Development of Thin Film Composite Hollow Fiber Membranes for Direct Ocean Capture

We propose to develop a thin film composite (TFC) hollow fiber membrane-based gas-liquid contactor (GLC) in an electrochemical pH swing based direct ocean capture (DOC) system. The objective of the proposal is to demonstrate efficient CO2 stripping from oceanwater using low-cost TFC hollow fiber membranes. The key innovations in this project are i) using ultra-low-cost hollow fiber membranes, traditionally used in water filtration applications, as a structural support and modify the outer layer of hollow fiber membranes with a highly CO2 permeable polydimethylsiloxane (PDMS) layer to selectively strip CO2 from oceanwater; ii) computational model-assisted design and rapid prototyping of new GLC designs that use counter-flow and cross-flow design for efficient CO2 stripping from oceanwater. Because we use a low-cost hollow fiber membrane support and thin layer (10-15 m) of PDMS, we estimate 10x cost reduction in the capital expenditure (CapEx) for the CO2 stripping unit. Our technoeconomic analysis (TEA) indicates that the realization of this new, low cost GLCs will enable the new learning curve for electrochemical based DOC technologies and meet the $100 ton-CO2 cost target at a projected Mt-CO2 per yr scale. Our focus is on the CO2 stripping unit (highlighted in green) of the overall DOC system, where dissolved CO2 (aq.) with a concentration of ~2.2 mM in the acidified oceanwater is stripped to produce high purity CO2 (g). Besides the CO2 stripping unit, the overall DOC system also includes an electrodialysis unit for low-cost acid and base generation, shallow oceanwater intake and outfall ,and acid and base neutralization before discharging the effluent back to the ocean. Currently available membrane-based GLCs are expensive and are designed for specialized degassing applications, where ppb level control of gas in the effluent liquid is needed. However for DOC, the design and operational requirements indicate that a high extraction efficiency ( 90%) can be achieve even when 10 ppm level of un-stripped CO2 remains in the oceanwater. Our low-cost GLC addresses this requirement. It performs ultrafiltration and provides durable support and facile gas transport to the lumen side of the fiber, while the thin PDMS coating provides selective transport of CO2 with respect to water and other dissolved gas species such as O2 and N2. The proposed multi-stage baffled GLC design also leverages efficient cross-flow and counter-current flow to optimize the CO2 (aq.) to CO2 (g) transport at the module level, to achieve high extraction efficiency at low cost.