Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods

G Model JIEC 3261 No. of Pages 10

Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

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Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods Yi-Bo Lia,* , Ya-Fei Chena , Wan-Fen Pua,* , Hong Dongb , Hao Gaoa , Fa-Yang Jina , Bing Weia a b

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China Research Institute of Exploration and Development, Xinjiang Oilfield Company, PetroChina, 834000 Karamay, People’s Republic of China

A R T I C L E I N F O

Article history: Received 14 October 2016 Received in revised form 4 January 2017 Accepted 13 January 2017 Available online xxx Keywords: Air injection Low temperature oxidation Heavy oil Kinetic analysis Coke deposition

A B S T R A C T

To study the oxidation behavior of heavy oils, low temperature oxidation experiments under differential temperature and pressure conditions were carried out for both heavy oils (ordinary heavy and ultraheavy oils) with thermogravimetry (TG–DTG)/differential scanning calorimeter (DSC) analyses. And the results indicated that the differences of physical properties and oxidation reactions for the heavy oils ascribe to the SARA compositions, H/C ratio and specific oxidation path. Owing to the asphaltenes content, the ultra-heavy oil after the oxidation has more obvious coke deposition with lower activation energy demand to make it possible for the in-situ combustion without combustion adjuvants. © 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Introduction With the declining production of conventional oil reservoirs and the accompanied growing demand of crude oil market, an increasing attention around the world is paid to the unconventional oil reserves which mainly include heavy oils, ultra-heavy oils and bitumen [1,2]. Although the elevated viscosity and density of these non-conventional oils require additional efforts and more efficient measures to exploitation and upgrade, in consideration of them accounting for about 70% of the energy resource in the world, they show a much unemployed resource and promising energy supply, which is becoming attractive for more and more researchers and companies to research and optimize the improved oil recovery (IOR) techniques [3–5]. Differ from the conventional oils, how to effectively reduce the viscosity of heavy and/or ultraheavy oils is a key technology challenge. Owing to the unique mechanisms and promise in the laboratory and in the practice, it is believed that the in situ combustion (ISC), compared with other enhanced oil recovery (EOR) methods, plays a vital role in the exploitation of heavy and ultra-heavy oil reservoirs. Although its success in past field application has been spotty at best, an important reason of which could be attributed to lack of enough

* Corresponding authors. E-mail addresses: [email protected], [email protected] (Y.-B. Li), [email protected] (W.-F. Pu).

understanding on the specific mechanism process and the screening criterion [6–9]. For the ISC, the overlap of consecutive reactions and over-all oxidation mechanisms, related in differential temperatures, result in not only the unique flooding impetus and promising prospect but also the difficulty of well understanding and application feasibility despite decades of development efforts. Under this background, thermogravimetric techniques had been employed and are considered as indispensable tools to determine the changes in properties, such as composition, thermal effects, decomposition characteristics, oxidation mechanisms, kinetic analysis, etc [10–12]. And in this field, many predecessors [10,13–22], especially Kok et al. [23–26], have made much researches on the oxidation process with thermal analysis methods. Three continuous and distinct reaction regions, low temperature oxidation, fuel deposited and high temperature oxidation (LTO, FD and HTO), exist in combustion process for experimental heavy oils with thermal gravity analysis/differential scanning calorimetry (TGA/DSC) [11]. In the LTO stage, heterogeneous reaction existence and a formed reaction-front temperature of nearly 350
C lead to the generation of partial oxides, such as aldehydes, ketones and alcohols [27,28]. For the FD process, the negative temperature coefficient (NTC) region should be overcome for the heavy oils to generation enough coke to sustain the ISC, as studied by Pu and co-workers with the thermogravimetry– derivative thermogravimetry (TG–DTG) [29,30]. Then the deposited coke or residue subjects to HTO reactions to produce amount of heat to achieve the propagation of thermal front and consequent

http://dx.doi.org/10.1016/j.jiec.2017.01.017 1226-086X/© 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Please cite this article in press as: Y.-B. Li, et al., Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.01.017

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mobility enhancement, which is the key step to evaluate the performance of oxidation progress and ISC process. Owing to the FD, HTO, and thermal front dependent on the LTO progress, the LTO region, underlined with TG–DTG, is divided into four specific subranges each with its own dominant oxidation mechanisms [31]. In addition, by TG/DTG/DSC/PDSC, the oxidation characteristics of saturates, aromatics, resins and asphaltenes (SARA) of heavy oils were analyzed to explored the specific LTO mechanisms by many researchers [17,21,25–27], which present that the complex and continuous LTO process is related with various oxidation behaviors (for example, temperature ranges and pathways) of SARA fractions to some degree. And more viscosity increase, higher thermal stability and higher activation energy caused by the LTO process for the heavy oil with a higher content of asphaltenes fraction. For heavy and ultra-heavy oils, the degree and durative of HTO region depends on the amount of heat to boost the temperature, which is generated by the un-oxidized oil in the vicinity of thermal front or recharging the surrounding region from elsewhere in the formation. In addition, the products in LTO stage not only play an important role in the sustainability of the HTO or ISC process, but also potentially deteriorate the subsequent oxidation reactions and crude oil recovery if too much oxides are produced [31]. Although the LTO process increases the density and viscosity of the oxidation oil via the conversation of volatile components into non-volatile components, promoting the fuel supply. The SARA changes, such as resins and asphaltenes (heavy components), saturates and aromatics (light components), and/or individual component, remain a matter of debate. Hence, more attention should be paid to the debatable and uncertain aspects of the LTO process to better understand the specific mechanisms independent on the heavy oil species. In this paper, we accomplished the oxygen consumption and oxidation investigations of ordinary heavy and ultra-heavy oils in LTO stage through constant temperature oxidation tube experiments. Then employed the TG/DTG/DSC, original and oxidized heavy oils were analyzed at different heating rates (5, 15, 25 K/min) under air atmosphere to evaluate the oxidation characteristics and kinetic mechanisms of heavy oils. The main objective of this contribution is not only to explore the oxidation characteristics including the gas and oil phases of different heavy oils, but also to evaluate how the structure varieties of heavy oils affect the fuel deposition and the kinetic behavior. Experimental Materials The studied ordinary heavy and ultra-heavy oils were from a block of Tahe oilfield (Tarim Basin, China). This deep reservoir is a carbonate reservoir with the formation temperature and pressure of 120
C and 45 MPa respectively. The viscosity–temperature curves of targeted heavy oils were measured by the rotational rheometer (MCR 302 Anton Paar, Austria), shown in Fig. 1, indicating the studied oils are consistent with the crude oil classification [5]. In addition, the essential physical properties, specific SARA components(eluting chromatography method, ASTM D2549), and element analyses (Elementar, Vario EL III [32]) of selected heavy oil samples were tabulated in Table 1. Isothermal oxidation tube experiments To investigate the LTO behaviors of the ordinary heavy and ultra-heavy oils from Tahe oilfield, the isothermal oxidation tube experiments were implemented and the flow chart was presented in Fig. 2. The oxidation tube has an effective volume of 251.20 ml with an inner diameter of 2.0 experiment cm and a length of

Fig. 1. Viscosity–temperature curves of ordinary heavy oil (a) and ultra-heavy oil (b) measured by the Anton Paar rheometer.

80.0 cm. For the first group experiments, the ordinary heavy and ultra-heavy oils were respectively oxidized under a family of constant temperatures (110, 120 and 130
C) with the reservoir pressure 45 MPa to determine the influence of oxidation temperature on the oxidation behaviors of gas and oil phases. The second group experiments for the same oil samples were completed with the reservoir temperature 120
C under differential oxidation pressures (40, 45 and 50 MPa) condition to investigate the pressure influence. In addition, the both group experiments were conducted at the same oxidation time (7 days) and air-oil ratio of 4/1 (under experimental pressure), as shown in Fig. 2. To analyze the experiments, the elementary analyzer (Elementar, Vario EL III) and Agilent 7890B series gas chromatography (GC) were respectively employed to obtain the elemental composition of oxidized oil sand oxygen consumption as well as gas components in the effluent gases. Non-isothermal thermogravimetric analysis NETZSCH STA 449F3 PC/PG with DSC and TG–DTG modules was employed to analyze the thermal behavior of original and oxidized oils in air atmosphere from 30 to 650
C. Prior to the experiments, the DSC system was calibrated including cell and temperature calibrations respectively with sapphire and indium as the reference standard. The TG/DTG system was calibrated with calcium oxalate monohydrate for the temperature readings and

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Table 1 Physical properties and SARA analyses of ordinary- and ultra-heavy oils. Oil typea

API
/(20
C)

Viscosity/(mPa s, 60
C)

1 2 3c 4d

8.18 17.92 14.95 15.5

84,600 3160 51,935 14,340

a b c d

SARA composition (wt%)b

Element analysis (wt%)

S

A1

R

A2

C

H

O

N

S

H/C

13.45 40.11 18 41.19

21.95 21.94 31 18.18

15.48 22.5 22 20.92

49.12 13.45 29 0.56

81.5 82.5 81.8 85.18

9.73 12.2 10.3 12.45

5.09 3.28 – 1.14

0.64 0.23 0.324 0.83

3.04 1.79 5.4 0.39

1.43 1.77 1.51 1.75

1-Ultra heavy oil and 2-ordinary heavy oil. S—saturates, A1—aromatics, R—resins and A2—asphaltenes. The oil is from Ref. [27], the API
and viscosity were measured at 20
C. The oil is from Ref. [22], the API
and viscosity were respectively measured at 60 F and 50
C.

silver was used to correct for buoyancy effects. In addition, based on the recommendation of ICTAC committee, each kinetic analysis based on experiment should be accomplished at multiple heatingrates [33]. All thesamples (15 mg) were prepared according to the ASTM standards (D2013-72) and performed with constant heating rates of 5, 15, and 25 K/min in each experiment. And all experiments were performed twice to test the repeatability. Kinetic theory Owing to numerous physico-chemistry reactions, it remains vague to measure the oxidation and recovery improvement mechanism for a specific candidate reservoir without other analytical means. In addition, whether the combustion can be sustained depends on the rate at which the fuel is formed from the crude oil and the rate at which the fuel is burned [16]. For the reason, several models and theories derived from the Arrhenius model to stimulate and assess the oxidation kinetics [24,26], such as Coats–Redfern model, Ingraham–Marrier model and Ratio model. In this study, the Arrhenius model has been employed to calculate the kinetic parameters, in which the reaction rate of the

oil sample only relies on the temperature (T), the rate constant (k) and the remaining sample (w), to the activation energy (E) and Arrhenius constant (A). The specific equation of Arrhenius kinetic model is expressed as following. dW=dt ¼ kW n

ð2:1Þ

k ¼ Ar expðE=RT Þ

ð2:2Þ

Assuming the first-order kinetics, the derivation is in the following. dW=dt ¼ Ar expðE=RT ÞW

ð2:3Þ

ðdW=dtÞ=W ¼ Ar expðE=RT Þ

ð2:4Þ

Taking the logarithm of both sides, lg½ðdW=dtÞ=W  ¼ lgAr  E=ð2:303 RT Þ

ð2:5Þ

When lg(dW/dt/W) is plotted versus 1/T, the active energy E and Arrhenius constant A are calculated from the obtained straight line with the slope equal to E/(2.303R) and the intercept lgAr [11].

Fig. 2. The procedure diagram of the isothermal oxidation tube experiments under different pressure and temperature conditions with the same oxidation time (7 days) and air–oil ratio (4:1, under experimental pressure).

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Table 2 Effluent gas compositions of ordinary heavy oil and ultra-heavy oil at different oxidation temperatures.

C1–C6 (mol%) O2 (mol%) N2 (mol%) CO2 (mol%) CO (mol%) H/C R (CO2/O2) m-Ratio

Ultra-heavy oil

120

O2(mol%)

100

H/C m(%) R(CO2/O2)(%)

Ordinary heavy oil

110
C

120
C

130
C

110
C

120
C

130
C

6.10 6.92 83.6 2.42 0.96 15.02 0.172 0.284

6.47 6.72 82.28 3.68 0.85 10.00 0.258 0.192

7.03 6.47 81.56 4.21 0.73 8.84 0.290 0.148

5.09 11.4 83.14 0.05 0.32 116.47 0.005 0.865

5.25 9.13 84.23 1.03 0.36 35.50 0.087 0.259

6.5 8.53 82.26 2.18 0.53 16.49 0.175 0.196

80

Values

Compositions

(a)

60 40 20

Results and discussion

0

The viscosity–temperature curves and specific properties of selected heavy oils were respectively shown in Fig. 1 and Table 1. From Fig. 1, it can be seen that the viscosities of both heavy oils increase rapidly in low temperature ranges (<45
C and <65
C for the ordinary heavy and ultra-heavy oils respectively), indicating that the fluidity of heavy oils becomes very difficult into the wellbore and/or surface pipeline, especially for the ultra-heavy oil. In addition, the used exponential regression has well coherence with viscosity data for both heavy oils. And the viscosity and flow diversities for the ordinary heavy and ultra-heavy oils inherently ascribe the content difference in resins and asphaltenes (heavy components) and saturates and aromatics (light components). For generality, various heterocycle and condensed rings are mainly located in strong polar compounds, including ethers, amine, phenols, etc, which take advantage of the intermolecular force and hydrogen bond to form complex compounds for colloid molecules or colloid and asphaltene molecules. Then macromolecular aggregations are easily generated in the colloidal system to significantly increase the viscosity, specifically fused rings carboxylic acids existence. For the element analysis, obvious O and S content for the ultra-heavy oil may be due to the amount of heavy components (resins, and asphaltenes), which involve considerable quantities of sulfide and oxygen-containing groups, especially for asphaltenes [34]. In addition, the amount of resins and asphaltenes implies the degree of hydrogen atoms linked with carbon atoms, namely the atomic H/C ratio, as shown in Table 1, which comprises the objective and contrastive data [22,27]. A high H/C ratio reflects abundant straight chain hydrocarbons, while the presence of polynuclear aromatic compounds is a sign for the low H/C ratio [14]. Hence, a lower H/C ratio (1.49) for the ultra-heavy oil is presented in Table 1 compared with the ordinary heavy oil.

Table 3 Effluent gas compositions of the ordinary heavy oil and ultra-heavy oil at different oxidation pressures. Compositions

C1–C6 (mol%) O2 (mol%) N2 (mol%) CO2 (mol%) CO (mol%) H/C R (CO2/O2) m-Ratio

Ultra-heavy oil

Ordinary heavy oil

40 MPa

45 MPa

50 MPa

40 MPa

45 MPa

50 MPa

6.0 8.53 83.59 1.46 0.42 26.18 0.117 0.223

5.27 7.13 83.55 3.26 0.79 11.56 0.235 0.195

2.95 5.16 85.25 6.11 0.53 6.88 0.386 0.080

5.09 11.76 85.6 0.05 0.42 93.84 0.005 0.894

5.02 9.25 84.23 0.94 0.56 32.55 0.080 0.373

4.48 7.95 84.78 2.25 0.54 17.71 0.172 0.194

35

40

45

50

Pressure/MPa

(b)

110

120

130

140

Temperature/°C

45 O2(mol%)

40

H/C m(%) R(CO2/O2)(%)

35 30

Values

Property analyses of crude ordinary heavy and ultra-heavy oils

25 20 15 10 5 35

40

45

Pressure/MPa

50

110

120

130

140

Temperature/°C

Fig. 3. Specific analyses of effluent gas for ordinary heavy oil (a) and ultra-heavy oil (b) for the isothermal oxidation experiments.

Oxidation characteristics of ordinary heavy and ultra-heavy oils For isothermal oxidation tube experiments, the effluent gases were detected by GC, as tabulated in Table 2 for different oxidation temperature and Table 3 for differential oxidation pressure respectively. As the crucial factors of oxidation process, the oxidation temperature and pressure not only determine the LTO progress for oil and gas phases, but also has considerable influence on subsequent oxidation process and ultimate performance for ISC and/or high pressure air injection (HPAI). As seen in Table 2, there is obvious variation and tendency for ordinary heavy and ultra-heavy oils with oxidation temperature augment. To put it simply, more light hydrocarbons (C1–6) are extracted into the gas phase owing to the distillation enhancement. Meanwhile, the oxygen consumption and carbon dioxide production are both increasing for two oils, especially for ordinary heavy oil on O2 consumption from 9.6% to 14.5% and ultra-heavy oil on CO2 generation from 2.42% to 4.21%. While the decreasing of C1–6 with oxidation pressure increase was due to much less light hydrocarbons produced and volatilized into the gas phase. In addition, the residual O2 was much more than that under oxidation temperatures, indicating that the influence of oxidation pressure on consuming O2 is inferior to the oxidation temperature. But the amount of CO2 and CO variations is more obvious than that under differential temperatures, especially under the highest pressure (55 MPa) with CO2 6.11% and 2.25% for the ultra-heavy and ordinary heavy oils respectively. Accompanying with the effluent gas analysis, the apparent H/C (atom ratio) and m-ratio (molar ratio of [CO/(CO + CO2)]) were also

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(a)

(a) 5K/min 15K/min 25K/min

100

80

80

60

60

40

5K/min 15K/min 25K/min

100

TG/%

TG/%

5

40

20

20

— Ordinary heavy oil

—Ultra-heavy oil

0 0

100

200

300

400

500

600

0

700

0

100

200

Temperature/°C

(b) 1

300

400

500

600

700

Temperature/°C

(b) 1

0

0

-1 -1

DTG/(%/min)

DTG/(%/min)

-2 -3 -4 -5 -6

-2 -3 -4 -5

-7 -8

5K/min 15K/min 25K/min

—Ultra-heavy oil

-6

5K/min 15K/min 25K/min

— Ordinary heavy oil

-7

-9

0

0

100

200

300

400

500

600

700

100

200

300

400

500

600

700

Temperature/°C

Temperature/°C Fig. 4. TG (a) and DTG (b) curves of original ultra-heavy oil under different heating rates.

calculated to estimate the chemical nature of oxidation reactions in specific oxidation condition. And the apparent H/C ratio is defined as Eq. (3.1), Where gi and g0 respectively present the concentration of original and residual O2, mol%; ni and n0 respectively present the concentration of original and residual N2, mol%; CO, CO2: concentration of generated carbon oxides, mol%. Meanwhile, to investigate the conversion degree from O2 to CO2, the R-ratio (molar ratio of generation CO2/consumption O2) was further gained to quantify the extent of decarboxylation reactions [29,35,36]. 
 4 n0 g i =ni  CO2  0:5 CO  g 0 H=C ¼ ð3:1Þ CO þ CO2 Specifically the contrastive H/C, m and R values of super and ordinary heavy oil were plotted in Fig. 3. Owing to free radicals contained in macromolecules, active hydrogen atoms are displaced by oxygen atoms to generate various partial oxides, which results in a higher oxidation degree with lower H/C value. In addition, the further aromatization and dealkylation process would lead to condensation reactions with higher aromaticity and lower H/C. Hence there is an obvious decreasing trend for the H/C value for both heavy oils, especially for the ordinary heavy oil. While, compared with the ordinary heavy oil, much lower H/C can be seen for the ultra-heavy oil under parallel condition and the lowest H/C for the super and ordinary heavy oils were respectively 6.88 and 16.49. It implies that the highly alkylated condensed polycyclic

Fig. 5. TG (a) and DTG (b) curves of original ordinary heavy oil under different heating rates.

aromatic compounds in the ultra-heavy oil were further oxidized to form higher molecular-weight material with lower H/C, which makes the oxidation oil more coke-like [8,35]. On the other hand, carbon gases (CO2 and CO) along with aldehydes are generated for the heavy oil oxidation during 50– 150
C. In addition, the further decarboxylation reactions for the intermediate products, such as carboxylic acids, ketones, alcohols, hydroperoxides, etc, are also indispensable to promote CO2 and CO production. In view of free radical reactions and subjected to further oxidation process, the amount of CO2 is finally more obvious than CO [14,31,36]. Consequently the R value presented an increasing tendency with the temperature/pressure argument. In addition, the reason of higher R values for the ultra-heavy oil shown in Fig. 3(b) may be that more active fractions and polar components were oxidized into intermediates to further form smaller fragments and higher molecular-weight addition products increasing the CO2/O2 ratio. Conversely high values in lower temperature/pressure and relatively low values in high conditions of m-ratio are obtained for both heavy oils. The decreasing m-value reflects that the conversion of carbon to CO2 increases with the temperature/pressure and the oxidation degree of oil samples is promoted with a progression of similar reactions. The m value changes are consistent with the above discussed m ratio and previous researches [7,21,29].

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Table 4 Summary of TG/DTG reaction interval, peak temperature, and mass loss for the ordinary heavy oil and ultra-heavy oil under the heating rate of 15 K/min. Sample

Ordinary heavy oil Ultra-heavy oil

LTO

FD

HTO

Interval (
C)

Peak temperature (
C)

Mass loss (wt Interval %) (
C)

Peak temperature (
C)

Mass loss (wt Interval %) (
C)

Peak temperature (
C)

Mass loss (wt%)

30–324

300

32.30

324–421

414

24.45

421–551

445

33.92

32–348

103

31.28

348–404

395

6.61

404–696

456

52.24

Comparative thermal behavior for ordinary heavy and ultra-heavy oils by TG/DTG/DSC analysis The thermogravimetry (TG–DTG) results under differential heating rates are presented in Figs. 4 and 5 for the ultra- and ordinary heavy oils respectively. And compared with each other, the TG and DTG curves for both heavy oils under different heating rates (5 K/min, 15 K/min and 25 K/min) can be divided into three distinct and different stages, known as LTO (low temperature oxidation), FD (fuel deposition) and HTO (high temperature oxidation) [11], in which heavy oils are subjected to a progression of oxidation reactions with different oxidation mechanisms in the whole non-isothermal oxidation process. And the specific parameters for TG/DTG results were tabulated in Table 4 for both heavy oils. Speaking specifically, in case of 15 K/min, the first reaction region, called as LTO, is mainly detected in 32–348
C for the ultraheavy oil (30–324
C for the ordinary heavy oil) from TG/DTG curves. In addition, different from light and medium oil oxidation process, the evaporation and distillation effects are un-conspicuous and subordinate for the heavy oils, especially ultra-heavy oils, owing to higher viscosity and more heavy components. Hence it was negligible of the mass loss (2%) during the range of 33– 95.4
C for the ultra-heavy oil showing in Fig. 4. While the mass loss for the ordinary heavy oil was nearly 6% below 100
C. Then with the temperature increase, there respectively exists the first distinct peak valleys as presented in Figs. 4(b) and 5(b) for the ultra-heavy and ordinary heavy oils, which indicates obvious mass loss in the corresponding temperature ranges. And the divergences of oil compositions may induce the difference in relevant peak temperature (103.0
C, and 300
C) and value (3.55%/min, and 1.96%/min) of peak valleys for ultra-heavy and ordinary heavy oil respectively. Then shown in Figs. 4(a) and 5(a), TG curves continue to drop to 68.72% for the ultra-heavy oil (and 67.70% for the ordinary heavy oil) to the end of LTO process, whose corresponding DTG curves present a fluctuant trend. During this process, a series of heterogeneous reactions under differential temperature subranges could be divided into oxygen addition reaction and bond breaking reaction. The former is accomplished based on the free radical reaction, in which oxygen atoms attack and substitute active hydrogens in carbon chains. In addition, owing to the active hydrogen usually enriched in heavy and macromolecular components (asphaltenes and resins), the ultra-heavy oil consumed more oxygen as discussed in Section “Oxidation characteristics of ordinary heavy and ultraheavy oils” and has undergone more obvious oxygen addition reactions that the ordinary heavy oil. Consequently the formed partial oxides such as aldehydes, alcohols, ketones, hydroperoxides and carboxylic acids obviously deteriorates the viscosity, especially for the ultra-heavy oil, and remain reactive indeed to step into the latter stage (bond breaking reaction) [31]. For light oils, it is tend to produce smaller fractions during the chain scission process, and finally there is a slight increase in viscosity and flue gas formation to as the predominant mechanism of air injection to improve the recovery. While higher molecular-weight fractions are easily generated by the intermolecular interactions, such as

aromatization, de-alkylation and condensation reactions, for the heavy oils in the bond breaking stage, which promotes and effects the degree of subsequent FD and HTO processes to the sustainability of combustion front and performance of ISC method [6,8,28]. The decreasing H/C also reflects the similar consequences. After the LTO stage, there exist the second distinct peak valleys in DTG curves for both heavy oils indicating the appearance of FD stage. In this region, the LTO products remain reactive to oxidation and condensation with the oxygen to provide the deposited fuel for HTO stage. In addition, heavier compositions are more reactive to promote the fuel formation, in which the influence of saturates is tiny and negligible. Generally speaking, the asphaltenes presented in the oil plays a crucial role in the formation and degree of fuel formation. Owing to higher asphaltene content, the ultra-heavy oil underwent more obvious FD stage with distinct fluctuant DTG curve and mild mass loss trend in Fig. 4(a) and (b) respectively.

Fig. 6. DSC curves of original ordinary heavy oil (a) and ultra-heavy oil (b) under different heating rates.

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Fig. 7. The morphology variation of the ordinary heavy and ultra-heavy oils in the oxidation process.

(a) -0.5 Ordinary heavy oil before oxidation LTO linear fitting HTO linear fitting

-1.0

log(dw/dT/W)

Meanwhile the behavior of resins and aromatics are very similar in oxidation pathway and heat generation to sustain the contiguous HTO stage. And several peak valleys in 350–400
C presented in Fig. 4(b) could reveal that volatiles and/or carbon gases are released by some fractions which is not involved in the ordinary heavy oil, corresponding with multiple exothermic peaks in DSC curve from Fig. 5(b) [23,27]. The final reaction stage, HTO, inferred from TG/DTG curves takes place between 404
C and 696
C for ultra-heavy oil and from 421
C to 551
C for ordinary heavy oil. The least trigging temperature, peak temperature and temperature range of HTO process not only reflect the behavior of heavy oils and the quality of deposited fuel, but also dominate the sustainability of combustion front and whole performance of ISC process for heavy oils [24]. Although comprising more macromolecular components, the ultra-heavy oil had a higher exothermal peak (4.86 mW/mg) and generated more heat in HTO stage shown in Fig. 5(b). As well as, owing to more heavy components and obvious FD process, the mass loss in HTO for the ultra-heavy oil was nearly 6.61%, which was less than that (24.45%) for the ordinary heavy oil. Although the ultra-heavy oil requires higher trigging and peak temperatures of HTO process, the more distinct FD process and active fractions promote the HTO degree with only 9.87% residue, which was in close proximity to the residual proportion (9.33%) of the ordinary heavy oil.

-1.5 y=4.108-2097.400x 2 R =0.978

y=1.994-2146.603x R2=0.994 -2.0

-2.5

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

-1

1/T(K )

(b)

-0.5 Ultra-heavy oil before oxidation LTO linear fitting HTO linear fitting

-1.0

In consideration of the experiment procedure of thermal analyses, in which the sample is in the dimension of milligrams, the differential heating rates could verify the experimental result and investigate the oxidation mechanisms. And it has already reached an agreement over the world for the material thermal analysis [33]. By comparison the TG/DTG curves in different heating rate, it was presented that the heating rate has significant effects on the oxidation behavior of the heavy oils, thought each heavy oil has the similar variation trend. As the heating rate increases, the trigging temperature, peak temperature, and region intervals are

log(dw/dt/W)

-1.5

The influence of heating rate on thermal behavior for ultra- and ordinary heavy oils by thermogravimetry

-2.0

-2.5

-3.0

y=8.316-6638.516x 2 R =0.945

y=13.148-5507.174x R2=0.966

-3.5 0.0010

0.0015

0.0020 0.0025

0.0030

0.0035

-1

1/T(K ) Fig. 8. The kinetic parameter calculation for the original ordinary heavy oil (a) and ultra-heavy oil (b) by the Arrhenius method.

Please cite this article in press as: Y.-B. Li, et al., Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.01.017

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Y.-B. Li et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

(a) Ordinary heavy oil after oxidation LTO linear fitting HTO linear fitting

-1.0

-1.5

log(dw/dt/W)

-2.0

y=1.489-1889.460x 2 R =0.992

-2.5

-3.0

-3.5

y=4.966-3010.266x 2 R =0.981

The kinetic comparison analysis for the ultra- and ordinary heavy oils

-4.0 0.0012

0.0015

0.0018

0.0027

0.0030

-1

1/T(K )

(b) Ultra-heavy oil after oxidation LTO linear fitting HTO linear fitting

-1

log(dw/dt/W)

-2

y=8.921-4750.286x R2=0.990

-4

-5

0.0015

In consideration of the influence of heating rate and the accuracy and credibility of kinetic results, the Ozawa–Flynn–Wall (OFW) method, an integral iso-conversional method, is used to eliminate the influence of heating rate on activation energies compared with the model fitting method, Arrhenius method [37,38]. The OFW method is based on the following Eq. (3.2) and the detailed process has been referred in previous literatures [33,38]. ln b ¼ ln

y=2.827-2806.471x R2=0.989

-3

0.0012

in HTO process. Although it presented similar exothermal trend under different heating rate, the oil samples experience more slowly dominant to more sufficient heat transition from the ambient environment to the inner particles of heavy oils at lower heating rate. As a result, the more adequate cracking and wider exothermal temperature interval occurs with lower temperature limit. In addition, the heavier components with higher calorific power require a higher temperature range to more heat generation. Hence the temperature extent of distinct exothermal peak (420–600
C) for the ultra-heavy oil is much higher and reaction accomplishing temperature is higher [24,25]. As we know, the exothermal behavior of heavy oils, closely related to the specific content of SARA components, still need further detailed study in future.

0.0027

0.0030

-1

1/T(K ) Fig. 9. The kinetic parameter calculation for the oxidized ordinary heavy (a) and ultra-heavy (b) oils by the Arrhenius method.

respectively tend to higher temperature limits and narrower temperature range, corresponding greater loss rate, peak value for maximum rate of oxidation. This might be due to that, under high heating rate, the insufficiency of heat transfer and short exposure time make specific components undergoing insufficient oxidation process, which is in agreement with antecedent researches [10,24,25]. On the other hand, the distinct exothermal peak in HTO stage following an un-conspicuous LTO peak could be found for both experimental heavy oils in Fig. 6. The deposition of solid fuel has a key and positive influence on the exothermal ability and maximum

AE E  5:331  1:052 RgðaÞ RT

ð3:2Þ

The morphology variation before and after the oxidation for both heavy oils are shown in Fig. 7, and the linear fitting procedures for original and oxidized heavy oils were respectively presented in Figs. 8 and 9 using the Arrhenius method. In addition, the corresponding kinetic parameters in LTO and HTO stages, activation energy (E) and pre-exponential factor (A), were summarized in Table 5. And based on the OFW method, the specific activation energies on fixed conversion degrees and corresponding average activation energies in LTO and HTO regions for the ordinary heavy oil and ultra-heavy oil are respectively calculated in Tables 6 and 7. It was observed that the Arrhenius plots are performed in the LTO and HTO ranges with high quality linear regression. And the calculated Arrhenius constants of the ordinary heavy oil before and after the oxidation were respectively from 1.28  104 min1 to 98.6 min1 and from 9.25  104 min1 to 30.8 min1 for LTO and HTO regions. While for the ultra-heavy oil in LTO and HTO stages, varied from 1.41 1013 min1 to 2.07  108 min1 and 8.34  108 min1 to 671 min1 for the crude and oxidized oils respectively. And the values of Arrhenius constants for the ultra-heavy oils were much higher than those for the ordinary heavy oils, which may indicate the difference of the reaction intensity between the ordinary heavy and ultra-heavy oils, which was similar for the crude and oxidized oils. For the ordinary heavy oil, the activation energies of crude oil sample in LTO and HTO stages were respectively 40.159 kJ mol1 and 41.101 kJ mol1 using the Arrhenius method (12.455 kJ mol1 and 126.215 kJ mol1 using the OFW method). While after the

Table 5 The specific kinetic parameters of original and oxidized heavy oils based on Arrhenius method. Sample

Original ordinary heavy oil Original ultra-heavy oil Oxidized ordinary heavy oil Oxidized ultra-heavy oil

Arrhenius constant/(min1)

Activation energy/(kJ/mol) LTO

HTO

LTO

HTO

40.159 127.109 57.638 90.954

41.101 105.447 36.178 53.736

1.28E + 04 1.41E + 13 9.25E + 04 8.34E + 08

9.86E + 01 2.07E + 08 3.08E + 01 6.71E + 02

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Y.-B. Li et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx Table 6 The specific activation energies (kJ mol1) of original ordinary heavy and ultraheavy oils using Ozawa–Flynn–Wall (OFW) method.

a

Ordinary heavy oil LTO

Ultra-heavy oil

HTO

LTO

HTO

E

R2

E

R2

E

R2

E

R2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.721* 3.944 12.636 13.304 12.979 14.674 12.751 11.019 9.825

0.99 0.972 0.806 0.763 0.763 0.716 0.784 0.838 0.867

59.270 11.157* 55.030 116.536 143.532 149.488 153.891 161.197 170.775

0.902 0.997 0.918 0.613 0.236 0.038 0.128 0.216 0.353

131.171 92.488 44.314 46.646 65.765 83.181 97.239 106.455 114.685

0.973 0.811 0.957 0.992 0.981 0.978 0.984 0.987 0.993

281.641 287.275 302.514 302.410 300.133 221.435 148.230 112.914 84.101

0.930 0.972 0.999 0.996 0.944 0.617 0.306 0.364 0.497

Aver.

12.455

–

126.215

–

86.883

–

226.739

–

Notes: owing to obvious difference, the specific activation energy data with * are invalid to calculate the average activation energy in Table 6.

isothermal oxidation process, the LTO activation energy obviously increased to 57.638 kJ mol1 for the Arrhenius method (67.415 kJ mol1 for the OFW method) accompanied with a slight decrease of HTO activation energy, 36.178 kJ mol1 for the Arrhenius method (95.906 kJ mol1 for the OFW method). In addition, a significant variation in activation energy (E) with conversion (a) was noted from Tables 6 and 7, which is an indication of the complicated reaction mechanism of the heavy oil oxidation. The LTO process makes the heavy components (resins and asphaltenes) forming the incomplete oxides and/or some light components, which have good oxidability and are the main reactant for the heavy oil. This process elevates the difficulty and activation energy of LTO region for the oxidized oil. In addition the oxides tend to further react and polymerize with each other to produce heavier, more condensed fractions (coke or coke-like residues), which promotes the oil easily undergoing HTO stage with less activation energy demand. Similarly, owing to the more heavy components (resins and asphaltenes) to accelerate the coke deposition process for the ultra-heavy oil, the more energy demand (127.109 kJ mol1 and 105.447 kJ mol1 using the Arrhenius method and 86.883 kJ mol1 and 226.739 kJ mol1 using the OFW method) before the oxidation and a more distinct decrease of activation energies (90.954 kJ mol1 and 53.736 kJ mol1 using the Arrhenius method and 21.393 kJ mol1 and 129.551 kJ mol1 using the OFW method) after the oxidation in LTO and HTO regions respectively. In addition, it is noted that the LTO activation energy for the ultraTable 7 The specific activation energies (kJ mol1) of oxidized ordinary heavy and ultraheavy oils using Ozawa–Flynn–Wall (OFW) method.

a

Ordinary heavy oil LTO

Ultra-heavy oil

HTO

LTO

HTO

E

R2

E

R2

E

R2

E

R2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

76.557 46.560 27.279 27.720 46.425 63.444 85.860 109.785 123.104

0.993 0.776 0.331 0.620 0.731 0.859 0.955 0.999 0.924

70.691 92.777 116.103 142.826 139.922 86.170 77.949 72.165 64.552

0.932 0.999 0.917 0.941 0.834 0.528 0.486 0.621 0.699

19.475 7.921 22.093 24.699 24.734 24.636 21.923 23.520 23.534

0.845 0.979 0.765 0.541 0.393 0.289 0.358 0.247 0.267

30.965 37.078 34.716 1.199* 127.378 222.413 160.733 115.211 307.914

0.426 0.589 0.810 0.999 0.694 0.709 0.976 0.889 0.984

Aver.

67.415

–

95.906

–

21.393

–

129.551

–

Notes: owing to obvious difference, the specific activation energy data with * are invalid to calculate the average activation energy in Table 7.

9

heavy oil after the oxidation was also lower than that before the oxidation, which was different from the variation for the ordinary heavy oil. This difference may be related to the SARA compositions and the degree of LTO stage. There exists a large amount of alkyl side chains in the macromolecule substances of the heavy components for the ultra-heavy oil to neutralize the natural free-radical scavengers, oxidation inhibitors, which contains complex free radical reactions (formation, growth and destruction) and need to be further studied. Once the oxidation inhibitors are consumed, the oxidation reactions continue unfettered with lower energy demand [39]. It suggests from above analyses that the kinetic parameters of heavy oils should be the comprehensive effect of manifold causes between oil and experimental condition. In addition, the priority levels of influence factors, SARA compositions, H/C ratio, and viscosity in oxidation and kinetic characteristic need to be further studied. In addition, it is difficult to decide and choose the kinetic method for activation energy calculation of oils due to the differences between equation parameters and specific assumptions on variation kinetic methods. For this reason, we should focus on the variation trends of activation energies from different kinetic methods rather than the compare of specific activation energy values with different kinetic methods. In consideration of a variety of chemical reactions coupled with simultaneous heat, mass, and momentum transfer to in-situ upgrade the heavy oils for the air injection/ISC, it is difficult to fully investigate and understand the oxidation characteristic and the influence of LTO process on coke deposition for the heavy oils in this study. In addition the microcosmic composition and structure of heavy oils in oxidation process still remain many vacancies and deficiencies and should be further evaluated. For the quantitative comparison of activation energies in LTO and HTO stages, it could presented that the LTO experiment has a positive role in upgrading heavy oils to trigger and sustain the HTO process. But the opposite influence on LTO activation energy changes for the ordinary heavy and ultra-heavy oils may be related to the oil compositions, coke deposition and H/C ratio. Therefore, the further chemical analyses in LTO oxidation process and the microcosmic structure investigation of the heavy oil and deposited coke should be involved in subsequent experiments and studies. Conclusions On the basis of the isothermal oxidation experiments, effluent gas analyses, TG/DTG/DSC tests, thermal analyses, and kinetics, the LTO characteristic, kinetic behavior, and flammability property after the oxidation of the ordinary heavy and ultra-heavy oils were investigated comparatively. And the following conclusions are drawn from this study. (1) Because of more heavy fractions (asphaltenes and resins) and lower H/C ratio, the ultra-heavy oil with worse mobility consumed more oxygen compared with the ordinary heavy oil. In addition the dominated oxygen addition reaction could obviously increase the viscosity of heavy oils in all isothermal oxidation experiments. (2) When the pressure/temperature was increased, the more oxygen consumption, the lower H/C ratio and higher RðCO2 O2 Þ value could be observed, especially for the ultra-heavy oil in 50 MPa. And more distinct H/C ratio decrease (from 116.47 to 16.49) for the ordinary heavy oil and more obvious RðCO2 O2 Þ value increase (from 11.70 to 38.60) for the ultra-heavy oil suggested that the oxygen addition reaction upgrade the nature of heavy oils and the sequent condensation reactions and inter-reactions promote the coke deposition with CO2

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Y.-B. Li et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

generation. In addition, the H/C ratio for the effluent gas and oil phase indicates that the oxidation process makes the ultraheavy oil more coke like with better flammable ability, compared with the ordinary heavy oil. (3) In thermogravimetric analyses, three distinct reaction stage, LTO, FD, and HTO, are existed in consecutive temperature ranges for the ordinary heavy and ultra-heavy oils. Owing to more active components, the threshold LTO temperature of the ultra-heavy oil is slightly lower with higher peak rate of the mass loss from the DTG curves. For the DSC curves, except the effect of heating rates, the maximum rate of heat flow and the quantity of generated heat in HTO stage shifts to greater for the ultra-heavy oil with lower
API gravity and higher asphaltene content. In addition, the fluctuant DTG and DSC curves in FD stage for the ultra-heavy oil indicate that the deposited coke with better exothermal behavior could improve the energy demand and sustainability of HTO process. (4) After the oxidation, the HTO activation energies of both ordinary heavy and ultra-heavy oils present a decrease trend, while the LTO activation energy are respectively increase by 43.5% for the ordinary heavy oil and decrease by 28.4% for the ultra-heavy oil. In addition the Arrhenius constants for the ultra-heavy oil are obviously much higher. It is shown that the oxidation process improves the quality of the oxidized oil, especially for the ultra-heavy oil, which contains higher heavy fractions to promote the coke deposition. Formed from dehydrogenation polymerization and/or dehydrogenation aromatization processes, the flammable coke not only reflects the in situ upgrade for heavy oils, but also delivers the sign of the potential application of the air injection without igniters in heavy oil reservoirs.

Conflict of interest The authors declare no competing financial interest. Acknowledgment The authors thank Sinopec Northwest Company (China) for the financial and crude oils support for this research and permission to publish this paper.

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Please cite this article in press as: Y.-B. Li, et al., Low temperature oxidation characteristics analysis of ultra-heavy oil by thermal methods, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.01.017