Nonideal gaseous mixtures

In this section we discuss the ideal gas equation of state and show how it is applied to systems containing single gaseous substances and mixtures of gases. Section 5,3 outlines methods used for a single nonideal gas (by definition, a gas for which the ideal gas equation of state does not work well) and for mixtures of nonideal gases. 

In the present example involving a mixture (part d), ideal behavior was assumed. For handling nonideal gaseous mixtures, volumetric data are required, preferably in the form of an equation of state at the temperature under consideration and as a function of composition and density, from zero density (lower integration limit) to the density of interest. These computations often require trial-and-error solutions and consequently are tedious for hand calculation. 

Comparison of this with equation 7.9 shows that the final term merely allows for nonideality. It is no more than a correction term. (When gaseous mixtures are involved, it is the various partial pressures p or partial fugacities,, which become important. In such a case, f would represent the fugacity of the compound i in the particular environment in which it found itself.)... 

H -I- HD is the only mixture of compounds of hydrogen that has a separation factor as favorable as in conventional industrial distillation. In this case, however, the true separation factor is less favorable than here calculated from the vapor-pressure ratio, because of nonidealities in gaseous and liquid mixtures of hydrogen and HD. Moreover, it is desirable to operate above atmospheric pressure, to preclude in-leakage of air. Under practical conditions, at... 

The most common method for treatment of phase equilibria in nonideal mixtures is the previously discussed method to calculate the activity coefficients from the free excess enthalpy. It should be emphasized that this treatment is basically just an interpolation of measured data retaining the thermodynamic consistency. Only nonideal behavior of the liquid phase is comprehended within this approach. If nonideal behavior of the gaseous phase is encountered (e.g., at high pressures), the treatment by means of equations of state is recommended. 

Nonideal fluids, such as high pressure gaseous mixture or liquid mixture, have higher density than the ideal gaseous mixtures dealt with in die previous section (Section 8.2). Thus, when dealing with such fluids, it is not possible to assume that collision is taken place by the two molecules, but three or more molecules may occur simultaneously. In this case, the Maxwell-Stefan approach still applies, but the driving force d is replaced by the following equation written in terms of the chemical potential ... 

In low-pressure gaseous mixtures and ideal liquid mixtures, a has been found to be small. On the other hand, in nonideal liquid mixtures, 3c may be large it becomes very large in the near-critical region for both near-critical gas and liquid phases. At the critical point, however, it has a limiting value. In this respect, a and the molecular diffusion coefficient, D, have opposite trends. Molecular diffusion is pronounced for low pressure gaseous mixtures and becomes small as the critical region is approached. 

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