Spatial and temporal profiles in millisecond partial oxidation processes
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Catalysis Letters Vol. 110, Nos. 3–4, September 2006 ( 2006) DOI: 10.1007/s10562-006-0117-8
Spatial and temporal profiles in millisecond partial oxidation processes Raimund Horn, Nick J. Degenstein, Kenneth A. Williams, and Lanny D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455-0132, USA
Received 18 April 2006; accepted 14 June 2006
Methods are presented to measure axial species and temperature profiles within catalytic partial oxidation foam monoliths at atmospheric pressure with 0.3 mm spatial resolution using a capillary sampling technique with a quadrupole mass spectrometer. The system allows sampling within the catalyst with negligible interference in flow or temperature by using a 0.6 mm quartz capillary containing a thermocouple and possessing a 0.3 mm side orifice. The capillary tightly fills a concentric channel drilled within the 10 mm long ceramic foam minimizing gas bypass. This technique has been used to measure axial catalyst species profiles at temperatures up to 1300 C for catalytic partial oxidation of methane and ethane to synthesis gas and ethylene, respectively. CH4 and O2 conversion are approximately twice as fast on Rh than on Pt. For C2H6 the reaction products at the catalyst entrance are H2, H2O, CO, and CO2. Ethylene production begins only after 4 mm into the catalyst after most of the O2 has reacted. Transient operation where the feed composition is varied stepwise between different C/O ratios has also been used to characterize these systems. The capillary sampler has a time resolution of 0.05 s, and C/O step changes within 0.5 s have been achieved using mass flow controllers. For switches from C/O = 0.6 to 1.4, sharp overshoots are observed for syngas (H2 and CO) and similar undershoots for combustion products (H2O and CO2). By placing the sampling orifice at different positions and stepping the C/O ratio, spatio-temporal profiles can be obtained. Spatio-temporal profiles are extremely important in validating detailed reaction mechanisms because their information content is much higher compared to integral steady state measurements at the reactor outlet. The spatial profiles show where and how quickly different species are formed or consumed along the catalyst axis. Transient profiles provide additional diagnostics of mechanisms and surface coverages because they show how temperature and species concentrations follow a perturbation from steady state. KEY WORDS: catalytic partial oxidation; spatial profiles; spatio-temporal profiles; mass spectrometry; methane; ethane; syngas; ethylene; platinum; rhodium.
1. Introduction Autothermal millisecond catalytic partial oxidation (CPO) reactors have great promise to replace steam reformers and steam crackers for production of hydrogen and olefins, respectively, because they operate at much shorter residence times, require much simpler equipment, and can be scaled up or down for different applications [1–4]. In spite of several decades of research on millisecond reactors, many questions remai
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