Column Design

Before designing a batch column, it obviously is desirable to have as much detailed information on the system as is possible. Data on vapor liquid equilibria (VLE), vapor and liquid densities, liquid viscosity, and the boiling temperatures of the components are essential if a column is to be properly designed. Failure to have accurate VLE data means that it is necessary to run a small-scale experiment in order to characterize the system. In addition, the customer must identify the product feed composition, the required composition of the residue and distillate, the batch size, and the batch time. Having acquired this data, the vendor finally can commence design work.

To illustrate the design principles, it is useful to study the case of binary mixtures. A typical system is shown in Figure 10. Once the VLE data have been established, it becomes a straightforward task to calculate the number of theoretical stages and reflux ratios.

There are two main techniques for operating a batch column. One is to work with constant reflux ratio during the complete run. The effect of this method is charted in Figure 11. As the composition of the more volatile component (MVC) in the still, xw, decreases, the fraction of MVC in the top product decreases. For example, to obtain a set composition of 90% in the total amount of top product collected, it will always be necessary to collect initially at a higher composition of about 95% to compensate for a composition below specification at the end of the run. The advantage of constant reflux is that control and operation are very simple.

The second method is to increase the reflux ratio during the run in order to maintain a steady top-product composition. This is shown in Figure 12, where the increase in the slope of the operating line is obtained by increasing the reflux ratio. The gradient of the operating line (L/V) is obtained from the enrichment equation given below.

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