INSTABILITY OF DISTILLATION-COLUMNS

As recently recognized, distillation columns, operating with reflux an d boilup as independent inputs, may have multiple steady-state solutio ns, even in the ideal binary case. Two fundamentally different sources may cause the multiplicity, and in both cases some operating points a re found to be unstable. This article provides evidence for the instab ility and discusses the effect of operating conditions on stability. I ncreasing the internal flow rates increases the probability of instabi lity; when flows other than reflux and boilup are used as independent inputs, an operating point may become unstable if the level control is not sufficiently tight. In this case, a limit cycle, usually stable, appears as the steady state goes unstable.