Page 204 - Synthetic Fuels Handbook

190 CHAPTER SIX

oil may contain 10 to 40 percent gasoline, and early refineries directly distilled a straight-
run gasoline (light naphtha) of low-octane rating. A hypothetical refinery may “crack” a
barrel of crude oil into two-thirds gasoline and one-third distillate fuel (kerosene, jet, and
diesel), depending on the refinery’s configuration, the slate of crude oils refined, and the
seasonal product demands of the market.
Just as natural clay catalysts help transform kerogen to petroleum through catagen-
esis, metallic catalysts help transform complex hydrocarbons to lighter molecular chains
in modern refining processes. The catalytic-cracking process developed during the World
War II era enabled refineries to produce high-octane gasoline needed for the war effort.
Hydrocracking, which entered commercial operation in 1958, improved on catalytic-cracking
by adding hydrogen to convert residuum into high-quality motor gasoline and naphtha-
based jet fuel. Many refineries rely heavily on hydroprocessing to convert low-value gas
oils residuum to high-value transportation fuel demanded by the market. Middle-distillate
range fuels (diesel and jet) can be blended from a variety of refinery processing streams.
To blend jet fuel, refineries use desulfurized straight-run kerosene, kerosene boiling range
hydrocarbons from a hydrocracking unit, and light coker gas-oil (cracked residuum). Diesel
fuel can be blended from naphtha, kerosene, and light cracked-oils from coker and fluid
catalytic cracking units. From the standard 42-gal barrel of crude oil, United States refiner-
ies may actually produce more than 44 gal of refined products through the catalytic reaction
with hydrogen.
Oil derived from shale has been referred to as a synthetic crude oil and thus closely asso-
ciated with synthetic fuel production. However, the process of retorting shale oil bears more
similarities to conventional refining than to synthetic fuel processes. For the purpose of this
report, the term oil-shale distillate is used to refer to middle-distillate range hydrocarbons
produced by retorting oil shale. Two basic retorting processes were developed early on—
aboveground retorting and underground, or in situ, retorting (qv). The retort is typically a
large cylindrical vessel, and early retorts were based on rotary kiln ovens used in cement
manufacturing. In situ technology involves mining an underground chamber that functions
as a retort. A number of design concepts were tested from the 1960s through the 1980s.
Retorting essentially involves destructive distillation (pyrolysis) of oil shale in the
absence of oxygen. Pyrolysis (temperatures above 900°F) thermally breaks down (cracks)
the kerogen to release the hydrocarbons and then cracks the hydrocarbons into lower-weight
hydrocarbon molecules. Conventional refining uses a similar thermal cracking process,
termed coking, to break down high-molecular weight residuum.
As the demand for light hydrocarbon fractions constantly increases, there is much inter-
est in developing economic methods for recovering liquid hydrocarbons from oil shale
on a commercial scale. However, the recovered hydrocarbons from oil shale are not yet
economically competitive against the petroleum crude produced. Furthermore, the value of
hydrocarbons recovered from oil shale is diminished because of the presence of undesir-
able contaminants. The major contaminants are sulfurous, nitrogenous, and metallic (and
organometallic) compounds, which cause detrimental effects to various catalysts used in
the subsequent refining processes. These contaminants are also undesirable because of their
disagreeable odor, corrosive characteristics, and combustion products that further cause
environmental problems.
Accordingly, there is great interest in developing more efficient methods for converting
the heavier hydrocarbon fractions obtained in a form of shale oil into lighter-molecular-
weight hydrocarbons. The conventional processes include catalytic cracking, thermal crack-
ing, and coking. It is known that heavier hydrocarbon fractions and refractory materials can
be converted to lighter materials by hydrocracking. These processes are most commonly
used on liquefied coals or heavy residual or distillate oils for the production of substan-
tial yields of low-boiling saturated products, and to some extent on intermediates that are
used as domestic fuels, and still heavier cuts that are used as lubricants. These destructive