Distillation

Distillation

General principles

Applications

Distillation is the process of separating liquids by boiling them, and then condensing the resulting vapor. Islamic alchemist, astronomer, and physicist Abu Musa Jabir ibn Hayyan (c. 721–c. 815) is generally considered today as the inventor of the modern distillation process. 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 pure products from the oil refinery industry are processed via distillation.

General principles

Distillation has been used widely to separate volatile components from nonvolatile 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 removal of the heat. 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 involved. 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 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.

Applications

Distillation has long been used as the separation process in the chemical and petroleum industries because of its reliability, simplicity, and low-capital cost. It is employed to separate benzene from toluene, methanol or ethanol from water, acetone from acetic acid, and many multi-component mixtures. Fractionation of crude oil and the production of deuterium also rely on distillation.

Today, with over 40, 000 distillation towers in operation, distillation makes about 95% of all current industrial separation processes; however, distillation systems also have relatively high energy consumption. Significant effort, therefore, has been made to reduce the energy consumption and to improve the 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

KEY TERMS

Activity coefficient— The ratio of the partial pressure of a component in the gas phase to the product of its mole fraction in the liquid phase and its vapor pressure as a pure liquid which is an important factor encountered in many vapor-liquid separation processes.

Bubble point— For a saturated liquid, because any rise in temperature will form bubbles of vapor, the liquid is said at its bubble point.

Dew point— The point at which air or a gas begins to condense to a liquid.

Differential distillation— During distillation, only a very small portion of the liquid is flashed each time and the vapor formed on boiling the liquid is removed at once from the system.

Distillate— The product withdrawn continuously at the top of the distillation column.

Reflux— Part of the condensate from the condenser is returned to the top tray of the distillation column as reflux to provide liquid flow above the feed point for increasing separation efficiency.

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.

Pang-Jen Kung