Parameters for Aqueous and Water-Lean Solvents from literature

With the parameters shown in Table 1, one can start analyzing the absorption behavior of CO₂ in both aqueous and water-lean solvents. In an unloaded solution, the reaction will be far from reaching equilibrium and K can be set to a very high value (K ≈ ∞). For loaded solutions, we regressed K from the vapor–liquid equilibrium of CO₂ in aqueous solutions of DEA from the data points obtained by Lee et al. (43) and then updated K in water-lean solvents by employing the dielectric permittivity ε of the pure diluents and eq 19a–c. Notice that eq 19a–c was fitted for MEA data and one would have to be extremely cautious before extending it to DEA. Fortunately, this is not relevant when dealing with unloaded solutions or essentially any solution far from equilibrium loading.

One can now analyze Figures 1 and 2 together with Table 1 to investigate the effects that some parameters have on the absorption performance of the solvents. The first profile in Figure 1 is that of aqueous 1.5 M DEA. Following that, a series of alcohols is accounted for, from one to four carbons long (methanol, ethanol, and 2-propanol in Figure 1 to n-butanol in Figure 2). The behavior of the penetration profile following this series is evident. As the carbon chain gets longer, the viscosity of the alcohol increases, and consequentially the diffusivities of both CO₂ and amine decrease. Meanwhile, CO₂ solubility also decreases. The dielectric permittivity goes down, dragging together the kinetic coefficients k₂ and k_b/k_–1. Altogether, these effects have three consequences in the profiles: (i) the CO₂ physical absorption decreases from methanol to n-butanol, as evidenced by the red-dotted lines showing lower concentrations of molecular CO₂; (ii) the amount of DEA that reacts with CO₂ in that limited timespan also decreases, as seen in the green-dotted lines showing lower concentrations of carbamate; and (iii) the penetration depth is reduced, and the point in space in which all curves reach their equilibrium values clearly diminishes from methanol to n-butanol. Ethylene glycol + DEA is an extreme example of this behavior, having very high viscosity and very low CO₂ solubility. Propylene glycol has high CO₂ solubility, but high viscosity as well, and its penetration depth is the lowest of all solvents in both Figures 1 and 2. For these reasons, these are the two diluents with the worst performance of all of the ones considered for DEA-based water-lean solvents. Finally, propylene carbonate (PC) is able to couple high CO₂ solubility with moderate viscosity and kinetic coefficients, producing a profile similar to that of ethanol while having the big advantage of being comparatively nonvolatile.