Influence of Thermal Processing on the Properties of Cold Lake

23

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Influence of Thermal Processing on the Properties of Cold Lake Asphaltenes: The Effect of Distillation KENNETH A. GOULD and MARTIN L. GORBATY Corporate Research Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45, Linden, NJ 07036 A better understanding of the composition and properties of heavy feeds such as Cold Lake and Arabian Heavy oils is central to the development of improved upgrading technology. An important question which must be answered is to what extent these materials are thermally altered during refinery distillation. These heavy oils already contain large percentages of refractory materials such as asphaltenes, and it would be highly undesirable to increase the amount or degrade the quality of these components. We have, therefore, investigated the effect of heat treatment during distillation on the quantity and physical and chemical properties of asphaltenes. Cold Lake crude was chosen for this study since it is known to be a thermally sensitive material. Any changes caused by thermal treatment should, therefore, be more obvious than with a more stable feed. We report here the results of a variety of measurements made on the asphaltenes isolated from Cold Lake crude oil and from its vacuum distillation residue. It should be borne in mind that Cold Lake crude is subjected to the high temperatures of pressurized steam used in the production process and may conceivably have already undergone some thermal alteration. The present study, however, is designed primarily to learn if any further changes might occur during refining. Background The q u e s t i o n o f whether and t o what extent asphaltenes are formed or a l t e r e d during crude o i l h a n d l i n g and p r o c e s s i n g has remained unresolved. In one i n v e s t i g a t i o n , samples of a T a r t a r m i n e r a l o i l d i s t i l l a t i o n r e s i d u e were heated f o r f i v e hours to 163°C and then f o r another f i v e hours to 400°C t o simulate cond i t i o n s during d i s t i l l a t i o n . ( 1 ) Both an i n c r e a s e i n asphaltene content and a decrease i n asphaltene H/C r a t i o were observed. In a d d i t i o n , d i s t i l l a t i o n r e s i d u e s from v a r i o u s other crudes were heated to v a r i o u s temperatures f o r three hours and then pentane deasphaltened. I t was observed that asphaltene H/C r a t i o s decreased r a p i d l y above 6.

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Stauffer; Oil Shale, Tar Sands, and Related Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


348

OIL SHALE, TAR SANDS, AND RELATED

MATERIALS

These i n v e s t i g a t o r s a l s o heated maltenes i n sealed v i a l s to v a r i o u s temperatures. The asphaltene y i e l d s obtained a t 350°C, 400°C, and 450°C were 18, 32, and 36%, r e s p e c t i v e l y . Although the conversions were approximately f i r s t order a t the lower temperatures, they changed s i g n i f i c a n t l y a t 450°C, the r e g i o n of t e c h n i c a l i n t e r e s t f o r many r e f i n i n g o p e r a t i o n s . Significant formation o f new asphaltenes was seen t o occur. Deasphaltened maltenes were a l s o separated by alumina chromatography i n t o a non-aromatic " g a s o l i n e " e l u a t e , a s t r o n g l y aromatic benzene e l u a t e , and a resinous benzene-methanol e l u a t e . Pentane i n s o l ubles were obtained from a l l three f r a c t i o n s upon h e a t i n g a t r e l a t i v e l y low temperatures, although the r a t e s were q u i t e d i f f e r e n t . Resins gave the h i g h e s t y i e l d s a t the f a s t e s t r a t e s while the aromatic o i l s showed about the same y i e l d , but a t a much slower r a t e . The y i e l d and r a t e were lowest f o r the nonaromatic o i l s , and t h e i r pentane i n s o l u b l e s were mostly toluene i n s o l u b l e s and p y r i d i n e i n s o l u b l e s r a t h e r than asphaltenes. The r e p o r t a l s o claimed that asphaltenes were formed even a t 20°C i n the absence of a i r a t r e l a t i v e l y slow r a t e s . A study of the e f f e c t o f heat on asphaltene decomposition a t 350-380°C i n a helium flow system (2) r e s u l t e d i n the f o l l o w i n g observations: 1. decomposition was found t o be f i r s t order i n asphaltenes 2. the percent coke make expressed as a percent of asphaltenes decomposed d i d not vary w i t h the extent of c r a c k i n g , implying that the mechanism i s independent of the percent c r a c k i n g . 3. a "20,000-fold i n c r e a s e i n s u r f a c e area of the asphaltenes v i a i n t r o d u c t i o n of carbon b l a c k (manner not s p e c i f i e d ) d i d not change the r e a c t i o n r a t e 4. toluene i n s o l u b l e s were formed i n amounts that decreased with i n c r e a s i n g r e a c t i o n time, implying t h a t these products are intermediates i n p y r i d i n e i n s o l u b l e formation. These observations l e d to the p r o p o s a l o f a f r e e r a d i c a l , c h a i n r e a c t i o n mechanism. Aspects of the mechanism i n c l u d e : (1) formation o f s m a l l r a d i c a l fragments which could a b s t r a c t hydrogen and leave as l i g h t products, (2) r e a c t i o n of s t a b i l i z e d f r e e r a d i c a l s (formed by hydrogen a b s t r a c t i o n ) which could i n t e r a c t w i t h asphaltenes to form l a r g e r and l a r g e r condensation products, and (3) formation of toluene i n s o l u b l e s , i . e . l i n e a r condensation products, and p y r i d i n e i n s o l u b l e s , i . e . c r o s s - l i n k e d products. These c h a i n r e a c t i o n s could be terminated by formation of very s t a b l e r a d i c a l s that could not r e a c t f u r t h e r . ( 2 ) T h i s mechanism i s i n accord with the c o n c l u s i o n s of Speight, who has s t a t e d that formation of p a r a f f i n s d u r i n g p y r o l y s i s of Athabasca asphaltenes probably occurs v i a i n t e r a c t i o n of a l k y l r a d i c a l s with hydrogen produced during a r o m a t i z a t i o n and condensation o f p o l y c y c l i c s t r u c t u r e s . ( 3 ) Carbon-carbon bond breaking i n these asphaltenes was found to occur p r i m a r i l y $ to aromatic r i n g s . 11


23.

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Thermal

Processing

349

In other work (4), x-ray a n a l y s i s l e d to the c o n c l u s i o n that not only d i d asphaltene m e l t i n g p o i n t i n c r e a s e w i t h i n c r e a s i n g f r a c t i o n a l a r o m a t i c i t y , f , but thermal s e n s i t i v i t y i n c r e a s e d i n the same d i r e c t i o n . Thus, asphaltenes with f 0.32 were more s e n s i t i v e and were transformed to a l a r g e extent to toluene i n s o l u b l e s a f t e r one hour at 375°C. When f was 0.17 and 0.24, only 18-32% conversion to toluene i n s o l u b l e s was observed. a

a

a


Experimental P r e p a r a t i o n of Asphaltenes. Asphaltenes were obtained by n-heptane p r e c i p i t a t i o n from e i t h e r Cold Lake crude or vacuum residuum using t y p i c a l deasphaltening procedures. ( i . e . One p a r t of residuum was r e f l u x e d f o r one hour with 10 p a r t s of heptane. The mixture was then f i l t e r e d and the i n s o l u b l e asphaltenes washed s e v e r a l times with heptane and pentane and d r i e d i n vacuo at 80°C.) P y r o l y s i s of Asphaltenes. Pyrolyses were performed u s i n g the apparatus shown i n Figure 1.(5) The a p p r o p r i a t e m a t e r i a l was placed i n a quartz tube w i t h 24/40 ground j o i n t s and a dry i c e condenser was attached. A f t e r a l t e r n a t e l y evacuating and f l u s h i n g with n i t r o g e n s e v e r a l times, the m a t e r i a l was pyrolyzed at the a p p r o p r i a t e temperature f o r 10 min. Char and l i q u i d y i e l d s were c a l c u l a t e d from the weights of the p y r o l y s i s tubes and condensers before and a f t e r r e a c t i o n . A n a l y t i c a l Data. Instrumental analyses and s p e c t r a were made on the f o l l o w i n g equipment: i n f r a r e d spectroscopy, D i g i l a b FTS14 F o u r i e r transform spectrophotometer; vapor pressure osmometry, H i t a c h i - P e r k i n Elmer 115; g e l permeation chromatography, Waters Assoc. 200; nuclear magnetic resonance spectrometry, V a r i a n Assoc. A60 and XL100; thermogravimetric a n a l y s i s , modified Stanton thermobalance; d i f f e r e n t i a l scanning c a l o r i m e t r y , P e r k i n Elmer DSC 2; and e l e c t r o n s p i n resonance spectrometry, V a r i a n Assoc. Century spectrometer with E102 X band microwave b r i d g e o p e r a t i n g at 9.5 GHz. Results and D i s c u s s i o n The f i n d i n g s discussed above (1-4) i n d i c a t e that changes i n asphaltene q u a l i t y and q u a n t i t y during thermal treatment depend s t r o n g l y on both the o r i g i n of the o i l and the s e v e r i t y of the treatment. This means that s p e c i f i c questions concerning s t a b i l i t y can only be answered v i a s t u d i e s on the p a r t i c u l a r o i l at the p a r t i c u l a r c o n d i t i o n s of i n t e r e s t .




350

Figure 1.

Rapid heat-up pyrolysis

unit


MATERIALS


23.

GOULD A N D GORBATY

Thermal

351

Processing

To provide raw m a t e r i a l f o r t h i s comparative study of untreated and h e a t - t r e a t e d o i l s , asphaltenes from Cold Lake crude (crude asphaltenes) and from Cold Lake vacuum residuum (residuum asphaltenes) were prepared by n-heptane p r e c i p i t a t i o n as d e s c r i b e d i n the Experimental s e c t i o n . The Cold Lake residuum f r a c t i o n was prepared by Imperial O i l E n t e r p r i s e s , L t d . a t S a r n i a , Ontario, Canada. The d i s t i l l a t i o n h i s t o r y of t h i s bottoms f r a c t i o n i n d i c a t e s that the pot m a t e r i a l was subjected to temperatures as h i g h as 314-318°C d u r i n g atmospheric and vacuum distillation. The length of time at 300°C or higher was about two hours. T h i s i s w e l l i n excess of what would be experienced i n a p i p e s t i l l and should have provided ample time f o r any decomposition. I t should be noted, however, that s i n c e i t was poss i b l e to maintain the system vacuum a t 0.35 mm, the maximum temperature experienced by the residuum was not q u i t e as h i g h as i t might be during r e f i n e r y d i s t i l l a t i o n (e.g. ca 350°C). Table I shows the y i e l d s of asphaltenes obtained from s e v e r a l deasphaltening operations on crude o i l and bottoms. The y i e l d s on bottoms were normalized to y i e l d s on crude by c o r r e c t i n g f o r the q u a n t i t y of d i s t i l l a t e s i n the crude. Table I Asphaltene Y i e l d s from Cold Lake Crude and Residuum Source Residuum Residuum Crude Crude Crude

% Asphaltenes

(On Crude)

10.6 10.9 9.9 9.8 10.8

The average percentage of asphaltenes i n the bottoms i s 10.8% (based on crude) and i s thus s l i g h t l y higher than the average value of 10.2% f o r the crude o i l . The 0.6% d i f f e r e n c e i s , however, w i t h i n the observed experimental v a r i a t i o n of 1.0% and i s t h e r e f o r e not considered s i g n i f i c a n t . The average elemental compositions f o r s e v e r a l p r e p a r a t i o n s of crude and residuum asphaltenes are shown i n Table I I . As can be seen, the two asphaltenes are q u i t e s i m i l a r with the d i f ferences between them being l e s s than the t y p i c a l e r r o r s from a n a l y s i s to a n a l y s i s . The H/C r a t i o s are almost i d e n t i c a l .


352


MATERIALS

Table I I


Average Elemental Analyses f o r Crude and Residuum Asphaltenes Asphaltene Source

% C

Residuum Crude

81.81 82.14

% H 7.75 7.65

% N 1.42 1.28

% S 8.01 7.78

Ni (ppm)

V (ppm)

H/C

329 345

893 935

1.14 1.12

Both the number average molecular weights as determined by vapor pressure osmometry and e x t r a p o l a t e d to zero c o n c e n t r a t i o n and the g e l permeation chromatographic molecular weight d i s t r i b u t i o n s i n d i c a t e that the crude and residuum asphaltenes do d i f f e r i n molecular weight. The VPO r e s u l t s are summarized i n Table I I I and comparative GPC t r a c e s are shown i n F i g u r e 2. As can be seen from these data, both techniques i n d i c a t e that the crude asphaltenes have a s i g n i f i c a n t l y higher molecular weight than the residuum asphaltenes. T h i s r e s u l t i s somewhat s u r p r i s i n g s i n c e one would not a. p r i o r i expect thermal c r a c k i n g a t such low temperatures, ^320°C, even w i t h a thermally s e n s i t i v e crude such as Cold Lake. T h i s e x p l a n a t i o n , however, cannot be r u l e d out. Another p o s s i b i l i t y which could account f o r lower molecular weights i n the residuum asphaltenes, s i d e c h a i n d e a l k y l a t i o n , can be e l i m i n a t e d on the b a s i s of n u c l e a r magnetic resonance r e s u l t s (vide i n f r a ) . Another p o s s i b l e cause of the molecular weight r e d u c t i o n i s thermally induced d i s s o c i a t i o n of II- II complexes which may help to hold the asphaltene macros t r u c t u r e together. Deasphaltening done at higher s o l v e n t - t o o i l r a t i o s , i . e . from 20:1 to 40:1, showed s i m i l a r molecular weight d i f f e r e n c e s between crude and residuum asphaltenes, implying that the r a t i o used here, 10:1, d i d not cause the observed d i f f e r e n c e s . ( 6 ) Table I I I Number Average Molecular Weights (M^) f o r Crude and Residuum Asphaltenes Asphaltene Source Residuum

Crude

Average 5120 4400 5850 5850 8250 6120 6850 6600

5305

6955



23.

GOULD A N D GORBATY

Thermal

353

Processing

The two asphaltenes were a l s o examined by i n f r a r e d , n u c l e a r magnetic resonance, and e l e c t r o n s p i n resonance techniques. Figures 3, 4, and 5 show the r e s u l t s of the IR analysis. I t i s immediately apparent that the two asphaltene s p e c t r a (Figures 3 and 4) are q u i t e s i m i l a r , showing no obvious q u a l i t a t i v e d i f f e r e n c e s . To l e a r n i f more s u b t l e d i f f e r e n c e s e x i s t e d , a d i f f e r e n c e spectrum (Figure 5) was generated by computer u s i n g the data accumulated f o r Figures 3 and 4. T h i s demonstrates that v i r t u a l l y complete c a n c e l l a t i o n can be obtained. The only r e s i d u a l a b s o r p t i o n of any s i g n i f i c a n c e i n t h i s h i g h l y magnified spectrum i s the s m a l l peak at 2950 cm~l. T h i s may r e s u l t from t r a c e s of r e s i d u a l s o l v e n t or i t may represent a very minor d i f f e r e n c e between the two asphaltenes. In the case of the magnetic resonance c h a r a c t e r i z a t i o n , both ! 3 c NMR and proton NMR were employed to o b t a i n the percentages of aromatic carbon and hydrogen. The r e s u l t s are shown i n Table IV. Although the measured l e v e l s o f aromatic hydrogen are w i t h i n experimental u n c e r t a i n t y of each other, the d i f f e r e n c e i n aromatic carbon i s probably s i g n i f i c a n t . Nevertheless, t h i s d i f f e r e n c e i s s m a l l and i n d i c a t e s that the aromatic carbon contents are q u i t e s i m i l a r . In a d d i t i o n , attempts to d i s c e r n q u a l i t a t i v e d i f f e r e n c e s i n the 13C NMR were i n v a i n . These r e s u l t s imply that very l i t t l e , i f any, d e a l k y l a t i o n or aromatizat i o n has occurred d u r i n g the crude d i s t i l l a t i o n procedure. Table IV Aromatic Carbon and Hydrogen Contents of Cold Lake Asphaltenes Asphaltene Source Crude Residuum

%__C^ 52.0 + 1 50.4 + 1

% H^ 13.7 14.2

+ +

0.5 0.5

Petroleum asphaltenes e x h i b i t two general types of s i g n a l s when examined by e l e c t r o n s p i n resonance techniques. One i s the 1 6 - l i n e , a n i s o t r o p i c , vanadyl (V=CH"2) resonance of the s o l i d s t a t e while the other a r i s e s from unpaired e l e c t r o n s which are present i n the form of r e l a t i v e l y s t a b l e f r e e r a d i c a l s . The crude and residuum asphaltenes were examined by ESR, and the r e l e v a n t data are summarized i n Table V.


354


700

1

? 600 •o

c


|

500

l

Resid Asphaltenes

-

MATERIALS

Crude Asphaltenes

y 100

1000

10000

100000

Molecular weight (Polystyrene) Figure

2.

Molecular-weight

distributions of Cold asphaltenes

Lake

crude

and

residuum

W A V E L E N G T H (jim)

r

i

i

i

4000

3500

3000

2500

u

i

1

2000 1800

WAVENUMBER

Figure 3.

i

i

1

i

1400 1200 1000 800 600 (CM ) - 1

IR spectrum of Cold Lake crude

asphaltenes


23.

GOULD

Thermal

A N D GORBATY

355

Processing

W A V E L E N G T H (jim)


2.5

4000

3

3500

3.5

3000

4 4.5 5

2500

5.5

2000 1800

WAVENUMBER

Figure 4.

7

8

9 10

1400 1200 1000

12

15

800 600

(CM" ) 1

1R spectrum of Cold Lake residuum

asphaltenes

W A V E L E N G T H (\im)

4000

3500

3000

2500

2000 1800

WAVENUMBER

Figure 5.

Differential

1400 1200 1000 800

600

(CM ) - 1

IR spectrum of crude and residuum

asphaltenes



356

MATERIALS

Table V ESR Parameters f o r Cold Lake Asphaltenes Parameter

Crude Asphaltenes

Residuum Asphaltenes


Vanadyl: Ml

(G)

174.0

174.4

A,

(G)

56.3

56.7

g|

1.9632

1.9629

1.9837

1.9813

2.00308

2.00307

6.4

6.6

Free R a d i c a l : g linewidth

(G)

I t i s apparent from the chemical s h i f t s ( g - v a l u e s ) , the h y p e r f i n e c o u p l i n g constants (A-values), and the l i n e w i d t h s t h a t the f r e e r a d i c a l s and vanadyl species are i n very s i m i l a r e n v i r o n ments i n both samples. I t was not p o s s i b l e to o b t a i n meaningful values f o r the absolute numbers of spins per gram f o r e i t h e r s p e c i e s , but estimates of the r e l a t i v e concentrations obtained by measuring peak h e i g h t s i n d i c a t e that the vanadyl and f r e e - r a d i c a l concentrations do not d i f f e r s i g n i f i c a n t l y between the two asphaltenes. I t thus appears that heat treatment of Cold Lake asphaltenes to 320°C does not a l t e r the nature or abundance of paramagnetic c e n t e r s . Since most of the p h y s i c a l p r o p e r t i e s of the asphaltenes d i d not show any major d i f f e r e n c e s , thermal r e a c t i v i t y was i n v e s t i gated to d i s c e r n any d i f f e r e n c e s which might e x i s t i n chemical r e a c t i v i t y . D i f f e r e n t i a l scanning c a l o r i m e t r y and thermogravimetric a n a l y s i s as w e l l as r a p i d p y r o l y s i s were employed. The only n o t a b l e f e a t u r e s of the DSC analyses were what appeared to be g l a s s t r a n s i t i o n s o c c u r r i n g at 175°C and 172°C f o r the crude and residuum asphaltenes, r e s p e c t i v e l y . The TGA curves f o r the two m a t e r i a l s were a l s o v i r t u a l l y i d e n t i c a l , d i f f e r i n g by l e s s than one percent v o l a t i l e matter at any temperature. Both of these techniques thus i n d i c a t e e s s e n t i a l l y no d i s c e r n a b l e d i f f e r ences i n the two asphaltenes. S i m i l a r l y , when the p y r o l y s i s behavior was s t u d i e d i n a r a p i d h e a t i n g u n i t w i t h a heatup time of one to two minutes, v i r t u a l l y i d e n t i c a l r e s i d u e y i e l d s were obtained. Summary and

Conclusions

The c h a r a c t e r i s t i c s of Cold Lake crude and residuum a s p h a l t enes have been compared by a number of i n s t r u m e n t a l and p h y s i c a l techniques. The asphaltenes were e s s e n t i a l l y i d e n t i c a l i n q u a l i t y



23.

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357

and q u a n t i t y except that the crude asphaltenes e x h i b i t e d higher average molecular weights as w e l l as molecular weight d i s t r i b u t i o n s peaking a t higher molecular weights than d i d the residuum asphaltenes. The thermal h i s t o r y of these p a r t i c u l a r residuum asphaltenes i s much more severe i n terms of h e a t i n g time than would o r d i n a r i l y be the case f o r a r e f i n e r y product from a p i p e s t i l l s i n c e , i n the present i n s t a n c e , a pot d i s t i l l a t i o n was used. I t t h e r e f o r e seems l i k e l y that r e f i n e r y asphaltenes should be even l e s s d i f f e r e n t from t h e i r r e s p e c t i v e crude asphaltenes than i n t h i s i n v e s t i g a t i o n , assuming that p i p e s t i l l temperatures would be kept below the decomposition temperatures f o r the asphaltenes, i . e . l e s s than about 350°C. Furthermore, any d i f f e r e n c e s should be f u r t h e r diminished i n the event that a crude which i s l e s s thermally s e n s i t i v e than Cold Lake i s i n v o l v e d . Since the Cold Lake crude used i n t h i s i n v e s t i g a t i o n has been exposed to the temperature of the p r e s s u i z e d steam used i n the o i l p r o d u c t i o n , one cannot be c e r t a i n that some thermal changes had not a l r e a d y occurred i n the crude o i l . To study t h i s p o s s i b i l i t y the p r o p e r t i e s of c o l d b a i l e d ( i . e . recovered without steam i n j e c t i o n ) Cold Lake crude asphaltenes are being i n v e s t i g a t e d by many of these same techniques and w i l l be described i n a f u t u r e r e p o r t . Acknowled gments We would l i k e to thank R. B. Long f o r h i s a s s i s t a n c e i n the p r e p a r a t i o n of t h i s manuscript, R. R i f f o r help with the experimental work, and the f o l l o w i n g i n d i v i d u a l s f o r t h e i r a s s i s t ance i n the v a r i o u s a n a l y t i c a l measurements: L. Ebert, J . E l l i o t t , B. Hager, B. Hudson, M. M e l c h i o r , E. P r e s t r i d g e , W. Schulz, and B. S i l b e r n a g e l .

Literature Cited 1. Hrapia, H. Meyer, D., and Prause, M., Chem. Tech., 1964, 16, (12), 733. 2. Magaril, R. Z., and Aksenova, E. J., Khim. Technol. Top. Masel, 1970, 15, (7), 22. 3. Speight, J. G., Am. Chem. Soc., Div. Fuel Chem. Prepr., 1971, 15, (1), 57. 4. Bestougeff, M. A., and Genderel, P., Am. Chem. Soc., Div. Petrol. Chem. Prepr., 1964, 9, (2), B51. 5. Design supplied by R. J. Lang, Exxon Res. and Eng. Co., Baytown, Texas. 6. R. C. Schucker, private communication from these laboratories. RECEIVED February 18, 1981. Stauffer; Oil Shale, Tar Sands, and Related Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Influence of Thermal Processing on the Properties of Cold Lake

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