Workshop 7: Distillation control

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem then with many other unit operations, as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus, a distillation column provides an example of a multiple-input-multiple-output control problem. It is critical that variable pairing is done appropriately between controlled and manipulated variables. The overall control problem can usually be reduced to a 2 x 2 conposition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental mles of distillation control and show how a basic distillation control scheme can be selected. 

For the reduction of pump noise in the actual laboratory the vacuum puiu]>s art-placed in a separate pump room. The pumps P are close to the wall between this room and the laboratory, so as to make the vacuum lines as short as possible. The other aide of tliis wall is provided with a framework M and is reserved for vacuum distillation. Automatic pressure controllers and vacuum gauges are placed on or behind the framework where they are accessible and can easily be watched. The pump room can also serve as a small workshop. 

Buckley, P. S., "Material Balance Control in Distillation Columns, Paper presented at the AIChE Workshop on Industrial Process Control, Tampa, FI. Nov. 11-13, 1974. 

Most industrial distillation columns are operated similarly to the stabilizer that we studied in the previous workshop. One degree of freedom is used to control a product composition and the second available degree of freedom is used to control fractionation or energy consumption. This mode of operation is often called one-point or single composition control. 

Reconfigure the controllers on the stabilizer from Workshop 7 to provide two-point composition control. Assume that each product is equally important and that the control objectives are 0.01 per cent propane in the bottoms and 0.1 per cent isopentane in the distillate. Also assume that you have two perfect analysers (i.e. no dead time, no error) available so that the two compositions can be controlled directly. 

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