7042
Ind. Eng. Chem. Res. 2005, 44, 7042
CORRESPONDENCE Comments on “Performance of a Conductor-Like Screening Model for Real Solvents Model in Comparison to Classical Group Contribution Methods” Andreas Klamt COSMOlogic GmbH&CoKG, Leverkusen, Germany
Sir: Grensemann and Gmehling present in their paper1 a comparison between group contribution models and a reimplementation of the COSMO-RS method originally developed by me in 1995 and further developed since then as described in several papers. While an objective comparison of the group contribution method and COSMO-RS would be very helpful, the presented comparison suffers from a number of biased decisions that clearly favor the group contribution methods. My main concerns in this direction are as follows: (1) The COSMO-RS program used in the comparison is not a validated state-of-the-art implementation that makes use of the improvements since 1995. Instead, a nonvalidated reimplementation of COSMO-RS that does not even exploit the multiple-segment descriptor generalization published in 1998 is used. (2) The authors report some trends such as the strong overestimation of the activity coefficient of ethanol in water that have never been observed in other COSMORS implementations, not even in the simplest versions of COSMO-RS. This raises the question of whether the implementation and parametrization of COSMO-RS used in this paper are representative for the COSMORS method. (3) The selection of compounds for the comparison is made in a way that only compounds that could be described by all four group contribution methods [UNIFAC, modUNIFAC(Do), modUNIFAC(Ly), and ASOG] entered the data set. This restricts the data set to the simplest and best-explored compound classes. There can be no doubt that group contribution methods using a large number of parameters adjusted on these simple compounds will outperform a less-parametrized model such as COSMO-RS in this range of compounds. (4) In addition, systems with very high activity coefficients have been excluded. However, it is wellknown that group contribution methods typically have larger errors for such very nonideal systems, while COSMO-RS tends to be quite robust in this direction. Furthermore, aqueous systems have been excluded, while the γinf evaluation of COSMO-RS by Putnam et al.2 had shown that COSMO-RS tends to be more accurate than UNIFAC on aqueous systems. (5) It is well-known and out of any discussion that COSMO-RS cannot resolve the small excess enthalpies, especially in alkane-alkane mixtures. They are below
the energetic resolution of COSMO-RS, and they result from “shape interactions”, which presently are not taken into account in COSMO-RS. However, the definition of ∆HErel as a relative error (see eq 32) clearly emphasizes large relative deviations on small excess enthalpies. (6) Because the data are taken from the DDB databank exclusively, most likely a big portion of the data used for the comparison have already been used in the fitting of the group interaction parameters of UNIFAC and modUNIFAC(Do). Thus, for this portion of data, UNIFAC did not perform a prediction at all but just reproduced the fit data. Because the data set is not provided as Supporting Information, we have no chance to make a comparison of the performance of more advanced COSMO-RS parametrizations on this data set nor do we have the chance to analyze in more detail how many compounds out of which classes have been used. Thus, for an objective comparison of the methods, I suggest that a group that is not involved in the development of either method selects a reasonably balanced data set of industrial importance and performs calculations with different models. As of the group contribution methods, different COSMO-RS implementations should be taken into account, including the most advanced COSMOtherm implementations. Finally, we mention at least two substantial errors in the description of the COSMO-RS theory. The authors write that “the difference of the screening charge densities σ and σ′ of a contact is a measure of the misfit ...”. However, the sum, not the difference, of the screening charge densities is a measure of the misfit, as is correctly expressed in eq 1. Further, the description of the “dual σ-profile” concept in eqs 22-25 is incomplete because no description of the treatment of cross terms is given and because the treatment of the integration symbols η and η′ is at least inconsistent in eq 25. Literature Cited (1) Grensemann, H.; Gmehling, J. Performance of a ConductorLike Screening Model for Real Solvents Model in Comparison to Classical Group Contribution Methods. Ind. Eng. Chem. Res. 2005, 44, 1610-1624. (2) Putnam, R.; Taylor, R.; Klamt, A.; Eckert, F.; Schiller, M. Prediction of Infinite Dilution Activity Coefficients Using COSMOSRS. Ind. Eng. Chem. Res. 2004, 42, 3635-3641.
IE050347G
10.1021/ie050347g CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
