Mathematical modeling of transport and separation properties of permselective membranes
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: concentration polarization, electroosmotic flow, reconstructed porous media, ionic permselectivity
Porous media with fixed surface charge are involved in many practical applications which utilize externally applied electric field or where the induced electric field is important, e.g., capillary electrochromatography, electrodialysis, nanofiltration and other membrane separation processes. In electric field driven applications the electroosmotic flow is an important mechanism of convective transport of the electrolyte solution. Although one of the practical problems is to find the relation between concentrations of charged species in electrolyte or the electroosmotic flow of the electrolyte and the porous structure of the medium, most current case studies assume the so called pseudohomogeneous (space-averaged) porous medium represented by space-averaged parameters, for example by effective diffusivity of species or averaged fixed charge on the surfaces of permselective membranes. We have developed the software which allows to predict the transport and separation properties of porous medium (e.g., perfusion flow and ionic permselectivity) in dependence on: (i) the spatially 3D porous micro-structure, (ii) type and distribution of the fixed charge, and (iii) type and concentration of the electrolyte. We employ the concept of the reconstructed porous medium and the simultaneous solution of the Navier-Stokes equations describing the velocity and pressure distribution, Poisson equation describing the electrostatic potential distribution and local mass balances of charged species to address the above mentioned problem. Finite element method and multigrid algorithms were employed to solve the resulting system of equations.
The subject of our study is the predictive modeling of the electroosmotic (perfusive) flow passing through the spatially 2D or 3D reconstructed porous media in the case of both negligible and significant double layer thickness. We also investigate the distribution of ions in the general-topology porous media with the distribution of fixed surface charge. Our model allows to calculate the concentration polarization and the ionic permselectivity of membranes in dependence on their micro-structure and other conditions. Calculated ionic permselectivities are compared to experimental measurements for several types of membranes.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
