MOLECULAR SIMULATION STUDIES OF DIESEL AND DIESEL ADDITIVES

Abstract

As diesel fuel is cooled down, waxes are deposited, which are made up from crystals of long chain n-alkanes. Wax depositions are undesirable, since they can block anything from filters in diesel engines to pipelines. It is already known that wax formation can be inhibited by the addition of wax crystal modifiers to diesel fuel. This thesis em- ploys computational models at atomistic and coarse-grained levels to investigate the crystallisation of diesel fuel and the effect of additives upon the crystallisation process.

In the first results section, a model for diesel fuel is introduced and a strategy for investigating its crystallisation is developed. Crystallisation was observed from pure n-tricosane, binary and tertiary mixtures of paraffins of similar chain lengths. These systems were found to crystallise into hexagonally arranged lamellae. The presence of different length alkanes was found to create gauche disorders, leading to the formation of lamellar layers with softer edges. It was also found that crystal growth could be simulated more efficiently in the presence of a positionally restrained crystal, acting as a nucleation centre.

Subsequently, crystallisation of paraffins, and the solvent effect upon it, was studied. This allowed to establish behavioural trends characteristic for aromatic and aliphatic solvents. Finally, paraffin crystallisation in the presence of four common additives was investigated. A common mode of action for these additives was identified, based upon partial co-crystallisation of additive alkyl chains and paraffin molecules.

The main drawback of atomistic simulation is the computational cost, which limits both the time and length scales accessible on modern computers. In order to overcome these inherent limitations, a coarse grained model was developed for a range of n-alkanes. Remarkably, the model shows transferability over 120 K, preserving thermodynamic and structural properties of both melt and crystal.

In summary, this thesis provides a detailed picture of diesel crystallisation at a molecular level, and provides new insights into the mechanism of action of a number of common diesel additives.