Evaluation of Alkylimidazoles as Physical Solvents for CO₂/CH₄ Separation

Abstract

1-n-Alkylimidazoles are a class of tunable solvents with low volatility and low viscosities. Although imidazoles have been known for some time in the pharmaceutical industry, and as convenient precursors for synthesizing imidazolium-based ionic liquids (ILs), only recently have they been given consideration in some of the same solvent-based separations applications that ILs have been studied for, such as post-combustion CO 2 capture and natural gas treating. "Sweetening", the removal of CO 2 , H 2 S, and other "acid" gases from natural gas (CH 4 ), is an existing industrial application where low volatility, low viscosity physical solvents are already applied successfully and economically at large scale. Physical solvents are also used for syngas cleanup and in the emerging application of pre-combustion CO 2 capture. Given the similarities in physical properties between 1-n-alkylimidazoles, and physical solvents currently used in industrial gas treating, the 1-n-alkylimidazole class of solvents warrants further investigation. Solubilities of CO 2 and CH 4 in a series of 1-n-alkylimidazoles were measured under conditions relevant to the use of physical solvents for natural gas treating: ∼5 atm partial pressure of CO 2 and temperatures of 30À75 °C. Solubilities of CO 2 and CH 4 were found to be strongly dependent on temperature, with the solubility of each gas in all solvents diminishing with increasing temperature, although CO 2 exhibited a stronger temperature dependence than CH 4 . Ideal CO 2 /CH 4 solubility selectivities were also more favorable at lower temperatures in 1-n-alkylimidazole solvents with shorter chain lengths. CO 2 solubility decreased with increasing chain length, while CH 4 solubility exhibited a maximum in 1-hexylimidazole. The solubility trends observed with temperature and chain length can be explained through calculation of solution enthalpies and solvent fractional free volume as approximated from van der Waals volumes as calculated via atomic contributions. Of the solvents examined, 1-methylimidazole displays the most favorable CO 2 solubility and CO 2 /CH 4 selectivity, and has the lowest viscosity. A comparison of 1-methylimidazole to commercially used solvents reveals similar physical properties and the potential for use in industrial gas processing. Imidazolium-based ILs are also compared, although they appear less favorable for use within established process schemes given their higher viscosities and reduced capacity for CO 2 .

Absorption degree analysis on biogas separation with ionic liquid systems

“…The CH 4 solubility decreases 15% on average from 303 to 323 K, while the value is 30% for the CO 2 solubility at the same temperature range. Similar observation has also been mentioned in the literature (Shannon et al, 2011(Shannon et al, , 2013.…”

“…It has been reported that the CO 2 solubility is much higher than the CH 4 solubility in ILs [bmim][Tf 2 N] and [bmim] [BF 4 ] (Mortazavi-Manesh et al, 2013;Raeissi and Peters, 2010). Therefore, ILs exhibit a high CO 2 /CH 4 selectivity which can compete with the most commercial physical solvents (Althuluth et al, 2012;Anthony et al, 2002;Karadas et al, 2010) approach to make ILs applicable in the CO 2 /CH 4 separation system is to blend ILs with other commercially available solvents (Shannon et al, 2011). These mixtures can retain the desired properties of ILs but keep the viscosity relatively low as well as reduce the overall solvent costs.…”

Influence of different alkyl‐methylimidazolium tetrafluoroborate ionic liquids on the structure of pebax® films. Consequences on thermal, mechanical, and water sorption and diffusion properties

“…ILs have emerged as a new class of physical solvents. They combine different interesting properties: they exhibit the lowest vapor pressures of any physical solvents, they can be easily introduced in various amounts in a wide variety of polymers and they display high thermal stability . Moreover some of them, such as imidazolium‐based ILs have been at the forefront of research into new solvents for gas processing …”

Imidazole-Based Solvents and Membranes for CO2 Capture Applications

“…We demonstrated that a solution containing both an alkylimidazole and an amine could exceed the conventional 1:2 (CO 2 :amine) stoichiometry as the alkylimidazole could serve as a H + acceptor, and increase the overall CO 2 capacity per volume of solvent. [30] A concern for all amine-based solvents in post-combustion CO 2 capture from coal-fired power plants is their degradation by flue gas components, especially SO 2 . [22] Yet for imidazole-amine solutions, we observed that viscosity was not negatively impacted in a significant manner when the imidazole was present and that anhydrous viscosities of < 40 cP at ambient temperature were observed for CO 2 -rich samples, which could be even further reduced upon introduction of water or via choice of imidazole.…”