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Investigations of Structure-Properties-Thermal Degradation Kinetics Alterations of Tahe Asphaltenes Caused by Low Temperature Oxidation Bing Wei, Peng Zou, Xiang Zhang, Xingguang Xu, Colin D. Wood, and Yibo Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03565 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018
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Investigations of Structure-Properties-Thermal Degradation Kinetics Alterations of Tahe Asphaltenes Caused by Low Temperature Oxidation Bing Wei,1,* Peng Zou,1 Xiang Zhang,1 Xingguang Xu,2 Colin Wood,2 Yibo Li3 1)
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University,
Chengdu, Sichuan 610500, China 2)
Energy Business Unit, The Commonwealth Scientific and Industrial Research Organization, 26 Dick Perry
Avenue, Kensington 6152, Perth, Australia 3)
Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and
Technology, Rolla, 65401, USA
ABSTRACT: In-situ combustion (ISC) is advantageous for (ultra) heavy reserves due to its high heating efficiency and small surface footprint compared to steam injection. Herein, the focus of this work was given to the key factor in sustaining the continuity of the combustion front, the asphaltene fraction. Structure-properties alterations of Tahe asphaltenes caused by low temperature oxidation (LTO) were thoroughly examined. Particular attentions were placed on its combustion and pyrolysis kinetics. The results showed that after LTO 10.35 wt% of coke was formed. SEM observations indicated that the surfaces of the oxidative products were fairly rough as a result of air attack and the caused reactions on site, and these alterations promoted the subsequent combustion. The textures of the products were observed to be further compacted and condensated after LTO. As anticipated, distinguished reaction regions were clearly identified on the TG/DSC curves in this work. The results of TG and activation energy revealed the differences of the reaction sites in the combustion and pyrolysis processes. The coke formed after LTO exhibited the highest reaction activity and exothermic effect compared to the fresh asphaltenes and residue. It is believed that this work can add new insights to ISC with regard to the mechanisms and reaction models, which are highly valuable for field applications. 1. INTRODUCTION 1
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With the depletion of easy-to-produce light oils together with the ever-increasing energy demand, considerable attentions have been given to heavy crude reserves over the past few decades. However, even after numerous efforts, the recovery of heavy crudes remains a challenge due to their high viscosity, high density and the caused poor fluidity in porous media.1, 2 Since the viscosity of heavy crudes strongly depends on temperature, thermal recovery techniques are therefore proposed. Based on the sources of the energy supplied, thermal recovery methods can be generally categorized into two main types, i.e. steam injection and in-situ combustion (ISC). With regard to the latter, air or oxygen-enriched gas is injected into the targeted formation with the intent of oxidizing crude oil in place. The combustion front created by self-ignition or external burner would propagate downstream slowly by the subsequent air flow. The ISC process in field scale is illustrated in Fig. 1(a). The significant heat generated by combustion leads to the improved production of heavy oils mainly via viscosity reduction and other relevant mechanisms.3 In comparison with the steam based methods, ISC generates the energy inside reservoirs, accordingly leading to extremely high heat utilization efficiency. This is particularly critical for (ultra) heavy oil reservoirs because of the significant heat loss at the surface and in the wellbore when steam injection is used. In recent years, extensive works have been performed to study the thermal behaviors of crude oils using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), ramped-temperature-oxidation tests (RTO), combustion tube (CT), etc., with the aim of precisely defining the reaction models and kinetics.4-7 As agreed by most peers, the ISC process can be divided into three temperature regions, namely Low Temperature Range (LTR), Negative Temperature Gradient Range (NTGR), and High Temperature Range (HTR), as shown in Fig. 1(b). LTR is largely governed by low temperature oxidation (LTO) and other bond-scission reactions with the production of oxygenated hydrocarbons (i.e. aldehydes, ketones and alcohol) and carbon oxides, while NTGR plays an integral role in determining the success of ISC because of the coke deposited. HTR corresponds to the region of exothermic reactions or high temperature oxidation (HTO) and burning zone, in which significant heat is 2
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generated and then transmits downstream.8-10
a
b Figure 1. (a) Schematic diagram of the ISC process; (b) Oxidation behaviors of crude oil (adapted from Ref.11). Despite the intensive studies of ISC, its underlying mechanisms remain unclear as a result of the complexity of hydrocarbon mixtures and their consecutive chemical reactions. Therefore, worldwide attention has focused on the individual fraction of crude oil such as saturates (S), Aromatics (A), Resin (R) and Asphaltenes (A). For instance, Karacan and Kok analyzed the pyrolysis behaviors of a crude oil and its fractions with TGA and DSC methods. It was concluded that asphaltenes and resin were two dominant contributors to coke deposition and each component followed its 3
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own reaction pathway without any interference.12 After the above work, Kok and Gul further investigated the combustion kinetics of SARA.13 Boytsova et al. conducted a TG study to determine the pyrolysis thermodynamic parameters of heavy oils and asphaltenes. It was claimed that the size of asphaltenes decreased with density, and the activation energy and pre-exponential factor of asphaltenes were lower than those of the liquid systems.14 Ambalae et al. proved that of the SARA fractions asphaltenes contributed the most to coke formation, which suggested the strong relationship between asphaltenes and fuel deposition.8,
15
Moreover, Liu et al. examined the
interactions of SARA fractions during oxidation and pyrolysis. Few changes were observed for the binary systems during pyrolysis, whereas the interactions did occur when co-oxidizing these compounds.16 Varfolomeev et al. compared the thermal decomposition behaviors of the SARA fractions of a heavy crude oil using TGA/DSC techniques. The temperature ranges of LTO and HTO, reaction intervals, activation energy, burning temperatures, etc., were summarized in their work.17
Tahe oilfield, locating in western China, produces different types of crude oils from light to heavy. The ISC technique has been proposed to promote the production of a heavy crude reservoir at a known depth. Therefore, this paper first focused on the vital part in this heavy oil for ensuring combustion continuity, i.e. asphaltenes. Our primary interest of this work was to determine the structure-properties alterations of Tahe asphaltenes induced by low temperature oxidation, which was performed under reservoir conditions. Particular emphasis was placed on the changes of the combustion and pyrolysis kinetics of this fraction. The results derived from this work should be of great significance for assessing the feasibility of ISC in this heavy oil reservoir.
2. EXPERIMENTAL SECTION 2.1. Materials The n-heptane insoluble asphaltenes evaluated in this work were split from Tahe heavy crude oil according to the standard method of NB/SH/T 0509-2010. Figure 2 plots the oil viscosity as a function temperature. Table 1 is presented to show the SARA fractions and basic properties of this heavy oil. The chemicals used in this 4
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work including hydrochloric acid, toluene, heptane, ethanol, etc., were all purchased from Chengdu Kelong Chemical Co., China, and used as received. Table 1. SARA fractions and basic properties of Tahe heavy oil. Viscosity (mPa·s) o
Density (g/cm3)
SARA (wt%)
API Gravity
o
o
@110 C
@110 C
( API )
Saturates
Aromatics
Resin
Asphaltenes
9620
0.967
6.6
17
39
8
36
80
60
Viscosity (Pa⋅s)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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40
20
0 90
95
100
105
110
115
120
125
130
135
Temperature (oC)
Figure 2. Dependence of oil viscosity on temperature.
2.2. Static oxidation experiment The experimental set-up for the static oxidation is shown in Fig. 3. The operating procedures of this experiment were briefly introduced as follows. 1. A high-temperature-pressure cell (40 mL) was pressurized to 15 MPa with and then placed in an air bath (110 oC) for 5 hours to check leakage; 2. Given amount (2.0 g in our test) of the asphaltenes was loaded into the cell; 3. The sealed high pressure cell was pressurized to 15 MP again by air at a constant temperature of 110 oC; 4. The oxidation reaction was terminated after ten days; 5. The solid products were washed with a large quantity of toluene to separate the formed coke and residue followed by drying in nitrogen environment at 100 oC. It was observed that after this oxidation process 1.78 g of residue and 0.21 g of coke were produced, which corresponded to the coke yield of approximately 10.5 wt%. Figure 4 shows the workflow of this work.
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Figure 3. Simplified schematic of the static oxidation experimental set-up.
Figure 4. Workflow of this work. 2.3. Scanning Electron Microscopy (SEM) The microscopic structures of the solid samples including the asphaltenes, formed coke and residue were studied using a scanning electron microscopy (SEM) (ZEISS EV0 MA15). A small amount of the samples was first loaded on a cuprum holder, the SEM images were then taken at an accelerating voltage of 20 kV. 6
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2.4. Elemental analysis To investigate the composition variation of the solid samples caused by the oxidation, a comparison of the elements in the asphaltenes, residue and coke was conducted using an Euro Vector EA3000 elemental analyzer in Sichuan University. 2.5. Thermogravimetry (TG) and Differential Scanning Calorimeter (DSC) The procedures of TG/DSC measurements have been widely reported.10, 18 In this work, the TG/DSC experiments were carried out from ambient temperature up to 800 o
C at a constant heating rate of 5 oC/min using a NETZSCH STA 449F3 thermal
analysis system. Prior to each test, the system was calibrated to ensure reliability (T±0.5 oC). Briefly, a small amount of the solid (~15 mg) was heated in the presence of air (combustion) and argon (pyrolysis), respectively. The gas flow was kept constant at a flow rate of 50 mL/min throughout the tests. The weight loss and heat flow of the samples as a function of temperature were precisely monitored and recorded by a data acquisition system. 3. RESULTS AND DISCUSSION 3.1. Morphology Observations To examine the alterations of the solid samples caused by the oxidation, SEM was employed for visually determining the surface morphology of the fresh asphaltenes, formed coked and residue. Table 2 is presented to show the SEM images at two magnifications. It is apparent that the oxidative products of the residue and coke were smaller in size than the fresh asphaltenes. This was thought to be largely caused by the occurrence of polycondensation reactions in the oxygen-enriched products that were produced by the oxidation of alkyl side chains attaching to asphaltenes. Therefore, shrinkage took place and the density of the products and coke deposition increased.19, 20 It is believed that this change is the predominant reason making oil viscosity to increase during air injection as we previously reported.21 A close examination of the morphology by SEM magnified 2000 times clearly revealed the texture differences of the three samples. In addition to the above observations, the SEM images indicated that the surface of the fresh asphaltenes was fairly smooth as a consequence of the homogenous aggregation and precipitation in n-heptane during 7
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SARA separation.22 In contrast, as for the residue and coke, the solid surfaces were noticeably rougher especially the coke as a result of air attack and the caused heterogeneous reactions on site on the surfaces. The resultant lumpy surface is expected to provide large surface area and thus promote combustion reactions compared to asphaltenes and residue as TG/DSC data evidenced below.23 Table 2. SEM images of the three samples. Samples
SEM
Asphaltenes
Residue
Coke
3.2. Elemental Analysis The elemental analysis of the fresh asphaltenes, residue and coke were presented in Table 3. Four elements composed of the solid samples including C, H, N and S were included. It is clear that the elements of C and H were predominant for all the three samples, while the content of the heteroatom (N and S) was fairly low. Furthermore, 8
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Table 3 shows that the contents of C and H were slightly changed after the oxidation and that the formed coke had the highest C and lowest H contents. This tendency was also revealed by a widely used ratio, H/C atomic ratio, as listed in the last column of Table 3. During the static oxidation process, the asphaltenes reacted with excessive air, which thus produced a series of oxygen-added compounds and side products such as carbon oxides and water at an uncertain stoichiometry.19 The aforementioned reactions would essentially lead the H/C atomic ratio to change. As discussed early (Section 3.1), the polycondensation of the oxygen-enriched compounds caused the products to further condensate and then deposit. Therefore, the H/C ratio of the formed coke is supposed to be smaller than that of the fresh asphaltenes and residue (given in Table 3) due to the release of water molecules during the oxidation reactions.24 Table 3. Elemental analysis results of the fresh asphaltenes, residue and coke. Samples
C (%)
H (%)
N (%)
S (%)
H/C (atomic ratio)
Asphaltenes
84.56
13.18
1.29
0.98
1.87
Residue
87.30
9.37
2.32
1.00
1.29
Coke
88.69
7.54
2.71
1.06
1.02
3.3. TG/DSC Analysis In this section, TG/DSC techniques were used to investigate the alterations of the thermal degradation behaviors of the three samples. The TG/DSC tests were conducted in the environments of argon and air with the intent of simulating the pyrolysis and combustion processes, respectively. 3.3.1. TG/DSC Pyrolysis The complexity of compound mixtures makes the understanding of the thermal behaviors of crude oil and SARA fractions a challenging job. Figure 5 shows the TG/DTG curves of the three samples under pyrolysis. Two distinct regions were identified from the curves for the asphaltenes and residue, suggesting their similar thermogravimetric pathways with the elevated temperature. In contrast, the pyrolysis of the coke was a one-stage process.25 9
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The first stage fell into the temperature range of 200-380 oC, in which a relatively low mass loss was observed. This mass loss was caused by the following reasons, 1. The distillation of the volatile species upon heating; 2. The breakage of the long side alkyl chains on the peripheral sites of asphaltenes. The second region appeared at the temperature ranging from 380 to 500 oC, which resulted in a significant mass loss of the asphaltenes and residue due to the destruction of the intermolecular associations and chemical bonds (C-C,C-H, C-S, etc.).26 In the case of the coke, as indicated by the TG/DTG results, a slight mass loss was observed in the second stage (between 380 and 500 oC) and nearly 50% of mass was remained at the end of pyrolysis. This fact was to be expected from the condensed or compacted texture of the coke as observed in the SEM images (Table 2). The DSC pyrolysis results of the three samples are presented in Fig. 6. On the DSC curves, two regions can be clearly recognized. The first region (25-380 oC) was associated with the distillation of volatile compounds and/or primary cracking of alkyl chains. The second region corresponded to a narrow area occurring around 450 oC. It was claimed that visbreaking and heavy thermal cracking were the leading reactions in this region,27 which thus made the majority of pyrolysis present endothermic behaviors. Table 4 summarized the temperature and thermal effect of the pyrolysis.
100
0
80 -1
60 -2
40
Asphaltenes Residue Coke
Derivative weight (%/min)
Weight loss (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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-3
20 0
100
200
300
400
500
600
700
800
Temperature (oC)
Figure 5. TG-DTG non-isothermal curves of the three samples under pyrolysis.
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2
Heat flow (mW/mg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0
-2
-4
Asphaltene Residue Coke
-6 0
100
200
300
400
500
o
600
700
800
Temperature ( C)
Figure 6. DSC non-isothermal curves of the three samples under pyrolysis. Table 4. Summary of the pyrolysis processes of the three samples. Asphaltenes
Residue
Coke
Peak temperature (oC)
452
502
494
Peak heat flow (mW/mg)
-0.64
-0.34
-0.72
Heat of reaction (J/g)
-11.2
-61.6
-26.6
Mass remaining (%)
27.4
28.1
51.6
"-" represents endothermic effect
3.3.1. TG/DSC Combustion The TG/DTG non-isothermal combustion curves of the tested samples are shown in Fig. 7. It was found that the combustion process was much more complex than pyrolysis, suggested by the fluctuating signals as temperature increased. Consistent with previous reports, three regions, named LTO, FD (fuel deposition) and HTO, were identified on the TG/DTG curves. Nevertheless, the FD region on the coke TG/DTG curves was found to be extremely narrow due to a fact that fuel had been partially formed during the oxidation. For the three samples, the LTO region generally started from ambient temperature until 350 oC was reached followed by the FD region. As stated above, the LTO caused oxygen-addition reactions (hydrocarbon oxygenation) between liquid/solid phase and gas phase, whereas the HTO burned the deposited fuel and resulted in significant mass loss. As shown in Fig. 7, at the termination of the HTO, more than 95% of the samples had been burned out. 11
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The DSC non-isothermal combustion curves of the three sample are shown in Fig. 8, in which two exothermic regions (LTO and HTO) were clearly observed. The first region occurred at the temperature from 200-380 oC, and the second activity appeared from 400 oC to 630 oC. However, the magnitude of the heat release that is the area under the DSC curves was noticeably smaller for the LTO reactions than that of HTO reactions. The quantity of the heat release was proven to be in the order of coke>asphaltenes>residue. It was also observed that the DSC curves of the asphaltenes and residue were not as smooth as that of the coke in the LTO region. This result might be the consequence of the negative effects of the endothermic reactions such as thermal cracking that occasionally occurred in this temperature range. However, for HTO, the exothermic reactions dominated the process.28 Table 5 shows the temperature and heat of the combustion process. 100
Weight loss (%)
-1 60
40
-2
20
Asphaltenes Residue Coke
0 0
100
200
Derivative weight (%/min)
0
80
-3
300
400
500
600
700
800
Temperature (oC)
Figure 7. TG-DTG non-isothermal curves of the three samples under combustion. HTO
Asphaltene Residue Coke
12
Heat Flow(mW/mg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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8
LTO 4
0 0
100
200
300
400
500 o
600
Temperature ( C) 12
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700
800
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Figure 8. DSC non-isothermal curves of the three samples under combustion. Table 5. Summary of the combustion process of the three samples. Asphaltenes
Residue
Coke
LTO
HTO
LTO
HTO
LTO
HTO
Peak temperature (oC)
310
547
341
577
344
538
Peak heat flow (mW/mg)
0.1
7.67
0.1
5.12
4.11
12.64
Heat of reaction (J/g)
14.11
663.86
17.72
549.0
77.08
1046.2
Mass remaining (%)
83.1
2.5
82.5
1.2
88.9
4.8
3.4. Thermal Degradation Kinetics Thermal analysis is widely performed in order to obtain the kinetic parameters in different temperature regions. Since the thermal degradation of crude oils or their fractions is a complex process involving a series of parallel and consecutive reactions, the evaluated kinetic parameters are therefore regarded as apparent values, which can be used to represent the overall reaction history. For an n-order kinetics, the reaction rate can be described simply by the following equation (Eq. 1), w is the reaction conversion, n is the reaction order and k is the kinetic constant, which can be expressed by the Arrhenius law (Eq. 2)
− = = exp (−
(1)
)
(2)
where dw/dt is the rate of mass change, E is the activation energy, A is the pre-exponential factor or Arrhenius constant, R is the universal gas constant (8.314 J/mol K) and T is the temperature (k). The thermal degradation process is usually assumed as a first-order kinetics (n=1); therefore, if Eq. 1 and Eq. 2 are combined, the following form can be taken (Eq. 3).29-31
ln −
= ln −
(3)
If ln[(- dw / dt )/w] against 1/T is plotted, a straight line is obtained for each reaction region with the slope of -Ea/R, and the activation energy and pre-exponential factor 13
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can be therefore determined by the linearity of the curves. Figure 9 and Figure 10 show the model fitting of the TG data using Eq. 3. Table 6 and Table 7 list the summary the fitting results and kinetic parameters. It should be pointed out that the slope of the fitting line is highly sensitive to the starting and end points of the region; thus, the reported Ea and A usually varied considerably. In this work, we found that the TG data were well fitted to Arrhenius model with high correlation coefficients. With regard to the non-isothermal pyrolysis, the values of Ea and A were fairly close in two temperature regions. Due to the nature of the samples (the heaviest fraction of crude), the Ea values of the three samples obtained in this work were larger than those of other reports,32, 33 indicating that higher energy was required to proceed the cracking reactions. As for the combustion kinetics, as shown in Table 7, it was interesting that the Ea of the combustion reaction was higher compared to that of the pyrolysis probably caused by differences of the reaction sites or species in these two processes. For the pyrolysis, the majority of the reactions occurred on the peripheral sites of the samples leaving more than 20% of mass unreacted (50% for coke) as observed in Fig. 5. In contrast, nearly all the species participated in the reactions when air was present. Consequently, as shown in Table 5 and Table 7, more than 95% of mass was consumed at the end of the combustion, which accordingly generated a greater apparent Ea compared to the pyrolysis. As shown in Table 7, after the static oxidation, the formed coke became highly reactive, suggested by the lower Ea of the two regions especially the HTO. This change would promote the combustion process and heat release. -2
Asphaltenes
Residue
-4
y=-9.83x+9.19
y=-11.49x+11.18 -4
-6
-6
ln[(dw/dt)/w]
ln[(dw/dt)/w]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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y=-5.56x+4.76
y=-7.34x+6.53 -8
-10
-8
-12
-10 1.0
1.5
2.0
2.5
3.0
3.5
1.0
1.5
2.0
3 -1 -1 10 T (K )
3 -1 -1 10 T (K )
14
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3.0
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Coke -5.2
y=-10.03x+8.923 ln[(dw/dt)/w]
-6.5
y=-3.05x-0.38
-7.8
-9.1
-10.4
1.0
1.5
2.0
2.5
3.0
3 -1 -1 10 T (K )
Figure 9. Arrhenius model fitting of the pyrolysis TG data. -2
Asphaltenes
y=-19.83x+22.53
Residue
-2
y=-17.04x+16.43
y=-14.65x+17.66 -4
-4
y=-9.64x+8.71
y=-3.52x+0.07 ln[(dw/dt)/w]
ln[(dw/dt)/w]
-6
y=-6.59x+6.79
y=-2.56x-2.354
-6
y=-6.42x+4.62 -8
-10
-8
-12
-10 -14
1.0
1.5
2.0
2.5
1.0
3.0
1.5
2.0
2.5
3.0
3 -1 -1 10 T (K )
3 -1 -1 10 T (K )
Coke -2
y=-14.45x+14.36 -4
ln[(dw/dt)/w]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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y=-7.98x+5.74 y=-2.99x-1.99
-6
y=-4.67x+3.79 -8
-10
1.0
1.5
2.0
2.5
3.0
3 -1 -1 10 T (K )
Figure 10. Arrhenius model fitting of the combustion TG data. Table 6. Reaction intervals and kinetic parameters under pyrolysis. Samples
Mass loss
Act. energy
Arrhenius
(%)
(kJ/mol)
cons. (1/min)
170-390
27.58
46.2
116.51
II
390-460
30.94
95.53
71682.36
I
160-400
32.12
40.99
681.98
Region
Reaction intervals
I Asphaltenes
Residue
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II
400-460
27.21
81.69
9808.45
I
170-390
8.88
25.34
0.68
II
390-480
28.49
83.39
7502.56
Coke
Table 7. Reaction intervals and kinetic parameters under combustion. Samples
Mass loss
Act. energy
Arrhenius cons.
(%)
(kJ/mol)
(1/min)
160-310
9.63
57.74
896.05
310-340
6.03
29.27
1.07
340-450
36.63
121.8
4.67E+07
450-580
18.72
164.86
6.09E+09
LTO
190-310
2.79
53.38
101.9
FD
310-360
4.18
21.27
0.09
360-470
27.21
80.17
6093.63
470-610
58.68
141.67
1.37E+07
LTO
160-300
6.08
38.86
44.57
FD
300-360
7.87
24.83
0.14
360-450
29.98
66.36
311.99
450-570
34.67
120.14
1.72E+06
Region
Reaction intervals
LTO FD Asphaltenes HTO
Coke HTO
Residue HTO
4. CONCLUSION Due to the significance of the asphaltene fraction in ISC, this work thoroughly examined the structure-properties-thermal degradation kinetics alterations of Tahe asphaltene after a static oxidation conducted under reservoir conditions. The implications of these alterations to ISC were emphasized particularly. Based on the experimental results, the following conclusions can be generally drawn: 1. After the static oxidation, 10.53 wt% of Tahe asphaltenes were coked under our experimental conditions. The deposited coke promoted the subsequent combustion. 2. Compared to the fresh asphaltenes, the surface morphology of the residue and coke was extremely rough as a result of air attack and the caused reactions. The products especially the coke were further condensated and compacted as revealed by the SEM observations. 16
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3. The results of the elemental analysis revealed the composition changes of the products due to the generations of side products such as water and carbon oxides. 4. Compared to the pyrolysis reactions, the combustion of the samples was more complicate, as revealed by the TG/DSC curves. The magnitudes of the mass remaining and activation energy indicated the differences of the reaction sites and/or species between pyrolysis and combustion. 5. Two exothermic regions were clearly observed on the combustion DSC curves. The coke generated the most significant heat in HTO region, whereas the pyrolysis reactions presented endothermic effect in two regions. 6. After the static oxidation, the Ea of the deposited coke was noticeably lower than that of the fresh asphaltenes, revealing its high reactivity. These results are expected to add new insights to the studies of ISC mechanisms.
AUTHOR INFORMATION Corresponding Authors *B. Wei. Email:
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Key Science & Technology Projects during 13th Five-Year Plan (2016ZX05053), National Natural Science Foundation of China (51704245) and Youth Science and Technology Innovation Team of SWPU (2017CXTD04). The authors also thank the anonymous reviewers for their valuable comments.
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