https://doi.org/10.1016/j.seppur.2020.118193
“There are three main factors that help defining the locus of the cross-over point. The first one is amine concentration. Some works have shown that, the less amine in a solvent, the lesser is its capacity for chemical absorption, and the lower the CO2 partial pressure required for crossing-over. An example can be found in the data of Huang et al. [73]. In their water-free DEGDME-MEA solvent, cross-over happens at around 500 kPa for 15 %wt. MEA but only at 1000 kPa for 30 %wt. MEA (T = 40 °C). This phenomenon can be seen in Fig. 5. Something similar has been observed with regards to water-free THFA-MEA formulations by Wanderley et al. [59].”
“Fig. 5. Crossing-over in VLE data for solvents containing MEA at 40 °C. The data in blue refers to aqueous MEA whereas the data in red refers to water-free DEGDME + MEA. On the right-hand side, one can see the VLE for solutions 15 %wt. MEA. On the left-hand side, the VLE for solutions 30 %wt. MEA. Data adapted from Aronu et al. [74], Lee et al. [75], [76], Jou et al. [77] and Huang et al. [73].”
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The second factor is the diluent itself, both in terms of its CO2 physical solubility and its solvating properties. Looking again at the report of Huang et al. [73], the cross-over for water-free NMP-MEA solvents happens before that of DEGDME. However, at even higher CO2 partial pressures, the DEGDME-MEA curve crosses over that of NMP-MEA itself. This is an interesting case in which both electrostatic phenomena and physical solubility play opposing roles. NMP has better electrostatic properties than DEGDME, and its curve crosses over that of aqueous MEA first. However, DEGDME has higher CO2 physical solubility than both water and NMP, surpassing NMP in capacity afterwards.
It might be instructive to mention other instances of observable crossing-over. Ai et al. [78] observed the cross-over in acetoacetamide-MDEA-water (5/48/47 %wt.) above 100 kPa at 25 °C. Macgregor and Mather [60] identified a cross-over for TMS-MDEA-water (30.5/20.9/48.6 %wt.) above 2000 kPa at 40 °C. Roberts and Mather [79] reported a cross-over for TMS-AMP-water (32.2/16.5/51.3 %wt.) above 60 kPa at 40 °C. Finally, Isaacs et al. [25] mark the cross-over for the proprietary Sulfinol-D® solution (TMS-DIPA-water 40/40/20 %wt.) over about 4000 kPa at 40 °C. With the exception of the example from Roberts and Mather [79], all of the crossing-overs mentioned above happen at pretty high CO2 partial pressures.
The third and final factor is temperature. Rivas and Prausnitz [34] show that water-lean solvents experience a stronger VLE dependence on temperature than in their aqueous counterparts. They convincingly demonstrate that with example of 15 %wt. MEA + water-free NMP, suggesting that this could lead to easier solvent recovery. Observations of this kind have been made repeatedly by several authors [59], [60], [80], [81]. What this means in practical terms is that the cyclic capacity, as defined by the CO2 loading at absorber temperatures minus the CO2 loading at desorber temperatures and fixed CO2 partial pressure, is higher in water-lean solvents. And that is true even despite the fact that, due to the equilibrium shift mentioned previously, rich loadings in water-lean solvents are almost inevitably lower than in aqueous solvents.
We must mention that these larger cyclic capacities verified experimentally seem to contradict the argument given by Moore [82] that, due to the low entropy of absorption in water-lean solvents, solvent regeneration should become less thermodynamically favorable. Their observation stems from the fact that water molecules form more structured lattices than most organic diluents, making it so that CO2 experiences a larger drop in entropy while being absorbed by aqueous solvents than when it is absorbed by non-aqueous ones. We are in no position to give a thermodynamic explanation regarding why desorption in water-lean solvents seems to be facilitated. From a molecular point of view, one could suggest that the hydrogen bonds connecting water molecules are less susceptible to breaking/shifting due to increases in temperature than the looser, weaker dipole–dipole ones connecting organic diluents. Perhaps this translates into a more temperature-dependent entropy of absorption than Moore [82] initially assumes.
4.3. Literature data for VLE in water-lean solvents
What follows in Table 2, Table 3 and Table 4 is a comprehensive list of all the published data regarding vapor–liquid equilibrium in water-lean solvents that we were able to find in the open literature. We have appended the list with some remarks regarding the conditions in which the datasets were obtained.
Unfortunately, a certain amount of data published in Russian in the Soviet Union regarding water-lean solvents (including supposedly even pilot plant data) is now hardly accessible to most Western investigators. One can see references to it in the work of Roberts and Mather [79] and, more recently, a quick summary of findings and conclusions elaborated by Leites [31]. We have been unable to assess most of these referred studies, with the sole exception of the book published by Semenova and Leites [17], of which we have translated and discussed one chapter.
Curiously, we have found only one instance in literature where the solubility of methane has been measured in water-lean solvents [22]. As these solvents are formulated with organic diluents, one would think that methane solubility (and thus the possibility of methane slip) should be properly assessed. Fortunately, Murrieta-Guevara and Trejo Rodriguez [22] report very low methane solubility in mixtures of N-methyl-2-pyrrolidone and DEA, so that this might indeed not be a cause for concern even for those working with biogas upgrading or natural gas treating.”