Vapor-Liquid Equilibrium for Non-Ideal Solutions: Summary



Key points from this module:
  1. When a vapor mixture is cooled or its pressure increases, both components condense when the dew point is reached. 
  2. Bubble pressure is the pressure where the first bubble of vapor forms as the pressure above a liquid decreases at constant temperature.
  3. Bubble temperature is the temperature where the first bubble of vapor forms as the temperature of a liquid increases at constant pressure. 
  4. Dew pressure is the pressure where the first drop of liquid forms as the pressure of a vapor increases at constant temperature.
  5. Dew temperature is the temperature where the first drop of liquid forms as the temperature of a vapor decreases at constant pressure. 
  6. A positive deviation from Raoult's law means the pressure above the liquid mixture is greater than that calculated from Raoult's law because the interactions between A and B molecules are weaker than the average A-A and B-B interactions.
  7. A negative deviation from Raoult's law means the pressure above the liquid mixture is less than that calculated from Raoult's law because the interactions between A and B molecules are stronger than the average of A-A and B-B interactions.
  8. An azeotrope corresponds to a maximum or minimum in the pressure versus mole fraction plot. At an azeotrope, the liquid and vapor compositions are identical, which makes separations by distilliation difficult.
  9. A system with maximum-pressure azeotrope will have a minimum-temperature azeotrope.
  10. A system with minimum-pressure azeotrope will have a maximum-temperature azeotrope. 

From studying this module, you should now be able to:
  1. Explain the meaning of positive and negative deviations from Raoult's law and explain why they occur in terms of molecular interactions.
  2. Calculate vapor-liquid equilibrium using the modified Raoult's law.
  3. Calculate activity coefficients from vapor-liquid equilibrium data.
  4. Determine what phases are present at specified conditions, given a phase diagram.
  5. Calculate pressure bubble points and dew points using the modified Raoult's law and a model for the liquid phase activity coefficients.
  6. Calculate temperature bubble points and dew points using the modified Raoult's law and a model for the liquid phase activity coefficients.
  7. Calculate activity coefficients for liquid phases from excess Gibbs free energy expressions.
  8. Calculate Gibbs free energy of mixing and excess Gibbs energy from VLE equilibrium compositions.
  9. Construct a P-x-y diagram at a given temperature for two miscible liquids that form a non-ideal solution, given saturation pressures at that temperature and equations that model the non-ideal behavior of the liquid solution.
    1. Construct a T-x-y diagram at a given pressure for two miscible liquids that form a non-ideal solution, given Antoine equations (saturation pressure versus temperature) for each component and equations that model the non-ideal behavior of the liquid solution.
    2. Explain the properties of azeotropes, and explain why they make separations by distillation difficult.
    Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder