Reaction Rates and Chemical Equilibrium

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

The EQ model requires reaction kinetic parameters and thermodynamic properties the latter for the calculation of phase equilibrium and taking into account the effect of non-ideal component behavior in the calculation of reaction rates and chemical equilibrium constants. 

Table 2. The school dance analog for chemical reaction, reaction rate, and chemical equilibrium conditions ...

This chapter is meant as a brief introduction to chemical kinetics. Some central concepts, like reaction rate and chemical equilibrium, have been introduced and their meaning has been reviewed. We have further seen how to employ those concepts to write a system of ordinary differential equations to model the time evolution of the concentrations of all the chemical species in the system. The resulting equations can then be numerically or analytically solved, or studied by means of the techniques of nonlinear dynamics. A particularly interesting result obtained in this chapter was the law of mass action, which dictates a condition to be satisfied for the equilibrium concentrations of all the chemical species involved in a reaction, regardless of their initial values. In the forthcoming chapters we shall use this result to connect different approaches like chemical kinetics, thermodynamics, etc. 

A temperature of 390 K is chosen because it gives reasonable reaction rates and chemical equilibrium constants for the numerical example under consideration. Different cases are studied for a range of values. When a39o = 2, the relative volatilities are independent of the temperature. When asgo = 0.95, the adjacent component switches volatilities as the temperature approaches 390 K, so the desired separation would be infeasible. 

---