Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae

Engineering analyses combined with experimental observations in horizontal tubular photobioreactors and vertical bubble columns are used to demonstrat e the potential of pneumatically mixed vertical devices for large-scale out door culture of photosynthetic microorganisms. Whereas the horizontal tubul ar systems have been extensively investigated, their scalability is limited . Horizontal tubular photobioreactors and vertical bubble column type units differ substantially in many ways, particularly with respect to the surfac e-to-volume ratio, the amount of gas in dispersion, the gas-liquid mass tra nsfer characteristics, the nature of the fluid movement and the internal ir radiance levels. As illustrated for eicosapentaenoic acid production from t he microalga Phaeodactylum tricornutum, a realistic commercial process cann ot rely on horizontal tubular photobioreactor technology. In bubble columns , presence of gas bubbles generally enhances internal irradiance when the S un is low on the horizon. Near solar noon, the bubbles diminish the interna l column irradiance relative to the ungassed state. The optimal dimensions of vertical column photobioreactors are about 0.2 m diameter and 4 m column height. Parallel east-west oriented rows of such columns located at 36.8 d egrees N latitude need an optimal inter-row spacing of about 3.5 m. In vert ical columns the biomass productivity varies substantially during the year: the peak productivity during summer may be several times greater than in t he winter. This seasonal variation occurs also in horizontal tubular units, but is much less pronounced. Under identical conditions, the volumetric bi omass productivity in a bubble column is similar to 60% of that in a 0.06 m diameter horizontal tubular loop, but there is substantial scope for raisi ng this value. (C) 1999 Elsevier Science B.V. All rights reserved.