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Thermogravimetric Studies on Pyrolysis and Combustion Behavior of a Heavy Oil and Its Asphaltenes Aprameya Ambalae,† Nader Mahinpey,*,† and Norman Freitag‡ Faculty of Engineering, UniVersity of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada, S4S 0A2, and Saskatchewan Research Council (SRC), 6 Research DriVe, Regina, Saskatchewan, Canada, SK S4S 7J9 ReceiVed September 1, 2005. ReVised Manuscript ReceiVed January 24, 2006
A thermogravimetric analyzer (TGA) was used to obtain information on the pyrolysis and combustion behavior of both crude oil (Neilburg) and its asphaltenes, each mixed with reservoir sand. Of all the saturate, aromatic, resin, and asphaltene fractions, asphaltenes contribute the most to the formation of coke (fuel). Temperatureramped as well as the isothermal pyrolysis experiments on whole oil and asphaltenes were analyzed to determine the temperature at which coke formation was maximized. Furthermore, isothermal combustion curves for coke derived from whole oil and asphaltenes were obtained to provide reliable data for calculating the kinetics of the reactions. The classical Arrhenius model was applied, and the activation energy for the combustion of coke formed from pure asphaltenes and from the whole oil was calculated. The results showed that the Arrhenius model fitted the data well in the entire range of temperatures the experiments were conducted. The source material for the coke led to modest differences in its reactivity. The observed activation energy for asphaltenes was 117.7 kJ/mol, and for the whole oil it was 129.5 kJ/mol, which indicates that they were in close agreement. Also, the combustion of coke from asphaltenes showed a reaction order of 0.4 at 375°C, which gradually increased to 0.9 at 525°C. For whole oil, it increased from 0.5 at 375°C to 0.7 at 500°C.
Introduction In situ combustion (ISC) is an enhanced oil recovery (EOR) technique. The heat required to reduce the viscosity of the heavy crude is obtained mostly from the combustion of coke formed during cracking in the reservoir. ISC is a complex process involving simultaneous heat and mass transfer in a multiphase environment coupled with chemical reactions of the crude oil combustion.1 To develop the ability to forecast and improve the design and field performance of the ISC processes, the development of an accurate reaction kinetics model for numerical simulation is required. It is believed that much of the coke is formed from asphaltenes. However, some of the coke is formed from other fractions of whole oil. Hence, it is necessary to know whether coke formed from asphaltenes fraction oxidizes in the same manner as coke from whole oil. If so, then a reliable coke combustion model could be obtained on data based only on asphaltenes pyrolysis, which can generally be conducted more easily and more accurately than pyrolysis of the whole oil. Moreover, coke formed from only the asphaltene fraction of oil could be treated as a representative of coke formed from whole oil in ISC field projects. Two other major reactions occur in ISC, namely, low-temperature oxidation (LTO) and hightemperature oxidation (HTO).2 However, our focus in this article * Corresponding author. Phone: (306) 585-4490. Fax: (306) 585-4855. E-mail:
[email protected]. † University of Regina. ‡ Saskatchewan Research Council. (1) Kok, M. V.; Okandan, E. Kinetic analysis of in situ combustion process with thermogravimetric and differential thermogravimetric analysis and reaction tube experiments. J. Anal. Appl. Pyrolysis 1995, 31, 63-73. (2) Prats, M. Thermal RecoVery; Society of Petroleum Engineers of AIME: New York, 1982.
will be on pyrolysis of and reactions occurring in asphaltenes and whole oil and on HTO of the resulting coke. Use of thermal analysis techniques dates back to 1959, when Tadema3 conducted experiments to understand and analyze combustion of crude oil. One of the earliest applications of thermogravimetry to test and characterize heavy fuel oils was conducted by Ciajolo and Barbella.4 In recent years, study on combustion behavior of fossil fuels using thermal analysis and differential scanning caorimetry (DSC) has gained wide acceptance among researchers.5 Freitag and Exelby6 conducted reactor experiments using the saturate, aromatic, resin, and asphaltene (SARA) fractions to model the pyrolysis reactions occurring in high temperature enhanced oil recovery processes. In their experiments, oils from two different reservoirs were used, and they discovered that the results obtained from the two samples were similar. This suggested that the model might apply to a broad range of oils. Reservoir sand has shown significant catalytic activity during ISC.7 Thermal analysis experiments examining pyrolysis and combustion have demonstrated enhancement in the conversion rate of oil to coke in the presence of reservoir rock. (3) Tadema, H. J. Mechanism of oil production by underground combustion. Proceedings of the 5th World Petroleum Congress, 1959; pp 279-287. (4) Ciajolo, A.; Barbella, R. Pyrolysis and oxidation of heavy fuel oils and their fractions in a thermogravimetric apparatus. Fuel 1984, 63, 657661. (5) Karacan, O.; Kok, M. V. Pyrolysis Analysis of Crude Oils and Their Fractions. Energy Fuels 1997, 11, 385-391. (6) Freitag, N. P.; Exelby, D. R. A SARA-based model for simulating the pyrolysis reactions that occur in high-temperature EOR processes. Proceedings of the Canadian International Petroleum Conference, Calgary, Alberta, 2002. Canadian Institute of Mining, Metallurgy & Petroleum: Calgary, Canada, 2002. (7) Ranjbar, M. Influence of reservoir rock composition on crude oil pyrolysis and combustion. J. Anal. Appl. Pyrolysis 1993, 27, 87-95.
10.1021/ef0502812 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/18/2006
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Figure 2. Typical comparison curve for pyrolysis of asphaltenes at 450, 500, and 550°C.
Figure 1. Schematic diagram of the TGA apparatus.9
Verkoczy8 conducted experiments on the formation of coke and was able to predict the temperature at which coke formation would be complete. He also showed that addition of mineral matrix increased the coke formation from an initial 37% to a final 315% of the initial weight. Experimental Section Equipment. All pyrolysis and oxidation experiments were conducted in a DuPont Instruments 951 TGA-Thermal Analyzer 2100 thermogravimetric analyzer. This instrument is capable of directly measuring and recording the weight loss of any of the SARA fractions or whole oil and their products during experiments. A Varian Analytical Instruments Intelligent Digital Flow Meter 13101 gas flow meter was used to measure the flow rate of nitrogen and air supplied to the TGA. A schematic diagram of the apparatus used for the study is shown in Figure 1. Materials. Oil from the Neilburg pool on the Saskatchewan side of the Lloydminster heavy oil region was used in the experiments, along with the asphaltenes separated from this oil. The Neilburg oil had a density of 959.5 kg/m3 at 15°C (15.8° API) and a viscosity of 1700 mPa at 20°C. Asphaltenes were recovered from the oil by ultrasonic dispersion in 40 volumes of n-pentane, overnight flocculation, and vacuum filtration through 0.8-µm filter paper. The remaining solvent was removed by evaporation in a vacuum oven. Neilburg oil occurs in naturally unconsolidated sands, and a sample of this sand was obtained from the oilfield cores, which had been cleaned by toluene extraction followed by heating in air to approximately 350°C for 4 h. The sand was cleaned further in the same manner as reported by Freitag and Exelby.6 The light ends with boiling points up to 340°C were removed from the reservoir oil by distillation. The resulting viscous liquid was used in the experiments. Reservoir sand was premixed with whole oil and asphaltenes in the ratios of 1:4 and 1:5, respectively. The samples were designated as Sample A for asphaltenes + sand and Sample B for whole oil + sand. The effect of sand concentration differences in the end product was subsequently eliminated when the resulting data were normalized to 1 mg of pure sample. Procedures for Temperature-Ramped Screening Tests. Pyrolysis and oxidation experiments were conducted on Samples A and B in the TGA to determine the temperature for coke preparation. (8) Verkoczy, B. Factors affecting coking in heavy oil cores, oils and SARA fractions under thermal stress. J. Can. Pet. Technol. 1993, 32(7), 25-33. (9) Ren, Y.; Freitag, N. P.; Mahinpey, N. A simple kinetic model for coke combustion during an in-situ combustion (ISC) process. Proceedings of the Canadian International Petroleum Conference, Calgary, Alberta, 2005. Canadian Institute of Mining, Metallurgy & Petroleum: Calgary, Canada, 2005.
The temperature was ramped at the rate of 10°C/min. The flow rate of nitrogen for pyrolysis and air for combustion was fixed at 45 mL/min for all the experiments. All the experiments were started at room temperature. Pyrolysis (Formation of Coke). About 40 mg of Sample A, stored in an airtight bottle at room temperature, was placed on the cleaned platinum TGA pan. Nitrogen was flushed through the TGA tube, and the temperature was ramped. Three such runs were conducted up to temperatures of 450, 500, and 550°C, respectively (Figure 2). After the final temperature was reached, Sample A was allowed to remain at that temperature for 45 min to complete the reactions for the formation of coke. In each of the cases, the TGA, still under nitrogen atmosphere, was cooled back to room temperature. The weight loss versus temperature data for each of the experiments were saved for further analysis. Similar experiments were conducted for Sample B. Experiments were repeated for both the samples, and it was observed that the error was less than 2% in terms of the overall weight loss. Procedures for Isothermal Pyrolysis Experiments. In addition to the temperature-ramped tests described above, isothermal pyrolysis and oxidation experiments were conducted on both Samples A and B. The initial setup was the same as that for the temperatureramped experiments. In all the isothermal runs, a plunging technique, wherein the sample was placed on the pan and only the furnace was heated to the predetermined temperature, was used. Once this temperature was achieved, the sample pan was moved sideways into the heated zone of the furnace where, after the sample was heated rapidly to the furnace temperature, the reactions proceeded isothermally. The weight loss of the sample versus time/ temperature was monitored in a real-time plot, and the data were recorded. The isothermal period for all runs was fixed at 45 min to ensure that the coking reactions reached completion. Once the isothermal period had elapsed, the TGA was cooled to room temperature under continuous nitrogen flush. Isothermal Oxidation. The main objective of the oxidation experiments was to obtain sufficient data to establish the reaction kinetics and determine the Arrhenius parameters. Isothermal experiments were conducted at 25°C intervals from 375 to 525°C on the coke formed from asphaltenes and from 375 to 500°C on the coke from whole oil. At higher temperatures, the reaction was too rapid for any data points to be gathered. Below 375°C, the combustion rate became prohibitively slow for TGA tests. The time duration, for which the sample was held constant at any given temperature, varied. At the lowest temperature, the sample was held for about 180-250 min, whereas at the highest temperature, only 4-5 min were required for the complete oxidation to take place.
Results and Discussion Analysis of the derivative thermogram curve of the preliminary temperature-ramped pyrolysis experiments showed that, for Sample A, the steepest slope in terms of percent weight loss of the sample occurred at around 450°C. At lower temperatures (450°C), much of the whole oil had evaporated, leaving behind much less oil to be pyrolyzed into coke. Conducting combustion runs on the coke obtained from such high temperatures was not possible as it was difficult to collect reliable data points for determining the kinetics. The case was the same for coke formed from asphaltenes. To facilitate direct comparison, the coking temperature was made the same for both the asphaltenes and the whole oil. An isothermal pyrolysis temperature of 425°C was chosen for both the samples because, at this temperature, production of coke was maximized. Isothermal combustion runs for coke formed from asphaltenes and whole oil were conducted in a predetermined temperature range. It was observed that the thermal traces of Samples A and B appeared to be smooth, which implied that a homoge(10) Khulbe, K. C.; Sachdev, A. K.; Mann, R. S.; Davis, S. TGA studies of asphaltenes derived from Cold-Lake (Canada) bitumen. Fuel Process. Technol. 1984, 8, 259-266.
Figure 7. Comparison plot for coke combustion from asphaltenes and whole oil at 375 and 400°C.
neous residue was formed at the end of each run. Furthermore, all data were normalized to 1 mg of pure sample using available, custom-written software. Figures 5 and 6 show the isothermal TGA traces produced at each of the studied temperatures for coke derived from asphaltenes and whole oil, respectively. Comparison plots of coke combustion for both the samples elaborate the combustion pattern. These plots at temperatures of 375 and 400°C and higher temperature of 500°C are shown in Figures 7 and 8, respectively. Observations from Figure 7 indicate that, at 375°C, the combustion rate of coke formed from whole oil was slower than that from asphaltenes. At 400 °C, during the first 25 min, the curves nearly coincided, indicating a similar combustion pattern. Furthermore, the coke formed from asphaltenes showed a slightly higher rate of combustion. Hence, at lower temperatures, the combustion of coke formed from the two samples was moderately different. At 500°C, the combustion curves of coke obtained from both the samples overlapped in the first 4 min, showing a nearly similar combustion pattern. After 4 min, in the case of coke formed from asphaltenes, combustion occurred faster than with coke from whole oil, as shown in Figure 8. A plausible
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Figure 8. Comparison plot for coke combustion of asphaltenes and whole oil at 500°C. Figure 10. Arrhenius plot for combustion of coke formed from whole oil.
Figure 9. Arrhenius plot for combustion of coke formed from asphaltenes. Table 1. Rate Constants for Combustion of Coke Formed from Pure Asphaltenes temp (°C)
375
400
425
450
475
500
525
rate constant (min -1) regression coefficient
0.013
0.032
0.058
0.142
0.322
0.454
0.812
0.96
0.97
0.97
0.97
0.98
0.98
0.98
Table 2. Rate Constants for Combustion of Coke Formed from Whole Oil temp (°C)
375
400
425
450
475
500
rate constant (min -1) regression coefficient
0.009 0.98
0.024 0.99
0.056 0.99
0.127 0.99
0.242 0.99
0.443 0.98
Table 3. Arrhenius Parameters for Asphaltenes and Whole Oil sample
activation energy (kJ/mol)
pre-exponential factor (1/min)
asphaltenes whole oil
117.7 129.5
0.44 × 108 2.69 × 108
explanation for this would be that the coke formed from asphaltenes tends to oxidize faster than that formed from the whole oil. Literature suggests that this phenomenon is found in asphaltene coke derived from different samples, and therefore, it may suggest that it is an inherent property of the matrix. In our studies, this type of trend was observed in the entire range of temperature in which the experiments were conducted. Data Analysis. Kisler and Shallcross11 have shown that the least accurate data points on a thermogravimetric weight loss curve are those at the beginning and end of the run. Data points from each run were chosen from the portion of the data after the oxygen concentration around the TGA pan attained a steady state and when the temperature around the TGA pan became constant. For each of the runs, the time at which the temperature around the sample became isothermal was chosen as the start (11) Kisler, J. P.; Shallcross, D. C. An improved model for the oxidation process of light crude oil. Trans. Ind. Chem. Eng. 1997, 75(A), 392-400.
Figure 11. Curve fitting for combustion (at 375°C) of coke formed from asphaltenes.
time. Accordingly, for combustion of coke from asphaltenes at 375 and 500°C, data were chosen in the time range of 15-120 min and 1-6 min, respectively. For combustion of coke from whole oil at 375 and 500°C, the time range was between 25 and 200 min and 1-3.5 min, respectively. The same procedure was adopted for intermediate temperatures. The Arrhenius model does not apply to varying order of reaction. For simplicity and for modeling purposes, a first-order reaction in the coke concentration was assumed for both the samples in the entire temperature range, which is expressed by the following rate equation:
dCA ) -knCA dt
(1)
Integration of the above equation yields
ln
( )
CA0 ) kn(t2 - t1) CA
(2)
where CA is the concentration of the reactant i, kn is the rate constant, and (t2 - t1) is the elapsed time. The rate constants at the different temperatures were determined by linear regression of eq 2, given above. The natural logarithm of the weight percent of the concentration was regressed against the difference in the initial time to the time corresponding to that concentration. Tables 1 and 2 contain the set of these rate constants. The regression coefficient for the combustion of coke was in the range of 0.96-0.98 for Sample A and 0.98-0.99 for Sample B.
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Figure 12. Variation of regression coefficient value with respect to reaction order n for combustion (at 375°C) of coke from asphaltenes.
Figure 15. Curve fitting for combustion (at 525°C) of coke formed from asphaltenes.
Figure 13. Curve fitting for combustion (at 375°C) of coke formed from whole oil.
Figure 16. Variation of regression coefficient value with respect to reaction order n for combustion (at 525°C) of coke from asphaltenes.
Order of the Reaction. The effect of the order of equation n with respect to the concentration of the sample CA was given by Wilson12 and is: Figure 14. Variation of regression coefficient value with respect to reaction order n for combustion (at 375°C) of coke from whole oil.
The dependence of the rate constant upon temperature was assumed to be given by the well-known Arrhenius equation:
kn ) Ai exp(-Ea/RT)
(3)
which in logarithmic form is:
ln(kn) ) ln(Ai ) - Ea/RT
(4)
The energy of activation, Ea, derived for coke combustion of Sample A was 117.7 kJ/mol, and for Sample B it was 129.5 kJ/mol. The pre-exponential factors for Samples A and B were 0.44 × 108/min and 2.69 × 108/min, respectively. These values are tabulated in Table 3. Figures 9 and 10 show the Arrhenius plots for coke combustion of Samples A and B, respectively. In both cases, an Arrhenius plot fits the data well, which indicates that the Arrhenius model provides a good description of the kinetics of coke combustion.
[( ) ]
CA0 1 (n - 1)kn(CA0)n-1 CA
n-1
- 1 ) τ for n * 1
(5)
where CA0 is the initial concentration of the sample on the TGA pan, CA is the concentration of the sample at time τ, n is the order of the reaction, and kn is the rate constant determined from the regression of the data. The values of the initial and final concentration (weight percent) of the sample, the rate constant, and the time interval were entered into the above equation. The results were then fitted by linear regression to eq 5 to reveal the best order of reaction. From the experimental data at 375°C, the combustion of coke formed from Sample A showed an order of 0.4. Figures 11 and 12 show the curves for different values of n and the variation of the regression coefficient plotted against the different orders of reaction. At 375°C, the reaction was half-order for Sample (12) Wilson, J. W. Fluid catalytic cracking technology and operations; PennWell Books: Tulsa, OK, 1997.
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Sample B at 500°C, the reaction order was 0.7. Comparison curves and plots of the regression coefficient against order of reaction for Samples A and B are shown in Figures 15-18, respectively. Conclusions
Figure 17. Curve fitting for combustion (at 500°C) of coke formed from whole oil.
1. In the HTO region, between 475 and 500°C, the combustion rates of coke from asphaltenes and whole oil were in better agreement. At lower temperatures of 375-400°C, the combustion pattern showed moderate differences. 2. The Arrhenius reaction model described the combustion behavior of coke from the asphaltenes and whole oil samples in the temperature range of 375-525°C and 375-500°C, respectively. 3. The activation energy calculated for the combustion of coke formed from asphaltenes was 117.7 kJ/mol, and the associated pre-exponential factor was 0.44 × 108/min. For the combustion of coke from whole oil, the activation energy was equal to 129.5 kJ/mol and the pre-exponential factor was 2.69 × 108/min. 4. The observed order of reaction with respect to the coke concentration for asphaltenes varied from 0.4 to 0.9 in the range of 375-525°C, and for whole oil, from 0.5 to 0.7 in the range of 375-500°C. Acknowledgment. We are grateful for the help and technical assistance provided by Mr. Ray Exelby. Help by Yan Ren in conducting some of the experiments is appreciated. We wish to thank the Saskatchewan Research Council (SRC) for providing an opportunity to work in their laboratory and for making the data analysis software available. Thanks are also extended to the Petroleum Technology Research Centre (PTRC) and Natural Science and Engineering Research Council (NSERC) for providing funding.
Nomenclature
Figure 18. Variation of regression coefficient with respect to order n for combustion (at 500°C) of coke formed from whole oil.
B, and Figures 13 and 14 show comparison curves and plots of the regression coefficient value versus n, respectively. It was observed that the order of reaction of the two samples crossover at a certain temperature, indicating that they are fairly similar but not identical, as they vary at other temperatures. The order of the reaction gradually increased with increasing temperature. For Sample A at 525°C, it was equal to 0.9. For
n ) Order of the reaction kn ) Rate constant (min -1) τ ) Time (min) CA ) Concentration of the sample at time τ(mg/cm3) CA0 ) Initial concentration of the sample (mg/cm3) dCA/dt ) First derivative of the concentration with respect to time (mg/min‚cm3) (t2 - t1) ) Elapsed time (min) Ai ) Pre-exponential factor (min -1) Ea ) Activation energy (kJ/mol) R ) Universal gas constant (J/mol K) T ) Temperature (°C) EF0502812
