Supported molten-carbonate and - hydroxide membranes for CO 2 permeation in the presence of H2O Jacqueline A. Penn, Wenting Hu, Ian S. Metcalfe, Greg A. Mutch Newcastle University, Newcastle Upon Tyne, UK Background Carbon capture is likely to play a key role in the transition to net-zero, including decarbonising the energy sector and implementing negative emission technologies. [1] The most mature carbon capture technology uses amine- based solvents, which incur a large energy penalty. Membranes do not yet meet the permeability and selectivity requirements for post combustion CO 2 capture; if they could, they may offer cost and efficiency advantages. [2] Introduction In recent years, extremely high CO 2 selectivity and permeabilities have been achieved using supported molten- salt membranes. These comprise a porous ceramic or metal support, with molten salts suspended in the pores. [3] Most research to date has used carbonate salts as the molten phase in various solid support materials. Adding steam to these systems was found to increase CO 2 permeability. [4][5] It has been hypothesised that this is due to the presence of OH - ions in the molten salt. One study investigating hydroxide salts as the molten phase achieved CO 2 permeabilities one order of magnitude higher than generally observed with carbonates, whilst maintaining high selectivity. [6] Here, we seek to understand CO 2 permeation in molten carbonate and hydroxide salts in more detail, by using inert membrane supports which likely restrict permeation to the salts alone. Laser-drilled tubular supports are used to provide a well-defined geometry, and enable higher quality sealing than previously published geometries. This allows high precision permeation experiments under varying temperature and humidity. Results and Discussion In supported molten-carbonate membranes with a feed gas containing 50% CO 2 and a dry sweep gas, CO 2 permeability increased by a factor of 1.3 when the temperature was increased from 500 to 700 °C. In supported molten-hydroxide membranes with the same feed gas, and a sweep gas containing 1% H 2 O, CO 2 permeability increased by a factor of 13 over the same temperature increase. CO 2 permeability of molten-carbonate membranes increased by a factor of 3.5 with the addition of H 2 O to the sweep gas. Characterisation of the salt showed that it contained 80% CO 3 2- and 20% OH - ions, supporting the hypothesis that increased permeability is due to the presence of OH- ions in the salt. The impact of H 2 O on supported molten hydroxide membranes was also investigated. This will be discussed, and an initial mechanistic model proposed. References 1. International Energy Agency (IEA), “Net Zero by 2050 A Roadmap for the Global Energy Sector,” 2021. 2. M. Kárászová, et al, “Post-combustion carbon capture by membrane separation, Review”, vol. 238, 116448, 2020. 3. G. A. Mutch et al., “Supported molten-salt membranes for carbon dioxide permeation,” vol. 7, no. 21, 12951- 12973, 2019. 4. W. Xing, et al., “Steam-promoted CO2 flux in dual-phase CO2 separation membranes,” vol. 482, 115 – 119, 2015. 5. K. Zhang, et al., “H2O-enhanced CO2 transport through a proton conducting ceramic- molten carbonate dual-phase membrane,” vol. 650, 120421, 2022. 6. M. R. Cerón, et al., “Surpassing the conventional limitations of CO2 separation membranes with hydroxide/ceramic dual- phase membranes,” vol. 567, 191-198, 2018.
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