Role of oxygen in tubular photobioreactors: Model-Based design and operating conditions to minimize productivity losses

Tubular photobioreactors (PBRs) guarantee high microalgal productivities but suffer from oxygen accumulation. It is known that the tube length must be limited to prevent build-up of high oxygen levels, but the combined effect of other variables (light intensity and biomass concentration) was not fully addressed. In this work, a mathematical model is developed to understand the influence of oxygen on biomass productivity in a continuous tubular PBR. Material balances are applied to investigate the behavior of a single tube reactor and of a complete process flowsheet of a commercial plant. Biomass concentration at the inlet resulted the key variable to minimize oxygen inhibition, confirming the solid retention time (SRT) as the main operating variable. However, an optimized length of the tube can minimize the effect of biomass concentration. Finally, it was observed that measuring the O2 concentration alone is not a reliable index of the overall productivity in a PBR.