Tan, Lianghui (Dr.)

Simulation studies on membrane-assisted micro-fluidized bed reactors

Introduction

The conventional process for the production of hydrogen is steam reforming of natural gas, which consists of a large sequence of different process units (reformers, shift reactors etc) with complex heat integration. This process is not efficient for scale-down applications, such as ultra-pure hydrogen reactor for PEMFC applications at smaller scales (typically 1-50kW). Membrane-assisted fluidized bed reactors provide a good opportunity for process integration and intensification, in particular for the production of ultra-pure hydrogen because of integrated shift and improved heat and mass transfer.  To overcome the limitation of the hydrogen permeation rate through the membranes and concentration polarization, the concept of micro-structured membrane-assisted fluidized bed reactors (MMAFBR) has been proposed for ultra-pure hydrogen production.

Proper MMAFBR design

Towards a proper MMAFBR design, it is crucial to investigate the optimal distance between two vertical membrane walls confining the gas-solids suspension, which constitute the small substructures. To approach that goal, studies on the hydrodynamics and mass transfer characteristics of one membrane-assisted micro-fluidized bed compartment have been carried out with detailed simulations and experiments (by T.Y.N. Dang). It has been found that the gas permeation through the membrane has a striking influence on the fluidization characteristics; such as a reversed solids circulation pattern in case of gas addition and the formation of densified zones close to membrane walls in case of gas extraction (Figure 1), which indicates a decreased gas-solid contacting. Detailed discrete particle simulations on the formation of the densified zones have been carried out as a function of the operating conditions, and a method to quantify the extent of densified zones has been proposed. The results show that in general the extent of densified zones increase with increasing gas extraction velocity (Figure 2). Based on the simulation results, it is possible to overcome possibly adverse hydrodynamic effects of gas permeation on the fluidization by optimizing the operating conditions and the particle size which correspondingly requires the adjustment of the distance between the two vertical membrane walls (Figure 3). Moreover, investigations on the mass transfer rate in MAMFBs have been started and preliminary results on the mass transfer between the bubble and emulsion phases are qualitatively in a good correspondence with experimental results.

Fig.2 Extend of densified zones

Fig.3a Extend of densified zones

Fig.3b Extend of densified zones