Page 335 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.8 Batch distillation 337

11.8.1 Design
Choice of the mode of operation is the first step in designing a batch distillation process. Single-stage
distillation leads to maximum recovery of the light component but the concentration of the same in the
distillate is low. Due to the interaction of the rising vapour with the liquid reflux falling down the
column, the rate of change of distillate composition is much slower when the operation is with multiple
stages and reflux. Operation with refluxing requires more energy compared to ‘no reflux’, i.e., single-
stage operation. Operation with varying reflux ratio needs highest energy requirement though the latter
permits highest purity of the overhead product. The reflux flow rate (and in turn the reflux ratio) is
manipulated to control the temperature of the vapour to the condenser (and in turn the distillate
composition). The highest fraction of more volatile is also left in the still for varying reflux ratio.
Constant reflux ratio operation is preferred due to its operational simplicity and the operation with
varying reflux ratio to obtain constant distillate composition is selected when the quality requirement
of distillate is very strict.
Batch distillation design problems are usually posed as the feed and product specifications being
specified, along with feed processing rate in terms of quantity per day. Operational constraints like
single/two/three shift operation of the plant may also be specified based on which the size and the
number of batches per day may be decided. The feed charge volume per batch decides the still size
(volume) and is considered while deciding in favour of batch distillation. The batch final volume left in
the still should not be too small so that its practical design considering submergence of the heating
surface is difficult. Also it should not be too large. The designer also decides whether a column with
contacting stages is needed in addition to the vapoureliquid contacting in the still. As already
mentioned, a column is required when the difference in purity of the distillate and the feed is large and
attainment of the desired product purity requires two or more stages. A McCabeeThiele diagram
drawn for the rectification section provides information to design the column. The minimum number
of stages is estimated under total reflux conditions with bottom product composition x B ¼ x F and top
product composition x D . In order to draw a finite quantity of top product, the reflux ratio needs to be
decreased. Alternatively the x-y plot can be altered by selecting a lower operating pressure. However,
this is rarely adopted from cost and safety considerations as discussed in Section 11.2.
The details of the design procedure for a simple batch with no reflux and a batch with column for
both constant reflux and constant top product composition are provided in the following section.
Design equations
Binary system with no reflux: The design for a binary system with no reflux is based on Rayleigh
equation relating the liquid amount (moles) and its composition (mole fraction) in the still at any
instant of time with the instantaneous composition of the vapour generated during differential
distillation.
x
Z B
B dx
(11.44)
F y x
ln ¼

x F
where B and x B are the amount (in moles) and mole fraction of the more volatile component in the
still at the end of distillation and F and x F are the moles and corresponding composition of the feed
(initial charge). y is the vapour composition in equilibrium with x at any instant of time. For ideal