6634
Ind. Eng. Chem. Res. 2002, 41, 6634
CORRESPONDENCE Comments on “Development of a Universal Group Contribution Equation of State. 2. Prediction of Vapor-Liquid Equilibria for Asymmetric Systems” by Jens Ahlers and Ju 1 rgen Gmehling [Ind. Eng. Chem. Res. 2002, 41, 3489-3498] Ilya Polishuk,*,† Jaime Wisniak,† and Hugo Segura‡ Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel, and Department of Chemical Engineering, Universidad de Concepcio´ n, Concepcio´ n, Chile
Sir: The paper of Ahlers and Gmehling proposes a new modification of the UNIFAC-based predictive cubic equation of state model. However, unfortunately, the robustness and reliability of some of the results presented in this paper seem questionable. In particular, Figure 14 in the paper compares the experimental data of Scheidgen1 for the system carbon dioxide (1) + n-pentadecane (2) with those predicted by the original PSRK model,2 Li et al.’s3 modification of the same, and the new equation. Figure 14 of Ahlers and Gmehling clearly demonstrates that the results of a new model proposed by the authors (GCEOS) are in qualitative disagreement with the experimental data of carbon dioxide (1) + npentadecane (2). In particular, Scheidgen1 has studied the phase behavior in these systems with great detail and found that it exhibits phase behavior of type IIIm, according to classification of van Konynenburg and Scott.4 Despite that, Figure 14 shows that the proposed new equation leads to behavior of type IV. Moreover, according to the results of Ahlers and Gmehling, the modification of Li et al.3 predicts an absolute liquid-liquid immiscibility at 316 K and no liquid-liquid immiscibility at the lower temperature of 292 K. Such results are absolutely nonrealistic for the system under consideration. It should be realized that all regions of the thermodynamic phase space are closely interrelated. In other words, one cannot achieve a reliable prediction of phase equilibria by developing the models by way of a local fit of numerous vapor-liquid equilibirum (VLE) data points in restricted pressure and temperature ranges, neglecting the other data, such as liquid-liquid equilibirum (LLE), liquid-liquid-vapor (LLV), and critical lines. Despite this, Ahlers and Gmehling present in their figure only the four VLE experimental points reported by Scheidgen1 at 292 K and neglect the other 16 LLE points, which do not match their predictions. Figure 1 here compares the complete set of experimental data given at 292 K with data predicted by different models. Although all of the models fail to predict LLE quantitatively, the doubless advantage of the theoretically * Corresponding author. E-mail:
[email protected]. † Ben-Gurion University of the Negev. ‡ Universidad de Concepcio´n.
Figure 1. VLE of carbon dioxide (1) + n-pentadecane (2) at 292 K: (circles) the experimental points of Scheidgen1 shown by Ahlers and Gmehling; (triangles) the experimental points of Scheidgen1 not shown by Ahlers and Gmehling; (gray line) the results of GCEOS, as presented by Ahlers and Gmehling; (black solid lines) data predicted by the model of Coutinho et al.;5 (dotted lines) data predicted by PSRK.2
based predictive approach of Coutinho et al.,5 which implements the van der Waals classical mixing rules, appears to be evident. This result contradicts a widely accepted opinion regarding the superiority of the GEbased mixing rules over the classical ones in predicting the data in asymmetric systems. Literature Cited (1) Scheidgen, A. Fluidphasengleichgewichte bina¨rer und terna¨rer Kohlendioxid mischungen mit schwerfluchtigen organischen Substanzen bis 100 MPa. Ph.D. Thesis, University of Bochum, Bochum, Germany, 1997. (2) Holderbaum, T.; Gmehling, J. PSRK: A Group Contribution Equation of State Based on UNIFAC. Fluid Phase Equilib. 1991, 70, 251. (3) Li, J.; Fischer, K.; Gmehling, J. Prediction of Vapor-Liquid Equilibria for Asymmetric Systems at Low and High Pressures with the PSRK Model. Fluid Phase Equilib. 1998, 143, 71. (4) van Konynenburg, P. H.; Scott, R. L. Critical Lines and Phase Equilibria in Binary van der Waals Mixtures. Philos. Trans. R. Soc. London 1980, 298, 495. (5) Coutinho, J. A. P.; Kontogeorgis, G. M.; Stenby, E. H. Binary Interaction Parameters for Nonpolar Systems with Cubic Equations of State: a Theoretical Approach. 1. CO2/Hydrocarbons Using SRK Equation of State. Fluid Phase Equilib. 1994, 102, 251.
IE0205804
10.1021/ie0205804 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
