Permeability in Dense Membranes

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

In the solution-diffusion separation mechanism, the permeating species dissolves in the membrane material and then diffuses responding to the chemical potential gradient. Consequently, the equation governing the solute flux is [24,25] 

Ds is the diffusivity of the solute in the membrane Cj is the solute concentration in the reject side ks is the solute distribution coefficient C2 is the solute concentration in the permeate side 

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The concentration gradient causes the diffusion in the direction of decreasing activity. Differences in the permeability in dense membranes are caused not only by diffusivity differences of the various species but also by differences in the physicochemical interactions of the species within the polymer. The solution-diffusion model assumes that the pressure within a membrane is uniform and that the chemical potential gradient across the membrane is expressed only as a concentration gradient. This mechanism controls permeation in polymeric membranes for separations. 

The permeability of dense membranes is low because of the absence of pores, but the permeance of Component i in Equation 10.20 can be high if SM is very small, even though the permeability is low. Thickness of the permselective layer is typically in the range 0.1 to 10 tm for gas separations. The porous support is much thicker than this and typically more than 100 tm. When large differences in PM exist among species, both high permeance and high selectivity can be achieved in asymmetric membranes. 

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. 

Transport and therefore separation mechanisms in porous inorganic membranes are distinctively different from and more varying than those prevailing in dense membranes. Because of the variety of the mechanisms that can be operative, these membranes in principle are capable of separating more varieties of compound mixtures. Compared to dense membranes, these porous membranes generally exhibit permeabilities of one or two orders of magnitude higher. For example, Pd-based membranes typically have a... 

Before we move on to more special cases, we take this opportunity to remind the reader of the many different pressure dependences we get for fluxes of dense membranes, and mixed conducting ones in particular. This is why permeabilities in dense mixed conducting membranes most often cannot be given as simply and in the same units as permeabUity in other types of membranes. We will return to the consequences of this when we quote example literature values in Section 1.9.1. 

In any case, what can be understood as dense membranes depends on the scale to which they are studied. In dense membranes, the performance (permeability, selectivity) is determined by the intrinsic properties of the material by solution-diffusion through the molecular interstices in the membrane material. When this is not the main mechanism, the membranes can be called porous. In this case, the selectivity is mainly determined by the dimension of the pores. Actually it can also... 

Among the dense membranes, those designed to perform gas separations are very important. In effect, there is a big market for gas separation through membranes. Of course, the material structure and, thus, synthesis processes must be considerably improved to reach optimization of selectivity and permeability for target gases. Long-term research has been dedicated to this aim. The fractional free volume in the most restrictive layer, usually that just on the active surface layer, plays a key role in the selectivity and permeability of dense membranes in gas separation. As a consequence it is very important to characterise the membrane surface. 

For this reason much work has been done at the ALZA Corporation and elsewhere to increase the water permeation rates by various technologies. For example, ALZA scientists utilized a composite membrane in the development of their first commercial product with this technology [21,22], In this system they first applied a CA membrane containing a high concentration of porosigens. A second dense membrane containing only CA was added. In this way the overall fluid permeability was increased, since the thickness of the dense portion of the film could be proportionately reduced. 

In Table 3, the membranes of capsules 2,7,8, and 10 are quite dense and have low permeability. In Table 4, capsule entries 2 and 4 are again relatively impermeable and are probably unsuitable for xenogeneic cell encapsulation. By comparison the alginate/cellulose sulfate//polydimethylene-co-guanidine/calcium chloride capsules seem to offer the most suitable MWCO (approximately 100 kD). This type of capsule is photographed in Fig. 2, with... 

A second class of membranes are described as dense membranes. They may consist of thin plates of metals (Pd and its alloys, Ag and some alloys) or oxides (stabilized zirconia or bismuth oxides, cerates). These membranes are permeable to atomic (for metals) or ionic (for oxides) forms of hydrogen or oxygen and have been studied, especially, in conjunction with chemical... 

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). 

As discussed earlier, many composite porous membranes have one or more intermediate layers to avoid substantial penetration of fme particles from the selective layer into the pores of the bulk support matrix for maintaining adequate membrane permeability and sometimes to enhance the adhesion between the membrane and the bulk support The same considerations should also apply when forming dense membranes on porous supports. This is particularly true for expensive dense membrane materials like palladium and its alloys. In these cases, organic polymeric materials are sometimes used and some of them like polyarilyde can withstand a temperature of up to 350X in air and possess a high hydrogen selectivity [Gryaznov, 1992]. 

For many industrial bulk processing applications, the purity of the hydrogen required can not justify the use of dense palladium or its alloy membranes due to their low permeabilities. In these cases, porous inorganic membranes are more often considered. 

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