combined-cycle power g e n e r a t i o n ) . S e v e r a l l a b o r a t o r i e s are working on systems f o r t h i s o p e r a t i o n . In most cases, the sulfur is recovered from the system as sulfur d i o x i d e , which would probably be upgraded to s u l f u r i c a c i d . AT IGT a system is being developed that removes the sulfur from the gas at elevated temp e r a t u r e , but the sulfur is discharged from the process in e l e mental form. E f f o r t s to develop modified Claus processes that w i l l operate w i t h low c o n c e n t r a t i o n s of sulfur in the feed gas have been p r e v i o u s l y mentioned. A s i m i l a r system is being l a b o r a t o r y t e s t ed at IGT which does not r e q u i r e p r e c i s e , instantaneous c o n t r o l of the oxygen/sulfur feed r a t i o . In the acid-gas removal area, process engineers continue to work on systems t h a t have lower energy demands and produce cleaner



34

SULFUR: NEW

SOURCES AND

USES

gas. In the d i r e c t o x i d a t i o n processes, developers are s t r i v i n g toward increased sulfur-carrying c a p a c i t y f o r the s o l u t i o n and regeneration techniques t o minimize system c o s t s . Improved COS h y d r o l y s i s systems are being developed to convert COS i n t o H2S, which is more r e a d i l y recovered. A d d i t i o n a l work of course is r e q u i r e d at a l l of these areas, t o f u r t h e r improve p l a n t economics. A l s o , as discussed e a r l i e r , proper e v a l u a t i o n of the sulfur problem from a t o t a l systems b a s i s , is r e q u i r e d . A d d i t i o n a l work should focus upon energy demands of t a i l - g a s t r e a t ment processes and the need, or d e s i r a b i l i t y , of employing h i g h energy consumption f o r minimal improvements in sulfur recovery. References Cited 1.

International Gas Technology Highlights IX No. 16 (1979) July 30.

2.

Fleming, D.K. and Smith, R.D., "Pilot Plant Study of Conversion of Coal to Low Sulfur Fuel", Chicago: The Inst. Gas Technology, EPA-600/2-77-206 October 1977.

3.

Vestal, M.L. and Johnston, W.H., "Desulfurization Kinetics of Ten Bituminous Coals", Report No. SRIC 69-10. Baltimore: Scientific Research Instruments Corp., 1969.

4.

Rosenquist, T., "A Thermodynamic Study of the Iron, Cobalt, and Nickel Sulfides", J. Iron Steel Inst. London 176, 37-57 (1954) January.

5.

Fleming, D.K., Smith, R.D., "Evaluation of the Flash Desulfurization Process for Coal Cleaning", EPA-600/7-79-016 Chicago: Institute Gas Technology, January 1979.

6.

Anderson, G.L., H i l l , A.H. and Fleming, D.K., "Predictions of the Disposition of Select Trace Constituents in Coal Gasification Processes", Paper presented at Environmental Aspects of Fuel Conversion Technology Symposium, Hollywood, Florida, April 17-20, 1979.

7.

Kohl A. and Riesenfeld, F., Gas Purification, 3rd Ed. Houston: Gulf Publishing Co., 1979.

8.

Maddox, R.N., "Gas-Liquid Sweetening", 2nd Edition, Campbell Petroleum Series, Norman, Oklahoma (1974).

9.

Fleming, D.K. and Primack, H.S., "Purification Processes for Coal Gasification", Paper presented at the American Institute of Chemical Engineers, 81st National Meeting, Kansas City, Mo., April 11-14, 1976.



2.

FLEMING


Energy Sources

35

10.

Hyne, J.B., "Methods f o r D e s u l f u r i z a t i o n of E f f l u e n t Gas Streams", Oil & Gas J o u r n a l , August 28 (1972), 64-67.

11.

Beavon D.K. and Leeper J.E., " S u l f u r Recovery from Very Lean Hydrogen S u l f i d e " , 27th Canadian Chem. Eng. Conf., Calgary, A l b e r t a , Oct. 1977.

12.

C h r i s t e n s e n , K.G. and S t u p i n , W.J., "Comparison of A c i d Gas Removal Processes", Final Report No. FE-2240-49, prepared f o r DOE and GRI under Contract No. EX-76-C-01-2240 C.F. Braun & Co., Alhambra, Calif.: April 1978.

13.

Benson, H.E. and P a r r i s h , R.W., "HiPure Process Removes H S", Hydrocarbon Process. 53, 81-82 (1974) April.

CO / 2

2

14.

Anon, "The TRW S-100 Process", TRW Systems L t d . , Redondo Beach Calif., (1971).

15.

Grancher P., "Recent Advances in Claus Techniques f o r S u l phur Recovery from A c i d Gases", 27th Canadian Chem. Eng. Conf., Calgary, A l b e r t a , Oct. 1977.

16.

Hass, R.H. (assigned to Union Oil Company of California), "Catalytic I n c i n e r a t i o n of Hydrogen S u l f i d e from Gas Streams", U.S. Patent 4,171,347 (1977) April 15.

17.

Beavon, D.K., Hass, R.H. and Muke, B., "High Recovery, Lower Emission Promised f o r Claus-Plant Tail Gas", Oil & Gas J o u r n a l , (1979) March 12.

18.

Hass, R.H., et al., II, 76-80 "Process Meets S u l f u r Recovery Needs", Hydrocarbon Process. 60, 104-107 (1981) May.

RECEIVED October 5,

1981.


Sulfur: New Sources and Uses - American Chemical Society

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