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Comparative evaluation on the thermal behaviors and kinetics of combustion of heavy crude oil and its SARA fractions
Fuel 239 (2019) 117–125
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Full Length Article
Comparative evaluation on the thermal behaviors and kinetics of combustion of heavy crude oil and its SARA fractions ⁎
T
⁎
Shuai Zhaoa, , Wanfen Pua,b, , Boshuai Sunc, Fei Gua, Liangliang Wanga a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia c Shenghong Petrochemical Co. Ltd., Jiangsu 222002, People’s Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Combustion Heavy crude oil Saturates-aromatics-resins-asphaltenes Thermogravimetry Differential scanning calorimeter Activation energy
In situ combustion (ISC) plays a significant role in the exploitation of heavy crude oil. In this study, the thermal behaviors and kinetics of combustion of heavy crude oil and its saturates-aromatics-reins-asphaltenes (SARA) fractions were comparatively evaluated using thermogravimetry (TG) and differential scanning calorimeter (DSC) techniques. The results indicated that saturates and aromatics were vulnerable to severe mass loss at the low-temperature oxidation (LTO) stage; however, resins and asphaltenes encountered appreciable mass loss in the fuel deposition (FD) interval to form a great amount of coke, which consequently contributed to hightemperature oxidation (HTO) reactions. Saturates made an inconsequential thermal release to the HTO reactions. Aromatics showed apparent exothermic effect both at the LTO and HTO stages, which was consistent with crude oil. Compared with saturates and aromatics, resins and asphaltenes gave considerably higher heat release in the HTO region, especially asphaltenes. By comparing the DSC curve of heavy crude oil with fitting curve of SARA superposition based on their proportion in crude oil, apparent inhibiting (promoting) effect among SARA fractions at the LTO stage (FD and HTO stages) were detected in terms of heat release, which should be of much significance for using SARA fractions to model ISC process. The kinetic parameters determined by Ozawa–Flynn–Wall (OFW) method were almost identical to those obtained by distributed activation energy model (DAEM). The activation energies of crude oil fluctuated and varied in the range of 45–95 kJ/mol during combustion, disclosing its intricate combustion processes and multiple reaction mechanisms. Unlike other three fractions, the FD and HTO activation energies of asphaltenes descended consecutively with conversion degree, which is favorable for boosting the sustainability of combustion front.
1. Introduction With the depletion of conventional oil reservoirs coupled with the ever-increasing energy demand, considerable attention has been focused on the heavy oil resources with a huge amount of reserves [1,2]. Among a series of enhanced oil recovery technologies, thermal recovery methods, including ISC, steam injection, in situ electrical heaters, binary mixtures, supercritical water, etc., have been proven to be significantly promising for the exploitation of heavy oil reservoirs [3,4]. With regard to ISC, after air or oxygen-enriched gas is injected into the crude oil reservoirs, numerous oxidation reactions occur between crude oils and oxygen, which produce appreciable thermal energy, hot water, steam and carbon oxides to displace the oil into production wells by multiple flooding mechanisms [5–7]. Accordingly, since the first ignition operation in 1933, hundreds of ISC projects have been deployed in
some crude oil reservoirs. Unfortunately, nearly 80% of the ISC projects turned out to be unsuccessful economically and technologically, mostly as a consequence of a limited understanding concerning the thermal behaviors and kinetics of combustion of crude oils [8]. Early investigations related to crude oil combustion were carried out using TG, DSC, accelerating rate calorimeter (ARC), combustion tube (CT), small batch reactor (SBR), etc. [9–14] It is widely accepted that the combustion behaviors and kinetics are predominantly determined by the reactivity of crude oils that mostly depends on their fractions. Typically, crude oils were divided into four fractions, socalled saturates, aromatics, resins and asphaltenes, shortly SARA fractions [15,16]. Therefore, quite a few researchers focused on the studies regarding the thermal behaviors and kinetics of crude oils and their SARA fractions during combustion. Kok et al. [17] studied the combustion behaviors of SARA components derived from medium and
⁎ Corresponding authors at: State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China and Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia (W. Pu). E-mail addresses: [email protected] (S. Zhao), [email protected] (W. Pu).
https://doi.org/10.1016/j.fuel.2018.11.014 Received 14 August 2018; Received in revised form 8 October 2018; Accepted 1 November 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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China. Table 1 is presented to show the density, viscosity, SARA fractions and elemental compositions of the crude oil used. The industrial standard method of China Petroleum NB/SH/T 0509-2010 was utilized to split SARA fractions from crude oil. A high performance liquid chromatograph UltiMate 3000 (Thermo Scientific) was deployed for testing the aromatic component and corresponding experimental details had been given by Yuan et al. [5] The main components of aromatics used in this study contained triaromatics (49%) and diaromatics (35%).
heavy oils by TG technique. It was claimed that saturates could be oxidized more readily owing to its lower activation energy as compared to others fractions, whereas asphaltenes with high activation energy was not oxidized easily. Simultaneously, the weight loss and kinetic parameters of individual SARA fraction at each reaction stage were provided. Kuppe et al. [18] examined the heat release of different crude oils and SARA fractions during combustion. It was found that the heat liberated of saturates and aromatics was greater than that of resins and asphaltenes. Varfolomeev et al. [19] investigated the thermal decomposition of Tatarstan Ashal’cha heavy crude oil and its SARA fractions. Two main reaction stages, LTO and HTO, were observed from DSC profiles, while only one primary stage appeared in differential thermogravimetry (DTG) curves. Ambalae et al. [20] analyzed the combustion kinetics of crude oil and its asphaltenes. It was reported that the activation energy of crude oil was comparable to its asphaltenes. The concise review shows that much emphasis has been devoted to acquiring some significant information such as mass loss, heat effect and kinetic parameters of crude oils and their SARA fractions during combustion. Meanwhile, numerous studies concerning the dependence of kinetic parameters upon conversion degree during the whole heating process for crude oils have been deployed using different isoconversional models [21–24], but those for SARA fractions were rarely presented for comparative analysis. Very recently, the interactions between crude oils and SARA components in terms of combustion behaviors have been studied by some researchers. For instance, Liu et al. [25] indicated that no interactions were found among saturates, aromatics and resins in the process of copyrolysis, whereas the interactions were pronounced during co-combustion. In addition, they gave a detailed description about the variation of mass loss and DTG peak temperature during co-combustion. Yuan et al. [5] performed an interesting investigation about whether there were some specific connections in combustion behaviors between light crude oil and its SARA fractions. At the LTO stage, the DSC curve of light oil was approximately the same as the predicted curve calculated by the DSC data of SARA fractions based on their proportion in crude oil. However, the measured DSC curve gradually deviated from the predicted curve after 350 °C. The results unravelled that individual SARA fraction followed its own reaction pathway at the LTO stage but was distinctly affected by the presence of other fractions at the FD and HTO regions. Nevertheless, to the best of our knowledge, whether this finding can apply to heavy crude oils and their SARA fractions is not validated by further investigation. In this work, the thermal behaviors of combustion of heavy crude oil and its SARA fractions were comparatively evaluated using TG-DSC technique. The primary objective of this research was intended to disclose the SARA interactions in the process of combustion. Additionally, special attention was placed on the relation of activation energy versus conversion rate for heavy oil and its SARA fractions. It is believed that the results can accelerate to understand action mechanisms among SARA fractions during combustion, which should be of much significance for utilizing SARA fractions to model ISC process.
2.2. TG/DSC analysis The thermal behaviors of combustion of heavy crude oil and its SARA fractions were employed using NETZSCH STA 449F3 PC/PG with TG-DTG and DSC modules. It was noted that prior to the tests, the thermal analysis systems should be calibrated as the methods described by Li et al. [2] A minor amount of sample (10 ± 0.1 mg) was heated up from ambient temperature to 700 °C in an air flow rate of 50 mL/min. The reliable kinetic tests should include three to five varying heating rates, all less than 20 °C/min, based on the recommendations of the International Confederation of Thermal Analysis and Calorimetry (ICTAC) [26]. Accordingly, three linear heating rates (5, 10 and 15 °C/ min) were used. The experiments were performed at least two times for each sample to guarantee the accuracy and repeatability of experimental data. The error of mass loss and temperature was less than ± 0.5 wt% and ± 1 °C, respectively. 2.3. Kinetics model The non-isothermal kinetic study of combustion process is extraordinarily complicated as a consequence of the presence of various components and their parallel, consecutive and competitive reactions. In this research, two kinetic models, named Ozawa–Flynn–Wall (OFW) and distributed activation energy model (DAEM), were utilized for investigating the combustion kinetics. The main advantages of these isoconversional models embody as follows. (1) They allow for evaluating the kinetic parameters without ascertaining the reaction model to avoid the compensation effect [26,27]. (2) They contribute to reveal the complex thermal degradation process and recognize the corresponding reaction mechanisms because of the dependence of kinetic parameters upon conversion degree [28]. The equation of OFW model is given as [26]:
AE ⎞ E ln (β ) = ln ⎛⎜ ⎟ − 5.331 − 1.052 Rg ( α ) RT ⎝ ⎠
(1)
The simplified DAEM equation is written as: [29]
β AR E ⎞ + 0.6075 − ln ⎛ 2 ⎞ = ln ⎛ RT ⎝ E ⎠ ⎝T ⎠
(2)
In this study, α can be calculated as follows:
α =
m o − mt mo − m∞
(3)
where β is the heating rate, E is the activation energy, A is the frequency factor, R is the universal gas constant, α is the conversion rate, g(α) is the integral form of reaction model, T is the absolute temperature, mt is the weight percentage of the sample left in the crucible at reaction time t, mo and m∞ are the initial and final weight percentage of sample,
2. Experimental 2.1. Materials The heavy crude oil was supplied from Xinjiang oilfield in northwest Table 1 SARA fractions and basic properties of heavy crude oil. Density (g/cm3, 20 °C)
0.9331
Viscosity (mPa s, 50 °C)
610
SARA fractions (wt%)
Element analysis (wt%)
Saturates
Aromatics
Resins
Asphaltenes
C
H
O
N
S
50.69
30.58
14.81
3.92
80.7
13.2
2.76
0.72
0.91
118
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6
of heat, CO2, CO, H2O, etc [21,37]. Fig. 2(a)–(d) presents the combustion TG and DTG profiles of SARA fractions at different heating rates. Different from the heavy crude oil, only two distinctive regions, LTO and HTO, can be observed for saturates and aromatics. In this scenario, FD was usually considered as a sub-region of LTO reactions [38]. Owing to the fact that evaporation dominated the total mass loss in the LTO interval as we previously discussed, saturates and aromatics were vulnerable to severe mass loss. Consequently, there was a small amount of coke being formed and weak HTO reactions. Considering resins, the temperature ranges at 30–361 °C, 361–508 °C and 508–614 °C represented for the LTO, FD and HTO regions, respectively, which corresponded to 30–353 °C (LTO region), 353–504 °C (FD region) and 504–648 °C (HTO region) for asphaltenes. Since the evaporation effect of heavy components was fairly mild, the mass loss at the LTO stage for resins and asphaltenes was only 7.6% and 3.5%, respectively. Unlike saturates and aromatics, resins and asphaltenes were subjected to distinct FD region, suggested by appreciable mass loss and pronounced DTG peak (given in Fig. 2(c), (d)). This result verified that both asphaltenes and resins were the prime fractions that contributed to the formation of coke. The apparent mass loss at the FD stage can be attributed to the violent occurrence of intricate reactions such as the cleavage of C-heteroatom and CeC bonds in alkanes and alkyl side chains connected to aromatic rings and/or naphthenes, as well as the dehydrogenation and ring-opening of the aromatic structure and/or naphthenes [39,40]. The mass loss at the HTO stage for resins and asphaltenes was accounted for 29.7% and 38.9%, respectively. This fact was to be expected from the combustion of much coke formed at the FD stage as we previously discussed. For the convenience of comparison, the TG/DTG profiles of heavy crude oil and its SARA fractions at a constant heating rate of 10 °C/min are presented in Fig. 3. It can be seen that saturates was drastically sensitive to the elevated temperature, indicated by appreciable mass loss and low onset temperature of HTO events as compared to other three fractions. In addition, resins showed analogous DTG behaviors with asphaltenes, largely as a consequence of their close chemical structures [41].
80 4
60 40
TG/%
0
0 -20
-2
-40 -60
0
5 C/min 0 10 C/min 0 15 C/min
-80 -100
DTG/(%/min)
2
20
100
200
300
400
500
600
-4
-6 700
0
Temperature/ C
Fig. 1. TG/DTG curves for heavy crude oil at the heating rates of 5, 10 and 15 °C/min.
respectively. The activation energy can be determined by the slope of the regression lines of ln(β) versus 1/T for OFW method and ln(β/T2) versus 1/T for DAEM method under the same conversion degree at different heating rates. 3. Results and discussion 3.1. Thermal behaviors of heavy crude oil and its SARA fractions characterized by TG/DTG technique The combustion TG and DTG curves of heavy crude oil at different heating rates are shown in Fig. 1. Consistent with previous reports [21,30,31], the reaction process of heavy crude oil can be classified into three varying intervals on the basis of TG/DTG curves, known as LTO, FD and HTO. It was found that the onset, peak and ending temperatures of three reaction regions were all shifted to right hand with the heating rate. The similar tendency was also observed for SARA fractions (see Fig. 2(a)– (d)) as a consequence of more intense thermal hysteresis effect at higher heating rate [32]. Accordingly, for the convenience of descriptions, the data for the heating rate of 10 °C/min were used as reference when it comes to the analysis covering TG/DTG/DSC characteristic parameters such as temperature range, mass loss, heat flow, etc. As shown in Fig. 1 and Table 2, the LTO reactions occurred from ambient temperature to 395 °C, and FD occurred from 395 °C to 499 °C, and HTO reactions took place over 499 °C. Correspondingly, three weight loss peaks observed from DTG curve appeared at 263, 463 and 535 °C, respectively. A certain amount of mass loss (65.9%) in the LTO interval was mostly attributed to the evaporation of light components as Yuan et al. [21] previously reported. Two types of oxidation reactions, named oxygen addition reactions and isomerization and decomposition reactions, were believed to be dominant LTO reaction pathways [33,34]. It was reported that the oxygen addition reactions to produce hydroperoxides were remarkable pathways at the initial stage of LTO, followed by the isomerization and decomposition reactions that produced numerous oxidized products (alcohols, ketones, carboxylic acids, aldehydes, etc.), H2O, CO2 and CO [21,34]. The mass loss at the FD stage was accounted for 17.6%, which indicated that some LTO residue (oxidized products and their condensation compounds) formed in the LTO reactions took part in thermo-oxidative cracking reactions to form coke. It should be noted as well that in actual ISC process, coke can also be formed by the pyrolysis of those non-volatile hydrocarbons that originally existed in crude oils [35,36]. The third reaction interval in the temperature range of 499–627 °C was considered as HTO. Nearly 16.5% of mass loss was found in this interval. As accepted by most researchers, the dominative reaction at the HTO stage was the combustion of formed coke, which consequently produced a great amount
3.2. Thermal behaviors of heavy crude oil and its SARA fractions characterized by DSC technique Fig. 4 shows the DSC curves of combustion of crude oil at the heating rates of 5, 10 and 15 °C/min, respectively. Consistent with TG profiles, the reaction zones reflected by DSC curves at higher heating rates were shifted to the right hand. The DSC peak heat flows and heat enthalpies of each reaction region for heavy oil and its SARA fractions are summarized in Table 3. According to Fig. 4, two apparent exothermic regions, LTO and HTO, existed during the whole heating process. The heat flow in the LTO interval peaked at 300 °C with a value of 1.9 mW/mg. Regarding the HTO interval, the heat flow peaked at 530 °C with a value of 3.88 mW/mg, and the cumulative heat liberated was accounted for 2.615 kJ/g, which was approximately 2.05 times that of LTO interval (1.279 kJ/g). The fact was due to the nature of heavy oil that covered a large amount of resins and asphaltenes, particularly for asphaltenes that released noticeably higher heat in the HTO interval relative to LTO interval as DSC data corroborated below. The DSC profiles for SARA fractions at the heating rates of 5, 10 and 15 °C/min are presented in Fig. 5. Two main exothermic regions observed from DSC profiles of saturates and aromatics were considered as LTO and HTO. It was apparent that saturates made a dominating contribution to the LTO reactions in terms of heat release, in accordance with the results obtained by Varfolomeev et al. [19] Aromatics exhibited pronounced exothermic activity both at the LTO and HTO stages. Its thermal enthalpy in the LTO region was lower than that in the HTO region, but an adverse tendency was obtained in a recent publication [5]. It was believed that different results were mainly related to the molecular structure of aromatics used. In that work, the main components of the aromatics separated from light crude oil 119
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(a)
DTG/(%/min)
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Temperature/ C
Temperature/ C
Fig. 2. TG/DTG curves for SARA fractions at the heating rates of 5, 10 and 15 °C/min: (a) saturates, (b) aromatics, (c) resins, (d) asphaltenes.
corresponding DSC peak of HTO region was tremendously prominent. Therefore, FD was considered as a sub-region of LTO reactions as mentioned before. Unlike saturates and aromatics, DSC curves of resins and asphaltenes presented the wavy shape in the temperature range from 350 to 500 °C. This was more likely due to the negative effects caused by endothermic reactions such as thermal cracking that
contained diaromatics (54%) and monoaromatics (23%), whereas that used in this study included triaromatics (49%) and diaromatics (35%) as stated in Section 2.1. The aromatics fraction with more benzene rings underwent stronger heat release caused by HTO rather than LTO [42]. As shown in Fig. 5(c), (d), it was impossible to designate a distinctive DSC peak of resins and asphaltenes to LTO and FD regions, while
Table 2 TG/DTG reaction regions, peak temperatures and mass losses of heavy crude oil and its SARA fractions at different heating rates. Sample
Heating rate (°C/min)
LTO
FD
Region (°C)
Tpa
(°C)
HTO
MassLoss (%)
Region (°C)
Tp (°C)
MassLoss (%)
Region (°C)
Tp (°C)
MassLoss (%)
Oil
5 10 15
30–383 30–395 30–403
258 263 280
70.6 65.9 63.9
383–475 395–499 403–520
443 463 478
17.2 17.6 20.3
475–558 499–627 520–658
506 535 549
12.2 16.5 15.7
Saturates
5 10 15
30–342 30–363 30–371
298 300 440
82.9 81.8 80.6
– – –
– – –
– – –
342–442 363–492 371–509
403 427 452
16.5 16.7 16.8
Aromatics
5 10 15
30–446 30–471 30–486
366 382 430
88.8 88.4 88.2
– – –
– – –
– – –
446–561 471–584 486–598
514 523 530
11.2 10.8 10.3
Resins
5 10 15
30–355 30–361 30–367
– – –
10.5 8.5 7.6
355–481 361–508 367–521
441 446 452
56.1 61.6 58.8
481–572 508–614 521–684
535 552 575
33.4 29.7 32.3
Asphaltenes
5 10 15
30–349 30–353 30–355
– – –
3.8 3.5 3.4
349–495 353–504 355–516
441 453 460
60.1 56.7 55.8
495–606 504–648 516–679
552 570 590
36.1 38.9 38.7
a
The temperature at which the maximum mass loss rate occurred. 120
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occasionally took place in this temperature interval [6]. Conversely, owing to the fact that the composition of reactant at the HTO stage (i.e. coke) was quite uniform and analogous reactions (i.e. combustion of coke) occurred at the HTO stage, all DSC curves for heavy crude oil and its SARA fractions were smooth and apparent. The DSC profiles of heavy crude oil and its SARA components at the heating rate of 10 °C/min are compared in Fig. 6. It was found that the exothermic behaviors of SARA fractions differed drastically. The descending order of the cumulative heat release during the whole heating process was the following: asphaltenes (7.683 kJ/g) > resins (6.997 kJ/g) > aromatics (4.423 kJ/g) > crude oil (3.894 kJ/g) > saturates (1.029 kJ/g). Resins and asphaltenes gave far higher heat release in the HTO region relative to saturates, especially asphaltenes. The phenomenon was due to that resins and asphaltenes were the main components that participated in a series of thermo-oxidative cracking reactions to produce coke, as documented by the TG and DTG results of SARA fractions, which in turn contributed to a great amount of heat generation in the HTO region. With regard to aromatics, the heat liberated in the HTO region was weaker than that of resins and asphaltenes but stronger than that of saturates, revealing that some coke was formed during LTO of aromatics to offer fuel for HTO reactions. Simultaneously, it should be noted that a little bit difference of heat enthalpy in the LTO stage existed among SARA fractions as listed in Table 3. This result gave a hint that given the enhanced oil recovery (EOR) mechanisms caused by the heating effect of LTO reactions during air injection for light oils involving plenty of light fractions, the LTO mechanisms can also be applied for heavy oils with abundant heavy components to make full use of the heating effect.
Fig. 3. TG/DTG curves for heavy crude oil and its SARA fractions at the heating rate of 10 °C/min.
5
o
5 C/min o 10 C/min o 15 C/min
Exo
DSC/(mW/mg)
4
3
2
3.3. Interactions between heavy crude oil and its SARA fractions 1
To investigate the interactions between crude oil and its SARA fractions considering heat release during combustion, the DSC curve of heavy crude oil and fitting curve of SARA superposition based on their proportion in crude oil (see Table 1) at the heating rate of 10 °C/min are presented in Fig. 7. Large discrepancies existed between the two curves both in the LTO and HTO regions, which did not accord with previous research obtained by Yuan et al. in which the two curves fitted well at the LTO stage but deviated from each other at the HTO stage [5]. As mentioned before, the oil properties and molecular structures of SARA fractions for two crude oils existed prominent differences, which was assumed to be the main reason that caused varying results. In this work, we could find that there were strong interactions among SARA fractions. For the temperature below 331 °C, the heat release of fitting curve was higher than that of measurement curve. This phenomenon disclosed that the evident inhibiting effect primarily existed among SARA fractions in the LTO interval, which was closely related to the findings concluded by some researchers. Freitag [43] claimed that saturates involved a few naturally occurring oxidation inhibitors that restrained the oxidation rates by rapidly consuming an essential intermediate in the reaction chain within low temperature. Alexandra et al. [33] indicated that some aromatic compounds such as phenol derivatives acted as inhibitors at the initial stage of oxidation, and oxidation reactions did not proceed unless the rate of radicals formation surpassed the rate of inhibition or all the inhibitors became oxidized and inert. With these in mind, it was believed that saturates and/or aromatic compounds generated a negative influence on LTO of crude oil in terms of heat release. Nevertheless, the direct evidence of inhibiting effect of saturates and/or aromatic compounds on crude oil oxidation at molecular scale is still undetectable and needs to be further studied. As the temperature exceeded 331 °C, the heat flow of measurement curve was higher than that of fitting curve, elucidating that there was the promoting effect among SARA fractions on the whole at the FD and HTO stages. The phenomenon might be explained that intermediate products of resins and asphaltenes in LTO reactions can promote fuel deposition via polycondensation reactions (exothermic effect). Consequently, the
0
-1
100
200
300
400
500
600
700
o
Temperature/ C Fig. 4. DSC curves for heavy crude oil at the heating rates of 5, 10 and 15 °C/ min. Table 3 DSC peak heat flows and heat enthalpies of heavy oil and its SARA fractions at different heating rates. Heating rate (°C/ min)
LTO Peak heat flow (mW/mg)
Heat enthalpy (kJ/g)
Peak heat flow (mW/mg)
Heat enthalpy (kJ/g)
Oil
5 10 15
1.52 1.90 2.23
1.924 1.279 1.065
2.54 3.88 4.43
2.546 2.615 2.519
Saturates
5 10 15
0.85 1.14 1.53
1.167 0.802 0.767
0.26 0.42 0.62
0.249 0.227 0.244
Aromatics
5 10 15
1.04 2.79 3.90
1.437 2.174 2.245
2.57 3.81 5.30
2.578 2.255 2.629
Resins
5 10 15
– – –
2.120 1.642 1.855
6.32 7.81 9.60
5.813 5.355 4.824
Asphaltenes
5 10 15
– – –
2.343 2.038 1.944
7.52 10.55 12.51
6.786 5.645 6.549
Sample
HTO
121
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Exo
1.6
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Fig. 5. DSC curves for SARA fractions at the heating rates of 5, 10 and 15 °C/min: (a) saturates, (b) aromatics, (c) resins, (d) asphaltenes. 12
4
Exo
DSC/(mW/mg)
8
Exo
crude oil saturates aromatics resins asphaltenes
3
DSC/(mW/mg)
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6
4
crude oil fitting value
2
1 2
0 100
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400
500
600
0
700
100
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o
300
400
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o
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Temperature/ C
Fig. 6. DSC curves for heavy crude oil and its SARA fractions at the heating rate of 10 °C/min.
Fig. 7. DSC curve of heavy crude oil and fitting curve of SARA superposition at the heating rate of 10 °C/min.
fact would lead to more heat release and coke formation at the FD stage and stronger combustion of coke at the HTO stage. When it comes to utilizing SARA fractions to model the combustion of heavy crude oil, the aforementioned interactions must be taken into full consideration.
3.4. Combustion kinetics The TG data under three heating rates (5, 10 and 15 °C/min) were adopted to calculate the kinetic parameters using DAEM and OFW models. Figs. S1–S5 in the Supporting information present the DAEM and OFW plots for heavy crude oil and its SARA components during the 122
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150
600
(b)
(a)
200
DAEM OFW o T (ȕ 10 C/min)
40
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o
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DAEM OFW o T (ȕ 10 C/min)
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o
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o
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o
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Temperature/ C
0.0
120
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o
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80
Temperature/ C
Activation energy/(kJ/mol)
600
120 0 0.0
0.2
0.4
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0.8
1.0
Conversion rate/%
Fig. 8. E-α and T-α curves: (a) heavy crude oil, (b) saturates, (c) aromatics, (d) resins, (e) asphaltenes.
activation energy increased all the time, suggesting that more energy were needed for initiating thermo-oxidative cracking reactions with conversion rate. On the whole, the activation energies of crude oil fluctuated and varied in the range of 45–95 kJ/mol during combustion, disclosing its intricate combustion processes and multiple reaction mechanisms. As shown in Fig. 8(b), (c), the conversion rate of 0.1–0.83 for saturates represented the LTO + FD stage, which corresponded to 0.1–0.89 (LTO + FD stage) for aromatics. Considering saturates, an increasing trend of activation energy with conversion degree was presented as a whole, mainly caused by the negative temperature coefficient (NTC) behaviors [34,44]. Nevertheless, a descending trend of activation energy was observed roughly after conversion degree
whole heating process. And Tables S1–S5 (see Supporting information) list the activation energies at varying conversion levels (α = 0.1–0.9). Additionally, E-α and T-α (β = 10 °C/min) curves of combustion of heavy crude oil and its SARA fractions were depicted in Fig. 8(a)–(e), respectively. It was seen that the activation energies obtained by DAEM were comparable to those obtained by OFW method. Regarding the heavy crude oil, the conversion rates at 0.1–0.66 and 0.66–0.85 stood for the LTO and FD stages, respectively. In the LTO region, the activation energy firstly increased and then decreased approximately after conversion rate reached 0.4. This fact indicated that at the initial stage of LTO, the reaction difficulty increased as the LTO reactions proceeded; but after a certain conversion degree was reached, the following LTO reactions became more readily to occur. For FD, the 123
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(4) Large differences in the dependence of activation energy upon conversion degree existed among SARA fractions, disclosing their different reaction pathways and mechanisms. Different from other three fractions, the FD and HTO activation energies of asphaltenes descended consecutively with conversion degree as a whole, which is a positive signal to boost the sustainability of combustion front and ISC performance.
attained 0.75, presumably owing to that oxidation inhibitors that originally existed in saturates were gradually consumed at the initial stage of LTO as we discussed before. As for aromatics, the activation energy increased slightly with conversion rate on the whole, except for conversion rate at 0.85–0.9. The activation energy ascended drastically as conversion degree was increased from 0.85 to 0.9, which might imply that the LTO products of aromatics needed considerably high energy to trigger thermo-oxidative cracking reactions in this temperature range (440–482 °C). The conversion rates at 0.1–0.7 and 0.7–0.9 stood for the FD and HTO stages for resins, respectively, which corresponded to 0.1–0.61 (FD stage) and 0.61–0.9 (HTO stage) for asphaltenes. The activation energy of resins increased quickly with an increasing conversion degree from 0.1 to 0.2, followed by a descending trend. This phenomenon manifested that higher energy demand was utilized to touch off reactions at the initial stage of FD, but the subsequent thermooxidative cracking and HTO reactions became more facilely to take place after conversion rate attained 0.2. As agreed by most peers, asphaltenes was the most resistant fraction in the crude oil [17,45]. So the energy demand to trigger thermo-oxidative cracking reactions at the initial stage of FD for asphaltenes was much higher than that for crude oil and other three fractions. The activation energy of asphaltenes reduced consecutively with conversion degree as a whole, which facilitated the FD and HTO processes to enhance the sustainability of combustion front. Fig. 8(b), (e) shows that large differences in the dependence of activation energy upon conversion degree existed among SARA fractions, elucidating their different reaction pathways and mechanisms. Compared with SARA fractions, the heavy crude oil had the greatest degree of oscillations regarding the variation of activation energy versus conversion degree. The fact evidenced that there were some interactions among SARA fractions during combustion, which was consistent with the findings obtained in Section 3.3.
Acknowledgements The authors gratefully acknowledge the financial support of the National Key Basic Research Program of China (2015CB250904) and Natural Science Foundation of Sichuan Province (2017JY0122). The authors also thank the anonymous reviewers for their constructive and valuable comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.11.014. References [1] Turta AT, Chattopadhyay SK, Bhattacharya RN, Condrachi A, Hanson W. Current status of commercial in situ combustion projects worldwide. J Can Pet Technol 2007;46(11):8–14. [2] Li YB, Chen YF, Pu WF, Dong H, Gao H, Jin FY, et al. Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods. J Ind Eng Chem 2017;48:249–58. [3] Shah A, Fishwick R, Wood J, Leeke G, Rigby S, Greaves M. A review of novel techniques for heavy oil and bitumen extraction and upgrading. Energy Environ Sci 2010;3(6):700–14. [4] Thomas S. Enhanced oil recovery – an overview. Oil Gas Sci Technol 2008;63(1):9–19. [5] Yuan CD, Varfolomeev MA, Emelianov DA, Eskin AA, Nagrimanov RN, Kok MV, et al. Oxidation behavior of light crude oil and its SARA fractions characterized by TG and DSC techniques: differences and connections. Energy Fuels 2018;32:801–8. [6] Wei B, Zou P, Zhang X, Xu XG, Wood C, Li YB. Investigations of structure-propertythermal degradation kinetics alterations of Tahe asphaltenes caused by low temperature oxidation. Energy Fuels 2018;32:1506–14. [7] Zhang L, Deng JY, Wang L, Chen ZY, Ren SR, Hu CH, et al. Low temperature oxidation characteristics and its effect on the critical coking temperature of heavy oils. Energy Fuels 2015;29(2):538–45. [8] Xu Q, Hang J, Cheng Z, Tang W, Ran X, Jia H, et al. Coke formation and coupled effects on pore structure and permeability change during crude oil in situ combustion. Energy Fuels 2016;30(2):933–42. [9] Santos RGD, Vargas JAV, Trevisan OV. Thermal analysis and combustion kinetic of heavy oils and their asphaltene and maltene fractions using accelerating rate calorimetry. Energy Fuels 2014;28(11):7140–8. [10] Larter SR, Adams J, Gates ID, Bennett B, Huang H. The origin, prediction and impact of oil viscosity heterogeneity on the production characteristics of tar sand and heavy oil reservoirs. J Can Pet Technol 2008;47(1):40–9. [11] Varfolomeev MA, Nurgaliev DK, Kok MV. Calorimetric study approach for crude oil combustion in the presence of clay as catalyst. Pet Sci Technol 2016;34:1624–30. [12] Vargas JAV, Santos RGD, Trevisan OV. Evaluation of crude oil oxidation by accelerating rate calorimetry. J Therm Anal Calorim 2013;113(2):897–908. [13] Murugan P, Mahinpey N, Mani T, Asghari K. Effect of low-temperature oxidation on the pyrolysis and combustion of whole oil. Energy 2010;35(5):2317–22. [14] Niu BL, Ren SR, Liu YH, Wang DZ, Tang LZ, Chen BL. Low-temperature oxidation of oil components in an air injection process for improved oil recovery. Energy Fuels 2011;25(10):4299–304. [15] Freitag NP, Verkoczy B. Low-temperature oxidation of oils in terms of SARA fractions: why simple reaction models don’t work. J Can Pet Technol 2005;44:54–61. [16] Alsaffar HB, Hasanin H, Price D, Hughes R. Oxidation reactions of a light crude oil and its SARA fractions in consolidated cores. Energy Fuels 2001;15(1):182–8. [17] Kok MV, Ozgen K, Pamir R. Kinetic analysis of oxidation behavior of crude oil SARA constituents. Energy Fuels 1998;12:580–8. [18] Kuppe G, Mehta S, Moore R, Ursenbach M, Zalewski E. Heats of combustion of selected crude oils and their SARA fractions. J Can Pet Technol 2008;47(1):38–42. [19] Varfolomeev MA, Galukhin A, Nurgaliev DK, Kok MV. Thermal decomposition of Tatarstan Ashal’cha heavy crude oil and its SARA fractions. Fuel 2016;186:122–7. [20] Ambalae A, Mahinpey N, Freitag N. Thermogravimetric studies on pyrolysis and combustion behavior of a heavy oil and its asphaltenes. Energy Fuels 2006;20(2):560–5. [21] Yuan CD, Emelianov D, Varfolomeev MA. Oxidation behaviour and kinetics of light, medium and heavy crude oils characterized by thermogravimetry coupled with fourier-transform infrared spectroscopy (TG-FTIR). Energy Fuels 2018;32(4):5571–81. [22] Zhao RB, Wei YG, Wang ZM, Yan W, Yang HJ, Liu SJ. Kinetics of low-temperature
4. Conclusion In this work, the thermal behaviors of combustion of heavy crude oil and its SARA fractions were comparatively analyzed using TG-DSC technique, followed by the investigation regarding SARA interactions during combustion. The dependence of activation energy upon conversion rate, which was determined by DAEM and OFW method, for heavy crude oil and its SARA fractions was presented in this paper. The conclusions derived from this study are as follows. (1) Saturates and aromatics were vulnerable to severe mass loss at the LTO stage. Resins and asphaltenes were subjected to appreciable mass loss and prominent DTG peak in the FD interval to form a great amount of coke, which in turn contributed to HTO reactions. (2) Saturates made an inconsequential contribution to the HTO reactions. Aromatics showed distinctive exothermic effect both at the LTO and HTO stages, which was close to crude oil. Compared with saturates and aromatics, resins and asphaltenes gave considerably higher heat release in the HTO region, especially asphaltenes. A little bit difference of heat enthalpy in the LTO stage existed among SARA fractions, implying that given the EOR mechanisms resulted from the thermal effect of LTO reactions during air injection for light oil, the LTO mechanisms can also be suitable for heavy oil to take full advantage of the thermal effect. (3) The inhibiting effect primarily existed among SARA fractions in the LTO interval, which was believed that saturates and/or aromatic compounds generated a negative influence on LTO of crude oil in terms of heat release. On the whole, there was the promoting effect among SARA fractions at the FD and HTO stages, mostly owing to that intermediate products of resins and asphaltenes in LTO reactions can promote fuel deposition via polycondensation reactions. These interactions must be taken into full consideration when using SARA fractions to model ISC process. 124
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2017;11:1–11. [34] Freitag NP. Chemical reaction mechanisms that govern oxidation rates during insitu combustion and high-pressure air injection. SPE Reserv Eval Eng 2016;19:645–54. [35] Murugan P, Mahinpey N, Mani T. Thermal cracking and combustion kinetics of asphaltenes derived from Fosterton oil. Fuel Process Technol 2009;90(10):1286–91. [36] Murugan P, Mahinpey N, Mani T, Freitag N. Pyrolysis and combustion kinetics of Fosterton oil using thermogravimetric analysis. Fuel 2009;88(9):1708–13. [37] Li J, Mehta S, Moore R, Ursenbach M, Zalewski E, Ferguson H, et al. Oxidation and ignition behaviour of saturated hydrocarbon samples with crude oils using TG/DTG and DTA thermal analysis techniques. J Can Pet Technol 2004;43(7):45–51. [38] Kok MV. Characterization of medium and heavy crude oils using thermal analysis techniques. Fuel Process Technol 2011;92(5):1026–31. [39] Kapadia PR, Kallos MS, Gates ID. A review of pyrolysis, aquathermolysis, and oxidation of Athabasca bitumen. Fuel Process Technol 2015;131:270–89. [40] Hao JH, Che YJ, Tian YY, Li DW, Zhang JH, Qiao YY. Study on thermal cracking characteristics and kinetics of oil sand bitumen and its SARA fractions by TG–FTIR. Energy Fuels 2017;31(2):1295–309. [41] Xu C, Gao J, Zhao S, Lin S. Correlation between feedstock SARA components and FCC product yields. Fuel 2005;84(6):669–74. [42] Li J, Mehta S, Moore R, Zalewski E, Ursenbach M, Van Fraassen K. Investigation of the oxidation behaviour of pure hydrocarbon components and crude oils utilizing PDSC thermal technique. J Can Pet Technol 2006;45(1):48–53. [43] Freitag NP. Evidence that naturally occurring inhibitors affect the low-temperature oxidation kinetics of heavy oil. J Can Pet Technol 2010;49(7):36–41. [44] Huang SY, Sheng JJ. An innovative method to build a comprehensive kinetic model for air injection using TGA/DSC experiments. Fuel 2017;210:8–106. [45] Kok MV, Gul KG. Thermal characteristics and kinetics of crude oils and SARA fractions. Thermochim Acta 2013;569(40):66–70.
oxidation of light crude oil. Energy Fuels 2016;30(4):2647–54. [23] Fan C, Zan C, Zhang Q, Ma D, Chu Y, Jiang H, et al. The oxidation of heavy oil: Thermogravimetric analysis and non-isothermal kinetics using the distributed activation energy model. Fuel Process Technol 2014;119(1):146–50. [24] Gundogar AS, Kok MV. Thermal characterization, combustion and kinetics of different origin crude oils. Fuel 2014;123(1):59–65. [25] Liu D, Song Q, Tang JS, Zheng RN, Yao Q. Interaction between saturates, aromatics and resins during pyrolysis and oxidation of heavy oil. J Pet Sci Eng 2016;154:543–50. [26] Vyazovkin S, Burnham AK, Criado JM, Pérez-Maqueda LA, Popescu C, Sbirrazzuoli N. ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data. Thermochim Acta 2011;520(1):1–19. [27] Karimian M, Schaffie M, Fazaelipoor MH. Estimation of the kinetic triplet for in-situ combustion of crude oil in the presence of limestone matrix. Fuel 2017;209:203–10. [28] Opfermann JR, Kaisersberger E, Flammersheim HJ. Model-free analysis of thermoanalytical data-advantages and limitations. Thermochim Acta 2002;39(1–2):119–27. [29] Pu WF, Chen YF, Li YB, Zou P, Li D. The comparison of different kinetic models for heavy oil oxidation characteristic evaluation. Energy Fuels 2017;31(11):12665–76. [30] Yuan CD, Pu WF, Jin FY, Zhang JJ, Zhao QN, Li D, et al. Characterizing the fuel deposition process of crude oil oxidation in air injection. Energy Fuels 2015;6(3):7622–9. [31] Pu WF, Yuan CD, Jin FY, Wang L, Qian Z, Li YB, et al. Low-temperature oxidation and characterization of heavy oil via thermal analysis. Energy Fuels 2015;29:1551–11159. [32] Mothé CG. Study of kinetic parameters of thermal decomposition of bagasse and sugarcane straw using Friedman and Ozawa–Flynn–Wall isoconversional methods. J Therm Anal Calorim 2013;113(2):497–505. [33] Ushakova A, Zatsepin V, Varfolomeev M, Emelyanov D. Study of the radical chain mechanism of hydrocarbon oxidation for in situ combustion process. J Combust
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Full Length Article
Comparative evaluation on the thermal behaviors and kinetics of combustion of heavy crude oil and its SARA fractions ⁎
T
⁎
Shuai Zhaoa, , Wanfen Pua,b, , Boshuai Sunc, Fei Gua, Liangliang Wanga a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia c Shenghong Petrochemical Co. Ltd., Jiangsu 222002, People’s Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Combustion Heavy crude oil Saturates-aromatics-resins-asphaltenes Thermogravimetry Differential scanning calorimeter Activation energy
In situ combustion (ISC) plays a significant role in the exploitation of heavy crude oil. In this study, the thermal behaviors and kinetics of combustion of heavy crude oil and its saturates-aromatics-reins-asphaltenes (SARA) fractions were comparatively evaluated using thermogravimetry (TG) and differential scanning calorimeter (DSC) techniques. The results indicated that saturates and aromatics were vulnerable to severe mass loss at the low-temperature oxidation (LTO) stage; however, resins and asphaltenes encountered appreciable mass loss in the fuel deposition (FD) interval to form a great amount of coke, which consequently contributed to hightemperature oxidation (HTO) reactions. Saturates made an inconsequential thermal release to the HTO reactions. Aromatics showed apparent exothermic effect both at the LTO and HTO stages, which was consistent with crude oil. Compared with saturates and aromatics, resins and asphaltenes gave considerably higher heat release in the HTO region, especially asphaltenes. By comparing the DSC curve of heavy crude oil with fitting curve of SARA superposition based on their proportion in crude oil, apparent inhibiting (promoting) effect among SARA fractions at the LTO stage (FD and HTO stages) were detected in terms of heat release, which should be of much significance for using SARA fractions to model ISC process. The kinetic parameters determined by Ozawa–Flynn–Wall (OFW) method were almost identical to those obtained by distributed activation energy model (DAEM). The activation energies of crude oil fluctuated and varied in the range of 45–95 kJ/mol during combustion, disclosing its intricate combustion processes and multiple reaction mechanisms. Unlike other three fractions, the FD and HTO activation energies of asphaltenes descended consecutively with conversion degree, which is favorable for boosting the sustainability of combustion front.
1. Introduction With the depletion of conventional oil reservoirs coupled with the ever-increasing energy demand, considerable attention has been focused on the heavy oil resources with a huge amount of reserves [1,2]. Among a series of enhanced oil recovery technologies, thermal recovery methods, including ISC, steam injection, in situ electrical heaters, binary mixtures, supercritical water, etc., have been proven to be significantly promising for the exploitation of heavy oil reservoirs [3,4]. With regard to ISC, after air or oxygen-enriched gas is injected into the crude oil reservoirs, numerous oxidation reactions occur between crude oils and oxygen, which produce appreciable thermal energy, hot water, steam and carbon oxides to displace the oil into production wells by multiple flooding mechanisms [5–7]. Accordingly, since the first ignition operation in 1933, hundreds of ISC projects have been deployed in
some crude oil reservoirs. Unfortunately, nearly 80% of the ISC projects turned out to be unsuccessful economically and technologically, mostly as a consequence of a limited understanding concerning the thermal behaviors and kinetics of combustion of crude oils [8]. Early investigations related to crude oil combustion were carried out using TG, DSC, accelerating rate calorimeter (ARC), combustion tube (CT), small batch reactor (SBR), etc. [9–14] It is widely accepted that the combustion behaviors and kinetics are predominantly determined by the reactivity of crude oils that mostly depends on their fractions. Typically, crude oils were divided into four fractions, socalled saturates, aromatics, resins and asphaltenes, shortly SARA fractions [15,16]. Therefore, quite a few researchers focused on the studies regarding the thermal behaviors and kinetics of crude oils and their SARA fractions during combustion. Kok et al. [17] studied the combustion behaviors of SARA components derived from medium and
⁎ Corresponding authors at: State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China and Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia (W. Pu). E-mail addresses: [email protected] (S. Zhao), [email protected] (W. Pu).
https://doi.org/10.1016/j.fuel.2018.11.014 Received 14 August 2018; Received in revised form 8 October 2018; Accepted 1 November 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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China. Table 1 is presented to show the density, viscosity, SARA fractions and elemental compositions of the crude oil used. The industrial standard method of China Petroleum NB/SH/T 0509-2010 was utilized to split SARA fractions from crude oil. A high performance liquid chromatograph UltiMate 3000 (Thermo Scientific) was deployed for testing the aromatic component and corresponding experimental details had been given by Yuan et al. [5] The main components of aromatics used in this study contained triaromatics (49%) and diaromatics (35%).
heavy oils by TG technique. It was claimed that saturates could be oxidized more readily owing to its lower activation energy as compared to others fractions, whereas asphaltenes with high activation energy was not oxidized easily. Simultaneously, the weight loss and kinetic parameters of individual SARA fraction at each reaction stage were provided. Kuppe et al. [18] examined the heat release of different crude oils and SARA fractions during combustion. It was found that the heat liberated of saturates and aromatics was greater than that of resins and asphaltenes. Varfolomeev et al. [19] investigated the thermal decomposition of Tatarstan Ashal’cha heavy crude oil and its SARA fractions. Two main reaction stages, LTO and HTO, were observed from DSC profiles, while only one primary stage appeared in differential thermogravimetry (DTG) curves. Ambalae et al. [20] analyzed the combustion kinetics of crude oil and its asphaltenes. It was reported that the activation energy of crude oil was comparable to its asphaltenes. The concise review shows that much emphasis has been devoted to acquiring some significant information such as mass loss, heat effect and kinetic parameters of crude oils and their SARA fractions during combustion. Meanwhile, numerous studies concerning the dependence of kinetic parameters upon conversion degree during the whole heating process for crude oils have been deployed using different isoconversional models [21–24], but those for SARA fractions were rarely presented for comparative analysis. Very recently, the interactions between crude oils and SARA components in terms of combustion behaviors have been studied by some researchers. For instance, Liu et al. [25] indicated that no interactions were found among saturates, aromatics and resins in the process of copyrolysis, whereas the interactions were pronounced during co-combustion. In addition, they gave a detailed description about the variation of mass loss and DTG peak temperature during co-combustion. Yuan et al. [5] performed an interesting investigation about whether there were some specific connections in combustion behaviors between light crude oil and its SARA fractions. At the LTO stage, the DSC curve of light oil was approximately the same as the predicted curve calculated by the DSC data of SARA fractions based on their proportion in crude oil. However, the measured DSC curve gradually deviated from the predicted curve after 350 °C. The results unravelled that individual SARA fraction followed its own reaction pathway at the LTO stage but was distinctly affected by the presence of other fractions at the FD and HTO regions. Nevertheless, to the best of our knowledge, whether this finding can apply to heavy crude oils and their SARA fractions is not validated by further investigation. In this work, the thermal behaviors of combustion of heavy crude oil and its SARA fractions were comparatively evaluated using TG-DSC technique. The primary objective of this research was intended to disclose the SARA interactions in the process of combustion. Additionally, special attention was placed on the relation of activation energy versus conversion rate for heavy oil and its SARA fractions. It is believed that the results can accelerate to understand action mechanisms among SARA fractions during combustion, which should be of much significance for utilizing SARA fractions to model ISC process.
2.2. TG/DSC analysis The thermal behaviors of combustion of heavy crude oil and its SARA fractions were employed using NETZSCH STA 449F3 PC/PG with TG-DTG and DSC modules. It was noted that prior to the tests, the thermal analysis systems should be calibrated as the methods described by Li et al. [2] A minor amount of sample (10 ± 0.1 mg) was heated up from ambient temperature to 700 °C in an air flow rate of 50 mL/min. The reliable kinetic tests should include three to five varying heating rates, all less than 20 °C/min, based on the recommendations of the International Confederation of Thermal Analysis and Calorimetry (ICTAC) [26]. Accordingly, three linear heating rates (5, 10 and 15 °C/ min) were used. The experiments were performed at least two times for each sample to guarantee the accuracy and repeatability of experimental data. The error of mass loss and temperature was less than ± 0.5 wt% and ± 1 °C, respectively. 2.3. Kinetics model The non-isothermal kinetic study of combustion process is extraordinarily complicated as a consequence of the presence of various components and their parallel, consecutive and competitive reactions. In this research, two kinetic models, named Ozawa–Flynn–Wall (OFW) and distributed activation energy model (DAEM), were utilized for investigating the combustion kinetics. The main advantages of these isoconversional models embody as follows. (1) They allow for evaluating the kinetic parameters without ascertaining the reaction model to avoid the compensation effect [26,27]. (2) They contribute to reveal the complex thermal degradation process and recognize the corresponding reaction mechanisms because of the dependence of kinetic parameters upon conversion degree [28]. The equation of OFW model is given as [26]:
AE ⎞ E ln (β ) = ln ⎛⎜ ⎟ − 5.331 − 1.052 Rg ( α ) RT ⎝ ⎠
(1)
The simplified DAEM equation is written as: [29]
β AR E ⎞ + 0.6075 − ln ⎛ 2 ⎞ = ln ⎛ RT ⎝ E ⎠ ⎝T ⎠
(2)
In this study, α can be calculated as follows:
α =
m o − mt mo − m∞
(3)
where β is the heating rate, E is the activation energy, A is the frequency factor, R is the universal gas constant, α is the conversion rate, g(α) is the integral form of reaction model, T is the absolute temperature, mt is the weight percentage of the sample left in the crucible at reaction time t, mo and m∞ are the initial and final weight percentage of sample,
2. Experimental 2.1. Materials The heavy crude oil was supplied from Xinjiang oilfield in northwest Table 1 SARA fractions and basic properties of heavy crude oil. Density (g/cm3, 20 °C)
0.9331
Viscosity (mPa s, 50 °C)
610
SARA fractions (wt%)
Element analysis (wt%)
Saturates
Aromatics
Resins
Asphaltenes
C
H
O
N
S
50.69
30.58
14.81
3.92
80.7
13.2
2.76
0.72
0.91
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6
of heat, CO2, CO, H2O, etc [21,37]. Fig. 2(a)–(d) presents the combustion TG and DTG profiles of SARA fractions at different heating rates. Different from the heavy crude oil, only two distinctive regions, LTO and HTO, can be observed for saturates and aromatics. In this scenario, FD was usually considered as a sub-region of LTO reactions [38]. Owing to the fact that evaporation dominated the total mass loss in the LTO interval as we previously discussed, saturates and aromatics were vulnerable to severe mass loss. Consequently, there was a small amount of coke being formed and weak HTO reactions. Considering resins, the temperature ranges at 30–361 °C, 361–508 °C and 508–614 °C represented for the LTO, FD and HTO regions, respectively, which corresponded to 30–353 °C (LTO region), 353–504 °C (FD region) and 504–648 °C (HTO region) for asphaltenes. Since the evaporation effect of heavy components was fairly mild, the mass loss at the LTO stage for resins and asphaltenes was only 7.6% and 3.5%, respectively. Unlike saturates and aromatics, resins and asphaltenes were subjected to distinct FD region, suggested by appreciable mass loss and pronounced DTG peak (given in Fig. 2(c), (d)). This result verified that both asphaltenes and resins were the prime fractions that contributed to the formation of coke. The apparent mass loss at the FD stage can be attributed to the violent occurrence of intricate reactions such as the cleavage of C-heteroatom and CeC bonds in alkanes and alkyl side chains connected to aromatic rings and/or naphthenes, as well as the dehydrogenation and ring-opening of the aromatic structure and/or naphthenes [39,40]. The mass loss at the HTO stage for resins and asphaltenes was accounted for 29.7% and 38.9%, respectively. This fact was to be expected from the combustion of much coke formed at the FD stage as we previously discussed. For the convenience of comparison, the TG/DTG profiles of heavy crude oil and its SARA fractions at a constant heating rate of 10 °C/min are presented in Fig. 3. It can be seen that saturates was drastically sensitive to the elevated temperature, indicated by appreciable mass loss and low onset temperature of HTO events as compared to other three fractions. In addition, resins showed analogous DTG behaviors with asphaltenes, largely as a consequence of their close chemical structures [41].
80 4
60 40
TG/%
0
0 -20
-2
-40 -60
0
5 C/min 0 10 C/min 0 15 C/min
-80 -100
DTG/(%/min)
2
20
100
200
300
400
500
600
-4
-6 700
0
Temperature/ C
Fig. 1. TG/DTG curves for heavy crude oil at the heating rates of 5, 10 and 15 °C/min.
respectively. The activation energy can be determined by the slope of the regression lines of ln(β) versus 1/T for OFW method and ln(β/T2) versus 1/T for DAEM method under the same conversion degree at different heating rates. 3. Results and discussion 3.1. Thermal behaviors of heavy crude oil and its SARA fractions characterized by TG/DTG technique The combustion TG and DTG curves of heavy crude oil at different heating rates are shown in Fig. 1. Consistent with previous reports [21,30,31], the reaction process of heavy crude oil can be classified into three varying intervals on the basis of TG/DTG curves, known as LTO, FD and HTO. It was found that the onset, peak and ending temperatures of three reaction regions were all shifted to right hand with the heating rate. The similar tendency was also observed for SARA fractions (see Fig. 2(a)– (d)) as a consequence of more intense thermal hysteresis effect at higher heating rate [32]. Accordingly, for the convenience of descriptions, the data for the heating rate of 10 °C/min were used as reference when it comes to the analysis covering TG/DTG/DSC characteristic parameters such as temperature range, mass loss, heat flow, etc. As shown in Fig. 1 and Table 2, the LTO reactions occurred from ambient temperature to 395 °C, and FD occurred from 395 °C to 499 °C, and HTO reactions took place over 499 °C. Correspondingly, three weight loss peaks observed from DTG curve appeared at 263, 463 and 535 °C, respectively. A certain amount of mass loss (65.9%) in the LTO interval was mostly attributed to the evaporation of light components as Yuan et al. [21] previously reported. Two types of oxidation reactions, named oxygen addition reactions and isomerization and decomposition reactions, were believed to be dominant LTO reaction pathways [33,34]. It was reported that the oxygen addition reactions to produce hydroperoxides were remarkable pathways at the initial stage of LTO, followed by the isomerization and decomposition reactions that produced numerous oxidized products (alcohols, ketones, carboxylic acids, aldehydes, etc.), H2O, CO2 and CO [21,34]. The mass loss at the FD stage was accounted for 17.6%, which indicated that some LTO residue (oxidized products and their condensation compounds) formed in the LTO reactions took part in thermo-oxidative cracking reactions to form coke. It should be noted as well that in actual ISC process, coke can also be formed by the pyrolysis of those non-volatile hydrocarbons that originally existed in crude oils [35,36]. The third reaction interval in the temperature range of 499–627 °C was considered as HTO. Nearly 16.5% of mass loss was found in this interval. As accepted by most researchers, the dominative reaction at the HTO stage was the combustion of formed coke, which consequently produced a great amount
3.2. Thermal behaviors of heavy crude oil and its SARA fractions characterized by DSC technique Fig. 4 shows the DSC curves of combustion of crude oil at the heating rates of 5, 10 and 15 °C/min, respectively. Consistent with TG profiles, the reaction zones reflected by DSC curves at higher heating rates were shifted to the right hand. The DSC peak heat flows and heat enthalpies of each reaction region for heavy oil and its SARA fractions are summarized in Table 3. According to Fig. 4, two apparent exothermic regions, LTO and HTO, existed during the whole heating process. The heat flow in the LTO interval peaked at 300 °C with a value of 1.9 mW/mg. Regarding the HTO interval, the heat flow peaked at 530 °C with a value of 3.88 mW/mg, and the cumulative heat liberated was accounted for 2.615 kJ/g, which was approximately 2.05 times that of LTO interval (1.279 kJ/g). The fact was due to the nature of heavy oil that covered a large amount of resins and asphaltenes, particularly for asphaltenes that released noticeably higher heat in the HTO interval relative to LTO interval as DSC data corroborated below. The DSC profiles for SARA fractions at the heating rates of 5, 10 and 15 °C/min are presented in Fig. 5. Two main exothermic regions observed from DSC profiles of saturates and aromatics were considered as LTO and HTO. It was apparent that saturates made a dominating contribution to the LTO reactions in terms of heat release, in accordance with the results obtained by Varfolomeev et al. [19] Aromatics exhibited pronounced exothermic activity both at the LTO and HTO stages. Its thermal enthalpy in the LTO region was lower than that in the HTO region, but an adverse tendency was obtained in a recent publication [5]. It was believed that different results were mainly related to the molecular structure of aromatics used. In that work, the main components of the aromatics separated from light crude oil 119
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DTG/(%/min)
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Temperature/ C
Temperature/ C
Fig. 2. TG/DTG curves for SARA fractions at the heating rates of 5, 10 and 15 °C/min: (a) saturates, (b) aromatics, (c) resins, (d) asphaltenes.
corresponding DSC peak of HTO region was tremendously prominent. Therefore, FD was considered as a sub-region of LTO reactions as mentioned before. Unlike saturates and aromatics, DSC curves of resins and asphaltenes presented the wavy shape in the temperature range from 350 to 500 °C. This was more likely due to the negative effects caused by endothermic reactions such as thermal cracking that
contained diaromatics (54%) and monoaromatics (23%), whereas that used in this study included triaromatics (49%) and diaromatics (35%) as stated in Section 2.1. The aromatics fraction with more benzene rings underwent stronger heat release caused by HTO rather than LTO [42]. As shown in Fig. 5(c), (d), it was impossible to designate a distinctive DSC peak of resins and asphaltenes to LTO and FD regions, while
Table 2 TG/DTG reaction regions, peak temperatures and mass losses of heavy crude oil and its SARA fractions at different heating rates. Sample
Heating rate (°C/min)
LTO
FD
Region (°C)
Tpa
(°C)
HTO
MassLoss (%)
Region (°C)
Tp (°C)
MassLoss (%)
Region (°C)
Tp (°C)
MassLoss (%)
Oil
5 10 15
30–383 30–395 30–403
258 263 280
70.6 65.9 63.9
383–475 395–499 403–520
443 463 478
17.2 17.6 20.3
475–558 499–627 520–658
506 535 549
12.2 16.5 15.7
Saturates
5 10 15
30–342 30–363 30–371
298 300 440
82.9 81.8 80.6
– – –
– – –
– – –
342–442 363–492 371–509
403 427 452
16.5 16.7 16.8
Aromatics
5 10 15
30–446 30–471 30–486
366 382 430
88.8 88.4 88.2
– – –
– – –
– – –
446–561 471–584 486–598
514 523 530
11.2 10.8 10.3
Resins
5 10 15
30–355 30–361 30–367
– – –
10.5 8.5 7.6
355–481 361–508 367–521
441 446 452
56.1 61.6 58.8
481–572 508–614 521–684
535 552 575
33.4 29.7 32.3
Asphaltenes
5 10 15
30–349 30–353 30–355
– – –
3.8 3.5 3.4
349–495 353–504 355–516
441 453 460
60.1 56.7 55.8
495–606 504–648 516–679
552 570 590
36.1 38.9 38.7
a
The temperature at which the maximum mass loss rate occurred. 120
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occasionally took place in this temperature interval [6]. Conversely, owing to the fact that the composition of reactant at the HTO stage (i.e. coke) was quite uniform and analogous reactions (i.e. combustion of coke) occurred at the HTO stage, all DSC curves for heavy crude oil and its SARA fractions were smooth and apparent. The DSC profiles of heavy crude oil and its SARA components at the heating rate of 10 °C/min are compared in Fig. 6. It was found that the exothermic behaviors of SARA fractions differed drastically. The descending order of the cumulative heat release during the whole heating process was the following: asphaltenes (7.683 kJ/g) > resins (6.997 kJ/g) > aromatics (4.423 kJ/g) > crude oil (3.894 kJ/g) > saturates (1.029 kJ/g). Resins and asphaltenes gave far higher heat release in the HTO region relative to saturates, especially asphaltenes. The phenomenon was due to that resins and asphaltenes were the main components that participated in a series of thermo-oxidative cracking reactions to produce coke, as documented by the TG and DTG results of SARA fractions, which in turn contributed to a great amount of heat generation in the HTO region. With regard to aromatics, the heat liberated in the HTO region was weaker than that of resins and asphaltenes but stronger than that of saturates, revealing that some coke was formed during LTO of aromatics to offer fuel for HTO reactions. Simultaneously, it should be noted that a little bit difference of heat enthalpy in the LTO stage existed among SARA fractions as listed in Table 3. This result gave a hint that given the enhanced oil recovery (EOR) mechanisms caused by the heating effect of LTO reactions during air injection for light oils involving plenty of light fractions, the LTO mechanisms can also be applied for heavy oils with abundant heavy components to make full use of the heating effect.
Fig. 3. TG/DTG curves for heavy crude oil and its SARA fractions at the heating rate of 10 °C/min.
5
o
5 C/min o 10 C/min o 15 C/min
Exo
DSC/(mW/mg)
4
3
2
3.3. Interactions between heavy crude oil and its SARA fractions 1
To investigate the interactions between crude oil and its SARA fractions considering heat release during combustion, the DSC curve of heavy crude oil and fitting curve of SARA superposition based on their proportion in crude oil (see Table 1) at the heating rate of 10 °C/min are presented in Fig. 7. Large discrepancies existed between the two curves both in the LTO and HTO regions, which did not accord with previous research obtained by Yuan et al. in which the two curves fitted well at the LTO stage but deviated from each other at the HTO stage [5]. As mentioned before, the oil properties and molecular structures of SARA fractions for two crude oils existed prominent differences, which was assumed to be the main reason that caused varying results. In this work, we could find that there were strong interactions among SARA fractions. For the temperature below 331 °C, the heat release of fitting curve was higher than that of measurement curve. This phenomenon disclosed that the evident inhibiting effect primarily existed among SARA fractions in the LTO interval, which was closely related to the findings concluded by some researchers. Freitag [43] claimed that saturates involved a few naturally occurring oxidation inhibitors that restrained the oxidation rates by rapidly consuming an essential intermediate in the reaction chain within low temperature. Alexandra et al. [33] indicated that some aromatic compounds such as phenol derivatives acted as inhibitors at the initial stage of oxidation, and oxidation reactions did not proceed unless the rate of radicals formation surpassed the rate of inhibition or all the inhibitors became oxidized and inert. With these in mind, it was believed that saturates and/or aromatic compounds generated a negative influence on LTO of crude oil in terms of heat release. Nevertheless, the direct evidence of inhibiting effect of saturates and/or aromatic compounds on crude oil oxidation at molecular scale is still undetectable and needs to be further studied. As the temperature exceeded 331 °C, the heat flow of measurement curve was higher than that of fitting curve, elucidating that there was the promoting effect among SARA fractions on the whole at the FD and HTO stages. The phenomenon might be explained that intermediate products of resins and asphaltenes in LTO reactions can promote fuel deposition via polycondensation reactions (exothermic effect). Consequently, the
0
-1
100
200
300
400
500
600
700
o
Temperature/ C Fig. 4. DSC curves for heavy crude oil at the heating rates of 5, 10 and 15 °C/ min. Table 3 DSC peak heat flows and heat enthalpies of heavy oil and its SARA fractions at different heating rates. Heating rate (°C/ min)
LTO Peak heat flow (mW/mg)
Heat enthalpy (kJ/g)
Peak heat flow (mW/mg)
Heat enthalpy (kJ/g)
Oil
5 10 15
1.52 1.90 2.23
1.924 1.279 1.065
2.54 3.88 4.43
2.546 2.615 2.519
Saturates
5 10 15
0.85 1.14 1.53
1.167 0.802 0.767
0.26 0.42 0.62
0.249 0.227 0.244
Aromatics
5 10 15
1.04 2.79 3.90
1.437 2.174 2.245
2.57 3.81 5.30
2.578 2.255 2.629
Resins
5 10 15
– – –
2.120 1.642 1.855
6.32 7.81 9.60
5.813 5.355 4.824
Asphaltenes
5 10 15
– – –
2.343 2.038 1.944
7.52 10.55 12.51
6.786 5.645 6.549
Sample
HTO
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Fig. 5. DSC curves for SARA fractions at the heating rates of 5, 10 and 15 °C/min: (a) saturates, (b) aromatics, (c) resins, (d) asphaltenes. 12
4
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crude oil saturates aromatics resins asphaltenes
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500
600
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100
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o
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o
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Temperature/ C
Fig. 6. DSC curves for heavy crude oil and its SARA fractions at the heating rate of 10 °C/min.
Fig. 7. DSC curve of heavy crude oil and fitting curve of SARA superposition at the heating rate of 10 °C/min.
fact would lead to more heat release and coke formation at the FD stage and stronger combustion of coke at the HTO stage. When it comes to utilizing SARA fractions to model the combustion of heavy crude oil, the aforementioned interactions must be taken into full consideration.
3.4. Combustion kinetics The TG data under three heating rates (5, 10 and 15 °C/min) were adopted to calculate the kinetic parameters using DAEM and OFW models. Figs. S1–S5 in the Supporting information present the DAEM and OFW plots for heavy crude oil and its SARA components during the 122
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(b)
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120 0 0.0
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0.4
0.6
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1.0
Conversion rate/%
Fig. 8. E-α and T-α curves: (a) heavy crude oil, (b) saturates, (c) aromatics, (d) resins, (e) asphaltenes.
activation energy increased all the time, suggesting that more energy were needed for initiating thermo-oxidative cracking reactions with conversion rate. On the whole, the activation energies of crude oil fluctuated and varied in the range of 45–95 kJ/mol during combustion, disclosing its intricate combustion processes and multiple reaction mechanisms. As shown in Fig. 8(b), (c), the conversion rate of 0.1–0.83 for saturates represented the LTO + FD stage, which corresponded to 0.1–0.89 (LTO + FD stage) for aromatics. Considering saturates, an increasing trend of activation energy with conversion degree was presented as a whole, mainly caused by the negative temperature coefficient (NTC) behaviors [34,44]. Nevertheless, a descending trend of activation energy was observed roughly after conversion degree
whole heating process. And Tables S1–S5 (see Supporting information) list the activation energies at varying conversion levels (α = 0.1–0.9). Additionally, E-α and T-α (β = 10 °C/min) curves of combustion of heavy crude oil and its SARA fractions were depicted in Fig. 8(a)–(e), respectively. It was seen that the activation energies obtained by DAEM were comparable to those obtained by OFW method. Regarding the heavy crude oil, the conversion rates at 0.1–0.66 and 0.66–0.85 stood for the LTO and FD stages, respectively. In the LTO region, the activation energy firstly increased and then decreased approximately after conversion rate reached 0.4. This fact indicated that at the initial stage of LTO, the reaction difficulty increased as the LTO reactions proceeded; but after a certain conversion degree was reached, the following LTO reactions became more readily to occur. For FD, the 123
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(4) Large differences in the dependence of activation energy upon conversion degree existed among SARA fractions, disclosing their different reaction pathways and mechanisms. Different from other three fractions, the FD and HTO activation energies of asphaltenes descended consecutively with conversion degree as a whole, which is a positive signal to boost the sustainability of combustion front and ISC performance.
attained 0.75, presumably owing to that oxidation inhibitors that originally existed in saturates were gradually consumed at the initial stage of LTO as we discussed before. As for aromatics, the activation energy increased slightly with conversion rate on the whole, except for conversion rate at 0.85–0.9. The activation energy ascended drastically as conversion degree was increased from 0.85 to 0.9, which might imply that the LTO products of aromatics needed considerably high energy to trigger thermo-oxidative cracking reactions in this temperature range (440–482 °C). The conversion rates at 0.1–0.7 and 0.7–0.9 stood for the FD and HTO stages for resins, respectively, which corresponded to 0.1–0.61 (FD stage) and 0.61–0.9 (HTO stage) for asphaltenes. The activation energy of resins increased quickly with an increasing conversion degree from 0.1 to 0.2, followed by a descending trend. This phenomenon manifested that higher energy demand was utilized to touch off reactions at the initial stage of FD, but the subsequent thermooxidative cracking and HTO reactions became more facilely to take place after conversion rate attained 0.2. As agreed by most peers, asphaltenes was the most resistant fraction in the crude oil [17,45]. So the energy demand to trigger thermo-oxidative cracking reactions at the initial stage of FD for asphaltenes was much higher than that for crude oil and other three fractions. The activation energy of asphaltenes reduced consecutively with conversion degree as a whole, which facilitated the FD and HTO processes to enhance the sustainability of combustion front. Fig. 8(b), (e) shows that large differences in the dependence of activation energy upon conversion degree existed among SARA fractions, elucidating their different reaction pathways and mechanisms. Compared with SARA fractions, the heavy crude oil had the greatest degree of oscillations regarding the variation of activation energy versus conversion degree. The fact evidenced that there were some interactions among SARA fractions during combustion, which was consistent with the findings obtained in Section 3.3.
Acknowledgements The authors gratefully acknowledge the financial support of the National Key Basic Research Program of China (2015CB250904) and Natural Science Foundation of Sichuan Province (2017JY0122). The authors also thank the anonymous reviewers for their constructive and valuable comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.11.014. References [1] Turta AT, Chattopadhyay SK, Bhattacharya RN, Condrachi A, Hanson W. Current status of commercial in situ combustion projects worldwide. J Can Pet Technol 2007;46(11):8–14. [2] Li YB, Chen YF, Pu WF, Dong H, Gao H, Jin FY, et al. Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods. J Ind Eng Chem 2017;48:249–58. [3] Shah A, Fishwick R, Wood J, Leeke G, Rigby S, Greaves M. A review of novel techniques for heavy oil and bitumen extraction and upgrading. Energy Environ Sci 2010;3(6):700–14. [4] Thomas S. Enhanced oil recovery – an overview. Oil Gas Sci Technol 2008;63(1):9–19. [5] Yuan CD, Varfolomeev MA, Emelianov DA, Eskin AA, Nagrimanov RN, Kok MV, et al. Oxidation behavior of light crude oil and its SARA fractions characterized by TG and DSC techniques: differences and connections. Energy Fuels 2018;32:801–8. [6] Wei B, Zou P, Zhang X, Xu XG, Wood C, Li YB. Investigations of structure-propertythermal degradation kinetics alterations of Tahe asphaltenes caused by low temperature oxidation. Energy Fuels 2018;32:1506–14. [7] Zhang L, Deng JY, Wang L, Chen ZY, Ren SR, Hu CH, et al. Low temperature oxidation characteristics and its effect on the critical coking temperature of heavy oils. Energy Fuels 2015;29(2):538–45. [8] Xu Q, Hang J, Cheng Z, Tang W, Ran X, Jia H, et al. Coke formation and coupled effects on pore structure and permeability change during crude oil in situ combustion. Energy Fuels 2016;30(2):933–42. [9] Santos RGD, Vargas JAV, Trevisan OV. Thermal analysis and combustion kinetic of heavy oils and their asphaltene and maltene fractions using accelerating rate calorimetry. Energy Fuels 2014;28(11):7140–8. [10] Larter SR, Adams J, Gates ID, Bennett B, Huang H. The origin, prediction and impact of oil viscosity heterogeneity on the production characteristics of tar sand and heavy oil reservoirs. J Can Pet Technol 2008;47(1):40–9. [11] Varfolomeev MA, Nurgaliev DK, Kok MV. Calorimetric study approach for crude oil combustion in the presence of clay as catalyst. Pet Sci Technol 2016;34:1624–30. [12] Vargas JAV, Santos RGD, Trevisan OV. Evaluation of crude oil oxidation by accelerating rate calorimetry. J Therm Anal Calorim 2013;113(2):897–908. [13] Murugan P, Mahinpey N, Mani T, Asghari K. Effect of low-temperature oxidation on the pyrolysis and combustion of whole oil. Energy 2010;35(5):2317–22. [14] Niu BL, Ren SR, Liu YH, Wang DZ, Tang LZ, Chen BL. Low-temperature oxidation of oil components in an air injection process for improved oil recovery. Energy Fuels 2011;25(10):4299–304. [15] Freitag NP, Verkoczy B. Low-temperature oxidation of oils in terms of SARA fractions: why simple reaction models don’t work. J Can Pet Technol 2005;44:54–61. [16] Alsaffar HB, Hasanin H, Price D, Hughes R. Oxidation reactions of a light crude oil and its SARA fractions in consolidated cores. Energy Fuels 2001;15(1):182–8. [17] Kok MV, Ozgen K, Pamir R. Kinetic analysis of oxidation behavior of crude oil SARA constituents. Energy Fuels 1998;12:580–8. [18] Kuppe G, Mehta S, Moore R, Ursenbach M, Zalewski E. Heats of combustion of selected crude oils and their SARA fractions. J Can Pet Technol 2008;47(1):38–42. [19] Varfolomeev MA, Galukhin A, Nurgaliev DK, Kok MV. Thermal decomposition of Tatarstan Ashal’cha heavy crude oil and its SARA fractions. Fuel 2016;186:122–7. [20] Ambalae A, Mahinpey N, Freitag N. Thermogravimetric studies on pyrolysis and combustion behavior of a heavy oil and its asphaltenes. Energy Fuels 2006;20(2):560–5. [21] Yuan CD, Emelianov D, Varfolomeev MA. Oxidation behaviour and kinetics of light, medium and heavy crude oils characterized by thermogravimetry coupled with fourier-transform infrared spectroscopy (TG-FTIR). Energy Fuels 2018;32(4):5571–81. [22] Zhao RB, Wei YG, Wang ZM, Yan W, Yang HJ, Liu SJ. Kinetics of low-temperature
4. Conclusion In this work, the thermal behaviors of combustion of heavy crude oil and its SARA fractions were comparatively analyzed using TG-DSC technique, followed by the investigation regarding SARA interactions during combustion. The dependence of activation energy upon conversion rate, which was determined by DAEM and OFW method, for heavy crude oil and its SARA fractions was presented in this paper. The conclusions derived from this study are as follows. (1) Saturates and aromatics were vulnerable to severe mass loss at the LTO stage. Resins and asphaltenes were subjected to appreciable mass loss and prominent DTG peak in the FD interval to form a great amount of coke, which in turn contributed to HTO reactions. (2) Saturates made an inconsequential contribution to the HTO reactions. Aromatics showed distinctive exothermic effect both at the LTO and HTO stages, which was close to crude oil. Compared with saturates and aromatics, resins and asphaltenes gave considerably higher heat release in the HTO region, especially asphaltenes. A little bit difference of heat enthalpy in the LTO stage existed among SARA fractions, implying that given the EOR mechanisms resulted from the thermal effect of LTO reactions during air injection for light oil, the LTO mechanisms can also be suitable for heavy oil to take full advantage of the thermal effect. (3) The inhibiting effect primarily existed among SARA fractions in the LTO interval, which was believed that saturates and/or aromatic compounds generated a negative influence on LTO of crude oil in terms of heat release. On the whole, there was the promoting effect among SARA fractions at the FD and HTO stages, mostly owing to that intermediate products of resins and asphaltenes in LTO reactions can promote fuel deposition via polycondensation reactions. These interactions must be taken into full consideration when using SARA fractions to model ISC process. 124
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