Energy & Fuels 1987, 1 , 2-11
2
Articles Composition of Heavy Petroleums. 1. Molecular Weight, Hydrogen Deficiency, and Heteroatom Concentration as a Function of Atmospheric Equivalent Boiling Point up to 1400 OF (760 "C) Mieczyslaw M. Boduszynski Chevron Research Company, Richmond, California 94802 Received September 16, 1986. Revised Manuscript Received October 10, 1986 The objective of this paper is to illustrate the variation of molecular weight, hydrogen deficiency, and heteroatom concentrations as functions of the atmospheric equivalent boiling point (AEBP). Short-path distillation (DISTACT) combined with the sequential elution fractionation (SEF) method was used to separate atmospheric residues derived from various petroleums into fractions having progressively higher AEBPs extending up to approximately 1400 "F (760 "C). Molecular weight measurements by field ionization (FI) and field desorption (FD) mass spectrometry (MS), concentrations of S, N, 0, V, Ni, and Fe, and the atomic H/C ratio are all reported as a function of the AEBP. The experimental evidence contradicts a common opinion that heavy petroleums, and residues in particular, are composed mostly of very high molecular weight components. The results reveal a broad molecular weight distribution pattern for the atmospheric residue from each crude oil and demonstrate that most heavy petroleum components do not exceed a molecular weight of approximately 2000. Data show that the heteroatom concentrations and hydrogen deficiency both increase with increasing AEBP. Significant bimodal distribution patterns for V and Ni were observed. The distribution profiles for S, N, and metals suggest that these constituents probably occur in the same molecular structures.
Introduction The effectiveness of the conversion of heavy petroleum feedstocks into more valuable products by using such processes as desulfurization, denitrogenation, demetalation, and hydrocracking can be significantly improved by adequate compositional information on the chemistry of reactions that are involved. Monitoring compositional changes by a mere comparison of operationally defined fractions such as, for example, "oils", "resins", and "asphaltenes" or by a determination of "average structures" for feedstock and product components provides inadequate and sometimes misleading information. Much more detailed compostional data are needed to unravel the structural transformations that occur during processing or to explain product properties. In recent years, a research effort was undertaken in this laboratory to develop an analytical scheme for detailed molecular characterization of heavy crude oils and petroleum residues. The objective of this work was to delineate the complexity of heavy petroleums and provide information on the variation of their composition as a function of atmospheric equivalent boiling point (AEBP). The analytical approach involved a combination of volatility and solubility fractionations to produce operationally well-defined fractions having progressively higher AEBPs. The fractions were then subjected to detailed characterization. This is the first paper in a series on composition of heavy petroleums. The objective of this paper is to illustrate the variation of molecular weight, hydrogen deficiency, and 0887-0624/87/2501-0002$01.50/0
heteroatom concentrations as a function of the AEBP up to approximately 1400 "F (760 "C). Results obtained for atmospheric residues (AR) derived from different crude oils are used as examples.
Experimental Section OF+) from Altamont (AL), Arabian Heavy (AH),Offshore California (OC),Maya (MA), Kern River (KR),and Boscan (BO) crude oils were used in this study. Volatility Fractionation. The short-path distillation apparatus (DISTACT, Leybold-Heraeus GmbH) was used to fractionate AR samples on the basis of their volatility. A detailed description of the DISTACT apparatus can be found elsewhere.'** The DISTACT fractionation involved a multistep distillation under reduced pressure of less than 0.002 Torr. Each distillation step was conducted at a constant evaporator temperature and produced one distillate cut and one residue. The actual distillation temperatures ranged from 130 "C (266 O F ) to 330 "C (626 O F ) . The thermal exposure of the material was minimized by a very short residence time of less than 1 min. The first distillation step was conducted at the evaporator temperature of 130 "C and produced a distillate (cut 1) and a residue that was used as a feed to the next experiment. The second distillation step was carried out at the evaporator temperature of 330 "C and produced a "nondistillable" residue and a distillate that was used as a feed to the third experiment. The evaporator temperature for the subsequent distillation steps was reduced by 25 "C increments as follows: 305 (cut 5B), 280 (cut Materials Studied. ARs (650
(1) Vercier, P.; Mouton, M. Analusis 1982, 101, 57-70. (2) Fischer, W. Technical Publication 28-220.1/2, 1982; LeyboldHeraeus GmbH, Hanau, Federal Republic of Germany.
0 1987 American Chemical Society
Composition of Heavy Petroleums 5A), 255 (cut 4B), 230 (cut 4A), 205 (cut 3B), 180 (cut 3A), and 155"C (cut 2B and cut 2A). Experiments at 305 "C were omitted for KR and OC AFb, thus the total cut 5 was produced from each. The AL and BO ARs were first fractionated at the evaporator temperature of 130 "C (cut l), followed by the experiment at 330 "C, and then by decreasing the evaporator temperature every 50 "C, namely, to 280 (cut 5), 230 (cut 4), and 180 OC (cut 3 and cut 2).
Solubility Fractionation. The sequential elution fractionation (SEF) method was used to further fractionatethe DISTACT "nondistillable" residues on the basis of their solubility in a sequence of solvents. A "nondistillable" residue sample of approximately 1 g was weighed accurately and was dissolved in about 5-10 mL of methylene chloride. The solution was introduced onto approximately 200 g of a 50:50 w/w mixture of glass beads (40 mesh, BDH Chemicals Ltd.) and Chromosorb-T (30/60 mesh, Manville) in a 1-L pear-shaped flask. The flask was placed on a Rotovap and was rotated very slowly at ambient temperature and under vacuum. The dry residue-coated support material was then packed into a glass liquid chromatographic column (25 mm i.d. X 50 cm, Altex). The column was connected to a programmable pump (Model 590, Waters), a solvent selection valve (Waters), a UV/vis diode array detector (Model 8451A, HP), which collected spectra from 350 to 650 nm at 30-s intervals, and a programmable fraction collector (Foxy, ISCO). Four solvents, (1)n-pentane, (2) cyclohexane, (3) toluene, and (4) a mixture of methylene chloride/methanol (4:l v/v), were pumped through the column at a constant flow rate of 20 mL/min. The four following solvent-derived fractions were collected: (1) n-pentane-soluble SEF-1; (2) cyclohexane-soluble, n-pentaneinsoluble SEF-P;(3) toluene-soluble,cyclohexane-insolubleSEF-3; (4) methylene chloride/methanol-soluble,toluene-insoluble SEF-4. Solvents were removed from the fractions, and the recovered material was determined gravimetrically. AEBP Determinations. The AEBP distributions for DISTACT cuts and SEF fractions were determined by using a vacuum thermal gravimetric analysis (VTGA) m e t h ~ d . ~A simulated distillation capillary supercritical fluid chromatography (SFC) method was also used for selected samples.* Molecular Weight Measurements. The molar mass distribution profiles of various DISTACT cuts and SEF fractions were obtained by using field ionization (FI) and field desorption (FD) mass spectrometry (MS). The FIMS measurments were obtained at SRI International by using procedures previously described! The FDMS experiments were performed at Chevron Research.6 Elemental Analysis. Carbon, hydrogen, sulfur, total and basic nitrogen, and oxygen were determined by using standard procedures. Vanadium, nickel, and iron were determined by an inductively coupled plasma-atomic emission spectroscopy (ICPAES) method.
Results and Discussion The adjectives "heavy", "high boiling", and "high molecular weight" are commonly but inappropriately used as equivalent terms to describe crude oils or their fractions. The term "heavy" refers to crude oil density. Heavy crudes, of which BO petroleum is a classic example (10.1 "API gravity in Figure l),have high densities (low API gravities) because they are rich in high-density naphthenes, aromatics, and compounds containing heteroatoms but are poor in alkanes. They are commonly either immature or degraded.7 Crude oils having gravity between 10 and 20 (3) SU, F., Chevron Research Co., Richmond, CA, private communication, 1984. (4) Schwartz, H. E.; Higgins, J. W.; Brownlee, R. G. Abstracts of Papers, 10th International SvmDosium on Column Liauid Chromatoaraphy, San Francisco, CA, 1986; Abstract 308. (5) Buttrill, S. E., Jr. Final Technical Report, SRI Project PYU 8903, 1981; SRI International, Menlo Park, CA. (6)Rechsteiner, C. E.; Attoe, T. H.; Boduszynski, M. M. Proceedings ASMS, The 33rd Annual Conference on Mass Spectrometry and Allied Topics, ASMS: East Lansing, MI, 1985, pp 937-938.
e
fl
Energy & Fuels, Vol. I , No. 1, 1987 3 K
B
COllshoreB
Arabian
22.5"APl
27.7"API
CrudeOii
I
Distillallon
1
Residue 650°F343°C.
1
+
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-
Vacuum Residue lOOO"F+ 538"C* 2.O"API Short-Path Distillation
-"l,'"d;:;+
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42.2"API
808 88
Vacuum Dlstillation
t "NondlstIIIable"
I3.B'API
8 -2.8'API
9.7"API
12.6'API
35.5'API
@ @ @ 5.8"API 2.3"API 0.4"API
@
@
@ @ @
@
0.9"API
8.8"API
-5.O'API
1O.I"API
-1.3"API
5.2'API
.2.6"API
30.6"API
27.5"API
'Wl h Irom Crude Oil
Figure 1. Effect of distillation on API gravity for various crude oils.
OAPI have been traditionally considered heavy. Light crudes, which have low densities (high API gravities) are rich in alkanes. AL petroleum (42.2 "API gravity in Figure l),an extreme example, has an exceptionally high alkane content because its predominant source is lacustrine algae, the same source as that of the nearby Green River shale.' Distillation is the primary refinery operation that separates petroleum into fractions varying in boiling point and composition. Distillation under atmospheric pressure removes fractions boiling below approximately 650 O F (343 "C) and produces AR. Distillation of the AR requires a reduced pressure to prevent thermal decomposition of petroleum components. A conventional vacuum distillation produces vacuum gas oil (VGO) distillate and vacuum residue (VR) boiling above approximately 10oO O F (538 "C) AEBP. Further fractionation of the VR can be accomplished by using a high vacuum short-path distillation also referred to as "molecular distillation", which allows for decomposition-free distillation up to approximately 1300 "F (704 "C) AEBP. For each of the crudes considered in Figure 1,the API gravity decreases with increasing depth of distillation. Hence, the term "heavy ends" tends to correlate with "high boiling" within a given crude. However, the correlation between "heavy" and "high boiling" does not necessarily hold if different crudes are being compared. For example, the "nondistillable" residue (1300 O F + ) from AL crude oil has a lower density (higher API gravity) than whole BO, KR, OC, or MA petroleum because it consists mainly of low-density alkanes. The dramatic effect of molecular structure on density (API gravity) is illustrated by the following examples: perhydron-hexadecane pyrene pyrene mol wt 226 218 202 atomic H/C ratio 2.125 1.625 0.625 density at 20 "C, g/mL 0.773" 0.983 1.271 gravity, "API 51.5 12.4 -20.2 "n-Hexadecane (CH,(CH2),,CH,) is a liquid at 20 "C (mp 18 "C). The density at 0 O C for n-CI6 is 0.787 g/mL (48.3 OAPI). Density increases remarkably with decreasing H/C ratio. This is due to the increasing hydrogen deficiency of a molecule (the Z value in a general formula CnHZn+Z, which (7) Tissot, B. P.; Welte, D. H. Petroleum formation and occurence, 2nd ed.; Springer-Verlag: West Berlin, 1984.
4 Energy &Fuels, Vol. 1, No. 1, 1987
Boduszynski Atmospheric Residue Feed Run at 130°C
30 28 26
24
/Run at 330'C
22
20 18
111,1.
16
Fraclton "SEF-1"
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n-pentane-
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"255°C Residue" CUI 4s
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280°C Distltiete"
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93
400 204
800 315
000 421
1000
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649
160
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Atmosph8rlc EquI~alenlBolllng Point
Figure 2. Effect of molecular structure on boiling point.
changes in the following order: paraffin (CnH2n+2)tetracyclic naphthene (C,H2,+)-tetraaromatic (C,H2n-z). All three compounds, however, have the same carbon number (C16),and their molecular weight decreases only slightly with increasing hydrogen deficiency. The terms "heavy" and "high boiling" are frequently but incorrectly used as if they were synonymous with "high molecular weight". The boiling point of a compound a t a given pressure is a rough measure of the attractive forces between the molecules. These forces vary with the structure of molecules, leading to great differences in the boiling point for compounds of a given molar mass but a different chemical structure. This is illustrated in Figure 2. Compounds having similar molar masses cover a broad boiling point range and, conversely, a narrow boiling point cut contains a wide molar mass range. For a given homologous series of compounds, the boiling point increases with molar mass as illustrated by the curve for paraffins (CnH2n+2) in Figure 2. This is due to the increase of the weak, van der Waals attractive intermolecular forces as molecules of a given type become larger. However, compounds having fused aromatic rings and functional groups capable of hydrogen bonding or other types of polar interactions have additional attractive intermolecular forces and may have a relatively low molar mass but a high boiling point and thus would be expected to concentrate in the "heavy ends". For a complex mixture, the molar mass range widens rapidly with increasing boiling point as illustrated in Figure 2. In order to investigate the effect of boiling point on molecular weight distribution of heavy petroleum components, the ARs derived from several crude oils were separated into fractions having progressively higher AEBPs. A schematic diagram of the volatility and solubility fractionations of ARs using DISTACT short-path distillation and the SEF method is shown in Figure 3. An approach similar to the SEF method has been previously reported for fractionation of solvent-refined coal and conventional petroleum The SEF method should not be confused with the conventional precipitation (8) Boduszynski, M. M.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1982,54, 372-375.
(9) Duffy, L., Amoco Research Center, Naperville, IL, private com-
munication.
Figure 3. Separation diagram for atmospheric residues using DISTACT short-path distillation and SEF method. Arabian Heavy AR 12.6'API
Bolcan AR DISTACT Evaporator Temp., 'C 1-130-CUI 1 2A-155-
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1
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24.3
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Figure 4. Examples of the "volatility-solubility" fractionation for AH and BO atmospheric residues.
of so-called asphaltenes. SEF is based on properties of solvent "philia" rather than solvent "phobia", which has been the basis for the traditional precipitation methods. We have found that the SEF method gives considerably higher yields of n-pentane-soluble fractions (SEF-1) than those obtained by using the ASTM D 2007 n-pentane precipitation method. Furthermore, SEF separates the n-pentane-insoluble portion into three well-defined solubility fractions, providing further insight into the composition of this most refractory portion of petroleum. Figure 4 shows the volatility and solubility fractionation results for AH (12.6 OAPI) and BO (6.3 OAPI) ARs as
Composition of Heavy Petroleums
Energy &Fuels, Vol. 1, No. 1, 1987 5
m
\
I
500 700 900 1100 1300 1500 "F 260 371 482
593 704 815 OC
50% REBP, OF
Atmospheric Equivalent Boiling Point
Figure 5. Examples of VTGA AEBP curves for fractions derived from AH atmospheric residue.
examples. "Nondistillable" residues, which accounted for 27.7 (AH) and 50.1 wt % (BO), had about the same API gravity (-2.6 and -2.8 OAPI, respectively) but different concentrations of SEF fractions. The volatility and solubility fractions were analyzed by using the VTGA method to determine their AEBP distributions. Preliminary results were also obtained with the SFC method. The SFC approach shows promise for characterization of heavy crudes, as it provides the AEBP distribution data, and a t the same time it is capable of producing fractions having progressively higher AEBPs for further off-line or on-line characterization. More details on a comparison between the two simulated distillation methods can be found elsewhere.1° Figure 5 shows an example of the VTGA AEBP curves for fractions derived from AH AR. The data demonstrate that the separation procedure produced a sequence of fractions having progressively higher AEBPs extending up to approximately 1400 O F (760 "C). The AEBP curves, however, reveal a considerable overlapping between the fractions. This is due to the very low efficiency of short-path distillation, which is typically less than one theoretical plate. Interestingly, the SEF-1 solubility fractions derived from different "nondistillable" residues were all volatile under the VTGA conditions and had similar AEBP distribution patterns with the 50% AEBP value of approximately 1370 O F (743 OC). The SEF-2, SEF-3, and SEF-4 fractions could not be volatilized under the VTGA conditions without thermal decomposition. The 50% AEBP values for each fraction were used to plot the results of volatility and solubility fractionations. This is illustrated in Figure 6 with results for AL,KR, AH, OC, MA, and BO ARs. The ordinate gives the cumulative weight percent from AR, and the abscissa gives the 50% AEBP. The labels for fractions are given on the right side of each plot. In the case of AL AR, solubility fractions SEF-3 and SEF-4 and a small amount of waxy material that was not soluble in any of the solvents used accounted together for 2.1 wt % and were not labeled for clarity. Plots in Figure 6 demonstrate that most of petroleum components do not exceed approximately 1400 O F (760 "C) AEBP. However, the extension of AEBP curves to solubility fractions (SEF-1)should be interpreted with caution. It is possible that the solubility separation increased the volatility of SEF-1 fractions by isolating their components from the remaining constituents of a "nondistillable" residue and by reducing intermolecular interactions. The DISTACT cuts and SEF fractions were analyzed by FIMS and FDMS to determine their apparent molar (10) Schwartz, H. E.; Brownlee, R. G.; Su, F.; Boduszynski, M. M., manuscript in Preparation.
50% REEP, "F
YRYR
PR
Figure 6. The 50% AEBP distribution curves for atmospheric residues derived from various crude oils.
SEF-3
SEF-1
so0
1w sw 1w
low
lKl0
low
5w
MI2
Figure 7. Examples of FIMS molecular weight profiles for fractions derived from BO atmospheric residue.
mass profiles. Both techniques gave consistent results. All DISTACT cuts and SEF-1 fractions were 100% volatile under the mass spectral analysis conditions. Fractions SEF-2 and SEF-3 were about 70% + and 40% + volatile, respectively. Fractions SEF-4 represented a very small portion of ARs (0.4-1.9 wt % ) and were not analyzed. Figure 7 shows FIMS profiles for BO fractions as an example. A significant trend can be observed. The molar mass range of the successive fractions widens in a fashion that is consistent with the trend indicated by the curves in Figure 2. The considerable molar mass overlapping
6 Energy & Fuels, Vol. 1, No. 1, 1987
Boduszynski
DISTACT CUT 5 FROM KERN RIVER AR
CNd.
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300
700
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Figure 8. FIMS molecular weight profiles for DISTACT cut 5 fractions from KR and BO atmospheric residues.
-18
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,
db: 343
,
,
,
1000 530
,
,
, ,, 1300
,
704
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,
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Atmoipheric Equivalent Bolllng Point
2%-%2k;w 011 Figure 9. Molecular weight distribution of petroleum components as a function of the AEBP.
between the fractions is mostly due to the effect of mocould be attributed to so-called "polymeric" asphaltenes. lecular structure on boiling point, as discussed earlier, and The results in Figure 9 show that fractions with the only in part due to the low efficiency of DISTACT disAEBP below approximately 1300 O F (the upper limit for tillation and SEF fractionation. a DISTACT distillation) account for 57.2 (BO) to 91.1 wt The molar mass profiles for fractions derived from other % (AL) of the whole crude oil and do not exceed a mocrude oils showed a similar trend. The molecular weight lecular weight of 1400. The addition of SEF-1 fractions range and average molecular weight values calculated from having a 50% AEBP of approximately 1370 OF brings the the spectra for a given AEBP fraction were similar, recumulative weight percent of crude oil components to 77.8 gardless of the crude oil origin. The shapes of the FIMS w t % for BO and 97.9 wt % for AL and extends the moenvelopes, however, varied among fractions derived from lecular weight range up to approximately 2000. All those different petroleums, indicating differences in composition. fractions (i.e., cut 1 through SEF-1) were 100% volatile This is illustrated in Figure 8, which shows FIMS molar under the FIMS analysis conditions. Interestingly, the mass profiles for the cut 5 fractions derived from KR and molar maw profiles for the SEF-2 and SEF-3 fractions also BO ARs. The FIMS profile for the KR fraction exhibits revealed a broad molecular weight distribution extending a broader distribution of molecular ion peaks than that to relatively low molecular weights (Figure 7). However, of the BO cut. Interestingly, the spectrum of the BO cut not all components of those fractions were completely 5 reveals the presence of relatively low molar mass peaks volatile under the maw spectral analysis conditions (SEF-2 with a maximum a t m/z 541, which were found to be was 70%+ and SEF-3 was 40%+ volatile) and the unacrepresented by vanadyl porphyrins (see further discussion counted material could involve compounds having mobelow on the distribution of metals). lecular weights higher than 2000. These findings are sigThe data obtained for fractions derived from several nificant because of the existing controversy over whether crude oils illustrate in Figure 9 the molecular weight disthere is an appreciable concentration of molecules in petribution of petroleum components as a function of the troleum having molecular weights greater than 2000. Data AEBP. Figure 9 reveals that most of petroleum compoin Figure 9 show there is not. The observed molecular nents do not exceed a molecular weight of about 2000. The weight distribution pattern indicates that the molecular data provide further support for the early speculations by structure, as well as the molecular weight, determines Dean and Whitehead who suggested a molecular weight volatility and solubility of petroleum components. maximum of 2000 for all compounds in petroleum." The atomic ratio of hydrogen-to-carbon reflects hydroThese results are also consistent with the FIMS measurements previously reported for other crude o i l ~ . ~ ~ Jgen ~ deficiency of the molecules and is frequently used as a simple measure of the "aromaticity" of petroleum fracThe experimental evidence contradicts a common tions. Figure 10 shows how the H/C ratio changes with opinion that heavy crude oils, and residues in particular, increasing boiling point for ARa derived from KR, AH, OC, are composed mostly of very high molecular weight comand BO crude oils. ponents. Actually, these materials have a wide molecular The results for all four crudes in Figure 10 show that weight distribution, which extends to relatively small molecules and is a continuum with no discrete fraction that the H/C ratio varies from high values of approximately 1.6-1.8 for cut 1fractions to the values of 1.1-1.2 for SEF-3 fractions. However, the H/C ratio decreases with de(11) Dean, R. A.; Whitehead, E. V. Proc. World Pet. Congr. 1983, creasing volatility and solubility (increasing AEBP) a t a 34(6),261-276. different rate for different crudes. Data for KR, AH, and (12) Boduszynski,M. M.; McKay, J. F.; Latham, D. R. Prep.-Am. OC A R s in Figure 10 exhibit a dramatic decrease of the Chem. SOC.,Diu. Pet. Chem. 1981,885-881. (13) McKay, J. F.; Latham, D.R.; Haines,W. E. Fuel 1981,60,27-32. H/C ratio between SEF-1 and SEF-2 fractions, while a
Energy & Fuels, Vol. 1, No. 1, 1987 7
Composition of Heavy Petroleums KERN R I V E R RR
RRRBIRN HERVY
FRRCTION
REBP, T
RR
0
FRRCTION
REBP. OF
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CUT 2R
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I
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v
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RTOMIC RRTIO
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2
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H/C
1.2 1.4 1.6 RTOMIC RRTIO
1.E
Figure 10. Atomic H/C ratio as a function of the AEBP.
the A€& shown in Figure 11was as follows: 1.21 (KR), 4.17 curve for the BO AR follows an approximately linear (MA), 4.4 (AH), and 5.9 wt % (BO). The concentration pattern. of sulfur in the low-sulfur KR AR increases only a little The increasing hydrogen deficiency of petroleum comwith increasing boiling point from approximately 1.0 wt ponents with increasing AEBP provides additional support % in cut 1to 1.4 wt % in the SEF-3 fraction, while that for the molecular weight distribution pattern shown in in the high-sulfur AH AR increases almost threefold from Figures 2 and 9. Namely, the hydrogen-poor, polycyclic 2.7 wt % in cut 1to 8.0 w t % in the SEF-3 fraction. The aromatic structures are likely to have limited volatility and sulfur concentrations in MA and BO ARs increase with solubility but may involve relatively small molecules. increasing AEBP from 2.6 to 7.2 wt % and 4.5 to 7.1 wt Heavy crude oils are generally associated with high % , respectively. The dramatic increase of sulfur concenheteroatom (S, N, 0, V, Ni, Fe) content. Recent studies trations in fractions SEF-2 and SEF-3 from AH and MA on the heteroatoms speciation involved size-exclusion is of particular interest because it coincides with a similar chromatography (SEC) with element-specific detection using inductively coupled plasma (ICP) s p e ~ t r o m e t r y . ~ " ~ ~ distribution pattern for metals (see further discussion on V, Ni, and Fe distribution). The data suggest that sulfur This approach provides information on heteroatom (V, Ni, and metals exist in the same molecular structures. The Fe, S) distributions as a function of a retention time in a results in Figure 11show that a relatively high percentage chromatographic column that is usually converted to a of the total sulfur in those three ARs (AH, MA, BO) is "molecular size". Despite the recent contributions from represented by the SEF-2 and SEF-3 fractions. SEC-ICP results, the available information on distribution The nitrogen content in petroleum is usually much lower of heteroatoms in petroleum as a function of boiling point than the sulfur content, and the average value in crude oils has been rather limited. is 0.094 wt % . 7 Crudes having more than 0.25 wt % niSulfur is the third most abundant atomic constituent trogen are considered nitrogen-rich. of petroleum, following carbon and hydrogen. A majority The distribution of nitrogen as a function of AEBP is of crude oils contain less than 1% sulfur. High sulfur illustrated in Figure 12. The four ARs shown in Figure crudes having more than 1% sulfur are represented by a 12 have a progressively higher nitrogen content as follows: smaller group.7 0.25 (AH),0.47 (MA),0.83 (OC), and 0.94 wt % (KR). The Figure 11 illustrates the distribution of sulfur as a concentration of nitrogen increases with decreasing volafunction of AEBP. The initial concentration of sulfur in tility and solubility of heavy oil components for all four crudes. (14)Hausler, D.;Taylor, L. Anal. Chem. 1981,53, 1223-1227. The nitrogen concentration in a low-nitrogen AH AR (15)Hausler, D.;Taylor, L. Anal. Chem. 1981,53,1227-1231. covers a range from 0.03 wt % for cut 1to 1.0 wt % for Hausler, D.W.Spectrochimica Acta, Part B 1985,40B,389-396. . (16) (17)Fish, R. H.; Komlenic, J. J. Anal. Chem. 1984,56,510-517. the SEF-3 fraction, while that in a nitrogen-rich KR AR (18)Biggs, W.R.;Fetzer, J. C.; Brown, R. J.; Reynolds, J. G. Li9. Fuels ranges from 0.3 wt % for cut 1to 2.4 w t % for the SEF-3 Technol. 1985,3,397-421. fraction. Of particular interest is the dramatic increase (19)Biggs, W.R.;Brown, R. J.; Fetzer, J. C., to be submitted for of the nitrogen concentration in SEF-2 and SEF-3 fractions publication in Energy Fuels.
Boduszynski
8 Energy &Fuels, Vol. 1, No. 1, 1987 K E R N R I V E R RR
1
'
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Figure 11. Sulfur distribution as a function of the AEBP.
I
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Figure 12. Nitrogen distribution as a function of the AEBP.
derived from all four residues. It will be shown later that this unique distribution pattern coincides with that for metals. The data suggest that nitrogen, as well as sulfur, occurs with metals in the same molecular structures. The percent of nitrogen that is represented by the SEF-2 and SEF-3 fractions varies from approximately 2550% of the total nitrogen in a given AR. Basic nitrogen was found to account for 12-35% of the total nitrogen, depending on boiling point and crude oil
origin. The percent of basic nitrogen decreases with increasing boiling point for all crudes. However, the distribution of basic nitrogen as a function of the AEBP varies for different oils. The low-to-high values for basic nitrogen as a percent of total nitrogen are as follows: 17-21% (AH), 12-33% (MA), 13-25% (OC),and 24-35% (KR). Literature data on the oxygen concentration in petroleum are very scarce, mainly because of a very low oxygen content in most crudes and also because of a relative
Energy &Fuels, Vol. 1, No. 1, 1987 9
Composition of Heavy Petroleums
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Figure 13. Oxygen distribution as a function of the AEBP. KERN R I V E R RR FRICTION
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Figure 14. Vanadium distribution as a function of the AEBP.
difficulty in obtaining reliable results. It is known, however, that young and immature crudes such as California crude oils are relatively rich in carboxylic acids, which seem to be the most common oxygen-containing compounds in petroleum.' Figure 13 shows how oxygen concentration changes with decreasing volatility and solubility of oil components by using results obtained for two California crude oils: KR and OC. The oxygen content increases with increasing AEBP in a fairly similar fashion for both residues from approximately 0.3-0.4 w t % for cut 1 fractions to 1.2-1.35 wt % for SEF-3 fractions. The results on distribution of sulfur, nitrogen, and oxygen show that all three of these nonmetallic heteroatomic constituents of petroleum increase in concentration with increasing AEBP. It is important to remember, however, that on the molecular level, the concentration of these heteroatoms increases in a much more dramatic way because of the simultaneously increasing molecular weight. This is illustrated in Table I. Data in Table I should be interpreted as average estimates only. The actual number of heteroatoms per molecule may vary. For example, the
value of 1.07 nitrogen atoms for a molecular weight of 600 and a nitrogen concentration of 2.5 wt % may indicate either that approximately every molecule is a mononitrogen compound or that about every fourth molecule contains four nitrogen atoms (e.g., in the form of a porphyrin). Although the actual distribution of sulfur, nitrogen, and oxygen on a molecular level is not known, the observed changes in heteroatom concentration and molecular weight with increasing AEBP imply a high concentration of compounds having several heteroatoms per molecule. Those compounds concentrate particularly in SEF-2 and SEF-3 fractions. Organometallic compounds in petroleum contain predominantly vanadium and nickel and, to a lesser extent, iron. Other metals have also been reported.20 However, systematic data are available only for vanadium and nickel. Vanadium and nickel are present in variable amounts from less than 1ppm up to 1200 ppm of vanadium and 150 ppm (20) Yen, T. In The Role of Trace Elements in Petroleum; Yen, T., Ed.; Ann Arbor Science: Ann Arbor, Michigan, 1975;pp 1-30.
10 Energy & Fuels, Vol. 1, No. 1, 1987
Boduszynski Table I av no. of atoms per molecule nitrogen
sulfur mol w t
3.0 wt %
8.0 w t %
0.3 w t %
2.5 w t %
200 600 1000 1400 2000
0.19 0.56 0.94 1.31 1.87
0.50 1.50 2.50 3.50 5.00
0.04 0.13 0.21 0.30 0.43
0.36 1.07 1.79 2.50 3.57
oxygen 0.2 wt % 0.02 0.07 0.12 0.17 0.25
1.5 wt % 0.19 0.56 0.94 1.31 1.87
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Figure 15. Nickel distribution as a function of the AEBP.
Figure 16. Iron distribution as a function of the AEBP. of nickel, depending on the crude oil 0rigin.I Figure 14 shows the distribution of vanadium as a function of AEBP by using data for KR, AH, MA, and BO A R s as examples. The initial concentrations of vanadium in the four ARs were as follows: 38 (KR), 79 (AH), 343 (MA), and 1409 ppm (BO). Data in Figure 14 show that the concentration of vanadium increases with decreasing volatility and solubility of heavy oil components up to a maximum value of 274 ppm for KR, 511 ppm for AH, 1860 ppm for MA, and 4430 ppm for BO. However, the plots also reveal a significant bimodal distribution pattern with a minimum a t approximately 1370 OF (SEF-1).
Vanadium represented by the DISTACT-distillablecuts (first envelope in Figure 14) accounts for a small percentage of the total vanadium in a given AR, namely 11.3% (MA), 11.8% (BO), 11.6% (AH), and 21.9% (KR). Vanadium in this portion of each AR was found to be present mostly in the form of metalloporphyrins (identified by the Soret band a t 408 nm). Most of the vanadium was present in the SEF fractions, particularly SEF-2 and SEF-3, derived from DISTACT"nondistillable" residues. Data in Figure 14 show significant differences among the four crudes. Interestingly, W/vis spectra collected during the SEF separations reveal
Composition of Heavy Petroleums
no Soret bands for any residue except that from BO. All four SEF fractions derived from the BO "nondistillable" residue exhibited the presence of petroporphyrins. The nickel distribution as a function of the AEBP follows a pattern similar to that for vanadium. However, lower concentrations of nickel are involved and the bimodal distribution is less pronounced. This is illustrated in Figure 15. The initial concentrations of nickel in the four ARs were as folows: 24 (AH), 72 (MA), 81 (KR), and 124 ppm (BO). Nickel concentrations increase with increasing AEBP up to maximum values of 186 (AH), 333 (MA), 397 (BO), and 436 ppm (KR). The DISTACT-distillable cuts account for a relatively small portion of the totalnickel, namely, 6.2% (AH), 7.3% (MA), 9.3% (BO), and 27.3% (KR). Most of the nickel was present in the SEF-2 and SEF-3 fractions. The iron distribution in OC and KR ARs is illustrated in Figure 16. The plots show a pattern similar to that observed for nickel and vanadium. However, the concentration of iron in DISTACT-distillablecuts is very low-13 ppm or less. The iron concentration increases with decreasing solubility of residue components and reaches maximum values of 300 (OC) and 419 ppm (KR) for SEF-3 fractions. The low recovery of iron in the case of the OC AR (55.8% in Figure 16) suggests that some iron in this crude oil is not of an organometallic nature.
Conclusions The results presented in this paper demonstrate that heavy petroleums can be successfully separated into fractions having progressively higher AEBP extending up to approximately 1400 OF (760 "C). The advantage of this fractionation approach is that it produces operationally well-defined fractions in sufficient quantities for further detailed characterization. Data derived from this study dispel many misconceptions about the molecular weight of heavy petroleum components. The results reveal a unique molecular weight distribution pattern as a function of the AEBP. The experimental evidence contradicts a common opinion that heavy crudes, and petroleum residues in particular, are composed mostly of very high molecular weight components. The data show that these materials have a wide molecular weight distribution that extends to relatively small molecules. Quantitative data are presented to demonstrate that most of the heavy petroleum components do
Energy &Fuels, Vol. 1, No. 1, 1987 11
not exceed a molecular weight of approximately 2000. For the first time, distributions of sulfur, nitrogen, oxygen, vanadium, nickel, and iron as a function of the AEBP up to approximately 1400 OF (760 "C) are presented. Concentrations of heteroatoms vary over a wide range depending on boiling point and crude oil origin. The heteroatom content and hydrogen deficiency of petroleum components increase with increasing AEBP. Significant bimodal distribution patterns for vanadium and nickel are observed. The distribution profiles for sulfur and nitrogen and those for metals suggest that these constituents probably occur in the same molecular structures.
Acknowledgment. The author expresses his appreciation to R. Malhotra and G. St. John of SRI International for their assistance in providing FIMS data and to C. E. Rechsteiner and E. J. Gallegos of Chevron Research Co. for FDMS measurements. Thanks are also due to F. Su and J. B. Newman for their assistance in providing VTGA data. Technical assistance from T. H. Attoe and J. R. Richter is also greatly appreciated. The author is grateful to Chevron Research Co. for supporting this research and allowing publication of this paper. Glossary AEBP atmospheric equivalent boiling point AH Arabian heavy crude oil AL Altamont crude oil AR atmospheric residue (650 O F + ) BO Boscan crude oil DISshort-path distillation apparatus TACT FDMS field desorption mass spectrometry FIMS field ionization mass spectrometry ICPinductively coupled plasma-atomic emission AES spectrometry KR Kern River crude oil MA Maya crude oil Mn number average molecular weight M w weight average molecular weight oc Offshore California crude oil SECsize-exclusion chromatography-inductively coupled ICP plasma SEF sequential elution fractionation SFC supercritical fluid chromatography VR vacuum residue (1000 OF+) VTGA vacuum thermal gravimetric analysis Registry No. V, 7440-62-2; Ni, 7440-02-0; Fe, 7439-89-6; S, 7704-34-9; N, 7727-37-9.
