Distillation stability using passivity and thermodynamics
Distillation is one of the most important unit operations in the chemical process industry. The stability of distillation systems has always been difficult to determine due to the nonlinear natural of the unit operation. Previous work in the area has had limited success and no theory has been able to show the stability criteria for a general multicomponent distillation column. With a better understanding of the stability criteria, distillation control systems can be implemented to ensure stability and greater control performance. In this work a detailed model is used to evaluate the stability criteria of multicomponent distillation. The model uses both temperature and pressure as driving forces for energy and mass flows, along with formulas for the flow of liquid between stages. The temperature and pressure driving forces are then linked to a passivity based approach to analysis using a thermodynamics based storage function. The analysis is done by algebraically decoupling the mass flows from the energy and component flows. The stability criteria for the mass flows are shown from a Eulerian, or fixed reference frame, and the energy and component flow criteria from a Lagrangian, or moving reference frame. The stability criteria for the flows within the column can be used in the design stages to ensure stable dynamic behavior. The results show that as long as the resistance to liquid flow is equal or decreases down the column the system will be stable. These results differ from work that has shown output multiplicities in open-loop distillation. The conclusions will focus on the differences between these results and why they appear. (C) 1000 Elsevier Science Ltd. All rights reserved.
distillation stability passivity thermodynamics