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PETE 310
  • Lecture # 26


  • Chapter 12
  • Gas-Liquid Equilibrium
  • Non-ideal VLE
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Gas-Liquid Equilibrium 
Non Ideal Behavior
  • Applications to low & high pressures


    • Use models for the equilibrium ratios (ki-values)
      • Simpler models ki = f (P,T)
      • More complex models
      • ki = f(P,T,compositions)
      • use equations of state EOS, convergence pressure
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VLE general (same)

  • The equilibrium criteria between 2 phases a and b is,
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Statement of Equilibrium
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Mathematical Relationships


  • with



  • In general
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Bubble Point Evaluation
(Non-Ideal Behavior Model)
  • The bubble point pressure at a given T is
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Bubble Point
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Dew Point Calculation
  • At the dew point the overall fluid composition coincides with the gas composition. That is.
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Dew Point Calculation
(Non-Ideal Behavior Model)
  • Find DP pressure and equilibrium liquid compositions
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Dew Point
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Flash Calculations
  • In this type of calculations the objective is to:


    • find fraction of vapor vaporized (fv) and equilibrium gas and liquid compositions


    • given the overall mixture composition, P and T.
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Flash Calculations
(Non-Ideal Behavior Calculations)
  • Start with the equilibrium equation




  • Material balance
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Flash Calculations
  • Now replace either liquid or gas compositions using Ki values (exercise derive the expression)
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Flash Calculations
  • Objective function (flash function) is
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Flash Calculations
  • There are several equivalent expressions for the flash function


  • (a)


  • (b)


  • (c)
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One of the Models for Equilibrium Ratios
  • 1. Require critical properties & acentric factors
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