Globally Optimal Networks for Multi-Pressure Distillation of Homogeneous Azeotropic Mixtures: Application to Ternary Mixtures

In this paper, a methodology for the globally optimal synthesis of a network of vapor-liquid equilibrium flash separators that can operate at multiple pressures and separate an azeotropic mixture is presented. The objective function minimized is the total flow entering the network flashes. The proposed synthesis methodology employs the Infinite DimEnsionAl State-Space (IDEAS) conceptual framework, which is shown to be applicable to the problem under consideration. The resulting infinite linear programming (ILP) IDEAS formulation is shown to possess several properties that allow its simplification. The approximate solution of this IDEAS ILP is pursued through the solution of a number of finite dimensional linear programs (FLP) of ever increasing size, whose optimum values form a sequence that converges to the ILP’s infimum. The proposed optimal design methodology is general in nature and can be used to separate any number of pressure-sensitive azeotropic mixtures, with or without using an entrainer. The method is demonstrated on a case study involving the dual pressure separation of a ternary mixture of Methanol-Acetone-Water which exhibits a minimum boiling azeotrope, without using an entrainer. Comparison with a traditional two-column design reveals the benefits of employing the IDEAS framework not only to realize the separation with minimum total flow but also to reach outlet concentrations and flow rates combinations that are unattainable with the traditional design.