Fluid catalytic crackers (FCC) units upgrade heavier fractions into lighter,
more valuable products. Long chain molecules (high carbon numbers) are
split into shorter molecules to achieve more of the high value fuel
components. A typical design uses a reactor and a regenerator. A fine
powdered porous catalyst with zeolite (silicate and alumina) is fluidized in the
hydrocarbon vapor, where a reaction takes place at 535 °C and 0.172 MPa.
The catalytic reaction takes place within a few seconds, after which the
reformate and catalyst is separated in a cyclone. The spent catalyst then
goes back to a regenerator that heats it to 715 °C at 0.241 MPa and
releases flue gas. The catalyst powder can then be reused. The reformates
go to a distillation column for separation into fractions.
A hydrocracker unit performs essentially the same function as the FCC when
more saturated hydrocarbons are desirable in the product. This means
alkane carbon chains with single bonds, not double bonds or cyclic rings like
aromatics, or more complex molecules. For this, additional hydrogen is
needed. The reaction takes place with hydrogen under pressure over a
catalyst.
The relative market need for diesel, kerosene and gasoline will
influence the choice of FCC versus hydrocracker. In the US, with a higher
relative volume of gasoline, more FCC capacity is needed, while in Europe
and Asia, with higher diesel consumption, more hydrocracking is used.
Visbreaking units upgrade heavy residual oils by thermally cracking them
into lower viscosity product that can be blended into lighter, more valuable
products. Visbreaking is characterized by its thermal severity, ranging from
mild cracking at 425 °C to severe cracking at 500 °C. Depending on the
residual oil, as much as 15-25% lighter fractions like diesel, kerosene and
gasoline could be obtained. The residue is tar and coke.
The Merox unit treats LPG, kerosene or jet fuel by oxidizing thiols
(mercaptans) to organic disulfides. The purpose is to reduce strong odors
caused by thiol presence.
Coking units (delayed coking, fluid coker, and flexicoker), like the visbreaker,
uses thermal cracking of very heavy residual oils into gasoline and diesel
fuel. The residue is green coke, and is further processed to fuel coke or, if
too low in sulfur and contaminants, to anode coke for the metallurgic
industries.
An alkylation unit produces high-octane components for gasoline blending.
The main use is to convert isobutane (C4H10, but arranged differently than n-
84
butane) to alkylates, mainly isooctane or isoheptane by adding an alkyl
group such as propene or butene over a strong acid catalyst, such as
sulfuric or hydrofluoric acid.
Dimerization is similar to alkylation, but uses a dimer group instead of an
alkyl group. For example, butenes can be dimerized into isooctene, which
may be hydrogenated to form isooctane.
Isomerization units convert linear molecules to higher-octane branched
molecules by rearranging the same atoms arranged in a different way.
For
example, C4H10 n-butane has the carbon atoms in a chain, while isobutane
has a central carbon with one hydrogen and three CH3 groups attached. The
isobutane can then be fed to the alkylation unit.
Steam reforming produces hydrogen for the hydrotreaters or hydrocracker.
Typical is the steam methane reformer (SMR), where steam reacts with
Methane at 425 °C with a nickel catalyst to produce syngas, which is a
source for many different reactions:
CH4 + H2O ↔ CO + 3 H2
If more hydrogen is needed, followed by a gas shift reaction with CO:
CO + H2O ↔ CO2 + H2
Amine gas treater, Claus unit, and tail gas treatment converts hydrogen
sulfide from hydrodesulfurization into elemental sulfur, which is a valuable
traded product.
Figure 22. Claus process (Source: Wikipedia)
The Claus process is the most common with the overall reaction:
85
2 H2S + O2 → S2 + 2 H2O
The reactor runs at 1,000° C and 0.15 MPa, with three steps: one thermal
and two catalytic to improve yield.
Using these processes, a modern refinery can raise the basic gasoline yield
depending on crude quality from 10-40% to around to 70%.
