Fluid flow in highly porous anisotropic graphites

In a previous work (Celzard A and Mareche J F 2001 J. Phys.: Condens. Matter 13 4387), the properties of transport in the pore space of a sample of expanded graphite compressed to a density of 140 kg m(-3) were investigated. It was found from gas permeability, ion diffusion and mercury porosimetry that the porous structure could be modelled by cylindrical pores of length l and diameters delta such that l = delta. In this context, the theory of Katz and Thomson (Katz A J and Thompson A H 1986 Phys. Rev. B 34 8179) and that of Johnson et al (Johnson D L, Koplik J and Schwartz L M 1986 Phys. Rev. Lett. 57 2564) were both shown to lead to very good agreement between the estimates and the experimental results. In the present paper, these studies are continued for a wide range of highly porous monohthic cubes of different densities. The latter are seen to have strongly porosity-dependent permeabilities (k), formation factors (F) and anisotropies. It is evidenced that the calculation of the formation factors may be achieved from the experimental values of the permeability in the framework of Johnson, Koplik and Schwartz theory, based on the l = delta model. The calculated values are found to be in good agreement with the measured ones. Also, the expected relationship k similar to F-2 is recovered. Hence, it is suggested that the properties of transport in the pore space of such highly porous graphites may be completely accounted for by the above models. Finally, the critical pore radii delta(c) as defined by Katz and Thomson are calculated; the theoretical expression kF similar to delta(c)(2) is checked with our materials.