Scale-up of tubular photobioreactors

The effect of the light/dark cycle frequency on the productivity of algal culture at different day-averaged irradiance conditions was evaluated for Phaeodactylum tricornutum grown in outdoor tubular photobioreactors. The photobioreactor scale-up problem was analyzed by establishing the frequency of light-dark cycling of cells and ensuring that the cycle frequency remained unchanged on scale-up. The hydrodynamics and geometry related factors were identified for assuring an unchanged light/dark cycle. The light/dark cycle time in two different tubular photobioreactors was shown to be identical when the linear culture velocity in the large scale device (U-LL) and that in the small scale unit (U-LS) were related as follows: U-LL = f(9/7)/alpha U-8/7(LS). Here f is the scale factor (i.e., the ratio of large-to-small tube diameters), alpha is a function of the illuminated volumes in the two reactors, and 'dark' refers to any zone of the reactor where the light intensity is less than the saturation value. The above equation was tested in continuous cultures of P. tricornutum in reactors with 0.03 m and 0.06 m diameter tubes, and over the workable culture velocity range of 0.23 to 0.50 m s(-1). The predicted maximum realistic photobioreactor tube diameter was about 0.10 m for assuring a culture performance identical to that in reactors with smaller tubes.