Ideal/real gases, property data

Significant errors in thermodynamic properties calculated as previously described may arise from the uncertainty in the specific-heat data. These errors may be reduced significantly by using zero-pressure specific heats calculated by the methods of statistical mechanics with spectroscopic data, since ideal-gas properties are generally about an order of magnitude more accurately known than the real-gas properties determined by calorimetric methods. ... 

The data of IVTANTHERMO are used mainly by the branches of industry connected with high temperatures, and by research institutes which deal with calculation of real-gas properties and need data on their prc rties in the ideal-gas state. 

Equilibrium data correlations can be extremely complex, especially when related to non-ideal multicomponent mixtures, and in order to handle such real life complex simulations, a commercial dynamic simulator with access to a physical property data-base often becomes essential. The approach in this text, is based, however, on the basic concepts of ideal behaviour, as expressed by Henry s law for gas absorption, the use of constant relative volatility values for distillation and constant distribution coeficients for solvent extraction. These have the advantage that they normally enable an explicit method of solution and avoid the more cumbersome iterative types of procedure, which would otherwise be required. Simulation examples in which more complex forms of equilibria are employed are STEAM and BUBBLE. 

Tests of this prediction against experimental critical-point data of Table 2.4 reveal large deviations (e.g., an approximately 20% error even in the most favorable case of He) that reflect serious quantitative defects of the Van der Waals description. This is but one of many indications that the Van der Waals equation, although a distinct improvement over the ideal gas equation, is still a significantly flawed representation of real fluid properties. 

As shown in Chap. 6, ideal-gas heat capacities, rather than the actual heat capacities of gases, are used in the evaluation of thermodynamic properties such as internal energy and enthalpy. The reason is that thermodynamic-property evaluation is conveniently accomplished in two steps first, calculation of ideal-gas values from ideal-gas heat capacities second, calculation from PVT data of the differences between real-gas and ideal-gas values. A real gas becomes ideal in the limit as P - 0 if it were to remain ideal when compressed to a finite pressure, its state would remain that of an ideal-gas. Gases in these hypothetical ideal-gas states have properties that reflect their individuality just as do real gases. Ideal-gas heat capacities (designated by Cf and Cy) are therefore different for different gases although functions of temperature, they are independent of pressure. 

Property values in the standard state are denoted by the degree symbol. For example, Cp is the standard-state heat capacity. Since the standard state for gases is the ideal-gas state, Cp for gases is identical with Cp , and the data of Table C.l apply to the standard state for gases. All conditions for a standard state are fixed except temperature, which is always the temperature of the system. Standard-state properties are therefore functions of temperature only. The standard state chosenfor gases is a hypothetical one, for at 1 bar actual gases are not ideal. However, they seldom deviate much from ideality, and in most instances enthalpies for the real-gas state at 1 bar and the ideal-gas state are little different. 

As to liquid mixtures, it is even more difficult to predict the p-V-T properties of liquid mixtures than of real gas mixtures. Probably more experimental data (especially at low temperatures) are available than for gases, but less is lcnown bburth estimation of the p-V T properties of liquid mixtures. For compounds with like molecular structures, such as hydrocarbons of similar molecular weight, called ideal liquids, the density of a liquid mixture can be approximated by assuming that the specific volumes are additive ... 

Of lesser importance are the properties of the pure compounds in the real gas state. These are used to calculate volrrmes needed to interpret flow measurements and to correct ideal- to real-gas enthalpies. Only simple equations of state are required. For pressures up to about one atmosphere, these PVT data can be represented by the second virial coefBcients of the prrre components and their interaction coefficients with methane. 

The conversion of p, V, T data published earlier on to the 1968 International Practical Temperature Scale will require a painstaking review of the whole literature a method of proceeding has been discussed by Angus. In this article we mainly deal with / , K, T data that relate to differences between real- and ideal-gas properties and as these differences are generally not established to an accuracy such that changes in definition of temperatures are relevant, we avoid the problem of revision here. The larger problem of converting measured thermodynamic properties that depend on temperature from IPTS 48 to IPTS 68 has been discussed by Rossini. ... 

The purpose of this chapter, in a book about transport properties, is to give advice to the reader on the best methods for converting the data, which are usually measured as a function of P and T, to a function of p and T, which is the form required for the correlating equations and, in addition, to provide sources for values of the ideal-gas isobaric heat capacities, which are also required for the transport-property calculations. Both of these purposes can be fulfilled by calculations from a single equation of state which has been fitted to the whole thermodynamic surface. Heat capacities of the real fluid are required only for the calculation of the critical enhancement of the thermal conductivity and viscosity, as described in Chapter 6 discussion of these properties in this chapter will be restricted to Section 8.4.4. 

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