Gas permeation in porous

GAS PERMEATION IN POROUS MEMBRANES 2.2.1 Types of Porous Membranes... 

For the transport of gas mixtures, the generalised Maxwell-Stefan equation (Krishna and WesseUngh, 1997) has been widely adopted to describe multi-component diffusion. Although quantitative descriptions of gas diffusion in various microporous or mesoporous ceramic membranes based on statistical mechanics theory (Oyama et al., 2004) or molecular dynamic simulation (Krishna, 2009) have been reported, the prediction of mixed gas permeation in porous ceramic membranes remains a challenging task, due to the difficulty in generating an accurate description of the porous network of the membrane. 

Figure 2.5 Gas permeation in the composite porous membranes (a) planar membrane (b) tubular membrane.

In the third part of the chapter the solid state properties of our block copolymer are examined. The surface energies of these materials are characterized by contact angle measurements. The organization of the polymer chains in the solid state phase is investigated by small-angle X-ray scattering (SAXS) and the gas selectivity of porous membranes coated with these block copolymers is characterized by some preliminary permeation measurements. 

Encouraged by the X-ray and contact angle results, we performed some preliminary gas-permeation measurements. Here, a self-supporting film is required and only the longer block copolymers were used. Fluoro-PSB-II and Fluoro-triblock were coated on porous Celgard 2400 membranes the measurements were taken at room temperature at a driving pressure of 5 bars. Since no absolute polymer layer thickness has been determined, only relative values of the permeability are given (Table 10.6). In the case of the separation of C02 from... 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

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. 

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

Membranes can be classified as porous and nonporous based on the structure or as flat sheet and hollow fiber based on the geometry. Membranes used in pervaporation and gas permeation are typically hydrophobic, nonporous silicone (polydimethylsiloxane or PDMS) membranes. Organic compounds in water dissolve into the membrane and get extracted, while the aqueous matrix passes unextracted. The use of mircoporous membrane (made of polypropylene, cellulose, or Teflon) in pervaporation has also been reported, but this membrane allows the passage of large quantities of water. Usually, water has to be removed before it enters the analytical instrument, except when it is used as a chemical ionization reagent gas in MS [50], It has been reported that permeation is faster across a composite membrane, which has a thin (e.g., 1 pm) siloxane film deposited on a layer of microporous polypropylene [61],... 

In order to predict correctly the fluxes of multicomponent mixtures in porous membranes, simplified models based solely on Fields law should be used with care [28]. Often, combinations of several mechanisms control the fluxes, and more sophisticated models are required. A well-known example is the Dusty Gas Model which takes into account contributions of molecular diffusion, Knudsen diffusion, and permeation [29]. This model describes the coupled fluxes of N gaseous components, Ji, as a function of the pressure and total pressure gradients with the following equation ... 

The permeation of a gas through a porous polymer is generally described by equations based on the kinetic theory of gases. The sorption isotherm described by Eq. 1 is concave to the pressure axis and is commonly observed for a penetrant gas in a glassy polymer. It is composed of Henry s law and Langmuir-terms [20] ... 

The constitutive equations of transport in porous media comprise both physical properties of components and pairs of components and simplifying assumptions about the geometrical characteristics of the porous medium. Two advanced effective-scale (i.e., space-averaged) models are commonly applied for description of combined bulk diffusion, Knudsen diffusion and permeation transport of multicomponent gas mixtures—Mean Transport-Pore Model (MTPM)—and Dusty Gas Model (DGM) cf. Mason and Malinauskas (1983), Schneider and Gelbin (1984), and Krishna and Wesseling (1997). The molar flux intensity of the z th component A) is the sum of the diffusion Nc- and permeation N contributions,... 

The design of a typical commercial reactor for large-scale production of materials is similar to the laboratory setup, except that the capacity of the former is larger, up to 30 liters. Since the synthesis of materials produced commercially is well understood, most reactors are not equipped with optical windows to monitor the process. A schematic diagram of such a reactor is shown in Fig. 5. Typically, it is a thick-walled stainless steel cylinder that can be water cooled (Borovinskaya et al., 1991). The green mixture or pressed compacts are loaded inside the vessel, which is then sealed and evacuated by a vacuum pump. After this, the reactor is filled with inert or reactive gas (Ar, He, Nj, O2, CO, C02). Alternatively, a constant flow of gas can also be supplied at a rate such that it permeates the porous reactant mixture. The inner surface of the reactor is lined with an inert material to... 

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. 

In both explosives and propellants the materials are relatively non-porous. The binders and plasticizers used effectively fill the pore spaces. Pyrotechnics are porous and the heat transfer related to the hot gas permeation into the reactant material mixture becomes important. In many pyrotechnics no binders or plasticizers are used. The explosives and propellants have burning (or detonation) rates that depend on density, temperature, and pressure. However, the burning rates of pyrotechnics are, in addition, affected by porosity, particle sizes, purity, homogeneity (degree of mixing), and stoichiometry (fuel or oxidizer ratio). 

Gavalas et al. [7] prepared ZSM-5 membranes onto porous a-alumina disks by in-situ hydrothermal synthesis at 175°C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 pm thick and consisted of well-intergrown crystals of about 2 pm in size Pure gas permeation measurements of the best preparations yielded hydrogen isobutane and butane isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185°C, respectively. 

Basic mechanisms involved in gas and vapor separation using ceramic membranes are schematized in Figure 6.14. In general, single gas permeation mechanisms in a porous ceramic membrane of thickness depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. In membranes with large mesopores and macropores the separation selectivity is weak. The number of intermolecular collisions is strongly dominant and gas transport in the porosity is described as a viscous flow that can be quantified by a Hagen-Poiseuille type law ... 

In summary, one can see that separation selectivity for gas and vapor molecules depends on the category of pores (mesopores, supermicropores, and ultramicropores) and on the related transport mechanisms. Either size effect or preferential adsorption effect (irrespective of molecular dimension) is involved in selective separation of multicomponent mixtures. The membrane separation selectivity for two gases is usually expressed either as the ratio between the two pure gas permeation fluxes (ideal selectivity) or between each gas permeation flux measured from the mixture of the two gases (real selectivity). More detailed information on gas and vapor transport in porous ceramic membranes can be found in Ref. [24]. 

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