The nucleation mechanism of diesel fuels within fractionation driven crystallisation

The formation of n-alkane waxes in solution during cold weather is a recurrent problem in the automotive industry. However, the variable multicomponent nature of fuels and the fractionation behaviour it displays as it crystallises, provides a complex challenge for the additive manufacturer to meet, as current additive development is largely empirical and cannot account for this change in composition and its influence on the nucleation of these waxy deposits. In particular the poorly understood concept of fractionation in multicomponent fuels has yet to be investigated in terms of its influence on nucleation kinetics and the effectiveness of additives.

This work studies the n-alkanes C16H34-C31H64 in both melt and solution phase in representative solvents and mixtures of middle distillate fuels, providing an overall assessment of their solubility, solution chemistry and nucleation kinetics. An in-depth analysis of the solid-state structure, intrinsic and extrinsic solid-state and solution state molecular interactions are also presented.

The polythermal method is applied to both solution and melt crystallisation. Methodologies for isothermally separating and quantifying the composition of a partially crystallised multicomponent fuel, are developed for assessing the compositional change for the assessment of solution composition as a function of temperature during fractionation and integrated with the novel polythermal KBHR approach to modelling nucleation kinetics.

The solid-state structure, morphology and molecular interactions are found to be highly dependent upon the molecular properties, specifically the chain parity, terminal methyl group arrangement and chain length. Cis-oriented terminal methyl groups in the odd n- alkanes disrupt optimal packing and intermolecular interactions, likely due to subtle stearic effects and the differences in charge distributions relative to the even n-alkanes.

The thermodynamic properties and phase behaviour of the melt alkanes are found to be influenced by the parity differences in molecular structure, with the odd alkanes forming stable rotational polymorphs. Analysis of the nucleation processes for this temperature dependant phase behaviour found the transition from the liquid to rotator state proceeds down an instantaneous pathway, whereas the transition from the rotator to the final crystal occurs via a progressive pathway, suggesting the nucleation pathway is influenced by the energetic cost of the transition.

The application of the KBHR approach in solution crystallisation of single and mixed n-alkanes suggests an instantaneous nucleation mechanism in almost all cases, which is associated with clustering of the alkanes in solution, consistent with a two-step pathway. The nucleation kinetic parameter the concentration of instantaneously nucleated crystallites (C0) displays positive correlations with both the nucleation mechanism parameter (ω) and supersaturation, due to the inverse relationship of supersaturation with critical cluster size, until nucleation switches over to a progressive mechanism.

Isothermal fractionation of a 16-alkane model middle distillate, at a range of temperatures from 20°C to 10°C, shows that whilst initially the heavier n-alkanes dominate crystallisation. Overall, the middle alkanes crystallise preferentially due to competing factors of solubility and concentration, with all components present in a solid solution. Nucleation is shown to be instantaneous at all temperatures but becomes more so at lower fractionation temperatures when the filtrate is dominated by lighter alkanes which limit the formation of clusters due to similarity between the solute and solvent. An industrial additive is also shown to marginally enhance the rate of crystallisation of the heavy alkanes at high fractionation temperatures, where the nucleation became more instantaneous and kinetically limited with a lower value of C0.