Studies on dynamics and control of distillation columns

摘要

Distillation is an important unit operation in which a given feed is separated into two or more products of different compositions. The separation requires input of relatively large amounts of energy. Improved composition control of distillation columns may reduce the energy requirements significantly, and will in addition yield improved product recovery. A prerequisite for obtaining tighter composition control is a basic understanding of the column behavior. This thesis addresses different aspects of the dynamic behavior as well as control of distillation columns. It is shown that simple distillation columns, even with ideal vapor-liquid equilibrium, may have multiple steady- state solutions and unstable operating points. Two fundamentally different sources for the multiplicity and instability are presented. The dynamics and control of columns with unstable operating points are also addressed. Low-order dynamic models are frequently used in control studies of distillation columns. Such models are studied in this thesis, and it is shown that they in many cases have fundamental shortcomings which make them poor for feedback control studies. It is also shown that it is the high-frequency behavior (initial time response) which is most important to capture in a dynamic model for feedback control studies. One important conclusion is that the flow dynamics should be included in the dynamic model of a distillation column. Most researchers aming at improving the composition control of distillation column, focus on improving the control algorithms. However, distillation columns have inherent control limitations which limit the performance, even of the ''best'' controller. In this thesis it is shown that the control limitations may be significantly reduced by modifying the column design.