Power consumption, gas holdup, and mass‐transfer coefficient of triple‐impeller configurations in a stirred vessel with vertical tubular coils

Abstract

Three kinds of impeller combinations (3 × 4WHd, HB + 2 × 4WHd, 2 × HB + 4WHd) were used to investigate the effects of coil baffles on system power consumption, gas holdup, and mass transfer coefficient. The experiments were performed in a dish‐bottomed vessel with a diameter of 0.29 m. Results of the power consumption experiments show that at the same rotation speed and an increasing rate of gas flow, the power consumptions of 3 × 4WHd and 2 × HB + 4WHd with coil baffles are almost the same as that of a system with four planar baffles, but there is a clear power draw for the HB + 2 × 4WHd combination. At rotational speeds of 250–350 rpm, the largest power draws for the 2 × HB + 4WHd, HB + 2 × 4WHd, and 3 × 4WHd impeller combinations equipped with coil baffles are 27, 25, and 15 %, respectively. The gas holdup experiment shows that with installation of coil baffles in the vessel, axial and radial impeller combinations can achieve gas dispersion identical to that attained with planar baffles. At the same power input and superficial gas rate over a wide range of gas velocities, the kLa values of the coil baffles for the 3 × 4WHd and 2 × HB + 4WHd combinations are almost identical to those of a conventional system with four baffles. However, in the HB + 2 × 4WHd system, kLa values of the coil baffles are 15 % higher than for planar baffles at low gas velocities and 30 % higher at high superficial gas rates. Finally, all data from the experiments on gas holdup and kLa values may be correlated through the equations ϵ=α(Pg/V)β(Vs)γ and KL a=K1(PG/V)K2(VS)K3.

Investigation of Local and Overall Volumetric Mass Transfer Coefficient and Gas Holdup in Aerated Coaxial Mixing Systems Containing Non-Newtonian Fluids through Tomography and Computational Fluid Dynamics

“…The reason for the preference of multiple impellers over a single-impeller is that multiple impellers provide better gas utilization in gasliquid systems. Moreover, using multi-impellers leads to an increase in the gas residence time [19],…”

“…In these large-scale mixing systems, multiple impellers are installed on the single shaft [11][12][13]. The use of multiple impellers not only helps to unify the shear distribution throughout the tank but also enhances gas-liquid contact, gas residence time, and mass transfer per specific power consumption compared to the single-impeller systems [14,[16][17][18][19]100].…”