Simple and countercurrent distillation under reduced pressure

Simple and countercurrent distillation under reduced pressure 

Rectified distillation, with a column, may be carried out at pressures down to about 0.5 mm Hg. At lower pressures special forms of apparatus must be employed. 

The basic advantage associated with the use of reduced pressure in distillation is the lowering of the boiling point and the consequent possibility of separating compounds below the temperature at which they decompose or undergo chemical alteration, such as polymerization. Examples from industry that may be quoted are the di.stillation of lubricating oils, the fractionation of crude phenols (which is performed 

An azeotrope frequently becomes richer in the low-boiling component as the ])ressure is reduced. By continuing the reduction, a j)ressure is reached at which the azeotropic point vanishes. As an example, the mixture ethanol-water may be quoted at 70 mm Hg this system no longer has an azeotropic point (c/. section 6.2.1). Hence it is possible to prepare absolute alcohol by the distillation of dilute spirits at a pressure below 70 mm, without the addition of auxiliar substances. The boiling point, however, is then rather low (28°C). In this connection it is obvious that the choice of distillation pressure may also depend on the temperature of the cooling medium. 

Another motive for using reduced pressure in distillation — especially on an industrial scale — may be that low-pressure steam is to be used as heating agent and that its temperature would be too low for distillation at normal pressure. In practice, furthermore, the corrosion of a metal still can be an important consideration, and on this account it may be necessary not to exceed a certain temperature. 

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