Section 3.7.1.2: Example Questions

1. It is required to determine the pressure at which a liquid mixture of propane and butane will be at its bubble point at a temperature of 293K. Prepare an incidence table for this system of equations and show that the problem can be reduced to iteration on a single variable. (Most of this problem has already been covered in the course material.)

   Calculate the bubble point pressure for a an equimolar mixture of the species and the composition of the vapour phase formed.

2. Repeat the above analysis and calculation for the dewpoint pressure calculation at 293K.

3. Repeat the analysis of question 1 for the case of calculating the bubble point temperature given the pressure. Show that the problem can be reduced to iteration on a single variable. (Most of this has also been described in the course material.) Calculate the bubble point temperature of an equimolar butane-pentane mixture at 2 bara.

4. Water and pentane form two immiscible liquid phases. In these circumstances it can be assumed that each liquid phase contains only a single pure component and the equilibrium condition is given by:

   (P^*₁(T) + P^*₂(T)) - P = 0

   Set up the equations, incidence table, etc. for this problem and show how it can be solved by iteration in a single variable. Show also that the composition of the vapour can be calculated. Note that this is technically a minimum boiling heterogeneous azeotrope. Determine the azeotrope temperature and composition at 2 atm.

5. The vapour pressures of butane and pentane at 300K are 2.73 atm and 0.713 atm respectively. Show that if the pressure is specified the problem can be described by 4 linear equations in the 4 mole fractions (two liquid and two vapour). find the two phase compositions at 2 atm and 300K by setting up and solving the 4 equations.

Data

• Propane: 231K
• Butane: 272.6K
• Pentane: 309.2K
• Hexane: 341.9K

Next - Section 3.7.1.3: Solutions to Questions
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