Fractional freezing is a process used in process engineering and chemistry to separate two liquids with different melting points. It can be done by partial melting of a solid, for example in zone refining of silicon or metals, or by partial crystallization of a liquid, for example "freeze distillation", also called "normal freezing" or "progressive freezing". Partial crystallization can also be achieved by adding a diluent solvent to the mixture, and cooling and concentrating the mixture by evaporating the solvent, a process called "solution crystallization".[1] Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.
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Freeze distillation is a term for a process of enriching a solution by partially freezing it and removing frozen material that is poorer in the dissolved material than is the liquid portion left behind. Such enrichment parallels enrichment by true distillation, where the evaporated and recondensed portion is richer than the liquid portion left behind.
Such enrichment by freezing of a solution in water is sometimes oversimplified by saying that, for instance, because of the difference in freezing points of water (0 °C/32 °F), and ethyl alcohol (-114 °C/-173 °F), "the water freezes into ice...while the ethyl alcohol remains liquid." This is false, and although some of the implications of that description are true and useful, other conclusions drawn from it would be false.
The detailed situation is the subject of thermodynamics, a subdivision of physics of importance to chemistry. Without resorting to mathematics, the following can be said:
The best-known freeze-distilled beverages are applejack and ice beer. Ice wine is the result of a similar process, but in this case, the freezing happens before the fermentation, and thus it is sugar, not alcohol, that gets concentrated. For an in depth discussion of the physics and chemistry, see eutectic point.
When a pure solid is desired, two possible situations can occur. If the contaminant is soluble in the desired solid, a multiple stage fractional freezing is required, analogous to multistage distillation. If, however, a eutectic system forms (analogous to an azeotrope in distillation), a very pure solid can be recovered, as long as the liquid is not at its eutectic composition (in which case a mixed solid forms, which can be hard to separate) or above its eutectic composition (in which case the undesired solid forms).
When the requirement is to concentrate a liquid phase, fractional freezing can be useful due to its simplicity. Fractional freezing is also used in the production of fruit juice concentrates and other heat-sensitive liquids, as it does not involve heating the liquid (as happens during evaporation).
Fractional freezing can be used to desalinate sea water. In a process that naturally occurs with sea ice, frozen salt water, when partially melted, leaves behind ice that is of a much lower salt content. Because sodium chloride lowers the melting point of water, the salt in sea water tends to be forced out of pure water while freezing. Likewise, the frozen water with the highest concentration of salt melts first. Either method decreases the salinity of the frozen water left over, and with multiple runs can be drinkable.
Fractional freezing can be used as a simple method to increase the alcohol concentration in fermented alcoholic beverages, a process sometimes called freeze distillation. Examples are applejack, made from hard cider, and ice beer. Freeze distillation of alcoholic beverages is illegal in many countries, as it can concentrate poisonous compounds, for example fusel alcohols, in the original fermented beverage to unhealthy levels; heated distillation methods tend to eliminate these more harmful fermentation products. In practice, while not able to produce an alcohol concentration comparable to distillation, this technique can achieve some concentration with far less effort than any practical distillation apparatus would require.
Scottish independent brewery BrewDog uses fractional freezing to raise the alcohol content of their strongest beers.[2]