Modeling Closed- and Open-End Gas-Transfer Kinetics for Syngas Components in Nonporous Hollow-Fiber Membranes

Syngas fermentation using hollow-fiber membrane (HFM) bioreactors is promising for converting its CO, H2, and CO2 into valuable organic chemicals. While operating the membranes in a closed-end configuration delivers 100% of the supplied syngas to the biofilm on the membranes’ outer surface, back-diffusion of water vapor into the membrane lumen dilutes CO, H2, and CO2, which slows the gas-transfer rate. This study investigated two partial open-end configurations designed to mitigate water-vapor accumulation in the lumen: periodic venting and a slow bleed flow. Experiments quantified the effects of these configurations on substrate–gas fluxes and gas-transfer efficiency. Venting and bleed configurations increased syngas component fluxes by approximately 2-fold and up to 3.6-fold, respectively. Mechanistic modeling explained how venting and bleed prevented a large accumulation of water vapor, which maintained lumen concentrations of CO, H2, and CO2 at their maximal levels. However, venting and bleed also led to advective losses that lowered the gas-transfer efficiency. For the conditions tested experimentally, experiments and modeling identified that the best combination for achieving high fluxes and gas-transfer efficiency was the bleed configuration with a flow rate of 0.025%–0.05% of total open-end flow rate. These results demonstrate that partial open-end operation can improve gas-transfer kinetics while maintaining low advective losses, providing a rational basis for process optimization and scale-up of HFM-based syngas fermentation.