Kolmetz.com Thermo Paper Page 7

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.

**SIMULATION MODEL **

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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