Design of Hybrid Distillation-Vapor Membrane Separation Systems

Currently the separation of olefins (ethylene, propylene) from (ethane, propane) on a commercial scale is performed almost exclusively by cryogenic distillation in petrochemical industries. Since this technology is highly energy intensive, there is a strong economic incentive to explore alternative separation technologies with lower energy consumption. In this work, using the separation of ethylene and ethane as a representative case, a mathematical programming approach is proposed to optimize and retrofit a hybrid separation system consisting of a distillation column and a parallel membrane separation unit. A two-stage approach is used. First, a shortcut model is introduced that allows determining whether the hybrid system could be of interest and the order of magnitude of the energy savings that call be expected. Second, a superstructure optimization approach is proposed that uses rigorous models for both the column and the membrane using a process simulator and state of the art MINLP solvers. The results presented in the case study show that significant savings in total costs and energy (up to 30%) can be obtained with the hybrid system.