Page 271 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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6. THERMODYNAMIC RELATIONS FOR PROPERTY ESTIMATIONS 251
method recommended by the API for calculation of volume
COMPONENTS—CONCEPT
of petroleum blends is based on such theory [5]. 20:46 6.5 PHASE EQUILIBRIA OF PURE
OF SATURATION PRESSURE
6.4.3 Volume of Petroleum Blends
As discussed in Section 5.2, a pure substance may exist in
One of the applications of partial molar volume is to calculate a solid, liquid or vapor phases (i.e., see Fig. 5.1). For pure
volume change due to mixing as shown by Eq. (6.86). How- substances four types of equilibrium exist: vapor–liquid (VL),
ever, for practical applications a simpler empirical method vapor–solid (VS), liquid–solid (LS) and vapor–liquid–solid
has been developed for calculation of volume change when (VLS) phases. As shown in Fig. 5.2 the VLS equilibrium oc-
petroleum products are blended. curs only at the triple point, while VL, VS, and LS equilibrium
Consider two liquid hydrocarbons or two different petrol- exist over a range of temperature and pressure. One important
eum fractions (products) which are being mixed to produce type of phase equilibria in the thermodynamics of petroleum
a blend of desired characteristics. If the mixture is an ideal fluids is vapor–liquid equilibria (VLE). The VLE line also called
solution, volume of the mixture is simply the sum of vol- vapor pressure curve for a pure substance begins from triple
umes of the components before the mixing. This is equiva- point and ends at the critical point (Fig. 5.2a). The equilib-
lent to “no volume change due to mixing.” Experience shows rium curves between solid and liquid is called fusion line and
that when a low-molecular-weight hydrocarbon is added to a between vapor and solid is called sublimation line. Now we
heavy molecular weight crude oil there is a shrinkage in vol- formulate VLE; however, the same approach may be used to
ume. This is particularly the case when a crude oil API grav- formulate any type of multiphase equilibria for single com-
ity is improved by addition of light products such as gasoline ponent systems.
or lighter hydrocarbons (i.e., butane, propane). Assume the Consider vapor and liquid phases of a substance coexist
volume of light and heavy hydrocarbons before mixing are in equilibrium at T and P (Fig. 6.7a). The pressure is called
V light and V heavy , respectively. The volume of the blend is then saturation pressure or vapor pressure and is shown by P sat .As
calculated from the following relation [5]: shown in Fig. 5.2a, vapor pressure increases with temperature
and the critical point, normal boiling point and triple point
V blend = V heavy + V light (1 − S) are all located on the vapor pressure curve. As was shown in
S = 2.14 × 10 C −0.0704 G 1.76 Fig. 2.1, for hydrocarbons the ratio T b /T c known as reduced
−5
(6.95) boiling point varies from 0.6 to more than one for very heavy
G = API light − API heavy
compounds. While the triple point temperature is almost
C = vol% of light component in the mixture
the same as the freezing point temperature, but the triple
point pressure is much lower than atmospheric pressure at
where S is called shrinkage factor and G is the API gravity
difference between light and heavy component. The amount
of shrinkage of light component due to mixing is V light (1 − S).
The following example shows application of this method.

Vapor (V)
Example 6.5—Calculate volume of a blend and its API gravity at T, P sat
produced by addition of 10000 bbl of light naphtha with API
gravity of 90 to 90000 bbl of a crude oil with API gravity of 30. V sat L sat
f (T P )=f (T P )
Solution—Equation (6.95) is used to calculate volume of
blend. The vol% of light component is 10% so C = 10. G =

90 − 30 = 60. S = 2.14 × 10 −5 × (10 −0.0704 ) × 60 1.76 = 0.025. Liquid (L)
sat
V Blend = 90000 + 10000(1 − 0.025) = 99750 bbl. The amount at T, P

of shrinkage of naphtha is 10000 × 0.025 = 250 bbl. As can
be seen from Eq. (6.95) as the difference between densities a. Pure Component System
of two components reduces the amount of shrinkage also
decreases and for two oils with the same density there is
no shrinkage. The percent shrinkage is 100S or 2.5% in this Vapor
example. It should be noted that for calculation of density sat
at T, P , y i
of mixtures a new composition should be calculated as: f i (T, P , y i ) = f i (T, P , ) x
sat
V
sat
L
x vi = 9750/99750 = 0.0977 which is equivalent to 9.77% in- i
stead of 10% originally assumed. For this example the mixture
API gravity is calculated as: SG L = 0.6388 and SG H = 0.8762
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
where L and H refer to light and heavy components. Now Liquid
sat
using Eq. (3.45): SG Blend = (1 − 0.0977) × 0.8762 + 0.0977 × at T, P , x i
0.6388 = 0.853 which gives API gravity of blend as 34.4 while
direct application of mixing rule to the API gravity with orig-
b. Multi Component System
inal composition gives API Blend = (1 − 0.1) × 30 + 0.1 × 90 =
36. Obviously the more accurate value for the API gravity of FIG. 6.7—General criteria for
blend is 34.4. vapor–liquid equilibrium.

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