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328 Y. Xie et al. / Applied Energy 136 (2014) 325–335 the CO2 solubility data up to 10 MPa was surveyed and evaluated in this work. The available sources of the experimental CO2 solubility data in imidazolium-based ILs were listed in Table 1. As shown in Table 1, the CO2 solubility in [BF4]-, [PF6]- and [Tf2N]-series of ILs has been measured extensively in a wide temperature range, while the CO2 solubility data in ILs with other anions is still scare, for example, [DCA]- and [TfO]-based ILs. For [BF4]- and [PF6]-based ILs, [bmim][BF4] and [bmim][PF6] are the mostly investigated ILs at both low and high pressures. For [Tf2N]-based ILs, [emim][Tf2N], [bmim][Tf2N] and [hmim][Tf2N] were all investigated extensively. The available experimental CO2 solubility data at different tem- peratures and at pressures up to 10 MPa was evaluated by NRTL- RK model. Firstly, all the experimental CO2 solubility in one IL from different sources was fitted by NRTL-RK model, and the data which shows large discrepancies from others was considered unreliable. Then only the reliable data was fitted by the model to obtain the model parameters for the further prediction of enthalpy and energy consumption analysis. As the evaluation depends on the number of groups of available experimental data, not all the ILs listed in Table 1 can be evaluated. The ILs with evaluation were marked as ‘‘Y’’ and those without evaluation were marked as ‘‘N’’ in Table 1. The evaluation shows that most of the available experimental results are reliable, and only 3 groups of data are unreliable as listed in Table 2. The inconsistency of the experimental data from different sources is shown in Figs. 2–4. Fig. 2 illustrates the CO2 sol- ubility in [emim][BF4] from three sources. It can be seen that the data measured by Hwang et al. [28] is lower than those by others at 298 K and shows a different tendency at 313 K. Fig. 3 depicts the comparison of the experimental CO2 solubility in [bmim][PF6] at 313 and 333 K. As shown in Fig. 3, the data measured by Blanchard et al. [39] is higher compared to the data from other sources, while the data from other sources agrees well with each other. The com- parison of the CO2 solubility in [bmim][Tf2N] at 313 and 333–334 K is shown in Fig. 4. Jacquemin et al. [50] measured the CO2 solubility in [bmim][Tf2N] at low pressures, but their data is much lower compared to the data measured by others. It is found that the Henry’s constant of CO2 in an IL is sensitive to the experimental CO2 solubility data, and the data from different Table 1 Sources of CO2 solubility in imidazolium-based ILs measured experimentally up to 10 MPa. Table 2 Unreliable sources of CO2 solubility in imidazolium-based ILs. IL [emim][BF4] [bmim][PF6] [bmim][Tf2N] T, K 293.2–323.2 313.15–333.15 283–343 P, MPa 2.62–9.25 0.1–10 0–0.1 Year Ref. 2011 [28] 2001 [39] 2007 [50] First author Hwang Blanchard Jacquemin IL [emim][BF4] [bmim][BF4] [hmim][BF4] [omim][BF4] [emim][PF6] [bmim][PF6] [hmim][PF6] [omim][PF6] [emim][Tf2N] [bmim][Tf2N] [hmim][Tf2N] [omim][Tf2N] [emim][FAP] [emim][TfO] [emim][EtSO4] [emim][MeSO3] [emim][DEP] [emim][SCN] [emim][HSO4] [bmim][DCA] [bmim][TfO] [bmim][NO3] T,K 298–343 278–383 293–373 303–363 308–366 283–413 298–373 313–333 293–450 273–450 282–413 293–353 283–364 303–343 303–353 298–343 298–333 298–333 298–333 293–363 298–333 298–333 Ref. Evaluated [25–28] Y [29–36] Y [25,28,35,37,38] Y [35,39,40] Y [41] N [25,30,34,39,42–48] Y [25,38,49] Y [39] N [25,31,50–54] Y [30,32,50,52,55–58] Y [25,29,38,52,54,59–63] Y [30,52] N [64,65] N [66] N [39,67] N [68,69] N [69] N [69] N [69] N [57] N [30] N [30] N sources may result in different Henry’s constant. In case that the Henry’s constant of CO2 in ILs is unreliable, the ILs will be excluded in the following calculation of the CO2 absorption enthalpy. For example, the Henry’s constant of CO2 in [omim][PF6] [39] decreases with increasing temperature, which is considered unreliable. In addition, the Henry’s constants of CO2 in ILs are temperature-dependent. In case that the experimental data is mea- sured at one or two temperatures, the ILs will also be excluded. For example, Mejía et al. [69] measured the CO2 solubility in [emim][SCN] and [emim][HSO4] at two temperatures, and the tem- perature-dependent Henry’s constant is unreliable. Therefore, these three ILs are excluded thereafter. In summary, the evaluation of the experimental CO2 solubility data is important in order to get reliable knowledge of the CO2 absorption enthalpy and energy consumption analysis. After the evaluation, three groups of data are unreliable and were excluded in parameter fitting for further energy consumption analysis. In addition, [omim][PF6], [emim][SCN] and [emim][HSO4] were fur- ther excluded from energy consumption analysis due to the unre- liable Henry’s constant. 3.2. CO2 absorption enthalpy After the evaluation, the reliable experimental data was re- fitted by NRTL-RK model. The NRTL-RK model with the fitted parameters was used to calculate the CO2 absorption enthalpy. As the vapor pressure of ILs is negligible, the vaporization enthalpy of ILs was set to be zero in this work. The CO2 absorption enthalpy consists of the CO2 dissolution enthalpy and excess enthalpy. The CO2 dissolution enthalpy was calculated from the Henry’s constant according to Eq. (10), and the excess enthalpy was calculated from the activity coefficient of each component in the liquid phase according to Eq. (11). The CO2 absorption enthalpy for all ILs was calculated and compared. The parameters used in calculation are listed in Table 3. For the studied ILs, the magnitude of CO2 dissolution enthalpy is much larger than the excess enthalpy. The contribution of excess enthalpy is extremely small at low pressures due to the low CO2 solubility, but it increases with increasing pressure. Fig. 5 illus- trates the CO2 dissolution enthalpy and excess enthalpy in [bmim][BF4], [hmim][BF4] and [omim][BF4], respectively, at 298 K as an example. The magnitude of excess enthalpy is 0.2–20% of the CO2 dissolution enthalpy when the pressure is lower than 6 MPa. For these three ILs, the contribution of excess enthalpy is less than 5% to the CO2 absorption enthalpy at pressures lower than 1 MPa. This observation implies that for imidazolium-based ILs, it is reasonable to neglect the excess enthalpy as the CO2 sep- aration process is often operated at pressures lower than 1 MPa (CO2 partial pressure). The effects of the chain length in cation and the type of anion on the CO2 solubility in imidazolium-based ILs have been studied [30,32,64]. However, the influences of cation and anion on the CO2 absorption enthalpy have not yet been studied. In this section, it was studied systematically. 3.2.1. Influence of chain length in cation The available experimental CO2 solubility data reveals that for the studied ILs the CO2 solubility increases with increasing chainPDF Image | CO2 Separation with Ionic Liquids
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