Membrane separation, general description

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

In this chapter, we will briefly present basic mass transfer mechanisms in polymer membranes and the mechanisms dominating in some membrane separations. In the first part of the chapter, we give a brief description of general transport mechanisms of solutes (charged or not) through a porous media (charged or not). In the second part, we discuss in detail some membranes processes and the transfer mechanisms, interesting from the theoretical point of view. 

This chapter presents a general description of the three kinds of measurements (SP, MP, IS) used for electrical characterization of membrane in working conditions , that is, with the membranes in contact with dectrolyte solutions and the information achieved from them is briefly indicated. The main attention focuses on the characterization of those membranes commonly used in traditional separation processes (from diverse materials and with different structures), and IS measurements are considered in more detail. Membrane and matrix material electrical parameters are obtained, but the measurements also provide thermodynamics and geometrical/structural information. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

This chapter starts with a general description of porous membranes and separation mechanisms related to the pore structure. A variety of porous membranes - including glass, ceramic, carbon, and silica - and their MRs will be illustrated individually. Considerable attention will be focused on... 

The second set of simulations is oriented towards the analysis of the simultaneous heat and mass transfer when two fluids are separated by a porous wall (membrane). The interest here is to couple the species transport through a wall associated with the heat transfer and to consider that the wall heat conduction is higher than the heat transported by the species motion. The process takes place through a cylindrical membrane and we assume the velocity to be quite slow in the inner compartment of the membrane. The process is described schematically in Fig. 3.65. The transformation of the above general model in order to correspond to this new description gives the following set of dimensionless equations ... 

Transport of mixtures is more complicated, especially in membrane systems with a more complex architecture and operated with large pressure gradients. In such cases quantitative solutions for permeation and separation efficiency (selectivity) are not available in a generally applicable form. Specific solutions have to be obtained by approximations and by combiiung solutions for limiting cases. The description in this chapter takes account of this situation. 

Membrane-based reactive separation (otherwise also known as membrane reactor) processes, which constitute the subject matter of this book, are a special class of the broader field of membrane-based separation processes. In this introduction we will first provide a general and recent overview on membranes and membrane-based separation processes. The goal is to familiarize those of our readers, who are novice in the membrane field, with some of the basic concepts and definitions. A more complete description on this topic, including various aspects of membrane synthesis can be obtained from a number of comprehensive books and reviews that have already been published in this area [1.1, 1.2, 1.3,... 

The description given here can be applied in general. However, a distinction must be made for pressure-driven processes such as microfiltration, ultrafiliration and reverse osmosis. Here the feed consists of a solvent (usually water) and one or more solutes. In general, the concentration of the solute(s) is low and the separation characteristics of the membrane are always related to the solute(s). On the other hand, in liquid separation (pervaporation) and gas separation the terms solvent and solute are best avoided. 

The water phase can also be recharged with fresh substrate in an emulsion reactor, where a hydrophilic membrane is used to cleave the emulsion (Fig. lOd) [49]. In this type of reactor as well as in the bimembrane reactor the enzyme is well separated from the organic phase. This is important as interphases, e.g., between two immiscible solvents or between a liquid and a gas differing widely in the dielectric constants, may lead to protein denaturation. A more detailed description of these reactors can be found in the references given or for membrane reactors in general [35,106-108]. 

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