Plantwide Control Issues for Distillation Columns

We briefly discussed in Chap. 2 (Sec. 2.5) one important aspect of distillation column control when one of the products leaving the column is a recycle stream. Should the composition of this recycle stream be controlled Probably not from the perspective of the isolated column because the control loop holding the composition of the other stream leaving the column, which is a product from the plant, could then be more tightly tuned. However, the plantwide control perspective may show that the performance of the reactor can be improved by holding the purity of the recycle stream more constant, and this could result in smaller disturbances to the column. The overall effect may be better product quality control even though the product-quality loop is less tightly tuned. 

There are many other aspects of distillation column control in a plantwide context. One of the earliest studies of these issues is the work of Downs (1992). According to Downs, the control strategy for each unit operation must be developed within the framework of the overall component inventory control structure. 7 He presents several 

In addition to the question of composition control of recycle streams, there are many other examples of plantwide issues affecting the choice of the best control structure for a distillation column. Several of these are discussed below. 

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