The selective removal of components from gasoline using membrane technology
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 binary mixtures. 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.