Thomas Lüder

Model Based Observation and Control of Distillation Columns

Abstract Two applications of mathematical process modeling to industrial scale distillation columns are discussed. In the first part the stripping section of a methanol-water separation column is described with the help of the location of a temperature front. A simple linear state space model is used to design an observer and state controller for the reboiler duty of the column. Its application is compared with experimental data obtained from an industrial distillation column with a PID-controller. In the second part an analytically reduced mathematical model for the estimation of the product composition in multicomponent distillation is presented. It is applied successfully to a fractionation plant consisting of three interlinked columns and is used for the purpose of diagnosis.

An Analytical Approach to Modelling in Distillation Control

Abstract The development of an appropriate low order dynamic process model of a distillation column is the most important prerequisite for the design of a successful control concept. In this paper we present a second order linear process model for a distillation column that can be treated fully analytically. It gives a steady- state exact representation of the column for any control configuration in the time as well as in the frequency domain and preserves the physical significance of all parameters. Because of the low computational effort and the consistency of the modelling information it is an effective tool for control structure analysis and controller design. A systematic comparison with related work of other authors reveals the analytical strength of our unified approach. A comparison to simulation results shows the very satisfactory reproduction of the column behaviour in the low to mid frequency range as well as the high frequency deficiencies of the model. These can be attributed to the fact that higher frequency effects such as level control and hydrodynamics have been neglected in its derivation in order to ensure its analytical tractability.