Membrane porosity

Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electro dialysis (ED), and electro dialysis reversal (EDR). These processes (with the exception of UF) remove most ions RO and UF systems also provide efficient removal of nonionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to remove contaminants, such as oil and grease, and suspended soHds. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

The object of the foregoing discussion is two-fold eq 19 to 24, together with Figure 12, show how one can obtain the values of Daji/k6 of solutes for a very large number of membrane-solution systems from Dam/k6 data for a single reference solute such as sodium chloride they also show how the physicochemical parameters characterizing solutes, membrane-materials and membrane-porosities are integrated into the transport equations in the overall development of the science of reverse osmosis. 

It has been shown that when the ABS content is increased in the blend, the surface of the membranes become dense, and the membrane porosity decreases (90). Concomitantly, a higher ABS content decreases the amorphous nature, which in turn lowers the permeate flux of the membranes. 

The parameters available to characterize the complexity of microporous membranes are also imperfect. Some widely used parameters are illustrated in Figure 2.30. The membrane porosity (e) is the fraction of the total membrane... 

The performance of a hollow-fiber or sheet bioreactor is primarily determined by the momentum and mass -transport rate [15,16] ofthe key nutrients through the biocatalytic membrane layer. Thus, the operating conditions (transmembrane pressure, feed velocity), the physical properties of membrane (porosity, wall thickness, lumen radius, matrix structure, etc.) can considerably influence the performance of a bioreactor, the... 

The value AP can change in the axial direction in the hollow fiber (AP is the pressure drop in the membrane matrix due to the momentum transfer, the velocity through the membrane is u0 , where e is the membrane porosity). Kelsey etal. [11] have solved the equation system in all three cases, namely for closed-shell operation, partial ultrafiltration and complete ultrafiltration and have plotted the dimensionless axial and radial velocities as well as the flow streamlines. Typical axial and radial velocity profiles are shown in the hollow-fiber membrane bioreactor at several axial positions in Figure 14.8 plotted by Kelsey etal. [ 11]. This figure illustrates clearly the change of the relative values of both the axial and the radial velocity [V=vL/(u0Ro), U=u/u0 where uc is the inlet centerline axial velocity]. 

The variable quantity within the braces of equation (7.25) describes the loss of reduced mediator from the biological layer and its consequent effect on the steady-state current. It indicates that the smallest current is obtained in the absence of a membrane layer—in which case there are equal fluxes of reduced mediator to the working electrode and into the external medium —and the current is one-half of the maximum value. The largest steady-state current is obtained with a biological layer that is negligibly thin. However, microbial whole cells are of a significant size (typically 0.2-10 jxm), so there may be an appreciable loss of reduced mediator. The steady-state current is insensitive to the membrane porosity (0), but it is sensitive to the thicknesses of the biological and membrane layers (/ and m) and to the ratio of the diffusion coefficients (D/Df) in the two layers, Fig. 7.6. 

Fig. 7.6. Effect of variation in the thicknesses of the microbiol layer, /, and the membrane layer, m, on the steady-state sensor current, which was normalized by the maximum attainable value /max. The range of the IIq axis takes cognisance of the fact that the thickness of the biological layer cannot be less than the smallest microbiol dimension, q. The data indicate that the current is insensitive to the membrane porosity ((1=1, top 0 = 0.5, bottom), but sensitive to the diffusion barrier presented by the biological material (D/D/ =...

Photochromic control of the polymer properties leads to potential applications involving the mechanical properties of a solution (viscosity, photogelation), polymer fiber (extensibility, photomuscle ), or membrane (porosity). More important, however, the ability to control the activity of enzymes and other biologically important macromolecules leads to potential applications in clinical phototherapy. 

Three different membrane processes, ultrafiltration, reverse osmosis, and electrodialysis are receiving increased interest in pollution-control applications as end-of-pipe treatment and for inplant recovery systems. There is no sharp distinction between ultrafiltration and reverse osmosis. In the former, the separation is based primarily on the size of the solute molecule which, depending upon the particular membrane porosity, can range from about 2 to 10,000 millimicrons. In the reverse-osmosis process, the size of the solute molecule is not the sole basis for the degree of removal, since other characteristics of the... 

This abstract definition will be explained with the actual example of gaseous permeation through a zeolite/alumina composite membrane. Here, we must investigate the effect of the five following factors on the rate of permeation the temperature (T) when the domain is between 200 and 400 °C, the trans-membrane pressure (Ap) when the domain is between 40 and 80 bar, the membrane porosity (s) ranging from 0.08 to 0.18 m /m, the zeolite concentration within the porous structure (c ) from 0.01 to 0.08 kg/kg and the molecular weight of the permeated gas (M) which is between 16 and 48 kg/kmol. With respect to the first... 

It is not difficult to observe that, by using this system of dimensionless coordinates for each factor, the upper level corresponds to -i-l, the lower level is -1 and the fundamental level of each factor is 0. Consequently, the values of the coordinates of the experimental plan centre will be zero. Indeed, the centre of the experiments and the origin of the system of coordinates have the same position. In our current example, we can consider that the membrane remains unchanged during the experiments, i.e. the membrane porosity (e) and the zeolite concentration (Cj.) are not included in the process factors. 

The performance of a given membrane may be characterized according to its product flux and purity of product. Flux, which is a rate of flow per unit area of membrane, is a function of membrane thickness, chemical composition of feed, membrane porosity, time of operation, pressure across membrane, and feedwater temperature. Product purity, in turn, is a function of the rejection ability of the particular membrane. 

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

The membrane material varies from alumina, zirconia, glass, titania, cordierite, mulUte, carbon to such metals as stainless steel, palladium and silver. The resulting pore diameter ranges from 10pm down to4 nm and the membrane thickness varies from 3 to 10 pm. The membrane porosity depends on the pore size and is 40-55%. 

It should be cautioned that, for a given membrane porosity, it is entirely possible that two membranes can have different pore sizes or pore size distributions which, in turn, can exhibit varying strength properties. Given in Table 5.6 are two mechanical properties of homogeneous porous stainless steel membranes as functions of the membrane pore diameters. It is evident that, as the pore size increases, both the yield strength and the minimum ultimate tensile strength deteriorate drastically. The rate of decline is particularly pronounced for pore diameters less than 5 pm. Therefore, the use of Eq.(5>5) should be limited to crude estimation in the absence of test data and the potential interdependence of pore size and porosity can not be overlooked. 

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