Fractional freezing

Fractional freezing is a separation technique used in chemistry and process engineering that relies on differences in melting points. It works either by partially melting a solid—as in zone refining of silicon or metals—or by partially crystallizing a liquid, a method often called freeze distillation. In both cases, the starting material is split into distinct fractions. This process is mainly applied to create extremely pure solids or to concentrate liquids that would be damaged by heat.

Freeze distillation is technically a condensation process, not true distillation. It works by removing frozen solid, which separates a dissolved substance from the remaining liquid. This is similar to regular distillation, where the evaporated and re-condensed portion is enriched. Ethanol and liquid water mix completely, but ethanol does not dissolve in water ice. So, cooling a lean ethanol-water mixture can precipitate nearly pure water ice, enriching the leftover liquid in ethanol. The two phases are then separated by filtration or decanting. The temperature at which water ice begins to form depends on the ethanol concentration. At a given temperature and concentration, the freezing process reaches equilibrium at a specific ratio of water ice to enriched ethanol solution. These ratios and temperatures can be found on the ethanol-water phase diagram. The maximum ethanol enrichment in the liquid occurs at the eutectic point, which is about 92.4% ethanol by weight at -123 °C. Well-known freeze-distilled drinks include applejack and ice beer. Ice wine is made by a similar process, but freezing happens before fermentation, so sugar, not alcohol, becomes concentrated.

When purifying a solid, two scenarios arise. If the contaminant dissolves in the desired solid, multiple stages of fractional freezing are needed, much like multistage distillation. If a eutectic system forms (similar to an azeotrope in distillation), a very pure solid can be obtained, as long as the liquid is not at its eutectic composition—where a mixed solid forms that is hard to separate—or above it, where the unwanted solid forms.

For concentrating a liquid, fractional freezing is valued for its simplicity. It is used to make fruit juice concentrates and other heat-sensitive liquids because it avoids heating, unlike evaporation.

Fractional freezing can also desalinate seawater. This happens naturally with sea ice: when frozen salt water partially melts, the leftover ice has much less salt. Because sodium chloride lowers water’s melting point, salt is pushed out of pure water during freezing—a process called brine rejection. Large lakes of saltier water, called polynyas, form in the middle of ice floes and eventually sink. Similarly, the frozen water with the highest salt content melts first. Either method reduces the salinity of the remaining ice, and after several cycles, the water can become drinkable.

In alcoholic beverages, fractional freezing is a simple way to boost alcohol concentration, often called freeze distillation. Examples include applejack from hard cider and ice beer. While it cannot reach the alcohol levels of heat distillation, this method achieves some concentration with much less effort. A danger is that it does not remove impurities like heat distillation does. So, fractional freezing can increase the ratio of impurities to total beverage volume (though not necessarily to ethanol volume). This concentration may cause worse hangovers and a condition called "apple palsy"—though that term has also simply meant intoxication, especially from applejack.

For alternative fuels, fractional freezing is a common method to lower the gel point of biodiesel and other diesel fuels. Esters with higher gel points are removed from those with lower gel points through cold filtering or other techniques, reducing the fuel blend’s gel point. This process uses fuel stratification: components in the blend develop a higher specific gravity as they approach their gel points, sinking to the bottom of the container where they can be removed.