Effects of process design on recycle dynamics and its implication to control structure selection

In this paper we explore the dynamics of simple recycle plants under different process designs using different control structures. A simple reactor/separator process with a first-order irreversible reaction is studied. From transfer-function-based linear analysis, we are able to derive recycle dynamics for the entire range of conversion, the most important design parameter for the simple recycle plant. Under the conventional control structure (constant reactor holdup practice), the results indicate that we have exactly the same input/output (fresh feed to the production rate dynamics), irrespective of reactor conversions. The linear analysis is validated using rigorous nonlinear simulation via a step-by-step relaxation on model assumptions. It turns out that reactor level control plays the most important role in the input/output dynamics. The analysis is extended to the balanced control structure, a control strategy with variable reactor holdup. The results show that, different from the previous case, a larger conversion implies slower input/output dynamics. Ongoing analyses indicate that the inherent dynamics of the recycle plant depend on the process design as well as the fundamental control principle. Finally, implications to control structure design are also given for different levels of reactor conversions.

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