Apparatus

A wide variety of apparatus is available to satisfy the different distillation techniques. The appropriate design of apparatus depends upon the type of distillation to be performed, considering, for example, whether a vacuum is required or steam is needed. Descriptions of apparatus are to be found in a number of different texts (see the Further Reading). Standards referring to the design and use of distillation apparatus have been published by the British Standards Institute and the American Society for Testing and Materials. Simulated distillation, which is a gas chromatographic technique, is dealt with in a [recent] review by Robillard et al. and referred to in several standards.

Apparatus may be discussed in terms of the distillation flask, the distillation column, the condenser and the collecting flask(s). By far the most effort has been expended in the design and operation of the distillation column, which is at the heart of the separation efficiency. The form of the column, its size and the packing used are very influential upon the results that are achievable. A summary of some different types of columns is given in Table 1 and of packings in Table 2.

Once apparatus has been chosen carefully to compare with previously used apparatus or to conform to standards, the operation of the equipment must be considered. The following factors are among the most important to be controlled:

• The heating of the distillation flask must be carefully controlled.

• The distillation column must be operated so that it does not become flooded.

• The reflux ratio, that is the ratio of material returning via reflux to the distillation column or the distillation flask compared to the amount presented to the condenser in unit time must be carefully controlled. The higher the reflux ratio, the purer the material collected from the distillation. Reflux ratios are controlled in simple distillation appar-

Table 1 Types of distillation column

Column type

Description/comments

Dufton An open tube into which a glass spiral fits closely

Hempel A simple tube normally filled with a suitable packing (rings/helices) and having a side-arm near the top Oldershaw A column with fixed but perforated plates that maintains a fixed amount of liquid on each plate

Podbielniak A simple tube with a wire packing to provide large contact area between liquid and vapour to effect high efficiencies Spinning band A tube fitted with a closely fitting spiral of PTFE or metal gauze that can be rotated at typical speeds of 600 to 3000 rev min~1 as the vapour-liquid equilibrium is maintained in the column Vigreux A tube having pairs of indentations down its length that slope downwards and provide a large and designed surface area to enhance the liquid-vapour equilibrium atus by adjustment of the heating rate and by maintaining stable thermal conditions throughout the apparatus.

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