Advanced CFD model of multiphase photobioreactors for microalgal derived biomass production
Development of more efficient algal photobioreactors (PBRs) is driven by increasing interest in algaculture for the production of fuels, chemicals, food, animal feed, and medicine, as well as carbon capture. While at present, the cost and microalgae production capacity are one of its restrictions when competition with other biodiesel feedstock. The objective of the present work is to develop and validate better computational models to investigate the interplay between fluid hydrodynamics, radiation transport and algae growth, which is crucial to determine the performance and scalability of algae photobioreactors.
First, a detailed review of the pertinent information required for developing a comprehensive computation model for photobioreactors was conducted. The current status of the submodels, including hydrodynamics and mass transfer multiphase CFD models, radiation transport models, microalgae growth rate models, and coupling method for developing a comprehensive model for PBRs was outlined.
Second, an Eulerian two-fluid model for gas-liquid Taylor-Couette flow was proposed and validated. The CFD was based on the RANS approach with constitutive closures for interphase forces and liquid turbulence. The model was validated by comparison with previously published experimental data. The mechanism of bubble radial non-uniformity distribution was discussed and the relative importance of various interphase forces was demonstrated.
Third, the validated two fluid CFD model was employed to simulate the local values of the mass transfer coefficient based on the Higbie theory. A novel approach was proposed to estimate the mass transfer exposure time. This approach automatically selects the appropriate expression (either the penetration model or eddy cell model) based on local flow conditions. The simulation predictions agree well with experimental foundlings, which demonstrates that the adaptive mass transfer model has the ability to correctly description of both local and global mass transfer of oxygen in a semi-batch gas–liquid Taylor–Couette reactor.
Forth, microalgae culture experiment was conducted to identify the limiting factor in the Taylor-Couette photobioreactor. The characteristic time scales for mixing, mass transfer and biomass growth was compared. It is found that algal growth rate in Taylor vortex reactors is not limited by fluid mixing or interphase mass transfer, and therefore the observed biomass productivity improvements are likely attributable to improved light utilization efficiency (high-frequency light/dark cycles).
Fifth, a commonly used Lagrangian strategy for coupling the various factors influencing algal growth was employed whereby results from computational fluid dynamics and radiation transport simulations were used to compute numerous microorganism light exposure histories, and this information, in turn, was used to estimate the global biomass specific growth rate. The simulation predictions were compared with experimental measurements and the origin of weaknesses of the commonly used Lagrangian approach model was traced.
Sixth, an alternative Eulerian computational approach for predicting photobioreactor performance was proposed, wherein a transport equation for algal growth kinetics is solved, thereby obviating the need to carry out thousands of particle tracking simulations. The simulation predictions were compared with experimental measurements and commonly used Lagrangian approach model.