Distillation is a process where liquids are heated until they turn into vapor. They are then cooled and condensed back into a liquid to form a pure sample, called the distillate.
A simple distillation apparatus consists of a pot to heat the liquid, a condenser to cool the vapor and a receiver to collect the distillate. Many variations of this basic equipment are used in industrial and laboratory settings.
Definition
Distillation is the process of separating a mixture of liquids into separate fractions by heating them to their boiling point and then condensing the vapors. This is based on the principle that different components of the mixture have different boiling points and vapor pressures, so when they are heated they will vaporize in a different order. The fraction that has the lowest boiling point will vaporize first and be separated from the remainder of the mixture, resulting in a distillate with a higher concentration of the desired component. The distillation process can be repeated as necessary to increase the purity of the output fractions.
The boiling point of a substance can also be manipulated by increasing or decreasing the external pressure over the liquid. This technique is often used to purify compounds that cannot be distilled at atmospheric pressures, as they decompose at their normal boiling points.
Batch distillation is the most common method of distillation, and involves a set of fixed conditions that are repeated over a period of time to obtain a specific output fraction. This type of distillation can be used to prepare alcoholic beverages, chemical products such as formaldehyde and phenol, or even to desalinate seawater. Continuous distillation is a more advanced form of this process, in which the composition of the source material and the vapors and distillates are continuously monitored. This allows for better control of the separation process and can provide a higher level of purity than batch distillation.
Purification
Distillation exploits the physical properties of compounds to separate them. One of these properties is their boiling point, which allows the vapors of different compounds to collect separately. This is possible because different compounds have different boiling points. However, the liquids cannot mix because their molecules have different polarity. For this reason, they need to be separated by distillation or a separation funnel.
A separation funnel is a large vessel that separates two liquids that don’t mix, such as water and oil. A vacuum is used to create negative pressure and energy is required for the process. Using a separation funnel is more expensive than relying on unidirectional positive pressure changes, and standard glassware must be used as it can’t handle the high temperatures needed to produce boiling.
In a batch distillation, heating an ideal mixture of volatile substances A and B until they boil produces a vapor above the liquid that is enriched in A relative to B (due to Raoult’s Law). The composition of the condensed liquid, which is now a mixture of pure A and methylcyclohexane, will gradually approach the composition of the less-volatile pure cyclohexane because the vapor from each new equilibration is enriched with more of the less-volatile component.
If the liquid from each equilibration is allowed to return to the pot, this will continue until the composition of the liquid approaches that of the more-volatile pure compound. This process is known as fractionating the mixture and can be improved by utilizing a distillation column. These columns consist of chambers with a central opening that connect to the chamber directly above it. The vapor from the first chamber cools slightly and is then condensed, and the liquid from the second chamber volatilizes and rises to fill the space above it. As the vapor flows through each chamber it becomes progressively more enriched with the less-volatile compound and, at equilibrium, the composition of the liquid is that of the pure, distilled substance.
Types
Many distillation processes exist, based on differences in the boiling point of components in a mixture. These include fractional, vacuum, continuous flash and steam distillation.
Fractional distillation is a process in which liquid mixtures that have close boiling points are separated into a series of different fractions using a separating column. This process is sometimes called rectification. A common example of a fractional distillation is ethanol (ethyl alcohol) production from water, grain or potatoes in a classic brewing procedure.
Several laboratory scale and industrial scale devices exist for conducting batch distillation. The basic apparatus consists of a reboiler or still pot in which the source material is heated to vaporize it, a condenser where the heated vapor cools back into a liquid, and a receiver in which the concentrated, purified liquid, known as the distillate, is collected.
Distillation can also be conducted under reduced atmospheric pressure, a process known as vacuum distillation. This is useful for extracting some compounds from natural sources that degrade under normal boiling conditions. For example, a simple vacuum distillation system can be used to obtain perfumes and some medicinal herbs from plants.
Some compounds do not boil at their normal boiling point under atmospheric pressure and require special equipment to vaporize them. These compounds include sulfuric acid, cyanide, and some pharmaceuticals. Typically these compounds are boiled under a vacuum in a fractional or vacuum distillation device, such as the Perkin Triangle apparatus.
Applications
Distillation is used in a variety of industrial applications. Many fermented beverages are distilled, as well as perfumes and food flavorings. It is also used to purify chemical products from synthesis reactions. The oil industry uses it for separating crude oil into the fuels kerosene, gasoline and diesel. It is also used in ocean desalination to create fresh water.
The principle behind distillation is that liquids with different boiling points evaporate at different rates. As the vapor cools, it separates into pure liquid components (distillate). The composition of the source material, vapor and distillate changes with the temperature of the process. Distillation is usually performed on a batch basis, but it can be done continuously.
Because the composition of a mixture changes with temperature, it is impossible to completely purify a mix using this method. It is also not possible to break an azeotrope by distillation alone, though some techniques have been developed to do this.
The simplest example of distillation is steam from a kettle evaporating and condensing on a cold surface. Other examples include the separation of salt and water in a seawater desalination plant or the removal of alcohol from wine by distillation. The distillation process is widely used because it provides a way to separate mixtures into components with distinct properties. It is a key part of chemistry and the physical sciences, and has been used since the time of medieval Muslim chemists like Jabir ibn Hayyan and Abu Bakr al-Razi (c. 865–925).