Page 134 - Synthetic Fuels Handbook

120 CHAPTER FOUR

In the modified in situ extraction processes, combinations of in situ and mining tech-
niques are used to access the reservoir. A portion of the reservoir rock must be removed
to enable application of the in situ extraction technology. The most common method is to
enter the reservoir through a large-diameter vertical shaft, excavate horizontal drifts from
the bottom of the shaft, and drill injection and production wells horizontally from the drifts.
Thermal extraction processes are then applied through the wells. When the horizontal wells
are drilled at or near the base of the tar sand reservoir, the injected heat rises from the injec-
tion wells through the reservoir, and drainage of produced fluids to the production wells is
assisted by gravity.

4.6 UPGRADING, REFINING, AND FUEL
PRODUCTION

The limitations of processing these heavy oil and bitumen depend to a large extent on the
amount of nonvolatile higher molecular weight constituents, which also contain the majority
of the heteroatoms (i.e., nitrogen, oxygen, sulfur, and metals such as nickel and vanadium).
These constituents are responsible for high yields of thermal and catalytic coke. The major-
ity of the metal constituents in crude oils are present as organometallic complexes, such as
porphyrins. The rest are found in organic or inorganic salts that are soluble in water or in
crude. In recent years, attempts have been made to isolate and to study the vanadium present
in petroleum porphyrins.
When catalytic processes are employed, complex molecules (such as those that are present
in the nonvolatile fraction) or those formed during the process, are not sufficiently mobile (i.e.,
they are strongly adsorbed by the catalyst) to be saturated by hydrogenation. The chemistry of
the thermal reactions of some of these constituents dictates that certain reactions, once initiated,
cannot be reversed and proceed to completion. Coke is the eventual product. These deposits
deactivate the catalyst sites and eventually interfere with the hydroprocess.
Technologies for upgrading heavy crude feedstocks, such as residua and tar sand bitu-
men, can be broadly divided into carbon rejection and hydrogen addition processes.
Carbon rejection processes are those processes in which hydrogen is redistributed
among the various components, resulting in fractions with increased hydrogen/carbon
atomic ratios (distillates) and fractions with lower hydrogen/carbon atomic ratios (coke).
On the other hand, hydrogen addition processes involve the reaction of heavy crude oils
with an external source of hydrogen and result in an overall increase in hydrogen/carbon
ratio. Within these broad ranges, all upgrading technologies can be subdivided as follows:
1. Carbon rejection: Visbreaking, steam cracking, fluid catalytic cracking, coking, and
flash pyrolysis.
2. Hydrogen addition: Catalytic hydroconversion (hydrocracking) using active hydrode-
sulfurization catalysts, fixed-bed catalytic hydroconversion, ebullated catalytic-bed
hydroconversion, thermal slurry hydroconversion, hydrovisbreaking, hydropyrolysis,
donor solvent processes, and supercritical water upgrading.
3. Separation processes: Distillation, deasphalting, and supercritical extraction.
Thermal-cracking processes offer attractive methods of conversion of heavy oil
and bitumen because they enable low operating pressure, while involving high operat-
ing temperature, without requiring expensive catalysts. Currently, the widest operated
heavy oil and bitumen upgrading or conversion processes are visbreaking and delayed
coking. And, these are still attractive processes for refineries from an economic point
of view.