Phase and Reaction Equilibrium

The previous description illustrates well the complications that may arise in second law studies when phase and reaction equilibria occur simultaneously. A number of assumptions are usually made, some of which may influence the final thermochemical results. For instance, it is possible that the equilibrium concentration of hydrogen obtained in the study by Bercaw and co-workers is not very accurate, because no may be underestimated (the cooling to — 196 °C will increase the amount of H2 in the condensed phase). Nevertheless, this error, which is constant for all the measurements at different temperatures (average T = 316 K), will probably have a negligible effect on the calculated Ar//j16 and Ar,V) l6 values, 28.3 1.9 kJ mol-1 and —5.3 6.1 J K-1 mol-1, respectively. These values were obtained from equation 14.20, which is a linear fit of the... 

Phase and Reaction Equilibria Considerations in the Evaluation and Operation of Supercritical Fluid Reaction Processes... 

One of the first questions that arises in the development of SCF reaction schemes concerns the criteria for selecting an SCF medium for a given reaction. The selection depends upon several factors, viz., density, transport properties, inertness or reactivity of the chosen solvent, toxicity, and phase and reaction equilibria, to mention a few. The two latter factors, viz., phase and reaction equilibria, may be as important as the other selection factors and have, as yet, received little attention. 

The objective of this paper is to demonstrate the importance of phase and reaction equilibria considerations in the rational development of SCF reaction schemes. Theoretical analysis of phase and reaction equilibria are presented for two relatively simple reactions, viz., the isomerizations of n-hexane and 1-hexene. Our simulated conversion and yield plots compare well with experimental results reported in the literature for n-hexane isomerization (4) and obtained by us for 1-hexene isomerization. Based on our analysis, the choice of an appropriate SCF reaction medium for each of these reactions is discussed. Properties such as viscosity, surface tension and polarity can affect transport and kinetic behavior and hence should also be considered for complete evaluation of SCF solvents. These rate effects are not considered in our equilibrium study. 

Our research program is thus aimed at gaining a fundamental understanding of the behavior of heterogeneous catalytic reactions at SCF conditions based on phase and reaction equilibria considerations as well as kinetic and mass-transfer rate considerations. Such an understanding is essential to the rational design of SCF reaction schemes. 

The concentrations of solute A and product P in the Internal phase, Cfti and Cpj, are restricted by reagent conservation and phase and reaction equilibria ... 

Chapters 4 and 5 are dedicated to the thermodynamic and kinetic fundamentals of RD processes. In Chapter 4, Hasse reviews the fundamentals of thermodynamic modeling of simultaneous phase and reaction equilibria. The author emphasizes the importance of consistency of phase equilibrium models. Thermodynamic consistency provides a sound basis for developing predictive reaction models for RDs, which are valid over a wide range of concentrations. To develop phase equilibrium models, reliable experimental data of phase equilibria in reactive systems have to be available. For successful measurements, suitable experimental techniques are needed, which are briefly summarized in this chapter. Criteria for their selection are also given. 

Here S counts any additional internal constraints besides those for phase and reaction equilibria. When S = 0 (10.3.1) reduces to the traditional form of the Gibbs phase rule extended to reacting systems. 

In previous chapters we developed the thermodynamics of phase and reaction equilibria, and we illustrated certain principles using straightforward computational procedures. We used only simple procedmes so as not to detract from thermodynamic issues. In this chapter we consider more complex situations and therefore give more attention to computational techniques. No new thermodynamics is introduced in this chapter instead, we try to show how the thermodynamics already developed can be used in multicomponent phase and reaction-equilibrium situations. 

When we write algorithms for computing phase and reaction equilibria, we should try to implement guards that reduce the chances of search routines entering indifferent situations. When we use those algorithms, we should be aware that indifferent situations exist, that no guard is likely to protect against all eventualities, and therefore when a particular solution is found, it should not be accepted blindly. 

Thermodynamic effects on biocatalysts working in the presence of non-conventional media have an impact on two levels i) phase and reaction equilibria and ii) biocatalyst stability and activity [34]. The thermodynamic effects on the first level are by now relatively well understood. It is probably safe to say that a certain scientific foundation for rational phase and reaction equilibrium engineering exists. Based on this knowledge, it is possible to conceive, if not to design, bio catalytic systems with tailored selectivities and/or improved product yields due to low water activity, the presence of non-aqueous non-conventional solvents [33], or characterized by a very high solid content [35, 36]. It has been shown for particular cases that this type of engineering may be based directly on standard thermodynamic tools such... 

For complicated equilibrium calculations, the G-minimization technique is a useful option for the evaluation of phase equilibria, especially if both phase and reaction equilibria are involved. In contrast to the equilibrium conditions, the minimum of G is not only a necessary but a sufficient equilibrium condition. As well, for complicated equilibria it is often the only way to keep the overview. The only knowledge that must be available is the functional relationship for the Gibbs energy g and a clear concept for the minimum evaluation task. The following example shall illustrate the method. 

S Saim, DM Ginosar, B Subramaniam. Phase and reaction equilibria considerations in the evaluation and operation of supercritical fluid reaction processes. In KP Johnston, JML Penninger, eds. Supercritical Fluid Science and Technology. ACS Symposium Series No. 406. Washington, D.C. American Chemical Society, 1989, pp 301-316. 

S. Lee and M. Z. Kuo, Phase and reaction equilibria of the acetic acid-isopropanol-isopropyl acetate-water system at 760 mmUg, Fluid Phase Equilib. 123, 147-165 (1996). 

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