Electrodialysis with Bipolar Membranes

Abstract

Quite often during the operation of a system for the treatment of gases, it is necessary to expand it for treating greater streams. In some cases, future expansion is contemplated even during the initial phase of a project. In other cases, it could be a necessity not foreseen during system design phase (Miller and Stöcker 1989; Brunetti et al. 2010).Membrane system expansion is very easy, since this only requires the addition of identical modules. This is the advantage offered by the modularity of membrane units and the reduced equipment and control systems required for operating it. In comparison, considering the other reference technologies for gas separation, PSA and absorption systems can also be expanded, but it requires additional design considerations and adds cost in the initial phase of the project.The cryogenic units cannot be expanded if it is not foreseen during the design phase. Generally they can be over-dimensioned, and a capacity increase is often obtained without modification to the cold box itself through addition of a tail gas compressor. ReferencesBrunetti A, Bernardo P, Drioli E, Barbieri G (2010) Membrane engineering progresses and potentialities in gas separations. In: Yampolskii Y, Freeman B (eds) Membrane gas separation. Wiley, New York, pp 281-312 Miller GQ, Stöcker J (1989) Selection of a hydrogen separation process NPRA annual meeting, 19-21 Mar, San Francisco Effective DiffusivityRenzo Di Felice University of Genova, Genova, ItalyEffective diffusivity is a convenient parameter which is introduced when diffusion takes place in non-homogenous media. Consider, for example, the case where a species is diffusing through porous particles, such as reactant diffusing inside a catalytic solid. In this case, the molecules have to travel for a longer distance given that the pores of the catalytic particles are not straight, and moreover diffusion takes place over a smaller area due to the solid being wall impermeable. These effects are taken into account by defining an effective diffusivity as where e is the particle void fraction and thereby takes into account the reduced flow area available and t is the tortuosity which considers the longer distance traveled by the molecules. It should be stressed, however, that more often than not, the parameter inside the parenthesis is utilized as a fitting factor derived from experimental data rather than predicting factor from the solid physical characteristics. These result in the reported tortuosity factor, for example, to assume rather wild values, as high as ten, which are difficult to justify with geometrical arguments alone. The concept of effective diffusivity is also applied for the case of species diffusing in heterogeneous media, such a polymer filled with a second, less permeable, material. Maxwell (1873) has obtained an exact expression, in the case of composite media filled with spheres, for the effective diffusivity given aswhere D is the diffusion coefficient in the primary media, D s the diffusion coefficient through the spheres, and f th...