Preliminary study on a counter-current bubble column near flooding point and an inverse gas-liquid-solid circulating fluidized bed

The hydrodynamics of an inverse gas-liquid-solid circulating fluidized bed (IGLSCFB) with downflow liquid and upflow gas was studied for the first time as a promising candidate bioreactor for biological processes. Experiments were carried out in a 0.076 m ID column with 5.4 m height using low-density particles. The hydrodynamics of the rarely studied high liquid velocity counter-current bubble column is also investigated. In IGLSCFB, both gas and solids holdups are axially uniform under lower gas and liquid velocities, but holdups become axially non-uniform at higher operating velocities. The gas holdup increases with superficial gas velocity and superficial liquid velocity but is not sensitive to solids circulation rate. Solids holdup increases with solids circulation rate and superficial gas velocity but decreases with superficial liquid velocity. In comparison with inverse liquid-solid circulating fluidized bed, solids holdup in IGLSCFB is significantly higher, potentially enhancing liquid-solid phase contact in the fluidized bed.