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Composition of petroleum heavy ends. 3. Comparison of the composition of high-boiling petroleum distillates and petroleum >675 °C residues
Composition of petroleum heavy ends. 3. Comparison of the composition of highboiling petroleum distillates and petroleum > 675°C residues John
F. McKay,
Dewitt
R. Latham
Laramie Energy Technology Center, Wyoming 82071, USA (Received 20 December 1979)
and William
US Department
E. Haines
of Energy,
PO Box
3395,
Laramie,
Three ‘heavy ends’ fractions of petroleum are compared and defined as 370-535X distillates, 535675°C distillates, and >675”C residues. The distributions of classes of compounds, compound types, molecular weights, and heteroatom content of individual molecules in the heavy ends fractions are discussed.
America1 Petroleum Institute Research Project 60 was a joint government-industry-university research programme begun in 1966 to develop methods for analysing the heavy ends of petroleum according to chemical compound type. In that investigation, crude oils having different geological classifications were carefully sampled and analysed by a standardized separationcharacterization scheme so that the compositions of distillates boiling in the range of 370-675°C could be compared. None of the distillates was subjected to a deasphaltening treatment prior to being separated and analysed. The results of the API 60 study have been reported’.‘. Parts 1 and 2 of this series3,4 describe composition studies on the residues from four of the API 60 crude oils and represent the work of this laboratory in extending the API 60 separation-characterization scheme to the analyses of >675”C petroleum residues. By analysing both high-boiling distillates and residues from the same crude oils and by using the same general separationcharacterization scheme throughout the study, the composition of the high-boiling distillates and residues can be compared. As in the previous studies, the samples were not subjected to a deasphaltening treatment before being separated and analysed. This paper discusses the distribution of acids, bases, neutral nitrogen compounds, saturate hydrocarbons, and aromatic hydrocarbons in the high-boiling distillates and residues from the four crude oils, using the data from previous studies. Also discussed are the kinds and, where possible, the amounts of major compound types in the distillates and residues. Molecular-weight distributions in the distillates and residues are compared. In addition, molecular-weight data and elemental analyses are used together to show how the number of heteroatoms in the average molecule increases with increase in distillation temperature. OOIS-2361/81/010027~06$2.00 01981 IPC Business Press
DISCUSSION The term ‘heavy ends’ is widely used in the petroleum industry as a general description for the high-molecularweight, high-boiling components of crude oil. In this work ‘heavy ends’ is defined as material boiling above 37O”C, and was examined in three fractions: (1) distillates boiling between 370-535°C; (2) distillates boiling between 535-675°C; and (3) residues that do not distill at 675’C. The percentage of a crude oil in these three fractions varies. Table 1 shows the four crude oils studied and the weight per cents of the oils in each of the three heavy ends fractions. The data in Table 1 show that the Wilmington and Gach Saran crude oils have a much higher percentage of material in the heavy end fractions than the other two oils. Over 61% of the Wilmington oil is found in the highboiling distillate and residue fractions. By comparison the Recluse and South Swan Hills oils have only ~227:; of the crude oil in these fractions. However, the heavy ends make up a significant portion of all of these crude oils. Distribution
in heavy ends
of classes of compounds
The separation scheme described earlier’,3 separates the distillates and residues into five major classes: acids, bases, neutral nitrogen compounds, saturate hydrocarTable 7 Distribution
of heavy ends in crude oil Crude oil (wt %)
Crude oil
370-535” distillate
Wilmington, Calif. Gach Saran, Iran South Swan Hills, Alta. Recluse, Wyo.
24.7 19.7 18.4 18.6
FUEL,
1981,
C
535-675’C distillate
>675’C residue
14.5 12.0 6.0 5.2
22.1 16.0 3.0 3.6
Vol 60, January
27
Composition Table 2
of petroleum
Distribution
heavy ends (3): J. F. McKay et al.
of compound
classes in petroleum
heavy ends Weight per cent of distillate
or residue
(“C)
Acids
Bases
Neutral nitrogen compounds
Saturate
Crude oil
hydrocarbons
Aromatic hydrocarbons
Wilmington, Calif.
370-535 535-675 >675
5.6 9.3 18.0
6.8 12.7 19.0
4.2 21.3 41.0
36.9 20.8 4.0
46.5 36.0 15.0
Gach Saran, I ran
370-535 535-675 >675
1.7 5.4 17.0
2.1 8.7 25.0
2.3 8.9 14.0
48.5 31.6 8.0
46.5 45.4 30.0
South Swan Hills, Alta.
370-535 535-675 >675
1.8 3.5 12.0
2.2 4.5 13.0
1.9 5.6 10.0
65.9 57.5a 34.0
29.7 28.9 27.0
Recluse, wyo.
370-535 535-675 >675
1.4 2.9 9.0
1.1 3.3 10.0
0.9 3.0 8.0
74.1 68.8a 44.0
22.5 21.9 26.0
Boiling range,
a Values corrected sulphur
from wax-free
distillate
to whole distillate
basis. Waxes considered
bons, and aromatic hydrocarbons. The term ‘acids’, ‘bases’, and ‘neutral nitrogen compounds’ are operational terms rather than chemical terms; for example, ‘acids’ are defined as compounds that are removed from the oil by an anion exchange resin. The acid fraction contains compounds that are generally recognized as being strong organic acids, such as phenols and carboxylic acids, but may also contain compounds that are not normally considered to be organic acids, such as pyrrolic compounds and amides. The latter compounds appear in the acid fraction probably because they hydrogen bond to the anion resin. Similarly, bases are defined as compounds that are removed from the oil by a cation exchange resin. The base fraction contains compounds such as pyridines that titrate as strong nitrogen bases, and amides that titrate as weak bases. The base fraction may also contain pyrrolic compounds that are not basic by a titration definition. Pyrrolic compounds, such as carbazoles, appear in the base fraction probably because they hydrogen bond to the cation resin. The neutral nitrogen fraction contains pyrrolic compounds and amides that are unreactive to either anion or cation resin but which coordinate with ferric chloride to form a coordination complex. In addition, this fraction may contain some aromatic hydrocarbons. Table 2 shows the distribution of the classes of compounds in heavy ends fractions from four crude oils. The weight per cents of acids, bases, and neutral nitrogen compounds increase with increase in boiling point of the fractions. The data show that the amounts of acids, bases and neutral nitrogen compounds in the residues are from two to ten times the amounts found in the 37&535”C distillates, depending on the oil and the compound classes being compared. This increase in polar compounds is accompanied by a corresponding decrease in the saturate and aromatic hydrocarbon fractions. The Wilmington crude oil shows a decrease in saturate hydrocarbon content with increasing boiling point to theextent that the >675”C residue contains only 4% saturates. The paraffinic Recluse oil shows a saturate hydrocarbon content of 44”,, in the residue fraction, roughly half the percentage found in the 37&535”C distillate. A comparison of the aromatic hydrocarbons in the three fractions usually shows a slight decrease in weight per cent with increase in boiling point, but the decrease is not dramatic.
28
FUEL,
1981,
Vol 60, January
entirely
saturate and essentially
free of nitrogen
and
Comparison of compound types in high-boiling distillates and residues Classes of compounds; i.e., acids and bases, may be further separated to isolate compound types. For example, bases may be separated into subfractions of pyridine, pyrrole, and amide compound types. In this work terms such as pyridines, pyrroles, and phenols are used to define the functional group characteristics of compound types. The molecules in the distillates and residues are actually high-molecular-weight homologs and benzologs of parent compound types. Qualitatively, the major compound types identified in >675’C residues are the same as those identified in the 37s535°C and 535-675°C distillates: phenols, carboxylic acids, amides, pyrroles, pyridines, alkanes, and aromatic hydrocarbons. Compound types unique to either the distillates or the residues may be present but have not been identified. The major qualitative differences in compounds found in the distillates and the residues appear to be (1) the average molecular weight of the individual molecules; and (2) the average number of heteroatoms per molecule. Although the same compound types have been identified in all of the oils studied, the amounts vary considerably. Quantitative comparisons of the distribution of major compound types in the distillates and residues are limited by the ability to measure the amounts of compound types in the residues; residue data are subject to more error than are distillate data, both in the measurement of compound types and in total material balance. In this work, only the acid fractions from residues could quantitatively analysed according to compound type; consequently, only the distributions of acid compound types in the distillates and residues may be compared. Tuble 3 shows the distribution of carboxylic acids, phenols, amides, and pyrroles in the acid fractions from four high-boiling petroleum distillates and residues. The data show that the carboxylic acid content of the Wilmington oil decreases with increase in distillation temperature while that of the other oils increases. Phenols either remain constant, as in the Wilmington and Recluse oils, or decrease as in the Gach Saran and South Swan Hills oils. The Wilmington acids show a steady increase in weight per cent nitrogen compounds, amides, and pyr-
Composition
Table 3
Distribution
of acid compound
types in petroleum
high-boiling
distillates
of petroleum and >675”C
heavy
ends
(3): J. F. McKay
et al.
residues
Weight per cent of acid fraction= Boiling range, Crude oil
(“C)
Carboxylic acids
Phenols
Amides
Pyrroles
Recovery
Wilmington, Calif.
370-535 535-675 >675
62 48 33
15 12 12
3 13 25
13 22 30
93 95 100
Gach Saran, Iran
370-535 535-675 >675
4 4 14
31 21 7
26 24 47
36 41 16
97 90 84
South Swan Hills, Alta.
370-535 535-675 >675
4 7 24
34 25 13
15 24 30
47 37 16
95 93 83
Recluse, wyo.
370-535 535-675 >675
11 4 14
24 24 24
3 8 23
56 58 25
94 94 86
a Weight per cents reported for distillate acids were determined by gravimetric analyses; weight per cents reported for residue acids were determined by quantitative infrared analyses’. In all cases, the data shown are raw data rather than data normalized to 100%
Tab/e 4
Distribution
of heteroatoms
in heavy ends
Distribution
Nitrogen
Sulphur
Wilmington, Calif.
370-535 535-675 >675
0.46 0.86 1.62
1.58 2.15 2.57
1.46
Gach Saran, Iran
370-535 535-675 >675
0.22 0.44 1.18
1.85 2.61 3.69
0.76
South Swan Hills, Alta.
370-535 535-675 >675
0.15 0.27 0.55
0.28 0.46 0.53
1.20
Recluse, wyo.
370-535 535-675 >675
0.07 0.16 0.56
0.12 0.34 0.37
Crude oil
of heteroatoms
(N, S, 0) in heuvy ends of
petroleum
Boiling range, (“C)
(wt %I Oxygen _ _
_
_
_ 0.78
roles, with increase in distillation temperature: both types increase with increase in distillation temperature. In contrast, the sum of the nitrogen compounds in the Gach Saran and South Swan Hills heavy ends remains constant with increase in distillation temperature; however, amide content increases as pyrrole content decreases. Thus the distributions of acidic nitrogen types are variable from oil to oil, which is useful information to refiners because different nitrogen types are converted to hydrocarbons under different reaction conditions, and refining conditions suitable for removing nitrogen from one oil may not be suitable for removing nitrogen from another oil. An important conclusion from the data in Tuhle 3 is that it is not possible to predict the trend or distribution of acid compound types in the heavy ends of crude oils. Some separation and detailed analysis is necessary to determine the distribution of acid compound types. Elemental analyses or easily obtainable physical property data for distillates and residues do not provide information from which the distributions of compound types can be deduced; however, knowledge of the distributions of compound types in the heavy ends fractions is important in chemical processes for converting these materials to useful hydrocarbon products.
In these studies of petroleum distillates and residues, an attempt was made to analyse the mixtures at the compound-type level, for the heavy ends of petroleum are much too complex to be analysed at the molecular level. However, for some purposes, such as catalyst design, it is important to know as much detail as possible about the chemical structure of the molecules in the mixture. One alternative to characterizing molecules is to describe an ‘average’ molecule based on gross chemical properties of the mixture. The simpler the mixture, the more accurate the description of the ‘average’ molecule. In our work, elemental analyses and molecular-weight data are used to describe average molecules in the acid, base, and neutral nitrogen fractions from high-boiling distillates and residues. Table 4 shows the distribution of heteroatoms in the heavy end distillates and residues from four crude oils. In each case the weight per cents of heteroatoms increase as the distillation temperature ofthe fractions increases. This trend correlates well with the data presented earlier which showed that the amounts of acids, bases, and neutral with distillation compounds increased nitrogen temperature. Table 5 shows the distribution of heteroatoms in the acid, base, neutral nitrogen, saturate, and aromatic hydrocarbon fractions of the Wilmington oil with change in distillation temperature. The nitrogen in the Wilmington acids increased from 1.41 wt ‘A in the 370-535’C distillate to 2.33 wt yi, in the >675”C residue. This increase with distillation temperature is interpreted as originating from two sources: (1) the increased amounts of nitrogen compound types such as pyrroles and amides in the Wilmington acids, as shown previously in Table 3; and (2) the increase in the number of acid compound types that contain more than one nitrogen atom per molecule. A reverse trend is noted in both the Wilmington bases and neutral nitrogen compounds; for example, the weight per cent nitrogen decreases with increase in distillation temperature. These data indicate that, in these two classes, the per cent of nitrogen compounds is constant and that the number of nitrogen atoms per molecule is constant. Thus
FUEL,
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Vol 60, January
29
Composition Tab/e 5
of petroleum
Distribution
in Wilmington
Compound class
heavy ends (3): J. F. McKay
of heteroatoms
according to compound
(wt %)
Boiling range, (“C)
Nitrogen
Sulphur
Acids
370-535 535-675 >675
1.41 1.51 2.33
1.40 1.78 2.53
Bases
370-535 535-675 >675
3.87 2.90 1.96
1.24 1.79 2.31
Neutral nitrogen compounds
370-535 535-675 >675
3.80 1.88 1.78
3.18 2.71
Saturates
370-535 535-675 >675
0.11
0.82
_ _
_ -
0.63
2.37
Aromatics
ClaSS
heavy ends
370-535 535-675 >675
Table 6 Distribution heavy ends
_
of average molecular
-
_ _
Oxygen 2.67 1.30 0.81 0.74
weights in petroleum
Heavy ends fraction
Average molecular
370-535°C
distillate
400
535-675°C distillate >675”C Residue
550 900
weight
the decrease in per cent nitrogen is due to the increase in molecular weight of individual molecules in each fraction. The distribution of sulphur in the acid, base, and neutral nitrogen fractions is more difficult to interpret because sulphur compounds were not specifically isolated. Sulphur was generally found to be evenly distributed among compound classes, with the exception that the saturated hydrocarbons contained only traces of sulphur. Distribution of molecular weights in high-boiling distillates und residues An understanding of the molecular weight distribution of a complex mixture such as a crude oil is essential to an understanding of the chemistry of the mixture. Specifically, information concerning molecular weight distribution is useful in designing chromatographic separation schemes, in characterizing compound types, and in correlating chemical and physical properties. In this work the determination of average molecular weights for the heavy ends fractions was important because the quantitative infrared analyses applied to the characterization of the polar fractions required this information. In addition, in working with molecules of high molecular weight, intermolecular bonding interactions are commonly observed that greatly influence the solution properties of the mixture, sometimes imparting polymer-like properties to the mixture. It is important to know when the mixture contains small molecules that behave like large molecules owing to intermolecular bonding and when the mixture contains truly large molecules. The four different methods used to determine average molecular weights: vapour pressure osmometry, quantitative infrared spectrometry, electron impact mass spectrometry, and field ionization mass spectrometry, are described in Part 24. Table 6
30
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Vol
60, January
et al. summarizes average molecular weight data for the heavy end fractions from four crude oils. The average molecular weights ofdistillates from different oils are similar because the oils were distilled in the same manner. However, the weight of 900 shown as an average molecular weight for different residues could differ by as much as 100 or 200 mass units because of varying amounts of high-molecularweight components in the oil. Field ionization mass spectrometry is considered the most reliable technique for examining the molecular weights of heavy ends molecules. Figure 1 shows field ionization mass spectra ofdistillates and residues from the Wilmington and Recluse oils. The shapes of the curves indicate that the weights of individual molecules in the heavy ends increase in a uniform manner with distillation temperature. The molecular weight spread is very large in any one distillate or residue. The molecular weight spread of the 37@~535”C distillates is from =20~-800, the 535 675°C distillates is from &30&l 100, and that of the residues is from 40&> 1600. All high-boiling distillates were 100 wt ‘Y;distillable into the field ionization instrument; the Wilmington residue was 78 wt % distillable, and the Recluse residue was 99 wt ‘x distillable. Approximately 95% of the molecules in the Wilmington oil and essentially all of the molecules in the Recluse oil can be observed by field ionization mass spectrometry. Figure 2 shows molecular-weight-distribution curves obtained for the Wilmington and Recluse oils by plotting the average molecular weights of distillate and residue fractions versus the weight per cent of the crude oils. The molecular weight data for the high-boiling distillates and residues were determined by field ionization mass spectrometry (see Figure 1). The molecular weight data for the low-boiling distillates were estimated from previous work’. The purpose of the Figure is to show graphically the distribution of molecular weights throughout the entire barrel ofcrude oil. The graphs for the two crude oils are parallel, showing that the distributions of molecular weights are similar in the two oils, but the average molecular weight of the Wilmington oil is higher than that of the Recluse oil. The transition in molecular weight from low-molecular-weight molecules to high-molecularweight molecules is continuous and changes rapidly in the high-boiling distillate and residue portions of the oil. Extrapolation of the data to the heaviest portion of the crude oil suggests that (1) =55”,:, of the Wilmington crude oil is much higher in molecular weight than any material in the Recluse oil and that (2) the molecular weights of those Wilmington molecules are above 1800. Characteristics of ‘average’ molecules in distillates and residues Average property data such as elemental analyses (Table 5) and average molecular weights (Table 6) can be used together to describe the chemical characteristics of ‘average’ molecules in distillate and residue fractions. The combined data show in general how the heteroatom content of molecules changes with change in molecular weight. This information can then be used to estimate the number of functional groups in an average distillate or residue molecule. To show how average property data are used to describe chemical characteristics of ‘average’ molecules the following example, which describes the changes in heteroatom content of an average Wilmington base molecule with increase in boiling range, is given. The
Composition
of petroleum
heavy ends (3): J. F. McKay et al.
d
a
J
200
400
600
800
1000
1200
1400
1600
Molecular
200
400
600
-
2000
r
.w c
=
(
Average moLecuLar determined by field
1000
1200
1400
1600
weight
Figure 1 Field ionization mass spectra of (a) Wilmington 370-535°C distillate; (b) Wilmington residue; (d) Recluse 370435°C distillate; (e) Recluse 535-675°C distillate; (f) Recluse >675’C
Average molecular determined from
800 535-675°C residue
distillate;
(c) Wilmington
>675’C
weight of fraction boiling point data weight of fraction ionization mass spect romet ry
-: 1600 3
E 800 Q1 t m 0L
WiLmingto c
40 Weight Figure 2
Molecular
weight distribution
of Wilmington
60 per cent
of crude
oils
and Recluse crude oils
FUEL, 1981,
Vol 60, January
31
Composition
of petroleum
heavy ends (3): J. F. McKay et al
heteroatom content of an average base molecule in the 370-535°C distillate and, for comparison, the heteroatom content of an average base molecule in the >675”C residue are calculated. The Wilmington 37&535”C bases have an average molecular weight of 400 and a nitrogen content of 3.87 wt ;! (Table 5). If the average base molecule contains one nitrogen atom and a molecular weight of 400, the calculated weight per cent nitrogen is 3.50 wt T! (14/400 x 100=3.50 wt ‘/“). Dividing the observed by the calculated weight per cent nitrogen (3.87/3.50 x lOO= 119%) shows that 119% of the molecules contain one nitrogen atom, or stated differently, on average one of every five molecules contains two nitrogen atoms. If every molecule contains one sulphur atom, the calculated sulphur content is: 32/400 x 100 = 8.0 wt :/,. The observed value of 1.24 wt “,, sulphur means that an average of * 16 per cent (1.24/8.00 x lOO= 15.5%) or one in every six molecules contains a sulphur atom. Oxygen data were not obtained for this sample and are estimated. Assuming that oxygen atoms are found in half of the base molecules (amides observed4 by i.r. and mass spectrometry), the calculations indicate that (1) the average base molecule contains one nitrogen atom; and that (2) in addition to the nitrogen atom. *851< of the molecules contain an additional heteroatom; the other heteroatom could be nitrogen or sulphur but is usually oxygen. The same calculations applied to the > 675’,C residue bases show significant differences in the composition of the average base molecule. Using a value of 900 for the average molecular weight of the residue bases and 1.96 wt ‘:;, nitrogen, 2.31 wt ‘>I:) sulphur, and 2.67 wt ‘4) oxygen, the following estimates are made: (1) the average residue base molecule contains about 3.4 heteroatoms, almost twice the number for the 37G535”C bases; and (2) every molecule contains one nitrogen atom, one in every four contains two nitrogen atoms, two ofevery three molecules contain one sulphur atom, every molecule contains one oxygen atom, and one of every two molecules contains two oxygen atoms. These estimates are for average molecules and may or may not be an accurate description of the molecules actually present in the heavy ends because the distributions of heteroatoms in individual molecules are not known. However, the calculations do show that with increase in boiling point the probability of a molecule having an increased heteroatom content greatly increases. This trend is also observed in the acid and neutral nitrogen fractions. The calculations also place limits on the number of functional groups one might expect to find in molecules of the heavy end fractions. For this discussion, functional groups aredefined as heteroatom-containing moieties in a molecule that usually impart acid or base properties to the molecule. For example, the hydroxyl group in a phenolic compound would be a functional group, but ether-type oxygen in a compound such as dibenzofuran would not be a functional group. In the distillate fractions, molecular weight and elemental analyses data could be correlated with compound type data to show that most molecules were monofunctional even though they contained about two heteroatoms per molecule. In the residues, quanti-
32
FUEL,
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Vol 60,
January
tative compound type data cannot be obtained in most cases and so one cannot correlate molecular weight and elemental analyses data with compound type data. But calculations using average properties show that average residue molecules contain three to five heteroatoms per molecule. With the increase in heteroatom content of residue molecules, some residue molecules probably contain more than one functional group.
SUMMARY
AND CONCLUSIONS
Qualitatively, the heavy ends fractions of the four oils studied contain the same compound types, but the amounts vary from oil to oil. Compound types unique to any one oil or heavy ends fraction were not observed. In general, all distributions related to composition show gradual and orderly changes with increase in distillation temperature. Amounts of acid, base, and neutral nitrogen classes of compounds increase with increase in distillation temperature, with a concomitant decrease in hydrocarbons, especially the saturates. Compound types within the classes vary in amount with boiling range, and the distribution of compound types cannot be predicted from oil to oil. Average molecular weights of distillate fractions increase gradually with increase in distillation temperature throughout the crude oil and then increase rapidly in the residue fraction. The molecular weights of the largest residue molecules vary for the different oils. The highest molecular weight molecules ‘seen’ in the Wilmington crude oil have a molecular weight of about 1800, but the largest molecules in the oil have not been observed. The largest molecules in the Recluse oil appear to be in the 1800 to 2000 molecular-weight range. The heteroatom content of individual molecules in the Wilmington oil increase gradually with boiling range until the heteroatom content per molecule of molecules in the > 675°C residue is about double that of molecules in the 37s535°C distillate. Most molecules in the 37(r 535°C distillate are monofunctional. With the gradual increase in heteroatom content in the 535-675°C distillate and >675”C residue, the probability of bifunctional or polyfunctional molecules increases, but the amounts of these compounds have not been determined.
REFERENCES Haines, W. E. and Thompson, C. J. ‘Separating and Characterizing High-Boiling Petroleum-Distillates: Tie USBM-API Procedure: Rept. of Inv. LERCIRI-75/S. BERG/RI-75/2. Julv 1975 Thbmpson, C. J., Ward, c. C. and Ball, ‘J.‘S. ‘Characteristics of World’s Crude Oils and Results of API Research Project 60,’ Rept. of Inv., BERG/RI-76/8, July 1976 McKay, J. F., Amend, P. J., Harnsberger, P. M., Cogswell, T. E. and Latham, D. R. Fuel 1981, 60, 14 McKay, J. F., Harnsberger, P. M., Erickson, R. B., Cogswell, T. E. and Latham, D. R. Fuel 1981, 60, 17 McKay, J. F., Cogswell, T. E., Weber, J. H. and Latham, D. R. Fuel 1975, 54, 50
F. McKay,
Dewitt
R. Latham
Laramie Energy Technology Center, Wyoming 82071, USA (Received 20 December 1979)
and William
US Department
E. Haines
of Energy,
PO Box
3395,
Laramie,
Three ‘heavy ends’ fractions of petroleum are compared and defined as 370-535X distillates, 535675°C distillates, and >675”C residues. The distributions of classes of compounds, compound types, molecular weights, and heteroatom content of individual molecules in the heavy ends fractions are discussed.
America1 Petroleum Institute Research Project 60 was a joint government-industry-university research programme begun in 1966 to develop methods for analysing the heavy ends of petroleum according to chemical compound type. In that investigation, crude oils having different geological classifications were carefully sampled and analysed by a standardized separationcharacterization scheme so that the compositions of distillates boiling in the range of 370-675°C could be compared. None of the distillates was subjected to a deasphaltening treatment prior to being separated and analysed. The results of the API 60 study have been reported’.‘. Parts 1 and 2 of this series3,4 describe composition studies on the residues from four of the API 60 crude oils and represent the work of this laboratory in extending the API 60 separation-characterization scheme to the analyses of >675”C petroleum residues. By analysing both high-boiling distillates and residues from the same crude oils and by using the same general separationcharacterization scheme throughout the study, the composition of the high-boiling distillates and residues can be compared. As in the previous studies, the samples were not subjected to a deasphaltening treatment before being separated and analysed. This paper discusses the distribution of acids, bases, neutral nitrogen compounds, saturate hydrocarbons, and aromatic hydrocarbons in the high-boiling distillates and residues from the four crude oils, using the data from previous studies. Also discussed are the kinds and, where possible, the amounts of major compound types in the distillates and residues. Molecular-weight distributions in the distillates and residues are compared. In addition, molecular-weight data and elemental analyses are used together to show how the number of heteroatoms in the average molecule increases with increase in distillation temperature. OOIS-2361/81/010027~06$2.00 01981 IPC Business Press
DISCUSSION The term ‘heavy ends’ is widely used in the petroleum industry as a general description for the high-molecularweight, high-boiling components of crude oil. In this work ‘heavy ends’ is defined as material boiling above 37O”C, and was examined in three fractions: (1) distillates boiling between 370-535°C; (2) distillates boiling between 535-675°C; and (3) residues that do not distill at 675’C. The percentage of a crude oil in these three fractions varies. Table 1 shows the four crude oils studied and the weight per cents of the oils in each of the three heavy ends fractions. The data in Table 1 show that the Wilmington and Gach Saran crude oils have a much higher percentage of material in the heavy end fractions than the other two oils. Over 61% of the Wilmington oil is found in the highboiling distillate and residue fractions. By comparison the Recluse and South Swan Hills oils have only ~227:; of the crude oil in these fractions. However, the heavy ends make up a significant portion of all of these crude oils. Distribution
in heavy ends
of classes of compounds
The separation scheme described earlier’,3 separates the distillates and residues into five major classes: acids, bases, neutral nitrogen compounds, saturate hydrocarTable 7 Distribution
of heavy ends in crude oil Crude oil (wt %)
Crude oil
370-535” distillate
Wilmington, Calif. Gach Saran, Iran South Swan Hills, Alta. Recluse, Wyo.
24.7 19.7 18.4 18.6
FUEL,
1981,
C
535-675’C distillate
>675’C residue
14.5 12.0 6.0 5.2
22.1 16.0 3.0 3.6
Vol 60, January
27
Composition Table 2
of petroleum
Distribution
heavy ends (3): J. F. McKay et al.
of compound
classes in petroleum
heavy ends Weight per cent of distillate
or residue
(“C)
Acids
Bases
Neutral nitrogen compounds
Saturate
Crude oil
hydrocarbons
Aromatic hydrocarbons
Wilmington, Calif.
370-535 535-675 >675
5.6 9.3 18.0
6.8 12.7 19.0
4.2 21.3 41.0
36.9 20.8 4.0
46.5 36.0 15.0
Gach Saran, I ran
370-535 535-675 >675
1.7 5.4 17.0
2.1 8.7 25.0
2.3 8.9 14.0
48.5 31.6 8.0
46.5 45.4 30.0
South Swan Hills, Alta.
370-535 535-675 >675
1.8 3.5 12.0
2.2 4.5 13.0
1.9 5.6 10.0
65.9 57.5a 34.0
29.7 28.9 27.0
Recluse, wyo.
370-535 535-675 >675
1.4 2.9 9.0
1.1 3.3 10.0
0.9 3.0 8.0
74.1 68.8a 44.0
22.5 21.9 26.0
Boiling range,
a Values corrected sulphur
from wax-free
distillate
to whole distillate
basis. Waxes considered
bons, and aromatic hydrocarbons. The term ‘acids’, ‘bases’, and ‘neutral nitrogen compounds’ are operational terms rather than chemical terms; for example, ‘acids’ are defined as compounds that are removed from the oil by an anion exchange resin. The acid fraction contains compounds that are generally recognized as being strong organic acids, such as phenols and carboxylic acids, but may also contain compounds that are not normally considered to be organic acids, such as pyrrolic compounds and amides. The latter compounds appear in the acid fraction probably because they hydrogen bond to the anion resin. Similarly, bases are defined as compounds that are removed from the oil by a cation exchange resin. The base fraction contains compounds such as pyridines that titrate as strong nitrogen bases, and amides that titrate as weak bases. The base fraction may also contain pyrrolic compounds that are not basic by a titration definition. Pyrrolic compounds, such as carbazoles, appear in the base fraction probably because they hydrogen bond to the cation resin. The neutral nitrogen fraction contains pyrrolic compounds and amides that are unreactive to either anion or cation resin but which coordinate with ferric chloride to form a coordination complex. In addition, this fraction may contain some aromatic hydrocarbons. Table 2 shows the distribution of the classes of compounds in heavy ends fractions from four crude oils. The weight per cents of acids, bases, and neutral nitrogen compounds increase with increase in boiling point of the fractions. The data show that the amounts of acids, bases and neutral nitrogen compounds in the residues are from two to ten times the amounts found in the 37&535”C distillates, depending on the oil and the compound classes being compared. This increase in polar compounds is accompanied by a corresponding decrease in the saturate and aromatic hydrocarbon fractions. The Wilmington crude oil shows a decrease in saturate hydrocarbon content with increasing boiling point to theextent that the >675”C residue contains only 4% saturates. The paraffinic Recluse oil shows a saturate hydrocarbon content of 44”,, in the residue fraction, roughly half the percentage found in the 37&535”C distillate. A comparison of the aromatic hydrocarbons in the three fractions usually shows a slight decrease in weight per cent with increase in boiling point, but the decrease is not dramatic.
28
FUEL,
1981,
Vol 60, January
entirely
saturate and essentially
free of nitrogen
and
Comparison of compound types in high-boiling distillates and residues Classes of compounds; i.e., acids and bases, may be further separated to isolate compound types. For example, bases may be separated into subfractions of pyridine, pyrrole, and amide compound types. In this work terms such as pyridines, pyrroles, and phenols are used to define the functional group characteristics of compound types. The molecules in the distillates and residues are actually high-molecular-weight homologs and benzologs of parent compound types. Qualitatively, the major compound types identified in >675’C residues are the same as those identified in the 37s535°C and 535-675°C distillates: phenols, carboxylic acids, amides, pyrroles, pyridines, alkanes, and aromatic hydrocarbons. Compound types unique to either the distillates or the residues may be present but have not been identified. The major qualitative differences in compounds found in the distillates and the residues appear to be (1) the average molecular weight of the individual molecules; and (2) the average number of heteroatoms per molecule. Although the same compound types have been identified in all of the oils studied, the amounts vary considerably. Quantitative comparisons of the distribution of major compound types in the distillates and residues are limited by the ability to measure the amounts of compound types in the residues; residue data are subject to more error than are distillate data, both in the measurement of compound types and in total material balance. In this work, only the acid fractions from residues could quantitatively analysed according to compound type; consequently, only the distributions of acid compound types in the distillates and residues may be compared. Tuble 3 shows the distribution of carboxylic acids, phenols, amides, and pyrroles in the acid fractions from four high-boiling petroleum distillates and residues. The data show that the carboxylic acid content of the Wilmington oil decreases with increase in distillation temperature while that of the other oils increases. Phenols either remain constant, as in the Wilmington and Recluse oils, or decrease as in the Gach Saran and South Swan Hills oils. The Wilmington acids show a steady increase in weight per cent nitrogen compounds, amides, and pyr-
Composition
Table 3
Distribution
of acid compound
types in petroleum
high-boiling
distillates
of petroleum and >675”C
heavy
ends
(3): J. F. McKay
et al.
residues
Weight per cent of acid fraction= Boiling range, Crude oil
(“C)
Carboxylic acids
Phenols
Amides
Pyrroles
Recovery
Wilmington, Calif.
370-535 535-675 >675
62 48 33
15 12 12
3 13 25
13 22 30
93 95 100
Gach Saran, Iran
370-535 535-675 >675
4 4 14
31 21 7
26 24 47
36 41 16
97 90 84
South Swan Hills, Alta.
370-535 535-675 >675
4 7 24
34 25 13
15 24 30
47 37 16
95 93 83
Recluse, wyo.
370-535 535-675 >675
11 4 14
24 24 24
3 8 23
56 58 25
94 94 86
a Weight per cents reported for distillate acids were determined by gravimetric analyses; weight per cents reported for residue acids were determined by quantitative infrared analyses’. In all cases, the data shown are raw data rather than data normalized to 100%
Tab/e 4
Distribution
of heteroatoms
in heavy ends
Distribution
Nitrogen
Sulphur
Wilmington, Calif.
370-535 535-675 >675
0.46 0.86 1.62
1.58 2.15 2.57
1.46
Gach Saran, Iran
370-535 535-675 >675
0.22 0.44 1.18
1.85 2.61 3.69
0.76
South Swan Hills, Alta.
370-535 535-675 >675
0.15 0.27 0.55
0.28 0.46 0.53
1.20
Recluse, wyo.
370-535 535-675 >675
0.07 0.16 0.56
0.12 0.34 0.37
Crude oil
of heteroatoms
(N, S, 0) in heuvy ends of
petroleum
Boiling range, (“C)
(wt %I Oxygen _ _
_
_
_ 0.78
roles, with increase in distillation temperature: both types increase with increase in distillation temperature. In contrast, the sum of the nitrogen compounds in the Gach Saran and South Swan Hills heavy ends remains constant with increase in distillation temperature; however, amide content increases as pyrrole content decreases. Thus the distributions of acidic nitrogen types are variable from oil to oil, which is useful information to refiners because different nitrogen types are converted to hydrocarbons under different reaction conditions, and refining conditions suitable for removing nitrogen from one oil may not be suitable for removing nitrogen from another oil. An important conclusion from the data in Tuhle 3 is that it is not possible to predict the trend or distribution of acid compound types in the heavy ends of crude oils. Some separation and detailed analysis is necessary to determine the distribution of acid compound types. Elemental analyses or easily obtainable physical property data for distillates and residues do not provide information from which the distributions of compound types can be deduced; however, knowledge of the distributions of compound types in the heavy ends fractions is important in chemical processes for converting these materials to useful hydrocarbon products.
In these studies of petroleum distillates and residues, an attempt was made to analyse the mixtures at the compound-type level, for the heavy ends of petroleum are much too complex to be analysed at the molecular level. However, for some purposes, such as catalyst design, it is important to know as much detail as possible about the chemical structure of the molecules in the mixture. One alternative to characterizing molecules is to describe an ‘average’ molecule based on gross chemical properties of the mixture. The simpler the mixture, the more accurate the description of the ‘average’ molecule. In our work, elemental analyses and molecular-weight data are used to describe average molecules in the acid, base, and neutral nitrogen fractions from high-boiling distillates and residues. Table 4 shows the distribution of heteroatoms in the heavy end distillates and residues from four crude oils. In each case the weight per cents of heteroatoms increase as the distillation temperature ofthe fractions increases. This trend correlates well with the data presented earlier which showed that the amounts of acids, bases, and neutral with distillation compounds increased nitrogen temperature. Table 5 shows the distribution of heteroatoms in the acid, base, neutral nitrogen, saturate, and aromatic hydrocarbon fractions of the Wilmington oil with change in distillation temperature. The nitrogen in the Wilmington acids increased from 1.41 wt ‘A in the 370-535’C distillate to 2.33 wt yi, in the >675”C residue. This increase with distillation temperature is interpreted as originating from two sources: (1) the increased amounts of nitrogen compound types such as pyrroles and amides in the Wilmington acids, as shown previously in Table 3; and (2) the increase in the number of acid compound types that contain more than one nitrogen atom per molecule. A reverse trend is noted in both the Wilmington bases and neutral nitrogen compounds; for example, the weight per cent nitrogen decreases with increase in distillation temperature. These data indicate that, in these two classes, the per cent of nitrogen compounds is constant and that the number of nitrogen atoms per molecule is constant. Thus
FUEL,
1981,
Vol 60, January
29
Composition Tab/e 5
of petroleum
Distribution
in Wilmington
Compound class
heavy ends (3): J. F. McKay
of heteroatoms
according to compound
(wt %)
Boiling range, (“C)
Nitrogen
Sulphur
Acids
370-535 535-675 >675
1.41 1.51 2.33
1.40 1.78 2.53
Bases
370-535 535-675 >675
3.87 2.90 1.96
1.24 1.79 2.31
Neutral nitrogen compounds
370-535 535-675 >675
3.80 1.88 1.78
3.18 2.71
Saturates
370-535 535-675 >675
0.11
0.82
_ _
_ -
0.63
2.37
Aromatics
ClaSS
heavy ends
370-535 535-675 >675
Table 6 Distribution heavy ends
_
of average molecular
-
_ _
Oxygen 2.67 1.30 0.81 0.74
weights in petroleum
Heavy ends fraction
Average molecular
370-535°C
distillate
400
535-675°C distillate >675”C Residue
550 900
weight
the decrease in per cent nitrogen is due to the increase in molecular weight of individual molecules in each fraction. The distribution of sulphur in the acid, base, and neutral nitrogen fractions is more difficult to interpret because sulphur compounds were not specifically isolated. Sulphur was generally found to be evenly distributed among compound classes, with the exception that the saturated hydrocarbons contained only traces of sulphur. Distribution of molecular weights in high-boiling distillates und residues An understanding of the molecular weight distribution of a complex mixture such as a crude oil is essential to an understanding of the chemistry of the mixture. Specifically, information concerning molecular weight distribution is useful in designing chromatographic separation schemes, in characterizing compound types, and in correlating chemical and physical properties. In this work the determination of average molecular weights for the heavy ends fractions was important because the quantitative infrared analyses applied to the characterization of the polar fractions required this information. In addition, in working with molecules of high molecular weight, intermolecular bonding interactions are commonly observed that greatly influence the solution properties of the mixture, sometimes imparting polymer-like properties to the mixture. It is important to know when the mixture contains small molecules that behave like large molecules owing to intermolecular bonding and when the mixture contains truly large molecules. The four different methods used to determine average molecular weights: vapour pressure osmometry, quantitative infrared spectrometry, electron impact mass spectrometry, and field ionization mass spectrometry, are described in Part 24. Table 6
30
FUEL,
1981,
Vol
60, January
et al. summarizes average molecular weight data for the heavy end fractions from four crude oils. The average molecular weights ofdistillates from different oils are similar because the oils were distilled in the same manner. However, the weight of 900 shown as an average molecular weight for different residues could differ by as much as 100 or 200 mass units because of varying amounts of high-molecularweight components in the oil. Field ionization mass spectrometry is considered the most reliable technique for examining the molecular weights of heavy ends molecules. Figure 1 shows field ionization mass spectra ofdistillates and residues from the Wilmington and Recluse oils. The shapes of the curves indicate that the weights of individual molecules in the heavy ends increase in a uniform manner with distillation temperature. The molecular weight spread is very large in any one distillate or residue. The molecular weight spread of the 37@~535”C distillates is from =20~-800, the 535 675°C distillates is from &30&l 100, and that of the residues is from 40&> 1600. All high-boiling distillates were 100 wt ‘Y;distillable into the field ionization instrument; the Wilmington residue was 78 wt % distillable, and the Recluse residue was 99 wt ‘x distillable. Approximately 95% of the molecules in the Wilmington oil and essentially all of the molecules in the Recluse oil can be observed by field ionization mass spectrometry. Figure 2 shows molecular-weight-distribution curves obtained for the Wilmington and Recluse oils by plotting the average molecular weights of distillate and residue fractions versus the weight per cent of the crude oils. The molecular weight data for the high-boiling distillates and residues were determined by field ionization mass spectrometry (see Figure 1). The molecular weight data for the low-boiling distillates were estimated from previous work’. The purpose of the Figure is to show graphically the distribution of molecular weights throughout the entire barrel ofcrude oil. The graphs for the two crude oils are parallel, showing that the distributions of molecular weights are similar in the two oils, but the average molecular weight of the Wilmington oil is higher than that of the Recluse oil. The transition in molecular weight from low-molecular-weight molecules to high-molecularweight molecules is continuous and changes rapidly in the high-boiling distillate and residue portions of the oil. Extrapolation of the data to the heaviest portion of the crude oil suggests that (1) =55”,:, of the Wilmington crude oil is much higher in molecular weight than any material in the Recluse oil and that (2) the molecular weights of those Wilmington molecules are above 1800. Characteristics of ‘average’ molecules in distillates and residues Average property data such as elemental analyses (Table 5) and average molecular weights (Table 6) can be used together to describe the chemical characteristics of ‘average’ molecules in distillate and residue fractions. The combined data show in general how the heteroatom content of molecules changes with change in molecular weight. This information can then be used to estimate the number of functional groups in an average distillate or residue molecule. To show how average property data are used to describe chemical characteristics of ‘average’ molecules the following example, which describes the changes in heteroatom content of an average Wilmington base molecule with increase in boiling range, is given. The
Composition
of petroleum
heavy ends (3): J. F. McKay et al.
d
a
J
200
400
600
800
1000
1200
1400
1600
Molecular
200
400
600
-
2000
r
.w c
=
(
Average moLecuLar determined by field
1000
1200
1400
1600
weight
Figure 1 Field ionization mass spectra of (a) Wilmington 370-535°C distillate; (b) Wilmington residue; (d) Recluse 370435°C distillate; (e) Recluse 535-675°C distillate; (f) Recluse >675’C
Average molecular determined from
800 535-675°C residue
distillate;
(c) Wilmington
>675’C
weight of fraction boiling point data weight of fraction ionization mass spect romet ry
-: 1600 3
E 800 Q1 t m 0L
WiLmingto c
40 Weight Figure 2
Molecular
weight distribution
of Wilmington
60 per cent
of crude
oils
and Recluse crude oils
FUEL, 1981,
Vol 60, January
31
Composition
of petroleum
heavy ends (3): J. F. McKay et al
heteroatom content of an average base molecule in the 370-535°C distillate and, for comparison, the heteroatom content of an average base molecule in the >675”C residue are calculated. The Wilmington 37&535”C bases have an average molecular weight of 400 and a nitrogen content of 3.87 wt ;! (Table 5). If the average base molecule contains one nitrogen atom and a molecular weight of 400, the calculated weight per cent nitrogen is 3.50 wt T! (14/400 x 100=3.50 wt ‘/“). Dividing the observed by the calculated weight per cent nitrogen (3.87/3.50 x lOO= 119%) shows that 119% of the molecules contain one nitrogen atom, or stated differently, on average one of every five molecules contains two nitrogen atoms. If every molecule contains one sulphur atom, the calculated sulphur content is: 32/400 x 100 = 8.0 wt :/,. The observed value of 1.24 wt “,, sulphur means that an average of * 16 per cent (1.24/8.00 x lOO= 15.5%) or one in every six molecules contains a sulphur atom. Oxygen data were not obtained for this sample and are estimated. Assuming that oxygen atoms are found in half of the base molecules (amides observed4 by i.r. and mass spectrometry), the calculations indicate that (1) the average base molecule contains one nitrogen atom; and that (2) in addition to the nitrogen atom. *851< of the molecules contain an additional heteroatom; the other heteroatom could be nitrogen or sulphur but is usually oxygen. The same calculations applied to the > 675’,C residue bases show significant differences in the composition of the average base molecule. Using a value of 900 for the average molecular weight of the residue bases and 1.96 wt ‘:;, nitrogen, 2.31 wt ‘>I:) sulphur, and 2.67 wt ‘4) oxygen, the following estimates are made: (1) the average residue base molecule contains about 3.4 heteroatoms, almost twice the number for the 37G535”C bases; and (2) every molecule contains one nitrogen atom, one in every four contains two nitrogen atoms, two ofevery three molecules contain one sulphur atom, every molecule contains one oxygen atom, and one of every two molecules contains two oxygen atoms. These estimates are for average molecules and may or may not be an accurate description of the molecules actually present in the heavy ends because the distributions of heteroatoms in individual molecules are not known. However, the calculations do show that with increase in boiling point the probability of a molecule having an increased heteroatom content greatly increases. This trend is also observed in the acid and neutral nitrogen fractions. The calculations also place limits on the number of functional groups one might expect to find in molecules of the heavy end fractions. For this discussion, functional groups aredefined as heteroatom-containing moieties in a molecule that usually impart acid or base properties to the molecule. For example, the hydroxyl group in a phenolic compound would be a functional group, but ether-type oxygen in a compound such as dibenzofuran would not be a functional group. In the distillate fractions, molecular weight and elemental analyses data could be correlated with compound type data to show that most molecules were monofunctional even though they contained about two heteroatoms per molecule. In the residues, quanti-
32
FUEL,
1981,
Vol 60,
January
tative compound type data cannot be obtained in most cases and so one cannot correlate molecular weight and elemental analyses data with compound type data. But calculations using average properties show that average residue molecules contain three to five heteroatoms per molecule. With the increase in heteroatom content of residue molecules, some residue molecules probably contain more than one functional group.
SUMMARY
AND CONCLUSIONS
Qualitatively, the heavy ends fractions of the four oils studied contain the same compound types, but the amounts vary from oil to oil. Compound types unique to any one oil or heavy ends fraction were not observed. In general, all distributions related to composition show gradual and orderly changes with increase in distillation temperature. Amounts of acid, base, and neutral nitrogen classes of compounds increase with increase in distillation temperature, with a concomitant decrease in hydrocarbons, especially the saturates. Compound types within the classes vary in amount with boiling range, and the distribution of compound types cannot be predicted from oil to oil. Average molecular weights of distillate fractions increase gradually with increase in distillation temperature throughout the crude oil and then increase rapidly in the residue fraction. The molecular weights of the largest residue molecules vary for the different oils. The highest molecular weight molecules ‘seen’ in the Wilmington crude oil have a molecular weight of about 1800, but the largest molecules in the oil have not been observed. The largest molecules in the Recluse oil appear to be in the 1800 to 2000 molecular-weight range. The heteroatom content of individual molecules in the Wilmington oil increase gradually with boiling range until the heteroatom content per molecule of molecules in the > 675°C residue is about double that of molecules in the 37s535°C distillate. Most molecules in the 37(r 535°C distillate are monofunctional. With the gradual increase in heteroatom content in the 535-675°C distillate and >675”C residue, the probability of bifunctional or polyfunctional molecules increases, but the amounts of these compounds have not been determined.
REFERENCES Haines, W. E. and Thompson, C. J. ‘Separating and Characterizing High-Boiling Petroleum-Distillates: Tie USBM-API Procedure: Rept. of Inv. LERCIRI-75/S. BERG/RI-75/2. Julv 1975 Thbmpson, C. J., Ward, c. C. and Ball, ‘J.‘S. ‘Characteristics of World’s Crude Oils and Results of API Research Project 60,’ Rept. of Inv., BERG/RI-76/8, July 1976 McKay, J. F., Amend, P. J., Harnsberger, P. M., Cogswell, T. E. and Latham, D. R. Fuel 1981, 60, 14 McKay, J. F., Harnsberger, P. M., Erickson, R. B., Cogswell, T. E. and Latham, D. R. Fuel 1981, 60, 17 McKay, J. F., Cogswell, T. E., Weber, J. H. and Latham, D. R. Fuel 1975, 54, 50