Physical properties group contribution methods, 47-59

Group Contribution Methods. It has been shown that many macroscopic physical properties, ie, those derived from experimental measurements of bulk solutions or substances, can be related to specific constituents of individual molecules. These constituents, or functional groups, are usually composed of commonly found combinations of atoms. One procedure for correlating functional groups to a property is as foUows. (/) A set of... 

Critica.1 Properties. Several methods have been developed to estimate critical pressure, temperature, and volume, U). Many other properties can be estimated from these properties. Error propagation can be large for physical property estimations based on critical properties from group contribution methods. Thus sensitivity analyses are recommended. The Ambrose method (185) was found to be more accurate (186) than the Lyderson (187) method, although it is computationally more complex. The Joback and Reid method (188) is only slightly less accurate overall than the Ambrose method, and is more accurate for some specific substances. Other methods of lesser overall accuracy are also available (189,190) (T, (191,192) (T, P ),... 

In many cases, it is possible to replace environmentally hazardous chemicals with more benign species without compromising the technical and economic performance of the process. Examples include alternative solvents, polymers, and refrigerants. Group contribution methods have been conunonly used in predicting physical and chemical properties of synthesized materials. Two main frameworks have... 

In general, procedures for estimating physical and thermodynamic properties and functions can be divided into two categories, namely, group contribution methods and semi-empirical correlations. It is usually difficult, if not impossible, to employ a semi-empirical correlation for predicting the properties of a new material or those of an existing material at a condition different from that under which the available data were obtained. In contrast, the group contribution method, which is based on the assumption that the property of a material is contributed from... 

A group contribution method, specifically, the second-order law, Is adopted In this work for estimating the physical and thermodynamic properties and functions of organic chemicals. 

Further, molecular simulation and computational chemistry have evolved, and are evolving, into important tools for developing better characterization techniques where it is not possible to measure all data. Even so, it is precisely the molecular complexity of petroleum fluids that seems to be an inhibiting factor in the use of these methods for developing better characterization methods. However, identification of important functional groups in petroleum fluids applying molecular simulation and/or computational chemistry for use with group contribution methods to predict thermo-physical properties may be an area for further research. 

Group contribution methods, such as the one discussed above, have been reasonably successful for estimating many physical and thermodynamic properties of pure substances and mixtures, especially when each molecule contains no more than one nonalkyl functional group. These methods dissect a molecule into functional groups that are assumed to be independent of each other. Tliat is, a functional group is assumed to behave the same in its interactions with other functional groups independent of the molecule of which it is... 

Additive (group contribution) methods have a long tradition of successful use in predicting the properties of both ordinary molecules and macromolecules (polymers). They have formed the backbone of the quantitative structure-activity relationships (QSAR) [1,2] used to predict the chemical reactivity and the biological activity of molecules in medicinal and agricultural chemistry. They have also been used extensively in many quantitative structure-property relationships (QSPR) developed for the physical and chemical properties of polymers. 

The determination of the physical and chemical interaction properties of a polymer, or even better, their prediction from the simple knowledge of the polymer structure has been successfully achieved using the concept of molar additivity of the groups forming the polymer molecule by the so-called group contribution methods. These are extensively treated in the book by Van Krevelen and Ni-jenhuis [6], where the prediction of physical properties such as density, T and other physical transitions, heat... 

In addition to matching bulk physical properties as already mentioned, it is also necessary to consider the activity coefficients to insure that the molecular interactions between the solutes and the solvent in the original and the substitute are generally similar. This insures that proposed substitute solvents will likely dissolve the same solutes and have similar effects to those of the original solvent. However, it is important to match only the activity coefBcients of the solutes in the solvents at in te dilution (zero solute concentration), so as not to include solute-solute interactions. The authors matched the activity coefficients at infinite dilution of a representative from six chemical families alcohols, ethers, ketones, water, normal alkanes, and aromatics, i.e., they have matched these activity coefficients in the solvent to be replaced to those in the replacement solvent. The particular components used are ethanol, diethyl ether, acetone, water, normal octane, and benzene. However, one could conceivably use different compounds successfully. Activity coefficients can be estimated from group contribution methods (77). 

Theoretical approaches to molecular structure design require accurate estimates of physical and transport properties. These are derived commonly iiom the principles of thermodynamics and transport phenomena, and using molecular simulations. Since the literature abounds with estimation methods, reference books and handbooks are particularly useful sources. One of the most widely used. Properties of Gases and Liquids (Poling et al., 2001), provides an excellent collection of estimation methods and data for chemical mixtures in the vapor and liquid phases. For polymers. Properties of Polymers (van Krevelen, 1990) provides a collection of group-contribution methods and data for a host of polymer properties. 

The normal boiling point is an easily accessible physical property and has been measured for a large number of substances. In case that it is not available, the normal boiling point can be estimated with group contribution methods, for example, Joback and Reid [4] and Constantinou and Gani [5], analogously to the estimation of the critical point. The estimation formula for the Joback method is... 

Group contribution methods have been applied to the problem of estimating the solubility parameter without physical measurements [110-117,39,118,119]. Small [39] was one of the first to recognize the additive properties of... 

TABLE 1 gives the reported values or ranges of the physical-chemical properties of chlorobenzenes (CBs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Fugacity ratios were obtained from a single estimated entropy of fusion of 56 J mol °K (Yalkowsky 1979), molar volumes were calculated by the Le Bas method, an additive group contribution method (Reid et al. 1977). Total surface area (TSA) values were obtained from Yalkowsky et al. (1979 a,b). Solubilities, vapour pressures and octanol/water partition coefficients (Andren et al. 1986 Shiu and Mackay 1986 Bobra et al. 1985) are also tabulated. Henry s law constants were calculated as PSl/C and the octanol solubility Q as C Kq, . 

Physical properties such as density, viscosity, thermal conductivity, and heat capacity are generally not a serious problem in simulation. The group-contribution methods are reasonably good, and simulator databank include experimental heat capacity data for more than a thousand substances. Although these correlations have random and systematic errors of several percent, this is close enough for most purposes. (However, they are not sufficient when you are paying for a fluid crossing a boundary based on volumetric flowrate.) As noted in Section 13.2.2. one should always be aware of which properties are estimated and which are from experimental measurements. 

Finally, there are models that permit analysis of the fate of chemicals in the atmosphere that might be released from a chemical process. The most popular model is the Mackay Level HI model [1.111. In this model, air, water, soil, and sediment are considered to be separate, well-mixed conpartments in equilibrium with each other. Additional inputs and outputs are allowed to model air flow, river flow, and so on, for certain of these conpartments. The equilibrium between conpartments is modeled using fugacity. Therefore, for an emission into, for exanple, air, by knowing the physical properties of the chemicals involved (calculated from the group contribution method described in Section 25.2T the ultimate fate of all chemicals emitted can be predicted. 

From an application point of view, linear relationships allow for the simple prediction of the physical bulk properties from linear equations (see above). Moreover, they should allow for the prediction of many properties from group contribution methods. For single (pure) ionic liquids, this approach has been impressively shown by Deetlefs et al. [119]. For example, the parachor P, [120, 121] which correlates surface tension a to density p irrespective of temperature, can be obtained experimentally (1) or from a group contribution approach [122]. It can be used to predict either a ox p from existing data collections (M = molar mass) ... 

Parameters describing the effect of individual groups (substructures) on the physical and thermodynamic properties of a chemical compound. Group contribution methods usually determine properties as linear combinations of group contributions. 

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