Membrane technology is a potential method for upgrading gasoline quality, with respect
to its tendency to promote fouling of engine inlet-systems. This thesis investigates the
transport and separation mechanisms of dense polydimethylsiloxane (PDMS)
membranes in nanofiltration applications relating to the filtration of gasoline fuels.
Simulated fuels were created which comprised representative organic solvents with
organometallic and poly-nuclear aromatic solutes. The flux and separation behaviour of
the solvent-solute systems were studied using several apparatus and a range of operating
regimes. Tests were performed with real fuels and refinery components to verify the
mechanisms observed with the model solvent-solute systems, and several strategies
were developed by which the process could be optimised or improved. Parallel to this
work, a project was undertaken to assess the suitability of the technology on an
industrial scale and to identify any scale-up issues.
The key factors influencing flux were found to be the viscosity and swelling-effect of
the solvent or solvent mixture. The dense membrane was shown to exhibit many
characteristics of a porous structure when swollen with solvents, with the separation of
low-polarity solutes governed principally by size-exclusion. It is postulated that
swelling causes expansion of the polymer network such that convective and diffusive
flow can take place between polymer chains. In general terms, a higher degree of
swelling resulted in a higher flux and lower solute rejection. The separation potential of
the membrane could be partly controlled by changing the swelling-effect of the solvent
and the degree of membrane crosslinking.
The transport of polar/non-polar solvent mixtures through PDMS was influenced by
swelling equilibria, with separations occurring upon swelling the membrane. Separation
of the more polar solvent occurred in this manner, and the solute rejection in
multicomponent polar/non-polar mixtures deviated significantly from the behaviour in
The results obtained from a pilot-plant scale apparatus were largely consistent with
those from laboratory-scale equipment, and engine tests showed that fuel filtration with
PDMS is a technically-viable means of upgrading gasoline quality.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.