(180u) Position Group Contribution Method for the Prediction of Vaporization Heat of Organic Compounds

1. Introduction Enthalpy of vaporization is an important parameter that is utilized for the design and operation of vapor?liquid equilibrium-based processes, where enthalpy of vaporization must be known for at least one temperature. It is not always possible, however, to find experimental values of properties for the compounds of interest in the literature. Since, it is not practical to measure them as the need arises, estimation methods are generally employed in this and other similar situations. There exist many empirical correlations that allow one to calculate the enthalpy of vaporization of pure fluids (Carruth and Kobayashi (1972), Svoboda and Smolova, 1994; Liley, 2003; Meyra et al., 2004).1-4 Certainly, the property may also be calculated by means of group contribution models (Basarova and Svoboda, 1995; Tu and Liu, 1996; Li et al., 1997; Fau xndez et al., 2000; Cuadros et al., 2003).5-10 However, most group contribution methods have a serious problem that they cannot distinguish among structural isomers. To overcome these limitations, esveral attemptes have been reported in the literature. Constantinou and Gani,11 Marrero and Gani,12 and Dalmazzone et al.13developed methods which perform estimations at different levels. Recently, Depends on three parameters related to the shape and size of the molecule and to the intermolecular interactions, Mulero et al.14 have proposed a model for the prediction of the enthalpy of vaporization at 273.15K of organic compounds. Recently, our Laboratory proposed a model for the prediction of critical properties of organic compounds. The higher prediction accuracy of the proposed method shown in our previous works suggests that it is possible to use a similar framework to predict the three critical properties, of organic compounds containing various functionalities. 15-19 Therefore, the objective of this work is the application our new model in the prediction of the enthalpy of vaporization of organic compounds at the normal boiling point and 298.15K using a database that covers a variety of chemical classes. 2. Method proposed in this work The standard enthalpies of vaporization function is constructed by all groups' contribution as well as the position correlation factor. The position correlation factor were used to take into account longer distance interactions, which could distinguish most isomer include cis- and trans- or Z- and E- structure of organic compounds for their thermodynamics properties. Here, the position distribution function for the estimation of standard enthalpies of vaporization is expressed as: Parameter or stands for or group contributions, for the number of each group that carbon element forms the centre of the group in the molecular formula, for the number of each group that non-carbon element forms the centre, for total number of groups, for the position correlation factor, and , for parameters of the model. The set of contributions that allowed to minimize the residual estimation difference was then computed by regression. is constant and is molecular weight. According to the IUPAC nominating method, we draw the structures and assign the values for the relevant positional correction. Table 1 reports the values computed for the group contributions . And our method developed is applicable only to comparatively low-molar-mass compounds involving carbon chain from C2 to C18. Conclusion Based on position group contribution method, a new method recently proposed for the estimation of the critical properties and boiling point as well as melting point , is extended to the prediction of standard enthalpies of vaporization. Contributions for compounds containing carbon, hydrogen, oxygen, nitrogen, chlorine and sulphur were reported, and that position distribution function has been developed which could distinguish between the thermodynamic properties of most isomers of organic compounds including cis- and trans- or Z- and E- structures. The results indicate that our model provides highly significances. The overall means absolute difference for enthalpies of vaporization predictions of 223 organic compounds is 2.78%. 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