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The liquid activity models are designed to have more flexibility by having more
adjustable parameters and allow the free energy curve to be accurately tuned for
magnitude and skewness. One of the key features of these models is treating liquids
differently than vapors. The liquids are evaluated as deviations from ideal-solution
behavior, where as the vapors are evaluated as deviations from ideal-gas behavior. (4)

Peng Robinson Equation

The Peng Robinson equation is an equation of state that is generally good for ideal
hydrocarbon systems not operating near the critical region. The Peng Robinson equation
does not do a good job of matching the experimental data in the critical region. The
equation calculates a larger two-phase region than what was generated through
experimental data. One reason that might explain this deviation is in the critical region
density fluctuations contributes strongly to thermodynamic properties. These
fluctuations are ignored in the Peng Robinson equation. To incorporate this into this
equation would be extremely difficult. (3)

NRTL & Van Laar Equations

The NRTL (Non-Random Two Liquid) equation is a three-parameter equation that can be
used for both liquid-liquid and vapor-liquid equilibria correlations. The strength of the
NRTL equation is for highly non-ideal systems. The NRTL equation often provides a
good representation of highly non-ideal mixtures, polar compounds and partially
immiscable systems. The NRTL equation provides good representation of experimental
data if care is exercised in data reduction to obtain the adjustable parameters in the
equation. (3)

The Van Laar is a two-parameter equation that is good for moderately non-ideal systems.
The Van Laar equation should be used for relatively simple non-polar liquids. However,
this equation has been empirically fit to represent the activity coefficients of complex
liquids. The Van Laar equation is a popular equation to use because it is easy to
mathematically manipulate. (3)

For moderately non-ideal systems, the NRTL equation does not offer a significant
advantage over the Van Laar and the three-suffix Margules equation.


Many times when simulating a distillation column the simulation model may not
converge or, if it does converge, the model does not match plant data. After confirming
that the input data are correct one may determine that the immediate answer to the
problem is that "I have chosen the wrong thermodynamic package for my system". Then
you begin switching between thermodynamic packages to try to get your simulation
model to work. However, once the simulation has converged or you have matched your
plant data does not mean that your model is correct. Thermodynamic packages are
constructed for

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