Gas fluidized beds operating at high velocities: a critical review of occurring regimes

Abstract

Industrial processes often involve the interaction of a gas and a solid phase. Low gas velocity processes are characterised by limited carry-over and are performed in packed, fluidized or slugging beds. At higher gas velocities, the heterogeneous two-phase character of the fluidized bed is replaced by a condition of uniformity where large discrete bubbles or voids are on the whole absent. This so-called ‘turbulent’ fluidized bed regime has been described in many contradictory and non-decisive ways, as will be reviewed in the paper. When the gas velocity through the fluidized bed is increased above a critical velocity (transport velocity), entrainment becomes so important that the bed can no longer be maintained in the column unless entrained particles are collected and returned to the bed. Reactors with an external recycling system have a means to control the solids hold-up and are referred to as ‘Circulating Fluidized Beds’ (CFB). The CFB normally operates in the so-called ‘fast fluidization’ regime, which is situated between the dilute and dense pneumatic transport regimes. However, the terms ‘fast fluidization’ and ‘transport velocity’ are used in different and controversial ways. A critical literature assessment is included in the paper. Special attention is focussed on the axial density profile as being the key parameter of the riser. Experimental work was performed in a lab-scale CFB with a riser of 0.1 m i.d. The transport velocity is the minimum gas velocity required to develop a stable fast fluidization mode. Several techniques to measure this velocity are reviewed and a correlation, based on own experimental results and literature data is included. Comparison with other correlations is performed. Since there is a literature discrepancy about the fast fluidization regime, the transition between dilute and S-shaped profile will be determined as function of the solids circulation rate and the gas velocity, based on own experimental work. The new correlation is thereafter applied to literature data and good agreement is obtained.