Arrangement of Throughput Manipulator and Inventory Control in Plantwide Control

Abstract

The arrangement of the throughput/inventory control in plantwide control is studied in this article. Based on the way the production rate change is handled, two basic product quality control principles are explored. For the case of the conventional control structure, i.e., constant reactor level and temperature, the importance of the interaction between the flow and composition dynamics is pointed out. A steady-state interaction measure, relative disturbance gain, is employed to evaluate the appropriateness of the material balance control. Moreover, the relative disturbance gain can be expressed in terms of the design parameters, e.g., composition and recycle ratio. This serves as a basis for the selection of the throughput manipulator. The results show that the external flow, i.e., feed or product flow, is preferred over the internal flows. For the cases of variable reactor holdup, the dynamic interaction becomes important. That means, despite having a perfect steady-state interaction measure, schemes with significant dynamic interaction lead to poor closed-loop flow dynamics. Thus, the material balance control should be designed such that the dynamic interaction is minimized. Therefore, the tuning of the flow dynamics becomes important. Results also show that we should use the external flow as the throughput manipulator and tune the inventory control loops such that the flow/composition interaction is minimized. Since flow/composition interaction is inherent in recycle processes, the results can be extended to more complex recycle processes, e.g., systems with more than one recycle streams, in a straightforward manner.