THERMOPHYSICAL PROPERTIES OF PETROLEUM FRACTIONS AND CRUDE OILS
3.1 Introduction A petroleum refinery is a collection of unit operations, such as fractionation towers, pumps, and heat exchangers. Analysis and design of these units require knowledge of the thermodynamic and physical properties of the petroleum fluids. Designing a crude distillation tower require knowledge on how the hydrocarbons in the crude oil distribute themselves on each tray on the tower, i.e. vapor‐liquid distribution, and the densities of the mixture. Heat exchanger design depends on enthalpies, thermal conductivity and viscosity of the flowing streams. Table 3.1 lists the thermophysical properties required for the design and operation of almost every piece of processing equipment in the refinery. Table 3.1 Thermopysical Property Prediction ___________________________________________________ Thermodynamic Properties: Enthalpy Heat Capacity Compressibility Factors Equilibrium K‐ values Flash Curves Transport Properties: Viscosity Thermal Conductivity Diffusivity Physical Properties: Densities Volumes Due to the complexity of the composition of petroleum fractions and crude oils it is not possible to measure or calculate accurately all of these properties. Furthermore, calculation methods developed for pure hydrocarbons are not always applicable. Therefore chemical and petroleum refining engineers over the years have developed special methods or correlations to estimate the 1
properties of petroleum fraction from easily measured properties like the normal boiling point and specific gravity. Such methods and schemes characterize these petroleum fractions. Traditionally these correlations are developed to be simple to use, requiring minimum input data, and are usually presented in graphical form. At this stage it is instructive to point out that there is a difference between the characterization of crude oils and narrow boiling petroleum fractions. For crude oils, mixture bulk properties cannot directly be used to estimate the properties of the mixture. While for narrow boiling fractions, mixture properties such as the average boiling point, specific gravity, molecular weight may directly be used to predict many other properties as outlined in the API Technical Data Book‐Petroleum Refining (1).
3.2 Basic Input Data The characterization of petroleum fractions require several laboratory measurable properties: a‐ Specific gravity (S ) b‐ Boiling point curve (ASTM or True Boiling Point distillation) c‐ Kinematic viscosity at 100 and 210 F (ν 100 ,ν 210 ) d‐ Refractive index ( n ) e‐ Molecular weight ( M ) With the exception of True Boiling Point distillation and molecular weight, these properties can be readily measured in any petroleum characterization laboratory. The meaning of these properties will be explained later. 3.2.1 Gravity: Specific gravity for liquid oils is defined as: S liq =
ρ liq ρ water
3.1
Both densities of oil and water are at some standard temperature and pressure. These standard conditions for specific gravity of petroleum liquids are 1 atm and 60 F. Since under the same conditions, most petroleum fractions are lighter than water, γ liq ≤ 1 . Another parameter for oil density is API gravity defined as: 141.5 API = − 131.5 S Heavy oils have low API and light oils high API gravities.
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3.2
Density measurement of petroleum fractions and crude oils are carried out using either a pycnometer or a Mettler/Parr densitometer. The latter method is based on density‐dependent frequency changes of an oscillating glass U‐ tube. 3.2.2 Boiling Point Curves ASTM or true boiling point distillations (TBP) characterize the volatility of petroleum fractions and crude oils. Both are batch distillations, which differ mainly in the degree of fractionation obtained during the distillation. 3.3.3 ASTM Distillation: This is carried out in a relatively simple apparatus consisting of a flask holding the sample connected to an inclined condenser, which condensed the rising vapors. The fractions distilled are collected in a graduated cylinder. The temperature of the rising vapors is recorded at specific interval of the collected distillates. This is essentially a batch distillation with one equilibrium stage and no reflux and minimum separation of the components of the fractions. For gasolines, kerosines, gas oil and similar light and middle distillates the ASTM method D86 which carried out at atmospheric pressure is used. Heavy petroleum products which tend to decompose in the ASTM D86 method but can be partially or completely vaporized at a maximum temperature of 750 F (400 °C) at pressures down to 1mm Hg are distilled using the ASTM D1160 method. It is carried out at pressures between 1 mmHg and 760 mmHg. The temperature at which the first drop of condensate is collected is called the initial boiling point (IBP). The end point (EP) is the maximum vapor temperature when almost the entire sample is distilled (above 95%). Boiling temperatures at sub‐atmospheric pressures (less than 760 mmHg) can be converted to normal boiling points (at 760 mmHg) using procedure 5A1.13 of API –TDB. Since minimum fractionation occurs in ASTM distillation, components in the mixture do not distill one by one in the order of their boiling points, but as mixtures of successively higher boiling points. Thus, components boiling below the IBP and above the EP are present in the sample. Nevertheless, because ASTM distillations are quickly conducted and have been successfully automated, require only a small sample, and are quite reproducible, they are widely used for comparison and specification of petroleum fractions.
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3.2.3 True Boiling Point Distillation: Data from TBP distillation provides a more detailed characterization of the volatility of the crude oil or petroleum fraction. It is performed in columns with 15 to 100 theoretical plates or equilibrium stages at relatively high reflux. The rising vapors are condensed and collected either at a constant rate of boiling points or constant rate of sample vaporized. Operation is at 760 mmHg for boiling points below 750 F. For higher boiling point fractions; the distillation is conducted at reduced pressures as low as0.5 mmHg by pulling vacuum at the top of the column. Results from vacuum operation are extrapolated to atmospheric pressure by the vapor pressure correlation of Maxwell and Bonner. The high degree of fractionation in this test gives accurate component distribution. Because the degree of separation for a TBP distillation test is much higher than for an ASTM distillation test, the IBP is lower and the EP is higher for the TBP method as compared with the ASTM method. The TBP curve which is a plot of the normal boiling point versus the percent sample distilled is usually used as basis for the characterization of the crude oil or petroleum product for the purposed of design and analysis. 3.2.4 Conversion between ASTM and TBP distillation: Since TBP distillation is both tedious and time‐consuming vis‐à‐vis the ASTM method, there has been an incentive to develop correlation to convert ASTM to TBP distillation and achieving the benefit of the detailed separation of TBP with the little effort in ASTM distillation. The following equations suggested by Riazi and Duabert and published by the API(1) are used for the interconversion: TBP = a( ASTM D86) b 3.3 a, b = constants varying with percent of liquid sample distilled as given in Table 3.2. TBP = true boiling point temperatures at 0, 10, 30, 50, 70, 90, and 95 volume percent distilled, in degrees Rankine. ASTM D86 = observed ASTM D86 temperatures at corresponding volume percent distilled, in degrees Rankine. Average error between the calculated and measured TBP is in the range of 5 °C.
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Table 3.2 Constants for equation 3.3
Volume % distilled 0 10 30 50 70 90 95
a
b
0.9167 0.5277 0.7429 0.8920 0.8705 0.9490 0.8008
1.0019 1.0900 1.0425 1.0176 1.0226 1.0110 1.0355
More recently, Daubert (2) published a new method for distillation curves interconversion using the following equations: T50' = A4 (T50 ) B 4 T30' = T50' − ∆T3' T10' = T30' − ∆T2'
T0' = T10' − ∆T1' T95' = T90' + ∆T7' where
T70' = T50' + ∆T5'
T90' = T70' + ∆T6'
∆Ti = Ai (∆Ti ) ∆T2 = T30 − T10 ∆T6 = T90 − T70 '
Bi
∆T1 = T10 − T0 ∆T3 = T50 − T30 ∆T7 = T f − T90
3.4
∆T2 = T30 − T10 ∆T5 = T70 − T50
The symbol T and Tʹ stands for ASTM D86 and TBP temperatures respectively, both in °F. The subscript 0 and f stands for the initial and final temperatures respectively. Ai and Bi are constants given in Table 3.3 Table 3.3 Constants for Daubertʹs Distillation Curves Interconversion Method Index Ai Bi number i 1 7.4012 0.6024 2 4.9004 0.7164 3 3.0305 0.8008 4 0.8718 1.0258 5 2.5282 0.8200 6 3.0419 0.7750 7 0.1180 1.6606 The reported average error for this method is about 3°C Example 3.1:
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A petroleum cut has the following ASTM D86 Distillation data: Volume % 0 10 30 50 70 90 95 distilled Temperature, 36.5 54 77 101.5 131 171 186.5 °C Convert these data to TBP data using the API method of Riazi and Daubert and Daubertʹs method. Plot the results and compare. Solution: Application of API method is straightforward using the constants in Table 3.2. For the Dauberʹs method, the constants in Table 3.3 are used to calculate TBP at 50 vol%. the ΔT are calculated from the ASTM data, the ΔTʹ for the TBP are calculated. Then Tʹ for TBP are calculated using the formulas in equation 3.2. The results are shown in Table 3.4 Table 3.4 : Example 3.1 Volume % D86 T, TBP, °C TBP, °C distilled °C API Daubert 0 36.5 14.1 ‐5.3 10 54 33.4 27.5 30 77 69.0 66.7 50 101.5 101.6 101.7 70 131 135.2 138.1 90 171 180.5 180.8 95 186.5 194.1 197.3
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250.0
200.0
Temperature, C
API TBP Curve ASTM D86
150.0
Duabert TBP 100.0
50.0
0.0 0
10
20
30
40
50
60
70
80
90
100
‐50.0
Volume % Distilled
Figure 3.1 Conversion of ASTM D86 into TBP (Example 3.1) As can be seen from Figure 3.1, The TBP distillation curve is below the ASTM curve at volume distilled below 50% and above it for volume distilled above 50%. The API and Duabertʹs methods give comparable results except for the low range, less than 10% distilled. Based on the distillation curve, five different average boiling points can be estimated. Among these, the volume average boiling point (VABP) and the mean average boiling points (MeABP) are the most widely used in property estimation and design. The mean average boiling point is utilized in the definition of an important parameter, the Watson characterization factor K given by: 1
( MeABP) 3 3.5 K = S Where MeABP is in degrees Rankine. The following is the procedure for estimating the average boiling point when the ASTM D86 distillation data is available. The VABP is calculated from the boiling temperatures at each of the 10,30,50,70, and 90 percent distilled: T + T30 + T50 + T70 + T90 VABP = 10 3.6 5
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Where all temperatures are in degrees ° F. The MeABP is calculated using the following equation: MeABP = VABP − ∆ 3.7 Where ∆ is given by: ln ∆ = −0.94402 − 0.00865(VABP − 32) 0.6667 + 2.99791SL0.333 T − T10 SL = 90 90 − 10 As mentioned before ASTM and TBP distillation can be performed on crude oils and petroleum products. The petroleum fractions are “cuts” from the crude oil with specific boiling point range and with special specification of properties such as API gravity and viscosity. Each of these cuts can be further defined by dividing them into narrow boiling fractions, called pseudo‐ (not real) components. For these pseudo‐components, the average boiling point can be estimated as either mid‐boiling point or mid‐percentage boiling point. The TBP curve is divided into an arbitrary number of pseudo‐components or narrow boiling cuts. The average boiling point is either the average between the IBP and the EP of that pseudo‐component. The mid‐volume percentage point is the temperature at the arithmetic average of the volume distilled at the IBP and EP of that pseudo‐component. Since boiling range is small, both averages are close to each other and can be considered the VABP or the MeABP for that pseudo‐component. Example 3.2: Calculate the mean average boiling point of the petroleum fraction of example 3.1. If the API gravity of this fraction is 66, calculate the Watsonʹs characterization factor. Solution: The D86 distillation temperatures are converted to degrees F. The volume average boiling point is obtained from equation 3.6 129.2 + 170.6 + 214.7 + 267.8 + 339.8 = 224.4 °F VABP = 5 SL =2.6325 Δ = 18.279
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MeBP = 224.4 – 18.3 =206.1 °F or 96.8 °C From equation 3.2 141.4 S= = 0.7313 62 + 131.5
(206.1 + 459.6) K = 0.7313
1
3
= 11.94
3.3 Pseudo Components Calculations involving crude oil and petroleum fractions require the composition of each process stream. Since most of the actual components are not known, the petroleum fractions is characterized as a mixture of discrete pseudo components with defined boiling point ranges or cut points on the TBP distillation curve. Each pseudo‐component corresponds to several unknown actual compounds (e.g. paraffins, naphthenes and aromatics) which boil in a given temperature range. Usually each pseudo‐component is characterized by an average normal point, specific gravity and molecular weight. The first properties are obtained experimentally from the TBP curve and gravity versus volume distilled curve. In some cases only the overall specific gravity of the fraction is measured. The molecular weight is usually calculated through a correlation. Once these parameters are determined the pseudo components can be treated as any defined component for the calculation of thermophysical and thermodynamic property like enthalpy, entropy, and transport properties like viscosity, thermal conductivity and diffusivity. Some properties like pour point depend on the chemical nature of the compounds represented in the pseudo‐ components and the information on the chemical compositions in terms of percentage of paraffins, naphthenes and aromatics become necessary. 3.3.1 Breakup of TBP curve into Pseudo‐components The TBP for the crude oil or the petroleum fraction has to be available, either by direct laboratory measurements through ASTM D1160 distillation or through the conversion of ASTM D86 distillation into TBP distillation curve. TBP cut point ranges are used to define pseudo‐components , with the average temperature of the cut or the mid point normal boiling point (NBP). If the petroleum fraction contains components lighter than pentanes, the composition of the lighter ends has to be available experimentally through chromatographic analysis of the vapors. Otherwise the lighter ends are lumped with lightest pseudo component. The number of such pseudo components depends on the boiling point range of the whole petroleum
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fraction. This number is a trade off between producing a smooth calculated property curve and having too many components which leads to excessive computation time. The following cut‐point ranges are reasonable for most refining calculations: TBP range, °C Number of cuts