Vapor-liquid equilibrium using salt added MEA solvent

https://doi.org/10.1016/j.cesx.2021.100096

“Fig. 10 shows a comparison between the vapor–liquid equilibria (VLE) of unsalted 30 %wt. MEA with that of SAS both at 40 °C and at 120 °C. The data referring to the unsalted solvent comes from Wanderley et al. (2020). Though the curves shown in Fig. 10 are for CO₂ partial pressure (p_CO2) versus loadings (α), we must reinforce that the methodology described in Section 2.3 is able to produce only total pressure versus loading, meaning that the CO₂ partial pressures have to be estimated from the original data set. As a result, some data points referring to lower CO₂ partial pressures have to be eliminated since their values end up being smaller than the inherent propagated uncertainties of ±0.34 kPa. Complete disclosure of the total pressures and loadings measured throughout our experiments, together with their uncertainties, can be found in the Appendix to this study.”

“Fig. 10. Estimated CO₂ partial pressure versus CO₂ loading for salted 30 %wt. MEA compared to the VLE for the unsalted amine obtained from Wanderley et al. (Wanderley et al., 2020).”

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Once we account for the uncertainties, which are particularly meaningful for the lower CO₂ partial pressure data points, what results from Fig. 10 is nearly an overlap between the VLE curves for the unsalted 30 %wt. MEA (aq.) and the SAS. It is even unclear whether the SAS experience a loss of CO₂ solubility or not. Looking at the blue scattered data points, both for solutions with NaCl and KI, it seems that a small shift in equilibrium towards less CO₂ conversion might be observed with the SAS at 40 °C. As such, both the blue △ and ▽ data points and the blue □ and ♢ data points are slightly to the left of the curves for unsalted MEA in top and bottom images alike. There are three outlying points for the SAS with 7.5 %wt. NaCl at 40 °C (uppermost blue ▽ markers at the top image). In this experiment, we observed precipitation of white solids at high CO₂ pressures, likely of sodium bicarbonate (NaHCO₃). This demonstrates a clear risk of employing too concentrated SAS for CO₂ absorption. At 120 °C, on the other hand, there are indications of a slight increase in CO₂ solubility for the SAS, with both the red △ and ▽ data points and the red □ and ♢ data points falling a bit to the right of the curves for unsalted MEA.

These contradictory small shifts in equilibrium at 40 °C and at 120 °C could imply a very small reduction in cyclic capacity for the SAS. However, it is difficult to interpret Fig. 10 as anything but an overlap of VLEs curves. It appears to us that, other than the caveat of precipitation issues at high NaCl concentrations, one can conclude an insignificant shift in absorption capacity when employing the SAS.”