Solubility of CO2/CH4 gas mixtures in ionic liquids
Introduction
Natural gas is usually contaminated with several impurities, including the acid gases carbon dioxide (CO2) and hydrogen sulfide (H2S), which should be removed at the well to avoid technological problems during transportation and liquefaction of the gas [1]. The removal of the acid gases is typically achieved in an absorber utilizing a selective solvent. Selection of a proper solvent is extremely important in separation processes, since a poor selective solvent will inevitably require additional separation steps to prevent/reduce the loss of valuable products in the output stream [2]. Therefore, the natural gas sweetening process demands a solvent with a high acid gas/methane (e.g., CO2/CH4) selectivity. Chemical solvents (e.g., MEA) used in the natural gas industry generally exhibit high CO2/CH4 selectivities, but amine-based processes suffer from some serious drawbacks. The used amine-solvents are volatile, corrosive, degradation sensitive, and energy inefficient [3]. The energy penalty to release the CO2 in the desorber is very high, since CO2 is captured through chemical complexation with the amine-solvent [4]. Ionic liquids (ILs) have been proposed as an alternative to overcome some of the problems associated with amines [5], [6]. In our previous study, we focused on the application potential of ionic liquids for CO2 removal from natural gas [7]. Ideal CO2/CH4 selectivities, defined as the ratio of the Henry constant of CH4 over that of CO2, were calculated for 10 different ILs using pure component solubility data. In practice, the real CO2/CH4 selectivity (i.e., the selectivity of CO2 in the presence of CH4 in the ternary mixture CO2–CH4–IL) may differ from the ideal selectivity due to solute–solute interactions as both gases are simultaneously being dissolved in the IL. The evaluation of real selectivities requires mixed-gas solubility data, which are extremely scarce in the literature [7]. Hert et al. [8] investigated the ternary system CO2 + CH4 + [hmim][Tf2N] and they reported a substantial enhancement of CH4 solubility in the presence of CO2 compared to the binary system at the same conditions. In this case, it is obvious that the ideal CO2/CH4 selectivity will differ from the real selectivity. Petermann et al. [9] studied the system CO2−CH4−[emim][EtSO4] and their experimental data show a slight enhancement (deterioration) of the CH4 (CO2) solubility relative to the pure component solubilities. These two studies are to the best of our knowledge the only literature dealing with the ternary CO2–CH4–IL system.
The aim of the present study is to investigate the effect of the presence of one gas (CO2) on the other (CH4) as both are simultaneously being dissolved in a single IL. More specifically, the solubility of gas-mixtures containing carbon dioxide (CO2) and methane (CH4) has been measured in the following ILs: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N], 1-ethyl-3-methylimidazolium diethylphosphate [emim][dep], trihexyltetradecylphosphonium dicyanamide [thtdp][dca], and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate [thtdp][phos]. The ILs were selected based either on their relatively low price ([emim][dep]), good CO2 solubility ([thtdp][dca] and [thtdp][phos]) or as a reference ([bmim][Tf2N]). The experiments involve bubble-point measurements in the temperature range 303.15–363.15 K and at pressures up to 14 MPa using a visual synthetic method. The effect of gas composition on the bubble-point pressure has been studied systematically by using three different gas-mixtures with a composition of 25 mol% CO2–75 mol% CH4, 50 mol% CO2–50 mol% CH4 and 75 mol% CO2–25 mol% CH4. Real CO2/CH4 selectivities (i.e., the selectivity of CO2 in the presence of CH4) were calculated using the Peng–Robinson equation of state in combination with van der Waals mixing rules. A comparison between ideal and real CO2/CH4 selectivities in the investigated ILs are presented. The results show that the real selectivity does not differ significantly from the ideal selectivity (i.e., the ratio of the pure gas Henry constants) regardless of the gas phase composition.
Section snippets
Experimental
Three different gas cylinders containing CO2–CH4 mixtures with a fixed composition (in mol% CO2-mol% CH4) of 25.1 ± 0.5–74.9 ± 0.5, 50.2 ± 1.0–49.8 ± 1.0 and 74.7 ± 0.5–25.3 ± 0.5, henceforth referred as 25–75, 50–50 and 75–25 mixtures, were purchased from the Linde Group. The ILs 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium diethylphosphate, trihexyltetradecylphosphonium dicyanamide, and trihexyltetradecylphosphonium
Thermodynamic modeling
The experimental data have been modeled using the Peng–Robinson (PR) equation of state [13] (EoS),where is the molar volume, am and bm are mixture constants accounting for the molecular interaction and covolume, respectively. The PR EoS is combined with the quadratic van der Waals (vdW) mixing rules [14]:
where kij and lij denote the binary interaction parameters and ai and bi
Results and discussion
The experimental data of the systems CO2 + CH4 + [bmim][Tf2N], CO2 + CH4 + [emim][dep], CO2 + CH4 + [thtdp][dca] and CO2 + CH4 + [thtdp][phos] are listed in Table 4, Table 5, Table 6, Table 7, respectively. The bubble-point isopleths of the ternary systems are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13. Note that for every gas mixture (i.e., 25–75, or 50–50 or 75–25) at least three ternary mixtures with a constant CH4/IL ratio
Conclusions
The solubility of the binary gas mixtures containing carbon dioxide (CO2) and methane (CH4) has been measured in the following ionic liquids (ILs): 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium diethylphosphate, trihexyltetradecylphosphonium dicyanamide and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate. The experiments involved bubble-point measurements for a temperature range of 303.15–363.15 K and for pressures up to 14 MPa
Acknowledgments
Financial support by the ADEM, A green Deal in Energy Materials (TUD-P10), program of the Dutch Ministry of Economic Affairs, Agriculture and Innovation. The authors thank Ir. E.J.M. Straver for his assistance with the experimental work.
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