Influence of Feed Impurity on the Design and Operation of an Industrial Acetic Acid Dehydration Column

In this work, the design and operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. Multiple column feed streams from various parts of the upstream process are fed into this column. The feed components besides acetic acid and water also contain small amounts of methyl acetate and another component. For proprietary reasons, this component will not be specified in this paper. This component during normal operation will not leave the column system through either a top decanter aqueous outlet stream or a column bottom stream. The accumulation of this component inside the column system is illustrated via rigorous dynamic simulation. A side stream is proposed to solve the accumulation problem of this component. The appropriate side-stream location and its flow rate are determined in this paper by total annual cost analysis. There is a great influence of this small feed impurity on the design and operation of the column. With the same purity specifications on both the top and bottom products, the column system with feed impurity requires much larger total annual cost and operating cost in comparison with the design of the no feed impurity case. A very simple overall control strategy is proposed for the column system with feed impurity and continuous side-stream draw-off. Because the optimal flow rate of the side stream is quite small, an alternative operating strategy in industry is to periodically purge out this impurity through the side stream. A practical automatic purging strategy is thus proposed in the paper. This automatic purging strategy does not rely on any online composition measurement but just needs information on some tray temperature measurements. Closed-loop dynamic simulation demonstrates that the proposed purging strategy can successfully keep the column under normal operation with both the top and bottom product purities at their specifications.