Economics and Exergy Efficiency in the Conceptual Design of Reactive Distillation Processes

The interactions between economics and thermodynamic efficiency in the design of reactive distillation (RD) processes are explored. Our motivation derives from taking a sustainable lifespan perspective, where not only economics, but also exergy-efficiency and controllability should be borne in mind. Furthermore, novel process configurations might be generated and optimized (e.g. diabatic RD units) when an increased number of design variables is exploited. Our approach involves the development of rigorous dynamic, non-equilibrium model of a generic lumped RD volume element, comprising (species) mass, energy and entropy balances. The entropy production rate is important both for the exergy efficiency and a controllability analysis. As multiple conjugated fluxes and forces are considered, the classical theory of non-equilibrium thermodynamics is applied, to link fluxes with forces and to establish the entropy production rate. The key design variables considered for the lumped RD volume element are: liquid phase reaction volume (for homogeneously catalyzed reaction(s) or catalyst mass for heterogeneously catalyzed reactions(s)), external heating/cooling flow-rate and the areas for heat and mass transfer between the phases. A fundamental understanding of the strengths and shortcomings of this approach is developed by considering two applications: the simulation of a steady state MTBE column to analyze the contributions to entropy production rate; and the multiobjective optimization of a single diabatic RD stage. In the second application a trade-off has been found between economics and exergy criteria. Significant deviations from the utopia point have been encountered.