Pore types constricted

Fig. 2.3. Schematic picture of pore types in a porous membrane, a Isolated pore b,f dead end pore c,d tortuous and/or rough pores (d) with constrictions (c) e conical pore.

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

Fig. 26. Screen filters contain pores of a uniform size and retain all particulates greater than the pore diameter at the surface of the membrane. Depth filters contain a distribution of pore sizes. Particulates entering the membrane are trapped at constrictions within the membrane. Both types of filters are rated 10...

For other types of pores other mechanisms may prevail. For Instance, for interconnected pores, liquid bridges may form around necks or other constrictions and hence shield the interior from the outside, in this way provoking hysteresis. 

The mechanism of gas mixture separation depends on the type of adsorbent. Two different mechanisms are distinguished. The first mechanism is based on the kinetically controlled gas diffusion, caused by constrictions of the pore apertures. Here the diameters of pores are in the same range as those of the gas molecules. The particles having smaller diameter can penetrate much quicker into the pores than larger molecules. In the second separation mechanism, the pore system is sufficiently wide to enable fest diffusion, while the separation is caused by the selective adsorption dependent upon different van der Waals forces of the gas species [5]. 

Filtration Model. A model based on deep-bed filtration principles was proposed by Soo and Radke (12), who suggested that the emulsion droplets are not only retarded, but they are also captured in the pore constrictions. These droplets are captured in the porous medium by two types of capture mechanisms straining and interception. These were discussed earlier and are shown schematically in Figure 22. Straining capture occurs when an emulsion droplet gets trapped in a pore constriction of size smaller than its own diameter. Emulsion droplets can also attach themselves onto the rock surface and pore walls due to van der Waals, electrical, gravitational, and hydrodynamic forces. This mode of capture is denoted as interception. Capture of emulsion droplets reduces the effective pore diameter, diverts flow to the larger pores, and thereby effectively reduces permeability. 

Membrane structures for MF include screen filters, which collect retained matter on the surface, and depth filters, which trap particles at constrictions within the membrane. Depth filters have a much sharper cutoff, resulting in enhanced separation factors. For example, a Nuclepore membrane of type 2 can separate a male-determining sperm from a female-determining sperm (Seader and Henley, 2006). Nuclepore MF membranes come in pore sizes from 0.03 to 8.0 microns with water permeate flux rates, at 294 K and a transmembrane pressure difference of 70 kPa, ranging from 15 to 350,000 L/m2-h. 

The AIPO4-I4 structure is a novel structure type, which contains a 3-D eight-ring channel system. The eight rings are distorted and constrict the pore openings to approximately 0.25 nm x 0.56 nm. (40). 

Miscellaneous effects A number of factors can influence the effectiveness factor, some of which are particle size distribution in a mixture of particles/pellets, change in volume upon reaction, pore shape and constriction (such as ink-bottle-type pores), radial and length dispersion of pores, micro-macro pore structure, flow regime (such as bulk or Knudsen), surface diffusion, nonuniform environment around a pellet, dilution of catalyst bed or pellet, distribution of catalyst... 

Figure 6.3.27. (a) UF membrane as a bundle of size-distributed tortuous capillaries, (b) Macrosolute retention behavior of two types of UF membranes having a narrow or a broad pore size distribution, (c) Macrosolute retention via smaller membrane pore size, pore mouth adsorption, pore blockage due to pore constriction. 

The CFP and Darcy air-permeability data discussed in Sect. 5.1 were correlated with mercury porosimetry (total PSD) and water porosimetry (hydrophobic PSD) before and after the consecutive aging/durability-testing experiments for cell M2. Mercury porosimetry can be effectively used to measure the total porosity and PSD of a GDL. This technique measures all porosity that exists (including constricted or dead-ended pores). The mercury intrusion volume also represents the hydrophobic plus hydrophilic surface domains because mercury is nonwetting for both types of pores. 

Different types of pores may co-exist in the porous membranes isolated pore, dead-end pore, cylindrical pore, constricted pore, and conical... 

Figure 2.2 Schematic representation of the main types of membrane pores (a) isolated (b) dead-end (c) straight cylindrical (d) constricted (e) conical.

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