Algal Reactor Design Based on Comprehensive Modeling of Light and Mixing

Abstract

The prospect of autotrophic (or light-driven) algal biomass production as a sustainable substitute for fossil feedstocks has yet to fulfill its potential. As a likely cause, the inability to robustly account for algal biomass production rates has prevented the derivation of satisfactory mass balances for the simple parameterization of bioreactors. The methodology presented here aims at resolving this shortcoming. Treating photons as a substrate continuously fed to algae provides the grounds to define an autotrophic yield Ф^DW, in grams of dry weight per mole of photons absorbed, as an operating parameter. Under low irradiances, the rate of algal biomass synthesis is the product of the yield Ф^DW and the flux of photons absorbed by the culture, modeled using a scatter-corrected polychromatic Beer-Lambert law. This work addresses the broad misconception that Photosynthesis-Irradiance curves, or the equivalent use of specific growth rate expressions independent of the biomass concentration, can be extended to adequately model biomass production under light-limitation. Since low photon fluxes per cell maximize Ф^DW, the photosynthetic units mechanistic model was adapted to determine a corresponding maximum residence time under high light. Such high speeds in the photic zone, which call for fundamental changes in bioreactor design, enable the use of Ф^DW to describe biomass productivity under otherwise inhibitory irradiances. Nitrogen limitation-induced lipid accumulation corresponds to a photon flux excess with respect to the rate of nitrogen uptake, such that continuous lipid production can be achieved using the Ф^DW and nitrogen quotient parameters. Additionally, energy to photon-counts conversion factors are derived.