(430c) New Reactor Configurations for Compact Methane Steam Reformers

In the current energy scenario, distributed hydrogen production (e.g. via biogas upgrade using compact fuel processors) represents a technological challenge. Methane steam reforming (MSR) is a well-established process, that is carried out industrially at severe operative conditions (i.e. high temperature, high pressure and very high flow rates) in order to fulfill the high thermal demand of the endothermic reaction. The direct downscale of the current industrial reactor technology, however, induces severe heat transfer limitations in the process, since the main heat transfer mechanism in packed bed reactors is associated with convection. The adoption of thermally conductive structured catalysts aimed at enhancing heat and mass transfer in non-adiabatic reactors is being investigated as a promising strategy to overcome these limitations. Even if application of conductive structured catalysts to the MSR process enables higher thermal process efficiency, as shown in the literature, particular attention must be paid to the design and manufacturing of the structured substrate due to the high temperatures required by the reaction.

In this context, another point of interest is represented by the substantial increase of distributed electric energy production observed in the last decade worldwide. The large availability of relatively low-cost renewable electric energy provides in fact a significant opportunity to decrease the carbon footprint of endothermic chemical processes [1]. In particular, the application of this concept to the MSR process would provide a twofold beneficial effect in terms both of reduction of CO₂ emissions and of incremented process effectiveness, thus making compact reformers a competitive alternative to hydrogen production by electrolysis.

Based on this scenario, we investigated in parallel the application of two novel and different MSR reactor layouts: the former one is based on highly thermal conductive open-cell foams for the intensification of conventional externally heated reformers, while the latter one is based on the possibility to exploit Joule effect for the generation of heat inside the reactor tubes with the adoption of electrically-resistive SiC foams. In both cases, the geometric supports (internals) can be made catalytically active either by packing small catalytic pellets into the open and interconnected porosity of the foam (packed foam configuration [2]) or by depositing a thin washcoat layer onto the surface of the foam (washcoated foam configuration [3]).

Conductive packed foam configurations were tested and compared extensively against a conventional packed bed reactor operated at lab-scale conditions. The presence of a conductive matrix flattens the internal temperature profiles, providing an advantage in terms of heat supplied to the system for the reaction. Both in kinetic regime or close to the thermodynamic equilibrium this phenomenon yields in higher conversions and reduced risk of deactivation with respect to the conventional configuration, thus enabling higher hydrogen productivity. The choice of the foam material represents a key aspect for the successful implementation of the solution. In this configuration, copper foams enabled very small radial and axial temperature gradients inside the reactor, thanks to the high thermal conductivity of the bulk material; as a result, an increase of the conversion at fixed reactor wall temperature was demonstrated.

In the configuration based on resistive foams, the electrified reactor exploits Joule effect to generate uniform heat supply in the reactor. Silicon carbide was found to be the material of choice to guarantee adequate electrical resistivity and thermal conductivity for stable operations of this reactor configuration. It is possible to supply 2 to 5 times the heat demand of conventionally fired reformers by imposing a voltage differential (in the range of 10-50 V at lab scale configuration) at the premises of the structured support. By this configuration, radially flat temperature profiles are present and, thanks to the adoption of highly active Rh catalyst, it is possible to reach the almost unitary methane conversion at GHSV in excess of 10000 h^-1. In this configuration, silicon carbide is a promising material for foam heating elements, as it can handle high specific power, in accordance with the high thermal process demand. Moreover, the electrical resistivity of the materials allows for an adequate voltage drop across the reactor brackets.

Experimental activities on lab-scale setups were coupled with numerical modelling of the different process configurations to obtain a model-based design of the optimal configuration are ongoing. We think that both these reactor configurations could be at the base of efficient design of small scale reformers for hydrogen and syngas production with reduced CO₂ footprint.

References

[1] S.T. Wismann et al., Ind. Eng. Chem. Res. 2019 58 23380-23388

[2] R. Balzarotti et al., Chem. Eng. J., 2019 123494

[3] R. Balzarotti et al., React. Chem. Eng. 2019 4 1387-1392

Acknowledgements

This project has received funding from the European Research Council (694910/INTENT).