(697d) Computational Fluid Dynamics Simulation of Biomethanation in a Large-Scale Gas Bioreactor

Biomethanation (the process of methane formation by anaerobic microbes from hydrogen and carbon dioxide) at industrial scales is a promising solution to more sustainable and carbon-neutral hydrocarbon fuel production. However, gas fermentation at large scales have technical challenges that can unfavorably impact the productivity and operating costs. Low concentration of low-solubility dissolved gaseous components in the water, e.g., hydrogen in our case, can inhibit the fermentation if not thoroughly distributed throughout the reactor. On the other hand, if we oversupply substrate gas to the bioreactor, we will have unconsumed reactants that need to be captured and separated from the product gas (i.e., methane) at the gas exit line of the bioreactor. Reactor designs with optimal size and mixer positioning and actuation, gas sparging flow rates, mixer speed, etc., can alleviate these challenges and achieve high productivity and efficiency. Predictive computational fluid dynamics (CFD) simulations can significantly help with design and identification of optimal operating conditions for such bioreactors at large scales.

In this work, we have performed CFD simulations for a 30 L bioreactor. A previously developed multiphase Euler-Euler model [1] was modified to incorporate the additional gaseous species and multi-species reaction kinetics. This model is a customized solver derived from OpenFOAM [2], an open-source CFD toolbox. Our simulations include interphase mass transfer as well as species transfer between the phases. The microorganism activity is represented by simple phenomenological reaction kinetics for the following reaction:

CO₂ + 4 H₂ → CH₄+ 2 H₂O

The dissolved hydrogen and carbon dioxide uptake rate and the methane production rate were incorporated in the kinetic reaction model. The mixer is a group of axially separated Rushton impellers whose parameters (number of blades, spacing between impellers, rotational speeds) can be varied at run time allowing for optimization of mixer design. The bioreactor is pressurized to 9 bar and preheated to 60 °C. We investigated the effect of gas flow rate and mixer speed on the performance of overall methane production and have also identified the regions in the bioreactor with low dissolved hydrogen and carbon dioxide concentrations. The simulation results will be used to help with the design of a future large-scale methane fermenting bioreactor.

References:

[1] Rahimi, M. J., Sitaraman, H., Humbird, D., and Stickel, J. J. (2018). Computational fluid dynamics study of full-scale aerobic bioreactors: Evaluation of gas–liquid mass transfer, oxygen uptake, and dynamic oxygen distribution. Chemical Engineering Research and Design, 139, 283–295.

[2] Weller, H., Tabor, G., Jasak, H. and Fureby, C., A tensorial approach to computational continuum mechanics using object-oriented techniques. Computers in physics 12, no. 6 (1998): 620-631.