INCONSISTENCIES IN DYNAMIC-MODELS FOR ILL-CONDITIONED PLANTS - APPLICATION TO LOW-ORDER MODELS OF DISTILLATION-COLUMNS

The paper addresses the problem of obtaining consistent dynamic models for certain ill-conditioned plants like distillation columns. Due to strong interactions there is often a single dominating slow mode (pole ) that results in similar first-order responses for all outputs. Typic ally, models are identified on the basis of fitting individual respons es, but unless special care is taken this will result in an inconsiste nt overall model with the slow pole repeated. It is shown that such mo dels with excessive slow poles, although a reasonable approximation fo r open-loop dynamics, yield a poor prediction of the closed-loop behav ior of the process, in particular under partial control.