Page 205 - Synthetic Fuels Handbook

FUELS FROM OIL SHALE 191

hydrogenation or hydrocracking processes may be operated on a strictly thermal basis or in
the presence of a catalyst. Thermodynamically speaking, larger hydrocarbon molecules are
broken into lighter species when subjected to heat.
The hydrogen/carbon atomic ratio of such molecules is lower than that of saturated
hydrocarbons, and abundantly supplied hydrogen improves this ratio by saturating reac-
tions, thus producing liquid species. These two steps may occur simultaneously. However,
the application of hydrocracking process has been hampered by the presence of certain
contaminants in such hydrocarbons. The presence of sulfur- and nitrogen-containing com-
pounds along with organometallic compounds in crude shale oils and various refined petro-
leum products has long been considered undesirable. Desulfurization and denitrification
processes have been developed for this purpose.
The thermal cracking process is directed toward the recovery of gaseous olefins as the
primarily desired cracked product, in preference to gasoline range liquids. By this process,
it is claimed that at least 15 to 20 percent of the feed shale oil is converted to ethylene, which
is the most common gaseous product. Most of the feed shale oil is converted to other gas-
eous and liquid products. Other important gaseous products are propylene, l,3-butadiene,
ethane, and butanes. Hydrogen is also recovered as a valuable non-hydrocarbon gaseous
product. Liquid products can comprise 40 to 50 weight percent or more of the total prod-
uct. Recovered liquid products include benzene, toluene, xylene, gasoline-boiling-range
liquids, and light and heavy oils. Coke is a solid product of the process and is produced by
polymerization of unsaturated materials. Coke is typically formed in an oxygen-deficient
environment via dehydrogenation and aromatization. Most of the formed coke is removed
from the process as a deposit on the entrained inert heat carrier solids.
The thermal cracking reactor does not require a gaseous hydrogen feed. In the reactor,
entrained solids flow concurrently through the thermal riser at an average riser temperature
of 700 to 1400°C. The preferred high length-to-diameter (L-to-D) ratio is in the range of a
high 4:1 to 40:1, or 5:1 to 20:1 preferably.
The moving bed hydroprocessing reactor is used to produce crude oil from oil shale or
tar sands containing large amounts of highly abrasive particulate matter, such as rock dust
and ash. The hydroprocessing takes place in a dual-function moving bed reactor, which
simultaneously removes particulate matter by the filter action of the catalyst bed. The efflu-
ent from the moving bed reactor is then separated and further hydroprocessed in fixed
bed reactors with fresh hydrogen added to the heavier hydrocarbon fraction to promote
desulfurization.
A preferred way of treating the shale oil involves using a moving bed reactor followed
by a fractionation step to divide the wide-boiling-range crude oil produced from the shale
oil into two separate fractions. The lighter fraction is hydrotreated for the removal of resid-
ual metals, sulfur, and nitrogen, whereas the heavier fraction is cracked in a second fixed
bed reactor normally operated under high-severity conditions.
The fluidized bed hydroretort process eliminates the retorting stage of conventional
shale upgrading, by directly subjecting crushed oil shale to a hydroretorting treatment in
an upflow, fluidized bed reactor such as that used for the hydrocracking of heavy petroleum
residues. This process is a single stage retorting and upgrading process. Therefore, the
process involves: (a) crushing oil shale, (b) mixing the crushed oil shale with a hydrocarbon
liquid to provide a pumpable slurry, (c) introducing the slurry along with a hydrogen-containing
gas into an upflow, fluidized bed reactor at a superficial fluid velocity sufficient to move
the mixture upwardly through the reactor, (d) hydroretorting the oil shale, (e) removing
the reaction mixture from the reactor, and (f ) separating the reactor effluent into several
components.
The mineral carbonate decomposition is minimized, as the process operating tempera-
ture is lower than that used in retorting. Therefore, the gaseous product of this process has
a greater heating value than that of other conventional methods. In addition, owing to the