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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)

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