Processing of Primary Fischer-Tropsch Products - University of Alberta

(RON=55 and MON=51) content and the presence of oxygenates, it is not
suitable for direct blending. Unlike the even numbered molecules, 1-heptene is
not a high valued product either, although it is possible to convert 1-heptene to 1-
octene [50]. Reforming does not convert C7 molecules well and CJC6 skeletal
isomerisation technology can tolerate C7's in low concentration only.
Oligomerisation to produce diesel is practised commercially, but it is currently
expensive. The alternatives are limited and the preferred refining option for
petrol production is:
9 Aromatisation: The advantages of non-acidic Pt/L-zeolite aromatisation cited
for the conversion of C6 hydrocarbons, apply equally well to C7 hydrocarbon
conversion. The only difference is that the main product is toluene
(RON=120 and MON-103), not benzene. In commercial applications the C6's
and C7's are processed together, with subsequent benzene and toluene
separation. Despite the absence of sulphur in the feed, hydrogenation of
oxygenates in the feed is required as pre-processing step. The large volume of
hydrogen generated also makes hydrogen recovery desirable as a post-
processing step.
Future processing of the C7 hydrocarbons at the Sasol Secunda refineries
will entail catalytic cracking [16]. Although this is a costly refining approach, it
was justified based on the local conditions and existing on-site infrastructure for
propylene processing.
3.2.6 Cs hydrocarbons
The linear 1-0ctene present in high concentration in the C8 hydrocarbons
has the same commercial benefit as 1-hexene as a valuable co-monomer, but it is
considerably more difficult to purify. The oxygenates are not only more
concentrated, but they also contain some carboxylic acids. However, once a
decision has been made to install a process to separate the oxygenates from the
hydrocarbons (e.g. by extractive distillation) then the remaining olefins are also
worth recovering. Since the FT product is very olefinic and has a low degree of
branching, the olefins can alternatively be used as feed for hydroformylation to
produce plasticizer alcohols. It is possible to upgrade more than 60% of the C8
hydrocarbons to high value chemical products once the oxygenates have been
removed. Irrespective of whether these opportunities are exploited, the
oxygenates must be removed prior to further refining and the alternative is
hydrotreating. The preferred fuel refining options are:
9 Aromatisation: Like C6's and C7's it is possible to use non-acidic Pt/L-zeolite
based technology to convert C8 hydrocarbons to aromatics. The xylenes and
ethyl benzene have high octane values and are good fuel blending
components. Since the feed is low in naphthenics (cyclo-paraffins), better
yields can be expected than with normal reforming, but this advantage is
495