Long-Term Stability of Thin-Film Pd-Based Supported Membranes

Membrane reactors have demonstrated a large potential for the production of hydrogen via reforming of different feedstocks in comparison with other reactor types. However, the long-term performance and stability of the applied membranes are extremely important for the possible industrial exploitation of these reactors. This study investigates the long-term stability of thin-film Pd-Ag membranes supported on porous Al2O3 supports. The stability of five similarly prepared membranes have been investigated for 2650 h, up to 600 °C and in fluidized bed conditions. Results show the importance and the contribution of the sealing of the membranes at temperatures up to 500 °C. At higher temperatures the membranes surface deformation results in pinhole formation and a consequent decrease in selectivity. Stable operation of the membranes in a fluidized bed is observed up to 450 °C, however, at higher temperatures the scouring action of the particles under fluidization causes significant deformation of the palladium surface resulting in a decreased selectivity.The presented work is funded within BIONICO. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671459. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme, Hydrogen Europe and N.ERGHY

Mass transfer in fluidized bed reactors using a novel non-invasive, whole field and high temporal resolution infra-red technique

Mass transfer rates between bubble and emulsion phases in fluidized beds, together with solids motion, determine the performance of fluidized bed reactors. Mass transfer has widely been studied in the literature using many different experimental techniques, such as colored gases, X-Ray or MRI. They show, however, important disadvantages like high costs or the use of unsafe gases. Moreover, they often lack the required spatial and temporal resolution and many assumptions have to be taken in order to determine mass transfer coefficients, e.g. the concentration is measured in a single point and this concentration is subsequently assumed to be representative for the entire bubble. In a recent work by Dang et al. (1) a novel technique has been developed for non-invasive, whole-field concentration measurements in the dilute regions of a fluidized bed with a high temporal resolution. The technique is based on the measurement of the decrease in the IR intensity at a specific wavelength inside the bubbles due to the absorption by a tracer gas using a high-speed IR camera. Because of the required IR accessibility, the application is limited to pseudo 2D columns and the walls of the column should be made of a material with a high transmittance for IR radiation. This novel technique has shown very promising results, but it is limited to small reactors due to the use of expensive materials (viz. sapphire) causing the results to be influenced by wall effects. In this work, this technique has been further developed in order to allow scaling-up of the system through the use of less expensive materials. The developed system is based on selective IR absorption by propane as tracer gas in a mixture with N2 in a pseudo-2D quartz column (see Figure 1). The decrease in the IR intensity is measured and calibrated, so that the instantaneous propane concentration profiles inside the bubbles are determined. By solving the component mass balances for the bubble phase, the mass transfer coefficients are calculated. Please click Additional Files below to see the full abstract

On concentration polarisation in a fluidized bed membrane reactor for biogas steam reforming: modelling and experimental validation

The production of pure hydrogen through the steam reforming of biogas in a fluidized bed membrane reactor has been studied. A phenomenological one-dimensional two-phase fluidized bed reactor model accounting for concentration polarisation with a stagnant film model has been developed and used to investigate the system performance. The validation of the model was performed with steam reforming experiments at temperatures ranging from 435 °C up to 535 °C, pressures between 2 to 5 bar and CO2/CH4 ratios up to 0.9. The permeation performance of the ceramic-supported PdAg thin-film membrane was first characterized separately for both pure gas and gas mixtures. Subsequently, the membrane was immersed into a fluidized bed containing Rh supported on alumina particles and the reactor performance, viz. the methane conversion, hydrogen recovery and hydrogen purity, was evaluated under biogas steam reforming conditions. The resulting hydrogen purity under biogas steam reforming conditions was up to 99.8%. The model results were in very good agreement with the experimental results, when assuming a thickness of the stagnant mass transfer boundary layer around the membrane equal to 0.54 cm. It is shown that the effects of concentration polarisation in a fluidized bed membrane reactor can be well described with the implementation of a film layer description in the two-phase model.The presented work is funded within BIONICO. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 671459. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme, Hydrogen Europe and N.ERGHY

Advancement of an infra-red technique for whole-field concentration measurements in fluidized beds

For a better understanding and description of the mass transport phenomena in dense multiphase gas-solids systems such as fluidized bed reactors, detailed and quantitative experimental data on the concentration profiles is required, which demands advanced non-invasive concentration monitoring techniques with a high spatial and temporal resolution. A novel technique based on the selective detection of a gas component in a gas mixture using infra-red properties has been further developed. The first stage development was carried out using a very small sapphire reactor and CO2 as tracer gas. Although the measuring principle was demonstrated, the real application was hindered by the small reactor dimensions related to the high costs and difficult handling of large sapphire plates. In this study, a new system has been developed, that allows working at much larger scales and yet with higher resolution. In the new system, propane is used as tracer gas and quartz as reactor material. In this study, a thorough optimization and calibration of the technique is presented which is subsequently applied for whole-field measurements with high temporal resolution. The developed technique allows the use of a relatively inexpensive configuration for the measurement of detailed concentration fields and can be applied to a large variety of important chemical engineering topics

Influence of H2S on the hydrogen flux of thin-film PdAgAu membranes

Pd-based membranes have the potential to be used for hydrogen purification and production in membrane reactors. However, the presence of impurities in the feedstock, such as H2S can poison the membrane, thus decreasing the hydrogen permeation by blocking and deactivating active sites of the Pd-alloy on the membrane surface. H2S at high concentrations can even destroy the membrane by the formation of Pd4S. It is known that alloying of Pd with Au enhances the membrane resistance to H2S. This work reports the performance of six PdAgAu/Al2O3 supported membranes, prepared by electroless plating combined with PVD under exposure to trace amounts (<2 ppm) of H2S. The Au content shows not to play a significant role at these low concentrations. Exposure results suggest a dual influence of physisorbed H2S and chemisorbed S species. A Langmuir description shows that the influence of the partial pressure of hydrogen is negligible on the flux inhibition. In the post characterization the absence of Pd4S was shown, however the surface was affected by the exposure to H2S