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Chapter 6 Kinetics and Mechanisms of Asphaltene Cracking During Petroleum Recovery and Processing Operations
A.sphriltcnc\ rind Avpholtv, 2. D~wdiipiiiriitsin Pivoleurn Science.40 B rdiled by T.F. Yen and G.V. Chilingarim 0 2000 Elsevier Science B.V. All rights reserved
129
Chapter 6
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING DURING PETROLEUM RECOVERY AND PROCESSING OPERATIONS A. CHAKMA
INTRODUCTION
Many bitumen and heavy oils produced around the world contain asphaltenes. Asphaltenes play an important role in the recovery, transportation, processing and utilization of bitumen and heavy oil-derived products. For example, asphaltenes may precipitate during recovery and transportation of bitumens/heavy oils [ 11. During the upgrading operations, the presence of bitumen may result in the formation of excessive coke in the upgrading reactor. In addition to the above-mentioned implications, asphaltenes are basically heavy-molecular-weight aromatic hydrocarbons with other impurities. Therefore, they represent certain energy values, which should be recovered. This can be achieved by cracking the complex asphaltene structure to obtain useful fuels. In petroleum recovery operations, such as in situ combustion, cracking of asphaltene not only results in lighter useful oil fractions, but the heavier components combine to form coke which serves as the fuel to sustain the in situ combustion process [2-61. On the processing front, cracking of asphaltenes into lighter useful forms requires innovative processing of the feedstock, frequently in the presence of catalysts [7-111. Alberta bitumens and heavy oils may be considered to be composed of macroand micro-structures, as well as chemical constitutive molecules. The macro- and micro-structural arrangements determine the viscosity of the bitumen. Dickie and Yen [I21 investigated the macro-structures of the asphaltenes and found them to be composed of polynuclear aromatic molecules with alkyl chains as attachments. These constitutive ‘asphaltene unit molecules’ are grouped in layers having several unit molecules (typically 5 or 6) surrounded or immersed into the maltene (fraction soluble in 40 volumes of n-pentane) fluid. The latter is composed of free saturates, mono- and di-aromatics, and resins that may be associated with the asphaltenes. This structural organization may be considered to be the micro-structure of the system. The micro-structure may form aggregates to reduce the free energy of the system. These aggregates form micellar structures that consist of several unit layers of asphaltenes surrounded by or associated with the maltenes. The micellar structures thus formed may be classified as the macro-structures of the system. The macro-structural arrangement to a large extent depends on the size of the unit asphaltene layers. Low-molecular-weight asphaltenes (MW up to about 1000) consist of single sheets of condensed aromatic and naphthenic rings with relatively alkyl chain attachments. Larger-molecular-weight asphaltenes may consist of several sheets of condensed ring systems. Speight [ 131 proposed a hypothetical structure consisting
130
A. C H A K M A
TABLE 6- I Structural parameters for asphaltene (after Savage and Klein [ 2 6 ] ) Atomic H/C ratio Fraction of C atoms in aromatic rings Fraction of C atoms in saturated rings Fraction of H atoms in aromatic rings Fraction of H atoms in saturated rings Ratio of peripheral aromatic C atoms over total aromatic carbon atoms
1.09-1.29 0.30-0.61 0.060.24 0.04-0. I I 0.164.19 0.314.55
of 12 condensed aromatic rings for Athabasca asphaltene. These individual structures may be connected together by various aliphatic, naphthenic or heteroatomic linkages [14-161. A review on the molecular structure of asphaltenes has been provided by Speight [ 171. Mojelsky et al. [ 181 have utilized Ru ion catalyzed oxidation reaction to determine various structural features of asphaltenes derived from different Alberta Oil Sand bitumen and heavy oils. The authors report on the determination of total number of alkyl chains, chain length and the number of carbon atoms with each chain and also on the total number and length of the bridge connecting two aromatic rings in the asphaltene molecule. The length of the alkyl chain has been the subject of numerous studies. Spectrometric studies of asphaltene structure suggests the alkyl chains to be 3 to 6 carbon atoms long [ 19-22]. However, analysis of the asphaltene pyrolysis products reveal the presence of alkyl chains of up to carbon number C3,, [13,23-251. These alkyl chains are thought to be covalently bound to the core asphaltene unit. These bonds can be broken with activation energies of about 50 kcal/mol [26]. A summary of the structural parameters of asphaltenes is given in Table 6-1. C-C bonds in aromatic rings are the most abundant. They are the most stable ones. However, the aliphatic C-C bonds found in alkylaromatic, alkylhydroaromatic and alkylnaphthenic positions are the most reactive ones in the temperature range of 300" to 550°C [24].
KINETICS
Thermal cracking of hydrocarbons has generated much interest since the early days of the industrial revolution. The first systematic theory on thermal cracking was postulated by Rice in 1931 [27]. He proposed the formation of free radicals for the decomposition of the hydrocarbons. For light hydrocarbons, the theory of Kossiakoff and Rice [28] can predict the product distributions very well. For example, product distribution arising from the cracking of n-nonane [29] can be predicted within 20% of the experimental data using the Kossiakoff and Rice [28] theory. Gates et al. [30] found the theory to be able to predict product distribution resulting from the thermal cracking of n-hexadecane well. However, the predictions for the cracking of high-molecular-weight hydrocarbons deviate considerably [3I]. Blouri et al. [31] studied the cracking of n-hexadecane, 6-methyleicosane
131
KINETICS A N D MECHANISMS OF ASPHALTENE CRACKING
and 1 -phenyldodecane under relatively mild temperature (350" to 440°C) under high pressure (20 bar). They found the cracking process to follow a molecular mechanism; however, the kinetic data were similar to those of radical cracking.
ASPHALTENE
A number of studies on the cracking of asphaltenes have been reported in the literature. Most of them, however, dealt with the determination of the asphaltene structures. Moschopedis et al. [32] carried out thermal decomposition of asphaltenes in a horizontal tube reactor and analyzed the gaseous and liquid products. Ritchie et al. [23] pyrolyzed Athabasca asphaltenes and analyzed the volatile products by GC/MS. Schucker [33] reported a thermogravimetric study of the coking kinetics of Arab heavy vacuum residuum. As a part of his investigation, he also studied the coking kinetics of the asphaltene fraction derived from the Arab heavy vacuum residuum. Inasmuch as the kinetic study was purely based on the weight loss measurement, it was not possible to analyze the reaction products. Therefore, the kinetic data obtained were for the overall disappearance of the asphaltenes. The authors found the activation energies to increase with conversion ranging from 176 kJ/mol at 20% conversion to 336.5 kJ/mol at 90% conversion. This is quite understandable. First, the lighter fractions of the asphaltenes volatilize followed by the heavier fractions. Therefore, the initial activation energies are lower, representing the lighter fractions; however, they increase when the heavier fractions are cracked. Major constitutive asphaltene groups and their primary cracking products are listed in Table 6-2.
TABLE 6-2 Principal reaction and reaction products obtained from the cracking of major hydrocarbon groups belonging to the asphaltene molecule (adapted from Voge, [34]) Hydrocarbon class
Structure
Aromatic compounds without side chains
Aromatic compounds with side chains
Principal reactions
Principal products
Negligible cracking
Coke
a-
Cleavage of peripheral chains
Aromatic compounds and olefins
Cleavage of peripheral chains and opening of napthenic ring
Aromatic compounds, olefins and paraffins
-\
Naphtheno-aromatic compounds
132
A . C'IIAKMA
The groups shown in Table 6-2 are typical ring structures present in the asphaltenes. Whereas some information on asphaltene cracking characteristics may be obtained by studying the cracking mechanisms of these typical structures, it is not practical to determine reaction kinetics for asphaltene cracking from the cracking studies involving individual groups due to the complexity of the asphaltene structure and the presence of many other minor groups. As a result, kinetic studies on asphaltene cracking usually involve an easily quantifiable pseudo-component approach, where reaction products are grouped according to solubility and/or boiling point characteristics. Hayashitani et al. [35] were the first investigators to carry out extensive kinetic experiments on the cracking of Athabasca bitumen using a pseudo-component approach. Although the focus of their study was on Athabasca bitumen, various reaction mechanisms postulated by the authors centered around asphaltenes. Hence, the kinetic models proposed can also be classified as asphaltene cracking models. These authors carried out thermal cracking of Athabasca bitumen, asphaltenes, and heavy oils (BP 400"Cf) and oils (BP 200" to 400°C) fractions separated from the original bitumen, at constant temperatures in a batch reactor. The reactor was equipped with a quartz tube liner, which prevented the reactants and the reaction products from directly contacting the vessel surface. An inert atmosphere was maintained inside the reactor by pressurizing it with helium. The temperature range studied was from 303°C to 452°C. The reaction products were separated into six pseudo-components: coke, asphaltenes, heavy oils (BP 400"C+), middle oils (BP 200" to 400"C), light oils (BP 20" to 200°C) and gases. The cracking of bitumen was found to yield all the above six pseudo-components. When asphaltenes were used as feed material, large amounts of coke and heavy oils were obtained along with small amounts of middle and light oils and gases. Cracking of the heavy oil fraction led to the formation of mainly middle and light oils and some asphaltenes. Coke formation from the heavy oil fraction was found to be negligible. Finally, the middle oil fraction yielded mostly light oils. Based on the above observations, Hayashitani et al. [36] proposed four reaction mechanisms. They correlated the reaction rate constants for each of the reaction mechanisms by an Arrhenius type expression. The activation energies and the frequency factors are given in Tables 6-3 and 6-4.
TABLE 6-3 Frequency factors
(in
s
I)
for the diffcrent models
~
Model Model Model Model
HI H2 H3 H4
5.04 x 10" 1.41 x 10" 2.04 x 10"' 1.01 x 10"
1.17 x 1.53 x 6.07 x 1.40 x
10''
10'' 10" 10'"
1.91 x 8.44 x 1.18 x 7.28 x
10''
10" 10'' 10''
4.42 x 10" 4.50 x l0li 4.45 x 10'' 1.43 x 10"
4.44 x 10''
-
4.55 x 10" 8.18 x 10'' 1.51 x lo"
1.49 x 10'' 2.90 x 10" 3.00 x lo"
133
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6-4 Activation energy (in kcal/mol) for the different rate models
Model HI Model H2 Model H3 Model H4
48. I 46.5 44.2 46.3
67.8 65.2 65. I 63.8
64.I 62.8 60.3 59.6
57.5 57.5 57.4 56.2
59. I 58.9 53.6 57.6
-
57.4 55.9 55.6
The corresponding reaction mechanisms are summarized as follows: Model H1
In their first model, Hayashitani et al. [36] lumped middle oils, light oils and gases into a single pseudo-component, called distillable oil, and postulated the scheme shown in Fig. 6-1. This model was able to provide reasonable predictions of the concentration versus time curves for the various pseudo-components. Addition of an additional reaction, asphaltene --f distillable oil, did not improve the model’s performance. Among the major drawbacks of this model was its inability to predict (i) the initial sharp decrease in the asphaltene concentration, (ii) initial increase in the heavy oil production, and (iii) initial delay in the coke production. Model H2
Because gases belong to a distinct group themselves, in their second model, Hayashitani et al. [36] separated gases from the distillable oil fractions. The model presented in Fig. 6-2 is the result. This model predicted the formation of gases very well, whereas the prediction for
ASPHALTENE
c-223 Fig. 6- I . Model HI - asphaltene cracking model of Hayashitani et al. 136J.
134
A CHAKMA
<_> HEAVY OIL
Fig. 6-2. Model H2
~
asphaltene cracking model of Hayashitani et al. 1351.
other pseudo-components were similar to Model H1. However, it was not able to overcome the earlier-mentioned limitations of Model H 1 .
Model H 3
In their third model, Hayashitani et al. [35] subdivided the asphaltene fraction into two subfractions, asphaltene-1 and asphaltene-2, in an effort to improve the model performance in describing the initial sharp decrease in asphaltene concentration. Otherwise, Model H3 was similar to Model H1. Asphaltene-1 subfraction was assumed to yield only heavy oil, whereas the asphaltene-2 subfraction was assumed to produce both heavy oil and coke upon cracking. Model H3 is summarized in Fig. 6-3. Model H3 was able to provide a somewhat better representation of the concentration versus time curves for the asphaltene and coke fractions compared to Models HI and H2.
k41 f
k5
Fig. 6-3. Model H3 - asphaltene cracking model of Hayashitani el al. 1351.
a <->
KINETICS A N D MECHANISMS OF ASPHALTENE CRACKING
k5
135
ASPHALTENE 2
Fig. 6-4. Model H4 - asphaltene cracking model of Hayashitani et al. 1361.
Model H4
Model H4 is a combination of Models H2 and H3 and as such incorporates all the favorable features of these two models. It is summarized in Fig. 6-4. Savage et al. [26] pyrolyzed asphaltenes separated from offshore Californian crude oil by precipitation with 40 vol of n-heptane at temperatures ranging from 350" to 565°C in batch reactors to study asphaltene reaction mechanisms. They assumed an asphaltene unit to consist of condensed aromatic ring systems with heleroatomic, alkyl and naphthenic substituents. These single units were linked through linkages located along their peripheries. At low temperatures, they found that HzS and CO? are the primary products and attributed their formation to the fission of peripheral thioether and carboxylic acid moieties, respectively. Hydrocarbon gases, cycloalkanes, and paraffins were the primary products for high-temperature conditions arising from the fission of C-C bonds. Based on their experimental observations, they concluded that coke, maltenes and gas are produced from the primary reactions. Maltenes are cracked into lower-molecular-weight maltenes and additional gas is produced from the coke and lower-molecular-weight maltene fractions. The reaction scheme is shown in Fig. 6-5. However, these authors do not provide any rate constants. Phillips et al. [37] studied the thermal cracking of Athabasca bitumen and its components in a batch reactor at temperatures ranging from 360" to 420°C in the presence of sand. They divided the products into six pseudo-components: coke, asphaltenes, heavy oils, middle oils, light oils and gases. They proposed the following two kinetic models. Model PI
In the first model (Model Pl), following Hayashitani's Model H1, they combined the middle oils, light oils and the gases into a single pseudo-component called distillables. They proposed the reaction mechanism shown in Fig. 6-6.
136
A CHAKMA
Fig. 6-5. Asphaltene cracking mechanism of Savage et al. [26).
D ASPHALTENE
D HEAVY OILS
DISTILLABLES
Fig. 6-6. Model PI
~
asphaltene cracking mechanism of Phillips et al. 1371
They found the reaction kinetics to be of first order and to follow the Arrhenius relationship. The presence of sand was found to affect the distribution of products and resulted in the lowering of activation energies of the reactions. The authors attribute this effect to the catalytic role played by the sand matrix. Table 6-5 shows a comparison of the kinetic parameters for the model in the presence of sand with the thermal cracking data of Hayashitani et al. [36].
Model P2 The second model of Phillips et al. [37] included all the six pseudo-components, as shown in Fig. 6-7. In Table 6-6, the kinetic constants for this model are compared with those obtained from thermal cracking studies of Hayashitani et al. [36]. Hayashitani et al. [36] obtained high activation energies for the reactions asphaltene
137
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6-5 Comparison of kinetic parameters of Model P1 with the thermal cracking data of Hayashitani et al. [36] Rate constant
Activation energy, E , with sand (kJ/mol)
Activation energy, E , without sand (kJ/mol)
Frequency factor
20 I 284 268 24 1 248
1.16 x 6.37 x 5.61 x 5.22 x 2.04 x
W I )
10" 10"
10Ih 10"
10l3
c
kg
Fig. 6-7. Model P2 - asphaltene cracking mechanism presented by Phillips et al. [37]. TABLE 6-6 Kinetic constants for Model P2 with or without the presence of sand Rate constant
Activation energy, E , with sand (kl/mol)
Activation energy, E , without sand
Frequency factor
( kl /mol)
W I )
~
195 273 263 24 I 247 240
1.16 x 4.99 x 1.41 x 1.91 4.04 x 3.79 x
10" 10'9 10'' 1017 10'4
1OlX
+ maltenes (224 f 15 kJ/mol) and maltenes + asphaltene (284 f 28 kJ/mol). They attributed these high levels of activation energy to the heterogeneity of the maltene fraction.
138
A CtIAKMA
ASPHALTENE
a MALTENE
D GASES
Fig. 6-8. Non-catalytic hydrocracking model of Kiiseoglu and Phillips [38]
Model KPI
In a follow-up work, Koseoglu and Phillips [38] on the hydrocracking of Athabasca bitumen proposed a kinetic model for bitumen decomposition by dividing the bitumen into four pseudo-components, namely, coke, asphaltenes, maltenes and gases. They found the activation energy for the asphaltene -+ coke reaction to be lower than that of the thermal cracking. They also concluded that coke formation from resins occurs via asphaltene intermediates. The model is summarized in Fig. 6-8. The kinetic constants for the above model are listed in Table 6-7 and are compared with those obtained from thermal cracking studies of Hayashitani [36]. As can be seen from Table 6-7, the activation energies for kz and k3 are higher than those encountered for the hydrocracking of simple hydrocarbon molecules. The authors attribute the high value of the activation energy for k 3 , representing the reaction maltene + asphaltene to the possible inhibition of polymerization and condensation reactions by excess hydrogen present. According to Belinko and Denis [39], unsaturated asphaltene molecules prefer to undergo polymerization and condensation reactions over TABLE 6-7 Kinetic constants for Model P2 with or without the presence of sand Rate constant
ki k? ki
k4
Thermal hydrocracking
Thermal cracking
Activation energy, E (kJ/mol)
Frequency factor. A
142 224 284 186
2.44 x 6.72 x 4.36 x 8.14 x
(SCI)
loh 10” 10’’ 10’
Activation energy. E (kJ/mol)
Frequency factor. A
178 I04 107 171
4.53 x 2.46 x 6.78 6.31 x
(s ’ ) 10”
10‘ lo7 107
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
139
hydrocracking reactions. Hence, the activation energies for the asphallene + maltene reaction (k2) is higher than that of asphaltene += coke reaction (kl ). A comparison of the kinetic data for the thermal and hydrocracking reactions suggest that the reversible reaction asphaltene + maltene requires less activation energy in the case of thermal craclung compared to thermal hydrocracking. The reaction asphaltene -+ coke, on the other hand, requires less activation energy for the thermal hydrocracking case. Koseoglu and Phillips [38] also proposed six kinetic models for the non-catalytic hydrocracking of Athabasca bitumen based on pseudo-components and lumped fractions. The first model is based on three pseudo-components: heavy ends, light oils and gases and is of the following form: heavy ends -+ light oils
-+
gases
The heavy ends included both coke and asphaltenes. In the second model, the heavy ends of the first model was divided into coke and heavy oils. The resulting model is as follows: coke
t
heavy oils + light oils + gases
The heavy oil fraction was further subdivided into asphaltenes and resins in the third model as follows: coke
t asphaltenes
+ resins -+ lightoils + gases
The light oil fraction can be subdivided into aromatics and saturates. Inasmuch as all the gases produced were saturated, the saturates fraction of the light oils was lumped with the gases into a single pseudo-component called ‘saturates’ in the fourth model. The model thus becomes: coke
t asphaltenes
+ resins += aromatics -+ saturates
The lumping of liquid saturates and gases into a single pseudo-component is rather questionable and perhaps unnecessary. The model prediction in this case was not as good as that of the previous model, the general trends, however, can be reproduced. The authors also examined the other variations of the previously described models. The final model, however, was an extension of the fourth model. The saturates were simply broken down to liquid saturates and gases. The final model (Model KP2), thus, included six pseudo-components as shown in Fig. 6-9. All the reactions, in general, followed the Arrhenius relationship, the reaction asphaltenes + resins, however, showed some deviation from the Arrhenius behavior. The relevant kinetic parameters are given in Table 6-8. Coal-derived asphaltenes
Szczygiel and Stolarski [40] studied the hydrocracking of asphaltenes extracted from coal in the presence of a natural aluminosilicate catalyst in a batch autoclave at temperatures of 673 to 733 K and under hydrogen pressure of 9.8 to 29.4 MPa. They found coke to form mainly during the preheating period and oil to form from asphaltene. Their proposed reaction mechanism is presented in Fig. 6-10.
140
c-1
CzF-3k1
A. C'HAKMA
ASPHALTENE
a AROMATICS
Fig. 6-9. Asphaltene cracking mechanism of Koseoglu and Phillips 1381. TABLE 6-8 Kinetic parameters of the asphaltene cracking model of Kdseoglu and Phillips [38] Rate constant
Activation energy. E (kl/mol)
(s-' )
Frequency factor, In A
168 f 17 103 f 7 96 f 9 85f8 I36 f 12 175 f 12
27.54 f 2.78 19.34 f 1.39 17.07% 1.51 13.57 f 1.27 24.43 =!r2.03 29.90 f 2.10
An analysis of the rate data that Szczygiel and Stolarski [40] provided for their model suggests Arrhenius behavior for all the reactions. Activation energies calculated from the data are shown in Table 6-9. The lower values of activity energies indicate the effectiveness of the natural aluminosilicates as catalysts for the cracking of coal-derived asphaltenes. Phillippopoulos and Papayannakos [41] studied the cracking and desulfurization of asphaltenes obtained from a Greek atmospheric residue in an integral trickle bed reactor using Co-Mo/Al?O3 catalysts. They found the asphaltene cracking reaction to follow a second-order behavior with respect to the asphaltene concentration. Schucker [33] determined coking kinetics of Arab heavy vacuum residuum and its constituent fractions, including asphaltene, by thermogravimetry. The residuum was divided into four fractions, namely: asphaltenes, polar aromatics, aromatics and saturates. The coking characteristics of the residue and its four fractions were determined.
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
141
ASPHALTENE
Fig. 6- 10. Coal-derived asphaltene crackmg mechanism TABLE 6-9 Kinetic parameters for the cracking of asphaltene derived from coal Rate constant
Activation energy, E (kJ/mol) 65 37 56 95 17
Activation energies and frequency factors were determined at various conversion levels and were found to increase with conversion level. Kinetic parameters for the asphaltene fraction are given in Table 6-10.
MODEL COMPONENT STUDIES
Kinetic models based on lumped solubility class or boiling point range groups are useful tools. They, however, do not provide much insight into the reaction mechanisms. Inasmuch as asphaltene is an ill-defined solubility class consisting of a number of different compounds of differing structures and chemical properties, it is not possible to study the fundamentals of the reaction mechanisms using asphaltenes directly. Model compounds, representing a given group of asphaltene constituent, are used in studies attempting to elucidate the reaction mechanism. The major constituents of petroleum asphaltenes are variable sizes of condensed aromatic and naphthenic ring systems with alkyl side chains, sulfide linkages and bridges between the rings [7,42-44,17,45,46]. Alkylaromatic compounds can be con-
142
A ('IIAKMA
TABLE 6-10 Kinetic parameter\ for the cracking of asphaltene fractions derived from Arabian vacuum residuuni (after Schuckcr [ 33 I )
20 30 40
SO 60 70 XO
90
Activation energy, E (kJ/mol)
(s
176.0 173.5 180.6 203.6 213.2 211.9 280.9 336.5
2.27 x 1.47 x s.21 x 2.53 x 1.40 x 1.22 x 7.90 3.12 x
Frequency factor 1)
10"' 10"' 10"'
lo'? 10" 10" 1017
10''
sidered to be the key components of the asphaltene molecule. Among them, polycyclic alkylaromatic compounds are probably the most prevalent ones. Pyrolysis of singly substituted alkylcyclohexane and long-chain ti-alkylaromatic compounds have been the subject of a number of studies. Fabuss et al. [47] studied the pyrolysis of a number of saturated hydrocarbons including cyclohexane and methyl-, ethyl-, propyl-, and n-butylcyclohexane and determined the activation energies for their formation. Mushrush and Hazlett [48] pyrolyzed tridecylcyclohexane at 450°C and found n-alkanes, 1 -alkenes, alkylcyclohexanes, toluene, and benzene as the reaction products. At high-temperature and low-pressure conditions the reactions were found to follow the free-radical mechanism. The authors did not provide enough kinetic information. Trahanovsky and Swenson [49] carried out flash pyrolysis of 1-methyl- and 2-methyltetralin at 700-900°C under vacuum (0.1 Torr) conditions. They found napthalene and 1.2-dihydronaphthalene to be the major reaction products indicating the ease of cleavage of the alkyl groups. n-pentadecylbenzene (PDB) is a typical prototype of the alkylaromatic moiety present in asphaltene. Savage and Klein [50] studied the pyrolysis of PDB to gain insight into the reaction mechanism of the alkylaromatic constituent of the asphaltenes. The temperature was varied from 375"to 450°C. Pyrolysis resulted in the production of two major product pairs: toluene plus 1-tetradecene and styrene plus n-tridecene. They found the thermolysis reaction to be of free-radical type and the reaction to be of first order with an activation energy of 55.45 kcal/mol and a frequency factor of 14.04 s-' . They proposed the reaction mechanism shown in Fig. 6- 1 1.
r +-@[+IT -'+@[ k21
1k31
k41
Fig. 6-1 I . Reaction mechanism for PDB cracking (after Savage and Klein [SO]).
143
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6- I I Kinetic parameters for PDB cracking. (After Savage and Klein [ 5 0 ] ) Rate constant
Activation energy, E (kcal/mol)
Frequency factor, log A
ki
55.5 47.3
14.0
k2
ki
kJ
16.6 18.8
(SKI)
12.3 2.5 53.9
The corresponding kinetic constants for the first-order rate equations are given in Table 6- 1 1. The rate equations based on these kinetic constants may not be directly applicable to the alkylaromatic constituent of the asphaltene due to likely diffusional limitations provided by the large asphaltene structure to the movement of free radicals generated during the decomposition. The same authors [50]also studied the thermal decomposition of 1-phenyldodecane (PDD) and concluded that PDD thermolysis followed the free-radical mechanism, contradicting the earlier findings of Blouri et al. [51], who suggested that it was molecular a reaction. Savage and Klein [52] also studied the reaction mechanisms of the alkylhydroaromatic and alkylnaphthenic groups of asphaltene by pyrolyzing model compounds representing these two groups. 2-ethyl tetralin (ET) was chosen as a model compound to represent the alkylhydroaromatic group, whereas n-tridecylcyclohexane (TDC) was the model compound for the alkylnaphthenic group. Pyrolysis of TDC yielded cyclohexane, methylenecyclohexane, I-tridecene, and n-dodecane as major products. Cyclohexane, methylcyclohexane, n-tridecane, other n-alkanes, a-olefins, alkylcyclohexane, and cyclohexylalkenes were formed as minor products. The disappearance of TDC followed first-order kinetics with an activation energy of 59.4 kcal/mol and a frequency factor of 7.94 x IOl4. Pyrolysis of ET-produced napthalene, tetralin, dialin, 2-ethylnaphthalene, and 2-ethyldialins as major products. Toluene, methylindan and substituted benzenes were the minor reaction products. Like TDC, the disappearance of ET also followed first-order kinetics. The activation energy was 53.5 kcal/mol and the frequency factor was5.01 x 10”. Savage and Klein [52] concluded that pyrolysis of TDC and ET follow the freeradical reaction mechanisms. The dominant reactions involved dealkylation near or at the ring. Ring opening reactions were considered to be minor reactions. Pyrolysis also resulted in significant dehydrogenation of the model compounds as indicated by the formation of napthalene and toluene from ET and TDC, respectively. These observations, when applied to asphaltenes, clearly suggest that asphaltene cracking primarily involves the cleavage of the peripheral aliphatic substituents followed by aromatization of the saturated rings. As a result of thermal cracking, the remaining asphaltene will become hydrogen-deficient and more aromatic. Zou et al. [53] studied the structural changes in the Cold Lake bitumen due to in situ combustion and also found the aromaticity of the
144
A. CIIAKMA
simulated fire-flood-produced oil to be higher than that of the original feedstock due to the loss of alkyl side chains and the dehydrogenation of the alkyl ring systems. Smith and Savage [54] pyrolyzed I-dodecylpyrene (DDP) and found the major reactions products to be similar to those observed for alkylbenzene pyrolysis. At higher conversion levels, however, n-dodecane and pyrene were found to be the major reaction products, suggesting the cleavage of the strong alkyl-aryl C-C bond. Smith and Savage [ 5 5 ] also pyrolyzed I-methylpyrene and 1-ethylpyrene and found hydrogenolysis to be the dominant mechanism. Pyrene and dimethylpyrene were found to be the major reaction products due to the pyrolysis of 1-methylpyrene, whereas pyrene and methylpyrene were the major pyrolysis products of 1 -ethylpyrene. The formation of these major reaction products suggested that radical hydrogen transfer was not a valid mechanism for these systems.
IN SITU COMBUSTION
Adegbesan et al. [3,56]have studied the reactions of Athabasca bitumen with oxygen and found the maltene fraction to oxidize to asphaltene and coke fractions, whereas the asphaltene fraction oxidized to coke fraction. They proposed a series of models describing formation of asphaltenes/coke from bitumen and its various components. The first model, as shown below, was formulated to provide the oxygen consumption rate for the overall oxidation reaction.
MODEL 1 Bitumen + Oxygen
k,
Products
The rate constant k l can be calculated from:
k , = 2.19 x 10”exp
~
I
125~44
where R = the universal gas constant, Pa m3/kmol K, and T = temperature, K. The above model was able to predict the oxygen consumption rate quite well. In addition, the authors also proposed four kinetic models for the liquid-phase reactions. They are as follows:
145
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
-
MODEL 2
k*
Maltenes
Asphaltenes-coke
MODEL 3 k3
Oils
k4
Resins
Asphaltenes-coke
---+
> <
MODEL 4
Resins1
> - Oils
Asphaltenes-coke
Resins11
MODEL 5
k,
k,,
kg
oils
A
Resins I
kl1
Resins II
Asphaltenes-coke
The authors found the above reactions to be of first order with respect to the pseudo-components and fractional order with respect to oxygen partial pressure. All the reactions exhibited Arrhenius behavior. Inasmuch as it was not possible to distinguish the effect of oxygen solubility, the authors calculated pseudo-activation energies in which the solubility effects were also lumped. The rate parameters for the above reactions are presented in Table 6-12. Except for klo. all the activation energies seem to be quite reasonable. The very high value of activation energy for klo suggests Model 5 to be unrealistic.
TABLE 6-12 Kinetic parameters for various reactions involving asphaltene formation during in situ combustion (after Adegbesan et al. [56]) Rate constant Pseudo-activation energy (J/mol) 8 I .93 77.23 86.73 77.75 52.37 89.04 79.14 74.92 4877.27 89.14
Pseudo-frequency factor Reaction order with respect to oxygen (h-' kPa-'I) n 1.00 x 2.64 7.69 x 1.81 4.64 x 1.94 x 4.48 x 1.16 x 7.78 7.78 x
108 107
lox 107 10' 10'
107 10' 107 107
0.41 0.41 0.50 0.41 0.50 0.50 0.50 0.41 0.50 0.50
146
A CIIAKMA
CONCLUSIONS
A review of asphaltene cracking kinetics has been made. Inasmuch as asphaltenes constitute a complex solubility class, it is not possible to determine a comprehensive reaction mechanism based on reaction fundamentals. Model compound studies can provide some insight into the reaction mechanisms involved in the cracking of various constituent groups of the asphaltenes. It is still not possible, however, to use kinetic data obtained from model compound studies to predict the formation of various groups due to asphaltene cracking. Kinetic models based on pseudo-component approach, in which the reaction products are divided into a number of easily quantifiable pseudo-components, are available. These models are practical tools for engineering calculations involving asphaltene cracking in petroleum recovery and processing applications.
REFERENCES
[ I ] Danesh, A.. Krinis, D., Henderson, G.D. and Penden, J.M.. Asphaltene deposition in miscible gas flooding of oil reservoirs. Chm. Eng. Rcs. Des.. 66: 339 ( 1988). [2] Abu-Khamsim. S.A.. Brigham. W.E. and Ramey, H.J., Reaction kinetics of fuel formation for in situ combustion. SPE Reservoir Engineering. Nov.: I308 ( 1988). [ 3 ] Adegbesan. K.O.. Donnelly. J.K., Moore, R.G. and Bennion. D.W.. Low-temperature oxidation kinetic parameters for in situ combustion: numerical simulation. S P E Reserwrr Enginwring. Nov.: 574 ( 1987). [4] Alexander. J.D., Martin, W.W. and Dews, J.N.. Factors affecting fuel availability and composition during in situ combustion. J. Pet. Techno/.,Oct.: I154 (1962). [ S ] Bennion. D.W., Donnelly, J.K. and Moore, R.G., A laboratory investigation of wet combustion in the Athabasca oil sands. The Oil Sands of Canada-Venezuela. Cm. Inst. Min. Metull. Spec. Ed., 17: 554 ( 1977). 161 Jha. K.N. and Verkoczy, B.. The role of thermal analysis techniques in the in situ combustion process. SPE Reservoir Engineering, July: 329 ( 1986). [71 Asakoa. S.. Nakata. S., Shiroto. Y. and Takeuchi, C., Asphaltene cracking in catalytic hydrotreating ~ of heavy oils. 2. Study of changes in asphaltene structure during catalytic hydroprocessing. I J I Eng. C/WJll. PrOC.CJ.S.SD ~ v .DPV., 22: 242 ( 1983). 181 Cebolla, V.L.. Diack. M., Oberson, M., Bacaud. R.. Cagniant, D. and Nickel-Pepin-Donat. B., Effect of various catalysts on the chemical structure of oils and asphaltenes obtained from the hydroliquetaction of a highly volatile bituminous coal. Furl Procrss. Techno/., 28: 183 ( I991 ). [ 9 ] Chakma. A., Chornet, E., Overend. R. and Dawson, W.H.. Hy/mvisbreuking of Arhubusca hirunten in (I high .shcwr jet r w c t o r . Department of Energy. Mines and Resources. Ottawa. CANMET Report 88-58 (OP) (1988). [ 101 Nomura, M.. Terao. K. and Kikkawa. S., Hydrocracking of athabasca asphaltene over ZnCIZ-KClNaCl melts with or without an additive of some transition metal chlorides. F d , 60: 699 (198 I ). [ 11J Takeuchi, C., Fukul. Y.. Nakamura, M. and Shiroto, Y., Asphaltene cracking in catalytic hydrotreating of heavy oils, 1. Processing of heavy oils by catalytic hydroprocessing and solvent dcasphalting. Ind. EJIK.C h ~ iProcess . Dec, 22: 236 ( 1983). [ 121 Dickie. J.P. and Yen. F.T., Macrostructures of the asphaltic fractions by various instrumental methods. A n d . Chfwf.. 39: I847 ( 1967). [ I31 Speight. J.G., Thermal cracking of Athabasca bitumen. Athabasca asphaltenes and Athabasca deasphalted heavy oil. Fuel, 49: 134 ( 1970). [ 141 Could, K.A.. Chemical depolymerization of petroleum asphaltenes. FIIPI,57: 756 (1978). [ I S ] lgnasiak. T.. Bimer. J . . Samman, N., Montgomery. D.S. and Strausz. O.P.. In: J.W. Bunger and N.C. Li (Editors). Cltemisrn of Asphaltcwes. Advances in Chemistry Serie\ 195, American Chemical Society, Washington. DC, p. I83 (I98 I ).
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
147
[I61 Yen. F.T., Structure of petroleum asphaltene and its significance. Energy Sources, I: 447 (1974). 1171 Speight, J.G., Latest thoughts on the molecular nature of petroleum asphaltenes. P r e p Am. Chem. Soc., Div. Fuel Chem., 34 (2): 321 (1989). [I81 Mojelsky, T.W., Ignasial, T.M., Frakman, Z., McIntyre, D.D., Lown, E.M., Montgomery, D.S. and Strausz, O.P., Structural features of Alberta oil sand bitumen and heavy oil asphaltenes. Energy Fuels, 6: 83 (1992). [ 191 Ali, L.H., Al-Ghannam, K.A. and Al-Rawi, J.M., Chemical structure of asphaltenes in heavy crude oils investigated by n.m.r. Fuel, 69: 5 19 (1990). [20] Boduszynski, M., Chada, B.R. and Szkuta-Pochopien, T., Investigations on Romashkino asphaltic bitumen, 3. Fractionation of asphaltenes using ion-exchange chromatography. Fuel, 56: 432 ( 1977). [21] Koots, J.A. and Speight, J.G., Relation of petroleum resins to asphaltenes. Fuel, 54: 179 (1975). [22] Speight, J.G., Structural analysis of Athabasca asphaltenes by proton magnetic resonance spectroscopy. Fuel, SO: 102 (1971). [23] Ritchie, R.G.S., Roche, R.S. and Steedman, W., Pyrolysis of Athabasca tar sands: analysis of the condensible products from asphaltene. Fuel, 58: 523 (1979). [24] Savage, P.E. and Klein, M.T., Asphaltene reaction pathways, 5. Chemical and mathematical modelling. Cheni. Eng. Sci., 44 (2): 393 (1989). [25] Simm, I. and Steedman, W., Straight-chain alkyl chain lengths in petroleum asphaltenes. Fuel, 59: 669 ( 1980). [26] Savage, P.E.. Klein, M.T. and Kukes, S.G., Asphaltene Reaction Pathways, I . Thermolysis. Ind. Eng. Chem. Process Des. Dev., 24: 1 169 ( 1985). [27] Rice, F.O., The thermal decomposition of organic compounds from the standpoint of free radicals, 111. The calculation of the products formed from paraffin hydrocarbons. J. Am. Cheni. Soc., 55: 3035 (1933). [28] Kossiakoff, A. and Rice, F.O., Thermal decomposition of hydrocarbons, resonance stabilization and isomerization of free radicals. J. Am. Chem. Soc., 65: 590 (1943). [29] Kunzru, D., Shah, Y.T. and Stuart, E.B., Thermal cracking of n-nonane. Ind. Eng. Cheni. Process Des. Dev., 11: 605 (1972). [30] Gates, B.C., Katzer, J.R. and Schuit, G.C., Chemistry of Catalytic Processes. McGraw Hill, New York, p. 10 (1979). [31] Bloun, B., Giraud, J., Nouri, S. and Herault, D., Steam cracking of high-molecular weight hydrocarbons. Ind. Eng. Chem. Process Des. Dev., 20: 307 (1981). [32] Moschopedis, S.E., Prakash, S. and Speight, J., Thermal decomposition of asphaltenes. Fuel, 57: 431 ( 1978). [33] Schucker, R.C., Thermogravimetric determination of the coking kinetics of Arab Heavy Vacuum Residuum. Ind. Eng. Chem. Process Des. Dev., 22: 615 (1983). [34] Voge, H.H., Catalytic Cracking. Catalysis, Vol. VI, Nostrand Reinhold, New York. p. 407 (1958). [3S] Hayashitani, M., Bennion, D.W., Donnelly, J.K. and Moore, R.G., Thermal cracking models for Athabasca oil sands oil. Proc. of Oil Sands of Canada-Venezuela Symposium, CIM Spec. Publ. 17, 233, 1977. [36] Hayashitani, M., Bennion, D.W., Donnelly, J.K. and Moore, R.G., Thermal cracking models for Athabasca oil sands oil. SPE 7549, paper presented at the 53rd Annual Fall Technical Conference and Exhibition of the SPE, Houston, October 1-3 (1978). [37] Phillips, C.R., Haidar, N.I. and Poon, Y.C., Kinetic models for the thermal cracking of Athabasca bitumen -the effect of the sand matrix. Fuel, 64:678 (1985). [38] Kiiseoglu, R.O. and Phillips, C.R., Kinetics of non-catalytic hydrocracking of Athabasca bitumen. Fuel, 66: 741 ( 1987). [39] Belinko, K. and Denis, J.M., A review of some chemical aspects of the f o m t i o n cf coke during thermal hydrocracking of bitumen. Department of Energy, Mines and Resources, Ottawa, CANMET Report 43 (1977). [401 Szczygiel, J. and Stolarski, M., Mathematical analysis of hydrogenating depolymerization of asphaltenes from coal extracts. Fuel, 67: 1292 (1988). [4 I] Phillippopoulos, C. and Papayannakos, N., Intraparticle diffusional effects and kinetics of desulfuriza-
148
A. C H A K M A
tion reactions and asphaltenes cracking during catalytic hydrotreatment of a residue. Ind. Eng. Choni. Res.. 27: 415 (1988). 1421 Cyr. N., Mclntyre, D.D., Toth, G. and Strausz, O.P.. Hydrocarbon structural group analysis of Athabasca asphaltene and its g.p.c fractions by "C n.m.r. Fuel, 66: 1709 (1987). 1431 Payzant, J.D., Lown, E.M. and Strausz, O.P., Structural units of Athabasca asphaltene: the aromatics with a linear carbon framework. Energy Fuels. 5 : 445 (1991). 1441 Speight, J.G. and Moschopedis, S.E., In: C h e n i I s ~qf Asphu1tene.s. Advances in Chemistry Series 195, American Chemical Society, Washington, DC, I (1983). 1451 Takegami. Y..Watanabe. Y., Suzuki, T., Mitsudo, T. and Itoh. M.. Structural investigation on column chromatographed vacuum residues of various petroleum crudes by "C nuclear magnetic spectroscopy. Fuel. 59: 253 ( 1980). 1461 Yen. T.F., Wu, W.H. and Chilingar, G.V., A study of the structure of petroleum asphaltenes and related substances by infrared spectroscopy. Energy Sources, 7: 203 (1984). 1471 Fabuss. B.M.. Kafesjian, R., Smith, J.O. and Satterfeld, C.N.. Thermal decomposition of saturated cyclic hydrocarbons. Ind. Eng. Chmi. Process Des. D w , 3 : 248 ( 1964). [48] Mushrush, G.W. and Hazlett, R.N.. Pyrolysis of organic compounds containing long unbranched alkyl groups. Ind. Eng. Cheni. Funduni., 23: 288 (1984). [49] Trahanovsky, W.S. and Swenson, K.E., Flash vacuum pyrolysis of 2,3 dialkyltetralins. J. Org. Chcwi., 46: 2984 ( 198 I ). [SO] Savage, P.E. and Klein, M.T., Discrimination between molecular and free-radical models of l-phenyldodecane pyrolysis. Ind. Eng. Chern. R e x , 26: 374 (1987). [ S I ] Blouri, B., Hamdan, F. and Herault, D., Mild cracking of high molecular weight hydrocarbons. Ind. Eng. Cheni. Process Drs. Dev., 24: 30 (1985). 1521 Savage, P.E. and Klein, M.T., Asphaltene reaction pathways, 4. Pyrolysis of tridecylcyclohexane and 2-ethyltetralin. Ind. Eng. Chern. Res., 27: 1348 (1988). 1531 Zou. J., Gray, M.R. and Thiel, J., Structural changes in Cold Lake Bitumen due to in situ combustion. AOSTRA J. Res.. 5: 75 (1989). [54] Smith. C.M. and Savage, P.E., Reactions of polycyclic alkylaromatics, I . Pathways, kinetics and mechanisms for I -dodecylpyrene pyrolysis. Ind. Eng. Chmi. Rev.. 30: 33 1 ( I99 I ). [ S S ] Smith, C.M. and Savage. P.E., Reactions of polycyclic alkylaromatics. 4. Hydrogenolysis mechanisms in I-alkylpyrene pyrolysis. Energy Fuels, 6: 195 (1992). 1561 Adegbesan, K.O., Donnelly, J.K., Moore, R.G. and Bennion, D.W., Liquid phase oxidation kinetics of oil sands bitumen: models for in situ combustion numerical simulators. AlChEJ., 32 (8): 1242 (1986).
129
Chapter 6
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING DURING PETROLEUM RECOVERY AND PROCESSING OPERATIONS A. CHAKMA
INTRODUCTION
Many bitumen and heavy oils produced around the world contain asphaltenes. Asphaltenes play an important role in the recovery, transportation, processing and utilization of bitumen and heavy oil-derived products. For example, asphaltenes may precipitate during recovery and transportation of bitumens/heavy oils [ 11. During the upgrading operations, the presence of bitumen may result in the formation of excessive coke in the upgrading reactor. In addition to the above-mentioned implications, asphaltenes are basically heavy-molecular-weight aromatic hydrocarbons with other impurities. Therefore, they represent certain energy values, which should be recovered. This can be achieved by cracking the complex asphaltene structure to obtain useful fuels. In petroleum recovery operations, such as in situ combustion, cracking of asphaltene not only results in lighter useful oil fractions, but the heavier components combine to form coke which serves as the fuel to sustain the in situ combustion process [2-61. On the processing front, cracking of asphaltenes into lighter useful forms requires innovative processing of the feedstock, frequently in the presence of catalysts [7-111. Alberta bitumens and heavy oils may be considered to be composed of macroand micro-structures, as well as chemical constitutive molecules. The macro- and micro-structural arrangements determine the viscosity of the bitumen. Dickie and Yen [I21 investigated the macro-structures of the asphaltenes and found them to be composed of polynuclear aromatic molecules with alkyl chains as attachments. These constitutive ‘asphaltene unit molecules’ are grouped in layers having several unit molecules (typically 5 or 6) surrounded or immersed into the maltene (fraction soluble in 40 volumes of n-pentane) fluid. The latter is composed of free saturates, mono- and di-aromatics, and resins that may be associated with the asphaltenes. This structural organization may be considered to be the micro-structure of the system. The micro-structure may form aggregates to reduce the free energy of the system. These aggregates form micellar structures that consist of several unit layers of asphaltenes surrounded by or associated with the maltenes. The micellar structures thus formed may be classified as the macro-structures of the system. The macro-structural arrangement to a large extent depends on the size of the unit asphaltene layers. Low-molecular-weight asphaltenes (MW up to about 1000) consist of single sheets of condensed aromatic and naphthenic rings with relatively alkyl chain attachments. Larger-molecular-weight asphaltenes may consist of several sheets of condensed ring systems. Speight [ 131 proposed a hypothetical structure consisting
130
A. C H A K M A
TABLE 6- I Structural parameters for asphaltene (after Savage and Klein [ 2 6 ] ) Atomic H/C ratio Fraction of C atoms in aromatic rings Fraction of C atoms in saturated rings Fraction of H atoms in aromatic rings Fraction of H atoms in saturated rings Ratio of peripheral aromatic C atoms over total aromatic carbon atoms
1.09-1.29 0.30-0.61 0.060.24 0.04-0. I I 0.164.19 0.314.55
of 12 condensed aromatic rings for Athabasca asphaltene. These individual structures may be connected together by various aliphatic, naphthenic or heteroatomic linkages [14-161. A review on the molecular structure of asphaltenes has been provided by Speight [ 171. Mojelsky et al. [ 181 have utilized Ru ion catalyzed oxidation reaction to determine various structural features of asphaltenes derived from different Alberta Oil Sand bitumen and heavy oils. The authors report on the determination of total number of alkyl chains, chain length and the number of carbon atoms with each chain and also on the total number and length of the bridge connecting two aromatic rings in the asphaltene molecule. The length of the alkyl chain has been the subject of numerous studies. Spectrometric studies of asphaltene structure suggests the alkyl chains to be 3 to 6 carbon atoms long [ 19-22]. However, analysis of the asphaltene pyrolysis products reveal the presence of alkyl chains of up to carbon number C3,, [13,23-251. These alkyl chains are thought to be covalently bound to the core asphaltene unit. These bonds can be broken with activation energies of about 50 kcal/mol [26]. A summary of the structural parameters of asphaltenes is given in Table 6-1. C-C bonds in aromatic rings are the most abundant. They are the most stable ones. However, the aliphatic C-C bonds found in alkylaromatic, alkylhydroaromatic and alkylnaphthenic positions are the most reactive ones in the temperature range of 300" to 550°C [24].
KINETICS
Thermal cracking of hydrocarbons has generated much interest since the early days of the industrial revolution. The first systematic theory on thermal cracking was postulated by Rice in 1931 [27]. He proposed the formation of free radicals for the decomposition of the hydrocarbons. For light hydrocarbons, the theory of Kossiakoff and Rice [28] can predict the product distributions very well. For example, product distribution arising from the cracking of n-nonane [29] can be predicted within 20% of the experimental data using the Kossiakoff and Rice [28] theory. Gates et al. [30] found the theory to be able to predict product distribution resulting from the thermal cracking of n-hexadecane well. However, the predictions for the cracking of high-molecular-weight hydrocarbons deviate considerably [3I]. Blouri et al. [31] studied the cracking of n-hexadecane, 6-methyleicosane
131
KINETICS A N D MECHANISMS OF ASPHALTENE CRACKING
and 1 -phenyldodecane under relatively mild temperature (350" to 440°C) under high pressure (20 bar). They found the cracking process to follow a molecular mechanism; however, the kinetic data were similar to those of radical cracking.
ASPHALTENE
A number of studies on the cracking of asphaltenes have been reported in the literature. Most of them, however, dealt with the determination of the asphaltene structures. Moschopedis et al. [32] carried out thermal decomposition of asphaltenes in a horizontal tube reactor and analyzed the gaseous and liquid products. Ritchie et al. [23] pyrolyzed Athabasca asphaltenes and analyzed the volatile products by GC/MS. Schucker [33] reported a thermogravimetric study of the coking kinetics of Arab heavy vacuum residuum. As a part of his investigation, he also studied the coking kinetics of the asphaltene fraction derived from the Arab heavy vacuum residuum. Inasmuch as the kinetic study was purely based on the weight loss measurement, it was not possible to analyze the reaction products. Therefore, the kinetic data obtained were for the overall disappearance of the asphaltenes. The authors found the activation energies to increase with conversion ranging from 176 kJ/mol at 20% conversion to 336.5 kJ/mol at 90% conversion. This is quite understandable. First, the lighter fractions of the asphaltenes volatilize followed by the heavier fractions. Therefore, the initial activation energies are lower, representing the lighter fractions; however, they increase when the heavier fractions are cracked. Major constitutive asphaltene groups and their primary cracking products are listed in Table 6-2.
TABLE 6-2 Principal reaction and reaction products obtained from the cracking of major hydrocarbon groups belonging to the asphaltene molecule (adapted from Voge, [34]) Hydrocarbon class
Structure
Aromatic compounds without side chains
Aromatic compounds with side chains
Principal reactions
Principal products
Negligible cracking
Coke
a-
Cleavage of peripheral chains
Aromatic compounds and olefins
Cleavage of peripheral chains and opening of napthenic ring
Aromatic compounds, olefins and paraffins
-\
Naphtheno-aromatic compounds
132
A . C'IIAKMA
The groups shown in Table 6-2 are typical ring structures present in the asphaltenes. Whereas some information on asphaltene cracking characteristics may be obtained by studying the cracking mechanisms of these typical structures, it is not practical to determine reaction kinetics for asphaltene cracking from the cracking studies involving individual groups due to the complexity of the asphaltene structure and the presence of many other minor groups. As a result, kinetic studies on asphaltene cracking usually involve an easily quantifiable pseudo-component approach, where reaction products are grouped according to solubility and/or boiling point characteristics. Hayashitani et al. [35] were the first investigators to carry out extensive kinetic experiments on the cracking of Athabasca bitumen using a pseudo-component approach. Although the focus of their study was on Athabasca bitumen, various reaction mechanisms postulated by the authors centered around asphaltenes. Hence, the kinetic models proposed can also be classified as asphaltene cracking models. These authors carried out thermal cracking of Athabasca bitumen, asphaltenes, and heavy oils (BP 400"Cf) and oils (BP 200" to 400°C) fractions separated from the original bitumen, at constant temperatures in a batch reactor. The reactor was equipped with a quartz tube liner, which prevented the reactants and the reaction products from directly contacting the vessel surface. An inert atmosphere was maintained inside the reactor by pressurizing it with helium. The temperature range studied was from 303°C to 452°C. The reaction products were separated into six pseudo-components: coke, asphaltenes, heavy oils (BP 400"C+), middle oils (BP 200" to 400"C), light oils (BP 20" to 200°C) and gases. The cracking of bitumen was found to yield all the above six pseudo-components. When asphaltenes were used as feed material, large amounts of coke and heavy oils were obtained along with small amounts of middle and light oils and gases. Cracking of the heavy oil fraction led to the formation of mainly middle and light oils and some asphaltenes. Coke formation from the heavy oil fraction was found to be negligible. Finally, the middle oil fraction yielded mostly light oils. Based on the above observations, Hayashitani et al. [36] proposed four reaction mechanisms. They correlated the reaction rate constants for each of the reaction mechanisms by an Arrhenius type expression. The activation energies and the frequency factors are given in Tables 6-3 and 6-4.
TABLE 6-3 Frequency factors
(in
s
I)
for the diffcrent models
~
Model Model Model Model
HI H2 H3 H4
5.04 x 10" 1.41 x 10" 2.04 x 10"' 1.01 x 10"
1.17 x 1.53 x 6.07 x 1.40 x
10''
10'' 10" 10'"
1.91 x 8.44 x 1.18 x 7.28 x
10''
10" 10'' 10''
4.42 x 10" 4.50 x l0li 4.45 x 10'' 1.43 x 10"
4.44 x 10''
-
4.55 x 10" 8.18 x 10'' 1.51 x lo"
1.49 x 10'' 2.90 x 10" 3.00 x lo"
133
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6-4 Activation energy (in kcal/mol) for the different rate models
Model HI Model H2 Model H3 Model H4
48. I 46.5 44.2 46.3
67.8 65.2 65. I 63.8
64.I 62.8 60.3 59.6
57.5 57.5 57.4 56.2
59. I 58.9 53.6 57.6
-
57.4 55.9 55.6
The corresponding reaction mechanisms are summarized as follows: Model H1
In their first model, Hayashitani et al. [36] lumped middle oils, light oils and gases into a single pseudo-component, called distillable oil, and postulated the scheme shown in Fig. 6-1. This model was able to provide reasonable predictions of the concentration versus time curves for the various pseudo-components. Addition of an additional reaction, asphaltene --f distillable oil, did not improve the model’s performance. Among the major drawbacks of this model was its inability to predict (i) the initial sharp decrease in the asphaltene concentration, (ii) initial increase in the heavy oil production, and (iii) initial delay in the coke production. Model H2
Because gases belong to a distinct group themselves, in their second model, Hayashitani et al. [36] separated gases from the distillable oil fractions. The model presented in Fig. 6-2 is the result. This model predicted the formation of gases very well, whereas the prediction for
ASPHALTENE
c-223 Fig. 6- I . Model HI - asphaltene cracking model of Hayashitani et al. 136J.
134
A CHAKMA
<_> HEAVY OIL
Fig. 6-2. Model H2
~
asphaltene cracking model of Hayashitani et al. 1351.
other pseudo-components were similar to Model H1. However, it was not able to overcome the earlier-mentioned limitations of Model H 1 .
Model H 3
In their third model, Hayashitani et al. [35] subdivided the asphaltene fraction into two subfractions, asphaltene-1 and asphaltene-2, in an effort to improve the model performance in describing the initial sharp decrease in asphaltene concentration. Otherwise, Model H3 was similar to Model H1. Asphaltene-1 subfraction was assumed to yield only heavy oil, whereas the asphaltene-2 subfraction was assumed to produce both heavy oil and coke upon cracking. Model H3 is summarized in Fig. 6-3. Model H3 was able to provide a somewhat better representation of the concentration versus time curves for the asphaltene and coke fractions compared to Models HI and H2.
k41 f
k5
Fig. 6-3. Model H3 - asphaltene cracking model of Hayashitani el al. 1351.
a <->
KINETICS A N D MECHANISMS OF ASPHALTENE CRACKING
k5
135
ASPHALTENE 2
Fig. 6-4. Model H4 - asphaltene cracking model of Hayashitani et al. 1361.
Model H4
Model H4 is a combination of Models H2 and H3 and as such incorporates all the favorable features of these two models. It is summarized in Fig. 6-4. Savage et al. [26] pyrolyzed asphaltenes separated from offshore Californian crude oil by precipitation with 40 vol of n-heptane at temperatures ranging from 350" to 565°C in batch reactors to study asphaltene reaction mechanisms. They assumed an asphaltene unit to consist of condensed aromatic ring systems with heleroatomic, alkyl and naphthenic substituents. These single units were linked through linkages located along their peripheries. At low temperatures, they found that HzS and CO? are the primary products and attributed their formation to the fission of peripheral thioether and carboxylic acid moieties, respectively. Hydrocarbon gases, cycloalkanes, and paraffins were the primary products for high-temperature conditions arising from the fission of C-C bonds. Based on their experimental observations, they concluded that coke, maltenes and gas are produced from the primary reactions. Maltenes are cracked into lower-molecular-weight maltenes and additional gas is produced from the coke and lower-molecular-weight maltene fractions. The reaction scheme is shown in Fig. 6-5. However, these authors do not provide any rate constants. Phillips et al. [37] studied the thermal cracking of Athabasca bitumen and its components in a batch reactor at temperatures ranging from 360" to 420°C in the presence of sand. They divided the products into six pseudo-components: coke, asphaltenes, heavy oils, middle oils, light oils and gases. They proposed the following two kinetic models. Model PI
In the first model (Model Pl), following Hayashitani's Model H1, they combined the middle oils, light oils and the gases into a single pseudo-component called distillables. They proposed the reaction mechanism shown in Fig. 6-6.
136
A CHAKMA
Fig. 6-5. Asphaltene cracking mechanism of Savage et al. [26).
D ASPHALTENE
D HEAVY OILS
DISTILLABLES
Fig. 6-6. Model PI
~
asphaltene cracking mechanism of Phillips et al. 1371
They found the reaction kinetics to be of first order and to follow the Arrhenius relationship. The presence of sand was found to affect the distribution of products and resulted in the lowering of activation energies of the reactions. The authors attribute this effect to the catalytic role played by the sand matrix. Table 6-5 shows a comparison of the kinetic parameters for the model in the presence of sand with the thermal cracking data of Hayashitani et al. [36].
Model P2 The second model of Phillips et al. [37] included all the six pseudo-components, as shown in Fig. 6-7. In Table 6-6, the kinetic constants for this model are compared with those obtained from thermal cracking studies of Hayashitani et al. [36]. Hayashitani et al. [36] obtained high activation energies for the reactions asphaltene
137
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6-5 Comparison of kinetic parameters of Model P1 with the thermal cracking data of Hayashitani et al. [36] Rate constant
Activation energy, E , with sand (kJ/mol)
Activation energy, E , without sand (kJ/mol)
Frequency factor
20 I 284 268 24 1 248
1.16 x 6.37 x 5.61 x 5.22 x 2.04 x
W I )
10" 10"
10Ih 10"
10l3
c
kg
Fig. 6-7. Model P2 - asphaltene cracking mechanism presented by Phillips et al. [37]. TABLE 6-6 Kinetic constants for Model P2 with or without the presence of sand Rate constant
Activation energy, E , with sand (kl/mol)
Activation energy, E , without sand
Frequency factor
( kl /mol)
W I )
~
195 273 263 24 I 247 240
1.16 x 4.99 x 1.41 x 1.91 4.04 x 3.79 x
10" 10'9 10'' 1017 10'4
1OlX
+ maltenes (224 f 15 kJ/mol) and maltenes + asphaltene (284 f 28 kJ/mol). They attributed these high levels of activation energy to the heterogeneity of the maltene fraction.
138
A CtIAKMA
ASPHALTENE
a MALTENE
D GASES
Fig. 6-8. Non-catalytic hydrocracking model of Kiiseoglu and Phillips [38]
Model KPI
In a follow-up work, Koseoglu and Phillips [38] on the hydrocracking of Athabasca bitumen proposed a kinetic model for bitumen decomposition by dividing the bitumen into four pseudo-components, namely, coke, asphaltenes, maltenes and gases. They found the activation energy for the asphaltene -+ coke reaction to be lower than that of the thermal cracking. They also concluded that coke formation from resins occurs via asphaltene intermediates. The model is summarized in Fig. 6-8. The kinetic constants for the above model are listed in Table 6-7 and are compared with those obtained from thermal cracking studies of Hayashitani [36]. As can be seen from Table 6-7, the activation energies for kz and k3 are higher than those encountered for the hydrocracking of simple hydrocarbon molecules. The authors attribute the high value of the activation energy for k 3 , representing the reaction maltene + asphaltene to the possible inhibition of polymerization and condensation reactions by excess hydrogen present. According to Belinko and Denis [39], unsaturated asphaltene molecules prefer to undergo polymerization and condensation reactions over TABLE 6-7 Kinetic constants for Model P2 with or without the presence of sand Rate constant
ki k? ki
k4
Thermal hydrocracking
Thermal cracking
Activation energy, E (kJ/mol)
Frequency factor. A
142 224 284 186
2.44 x 6.72 x 4.36 x 8.14 x
(SCI)
loh 10” 10’’ 10’
Activation energy. E (kJ/mol)
Frequency factor. A
178 I04 107 171
4.53 x 2.46 x 6.78 6.31 x
(s ’ ) 10”
10‘ lo7 107
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
139
hydrocracking reactions. Hence, the activation energies for the asphallene + maltene reaction (k2) is higher than that of asphaltene += coke reaction (kl ). A comparison of the kinetic data for the thermal and hydrocracking reactions suggest that the reversible reaction asphaltene + maltene requires less activation energy in the case of thermal craclung compared to thermal hydrocracking. The reaction asphaltene -+ coke, on the other hand, requires less activation energy for the thermal hydrocracking case. Koseoglu and Phillips [38] also proposed six kinetic models for the non-catalytic hydrocracking of Athabasca bitumen based on pseudo-components and lumped fractions. The first model is based on three pseudo-components: heavy ends, light oils and gases and is of the following form: heavy ends -+ light oils
-+
gases
The heavy ends included both coke and asphaltenes. In the second model, the heavy ends of the first model was divided into coke and heavy oils. The resulting model is as follows: coke
t
heavy oils + light oils + gases
The heavy oil fraction was further subdivided into asphaltenes and resins in the third model as follows: coke
t asphaltenes
+ resins -+ lightoils + gases
The light oil fraction can be subdivided into aromatics and saturates. Inasmuch as all the gases produced were saturated, the saturates fraction of the light oils was lumped with the gases into a single pseudo-component called ‘saturates’ in the fourth model. The model thus becomes: coke
t asphaltenes
+ resins += aromatics -+ saturates
The lumping of liquid saturates and gases into a single pseudo-component is rather questionable and perhaps unnecessary. The model prediction in this case was not as good as that of the previous model, the general trends, however, can be reproduced. The authors also examined the other variations of the previously described models. The final model, however, was an extension of the fourth model. The saturates were simply broken down to liquid saturates and gases. The final model (Model KP2), thus, included six pseudo-components as shown in Fig. 6-9. All the reactions, in general, followed the Arrhenius relationship, the reaction asphaltenes + resins, however, showed some deviation from the Arrhenius behavior. The relevant kinetic parameters are given in Table 6-8. Coal-derived asphaltenes
Szczygiel and Stolarski [40] studied the hydrocracking of asphaltenes extracted from coal in the presence of a natural aluminosilicate catalyst in a batch autoclave at temperatures of 673 to 733 K and under hydrogen pressure of 9.8 to 29.4 MPa. They found coke to form mainly during the preheating period and oil to form from asphaltene. Their proposed reaction mechanism is presented in Fig. 6-10.
140
c-1
CzF-3k1
A. C'HAKMA
ASPHALTENE
a AROMATICS
Fig. 6-9. Asphaltene cracking mechanism of Koseoglu and Phillips 1381. TABLE 6-8 Kinetic parameters of the asphaltene cracking model of Kdseoglu and Phillips [38] Rate constant
Activation energy. E (kl/mol)
(s-' )
Frequency factor, In A
168 f 17 103 f 7 96 f 9 85f8 I36 f 12 175 f 12
27.54 f 2.78 19.34 f 1.39 17.07% 1.51 13.57 f 1.27 24.43 =!r2.03 29.90 f 2.10
An analysis of the rate data that Szczygiel and Stolarski [40] provided for their model suggests Arrhenius behavior for all the reactions. Activation energies calculated from the data are shown in Table 6-9. The lower values of activity energies indicate the effectiveness of the natural aluminosilicates as catalysts for the cracking of coal-derived asphaltenes. Phillippopoulos and Papayannakos [41] studied the cracking and desulfurization of asphaltenes obtained from a Greek atmospheric residue in an integral trickle bed reactor using Co-Mo/Al?O3 catalysts. They found the asphaltene cracking reaction to follow a second-order behavior with respect to the asphaltene concentration. Schucker [33] determined coking kinetics of Arab heavy vacuum residuum and its constituent fractions, including asphaltene, by thermogravimetry. The residuum was divided into four fractions, namely: asphaltenes, polar aromatics, aromatics and saturates. The coking characteristics of the residue and its four fractions were determined.
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
141
ASPHALTENE
Fig. 6- 10. Coal-derived asphaltene crackmg mechanism TABLE 6-9 Kinetic parameters for the cracking of asphaltene derived from coal Rate constant
Activation energy, E (kJ/mol) 65 37 56 95 17
Activation energies and frequency factors were determined at various conversion levels and were found to increase with conversion level. Kinetic parameters for the asphaltene fraction are given in Table 6-10.
MODEL COMPONENT STUDIES
Kinetic models based on lumped solubility class or boiling point range groups are useful tools. They, however, do not provide much insight into the reaction mechanisms. Inasmuch as asphaltene is an ill-defined solubility class consisting of a number of different compounds of differing structures and chemical properties, it is not possible to study the fundamentals of the reaction mechanisms using asphaltenes directly. Model compounds, representing a given group of asphaltene constituent, are used in studies attempting to elucidate the reaction mechanism. The major constituents of petroleum asphaltenes are variable sizes of condensed aromatic and naphthenic ring systems with alkyl side chains, sulfide linkages and bridges between the rings [7,42-44,17,45,46]. Alkylaromatic compounds can be con-
142
A ('IIAKMA
TABLE 6-10 Kinetic parameter\ for the cracking of asphaltene fractions derived from Arabian vacuum residuuni (after Schuckcr [ 33 I )
20 30 40
SO 60 70 XO
90
Activation energy, E (kJ/mol)
(s
176.0 173.5 180.6 203.6 213.2 211.9 280.9 336.5
2.27 x 1.47 x s.21 x 2.53 x 1.40 x 1.22 x 7.90 3.12 x
Frequency factor 1)
10"' 10"' 10"'
lo'? 10" 10" 1017
10''
sidered to be the key components of the asphaltene molecule. Among them, polycyclic alkylaromatic compounds are probably the most prevalent ones. Pyrolysis of singly substituted alkylcyclohexane and long-chain ti-alkylaromatic compounds have been the subject of a number of studies. Fabuss et al. [47] studied the pyrolysis of a number of saturated hydrocarbons including cyclohexane and methyl-, ethyl-, propyl-, and n-butylcyclohexane and determined the activation energies for their formation. Mushrush and Hazlett [48] pyrolyzed tridecylcyclohexane at 450°C and found n-alkanes, 1 -alkenes, alkylcyclohexanes, toluene, and benzene as the reaction products. At high-temperature and low-pressure conditions the reactions were found to follow the free-radical mechanism. The authors did not provide enough kinetic information. Trahanovsky and Swenson [49] carried out flash pyrolysis of 1-methyl- and 2-methyltetralin at 700-900°C under vacuum (0.1 Torr) conditions. They found napthalene and 1.2-dihydronaphthalene to be the major reaction products indicating the ease of cleavage of the alkyl groups. n-pentadecylbenzene (PDB) is a typical prototype of the alkylaromatic moiety present in asphaltene. Savage and Klein [50] studied the pyrolysis of PDB to gain insight into the reaction mechanism of the alkylaromatic constituent of the asphaltenes. The temperature was varied from 375"to 450°C. Pyrolysis resulted in the production of two major product pairs: toluene plus 1-tetradecene and styrene plus n-tridecene. They found the thermolysis reaction to be of free-radical type and the reaction to be of first order with an activation energy of 55.45 kcal/mol and a frequency factor of 14.04 s-' . They proposed the reaction mechanism shown in Fig. 6- 1 1.
r +-@[+IT -'+@[ k21
1k31
k41
Fig. 6-1 I . Reaction mechanism for PDB cracking (after Savage and Klein [SO]).
143
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
TABLE 6- I I Kinetic parameters for PDB cracking. (After Savage and Klein [ 5 0 ] ) Rate constant
Activation energy, E (kcal/mol)
Frequency factor, log A
ki
55.5 47.3
14.0
k2
ki
kJ
16.6 18.8
(SKI)
12.3 2.5 53.9
The corresponding kinetic constants for the first-order rate equations are given in Table 6- 1 1. The rate equations based on these kinetic constants may not be directly applicable to the alkylaromatic constituent of the asphaltene due to likely diffusional limitations provided by the large asphaltene structure to the movement of free radicals generated during the decomposition. The same authors [50]also studied the thermal decomposition of 1-phenyldodecane (PDD) and concluded that PDD thermolysis followed the free-radical mechanism, contradicting the earlier findings of Blouri et al. [51], who suggested that it was molecular a reaction. Savage and Klein [52] also studied the reaction mechanisms of the alkylhydroaromatic and alkylnaphthenic groups of asphaltene by pyrolyzing model compounds representing these two groups. 2-ethyl tetralin (ET) was chosen as a model compound to represent the alkylhydroaromatic group, whereas n-tridecylcyclohexane (TDC) was the model compound for the alkylnaphthenic group. Pyrolysis of TDC yielded cyclohexane, methylenecyclohexane, I-tridecene, and n-dodecane as major products. Cyclohexane, methylcyclohexane, n-tridecane, other n-alkanes, a-olefins, alkylcyclohexane, and cyclohexylalkenes were formed as minor products. The disappearance of TDC followed first-order kinetics with an activation energy of 59.4 kcal/mol and a frequency factor of 7.94 x IOl4. Pyrolysis of ET-produced napthalene, tetralin, dialin, 2-ethylnaphthalene, and 2-ethyldialins as major products. Toluene, methylindan and substituted benzenes were the minor reaction products. Like TDC, the disappearance of ET also followed first-order kinetics. The activation energy was 53.5 kcal/mol and the frequency factor was5.01 x 10”. Savage and Klein [52] concluded that pyrolysis of TDC and ET follow the freeradical reaction mechanisms. The dominant reactions involved dealkylation near or at the ring. Ring opening reactions were considered to be minor reactions. Pyrolysis also resulted in significant dehydrogenation of the model compounds as indicated by the formation of napthalene and toluene from ET and TDC, respectively. These observations, when applied to asphaltenes, clearly suggest that asphaltene cracking primarily involves the cleavage of the peripheral aliphatic substituents followed by aromatization of the saturated rings. As a result of thermal cracking, the remaining asphaltene will become hydrogen-deficient and more aromatic. Zou et al. [53] studied the structural changes in the Cold Lake bitumen due to in situ combustion and also found the aromaticity of the
144
A. CIIAKMA
simulated fire-flood-produced oil to be higher than that of the original feedstock due to the loss of alkyl side chains and the dehydrogenation of the alkyl ring systems. Smith and Savage [54] pyrolyzed I-dodecylpyrene (DDP) and found the major reactions products to be similar to those observed for alkylbenzene pyrolysis. At higher conversion levels, however, n-dodecane and pyrene were found to be the major reaction products, suggesting the cleavage of the strong alkyl-aryl C-C bond. Smith and Savage [ 5 5 ] also pyrolyzed I-methylpyrene and 1-ethylpyrene and found hydrogenolysis to be the dominant mechanism. Pyrene and dimethylpyrene were found to be the major reaction products due to the pyrolysis of 1-methylpyrene, whereas pyrene and methylpyrene were the major pyrolysis products of 1 -ethylpyrene. The formation of these major reaction products suggested that radical hydrogen transfer was not a valid mechanism for these systems.
IN SITU COMBUSTION
Adegbesan et al. [3,56]have studied the reactions of Athabasca bitumen with oxygen and found the maltene fraction to oxidize to asphaltene and coke fractions, whereas the asphaltene fraction oxidized to coke fraction. They proposed a series of models describing formation of asphaltenes/coke from bitumen and its various components. The first model, as shown below, was formulated to provide the oxygen consumption rate for the overall oxidation reaction.
MODEL 1 Bitumen + Oxygen
k,
Products
The rate constant k l can be calculated from:
k , = 2.19 x 10”exp
~
I
125~44
where R = the universal gas constant, Pa m3/kmol K, and T = temperature, K. The above model was able to predict the oxygen consumption rate quite well. In addition, the authors also proposed four kinetic models for the liquid-phase reactions. They are as follows:
145
KINETICS AND MECHANISMS OF ASPHALTENE CRACKING
-
MODEL 2
k*
Maltenes
Asphaltenes-coke
MODEL 3 k3
Oils
k4
Resins
Asphaltenes-coke
---+
> <
MODEL 4
Resins1
> - Oils
Asphaltenes-coke
Resins11
MODEL 5
k,
k,,
kg
oils
A
Resins I
kl1
Resins II
Asphaltenes-coke
The authors found the above reactions to be of first order with respect to the pseudo-components and fractional order with respect to oxygen partial pressure. All the reactions exhibited Arrhenius behavior. Inasmuch as it was not possible to distinguish the effect of oxygen solubility, the authors calculated pseudo-activation energies in which the solubility effects were also lumped. The rate parameters for the above reactions are presented in Table 6-12. Except for klo. all the activation energies seem to be quite reasonable. The very high value of activation energy for klo suggests Model 5 to be unrealistic.
TABLE 6-12 Kinetic parameters for various reactions involving asphaltene formation during in situ combustion (after Adegbesan et al. [56]) Rate constant Pseudo-activation energy (J/mol) 8 I .93 77.23 86.73 77.75 52.37 89.04 79.14 74.92 4877.27 89.14
Pseudo-frequency factor Reaction order with respect to oxygen (h-' kPa-'I) n 1.00 x 2.64 7.69 x 1.81 4.64 x 1.94 x 4.48 x 1.16 x 7.78 7.78 x
108 107
lox 107 10' 10'
107 10' 107 107
0.41 0.41 0.50 0.41 0.50 0.50 0.50 0.41 0.50 0.50
146
A CIIAKMA
CONCLUSIONS
A review of asphaltene cracking kinetics has been made. Inasmuch as asphaltenes constitute a complex solubility class, it is not possible to determine a comprehensive reaction mechanism based on reaction fundamentals. Model compound studies can provide some insight into the reaction mechanisms involved in the cracking of various constituent groups of the asphaltenes. It is still not possible, however, to use kinetic data obtained from model compound studies to predict the formation of various groups due to asphaltene cracking. Kinetic models based on pseudo-component approach, in which the reaction products are divided into a number of easily quantifiable pseudo-components, are available. These models are practical tools for engineering calculations involving asphaltene cracking in petroleum recovery and processing applications.
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