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Oxidation kinetic evaluation of the low temperature oxidized products of Tahe heavy oil characterized by the distributed activation energy model
Journal of Petroleum Science and Engineering 181 (2019) 106155
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Oxidation kinetic evaluation of the low temperature oxidized products of Tahe heavy oil characterized by the distributed activation energy model
T
Wan-fen Pua,c,∗∗, Xiao-long Gonga, Ya-fei Chena,∗, Xue-li Liub, Jian Huib, Chen Guob, Mikhail A. Varfolomeevc a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People's Republic of China Northwest Oilfield Branch Company, SINOPEC, Urumqi, Xinjiang, 830011, People's Republic of China c Department of Physical Chemistry, Kazan Federal University, Kazan, 420008, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Heavy oil Low temperature oxidation Oxidized product Oxidation kinetic Coke deposition DAEM
In view of the significance of low temperature oxidation (LTO) for in-situ combustion (ISC) of heavy crude oil and to provide further data for Tahe ISC numerical simulation, this research aimed to investigate the nonisothermal oxidation behavior of heavy oil after static LTO reactions. Thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques were used to characterize oxidation behavior, and distributed activation energy method (DAEM) was used to calculate kinetic parameters. The results showed that the non-isothermal oxidation process of static LTO oxidized oil was the result of the interaction between residue and coke, and coke can significantly reduce the activation energy of oxidized oil. Only one HTO region was identified for the coke from DTG/DSC curves, and it had the lowest activation energy (114.57 kJ/mol) and the highest heat enthalpy (24.3 kJ/g), indicating high oxidative activity with maximum heat release potential. A satisfactory corresponding relation between DTG and DSC curves was presented for all samples in FD and HTO regions, which indicated that the sequential reaction mechanisms, fuel deposition and combustion, were undergone. In LTO region, there was a temperature difference between the peaks of the mass loss and heat flow, indicating that the LTO process was more complex with a multi-step control mechanism should be considered to analyze and simulate the LTO stage.
1. Introduction As an unconventional crude oil resource, the heavy crude oil has been verified to be more than twice than the conventional light oil in the world (Shah et al., 2010). With the reduction of recoverable reserves of light crude oil and the ever-increasing energy demand, it is crucial and necessary to increase the production of heavy crude oil. Although many efforts have been made to decrease the heavy oil viscosity and enhance the recovery, there still remains enormous challenges to exploit the heavy oil (Larter et al., 2006; Li et al., 2017). Since the viscosity of heavy crude oil is greatly sensitive to the temperature, thermal recovery techniques including in-situ combustion (ISC), steam flooding, and cyclic steam injection, etc. are proposed. With regard to the ISC, air or oxygen-enriched gas is injected into the target formation, self-ignition or artificial ignition is adopted to make the oil reservoir temperature reach the crude oil ignition point, the
viscosity of heavy oil is reduced due to the large amount of heat generated by combustion, combined with other relevant mechanisms, then the oil recovery is enhanced (Kovscek et al., 2013). In general, heavy crude oil oxidation was divided into three stages: low temperature oxidation (LTO), fuel deposition (FD), and high temperature oxidation (HTO). In LTO stage, the light hydrocarbon components in the heavy oil mainly undergo oxidation addition reactions to form partial oxides such as aldehydes, ketones and alcohols. Then enter FD period, where fuel (coke) deposition occurs primarily in preparation for high temperature oxidation. Finally, the crude oil will enter the HTO stage, and the coke deposited in the FD stage will undergo a violent combustion reaction, generating CO2 and H2O, and releasing a large amount of heat. Hence, the amount of coke deposited is an important factor in determining the success of ISC (Akin et al., 2000; Li et al., 2013). In recent years, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ramped-temperature-oxidation tests (RTO)
∗
Corresponding author. Corresponding author.State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, People's Republic of China. E-mail addresses: [email protected] (W.-f. Pu), [email protected] (Y.-f. Chen). ∗∗
https://doi.org/10.1016/j.petrol.2019.06.019 Received 10 March 2019; Received in revised form 4 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
Journal of Petroleum Science and Engineering 181 (2019) 106155
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and combustion tube (CT), etc. have been widely recognized as the thermal analysis methods to study the oxidation characteristics and kinetics of crude oil (Ambalae et al., 2006; Anto-Darkwah and Cinar, 2016; Cinar et al., 2011a, 2011b; Lee and Noureldin, 1989; Pu et al., 2017b). By reason of coke plays an important role in the kinetics of oxidized heavy oil, recent emphasis has focused on the mechanisms and characteristics of coke deposition and its changes in the oxidation kinetic parameters of crude oil. For instance, Cinar et al. (2011a) studied the role of oxygen in coke formation by X-ray photoelectron spectroscopy (XPS) and ramped temperature oxidation (RTO) tests. The results showed that the quality (or reactivity) of coke produced was a function of oxygen presence/absence, compared to coke produced in an inert atmosphere, since the surface of the coke formed after oxidation had additional oxygen-containing functional groups, coke produced under oxygen conditions was significantly more reactive. Ren et al. (2007) proposed a two-coke model to describe the combustion process of coke, under low temperature conditions, this model matched the experimental data more reasonably than the classical Arrhenius model. It was concluded that coke from whole oil was more reactive than coke from asphaltenes. Verkoczy (1993) studied the coking process of heavy oil in the oxidation stage and found that the coke production of heavy oil was between 5 and 10%, which was slightly lower than the sum of coke produced by SARA oxidation. It indicated that the interaction between SARA affected the coking process of heavy oil. Chen et al., 2017 conducted oxidation experiments of heavy oil at 250–500 °C in sealed capillaries. It was found that chlorobenzene insolubles (CI, hard coke) and toluene insolubles (TI, soft coke) were formed at 440 °C and 350 °C, respectively. At the same time, it was measured by electron spin resonance (ESR) that the radical concentration of CI was higher than TI. In previous work, Li et al. (2015) conducted oxidation experiments under different pressures for Tahe heavy crude, and carried out kinetic analysis of heavy crude and oxidized oil samples. It was found that there was obvious coke formation after oxidation of heavy oil, and with the increased of pressure, the activation energy of oxidized oil samples showed a decreasing trend (Chen et al., 2019a, 2019b; Liu et al., 2017, 2019). Despite extensive research has been done on coke and heavy oil, the oxidation kinetics of the effect of coke on ISC under non-isothermal oxidation conditions are not well understand. Hence, in this work, the oxidation mechanism and coke deposition process of Tahe heavy oil were studied by TG/DSC analysis. Particular attention was placed on the kinetics of oxidized oil, coke, residue, which formed by static oxidation experiments under reservoir conditions. And the differences in non-isothermal oxidation processes and changes in kinetic parameters were investigated. The results of this research work were of great significance to model Tahe in-situ combustion process.
Fig. 1. Viscosity−temperature curve for the heavy oil.
pressure oxidation unit was pressurized to 40 MPa, and placed in an air bath at 120 °C (reservoir conditions) for 10 h to check leakage. (2) Add 5 g of heavy oil to the unit, and then pressurized again to 40 MPa by air and set the temperature to 120 °C. (3) The oxidation reaction was terminated after 7 days. (4) Because coke was insoluble in toluene, the solid products (∼4 g) were extracted by a large amount of toluene to obtain coke and residue, and then the separation was dried at 100 °C in nitrogen atmosphere. It was observed that 3.02 g of residue and 0.98 g of coke were formed after the oxidation reactions, corresponding to the coke yield of 24.5 wt%. 2.3. Non-isothermal thermogravimetric analysis In this work, NETZSCH TG 209F3/DSC 214 (NETZSCH Ltd., Germany) were used to perform TG-DSC testing on samples from this experiment to study the kinetic characteristics of oxidized oil, coke and residue. In order to eliminate the influence of the single heating rate on the kinetic parameters, the samples were tested at various heating rates according to the recommendations of the ICTAC Committee (International Confederation for Thermal Analysis and Calorimetry) (Vyazovkin et al., 2011). A small amount of sample (∼7 mg) was heated at a rate of 5, 15, 25 °C/min, respectively, from ambient temperature to 800 °C at an air flow rate of 50 ml/min. The system was calibrated to ensure reliability prior to each test. In the end, the mass loss and heat flow of the samples against temperature changes were recorded by the data acquisition system on the computer-side.
2. Experimental section 3. Results and discussion 2.1. Materials 3.1. Non-isothermal oxidation TG/DTG analysis The crude oil in this experiment was provided by Tahe Oilfield, located in western China. Reservoir pressure and temperature are 40 MPa, 120 °C respectively. Its viscosity-temperature curve is shown in Fig. 1. Moreover, Table 1 shows the basic properties of this heavy oil. Obviously, the viscosity and density of heavy oil was much higher than ordinary crude oil, and SARA fractions analysis also verify this result from the side because it contains the most abundant asphaltenes (47.68 wt%). Toluene was used in the experiment to separate coke and residue.
The non-isothermal oxidation TG/DTG profiles of oxidized oil, residue and coke at different heating rates are shown in Figs. 3–5, respectively. According to different trends of the curves, three reaction stages for oxidized oil and residue, named LTO, FD, and HTO, were identified. Nevertheless, coke as fuel formed after static low temperature oxidation, only one exact stage which named HTO, was identified. This result was consistent with previous research findings (Yuan et al., 2015; Zhao et al., 2018). The reaction intervals, peak temperatures, and mass losses of the three samples are listed in Table 2. For oxidized oil and residue, there was a small amount of mass loss before the onset of low temperature oxidation, averaging about 1.64% and 1.97%, respectively, which primarily reflected the volatilization of light components. Thereafter, the two samples entered the LTO stage with a temperature range of approximately 130 °C–350 °C. During this
2.2. Oxidation experiments The heavy oil oxidation experiment and analysis equipment are shown in Fig. 2, and the following is a brief introduction to the operation process of this experiment. (1) A high temperature and high 2
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Table 1 Basic properties, elemental composition and SARA fractions of heavy oil. Physical properties (25 °C) 3
Density (g/cm ) 1.045
Viscosity (mPa·s) 1.11 × 105
Element (wt%) C 82.1
H 10.67
SARA (wt%) O 3.36
N 0.72
S 3.15
Saturates 16.86
Aromatics 26.11
Resins 9.35
Asphaltenes 47.68
result during the ISC process. As the TG curves shown in Fig. 5, after undergoing high temperature combustion reactions, the remaining mass of oxidized oil, residue and coke were 2.79%, 0.56% and 7.81%, respectively. Coke molecule contained heterocyclic ring, metal clathrate, and condensed ring, which were classified into soft coke and hard coke depending on the molecular weight and molecular structure (Chen et al., 2017; Pu et al., 2017a; Zhao et al., 2018). Among them, because of the larger microscopic surface area and molecular activity, soft coke was easier to burn in the relatively low temperature range, while part of the hard coke was not completely consumed under the experimental conditions, and became the leftover after the combustion reaction. In addition, another noteworthy phenomenon was that, as shown in Table 2, the initial temperature, the termination temperature, and the peak temperature of coke in HTO were much lower than the other two samples, which indicated that coke had higher reactivity during HTO. This high oxidative reactivity reflected the degree and ability of oxygen consumption in HTO region. Therein the C–H and C–C bonds were cleaved by the O atom to form CO, CO2, and H2O, during which released a large amount of heat.
period, the oxygen addition reactions occurred, oxygen atom and hydrocarbon molecule combined to form alcohol, aldehyde, ketone, and hydroperoxide, although this increased the viscosity of heavy oil, it was a crucial step to form coke, which was the direct material of the combustion reaction in HTO region (Kök et al., 1998; Noureldin et al., 1987). Subsequently, both the condensation and cleavage reactions involved the above oxides, the former produced more macromolecules with a longer carbon chain and a higher degree of aromatization, while the latter produced small molecular weight hydrocarbons and small amounts of H2O, CO2 and CO. The mass loss of the residue (20.95%) in the LTO stage was higher than that of the oxidized oil (16.43%), since the oxidized oil had formed a part of coke under static oxidation conditions, resulting in fewer hydrocarbons were involved in the LTO stage reaction. At the same time, it was worth noting that as the heating rate increased, the peak temperature of the mass loss of the two samples also increased during this period, which indicated that the more obvious reaction lag was caused by higher heating rate. It was believed that the molecules had more time to accumulate energy at a low heating rate, followed by a chemical collision reaction in the activated state. As the oxidation temperature increased, DTG curves began to enter the FD stage. Since oxidized oil had formed part of coke, the high activity of coke allowed oxidized oil to show higher mass loss at this stage. For the residue, it was mainly through the reoxidation, condensation and aromatization of the LTO stage product to produce coke (Kök et al., 2017), preparing the fuel required for the subsequent HTO stage. In addition, some small molecular hydrocarbons were further oxidized or cleaved to form H2O, CO2 and CO into the atmosphere. Following the FD stage, due to the intensification of intermolecular collisions and the accumulation of fuel, the combustion reaction occurred in the HTO stage, which was widely considered to be the only important reaction during this period. The coke combustion reaction released a large amount of heat to reduce the viscosity and increase fluidity of the heavy oil in the formation, which was the most desirable
3.2. Non-isothermal oxidation DSC analysis Due to the different hydrocarbons involved in the reactions at different stages, the enthalpy of different stages of crude oil was different. In order to further study the thermal behavior of oxidized oil, residue and coke formed after static LTO reaction, DSC tests and analyses were performed for these three samples. Figs. 6–8 show the heat flow curves of the samples at different heating rates of 5, 10, and 15 °C/min. The exothermic peaks, peak temperatures, and heat enthalpy obtained by integrating the curves were given in Table 3. For the oxidized oil, three distinct exothermic reaction regions were observed from the DSC curves, corresponding to the three stages of the LTO, FD, and HTO in the DTG curves. However, it was particularly
Fig. 2. Simplified schematic for static LTO experiment and TG/DSC test. 3
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Fig. 3. TG−DTG curves for oxidized oil at different heating rates of 5, 10, and 15 °C/min.
Fig. 4. TG−DTG curves for residue at different heating rates of 5, 10, and 15 °C/min.
observed that the LTO ranges and peak temperatures determined in the DSC curves were slightly higher than the DTG curves, that is, the highest heat release took place after the fastest mass loss. And the corresponding relation between the two curves in the LTO stage was not very good, which meant that there was a significant difference between heat release and mass loss. Because the residue and oxidized oil had some of the same composition, same phenomenon also occurred in the DTG/DSC curves of the residue. This indicated that a large mass loss did not mean that the reactants were active, and it was not reasonable to determine the reactivity of the oxidized oil and the residue only by the heat flow rate. In fact, this phenomenon should be attributed to the low reactivity of the relatively light components in the residue and oxidized oil in the low temperature range. Considering that both samples had undergone high pressure LTO reaction, the heavy components such as asphaltenes and resins in the two samples were partially converted to coke (Murugan et al., 2011). Therefore, the samples mainly contained relatively light components such as saturates, aromatics, as well as some heavy components such as oxygenated compound, resins and asphaltenes. In many previous reports, it has been pointed out that heavy components such as asphaltenes were more reactive than light components such as saturates (Moschopedis and Speight, 1973; Pu et al., 2015; Verkoczy and Freitag, 1997). This reactivity referred to the ability to absorb oxygen by oxygen addition reaction, in which case, the oxygen atoms were incorporated into the molecular structure of heavy components to form various oxygenated compound and released some
heat. Therefore, at about 282 °C, the DTG curves reached the mass loss peak due to oxidation of relatively light components, and at about 373 °C, the exothermic peak was reached due to the oxygen addition reaction of the heavy components. For the FD and HTO stages, there was a good corresponding relation between the two curves for oxidized oil, residue and coke. This corresponding relation meant that the heat flow and mass loss in the FD and HTO stages were caused by the sequential reaction mechanisms considered to be fuel deposition and coke combustion. The same phenomenon also reported in the study of Yuan et al. (2017). Hence, this indicated that the LTO stage of oxidized oil and residue should be analyzed and modeled by a multi-step mechanism. Simultaneously, another noteworthy phenomenon was that the heat flow curves of oxidized oil at 10 °C/min and 15 °C/min were completely different from the performance at 5 °C/min, as shown in Fig. 6, the peak temperatures of the first two relatively high heating rate curves appeared in the FD stage, while the last curve appeared in the HTO stage. The same results were obtained by the repeated experiments under same conditions, which indicated that the reaction of oxidized oil at low heating rate was similar to the residue, both of which had a violent combustion reaction during the HTO stage. However, at relatively high heating rates, coke had undergone combustion reactions in FD stage of the oxidized oil and released the highest heat flow due to its high reactivity. Fig. 9 shows a comparison of the mass loss, differential thermal 4
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Fig. 6. DSC curves for oxidized oil at different heating rates of 5, 10, and 15 °C/ min.
Fig. 5. TG−DTG curves for coke at different heating rates of 5, 10, and 15 °C/ min.
weight and heat flow for three samples at heating rate of 5 °C/min. It was found that the mass loss path and the heat flow curves of the oxidized oil were always between the two samples, indicating that the oxidation process of the oxidized oil was the result of the interaction of coke and residue. As mentioned above, because of high activity of the coke, the DTG/DSC curves for coke showed that the mass loss peak temperature and the heat flow peak temperature were lower than the other two samples. In addition, it was observed that the DSC curves of the oxidized oil and residue in the LTO stage were not as smooth as the HTO stage, and the result may be due to the negative effect of the
Fig. 7. DSC curves for residue at different heating rates of 5, 10, and 15 °C/min.
endothermic reaction on the curves, such as evaporation and thermal cracking reaction in this interval. The DSC curves were integrated to show that the pure coke released the most heat enthalpy during the oxidation process, about 24.3 kJ/g, which was extremely advantageous for the stable combustion front in the ISC process.
Table 2 Reaction intervals, peak temperatures and mass loss of the samples. Samples
Oxidized oil
Residue
Coke
Heating rate (°C/min)
5 10 15 5 10 15 5 10 15
LTO
FD
HTO
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
132.36–309.25 134.36–326.73 143.31–340.75 136.75–325.97 141.64–345.37 146.98–355.63 / / /
265.52 276.84 286.93 272.84 293.82 303.23 / / /
18.87 15.54 14.89 20.89 21.18 20.78 / / /
309.73–449.51 326.12–473.63 340.27–477.46 325.64–457.31 345.44–478.36 355.72–489.43 / / /
402.27 433.72 431.88 418.53 401.01 455.64 / / /
46.69 50.32 43.75 38.06 41.02 46.34 / / /
449.64–525.73 473.35–565.57 477.12–569.31 457.12–521.31 478.43–552.69 489.14–577.78 347.32–457.38 379.48–504.92 380.34–513.68
480.45 506.02 509.92 485.03 510.97 527.63 403.24 411.67 398.83
31.97 31.70 38.15 40.26 37.26 32.47 69.29 67.46 65.05
5
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loss of the sample, A is the Arrhenius constant or pre-exponential factor, E is the activation energy corresponding to A, R is the universal gas constant (8.314 J/mol·K), T is the temperature, f (E) is defined as a function of activation energy E in each reaction, and β is the heating rate. f (E) is the normalized activation energy distribution function, and it is obvious that it follows the following relationship (Eq. (3)).
∫0
∞
f (E )dE = 1
(3)
Combine equations (1)–(3), and after appropriate approximation, derive the integral expression of DAEM as follows (Eq. (4)).
ln
3.3.1. Distributed activation energy method Due to the complex composition of heavy oil, the kinetic parameters calculated by the isothermal kinetic model have large errors. Therefore, in this work, Distributed Activation Energy Method (DAEM) was used to describe the thermal behavior. As a model-free method widely used to analyze complex reaction kinetic parameters, DAEM is based on two assumptions. On the one hand, the reaction system occurs simultaneously by a finite number of irreversible independent first-order reactions, and each reaction has a corresponding activation energy and a pre-exponential factor. On the other hand, the activation energy of each reaction is in the form of a function of a continuous distribution (Hashimoto et al., 1982). After long-term development, the DAEM method has made great progress in kinetics research. Miura linked the weight loss curves at different heating rates, eliminating the effect of heating rate on the kinetic parameters (Miura and Maki, 1998). The weight loss process of heavy oil can be described by Eq. (1).
∫0
∞
∫0
exp ⎛–A ⎝
t
E ⎞ dt ⎞ f (E ) dE = exp ⎛– ⎝ RT ⎠ ⎠
∫0
∞
Φ (E , T ) f (E )dE (1)
Φ(E,T) can be approximated by the following formula (Eq. (2)).
ART 2 −E / RT ⎞ Φ(E , T ) ≅ exp ⎜⎛− e ⎟ βE ⎝ ⎠
⎟
(4)
3.3.2. Calculation results and analysis Figs. 10–12 show the ln(β/T2) versus 1/T curves for the non-isothermal oxidation process of the three samples at same conversion rate using the DAEM model. Since the kinetic parameters of the LTO and HTO stages are more concerned in the actual analysis, the FD stage of oxidized oil and residue was incorporated into the HTO stage for kinetic calculation. The activation energy (E) and coefficient of determination (R2) at different conversion rates, as well as the average activation energy and pre-exponential factor at each stage were shown in Fig. 13 and Table 4, respectively. According to the calculation results, coke had the lowest activation energy (114.57 kJ/mol) among the three samples, which was direct evidence that the coke formed after static oxidation was highly reactive. This change was beneficial to the combustion process and heat release of the ISC. However, for the residue, it was observed that the activation energy in HTO stage was significantly higher than that in LTO, indicating a high threshold energy to trigger the HTO reaction. Interestingly, the activation energy of the HTO stage of the oxidized oil was lower than that of the LTO stage, which obviously resulted from the coke effect. In addition, it was worth noting that the activation energy obtained in this study was significantly higher than that reported in other studies, which may be due to the fact that asphaltenes and aromatics account for the largest proportion in this crude oil. It was reported that aromatics, as oxidation inhibitors, can significantly reduce the oxidation rate even at ultra-low levels (1%), making the oxidation process difficult and leading to higher energy requirements (Freitag, 2014; Wei et al., 2018). Fig. 13 shows the relationship between activation energy and conversion rate. For all samples, the points with poor fitting accuracy were
3.3. Non-isothermal oxidation kinetic analysis
w = w0
⎜
According to the TG data at different heating rates, the activation energy and the pre-exponential factor can be estimated from the slope and intercept respectively by plotting the Arrhenius curve of ln (β/T2) against 1/T under the same conversion rate.
Fig. 8. DSC curves for coke at different heating rates of 5, 10, and 15 °C/min.
1−
β AR w ⎞⎞ E = ln − ln ⎜⎛−ln ⎛1 − ⎟ − T2 E w RT 0 ⎝ ⎠⎠ ⎝
(2)
where w is the weight loss of the sample at time t, w0 is the total weight Table 3 Peak temperature, peak heat flow and heat enthalpy of the samples. Samples
Oxidized oil
Residue
Coke
Heating rate (°C/min)
5 10 15 5 10 15 5 10 15
LTO
FD
HTO
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/ g)
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/ g)
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/g)
353.59 363.46 359.43 387.11 384.64 393.15 / / /
4.53 8.22 15.88 6.96 7.29 5.34 / / /
3.21 2.80 3.63 2.22 1.37 0.74 / / /
414.19 446.20 443.39 431.15 448.51 419.03 / / /
12.06 19.26 42.29 8.93 13.37 10.82 / / /
8.46 11.58 14.51 1.68 2.12 1.63 / / /
488.80 510.73 523.04 490.64 507.96 519.42 417.72 461.13 474.86
15.85 17.63 24.80 32.81 33.29 41.21 15.99 21.96 29.35
8.03 6.35 6.69 15.89 11.09 10.65 24.03 24.17 23.08
6
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Fig. 10. DAEM plots for oxidized oil non-isothermal oxidation process at different conversion rates.
the remaining HTO reactions became much more easily to occur. 4. Conclusions In this work, the main focus was on the non-isothermal oxidation behavior of oxidized oil, residue, and coke resulting from static oxidation under reservoir conditions. Based on TG/DSC experimental data and DAEM calculation results, the following conclusions can be summarized. (1) For coke formed after static LTO reaction, only one region HTO was identified on the DTG/DSC curves, and the HTO temperature range was much lower than the other two samples. The activation energy was only 114.57 kJ/mol, indicating high reactivity with oxygen, and the maximum heat enthalpy was released during the non-isothermal oxidation process (24.3 kJ/g). (2) For oxidized oil, the non-isothermal oxidation process was the result of the interaction of coke and residue, and coke can significantly reduce the activation energy in HTO stage. (3) The fastest mass loss in the LTO stage of oxidized oil and residue occurred before the highest heat release, so a multi-step mechanism should be considered to analyze and model the LTO stage of the two samples. However, for FD and HTO (including coke) regions, heat flow and mass loss shown good corresponding relation, which meant they had the sequential reaction mechanisms, namely fuel deposition and coke combustion. (4) In LTO stage, the activation energy of the residue and oxidized oil
Fig. 9. TG-DTG-DSC curves for three samples at the heating rate of 5 °C/min.
excluded, different trends of change were observed in the range of α = 0.1–0.9. For LTO, the activation energy gradually increased with the conversion rate increasing, which indicated that the reaction became more difficult as the LTO reaction proceeded. On the contrary, in HTO period, the activation energy of oxidized oil and coke did not change significantly, while the activation energy of residue decreased distinctly with the conversion rate increasing. This phenomenon indicated that during the HTO period, with the increase of temperature, 7
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Fig. 13. Coefficient of determination and activation energy plots for three samples non-isothermal oxidation process at different conversion rates.
Fig. 11. DAEM plots for residue non-isothermal oxidation process at different conversion rates.
Table 4 Mean activation energy and mean pre-exponential factor of the samples. Samples
LTO E (kJ/mol)
Oxidized oil Residue Coke
152.43 151.13 /
HTO A (1/min) 18
4.10 × 10 1.44 × 1027 /
E (kJ/mol)
A (1/min)
136.99 214.62 114.57
2.62 × 1011 1.36 × 1035 1.09 × 1010
oxidation reactions became much easier as the temperature increased. Notes The authors declare no competing financial interest. Acknowledgments Fig. 12. DAEM plots for coke non-isothermal oxidation process at different conversion rates.
The authors wish to recognize the financial support received from the National Key Science & Technology Projects during 13th Five-Year Plan (2016ZX05053-009). In addition, the authors also thank the Northwest Oilfield Company, Sinopec (China), for providing the crude oil and reservoir core samples.
gradually increased with the conversion rate increased, indicating that the oxidation reactions became more difficult as the LTO reaction proceeded. During HTO period, the activation energy of oxidized oil and coke were both at a relatively low level and did not change much, while the activation energy of the residue decreased from the highest 482.3 kJ/mol to 106.2 kJ/mol, it indicated that the
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 8
Journal of Petroleum Science and Engineering 181 (2019) 106155
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doi.org/10.1016/j.petrol.2019.06.019.
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References Akin, S., Kok, M.V., Bagci, S., Karacan, O., 2000. Oxidation of heavy oil and their SARA fractions: its role in modeling in-situ combustion. In: SPE Annual Technical Conference and Exhibition, Dallas, Texas, pp. 1–4 October. Ambalae, A., Mahinpey, N., Freitag, N., 2006. Thermogravimetric studies on pyrolysis and combustion behavior of a heavy oil and its asphaltenes. Energy Fuels 20 (2), 560–565. Anto-Darkwah, E., Cinar, M., 2016. Effect of pressure on the isoconversional in situ combustion kinetic analysis of Bati Raman crude oil. J. Pet. Sci. Eng. 143, 44–53. Chen, Y.F., Pu, W.F., Liu, X.L., Li, Y.B., Varfolomeev, M.A., Hui, J., 2019a. A preliminary feasibility analysis of in situ combustion in a deep fractured-cave carbonate heavy oil reservoir. J. Pet. Sci. Eng. 174, 446–455. Chen, Y.F., Pu, W.F., Liu, X.L., Li, Y.B., Gong, X.L., Hui, J., Guo, C., 2019b. Specific kinetic triplet estimation of Tahe heavy oil oxidation reaction based on non-isothermal kinetic results. Fuel 242, 545–552. Chen, Z., Yan, Y., Zhang, X., Shi, X., Liu, Q., Liu, Z., Xu, T., 2017. Behaviors of coking and stable radicals of a heavy oil during thermal reaction in sealed capillaries. Fuel 208, 10–19. Cinar, M., Castanier, L.M., Kovscek, A.R., 2011a. Combustion kinetics of heavy oils in porous media. Energy Fuels 25 (10), 4438–4451. Cinar, M., Hasçaki, B., Castanier, L.M., Kovscek, A.R., 2011b. Predictability of crude oil in-situ combustion by the isoconversional kinetic approach. SPE J. 16 (03), 537–547. Freitag, N.P., 2014. Chemical reaction mechanisms that govern oxidation rates during insitu combustion and high-pressure air injection. In: SPE Heavy Oil ConferenceCanada, Alberta, Canada, pp. 10–12 June. Hashimoto, K., Miura, K., Watanabe, T., 1982. Kinetics of thermal regeneration reaction of activated carbons used in waste water treatment. AIChE J. 28 (5), 737–746. Kök, M.V., Karacan, Ö., Pamir, R., 1998. Kinetic analysis of oxidation behavior of crude oil SARA constituents. Energy Fuels 12 (3), 580–588. Kök, M.V., Varfolomeev, M.A., Nurgaliev, D.K., 2017. Thermal characterization of crude oils in the presence of limestone matrix by TGA-DTG-FTIR. J. Pet. Sci. Eng. 154, 495–501. Kovscek, A., Castanier, L.M., Gerritsen, M., 2013. Improved predictability of in-situcombustion enhanced oil recovery. SPE Reservoir Eval. Eng. 16 (02), 172–182. Larter, S., Adams, J., Gates, I., Bennett, B., Huang, H., 2006. The origin, prediction and impact of oil viscosity heterogeneity on the production characteristics of tar sand and heavy oil reservoirs. In: Canadian International Petroleum Conference, Calgary, Canada, pp. 13–15 June. Lee, D.G., Noureldin, N.A., 1989. Effect of water on the Low-Temperature Oxidation of heavy oil. Energy Fuels 3 (6), 713–715. Li, Y.B., Chen, Y.F., Pu, W.F., Jin, F.Y., Yuan, C.D., Li, D., Zhao, J.Y., Zhou, W., 2015. The kinetic analysis of oxidized oil during the high pressure air injection by thermal kinetic analysis. Petrol. Sci. Technol. 33 (3), 319–326. Li, Y.B., Chen, Y.F., Pu, W.F., Gao, H., Bai, B.J., 2017. Viscosity profile prediction of a heavy crude oil during lifting in two deep artesian wells. Chin. J. Chem. Eng. 25 (7), 976–982. Li, Y.B., Pu, W.F., Jia, H., Peng, H., Zhong, D., Wang, S.K., 2013. Catalytic effect analysis of metallic additives on light crude oil by TG and DSC tests. J. Therm. Anal. Calorim.
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Oxidation kinetic evaluation of the low temperature oxidized products of Tahe heavy oil characterized by the distributed activation energy model
T
Wan-fen Pua,c,∗∗, Xiao-long Gonga, Ya-fei Chena,∗, Xue-li Liub, Jian Huib, Chen Guob, Mikhail A. Varfolomeevc a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People's Republic of China Northwest Oilfield Branch Company, SINOPEC, Urumqi, Xinjiang, 830011, People's Republic of China c Department of Physical Chemistry, Kazan Federal University, Kazan, 420008, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Heavy oil Low temperature oxidation Oxidized product Oxidation kinetic Coke deposition DAEM
In view of the significance of low temperature oxidation (LTO) for in-situ combustion (ISC) of heavy crude oil and to provide further data for Tahe ISC numerical simulation, this research aimed to investigate the nonisothermal oxidation behavior of heavy oil after static LTO reactions. Thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques were used to characterize oxidation behavior, and distributed activation energy method (DAEM) was used to calculate kinetic parameters. The results showed that the non-isothermal oxidation process of static LTO oxidized oil was the result of the interaction between residue and coke, and coke can significantly reduce the activation energy of oxidized oil. Only one HTO region was identified for the coke from DTG/DSC curves, and it had the lowest activation energy (114.57 kJ/mol) and the highest heat enthalpy (24.3 kJ/g), indicating high oxidative activity with maximum heat release potential. A satisfactory corresponding relation between DTG and DSC curves was presented for all samples in FD and HTO regions, which indicated that the sequential reaction mechanisms, fuel deposition and combustion, were undergone. In LTO region, there was a temperature difference between the peaks of the mass loss and heat flow, indicating that the LTO process was more complex with a multi-step control mechanism should be considered to analyze and simulate the LTO stage.
1. Introduction As an unconventional crude oil resource, the heavy crude oil has been verified to be more than twice than the conventional light oil in the world (Shah et al., 2010). With the reduction of recoverable reserves of light crude oil and the ever-increasing energy demand, it is crucial and necessary to increase the production of heavy crude oil. Although many efforts have been made to decrease the heavy oil viscosity and enhance the recovery, there still remains enormous challenges to exploit the heavy oil (Larter et al., 2006; Li et al., 2017). Since the viscosity of heavy crude oil is greatly sensitive to the temperature, thermal recovery techniques including in-situ combustion (ISC), steam flooding, and cyclic steam injection, etc. are proposed. With regard to the ISC, air or oxygen-enriched gas is injected into the target formation, self-ignition or artificial ignition is adopted to make the oil reservoir temperature reach the crude oil ignition point, the
viscosity of heavy oil is reduced due to the large amount of heat generated by combustion, combined with other relevant mechanisms, then the oil recovery is enhanced (Kovscek et al., 2013). In general, heavy crude oil oxidation was divided into three stages: low temperature oxidation (LTO), fuel deposition (FD), and high temperature oxidation (HTO). In LTO stage, the light hydrocarbon components in the heavy oil mainly undergo oxidation addition reactions to form partial oxides such as aldehydes, ketones and alcohols. Then enter FD period, where fuel (coke) deposition occurs primarily in preparation for high temperature oxidation. Finally, the crude oil will enter the HTO stage, and the coke deposited in the FD stage will undergo a violent combustion reaction, generating CO2 and H2O, and releasing a large amount of heat. Hence, the amount of coke deposited is an important factor in determining the success of ISC (Akin et al., 2000; Li et al., 2013). In recent years, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ramped-temperature-oxidation tests (RTO)
∗
Corresponding author. Corresponding author.State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, People's Republic of China. E-mail addresses: [email protected] (W.-f. Pu), [email protected] (Y.-f. Chen). ∗∗
https://doi.org/10.1016/j.petrol.2019.06.019 Received 10 March 2019; Received in revised form 4 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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and combustion tube (CT), etc. have been widely recognized as the thermal analysis methods to study the oxidation characteristics and kinetics of crude oil (Ambalae et al., 2006; Anto-Darkwah and Cinar, 2016; Cinar et al., 2011a, 2011b; Lee and Noureldin, 1989; Pu et al., 2017b). By reason of coke plays an important role in the kinetics of oxidized heavy oil, recent emphasis has focused on the mechanisms and characteristics of coke deposition and its changes in the oxidation kinetic parameters of crude oil. For instance, Cinar et al. (2011a) studied the role of oxygen in coke formation by X-ray photoelectron spectroscopy (XPS) and ramped temperature oxidation (RTO) tests. The results showed that the quality (or reactivity) of coke produced was a function of oxygen presence/absence, compared to coke produced in an inert atmosphere, since the surface of the coke formed after oxidation had additional oxygen-containing functional groups, coke produced under oxygen conditions was significantly more reactive. Ren et al. (2007) proposed a two-coke model to describe the combustion process of coke, under low temperature conditions, this model matched the experimental data more reasonably than the classical Arrhenius model. It was concluded that coke from whole oil was more reactive than coke from asphaltenes. Verkoczy (1993) studied the coking process of heavy oil in the oxidation stage and found that the coke production of heavy oil was between 5 and 10%, which was slightly lower than the sum of coke produced by SARA oxidation. It indicated that the interaction between SARA affected the coking process of heavy oil. Chen et al., 2017 conducted oxidation experiments of heavy oil at 250–500 °C in sealed capillaries. It was found that chlorobenzene insolubles (CI, hard coke) and toluene insolubles (TI, soft coke) were formed at 440 °C and 350 °C, respectively. At the same time, it was measured by electron spin resonance (ESR) that the radical concentration of CI was higher than TI. In previous work, Li et al. (2015) conducted oxidation experiments under different pressures for Tahe heavy crude, and carried out kinetic analysis of heavy crude and oxidized oil samples. It was found that there was obvious coke formation after oxidation of heavy oil, and with the increased of pressure, the activation energy of oxidized oil samples showed a decreasing trend (Chen et al., 2019a, 2019b; Liu et al., 2017, 2019). Despite extensive research has been done on coke and heavy oil, the oxidation kinetics of the effect of coke on ISC under non-isothermal oxidation conditions are not well understand. Hence, in this work, the oxidation mechanism and coke deposition process of Tahe heavy oil were studied by TG/DSC analysis. Particular attention was placed on the kinetics of oxidized oil, coke, residue, which formed by static oxidation experiments under reservoir conditions. And the differences in non-isothermal oxidation processes and changes in kinetic parameters were investigated. The results of this research work were of great significance to model Tahe in-situ combustion process.
Fig. 1. Viscosity−temperature curve for the heavy oil.
pressure oxidation unit was pressurized to 40 MPa, and placed in an air bath at 120 °C (reservoir conditions) for 10 h to check leakage. (2) Add 5 g of heavy oil to the unit, and then pressurized again to 40 MPa by air and set the temperature to 120 °C. (3) The oxidation reaction was terminated after 7 days. (4) Because coke was insoluble in toluene, the solid products (∼4 g) were extracted by a large amount of toluene to obtain coke and residue, and then the separation was dried at 100 °C in nitrogen atmosphere. It was observed that 3.02 g of residue and 0.98 g of coke were formed after the oxidation reactions, corresponding to the coke yield of 24.5 wt%. 2.3. Non-isothermal thermogravimetric analysis In this work, NETZSCH TG 209F3/DSC 214 (NETZSCH Ltd., Germany) were used to perform TG-DSC testing on samples from this experiment to study the kinetic characteristics of oxidized oil, coke and residue. In order to eliminate the influence of the single heating rate on the kinetic parameters, the samples were tested at various heating rates according to the recommendations of the ICTAC Committee (International Confederation for Thermal Analysis and Calorimetry) (Vyazovkin et al., 2011). A small amount of sample (∼7 mg) was heated at a rate of 5, 15, 25 °C/min, respectively, from ambient temperature to 800 °C at an air flow rate of 50 ml/min. The system was calibrated to ensure reliability prior to each test. In the end, the mass loss and heat flow of the samples against temperature changes were recorded by the data acquisition system on the computer-side.
2. Experimental section 3. Results and discussion 2.1. Materials 3.1. Non-isothermal oxidation TG/DTG analysis The crude oil in this experiment was provided by Tahe Oilfield, located in western China. Reservoir pressure and temperature are 40 MPa, 120 °C respectively. Its viscosity-temperature curve is shown in Fig. 1. Moreover, Table 1 shows the basic properties of this heavy oil. Obviously, the viscosity and density of heavy oil was much higher than ordinary crude oil, and SARA fractions analysis also verify this result from the side because it contains the most abundant asphaltenes (47.68 wt%). Toluene was used in the experiment to separate coke and residue.
The non-isothermal oxidation TG/DTG profiles of oxidized oil, residue and coke at different heating rates are shown in Figs. 3–5, respectively. According to different trends of the curves, three reaction stages for oxidized oil and residue, named LTO, FD, and HTO, were identified. Nevertheless, coke as fuel formed after static low temperature oxidation, only one exact stage which named HTO, was identified. This result was consistent with previous research findings (Yuan et al., 2015; Zhao et al., 2018). The reaction intervals, peak temperatures, and mass losses of the three samples are listed in Table 2. For oxidized oil and residue, there was a small amount of mass loss before the onset of low temperature oxidation, averaging about 1.64% and 1.97%, respectively, which primarily reflected the volatilization of light components. Thereafter, the two samples entered the LTO stage with a temperature range of approximately 130 °C–350 °C. During this
2.2. Oxidation experiments The heavy oil oxidation experiment and analysis equipment are shown in Fig. 2, and the following is a brief introduction to the operation process of this experiment. (1) A high temperature and high 2
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Table 1 Basic properties, elemental composition and SARA fractions of heavy oil. Physical properties (25 °C) 3
Density (g/cm ) 1.045
Viscosity (mPa·s) 1.11 × 105
Element (wt%) C 82.1
H 10.67
SARA (wt%) O 3.36
N 0.72
S 3.15
Saturates 16.86
Aromatics 26.11
Resins 9.35
Asphaltenes 47.68
result during the ISC process. As the TG curves shown in Fig. 5, after undergoing high temperature combustion reactions, the remaining mass of oxidized oil, residue and coke were 2.79%, 0.56% and 7.81%, respectively. Coke molecule contained heterocyclic ring, metal clathrate, and condensed ring, which were classified into soft coke and hard coke depending on the molecular weight and molecular structure (Chen et al., 2017; Pu et al., 2017a; Zhao et al., 2018). Among them, because of the larger microscopic surface area and molecular activity, soft coke was easier to burn in the relatively low temperature range, while part of the hard coke was not completely consumed under the experimental conditions, and became the leftover after the combustion reaction. In addition, another noteworthy phenomenon was that, as shown in Table 2, the initial temperature, the termination temperature, and the peak temperature of coke in HTO were much lower than the other two samples, which indicated that coke had higher reactivity during HTO. This high oxidative reactivity reflected the degree and ability of oxygen consumption in HTO region. Therein the C–H and C–C bonds were cleaved by the O atom to form CO, CO2, and H2O, during which released a large amount of heat.
period, the oxygen addition reactions occurred, oxygen atom and hydrocarbon molecule combined to form alcohol, aldehyde, ketone, and hydroperoxide, although this increased the viscosity of heavy oil, it was a crucial step to form coke, which was the direct material of the combustion reaction in HTO region (Kök et al., 1998; Noureldin et al., 1987). Subsequently, both the condensation and cleavage reactions involved the above oxides, the former produced more macromolecules with a longer carbon chain and a higher degree of aromatization, while the latter produced small molecular weight hydrocarbons and small amounts of H2O, CO2 and CO. The mass loss of the residue (20.95%) in the LTO stage was higher than that of the oxidized oil (16.43%), since the oxidized oil had formed a part of coke under static oxidation conditions, resulting in fewer hydrocarbons were involved in the LTO stage reaction. At the same time, it was worth noting that as the heating rate increased, the peak temperature of the mass loss of the two samples also increased during this period, which indicated that the more obvious reaction lag was caused by higher heating rate. It was believed that the molecules had more time to accumulate energy at a low heating rate, followed by a chemical collision reaction in the activated state. As the oxidation temperature increased, DTG curves began to enter the FD stage. Since oxidized oil had formed part of coke, the high activity of coke allowed oxidized oil to show higher mass loss at this stage. For the residue, it was mainly through the reoxidation, condensation and aromatization of the LTO stage product to produce coke (Kök et al., 2017), preparing the fuel required for the subsequent HTO stage. In addition, some small molecular hydrocarbons were further oxidized or cleaved to form H2O, CO2 and CO into the atmosphere. Following the FD stage, due to the intensification of intermolecular collisions and the accumulation of fuel, the combustion reaction occurred in the HTO stage, which was widely considered to be the only important reaction during this period. The coke combustion reaction released a large amount of heat to reduce the viscosity and increase fluidity of the heavy oil in the formation, which was the most desirable
3.2. Non-isothermal oxidation DSC analysis Due to the different hydrocarbons involved in the reactions at different stages, the enthalpy of different stages of crude oil was different. In order to further study the thermal behavior of oxidized oil, residue and coke formed after static LTO reaction, DSC tests and analyses were performed for these three samples. Figs. 6–8 show the heat flow curves of the samples at different heating rates of 5, 10, and 15 °C/min. The exothermic peaks, peak temperatures, and heat enthalpy obtained by integrating the curves were given in Table 3. For the oxidized oil, three distinct exothermic reaction regions were observed from the DSC curves, corresponding to the three stages of the LTO, FD, and HTO in the DTG curves. However, it was particularly
Fig. 2. Simplified schematic for static LTO experiment and TG/DSC test. 3
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Fig. 3. TG−DTG curves for oxidized oil at different heating rates of 5, 10, and 15 °C/min.
Fig. 4. TG−DTG curves for residue at different heating rates of 5, 10, and 15 °C/min.
observed that the LTO ranges and peak temperatures determined in the DSC curves were slightly higher than the DTG curves, that is, the highest heat release took place after the fastest mass loss. And the corresponding relation between the two curves in the LTO stage was not very good, which meant that there was a significant difference between heat release and mass loss. Because the residue and oxidized oil had some of the same composition, same phenomenon also occurred in the DTG/DSC curves of the residue. This indicated that a large mass loss did not mean that the reactants were active, and it was not reasonable to determine the reactivity of the oxidized oil and the residue only by the heat flow rate. In fact, this phenomenon should be attributed to the low reactivity of the relatively light components in the residue and oxidized oil in the low temperature range. Considering that both samples had undergone high pressure LTO reaction, the heavy components such as asphaltenes and resins in the two samples were partially converted to coke (Murugan et al., 2011). Therefore, the samples mainly contained relatively light components such as saturates, aromatics, as well as some heavy components such as oxygenated compound, resins and asphaltenes. In many previous reports, it has been pointed out that heavy components such as asphaltenes were more reactive than light components such as saturates (Moschopedis and Speight, 1973; Pu et al., 2015; Verkoczy and Freitag, 1997). This reactivity referred to the ability to absorb oxygen by oxygen addition reaction, in which case, the oxygen atoms were incorporated into the molecular structure of heavy components to form various oxygenated compound and released some
heat. Therefore, at about 282 °C, the DTG curves reached the mass loss peak due to oxidation of relatively light components, and at about 373 °C, the exothermic peak was reached due to the oxygen addition reaction of the heavy components. For the FD and HTO stages, there was a good corresponding relation between the two curves for oxidized oil, residue and coke. This corresponding relation meant that the heat flow and mass loss in the FD and HTO stages were caused by the sequential reaction mechanisms considered to be fuel deposition and coke combustion. The same phenomenon also reported in the study of Yuan et al. (2017). Hence, this indicated that the LTO stage of oxidized oil and residue should be analyzed and modeled by a multi-step mechanism. Simultaneously, another noteworthy phenomenon was that the heat flow curves of oxidized oil at 10 °C/min and 15 °C/min were completely different from the performance at 5 °C/min, as shown in Fig. 6, the peak temperatures of the first two relatively high heating rate curves appeared in the FD stage, while the last curve appeared in the HTO stage. The same results were obtained by the repeated experiments under same conditions, which indicated that the reaction of oxidized oil at low heating rate was similar to the residue, both of which had a violent combustion reaction during the HTO stage. However, at relatively high heating rates, coke had undergone combustion reactions in FD stage of the oxidized oil and released the highest heat flow due to its high reactivity. Fig. 9 shows a comparison of the mass loss, differential thermal 4
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Fig. 6. DSC curves for oxidized oil at different heating rates of 5, 10, and 15 °C/ min.
Fig. 5. TG−DTG curves for coke at different heating rates of 5, 10, and 15 °C/ min.
weight and heat flow for three samples at heating rate of 5 °C/min. It was found that the mass loss path and the heat flow curves of the oxidized oil were always between the two samples, indicating that the oxidation process of the oxidized oil was the result of the interaction of coke and residue. As mentioned above, because of high activity of the coke, the DTG/DSC curves for coke showed that the mass loss peak temperature and the heat flow peak temperature were lower than the other two samples. In addition, it was observed that the DSC curves of the oxidized oil and residue in the LTO stage were not as smooth as the HTO stage, and the result may be due to the negative effect of the
Fig. 7. DSC curves for residue at different heating rates of 5, 10, and 15 °C/min.
endothermic reaction on the curves, such as evaporation and thermal cracking reaction in this interval. The DSC curves were integrated to show that the pure coke released the most heat enthalpy during the oxidation process, about 24.3 kJ/g, which was extremely advantageous for the stable combustion front in the ISC process.
Table 2 Reaction intervals, peak temperatures and mass loss of the samples. Samples
Oxidized oil
Residue
Coke
Heating rate (°C/min)
5 10 15 5 10 15 5 10 15
LTO
FD
HTO
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
Interval (°C)
Peak temperature (°C)
Mass loss (wt/%)
132.36–309.25 134.36–326.73 143.31–340.75 136.75–325.97 141.64–345.37 146.98–355.63 / / /
265.52 276.84 286.93 272.84 293.82 303.23 / / /
18.87 15.54 14.89 20.89 21.18 20.78 / / /
309.73–449.51 326.12–473.63 340.27–477.46 325.64–457.31 345.44–478.36 355.72–489.43 / / /
402.27 433.72 431.88 418.53 401.01 455.64 / / /
46.69 50.32 43.75 38.06 41.02 46.34 / / /
449.64–525.73 473.35–565.57 477.12–569.31 457.12–521.31 478.43–552.69 489.14–577.78 347.32–457.38 379.48–504.92 380.34–513.68
480.45 506.02 509.92 485.03 510.97 527.63 403.24 411.67 398.83
31.97 31.70 38.15 40.26 37.26 32.47 69.29 67.46 65.05
5
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loss of the sample, A is the Arrhenius constant or pre-exponential factor, E is the activation energy corresponding to A, R is the universal gas constant (8.314 J/mol·K), T is the temperature, f (E) is defined as a function of activation energy E in each reaction, and β is the heating rate. f (E) is the normalized activation energy distribution function, and it is obvious that it follows the following relationship (Eq. (3)).
∫0
∞
f (E )dE = 1
(3)
Combine equations (1)–(3), and after appropriate approximation, derive the integral expression of DAEM as follows (Eq. (4)).
ln
3.3.1. Distributed activation energy method Due to the complex composition of heavy oil, the kinetic parameters calculated by the isothermal kinetic model have large errors. Therefore, in this work, Distributed Activation Energy Method (DAEM) was used to describe the thermal behavior. As a model-free method widely used to analyze complex reaction kinetic parameters, DAEM is based on two assumptions. On the one hand, the reaction system occurs simultaneously by a finite number of irreversible independent first-order reactions, and each reaction has a corresponding activation energy and a pre-exponential factor. On the other hand, the activation energy of each reaction is in the form of a function of a continuous distribution (Hashimoto et al., 1982). After long-term development, the DAEM method has made great progress in kinetics research. Miura linked the weight loss curves at different heating rates, eliminating the effect of heating rate on the kinetic parameters (Miura and Maki, 1998). The weight loss process of heavy oil can be described by Eq. (1).
∫0
∞
∫0
exp ⎛–A ⎝
t
E ⎞ dt ⎞ f (E ) dE = exp ⎛– ⎝ RT ⎠ ⎠
∫0
∞
Φ (E , T ) f (E )dE (1)
Φ(E,T) can be approximated by the following formula (Eq. (2)).
ART 2 −E / RT ⎞ Φ(E , T ) ≅ exp ⎜⎛− e ⎟ βE ⎝ ⎠
⎟
(4)
3.3.2. Calculation results and analysis Figs. 10–12 show the ln(β/T2) versus 1/T curves for the non-isothermal oxidation process of the three samples at same conversion rate using the DAEM model. Since the kinetic parameters of the LTO and HTO stages are more concerned in the actual analysis, the FD stage of oxidized oil and residue was incorporated into the HTO stage for kinetic calculation. The activation energy (E) and coefficient of determination (R2) at different conversion rates, as well as the average activation energy and pre-exponential factor at each stage were shown in Fig. 13 and Table 4, respectively. According to the calculation results, coke had the lowest activation energy (114.57 kJ/mol) among the three samples, which was direct evidence that the coke formed after static oxidation was highly reactive. This change was beneficial to the combustion process and heat release of the ISC. However, for the residue, it was observed that the activation energy in HTO stage was significantly higher than that in LTO, indicating a high threshold energy to trigger the HTO reaction. Interestingly, the activation energy of the HTO stage of the oxidized oil was lower than that of the LTO stage, which obviously resulted from the coke effect. In addition, it was worth noting that the activation energy obtained in this study was significantly higher than that reported in other studies, which may be due to the fact that asphaltenes and aromatics account for the largest proportion in this crude oil. It was reported that aromatics, as oxidation inhibitors, can significantly reduce the oxidation rate even at ultra-low levels (1%), making the oxidation process difficult and leading to higher energy requirements (Freitag, 2014; Wei et al., 2018). Fig. 13 shows the relationship between activation energy and conversion rate. For all samples, the points with poor fitting accuracy were
3.3. Non-isothermal oxidation kinetic analysis
w = w0
⎜
According to the TG data at different heating rates, the activation energy and the pre-exponential factor can be estimated from the slope and intercept respectively by plotting the Arrhenius curve of ln (β/T2) against 1/T under the same conversion rate.
Fig. 8. DSC curves for coke at different heating rates of 5, 10, and 15 °C/min.
1−
β AR w ⎞⎞ E = ln − ln ⎜⎛−ln ⎛1 − ⎟ − T2 E w RT 0 ⎝ ⎠⎠ ⎝
(2)
where w is the weight loss of the sample at time t, w0 is the total weight Table 3 Peak temperature, peak heat flow and heat enthalpy of the samples. Samples
Oxidized oil
Residue
Coke
Heating rate (°C/min)
5 10 15 5 10 15 5 10 15
LTO
FD
HTO
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/ g)
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/ g)
Peak Temperature(°C)
Peak heat flow (mW/ mg)
Heat enthalpy (kJ/g)
353.59 363.46 359.43 387.11 384.64 393.15 / / /
4.53 8.22 15.88 6.96 7.29 5.34 / / /
3.21 2.80 3.63 2.22 1.37 0.74 / / /
414.19 446.20 443.39 431.15 448.51 419.03 / / /
12.06 19.26 42.29 8.93 13.37 10.82 / / /
8.46 11.58 14.51 1.68 2.12 1.63 / / /
488.80 510.73 523.04 490.64 507.96 519.42 417.72 461.13 474.86
15.85 17.63 24.80 32.81 33.29 41.21 15.99 21.96 29.35
8.03 6.35 6.69 15.89 11.09 10.65 24.03 24.17 23.08
6
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Fig. 10. DAEM plots for oxidized oil non-isothermal oxidation process at different conversion rates.
the remaining HTO reactions became much more easily to occur. 4. Conclusions In this work, the main focus was on the non-isothermal oxidation behavior of oxidized oil, residue, and coke resulting from static oxidation under reservoir conditions. Based on TG/DSC experimental data and DAEM calculation results, the following conclusions can be summarized. (1) For coke formed after static LTO reaction, only one region HTO was identified on the DTG/DSC curves, and the HTO temperature range was much lower than the other two samples. The activation energy was only 114.57 kJ/mol, indicating high reactivity with oxygen, and the maximum heat enthalpy was released during the non-isothermal oxidation process (24.3 kJ/g). (2) For oxidized oil, the non-isothermal oxidation process was the result of the interaction of coke and residue, and coke can significantly reduce the activation energy in HTO stage. (3) The fastest mass loss in the LTO stage of oxidized oil and residue occurred before the highest heat release, so a multi-step mechanism should be considered to analyze and model the LTO stage of the two samples. However, for FD and HTO (including coke) regions, heat flow and mass loss shown good corresponding relation, which meant they had the sequential reaction mechanisms, namely fuel deposition and coke combustion. (4) In LTO stage, the activation energy of the residue and oxidized oil
Fig. 9. TG-DTG-DSC curves for three samples at the heating rate of 5 °C/min.
excluded, different trends of change were observed in the range of α = 0.1–0.9. For LTO, the activation energy gradually increased with the conversion rate increasing, which indicated that the reaction became more difficult as the LTO reaction proceeded. On the contrary, in HTO period, the activation energy of oxidized oil and coke did not change significantly, while the activation energy of residue decreased distinctly with the conversion rate increasing. This phenomenon indicated that during the HTO period, with the increase of temperature, 7
Journal of Petroleum Science and Engineering 181 (2019) 106155
W.-f. Pu, et al.
Fig. 13. Coefficient of determination and activation energy plots for three samples non-isothermal oxidation process at different conversion rates.
Fig. 11. DAEM plots for residue non-isothermal oxidation process at different conversion rates.
Table 4 Mean activation energy and mean pre-exponential factor of the samples. Samples
LTO E (kJ/mol)
Oxidized oil Residue Coke
152.43 151.13 /
HTO A (1/min) 18
4.10 × 10 1.44 × 1027 /
E (kJ/mol)
A (1/min)
136.99 214.62 114.57
2.62 × 1011 1.36 × 1035 1.09 × 1010
oxidation reactions became much easier as the temperature increased. Notes The authors declare no competing financial interest. Acknowledgments Fig. 12. DAEM plots for coke non-isothermal oxidation process at different conversion rates.
The authors wish to recognize the financial support received from the National Key Science & Technology Projects during 13th Five-Year Plan (2016ZX05053-009). In addition, the authors also thank the Northwest Oilfield Company, Sinopec (China), for providing the crude oil and reservoir core samples.
gradually increased with the conversion rate increased, indicating that the oxidation reactions became more difficult as the LTO reaction proceeded. During HTO period, the activation energy of oxidized oil and coke were both at a relatively low level and did not change much, while the activation energy of the residue decreased from the highest 482.3 kJ/mol to 106.2 kJ/mol, it indicated that the
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 8
Journal of Petroleum Science and Engineering 181 (2019) 106155
W.-f. Pu, et al.
doi.org/10.1016/j.petrol.2019.06.019.
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