Transport microporous membrane

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

For single-component gas permeation through a microporous membrane, the flux (J) can be described by Eq. (10.1), where p is the density of the membrane, ris the thermodynamic correction factor which describes the equilibrium relationship between the concentration in the membrane and partial pressure of the permeating gas (adsorption isotherm), q is the concentration of the permeating species in zeolite and x is the position in the permeating direction in the membrane. Dc is the diffusivity corrected for the interaction between the transporting species and the membrane and is described by Eq. (10.2), where Ed is the diffusion activation energy, R is the ideal gas constant and T is the absolute temperature. 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

Microporous membranes - pore size in these membranes ranges from 50 to 200 A. In this case, the pores are usually only slightly larger than the solutes and this results in hindered transport through the pores. 

Various theories have been proposed to describe the transport in all of these types of polymer membranes. Theories for macroporous and microporous membranes have been based on hydrodynamic and frictional considerations while those for nonporous gels have been based on Eyring s theory and use a free volume approach to describe the movement of solute through the mesh of the polymer. 

The case of transport through microporous membranes is different from that of macroporous membranes in that the pore size approaches the size of the diffusing solute. Various theories have been proposed to account for this effect. As reviewed by Peppas and Meadows [141], the earliest treatment of transport in microporous membranes was given by Faxen in 1923. In this analysis, Faxen related a normalized diffusion coefficient to a parameter, X, which was the ratio of the solute radius to the pore radius... 

Other modifications to the theory of Anderson and Quinn [142] have been reviewed by Deen [146]. Malone and Quinn [147] modified the above theory to include the effect of electrostatic interactions on transport in microporous membranes. Smith and Deen [148] have also looked at these electrostatic or double layer interactions. More recently, Kim and Anderson [149] investigated the hindrance of solute transport in polymer lined micropores. Also, as briefly mentioned above, an excellent review of the theories presented for transport in microporous membranes has been given by Deen [146]. 

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. 

As seen, diffusion in nonporous gel membranes differs from that in macro-porous or microporous membranes. Various theories based on solute diffusion through the macromolecula r free volume in the membrane have been proposed. It is clear from these theories that structural parameters of the polymer network such as degree of swelling, molecular weight between crosslinks, and crystallinity in addition to factors such as solute size and solvent free volume play important roles in this type of transport. 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

A membrane is usually seen as a selective barrier that is able to be permeated by some species present into a feed while rejecting the others. This concept is the basis of all traditional membrane operations, such as microfiltration, ultrafiltration, nanofil-tration, reverse osmosis, pervaporation, gas separation. On the contrary, membrane contactors do not allow the achievement of a separation of species thanks to the selectivity of the membrane, and they use microporous membranes only as a mean for keeping in contact two phases. The interface is established at the pore mouths and the transport of species from/to a phase occurs by simple diffusion through the membrane pores. In order to work with a constant interfacial area, it is important to carefully control the operating pressures of the two phases. Usually, the phase that does not penetrate into the pores must be kept at higher pressure than the other phase (Figure 20.1a and b). When the membrane is hydrophobic, polar phases can not go into the pores, whereas, if it is hydrophilic, the nonpolar/gas phase remains blocked at the pores entrance [1, 2]. 

Osmotic distillation also removes the solvent from a solution through a microporous membrane that is not wetted by the liquid phase. Unlike membrane distillation, which uses a thermal gradient to manipulate the activity of the solvent on the two sides of the membrane, an activity gradient in osmotic distillation is created by using a brine or other concentrated solution in which the activity of the solvent is depressed. Solvent transport occurs at a rate proportional to the local activity gradient. Since the process operates essentially isothermally, heat-sensitive solutions may be concentrated quickly without an adverse effect. Commercially, osmotic distillation has been used to de-water fruit juices and liquid foods. In principle, pharmaceuticals and other delicate solutes may also be processed in this way. 

Membrane extraction encompasses a class of liquid-phase separations where the primary driving force for transport stems from the concentration difference between the feed and extractant liquids rather than a pressure gradient, as is the case with most of the other processes discussed above. A microporous membrane placed between the feed and the extractant liquids functions primarily as a phase separator. The degree of separation achievable is determined by the relative partition coefficients among individual solutes. This operationx is known as membrane solvent extraction. If a nonporous, permselective membrane is used instead, however, the selectivity of the membrane would be superimposed on the partitioning selectivity in this case the process may be referred to as perstraction. These process concepts are illustrated in Fig. 34. 

Using a microcentric nebulizer at low flow rates (typically about 50 pJL/min), a heated spray chamber, and a heated microporous membrane desolva-tor, the Cetac MCN-6000 system can provide analyte transport efficiencies of 50% to 90%. This system is made completely of HF resistance materials. 

Benes and Verweij provide a thorough theoretical description of the multi-component mass transport in microporous systems [54], Lately, some systematic gas transport data has been obtained for different microporous membranes in our group [50], but more extensive measurements are necessary to get a good insight in the detailed transport properties of the different types of silica membranes. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

In recent years, the fractionation and purification of blood and blood products has emerged as a significant enterprise. Separation of blood and plasma into various cellular and protein fractions has become more of a necessity, given the specific requirements of newer therapies. The first step, the separation of plasma from whole blood (a procedure known as plasmapheresis), is now carried out with a filtration process using synthetic microporous membranes. Chemical engineers pioneered the development of this process and have provided the understanding of what determines its performance in terms of fundamental transport principles. 

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

Wu et al. (1993] have developed a mathematical model based on Knudsen diffusion and intermolecular momentum transfer. Their model applies the permeability values of single components (i.e., pure gases) to determine two parameters related to the morphology of the microporous membranes and the reflection behavior of the gas molecules. The parameters are then used in the model to predict the separation performance. The model predicts that the permeability of carbon monoxide deviates substantially from that based on Knudsen diffusion alone. Their model calculations are able to explain the low gas separation efficiency. Under the transport regimes considered in their study, the feed side pressure and pressure ratio (permeate to feed pressures) are found to exert stronger influences on the separation factor than other factors. A low feed side pressure and a tow pressure ratio provide a maximum separation efficiency. 

Figure 7.4 Schematic diagram of capillary condensation and transport of water vapor through a microporous membrane [Asaeda et al. 19S4]...

D. Casanave, A. Giroir-Fendler, J. Sanchez, R. Loutaty, and J.A. Dalmon, Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions, Cat. Today 25 309 (1995). 

FIGURE 4.2 Illustration of transport mechanisms in microporous membranes. (From Koros W.J., Macromol. Symp., 188, 13, 2002. With permission.)... 

Selective surface flow is, as Knudsen diffusion, associated with transport through microporous membranes, usually inorganic materials. The mechanism of surface diffusion is disputed and several different approaches have been proposed in the literature. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

This brief overview of mass transfer and separation mechanisms involved in ceramic membrane processes will be useful not only for a better understanding of actual operating conditions of ceramic membranes, but also for anticipating future applications. For example, a same microporous membrane can serve theoretically as liquid or gas separation membrane. However, transport mechanisms and operating conditions being totally different, a good membrane permeability and selectivity in the former case cannot be systematically transposed to the second case. 

Romero J, Gijiu C, Sanchez J, and Rios GM. A unified approach of gas, Uquid and supercritical solvent transport through microporous membranes. Chem. Eng. Sci. 2004 59(7) 1569-1576. 

Barrer proposed a qualitative model of single-gas transport through microporous materials [62]. A five-step model can qualitatively describe the transport through a microporous membrane at moderate temperatures (see Figure 10.14) ... 

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