Membrane materials diffusivity selectivity

Water vapor molecules selectively dissolve into the coating or membrane material, diffuse across it, and desorb into the environment, while organic vapor molecules are prevented from permeating through its polymeric material structure. 

DJD is the ratio of the diffusion coefficients of the two molecules and can be viewed as the mobil-ity or diffusivity selectivity, reflecting the different sizes of the two molecules is the ratio of the Henry s law sorption coefficients of the two molecules and can be viewed as the sorption or solubility selectivity of the two molecules. The balance between the solubility selectivity and the diffusivity selectivity determines whether a membrane material is selective for molecule A or molecule B in a feed mixture. Either the diffusivity or the solubility needs to be enhanced to increase membrane selectivity however, polymers that are more permeable are generally less selective and vice versa [19]. The schematic diagram of polymer membrane is given in Figure 6.1. The driving force behind the transport process which involves sorption, diffusion and permeation is the concentration difference between the two phases [21]. 

In most cases, hoUow fibers are used as cylindrical membranes that permit selective exchange of materials across their waUs. However, they can also be used as containers to effect the controUed release of a specific material (2), or as reactors to chemically modify a permeate as it diffuses through a chemically activated hoUow-fiber waU, eg, loaded with immobilized enzyme (see Enzyme applications). 

As the pore size is reduced to 1 nm or less, gas permeation may exhibit a thermally activated diffusion phenomena. For example, in studies at Oak Ridge National Laboratory, for a certain proprietary membrane material and configuration, permeation of helium appeared to increase much faster than other gases resulting in an increase in Helium to C02 selectivity from 5 at 25°C to about 48.3 at 250°C (Bischoff and Judkins, 2006). Hydrothermal stability of this membrane in the presence of steam, however, was not reported. 

Approaches to make a polymeric membrane selective to C02 attempt to enhance the solubility selectivity of the polymer material for C02 and reduce the diffusivity selectivity of the polymer that favors smaller hydrogen molecule. The permeability of a polymer membrane for species A, PA, is often expressed as (Ghosal and Freeman, 1994)... 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

Several different membrane materials have been used, namely Teflon, polyethylene, and silicon rubber among others. It is possible to obtain some degree of selectivity by choosing the material of this membrane according to the conditions of the application. The diffusion through such a structure is more complicated. For radial geometry, the steady-state current is given as... 

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

There are several future trends for the development of passive sampling techniques. The first is the development of devices that can be used to monitor emerging environmental pollutants. Recently, attention has shifted from hydrophobic persistent organic pollutants to compounds with a medium-to-high polarity, for example, polar pesticides, pharmaceuticals, and personal care products.82 147148 Novel materials will need to be tested as selective receiving phases (e.g., ionic liquids, molecularly imprinted polymers, and immunoadsorbents), together with membrane materials that permit the selective diffusion of these chemicals. The sample extraction and preconcentration methods used for these devices will need to be compatible with LC-MS analytical techniques. 

The most important characteristic of nonporous membranes is that they are hydrophobic and contain no pores in the polymeric structure. This means that these membranes not only selectively act as a barrier to particles and polar species, but they also provide unique selectivity and specificity for the permeation and transport of a specific group of compounds that can readily solubilize and diffuse in the membrane material. The analyte extraction rate (permeability) in a nonporous membrane separation process is governed by the solution-diffusion mechanism, as commented on earlier. 

Various analytical techniques make use of both porous and nonporous (semipermeable) membranes. For porous membranes, components are separated as a result of a sieving effect (size exclusion), that is, the membrane is permeable to molecules with diameters smaller than the membrane pore diameter. The selectivity of such a membrane is thus dependent on its pore diameter. The operation of nonporous membranes is based on differences in solubility and the diffusion coefficients of individual analytes in the membrane material. A porous membrane impregnated with a liquid or a membrane made of a monolithic material, such as silicone rubber, can be used as nonporous membranes. 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

Membranes used for separation are thin selective barriers. They may be selective on the basis of size and shape, chemical properties, or electrical charge of the materials to be separated. As discussed in previous sections, membranes that are microporous control separation predominantly by size discrimination, charge interaction, or a combination of both, while nonporous membranes rely on preferential sorption and molecular diffusion of individual species. This permeation selectivity may, in turn, originate from chemical similarity, specific complexation, and/or ionic interaction between the permeants and the membrane material, or specific recognition mechanisms such as bioaffinity. 

NF is used when high molecular weight solutes have to be separated from a solvent. It is effective in the production of drinking water, especially in the case of water softening. Compared to RO, a lower retention is found for monovalent ions. But very recently [9], it has been found that NF separates the ions of the same valency for a selective defluorination of brackish water. RO and UF have shown, respectively, solution-diffusion and convection mass transfers. In NF, a synergism between both can be observed but strongly depends on the operational conditions (pH, ionic strength, flow rate, transmembrane pressure) and on the membrane material used. 

Membrane cell (Fig. 15.3c). This is similar to the diaphragm cell, except that a selective membrane, which lets only certain ions through, is employed instead of a physical separator. In this way higher concentrations of caustic soda can be produced than in the diaphragm cell. Membranes used are of Nation (a tetrafluoroethylene copolymer), Flemion , and other similar materials, as well as bilayer membranes which improve selectivity and impede hydroxide diffusion, so that up to 40 per cent sodium hydroxide solution can be produced. The cell potential is —2.95 V (reversible potential —2.20 V). Energy consumption is the lowest of the three processes, and product purity is high. 

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