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No.1998.168
Molecular Weight Distributions in Heavy Crudes
M.M. Boduszynski and C.E. Rechsteiner, Chevron Research and Technology,
Richmond, California, USA; and R.M. Carlson, Chevron Petroleum Technology,
Richmond, California, USA
Abstract
While the average molecular weight of any crude is a useful
correlation parameter, the true molecular weight distribu-
tion of all of the compounds in petroleum can not be readily
measured. This paper describes progress in the use of Field
Ionization and Field Desorption Mass Spectrometry to
enhance our understanding of the molecular weight distribu-
tion, and hence the composition, of heavy crudes and the
heavy ends of conventional crudes.This technology provides a direct measure of the molecular
weight distribution in a crude or crude fractions. Coupled
with appropriate separation technology, this technology pro-
vides extensive information of the composition of crude and
crude fractions, including deep-cut VGOs and Resids. Appli-
cation of this technology provides clear support for the thesis
that crude oil is a continuous mixture of organic compounds.
Concurrent development of practical measures for the atmo-
spheric equivalent boiling point (AEBP) scale concept pro-
vides linkage between molecular weight distributions and
boiling point distributions consistently across crudes.
Introduction
The average molecular weight of any crude fraction is a useful
correlation parameter. Its value can be obtained in a variety of
ways, for example, vapor phase osmometry (VPO), size
exclusion chromatography, or mass spectrometry. However, it
is only a single number. Thus, it can provide only limited
insight into the complexity that is crude.
Questions about the distribution of molecular weight (and
its surrogate, carbon number) in petroleum abound. A small
set of questions include:
1. What is the true molecular weight distribution for any
crude?
2. How does the average molecular weight change as a func-
tion of boiling point within a crude?
3. What and how wide is the distribution of molecular
weight as a function of boiling point?
4. How does the molecular weight distribution change as a
function of the composition within crude?
5. What is the maximum molecular weight of compounds in
petroleum?
The answers obtained to these questions depends upon the
measurement technology used to study molecular weight and
their distribution in petroleum. Progress on resolving these
issues is the subject of this presentation. In particular, thispaper discusses progress in the application of field ionization
mass spectrometry (FIMS) and allied technologies to build
understanding of heavy oils and heavy fractions of petroleum
In particular, the application of FIMS to address issues con
cerning molecular weight distributions is illustrated.
Approach
The general approach used to gain insight into the composi
tion of heavy crudes and the heavy fractions of crudes is
reported elsewhere.
1,2
The basic characterization strategycould be summarized as divide and conquer.
The initial step involves fractionation of the crude under
study into discreet boiling point ranges, usually by distillation
or molecular distillation as appropriate. For non-distillable
residua fractions, a set of analogous materials are obtained
through a sequential extraction fractionation (SEF) protocol.
Depending upon the information needed and the level of detai
sought, chromatographic separation is applied on a consistent
basis to obtain compound class fractions. These are subse-
quently analyzed by a combination of techniques to provide
consistent compositional information for each compound
class. Field ionization (FIMS) or field desorption mass spec-
trometry, non-fragmenting mass spectrometry techniques
produce a spectrum of the molecular ions in the sample. These
molecular ion distributions reveal information about the car-
bon number distribution of compositional subclasses. These
compositional subclasses vary by their hydrogen deficiency, z
in the formula CnH2n-zX, where X can be nitrogen, oxygen
sulfur, nickel, iron or vanadium.
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Experimental
This work used a VG Analytical (now Micromass) 70SE high-
resolution mass spectrometer modified in the factory for opti-
mum field ionization/field desorption performance. It was
operated in the unit resolution mode. That is, the resolution
was sufficient for baseline separation of the highest mass ions
collected.
The mass spectrometer was constructed with an opensource design where all parts of the source are exposed to high
vacuum. This was done to reduce potential fragmentation by
minimizing contact between the sample and heated parts of
the source. Collisions between the sample molecules and
heated surfaces will raise the internal energy of the sample
molecules increasing fragmentation. Electric current was
passed through the emitting surface during the interscan delay
between successive data collections. This cleaning current
helps maintain the performance and extend the usable lifetime
of the emitting surface.
Other modifications were made to the system locally. They
included redesign of the temperature control circuitry to
improve the temperature stability; a flow-through batch inlet
4
to improve sample delivery for materials boiling below
1,000F; and improved detection subsystems to provide
higher sensitivity and lower background noise.
Insights into Crude Composition
The complexity of crude precludes the possibility that any sin-
gle measurement technology will be sufficient to unravel its
composition. Mass spectrometry, when used in combination
with other targeted separations and measurement techniques,
can provide information on the distribution of compoundcompositions in crude.
Distillation was used to produce a set of 14 contiguous
fractions and the non-distillable residum for a 21API crude.
The residum was further separated into 4 sequential extraction
fractions using the protocol referenced earlier.
1
Table 1 pro-
vides some basic information about these 16 fractions. These
fractions were used throughout this work.
Figure 1 shows an example FIMS spectrum. It is basically a
histogram with axes of amount and molecular weight. Another
way to display the molecular weight distribution data is also
shown in Figure 1, integration of the FIMS spectrum. The lat-
ter will be used to show trends across a crude and to comparewith simulated distillation (SimDis) results.
Figure 2 shows that the average molecular weight and the
molecular weight range increase as one moves to the heavy
ends of petroleum. To illustrate trends across the crude, this
figure uses the yield from the crude as the x-axis. Thus, the
distillable range of this crude (from cut 1 through cut 14)
spans about 72% of the crude. As the mid-boiling point of the
crude fraction increases (increasing cut number), the number
average molecular weight increases. However, even for the
highest boiling distillable fraction, cut 14 nominally boiling
between 1,100 and 1,250F, the average molecular weight
doesnt exceed 700 daltons.
The molecular weight distributions broaden as the boiling
point increases. For cut 14, the molecular weight range is from
about 200 to almost 1,400. Significantly, compounds whose
molecular weights are between 300 and 600 represent an
appreciable portion of the sample.
This figure also includes average molecular weights as
determined by VPO for the SEF fractions (only the first two
are shown on the graph). These data are shown by filled trian-
gles and connected by the dotted line. The first SEF fraction,
SEF-1, completely volatilizes in the mass spectrometer. Its
distribution curve, as measured by FIMS, is also shown in
Figure 2. The FIMS average molecular weight and the major-
ity of the molecular weight distribution is lower than the cor-
responding VPO number average molecular weight. This
illustrates that how one measures molecular weight can affect
the results. Further, as one progresses from SEF1 to the
higher boiling SEF fractions, the samples become increas-
ingly more difficult to volatilize. The inability to access thosemost difficult to analyze molecules of the crude prevents com-
plete characterization of the molecular weight distribution of
the crude.
Figures 35 illustrate the use of combined separations with
FIMS with a heavy VGO from the same 21API crude to help
understand crude composition. Molecular distillation was
used to produce a material with cut points from nominally 850
to 1,000F. Its FIMS spectrum is shown in
Figure 3. While
complex, the spectrum actually provides limited information,
primarily average molecular weight and the molecular weight
range for this sample. This is due to two fundamental issues;
the immense number of isomers possible at any mass in the
observed carbon number range and the differing response fac-
tors for various compound classes whose members overlap in
mass. That is, at mass 422 compositions such as C30H62,
C31H50, C32H38, C28H54S, etc. can occur and will contrib-
ute to the total intensity at mass 422 based on their relative
abundance multiplied by a response factor specific for each
composition.
Figure 4 shows the FIMS spectra for chromatographic frac-
tions of this heavy VGO. This sample separation step reduces
the ambiguity in assignment of compound classes. For exam-
ple, separation to produce the saturates fraction limits the
compositions therein, to only saturate hydrocarbons and a few
classes of sulfur containing species. Similar simplificationoccurs for the other fractions. Additionally, complementary
chromatographic separations could be performed to further
simplify the sample and increase the specificity of the analy-
sis. For example, the aromatics fractions could be separated
based on the degree of unsaturation, primarily in the ring
core.
5
Sulfur containing species could be isolated using proce-
dures such as those reported by NIPER.
6
The goal of such
efforts is to minimize the ambiguity in interpretation of the
mass spectral results.
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Table 2 shows the number and weight average molecular
weights and the mass range for the crude fraction and the
chromatographic fractions. Saturates have the highest average
molecular weight and extend to the highest mass. The pyrollic
fraction on the other hand has the lowest average molecular
weight and the lowest starting mass.
The pyrollic nitrogen-containing fraction is used in Figure
5 to illustrate the fine detail potentially achievable with this
approach. It shows the distribution of a single homologous
series of compounds starting at 195 daltons. This corresponds
to dimethyl carbazole. This identification is corroborated by
several pieces of complementary information. In this fraction
and one that was slightly lower boiling, ultraviolet-visible
spectroscopy indicated the presence of the carbazole ring
core. FIMS measurement of the lower boiling fraction showed
a homologous series of compounds starting at 167 daltons, the
mass of the unsubstituted carbazole, and contained apprecia-
ble amounts of the dimethyl homologue at 195 daltons.
A useful comparison to measure progress with FIMS is to
compare it with simulated distillation (SimDis). This is illus-
trated on Figures 67. Figure 6 shows a comparison betweenFIMS measurements and SimDis for a series of 14 sequential
distillation fractions from the 21API crude. For the low boil-
ing cuts, neither the SimDis measurements nor the FIMS mea-
surement produce smooth integral curves. In the lowest
boiling ranges, discrete compounds can be seen in the raw GC
trace. As one moves to higher boiling distillation fractions, the
diversity of components within each fraction increases and the
contribution of any discrete component is lessened. This will
smooth the SimDis curves. A similar thing happens with
FIMS. However, FIMS requires a much higher boiling point
before the curves appear as smooth as for SimDis.
The equivalent FIMS and SimDis curves do not overlay.
The FIMS data consistently starts at lower mass and has a
somewhat lower average molecular weight. This is not sur-
prising since the SimDis boiling point is calibrated versus nor-
mal paraffins. Hence, conversion to molecular weight was
based on the molecular weight of normal paraffins. Given the
fact that increased hydrogen deficiency and polarity in a com-
pound leads to lower molecular weight for the same boiling
point, one would expect the actual molecular weight distribu-
tion in a crude fraction to trend lower than the normal paraffin
molecular weight corresponding to the same boiling point.
Figure 7 shows one additional insight. It compares the
molecular weight-yield distribution as obtained by SimDis
and FIMS across the distillable range of this crude. It showsthe behavior just noted but the highest molecular weight as
measured by FIMS is actually higher than that measured by
SimDis. This is consistent with the observation that branched
paraffins, which may be present in this crude, have higher
boiling points than normal paraffins of the same molecular
weight.
Conclusion
This presentation illustrates the use of FIMS to aid under-
standing of crude composition. Advances in answering ques-
tions concerning crude molecular weight and molecular
weight distributions have been made. When coupled with dis
tillation and chromatographic separations, FIMS provides
valuable information on the distribution of compound classes
in crude fractions. Complementary information from other
measurement technologies enhances the specificity of such
information. However, a number of tough questions like
What is the true molecular weight distribution for any
crude? still can not be answered.
References
1. K.H. Altgelt and M.M. Boduszynski, Composition and
Analysis of Heavy Petroleum Fractions, Marcel Dekker
Inc., New York, 1994.
2. M.M. Boduszynski, et al, Deep-Cut Assay reveals addi
tional yield of high-value VGO, Oil & Gas Journal, Sept
11, 1995, p. 39.
3. K.H. Altgelt and M.M. Boduszynski, Composition and
Analysis of Heavy Petroleum Fractions, Marcel Dekker
Inc., New York, 1994, pp. 241248.
4. E.J. Gallegos and E.C. Pazzi, A dynamic batch inlet fo
group type analysis by mass spectrometry, presentation a
the 39
th
ASMS conference on Mass Spectrometry and
Allided Topics, Nashville, TN, May 1924, 1991.
5. M.M. Boduszynski, 1988, Composition of Heavy Petro
leums. 2. Molecular Characterization, Energy and Fuels
2, 597.
6. J.B. Green, et al, 1989, Liquid chromatographic separa
tions as a basis for improving asphalt composition-physi
cal property correlations. Fuel Sci. Tech. Int., 7 (9)
13271363.
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Table 1: Crude Fraction Properties from a 21API Crude
Table 2: Molecular Weight Information for 850-1000F VGO from 21API Crude and Its Chemical Class Fractions
Crude
Fraction
Yield
(wt%)
Yield
( wt%)
mid-AEBP
(SimDis, F)
Gravity
(API)
NumberAverage
Molecular
Weight
1 4.1 4.1 222 71.8 97
2 4.1 8.2 282 57.6 1143 4.1 12.3 345 50.8 134
4 4.0 16.3 410 46.2 160
5 4.2 20.5 465 40.4 179
6 4.2 24.5 540 34.4 210
7 4.2 28.8 558 32.3 2228 4.1 33.0 652 29.6 261
9 10.2 43.1 719 23.5 313
10 5.3 48.4 822 19.9 380
11 4.2 52.6 886 18.1 424
12 5.8 58.5 954 16.6 488
13 6.7 65.2 1036 11.3 55514 6.8 72.0 1147 10.2 700
N o n-distillable
Residue
28.0 100.0 1310 -1.5 -
SEF-1 14.0 85.9 1276 - 924
SEF-2 6.2 92.1 >1382 - -
SEF-3 6.8 98.9 >1382 - -
SEF-4 1.1 100.0 >1382 - -
Number Weight Nom ina l
Average A ve ra g e Molecular
Molecular Molecular Weight
Weight Weight Range
850-1000F Distillate Fraction 441.2 456.1 190 to 760
Compositional Class Fractions
Saturates 495.3 506.2 320 to 760
Aromatics 421.8 433.6 210 to 630
Basic Nitrogen 389.2 404.3 200 to 610
Pyrollic Nitrogen 343.1 356.3 190 to 570
Acidic 386 400.6 190 to 630
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Figure 1: Transformation of FIMS Spectrum into an Integrated Curve
0
1
2
3
4
5
6
7
8
9
10
11
150 250 350 450 550 650 750Molecular Weight
Integrated FIMS Spectrum
FIMS Spectrum
0.00
0.010.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
0.14
0.15
Molecular Weight
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Figure 2: Molecular Weight Distributions for Distillable Fractions from 21API Crude
Figure 3: Molecular Weight Distribution for 850-1000F VGO Fraction from 21API Crude
0
500
1000
1500
2000
2500
3000
0 10 20 30 40 50 60 70 80 90 100
Yield from Crude (wt %)
MolecularWeight
1 23 4 5
6 78
910 11
1213
14SEF-1
SEF-2
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
150200250300350400450500550600650700750800
Molecular Weight
wt%from
Crude
Oil
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Figure 4: Molecular Weight Distributions for Compound Class Fractions from 850-1000F VGO of 21API Crude
Molecular Weight
Saturates
Molecular Weight
Aromatics
Pyrrolic Cmpds
Molecular Weight
Basic Cmpds
Molecular Weight
AcidicCmpds
Molecular Weight
0.000
0.020
0.040
0.060
0.080
0.100
0.120
0.000
0.020
0.040
0.060
0.080
0.100
0.120
0.000
0.001
0.001
0.002
0.002
0.003
0.003
0.001
0.003
0.005
0.007
0.009
0.011
0.013
0.015
0.000
0.001
0.001
0.002
0.002
0.003
0.003
40x
8x
40x
1x
1x
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Figure 5: Molecular Weight Distribution for Carbazole Homologous Series in 850-1000F VGO of 21API Crude
Figure 6: Molecular Weight Distributions for Distillable Fractions from 21API Crude
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
Molecular Weight
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50 60 70
Yield from Crude (cum. wt%)
(IBP - 1250 F AEBP)
1 2 34
5 67
8
9
1011
12From Sim Dist
From FIMS
14
13
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Figure 7: Molecular Weight Distribution for Distillable Portion of a 21API Crude
0100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
0 10 20 30 40 50 60 70 80
Yield, cum. wt%
From Sim Dist
From FIMS
(IBP - 1250oF AEBP, Composite)