Fractional freezing
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Fractional freezing is a process used in
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
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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-
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
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
Desalination
Fractional freezing can be used to
Alcoholic beverages
Fractional freezing can be used as a simple method to increase the alcohol concentration in
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
- Fractional crystallization (chemistry)
- Laser-heated pedestal growth
- Pumpable ice technology
- Zone melting
References
- ISBN 978-0-07-142294-9.
- ^ https://serc.carleton.edu/files/research_education/equilibria/alcohol-ice.pdf [bare URL PDF]
- ISBN 9781861899583. Retrieved 3 October 2014.
- ISBN 9780874511482. Retrieved 3 October 2014.
- ISBN 9781581124040. Retrieved 3 October 2014.