Computer simulation of diesel and diesel additives

Abstract

The formation of n-alkane (paraffin) wax crystals in diesel fuel in winter time is an important problem for many oil industries. This thesis investigates the possibilities of making a simulation model, which can be used to study the behaviour of diesel fuel, and can be used to investigate crystallization. Initial work focused on finding a suitable molecular mechanics force field. Hexane molecules were modelled with three different force field and the TraPPE force field was chosen. A model of a liquid diesel fuel was produced composed of 78.7 % of a branched alkane solvent (2,4,6,10 tetramethyl dodecane) plus a distribution of ท-alkanes from C(_10) to C(_26) After technical problems, due to the inhomogeneity of the system leading to a formation of clusters molecules with gaps between them, equilibration of a model liquid diesel was achieved starting from an initial gas phase simulation. Calculated liquid density was in good agreement with experiment. Volume, total energy, enthalpy, radial distribution functions and dihedral angle distributions of the solvent molecules all confirmed equilibration of the model diesel. A crystallization inhibitor consisting of a random co-polymer of ethylene vinyl acetate, EVA, was simulated. Starting from an extended configuration led to the formation of a coiled chain in the gas phase. The chain was added to the liquid diesel model and it was found that the diesel can be considered as a good solvent for EVA polymer, with the radius of gyration for EVA increasing slightly in the diesel. An orthorhombic structure with P(_bcm) space group of tricosane (C(_23)H(_48)) was built and incorporated into the liquid diesel and the liquid diesel I EVA models. In these models it was found that edges of the crystal started to melt into the liquid diesel as a function of time. The EVA was found to migrate to the surface of the crystal.