Development of phase-change absorbent for hydrogen chloride with hydrogen and chlorine generation by electrolysis

To reduce the energy consumption of regeneration for HCl absorption, and to recover HCl to high-value products, a sustainable phase-change system was constructed with N-methyldiethanolamine (MDEA) and polyethylene glycol dimethyl ether (NHD) in this paper. The MDEA/NHD phase change absorbent for HCl absorption is optimized by the ternary phase diagram and phase behavior of MDEA/NHD/HCl system. The results demonstrate that the biphasic system can be constructed with the mass fraction of HCl less than 28% and NHD component in the range of 23.0% similar to 95.3%. Because of the higher stability of protonated MDEA center dot HCl than that of NHD center dot HCl, HCl molecules are mainly enriched in MDEA phase containing the volume ratio of less than 1/3 and partition co-fficient over 100 at the neutral point. Therefore, only MDEA phase needs to be recycled, which results in reducing the circulation of regeneration by approximately 68%. Moreover, concentrated HCl in anhydrous MDEA phase could further prevent the side reaction of OER and improve the Faradaic efficiency toward HER and CER to reach 98.5% and 90.5% at 55 degrees C, respectively. And the energy consumption of regeneration is reduced to 0.37 GJ/t(absorbed HCl) in MDEA/NHD biphasic absorbent compared with that of 2.3 GJ/tCO(2) by DMX (TM) phase change absorption for CO2. By enhancing the conductivity and decreasing the viscosity of MDEA phase, introducing water in MDEA/NHD/HCl solution within 5% could significantly promote the current density and maintain high Faradaic efficiency toward HER and CER as 89.0% and 86.8%, respectively. Therefore, it is a promising technology to be applied in industry.