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Abstract
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Distillate
cracks to propylene more readily than VGO. Additionally, less branched
hydrocarbons crack to propylene more readily than more branched hydrocarbons.
Oligomerization to diesel range oligomers followed by catalytic cracking with
less branched oligomers can provide more propylene.
BACKGROUND
When oligomerizing light olefins within a refinery, there is frequently a
desire to have the flexibility to make high octane gasoline, high cetane
diesel, or combination of both. However, catalysts that make high octane
gasoline typically make product that is highly branched and within the
gasoline boiling point range. This product is very undesirable for diesel. In
addition, catalysts that make high cetane diesel typically make product that
is more linear and in the distillate boiling point range. This results in
less and poorer quality gasoline due to the more linear nature of the product
which has a lower octane value. The oligomerization of butenes is often
associated with a desire to make a high yield of high quality gasoline
product. There is typically a limit as to what can be achieved when
oligomerizing butenes. When oligomerizing butenes, dimerization is desired to
obtain gasoline range material. However, trimerization and higher
oligomerization can occur which can produce material heavier than gasoline
such as diesel. Efforts to produce diesel by oligomerization have failed to
provide high yields except through multiple passes. When oligomerizing
olefins from a fluid catalytic cracking (FCC) unit, there is often the desire
to maintain a liquid phase within the oligomerization reactors. A liquid
phase helps with catalyst stability by acting as a solvent to wash the
catalyst of heavier species produced. In addition, the liquid phase provides
a higher concentration of olefins to the catalyst surface to achieve a higher
catalyst activity. Typically, this liquid phase in the reactor is maintained
by hydrogenating some of the heavy olefinic product and recycling this
paraffinic product to the reactor inlet. To maximize propylene produced by
the FCC unit, refiners may contemplate oligomerizing FCC olefins to make
heavier oligomers and recycling heavier oligomers to the FCC unit. However,
some heavy oligomers may be resistant to cracking down to propylene. The
products of olefin oligomerization are usually mixtures of, for example,
olefin dimers, trimers, and higher oligomers. Further, each olefin oligomer
is itself usually a mixture of isomers, both skeletal and in double bond
location. Highly branched isomers are less reactive than linear or lightly
branched materials in many of the downstream reactions for which oligomers
are used as feedstocks. This is also true of isomers in which access to the
double bond is sterically hindered. Olefin types of the oligomers can be
denominated according to the degree of substitution of the double bond, as
follows: TABLE 1 Olefin Type Structure Description I R—HC═CH2 Monosubstituted
II R—HC═CH—R Disubstituted III RRC═CH2 Disubstituted IV RRC═CHR
Trisubstituted V RRC═CRR Tetrasubstituted wherein R represents an alkyl
group, each R being the same or different. Type I compounds are sometimes
described as α- or vinyl olefins and Type III as vinylidene olefins. Type IV
is sometimes subdivided to provide a Type IVA, in which access to the double
bond is less hindered, and Type IVB where it is more hindered.
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