Characterization of CO2 flux through hollow-fiber membranes using pH modeling

CO2 must be delivered efficiently during large-scale microalgal cultivation. Bubbleless mass-transfer via diffusion through hollow-fiber membranes (HFM) can achieve much higher CO2-transfer efficiency than traditional sparging systems. This study developed and used a model to compute accurate values for the CO2 flux (J(CO2)), overall mass-transfer coefficient (K-L), and overall volumetric mass-transfer coefficient (K(L)a) based on the rate of change of pH in batch experiments. A composite HFM comprised of two macroporous polyethylene layers and a nonporous polyurethane layer was tested for CO2 transfer to a sodium carbonate solution using a range of total pressures, inlet CO2 concentrations, and open-end versus closed-end modes of operation. The model accurately computed J(CO2) and K(L)a for pH values above 8. Key trends are that (i) J(CO2) and K(L)a increased with increasing average inlet CO2 partial pressure; (ii) open-end HFMs performed better than closed-end HFMs when the supplied CO2 was less than 100%; and (iii) the available membrane area used for CO2 mass-transfer decreased as the inlet CO2 partial pressure decreased due to depletion of CO2 inside the membrane, especially for closed-end HFMs, since inert gases could not be vented.