Viscosity and CO2 Solubility in Hypothetical Water-Lean Solvents

Figure 7 addresses this issue with the results of the model developed throughout Section 2. The lines in Figure 7 designate values of η and H_A for which a solvent of fixed ε, varying between ε = 20 and 60, can deliver the same mass transfer coefficient as aqueous MEA. For these calculations, the diffusivities D_A had to be evaluated with the Wilke–Chang expression (eq 13), which is a weak function of the diluent molar weight M. Therefore, we have fixed M = 100 g·mol^–1, as this value is close to the average of that for most interesting diluent candidates (ethylene glycol, N-methyl-2-pyrrolidone, sulfolane), and χ = 1.0.

To read Figure 7, one must pay notice to the fact that, below each line, the proposed diluent is able to deliver mass transfer coefficients above those of aqueous 30 wt % MEA. In this sense, it becomes evident that the “room for maneuver” is very limited.

In case-A-type analyses, the curves for different dielectric permittivities ε fall very closely by, this being the most optimistic scenario. Still, for diluents above the viscosity of η = 5 mPa·s, there are meager chances of finding corresponding Henry’s coefficients low enough so as to make a feasible water-lean solvent. In fact, the corresponding H_A for a valid trade-off decreases exponentially with η. In other words, the viscosity of a diluent is a lot more impactful than its CO₂ solubility in terms of enhancing mass transfer rates. Meanwhile, case-B-type analyses are even more pessimistic. Due to the viscosity increase with loading, and now unaided by carbamate equilibrium shifts, organic diluents with low ε are barely interesting for solvent formulation. This result reinforces how important the equilibrium shift is to elevate the mass transfer coefficients of water-lean solvents.

Perhaps a better way of making our point is considering that what is understood as CO₂ solubility is the inverse of Henry’s coefficient as defined in this study. Taking case A as an example, Figure 8 shows the trade-off directly in terms of viscosity and CO₂ solubility. This time, the value referring to water is plotted as a black star in the image. Additionally, the data is shown in a semilog plot.