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Development of the Theory of Catalytic Cracking ROWLAND C. HANSFORD Yorba Linda, CA 92686 The theory of catalytic cracking was developed in the 1940's almost simultaneously in the research laboratories of Socony-Vacuum Oil Company (now Mobil Oil Corporation), Shell Development Company, and Universal Oil Products Company (now UOP, Inc). This review records the history of that portion of the development which occurred at Mobil's laboratories in Paulsboro, New Jersey, beginning about 1940. Mobil was one of the early pioneers in the commercial development and application of catalytic cracking through its collaboration with Eugene Houdry in the middle 1930's. The first commercial catalytic cracking unit employing Houdry's concept of a solid regenerable catalyst went into operation at Mobil's Paulsboro Refinery in 1936 (2). This was a 2000-barrel plant which served to demonstrate and prove out the process. Soon a larger 15,000-barrel unit was built at Sun Oil's Marcus Hook, Pennsylvania refinery across the Delaware River from Mobil's Paulsboro Refinery where a similar larger Houdry-type plant also was built beside the prototype unit. By the beginning of World War II in Europe in 1939, approximately 100,000 barrels of Houdry capacity was in operation across the United States. It has been said that this capacity to produce high-grade aviation gasoline components enabled victory in the Battle of Britian in the early 1940's. With the expansion of the war in 1941 by our entry into the conflict against both Japan and Germany, it was obvious that larger and better cracking plants were going to be necessary to support the war effort of our allies and ourselves. Already new types of catalytic cracking processes were being developed. These were the Fluid Catalytic Cracking (FCC) Process and the Thermofor Catalytic Cracking (TCC) Process. Prototypes of both process units went into commercial operation in the early years of American involvement in the war. In addition to better and more efficient cracking plants, the exigencies of the expanding war effort required higher yields of high quality aviation gasoline. This meant improved 0097-615 6/8 3/0222-0247$06.00/0 © 1983 American Chemical Society
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In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Washington, 0. C. Society: 20030Washington, DC, 1983.
248
HETEROGENEOUS
CATALYSIS
c a t a l y s t s had to be developed q u i c k l y to f i l l the new cracking units. At Mobil's research l a b o r a t o r i e s i n Paulsboro, M i l t o n M a r i s i c and coworkers soon came up with a new c a t a l y s t . This became known as "bead c a t a l y s t " since i t was made in the form of opalescent spheres. The improved c a t a l y s t was used e x t e n s i v e l y both i n Houdry-type plants and TCC plants during the c r i t i c a l l a t t e r part of the war. M a r i s i c s basic concept was to make a true homogeneous hydrosol of s i l i c a and alumina, which could be formed into s p h e r i c a l droplets i n an immiscible f l u i d , l i k e o i l . The concentration of hydrous oxides and the pH of the hydrosol were c o n t r o l l e d to cause the droplets to set to a firm hydrogel before reaching the bottom of the o i l column. The r e s u l t i n g hydrogel spheres were washed, ion exchanged to remove sodium, d r i e d , and c a l c i n e d to produce hard, microporous, opalescent "beads" (_3 ). The process developed for the manufacture of the c a t a l y s t was continuous and therefore capable of a high rate of production. The s p h e r i c a l shape made the c a t a l y s t very s u i t a b l e for moving bed operation i n the TCC Process, as w e l l as for fixed-bed operation i n the Houdry Process. But most important from the viewpoint of t h i s d i s c u s s i o n of c a t a l y s t mechanism development i s the chemistry of the process and of i t s product. M a r i s i c recognized that the g e l a t i o n of the homogeneous hydrosol of hydrous s i l i c a and alumina produced a more or less uniform c r o s s l i n k e d s t r u c t u r e i n which nearly every aluminum ion i s t e t r a h e d r a l l y coordinated with s i l i c o n ions through oxygen ions. Each such aluminum ion possesses a net negative charge r e q u i r i n g n e u t r a l i z a t i o n by a c a t i o n . In short, the hydrogel i s an amorphous " z e o l i t e " having ion-exchange capacity equivalent to i t s aluminum ion content. The hydrogel, when washed free of soluble s a l t s formed i n the n e u t r a l i z a t i o n process of s o l formation, s t i l l contains approximately one Na for every aluminum i n the hydrogel s t r u c t u r e . This " z e o l i t i c " sodium i s easily replaced by other ions such as NH^"", H , A10 , etc. F u l l ion-exchange capacity equivalent to the content of aluminum of the hydrogel e x i s t s before c a l c i n a t i o n . However, i f the Na i s exchanged out with NH4 and the product is heated to drive o f f N H 3 , the ion-exchange capacity w i l l drop to about 10% of t h e o r e t i c a l . But t h i s r e s i d u a l capacity, now present as hydrogen ions, i s very important for the functioning of the c a l c i n e d xerogel as a cracking c a t a l y s t . Edward G r i e s t i n M a r i s i c s laboratory discovered that when the c a l c i n e d beads were contacted with an a l c o h o l i c s o l u t i o n of methyl orange the a c i d c o l o r of the i n d i c a t o r developed. Unfortunately, he never published t h i s observation, although i t was made s e v e r a l years before s i m i l a r r e s u l t s were published by others (4)· In 1944, the w r i t e r became head of the c a t a l y s t research group that had worked with M a r i s i c on the development of bead catalyst. Among t h i s group was Charles Plank, who much l a t e r
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In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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21.
HANSFORD
Theory of Catalytic Cracking
249
was responsible f o r the important invention of the v a s t l y superior c r y s t a l l i n e a l u m i n o s i l i c a t e z e o l i t e cracking c a t a l y s t s employed i n p r a c t i c a l l y a l l commercial c a t a l y t i c cracking today. Others i n the group were Leonard Drake and Howard R i t t e r , who j o i n t l y invented the mercury porosimeter. This device has been an important research t o o l i n determining the porous s t r u c t u r e of a l l s o r t s of s o l i d s ( 5 ) . Plank made many fundamental studies of the p r o p e r t i e s o f silica and s i l i c a - a l u m i n a hydrogels and xerogels. Drake contributed to the research on p h y s i c a l s t r u c t u r e of these m a t e r i a l s using h i s mercury porosimeter. It i s probable that much of t h i s background l e d Plank to h i s p r a c t i c a l development of z e o l i t e cracking c a t a l y s t s i n l a t e r years. But the group was also concerned with improving the p r o p e r t i e s of the bead c a t a l y s t , such as i t s pore s t r u c t u r e and i t s regeneration. A practical problem associated with "after-burning" of regeneration gases a f t e r leaving the dense c a t a l y s t bed i n TCC u n i t s was solved by adding small amounts of chromia to the c a t a l y s t (6). This was c a l l e d the chrome bead c a t a l y s t . It i s i n t e r e s t i n g that s i m i l a r problems i n modern c a t a l y t i c cracking p l a n t s have been solved by adding traces of platinum to the catalyst. A major e f f o r t of the group a f t e r the war was devoted to research aimed at an understanding of the o r i g i n of c a t a l y s t a c t i v i t y and of the mechanism of c a t a l y t i c c r a c k i n g . Griest's observation of the a c i d i c nature of s i l i c a - a l u m i n a c a t a l y s t s l e d the w r i t e r to consider the o r i g i n and nature of these a c i d centers. Further, the questions of whether a c i d i t y plays a r o l e i n the cracking mechanism and how i t may do so became a subject for serious i n v e s t i g a t i o n . In a study of the cracking of η-butane over a commercial s i l i c a - a l u m i n a c a t a l y s t , i t was e a r l y observed that a c t i v i t y decreased as the c a t a l y s t was dehydrated at high temperature under vacuum (_7). The a c t i v i t y could be restored simply by allowing water vapor to readsorb on the c a t a l y s t at elevated temperature. As l i t t l e as 0.1% of adsorbed water s i g n i f i c a n t l y affected a c t i v i t y . It seemed c l e a r that since only small amounts of water are e f f e c t i v e , and i f a c i d i t y i s also involved i n the c a t a l y t i c f u n c t i o n , then the nature of the a c i d i t y probably i s p r o t o n i c . Removal of water reduces proton a v a i l a b i l i t y and also c a t a l y s t activity. I f protons are indeed involved i n the mechanism of hydrocarbon a c t i v a t i o n and r e a c t i o n , then i t was reasoned that an exchange of hydrogen between c a t a l y s t and hydrocarbon might occur. By hydrating the c a t a l y s t with D2O i t was shown that indeed such exchange does occur (_7). The ease of exchange was found to follow more or less the expected r e l a t i v e ease of protonation or carbonium ion formation. Thus, the lowest temperature at which exchange was observed f o r a number of pure hydrocarbons was found to be as follows:
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
250
HETEROGENEOUS
2-Butene Isobutane Benzene n-Butane n-Hexane Cyclohexane
CATALYSIS
Below 30°C 35° 70° 205° 260° 315°
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A p o s s i b l e explanation f o r the o r i g i n of c a t a l y s t a c i d i t y was proposed by the w r i t e r , based on Pauling's e l e c t r o s t a t i c valence r u l e :
OH HO-AK HO
0
O-H*
- Si - 0 - Si - 0 - Si 1
0
0
0I
I
Aluminum i s four-coordinated i n the s t r u c t u r e , but having a valence o f 3 the r e s i d u a l bonding power f o r one of the hydroxy1 hydrogens ( i n t h i s case hydroxyl bonded to S i ) i s reduced enough to l a b i l i z e that hydrogen as a proton. Tamele (8) l a t e r proposed a s i m i l a r , but probably b e t t e r , p i c t u r e i n which the l a b i l e proton comes from the hydroxyl attached to A l : +
I
..
..
..
I
- Si : Ο : ΑΙ : Ο : Si ι
ι
: Ο: 1
The advantage o f Tamele s concept i s that i t more c l e a r l y shows how the acid s i t e associated with the aluminum ion i s i n t e r c o n v e r t i b l e t o Lewis acid and Br'onsted acid by removing or adding a water molecule:
HO I
. . I
*H 0
Si : Ο : ΑΙ : Ο : Si '
" LEWIS ACID
I
2
•
^ - S i : Ο : Al : Ο : Si ^ 5 "
1
1
B R O N S T E D ACID
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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21.
HANSFORD
Theory of Catalytic Cracking
251
If the Lewis acid s i t e i s assumed to be i n a c t i v e i n cracking, t h i s explains the e f f e c t of dehydration on a c t i v i t y of the catalyst. The mechanism o r i g i n a l l y proposed by the w r i t e r f o r the cracking r e a c t i o n invoked both carbonium ions and carbanions, depending on whether the hydrocarbon had unsaturated or saturated carbon-carbon bonds. With olefins or aromatics protonation to form a carbonium ion i s r e l a t i v e l y easy. However, at that time i t was not obvious how a p a r a f f i n can be converted to a carbonium i o n . So i t was postulated that water might extract a proton from the p a r a f f i n to form i n t h i s case a carbanion. However, t h i s concept was very soon abandoned because i t was inconsistent with the observed rearrangements (isomerization) i n reactants and products of cracking. An a l l - c a t i o n i c mechanism was then proposed (9) i n which the a c t i v a t i o n of p a r a f f i n s occurs by hydrogen t r a n s f e r to form a carbonium i o n intermediate. Convincing proof of the carbonium ion mechanism o f a c t i v a t i o n of p a r a f f i n s was obtained from a d e t a i l e d study of hydrogen exchange between c a t a l y s t and isobutane (10). I f the exchange occurs through a carbonium ion mechanism, the theory requires that only primary hydrogens w i l l exchange. The t e r t i a r y hydrogen w i l l not exchange. The r e s u l t s c l e a r l y showed that nine of the hydrogens of isobutane r e a d i l y exchanged with hydrogen (deuterium) on the c a t a l y s t . Only traces of product containing ten deuterium atoms were found. Proof that i t was the t e r t i a r y hydrogen that does not exchange was provided by an experiment with 2-methylpropane-2-d on a non-deuterated silica-alumina catalyst. No s i g n i f i c a n t exchange was observed, i . e . , less than 0.4% of nondeuterated isobutane was observed i n the product a f t e r 16 hours of contact at 120°C. Under the same c o n d i t i o n s , approximately 80% t o t a l exchange (disappearance of mass 58 isobutane) was observed with undeuterated isobutane on a deuterated c a t a l y s t . The carbonium ion theory o r i g i n a l l y proposed by Frank Whitmore (11) i s thus as a p p l i c a b l e to the mechanism of c a t a l y t i c cracking as i t i s to other a c i d - c a t a l y z e d reactions l i k e isomerization or a l k y l a t i o n . The question of carbonium ion formation from saturated hydrocarbons was considered i n (_7) by the w r i t e r when the p o s s i b i l i t y of p a r t i c i p a t i o n by o l e f i n s from thermal cracking was mentioned. However, i t was only somewhat l a t e r that t h i s suggestion was s e r i o u s l y adopted ( 9 ) . Then i t was postulated that even traces of unsaturated hydrocarbons can a c t i v a t e saturated hydrocarbons by f i r s t forming a carbonium ion by proton a d d i t i o n . This ion can then extract a hydride ion by hydrogen t r a n s f e r from the p a r a f f i n or c y c l o p a r a f f i n . This i n i t i a t e s a sort of chain r e a c t i o n i n which new carbonium ions are formed by hydrogen t r a n s f e r with a steady-state population of ions on the c a t a l y s t surface.
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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252
HETEROGENEOUS
CATALYSIS
The hydrogen exchange study reported by the author and co-workers (10) showed that indeed only small amounts of o l e f i n are required to a c t i v a t e isobutane. The e f f e c t of traces of o l e f i n s on the exchange rate was greatest at low temperatures (60-80°C). At 120°C the a d d i t i o n o f o l e f i n had no e f f e c t on the exchange rate of the undoped hydrocarbon, i n d i c a t i n g that at t h i s temperature enough o l e f i n was being formed by thermal cracking (or dehydrogenation) to maintain the i n i t i a t i o n and propagation steps. The w r i t e r prefers t h i s mechanism of carbonium ion formation from saturated hydrocarbons over the d i r e c t hydride i o n e x t r a c t i o n by the c a t a l y s t (H +H~ H2). It i s w e l l known that s a t u r a t i o n of o l e f i n s by hydrogen t r a n s f e r i s a major r e a c t i o n occurring i n commercial c a t a l y t i c cracking. The o r i g i n a l carbonium ion theory (now c a l l e d the carbeniura ion theory by modern p u r i s t s ) was applied i n a more q u a n t i t a t i v e way by Charles L. Thomas (12) o f UOP and i n p a r t i c u l a r by Bernard Greensfelder and Hervey Voge of S h e l l Development Company ( 13). I t s success has endured few challenges now f o r some 35 years, and i t seems u n l i k e l y that a b e t t e r theory as applied to c a t a l y t i c cracking will ever replace i t . +
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11. 12. 13.
Formerly Research Associate, Mobil Research and Development Corporation; presently Retired Staff Consultant, Science and Technology Division, Union Oil Company of California. Houdry, E . , Burt, W. F . , Pew, Α. Ε . , and Peters, J r . , W. A Petroleum Refiner (1938) 17, No. 11, 574. Marisic, M. M. U. S. Patents 2,384,942 and 2,384,946, (1945); Porter, R. W. Chem. & Met. Eng. 53, 94 (1946). See for example: Walling, C. J . Am. Chem. Soc. 1950, 72, 1164. Ritter, H. L. and Drake, L. C. Ind. Eng. Chem., Anal. Ed., 1945, 17, 782. L. C. Drake, Ind. Eng. Chem. 1949, 41, 780. Plank, C. J . and Hansford, R. C., U. S. Patent 2,647,860 (1953). Hansford, R. C. Ind. Eng. Chem. 1947, 39, 849. Tamele, M. W. Faraday Soc. Discussion 1950, 8, 270. Hansford, R. C. Advances in Catalysis 1952, 4, 14. Hansford, R. C . , Waldo, P. G., Drake, L. C., and Honig, R. E. Ind. Eng. Chem. 1952, 44, 1108. Whitmore, F. C. J . Am. Chem. Soc. 1,932, 54, 3274. Thomas, C. L. Ind. Eng. Chem. 1949, 41, 2564. Greensfelder, B. S., Voge, H. H. and Good, G. M. Ind. Eng. Chem. 1949, 41, 2573.
RECEIVED December
2, 1982
In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
