Steady-state analysis for the control of a benzene-toluene-o-xylene distillation column with a side stream

The objective of this thesis is to present a steady-state analysis of the first-level structure of a control system for a benzene-toluene-o-xylene distillation column. This analysis uses Singular Value Decomposition as well as the Relative Gain Array and is based entirely on composition as a controlled variable. The analysis assumes that composition can be measured at selected points in the column as well as on the three product streams.

Presented in this work are new analysis procedures to determine where the analyzers should be placed and which are the most important components to analyze. Included in this procedure is also the possibility to present either composition or component ratios to the first-level control system.

The analysis of the benzene-toluene-o-xylene column strongly favors two first-level steady-state control structures. The first is an LQS control structure which controls the liquid compositions of benzene at tray 40, o-xylene at tray 24, and toluene at tray 13. The other is also an LQS control structure which controls the liquid component ratios of toluene/benzene at tray 40, toluene/benzene at tray 24, and the component o-xylene at tray 13.