5058
2007, 111, 5058-5061 Published on Web 04/19/2007
SO2 Gas Separation Using Supported Ionic Liquid Membranes Ying-Ying Jiang, Zheng Zhou, Zhen Jiao, Lei Li, You-Ting Wu,* and Zhi-Bing Zhang School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, China ReceiVed: March 3, 2007; In Final Form: April 3, 2007
Measurements of permeability of sulfur dioxide (SO2) in five imidazolium-based ionic liquids supported on the polyethersulfone microfiltration membranes at temperatures from 25 to 45 °C and atmospheric pressure indicate that under the same conditions, the SO2 selectivity of separations using supported ionic liquid membranes are 9-19 times that of CO2.
Introduction Combustion of fossil fuels (petroleum, coal, and natural gas) results in the formation of acidic gases, such as carbon dioxide (CO2) and sulfur dioxide (SO2). Methods for avoiding the emission of CO2 and SO2 have been paid increasing attention all over the world. One of the most attractive approaches for the separation of a target compound from a mixture of gases is the selective permeation through supported liquid or ionic liquid membranes (SLMs or SILMs).1 These membranes, being impregnated with ionic liquids in their porous supports, have distinct advantages over conventional supported liquid membranes. The nonvolatility and incredible stability of ionic liquids are among the most important characteristics that enable the membrane separation process to avoid the loss of the supported liquid and the contamination of the gas streams.2 In particular, CO2, as a kind of acidic gas, has been recently shown to have remarkable selectivity and permeability in SILMs. For instance, the CO2 permeability is 350-1000 barrers in the ionic liquids that consist of common [emim]+ and different water stable anions, [Tf2N]-, [CF3SO3]-, and [dca]-, whereas the CO2/N2 ideal selectivities range from 15 to 61, and the CO2/CH4 ideal selectivities are from 4 to 20.3 Therefore, if SO2 can be found to have higher selectivity and permeability in SILMs, it may be possible to remove and recover SO2 and CO2 from flue gas simultaneously or separately in an integrated SILM system or integrated system composed of SILMs and absorption processes. Even though there are several recent papers concerning the absorption of SO2 in ionic liquids (ILs),4-6 the use of supported ionic liquid membranes for SO2 separation are scarcely found. In this work, we present the experimental selectivities and permeabilities of SO2 in five SILMs at temperatures from 25 to 45 °C and atmospheric pressure. The five ILs used in this paper are all imidazolium-based: 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([bmim][Tf2N]). To * Corresponding author. Phone: 86-25-81638601. Fax: 86-25-83593772. E-mail:
[email protected].
10.1021/jp071742i CCC: $37.00
our knowledge, this is the first report of SO2 selectivity and permeability in the SILM process. Experimental Section The ILs were synthesized in our laboratory using established procedures.7 Table 1 lists the ILs used in this study, along with their relevant properties: viscosity, density, and water content. The ILs were purified using SiO2 gel filtration and then dried under vacuum for at least 48 h at 40-80 °C to remove inorganic salt, organic solvents, and water before use. The approximate water contents before use were determined by Karl Fischer titration. SO2 with a minimum purity of 99.95% was purchased from Nanjing Gas Supply Inc. In addition, the SO2 solubility in ILs in this study was also measured using the standard procedures that were established by Wu, et al.4 The porous supports for the IL membranes were hydrophilic polyethersulfone (PES) with the following specifications: 0.22 µm pore size, 80% porosity, 150 µm thick, and 64 cm2 cross-sectional area. The primitive PES membrane was submerged in a given ionic liquid and put in a vacuum oven at 40 °C for overnight, allowing it to soak up the liquid, followed by the removal of excessive ionic liquid on the membrane surfaces using soft tissue prior to the test. Before performing the experiments, the SILMs were tested for leaks using nitrogen. The permeability experiments of SO2 in SILMs were performed with a stainless steel dual-chamber cell. The membrane is a barrier between two sealed chambers that contain thermodynamically equilibrated amounts of nitrogen (i.e., at the same pressure and temperature). The upper (feed) and lower (permeate) chambers have volumes of 245 and 215 mL, respectively. Two absolute pressure transducers (Omega PX811005AV) of 0.2% precision and 0.6 MPa scale were calibrated and used for the pressure measurement of each chamber. A computer data acquisition system recorded the data from the sensors at an elapsed time interval (usually 1 min). In a typical experiment, the upper and lower chambers were charged to have the same pressure (1 bar) of nitrogen, followed by the additional loading of a known amount of solute gas sample in the feed chamber. This loading produces a partial pressure difference of the solute gas across the membrane. The solute gas flux across the membrane was measured indirectly from the total pressure © 2007 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5059
TABLE 1: Room-tcemperature Ionic Liquids Used in This Study IL [emim][BF4] [bmim][BF4] [hmim][BF4] [bmim][PF6] [bmim][Tf2N]
viscosity (cp) density (gcm-3) water content (ppm) 439 2199 2469 4509 529
1.2810 1.2610 1.1510 1.369 1.439
1320 1200 410 50-100 460
TABLE 2: SO2 Gas Solubilities at 1.0 bar and Henry’s Law Constants for the Tested Ionic Liquids at 25 °C IL
solubility (mole fraction)
Henry’s law (bar)
[emim][BF4] [bmim][BF4] [hmim][BF4] [bmim][PF6] [bmim][Tf2N]
0.550 ( 0.007 0.570 ( 0.013 0.585 ( 0.009 0.536 ( 0.012 0.552 ( 0.013
1.02 1.04 1.05 1.12 1.10
Figure 1. Permeability of SO2 and CO2 in the tested SILMs at 25 °C. For the left axis: 9, [emim][BF4]; [, [bmim][Tf2N]; b, [bmim][BF4]; 1, [hmim][BF4]; and 2, [bmim][PF6]. For the right axis: O, [bmim][Tf2N]-CO2.
increase with time in the permeate chamber. This change in pressure is due to the flux of the solute gas, since the partial pressures of N2 on both sides of the membrane are equal. To measure the flux rate of SO2, the known amount of SO2 was additionally loaded. The initial slope of the raw pressure-time data over 20-30 min for SO2, after a lag time of about 3 min, was used to calculate the flux rate under the test conditions. To measure the flux rate of N2, the procedure is the same except that a known pressure of N2 is loaded in the feed chamber as the solute gas. Due to the lower N2 flux, the initial slope data is taken over ∼700 min. The same procedure as that for N2 is followed to measure the CH4 flux, except the initial slope data is taken over ∼270 min. All the permeability data were calculated on the basis of the cross-sectional area and the thickness of the primitive PES membrane. The entire experimental unit was thermostated in a constant temperature water bath. The system was validated by measuring the CO2 permeability of an ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amid ([emim][Tf2N]) supported membranes at 20 kPa of CO2 driving force and 25 °C. The measured CO2 permeability was 1002 barrers, consistent with that already reported.3 Results and Discussion Table 2 gives the SO2 gas solubilities at 1 bar for the tested ILs of this study. According to the standard procedures as adopted in literature,5 all the Henry’s law constants shown in Table 2 were calculated from the solubility data at the SO2 partial pressures between 0.2 and 2.5 bar (see the Supporting Information) using eq 1,
H ) lim ) xf0
p x
(1)
where p is the pressure of gas, x is the mole fraction of gas, and H is the Henry’s law constant. The SO2 solubilities in ILs are found to be extremely high, that is, the mole fraction of SO2 at 25 °C in [bmim][Tf2N] approaches 0.552 ( 0.013 at 1.0 bar. By contrast, the mole fraction of CO2 in the same IL under similar conditions is only 0.031.11 On a molar basis, [hmim][BF4] has the highest SO2 solubility (0.585 ( 0.009 mole fraction), whereas [bmim][PF6] has the lowest SO2 solubility (0.536 ( 0.012 mole fraction). The Henry’s law constants for five ILs are all located at 1.0-1.2 bar-1 and very close to each other. The SO2 permeabilities versus driving force (cross-membrane SO2 partial pressure difference) for the SILMs tested at 25 °C are shown in Figure 1. There is an explicit difference of SO2
permeability in the SILMs. [emim][BF4] has the highest SO2 permeability, 9350 ( 230 barrers, whereas [bmim][PF6] has the lowest SO2 permeability, 5200 ( 117 barrers (measured at 20 kPa of SO2 driving force). The relative SO2 permeabilities between the SILMs tested appear to be related to the viscosities of the ILs (see Table 1), with the tendency of the highest to lowest SO2 permeabilities, [emim][BF4] > [bmim][Tf2N] > [bmim][BF4] > [hmim][BF4] > [bmim][PF6] being similar to the case of the lowest to highest viscosities of ILs. [emim][BF4] < [bmim][Tf2N] < [bmim][BF4] < [hmim][BF4] < [bmim][PF6]. The permeability of gas can be explained on the basis of a dissolution-diffusion transport mechanism that consists of two steps: (1) the dissolution of the separate molecules in the membrane material and (2) the SO2 diffusion across the membrane toward the downstream boundary. The permeability, P, can be approximated as10
P ) D12Scoeff
(2)
where P is permeability in barrer, Scoeff is the solubility coefficient in cm3/(cm3 ionic liquid cmHg) that can be calculated from the Henry’s law constant, and D is the diffusivity in cm2/s of the gas molecule (1) in the ionic liquid (2). The diffusivity of gases such as CO2, O2, and C2H4 in imidazolium-based ionic liquids is found to be dependent on the viscosity and density,13
D12 ) 3.7 × 10-3
1 µ2
(0.59(0.02)
V h1
(1.00(0.07)
F2(2.0(0.1)
(3)
where µ2 is the ionic liquid viscosity in cP, V h 1 is the solute molar volume in cm3/mol and F2 is the ionic liquid density in g/cm3. As shown in eqs 1-3, the difference of the SO2 permeability in different ionic liquids can be explained from the behaviors of solubility, viscosity, and density. However, since the solubilities of SO2 in these ionic liquids used in this study are very close to each other, the viscosity and density of the ionic liquid play a key role in the SO2 permeability. For example, the solubilities of SO2 in [bmim][BF4] and [bmim][PF6] are 0.570 ( 0.013 and 0.536 ( 0.012 mole fraction, respectively, which means similar Scoeff values. On the contrary, the viscosities of the two ionic liquids are rather different: at 25 °C, [bmim][PF6] is ∼2 times more viscous than [bmim][BF4] (450 vs 219 mPas) and [bmim][PF6] has a larger density than [bmim][BF4] (1.36 vs 1.26 gcm-3). The viscosity and density become the main effect, resulting in a much higher SO2 permeability in the [bmim][BF4] membrane than the [bmim][PF6] membrane. This qualitative analysis agrees well with the experimental results,
5060 J. Phys. Chem. B, Vol. 111, No. 19, 2007
Letters
TABLE 3: Gas Permeabilities (barrers) with Standard Errors and Ideal Selectivities for the Tested SILMs under 20 kPa Transmembrane Pressure Difference room-temperature ionic liquids gas sulfur dioxide carbon dioxide nitrogen methane SO2/N2 selectivity SO2/CH4 selectivity SO2/CO2 selectivity a
[emim][BF4]
[bmim][BF4]
[bmim][PF6]
[hmim][BF4]
[bmim][Tf2N]
9350 ( 230 126 760a 480 ( 15 1300a 42 ( 3 73 ( 3 223 128 19
8070 ( 260 47 680a 460 ( 12 1280a 37 ( 2 56 ( 2 218 144 18
5200 ( 117 20 130a 360 ( 15 550a 26 ( 1 49 ( 2 200 106 14
7280 ( 150 46 440a 520 ( 11 1310a 45 ( 2 68 ( 3 161 107 14
8560 ( 112 57 600a 980 ( 23 1850a 68 ( 4 98 ( 3 126 87 9
Predicted permeabilities using eqs 1-3.
and the SO2 permeabilities in other SILMs can be explained in the same way. However, the quantitative calculation of both SO2 and CO2 permeability, as shown in Table 3, indicates that the predicted data using correlations 1-3 are ∼2 to 10 times the experimental values. The main reason may be due to the fact that the fast dynamic permeation of the solute gas across the SILM causes a much smaller solubility coefficient than that measured at equilibrium conditions (or static conditions). The use of saturated solubility for the permeability prediction is questionable, especially when the solute gas, such as SO2, has an extremely large solubility in the ILs. The higher the saturated solubility, the larger the difference between the actual concentration and the saturated solubility of the solute gas in the ILs (the actual concentration is dependent on the dissolution and diffusion rates). The overestimated permeability of both SO2 and CO2 in the SILMs is in good agreement with such a conclusion. Therefore, further examination is required for the quantitative prediction of permeability of the extremely soluble gases in the ILs. The SO2 permeability across five types of SILMs increases with the increasing gas partial pressure, that is, the results in Figure 1 demonstrate that the SO2 permeability in [bmim][Tf2N] increases by 1206 barrers when the transmembrane pressure rises from 10 to 50 kPa. This is consistent with the SO2 solubility’s increasing with the driving force. However, the CO2 permeabilities across [bmim][Tf2N] membranes, done at the same conditions for the purpose of comparison, are around 1000 barrers and change very little with the increasing driving force (10-50 kPa). Thus, the SO2 permeability in [bmim][Tf2N] is 7-10 times as large as that of CO2. In [emim][BF4] membranes, SO2 permeates 18 times faster than CO2 (measured at 20 kPa of driving force; see Table 3), which indicates that the supported [emim][BF4] membrane has a higher SO2 selectivity than other SILMs. Because of this, even for industrial flue gases of low SO2 partial pressures (usually 5-10% of the total pressure), it may be possible to remove SO2 and CO2 in separate steps for the purpose of resource recovery. In addition, it is shown in Figure 2 that the increasing temperature has a positive effect on the SO2 permeability across the [bmim][Tf2N] membrane. The SO2 permeation rate increase from 35 to 45 °C, much faster than that from 25 to 35 °C. This trend is consistent with that seen for other gases, that is, CO2 in polyionic membranes.12 A careful selection of the combination of ionic liquid species and operation temperature may be expected to realize the isolation and recovery of SO2 from the industrial gas stream in the SILM process. Table 3 also summaries the gas permeabilities and ideal selectivities for the tested SILMs under 20 kPa transmembrane pressure. The SO2/N2 ideal selectivities range from 126 ([bmim]-
Figure 2. Permeability of SO2 in the [bmim][Tf2N] membrane at 25 (-9-), 35 (-b-), and 45 °C (-2-).
[Tf2N]) to 223 ([emim][BF4]), whereas the SO2/CH4 ideal selectivities range from 87 ([bmim][Tf2N]) to 144 ([bmim][BF4]). Although these ideal selectivities may not reflect the actual performance of SO2 permeation in a gas mixture, they do indicate the potential of ILs to be a viable membrane material for acidic gas separation. In all of these gas pair selectivities, the [bmim][Tf2N] membranes had the lowest SO2 selectivities as compared to the other ionic liquids of this study. The trend for the highest to lowest SO2/N2 and SO2/CH4 ideal selectivities is [BF4]- > [PF6]- > [Tf2N]-. The relationship between gas selectivity and permeability in ionic liquids is somehow in contrast and more complicated: [bmim][BF4] has the best SO2/ CH4 selectivity, but offers a permeation rate less than [bmim][Tf2N], whereas the latter has the better permeation performance and the worst selectivity. Further research is ongoing in our group to find an ionic liquid of both good selectivity and permeability. Conclusions In summary, this work provides the first insight into the permeability and selectivity of SO2 in supported ionic liquid membranes. It has been found that the dissolution-diffusion transport mechanism can be qualitatively applied to analyze the SO2 permeation. The quantitative prediction fails, very probably due to the use of saturated solubility in the calculation, which needs further examination. Nevertheless, the experimental results show that the SILMs not only offer very good permeability of SO2 but also provide ideal SO2/CH4 and SO2/N2 selectivities up to 144 and 223, respectively. When compared to CO2 in the tested SILMs, there is also over an order of magnitude increase in the permeability and selectivity of SO2. Therefore, the SILM process may be expected to have potential application in implementing or replacing traditional chemical absorption processes for separation and recovery of SO2 from a flue gas stream or a gas mixture.
Letters Acknowledgment. We thank the Natural Science Foundation of Jiangsu Province (JSNSF), China, for financial support (BK2005077). Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gorri, D.; Ibanez, R.; Ortiz, I. J. Membr. Sci. 2006, 280, 582593. (2) Gan, Q.; Rooney, D.; Xue, M.; Thompson, G.; Zou, Y. J. Membr. Sci. 2006, 280, 948-956. (3) Scovazzo, P.; Kieft, J.; Finan, D. A.; Koval, C.; DuBois, D. L.; Noble, R. D. J. Membr. Sci. 2004, 238, 57-63. (4) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Angew. Chem., Int. Ed. 2004, 43, 2415-2417.
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