Swirling flow implementation in a photobioreactor for batch and continuous cultures of porphyridium cruentum

Abstract

Light is the main limiting factor in photoautotrophic-intensive production of microorganisms, and improvement of its use is an important concern for photobioreactor design and operation. Swirling flows, which are known to improve mass and photon transfers, were applied to annular light chambers of a photobioreactor and studied by simulation and microalgal culture. Two hydrodynamic conditions were compared: axial flow generating poor radial mixing, and tangential flow generating three-dimensional swirling motion. Batch and continuous cultures of the Rhodophyte Porphyridium cruentum were performed in a 100-L, 1.5-m(2), fully controlled photobioreactor with eight light chambers. The inlet design of these chambers was modified to create the hydrodynamic conditions for comparison. Various intensities of swirling motion were used, characterized by the velocity factor (VF), defined as the ratio between annular chamber flow and inlet aperture sections. Experiments were performed within the range of photon flux densities (PFD) optimizing the yield of light energy transformation into living substance for the species and the temperature used. Culture kinetics with swirling flows generated by apertures of VF = 2, 4, and 9 were compared with pseudoaxial VF = 2 chosen as reference. Batch cultures with VF = 4 swirling flow showed no significant difference, whereas continuous cultures proved more discriminating. Although no significant difference was obtained for VF = 2, a 7% increase of steady-state productivity and a 26% decrease in time required to reach this steady state were obtained with VF = 4 swirling flow. This beneficial effect of swirling flow could have accounted for increased mixing. Conversely, VF = 9 swirling flow resulted in a 9% decrease of steady-state productivity and a 9% increase in the time required to reach this steady state, a negative effect that could have accounted for increased shear stress. CO(2) bioconversion yield at steady state showed a 34% increase for VF = 4. These results suggest that swirling motion makes microalgal cultures more efficient, provided that the resulting adverse effects remain acceptable. Experimental investigation was completed by a theoretical approach in which simulation of continuous cultures of P. cruentum was based on the hydrodynamic conditions achieved in the photobioreactor. Although the results obtained with pseudoaxial flow were correctly predicted, simulations with swirling flow showed a marked enhancement of productivity not observed experimentally. The influence of side effects induced by increased mixing (particularly hydrodynamic shear stress) was considered with respect to modeling assumptions. Comparison of experimental results with theoretical simulation provided a better understanding of the mixing effect, a key factor in improving the efficiency of such bioprocesses.

Design principles of photo‐bioreactors for cultivation of microalgae

“…This could be an option for improved annular reactors, although the technical expenditure is quite high. The employment of tangential flow to generate a threedimensional swirling motion is given by [33].…”

Exploitation of marine algae: biogenic compounds for potential antifouling applications

“…Barahona and Rorrer (2003) have shown that bioreactor tissue culture can be used successfully for production of secondary metabolites from marine algae within a controlled environment using species of Ochtodes secundiramea and Portieria hornemannii. Muller-Feuga et al (2003) have implemented swirling flow in a photobioreactor for batch cultures of the red alga Porphyridium cruentum. Since light is one of the principal factors for the growth of photoautotrophic organisms such as marine algae, swirling-flow modifications help in moving biomass into areas with higher Fig.…”

Industrial Cultivation Systems for Intensive Production of Microalgae

“…12.12A) or another type of swirling flow inducer (Fig. 12.12B) can enhance the turbulence mixing degree in the photobioreactor [60,66,67,83,84,85,86,87]. In photobioreactors operated with gas-liquid two-phase flow, the turbulent mixing is bubble induced [88].…”