Inorganic Microporous Membranes for Gas Separation in Fossil Fuel ...

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2 Theoretical background Figure 15 Flux resistance in each layer including defects in the functional membrane layer and in the optional intermediate layer Sol-gel derived microporous membrane mass transport is predominantly micropore diffusion. Mass transport in zeolite membranes is a combination of micropore diffusion and intercrystalline diffusion. Microporous membranes exhibiting the Knudsen permselectivity can be a result of mesoporous defects. Macroporous defects such as defects in the intermediate layer or intercrystalline pores can be identified by observing the contribution of viscous flow. A simplified overview of transport mechanisms is outlined in Table 10 and will be discussed below. Table 10 Transport mechanisms of gasses in different porous media. 38 Transport mechanism Pore size and classification 36 Selectivity Viscous Flow > 20 nm Non-selective Knudsen Diffusion 2-100 nm 33 = M M 2 α ; M1
2 Theoretical background size, (viscous) properties of the transported species, activity of the pore or particle wall, applied pressure and temperature, texture and microstructure of the supports such as the connectivity of the pores, the porosity, the concentration and the connectivity of the defects, and the thickness of the support and supporting layers. Viscous flow Viscous flow occurs when mutual gas molecule interactions are more frequent than molecule-wall collisions. Convective flow of the gas molecules occurs when a pressure gradient is applied over the membrane. This transport mechanism is referred as viscous flow due to the dominant transport factor relating to the collisions between the gas molecules as a function of their viscosity η. The flux can be described using the modified Poiseuille type law, 45 as seen in equation (9). 2 ε r P dP N = − (9) τ 8η RT dz The term N is the molar flux in mol/m 2 s, r is the pore radius in m, P is the pressure in Pa, R is the gas constant, and T is the gas temperature in K. The geometric factors are the porosity ε and the tortuosity τ, in which the latter can present a ratio of the effective distance that the species travels per membrane thickness. No significant gas selectivity can be obtained using viscous flow. Knudsen diffusion Knudsen diffusion occurs in the regime where the mean free path length of the gas is largely compared to the pore size, which is typical in mesoporous materials. 45 The molecule-wall collisions are more frequent than the gas molecule interactions. Knudsen diffusion is dependent on the molar mass (M in kg/mol) of the gas species (Table 10) and the thickness of the membrane layer (L in m), see equation (10) (valid for ideal gasses in cylindrical pores). ε 2 r 8 N = − ∆P (10) τ 3 L πMRT Defects contribution Intercrystalline defects in zeolite membranes and defects in sol-gel derived membrane layers can be large. The combination of both transport mechanisms where the gas molecule to molecule collisions are in the same order of magnitude as the gas molecule to 34
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Page 6 and 7: List of Abbreviations Ac Acetyl Ace
Page 8 and 9: Summary CO2 capture and storage mig
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Page 14 and 15: 1 Introduction and aims 1 Introduct
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4 Results and discussion In summary
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4 Results and discussion Intensity
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4 Results and discussion nanonic YS
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4 Results and discussion ~50nm TiO2
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4 Results and discussion Raman Inte
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4 Results and discussion Tungsten 5
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4 Results and discussion Mol% Carbo
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4 Results and discussion Permeance
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4 Results and discussion He Permean
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5 Conclusions and recommendations m
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6 References 22. Shao, Z., Dong, H.
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6 References 65. Van den Berg, A. W
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6 References 108. Wen, T. L., Heber
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Curriculum Vitae - George van der D
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Last but not least, I would like to
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Band | Volume 8 ISBN 978-3-89336-52
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