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Palladium-based membrane-assisted fluidized bed reactors have been proposed for the production of ultra-pure hydrogen at small scales. Due to the improved heat and mass transfer characteristics inside such reactors, it is commonly believed that they can outperform packed bed membrane reactor configurations. It has been widely shown that the performance of packed bed membrane reactors can suffer from serious mass transfer limitations from the bulk of the catalyst bed to the surface of the membranes (concentration polarization) when using modern highly permeable membranes. The extent of concentration polarization in fluidized bed membrane reactors has not yet been researched in detail. In this work, we have quantified the concentration polarization effect inside fluidized bed membrane reactors with immersed vertical membranes with high hydrogen fluxes. A Two-Fluid Model (TFM) was used to quantify the extent of concentration polarization and to visualize the concentration profiles near the membrane. The concentration profiles were simplified to a mass transfer boundary layer (typically 1 cm in thickness), which was implemented in a 1D fluidized bed membrane reactor model to account for the concentration polarization effects. Predictions by the TFM and the extended 1D model showed very good agreement with experimental hydrogen flux data. The experiments and models show that concentration polarization can reduce the hydrogen flux by a factor of 3 even at low H 2 concentrations in the feed (10%), which confirms that concentration polarization can also significantly affect the performance of fluidized bed membrane reactors when integrating highly permeable membranes, but to a somewhat lesser extent than packed bed membrane reactors. The extraction of hydrogen also affects the gas velocity and solids hold-up profiles in the fluidized bed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The role of mesoscale drag modeling in both TFM and DPM methods is analyzed. • The effect of mesoscale drag in TFM is more significant than for DPM simulation. • DPM itself considers sub-grid heterogeneity but over-predicts particle collisions. • TFM with mesoscale modeling better predicts solid concentration and conversion. The coarse-grained discrete particle model (DPM) is fast growing into a powerful tool and a useful counterpart of the widely used two-fluid model (TFM) in simulation of large-scale reactors. This work aims to study the role of mesoscale modeling in both TFM and DPM approaches to understand the advantage and disadvantage of each approach for further development. Both simulation approaches with and without considering mesoscale structures in drag modeling are systematically investigated through simulations of an industrial diameter-transformed fluidized bed reactor with complex reactions. It is found that considering mesoscale drag can obviously improve the prediction in solid concentration for both approaches, and the effect of mesoscale drag for TFM modeling is more significant than for DPM approach. Besides, the DPM approach can reveal local heterogeneous structures without using mesoscale drag because it can distinguish different parcels in each fluid cell, but it overestimates the accumulation of solid particles below the distributor, as the large coarse-grain ratio may over-enhance the particle collision. For reaction, the coke content can be better predicted by both approaches with mesoscale drag, and the DPM simulation can capture more heterogeneous distribution of coke content than TFM modeling. The predicted temperature and product distribution still have obvious deviation from industrial data, suggesting a need of mesoscale heat and mass transfer modeling. The underlying mechanisms are further analyzed together with proposing future work. [ABSTRACT FROM AUTHOR]

For ultra-pure hydrogen production packed-bed and fluidized bed membrane reactor concepts have been proposed. Due to the development of novel ultra-thin Pd-membranes with higher selective hydrogen permeabilities, packed-bed membrane reactors suffer from inefficiencies related to concentration polarization and heat management. These problems can largely be overcome in fluidized bed membrane reactors. The focus of this work is the better understanding of the hydrodynamics in the vicinities of membranes immersed in fluidized beds. In this study, it has been proven that a new effect might be detrimental for the performance of a fluidized bed membrane reactor: the formation of bubbles, or better ‘gas pockets’, surrounding (and attached to) the membranes. A non-invasive PIV–DIA technique has been employed to study the gas pockets behavior in detail in a pseudo 2D column with horizontally immersed membranes. Experimental results have confirmed that gas pockets are free of solids, do not rise like bubbles but stay attached to the membranes thus reducing the effective contact area with the emulsion phase, have very short life times and appear frequently in the bed. A gas pocket is formed with gas from the emulsion phase and thus no gas exchange takes place between this gas pocket and the surroundings, implying an important mass transfer limitation. Due to these characteristics, unlike previously done in literature gas pockets should not be considered as bubbles when studying hydrodynamics. A new method, able to distinguish between a gas pocket and a bubble, has been developed with accuracy over 95%. The effect of different experimental conditions on gas pockets is also reported. Furthermore, a simulation study has been performed with a Two-Fluid Model including the Kinetic Theory of Granular Flow (KTGF) that has shown good agreement with the experimental observations and has provided a more detailed insight into the formation of the gas pockets. For a similar configuration, average bubble diameter after gas pocket removal from the analysis is 2.4 and 2.5 cm for experimental and simulated results. Moreover, gas pockets characteristics are in a good agreement, being 1.3 and 1.1 cm on average for experimental and TFM simulations, respectively. Finally the mechanism of formation of a gas pocket and its influence on the behavior of a fluidized bed membrane reactor are discussed in detail. [ABSTRACT FROM AUTHOR]

• Numerical modelling of novel indirectly heated bubbling fluidized bed steam reformer. • Hydrodynamics investigation in a complex geometry (immersed vertical tube). • Experimental validation in a 50 kW th pilot scale reactor. • Simulation of additional N 2 side-flow in the reactor. • Favourable comparison of experimental and computational global hydrodynamic metrics. This work focuses on the numerical modelling and experimental validation of the hydrodynamic behaviour of a novel 50 kW th indirectly heated bubbling fluidized bed steam reformer. The hydrodynamic behaviour of fluidized beds with immersed vertical tubes and complex fluidized bed geometries in general have not been thoroughly investigated in terms of numerical modelling coupled with experimental validation for pilot scale reactors. Therefore, the present study contributes to the fluidized bed hydrodynamics numerical modelling field, while investigating a novel reactor concept. Simulations were performed employing the Two-Fluid Model approach, using the Kinetic Theory of Granular Flows (KTGF) and the adjusted Syamlal O'Brien drag model. The reactor's hydrodynamic behaviour was simulated successfully, as showcased by a comparison of global hydrodynamic metrics (bed height, pressure drop) between computational and experimental results. Simulations were performed with and without considering an additional nitrogen gas feed on the side of the reactor (feeding system pressurization). Overall, for both cases, for realistic values of the particle restitution coefficient channelling of the gas flow near the reactor walls was observed. Larger bubbles appeared to be forming near the outer wall of the reactor for the no side-flow simulations. The opposite behaviour was encountered for the side-flow simulations due to stream-like behaviour of the side-flow moving up against the reactor's outer wall. The choice to limit the simulations to a 72° symmetry domain was validated, indicating the possibility of further reduction. Finally, it was argued that increasing the reactor's diameter could potentially lead to a reduction of the observed channelling of the fluidization media and improve the mixing achieved in the reactor and thus the conversion efficiency of the IHBFBSR during gasification applications. [ABSTRACT FROM AUTHOR]