Distillation

DISTILLATION, an operation involving the conversion of a substance into vapour which is subsequently condensed to the liquid form. The process is exemplified at its simplest when steam from a kettle plays upon a cold surface, producing drops of dis tilled water. The term, which was originally applied to the sepa ration of spirituous liquors from fermentation processes, now has a far wider application, and distillation is an integral part of many manufacturing processes. It has for its object the separation of liquids from solids, as pure water from sea water, or the separation of two or more liquids, petrol, or gasolene, and paraffin, or kero sene, from crude petroleum.

The simplest distillation apparatus consists of three parts : the still or retort in which the liquid is heated, the condenser, to cool the vapours, and the receiver, to collect the distillate. This is satisfactory for the purification of liquids containing solids.

For the separation of several liquids, of which the components,

in normal cases, distil in the order of their boiling points, the appa ratus requires elaboration. These components do not usually distil exactly at their boiling points, but over a range of temperature, so that some overlapping of the boiling ranges is usual. To come this difficulty the vapours are led through a still head or column in which a sorting out of the components occurs. In this way the distillate may be lected in fractions (technically cuts), and the operation is termed fractional distillation.

Some substances decompose when heated to their boiling points. This may often be avoided by taking advantage of the fact that a lowering of pressure over the surface of the liquid lowers the temperature at which that liquid boils. This may be achieved by vacuum distillation, or by steam distillation, when steam is passed into the substance, which then volatilizes with the steam at a much lower temperature. When solid substances are heated to give saseous or liquid products the operation is termed dry distillation. If the procedure involves a breakdown of complex materials into simpler substances (ele mentary or compound) the process is described as destructive distillation. (See COAL TAR and TARS, LOW-TEMPERATURE CAR