Assay Pseudo-component Criticals & Ideal Gas Enthalpy Estimation Methods

Critical temperature, critical pressure, acentric factor, and vapor pressure are required for many of the thermo-physical property methods. Ideal gas enthalpies are required for most of the enthalpy calculation methods, which are based on application of an enthalpy departure term to the ideal gas enthalpy. Keywords support these options using the CHARACTERIZE entry on the ASSAY statement.

Several methods are available:

SimSci (TWU)

Correlation procedures developed by SimSci [1] are used to estimate the above properties for each pseudo-component, based on its normal boiling point and gravity. The component properties are derived by applying a correction factor to the properties for normal alkanes. This is the default method.

Heavy Oil (HOIL)

This is the SimSci method (see immediately above) modified to better represent components having a normal boiling point above 1000K, Below 1000K, characterization essentially is identical to the SimSci method.

ExtAPI (Extended 1987 API)

A version of the API 87 correlation (see immediately below) enhanced with adjustments for components boiling below 300F. This generally results in a better match to known pure component data.

CAV8 (API87)

This method comes from the 1987 API Technical Data Book and in most cases is preferred to using the older correlations. The older cracking options should not be used with this method.

Lee Kesler (LK)

Methods developed For Mobil Oil Company by M.G. Kesler and B.I. Lee [5] are used to predict all of the properties above, based on the component normal boiling points, gravities, and molecular weights.

Cavett (API63)

This method comes from the 1963 API Technical Data Book. Equations developed by R.H Cavett [2] are used to estimate the critical properties and ideal gas enthalpies for each pseudocomponent. Critical properties are correlated with normal boiling point and gravity. Ideal gas enthalpy constants are correlated with normal boiling point, gravity, and molecular weight. Vapor pressures are estimated with the method of L.C. Yen [3]. The acentric factor is determined with the Edmister [4] equation. A later development added a correction for cracking to better characterize crude feedstocks. However, all forms of this method generally have been replaced by the API87 method.

Additional Information

Assay Processing Reference Help

References

[1] Twu, C.H., "An Internally Consistent Correlation for Predicting Critical Properties and Molecular Weight of Petroleum and Coal Tar Liquids", Fluid Phase Equilibria, 16, 137-150, (1984).

[2] Cavett, R.H., "Physical Data for Distillation Calculations- Vapor-Liquid Equilibria", 27th Mid-year Meeting API Division of Refining, San Francisco, May 15, 1962.

[3] Yen, L.C., Reference Unknown. The constants A and B are determined by fitting the Antoine equation from the normal boiling point to the critical point:

[4] Edmister, W.C., Applied Hydrocarbon Thermodynamics, Gulf publishing Company, page 23, (1961).

[5] Kesler, M.G. and Lee, B.I., Hydrocarbon Processing, Vol. 53, No.3, pp 153, 1976.

Kesler, M.G. and Lee, B.I., A I Ch E Journal, Vol. 21, No.3, 510, (1975).