Page 102 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 89

Carbon rejection processes operate at moderate to high temperatures and low pres-
sures and suffer from a lower liquid yield of transportation fuels than hydrogen addition
processes, because a large fraction of the feedstock is rejected as solid coke high in sulfur
and nitrogen (and gaseous product). The liquids are generally of poor quality and must be
hydrotreated before they can be used as reformer or fluid catalytic-cracking (FCC) feeds
to make transportation fuels.
In the delayed coking process, heavy oil is heated to above 480°C (896°F) and fed to a
vessel where thermal cracking and polymerization occur. A typical product slate would be
10 percent gas, 30 percent coke, and only 60 percent liquids, the coke percentage increas-
ing at the expense of liquid products as feeds become heavier. Since sulfur is concentrated
in the coke, the coke market is limited to buyers that can control, or are not restricted by,
emissions of sulfur oxides (SO ).
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In the fluid-coking process, heavy oil is fed to a reactor containing a 480 to 540°C
(896–1004°F) bed of fluidized coke particles, where it cracks to produce lighter liquids,
gases, and more coke. The coke is circulated to a burner vessel where a portion of the
coke is burned to supply the heat required for the endothermic coking reactions. A portion
of the remaining coke is returned to the reactor as fluidizing medium, and the balance
is withdrawn as product. The net coke yield is only about 65 percent of that produced
by delayed coking, but the liquids are of worse quality and the flue gas from the burner
requires SO control.
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The Flexicoking process is an extension of fluid coking. All but a small fraction of the
coke is gasified to low-Btu gas (120 Btu/standard cubic feet) by addition of steam and air in
a separate fluidized reactor. The heat required for both the gasification and thermal cracking
is generated in this gasifier. A small amount of net coke (about 1 percent of feed) is with-
drawn to purge the system of metals and ash. The liquid yield and properties are similar to
those from fluid coking. The need for a coke market is eliminated or markedly reduced. The
low-Btu gas can be burned in refinery furnaces and boilers or probably could also be used in
cogeneration units to generate power and steam; but it must be used near the refinery since
its heating value is too low to justify transportation. Unlike with fluid coking, SO is not an
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issue since sulfur is liberated in a reducing atmosphere (carbon monoxide and molecular
hydrogen) inside the gasifier but hydrogen sulfide removal is required.
The Resid FCC process is an extension of gas oil FCC technology. In the process, resid
(usually above 650°F boiling point, not vacuum resid) is fed to a 480 to 540°C (896–1004°F)
fluidized bed of cracking catalyst. It is converted to predominantly gasoline-range boiling
materials, and the carbon residue in the feed is deposited on the catalyst. The catalyst activity
is then restored by burning the deposited coke in the regenerator. This also supplies the heat
required to crack the feed in the next contacting cycle. The sulfur emissions are typically
controlled by additives that bind the sulfur to the catalyst for later reduction to hydrogen
sulfide in the fluid catalytic cracking reactor. The hydrogen sulfide is later processed to sulfur
for sale as low-value by-product.
Ebullating bed processes (catalytic hydrocracking processes) such as the LC-Fining
(developed by the Lummus Company) and the H-Oil process (developed by Hydrocarbon
Research, Inc.) can be used to demetallize, desulfurize, and hydrocrack any heavy oil.
Both processes involve high-pressure catalytic hydrogenation but runs at higher temper-
atures than fixed bed resid desulfurization processes [about 426–441°C (799–825°F)]. The
feed passes upflow, expanding the catalyst bed with the ebullation and producing a back-
mixed isothermal system. Reactors are very large relative to fixed bed and are frequently
staged to overcome the kinetic penalties associated with back mixing. A big advantage
is the ability to add and withdraw catalyst while the unit is onstream, which allows the
processing of oils with high metal concentrations than is practical with conventional fixed
beds. Also, the ebullating bed eliminates coke plugging problems and allows high-temperature
operation and high (70–90 percent) conversion of the vacuum resid. The disadvantages