Membranes Nonideal separations

For a defect-free ideal membrane, the selectivity is independent of thickness, and either permeability ratios or permeance ratios can be used for comparison of selectivi-ties of different materials. Nonideal module flow patterns, defective separating layers, impurities in feeds, and other factors can lower the actual selectivity of a membrane compared to tabulated values based on ideal conditions (Koros and Pinnau, 1994). 

Bausa, J. and W. Marquardt, 2000, Shortcut Design Methods for Hybrid Membrane/Distillation Processes for the Separation of Nonideal Multicomponent Mixtures, Ind. Eng. Chem. Res., 39, 1658-1672. 

Maxwell-Stefan (dusty gas) approach by taking the membrane to be the additional component in the mixture. When the model is extended to account for thermodynamic nonidealities (what may be considered to be a dusty fluid model) almost all membrane separation processes can be modeled systematically. Put another way, the Maxwell-Stefan approach is the most promising candidate for developing a generalized theory of separation processes (Lee et al., 1977 Krishna, 1987). 

Bausa, J. and W. Marquardt, Shortcut design methods for hybrid membrane/distillation processes for the separation of nonideal multicomponent mixtures. Industrial and Engineering Chemistry Research, 2000, 39(6) 1658 1672. 

Laboratory experiments may also be necessary to aid in the selection and preliminary design of separation operations. The separation of gas mixtures requires consideration of absorption, adsorption, and gas permeation, all of which may require the search for an adequate absorbent, adsorbent, and membrane material, respectively. When nonideal liquid mixtures are to be separated, laboratory distillation experiments should be conducted early because the possibility of azeotrope formation can greatly complicate the selection of adequate separation equipment, which may involve the testing of one or more solvents or entrainers. When solids are involved, early laboratory tests of such operations as crystallization, filtration, and drying are essential. 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

It has been indicated how interfacial morphology plays a crucial role in the transport properties of hybrid membranes, and conditions as close as possible to the ideal case must be achieved to ensure advanced separation performances. For this reason, several fabrication techniques have been developed to reduce as much as possible the influence of a nonideal interfacial morphology on the transport characteristics of the hybrid membranes. Among the most frequently used procedures there are solution blending, the sol-gel procedure, surface modification, and interfacial polymerization. In some cases a combination of these techniques is required to reach the best dispersion of the filler within the matrix. 

The membrane serves as a phase separator, which is made selectively permeable to those ions for which a concentration value (activity) is to be determined. Using the Nernst relation, with necessary correction factors for temperature and nonideality of the membrane, the potential between the known and unknown concentration can be measured and the unknown ionic concentration thus determined. 

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