Use of rotating membranes for air-to-liquid mass transfer of carbon dioxide to enhance algal growth

A novel air-to-liquid mass transfer system using wetted rotating membranes was designed to enhance air-to-liquid carbon dioxide (CO₂) mass transfer efficiency. Traditional methods, such as sparging, are energy-intensive, but the rotating membrane reduces energy demands by optimising membrane wetting via rotational motion. Experimental tests were conducted using a small-scale system with a membrane width of 0.64 m and loop size of 2 to 5 m, with rotational speeds between 0.0 and 0.78 m/s. CO₂ flux increased by up to 45%, achieving maximum uptake rate of 9.14 mg CO₂/min/m² at 100% speed. An empirical model was developed to predict mass transfer rates under varying operational conditions, and model validation showed a strong correlation with experimental data (<i>R</i>² = 0.9668). Preliminary techno-economic analysis estimated that scaling the system to meet the CO₂ demands of a hypothetical 500,000 L raceway, 915 membranes would be required, utilising ∼223 m² (13.4%) of 1667 m² surface area, assuming a 0.3 m depth, 12 g/m²/day growth rate, and algae with 50% carbon by weight. The system's energy consumption was measured at 17.1 J/g CO₂ captured, representing a 90% reduction in power usage compared to conventional sparging systems, which typically require ∼627 W per 8.3 m² of membrane surface area. Based solely on electricity costs of $0.10/kW-hr, the cost of capturing atmospheric CO₂ was estimated at $1550 per ton. This marks a significant improvement over existing technologies, enhancing commercial viability. Future work will validate the system with <i>Chlorella vulgaris</i> and scale to optimise CO₂ capture and reduce costs.