Processing of Primary Fischer-Tropsch Products - University of Alberta

is limited by the fuel specifications and it is therefore important to preferentially
include molecules with a large octane differential between the olefinic and
hydrogenated (paraffinic) species. Although this does not preclude direct
blending of the C5' s, the preferred refining options are:
9 Skeletal isomerisation: Both pentane [23, 24] and pentene [21, 22] are readily
converted to the methyl branched species and a number of commercial
technologies exist. Metal promoted alumina, chlorided alumina, zeolites and
metal oxide catalysts are used for pentane skeletal isomerisation, but pentene
skeletal isomerisation is limited to alumina and non-zeolitic molecular sieve
catalysts. The skeletal isomerisation reaction is equilibrium limited [41], but
high conversions can be achieved by recycling the unconverted product. In a
FT environment the high linearity of the feed implies that a large benefit can
be derived from skeletal isomerisation. Iso-pentane has an acceptable octane
(RON=92 and MON=90) for a paraffin. Unlike C4's, pentene and pentane
skeletal isomerisation generally requires less severe operating conditions,
thereby favouring the equilibrium for isomerisation that can be closely
approached in commercial operation. The acceptable paraffinic octane, large
octane gain with respect to the linear feed, ease of conversion and further
beneficiation possibilities of the olefin by etherification, makes it the most
preferred refining option.
9 Etherification: The etherification of 2-methyl-l-butene and 2-methyl-2-
butene with methanol yields tertiary amyl methyl ether (TAME). The
importance of TAME as octane booster was recognised in the late 1970's and
has been thoroughly studied since then [42, 43]. A number of commercial
technologies exist. Although TAME has a slightly lower octane (RON-115
and MON=100) compared to the iso-butene derived ethers, it has a lower
vapour pressure. The low degree of branching in the FT material presupposes
pentene skeletal isomerisation and etherification is therefore not a primary
refining option. The decision to invest in etherification technology should be
weighed up against purifying ethanol from the aqueous FT product stream as
alternative oxygenate-containing fuel additive.
The high olefin content makes oligomerisation a possibility. This can be
considered if it is necessary to increase the diesel to petrol ratio or reduce the
vapour pressure of the petrol.
3.2.4 C6 hydrocarbons
The most abundant C6 hydrocarbon is 1-hexene, which is a high value co-
monomer used in the polymer industry. It makes sense to extract and purify it,
since 1-hexene also happens to be a poor fuel component (RON=76 and
MON=63). Some oil soluble oxygenates start appearing in the FT product, but
these are mainly non-acid chemicals (mostly carbonyl compounds) and do not
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