Selectivity versus conversion and optimal operating policies for plants with recycle



A simple and fast procedure is developed for determining optimal steady state operating policies for chemical plants with recycle. This article improves upon the previous findings by Griffin et al. (Ind Eng Chem Res. 2006;45:8056-8062) and now provides a more general classification procedure for process chemistries that have reversible reactions or reactions of unequal overall order or both. Selectivity vs. conversion relationships are indirectly determined herein, requiring only the reaction orders of reaction species. The effect of these relationships allows process chemistries to be classified into two equivalence classes: bounded and nonbounded. This classification has important implications for the optimal operation and plantwide control structure of a flexible plant with spare equipment capacity. For instance, using the classification procedure and decision tree developed in this article, it can be determined quickly whether or not it is economically optimal to operate the reactor at the holdup constraint under all operating conditions (bounded) or away from the constraint with a variable holdup operating policy (nonbounded). © 2008 American Institute of Chemical Engineers AIChE J, 2008