Thermodynamic based response time as controllability indicator on heat exchanger networks

Abstract

Nowadays, the interaction between process design and control has become the focus of research and development, because not always the best process design features the best dynamic performance, with implications in the controllability of the plant. The necessity in minimizing fixed investment and utilities consumption promotes integrated process design in energy and mass terms. These integrations are often designed without consideration of controllability and flexibility of these projects; creating difficulties in process control and resulting in more efficient designs with smaller gradients and reduced driving forces, which brings complications with disturbances rejection. Thermodynamics theory allows establishing relationships between physical quantities, giving an idea of how a system evolves in time. In this way, a thermodynamic analysis with a dissipative approach, can lead to an optimal point between process integration and controllability. The present paper proposes a simultaneous approach relating reversibility and control, obtaining a method to determine a thermodynamic index, relating the entropy and energy production in a new state function and establishing a response time index that serves as a guide to measure the process controllability. The method was applied to a known Heat Exchanger Network (HEN), resulting in smaller values for stages with high entropy production, when exposed to disturbance. The results were compared with the given by Relative Gain Array (RGA) and Disturbance Cost (DC) methods, showing consistency. With the proposed methodology, it is possible to relate the stages of process control and process design, with a base analysis of controllability based on the system thermodynamics.