Batch and semi-continuous distillation

The basic difference between batch and continuous distillation is that the composition of the liquid in the still pot and that of the distillate change progressively during the former operation, but are constant during the latter. 

In batch distillation (whether of the simple or countercurrent type) a definite charge is put into the still and treated. A distinction should be made between fractional and unfractionated distillation. In the latter, the whole distillate is collected in one lot, and the initial material is thus separated into two fractions the distillate and 

Apparatus for semi-continuous vacuum distillation with column and heating of the feed 

After a certain time the feed is stopped and the liquid in the boiler is further evaporated batohwise, or drawn off. Continuous withdrawal is impossible since the feed steadih introduces low-boiUng material which has to be evaporated before the residue is drawn off. The apparatus illustrated in Fig. 161 is built up from standard parts, is provided with a column and allows the feedstock to be preheated in a jacketed vessel. 

In continuous distillation the feedstock — usually preheated to a certain degree — is uninterruptedly passed into the apparatus and is separated into an overhead distillate and a bottom product, both of which are continuously removed from the process. After the ratio and compositions of these two products, and other predetermined conditions, have become established, they should not change during the further course of the operation (see chap. 4.7.2). Continuous countercurrent distillation, like the corresponding batch process, is carried out with a column, and the feed is usually fed into the column at some point between its base and top (Fig. 162). 

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