Chapter 3 - Mass Transport Through a Membrane Layer

Abstract

This chapter surveys the general consideration of the different mass transport mechanisms depending on the component physical/chemical properties, the interaction between the transported molecules and the molecules of the membrane matrix, and the membrane structure and operating conditions. The chapter first views the thermodynamic mathematical expressions, originated by means of the chemical potential change as the driving force of all transport processes within a fluid and membrane layer, considering primarily the laws of nonequilibrium thermodynamics, which are fundamentally important in membrane separation processes. Then three types of mass transport are reviewed, depending on the membrane characteristics, namely solution/diffusion mass transport within the dense membrane layer (Section 3.3.1), convective (or convective/diffusive) transport in the porous membrane (Sections 3.3.2 and 3.3.3), and molecular sieving transport, namely component transport in the membrane with narrow pores, where the pore size is similar to the molecule size (Sections 3.3.4, 3.3.6.2, and 3.3.8). Different mass transfer theories are then briefly discussed, namely the Maxwell–Stefan approach (Section 3.3.6), the Flory–Huggins mixing theories for prediction of the chemical potential change and for prediction of the mass transfer rate (Sections 3.3.7 and 3.3.8), and the UNIQUAC model, which describes solute/solvent dissolution in the membrane and consequently the mass transfer rate (Section 3.3.9). The roughly 300 expressions given in this chapter should orient the user to find a solution to a membrane separation problem and help in the description of the process, which enables process engineers to predict the separation performance and thus plan the scale of a given separation task.