Crystallisation in Diesel and Biodiesel Fuel in Relation to their Solution Composition

Wax formation in diesel and biodiesel fuels at low temperatures is one of the major problems faced by the fuels industry as crystallisation of the saturated compounds present in these solutions can plug up filters and obstruct pipelines. Preventing wax formation in these multicomponent mixtures requires a good understanding of the crystallisation behaviour of both n-alkanes and Fatty Acid Methyl Esters (FAMEs). This work studies solutions of methyl palmitate and stearate in representative mixtures of diesel fuels and unsaturated esters solvents, providing an overall assessment of their solubility and nucleation kinetics. An in-depth analysis of the growth kinetics and crystal morphology for methyl stearate crystallising from n-dodecane, kerosene and toluene solvents is also presented. To perform this study methodologies are developed for the assessment of nucleation kinetics and morphological indexation of crystal specific faces. Models for the analysis of crystal growth kinetics are also derived. These describe the effect in series of the diffusion of growth units within the bulk of the solution and a rate of incorporation of these units which is associated to the molecular structure of the crystal faces´ surface. It was found that the solubility and crystallisation kinetics of methyl palmitate and stearate is very dependent on the solution environment in particular solvent type. Solubilities are higher in solvents whose molecules have more compact structures, such as in the case of toluene and methyl-cyclohexane and are lower in unsaturated methyl ester type solvents. The nucleation process in these systems is found to be much more thermodynamically controlled in the case of diesel type solvents and show a greater kinetic influence in the case of unsaturated methyl esters solvents. The growth kinetics of methyl stearate crystals is found to be strongly associated to the level of solvation of these solutes molecules as well as to the chemical structure of the crystal-solution interface. Accordingly, the rate limiting step is suggested to be the integration of growth units to the surface of the crystal in the case of methyl stearate growing from n-dodecane solvent, and more diffusional controlled in the case of methyl stearate growing from kerosene and toluene solvents. The morphology of methyl stearate crystals however is not found to be strongly influenced by solvent type, only showing changes as a function of supersaturation in the case of kerosene solvent