Composition of Heavy Petroleums. 1. Molecular Weight,

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2 Energy & Fuels 1987, 1 , 2-11 
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), concen- 
trations 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 approx- 
imately 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 ade- 
quate compositional information on the chemistry of re- 
actions 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 de- 
tailed 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 petro- 
leum residues. The objective of this work was to delineate 
the complexity of heavy petroleums and provide infor- 
mation 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 character- 
ization. 
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 
Materials Studied. ARs (650 O F + ) 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 appa- 
ratus (DISTACT, Leybold-Heraeus GmbH) was used to frac- 
tionate 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 tem- 
perature 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 
(1) Vercier, P.; Mouton, M. Analusis 1982, 101, 57-70. 
(2) Fischer, W. Technical Publication 28-220.1/2, 1982; Leybold- 
Heraeus 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 fraction- 
ation (SEF) method was used to further fractionate the DISTACT 
"nondistillable" residues on the basis of their solubility in a se- 
quence 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 approx- 
imately 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-pentane- 
insoluble SEF-P; (3) toluene-soluble, cyclohexane-insoluble SEF-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 DIS- 
TACT 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 dis- 
tribution 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 
obtainedat 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 pro- 
cedures. Vanadium, nickel, and iron were determined by an 
inductively coupled plasma-atomic emission spectroscopy (ICP- 
AES) method. 
Energy & Fuels, Vol. I , No. 1, 1987 3 
CrudeOii 
Distillallon 
K B e C B Ollshore Arabian fl 
I 
1 10.1'APi I3.B'API 22.2'API 22.5"APl 27.7"API 42.2"API 
Results and Discussion 
The adjectives "heavy", "high boiling", and "high mo- 
lecular 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 communi- 
cation, 1984. 
(4) Schwartz, H. E.; Higgins, J. W.; Brownlee, R. G. Abstracts of 
Papers, 10th International SvmDosium on Column Liauid Chromatoa- 
raphy, 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. 
1 8 6.3"API 0 9.7"API 8 8.8"API 1O.I"API 8 12.6'API 8 35.5'API Residue 650°F- 343°C. 
Vacuum Dlstillation - + 
Vacuum Residue 
lOOO"F+ 
538"C* 
Short-Path Distillation 
@ @ @ @ @ 
2.O"API 5.8"API 2.3"API 0.4"API 5.2'API 30.6"API 
t 
"NondlstIIIable" 
-"l',"d;:;+ -704°C- 8 @ @ @ @ 
-2.8'API 0.9"API -5.O'API -1.3"API .2.6"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 grav- 
ities) 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 sep- 
arates petroleum into fractions varying in boiling point and 
composition. Distillation under atmospheric pressure re- 
moves 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 distilla- 
tion 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 accom- 
plished 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 OF+) from AL crude oil 
has a lower density (higher API gravity) than whole BO, 
KR, OC, or MA petroleum because i t consists mainly of 
low-density alkanes. 
The dramatic effect of molecular structure on density 
(API gravity) is illustrated by the following examples: 
n-hexadecane pyrene pyrene 
perhydro- 
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 
30 
28 
26 
24 
22 
20 
18 
16 
14 
12 
10 
8 
6 
4 
2 
0 
0 200 400 800 000 1000 12W 1400 
.18 93 204 315 421 538 649 160 
'F 
'C 
Atmosph8rlc EquI~alenl Bolllng 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 ho- 
mologous 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, com- 
pounds having fused aromatic rings and functional groups 
capable of hydrogen bonding or other types of polar in- 
teractions 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 com- 
ponents, the ARs derived from several crude oils were 
separated into fractions having progressively higher 
AEBPs. A schematic diagram of the volatility and solu- 
bility 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. 
(9) Duffy, L., Amoco Research Center, Naperville, IL, private com- 
1982,54, 372-375. 
munication. 
Atmospheric 
Residue Feed 
Run at 130°C 
/Run at 330'C 
111, n-pentane- Fraclton "SEF-1" 1 
1. 280°C Distltiete" 
I lRun at 255°C C y c l o h e x a n e 4 Fracllon "SEF-2 I 
Fraction "SEF-3" 
4 "255°C Residue" ,,255rC Distillate,, CUI 4s ' ' ,Run at 2 3 W j 
Figure 3. Separation diagram for atmospheric residues using 
DISTACT short-path distillation and SEF method. 
Arabian Heavy AR Bolcan AR 
DISTACT 6'3"AP1 
12.6'API 
7.0 -ut 
6.5 -ut 
Evaporator 
Temp., 'C 
1-130- -CUI 1 s 1 
2A-155- 
38-230- - C u t 3- t 3A-205- 
w t Qh 
7.7 
16.5 
9.93.2 -ut 4B-260- -Cut 4- 9.3 
4.9 a u t 4A-255- 
6.8 B u t 5A-305- t 
2.8 =-Cut 5 B - 3 3 O k C u l 5 ~ 6.5 
"Nondl8llli bl " Restdue 
27.7 s/ 50.1 
-2.6"APi -2.8'API 
\ / 
"SEF" Separation -6 24.3 
SEI-2 
SEk-4 
0.6 0 - M e C i 2 : M e O H - a 2.0 
( 4 1 V N ) 
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 solu- 
bility fractions, providing further insight into the compo- 
sition 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 
500 700 900 1100 1300 1500 "F 
260 371 482 593 704 815 OC 
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 dis- 
tributions. 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 solu- 
bility 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. 
m 
\ I 
50% REEP, "F 
Y R Y R P R 
50% REBP, OF 
Figure 6. The 50% AEBP distribution curves for atmospheric 
residues derived from various crude oils. 
SEF-3 
SEF-1 
so0 low lKl0 
1w sw low 
1w 5w 
M I 2 
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 
Mass, M/Z 
DISTACT CUT 5 FROM BOSCAN AR 
0.3 7 I 
Mn = 869 
300 700 1100 1500 1900 
Mass, M/Z 
Figure 8. FIMS molecular weight profiles for DISTACT cut 5 
fractions from KR and BO atmospheric residues. 
between the fractions is mostly due to the effect of mo- 
lecular structure on boiling point, as discussed earlier, and 
only in part due to the low efficiency of DISTACT dis- 
tillation and SEF fractionation. 
The molar mass profiles for fractions derived from other 
crude oils showed a similar trend. The molecular weight 
range and average molecular weight values calculated from 
the spectra for a given AEBP fraction were similar, re- 
gardless of the crude oil origin. The shapes of the FIMS 
envelopes, however, varied among fractions derived from 
different petroleums, indicating differences in composition. 
This is illustrated in Figure 8, which shows FIMS molar 
mass profiles for the cut 5 fractions derived from KR and 
BO ARs. The FIMS profile for the KR fraction exhibits 
a broader distribution of molecular ion peaks than that 
of the BO cut. Interestingly, the spectrum of the BO cut 
5 reveals the presence of relatively low molar mass peaks 
with a maximum a t m/z 541, which were found to be 
represented by vanadyl porphyrins (see further discussion 
below on the distribution of metals). 
The data obtained for fractions derived from several 
crude oils illustrate in Figure 9 the molecular weight dis- 
tribution of petroleum components as a function of the 
AEBP. Figure 9 reveals that most of petroleum compo- 
nents do not exceed a molecular weight of about 2000. The 
data provide further support for the early speculations by 
Dean and Whitehead who suggested a molecular weight 
maximum of 2000 for all compounds in petroleum." 
These results are also consistent with the FIMS mea- 
surements previously reported for other crude o i l ~ . ~ ~ J ~ 
The experimental evidence contradicts a common 
opinion that heavy crude oils, and residues in particular, 
are composed mostly of very high molecular weight com- 
ponents. Actually, these materials have a wide molecular 
weight distribution, which extends to relatively small 
molecules and is a continuum with no discrete fraction that 
(11) Dean, R. A.; Whitehead, E. V. Proc. World Pet. Congr. 1983, 
(12) Boduszynski, M. M.; McKay, J. F.; Latham, D. R. Prep.-Am. 
(13) McKay, J. F.; Latham, D. R.; Haines, W. E. Fuel 1981,60,27-32. 
34(6), 261-276. 
Chem. SOC., Diu. Pet. Chem. 1981,885-881. 
CNd. 
011 Z W t K , 
AL 39.2 
-- 
AH =.a 
MA 36.0 
OC 38.1 
KR 18.0 
80 14.5 
\ 
Z W I % 
79.1 
68.8 
85.7 
80.8 
58.7Y.4 
- 
\ 
\ 
Z W l % l W l % 
91.1 97.9 
0a.2 92.7 
77.2 88.7 
76.4 36.4 
79.1 92.3 
57.2 77.8 
-- Z W l l l W l % Z W l * 
1 . 4 1 . 5 1 . 6 
97.7 99.8 1 w 
91.0 99.4 1 w 
92.5 99.3 100 
97.0 89.5 1 w 
89.0 1 . 1 100 
--- 
"I :{ f , 3 0 0 , , b:d , , , , , , ,, , , , 
1000 1300 1700 'F 
-18 149 343 530 704 927 "C 
0 
Atmoipheric Equivalent Bolllng Point 
2%-%2k;w 011 
Figure 9. Molecular weight distribution of petroleum components 
as a function of the AEBP. 
could be attributed to so-called "polymeric" asphaltenes. 
The results in Figure 9 show that fractions with the 
AEBP below approximately 1300 O F (the upper limit for 
a DISTACT distillation) account for 57.2 (BO) to 91.1 wt 
% (AL) of the whole crude oil and do not exceed a mo- 
lecular weight of 1400. The addition of SEF-1 fractions 
having a 50% AEBP of approximately 1370 OF brings the 
cumulative weight percent of crude oil components to 77.8 
w t % for BO and 97.9 wt % for AL and extends the mo- 
lecular weight range up to approximately 2000. All those 
fractions (i.e., cut 1 through SEF-1) were 100% volatile 
under the FIMS analysis conditions. Interestingly, the 
molar maw profiles for the SEF-2 and SEF-3 fractions also 
revealed a broad molecular weight distribution extending 
to relatively low molecular weights (Figure 7). However, 
not all components of those fractions were completely 
volatile under the maw spectral analysis conditions (SEF-2 
was 70%+ and SEF-3 was 40%+ volatile) and the unac- 
counted material could involve compounds having mo- 
lecular weights higher than 2000. These findings are sig- 
nificant because of the existing controversy over whether 
there is an appreciable concentration of molecules in pe- 
troleum having molecular weights greater than 2000. Data 
in Figure 9 show there is not. The observed molecular 
weight distribution pattern indicates that the molecular 
structure, as well as the molecular weight, determines 
volatility and solubility of petroleum components. 
The atomic ratio of hydrogen-to-carbon reflects hydro- 
gen deficiency of the molecules and is frequently used as 
a simple measure of the "aromaticity" of petroleum frac- 
tions. Figure 10 shows how the H/C ratio changes with 
increasing boiling point for ARa derived from KR, AH, OC, 
and BO crude oils. 
The results for all four crudes in Figure 10 show that 
the H/C ratio varies from high values of approximately 
1.6-1.8 for cut 1 fractions to the values of 1.1-1.2 for SEF-3 
fractions. However, the H/C ratio decreases with de- 
creasing volatility and solubility (increasing AEBP) a t a 
different rate for different crudes. Data for KR, AH, and 
OC A R s in Figure 10 exhibit a dramatic decrease of the 
H/C ratio between SEF-1 and SEF-2 fractions, while a 
Energy & Fuels, Vol. 1, No. 1, 1987 7 Composition of Heavy Petroleums 
K E R N R I V E R R R 
FRRCTION 
CUT 28 
CUT 3R 
CUT 3B 
CUT 4 R 
CUT 48 
CUT 5 
SEF-I 
50 
75 
BEF-2 
I00 %E-I 
I 1 . 2 1 . 4 1.6 1.9 
H/C R T O M I C R R T I O 
REBP, T 
722 
E25 
867 
955 
1023 
1091 
I156 
1239 
1365 
O F F S H O R E C R L I F O R N I R R R 
FRRCTION REBP. O F 
CUT 2R 604 
CUT 28 
CUT 3 R 
CUT 3 B 
CUT 4 R 
v CUT 4B 
c CUT 5 
687 
955 
1026 
I100 
1 1 5 3 
1 2 4 3 
Figure 10. Atomic H/C ratio as a function of the AEBP. 
curve for the BO AR follows an approximately linear 
pattern. 
The increasing hydrogen deficiency of petroleum com- 
ponents with increasing AEBP provides additional support 
for the molecular weight distribution pattern shown in 
Figures 2 and 9. Namely, the hydrogen-poor, polycyclic 
aromatic structures are likely to have limited volatility and 
solubility but may involve relatively small molecules. 
Heavy crude oils are generally associated with high 
heteroatom (S, N, 0, V, Ni, Fe) content. Recent studies 
on the heteroatoms speciation involved size-exclusion 
chromatography (SEC) with element-specific detection 
using inductively coupled plasma (ICP) spe~t rometry .~"~~ 
This approach provides information on heteroatom (V, Ni, 
Fe, S) distributions as a function of a retention time in a 
chromatographic column that is usually converted to a 
"molecular size". Despite the recent contributions from 
SEC-ICP results, the available information on distribution 
of heteroatoms in petroleum as a function of boiling point 
has been rather limited. 
Sulfur is the third most abundant atomic constituent 
of petroleum, following carbon and hydrogen. A majority 
of crude oils contain less than 1% sulfur. High sulfur 
crudes having more than 1 % sulfur are represented by a 
smaller group.7 
Figure 11 illustrates the distribution of sulfur as a 
function of AEBP. The initial concentration of sulfur in 
(14) Hausler, D.; Taylor, L. Anal. Chem. 1981, 53, 1223-1227. 
(15) Hausler, D.; Taylor, L. Anal. Chem. 1981,53, 1227-1231. 
(16) Hausler, D. W. Spectrochimica Acta, Part B 1985,40B, 389-396. 
(17) Fish, R. H.; Komlenic, J. J. Anal. Chem. 1984,56, 510-517. 
(18) Biggs, W. R.; Fetzer, J. C.; Brown, R. J.; Reynolds, J. G. Li9. Fuels 
(19) Biggs, W. R.; Brown, R. J.; Fetzer, J. C., to be submitted for 
. 
Technol. 1985, 3, 397-421. 
publication in Energy Fuels. 
R R R B I R N H E R V Y R R 
0 FRRCTION REBP. OF 
(2: CUT I 662 
+ LL 
m (0 
E 
0 - 50 
v 
I 
25 CUT 2R 796 
CUT 28 689 
CUT 3R 972 
CUT 38 1024 
CUT I R I092 
CUT 18 1 1 4 3 
CUT 5R 1206 
CUT 58 1307 
c SEF-I 1587 
3 
3 
I 75 
- 
SEF-2 - 
U 188 %E-I : 
I 1 . 2 1 . 4 1 . 6 1.9 
H / C R T O M I C R R T I O 
B O S C R N R R 
FRACTION REBP, OF 
25 
u1 
E 0 
CUT 5 I 1 9 7 
SEF- I 
5 7 5 1 f i SEF-2 
U I00 5EF-4 
1 1 . 2 1 . 4 1.6 1 . E 
H / C R T O M I C R R T I O 
- 
2 SEF-3 
! 3 6 5 
the A€& shown in Figure 11 was as follows: 1.21 (KR), 4.17 
(MA), 4.4 (AH), and 5.9 wt % (BO). The concentration 
of sulfur in the low-sulfur KR AR increases only a little 
with increasing boiling point from approximately 1.0 wt 
% in cut 1 to 1.4 wt % in the SEF-3 fraction, while that 
in the high-sulfur AH AR increases almost threefold from 
2.7 wt % in cut 1 to 8.0 w t % in the SEF-3 fraction. The 
sulfur concentrations in MA and BO ARs increase with 
increasing AEBP from 2.6 to 7.2 wt % and 4.5 to 7.1 wt 
% , respectively. The dramatic increase of sulfur concen- 
trations in fractions SEF-2 and SEF-3 from AH and MA 
is of particular interest because it coincides with a similar 
distribution pattern for metals (see further discussion on 
V, Ni, and Fe distribution). The data suggest that sulfur 
and metals exist in the same molecular structures. The 
results in Figure 11 show that a relatively high percentage 
of the total sulfur in those three ARs (AH, MA, BO) is 
represented by the SEF-2 and SEF-3 fractions. 
The nitrogen content in petroleum is usually much lower 
than the sulfur content, and the average value in crude oils 
is 0.094 wt % . 7 Crudes having more than 0.25 wt % ni- 
trogen are considered nitrogen-rich. 
The distribution of nitrogen as a function of AEBP is 
illustrated in Figure 12. The four ARs shown in Figure 
12 have a progressively higher nitrogen content as follows: 
0.25 (AH), 0.47 (MA), 0.83 (OC), and 0.94 wt % (KR). The 
concentration of nitrogen increases with decreasing vola- 
tility and solubility of heavy oil components for all four 
crudes. 
The nitrogen concentration in a low-nitrogen AH AR 
covers a range from 0.03 wt % for cut 1 to 1.0 wt % for 
the SEF-3 fraction, while that in a nitrogen-rich KR AR 
ranges from 0.3 wt % for cut 1 to 2.4 w t % for the SEF-3 
fraction. Of particular interest is the dramatic increase 
of the nitrogen concentration in SEF-2 and SEF-3 fractions8 Energy &Fuels, Vol. 1, No. 1, 1987 Boduszynski 
KERN R I V E R RR 
1 ' 1 C U I I 
Figure 11. Sulfur distribution as a function of the AEBP. 
I R R R B I R h HERVY RR 
\ 
f ~ 5EF-1 
B . 6 I . ? , . E 2 . 4 B . 6 , . 2 , . a ?., I N I T R O G E N , WTX 1 I N I T R O G E N , WT% 
I 
Figure 12. Nitrogen distribution as a function of the AEBP. 
derived from all four residues. I t 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 in- 
creasing boiling point for all crudes. However, the dis- 
tribution 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 petro- 
leum are very scarce, mainly because of a very low oxygen 
content in most crudes and also because of a relative 
Composition of Heavy Petroleums 
R 
Energy &Fuels, Vol. 1, No. 1, 1987 9 
O F F S H O R E C R L I F O R N I R RR 
FllRCTlDN REBP,OF CUM. x 0 
E 'r] CUT I 697 9.3 
25 CUT 2 I 804 16 .2 
C U T 2 8 887 2 1 . 1 
CUT 38 955 2 6 . 1 
CUT 3B 1826 28 .5 
C U T I B 1153 3 4 , s 
CUT 5 I 2 4 3 39.4 
CUT 48 lis0 3 2 . 3 
I 
Figure 13. Oxygen distribution as a function of the AEBP. 
K E R N R I V E R RR 
FRICTION REBP.aF CUM. X V 
CUT ZR $25 - 
, CUT 28 8 6 7 - 
E 
955 
1023 
le9 l 
1158 
L239 
0. I 
1 . 1 
7 . 7 
15 .5 
I I . 9 
K E R N R I V E R RR 
OXYGEN, WT% 
CUT 2R 825 I S . 3 
CUT 28 887 S 7 . 5 
CUT 3R 855 31.6 
C U T 3 8 1023 4 1 . 3 
CUT 4R 109) 4 8 . 1 
CUT 5 I 2 3 9 6 0 . 6 
5EF-1 1365 8 3 . 8 
5EF-2 - 93 .2 
CUT 18 1158 53,s 
I&:? : SS.8 
756 - 
888 - 
$72 .I 
Ins, . B 
1092 3 . 3 
11.3 5 . 4 
1206 s .5 
I307 11.6 
l 3 B 7 1 1 . 7 
67 .6 
z s 7 . 5 
1 CUT 2 816 . 0 3 
1 CUT 3 957 . I 
I. I , 
Lo 1 . 
Figure 14. Vanadium distribution as a function of the AEBP. 
difficulty in obtaining reliable results. I t is known, how- 
ever, 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 ox- 
ygen 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 be- 
cause 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 mono- 
nitrogen compound or that about every fourth molecule 
contains four nitrogen atoms (e.g., in the form of a por- 
phyrin). 
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 com- 
pounds having several heteroatoms per molecule. Those 
compounds concentrate particularly in SEF-2 and SEF-3 
fractions. 
Organometallic compounds in petroleum contain pre- 
dominantly 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 1 ppm 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 
200 
600 
1000 
1400 
2000 
av no. of atoms per molecule 
sulfur nitrogen oxygen 
mol w t 3.0 wt % 8.0 w t % 0.3 w t % 2.5 wt % 0.2 wt % 1.5 wt % 
0.19 0.50 0.04 0.36 0.02 0.19 
0.56 1.50 0.13 1.07 0.07 0.56 
0.94 2.50 0.21 1.79 0.12 0.94 
1.31 3.50 0.30 2.50 0.17 1.31 
1.87 5.00 0.43 3.57 0.25 1.87 
R R R B I R N HERVY RR 
FRRCTIOh REBP,OF CUM i( N I 
0 7 
6 6 2 
8 1 ' 
I BOSCRN RR 1 I Y E R L R!LER RR 
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 at approximately 1370 OF (SEF-1). 
tun. i( N I 
0 . 2 
2 . 7 
, , .* 
20 .5 
2 1 . 3 
5 2 . 8 
u2 ,7 
$0. I 
- 
Vanadium represented by the DISTACT-distillable cuts 
(first envelope in Figure 14) accounts for a small per- 
centage of the total vanadium in a given AR, namely 11.3% 
(MA), 11.8% (BO), 11.6% (AH), and 21.9% (KR). Va- 
nadium in this portion of each AR was found to be present 
mostly in the form of metalloporphyrins (identified by the 
Soret band at 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 signif- 
icant 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 fol- 
lows a pattern similar to that for vanadium. However, 
lower concentrations of nickel are involved and the bi- 
modal 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 in- 
creasing 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 total nickel, 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 plotsshow a pattern similar to that 
observed for nickel and vanadium. However, the concen- 
tration of iron in DISTACT-distillable cuts is very low-13 
ppm or less. The iron concentration increases with de- 
creasing 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. 
Energy &Fuels, Vol. 1, No. 1, 1987 11 
not exceed a molecular weight of approximately 2000. 
For the first time, distributions of sulfur, nitrogen, ox- 
ygen, 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 struc- 
tures. 
Acknowledgment. The author expresses his appreci- 
ation 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 OF+) 
BO Boscan crude oil 
DIS- short-path distillation apparatus 
FDMS field desorption mass spectrometry 
FIMS field ionization mass spectrometry 
ICP- inductively 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 
SEC- size-exclusion chromatography-inductively coupled 
SEF sequential elution fractionation 
SFC supercritical fluid chromatography 
VR vacuum residue (1000 OF+) 
VTGA vacuum thermal gravimetric analysis 
TACT 
ICP plasma 
Registry No. V, 7440-62-2; Ni, 7440-02-0; Fe, 7439-89-6; S, 
7704-34-9; N, 7727-37-9. 
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 misconcep- 
tions about the molecular weight of heavy petroleum 
components. The results reveal a unique molecular weight 
distribution pattern as a function of the AEBP. The ex- 
perimental evidence contradicts a common opinion that 
heavy crudes, and petroleum residues in particular, are 
composed mostly of very high molecular weight compo- 
nents. The data show that these materials have a wide 
molecular weight distribution that extends to relatively 
small molecules. Quantitative data are presented to dem- 
onstrate that most of the heavy petroleum components do