Effect of the Characteristic Dimension of Catalytic Wall Microchannels and Microslits on the Performance of Microreactors Coupling the Methane Steam Reforming and Combustion Reactions: A CFD Simulation Study

Effect of the Characteristic Dimension of Catalytic Wall Microchannels and Microslits on the Performance of Microreactors Coupling the Methane Steam Reforming and Combustion Reactions: A CFD Simulation Study

G. Arzamendi¹, I. Uriz¹, P.M. Diéguez¹, M. Montes², M.A. Centeno², J.A. Odriozola³, and L.M. Gandía^1,*

¹Departamento de Química Aplicada. Universidad Pública de Navarra, Campus de Arrosadía s/n, E-31006 Pamplona. Spain

²Grupo de Ingeniería Química, Departamento de Química Aplicada, Facultad de Ciencias Químicas de San Sebastián, UPV/EHU, Paseo Manuel de Lardizábal 3, 20018 San Sebastián, Spain

³Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, 41092 Sevilla, Spain

^* Corresponding author. E-mail address: lgandia@unavarra.es (L.M. Gandía)

Both H₂ and syngas (synthesis gas, a mixture of H₂ and CO) are expected to play an increasingly important role in our energetic system. H₂/syngas production technologies can be integrated in Natural Gas Combined Cycle plants for precombustion CO₂ capture, in advanced systems as the Integrated Gasification Combined Cycle with or without capture of CO₂ for future coal-based power plants and, of course, for synthetic liquid fuels production through the gas-to-liquids (GTL) and coal/biomass-to-liquids (CTL/BTL) processes [1]. Syngas can be used also as fuel for high-temperature solid oxide and molten carbonate fuel cells [2].

Steam reforming of natural gas in conventional packed-bed reactors is the preferred technology for commercial large-scale H₂ and syngas manufacture. However, for small-scale stationary or mobile/portable applications as well as production offshore or in remote areas, microreaction technology offers a convenient solution [3-5].

In this work, a Computational Fluid Dynamics (CFD) simulation study on the thermal integration of the endothermic steam reforming of methane (SRM) and exothermic methane combustion reactions in microstructured devices is presented. In a previous study on this system, the effects on the performance of catalytic wall microchannels of the gas streams space velocities, SRM catalyst loading and steam-to-carbon (S/C) ratio were investigated [6]. In this work, the effect of the characteristic dimension (d = 0.35, 0.70, 1.40 and 2.80 mm) of square microchannels of 20 mm of length on the microrreactor performance is shown. Moreover, microslides are also considered as a new geometry with very different aspect ratio compared with square microchannels. The characteristic dimension of microslides has been also varied between 0.35 and 2.80 mm whereas their length has been kept at 20 mm. CFD models of these geometries have been developed with commercial ANSYS^® CFX software considering both parallel and cross flow arrangement. It has been assumed that thin layers of typical Ni and Pd catalysts for the SRM and methane combustion reactions, respectively, have been deposited onto the walls of the microchannels or microslides. The loadings were established at 2-4 mg/cm² and 1 mg/cm² for the Ni and Pd catalysts, respectively. Heterogeneous catalytic reactions (SRM, water-gas shift-WGS, and methane combustion in air) have been modeled considering the inner walls as sources of products and sinks of reactants. Kinetic expressions of the relevant catalytic reactions have been taken from the literature [6]. The feedstreams consisted of methane and steam with S/C = 2 and 2 wt.% methane in air for the SRM and combustion reactions; the inlet temperature was set at 600ºC for both streams.

Steady state simulations have evidenced a marked influence of the characteristic dimension of both channels and slides on the microrreactor performance. For example, at GHSV of 10000 h^-1 the SRM methane outlet conversion decreases according to 99.9, 99, 96 and 87% as the microchannel characteristic dimension increases as 0.35. 0.70, 1.40 and 2.80 mm, respectively. In contrast, the selectivity was almost unaffected, resulting a reformate with a H₂/CO ratio close to 3.8. To take into account the very different surface-to-volume ratio associated to each dimension the performance was compared also on a WHSV basis. Although the differences decreased the performance still improved with the decrease of the characteristic dimension. For example, at WSHS of 13542 h^-1 the microchannels with d = 0.35 mm yielded 93% methane conversion while those with d = 2.80 mm gave 87%. An analysis of the results has shown that mass transport limitations became an issue for the largest characteristic size as evidenced by concentration profiles as the ones that can be seen in Figure 1. Therefore, decreasing the characteristic dimension has the advantages of a higher surface-to-volume ratio and a lower influence of transport limitations.

When comparing the two geometries considered in this work it is found that the performance of the microchannels is better than that of the microslides; however, the differences are small, of the order of 3-6 percent units of methane conversion at the reactor exit. This is due to the improved heat transfer characteristics and surface-to-volume ratio of microchannels compared with the microslides. Nevertheless, the microslides can be manufactured more easily and at lower cost so they are an interesting option to develop steam methane microreformers.

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Figure 1. Methane mass fraction normalized to the methane mass fraction in the MSR microchannels feed in the planes indicated.

Literature cited

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