(170e) Impact of Tracer Density and Injection Location on Blend Time Determination of Turbulent Newtonian Fluids in Stirred Tanks

Blending, the process of combining miscible components into homogenous solutions, is a common unit operation performed by stirred tanks. Measuring and predicting blend time in stirred tanks for a wide variety of conditions has been investigated by countless researchers (Grenville, 1992; Nienow, 1997). In prior work, the submitting author experimentally measured blend time by introducing a concentrated NaCl tracer solution into a stirred tank of water and monitored the response of conductivity probes (Strand, 2017). The prior work focused on predicting the blend time of turbulent Newtonian fluids in stirred tanks and correlations were derived, compared, and presented (Strand, 2018). A frequent curiosity that arose during feedback on the prior work was whether the elevated density of the tracer decreased the blend time since the tracer was injected near the liquid surface and sank towards the impeller. The low volume fraction of the injection (0.12%) and mild density difference (200 kg/m³) suggested minimal impact, but past studies have indicated the potential for density dependence (Bouwmans et al., 1997; Gogate and Pandit, 1999). The impact of tracer density and injection location on blend time determination of turbulent Newtonian fluids in stirred tanks will be investigated computationally using M-Star CFD.

M-Star CFD, a transient Lattice Boltzmann solver with large eddy simulation filter, was previously validated for predicting the blend time for a subset of the experimental body of work (Strand and Thomas, 2019). This study will utilize one of the experimental configurations within the validated subset. The tank diameter (T) will be fixed at 1.22 m, water level (H) will be fixed at H = T, impeller diameter (D) will be fixed at D = T/3, impeller off-bottom (C) will be fixed at C = T/3, mean specific energy dissipation will be fixed at 0.010 W/kg, and the impeller type will be an A310 Hydrofoil. If time allows, a Rushton Turbine will also be investigated in the same configuration. The tracer density will be varied at three levels (ρ = 800, 1000, and 1200 kg/m³) and the tracer injection location (TIL) will be varied at two levels (TIL = experimental location near liquid surface and translated to the lower-tangent-line). This will create an experimental design of six configurations. Six blend time measurements will be performed for each configuration’s simulation by staggering six tracer injections across the simulation duration and determining the time required for each of the six tracer injections to reach 95% homogeneity within the bulk fluid. The results will be analyzed and compared to determine if tracer density and/or injection location significantly impacted the blend time determination in past studies pertaining to the blend time of turbulent Newtonian fluids in stirred tanks.

References:

Bouwmans, I., Bakker, A., and Van Den Akker, H. E. A., Blending Liquids of Differing Viscosities and Densities in Stirred Vessels. Chemical Engineering Research and Design, Volume 75, Issue 8. 1997.

Gogate, P. and Pandit, A., Mixing of Miscible Liquids with Density Differences: Effect of Volume and Density of the Tracer Fluid. The Canadian Journal of Chemical Engineering, Volume 77. October, 1999.

Grenville, R. K., Blending of Viscous Newtonian and Pseudo-Plastic Fluids. Cranfield Institute of Technology. April, 1992.

Nienow, A. W., On impeller circulation and mixing effectiveness in the turbulent flow regime. Chemical Engineering Science, Volume 52, Number 15. 1997.

Strand, A. Investigation of Blend Time for Turbulent Newtonian Fluids in Stirred Tanks. Rochester Institute of Technology. ProQuest Dissertations Publishing. 2017.

Strand, A. Investigation of Blend Time for Turbulent Newtonian Fluids in Stirred Tanks: A Second Analysis. NAMF Mixing XXVI. 2018.

Strand, A. and Thomas, J., Comparison of CFD Simulations to Experimental Results for Blend Time of Turbulent Newtonian Fluids in Stirred Tanks. AIChE Annual Meeting 2019. 2019.