Microstructural aspects of the flow behaviour in a liquid-solids circulating fluidized bed

The local instantaneous and time-average suspension densities were determined in a 76 mm diameter by 3 m tall liquid-solids circulating fluidized bed riser using a fibre-optic probe. Attempts were made to qualify the microflow structure through statistical analysis of the local bed voidage fluctuations obtained under different operating conditions for the first time. The results show that local microflow structure is uniform in the axial direction but non-uniform in the radial direction with more flow fluctuation near the wall than in the core of the column for a given axial position. The standard deviation and intermittency index tend to increase with increasing solids circulating rates. Comparing with the gas-solids CFB, the liquid-solids CFB shows much more homogeneous flow structure in both the axial and radial microscopic flow behaviours. The microflow behaviours in the conventional liquid-solids fluidization, liquid-solids circulating fluidization and dilute-phase liquid transport regimes are also characterized by examining the probability distribution and the intermittency index of the solids holdup.