A general correlation for mass transfer in isotropic and anisotropic solid foams G. Incera Garrido, S. Lang, F. Patcas, B. Kraushaar-Czarnetzki, Institute of Chemical Process Engineering, University of Karlsruhe, Germany The aim of this research was the analysis of mass transfer from the gas phase to the outer surface of foams in a wide hydrodynamic range. We investigated ceramic and metallic foams with three different porosities and four pore densities as well as a honeycomb and packings of spheres for comparison. The catalytic oxidation of CO was chosen as a diagnostic tool. This reaction was carried out at 220 °C, at which temperature external mass transfer is rate limiting. The reaction unit was a plug flow system with external recycle allowing for the variation of fluid velocity. The first step was a detailed analysis of the network structures of the various foams because any further property, be it mass (or heat) transfer or pressure drop is related to the morphology. Finally, mass transfer and pressure drop data were assessed from all types of carriers, and a dimensional analysis was performed to obtain relations between mass transfer and hydrodynamic properties. A dimensional analysis was performed, aiming at the development of general quantitative relations between the mass transfer properties in foams and the related dimensionless products of the phenomenon-influencing variables. The Sh-values of all structures were compared at equal fluid properties (Sc) by plotting over Re. The data for packed beds and honeycombs were in good agreement with literature correlations and served as a validation of the experimental method used (Fig. 1). The Sh-values of the foams increase with the size of the pores. In contrast to particle packings, who's Sh-numbers can be generally described with Re and Sc alone, an additional geometric parameter is effective in foams. The reason for this phenomenon is the anisotropy of the cells, i.e. the deformation of the strut network caused upon fabrication of the foams by replication which includes squeezing of the greenbodies. With the additional characterization of pore diameters in the x-z and y-z planes, the anisotropy of the pores in space could be quantified. A pore anisotropy ratio was defined by dividing the longest pore length by the two shorter ones, a2/(b∙c) and included in the dimensionless description of the mass transfer properties of the sponges (Fig. 2). 100 Dwivedi & 100 3,9 mm beads Correlation 10ppi_0.85 Upadhyay CI Upper L 20ppi_0.85 CI Lower L Shsponges/(Sc1/3*Fg,A) [-] 30ppi_0.85 45PPI_0.85 45ppi_0.85 30PPI_0.85 20PPI_0.85 10ppi_0.8 10PPI_0.85 20ppi_0.8 45PPI_0.8 Sh [-] 10 30ppi_0.8 10 30PPI_0.8 45ppi_0.8 20PPI_0.8 20ppi_0.75 Hawthorn 10PPI_0.8 20PPI_0.75 anisotropy function 400CPSI HC 0.84 a2 Fg,A bc εh 0.43 1 1 1 10 100 1000 10000 1 10 100 1000 10000 Re [-] Re [-] Fig. 1. Experimental Sh and Re data of all analyzed Fig.2. General correlation for the prediction structures and literature correlations. of mass transfer properties of foams including an anisotropy function Based on the Lévêque-analogy, mass transfer data could be correlated with the experimental pressure drop of the structures within 20 %, confirming the possibility to use this analogy for a first estimation of mass transfer coefficients from pressure drop data. However, more accurate values of mass transfer coefficients are determined with the correlation presented above.
Pages to are hidden for
"Development-of-a-general-correlation-for-the-prediction-of-mass-"Please download to view full document