Page 90 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 77

Natural clays have long been known to exert a catalytic influence on the cracking of oils,
but it was not until about 1936 that the process using silica-alumina catalysts was developed
sufficiently for commercial use. Since then, catalytic cracking has progressively supplanted
thermal cracking as the most advantageous means of converting distillate oils into gasoline.
The main reason for the wide adoption of catalytic cracking is the fact that a better yield of
higher octane gasoline can be obtained than by any known thermal operation. At the same
time the gas produced consists mostly of propane and butane with less methane and ethane.
The production of heavy oils and tars, higher in molecular weight than the charge material,
is also minimized, and both the gasoline and the uncracked “cycle oil” are more saturated
than the products of thermal cracking.
Cracking crude oil fractions to produce fuels occurs over many types of catalytic mate-
rials, but high yields of desirable products are obtained with hydrated aluminum silicates.
These may be either activated (acid-treated) natural clays of the bentonite type of synthe-
sized silica-alumina or silica-magnesia preparations. Their activity to yield essentially the
same products may be enhanced to some extent by the incorporation of small amounts of
other materials such as the oxides of zirconium, boron (which has a tendency to volatilize
away on use), and thorium. Natural and synthetic catalysts can be used as pellets or beads
and also in the form of powder; in either case replacements are necessary because of attri-
tion and gradual loss of efficiency. It is essential that they be stable to withstand the physical
impact of loading and thermal shocks, and that they withstand the action of carbon dioxide,
air, nitrogen compounds, and steam. They also should be resistant to sulfur and nitrogen
compounds and synthetic catalysts, or certain selected clays, appear to be better in this
regard than average untreated natural catalysts.
The catalysts are porous and highly adsorptive and their performance is affected mark-
edly by the method of preparation. Two chemically identical catalysts having pores of dif-
ferent size and distribution may have different activity, selectivity, temperature coefficients
of reaction rates, and responses to poisons. The intrinsic chemistry and catalytic action
of a surface may be independent of pore size but small pores produce different effects
because of the manner in which hydrocarbon vapors are transported in and out of the pore
systems.

3.3.5 Hydroprocesses

Hydroprocesses use the principle that the presence of hydrogen during a thermal reac-
tion of a petroleum feedstock will terminate many of the coke-forming reactions and
enhance the yields of the lower boiling components such as gasoline, kerosene, and jet
fuel (Table 3.4).
Hydrogenation processes for the conversion of petroleum fractions and petroleum
products may be classified as destructive and nondestructive. Destructive hydrogenation
(hydrogenolysis or hydrocracking) is characterized by the conversion of the higher molecu-
lar weight constituents in a feedstock to lower boiling products. Such treatment requires
severe processing conditions and the use of high hydrogen pressures to minimize polymer-
ization and condensation reactions that lead to coke formation.
Nondestructive or simple hydrogenation is generally used for the purpose of improv-
ing product quality without appreciable alteration of the boiling range. Mild process-
ing conditions are employed so that only the more unstable materials are attacked.
Nitrogen, sulfur, and oxygen compounds undergo reaction with the hydrogen to remove
ammonia, hydrogen sulfide, and water, respectively. Unstable compounds which might
lead to the formation of gums, or insoluble materials, are converted to more stable
compounds.