Applications – Distillation Control

The fourth workshop in chapter 16 of Control Loop Foundation – Batch and Continuous Processes allows the reader to explore the different features of distillation control. Continuous distillation is used in many industries to separate mixtures of liquids based on differences in their volatility. Within a distillation column, the vapor rising from the bottom of the column is used to heat the feed entering the column and the reflux material. Heat transfer between the vapor and liquid is increased by metal trays or by packing that fills the column. Liquid that falls to the bottom of the column is reheated to boiling using an external heater. The lighter material that reaches the top of the column as a vapor is condensed using a heat exchanger and collected in the accumulator. A portion of the condensate is removed from the accumulator as overhead distillate and the remaining liquid is returned to the column as reflux. One example of the controls that may be provided on a distillation column is shown below.

The extent to which the lighter components may be separated from the heavier component of the feed to a distillation column is determined by the column design, the properties of the feedstock and the energy input. By implementing a steam/feed ratio control the energy input per unit of feed may be maintained constant. As the feed rate increases, the overhead distillate flow should also be increased proportionately to achieve the same distillate composition. This balance is achieved using a distillate/feed ratio control. To compensate for changes in the feedstock composition, the distillate/feed ratio may be automatically adjusted based on an analysis of the distillate.

A balance between the condensed vapor flow to the accumulator and flow from the accumulator is maintained by regulating the reflux flow based on accumulator level. Similarly, a balance between the feedstock flow and the material removed from the column as overhead distillate flow is maintained by regulating the removal of liquid from the bottom of the column, through the bottoms flow, to maintain a constant level in the bottom of the column. Any imbalance between the rate of vapor generation in the column and the rate of vapor condensation in the heat exchanger is reflected in the column pressure. Thus, the vapor flow that bypasses the heat exchanger is regulated to maintain a constant column pressure. The control module provided for distillation control workshop is shown below.

By accessing the book’s web site, you may complete this distillation control workshop using your web browser. The viewer below may be used to see the solution to this exercise.