Two relay feedback control schemes of a periodic distillation column which keep the compositions of the overhead and bottoms products constant were investigated together with the open-loop control scheme by computer simulation. The instability of the direct feedback scheme in which two switchings of the operational mode are carried out when the overhead and bottoms compositions reach the respective desired values was shown by stability analysis. So, the indirect feedback scheme was introduced. In this scheme both switchings are carried out when the bottoms composition reaches the respective desired values and a cascade controller is provided to modify the desired value of the bottoms composition for the overhead composition control. It was ascertained that this scheme indicates both enough stability and good control performance.
Some considerations on the local stability of the above two schemes were also made by using the transition graph.