NOVEL INTERNALLY ILLUMINATED STIRRED-TANK PHOTOBIOREACTOR FOR LARGE-SCALE CULTIVATION OF PHOTOSYNTHETIC CELLS

A new concept in photobioreactor design with reactor scale-up as a pri mary design criterion is proposed for the development of large-scale s tirred tank photobioreactors. A photobioreactor is considered as consi sting of units, with each unit being composed of a light source (or li ght distributing object) and its surroundings. The light supply coeffi cient of each unit depends on its size and the light intensity of the light source. At a given light intensity, the optimum unit size which gives the desired light supply coefficient for the target process is e xperimentally determined. A large photobioreactor with a desired light supply coefficient is obtained by increasing the number of units. A p rototype photobioreactor, consisting of 4 units, was constructed for t he cultivation of Chlorella pyrenoidosa. Each unit was equipped with a centrally fixed glass tube into which the light source was inserted. The illuminating system consisted of either 4-W fluorescent or halogen lamps with controllable light intensity. By changing the light intens ity, it is possible to use the photobioreactor for the cultivation of various cells with different optima light supply coefficients. Mixing was achieved by means of an impeller, designed in such a way that whil e rotating it does not touch the glass tubes, which also serve as baff le plates. Although the hydrodynamic stress generated by the impeller was low, a high degree of mixing was achieved even at low rotation spe eds. Since the light distributing objects were not mechanically fixed to the reactor, and were separated from the broth by the glass tubes, the reactor could be sterilized by autoclaving and the light distribut ing objects inserted to the glass tubes after cooling. The photobiorea ctor was equipped with a ring sparger for aeration. When C. pyrenoidos a was cultivated in the new photobioreactor at low and moderately high light supply coefficients, both the linear growth rates and the final cell concentrations increased linearly with the light supply coeffici ent of the reactor. A comparison of the results obtained in this new p hotobioreactor with those of tbe commercially available photobioreacto rs with either external or internal illumination showed that the cell yield from the supplied light energy was highest in the new photobiore actor.