Fractional freezing

Lore & Background

Fractional freezing can be achieved through partial crystallization of a liquid, as in freeze distillation, also called normal freezing or progressive freezing. Partial crystallization can also be achieved by adding a dilute solvent to the mixture, cooling, and concentrating by evaporating the solvent, a process called solution crystallization. In freeze distillation, frozen solid is removed, separating a dissolved material from the liquid left behind, analogous to true distillation. For ethanol and water, almost pure water ice can be precipitated from a lean ethanol-water mixture by cooling, enriching ethanol in the remaining liquid. The maximum enrichment of ethanol in the liquid phase is reached at the eutectic point of ethanol and water, approximately 92.4% ethanol by weight at −123 °C.

Reader's Guide

Fractional freezing holds significance in multiple fields. In purification of solids, if a contaminant is soluble in the desired solid, a multiple stage fractional freezing is required, analogous to multistage distillation. If a eutectic system forms, a very pure solid can be recovered as long as the liquid is not at its eutectic composition. For concentrating liquids, fractional freezing is useful due to its simplicity and is used in producing fruit juice concentrates and other heat-sensitive liquids, as it does not involve heating. It can desalinate sea water through brine rejection, where frozen salt water, when partially melted, leaves behind ice of much lower salt content. In alcoholic beverages, it increases alcohol concentration in fermented beverages, as seen in applejack and ice beer, though it does not remove impurities, potentially causing intense hangovers. In alternative fuels, it reduces the gel point of biodiesel by removing esters of higher gel point through cold filtering.

Did You Know?

• Freeze distillation is actually a condensation, not distillation per se.
• The maximum enrichment of ethanol in the liquid phase is approximately 92.4% ethanol by weight at −123 °C.
• Ice wine results from freezing before fermentation, concentrating sugar, not alcohol.
• Fractional freezing can desalinate sea water through a process that naturally occurs with sea ice.