The paper consists of new material supported by background information tutorial in nature. These latter include, among others, refractive index measurements assembled from various sources and estimates of liquid density for ethanol-water mixtures extended beyond the range of published data. A summary of VLE relationships for ideal and non-ideal systems plus temperature- dependent methods for analysis of constituent composition specific to the ethanol-water system are discussed in separate appendices. Results compare well with expected values. The method is then utilized successfully in a computer-simulated batch distillation of an ethanol-water solution at 1 atm pressure, a popular student laboratory exercise and one of the original systems studied by Lord Rayleigh. The new method has proven especially useful for the non-ideal systems investigated, in which α varies widely with composition. Both the traditional expression at constant α and the new equation are evaluated against numerical integration for a number of ideal and non-ideal binary vapor-liquid equilibrium (VLE) systems. The newly developed equation reduces algebraically to the traditional function when α is absolutely constant. A new technique presented in this paper produces a continuous analytical function for the Rayleigh Equation integral whether α is constant or not. Hillsborough, North Carolina Paper 118 b 2012 AIChE Spring Meeting April 1-5, 2012 Houston, Texas Abstract Integration of the Rayleigh Equation for batch distillation in closed analytical form has heretofore required that relative volatility ( α ) be assumed constant. Kunz RGK Environmental Consulting, L.L.C. The Rayleigh Equation Revisited What to Do When Alpha ( α ) Isn’t Constant Robert G.
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