(548c) NAMF Student Award Presentation 2: Local and Overall Gas Holdup in an Aerated Coaxial Mixing System Containing a Non-Newtonian Fluid

Gas dispersion in non-Newtonian fluids is a non-separable part of biotechnology, wastewater treatment, cosmetic, and pharmaceutical industries. The present study intends to ameliorate the shortcomings of the conventional mixing systems in the gas dispersion in non-Newtonian fluids through the use of a coaxial mixing system comprised of two pitched blade impeller and a close clearance impeller. The gas holdup, which is a critical parameter in gas-liquid mixing, was assessed for this aerated coaxial mixing system via electrical resistance tomography (ERT) and computational fluid dynamics (CFD). The investigation was conducted aiming to find the effects of the impellers speed, fluid rheology, impeller pumping direction, rotational mode, and gas flow rate on the aeration efficiency of the coaxial mixers. It was found that the downward pumping and co-rotating mode of the impellers resulted in having the highest gas holdup. Moreover, the anchor speed of 10 rpm generated the highest gas holdup in downward pumping mode at a constant specific power consumption under all the aeration rates studied in this work. For the upward pumping system, the CFD simulations confirmed the results of experiment showing that the anchor rotation deteriorated the gas circulation inside the mixing tank by decreasing the number of circulation loops and increasing the upward axial velocity (Figure 1) resulting in a lower gas holdup compared to that achieved for the system without anchor rotation. The highest efficiency of the aerated coaxial mixing system in terms of gas holdup per specific power consumption was achieved for the downward pumping co-rotating system at different concentrations of CMC solution as well as at various central impellers and anchor speeds (Figure 2). The downward pumping and co-rotating mode generated the highest gas holdup among all mixing configurations utilized in this study for three CMC concentrations of 0.50, 0.75 and 1.00 wt%.

Captions

Figure 1: Air dimensionless axial velocity profiles along the tank height at 2r/T = 0.5 for the upward pumping and co-rotating mode, a central impeller speed of 226 rpm, and an aeration rate of 0.1 vvm.

Figure 2. Efficiency of coaxial mixer (gas holdup per unit of specific power consumption) at N_c = 284 rpm and 0.15 vvm aeration rate for different configurations at 0.5 wt% CMC solution.