Permeability dense membranes

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). 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

Membranes made by interfacial polymerization have a dense, highly crosslinked polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less crosslinked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. The dense, crosslinked polymer layer, which can only form at the interface, is extremely thin, on the order of 0.1. im or less, so the membrane permeability is high. Because the polymer is highly crosslinked, its selectivity is also high. Although the crosslinked interfacial polymer layer determines membrane selectivity, the nature of the microporous support film affects membrane flux... 

Deposition from having both reactants introduced to the same side of the support yields thinner and more permeable dense membranes than that from the opposing-reactant geomeuy. 

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]. 

Compared to liquid-phase applications, commercial gas- or vapor-phase applications of inorganic membranes have been limited. Due to their low permeabilities, dense inorganic membranes arc utilized only in special and low-volume cases. Hydrogen... 

Microporous membranes. While dense metal or metal oxide membranes possess exceptionally good peimselectivities, their permeabilities are typically lower than those of porous inorganic membranes by an order of magnitude or more. Commercial availability of porous ceramic membranes of consistent quality has encouraged an ever... 

The flux of each component is proportional to the concentration gradient and the diffiisivity in the dense layer. However, the concentration gradient is often nonlinear because the membrane swells appreciably as it absorbs liquid, and the diffusion coefficient in the fully swollen polymer may be 10 to 100 times the value in the dense unswollen polymer. Furthermore, when the polymer is swollen mainly by absorption of one component, the diffusivity of other components is increased also. This interaction makes it difficult to develop correlations for membrane permeability and selectivity. 

This chapter aims to keep these challenges in mind as we review the defect chemistry, transport theory and aspects of characterization of hydrogen permeation in dense ceramics. We will first look at some applications and simple schemes of operation of hydrogen-permeable membranes and then, briefly, at the literature and status of hydrogen-permeable dense ceramics. 

Bhanushali et al. [22] showed differences between porous UF polymer membranes and dense reverse osmosis/NF membranes. According to these authors, permeability can be correlated with the inverse of the solvent viscosity for UF membranes whatever the nature of the solvent. For reverse osmosis/NF membranes, a permeation model is proposed in which the flux relates to a solvent permeability coefficient, accounting for a number of solvent intrinsic parameters, like molar volume V , the viscosity p, the sorption value O, and to an intrinsic parameter of the membrane (the solid-vapor surface tension ysv). 

The seminal discovery that transformed membrane separation fi-om a laboratory to an industrial process was the development in the 1960s of the Loeb-Sourirajan process to make defect-free ultrathin cellulose acetate membranes [1]. Loeb and Sourirajan were trying to use membranes to desalt water by reverse osmosis (RO). The concept of using a membrane permeable to water and impermeable to salt to remove salt from water had been known for a long time, but the fluxes of aU the membranes then available were far too low for a practical process. The Loeb-Sourirajan breakthrough was the development of an anisotropic membrane. The membrane consisted of a thin, dense polymer skin 0.2-0.5 pm thick sup-... 

In the case of the PDMS gas, the membrane permeability of CO2 decreased, but the selectivity of CO2 over CH4 was found to be remarkably improved irrespective of the plasma gas used (NH3, Ar, Nj, O2). The nitrogen plasma treatment seemed to give better selectivity than the ammonia plasma (Matsuyama et al. 1995). The NH3 and N2 plasma treatment of the dense PE (Nakata and Kumazawa 2006) and PP (Teramae and Kumazawa 2007) membranes increased both the permeation coefficient for CO2 and the ideal separation factor for CO2 relative to N2. The effects of both plasma gases are very similar. 

Lu Y P, Dixon A G, Moser W R, Ma Y H and Balachandran U (2000a), Oxidative couphng of methane using oxygen-permeable dense membrane reactors , Catal Today, 56,297-305. 

Kniep, J., Lin, Y. S. (2011). Oxygen- and hydrogen-permeable dense ceramic membranes. In V. V. Kharton (Ed.), Solid state electrochemistry I, fundamentals, materials and their applications (pp. 467—500). Springer (Chapter 10). 

RO membranes are dense semi-permeable membranes mainly used for desalination of sea water [38], Contrary to MF and UF membranes, RO membranes have no distinct pores. As a result, high pressures are applied to increase the permeability of the membranes [16]. The properties of the various types of membranes are summarized in Table 9.2. 

The permeation performance of dense membranes is often characterized in terms of a transmembrane flux, ji, at a given temperature and pressure gradient. Alternatively, a specific flux, /, can be defined as a normalized membrane flux based on the thicloiess and logarithm of the partial pressure gradient as a reminiscence of the Wagner equation. The specific flux can be related to the membrane permeability as follows ... 

Lu, Y, Dixon, A., Moser, W., et al (2000). Oxidative Coupling of Methane Using Oxygen-Permeable Dense Membrane Reactors, Catal. Today, 56, pp. 297-305. 

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