The Reactivity of Cold Lake Asphaltenes - ACS Symposium Series

22 The Reactivity of Cold Lake Asphaltenes 1

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch022

R. C.SCHUCKER and C. F. KEWESHAN Corporate Research Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45, Linden, NJ 07036

As known reserves of light crudes become depleted, the conversion of heavy crudes and residua to distillate fuels is becoming increasingly important. While reserves of Canadian and Venezuelan bitumen and Arabian heavy oils represent vast, largely untapped resources, their usefulness to a large extent depends on our ability to chemically convert macromolecules such as asphaltenes and polar aromatics to smaller molecules boiling typically in the mid-distillate/naphtha range. To optimize the utilization of these feedstocks, we need a much better understanding of the structure and reactivity of petroleum macromolecules, particularly asphaltenes. While there has been a significant amount of research done to date to elucidate structural characteristics of asphaltenes, there appears to be no consensus of opinion even on major issues such as the average size of asphaltene aromatic units. For example, proposed structures for Athabasca pentane asphaltenes vary from the twelve-ring naphtho-ovalene structure of Speight (1) to the two-ring sulfur polymer structure of Ignasiak et al (2). With this kind of disagreement regarding crude asphaltene structures, it is easy to see why little progress has been made in the area of asphaltene reactivity. Recently i t has been reported by Bearden and A l d r i d g e (3) that c e r t a i n molybdenum c a t a l y s t s can s u b s t a n t i a l l y reduce coke formation i n the hydroconversion o f asphaltene - c o n t a i n i n g feeds under thermal c r a c k i n g c o n d i t i o n s . We have now a p p l i e d t h i s method to o b t a i n asphaltene fragments i n high y i e l d s f o r charact e r i z a t i o n of the s t r u c t u r e of asphaltenes from Cold Lake crude. The goal of our work has been to d e f i n e the major b u i l d i n g blocks i n Cold Lake asphaltenes i n order to begin to b r i n g together the concepts of s t r u c t u r e and r e a c t i v i t y . We have approached the problem by c a r r y i n g out m i l d , thermal hydroconversion r e a c t i o n s on neat asphaltenes and c h a r a c t e r i z i n g both reactant and r e a c t i o n 1

C u r r e n t address: Exxon Research and Development L a b o r a t o r i e s , P. 0. Box 2226, Baton Rouge, L o u i s i a n a 70821. 0097-6156/81/0163-0327$05.00/0 © 1981 American Chemical Society

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

OIL SHALE, TAR SANDS, AND RELATED

328

MATERIALS

products i n d e t a i l . While previous research has concentrated on crude asphaltenes, we have focused on reacted asphaltenes. By combining s t r u c t u r a l and k i n e t i c i n f o r m a t i o n obtained i n t h i s study, we have been able to p o s t u l a t e a g l o b a l asphaltene react i o n mechanism which i s c o n s i s t e n t with a l l of our observations.


Experimental Cold Lake asphaltenes f o r t h i s study were prepared by prec i p i t a t i o n with n-heptane using a s o l v e n t to crude r a t i o of 20:1. The p r e c i p i t a t e d asphaltenes were separated from the maltenes (n-heptane s o l u b l e s ) by f i l t r a t i o n , then washed with an equal volume of n-heptane and d r i e d at 100°C i n vacuo. T o t a l y i e l d of asphaltenes on crude was 12.6% (by weight). These asphaltenes contained approximately 0.8% (by weight) toluene insoluble, i n o r g a n i c matter which was c o r r e c t e d f o r i n subsequent y i e l d calculations. Elemental composition was measured by r o u t i n e a n a l y t i c a l techniques. Oxygen was measured d i r e c t l y by neutron a c t i v a t i o n a n a l y s i s and not obtained by d i f f e r e n c e . Number average molecular weights were obtained by vapor pressure osmometry (VPO) i n toluene at 50°C. N i c k e l and vanadium concentrations were measured by atomic absorption spectrophotometry. Mole f r a c t i o n s of aromatic carbon and hydrogen were determined d i r e c t l y by pulsed F o u r i e r transform nmr techniques. A summary of a n a l y t i c a l data f o r the reactant asphaltenes i s given i n Table I. A l l r e a c t i o n s were c a r r i e d out as batch experiments i n tubing bomb r e a c t o r s (30cc) using e i t h e r Cold Lake asphaltenes as p r e c i p i t a t e d or Cold Lake asphaltenes impregnated with a s o l u b l e molybdenum c a t a l y s t which had been p r e v i o u s l y shown to reduce coke formation i n the hydroconversion of heavy hydrocarbons (3). In a t y p i c a l experiment the r e a c t o r was charged with f i v e grams of asphaltenes and p r e s s u r i z e d to 6 MPa with hydrogen. I t was then plunged i n t o a preheated, f l u i d i z e d sandbath, held f o r the d e s i r e d r e a c t i o n time while a g i t a t i n g , removed from the bath and quenched i n c o l d water. The temperature of the r e a c t i o n mixture was monitored at a l l times using a thermocouple l o c a t e d i n the bomb. T y p i c a l heatup time from ambient temperatures to 95% of r e a c t i o n temperature was three minutes. At the end of a run gases were vented through an H S scrubber i n t o a gas bag. Hydrocarbon gases ( C - C ) were analyzed by gas chromatography using flame i o n i z a t i o n d e t e c t i o n and 1,1-difluoroethane as an i n t e r n a l standard. Toluene was used to remove the l i q u i d and s o l i d products from the bomb and the toluene s o l u t i o n was filtered to determine the coke (toluene i n s o l u b l e s ) y i e l d . A f t e r removing toluene from the f i l t r a t e by vacuum, n-heptane was added to separate asphaltenes (toluene s o l u b l e , n-heptane i n s o l u b l e ) from maltenes (n-heptane s o l u b l e ) . Coke and asphaltene f r a c t i o n s were d r i e d overnight at 100°C i n vacuo. An o v e r a l l m a t e r i a l balance was obtained by summing the coke, asphaltene, maltene and gas f r a c t i o n s and f o r lower s e v e r i t y runs averaged between 97-101%. 2

1

4



22.

SCHUCKER

Cold

A N D KEWESHAN

Lake

T A B L E

ANALYSIS

O FCOLD

Asphaltenes

I

L A K E CRUDE

ASPHALTENES

C (WT.%)

80.64

H (WT.%)

7.64

0(WT.%)

1.84

N (WT.%)

1.60

S (WT.%)

7.95

Ni (PPM)

310

V

815

(PPM)

Mn (VPO, T O L U E N E , 50°C)

C H

A

A

(MOLE

%)

47.3

( M O L E % )

11.4

( H / C ) (H/C) (H/C) T

M

6640 ± 120

T

A

s

(°C)

0

T

A

L

1.14 0.274 1.91 209


329


330

MATERIALS

F u r t h e r s e p a r a t i o n of maltenes i n t o r e s i n s (polar aromatics) and o i l s was achieved i n s e l e c t e d cases by adsorption onto Attapulgus c l a y using a m o d i f i c a t i o n of ASTM D2007 ( c l a y - g e l s e p a r a t i o n ) .


Results And D i s c u s s i o n The thermal hydroconversion of Cold Lake asphaltenes was studied i n i t i a l l y to provide a b a s i s f o r e v a l u a t i o n of c a t a l y t i c e f f e c t i v e n e s s i n subsequent work. S e r i e s of thermal runs were made at 335°C, 365°C and 400°C and the r e a c t i o n products were separated as described p r e v i o u s l y . Several k i n e t i c models were t r i e d , but a f t e r examining the v a r i a b i l i t y of our data, we decided on the simple f i r s t - o r d e r asphaltene decomposition model shown below: A where A

->

aA* + bM

(1)

=

weight f r a c t i o n reactant asphaltenes

A*

=

weight f r a c t i o n reacted asphaltenes

M

=

weight f r a c t i o n maltenes

a,b = s t o i c h i o m e t r i c c o e f f i c i e n t s

(based on weight)

Rate expressions f o r t o t a l asphaltenes grated to y i e l d Equations (2) and (3). A

= a + (l-a)e"

t

M

= b(l-e"

k t

and

maltenes

were

k t

)

inte-

(2) (3)

Equations (2) and (3) were f i t to experimental data using nonl i n e a r r e g r e s s i o n to o b t a i n values of the f i r s t - o r d e r r e a c t i o n rate constants and the s t o i c h i o m e t r i c c o e f f i c i e n t s at each temperature. The conversion data from the 400°C thermal run and the best f i t of the k i n e t i c model are shown i n F i g u r e 1. It is intere s t i n g to note that at the time of i n c i p i e n t coke formation (^60 minutes) the asphaltene and maltene data deviate from p r e d i c t e d f i r s t - o r d e r behavior. From t h i s we concluded that both asphaltenes and maltenes were p a r t i c i p a t i n g i n secondary coke-forming r e a c t i o n s . F u r t h e r s e p a r a t i o n of the maltenes i n t o r e s i n s (polar aromatics) and o i l s confirmed t h i s to be true and showed t h a t i t was the r e s i n f r a c t i o n t h a t was i n v o l v e d i n coke formation. The same experiments were run using asphaltenes impregnated with molybdenum. The conversions and best f i t of the k i n e t i c model at 400°C are shown i n F i g u r e 2. Previous work (4) had suggested that the molybdenum would s u l f i d e and thus be able to a c t i v a t e hydrogen r e s u l t i n g i n improved hydrogen t r a n s f e r to radical fragmentation products. Increased hydrogen transfer



Figure

1.

2

Thermal hydroconversion of Cold Lake asphaltenes at 400 °C and 6 MPa H : tenes (A); maltenes (%); coke (H); model ( ).


asphal-


Figure

2. t

Hydroconversion of Cold Lake crude asphaltenes with 200 ppm molybdenum and 6 MPa H : asphaltenes (J^); maltenes (• ); coke (J^); model ( ).


at 400°C

>

£ H

a

m

H

>

m r

D

>

70

>

X > r

O r

to

22.

SCHUCKER

AND KEWESHAN

Cold

Lake

Asphaltenes

333

above the thermal base case would e x p l a i n the s t a b i l i z a t i o n of r e s i n fragments shown i n F i g u r e 2 and the lower o l e f i n / p a r a f f i n r a t i o i n the gas i l l u s t r a t e d i n Table II f o r the C gases. The temperature dependence of the r e a c t i o n rate constants obtained f o r the thermal and c a t a l y t i c runs was assumed to f o l l o w the Arrhenius r e l a t i o n s h i p and the r e s u l t i n g p l o t of these data i s shown as Figure 3. As we can see, the c a t a l y s t had no r e a l e f f e c t on the a c t i v a t i o n energy. I t d i d , however, increase the rate of r e a c t i o n at a l l temperatures. I n t e r p r e t a t i o n of these data, though, i s at best somewhat s u b j e c t i v e . In complex react i o n systems l i k e these, the measured rate i s g e n e r a l l y considered to be that of the slowest or r a t e - l i m i t i n g step i n the r e a c t i o n sequence. The low values of the a c t i v a t i o n energies obtained s t r o n g l y suggest that primary bond breaking i s not the r a t e - l i m i t i n g step and that some other step such as hydrogen t r a n s f e r might be. This i s supported by the f a c t that the observed rate increased under improved hydrogen t r a n s f e r conditions . While the y i e l d of asphaltenes and other products during the e a r l y stages of r e a c t i o n are s i m i l a r (as shown i n Figures 1 and 2), the thermal asphaltenes e x h i b i t e d lower H/C r a t i o s and higher number average molecular weight (M ) and r e s u l t e d i n s u b s t a n t i a l coke formation. The unique behavior of the asphaltenes i n the presence of molybdenum on the other hand provided us with an e x c e l l e n t opportunity to look c l o s e r at the s t r u c t u r e of the reacted asphaltenes. Since these r e a c t i o n s were c a r r i e d out neat, maltenes could be separated and analyzed d i r e c t l y . There were no maltenes i n i t i a l l y so these molecules must at one time have been attached to the reactant asphaltene molecules. Furthermore, the reacted asphaltenes could a l s o be analyzed to determine what chemical changes were t a k i n g place during reaction. Elemental a n a l y s i s showed some i n t e r e s t i n g r e s u l t s with regard to H/C, s u l f u r and n i t r o g e n l e v e l s . Figure 4 shows a p l o t of the (H/C) values i n the reacted asphaltenes and the product maltenes. As can be seen, the (H/C) r a t i o i n the reacted asphaltenes drops continuously while that of the product maltenes r i s e s continuously. The weighted average of the measured asphaltene and maltene f r a c t i o n s r i s e s s l i g h t l y i n d i c a t i n g the a d d i t i o n of some hydrogen to the system. This i s the kind of behavior that might be expected of an asphaltene s t r u c t u r e c o n t a i n i n g a l a r g e , hydrogen d e f i c i e n t core to which are attached smaller, hydrogen-rich molecules. I t i s not c o n s i s t e n t with the smaller asphaltene s t r u c t u r e proposed by Ignasiak et a l (2) f o r Athabasca asphaltenes. Next the question of s u l f u r d i s t r i b u t i o n was addressed. S u l f u r i n the asphaltene and maltene f r a c t i o n s was measured d i r e c t l y and that i n the gas was obtained by d i f f e r e n c e . The r e s u l t f o r t h i s same s e r i e s of runs i s shown i n Figure 5. What we found was that approximately 50% of the s u l f u r remained i n


3


OIL


334

SHALE,

TAR

T A B L E

SANDS, AND

RELATED MATERIALS

II

EFFECT OF HYDROGEN TRANSFER ON OLE

FIN/PARAFFIN

R A T I O IN G A S P R O D U C T S

C

3 ^

C

3

REACTION TEMP. (°C)

T I M E (MIN.)

THERMAL

365

42

0.18

0.08

335

200 p p m

MOLYBDENUM

87

0.13

0.05

177

0.07

0.03

357

0.04

0.02

87

0.26

0.14

177

0.15

0.06

357

0.08

0.03

747

0.04

0.02


SCHUCKER

AND KEWESHAN

Cold

Lake

Asphaltenes

335


22.

Figure 3.

Arrhenius

plot for hydroconversion of Cold Lake asphaltenes: molybdenum (A A' thermal (

200


ppm

336

MATERIALS



Figure

4.

Hydrogen-to-carbon ratios in reaction products: maltenes product (calculated) ( ); asphaltenes (AA

(%);


total

Cold

Lake

Asphaltenes


SCHUCKER AND KEWESHAN


337

338


the reacted asphaltene, 28% was found i n the maltenes while 22% wound up i n the gas (presumably as H S ) . Studies with model s u l f u r compounds ( i n c l u d i n g dibenzothiophene, diphenyl s u l f i d e , benzyl phenyl s u l f i d e and d i b e n z y l d i s u l f i d e ) under the same r e a c t i o n c o n d i t i o n s l e d us to conclude that the m a j o r i t y of the s u l f u r i n both the asphaltene and maltene products was e i t h e r h e t e r o c y c l i c or an intermediate r e a c t i o n product from the c l e a v age of d i a r y l or a l k y l - a r y l s u l f i d e l i n k a g e s . More e a s i l y cleaved bonds such as those i n d i a l k y l s u l f i d e s or d i s u l f i d e s were found to be converted very q u i c k l y . Nitrogen was a l s o measured i n the asphaltenes and maltenes and the r e s u l t s are shown i n Table I I I . What we found was t h a t , u n l i k e s u l f u r which i s d i s t r i b u t e d p r e t t y evenly between the asphaltene core and the p e r i p h e r a l groups, n i t r o g e n i s p r i m a r i l y i n the core s t r u c t u r e . In a d d i t i o n , during r e a c t i o n very l i t t l e i f any of the n i t r o g e n i s removed from the system. This suggests that n i t r o g e n i s i n predominantly condensed h e t e r o c y c l i c s t r u c tures i n the core with only about 12-14% e x i s t i n g as smaller condensed n i t r o g e n s t r u c t u r e s on the periphery. Oxygen was measured only i n the asphaltenes due to sample size limitations. Combined r e s u l t s i n d i c a t e d that over 50% of the oxygen was l i b e r a t e d during these r e a c t i o n s as gaseous species and t h i s i s i n good agreement with r e c e n t l y p u b l i s h e d work of Moschopedis et a l (5) suggesting the presence of carb o x y l i c a c i d and aldehyde f u n c t i o n a l i t y . In a d d i t i o n to elemental analyses, number average molecular weights (M ) were obtained on both asphaltene and maltene f r a c t i o n s from s e r i e s . The r e s u l t i n g curves are shown i n F i g u r e 6. The s t a r t i n g asphaltenes are observed to have a number average molecular weight of 6640 ± 120. This decreases monotonically to an apparent asymptote of 3400. At the same time, maltenes which are produced e x h i b i t much lower molecular weights s t a r t i n g at 645 and decreasing to 415. I t i s not unreasonable at t h i s p o i n t to p o s t u l a t e that the maltenes, once formed, continue to break down. Here again, the observed v a r i a t i o n i n average molecular weight i s c o n s i s t e n t with the concept that asphaltenes have a l a r g e r core s t r u c t u r e to which are attached smaller (M/10 the s i z e of the core) groups. We are not saying t h a t 3400 represents the molecular weight of the core s t r u c t u r e . Experimental nmr and other VPO evidence p o i n t s to the contrary. We are saying that at 400°C we have broken a l l bonds that can be thermally broken at a reasonable rate and are l e f t with the core plus p e r i p h e r a l groups attached by much stronger bonds ( i . e . b i p h e n y l l i n k a g e s , e t c . ) and some a l k y l s i d e chains. One of the most powerful t o o l s a v a i l a b l e to us f o r charact e r i z a t i o n of these f r a c t i o n s i s nuclear magnetic resonance spectroscopy. Proton and C F o u r i e r transform nmr spectra were run i n deuterochloroform on these same asphaltene and maltene samples and some of the spectra are shown i n F i g u r e s 7 and 8. One of the f i r s t i n t e r e s t i n g p o i n t s we f i n d i s t h a t the asphal2


MATERIALS

1 3



22.

SCHUCKER

Cold

AND KEWESHAN

T A B L E

NITROGEN CONTENT

REACTION

% ASPHALTENE

T I M E (MIN)

CONVERSION

0

0

Lake

339

Asphaltenes

III

O F REACTION

PRODUCTS

N / N ASPHALTENES

n

MALTENES

1.0

0

27

26.0

0.83

0.12

57

38.3

0.83

0.12

117

42.7

0.76

0.14

4 0 0 ° C , 200 ppm Mo



340

80001

,

,

1

I 20

I 40

I

I

60

80

MATERIALS

r


7000r-

3001 0

TIME a) 400°C

Figure 6.

Molecular

I 1

I 00

(MIN)

weights of reaction

products


1

20


SCHUCKER

A N D KEWESHAN

Cold

Lake

Asphaltenes

, i • i . i i i •i . i . i • i • i • i • i • i • i 10 9 8 7 6 5 4 3

i | i | i | i | i | ' i ' i i i i i ' I • I ' I • I

i • i . i • i , i , i , i • i • I . I 10 9 8 7 6 5

4

3

1

2

I

1

. i . i . l 0

1

i

1

I

1

I

1

I

1

I

1

I

I i i . i • i . i i I . I i I 2 1 0

6 (PPM FROM TMS)

Figure

7. H-l NMR spectra of (top) crude and (bottom; reacted (400°C, asphaltenes


2 h)


342

I

I

I ' I I I | I I I I

11

| ' M II

I I | I I I I | I I I I | I I I I | I II I | I I I I | I I I I | I I I I | I I I I | I I I I | I I I I | M M |


1

MATERIALS

6 (PPM FROM TMS)

Figure

8.

C-13 NMR spectra of (top) crude and (bottom; reacted (400°C, asphaltenes


2 h)

SCHUCKER

AND KEWESHAN

Cold

Lake

Asphaltenes


22.


343



344


MATERIALS

22.

SCHUCKER

A N D KEWESHAN

Cold

Lake

Asphaltenes

345

1 3

tenes with M = 3400 s t i l l have 40% a l i p h a t i c carbon. Both C and H specfra confirm these as predominantly p a r a f f i n i c s i d e chains although some naphthenic character s t i l l remains. These side chains need not be connected to the asphaltene core s i n c e the smaller p e r i p h e r a l groups are a l s o known to be h i g h l y a l k y l substituted. In general we can say that the increase i n the f r a c t i o n o f aromatic carbon and hydrogen during r e a c t i o n i s c o n s i s t e n t with (1) the loss of a l k y l side chains, (2) l o s s o f h i g h l y s u b s t i t u t e d aromatic and naphthenic groups, and (3) l o s s of naphthenic hydrogen. We b e l i e v e that to a c e r t a i n degree a l l of these are o c c u r r i n g but that (2) i s the dominant r e a c t i o n . We can a l s o say based on subsequent experimental work using n-decyl benzene as a model a l k y l aromatic that under these condit i o n s (400°C, 120 min, 7 MPa H ) p - s c i s s i o n of a l k y l side chains i s p r e f e r r e d 20:1 over Of. One maltene sample generated under somehwat milder condit i o n s (3 h r s . , 365°C, CoMo/y-Al 0 ) to minimize secondary cracking r e a c t i o n s was analyzed by gas chromatography and the r e s u l t i n g chromatogram i s shown i n F i g u r e 9. I t i s c l e a r that while the vast m a j o r i t y o f the area i s contained i n the lower envelope, a d e f i n i t e p a t t e r n o f r e g u l a r l y - s p a c e d peaks i s observa b l e above the base. These were i d e n t i f i e d by gas chromatography/mass spectrometry as n - p a r a f f i n s ranging i n length from C11 to C 3 9 . The smaller peaks i n between were i d e n t i f i e d as p r i m a r i l y i s o - p a r a f f i n s which may have been formed by i s o m e r i z a t i o n during hydroconversion over the somewhat a c i d i c CoMo/v-Al 0 or which may represent the n a t u r a l d i s t r i b u t i o n of i s o p a r a f f i n s i n the a l k y l side chains. In summary, we have presented experimental evidence which supports the concept that Cold Lake asphaltenes have somewhat l a r g e , hydrogen-deficient core s t r u c t u r e s to which are attached a l k y l side chains and h i g h l y s u b s t i t u t e d aromatic groups. We have shown that s u l f u r tends to be r e l a t i v e l y evenly d i s t r i b u t e d between the core s t r u c t u r e s and the p e r i p h e r a l groups and that n i t r o g e n i s concentrated predominantly i n the cores. The o v e r a l l p i c t u r e of asphaltene r e a c t i v i t y that has emerged from t h i s i s shown s c h e m a t i c a l l y i n F i g u r e 10. During m i l d hydroconversion, weaker linkages are thermally broken r e s u l t i n g i n the formation of maltenes having a higher (H/C) and reacted asphaltenes having a lower (H/C). Some a l k y l s i d e chains are a l s o l o s t predomi n a n t l y by p - s c i s s i o n . In the absence o f e f f e c t i v e hydrogen t r a n s f e r , some o f these r e a c t i o n fragments can recombine to form coke. With improved hydrogen t r a n s f e r the coking r e a c t i o n s can be s i g n i f i c a n t l y delayed. T o t a l conversion o f these asphaltenes to maltenes would a t t h i s p o i n t seem to be an improbable g o a l ; however, more research i s needed i n order to see how f a r the s t r u c t u r a l concepts developed here f o r Cold Lake asphaltenes can be g e n e r a l i z e d to others.


X

2

2

3

2


3

346

OIL SHALE, TAR SANDS, AND RELATED MATERIALS

Literature Cited 1. Speight, J. G., "A Structural Investigation of the Constituents of Athasbaca Bitumen by Proton Magnetic Resonance Spectroscopy," Fuel, 1970, 49, 76-90.


2. Ignasiak, T., Kemp-Jones, A. V. and Strausz, O. P., "The Molecular Structure of Athabasca Asphaltenes. Cleavage of the Carbon-Sulfur Bonds by Radical Ion Electron Transfer Reactions," J. Org. Chem., 1977, 42(2), 312-320. 3. Bearden, R., Jr. and Aldridge, C. L., U.S. Patent 4,134,825 (1979). 4. Bearden, R., Jr., private communication. 5. Moschopedis, S. E., Parkash, S. and Speight, J. G., "Thermal Decomposition of Asphaltenes," Fuel, 1978, 57, 431-434. RECEIVED January 29, 1981.


The Reactivity of Cold Lake Asphaltenes - ACS Symposium Series

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