Modeling of bioreactors

Chemicals and biofuel components can be produced with the help of microorganisms in bioreactors. Microorganisms have very specific demands for optimal growth which makes the design and especially scale up of bioreactors challenging. Bioreactors are multiphase systems with complex hydrodynamics. Mathematical modeling can provide information on the functioning of bioreactors without the need for experimentation. Computational fluid dynamic (CFD) calculations are combined with a compartmental modeling approach for the calculation of 1) bubble column photobioreactors for the cultivation of algal cells and 2) large aerated stirred tanks for the cultivation of aerobic bacteria. Inclusion of dispersed phase population balances, mass transfer and reaction models makes the compartmental model very comprehensive. Local values of variables such as oxygen concentrations are attained. Simulation of bubble columns and heterogeneous conditions is enhanced with a new model for the calculation of dispersed phase flows that allows for bubble induced flow. New models for the consideration of light distribution and algal growth kinetics in the compartmental model facilitate the comprehensive simulation of algal growth. The results correspond well with measurements taken from literature and provide new insights to the design and run strategies of photobioreactors. It is shown that large industrial-sized stirred tank bioreactors operate at heterogeneous conditions due to the high oxygen demand of microorganisms. A new simple model estimates gas holdup and mass transfer rates at these conditions. Furthermore, a large stirred tank bioreactor is modeled at homogeneous and heterogeneous conditions with the compartmental modeling approach. Models accounting for the increase in coalescence at high volume fractions are required, without them the mass transfer results are too optimistic. A new reaction model for the uptake of oxygen enables the calculation of local reactor oxygen mass transfer capability at different hydrodynamic conditions. Dead spaces in the reactor are found and suggestions made for reactor design. The importance of considering local conditions in bioreactor modeling is shown by comparison to model runs where local differences are neglected. Omission of local detail leads to different growth dynamics in photobioreactors and overestimated mass transfer in stirred tank bioreactors. The study of outside cultivation in photobioreactors shows that the existence of the night, rather than the light extinction due to biomass during the day, decreases the maximum achievable cell density. Therefore, further modeling studies to the feasibility and optimum run strategy of outside cultivation are warranted. The study of large stirred tank bioreactors presents several issues concerning the use of these reactors in large scale. Designs of vessels and mixers can be altered, but eventually, a change to bubble driven systems such as airlift reactors should be considered.