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VLE of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol)
Fluid Phase Equilibria 303 (2011) 180–183
Contents lists available at ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
VLE of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol) G.V.S.M. Carrera, Z. Visak 1 , R. Bogel-Lukasik 2 , M. Nunes da Ponte ∗ REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 20 October 2010 Received in revised form 18 January 2011 Accepted 24 January 2011 Available online 31 January 2011 Keywords: Carbon dioxide Glycerol Ethanol 1-Propanol 1-Butanol
a b s t r a c t Vapour–liquid equilibrium of CO2 + [0.00871 glycerol + 0.99129 (ethanol or 1-propanol or 1-butanol)] mixtures was measured at the temperatures of 313.15 K and 333.15 K, and close to the critical line, at pressures up to 12 MPa. On the liquid side, the bubble points measured for these ternary mixtures follow closely the behaviour of VLE reported by several authors for the corresponding binary mixtures without glycerol. On the vapour side, however, dew points for the ternary mixtures deviate significantly from VLE results for the binaries. A correlation of the results obtained for the CO2 + glycerol + ethanol mixture with the Peng–Robinson equation of state, admitting quasi-binary behaviour, equally yields good agreement on the liquid side, and significant deviations on the vapour side. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is a by-product of the transesterification of triglycerides in the production of biodiesel. Although it is already being used in medical, pharmaceutical and personal care applications, in the food industry and as a raw material for production of value added compounds [1–3], the large increases in biodiesel demand during the last decade have led to glycerol overproduction. Recent research studies have been proposing new applications for glycerol. One interesting application would combine the need for alternative ways to balance the production and demand of glycerol with finding strategies to capture and store CO2 by direct synthesis of 1,2-glycerol carbonate from glycerol and CO2 . The glycerol carbonate universe of applications is diverse. It may be used as organic solvent [4], as component of gas separation membranes, surfactants, coatings, detergents, as source of new polymeric materials [5] and as fuel component [6]. Several papers describe the synthesis of 1,2-glycerol carbonate from glycerol and CO2 [7–9]. George et al. [9] reported the highest yield obtained so far. The authors use lower alcohols (methanol, ethanol, 1-propanol and 1-butanol) as solvents, indicating that the highest rate of reaction was obtained with methanol.
In a study on dimethyl carbonate formation from methanol and high pressure carbon dioxide, Ballivet-Tkatchenko et al. [10] correlate the reaction mixture phase behaviour and the yield of dimethyl carbonate with monophasic reacting mixtures, leading to higher yields than biphasic ones. This study highlights the importance of knowing the phase behaviour of a system in order to optimize reaction and process conditions. In the case of mixtures of CO2 + glycerol + short chain alcohol, only data on binary mixtures are available in the literature. The phase behaviour of mixtures containing glycerol and another alcohol is presented by several authors in the context of biodiesel production [2,11–13]. The phase behaviour of mixtures of CO2 + alcohol (methanol, ethanol, 1-propanol or 1-butanol) is well documented, as for instance in [14–26]. In this paper, bubble and dew points of ternary mixtures of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol) were measured at 313.15 and 333.15 K, and at pressures relatively close to the critical line. The molar ratio of glycerol to other alcohol was 0.00871, the same as used by George et al. [9]. To the best of our knowledge, this is the first time that the phase behaviour of the ternary system CO2 + glycerol + alcohol is studied.
2. Materials and methods ∗ Corresponding author. Tel.: +351 2129 48353; fax: +351 2141 44187. E-mail addresses: [email protected] (Z. Visak), [email protected] (R. Bogel-Lukasik), [email protected] (M. Nunes da Ponte). 1 Current address: Instituto Superior Tecnico, Centro de Química Estrutural, P1049001 Lisboa, Portugal. 2 Current address: Laboratório Nacional de Energia e Geologia, I.P., Unit of Bioenergy, Estrada do Pac¸o do Lumiar 22, 1649-038 Lisboa, Portugal. 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.01.019
2.1. Experimental apparatus The apparatus used for vapour–liquid equilibrium (VLE) measurements is based on a high-pressure sapphire cell. The apparatus was described in detail elsewhere [27,28], and only a brief description follows herein. The cell, with an internal volume of
G.V.S.M. Carrera et al. / Fluid Phase Equilibria 303 (2011) 180–183
approximately 34.6 cm3 , is a thick-walled sapphire tube placed inside an air bath. The measurement uncertainties are: temperature ± 0.01 K and pressure ± 0.01 MPa. The data points for the ternary (CO2 + glycerol + alcohol) mixtures were obtained visually by the cloud-point method. Each cloud point was determined in accordance with the following procedure: a measured volume (dependent on the targeted overall composition) of a liquid mixture of 0.00871 glycerol + 0.99129 (ethanol or 1-propanol or 1-butanol) was introduced in the cell. CO2 was added from a manually driven screw-injector connected to the bottom of the cell, until one single fluid phase was observed by naked eye. During the experiment, three different scenarios might be envisaged with the addition of CO2 , leading to the occurrence of a cloud point: (1) a progressive disappearance of the liquid phase in the bottom of the cell leads to a single gaseous phase and a dew point is obtained, with a precise composition of glycerol, mono-alcohol, CO2 and an associated pressure; (2) a bubble point is obtained when a homogeneous single liquid phase is reached, after a progressive increment of the volume of the liquid phase from the bottom up, with the addition of CO2 ; (3) occurrence of a critical point that consists in the disappearance of the meniscus between the liquid and gaseous phase and changes of the optical properties of the mixture (change of colour to orange or dark blue). The composition of the cloud point was calculated from the initial quantity of liquid mixture and the quantity of CO2 introduced in the cell. CO2 was added from a screw injector kept at a temperature of 273 K. The variation of volume per turn of the manual handle of this screw injector was previously calibrated. The densities given by the equation of state of Span and Wagner [29] were used to calculate the total amount of carbon dioxide fed to the cell. Errors in these calculations are the main source of uncertainty in the composition, which was estimated to be ±0.0015 mole fraction. 2.2. Materials The chemicals 1-propanol (>99.5%, GC – water ≤ 0.2%, v/v) and glycerol (>99.5%, GC – water ≤ 0.1%, v/v) were obtained from Sigma–Aldrich (Steinheim, Germany), 1-butanol (>99.5%, GC – water ≤ 0.1%, v/v) was obtained from Merck (Darmstadt, Germany), and ethanol (>99.8%, GC – water ≤ 0.02%, v/v) was obtained from Panreac (Barcelona, Spain). CO2 N48 (99.9998% CO2 purity) was supplied by Air Liquide. 3. Results Dew, bubble and (in some cases) critical points of mixtures CO2 + (0.00871 glycerol + 0.99129 alcohol), where alcohol stands for ethanol, 1-propanol or 1-butanol, were obtained at 313.15 and 333.15 K and are given in Tables 1–3. In Fig. 1, cloud point pressures are represented versus carbon dioxide composition (mole fraction), at 333.15 K, for mixtures of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol). The diagram corresponds to a cut at a constant molar ratio of 0.00871 glycerol to 0.99129 short-chain alcohol. It should be noted that the vapour–liquid tie-lines do not lie on the plane of the cut. Therefore the liquid (vapour) mixtures at each bubble (dew) point are in equilibrium with vapour (liquid) mixtures that cannot be represented in the figure. 4. Discussion 4.1. Comparison with binary mixtures CO2 + alcohol The effect of glycerol on the location and shape of the phase equilibrium diagram may be observed by comparison of the
181
Table 1 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + ethanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 ethanol. xCO2 T = 313.15 K 0.9865 0.9793 0.9720 0.9624 0.9492 0.9304 0.9108 0.8211 0.7350 0.4976 T = 333.15 K 0.9838 0.9655 0.9361 0.8806 0.8524 0.7867 0.7329 0.7204 0.6899
xethanol
xglycerol
p/MPa
0.0134 0.0205 0.0278 0.0373 0.0504 0.0690 0.0884 0.1773 0.2627 0.4980
0.0001 0.0002 0.0002 0.0003 0.0004 0.0006 0.0008 0.0016 0.0023 0.0044
7.508 8.108 8.143 8.067 8.084 7.946 7.884 7.746 7.564 6.591
Dew point Dew point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0161 0.0342 0.0633 0.1184 0.1463 0.2114 0.2648 0.2772 0.3074
0.0001 0.0003 0.0006 0.0010 0.0013 0.0019 0.0023 0.0024 0.0027
10.340 10.630 10.760 10.780 10.750 10.670 10.470 10.410 10.220
Dew point Dew point Dew point Critical point Bubble point Bubble point Bubble point Bubble point Bubble point
Table 2 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + 1propanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 1-propanol. xCO2 T = 313.15 K 0.9911 0.9859 0.9824 0.9794 0.9630 0.9272 0.9006 0.8325 0.7695 0.6695 T = 333.15 K 0.9772 0.9527 0.9469 0.9262 0.8752 0.8307 0.7673
x1-propanol
xglycerol
p/MPa
0.0088 0.0140 0.0174 0.0204 0.0367 0.0722 0.0985 0.1660 0.2285 0.3276
0.0001 0.0001 0.0002 0.0002 0.0003 0.0006 0.0009 0.0015 0.0020 0.0029
7.963 8.205 8.219 8.198 8.101 7.970 7.929 7.798 7.626 7.384
Dew point Dew point Critical point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0226 0.0469 0.0526 0.0732 0.1237 0.1678 0.2307
0.0002 0.0004 0.0005 0.0006 0.0011 0.0015 0.0020
10.790 10.940 11.000 11.030 11.050 10.980 10.840
Dew point Dew point Dew point Dew point Bubble point Bubble point Bubble point
Table 3 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + 1-butanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 1-butanol. xCO2 T = 313.15 K 0.9934 0.9697 0.9439 0.9173 0.8683 0.8121 0.7169 T = 333.15 K 0.9846 0.9767 0.9662 0.9417 0.9235 0.8943 0.8364 0.8077 0.7264
x1-butanol
xglycerol
p/MPa
0.0065 0.0300 0.0556 0.0820 0.1305 0.1863 0.2806
0.0001 0.0003 0.0005 0.0007 0.0012 0.0016 0.0025
8.184 8.136 8.088 8.019 7.963 7.826 7.598
Dew point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0153 0.0231 0.0335 0.0578 0.0758 0.1048 0.1622 0.1906 0.2712
0.0001 0.0002 0.0003 0.0005 0.0007 0.0009 0.0014 0.0017 0.0024
10.950 11.220 11.430 11.730 11.740 11.650 11.530 11.490 11.360
Dew point Dew point Dew point Dew point Bubble point Bubble point Bubble point Bubble point Bubble point
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11.0
10.8
p/MPa
10.6
10.4
10.2
10.0
9.8 0.6
0.7
0.8
0.9
1.0
x CO2 Fig. 1. Cloud point pressure versus CO2 mole fraction for CO2 + (0.00871 glycerol + 0.99129 alcohol) mixtures at 333.15 K; open symbols – bubble points, closed symbols – dew points, grey symbol – critical point: , ♦ – ethanol; , – 1-propanol and , – 1-butanol.
pseudo-binary phase diagrams of Fig. 1 with the actual binaries CO2 + alcohol, where glycerol is absent. Comparisons were made, at 313 K and 333 K, with the following published VLE data for mixtures of CO2 + an alcohol: ethanol at 313 K [14–16] and at 333 K [14,15,17,18]; 1-propanol at 313 K [14,19–21] and at 333 K [14,19]; 1-butanol at 313 K [22,23] and at 333 K [24–26]. At 313 K, the cloud point results for the glycerol-containing mixtures are within experimental error of the VLE data for the binary mixtures with the corresponding alcohol. Fig. 2 illustrates this behaviour by comparison with available literature data on CO2 + ethanol. In the case of CO2 + glycerol + 1-butanol at 333 K, only the data of Chen et al. [25] on mixtures of CO2 + 1-butanol extend to pressures close to the critical area. Their equilibrium pressures are higher than the results obtained in this work, and no meaningful conclusions can be drawn. However, at the same temperature for ethanol 8.4
8.2
p/MPa
8
7.8
7.6
7.4
7.2 0.6
0.7
0.8
0.9
1.0
x CO2 Fig. 2. Comparison of phase diagrams at 313 K: – CO2 + glycerol + ethanol, this work; CO2 + ethanol: – [14], x – [15], – [16].
Fig. 3. Comparison of phase diagrams at 333 K: – CO2 + glycerol + ethanol, this work; CO2 + ethanol: – [14], x – [15], – [17], – [18].
and propanol-containing ternary mixtures, the comparisons show that the liquid side behaves differently from the vapour side. The bubble points, that is, the liquid side, continue to display no effect of glycerol, as they lie very close to the bubble points of binary mixtures of carbon dioxide with the corresponding alcohol. But on the vapour side, as seen in Fig. 3, the ternary mixtures with glycerol and ethanol require higher concentrations of carbon dioxide at a given pressure to exhibit a homogeneous single-phase. As the concentration of glycerol in the liquid mixtures is very small, it will scarcely affect the concentration of the CO2 dissolved in the liquid phase. It may be expected then that the composition of bubble points will be similar for ternary (glycerol-containing) and binary mixtures. On the contrary, as glycerol is much less soluble in gaseous carbon dioxide than any of the other alcohols, the first drop of liquid to appear when a dew point is reached should be almost pure glycerol. Even taking into consideration the co-solvent effect of the presence of the other alcohols, the condensation of that liquid drop from the vapour will naturally happen at a higher concentration of CO2 than in a binary mixture without glycerol. It is reasonable to assume that the same may happen at the lower temperature of 313.15 K. However, the critical compositions at this temperature lie at higher concentrations of carbon dioxide than at 333.15 K (0.881 at 333 K and between 0.972 and 0.979 at 313 K for the CO2 + glycerol + ethanol system). Therefore, dew points at 313.15 K only exist in a very narrow range of compositions, and the imprecision of experimental data does not allow the differences to stand out as visibly in Fig. 2 as in Fig. 3. As stated above, the vapour–liquid tie-lines do not lie on the pressure-composition plane of Figs. 1–3. The above commented differences between bubble and dew point compositions suggest that the tie-lines starting at dew points connect to liquid mixtures that are much richer in glycerol than the starting 0.00871 glycerol + 0.99129 alcohol one, while those starting at bubble points connect to vapour phases that are essentially composed of carbon dioxide and the corresponding short chain alcohol. This is a good indication that three-phase equilibrium may appear for these mixtures at higher glycerol concentrations, similarly, for example, to the behaviour of CO2 + water + 1-propanol mixtures described by Adrian et al. [30]. 4.2. Correlation with the Peng–Robinson equation of state The experimental results for (CO2 + 0.00871 glycerol + 0.99129 ethanol) were correlated using the Peng–Robinson equation of
G.V.S.M. Carrera et al. / Fluid Phase Equilibria 303 (2011) 180–183 Table 4 Critical properties and acentric factors for the components of the quasi-binary mixture (CO2 + 0.00871 glycerol + 0.99129 ethanol). Compound
Tc /K
Pc /bar
ω
CO2 Glycerol + ethanol
304.10 523.2
73.80 62.0
0.225 0.644
12.0
Pressure (MPa)
10.0 8.0 6.0 4.0
5. Conclusions The results obtained in this work show that the addition of small amounts of glycerol to mixtures of carbon dioxide + an alcohol has different effects on vapour–liquid equilibrium on the liquid and vapour sides. While the bubble points, on the liquid side, are scarcely affected, dew points are noticeably dragged to higher concentrations in carbon dioxide. The two phase region is therefore enlarged on the vapour side. A correlation of the experimental results with the Peng–Robinson equation of state, using a quasi-binary approach, fits very well the liquid side, but it fails to reproduce this enlargement on the vapour side. Acknowledgments
2.0 0.0
183
0
0.2
0.4
x CO2
0.6
0.8
1
Fig. 4. Pressure-composition phase-equilibrium diagrams for the ternary systems (CO2 + ethanol + glycerol): calculated with Peng–Robinson equation of state at – 333.15 K and 313.15 K. , experimental. This work at 333.15 K. , experimental, this work at 313.15 K.
Table 5 Binary interaction parameters kij , lij , ij estimated for the Mathias–Klotz–Prausnitz mixing rule concerning the ternary system CO2 + ethanol + glycerol at 313.15 and 333.15 K. T/K
kij
lij
ij
313.15 333.15
0.05534 0.14708
−0.02122 0.06807
−0.03272 0.10776
state [31] coupled to the Mathias–Klotz–Prausnitz mixing rule [32]. The correlations were carried out using the program PE developed by Pfohl et al. [33]. As glycerol in the starting mixture is in a considerable lower amount than ethanol, the ternary mixtures were taken as quasi-binary systems. In this case the mixtures were treated as consisting of two “pure” components: glycerol/alcohol and CO2 . The objective of this calculation was to check whether this type of simplified correlation could reproduce the actual phase equilibrium behaviour of the ternary mixture. Tc and Pc were estimated for the “pure” component (0.00871 glycerol + 0.99129 ethanol) by linear extrapolation from the experimental VLE results of Shimoyama et al. [13] for ethanol + glycerol at 493–573 K. The acentric factor ω was the same as for pure ethanol. The resulting parameters are given in Table 4. The experimental and correlated data are compared in Fig. 4 (CO2 + ethanol + glycerol at 313.15 and 333.15 K). The obtained binary interaction parameters are given in Table 5. The standard deviation for the correlations obtained at 313.15 and 333.15 are 0.0365 and 0.0337 in mole fraction, respectively. Although the correlated and experimental results plotted in Fig. 4 seem to be in good agreement, it is apparent from the figure that the correlated quasi-binary mixtures do not reproduce the above-mentioned distinct behaviour of the dew points of the ternary mixture.
Alexandre Paiva is acknowledged for fruitful discussions and advice. This work was supported by Fundac¸ão para a Ciência e Tecnologia, through project ERA-CHEM/0001/2007 and fellowships SFRH/BPD/33986/2009 (GVSMC), SFRH/BPD/33448/2008 (ZV) and SFRH/BPD/34577/2007 (RBL). References [1] Y. Zheng, X. Chen, Y. Shen, Chem. Rev. 108 (2008) 5253–5277. [2] M.B. Oliveira, A.R.R. Teles, A.J. Queimada, J.A.P. Coutinho, Fluid Phase Equilibr. 280 (2009) 22–29. [3] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem. 10 (2008) 13–30. [4] A. Behr, P. Bahke, B. Klinger, M. Becker, J. Mol. Catal. A 267 (2007) 149–156. [5] S.C. Kim, Y.H. Kim, H. Lee, D.Y. Yoon, B.K. Song, J. Mol. Catal. B 49 (2007) 75–78. [6] D. Fabbri, V. Bevoni, M. Notari, F. Rivetti, Fuel 86 (2007) 690–697. [7] C. Vieville, J.W. Yoo, S. Pelet, Z. Mouloungui, Catal. Lett. 56 (1998) 245–247. [8] M. Aresta, A. Dibenedetto, F. Nocito, C. Pastore, J. Mol. Catal. A 257 (2006) 149–153. [9] J. George, Y. Patel, S.M. Pillai, P. Munshi, J. Mol. Catal. A 304 (2009) 1–7. [10] D. Ballivet-Tkatchenko, S. Chambrey, R. Keiski, R. Ligabue, L. Passeraud, P. Richard, H. Turunen, Catal. Today 115 (2006) 80–87. [11] B.B. Franc¸a, F.M. Pinto, F.L.P. Pessoa, A.M.C. Uller, J. Chem. Eng. Data 54 (2009) 2359–2364. [12] H. Zhou, H. Lu, B. Liang, J. Chem. Eng. Data 51 (2006) 1130–1135. [13] Y. Shimoyama, T. Abeta, L. Zhao, Y. Iwai, Fluid Phase Equilibr. 284 (2009) 64–69. [14] K. Suzuki, H. Sue, M. Itou, R.L. Smith, H. Inomata, K. Arai, S. Saito, J. Chem. Eng. Data 35 (1990) 63–66. [15] S.N. Joung, C.W. Yoo, H.Y. Shin, S.Y. Kim, K.-P. Yoo, C.S. Lee, W.S. Huh, Fluid Phase Equilibr. 185 (2001) 219–230. [16] M. Kato, D. Kodama, T. Ono, M. Kokubo, J. Chem. Eng. Data 54 (2009) 2953–2956. [17] S.-D. Yeo, S.-J. Park, J.-W. Kim, J.-C. Kim, J. Chem. Eng. Data 45 (2000) 932–935. [18] C. Secuianu, V. Feroiu, D. Geana, J. Supercrit. Fluids 47 (2008) 109–116. [19] C. Secuianu, V. Feroiu, D. Geana, J. Chem. Eng. Data 53 (2008) 2444–2448. [20] R. Yaginuma, T. Nakajima, H. Tanaka, M. Kato, Fluid Phase Equilibr. 144 (1998) 203–210. [21] L.A. Galicia-Luna, O. Elizalde-Solis, Fluid Phase Equilibr. 296 (2010) 46–52. [22] T. Hiaki, H. Miyagi, T. Tsuji, M. Hongo, J. Supercrit. Fluids 13 (1998) 23–27. [23] O. Elizalde-Solis, L.A. Galicia-Luna, Fluid Phase Equilibr. 296 (2010) 66–71. [24] J.S. Lim, C.H. Yoon, K.-P. Yoo, Korean J. Chem. Eng. 26 (2009) 1754–1758. [25] H.-I. Chen, H.-Y. Chang, P.-H. Chen, J. Chem. Eng. Data 47 (2002) 776–780. [26] R.M.M. Stevens, X.M. Shen, T.W. de Loos, J. de Swaan Arons, J. Supercritical Fluids 11 (1997) 1–14. [27] J.A. Lopes, D. Gourgouillon, P.J. Pereira, A.M. Ramos, M. Nunes da Ponte, J. Supercrit. Fluids 16 (2000) 261–267. [28] P.J. Pereira, M. Goncalves, B. Coto, E.G. de Azevedo, M. Nunes da Ponte, Fluid Phase Equilibr. 91 (1993) 133–143. [29] R. Span, W. Wagner, J. Phys. Chem. Ref. Data 25 (1996) 1509–1596. [30] T. Adrian, S. Oprescu, G. Maurer, Fluid Phase Equilibr. 132 (1997) 187–203. [31] D. Peng, D.B. Robinson, Ind. Eng. Chem. Fundam. 15 (1976) 59–64. [32] P.M. Mathias, H.C. Klotz, J.M. Prausnitz, Fluid Phase Equilibr. 67 (1991) 31–44. [33] O. Pfohl, S. Petkov, G. Brunner, PE V2.9.9a-Software for Phase Equilibria Calculations, Technische Universitat Hamburg-Harburg, Hamburg, 1998.
Contents lists available at ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
VLE of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol) G.V.S.M. Carrera, Z. Visak 1 , R. Bogel-Lukasik 2 , M. Nunes da Ponte ∗ REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 20 October 2010 Received in revised form 18 January 2011 Accepted 24 January 2011 Available online 31 January 2011 Keywords: Carbon dioxide Glycerol Ethanol 1-Propanol 1-Butanol
a b s t r a c t Vapour–liquid equilibrium of CO2 + [0.00871 glycerol + 0.99129 (ethanol or 1-propanol or 1-butanol)] mixtures was measured at the temperatures of 313.15 K and 333.15 K, and close to the critical line, at pressures up to 12 MPa. On the liquid side, the bubble points measured for these ternary mixtures follow closely the behaviour of VLE reported by several authors for the corresponding binary mixtures without glycerol. On the vapour side, however, dew points for the ternary mixtures deviate significantly from VLE results for the binaries. A correlation of the results obtained for the CO2 + glycerol + ethanol mixture with the Peng–Robinson equation of state, admitting quasi-binary behaviour, equally yields good agreement on the liquid side, and significant deviations on the vapour side. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is a by-product of the transesterification of triglycerides in the production of biodiesel. Although it is already being used in medical, pharmaceutical and personal care applications, in the food industry and as a raw material for production of value added compounds [1–3], the large increases in biodiesel demand during the last decade have led to glycerol overproduction. Recent research studies have been proposing new applications for glycerol. One interesting application would combine the need for alternative ways to balance the production and demand of glycerol with finding strategies to capture and store CO2 by direct synthesis of 1,2-glycerol carbonate from glycerol and CO2 . The glycerol carbonate universe of applications is diverse. It may be used as organic solvent [4], as component of gas separation membranes, surfactants, coatings, detergents, as source of new polymeric materials [5] and as fuel component [6]. Several papers describe the synthesis of 1,2-glycerol carbonate from glycerol and CO2 [7–9]. George et al. [9] reported the highest yield obtained so far. The authors use lower alcohols (methanol, ethanol, 1-propanol and 1-butanol) as solvents, indicating that the highest rate of reaction was obtained with methanol.
In a study on dimethyl carbonate formation from methanol and high pressure carbon dioxide, Ballivet-Tkatchenko et al. [10] correlate the reaction mixture phase behaviour and the yield of dimethyl carbonate with monophasic reacting mixtures, leading to higher yields than biphasic ones. This study highlights the importance of knowing the phase behaviour of a system in order to optimize reaction and process conditions. In the case of mixtures of CO2 + glycerol + short chain alcohol, only data on binary mixtures are available in the literature. The phase behaviour of mixtures containing glycerol and another alcohol is presented by several authors in the context of biodiesel production [2,11–13]. The phase behaviour of mixtures of CO2 + alcohol (methanol, ethanol, 1-propanol or 1-butanol) is well documented, as for instance in [14–26]. In this paper, bubble and dew points of ternary mixtures of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol) were measured at 313.15 and 333.15 K, and at pressures relatively close to the critical line. The molar ratio of glycerol to other alcohol was 0.00871, the same as used by George et al. [9]. To the best of our knowledge, this is the first time that the phase behaviour of the ternary system CO2 + glycerol + alcohol is studied.
2. Materials and methods ∗ Corresponding author. Tel.: +351 2129 48353; fax: +351 2141 44187. E-mail addresses: [email protected] (Z. Visak), [email protected] (R. Bogel-Lukasik), [email protected] (M. Nunes da Ponte). 1 Current address: Instituto Superior Tecnico, Centro de Química Estrutural, P1049001 Lisboa, Portugal. 2 Current address: Laboratório Nacional de Energia e Geologia, I.P., Unit of Bioenergy, Estrada do Pac¸o do Lumiar 22, 1649-038 Lisboa, Portugal. 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.01.019
2.1. Experimental apparatus The apparatus used for vapour–liquid equilibrium (VLE) measurements is based on a high-pressure sapphire cell. The apparatus was described in detail elsewhere [27,28], and only a brief description follows herein. The cell, with an internal volume of
G.V.S.M. Carrera et al. / Fluid Phase Equilibria 303 (2011) 180–183
approximately 34.6 cm3 , is a thick-walled sapphire tube placed inside an air bath. The measurement uncertainties are: temperature ± 0.01 K and pressure ± 0.01 MPa. The data points for the ternary (CO2 + glycerol + alcohol) mixtures were obtained visually by the cloud-point method. Each cloud point was determined in accordance with the following procedure: a measured volume (dependent on the targeted overall composition) of a liquid mixture of 0.00871 glycerol + 0.99129 (ethanol or 1-propanol or 1-butanol) was introduced in the cell. CO2 was added from a manually driven screw-injector connected to the bottom of the cell, until one single fluid phase was observed by naked eye. During the experiment, three different scenarios might be envisaged with the addition of CO2 , leading to the occurrence of a cloud point: (1) a progressive disappearance of the liquid phase in the bottom of the cell leads to a single gaseous phase and a dew point is obtained, with a precise composition of glycerol, mono-alcohol, CO2 and an associated pressure; (2) a bubble point is obtained when a homogeneous single liquid phase is reached, after a progressive increment of the volume of the liquid phase from the bottom up, with the addition of CO2 ; (3) occurrence of a critical point that consists in the disappearance of the meniscus between the liquid and gaseous phase and changes of the optical properties of the mixture (change of colour to orange or dark blue). The composition of the cloud point was calculated from the initial quantity of liquid mixture and the quantity of CO2 introduced in the cell. CO2 was added from a screw injector kept at a temperature of 273 K. The variation of volume per turn of the manual handle of this screw injector was previously calibrated. The densities given by the equation of state of Span and Wagner [29] were used to calculate the total amount of carbon dioxide fed to the cell. Errors in these calculations are the main source of uncertainty in the composition, which was estimated to be ±0.0015 mole fraction. 2.2. Materials The chemicals 1-propanol (>99.5%, GC – water ≤ 0.2%, v/v) and glycerol (>99.5%, GC – water ≤ 0.1%, v/v) were obtained from Sigma–Aldrich (Steinheim, Germany), 1-butanol (>99.5%, GC – water ≤ 0.1%, v/v) was obtained from Merck (Darmstadt, Germany), and ethanol (>99.8%, GC – water ≤ 0.02%, v/v) was obtained from Panreac (Barcelona, Spain). CO2 N48 (99.9998% CO2 purity) was supplied by Air Liquide. 3. Results Dew, bubble and (in some cases) critical points of mixtures CO2 + (0.00871 glycerol + 0.99129 alcohol), where alcohol stands for ethanol, 1-propanol or 1-butanol, were obtained at 313.15 and 333.15 K and are given in Tables 1–3. In Fig. 1, cloud point pressures are represented versus carbon dioxide composition (mole fraction), at 333.15 K, for mixtures of CO2 + glycerol + (ethanol or 1-propanol or 1-butanol). The diagram corresponds to a cut at a constant molar ratio of 0.00871 glycerol to 0.99129 short-chain alcohol. It should be noted that the vapour–liquid tie-lines do not lie on the plane of the cut. Therefore the liquid (vapour) mixtures at each bubble (dew) point are in equilibrium with vapour (liquid) mixtures that cannot be represented in the figure. 4. Discussion 4.1. Comparison with binary mixtures CO2 + alcohol The effect of glycerol on the location and shape of the phase equilibrium diagram may be observed by comparison of the
181
Table 1 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + ethanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 ethanol. xCO2 T = 313.15 K 0.9865 0.9793 0.9720 0.9624 0.9492 0.9304 0.9108 0.8211 0.7350 0.4976 T = 333.15 K 0.9838 0.9655 0.9361 0.8806 0.8524 0.7867 0.7329 0.7204 0.6899
xethanol
xglycerol
p/MPa
0.0134 0.0205 0.0278 0.0373 0.0504 0.0690 0.0884 0.1773 0.2627 0.4980
0.0001 0.0002 0.0002 0.0003 0.0004 0.0006 0.0008 0.0016 0.0023 0.0044
7.508 8.108 8.143 8.067 8.084 7.946 7.884 7.746 7.564 6.591
Dew point Dew point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0161 0.0342 0.0633 0.1184 0.1463 0.2114 0.2648 0.2772 0.3074
0.0001 0.0003 0.0006 0.0010 0.0013 0.0019 0.0023 0.0024 0.0027
10.340 10.630 10.760 10.780 10.750 10.670 10.470 10.410 10.220
Dew point Dew point Dew point Critical point Bubble point Bubble point Bubble point Bubble point Bubble point
Table 2 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + 1propanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 1-propanol. xCO2 T = 313.15 K 0.9911 0.9859 0.9824 0.9794 0.9630 0.9272 0.9006 0.8325 0.7695 0.6695 T = 333.15 K 0.9772 0.9527 0.9469 0.9262 0.8752 0.8307 0.7673
x1-propanol
xglycerol
p/MPa
0.0088 0.0140 0.0174 0.0204 0.0367 0.0722 0.0985 0.1660 0.2285 0.3276
0.0001 0.0001 0.0002 0.0002 0.0003 0.0006 0.0009 0.0015 0.0020 0.0029
7.963 8.205 8.219 8.198 8.101 7.970 7.929 7.798 7.626 7.384
Dew point Dew point Critical point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0226 0.0469 0.0526 0.0732 0.1237 0.1678 0.2307
0.0002 0.0004 0.0005 0.0006 0.0011 0.0015 0.0020
10.790 10.940 11.000 11.030 11.050 10.980 10.840
Dew point Dew point Dew point Dew point Bubble point Bubble point Bubble point
Table 3 Cloud point compositions, in mole fraction, for carbon dioxide + glycerol + 1-butanol mixtures, at 313.15 and 333.15 K, and the given pressures. The initial composition of the liquid mixture, before adding CO2 , is 0.00871 glycerol + 0.99129 1-butanol. xCO2 T = 313.15 K 0.9934 0.9697 0.9439 0.9173 0.8683 0.8121 0.7169 T = 333.15 K 0.9846 0.9767 0.9662 0.9417 0.9235 0.8943 0.8364 0.8077 0.7264
x1-butanol
xglycerol
p/MPa
0.0065 0.0300 0.0556 0.0820 0.1305 0.1863 0.2806
0.0001 0.0003 0.0005 0.0007 0.0012 0.0016 0.0025
8.184 8.136 8.088 8.019 7.963 7.826 7.598
Dew point Bubble point Bubble point Bubble point Bubble point Bubble point Bubble point
0.0153 0.0231 0.0335 0.0578 0.0758 0.1048 0.1622 0.1906 0.2712
0.0001 0.0002 0.0003 0.0005 0.0007 0.0009 0.0014 0.0017 0.0024
10.950 11.220 11.430 11.730 11.740 11.650 11.530 11.490 11.360
Dew point Dew point Dew point Dew point Bubble point Bubble point Bubble point Bubble point Bubble point
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11.0
10.8
p/MPa
10.6
10.4
10.2
10.0
9.8 0.6
0.7
0.8
0.9
1.0
x CO2 Fig. 1. Cloud point pressure versus CO2 mole fraction for CO2 + (0.00871 glycerol + 0.99129 alcohol) mixtures at 333.15 K; open symbols – bubble points, closed symbols – dew points, grey symbol – critical point: , ♦ – ethanol; , – 1-propanol and , – 1-butanol.
pseudo-binary phase diagrams of Fig. 1 with the actual binaries CO2 + alcohol, where glycerol is absent. Comparisons were made, at 313 K and 333 K, with the following published VLE data for mixtures of CO2 + an alcohol: ethanol at 313 K [14–16] and at 333 K [14,15,17,18]; 1-propanol at 313 K [14,19–21] and at 333 K [14,19]; 1-butanol at 313 K [22,23] and at 333 K [24–26]. At 313 K, the cloud point results for the glycerol-containing mixtures are within experimental error of the VLE data for the binary mixtures with the corresponding alcohol. Fig. 2 illustrates this behaviour by comparison with available literature data on CO2 + ethanol. In the case of CO2 + glycerol + 1-butanol at 333 K, only the data of Chen et al. [25] on mixtures of CO2 + 1-butanol extend to pressures close to the critical area. Their equilibrium pressures are higher than the results obtained in this work, and no meaningful conclusions can be drawn. However, at the same temperature for ethanol 8.4
8.2
p/MPa
8
7.8
7.6
7.4
7.2 0.6
0.7
0.8
0.9
1.0
x CO2 Fig. 2. Comparison of phase diagrams at 313 K: – CO2 + glycerol + ethanol, this work; CO2 + ethanol: – [14], x – [15], – [16].
Fig. 3. Comparison of phase diagrams at 333 K: – CO2 + glycerol + ethanol, this work; CO2 + ethanol: – [14], x – [15], – [17], – [18].
and propanol-containing ternary mixtures, the comparisons show that the liquid side behaves differently from the vapour side. The bubble points, that is, the liquid side, continue to display no effect of glycerol, as they lie very close to the bubble points of binary mixtures of carbon dioxide with the corresponding alcohol. But on the vapour side, as seen in Fig. 3, the ternary mixtures with glycerol and ethanol require higher concentrations of carbon dioxide at a given pressure to exhibit a homogeneous single-phase. As the concentration of glycerol in the liquid mixtures is very small, it will scarcely affect the concentration of the CO2 dissolved in the liquid phase. It may be expected then that the composition of bubble points will be similar for ternary (glycerol-containing) and binary mixtures. On the contrary, as glycerol is much less soluble in gaseous carbon dioxide than any of the other alcohols, the first drop of liquid to appear when a dew point is reached should be almost pure glycerol. Even taking into consideration the co-solvent effect of the presence of the other alcohols, the condensation of that liquid drop from the vapour will naturally happen at a higher concentration of CO2 than in a binary mixture without glycerol. It is reasonable to assume that the same may happen at the lower temperature of 313.15 K. However, the critical compositions at this temperature lie at higher concentrations of carbon dioxide than at 333.15 K (0.881 at 333 K and between 0.972 and 0.979 at 313 K for the CO2 + glycerol + ethanol system). Therefore, dew points at 313.15 K only exist in a very narrow range of compositions, and the imprecision of experimental data does not allow the differences to stand out as visibly in Fig. 2 as in Fig. 3. As stated above, the vapour–liquid tie-lines do not lie on the pressure-composition plane of Figs. 1–3. The above commented differences between bubble and dew point compositions suggest that the tie-lines starting at dew points connect to liquid mixtures that are much richer in glycerol than the starting 0.00871 glycerol + 0.99129 alcohol one, while those starting at bubble points connect to vapour phases that are essentially composed of carbon dioxide and the corresponding short chain alcohol. This is a good indication that three-phase equilibrium may appear for these mixtures at higher glycerol concentrations, similarly, for example, to the behaviour of CO2 + water + 1-propanol mixtures described by Adrian et al. [30]. 4.2. Correlation with the Peng–Robinson equation of state The experimental results for (CO2 + 0.00871 glycerol + 0.99129 ethanol) were correlated using the Peng–Robinson equation of
G.V.S.M. Carrera et al. / Fluid Phase Equilibria 303 (2011) 180–183 Table 4 Critical properties and acentric factors for the components of the quasi-binary mixture (CO2 + 0.00871 glycerol + 0.99129 ethanol). Compound
Tc /K
Pc /bar
ω
CO2 Glycerol + ethanol
304.10 523.2
73.80 62.0
0.225 0.644
12.0
Pressure (MPa)
10.0 8.0 6.0 4.0
5. Conclusions The results obtained in this work show that the addition of small amounts of glycerol to mixtures of carbon dioxide + an alcohol has different effects on vapour–liquid equilibrium on the liquid and vapour sides. While the bubble points, on the liquid side, are scarcely affected, dew points are noticeably dragged to higher concentrations in carbon dioxide. The two phase region is therefore enlarged on the vapour side. A correlation of the experimental results with the Peng–Robinson equation of state, using a quasi-binary approach, fits very well the liquid side, but it fails to reproduce this enlargement on the vapour side. Acknowledgments
2.0 0.0
183
0
0.2
0.4
x CO2
0.6
0.8
1
Fig. 4. Pressure-composition phase-equilibrium diagrams for the ternary systems (CO2 + ethanol + glycerol): calculated with Peng–Robinson equation of state at – 333.15 K and 313.15 K. , experimental. This work at 333.15 K. , experimental, this work at 313.15 K.
Table 5 Binary interaction parameters kij , lij , ij estimated for the Mathias–Klotz–Prausnitz mixing rule concerning the ternary system CO2 + ethanol + glycerol at 313.15 and 333.15 K. T/K
kij
lij
ij
313.15 333.15
0.05534 0.14708
−0.02122 0.06807
−0.03272 0.10776
state [31] coupled to the Mathias–Klotz–Prausnitz mixing rule [32]. The correlations were carried out using the program PE developed by Pfohl et al. [33]. As glycerol in the starting mixture is in a considerable lower amount than ethanol, the ternary mixtures were taken as quasi-binary systems. In this case the mixtures were treated as consisting of two “pure” components: glycerol/alcohol and CO2 . The objective of this calculation was to check whether this type of simplified correlation could reproduce the actual phase equilibrium behaviour of the ternary mixture. Tc and Pc were estimated for the “pure” component (0.00871 glycerol + 0.99129 ethanol) by linear extrapolation from the experimental VLE results of Shimoyama et al. [13] for ethanol + glycerol at 493–573 K. The acentric factor ω was the same as for pure ethanol. The resulting parameters are given in Table 4. The experimental and correlated data are compared in Fig. 4 (CO2 + ethanol + glycerol at 313.15 and 333.15 K). The obtained binary interaction parameters are given in Table 5. The standard deviation for the correlations obtained at 313.15 and 333.15 are 0.0365 and 0.0337 in mole fraction, respectively. Although the correlated and experimental results plotted in Fig. 4 seem to be in good agreement, it is apparent from the figure that the correlated quasi-binary mixtures do not reproduce the above-mentioned distinct behaviour of the dew points of the ternary mixture.
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