(297e) Bubble-Scale Modeling of Gasified Reactors

Aerobic and microaerobic fermentation processes require significant aeration for proper organism performance in both stirred tanks and air-lift columns. Aeration provides oxygen for fermentation as well as mixing for the bulk fermenter. Air flows are high and have a significant impact on the overall fluid flow. In the case of air-lift columns, air flow is the only mechanism for fluid flow.

Fermentation productivity is often limited by the rate of oxygen transport into the system, characterized by the bulk mass transfer coefficient k_La, a product of the local mass transfer coefficient k_L and the interfacial area a, both strongly dependent on the turbulent dissipation. Both terms are difficult to predict by standard CFD methodology.

In this work, we use lattice-Boltzmann simulations to model gasified reactors with a two-way coupling between the fluid and gas bubbles. We begin by showing how it is practical to track the trajectories of hundreds of millions of individual bubbles—populations that are comparable to the total number of bubbles within typical industrial-scale gasified systems. Next, we review experimental correlations used to predict the coalescence behavior of bubble pairs, as a function of the approach velocity and associated Weber number. We then review experimental data related to bubble break-up, as a function of fluid shear and power dissipation. Then, by superimposing these relationships on to the explicit bubble trajectories/collisions captured in the simulation, we model and simulate the real-time, spatiotemporal variation in bubble diameter. Finally, we use various approaches to connect bubble size and turbulence to k_La. We compare these predictions to available experimental data, including gas hold-up, gasified power draw, and mass transfer rates.