Ammonium-Based Ionic Liquid as an Entrainer for ... - ACS Publications

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Ammonium-Based Ionic Liquid as an Entrainer for the Separation of n‑Propanol + Water and Isopropanol + Water Mixtures V. K. P. Janakey Devi, P. S. T. Sai, and A. R. Balakrishnan* Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India ABSTRACT: The effect of the ionic liquid tetramethylammonium chloride [TMACl] on the vapor−liquid equilibrium of n-propanol + water and isopropanol + water was studied experimentally. The ionic liquid [TMACl] was identified as a suitable entrainer for breaking the azeotrope of these two alcohol−water mixtures. This work reports the influence of an ammonium-based ionic liquid on the separation of azeotropic mixtures. The isobaric vapor liquid equilibrium data of npropanol + water + [TMACl] and isopropanol + water + [TMACl] were measured at atmospheric pressure. The amount of [TMACl] required for eliminating the azeotrope for both the systems was found to be low compared to imidazolium ionic liquids reported earlier. For n-propanol + water system, the azeotrope was eliminated at [TMACl] mole fraction of 0.05, and for isopropanol + water system, the azeotrope was eliminated at a mole fraction of 0.026. The effect of [TMACl] on both the alcohol−water systems was explained in terms of relative volatility and activity coefficient. their unique properties such as low flammability; low vapor pressures; good thermal and chemical stability; good dissolving capacity of polar, nonpolar, organic, and inorganic compounds; low causticity; and task-specific tunable properties.6−11 Although the use of ionic liquids as entrainers for extractive distillation is very promising, the reusability of ionic liquids as reflux to the extractive distillation column is a big concern. The bottom stream of the extractive distillation column containing ionic liquid can be flash distilled at mild or deep vacuum conditions to recover and reuse the ionic liquid as reflux stream. The replacement of conventional entrainers for extractive distillation with ionic liquids is promising for separation of azeotropes; however, the design of such processes requires the availability of phase equilibrium data of systems containing the ionic liquids. Lei’s group12,13 first reported the possible use of ionic liquids as promising entrainers for extractive distillation. Since then, ILs have gained considerable importance for the separation of azeotropic mixtures such as ester−alcohol,3,7,14−21 alcohol−water,4,5,22−35 and so on. In the present work, the phase equilibrium data of alcohol−water systems such as npropanol + water and isopropanol + water with the addition of ionic liquid have been studied experimentally. The experimental measurements of ternary VLE data of npropanol−water and isopropanol−water systems using different ionic liquids have been reported in the past.4,5,22−27 Much of the VLE data on the separation of n-propanol−water and isopropanol−water systems with ionic liquid entrainers has been focused on imidazolium-based ILs. The amount of imidazolium ionic liquids required to break the azeotrope

1. INTRODUCTION Alcohols such as n-propanol and isopropanol are very important products of the chemical process industries due to their extensive use as fuel additives and replacements. The addition of such alcohols increases the octane level of the fuels and promotes a more complete fuel burning that reduces harmful exhaust pipe emissions. The presence of water in these alcohols leads to phase-splitting and thereby results in engine damage.1 Due to the presence of unfavorable interaction between alcohols and water, these alcohols form minimum boiling azeotrope with water. The separation of such closeboiling mixtures into its constituent pure components is a challenging problem. Conventional distillation cannot be used to separate such mixtures as the separation beyond the azeotropic point is not possible. Hence, advanced distillation techniques such as pressure-swing distillation, azeotropic distillation, extractive distillation, etc. have to be employed to enhance distillation and facilitate separation of such mixtures.2−5 Extractive distillation requires the addition of an entrainer which enhances the relative volatility of the mixture and facilitates the separation. The added entrainer should have higher boiling point than the components to be separated and so evaporation of the entrainer is not required. This saves energy required for the separation process, making extractive distillation an efficient and widely preferred method. The important aspect in the extractive distillation process is the selection of a suitable entrainer.6 Traditional entrainers used are solid salts, which cause corrosion problems in the pipelines, and liquid solvents, which cause the formation of volatile organic compounds, resulting in environmental problems.7 Ionic liquids (ILs) replacing the traditional entrainers for the extractive distillation process has been gaining popularity recently due to © XXXX American Chemical Society

Received: June 8, 2017 Accepted: February 2, 2018

A

DOI: 10.1021/acs.jced.7b00523 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

completely is very high in these studies. Therefore, the need for identifying ionic liquids which can result in the separation of azeotropes at minimal IL concentration is required. Recently, Govinda et al.36 reviewed ammonium-based ionic liquids and their applications in different areas such as fuel cells, carbon sequestration, gas hydrate formations, anticorrosive agents, and numerous technical applications. However, there is no experimental study reported in the literature regarding the use of ammonium-based ionic liquids as possible entrainers for the separation of azeotropic mixtures. In cases where experimental ternary VLE or liquid−liquid equilibrium data are available, the behavior of mixtures containing ionic liquids can be predicted by NRTL or UNIQUAC thermodynamic models. Recently some researchers used the UNIFAC37,38 model and modified UNIFAC39 model to predict the thermodynamic behavior of the ternary systems containing ILs by measuring the activity coefficients at infinite dilution. In the case of mixtures containing ILs, the numbers of functional groups required for performing calculations are limited in the UNIFAC model. Hence, a lot of accurate experimental information is required for further development of this predictive model. The COSMO-RS model was used to screen potential ionic liquids for the separation of n-propanol + water and isopropanol + water azeotropic mixtures. More details on COSMO-RS model for ionic liquids (ADF version) are provided by Lei’s group.40 The study suggested that AILs have better extraction ability for separating alcohol−water mixtures than imidazolium, pyridinium, and pyrrolidinium ILs. Additionally, ILs with short alkyl chain cations and halide anions such as chloride [Cl−] and bromide [Br−] were found to be promising in the separation of the azeotropic mixtures. Because of these reasons, tetramethylammonium chloride [TMACl] was chosen as a possible ionic liquid entrainer for the separation of n-propanol + water and isopropanol + water system. Li et al.41 reported [TMACl] as an effective thermodynamic inhibitor compared to the other ILs in a study which deals with equilibrium hydrate formation conditions for methane. To this end, the present study aims at examining the effect of [TMACl] on the separation of azeotropic mixtures n-propanol + water and isopropanol + water. This is a pioneering work reporting the influence of an ammonium-based ionic liquid on the separation of an azeotropic mixture. Initially, the isobaric vapor liquid equilibrium data of n-propanol + water + [TMACl] and isopropanol + water + [TMACl] were measured at atmospheric pressure. Then, the separation ability of [TMACl] was studied by comparing the relative volatility and activity coefficient for both the systems and with the ILs reported in the literature.

Table 1. Specifications of the Chemicals Used chemicals

source

n-propanol isopropanol [TMACl]

Merck Merck SigmaAldrich Merck

acetonitrile

CAS registry no.

initial mass fraction purity

purification method

analysis method

71-23-8 67-63-0 75-57-0

0.990 0.990 0.980

none none none

GC GC GC

75-05-08

0.998

none

GC

2.2. Apparatus. To obtain the VLE data of binary and ternary systems, a simple still was fabricated2,42 and is shown in Figure 1. It is a continuous distillation still that recirculates the vapor phase alone, and accurate VLE data is obtained in a moderately short time. The still, which is made from high quality borosil glass, is based on the Othmer-type still.43 The boiling of the liquid in the still was achieved by a constant temperature oil bath heater, and a magnetic stirrer ensured proper mixing of the liquid in the still. The vapor and liquid temperature in the still was measured using the PT100 RTD (resistance temperature detector) sensor, which was inserted into the still. The temperature probe was checked against the ice and steam points of distilled water, and the standard uncertainty was found to be 0.1 K. The vapor space above the solution should be kept at a higher temperature than the solution boiling point to prevent refluxing, and this was accomplished by providing external heating using a jacketed heating coil. PID controller was used to control both liquid and vapor temperatures. The vapors generated were condensed and collected in the condensate receiver which is provided below the condenser. The other end of the condenser is open to the atmosphere to keep the system at atmospheric pressure. The condensate was circulated back to the still through a three-way stopcock. The condensate samples are withdrawn from the three way stopcock, and the liquid samples are drawn from the still using a pipet pump from the sampling port. 2.3. Procedure. The procedure adopted for the measurement of VLE is as follows: An initial feed mixture of about 50 mL was prepared and introduced into the still. The ionic liquid was added to the feed according to the mole fraction required. The heater was turned on to boil the liquid in the still, and the magnetic stirrer ensured the proper mixing of the solution and the condensate which was recirculated. The steady state was obtained in about 30−45 min, which was observed by the constant temperature of the liquid mixture. After observing a steady temperature reading for 20 min, the reading was noted, and the samples of liquid and condensate were collected into a sample vial. After the required number of samples was taken, the power source was switched off. The experiments were repeated as described above for various ionic liquid concentrations and feed compositions. 2.4. Sample Analysis. The analysis of all the samples was carried out with HPLC. The HPLC (Shimadzu, LC-20AD) was equipped with a refractive index detector (Shimadzu, RID10A), which is connected to a desktop computer. The separation of the components was achieved using the column HIQ sil C18HS (Kromatek, 4.6 mm diameter, 250 mm length). The operating conditions were the following: mobile phase, acetonitrile:water (60:40); injection volume, 20 μL; flow rate, 0.5 mL/min. The compositions of 1-propanol and 2-propanol were calculated using the area normalization method. To calculate the amount of water in each sample, analysis of the samples was performed using Karl Fischer Titrator (Metrohm

2. EXPERIMENTAL SECTION 2.1. Materials Used. The organic solvents used were npropanol (Analysis grade, Merck, >0.990 mass fraction), 2propanol (Analysis grade, Merck, >0.990 mass fraction), acetonitrile (HPLC grade, >0.998, Merck) and tetramethylammonium chloride [TMACl] (reagent grade, >0.98 mass fraction, Sigma-Aldrich). The water used was Millipore (MilliQ Ultrapure System, Merck). The reagents were used without purification after GC failed to show any impurities. The water content, determined by Karl Fischer titrator (Metrohm 870 KF Titrino Plus), was small in all chemicals ( 0.11), a liquid phase split was noticed at higher concentrations of n-propanol, and this may be due to the increase in the unfavorable interaction between water and npropanol caused by the increased IL concentration. 3.3. Ternary Systems: Isopropanol + Water + [TMACl]. The isobaric VLE data for the ternary system of isopropanol F



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Figure 9. Equilibrium phase diagram of isopropanol (1)−water (2)− [TMACl] (3) for different IL concentrations at atmospheric pressure: ■, x3 = 0; ●, x3 = 0.026; ▲, x3 = 0.05.

Figure 11. Boiling point diagram of isopropanol (1)−water (2)− [TMACl] (3) at atmospheric pressure: ■, x1′ (x3 = 0); □, y1 (x3 = 0); ▲, x1′ (x3 = 0.05); Δ, y1 (x3 = 0.05).

Figure 10. Boiling point diagram of isopropanol (1)−water (2)− [TMACl] (3) at atmospheric pressure: ■, x1′ (x3 = 0); □, y1 (x3 = 0); ●, x1′ (x3 = 0.026); ○, y1 (x3 = 0.026).

Figure 12. Relative volatility of isopropanol (1) to water (2) at atmospheric pressure: ■, x3 = 0; ●, x3 = 0.026; ▲, x3 = 0.05.

To study further the salt effect of IL on isopropanol + water system, activity coefficients of isopropanol and water are plotted and shown in Figures 13 and 14. With the addition of ionic liquid to the binary system, salt in effect occurs at low isopropanol concentrations indicated by the reduced activity coefficient values. As the isopropanol concentration increases the activity coefficient of isopropanol increases, indicating the salting out effect. The activity coefficient of water decreases significantly as the IL concentration is increased. The interaction between water and ionic liquid is stronger than that between isopropanol and ionic liquid, which results in the separation of the azeotropic mixture. The salting in effect of ionic liquid with isopropanol + water system was reported earlier for [EMIM][BF4].27 The liquid phase split was observed in the isopropanol + water + [TMACl] system at higher concentrations of isopropanol and at higher IL concentration (x3 > 0.11), and this may be due to the increase in the

unfavorable interaction between water and isopropanol caused by the increased IL concentration. 3.4. Comparison of Separation Effect of [TMACl] with Various ILs. The separation effect of [TMACl] on the azeotropic systems n-propanol + water and isopropanol + water was compared with the ILs reported earlier and given in Tables 6 and 7, respectively. Lei et al.6 studied the structure−property relations between the molecular structures of ILs and their separation performance for polar−polar systems. That study suggested that the structural factors such as short alkyl chain length, unbranched group, and the hydroxylation and cyaniding on the cation has positive effect on the selectivity and solvent capacity of ionic liquids for polar−polar systems, whereas steric shielding effect around anion charge center, alkoxyl and benzyl substitutions on the anions, fluorination on the anion, and metal ion inclusion on the anion has negative effect on the G



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Table 7. Minimum Mole Fraction of IL Required for Elimination the Azeotrope of Isopropanol−Water System ionic liquid

Table 6. Minimum Mole Fraction of IL Required for Elimination the Azeotrope of n-Propanol−Water System

[EMIM][BF4] [BMIM][BF4] [EMIM] [Triflate] [BEIM] [Triflate] [BMPYR] [Triflate] [TMACl]

0.41 large amount required (>0. 38) 0.34

equilibrium conditions

ref

et isobaric, 100 kPa Zhang al.23

0.05

[EMIM][BF4]

isobaric, 101.325 kPa

Zhang et al.4

Li et al.5

0.23

isobaric, 101.325 kPa isobaric, 100 kPa

[BMIM][BF4]

0.27

isobaric, 100 kPa

[EMIM][BF4]

0.1

[TMACl]

0.026

isobaric, 101.325 kPa isobaric, 101.058 kPa

isobaric, 100 kPa

Orchilles et al.25

isobaric, 100 kPa

Orchilles et al.26

■

present data

ORCID

0.72 0.52

0.1

isobaric, 101.058 kPa

ref

0.1 0.2 0.2

Zhang et al.23 Zhang et al.24 Li et al.27 present data

4. CONCLUSION The effect of the ionic liquid tetramethylammonium chloride [TMACl] on the vapor−liquid equilibrium of n-propanol + water and isopropanol + water was studied experimentally. This work reports the influence of an ammonium-based ionic liquid on the separation of an azeotropic mixture. The isobaric vapor liquid equilibrium data of n-propanol + water + [TMACl] and isopropanol + water + [TMACl] were measured at different IL concentrations (x3 = 0.026 and 0.05) at atmospheric pressure. The amount [TMACl] required for eliminating the azeotrope for both systems were found to be low compared to the reported ionic liquids. For the n-propanol + water system, the azeotrope was eliminated at a mole fraction of 0.05 of [TMACl], and for the isopropanol + water system, the azeotrope was eliminated at a mole fraction of 0.026 of [TMACl]. The effect of [TMACl] on the two alcohol−water systems was explained in terms of relative volatility and activity coefficient.

Figure 14. Activity coefficient of water at atmospheric pressure: ■, x3 = 0; ●, x3 = 0.026; ▲, x3 = 0.05.

minimum IL mole fraction

[EMIM] [OAc] [BMIM] [OAc] [EMIM][Br] [BMIM][BF4]

equilibrium conditions

selectivity and solvent capacity of ionic liquids for polar−polar systems. From Tables 6 and 7, it can be seen that the comparison of the separation effect of different ionic liquids on both the alcohol−water systems is consistent with the study reported by Lei et al.6 The short alkyl chain length and unbranched group of the cation has a positive effect and steric shielding effect around the anion charge center; alkoxyl substitutions and fluorination on the anion has a negative effect on the separation effect of ionic liquids. From the tables, it is seen that the separation effect of the ammonium-based ionic liquid [TMACl] is superior to those of all other ionic liquids. The amount of [TMACl] required for eliminating the azeotrope for both the systems was found to be low compared to ionic liquids reported earlier. Hence, [TMACl] may be a potential entrainer for the separation of the azeotropic mixtures n-propanol + water and isopropanol + water.

Figure 13. Activity coefficient of isopropanol at atmospheric pressure: ■, x3 = 0; ●, x3 = 0.026; ▲, x3 = 0.05.

ionic liquid

minimum IL mole fraction

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

V. K. P. Janakey Devi: 0000-0003-4147-1506 A. R. Balakrishnan: 0000-0003-0272-3491 H



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Funding

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This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Notes

The authors declare no competing financial interest.

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NOMENCLATURE tetramethylammonium chloride vapor liquid equilibrium nonrandom two-liquid universal quasichemical universal functional-group activity coefficients ionic liquid ammonium ionic liquid conductor like screening model - real solvents gas chromatography high-performance liquid chromatography normal boiling point (K) total pressure of the system (Pa) saturated vapor pressure of component i at temperature T (Pa) universal gas constant (J mol−1 K−1) liquid phase composition of pure component i vapor phase composition of pure component i activity coefficient relative volatility

[TMACl] VLE NRTL UNIQUAC UNIFAC IL AIL COSMO-RS GC HPLC T P Psat i R xi yi γ α

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REFERENCES

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