Processing of Primary Fischer-Tropsch Products - University of Alberta

oligomerisation due to the low olefin content of the primary product. If the
olefins lighter than diesel are converted to the diesel boiling range then the
remaining naphtha may not require hydrogenation to improve the storage
stability. Oligomerisation has already been discussed in relation to the refining of
HTFT products so the remainder of the discussion on the processing of LTFT
products will focus on the hydroprocessing operations.
The hydrocracking of the heavy paraffins serves two purposes, to reduce
the boiling range to middle distillates and to improve the cold properties as the
hydrocracked products are mostly branched.
Currently the production of a high quality diesel fuel is a preferred option
to the production of gasoline. This is because the very factors which count
against FT gasoline, viz. product linearity and low aromatic content, are very
positive factors in favour of high quality, i.e. high cetane number, diesel fuel.
For maximum production of high quality diesel fuel the slurry bed reactor
operating in the high wax selectivity mode with either iron or cobalt based
catalysts, between 210 and 260 ~ and about 3Mpa, is the recommended route.
The straight run FT diesel makes up about 20% of the total FT product and
because it is predominantly linear it has a cetane number of about 75. Note that
at present the specified cetane number of diesel fuels varies from about 40 to 50,
depending on the location. The FT slurry reactors are operated for maximum
wax production because subsequent down stream hydrocracking of the wax
under relatively mild conditions makes the largest contribution to the final diesel
fuel pool. In the case of cobalt catalysts there are two reasons for operating the
FT process at high pressures, the wax selectivity increases with pressure (see
Chapter 3, Section 9.3.2) and the degree of branching decreases [64]. The
hydrocracking of wax with standard bi-functional catalysts was investigated at
Sasol during the 1970's [65, 66]. Mild catalytic hydrocracking of the wax yielded
about 80% diesel, 15% naphtha and 5% C1 to C4 gases. The product cut heavier
than diesel was recycled to extinction. Simple calculation shows that the above
product yields are the result of random beta scission along the linear wax chains.
There is therefore a big incentive to improve the selectivity of the wax
hydrocracking operation in order to increase the diesel cut yield. Some chain
branching does occur during the wax hydrocracking operation and so the cetane
number of the diesel fuel produced is somewhat lower than that of the straight-
run FT diesel. The final diesel pool nevertheless has a cetane number of above
70. The naphtha produced in the wax hydrocracking process consists only of
alkanes. The naphtha produced in the FT process also consists predominantly of
linear alkanes. To convert these two naphtha cuts to in- specification gasoline
would require a considerable amount of further octane number upgrading.
However, since these naphthas consist essentially of linear alkanes they would
be an excellent feedstock for the production of ethylene by steam cracking,
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