The aim of the present work is to study the transfer processes in a cross-flow filtration cell, in order to determine the conditions for stable and efficient operation of a side-stream filtration module, integrated with a bioreactor. The current interest in membrane integrated bioreactors is connected with the pursuit of energy and cost efficiency in a wide area of industrial applications, including wastewater treatment, food industry, pharmaceutical industry, and fuel production. A numerical CFD model is employed, based on previous experience with experimental concentration of antioxidants, such as polyphenols and flavonoids from extracts of natural products by nanofiltration. The geometry under investigation is a 3D model of the experimental flat-sheet cell with tangential orientation of the feed inlet. The swirling turbulent flow in the feed channel is favourable for reducing the concentration polarization layer on the membrane surface and preventing fouling. The main factors, affecting the filtration process, are the shear stress distribution and the concentration profiles in the vicinity of the membrane surface. The CFD models of mass transfer in cross-flow nanofiltration are scarce and there are none for the reference experimental filtration cell. The present CFD simulation reveals the concentration distribution in the feed channel. It complements previous data for the flow pattern with new knowledge on the mass transfer there, directed to understanding and control of the concentration polarization phenomenon. The numerical study uses the tools of ANSYS Fluent R13, based on the finite volume method for solving the Reynolds-Averaged Navier-Stokes (RANS) equations. The obtained results are analysed in rapport with available experimental data.