Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Phase Equilibrium Behavior in Mixtures Containing Tributyl Citrate, Citric Acid, Butan-1-ol, and Water Miguel A. Santaella, Andrea Suaza, Claudia E. Berdugo, Jose L. Rivera, and Alvaro Orjuela* Department of Chemical and Environmental Engineering, Universidad Nacional de Colombia, 111321 Bogotá D.C., Colombia
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S Supporting Information *
ABSTRACT: This work focused on the development of the phase equilibria models required to describe the behavior of mixtures involved in the synthesis of tributyl citrate (TBC) via esterification of citric acid (CA) and butan-1-ol (BuOH). Vapor−liquid equilibrium (VLE) for the mixture TBC−BuOH, liquid−liquid equilibrium (LLE) for the ternary mixture TBC− H2O−BuOH, and solubility data for the mixture CA−BuOH−TBC were measured at different temperatures. The thermodynamic consistency was verified with the Wisniak test for VLE data, and the LLE data exhibited linear behavior in an Othmer and Tobias plot. Anhydrous citric acid was characterized by differential scanning calorimetry exhibiting a melting point of 424.9 K, an enthalpy of fusion of 59.2 kJ/mol, an average heat capacity of 255.48 J/mol·K in the evaluated temperature range (320−375 K), and a change of heat capacity from solid to liquid of 236 J/mol·K. Together with reported equilibrium data from the open literature, and the evaluated physicochemical properties, the measured equilibrium data were regressed with the UNIQUAC equation to fit the binary interaction parameters of the components in the mixture. The obtained model agrees well with the whole set of experimental data and can be used for further process design.
1. INTRODUCTION
From the different commercially available citrates, special interest has been given to acetyl tributyl citrate (ATBC) and its precursor tributyl citrate (TBC). These have been successfully used as phthalate substitutes in the manufacturing of sensible products (i.e., medical devices, food wraps, pharmaceutical containers, toys, etc.). Particularly, TBC is produced via direct esterification of citric acid (CA) with butan-1-ol (BuOH) using acid catalysts. The process requires the initial dissolution of CA in an excess of BuOH, while water (H2O) is formed as the main side product of the reaction. The partial esterification of CA to monobutyl citrate (MBC) and the subsequent step to dibutyl citrate (DBC) are the intermediate steps toward TBC. The overall stoichiometry of reaction is presented in Figure 1. The phase equilibria data for component mixtures involved in the TBC production process are scarce, and there are only few open literature reports mainly for mixtures containing BuOH, CA, and H2O. These include vapor−liquid equilibria (VLE) and liquid−liquid equilibria (LLE) for the binary
In different countries, some petrochemical derived plasticizers, such as certain phthalates, have been banned due to safety concerns over the human health and the ecosystems.1 Consequently, biobased plasticizers, especially citric acid esters, have attracted attention, since they are safer in terms of biodegradability and biocompatibility.2 Nevertheless, the current production processes of citric acid esters encounter a variety of technical challenges that prevent them from gaining a higher market share. These include batch-operating policies, long reaction times, the use of a large excess of alcohol, the use of homogeneous catalysts, etc.; therefore, the required separation process is complex, energy intensive, and costly. In this regard, there is an industrial interest in exploring new technologies and integrated processes to improve the production of citrates. Among the different intensification alternatives, reactive distillation has already been successfully applied in this field.3−5 In order to assess the reactive distillation performance in the production of citrates, reliable thermodynamic models are required for the computer-aided process design. © XXXX American Chemical Society
Received: January 19, 2018 Accepted: August 10, 2018
A
DOI: 10.1021/acs.jced.8b00064 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
BuOH mixtures were carried out using a similar setup to the one proposed by Gibbs and Van Ness.13 This has been successfully used by other authors to obtain reliable VLE data, with slightly modified configurations.14−17 In brief, the VLE experiments were obtained under isothermal conditions using an agitated-jacketed glass cell connected to a high accuracy pressure gauge (MKS Baratron 41B Pressure Switch) adapted to a Digital Power supply and readout (MKS PDR2000). The isothermal conditions were maintained by circulating a temperature-controlled heating fluid through the cell jacket, which was pumped from an isothermal bath. The temperature of the liquid mixture inside the cell was measured with a high accuracy thermo-probe (Precision RTD Thermometer 407907 - EXTECH). The external lines were wrapped with heating tapes that were set just above the experiment temperature to avoid vapor condensation. Separate valves allowed the liquid feed and the exhaust to the vacuum line to be controlled. The configuration of the experimental setup is presented in Figure 2.
Figure 1. Esterification reaction for the production of tributyl citrate.
BuOH−H2O mixtures, including azeotropic conditions.6−8 In the case of BuOH−CA interaction, the solubility of CA in BuOH has been explored at various temperatures.9 For CA− H2O mixtures, the solubility of CA and its effect on the water vapor pressure have been studied.10,11 Regarding TBC, there are few data available but some vapor pressure reports.12 In the case of the intermediate esters (i.e., MBC and DBC), there are no equilibria data available in the open literature, mainly due to the difficulties of separation and isolation, and because they are not commercially available. In this regard, this work focused on obtaining phase equilibrium data for the mixtures involved in the TBC production process, and developing a reliable model for the description of the phase equilibria of the multicomponent mixtures. Literature and collected equilibria data were used to fit a set of binary interaction parameters for the UNIQUAC equation. Specifically, this work was limited to study the phase equilibria of mixtures containing the main components of the overall reaction (Figure 1). The experimental evaluation of phase equilibria involved VLE tests at constant temperature for the TBC−BuOH mixture, LLE for the ternary TBC−H2O− BuOH mixture, solubility data for CA in the BuOH−TBC mixture, and the thermal properties of CA required in the solid−liquid equilibrium (SLE) modeling. Experiments were carried out at different temperatures, and the corresponding thermodynamic consistency tests were applied to the data sets. The binary interaction parameters for the UNIQUAC model were obtained by regression of experimental data, using a differential evolution algorithm programmed in VBA. Finally, the binary parameters for BuOH−H2O, CA−BuOH, CA− H2O, CA−TBC, BuOH−TBC, and H2O−TBC were obtained.
Figure 2. Isothermal glass cell for vapor−liquid equilibrium data determination.
During the VLE experiments, the cell was initially loaded with the more volatile component, and it was maintained under agitation at constant temperature. Once the cell reached the desired temperature (333.2 or 348.2 K), which took ca. 20 min, the vacuum pump was momentarily turned on to degas the cell. Thereafter, the pressure and temperature of the system were allowed to stabilize again for ca. 15 min, and the pressure was recorded. Then, the system was depressurized, and a sample of liquid phase was taken for chromatographic analysis. For further points, the liquid mixture composition was slightly modified by adding TBC. The procedure was repeated for each different concentration of the binary mixture until there was not enough liquid sample in the cell to cover the thermowell. 2.2.2. Liquid−Liquid Equilibrium. The LLE data for the TBC−H2O−BuOH system were obtained by contacting the components within centrifuge tubes at different concentrations under isothermal conditions. Each tube was loaded with ca. 5 g of the mixture, stirred in a vortex, and located in an isothermal bath for 2 or 3 days at 298, 313, or 343 K. After stabilization, samples from each phase were withdrawn using 1 cm3 syringes with 40 mm SS316 needles. Samples were processed through HPLC analysis to determine TBC concentrations and through GC for butan-1-ol quantification in the aqueous phase. The two liquid phases were separated and weighted to verify the mass balance closure. This procedure was used to obtain the binodal curve compositions of the ternary diagram and the
2. MATERIALS AND METHODS 2.1. Materials. The list of chemicals and purities is presented in Table 1. The concentration of purchased Table 1. Characteristics of Chemicals Used during Experimentation chemical name
source
purity (wt %)
CAS number
tributyl citrate citric acid butan-1-ol acetonitrile 1,4-dioxane
Alfa Aesar Sucroal Panreac Panreac, HPLC grade EMSURE
>99 99.9 99.5 99.9 99.5
77-94-1 77-92-9 71-36-3 75-05-8 123-91-1
reactants was confirmed by gas and liquid chromatography, and they were used without further purification. Tributyl citrate, butan-1-ol, citric acid, and distilled water were used for equilibrium experiments. HPLC grade acetonitrile was used in chromatographic analyses. The 1,4-dioxane was used as an internal standard in the gas chromatography analysis for butan1-ol determination. 2.2. Methods. 2.2.1. Vapor−Liquid Equilibrium. VLE data for the binary BuOH−H2O mixtures were obtained from the DDBST database6 and used to fit the corresponding binary interaction parameters. The VLE experiments for the TBC− B
DOI: 10.1021/acs.jced.8b00064 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Figure 3. Calorimetric curves obtained by DSC of a CA sample measured under N2 atmosphere at 101.3 kPa from 298 to 472 K: (a) calorimetry; (b) heat capacity (□, experimental data; , third grade polynomial regression).
Table 2. Thermal Properties of Citric Acid Determined by DSC Analysis and from Reported Literature20,22 (Pressure: 74.66 ± 0.3 kPa) property
units
value
melting point peak (onset point) literature melting point enthalpy of fusion experimental solid specific heat capacity (325−395 K) literature solid specific heat capacity (300 K) solid−liquid heat capacity change
K K kJ/mol J/mol K J/mol K J/mol K
424.9 (±1) 427.15 (±1.5) 59.2 (±0.3a) cpS = 4.31 × 10−5(T3) − 0.0489(T2) + 19.01(T) − 2258.67 (±14.9b; T, K) cpS = 226.5 Δcp = 236.2 (±3b)
a
Standard uncertainty calculated from the instrument. bStandard uncertainty calculated from the standard deviation of the measurement.
standard. A 30 m BP-20 column (0.53 mm i.d., 0.5 μm film thickness) was used for separation which operated with a temperature-programmed analysis as follows: an initial temperature of 323.15 K, a ramp rate of 15 K/min, and a final temperature of 533.15 K (13 min). The injection port was maintained at 543.15 K in a split mode. The detector temperature was kept at 573.15 K. Helium was used as carrier gas (2 μL/min), and hydrogen (40 μL/min) and air (400 μL/ min) were used as combustion gases. Volume injections of 1.0 μL were used. The method was calibrated by triplicate analysis of samples prepared within the range of experimental concentrations. The maximum uncertainty of BuOH concentration was 0.001 in mass fraction. High performance liquid chromatography was used for TBC and CA quantification. Samples were analyzed using a DionexUltiMate 3000 HPLC system equipped with a reversed phase C18 column (Acclaim 120, 3 × 150 nm, 3 μm) operating under isothermal conditions at 313 K. The mobile phase operated with a gradient, using microfiltrated and degasified acetonitrile and ∼2 mmol of sulfuric acid solution in deionized water (pH ∼ 2.1). The technique lasted 28 min, with a programmed gradient of acetonitrile (ACN)/water (pH 2.1) as follows: 0% ACN (t = 0 min), 60% ACN (t = 20 min), 90% ACN (t = 25 min), and 0% ACN (t = 28 min). A diode array detector was used with a wavelength of 210 nm. The method was calibrated by analyzing in triplicate prepared samples within the rage of experimental concentrations. The maximum uncertainty of TBC or CA concentration was 0.001 in mass fraction.
corresponding tie lines. LLE data for H2O−BuOH binary mixtures and H2O−CA−BuOH ternary mixtures were obtained from the open literature.7,8,18 2.2.3. Solid−Liquid Isothermal Equilibrium Method. The SLE data for the CA−BuOH−TBC ternary mixture were obtained similarly to the LLE experiments. Samples of ca. 3g were prepared with an excess of CA and mixed in centrifuge tubes using a vortex stirrer. Once homogenized, the tubes were placed in an isothermal bath for one or 2 days at 296 or 311 K. Then, small aliquots were withdrawn from the liquid phase with a syringe and processed through HPLC analysis for TBC and CA quantification. The solid phase was not analyzed, as it was assumed as pure CA. Solubility data for CA in water and butan-1-ol were obtained from the literature.9,11 To confirm the reliability of the method, the solubility of citric acid in water was measured at 313.15 and 323.15 K, and the citric acid content in the samples was measured by volumetric titration. Differential scanning calorimetry (DSC) was used to measure the CA thermal properties required in the SLE modeling. DSC analysis to determine melting temperature for CA was carried out using a DSC 1 stare system (Mettler Toledo) measuring from 243 to 457 K under a nitrogen atmosphere (50 mL/min) at a heating rate of 10 K/min. The solid specific heat capacity was measured with DSC according to the standard test method ASTM E-1269-05.19 The method consists of an initial isothermal stabilization for 5 min at 298 K followed by a heating ramp from 298 to 398 K. Finally, an isothermal stabilization for 5 min at 398 K was allowed. A previous calibration using a sapphire disc standard was used according with the referenced standard method. The average uncertainty of heat capacity with respect to the calibration was 0.5%. 2.2.4. Chromatographic Analysis. Gas chromatography was used to quantify BuOH in equilibrium samples. The liquid samples were analyzed using a Shimadzu 2010 GC, equipped with an ionization flame detector (FID). The samples were dissolved in acetonitrile, and 1,4-dioxane was used as internal
3. RESULTS AND DISCUSSION The differential scanning calorimetry curve for CA is presented in Figure 3, and the corresponding thermal properties are listed in Table 2. The solid heat capacity dependence with temperature from 305 to 395 K of CA is also presented in Figure 3 with the corresponding third grade polynomial regression (R2 = 0.9999). The heat capacity change between C
DOI: 10.1021/acs.jced.8b00064 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 5. Finally, the SLE data for CA−BuOH−TBC mixtures determined at 298 and 313 K are listed Table 6. As observed from the VLE data, even at high molar concentrations of BuOH (∼40%), the bubble pressure of the TBC−BuOH mixtures was very low. This can be explained from the low temperature of the experiments and the large molecular weight differences between BuOH (74.12 g/mol) and TBC (360.44 g/mol); even at an equimolar condition, the mixture contains more than 82 wt % TBC. This behavior did not allow vapor pressures to be measured at higher TBC concentrations with the implemented method due to the minimum absolute pressure achieved with the available vacuum pump. Regarding the LLE, it was verified that TBC solubility in water is negligible, while some H2O is able to dissolve in TBC. This is caused by the affinity of water with the hydroxyl group in the citrate backbone of TBC. This is important to consider during processing and storage to avoid potential TBC hydrolysis. With respect to the SLE, the solubility of CA in BuOH slightly changed when TBC was present in the liquid phase. The obtained VLE and LLE data were subjected to the corresponding thermodynamic consistency assessment. This was verified by applying the Wisniak test for VLE data23 and the Othmer and Tobias correlation for LLE data.24 The consistency of SLE data was not evaluated. 3.1. VLE Data Thermodynamic Consistency. The classic approach of performing the PAI consistency test25 for the isothermal TBC−BuOH VLE data was not possible. This method requires high quality vapor phase equilibrium data, which in this case were not measured. For this, it would be
solid and liquid was calculated using the software of the equipment. The whole set of experimental data is listed in Table S.1 in the Supporting Information. The obtained physicochemical properties from the calorimetric analysis were in good agreement with previous reports in the literature.20−22 The obtained solubility data of citric acid in water are summarized and compared with literature reports in Table 3. Table 3. Citric Acid Solubility in Water Determined by the Solid−Liquid Equilibrium Method and from the Literature11 (Pressure: 74.66 ± 0.3 kPa)a temperature (K)
citric acid solubility (mol fraction)
literature solubility (mol fraction)
error (%)
313.15 323.15
0.178 0.194
0.170 0.190
4.66% 2.15%
a
Standard uncertainties u are u(T) = 1 K and u(X) = 0.02.
The maximum error with respect to previous measurements was less than 1.7 wt %; however, when reported in a molar basis, and taking into account the large difference in molecular weight between citric acid and butanol, the maximum error is 4.66 mol % as reported. These measurements allowed to verify the reliability of the method used to quantify the solubility of citric acid in the studied solutions. The obtained set of equilibrium data is summarized in Tables 4, 5, and 6. The isothermal VLE data for the TBC− BuOH binary mixture measured at 333.2 and 348.2 K are listed in Table 4. The LLE data for the TBC−BuOH−H2O ternary mixtures obtained at 298, 313, and 343 K are presented in
Table 4. Isothermal Vapor−Liquid Equilibria for BuOH−TBC (Mole Fractions)a T (K)
xTBC
xBuOH
P (kPa)
T (K)
xTBC
xBuOH
P (kPa)
T (K)
xTBC
xBuOH
P (kPa)
333.2
0 0.111 0.112 0.140 0.141 0.183 0.285 0.287 0.295 0.295 0.313 0.313 0.315 0.315 0.343 0.443 0.548 0.556 0.614 0.710 0.740 0.753
1 0.889 0.888 0.860 0.859 0.817 0.715 0.713 0.705 0.705 0.687 0.687 0.685 0.685 0.657 0.557 0.452 0.444 0.386 0.290 0.260 0.247
8.10 7.31 7.31 7.07 7.07 6.75 6.11 6.11 6.29 6.29 6.07 6.07 6.07 6.07 5.77 5.28 4.19 4.19 3.89 3.19 2.71 2.71
348.2
0 0.045 0.047 0.049 0.052 0.052 0.052 0.054 0.056 0.056 0.057 0.058 0.060 0.060 0.062 0.063 0.066 0.068 0.071 0.072 0.074 0.075 0.087 0.094 0.104 0.109
1 0.955 0.953 0.951 0.948 0.948 0.948 0.946 0.944 0.944 0.943 0.942 0.940 0.940 0.938 0.937 0.934 0.932 0.929 0.928 0.926 0.925 0.913 0.906 0.896 0.891
17.50 16.61 16.41 16.35 16.27 16.41 16.51 16.27 16.16 16.24 16.28 16.11 15.97 16.04 15.96 15.91 15.84 15.84 15.84 15.75 15.79 15.83 15.63 15.45 15.31 15.23
348.2
0.118 0.124 0.135 0.142 0.167 0.168 0.472 0.482 0.536 0.538 0.548 0.558 0.581 0.606 0.614 0.618 0.622 0.632 0.709 0.723 0.725 0.749 0.752 0.761 0.793
0.882 0.876 0.865 0.858 0.833 0.832 0.528 0.518 0.464 0.462 0.452 0.442 0.419 0.394 0.386 0.382 0.378 0.368 0.291 0.277 0.275 0.251 0.248 0.239 0.207
15.12 15.12 14.96 14.75 14.41 14.32 9.72 9.55 8.83 8.68 8.6 8.24 7.72 7.44 7.17 7.08 7.31 7.31 5.39 5.13 5.04 5.12 5.12 5.28 5.28
a
Standard uncertainties u are u(T) = 0.1 K, u(P) = 0.3 kPa, and u(X) = 0.05. D
DOI: 10.1021/acs.jced.8b00064 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Table 5. Isothermal Liquid−Liquid Equilibrium Data for TBC−BuOH−H2O at 74.6 ± 0.3 kPa (Mole Fractions)a aqueous phase T (K) 298.2
313.2
343.2
XTBC 0 1.81 0 0 0 0 0 0 0 1.21 0 0 0 0 0 0 0 0 7.97 0 2.56 0 0 0 0 0 1.59 0 0 0 0 0 0 0
× 10−4
× 10−4
× 10−5 × 10−4
× 10−3
XBuOH 0.006 0.014 0.017 0.017 0.016 0.013 0.015 0.017 0.016 0.016 0.011 0.013 0.005 0.000 0.013 0.015 0.015 0.014 0.010 0.013 0.000 0.013 0.016 0.015 0.014 0.009 0.013 0.008 0.007 0.005 0.013 0.013 0.011 0.012
Table 6. Isothermal Solid−Liquid Equilibria for the Mixture BuOH−TBC−CA at 74.6 ± 0.3 kPa (Mole Fractions)a
organic phase XH2O 0.993 0.985 0.983 0.983 0.984 0.987 0.985 0.983 0.984 0.983 0.989 0.987 0.995 1.000 0.987 0.985 0.985 0.986 0.990 0.987 1.000 0.987 0.984 0.985 0.986 0.991 0.985 0.992 0.993 0.995 0.987 0.987 0.989 0.988
XTBC 0.568 0.144 0.029 0.045 0.056 0.202 0.106 0.021 0.065 0.042 0.249 0.185 0.651 0.820 0.122 0.019 0.038 0.064 0.272 0.013 0.810 0.113 0.021 0.040 0.068 0.280 0.226 0.438 0.477 0.465 0.054 0.049 0.088 0.096
XBuOH 0.212 0.473 0.507 0.511 0.480 0.462 0.495 0.504 0.514 0.510 0.418 0.464 0.170 0.000 0.536 0.523 0.552 0.530 0.427 0.543 0.000 0.518 0.581 0.586 0.600 0.445 0.487 0.302 0.181 0.187 0.478 0.461 0.488 0.498
liquid phase XH2O
T (K)
xTBC
xBuOH
xCA
xCA
0.220 0.382 0.464 0.444 0.464 0.335 0.399 0.475 0.422 0.448 0.333 0.352 0.179 0.180 0.342 0.458 0.410 0.407 0.300 0.444 0.190 0.370 0.398 0.374 0.332 0.274 0.287 0.260 0.342 0.348 0.468 0.490 0.424 0.406
298.2
0.028 0.022 0.063 0.060 0.159 0.197 0.352 0.412 0.026 0.031 0.660 0.047 0.172 0.170 0.181 0.255
0.928 0.933 0.888 0.893 0.788 0.751 0.608 0.550 0.931 0.925 0.299 0.901 0.775 0.773 0.752 0.697
0.044 0.045 0.049 0.047 0.053 0.052 0.040 0.038 0.043 0.044 0.042 0.052 0.053 0.058 0.067 0.048
1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000
313.2
a
Wi =
GE − RTw/Δs ΔS
(3)
1
L=
∫0
W=
∫0
Li dx1
(4)
1
Wi dx1
(5)
|L − W |
