Capturing volatile ester compounds from gas mixture with ionic liquids

Journal of Molecular Liquids 281 (2019) 517–527

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Capturing volatile ester compounds from gas mixture with ionic liquids Zhigang Lei, Hui Gao, Gangqiang Yu, Yifan Jiang ⁎ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 7 November 2018 Received in revised form 20 January 2019 Accepted 10 February 2019 Available online 27 February 2019 Keywords: Ionic liquids Volatile ester compounds UNIFAC-Lei model Process intensification

a b s t r a c t As a promising absorbent, ionic liquid (IL) was first used for capturing the volatile ester compounds from gas mixture. Considering the solubility of ester in IL and the selectivity of ester to feeding gas, the hydrophobic IL [BMIM] [Tf2N] was selected. In combination with the quantum chemistry calculation, the separation mechanism was revealed at the molecule level. The vapor-liquid equilibrium (VLE) data of the binary mixtures of ester and [BMIM] [Tf2N] were obtained and compared with the calculated results by UNIFAC-Lei model. It was found that the popular UNIFAC-Lei model can predict the VLE of ester and [BMIM][Tf2N] well. Moreover, the ester vapor capture experiment with [BMIM][Tf2N] as absorbent was conducted, demonstrating the good separation performance. Furthermore, the process simulation on continuous ester vapor capture with [BMIM][Tf2N] as absorbent was carried out using the rigorous equilibrium (EQ) stage model built with the UNIFAC-Lei model as property method. This work confirmed that capturing ester vapor with ILs is a typical process intensification technology. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Nowadays volatile organic compounds (VOCs) are playing an increasingly important role in environmental pollution and health hazard due to the high vapor pressures and special chemical and physical properties [1]. The latest findings suggest that VOCs can be generated by a variety of ways like automobile exhaust, industrial flue gas, incomplete combustion of domestic fuel as well as solvent evaporation in interior decoration materials. Among others, the industrial flue gas has a major contribution to the production of VOCs in the entire atmosphere [2]. Therefore, systematic investigation on capturing or recycling VOCs from industrial gas mixture is needed to improve the current environmental status and recycle raw materials [3]. At present, the existing methods for VOCs abatement include catalytic combustion method by excellent noble metal or transition metal catalysts [4,5], adsorption with porous materials like activated carbon, zeolites [6], and metal-organic frameworks (MOFs) [7], membrane separation [8], and absorption with liquid solvents [9]. For catalytic combustion method, although it is an effective and economically feasible route for the abatement of VOCs, a huge energy consumption and the generation of CO2 (as an important contribution to global warming) and NOx (as secondary pollutants to environment) as well as the problem of catalyst deactivation can be produced in this process [10]. Moreover, VOCs as a kind of chemical raw materials can't be recovered and recycled. For the method of adsorption with porous materials like zeolites, the high selectivity exhibits in the VOCs capture processes due to ⁎ Corresponding author. E-mail address: [email protected] (Y. Jiang).

https://doi.org/10.1016/j.molliq.2019.02.052 0167-7322/© 2019 Elsevier B.V. All rights reserved.

their large specific surface area and many holes [11,12]. However, they would be quickly saturated with a little water in feeding gas, resulting in the decrease of VOCs adsorption capacity [13]. In addition, MOFs have a large VOCs adsorption capacity because of their ultra large surface area, continuous storage behavior, the uniform crystal size and structure, and extra-high porosity [14]. Unfortunately, the presence of the moisture or the deposition of other compounds in VOCs can cover the adsorption sites at the surface of MOFs, leading to the adsorption capacity decreasing sharply [15]. Removal of VOCs from industrial gas mixture by membrane separation is desired, whereas the service life and mechanical strength of membrane usually limit its application in chemical industry [16,17]. The method of absorption with liquid solvents as the most feasible method has been used for capturing VOCs from various kinds of industrial gases, because of the very simple and efficient absorption process [9,18]. However, the selection of a proper absorbent is important and urgent to ensure an economical and effective separation process. So far, some high boiling organic solvents, such as di-(2-ethyl) hexyladipate (DEHA), diisobutyl phthalate [19], and polyethylene glycol (PEG) [20], as absorbents have been applied for capturing VOCs and water from industrial gases. Unfortunately, there are still many disadvantages for the use of these solvents, e.g., unavoidable volatile losses, huge energy consumption for absorbent regeneration, and solvent degradation, thus decreasing the green degree of absorption process. Thus, it is urgent to develop a more appropriate absorbent for capturing VOCs from industrial gases. Recently, ionic liquids (ILs) have been more and more popular for the adjustable structures, almost no vapor pressures, and good chemical and thermodynamic stability [21,22]. It is wellknown that ethyl acetate, ethyl propionate, and butyl acetate as three

518

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

representative ester members are important raw materials in the pharmaceutical and textile industries along with synthetic fibers, which have a high recycling value from gas mixture. Although a few researchers reported several methods and technologies on capturing aromatic hydrocarbons and sulfur compounds with some specific ILs with good absorption performance, there has been no publication on ester capture so far. So it is interesting to study the separation technology of capturing ethyl acetate, ethyl propionate, and butyl acetate simultaneously with ILs as absorbent to replace the conventional high boiling point volatile solvents [23,24]. The objectives of our research are resolving the following critical issues: (1) screening out the potential and suitable ILs for capturing ester vapor by COSMO-RS model and identifying the effect of different molecule structures of ILs on separation performance; (2) measuring the vapor-liquid equilibrium (VLE) data of ester-IL systems; (3) revealing the separation mechanism by combining COSMO-RS model with density functional theory (DFT) calculations; (4) putting the UNFAC-Lei model parameters into the Aspen Plus (version 8.4) and establishing the rigorous equilibrium (EQ) stage mathematical model. Herein, CO2 gas was regarded as the simulated gas mixture.

respectively. The details on experimental equipment are given in Supporting Information Fig. S1. The experiment for capturing ester vapor was carried out in an absorption column, which is 1 m high, 0.03 m diameter, and filled with θ-shaped random packings (4 × 4 × 0.6 mm). The flowsheet is illustrated in Fig. 1. The IL flowed into the absorption column via a constant-flux pump (Type 2 PB3020, Beijing Satellite Manufacturing Factory). Subsequently, the feeding CO2 with ester vapor flowed into the absorption column. The ester vapor content in CO2 was measured by a GC 4002A chromatograph (Beijing East-West Analytical Instruments Co., Ltd. China) with a HJ-10 capillary column (50 m × 0.53 mm × 7.0 μm). The ester vapor content in CO2 product gas was detected online by the GC 4002A chromatograph. After absorption, the ester-rich IL was collected and regenerated by the vacuum drying oven at 140 °C and 0.05 atm for several hours. Moreover, the use of recovered IL has no effect on the absorption experiments.

3. Theoretical model 3.1. COSMO-RS model

2. Experimental section 2.1. Materials Ethyl acetate, ethyl propionate, butyl acetate, CO2, and the IL [BMIM] [Tf2N] were purchased from chemical market and used in this work. The details on the purity and supplier of reagents are given in Supporting Information Table S1.

2.2. Equipment and experiment The vapor pressures of binary ester + IL systems over a wide temperature range were measured using a modified equilibrium still. The temperature and pressure precisions were 0.01 K and 0.01 kPa,

The COSMO-RS model is a prior model for predicting the thermodynamic properties because it doesn't depend on the experimental data [25,26]. Moreover, thermodynamic properties (i.e., activity coefficients, vapor pressure, and gas solubility) were obtained by screening charge densities σ on the surfaces of molecular cavities based on quantum chemical calculations [27]. The interaction between molecules can be deduced from the corresponding molecular surface screening charge density [28]. In this work, the COSMOthermX software (version C30_1301) was used for screening the ILs, theoretical analysis, and excess enthalpy calculation. The conventional compounds are taken from the built-in Database-TZVP, while the IL [BMIM][Tf2N] as a nonconventional compound isn't included in the database, and thus the molecular structure optimization needs to be conducted by TURBOMOLE package (version 6.4). Then, the optimized IL molecule was successfully imported into the COSMOthermX software.

7 8 4

5 1

2 7.00 set

6

ml/sec run

3

Fig. 1. The flowsheet of experimental equipment. (1) CO2 resource, (2) gas flow controller, (3) buffer tank, (4) absorption column (θ-shaped random packings), (5) pump, (6) IL storage, (7) gas chromatograph, (8) computer screen.

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

3.2. UNIFAC-Lei model

4. Results and discussion

The thermodynamic phase equilibrium behavior of the systems containing ILs can be well predicted by the popular UNIFAC-Lei model [29]. The anion and cation skeleton are regarded as a whole group and then the entire IL molecule is disassembled into different functional groups. Functional group decomposition method for conventional compounds is similar to that in the original UNIFAC model as proposed by Fredenslund et al. [30,31]. The activity coefficient was calculated by

4.1. Screening ILs

ln γi ¼

ln γCi

þ

ln γRi

ð1Þ

where lnγCi represents the combinatorial contribution to activity coefficient including two group parameters Qk and Rk for each group, resulting from different size and shape of the molecules; and lnγRi as a function of group interaction parameters anm and amn represents the residual contribution, attributed to energetic interactions between functional groups. The system investigated in this work contains five groups (i.e., CO2, [MIM][Tf2N], CH2, CH3, and CH3COO). Moreover, the group parameters (Rk and Qk) and binary group interaction parameters (anm and amn) are listed in Supporting Information Table S2 and Table 1, respectively. Notably, most of the binary group interaction parameters are available in previous publications [32,33], but several group interaction parameters are obtained by correlating the measured VLE data as well as the data coming from literature [34–37]. The following minimized average relative deviation (ARD) was used as objective function (OF): OF ¼

   N γ 1 X  i;cal −γi; exp     N 1  γ i; exp

ð2Þ

where γi,exp and γi,cal are the experimented and calculated activity coefficients of solute i in the mixture, respectively, and N is the number of data points. The details on all data points including the new data obtained from this work and those from previous literature are listed in Supporting Information Tables S3–S6. The detailed fitting procedure for obtaining the new binary group interaction parameters is given in our previous work [29]. The VLE of ester + IL systems at low or medium pressures can be written as yi P∅1 ðT; P; yi Þ ¼ xi γ i P Si

ð3Þ

where x1 and y1 are the mole fractions of ester in liquid and vapor phases, respectively; T and P are the system temperature and pressure, respectively; Psi represents the saturated vapor pressure of pure ester as calculated by Antoine equation [38,39]; γ1 represents the activity coefficient of ester in liquid phase, which can be calculated by the UNIFACLei model; and ∅1(T, P, y1) is the fugacity coefficient of ester in vapor phase at pressure P and temperature T. Notably, the vapor phase can be assumed as ideal gas (i.e., ∅1(T, P, y1)= 1) under the operating temperature and pressure, consisting of pure component (i.e., y1 = 1) due to the IL non-volatile property. Table 1 Group interaction parameters of the UNIFAC-Lei model used in this work. m

n

αmn

αnm

CO2 CO2 CO2 CH2(CH3) CH2(CH3) CH3COO

CH2(CH3) CH3COO [MIM][Tf2N] CH3COO [MIM][Tf2N] [MIM][Tf2N]

6339 165.7128a 14.56 231.1 400.89 7.466a

107.7 −286.6103a 81.47 114.8 145.8 39.8337a

a Group binary interaction parameters are obtained in this work. Other group binary interaction parameters are from reference.

519

It is necessary to select the appropriate IL as absorbent before the ester capture experiment. The solubility of solutes in IL and the selectivity of solutes to CO2 are two important indicators. The solubility of solutes can be represented by the Henry's constants Hi (i = solutes), whereas the selectivity is usually denoted with the ratio of Henry coefficients between two solutes in IL (Si=CO2 ¼ HHCOi , i = solutes). In this work, 2

we investigated 270 kinds of ILs containing 18 anions combined with 15 cations (see Supporting Information Table S7) to screen out the suitable IL. It is evident that ester solubility and the selectivity of ester to CO2 in ILs have the identical trend (see Fig. 2a–f). In addition, it was found that ester solubility and selectivity are influenced by both anions and cations. On the other hand, it seems that both anions and cations also affect the CO2 solubility in ILs (see Fig. 2g). It is worth noting that the solubility of CO2 is far less than that of ester in the same IL. As a result, the selectivity of ester to CO2 is large, which is the separation basis of capturing ester from gas mixture with ILs. It is clear that the solubility of both ester and CO2 increases with the increase of carbon chain length on cations. Furthermore, thermodynamic stability of IL should be considered as well. Thus, the common hydrophobic IL [BMIM][Tf2N] is suggested to capture the ester vapor, because in this case the selectivities of ethyl acetate, ethyl propionate, and butyl acetate to CO2 in [BMIM][Tf2N] are up to 162, 253, and 584, respectively. It is enough to meet the needs of capturing the three kinds of ester vapors from CO2 gas. 4.2. Analysis of the sigma profile The sigma profile (σ-profiles) as a surface composition function represents the probability distribution of screening charge density on molecular surfaces, reflecting the polar size on the molecular surface. Fig. 3 illustrates the σ-profiles curves of ester, CO2, [BMIM]+, and [Tf2N]−. The overall σ-profile zone is composed of three sections: Hbond acceptor zone (σ N 0.0082 e/Å2), non-polar zone (−0.0082 b σ b 0.0082 e/Å2), and H-bond donor zone (σ b −0.0082 e/Å2). All these σprofile peaks of CO2 molecule locate in the non-polar zone, indicating the strong non-polar characteristics of linear CO2 molecule. For the σprofile of ethyl acetate, there is a strong peak appearing in the nonpolar zone resulting from the alkyl chain on ester molecules. In addition, one peak appears in the H-bond acceptor zone resulting from the oxygen atoms on carbonyl, indicating that the ethyl acetate molecule has a certain affinity with H-bond donor. It should be noted that the peaks of ethyl propionate and butyl acetate locate in the identical positions with that of ethyl acetate in the non-polar zone, while the strengths of their peaks are different, following the order of ethyl acetate b ethyl propionate b butyl acetate, indicating that molecular nonpolarity increases with the increase of alkyl chain length. Moreover, the peaks of ethyl propionate and butyl acetate coincide with that of ethyl acetate in the Hbond acceptor zone. This indicates that ethyl acetate, ethyl propionate, and butyl acetate have the similar capacity as H-bond acceptors. For the σ-profile of [BMIM]+, although most of the zones locate in the non-polar zone, a peak appears in the H-bond donor zone, indicating that [BMIM]+ can be treated as an H-bond acceptor, thus forming the H-bond interaction with ester. The σ-profile of [Tf2N]− has two strong peaks. One is in the non-polar zone, indicating that the relatively strong interaction can be formed between [Tf2N]− and ester, while the other is in the H-acceptor zone, indicating the strong affinity with H-bond donor. 4.3. Excess enthalpies analysis In this work, the COSMO-RS model was used to calculate the excess enthalpies of ethyl acetate + [BMIM][Tf2N], ethyl propionate + [BMIM]

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

1.1-1.4 0.8-1.1 0.5-0.8 0.2-0.5

P666,14

2.4-2.8

P4,4,4,4 OMIM

2-2.4 1.6-2

OPY

1.2-1.6

HMPY MMIM N4,1,1,1 HMIM BMIM BMPY BMPYR BPY C3MIM

EPY

pFAP

bFAP

PF6

Tf2N

SbF6

TOS

CF3SO3

Cl

TFA

Ac

MeSO4

BF4

MeSO3

Br

NO3

D CA

TCB

EMIM

Ac B F4 MeSO 3 MeSO 4 NO3 TFA CF3SO3 PF6 SCN TOS SbF6 Br Cl Tf2N bFAP DCA pFAP TCB

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

SCN

520

(b)

(a) P4,4,4,4

2.9-3.2

P6,6,6,14 N4,1,1,1

2.6-2.9 2.3-2.6

EPY EMIM BPY

2-2.3 1.7-2

BMIM HMIM BMPY C3MIM BMPYR MMIM HMPY OMIM

-0.5-0 -1--0.5 -1.5--1 -2--1.5

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

-0.7--0.3

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

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

OPY

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

(d)

(c) EPY EMIM BMIM BPY

-0.5-0 -1--0.5 -1.5--1

C3MIM

-2--1.5

MMIM

-2.5--2

BMPY HMIM OPY OMIM HMPY BMPYR N4,1,1,1 P4,4,4,4

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

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

P666,14

(e)

(f)

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

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

(g)

2.4-2.7 2.1-2.4 1.8-2.1 1.5-1.8 1.2-1.5

-1.1--0.7 -1.5--1.1 -1.9--1.5 -2.3--1.9

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

32

24 20 p (σ)

4.5. Visualization of weak interaction types for the IL and ester systems

CO2 ethyl acetate butyl acetate ethyl propionate + [BMIM] [Tf2N]

28

16 12 8 4 0 -0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

σ (e/Å ) 2

Fig. 3. σ-Profiles of CO2, ethyl acetate, ethyl propionate, butyl acetate, [BMIM]+, and [Tf2N]−.

[Tf2N], butyl acetate + [BMIM][Tf2N], and CO2 + [BMIM][Tf2N]. The excess enthalpy (HE) was calculated by [40] H E ¼ HE ðMF Þ þ H E ðHBÞ þ H E ðvdW Þ

ð4Þ

As shown in Fig. 4a, the van der Waals interaction has an important impact on HE of CO2 + [BMIM][Tf2N]. However, for the ethyl acetate + [BMIM][Tf2N] system, the H-bond interaction is the main contribution to HE as shown in Fig. 4b. This also holds for the ethyl propionate + [BMIM][Tf2N] and butyl acetate + [BMIM][Tf2N] systems (see Fig. 4c– d). It is worth noting that the vdW interaction of butyl acetate + [BMIM][Tf2N] system is a little stronger than others, following the order of butyl acetate N ethyl propionate N ethyl acetate, which is attributed to the longer alkyl chain in butyl acetate molecule. This indicates that the H-bond interaction of ester and [BMIM]+ along with the vdW interaction of ester and [Tf2N]− jointly constructs the separation basis for capturing ester vapor from gas mixture at the molecule level. 4.4. Interaction energies analysis In this work, the lowest energy geometries of ester, CO2, and IL were obtained by Gaussian 09 software with density functional theory (DFT) [41] and Grimme's DFT-D3 dispersion correction function [42]. The most stable structures of the binding mode between molecules are shown in Fig. 5. The interaction energy can measure the strength of intermolecular interaction, and is written as   −1 ΔEA−B KJ∙mol ¼ ΔEA−B −ðΔEA þ ΔEB Þ

521

ð5Þ

where EA, EB, and EA-B represent the energy of single molecule A, the energy of single molecule B, and the overall energy of A + B system, respectively. The calculated results on interaction energies between molecules are given in Supporting Information Table S8. The interaction energy between CO2 and [BMIM]+ is almost identical with that between CO2 and [Tf2N]−. However, the interaction energies between ester and [BMIM]+ are slightly larger than those between ester and [Tf2N]−. Moreover, the interaction energy of ester and [BMIM][Tf2N] is much bigger than that of CO2 and [BMIM][Tf2N]. In addition, the stronger interaction of ester and [BMIM][Tf2N] results in the large selectivity of ester to CO2 in IL.

The reduced density gradient (RDG) analysis has been widely adopted for describing the weak interactions among molecules. In this work, Multiwfn program [43] combined with the VMD program [44] was used for the RDG analysis. The advantage of this analysis is that the areas of weak interactions in systems can be clearly displayed in the form of isosurfaces. The isosurfaces coloring maps of weak interaction for the IL and ester/CO2 systems are illustrated in Fig. 6. For the CO2 + [BMIM]+ system, Fig. 6a illustrates that the green isosurface region indicates the vdW interaction between neutral CO2 molecule and [BMIM]+, and the yellow region represents the formation of spatial steric effect. Besides, the strong steric repulsion in the imidazolium ring resulting from the ring tension is represented in the red isosurface. For the ethyl acetate and [BMIM]+ system (see Fig. 6b), there is a blue region, which lies between the most positively charged hydrogen atom in the imidazolium ring and the most negatively charged oxygen atom on carbonyl in ethyl acetate molecule, indicating the strong H-bond formation. Moreover, the vdW interaction and spatial steric effect represented by the green and red (or yellow) isosurfaces, respectively, are also observed. The similar phenomenon is also displayed for the ethyl propionate (or butyl acetate) and [BMIM]+ system as shown in Fig. 6c–d. The CO2 and [Tf2N]− molecules are combined together by the vdW interaction, but the steric repulsion is also observed in the [Tf2N]− represented by yellow isosurfaces (see Fig. 6e). For the ester molecule and the anion [Tf2N]− systems (see Fig. 6f–h), some green isosurfaces with large areas between ester molecule and the anion [Tf2N]− indicate that the vdW interaction plays the predominant role in these systems. It should be mentioned that the areas of green isosurfaces follow the order of ethyl acetate + [Tf2N]− b ethyl propionate + [Tf2N]− b butyl acetate + [Tf2N]−. Meanwhile, the strength of the vdW interactions is consistent with the area of green isosurfaces in these systems. This explains why the absorption ratio of ethyl acetate is smaller than those of other two esters which will be mentioned below. The RDG isosurfaces for the ester and whole IL molecule systems are shown in Supporting Information Fig. S2. 4.6. Vapor-liquid equilibrium of IL and ester The vapor pressure experiments of the [BMIM][Tf2N] and ester systems were carried out using a modified equilibrium still as mentioned above. The experiment results are compared with the prediction of UNIFAC-Lei model. Fig. 7 shows that the UNIFAC-Lei model prediction is generally consistent with the experiment data with the small ARDs of 4.69%, 7.09%, and 7.39% for the ethyl acetate + [BMIM][Tf2N], ethyl propionate + [BMIM][Tf2N], and butyl acetate + [EMIM][Tf2N] systems, respectively (detailed data are given in Supporting Information Tables S4–S6). Notably, the new group interaction parameters between CH3COO (or CH2COO) and [BMIM][Tf2N] were calculated from correlating the experimental activity coefficients of ethyl acetate + [BMIM] [Tf2N] mixture system. The parameter pair were successfully extended to predict the VLE of ethyl propionate + [BMIM][Tf2N] and butyl acetate + [BMIM][Tf2N] systems. This demonstrates that UNIFAC-Lei model can predict the VLE of a verity of ester and IL systems well. Fig. 7a illustrates that the vapor pressure for ethyl acetate + [BMIM] [Tf2N] system exhibits a linear increase with the increase of ester content in solution. For the systems of ethyl propionate + [BMIM][Tf2N] and butyl acetate + [EMIM][Tf2N], there also exhibits the linear relationship between vapor pressure and ester content when ethyl propionate and butyl acetate are at the middle or low concentrations in these binary systems (see Fig. 7b–c). However, at high ester concentrations the curve slope as a function of ester content reduces slowly. This is

Fig. 2. Common logarithms values of the selectivity of ethyl acetate (a), ethyl propionate (b), and butyl acetate (c) to CO2 along with the Henry's constants (bar) of ethyl acetate (d), ethyl propionate (e), and butyl acetate (f), and CO2 (g) in 270 ILs at 298.15 K.

522

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

200

500

H (HB)

(a) 0

HE(vdW)

0

HE(MF)

-500

HE (kJ·mol)-1

HE (kJ·mol)-1

(b)

E

-200

-1000

HE(MF)

-400

-1500

HE

-2000

HE(vdW)

HE(HB)

-600

HE

-800

-2500 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

(c)

HE(vdW)

100

HE (kJ·mol)-1

200

1.0

(d) HE(vdW)

100

0

HE(MF)

-100 -200

HE

-300 -400

0.2

0.4

0.6

0.8

0

HE(MF)

-100

HE

-200 -300

HE(HB)

HE(HB)

-400

-500 0.0

0.8

300

HE (kJ·mol)-1

200

0.6

x ethyl acetate

xCO2

1.0

x ethyl propionate

-500

0.0

0.2

0.4

0.6

0.8

1.0

x butyl acetate

Fig. 4. Excess enthalpies of CO2 + [BMIM][Tf2N] (a), ethyl acetate + [BMIM][Tf2N] (b), ethyl propionate + [BMIM][Tf2N] (c), and butyl acetate + [BMIM][Tf2N] (d) at T = 298.15 K.

due to the different interaction strengths between ester and IL. When ester is at the low concentrations, the oxygen atom on carbonyl in ester molecules can completely form the strong H-bond interaction with the hydrogen atom on the imidazole ring in [BMIM][Tf2N] molecule. However, ester molecules at the high concentrations can only partly form the H-bond interaction with IL. In this case, the vdW interaction between ethyl propionate (or butyl acetate) and [BMIM][Tf2N] is larger than that between ethyl acetate and [BMIM][Tf2N], with the result that vapor pressure for the ethyl propionate (or butyl acetate) and [BMIM][Tf2N] system increases more slowly than that for the ethyl acetate and [BMIM][Tf2N] system. 4.7. Experimental results of capturing ester vapor with IL For capturing ester vapor with the IL [BMIM][Tf2N], the operating temperature and pressure are 25 °C and ambient pressure, respectively. Moreover, CO2 gas with saturated ester content, in which the contents of ethyl acetate, ethyl propionate, and butyl acetate are 35,500, 18,500, and 4980 ppm, respectively, is treated as the simulated flue gas. Notably, the gas flow rate is stably 500 mL·min−1. The ester contents in the simulated flue gas were measured online by gas chromatography, and the ester-rich IL was regenerated by evaporation using the vacuum drying oven. The IL flow rate significantly affects the ester content and separation efficiency of the absorption column. In this work, a wide IL flow rate

ranging from 1 to 30 mL·min−1 was chosen to identify the relationship between the flow rate of IL and the ester content of outlet CO2. As shown in Fig. 8, the ester content of outlet CO2 gas firstly reduces rapidly with the increase of flow rate of IL, and then the degree of ester content reduction will be gradually weaken at the middle flow rate of IL. Finally, the ester content remains stable. It should be mentioned that the effect of flow rate of IL on the ester content of outlet CO2 is similar among ethyl acetate, ethyl propionate, or butyl acetate. When the IL flow rate reaches 15 mL·min−1, the contents of ethyl acetate, ethyl propionate, and butyl acetate (mole fraction) in the outlet CO2 gas decrease to about 1580, 379, and 184 ppm, respectively (i.e., the total ester content decreases to 2673 ppm in mole fraction). In this case, the absorption ratios of ethyl acetate, ethyl propionate, butyl acetate, and the total ester are up to 94.79, 96.33, 97.11, and 95.47%, respectively. In particular, the absorption ratio of butyl acetate is the highest among all the ester components, implying that the interaction between butyl acetate and IL is larger than that between ethyl acetate or ethyl propionate and IL as obtained by molecular microstructural analysis using the COSMO-RS model and quantum chemical calculations aforementioned. 4.8. Process simulation for capturing ester vapor with IL In this work, Aspen Plus software was used for the process simulation on capturing ester vapor with IL as absorbent from CO2 gas at laboratory scale. In the simulation, the rigorous equilibrium stage model was

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

built, the UNIFAC model was selected for the property method, and these UNIFAC-Lei model parameters associated with IL (see Supporting Information Table S2 and Table 1) were input. Notably, all of the property parameters of the IL [BMIM][Tf2N] are not within the Aspen database and must be manually entered, while the property parameters of other components (i.e., CO2, ethyl acetate, ethyl propionate, and butyl acetate) except for the UNIFAC-Lei model parameters associated with IL were derived from the built-in Aspen database. Fig. 8 shows that the simulation results by EQ stage model agree well with the experiment data with the small ARDs, thus validating the reliability and capacity of EQ stage model with UNIFAC-Lei model parameters for process simulation on capturing ester vapor with IL.

(a) CO2 + [BMIM]+

523

5. Conclusions In this work, ILs were suggested for capturing ester vapor from gas mixture for the first time. Considering the solubility of ester in IL and the selectivity of ester to feeding gas, the hydrophobic IL [BMIM][Tf2N] was selected as the appropriate absorbent. The COSMO-RS model in combination with the quantum chemistry calculation revealed the separation mechanism at the microscopic molecular level. It was found that the separation performance of capturing ester vapor with IL is dependent on the types of both cation [BMIM]+ and anion [Tf2N]− due to the H-bond interaction between [BMIM]+ and ester along with the vdW interaction between [Tf2N]− and ester. In addition, the vapor-

(b) CO2 + [Tf2N]-

2.0232 Å

(c) CO2 + [BMIM][Tf2N]

(e) Ethyl acetate + [Tf2N]-

(d) Ethyl acetate + [BMIM]+

(f) Ethyl acetate + [BMIM][Tf2N]

Fig. 5. The optimized molecule geometries with the lowest energies. Dashed lines represent H-bonds.

524

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

1.9879 Å

(h) Ethyl propionate + [Tf2N]-

(g) Ethyl propionate + [BMIM]+

2.0219 Å

(i) Ethyl propionate + [BMIM][Tf2N]

(j) Butyl acetate + [BMIM]+

(k) Butyl acetate + [Tf2N]-

(l) Butyl acetate + [BMIM][Tf2N]

C

H

O

F

N

S

Fig. 5 (continued).

Fig. 6. The density gradient (RDG) maps of ester and CO2 combined with the cation [BMIM]+ or the anion [Tf2N]−. Isovalue of RDG is set to 0.5, and the value of sign (λ2)ρ on the surfaces is represented by filling color ranging from −0.04 to 0.02 au. Blue means the strong attractive interactions, and red means the strong nonbonded overlap. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

(b) Ethyl acetate + [BMIM]+

(a) CO2+ [BMIM]+

(d) Butyl acetate + [BMIM]+

(c) Ethyl propionate + [BMIM]+

(f) Ethyl acetate + [Tf2N]-

(e) CO2 + [Tf2N]-

(g) Ethyl propionate + [Tf2N]-

(h) Butyl acetate + [Tf2N]-

0.02

-0.04

H-bond

vdW

Steric

525

526

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

80 80

(a)

70 (b) 60 50

P/kPa

P/kPa

60

40

40 30 20

20

10 0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

0.4

0.6

x1

0.8

1.0

x1

35

(c) 30 25

P/kPa

20 15 10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

x1 Fig. 7. Vapor pressure of ethyl acetate (1) + [BMIM][Tf2N] (2) (a); ethyl propionate (1) + [BMIM][Tf2N] (2) (b), and butyl acetate (1) + [BMIM][Tf2N] (2) (c). Solid lines, predicted results by the UNIFAC-Lei model; dashed lines, vapor pressures of pure solute; scattered points, experimental measurements. ■, 313.15 K; , 318.15 K; , 323.15 K; , 328.15 K; , 333.15 K; , 338.15 K; , 343.15 K; , 348.15 K; , 353.15 K; ●, 358.15 K; , 363.15 K.

0.020

0.040 0.035

0.015

0.030

y1

y1

0.025 0.020

0.010

0.015 0.005

0.010

0.000

(b)

(a)

0.005

0.000

0

5

10

15

20

25

30

0

5

10

15

20

25

30

V (ml·min-1)

V (ml·min-1)

IL

IL

0.06

0.005

0.05

0.004

0.04

y1

ytotal

0.003 0.002

0.03 0.02 0.01

0.001

(d)

(c) 0.000

0.00 0

5

10

15

20

V (ml·min-1) IL

25

30

0

5

10

15

20

25

30

V (ml·min-1) IL

Fig. 8. Effect of the IL volume flow rate VIL on the contents of ethyl acetate (a), ethyl propionate (b), butyl acetate (c), the total ester (d) in gas product. Solid lines, calculated results by the EQ stage model; Scattered points, experimental measurements.

Z. Lei et al. / Journal of Molecular Liquids 281 (2019) 517–527

liquid phase equilibrium (VLE) data of ester + [BMIM][Tf2N] were measured and compared with the UNIFAC-Lei model calculation. The results demonstrated that the UNIFAC-Lei model can predict the VLE of such systems well. Furthermore, the ester capture experiment with [BMIM] [Tf2N] as absorbent was conducted at laboratory scale, exhibiting the good separation performance. On this basis, the process simulation on continuous ester vapor capture with [BMIM][Tf2N] as absorbent was carried out. It was found that in comparison with traditional organic solvent process, the IL process demonstrates some better characteristics, e.g., the lower energy consumption, the smaller equipment investment, and the simpler process flow in gas dehydration and volatile organic compounds capture [45,46]. Thus, this work demonstrates that the ester vapor capture process with IL as absorbent is a typical process intensification technology. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. U1862103). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.02.052. References [1] R. Atkinson, J. Arey, Atmospheric degradation of volatile organic compounds, Chem. Rev. 103 (2003) 4605–4638. [2] Z. Shayegan, C. Lee, F. Haghighat, TiO2 photocatalyst for removal of volatile organic compounds in gas phase – a review, Chem. Eng. J. 334 (2018) 2408–2439. [3] R. Atkinson, J. Arey, Gas-phase tropospheric chemistry of biogenic volatile organic compounds: a review, Atmos. Environ. 37 (2003) 197–219. [4] J. Bedia, J.M. Rosas, J. Rodríguez-Mirasol, T. Cordero, Pd supported on mesoporous activated carbons with high oxidation resistance as catalysts for toluene oxidation, Appl. Catal. B Environ. 94 (2010) 8–18. [5] B. de Rivas, R. LópezFonseca, M.Á. GutiérrezOrtiz, J.I. GutiérrezOrtiz, Combustion of chlorinated VOCs using κ-CeZrO catalysts, Catal. Today 176 (2011) 470–473. [6] X.S. Zhao, A.Q. Ma, G.Q. Lu, VOC removal: comparison of MCM-41 with hydrophobic zeolites and activated carbon, Energy Fuel 12 (1998) 1051–1054. [7] J. Sherwood, B.M. De, A. Constantinou, L. Moity, C.R. Mcelroy, T.J. Farmer, T. Duncan, W. Raverty, A.J. Hunt, J.H. Clark, Dihydrolevoglucosenone (Cyrene) as a bio-based alternative for dipolar aprotic solvents, Chem. Commun. 50 (2014) 9650–9652. [8] J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, ChemInform abstract: MOF-5/n-Bu 4 NBr: an efficient catalyst system for the synthesis of cyclic carbonates from epoxides and CO2 under mild conditions, Green Chem. 11 (2009) 1031–1036. [9] F. Heymes, P.M. Demoustier, F. Charbit, J.L. Fanlo, P. Moulin, Treatment of gas containing hydrophobic VOCs by a hybrid absorption–pervaporation process: the case of toluene, Chem. Eng. Sci. 62 (2007) 2576–2589. [10] F.N. Aguero, B.P. Barbero, O. Sanz, F.J.E. Lozano, M. Montes, L.E. Cadús, Influence of the support on MnOx metallic monoliths for the combustion of volatile organic compounds, Ind. Eng. Chem. Res. 49 (2010) 1663–1668. [11] X. Peng, D. Cao, Computational screening of porous carbons, zeolites, and metal organic frameworks for desulfurization and decarburization of biogas, natural gas, and flue gas, AICHE J. 59 (2013) 2928–2942. [12] B. Cardoso, A.S. Mestre, A.P. Carvalho, J. Pires, Activated carbon derived from cork powder waste by KOH activation: preparation, characterization, and VOCs adsorption, Ind. Eng. Chem. Res. 47 (2008) 5841–5846. [13] Y.H. Chu, H.J. Kim, K.Y. Song, Y.G. Shul, K.T. Jung, K. Lee, M.H. Han, Preparation of mesoporous silica fiber matrix for VOC removal, Catal. Today 74 (2002) 249–256. [14] M. Du, Z.H. Zhang, L.F. Tang, X.G. Wang, X.J. Zhao, S.R. Batten, Molecular tectonics of metal-organic frameworks (MOFs): a rational design strategy for unusual mixedconnected network topologies, Chem. Eur. J. 13 (2010) 2578–2586. [15] F. Xu, G. Cheng, S. Song, Y. Wei, R. Chen, Insights into promoted adsorption capability of layered BiOCl nanostructures decorated with TiO2 nanoparticles, ACS Sustain. Chem. Eng. 4 (2016) 7013–7022. [16] H. Xu, Y. Li, M. Ding, W. Chen, K. Wang, C. Lu, Engineered photocatalytic material membrane assemblies for removing nitrate from water, ACS Sustain. Chem. Eng. 6 (2018) 7042–7051.

527

[17] G.B.V.D. Berg, C.A. Smolders, Diffusional phenomena in membrane separation processes, J. Membr. Sci. 73 (1992) 103–118. [18] H. Sui, T. Zhang, J. Cui, et al., Novel off-gas treatment technology to remove volatile organic compounds with high concentration[J], Ind. Eng. Chem. Res. 55 (9) (2016) 2594–2603. [19] F. Heymes, P.M. Demoustier, F. Charbit, J.L. Fanlo, P. Moulin, Recovery of toluene from high temperature boiling absorbents by pervaporation, J. Membr. Sci. 284 (2006) 145–154. [20] F. Cotte, J.L. Fanlo, P.L. Cloirec, P. Escobar, Absorption of odorous molecules in aqueous solutions of polyethylene glycol, Environ. Technol. Lett. 16 (1995) 127–136. [21] T. Jiang, X. Ma, Y. Zhou, S. Liang, J. Zhang, B. Han, ChemInform abstract: solvent-free synthesis of substituted ureas from CO2 and amines with a functional ionic liquid as the catalyst, Green Chem. 10 (2008) 465–469. [22] J.A. Lazzús, F. Cuturrufo, G. Pulgarvillarroel, I. Salfate, P. Vega, Estimating the temperature-dependent surface tension of ionic liquids using a neural networkbased group contribution method, Ind. Eng. Chem. Res. 56 (2017) 6869–6886. [23] K.K. Laali, V.J. Gettwert, Electrophilic nitration of aromatics in ionic liquid solvents, J. Org. Chem. 66 (2001) 35–40. [24] W. Li, Z. Zhang, B. Han, S. Hu, J. Song, Y. Xie, X. Zhou, Switching the basicity of ionic liquids by CO2, Green Chem. 10 (2008) 1142–1145. [25] A. Klamt, Conductor-like screening model for real solvents: a new approach to the quantitative calculation of solvation phenomena, J. Phys. Chem. 99 (1995) 2224–2235. [26] M. Buggert, L. Mokrushina, I. Smirnova, R. Schomäcker, W. Arlt, Prediction of equilibrium partitioning of nonpolar organic solutes in water- surfactant systems by UNIFAC and COSMO-RS models, Chem. Eng. Technol. 29 (2010) 567–573. [27] Z. Lei, C. Dai, J. Zhu, B. Chen, Extractive distillation with ionic liquids: a review, AICHE J. 60 (2014) 3312–3329. [28] M. Wlazło, E.I. Alevizou, E.C. Voutsas, U. Domańska, Prediction of ionic liquids phase equilibrium with the COSMO-RS model, Fluid Phase Equilib. 42 (2016) 16–31. [29] Z. Lei, J. Zhang, Q. Li, B. Chen, UNIFAC Model for Ionic Liquids, Ind. Eng. Chem. Res. 48 (2009) 2697–2704. [30] A. Fredenslund, R.L. Jones, J.M. Prausnitz, Group-contribution estimation of activity coefficients in nonideal liquid mixtures, AIChE J 21 (2010) 1086–1099. [31] A. Akbari, M.R. Rahimpour, Prediction of the solubility of carbon dioxide in imidazolium based ionic liquids using the modified scaled particle theory, J. Mol. Liq. 25 (2018) 135–147. [32] S. Skjoldjorgensen, B. Kolbe, J. Gmehling, P. Rasmussen, Vapor-liquid equilibria by UNIFAC group contribution. Revision and extension, Ind. Eng. Chem. Process. Des. Dev. 18 (1979) 2352–2355. [33] Z. Lei, C. Dai, X. Liu, X. Li, B. Chen, Extension of the UNIFAC model for ionic liquids, Ind. Eng. Chem. Res. 51 (2012) 12135–12144. [34] Y.L. Tian, H.G. Zhu, Y. Xue, A. Zhihua Liu, L. Yin, Vapor−liquid equilibria of the carbon dioxide + ethyl Propanoate and carbon dioxide + ethyl acetate Systems at Pressure from 2.96 MPa to 11.79 MPa and temperature from 313 K to 393 K, J. Chem. Eng. Data 49 (2004) 1554–1559. [35] S. Sima, V. Feroiu, G. Dan, New high pressure vapor–liquid equilibrium and density predictions for the carbon dioxide + ethanol system, J. Chem. Eng. Data 56 (2011) 5052–5059. [36] C.H. Cheng, Y.P. Chen, Vapor–liquid equilibria of carbon dioxide with isopropyl acetate, diethyl carbonate and ethyl butyrate at elevated pressures, Fluid Phase Equilib. 234 (2005) 77–83. [37] H. Li, R. Zhu, W. Xu, Y. Li, Y. Su, Y. Tian, Vapor−liquid equilibrium data of the carbon dioxide + ethyl butyrate and carbon dioxide + propylene carbonate systems at pressures from (1.00 to 13.00) MPa and temperatures from (313.0 to 373.0) K, J. Chem. Eng. Data 56 (2011) 1148–1157. [38] J. Polák, I. Mertl, Saturated vapor pressure of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, and ethyl propionate, Collect. Czechoslov. Chem. Commun. 30 (1965) 3526–3528. [39] V. Kliment, V. Fried, J. Pick, Systeme butylacetat-phenol und wasser-phenol, Collect. Czechoslov. Chem. Commun. 29 (1964) 2008–2015. [40] G. Yu, C. Dai, L. Wu, Z. Lei, Natural gas dehydration with ionic liquids, Energy Fuel 31 (2017) 1429–1439. [41] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B Condens. Matter 37 (1988) 785–789. [42] S. Grimme, S. Ehrlich, L. Goerigk, Effect of the damping function in dispersion corrected density functional theory, J. Comput. Chem. 32 (2011) 1456–1465. [43] T. Lu, F. Chen, Multiwfn: a multifunctional wavefunction analyzer, J. Comput. Chem. 33 (2012) 580–592. [44] W. Humphrey, A. Dalke, K. Schulten, VMD: visual molecular dynamics, J. Mol. Graph. 14 (1996) 33–38. [45] J. Han, C. Dai, Z. Lei, B. Chen, Gas drying with ionic liquids, AICHE J. 64 (2018) 606–619. [46] G. Yu, C. Dai, H. Gao, R. Zhu, X. Du, Z. Lei, Capturing condensable gases with ionic liquids, Ind. Eng. Chem. Res. 57 (2018) 12202–12214.