Low–energy–consuming CO2 capture by liquid–liquid biphasic absorbents of EMEA/DEEA/PX

Non-aqueous biphasic solvents, which combine the advantages of a non-aqueous absorbent and biphasic absorbent, have attracted increasing attention for CO2 capture because of their great potential to reduce the energy consumption. This study proposed a non-aqueous N-ethylethanolamine (EMEA)/N,N-diethylethanolamine (DEEA)-based liquid–liquid biphasic absorbent with p-xylene (PX) to replace 45–65 % of DEEA. The biphasic absorbent comprising 30 wt% EMEA + 5 wt% DEEA + 65 wt% PX (30E5D65P) had the best phase-change behaviour. In this absorbent, 98.8 % of the absorbed CO2 was in the lower phase of the solvent, with a volume percentage of 35.1 %. This absorbent also showed the best CO2-desorption performance, with almost double the CO2-desorption amount and maximum CO2-desorption rate compared with those achieved using the 30 wt% EMEA + 70 wt% DEEA (30E70D) non-aqueous absorbent. The regeneration energy of 30E5D65P was 1.71 GJ∙t−1 CO2, which was reduced by 31 % and 55 % compared with that of the non-aqueous 30E70D absorbent and the aqueous 30 wt% monoethanolamine absorbent, respectively. 13C NMR analysis and quantum chemical calculations showed that carbamate and PX have a better separation effect than that of EMEA and DEEA, owing to the presence of both polarity and hydrogen bonding. PX with low polarity was beneficial for the separation of carbamate. The carbamate interacted with (protonated) EMEA, (protonated) DEEA and carbamic acid through hydrogen bonds, resulting in the appearance of these components in the lower phase. The carbamate had no hydrogen bond self-aggregation, which may be the reason why there was no crystal precipitation.