Avoiding the Carbamate Route: VLE Investigations Into Alkylcarbonate Formation in Water-Lean Solvents

Abstract

Alkylcarbonate formation in water-lean solvents might be an alternative route to fixate CO2 into liquid absorbents. This is a relatively unexplored and overlooked path of CO2 conversion. Many recent studies in this area have been following a trial-and-error approach. This work attempts to suggest a theoretical framework to explore alkylcarbonate-forming solvents by looking at the pKa of their amine reactants and the pKs of their diluents. We have obtained vapor-liquid equilibrium curves in a series of amines with an increasing order of pKa (TEA 7m, MDEA 7m, DEEA 7m) and a series of diluents with an increasing order of pKs (water, MEG, methanol, ethanol, NMP, TMS and THFA) at 40 °C. Our results verify that CO2 solubility increases with pKa and decreases with pKs. However, this correlation does not hold for diluents with pKs below that of water (e.g., acidic diluents). We have also explored the solubility of CO2 in MEG + DEEA as a function of temperature, finding out that this solvent has lower cyclic capacity than aqueous DEEA, and thus lower enthalpy of absorption. At the same time, MEG + DEEA can plausibly be regenerated at 80 °C. This suggests that, besides their several shortcomings, alkylcarbonate-forming water-lean solvents might be practical for CO2 desorption using low-grade heat.

Keywords: alkylcarbonate, autoprotolysis, water-lean solvent, hybrid solvent, co2 capture