High-temperature combustion of CH4 over PdO/Al2O3: kinetic measurements ina structured annular reactor

The potentialities of a structured annular reactor for kinetic measurements of very fast catalytic combustion reactions have been theoretically and ex perimentally investigated. An optimal design of the reactor geometry, consi sting of one internal ceramic tube coated with a thin catalyst layer and on e external quartz tube co-axially placed, has been preliminarily defined th rough mathematical modelling to achieve a minimal impact of diffusion pheno mena on kinetic measurements. The theoretical analysis has identified the i nternal diffusion as the most critical phenomenon, whose effect can be mini mized only by using a very thin catalyst layer( less than or equal to 10 mu m). Such a 10 mum Pd(10% w/w)/Al2O3 coating has been obtained through a pro per washcoat technique and preliminary tests at GHSV = 3 x 10(6) cm(3)/(gh) STP have demonstrated the possibility to obtain kinetic data at partial co nversion up to 600 degreesC with negligible pressure drop and very small te mperature gradients across the catalyst. An extensive kinetic study has the n been performed in the 400-600 degreesC temperature range including the ef fects of H2O,CH4 and O-2. The data have shown that a strong inhibiting effe ct of water, already reported in the literature, still prevails up to 600 d egreesC. Experimental data have been satisfactorily fitted through the foll owing pseudo-LHHW rate expression: R-w = KrCCH4/(1 + KH2OCH2O). (C) 2001 El sevier Science Ltd. All rights reserved.