Abstract
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 and 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.
Highlights
The increasing energy demand over the last decades, in combination with the need to reduce greenhouse gas (GHG) emissions, has given rise to the development of more efficient conversion technologies and alternative energy carriers
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
Analysis of the conditions of these results show that the decrease in δ with a decrease of the partial pressure of hydrogen and fluidization velocity is related to the increase in the Reynolds number of the system, as shown in Fig. 5b, demonstrating the strong influence of the hydrodynamics on the extent of the concentration polarisation
Summary
The increasing energy demand over the last decades, in combination with the need to reduce greenhouse gas (GHG) emissions, has given rise to the development of more efficient conversion technologies and alternative energy carriers. Pure hydrogen is obtained directly from the membranes without the requirement of downstream separations, reducing the process complexity and the associated capital costs These advantages of membrane reactors can make hydrogen production on smaller scales from a decentralized source such as biogas attractive. When describing the steam reforming of biogas in a fluidized bed membrane reactor, the concentration polarisation is still expected to influence the system performance significantly. These effects are a result of the low methane and hydrogen concentrations and cannot be ignored. The present work evaluates a fluidized bed membrane reactor for biogas steam reforming and the influence of concentration polarisation on the system performance. The model is used to quantify the influence of the concentration polarisation and its significance for the design of fluidized bed membrane reactors for biogas reforming
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