High pressure phase behavior of the binary system (ethyl lactate + carbon dioxide)

Accepted Manuscript Title: High pressure phase behaviour of the binary system (ethyl lactate + carbon dioxide) Author: Ana B. Paninho Ana.V.M. Nunes Alexandre Paiva Vesna Najdanovic-Visak PII: DOI: Reference:

S0378-3812(13)00522-0 http://dx.doi.org/doi:10.1016/j.fluid.2013.09.024 FLUID 9772

To appear in:

Fluid Phase Equilibria

Received date: Revised date: Accepted date:

5-7-2013 6-9-2013 11-9-2013

Please cite this article as: A.B. Paninho, Aa.V.M. Nunes, A. Paiva, V. Najdanovic-Visak, High pressure phase behaviour of the binary system (ethyl lactate + carbon dioxide), Fluid Phase Equilibria (2013), http://dx.doi.org/10.1016/j.fluid.2013.09.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High pressure phase behaviour of the binary system (ethyl lactate + carbon dioxide) Ana B. Paninho1, Ana.V.M. Nunes2, Alexandre Paiva2, Vesna Najdanovic-Visak3* Instituto de Biologia Experimental e Tecnológica (IBET), Apartado 12, Oeiras 2781-901,

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1

Portugal.

Requimte/CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia,

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2

Universidade Nova de Lisboa, Campus de Caparica, Caparica 2829-516, Portugal. Energy Lancaster, Engineering Department, Lancaster University, Lancaster LA1 4YR,

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United Kingdom.

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3

*[email protected]

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Abstract

In this work the phase behaviour of the binary system (ethyl lactate+CO2) was studied in the

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pressure range 0.4–17 MPa and at temperatures of 313.2 K, 333.2 K, 353.2 K, 373.2 K and

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393.2 K. Experiments were performed using the static synthetic method in a high pressure

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variable-volume view cell. Equilibrium data were correlated with the Peng–Robinson equation of state combined with the Mathias–Klotz–Prausnitz mixing rule. Additionally, density-based correlations, namely Chrastil’s and Fornari’s equations were used in order to correlate solubilities in both liquid and vapour phases.

Keywords: Density-based correlations; Chrastil; Supercritical; Solubility; Green solvent.

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1. INTRODUCTION Ethyl lactate (ethyl 2-hydroxypropanoate) is a biomass-derived and generally

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recognized as safe (GRAS) solvent. Although known for a long time, it has only become an economical viable alternative to traditional organic solvents, after the introduction of an

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innovative separation step on its production process.[1] This technological intensification, was awarded by the US Environmental Protecting Agency in 1998 and is actually a common

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referred example, used to illustrate most of the 12 Principles of Green Chemistry. [2] Due to extremely low toxicity, ethyl lactate is approved by the US Food and Drug

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Administration as a pharmaceutical and food additive.[1] It is further fully biodegradable and

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easily recyclable; it has low volatility, broad liquid temperature range, and low viscosity.[1] As solvent, it has the unusual capacity of being miscible either with water and e.g. paraffin

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oil, which reveals it hybrid properties and potential to cover a large number of solutes.

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A recent review paper published in 2011 by Pereira et al., [1] highlights the increasing attention devoted by the scientific community to ethyl lactate as an alternative benign solvent.

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Indeed, several scientific studies related with ethyl lactate properties,[3,4] preparation [5-8], process intensification [9-12] and applications [13-27] have been reported. Furthermore, ethyl lactate mixtures with other green solvents, have also been subject of interest, as for example, with alcohols [28,29], water [30] and high pressure CO2 [31-34]. In the latter case, very few specific applications regarding chemical synthesis [31], extraction and particle formation [33], were investigated. Our group has recently reported the successful preparation of the mesoporous gelatine particles using (ethyl lactate)-gel high pressure CO2 extraction [35]. Nevertheless, due to the reported high affinity between CO2 and ethyl lactate, other promising high pressure applications are expected to emerge in the near future.

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Indeed, high pressure CO2-based processes are, in many cases, limited by inadequate solubilities. A classic technique to overcome this limitation, is the use of a small amount (typically 5% to 20%) of a polar co-solvent also called entrainer,[36] or in a different

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approach, expanded liquids (an organic liquid in which CO2 has been dissolved) can also be applied.[37] The exploitation of new solvents, whose mixtures with CO2, allow for different

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physical properties, as viscosity, surface tension, diffusion rates, solubility of reagents, catalysts and substrates, density and polarities, are of crucial importance, as they can boost

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high pressure CO2 applications. In this context, ethyl lactate is a good candidate to modify or to be modified by high pressure CO2, with the additional advantage of being itself a green

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solvent.

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To make beneficial use of dense CO2 mixtures and its specific properties, a comprehensive understanding of the phase behaviour is a necessary prerequisite, especially at

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high pressures and high temperatures. Villanueva et al. [34] reported data on solubility of CO2

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in the ethyl lactate liquid phase at temperatures of 311 K, 318 K, and 323K and pressures ranging from 1 MPa to 8.1 MPa. Earlier, Chylinski and Gregorowicz [31] reported data on

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solubility of ethyl lactate in CO2 at the same temperatures. Moreover, Cho et al.[32] studied high-pressure equilibrium of the binary system (ethyl lactate+CO2) from 323.2 K to 363.2 K. In this work experimental phase behaviour measurements of the binary system (ethyl lactate+CO2) are presented up to 393 K. In order to predict the phase behaviour of the system at hand for conditions other than the experimental, a correlation must be found that accurately represents the phase behaviour. For that purpose phase equilibrium data were correlated with the Peng–Robinson equation of state (PR) [38] combined with the Mathias–Klotz–Prausnitz mixing rule (MKP) [39] using PE Software [40].

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2 . MATERIALS AND METHODS 2.1 Materials The specifications of chemicals used in this work are presented in Table 1. High purity

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carbon dioxide 99.998 mass % was supplied by Air Liquide and was used as received. Ethyl lactate was purchased from Sigma-Aldrich and dried on 3 Å molecular sieves for at least 48 h.

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The water content after drying was determined regularly by Karl-Fischer Coulometric titration

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(Metrohm 831 KF Coulometer) to less than 130 ppm.

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2.2.1 Phase Equilibrium Measurements

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2.2 Methods

Phase equilibrium measurements were performed using a high pressure apparatus (New

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Ways of Analytics GmbH, Germany), described in detail elsewhere. [41] The apparatus is

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composed by a cell, equipped with two sapphire windows positioned at the front and at the back of the cell, allowing the visual observation of phase transitions. The back sapphire acted

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as a piston, moving inside and along the stainless steel cylinder by means of a hydraulic fluid pump, varying the internal volume of the cell (between 38 and 70 mL). The apparatus operates between ambient temperature and 453 K and pressures between atmospheric up to 70 MPa. The temperature control is achieved by means of a PID controller (Eurotherm 2216e), connected to a temperature sensor in direct contact with the fluid mixture inside the cell body, (that measure temperature with an accuracy of 0.1K) and two electrical band heaters. Pressure is measured by an Omega DP41-E230 transducer with an accuracy of 0.05 MPa. Each cloud point was determined using the same procedure as follows. Depending on the desired composition, known amounts of ethyl lactate and CO2 were loaded into the

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equilibrium cell. The addition of CO2 was performed using a manual screw injector and calculated by the variation of volume per rotation as described by Podilla et al. [41] Briefly, the mixture inside the cell was vigorously stirred using a magnetic drive propeller.

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After attaining the desired temperature, the cell pressure was increased by applying pressure on the back sapphire piston with the hydraulic pump. When a single phase was reached, the

until visual observation of a new phase formation.

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system was stirred for more 30 min and then the cell pressure was decreased very slowly, Experimental results were obtained

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through repeated measurements and the uncertainty in pressure values was better than 0.1

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MPa.

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2.2.2 Thermodynamic Modeling

The experimental binary pTxy data were correlated using the Peng-Robinson equation of state

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[37] with the Mathias-Klotz-Prausnitz mixing rule [39],[40]. Fitting the PR – MKP to the

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binary system was made by finding the best set of interaction parameters, kij, lij and ij, which minimized the deviations between the calculated and experimentally determined liquid and

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vapour phase compositions. The objective function used to calculate the deviation between the experimental and the correlated data is: deviation 

2 1 n exp  zi  ziEOS   n i1

(1)

with z stands for liquid and vapour fraction of component i and n for number of data points. The pure component parameters used in the calculations are taken from the literature [42],[43] and presented in Table 2.

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Another approach to correlate the solubility data in supercritical fluid is the density-based correlation proposed by Chrastil [44]. That is a semi-empirical solubility correlation based on assumption that the molecules of the solute (B) associate with the molecules of the gas (C),

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forming solvato-complex: B+kC↔BCk. Equilibrium concentration can be calculated from the mass action law. At the constant temperature, the semi-empirical Chrastil equation has the

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form: ln S s  k1  ln  CO2  A1

(2)

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where Ss is the concentration of the solute in gas (g∙L-1), ρCO2 is the density of gas (kg∙m-3), k1

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(association number) and A1 are parameters.

Similarly, Fornari at al. [45] demonstrated that the supercritical fluid density also defines the

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liquid phase composition of binary systems. The authors successfully correlated the isothermal solubility of various gases (carbon dioxide, methane, and ethane) in different

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liquid phases (alkenes, alcohols, acids, ketones, esters, terpenes and aromatic compounds) as a

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function solely of the pure supercritical fluid density. At the constant temperature, the semiempirical Fornari`s equation has the form:

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ln xCO 2  k 2  ln  CO2  A2

(3)

where xCO2 is a mole fraction of CO2 in liquid phase, ρCO2 is the density of carbon dioxide (kg∙m-3), k2 and A2 are parameters of the model. For both Peng-Robinson equation of state and density-based correlations given by equations (2) and (3), the experimental and fitted results were compared in terms of the absolute average deviations (AAD) of the solubilities: xicalc  xiexp 1   100 AAD%   NP xiexp

where xicalc

(4)

and xiexp are calculated and experimental solubility of either ethyl lactate in

vapour phase (dew point) or carbon dioxide in liquids phase (bubble point).

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3. RESULTS AND DISCUSSION Bubble and dew points for the binary system (ethyl lactate+CO2) at the temperatures of 313.2

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K, 333.2 K, 353.2 K, 373.2 K, 393.2 K and pressures between 0.4 MPa and 17 MPa, are

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presented in Table 3. Furthermore cloud point pressures are represented versus carbon dioxide composition (mole fraction) in Figure 1 and compared with literature data in Figure 2 and 3.

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As expected solubilities of CO2 in ethyl lactate liquid phase as well as the solubility of ethyl lactate in the CO2 vapour phase, were enhanced by pressure increase and by temperature

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decrease. Furthermore, results obtained in this work are in good agreement with data reported using an identical synthetic visual method by Cho et al.[32], to which is possible to directly

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compare experimental data for two temperatures, 313.2 K and 333.2 K (Fig.2). Villanueva et

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al. [34] reported data on the solubility of CO2 in the ethyl lactate liquid phase at 311 K, 318 K and 323 K, and pressures ranging from (1 to 8.1) MPa using different analytical method.

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Although at different temperatures and lower pressures, it is possible to recognize a considerable deviation, which could be due to the difference in the methodology used or due to the pretreatment procedure. In this work, samples of ethyl lactate were dried in order to reduce water content while Villanueva et al. [34] used ethyl lactate as received. It is possible that different water content in ethyl lactate is the cause of discrepancies between data. It is interesting to compare solubility of CO2 in ethyl lactate with solubility of CO2 in similar compounds, such as ethanol and ethyl acetate. This comparison is presented in Figure 4 for solubilities at 313 K, data from this work and from literature [46],[47]. The relative affinity of carbon dioxide follows the order: ethyl acetate > ethyl lactate > ethanol. This difference in solubilities is more pronounced at lower pressures (approximately less than 8 MPa). As

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expected, the highest solubility is observed for ethyl acetate, compound with the lowest polarity. Dashed lines in Figure 1 present correlations of the experimental data using the Peng–

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Robinson equation of state with the Mathias–Klotz–Prausnitz mixing rule using PE Software. The model parameters obtained in the calculations along with the average absolute deviations

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(AAD) are presented in Table 4. The vapour-liquid data were more accurately predicted for the higher temperature and pressure regions of the phase envelope, which can be concluded

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from both Table 4 and Figure 1.

The density-based correlations given by equations (2) and (3) were applied. The obtained

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solubility curves are also shown in Figure 1 (solid line) for comparison with the equation of

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state model applied before (PR-MKP). The model parameters are listed in Table 5, along with the absolute average deviations (AAD) calculated according to equation (3). It should be

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noted that the dew point curve at 393.2 K was not correlated due to a small number of data

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points. As it can be concluded from Table 5, relatively low deviations were observed with the

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maximum value of 2.92 % for bubble point curve at 313.2 K.

4. CONCLUSION

Mutual solubility of the (CO2 + ethyl lactate) system was enhanced by pressure increase and by temperature decrease. Solubility of CO2 in ethyl lactate is higher than in ethanol, but lower than in ethyl acetate. Equilibrium data were correlated with the Peng–Robinson equation of state combined with the Mathias–Klotz–Prausnitz mixing rule and a good correlation of was obtained with an average absolute deviation (AAD) of 5.7 %. Even better deviations were observed when using density-based correlations (maximum value for AAD was 2.92 %), namely Chrastil`s and Fornari`s equation.

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5. ACKNOWLEDGEMENTS This work was supported by Fundação para a Ciência e a Tecnologia – FCT (Portugal)

to

FCT

for

the

post-doctoral

fellowships

SFRH/BPD/74994/2010

and

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thankful

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through project PTDC/EQU-EQU/104552/2008. A.V.M. Nunes and Alexandre Paiva are

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SFRH/BPD/44946/2008, respectively.

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[32] D.W. Cho, M.S. Shin, J. Shin, W. Bae, H. Kim, J. Chem. Eng. Data 56 (2011) 35613566.

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[35] A.B. Paninho, C. Barbosa, I.D. Nogueira, V. Najdanovic-Visak, A.V.M. Nunes, J. Supercrit. Fluids (2013), DOI: http://dx.doi.org/10.1016/j.supflu.2013.08.002, in press. [36] B. C. Y. Lu, D. G. Zhang, and W. Sheng, Pure Appl. Chem. 62 (1990) 2277-2285. [37] P.G. Jessop, B. Subramaniam, Chem. Rev. 107 (2007) 2666-2694. [38] D.Y. Peng, D.B. Robinson, Ind. Eng. Chem. Fundam. 15 (1976) 59–64. [39] P.M. Mathias, H.C. Klotz, J.M. Prausnitz, Fluid Phase Equilib. 67 (1991) 31–44. [40] O. Pfohl, S. Petkov, G. Brunner, Usage of PE—A Program to Calculate Phase Equilibria, TU Hamburg-Harburg, Hamburg, 1998.

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[41] S. Podila, L. Plasseraud, H. Cattey, D. Ballivet-Tkatchenko, G.V.S.M. Carrera, M. Nunes da Ponte, S. Neuberg, A. Behr, Indian J. Chem. Sect A 51 (2012) 1330-1338. [42] R. L. Rowley, W. V. Wilding, J. L. Oscarson, Y. Yang, N. F. Giles, DIPPR® Data

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Compilation of Pure Chemical Properties, Design Institute for Physical Properties, AIChE, New York, 2010.

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[43] S. Angus, B. Armstrong, K.M. Reuck, IUPAC—International Thermodynamics Tables

[44] J. Chrastil, J. Phys. Chem. 86 (1982) 3016-3021.

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of the Fluid State Carbon Dioxide, Pergamon Press, Oxford, UK, 1976.

[45] T. Fornari, E.J. Hernández, G. Reglero, J. Supercrit. Fluids 51 (2009), 115–122.

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[46] D.W. Jennings, R.-J. Lee, A.S. Teja, J. Chem. Eng. Data 36 (1991), 303-307.

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[47] G.R. Borges, A. Junges, E. Franceschi, F.C. Corazza, M.L. Corazza, J.V. Oliveira, C.

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Dariva, J. Chem. Eng. Data. 52 (2007), 1437.

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Table 1. Specifications of chemicals used in this work. Chemical name

Certified purity 98.0 mass% 99.998 mass%

Purification method Molecular sieves None

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Ethyl lactate CO2

Source Sigma Air Liquide

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Table 2. Pure component parameters. MW - molecular weight; Tb - boiling point; Tc - critical temperature; pc - critical pressure;  - acentric factor. Tb (K) 426 195

Tc (K) 607 304

pc (MPa) 3.7 7.4

 0.362 0.255

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Ethyl lactate [42] CO2 [43]

MW (g∙mol-1) 118 44

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Component

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d

353.2

373.2

393.2

cr

Phase transitionb BP BP BP BP BP BP BP BP BP DP DP DP DP BP BP BP BP BP BP BP CP DP DP DP DP DP BP BP BP BP BP BP BP DP DP DP DP BP BP BP BP BP BP CP DP DP DP DP BP BP BP BP BP BP BP DP DP DP

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xCO2 0.4098 0.4595 0.5331 0.7049 0.7942 0.9520 0.9637 0.9744 0.9861 0.9937 0.9949 0.9955 0.9965 0.4098 0.5331 0.5659 0.6153 0.6881 0.7942 0.9520 0.9637 0.9744 0.9861 0.9901 0.9934 0.9951 0.4098 0.4595 0.5331 0.6153 0.6885 0.7942 0.9520 0.9637 0.9744 0.9861 0.9901 0.4098 0.4595 0.5331 0.6153 0.7049 0.7942 0.9520 0.9637 0.9744 0.9861 0.9901 0.4098 0.4595 0.5331 0.6153 0.6277 0.7049 0.7942 0.9520 0.9861 0.9744

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333.2

p / MPa 3.61 4.18 4.98 6.38 6.81 7.80 7.86 8.04 8.10 8.17 8.04 7.97 7.79 4.55 6.35 6.83 7.42 8.34 9.61 10.93 10.81 10.90 10.68 10.46 8.92 7.38 6.10 6.83 8.21 9.53 10.67 12.48 13.59 13.39 12.15 12.06 10.83 7.38 8.26 9.91 11.62 13.88 14.55 15.45 14.55 12.00 12.34 11.59 7.91 9.72 11.10 13.50 13.62 15.95 16.86 15.86 12.62 12.52

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T/K 313.2

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Table 3. Experimental high pressure phase equilibrium of the binary system CO2 (1) + ethyl lactate (2) expressed in mole fraction of carbon dioxide (xCO2).a

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a b

Estimated uncertainties u are u(T) = 0.1 K, u(p) = 0.07 MPa, u(xCO2) = 0.005 BP - bubble point; CP - estimated critical point; DP - dew point.

Table 4. Parameters of PR-MKP fitted to the liquid and vapour phase compositions obtained from the solubility of carbon dioxide in vapour phase (dew point) and solubility of carbon dioxide in liquid phase (bubble point) at different temperatures. 333.2

353.2

373.2

393.2

-0.2131 -0.2084 -0.2864 6.1

-0.2360 -0.1618 -0.3157 14.4

0.3296 0.2037 0.4103 2.0

0.5510 0.3775 0.7501 3.6

0.5782 0.4317 0.7891 2.5

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kij lij ij AAD (%)

313.2

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T/K

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Table 5. Parameters of density-based correlations fitted to the solubility of carbon dioxide in vapour phase – dew point (Equation (2)) and solubility of carbon dioxide in liquid phase – bubble point (Equation (3)) at different temperatures.

0.6178 -3.5019 2.92

k1 A1 AAD (%)

4.7883 -32.602 0.49

393.2 0.7013 -4.3141 -

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k2 A2 AAD (%)

333.2 353.2 373.2 Equation (3) – bubble points 0.6566 0.6773 0.679 -3.849 -4.089 -4.1787 2.75 1.72 1.61 Equation (2) – dew points 2.853 4.8447 4.2977 -20.868 -31.794 -28.185 1.23 2.20 2.20

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313.2

cr

T/K

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Captions to Figures:

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Figure 1. Experimental vapour-liquid equilibrium data determined in this work at 313.2 K (◊), 333.2 K(○), 353.2 K(Δ),373.2 K (□) and 393.15 K (+) for the binary system ethyl lactate + CO2. Solid lines present correlations obtained by density-based equations – Chartil’s Eq. (2) and Fornari’s Eq. (3). Dashed lines stand for the correlation obtained by the Peng–Robinson equation of state combined with the Mathias–Klotz–Prausnitz mixing rule.

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Figure 2. Comparison of vapour-liquid equilibrium data obtained in this work at: 313.2 K (◊), 333.2 K(○), 353.2 K(Δ),373.2 K (□) and 393.15 K (+), and data reported by Cho et al.[32] at: 323.2 K (*), 333.2 K(●), 343.2 K (- ), 353.2 K (▲), 363.2 K (× ).

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Figure 3. Comparison of vapour-liquid equilibrium data obtained in this work at: 313.2 K (◊), 333.2 K(○), 353.2 K(Δ),373.2 K ( □) and 393.15 K (+), and data reported by Villaneuva et al. [34] at: 311.2 K ( ×), 318.2 K(* ), 323.2 K (- ).

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Figure 4. Comparison of solubility of carbon dioxide in ethyl lactate from this work (), in ethanol [43] (●), and in ethyl acetate [44] () at 313.2 K.

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