Constant Distillate Composition Operation

For the separation of a binary mixture under the quasi-steady-state assumption, a quick glance at the process can be obtained again by using a McCabe-Thiele representation, as shown in Figure 7.

In order to get the desire specification xD>ec from the very beginning of the operation, the initial value of the reflux ratio must be such that the feed composition z is exactly obtained in the still when stepping off N stages from xDTc. Then, the reflux ratio (hence, the

Figure 7 Binary batch distillation at constant distillate composition (three-ideal-stage column).

slope of the operating line) is continuously increased during the batch in order to keep the distillate composition on specification. The operation is interrupted when either the reboiler composition has reached the desired composition, or the reflux ratio is so high that continuing the operation would be uneconomical (or even impossible, in an actual column).

The determination of the total amount of energy that needs to be supplied to the still is obtained by the following equation:

whose integration requires knowing r as a function of Hd. By combining eqns [4] and [5], with xD = xD"*, it is found that:

Figure 7 Binary batch distillation at constant distillate composition (three-ideal-stage column).

which enables relating the instantaneous reboiler composition (xB) to the amount of product (HD) collected up to the same instant. Therefore, since the reflux ratio can be related to xB at any instant of time, eqn [21] can be easily integrated.

In a multicomponent separation, product 1 is collected until the reflux ratio reaches the prespecified maximum value. Then, the reflux rate is decreased in order to allow a quick removal of the remaining traces of the most volatile component (these traces are accumulated in slop cut 1). At the same time, this enables the front of the second lightest component to build up in the column so that, when the reflux drum composition meets the specification for product 2, the accumulation of this product is started. The production phase and the slop removal phase are then repeated sequentially; product Nc is eventually recovered from the reboiler.

Since operation at constant distillate composition is inherently a feedback operation, some kind of feedback from the plant is needed in order to be able to determine the correct value of the reflux ratio at any instant. When the distillate composition measurement is not available online, the most convenient feedback signal that can be used is a pilot tray temperature measurement. This strategy is similar to the one applied for one-point composition control in continuous columns. In principle, temperature control should be applied to binary mixtures only, but it can be extended to multicomponent mixtures, provided that the column has enough trays. Note that, similarly to continuous distillation, the measuring element must be placed somewhat down from the top of the column in order to ensure sensitivity of the temperature

response to changes in the distillate composition. Therefore, if good control at a constant set point is provided, the product purity usually increases while this product is being removed from the column. This is because the separating capacity of the column increases during the batch, due to the increase of the reflux rate.

From a practical point of view, this kind of operation is harder to implement than constant reflux ratio, even if a measure or an estimate of the distillate composition is available online. Because the dynamics of the process may change significantly as the batch proceeds, conventional proportional plus integral controllers often prove to be inadequate for tight composition control. Care should be taken when the reflux rate increases widely, because the liquid loading to the column might increase to a point that flooding conditions are reached.

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