Gas transport mechanism

Measurements made in the Type I pile and the Type III pile show no depletion in oxygen or increase in carbon dioxide concentrations, indicating gas transport mechanisms are fast relative to the rate of the oxygen consumption and carbon dioxide production due to geochemical reactions. In the Covered pile, significant depletions in oxygen and increases in carbon dioxide concentration have been observed at some locations, suggesting gas transport rates are limited by the till... 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

Chanton, J.P., and Whiting, GJ. (1996) Methane stable isotopic distributions as indicators of gas transport mechanisms in emergent aquatic plants. Aquat. Bot. 54, 227-236. 

Isothermal chemistry in fuel cells. Barclay (2002) wrote a paper which is seminal to this book, and may be downloaded from the author s listed web site. The text and calculations of this paper are reiterated, and paraphrased, extensively in this introduction. Its equations are used in Appendix A. The paper, via an equilibrium diagram, draws attention to isothermal oxidation. The single equilibrium diagram brings out the fact that a fuel cell and an electrolyser which are the thermodynamic inverse of each other need, relative to existing devices, additional components (concentration cells and semi-permeable membranes), so as to operate at reversible equilibrium, and avoid irreversible diffusion as a gas transport mechanism. The equilibrium fuel cell then turns out to be much more efficient than a normal fuel cell. It has a greatly increased Nernst potential difference. In addition the basis of calculation of efficiency obviously cannot be the calorific value of the... 

West GD, Diamond GG, Holland D, Smith ME, and Lewis MH. Gas transport mechanisms through sol-gel derived templated membranes. J. Membr. Sci. 2002 203 53-69. 

For characterization of the micro structure small angle X-ray scattering (SAXS) experiments were carried out. The gas permeability was determined by measuring the stationary gas flow for helium and nitrogenwhich gives important information about the gas transport mechanisms. The electrochemical behaviour was studied by cj clic voltammetry and impedance spectroscopy. 

The measurement of the permeability of non adsorbed gases is classically used to determine the range of pore size in membranes (macro, meso or micropores). Indeed by plotting the permeability as a function of gas pressure, a straight line is usually obtained whose slope gives an indication of the gas transport mechanism in the membrane. A quantitative description of pore structure can be attempted from the results. 

The effects of molecular order on the gas transport mechanism in polymers are examined. Generally, orientation and crystallization of polymers improves the barrier properties of the material as a result of the increased packing efficiency of the polymer chains. Liquid crystal polymers (LCP) have a unique morphology with a high degree of molecular order. These relatively new materials have been found to exhibit excellent barrier properties. An overview of the solution and diffusion processes of small penetrants in oriented amorphous and semicrystalline polymers is followed by a closer examination of the transport properties of LCP s. 

What is the difference between diffusion and mass flow gas transport mechanism ... 

Clearly, a better understanding of the gas transport mechanisms in polymers would greatly facilitate the development of polymer membranes that exhibit both a higher selectivity and a higher (or lower) permeability to specified gases. It is beyond the scope of the present chapter to review this area of research, particularly since a number of extensive reviews are available [1,3-9,11-15,17,18]. 

Figure 3.4 Gas transport mechanisms for membrane-based gas permeation. Reproduced with permission from Ref. [156],...

Concerning the gas transport mechanisms, for dense membranes the permeation of a gas takes place through the bulk of its material and the transport can be described by a solution/diffusion mechanism. Otherwise, in the case of porous membranes, it takes place through its porous stmcture and the gas transport can be described by... 

Chang KS, Tung CC, Wang KS, Tung KL (2009) Free Volume Analysis and Gas Transport Mechanisms of Aromatic Polyimide Membranes A Molecular Simulation Study). Phys. Chem. B. 113 9821-9830. 

West G.D., Diamond G.G., Holland D., Smith M.E., Lewis M.H. Gas transport mechanisms through sol-gel derived templated membranes. J. Membr. Sci. 2002 203 53-69 Wu J.C.S., Sabol H., Smith G.W., Flowers D.L. Characterization of hydrogen-permselective microporous ceramic membranes. J. Membr. Sci. 1994 96 275-287 Xomeritakis G., Naik S., Braunbarth C.M., Cornelius C.J., Pardey R., Brinker C.J. Organic-templated silica membranes I. Gas and vapor transport properties. J. Membr. Sci. 2003 215 225-233... 

The typical gas transport mechanisms in porous membranes are molecular diffusion and viscous flow, capillary condensation, Knudsen diffusion, surface diffusion, and configurational or micropore-activated diffusion. The contributions of these different mechanisms depend on the properties of both the membrane and the gas under the operating temperature and pressure. Figure 2.3 illustrates schematically the gas transport mechanisms in a single membrane pore. 

Piroux, F., Fspuche, F., Mercier, R., Pineri, M., Gebel, G. (2002) Gas transport mechanism in sulfonated polyimides consequences on gas selectivity. Journal of... 

The TST is a well-established methodology for the calculation of the kinetics of infrequent events in numerous physical systems. According to the TST method the gas transport mechanism through a dense polymer system is described as a series of activated jumps. For each transition, a reaction trajectory leading from a local energy minimum to another through a saddle point in the configuration space is tracked, and the transition rate constant is evaluated. 

Therefore, there is a strong requirement to develop high-performance polymer membranes with superior thermal, chemical, mechanical and long-term stabilities for CO2 separation. Thus recently developed polymer materials are suggested to be the excellent candidates. In this chapter, gas transport mechanism with respect to sorption and diffusion through polymeric membrane materials will be introduced followed by a recent literature review on the chemical structure, physical properties, gas permeabilities and selectivities of membrane materials as well as up-to-date high performance and highly permeable membranes especially for CO2 separation. 

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