Continuous equilibrium vaporisation flash distillation

Flash distillation has proved particularly convenient for distilling off low-boiling tops at a pressure of 1 to 20mm Hg a throughput of 800 to 1500g/h may be attained. In the separation, for instance, of a crude fatty acid mixture, a C4—Cjo fraction could be taken off with a feed rate of 1200 g/h at 15 mm pressure. 

Distillation analyses of the top and bottom products from a flash distillation of crude fatty acids at 15 torr 

A mixture of phenol containing 9% of water could be dehydrated in the same apparatus, with the omission of the column below the flash vessel. At a pressure of 23 mm and a feed temperature of 80°C the optimum rate of input proved to be 41/h. Such a large throughput could scarcely be attained in the laboratorj by the normal method of distillation from a flask, if only on account of foaming. 

Flash distillation has proved valuable in preparative work as a preliminary operation in the separation of multicomponent mixtures into their constituents. 

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Flash or equilibrium distillation, frequently carried out as a continuous process, consists of vaporising a definite fraction of the liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid. The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. The still is essentially a separator in which the liquid and vapour produced by the reduction in pressure have sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom. 

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