An experimental investigation on the onset from bubbling to turbulent fluidization regime in micro-structured fluidized beds

Membrane-assisted fluidized beds have been investigated for a number of industrially important applications. Recently, micro-structured membrane-assisted fluidized beds have been proposed and particular for efficient hydrogen production in particular for maximising the volumetric production rate by maximising the installed specific membrane areas. Another important advantage of micro-structuring became evident from computational studies. CFD calculations have shown that micro-structured fluidized beds can be operated in the turbulent fluidization regime at much lower superficial gas velocities compared with larger fluidized bed [1]. This work focuses on an experimental validation of the transition velocity in micro-structured fluidized beds. The influence of the reactor scale and the particle diameter on the transition gas velocity from the bubbling to the turbulent fluidization regime in micro-structure fluidized beds was measured in pseudo 2D column using different experimental techniques: in particular, the pressure drop fluctuation measured by pressure sensors and the local solid holdup fluctuation measured by digital image analysis on images recorded by high speed camera. It was found that smaller reactors filled with particles of smaller diameter can be operated in the turbulent fluidization at a relatively lower gas velocity compared with larger reactors. Interestingly, the different measurement techniques result in somewhat different transition velocities, i.e. the pressure drop fluctuation method generated higher critical velocities than the local solidsʹ holdup fluctuation method. This difference is discussed in this paper. The experimental results confirmed that micro-structured fluidized beds can indeed be operated in the turbulent fluidization regime with lower fluidization velocities thus anticipating increased energy efficiencies and enhanced mass transfer rates.