(218d) Effect of Gas Sparge Type on Oxygen Mass Transfer in an Agitated Vessel (comparing CFD versus Experimental Modeling Techniques)

Mixers used in minerals processing often require the suspension of solids in combination with gas dispersion and oxygen mass transfer. Some examples of these types of gas-liquid-solid applications include cyanide destruction, cyanide leach tanks, biological leaching, neutralization, and pre-aeration, among others.

Gas is typically injected in a vessel through a sparge system. As presented previously (Richard Kehn and Antonio Iniguez, “Improvements in Mass Transfer by Sparge System Design Optimization for Minerals Processing,” Society of Mining Engineers Conference, Minneapolis, MN, February 2018), it has been shown that an agitated vessel operating under the same air injection rate and gassed power density will yield different oxygen mass transfer rates when different sparging devices are used. The observed change in oxygen mass transfer is due to how the gas interacts with the impeller system, with the physical dispersion and bubble size changing as sparge type is varied. In addition, the mechanical response of the agitator system will also vary by a measurable amount depending on the sparge system chosen. Knowing these variables can allow a plant to optimize both the performance of the agitator system while also considering the cost to implement. The experimental set up consisted of a 1.1 m clear acrylic tank with a flat bottom and four (4) anti-swirl wall baffles and a 406 mm diameter Lightnin A320 wide blade hydrofoil. The wide blade hydrofoil design is typically used for large gassed reactors in minerals processing.

Performing CFD modeling on two phase systems is particularly challenging. It has been shown in previous work (Christopher Tyler and John A. Thomas, “Bubble-Scale Modeling of Gasified Reactors.” AIChE Annual Meeting, Pittsburgh, PA, October 2018.) that the Lattice Boltzmann approach can be used to predict both oxygen mass transfer rates and physical dispersion phenomena. Within this approach, the fluid is modeled as a continuous phase and the bubbles are modeled explicitly as Lagrangian point objects that dynamically break-up and coalesce in response to varying local fluid environments. This approach provides direct access to the bubble size distribution and any spatial variations in the gas transfer rates.

In this work, the Lattice Boltzmann modeling technique is used to model three sparge types: an open pipe versus a plate sparge located above the open pipe versus a ring sparge. The CFD modeling results are then compared to the experimental modeling results for validation and comparison purposes.