Surface extended catalyst support structures for hydrogen energy applications

Abstract

In the current study, metal foams are evaluated as process-intensifying, surface extended catalyst support structures for three hydrogen (H2)-based applications, namely the dehydrogenation of perhydro-dibenzyltoluene (H18DBT), the passive autocatalytic recombination (PAR) of H2 and the direct-CO2-to-methanol (MeOH) synthesis. For these applications, suitable in-house developed or commercial catalysts were washcoated on the surface of metal foams. They were investigated during catalytic activity tests using methods selected from the available literature. Their performances were then compared experimentally or using computational fluid dynamics (CFD) simulation to those of typical catalyst systems used in reactors conventionally used for the same processes. The dehydrogenation of H18DBT was conducted in an unstirred tank rector and a fixed-bed reactor (FBR). An in-house developed Pt/Al2O3 catalyst was coated on aluminium (Al) foams and tested in the tank reactor. The performance of the foam-supported catalyst was later compared to that of the same amount of Pt/Al2O3 pellets. Experiments were conducted at 300°C, and Pt/H18DBT mol. ratio of 0.05-0.4 mol.%. H2 productivities were 12-59% higher in the foam-based than the pellet-based tank reactor. The higher performance of the foam-based reactor was attributed to the geometric properties of the coated foam. The Pt/Al2O3-coated foams also demonstrated a high catalytic stability for more than 16 h on stream in a FBR. The relatively low degree of dehydrogenation obtained in a single pass in the FBR was improved by recycling the partially dehydrogenated product three times in the reactor at the same temperature and flow conditions. However, the H2 productivity significantly decreased with the number of dehydrogenation cycles. The PAR tests were performed in a recombiner consisting of Pt/Al2O3-coated Al foams in the catalytic section. The feed gas, made of 2-6% H2 in air, was introduced to the recombiner at inlet velocities of 2-10 m/s. H2 conversions between 57 and 89% were achieved in the recombiner, while the highest temperature recorded during the operations was 172°C lower than the self-ignition limit of a H2 air mixture. A comparison between the foam-based, plate-based and pellet-based recombiners using the same amount of catalyst revealed that the H2 conversions achieved on the coated foams were similar to those obtained on the coated plates. The pellet-based recombiner exhibited the lowest performance of the three systems. Non-uniform temperature profiles were observed on the coated foams/plates due to the unpredictable, uneven gas flow distribution in the recombiner. Three-dimensional (3D) heterogeneous CFD simulations of the foam-based recombiner were successfully validated using experimental data, including H2 conversions and maximum temperatures. The model can successfully be used as a numerical tool for the prediction of the recombiner performance at various conditions. The direct-CO2-to-MeOH synthesis was performed in a two-module, bench-scale FBR containing a commercial Cu/ZnO/Al2O3 coated on copper foams. Activity tests were conducted at weight hourly space velocities of 1.125-2.925 NL/gcat/h, pressures of 30 and 50 bar and temperatures of 190-250°C. CO2 conversions, MeOH selectivities and MeOH productivities up to 27.46%, 82.97% and 30 gMeOH/h, respectively, were achieved in the reactor. This demonstrated the suitability of the coated foam for the direct CO2 hydrogenation to MeOH. Temperatures measured at different points in the reactor revealed that the nearly isothermal conditions were achieved in the reactor. The experimental results of the direct-CO2-to-MeOH synthesis were used to validate a 3D pseudo-homogeneous CFD model of the foam-based modular reactor. The model included simplified and optimised kinetic expressions which were successfully used to predict the CO2 conversions, MeOH selectivity, MeOH weight-time yield (WTY) and CO outlet concentration. The 3D model was also used as prediction tool to simulate a modular packed bed reactor (PBR). Based only on the difference in geometric properties between the two reactors, the model predicted lower CO2 conversions, MeOH selectivities and MeOH WTY in the PBR compared to the foam-based reactor. The performance of the coated foams varied according to the application used. This study demonstrated that the selection of a foam-based catalytic system can be motivated by its ability to enhance processes which are more strongly affected by mass transfer than kinetic limitations.