Equations of State in Chemical Reacting Systems

Abstract

Phase and chemical equilibrium calculations are essential for the design of processes involving chemical transformations. Even in the case of reactions that cannot reach chemical equilibrium, the solution of this problem gives information on the expected behaviour of the system and the potential thermodynamic limitations. There are several problems in which the simultaneous calculation of chemical and phase behaviour is mandatory. This is the case, for example, of reactive distillations where phase separation is used to shift chemical equilibrium. Also, the calculation of gas and solid solubility in liquids of high dielectric constants requires at times the resolution of chemical equilibrium between the different species that are formed in the liquid phase. Several algorithms have been proposed in the literature to solve the complex non-linear problem; however, proper thermodynamic model selection has not received much attention. In recent times, the use of supercritical solvents has emerged as an important technique to improve rates and selectivities in diffusion-controlled reactions. Phase behaviour near the critical point of mixtures is very sensitive to process operating conditions and mixture compositions. The selection and design of the appropriate phase conditions to exploit process potential require thermodynamic models able to deal with highly asymmetric mixtures involving permanent gases, supercritical solvents and non-volatile substrates. This chapter presents a phase equilibrium engineering approach to analyze the phase behaviour of chemical reacting systems. The use of group contribution equations of states in these systems is discussed. The main advantage of these models is that they have predictive capability for compounds that were not included in the parameterization process. The lack of equilibrium data in reactive mixtures is quite common; therefore, group contribution methods allow designers to gain knowledge on the changes in phase behaviour as the reaction proceeds.