Page 152 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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AT029-Manual
AT029-03
June 22, 2007
132 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
blending RVP of each cut is then calculated by the following
pressure may be estimated from T b and SG using methods
equation: AT029-Manual-v7.cls T1: IML 14:23 and specific gravity of the fraction. Critical temperature and
presented in Chapter 2. This equation is based on data with
7.641
B i = RVP in the range of 0.0007–1.207 bar (0.01–17.5 psia), normal
exp (0.03402T bi + 0.6048) boiling point range of 305–494 K, and specific gravity range
i=28 of 0.65–1.08. The average absolute deviation for 52 samples is

(3.101) P a = B i x vi 0.061 bar (0.88 psia). The above equation may be used for cal-
i=1 culation of RVP to determine quality characteristics of a fuel.
f = 1.0 + 0.003744 (VAPB − 93.3) The calculated RVP value should not be used for calculation
RVP = fP a of TVP when very accurate values are needed. (Appropriate
methods for direct estimation of TVP of petroleum fractions
where B i = RVP is the blending number for cut i and T bi = are discussed in Section 7.3.3.) Vapor pressure of a petroleum
normal boiling point of cut i in C. x vi is the volume fraction mixture depends on the type of its constituents and with use
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of cut i, VABP is the volume average boiling point in C, and of only two bulk properties to predict RVP is a difficult task.
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RVP is the Reid vapor pressure in bars. The constants were ob- This equation is recommended for a quick and convenient
tained from the original constants given in the English units. estimation of RVP, but occasionally large errors may be ob-
The average error for this method for 51 samples was 0.12 tained in use of this equation. For more accurate estimation
bar or1.8 psi [67, 68]. of RVP the sophisticated method suggested in the API-TDB
Recently some data on RVP of gasoline samples have been [2] may be used. In this method RVP is calculated through a
reported by Hatzioznnidis et al. [69]. They measured vapor series of vapor-liquid-equilibrium calculations.
pressure according to ASTM D 5191 method and related to RVP is one of the main characteristics that is usually used
RVP. They also related their measured vapor pressure data to to blend a fuel with desired specifications. The desired RVP
TVP thus one can obtain RVP from TVP, but their relations of a gasoline is obtained by blending naphtha with n-butane
have not been evaluated against a wide range of petroleum (M = 58, RVP = 3.58 bar or 52 psia) or another pure hydro-
fractions. Other relations for calculation of TVP from RVP carbon with higher RVPs than the original fuel. For condi-
for petroleum fractions and crude oils are given in Section tions where RVP should be lowered (hot weather), heavier
7.3.3. TVP at 100 F (311 K) can be estimated from Eq. (3.33) hydrocarbons with lower RVP are used for blending purposes.
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as --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`--- RVP of several pure hydrocarbons are given as follows: i-C 4 :
T b − 41
4.896 bar (71 psia); n-C 4 : 3.585 (52); i-C 5 :1.338 (19.4); n-C 5 :
(3.102) log (TVP) 100 = 3.204 × 1 − 4 × 1.0135 (14.7); i-C 6 : 0.441 (6.4); n-C 6 : 0.34 (5.0); benzene: 0.207
10
1393 − T b
(3.0); and toluene: 0.03 (0.5), where all the numbers inside the
where T b is the normal boiling point in K and TVP 100 is the parentheses are in psia as given in Ref. [63]. However in the
true vapor pressure at 100 F (311 K). Once TVP is calculated same reference in various chapters different values of RVP
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it may be used instead of RVP in the case of lack of sufficient for a same compound have been used. For example, values of
data. When this equation is used to estimate RVP of more than 4.14 bar (60 psi) for n-C 4 , 1.1 bar (16 psi) for n-C 5 , and 0.48
50 petroleum products an average error of 0.13 bar (∼1.9 psi) bar (7 psi) for i-C 6 are also reported by Gary and Handwerk
and a maximum error of 5.9 psi were obtained [67, 68]. [63]. They also suggested two methods for calculation of RVP
RVP data on 52 different petroleum products (light and of a blend when several components with different RVPs are
heavy naphthas, gasolines, and kerosenes) from the Oil and blended. The first method is based on the simple Kay’s mixing
Gas Journal data bank [46] have been used to develop a sim- rule using mole fraction (x mi ) of each component [63]:
ple relation for prediction of RVP in terms of boiling point
and specific gravity in the following form [67]: (3.104) RVP(blend) = x mi (RVP) i
i
RVP = P c exp(Y) where (RVP) i is the RVP of component i in bar or psia. The
T b SG 5
second approach is to use blending index for RVP as [63]:
Y =−X (1 − T r) 1.25
T r (RVPBI) i = (RVP) i

X =−276.7445 + 0.06444T b + 10.0245SG − 0.129T b SG (3.105) RVPBI (blend) = x vi (RVPBI) i
i
9968.8675 0.8
+ + 44.6778 ln T b + 63.6683 ln SG RVP (blend) = RVPBI (blend)
T b SG
where (RVPBI) i is the blending index for (RVP) i and x vi is
T r = 311/T c the volume fraction of component i. Both units of bar or
(3.103) psia may be used in the above equation. This relation was
originally developed by Chevron and is also recommended
where T b is the mid boiling point and T c is the pseudocritical in other industrial manuals under Chevron blending number
temperature of the fraction in kelvin. P c is the pseudocritical [61]. Equations (3.104) and (3.105) may also be applied to
pressure and RVP is the Reid vapor pressure in bars. The basis TVP; however, methods of calculation of TVP of mixtures are
for development of this equation was to use Miller equation discussed in Section 7.3 through thermodynamic relations.
for TVP and its application at 311 K (100 F). The Miller equa-
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tion (Eq. 7.13) is presented in Section 7.3.1. The constants Example 3.19—Estimate RVP of a gasoline sample has
of vapor pressure correlation were related to boiling point molecular weight of 86 and API gravity of 86.

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