(218f) Modeling Mixing in Aerated Systems: Mass Transport and Flooding

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.

Oxygen transport also depends on the partial pressure of oxygen in the bubbles. In bubble columns, the oxygen content decreases as a bubble rises through the column and oxygen is transferred into the liquid phase, and the pressure decreases as well. Both effects contribute to the partial pressure of oxygen and need to be considered in predicting oxygen transport.

In this work, we use lattice-Boltzmann simulations to model gasified reactors with a two-way coupling between the fluid and gas bubbles. Within the context of this solver, it is practical to track the trajectories of hundreds of millions of individual bubbles—populations that are comparable the total number of bubbles within typical industrial-scale gasified systems. We compare gas hold-up and free surface height in response to gas injection for airlift columns. For agitated systems, we compare predicted power draw, residence time distribution, and interfacial area to experimental values. We show that we can predict the onset of flooding. We conclude by discussing means of modeling explicitly the oxygen transport, accounting for changes in partial pressure due to hydraulic head and oxygen consumption, and compare the results to experimental data.