Page 346 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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326 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
of the fuel but also decreases amount of unburned hydrocar-
bon and CO/CO 2 production. For these reasons, natural gas can be estimated with greater accuracy than for properties of
liquids. This is mainly due to better understanding of inter-
is considered to be a clean fuel but the cleanest and most molecular forces in gaseous systems. Similarly properties of
valuable fuel is hydrogen. This is the reason for global ac- gases at low pressures can be estimated with better accuracy
celeration of development of hydrogen fuel cells as a clean in comparison with gases at high pressures. Effect of pres-
energy, although still production of energy from hydrogen is sure on properties of gases is much greater than the effect
very costly [33]. of pressure on properties of liquids. At high pressures as we
HHV can be measured in the laboratory through combus- approach the critical region properties of gases and liquids
tion of the fuel in a bomb calorimeter surrounded by water. approach each other and under such conditions a unique gen-
Heat produced can be calculated from the rise in the tempera- eralized correlation for both gases and liquids termed dense
ture of water. The experimental procedure to measure heating fluids may be used for prediction of properties of both gases
value is explained in ASTM D 240 test method. The heating and liquids. For wide boiling range fractions or crude oils the
value of a fuel is one of the characteristics that determines mixture should be split into a number of pseudocomponents
price of a fuel. and treat the fluid as a defined mixture. A more accurate ap-
proach would be to consider the fluid as a continuous mixture
When using a thermodynamic model, cubic equations of
7.5 SUMMARY AND RECOMMENDATIONS state (i.e., PR or SRK) should be used for calculation of
PVT and equilibrium properties at pressures greater than
In this chapter, application of methods and procedures pre- about 13 bars (∼200 psia). At low pressures and especially
sented in the book for calculation and estimation of various for liquids, properties calculated from a cubic EOS are not
thermophysical properties are shown for pure hydrocarbons reliable. For liquid systems specific generalized correlations
and their defined mixtures, natural gases and nonhydrocar- developed based on liquid properties are more accurate than
bon gases associated with them (i.e. H 2 S, CO 2 ,N 2 ,H 2 O), other methods. It is on this basis that Rackett equation pro-
defined and undefined petroleum fractions, crude oils, coal vides more accurate data on liquid density than any other
liquids, and reservoir fluids. Characterization methods of correlation. In application of EOS to petroleum mixtures the
Chapters 2–4 and thermodynamic relations of Chapters 5 and BIPs especially for the key components have significant im-
6 are essential for such property calculations. Basically, ther- pact on accuracy of predicted results. Wherever possible BIP
mophysical properties can be estimated through equations of of key components (i.e., C 1 –C 7+ in a reservoir fluid) can be
state or generalized correlations. However, for some special tuned with available experimental data (i.e., density or satu-
cases empirical methods in the forms of graphical or analyt- ration pressure) to improve prediction by an EOS model [34].
ical correlations have been presented for quick estimation of
certain properties. TABLE 7.14—Summary of recommended methods for various
Methods of prediction of properties introduced in the pre- properties.
vious chapters such as density, enthalpy, heats of vaporization Property Methods of estimation for various fluids
and melting, heat capacity at constant pressure and volume, Density • Eq. (7.3) for gases with Lee–Kesler
vapor pressure, and fuels’ heating values are presented. generalized correlation for calculation
of Z (Ch 5), also see Section 7.2.1.
For calculation of properties of pure components when a • For pure liquid hydrocarbons, Table
correlation for a specific compound is available it must be 2.1 and Eq. (7.5) or Rackett equation.
used wherever applicable. Generalized correlations should • Eq. (7.4) for defined liquid mixture.
be used for calculation of properties of pure hydrocarbons • For petroleum fractions use Rackett
equation, Eq. (7.5), or Figs. 7.1–7.3.
when specific correlation (analytical or graphical) for the • See Section 7.2.2 for other cases.
given compound is not available. For defined mixtures the Vapor pressure • Eq. (7.8) for pure compounds and
best way of calculation of mixture properties when experi- if coefficients are not known use
mental data on properties of individual components of the Eqs. (7.18) or (7.19).
mixture are available is through appropriate mixing rules for • Use Eqs. (7.20) and (7.22) for
petroleum fractions and Eqs. (7.21)
a given property using pure components properties and mix- and (7.23) for coal liquids.
ture composition. For defined mixtures wherein properties • For crude oils use Eq. (7.26) or
of pure components are not available, the basic input pa- Fig. 7.11.
rameters for equations of states or generalized correlations Enthalpy • Use Eq (7.32) and Fig. 7.13 with
should be calculated from appropriate mixing rules given in Lee–Kesler correlations of Ch 6 for
petroleum fractions.
Chapter 5. These basic properties are generally T c , P c , V c , ω, • Use Eq. (7.34) for H .
ig
ig
M, and C , which are known for pure components. For • For special cases see Section 7.4.1.
P
petroleum fractions these parameters should be estimated Liquid heat capacity, C P L • Eq. (7.40) for pure compounds.
and the method of their estimations has a great impact on • Eq. (7.43) for petroleum fractions.
accuracy of predicted physical properties. In fact the impact H vap • Eq. (7.45) for coal liquids.
• Eq. (7.50) or Eqs. (7.54) and (7.57) for
of estimation of basic input properties is greater than the im- pure compounds.
pact of selected thermodynamic method on the accuracy of • Eqs. (7.54) and (7.57) for petroleum
property predictions. fractions.
For prediction of properties of petroleum fractions, spe- • Eqs. (7.58) and (7.57) for coal liquids.
cial methods are provided for undefined mixtures. For both Heating value See Section 7.4.4.
These recommendations are not general and for special cases one should see
pure compounds and petroleum mixtures, properties of gases specific recommendations in each section.

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