Page 52 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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32 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
Petroleum Institute (API) defined the API gravity (degrees
◦
index is called a refractometer and is discussed in Chapter 3.
API) to quantify the quality of petroleum products and crude at 20 C and 1 atm. The instrument to measure the refractive
oils. The API gravity is defined as [2] In some references the values of refractive index are reported
at 25 C; however, in this book the refractive index at 20 C and
◦
◦
141.5
(2.4) API gravity = − 131.5 1 atm is used as a characterization parameter for hydrocar-
SG (at 60 F)
◦
bons and petroleum fractions. As is shown in this chapter and
Degrees API was derived from the degrees Baum´e in which Chapter 3, refractive index is a very useful characterization
it is defined in terms of specific gravity similar to Eq. (2.4) parameter for pure hydrocarbons and petroleum fractions,
except numerical values of 140 and 130 were used instead especially in relation with molecular type composition. Val-
of 141.5 and 131.5, respectively. Liquid hydrocarbons with ues of n vary from about 1.3 for propane to 1.6 for some
lower specific gravities have higher API gravity. Aromatic hy- aromatics. Aromatic hydrocarbons have generally higher
drocarbons have higher specific gravity (lower API gravity) n values than paraffinic compounds as shown in Table 2.1.
than do paraffinic hydrocarbons. For example, benzene has
SG of 0.8832 (API of 28.72) while n-hexane with the same car-
bon number has SG of 0.6651 (API gravity of 81.25). A liquid 2.1.5 Critical Constants (T c , P c , V c , Z c )
with SG of 1 has API gravity of 10. Once Eq. (2.4) is reversed it The critical point is a point on the pressure–volume–
can be used to calculate specific gravity from the API gravity. temperature diagram where the saturated liquid and satu-
141.5 rated vapor are identical and indistinguishable. The temper-
(2.5) SG = ature, pressure, and volume of a pure substance at the critical
API gravity + 131.5
point are called critical temperature (T c ), critical pressure (P c ),
The definition of specific gravity for gases is somewhat dif- and critical volume (V c ), respectively. In other words, the crit-
ferent. It is defined as relative density of gas to density of air at ical temperature and pressure for a pure compound are the
standard conditions. In addition, density of gases is a strong highest temperature and pressure at which the vapor and liq-
function of pressure. Since at the standard conditions (15.5 C uid phase can coexist at equilibrium. In fact, for a pure com-
◦
and 1 atm) the density of gases are estimated from the ideal pound at temperatures above the critical temperature, it is
gas law (see Chapter 5), the specific gravity of a gas is pro- impossible to liquefy a vapor no matter how high the pres-
portional to the ratio of molecular weight of gas (M g )tothe sure is. A fluid whose temperature and pressure are above
molecular weight of air (28.97). the critical point is called supercritical fluid. For pure com-
pounds, critical temperature and pressure are also called true
M g
(2.6) SG g = critical temperature and true critical pressure. However, as
28.97
will be discussed in Chapter 3, pseudocritical properties are
Therefore, to obtain the specific gravity of a gas, only its defined for mixtures and petroleum fractions, which are dif-
molecular weight is needed. For a mixture, M g can be deter- ferent from true critical properties. Pseudocritical properties
mined from the gas composition, as discussed in Chapter 3.
are important in process calculations for the estimation of
thermophysical properties of mixtures.
2.1.4 Refractive Index The critical compressibility factor, Z c , is defined from T c , P c ,
and V c according to the general definition of compressibility
Refractive index or refractivity for a substance is defined as factor.
the ratio of velocity of light in a vacuum to the velocity of
light in the substance (fluid) and is a dimensionless quantity (2.8) Z c = P c V c
shown by n: RT c
velocity of light in the vacuum where R is the universal gas constant. According to Eq. (2.8),
(2.7) n =
velocity of light in the substance Z c is dimensionless and V c must be in terms of molar vol-
3
ume (i.e., cm /mol) to be consistent with R values given in
In other words, when a light beam passes from one substance Section 1.7.24. Critical temperature, pressure, and volume
(air) to another (a liquid), it is bent or refracted because of (T c , P c ,V c ) are called the critical constants or critical properties.
the difference in speed between the two substances. In fact, Critical constants are important characteristics of pure com-
refractive index indicates the degree of this refraction. Refrac- pounds and mixtures and are used in corresponding states
tive index is a state function and depends on the temperature correlations and equations of state (EOS) to calculate PVT and
and pressure of a fluid. Since the velocity of light in a fluid is many other thermodynamic, physical, and transport proper-
less than the velocity of light in a vacuum, its value for a fluid ties. Further discussion on the critical point of a substance
is greater than unity. Liquids have higher values of refractive is given in Chapter 5. As was discussed in Section 1.3, the re-
index than that of gases. For gases the values of refractive sults of EOS calculations very much depend on the values of
index are very close to unity. critical properties used. Critical volume may be expressed in
All frequencies of electromagnetic radiation (light) travel terms of specific critical volume (i.e., m /kg), molar critical
3
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
8
at the same speed in vacuum (2.998 × 10 m/s); however, in a volume (i.e., m /kmol), or critical density d c (i.e., kg/m )or
3
3
substance the velocity of light depends on the nature of the critical molar density (i.e., kmol/m ). Critical density is re-
3
substance (molecular structure) as well as the frequency of lated to the critical molar volume as
the light. For this reason, standard values of refractive index
must be measured at a standard frequency. Usually the refrac- (2.9) d c = M
tive index of hydrocarbons is measured by the sodium D line V c

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