small crystal zeolites improve gasoline selectivity & maintain octane

8/11/2019 Small Crystal Zeolites Improve Gasoline Selectivity & Maintain Octane

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Smaller Zeolite Crystals Reduce Coke And Gas Make

t 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 aasoline sized hydrocarbon fragment to escape intact. This reduces the chance that coke or gas will be formed from gasoline,

hus reducing gasoline recracking.

Researchers have recently demonstrated(Ref. 5) that smaller zeolitic crystals are effective in improving gasoline yields as theiffusion models predict. Samples of zeolite crystals were prepared with average sizes that measured 0.9 and 0.3 microns iniameter. These crystals were incorporated into experimental catalysts that were otherwise identical. Results of this study are

eproduced in Figures 1, 2 and 3. The smaller crystals yield from 1 to 10 weight percent more gasoline and 3 to 4 weight percenmore LCO at constant conversion. Heavy cycle oils and light gases were shown to be reduced.


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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 islso evidence that gasoline octane is maintained or improved by reducing crystal size. In another recent experiment(Ref. 6), twoatalysts were prepared by methods that left the zeolite crystals in one catalyst intact and produced 30 angstrom holes in therystals in the second. The researchers concluded that the treatment that altered the crystals reduced their effective crystal sizehus improving gasoline yield at the expense of light gases. They also demonstrated the C 4 products from crystals with the smaffective diameter had a higher ratio of butylenes to butanes, which implies that gasoline olefinicity and gasoline octane were

mproved.


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The reason small crystal zeolites increase gasoline olefinicity is related to the shorter time that gasoline boiling range productspend inside the crystals. Gasoline sized hydrocarbon fragments formed by the initial cracking reactions are olefinic(Ref. 4). Thehorter the time they spend in contact with the zeolite interior, the less likely they are to receive hydrogen from a hydrogen donoruch as a coke percursor. This reduces the chance they will become saturated and increases gasoline olefinicity. Higher olefinic

s 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

iesel selectivity can be attributed to the higher percentage of zeolitic surface area that is available on the exterior of small zeoliterystals. This surface area is active for cracking heavy molecules, while the interior surface area is not(Ref. 3,4). Figure 4(Ref. 7hows that a significant percentage of the surface area of zeolite crystals with diameters below .1 micron is external surface areand thus is available to promote bottoms cracking to diesel.

Conclusions

Measurements of the crystallite size of commercially prepared FCC catalysts show Engelhard's "in-situ" manufacturing processroduces 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 bysing Engelhard'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 le one weight percent increase in gasoline is expected, which will translate to a four cent per barrel of feed increase in productalue 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 productioneason, higher diesel yields could contribute as much to refinery profit margins as gasoline selectivity improvements.

References

. Unzelman, G.H., Oil and Gas Journal, April 4, 1988, Page 35.


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small crystal zeolites improve gasoline selectivity & maintain octane

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