Viscosity of aqueous MEA with added NaCl salt

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

“Fig. 11 shows how the viscosities of SAS increase with loading at 25 °C when compared to that of unsalted 30 %wt. MEA as measured by Amundsen et al. (2009). Once again, the full data set together with its uncertainties is found in the Appendix to this study.”

“Fig. 11. Viscosity versus CO₂ loading for salted 30 %wt. MEA compared to that for the unsalted amine obtained from Amundsen et al. (Amundsen et al., 2009) at 25 °C.”

“As expected, the addition of inorganic salts promotes an increase in solvent viscosity. This seems to happen both with NaCl, which is structure-making at 25 °C, and with KI, which is structure-breaking (Marcus, 2009b). However, NaCl does indeed increase the viscosity of aqueous MEA more than KI. This is particularly evident in the SAS with 7.5 %wt. salt, wherein the 7.5 %wt. NaCl SAS (yellow ▽ markers) clearly has the highest viscosity at all loadings while that of the 7.5 %wt. KI SAS (purple ♢ markers) almost overlaps with both the 2.0 %wt. SAS. This might be a consequence of the SAS with KI having lower ionic strength than the SAS with NaCl on account of KI having higher molecular mass than NaCl. KI has a molecular mass of 166.00 g mol⁻¹ compared to 58.44 g mol⁻¹ of NaCl, meaning that the ionic strength of the unloaded NaCl SAS is almost three times as big as that of the unloaded KI SAS. As shown by Esteves et al. (2001), higher viscosities might be attributed to higher ionic strengths.

The viscosity of the SAS with 2.0 %wt. salt is very similar to that of aqueous MEA, and there appears to be even a crossing-over for both 2.0 %wt. NaCl and KI SAS at around α ≈ 0.4 and 0.5 mol _CO2mol_MEA⁻¹ respectively (see where the yellow △ markers and blue □ markers overlap with the red stars in Fig. 4). This is a very interesting behavior. In fact, except for the 7.5 %wt. NaCl SAS, the viscosities of the SAS apparently increase less steeply with loading than that of aqueous 30 %wt. MEA. To our knowledge, there is no explanation for this observed behavior in the Debye-Hückel theory (Esteves et al., 2001), which suggests that such phenomena might be attributed to unexpected solute–solute interaction between the inorganic salt electrolytes and the products of the MEA–CO₂ reaction.

In other words, the impact of mixing both inorganic salts to aqueous 30 %wt. MEA (aq.) on solvent viscosity is not nearly as noticeable as one would expect. This is good, as it signalizes that transport phenomena in SAS will probably not be significantly depressed due to the addition of salts.”