Small Crystal Zeolites Improve Gasoline Selectivity and Maintain Octane
According to a recent survey(Ref. 1), 35 percent of the gasoline blending stock manufactured in North America is produced by fluid catalytic crackers. The high proportion of the gasoline pool contributed by FCC units makes these units the workhorses of a gasoline refinery. The favorable economics of upgrading gas oils to gasoline provides substantial incentive to increase gasoline yields from the fluid cat cracker. However, the current demand for high octane unleaded fuels also requires that FCC gasoline octane be maintained or increased.
These two important FCC operating goals - increasing gasoline volume and maintaining or raising octane - are often in conflict. Increasing reactor severity can increase both gasoline yield and improve octane if a commercial unit can handle the by-products of the cracking reaction, but a well run unit is usually handling as much coke and gaseous by-products as possible. Catalyst and operating changes that do not increase conversion either improve gasoline yields at the expense of octane or improve octane at the expense of gasoline yield. This trade off between gasoline selectivity and gasoline octane is called the gasoline yield-octane curve. A catalyst that can improve gasoline volume at constant conversion and that can maintain octane is said to improve the yield-octane curve.
BASF's FCC catalysts have always had the characteristics of improving the yield-octane curve. Our insitu manufacturing process accomplishes this by reducing the average crystalite size of our zeolite. Smaller crystal zeolite increase gasoline selectivity without adversely affecting octane. The reasons that small crystal zeolites are effective in improving FCC yields are discussed below.
Reducing Diffusional Effects Improves Gasoline Selectivity
Chemical engineers who study the fundamentals of catalysis have known since the 1950's(Ref. 2) that diffusional effects can reduce the selectivity of chemical reactions for desired intermediate products. In catalytic cracking, gasoline and light fuel oil are both intermediate products. Light gases and coke are by-products that are formed if the cracking reactions proceed too far (overcracking). It follows that gasoline yields will be improved at constant conversion if diffusional limitations are minimized.
Since the early development of zeolitic FCC catalysts, researchers have also known that it was difficult for gas oil molecules to diffuse into the center of the zeolitic crystals that catalyze the cracking reaction(Ref. 3). The reason for this diffusional limitation is that a regular network of 8 A diameter pores provides the only access to the interior of a zeolite crystal, and many gas oil molecules are sufficiently large that they do not easily fit into these pores. As a result, gas oil molecules are believed to crack primarily on the external surface of the crystals(Ref. 3,4).
The hydrocarbon fragments produced by cracking on the crystal exterior will be able to diffuse into the crystal interior due to their smaller size. Inside the crystal, they will be free from competition with gas oil molecules for cracking sites(Ref. 5). For this reason, gasoline recracking to coke and gas occurs preferentially in the interior. If gas oil diffusion into the interior of the particle were improved, gasoline selectivity would also be improved because gas oil cracking would compete with gasoline recracking. The result of this competition would be less gasoline recracking.
The Pac Man Process
Corma and Wojciechowski described in everyday language the reason diffusional limitations lead to coke and gas production. Gas oil molecules penetrate into the zeolite crystals like snakes crawling into holes. The acid sites on the inside of the pores cannot promote cracking readily at the end of the molecule; cracking is initiated on the fourth or fifth carbon from the end of the chain. The hydrocarbon fragment that breaks off is often 5 or 6 carbon atoms long. This molecule is in the gasoline boiling range. However, it cannot diffuse out of the crystal because the other, larger fragment of the gas oil molecule is blocking the pore opening.
From here on, the gasoline range hydrocarbon is subject to the "Pac Man Process". It diffuses through the pores of the zeolite crystal until it finds its way out. Along the way, it may react on acid sites in the interior to isomerize or to form a cyclic aromatic molecule. These processes increase its octane. It may also encounter a strong acid site that destroys the molecule by cracking it to gas or polymerizing it to coke. These "bad" sites represent the "ghosts" in the Pac Man video game that destroy your man and end your turn.
Smaller Zeolite Crystals Reduce Coke And Gas Make
It is easier to escape a small maze than it is to escape from a large one. Likewise, a smaller zeolite crystal makes it easier for a gasoline sized hydrocarbon fragment to escape intact. This reduces the chance that coke or gas will be formed from gasoline, thus reducing gasoline recracking.
Researchers have recently demonstrated(Ref. 5) that smaller zeolitic crystals are effective in improving gasoline yields as the diffusion models predict. Samples of zeolite crystals were prepared with average sizes that measured 0.9 and 0.3 microns in diameter. These crystals were incorporated into experimental catalysts that were otherwise identical. Results of this study are reproduced in Figures 1, 2 and 3. The smaller crystals yield from 1 to 10 weight percent more gasoline and 3 to 4 weight percent more LCO at constant conversion. Heavy cycle oils and light gases were shown to be reduced.
Small Zeolite Crystals Improve The Yield-Octane Curve
The gasoline selectivity improvement shown by small zeolite crystals in FCC catalysts would be impressive by itself, but there is also evidence that gasoline octane is maintained or improved by reducing crystal size. In another recent experiment(Ref. 6), two catalysts were prepared by methods that left the zeolite crystals in one catalyst intact and produced 30 angstrom holes in the crystals in the second. The researchers concluded that the treatment that altered the crystals reduced their effective crystal size, thus improving gasoline yield at the expense of light gases. They also demonstrated the C4 products from crystals with the smaller effective diameter had a higher ratio of butylenes to butanes, which implies that gasoline olefinicity and gasoline octane were improved.
The reason small crystal zeolites increase gasoline olefinicity is related to the shorter time that gasoline boiling range products spend inside the crystals. Gasoline sized hydrocarbon fragments formed by the initial cracking reactions are olefinic(Ref. 4). The shorter the time they spend in contact with the zeolite interior, the less likely they are to receive hydrogen from a hydrogen donor such as a coke percursor. This reduces the chance they will become saturated and increases gasoline olefinicity. Higher olefinicity is known to increase octane.
Diesel Selectivity Is Also Improved
The yield of diesel boiling range hydrocarbons is also expected to increase when diffusional resistance is reduced. The higher diesel selectivity can be attributed to the higher percentage of zeolitic surface area that is available on the exterior of small zeolite crystals. This surface area is active for cracking heavy molecules, while the interior surface area is not(Ref. 3,4). Figure 4(Ref. 7) shows that a significant percentage of the surface area of zeolite crystals with diameters below .1 micron is external surface area, and thus is available to promote bottoms cracking to diesel.
Measurements of the crystallite size of commercially prepared FCC catalysts show BASF's "in-situ" manufacturing process produces zeolite crystals with a 0.3 micron average diameter. A typical product by another FCC manufacturing process has a 0.9 micron average diameter. Research efforts have indicated that the reduction in zeolite crystal size that can be accomplished by using BASF's catalyst has been effective in increasing FCC gasoline yields at constant conversion without reducing octane These selectivity benefits can translate directly to higher gasoline yields in commercial FCC units. Experimental data shows at least a one weight percent increase in gasoline is expected, which will translate to a four cent per barrel of feed increase in product value given a 4 dollar difference between gasoline and feed.
Similar improvements in diesel yields at constant conversion are expected from small crystal zeolites. During fuel oil production season, higher diesel yields could contribute as much to refinery profit margins as gasoline selectivity improvements.
1. Unzelman, G.H., Oil and Gas Journal, April 4, 1988, Page 35.
2. Wheeler, A., Adv. Catal. 3, 250 (1951).
3. Thomas, C. L. and Barmby, D.S., Journal of Catalysis 12, 341 (1968).
4. Corma, A. and Wojciechowski, B. W., Catal. Rev. Sci. Eng. 27, 29 (1985).
5. Rajagopalan, K., Peters, A.W., and Edwards, G.C., Applied Catalysis 23, 69 (1986).
6. Corma, A., Herrero, E., Martinez, A., and Prieto, A.C.S. Symp. on Advances in FCC, New Orleans, 8/30-9/4/87.
7. Farcasiu, M. and Degnan, T.F., Ind. Eng. Chem. Res. 27, 45 (1988).