https://doi.org/10.1016/j.seppur.2022.120786
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At full-scale, crystalline ammonium bicarbonate will form in the solvent along the length of the membrane contactor, as CO2 absorbed in the solvent quickly reacts with NH3 to produce carbamic acid. This is part of an equilibrium dependent reaction that will determine reactivity and is the step preceding nucleation (Eq. (9), (10)), where formation of solid NH4HCO3 may therefore also limit NH3 slip into the gas phase [15], [2]. To evaluate whether this mechanism helps shift crystallisation from lumen side (gas phase) to shell-side (solvent), solvent recirculation was employed to mimic analogous conditions. Initial CO2 flux was highest for the 3 M NH3 solvent, while all three solvents exhibited a similar decline in CO2 flux following solvent recirculation due to the decline in reactivity as NH3 was progressively consumed through reaction (Fig. 4).
Fig. 4. Impact of recycling ammonia absorbent on wetting and lumen side crystallisation (observed immediately after wetting, t + 1) for PTFE membrane. Cumulative carbonate ratio compares CO2 absorbed to HCO3 concentration at saturation. Conditions: G/L 11, VG 0.2 m s−1, VL 1.4 × 10−3 m s−1, gas and absorbent temperature, 20 °C. Error bars indicate standard deviation. Red line indicates ammonium bicarbonate solubility at 20 °C.
Blockage of the lumen by wetting or crystallisation was not observed for the 0.6 and 2.3 M solvents following recirculation (Fig. 4), despite the membrane having been in contact with the solvent for longer than in single pass, which supports the hypothesis that wetting is not the sole criteria governing gas phase crystallisation (Fig. 1). During the progressive absorption of CO2, carbamic acid forms through the reaction between NH3 and CO2. The carbamic acid liberates [H+] to reduce the solvent pH, shifting the ammonia-ammonium equilibrium from NH3 to non-gaseous NH4+ [2](Fig. 5). This indirectly reduces the solvent ammonia vapour pressure, lowering the transmission of reactive NH3 into the gas phase. By avoiding gas-phase blockages, prolonged CO2 absorption was achieved, where the cumulative mass of CO2 absorbed into the 2.3 M NH3 solvent through recirculation was sufficient to theoretically achieve supersaturation (Fig. 4b). To illustrate, based on the ratio between the CO2 absorbed (C, mol) and the CO2 required to initiate nucleation (C*, mol of CO2 in NH4HCO3 at saturation, 2.32 M L−1 at 20 °C), solvent supersaturation (C/C*) just exceeded 1. However, nucleation in the solvent was not observed. This can be explained by a shift in the carbonate equilibrium from HCO3− toward H2CO3 following acidification of the solvent, which reduced the actual NH4HCO3 solvent concentration to 2.05 M L−1 (88% of C/C*). This is below the concentration required to initiate shell-side (solvent) crystallisation (Fig. 5) and indicates that either a higher NH3 solvent concentration or a reduction in solvent temperature (to reduce the solubility of NH4HCO3) is required to initiate shell-side crystallisation.
Fig. 5. Dynamic equilibria following progressive CO2 absorption into 2.3 M aqueous ammonia in recirculation. Absorbent temperature 20 °C. Red line indicates ammonium bicarbonate solubility at 20 °C.
Lumen-side crystallisation was observed for the 3 M NH3 solvent after 4 h, which is only slightly longer than when operated in single pass (Fig. 4). This was coincident with a solvent CO2 saturation concentration of C/C* 0.54, which is insufficient to initiate shell-side crystallisation. At the same solvent temperature, the NH3 vapour pressure at 3 M is 65% higher than at 2.3 M (Appendix B, Fig. B1) [30]. Consequently, higher NH3 transport from the solvent to the gas phase (slip) can be expected at the outset of solvent recirculation, where the unreacted nitrogen is primarily available in the NH3 (gaseous) form. These observations are supported by McLeod et al. [26] noting lumen side (gas-phase) crystallisation to occur at solvent concentrations for 3 to 5 M but not at 2 M (Table 1). This analysis therefore implies that by reducing ammonia vapour pressure, the probability for gas-phase crystallisation can be reduced.
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