Enhanced carbon-transfer and -utilization efficiencies achieved using membrane carbonation with gas sources having a range of CO₂ concentrations

The economic viability of microalgal biofuels relies on increasing productivity in a cost-effective manner. As microalgal biomass contains >50% carbon, a high rate of CO₂ delivery is required for high productivity, and inefficient CO₂ delivery amplifies operating costs. Membrane carbonation using non-porous hollow fiber membranes can ideally deliver CO₂ without bubble formation and high carbon transfer efficiency. Because CO₂ streams from industrial resources are not 100% CO₂, the buildup of inert gasses can significantly lower the CO₂ delivery rate when the distal end of the membrane is closed. To overcome the buildup of inert gases, we managed the distal end of the membranes with three different approaches: fully open end, restricted bleed valve, and restricted bleed valve with pH-actuated venting. For all approaches, CO₂ was delivered to membranes on-demand based on a pH set point. Evaluating a wide range of CO₂ concentrations (10% to 100%), we found that all approaches eliminated the buildup of inert gases, could maintain target pH values and gave the same biomass productivities and carbon distributions. However, carbon transfer efficiency depended on the operation of the distal end. Fully open-end operation gave a poor carbon transfer efficiency due to excessive loss of CO₂ from the distal end. However, restricting the exit flow rate to ≤4 cm³/min mitigated the problems of excessive CO₂ loss, but without incurring a large loss of CO₂-delivery flux. For the continuous cultivation, combining a restricted bleed valve with pH-actuated venting improved the carbon-transfer efficiency and -utilization efficiencies up to 85% and 67%, respectively, with a sufficient CO₂ delivery flux.