The Rayleigh Equation Revisited What to Do When Alpha Isn't Constant
- Type: Conference Presentation
- Conference Type:
AIChE Spring Meeting and Global Congress on Process Safety
- Presentation Date:
April 4, 2012
- Skill Level:
Integration of the Rayleigh Equation for batch distillation in closed analytical form has heretofore required that relative volatility (α) be assumed constant. A new technique presented in this paper produces a continuous analytical function for the Rayleigh Equation integral whether α is constant or not. The newly developed equation reduces algebraically to the traditional function when α is absolutely constant.
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 new method has proven especially useful for the non-ideal systems investigated, in which α varies widely with composition.
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. Results compare well with expected values.
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. 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.
The paper consists of new material supported by background information tutorial in nature.
Keywords. Alcohol “Proof” Levels; Boiling Point Curve; Density of Ethanol-Water Solutions; Ethanol-Water Flash Points; Ethanol-Water Refractive Index; Gas Chromatography (GC); Numerical Integration; Rayleigh Equation; Simpson's Rule; Systems: Acetone-Water, Benzene-Toluene, Ethanol-Water, Ethylene Dichloride (EDC)-Toluene; Vapor-Liquid Equilibrium.
Summary and Conclusions
• Batch, or differential, distillation without reflux is described by the Rayleigh Equation:
+ Relates composition and amount of material remaining in distilling flask
+ Other quantities determined by material balance
+ Illustrative example from the literature presented.
• Numerical integration required for the Rayleigh Equation in its basic form.
• Substitution of relative volatility (α) allows analytical integration for constant α.
• New equation derived here allows analytical integration whether α is constant or not.
• Analytical integration utilizing α has been evaluated here against numerical integration:
+ For constant or nearly constant α
+ For a number of real vapor-liquid systems, where α varies with composition.
• For ideal systems, with α not varying widely, use of an “average” α is adequate.
• For non-ideal systems, only the new equation follows the track of numerical integration.
• The new function was used in simulation of batch distillation of pure ethanol and water.
• Ethanol-water results compare favorably with expectations, for example:
+ Boiling point vs. composition curve
+ Still pot temperature vs. cumulative volume of distillate collected
+ Density of solution remaining in still pot.
• Methods to analyze liquid composition are discussed for the ethanol-water system:
+ Gas chromatography (GC)
+ Refractive index data compiled from various sources
+ Densities of ethanol-water solutions extended beyond range of published data
+ Qualitative ignition test / flash points of ethanol-water solutions.