Decoking and Regeneration of a Hydrotreating Catalyst by

Chapter 9

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch009

Decoking and Regeneration of a Hydrotreating Catalyst by Supercritical Extraction M . Seapan and Z . Guohui School of Chemical Engineering, Oklahoma State University, Stillwater, O K 74078

An industrially spent hydrotreating catalyst from naphtha service was extracted with tetrahydrofuran, carbon dioxide, pyridine and sulfur dioxide under subcritical and supercritical conditions. After extraction, the catalyst activity, coke content, and pore characteristics were measured. Tetra hydrofuran was not effective in the removal of coke from catalyst, but the other three solvents could remove from 18% to 54% of the coke from catalyst. The coke was removed predominantly from pores in the range of 4-12 nm, resulting in a bimodal pore size distribution and an increase in the pore volume and surface area. The amount of coke removed depended on the extraction temperature, pressure and duration. In the most severe extraction conditions, the s i l i c a foulant of the catalyst could also be removed as fine particles. Pyridine poisoned the catalyst during extraction, however its removal by acetone wash could restore the catalyst activity. H y d r o t r e a t i n g c a t a l y s t s are e x t e n s i v e l y used i n p e t r o l e u m p r o c e s s i n g , c o a l l i q u e f a c t i o n , and g e n e r a l l y i n u p g r a d i n g o f l i q u i d f o s s i l f u e l s . These c a t a l y s t s d e a c t i v a t e b y coverage o f t h e i r a c t i v e s i t e s w i t h p o i s o n i n g compounds and b y b l o c k a g e o f t h e i r p o r e s b y d e p o s i t i o n o f carbonaceous s o l i d s and m e t a l l i c f o u l a n t s . Depending on the type o f f e e d s t o c k , the c a t a l y s t l i f e may v a r y from a few months f o r heavy o i l s t o a few y e a r s f o r naphthas. A t the end o f t h e i r a c t i v e l i v e s , the spent c a t a l y s t s are removed from the r e a c t o r and r e p l a c e d by f r e s h c a t a l y s t s . Most o f the s p e n t c a t a l y s t s are d i s p o s e d o f as s o l i d wastes i n l a n d f i l l s o r u s e d f o r m e t a l s r e c l a m a t i o n . Only a s m a l l f r a c t i o n o f t h e s e c a t a l y s t s i s r e g e n e r a t e d by c o n t r o l l e d o x i d a t i o n and r e u s e d as low a c t i v i t y c a t a l y s t i n guard chambers. A t p r e s e n t , no i n - s i t u r e g e n e r a t i o n t e c h n i q u e i s a v a i l a b l e f o r t h e s e c a t a l y s t s . T h i s work d i s c u s s e s the m e r i t s o f i n - s i t u c a t a l y s t r e g e n e r a t i o n and i n v e s t i g a t e s t h e 0097-6156/89/0411-0079$06.00/0 © 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

80

CHARACTERIZATION AND CATALYST DEVELOPMENT


p o t e n t i a l o f s u p e r c r i t i c a l f l u i d e x t r a c t i o n as an i n - s i t u c a t a l y s t d e c o k i n g and r e g e n e r a t i o n t e c h n i q u e . Advantages o f I n - s i t u C a t a l y s t R e g e n e r a t i o n . Replacement o f h y d r o t r e a t i n g c a t a l y s t s i s an e x p e n s i v e o p e r a t i o n i n r e f i n e r i e s . I n a d d i t i o n t o the purchase p r i c e o f c a t a l y s t s , t h e r e f i n i n g o p e r a t i o n has t o be s h u t down f o r s e v e r a l days t o r e p l a c e t h e c a t a l y s t , w h i c h r e p r e s e n t s a l o s t revenue. R e f i n e r s always t r y t o s c h e d u l e c a t a l y s t r e p l a c e m e n t s a t t h e time o f r o u t i n e maintenance shutdowns. However, the need f o r c a t a l y s t r e p l a c e m e n t sometimes n e c e s s i t a t e s c o s t l y u n s c h e d u l e d shutdowns. An i n - s i t u c a t a l y s t regeneration process not only can increase the l i f e o f the c a t a l y s t b u t a l s o c a n a l l e v i a t e the u n s c h e d u l e d shutdowns. H y d r o t r e a t i n g c a t a l y s t s are u s u a l l y a l u m i n a s u p p o r t e d molybdenum b a s e d c a t a l y s t s w i t h c o b a l t o r n i c k e l p r o m o t o r s . By 1990, t h e demand f o r h y d r o t r e a t i n g c a t a l y s t s i s e x p e c t e d t o r e a c h 80,000,000 pounds a n n u a l l y ( 1 ) . The i n c r e a s e d demand f o r t h e s e c a t a l y s t s and the l i m i t a t i o n s on t h e a v a i l a b i l i t y and s u p p l y o f t h e a c t i v e m e t a l s i n c r e a s e the urgency t o d e v e l o p e f f e c t i v e c a t a l y s t regeneration techniques. Recent developments i n o x i d a t i v e r e g e n e r a t i o n have r e s u l t e d i n c a r e f u l l y c o n t r o l l e d combustion t e c h n i q u e s w h i c h c a n remove t h e coke and cause t h e g a i n o f a s i g n i f i c a n t p o r t i o n o f t h e c a t a l y s t a c t i v i t y w i t h o u t s i n t e r i n g . However, t h e s e p r o c e s s e s a r e u s e f u l o n l y when the m e t a l s laydown on the c a t a l y s t i s m i n i m a l . F u r t h e r m o r e , they r e q u i r e t h e c a t a l y s t t o be removed from t h e r e a c t o r and s e n t t o c a t a l y s t r e g e n e r a t i o n companies. In-situ c a t a l y s t d e c o k i n g and r e g e n e r a t i o n c a n be an a t t r a c t i v e a l t e r n a t i v e t o an o f f - s i t e c a t a l y s t r e g e n e r a t i o n p r o c e s s . S u p e r c r i t i c a l f l u i d e x t r a c t i o n i s a novel technique, which can p o t e n t i a l l y r e g e n e r a t e t h e c a t a l y s t s i n - s i t u i n the h y d r o t r e a t i n g r e a c t o r . Most s o l v e n t s t h a t a r e s u i t a b l e f o r d e c o k i n g have c r i t i c a l p o i n t s l o w e r t h a n the o p e r a t i n g c o n d i t i o n s o f h y d r o t r e a t i n g p r o c e s s e s , as shown i n T a b l e I . T h e r e f o r e , t h e s u p e r c r i t i c a l c a t a l y s t d e c o k i n g and r e g e n e r a t i o n can be c o n d u c t e d i n s i d e t h e e x i s t i n g h y d r o t r e a t i n g r e a c t o r s . The r e g e n e r a t i o n p r o c e s s would o n l y r e q u i r e t h e r e a c t o r t o be d e p r e s s u r i z e d and cooled t o the conditions o f e x t r a c t i o n . A f t e r the e x t r a c t i o n i s completed, t h e r e a c t o r c o n d i t i o n s c a n be e a s i l y changed t o those o f h y d r o t r e a t i n g and the r e f i n e r y o p e r a t i o n resumed. Thus t h e time l o s t w i l l be m a i n l y t h e time r e q u i r e d f o r e x t r a c t i o n , w h i c h c a n be o n l y a few h o u r s under p r o p e r c o n d i t i o n s , as shown i n t h i s work. S u p e r c r i t i c a l f l u i d r e g e n e r a t i o n c a n a l s o be done p e r i o d i c a l l y d u r i n g r e f i n e r y o p e r a t i o n s w i t h o u t a l l o w i n g t h e c a t a l y s t t o become completely deactivated. This p e r i o d i c regeneration w i l l allow the c a t a l y s t t o be o p e r a t e d a t lower temperatures b y m a i n t a i n i n g t h e c a t a l y s t a t i t s peak a c t i v i t y f o r a l o n g e r p e r i o d o f t i m e , thus reducing the operating costs o f r e f i n i n g . P r i n c i p l e s o f E x t r a c t i v e De-Coking. H y d r o t r e a t i n g d e a c t i v a t e b y t h r e e major mechanisms: 1. P o i s o n i n g o c c u r s by s t r o n g a d s o r p t i o n o f s t a b l e active s i t e s o f c a t a l y s t s . Basic h e t e r o c y c l i c compounds c a n u s u a l l y p o i s o n t h e a c i d i c s i t e s .

catalysts m o l e c u l e s on t h e nitrogen Adsorption o f


9.

SEAPAN & GUOHUI

Decoking and Regeneration of a Hydrotreating Catalyst

Table I. C r i t i c a l P r o p e r t i e s of Solvents and T y p i c a l H y d r o t r e a t i n g C o n d i t i o n s

Compound


Tetrahydrofuran

T (THF)

K

p

c

, atm

535, .6

41

Pyridine

620

55, .6

co

2

304. .2

72, .8

so

2

430, .8

77, .8

590- -680

70- -170

Typical Hydrotreating Conditions

b a s i c compounds on c a t a l y s t s i s u s u a l l y f a s t , r e a c h i n g an e q u i l i b r i u m l e v e l i m m e d i a t e l y a f t e r the o i l c o n t a c t s the c a t a l y s t . The o n l y l o n g - t e r m s i g n i f i c a n c e o f p o i s o n i n g i n c a t a l y s t d e a c t i v a t i o n i s the p o t e n t i a l o f n i t r o g e n compounds t o a c t as coke p r e c u r s o r s . 2. C o k i n g o r f o r m a t i o n o f s o l i d carbonaceous d e p o s i t s i n the c a t a l y s t p o r e s o c c u r s by d e c o m p o s i t i o n and p o l y m e r i z a t i o n o f h y d r o c a r b o n s on the c a t a l y s t s u r f a c e . Carbonaceous m a t e r i a l c o n t i n u o u s l y forms on the c a t a l y s t , c a u s i n g a c t i v e s i t e coverage and pore p l u g g i n g . Coking i s a long-term d e a c t i v a t i o n phenomenon. A f t e r an i n i t i a l r a p i d c o k i n g , the coke c o n t i n u o u s l y b u i l d s up on the c a t a l y s t , w h i l e i t s c o m p o s i t i o n and s t r u c t u r e change. A c c u m u l a t i o n o f coke causes a s l o w d e a c t i v a t i o n o f the c a t a l y s t . I n i n d u s t r i a l p r a c t i c e , the o p e r a t i n g temperature i s g r a d u a l l y r a i s e d t o compensate f o r the a c t i v i t y l o s s . The i n c r e a s e d t e m p e r a t u r e , i n t u r n , i n c r e a s e s the r a t e o f c o k i n g and the r a t e o f change o f coke c o m p o s i t i o n , a c c e l e r a t i n g the c a t a l y s t d e a c t i v a t i o n u n t i l the maximum a l l o w a b l e temperature i s reached. 3. F o u l i n g o c c u r s by d e c o m p o s i t i o n o f o r g a n o m e t a l l i c compounds o f the o i l and d e p o s i t i o n o f the g e n e r a t e d m e t a l s i n the porous s t r u c t u r e o f the c a t a l y s t , b l o c k i n g the a c t i v e s i t e s and p l u g g i n g the p o r e s . Long-term a c c u m u l a t i o n o f these m e t a l s i n the c a t a l y s t p o r e s can r e s u l t i n permanent d e a c t i v a t i o n o f the catalyst. V e r y l i t t l e i s known about the c h e m i c a l s t r u c t u r e o f coke. The c h e m i c a l a n a l y s e s o f d i f f e r e n t coked c a t a l y s t s have i n d i c a t e d t h a t coke i s n o t pure c a r b o n , b u t i s composed o f 90-95% c a r b o n , 5-7% hydrogen, 0-2% n i t r o g e n , and t r a c e s o f o t h e r elements f o u n d i n the processed o i l . The e x i s t e n c e o f d i f f e r e n t o r g a n i c bonds i n d i c a t e s t h a t coke has a m a c r o m o l e c u l a r s t r u c t u r e . T h i s m a c r o m o l e c u l a r s t r u c t u r e can be b r o k e n down by c e r t a i n r e a c t i v e and s t r o n g solvents. F l u i d s around t h e i r c r i t i c a l p o i n t show u n u s u a l p h y s i c a l p r o p e r t y changes. Under s u p e r c r i t i c a l c o n d i t i o n s , f l u i d s have lower v i s c o s i t i e s and s u r f a c e t e n s i o n s and h i g h e r d i f f u s i v i t i e s


81


82


t h a n t h o s e under normal c o n d i t i o n s . H i g h s o l u b i l i t y o f s u p e r c r i t i c a l f l u i d s can be used t o s o l u b i l i z e h a r d t o d i s s o l v e solids. T h i s h a s r e s u l t e d i n the development o f s u p e r c r i t i c a l f l u i d e x t r a c t i o n t e c h n o l o g y i n f o o d p r o c e s s i n g , p h a r m a c e u t i c a l and other i n d u s t r i e s . S u p e r c r i t i c a l f l u i d e x t r a c t i o n can be used t o remove carbonaceous m a t e r i a l from spent c a t a l y s t s . De F i l i p p i and Robey (2) u s e d s u p e r c r i t i c a l c a r b o n d i o x i d e e x t r a c t i o n t o r e g e n e r a t e a d s o r b e n t s . A b e l (3) t r i e d s u p e r c r i t i c a l c a r b o n d i o x i d e e x t r a c t i o n t o r e g e n e r a t e a c e r t a i n c a t a l y s t . T i l t s c h e r e t a l . (4,5) s t u d i e d the i s o m e r i z a t i o n o f 1-hexene on an a l u m i n a c a t a l y s t and showed t h a t under s u p e r c r i t i c a l c o n d i t i o n s , 1-hexene was a b l e t o remove the o l i g o m e r i c compounds (C12-C30) from the c a t a l y s t s u r f a c e and prevent coking. S o l u b i l i t i e s o f heavy h y d r o c a r b o n s i n s u p e r c r i t i c a l f l u i d s depend on the type o f s o l v e n t ( 6 ) . M o r a d i n i a and T e j a (7) showed t h a t the s o l u b i l i t i e s o f s o l i d n - a l k a n e s (n-C2g, n-C^Q, n - C ^ ) about t e n times h i g h e r i n s u p e r c r i t i c a l ethane t h a n i n c a r b o n d i o x i d e . T h e r e f o r e , i t i s r e a s o n a b l e t o s e a r c h and f i n d an a p p r o p r i a t e s o l v e n t w h i c h can d i s i n t e g r a t e and d i s s o l v e t h e carbonaceous d e p o s i t s from h y d r o t r e a t i n g c a t a l y s t s , r e s u l t i n g i n t h e i r d e c o k i n g and r e g e n e r a t i o n . I n t h i s p r o j e c t , the f e a s i b i l i t y o f c a t a l y s t r e g e n e r a t i o n b y s u p e r c r i t i c a l f l u i d e x t r a c t i o n was s t u d i e d . A spent c a t a l y s t from an i n d u s t r i a l naphtha h y d r o t r e a t e r was e x t r a c t e d w i t h t e t r a h y d r o f u r a n , p y r i d i n e , c a r b o n d i o x i d e , and s u l f u r d i o x i d e under s u b c r i t i c a l and s u p e r c r i t i c a l c o n d i t i o n s . The coke r e d u c t i o n and changes i n the c a t a l y s t pore c h a r a c t e r i s t i c s were measured and t o a l i m i t e d e x t e n t the c a t a l y s t a c t i v i t y was e v a l u a t e d . I t i s shown t h a t by s u p e r c r i t i c a l e x t r a c t i o n , the coke c o n t e n t o f spent h y d r o t r e a t i n g c a t a l y s t s can be reduced and the c a t a l y s t pore volume and s u r f a c e a r e a can be i n c r e a s e d . a

r

e

E x p e r i m e n t a l Work E x p e r i m e n t a l a p p a r a t u s used i n t h i s s t u d y c o n s i s t e d o f a 12.5 mm i n t e r n a l diameter s t a i n l e s s s t e e l tubular r e a c t o r w i t h a length o f 864 mm (34 i n ) . I t was o r i g i n a l l y c o n s t r u c t e d and u s e d as a t r i c k l e b e d downflow r e a c t o r . W i t h p r o p e r m o d i f i c a t i o n o f the p l u m b i n g i t was used t o r e g e n e r a t e the c a t a l y s t i n e i t h e r an u p f l o w or downflow mode. An i n d u s t r i a l l y spent nickel-molybdenum c a t a l y s t from a naphtha h y d r o t r e a t m e n t s e r v i c e was l o a d e d i n the r e a c t o r . A p p r o x i m a t e l y 80 g o f the spent c a t a l y s t was packed between two l a y e r s o f g l a s s beads i n the r e a c t o r . The i n i t i a l c a t a l y s t a c t i v i t y was t e s t e d b y h y d r o g e n a t i o n o f a m i x t u r e o f 10% q u i n o l i n e i n T e t r a l i n i n the temperature range o f 355-380°C and 10.3 MPa (1500 p s i a ) . A f t e r s t o p p i n g the f l o w o f the r e a c t a n t s , t h e o p e r a t i n g c o n d i t i o n s were a d j u s t e d t o those r e q u i r e d f o r e x t r a c t i o n e x p e r i m e n t s . The e x t r a c t i o n was c o n d u c t e d b y pumping the s o l v e n t i n t o the r e a c t o r . A f t e r c o m p l e t i o n o f the e x t r a c t i o n p r o c e s s , t h e c a t a l y s t a c t i v i t y was measured a g a i n . The c a t a l y s t h y d r o g e n a t i o n a c t i v i t y was c a l c u l a t e d as the p e r c e n t a g e o f T e t r a l i n c o n v e r t e d t o t r a n s - a n d c i s - d e c a l i n s . The h y d r o d e n i t r o g e n a t i o n (HDN) activity was c a l c u l a t e d as the p e r c e n t a g e o f q u i n o l i n e c o n v e r t e d t o



9.

SEAPAN & GUOHUI


p r o p y l b e n z e n e and p r o p y l c y c l o h e x a n e , the major h y d r o c a r b o n s formed from q u i n o l i n e . N e i t h e r o f t h e s e r e a c t a n t s were s u i t a b l e t o measure the p o t e n t i a l a c t i v i t y g a i n a s s o c i a t e d w i t h the coke r e m o v a l . Coke removal o c c u r r e d m a i n l y from l a r g e p o r e s (- 10 nm) w h i c h a r e n o t e x p e c t e d t o a f f e c t the r e a c t i o n r a t e s o f such s m a l l m o l e c u l e s as T e t r a l i n and q u i n o l i n e . However, t h e s e a c t i v i t y t e s t s were u s e f u l i n i d e n t i f i c a t i o n o f the p o t e n t i a l p o i s o n i n g e f f e c t s o f the s o l v e n t s . The c a t a l y s t was removed and a n a l y z e d f o r i t s coke c o n t e n t , p o r e s i z e d i s t r i b u t i o n , pore volume, and s u r f a c e a r e a . A l l the c a t a l y s t samples were a n a l y z e d a f t e r a S o x h l e t e x t r a c t i o n w i t h t e t r a h y d r o f u r a n f o r 24 h and d r y i n g i n oven a t 100 C. The c a t a l y s t w e i g h t l o s s due t o combustion a t 550 C was c o n s i d e r e d as the coke on c a t a l y s t . The c a t a l y s t pore c h a r a c t e r i s t i c s were measured by a Quantachrome A u t o s c a n P o r o s i m e t e r . The spent c a t a l y s t c o n t a i n e d an average o f about 17.5% coke. I t had a monomodal pore d i s t r i b u t i o n i n the range o f 4.4 t o 12 nm pore d i a m e t e r , w i t h a most p r o b a b l e pore s i z e o f 5.4 nm. The pore volume and s u r f a c e a r e a o f the c a t a l y s t were 0.17 cm /g and 106 m /g r e s p e c t i v e l y . The spent c a t a l y s t a l s o c o n t a i n e d s i g n i f i c a n t amounts o f s i l i c a f o u l i n g , w h i c h had d e p o s i t e d on the c a t a l y s t from de-foaming agents added t o the o i l d u r i n g the i n d u s t r i a l use o f the catalyst. R e s u l t s and D i s c u s s i o n The e f f e c t i v e n e s s o f t e t r a h y d r o f u r a n , p y r i d i n e , c a r b o n d i o x i d e , and s u l f u r d i o x i d e as s o l v e n t s t o remove the coke from c a t a l y s t under s u p e r c r i t i c a l and s u b c r i t i c a l c o n d i t i o n s was s t u d i e d . The c r i t i c a l p r o p e r t i e s o f t h e s e s o l v e n t s a r e l i s t e d i n T a b l e I and the e x t r a c t i o n c o n d i t i o n s a r e shown i n T a b l e I I . Two e x t r a c t i o n r u n s , runs #1 and #2, were c o n d u c t e d w i t h t e t r a h y d r o f u r a n a t a reduced p r e s s u r e o f 1.05 and a r e d u c e d t e m p e r a t u r e o f about 1.35 f o r 5 and 11 h r e s p e c t i v e l y . The r e d u c t i o n o f the coke c o n t e n t o f c a t a l y s t was i n s i g n i f i c a n t . The c a t a l y s t a c t i v i t i e s b e f o r e and a f t e r e x t r a c t i o n were measured a t 355 C. The h y d r o g e n a t i o n a c t i v i t y o f the c a t a l y s t d i d n o t change, whereas, the q u i n o l i n e HDN a c t i v i t y o f the c a t a l y s t was d e c r e a s e d from 50% t o about 20% when the c a t a l y s t was e x t r a c t e d f o r o n l y 5 h. T h i s i n i t i a l low a c t i v i t y g r a d u a l l y i n c r e a s e d t o 50% i n about 4 h. However, the c a t a l y s t t h a t was e x t r a c t e d f o r 11 h, a f t e r s t a b i l i z a t i o n showed an HDN a c t i v i t y o f about 80%. These changes i n the c a t a l y s t a c t i v i t y a r e n o t r e l a t e d t o the coke. They a r e r a t h e r a t t r i b u t e d t o the i n t e r a c t i o n o f t e t r a h y d r o f u r a n w i t h the a d s o r b e d s p e c i e s on the c a t a l y t i c s i t e s r e s p o n s i b l e f o r HDN reactions. Three e x t r a c t i o n e x p e r i m e n t s , runs #11-13, were c o n d u c t e d w i t h c a r b o n d i o x i d e . Run #12 was c o n d u c t e d a t a r e d u c e d p r e s s u r e o f 0.93 and a reduced temperature o f 1.05 f o r 13 h. The c a t a l y s t coke c o n t e n t was reduced from 17.5% t o 11%, where the coke was p r i m a r i l y removed from p o r e s o f 9.6 nm d i a m e t e r . T h i s r e p r e s e n t e d a 37% removal o f coke from the c a t a l y s t and r e s u l t e d i n a b i m o d a l pore s i z e d i s t r i b u t i o n w i t h a pore volume o f 0.22 m*Vg ^ a s u r f a c e a r e a o f 137 m /g. The changes i n the pore s i z e d i s t r i b u t i o n a r e shown i n F i g . 1. The o t h e r two e x t r a c t i o n s w i t h c a r b o n d i o x i d e a n


83

84


Table I I .


RUN #

Experimental Conditions o f Catalyst

SOLVENT

Zr

T r

1

THF*

1.05

1. 35

5

2

THF

1.03

1. 38

11

3

Pyridine

1.1 - 1.6

1. 07

9

4B

Pyridine

Atmospheric

--

4A

Pyridine

1.02 1.36 1.85

0.,99 1.,03 1.,07

7

Pyridine

Atmospheric

8

Pyridine

0.63 1.00 1.36

9

--

Pyridine

Atmospheric

11

co

2

1.14

12

co

2

13

co

14

COMMENTS

24 5 3 3

Run 4A i s t h e continuation of Run 4B

24

Presulfided after extraction

5 17 7.5

After extraction, c a t a l y s t was removed, washed w i t h acetone, d r i e d , r e l o a d e d and s u l f i d e d Presulfided only (No E x t r a c t i o n )

-•

10

15

0.,92 1.,00 1.,06

TIME, h

Extraction

12

Extracted, dried i n oven l o a d e d and sulfided

1..06

22

Batch

0.93

1.,05

13

Semi-Batch

2

0.95

1..06

8

Flow e x t r a c t i o n followed by sulfidation

so

2

0.05

1..05

8

Batch

so

2

2.22

1..04

4

0.05

1..75

4

Pyridine extraction f o l l o w e d by S 0 extraction

+ Pyridine

extractions

2

* Tetrahydrofuran


9.


SEAPAN & GUOHUI

were c o n d u c t e d a t s i m i l a r p r e s s u r e s and t e m p e r a t u r e s , b u t f o r 8 and 22 h p e r i o d s . The c a t a l y s t from 8 h e x t r a c t i o n was f u r t h e r s u l f i d e d w i t h a 5% H S i n H gas t o s t u d y the e f f e c t o f c a t a l y s t s u l f i d a t i o n . T e t r a l i n h y d r o g e n a t i o n a c t i v i t y was n o t changed measurably i n any o f t h e s e c a s e s . Q u i n o l i n e HDN a c t i v i t y was s l i g h t l y r e d u c e d i n the p l a i n c a r b o n d i o x i d e e x t r a c t i o n s and s l i g h t l y improved when the c a t a l y s t was s u l f i d e d a f t e r extraction. T h i s i n d i c a t e d t h a t c a r b o n d i o x i d e has no p o i s o n i n g e f f e c t on the c a t a l y s t . S e v e r a l c a t a l y s t r e g e n e r a t i o n e x p e r i m e n t s were c o n d u c t e d w i t h p y r i d i n e as the s o l v e n t u s i n g d i f f e r e n t e x t r a c t i o n c o n d i t i o n s and the c a t a l y s t a c t i v i t i e s were measured. The c o n d i t i o n s o f c a t a l y s t e x t r a c t i o n s are l i s t e d i n T a b l e I I and the s t a b i l i z e d a c t i v i t i e s o f the c a t a l y s t s a f t e r 5-6 h o f a c t i v i t y t e s t s are shown i n F i g u r e s 2 and 3. The c a t a l y s t c h a r a c t e r i s t i c s were examined o n l y i n one r u n , w h i c h are shown i n T a b l e I I I .


2

Table I I I .

Run

#

2

E f f e c t o f E x t r a c t i o n on C a t a l y s t

Solvent

Coke Content Wt%

Characteristics

Pore Volume 3 cm /g

Surface Area 2 cm /g

C a t a l y s t as R e c e i v e d

17. .5

0. ,17

106

4

Pyridine

14. .3

0. ,22

119

12

co

2

11. .0

0. ,22

137

14

so

2

10, .3

0. .23

129

15

Pyridine +

8,.1

0. .22

132

so

2

I n run 4A, the c a t a l y s t a f t e r S o x h l e t e x t r a c t i o n w i t h p y r i d i n e f o r 24 h was l o a d e d i n the r e a c t o r and f u r t h e r e x t r a c t e d under a r e d u c e d temperature o f about 1.0 and a t s u c c e s s i v e l y increasing r e d u c e d p r e s s u r e s o f 1.02, 1.36, and 1.85 f o r 5, 3, and 3 h respectively. The c o l o r o f the e x t r a c t was m o n i t o r e d d u r i n g t h i s p r o c e s s . The f i r s t e x t r a c t had a m o d e r a t e l y d a r k brown c o l o r , w h i c h t u r n e d i n t o y e l l o w a f t e r 5 h. When the p r e s s u r e was i n c r e a s e d , the c o l o r o f the e x t r a c t became brown a g a i n w h i c h g r a d u a l l y changed t o l i g h t brown i n 3 h. The l a s t i n c r e a s e o f p r e s s u r e caused a major d a r k e n i n g o f the c o l o r o f the e x t r a c t , f o l l o w e d by a l i g h t e n i n g o f c o l o r i n 3h. Supercritical extraction removed more coke t h a n s u b c r i t i c a l e x t r a c t i o n and the e x t e n t o f coke removal depended on the d u r a t i o n and s e v e r i t y o f c o n d i t i o n s o f extraction. T h i s p r o c e s s reduced the coke c o n t e n t o f the c a t a l y s t from 17.5% t o 14.3%, a r e d u c t i o n o f about 18% i n the coke c o n t e n t o f the c a t a l y s t . The coke was p r i m a r i l y removed from p o r e s o f 8 to


85

86


As Received

Characteristic Surface Area, m2/g Pore Volume, cm3/g Percent Coke, Wt. %

~i r Extracted (Run 12)

106 0.17 17.5

137 0.22 11


>O

z

UJ

o

111

cn

EXTRACTED

AS RECEIVED

12

6 8 10 PORE DIAMETER, nm

14

F i g u r e 1. E f f e c t s o f CO2 E x t r a c t i o n on C a t a l y s t C h a r a c t e r i s t i c s .

30 h SCE + Acetone Wash + Dry + Presulfide 12h Atm. Ext. + Dry + Presulfide (Run 10) 380°C* Presulfide - No Extraction (Run 9)

24h Atm. Ext. + Presulfide (Run 7) 24h Atm. Ext. + tin SCE (Run 4A)

I

24h Atm. Ext. (Run 4B) 370°C 9h SCE (Run 3A) As Received (Run 3B)

5

10

_!_

15

20

25

30

% HYD OF TETRALIN F i g u r e 2. E f f e c t s o f P y r i d i n e E x t r a c t i o n on H y d r o g e n a t i o n Activity of Catalyst.



9.

SEAPAN & GUOHUI


12 nm, r e s u l t i n g i n a b i m o d a l pore s i z e d i s t r i b u t i o n f o r the e x t r a c t e d c a t a l y s t . C o n s e q u e n t l y , the pore volume and s u r f a c e a r e a i n c r e a s e d t o 0.22 cm^/g and 119 m /g r e s p e c t i v e l y . As F i g u r e s 2 and 3 show the change i n the h y d r o g e n a t i o n a c t i v i t y o f the c a t a l y s t was i n s i g n i f i c a n t , w h i l e the c a t a l y s t l o s t i t s HDN a c t i v i t y from a p p r o x i m a t e l y 90% t o 50%. T h i s showed the s t r o n g p o i s o n i n g e f f e c t of p y r i d i n e e x t r a c t i o n . As F i g u r e 2 shows, n e i t h e r e x t r a c t i o n w i t h p y r i d i n e (runs #3 and #4) nor s u l f i d a t i o n (runs #7 and #9) had any s i g n i f i c a n t e f f e c t on the h y d r o g e n a t i o n a c t i v i t y o f the c a t a l y s t . Only i n r u n #8 where the p y r i d i n e e x t r a c t e d c a t a l y s t was washed w i t h a c e t o n e , d r i e d , and p r e s u l f i d e d , the g a i n i n h y d r o g e n a t i o n a c t i v i t y became significant. S i m i l a r phenomena were o b s e r v e d f o r the c a t a l y s t HDN a c t i v i t y , as shown i n F i g u r e 3. E x t r a c t i o n w i t h p y r i d i n e reduced the HDN a c t i v i t y o f the c a t a l y s t by p o i s o n i n g o f the HDN s i t e s . A c o m p a r i s o n o f runs 3B, 3A, 4B and 4A o f F i g u r e 3 shows t h a t the c a t a l y s t HDN a c t i v i t y d e c r e a s e d w i t h an i n c r e a s e i n the t o t a l c a t a l y s t - p y r i d i n e c o n t a c t time. However, removal o f the adsorbed p y r i d i n e by s u l f i d a t i o n ( r u n #7) o r by washing w i t h acetone and subsequent s u l f i d a t i o n o f c a t a l y s t ( r u n #8) i n c r e a s e d the c a t a l y s t HDN a c t i v i t y t o about 90%. I n summary, t h e s e a c t i v i t y t e s t s showed t h a t even though p y r i d i n e can remove some o f the coke from c a t a l y s t , i t can p o i s o n the d e n i t r o g e n a t i o n s i t e s o f the c a t a l y s t . T h e r e f o r e any c a t a l y s t d e c o k i n g by p y r i d i n e s h o u l d be f o l l o w e d by a p y r i d i n e removal p r o c e s s . E x t r a c t i o n of c a t a l y s t w i t h s u l f u r d i o x i d e at a reduced p r e s s u r e o f 0.05 and a reduced temperature o f 1.05 r e s u l t e d i n a r e d u c t i o n i n coke c o n t e n t o f c a t a l y s t from 17.5% t o 10.3%, w h i c h i n c r e a s e d the pore volume and s u r f a c e a r e a t o 0.23 cm /g and 129 m /g r e s p e c t i v e l y ( r u n #14). When the c a t a l y s t was f i r s t e x t r a c t e d w i t h p y r i d i n e t h e n w i t h s u l f u r d i o x i d e , the coke c o n t e n t was r e d u c e d to 8.1%, w h i c h r e p r e s e n t s a 54% r e d u c t i o n i n the coke c o n t e n t o f the c a t a l y s t ( r u n #15). The coke was a g a i n p r i m a r i l y removed from the p o r e s o f about 10 nm, r e s u l t i n g i n a b i m o d a l d i s t r i b u t i o n o f p o r e s i n the r e g e n e r a t e d c a t a l y s t , as shown i n F i g u r e 4. The c a t a l y s t a c t i v i t i e s d i d not s t a b i l i z e i n the 6 h e x p e r i m e n t s c o n d u c t e d a f t e r t h e s e e x t r a c t i o n s , however the g e n e r a l t r e n d showed an i n c r e a s e i n the HDN a c t i v i t y and i n s i g n i f i c a n t g a i n s i n the h y d r o g e n a t i o n a c t i v i t y o f the c a t a l y s t . I n summary, e x t r a c t i o n w i t h c a r b o n d i o x i d e , p y r i d i n e and s u l f u r d i o x i d e can remove the coke from c a t a l y s t . The amount o f coke removed depends on the e x t r a c t i o n t e m p e r a t u r e , p r e s s u r e and duration. C o n s e c u t i v e e x t r a c t i o n s w i t h two s o l v e n t s appear t o remove more coke t h a n the i n d i v i d u a l s o l v e n t s do. Adsorption of c e r t a i n s o l v e n t s on the c a t a l y s t d u r i n g e x t r a c t i o n can p o i s o n the c a t a l y s t . T h e r e f o r e , i f p o i s o n i n g s o l v e n t s are used f o r d e c o k i n g , t h e i r remains must be removed from the e x t r a c t e d c a t a l y s t t o r e s t o r e the c a t a l y s t a c t i v i t y . I n two o f t h e s e e x t r a c t i o n e x p e r i m e n t s (runs #8 and #15), i n a d d i t i o n t o coke r e d u c t i o n the s i l i c a f o u l a n t o f the c a t a l y s t was a l s o removed as g l a s s y w h i t e f i n e powder. I n t h e s e c a s e s , the c a t a l y s t , a f t e r e x t r a c t i o n w i t h p y r i d i n e , was e i t h e r washed w i t h acetone or e x t r a c t e d w i t h s u l f u r d i o x i d e . O b v i o u s l y , when the e x t r a c t i o n i s s e v e r e the s t r u c t u r e o f coke d i s i n t e g r a t e s , a l l o w i n g the l o o s e n e d f o u l a n t p a r t i c l e s t o be s e p a r a t e d from the c a t a l y s t .


87

88


30h SCE + Acetone Wash + Dry + Presulfide 12h Atm. Ext. + Dry + Presulfide (Run 10) 380°C
O

z

UJ D

o UJ

cc

SO2 - EXTRACTED

6

8 10 PORE DIAMETER, nm

SO2/PYRIDINE EXTRACTED

12

14

F i g u r e 4. E f f e c t s o f S 0 2 / P y r i d i n e E x t r a c t i o n on C a t a l y s t Characteristics.


16

9. SEAPAN & GUOHUI


Conclusions 1.


2.

3.

4.

Supercritical fluid extraction i s a potentially viable t e c h n i q u e i n removing carbonaceous coke d e p o s i t s from hydrotreating catalysts. T e t r a h y d r o f u r a n i s not a good s o l v e n t f o r coke r e m o v a l , b u t p y r i d i n e , c a r b o n d i o x i d e , and s u l f u r d i o x i d e , can remove s i g n i f i c a n t amounts o f coke from the c a t a l y s t s . The e x t e n t o f coke removal depends on d u r a t i o n and s e v e r i t y o f e x t r a c t i o n . I n the b e s t c a s e , 54% o f the c a t a l y s t coke was removed from a spent h y d r o t r e a t i n g c a t a l y s t . T h i s was a c h i e v e d by a s u p e r c r i t i c a l p y r i d i n e e x t r a c t i o n f o l l o w e d by a s u b c r i t i c a l sulfur dioxide extraction. I n a d d i t i o n t o coke r e m o v a l , i n two s e v e r e l y e x t r a c t e d c a s e s , the s i l i c a f o u l a n t o f the c a t a l y s t c o u l d a l s o be removed from the c a t a l y s t . P y r i d i n e e x t r a c t i o n p o i s o n s the c a t a l y s t , r e d u c i n g i t s HDN a c t i v i t y . Removal o f the adsorbed p y r i d i n e b y acetone o r a c i d i c gases such as s u l f u r d i o x i d e can r e g e n e r a t e the HDN activity.

Acknowledgments T h i s p r o j e c t was s u p p o r t e d by the U n i v e r s i t y C e n t e r f o r Energy R e s e a r c h o f Oklahoma S t a t e U n i v e r s i t y . Literature Cited 1. 2.

3. 4. 5. 6.

7.

Stinson, S. C. Chem. Eng. News, Feb. 1986, 64(7), 27. De F i l i p p i , R. P.; Robey, R. J . Supercritical Fluid Regeneration of Adsorbents; Environmental Protection Agency EPA-600/52-83-038, 1983. Abel, H. West German Patent 28-53-065, 1978. Tiltscher, H . ; Wolf, H . ; Schelchshorn, J . Angew. Chem. Int. Ed. Engl., 1981, 20, 892. Tiltscher, H . ; Wolf, H . ; Schelchshorn, J . Ber. Bunsenges. Phy. Chem., 1984, 88, 897. Schmitt, W. J.; Reid, R. C. In Supercritical Fluid Technology; Penninger, J . M. L.; Radosz, M.; McHugh, M. A . ; Krukonis, V. J. Ed., Elsevier Science Publishers, Amsterdam, 1985. Moradinia, I . ; Teja, A. S. Fluid Phase Equil. 1986, 28, 199.

R E C E I V E D April 27, 1989


89

Decoking and Regeneration of a Hydrotreating Catalyst by

Recommend Documents