Page 412 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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392 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
and amount of solid formation in petroleum fluids. Some
addition to data on petroleum mixtures from oils around the
guidelines for quick convergence in the calculation and de- of pure compounds have been used in their development in
termination of key interaction parameters are given. Various world. The weakest predictive methods are perhaps those em-
models available in most recent publications for calculation pirically developed correlations that are based on a set of data
of amounts of wax and asphaltene precipitations and their on- for oils from a certain part of the world.
sets are presented. Mechanism of solid formation, their neg- As it is shown in this book the main difficulty in prediction
ative effects in the petroleum industry and methods of their of properties of petroleum fractions relies on properties of
prevention are also discussed. Results of calculations from heavy fractions containing polar multiring compounds with
various models when applied to different petroleum mixtures few experimental data available on their properties. As heavy
are given. Effects of temperature and pressure on the wax and compounds are generally polar with high boiling points, data
asphaltene precipitation for different oils are demonstrated. on specific gravity and molecular weight alone are not suffi-
Methods of calculation of the conditions at which hydrates cient for their property predictions. For such compounds it
may be formed are shown. The impact of characterization is not possible to measure critical properties or even boiling
methods of Chapters 2–4 on property and phase behavior point. Boiling points of such compounds or their mixtures are
predictions as well as methods of calculation of true critical not measurable and estimated boiling points based on distil-
properties are also presented. The chapter ends with another lation data at low pressures have little practical applications
application of methods presented in the book in evaluation as they do not represent true boiling points. For such com-
of gas injection projects for EOR. pounds one has to look at other properties that are directly
In VLE calculations, accuracy of the results basically de- measurable and represent their characteristics.
pends on the method chosen for calculation of equilibrium Reported values of critical properties of heavy compounds
ratios. In this regards suggestions given in Table 6.14 should are usually predicted from methods developed for lighter hy-
be used as a guide. For calculation of CPT and WFT the drocarbons. For example, in the API-TDB [12] reported values
multisolid-phase model provides a reliable method without of critical constants for heavy compounds are calculated from
the need for adjustable parameters. A good prediction of onset group contribution methods. Kesler–Lee method for calcula-
of asphaltene precipitation is possible through measurement tion of critical properties of heavy hydrocarbons are based
of kinematic viscosity. on calculated values from vapor pressure data [58]. Predicted
values of critical constants and boiling point from different
methods for heavy compounds differ significantly from each
9.8 FINAL WORDS other, especially as carbon number increases. This leads to an
even greater difference in predicted thermodynamic proper-
Variety of methods for prediction and calculation of various ties. Presence of very heavy compounds in a mixture requires
thermophysical properties for petroleum and related fluids is a rigorous mixing rule for calculation of mixture properties.
much wider than the methods presented in this book. How- Attempts in this area should be focused on standardization
ever, attempts were made to include the most accurate and of values of critical constants for heavy hydrocarbons and
widely used methods by the people from industry and re- characterization of heavy oils.
searchers. Limitations of application of methods, points of Use of directly measurable properties in calculation of ther-
strength and weaknesses, and their degrees of accuracy have modynamic properties of heavy petroleum mixtures is an ap-
been discussed for different systems. Furthermore, the basis propriate approach as it was discussed in Chapters 5 and 6.
of development of nearly all methods discussed in this book Use of velocity of sound to determine EOS parameters was
have been discussed so the students and new researchers in demonstrated in Section 6.9 and new developments in this
this area can understand the basic concepts and fundamen- area are highly desirable [59]. Measurement and reporting
tals of property calculations. In addition, the approaches pre- of this thermodynamic property on heavy petroleum frac-
sented in the book should help researchers in expansion of the tions and crude oils would help researchers to find methods
existing methods and be used as a guide in the development of of calculation of EOS parameters from measurable proper-
new predictive methods. The methods presented in the book ties. Other useful and measurable properties for heavy oils
should also help users of various simulators (process, PVT, include molecular weight, density, and refractive index. Use of
phase behavior, etc.) to be able to select the most appropriate refractive index in determination of EOS parameters has been
method for their property prediction purposes. shown in Section 5.9. It seems that more advanced equations
Empirical correlations should be used with caution and of state such as SAFT equations would be more appropriate
as a last option in absence of experimental data or accurate for prediction of thermodynamic properties of heavy oils such
fundamentally based thermodynamic models. In use of these as those containing heavy residues, asphaltenes, and complex
correlations their limitations and sensitivity to the input pa- polar compounds. Investigation of this approach should be
rameters must be considered. Some of these methods are reli- continued for more accurate estimation of thermophysical
able when the input parameters are determined through rec- properties. Newly developed methods for phase equilibrium
ommended methods. Perhaps the most accurate methods are calculations and phase determination of many-component
those based on fundamental theoretical approach combined systems are useful tools in formulation and efficient predic-
with empirically determined coefficients and parameters. In tion of hydrocarbon phase behavior and should be pursued
development of such relations availability of input parame- [60].
ters and accuracy of their measurements should be consid- Another appropriate approach in characterization of heavy
ered. Furthermore, predictive methods can have general ap- oils was taken by Goual and Firoozabadi [23] to mea-
plication for a wide range of petroleum fluids if properties sure dipole moments of such complex systems. Attempts in

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