Steady State Analysis for the Synthesis of Reactive Distillation Column Control Structures

The synthesis of effective control structures for reactive distillation (RD) columns through a systematic analysis of the steady state relationship between potential manipulated variables (inputs) and potential controlled variables (outputs) is demonstrated. The industrially important double feed RD columns for MTBE and methyl acetate production are used as examples. In order to regulate product purity and reaction conversion, two control loops are required. The first is to balance the fresh feeds as per the reaction stoichiometry and the second is to adjust the column internal flows for the desired separation. The input-output pairings for the two control loops are chosen so that they are sensitive and avoid steady state multiplicities as well as instability problems due to loop interaction. The proposed MTBE control structure uses the reboiler duty to maintain a stripping tray temperature and adjusts the fresh butene feed to maintain a reactive tray composition. The proposed control structures for the methyl acetate column use the fresh acetic acid feed to maintain a reactive tray temperature / composition and the reboiler duty to regulate a stripping tray temperature. Alternatively, the fresh methanol flow may be used to regulate a stripping tray temperature. Results show that the proposed control structures maintain the steady state purity and conversion for production rate and feed composition changes near the design values for both the example columns.