Page 128 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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108 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
3.2.2.3.2 Conversion of a Distillation Curve from Sub- or
◦
kelvin.
Super- Atmospheric Pressures to a Distillation Curve at in the above relations all temperatures are either in Corin
Atmospheric Pressure—The method of conversion of boil-
ing points through Eqs. (3.29)–(3.32) can be used to every 3.2.2.4 Summary Chart for Interconverion of Various
point on a distillation curve under either sub- or superatmo- Distillation Curves
spheric pressure conditions. In these equations T b or T (760 A summary of all conversion methods is shown in Fig. 3.20.
mm Hg) represent a point along the distillation curve at at- It should be noted that any distillation curve at low pressure
mospheric pressure. It can be applied to any of TBP, EFV, (i.e., ASTM D 1160 or EFV at 1, 10, 50, mm Hg or TBP at
or ASTM D 1160 distillation curves. However, it should be 1 mm Hg) should be first converted to TBP distillation curve
noted that these equations convert distillation curves from at 10 mm Hg before they are converted to TBP at atmospheric
one pressure to another within the same type. For example, it pressure.
is not possible to use these equations to directly convert ASTM
D 1160 at 10 mm Hg to TBP at 760 mm Hg. Such conversions Example 3.6—For a petroleum fraction the ASTM D 1160
require two steps that are discussed in the following section. distillation data at 10 mm Hg are given in Table 3.13. Predict
The only distillation curve type that might be reported un- the TBP curve at atmospheric pressure.
der superatmospheric pressure (P > 1.01325 bar) condition
is the EFV distillation curve. TBP curve may be at 1, 10, 100, Solution—ASTM D 1160 data have been converted to TBP
or 760 mm Hg pressure. Experimental data on ASTM D 1160 at 10 mm Hg by Eq. (3.34). Then Eq. (3.29) with P = 10
are usually reported at 1, 10, or 50 mm Hg. ASTM D 86 distil- mm Hg and Q = 0.001956 is used to convert TBP from 10
lation is always reported at atmospheric pressure. It should to 760 mm Hg. A summary of results is given in Table 3.13.
be noted that when ASTM D 1160 distillation curve is con- The second and less accurate method to convert TBP from 10
verted to or reported at atmospheric pressure (760 mm Hg) to 760 mm Hg is through Eq. (3.32), which in its reverse form
it is not equivalent to or the same as ASTM D 86 distillation becomes T (760 mm Hg) = 1.17T (10 mm Hg) + 67.51. Esti-
data. They are different types of distillation curves and there mated TBP at 760 mm Hg through this relation is presented
is no direct conversion between these two curves.
in the last column of Table 3.13.
3.2.2.3.3 Conversion of ASTM D 1160 at 10 mm Hg to
TBP Distillation Curve at 10 mm Hg—The only method 3.2.3 Prediction of Complete Distillation Curves
widely used under subatmospheric pressure condition for In many cases distillation data for the entire range of percent
conversion of distillation curves is the one developed by distilled are not available. This is particularly the case when a
Edmister–Okamoto [17], which is used to convert ASTM fraction contains heavy compounds toward the end of distilla-
D 1160 to TBP, both at 10 mm Hg. This method is graphi- tion curve. For such fractions distillation can be performed to
cal and it is also recommended by the API-DTB [2]. In this a certain temperature. For example, in a TBP or ASTM curve,
method it is assumed the at 50% points ASTM D 1160 and distillation data may be available at 10, 30, 50, and 70% points
TBP temperatures are equal. The Edmister–Okamoto chart but not at 90 or 95% points, which are important for process
is converted into equation form through regression of values engineers and are characteristics of a petroleum product. For
read from the figure in the following form [2]: heavier fractions the distillation curves may even end at 50%
point. For such fractions it is important that values of temper-
TBP(100%) = ASTM D 1160(100%) atures at these high percentage points to be estimated from
TBP(90%) = ASTM D 1160(90%) available data. In this section a distribution function for both
TBP(70%) = ASTM D 1160(70%) boiling point and density of petroleum fractions is presented
so that its parameters can be determined from as few as three
(3.34) TBP(50%) = ASTM D 1160(50%)
data points on the curve. The function can predict the boiling
TBP(30%) = ASTM D 1160(50%) − F 1 point for the entire range from initial point to 95% point. This
function was proposed by Riazi [31] based on a probability
TBP(10%) = ASTM D 1160(30%) − F 2
distribution model for the properties of heptane plus fractions
TBP(0%) = ASTM D 1160(10%) − F 3
in crude oils and reservoir fluids and its detailed characteris-
tics are discussed in Section 4.5.4. The distribution model is
where functions F 1 , F 2 , and F 3 are given in terms of temper-
ature difference in the ASTM D 1160: presented by the following equation (see Eq. 4.56):

1/B

F 1 = 0.3 + 1.2775( T 1 ) − 5.539 × 10 ( T 1 ) + 2.7486 (3.35) T − T ◦ = A ln 1
−3
2
×10 ( T 1 ) 3 T ◦ B 1 − x
−5
F 2 = 0.3 + 1.2775( T 2 ) − 5.539 × 10 ( T 2 ) + 2.7486 in which T is the temperature on the distillation curve in
2
−3
−5
×10 ( T 2 ) 3 kelvin and x is the volume or weight fraction of the mixture
2
−4
F 3 = 2.2566( T 3 ) − 266.2 × 10 ( T 3 ) + 1.4093 distilled. A, B, and T ◦ are the three parameters to be deter-
−4
×10 ( T 3 ) 3 mined from available data on the distillation curve through a
T 1 = ASTM D 1160(50%) − ASTM D 1160(30%) linear regression. T ◦ is in fact the initial boiling point (T at x =
0) but has to be determined from actual data with x > 0. The
T 2 = ASTM D 1160(30%) − ASTM D 1160(10%) experimental value of T ◦ should not be included in the regres-
T 3 = ASTM D 1160(10%) − ASTM D 1160(0%) sion process since it is not a reliable point. Equation (3.35)

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