Membrane nonporous

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

Figure 2.1 Exploded views showing the nonporous membrane size-exclusion phenomenon in the uptake and loss of organic compounds. Middle illustration shows the movement of contaminant molecules through transient pores in the membrane and retention (membrane exclusion) of much larger lipid molecules. Upper illustration shows similarly scaled space-filled molecular models of some organic contaminants and triolein, along with the hypothetical polymer pore (transient) size. Reprinted with permission from the American Petroleum Institute (Huckinset al., 2002).

First, porous membranes will be discussed. Gases can be separated due to differences in their molecular masses (Knudsen diffusion), due to interaction (surface diffusion, multilayer diffusion and capillary condensation) and due to their size (molecular sieving). All these mechanisms and their possibilities will be discussed. For the sake of simplicity, theoretical aspects are not covered in detail, but examples of separations in literature will be given. The next section deals with nonporous membranes. Here the separation mechanism is solution-diffusion, e.g. solution and diffusion of hydrogen through a platinum membrane. This section is followed by an outline of some new developments and conclusions. 

The theories developed for transport in microporous membranes cannot be applied to nonporous gel membranes. The pore structure in microporous membranes is not analogous to the mesh of the nonporous gels. Thus a different set of theories had to be developed for the treatment of nonporous polymer gel membranes. These theories are based on the idea of the existence of free volume in the macromolecular mesh. As a result, diifusion through nonporous membranes is said to occur through the space in the polymer gel not occupied by polymer chains. 

In nonporous membranes, diffusion occurs as it would in any other nonporous solid. However, the molecular species must first dissolve into the membrane material. This step can oftentimes be slower than the diffusion, such that it is the rate-limiting step in the process. As a result, membranes are not characterized solely in terms of diffusion coefficients, but in terms of how effective they are in promoting or limiting both solubilization and diffusion of certain molecular species or solutes. When the solute dissolves in the membrane material, there is usually a concentration discontinuity at the interface between the membrane and the surrounding medium (see Figure 4.55). The equilibrium ratio of the solute concentration in one medium, c, to the solute concentration in the surrounding medium, C2, is called the partition coefficient, K12, and can be expressed in terms of either side of the membrane. For the water-membrane-water example illustrated in Figure 4.55,... 

One way to apply MIP membranes for separations is to construct them as selectively permeable membranes. Since a membrane with large pores (compared to the molecular size of the template and the interferents) is necessarily easily permeable for all these substances, selective permeability may only be expected from nonporous membranes or membranes with very small pores. Work with selectively permeable nonporous MIP membranes has been limited and the underlying mechanisms may hardly be categorized as adsorption. Therefore a more specific recent review (M. Ulbricht in [3], p 455) should be consulted in this respect. 

The types of polymeric membranes that have attracted much interest for analytical applications and are nowadays in common use are characterized as nonporous membranes such as low-density polyethylene (LDPE), dense PP and PDMS silicone rubbers, and asymmetric composite membranes... 

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. 

The membrane extraction process mostly makes use of nonporous membranes. Such a membrane can be a liquid or a solid phase (a polymer impregnated with a liquid), which is placed between two... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

The hydrophilicity-hydrophobicity balance of the membrane polymer is another important parameter that is mainly influenced by the functional groups of the polymer. Hydrophilic polymers have high affinity to water, and therefore they are suited as a material for nonporous membranes that should have a high permeability and selectivity for water (e.g., in RO or hydrophilic PV). In addition, hydrophilic membranes have been proven to be les s prone to fouling in aqueous systems than hydrophobic materials. 

Figure 6.2 depicts 3 ways in which microporous membranes can foul (a) pores can suffer closure or restriction, (b) pores or porosity can be blocked or plugged and (c) a surface cake or layer can cover the membrane. All three mechanisms could apply, probably in sequence (a) then (b) followed by (c). Nonporous membranes are fouled by cake or surface layers (c). 

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. 

Dialysis operates by the diffusion of selected solutes across a nonporous membrane from high to low concentration. An early industrial application of dialysis was caustic soda recovery from rayon manufacturing. It had been a viable process because inexpensive but alkali-resistant cellulose membranes were available that were capable of removing polymeric impurities from the caustic. Gradually however, dialysis is being replaced by dynamic membrane technology for caustic soda recovery because of the latter s much higher productivity. 

Which polymer is most extensively used as non-degradable nonporous membrane to develop reservoir-type polymeric implants ... 

Mitochondria are encased in outer (porous) and an inner (nonporous) membrane. 

In the latter case (nonporous membrane), the space in which the transport occurs is not fixed in size and location. The free volume is the volume that is not occupied by the polymer molecules in the solid phase, and its size and location fluctuate with time at a given temperature. Accordingly, the transport through such a membrane is completely different from the transport through fixed pores, and can be expressed by the solution-diffusion mechanism. The permeant is first dissolved in the membrane phase, and the dissolved permeant diffuses through the membrane following the chemical potential gradient. 

Figure 34.3 Modes of plasma polymerization coating, (a) on nonporous membrane, and (b) on porous membrane.

Luminous Gas Treatment of Nonporous Membrane to Increase the Selectivity... 

Oxygenation of water or water suspension such as blood can be done by (1) blowing oxygen gas into the liquid via a porous membrane and (2) bubbleless oxygenation via a gas-permeable (nonporous) membrane. Both the methods have... 

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