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% ∼ 95.3%. Because of the higher stability of protonated MDEA·HCl than that of NHD·HCl, HCl molecules are mainly enriched in MDEA phase containing the volume ratio of less than 1/3 and partition coefficient 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 °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/tCO2 by DMX™ 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.