Heat transfer effects in microwave-heated heterogeneous catalytic reactions.

摘要

The kinetics of the CO oxidation reaction on Pt/{dollar}rm Alsb2Osb3{dollar} as well as Pd/{dollar}rm Alsb2Osb3{dollar} have been measured in a packed bed reactor heated by microwaves. The reaction rate was also measured in an identical reactor that was heated conventionally. Insertion of a thermocouple after the microwaves were switched off provided the best method for temperature measurement in this study. However, large-scale temperature gradients must be eliminated before this method can provide accurate temperature measurements. After modifications in our packed bed reactor to eliminate non-isothermality, the reaction kinetics for the microwave heated CO oxidation reaction were comparable to those for the conventionally heated reactor. The CO oxidation reaction therefore serves as an in-situ probe of metal surface temperature. These observations suggest the Pt and Pd crystallites in a supported catalyst are not significantly hotter than the alumina support in a microwave heated reactor.; A simple model is presented which estimates the temperature rise of 1 and 100 nm metallic particles typically found in a supported metal catalyst structure. The model, based on a simple steady-state energy balance, assumes that the particles only lose heat to the gas-phase, not the support matrix. This represents a best-case scenario for a temperature gradient relative to the surroundings. The model indicates that the temperature gradient is insignificant and this conclusion is supported by an experiment in which the microwave-driven carbon monoxide reaction acts as an in-situ temperature probe.; The methanol-steam reaction was used as a test reaction to study the effect of microwave heating on endothermic catalytic reactions. Mathematical models were derived and solved for: (1) a single catalyst pellet; (2) a one-dimensional tubular reactor; and (3) the two-dimensional tubular reactor where radial heat transfer effects were not negligible. The single catalyst pellet model indicated at gas temperatures near the operating limit of the catalyst, a significant increase in productivity could be achieved using microwave heating. The one-dimensional model also showed how microwaves could offer an advantage over interstage heating of adiabatic reactors for this endothermic reaction. Finally, the two-dimensional model indicated the homogeneous nature of microwave heating could minimize radial heat transfer effects. An energy balance on a conventional reactor versus a microwave heated reactor indicated an efficiency advantage for the microwave as axial heat transfer rates increased relative to the radial heat transfer case.