Capturing Condensable Gases with Ionic Liquids - Industrial

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Capturing Condensable Gases with Ionic Liquids Gangqiang Yu, Chengna Dai, Hui Gao, Ruisong Zhu, Xiaoxiao Du, and Zhigang Lei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02420 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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Industrial & Engineering Chemistry Research

Capturing Condensable Gases with Ionic Liquids

Gangqiang Yu, Chengna Dai, Hui Gao, Ruisong Zhu, Xiaoxiao Du, and Zhigang Lei* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China

ABSTRACT: Ionic liquids (ILs) have been proposed to simultaneously capture a variety of condensable gases for the first time. The hydrophobic IL [EMIM][Tf2N] was selected as a suitable absorbent screened by COSMO–RS model, which combination with the quantum chemistry calculation provided some theoretical insights into mechanism with respect to condensable gases capture. Considering simultaneously capturing volatile organic compounds (VOCs) and water, we selected benzene, toluene, and p–xylene (BTX) as three kinds of VOCs representatives, measured the vapor–liquid equilibrium (VLE) of BTX + [EMIM][Tf2N] mixture systems, and compared with the predicted results by the UNIFAC–Lei model. The experiment of capturing condensable gases with [EMIM][Tf2N] as absorbent was conducted. Furthermore, the conceptual design of continuous processes with [EMIM][Tf2N] and the conventional benchmark solvent triethylene glycol (TEG) as absorbents at industrial scale was performed using a rigorous equilibrium (EQ) stage model with the UNIFAC–Lei model parameters input. It reveals that condensable gases capture with IL belongs to a typical process intensification technology in chemical engineering.

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1. INTRODUCTION At present, volatile organic compounds (VOCs) as a class of hazardous substances of greatest concern are playing an increasingly significant role in environmental pollution, health damage, and economical losses. Nowadays, the popular methods used in VOCs treatment include catalytic combustion method,1 adsorption on porous materials,2 membrane separation,3 condensation,4 biological treatments,5 and absorption with conventional solvents.6 In principle, the absorption treatment method with liquid solvents is the most simple and efficient for capturing VOCs from various kinds of industrial gases, and has also been widely used in industry.6 In absorption process, how to select an appropriate absorbent is the most critical and significant step to achieve an economical, effective, and green separation process. For this purpose, some conventional organic solvents with high boiling points, e.g., diisobutyl phthalate, di–(2–ethyl) hexyladipate (DEHA),7 silicone oils, perfluorocarbons,8 polyethylene glycol (PEG),9 and triethylene glycol(TEG),10 were suggested previously as absorbents. Among others, TEG with a good thermal stability is used to capture VOCs as well as water from industrial gases. Unfortunately, there are still many disadvantages for the use of TEG, e.g., high cost of energy requirement for regenerating absorbents, unavoidable volatile entrainments along with losses, solvent degradation, and equipment corrosion resulting from the TEG foaming behavior, thus decreasing the green degree of absorption process. Therefore, it is imperative and urgent to discover a more satisfactory, economic, and efficient absorbent so as to avoid or eliminate these shortcomings of conventional organic solvents for capturing VOCs from industrial gases.

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Currently, room temperature ionic liquids (ILs) with the ultralow melting point as a significantly popular and designable kind of alternatives to traditional “green liquid solvents” have been widely applied to a variety of chemical, materials, and environmental industries such as new material preparation,11 chemical reaction,12 biomass dissolution,13,14 liquid–liquid extraction,15–17 and gas separation18,19 attributed to their well–known properties, e.g., excellent chemical with thermodynamic stability, high solubility for organic or inorganic compounds, negligible vapor pressure, appropriate viscosity, and designable characteristics along with modifiable structures.20,21 More specially, the negligible vapor pressure character has greatly simplified the solvent recovery process in gas separation with ILs , in which only a simple flash tank is needed to replace a complex distillation column to recover and recycle IL. Herein, we like to go a further step to use the COSMO–RS (conductor–like screening model for real solvents) model to predict the Henry's constants of a wide variety of VOCs and selectivities (S = HN2/Hi) in the common IL [EMIM][Tf2N] at 298.15 K (see Supporting Information Figure S1). It is clear that not only the selectivities of condensable VOCs/N2 gas pairs are much higher than those of non–condensable VOCs/N2 gas pairs, but also the Henry's constants of condensable VOCs in IL are much lower than those of non–condensable VOCs (low Henry's constants correspond to high solubility). This manifests that ILs are more effective and efficient for capturing condensable VOCs than non–condensable VOCs from gas mixtures. However, by far almost all of the studies on gas separation with ILs focus on the absorption of acidic gases and non–condensable VOCs (e.g., CH4), except that a few researchers recommended the promising application of ILs for capturing condensable VOCs. For instance, Bedia et al.22 revealed that the anion [Tf2N]––based ILs can be used for

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capturing toluene from N2 gas, but the authors used a continuous dry nitrogen flow for IL regeneration so that toluene couldn’t be directly obtained as liquid product. On the other hand, it is noting that the most real industrial gases usually contain condensable VOCs and water together. Thus, in this work, benzene, toluene, p–xylene (BTX) (as the representatives of multi–component condensable VOCs), and water are concerned in the feeding gases to simulate the real condition. The aims of our work are focused on addressing the following crucial issues on capturing condensable gases with ILs as absorbents: (1) understanding the effect of molecular structures of ILs on separation performance (i.e., Henry’s constant of BTX with H2O and selectivity of BTX with H2O to N2 in various ILs), and finding out the appropriate IL for capturing condensable gases; (2) providing theoretical insights into separation mechanism on capturing condensable gases at the molecular level by combining quantum chemistry calculation with the molecular thermodynamic model COSMO–RS; (3) measuring the vapor–liquid equilibrium (VLE) experimental data for the systems of IL + BTX and validating the accuracy and reliability of UNIFAC–Lei model; (4) establishing the rigorous equilibrium (EQ) stage mathematical model using Aspen Plus software for process simulation with the UNIFAC–Lei model parameters input; (5) calculating and estimating the energy consumption in the condensable gases capture processes with IL and the conventional benchmark solvent triethylene glycol (TEG) as absorbents at industrial scale. Herein, the details on ILs (e.g., full name, abbreviation, and structural formula) are given in Supporting Information Table S1.

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2. EXPERIMENTAL SECTION 2.1. Materials. The experimental materials used in this work were N2, benzene, toluene, p–xylene, and [EMIM][Tf2N]. The details on purity, mass weight, and source are given in Supporting Information Table S2. Before experiments, [EMIM][Tf2N] was fully purified and dried at 413.2 K for 4h using a vacuum drying oven (type DZF–6020, Shanghai Yiheng Scientific Instrument Co. Ltd.) to eliminate traces of water and other volatile impurities. Moreover, the Karl Fischer titration (model KLS701) was employed to measure the mass fraction of residual water content in IL (less than 200 ppm). In this work, N2, benzene, toluene, and p–xylene were used directly without further purification. 2.2. Apparatus and procedure. 2.2.1. VLE Experiment for IL + BTX systems. A modified equilibrium still was used to measure the vapor pressures of BTX + [EMIM][Tf2N] systems. The details on experimental apparatus and flowsheet are illustrated in our previous work.23 Firstly, the U–type equilibrium still should be fully cleaned using deionized water, and further sufficiently dried by a vacuum drying oven. The mixed solutions of BTX + [EMIM][Tf2N] over the entire concentration range were prepared using an electronic balance. Approximate 25 mL of the mixed solution was added into the equilibrium still, which was soaked in the water–bath. Next, the equilibrium still was evacuated until the liquid boiled in the still. The liquid level on both sides of the equilibrium still should be kept on the same horizontal line by controlling the needle valve. It was considered to reach equilibrium when the liquid level on the same horizontal line lasted for more than 30 min. In this case, the readings on pressure gauge should be recorded as the experimental vapor pressure data. 2.2.2. Condensable Gases Capture Experiment. An absorption column consisting of a

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stainless steel tube, in which the height is 100 cm, the diameter is 3 cm, and a large amount of randomly θ–type packings with the specification of 4 × 4 × 0.6 mm are filled, was used to conduct the condensable gases capture experiment (see Figure 1). Firstly, the purified IL flowed into the top of absorption column at a designated volume flowrate by a IL pump (model 2PB3020, Beijing Satellite Manufacturing Factory). Then, the high–purity dry N2 stream coming from a N2 tank flowed into a buffer vessel containing BTX and water to be saturated. Consequently, the N2 stream with saturated water and BTX was treated as the experimental feed gas, and then introduced into the absorption column from the bottom. During the experiment, the humidity meter of gas (model RHD–601) and a GC 4002A chromatograph (Beijing East–West Analytical Instruments Co., Ltd. China) with a capillary column (type PEG–20M, size 30 m × 0.32 mm × 1.0 µm) using a reliable standard curve were applied to determine the saturated water and BTX contents in N2 gas stream, respectively. Notably, the product N2 stream left from the absorption column, the water content being first detected by another humidity meter of gas, and then the BTX content being detected by the GC 4002A chromatograph. As a result, an online monitoring was achieved. After absorption, the IL with high water content collected from the bottom was regenerated by the vacuum drying oven (model DZF–6020). 3. UNIFAC–LEI MODEL 3.1. Model Introduction. The popular UNIFAC–Lei model as proposed by the Lei et al.,24 is an efficient and widely used predictive molecular thermodynamic model in chemical engineering fields. More especially, this predictive model has been applied for predicting a large number of phase equilibrium properties based on activity coefficients (e.g., vapor

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pressure and gas solubility) for multi–component systems containing ILs. It should be noted that the entire IL can be subdivided into several separate functional groups, in which the combination of cation skeleton together with the anion is considered as only one specific IL functional group in the UNIFAC–Lei model.24,25 In the calculation, the activity coefficient of component i ( γ i ) derived from the two parts of contributions, namely the combinatorial and residual contributions, can be represented by

ln γ i = ln γ i C + ln γ i R

(1)

where ln γ i C denotes the combinatorial contribution as a function of two group parameters (Rk and Qk); and ln γ i R stands for the residual contribution as a function of two temperature–independent binary interaction parameters (amn and anm). Detailed description on this model can be found in our previous work.24,25 In particular, all of group interaction parameter pairs (amn and anm) and group parameters (Rk and Qk) with respect to BTX–H2O–IL mixture systems are given in Supporting Information Tables S3 and S4, respectively, which come from previous work.24,26 3.2. VLE Prediction. For the VLE of BTX (1) + IL (2) mixture systems, phase equilibrium equation can be written as y1 Pφ1 (T , P, y1 ) = x1γ 1 P1S

(2)

where y1 and x1 stand for the BTX mole compositions in vapor and liquid phases, respectively; γ1 represents the calculated BTX activity coefficient using the UNIFAC–Lei model;

φ1 (T , P, y1 ) denotes the BTX fugacity coefficient in vapor phase, which was treated as ideal gas ( φ1 =1 ) due to the low system pressure P; P1S stands for the vapor pressure of pure BTX as determined by Antoine equation;27–30 and T is the system temperature. It should be

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mentioned that the vapor phase is considered as the single BTX composition (i.e., y1 = 1) under the operating temperature and pressure due to the negligible vapor pressure of IL.

4. COSMO–RS MODEL The COSMO–RS model31 as one priori predictive molecular thermodynamic model and independent of the experimental data, based on the quantum chemistry calculation combining with a statistical thermodynamic method, has been widely used to calculate various kinds of thermodynamic properties for single component and mixture systems, e.g., gas solubility, Henry’s constants, activity coefficients, and vapor pressure. It can also be applied to screen the suitable ILs in separation processes.32–36 In this work, we employed the COSMOthermX software (version C30_1301) to screen out the appropriate ILs and conduct the analysis of σ–profiles and excess enthalpy for benzene, toluene, p–xylene, H2O, and IL systems concerned in this work. Notably, the COSMO files of BTX and H2O can be found in built–in database of COSMOthermX software, whereas those of cations and anions for the corresponding ILs are acquired by carrying out the geometry optimization using TURBOMOLE (version 6.4) program. More details on the COSMO–RS calculation and analysis are given in the Users Manual of COSMOthermX software.

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5. QUANTUM CHEMISTRY CALCULATION Density functional theory (DFT) as the most widely used computational chemistry method was used for the molecular microstructural analysis of the compounds of [EMIM][Tf2N], BTX, N2, and H2O using Gaussian 09 software.37 The geometry optimizations were conducted by the B3LYP/6–31+G**,38,39 along with Grimme’s DFT–D3 dispersion correction40 to obtain the most stable molecular structures. When precisely analyzing the interaction energies between molecules, the basis set superposition error (BSSE) was employed.

6. RESULTS AND DISCUSSIONS 6.1. Screening of ILs for Condensable Gases Capture. It is necessary to select a suitable IL for condensable gases capture from N2 gas before conducting the gas absorption experiment. The solubility of solutes (BTX, H2O, and N2) in ILs and the selectivity of key components (BTX or H2O) to N2, as two important physical quantities calculated by the COSMO–RS model, can measure whether the separation process is effective or not. The Henry’s constant is considered as a measure of gas solubility and obtained by H i (T ) = lim(γ i Pi s ) = γ i∞ Pi s

(3)

xi → 0

where the Henry’s constant Hi(T) of solute i in ILs is the product of activity coefficients at infinite dilution γ i∞ multiplied by the saturated vapor pressure Pi S . Herein, the selectivity of key components i (BTX or H2O) to N2 ( S i / N ) in various kinds 2

of ILs is calculated by

Si / N 2 =

H i (T ) H N2 (T )

(4)

It should be mentioned that the selectivity ( S i / N ) is regarded as an “idealized” selectivity, not 2

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considering the interaction between different gases. To screen out the suitable ILs, 255 kinds of ILs containing 15 common cations and 17 common anions were concerned in this work. The effect of IL molecular structures on separation performance (i.e., the solubility of BTX and H2O as well as the selectivity of BTX or H2O to N2 in ILs) is shown in Figure 2. It can be seen that the selectivity of BTX to N2 in ILs are dependent on the types of both cations and anions, whereas the anions mainly determine the selectivity of H2O to N2 in ILs and the cations only play a secondary role. The color–filled maps for Henry’s constants of H2O, benzene, toluene, p–xylene, and N2 in various ILs at 298.15 K are displayed in Supporting Information Figures S2. It shows that the solubility of both BTX and N2 in ILs are dependent on the structures and types of both anions and cations. Besides, the solubility of H2O in ILs exhibits the almost consistency with the selectivity of H2O to N2 in ILs. In most cases, the higher solubility of H2O and BTX in ILs corresponds to the higher selectivity. Moreover, those ILs with the longer alkyl side–chain on the cationic skeletons have the higher solubility of both H2O and BTX, but the longer alkyl side–chain on cations would lead to the higher IL viscosity, which is unfavorable for improving the separation efficiency due to the mass and heat transfer limitation of vapor–liquid phase in absorption column. Thus, the common cation [EMIM] with a short alkyl side–chain was selected in this work as a part of candidate ILs. On the other hand, the thermal stability as a very significant property should be taken into account for the employment of ILs as suitable absorbents, which can be estimated using the thermogravimetrical analysis (TGA) methods. The onset thermal decomposition temperature (Tonset) is often treated as an extremely important parameter to estimate the

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thermal stability of ILs. The Tonset is significantly affected by the types of anions since there is a decreasing trend with the increasing hydrophilic capacity of anions. Some authors reported that the [Tf2N]––based ILs are not easily thermally decomposed accompanying with the relatively high Tonset.41,42 It is worth noting that the Tonset of [EMIM][Tf2N] is up to 419 ℃ as measured by Mu et al.41 Furthermore, the investigation by Ferreira et al.42 also demonstrated that the IL [EMIM][Tf2N] has a high thermodynamic stability. For those ILs with the same imidazolium–based cations, the trend of Tonset is in the order of [Tf2N]– > [BF4]– > [DCA]– > [Br]– > [Cl]– > [Ac]–. In addition, [EMIM][Tf2N] exhibits the relatively small viscosity.43 Thus, the hydrophobic IL [EMIM][Tf2N] with the good thermodynamic stability was selected as an appropriate absorbent in this work used for capturing condensable gases because it can endure the high temperature during the IL regeneration to obtain the high–purity recycled IL. Moreover, it can be found from Supporting Information Table S5 that [EMIM][Tf2N] presents the higher solubility of BTX as well as the higher selectivity of either H2O or BTX to N2 in IL than the conventional solvent TEG. It is worth noting that although [EMIM][Tf2N] is considered as a “hydrophobic” IL, it exhibits a considerable amount of solubility for water with the experimental solubility value of 0.00676±0.0002 in mass fraction for water in [EMIM][Tf2N] at ambient pressure and temperature,44 resulting in a high absorption performance for water, which will be mentioned in the subsequent condensable gases capture experiment. Therefore, it suffices for [EMIM][Tf2N] as a suitable absorbent to replace the conventional TEG for simultaneous capture of water and BTX from N2 gas.

6.2. Theoretical Insights into Solute–IL Systems by COSMO–RS Model. 6.2.1. Analysis of the σ–Profiles. In the COSMO–RS Model, various thermodynamic behaviors of

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components are related to σ–profiles, which denotes with the molecular surface polarity distributions.45 In the σ–profile curves, the whole σ region consists of three parts, namely the non–polar region in the range of (–0.0082 < σ < 0.0082 e/Å2) presented at the middle position, the hydrogen bond (HB) donor region located at the leftmost position σ < –0.0082 e/Å2, and the HB acceptor region appearing in the rightmost position σ > 0.0082 e/Å2. The σ–profiles curves for all the components concerned in this work are shown in Figure 3a. It is observed that almost the entire σ–profile curves for benzene, toluene, and p–xylene locate in the non–polar region. This means that BTX have the strong affinity with other non–polar components. In particular, all peaks in the σ–profile of N2 are relatively weak and locate in the position close to σ = 0, showing the strong non–polarity as well as the neutral N≡N molecular characteristics of N2. For the σ–profile of the cation [EMIM]+, one strong peak lies in the non–polar region, indicating a strong non–polar capacity, while one weak peak appears around –0.0084 e/Å2, indicating that it has a weak HB donor ability. Therefore, the relatively strong interaction can be expected between [EMIM]+ and BTX molecules. Two very strong peaks in the σ–profile of [Tf2N]– appear at σ = 0.0017 e/Å2 attributed to the lone pairs on nitrogen atom and at σ = 0.011 e/Å2 due to the strongly electronegative fluorine and oxygen atoms, indicating that the anion [Tf2N]– can be used as a relatively strong HB acceptor. Thus, there exists the strong interaction between BTX and [EMIM]+ or [Tf2N]– in accordance with the rule–of–thumb of “like dissolves like.” The σ–profile curve of H2O covers a broad scope, exhibiting two strong peaks appearing at σ = –0.016 and σ = 0.018 e/Å2, showing that H2O can be treated as both a strong HB donor and acceptor. Thus, the strong HB can be formed between H2O and the anion [Tf2N]– (i.e., the so–called anion effect). That explains why the IL

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[EMIM][Tf2N] is effective for simultaneous capture of water and BTX from gas mixture. 6.2.2. Analysis of the Excess Enthalpies. In the classic theory of chemical thermodynamics, the excess enthalpy HE plays a significant role in evaluating the non–ideal behavior of mixture solution, and is composed of three parts of contributions from the perspective of molecular thermodynamics, i.e., electrostatic–misfit HE(MF), van der Waals interactions HE(vdW), and hydrogen bond interaction HE(HB), which can be written as46

H E = H E (MF) + H E (HB) + H E (vdW)

(5)

As seen from Figure 3b, the excess enthalpy of H2O + [EMIM][Tf2N] mixture system is mainly contributed by the HB interaction between H2O and [EMIM][Tf2N], while both electrostatic–misfit interaction and van der Waals interaction are almost negligible. Figures 3c–e show the excess enthalpies of BTX–IL mixture systems (benzene + [EMIM][Tf2N], toluene + [EMIM][Tf2N, and p–xylene + [EMIM][Tf2N). These curves exhibit a similar trend, in which the electrostatic–misfit interaction is the most important contribution to HE. It should be mentioned that the electrostatic–misfit interaction between BTX and IL would reduce when the solute content in IL exceeds a certain amount, leading to the occurrence of demixing phenomenon at high solute concentrations. For the binary systems of N2 + [EMIM][Tf2N] (see Figure 3f), it can be clearly observed that the HB interaction is almost zero, and the van der Waals interaction dictates.

6.3. Molecular Microstructural Analysis by Quantum Chemistry Calculation. 6.3.1. Analysis of the Interaction Energies. The most stable structure selected from a number of possible configurations optimized with Gaussian 09 software,37 was used to analyze the intermolecular interaction energies. The specific intermolecular interaction modes and types

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along with the most stable structures are illustrated in Figure 4. The interaction energies reflect the strength of bond between molecules, and is represented with the following equation:

∆EA-B (kJ⋅ mol −1 ) = 2625.5 {EA-B (au) − [ EA (au) + EB (au)]}

(6)

where EA–B, EA, and EB (au) represent the energy of A together with B, the energy of single A, and the energy of single B, respectively, and the symbol “au” means the atom unit. Accordingly, the interaction energies of BTX (H2O, N2) with the cation [EMIM]+, the anion [Tf2N]–, and the IL [EMIM][Tf2N] were calculated. The calculated results are given in Supporting Information Table S6. It is clear that the interaction energies between the cation [EMIM]+ and BTX are somewhat greater than those between the anion [Tf2N]– and BTX, but the interaction energy between the cation [EMIM]+ and H2O (–3.53 kJ·mol–1) is much less than that between the anion [Tf2N]– and H2O (–37.67 kJ·mol–1). Notably, it can be clearly found that the interaction energies of N2 with the cation [EMIM]+, the anion [Tf2N]–, and the IL [EMIM][Tf2N] are almost identical but very small when compared to those of BTX or H2O. This conclusion is consistent with the predicted results by COSMO–RS model aforementioned that the solubility of BTX and H2O in ILs is much larger than that of N2, leading to the large selectivity of BTX/N2 and H2O/N2 gas pairs in gas separation (see Figure 2 and Supporting Information Figures S2). 6.3.2. Recognition of the Weak Interaction of Solute–IL Systems. The weak interaction, namely noncovalent interaction refers to the van der Waals interaction included in almost all molecules or ion, HB interaction occurring at the inside or between molecules containing hydrogen bond acceptors and donors, and steric repulsion interaction presented in some

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molecules with relatively large volume and cyclic structure. The method of reduced density gradient (RDG) analysis have been applied to identify these interactions, resulting in intuitive visual pictures as reported by Johnson et al.47 In this work, Multiwfn program48 as a very powerful tool was used to carry out the RDG analysis, in which the color–filled gradient isosurfaces in broad regions of real space can conveniently provide a rich and reliable insight into weak interactions. As shown in Figure 5a, the strong steric effect represented by the red isosurface in the five–membered ring of the cation [EMIM]+ is clearly observed due to the strong ring tension. For the anion [Tf2N]– (see Figure 5b), the isosurface lies between the oxygen and the fluorine along with between the two oxygens in both sides of the nitrogen atom on the central O–O and O–F connected lines. This indicates the repulsive interaction between O–O and O–F when close to each other, resulting from the extremely strong electronegativity of oxygen and fluorine. For the IL [EMIM][Tf2N], there are many isosurfaces formed in the space composed of the cation [EMIM]+ and the anion [Tf2N]– (see Figure 5c). In this case, the blue regions inside the molecule represent the intramolecular weak HB and strong electronegativity between [EMIM]+ and [Tf2N]–, while other green disk–shaped isosurfaces represent the dispersion interaction caused by the large vdW surface. For the [EMIM]+ + H2O system (see Figure 5d), the light blue isosurface shows the characteristic of H–bond formed between the hydrogen (hydrogen donor) with the most positive charge in the imidazole ring and the oxygen (hydrogen acceptor) in H2O. Moreover, the green isosurface indicates the vdW dispersion interaction between one hydrogen of the alkyl side chains on the imidazole ring and one afforded by H2O. Two blue disk–shaped isosurfaces (see Figure 5e) reveal the

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stronger H–bonds in [Tf2N]– + H2O than in [EMIM]+ + H2O. As shown in Figure 5f, the stronger H–bonds are formed between oxygen and hydrogen atoms in [EMIM][Tf2N] + H2O than those in [Tf2N]– + H2O or [EMIM]+ + H2O. The reason is due to the synergistic effect among H2O, [EMIM]+, and [Tf2N]–. That is, water molecule can enter into a space cage formed by the strong electronegativity between [EMIM]+ and [Tf2N]–, and thus the distance between H2O and [EMIM]+ or [Tf2N]– decreases. For the [EMIM]+ + benzene system (see Figure 5g), a red isosurface locates at the most central position of benzene ring, which is similar to the isosurface on the imidazolium ring of [EMIM]+. Besides, a large dark green conical isosurface locates at the overlapping portion between the electron–deficient H7 and the high π electron density region above the benzene ring, where the C–H···π bond is discovered. Notably, the essence of C–H···π bond should be taken on as electrostatic interaction.49 However, for the [Tf2N]– + benzene system (see Figure 5h), the curved isosurface reveals the weak vdW attraction between the large electronegative oxygen in [Tf2N]– and the electron–deficient hydrogen on the benzene ring. Figure 5i shows that the strong electrostatic and vdW attraction interactions between [EMIM]+ and [Tf2N]– make the isosurface of C–H···π bonds remove from the overlapping portion between the H7 atom and the benzene ring to the alkyl side chains on the imidazole ring, which weakens the steric effect as a whole. It is evident that the vdW attraction interaction dominates for [EMIM]+ + N2, [Tf2N]– + N2, and [EMIM][Tf2N] + N2, with the green isosurfaces clearly observed in Figures 5j–l. Anyway, the conclusions on molecular microstructural analysis obtained by quantum chemistry calculation are consistent with those coming from the theoretical analysis as obtained by the COSMO–RS model.

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6.4. VLE Experimental Results for BTX + [EMIM][Tf2N] Systems. The VLE experimental data in mixture systems of BTX and [EMIM][Tf2N] at different temperatures and BTX concentrations (from 0.1 to 1 in mole fraction) were obtained by measuring the vapor pressures of BTX–[EMIM][Tf2N] mixture systems. In addition, the UNIFAC–Lei model was employed to predict the vapor pressures. It can be seen from Figure 6 that a good agreement with the extremely small ARDs (2.40%, 3.39%, and 4.99% for the mixture system of benzene + IL, toluene + IL, and p–xylene + IL, respectively) (see Supporting Information Table S7) between experimental data and the predicted results is observed. Besides, the relation between temperature and vapor pressure is shown in Supporting Information Figure S3. This confirms the predictive capacity of UNIFAC–Lei model extended to depict the VLE behavior for BTX–IL systems. On the other hand, the predictive capacity of UNIFAC–Lei model for liquid–liquid equilibrium (LLE) is also validated in comparison with the LLE data of BTX–[EMIM][Tf2N] systems collected from the literature,50–53 in which the corresponding group binary interaction parameters are derived from previous publications by correlating the experimental VLE data. The result shows that the UNIFAC–Lei model can also be successfully extended to predict the LLE data (see Supporting Information Figure S4). It is obvious that with the increase of BTX contents in solution, vapor pressure first exhibits a linear increase, and then levels off at high BTX contents near the vapor pressure of pure solvent at the same temperature. This is related to the liquid–liquid phase demixing at high BTX contents. In accordance with the molecular microstructural analysis aforementioned, at low BTX contents each benzene ring in BTX molecule can form the C–H···π bond with the imidazole ring in [EMIM][Tf2N] molecule, which fully encloses the

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BTX molecules, while at high BTX contents a part of BTX molecules can’t form the C–H···π bonds, leading to the occurrence of liquid–liquid demixing phenomenon.

6.5. Experimental Results of Condensable Gases Capture. The absorption experiment for capturing condensable gases with [EMIM][Tf2N] was conducted under the ambient condition (20 ℃ and 1 atm). In the process, the flow rate of N2 gas with saturated water and BTX contents (water content around 2.30%, benzene content around 9.86%, toluene content around 2.89%, and p–xylene content around 0.86% in mole fraction) was specified at a constant value of 550 mL·min–1 under normal temperature and pressure (NTP). Meanwhile, the water–loaded and BTX–loaded IL was regenerated by using the vacuum drying oven at 140 ℃ and 0.05 atm for a few hours. Moreover, the effect of regeneration time on the water and BTX contents of regenerated IL is given in Supporting Information Table S8. In absorption, there are two important operating parameters, that is, the IL volume flow rate VIL and IL water content entering into the absorption column, which significantly influence the purity of gas product. The relationship between VIL and the water content in gas product yH 2O at several different water contents in the entry IL (70, 550, and 1000 ppm, mass fraction) is illustrated in Figure 7a. It is clear that yH 2O first reduces rapidly, and then levels off with the increase of VIL. The same phenomenon also holds for the BTX contents in gas product (see Figures 7b–d). Furthermore, it should be mentioned that the existence of small amount of water in the entry IL has almost no influence on the BTX contents in gas product, because the three curves at different water contents seem overlapping. In particular, when the VIL is more than 15 mL·min–1 (NTP), the contents (mole fraction) of water, benzene,

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toluene, and p–xylene in gas product will decrease to less than 146, 1165, 505, and 132 ppm, respectively. In addition, the relationship between the IL initial water content and the water and BTX contents in gas product is shown in Figure 8, in which the VIL is fixed at 15 mL·min–1 (NTP). A linear relationship between the IL initial water content and yH 2O is clearly observed (see Figure 8a). That is, a low IL initial water content is favorable for improving the purity of gas product. Notably, the relationship between the IL initial water content and the BTX contents in gas product exhibits a horizontal line (see Figure 8b). This demonstrates that the BTX contents in gas product are independent on the IL initial water contents.

6.6. Process Simulation of Condensable Gases Capture at Laboratory Scale. The process simulation of condensable gases capture at laboratory scale with IL as absorbent was conducted by commercial process simulation software Aspen Plus (version 7.2). It should be mentioned that the IL [EMIM][Tf2N] isn’t included in the original database and has to be added into Aspen database using User Defined method, while other compounds (e.g., water, N2, and BTX) as conventional substances are included in the built–in Aspen database. In addition, the UNIFAC model was selected to carry out the process simulation with the EQ stage model (RadFrac module). Moreover, the number of theoretical stages of experimental absorption column as calculated by the Strigle relation54 is equal to 2. It is evident that the simulated results by the EQ stage model agree well with the experimental data (see Figures 7 and 8). This verifies the reliability of UNIFAC–Lei model embedded in Aspen Plus software for simulating the condensable gases capture process.

6.7. Process Simulation and Optimization of Capturing Condensable Gases at

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Industrial Scale. In this work, we further simulate and compare the continuous condensable gases capture processes with [EMIM][Tf2N] and TEG as absorbents at industrial scale. The flowsheet with IL as absorbent are mainly composed of three parts, that is, a packing absorption column, heat exchangers, and a flash drum (see Supporting Information Figure S5a). The N2 stream with the saturated water content of 2.30% and the total BTX content of 13.61% (i.e., benzene 9.86%, toluene 2.89%, and p–xylene 0.86%) in mole fraction, along with a mass flow of mFeed (1000 kg·h–1) was treated as the simulated feed gas at industrial scale. It is required that water content in gas product can’t exceed 100 ppm, the total BTX content can’t exceed 1.50%, and the recovery ratio of N2 is more than 99.0%. To achieve this specific separation target, some operating and design parameters should be optimized comprehensively. The influence of these parameters on separation performance (the water content yH 2O and the total BTX content yBTX in gas product) was investigated by sensitivity analysis. It can be seen that yH 2O shows a significant decreasing trend as the IL flow rate mIL increases, and then tends to be stable (see Supporting Information Figure S6a). Besides, a high number of theoretical stages Nt will be helpful in enhancing the purity of gas product, but when Nt is over 6, the influence is not predominant. The similar tendency can also be found as for the influence of mIL and Nt on the total BTX content in gas product (see Supporting Information Figure S6b). Therefore, the optimal Nt is 6, because too large Nt will increase the cost of equipment, while too small Nt will not meet the separation requirement. Moreover, with the increase of absorption temperature (T1), both yH 2O and yBTX increase gradually (see Supporting Information Figures S5c–d). On the other hand, as shown in

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Supporting Information Figures S5e–h, the higher operating temperature (T2) and lower pressure (P2) for regenerating the BTX–rich and water–rich IL in flash drum will lead to the lower water and BTX contents in the entry IL, which is beneficial to simultaneous capture of water and BTX. Thus, the optimized specifications are given as follows: mIL = 4000 kg·h–1 T1 = 20 ℃, Nt = 6, T2 = 150 ℃, and P2 = 0.05 bar. As a result, the high–purity gas product with

yH 2O = 84 ppm and yBTX = 1.32% is obtained, while the N2 recovery ratio is as high as 99.67%. For comparison, process simulation and optimization for condensable gases capture with TEG as the benchmark absorbent was also systematically carried out. The flowsheet of TEG process consists of an absorption column similar as that of IL and a distillation column for TEG regeneration in place of a flash drum (see Supporting Information Figure S5b). Similarly, the influence of design and operating parameters on separation performance was also investigated (see Supporting Information Figure S7). It should be noted that in the TEG process, the operating conditions of absorption column are identical to those in the IL process, that is, T1 = 20 ℃ and P1 = 1 bar, but it is noting that the reboiler temperature of desorption column for TEG regeneration should not over 203 ℃, because the TEG decomposition will take place at high temperature. To obtain high–purity gas product in the TEG process, the selected optimal conditions are listed as follows: the number of theoretical stages in absorption column N abs = 6, the mass flow rate for the conventional benchmark solvent TEG mTEG = 4000 kg·h–1, boiler temperature in the desorption column Treb = 202 ℃, and the number of theoretical stages of desorption column N des = 5. In this case, yH 2O in the TEG process (218 ppm) is higher than that in the IL process (84 ppm). In summary, the optimized

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operating and design parameters are given in Supporting Information Tables S9, and the simulated results for the two different absorbents (i.e., [EMIM][Tf2N] and TEG) are given in Supporting Information Tables S10. Furthermore, the estimation and analysis of energy consumptions for capturing condensable gases processes with different absorbents were also conducted. The heating duty required for absorbent regeneration provided by the flash drum in the IL process is 70.32 kW, while it is 109.94 kW provided by the boiler of desorption column in the TEG process. Besides, there is an extra energy consumption of 6.76 kW presented in the IL process, resulting from the vacuum operation in flash drum. The total cooling duty in the IL process comprising the energy supplied by cooler 1 and cooler 2 is –86.78 kW, while that in the TEG process supplied by the condenser in desorption column in addition to the same part as in the IL process is –131.46 kW. Taken together, when compared to the TEG process, the total heating and cooling duties in the IL process can be saved by 36.04% and 33.99%, respectively. Moreover, the exergy consumption in the IL and TEG processes was calculated using the method as proposed by Kutas.55 It was found that the exergy consumption in the IL process can be saved by 51.51% (see Supporting Information Table S10 for more energy consumption details). Thus, the condensable gases capture process with [EMIM][Tf2N] as absorbent exhibits more efficient and effective separation performance like no solvent loss, lower energy consumption, and simpler flowsheet, when compared to that with TEG as absorbent.

7. CONCLUSIONS In this work, the ILs have been proposed to simultaneously capture a variety of

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condensable gases (BTX and water) for the first time. The hydrophobic IL [EMIM][Tf2N] was selected for simultaneous capture of BTX and water from N2 gas among 255 ILs when taking into account separation performance (i.e., the solubility of BTX and H2O as well as the selectivity of BTX or H2O to N2 in ILs) and other properties (i.e., viscosity, market price, and thermodynamic stabilities) together. The quantum chemistry calculation revealed that both cation [EMIM]+ and anion [Tf2N]– have mutual contribution to capturing BTX due to the C–H···π interaction between the H7 atom in imidazole ring on [EMIM]+ and the benzene ring on BTX, as well as the vdW attraction interaction between [Tf2N]– and BTX. However, the anion [Tf2N]– dictates in capturing water due to the H–bond interaction between [Tf2N]– and H2O. Moreover, the VLE data of BTX + [EMIM][Tf2N] mixture systems over a wide BTX concentration range (from 0.1 to 1 in mole fraction) were measured. The popular UNIFAC–Lei model has been successfully extended to describe the thermodynamic behavior of VLE for IL–BTX systems. Furthermore, the experiment of condensable gases capture with [EMIM][Tf2N] was conducted, exhibiting the excellent separation performance as expected. On this basis, the conceptual design and optimization of continuous condensable gases capture processes with [EMIM][Tf2N] and the conventional benchmark solvent TEG as absorbents at industrial scale was performed by the EQ stage model using Aspen Plus software. This work demonstrates that the new condensable gases capture technology with IL as absorbent is efficient and effective, with the higher VOCs and water absorption capacity, the lower energy consumption, and no solvent loss, when compared to that with the conventional TEG as absorbent.

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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the website. Experimental VLE data for BTX + [EMIM][Tf2N] mixture systems along with the uncertainties of temperature, pressure, and BTX composition, as well as the optimized parameters and simulation results for capturing condensable gases processes with different absorbents (i.e., IL and TEG) at industrial scale, can be found in the online version. (.doc file).

■ AUTHOR INFORMATION Corresponding Author *Tel.: +86–1064433695. E–mail: [email protected].

ORCID Zhigang Lei: 0000–0001–7838–7207

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work is financially supported by the National Key R&D Program of China (No. 2018YFB0604902) and the National Natural Science Foundation of China under Grant (No. 21476009).

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Figure Captions Figure 1. The flow chart of experimental apparatus for condensable gases capture. (1) N2 tank; (2) pressure–reducing valve; (3) humidity meter of gas; (4) flow regulator; (5) gas controlling valve; (6) buffer vessel; (7) absorption column; (8) IL pump; (9) gas chromatograph; (10) IL storage tank.

Figure 2. Common logarithms of the selectivity of H2O (a), benzene (b), toluene (c), and p–xylene (d) to N2 in 255 ILs as calculated by the COSMO–RS model at 298.15 K.

Figure 3. σ–Profile curves of N2, H2O, benzene, toluene, p–xylene, [EMIM]+, and [Tf2N]– (a) as well as the excess enthalpies of the mixtures of H2O + [EMIM][Tf2N] (b), benzene + [EMIM][Tf2N] (c), toluene + [EMIM][Tf2N] (d), p–xylene + [EMIM][Tf2N] (e), and N2 + [EMIM][Tf2N] (f) at T = 298.15 K.

Figure 4. The most stable structures of [EMIM]+ (a), [Tf2N]– (b), [EMIM][Tf2N] (c), [EMIM]+ + H2O (d), [Tf2N]– + H2O (e), [EMIM][Tf2N] + H2O (f), [EMIM]+ + benzene (g), [Tf2N]– + benzene (h), [EMIM][Tf2N] + benzene (i), [EMIM]+ + N2 (j), [Tf2N]– + N2 (k), and [EMIM][Tf2N] + N2 (l) optimized by Gaussian 09 software. Dashed lines represent H–bonds.

Figure 5. Color–filled three–dimensional visualization maps of reduced density gradient (RDG). The isosurface value of RDG is specified to 0.5, and the value of sign(λ2)ρ ranging from –0.03 to 0.02 au is denoted with filling color. Blue refers to the strong hydrogen interactions, and red refers to the strong steric repulsive effect.

Figure 6. Vapor pressures of the binary mixtures of benzene (1) + [EMIM][Tf2N] (2) (a), toluene (1) + [EMIM][Tf2N] (2) (b), and p–xylene (1) + [EMIM][Tf2N] (2) (c). Solid lines, predicted results by the UNIFAC–Lei model; dashed lines, saturated vapor pressures of pure BTX at the same temperature; scattered points, experimental data. ■, 313.15 K; ●, 323.15 K; ▲, 333.15 K; ◆, 343.15 K; ★, 353.15 K; ▼, 363.15 K.

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Figure 7. Influence of the IL volume flow rate VIL (NTP) on the water yH2O (a), benzene ybenzene (b), toluene ytoluene (c), and p–xylene yp–xylene (d) contents in gas product under different IL initial water contents. Scattered points, experimental data; solid lines, predicted results by the EQ stage model; ■, wH 2O, IL = 70 ppm; ●, wH 2O, IL = 550 ppm; ▲, wH 2O, IL = 1000 ppm.

Figure 8. Influence of the IL initial water content wH2O, IL on the water yH2O (a) and BTX yBTX (b) contents in gas product with the IL volume flow rate VIL = 15 mL·min–1 (NTP). Scattered points, experimental data; solid lines, predicted results by the EQ stage model.

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3 2 7

9 5 1

4 Water–load IL

8

10

6

Figure 1. The flow chart of experimental apparatus for condensable gases capture. (1) N2 tank; (2) pressure–reducing valve; (3) humidity meter of gas; (4) flow regulator; (5) gas controlling valve; (6) buffer vessel; (7) absorption column; (8) IL pump; (9) gas chromatograph; (10) IL storage tank.

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EPY MMIM EMIM C3MIM BMIM BPY BMPY HMIM HMPY OPY OMIM N4,1,1,1 BMPYR P4,4,4,4 P6,6,6,14

EPY MMIM BPY EMIM C3MIM BMIM BMPY HMPY HMIM OMIM OPY BMPYR N4,1,1,1 P4,4,4,4 P6,6,6,14

5-6 4-5 3-4

BF4 DCA MeSO4 NO3 PF6 SCN TFA CF3SO3 SbF6 TOS Ac Br Cl MeSO3 Tf2N bFAP pFAP

EPY BPY MMIM EMIM C3MIM BMIM N4,1,1,1 OPY HMIM OMIM BMPY HMPY BMPYR P4,4,4,4 P6,6,6,14

pFAP

Tf2N bFAP

SbF6

CF3SO3

TFA TOS

PF6

4.0 -4.5 3.5 -4.0 3.0 -3.5

(b)

SCN

NO3

BF4

DCA MeSO4

MeSO3

Cl

4.5 -5.0

Ac Br Cl MeSO3 DCA MeSO4 NO3 TOS TFA CF3SO3 SCN PF6 BF4 SbF6 Tf2N bFAP pFAP

6-8 4-6 2-4

pFAP

Cl Br DCA MeSO3 MeSO4 NO3 SCN TOS TFA CF3SO3 BF4 PF6 Tf2N SbF6 bFAP

MMIM EPY EMIM C3MIM BMIM BMPYR HMIM HMPY OPY OMIM BPY BMPY N4,1,1,1 P4,4,4,4 P6,6,6,14

(a)

Br Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ac

Page 33 of 42

(c)

(d)

Figure 2. Common logarithms of the selectivity of H2O (a), benzene (b), toluene (c), and p–xylene (d) to N2 in 255 ILs as calculated by the COSMO–RS model at 298.15 K.

33


5-6 4-5 3-4


2.5

35 -

[Tf2N]

(a)

Benzene Toluene P-xylene + [EMIM] H2O

25

p (σ)

2.0

20

HE (kJ·mol)–1

30

N2

15 10 5 0 -0.03

(b)

HE(HB) HE

1.5 1.0 0.5

HE(MF)

0.0

HE(vdW)

-0.5 -0.02

-0.01

0.00

0.01

0.02

0.03

2

-1.0 0.0

0.2

0.4

1.8

(c)

1.0

2.0

H

1.4 1.2 1.0

E

0.8

H (MF)

0.6

HE(HB)

0.4

HE

(d)

E

HE (kJ·mol)–1

HE (kJ·mol)–1

0.8

2.4

2.0 1.6

0.6

xH 2 O

σ (e/Å )

1.6 1.2

HE(MF)

0.8 0.4

HE(HB)

0.0

HE(vdW)

0.2

HE(vdW)

0.0 -0.2 0.0

0.2

0.4

0.6

0.8

0.0

1.0

0.2

0.4

3.2

(e) 1

HE

2.4 2.0 1.6

HE(MF)

1.2 0.8

H (HB)

0.0

HE(vdW) 0.2

0.4

xp-xylene

0.6

(f)

1.0

HE(MF)

0

HE(HB)

-1

HE

-2

E

0.4

-0.4 0.0

0.8

2

HE (kJ·mol)–1

2.8

0.6

xtoluene

xbenzene

HE (kJ·mol)–1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

-3

0.8

1.0

0.0

HE(vdW) 0.2

0.4

0.6

0.8

1.0

xN 2

Figure 3. σ–Profile curves of N2, H2O, benzene, toluene, p–xylene, [EMIM]+, and [Tf2N]– (a) as well as the excess enthalpies of the mixtures of H2O + [EMIM][Tf2N] (b), benzene + [EMIM][Tf2N] (c), toluene + [EMIM][Tf2N] (d), p–xylene + [EMIM][Tf2N] (e), and N2 + [EMIM][Tf2N] (f) at T = 298.15 K. 34


Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

2.3996 Å 2.3216 Å 2.0489 Å

(c)

(d)

2.2126 Å

2.0691 Å

2.2424 Å 1.8448 Å

(e)

(f)

35



2.589 Å

(g)

(h)

(i)

(j)

(l)

(k)

C

H

O

F

N

S

Figure 4. The most stable structures of [EMIM]+ (a), [Tf2N]– (b), [EMIM][Tf2N] (c), [EMIM]+ + H2O (d), [Tf2N]– + H2O (e), [EMIM][Tf2N] + H2O (f), [EMIM]+ + benzene (g), [Tf2N]– + benzene (h), [EMIM][Tf2N] + benzene (i), [EMIM]+ + N2 (j), [Tf2N]– + N2 (k), and [EMIM][Tf2N] + N2 (l) optimized by Gaussian 09 software. Dashed lines represent H–bonds. 36


Page 36 of 42

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) [EMIM]+

(d) [EMIM]+ + H2O

(g) [EMIM]+ + benzene

(j) [EMIM]+ + N2

(b) [Tf2N]–

(c) [EMIM][Tf2N]

(e) [Tf2N]– + H2O

(f) [EMIM][Tf2N] + H2O

(h) [Tf2N]– + benzene

(k) [Tf2N]– + N2

37


(i) [EMIM][Tf2N] + benzene

(l) [EMIM][Tf2N] + N2


0.02

–0.03 H–bond

Dispersion

Steric

Figure 5. Color–filled three–dimensional visualization maps of reduced density gradient (RDG). The isosurface value of RDG is specified to 0.5, and the value of sign(λ2)ρ ranging from –0.03 to 0.02 au is denoted with filling color. Blue refers to the strong hydrogen interactions, and red refers to the strong steric repulsive effect.

38


Page 38 of 42

Page 39 of 42

60 100 80

50

(a)

(b)

40 60

P/kPa

P/kPa

40 20 0 0.0

30 20 10

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

0.4

x1

0.6

0.8

x1

25

20

(c)

15

P/kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

x1

Figure 6. Vapor pressures of the binary mixtures of benzene (1) + [EMIM][Tf2N] (2) (a), toluene (1) + [EMIM][Tf2N] (2) (b), and p–xylene (1) + [EMIM][Tf2N] (2) (c). Solid lines, predicted results by the UNIFAC–Lei model; dashed lines, vapor pressures of pure BTX; scattered points, experimental data. ■, 313.15 K; ●, 323.15 K; ▲, 333.15 K; ◆, 343.15 K; ★, 353.15 K; ▼, 363.15 K.

39


1.0


8000 60000

(a)

7000

ybenzene (ppm)

yH2O (ppm)

(b)

50000

6000 5000 4000 3000

40000 30000 20000

2000 10000

1000 0

0

0

5

10

15

20

V IL ( mL ⋅ min

-1

25

0

30

10

)

15

20

25

30

900

8000

(c)

800 700

6000

600

yp–xylene (ppm)

7000

5000 4000 3000

400 300 200

1000

100 0

0

5

10

15

20

25

30

(d)

500

2000

0

5

V IL ( mL ⋅ min -1 )

9000

ytoluene (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

0


5

10

15

20

25

30


Figure 7. Influence of the IL volume flow rate VIL (NTP) on the water yH 2O (a), benzene ybenzene (b), toluene ytoluene (c), and p–xylene yp–xylene (d) contents in gas product under different IL initial water contents. Scattered points, experimental data; solid lines, predicted results by the EQ stage model; ■, wH 2O, IL = 70 ppm; ●, wH 2O, IL = 550 ppm; ▲, wH 2O, IL = 1000 ppm.

40


Page 41 of 42

3500

(a)

3000

yH2O(ppm)

2500 2000 1500 1000 500 0

0

500

1000

1500

2000

2500

2000

2500

wH2 O, IL (ppm)

1400 1200

Benzene

1000

yBTX (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 800 600

Toluene

400

P–xylene

200 0

0

500

1000

1500

wH2 O, IL (ppm)

Figure 8. Influence of the IL initial water content wH2O, IL on the water yH2O (a) and BTX yBTX (b) contents in gas product with the IL volume flow rate VIL = 15 mL·min–1 (NTP). Scattered points, experimental data; solid lines, predicted results by the EQ stage model.

41



Page 42 of 42

Table of Content (TOC) Graphic

Product gas N2 Ionic liquid

VOCs + H2O + N2

Feed gas VOCs + H2O

IL + VOCs + H2O

42


Capturing Condensable Gases with Ionic Liquids - Industrial

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