Process Intensification on the Supercritical Carbon Dioxide Extraction

Jan 11, 2012 - The stimulants prohibited by the WADA (World Anti-Doping. Agency)5 in the year 2011 can be divided into two types: one is the type of w...
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Process Intensification on the Supercritical Carbon Dioxide Extraction of Low-Concentration Ethanol from Aqueous Solutions Zhigang Lei,* Jingli Han, Qunsheng Li, and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, People's Republic of China ABSTRACT: Supercritical carbon dioxide (SC CO2) extraction of low-concentration ethanol from aqueous solutions was intensified by the addition of salts including inorganic solid salts and ionic liquids. The influence of operating conditions, e.g., the initial aqueous ethanol mass fraction, phase volume ratio, temperature, pressure, and the kinds of salts added on the extraction process was investigated in this work. It was found that KHCO3 is the best among all of the salts investigated, and the salting-out effect of ionic liquids is weaker than that of solid salts. The experimental results are consistent with the Hofmeister series and the prediction of COSMO-RS (conductor-like screening model for real solvent) model. This work also tried to explain the separation mechanism at the microscopic scale. FTIR (Fourier transform infrared) spectrometry and density functional theory were used to explore the complex formation and interaction force of the systems. The pretreatment process could be directly used to detect the prohibited stimulants in athletes or monitor the environmental pollutants at low concentration. This work also opens a new window for the application of ionic liquids and solid salts in sample pretreatment in laboratory rather than only limited in industry.

1. INTRODUCTION The misuse of stimulants for enhancement of performance by athletes is banned by the majority of sporting federations.1−4 The stimulants prohibited by the WADA (World Anti-Doping Agency)5 in the year 2011 can be divided into two types: one is the type of weak polar compounds having the similar chemical structures as aniline with at least a benzene ring and a nitrogen atom, and the other is the type of strong polar compounds like ethanol prohibited in particular sports. Stimulant detection starts with a urine sample in which stimulants are at low concentration.6,7 Determination of stimulants in aqueous samples requires separation treatment prior to detection. However, in the field of environmental science, the wastewater coming from agricultural, biological, and pharmaceutical industries contains amines, phenols, and chloride ions, as well as a certain amount of ethanol.8 These pollutants are also at low concentration and should be detected for pollutant monitoring. However, pretreatment processes are required to concentrate organics from aqueous solutions prior to analysis, and should be further intensified based on the following two reasons: (i) the detection sensitivity of analytical apparatuses is sometimes limited especially at low concentration, but it is difficult or impossible to improve the hardware performance in a short period of time; and (ii) most of the task on environmental and stimulant sample detection is to answer whether some prohibited chemicals are contained or not, no matter how much. The separation of ethanol and water is the most difficult due to their strong polar. But the separation mechanism of ethanol and water is similar to that of other organic polar compounds from aqueous solutions, and thus the results can also be applied to the detection of other priority pollutants and stimulants in environmental and sports sciences. Therefore, the study on intensifying the extraction of low-concentration ethanol from aqueous solutions is important to provide © 2012 American Chemical Society

valuable information for understanding this type of separation problem. Several sample pretreatment techniques, i.e., liquid−liquid extraction (LLE),9,10 solid-phase extraction (SPE),11 solid phase microextraction (SPME),12 liquid-phase microextraction (LPME), 1 3 dispersive liquid−liquid microextraction (DLLME),14 and supercritical fluid extraction (SFE),15 have been developed over the past decades aimed to improve the separation performance as well as to reduce overall analysis time and costs. Among others, supercritical fluid extraction is relatively rapid for the separation of aqueous solutions, because of the low viscosities and high diffusivities associated with supercritical fluids. CO2 is the most common supercritical solvent, because of its low critical properties, low toxicity and cost, and its ability to solvate a wide range of organics including high molecular weight and moderately polar organics. In comparison with LLE using MTBE (methyl tert-butyl ether) as the separating agent that is currently used for practical measurement, SC CO2 extraction is more environmentfriendly. But in this case, the optimum operating conditions for the extraction of low-concentration ethanol from aqueous solutions should be investigated. In order to improve the SC CO2 extraction efficiency, a certain amount of salts including inorganic solid salts and ionic liquids can be added to intensify the extraction system in light of salting effect. Ionic liquids (ILs) are molten salts, and have received significant attention by the academic and industrial chemical community in recent years. They have come to be viewed as green and potentially environmentally friendly Received: Revised: Accepted: Published: 2730

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Figure 1. Experimental apparatus for SC−CO2 extraction: 1, CO2 cylinder; 2, syringe pump; 3, valve; 4, thermostat cover; 5, high-pressure view cell; 6, magnetic stirrer; 7, pressure gauge; 8, temperature control meter; 9, gas chromatograph; 10, FT-IR spectrometer; and 11, computer.

mass fraction in ionic liquids determined by Karl Fisher titration was less than 0.002. Deionized water was obtained from a Milli-Q reverse osmosis purification system. 2.2. Equipment. Schematic diagram of SC−CO2 extraction apparatus used in this work is shown in Figure 1. The SFE apparatus consisted mainly of a CO2 cylinder, a syringe pump, a pressure gauge, a magnetic stirred equilibrium still and various kinds of valves and fittings. The syringe pump was model DB80 (made by Beijing Satellite Manufacturing Factory) which was used to charge CO2 into the system. The accuracy of pressure gauge was ±0.01 MPa in the pressure range of 0−20 MPa. 2.3. Experimental Procedure. Before the experiment, the equilibrium still was washed thoroughly using deionized water and then dried. A known volume of aqueous solution containing ethanol was first loaded into the still. The subsidiary separating agent (i.e., inorganic solid salt or ionic liquid), if used, was also added at this time. Then air was purged from the system with CO2 several times. Finally, CO2 were charged into the system at a fixed temperature until the desired pressure was reached. Each sample system maintained equilibrium for more than 2 h. The equilibrium temperature was measured by a resistance thermometer with an uncertainty of 0.1 K. 2.4. Analysis. The concentration of ethanol in the aqueous solutions was determined by gas chromatography (GC4000A Series) equipped with a thermal conductivity detector (TCD). The chromatographic column (3 m × 3 mm) was a Porapak.Q type. The operating conditions were as follows: flow rate of hydrogen (H2) as carrier gas, 30 mL min−1; detector and injector temperatures, 150 °C; and oven temperature, 120 °C. The injection volume was 0.5 μL. The gas chromatograph was calibrated using mixtures of known compositions of reagents. The maximum uncertainty in the mass fractions of the components was 0.001. Each analysis was done at least twice to ensure repeatability.

separating agents. Their distinctive properties, such as negligible vapor pressure, wide temperature range at liquid state, and stability at high temperatures, make them good replacements for conventional volatile, flammable, and toxic organic solvents in separation science.16,17 It is also interesting for us to compare the salting effects between inorganic solid salts and ionic liquids on the extraction process. Moreover, due to the small amounts of solid salts or ionic liquids added into the aqueous solutions in the sample pretreatment, there wil actuallyl be no such problems as high prices of ionic liquids bought from chemical markets and equipment corrosion brought on by some salts, which is often encountered in traditional chemical industry. That is to say, no scaling-up takes place in practice. Therefore, the focus of this work is on (i) finding the optimum operating conditions for concentrating low-concentration ethanol from aqueous solutions in the pretreatment process; (ii) identifying the relationship between molecular structure of the salts and separation performance by comparison of the experimental data with predicted results so as to guide the selection of suitable salts; and (iii) exploring the separation mechanism by means of the combination of Fourier Transform Infrared (FTIR) and Density Functional Theory (DFT).

2. EXPERIMENTAL SECTION 2.1. Samples and Chemicals. The chemicals of 99.7% ethanol, 99.5% sodium chloride, 92.0% potassium acetate, 99.5% potassium bicarbonate, 99.0% sodium bromide, and 99.0% sodium iodide were purchased from Beijing Chemical Workshop. The inorganic solid salts were dried for 12 h at 333 K for further purification. High purity CO2 (>99.9%) was supplied by Beijing Zhongke Technology Co. Ltd. 98 wt % 1ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[BF4]−), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]−), 1-butyl-3-methylimidazolium acetate ([BMIM]+[Ac]−) and 1-octyl-3-methylimidazolium acetate ([OMIM]+[Ac]−) were supplied by ShenLan Science and Technology Development Co., Ltd. The purities of ionic liquids (ILs) were determined by liquid chromatography and no Cl− anion was found (the detection limit 0.1 g·L−1). All ionic liquids were evaporated for 24 h at 333 K under a vacuum by rotating evaporator to remove volatile impurities. The water

3. RESULTS AND DISCUSSION The concentration of ethanol in SC CO2 phase (i.e., mass fraction wsc or molar concentration csc) is an important indicator for the selection of suitable salts in the pretreatment process and deduced by the material balance. In addition, the 2731

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extraction efficiency E and distribution coefficient D are also important physical quantities in SFE and defined as follows: c V cscVsc E = sc sc = ms cscVsc + caqVaq D=

csc =

Csc Caq

(1)

(2)

ms − caqVaq Vsc

(3)

Figure 3. Influence of operating pressure P on the ethanol mass fraction in SC CO2 phase wsc with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503 and temperature T = 319.2 K.

where ms is the total mass of ethanol loaded into the equilibrium still; Caq and Csc are molar concentrations of aqueous phase (determined by gas chromatography) and SC CO2 phase (obtained by the mass balance), respectively; Vaq and Vsc are the corresponding phase volumes. It is assumed that the two phase volumes remain unchanged before and after equilibrium because SC CO2 and water are immiscible, and ethanol is at low concentration, the transferring of which from one phase to another does not influence the phase volume. 3.1. Influence of Initial Aqueous Ethanol Concentration on SC CO2 Extraction. The experiments were done with different initial ethanol mass fraction in the aqueous phase, i.e., win = 0.810, 1.022, 2.078, 3.011, 4.025, and 5.030 wt %. As shown in Figure 2, there is a good linear relationship between

Figure 4. Influence of operating temperature T on the ethanol mass fraction in SC CO2 phase wsc (■) and on the extraction efficiency E (▲) with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503 and pressure P = 14 MPa.

case, the operating temperature is a sensitive parameter. At a given pressure, as temperature increases, the density of SC CO2 decreases but the affinity between ethanol and water becomes weaker. The latter is favorable for extraction process and predominates in increasing wsc. However, it should be noted that the operating temperature cannot be too high because heat-sensitive materials may be contained in the common stimulant or wastewater sample. In addition, the extraction equilibrium of ethanol between aqueous and SC CO2 phases can be written as follows:

Figure 2. Influence of initial aqueous ethanol mass fraction win on the ethanol mass fraction in SC CO2 phase wsc with phase volume ratio Vsc/Vaq = 0.5, temperature T = 319.2 K and pressure P = 14 MPa.

D

the initial aqueous ethanol mass fraction in aqueous phase win and the ethanol mass fraction in SC CO2 phase wsc. It is demonstrated that the initial aqueous ethanol mass fraction can be deduced from the ethanol mass fraction in the SC CO2 phase after extraction in quantitative analysis. In this work, we selected the initial ethanol mass fraction 5.030 wt % in the following experiments to investigate the separation performance. 3.2. Influence of Operating Pressure on SC CO2 Extraction. Figure 3 shows that the ethanol mass fraction in SC CO2 phase wsc remains almost unchanged in the pressure range of 9.00−15.00 MPa. This indicates that operating pressure is not a sensitive parameter influencing SC CO2 extraction. The reason may be attributed to the strong affinity between ethanol and water. Only increasing the amount of CO2 by increasing the operating pressure cannot extract more ethanol from aqueous phase to SC CO2 phase. 3.3. Influence of Operating Temperature on SC CO2 Extraction. Figure 4 shows that both the ethanol mass fraction in SC CO2 phase wsc and extraction efficiency increase apparently with increasing temperature. Therefore, in this

CH3CH2OH(aq) → CH3CH2OH(sc)

(4)

According to the Van’t hoff equation, distribution coefficient D can be obtained by the following: −ln D =

ΔH +C RT

(5)

where C is a constant, and ΔH is the enthalpy change of extraction process. As shown in Figure 5, the logarithmic distribution coefficient exhibits a linear trend with inverse temperature. It can be derived that ΔH = 19.73 k·J·mol−1, which reveals that the extraction process is endothermic. 3.4. Influence of Phase Volume Ratio on SC CO2 extraction. It can be seen from Figure 6 that with the increase of volume ratio of SC CO2 phase to aqueous phase, the ethanol mass fraction in SC CO2 phase wsc decreases. This is due to the dilution effect on wsc when SC CO2 phase volume increases in a certain volume of equilibrium still. Therefore, a low phase volume ratio is favorable for increasing wsc in the pretreatment process although in this case extraction efficiency may decrease. For the convenience of sampling from the SC 2732

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their ability to salt-out or salt-in proteins and is also extended to the polar and nonpolar compounds with relatively low solubility in water. According to the Hofmeister series, anions appear to have a larger effect than cations, and are usually in the order of HCO3−> Ac− > Cl− > Br− > I−. The order of cations is usually given as K+ > Na+. Moreover, the Hofmeister series speculated that the cation and anion effects are independent and additive. Therefore, the salting effect of KHCO3 is the largest among all the inorganic solid salts investigated in this work. As the mass fraction of KHCO3 in aqueous phase increases, wsc also increases. This indicates that the best KHCO3 mass fraction would be as large as possible, which makes free water molecules reduce and promotes ethanol molecules migrated from aqueous phase to SC CO2 phase, thereby achieving the enrichment of ethanol concentration in SC CO2 phase. But the amount of KHCO3 added into the system is limited by its saturated solubility in water. 3.6. Influence of Ionic Liquids on SC CO2 Extraction. Room-temperature ionic liquids consist of a large organic cation and an inorganic polyatomic anion, and have been very popular in recent years for their potential as “green solvents”.16 The structure of organic cation can affect the hydrophobicity and hydrogen bonds of ionic liquids, while the structure of the inorganic anions can affect the solubility and dissolution capacity.22−25 The solubility between ionic liquids and water is related to cationic substituents and reduces with the increasing of alkyl chain length.26−29 It can be seen from Figure 8 that

Figure 5. Logarithmic distribution coefficient as a linear function of inverse temperature with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503 and pressure P = 14 MPa.

Figure 6. Influence of phase volume ratio on the ethanol mass fraction in SC CO2 phase wsc with initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K, and pressure P = 14 MPa.

CO2 phase, the phase volume ratio 0.5 was selected in this work. 3.5. Influence of Inorganic Solid Salts on SC CO2 Extraction. One strategy to intensify the SC CO2 extraction is to add inorganic solid salt to the separation system since the salt has a significant effect on the components to be separated which is induced by salting-in or salting-out effect. Five kinds of inorganic solid salts, i.e., NaBr, NaCl, NaI, KHCO3, and KAc, were selected because they are generally used for the distillation separation for organic aqueous solutions. The influence of NaBr, NaCl, NaI, KHCO3, and KAc at 0, 5, 10, 15, 20, and 25 wt % on the ethanol mass fraction in SC CO2 phase wsc was investigated. It can be seen from Figure 7 that

Figure 8. Influence of the amount of ionic liquids [BMIM]+[Ac]−, [OMIM]+[Ac]−, [EMIM]+[BF4]− and [BMIM]+[BF4]− on the ethanol mass fraction in SC CO2 phase wsc with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K and pressure P = 14 MPa. ■, [BMIM]+[Ac]−; ●, [OMIM]+[Ac]−; Δ, [EMIM]+[BF4]−; ⧫, [BMIM]+[BF4]−.

with the increase of mass fraction of ionic liquid, the salting effect is in the order of [BMIM]+[Ac]− > [OMIM]+[Ac]−> [EMIM]+[BF4]− > [BMIM]+[BF4]−. This may be due to the polarity of ionic liquids decreased and thus the hydrophobicity enhanced with the increasing length of alkyl carbon chain on the imidazolium ring from C2 to C8. The hydrogen bonds between H atoms of imidazolium cations and anions play an important role in the space steric interaction.30−32 With the growth of carbon chain on the alkyl cations, the steric effect increases, which leads to the decrease of extraction performance. Therefore, the optimum ionic liquids is [BMIM]+[Ac]− among all the ionic liquids investigated in this work. However, the COSMO-RS model33−36 is used for evaluating the inorganic solid salts and ionic liquids. It is a novel and efficient method for the prior prediction of thermodynamic properties based on the density functional theory (DFT)

Figure 7. Influence of the amount of inorganic solid salts NaBr, NaCl, NaI, KHCO3, and KAc on the ethanol mass fraction in SC CO2 phase wsc with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K, and pressure P = 14 MPa. ■, KHCO3; Δ, KAc; ▽, NaBr; ●, NaCl; ▲, NaI.

potassium salts are more effective than sodium salts in increasing wsc. This is consistent with the Hofmeister series,18−21 which is a classification of ions in the order of 2733

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level.37 The calculation was performed as follows: (i) the initial structure of all the cations and anions were drawn with the help of Amsterdam Density Functional (ADF) commercial software, in which the exchange-correlation (XC) potential model for the interaction of two atoms in self-consistent field (SCF) is generalized gradient approximation (GGA) and Becke’s exchange and Perdew’s correlation functional (BP), and calculated by employing basis sets of TZP (triple-ζ potential); (ii) the equilibrium geometry of molecules was optimized in gas phase and solvent phase, and then a COSMO result file is obtained as a tape21 (.t21) file; (iii) the activity coefficient of solute at infinite dilution was obtained by COSMO-RS calculation. In this calculation, it is assumed that ethanol is at infinite dilution since its concentration is very low. The distribution coefficient D of ethanol between aqueous phase and SC CO2 phase is rewritten as follows: γaq, i c x D = sc = C0 sc = C0 caq xaq γsc, i (6)

Figure 10. FTIR spectra in the aqueous phase after extraction with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K and pressure P = 14 MPa: (1) saltfree, (2) 10 wt % KHCO3, and (3) 25 wt % KHCO3.

and becomes broad at the same time. This indicates that the interaction between KHCO3 and water is greatly enhanced after the addition of KHCO3. The FTIR spectra for the addition of [BMIM]+[Ac]− and [OMIM]+[Ac]− at different mass fractions are shown in Figures 11 and 12, respectively. It can be seen that after the addition of

where C0 is a constant, x is the mole fraction of ethanol, γaq,i and γsc,i are activity coefficients of ethanol at infinite dilution in aqueous phase and SC CO2 phase, respectively, which are obtained by the COSMO-RS model. Figure 9 shows the

Figure 11. FTIR spectra in the aqueous phase after extraction with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K and pressure P = 14 MPa: (1) saltfree, (2) 10 wt % [BMIM]+[Ac]−, and (3) 25 wt % [BMIM]+[Ac]−.

Figure 9. Comparison of distribution coefficients D between experimental and calculated results by the COSMO-RS model for different salts at T = 329.2 K: (1) KHCO3; (2) [BMIM]+[Ac]−; (3) [OMIM]+[Ac]−; (4) [EMIM]+[BF4]− ; and (5) [BMIM]+[BF4]−.

comparison of distribution coefficients between experimental and calculated results. It is evident that the calculated results from the COSMO-RS model are qualitatively consistent with the experimental results. Moreover, it can be found that the salting effect of ionic liquids is less effective for improving the extraction performance than inorganic solid salt. This may be attributed to the large monovalent ions causing poor salting-out effect.38 Although the separation ability of ionic liquids is weaker than that of inorganic solid salts, they are in the liquid state at room temperature and thus have no problems of dissolution, tub-jam, and transport brought on by inorganic solid salts in industry. Therefore, ionic liquids are regarded as green separating agents and suitable for application at industrial scale for intensifying the conventional separation processes. 3.7. Theoretical Analysis of Salting Effect. FTIR (Fourier transform infrared) spectrometry (Bruker Tensor 27, Germany) was used to qualitatively analyze the interaction between the salt and the component to be separated in this work. Figure 10 shows the infrared spectra in the aqueous phase after extraction, where the absorption peak near 3300 cm−1 corresponds to the stretching vibration of OH group. As mass fraction of KHCO3 in the aqueous phase increases, the absorption peak near 3300 cm−1 shifts to a higher wavenumber,

Figure 12. FTIR spectra in the aqueous phase after extraction with phase volume ratio Vsc/Vaq = 0.5, initial ethanol mass fraction win = 0.0503, temperature T = 329.2 K and pressure P = 14 MPa: (1) saltfree, (2) 10 wt % [OMIM]+[Ac]−, and (3) 25 wt % [OMIM]+[Ac]−.

the ionic liquids, the intensity of absorption peak near 3300 cm−1 increases. But compared to KHCO3, the change caused by ionic liquids is relatively small. This confirms that the salting effect of ionic liquids is weaker than that of solid salts. Quantum chemistry calculation was carried out as a theoretical study at the electronic scale to explore the interaction between molecules. In this work, density functional theory (DFT) was used for geometry optimization and energy calculation of the molecules concerned.39 The B3LYP (Becke, 2734

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Figure 13. Optimized geometric structures of the systems of (a) water + ethanol + CO2, (b) water + ethanol + CO2 + KHCO3, (c) water + ethanol + CO2 + [BMIM]+[Ac]−, and (d) water + ethanol + CO2 + [OMIM]+[Ac]−.

water molecular and is far away from ethanol molecule. This is consistent with the FTIR analysis, and the shift of OH group from low to high wavenumber mainly reflects the interaction between KHCO3 and water instead of KHCO3 and ethanol. In addition, binding energy is calculated by

three-parameter, and Lee−Yang−Parr) function was used to describe electron exchange and correlation, and the 6-31G** which is a standard split-valence double-ζ polarization basis set was used to locate optimized ground-state and transition-state structures.40 All of the geometry optimizations were performed using Gaussian 03 software.41−43 At the same time, it should be mentioned that all of the chemical structures were built in the ideal gas phase and thus CO2 in supercritical state cannot be considered in this software. Besides, in principle, more molecules existing in actual solutions should be included for describing the interaction sites. But in order to support the above experimental results and save computation time, single molecule (e.g., water, ethanol, CO2, and salt) was chosen in this work as adopted by some researchers44 for other separation processes with ionic liquids to qualitatively clarify the separation mechanism in light of bond length and binding energy with the zero-point correction. Figure 13 shows the calculated results by quantum chemistry. It can be seen from Figure 13a,b that, after the addition of KHCO3, the bond length of OH group of water increases from (0.96411, 0.96411) Å to (1.65871, 0.97521) Å, whereas the bond length of OH group of ethanol does not change apparently, being (0.97174) Å and (0.97241) Å, respectively. Moreover, KHCO3 (especially the oxygen atom) approaches

ΔE = E(A + B) − E(A) − E(B)

(7)

where ΔE is the binding energy of binary system; E(A) and E(B) are the sum of electronic and zero-point energies of single component, and E(A + B) is that of binary system. The binding energy between KHCO3 and water is −148.31 kJ mol−1, the absolute value of which is much higher than that between ethanol and water (−26.08 kJ mol−1). Therefore, more water molecules are constrained in the aqueous phase and thus promote the transfer of ethanol from aqueous phase to SC CO2 phase. The theoretical calculations based on DFT have also been performed on ionic liquids, as shown in Figure 13c,d. It can be seen that after the addition of ionic liquids, the change of bond length of OH group in water is more apparent than that in ethanol. The bond length of OH group in water changes from (0.96411, 0.96411) Å to (1.14685, 0.96452) Å for [BMIM]+[Ac]− and to (1.06593, 0.97531) Å for [OMIM]+[Ac]−. The bond length of OH group in ethanol changes from 2735

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(0.97241) Å to (0.97302) Å for [BMIM]+[Ac]− and to (0.97291) Å for [OMIM]+[Ac]−. The binding energy is −65.02 kJ mol−1 between [BMIM]+[Ac]− and water, and −60.94 kJ mol−1 between [OMIM]+[Ac]− and water. The absolute values of binding energy for ionic liquids follows the order of KHCO3 > [BMIM]+[Ac]− > [OMIM]+[Ac]−. Therefore, the salting effect of hydrophilic imidazolium-based ionic liquids is weaker than that of solid salts for intensifying the extraction process discussed in this work.

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4. CONCLUSIONS This work focuses on concentrating ethanol at low concentration from aqueous solutions by the double action of SC CO2 extraction and salting effect. The influence of operating conditions on the extraction process was investigated, and it was found that the initial aqueous ethanol mass fraction in aqueous phase win has a linear relationship with the ethanol mass fraction in SC CO2 phase wsc, which can become the basis of quantitative analysis; the operating pressure has almost no influence on wsc, while the operating temperature has significant influence on increasing wsc; and a low phase volume ratio Vsc/ Vaq is favorable for increasing wsc. The inorganic solid salts and ionic liquids as additives were used to intensify the SC CO2 extraction. After addition of the salts, wsc increases. Moreover, for inorganic solid salts, the salting effect follows the order of KHCO3 > KAc > NaCl ≈ NaBr > NaI which conforms to the Hofmeister series, while it follows the order of [BMIM]+[Ac]− > [OMIM]+[Ac]−> [EMIM]+[BF4]− > [BMIM]+[BF4]− for ionic liquids. The salting effect of ionic liquids is weaker than that of solid salts, and KHCO3 is the best among all of the salts investigated, with the result that wsc is almost three times higher after the addition of 20 wt % KHCO3 than without addition. The results coming from experiment (by SC CO2 extraction), calculation (by the COSMO-RS model), and theoretical analysis (by the combination of FTIR and DFT) are consistent. This work tried to link the separation technology in chemical engineering and sample detection in the pretreatment process, and the process intensification methods and results present here can also be extended to the separation of trance amount of organic substances from aqueous solutions for other purposes.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-1064433695. Fax: +86-1064419619. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China under Grant Nos. 21121064 and 21076008, the Projects in the National Science & Technology Pillar Program during the twelfth Five-Year Plan Period (No. 2011BAC06B04), and the Fok Ying Tong Education Foundation (No. 111074).

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