Characterization of a flat plate photobioreactor for the production of microalgae

Abstract

Abstract This paper presents the characterization of a flat panel photobioreactor (0.07 m wide, 1.5 m height and 2.5 m length) for the production of microalgae. Several factors are considered. The orientation was studied first resulting east/west the most favourable because the total solar radiation intercepted was maximum, increasing 5% with regard to horizontal placement, and the exposure resulted to be the most homogeneous over the year. Then, gas holdup, mass transfer, mixing and heat transfer were studied as a function of the aeration rate. This is a key operating variable because it determines the power supply, which governs the fluid-dynamics of the system and subsequently influences other transport phenomena. The gas holdup and mass transfer coefficient found were consistent with referenced values for bubble columns observed in tubular photobioreactor. A power supply of 53 W/m 3 promoted a mass transfer rate high enough to avoid the excessive accumulation of dissolved oxygen in this flat panel photobioreactor. This is similar to the 40 W/m 3 necessary in bubble columns and much lower than the 2000–3000 W/m 3 required in tubular photobioreactors. However, this power supply is in the order of magnitude of 100 W/m 3 , which has been reported to damage some microalgal cells, whereas no damage has been referenced in tubular photobioreactors. Even at low power supplies the mixing time was shorter than 200 s, longer than the 60 s measured for bubble columns, but quite faster than the typical values found for tubular photobioreactors (1–10 h). With regard to heat transfer, global coefficients were determined for the internal heat exchanger and for the external surface of the photobioreactor. The observed behaviour was similar to that referenced for bubble columns, although the values of heat transfer coefficients measured were lower than in bubble columns. The heat transfer coefficient of the internal heat exchanger (over 500 W/m 2  K) was much higher than the coefficient of the external surface of the reactor (30 W/m 2  K). Internal heat exchangers are therefore useful to control the temperature of the culture in this type of photobioreactor. The major disadvantage of this reactor is the potential high stress damage associated with aeration. The main advantages are the low power consumption (53 W/m 3 ) and the high mass transfer capacity (0.007 1/s). The characterization carried out allows improving the design and establishing the proper operating conditions for the production of microalgae using this type of photobioreactor.