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Nonclassical Gasdynamics of Vapour Mixtures No fundamental and general theory on the interaction of molecules of different type exists yet, therefore no accurate model is available. For these reasons the estimation of mixture properties is affected in general by larger uncertainties, if compared to the estimation of pure-fluid properties. Mixtures of simple molecules, e.g., light gases and simple hydrocarbons, can be modelled with relatively high accuracy, and reference equations of state have been developed [66]. These semi- empirical models rely on large sets of accurate fluid property measurements. Unfortunately, accu- rate property measurements of complex organic compounds are not available. In order to estimate dense-vapour thermodynamic properties of mixtures of complex organic fluids, simpler so-called predictive equations of state must be adopted, see, e.g., Refs. [67, 68]. These models rely on a small set of data related to the pure constituents, and to parameters describing the interaction between dif- ferent molecules; these parameters can be determined either experimentally or estimated. Predictive models applicable to mixtures are thermodynamically consistent, but calculated property values are affected by much larger uncertainties if compared to the estimation of pure-fluid properties. In this study the properties of mixtures of siloxanes and perfluorocarbons are evaluated with either the improved Peng-Robinson Stryjek-Vera cubic equation of state [69], complemented by the Wong-Sandler mixing rules (iPRSV-WS), see Ref. [70, 71], or the PC-SAFT model [72], which is formulated in terms of molecular parameters whose value depends on the molecular arrangement. Since most of the treatment in this chapter is based on the use of the iPRSV-WS model, both the functional form of the equation of state and the derivation of the adopted mixing-rules are recalled in Appendix A.1. These models, together with others, are implemented in an in-house computer library for the calculation of primary and secondary thermodynamic properties of fluids [73]. Information on how the data for the iPRSV-WS model applied to siloxane mixtures were ob- tained can be found in Ref. [55]. The analytical expression of Γ for this thermodynamic model is reported in Ref. [61]. The application of the PC-SAFT model to linear siloxanes is documented in Ref. [74], and has been extended by the authors to model also cyclic siloxanes. Siloxane/perfluoro- cabon mixtures are modelled with the iPRSV-WS equation of state starting from experimental val- ues of the critical point of these mixtures [75], and compared to results from the PC-SAFT model. Such a comparison is the only possible assessment at the moment, since no other experimental values are available. Values of Γ for the PC-SAFT mixture model are calculated with analytical ex- pressions obtained by derivation from the equation of state and the isobaric ideal-gas heat capacity relation, see for example Ref. [48] and [21]. Figure 8.2 shows a comparison between the values of Γ calculated along the dew line for the equimolar mixture of propane and pentane using a reference model [66], the iPRSV-WS and the PC-SAFT models. As it is known, see Refs. [76, 77], predictive models fail to accurately estimate properties close to the vapour-liquid critical point, therefore also Γ values at high reduced temper- ature T ̃ = T/Tc deviate from those obtained with the reference model, cf. figure 8.2. A number of evaluations for various fluids modelled by the reference model presented in Ref. [66] revealed that the iPRSV-WS model performs better than the PC-SAFT model in the critical-point region, therefore it has been chosen for the analysis presented in §8.3. Figure 8.3 shows the negative-Γ region (also termed BZT region) in the P-T thermodynamic plane for several selected organic compounds of the family of siloxanes, cloro-fluorocarbons, per- fluorocarbons and their mixtures, calculated with the iPRSV-WS model. The ensemble of fluid thermodynamic states featuring a negative value of Γ in the dense vapour phase is delimited by the dew line on the left and by the concave-upward Γ = 0 line on the right. An estimate of the temperature at which thermal break-down in stainless steel is likely to occur is also indicated (TSL, Thermal Stability Limit). As an example, in table 8.1, the molar fraction x, average molecular weight MW, critical pres- 207PDF Image | New Concepts FOR Organic Rankine Cycle Power Systems
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