Processing of Primary Fischer-Tropsch Products - University of Alberta

product more amenable to refining [17], but this train of thought will not be
explored further.
3.1.1 Hetero-atom constraints
The most obvious advantage is the low sulphur content. There is no need to
worry about deep hydrodesulphurisation or other sulphur reduction technologies
to meet fuel sulphur specifications. There is also no need to worry about the
sulphur sensitivity of catalysts and high activity tmsulphided catalysts can in
principle be used for hydrotreating.
The bugbear of a FT refinery is oxygenates and most refining processes are
affected. Only the lighter than C5 boiling range material is oxygenate free or
have oxygenates present in very low concentration. Oxygenates can therefore be
considered critical in HTFT refining, with carboxylic acids being especially
troublesome. They restrict the metallurgy of processing equipment and cause
metal leaching of some classes of unsulphided catalysts, most notably nickel
promoted catalysts. Furthermore, the carboxylic acids tend to be difficult to
remove by hydrotreating.
Unlike hydrodesulphurisation (HDS) and hydrodenitrogenation (HI)N) that
produces hydrogen sulphide and ammonia, hydrodeoxygenation (HDO)
produces water. This not only complicates product work-up, but also creates
problems in catalysis. Hydrothermal dealumination can lead to changed catalyst
behaviour [18] and faster catalyst deactivation [19]. In chlorided catalyst systems
severe corrosion may also result from the presence of water.
3.1.2 Petrol component properties
It is instructive to look at the hydrocarbon component classes in relation to
their octane value and abundance in FT products. As an example, the C7
hydrocarbon compound classes have been listed, with their relative abundance
on an oxygenate-free basis in a high temperature FT product (Table 1).
It is clear that an unrefined HTFT product has a low octane and that the
only saving grace is its high olefin content. Exploiting the high olefin content
and redressing both the low degree of branching and low aromatics content, are
therefore central to the fuels refining strategy. This can typically be done by
skeletal isomerisation of the olefins [21, 22] (with the possibility for
etherification), skeletal isomerisation of the paraffins [23, 24], selective
aromatisation or reforming [25, 26, 27]. Similarly, the extraction of alpha-olefins
as chemicals is desirable from a fuels point of view. In some cases it may also be
useful to consider double bond isomerisation as a cheap way to improve the
octane of alpha-olefin-rich material.
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