Importance of Process Chemistry in Selecting the Operating Policy for Plants with Recycle

A novel methodology using an economic objective function and employing recycle flow rates as independent degrees of freedom is proposed for the selection of a steady-state operating policy for chemical plants with recycle streams. Recycle flow rates are chosen as degrees of freedom instead of traditional decision variables such as conversion and molar ratio because the analysis is considerably simplified, allowing for greater insight into the nature of the solution of the problem. The methodology makes it clear that the process chemistry is of paramount importance in selecting a plantwide operating policy. Based on the kinetics of the process reactions, plants can be assigned to one of two equivalence classes of operating policies. One class requires constant maximal reactor holdup for optimal operation, and the other class requires a variable reactor holdup (assuming that the process has sufficient flexibility). Results are illustrated with simple examples and a case study based on a process to produce c...