Computational-fluid-dynamics study of full-scale aerobic bioreactors: Evaluation of dynamic oxygen distribution, gas-liquid mass transfer, and reaction.

Several potential routes for the biological conversion of sugars to fungible biofuels or bioproducts utilize aerobic metabolic pathways. Submerged aerobic bioreaction presents several engineering and economic challenges for producing commodity products. Very large reactor vessels (~500 m³ or bigger) are proposed in order to achieve sufficient economies of scale. Our initial evaluation indicates that bubble-column reactors are more economical than stirred-tank reactors. Nonetheless, the cost of sparger operation could be substantial. Existing engineering models are inadequate for determining the optimal sparge rate with reasonable confidence.

In order to make progress towards designing economical bioreactors, we are performing computational-fluid-dynamics (CFD) simulations of bubble-column reactors. A two-phase Euler model is used to explicitly account for the spatial distribution of air (i.e., gas bubbles) in the reactor. Mass transfer of oxygen (O₂) and carbon dioxide (CO₂) between the gas and liquid phases is included in the simulation.  A simple phenomenological reaction model that converts oxygen to carbon dioxide in the liquid phase is used to represent the aerobic bioreaction. The simulations reproduce the experimentally observed net upward flow of liquid in the center of column and downward flow near the wall. At high sparge rates, the recirculating turbulent flow provides adequate mixing and sufficient dissolved oxygen content throughout the reactor. Regions of depleted oxygen are observed at low sparge rates. Sparge rates necessary to avoid oxygen limitation are determined for various reactor sizes and aspect ratios. Finally, the costs of the reaction vessel and sparger operation are estimated for a prototypical bubble-column reactor.