Mass transfer and pressure drop in ceramic foams: A description for different pore sizes and porosities

Mass transfer and pressure drop properties of alumina open-cell foams with pore counts between 10 and 45 PPI and porosities between 75% and 85% were studied in connection with their morphology. A combination of microscopic imaging, mercury porosimetry and magnetic resonance imaging allowed the determination of the pore sizes, strut diameters, void fractions and geometric surface areas of the foams. The mass transfer coefficients of the foams were measured by monitoring the CO oxidation over Pt/SnO2Pt/SnO2-coated foams in the temperature regime where external mass transfer is rate determining. The dimensionless correlation Sh=A·ReB·Sc1/3Sh=A·ReB·Sc1/3 showed a systematic variation of AA and BB parameters with the foam pore size and porosity. The observed anisotropy of the foam structures required the implementation of an additional geometrical factor to obtain a unifying description of the dimensionless mass transfer coefficients for foams with different pore densities and void fractions: Shfoam=1.00·Re0.47·Sc1/3·(Dp/0.001m)0.58·εh0.44. This equation was also applicable to a metallic foam (40 PPI, 95% porosity). Pressure drop data could be correlated with the superficial velocity by means of the Forchheimer equation. Empirical equations for the calculation of the viscous and inertial permeability as a function of the morphological parameters were derived. The pressure drop was very sensitive to imperfections within the foam packing, leading to considerable deviations of up to ±20%±20% for foam samples having apparently the same morphology. The analogy between the mass and momentum transfer within foams was successfully evaluated with the Lévêque equation. Hence, mass transfer coefficients of foams can be estimated from pressure drop data, the latter being available with much less experimental effort.