Distillable Protic Ionic Liquid 2-(Hydroxy)ethylammonium Acetate (2

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Distillable Protic Ionic Liquid 2‑(Hydroxy)ethylammonium Acetate (2HEAA): Density, Vapor Pressure, Vapor−Liquid Equilibrium, and Solid−Liquid Equilibrium Anne Penttila,̈ * Petri Uusi-Kyyny, and Ville Alopaeus Department of Biotechnology and Chemical Technology, School of Chemical Technology, Aalto University, P.O. Box 16100, 00076 Aalto, Finland S Supporting Information *

ABSTRACT: Recently it has been found that certain ionic liquids (ILs) have notable vapor pressures (Earle et al. Nature 2006, 439, 831−834). These ILs may be important in various novel technologies, but they may also be important in postcombustion carbon captures as side products. In this work a distillable protic ionic liquid (PIL) 2-(hydroxy)ethylammonium acetate (2HEAA) was prepared from monoethanolamine (MEA) and acetic acid (HAc) and it was purified with a Vigreaux type distillation column under vacuum. Density was measured for the MEA + 2-HEAA and HAc + 2-HEAA systems with a DMA HP densimeter from 293 to 363 K. The Redlich−Kister polynomial was used to model the density data. Vapor−liquid equilibrium was measured for the H2O + HAc + 2-HEAA system with a static total pressure apparatus at 347 K. Solid−liquid equilibrium was measured for the H2O + HAc + 2-HEAA system with a visual method. The NRTL activity coefficient model was used to model the vapor− liquid and solid−liquid equilibrium data.

1. INTRODUCTION Alkanolamines are widely used for absorbing sour gases from many gas streams such as flue gases, natural gases, and refinery gases. Monoethanolamine (MEA) is the most used alkanolamine for carbon dioxide (CO2) capture from flue gases, which may contain oxygen (O2), nitrogen oxides (NOx), and sulfur oxides (SOx).1 These compounds cause degradation of MEA in the CO2 capture processes, which is a significant problem. Due to the basic nature of amines, they react rapidly with acidic compounds, such as acetic acid, formic acid, and oxalic acid, to form heat stable salts (HSS) and other degradation products. Carboxylic acids are stronger than sour gases, and thus they react easily with amines instead of sour gases. Heat stable salts are not thermally regenerable leading to reduced amine capacity to absorb acid gases. Heat stable salts also cause corrosion, foaming, and fouling in process equipment.1,2 This study is a continuation of our previous research.3 In both this work and the previous work, 2-(hydroxy)ethylammoniun acetate (2-HEAA) was prepared from MEA and acetic acid (HAc). 2-HEAA is a heat stable salt but also a distillable protic ionic liquid (PIL). As stated through a literature survey in the work of Penttilä et al.,3 only a limited amount of experimental data is available for 2-HEAA. In the same research, it was also concluded that 2-HEAA containing systems are rather cumbersome to investigate since the melting point of 2-HEAA is above the room temperature. However, physical properties and phase equilibrium data are needed for these kinds of systems when CO2 capture processes have to be modeled accurately. Phase equilibrium data for 2-HEAA containing systems are also needed when thermal reclamation units or the sweetening of natural gas processes are designed. The objective of this and our previous work was to produce new experimental data related to the systems that contain 2HEAA. © 2014 American Chemical Society

ILs have been traditionally considered to be nonvolatile by having negligible vapor pressure. Earle et al.,4 however, demonstrated that many ILs can be distilled under vacuum without decomposition. PILs can be considered to be distillable, so long as they do not undergo decomposition before vaporization. When temperature is increased, a distillable PIL dissociates into its original Brønsted acid and Brønsted base. The molecular components then vaporize, and on condensing, the PIL is reformed. The mechanism involved in distillation is a proton transfer from the protonated cation to the anion and then on condensing back to the Brønsted base. This phenomenon is assumed to occur since a distillable ionic liquid forms a weak ionic bond between acid and base species. In a strong ionic bond between acid and base species, the proton from the Brønsted acid become more tightly locked on the Brønsted base so that the formed ionic liquid may evaporate partially as ion clusters. In the former case, the vapor pressure has also been reported to be higher than in the latter case. This has been explained to be due to both interactions between ions and ionicity. Ionic liquids with weak ionic bonds and low ionicity evaporate easier than ionic liquids with strong ionic bonds and high ionicity.5,6 The pKa values of acids and bases may give an indication how strongly a proton is transferred from the acid to a base. pKa values are normally given for aqueous solutions and thus may not be appropriate for nonaqueous ILs. However, in the investigation by Greaves and Drummond, the ΔpKa value between the base and acid species is mentioned to be higher than 8 for ILs possessing strong ionic bond and high ionicity.5 Received: Revised: Accepted: Published: 19322

September 27, 2014 November 17, 2014 November 21, 2014 November 21, 2014 dx.doi.org/10.1021/ie503823a | Ind. Eng. Chem. Res. 2014, 53, 19322−19330

Industrial & Engineering Chemistry Research

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The pKa value for aqueous MEA solution is 9.50 at 298.15 K,7 and it is 4.756 for aqueous HAc solution at 298.15 K.8 The ΔpKa value is 4.744, and thus it can be considered as an IL with a weak bond and low ionicity.5 Vapor pressures for ILs have been earlier measured with methods such as Knudsen effusion method, transpiration method, and thermogravimetric analysis. As already mentioned above, Earle et al.4 showed in their work that it is possible to directly measure very low vapor pressures of ILs. They used two separate methods in their experiments: a Kugelrohr distillation and a sublimation apparatus. The Kugelrohr apparatus consists of an oven and a collection flask with glass wool in between to avoid splashing as a result of bubbles bursting in the IL. The sublimation apparatus was not described. In this work protic IL 2-HEAA was purified by distilling it with a Vigreaux type distillation column under vacuum. 2-HEAA vaporizes in the form of MEA and HAc, since 2-HEAA dissociates into these compounds before vaporization. Therefore, 2-HEAA does not possess pure component’s vapor pressure. Instead the pressure at which 2-HEAA distillates can be called apparent vapor pressure. As discussed above, all PILs are not distillable since they undergo decomposition before vaporization. 2-HEAA studied in this work is a distillable PIL since 2-HEAA is obtained almost as a pure distillate. In vacuum distillation, 2-HEAA dissociates mainly into MEA and HAc but some of 2-HEAA also decomposes into water (H2O) and N-(2-hydroxyethyl)acetamide (2-HEAM) due to the effect of temperature in the bottom flask. This means that MEA, HAc, and small quantities of H2O vaporize and 2-HEAM remains in the bottom flask. On the condensing section, MEA and HAc condense into the liquid phase and then react by reforming 2-HEAA. H2O also condenses into the liquid phase and thus it is collected as a distillate among 2-HEAA. Since the vapor phase inevitably contains traces of H2O due to the decomposition reaction, the apparent vapor pressure of 2-HEAA is not possible to measure directly. The main goal of this work was to measure physical properties of 2-HEAA containing systems. Density was measured for the MEA + 2-HEAA and HAc + 2-HEAA systems. The densities were measured at 293 to 363 K and modeled with the Redlich−Kister expression. In addition, VLE and SLE were measured for the H2O + HAc + 2-HEAA system. The VLE measurements were conducted with a static total pressure apparatus at 347 K and the SLE measurements were determined with a visual method. The measured data were modeled by using the NRTL activity coefficient model.9 The vapor phase was assumed ideal.

titration method. 2-HEAM was used as supplied. Deuterated dimethyl sulfoxide (DMSO-d6, 99% deuterated) was stored over 0.3 nm molecular sieves. The water content in 2-HEAM and DMSO-d6 was not determined. 2.2. Preparation of 2-HEAA. 2-HEAA was synthesized by neutralizing MEA with an equimolecular amount of HAc. A 305 g portion of MEA was first placed in a triple necked flask equipped with a reflux condenser, a dropping funnel, and N2 gas. Then, 300 g of HAc was added dropwise into the flask and the reaction mixture was stirred vigorously with a magnetic bar during HAc additions. The reaction mixture was cooled in an ice bath. Although, HAc was added slowly into the flask and the reaction mixture was stirred well, the formed 2-HEAA was not completely pure but the reaction mixture consisted of MEA, HAc, H2O, 2-HEAM, and 2-HEAA. H2O and 2-HEAM are formed in the salt preparation when 2-HEAA undergoes dehydration reaction. 2-HEAM also gives the reaction product a yellowish color. The reaction mixture solidified either during or after its preparation, and therefore, it was melted in an oven before purification. The temperature of the oven was set to slightly below 343 K since according to Temin10 the melting point of 2-HEAA is 338−339 K. Keeping the temperature of liquid 2-HEAA as low as possible, further dehydration reaction of 2-HEAA was avoided. 2-HEAA was purified by distilling the reaction mixture under vacuum. 2.3. Purification of 2-HEAA: Vacuum Distillation. 2HEAA was purified by using a Vigreaux type distillation column under vacuum with the reflux ratio of 2. The length of the column is 40 cm, and the diameter of the column is 2.5 cm. A schematic diagram of the distillation unit is shown in Figure 1.

Figure 1. Schematic diagram of the distillation unit: 1 = bottom flask, 2 = distillate flask, 3 = distillation column, 4 = column adapter for reflux temperature measurement, 5 = condensing unit, T = temperature meter, P = pressure meter, and N2 = liquid nitrogen bath.

2. EXPERIMENTAL SECTION 2.1. Materials. The suppliers and purities of the materials used in this work are presented in Table 1. MEA and HAc were dried over Merck 0.3 nm molecular sieves. The water content in both MEA and HAc was determined by the Karl Fischer

In vacuum distillation, unreacted MEA and HAc as well as in the decomposition reaction formed H2O were collected in a trap cooled by liquid nitrogen before the system pressure at which the distillation occurred, was reached. 2-HEAA and H2O were obtained as a distillate and the mixture was distilled twice in order to get as pure 2-HEAA as possible. The distilled 2HEAA + H2O mixture was a viscous and colorless liquid. It was recovered as subcooled liquid and poured from the 1000 cm3 flask into 25 cm3 vials since in that way the mixture was easier to handle without crystallization. The mixture of 2-HEAA + H2O was stored at room temperature.

Table 1. Materials, Their Purities, and Suppliers component

supplier

purity in mass percent

monoethanolamine acetic acid N-(2-hydroxyethyl)acetamide deuterated dimethyl sulfoxide

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Euriso-Top

>99.9 >99.9 not determined not determined 19323

dx.doi.org/10.1021/ie503823a | Ind. Eng. Chem. Res. 2014, 53, 19322−19330


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calibration uncertainty in density is ±0.3 kg/m3 at 293−353 K. The calibration uncertainty in density is ±3.0 kg/m3 at 363 K. The uncertainty at 363 K is higher than below 363 K since the probability of bubble formation increases when the temperature approaches the boiling point of water. The formed bubbles disturb density measurements. The samples for the density measurements were prepared gravimetrically and the air dissolved in the samples was removed by using a vacuum pump. Each sample was subjected to five subsequent measurements and the average was reported. 2.6. Vapor−Liquid Equilibrium: Static Total Pressure Apparatus. The static total pressure apparatus was used for the VLE measurements in this work. The construction of the apparatus can be found in the work of Uusi-Kyyny et al.12 and the automation of the apparatus is described in the work of Ouni et al.13 2.6.1. Equipment. The pressure transducer was calibrated against a BEAMEX MC2-PE calibrator (0−300 kPa pressure range). The resolution is 0.01 kPa, and the accuracy is 0.05% of the full scale. The BEAMEX pressure calibrator was calibrated at the Finnish National Standards Laboratory. The obtained calibration uncertainty is ±0.01 kPa. The pressure of the equilibrium cell was measured with a UNIK 5000 pressure transducer (with pressure range from 0 to 150 kPa and temperature range from 218 to 398 K) manufactured by GE. The accuracy in pressure transducer is 0.04% of the full scale, and the resolution is 0.01 kPa. The pressure transducer was maintained at a 5 K higher temperature than the equilibrium cell in order to prevent sample condensation in the pressure transducer line and in the pressure transducer. The pressure transducer was calibrated against the BEAMEX MC2-PE calibrator. The calibration uncertainty is ±0.08 kPa. The total calibration uncertainty of the pressure measurements is ±0.09 kPa. A thermolyzer S2541 (Frontek) temperature meter equipped with four Pt-100 probes was used to measure the temperature of the equilibrium cell, the pump, and the pressure transducer. The resolution and accuracy in temperature are 0.005 and ±0.01 K, respectively. The temperature meter was also calibrated at the Finnish National Standards Laboratory. The calibration uncertainty of each probe is ±0.02 K. The volume of the equilibrium cell was determined with distilled water. The volume consists of the volume of the equilibrium cell, the lines from the cell to the valves and the pressure transducer. The volume of the equilibrium cell and the lines were determined with the syringe ISCO pump (model 260D). The pump was pressurized to 500 kPa. The pressure transducer was detached from the equilibrium cell for the duration of the calibration in order to prevent exceeding the maximum pressure of the pressure transducer. The volume of the pressure transducer was obtained by pipetting water into the pressure transducer manually. The volume obtained for the equilibrium system is 105.2 cm3. The uncertainty of the volume was estimated to be ±0.1 cm3 based on the uncertainty of pipetting water. The uncertainty due to pipetting water is higher than the uncertainty due to the volumetric ISCO pump. 2.6.2. Experimental Procedure. HAc was degassed in an ultrasonic bath with ice for 2.5−3 h. Meanwhile the static apparatus was evacuated. The degassed HAc was introduced into the syringe pump, and then, the pump was pressurized to 500 kPa. Since 2-HEAA is extremely viscous and crystallizes easily, it would have not been possible to insert it into the pump through narrow lines. Therefore, 2-HEAA was injected

The apparent vapor pressure of 2-HEAA was not possible to measure due to the formation of H2O during the distillation. However, it was considered interesting to know how the system pressure varies as a function of temperature. The pressure measurements were operated with total reflux except in the cases when samples were taken for the water content determination. In those cases, the reflux ratio was 2. Experimental arrangement was such that the samples for water content determination were obtained without interruption of the distillation under vacuum. The distillation conditions were changed by raising the system pressure approximately 0.1 kPa at a time with consequence of the temperature increase. The system reached the equilibrium when the temperature stabilized to a specific level at a specific pressure. The temperature and pressure readings were recorded manually. The water content in the vapor and liquid phases depended on the temperature in the distillation column since the decomposition of 2-HEAA into H2O and 2-HEAM is temperature dependent. The temperature was measured with an Ametek DTI 100 device equipped with two high precision Pt-100 sensors. The accuracy and resolution in temperature are ±0.015 and 0.01 K, respectively. The Ametek temperature meter was calibrated at the Finnish National Standards Laboratory. The calibration uncertainty is ±0.02 K. The pressure transducer was calibrated against a BEAMEX MC2-PE pressure calibrator (0 to 300 kPa pressure range). The accuracy and resolution of the BEAMEX pressure calibrator are ±0.05% of the full scale and 0.01 kPa, respectively. The BEAMEX pressure calibrator was calibrated at the Finnish National Standards Laboratory. The obtained calibration uncertainty is ±0.01 kPa. The pressure was measured with a Vacuubrand VAP 5 pressure transducer. The accuracy is ±10% of the indicated value within 10−10−3 kPa range. The resolution depended on the range of the pressure measurement. It was 0.001 kPa (in the pressure range of 10−3 to below 10−2 kPa), 0.01 kPa (in the pressure range from 10−2 to below 1 kPa), or 0.1 kPa (in the pressure range from 1 to 100 kPa). The calibration uncertainty in pressure is ±0.03 kPa. The total calibration uncertainty is ±0.04 kPa. Temperature and pressure readings were recorded manually. During the distillation, vacuum was adjusted either by a metering valve or by a vacuum control unit (Vacuubrand CVC 2 II). The distillation column has been successfully used for vapor pressure measurements in our previous studies.11 2.4. Characterization of 2-HEAA. The purity of 2-HEAA was determined by a proton nuclear magnetic resonance (1H NMR) analysis. The procedure of the 1H NMR characterization of 2-HEAA is described in great detail in our previous work,3 including both the figures of 1H NMR spectra of MEA, 2HEAM, and 2-HEAA, and the calculations of the purity of 2HEAA. The water content in 2-HEAA was determined with a Karl Fischer titration due to the limitation of the 1H NMR analysis, as described in the work of Penttilä et al.3 The purity of 2-HEAA was confirmed with a differential scanning calorimetry (DSC) analysis. The DSC 6000 apparatus is presented in our previous work.3 2.5. Density: DMA HP densimeter. The densities of MEA + 2-HEAA and HAc + 2-HEAA were determined with a DMA HP connected to a DMA 5000 M densimeter by Anton Paar GmbH. The resolution in temperature is 0.001 K, and the resolution in density is 0.0005 g/m3. The accuracy in temperature is ±0.05 K. The densimeter was calibrated with both deionized, degassed water, and silica gel dried air. The 19324



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Table 2. Density (ρ) Parameters for the Pure Components of MEA, HAc, and 2-HEAA and for the MEA + 2-HEAA and HAc + 2-HEAA Systems ρ (kg/m3) a0 a1 a2 a3 no. points T range (K) w range ave. abs. dev. model a

HAc

MEA

86.99068b 0.259a 591.950a 0.253a 42a 289.81−591.95 pure 1.34 D A/B{1+[1−((T/K)/C)] }

61.11208b 0.22523a 678.2a 0.21515a 26a 283.65−413.15 pure 1.16 D A/B{1+[1−((T/K)/C)] }

2-HEAA

MEA + 2-HEAA

HAc + 2-HEAA

1316.15887c −0.54553c

17.49223b −0.99278b

134.30112b 57.24600b 51.73059b

7c 303.15−363.15 pure 0.24 A + B(T/K)

48b 303.15−363.15 0.18−0.66 0.97 eq 1

48b 303.15−363.15 0.19−0.98 1.10 eq 1

DIPPR.14 bThis work. cPenttilä et al.3

total uncertainty of the measured temperatures was estimated to be ±0.5 K, and it is based on the calibration of the temperature meter and the used method. The mixture in the equilibrium cell was heated with a thermostated 2-propanol bath. The H2O + HAc + 2-HEAA mixture melted gradually when the temperature was increased forming first a mixture of solid and liquid. Near the freezing point of the mixture, the temperature was increased at 0.1 K intervals and between the temperature changes the mixture was let to reach the equilibrium for 5−10 min. The temperature at which all the solid material had melted was recorded. Two sequential measurements were conducted in each composition, and the average of the results was reported. The detection of the freezing point was based on visual observation.

with a classical syringe directly into the equilibrium cell. Subcooled 2-HEAA, stored in a 25 cm3 vial, was heated in an oven before the injection. 2-HEAA was warmed up to approximately 5 K above its melting point in order to avoid the crystallization. The heating time was approximately 15 min. The syringe was weighted before and after the injection of 2HEAA with a Precisa 410AM-FR balance. The resolution is 0.0001 g, and the accuracy is ±0.0002 g. After the injection of 2-HEAA, the equilibrium cell was sealed and then immersed into a temperature controlled water bath. Continuous stirring of both 2-HEAA in the equilibrium cell and water in the temperature controlled bath was started, and it was continued until the temperature of the equilibrium cell reached the isothermal temperature. In the meantime, the equilibrium cell was evacuated until the vapor pressure of the 2-HEAA + H2O mixture was achieved. The measurements were initiated by adding a known amount of HAc from the ISCO pump into the cell. Between each addition, the system reached the equilibrium approximately in 20−40 min depending on the added amount of HAc. The equilibrium was observed from the stabilized pressure in the cell. The equilibrium temperature, pressure, and volume were recorded automatically. The temperature of the pump was also recorded in order to calculate the correct amount of HAc added through the syringe pump into the cell. After the measurement, the apparatus was first washed with water and then with acetone. After cleaning, the system was evacuated. 2.7. Solid−Liquid Equilibrium: Visual Method. Solid− liquid equilibrium of the H2O + HAc + 2-HEAA system was measured with a visual method. The apparatus is described in our previous work.3 The samples were prepared gravimetrically just prior to measurements. The well-mixed liquid mixture was first poured into the equilibrium cell and then the liquid mixture was crystallized. The samples with high 2-HEAA concentrations were crystallized by using a tiny amount of 2-HEAA seed crystals and submerging it into liquid nitrogen. The effect of the crystals on the concentration of a sample was ignored since the amount of added 2-HEAA was considered to be negligible. The samples with low 2-HEAA concentrations crystallized in liquid nitrogen. When the temperature probe was installed into the equilibrium cell and the sample was in a solid state, the cell was placed into the apparatus. The temperature was measured with an Ametek DTI 100 temperature meter, which is the same apparatus as used in vacuum distillation. Therefore, the information related to the calibration of the temperature meter is given in the section of the purification of 2-HEAA. The

3. CALCULATION AND MODELING 3.1. Purity of 2-HEAA: Determination by NMR, Karl Fischer Titration, and DSC 6000. The first method used to determine the purity of 2-HEAA was a proton nuclear magnetic resonance (1H NMR) analysis. The analysis of experimental data is shown in great detail in our previous work.3 However, to put it briefly, the 1H NMR spectra of commercial MEA and 2HEAM were compared with the 1H NMR spectrum of 2-HEAA synthesized in this work. The aim of the comparison of the spectra was to figure out if the synthesized 2-HEAA sample contained any MEA or 2-HEAM. Our previous work3 also presents how the purity of 2-HEAA was calculated. The second method was Karl Fischer titration, and it was used to determine the water content in 2-HEAA samples. Karl Fischer titration gives more reliable results for the water content in 2-HEAA samples than the 1H NMR analysis due to the limitations as described in the work of Penttilä et al.3 The third method was differential scanning calorimetry. The DSC 6000 apparatus was used to confirm the results obtained for the purity of 2-HEAA by means of NMR and Karl Fischer titration. The DSC melting curve of water containing 2-HEAA sample was first measured with the DSC 6000 apparatus, and then, the purity of the sample was analyzed with a Pyris Series software which calculated pure component properties based on the calibrations and the DSC melting curve of water containing 2-HEAA. Therefore, as a result of the analysis, the purity of 2HEAA as well as the melting point and the enthalpy of fusion of water free 2-HEAA were obtained. 3.2. Density. Density data for the systems of MEA + 2HEAA and HAc + 2-HEAA were not found in the literature. The densities of MEA and HAc were obtained from DIPPR,14 and the density for 2-HEAA was taken from the work by 19325



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Table 3. VLE and HE Data Used for Fitting the NRTL Binary Parameters of H2O + HAc data type

T/K

P/kPa

no. data points

avg. abs. deviation, %

ref

VLE (isobaric), xyTP VLE (isobaric), xyTP VLE (isobaric), xyTP VLE (isobaric), xyTP VLE (isobaric), xyTP VLE (isobaric), xyTP VLE (isothermal), xyTP HE (isobaric) HE (isobaric) HE (isobaric)

329.6−391.6 391.5−420.8 317.8−389.7 295.3−473.9 373.3−386.5 373.2−391.3 412.6 313.15−353.15 298.15−328. 15 313.15−353.15

16.7−101.3 179−273 9.3−101.3 2.7−790 101.3 101.3 188.9−350.3 101.3 101.3 101.3

40 22 45 65 24 15 13 15 46 31

0.43 4.85 0.47 0.53 0.45 0.36 3.24 24.87 15.68 17.23

20 21 22 23 24 25 26 27 28 29

Table 4. Binary NRTL Interaction Parameters for H2O + HAc, H2O + 2-HEAA, and HAc + 2-HEAA

NRTL, a1,2≠2,1 (K) NRTL, b1,2≠2,1 NRTL, α2,1≠2,1 (K) NRTL, b2,1≠2,1 NRTL (α12= α21) T range (K) |ΔT| (K) |Δp| (kPa) |ΔHE| (kJ/mol) no. data points

VLE + HE

VLE + SLE

VLE + SLE

H2O + HAc

H2O + 2-HEAA

HAc + 2-HEAA

−255.732a (22.85)b 0.71467a (0.18)b 621.415a (15.59)b −1.21813a (0.070)b 0.4a 295.3−473.9 VLE (0.93) VLE (0.003) HE (17.70) 316

167.397543c −0.020558c −465.72452c 0.030143c 0.9c 244.8−339,3 SLE (4), VLE (1.45) VLE (0.4)

164.115a (81.17 ± 0.34) 0.2101a (81.17 ± 0.11) 1037.5468a (81.17 ± 19.27) −5.3494a (81.17 ± 12.66) 0.9a 268.8−346.7 SLE (5.79) VLE (0.16)

44

26

a

Parameters regressed in this work. bConfidential limit in the parentheses obtained from in-house VLEFIT program.30 cFrom the work of Penttilä et al.3 dA ±5% change in a regressed parameter gave the average value for the total error of ABS(Tmeasured − Tcalculated). The value is given beside the parameter in the parentheses. The same error calculated with the parameters given in Table 4 is 81.17.

Penttilä et al.3 Equations and their parameters are given in Table 2. A Redlich−Kister correlation was used to describe the excess part of the MEA + 2-HEAA and HAc + 2-HEAA densities. The correlation is given in eq 1:

the compressed molar volume of HAc with respect to the pressure and this value was used to calculate the correction factor of the HAc density. In the VLE calculations, the density of HAc was obtained from DIPPR14 and it was corrected as described above. In the case of carboxylic acids, the system forms dimers in the vapor phase and therefore the fugacity coefficients are suggested to be modeled by means of the Hayden−O’Connell method,16 including the chemical theory. However, in this work the total pressure was very low in the H2O + HAc + 2-HEAA measurements and thus the fugacity coefficients in the vapor and liquid phases were assumed to be equal to unity. The VLE is presented in eq 3:

n

ρ E (kg/m 3) = w1w2 ∑ ai(w1 − w2)i i=0

(1)

w1 is the mass fraction of MEA or HAc, w2 is the mass fraction of 2-HEAA, ai is the parameter and n is the number of the ith parameter starting from zero. The objective function, obj, used in the parameter regression is obj =

ρexp − ρcalc ρexp

yP = xiγiPivap i

(2)

(3)

yi is the mole fraction of the component i in the vapor phase, P is the total system pressure, xi is the mole fraction of the component i in the liquid phase, γi is the activity coefficient of the component i, and Pvap is the vapor pressure of the i component i at the system temperature. The NRTL activity coefficient model9 was used to regress the parameters as it was done in our previous work.3 The NRTL model was selected since it has been successfully applied to ILs by Döker and Gmehling,17 Li et al.,18 and Wang et al.19 The binary interaction parameters τij and τji are presented as functions of temperature according to eq 4:

3.3. Vapor−Liquid Equilibrium. VLE data were measured for the H2O + HAc + 2-HEAA system. The measured VLE data consisted of the mass of 2-HEAA + H2O, the volume of HAc, the temperature, and the total pressure of the cell. The water content in 2-HEAA was easily taken into account in the calculations and modeling, and thus its effects on phase equilibria were considered. HAc was pressurized to 500 kPa in the ISCO pump during the measurements. The effect of the pressure on the density of HAc was taken into account in the calculation of HAc moles added into the equilibrium cell. The Hankinson−Brobst−Thomson method (HBT)15 was used to estimate the saturated molar volume of HAc. The predicted saturated molar volume obtained in this way was used to calculate the compressed molar volume of HAc at the ISCO pump pressure of 500 kPa with the Tait equation.15 The isothermal compressibility was obtained from the derivative of

τij ≠ ji =

aij ≠ ji + bij ≠ jiT T

(4)

i and j refer to the components in the binary system, and aij, bij, aji, and bji are adjustable parameters. The binary NRTL 19326



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Figure 2. Logarithm of the pressure of the 2-HEAA + H2O system as a function of the reciprocal of temperature. Experimental data were measured from seven different batches, which are indicated with the symbols of ○, ●, Δ, □, ▲, ×, and ◊. Water content in 2-HEAA was determined only from the experimental points of ●.

mol, respectively. The objective function used to regress the NRTL parameters of the binary HAc + 2-HEAA system is given in eq 9:

interaction parameters for H2O + HAc were regressed against the VLE and HE data given in Table 3. The experimental data were selected to cover as large temperature and pressure ranges as possible. The objective functions based on the total pressure and the excess enthalpy are given in eqs 5 and 6: ⎛ P calc − P exp ⎞2 obj = ∑ ⎜⎜ k exp k ⎟⎟ Pk ⎠ k=1 ⎝

N

obj =

N

∑ k=1

Tkexp − Tkcalc Tkexp

(9)

The nonrandomness factor for the HAc + 2-HEAA system is 0.9.

(5)

N

obj =

∑ |HkE ,calc − HkE ,exp| k=1

4. RESULTS AND DISCUSSION 4.1. Conditions in 2-HEAA Distillation. The apparent vapor pressure of 2-HEAA was not possible to measure since H2O was continuously formed under vacuum distillation. In spite of that, the temperature changes were examined as the system pressures were changed. Therefore, the system pressure was measured as a function of temperature by using seven different batches. The measured data are presented in the Supporting Information and plotted in Figure 2. The water content in 2-HEAA was determined from six samples, and the results are tabulated in the Supporting Information. The water contents are shown as a function of temperature in Figure 3. In Figure 3 it can be seen that the water content in 2-HEAA increases when the temperature rises in a distillation unit due to the elevated pressure. This phenomenon is explainable with the fact that the temperature

(6)

The nonrandomness factor was set equal to 0.4 since it gave the best fit in terms of the average absolute deviation compared to other nonrandomness parameter values. The model parameters for the H2O + 2-HEAA system, and the nonrandomness factor were taken from the work by Pentilä et al.3 The parameters are given in Table 4, and the nonrandomness factor for this system is 0.9. The binary NRTL interaction parameters for the HAc + 2-HEAA system were obtained by regressing the parameters against the ternary VLE and SLE data of H2O + HAc + 2HEAA. The SLE system is discussed in the next section. The objective function for the VLE data is given based on the total pressure in eq 7: ⎛ P exp − P calc ⎞2 ∑ ⎜⎜ k exp k ⎟⎟ Pk ⎠ k=1 ⎝ N

obj =

(7)

3.4. Solid−Liquid Equilibrium. Solid−liquid equilibrium of the H2O + HAc + 2-HEAA system was modeled in this work by calculating the freezing point curves for pure HAc and 2HEAA by means of a simplified SLE eq 8: ln(γixi) = −

ΔHifus ⎛ T ⎞ ⎜⎜1 − fus ⎟⎟ RT ⎝ Ti ⎠

(8)

xi is the mole fraction of the component i, γi is the activity coefficient of the component i, ΔHfus i is the enthalpy of fusion of the pure component i, Tfus i is the melting temperature of the pure component i, T is the freezing point temperature of the mixture, and R is the universal gas constant. The melting point and the enthalpy of fusion of 2-HEAA were measured in this work. The melting point and the enthalpy of fusion of HAc were taken from DIPPR,14 and they are 289.7 K and 11.7 kJ/

Figure 3. Mole percent of water in 2-HEAA as a function of temperature. The experimental points were taken from the distillation column as a distillate. 19327



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rise in the bottom flask results in more rapid decomposition of 2-HEAA than at lower temperatures. A larger amount of H2O and 2-HEAM forms in the bottom flask and thus the amount of H2O in the vapor phase is higher than before the elevation of the pressure in the column. 4.2. Purity of 2-HEAA. The purity determination of 2HEAA carried out with the proton nuclear magnetic resonance (1H NMR) analysis indicated that a 2-HEAA sample consists mainly of 2-HEAA but also in very small quantities of 2-HEAM. The mole fraction of 2-HEAM is ≤0.01. No water was detected in the 2-HEAA sample due to the limitation of NMR as described in our previous work.3 The water content in 2-HEAA was determined with Karl Fischer titration. The water content in density and VLE samples was ≤0.053 mole fractions of H2O and in SLE samples ≤0.049 mole fractions of H2O. The purity of 2-HEAA was confirmed with the DSC 6000 apparatus, and it is 95.7 mol percent. Alongside the purity determination, the melting point and the enthalpy of fusion of pure 2-HEAA was obtained. The melting point and the enthalpy of fusion of pure 2-HEAA are 340.3 K and 18.08 kJ/ mol, respectively. The melting point of pure 2-HEAA obtained in this work is a bit above the value obtained by Temin.10 According to Temin the melting point of crystallized 2-HEAA was taken after drying under a vacuum on a microscopic hot plate due to the extreme hygroscopicity of 2-HEAA. In this work the melting point of pure 2-HEAA was analyzed with a Pyris Series software as discussed in section 3.1. The results above indicate that the 2-HEAM content in 2HEAA was very small and thus it was reasonable to consider its effect on the measurements negligible. The results also show that the water contents in 2-HEAA obtained with Karl Fischer titration are in line with the results obtained from the DSC 6000 apparatus. Therefore, it can be concluded that the purity of 2-HEAA was determined successfully. 4.3. Density. Densities were measured for the MEA + 2HEAA and HAc + 2-HEAA systems, and the results are tabulated in the Supporting Information and plotted in Figures 4 and 5. No density data were found for the studied systems in

Figure 5. Density of HAc (1) + 2-HEAA (2) as a function of 2-HEAA mass fraction. Lines represent densities calculated with eq 1 and using parameters reported in Table 2: () 293.15, (▲) 303.15, (+) 313.15, (●) 323.15, (×) 333.15, (■) 343.15, (⧫) 353.15, and (○) 363.15 K. Pure HAc points are the model points from DIPPR,14 and pure 2HEAA data are from Penttilä et al.3 The density data of HAc + 2HEAA are from this work.

the literature. The densities were measured at 293 to 363 K at intervals of 10 K in this work. The 2-HEAA mass fraction varied from 0.18 to 0.66 in the case of the MEA + 2-HEAA system. Correspondingly, the 2-HEAA mass fraction ranged from 0.19 to 0.98 in the measurements of the HAc + 2-HEAA system. The densities of both of the systems were modeled with the Redlich−Kister equation. The model parameters are given in Table 2 along with the parameters and equations for pure components. The average absolute deviations for all the models are also given in Table 2. The average absolute deviation is 0.97 for MEA + 2-HEAA, and it is 1.10 for HAc + 2-HEAA. The highest deviation value was obtained for pure HAc, and it is 1.34. 4.4. VLE and SLE Measurements. VLE and SLE data were measured for the H2O + HAc + 2-HEAA system. Experimental data for both of the systems are given in the Supporting Information. In addition, the melting point and the enthalpy of fusion of pure 2-HEAA were measured and they are 340.3 K and 18.08 kJ/mol, respectively. The melting point of pure HAc was taken from DIPPR,14 and it is 289.7 K. Twelve experimental VLE points were measured at 347 K. SLE data were not measured for the whole composition range since there were difficulties to crystallize some of the samples. The sample at 0.55 mol fraction of HAc did not crystallize even though a tiny amount of pure 2-HEAA was used together with liquid nitrogen. The sample at 0.80 mole fraction of HAc did crystallize, but the freezing point of the sample was not reliably observed due to very small air bubbles in the sample. For this reason, it was not easy to separate the air bubbles from the solid particles and to detect the freezing point of the mixture. The VLE and excess enthalpy data for the H2O + HAc system were collected from the literature and are summarized in Table 3. The data were gathered so that it covered large concentration, temperature, and pressure ranges. 224 VLE points and 92 excess enthalpy points were used to identify the NRTL parameters. The parameters and average absolute deviations for each data set are given in Table 4. The average absolute deviation is 0.93 K for the isobaric VLE data, 0.003 kPa for the isothermal VLE data, and 17.70 kJ/mol for the

Figure 4. Density of MEA (1) + 2-HEAA (2) as a function of 2-HEAA mass fraction. Lines represent densities calculated with eq 1 and using parameters reported in Table 2: () 293.15, (▲) 303.15, (+) 313.15, (●) 323.15, (×) 333.15, (■) 343.15, (⧫) 353.15, and (○) 363.15 K. Pure MEA points are the model points from DIPPR,14 and pure 2HEAA data are from Penttilä et al.3 The density data of MEA + 2HEAA are from this work. 19328



Article

5. CONCLUSION The melting point of 2-HEAA is well above room temperature, 338.5 ± 0.5 K.10 Therefore, 2-HEAA was handled as a subcooled liquid instead of a solid. 2-HEAA was easier to handle in a metastable state. Densities of MEA + 2-HEAA and HAc + 2-HEAA were measured as a function of temperature in this work. In addition, the system pressure of 2-HEAA + H2O was measured as a function of temperature. The water content in 2-HEAA was also determined. The VLE and SLE data for the H2O + HAc + 2-HEAA system were measured and modeled with the NRTL activity coefficient model. This work is a continuation of our previous work3 and gives new data that are needed to study the effects of heat stable salts on phase equilibria. The data are especially useful if the chemical solubility of CO2 is investigated in systems that contain 2-HEAA. In addition, the experimental information can be used for designing a reclamation unit.

excess enthalpy data. The parameters for the H2O + 2-HEAA system were obtained from the work of Penttilä et al.,3 and they are listed in Table 4. The binary NRTL interaction parameters of the H2O + HAc and H2O + 2-HEAA systems were used to model the ternary H2O + HAc + 2-HEAA system. The binary parameters for the HAc + 2-HEAA system were regressed against the ternary VLE and SLE data. The NRTL parameters and average absolute deviations are shown in Table 4. The average absolute deviation is 0.18 kPa for the VLE data and 2.82 K for the SLE data. In Figure 6 the total pressure is plotted with the experimental data

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental conditions in 2-HEAA distillation, the density of the MEA + 2-HEAA and HAc + 2-HEAA systems, VLE data measured for the H2O + HAc + 2-HEAA system at 347 K, and SLE data measured for the H2O + HAc + 2-HEAA system at 101.325 kPa. This material is available free of charge via the Internet at http://pubs.acs.org/.

Figure 6. VLE data measured for the H2O + HAc + 2-HEAA system at 347 K: (⧫) measured points, () the model, bubble point curve, and (- - -) the model, dew point curve.

■

as a function of HAc mole fraction. Figure 6 shows that the model describes the experimental points well. The SLE data are expressed along with the model in Figure 7. As can be seen in

AUTHOR INFORMATION

Corresponding Author

*E-mail: anne.penttila@aalto.fi. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors acknowledge Arto Salminen for the help in this work.

Figure 7. SLE data for the H2O + HAc + 2-HEAA system: (◆) measured points and () NRTL activity coefficient model.

Figures 6 and 7, the model describes the VLE data a little bit better than the SLE data. In the parameter regression, the VLE data were emphasized more than the SLE data through the objective function since the VLE data were considered to be more important than the SLE data when modeling CO2 capture processes. The objective function for VLE data is given in eq 7 and for SLE data in eq 9. From Figure 6 it can also be seen that the VLE data for the H2O + HAc + 2-HEAA system shows a negative deviation from the Raoult’s law when the total pressure is plotted as a function of HAc mole fraction.

NOMENCLATURE a0, a1, a2 = parameters in eq 1 aij, bij, aji, bji = parameters in eq 4 H = enthalpy (kJ/mol) IL = ionic liquid obj = objective function P = pressure (kPa) PIL = protic ionic liquid R = gas constant (J/Kmol) T = temperature (K) w = mass fraction x = mole fraction in the liquid phase y = mole fraction in the vapor phase

Greek Letters

α = nonrandomness parameter of the NRTL activity coefficient model γ = activity coefficient ρ = density (kg/m3) τij, τji = parameters in the NRTL activity coefficient model

Superscripts

calc = calculated exp = experimental fus = fusion 19329



Article

(20) Gilmont, R.; Othmer, D. F. Composition of vapors from boiling binary solutions; H2O-AcOH system at atmospheric and subatmospheric pressures. Ind. Eng. Chem. 1944, 36, 1061−1064. (21) Houzelle, C.; Legret, D.; Richon, D.; Renon, H. Vapor-liquid equilibriums of corrosive components using a dynamic method: a new flow apparatus. Fluid Phase Equilib. 1983, 11, 179−185. (22) Ito, T.; Yoshida, F. Vapor-liquid equilibriums of water-lower fatty acid systems. Water-formic acid, water-acetic acid, and waterpropionic acid. J. Chem. Eng. Data 1963, 8, 315−320. (23) Othmer, D. F.; Silvis, S. J.; Spiel, A. Composition of vapors from boiling binary solutions. Pressure equilibrium still for studying wateracetic acid system. Ind. Eng. Chem. 1952, 44, 1864−1872. (24) Calvar, N.; Dominguez, A.; Tojo, J. Vapor-liquid equilibria for the quaternary reactive system ethyl acetate+ethanol+water+acetic acid and some of the constituent binary systems at 101.3 kPa. Fluid Phase Equilib. 2005, 235, 215−222. (25) Conti, J. J.; Othmer, D. F.; Gilmont, R. Composition of vapors from boiling binary solutions; systems containing formic acid, acetic acid, water, and chloroform. J. Chem. Eng. Data 1960, 5, 301−307. (26) Freeman, W. G. M. Experimental results from the design institute for physical property data I. Fluid Phase Equilib. 1985, 81, 14. (27) Haase, R.; Steinmetz, P.; Duecker, K. H. Heats of mixing for the liquid water-acetic acid system. Z. Naturforsch. 1972, A 27, 1526− 1528. (28) Haase, R.; Pehlke, M. Thermodynamic excess functions for the liquid system water + acetic acid from calorimetric data. Z. Naturforsch. 1977, A 32A, 507−510. (29) Sabinin, V. E.; Belousov, V. P.; Morachevskii, A. G. Heat of miscibility and heat of evaporation in an acetic acid-water system. Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 1966, 9, 889−891. (30) Aittamaa, J.; Pokki, J.-P. VLEFIT user manual; 2001.

E = excess property vap = vapor pressure Subscripts

i = component i

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REFERENCES

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Distillable Protic Ionic Liquid 2-(Hydroxy)ethylammonium Acetate (2

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