Developing a Spiral-Ascending CO2 Dissolver to Enhance CO2 Mass Transfer in a Horizontal Tubular Photobioreactor for Improved Microalgal Growth

In order to enhance the CO2 dissolution rate and prolong gas–liquid contact time to improve microalgal growth, a spiral-ascending CO2 dissolver was developed to enhance CO2 mass transfer in a horizontal tubular photobioreactor. A spiral-ascending flow pattern of CO2 bubbles was formed with a helical baffle and central hollow tube in the CO2 dissolver, which markedly extended flow trajectory of CO2 bubbles and intensified the mixing effect. This novel CO2 dissolver with the helical baffle shortened the bubble generation time by 30.6% and decreased the bubble generation diameter by 23.4%, thereby enhancing the mass transfer coefficient by 69.2%. The bubble retention time dramatically increased by 190.2%, which prolonged CO2 gas–liquid contact time to enhance dissolved CO2 concentration. The liquid phase mixing time decreased by 15.8% to give a uniform status in microalgal suspension, allowing for sufficient nutrient supply for photosynthesis during microalgal circulation in horizontal tubes. This led to an increased microalgal biomass accumulation by 40.8% with 15% CO2 in the horizontal tubular photobioreactor.