Porous membranes transport mechanisms

Selective barrier structure. Transport through porous membranes is possible by viscous flow or diffusion, and the selectivity is based on size exclusion (sieving mechanism). This means that permeability and selectivity are mainly influenced by membrane pore size and the (effective) size of the components ofthe feed Molecules... 

Transport in porous membranes occurs via diffusion of gaseous molecules within the porous framework this transport may involve different mechanisms (Section A9.3.2.4) which are more or less dependent on the nature of the gaseous molecules, and hence more or less efficient for the separation of a gas mixture. Porous membranes are therefore generally less permselective when compared to dense ones however, their permeability is higher (a conventional mesoporous y-Al2C>3 membrane has a permeability for hydrogen which is 10 to 100 times higher than a conventional Pd dense membrane. More detailed permeability data can be found in Ref. 9). 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

The membranes used in GS can be distinguished in two main categories, polymeric and inorganic. Polymeric membranes, specifically used for GS, are generally asymmetric or composite and based on a solution-diffusion transport mechanism. These membranes, made as flat sheet or hollow fibres, have a thin, dense skin layer on a micro-porous support that provides mechanical strength [7]. Typically, polymeric membranes show high, but finite, selectivities with respect to porous inorganic materials due to their low free-volume... 

As mentioned, membranes ean be classified according to their material components and structure as being porous or dense. What can be understood as dense membranes depends on die scale to which they are studied. The performance of dense membranes (permeability and selectivity) is determined by the intrinsic properties of the material by solution-diffusion flirough the molecular interstices in the membrane material. When solution-diffusion is not the main transport mechanism, the membranes are considered porous. 

Figure 12.4 Schema of DCMD mechanism of transport through porous membranes (a) homogenous hydrophobic and (b) composite bydropbobic/hydiophilic. Tbe concentration profiles correspond to nonvolatile solutes. (Adapted liom Khayet et al., 2005b.)...

J. L. Anderson, D. M. Malone. Mechanisms of osmotic flow in porous membranes transport. Biophys 14 951, 1974. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

In dense membranes, no pore space is available for diffusion. Transport in these membranes is achieved by the solution diffusion mechanism. Gases are to a certain extent soluble in the membrane matrix and dissolve. Due to a concentration gradient the dissolved species diffuses through the matrix. Due to differences in solubility and diffusivity of gases in the membrane, separation occurs. The selectivities of these separations can be very high, but the permeability is typically quite low, in comparison to that in porous membranes, primarily due to the low values of diffusion coefficients in the solid membrane phase. 

However, a distinction should be made in that Eq. (12) is purely phenomenological and does not require any transport mechanism model while the Nermst-Planck equation used in the previous finely-porous membrane model requires a specific pore model. Another difference is that the salt concentration in Eq. (12) is that in the membrane while the quantity appearing in the Nernst-Planck equation refers to the salt concentration in the membrane pores. 

This is identical to the Spiegler-Kedem relationship, Eq. (2), and that of finely-porous membrane model, Eq. (3), with a = r. However, it should be noted that Eq. (28) is derived phenomenologically without any assumptions on the transport mechanism. 

Pervaporation is a membrane separation process in which a dense, non-porous membrane separates a liquid feed solution from a vapour permeate (Fig. 19.2c). The transport across the membrane barrier is therefore based, generally, on a solution-difliision mechanism with an intense solute-membrane interaction. It... 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

Synthetic membranes for molecular liquid separation can be classified according to their selective barrier, their structure and morphology and the membrane material. The selective barrier- porous, nonporous, charged or with special chemical affinity -dictates the mechanism of permeation and separation. In combination with the applied driving force for transport through the membrane, different types of membrane processes can be distinguished (Table 2.1). 

The separation performance of membranes with nonporous barriers is - because of the transport via solution-diffusion (cf. Section 2.2) - predominantly influenced by the polymer material itself. Therefore, the material selection is directly related to the intrinsic (bulk) properties of the polymer, but - as for porous membranes - filmforming properties, mechanical and thermal stability form the basis of applicability (cf. Section 2.3.2.1). The following characteristics should be considered ... 

Comprehensive monographs are also available detailing the analysis of mass transfer though porous and dense membranes. Standard textbooks [e.g., Refs. 26, 27] provide the basis for discriminating between various possible transport mechanisms and the selection of models capable of describing the processes in quantitatively. 

In the fourth subtechnique, flow FFF (F/FFF), an external field, as such, is not used. Its place is taken by a slow transverse flow of the carrier liquid. In the usual case carrier permeates into the channel through the top wall (a layer of porous frit), moves slowly across the thin channel space, and seeps out of a membrane-frit bilayer constituting the bottom (accumulation) wall. This slow transverse flow is superimposed on the much faster down-channel flow. We emphasized in Section 7.4 that flow provides a transport mechanism much like that of an external field hence the substitution of transverse flow for a transverse (perpendicular) field is feasible. However this transverse flow—crossflow as we call it—is not by itself selective (see Section 7.4) different particle types are all transported toward the accumulation wall at the same rate. Nonetheless the thickness of the steady-state layer of particles formed at the accumulation wall is variable, determined by a combination of the crossflow transport which forms the layer and by diffusion which breaks it down. Since diffusion coefficients vary from species to species, exponential distributions of different thicknesses are formed, leading to normal FFF separation. 

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. 

The permeation of a simple permeant, e.g., O2, through a polymeric membrane could occur, in principle, by two different mechanisms. One is the transport through pores, and the other is the transport through the free volume of polymer solid. The size of pore and its distribution is the most crucial parameter in the former case (porous membrane), and the value of a is determined by the molecular sizes of permeants A and B. The values of Ps are in the reverse order of the size of permeant, i.e., Pn2 > -P02 > -Pco2, and P can be dealt as a kinetic parameter. 

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