Gas separation in porous membranes

When gas iranspon takes place by viscous flow (as in the case of a microfiltration membrane, for example), no separation is achieved because the mean free path of the gas molecules is very small relative to the pore diameter. By decreasing the pore diameter of 

Pick s law is the simplest description of gas diffusion through a nonporous structure, i.e. 

The concentrations are related to the partial pressures by Henry s law which states that a linear relationship exists between the concentration inside the membrane (Cj) and tbe (partial) pressure of gas outside the membrane (pj), Le. 

VI - 46 show s that the flow rate across a membrane is proponional to the difference in (partial) pressure and inversely proportional to the membrane thickness. The ideal selectivity is given by the ratio of the permeability coefficients  

With a number of gaseous mixtures, the real separation factor is not equal to the ideal separation factor because of plasticisation which may occur at high (partial) pressures when a permeating gas exhibits a high chemical affinity for the polymer. Because of such plasdcisation, the permeability increases but the selectivity decreases generally. 

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Modelling Gas Separation in Porous Membranes 95 5.7 J, Resistance in Series Transport Model... 

Modelling Gas Separation in Porous Membranes 99 Table 5.2 Lennard-jones constants, molecular masses and average velocities at room... 

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 this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Separative flow often occurs in a packed bed—typically a tube filled with a granular material. Chromatography in packed columns is the most important example of packed-bed flow. Similar flow is found in porous membranes used for membrane separation. The fluid flowing through such media can be a gas, a liquid, or a supercritical fluid. 

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). 

Ceramic membrane is the nanoporous membrane which has the comparatively higher permeability and lower separation fector. And in the case of mixed gases, separation mechanism is mainly concerned with the permeate velocity. The velocity properties of gas flow in nanoporous membranes depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collision. The Knudsen number Kn Xydp is characteristic parameter defining different permeate mechanisms. The value of the mean free path depends on the length of the gas molecule and the characteristic pore diameter. The diffusion of inert and adsorbable gases through porous membrane is concerned with the contributions of gas phase diffusion and sur u e diffusion. 

Considering at first transport mechanisms in porous membranes, viscous flow (Fig. 9), also called Poiseuille flow, takes place when the mean pore diameter is larger than the mean free path of gas molecules (pore diameter higher than a few microns), so that collisions between different molecules are much more frequent than those between molecules and pore walls. In such conditions, no separation between different molecules can be attained [45]. 

Although these two expressions have the same form, the coefficients are different. In particular, the solubility constant varies much more between various gas solutes than does the diffusion coefficient D. This implies that polymer membranes tend to be much more selective in separation various gas species than porous membranes. Unfortunately, diffusion coefficients in solids and liquids are much smaller than for gases so this increase in selectivity is often traded off with lower permeation rates. 

Sircar, S. and Rao, M.B. (2000). Nano-porous carbon membranes for gas separation. In Recent Advances on Gas Separation by Micro-Porous Membranes (N. KaneUopoulos, ed.). Elsevier, pp. 473-6. 

Solution diffusion — gas dissolves in the membrane material and diffuses across it. The membranes used in most commercial appHcations are non-porous in structure where separation is based on the SD mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The model assumes that each component is sorbed by the membrane at one interface, transported by diffusion across the membrane through the voids between the polymeric chains (the so-called free volume ) and desorbed at the other interface. According to the SD model, the flux of gas through a membrane is given by... 

In comparison to other polymer membranes, soluble porous networks hold an advantage in gas separations in the form of thin solution-cast films. Linear PIMs are soluble and thus good candidates in this regard. The introduction of triazole groups in PIMs (TZPIMs) by post-synthetic modification favors CO2 uptake and greater selectivity for gas separation. TZPIM membranes exhibit exceptional selective permeation as polymeric membranes for gas mixtures like CO2 and N2. 

Fouling occurs mainly in micronitration/ultraiiltration where porous membranes which are implicitly susceptible to fouling are used. In pervaporation and gas separation with dense membranes, fouling is vinually absent. Therefore, pressure driven processes will be emphasized but also here the type of separation problem and the type of membrane used in these processes detemiine the extent of fouling. Roughly three types of foulant can be distinguished ... 

The efficiency of membrane separation increases with the permeability and the selectivity. Thin membranes are economic, since according to Equation (2.1) the gas flow is inverse proportional to the layer thickness. However thin polymeric films, which have favorable permeability and selectivity, are too weak to withstand the high pressure difference between permeate and retentate side. The economic breakthrough set in with the production of ultrathin compound polymeric membranes. These are designed as hollow fibres with a thick porous back-up layer for mechanical stability and a thin dense non porous membrane layer for gas separation. The porous layer only has a slight influence on gas separation. These hollow fibres are combined in a bundle, which is arranged in a cylindrical container [2.13]. Several of these bundles, also called modules, can be added to... 

Non-porous polymeric membranes are usually employed for gas separation, although porous ones can also be used. Composite polymeric membranes developed in the 1970s made the separation of gas streams commercially feasible. The first large-scale gas separation modules were developed by DuPont in early 1970s, but the first successful commercial membrane gas separation processes (PRISM) were announced by Monsanto in late 1970s. 

The diffusing flux through different membranes can be adequately described by Tick s law (Equation [8.4]), indicating that gas transport through porous membranes is driven by a cross membrane pressure gradient. Based on the differences in partial pressures, gas diffusivities, molecular sizes and shapes, gases can be separated when they flow through a membrane. 

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