Page 65 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 45
are calculated by Eqs. (2.14) and (2.15) and will be used to
cific gravity were used in most correlations based on expe-
develop predictive methods for the composition of heavy frac- without any theoretical explanation. Boiling point and spe-
tions discussed in Chapter 3 (Section 3.5). rience and their availability. However, the characterization
methods proposed by Riazi and Daubert [28, 29, 35] are based
on the theory of intermolecular forces and EOS parameters
2.3 CHARACTERIZATION [36]. Although EOS are discussed in Chapter 5, their applica-
OF HYDROCARBONS tion in the development of analytical correlations to charac-
terize hydrocarbons are discussed here. In the following parts
The work on characterization of pure hydrocarbons began in in this section several characterization schemes developed by
1933 when Watson and Nelson for the first time developed Riazi et al. [28, 29, 31, 35, 37] are presented along with other
two empirical charts relating molecular weight to either boil- methods.
ing point and K W or boiling point and API gravity [24]. In
these charts boiling point and specific gravity (or API grav-
ity) are used as the two independent input parameters. Since 2.3.1 Development of a Generalized Correlation
then the work on characterization and methods of estimation for Hydrocarbon Properties
of basic properties of pure hydrocarbons and petroleum frac- Properties of a fluid depend on the intermolecular forces that
tions has continued to the present time. Methods developed exist between molecules of that fluid [38, 39]. As summarized
in 1930s till 1960s were mainly graphical, while with the use by Prausnitz et al. [39] these forces are grouped into four cat-
of computer, methods developed in 1970s till present time are egories. (1) Electrostatic forces between charged molecules
in the forms of analytical correlations. The best example of (ions) and between permanent dipoles or higher multipoles.
chart-type correlations, which has been in use by the industry, These forces result from the chemical structure of molecules
is the Winn nomogram that relates molecular weight, critical and are important in polar compounds (i.e., water, methanol,
pressure, aniline point, and CH weight ratio to boiling point ethanol, etc.). (2) Induction forces on molecules that are po-
and specific gravity [25]. A version of Winn nomograph as larizable when subjected to an electric field from polar com-
used by the API [2] is presented in Fig. 2.12. Some of the an- pounds. These forces are also called dipole forces and are de-
alytical correlations that are used in the industry are Cavett termined by dipole moment of molecules (μ), which is propor-
[26], Kesler–Lee [12], Lee–Kesler [27], Riazi–Daubert [28, 29], tional to polarizability factor, α, and the field strength. These
Twu [30], and Riazi–Sahhaf [31]. Most of these correlations forces are proportional to μ × α. (3) The third type of forces
2
use boiling point and specific gravity as the input parame- are attraction (dispersion forces) and repulsion between non-
ters to estimate parameters such as molecular weight, critical polar molecules. These forces, also called London forces, are
constants, and acentric factor. Most recently Korsten [32] has static in nature and are proportional to α . (4) The last are spe-
2
developed a characterization scheme that uses boiling point cial (chemical) forces leading to association or complex for-
and a parameter called double-bond equivalent (DBE) as the mation such as chemical bonds. According to London these
input parameters. DBE can be estimated from H/C atomic forces are additive and except for very polar compounds, the
ratio. In another paper Korsten [33] lists and evaluates vari- strongest forces are of the London type. For light and medium
ous correlations for estimation of critical properties of pure hydrocarbon compounds, London forces are the dominant
hydrocarbons. Tsonopolus et al. [34] give the list of correla- force between the molecules.
tions developed for characterization of coal liquids in terms The intermolecular force, F, is related to the potential en-
of boiling point and specific gravity. There are some meth- ergy, , according to the following relation:
ods of estimation of properties of pure compounds that are
based on various group contribution techniques. The most d
accurate methods of group contributon for various proper- (2.19) F =− dr
ties with necessary recommendations are given in the fifth
edition of Properties of Gases and Liquids [4]. Even some of where r is the distance between molecules. The negative of
these group contribution methods require properties such as the potential energy, − (r), is the work required to sepa-
molecular weight or boiling point. Examples of such proce- rate two molecules from the intermolecular distance r to in-
dures are the Lydersen and Ambrose methods [4]. The prob- finite separation. Equation of state parameters can be esti-
lem with group contribution methods is that the structure of mated from the knowledge of the potential energy relation
the compound must be known. For this reason they are not [39]. Most hydrocarbon compounds, especially the light and
appropriate for undefined petroleum fractions. However, they medium molecular weight hydrocarbons, are considered as
can be used to predict properties of pure compounds when nonpolar substances. There are two forces of attraction (dis-
experimental data are not available (i.e., critical properties persion forces) and repulsion between nonpolar molecules.
of heavy pure hydrocarbons). In fact, on this basis the prop- The common convention is that the force of attraction is neg-
erties of hydrocarbons heavier than C 18 have been predicted ative and that of repulsion is positive. As an example, when
and reported by the API [2]. molecules of methane are 1 nm apart, the force of attraction
−8
As discussed, during the past 70 years many methods in between them is 2 × 10 dyne [39]. The following relation
the forms of charts and equations were proposed to esti- was first proposed by Mie for the potential energy of nonpolar
mate the basic properties of hydrocarbons from the knowl- molecules [39]:
edge of the boiling point and specific gravity or the molecular A ◦ B ◦
weight. Nearly all of these correlations are empirical in nature (2.20) = r n − r m

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