Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance

A methodology is presented for designing photobioreactors with tubular loop solar receivers in which the fluid is circulated by an airlift device. The design method effectively combines the relevant aspects of external irradiance-dependent cell growth, oxygen accumulation in the solar loop, oxygen removal in the airlift device, and hydrodynamics of the airlift system that determine the flow velocity through the solar receiver. The design approach developed was used to model and build a 0.2 m ³ outdoor photobioreactor. A compact degasser in the airlift section eliminated dead zones and dark zones, while achieving complete separation of gas and liquid. The measured gas-liquid hydrodynamics, mass transfer, and culture productivity were consistent with the model predictions. The reactor was tested with continuous culture of the microalga Phaeodactylum tricornutum at various liquid velocities through the tubular solar receiver. A biomass productivity of 1.20 g l ^-1 d ^-1 (or 20 g m ^-2 d ^-1) was obtained at a dilution rate of 0.050 h ^-1 . Solar receiver linear liquid velocities of 0.50 and 0.35 m s ^-1 gave similar biomass productivities, but the culture collapsed at lower velocities. An adverse effect of high dissolved oxygen concentration on productivity was observed. Oxygen accumulation could be reduced by increasing the liquid velocity and this enhanced the biomass yield.