Page 51 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 31
AS DISCUSSED IN CHAPTER 1, the characterization of petroleum
lation and methods for its calculation for petroleum fractions
fractions and crude oils depends on the characterization and pressure to normal boiling point requires a vapor pressure re-
properties of pure hydrocarbons. Calculation of the prop- are discussed in Chapter 3. The boiling point, when available,
erties of a mixture depends on the properties of its con- is one of the most important characterization parameters for
stituents. In this chapter, first basic parameters and properties hydrocarbons and is frequently used in property estimation
of pure compounds are defined. These properties are either methods.
temperature-independent or values of some basic properties
at a fixed temperature. These parameters are the basis for 2.1.3 Density, Specific Gravity, and API Gravity
calculation of various physical properties discussed in this
book. Reported values of these parameters for more than Density is defined as mass per unit volume of a fluid. Density
100 selected pure compounds are given in Section 2.2. These is a state function and for a pure compound depends on both
values will be used extensively in the following chapters, es- temperature and pressure and is shown by ρ. Liquid densities
pecially in Chapters 3 to determine the quality and properties decrease as temperature increases but the effect of pressure
of petroleum fractions. In Section 2.3, the characterization of on liquid densities at moderate pressures is usually negligible.
hydrocarbons is introduced, followed by the development of At low and moderate pressures (less than a few bars), satu-
a generalized correlation for property estimation that is a rated liquid density is nearly the same as actual density at the
unique feature of this chapter. Various correlations and meth- same temperature. Methods of the estimation of densities of
ods for the estimation of these basic parameters for pure hy- fluids at various conditions are discussed in Chapters 5 and 7.
drocarbons and narrow boiling range petroleum fractions are However, liquid density at the reference conditions of 20 C
◦
presented in different sections. Finally, necessary discussion (293 K) and 1 atm is shown by d and it is used as a characteri-
and recommendations for the selection of appropriate pre- zation parameter in this chapter as well as Chapter 3. Parame-
dictive methods for various properties are presented. ter d is also called absolute density to distinguish from relative
density. Other parameters that represent density are specific
volume (1/d), molar volume (M/d), and molar density (d/M).
2.1 DEFINITION OF BASIC PROPERTIES Generally, absolute density is used in this book as the charac-
teristic parameter to classify properties of hydrocarbons.
In this section, all properties of pure hydrocarbons presented Liquid density for hydrocarbons is usually reported in
in Section 2.2 are defined. Some specific characteristics of terms of specific gravity (SG) or relative density defined as
petroleum products, such as cetane index and pour point, are
defined in Chapter 3. Definitions of general physical proper- (2.1) SG = density of liquid at temperature T
ties such as thermal and transport properties are discussed in density of water at temperature T
corresponding chapters where their estimation methods are Since the standard conditions adopted by the petroleum in-
presented. dustry are 60 F (15.5 C) and 1 atm, specific gravities of liquid
◦
◦
hydrocarbons are normally reported at these conditions. At a
2.1.1 Molecular Weight reference temperature of 60 F (15.5 C) the density of liquid
◦
◦
3
3
water is 0.999 g/cm (999 kg/m ) or 8.337 lb/gal(U.S.). There-
The units and definition of molecular weight or molar mass, fore, for a hydrocarbon or a petroleum fraction, the specific
M, was discussed in Section 1.7.8. The molecular weight of a gravity is defined as
pure compound is determined from its chemical formula and
the atomic weights of its elements. The atomic weights of the ◦ ◦ density of liquid at 60 F in g/cm 3
◦
elements found in a petroleum fluid are C = 12.011, H = 1.008, (2.2) SG (60 F/60 F) = 0.999 g/cm 3
S = 32.065, O = 16.0, and N = 14.01, as given by the IUPAC 3
◦
standard [1]. As an example, the molecular weight of methane Water density at 60 F is 0.999 or almost 1 g/cm ; therefore,
(CH 4 ) is calculated as 12.011 + 4 × 1.008 = 16.043 kg/kmol or values of specific gravities are nearly the same as the density
3
◦
16.043 g/mol (0.01604 kg/mol) or 16.043 lb/lbmol. Molecular of liquid at 15.5 C (289 K) in g/cm . The Society of Petroleum
weight is one of the characterization parameters for hydro- Engineers usually uses γ for the specific gravity and in some
carbons. references it is designated by S. However, in this book SG de-
notes the specific gravity. Since most of hydrocarbons found
in reservoir fluids have densities less than that of water, spe-
2.1.2 Boiling Point
cific gravities of hydrocarbons are generally less than 1. Spe-
The boiling point of a pure compound at a given pressure cific gravity defined by Eq. (2.2) is slightly different from the
is the temperature at which vapor and liquid exist together specific gravity defined in the SI system as the ratio of the den-
at equilibrium. If the pressure is 1 atm, the boiling point is sity of hydrocarbon at 15 C to that of water at 4 C designated
◦
◦
3
15
called the normal boiling point. However, usually the term by d . Note that density of water at 4 C is exactly 1 g/cm and
◦
4
boiling point, T b , is used instead of normal boiling point and therefore d 4 15 is equal to the density of hydrocarbon at 15 C
◦
3
for other pressures the term saturation temperature is used. in g/cm . The relation between these two specific gravities is
In some cases, especially for heavy hydrocarbons in which approximately given as follows:
thermal cracking may occur at high temperatures, boiling (2.3) SG = 1.001 d 15
points at pressures other than atmospheric is specified. Boil- 4
ing points of heavy hydrocarbons are usually measured at 1, In this book specific gravity refers to SG at 60 F/60 F (15.5 C).
◦
◦
◦
10, or 50 mm Hg. The conversion of boiling point from low In the early years of the petroleum industry, the American
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