Design and control of acetic acid dehydration column with p-xylene or m-xylene feed impurity. 1. Importance of feed tray location on the process design

In the production of aromatic acids, such as terephthalic acid and isophthalic acid, tiny amounts of reactant p-xylene or m-xylene may also enter into the acetic acid dehydration column through the feed stream. In this work, the process design flow sheets both with and without these tiny impurities are considered. For the case with one of these tiny impurities in the feed stream in combination with a high-purity specification for the bottom stream, a side stream is necessary to purge the impurity; otherwise, accumulation of the impurity will occur inside the column. If only the optimum side stream location and its flow rate are considered in the optimization search, it is found that the total annual cost and the operating cost of this acetic acid dehydration column become much higher by just adding a tiny amount of p-xylene or m-xylene in the feed stream, compared to the no-impurity case. With careful reselection of the feed tray location, significant total annual cost and energy savings for the operation of this column can be realized. A separate case with lower bottom product specification has also been investigated. In this case, the side stream is not necessary because this impurity can leave the system from the bottom stream. The process design investigation reveals that the selection of the feed tray location is also extremely important. The distillation path of the column composition profile for the optimum design follows a path with more p-xylene in the bottom part of the column. This desirable distillation path should be maintained even though various disturbance changes inevitably could enter into the column system.