https://doi.org/10.1021/acs.iecr.0c00940
“Water-lean solvents have been often proposed as interesting alternatives to regular aqueous amine solvents for CO2 capture applications. (1−6) Due to the multitude of possibilities regarding water-lean solvent formulation, the issue of coming up with a brand-new solvent may be as cumbersome as blindly guessing which chemicals to put together. To address this complexity, the present study provides a qualitative and, to a certain extent, quantitative evaluation of what parameters one should keep in mind when looking for candidates for water-lean solvents.
Before carrying out this parametric evaluation, a short discussion on which advantages water-lean solvents can really bring to the table is warranted. Some authors have proposed that the capacity for CO2 absorption in water-lean solvents would be extended due to their coupling of physical and chemical absorption. (3,7,8) However, in previous experiments carried out by this group and others, (9−11) this was not observed to be the case for low-to-moderate CO2 partial pressure spans (0–600 kPa). Clearly, any advantages brought about by this supposed extended absorption capacity will hardly be yielded in regular postcombustion CO2 capture applications, where CO2 partial pressures are typically lower than 600 kPa.
Additionally, some authors have proposed that either the heat of absorption or the reboiler heat duties can be considerably lower in water-lean solvents. (3,4,12) Our previous research has shown that the heat of absorption, when employing monoethanolamine-based (MEA-based) water-lean solvents, is roughly the same as with aqueous solvents. (10) This does not contradict the studies that proposed that this should be the case for secondary or hindered amines, (4,12) but it suggests that one should not take for granted the fact that carbamate-forming amines will deliver reboiler duty savings through heat of absorption alone. In our previous work, (10) we have indicated that the reboiler duty savings might come from the low volatility of some organic diluents used in water-lean solvent formulations (N-methyl-2-pyrrolidone, monoethylene glycol (MEG), tetrahydrofurfuryl alcohol (THFA)) providing less vaporization heat expenditures in the reboiler, something that seems to be in agreement with the literature. (13)
Parallelly, some authors suggested that the shifting from aqueous to water-lean solvents can increase mass transfer rates in the absorber. (6,9,14) This enhancement in mass transfer rates is driven by increased CO2 physical solubility but depressed by increased viscosity, both of which are typical of water-lean solvents.”