DYNAMICS AND CONTROL OF UNSTABLE DISTILLATION-COLUMNS

The fact that ideal two-product distillation columns may have multiple steady states and unstable operating points has only recently been recognized. This paper discusses the dynamics and control implications. It is shown that columns operating with reflux and boilup as independent variables become unstable if the internal flows are sufficiently large. An open-loop unstable operating point may be stabilized by use of one-point control, i.e., feedback control of a column composition or temperature. If the control is not sufficiently tight, the column may go into a stable limit cycle. Finally, it is shown that with distillate flow and boilup as independent variables the operating points may become unstable if the level control is not sufficiently tight. The column may also in this case go into a stable limit cycle.