Investigation on the Mass Transfer Rates in Fluidized Bed Membrane Reactors

An interesting development in the field of reforming reactors is the application of membrane reactors, where the in-situ selective extraction of hydrogen results in a shift in the equilibrium, which allows operation at lower temperatures while still achieving higher efficiencies. Moreover, no downstream separation is required since pure hydrogen is directly obtained from the membrane. Thin-film palladium-based membranes have the largest potential for integration in membrane reactors for hydrogen production due to their extremely high permeation fluxes and perm-selectivities. The permeation of hydrogen through the membrane depends only on the hydrogen partial pressure difference across the membrane. However, due to the high hydrogen flux, the mass transfer towards the membrane can become limiting, which effect is known as concentration polarization. In the case of fixed bed reactor configurations, the concentration polarization can significantly increase the required membrane area. The application of a gas-solid fluidized bed reactor configuration can reduce the concentration polarization (CP) effects due to its higher gas-solids mixing. Despite the decrease in the CP, it is still important to account for these mass transfer limitations, also for fluidized bed configurations; However, the prediction of CP in fluidized beds remains difficult. In this work the mass transfer rates in fluidized bed membrane reactors was studied, both the concentration polarization and the bubble-to-emulsion phase mass transfer rates.

Methods

Experiments were performed to quantify the extent of concentration polarization in a fluidized bed membrane reactor. The system used was a tube-in-shell system with a single thin-film (4 µm) PdAg membrane immersed into a fluidized bed of 250 µm catalyst particles, operated in the bubbling fluidization regime (3 to 7 times the minimum fluidization velocity). Experiments with mixtures, also under reactive conditions, were performed and used to develop a phenomenological description based on a stagnant film model to describe the CP in fluidized bed membrane reactors.

The bubble-to-emulsion phase and bulk-to-membrane mass transfer rates were studied in more detail with an in-house developed two-dimensional fluidized bed Infrared Radiation (IR) transmission system. This technique makes use of the IR absorbing properties of a tracer gas and is able to make instantaneous whole-field concentrations measurements of the dilute phase in the fluidized bed, from which the volumetric bubble-to-emulsion phase mass transfer coefficient can be determined.

Results and discussion

Unlike in fixed bed reactors, the gas passes through a fluidized bed consisting of Geldart B type particles via two phases, viz. the bubble and the emulsion phase, with different gas residence times, where there is mass transfer between the bubble and the emulsion phase. In the case of gas extraction through the membranes there is also mass transfer toward the membrane surface. Experimental results extracting hydrogen from a nitrogen hydrogen mixture show that fluidization can increase these mass transfer rates. A stagnant film layer description is shown to be able to predict these effects, once the film layer thickness has been estimated from experimental data.

The IR transmission technique has been applied to quantify bubble-to-emulsion mass transfer rates for single and multiple bubble injections into a fluidized bed at incipient fluidization conditions and bubble injections into a fluidized bed operated in the freely bubbling regime. The bubble-to-emulsion mass transfer is shown to be dominated by convective transport for larger particle sizes ranging from 300 to 700 µm. Extension of the image analyses by time averaging of instances allowed to study the mass transfer rates in beds consisting of smaller particles and to investigate the mass transfer rates from bubbles rising for longer times through the bed. The obtained results show that in beds consisting of smaller particles diffusive mass transfer becomes more pronounced and should be carefully considered.

Conclusions

Fluidized bed technology combined with membrane technology has proven to be an interesting concept for hydrogen production. The excellent heat and mass transfer of fluidized beds allow for an easy integration and improved performance combined with hydrogen extraction membranes. The mass transfer limitations towards the membrane can be described by a stagnant film model and are reduced due to the gas-solid mixing in the fluidized bed. A further understanding of the mass transfer in fluidized bed membrane reactors is, however, still required. With the use of a novel IR transmission technique the bubble-to-emulsion phase mass transfer could be studied in much more detail and the results have clearly indicated that the often-used existing correlations need to be revisited.

Acknowledgements

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.