Safe Operation of Catalytic Reactors for the Oxidation of VAM Operating Under Abnormal Reaction Conditions

The catalyst Pd/TS-1 has shown excellent activity in oxidising ventilation air methane (VAM) and excellent resistance to water vapor poisoning. This report outlines the project undertaken to evaluate the inherent safety of a catalytic system for the mitigation of fugitive VAM emissions. The two safety aspects of most interest when considering industrial implementation of the proposed technology are the potential for autoignition of the stream if methane concentration increases into the flammability range or if the bed temperature becomes too high, and if carbon monoxide is formed through partial or incomplete oxidation of the methane. These two conditions were examined using a microreactor with a typical total flowrate of 300 mL/min and methane concentrations ranging from 0.1% to 15%. The associated telemetry was measured via thermocouples and capacitance manometers, which were utilised to record temperature and pressure data from the reactor bed, with gas chromatography (GC) used to analyse the product stream and measure methane conversion under different conditions.

Pd/TS-1 has shown excellent stability and activity while under a high range of methane concentrations. When methane concentrations were increased, an expected commensurate reactor temperature was observed owing to the increased reaction exotherm, however this was noted as a slow change up to steady state. No significant pressure changes were observed, further demonstrating the stable environment in the reactor while the catalyst is oxidising methane. It was noticed that the history of the catalyst can change the conversion as the concentration decreases from elevated methane concentrations the recorded conversion is lower than the same point reached when increasing methane content. This was assumed to be due to a significant amount of water vapour being produced when methane concentrations were high, shifting the equilibrium on the surface and temporarily reducing catalyst activity. Once concentration returned to lower levels, excess water species on the catalyst surface were removed and the catalyst activity returned to initial values.

It was shown that even under elevated methane concentrations, the catalyst would not produce carbon monoxide unless the stoichiometric limit between methane and oxygen was reached. The catalyst showed excellent ability to oxidise very lean VAM streams, with above 90% conversion achieved for methane concentrations of 0.1% - 0.5%. The catalyst also exhibited an inherent safety when reactor temperatures exceeded autoignition temperatures with no obvious sign of spontaneous ignition being observed.