Fractional freezing

Overview

Fractional freezing is a process used in process engineering and chemistry to separate two liquids 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, 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

Freeze distillation is a term for 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 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 points 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, a subdivision of physics of importance to chemistry. Without resorting to mathematics, the following can be said:

• Freezing in this scenario would begin at a temperature significantly below 0 °C.
• The first material to freeze would not be water, but a dilute solution of alcohol in water.
• The liquid left behind would be richer in alcohol, and as a consequence, further freezing would take place at progressively lower temperatures, and the frozen material, while always poorer in alcohol than the (increasingly rich) liquid, would become progressively richer in alcohol.
• Further stages of removing frozen material and waiting for more freezing will come to nought 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.

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.

Today, freeze distillation of alcoholic beverages is illegal in many countries because a number of by-products of fermentation (fusel alcohols), which are mostly removed by heat distillation, tend to accumulate to an unhealthy level in freeze-distilled beverages.

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, for example when it is used 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. Freeze distillation of alcoholic beverages is illegal in many countries, as it concentrates poisonous compounds, for example fusel alcohols, in the original fermented beverage to unhealthy levels.

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).

See also

• Zone melting

External links

• Freeze Distillation of Vinegar