Membrane narrow pore size distribution

Surface media Captures particles on the upstream surface with efficiencies in excess of depth media, sometimes close to 100% with minimal or no off-loading. Commonly rated according to the smallest particle the media can repeatedly capture. Examples of surface media include ceramic media, microporous membranes, synthetic woven screening media and in certain cases, wire cloth. The media characteristically has a narrow pore size distribution. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

The quality of the support is especially critical if the formation of the top layer is mainly determined by capillary action on the support (see Section 2.3.2). Then, besides a narrow pore size distribution the wettability of the support system plays a role (see Equation 2.1). An example of the synthesis of a two-layer support and ultrafUtration membrane is given in the French Patent 2,463,636 (Auriol and Trittcn 1973). In many cases an intermediate layer, whose pore sizes and thickness lie between those of the main support and the top layer (see Figure 2.2), is used. This intermediate layer can be used to improve the quality of the support system. If large capillary pressures are used to form such an intermediate layer, defects (pinholes) in the support will be transferred to this layer. This can be avoided by decreasing the acting capillary pressures or even by eliminating them. This can be done in several ways. 

With anodic oxidation very controlled and narrow pore size distributions can be obtained. These membranes mounted in a small module may be suitable for ultrafiltration, gas separation with Knudsen diffusion and in biological applications. At present one of the main disadvantages is that the layer has to be supported by a separate layer to produce the complete membrane/support structure. Thus, presently applications are limited to laboratory-scale separations since large surface area modules of such membranes are unavailable. 

The pore size distribution determination is illustrated in Figure 10. Again, the pore size distribution determination method does not result in the actual membrane pore size and pore size determination. It does, however, give a means of comparing different fdter media. A narrow pore size distribution is required for effective filtration and filtration validation. 

Figure 11 shows the wet-flow properties of three hypothetical membrane filter media. Each filter medium is made of the same material and has the same thickness and total void fraction. Media A and B have the same oversized pore size, but A has a broader pore size distribution. Medium C has a pore size smaller than A and B with a narrow pore size distribution. 

Conventional filters, such as a coffee filter, termed depth filters , consist of a network of fibers and retain solute molecules through a stochastic adsorption mechanism. In contrast, most membranes for the retention of biocatalysts feature holes or pores with a comparatively narrow pore size distribution and separate exclusively on the basis of size or shape of the solute such membranes are termed membrane filters . Only membrane filters are approved by the FDA for sterilization in connection with processes applied to pharmaceuticals. Table 5.3 lists advantages and disadvantages of depth and membrane filters. 

Electrical-charge effects can be further exploited by using charged membranes (as referred to above) to increase retention of all species with like polarity. It is important to mention that it may be possible to exploit electrostatic interactions even for solutes with similar isoelectrical points, due to different charge-pH profiles for the different species present. The membrane pore-size distribution also affects selectivity by altering the solute sieving coefficients locally. Narrow pore-size distributions, especially for electrically charged membranes, will impact very positively on membrane selectivity and overall performance. 

Developing new, more effective membrane modules, and membrane material with the desired membrane structure that have narrow pore-size distribution and thus, better selectivity ... 

The membrane used was a Vycor glass tube (Corning Inc., code 7930), the dimensions of which were dinner = 7.8 mm, douter = 10 mm, and L = 100 mm. Vycor glass possesses a narrow pore size distribution, with a mean pore diameter of approximately 4 nm. 

Molecular sieving Fig. 4(e) where, due to steric hindrance, only small molecules will diffuse through the membrane, seems to be a useful principle for achieving good separations. To ensure this molecular sieving effect, ultramicroporous membranes have to be prepared. Moreover, such membranes should not only be defect free but must also present a very narrow pore size distribution to avoid any other (less selective) permeation mechanisms defect-free zeolite membranes appear to be good candidates for this type of separation. 

Droplet sizes and important proeess parameters The properties of the membrane are very important in this process. The membrane should have a relatively narrow pore size distribution, just as with cross-flow membrane emulsification, and it should be wetted by the phase that should be the continuous phase. 

The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 

Ultrathin porous glass membranes with variable texture properties were prepared from a Si02-rich sodium borosilicate initial glass by careful fine timing of the conditions of heat treatment for phase-separation. Pore sizes between < 1 and 120 nm can be realized. The membranes are characterized by a narrow pore size distribution. The transport, optical and mechanical properties vary with the pore size. The tailorable texture and transport characteris-... 

This method has been employed to commercialize polycarbonate membranes with extremely narrow pore size distributions and a wide range of pore size. The pore length (depth) is limited due to the energy constraints of the charged particles. No inorganic membranes are commercially produced this way but mica membranes with pore diameters of 6 nm to 1.2 mm have been prepared in the laboratory [Quinn et al., 1972 Riedel and Spohr, 1980]. Membranes prepared by this technique are good candidates for fundamental transport studies due to their very uniform pore shape. 

Figure 4.10 shows the pore size distribution data of an alumina membrane by mercury porosimetry. This particular sample has a three-layered structure. The support has a relatively narrow pore size distribution but the membrane layer and the intermediate support layer do not show a clear distinction on the mercury porosimetry data. Typical mercury porosimetry analysis involves intrusion and extrusion of mercury. The intrusion data are normally used because the intrusion step precedes the extrusion step and complete extrusion of mercury out of the pores during the de-pressurization step may... 

Due to their greater chemical and thermal stabilities and narrower pore size distributions compared to polymer membranes, ceramic membranes are attractive in a number of filtration applications related to the fermentation broths. They can be used for either upstream or downsueam processing. 

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