Extractive Distillation with Ionic Liquid Entrainers for the Separation of

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Extractive Distillation with Ionic Liquid Entrainers for the Separation of Acetonitrile and Water Jinlong Li, Tingting Li, Changjun Peng, and Honglai Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05907 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Industrial & Engineering Chemistry Research

Extractive Distillation with Ionic Liquid Entrainers for the Separation of Acetonitrile and Water Jinlong Li,a* Tingting Li,b Changjun Peng,b* Honglai Liub a

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology and School of

Petrochemical Engineering, Changzhou University, Changzhou 213164, China b

State Key Laboratory of Chemical Engineering and Department of Chemistry, East China

University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

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Industrial & Engineering Chemistry Research 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

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ABSTRACT Ionic

liquids

(ILs)

of

1-ethyl-3-methylimidazolium

acetate

([EMIM][OAC]),

1-ethyl-3-methylimidazolium proline ([EMIM][PRO]) and N,N,N-dimethyl-butylethanol amine glycolic acid ([N1,1,4C2OH][GAC]) are selected as entrainers for the extractive distillation to separate the mixture of acetonitrile and water based on COSMO-RS method. The isobaric vapor-liquid equilibrium (VLE) of ternary mixtures containing ILs at atmospheric pressure are determined, verifying that the used ILs can break the azeotropy of acetonitrile and water. Based on the thermodynamic analysis, a typical extractive distillation process is designed with Aspen Plus to evaluate the competitiveness of IL entrainers in separation performance. Significant savings of entrainer and energy, that the solvent ratios in mole reduce from 0.94 to 0.09, 0.12 and 0.12 and the overall heat duties on reboiler decrease 24.9%, 17.5% and 15.4% for [EMIM][OAC], [EMIM][PRO] and [N1,1,4C2OH][GAC] respectively, can be obtained as compared to that of ethylene glycol used as entrainer.


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1. INTRODUCTION As a kind of colorless, volatile liquid with certain toxicity and favorable chemical stability, acetonitrile is an excellent organic solvent. It has good solubility to both polar and non-polar substances and can dissolve a variety of inorganic and organic compounds and gases. Due to its high polarity, low viscosity and relatively low toxicity, acetonitrile boasts wide applications in pharmaceutical industry and high-performance liquid chromatography.1-4 Industrially, acetonitrile can mainly be produced through the one-step synthesis of acetic acid and ammonia, synthesis of ethanol and ammonia, propylene ammonia oxidation by-product method and so on. In these various industrial methods for acetonitrile production and recovery, the separation of acetonitrile and water is often encountered. However, the mixture of acetonitrile and water is a typically azeotropic system with a minimum boiling temperature at a certain condition5, for which it is impossible to separate the acetonitrile and water mixture through the ordinary distillation method. Ionic liquids (ILs), as a kind of new “green” solvent, is characterized by high boiling point, low vapor pressure, wide liquid range, favorable solubility, variably designed structures and other merits6 and has drawn much attention for being a potential extractant for separation process 7-14 in the past decades. Some special works using IL as entrainers have been contributed to the separation of different azeotropic mixtures15-24. It could effectively avoid the shortcomings of large amount of solvent consumption, corrosion and crystallization of aqueous inorganic salts solution or expensive equipment investments severally introduced by conventional solvents like ethylene glycol25-27, butyl acetate28 and n-propyl chloride27, extractive distillation with salts29 or pressure swing distillation30, and furthermore prevent the harms from certain organic solvent like DMSO easily penetrating into the skin and smelling awfully31, if ILs could be used as the additive for extraction distillation. Some ILs have been employed to break and improve the acetonitrile and water separation so ACS Paragon Plus Environment


far,

and

the

imidazole

1-butyl-3-methylimidazolium

based

ionic

dibutyl

ester

liquids

dominates.

([BMIM][DBP]),

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For

instance,

ILs

of

1-butyl-3-methylimidazolium

tetrafluoroborate ([BMIM][BF4]), 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) and 1-hexyl-3-methylimidazolium chloride ([HMIM][Cl]) were verified to be effective in improving the relative volatility of

acetonitrile in

its

mixture,

whereas

1-ethyl-3-methylimidazolium

tetrafluoroborate ([EMIM][BF4]) was noneffective in this regard.32-34 With the help of COSMO-RS method35-38 embedded in COSMOthermX software package39 and the experimental measurement, the authors analyzed the effects of 182 kinds of ILs formed by 14 cations and 13 anions on vapor-liquid equilibrium (VLE) of acetonitrile-water azeotropic system, resulting in that IL with acetate anion ([OAC]-) stands out in terms of the separation effects and the shorter is the carbon chain in the cation, the better the separation performance. The experimental VLE data verified that 1-ethyl-3-methylimidazolium acetate ([EMIM][OAC]) can break the azeotropy of acetonitrile and water mixture indeed19. However, the large-scale applications of the imidazolium based ILs are partially limited due to its relatively high toxicity and poor biodegradability.40,

41

Comparatively, the amino acid and

quaternary amine choline based ILs might have a better potential application in industry since their raw materials are easy to obtain and relatively low toxic and they could be easy to be tailored and synthesized according to specific requirement. Furthermore, higher biocompatibility and biodegradability of cholinium-based ILs42, 43 were also observed. In our previous work19, the effects of ILs containing amino acid anions on the phase behavior of acetonitrile-water mixture were systematically analyzed in virtue of COSMO-RS model, proving that 1-ethyl-3-methylimidazolium proline ([EMIM][PRO]) can effectively eliminate azeotropic point. But the imidazolium cation was still used. Kurzin et al.44-46 studied the effects of quaternary ammonium type ILs of


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tetrabutylammonium bromide ([N4444][Br]) and tetrapropylammonium bromide ([N3333][Br]) on the VLE of acetonitrile and water system. The isothermal phase equilibrium measured showed that [N4444][Br] works well at high concentration area of acetonitrile and can effectively eliminate the azeotropy, while [N3333][Br] can’t break the azeotropy and only makes it move towards high concentration area of acetonitrile in its mixture. The quaternary amine choline IL, that is synthesized based on choline and contains a hydroxyl group in cation, is a kind of biocompatible IL and features good thermal stability and solubility47-49. Just like the amino acid based ILs, it is also an environment friendly solvent.50, 51 Due to its hydroxyl group, it could make the disappearance of the UCST-type phase separation in choline IL and propanol system, while which exists in ordinary quaternary ammonium ILs mixtures52, indicating that the choline IL might combine with polar components well. In most of researches, the cholinium-based ILs (especially choline chloride) were used to mix with other components and obtain another IL analogues called as deep eutectic solvents53, 54 (DES), in which the different molecules associate with each other through hydrogen bonding interactions to form a eutectic mixture with a melting temperature lower than either of its individual substance.55 For instance, the DESs containing the choline substance were used to replace the conventional organic or IL solvents to separate different type mixtures54, 56, 57, of course including azeotropic mixture22, 58. Gjineci et al.22 showed that the DESs of choline chloride/urea (1:2) and choline chloride/triethylene glycol (1:3) could well improve the relative volatility and consequently eliminate the azeotropy of ethanol and water mixture at certain concentration of DESs. However, sparse research work is contributed to pure choline ILs and its applications in separation. In this work, we will try to employ the pure choline IL to separate the azeotropic mixture of acetonitrile and water. With the help of COSMO-RS method that has been extensively used for IL screening,59-62 the effects of pure choline IL on the phase behavior of the investigated system are



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firstly analyzed. The isobaric VLE data at atmospheric pressure are then measured through experiment to verify the separation performance of the sifted IL. Finally, the conceptual process design of extractive distillation with the IL entrainer is established with Aspen Plus package, into which the binary interaction parameters for the Non-Random Two Liquid Equation63 (NRTL) obtained from experimental VLE data are incorporated, to evaluate the possibility of heat duty and solvents saved in comparison with the ordinary organic solvent of ethylene glycol (EG). It is noted that three different type IL entrainers of [EMIM][OAC], [EMIM] [PRO] and choline based IL are used in process simulation, and the former of both ILs were reported as effective additives in the separation of acetonitrile and water mixture in other work.19 2. DETAILS OF COMPUTATION AND EXPERIMENT 2.1. COSMO-RS Calculations In COSMO-RS method39, only COSMO files that just depend on the molecular structure are required to complete the predictions for thermodynamic properties. The molecular structure can be optimized with different quantum chemical software packages like Gaussian, Materials Studio and Turbomole. In this work, the ionic structures of IL and ones of acetonitrile and water are optimized through Dmol3 module embedded in Materials Studio 7.0 with the density functional theory (DFT) at GGA/VWN-BP level. The atomic orbital basis set of DNP+ is used in energy calculation. The relative volatilities of acetonitrile to water are then predicted with COSMOthermX software39 at BP_TZVP_C21_0110 Version. 2.2. Screening of Ionic Liquids The separation ability is usually expressed by the relative volatility of one component to others in a mixture at equilibrium. Supposed that the vapor phase is an ideal system and the effect of pressure on the liquid volume is neglected at relatively low pressure, the VLE can be simply ACS Paragon Plus Environment

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calculated by:

pyi  piS xi i

(1)

where, p represents the total pressure of a system; piS is the saturated vapor pressure for component i; γi and xi represent the activity coefficient and molar fraction of component i in the liquid phase respectively; yi represents molar fraction of component i in the vapor phase. The saturated vapor pressure and activity coefficient can be provided through Antoine equation and COSMO-RS model respectively, so that the equilibrium vapor and liquid phase compositions are then predicted at certain temperature and pressure. In prediction, IL is considered as one composed of anions and cations with equimolar ratio. Due to its negligible vapor pressure, the IL in vapor phase can be neglected and all of them stay in liquid phase. To compare with the results of free-IL system, the relative volatility of acetonitrile to water in the ternary system of acetonitrile (1) + water (2) + ionic liquid (3) is defined as

12 =

y1 / y2 x'1 / x'2

(2)

where, x1’ and x2’ severally mean pseudo liquid phase composition normalized the concentration of acetonitrile and water after IL being deducted; y1 and y2 represent the real vapor phase composition of acetonitrile and water at phase equilibrium, respectively. Based on the relative volatility, the effects of ILs on acetonitrile and water separation can be determined. 2.3. Experimental materials and measurement Experimental materials in this work are Acetonitrile, deionized water, Karl Fischer reagent and the IL of [N1,1,4C2OH][GAC]. Acetonitrile, analytical grade ≥ 99.9% in mass fraction (same hereinafter), was purchased from J&K Scientific Ltd., Beijing, China. The deionized water was made in laboratory through the Hitech-Kflow reverse osmosis membrane separation method. The analytical Karl Fischer reagent was provided by Merck KGaA, Darmstadt, Germany. The IL of ACS Paragon Plus Environment


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[N114C2OH][GAC] with purity ≥ 99.0% was synthesized in laboratory as shown in Supporting Information (SI). The used materials of N,N-dimethylethanolamine, analytical grade ≥99.5%; butyl bromide, analytical grade ≥99.5%; silver nitrate, analytical grade ≥99.0% and sodium glycolate (Aladdin) were from Shanghai Civic Chemical Technology Co., Ltd., China. Before experiment, the purities of organic reagent were determined via gas chromatography (GC, TECHCOMP-7900) without impurity peak, and the IL was purified through vacuum evaporation (pressure, 0~5 kPa) for at least 24h at 343.15K to avoid water absorption and organic reagent residual. The water content in all used samples was measured via Karl Fischer method (METTLER TOLEDO C20) and was less than 500 ppm. The isobaric VLE data at atmospheric pressure is measured in a glass equilibrium still with a vapor cycle made by our laboratory. The reliability of the VLE apparatus and experimental procedure are verified by comparing our experimental and literature data of VLE for the mixture of acetonitrile + water 64 and one of methanol + water65. As shown in our previous work19, about 200 mL of the mixture samples are firstly prepared with an analytical balance (METTLER TOLEDO XP8002S) with an uncertainty of 0.1mg in each measurement. After putting the mixture sample into the equilibrium still, the heater at 150V heating voltage is then turn on and began to heat the sample. About thirty minutes are required to reach the VLE after boiling, when the temperature is kept approximately unchangeable. Liquid and vapor samples are then taken from the liquid phase staying in equilibrium chamber and the vapor phase condensing in a contact effusion slot in the vapor loop. At last, the analysis results of the samples and the equilibrium temperature, pressure are recorded to get a full VLE data. In experiment, the recovery of IL is made to save the cost. All samples are analyzed by the gas chromatograph (GC) combined with a Karl Fischer reagent. The GC used is equipped with a thermal conductivity detector (TCD, electric current 95mA) and a packed column


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(Techcomp-OV-101, 0.6m (l) × 3mm (o.d.) × 2mm (i.d.)). Hydrogen is used as carrier gas and the flow rate is 30mL/min. The temperatures of TCD, over and injection are controlled at 453.15, 423.15 and 453.15K, respectively. The equilibrium and ambient temperatures are determined by a thermometer with a standard uncertainty of 0.1K and the ambient pressure was recorded by a barometer with an uncertainty of 0.01kPa. 2.4. Process simulation The process simulation is performed and calculated by Aspen Plus software (version 10). The extractive distillation column (EDC) and the solvent recovery column (SRC) are modeled with RadFrac module. In the process simulation, the acetonitrile, water and EG are selected as conventional compounds from databank of Aspen Plus package, while ILs are defined as new conventional compounds, where the required thermodynamic properties like critical properties, density, boiling temperature and acentric factor for ILs are determined by group contribution method66-70. For the binary interaction parameters, the Non-Random Two Liquid Equation63 (NRTL) is used to fitting the experimental VLE data. The interaction parameters of water-IL and acetonitrile-IL are regressed from the corresponding ternary VLE data at fixed water-acetonitrile ones. 3. RESULTS AND DISCUSSION 3.1. Screening of IL Entrainers The detailed analysis and discussion for the effects of [EMIM][OAC] and [EMIM][PRO] on the phase behavior of acetonitrile + water mixture were made in a previous work.19 Both theoretical and experimental results have shown that they can effectively eliminate the azeotropy of acetonitrile and water. In this section, the focus is mainly contributed to the effects of choline based ILs on the phase equilibrium of the investigated mixture. ACS Paragon Plus Environment


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The cholinium-based ILs are usually obtained after that the cations of ordinary quaternary amine ILs are hydroxyl-functionalized. To compare the effect of different functional groups contained in quaternary ammonium cations, the quaternary amine cation of [N1,1,1,2]+ is firstly modified by the functional groups of chlorine (-Cl), cyano (-CN), sulfhydryl (-SH), hydroxy (-OH) and amine (-NH2) and then combined with the anion of [OAC]- to form new ILs. The structures of the related cations are summarized in Table S1 of SI. The effects of different functional group on the relative volatility of acetonitrile to water are illustrated in Figure 1. One can see that the amine-modified IL can effectively enhance the relative volatility of acetonitrile over the full concentration range, while the hydroxyl-modified one can significantly improve the relative volatility of acetonitrile at the relative low concentration zone of acetonitrile (x0.7). Other functional groups modified ILs seem to be no obvious impact on the relative volatility. In addition, comparing the effects introduced by amine- and hydroxyl-group modified ILs, it can be found that the relative volatilities for both cases are great than 1, and the later has stronger effect than the former at the low concentration area of acetonitrile, while opposite at high zone. On the other hand, the amine functionalized ILs might be relatively difficult to synthesize and the purity of the product obtained would also be low compared to other type ILs. Therefore, the quaternary amine cation hydroxyl-functionalized (cholinium-based) ILs might comparatively be more suitable for an industrial application, so that more research work here is contributed to choline ILs.


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Figure 1 Relative volatilities for the acetonitrile (1) + water (2) + IL (3) systems at 101.3 kPa calculated by COSMO-RS: x3=0.2

As known, the fluid properties would be affected by the molecular micro-structure like the length of carbon chain and functional group position. For example, the longer is the carbon chain length in imizolium cation, the weaker the enhancement of separation effect.19, 71 So, the effects of the carbon chain length and the position of hydroxyl group in quaternary amine cation on the phase equilibrium of acetonitrile and water mixture are made and illustrated in Figure 2, showing that the increasing carbon chain length in cation is not positive to the enhancement of separation performance. Moreover, the separation ability of [N1,1,1C2OH][OAC] is greater than one of [N1,1,2COH][OAC], indicating that the cation with a hydroxyl standing on a longer carbon chain would be more effective in improving separation for those choline cations containing same carbon branch chain. However, for the isomer containing the same branch chain, like [N1,1,1C3OH]+[OAC] and [N1,1,1C3OH][OAC], there is no obvious difference between them in general. In addition, the cholinium-based IL generally contains four carbon chains on ammonia ion. In order to show the effects caused by the number of carbon chains contained in the cation, [N1,1,1,2][OAC] was still used ACS Paragon Plus Environment


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as a reference and some three-carbon chains choline IL are designed. As shown in Figure 3, one can see the number of carbon chains on ammonia ion have little effects on separation (see the results of [N1,1,4C2OH][OAC] and [N1,4C2OH][OAC]). Furthermore, [N1,2C2OH][OAC] > [N1,3C2OH][OAC] > [N1,4C2OH][OAC] in terms of separation performance means that the effects of carbon chain length on separation are almost same regardless of three or four carbon chains connecting to quaternary ammonia ion.


To show the enhancement of choline IL to separation performance, two quaternary amine cations are selected to form new ILs with the anion of [OAC]- and their effects on the relative volatility compared to [EMIM][OAC], which was proved to be effective in breaking the azeotrope of acetonitrile and water, are illustrated in Figure 4. At the low concentration area of acetonitrile, the difference of promotion among three ILs is relatively small and [N1,1,4C2OH][OAC]


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performance best. The sequence is [N1,4C2OH][OAC] < [N1,1,4C2OH][OAC] < [EMIM][OAC] at high concentration rang. However, the promoting effect on separation is not only affected by cations, but by anions. It was found in our study61 on imidazole IL that the IL whose anions tailored by a single hydroxyl group like 1-ethyl-3-methylimidazolium glycolic acid ([EMIM][GAC]) can remarkably increase the relative volatility of acetonitrile. In other words, [EMIM][GAC] catches better separation effects of acetonitrile and water than [EMIM][OAC]. Therefore, the anion of [GAC]- is employed here to combine with hydroxylated quaternary ammonium cation to form a new choline IL and the results are given in Figure 5. One can see that the cholinium-based ILs can effectively improve the relative volatility of acetonitrile after the anion is modified by a single hydroxyl, and [N1,1,4C2OH][GAC] performances the best effects over full concentration range of acetonitrile. Thus, the IL of [N1,1,4C2OH][GAC] is finally selected as a new entrainer and synthesized to obtain the experimental VLE data.




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Figure 4 Comparison of relative volatilities for the acetonitrile (1) + water (2) + IL (3) systems at 101.3 kPa calculated by COSMO-RS: x3=0.2

Figure 5 Comparison of relative volatilities for the acetonitrile (1) + water (2) + IL (3) systems at 101.3 kPa calculated by COSMO-RS: x3=0.2

To well understand the enhancement of separation performance induced by the related ILs, we have tried to provide some theoretical insight at molecular level based on DFT calculations. The


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geometric structures are optimized with Dmol3 module embedded in Materials Studio 7.0. The optimized binding energy between the acetonitrile (or water) and the IL entrainer are listed in Table 1 and the corresponding optimized geometric structures and the total energy for different molecular clusters are given in Figure S3 and Table S2 of SI. The selected ILs can combine with water and acetonitrile through hydrogen bonds and the stronger hydrogen bond are found between IL and water than one between IL and acetonitrile. What’s more, the binding energy between acetonitrile and water is -10.16 kJ.mol-1, while ones between the entrainer of [EMIM][OAC], [EMIM][PRO] or [N1,1,4C2OH][GAC] and water are -25.96, -35.65 and -23.80 kJ.mol-1, showing that the interactions between IL and water were stronger than ones between acetonitrile and water, so that more acetonitrile molecules in liquid mixture tend to be free leading to higher relative volatility of acetonitrile to water. On the other hand, the local polarity of the molecular surface and the hydrogen bonding between different molecules or ions can be characterized by sigma profile in COSMO method, as shown in Figure 6, in which the two vertical dash lines represent the cutoff radius for the hydrogen bond donor (σ < -0.082 e.Å-2) and acceptor (σ > 0.082 e.Å-2). One can see that three IL anions have peaks in the region of σ > 0.082 e.Å-1 and far away from 0.082 e.Å-2, while water has peaks in the region of σ < -0.082 e.Å-2. Furthermore, no obvious hydrogen bond donators exist in acetonitrile molecules and IL cations, so that the IL entrainer and water can form stable and strong hydrogen bonds.72 Therefore, the selected IL entrainers can effectively enhance the relative volatility and further break the azeotropy of acetonitrile and water.

Table 1 Interaction energy between acetonitrile (or water) and IL entrainer component

acetonitrile/ kJ·mol-1

water/ kJ·mol-1

[EMIM][OAC] [EMIM][PRO] [N1,1,4C2OH][GAC]

-4.41 -13.79 -3.72

-25.96 -35.65 -23.80



water

30 25 20

p(σ)

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

-10.16

Page 16 of 30

-

water Acetonitrile [EMIM]+ [N114C2OH]+ [OAC][Pro][GAC]-

15 10 5 0 -0.03

-0.02

-0.01

0 σ, e/Å2

0.01

0.02

0.03

Figure 6 Comparisons of the σ profiles for acetonitrile, water, cations and anions

3.2. Experimental VLE Results The isobaric vapor-liquid equilibrium data of acetonitrile (1) + water (2) + [N1,1,4C2OH][GAC] (3) ternary system with IL mole fractions of 0.05 and 0.10 are measured at atmospheric pressure in this work. All experimental data including acetonitrile (1) + water (2) + [EMIM][OAC] (3) and + [EMIM][PRO] (3) systems, which were given in a previous work61, are summarized in Table S3-S6 of SI and in Figure 7. In Figure S7 of SI, the predicted results from COSMO-RS were also included. It can be seen that three different ILs could effectively enhance the relative volatility of acetonitrile to water and break the azeotropy only when 0.05 mole fraction of ILs in total composition are added, despite that the predicted separation performance from COSMO-RS is somewhat inconsistent with the experimental observation for [EMIM][OAC] and [N1,1,4C2OH][GAC]. What’s more, the enhancement of the separation ability becomes stronger with the increasing concentration of IL in mixtures. On the other hand, comparing the effects introduced by the three different ILs from the


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experimental VLE, the separation ability improved by [N1,1,4C2OH][GAC] was relatively weaker than ones of [EMIM][OAC] and [EMIM][PRO]. In addition, the VLE results for acetonitrile (1) + water (2) + ethylene glycol (EG) (3) produced from Aspen Plus based on its built-in NRTL parameters are also given in Figure 7 and the separation ability of EG is obviously weaker than ones of the investigated ILs, where the EG entrainer was selected as a benchmark solvent to compare ILs. To describe the ternary VLE through theoretical method and obtain the parameters for process simulation, the widely used NRTL equation63, that is summarized in Supporting Information, is employed to represent the experimental VLE. In NRTL, there are six energy parameters (∆gij, ∆gji) and three regulated parameters (supposed αij=αji) required for each ternary mixture. In order to reduce the number of fitting parameters, the binary interaction parameters of acetonitrile-water are firstly determined through fitting the experimental VLE data of binary acetonitrile and water system. Other four binary parameters are then regressed according to the experimental VLE of ternary system. In addition, all unknown regulated parameters except for ones of binary acetonitrile and water mixture are fixed as 0.3. Therefore, only there are four binary interaction parameters needed to obtain for each ternary system and the optimization objective function is: F

5 N

N

( i 1

T cal  T exp 2 1 )  T exp N

N

(y

exp

 y cal ) 2

(3)

i 1

where the superscripts of ‘cal’ and ‘exp’ severally represent the theoretical and experimental results. N is the number of experimental data. The number of ‘5’ in the above equation is used to balance the effect of temperature on fitting results. The required Antoine equation parameters of acetonitrile and water are given in Table S7 of SI. The obtained binary interaction parameters for NRTL model are listed in Table 2. The overall absolute relative average deviations of equilibrium temperature for ternary mixtures containing [EMIM][OAC], [EMIM][PRO] and [N1,1,4C2OH][GAC] are 1.12%, ACS Paragon Plus Environment


2.50% and 3.42% respectively. The comparisons between theoretical and experimental results are plotted in Figures S4-S6 and good agreements are observed. The binary interaction parameters listed in Table 2 are finally used in process simulation.

1.0

0.8 0.6

y1

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

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Entrainer Free [EMIM][OAC] 0.05 [EMIM][OAC] 0.10 [EMIM][PRO] 0.05 [EMIM][PRO] 0.10 [N114C2OH][GAC] 0.05 [N114C2OH][GAC] 0.10 EG 0.10 EG 0.50

0.4 0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

x'1 Figure 7 Isobaric VLE diagram for the acetonitrile (1) + water (2) + IL (3) systems at 101.3 kPa

Table 2 Binary interaction parameters for NRTL Equation component i

component j

(gij-gjj)/(J.mol-1)

(gji-gii)/(J.mol-1)

αij

acetonitrile

water

2214.21

5202.99

0.3545

acetonitrile

[EMIM][OAC]

11936.91

-4743.74

0.3

water

[EMIM][OAC]

24633.47

-13615.84

0.3

acetonitrile

[EMIM][PRO]

-928.85

-6026.64

0.3

water

[EMIM][PRO]

-4487.25

-12827.89

0.3

acetonitrile

[N1,1,4C2OH][GAC]

7055.37

-2985.88

0.3

water

[N1,1,4C2OH][GAC]

1557.40

-10097.50

0.3

3.3. Conceptual process design and simulation It is necessary that the basic thermo-physical properties of ILs should be firstly determined


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before process simulation due to be absent in Aspen Plus database. So the group distribution method66-70 is generally used to obtain the critical properties, boiling temperature, liquid density and so on in this work, as listed in Table S8 of SI. On the other hand, the isobaric heat capacities are necessary to calculate the energy consumption. Therefore, the isobaric heat capacities of all ILs in this work are also estimated through group contribution method embedded in Aspen Plus, as illustrated in Figure S8, in which the results predicted from another empirical method73 and experimental data74 of [EMIM][OAC] are also contained. One can see that the heat capacity for [EMIM][OAC] from different sources are basically consistent, while certain deviations for [EMIM][PRO] and [N1,1,4C2OH][GAC] predicted from different methods are observed.

Figure 8 Residual curves based on different entrainers

Based on the basic parameters obtained from experiment, the residual curves for the



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investigated ternary systems with entrainers of [EMIM][OAC], [EMIM][PRO], [N1,1,4C2OH][GAC] and EG are produced with Aspen Plus, as shown in Figure 8. One can see from the residual curves that the azeotropic point of acetonitrile and water mixture is an unstable point, pure acetonitrile and water are saddle points and the corresponding points of different solvents are stable points. It can also be seen from Figure 8 that no distillation boundary among all residual curves are observed, suggesting that the feasibility to separate the acetonitrile and water mixture in the presence of different solvents. Lots of typical examples of the extractive distillation process with IL entrainer for separating different mixtures could be found elsewhere75-78 so far. For a typical two-component mixtures, the extractive distillation process can be simply designed as a double-column process, which are basically consisted of one extractive distillation column (EDC) and one solvent recovery column (SRC), additionally coupled with reboilers, condensers, intermediate heat exchangers and pumps, as shown in Figure 9. The mixture liquids of acetonitrile and water enter into EDC from the middle position and the entrainers are introduced from the top of this column. The acetonitrile products are then produced after total condensation at the top of EDC. The mixtures of water and entrainers at the bottom of EDC are pumped into SRC, where the high purity of water is obtained from the top. The entrainer with high concentration obtained at the bottom of SRC is recycled back into the top of EDC at last after cooled by cooling water and mixed with the makeup of solvents.


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HEAT Acetonitrile

Entrainer

Water Feed

EDC

SRC

Figure 9 Extractive distillation process for acetonitrile and water with different entrainers

The process simulation for the separation of acetonitrile and water with EG entrainer has actually been reported in open literatures25-27. To directly compare the effects of different entrainers on the process simulation, the same operating parameters of the feed compositions and two product specifications are employed as ones after pre-concentrator column given in literatures25-27, as listed in Table 3. The constraints for all investigated cases are molar purity > 0.9999 for acetonitrile and water at the top of the EDC and SRC respectively, and the mole fraction > 0.99999 for entrainers contained in the recycling stream at the bottom of SRC. The process simulation with EG entrainer was firstly carried out and the duplicated simulation results in literatures25-27 are obtained, as shown in Table 4 and Figure S9, suggesting that our simulation method is reasonable and the results can be used as a benchmark for IL entrainers. The simulations and comparisons with different IL entrainers are then performed, where the optimized parameters like the theoretical stages, feed stages, mass reflux ratio and so on are determined with the Design Specs module and sensitivity analysis in Aspen Plus.



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Table 3 Operating parameters in process simulation for different entrainers Contents Columns

Streams

Value Extractive distillation column (EDC) Pressure (atm) Distillate rate (kmol/h) Solvent recovery column (SRC) Pressure (atm) Distillate rate (kmol/h) Feed stream Temperature (K) Pressure (atm) Flow rate (kmol/h) Composition (mole fraction) Acetonitrile Water Entrainer stream Temperature (K) Pressure (atm)

1.0 99.975 1.0 53.824 349.7 1.1 153.8 0.65 0.35 345.0 1.1

The sensitivity analysis of the theoretical stage, molar reflux rate, distillation rate, feed and entrainer feed stages and flow rate of entrainer on basis of acetonitrile purity for EDC are made and given in Figures S10-S15 of SI. As it is wanted that the purity of acetonitrile product is strongly affected by these operation parameters, and the optimal stages are observed with the changing of feed and entrainer feed stages respectively. It is also found that the entrainer feed stage is one rather than the first stage. It can be explained that the entrainer is required some stages to separate even though the volatility of IL entrainer could almost be neglected. In addition, the content of water will increase and go beyond a limitation with the decrease of theoretical stage if no IL entrainer is available. Therefore, an optimal position for entrainer feeding is obtained on balancing the above two opposing factors. In Table 4 and Figures S16-S18, the optimized operating parameters are given. One can see that the difference of the optimized parameters among three IL entrainers is very small. The entrainer flow rate of EG is far greater than ones of ILs due to EG having less effects on the acetonitrile and water mixtures (see Figure 7), leading that more energy is required to provide in ACS Paragon Plus Environment

Page 23 of 30 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


EG case, as illustrated in Figure 10. It can be seen that both heat and condensation duties and intermediate heating load (HEAT in Figure 9) can be saved if IL entrainers are used compared to the conventional

organic

solvent.

The

process

with

[EMIM][OAC],

[EMIM][PRO]

and

[N1,1,4C2OH][GAC] entrainers makes the solvent flow rate severally decreasing from 145 to 14.0, 18.0 and 19.0 kmol/h and the total reboiler heat duties of EDC and SRC reducing 24.9%, 17.5% and 15.4% respectively, compared to one with EG entrainer. Furthermore, it can be seen from Table 4 that the number of theoretical stage of EDC in EG case is 53 and far greater than ones in IL cases. It also explains that the enhancement of EG for the separation efficiency of acetonitrile and water is great weaker than ones of ILs. For the three different type IL entrainers, the solvent quantity of [EMIM][OAC] is less than ones of [EMIM][PRO] and [N1,1,4C2OH][GAC], indicating that the former has stronger improvement for the separation of acetonitrile and water than the two later, whereas the similar results of the simulations with [EMIM][PRO] and [N1,1,4C2OH][GAC] entrainers are observed, which are basically corresponding to the results of experimental VLE. Meanwhile, the total reboiler heat duty of EDC and SRC in [EMIM][OAC] case can decrease 8.9% and 11.3% compared with ones in [EMIM][PRO] and [N1,1,4C2OH][GAC] processes respectively. In general, the selected IL entrainers can well enhance the separation of acetonitrile and water and save energy and solvent consumptions, and [EMIM][OAC] shows the best performance. In view of environmental safety, however, ILs containing imidazolium group might capture relatively high ecotoxicity79 and poor biodegradability41, while the cholinium-based IL of [N1,1,4C2OH][GAC] might be considered as a better alternative due to its potentially benign characteristics of lower ecotoxicity and easier biodegradation43, 80, which would be required to verify further.



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Table 4 Comparison of process simulation results among different solvents Contents

[EMIM][OAC] [EMIM][PRO] [N1,1,4C2OH][GAC]

EG

Column EDC Total stages Feed stage Entrainer feed stage Molar reflux ratio SRC Total stages Feed stage Pressure (atm) Molar reflux ratio

17 11 3 0.36

19 14 3 0.38

20 13 3 0.35

53 46 6 0.48

10 5 1.0 0.09

21 3 1.0 0.08

11 6 1.0 0.04

18 10 1.0 0.25

14.0

18.0

19.0

145.0

354.8 0.9999

354.8 0.9999

354.8 0.9999

354.8 0.9999

395.9

414.4

401.4

411.7 198.8

75 ppm 0.7935 0.2064

70 ppm 0.7493 0.2506

67 ppm 0.7390 0.2609

25 ppm 0.2707 0.7293

373.1 0.9999

373.1 0.9999

373.1 0.9999

373.1 0.9999

560.8 14.0

584.0 18.0

585.7 19.0

470.2 145.0

0 11 ppm 0.99999 0.02

0 10 ppm 0.99999 4.56

Streams Entrainer Flow rate (kmol/h) Top stream of EDC Temperature (K) Acetonitrile purity in mole Bottom stream of EDC Temperature (K) Flow rate (kmol/h) Composition (mole fraction) Acetonitrile Water Solvent Top stream of SRC Temperature (K) water purity in mole Bottom stream of SRC Temperature (K) Flow rate (kmol/h) Composition (molar fraction) Acetonitrile Water Solvent Entrainer loss ×104 (kmol/h) EDC Condenser (kW) Reboiler (kW) SRC Condenser (kW) Reboiler (kW) Duty of HEAT (kW)

0 0 13 ppm 9 ppm 0.99999 0.99999 1.82 1.58 Heat duty -1169 1100

-1188 1212

-1163 1242

-1279 1762

-671 1091 -342

-670 1194 -539

-633 1227 -664

-761 1155 -868


Page 25 of 30

3500

Heat duty, kW

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


reboiler Condenser 3000 [N114C2OH][GAC] HEATER [EMIM][PRO] 2500 [EMIM][OAC]

EG

2000

1500 1000 500 0

Figure 10 Comparison of heat duty among different cases

4. CONCLUSIONS Coupled COSMO-RS theory and experimental measurement, three ionic liquids (ILs) of [EMIM][OAC], [EMIM][PRO] and [N1,1,4C2OH][GAC] are verified to be effective in separating the azeotropic mixture consisted of acetonitrile and water. The relative volatility of acetonitrile to water can be well enhanced by adding a small amount of ILs, and the more the IL entrainer is added, the higher the enhancement of the relative volatility is. The theoretical insights at molecular level by analyzing the binding energy and charge density distributions show that the strong hydrogen bonding between IL and water molecules weakens the interactions between water and acetonitrile molecules. The NRTL model is employed to represent the ternary VLE of acetonitrile + water + IL mixtures, based on which the conceptual process for extractive distillation is established with RadFrac module in Aspen Plus package. The process simulation results show that the ratio of entrainer to feed decreases from 0.94 to 0.09, 0.12 and 0.12, and the total reboiler heat duties reduce 24.9%, 17.5% and 15.4% by introducing the IL entrainer of [EMIM][OAC], [EMIM][PRO] and [N1,1,4C2OH][GAC] respectively, in comparison with EG entrainer, suggesting that the selected IL ACS Paragon Plus Environment


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entrainer are promising in saving energy and solvent consumption. ASSOCIATED CONTENTS The Supporting Information is available free of charge via the internet at http://pubs.acs.org. Synthesis of ionic liquid [N1,1,4C2OH][GAC], NRTL model, List of quaternary ammonium cations, Optimized geometries, Optimized energy, Experimental VLE data, Antoine constants, Thermophysical properties of ILs, Sensitivity analysis of process simulation and Process flow diagrams with simulation results. AUTHOR INFORMATION Corresponding Author. * Tel.: +86-0519-86330253. E-mail: [email protected]. Tel.: +86-021-64252630. E-mail: [email protected]. ACKNOWLEDGMENTS Financial support for this work was provided by the National Basic Research Program of China (2015CB251401) and the National Natural Science Foundation of China (No. 21878025, 21776069, 21476070). REFERENCES (1) Rosen, J.; Hellenas, K. E. Analysis of acrylamide in cooked foods by liquid chromatography tandem mass spectrometry. Analyst 2002, 127, 880-882. (2) Srinivasu, M. K.; Narasa, R. A.; Reddy, G. O. Determination of lovastatin and simvastatin in pharmaceutical dosage forms by MEKC. J. Pharmaceut. Biomed. 2002, 29, 715-721. (3) Acosta-Esquijarosa, J.; Rodriguez-Donis, I.; Jauregui-Haza, U.; Nuevas-Paz, L.; Pardillo-Fontdevila, E. Recovery of acetonitrile from aqueous waste by a combined process: solvent extraction and batch distillation. Sep. Purif. Technol. 2006, 52, 95-101. (4) McConvey, I. F.; Woods, D.; Lewis, M.; Gan, Q.; Nancarrow, P. The importance of acetonitrile in the pharmaceutical industry and opportunities for its recovery from waste. Org. Process Res. Dev. 2012, 16, 612-624. (5) Gmehling, J.; Menke, J.; Krafczyk, J.; Fischer, K. Azeotropic Data. Weinheim, Germany: VCH, 1994.


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