Dynamic Modelling and Control of Cryogenic AIR Separation Plants

Abstract

An air separation plant is an interesting and challenging system from a control standpoint. The process involves up to three highly interconnected, multicomponent, high-purity distillation columns coupled to a series of complex, efficient heat exchangers. The system can have over five liquid and gaseous product streams. A high degree of energy integration is achieved. The plant operating conditions can change over a wide range depending on the mode of operation, on the variations in demand of the on-line customer and on weather conditions. Plant optimization is sought in the face of these and other process changes.Together with an introductory overview of the air separation process and its main control issues, this paper describes our current work on dynamic modelling and simulation of cryogenic air separation units. Rigorous mathematical models were derived for the multicomponent-multiphase processes, including distillation column models with minimal a-priori assumptions and distributed/dynamic models for the reboiler/condenser units. The paper highlights special features of cryogenic distillation systems that distinguish them from more conventional distillation systems and that also give rise to special modelling needs. The paper presents the basis of our dynamic modelling/numerical solution approach which fully addresses these special needs. The benefits of a differential/algebraic model formulation and solution are discussed.