Gas transport through porous

The dusty gas model (DGM) [21] is used most frequently to describe multi component transport in between the two limiting cases of Knudsen and molecular diffusion. This theory treats the porous media as one component in the gas mixture, consisting of giant molecules held fixed in space. The most important aspect of the theory is the statement that gas transport through porous media (or tubes) can be divided into three independent modes or mechanisms ... 

The flux expressions for gas transport through porous membranes have been considered in Section 3.I.3.2.4. The steady state Knudsen diffusion flux expression (3.1.115a),... 

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. 

The mechanism of gas transport through the porous hydrophobic layer of the developed air gas-diffusion electrodes is theoretically and experimentally investigated [5],... 

Chapter 3 described a new model for transport through porous media, developed recently by Kerkhof [5] and called the binary friction model (BFM). It is of interest to see how this model can be applied to the description of available experiments and to compare the results with those of the dusty gas model (DGM). Kerkhof [5] took the experimental data of Evans et al. [6,7] for the permeation of He and Ar through a low-permeability porous graphite septum. The experimental set-up, similar to the Wicke-Kallenbach diffusion cell, is sketched in Figure 9.7. Of interest are the steady... 

To model the transport of a multicomponent mixture through porous catalysts, flux relations should be developed. There are two geometrical models of gas transport in porous media with different trends of flux relations (Jackson, 1977). These models are the capillary network model and the dusty gas model. 

Figure 24.8 shows the temperature dependence of viscosity for different fluids. In general, the viscosity of most liquid organic chemicals decreases by about 1% for a temperature increase of 1 °C (Davis, 1997 Poling, Prausnitz, and O Connell, 2001). Gas viscosities tend to be one to two orders of magnitude less than liquid viscosities but increase proportionally with temperature. Typically, the viscosity of a gas will increase about 30% with a temperature increase of 100°C (Davis, 1997), facilitating transport through porous media. 

Other properties related to fluid and energy transport are viscosity, sorption, vapor transport, and hydrolysis. Gas viscosities will increase about 30 % with an increase of the temperature around 100 °C [28], facilitating transport through porous media. 

The importance of structural effects in gas diffusion electrodes was realized long before the development of the current generation of CLs for PEFCs. The basic theory of gas diffusion electrodes, including the interplay of reactant transport through porous networks and electrochemical processes at highly dispersed electrode I electrolyte interfaces, dates back to the 1940s and 50s [13, 14]. Later work realized the importance of surface area and utilization of electrocatalysts in porous electrodes [15]. A series of seminal contributions by R. De Levie opened... 

The permeation of gas mixtures through porous membranes provides more stringent test of the transport mechanism than the single gas permeation. If the transport occurs by the Knudsen diffusion, the selectivity based on the single gas experiments and that for the gas mixture should be equal. 

The permeation flux expressions (3.4.76) and (3.4.81a) are valid for membranes whose properties do not vary across the thickness. Most practical gets separation membranes have an asymmetric or composite structure, in which the properties vary across the thickness in particular ways. Asymmetric membranes are made from a given material therefore the properties varying across Sm are pore sizes, porosity and pore tortuosity. Composite membranes are made from at least two different materials, each present in a separate layer. Not only does the intrinsic Qim of the material vary from layer to layer, but also the pore sizes, porosity and pore tortuosity vary across Sm- At least one layer (in composite membranes) or one section of the membrane (in asymmetric membranes) must be nonporous for efficient gas separation by gas permeation. The flux expressions for such structures can be developed only when the transport through porous membranes has been studied. 

Flow of trains of surfactant-laden gas bubbles through capillaries is an important ingredient of foam transport in porous media. To understand the role of surfactants in bubble flow, we present a regular perturbation expansion in large adsorption rates within the low capillary-number, singular perturbation hydrodynamic theory of Bretherton. Upon addition of soluble surfactant to the continuous liquid phase, the pressure drop across the bubble increases with the elasticity number while the deposited thin film thickness decreases slightly with the elasticity number. Both pressure drop and thin film thickness retain their 2/3 power dependence on the capillary number found by Bretherton for surfactant-free bubbles. Comparison of the proposed theory to available and new experimental... 

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. 

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