Optimization of air separation processes with a two-stage membrane system

In order to improve membrane separation profits, it is necessary to conduct process optimization on multi-stage structures and equipment. In this study, process optimization of two-stage membrane separation was performed for nitrogen production. Effects of permeability and selectivity of membrane material on membrane area per nitrogen product was investigated. Optimization of total annual cost was studied with membrane materials, flowsheet structure and membrane area as decision variables. A model based on differential algebraic equations was obtained. The results show that the profit of a single-stage membrane system is improved after retrofitted into a two-stage membrane system, and capital cost is decreased. Comparing with a two-stage membrane without recycle system, the two-stage membrane with recycle system provided higher recovery with higher capital and operational cost using the same membrane material. The two-stage membrane system employed CMS (high permeability and selectivity) with recycle C13 was the optimal solution. The two-stage membrane system composed of high selectivity material PC at the first stage and high permeability TMHFPSF at the second stage with recycle C20 was the optimal solution for traditional membrane materials. After the flowsheet structure rearrangement, driven force increased at the second stage results in the decrease of membrane area. © 2019, Editorial Board of Journal of Chemical Engineering of Chinese Universities. All right reserved.