(191j) Position Group Contribution Method for the Prediction of the Acentric Factor of Organic Compounds

Abstract Recently, authors have proposed a position group contribution method for the prediction of the critical and other thermodynamics parameters of organic compounds with a similar framework. The objective of this work was to develop and evaluate our new position group contribution method for predicting the acentric factor of a variety of pure organic compounds. Comparison results between experimental and calculated data indicate that our model provides very satisfactory results. The overall average absolute errors for the acentric factor prediction of 480 organic compounds is 0.025 and 5.72 % means absolute relative derivation. Comparing with the method of Constantinou, Gani and O'Connell, our method performed better both in accuracy and generality. Also, good prediction of the proposed method shown in our previous works and this work further demonstrates the universality of our proposed method. Keywords: Acentric factor; Prediction; Position distribution function Introduction The acentric factor is a parameter that was originally defined by Pitzer (Pitzer,1955) to improve accuracy of corresponding state correlations for heavier and more complex compounds. Values reported for the acentric factor of pure compounds are calculated based on its definition, which depends on the values of vapor pressure. In addition, since calculation of the acentric factor requires values of critical temperature and pressure, reported values for the acentric factor also depend on the values of critical temperature and pressure used (Reid, Prausnitz. and Poling, 1987). Owing to the lack of the critical temperature and pressure data, the acentric factor cannot be estimated from its definition. Therefore it is vital that prediction methods be developed to obtain the acentric factor data which are capable of reasonably accurate predictions. Lee and Kesler (Lee and Kesler, 1975) proposed a useful estimation technique for the acentric factor based on the normal boiling point, the critical pressure in bar and critical temperature. One can realize that the need for other primary properties significantly limits the applicability and reliability of such techniques, as experimental values of these properties may not be available. The other estimation methods use group contributions (GC) directly. Thus, Hoshino et al. (Hoshino, Naghama, and Hirata, 1982) presented a method for alkanes using only the molecular structure. Han and Peng (Han and Peng, 1993) proposed another group-contribution technique for both hydrocarbons and non-hydrocarbons. Constantinou, Gani and O'Connell (Constantinou, Gani and O'Connell, 1995) extended their second-order GC method to the estimation of the acentric factor for a variety of organic compounds. Recently, wang et al (Wang, Jia, et al. 2008, 2009) proposed a position group contribution method for the prediction of critical and other thermodynamics parameters of organic compounds with a similar framework and the proposed method performed well both in accuracy and generality. Therefore, the purpose of this study was to determine whether our proposed position group contribution method could be used directly for the acentric factor estimation. For this purpose, 480 organic compounds from literature were selected, and the performance of our new model had been compared with the method of Constantinou, Gani and O'Connell. Method proposed in this work Experimental data The sources of experiment data were from DIPPR database (DIPPR, 1996). When all the groups' contribution values have been determined, the recommended 480 experiment data from the literature was used to validate and evaluate the performance of our new method. The position group contributions for the acentric factor The acentric factor function is constructed by all groups' contribution as well as the position distribution factor. The position distribution factor were used to take into account longer distance interactions. The molecule structures were described according to the IUPAC nominating method, and thus, the only position distribution factor values could be obtained for the relevant positional correction factor , which could distinguish all isomers include cis- and trans- or Z- and E- structure of organic compounds for their thermodynamics properties. Here, the position distribution function for estimation is expressed as eq 1 and eq 2. And this expressions is similar in framework with our previous methods used for the prediction of the critical properties of organic compounds containing various functionalities. Comparison method Constantinou, Gani and O'Connell proposed their second-order GC method to the estimation of the acentric factor for a variety of organic compounds (shown as eq(3)). Results and discussion Results of this work indicate that the predicted the acentric factor agree well with the ?experimental results?, which demonstrates that our new position group contribution method for predicting the acentric factor has good overall accuracy. The AAD for the acentric factor prediction of 480 organic compounds is 0.025 and the mean absolute relative derivation is 5.72 %. And the results presented in Table 1 show that our new simple model gives lower deviations and can be used with confidence in thermodynamic and engineering calculations. Comparing with the method of Constantinou, Gani and O'Connell, our method performed better both in accuracy and generality. Table 1. The acentric factor prediction results for various class organic compounds.Conclusion The objective of this work was to develop and evaluate our new position group contribution method for predicting the acentric factor. In this study, contributions for compounds containing carbon, hydrogen, oxygen, nitro¬gen, chlorine and sulphur were reported, and that position distribution function has been developed which could distinguish between the thermodynamic properties of all isomers of organic compounds including cis- and trans- or Z- and E- structures. The results indicate that our model provides very satisfactory results. The overall average absolute difference and the relative derivation for the acentric factor predictions of 480 organic compounds are found to be 0.025 and 5.72 %, respectively. More importantly, the higher prediction accuracy of the proposed method shown in our previous works and this work suggests that it is possible to use a similar framework to predict not only the critical properties, , but also the acentric factor of organic compounds containing various functional groups, which further demonstrated the universality of our proposed method. Acknowledgement. Thanks for the fund support of the National Natural Science Foundation of China (No. 20976131) Literature Cited (1) Pitzer, K.S. The volumetric and thermodynamic properties of fluids, I: Theoretical basis and virial coefficients. J. Am. Chem. Soc. 1955. 77, 107-113 (2) Reid, R.C., Prausnitz, J.M. and Poling, B.E., 1987. The Properties of Gases and Liquids, 4th edn. McGraw-Hill, New York. (3) Lee, B.I. and Kesler, M.G. A generalised thermodynamic correlation based on three-parameter corresponding states. AIChE J. 1975, 17, 1412-1418 (4) Hoshino, D.; Naghama, K.; Hirata, M. Prediction of the acentric factor of alkanes by the group contribution method. J. Chem. Eng. Jpn. 1982, 15, 153-155 (5) Han, B. and Peng, D.Y. A group-contribution correlation for predicting the acentric factors of organic compounds. Can. J. Chem. Eng. 1993, 71, 332-334 (6) Constantinou, L.; Gani, R.; O'Connell, J. P. Estimation of the acentric factor and the liquid molar volume at 298 K using a new group contribution method, Fluid Phase Equilibria. 1995, 103, 11-22 (7) Wang, Q.; Ma, P. Sh.; Jia, Q. Zh.; Xia, Sh. Q. Position Group Contribution Method for the Prediction of Critical Temperatures of Organic Compounds. J. Chem. Eng. Data 2008, 53, 1103-1109 (8) Wang, Q.; Jia, Q. Zh.; Ma, P. Sh. Position Group Contribution Method for the Prediction of Critical Pressure of Organic Compounds. J. Chem. Eng. Data 2008，53，1877-1885 (9) Jia, Q. Zh.; Wang, Q.; Ma, P. Sh. Position Group Contribution Method for the Prediction of Critical Volume of Organic Compounds, J Chem Eng Data 2008, 53, 2606-2612 (10) Wang, Q.; Jia, Q. Zh.; Ma, P. Sh. Xie Sh. K, Chai J. W. Position Group Contribution Method for the Prediction of Vaporization Heat of Organic Compounds, 2009 AIChE Annual Meeting, Nashville, TN. (11) DIPPR, Dedign Institute for Physical Property Data; American Institute of Chemical Engineers, 1996.