A numerical study on concentration polarization in 3D cylindrical fluidized beds with vertically immersed membranes

Abstract

Concentration polarization is a serious issue for systems in which high-flux, palladium-based, hydrogen perm-selective membranes are used to extract hydrogen, not only for packed-bed reactor systems, but also for fluidized bed membrane reactors. Two-Fluid Model simulations of 3D cylindrical lab-scale fluidized beds with single and multiple vertically immersed cylindrical membranes with state-of-the-art hydrogen permeability were performed to quantify concentration polarization, to study the interaction between concentration polarization zones of multiple vertically immersed membranes and to link these findings to the bed hydrodynamics. Simulations of a fluidized bed consisting of 500 μm particles using a H2/N2 mixture as fluidization gas with a single immersed membrane, show that reduced hydrogen concentrations prevail mostly within 1 cm from the membrane surface, and have disappeared beyond about 2 cm distance from the membrane surface. A 3D simulation was compared to a Cartesian 2D simulation that represents a slice of the 3D system, to assess the accuracy of 2D simulations. The 2D simulation did not fully capture the hydrodynamics and radial mass transfer effects of 3D cylindrical fluidized bed membrane reactors and overestimated the severity of concentration polarization and the formation of densified zones near the membrane surface. In particular, the hydrogen fluxes were determined at 0.217 mol/(m2 s Pa0.5) for the 3D case, and 0.133 mol/(m2 s Pa0.5) for the 2D case. The severity of densified zones is also affected by the particle size, because for smaller particles (250 μm) the emulsion phase density near the membrane surface is higher than for systems with larger particles (500 μm), which results in increased concentration polarization and a reduced extractive hydrogen flux. Therefore, employing relatively large particles of at least 500 μm in fluidized beds with modern high-flux membranes is advised. In fluidized beds with multiple membranes, interaction between concentration polarization zones of each membrane was observed. The interaction becomes more significant at smaller inter-membrane distances, especially below 2 cm, and decreasing the bed diameter decreases the system performance even more due to hydrogen depletion. Vertically immersed membranes also affect the fluidized bed hydrodynamics by reducing bubble size and increasing the number of small bubbles.