Fractional freezing

Fractional freezing is a fundamental process used in chemistry and process engineering. It is a method used to separate substances based on their different melting points. This technique can be applied in two main ways. One way involves the partial melting of a solid, such as in the zone refining of metals or silicon. The other way involves the partial crystallization of a liquid, which is often called freeze distillation. By using these methods, a single sample is separated into different parts, or fractions. This allows scientists to produce ultra-pure solids or to concentrate liquids that are heat-sensitive.

Freeze distillation is a specific type of fractional freezing that works through condensation. It removes frozen solids to separate a dissolved material from the remaining liquid. This is very similar to true distillation, where an evaporated fraction is enriched. A common example involves a mixture of ethanol and liquid water. While ethanol and water are completely miscible, meaning they mix perfectly, ethanol is practically insoluble in water ice. When you cool a lean ethanol-water mixture sufficiently, almost pure water ice will precipitate. This process enriches the ethanol in the liquid phase that remains. You can then separate the two phases by using filtration or decanting.

The behavior of these mixtures is quite complex and follows specific rules. The temperature at which water ice starts to precipitate depends on the ethanol concentration. At a specific temperature and concentration, the freezing process reaches an equilibrium. This results in a specific ratio of water ice and enriched ethanol solution. These temperatures and mixing ratios can be mapped on a phase diagram. This diagram shows the relationship between the components of the mixture. For ethanol and water, there is a limit to how much they can be concentrated. The maximum enrichment of ethanol occurs at the eutectic point. This point is reached at approximately 92.4% ethanol by weight at a temperature of −123 °C.

Fractional freezing is used to create several well-known alcoholic beverages. One method involves increasing the alcohol concentration in fermented drinks. This is sometimes called freeze distillation and is used to make applejack from hard cider or to make ice beer. Historically, people would leave barrels of beer outdoors to partially freeze. They would then remove the ice to strengthen the drink. However, this method has risks compared to heat distillation. It does not remove impurities from the beverage. This can actually increase the ratio of impurities to the total volume. This concentration might lead to intense hangovers or a condition known as "apple palsy."

Another type of beverage uses a similar principle but at a different stage. Ice wine is produced through a process where freezing happens before fermentation. In this case, the process concentrates the sugar in the grapes rather than the alcohol. This is a way to create a very sweet and specialized drink.

Beyond food and drink, this process is very useful for industrial liquids. It is used to produce fruit juice concentrates. Because fractional freezing does not require heating the liquid, it is ideal for heat-sensitive substances. This ensures the quality of the concentrate is maintained.

Nature also uses fractional freezing in the oceans through a process called brine rejection. When sea ice forms, the salt in the seawater is forced out of the pure water. This happens because sodium chloride lowers the melting point of water. As the water freezes, the salt is pushed away, creating highly salty water. This can form large lakes of high salinity, called polynyas, in the middle of ice floes. This salty water eventually sinks to the bottom. This natural process can be used for desalination. After multiple runs of freezing and melting, the resulting water can become drinkable.

Finally, fractional freezing plays a role in the production of alternative fuels. It is commonly used to reduce the gel point of biodiesel and other alternative diesel fuels. This is achieved through a process called fuel stratification. In this process, different components in the fuel blend develop different specific gravities. As they approach their respective gel points, these components sink to the bottom of the container. They can then be removed through cold filtering or other methods. This helps create a better fuel blend by removing esters with higher gel points. This makes the fuel more reliable for use in various conditions.