Vacuum distillation is a process by which volatile compounds are separated from a liquid mixture by exploiting the tendency of such compounds to evaporate from the mixture at an increased rate at lower atmospheric pressures, often without increasing the temperature. By enclosing a mixture in a distillation tank and reducing the pressure, volatile compounds are induced to evaporate from the mixture. The vapor is then captured and condensed back into a liquid. Vacuum distillation can be carried out on a small scale in a laboratory or on an industrial scale as is done in petroleum refineries.
Any compound dissolved in a liquid will have a vapor pressure, which is usually expressed as a fraction of one standard atmosphere of pressure, at a specific temperature. It is the pressure at which a particular compound will evaporate readily from a liquid solution. Vacuum distillation uses this property to separate individual compounds from mixtures, particularly in the refining of petroleum products. The liquid to be separated is placed in a sealed container with a system for regulating pressure. By using known vapor pressures of specific elements of the mixture, the pressure is reduced, and sometimes the temperature is raised, until the desired compound begins to evaporate or boil out of the mixture.
After the desired compound is converted to vapor, it is collected, cooled, and returned to standard atmospheric pressure to condense it. It may even be pressurized to higher than standard atmospheric pressure if it is normally a gas, such as fuels like propane, to convert it into a liquid. By subjecting a mixture like raw crude to a variety of temperatures and pressures, many different components of the mixture can be separated and refined this way. Raw crude oil can contain hundreds of hydrocarbons, and in many cases, vacuum distillation is the only way to separate these from one another easily and efficiently.
Compounds that have a high boiling point or whose vapors are heavier than air at one standard atmosphere of pressure are particularly suited to separation by vacuum distillation, especially when dissolved in water or in a mixture containing water. Vacuum distillation also allows compounds to be distilled at lower temperatures than would otherwise be possible. That is useful for compounds that are altered or broken down by heating.
The apparatus for vacuum distillation can be relatively simple, such as those sometimes used in laboratories, or extremely complex, like the large towers found in petroleum refineries. A vacuum distillation tower in a petroleum refinery can be several stories tall and may be capable of separating dozens of hydrocarbon compounds. A small laboratory set-up can be very simple and take up no more space than a table top.