Solid-liquid equilibria in wax crystallization



The equilibria obtained during the crystallization of waxes has been studied theoretically and it has been found that the degree of separation between normal paraffins may be predicted, if it is assumed that the solid is an ideal solution. Results predicted from the theoretical equations have been compared with mass spectrometer analyses of recrystallized waxes and surprisingly close agreement has been found.

It has also been shown that ketone solvents have essentially the same selectivity for recrystallizing waxes as n-hexane. The reason for this is that the activity coefficients for various n-paraffins in a ketone solution are nearly identical even though they are all much higher than in the hydrocarbon solution.

The separation between normal paraffins in conventional single stage wax recrystallization is not very sharp. This may be improved by the use of a number of stages in a countercurrent cascade. Experimental results with such a cascade are in agreement with the theoretical predictions.

The presence of non n-paraffinic components in some waxes can cause the formation of eutectic mixtures. This affects the separation in a manner analogous to that of hetero-azeotropes in some distillation processes.

The crystallization of waxes from their, melts has also been studied both theoretically and experimentally. When mixtures of similar molecular weight n-paraffins, such as n-C21H44 and n-C23H48 are cooled, a solid solution is formed. This is also found with mixtures of similar recrystallized waxes. On the other hand, mixtures of n-paraffins of widely different molecular weight, such as n-C20H42 and n-C32H66, or hydrocarbons of different structure, such as n-paraffins and iso-paraffins, form separate crystals and a eutectic. Similar results are found with mixtures of commercial waxes of widely different melting point and type.