Properties of ceramic foam catalyst supports: one-dimensional and two-dimensional heat transfer correlations

Ceramic foam catalyst supports in structured reactors promise higher external surface areas, lower pressure drop and increased heat transfer than beds of packed particles, especially for reactions limited to the external surface. Heat transfer is an important factor in highly endothermic and exothermic reactions, with the latter more compelling since hot spots, once started, propagate through the bed and lead to catalyst deactivation and decreased selectivity. In this research, we report experimental data from which heat transfer parameters are determined for one- and two-dimensional models, suitable for reactor modeling and analysis. In the 2-D experiments, axial temperature profiles in a 1.27 cm bed of 30-PPI alpha-Al2O3 foam, covering a temperature range of 25-400degreesC and up to air flow rates of 7 SLPM, were used to derive a correlation for the overall heat transfer coefficient comprising radiation and convective terms. The convective term is proportional to the Reynolds number and is greater than that of an equivalent bed of glass spheres by about a factor of two. This convective term is also sensitive to the roughness of the surface, since adding gamma-Al2O3 washcoat resulted in a further increase by about a factor two. In the 2-D measurements, at 5 cm diameter cylinder was embedded with thermocouples to measure the radial temperature profile a various axial positions. Data for 150-550 degreesC and 3-20 SLPM were used to extract the effective bed conductivity and the wall heat transfer coefficient as functions of the temperature and Reynolds number. Comparison of these parameters with beds of particles and structured packing show the foam gives the highest effective bed conductivity and external surface areas, confirming that foams are excellent candidates for reactions occurring on the surface cf non-porous surfaces, resulting in high activities per unit volume, low-pressure drop and superior heat transfer. Enhanced mixing and turbulence indicates less susceptibility to hot spot development in exothermic reactions with foams, especially when compared to monolith supports. (C) 2004 Elsevier B.V. All rights reserved.