Crystal Structure, Phase Behaviour and Kinetics Associated with the Crystallisation of Octadecane, Hexadecane, and Mixtures Thereof

There is an ever-increasing use of sustainable, bio-derived feedstocks in fuel oils including hydrogenated vegetable oil (HVO) from agricultural or recycled sources. HVO fuels consist of a narrow alkane distribution mainly octadecane (C18) and hexadecane (C16). Such fuels crystallise easily, which can lead to vehicle failures in cold weather conditions. This thesis examines the interplay between crystallisation kinetics, crystal structure and phase behaviour using model C18/C16 alkane mixtures.
Computational systematic search modelling in combination with high resolution synchrotron X-ray powder data yields its crystallographic structure for C18 and C16 together with their volume thermal expansion coefficients: 4.39±(1.94)× 〖10〗^(-4) ˚C-1 and4.17±(2.48)× 〖10〗^(-4) ˚C-1. Morphological analysis of C18 crystals reveals a plate-like morphology dominated by the {001} habit plane with smaller side faces (depending upon supersaturation).
Analysis of C18/C16 binary phase diagram reveals five single phases (liquid, R1, Mdcp, Op and T18) and five three-phase-equilibrium invariants (eutectic and peritectic at high temperature, eutectoid and peritectoid at low temperature). Kinetic studies reveal the crossover behaviour of the R1 existed in a transient to metastable to stable phase, which the stability is affected by both composition and alkane chain length.
Studies of the ternary C18/C16/kerosene system reveal a variety of different structures with compositions close to that of its pure components forming solid-solution in triclinic structures, whilst high relative fractions result in its formation of multiple phases (triclinic and two higher symmetry structures). Rotator induced crystallisation observed in melt phase crystallisation is also formed in solution crystallisation but only for C18/C16 mixtures with its metastability depending on the composition.
Solubility studies reveal the closest to the ideal condition with highest solubility in n-dodecane followed by kerosene and toluene respectively. Calculated activity coefficients reflect the non-identical phase formation in the solution phase with higher deviations for C18 molar compositions of x=0.1, 0.5-0.7. Solvents type is not found to have any significant effect on its solid for structure behaviour for C18/C16 mixtures.
Crystallisation of C18/C16 mixtures for most compositions in the three solvents are found to be mediated by progressive nucleation mechanism. Cases of instantaneous nucleation are also seen in particularly compositions with higher C18 concentration. Progressive nucleation reveals well correlated with wider metastable zone width and concomitant larger interfacial tension (1.33-1.64 mJ/m^2 ). Higher solubility is found to accelerate the instantaneous nucleation by increasing effective solute concentration, whilst inhibit the progressive nucleation by causing larger interfacial tension.