Fractional freezing

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Fractional freezing of grapes to concentrate grape sugar for making ice wine Ice wine grapes.jpg
Fractional freezing of grapes to concentrate grape sugar for making ice wine
Crystallization
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Fundamentals
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Methods and technology

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).

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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. [1] Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.

Freeze distillation

Freeze distillation is a misnomer, because 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.

Ethanol 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 of ethanol and water, approximately 92.4 weight-% ethanol at -123 °C. [2]

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.

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).

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 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 polynyas, 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

Eisbock beer (12% alcohol) created via freeze distillation of doppelbock beer. Barrels of beer were originally left outdoors to partially freeze, then the ice removed. Aventinus Weizen-Eisbock.jpg
Eisbock beer (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 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 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 hangovers and a condition known as "apple palsy" [3] (although this term has also simply been used to refer to intoxication, [4] especially from applejack. [5] )

Alternative fuels

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.

See also

Related Research Articles

<span class="mw-page-title-main">Distillation</span> Method of separating mixtures

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 mixture and the condensation of the vapors in a still.

<span class="mw-page-title-main">Ethanol</span> Organic compound (CH₃CH₂OH)

Ethanol is an organic compound with the chemical formula CH3CH2OH. It is an alcohol, with its formula also written as C2H5OH, C2H6O or EtOH, where Et stands for ethyl. Ethanol is a volatile, flammable, colorless liquid with a characteristic wine-like odor and pungent taste. It is a psychoactive recreational drug, and the active ingredient in alcoholic drinks.

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.

<span class="mw-page-title-main">Eutectic system</span> Mixture with a lower melting point than its constituents

A eutectic system or eutectic mixture is 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 eutectic temperature. On a phase diagram, the eutectic temperature is seen as the eutectic point.

<span class="mw-page-title-main">Zone melting</span> Purification process by moving a molten zone along a metal bar

Zone melting 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 along the crystal. The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot. The impurities concentrate in the melt, and are moved to one end of the ingot. Zone refining was invented by John Desmond Bernal and further developed by William G. Pfann in Bell Labs as a method to prepare high-purity materials, mainly semiconductors, for manufacturing transistors. Its first commercial use was in germanium, refined to one atom of impurity per ten billion, but the process can be extended to virtually any solute–solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium. This process is also known as the float zone process, particularly in semiconductor materials processing.

<span class="mw-page-title-main">Phosphoric acid</span> Chemical compound (PO(OH)3)

Phosphoric acid is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. It is a major industrial chemical, being a component of many fertilizers.

<span class="mw-page-title-main">Freezing</span> Phase transition of liquid to solid

Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid or the liquid content of a substance, usually due to cooling.

<span class="mw-page-title-main">Supercooling</span> Lowering the temperature of a liquid below its freezing point without it becoming a solid

Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. It is achieved in the absence of a seed crystal or nucleus around which a crystal structure can form. The supercooling of water can be achieved without any special techniques other than chemical demineralization, down to −48.3 °C (−54.9 °F). Supercooled water can occur naturally, for example in the atmosphere, animals or plants.

<span class="mw-page-title-main">Freezing-point depression</span> Process in which adding a solute to a solvent decreases the freezing point of the solvent

Freezing-point depression is a drop in the maximum temperature at which a substance freezes, caused when a smaller amount of another, non-volatile substance is added. Examples include adding salt into water, alcohol in water, ethylene or propylene glycol in water, adding copper to molten silver, or the mixing of two solids such as impurities into a finely powdered drug.

In physics and chemistry, flash freezing is the process whereby objects are rapidly frozen. This is done by subjecting them to cryogenic temperatures, or it can be done through direct contact with liquid nitrogen at −196 °C (−320.8 °F). It is commonly used in the food industry.

<span class="mw-page-title-main">Liquor</span> Alcoholic drink produced by distillation

Liquor is an alcoholic drink produced by the distillation of grains, fruits, vegetables, or sugar that have already gone through alcoholic fermentation. Other terms for liquor include: spirit, distilled beverage, booze, spirituous liquor or hard liquor. The distillation process concentrates the liquid to increase its alcohol by volume. As liquors contain significantly more alcohol (ethanol) than other alcoholic drinks, they are considered "harder." In North America, the term hard liquor is sometimes used to distinguish distilled alcoholic drinks from non-distilled ones, whereas the term spirits is more commonly used in the UK. Some examples of liquors include vodka, rum, gin, and tequila. Liquors are often aged in barrels, such as for the production of brandy and whiskey, or are infused with flavorings to form flavored liquors, such as absinthe.

In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear thermodynamically in at least one chemical phase and can also be characterized by its one-component-phase diagram. Secondly, practically speaking, a pure chemical should prove to be homogeneous. The perfect pure chemical will pass all attempts and tests of further separation and purification. Thirdly, and here we focus on the common chemical definition, it should not contain any trace of any other kind of chemical species. In reality, there are no absolutely 100% pure chemical compounds, as there is always some minute contamination. Indeed, as detection limits in analytical chemistry decrease, the number of impurities detected tends to increase.

<span class="mw-page-title-main">Volatility (chemistry)</span> Tendency of a substance to vaporize

In chemistry, volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapour, while a substance with low volatility is more likely to be a liquid or solid. Volatility can also describe the tendency of a vapor to condense into a liquid or solid; less volatile substances will more readily condense from a vapor than highly volatile ones. Differences in volatility can be observed by comparing how fast substances within a group evaporate when exposed to the atmosphere. A highly volatile substance such as rubbing alcohol will quickly evaporate, while a substance with low volatility such as vegetable oil will remain condensed. In general, solids are much less volatile than liquids, but there are some exceptions. Solids that sublimate such as dry ice or iodine can vaporize at a similar rate as some liquids under standard conditions.

<span class="mw-page-title-main">Fractional crystallization (chemistry)</span> Method for refining substances based on differences in their solubility

In chemistry, fractional crystallization is a stage-wise separation technique that relies on the liquid-solid phase change. It fractionates via differences in crystallization temperature and enables the purification of multi-component mixtures, as long as none of the constituents can act as solvents to the others. Due to the high selectivity of the solid - liquid equilibrium, very high purities can be achieved for the selected component.

<span class="mw-page-title-main">Cooling bath</span> Liquid mixture used to maintain low temperatures

A cooling bath or ice bath, in laboratory chemistry practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 °C and −196 °C. These low temperatures are used to collect liquids after distillation, to remove solvents using a rotary evaporator, or to perform a chemical reaction below room temperature.

<span class="mw-page-title-main">Applejack (drink)</span> Alcoholic drink produced from apples

Applejack is a strong alcoholic drink produced from apples. Popular in the American colonial era, the drink's prevalence declined in the 19th and 20th centuries amid competition from other spirits.

<span class="mw-page-title-main">Pumpable ice technology</span> Type of technology to produce and use fluids or secondary refrigerants

Pumpable icetechnology (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, or gas bubbles of air, ozone, or carbon dioxide.

A separation process is a method that converts a mixture or a solution of chemical substances into two or more distinct product mixtures, a scientific process of separating two or more substance in order to obtain purity. At least one product mixture from the separation is enriched in one or more of the source mixture's constituents. In some cases, a separation may fully divide the mixture into pure constituents. Separations exploit differences in chemical properties or physical properties between the constituents of a mixture.

<span class="mw-page-title-main">Freeze-fracture</span> Freeze-fracture processes and methods

Freeze-fracture is a natural occurrence leading to processes like erosion of the earths crust or simply deterioration of food via freeze-thaw cycles. To investigate the process further freeze-fracture is artificially induced to view in detail the properties of materials. Fracture during freezing is often the result of crystallizing water which results in expansion. Crystallization is also a factor leading to chemical changes of a substance due to changes in the crystal surroundings called eutectic formation.

References

  1. Perry, Robert; Don Green (2007). Perry's Chemical Engineers' Handbook. McGraw-Hill International Editions. pp. 17–3 to 17–4. ISBN   978-0-07-142294-9.
  2. https://serc.carleton.edu/files/research_education/equilibria/alcohol-ice.pdf [ bare URL PDF ]
  3. Janik, Erika (2011). Apple a global history. London: Reaktion Books. ISBN   9781861899583 . Retrieved 3 October 2014.
  4. Kaufman, Martin (1979). The University of Vermont College of Medicine. [Burlington, Vt.]: University of Vermont College of Medicine. p. 12. ISBN   9780874511482 . Retrieved 3 October 2014.
  5. Nordegren, Thomas (2002). The A-Z encyclopedia of alcohol and drug abuse. Parkland, Fla.: Brown Walker Press. p. 78. ISBN   9781581124040 . Retrieved 3 October 2014.