Rigorous variable permeability modelling and process simulation for the design of polymeric membrane gas separation units: MEMSIC simulation tool

Membrane processes are, together with cryogeny, absorption and adsorption, a key technology for gas separation applications. For simulation purposes, a constant membrane permeability hypothesis is most often assumed for each gas compound, for sake of simplicity. In this study, the incidence of variable (i.e. pressure dependent) permeability of gases or vapors through dense polymeric membranes on module separation performances or design is investigated through a simulation study. A dedicated computer software (MEMSIC) including multicomponent mixtures computations and extendable to Process System Engineering (PSE) softwares through a CAPE OPEN tool is described. The modelling approach offers, for the first time, a rigorous computation of transmembrane fluxes based on different solution-diffusion models (dual mode, Flory Huggins, ENSIC) for a set of absolute upstream and downstream pressure. This strategy differs from a classical flux expression, based on the dependency of permeability either upon upstream pressure or transmembrane pressure, which has been proposed up to now but can generate computational errors. A series of simulation case studies through glassy and rubbery membranes is reported; it shows that the constant permeability hypothesis can lead in some cases to significant design errors (i.e. required membrane surface area for a given set of specifications). Furthermore, significant differences are observed compared to the classical variable permeability approach making use of the product of permeability with pressure difference. Tentative guidelines for the identification of the key characteristics of a given system which require a variable permeability behavior to be taken into account are finally proposed.