Thermal Stability of Alkylaromatics in Natural Systems: Kinetics of

Geology-Geochemistry Division, Institut Franc¸ais du Pe´trole (IFP), 1-4 Avenue de Bois Pre´au, ... the evolution of petroleum systems during geolo...
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Thermal Stability of Alkylaromatics in Natural Systems: Kinetics of Thermal Decomposition of Dodecylbenzene Franc¸ oise Behar,* Franc¸ ois Lorant,† He´le`ne Budzinski,‡ and Emmanuel Desavis† Geology-Geochemistry Division, Institut Franc¸ ais du Pe´ trole (IFP), 1-4 Avenue de Bois Pre´ au, 92852 Rueil-Malmaison, France, and Laboratoire de Photophysique et Photochimie Mole´ culaire, Universite´ de Bordeaux 1, Talence, France Received June 26, 2001. Revised Manuscript Received November 9, 2001

The aim of this work is to determine the apparent rate constants for dodecylbenzene (DDB) thermal cracking in laboratory conditions in order to evaluate alkylaromatic stability in geological conditions. Pyrolysis experiments were carried out under Argon atmosphere, in anhydrous closed system (gold bags) during times ranging from 1 to 72 h under isothermal conditions (325-425 °C). The global rate constants were determined based on the reactant conversions obtained at various temperatures. For all temperatures investigated, bulk decomposition of the DDB obeys first-order kinetics and the resulting apparent activation energy derived from an Arrhenius diagram is found at 53.3 kcal/mol and the corresponding frequency factor A at 1.3 × 1013 s-1. When these kinetic parameters are used for predicting global rate constants at lower temperatures, results show that the DDB should decompose below 160 °C assuming a constant thermal history at 1.25 °C/my. This means that, even within source rocks, the monoalkyl aromatics are likely to decompose during the late stage of kerogen cracking. Consequently, depending on the timing of petroleum expulsion, the expelled fluid may be depleted in alkyl aromatics due to their partial secondary cracking before the migration threshold is reached. The major products observed during DDB cracking are on one hand, a light C7-C14 fraction dominated by decane (n-C10) and undecene/undecane (R-C11/n-C11) toluene and ethyl benzene and on the other hand, heavy C14+ aromatic fraction. For DDB conversion lower than 88%, hydrocarbon gas generation was negligible and no insoluble residue was observed.

Introduction Oil and gas formation and accumulation result from the evolution of petroleum systems during geological time. Kerogen, the insoluble organic matter deposited into fine grained minerals of source rocks, is progressively thermally cracked and transformed into petroleum fluids during sediment burial. Due to compaction in source rocks, a part of these fluids migrates toward reservoirs. Further cracking of kerogen and/or petroleum produces lighter and finally gaseous compounds. It is widely accepted1 that the thermal evolution of oils is controlled by the kinetics of cracking reactions. This allows petroleum geochemists to simulate experimentally the low temperature, long residence time natural processes by operating at higher temperatures generally comprised between 250 and 550 °C.2-9 At * To whom correspondence should be addressed. E-mail: [email protected]. † Geology-Geochemistry Division. ‡ Laboratoire de Photophysique et Photochimie Mole ´ culaire. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: New York, 1984; p 699. (2) Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L. Rev. Inst. Fr. Pe´ t. 1991, a46, 151-181. (3) Behar, F.; Kressmann, S.; Rudkiewicz, J. L.; Vandenbroucke, M. Org. Geochem. 1991, b19, 173-189. (4) Behar, F.; Vandenbroucke, M.; Tang, Y.; Marquis, F.; Espitalie, J. Org. Geochem. 1997, a26, 321-339. (5) Lewan, M. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Publishing Corporation: New York, 1994; Chapter 18, pp 419-444.

these temperatures, reactions are rapid enough to monitor cracking with acceptable time i.e., a few minutes to a couple of months. These experimental simulations are presently the best way to elaborate mathematical models that describe oil cracking and contribute to petroleum evaluation in natural reservoirs. Nevertheless, due to the diversity and the complexity of the chemical composition of oils, the development of a mathematical model can be made presently only on an empirical basis through lumping individual molecules having similar structures, thus similar thermal stability, into chemical classes. In previous papers,2,3 a kinetic model was proposed which comprises the following chemical classes defined by a given elementary composition: (in the gas fraction) methane, ethane, and (propane+butane); (in the light hydrocarbon fraction) C9-C14 aromatics, C6-C14 saturates, and the mixture of aromatics benzene + toluene + xylenes + naphthalene (BTXN); (in the heavy hydrocarbon fraction) the C14+ saturates and the C14+ unstable aromatics comprising mainly alkyl and naphthenoaromatic structures with eventually heteroelements, such as N, S, and O, the more stable C14+ aromatics (6) Lewan, M. Geochim. Cosmochim. Acta 1997, 62, 2211-2216. (7) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985, 8, 275-292. (8) Burnham, A. K.; Braun, R. L. Org. Geochem. 1990, 16, 27-39. (9) Ungerer, P. Org. Geochem. 1990, 16, 1-25.

10.1021/ef010139a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/25/2002

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comprising the methylated compounds; (in a solid fraction) the precoke and the coke. In this kinetic scheme, methane, the BTXN fraction, and coke are considered as stable classes. The determination of kinetic parameters for each unstable chemical class cracking (apparent activation energy E, preexponential factor A, and stoichiometric coefficients) is made on the basis of a set of reference pyrolysis experiments. It consists of determining the minimum of an error function defined as the mean square residual of the model versus the experiments. To reduce the number of free parameters, it was decided to impose for all cracking reactions the same preexponential factor. This assumption is commonly used in kinetic models dealing with coal evolution10 or with other types of organic matter.4,8,9,11,12 It must be noticed however that such an assumption is arbitrary and not supported by any theoretical consideration. Consequently, to better constrain the kinetic parameters in the overall kinetic scheme of oil cracking and to assign specific frequency factors for the different reactions, we decided to determine kinetic parameters on model compounds which are representative of the main chemical classes found in oils. The proposed research strategy is to determine global rate constants based on the conversion of the reactant obtained under isothermal conditions and to derive the apparent activation energy and corresponding frequency factor by plotting the calculated global rate constants in an Arrhenius diagram. First, we studied the saturated hydrocarbon C25H52 (n-C25) in order to predict thermal stability of heavy paraffins in high-temperature/high-pressure reservoirs for which present temperatures may be higher than 180 °C.13 Extrapolation of the kinetic parameters to geological conditions has shown that the n-C25 is degraded above 180 °C for residence times higher than 10 millions years. This means that reservoir oil enriched in n-alkanes should be stable in very high-temperature conditions. These results were in very good agreement with observations done in petroleum reservoirs.14,15 Consequently, the global kinetic scheme proposed by Behar et al.3 was modified by including a specific reaction for heavy paraffin thermal cracking: the corresponding global kinetic parameters applied to that reaction were those of the n-C25.15 More recently, we studied, following the same research strategy as that proposed for the n-C25, a methylated aromatic compound: the 9-methylphenanthrene16 for which the frequency factor of its global decomposition was found at 4.5 × 1010 s-1 for a global activation energy at 49.0 kcal/mol. These parameters are indeed very different to those determined for the n-C25 (6.1 × 1017 s-1 with E ) 68.2 kcal/mol). As a direct (10) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, 1, 138-152. (11) Pepper, A. S.; Corvi, P. J. Mar. Pet. Geol. 1995, 12, 1-319. (12) Tissot, B. P.; Espitalie, J. Rev. Inst. Fr. Pe´ t. 1975, 30, 743777. (13) Behar, F.; Vandenbroucke, M. Energy Fuels 1996, 10, 932940. (14) McNeil, R. I.; Bement, W. O. Energy and Fuels 1996, 10, 6067. (15) Vandenbroucke, M.; Behar, F.; Rudkiewicz, J. L. Org. Geochem. 1999, 30, 1105-1125. (16) Behar, F.; Budzinski, H.; Vandenbroucke, M.; Tang, Y. Energy Fuels 1999, 13, 471-481.

Behar et al.

consequence, the methylated aromatics are much more stable than heavy paraffins in laboratory, whereas they start to decompose below 160 °C in natural reservoirs. Again these results are in good agreement with chemical composition of crude oils recovered in HP/HT reservoirs in which the ratio saturates over aromatics can be as high as 9,15 whereas in oils from shallower reservoirs this ratio is much lower.1 As for the specific degradation of the heavy paraffins, the kinetic parameters determined on the methylated aromatic were included in the global schema of oil cracking developed at the IFP. As a next step, we propose to extend this type of kinetic study to other aromatic compounds because methylated aromatics are not the only aromatic class in crude oils or in source rock extracts. These compounds are mixed to either naphtheno-aromatics, alkylated aromatics, or sulfur aromatic compounds.1 To get a complete description of the thermal degradation of the total aromatic fraction, it is necessary to select at least one model compound from among these four main classes. We decided to study, as second chemical class, the alkylated aromatics because, based on literature data, their thermal cracking will lead to the production of methylated aromatics already studied. Thus, by combining the global kinetics for the decomposition of both alkylated and methylated aromatics, it is possible to predict the overall thermal stability window of these two model compounds in geological conditions. We have selected the dodecylbenzene (DDB) because all its pyrolysis products can be quantified by GC except benzene which is coeluted with n-pentane chosen as solvent. It was shown, based on literature data, that thermal stability of alkyl aromatics depends at least on two parameters: the length of the side chain and the number of aromatic rings. Smith and Savage17 have demonstrated that for a same number of aromatic rings, the thermal instability of the alkyl aromatic increases with the side chain length. According to the authors, this relationship is only due to the extent of the hydrogenabstraction steps during the pyrolysis of such compounds. As the number of abstractable hydrogen atoms increases with the number of carbon atoms in the alkyl chain, the net rate of hydrogen abstraction increases, and thus the apparent reaction rate constant as well. Consequently, this effect is much more pronounced when the side chain contains less than five carbon atoms whereas it becomes more and more negligible as the chain length increases (Figure 1 after Smith and Savage.17 By comparing18-20 the thermal stability of the pentadecylbenzene (PDB) and the dodecyl pyrene (DDP), it was shown that the DDP is less stable than PDB. Since these two alkylated aromatics have very close side chain lengths (15 and 12 carbon atoms, respectively), these authors concluded that the thermal stability is due to the number of aromatic rings. These conclusions are in good agreement with the Dewar reactivity numbers21,22 of these two model compounds which are respectively 2.31 and 1.51.17 (17) Smith, C. M.; Savagem, P. E. AIChE J. 1991, a37, 1613-1624. (18) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 488494. (19) Smith, C. M.; Savage, P. E. Ind. Eng. Chem. Res. 1991, b30, 331-339. (20) Savage, P. E. Energy and Fuels 1995, 9, 590-598. (21) Dewar, M. J. S. J. Am. Chem. Soc. 1952, 74, 3357-3363.

Thermal Stability of Alkylaromatics

Energy & Fuels, Vol. 16, No. 4, 2002 833 Table 1. Geochemical Characterization of the Studied Kerogens4 type

origin

Tmax HI VR corg (°C) (mg/g of C) (%) (wt %) H/C O/C

I Green River Fm 438 II Paris Basin 419 III Mahakam Delta 419

918 600 194

nd 66.88 1.55 0.081 0.55 55.33 1.27 0.145 0.57 76.14 0.84 0.152

Table 2. Pyrolysis Conditions Selected for DDB Thermal Cracking temperature

time

325 °C 350 °C 375 °C 400 °C 425 °C

73, 121 h 6, 8, 10, 15, 24, 31, 48, 72, 96 h 3, 4, 5, 6, 7, 24, 48 h 1.2, 2.2, 3, 6, 9, 24 h 1.2, 2.2, 3.2, 6.2 h

Figure 1. Influence of the alkyl chain length on the thermal stability of monoalkylated aromatics.17

Figure 2. Description of the analytical procedure for the synthesis and labeled 1-13C dodecylbenzene. The intermediate products in brackets are unstable structures.

Consequently, the DDB may be considered as the most stable model compound for the global chemical class of alkyl aromatics with a side chain containing more than five carbon atoms. Thus, it will enable to define, in geological conditions, the upper limit of the thermal stability for that global aromatic class. In terms of kinetic scheme, the main products generated from the DDB decomposition were compared to those obtained by previous studies on other similar

compounds,17,18,23-29 and a simplified kinetic scheme was proposed. Finally, to determine the window of thermal stability for the alkylated aromatics in source rocks and reservoirs, we have compared the degradation curve of the DDB to those obtained for standard kerogens deriving from the three main types of organic matter.4 In fact, since the methylated aromatics were shown to be degraded before saturates in natural systems, it is

(22) Dewar, M. J. S.; Thiel, W. J. J. Am. Chem. Soc. 1977, 99, 48994907. (23) Freund, H.; Olsmtead, W. N. Int. J. Chem. Kinetics 1989, 21, 561-574. (24) Poutsma, M. Energy Fuels 1990, 4, 2, 114-131. (25) Billaud, F.; Chaverot, P.; Berthelin, M.; Freund, E. Ind. Eng. Chem. Res. 1988, 27, 1529-1536.

(26) Chaverot, P. 1985 Thesis. Ecole Nationale du Pe´trole et des Moteurs (ENSPM), Rueil-Malmaison, France. (27) Blouri, B.; Hamdan, F.; Herault, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 30-37. (28) Savage, P. E.; Korotney, D. J. Ind. Eng. Chem. Res. 1990, 29, 3, 499-502. (29) Savage, P. E. J. Anal. Appl. Pyr. 2000, 54, 109-126.

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expected that the alkyl aromatics are even less stable and may overlap kerogen cracking. If so, these compounds may be degraded before the migration threshold is reached, leading to significant changes in the chemical composition of the expelled fluid. Samples. The DDB was synthesized according to the analytical procedure presented in Figure 2. The isotopically marked carbon is used for another study in progress on mixture effect. It was purified by liquid chromatograhy on silica gel and its purity checked by gas chromatography. Three kerogens representative of the main organic matter types at the end of the diagenesis stage4 were selected: a Type I shale from the Green River Formation (Eocene), a Type II Toarcian shale from the Paris Basin (Toarcian), and a Type III coal from the Mahakam Delta (Miocene). For both shales and coal, kerogens were prepared by HF/HCl digestion according to the standard procedure described by Durand and Nicaise,30 then extracted with dichloromethane. Their geochemical parameters are given in Table 1. Kinetic parameters for the three kerogens were calculated from open pyrolysis experiments and were already published in a previous paper:4 they are given in Figure 3. Experimental Section Pyrolyses of the DDB were carried out in gold tubes (40 mm length, 5 mm i.d. and 0.5 mm thick) sealed by welding under an argon atmosphere and containing between 20 and 50 mg of initial sample.31 The gold tubes were placed in pressurized autoclaves in a furnace preheated at the chosen isothermal temperature and kept at a pressure of 14 MPa during the entire course of the experiment. At the end of the desired reaction time, the cells were taken out and cooled, the argon pressure was then vented and the gold tube removed from the autoclave and weighed. The temperature was recorded during each experiment in order to have the accurate value when calculating the apparent rate constants. The pyrolysis conditions selected for the DDB thermal cracking are reported in Table 2: temperature between 350 and 425 °C and heating time between 1.2 and 96 h. For the present study, no gas analysis was performed because, according to previous data,18 no significant gas amount was expected below 80%. This point is discussed in the “Results and Discussion Part”. Analysis of the C7+ Fraction. Each gold tube was opened in pentane. After extraction for 1 h by stirring under reflux, then filtration, the internal standard (n-C25) was added and the solution was accurately divided by weighing into three fractions. The first aliquot was injected as such i.e., without solvent evaporation into an on column gas chromatograph, equipped with an auto sampler, for identification and quantification of individual compounds. It was checked that both the reactant and the products were accurately quantified. In fact, at low conversion, the reactant is present in very large concentration in comparison to the generated products: in that case, it was necessary to (i) dilute the first aliquot for avoiding any saturation of the FID detector when quantifying the residual reactant amount and (ii) to concentrate the second aliquot for a good quantification of the products present in low concentration. By this way, individual peaks as well as the total C7-C14 fraction were quantified. However, partial loss (30) Durand, B.; Nicaise, G. In Kerogen; Durand, B., Ed.; Editions TECHNIP: Paris, 1980; pp 35-53. (31) Behar, F.; Leblond, C.; Saint-Paul, C. Rev. Inst. Fr. Pe´ t. 1989, 44, 387-411.

Figure 3. Global kinetic parameters for the three kerogens under study.4 of volatile compounds is likely as discussed in the “Results and Discussion Part”. As explained in the Introduction, the aim of the present work is to derive global kinetic parameters by following the reactant conversion at various temperatures. Consequently, it was necessary to evaluate this conversion, quantified by GC, as precisely as possible. To minimize the experimental error, the GC equipped with an auto sampler was calibrated with various concentrations of the internal standard (n-C25) diluted in pentane. The calibration curve was established 1 day before running the GC analyses and, for the present study, which includes 28 experiments, all GC quantification were performed during 3 weeks. Standard solutions at various concentrations were intercalated regularly with C7+ aliquots in order to verify the FID response. Following this procedure, individual peaks were quantified with a precision of ( 1.0%. However, for the evaluation of the total C7-C14 fraction, the measurement was less precise (( 2%), because a blank must be subtracted, leading to an additional experimental error.

Thermal Stability of Alkylaromatics The GC conditions were the following: on column injector, apolar column (50 m, 0.2 mm i.d.); initial temperature at 20 °C, final temperature at 320 °C during 20 min, and heating rate from 20 to 320 °C at 5 °C/min. The third aliquot was evaporated in order to evaluate the C14+ fraction by weighing. It was checked that, during filtration of the n-pentane, no insoluble fraction was formed in all our experimental conditions. By adding the C7-C14 fraction quantified by GC, to the weighed C14+ amount, it is possible to get the mass balance and to estimate the contribution of pyrolysis products which were not recovered such as gas and benzene. Molecular identification was done by gas chromatographymass spectrometry. The gas chromatograph was a HewlettPackard 5890 series II equipped with a splitless injector (purge delay, 1 min; purge flow rate, 60 mL/min) and an electronic pressure controller (EPC). The column used was a HP5-MS (5% phenyl-95% methypolysiloxane): 30 m × 0.25 mm ID × 0.25 µm film thickness. The column was kept at 50 °C during 2 min, programmed to 310 °C at 2 °C/min and finally kept at 310 °C during 15 min. The injector was kept at 270 °C. Helium was employed as the carrier gas at a 1.2 mL/min constant flow. The gas chromatograph was coupled to an HP 5972 mass selective detector (MSD) (electronic impact, 70 eV; voltage, 2000 V) in scan mode (from 50 to 600 amu at 1.3 scan/s). The interface temperature was maintained at 290 °C.

Energy & Fuels, Vol. 16, No. 4, 2002 835 Table 3. DDB Conversion (wt %) Obtained at 325, 350, 375, 400, and 425 °C for Various Heating Times (h) 325 °C

350 °C

375 °C

400 °C

425 °C

time time time time time (h) conv (h) conv (h) conv (h) conv (h) conv 73 121

10.2 15.8

6 8 15 24 31 48 72 96

6.5 7.5 14.4 20.4 28.8 32.9 46.4 61.6

3 4 5 6 7 24 48 48

16.8 21.7 25.9 27.8 31.5 71.5 87.6 89.0

1.2 2.2 3 6 9 24

29.0 52.5 61.8 82.4 89.9 98.0

1.2 2.2 3.2 6.2

78.7 91.8 96.0 99.1

Table 4. Comparison of the DDB Conversion (%) with Those Obtained on the PDB18 and on the DDP17 for Similar Pyrolysis Conditions T

t

375 °C

1h 2h 3h 1h 2h 3h 1h 2h 3h

400 °C 425 °C

DDB

16.9 29.0 52.5 61.8 78.7 91.8 96.0

PDB

DDP

6.3 12.2

18.5

45.5 49.8 63.0 82.0 94.6 98.9

37.4 32.5 74.8 94.1 95.5 99.2

Results and Discussion First-Order Plots and Kinetic Parameter Calculations for DDB Cracking. As indicated in Table 2, isothermal pyrolyses were performed at 325, 350, 375, 400, and 425 °C. DDB conversions are reported in Table 3. The conversion (conv), expressed in wt %, is defined as follows: conv ) 100(1 - residual DDB/initial DDB). As a general trend, DDB conversions are in the same range asthose previously obtained by Smith and Savage19 on the dodecylpyrene (DDP) and those obtained on the PDB18 as indicated in Table 4. As expected, except for the experiment at 400 °C/1h, PDB conversion is always higher than that of DDB and, except for the experiment at 400 °C/1h, the difference is much smaller than that observed between the DDP and DDB. When a reaction follows a first-order law, the plot of time versus the logarithm of (1-conv.) must give a linear relationship, the slope of which corresponds to the reaction rate. Thus, it is necessary for a precise slope determination at a given temperature, to cover a large conversion range. However, caution must be taken for data obtained above 95% conversion because the experimental error weight could lead to a first-order deviation which does not correspond to reality. First-order plots, given in Figure 4a, were determined only at temperatures higher than 325 °C, for which large conversion range is observed. They show an excellent linearity, and thus, it was possible to calculate 4 global apparent reaction rates. At 325 °C, assuming a first-order reaction, global rate constant can be also calculated, but the resulting value is less precise. All data are reported in Table 5 and compared to those already obtained in our previous papers13,16 for two other model compounds: the 9-methylphenanthrene (9MPh) and the n-C25. In laboratory conditions, the 9-MPh is the most stable compound and the DDB the least stable for all temperatures investigated. On the basis of the Arrhenius equation, one can write that

ln k ) -E/RT + ln A

Figure 4. First-order plots for the DDB degradation at 350, 375, 400, and 425 °C (Figure 4-) and corresponding Arrhenius diagram (Figure 4b).

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Table 5. Comparison of the Global Rate Constants for the DDB Degradation with Those of the 9-MPh16 and n-C2513 in the Temperature Range 325-425 °C T

DDB k (s-1)

9-MPh k (s-1)

n-C25 k (s-1)

325 °C 350 °C 375 °C 400 °C 425 °C

5.00 × 10-7 2.33 × 10-6 1.22 × 10-5 7.13 × 10-5 2.83 × 10-4

5.5 × 10-8 2.98 × 10-7 1.23 × 10-6 5.57 × 10-6 2.06 × 10-5

7.0 × 10-8 7.4 × 10-7 5.4 × 10-6 4.3 × 10-5 2.7 × 10-4

where k is the rate constant, T is the temperature, R is the universal gas constant, A is the frequency factor, and E the activation energy. Plotting 1/T versus ln k, one can determine both E and A through the slope and intercept of the linear relationship obtained. For DDB degradation, the Arrhenius diagram given in Figure 4b shows an excellent linearity (r2 equal to 0.9988). The corresponding kinetic parameters are respectively 53.3 kcal/mol for the activation energy and 1.3 × 1013 s-1 for the frequency factor. These parameters are in very good agreement to those determined on the PDB by Savage and Klein (1987): E ) 55.5 kcal/mol and A ) 1.0 × 1014 s-1. This similarity is very important because this means that it is possible to average these two sets of kinetic parameters for obtaining kinetic parameters representative of the global class of alkyl benzene compounds (with a side chain containing at least five carbon atoms) found in source rock extracts and reservoir oils. However, our parameters are not similar to those calculated by Blouri et al.:27 E ) 44.4 kcal/mol and A ) 1.57 × 1010 s-1. It is worth noting that the accuracy of kinetic parameters derived from an Arrhenius diagram depends on two parameters: the range of temperature and the uncertainty on the rate constant at each temperature. In Blouri et al.’s work,27 the temperature range is 50 °C and the total number of experiments is 11 with only 1 point at 325 °C, 2 points at 350 °C, and 4 points at 380 °C and 400 °C. In the present work, the temperature range is 75 °C and the total number is 28 with 2 points at 325 °C, 8 points for 350 °C and 375 °C, 6 points at 400 °C, and 4 points at 425 °C. Consequently, on the basis of good linearity obtained for the first-order plots (Figure 4a), kinetic parameters obtained in the present study seem to be more accurate than those of Blouri et al.27 Moreover, a good way to verify the validity of kinetic parameters obtained in laboratory conditions is to extrapolate their results to geological conditions, i.e., below 150 °C. In fact, geochemical studies1 have demonstrated that alkyl benzene compounds are present in both source rock extracts and reservoir oils, thus they are much more stable than the kerogen from which they are generated. Results presented in Table 6 show that, using kinetic parameters from Blouri et al.’s work,27 the ratio of the rate constant between DDB and kerogen decomposition is much larger than 1. This means that below 150 °C the DDB is decomposed before kerogen. With our rate constants, the DDB is more stable than kerogen, which is in good agreement with natural observations. When the Arrhenius plot for the DDB is compared to those for the 9-MPh and the n-C25 (Figure 5), DDB is more unstable than n-C25 and than 9-MPh in both laboratory and geological conditions. These results confirm that at least alkylaromatics and methylated aromatics are less stable than paraffins in natural systems. Consequently, as already observed,14,15 oils

Figure 5. Comparison of the Arrhenius diagram obtained for DDB degradation with those obtained13,16 on the n-C25. Table 6. Comparison in Both Laboratory and Geological Conditions of the Global Rate Constants for the DDB Degradation Obtained in the Present Work (DDB1) with Those (DDB2) of Blouri et. al.:27 Calculation of the Ratio between These Rate Constant and that of the Type I Kerogen Using Kinetic Parameters of Figure 3 T

DDB1

DDB2

350 °C 360 °C 380 °C 400 °C 120 °C 130 °C 140 °C 150 °C

2.62 × 10-6 5.17 × 10-6 1.89 × 10-5 6.42 × 10-4 2.98 × 10-17 1.62 × 10-16 8.13 × 10-16 3.77 × 10-15

DDB1/ DDB1/ DDB2/ DDB2 kerogen kerogen

3.80 × 10-6 0.69 6.74 × 10-6 0.77 -5 2.02 × 10 0.94 5.56 × 10-5 1.15 3.18 × 10-15 200 1.30 × 10-14 140 4.99 × 10-14 101 1.79 × 10-13 73

0.31 0.31 0.30 0.30 0.43 0.42 0.42 0.41

0.50 0.44 0.35 0.28 46 34 26 19

Table 7. Mass Balances and Quantification of the Toluene, the Ethylbenzene, and the n-C10 (wt %) for a Subset of Experiments: Data Are Classified with Increasing DDB Conversion T

t

350 °C 375 °C 375 °C 350 °C 350 °C 400 °C 350 °C 375 °C 375 °C 425 °C 425 °C 400 °C 425 °C

6h 3h 6h 48 h 72 h 2.2 h 96 h 24 h 48 h 2.2 h 3.2 h 24 h 6.2 h

conv DDB C7-C14 C14+ total CH3Bz EtBz n-C10 6.5 16.8 27.8 39.1 46.4 52.5 61.6 71.5 88.3 91.8 96.0 98.0 99.1

93.4 84.9 72.2 60.9 53.6 47.5 38.4 28.5 11.7 8.2 4.0 2.0 0.9

7.0 9.8 12.5 14.5 23.2 25.9 28.4 38.7 47.0 32.7 33.7 36.3 34.4

0.0 8.3 17.3 nd 25.5 27.9 35.3 36.7 38.6 31.9 25.2 19.6 15.2

100.4 103.0 102.0 nd 102.3 100.3 102.1 103.9 97.6 72.8 62.9 57.9 50.5

0.6 2.1 3.2 2.3 4.2 5.9 7.5 9.0 8.3 7.6 7.8 8.3 8.8

0.1 0.5 1.3 1.2 1.8 4.2 5.4 4.0 4.8 5.7 6.3

0.4 1.2 1.8 1.6 2.0 2.9 3.4 4.2 4.9 4.3 3.8 3.7 2.5

might be enriched in saturates in high pressure/hightemperature reservoirs. Distribution of the Pyrolysis Products. The mass balances obtained for selected temperature/time conditions together with the DDB conversion are given in Table 7. In this table, we have reported the yield of the C7-C14 fraction quantified by GC and the C14+ fraction evaluated by weighing which does not include the DDB contribution. Thus, the sum (DDB + C7-C14 + C14+) enables us to evaluate the total mass balance for the recovered products. Results presented in Table 7 show that up to DDB conversion at 88%, the total recovery of the C7+ liquid fraction represents, at least, 98% of the total products. This means, as indicated in the experimental part, that hydrocarbon gas and insoluble residue are negligible for

Thermal Stability of Alkylaromatics

Energy & Fuels, Vol. 16, No. 4, 2002 837

Figure 6. Selectivity diagram for the C7-C14 and C14+ fractions generated from DDB pyrolysis. The selectivity of the reaction product, here expressed in wt %, is the ratio of its amount over the reactant conversion measured in each experiment. Arrows indicate the graphical estimation of stoichiometric coefficients.

conversion below 88%. In fact, we did not observe, when opening the gold bags, any insoluble deposit on the inside walls. For the gas yield, on the basis of the previous study on a similar model compound, i.e., the pentadecyl benzene, Savage and Klein18 have shown that the total gas fraction, for PDB conversion equal to 82%, represents 11.1 mol % i.e., 2.17 wt %. With increased severity, an increasing amount of C7C14 and C14+ compounds is generated, up to 90% conversion, then the C14+ fraction decreases strongly whereas it seems to reach a plateau for the total C7C14 fraction. When a selectivity diagram is plotted, as indicated in Figure 6, it is clear that the amount of C7C14 and C14+ fractions are in similar proportion up to 71.5% conversion. Then, these two fractions undergo secondary cracking. From this figure, by extrapolating data to 0% DDB conversion, it is possible to estimate a general stoichiometric equation for the total DDB conversion as follows:

DDB f 45% C14- + 55% C14+

(1)

To better describe the different chemical families present in the C7-C14 fraction, molecular identification by GC-MS revealed that the saturate fraction contains only n-alkanes and their total amount was estimated to 30% by GC at 61.6% conversion (350 °C/96 h). This relative n-alkane proportion was then applied to eq 1:

DDB f 14% C7-C14 n-alkanes + 31% C7-C14 aromatics + 55% C14+ (2) Above 90% conversion, a sharp and continuous decrease of the total C7+ liquid fraction yield is observed (Table 7). This can be explained by three reasons. First, still based on the PDB experiments of Savage and Klein,18 gas contribution is more and more significant above 90% conversion and for a PDB conversion at 98%, 41 mol % or 13.3 wt % of hydrocarbon gases are generated. The second reason is the absence of quantification of benzene in our experiments because of its coelution with npentane chosen as solvent for GC analysis. This contribution, if significant, may not be due to primary reaction but to secondary or tertiary reactions. As benzene yield was not given in previous studies published by Savage

and Klein18 on the PDB or by Blouri et al.,27 it is not possible to propose a corresponding yield in our experiments. A third reason is a possible partial loss of the very light hydrocarbons in the carbon number range C7-C8 during concentration step of the mother solution before taking the aliquot for GC quantification of the pyrolysis products. This light fraction being more and more important along DDB conversion, this partial loss may be important. In fact, from data given in Table 7, toluene yields are more fluctuating than those of the n-C10 which, because of its higher molecular weight, is more accurately estimated. To summarize, at very high conversion, gas and benzene yield probably represent the main counterpart of the C7+ liquid fraction without excluding a possible loss by evaporation of the C7-C8 fraction. A mere estimation of the amount of non recovered products was done at 98 wt % conv (400 °C/24 h) at least for gas and toluene. The contribution of gas was estimated at 15.2 wt % from data on PDB.18 The theoretical yield for toluene was found at 18.7 wt % from data on PDB:18 by subtracting the recovered amount (8.3 wt % in Table 7), the calculated toluene loss by evaporation is 10.4 wt %. Consequently, the total estimation of the non recovered gas and toluene is 26 wt %. The total estimation of the product loss, including benzene, being 42 wt % as indicated in Table 7, the maximum contribution of benzene calculated by difference is estimated to 16 wt % but this estimation must be verified by complementary experiments. A subset of GC traces of the total C7+ extracts diluted in pentane is shown in Figure 7. The chromatographs are sorted by increasing DDB conversion and the series of C7-C14 n-alkanes and C9-C15 alkylbenzenes are indicated with different labels. Molecular analysis of the C7-C14 fraction shows that major products of the reaction after 30% conversion are toluene, ethylbenzene and n-decane (n-C10). We observe the presence of styrene, R-decene, and R-undecene in lower amounts and traces of other n-alkanes and alkylbenzenes. These analyses are fairly consistent with other experimental studies published in the literature, either based on the DDB25 or similar compounds.18,28,32 It is worth noticing that we do not observe as many unsaturated compounds as others did (i.e., numerous olefins and alkenylbenzenes), because in our experimental conditions these primary products are rapidly reused in addition reactions to generate alkyl and phenylalkyl radicals. Nevertheless, the occurrence of major pyrolysis products may be explained by the set of reactions displayed in Figure 8. DDB undergoes unimolecular homolysis to give benzyl and undecyl radicals. Both of these radicals abstract hydrogen atom from DDB to form 1-phenyldodecyl radical, toluene and undecane. Due to stabilization by resonance, hydrogen at the R-carbon is preferentially abstracted compared to hydrogen at other carbons. Hence, 1-phenyl-1-dodecyl radical formation is favored and consequently that of styrene and decane. At the pyrolysis conditions selected for this work, styrene is (32) Smith, P. E.; Jacobs, G. E.; Javamardjan, M. Ind. Eng. Chem. Res. 1989, 28, 645-654.

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Figure 7. GC traces of a subset of C7+ total extract diluted in n-pentane obtained during DDB thermal cracking.

Figure 8. Simplified kinetic scheme proposed for DDB thermal cracking.

likely to undergo disproportionation with DDB to generate 1-phenylethyl and 1-phenyldodecyl radicals. This process might explain the significant amounts of ethylbenzene in our experiments, while styrene was observed in very small amount. However, this set of reactions is not enough to explain our pyrolysis results, especially the formation of undecene and the large amounts of toluene. Undecene might come from the disproportionation of undecyl radical with other radicals in the medium. However, if this pathway was efficient to form undecene, it would be for decene as well. Hence the pyrolysis of DDB would generate large amounts of decene rather than decane, which is not observed in our experiments. Two other pathways have been discussed in the literature to explain the occurrence of toluene and

undecene. The first one is a molecular mechanism wherein DDB generates toluene and undecene in a single step.33 However, by pyrolyzing pentadecybenzene together with deuterated tetralin, Savage and Klein18 demonstrated that this type of intramolecular retro-ene reaction is not favored during the thermal cracking of alkylbenzenes. The second pathway is a free-radical mechanism wherein a 1-phenyl-3-dodecyl radical decomposes rapidly to produce undecene and benzyl radical.18,25 According to Savage and Klein,18 although the formation of 1-phenyl-3-dodecyl radical by hydrogen abstraction at the γ-position is not efficient compared to that of 1-phenyl-1-dodecyl radical by hydrogen abstraction at the R-position, this pathway is the most (33) Mushrush, G. W.; Hazlett, R. N. Ind. Eng. Chem. Fundam. 1984, 23, 288.

Thermal Stability of Alkylaromatics

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Figure 9. Comparison of DDB conversion in laboratory and geological conditions with those obtained13,16 on the n-C25, the 9-methylphenanthrene and Type I (Figure 9a), Type II (Figure 9b), and Type III (Figure 9c) kerogens.

plausible to explain the amounts of toluene recovered. We supported this pathway in the scheme of Figure 8. Comparisons between kerogen cracking and model compound degradation were done under non isothermal conditions in both laboratory (25 °C/min between 300 °C and 650 °C) and geological conditions (1.25 °C/my between 100 and 200 °C). Results are presented in Figures 9a, b, and c and in Table 8. It is generally admitted1 that petroleum is not expelled as soon as it is generated and there is a migration threshold which depends on various geochemical (organic carbon content and kerogen Type) and geological parameters (mineralogy, mineral permeability and porosity). From observations on case studies,34-35 this migration threshold is (34) De Barros Penteado, H.; Behar, F. In Petroleum Systems of South Atlantic Margins; AAPG Memoir 73; Mello, M. R., Katz, B. J., Eds.; 2000; pp 179-194.

reached for kerogen transformation (TR) higher than 50%. Thus, five kerogen TR were selected between 50 and 90% for the 3 kerogens under study and corresponding temperatures were determined using global kinetic parameters4 given in Figure 3. Conversions of model compounds were calculated at temperatures corresponding to the selected kerogen conversions: they were derived from the Arrhenius diagram obtained in the present study for the DDB, and already published data13,16 for the n-C25 and the 9-MPh. In laboratory conditions, results show that, for TR lower than 90%, the 9-MPh being the most stable compound, its cracking never overlaps that of kerogen whatever its type, nor (35) Hill, R. H.; Bence, E. A.; Behar, F.; Curry, D. J.; Symington, W. A.; Vandenbroucke, M.; Lacombre, D. Presented at the 19th International Meeting on Organic Geochemistry, 6-10 Sept 1999, Istanbul, Turkey.

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Table 8. Conversion Calculated for the DDB, the 9-MPh, and the n-C25 at Five Transformation Ratios of the Three Kerogens in Laboratory and Geological Conditions laboratory conditions (25 °C/min)

geological conditions (1.25 °C/my)

type

TR

T, °C

DDB

MPh

n-C25

T, °C

DDB

MPh

n-C25

I

50 60 70 80 90 50 60 70 80 90 50 60 70 80 90

479 484 490 496 503 450 458 466 475 487 462 473 485 501 529

18.0 23.0 29.0 36.6 47.8 4.3 6.5 9.9 15.4 26.3 8.1 13.5 23.4 44.8 87.7

0.7 0.9 1.2 1.5 2.2 0.1 0.2 0.4 0.6 1.0 0.3 0.5 0.9 2.0 7.2

28.8 38.3 49.3 62.3 78.3 4.9 8.4 14.1 24.1 44.7 10.9 20.6 39.4 74.7 99.9

145 147 149 150 153 132 137 141 144 149 153 158 166 174 191

25.1 31.6 39.2 48.5 61.1 3.7 7.5 13.5 22.4 40.0 63.3 88.2 99.9 100.0 100.0

0.5 0.7 0.9 1.2 1.7 0.1 0.1 0.3 0.5 0.9 1.8 3.9 11.7 35.0 98.4