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Fractional freezing is a process used in process engineering and chemistry to separate substances 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, as in freeze distillation, also called normal freezing or progressive freezing. The initial sample is thus fractionated (separated into fractions).
Partial crystallization can also be achieved by adding a dilute solvent to the mixture, and cooling and concentrating the mixture by evaporating the solvent, a process called solution crystallization. Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.
Freeze distillation is a misnomer, since it is not distillation but rather 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 re-condensed portion is richer than the liquid portion left behind.
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 for a mixture of water and alcohol:
- Freezing in this scenario begins at a temperature significantly below 0 °C.
- The first material to freeze is not the water, but a dilute solution of alcohol in water.
- The liquid left behind is richer in alcohol, and as a consequence, further freezing would take place at progressively lower temperatures. The frozen material, while always poorer in alcohol than the (increasingly rich) liquid, becomes progressively richer in alcohol.
- Further stages of removing frozen material and waiting for more freezing will come to naught once the liquid uniformly cools to the temperature of whatever is cooling it.
- If progressively colder temperatures are available,
- the frozen material will contain progressively larger concentrations of alcohol, and
- the fraction of the original alcohol removed with the solid material will increase.
- In practice, unless the removal of solid material carries away liquid, the degree of concentration will depend on the final temperature rather than on the number of cycles of removing solid material and chilling.
- Thermodynamics gives fair assurance, even without more information about alcohol and water than that they freely dissolve in each other, that
- even if temperatures somewhat below the freezing point of ethyl alcohol are achieved, there will still be alcohol and water mixed as a liquid, and
- at some still lower temperature, the remaining alcohol-and-water solution will freeze without an alcohol-poor solid being separable.
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.
Purification of solids
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).
Concentration of liquids
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, called brine rejection. Large lakes of higher salinity water, called polynas, form in the middle of floes, and the water eventually sinks to the bottom. 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. 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. The danger of freeze distillation of alcoholic beverages, is that unlike heat distillation, where the methanol and other impurities can be separated from the finished product, freeze distillation does not remove them. Thus the ratio of impurities may be increased compared to the total volume of the beverage. This concentration may cause side effects to the drinker, leading to intense hangovers and a condition known as "apple palsy" (although this term has also simply been used to refer to intoxication, especially from applejack.)
Fractional freezing is commonly used as a simple method to reduce the gel point of biodiesel and other alternative diesel fuels, whereby esters of higher gel point are removed from esters of lower gel point through cold filtering, or other methods to reduce the subsequent alternative fuel gel point of the fuel blend. This process employs fuel stratification whereby components in the fuel blend develop a higher specific gravity as they approach their respective gel points and thus sink to the bottom of the container, where they can be removed.
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