Transport Properties of Reconstructed Alumina and Vycor Membranes
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AA7.7.1
Transport Properties of Reconstructed Alumina and Vycor Membranes M.E. Kainourgiakis, E.S. Kikkinides1, Th. A. Steriotis and A.K. Stubos National Center for Scientific Research “DEMOKRITOS”, 15310 Ag. Paraskevi Attikis, Athens, GREECE. 1 Chemical Process Engineering Research Institute, CE.R.T.H., P.O. Box 361, Thermi-Thessaloniki 57001, GREECE.
ABSTRACT Aim of the present work is the investigation of the transport properties (Knudsen and molecular diffusivity, permeability) of 3-dimensional binary domains that represent two model mesoporous materials, namely Vycor glass and alumina membrane. A process-based technique producing a random packing of equal spheres is used for the reconstruction of the alumina membrane, while a stochastic reconstruction procedure is employed in the case of Vycor. The comparison between computed and reported transport coefficients confirms that the random sphere pack represents quite well the porous structure of the alumina membrane, while the stochastic reconstruction technique is sufficient for an accurate representation of the porous matrix of Vycor glass. INTRODUCTION The digital reconstruction of porous materials is a relatively new, powerful methodology that enables the reliable representation of the complex biphasic structure induced by porosity. The significant advances in both experimental and numerical practices have allowed the development of reconstruction techniques, the most advantageous being (a) the statistical methods [1-6] leading to the stochastic reconstruction of porous media in three dimensions based on statistical information obtained from one or several two dimensional images of thin sections of the actual material, and (b) the process-based methods [7,8], where the computational procedure tries to imitate the physical processes that commence during the formation of the medium. Although the latter seem to be closer to the physical system, they frequently exhibit severe computational requirements and hence are limited to the specific material considered in each case. In the present work, a sphere packing procedure is used as a process-based method for the representation of alumina mesoporous membranes made by compaction of non-porous monodisperse alumina microspheres. On the other hand, a stochastic reconstruction technique generates 3D images of Vycor porous glass with the same statistical content as the actual material, in terms of porosity and auto-correlation function. The resulting binary matrices are subsequently used, for the calculation of the corresponding transport properties, namely the diffusivity (in the Knudsen and the molecular regime) and the permeability. The diffusivity is determined by the application of random walk algorithm while the permeability by the numerical solution of Stokes’ equation.
AA7.7.2
RECONSTRUCTION OF POROUS STRUCTURE The way matter is distributed in a porous medium is reflected by the phase function Z(x), defined as follows: 1 , if x belongs to the pore space (1) Z (x ) = 0 , otherwise where x is the positi
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