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CHAPTER
11
Distillation
11.1 Introduction
Distillation is employed to separate a miscible binary or a multicomponent feed to two or more
fractions utilising the difference in volatility of the components. Distillation equipment ensures inti-
mate contact between the vapour and the liquid phase to achieve transfer of components (lighter
component(s) from liquid to vapour and the heavier one(s) from vapour to liquid) across the phase
boundary. This results in the vapour phase getting richer in more volatile component(s) and the liquid
phase richer in less volatile component(s).
The process can be carried out in batch or in continuous mode. It can involve one or more stages of
vapoureliquid contact in tray/plate columns, packed columns or just a heated still providing a single
stage of contact. The equipment, their configuration arrangements and operation differ in these cases.
‘Flash distillation’ is distillation in its simplest form. This involves flashing or partial vaporisation
of a feed stream in a flash drum to a vapour stream richer in more volatile component(s) and a liquid
stream that has lower concentration of more volatile components. Such systems are always continuous.
The other options of the distillation system are (a) batch distillation with or without reflux and
(b) continuous staged distillation (also called fractionation or rectification). The reboiled stripper is a
variation of the latter.
Typical configuration of flash distillation, batch distillation, fractionator and reboiled stripper are
schematically shown in Fig. 11.1. General features of these distillation arrangements are presented in
Table 11.1.
11.2 Conceptual design
Prior to design, the configuration of the system is conceived based on available knowledge and in-
formation in line with the general strategy of process design that has been discussed in Chapter 1. The
information contains the details of the problem and the solution options available with the designer.
At this stage, the options to be considered and their corresponding configurations are e Batch
versus Continuous. The batch process goes through cycles of start-up, production run and shutdown
that makes the productivity low compared to a continuous plant of same equipment size or same
investment. Substantial loss of heat energy makes the process inherently less thermally efficient
compared to continuous distillation. The batch distillation equipment is bigger in size since the still has
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