Page 203 - Synthetic Fuels Handbook

FUELS FROM OIL SHALE 189

Diesel fuel derived from oil shale is also subject to the degree of unsaturation, the effect
of diolefins, the effect of aromatics, and to the effect of nitrogen and sulfur compounds.
Jet fuel produced from shale oil would have to be subjected to suitable refining treat-
ments and special processes. The resulting product must be identical in its properties to
corresponding products obtained from conventional crude oil. This can be achieved by
subjecting the shale oil product to severe catalytic hydrogenation process with a subsequent
addition of additives to ensure resistance to oxidation.
If antioxidants are used for a temporary reduction of shale oil instability, they should be
injected into the shale oil (or its products) as soon as possible after production of the shale
oil. The antioxidant types and their concentrations should be determined for each particular
case separately.
The antioxidants combine with the free radicals or supply available hydrogen atoms
to mitigate the progress of the propagation and branching processes. When added to the
freshly produced unstable product, the antioxidants may be able to fulfill this purpose.
However, when added after some delay, that is, after the propagation and the branching
processes have advanced beyond controllable limits, the antioxidants would not be able to
prevent formation of degradation products.
Exposure to oxygen is a major factor contributing to degradation product formation in
shale oils. Peroxy radicals, that are readily formed when untreated shale oils or their prod-
ucts, are exposed to oxygen lead to rapid gum formation rate. Once oxygen is eliminated
from such a system, the polymerization chain reaction tends to arrive to its termination
stage. The termination stage of this polymerization chain reaction can take place by one
of several ways, as for example exhaustion of the reactive monomers or a combination of
two free radicals. Chain reaction termination can be so affected by radical combination or
disproportionation.
In all cases free radicals have to be eliminated from the system. The chain termination
can be also induced by certain constituents present naturally or added artificially in the
form of antioxidants.
Thus, shale oil is different to conventional crude oils, and several refining technolo-
gies have been developed to deal with this. The primary problems identified in the past
were arsenic, nitrogen, and the waxy nature of the crude. Nitrogen and wax problems
were solved using hydroprocessing approaches, essentially classical hydrocracking and the
production of high quality lube stocks, which require that waxy materials be removed or
isomerized. However, the arsenic problem remains.
In general, oil-shale distillates have a much higher concentration of high boiling-point
compounds that would favor production of middle-distillates (such as diesel and jet fuels)
rather than naphtha. Oil-shale distillates also had a higher content of olefins, oxygen, and
nitrogen than crude oil, as well as higher pour points and viscosities. Above-ground-retorting
processes tended to yield a lower API gravity oil than the in situ processes (a 25° API gravity
was the highest produced). Additional processing equivalent to hydrocracking would be
required to convert oil-shale distillates to a lighter range hydrocarbon (gasoline). Removal
of sulfur and nitrogen would, however, require hydrotreating.
By comparison, a typical 35° API gravity crude oil may be composed of up to 50 percent of
gasoline and middle-distillate range hydrocarbons. West Texas Intermediate crude (bench-
mark crude for trade in the commodity futures market) has 0.3 percent by weight sulfur,
and Alaska North Slope crude has 1.1 percent by weight sulfur. The New York Mercantile
Exchange (NYMEX) specifications for light “sweet” crude limits sulfur content to 0.42 percent
or less (ASTM D4294) and an API gravity between 37° and 42° (ASTM D287).
A conventional refinery distills crude oil into various fractions, according to boiling point
range, before further processing. In order of their increasing boiling range and density, the
distilled fractions are fuel gases, light and heavy straight-run naphtha (90–380°F), kerosene
(380–520°F), gas-oil (520–1050°F), and residuum (above 1050°F) (Speight, 2007). Crude