Solution crystallisation of single and mixed n-alkanes, within the homologous series C(16 )to C-23 from representative hydrocarbon fuel solvents

The unwanted crystallisation of n-alkanes from the solution phase under cold weather conditions is a recurrent problem in domestic heating and automotive hydrocarbon fuel systems, where mitigation is important to avoid mechanical failure due to fuel starvation. The solubility, crystallisability and solution nucleation behaviour for a homologous series of n-Alkanes, in the range of C16H34 to C23H48, is examined in representative hydrocarbon fuel solvents, dodecane (C12H26) and toluene (C7H8), using polythermal cycling crystallisation/dissolution kinetic analysis. Solubility is found to decrease with increasing solute chain lengths in both solvents, with dodecane displaying greater solubility than toluene for the same solute. Examination of an 8-alkane mixture is also found to display similar solubility behaviour with respect to solvent choice as the single alkanes acting to some extent as a pseudo alkane whose carbon number equals the average of the mixture. The activity coefficients for the alkanes in both solvents are consistent with less than ideal behaviour in the majority of cases, indicating a preference for solute/solute interactions with a greater solvation in "like" systems, i.e. with dodecane being closer to ideality. Kinetic analysis of the nucleation on-set data as a function of cooling rate mostly reveals an instantaneous mechanism, where all nuclei form at one instance, consistent with the initial concentration of nuclei, C0, lying between 4.24 x 1022-2.78 x 1019 mL-1. This process is found to be kinetically limited at the lower supersaturations, where C0 decreases with decreasing supersaturation. The change in supersaturation is also found to be associated with the nucleation mechanism, with a shift from instantaneous to progressive nucleation at high supersaturations.