Chemical Process and Thermodynamics

We now have the foundation for applying thermodynamics to chemical processes. We have defined the potential that moves mass in a chemical process and have developed the criteria for spontaneity and for equilibrium in terms of this chemical potential. We have defined fugacity and activity in terms of the chemical potential and have derived the equations for determining the effect of pressure and temperature on the fugacity and activity. Finally, we have introduced the concept of a standard state, have described the usual choices of standard states for pure substances (solids, liquids, or gases) and for components in solution, and have seen how these choices of standard states reduce the activity to pressure in gaseous systems in the limits of low pressure, to concentration (mole fraction or molality) in solutions in the limit of low concentration of solute, and to a value near unity for pure solids or pure liquids at pressures near ambient. 

Because it is usual to perform a chemical experiment with the top of the beaker open to the open air, the pressure p during most chemical reactions and thermodynamic processes is the atmospheric pressure p%. Furthermore, this pressure will not vary. In other words, we usually simplify A (pV) saying pAV because only the volume changes. Accordingly, Equation (3.17) becomes... 

Significance of Activity Coefficients. While we typically focus our attention on the analytical concentration of reactant(s) and product(s) for a given chemical process, the thermodynamic concept of equilibrium depends on the chemical potential of a species. This is shown by the following relationship... 

Although COSMO-RS generally provides good predictions of chemical potentials and activity coefficients of molecules in liquids, its accuracy in many cases is not sufficient for the simulation of chemical processes and plants, because even small deviations can have large effects on the behavior of a complex process. Therefore, the chemical engineer typically prefers to use empirical thermodynamic models, such as the UNIQUAC and NRTL, for the description of liquid-phase activity coefficients with... 

Applicability of the first approach suggested by Keiko and Zarod-nyuk is based on the unity of thermodynamics and kinetics which explain differently the same physical regularities. As was said above this unity was brilliantly revealed by Boltzmann in his "kinetic" and "thermodynamic" explanations of the second law. In our case, setting, for example, a constraint on the equilibrium constant value of an individual reaction S VjXj = 0 within complex chemical process and writing this constraint intone of the possible forms ... 

The microanalytical methods of differential thermal analysis, differential scanning calorimetry, accelerating rate calorimetry, and thermomechanical analysis provide important information about chemical kinetics and thermodynamics but do not provide information about large-scale effects. Although a number of techniques are available for kinetics and heat-of-reaction analysis, a major advantage to heat flow calorimetry is that it better simulates the effects of real process conditions, such as degree of mixing or heat transfer coefficients. 

There are many common concepts between Chemical and Metallurgical Thermodynamics. They differ in the ranges of temperature used (high temperatures in metallurgical processes and relatively lower temperatures in chemical processes) and also in the ranges of pressure applied (close to atmospheric and up to a few bars in metallurgical processes, while chemical processes very often use hundreds of atmospheres of pressure). 

The thermodynamic form of kinetic equations is helpful for providing the kinetic thermodynamic analysis of the effect of various thermodynamic parameters on the stationary rate of complex stepwise processes. Following are a few examples of such analyses in application to the noncatalytic reac tions. The analysis of the occurrence of catalytic transformations is more specific because the concentrations and, therefore, the chemical potentials and thermodynamic rushes of the intermediates are usually related to one another in the total concentrations of the catalyticaUy active centers. (Catalytic reactions are discussed in more detail in Chapter 4.)... 

Shinnar, R., Thermodynamic analysis in chemical processes and reactor design. Chem. Eng. Sci. 43, 2303 (1988). 

Thermophysical properties, phase equilibria, and solution chemistries are the underlying physical and chemical phenomena of industrial chemical processes. Rigorous thermodynamic modeling of such phenomena establishes a sound scientific foundation for simulation of industrial chemical processes and subsequent process development, optimization, and control. 

---