Chemical Engineering Science, Vol.84, 822-833, 2012
Membrane-assisted fluidized beds-Part 2: Numerical study on the hydrodynamics around immersed gas-permeating membrane tubes
Among many research studies on fluidized bed membrane reactors, only very few focus on the details of the effects of gas permeation through the membranes on the hydrodynamic properties of the membrane-assisted fluidized bed. For this reason, in the first part of this paper, a novel hybrid Immersed Boundary Method (IBM) Discrete Particle Model (DPM) has been developed. In the second part of the paper, this model is employed to investigate fluidized bed membrane reactors in more detail. Simulations without membrane inserts (having vertical membranes at the side walls instead), with membrane tubes in an in-line arrangement and membranes in a staggered arrangement are compared with each other. The time-averaged particle fraction maps display a lower bed height for the simulations with inserts. A very important aspect of the simulation with gas extraction/addition via the side walls is the solids circulation; an inversion of the solids circulation is observed when gas is added via the side walls with altered bubble properties. Because of this phenomenon, the bubble size counter-intuitively decreases when gas is added, and increases slightly when gas is extracted. The latter is a result of the existence of compacted zones near the membranes. For the simulation cases with membrane inserts, the solids circulation and bubble properties are different; in these cases they mainly depend on the local fluidization velocity, and not so much on the extent of permeation itself. Although the arrangement of the membrane tubes plays only a minor role in this matter, their presence has a pronounced influence on the bubble size. The system's energy and energy dissipation are both smaller for the simulations with inserts compared to those with permeation via the side walls. Also in this respect, the local fluidization velocity is found to be more important than the extent of permeation. (c) 2012 Elsevier Ltd. All rights reserved.