Processing of Primary Fischer-Tropsch Products - University of Alberta

Hydrogenation with an tmsulphided catalyst is more difficult, since the acids
tend to leach the metal from the catalyst. Hydrotreating technology is
therefore not the obstacle, but catalyst selection is difficult. The hydrogenated
product responds well to the addition of commercial quality improving
additives.
Hydroisomerisation: The main difference between hydrotreating and
hydroisomerisation is that the latter uses a catalyst that includes acid
functionality. This enables the isomerisation of the feed to take place while
doing the hydrotreating. This results in a product with better cold flow
properties, albeit at the potential loss of some Cetane. The reactive nature of
the feed needs to be taken into account when the catalyst and operating
conditions are specified. The catalysts used for hydrocracking and
hydroisomerisation are the same. At low processing severity
hydroisomerisation is the dominant reaction pathway, but as processing
severity increases more hydrocracking will take place, thereby increasing the
production of low octane naphtha. Gum formation on the acidic sites of the
hydroisomerisation catalyst is also possible if the feed in not hydrotreated.
The use of a hydroisomerisation catalyst as an intermediate bed in a
hydrotreater is therefore preferable to using hydroisomerisation as a refining
option on its own.
It is possible to extract some of the linear ct-olefins (typically C12-C15
range) to produce detergent alcohols by hydroformylation [53]. This has been
done in the Sasol Secunda refineries.
One shortcoming that is still left unaddressed is the low density of the
diesel. This can only be redressed by aromatics addition. Blending with biodiesel
improves the physical density but does not improve the energy density and high
energy density is actually the desired product characteristic for the fuel user.
Even Euro-4 does not impose a limit on aromatics for diesel, only polyaromatics
(< 11%). Some limitation on aromatics content is indirectly provided by the
reduction in upper density limit to 845 kg.m 3.
3.2.9 Heavier than Czz hydrocarbons
The fraction of the HTFT product with a boiling point higher than 360~
is small, but not negligible. Like the diesel fraction (Cll-C22) it consists mainly of
hydrocarbons, mostly olefins, with a low degree of branching and oxygenates.
Size reduction of the hydrocarbon chains is the most important objective, since
post-processing in the appropriate carbon number range is always possible.
Another important issue is the density of the diesel. The preferred refining option
is:
9 Hydrocracking: Hydrogen addition, rather than carbon rejection, underpins
the molecular size reduction. It is a clean, well-known and efficient
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