(431e) Comparison of CFD Simulations to Experimental Results for Blend Time of Turbulent Newtonian Fluids in Stirred Tanks

Blending, the process of combining miscible components into homogeneous solutions, is a common unit operation performed by rotating agitators. Industrialists are interested in minimizing the blend time to decrease operating periods and increase throughput while also minimizing capital and operating costs. Therefore, agitator manufacturers are driven to provide efficient and cost effective solutions for blending applications as well as accurate blend time predictions.

A blend time investigation for stirred tanks was conducted to study the effect of impeller diameter (D = T/2, T/3, and T/5), impeller off-bottom distance (C = T/2, T/3, and T/5), and impeller type (A310 Hydrofoil, Pitched-Blade Turbine, and Rushton Turbine) while considering tank diameter (T = 0.80 and 1.22 m) and mean specific energy dissipation (ε = 0.005 and 0.010 W/kg) for fully turbulent Newtonian fluids. The response of conductivity probes to the injection of a saturated NaCl tracer was monitored to experimentally measure blend time for 108 unique configurations produced by a full factorial experimental design. The results of the experimental blend time measurements were analyzed to determine trends and correlation coefficients were fit from the data. A growing body of findings has been presented at the 2017 AIChE Annual Meeting, published in the submitting author’s master’s thesis, “Investigation of Blend Time for Turbulent Newtonian Fluids in Stirred Tanks”, and presented at NAMF Mixing XXVI.

The latest findings presented at NAMF Mixing XXVI suggested that the optimum impeller type for turbulent blending was a function of the impeller diameter to tank diameter ratio (D/T) with an inflection point occurring at a D/T of 0.30. Below 0.30 D/T, the A310 hydrofoil was the most efficient impeller type. Above 0.30, the Rushton Turbine was the most efficient impeller type. At 0.30, all three impeller types were equivalent. This conclusion contradicted the two main turbulent blending theories: the first being impeller type independence and the second being that flow efficient impeller types, like the A310 hydrofoil, are more energy efficient at blending. A mechanism for explaining the findings was not proposed, but a combination of exploratory theoretical derivations and transient CFD simulations with LES filter were proposed to probe for an explanation.

Up to this point, no simulation-based approaches have been utilized in the investigation. Therefore, the first goal is to validate a simulation methodology that may then be used for mechanism exploration. A subset of the experimental plan will be simulated. All three impeller types (A310 Hydrofoil, Pitched-Blade Turbine, and Rushton Turbine) at all three impeller diameters (D = T/2, T/3, and T/5) will be simulated, but impeller off-bottom will be fixed at T/3, tank diameter fixed at 1.22 m, and mean specific energy dissipation fixed at 0.010 W/kg. Probe and tracer injection locations in the simulations will be identical to the experiments, and blend time will be determined using identical methodology. The validation criteria will be that simulation-derived blend times must fall within ±14% of the experimentally determined blend times as that was the average relative variation among experimental trials.

The simulation methodology that will be studied is a transient Lattice Boltzmann solver with LES filter. This methodology will be executed using M-Star CFD. The suitability of using M-Star CFD to predict blend time of turbulent Newtonian fluids in stirred tanks will be discussed and next steps will be proposed. Since this modeling approach is physics based, preliminary insights into the flow mechanisms driving this impeller-dependent blending behavior will be presented.