Distillation
Distillation is one of the most important processes for separating the components of a solution. The solution is heated to form a vapor of the more volatile components in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and condensation, individual components in the solution can be recovered in a pure state. Whiskey, essences, and many products from the oil refinery industry are processed via distillation.
Distillation has been used widely to separate volatile components from non-volatile compounds. The underlying mechanism of distillation is the differences in volatility between individual components. With sufficient heat applied, a gas phase is formed from the liquid solution. The liquid product is subsequently condensed from the gas phase by transferring heat from the vapor. Therefore, heat is used as the separating agent during distillation. Feed material to the distillation apparatus can be liquid and/or vapor, and the final product may consist of liquid and vapor. A typical apparatus for simple distillation used in chemistry laboratories is one in which the still pot can be heated with a water, steam, or oil bath. When liquids tend to decompose or react with oxygen during the course of distillation, the working pressure can be reduced to lower the boiling points of the substances and hence the temperature of the distillation process.
In general, distillation can be carried out either with or without reflux. For the case of single-stage differential distillation, the liquid mixture is heated to form a vapor that is in equilibrium with the residual liquid. The vapor is then condensed and removed from the system without any liquid allowed to return to the still pot. This vapor is richer in the more volatile component than the liquid that is removed as the bottom product at the end of the process. However, when products of much higher purity are desired, part of the condensate has to be brought into contact with the vapor on its way to the condenser and recycled to the still pot. This procedure can be repeated for many times to increase the degree of separation in the original mixture. Such a process is normally called "rectification."
Distillation has long been used as the separation process in the chemical and petroleum industries because of its reliability, simplicity, and low-capital cost. Recorded applications date back nearly 2,000 years. It is employed to separate benzene from toluene, methanol or ethanol from water, acetone from acetic acid, and many multicomponent mixtures. Fractionation of crude oil and the production of deuterium also rely on distillation.
Today, with 40,000 distillation towers in operation, distillation accounts for about 95% of all current industrial separation processes; even so, distillation systems also have relatively high energy consumption. Significant effort, therefore, has been made to reduce the energy consumption and to improve efficiency in distillation systems. This includes incorporating new analytical sensors and reliable hardware into the system to achieve advanced process control, using heat rejected from a condenser of one column to reboil other columns, and coupling other advanced process such as adsorption and crystallization with distillation to form energy-saving hybrid operation systems.
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