Fractional freezing

Fractional freezingis a process used inprocess engineeringandchemistrytoseparatesubstances with different melting points. It can be done by partial melting of a solid, for example inzone refiningofsiliconormetals,or by partialcrystallizationof a liquid, as infreeze distillation,also callednormal freezingorprogressive freezing.The initial sample is thusfractionated(separatedintofractions).

Fractional freezing of grapes to concentrate grape sugar for makingice wine

Crystallization
Fundamentals
Crystal Crystal structure Nucleation
Concepts
Crystallization Crystal growth Recrystallization Seed crystal Protocrystalline Single crystal
Methods and technology
Boules Bridgman–Stockbarger method Van Arkel–de Boer process Czochralski method Epitaxy Flux method Fractional crystallization Fractional freezing Hydrothermal synthesis Kyropoulos method Laser-heated pedestal growth Micro-pulling-down Shaping processes in crystal growth Skull crucible Verneuil method Zone melting

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

Freezedistillationis actually a condensation, not distillation per se.It removes frozen solid, separating a dissolved material from the liquid left behind. This is analogous to true distillation, where the evaporated and re-condensed fraction is enriched.

Ethanoland liquid water are completely miscible, but ethanol is practically insoluble in water ice. That means almost pure water ice can be precipitated from a lean ethanol-water mixture by cooling it sufficiently. The precipitation of water ice from the mixture enriches ethanol in the remaining liquid phase. The two phases can then be separated by filtration or decanting. The temperature at which water ice starts to precipitate depends on the ethanol concentration. Consequently, at a given temperature and ethanol concentration, the freezing process will reach an equilibrium at a specific ratio of water ice and enriched ethanol solution with a specific ethanol concentration. The temperatures and mi xing ratios of these phase equilibria can be read from thephase diagramof ethanol and water. The maximum enrichment of ethanol in the liquid phase is reached at theeutectic pointof ethanol and water, approximately 92.4 weight-% ethanol at -123 °C.

The best-known freeze-distilled beveragesareapplejackandice beer.Ice wineis the result of a similar process, but in this case, the freezing happensbeforethe fermentation, and thus it is sugar, not alcohol, that gets concentrated.

Purification of solids

When a pure solid is desired, two possible situations can occur. If the contaminant issolublein the desired solid, a multiple stage fractional freezing is required, analogous to multistagedistillation.If, however, aeutecticsystem forms (analogous to anazeotropein 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 offruit juiceconcentratesand other heat-sensitive liquids, as it does not involve heating the liquid (as happens duringevaporation).

Desalination

Fractional freezing can be used todesalinatesea 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. Becausesodium chloridelowers themelting pointof water, the salt in sea water tends to be forced out of pure water while freezing, calledbrine rejection.Large lakes of higher salinity water, calledpolynyas,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 remaining frozen water, and after multiple runs the results can be drinkable.

Alcoholic beverages

Eisbockbeer (12% alcohol) created via freeze distillation of doppelbock beer. Barrels of beer were originally left outdoors to partially freeze, then the ice removed.

Fractional freezing can be used as a simple method to increase the alcohol concentration infermentedalcoholic beverages, a process sometimes called freeze distillation. Examples areapplejack,made fromhard cider,andice beer.In practice, while not able to produce an alcohol concentration comparable todistillation,this technique can achieve some concentration with far less effort than any practical distillation apparatus would require. The danger of fractional freezing of alcoholic beverages is that it does not remove impurities, unlike (heat) distillation. Thus, fractional freezing may increase the ratio of impurities to the total volume of the beverage (though not necessarily the ratio of impurities to the amount of ethanol). This concentration may cause side effects to the drinker, leading to intensehangoversand a condition known as "apple palsy"(although this term has also simply been used to refer to intoxication,especially from applejack.)

Alternative fuels

Fractional freezing is commonly used as a simple method to reduce thegel pointofbiodieseland 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 fuelstratificationwhereby 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.

See also

• Fractional crystallization (chemistry)
• Laser-heated pedestal growth
• Pumpable ice technology
• Zone melting

External links

• Freeze Distillation of Vinegar