DYNAMICS AND CONTROL OF DISTILLATION-COLUMNS - A CRITICAL SURVEY

Distillation column dynamics and control have been viewed by many as a very mature or even dead field. However, as is discussed in this pape r significant new results have appeared over the last 5-10 years. Thes e results include multiple steady states and instability in simple col umns with ideal thermodynamics (which was believed to be impossible), the understanding of the difference between various control configurat ions and the systematic transformation between these, the feasibility of using the distillate-bottom structure, for control (which was belie ved to be impossible), the importance of flow dynamics for control stu dies, the fundamental problems in identifying models from open-loops r esponses, the use of simple regression estimators to estimate composit ion from temperatures, and an improved general understanding of the dy namic behavior of distillation columns which includes a better underst anding of the fundamental difference between internal and external flo w, simple formulas for estimating the dominant time constant, and a de rivation of the linearizing effect of logarithmic transformations. The se issues apply to all columns, even for ideal mixtures and simple col umns with only two products. In addition, there have been significant advances for cases with complex thermodynamics and complex column conf igurations. These include the behavior and control of azeotropic disti llation columns, and the possible complex dynamics of nonideal mixture s and of interlinked columns. However, both for the simple and more co mplex cases there are still a number of areas where further research i s needed.