Experimental approach for identifying hotspots in labscale fixed-bed reactors exemplified by the Sabatier reaction

Isothermal conditions are a prerequisite for the accurate determination of kinetics in heterogeneous catalysis. In this work, an integral fixed-bed lab reactor with 5 mm inner diameter placed inside an aluminum heating block is used. Temperature profiles were recorded with a single but axially movable thermocouple in a fixed bed of a highly loaded supported Ni catalyst (15.4 wt% Ni on gamma-Al2O3) for the Sabatier reaction, i.e., the methanation of CO2, under elevated pressures and without carrier gas dilution. The corresponding conversions of CO2 were determined at the reactor outlet. Depending on the reaction conditions, temperature profiles can show significant deviations from isothermal conditions and even a light-off phenomenon with fluctuating conversions seems to occur. Based on our results, an experimental approach based on the use of one single thermocouple in the fixed bed is recommended which allows to exclude the occurrence of hotspots with a high degree of certainty.