High-temperature combustion of CH_4 over PdO/Al_2O_3: kinetic measurements in a structured annular reactor

摘要

The potentialities of a structured annular reactor for kinetic measurements of very fast catalytic combustion reactions have been theoretically and experimentally investigated. An optimal design of the reactor geometry, consisting of one internal ceramic tube coated with a thin catalyst layer and one external quartz tube co-axially placed, has been preliminarily defined through mathematical modelling to achieve a minimal impact of diffusion phenomena on kinetic measurements. The theoretical analysis has identified the internal diffusion as the most critical phenomenon, whose effect can be minimized only by using a very thin catalyst layer ( ~ 10 ~). Such a 10 ~m Pd(10% w/w)/AI2O3 coating has been obtained through a proper washcoat technique and preliminary tests at GHSV = 3 x 106 cm3/(gh) STP have demonstrated the possibility to obtain kinetic data at partial conversion up to 600°C Wl'th negligible pressure drop and very small temperature gradients across the catalyst. An extensive kinetic study has then been performed in the 400-600°C temperature range including the effects ofH20,CH4 and O2. The data have shown that a strong inhibiting effect of water, already reported in the literature, still prevails up to 600°C. Experimental data have been satisfactorily fitted through the following pseudo-LHHW rate expression: R", = K,CCH./(l + KH,oCH,O).