Distillation stability using passivity and thermodynamics

Distillation is one of the most important unit operations in the chemical p rocess industry. The stability of distillation systems has always been diff icult to determine due to the nonlinear natural of the unit operation. Prev ious work in the area has had limited success and no theory has been able t o show the stability criteria for a general multicomponent distillation col umn. With a better understanding of the stability criteria, distillation co ntrol systems can be implemented to ensure stability and greater control pe rformance. In this work a detailed model is used to evaluate the stability criteria of multicomponent distillation. The model uses both temperature an d pressure as driving forces for energy and mass flows, along with formulas for the flow of liquid between stages. The temperature and pressure drivin g forces are then linked to a passivity based approach to analysis using a thermodynamics based storage function. The analysis is done by algebraicall y decoupling the mass flows from the energy and component flows. The stabil ity criteria for the mass flows are shown from a Eulerian, or fixed referen ce frame, and the energy and component flow criteria from a Lagrangian, or moving reference frame. The stability criteria for the flows within the col umn 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 dec reases down the column the system will be stable. These results differ from work that has shown output multiplicities in open-loop distillation. The c onclusions will focus on the differences between these results and why they appear. (C) 1000 Elsevier Science Ltd. All rights reserved.