Operating and hydrodynamic characteristics of a cocurrent downflow bubble column reactor

The hydrodynamic stability of the gas-liquid two-phase dispersion in a cocurrent downflow column (CDC) depends critically on the column configuration and such operating parameters as the inlet liquid velocity, column liquid velocity and bubble dispersion height. The hydrodynamic characteristics of the CDC were investigated using a column of 0.076 m (3 in) W. with liquids of different surface tension, viscosity and coalescence properties. The two-phase flow patterns, bubble formation, stability and entrainment were observed for oxygen with water and aqueous solutions of 1-propanol, glycerol and sodium alginate. The minimum inlet liquid velocity required to maintain a stable gas-liquid dispersion was determined, as was the maximum column liquid velocity which did not give bubble entrainment in the outlet. An empirical correlation defining stable column operating conditions was proposed. Strongly coalescing liquids, such as water and water containing low concentrations of I-propanol (<<0.015 wt%) or glycerol (<<5 wt%), gave large diameter bubbles (3-5 mm) and good gas disengagement which allowed the use of relatively high column liquid velocities (up to 0.14 m/s). Coalesccnce-inhibiting liquids such as aqueous 1-propanol (>0.015 wt%) and aqueous glycerol (>5 wt%) gave rise to very small bubbles (<1 mm) and required liquid outlet velocities in the range of 0.001-0.015 m/s in order to ensure bubble disengagement. The CDC compared with other types of downflow bubble columns was found to be capable of greater operational flexibility. No excess gas input or recycling was required in the CDC due to its ability to achieve 100% gas utilization. The CDC would be particularly suited to applications where high ratios of liquid-to-gas throughputs were required, such as in waste water or effluent treatment.