Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1169-1 174
1169
Asphaltene Reaction Pathways. 1. Thermolysis Phllllp E. Savage and Michael T. Kleln" Department of Chemical Engineering and Center for Catalyfic Science and Technology, University of Delaware, Newark, Delaware 19716
Slmon G. Kukes Research and Development Center, Phillips Petroleum Company, Bartlesvilie. Oklahoma 74004
Petroleum asphaltenes were pyrolyzed at temperatures ranging from 350 to 565 O C for times from 5 to 150 min in batch tubing bomb reactors. Gas, mattene, coke, and unreacted asphaltene product fractions were recovered by using a solvent extraction sequence and were subsequently analyzed by GC and GCIMS. The variation of these products' yields with time and temperature provided insight to asphaltene structure, thermal reaction pathways, and processing schemes. Primary pyrolysis products included light heteroatomic and hydrocarbon gases, alkanes, cycloalkanes, and single-ring aromatics, from which the presence of corresponding moieties in the asphattene was inferred. The product fractions' temporal variations were consistent with an asphaltene structure containing a hydrogendeficient, condensed aromatic core. These temporal variations also allowed discernment of primary and secondary thermal reaction pathways. The primary reaction of the asphaltene to each of its aromatic core and mattene anQgas fractions was followed by secondary degradation of these primary products to lighter compounds. These results suggest that catalyst deactivation during the hydroprocessingof heavy oils can be partially attributed to the precipitation and deposition of asphaltene and its aromatic core.
The thermal reactions of petroleum asphaltenes are of considerable practical interest. The industrial trend toward processing comparatively refractory heavy crudes and residua that can contain appreciable portions (10%-30 % ) of asphaltenes assures interest in their thermolysis. Being rich in heteroatoms and metals, these asphaltenes could rapidly deactivate common hydroprocessing catalysts. Asphaltenes can also react to or precipitate as substantial amounts of catalyst-deactivating coke during hydrotreating. Furthermore, much of what is nominally catalytic upgrading of asphaltenes may actually follow thermolysis in series since the pore structure of a hydroprocessing catalyst may deny asphaltenes access to the majority of the catalytically active surface. Most previous asphaltene pyrolyses have been oriented toward either elucidation of its structure (Speight and Pancirov, 1983; Schucker and Keweshan, 1980; Posadov et al., 1977; Posadov et al., 1975) or the identification of fragmentation products and the dependence of their yields on pyrolysis temperature (Cotte and Calderon 1981; Ritchie et al., 1979; Moschopedis et al., 1978; Strausz et al., 1977). Pyrolysis kinetics related to global product fractions (e.g., asphaltene, maltene, coke) have also been reported (Schucker, 1983; Schucker and Keweshan, 1980; Posadov et al., 1975). Little information exists, however, about the variation of the yields of the individual constituents of the product fractions with both time and temperature. The temporal variations of the products' yields allow discrimination between primary and secondary reaction products and thus provide a basis for reaction pathway resolution and subsequent kinetic analysis. Discrimination between primary and secondary products also assists the inference of structural features from pyrolysis data. The foregoing motivated the present investigation of asphaltene pyrolysis. Experimental Section Asphaltenes were pyrolyzed at isothermal temperatures of 350, 400, 450, and 565 "C for times ranging from 5 to 150 min. These experiments probed slow asphaltene 0 1 96-4305/85/ 1124-1 169$0 1.50/0
Table I. Asphaltene Elemental Analysis element i abundance atomic ratio i/C C 80.23 wt % 1.00 H 8.41 wt % 1.26 0 1.79 wt % 0.017 N 2.05 wt % 0.022 S 7.75 wt % 0.036 V 824 ppm Ni 316 ppm
thermolysis at low conversion as well as rapid thermolysis and complete asphaltene conversion. Chemicals. The asphaltenes were isolated from an off-shore California crude oil via a standard procedure of precipitation with 40 vol of n-heptane. The crude oil and paraffin were contacted for 5 h, and the solution was then allowed to settle for 16 h prior to decanting. Obtained in a yield of 14.6% of the crude by weight, these asphaltenes were ground in a mortar and pestle and checked to be fully benzene-soluble and heptane-insoluble prior to use. The elemental composition and metals content of the asphaltene are given in Table I. All other chemicals were obtained from Aldrich and used as received. Reactors. All pyrolyses were performed in batch tubing bomb reactors that comprised nominal 3/sin. stainless steel Swagelok tube fittings. Capped port connectors were attached to the two ends of a union tee, and a reducer connected the center port of the tee to a 1 2 in. length of in. 0.d. stainless steel tubing. A II4-in. Whitey severe service union bonnet valve interposed in the tubing served as a shut-off valve and also allowed attachment of a gauge for measurement of pressure. Procedure. The reactors were typically loaded with 1 g of asphaltene and purged with argon to provide an inert environment for reaction. The shut-off valve was then closed, and the reactor was plunged into a Tecam fluidized sandbath that had been preheated to the pyrolysis temperature. After the desired reaction time had elapsed, the 0 1985
American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
-------,
I
4 Asphaltene
I
02 18 0
I
1 i 1
80 t
20
1
lL
2a
Moltene
"
u
004
a 002 Gas
0
30
60
90
TIME ( minutes
120
I
c
QOO
150
0 TIME (minutes 1
30
60 90 TIME [ minutes
120
I50
Figure 1. Asphaltene pyrolysis at 350 O C : (a) temporal variations of product fractions, (b) temporal variations of gaseous products, (c) temporal variations of paraffin yield.
reactor was removed from the sandbath and immediately immersed in ice water to quench the reaction. After the reactor assembly had cooled and equilibrated at room temperature, the pressure was measured by attaching the pressure gauge to the free end of the tubing and opening the shut-off valve. This permitted estimation of the number of moles of gas produced during pyrolysis. The valve was then closed, disconnected from the pressure gauge, and connected to a gas sampling valve for chromatographic analysis of the gaseous products. Upon disconnecting the reactor assembly from the sampling valve and removing the tubing and the shut-off valve, a small amount of phenyl ether ( e l 0 mg but accurately weighed) was added to the reactor contents to serve as an external standard in the ensuing chromatographic analysis of the heptane-soluble products. The reactor contents were next subjected to a series of solvent extractions to separate and collect the pyrolysis product fractions. Maltenes were collected in sequential volumes of nheptane until the heptane extract remained colorless upon withdrawal from the reactor. The maltene fraction collected in this manner was subsequently analyzed by gas chromatography and its yield determined by evaporating the heptane at room temperature and weighing the material remaining. Unreacted asphaltenes were next collected in benzene. After several repetitions of the extraction procedure, one last volume of benzene was added, and the reactor's contents were allowed to stand overnight to ensure complete asphaltene recovery. The benzene solution was then placed in a beaker, and the unconverted asphaltene was recovered by evaporating the benzene at room temperature. The dry asphaltene was then weighed. The insoluble solids remaining in the reactor were collected, dried, and weighed for a determination of the yield of a product that will be hereafter referred to as coke. Since it was not possible to remove completely all the coke from the reactor walls, an accurate determination of the coke yield was difficult. However, pyrolyses at conditions where coke did not form indicated that the yields of gas, maltene, and unconverted asphaltene could be measured reasonably accurately. Material balances for these situations ranged from 96% to 101%. Therefore, for the more severe pyrolysis conditions, directly measured coke yields were complemented by calculations of the coke yields by difference. The experimentally determined coke yields were generally 55%-75% of the calculated yields. Analytical Chemistry. A Hewlett-Packard Model
5880A gas chromatograph was used in the analysis of both the gas and maltene fractions. Gases were separated over a stainless steel column, 6 ft in length and ' / a in. o.d., packed with 80/ 100 silica gel, and a UTI quadrupole mass spectrometer was connected to the chromatograph in order to identify the components of the gas fraction. Maltenes were separated over a 50-m fused-silica capillary column with an SE-54 stationary phase and were identified by retention time comparison with standard compounds. Results The delineation of results that follows is organized in order of ascending pyrolysis temperature. Asphaltene pyrolyses at 350 "C yielded gas, maltene, and unreacted asphaltene product fractions. Coke did not form, and material balance closure averaged 97%. The ultimate asphaltene conversion was less than 10%. Within the product fractions listed above, the most abundant of the individual products were the light non-hydrocarbon and hydrocarbon gases H2S,COz, CHI, C2Hs,C3H8,n-C4Hlo, and n-C5HI2. The maximum total gas yield observed was about 1.0% after 150 min. The maltene fraction (heptane-soluble products) yield was about 10% by weight of the original asphaltene after 150 min at this temperature. No individual product constituted a major portion of the maltene fraction, but a series of n-paraffins was identified with each present in trace amounts. Figure 1illustrates the temporal variations of the yields of the product fractions and the major gaseous and maltene products observed from asphaltene pyrolysis at 350 OC. Solid curves in Figure 1 and all figures to follow are illustrative and do not represent a statistical fit or theoretical significance. The asphaltene reaction rate illustrated in Figure l a is low, and the ultimate maltene yield is about 10 times the gas yield. Figure 1b shows that H,S was the most abundantly formed of the gases along with lesser amounts of COz and CH4. The ultimate yield of COz of about 1 mg/g observed at this temperature, as well as at higher temperatures, represents about 5% of the oxygen content of the original asphaltene listed in Table I. Figure ICshows that the nearly equal yields of octane, dodecane, and eicosane were low but increasing with time. Asphaltene thermolysis at 400 "C produced coke in addition to gas, maltene, and asphaltene fractions. Material balance closure varied from 79% to 96%. Major gaseous products at 400 "c were H2S, CH,, C2Hs,C&, n-C4H,,, and n-C5HI2along with lesser amounts of C 0 2 , CZH4, C,H,, and i-C4HIo.A typical GC trace of the maltene
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 1171 :lo
c 12 STD
I
!
.
Figure 2. Typical maltene chromatogram: pyrolysis at 400 OC for 90 min.
::I
E
50
a
$ 0 W
40
3 30 20
4
Aspholtene
2
1
1 4
1 1 2
1
1
A
a
w>
I5
30
45 6 0 75 90 TIME ( minutes )
105
120
I 0
5
0 8
2
0 6
LT
2
Maltene
0
16
v1
I0
0
18
4
b &p fFF-
60 w 0
& 20 z
0 4
0 2 00 0
15
30
45 60 75 90 TIME ( minutes I
105 I 2 0
0
15
30
45 60 75 90 TIME ( m i n u t e s I
I 0 5 120
Figure 3. Asphaltene pyrolysis at 400 OC: (a) temporal variations of product fractions, (b) temporal variations of gaseous products, (c) temporal variations of paraffin yield.
fraction, such as that given in Figure 2, contained about 300 integrated peaks. This complexity made identification and quantification of each of these compounds impractical. A salient feature of Figure 2 is the series of prominent peaks corresponding to the n-paraffin series up to C26, n-hexacosane. This paraffin series and the saturated compounds cyclopentane, methylcyclopentane, cyclohexane, and methylcyclohexane were collectively the most abundant products in the maltene fraction at this temperature. The single-ring aromatics, toluene and the xylenes, were also detected in smaller proportions. Figure 3a summarizes the temporal variation of the yields of pyrolysis product fractions at 400 "C. Maltenes were more abundant than gases at all times, and the nearly discontinuous rate of generation of coke at 30 min coincided with rapid asphaltene conversion. Note that during this period of apparently rapid asphaltene reaction the yields of gas and maltene did not increase dramatically. The mole fractions and yields of all gases identified after 90 min of pyrolysis at 400 "C are listed in Table 11. Figure 3b shows the effect of reaction time on the yield of the major products. H2S was once again the major product formed, and its yield reached an asymptotic value of about 18 mg/g asphaltene after 120 min. This represents about 23% of the sulfur content of the original asphaltene. Yields of hydrocarbon gases formed at 400 "C ranged from 1 to 8 mg/g, and saturated products were much more abundant than olefins. Further, and more generally observed in all pyrolyses, the yields of the alkanes decreased
Table 11. Gaseous Products from Asphaltene Pyrolysis at 400 "C for 90 min product
mol %
mmol/g
34.3 31.2 13.1
CZH,
1.7 0.5
0.50 0.45 0.19 0.10 0.03 0.03 0.03 0.02 0.01
unknowns"
2.2
0.03
HZS CH4 C!2H6
C3H8 n-C4H,o C3Hs and i-C,H,, COZ n-C5H1Z
7.0 2.3 2.2 1.9
Probably butenes, pentanes, and pentenes.
with increasing carbon number. Figure 3c depicts the temporal variation of selected n-paraffins at 400 "C. At 15 min, all paraffins were present in roughly equal yields, but the yields of the lower alkanes were greater than the higher n-alkanes at longer times. Note that the yields of eicosane and tetracosane decreased slightly beyond 60-min reaction time and that an individual paraffin's yield decreased with increasing carbon number. Asphaltene pyrolysis at 450 "C produced gas, maltenes, coke, and unconverted asphaltene. Material balances ranged from 60% to 74%. Asphaltene conversion, to predominantly coke, was rapid and complete after 30 min. The major constituents of the gaseous fraction were saturated hydrocarbons along with H2S,and the identified
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10
/
v
Moltene I
O 0
I5
30 45 60 TIME ( m i n u t e s I
-
75
99
3
15
30
45
60
TIME ( minutes 1
75
9C
0
I0 20
30
40
50
60
T I M E ( minutes
70
80 90
I
Figure 4. Asphaltene pyrolysis at 450 "C: (a) temporal variations of product fractions, (b) temporal variations of gaseous products, (c) temporal variations of paraffin yield.
\
r--
L
Figure 5. Typical maltene chromatogram: pyrolysis at 565 "C for 45 min.
maltenes included cyclopentane, methylcyclopentane, methylcyclohexane, toluene, cyclohexane, xylenes, naphthalene, and C5-CZsn-paraffins. Figure 4 summarizes the temporal variations of the yields of the product fractions and major gaseous and maltene produds observed at 450 "C. Figure 4a shows that the proportions of the product fractions at 90 min aligned in the descending order of coke, gas, and maltene. Within the gas fraction, Figure 4b illustrates that H2S was a major product, and its nearly constant yield of 217 mg/g was of the same magnitude as the yields of methane and ethane. The yields of saturated hydrocarbon gases increased steadily with time even after 30 min where asphaltene conversion was complete. Thus, in addition to primary routes from the asphaltene, light hydrocarbon gases were also produced by secondary reactions of the maltenes and/or coke. Figure 4c depicts the variation of the yield of selected n-alkanes with pyrolysis time at 450 "C. Each alkane exhibited an obvious maximum yield which occurred earlier for eicosane and tetracosane than for octane and dodecane. Note once again that the yield of paraffin decreased as the carbon number increased.
A t 565 "C, asphaltene conversion was complete after only 5-min reaction time. Only the maltene fraction was analyzed at this temperature. GC traces of the maltene fraction were much less complex at 565 "C than at the lower temperatures. This is illustrated in Figure 5. Aromatic compounds predominated, and the regular series of paraffi peaks evident at less severe conditions was not observed. Approximately 35 compounds were detected in the pyrolysate, the major products being the xylenes, toluene, benzene, and naphthalene. Several polynuclear aromatics including phenanthrene, pyrene, and dibenzothiophene were also observed at 565 "C. These products were not detected at the lower temperatures. Figure 6 illustrates the variations of the yields of major maltenes with time. Single-ring aromatics are clearly more abundant than the polycyclics. The yields of toluene and xylenes decreased with time while the yields of unsubstituted aromatics reached a nearly constant value after about 20 min. Note that the polynuclear aromatics detected were pericondensed. That is, pyrene formed, but naphthacene was not observed. Figure 6 also shows that the yield of aromatic products decreased as the number of condensed rings in the compound increased.
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 6
1' Xylene
5
W
z w
5 a
4
I
a (0
a
3
DI
m
Benzene
\
E I
n
d 2
Nophthalene
-
>
I
Phenonthrene
1
Pyrene
0
0
IO
20
30
40
50
60
TIME ( m i n u t e s )
Figure 6. Asphaltene pyrolysis at 565 major maltenes.
OC:
temporal variation of
Discussion The significance of these results has been apportioned into three discussion topics concerning the implications of the present results to asphaltene structure, asphaltene pyrolysis pathways, and, finally, heavy oil processing. Structural Implications. Asphaltenes are strictly defined as the response of an oil to a solvent addition protocol. The concept of an intrinsic chemical structure is nebulous, and structural scenarios advanced are equivocal. In fact, strikingly different hypothetical average structures have been proposed for the same asphaltene (Ignasiak et al., 1977; Speight, 1970). Generally, however, condensed aromatic, naphthenic, and paraffinic moieties are considered (Speight and Moschopedis, 1981; Yen, 1981; Yen, 1974) to be important structural features. The present results allow further discussion of the structure of the petroleum asphaltene used. Figures lb, 3b, and 4b collectively show that the major low-temperature products, H2S and C02, were produced in nearly constant yields that were apparently independent of temperature. This indicates that a finite fraction of the sulfur atoms in asphaltene are available for H2Sformation. These sulfur atoms probably exist as very reactive thioether moieties in the asphaltene since H2S yields were appreciable even at 350 "C. The carbon dioxide yield was also nearly constant and essentially independent of temperature. This suggests that the small fraction of the asphaltene's oxygen atoms that appear as C 0 2 is located in carboxylic acid groups. Thus, it appears that these heteroatomic moieties are the most thermally labile constituents of the asphaltene and that a measurable fraction of the sulfur and oxygen atoms are probably located at the periphery of the condensed aromatic cores. The nature of the substituents and linkages on the asphaltenic cores has been the subject of considerable previous investigation and discussion (Speight and Moschopedis, 1981; Yen, 1981;Takegami et al., 1980; Gould, 1978). The present results reveal that the asphaltene pyrolysate contains an n-alkane series including very long paraffins. Long alkanes up to C21,CZ6,C3*, Ca9, and C30were also detected in asphaltene pyrolysate by Speight and Pancirov (1983),Speight (1970),Ritchie et al. (1979), Schucker and
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Keweshan (1980), and Simm and Steedman (1980), respectively. These are in apparent conflict with spectrometric studies of asphaltene structure that indicated the alkyl chains to be on the average of three to six carbon atoms in length (Boduszynski et al., 1977; Koots and Speight, 1975; Ali and Al-Ghannam, 1981; Speight, 1971). The essential issue is whether these long alkyl chains are actually present in the asphaltene as chemically bonded moieties or whether they are merely artifacts due to absorbed resins as a result of incomplete separation and purification of the asphaltene. The present results indicate that these alkyl chains are chemically bonded constituents of, and not simply physically associated with, the asphaltene. For pyrolysis at 350 "C, the paraffin yield was always low up to 150-min reaction time. At 400 "C, the total yield of n-paraffins increased slowly but steadily with time, and at 450 "C, the total yield was somewhat higher than at 400 "C and exhibited a maximum value in time. If alkane appearance was through dissociation of weak, low-energy, physical bonds, then the maximum yield of paraffins would be expected a t low temperatures (certainly by 400 "C) and short reaction times. The yield of higher alkanes would then decrease as they cracked to lighter products. This was not the case. On the other hand, these results are more indicative of the presence of stronger chemical (covalent) bonds that would give rise to the appreciable energy barrier and thermal activation that was observed. Apparent activation energies of 52.2, 51.7, and 46.7 kcal/mol for the formation of octane, dodecane, and eicosane, respectively, are consistent. It is interesting that mild pyrolyses (350 and 400 "C for times less than 15 min) generated octane, dodecane, and eicosane in nearly equal yields. This is illustrated in Figures IC and 3c. The abundances of these primary products likely reflect the abundances of corresponding aliphatic moieties, to within two or three carbon numbers, in the asphaltene. Thus, not only are long alkyl chains present in the asphaltene structure, but they evidently are found in roughly equal proportions on a weight basis. Their molar, or number, proportion must then be correspondingly less. The present results also probed the aromatic portion of the asphaltene. The primary formation of toluene and xylenes at 400 "C indicates that single-ring aromatic moieties are linked to the asphaltene core, possibly by scissile aliphatic and heteroatom-containing bonds. The prevalence of pericondensed aromatics from the higher temperature pyrolysis at 565 "C is consistent with current concepts of asphaltene structure (Yen et al., 1984, Yen, 1981; Speight and Moschopedis, 1981; Yen, 1974). However, it would be unwarranted to assume that the observed distribution of aromatic products reflects that of the aromatic moieties in the original asphaltene, since pyrolysis of substituted aromatiqs and alkanes to condensed aromatics has been observed (Badger, 1965). Pyrolysis Pathways. The present results combine with the foregoing interpretation of asphaltene structure to permit resolution of asphaltene pyrolysis pathways as well as the secondary reactions of primary products. To summarize the above, we consider asphaltenes to contain condensed aromatic ring systems with heteroatomic, alkyl, and naphthenic substituents and interunit linkages located along their peripheries. Fission of these peripheral moieties is the likely first step in asphaltene conversion. Thus, the heteroatomic gases H2S and C 0 2were the primary products from low-temperature thermolysis of scissile thioether and carboxylic acid moieties, respectively.
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
ASPHALTENE
-
CORE
I
GAS
+
MALTENE
1
+
GAS
LOWER MOLECULAR WEiGHT MALTENES
+ GAS
Figure 7. Asphaltene t h e r m a l reaction pathways.
Higher temperature primary products that arose from fission of carbon-carbon bonds were hydrocarbon gases, cycloalkanes, paraffins, and possibly condensed polynuclear aromatics. It would thus be consistent that the coke observed is actually the asphaltenic core stripped of its peripheral substituents. This strictly primary product evidently transforms from benzene-soluble to benzene-insoluble over a very small "window" of degradation (see Figure 3a). In both the gas and maltene fractions observed from pyrolysis at 400 "C and 450 O C the yield of saturated hydrocarbons consistently decreased with increasing carbon number. This suggests that lighter paraffins formed from secondary reactions of higher aliphatics and not solely via primary asphaltene decomposition. These secondary reactions likely include the thermal cracking of higher n-alkanes and naphthenics, along with the dealkylation of substituted aromatics. Recall that Figures 4c and 6 illustrated that several paraffinic and substituted aromatic products passed through maximal yields in time at a given pyrolysis temperature. For example, the paraffin yields exhibited in Figure 4c are indicative of consecutive reactions of the type A-B-C where A is asphaltene, B an n-alkane, and C the thermally cracked products. Note that the time for this maximum yield was less for the longer paraffins, which suggests that the lighter paraffins are decomposition products of the higher paraffins. Figure 7 is a summary of the reaction pathways suggested above. Primary asphaltene reactions produce asphaltenic core (coke), maltenes, and gas. Secondary reactions include fission of maltenes to lower molecular weight maltenes and gases, as well as reaction of the core to gas. Note that severe overreaction of primary products is not required to predict the formation of large quantities of benzene-insoluble solid materials. This network is similar to that proposed by Schucker and Keweshan (1980) and supported by Speight and Pancirov (1983). Implications t o Petroleum Hydroprocessing. The presence of asphaltenes in heavy oils and residua is of considerable significance to hydroprocessing since asphaltenes have been implicated in metals and carbon deposition on catalysts. Mild pyrolysis a t 350 "C led to only maltenes and gas, and no coke, for asphaltene conversion of
