Real gas mixtures

Properties of mixtures as a real gas or as a liquid under pressure are determined starting from the properties of mixtures in the ideal gas state or saturated liquid after applying a pressure correction determined as a function of a property or a variable depending on pressure )... 

In mixtures of real gases the ideal gas law does not hold. The chemical potential of A of a mixture of real gases is defined in terms of the fugacity of the gas, fA. The fugacity is, as discussed in Chapter 2, the thermodynamic term used to relate the chemical potential of the real gas to that of the (hypothetical) standard state of the gas at 1 bar where the gas is ideal ... 

Equation 2.63 is valid for any homogeneous or heterogeneous reaction. The only difference is in the definition of activities. For a species in a perfect gas-phase mixture a = pi/p°, where pi is the partial pressure of species i andp° is the standard pressure (1 bar). For a real gas-phase mixture a =f/p°, where is the fugacity of i. The fugacity concept was developed for the same reason as the activity to extend to real gases the formalism used to describe perfect gas mixtures. In the low total pressure limit (p -> 0), fi = pi. 

Figure 9 provides a comparison of the predictions of empirical methods with Wormald s data for a 50/50 mole percent mixture of steam and methane. As can be seen, the frequently used artifices of calculating mixture enthalpies by blending the pure component enthalpies at either total or partial pressures are very inaccurate. Likewise, the assumption of ideal gas enthalpy for the real gas mixture, equivalent to a zero enthalpy departure on the diagram, is an equally poor method. 

Now that we have obtained expressions for the fugacity of a real gas and its temperature and pressure coefficients, let us consider the application of the concept of fugacity to components of a mixture of real gases. 

In defining activities in a real gas mixture, the partial pressure of each component is replaced by its fugacity, fx... 

In the absence of experimental measurements, it is usual to calculate the fugacities of components in a real gas mixture using the Lewis and Randall rule... 

Calculation of the equilibrium composition of a mixture, allowing for real gas effects... 

PHYSICAL NATURE OF CHEMICAL POTENTIAL IN IDEAL AND REAL GAS MIXTURES... 

Following the philosophy of Section 5.8.1, it is also possible (if uninformative) to express /jlt for real gas mixtures in a form that mimics the ideal gas expression (6.57), namely,... 

If equilibrium has not been reached between a mixture of components, the condition is referred to as partial saturation. At partial saturation the gas mixture obeys real gas laws. There are several ways to express the concentration of a vapor in a mixture of gases. Most often, weight or mole fraction is used. Other definitions are relative saturation (relative humidity), molal saturation (molal humidity) and absolute saturation (absolute humidity). 

The affinity of a chemical reaction is. I Xryr, where v, is the stoichiometric coefficient, which is negative for reactants and positive for products. For a real gas mixture, the affinity is determined by... 

Calculate the injection pressure for a 50-50 mixture of hydrogen sulfide and carbon dioxide. The reservoir is at a pressure of 2000 kPa, is at a depth of 750 m, and is isothermal at 20°C. Assume the acid gas will remain gaseous throughout the injection. Further assume (a) the gas is an ideal gas and (b) the gas is a real gas with properties described by the generalized compressibility chart. Take the properties of hydrogen sulfide and carbon dioxide from table 2.1. 

The real-gas equation in the form (115) is also applicable to a mixture of gases, provided the virial coefficients A, B, C are expressed as ... 

PVT, Vapor Density, and Virial Coefficient. In these three techniques the pressure (of a known volume and weight of gas) is measured over a temperature range. The data may be treated in two ways by assuming that the real gas is a mixture of perfect gases (each gas being one of the species), or by describing the behavior of the real gas by a virial equation. 

Property relations for mixtures of ideal gases are important as references in the treatment of real-gas irrixtures, aird tlrey fonrr tire basis forintroductionof yet anotherimpo riant property, ihefugacity. Related to tire chemical potential, it is vital in the fonrrulation of both phase- aird chenrical-reaction-equihbriunr relations. 

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