Factor size distribution, membrane

An investigator in this area typically has precise information on composition of casting solutions and other physicochemical factors affecting membrane formation. Functional measurements of transport in terms of convective permeability, selectivity or diffusive permeability are usually also available. However, without proper techniques for quantitative description of membrane pore structures, and their shape and size distributions, membrane development efforts remain largely empirical. 

Manh Thang et al. (2001) studied another important factor influencing the measured size distribution of HS by F1FFF, namely, the interactions of HS with channel components and with the ultrafiltration membrane used as the accumulation wall. 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

Besides higher unit costs, in both initial investment and replacement, another factor limiting inorganic membranes from wider usage is the control of their pore size distributions. Recent advances, however, such as the sol-gel process and anodic oxidation have started to be implemented in commercial production and have made significant impacts on the current and future market shares of the inorganic membranes. 

On the largest length scale, top picture of Fig. 2, the distribution of water in the membrane is depicted as a porous network. The latter is characterized by a pore size distribution (psd) and a tortuousity factor , which accounts for multiple interconnectivity and bending of pathways in the network. The distribution of pore radii translates into a distribution of pore conductivities. Via this correspondence, the distribution of water in the membrane finally determines its transport properties, namely, proton conductivity and water dif-fusivity. 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

Several authors introduce a shape factor (3 in (9.6) which accounts for the pore size distribution. After integration of (9.6) over the membrane thickness L the permeation is found to be ... 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Four types of diffusion mechanisms can be utilized to effect separation in porous membranes. In some cases, molecules can move through the membrane by more than one mechanism. These mechanisms are described below. Knudsen diffusion gives relatively low separation selectivities compared to surface diffusion and capillary condensation. Shape selective separation can yield high selectivities. The separation factor for these mechanisms depends strongly on pore-size distribution, temperature, pressure, and interactions between the solute being separated and the membrane surfaces. 

Otherwise, the separation factor will be smaller. The narrow pore-size distributions and the small pores of ceramic and glass membranes allow separation due to Knudsen diffusion (for the appropriate pressure range) by preferential diffusion of the lighter component through the membrane. In composite membranes, the thin permselective layer can be in the Knudsen diffusion regime and thus be responsible for all the separation. The support layers, with their larger-diameter pores, are usually in the viscous-flow regime. 

Pretreatment is often used to reduce fouling. Methods include heating, pH adjustment, chlorination, activated-carbon sorption, or chemical precipitation. Other factors such as membrane pore-size distribution, hydrophilicity/hydrophobicity, or surface charge can also reduce the effects of fouling. Methods which reduce concentration polarization, such as using higher axial flow velocities, lower flux membranes, or turbulence promoters, also help to reduce fouling. 

Equations [4.13] and [4.14] are also supported by some atmospheric measurements. Thus, E. Meszaros (1970) measured the size distribution of the mass of atmospheric sulfate particles by means of a cascade impactor backed up by membrane filters. He found that the geometric mean radius of the distribution averaged by humidity intervals varies as a function of the relative humidity as shown by the points in Fig. 38. The solid line of this figure gives the theoretical relation calculated from equation [4.14] for an ammonium sulfate particle with a dry radius of 0.14 /an, the value found for the geometric mean radius at low relative humidity. The line shows that the particle radius increases by a factor of two at a relative humidity of 80 %. Near 100 % the droplet radius is several times larger than the dry particle size. Comparison of the curve with the experimental points indicates that the behaviour of the atmospheric particle population is well approximated by the theory outlined. It cannot be excluded, however, that the real phase change is less sudden than the theory predicts. 

One of the most important factors affecting the hydrogen flux is the thickness of the membrane. Thin layers can generally be deposited on substrates with a relatively smooth surface and small pores with a uniform or narrow pore size distribution [36]. 

Technology development on the fabrication of asymmetric membranes with an ultrathin dense layer has received much attention due to the fact that the thinner the dense layer is, the higher is the productivity. The fabrication of a hollow fiber with a desirable pore-size distribution and performance is not a trivial process as many factors influence fiber morphology during the phase inversion. 

The instability of the system is a serious challenge in a gas-liquid membrane contactor, for instance, the wetting and bubbling problems occur when the pressure difference across a porous membrane is too high. Factors like pore size, pore size distribution, and hydrophobicity and hydrophilicity of the membrane will play a major role in determining the breakthrough of gas or liquid across the membrane [159]. 

The polymeric membrane has three important structural levels (1) the molecular, which is equivalent to the chemical nature of the polymer, is characterized by polar, steric, and ionic factors, and is also responsible for the membranes microcrystalline nature (2) the microcrystalhne, which affects both the transport and mechanical properties of the membrane and (3) the colloidal, which is concerned with the a e-gation of macromolecules and governs the statistics of pores (size, size distribution, density, and void volume). It is desirable to develop new characterization methods at each level to achieve a more rigorous understanding of the polymeric structure in the membrane. 

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