Studies of Solubility of CO2 in Ionic Liquids, Kinetics, and Heat of Reactions of CO2 in Promising Cyclic Amines
Tagiuri, Ali Moftah
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In many industrial processes such as in carbon capture, solvents play an important role, acting as a media for extraction of products or for chemical reactions. It is important to find more energy efficient physical or chemical solvents that meet industry needs while limiting the environmental and health risks. A new type of physical solvents (ionic liquid) was investigated for potential use in gas sweetening operations or in mixtures with amines for flue gas cleaning. Six ionic liquids were investigated for their capacities to absorb CO2 at various temperatures and pressures. Henry’s law constants, enthalpies, and entropies of absorption were calculated and reported. Solubility data were modeled using the standard Peng-Robinson, the Soave-Redlich-Kwong, and the Non-Random Two-Liquid (NRTL)] model with acceptable average absolute deviations (%AAD). N-methyl-n-pentylpyrrolidinium bis(trifluoro methyl sulfonyl)imide [P(5)mpyrr][Tf2N] was selected as the most promising ionic liquid studied. The molar heat capacity of pure and aqueous mixtures of the best ionic liquid, [P(5)mpyrr][Tf2N], were measured at (298.15, 313.15 and 333.15) K. The data were well correlated using the UNIQUAC and NRTL models. Excess heat from mixing aqueous solutions of [P(5)mpyrr][Tf2N] exhibited the highest positive values of HE (endothermic). Based on a comprehensive literature search, 1-(2-Hydroxyethyl) piperazine [HEP] was selected as a promising chemical solvent. High-pressure solubilities studies for CO2 in aqueous HEP (30 wt%) were performed up to 7000 kPa at 313.15 and 333.15 K. The E-NRTL model was used to correlate the solubility of CO2 in aqueous solutions of HEP. HEP was found to have a higher CO2 absorption capacity when compared to Monoethanolaime (MEA) but lower than Piperazine (PZ). The molar heat capacity of pure and aqueous mixtures of HEP were measured at (298.15, 313.15 and 333.15) K. Excess heat of mixing HE for HEP were very negative (exothermic), and were related to a high kinetic rates when reacting with CO2. The data were well correlated using the UNIQUAC and NRTL models. A rigorous thermodynamic model was proposed to find the temperature dependent heat of reaction and equilibrium constants for HEP. Data were well correlated with the Electrolyte-NRTL model and used to predict the heat of reaction. HEP was found to have a slightly lower heat of absorption than Monoethanolamine (MEA) and therefore capable of minimizing the cost of steam required for regeneration. The reaction kinetics of CO2 with five tertiary amines (including HEP) were studied using the direct stopped-flow method at various temperatures and amine concentrations. Cyclic amine HEP has higher pseudo-first-order reaction rates compared to the other linear tertiary diamines. 2-DMAEMAE has the highest reaction rate among tertiary amines studied. 2-DIPA had the highest reaction rate and 2-HEAN the lowest. The reaction rates with 2-DIPA and 2-DMAEMAE were found to be higher than MDEA. pKa values of nine amines were measured using potentiometric titration at (298, 305, 315, 325, and 335) K. The van’t Hoff equation was used to determine few thermodynamic properties such as standard enthalpy (ΔHo/kJˑmol-1) and entropy (ΔSo/kJˑmol-1) for the dissociation process. 2-DMAEMAE was identified as having a high pKa as well as a low enthalpy of dissociation, and with its high kinetic rates, it can be considered a promising amine for CO2 removal applications.