Page 135 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 121

The fluid catalytic cracking process using vacuum gas oil feedstock was introduced into
refineries in the 1930s. In recent years, because of a trend for low-boiling products, most refin-
eries perform the operation by partially blending residues into vacuum gas oil. However, con-
ventional fluid catalytic cracking processes have limits in when applied to processing heavy
oils and bitumen, so residue fluid catalytic cracking processes have lately been employed one
after another. Because the residue fluid catalytic cracking process enables efficient gasoline
production directly from residues, it will play the most important role as a residue cracking
process, along with the residue hydroconversion process. Another role of the residuum fluid
catalytic cracking process is to generate high-quality gasoline blending stock and petrochemical
feedstock. Olefins (propene, butenes, and pentenes) serve as feed for alkylation processes, for
polymer gasoline, as well as for additives for reformulated gasoline.
Residuum hydrotreating processes have two definite roles: (a) desulfurization to sup-
ply low-sulfur fuel oils and (b) pretreatment of feed residua for residuum fluid catalytic
cracking processes. The main goal is to remove sulfur, metal, and asphaltene contents
from residua and other heavy feedstocks to a desired level. The major goal of residuum
hydroconversion is cracking of heavy oil (and to some extent bitumen) with desulfurization,
metal removal, denitrogenation, and asphaltene conversion. Residuum hydroconversion
process offers production of kerosene and gas oil, and production of feedstocks for hydro-
cracking, fluid catalytic cracking, and petrochemical applications.
Finally, in terms of upgrading tar sand bitumen, solvent deasphalting processes have not
realized their maximum potential. With ongoing improvements in energy efficiency, such
processes would display its effects in a combination with other processes. Solvent deas-
phalting allows removal of sulfur and nitrogen compounds as well as metallic constituents
by balancing yield with the desired feedstock properties.
Upgrading residua that are similar in character to tar sand bitumen began with the intro-
duction of desulfurization processes that were designed to reduce the sulfur content of
residua as well as some heavy crude oils and products therefrom. In the early days, the goal
was desulfurization but, in later years, the processes were adapted to a 10 to 30 percent
partial conversion operation, as intended to achieve desulfurization and obtain low-boiling
fractions simultaneously, by increasing severity in operating conditions. Refinery evolution
has seen the introduction of a variety of heavy feedstock residuum cracking processes based
on thermal cracking, catalytic cracking, and hydroconversion. Those processes are different
from one another in cracking method, cracked product patterns, and product properties, and
will be employed in refineries according to their respective features.
In general terms, the quality of tar sand bitumen is low compared to that of conventional
crude oil and heavy oil. Upgrading and refining bitumen requires a different approach to
that used for upgrading heavy oil. In addition, the distance that the bitumen must be shipped
to the refinery and in what form as well as product quality must all be taken into account
when designing a bitumen refinery.
The low proportion of volatile constituents in bitumen [i.e., those constituents boiling
below 200°C (392°F)] initially precluded distillation as a refining step, are recognized by
thermal means and are necessary to produce liquid fuel streams. A number of factors have
influenced the development of facilities that are capable of converting bitumen to a synthetic
crude oil. A visbreaking product would be a hydrocarbon liquid that was still high in sulfur
and nitrogen with some degree of unsaturation. This latter property enhances gum formation
with the accompanying risk of pipeline fouling and similar disposition problems in storage
facilities and fuel oil burners. A high sulfur content in finished products is environmentally
unacceptable. In addition, high levels of nitrogen cause problems in the downstream pro-
cesses, such as in catalytic cracking where nitrogen levels in excess of 3000 ppm will cause
rapid catalyst deactivation; metals (nickel and vanadium) cause similar problems.
However, high-boiling constituents [i.e., those boiling in the range 200–400°C, (392–
752°F)] can be isolated by distillation but, in general terms, more than 40 percent by weight