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Catalytic Cracking: breaking down large molecules of
heavy heating oil into smaller gasoline molecules.
The most widely used conversion method is called catalytic
cracking because it uses heat and pressure to "crack"
heavy hydrocarbon molecules into lighter ones. The cracking reaction
is very endothermic, and requires a large amount of heat. Another
problem is that reaction quickly fouls the Silica (SiO2)
and alumina (Al2O3) catalyst by forming
coke on its surface.
For example if eicosane (C-20) is heated to about 500 C the
covalent carbon-carbon bonds begin to break during the cracking
process. Many kinds of compounds including alkenes are made during
the cracking process. Alkenes are formed because there are not
enough hydrogens to saturate all bonding positions after the
carbon-carbon bonds are broken.
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Hydroprocessing applies the same principles but
uses a different catalyst, slightly lower temperatures, much
greater pressure and hydrogen to obtain chemical reactions of
breaking down large molecules of heavy heating oil into smaller
gasoline molecules. It also converts aromatics to cyclic alkanes,
olefins to alkanes.
Hydroprocessing first involves the addition of hydrogen atoms
to molecules without actually breaking the molecule into smaller
pieces at temperatures of about 325 degrees Celsius and pressures
of about 50 atmospheres. Many catalysts will work, including;
nickel, palladium, platinum, cobalt, and iron. Then hydrocracking
breaks longer molecules into smaller ones at temperatures over
350 degrees Celsius and pressures up to 200 atmospheres. In both
cases, very long residence times (about an hour) are required
because of the slow nature of the reactions.
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Alkylation or "polymerization" - forming
longer molecules from smaller ones. Another process is isomerization
where straight chain molecules are made into higher octane branched
molecules.
The reaction requires an acid catalyst (sulfuric acid, H2SO4
or hydrofluoric acid, HF) at low temperatures (1-40 degrees Celsius)
and low pressures (1-10 atmospheres). The acid composition is
usually kept at about 50% making the mixture very corrosive.
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