High-rate CO2 sequestration using a novel venturi integrated photobioreactor and subsequent valorization to microalgal lipids

Recovery of waste CO2 from flue gas involves two phase gas-liquid interactions for application in enhanced microalgal growth, CO2 bio-fixation and increased production of microalgal lipids for 4(th) generation biofuels. Mineralized CO2 as bicarbonate between pH 10 and 7 constitutes the major inorganic carbon for phototrophic microalgae. Consolidating Carbon Capture and Usage (CCU) technology with microalgal lipid synthesis, the present work exhibits design and optimization of a microbubble assisted CO2 sequestration reactor to recover CO2 from low concentration sources (5% v/v) and maximize microalgal lipid productivity without any nutrient stress. A venturi microbubble generator was amalgamated with a bubble column and the Reynolds number was used to define the hydrodynamic flow regime. At a steady volumetric liquid flow rate Q(L), corresponding to laminar flow conditions, the gas flow rate Q(G) was varied in the range 0.1-1 L min(-1) to determine the volumetric mass transfer coefficient K(L)a and the CO2 mass transfer efficiency. K(L)a and Q(G) values were found to be linearly dependent in the Q(G) range 0.1-0.7 L min(-1), while CO2 capture efficiencies in the range 86% to 98% were obtained. The optimized medium recirculation sufficed for culture agitation and further aeration for mixing was not required. A correlation coefficient was derived, relating the change in gas-liquid interfacial area with the change in Q(G). The resultant system achieved a biomass yield of 0.454 g L-1 and a carbon content of 38.9% volatile suspended solids (VSS). Furthermore, a lipid accumulation of 35.9% on a dry cell weight basis and a lipid productivity of 11.64 mg L-1 d(-1) exhibited feasible valorisation of sequestered carbon.