Petroleum-Coke Desulfurization - ACS ... - ACS Publications

14 Petroleum-Coke Desulfurization A n Improved Thermal-Chemical Process 1

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Downloaded by GEORGETOWN UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch014

H. H. Brandt and R. S. Kapner 1

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Diamond West Energy Corp., 27 River Ridge Road, Lake Charles,LA70605 Chemical Engineering Department, The Cooper Union, 51 Astor Place, New York, NY 10003

Current and foreseeable future supplies of anode grade petroleum coke for the primary aluminum industry are subject to coker feedstock quality limitations, i . e . , to higher levels of metallic impurities (V, Ni, Fe and Si) and increasing sulfur content. The presence of metals in coke directly affects refined aluminum quality, while sulfur, though tolerated by producers, causes environmental problems that appear to be on the edge of stringent federal regulations. Steadily rising coke sulfur levels have stimulated interest in removal methods. This paper briefly reviews the technology of petroleum coke desulfurization and the effects of sulfur removal processes on coke physical properties. A new thermal-chemical process is described which uses hydrogen sulfide as a gaseous reactant to remove coke sulfur after calcining. The process appears able to maintain high bulk and real coke densities by balanc ing reaction temperature and contact time and is suitable for new calciner construction as well as a relatively easy retrofit to existing calciners. I m p u r i t i e s i n Coke - N a t u r e of the Problem F r e e w o r l d p e t r o l e u m coke p r o d u c t i o n has i n c r e a s e d d r a m a t i c a l l y i n the past decade. The U n i t e d S t a t e s c o n t i n u e s t o be a dominant f a c t o r i n the p e t r o l e u m coke i n d u s t r y , a c c o u n t i n g f o r more than 75% of t h e t o t a l p r o d u c t i o n . I n s p i t e of the phenomenal growth i n p r o d u c t i o n , t h e r e i s a r e a l and j u s t i f i e d c o n c e r n about a coke s h o r t a g e by the aluminum industry. The s h o r t a g e I s i n the s u p p l y of anode grade c o k e . This i s t h e coke t h a t i s used by t h e producers of p r i m a r y aluminum (about o n e - h a l f pound p e r pound o f aluminum). This industry consumes a p p r o x i m a t e l y 10 m i l l i o n tons of raw p e t r o l e u m coke a n n u a l l y o r about 35% o f t h e t o t a l f r e e w o r l d o u t p u t . Most o f t h e r e c e n t l y b u i l t c o k e r s , as w e l l as many of t h e o l d e r o n e s , a r e b e i n g 0097-6156/86/0303-0193$06.00/0 © 1986 American Chemical Society

In Petroleum-Derived Carbons; Bacha, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


194

PETROLEUM-DERIVED CARBONS

charged w i t h feedstocks t h a t are h i g h i n s u l f u r and m e t a l s c o n t e n t s ; the r e s u l t a n t cokes a r e c o r r e s p o n d i n g l y h i g h I n s u l f u r and m e t a l s c o n t e n t s . W h i l e e x p e r i e n c e i n d i c a t e s t h a t both of these contaminants can be t o l e r a t e d by the aluminum p r o d u c e r s , they do create serious problems. I n the case of m e t a l l i c i m p u r i t i e s ( i . e . , V , N i , Fe and S i ) , the problem can be r e f l e c t e d I n reduced anode e f f i c i e n c y ( e . g . , V o f t e n a c t s c a t a l y t i c a l l y to i n c r e a s e o x i d a tion). F u r t h e r m o r e , a l l m e t a l s contaminate the aluminum; the m e t a l l u r g y r e q u i r e d to remove such contaminants i s expensive. S u l f u r p r e s e n t s a n o t h e r problem whose s o l u t i o n i s l e s s c l e a r l y defined. The problem i s a i r p o l l u t i o n . Two elements i n t h i s s c e n a r i o a r e on a c o l l i s i o n c o u r s e . F i r s t , due t o s h r i n k i n g s u p p l i e s of low s u l f u r ( l e s s t h a n 2.5 wt%) c o k e s , c o n c u r r e n t w i t h i n c r e a s e d r e q u i r e m e n t s by the aluminum i n d u s t r y , the average s u l f u r c o n t e n t of s o - c a l l e d anode grade cokes has i n c r e a s e d s i g n i f i c a n t l y . Ten y e a r s ago the average was 2.0 wt%, w h i l e t o d a y ' s s p e c i f i c a t i o n i s 3.5 wt%. Second, SO e m i s s i o n l i m i t s a r e b e i n g t i g h t e n e d , and t h e r e a r e s t r o n g i n d i c a t i o n s t h a t we w i l l see a quantum jump i n the e n v i r o n m e n t a l r e s t r i c t i o n s on S 0 e m i s s i o n s i n the near f u t u r e . x

Technology of S u l f u r Removal D e s u l f u r i z a t i o n of coker f e e d s t o c k s to produce lower s u l f u r cokes has proven not t o be the s o l u t i o n to the p r o b l e m . Nor i s t h e r e cause f o r o p t i m i s m i n t h i s a r e a . Not o n l y i s t h i s t e c h n o l o g y extremely expensive, it doesn t always a c h i e v e the expected r e s u l t s . A 50% r e d u c t i o n i n f e e d s t o c k s u l f u r content u s u a l l y y i e l d s a l e s s e r r e d u c t i o n of the s u l f u r c o n t e n t of the c o k e . That l e a v e s us w i t h the c h a l l e n g e to d e s u l f u r i z e the coke itself. There has l o n g been an I n c e n t i v e to remove s u l f u r from coke s i n c e low s u l f u r coke has always commanded a premium p r i c e over i t s h i g h e r s u l f u r c o u n t e r p a r t . There have been dozens of patents filed on petroleum coke desulfurization technology (1-18). A l t h o u g h f a r from e x h a u s t i v e , the l i s t of 18 p a t e n t s c i t e d a r e those most p e r t i n e n t to t h i s d i s c u s s i o n . I t has been demonstrated i n the l a b o r a t o r y t h a t s u l f u r can be removed from p e t r o l e u m coke t h e r m a l l y , c h e m i c a l l y , or t h e r m a l chemically. Thermal s u l f u r removal i s , as might be e x p e c t e d , a time-temperature f u n c t i o n . A l o n g soak at r e l a t i v e l y low t e m p e r a t u r e s , e . g . , s e v e r a l days a t 2000°F (1095°C), can remove 50% o r more of the c o n t a i n e d s u l f u r . W h i l e no s p e c i f i c supporting literature reference can be s i t e d , this effect is commonly e x p e r i e n c e d when b a k i n g anodes i n r i n g f u r n a c e s where temperatures are h e l d from about 950-1050°C f o r p e r i o d s as l o n g as one week. Such t r e a t m e n t a p p l i e d to p e t r o l e u m coke per s e , however, is i m p r a c t i c a l because of e x c e s s i v e energy c o s t s and i t s i n a b i l i t y to process l a r g e t o n n a g e s . H i g h e r temperatures and s h o r t e r t i m e s , e . g . , 2600-2900°F (1425-1595°C) f o r 45-60 m i n u t e s , can a l s o remove 50% or more of the s u l f u r , but c r e a t e a h i g h degree of p o r o s i t y and reduce r e a l d e n s i t y ( 3 , 1 1 , 1 2 , 1 4 , 1 8 , 2 0 - 2 7 ) . Both of these p r o p e r t i e s a r e c r i t i c a l i n anode f a b r i c a t i o n . C h e m i c a l d e s u l f u r i z a t i o n t e c h n i q u e s have a common drawback. They r e q u i r e p u l v e r i z i n g the coke to expose maximum s u r f a c e a r e a t o the r e a c t a n t ( s ) . Thus, the end product i s low s u l f u r coke w i t h no f



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Petroleum-Coke

Desulfurization

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lump c o n t e n t . S i n c e anodes cannot be made w i t h o u t a c o a r s e f r a c t i o n , a g g l o m e r a t i o n of the f i n e s would be r e q u i r e d . This i n turn would r e q u i r e a c o a l t a r b i n d e r p i t c h and a b a k i n g s t e p . Even s o , the f i n a l p r o d u c t becomes a r e l a t i v e l y weak lump w i t h p r o h i b i t i v e cost. Some c h e m i c a l t r e a t m e n t s employ v a r i o u s a d d i t i v e s , e.g., a l k a l i e s , a c i d s , f l u o r i d e s or o r g a n i c s o l v e n t s , w h i c h , i n a d d i t i o n t o b e i n g e x p e n s i v e , may l e a v e r e s i d u e s t h a t a r e unacceptable (1,28-31). The accumulated d a t a appear t o suggest t h a t a t h e r m a l - c h e m i c a l p r o c e s s s h o u l d have the b e s t chance f o r s u c c e s s . A considerable amount of r e s e a r c h e f f o r t has been expended i n t h i s a r e a . The r e p o r t e d r e s u l t s , however, are f r e q u e n t l y i n c o n c l u s i v e and o c c a s i o n a l l y c o n f l i c t i n g . F o r example, most l a b o r a t o r y r e s u l t s support the c o n t e n t i o n t h a t ^ S , h y d r o g e n , methane and a v a r i e t y of o t h e r low m o l e c u l a r weight m a t e r i a l s such as hydrocarbons can i n d u c e d e s u l f u r i z a t i o n a t e l e v a t e d temperatures (2-7,11,14-17,24,26,28,34-38); y e t s e v e r a l s t u d i e s conclude ( 2 1 , 3 2 , 3 3 ) t h a t the s u l f u r content of p e t r o l e u m coke a c t u a l l y i n c r e a s e s when i t i s exposed to ï^S at elevated temperatures. Many s t u d i e s I n d i c a t e the need to p u l v e r i z e the coke p r i o r to treatment ( 9 , 1 2 , 3 9 ) , w h i l e o t h e r s do n o t . Still o t h e r s c l a i m ( 1 0 , 2 2 ) t h a t a two-stage temperature treatment i s beneficial. Two t h e r m a l - c h e m i c a l systems r e p o r t ( 1 1 , 1 8 ) 90% s u l f u r removal u s i n g a t h r e e - s t e p process. I n the f i r s t s t e p , the v o l a t i l e s a r e removed by treatment w i t h an i n e r t gas a t 1400°F (760°C). The temperature i s then reduced to 700°F (370°C) and the coke p a r t i a l l y o x i d i z e d (presumably t o a c t i v a t e i t ) . The f i n a l s t e p c o n s i s t s of r e h e a t i n g the coke to about 1700°F (925°C) and t r e a t i n g i t w i t h methane or a s i m i l a r h y d r o c a r b o n . Some of the i n c o n s i s t e n c i e s i n the r e s e a r c h a r e p r o b a b l y due to v a r i a t i o n i n p e t r o l e u m c o k e s . I t i s g e n e r a l l y conceded t h a t the coker i s the "garbage c a n " of the r e f i n e r y and as such i t seldom sees an i d e n t i c a l charge from day to day. Cokes w i t h v i r t u a l l y i d e n t i c a l c h e m i c a l a n a l y s e s can e x h i b i t markedly d i f f e r e n t b e h a v i o r when s u b j e c t e d to i d e n t i c a l t h e r m a l t r e a t m e n t s . A New T h e r m a l - C h e m i c a l D e s u l f u r i z a t i o n Process To d a t e , a l l of the r e s e a r c h e f f o r t has not produced a commercial p r o c e s s to d e s u l f u r i z e p e t r o l e u m coke f o r use i n aluminum c e l l anodes. However, t h e r e i s a new and unique p r o c e s s t h a t promises to succeed where so many p r e v i o u s attempts have f a i l e d . T h i s new p r o c e s s has been patented (14) by Diamond West Energy C o r p o r a t i o n , and the r e s e a r c h work, conducted at The Cooper Union i n New York C i t y , has been j o i n t l y funded by Kennedy Van Saun C o r p o r a t i o n and Diamond West. The p r o c e s s c o n s i s t s of c o n t a c t i n g c a l c i n e d coke w i t h a s u l f u r b e a r i n g gas at e l e v a t e d t e m p e r a t u r e s . The concept of t r e a t i n g the c a l c i n e d coke i m m e d i a t e l y a f t e r i t i s d i s c h a r g e d from the c a l c i n e r i s a key element i n m i n i m i z i n g energy c o s t s . The hot c o k e , at about 2300°F (1260°C), i s f e d i n t o a r e a c t o r d i r e c t l y from the c a l c i n e r , where i t may be f u r t h e r heated up to 2800°F (1540°C), and exposed t o a s u l f u r b e a r i n g gas f o r a p e r i o d of time up t o 90 m i n u t e s .


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The desulfurization chemistry appears to depend on the presence of f r e e s u l f u r i n the c o n t a c t i n g gas and works in c o n j u n c t i o n w i t h o r g a n i c s u l f u r p y r o l y t i c a l l y r e l e a s e d from the coke. Gases t h a t decompose to form f r e e s u l f u r or r e a c t to form f r e e s u l f u r are s u i t a b l e d e s u l f u r i z a t i o n agents and i n c l u d e , f o r example, r e f i n e r y sour g a s , pure and d i l u t e I ^ S , mercaptans, m i x t u r e s of CO and S 0 or COS and H 0 . L a b o r a t o r y t e s t s on two-pound sampl Ï of green p e t r o l e u m cokes w i t h s u l f u r c o n t e n t s of 4 wt% to 6 wt% have s u c c e s s f u l l y reduced the s u l f u r l e v e l s to as low as 0.7 wt%. The degree of d e s u l f u r i z a tion can r e a d i l y be c o n t r o l l e d by a d j u s t i n g the temperature ( n o m i n a l l y between 2500 and 2 8 0 0 ° F ) , the h o l d i n g time i n the r e a c t o r (between 5 and 90 m i n u t e s ) , and the r e a c t a n t concentrations and f l o w r a t e s . The e x p e r i m e n t a l work has been conducted i n h o r i z o n t a l furnace tube r e a c t o r s c o n t a i n i n g f i x e d beds of s i z e d (1/2 i n c h χ 4 mesh) coke e l e c t r i c a l l y heated and c o n t r o l l e d at p r e c i s e temperatures over the e n t i r e bed l e n g t h . The coke i s f i r s t c a l c i n e d , f o l l o w i n g a temperature p r o f i l e s i m i l a r to t h a t of commercial c a l c i n i n g . The c a l c i n i n g i s i m m e d i a t e l y f o l l o w e d by d e s u l f u r i z a t i o n w i t h pure or d i l u t e s u l f u r gases metered from c y l i n d e r s . The d a t a shown i n Table I were o b t a i n e d at r e a c t i o n t e m p e r a t u r e s of 2730 °F (1500°C) and are t y p i c a l of measured d e s u l f u r i z a t i o n


2

2

Table

I.

Thermal D e s u l f u r i z a t i o n H o l d i n g Time (min)* 0 10 20 30 45 60

W i t h Added S u l f u r Gas

% Desulfurization**

0 15.2 32.1 55.0 66.8 71.2

* H o l d i n g times are at constant **Percent desulfurization is c a l c i n e d coke.

% Desulfurization**

0 27.5 48.0 66.8 76.8 83.8 temperature a f t e r c a l c i n a t i o n . relative to sulfur content

in

r e s u l t s a t a l l temperatures between 2550°F (1400°C) and 2900°F (1595°C). A n a l y s i s of these and o t h e r d a t a suggests t h a t the d e s u l f u r i z a t i o n p r o c e s s i s a complex sequence of s t e p s t e n t a t i v e l y represented as:


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BRANDT AND KAPNER

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Desulfurization

(-CS-) S

v


2S

V

(1)

+ (-CS-)

-> c s

2

(2)

+ (-C-)

-> c s

2

(3)

R e a c t i o n (1) c o n s t i t u t e s a p u r e l y t h e r m a l i n i t i a t i o n of t h e d e s u l f u r i z a t i o n p r o c e s s d e s c r i b e d by (1) and ( 2 ) . Step (1) would appear to be s l o w , r a t e c o n t r o l l i n g and w i t h a h i g h a c t i v a t i o n e n e r g y . A l s o i t i s v e r y r e s p o n s i v e to temperature i n c r e a s e s Hence, the g e n e r a l l y observed r a p i d a c c e l e r a t i o n of t h e r m a l d e s u l f u r i z a t i o n as the temperature i n c r e a s e s from 2400°F (1315°C) to 2900°F ( 1 5 9 5 ° C ) . By a d d i n g s u l f u r from an e x t e r n a l s o u r c e , the d e s u l f u r i z a t i o n d e s c r i b e d i n s t e p (2) i s no l o n g e r l i m i t e d by the r a t e a t which s u l f u r i s s u p p l i e d by t h e coke i t s e l f and d e s u l f u r i z a t i o n proceeds more r a p i d l y than by a p u r e l y t h e r m a l p r o c e s s a t every t e m p e r a t u r e . E x p e r i m e n t a l d a t a show t h a t t h e d e s u l f u r i z a t i o n w i t h added e x t e r n a l s u l f u r , r e p r e s e n t e d by r e a c t i o n ( 2 ) , i s dependent on temperature and c o n c e n t r a t i o n of s u l f u r i n t h e r e a c t a n t g a s . I t has a s i g n i f i c a n t l y lower a c t i v a t i o n energy than r e a c t i o n ( 1 ) . The o b s e r v a t i o n t h a t t h e a c t i v a t i o n energy of r e a c t i o n (1) i s g r e a t e r than t h a t of r e a c t i o n (2) i s confirmed by e x p e r i m e n t a l d a t a w h i c h show t h a t t h e r e l a t i v e r a t e s of d e s u l f u r i z a t i o n by t h e r m a l p r o c e s s i n g and by the a d d i t i o n of a s u l f u r gas decrease as h o l d i n g temperature i s i n c r e a s e d . T h i s p o i n t s out one of t h e more v a l u a b l e f e a t u r e s of t h i s t e c h n o l o g y — a h i g h r a t e of d e s u l f u r i z a t i o n can be s u p p o r t e d a t t e m p e r a t u r e s where t h e r m a l d e s u l f u r i z a t i o n i s not generally achieved. A n o t h e r v a l u a b l e f e a t u r e i s the p r e s e r v a t i o n of the c r i t i c a l properties, r e a l d e n s i t y and b u l k d e n s i t y . Real density i s a s p e c i f i c g r a v i t y measurement of the c a l c i n e d coke (performed on a sample t h a t has been reduced t o minus 200 mesh), and b u l k density i s a p o r o s i t y d e t e r m i n a t i o n (performed on a 35 χ 65 mesh s a m p l e ) . Both of these p r o p e r t i e s tend t o d e t e r i o r a t e t o u n a c c e p t a b l e levels d u r i n g t h e r m a l d e s u l f u r i z a t i o n a t h i g h temperatures due t o the c r e a t i o n of m i c r o - and m a c r o - p o r o s i t y as s u l f u r i s v a p o r i z e d and l e a v e s the s o l i d coke m a t r i x . The p e r s i s t e n c e of h i g h r e a l and b u l k d e n s i t y v a l u e s I n cokes d e s u l f u r i z e d u s i n g t h i s new t e c h n o l o g y i s a p r o m i s i n g i n d i c a t i o n t h a t some s o r t of a n n e a l i n g p r o c e s s o c c u r s d u r i n g the t r e a t m e n t . U n f o r t u n a t e l y , we do not have an e x p l a n a t i o n f o r t h i s phenomenon. The d a t a from almost one hundred d e s u l f u r i z a t i o n runs s t r o n g l y support t h e c l a i m and a r e summarized i n Table I I . Table I I .

Comparison of P h y s i c a l P r o p e r t i e s of D e s u l f u r i z e d Cokes w i t h T y p i c a l C a l c i n e d Coke S p e c i f i c a t i o n s

Property

Commercial Specification

Real Density (g/cc) Bulk Density ( l b / c u f t )

2.05-2.11 49 (minimum)

S u l f u r Gas D e s u l f u r i z e d Coke

2.07-2.16 53-55



198

P i l o t p l a n t work i s p r e s e n t l y under way at Kennedy Van Saun's f a c i l i t i e s i n D a n v i l l e , Pennsylvania. We hope t o be a b l e t o develop hardware t h a t can be r e t r o f i t t e d to e x i s t i n g c a l c i n e r s .

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