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Hydrotreating and Desulfurization
Hydrogen is produced by catalytic reformers or hydrogen generation units (Chapter 23) and distributed to the hydrotreaters through a refinery-wide network.
In a hydrotreating unit feed and treating gas are combined and brought to the reaction temperature and pressure, prior to entering the reactor. The reactor is a vessel preloaded with solid catalyst,
which promotes the reaction. The catalyst is slowly deactivated by the continuous exposure to high
temperatures and by the formation of a coke layer on its surface. Refineries have to shut down the
units periodically and regenerate or replace the catalyst.
The severity of operation of an existing unit can be increased by increasing the reaction temperature but there is a negative impact on the catalyst life. The severity of operation can also be
increased by increasing the catalyst volume of the unit. In this case the typical solution is to add a
second reactor identical to the existing one, doubling the reactor volume. The pressure of an existing unit cannot be changed to increase its severity, because the pressure is related to material of
construction and thickness of metal surfaces. If higher pressure is required, the typical solution
is to install a new unit and use the existing one for a less severe service. In contrast there are also
processes which do not require either of the aforementioned technologies.
Biodesulfurization (q.v.) is only one of several concepts by which gasoline and diesel fuel might
be polished, that is, sulfur removed to an extremely low level, if not to a zero, level. At this point,
it is pertinent that a brief review of the potential methods for fuel polishing shoed be introduced.
One new technology is the use of adsorption by metal oxides in which the oxides either react by
physical adsorption or by chemical adsorption insofar as adsorption followed by chemical reaction
is promoted. The major distinction of this type of process from conventional hydrotreating is that
the sulfur in the sulfur-containing compounds adsorbs to the catalyst after the feedstock-hydrogen
mixture interacts with the catalyst. The catalyst does need to be regenerated constantly.
Another option involves sulfur oxidization in which a petroleum and water emulsion is reacted
with hydrogen peroxide (or another oxidizer) to convert the sulfur in sulfur-containing compounds
to sulfones. The sulfones are separated from the hydrocarbons for post-processing. The major
advantages of this new technology include low reactor temperatures and pressures, short residence
time, no emissions, and no hydrogen requirement. The technology preferentially treats dibenzothiophene derivatives, one of streams that are most difficult to desulfurize.
One way to add to the supply of ultra-low sulfur fuels is to turn to a non-oil-based diesel. The
Fischer-Tropsch process, for example, can be used to convert natural gas to a synthetic, sulfur-free
diesel fuel. Commercial viability of gas-to-liquids projects depends (in addition to capital costs) on
the market for petroleum products and possible price premiums for gas-to-liquids fuels as well as
the value of any byproducts.
A second way to avoid desulfurization is with biodiesel made from vegetable oil or animal fats.
Although other processes are available, most biodiesel is made with a base-catalyzed reaction. In
the process, fat or oil is reacted with an alcohol, such as methanol, in the presence of a catalyst to
produce glycerin and methyl esters or biodiesel. The methanol is charged in excess to assist in quick
conversion and recovered for reuse. The catalyst, usually sodium or potassium hydroxide, is mixed
with the methanol. Biodiesel is a strong solvent and can dissolve paint as well as deposits left in fuel
lines by petroleum-based diesel, sometimes leading to engine problems. Biodiesel also freezes at a
higher temperature than petroleum-based diesel.
Reactive adsorption desulfurization is a process using metal-based sorbent for sulfur capture to
form metal sulfide, usually in the presence of hydrogen; the sulfur atom is retained on the sorbent,
while the hydrocarbon portion of the molecule is released back into the process stream—ultra-deep
desulfurization for FCC gasoline can be achieved by this method (Babich and Moulijin, 2003;
Zhang et al., 2010).
Reactive adsorption has been implemented in several desulfurization processes. The reaction
is exothermic and the adsorbent (unlike a hydrogenation catalyst) cannot remain stable activity
during desulfurization for prolonged periods and is more suitable to be performed in a fluidized
bed reactor.
Hydrotreating and Desulfurization
Hydrogen is produced by catalytic reformers or hydrogen generation units (Chapter 23) and distributed to the hydrotreaters through a refinery-wide network.
In a hydrotreating unit feed and treating gas are combined and brought to the reaction temperature and pressure, prior to entering the reactor. The reactor is a vessel preloaded with solid catalyst,
which promotes the reaction. The catalyst is slowly deactivated by the continuous exposure to high
temperatures and by the formation of a coke layer on its surface. Refineries have to shut down the
units periodically and regenerate or replace the catalyst.
The severity of operation of an existing unit can be increased by increasing the reaction temperature but there is a negative impact on the catalyst life. The severity of operation can also be
increased by increasing the catalyst volume of the unit. In this case the typical solution is to add a
second reactor identical to the existing one, doubling the reactor volume. The pressure of an existing unit cannot be changed to increase its severity, because the pressure is related to material of
construction and thickness of metal surfaces. If higher pressure is required, the typical solution
is to install a new unit and use the existing one for a less severe service. In contrast there are also
processes which do not require either of the aforementioned technologies.
Biodesulfurization (q.v.) is only one of several concepts by which gasoline and diesel fuel might
be polished, that is, sulfur removed to an extremely low level, if not to a zero, level. At this point,
it is pertinent that a brief review of the potential methods for fuel polishing shoed be introduced.
One new technology is the use of adsorption by metal oxides in which the oxides either react by
physical adsorption or by chemical adsorption insofar as adsorption followed by chemical reaction
is promoted. The major distinction of this type of process from conventional hydrotreating is that
the sulfur in the sulfur-containing compounds adsorbs to the catalyst after the feedstock-hydrogen
mixture interacts with the catalyst. The catalyst does need to be regenerated constantly.
Another option involves sulfur oxidization in which a petroleum and water emulsion is reacted
with hydrogen peroxide (or another oxidizer) to convert the sulfur in sulfur-containing compounds
to sulfones. The sulfones are separated from the hydrocarbons for post-processing. The major
advantages of this new technology include low reactor temperatures and pressures, short residence
time, no emissions, and no hydrogen requirement. The technology preferentially treats dibenzothiophene derivatives, one of streams that are most difficult to desulfurize.
One way to add to the supply of ultra-low sulfur fuels is to turn to a non-oil-based diesel. The
Fischer-Tropsch process, for example, can be used to convert natural gas to a synthetic, sulfur-free
diesel fuel. Commercial viability of gas-to-liquids projects depends (in addition to capital costs) on
the market for petroleum products and possible price premiums for gas-to-liquids fuels as well as
the value of any byproducts.
A second way to avoid desulfurization is with biodiesel made from vegetable oil or animal fats.
Although other processes are available, most biodiesel is made with a base-catalyzed reaction. In
the process, fat or oil is reacted with an alcohol, such as methanol, in the presence of a catalyst to
produce glycerin and methyl esters or biodiesel. The methanol is charged in excess to assist in quick
conversion and recovered for reuse. The catalyst, usually sodium or potassium hydroxide, is mixed
with the methanol. Biodiesel is a strong solvent and can dissolve paint as well as deposits left in fuel
lines by petroleum-based diesel, sometimes leading to engine problems. Biodiesel also freezes at a
higher temperature than petroleum-based diesel.
Reactive adsorption desulfurization is a process using metal-based sorbent for sulfur capture to
form metal sulfide, usually in the presence of hydrogen; the sulfur atom is retained on the sorbent,
while the hydrocarbon portion of the molecule is released back into the process stream—ultra-deep
desulfurization for FCC gasoline can be achieved by this method (Babich and Moulijin, 2003;
Zhang et al., 2010).
Reactive adsorption has been implemented in several desulfurization processes. The reaction
is exothermic and the adsorbent (unlike a hydrogenation catalyst) cannot remain stable activity
during desulfurization for prolonged periods and is more suitable to be performed in a fluidized
bed reactor.