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Ind. Eng. Chem. Res. 2010, 49, 1370–1377
Chemical Absorption of Sulfur Dioxide in Room-Temperature Ionic Liquids Mark B. Shiflett*,† and A. Yokozeki‡ DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880, and 109-C Congressional DriVe, Wilmington, Delaware 19807
Gaseous solubilities of sulfur dioxide (SO2) in room-temperature ionic liquids (RTILs), 1-n-butyl-3methylimidazolium acetate and 1-n-butyl-3-methylimidazolium methyl sulfate, have been measured at four isothermal conditions (about 283, 298, 323, and 348 K) using a gravimetric microbalance. The observed pressure-temperature-composition (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been successfully applied for our previous works. Excess thermodynamic functions and Henry’s law constants have been obtained from the observed (PTx) data and our previous measurements of SO2 + 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide using the EOS correlation. All three RTILs show the chemical absorption. The classification of whether the absorption is the physical or chemical type is based on the excess Gibbs and enthalpy functions as well as the magnitude of the Henry’s constant. An ideal association model has been applied in order to interpret those excess thermodynamic functions. Then, two types of the chemical associations (AB and AB2, where A is RTIL and B is SO2) have been observed with the heat of complex formations of about -6 to -19 (for AB) and from -6 to -29 (for AB2) kJ · mol-1, respectively. Introduction
associations (AB and AB2, where A is RTIL and B is SO2) have been observed.
Petroleum refineries, fossil fuel burning power plants, sulfidebased metal smelters, and other SO2-emitting industries worldwide are under increasing regulatory pressure to reduce their sulfur emissions. The conventional removal techniques include using limestones1,2 or organic solvents.3,4 Room-temperature ionic liquids (RTILs) have also been proposed for such separations and may provide a more cost-effective and environmentally friendly alternative.5–16 Effectively capturing SO2 from flue gas requires very strong absorption because of the relatively low partial pressures (e.g., 0.2 vol % of SO2 at atmospheric pressure) of the gas in this stream. By strong absorption, we mean that the gas absorption needs to be “chemical” absorption (or reversible chemical complex formation), instead of the simple “physical” absorption (or no chemical reactions), which is only practical for high pressure gas absorption (e.g., CO2 partial pressure > 525 kPa17). Binary PTx (pressure-temperature-composition) data for SO2 have only been measured using a few ionic liquids,14 including our own previous work.18 In this report, we continue our recent investigations on the solubility of sulfur dioxide in the ionic liquids: 1-n-butyl-3-methylimidazolium acetate [bmim][Ac] and 1-n-butyl-3-methylimidazolium methyl sulfate [bmim][MeSO4]. The solubility behavior has been further examined using the thermodynamic excess functions, as well as Henry’s law constant at 298.15 K for the present systems; the binary system of SO2 + 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N] which was previously studied by us,18 has also been included for comparison. Finally, the excess functions from the EOS correlation are interpreted in terms of a chemical association model,19–25 and two types of chemical * To whom correspondence should be addressed. Tel.: 302-695-2572. Fax: 302-695-4414. E-mail:
[email protected]. † DuPont Central Research and Development. ‡ 109-C Congressional Drive.
Experimental Section Materials. Sulfur dioxide (mole fraction purity > 0.9998, CAS no. 7446-09-5) was purchased from MG Industries (Philadelphia, Pennsylvania). The [bmim][Ac] (assay > 95%, C10H18N2O2, lot and filling number S30543 42706B14, CAS no. 284049-75-8) and [bmim][MeSO4] (ECOENG 411, purity >99%, C9H18N2O4S, lot number 99/851, CAS no. 401788-985) were obtained from Fluka/Aldrich (Buchs, Switzerland) and Solvent Innovations (Cologne, Germany), respectively. Figure 1 provides the chemical structure and abbreviations. The asreceived mass fraction of water was measured by Karl Fischer titration (AquaStar Coulomat C and A) and the undried [bmim][Ac] and [bmim][MeSO4] samples contained 9200 and 3850 ppm H2O, respectively. The [bmim][Ac] and [bmim][MeSO4] ionic liquids were dried and degassed by first placing a sample in a borosilicate glass tube and pulling a course vacuum with a diaphragm pump (Pfeiffer, model MVP055-3) for about 3 h. Next, the samples were fully evacuated using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4 × 10-7 kPa while simultaneously heating and stirring the ionic liquid at a temperature of about 353 K for 5 days. Removal of water from these ionic liquids is difficult and requires sufficient time, vacuum, heating, and stirring. The final mass fraction of water was again measured by Karl Fischer titration and the dried [bmim][Ac] and [bmim][MeSO4] contained 1000 and 900 ppm H2O, respectively which corresponds to about 1.0 mol % of water. Binary VLE Measurements. We have conducted gas solubility experiments for SO2 + [bmim][Ac] and SO2 + [bmim][MeSO4] using a gravimetric microbalance26 (Hiden Isochema Ltd., IGA 003). Initially, about 67 and 69 mg of [bmim][Ac] and [bmim][MeSO4], respectively, were loaded into the sample container, but because of the high amount of gas absorption this led to the balance going off-scale at higher pressures. Therefore, a smaller sample size of about 30 mg of ionic liquid was loaded into the sample container and heated to
10.1021/ie901254f 2010 American Chemical Society Published on Web 12/17/2009
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 1. Chemical structures of the present ionic liquids (a) 1-n-butyl-3methylimidazolium acetate [bmim][Ac]; (b) 1-n-butyl-3-methylimidazolium methyl sulfate [bmim][MeSO4]; (c) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N].
348.15 K under a vacuum of about 10-9 MPa for 24 h to remove any further trace amounts of water and other volatile impurities. To prevent any corrosion or damage to the microbalance caused by exposure to the SO2, a molecular sieve trap was installed to remove trace amounts (
