Long-Residue Processing in a Riser Pilot Plant - ACS Symposium

Chapter 19

Long-Residue Processing in a Riser Pilot Plant

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 10, 2016 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch019

Vida J. Stripinis Oakville Research Centre, Shell Canada Limited, Oakville, Ontario, Canada

P i l o t plants are often used f o r studying the FCC process. In order f o r r e s u l t s to be meaningful, it i s necessary that p i l o t plant operation be consistent with that of commercial u n i t s . When processing feedstocks containing residue, t h i s becomes even more important. F a i l u r e to pay attention to d e t a i l s , such as feed/catalyst contacting, can lead to problems with data i n t e g r i t y and with coke buildup i n the equipment. A r i s e r p i l o t plant has been successfully used to process heavy feedstocks with Conradson carbon contents as high as 10%w.

A number of d i f f e r e n t types of laboratory scale units have been developed to simulate commercial c a t a l y t i c crackers. These include f i x e d bed (MAT), f l u i d i z e d bed, and r i s e r u n i t s . ( 1 , 2 , 3 ) In p a r t i c u l a r , f o r simulating commercial r i s e r FCC units which process residue, a r i s e r p i l o t plant i s the preferred choice. The c a t a l y s t and o i l are i n plug flow and the contact time i s short so that secondary reactions are avoided and c a t a l y s t deactivation by coke formation i s properly simulated. The r e s u l t i n g product s e l e c t i v i t y , then, i s s i m i l a r to commercial units. Experimental r e s u l t s from a laboratory scale unit can thus be translated to commercial u n i t s . This paper describes experience with residue processing i n c a t a l y t i c cracking units i n the industry, both on a commercial and p i l o t plant scale. S p e c i f i c changes which have been made to the design and operating procedures of the S h e l l Canada Oakville Research Centre (ORC) r i s e r p i l o t plant are also discussed. These have allowed processing of residues containing up to 9.9% Conradson Carbon Residue (CCR).

0097-6156/91/0452-0308$06.00y0 © 1991 American Chemical Society

Occelli; Fluid Catalytic Cracking II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

19.

STRIPINIS


Commercial

Long-Residue Processing

309

Experience

A number o f r e f i n e r s have p r o c e s s e d r e s i d u e c o n t a i n i n g f e e d s t o c k s i n commercial FCC u n i t s . Feeds w i t h as much as 5.1%w RCR (~6.5%w CCR) and 85 ppm N i + V have been p r o c e s s e d i n P h i l l i p s ' B o r g e r Refinery.(4) A s h l a n d has p r o c e s s e d f e e d s t o c k s o f up t o 7.1%w RCR (~8.5%w CCR) and 85 ppm N i + V i n t h e i r RCC (Reduced Crude Conversion) process.(5,6) A commercial s c a l e ART ( A s p h a l t R e s i d u a l T r e a t i n g ) u n i t has p r o c e s s e d r e s i d u e s c o n t a i n i n g l e v e l s of c o n t a m i n a n t s as h i g h as 13.5%w RCR and 300 ppm N i + V (7,8) . However, i n t y p i c a l d a y - t o - d a y o p e r a t i o n o f r e s i d u e c a t c r a c k e r s , f e e d s t o c k q u a l i t y i s not as extreme as t h o s e i l l u s t r a t e d above. The above d a t a i n d i c a t e s t h a t some commercial u n i t s can a l r e a d y p r o c e s s l o n g r e s i d u e from a number o f a v a i l a b l e w o r l d crudes. T h i s i s i l l u s t r a t e d i n F i g u r e 1 (9), a p l o t o f m e t a l s v e r s u s Conradson c a r b o n c o n t e n t f o r l o n g r e s i d u e s from v a r i o u s crudes. A box i s drawn i n d i c a t i n g t h e l i m i t a t i o n s o f c u r r e n t FCC t e c h n o l o g y f o r h a n d l i n g m e t a l s and Conradson c a r b o n . A number o f l o n g r e s i d u e s f a l l w i t h i n t h i s box, a l t h o u g h an even l a r g e r number f a l l o u t s i d e i t . The c h a l l e n g e i s t o d e v e l o p FCC technology t o enable economical p r o c e s s i n g of poorer q u a l i t y feedstocks. Pilot

Plant

Experience

Development o f t e c h n o l o g y i s g e n e r a l l y done u s i n g l a b o r a t o r y scale units. E x p e r i m e n t a l d a t a can be o b t a i n e d w i t h l e s s expense and w i t h more a c c u r a c y and f l e x i b i l i t y t h a n by a t t e m p t i n g t o do t h e same t e s t on a commercial s c a l e . In t h e l a b o r a t o r y , s m a l l e r q u a n t i t i e s o f f e e d s t o c k and c a t a l y s t a r e needed and lower manpower and c a p i t a l c o s t s a r e i n c u r r e d . By n e c e s s i t y , bench s c a l e u n i t s must p r o c e s s f e e d s t o c k s which a r e a t l e a s t as d i f f i c u l t as t h o s e p r o c e s s e d c o m m e r c i a l l y . P r o c e s s i n g r e s i d u e on a l a b o r a t o r y s c a l e p o s e s a number o f challenges. R e s i d u e - c o n t a i n i n g f e e d s t o c k s a r e more d i f f i c u l t t o v a p o u r i z e and have a h i g h e r coke f o r m i n g t e n d e n c y t h a n gas o i l s . P a r t i c u l a r a t t e n t i o n must be p a i d t o p i l o t p l a n t d e s i g n and o p e r a t i n g p r o c e d u r e s i n o r d e r t o a v o i d coke f o r m a t i o n and p l u g g i n g o f t h e s m a l l t u b i n g i n p i l o t p l a n t s and t o e n s u r e meaningful data. G u l f has r e p o r t e d problems c o n d u c t i n g r e s i d c r a c k i n g studies i n p i l o t scale units.(10) The d i f f i c u l t i e s i n c l u d e d f e e d i n j e c t o r p l u g g i n g and c a t a l y s t f l o w problems i n s m a l l lines in t h e i r 0.2 B/D g a s o i l c r a c k i n g u n i t . They have b u i l t a somewhat l a r g e r , 1.0 B/D p i l o t p l a n t , s p e c i f i c a l l y f o r h i g h c a r b o n r e s i d u e f e e d s t o c k s , and t h e y have p r o c e s s e d f e e d s t o c k s c o n t a i n i n g up t o 7.66%w CCR. E n g e l h a r d has o p e r a t e d a m o d i f i e d ARCO Lab u n i t w i t h a f e e d s t o c k o f 2.5% RCR (-3.5% CCR).(11) K e l l o g g has a 1/3 B/D u n i t u s e d f o r s t u d i e s i n c o n v e n t i o n a l gas o i l FCC, HOC (Heavy O i l C r a c k i n g ) and ART.(12) N o r t h Sea r e s i d u e , c o n t a i n i n g 3.9%w CCR, has been p r o c e s s e d i n t h e i r u n i t . ( 1 3 ) The ART p r o c e s s has, f o r t h e most p a r t , been d e v e l o p e d i n much l a r g e r u n i t s , namely, a 200



310

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

B/D demonstration plant and a 10,000 B/D commercial unit which have both processed poorer q u a l i t y feedstocks. Total (15) reports that operation of t h e i r p i l o t plant i s not s a t i s f a c t o r y for feedstocks with a v i s c o s i t y greater than 80 cSt at 1 0 0 ' C They state that thermal shock simulation and feedstock atomizing and evaporation are unsatisfactory i n t h e i r laboratory scale u n i t . S h e l l Canada's Oakville Research Centre has had a r i s e r p i l o t plant i n operation f o r about ten years. A s e r i e s of modifications have been made to the unit over the years i n order to improve i t s a b i l i t y to process heavier feedstocks. In s p i t e of a rather low feedrate of 0.52 g/s (approximately 0.3 B/D), poorer q u a l i t y feedstocks have been processed than those described above. To date, feedstocks with Conradson carbon contents up to 9.9%w and v i s c o s i t i e s up to 120 cSt at 100*C have been successfully run i n the ORC p i l o t plant. Operating experience with the p i l o t plant i s summarized i n Table I, and a schematic i s shown i n Figure 2. Experience has shown that there are four key aspects of design and operation of the p i l o t plant that must be considered i n order f o r i t to operate successfully with residue-containing feedstocks: 1) 2) 3) 4)

Feedstock atomization and vaporization Feed/catalyst mixing Spent c a t a l y s t s t r i p p i n g Product handling

These items w i l l each be discussed separately below. The resolution of problems encountered i n the ORC laboratory unit while processing heavy feedstocks w i l l be described. Feedstock Atomization

and

Vaporization

Feedstocks containing residue are more d i f f i c u l t to vapourize and tend to coke more e a s i l y . Atomizing the o i l feed to small droplets r e s u l t s i n more surface area for heat t r a n s f e r and quicker vapourization of the higher b o i l i n g materials i n i t . The atomizing nozzle i n use on the ORC p i l o t plant achieves average droplet sizes of less than 60 microns. This i s i l l u s t r a t e d i n Figure 3, which i s a p l o t of Sauter mean diameter of the droplets produced versus atomizing gas flow. The data was c o l l e c t e d at conditions chosen to simulate p i l o t plant operation on a residuecontaining feedstock. In c a t a l y t i c cracking, a large amount of heat needs to be supplied at the reactor i n l e t to vapourize the feed and provide the heat of reaction. In commercial units, t h i s heat i s provided by the hot c a t a l y s t r e c i r c u l a t e d from the regenerator. High heat transfer rates are achieved when the f l u i d i z e d c a t a l y s t i s mixed with the feed. In some experimental units, feed and c a t a l y s t are i n j e c t e d at reactor temperature. The heat of reaction must then be supplied by an external heating element, at much slower rates of heat t r a n s f e r . The product s e l e c t i v i t y from such laboratory units cannot be expected to simulate that of commercial units


STRIPINIS

250


Ni + V, ppm


200

150

limit of current technology

4

6

8

10

12

14

Conradson Carbon, %w

Figure 1 - Long Residue from Various Crudes

catalyst stripper

N

2

riser reactor condenser vent

r

catalyst hopper

gas

liquid receiver r—N

2

feed

Figure 2 - Schematic of Riser P i l o t Plant


312



Table I .

Riser Pilot

Plant Operation

0.52 g/s (approx 0.3 B/D) 2.5 t o 16 w/w up t o 540*C up t o 300*C up t o 800*C

F e e d Rate Cat/Oil Ratio R e a c t o r O u t l e t Temp. F e e d Temperature C a t a l y s t Temperature F e e d CCR up t o 9.9%w Feed V i s c o s i t y Reactor Pressure

up t o 120 c S t @ 100*C 1 bar

Sauter Mean Diameter (microns) 70 p 60 50 40 30 20 10 -

0

1

2

Air Flow (nL/min) F i g u r e 3 - A t o m i z i n g N o z z l e Performance


3


19.

STRIPINIS


313

very well. The preferred way to supply the heat required by the process i s using hot catalyst, the same as i n a commercial u n i t . Heating the feed to reactor temperature, usually greater than 500*C, causes an additional problem. Coke i s formed by thermal reactions of the feed i n the preheater and feed i n j e c t i o n system. The coking tendency i s more severe for heavy feedstocks which have higher concentrations of coke precursors. This can be avoided by l i m i t i n g the feed temperature to 300*C or l e s s . The practice at ORC has been to preheat the feedstock to a temperature s u f f i c i e n t l y high to reduce i t s v i s c o s i t y enough to ensure good atomization without causing coke formation i n the feed system. The heat of vaporization and reaction i s supplied by the much hotter c a t a l y s t . Good simulation of commercial FCC units has been achieved using t h i s operating strategy. Feed/Catalvst

Mixing

Once the feedstock has been atomized, i t must be mixed with the catalyst i n a uniform way to ensure e f f i c i e n t heat transfer and fast vapourization of the o i l . O i l droplets must not be allowed to h i t the wall of the reactor where they would deposit on the wall and form coke. They must come i n contact with a catalyst p a r t i c l e f i r s t . Coke formation on the wall can lead to flow r e s t r i c t i o n s i n the unit and an i n a b i l i t y to operate i t successfully. In the past, ORC experienced plugging from exactly t h i s type of phenomenon. When processing residue-containing feedstocks, coke would b u i l d up just above the feed i n j e c t i o n nozzle causing the flow to the r i s e r reactor to become r e s t r i c t e d . As a result, runs would have to be ended prematurely. The coke build-up tended to be the worst at low c a t a l y s t - t o - o i l ratios when catalyst flow rates were also low. It was thought that the density of the catalyst being supplied to the mixing zone was not constant. During moments of low density, feed droplets would come i n contact with the wall and form coke deposits. The catalyst i s supplied from.a hopper which i s f l u i d i z e d with nitrogen. Bubbles of gas from the f l u i d i z e d bed were getting entrained with the catalyst to the mixing zone and causing variations i n catalyst density. The operation of the catalyst hopper was examined keeping i n mind known facts about catalyst f l u i d i z a t i o n behavior. Figure 4 i l l u s t r a t e s the f l u i d i z a t i o n behavior of a t y p i c a l FCC c a t a l y s t . Catalyst bed height i s p l o t t e d against s u p e r f i c i a l v e l o c i t y of the f l u i d i z i n g gas. At gas v e l o c i t i e s between zero and minimum f l u i d i z a t i o n v e l o c i t y (Umf), the bed height remains unchanged. Once the minimum f l u i d i z a t i o n v e l o c i t y i s reached, the bed starts to expand. When gas flow i s increased further, the minimum bubbling v e l o c i t y (Umb) i s reached. At t h i s point, the gas starts to r i s e through the catalyst bed i n the form of bubbles. The bed height then starts to drop and slowly increases again with increasing s u p e r f i c i a l v e l o c i t y . The catalyst hopper had, at times, been operated at gas v e l o c i t i e s above the minimum bubbling v e l o c i t y . This caused bubbles as well as catalyst to be c a r r i e d to the feed/catalyst



314


Bed Height

«

U f m

.catalyst hopper operating region

U b m

Figure

4 -

Superficial Velocity FCC C a t a l y s t

Fluidization

Behaviour


19.

STRIPINIS


315

m i x i n g zone and r e s u l t e d i n v a r i a t i o n s i n c a t a l y s t d e n s i t y and t h e f o r m a t i o n o f t h e coke d e p o s i t s . To c o r r e c t t h i s , t h e gas f l o w was r e d u c e d t o e l i m i n a t e b u b b l e s from t h e c a t a l y s t b e i n g s u p p l i e d t o t h e r e a c t o r and t h e c a t a l y s t was i n s t e a d s u p p l i e d i n dense phase f l o w . T h i s change i n c a t a l y s t hopper o p e r a t i o n v i r t u a l l y e l i m i n a t e d t h e f o r m a t i o n o f coke d e p o s i t s i n t h e f e e d m i x i n g zone. C a t a l y s t - t o - o i l r a t i o s as low as 2.5 have been a c h i e v e d even w i t h v e r y heavy f e e d s t o c k s .


Catalyst

Stripping

Good s t r i p p i n g i s i m p o r t a n t i n o r d e r t o m i n i m i z e coke f o r m a t i o n . When c a t a l y s t i s s t r i p p e d e f f e c t i v e l y , h i g h e r b o i l i n g m a t e r i a l s a r e r e c o v e r e d as p r o d u c t r a t h e r t h a n b e i n g q u a n t i f i e d as qoke. In o r d e r t o a c h i e v e t h i s , equipment t o s t r i p t h e s p e n t c a t a l y s t must be c a r e f u l l y d e s i g n e d . S t r i p p i n g n i t r o g e n and c a t a l y s t r e s i d e n c e time must t h e n be o p t i m i z e d i n o r d e r t o a c h i e v e good stripping. I f more n i t r o g e n i s u s e d t h a n n e c e s s a r y , t h e p r o d u c t gas i s d i l u t e d , making a c c u r a t e gas a n a l y s i s f o r y i e l d d e t e r m i n a t i o n more d i f f i c u l t . Such a s t u d y t o o p t i m i z e spent c a t a l y s t s t r i p p i n g has been done f o r t h e ORC r i s e r p i l o t p l a n t . A f t e r making changes t o t h e d e s i g n o f t h e s t r i p p e r , a s e r i e s o f t e s t s were done w i t h v a r y i n g s t r i p p i n g n i t r o g e n flow while keeping other o p e r a t i n g c o n d i t i o n s c o n s t a n t t o d e t e r m i n e t h e optimum f l o w . The f l o w r a t e chosen f o r f u t u r e o p e r a t i o n was one f o r which t h e o b s e r v e d coke y i e l d had r e a c h e d a minimum. The u n i t o p e r a t e s i n a b a t c h mode w i t h t h e c a t a l y s t b e i n g c o l l e c t e d and s t r i p p e d i n t h e same v e s s e l . As a r e s u l t , a n o t h e r v a r i a b l e which a f f e c t s c a t a l y s t s t r i p p i n g i s t h e l e n g t h o f time t h a t t h e c a t a l y s t i s s t r i p p e d a f t e r t h e run i s finished. A s t u d y was done t o o p t i m i z e t h i s s t r i p p i n g t i m e . O v e r a l l , t h e changes i n s t r i p p e r d e s i g n and o p e r a t i o n r e s u l t e d i n a coke y i e l d r e d u c t i o n o f 1.5%w at constant c o n d i t i o n s . Product

Handling

R e s i d u e - c o n t a i n i n g feedstocks produce products t h a t are h e a v i e r and more v i s c o u s t h a n n o r m a l . Therefore, o p e r a t i n g temperature i n t h e p r o d u c t c o n d e n s e r must be chosen w i t h c a r e . I f the t e m p e r a t u r e i s t o o low, t h e h i g h b o i l i n g , v i s c o u s p o r t i o n o f t h e p r o d u c t d e p o s i t s on t h e condenser tube w a l l s and g e t s r e t a i n e d i n t h e c o n d e n s e r r a t h e r t h a n b e i n g r e c o v e r e d as p r o d u c t . On t h e o t h e r hand, i f t h e t e m p e r a t u r e i s t o o h i g h , l i g h t l i q u i d s a r e not condensed and g e t c a r r i e d o v e r i n t o t h e gas s t r e a m . I f t h e gas sample i s a l l o w e d t o c o o l , t h e s e l i g h t l i q u i d s can t h e n condense and not get a n a l y z e d . E x p e r i e n c e a t ORC has shown t h a t a c o n d e n s e r o p e r a t i n g t e m p e r a t u r e o f 0*C i s t o o low when p r o c e s s i n g r e s i d u e - c o n t a i n i n g feedstocks. The p r o d u c t r e c o v e r y i s r e d u c e d by 5% o r more due t o r e t e n t i o n o f heavy ends i n t h e c o n d e n s e r . A t e m p e r a t u r e o f 38*C i s s u f f i c i e n t l y h i g h t o a v o i d t h i s problem. In o r d e r t o e n s u r e t h a t , i n s p i t e o f t h i s , l i g h t l i q u i d s a r e condensed and not


316


c a r r i e d over to the gas stream, the product receiver i s cooled to -24 *C and the gas stream knock-out pot also to -24*C. Summary and

Conclusions


By paying p a r t i c u l a r attention to FCC p i l o t plant design and operating procedures, feedstocks containing s i g n i f i c a n t amounts of residue can be successfully processed i n a small-scale u n i t . The c r i t i c a l aspects can be itemized as follows: o

atomization of o i l to small droplet sizes;

o

feed temperature adequate to avoid coke formation i n the feed system;

o

use of hot c a t a l y s t to supply heat to the system, as i s done i n commercial units;

o

consistent c a t a l y s t flowrate to the feed/catalyst mixing zone by s e l e c t i n g f l u i d i z i n g nitrogen flow rate to stay below the minimum bubbling v e l o c i t y of the c a t a l y s t ;

o

uniform mixing of the feed and c a t a l y s t i n the bottom of the reactor to ensure quick vapourization and avoid coke formation i n the mixing zone;

o

optimized design and operation of the spent c a t a l y s t s t r i p p e r to ensure that coke y i e l d s are minimized without d i l u t i n g the product gas more than necessary; and

o

proper s e l e c t i o n of condenser operating temperature i n order to avoid retaining the heavy, viscous f r a c t i o n of the product on the condenser tube walls.

By focussing on the above points, cat cracking experiments can be performed i n laboratory equipment with residue-containing feedstocks. With proper choice of operating conditions, the r e s u l t i n g product y i e l d s and s e l e c t i v i t y simulate those i n commercial units quite well. Literature Cited 1)

P. O'Connor and M.B. Hartkemp, "A Microscale Simulation Test f o r FCC Development", ACS Los Angeles Meeting, Sept. 2 5 - 3 0 , 1 9 8 8 , pp.

2)

3) 4)

656-662.

J.E. Creighton, G.W. Young, " F l u i d Cracking Catalyst Evaluation: A Comparison of Testing Strategies", The Catalysis Society, 8th North American Meeting, May 1-4, 1 9 8 3 , Philadelphia, Pennsylvania. W.H. Humes, "ARCO's Updated Cat-Cracking P i l o t Unit", CEP, February 1 9 8 3 , 5 1 - 5 4 . J.B. Rush, P.V. Steed, "HDS + Cracking Ups Capacity, Yields", OGJ, May 2 8 , 1 9 8 4 , 9 6 - 1 0 3 .


19.

STRIPINIS

5)

6)

7)


8)

9) 10)

11)

12)

13) 14) 15)


317

L.E. Busch, W.P. Hettinger, J r . , Richard P. Krock, "Reduced Crude Oil Conversion i n Commercial RCC and ART Process Operations" (Paper No. AM-84-50), 1984 NPRA Annual Meeting, March 25-27, 1984, San Antonio, Texas. L.E. Busch, W.P. Hettinger, J r . , R.P. Krock, "RCC Complex Now Cornerstone of Ashland Refinery", OGJ, December 10, 1984, 79-84. D.B. Bartholic and R.P. Haseltine, "Development of Engelhard's Art Process", API, 46th Mid-Year Meeting, May 11-14, 1981, Chicago, I l l i n o i s , pp. 335-339. R.P. Haseltine, A.K. Logwinuk, D.L. Caldwell, The ART Process Offers Increased Refinery F l e x i b i l i t y " (Paper No. AM-83-43), 1983 NPRA Annual Meeting, March 20-22, 1983, San Francisco, C a l i f o r n i a . O i l & Gas Data Book, 1986 ed. Pennwell Publishing Co., Tulsa, Oklahoma. R.J. Campagna, A.S. Krishna, S.J. Yanik, "Research and Development Directed at Resid Cracking", OGJ, October 31, 1983, 128-134. D.B. Bartholic, R.P. Haseltine, " U t i l i z i n g Laboratory Equipment i n New Residual Oil Development" (Paper No. AM81-45), 1981 NPRA Annual Meeting, March 29-31, 1981, San Antonio, Texas. M. Schlossman et al, "Riser P i l o t Plant f o r C a t a l y t i c Cracking Studies", K a t a l i s t i k s 7th Annual FCC Symposium, Venice, Italy, May 12-13, 1986. H. Torgaard, "North Sea Resid Tested as FCC Feedstock", OGJ, January 10, 1983, 100-103. D.F. Barger, C . B . M i l l e r , "Convert Resid f o r F l e x i b i l i t y " , Hydrocarbon Processing, May 1983, 68-70. M. Denmar, A. T r i k i , J.P. Franck, "Advanced Analyses Improve Delta Coke Prediction f o r Resids", OGJ, Sept. 15, 1986.

RECEIVED June 8, 1990


Long-Residue Processing in a Riser Pilot Plant - ACS Symposium

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