Experimental and Numerical Investigation of Fluid Flow and Mixing in Pachuca Tanks

This study reports laboratory and computational work carried out to determine the effect of operating and design parameters on the motion of the liquid and the mixing of a solute in Pachuca tanks. In the laboratory tanks, the liquid velocity field was measured using particle image velocimetry (PIV) and mixing was characterized using a stimulus-response technique. Visualization of the air-water flow suggested the suitability of a one-phase variable density turbulent recirculating flow model coupled to the drift flux model, to describe the motion of the water phase and the gas holdup, in two dimensions (2-D) and in steady state. The dispersion of the solute tracer was simulated by solving the unsteady state turbulent mass transfer equation in three dimensions (3-D). The computational predictions give a good estimate of laboratory measurements of the influence that operating and design variables have on liquid circulation velocity, liquid flow pattern, gas holdup, and mixing evolution and time. Fluid flow simulation of industrial-scale tanks revealed that the recirculating loop that forms in their annular section is more intense and extends over a larger proportion of the reactor height as the draft tube/tank diameters ratio, d _d /d _t , decreases, at the same time the superficial liquid velocity in the draft tube increases. These features suggest that tanks with d _d /d _t ∼ 0.1 promote conditions for good particle suspension by hindering the settling of particles in the annulus and favoring their lifting in the draft tube; in laboratory-scale tanks, the flow characteristics that enhance particle suspension are not as apparent. The mathematical model also predicts different solute mixing behavior between the laboratory and industrial-scale tanks. At low superficial gas velocities (u _sg  ≤ 0.003 m/s) the effect of the increasing d _d /d _t on the decreasing mixing time is smaller in the last tanks. Hence, according to the calculations, it should be expected that industrial tanks with d _d /d _t  = 0.1 have advantages in regard to particle suspension in comparison to tanks with d _d /d _t  = 0.5 and, at the same time, they should be comparable in respect to solute mixing under low superficial air velocities, at which they also exhibit good energy efficiency.