Catalytic Control of SOx Emissions from Fluid Catalytic Cracking Units

Chapter 8

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Ε. H. Hirschberg and R. J. Bertolacini Amoco Research Center, Amoco Oil Company, Naperville, IL 60566

T h i s paper identifies alumina, rare earths, p l a t i n u m , and magnesia as i m p o r t a n t SOx c a p t u r e materials. A l u m i n a is e i t h e r i n c o r p o r a t e d directly into the m a t r i x of a c r a c k i n g c a t a l y s t o r added as a separate particle. Cerium i s shown to promote the c a p t u r e of SO on h i g h a l u m i n a c r a c k i n g catalyst, a l u m i n a , and m a g n e s i a . O t h e r r a r e e a r t h s a r e ranked by their e f f e c t i v e n e s s . The p r o m o t i o n a l e f f e c t o f p l a t i n u m is shown between 1200 and 1 4 0 0 ° F f o r SO2 c a p t u r e on a l u m i n a . Silica, from f r e e silica or silica-alumina in the matrix of c r a c k i n g catalyst, a c t s as a p o i s o n by m i g r a t i n g to the a d d i t i v e . Silica from zeolite m i g r a t e s l e s s readily. In the magnesia-alumina system, spinel, as identified by X-ray diffraction, i s i n a c t i v e f o r SO r e m o v a l . The e f f e c t o f temperature on steam stability, oxidative a d s o r p t i o n and r e d u c t i v e d e s o r p t i o n of SO a r e described. F i v e commercial catalyst types are ranked f o r SOx r e m o v a l . 2

2

2

I t has been t e n y e a r s s i n c e Amoco announced the U l t r a C a t p r o c e s s (1) f o r SOx c o n t r o l i n FCC u n i t s . In t h o s e t e n y e a r s , as w e l l as i n the y e a r s p r e v i o u s to the announcement, much work was done to d e v e l o p c a t a l y s t s t h a t would c o n t r o l SOx e m i s s i o n s . The e v i d e n c e i s the 80 o r more U . S . p a t e n t s t h a t have i s s u e d i n t h a t time to Amoco and o t h e r s . One o f the f i r s t p a t e n t s i s s u e d was to Amoco i n 1974 (2) f o r the a d d i t i o n of magnesia and o t h e r group I I A o x i d e s to c r a c k i n g c a t a l y s t . T h i s paper r e v i e w s the SOx c a t a l y s t developments and emphasizes the work done a t Amoco to i d e n t i f y the a c t i v e m a t e r i a l s , e x p l a i n the d e a c t i v a t i o n mechanism a n d , f i n a l l y , to make a s i d e - b y - s i d e c o m p a r i s o n o f v a r i o u s c a t a l y t i c systems t h a t are being pursued commercially today.

0097-6156/88/0375-0114$09.00/0 • 1988 American Chemical Society

Occelli; Fluid Catalytic Cracking ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

8. HIRSCHBERG AND BERTOLACINI

Control of SO Emissions

115

Historical E a r l y H i s t o r y . Some o f the e a r l i e s t work on the c a t a l y t i c c o n t r o l of SOx was s t a r t e d a t Amoco by H e a l y and H e r t w i g (3) who, i n 1949, reported that, with a silica-magnesia cracking c a t a l y s t , sulfur c o u l d be s h i f t e d from coke t o d r y - g a s w i t h a c o r r e s p o n d i n g d e c r e a s e i n s u l f u r e m i s s i o n s i n the f l u e gas. As V a s a l o s (_1) l a t e r i n d i c a t e d , o t h e r c a t a l y s t changes caused a s t e a d y d e c r e a s e i n f l u e gas e m i s s i o n s t h r o u g h improvements i n coke s e l e c t i v i t y . CO b u r n i n g t e c h n o l o g y (4) a l s o lowered SOx e m i s s i o n s by l o w e r i n g coke on regenerated c a t a l y s t . D u r i n g t h i s e a r l y p e r i o d , a new mechanism f o r c a t a l y t i c c o n t r o l o f SOx emerged, w h i c h was r e l a t e d t o the a b i l i t y o f the c a t a l y s t t o c a p t u r e SOx, and t h i s l e d t o the development o f an e n t i r e l y new approach f o r e m i s s i o n c o n t r o l from FCU's ( 1 ) . The U l t r a C a t P r o c e s s . The U l t r a C a t p r o c e s s f o r SOx o x i d a t i v e c a p t u r e by a m e t a l o x i d e , MO, of S 0 from s u l f u r i n coke on the r e g e n e r a t o r s i d e o f the FCU, 2

S (coke)

+

S0

+ MO

2

+ h0

2

0

>

0

>

S0

MS0

control entails the b u r n i n g o f

(1)

o

(2)

4

and r e l e a s e of the S 0 on the r e a c t o r s i d e by r e d u c t i o n w i t h hydrogen o r h y d r o c a r b o n : 2

4H

0

+

MSO.

ζ

>

MO

+ HS 0

4

+ 3H 0 I I o

(3)

The o x i d a t i v e a d s o r p t i o n of S 0 j as shown i n E q u a t i o n 2, can be enhanced by u s i n g an o x i d a t i o n promotor. I f m e t a l s u l f i d e , MS, i n s t e a d of m e t a l o x i d e forms from the r e d u c t i o n o f the s u l f a t e , the s u l f i d e must h y d r o l y z e t o MO by the a c t i o n o f steam i n the s t r i p p e r s e c t i o n of the FCC u n i t : 2

MS

+ H0

>

2

MO

+ HS 2

(4)

I f the s u l f i d e does not h y d r o l y z e , s u l f u r w i l l be c a r r i e d t o the r e g e n e r a t o r and r e a p p e a r as S 0 a f t e r o x i d a t i o n i n the f l u e gas w i t h no net change i n s u l f u r e m i s s i o n s . The FCU r e g e n e r a t o r o p e r a t e s a t temperatures from about 1250-1350°F, w h i l e the r e a c t o r and s t r i p p e r o p e r a t e a t about 900-1000°F. 2

S e l e c t i o n of Oxides. At Amoco, p r e v i o u s s t u d i e s i n the l i t e r a t u r e on S 0 removal from f l u e gas have been used t o g u i d e the s e l e c t i o n of o x i d e s f o r the U l t r a C a t p r o c e s s but they have been of l i m i t e d d i r e c t u s e f u l n e s s . T h i s was t r u e because o f the p e c u l i a r r e q u i r e m e n t s o f the U l t r a C a t p r o c e s s o f h i g h a d s o r p t i o n temperature, low r e g e n e r a t i o n temperature, and n o n - i n t e r f e r e n c e w i t h the c r a c k i n g r e a c t i o n s . The p r e v i o u s l i t e r a t u r e s t u d i e s g e n e r a l l y assumed t h a t S 0 would be a d s o r b e d a t temperatures c l o s e to a s t a c k gas temperature o f 600°F, and desorb a t e i t h e r the same temperature o r h i g h e r . The c o n d i t i o n s o f t h e s e s t u d i e s was s e t , 2

2


FLUID CATALYTIC CRACKING: R O L E IN M O D E R N REFINING

116

not by conformance t o an e x i s t i n g p r o c e s s , l i k e c a t a l y t i c c r a c k i n g , but by economic c o n s i d e r a t i o n s . The e a r l y work o f B i e n s t o c k (_5) a t 625°F showed manganese, copper and c o b a l t o x i d e s t o be a c t i v e . But t h e s e m a t e r i a l s have not been used f o r t h e U l t r a C a t P r o c e s s p r o b a b l y because o f the adverse e f f e c t on t h e c r a c k i n g r e a c t i o n s . S i m i l a r work, a g a i n a t lower temperatures o f about 600°F, was done by two groups who i n v e s t i g a t e d t h e a d s o r p t i o n o f S 0 on m e t a l o x i d e s s u p p o r t e d on h i g h a r e a a l u m i n a . A t G u l f , V o g e l e t a l . (6) found copper and s t r o n t i u m o x i d e s t o be e x c e p t i o n a l b u t a l s o recommended group IA and I I A o x i d e s . K o b a l l a and Dudukovic (7^) ranked N i and Mn as b e s t f o l l o w e d by Co, F e , and Zn o x i d e s . Of these m a t e r i a l s o n l y magnesia proved t o be s u c c e s s f u l i n t h e UltraCat process. In a t h e o r e t i c a l s t u d y , L o w e l l e t a l . (8) s e l e c t e d o x i d e s from thermodynamic c o n s i d e r a t i o n s f o r a p r o c e s s i n which S 0 was adsorbed a t temperatures g r e a t e r than 100°C and desorbed by d e c o m p o s i t i o n o f t h e s u l f a t e o r s u l f i t e formed, a t temperatures below 750°C. Under t h e s e c o n s t r a i n t s , a l l o f 47 o x i d e s c o n s i d e r e d had p o t e n t i a l f o r a d s o r p t i o n b u t o n l y 16 had low enough decomposit i o n temperatures t o make a p r o c e s s e c o n o m i c a l . Intuitively, s u l f a t e d e c o m p o s i t i o n temperature s h o u l d c o r r e l a t e l o o s e l y w i t h r e d u c i b i l i t y o f s u l f a t e s , so i t i s i n t e r e s t i n g t h a t many o f t h e 16 o x i d e s chosen by L o w e l l , which i n c l u d e d c e r i u m and aluminum, have been shown t o be u s e f u l i n t h e U l t r a C a t p r o c e s s . DeBerry and S l a d e k (9) f o l l o w e d up on t h e work o f L o w e l l by measuring t h e r a t e s o f S 0 a d s o r p t i o n on t h e o x i d e s s e l e c t e d by Lowell. Cerium o x i d e was found t o have one o f t h e h i g h e s t r a t e s . More r e c e n t l y , workers a t U n o c a l (10) have used thermodynamics to s e l e c t o x i d e s f o r SOx removal from F C U s and have i d e n t i f i e d t h e o x i d e s o f 20 elements as l i k e l y c a n d i d a t e s . T h i s l i s t was narrowed f u r t h e r (11) t o s e v e r a l c a n d i d a t e s , Ce, A l , Co, N i and F e , by matching t h e d e c o m p o s i t i o n and r e d u c t i o n temperatures o f t h e i r s u l f a t e s t o the c o n d i t i o n s o f the p r o c e s s . 2

2

2

f

S e l e c t i o n o f Promotors. L o w e l l e t a l . (8) ranked m e t a l o x i d e s by t h e i r a b i l i t y t o o x i d i z e S 0 and, t h e r e f o r e , t o promote t h e o x i d a t i v e a d s o r p t i o n o f S 0 a c c o r d i n g t o E q u a t i o n 2. The r a n k i n g was r e f e r e n c e d t o p l a t i n u m w h i c h has h i g h a c t i v i t y ; V 0 and F e 0 were ranked h i g h f o l l o w e d by t h e m o d e r a t e l y a c t i v e o x i d e s , CuO and T i 0 ; C e 0 , Mn0 and S n 0 were c o n s i d e r e d t o have o n l y s l i g h t a c t i v i t y ; B i 0 , P b 0 and U 0 were l i s t e d as h a v i n g no o x i d a t i o n activity. I n t h e e n s u i n g y e a r s s e v e r a l announcements o f SOx c a p t u r e c a t a l y s t s were made by c a t a l y s t and p e t r o l e u m companies (10,12-18) as shown h e r e : 2

2

2

2

2

2

2

3

5

2

2

2


2

3

8. HIRSCHBERG AND BERTOLACINI 1977 1978 1981 1981 1983 1984 1985 1985 1986

117

Control ofSO Emissions χ

Amoco — U l t r a C a t P r o c e s s Arco/Engelhard — Soxcat Unocal — Unisox Chevron — T r a n s o x A r c o -- HRD 276 Engelhard — Ultrasox A r c o — HRD 277 Davison — A d d i t i v e R Chevron — T r a n s c a t

In a d d i t i o n , many p a t e n t s were i s s u e d , which have been summarized up t o 1982 by Habib (19) . S i n c e 1982, an a d d i t i o n a l 30 or so p a t e n t s have appeared b u t , the i m p o r t a n t elements and compounds t h a t have s u r f a c e d from the p a t e n t l i t e r a t u r e s t i l l a r e a l u m i n a , r a r e e a r t h s , p l a t i n u m and magnesia. Experimental A p p a r a t u s and P r o c e d u r e . A s i m p l e g l a s s and T e f l o n f i x e d - b e d a p p a r a t u s , shown i n F i g u r e 1, was used t o t e s t f o r S 0 removal from a m i x t u r e o f 1000 ppm S 0 , 2.7% 0 and about 2% water i n h e l i u m f l o w i n g a t 10 cc/min. The charge t o the a p p a r a t u s was 1.00 g o f m a t e r i a l w h i c h , i n most c a s e s , i n c l u d e d a c r a c k i n g c a t a l y s t w i t h low c a p a c i t y f o r S 0 , a s d i l u e n t . S 0 was measured c o n t i n u o u s l y u s i n g a DuPont 400 UV a n a l y z e r and r e p o r t e d as u£ a t 70°F and 1 atm. C a t a l y s t s a r e compared e i t h e r from p l o t s o f time v s . % S 0 removed o r by i n t e g r a l amounts o f S 0 removed a f t e r an a r b i t r a r y t i m e , u s u a l l y 92 m i n u t e s . The dead space i n the a p p a r a t u s p r e c e d i n g , as w e l l as f o l l o w i n g , the c a t a l y s t was kept t o a minimum but about 10 minutes e l a p s e b e f o r e S 0 r e a c h e s the d e t e c t o r . I n a d d i t i o n , the S 0 f l o w i n g t h r o u g h the a p p a r a t u s forms a d i f f u s e front. Both t h e s e f a c t o r s , the d i f f u s e f r o n t o f S 0 and the dead space, l e a d t o a u n i t f a c t o r o r an amount o f S 0 t h a t seems t o be removed even w i t h an i n e r t m a t e r i a l o r an empty tube. This unit f a c t o r i s a c c o u n t e d f o r by s i m p l y s u b t r a c t i n g i t from the t o t a l amount o f S 0 removed. A d s o r p t i o n t e m p e r a t u r e s ranged from 1250 t o 1500°F. C a t a l y s t r e g e n e r a t i o n s were made w i t h 10 cc/min. hydrogen f o r 15 m i n u t e s . Steam d e a c t i v a t i o n o f c a t a l y s t s was done a t v a r i o u s t e m p e r a t u r e s i n 100% steam and f o r v a r i o u s times as i n d i c a t e d i n the t e x t . 2

2

2

2

2

2

2

2

2

2

2

2

Catalysts. A v a r i e t y o f commercial and i n - h o u s e c a t a l y s t s was used i n t h i s work. The p r e p a r a t i o n s o f the i n - h o u s e c a t a l y s t s a r e d e s c r i b e d i n the t e x t . Analytical. S i l i c o n a n a l y s e s o f d e a c t i v a t e d c a t a l y s t s were made on c r o s s - s e c t i o n e d samples w i t h SEM-EDAX. R e s u l t s and D i s c u s s i o n Cracking C a t a l y s t Composition. S e v e r a l workers (20-21) have r e p o r t e d d i f f e r e n c e s among c r a c k i n g c a t a l y s t s t o remove SOx which c o r r e l a t e d q u a l i t a t i v e l y w i t h a l u m i n a c o n t e n t . Our work c o n f i r m e d t h e s e r e p o r t s as shown on F i g u r e 2. P l o t t e d a r e % S 0 removal 2



118

F i g u r e 2.

E f f e c t of Cracking C a t a l y s t

Composition.



Control of SO Emissions χ

119

c u r v e s f o r f o u r c a t a l y s t s w i t h alumina c o n t e n t s from 30 t o 60%. The c u r v e shown f o r 60% A 1 0 a c t u a l l y r e p r e s e n t s two c a t a l y s t s : one w i t h and one w i t h o u t r a r e e a r t h exchange. A t f i r s t , the s i m i l a r i t y o f t h e s e two c a t a l y s t s u n d e r e s t i m a t e d the importance o f rare earth content. To show the a l u m i n a e f f e c t q u a n t i t a t i v e l y , a s e r i e s o f c a t a l y s t s was made i n w h i c h the amount o f alumina i n the m a t r i x was v a r i e d from 25 t o 100% by a d d i n g a l u m i n a s o l t o a 25% a l u m i n a , silica-alumina slurry. These c a t a l y s t s were f o r m u l a t e d w i t h REY molecular sieve. The r e s u l t s f o r S 0 removal a r e shown i n F i g u r e 3 where S 0 removal ( c o r r e c t e d f o r u n i t f a c t o r ) i n c r e a s e s w i t h i n c r e a s i n g alumina. Our c o n c l u s i o n t h a t alumina was i m p o r t a n t f o r S 0 a d s o r p t i o n a l s o c o n f i r m e d the r e s u l t s o f B l a n t o n and F l a n d e r s at Chevron ( 2 2 ) . The n o n - l i n e a r i t y o f the r e l a t i o n s h i p i m p l i e s an a n t a g o n i s t i c e f f e c t between s i l i c a and a l u m i n a . The s i l i c a - a l u m i n a antagonism w i l l be d i s c u s s e d r e l a t i v e t o d e a c t i v a t i o n s u b s e q u e n t l y . The S 0 removal c a p a b i l i t y o f c r a c k i n g c a t a l y s t s w i t h h i g h a l u m i n a m a t r i x can be enhanced. F o r i n s t a n c e , we found t h a t z e o l i t e t y p e c o u l d be i m p o r t a n t . F i g u r e 4 shows t h a t f o r c a t a l y s t s f o r m u l a t e d w i t h a m a t r i x c o n t a i n i n g 60% a l u m i n a , a w e l l - e x c h a n g e d r a r e e a r t h Y z e o l i t e , as measured by a sodium c o n t e n t o f 0.1%, gave better S0 removal than e i t h e r a m o d e r a t e l y exchanged r a r e e a r t h Y z e o l i t e , 2% sodium, o r an u l t r a s t a b l e Y z e o l i t e ( 2 3 ) . These c r a c k i n g c a t a l y s t s were f o r m u l a t e d by a d d i n g alumina s o l and z e o l i t e t o a low a l u m i n a s i l i c a - a l u m i n a g e l . Both REY c a t a l y s t s c o n t a i n e d 15% z e o l i t e ; the USY c a t a l y s t 25%. The improvement i n S 0 removal was o b s e r v e d o n l y i f the c a t a l y s t was f o r m u l a t e d w i t h a h i g h alumina m a t r i x . The u n u s u a l e n h a n c i n g e f f e c t o f the w e l l - e x c h a n g e d REY remains u n e x p l a i n e d , but the r e s u l t s l e d t o more e x p e r i m e n t s w i t h b o t h r a r e e a r t h s and a l u m i n a . 2

3

2

2

2

2

2

2

Rare E a r t h s and A l u m i n a . A much e a s i e r and cheaper way o f g e t t i n g the S 0 removal enhancement from r a r e e a r t h s t h a t was o b s e r v e d w i t h the w e l l - e x c h a n g e d r a r e e a r t h Y z e o l i t e was t o add r a r e e a r t h s , e s p e c i a l l y c e r i u m , by d i r e c t i m p r e g n a t i o n t o h i g h a l u m i n a c r a c k i n g c a t a l y s t (24). The e f f e c t o f c e r i u m was shown by measuring the s u l f a t e / c e r i u m m o l a r r a t i o o b s e r v e d when c e r i u m was added a t 5, 10 and 20 ppm by i m p r e g n a t i o n t o a h i g h a l u m i n a , commercial c r a c k i n g c a t a l y s t w h i c h c o n t a i n e d no r a r e e a r t h . A f t e r S 0 a d s o r p t i o n a t 1250°F f o r 140 m i n u t e s the c a t a l y s t s were a n a l y z e d f o r s u l f a t e . The r e s u l t s a r e shown i n F i g u r e 5 where the mole r a t i o i s p l o t t e d v s . c e r i u m on catalyst. The mole r a t i o s , which were c a l c u l a t e d t o e x c l u d e the s u l f a t e adsorbed by the c r a c k i n g c a t a l y s t w i t h o u t c e r i u m , a r e much g r e a t e r than the s t o i c h i o m e t r i c r a t i o o f 1.5 o r 2.0 f o r c e r o u s o r e e r i e s u l f a t e , demonstrating a l a r g e c a t a l y t i c e f f e c t . The s u l f a t e / c e r i u m mole r a t i o d e c l i n e s w i t h c e r i u m c o n t e n t i n t h i s experiment because the amount o f s u l f a t e accumulated was r e l a t i v e l y i n s e n s i t i v e t o the amount o f c e r i u m added t o the c a t a l y s t . A large e f f e c t was o b s e r v e d w i t h the f i r s t i n c r e m e n t a l amount added w i t h p r o p o r t i o n a t e l y s m a l l e r e f f e c t s w i t h subsequent amounts. This o b s e r v a t i o n s u p p o r t s the c o n c l u s i o n t h a t c e r i u m i s c a t a l y t i c . 2

2



Wt.

F i g u r e 3.

0

I

ι

Effect

ι

ι

20

%

of M a t r i x

ι

ι

40

ι

Composition.

ι

60

ι

ι

80

I 100

Time, minutes

F i g u r e 4.

Effect

of Z e o l i t e

Type.



121

Control of Submissions

Independence from c r a c k i n g c a t a l y s t c o m p o s i t i o n has been shown by a d d i n g c e r i u m and a l u m i n a t o c r a c k i n g c a t a l y s t ( 2 5 ) . The d a t a on T a b l e I demonstrate this. Table I. E f f e c t

o f Cerium

and Alumina

2

FCC

+ 25 ppm Ce (on

FCC

+ A1 0

FCC

44

FCC)

(5%)

54

+ 25 ppm Ce (on FCC) + A1 0 (5%)

127

+ 25 ppm Ce (on A 1 0 ) + A1 0 (5%)

130

2

FCC

3

3

2

2

Catalyst

Net S 0 Removed a t 1250°F, μΐ a f t e r 92 minutes 0

Catalyst Description FC C a t a l y s t

2

Added t o C r a c k i n g

3

3

R e s u l t s a r e shown as n e t g a i n over the c r a c k i n g c a t a l y s t which showed poor S 0 removal a b i l i t y . The c r a c k i n g c a t a l y s t was a low alumina-REY t y p e . A d d i t i o n o f 25 ppm c e r i u m t o the c r a c k i n g c a t a l y s t i n c r e a s e d S 0 a d s o r p t i o n even though the c a t a l y s t a l r e a d y contained r a r e earths i n c l u d i n g cerium. Cerium was added by i m p r e g n a t i o n from an aqueous s o l u t i o n o f e e r i e ammonium n i t r a t e . A d d i t i o n o f a l u m i n a , as a s e p a r a t e p a r t i c u l a t e , a l s o had a p o s i t i v e e f f e c t even though the c a t a l y s t c o n t a i n e d a l u m i n a . Adding both c e r i u m and a l u m i n a , whether the c e r i u m i s on the c r a c k i n g c a t a l y s t o r on the a l u m i n a , however, i n c r e a s e d S 0 a d s o r p t i o n disproportionately. A g a i n , from t h e s e r e s u l t s we c o n c l u d e d t h a t c e r i u m was a c a t a l y s t f o r the o x i d a t i v e a d s o r p t i o n o f S 0 . 2

2

2

2

The p r o m o t i o n a l e f f e c t o f c e r i u m i s not c o n f i n e d t o alumina as shown by the d a t a f o r magnesia (26) i n T a b l e I I . Table I I .

Effect

o f Cerium

Catalyst Description FCC + 500 ppm MgO FCC

and Magnesia Added t o C r a c k i n g C a t a l y s t

Net

+ 500 ppm MgO + 25 ppm Ce (on MgO)

S 0 Removed a t 1250°F, μΐ a f t e r 92 m i n u t e s 113 2

235

R e s u l t s , a g a i n , show n e t g a i n over the same c r a c k i n g c a t a l y s t used p r e v i o u s l y f o r cerium/alumina case. Cerium, moreover, seems t o a c t as a promotor f o r o t h e r r a r e e a r t h s as c o u l d be i m p l i e d from the s y n e r g i s t i c e f f e c t o b s e r v e d between c e r i u m and lanthanum (27) . Our c o n c l u s i o n s about the c a t a l y t i c e f f e c t o f c e r i u m have been c o n f i r m e d r e c e n t l y by o t h e r s ( 2 8 ) . Other r a r e e a r t h s , i n c l u d i n g y t t r i u m (29) and lanthanum (30) a r e a c t i v e f o r S 0 removal as shown on T a b l e I I I . 2



122

Table I I I .

Effect

o f Other Rare E a r t h s

Relative S0 Removal 1.0 1.0 0.9 0.8 0.6 0.6 0.4 0.4 0.4

Rare E a r t h Ce Y Mixed (Ce R i c h ) La Pr Dy Gd Nd Sm

2

These m a t e r i a l s were made t o c o n t a i n 10 wt% o x i d e s on gamma alumina. The p e r c e n t a g e o f S 0 removed a f t e r 50 minutes was measured, a t 1250°F, f o r t h e s e a d d i t i v e s a t the 1 wt% l e v e l mixed with cracking c a t a l y s t s . They were then ranked by the r a t i o o f the % removed t o t h a t removed by c e r i u m on a l u m i n a . 2

Platinum. Other m a t e r i a l s a r e e f f e c t i v e promotors f o r the o x i d a t i v e a d s o r p t i o n of S0 . F i g u r e 6, f o r i n s t a n c e , demonstrates the e f f e c t o f p l a t i n u m which i s the b e s t promotor and the e a r l i e s t one used f o r the U l t r a C a t p r o c e s s ( 3 1 ) . The f i g u r e , which compares S 0 removal c u r v e s f o r a l u m i n a a l o n e and w i t h 2 and 100 ppm Pt a t 1200, 1300 and 1400°F, i n d i c a t e s t h a t a l u m i n a promoted w i t h p l a t i n u m a t b o t h l e v e l s i s more e f f i c i e n t f o r removing S 0 than pure a l u m i n a . The c a t a l y t i c e f f e c t o f p l a t i n u m , not u n e x p e c t e d l y , becomes l e s s pronounced as the temperature i s i n c r e a s e d as can be seen by i n s p e c t i n g the c u r v e s and a l s o by comparing the p e r c e n t a g e o f S 0 removed a f t e r 100 minutes as shown on T a b l e IV. 2

2

2

2

T a b l e IV. C a t a l y t i c E f f e c t o f P l a t i n u m % SO Removed a t 100 M i n u t e s P l a t i n u m C o n c e n t r a t i o n , ppm 0 2 100 36 85 98 43 78 90 50 68 86 2

Temperature, 1200 1300 1400

°F

Without p l a t i n u m , a l u m i n a becomes more e f f e c t i v e f o r removing S 0 as the temperature i s i n c r e a s e d . In t h i s unpromoted c a s e , the r a t e o f o x i d a t i v e a d s o r p t i o n o f S 0 c o n t r o l s the amount o f S 0 removed. I n c r e a s i n g the temperature i n c r e a s e s t h a t r a t e . I n c o n t r a s t , w i t h p l a t i n u m , S 0 r e m o v a l , w h i l e always g r e a t e r than the unpromoted c a s e , tends t o d e c r e a s e w i t h i n c r e a s i n g temperature. The p r e s e n c e o f p l a t i n u m i n c r e a s e s the r a t e o f o x i d a t i v e a d s o r p t i o n of S0 t o the p o i n t t h a t the c a p a c i t y o f a l u m i n a becomes the l i m i t i n g f a c t o r r a t h e r t h a n the r a t e . The c a p a c i t y , l i m i t e d by thermodynamics, d e c r e a s e s w i t h i n c r e a s i n g temperature because o f the s t a b i l i t y of s u r f a c e s u l f a t e s p e c i e s 2

2

2

2

2


HIRSCHBERG AND BERTOLACINI

F i g u r e 5.

F i g u r e 6.


Catalytic

Catalytic

%

E f f e c t o f Cerium.

Effect

of Platinum.



124

b e i n g formed. Presumably, a t a h i g h enough temperature p l a t i n u m would have no c a t a l y t i c e f f e c t and t h e c a p a c i t y o f a l u m i n a would be lowered more. These r e s u l t s e x t e n d t h o s e found p r e v i o u s l y by Summers ( 3 2 ) . P l a t i n u m , as w e l l as o t h e r promotors, c a n cause problems such as NOx f o r m a t i o n o r o t h e r u n f a v o r a b l e s i d e e f f e c t s . The r a r e e a r t h s do n o t have t h e s e drawbacks so a good d e a l o f a t t e n t i o n has been g i v e n t o t h e i r development f o r SOx removal. Deactivation P i l o t Plant Testing. P i l o t p l a n t t e s t i n g o f 10% c e r i u m on a l u m i n a mixed w i t h c r a c k i n g c a t a l y s t showed a d i s c r e p a n c y i n t h e e x t e n t o f o b s e r v e d and p r e d i c t e d d e a c t i v a t i o n . D u r i n g the t e s t , which was made a t 1350°F, a common FCU r e g e n e r a t i o n temperature, t h e c a t a l y s t was sampled and t e s t e d i n s e v e r a l ways. We used o u r bench t e s t t o measure S 0 removal a b i l i t y on t h e samples b e f o r e and a l s o a f t e r r e d u c t i o n w i t h hydrogen a t 1350°F, t o remove s u l f a t e . Then, t h e a d d i t i v e p o r t i o n o f t h e samples was s e p a r a t e d by f l o a t - s i n k u s i n g tetrabromoethane. Estimates of the e f f i c i e n c y of the s e p a r a t i o n were made by m i c r o s c o p i c i n s p e c t i o n and found t o be g r e a t e r t h a n about 95% i n a l l c a s e s . The d i f f e r e n c e s i n shape and appearance made i t easy t o d i s t i n g u i s h v i s u a l l y c e r i u m / a l u m i n a a d d i t i v e from cracking catalyst. L o s s e s by a t t r i t i o n were c a l c u l a t e d from t h e weight o f t h e r e c o v e r e d a d d i t i v e . S u r f a c e a r e a l o s s e s were measured by comparing t h e s u r f a c e a r e a o f t h e f r e s h a d d i t i v e t o t h e s u r f a c e area of the separated a d d i t i v e . L o s s e s i n S 0 removal a b i l i t y were p r e d i c t e d from r e l a t i o n s h i p s between s u r f a c e a r e a , weight p e r c e n t o f a d d i t i v e , and S 0 r e m o v a l . R e d u c t i o n had no e f f e c t i n d i c a t i n g t h a t s u l f a t e a c c u m u l a t i o n was n o t i m p o r t a n t . The l a r g e d i f f e r e n c e s between p r e d i c t e d and o b s e r v e d l o s s e s shown on T a b l e V i n d i c a t e d t h a t o t h e r f a c t o r s were i n v o l v e d i n t h e deactivation. 2

2

2

T a b l e V.

Pilot

P l a n t D e a c t i v a t i o n o f Cerium/Alumina

Additive

% L o s s i n S 0 Removal Δ Observed Predicted 28 16 12 24 49 25 52 25 27 62 33 29 2

Time, H r s . 30 40 60 80

SEM-EDAX e x a m i n a t i o n o f c r o s s - s e c t i o n e d samples t a k e n from t h e p i l o t p l a n t t e s t a t 30, 60 and 80 hours show t h e p r e s e n c e o f silicon. T h i s i s i n d i c a t e d i n t h e p h o t o m i c r o g r a p h s i n F i g u r e 7. The o u t l i n e s a r e c e r i u m / a l u m i n a p a r t i c l e s ; t h e b r i g h t w h i t e d o t s represent s i l i c o n . Q u a l i t a t i v e l y , s i l i c o n contamination increases w i t h time. The p r e s e n c e o f s i l i c o n on t h e d e a c t i v a t e d a d d i t i v e r a i s e d s e v e r a l q u e s t i o n s about t h e s i l i c o n s o u r c e and how i t g e t s t o t h e additive. The e f f e c t o f steam was examined f i r s t .



Control of Submissions

F i g u r e 7. SEM-EDAX A n a l y s i s o f P i l o t P l a n t Samples: Silicon. Top l e f t , 30 h o u r s ; top r i g h t , 60 h o u r s ; and bottom, 80 h o u r s .


125


126

Bench S c a l e Steaming E x p e r i m e n t s . Steaming e x p e r i m e n t s showed t h a t steam d e a c t i v a t i o n o f the c e r i u m / a l u m i n a c a t a l y s t was more s e v e r e when d e a c t i v a t e d i n the p r e s e n c e o f c r a c k i n g c a t a l y s t . Figure 8 shows how S 0 removal d e c r e a s e s w i t h steaming time a t 1350°F. The r e s u l t s a r e shown f o r t h r e e e x p e r i m e n t s i n a f l u i d i z e d bed apparatus. I n the f i r s t , c e r i u m on alumina a d d i t i v e was steamed i n a Vycor tube w i t h o u t c r a c k i n g c a t a l y s t . The a d d i t i v e was sampled at v a r i o u s times d u r i n g the steaming and t e s t e d f o r S 0 removal ability. In the second and t h i r d e x p e r i m e n t s , the c e r i u m on a l u m i n a a d d i t i v e was steamed i n the p r e s e n c e o f two commercial FC c a t a l y s t s o f d i f f e r e n t c o m p o s i t i o n s : h i g h and low a l u m i n a . Again, the c a t a l y s t - a d d i t i v e m i x t u r e s were sampled a t v a r i o u s t i m e s . The a d d i t i v e p o r t i o n o f the samples, which was s e p a r a t e d by f l o a t - s i n k method, was t e s t e d f o r S 0 removal a b i l i t y . As the d a t a i n F i g u r e 8 i n d i c a t e , t h e r e was some d e a c t i v a t i o n i n a l l c a s e s , but a s t r o n g i n t e r a c t i v e e f f e c t between a d d i t i v e and c r a c k i n g c a t a l y s t i s e v i d e n t i n t h a t the r a t e s o f d e a c t i v a t i o n a r e much g r e a t e r f o r steam d e a c t i v a t i o n i n the p r e s e n c e o f c r a c k i n g c a t a l y s t . However, the e a r l y d e a c t i v a t i o n , e s p e c i a l l y , i s l e s s pronounced f o r steam d e a c t i v a t i o n i n the p r e s e n c e o f h i g h a l u m i n a c r a c k i n g c a t a l y s t compared t o a low a l u m i n a c r a c k i n g c a t a l y s t . The p r e s e n c e o f the h i g h a l u m i n a c r a c k i n g c a t a l y s t seems t o r e t a r d d e a c t i v a t i o n . S i m i l a r e x p e r i m e n t s w i t h d r y n i t r o g e n showed t h a t steam was n e c e s s a r y f o r the o b s e r v e d d e a c t i v a t i o n . 2

2

2

The steamed samples were a g a i n examined by SEM-EDAX. The r e s u l t s i n F i g u r e 9 a r e f o r samples t a k e n a t 2 and 24 hours f o r c e r i u m on a l u m i n a a d d i t i v e steamed w i t h o u t and w i t h h i g h and low alumina c r a c k i n g c a t a l y s t . A g a i n , the o u t l i n e d shapes a r e the c e r i u m on a l u m i n a a d d i t i v e and the b r i g h t s p o t s s i l i c o n . Some s i l i c o n i s p r e s e n t i n a l l the samples i n s p e c t e d , even the sample steamed i n the absence o f c r a c k i n g c a t a l y s t , showing t h a t the a d d i t i v e has been c o n t a m i n a t e d w i t h s i l i c o n from the V y c o r steaming tube. The l e v e l o f s i l i c o n , however, i s much g r e a t e r f o r samples s t e a m - d e a c t i v a t e d i n the p r e s e n c e o f a c r a c k i n g c a t a l y s t , even f o r those steamed f o r as l i t t l e as two h o u r s . A l s o , e s p e c i a l l y f o r the two-hour samples, the amount o f s i l i c o n on the a d d i t i v e d e a c t i v a t e d i n the p r e s e n c e o f h i g h a l u m i n a (low s i l i c a ) c r a c k i n g c a t a l y s t seems t o be s u b s t a n t i a l l y l e s s t h a n e i t h e r o f those samples d e a c t i v a t e d i n the p r e s e n c e o f low a l u m i n a ( h i g h s i l i c a ) c a t a l y s t s . S u r f a c e a r e a l o s s e s f o r a l l the samples were about the same. S i l i c a Added D i r e c t l y . D i r e c t a d d i t i o n o f s i l i c a t o c e r i u m on a l u m i n a a l s o d e a c t i v a t e s the a d d i t i v e . To show t h i s , the a d d i t i v e was impregnated w i t h aqueous s o l u t i o n s o f " s i l i c i c a c i d prepared by i o n exchange o f sodium s i l i c a t e w i t h IR-120 a c i d i o n exchange r e s i n , t h e n d r i e d and c a l c i n e d a t 1000°F f o r 5 h r s . S 0 removal a b i l i t y was measured on the a s - p r e p a r e d samples and a f t e r steaming a t 1400 and 1550°F f o r 5 h r s . The r e s u l t s shown i n F i g u r e 10 i n d i c a t e t h a t the d e a c t i v a t i n g e f f e c t o f s i l i c a i s much worse a f t e r the samples a r e steamed. The mere p r e s e n c e o f s i l i c a i s not enough. An i n t e r a c t i o n between the c e r i u m / a l u m i n a a d d i t i v e and s i l i c a b r o u g h t about by steam and/or h e a t i s n e c e s s a r y f o r the f u l l d e a c t i v a t i n g e f f e c t o f s i l i c a t o be o b s e r v e d . 1 1

2




300

—

— —

0

— — · —

—

No catalyst High alumina FCC

1

1

1

1

1

1

ι

ι

ι

ι

ι

2

4

6

8

10

12

14

16

18

20

22

I 24 26

Steaming time, hours

F i g u r e 8.

Steam D e a c t i v a t i o n o f Cerium on Alumina.


127


128

No

catalyst

F i g u r e 9.

High alumina c a t a l y s t

Low

alumina

SEM-EDAX A n a l y s i s of S t e a m - D e a c t i v a t e d Silicon.

catalyst

Samples:



F i g u r e 10.

Control of ΞΟ Emissions

Effect

χ

o f S i l i c a and Steam.


129


130

Source o f S i l i c a . S i l i c a can m i g r a t e e i t h e r from f r e e s i l i c a p r e s e n t i n the c r a c k i n g c a t a l y s t o r from the s i l i c a a l u m i n a m a t r i x but not as r e a d i l y from the z e o l i t e . F i g u r e 11 shows SEM-EDAX s i l i c o n s c a n s o f c e r i u m / a l u m i n a steamed i n the p r e s e n c e o f t h e s e three sources of s i l i c a . A g a i n , the b r i g h t d o t s r e p r e s e n t s i l i c o n . Q u a l i t a t i v e l y the sample steamed w i t h pure s i l i c a c o n t a i n s more s i l i c o n t h a n the sample steamed w i t h s i l i c a - a l u m i n a . The sample steamed w i t h z e o l i t e shows s i l i c o n a t the s u r f a c e of the c r o s s - s e c t i o n e d p a r t i c l e but l i t t l e i n the i n t e r i o r . The s u r f a c e s i l i c o n comes from d u s t i n g o f the p a r t i c l e w i t h v e r y f i n e l y d i v i d e d zeolite. We are not a l o n e i n i m p l i c a t i n g s i l i c a i n the d e a c t i v a t i o n mechanism o f SOx a d d i t i v e s . Our r e s u l t s s u p p o r t B l a n t o n ' s c o n c e r n about s i l i c a (33) and a l s o a g r e e s w e l l w i t h the work o f s e v e r a l o t h e r s (15,17,34). The M o b i l i t y of S i l i c a i n Steam. The r e a c t i v i t y of s i l i c a and s i l i c a - c o n t a i n i n g m a t e r i a l s t o steam has been assumed i n the l i t e r a t u r e t o e x p l a i n s e v e r a l phenomena, a few o f which a r e : the s i n t e r i n g o f s i l i c a ( 3 5 ) , the a g i n g of amorphous s i l i c a a l u m i n a c r a c k i n g c a t a l y s t s (36) and the f o r m a t i o n o f u l t r a s t a b l e m o l e c u l a r s i e v e s (37). The b a s i s of a l l t h e s e e x p l a n a t i o n s i s the i n t e r a c t i o n of s i l i c e o u s m a t e r i a l s w i t h water t o form m o b i l e , low m o l e c u l a r weight s i l i c o n compounds by h y d r o l y s i s (38) such a s : (Si0 ) 2

n

+ 2H 0 2

>

Si(0H)

4

+

(Si0 ) 2

(5)

n - 1

S i l i c a i s known t o be v o l a t i l e i n steam (39-40). For i n s t a n c e , the e q u i l i b r i u m c o n c e n t r a t i o n o f s i l i c a i n steam a t 1 atmosphere from 1200 t o 1450°F has been found t o range from 0.2 t o 0.5 ppm (by w e i g h t ) showing t h a t the a v a i l a b i l i t y o f s i l i c a i n the v a p o r phase can be s u b s t a n t i a l under the c o n d i t i o n s used i n our work. U n f o r t u n a t e l y , we cannot p r e d i c t how much s i l i c a w i l l be t r a n s f e r r e d i n our e x p e r i m e n t s s i n c e the r a t e s of e i t h e r h y d r o l y s i s o r a d s o r p t i o n on the c e r i u m on a l u m i n a a d d i t i v e a r e unknown. The r a t e s o f h y d r o l y s i s of s i l i c e o u s m a t e r i a l s w i l l be a f f e c t e d by s e v e r a l f a c t o r s . F o r i n s t a n c e , the r a t e w i l l be d i r e c t l y r e l a t e d t o s u r f a c e a r e a , e x p l a i n i n g the low r a t e s o b s e r v e d f o r s i l i c a d e p o s i t i o n from the V y c o r a p p a r a t u s . A l s o , the c o m p o s i t i o n of the s i l i c e o u s m a t e r i a l w i l l i n f l u e n c e the r a t e o f h y d r o l y s i s , e x p l a i n i n g the d i f f e r i n g amounts of s i l i c a t r a n s f e r r e d from pure s i l i c a , s i l i c a a l u m i n a , z e o l i t e , and the h i g h a l u m i n a cracking catalyst. Mechanism of D e a c t i v a t i o n . The d e t a i l e d mechanism f o r s i l i c a p o i s o n i n g o f c e r i u m on a l u m i n a a d d i t i v e i s unknown but we suggest t h a t s i l i c a , as v o l a t i l e s i l i c i c a c i d , a d s o r b s s t r o n g l y , and, under the i n f l u e n c e of steam and/or heat r e a c t s t o i r r e v e r s i b l y remove s i t e s t h a t a r e r e q u i r e d f o r the o x i d a t i v e a d s o r p t i o n o f S 0 . S i l i c i c a c i d r e a c t s i n what can be c o n s i d e r e d a s i m p l e a c i d base type r e a c t i o n and permanently l o w e r s the c a p a c i t y of the a d d i t i v e . More work would have t o be done to show whether s i l i c a i n t e r a c t s w i t h c e r i u m and/or a l u m i n a . l i e r ( 4 1 ) , however, has remarked t h a t a l u m i n a and s i l i c a a r e c o n s i d e r e d t o have a 2



Control ofSO Emissions %

F i g u r e 11. SEM-EDAX A n a l y s i s o f Cerium/Alumina: Silicon. Top l e f t , p u r e s i l i c a ; top r i g h t , s i l i c a a l u m i n a ; and bottom, m o l e c u l a r s i e v e .


131


132

" p e c u l i a r " a f f i n i t y towards each o t h e r . He goes on t o s t a t e t h a t w h i l e t h e r e a r e o n l y a few o b s e r v a t i o n s r e g a r d i n g the i n t e r a c t i o n w i t h s i l i c i c a c i d , i t has been e s t a b l i s h e d from s o l u t i o n c h e m i s t r y t h a t S i i O H ) ^ s t r o n g l y a d s o r b s onto the s u r f a c e of hydroxy alumina o x i d e and a l s o r e a c t s w i t h gamma a l u m i n a . I n some c a s e s an a l u m i n o s i l i c a t e w i t h the h a l o y s i t e c o m p o s i t i o n forms: Al Si 0 (OH) 2

2

5

4

On an anhydrous b a s i s the above c o m p o s i t i o n would c o n t a i n 46% alumina. From F i g u r e 3 such a m a t e r i a l would have lower S 0 removal c a p a c i t y compared t o a l u m i n a . 2

How t o S o l v e the D e a c t i v a t i o n Problem. S o l u t i o n s t o the d e a c t i v a t i o n problem a r e d i f f i c u l t . The p a t e n t l i t e r a t u r e (42) has c l a i m s t h a t e i t h e r sodium, manganese o r phosphorous added t o alumina p r e v e n t s d e a c t i v a t i o n by s i l i c a . I n a d d i t i o n , removal o f m a t r i x s i l i c a from c r a c k i n g c a t a l y s t f o r m u l a t i o n s s h o u l d p r e v e n t f u r t h e r d e a c t i v a t i o n because z e o l i t i c s i l i c a , as we have shown, m i g r a t e s more s l o w l y . There i s a t l e a s t one p a t e n t r e l a t i n g t o v e r y h i g h alumina m a t r i x c r a c k i n g c a t a l y s t s (43). Another s o l u t i o n i s t o use more a c t i v e SOx c a t a l y s t s such as magnesia-based materials. Magnesia The h i g h c a p a c i t y m a t e r i a l d e s c r i b e d i n the p a t e n t l i t e r a t u r e (44-45) i s a c e r i u m promoted " o v e r - b a s e d " magnesia-alumina i n which magnesia i s added i n e x c e s s o f the s t o i c h i o m e t r i c s p i n e l composition, MgAl 0i . To b e t t e r u n d e r s t a n d t h i s system, a s e r i e s o f c a t a l y s t s was p r e p a r e d w i t h v a r i o u s amounts o f magnesia and a l u m i n a from a l u m i n a s o l and magnesium hydroxide s l u r r y . A f t e r d r y i n g and c a l c i n i n g , t h e s e m a t e r i a l s were impregnated w i t h e e r i e ammonium n i t r a t e t o 6% C e 0 . The d a t a i n F i g u r e 12 show how S 0 removal changes w i t h c o m p o s i t i o n . The d a t a a r e e x p r e s s e d i n terms o f p e r c e n t a p p r o a c h t o the s t o i c h i o m e t r i c amount o f S 0 t h a t c o u l d have been adsorbed based on C e 0 , MgO, and A 1 0 assuming complete s u l f a t e f o r m a t i o n . The t e s t s were made a t 1350°F f o r 92 m i n u t e s . In a l l c a s e s , removal o f S 0 was measured on the same weight o f c a t a l y s t . Magnesia i s a t l e a s t an o r d e r o f magnitude more e f f i c i e n t f o r removing S 0 than a l u m i n a . In a d d i t i o n , t h e r e i s an u n f a v o r a b l e i n t e r a c t i o n between magnesia and a l u m i n a as i n d i c a t e d by the n o n - l i n e a r r e l a t i o n s h i p between S 0 removal and c o m p o s i t i o n w i t h the f u l l e f f e c t o f magnesia unobserved u n t i l w e l l above 50 mole%. There i s no c l e a r e v i d e n c e t o i d e n t i f y the a c t i v e m a t e r i a l f o r S0 removal i n a M g A l 0 " s t o i c h i o m e t r i c " system. F i g u r e 13 shows r e s u l t s f o r a 50-50 mole% magnesia-alumina m a t e r i a l p r e p a r e d from magnesium h y d r o x i d e and alumina s o l and c a l c i n e d a t v a r i o u s temperatures. An attempt was made t o c o r r e l a t e S 0 removal w i t h compound f o r m a t i o n , as measured by X - r a y d i f f r a c t i o n , and s u r f a c e area. As i n d i c a t e d i n the f i g u r e , S 0 removal a b i l i t y d e c r e a s e d w i t h i n c r e a s i n g c a l c i n a t i o n temperature as d i d s u r f a c e a r e a . X-ray d i f f r a c t i o n a n a l y s i s showed s p i n e l f o r m a t i o n i n c r e a s e s as 2

f

2

2

2

2

2

3

2

2

2

2

2

it

2

2



F i g u r e 12.

Control ο/ΞΟ Emissions χ

Magnesia-Alumina System: 6% C e r i a .

Calcination temperature, °C

F i g u r e 13.

Effect of Calcination MgO-Al2Û3.

Temperature: 50-50 Mole%


133


134

c a l c i n a t i o n temperature i n c r e a s e d . The o n l y o t h e r m a t e r i a l s found by XRD were minor amounts of gamma alumina i n some o f the samples c a l c i n e d a t low temperature and v e r y minor amounts o f f r e e magnesia i n the samples c a l c i n e d a t h i g h t e m p e r a t u r e . Temperature

Effects

An u n d e r s t a n d i n g o f temperature e f f e c t s i s i m p o r t a n t t o maximize the b e n e f i t from SOx removal a g e n t s . There a r e t h r e e a r e a s i n which temperature e f f e c t s a r e p a r t i c u l a r l y i m p o r t a n t : — — —

steam s t a b i l i t y , oxidative adsorption of S0 , reductive desorption of S0 2

and

2

Steam S t a b i l i t y . Steam s t a b i l i t y o f SOx removal agents i s s t r o n g l y a f f e c t e d by t e m p e r a t u r e . We have seen p r e v i o u s l y t h a t a t 1350°F d e a c t i v a t i o n o f c e r i u m / a l u m i n a a d d i t i v e , caused by s i l i c a p o i s o n i n g , was i n f l u e n c e d by how l o n g the a d d i t i v e was steamed and whether the a d d i t i v e was steamed i n the p r e s e n c e o r absence o f cracking catalyst. These r e s u l t s were extended t o o t h e r temperatures. Two s e t s of e x p e r i m e n t s were made t o show the e f f e c t o f steaming temperature on s t a b i l i t y . In the f i r s t s e t , s t e a m i n g was done n o n - i n t e r a c t i v e l y . Cerium/alumina a d d i t i v e was steamed (100% steam, 1 atm) f o r 5 h o u r s i n a f i x e d bed from 1200 t o 1450°F. S0 removal a b i l i t y was t h e n measured on t h e s e steamed samples d i l u t e d with cracking catalyst. The d a t a i n F i g u r e 14 show t h a t , f o r steamings done s e p a r a t e from c r a c k i n g c a t a l y s t , l o s s e s o f S 0 removal a b i l i t y a r e s m a l l but become more pronounced above 1350°F. L o s s e s i n c u r r e d i n the n o n - i n t e r a c t i v e s t e a m i n g s , however, were lower than t h o s e found i n the second s e t o f e x p e r i m e n t s where the c e r i u m / a l u m i n a a d d i t i v e was steamed t o g e t h e r w i t h a low alumina c r a c k i n g c a t a l y s t at v a r i o u s temperatures. The r e s u l t s from t h i s second s e t o f e x p e r i m e n t s , shown i n F i g u r e 14, i n d i c a t e t h a t l o s s e s are i m p o r t a n t a t t e m p e r a t u r e s above 1200°F. I t s h o u l d be n o t e d t h a t S 0 removal a b i l i t y was measured under the same c o n d i t i o n s i n b o t h s e t s o f e x p e r i m e n t s . A l s o , t h e s e f i x e d bed steaming seem t o be h a r s h e r t h a n f l u i d i z e d bed steamings because the l o s s e s i n c u r r e d are greater. 2

2

2

Oxidative Adsorption of S0 . Oxidative adsorption of S0 i s a l s o a s t r o n g f u n c t i o n o f temperature as shown on F i g u r e 15. Plotted i s the amount o f S 0 removed a f t e r 92 minutes from room temperature t o 1500°F. The m a t e r i a l used f o r t h e s e e x p e r i m e n t s was a r a r e e a r t h s t a b i l i z e d Rhone-Poulenc alumina which was t e s t e d w i t h o u t d i l u t i o n with cracking catalyst. A f r e s h charge o f a l u m i n a was used a t each temperature. As the d a t a i n F i g u r e 15 show, a p l o t o f S 0 a d s o r b e d v s . temperature y i e l d s a c u r v e w i t h the c l a s s i c a l shape o f an a d s o r p t i o n i s o t h e r m d i v i d e d , as i n d i c a t e d on the f i g u r e , i n t o t h r e e d i s t i n c t regimes. From the l i t e r a t u r e ( 4 6 ) , we can s p e c u l a t e about the c h e m i s t r y . Regime 1 i s a s s o c i a t e d w i t h the s t r o n g c h e m i s o r p t i o n o f S 0 as s u l f i t e and i s c o n t r o l l e d t h e r m o d y n a m i c a l l y 2

2

2

2

2


HIRSCHBERG AND BERTOLACINI

Control ofSO Emissions x

400

F i g u r e 14.

F i g u r e 15.

Effect

Effect

o f Steaming

of Adsorption

Temperature.

Temperature.



136

by t h e s t a b i l i t y o f the s u l f i t e s p e c i e s . As the t e m p e r a t u r e i s i n c r e a s e d t h e s u l f i t e s p e c i e s becomes l e s s s t a b l e b u t a t a h i g h enough t e m p e r a t u r e , Regime 2, o x i d a t i o n o f s u l f i t e t o s u l f a t e becomes f a s t enough so f o r m a t i o n o f s u l f a t e becomes t h e i m p o r t a n t process. I n Regime 2, a d s o r p t i o n o f s u l f a t e i s l i m i t e d by t h e oxidation kinetics. The amount o f S 0 a d s o r b e d i n c r e a s e s w i t h t e m p e r a t u r e i n Regime 2 u n t i l the s t a b i l i t y o f t h e s u r f a c e s u l f a t e s p e c i e s becomes i m p o r t a n t . A t t h a t p o i n t , Regime 3, a d s o r p t i o n b e g i n s t o d e c r e a s e w i t h temperature and once a g a i n t h e p r o c e s s i s l i m i t e d by thermodynamics. The shape o f t h e c u r v e d e s c r i b i n g t h e temperature dependence of S 0 a d s o r p t i o n i n t h e t e m p e r a t u r e range o f FCC r e g e n e r a t o r s , 1200-1350°F, i s c r i t i c a l as t o how a g i v e n S 0 a d s o r p t i o n m a t e r i a l w i l l p e r f o r m . As i n d i c a t e d i n F i g u r e 15, a maximum i s o b s e r v e d f o r the s t a b i l i z e d Rhone-Poulenc a l u m i n a i n t h a t range. Other m a t e r i a l s , as we have seen p r e v i o u s l y f o r p l a t i n u m promoted a l u m i n a , w i l l have a somewhat d i f f e r e n t shape i n t h a t temperature region. A c u r v e v e r y s i m i l a r t o t h e one shown i n F i g u r e 15 was p r e v i o u s l y reported f o r high alumina c r a c k i n g c a t a l y s t ( H s i e h , C. K., Amoco O i l Co., u n p u b l i s h e d d a t a , 1976). 2

2

2

R e d u c t i v e D e s o r p t i o n o f S 0 . The r e d u c t i v e d e s o r p t i o n o f o x i d a t i v e l y a d s o r b e d S 0 i s a l s o a s t r o n g f u n c t i o n o f temperature as shown g r a p h i c a l l y i n F i g u r e 16. Shown a r e the r e s u l t s f o r c y c l i c use o f a magnesia-based SOx a d d i t i v e . Each c y c l e , i n d i c a t e d by a c y c l e marker, c o n s i s t s o f a d s o r p t i o n o f S 0 a t 1350°F f o r 100 m i n u t e s f o l l o w e d by a 15 minute r e d u c t i o n w i t h hydrogen. I n a l l c a s e s , the a d s o r p t i o n temperature was h e l d c o n s t a n t a t 1350°F but t h e r e d u c t i o n temperature was changed from 1350° t o 950°F. F o r each c y c l e the amount o f S 0 a d s o r b e d i s then p l o t t e d on the Y-axis. The d a t a show a g r a d u a l d e c l i n e i n the amount o f S 0 removed i n s u c c e s s i v e c y c l e s o f a d s o r p t i o n - r e d u c t i o n from a r e d u c t i o n temperature o f 1350 t o 1150°F. T h i s minor g r a d u a l d e c l i n e i s caused by permanent d e a c t i v a t i o n o f t h e c a t a l y s t r a t h e r t h a n i n c o m p l e t e r e d u c t i o n as i n d i c a t e d by t h e f a i l u r e a t the end o f the t e s t t o r e s t o r e t h e a d s o r p t i o n c a p a c i t y o f the a d d i t i v e by a h i g h temperature r e d u c t i o n . T h e r e i s a sharp drop i n the amount o f S0 a d s o r b e d i n s u c c e s s i v e c y c l e s when t h e r e d u c t i o n temperature i s lowered t o 1050°F. The amount o f S 0 a d s o r b e d i n s u c c e s s i v e c y c l e s does n o t change s i g n i f i c a n t l y when the r e d u c t i o n temperature i s l o w e r e d f u r t h e r t o 950°F. The r e s u l t s from t h i s "temperature programmed r e d u c t i o n " i n d i c a t e t h a t t h e r e a r e two regimes as i n d i c a t e d i n F i g u r e 16. I n Regime 1, a t t e m p e r a t u r e s above 1050°F t h e r e i s v i r t u a l l y complete removal o f S 0 under t h e c o n d i t i o n s o f o u r t e s t . In Regime 2, c h a r a c t e r i z e d by the s t e p jump downward a t 1050°F i n t h e amount o f S0 a d s o r b e d i n s u c c e s s i v e c y c l e s , t h e r e i s o n l y p a r t i a l removal o f S0 . T h i s o b s e r v a t i o n s u g g e s t s two k i n d s o f s u r f a c e s u l f a t e s p e c i e s p r e s e n t on t h i s m a t e r i a l , one e a s i l y and a n o t h e r more d i f f i c u l t l y removed as i n d i c a t e d by A n d e r s s o n ( 4 7 ) . 2

2

2

2

2

2

2

2

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2


8. HIRSCHBERG AND BERTOLACINI Commercial


137

Catalysts

F i v e t y p e s o f commercial SOx c a t a l y s t s were t e s t e d f o r c o m p a r a t i v e ranking. Three o f t h e s e c o m m e r c i a l - t y p e a d d i t i v e s were w e l l defined materials: h i g h s u r f a c e a r e a gamma a l u m i n a ; 10% Ce, as c e r i u m o x i d e , on gamma a l u m i n a ; and 100 ppm P t on gamma a l u m i n a . Two o f t h e m a t e r i a l s were f u r n i s h e d by c a t a l y s t companies and a r e r e f e r r e d t o as magnesia-based o r magnesia and lanthanum-based o r lanthanum. These l a t t e r m a t e r i a l s a r e b o t h known t o c o n t a i n c e r i u m and a l u m i n a as w e l l . F i g u r e 17 shows a comparison o f t h e f r e s h S 0 removal a b i l i t y f o r t h e s e f i v e major t y p e s o f c o m m e r c i a l l y a v a i l a b l e SOx c a t a l y s t s . The m a t e r i a l s were t e s t e d a t 1350°F a t v a r i o u s c o n c e n t r a t i o n s w i t h a v e r y low c a p a c i t y c r a c k i n g c a t a l y s t . The magnesia-based c a t a l y s t i s much b e t t e r than lanthanum-based c a t a l y s t f o l l o w e d by p l a t i n u m or c e r i u m on a l u m i n a and f i n a l l y a l u m i n a a l o n e . The r e v e r s e o r d e r i n a c t i v i t y o b s e r v e d f o r t h e lanthanum-based and c e r i u m a d d i t i v e s , compared t o t h e r e l a t i v e r e s u l t s g i v e n p r e v i o u s l y f o r lanthanum and c e r i u m , was n o t i n v e s t i g a t e d , b u t may be r e l a t e d t o t h e p r e s e n c e o f c e r i u m on t h e lanthanum-based a d d i t i v e (27). F r e s h a c t i v i t y , however, o n l y p a r t i a l l y d e t e r m i n e s t h e e f f i c a c y o f t h e s e c a t a l y s t s . Among o t h e r i m p o r t a n t f a c t o r s , as we have seen, a r e how e a s i l y t h e c a t a l y s t r e l e a s e s S 0 d u r i n g t h e c r a c k i n g c y c l e and how r e s i s t a n t i t i s t o d e a c t i v a t i o n by steam. Commercial c a t a l y s t s v a r y i n t h e degree t o which they a r e r e g e n e r a b l e a t r e a c t o r t e m p e r a t u r e s as shown on F i g u r e 18. The i n i t i a l S 0 removal f o r a l l f i v e m a t e r i a l s was a d j u s t e d t o an e q u a l b a s i s by v a r y i n g t h e amount o f a d d i t i v e used: 0.8% magnesia-based, 3% lanthanum-based, 10% o f b o t h c e r i u m / a l u m i n a and p l a t i n u m / a l u m i n a , and f i n a l l y 30% o f a l u m i n a i n m i x t u r e s w i t h a low c a p a c i t y c r a c k i n g c a t a l y s t . S 0 removal was measured a t 1350°F f o r the f r e s h a d d i t i v e s and a f t e r r e g e n e r a t i o n w i t h hydrogen a t 930°F. S i x such a d s o r p t i o n - r e g e n e r a t i o n c y c l e s were made a f t e r which r e l e a s e was 76 and 70% f o r p l a t i n u m and c e r i u m on a l u m i n a r e s p e c t i v e l y and 38% f o r magnesia and lanthanum, and 42% f o r alumina. R a i s i n g t h e r e g e n e r a t i o n temperature above 1100°F r e s t o r e s t h e a d d i t i v e s t o near f r e s h c a p a c i t y a f t e r t h e s e v e r a l c y c l e s o f t h i s t e s t . The commercial e f f e c t o f i n c o m p l e t e r e l e a s e of S 0 would be t o r e q u i r e more a d d i t i v e t o a c h i e v e a g i v e n S 0 removal. In our t e s t , steaming f i v e commercial SOx a d d i t i v e s i n t h e p r e s e n c e o f c r a c k i n g c a t a l y s t , shown i n T a b l e V I , i n d i c a t e d t h a t d e a c t i v a t i o n by s i l i c a p o i s o n i n g i s i m p o r t a n t . 2

2

2

2

2

2

Table VI. A d d i t i v e Type Magnesia-Based Ρla tinum/Alumina Lanthanum-Based Cerium/Alumina Alumina

Steam D e a c t i v a t i o n

o f Commercial

Additives

% Loss o f S 0 52 52 55 57 79

2

Removal



138

Reduction temperature, °F 1350

1250 1

1150

1050

950

1

1350

1

•ν 3.

400

1ο I

300

1

200 2 J J . J _ l _ l . U l l . i l 1J 1 i M l J J j . l J J . J . l l l i . ι ι ι ι ι ι ι ι ι

I ιι ιι Iι 1ι

Cycle marker

F i g u r e 16.

I 0

1

I

2

I

E f f e c t of Reduction

I

4

I

I

6

I

I

I

8

I

10

I

Temperature.

I

12

I

I

14

I

I 16

% additive

F i g u r e 17.

F r e s h A c t i v i t i e s o f Commercial

Catalysts.



139


These d a t a were o b t a i n e d by m i x i n g the SOx a d d i t i v e w i t h a low a l u m i n a c r a c k i n g c a t a l y s t and steaming a t 1400°F i n 100% steam i n a f i x e d bed f o r 5 h o u r s . The c o n c e n t r a t i o n o f a d d i t i v e was a d j u s t e d so t h a t the i n i t i a l a c t i v i t y was a p p r o x i m a t e l y the same f o r a l l materials. (The amounts were the same as t h o s e used f o r the regenerability test.) The S 0 removal a b i l i t y was t h e n measured b e f o r e and a f t e r steaming and the % l o s s c a l c u l a t e d . The average d e v i a t i o n was ± 7 % . These t e s t s show t h a t c o m p o s i t i o n o f the a d d i t i v e has l i t t l e e f f e c t on c o n t r o l l i n g d e a c t i v a t i o n from s i l i c a p o i s o n i n g . On an e q u a l i n i t i a l a c t i v i t y b a s i s , f o u r o f the f i v e a d d i t i v e s t e s t e d showed v e r y s i m i l a r d e a c t i v a t i o n . A l u m i n a , however, had a much higher s u s c e p t i b i l i t y . A f i n a l r a n k i n g o f the f i v e SOx c o n t r o l c a t a l y s t s can be made by assuming t h a t the l o s s e s i n S 0 removal from i n c o m p l e t e r e g e n e r a t i o n (% L^) and s t e a m i n g (% L ) can be compensated by a d d i t i o n o f more c a t a l y s t assuming a l i n e a r r e s p o n s e : 2

2

EAC = IA • % L

where:

R

.

J£

+

%L

S

.

-M

Catalytic Control of SOx Emissions from Fluid Catalytic Cracking Units

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