Pure component vapor thermal conductivity

The physical property monitors of ASPEN provide very complete flexibility in computing physical properties. Quite often a user may need to compute a property in one area of a process with high accuracy, which is expensive in computer time, and then compromise the accuracy in another area, in order to save computer time. In ASPEN, the user can do this by specifying the method or "property route", as it is called. The property route is the detailed specification of how to calculate one of the ten major properties for a given vapor, liquid, or solid phase of a pure component or mixture. Properties that can be calculated are enthalpy, entropy, free energy, molar volume, equilibrium ratio, fugacity coefficient, viscosity, thermal conductivity, diffusion coefficient, and thermal conductivity. 

The vapor viscosity, thermal conductivity, and pure component capacities have been taken from the detailed calculations of Austin and Jeffreys (1979). For the purpose of this particular example these properties are assumed to be independent of temperature and composition. This may not be a particularly good assumption in this case since the temperature and composition changes are relatively large and the properties of the pure components also differ quite widely. 

Methods to estimate the thermal conductivity of liquid mixtures have been reviewed by Reid et al. (1977, 1987) and Rowley et al. (1988). Five methods are summarized by Reid et al. (1987), but three of these can be used only for binary mixtures. The two that can be extended to multicomponent mixtures are the Li method (Li 1976), and Rowley s method (Rowley et al. 1988). According to the latter the Li method does not accurately describe ternary behavior. Furthermore, it was indicated that the power law method (Reid et al. 1977 Rowley et al. 1988) successfully characterizes ternary mixture behavior when none of the pure component thermal conductivities differ by more than a factor of 2. But, the power law method should not be used when water is present in the mixture. Rowley s method is based on a local composition concept, and it uses NRTL parameters from vapor-liquid equilibrium data as part of the model. These parameters are available for a number of binary mixtures (Gmehling Onken 1977). When tested for 18 ternary systems, Rowley s method gave an average absolute deviation of 1.86%. 

The pure component databank only stores correlation coefficients for the ideal-gas or zero-density temperature dependency. In the vapor phase, properties are corrected by means of generalized equations. In the case of the thermodynamic properties, the equation of state developed by Lee Kesler (1975) is employed. For the transport properties the correlation of Stiel Thodos (1964a,b) is used for thermal conductivity and that of Jossi et al. (1962) for viscosity. Both transport property corrections employ mechanisms for differentiating between polar and nonpolar streams. 

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