Abstract
M.Sc.
The development of synthetic lubricants was the logical result of high demands
being made by modern technology, the exhaustion of natural mineral oil sources
and the search for environmentally friendly substances.
Alpha olefins are products of the Sasol Fischer-Tropsch process and are currently
part of the wider petrol pool. Substantial value can be added to the alpha olefins
by the production of high value polyalphaolefins (PADs). This project was aimed
at finding explanations for previous unsuccessful attempts to produce acceptable
PADs from Sasol feedstreams. High priority was given to the modification of
experimental procedures to produce a synthetic oil that conforms to specific
standards.
Oxygenates, aromatics and "other than alpha" olefins (branched, internal and cyclic
olefins) were identified as problem components in the Sasol feedstream. These
compounds led to early termination of oligomerisation and in the presence of BF3 ,
unwanted compounds were even incorporated into the PAD products. The
feedstream was thus purified by distillation and a MeDH/H2D extraction procedure.
As a result, better reaction control (with regard to oligomer distribution) was gained
and the product quality improved. Reaction conditions were optimised and basic
reaction models (to predict oligomer distributions) were developed.
It became clear however, that product quality did not only depend on oligomer
distribution, but also on the specific structures present in the various oligomers. GC
analyses showed that each oligomer consists of a great number of isomers - even
more than can be explained in terms of the classic cationic mechanism for
oligomerisation. GC-MS analyses could not successfully distinguish between
different branched isomers, because of the complexity of the mixture.
Summary
The question was asked whether the complex mixture has already been formed
during the primary oligomerisation process, or at a later stage by skeletal
rearrangements. Research work indicated that one can distinguish between a
primary (normal oligomerisation) process, and a secondary process (skeletal
rearrangement). It became clear that the secondary process is favoured by specific
reaction conditions (e.g. long reaction times and high co-catalyst concentrations).
The formation of isomers increased under these conditions and product quality was
adversely affected. The possibility to produce PAD products from Sasol
feedstreams identical to commercial products still exists, and necessitate further research work.