Distillation is used for the purification of fermented alcoholic beverages, perfumes, distilled water and many other products. It can also be used to separate gases such as nitrogen, oxygen and argon from air.
The distillation process involves a series of vaporization-condensation steps known as rectification. The goal of the process is to produce a pure sample of ethanol from an initial mixture of materials such as beer.
Evaporation
Evaporation is when liquid particles move away from each other and become gaseous. The rate at which this happens depends on how the liquid is heated. Generally, a substance will only be vaporized when its molecules can receive enough energy from the surrounding environment to overcome its own vapor pressure. Liquids that don’t evaporate visibly have too much vapor pressure, or have molecules that don’t transfer energy to each other in a way that allows them to break free and turn into a vapor.
Distillation relies on evaporation to separate impurities from water. In home distillation units, contaminated water is heated to steam and inorganic chemicals or large non-volatile organic compounds are left behind. This steam is then condensed into purified water. In the industrial setting, a fractionating column is used to improve separation.
A vacuum or very low positive pressure can also be used to help the process along. When a compound is boiled under negative pressure, its vapors can be concentrated on a small surface area, so they are less likely to lose energy to the surrounding environment and decompose.
If the vapors can be confined to one area, it’s possible to bias their composition by lowering the boiling point. This is called codistillation and is a technique commonly used to remove solvents from synthesis products, such as the Dean-Stark apparatus and the Bleidner apparatus.
Boiling
If enough heat is applied to a liquid, it will start to evaporate. The temperature at which this occurs is called the boiling point. When a mixture of liquids is heated, the vapors from each individual component will boil at different temperatures, but they will all boil at the same pressure (which is the same as atmospheric pressure). The ratio of the boiling points of each individual component is a measure of its volatility and therefore its ability to separate from the rest of the solution.
Some liquids are known as azeotropes, which exhibit boiling behavior similar to that of pure substances. In the laboratory, such compounds are boiled at very high temperatures and are decomposed by this method. It is possible to reduce the required temperature by lowering the pressure. This is a technique called vacuum distillation and is often used for compounds which would otherwise be decomposed at ambient atmospheric pressure.
During the distillation process, water will cycle in and out of the system to cool the condenser. This helps keep the boiling temperature relatively constant as there will be many cycles of vaporization and condensation. The cooled vapor that is collected from each cycle is referred to as a fraction and will be used for analysis. This fraction will be distilled again and again to collect more pure samples of the original liquid.
Condensation
Distillation is the process of converting a gaseous substance into a liquid form. Water vapor can only condense into liquid when it cools to a temperature lower than the vapor’s boiling point, and thus, condensation is an important step in the distillation process. The condensation of water vapor can happen naturally, as in the formation of dew. It can also be caused artificially, as in the case of fog fences and air wells for desert regions. Weather forecasters often talk about relative humidity, which is a measure of the amount of moisture in the air. This is based on data gathered with psychrometric equipment such as the sling psychrometer, which measures the amount of water vapor in the air.
During distillation, the vapor passes up through a column and as it reaches the top it cools and condenses into a liquid. This liquid is called the overhead product and part of it can be removed as a distillate, while the rest of it returns to the bottom of the column as reflux.
Distillation is effective in removing inorganic compounds, such as lead and nitrate, and nuisance particles from contaminated water supplies. However, it is less effective at removing organic contaminants that have boiling points lower than the boiling point of water. This is the reason other water treatment methods are used to treat drinking water.
Filtration
After distillation, the alcoholic beverage must be filtered again to remove particles that were introduced by the boiling process. These may include flecks of carbon powder or dust that were left behind on the inside of your distillation apparatus. The final filtration stage polishes your product to ensure that it looks as crisp and clean as possible before it goes into the bottling tank. This is important for impressing distributors, retailers and customers.
Distillation is one of the oldest water purification processes and, if operated properly, can remove several contaminants from household drinking water. It removes bacteria, volatile organic compounds (VOCs), metals and dissolved solids including fluoride and nitrate, and inactivates microorganisms such as viruses and protozoan cysts, though it does not eliminate them completely.
The boiling process also vaporizes some chemical pollutants, such as pesticides and VOCs that are present in some groundwater supplies. These are not removed by the distillation process, unless another process is used prior to condensation.
Although federal, state and local laws do not regulate home drinking water distillation systems, the industry is self-regulated by NSF (National Sanitation Foundation) and WQA (Water Quality Association). Distillation equipment should be carefully selected based on water analysis and evaluation of the individual homeowner’s needs and situation. Proper operation and maintenance can reduce operating costs for these systems.