Petroleum Refining-Practical/ 3rd Stage
Boiling points and Distillation curves
For any pure component, a single temperature generally represents the boiling
point for the component. However, a mixture of different components such as crude
oil or petroleum fractions, where the boiling point differs from more volatile
components to less volatile components. Therefore, the boiling points of a crude oil
of unknown composition can be represented by a curve of different boiling point
temperatures versus volume % of crude oil distilled (Riazi, Characterization and
properties of petroleum fractions, 2005). A typical TBP curve is shown in figure
below.
True boiling points curve is the presentation of volatility characteristics of petroleum
fractions or crude oils. The boiling point of the lightest hydrocarbon component is
called as initial boiling point (IBP). The boiling of the heaviest hydrocarbon
component is called as final boiling point (FBP). The temperature range between
initial boiling point and final boiling point is called boiling point range. For a
particular petroleum fraction or crude oil, IBP and FBP points are not as reliable as
the middle points of boiling range. In reality, it is very difficult to predict the final
boiling point for heavy hydrocarbons. However, the FBP is estimated for the crude
oil, the heaviest hydrocarbon may not vaporize at that temperature. If boiling points
are estimated up to 70% of total crude oil feed, then the remaining 30 % of crude is
referred as a residue.
There are several methods to determine boiling points and develop true boiling
point curve of petroleum fractions that are described below.
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Laboratory Test Methods
ASTM D86
ASTM D86 is a batch distillation operation for determining and reporting boiling
points of crude oils, and petroleum fractions. It is a simple and convenient method
mostly applied to the distillation of motor gasoline, aviation gasoline, aviation
turbine fuel, naphtha, kerosene, gas oil, distillate fuel oil, and similar petroleum
products.
The distillation is conducted at atmospheric pressure, which required a very small
amount of sample for the experiment. ASTM D86 operation is partly similar to a
simple distillation operation; the system consists of apparatus such as a flask with
attached glass tube condenser, temperature sensor on the top of the flask, and
heating source. The system does not consist of packed or tray column (of minimum
15 theoretical plates) and reflux arrangement causes the operation not to attain a
high degree of separation; therefore the boiling points from this standard method
are not true boiling points of petroleum fractions. The boiling points from ASTM D86
need to be converted to the true boiling points.
ASTM D1160
ASTM D1160 is nothing but the vacuum distillation frequently used for the
determination of boiling points of heavy hydrocarbons such as atmospheric residue
after ASTM D86 method. As mentioned earlier ASTM D86 is an atmospheric
distillation. All heavy hydrocarbons cannot be vaporized in the atmospheric
distillation. Actually above 300℃ in the atmospheric distillation, heavy hydrocarbons
usually undergo a cracking process in which they start to break down in to lighter
molecules. Therefore, there is a need to execute the further distillation at reduced
pressure in order to avoid the cracking process.
ASTM D1160 is the test method in which the distillation is carried out at reduced
pressures without any significant cracking
approximately one theoretical plate fractionation; therefore, the boiling point curves
from ASTM D1160 are closer to true boiling point curves.
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ASTM D2892
includes
an atmospheric distillation as well as a reduced pressure distillation.
ASTM D2892 system contains column of 14 to 18 theoretical plates and operates
on reflux ratio of 5:1 which gives high degree of fractionation. Therefore, the
distillation curves obtained from the system is in the form of true boiling point (TBP)
and it can be expressed with mass or volume.
ASTM D2892 is the laboratory standard which plays crucial part in estimation of
crude oil value, determination of yields of fractions of different boiling points, and
technical discussions for fixing the commercial values of petroleum products.
Because of high degree of separation, the products obtained from this laboratory
technique can be used for analytical studies, engineering, and products quality
evaluations.
True Boiling Point Method
The boiling curves from all ASTM test methods except ASTM D 2892 are not true
boiling point curves for crude oil or fractions.
TBP distillations require columns with 15-100 theoretical plates and high reflux
ratios (5:1 or greater) to achieve accurate separation of components in a mixture.
TBP distillations are difficult, time consuming, and expensive as compared to
Laboratory test methods.
ASTM D2892 is only laboratory test method gives high degree of fractionation
because the system has column with 15 theoretical stages and it operates at reflux
ratio of 5:1. This method offers the compromise between efficiency and time so that
the comparison of distillation data from different laboratories can occur (ASTM
D2892, 1998).
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Characterization Factors:
Watson K
Since the early days of the petroleum industry it was desired to define a
characterization parameter based on measurable parameters (some physical
properties of crude oil) to classify and identify hydrocarbon types.
The Watson characterization factor Kw is one of the oldest characterization factors
originally defined by Watson et al. of the Universal Oil Products (UOP) in mid
1930s. For this reason the parameter is sometimes called UOP characterization
factor and is defined as:
Where:
= mean average boiling point,
SG = specific gravity
The purpose of definition of this factor was to classify the type of hydrocarbons in
petroleum mixtures. Depending on the value of the Watson characterization factor,
crude oils are classified as
Paraffinic (Kw = 11-12.9),
Naphthenic (Kw = 10-11), or
Aromatic (Kw < 10)
Given the ASTM D86 distillation the volume average boiling point (VABP) can
be calculated as the average of the five boiling temperatures at 10, 30, 50, 70
and 90 percent distilled.
where all temperatures are in F.
The Mean Average Boiling Point (MeABP) is calculated using the following
equation:
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Example:
Calculate the MeABP of a petroleum fraction if the API gravity of this fraction
is 62 and calculate the Watson’s characterization factor.
Solution:
The D86 distillation temperatures are converted to F. The VABP is obtained
from equation:
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ASTM D 86 and TBP Conversion:
If aistillation data are available in the form of ASTM D 86 and desired distillation is
TBP, the following procedure can be followed:
The results of ASTM D 86 distillations can be converted into an atmospheric ASTM
2892 TBP by using an equation proposed by Riazi and published by the API
Riazi-Dauber Method
. . . . . . . . . . . . . . . . . (1)
Correlation Constants for Equation (1)
ASTM D 86
Vol.% a b
range, oC
0 0.9177 1.0019 20-230
10 0.5564 1.0900 35-305
30 0.7617 1.0425 50-315
50 0.9013 1.0176 55-320
70 0.8821 1.0226 65-330
90 0.9552 1.0110 75-345
95 0.8177 1.0355 75-400
Example:
A low boiling naphtha has the ASTM D86 temperatures of 77.8, 107.8, 126.7, 155
and 184.4 0C at 10, 30, 50, 70 and 90 vol.% distilled. Calculate VABP and MeABP
for this fraction.
Solution:
Using equations above VABP and SL are calculated as follows:
VABP = (77.8 + 107.8 + 126.7 + 155 + 184.5) / 5 = 130.3 oC = 403.5 oK
SL = (184.4 - 77.8) / 80 = 1.333 oC (K)/%
From the following Equation, correction temperature is calculated:
o
Δ = C
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MeABP =
MeABP = 121.7 oC
Example:
ASTM D 86 and TBP distillation data for a kerosene sample are given in the
following table. Predict the TBP curve from ASTM data using Riazi-Daubert method
and calculate AAD for the method.
Vol. % ASTM D 86 TBP
distilled Exp. oC Exp. oC
0 165.6 146.1
10 176.7 160.6
30 193.3 188.3
50 206.7 209.4
70 222.8 230.6
90 242.8 255.0
Solution: The Riazi-Daubert method for conversion of ASTM data is presented by
equation (1) and constants in table given above.
The summary of results is given in table below. The average absolute deviation is
calculated as 2.2 oC.
Vol. % ASTM D 86 TBP TBP
AD, oC
distilled Exp. oC Exp. oC Calculated, oC
0 165.6 146.1 134.1 12.0
10 176.7 160.6 160.6 0.0
30 193.3 188.3 188.2 0.1
50 206.7 209.4 208.9 0.5
70 222.8 230.6 230.2 0.4
90 242.8 255.0 254.7 0.3
AAD, oC 2.2
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Transformation of atmospheric TBP curve to ASTM D 86- A relationship
established by Riazi (1982)
. . . . . . . . . . . . . . . . . (2)
Where:
Temperature TBP [K] and Temperature of ASTM D 86 [K]
a, b = Transformation Coefficients (See application example)
Example:
Distillation ASTM D86 to TBP (Riazi Method)
Vol. % Coefficient Coefficient Temp. Temp.
distilled a b D 86, oC TBP, oC
0 0.9177 1.0019 36.5 14.1
10 0.5564 1.0900 54.1 33.4
30 0.7617 1.0425 76.9 68.9
50 0.9013 1.0176 101.5 101.6
70 0.8821 1.0226 131.0 135.1
90 0.9552 1.0110 171.0 180.5
95 0.8177 1.0355 186.5 194.1
The accuracy of method depends on the smoothness of curve. Errors affect the
points in the low % distilled ranges. Average error is in order of 5 °C for smooth
curve.
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