Model Lee-Kesler

The reduced correction for enthalpy employed in the preceding equation is obtained by the Lee Kesler model ... 

The average accuracy of the Lee and Kesler model is much better than that of all cubic equations for pressures higher than 40 bar, as well as those around the critical point. 

The Cpg of real gas is calculated using the equation derived from the Lee and Kesler model ... 

M = molecular weight dCp = reduced Cp correction calculated fromthe Lee and Kesler model From a practical point of view, as for liquids, it is possible to calculate dC... 

In Figure 10 are shown comparisons of the equation of state methods with the experimental data. The Lee-Kesler methods represent the data the best. Again, if the water acentric factor determined to best represent the pure steam enthalpy data is applied to the mixtures, further improvement is noted for the predictions by the Lee-Kesler method. Use of interaction constants within the Lee-Kesler, or other models, would undoubtedly provide even better representation of the data. 

Raoult s law K model Lee Kesler vapor pressure Ideal enthalpy... 

Commonly used EOS models include the ideal, virial, PengRobinson, Soave-RedUch Kwong, and Lee-Kesler. The reduced form of the EOS is particularly significant. Substances with the same reduced properties are in corresponding states. Van der Waal s EOS is a poor predictor of state properties, but the experimental data do correlate well with reduced conditions. Many of the cubic EOS models are based on the van der Waal equation. 

Table 3.2 presents a selection of the most used thermodynamic options for phase equilibrium with suitable enthalpy and entropy methods. The accuracy of both phase equilibrium and enthalpy/entropy computation must be examined when using EOS models. For example, often a cubic EOS underestimates the enthalpy of vaporisation. In this case other methods are more accurate, as those based on three-parameters corresponding states law (Lee-Kesler, Curl-Pitzer, etc.). Mixtures rich in components with particular behaviour, as or CH, need special methods for accurate simulation. When binary interaction parameters for liquid activity models are absent, the UNI FAC predictive method may be employed. It is worth to note that UNIFAC is suitable only for exploratory purposes, but not for final design. When high non-ideal mixtures are involved at higher pressure then the combination of EOS with liquid activity models is recommended (see Chapter 6). 

A cubic equation of state model cannot predict all the properties with equal accuracy. Usually there is a non-negligible error in estimating liquid volume, which produces also errors in computing enthalpies, frequently underestimated. More accurate methods for enthalpy and entropy are based on corresponding states correlation (Lee-Kesler). 

The vapor pressure of pseudocomponents is also an important property when an equation-of-state approach is not used. All other approaches to process thermo-dynamics require some form of vapor-pressure correlation. The vapor pressure for pure hydrocarbons has been extensively tabulated in many component databases such as DIPPR (Design Institute for Physical Property Research, American Institute of Chemical Engineers) and significant libraries are available in modern process modeling software. Several correlations are available in the literature for the vapor pressure of pseudocomponents. It is important to recall that the vapor pressure and heat vaporization are related through the Clausius-Clapeyron [17] Equation (Equation (1-46)). This relationship imposes a constraint if we wish the model to be thermodynamically consistent In general, most of the popular correlations for vapor pressure such as the Lee-Kesler [9,10] agree well with heat of vaporization correlations and maintain thermodynamic consistency. We present the Lee-Kesler vapor pressure correlation in Equation (1.47). 

The Lee-Kesler correlation for vapor pressure is quite accurate for low to medium boiling pseudocomponents. For very light components, we recommend using pure component properties directly. In the case of heavy components, Ambrose [17] has presented an additional term for the Lee-Kesler correlation. In practice, however, the additional term is not necessary for refinery modeling purposes. 

Hydrocarbon mixtures are most often modeled by the equations of state of Soave, Peng Robinson, or Lee and Kesler. 

---