Ethylene-vinyl acetate co-polymers are widely used as cold-flow improving additives in diesel fuels. Although their functionality is undisputed, the mechanisms of action at a molecular level are not fully understood. Theories that EVA acts as both a nucleating agent for small wax crystals and as a crystal growth inhibitor are generally acknowledged, however the understanding of the effect of structure on additive performance is limited. Studies have been carried out using molecular simulation techniques with the aims of improving the understanding of the interactions between EVA and diesel fuels, and assessing the potential of these methods for the analysis of different additive structures and fuel compositions. Firstly, the ability of Monte Carlo simulations to predict the chemical potentials of n-alkanes in organic solvents was studied. This information would be of use in determining the characteristics of different fuel compositions. It was found that the particle insertion method used becomes more efficient as the aspect ratio of the solvent increases relative to that of the solute. Despite the identification of expected linear trends in the chemical potential of n-alkanes, it was not possible to produce high quality quantitative data using this method. The effect of vinyl acetate spacing and solvent environment on the configuration of lone EVA molecules in the liquid phase was studied. Molecular dynamics showed that closely spaced vinyl acetate groups increase the tendency of the molecule to fold at that point, whilst molecules with 2 or 3 ethylene groups in between show resistance to folding. It was shown that n-hexane as a solvent produced more folded configurations than benzene and n-hexadecane respectively. Monte Carlo simulations were also performed but were largely unsuccessful. Finally, molecular dynamics simulations were performed to study the effect of different EVA structures on the spontaneous crystallisation of n-hexadecane, and the interaction of the EVA with the resulting structures at a molecular level. It was shown that different EVAs had varying effects on the crystallisation, with trends attributed to the spacing of the vinyl acetate groups and the length and position of extended aliphatic sections of the EVA molecules. One molecule, with evenly spaced side-groups and no extended aliphatic sections was shown to fully prevent the formation of any crystalline structure within the time of the simulation where all other EVA molecules allowed some degree of wax formation to occur.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:703001 |
| Date | January 2012 |
| Creators | English, Hugh Edmund |
| Contributors | Siperstein, Flor |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/molecular-simulations-of-ethylene-vinyl-acetatefor-the-improvement-of-the-cold-flow-propertiesof-diesel(735bf45e-acf0-4115-86a0-ea0c811c910a).html |
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