Interactive Self-Study Module: Immiscible Liquid Phase Diagrams

Department of Chemical and Biological Engineering, University of Colorado Boulder

Send email with questions, corrections, or suggestions to LearnChemE@gmail.com


Motivation:

The phase diagrams for immiscible liquids helps understand solid-liquid phase diagrams that have multiple solid phases.


This module is intended for a Thermodynamics course and may also be useful for a Materials course.


Pre-requisites:

  • Understand single-component vapor-liquid equilibrium
  • Be able to apply the Antoine equation to determine saturation pressure of a single component at a given temperature
  • Be able to calculate partial pressures for a mixture of ideal gases

After studying this module, you should be able to:
  1. Construct a pressure-composition diagram at a given temperature for two immiscible liquids, given saturation pressures at that temperature.
  2. Construct a temperature-composition diagram at a given pressure for two immiscible liquids, given Antoine equations
    (saturation pressure versus temperature) for each component.
  3. Determine what phases are present, given temperature, pressure, saturation pressures, and overall compositions.
  4. Given a vapor composition and saturation pressure data, determine the temperature (at constant pressure) or the pressure
    (at constant temperature) at which one or more components condenses.

Try to answer these ConcepTests before using this module.

     

Overview

This module uses screencasts and interactive simulations to explain the vapor-liquid phase equilibrium of two immiscible liquids.
Both pressure-composition and temperature-composition diagrams are explained. It then provides example problems and
step-by-step quiz simulations to allow the user to test themselves. We suggest using the learning resources in the following order:

  1. Watch the screencast that describe the phase diagrams and answer the questions within the screencasts
  2. Use the two interactive simulations to further understand the behavior of the phase diagrams
  3. Use the two quiz interactive simulations to test your understanding by carrying out step-by-step preparation of phase diagrams
  4. Use the two example problem screencasts to test your knowledge by reading the problem statement and try to solve the problem
    on your own and then watch the solution in the screencast.
  5. Answer the ConcepTests

Screencast

A Txy diagram is used to explain phases present for vapor-liquid-liquid equilibrium for two immiscible species.

Vapor-Liquid-Liquid Equilibrium ‎‎‎‎(VLLE)‎‎‎‎ for Immiscible Liquids ‎‎‎‎(6 minutes)‎‎‎‎


Important Equations
Interactive Simulations
These simulations were prepared using Mathematica.
To use them, download the free CDF player available here, download the simulation CDF file (click on the images below).
Then, try to predict the behavior when some parameter changes before using a slider to change the parameter.
For most simulations, a screencast is provided to explain how to use the simulation.

Immiscible Liquids on Pressure-Composition Diagram

This simulation shows a pressure-composition phase diagram for two immiscible liquids, benzene and water.
The temperature and the overall benzene mole fraction can be changed with a slider.
A bar graph shows the moles of liquid water (blue), liquid benzene (orange), and vapor (green).
Changing the piston height changes the amounts in each phase.
Selecting "piston-cylinder" instead of "amounts of each phase" shows the volume of each phase on a log scale.



Try to answer these questions before manipulating the simulation:

  1. For a constant temperature of 122°C, how many phases are present for a benzene mole fraction 0.30 and a pressure of 3.8 bar?
  2. How does the pressure-composition graph change as the temperature increases?
download simulation
 

Immiscible Liquids P-x-y Diagram
Download here. 

Temperature-Composition Diagrams for Immiscible Liquids 

This simulation shows a temperature-composition phase diagram for two immiscible liquids, benzene and water. 

The pressure and overall mole composition can be changed with sliders. 

The bar graph shows the moles of liquid water (blue), liquid benzene (orange), and vapor (green). 

The system contains one mole total.


Try to answer these questions before manipulating the simulation:

  1. How many phases are present at 12 bar and 160°C for a benzene mole fraction of 0.35?
  2. How does the graph change as pressure increases? 
 Click here to go to the simulation page  

Temperature-Composition Diagram for Immiscible Liquids
Download here. 

Quiz-yourself simulations

These simulations lead you through the construction of pressure-composition and temperature-composition
phase diagrams for immiscible liquids in a step-by-step procedure. Use these simulations to test your understanding.


Construct a Pressure-Composition Diagram for Immiscible Liquids


Construction of Temperature-Composition Diagram for Immiscible Liquids 


Example Problems

After reading the problem statements below, try to solve the problem before watching the screencast.

Example Problem 1

A gas mixture is 0.75% A and 25% B. As the pressure increases isothermally, component A condenses at 1.6 bar. At 2.4 bar, B starts to condense.

What are the saturation pressures of A and B?

Example Problem 2

6 mol A and 4 mol B are in equilibrium at 100°C and 2.0 atm.

A and B are completely immiscible in the liquid phase. 

Their vapor pressures at 100°C are:

What phases are present?

Immisicble Liquids VLLE (3 minutes)

Phases for Immiscible Liquids ‎(3 minutes)‎

ConcepTests

Try to answer these ConcepTests after using this module as a way to test your understanding. 

     


Summary of Phase Equilibrium for Immiscible Liquids

  1. If a liquid is present, it exerts its own saturation pressure, independent if other immiscible liquid phases are present.     
  2. The partial pressure of a component in the vapor phase is always less than or equal to its saturation pressure.
  3. When a vapor mixture is cooled or its pressure is increased, one component will condense first
    unless the vapor mole fractions are proportional to their saturation pressures.
  4. For a given temperature, two liquids are in equilibrium with the vapor phase at only one pressure.
  5. For a given pressure, two liquids are in equilibrium with the vapor phase at only one temperature.


Prepared by John L. Falconer and Kimberly R. Bourland

Department of Chemical and Biological Engineering, University of Colorado Boulder