Fractional freezing
Encyclopedia
Process engineering
Process engineering focuses on the design, operation, control, and optimization of chemical, physical, and biological processes through the aid of systematic computer-based methods...
and chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
to separate two liquids with different melting points. It can be done by partial melting of a solid, for example in zone refining
Zone melting
Zone melting is a group of similar methods of purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal...
of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
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". Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.
Freeze distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
, 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 point
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
s 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
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
, a subdivision of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
of importance to chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
. Without resorting to mathematics, the following can be said:
- 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 beverage
Distilled beverage
A distilled beverage, liquor, or spirit is an alcoholic beverage containing ethanol that is produced by distilling ethanol produced by means of fermenting grain, fruit, or vegetables...
s are applejack
Applejack (beverage)
Applejack is a strong alcoholic beverage produced from apples, popular in the American colonial period.Applejack was historically made by concentrating hard cider, either by the traditional method of freeze distillation or by true evaporative distillation. The term applejack derives from jacking, a...
and ice beer
Ice beer
Ice beer is a marketing term for pale lager beer brands which have undergone some degree of fractional freezing similar to the German Eisbock, which increases the alcohol content.-Process:...
. Ice wine
Ice wine
Ice wine is a type of dessert wine produced from grapes that have been frozen while still on the vine. The sugars and other dissolved solids do not freeze, but the water does, allowing a more concentrated grape must to be pressed from the frozen grapes, resulting in a smaller amount of more...
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
Eutectic point
A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature...
.
Purification of solids
When a pure solid is desired, two possible situations can occur. If the contaminant is solubleSolid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
in the desired solid, a multiple stage fractional freezing is required, analogous to multistage distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
. If, however, a eutectic system forms (analogous to an azeotrope
Azeotrope
An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....
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 concentrateConcentrate
A concentrate is a form of substance which has had the majority of its base component removed. Typically this will be the removal of water from a solution or suspension such as the removal of water from fruit juice...
s and other heat-sensitive liquids, as it does not involve heating the liquid (as happens during evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
).
Desalination
Fractional freezing can be used to desalinateDesalination
Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...
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
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
lowers the melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
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.
Alcoholic beverages
Fractional freezing can be used as a simple method to increase the alcohol concentration in fermentedBrewing
Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...
alcoholic beverages, a process sometimes called freeze distillation. Examples are applejack
Applejack (beverage)
Applejack is a strong alcoholic beverage produced from apples, popular in the American colonial period.Applejack was historically made by concentrating hard cider, either by the traditional method of freeze distillation or by true evaporative distillation. The term applejack derives from jacking, a...
, made from hard cider
Cider
Cider or cyder is a fermented alcoholic beverage made from apple juice. Cider varies in alcohol content from 2% abv to 8.5% abv or more in traditional English ciders. In some regions, such as Germany and America, cider may be termed "apple wine"...
, and ice beer. Freeze distillation of alcoholic beverages is illegal in many countries, as it can concentrate poisonous compounds, for example fusel alcohol
Fusel alcohol
Fusel alcohols, also sometimes called fusel oils, or potato oil in Europe, are higher-order alcohols formed by fermentation and present in cider, mead, beer, wine, and spirits to varying degrees....
s, 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
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
, this technique can achieve some concentration with far less effort than any practical distillation apparatus would require.
Scottish independent brewery BrewDog
BrewDog
BrewDog is a Scottish brewery located in the town of Fraserburgh, Aberdeenshire.- History :BrewDog was founded in 2006 by friends James Watt and Martin Dickie. The brewery at the Kessock Industrial Estate in Fraserburgh produced its first brew in April 2007...
uses fractional freezing to raise the alcohol content of their strongest beers.