(143h) An Impeller-Draft Tube Agitation System for Gas-Liquid Mixing in a Stirred-Tank Reactor

Stirred-tank reactors (STR) are widely used in chemical processes where intimate contact between a gas and a liquid is required to achieve good mass transfer. The design of the STR significantly affects gas bubble formation and dispersion, the gas-liquid interfacial area density, and the mass transfer coefficient. These in turn influence the rate of conversion, selectivity, and yield of the reaction. This work relates to the design of an agitation system for gas-liquid contacting in STRs that is critical to applications requiring higher gas flow rates. It incorporates a draft tube with one or more down-pumping axial-flow impellers located within. The draft tube has internal vertical baffles to disrupt tangential swirl and facilitate axial flow. A radial-flow impeller is located outside and below the bottom opening of the draft tube. The reactant gas is introduced below the radial-flow impeller in the conventional manner through a ring or nozzle sparger. The axial-flow impellers inside the draft tube pump the liquid downward in the core region of the STR, then the radial-flow impeller disperses the gas into the liquid and directs the resulting two-phase mixture radially outward so that it rises in the annular region and recirculates back into the draft tube at the top of the STR.

The performance of the agitation system described above was studied experimentally using water sparged with air in a 0.45 m diameter STR. The agitator consisted of two 0.13 m axial-flow hydrofoil impellers plus one 0.23 m radial-flow Rushton disk turbine at the bottom. In this study, the STR was configured both with and without a 0.30 m diameter draft tube for comparison of their flow characteristics and mass transfer efficacy. A transient method was used to determine the mass transfer coefficient k_La. The water was initially stripped free of oxygen; then after the air sparging began, the transient dissolved oxygen concentration was measured and used to calculate k_La. In addition to the mass transfer rate, the specific power was also measured. The results show that the incorporation of the draft tube in the STR yielded a system that has improved performance compared with conventional gas-liquid STRs without the draft tube, offering up to 20% increase in mass transfer at the same gas flow rate and agitation speed, or reducing the specific power draw by as much as 30% for similar k_La values at the same agitation speed.