Control of a Reactive Distillation Column in the Kinetic Regime for the Synthesis of n-Butyl Acetate

In a reactive distillation column in which the reaction is kinetically controlled, the steady-state temperature and composition profiles will change whenever there is a change in the production rate or the catalyst deactivates. Because of changes in the residence times on the catalytic stages, conversion changes. This poses a problem for controller design because the controller set points must be changed when disturbances in the feed flow rate and catalyst activity occur. For a column used for the synthesis of n-butyl acetate, we were able to locate points in the temperature and composition profiles that are relatively insensitive to throughput rate changes and catalyst deactivation. Reboiler duty is used as the manipulated variable in the temperature control loop. The feed rates of n-butanol and acetic acid are controlled, with the set point of the acetic acid flow loop ratioed to that of the n-butanol flow loop. The set point of the ratioed control is used as the manipulated variable in an inner cascade loop to control the acetic acid composition inside the column. This inner composition cascade loop guards against feed flow sensor bias in the feed-ratio control. The value of this composition set point is manipulated by an outer cascade loop that controls the composition of the bottom product. This outer cascade loop accounts for changes in reaction residence time relative to reaction time due to production rate changes or catalyst deactivation. The proposed scheme was able to reject various disturbances and maintain the desired high-purity and high-conversion operation.