Highly selective amino acid salt solutions as absorption liquid for CO₂ capture in gas-liquid membrane contactors

The strong anthropogenic increase in the emission of CO₂ and the related environmental impact force the developments towards sustainability and carbon capture and storage (CCS). In the present work, we combine the high product yields and selectivities of CO₂ absorption processes with the advantages of membrane technology in a membrane contactor for the separation of CO₂ from CH₄ using amino acid salt solutions as competitive absorption liquid to alkanol amine solutions. Amino acids, such as sarcosine, have the same functionality as alkanol amines (e.g., monoethanolamine=MEA), but in contrast, they exhibit a better oxidative stability and resistance to degradation. In addition, they can be made nonvolatile by adding a salt functionality, which significantly reduces the liquid loss due to evaporation at elevated temperatures in the desorber. Membrane contactor experiments using CO₂/CH₄ feed mixtures to evaluate the overall process performance, including a full absorption/desorption cycle show that even without a temperature difference between absorber and desorber, a CO₂/CH₄ selectivity of over 70 can be easily achieved with the sarcosine salt solution as absorption liquid. This selectivity reaches values of 120 at a temperature difference between absorber and desorber of 35 °C, compared to a value of only 60 for MEA under the same conditions. Although CO₂ permeance values are somewhat lower than the values obtained for MEA, the results clearly show the potential of amino acid salt solutions as competitive absorption liquids for the energy efficient removal of CO₂. In addition, due to the low absorption of CH₄ in sarcosine compared to MEA, the loss of CH₄ is reduced and significantly higher CH₄ product yields can be obtained.