Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae
Introduction
Photobioreactors for large-scale monoculture of microalgae have conventionally been designed as devices with large surface–to–volume ratios. Various types of tubular photobioreactors are examples of this approach (Lee, 1986, Borowitzka, 1996). These reactors occupy vast land areas: they are expensive to build; difficult to maintain; and only somewhat scaleable. Tubular photobioreactors can usefully satisfy only medium level production demands. Attempted, large–scale production in horizontal tubular loops has failed quite spectacularly in one case (Fig. 1); hence, other reactor configurations are needed for the production of larger quantities of pharmacologically active compounds that certain microalgae can potentially produce.
The areal productivity, i.e. productivity per unit land area, is low for conventional tubular photobioreactors and in large units as well as modular designs, sterile operation to the levels demanded in the pharmaceutical industry is difficult. Some low surface–to–volume, pneumatically agitated photobioreactors can potentially overcome these significant disadvantages. Examples of the latter type are bubble columns and airlift bioreactors. Large scale culture of microalgae in these systems has not been investigated as it has always been assumed that small surface–to–volume ratios of these devices would make them ineffective. This need not be so as reported in this work which deals with comparative outdoor evaluation of pilot scale bubble column photobioreactors with respect to performance in horizontal tubular loops.
Data are reported on three aspects of comparative characterization: (a) gas–liquid hydrodynamics and mass transfer; (b) internal irradiance levels as functions of Sun’s location relative to the photobioreactors; and (c) performance during culture of the microalga Phaeodactylum tricorntum. Also reported are the effects of hydrodynamics on survival behavior of algal cells.
Section snippets
Comparison of performance
The vertical and the horizontal tubular photobioreactors differ in several significant ways including differences in light regimens, gas–liquid hydrodynamics and mass transfer behavior. Some of these factors—e.g. hydrodynamics and light regimen—are interrelated. Their impact on culture performance is discussed below.
Conclusions
Horizontal tubular photobioreactors are generally believed to be the most practicable culture system for fully contained large-scale monoculture of microalgae, nevertheless, as discussed here, detailed analyses reveal severe limitations of tubular photobioreactors. Unless the concentration of the desired microalgal metabolite in the biomass is unusually high and the market size for the product is exceedingly small, the use of horizontal tubular photobioreactors would be impossible in commercial
Nomenclature
Ad cross-sectional area of downcomer zone (m2) Ar cross-sectional area of riser zone (m2) Cf Fanning friction factor d diameter (m) dB mean bubble diameter (m) E energy dissipation rate per unit mass (W kg−1) EPA eicosapentaenoic acid g gravitational acceleration (m s−2) hc height of the column (m) ℏ solar hour (h) kLaL overall volumetric gas–liquid mass transfer coefficient for oxygen (s−1) L length of tubing (m) Ls length of the shadow from the column’s base (m) length scale of microeddies (m) N day of the year NB number
Acknowledgements
This work was supported by the European Commission contract BRPR CT970537 and CICYT (BIO-98-0522), Spain.
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