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Chapter 9
Attainable Product Yield Distribution Curve: A Roadmap to Crude Oil Composition Carl E. Rechsteiner* CRechsteiner Consulting, LLC, 1712 Putnam Way, Petaluma, California 94954, United States *E-mail:
[email protected].
Crude oil is an exceedingly complex mixture of organic compounds. Advances in separations and measurement technologies, especially the utilization of Gas Chromatography (GC), allow robust measurement of the distribution of material within a crude oil from C3 up to ca. C110 to C120 calibrated against normal paraffin standards. The focus of this chapter is on the application of Detailed Hydrocarbon Analysis (DHA) for light ends and GC Simulated Distillation (SimDis) for distillable fractions and residuum. The chapter discusses the concepts behind these technologies, the robustness of these measurements, and the importance of the composite yield distribution curve obtained from combining these measurements.
Background Among the many contributions of Mieczyslaw M. Boduszynski (Mietek) to the understanding of petroleum composition, arguably the most significant is his “Petroleum Molecular Composition Continuity Model”. A concise description of this model is found in the references (1). This model was also presented at the 13th International Conference on Petroleum Phase Behavior, and Fouling (2). Mietek has also published numerous articles and a seminal book on this matter (3–8). The model proposes that, unlike conventional wisdom that petroleum transitions from individual molecules to extremely high molecular weight © 2018 American Chemical Society
Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
components (i.e., asphaltenes), the molecular composition of petroleum is a continuously variable series of molecules without abrupt transitions. Recent work by A.G.Marshall and R.P. Rodgers, et al. provides validation for this model (9, 10). Mietek defined a temperature scale (n-paraffin atmospheric equivalent boiling point, AEBP) as a yardstick for tracking these continuous changes. Some important concepts behind this choice are: 1. The boiling points for normal paraffins are well defined. 2. Mathematical techniques can be used to extend this scale far beyond what current technology can measure, e.g., to C200 (11). 3. This yardstick provides a mechanism to interpolate physical and chemical properties and allow direct comparison of different crudes and crude fractions on a consistent basis. Further, this choice allows compositional information to be discussed with members of Upstream operations (molecular weight or carbon number) or Downstream operations (boiling point) in terms that they prefer.
Historical Perspective The work reported here spans many years (decades, in fact). Several of the relevant measurement technologies were either non-existent or in a gestational phase and developed during these efforts. The gas chromatographic methodologies of Simulated Distillation and Detailed Hydrocarbon Analysis were developed, modified, improved, and standardized during this period. A refinery fractionates crude oil into a series of boiling point range cuts by distillation for subsequent processing into products, e.g., motor gasoline, jet fuel, diesel fuel, etc. Distillation is one of the few industrial scale processes that can handle the amount of crude oil converted into products in a refinery. For example, a small refinery may process 50,000 barrels per day of crude oil. That means that it is processing over 2 million gallons of crude every day of operation. The value of any crude oil to refiners is determined by that specific crude oil’s chemical and physical properties and its yield in each product boiling point range and that refinery’s specific configuration. A crude assay is used to measure the yield and properties of crude fractions for valuation purposes in a standardized way and then build a description of the whole crude oil.
Deep-Cut Distillation ASTM methods, D-2892 for atmospheric distillation and D-5236 for vacuum distillation (12, 13), are standard distillation methods for crude oil assays. ASTM method D-2892 is used to produce low boiling crude oil fractions up to 400°C AET (Atmospheric Equivalent Temperature) which can be further analyzed. The method specifies that the vapor temperature at the collection point must not exceed 310 °C, and as needed the vapor pressure can be reduced from atmospheric to 100 mm of mercury (Hg). Fractions are collected in a crude oil assay from the D-2892 method might be: Start to 350 °F, 350-450 °F, 450-550 °F, 550-650 °F, and atmospheric residuum. 206 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
ASTM D-5236 specifies a vacuum distillation that can start with the atmospheric residuum from the D-2892 distillation and then produce crude oil fractions up to 1050 °F under reduced pressure, 1 mm of Hg. However, the exact endpoint is highly crude dependent. The maximum pot still temperature is not to exceed 400 °C, and some crude oils must be well below this temperature to avoid thermal cracking of the residue. Use of a molecular distillation allows collecting fractions to much higher endpoints (14). In molecular distillation, the pressure is significantly reduced, coupled with a short residence time for contact between a thin film of the feed and the heated surface allows for efficient mass and heat transfer. This means that higher boiling components can be produced without experiencing thermal cracking. Figure 1 shows a molecular distillation unit from UIC GmbH that has been used to produce fractions with endpoints up to 1250-1350 °F.
Figure 1. Molecular Distillation Unit, model KDL-5.
Access to these much higher boiling point fractions allows a more accurate assay (interpolation of the physical and chemical properties rather than extrapolation) particularly for crude oil that cannot reach the 1050 °F target boiling point with D-5236 and facilitates evaluation of the higher boiling fractions. This topic will be illustrated in a later section. 207 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Detailed Hydrocarbon Analysis In an ideal world, one could compute the value of any crude by summing up the assigned value of each molecule in the crude. Due to the overwhelming number of compounds in crude (the number of isomers of C60H122 is on the order of 1022) (15), this is not possible. However, within limits, this can be done at much lower boiling points using the detailed hydrocarbon analysis methodology (DHA). In DHA, the fraction of interest is separated and quantified using high-resolution gas chromatography (GC). Prior to industrial standardization of the DHA methodology, a variety of approaches were used to measure the concentration of individual components in crude oil fractions, process intermediates, and finished products. Industry standardization took several forms. ASTM Method D-5134 was approved in 1990 for olefin-free samples up to n-nonane (ca. 300°F) (16). All species are eluting after n-nonane were lumped into a single group. Three methods, ASTM Methods D-6729 (17), D-6730 (18), and D-6733 (19), were originally approved in 2001. These extended DHA to include specific oxygen-containing compounds with boiling point ranges up to 225°C (about the boiling point for n-eicosane). Such methods come with limitations. 1) Coelution increases as the carbon number/boiling point increases. Table 1 shows the number of paraffin isomers as a function of carbon number/boiling point. 2) The inclusion of olefins in the method vastly increases the number of isomers and possible coelutions. 3) As the carbon number increases, there is a lack of standards for each isomer, along with their individual chemical and physical properties. For example, at C6 and above, olefins are only identified by their carbon number. Similarly, paraffins above C9 and aromatics (except for specific condensed ring cores like naphthalene) above C10 are only identified by their carbon number. Generic identifications, while used for economic assessments, are less precise and increase errors in valuations.
Table 1. Number of Isomers of Paraffins as a Function of Carbon Number Range Carbon Number Range
Number of Isomers
C1 - C9
35
C1 - C12
355
C1 - C15
4347
C1 - C18
60523
Figure 2 shows the GC traces for 3 consecutive crude oil fractions. This figure clearly shows the increase in coelutions as the carbon number/boiling point increases. For the 350-450°F fraction, even the distinct peaks are riding upon other coeluting compounds. Figures 3 and 4 expand and label specifically identified peaks in the Start to 250°F and 250-350°F fractions. 208 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 2. Impact of boiling point range on GC of consecutive boiling range fractions from a single crude oil. Start to 250°F about C8 (bottom), 250-350°F about C8 to C10 (middle), and 350-450°F about C10 to C13 (top).
Figure 3. Lowest boiling fraction (Start to 250°F) showing limited coelutions, mostly between C7 (toluene) and C8. 209 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 4. Middle boiling fraction (250 to 350°F) where components identified only by carbon number and type (paraffin, naphthene, and aromatic) starts between C9 and C10.
Simulated Distillation SimDis is a GC based method that provides yield distribution information comparable to physical distillation. Compared to physical distillation, it is faster, more reproducible, uses smaller sample sizes, and has lower workforce requirements. During the time of these studies, SimDis methodologies evolved to cover not only subsets of the distillable range up to 1000°F but also to include residuum containing materials (including certain crudes). Table 2 lists some of the ASTM SimDis methods, their initial date of approval, and their range of coverage. Note, ASTM Method D-6352 was reissued in 2005 with an expanded carbon number/ boiling point range.
Table 2. ASTM VGO SimDis Method Comparisons ASTM Method
BP Range °F
Carbon # Range
Initially Issued
D-2887
~100-1000
5-44
1973
D-6352
345-1292
10-90
1998
D-7213
~100-1140
7-60
2005
4 – 100/110/120
2005
D-6352
210 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
The limitations concerning residuum-containing fractions are of particular interest. To dissolve the residuum containing samples for measurement, carbon disulfide is used as a solvent. That limits the accuracy in the C4 to C8 range due to coelution of the hydrocarbons with the solvent. A second issue concerns the upper-temperature limit for calibration. Typically, a Polywax standard is used as a stationary phase but one may not be able to identify n-paraffins above C100, C110, or C120. Issues with the early versions of this method will be discussed later. Figure 5 compares the yield curve for a start to 350°F crude oil fraction obtained by DHA and by SimDis. Minor differences are seen between the two yield curves. These discrepancies are explainable due to the difference between the high-resolution GC data from DHA and the much lower resolution from the SimDis measurement.
Figure 5. Comparison of DHA versus SimDis Yield curves for a Start to 350°F crude fraction. Tag indicates whether SimDis measured temperature is above or below that from DHA.
Figure 6 shows the chromatographic data for fractions obtained from a 30°API crude. In addition to the data for the fractions, a composite yield curve is also overlaid. The top is on the boiling point scale (AEBP) while the bottom is on the chromatographic retention time scale. Note, the highest boiling fraction (950-1100°F) was obtained from the molecular distillation of the 950°F+ vacuum residua. 211 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. SimDis Chromatograms for 8 fractions from a 30°API crude and the composite from those fractions on the AEBP scale (top) and the retention time scale (bottom).
Figure 7 and 8 modify the data in Figure 6 by removing some of the fractions. In figure 7, the Start-to-350°F fraction is removed while in Figure 8 the fractions below 650oF are removed. The chromatographic data for the whole liquid product for the 350-650°F and the 650-1100°F fractions and the chromatogram from the compositing of the individual fractions are virtually identical. For reference, table 3 compares basic properties for the 11°API crude. 1) As measured on the crude oil, calculated from the ASTM D-2892 distillation (fractions below 650°F plus the atmospheric resid), and that calculated from all of the cuts including the 1100°F+ residua. 2) On the atmospheric residua and the calculation from the 650°F+ fractions. 3) On the wide 650-1100°F gas oil. Differences between the values are consistent with the experimental errors associated with the measurement methods. 212 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 7. GC chromatograms for consecutive VGOs from 350 to 1100°F and their composite yield distribution curve on the AEBP scale (top) and the retention time scale (bottom).
213 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 8. GC chromatograms for consecutive VGOs from 650 to 1100°F and their composite yield distribution curve on the AEBP scale (top) and the retention time scale (bottom).
214 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 3. 11.0°API Crude Oil Properties. Measured and Calculated from Fractions Properties / Fractions
Crude Measured
Crude Calculated D-2892 Basis
Crude Calculated Cut 1- Cut-n
Gravity, °API
11.0
11.1
11.5
Sp. Gr. 60F/60F, g/mL
0.9930
0.9921
0.9899
Sulfur, wt%
1.54
1.53
1.59
Nitrogen, wt%
0.86
0.88
0.87
Hydrogen, wt%
11.10
10.51
10.90
Vanadium, ppm
92.55
97.52
88.82
Nickel, ppm
93.57
95.50
87.08
9.44
9.35
MCR, wt%
Table 4 shows the same as measured versus as computed for this crude’s atmospheric residuum (AR) and its wide VGO.
Table 4. 11.0°API AR and VGO Properties. Measured and Calculated from Fractions Properties/Fractions
650°F+ Measured
650°F+ Calculated
650-1100 Measured
650-1100 Calculated
Gravity, °API
7.6
7.8
10.8
11.1
Sp. Gr. 60F/60F, g/mL
1.0173
1.0161
0.9944
0.9921
Sulfur, wt%
1,72
1.8
1.61
1.61
Nitrogen, wt%
1.09
1.08
0.75
0.74
Hydrogen, wt%
10.21
10.74
11.07
1.00
Vanadium, ppm
120.6
110.67
29.99
28.82
Nickel, ppm
118.10
109.16
49.35
47.25
MCR, wt%
11.68
11.53
3.79
3.85
Figure 9 compares the yield curve obtained on the VGO from the atmospheric residua from a 29.7°API crude with the composite of the cuts from a deep cut assay of that VGO (20). The VGO endpoint is above 1250°F, and the yield curves are identical. 215 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 9. Comparison of yield distribution curves for 650-1250°F VGO and a composite from 8 fractions.
Examples In 1995, Mietek and coworkers published a paper titled “Deep-cut assay reveals additional yields of high-value VGO.” Using the combination of distillation and chromatographic methods (ASTM D-2892, D-5236, molecular distillation and sequential extraction fractionation) with physical/chemical measurements (21), they illustrated the compositional changes that accompany cutting deeper into an atmospheric residuum. The use of the boiling point scale allows reporting information for these fractions on a consistent, comparable basis. As the cut-point between the respective VGO and the residua increases, the yield of the VGO increases. Other vital properties also increase but at a slower rate. For example, the sulfur content in a VGO with a cut-point of 1179°F is lower than that of the atmospheric residuum starting material. Likewise, the content of nickel, vanadium, and Microcarbon residue is all reduced compared to the starting material. Table 5 shows these changes across a series of VGOs with increasing cut points. 216 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 5. Changes to VGO Properties as VGO-Residuum Cut-Point Increases for a 30.9°API Crude Oil Cut-point, °F
650°F+
650-919°F
650-1066 °F
650-1179 °F
Gravity, °API
14.1
21.1
19.5
18.2
Sp. Gr., 60F/60F, g/mL
0.9718
0.9273
0.9371
0.9452
Carbon, wt%
86.25
86.32
86.33
86.61
Hydrogen, wt%
11.4
12.62
12.30
11.86
Sulfur, wt%
1.97
1.33
1.48
1.67
Nitrogen, wt%
0.37
0.12
0.17
0.21
Vanadium, ppm
54.9
