Effects of fluid flow on the rate limiting for CO2 capture by an AEEA containing membrane

Abstract

The efficiency of CO2 capture using a hollow fiber membrane containing 2-(2-Aminoethylamino) ethanol (AEEA) from the emission gas in the Integrated Gasification Combined Cycle (IGCC) was investigated by density functional theory and fluid dynamics. A reaction path for CO2 sorption to AEEA involving two H2O molecules with three – proton transfer was found to give the low barrier of the reaction due to 6 intermolecular hydrogen bonds which make the transition state structure more stable. The activation energy in the reaction path for CO2 sorption to AEEA was 7.11 kcalmol−1 which is approximately 10 kcalmol−1 lower than that involving one H2O molecule with two – proton transfer discussed in our previous study (Higashino et al., 2023). Since the activation energy is much lower than those in the other reaction paths we have investigated, it can occur much more frequently than others when sufficient moisture is maintained in the membrane. The estimated reaction rate k = 3.407 × 108 s−1 indicates that CO2 becomes carbamate by reaction with AEEA as soon as reaching the surface of the membrane, and thus, CO2 attachment to AEEA cannot be the rate limiting. Since flow in the hollow fiber membrane can be laminar, simulated CO2 permeance for the temperature of 358 K and the pressure of 2.4 MPa has a range from 10−10 to 10−9 m3 (STP) m−2s−1Pa−1, which is a few orders smaller than that for the flat sheet membrane, flow over which is turbulent, i.e.10−8 to 10−7 m3(STP) m−2s−1Pa−1 (Higashino et al., 2023). Overall, the reaction rate of CO2 attachment to AEEA is sufficiently large, and hence, any one of mass transfer of CO2 induced by turbulence (turbulent flow), molecular diffusion of CO2 from feed gas to the membrane surface (laminar flow), and molecular diffusion of bicarbonate (HCO3-) from the feed side to the permeate side in the membrane can be the rate limiting depending on which type of membrane, e.g. a flat sheet membrane module, a spiral membrane module, a hollow fiber membrane contactor, etc., is used for CO2 capture.