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Abstract The optimal structure and operation of a batch distillation column separating ternary mixture are discussed from the viewpoint of energy conservation. The optimal reflux operation, which minimizes energy consumption, is derived for three types of batch distillation columns: rectifying, stripping and total reflux columns. The separation performance measured in terms of energy consumption is calculated for each column under different conditions. The results of the example problems show that the separation performance of the total reflux column can be considerably improved by optimizing reflux flow rate, while those of the rectifying and the stripping columns can not be. In all example cases, the separation performance of the total reflux column is equal to or even better than those of the other two columns. It is experimentally verified using a pilot plant that the energy consumption of the total reflux column can be drastically reduced when the optimal operation is used. © 2001 Elsevier Science Ltd. All rights reserved.1. Introduction 20493
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Batch distillation is a flexible separation method,
which can be used even when the feed composition
and:or the number of products are frequently changed.
Therefore, this separation method is suitable for the
chemical industry, where the demand for and lifetime of
a product may often be uncertain. When a column does
not have any middle vessels, the batch distillation can
be classified into rectifying, stripping and total reflux
columns as shown in Fig. 1. Among the three types, the
rectifying column is most widely used in the present
chemical industry.
The optimal operation problems of a batch distilla-
tion column have been discussed by many authors in
the past. However, most of those study focus on the
optimal operation of a rectifying column. The different
types of column structures are also discussed by
Hasebe, Kurooka, Abdul Aziz, Hashimoto and Watan-
abe (1996), Wittgens, Litto, Sorensen and Skogestad,
(1996), Skogestad, Wittgens, Litto and Sorensen (1997).Thus, it is important to discuss the optimal structure
and the optimal operation simultaneously, because the
separation performance of a batch distillation column
depends on not only the operation but also the
structure.
Sorensen and Skogestad (1994) proposed the cyclic
operation policy of a total reflux column, where the
following three operations are repeated sequentially:
filling the product from the reboiler to the reflux drum,
total reflux operation, and dumping the product to the
product tank from the reflux drum. If the duration of
one cyclic operation is very small, the distillation using
a total reflux column is close to that using a rectifying
column. Therefore, the optimal cyclic operation policy
can be considered optimal for both columns.
In the previous papers (Hasebe, Noda & Hashimoto,
1997; Noda, Hasebe & Hashimoto, 1997), the separa-
tion performance of three batch distillation columns
separating binary mixture were discussed from the
viewpoint of energy conservation. The following results
are obtained through the simulation.
1. The effect of optimization for the rectifying column
is insignificant, even if the reflux flow rate is opti-
mized as a function of time.2. The separation performance of the total reflux
column can be considerably improved by optimizing
the reflux flow rate as a function of time. In most
separation problems studied, the optimal operation
of the total reflux column is that the holdup of the
reboiler is gradually decreased to the final holdup.
3. The separation performance of the total reflux
column is almost the same or even better than that
of the other columns when the optimal operation is
adopted.