ARTICLE pubs.acs.org/IECR
Evaluation of Alkylimidazoles as Physical Solvents for CO2/CH4 Separation Matthew S. Shannon,† Jason M. Tedstone,‡,§ Scott P. O. Danielsen,‡,|| and Jason E. Bara*,† Department of Chemical & Biological Engineering and ‡NSF-REU Site: Engineering Solutions for Clean Energy Generation, Storage and Consumption, Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States § Department of Chemical & Biomolecular Engineering, Clemson University, Clemson, South Carolina 29634, United States Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6315, United States
)
†
bS Supporting Information 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 CO2 capture and natural gas treating. “Sweetening”, the removal of CO2, H2S, and other “acid” gases from natural gas (CH4), 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 CO2 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 CO2 and CH4 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 CO2 and temperatures of 3075 °C. Solubilities of CO2 and CH4 were found to be strongly dependent on temperature, with the solubility of each gas in all solvents diminishing with increasing temperature, although CO2 exhibited a stronger temperature dependence than CH4. Ideal CO2/CH4 solubility selectivities were also more favorable at lower temperatures in 1-n-alkylimidazole solvents with shorter chain lengths. CO2 solubility decreased with increasing chain length, while CH4 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 CO2 solubility and CO2/CH4 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 CO2.
1. INTRODUCTION Physical (i.e., non-chemically reacting) solvents are used industrially for the removal of CO2, H2S, and other contaminants from natural gas (CH4) in “sweetening” processes.1,2 The use of physical solvents is advantageous for bulk removal of CO2 when the partial pressure(s) and concentration(s) of the acid gas(es) are high.1,2 Physical solvent processes are economically favorable under these conditions as high acid gas loadings in the solvent can be achieved in the absorber while regeneration is much less energy intensive than it is for chemical (reactive) solvents (i.e., aqueous amines).1,2 Selexol and Rectisol are examples of commercial processes that utilize physical solvents composed of dimethyl ethers of poly(ethylene glycol) (DMPEG) and chilled MeOH, respectively.3,4 Physical solvent processes can also be run to selectively recover separate H2S and CO2 streams when both gases are present, so as to convert H2S to elemental sulfur via the Claus process,1,2 and then compress a relatively pure CO2 stream for applications such as enhanced oil recovery (EOR) or geologic sequestration.5 Physical solvents are also used for precombustion CO2 capture in the integrated gasification combined cycle (IGCC) process and syngas cleanup (CO2/H2 separation).5,6 r 2011 American Chemical Society
Figure 1 presents useful guidelines for selecting the appropriate solvent type based on feed conditions and requisite product purity.2,7 As a point of reference, process conditions for postcombustion CO2 capture6,8 have been appended to the lower left corner of the figure. Recently, ionic liquids (ILs) have emerged as a new class of physical solvents that may have utility for the separation of CO2 from CH4 in natural gas sweetening,9,10 as certain ILs have been shown to exhibit selectivities for CO2/CH4 that rank among the most selective organic solvents.9,11 Furthermore, additional applications involving the removal of H2S may also be applicable to the use of IL solvents.6,1215 Solubilities of CO2 and CH4, and CO2/CH4 selectivity are known to be controlled by the chemical structures of the IL cation and anion,9,11,1619 as well as temperature.20 Although ILs typically exhibit ∼10 times higher viscosities21,22 and ∼60% of the CO2 capacity of MeOH and other organic solvents,3,4,9,23,24 ILs Received: September 14, 2011 Accepted: December 9, 2011 Revised: December 6, 2011 Published: December 22, 2011 515
dx.doi.org/10.1021/ie202111k | Ind. Eng. Chem. Res. 2012, 51, 515–522
Industrial & Engineering Chemistry Research
ARTICLE
van’t Hoff equation, were shown to be exothermic for both gases, with the dissolution of CO2 more thermodynamically driven. Both the CO2 solubility and the CO2/CH4 selectivity were most favorable in 1-methylimidazole, the smallest molecule in the series, while 1-hexylimidazole exhibited the maximum CH4 solubility among the six compounds. Gas solubility and selectivity trends can be explained in terms of 1-n-alkylimidazole solubility parameters as well as fractional free volumes (FFVs). Solubility parameters that we have previously calculated for 1-n-alkylimidazoles via group contribution methods21 have been refined to reflect recently published data for ΔHvap values of 1-n-alkylimidazoles.27,28 FFV as determined via an atomic contribution approach29 to calculating the van der Waals volumes (VvdW) of 1-n-alkylimidazoles helps to provide insight into the solubility and selectivity trends observed. Finally, a comparison of the thermophysical properties of commercially used physical solvents, 1-methylimidazole, and ILs is provided. Using the data developed in this and other works, 1-methylimidazole is revealed to exhibit several favorable properties for use in existing process configurations. Examination of ILs shows that while they exhibit the lowest vapor pressures of any physical solvents, their relatively large viscosities may be a hindrance to use within conventional process schemes.
Figure 1. Process selection guidelines for the removal of CO2 from CH 4 using absorptive (solvent-based) processes. Adapted from refs 2 and 7.
2. EXPERIMENTAL SECTION 2.1. Materials. 1-Methylimidazole (1) and 1-ethylimidazole (2) were obtained from Sigma-Aldrich (Milwaukee, WI, USA) and used without further purification. Other 1-n-alkylimidazoles (36) were synthesized in our laboratory from sodium imidazolate and a corresponding alkyl bromide, according to a previously outlined procedure.30 Research grades of CO2 and CH4 were purchased from AirGas (Radnor, PA, USA). 2.2. CO2 and CH4 Solubility Measurements. Solubilities of CO2 and CH4 in each 1-n-alkylimidazole were measured using the same apparatus with a methodology similar to that described in our previous work.21,31 Experiments were conducted at temperatures of 30, 45, 60, and 75 °C (as controlled by an oil bath) for both CO2 and CH4 measurements. An initial charge of gas was fed at 30 °C until the pressure equilibrated at ∼5 atm, which was selected as the target pressure based on Figure 1, which suggested that 5 atm (75 psia) would be a pressure where physical solvents would first be considered for use in CO2 removal. Solubility values for all temperatures were then calculated from this known mass of gas in the system and the change in pressure upon heating, similar to the methodologies outlined by Finotello.20On the basis of published data,27,28 the vapor pressure of the 1-n-alkylimidazole compound can be assumed as negligible under the experimental temperature and pressure conditions, as it is low (∼5 mmHg maximum and typically
