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

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Abstract

Engineering analyses combined with experimental observations in horizontal tubular photobioreactors and vertical bubble columns are used to demonstrate the potential of pneumatically mixed vertical devices for large-scale outdoor culture of photosynthetic microorganisms. Whereas the horizontal tubular 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 surface–to–volume ratio, the amount of gas in dispersion, the gas–liquid mass transfer characteristics, the nature of the fluid movement and the internal irradiance levels. As illustrated for eicosapentaenoic acid production from the microalga Phaeodactylum tricornutum, a realistic commercial process cannot rely on horizontal tubular photobioreactor technology. In bubble columns, presence of gas bubbles generally enhances internal irradiance when the Sun is low on the horizon. Near solar noon, the bubbles diminish the internal 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°N latitude need an optimal inter-row spacing of about 3.5 m. In vertical columns the biomass productivity varies substantially during the year: the peak productivity during summer may be several times greater than in the winter. This seasonal variation occurs also in horizontal tubular units, but is much less pronounced. Under identical conditions, the volumetric biomass productivity in a bubble column is ∼60% of that in a 0.06 m diameter horizontal tubular loop, but there is substantial scope for raising this value.

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

Adcross-sectional area of downcomer zone (m2)
Arcross-sectional area of riser zone (m2)
CfFanning friction factor
ddiameter (m)
dBmean bubble diameter (m)
Eenergy dissipation rate per unit mass (W kg−1)
EPAeicosapentaenoic acid
ggravitational acceleration (m s−2)
hcheight of the column (m)
solar hour (h)
kLaLoverall volumetric gas–liquid mass transfer coefficient for oxygen (s−1)
Llength of tubing (m)
Lslength of the shadow from the column’s base (m)
llength scale of microeddies (m)
Nday of the year
NBnumber

Acknowledgements

This work was supported by the European Commission contract BRPR CT970537 and CICYT (BIO-98-0522), Spain.

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