Comment/Reply pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Comment on “Determination and Correlation of Liquid−Liquid Equilibrium Data for the Ternary Dichloromethane + Water + N,N‑Dimethylacetamide System” Antonio Marcilla, María del Mar Olaya,* and Juan Reyes-Labarta Chemical Engineering Department, University of Alicante, Apdo. 99, 03080 Alicante, Spain
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3
s part of a study about the consistency of the model parameters obtained as a result of liquid−liquid equilibrium (LLE) data correlations of ternary systems, we have analyzed the paper by Wang et al.1 First, we focused on the nonrandom two liquid (NRTL) model parameters reported by the authors but after detecting some serious inconsistencies, the experimental procedure and the data reported in it were also reviewed resulting in some problematic aspects that are discussed in the present letter. Regarding the correlation procedure, the following objective function is the one used by the authors: n
F=
ln γi =
∑ j = 1 τijGijXj 3
∑k = 1 GkiXk
3
+
∑ j=1
3 ⎡ ∑ Xτ G ⎤ ⎢τ − l = 1 l lj lj ⎥ ij 3 3 ∑k = 1 GkjXk ⎢⎣ ∑k = 1 GkjXk ⎥⎦
XjGij
(2)
while the original NRTL equation, according to the original paper by Renon and Praustnitz5 is 3
ln γi =
∑ j = 1 τjiGjixj 3
∑k = 1 Gkixk
3
+
∑ j=1
3 ⎡ ∑ xτ G ⎤ ⎢τ − l = 1 l lj lj ⎥ ij 3 3 ∑k = 1 Gkjxk ⎢⎣ ∑k = 1 Gkjxk ⎥⎦
XjGij
(3)
3
∑ ∑ (XEij − XCij)2
The difference is the permutation of the subindexes in the numerator of the first term at the right-hand side of the equation resulting in the equation not being that accepted as the NRTL equation; it is similar but not identical. Because we cannot know if this is only a typographic error or if conversely calculations have been performed with this mistake in the equation, we have also checked the reported NRTL parameters using the incorrect eq 2 and the GM/RT surface obtained is similar to the one presented in Figure 1a). Consequently, the mistake in the NRTL model is not the reason that could justify the inconsistent results presented in the paper by Wang et al.1 In this specific case, the GM/RT surface is almost identical with the permutation cited due to the symmetry that it presents, but for many other systems the use of eqs 2 and 3 could lead to very different results. The authors state that LLE data for this ternary system has not been reported to date, so they do not compare their results with the data in the literature. The authors also report the mutual solubility of the dichloromethane (1) and water (2) in their Table 1, at atmospheric pressure as mass fractions: w1,F1 = 0.1, w2,F1 = 0.9, and w1,F2 = 0.946, w2,F1 = 0.054 at 298.15 K and w1,F1 = 0.092, w2,F1 = 0.908, and w1,F2 = 0.889, w2,F2 = 0.111, at 308.15 K. Nevertheless, the experimental LLE data for this binary system have indeed been previously studied and the result of this comparison shows an important inconsistency. For instance, in the DECHEMA Chemistry Data Series vol. V, Liquid−Liquid Equilibrium Data Collection,6 the solubilities in mole fraction reported at 298.15 K are x1,F1 = 0.00417 and x2,F2 = 0.00784 (corresponding to w1,F1 = 0.01937, w2,F1 = 0.98062, and w2,F1 = 0.99833, w2,F2 = 0.00167, in mass fractions). The differences are dramatic; however, no comment has been included in the paper. Comparison at 303.15 K shows equivalent results being the solubilities previously published are the
(1)
i=1 j=1
where XEij represents the mole fraction according to the experiments, and XCij represents the mole fraction based on calculations, and n means the number of data points. No mention is made in the paper of the required condition of isoactivity of the two liquid phases in equilibrium, and what is more important is that it seems like they have no use for it at all. This affirmation is based on the analysis of the GM/RT surface (as a function of the composition) obtained using the NRTL parameters reported by the authors as a result of their correlations at 298 and 303.15 K. For example, in Figure 1a the GM/RT surface has been represented at 298 K showing completely miscible behavior of the mixture in all the composition space at that temperature (similar results are obtained for 303.15 K). The stability analysis2−4 states that this surface should exhibit a shape as that in Figure 1b) where the conjugated points with a common tangent line or surface, for the binary or ternary mixtures, respectively, must exist in order to allow the liquid−liquid phase splitting. In other words, the GM/RT surface is convex in all the composition space while concavity changes are required for the existence of LLE according to the Gibbs stability condition. We have calculated the activities of the three components in the conjugated liquid phases using the calculated ELL compositions reported by Wang et al.1 together with the NRTL parameters obtained by them at 298 K. Figure 2 shows these results in which the activity of each component in one phase is plotted versus the activity of the same component in the conjugated phase. Obviously, these data should lie on the diagonal (that we have also plotted as reference) to fulfill the isoactivity LLE condition. It can be observed that the activities are far from being equal as required by the system exhibiting phase splitting, and consequently they do not represent anything related to the equilibrium of this system. Moreover, the authors indicate that they have used the following expression for the NRTL activity coefficient © XXXX American Chemical Society
Received: June 9, 2016 Accepted: March 8, 2018
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DOI: 10.1021/acs.jced.6b00473 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Comment/Reply
Figure 1. Gibbs energy surface (GM/RT) versus composition for the dichloromethane + water + N,N-dimethylacetamide ternary system at 298 K obtained with the NRTL model: (a) parameters obtained by Wang et al.1 (no LL splitting) and (b) parameters consistent with the experimental LLE data.
following: x1,F1 = 0.00416 x2,F2 = 0.00917 in molar fractions (corresponding to w1,F1= 0.01933, w2,F1 = 0.98067, and w2,F1 = 0.99804, w2,F2 = 0.001958, in mass fractions). These discrepancies in the binary data pose serious doubts on the validity of the binary and ternary data reported by Wang et al.1 In the experimental section, the authors indicate that they have measured the weight percentage of the three components by GC independently and also that they have run triplicates of each analysis. The data shown in the corresponding tables exactly match to the fourth significant figures the mass balance in each phase. This is very unlikely to occur except when the composition of one component is obtained by difference or normalizing the compositions by dividing each one by the sum of the three or using any other normalization procedure. None of these alternatives or the deviation of the three analyses is mentioned in the paper. In conclusion, the present paper presents serious inconsistencies both in the experimental part and in the fitting of the data with the NRTL model. With regard to the fitting results, it is quite surprising that the inconsistency is not due to metastable solutions or any other more likely cause, but simply
Figure 2. Representation of the activities for the three components in the two liquid phases in equilibrium for the dichloromethane + water + N,N-dimethylacetamide ternary system at 298 K according to the results obtained by Wang et al.1 B
DOI: 10.1021/acs.jced.6b00473 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Comment/Reply
because the parameters obtained as a result of the experimental tie-lines correlation do not show any possibility of LLE splitting at any composition.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
María del Mar Olaya: 0000-0001-8068-9562 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wang, C.; Liu, X.; Wang, P.; Yang, P. Determination and correlation of liquid-liquid equilibrium data for the ternary dichloromethane + water + N,N-dimethylacetamide system. J. Chem. Eng. Data 2014, 59, 1733−1736. (2) Wasylkiewicz, S. K.; Sridhar, L. N.; Doherty, M. F.; Malone, M. F. Global stability analysis and calculation of liquid-liquid equilibrium in multicomponent mixtures. Ind. Eng. Chem. Res. 1996, 35, 1395−1408. (3) Iglesias-Silva, G. A.; Bonilla-Petriciolet, A.; Eubank, P. T.; Holste, J. C.; Hall, K. R. An algebraic method that includes Gibbs minimization for performing phase equilibrium calculations for any number of components or phases. Fluid Phase Equilib. 2003, 210, 229−245. (4) Reyes-Labarta, J. A.; Olaya, M. M.; Velasco, R.; Serrano, M. D.; Marcilla, A. Correlation of the liquid-liquid equilibrium data for specific ternary systems with one or two partially miscible binary subsystems. Fluid Phase Equilib. 2009, 278, 9−14. (5) Renon, H.; Praustnitz, J. M. Local compositions in thermodynamic excess functions for liquid mixtures. AIChE J. 1968, 14, 135. (6) Arlt, W.; Macedo, M. E. A.; Rasmussen, P.; Sorensen, J. M. Liquid-Liquid Equilibrium Data Collection, Binary Systems. DECHEMA Chemistry Data Series; DECHEMA, 1979; Vol. V, Part 1.
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DOI: 10.1021/acs.jced.6b00473 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
