Novel mixing tank for improved gas–liquid mass transfer and solids suspension

Abstract

A novel, baffleless, mixing tank configuration presented provides greater mixing efficiency than conventional technologies under a wide variety of operating conditions. The concept uses a cylindrical tank with a contoured bottom and a shroud that surrounds an axial flow impeller and supports a set of stationary antiswirl fins positioned in close proximity to the impeller blades. The angle and pitch of the fins impart an angular component to the flow that is equal and opposite to the swirl induced by impeller rotation, thereby eliminating the need for wall baffles. The contoured bottom eliminates dead space beneath the impeller and n the corners of flat-bottom mixing tanks. This alternative mixing system, named the ASSET system, is compared to conventional mixing configurations through experiments of power consumption, gas-liquid mass transfer, and solids suspension. Measurements of Power number vs. Reynolds number reveal that it has power requirements similar to those of high-efficiency axial-flow turbines in a conventional tank geometry. Unsteady-state oxygen absorption and stripping experiments are used to determine the interphase mass-transfer coefficient as a function of power input for each configuration at superficial gas feed velocities of 0.057 to 0.17 cm/s. With this system, K{sub L}a values are on average 30 tomore » 50% greater than those obtained in a conventional mixing tank at the same power input. Measurements of the height of the well-mixed region of suspended solids in a 15 wt. % mixture of 0.1-cm-dia. glass beads in water indicate that the ASSET system requires {approximately} 30% less power to achieve equivalent extents of solids suspension than the conventional setup.« less