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

Abstract

A methodology is presented for designing photobioreactors with tubular loop solar receivers in which the #uid is circulated by an airlift device. The design method e!ectively 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 #ow velocity through the solar receiver. The design approach developed was used to model and build a 0.2 m 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\ d\ (or 20 g m\ d\) was obtained at a dilution rate of 0.050 h\. Solar receiver linear liquid velocities of 0.50 and 0.35 m s\ gave similar biomass productivities, but the culture collapsed at lower velocities. An adverse e!ect 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.

Design principles of photo‐bioreactors for cultivation of microalgae

“…1 cm [45] quite high biomass concentrations (see aspect 2) of more than 6 g/L can be obtained. Of course the arrangement of the tubes has to be calculated to achieve the most homogeneous incident light conditions [46]. Pumping of the medium with linear liquid velocities of 20 cm/s to 50 cm/s is either done by airlift circulators or by centrifugal pumps.…”

“…Aeration and degassing is also achieved in the airlift part, while a separate gas exchange station has to be supplied along with the pumps. A velocity of more than 1 m/s will cause micro eddies of less than 50 mm diameters which potentially can damage the cells [46]. The relatively high velocity is necessary to achieve turbulent conditions leading to acceptable light/dark cycles [31].…”

“…Nevertheless, tube lengths of more than 100 m are good enough to build up quite big outdoor photo-bioreactors. Large installations can be found in the Negev (Israel) [52] or in Almeria [46]. Actually, the world-wide largest closed photobioreactor was erected in Klötze (Saxon-Anhalt, Germany) by Bisantech, following the instructions of IGV [53], Fig.…”

Optimization of Hydraulic Retention Time and Biomass Concentration in Microalgae Biomass Production from Treated Sewage with a Membrane Photobioreactor

“…Secondary treatment effluent from activated sludge processes could serve as a good cultivation medium because it also contains high levels of carbonate, as well as nitrogen and phosphorus. However, nutrient contents in wastewater and treated wastewater are much lower than those in cultivation media that has typically been used in past studies of microalgae cultivation [4][5][6][7][8].…”

A Dynamic Optimization Model for Designing Open-Channel Raceway Ponds for Batch Production of Algal Biomass

“…Power required for maintaining flow in the raceway, i.e., power of paddlewheel is given by Equation (41) [36]. Power required for pumping the inflow water that is added to compensate for annual evaporation losses is given by Equation (42). Annual energy requirements of paddle wheel and pumping the inflow water are calculated in Equations (43) and (44).…”