Thermo-Physical Property Prediction Methods

Enthalpy is one of the most important thermo-physical properties required for calculating heat loads in process design. The most common models used for estimating enthalpies of petroleum and its fractions in the refining industry are based on the corresponding-states approach. Methods for calculating the necessary critical properties 

It is important to mention here that since it is not possible to estimate the absolute enthalpy of a compound, a reference temperature is generally chosen. Ideally, this reference temperature is such that all the enthalpies under consideration are zero at this temperature. In the absolute sense, unfortunately, different compounds have different molecular motions (and hence different enthalpies) at a given temperature. Hence, the solution lies in choosing a temperature that is sufficiently low so that different compounds may be assumed to have the same enthalpy at the reference point.Kesler-Lee defined the reference state for enthalpies of petroleum fluids as saturated liquid at — 200 °F ( 144 K), and it is reasonable to assume that enthalpies of all the petroleum fractions at this low temperature will be zero. 

In general, the estimation of enthalpy (departure) can be represented in the following manner  

The Kesler-Lee correlations for liquid and vapour phase heat capacities of petroleum fluids are used for estimating the respective enthalpies at temperatures of interest. The Lee-Kesler corresponding-states method is used for obtaining estimates of the heats of vaporization and for developing the saturation envelope enthalpies. This method uses the Curl and Pitzer approach and calculates various thermodynamic properties by representing the compressibility factor of any fluid in terms of a simple fluid and a reference fluid as follows  

The enthalpy departure (and other thermodynamic properties) is then calculated using equations obtained from the compressibility factor via the usual thermodynamic relationships. 

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Further, molecular simulation and computational chemistry have evolved, and are evolving, into important tools for developing better characterization techniques where it is not possible to measure all data. Even so, it is precisely the molecular complexity of petroleum fluids that seems to be an inhibiting factor in the use of these methods for developing better characterization methods. However, identification of important functional groups in petroleum fluids applying molecular simulation and/or computational chemistry for use with group contribution methods to predict thermo-physical properties may be an area for further research. 

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