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Fractional freezing is a process used in
process engineering Process engineering is a field of study focused on the development and optimization of industrial processes. It consists of the understanding and application of the fundamental principles and laws of nature to allow humans to transform raw mate ...
and
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
to separate substances with different melting points. It can be done by partial melting of a solid, for example in zone refining of
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
or
metals A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...
, or by partial
crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...
of a liquid, as in freeze distillation, also called normal freezing or progressive freezing. The initial sample is thus fractionated ( separated into
fractions A fraction (from , "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, thre ...
). 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

Freeze
distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
is 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.
Ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
and 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 mixing ratios of these phase equilibria can be read from the phase diagram of ethanol and water. The maximum enrichment of ethanol in the liquid phase is reached at the
eutectic point A eutectic system or eutectic mixture ( ) is a type of a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eute ...
of ethanol and water, approximately 92.4% ethanol by weight at −123 °C. The best-known freeze-
distilled beverage Liquor ( , sometimes hard liquor), spirits, distilled spirits, or spiritous liquor are alcoholic drinks produced by the distillation of grains, fruits, vegetables, or sugar that have already gone through alcoholic fermentation. While the ...
s are applejack and
ice beer Ice beer is a beer that has undergone some degree of freezing during production. These beers generally have a higher alcohol content, and lower price relative to it. The process of "icing" beer involves lowering the temperature until ice crystals ...
.
Ice wine Icewine (or ice wine; ) is a type of dessert wine produced from grapes that have been Freezing, frozen while still on the vine. The sugars and other dissolved solids do not freeze, but the water does, allowing for a more concentrated grape juice ...
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.


Purification of solids

When a pure solid is desired, two possible situations can occur. If the contaminant is
soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
in the desired solid, a multiple stage fractional freezing is required, analogous to multistage
distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
. If, however, a eutectic system forms (analogous to an
azeotrope An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This happens beca ...
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 Juice is a drink made from the extraction or pressing of the natural liquid contained in fruit and vegetables. It can also refer to liquids that are flavored with concentrate or other biological food sources, such as meat or seafood, such ...
concentrate A concentrate is a form of Chemical substance, substance that has had the majority of its diluting agent or diluent (in the case of a liquid: the solvent) removed, such that the substance becomes the majority of the composition. Typically, this w ...
s and other heat-sensitive liquids, as it does not involve heating the liquid (as happens during
evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
).


Desalination

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 Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
lowers the
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
of water, the salt in sea water tends to be forced out of pure water while freezing, called
brine rejection Brine rejection is a process that occurs when salty water freezes. The salts do not fit in the crystal structure of water ice, so the salt is expelled. Since the oceans are salty, this process is important in nature. Salt rejected by the forming ...
. Large lakes of higher salinity water, called
polynyas A polynya () is an area of open water surrounded by sea ice. It is now used as a geographical term for an area of unfrozen seawater within otherwise contiguous pack ice or fast ice. It is a loanword from the Russian (), which refers to a n ...
, 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

Fractional freezing can be used as a simple method to increase the alcohol concentration in
fermented Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic compound, Organic molecules, such as glucose or other sugars, are Catabo ...
alcoholic beverages, a process sometimes called freeze distillation. Examples are applejack, made from hard cider, and
ice beer Ice beer is a beer that has undergone some degree of freezing during production. These beers generally have a higher alcohol content, and lower price relative to it. The process of "icing" beer involves lowering the temperature until ice crystals ...
. In practice, while not able to produce an alcohol concentration comparable to
distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
, 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 intense
hangover A hangover is the experience of various unpleasant physiological and psychological effects usually following the consumption of alcohol (beverage), alcohol, such as wine, beer, and liquor. Hangovers can last for several hours or for more than ...
s and 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 the gel point of
biodiesel Biodiesel is a renewable biofuel, a form of diesel fuel, derived from biological sources like vegetable oils, animal fats, or recycled greases, and consisting of long-chain fatty acid esters. It is typically made from fats. The roots of bi ...
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.


See also

*
Fractional crystallization (chemistry) {{crystallization In chemistry, fractional crystallization is a stage-wise separation technique that relies on the liquid–solid phase change. This technique fractionates via differences in crystallization temperature and enables the purific ...
*
Laser-heated pedestal growth Laser-heated pedestal growth (LHPG) or laser floating zone (LFZ) is a crystal growth technique. A narrow region of a crystal is melted with a powerful Carbon-dioxide laser, CO2 or Nd:YAG laser. The laser and hence the zone melting, floating zone, i ...
*
Pumpable ice technology Pumpable ice technology (PIT) uses thin liquids, with the cooling capacity of ice. Pumpable ice is typically a slurry of ice crystals or particles ranging from 5 micrometers to 1 cm in diameter and transported in brine, seawater, food liquid ...
*
Zone melting Zone melting (or zone refining, or floating-zone method, or floating-zone technique) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved through the crystal. The molt ...


References


External links


Freeze Distillation of Vinegar
{{DEFAULTSORT:Fractional Freezing Fractionation Phase transitions Methods of crystal growth