Dang, Nhi (Dr.)

Dang, Nhi (Dr.): Membrane-assisted micro-fluidized bed membrane reactors: An experimental investigation into hydrodynamics and mass transfer

Hydrogen has recently been considered as a green energy carrier especially for fuel cell applications where ultra-pure hydrogen is required. Conventional processes for pure hydrogen production have been carried out in multi-tubular packed bed reactors from steam reforming methane, showing large amount of unit operation. For small scale application (typically 1-50 kW), these reactor concepts are not economic in terms of energy supply and process efficiency.

Novel membrane-assisted fluidized bed reactors for pure hydrogen production with integrated carbon dioxide capture have been patented by Kuipers et al. (2004) and van Sint Annaland et al. (2006) where chemical reactions, energy supply and separation steps have been integrated in a single reactor unit, showing complete process integration/intensification. In order to enhance mass transfer from the bubble-to-emulsion phase and the permeation flux of hydrogen via inserted palladium membranes, a novel membrane-assisted micro-structured fluidized bed reactor has recently been proposed and investigated within Chemical Process Intensification (SPI) research group.  

The main objective of this research is to investigate the hydrodynamic behaviour, mass transfer and gas mixing characteristics of micro-structured fluidized bed membrane reactors. The focus of this research was on a small fluidized bed compartment with flat membranes (i.e. porous filters) built into the left and right walls confining the fluidized suspension through which gas was added to or extracted from the gas-solid suspension, mimicking a single compartment of a micro-structured fluidized bed membrane reactor module. The detailed experimental results gave clear guidelines for the design, operation and optimization of micro-structured fluidized bed membrane reactors.

Hydrodynamic studies

The hydrodynamic characteristics of both gas and solid phases have been investigated non-invasively using a combined Particle Image Velocimetry (PIV) / Digital Image Analysis (DIA) technique. This experimental study has been carried out for both the bubbling and turbulent fluidization flow regimes and focuses on the investigation of the influence of gas permeation via flat membranes installed into the left and right walls of the column. It has been observed that the extraction of gas creates densified zones near the membrane walls with decreased solids mixing, which may result in increased mass transfer resistances towards the membranes (due to induced concentration polarization). In addition, more gas is forced towards the bed centre and by-passes the bed, resulting in reduced gas-solids contacting and decreased reactor performance. A very different behaviour has been observed for the case of gas addition through the membranes: in this case solids are pushed towards the centre of the bed with inversed solids circulation patterns compared to the reference case without gas permeation. In addition, gas by-pass is observed near the membrane walls for both bubbling/slugging and turbulent fluidization. These results are very important for the design and operation of membrane-assisted fluidized beds, since they indicate the limits on the membrane flux, but also for other fluidized bed operations (such as dryers), where the addition or extraction of gas through the membranes may be exploited to optimize the solids circulation patterns (Figure 1).

Figure 1: Influence of gas permeation on the solid circulation patterns for (a) gas extraction, (b) no gas permeation and (c) gas addition.

Experiments have been carried out for three different bed widths (80, 40, 20 mm) and two different particle diameters (d= 100-200 and 400-600 µm, ρp = 2500 kg.m-3) showing that the adverse effects of gas permeation through the membranes (formation of densified zones and gas by-passing) can be avoided by decreasing the gas velocity though the membranes (via increasing the membrane area or decreasing the membrane permeation ratio) and that optimal (hydrodynamic) performance was observed in relatively small beds with relatively large particles operated in the turbulent fluidization regime (Figure 2).

Figure 2: (a) Extent of densified zones as a function of the extraction velocity for three columns with different bed width and operated in bubbling and turbulent flow regimes 

fig. 2b comparison of extent of densified zones for the different bed widths and particle sizes, operated in different flow regime with 40% of gas extracted via the membranes in the left and right walls.

Mass transfer from the bubble-to-emulsion phase in a micro-fluidized bed

A novel experimental technique for the instantaneous whole-field, non-invasive gas concentration measurement in a gas-solid fluidized bed with high temporal and spatial resolution has been developed to be able to study mass transfer rates in fluidized bed membrane reactors. The technique is based on digital image analysis of images acquired with an infrared (IR) camera and a visual (VIS) high-speed camera to obtain the local gas-phase carbon dioxide concentration (Figure 3). The bubble-to-emulsion phase gas exchange coefficient computed from the measured concentration profiles deviates from literature correlations and it has been found to be dominated by convection of fluidization gas inside the bubble (Figure 4) in the first period of the mass transfer process and afterwards controlled by diffusion between the vortices at the right and left side of the bubble and the emulsion phase. This observation contradicts often made simplifying assumptions and clearly indicates the need for a better phenomenological mass transfer model for bubbling fluidized beds.

Figure 3: Combined IR/PIV/DIA technique for hydrodynamic and mass transfer measurements.

Figure 4: Snapshots of the concentration profile inside the bubbles at different moments in time. The equivalent bubble diameter is 31 mm with injection velocity 9.75 m.s-1.