Thermal Cracking Characteristics and Kinetics of Oil Sand Bitumen

Article pubs.acs.org/EF

Thermal Cracking Characteristics and Kinetics of Oil Sand Bitumen and Its SARA Fractions by TG−FTIR Junhui Hao, Yuanjun Che, Yuanyu Tian,* Dawei Li, Jinhong Zhang, and Yingyun Qiao* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China ABSTRACT: The thermal cracking process of oil sand bitumen was investigated via studying the thermal cracking behaviors and online-gas-releasing characteristics of its SARA fractions (saturates, aromatics, resins, asphaltenes) by using TG−FTIR. The results showed that the asphaltenes contributed the most to coke formation of oil sad bitumen according to its largest weighted coke yield compared with that of others. The online FTIR analysis for the gases evolved from the thermal cracking process of oil sand bitumen and its SARA fractions indicated that the gaseous products in the main reaction stage showed more complex composition than those in the volatilization stage, and predominantly consisted of CO2, CO, methane, ethylene, other light alkanes and olefins, light aromatics, and hydrogen sulfide. Moreover, the release behaviors of typical gaseous products (CO2, CO, methane, ethylene, and light aromatics) for oil sand bitumen and its SARA fractions were different due to the different composition and structure. The thermal-cracking kinetic parameters of oil sand bitumen and its SARA fractions were determined using the iso-conversional Friedman procedure at heating rates of 10 to 800 K/min. The activation energy of oil sand bitumen ranging from 93.74 to 215.99 kJ/mol in the whole thermal cracking process fell in between that of aromatics and resins. The variation of activation energy with the conversion rate for oil sand bitumen during the thermal cracking process was influenced by the interaction between SARA fractions.

1. INTRODUCTION With the rapid development of a world economy, the demand of human beings for petroleum resources is ever-increasing. However, the current oil reserves ensure a continuous supply of petroleum for no more than 50 years at the recent rate of consumption.1 Thus, it is significant to explore alternative unconventional oil resources to ensure a reliable and secure energy supply. In recent years, the utilization of the oil sand has received worldwide increasing attention, as the oil sand is an abundant, underdeveloped unconventional fossil fuel resource important for alleviating the energy crisis.2,3 The Indonesia oil sand features a high value of exploitation and utilization. Indonesia has substantial oil sand resources, most of which are deposited in Buton Island. The reserves of Indonesia oil sand resources are more than 3.0 × 109 tons, and the content of the bitumen in this oil sand exceeds 20%.4 Oil sand, also known as bituminous sand, naturally exists as a complex mixture of quartz sands, clay, water, and bitumen. The oil sand can be classified into oil-wet, water-wet, and neuter-wet oil sand according to its surface wettability.3 The utilization of oil sand requires separating the organic matters from the sand matrix with suitable methods such as pyrolysis, hot-water extraction, organic solvent extraction, supercritical solvent extraction, and ultrasonic-assisted solvent extraction.5−8 Among the methods, the direct pyrolysis of oil sand can be used to achieve the bitumen-derived liquid products,2 which feature lower carbon residue, viscosity, and heteroatom content, but better fraction distribution than native oil sand bitumen. However, the use of the above-mentioned extraction methods allows obtaining the bitumen whose chemical structure and physical properties are similar to those of the native oil sand bitumen. Because of the undesirable properties of oil sand bitumen, such as high acid value, density, viscosity, carbon residue, asphaltenes content, and heteroatom content, oil sand © XXXX American Chemical Society

bitumen is advised to be upgraded through coking to produce the light liquid products (also called oil sand oil), followed by refining the oil sand oil via catalytic hydrogenation or catalytic cracking.9 As a matter of fact, the two methods, namely the direct pyrolysis of oil sand and the coking upgrading of oil sand bitumen, are both the thermal conversion process of bitumen derived from oil sand. Therefore, it is necessary to study the thermal cracking process of oil sand bitumen to guide the recovery and upgrading of oil sand bitumen, and control the distribution of thermal products. In previous research, attention was mainly focused on studying the pyrolysis behaviors of oil sand and oil sand bitumen itself by thermogravimetric analysis (TG).10−17 However, in view of the complicated thermal cracking process of oil sand bitumen, it is necessary to further study the thermal cracking process of oil sand bitumen in terms of chemical group composition which is divided into saturates, aromatics, resins, and asphaltenes (SARA). In addition, it is known that TG can only reveal the variation of the weight loss of the sample with heating temperature or heating time. Thus, in order to obtain a deep insight into the thermal-cracking mechanism of oil sand bitumen and its SARA fractions, it is necessary to investigate the composition and releasing characteristics of thermal cracking products under different reaction conditions, such as temperature and heating rate. So far, some investigations related to products distribution during the thermal cracking process of oil sand bitumen have been carried out with laboratory-scale equipment, such as fluidized bed, fixed bed reactors, etc.2,8,12,18−20 Nevertheless, this equipment does not well allow real-time Received: October 10, 2016 Revised: December 30, 2016 Published: January 14, 2017 A

DOI: 10.1021/acs.energyfuels.6b02598 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

depending on the thermogravimetric experimental data at different heating rates of 10−800 K/min.

investigation of the composition and release behaviors of evolving gaseous products during the thermal cracking process of oil sand bitumen. Fortunately, a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer (TG−FTIR) can not only investigate the thermal cracking weight-loss behavior, but also identify the species in multiple gaseous compounds evolved in real-time during the thermal cracking process.21 The identification is based on the characteristic absorption peaks of the functional groups in the evolved gases.22 Thus, this technology has been adopted widely to study the pyrolysis mechanism and kinetics of coal, biomass, waste materials, and so on.23−28 However, few studies have been conducted on online detection of gaseous products evolved from thermal cracking of oil sand bitumen by TG−FTIR. The study of the thermal cracking kinetic of oil sand bitumen and its fractions is essential to guide the pyrolysis of oil sand and upgrading of oil sand bitumen. So far, gathering of the experimental data of nonisothermal thermogravimetric analysis has been extensively carried out by various kinetic methods with different approximations to evaluate the kinetics, which are used to predict and regulate the pyrolysis process of fossil fuels. These methods can be divided into model-fitting methods and model-free methods. Because of the assumption that the activation energies of the thermal decomposition process at different heating rates are the same at constant conversion rate, namely, the reaction rate at constant conversion rate is only a function of temperature, the reaction mechanism function is not necessary to be known before the model-free method to calculate the activation energy of the pyrolysis process.29,30 Therefore, model-free methods, such as the Friedman method, Flynn− Wall−Ozawa (FWO) method, and Kissinger−Akahira−Sunose (KAS) method, are more accurate than model-fitting methods, of which the Friedman procedure is very popular to be used to evaluate the activation energy of pyrolysis of fossil fuels. Ma et al.15 reported that the Friedman procedure could be suitably used to reflect the oil sand pyrolysis process. Liu et al.31 found good agreement between the integral model-free method and experimental data of the dependence of the conversion rate on temperature for the pyrolysis process of oil sand. Moreover, considering the fact that oil sand bitumen is a kind of oil resource, thus evaluation and analysis of its activation energy of the thermal cracking process can refer to that of other oil resources. Schucker32 evaluated the coking kinetics of an Arab heavy vacuum residuum by the iso-conversional Friedman procedure and concluded that the activation energy gradually increased with the conversion rate. Elena Alvarez et al.33 determined the pyrolysis activation energy of the atmospheric residue and its SARA fractions based on the iso-conversional Friedman procedure and indicated that the activation energy of asphaltenes as the main fraction to form coke ranged from 41.0 to 58.6 kcal·mol−1 whereas that of the atmospheric residue was in a range of 11.5 to 30.0 kcal·mol−1. This research aimed to investigate the thermal cracking characteristics and kinetics of oil sand bitumen and its SARA fractions by using TG−FTIR. To better analyze the thermal cracking process of oil sand bitumen and its fractions, the chemical structures of the samples were studied by the analysis of FTIR and average structure parameters. The study of the thermal cracking characteristics of the samples focused on the weight-loss behavior, and composition and evolving characteristics of gaseous products. Subsequently, the kinetic parameters were calculated by the iso-conversional Friedman procedure

2. EXPERIMENTAL SECTION 2.1. Materials. The oil sand used in this study was derived from Indonesia, with its physicochemical properties shown in Table 1. It can

Table 1. Physicochemical Properties of Indonesia Oil Sand Properties

Values

Proximate analysis, wt % ad moisture ash volatile fixed carbon Fischer assay, wt % oil gas + loss water semicoke a

2.33 50.56 45.35 1.76 16.28 2.41 1.43 79.88

Properties Ultimate analysis, wt % C H N S Oa Dean−Stark analysis, wt % bitumen moisture sand

Values 29.81 2.86 0.20 1.81 65.32 27.65 2.33 70.02

Calculated by difference.

be seen that the bitumen content of Indonesia oil sand reaches to 27.65%, which is significantly lager than that of Athabasca oil sand (10.4%)34 and Xinjiang oil sand (8.29%). The Indonesia oil sand is a kind of oil-wet oil sand and features strong hydrophobicity, whose properties are the opposite of Athabasca oil sand.35 2.1.1. Oil Sand Bitumen and Its SARA Fractions. According to the similarity−intermiscibility theory, oil sand bitumen was extracted from oil sand with toluene which was of analytical reagent grade. In this research, the thermal reflux extraction method was used to separate oil sand bitumen from oil sands. The filter paper wrapping 50 g of oil sands was placed into a reflux extractor, and then 250 mL of toluene was added into the boiling flask (500 mL) kept in an oil bath. As the oil bath pan was heated to the proper temperature, the toluene evaporated was condensed in the condenser pipe and dripped into the oil sands wrapped in the filter paper, and then the oil sand bitumen was extracted with toluene into the boiling flask. This solvent extraction was continued for 24 h. After this extraction process was completed, the bitumen−solvent mixture solution was transferred to a conical flask from the boiling flask so as to be separated by distillation. The conical flask with oil sand bitumen was placed in vacuum drying oven for 1 h to evaporate the remaining solvent. After that, the oil sand bitumen needed (properties shown in Table 2) was obtained.

Table 2. Physicochemical Properties of Oil Sand Bitumen Properties Density (20 °C), g/cm API gravity Viscosity (110 °C), mPa·s Carbon residue, wt % SARA fraction, wt % Saturates Aromatics Resins n-C7 Asphaltenes 3

Values

Properties

1.03 6.13 1019 15.64

Metal content, μg/g Ni V Simulated distillation, wt % 500 °C IBP, °C

14.05 38.23 24.51 23.21

Values 32.27 34.76 6.60 5.80 21.30 66.30 136

The SARA fractions (saturates, aromatics, resins, asphaltenes) of oil sand bitumen were separated by two steps, namely precipitation of asphaltenes and separation of saturates, aromatics, and resins from maltenes. One gram of oil sand bitumen added into 50 mL of n-heptane in a conical flask was fully mixed by means of hot reflux for 1 h, and then was cooled and left to stand for 24 h. After that, the mixture liquids were filtered to obtain the maltenes mixed with n-heptane in the new conical flask, and asphaltenes were precipitated B


Article

Energy & Fuels in the original conical flask. Residual asphaltenes and maltenes in the filter paper were recovered by hot reflux with n-heptane (in the new conical flask) and toluene (dissolving precipitated asphaltenes in the original conical flask), respectively. The maltenes with n-heptane concentrated to about 10 mL were loaded on the top of a chromatographic column packed with 40 g of neutral alumina (100−200 meshes). Then, 80 mL of n-heptane and 80 mL of toluene were, in turn, added into the chromatographic column to separate saturate and aromatic fraction, respectively. Afterward, resin fractions were separated with 40 mL of mixture of toluene and ethanol (1:1, v/v), followed by 40 mL of toluene, and then 40 mL of methanol. Finally, saturates, aromatics, resins, and asphaltenes after vacuum drying were weighed to calculate their content in oil sand bitumen. 2.2. Characterization and Analysis. The ultimate analyses for oil sand bitumen and its SARA fractions were carried out on an elemental analyzer (Elementar Vario Macro cube, Germany). The functional groups of oil sand bitumen and its fractions were studied by the Fourier transform infrared (FTIR) spectra recorded on a FTIR instrument (Bruker Tensor 27, Germany). The wavenumber of the spectra ranges from 4000 to 500 cm−1. Each sample was mixed with dried KBr powder at a mass ratio of 1:150 before the mixture was pressed under 12 MPa into a pellet. The average molecular weight of the Indonesia oil sand bitumen and its SARA fractions (saturates, aromatics, resins, and asphaltenes) were measured by vapor pressure osmometry (KNAUER K-7000, Germany). The 1H NMR spectra for oil sand bitumen and its SARA fractions were recorded on a Bruker AVANCEIII-600 NMR spectrometer with the resonant frequency of 300 MHz for proton. The concentration of sample dissolved in CDCl3 was 30 mg/mL. The tetramethylsilane (TMS) was used as the internal standard substance. The 1H NMR spectra were also required with a scanning number of 1600 accumulated in each spectrum, a spectral width of 3.6 kHz, and a relaxation delay time of 3 s. The 1H NMR spectra are divided into four regions, namely (1) 0.5−1.0 ppm, (2) 1.0−2.0 ppm, (3) 2.0−4.0 ppm, and (4) 6.0−9.0 ppm, which represent (1) the presence of γ- and further methyl hydrogen to aromatic ring, (2) β-methylene or methyne hydrogen to aromatic, (3) α-methyl, methylene, or methyne hydrogen to aromatic, and (4) aromatic hydrogen, respectively. 2.3. TG−FTIR Analysis. The thermal cracking experiments of oil sand bitumen and its SARA fractions were studied by using a fast speed thermogravimetric analyzer (NETZSCH STA 449 F3, Germany) coupled with a FTIR spectrometer (Bruker Tensor 27, Germany) over the temperature range of 28 to 820 °C in a N2 flow (99.99% purity, 100 mL/min). The heating rates investigated were 10, 20, 50, 80, 120, 200, 500, and 800 K/min. To minimize the influence of heat and mass transfer, the mass of sample placed in the platinum crucible was less than 5 mg. The FTIR spectrometer recorded infrared spectrum in the wavenumber range of 500−4000 cm−1 with a resolution of 4 cm−1. This spectrometer was connected to the thermogravimetric analyzer by the polytetrafluoroethylene transfer line and gas cell, which were both heated to 200 °C to avoid condensation of gaseous products evolved during the thermal cracking process of samples. 2.4. Kinetic Analysis. The thermal cracking process of oil sand bitumen and its SARA fractions can be described by14

dα = Ae(−E / RT )f (α) dt

bitumen and its SARA fractions. The differential form of the reaction mechanism function of the first-order reaction is

f (α) = 1 − α

The Friedman procedure can be described by the following eq 4, which is derived by taking the natural logarithm of eq 1, ⎛ dα ⎞ E 1 ln⎜ ⎟ = ln[A(1 − α)] − · ⎝ dt ⎠ R T

m 0 − mt m0 − m∞

(4)

In this study, eight heating rates, including 10, 20, 50, 80, 120, 200, 500, and 800 K/min, and the range of conversional rate α selected were all from 0.1 to 0.9. As is known from eq 4, under the same reaction conversional rate at different heating rates, the ln[A(1−α)] can be considered as a constant, so the plots of ln(dα/dt) as a function of 1/T are fitted as a straight line by the linear regression. The slope and intercept of the regression lines are used to determine the activation energy E and pre-exponential factors A, respectively, for a specific α.

3. RESULTS AND DISCUSSION 3.1. Structural Analysis of Oil Sand Bitumen and Its SARA Fractions. 3.1.1. FTIR Analysis. The FTIR spectra of oil sand bitumen and its SARA fractions were shown in Figure 1. The assigned peaks of a typical functional of the samples can be referred to the literature.2,36−38 The strong absorption peaks at 2926 cm−1, 2853 cm−1, 1458 cm−1, and 1377 cm−1 could all be observed in the FTIR spectra of oil sand bitumen and its SARA fractions. The absorption peaks at 2926 and 2853 cm−1 were due to the asymmetric and symmetric stretching vibration of C−H in methylene (−CH2−), while 1458 and 1377 cm−1 were ascribed to the bending vibration of methylene (−CH2−) and methyl (−CH3). These methylene (−CH2−) and methyl (−CH3) were contained in the chain alkanes, naphthenic rings, and the side chains attached to the aromatic rings and naphthenic rings. However, the absorbance peak corresponding to aromatic ring structure at 1604 cm−1 was rather weak and hardly observed in the FTIR spectrum of saturates. These results mentioned above indicated that the saturates mainly consisted of chain alkanes and cycloalkanes. In the FTIR spectra of aromatics, resins, asphaltenes, and oil sand bitumen, the presence of aromatic ring structure was indicated by the weak absorption peaks at 3058 cm−1 attributed to the stretching vibration of aromatic C−H, and 1601 cm−1 for aromatics or 1620 cm−1 for oil sand bitumen, resins, and asphaltenes due to the stretching vibration of the aromatic CC, respectively. Moreover, the intensity of the absorption peak near 3058 cm−1 in the spectrum of aromatics was larger than that of the others, which could indicate that the condensation degree of the aromatic ring contained in the average molecular structure of aromatics was lower than that of the others. In addition, several peaks in the region 900−700 cm−1 due to the out-of-phase bending vibration of the aromatic C−H also suggested the presence of aromatic ring structure in these FTIR spectra, as these peaks reflect the position of the substituents on the aromatic rings. The presence of the characteristic peak at 1708 cm−1 was attributed to the stretching vibration of the carbonyl (CO) groups. Moreover, the peaks of CO in aromatics and oil sand bitumen were observed more obviously than those in resins and asphaltenes, which might be related to the fact that these peaks in resins and asphaltenes overlapped the broad peaks in the region 1795−1524 cm−1 corresponding to the aromatic CC. Several weak absorption peaks located in the 1074−1209 cm−1 region were assigned to the stretching vibration of C−O in alcohols, esters, etc. The absorption peak at 1032 cm−1 was

(1)

The α in eq 1 can be calculated by

α=

(3)

(2)

In consideration of the complexity of the composition and average structure, and thermal cracking reactions of oil sand bitumen and its SARA fractions, the Friedman procedure as an isoconversional method (Model-free method) was used to calculate the activation energy of the thermal cracking process. Generally, the first-order reaction model can be carried out to describe the thermal cracking process of oil sand C


Article

Energy & Fuels

Figure 1. FTIR spectra of oil sand bitumen and its SARA fractions.

attributed to the stretching of the SO bond in sulfoxides, which might be formed via the oxidation of thiophenic compounds in samples. Besides, the broad and strong absorption peaks at around 3443 cm−1 predominantly originated from the stretching vibration of O−H. Moreover, the position and shape of these absorption peaks received the influence of hydrogen bonds originating from the O−H or N−H functional groups.2,39 Nevertheless, it was suggested that the peaks located at around 3443 cm−1 in oil sand bitumen, its resins, and asphaltenes were mainly related to the O−H functional groups, in view of the relatively high content of oxygen compared to that of nitrogen (Table 3) in these samples. It was noticed that the peaks near 3443 cm−1 were significantly observed in the IR spectra of oil sand bitumen, resins, and asphaltenes but absent in those of aromatics, implying that the number of species and the amount of polar functional groups in these three samples were both larger than those in saturates and aromatics. Moreover, the intensity of the absorption peaks at 3443 cm−1 was in the order asphaltenes > resins > oil sand bitumen, which was in accordance with the order of the oxygen content of the three samples.

Table 3. Ultimate Analysis of SARA Fractions of Oil Sand Bitumen

a

Sample

C (wt %)

H (wt %)

Bitumen Saturates Aromatics Resins Asphaltenes

82.39 86.18 82.31 80.00 78.28

9.58 12.73 9.73 9.09 8.08

N S Oa (wt %) (wt %) (wt %) 0.47 0.05 0.20 1.09 0.88

6.20 0.45 6.68 6.65 9.51

1.36 0.59 1.08 3.18 3.26

H/C (atomic ratio) 1.40 1.77 1.42 1.36 1.24

Calculated by difference.

3.1.2. Average Structural Parameters. The average structural parameters of oil sand bitumen and its SARA fractions were calculated by the modified B-L method, as detailed in the previous literature.40,41 This method would depend on the data of the VPO-measured average molecular weight, ultimate analysis, and 1H NMR analysis of these samples. Moreover, it was assumed that the average molecular structures for resins and asphaltenes consisted of several same structure unit sheets due to the rather large average molecular weight (M) of D


Article

Energy & Fuels resins and asphaltenes.42 Thus, the number of total rings (RT), aromatic rings (RA), and naphthenic rings (RN) for resins and asphaltenes determined by the modified B-L method could be represented by RT*, RA*, and RN* in one structural unit sheet, respectively. The average structural parameters of oil sand bitumen and its SARA fractions were shown in Table 4. For saturates, the

the range between 0.53 and 0.56. Therefore, it was indicated that the proportions of peripheral carbons in the aromatic ring system connecting with the alkyl chains or the naphthenic ring for these four samples were similar. 3.2. TG Analysis. The TG/DTG curves of the thermal cracking process for oil sand bitumen and its SARA fractions at the heating rate 50 K/min are shown in Figure 2(a) and 2(b).

Table 4. Average Structure Parameters of Oil Sand Bitumen and Its Fractions Structure parameters

Oil sand bitumen

M CT HT fA fN fP RT RA RN RA/RN HAU/CA σ n usw

990.66 67.96 94.16 0.36 0.12 0.52 9.56 6.88 2.67 2.58 0.65 0.53

saturates

aromatics

resins

asphaltenes

464.80 33.35 58.70 0.13 0.27 0.60 2.80 0.60 2.20

873.94 59.90 84.36 0.35 0.22 0.43 8.13 4.79 3.34 1.44 0.71 0.54

2048.67 136.46 177.23 0.38 0.07 0.55 5.55* 3.64* 1.91* 1.91 0.65 0.54 3.46 591.30

6557.03 427.38 525.63 0.44 0.08 0.48 7.96* 5.58* 2.38* 2.35 0.55 0.56 9.03 726.17

*

represents RT, RN, and RP in one structural unit sheet.

fraction of carbons in saturated structures (alkyl chains and naphthenic rings) reached 0.87, which was the sum of the fraction of naphthenic carbon (f P) of 0.60 and the fraction of paraffinic carbon (f N) of 0.27. The fraction of carbons in saturated structures indicated that the average molecular structures of saturates were mainly composed of aliphatic chains and naphthenic ring structure. Besides, we noted the presence of a small quantity of aromatics in saturates indicated by the fraction of the aromatic carbon (fA) of 0.13. These observations mentioned above were in accordance with the FTIR analytical results for saturates. The values of fA, f N, and f P were in the order saturates < aromatics < oil sand bitumen < resins < asphaltenes, saturates > aromatics > oil sand bitumen > asphaltenes ≈ resins, and saturates > resins > oil sand bitumen > asphaltenes > aromatics, respectively. These results, for one thing, confirmed that saturates could be seen as the constituent with the simplest average molecular structure compared to others. For another, these results, to a certain extent, indicated that the average molecular structures of asphaltenes consisted of a more condensed aromatic ring structure than oil sand bitumen and other fractions. Moreover, this conclusion could be further evidenced by the number of the structural unit sheet (n) and its unit sheet weight (usw) for aphaltenes, which were 9.03 and 726.17, respectively. The condensation degree of the aromatic ring system (HAU/CA) was in the order aromatics < oil sand bitumen ≈ resins < asphaltenes. This order could conclude that the condensation degree of asphaltenes and aromatics was the largest and smallest, respectively, according to the fact that the higher the HAU/CA value, the lower the condensation degree.42 In addition, the values of the degree of the substituent of the peripheral hydrogen in the aromatic ring system (σ) for oil sand bitumen and its fractions, except saturates, were close: in

Figure 2. TG and DTG curves for thermal cracking of oil sand bitumen and SARA fractions at 50 K/min.

According to these curves, the weight loss of oil sand bitumen and its SARA fractions could be divided into two stages, but this rule was not obviously observed in the thermal cracking process of saturates. The first one was the distillation stage, which occurred for the distillation of hydrocarbons with low boiling point and the rupture of the weak chemical bound. The second one was the main reaction stage, which featured intense E


Article

Energy & Fuels

the naphthenic ring, and the ring-opening of the naphthenic ring.42 However, a small quantity of naphthenic hydrocarbons might take part in the dehydroaromatization reaction to generate the macromolecular aromatics. Therefore, the asphaltenes and toluene insoluble which were susceptible to form coke were hardly produced during the thermal cracking process of saturates.46 This could explain why the coke yield of saturates was the lowest compared to those of others. For asphaltenes, Ti was the largest compared to those of oil sand bitumen and other fractions. Prior to Ti of 420 °C, the weight loss of asphaltenes was only 8.17%, which was mainly due to the release of small gaseous molecules originating from the cleavage of a weak chemical bond (C−C, C−O, and C−S, etc.) in alkyl groups at peripheral sites.47 Moreover, the cracking extent of alkyl side chains attached to naphthenic rings and aromatic rings in asphaltenes was lower than others in the same temperature region so as to cause the slight weight loss for saturates. The reason leading to this observation was the shorter alkyl side chains in the average molecular structure of asphaltenes compared to others, which was indicated by the low f P, which was second only to aromatics, the relatively large σ, and the low HAU/CA compared with oil sand bitumen and other fractions (Table 4). Subsequently, the C−C bounds in the short aliphatic chains formed from the cracking of long aliphatic chains, and naphthenic rings attached to polycyclic aromatics could be ruptured in the intense weight loss stage. Meanwhile, the bridge bonds were also broken in this stage. These bridge bonds linking with the unit sheets of asphaltenes mainly consisted of two aspects: (1) groups containing sulfur, oxygen, or nitrogen atom, which form intermolecular forces such as hydrogen bonding or ether-type bonding; and (2) aliphatic chains.48,49 In the higher temperature range, the dehydroaromatization of naphthenic rings and the condensation reaction of aromatic ring structures would take place to form the coke precursors with the high condensation degree of the aromatic unit sheet. Moreover, it was believed that the precursors in asphaltenes which tend to form coke were more than that in others due to the lowest HAU/CA and the largest fA for asphaltenes. Thus, the coke yield of asphaltenes was the largest compared to those of others. In comparison with these two extreme fractions mentioned above, it was found that the thermal reactivities of oil sand bitumen, aromatics, and resins fell in between asphaltenes and saturates. As shown in Figure 2(b), exhibiting the DTG curves of oil sand bitumen, aromatics, and resins, prior to 300 °C, their weight losses and the absolute values of the weight loss rate (dw/dt) all obeyed the order of oil sand bitumen > resins > aromatics. As the temperature increased to Ti, the variation of the weight and dw/dt for resins was slight, while that for oil sand bitumen and aromatics both continued to increase. The reason for this might be the relatively large amount of the components which were liable to volatilization and the long alkyl chains with poor thermal stability in oil sand bitumen and aromatics compared with that in resins. In the temperature range from Ti to Tmax, it was observed that the dw/dt for oil sand bitumen, aromatics, and resins all increased more remarkably than that in the volatilization stage, which was in agreement with the variation tendency of weight loss for these samples. This was attributed to the intense cracking reactions, such as the scission of alkanes and shorter alkyl side chains originating from the shallow cracking of long chains, etc. Moreover, resins showed the largest increment values of dw/dt, followed by aromatics and oil sand bitumen in the same

weight losses due to the occurrence of complex reactions such as the cleavage of the C−C and C−heteroatom bonds in alkanes and alkyl side chains attached to naphthenic rings or aromatic rings, the dehydrogenation and ring-opening of naphthenic rings, and the dehydrogenation of the aromatic ring structure.2,32,43,44 As the temperature further increased, the thermal cracking for oil sand bitumen and its fractions, except saturates, predominantly occurred during the condensation reaction of condensed nuclei aromatics to form coking precursors with much higher condensation degree, and then transformed into the coke as the final residue. Moreover, it was seen that the weight loss values above the final temperature (Tf) of the intense weight loss interval for oil sand bitumen and its SARA fractions all hardly varied. The coke yields for oil sand bitumen and its saturates, aromatics, resins, and asphaltenes shown in Table 5 were 11.32%, 0.52%, 3.11%, 17.01%, and Table 5. Characteristic Parameters of the Thermal Cracking Process of Oil Sand Bitumen and Its SARA Fractions at 50 K/min Sample Oil sand bitumen Saturates Aromatics Resins Asphaltenes a

Temp intervala, °C

(dw/dt)max, %/min

Tmax, °C

αM, wt %

Coke yield, %

413−508

33.51

479

74.05

11.32

233−467 409−511 409−512 420−516

21.40 42.25 45.83 45.39

357 485 488 484

51.47 72.61 62.42 61.43

0.52 3.11 17.01 33.31

Temperature interval corresponding to intense weight loss stage.

33.31%, respectively. Meanwhile, the weighted coke yields determined by the multiplication of the coke yield and the percent amount of corresponding fraction in oil sand bitumen for saturates, aromatics, resins, and asphaltenes were, in turn, 0.07%, 1.19%, 4.17%, and 7.73%, and added up to 13.16%. By the comparison of the coke yield of oil sand bitumen itself and the sum of weighted coke yields of the SARA fraction, it was suggested that the thermal cracking process of oil sand bitumen exhibited an interactive effect between SARA fractions, referring to the analysis of Yang.45 Moreover, the fact that asphaltenes contributed the most to coke formation was indicated on the basis of the weighted coke yield of asphaltenes. As listed in Table 5, It was found that the temperature interval corresponding to the intense weight loss stage for saturates in the range 233−467 °C was broader than those of oil sand bitumen, aromatics, resins, and asphaltenes, which were 413−508 °C, 409−511 °C, 409−512 °C, and 420−516 °C, respectively. Moreover, the initial (Ti) and final (Tf) temperatures of the intense weight loss interval of saturates were both lower than those of others; in other words, the thermal stability of the saturates was weaker than that of others. The reason for this observation was that the composition and structure of saturates were simpler than those of others, as evidenced by FTIR analysis and the average structure parameters for saturates. Besides, the conversion rates corresponding to the maximum weight loss rate (αM) at the Tmax of 357 °C reached 51.47%, indicating that the weight loss of saturates during the thermal cracking process mainly resulted from the distillation of compounds with low molecular weight. As the temperature further increased, the weight loss for saturates was as a result of the cracking reactions, such as the cleavage of C−C bound in the aliphatic hydrocarbons and the aliphatic chains attached to F


Article

Energy & Fuels

Figure 3. FTIR spectra of gaseous products from pyrolysis of oil sand bitumen and SARA fractions at the (a) volatilization stage and (b) main reaction stage.

originated from the naphthenic rings, instead of aromatic rings because the condensation reactions of polycyclic aromatics was not remarkable in the Ti to Tmax. Thus, on the basis of the order of resins < oil sand bitumen < aromatics for f N, the negative influence of the combination of alkyl free radicals and H free radicals on cracking reactions for resins was less than that on oil sand bitumen and aromatics, while the interaction between the SARA fraction mentioned above might be responsible for the lowest (dw/dt)max for asphaltenes compared with aromatics and resins. With further increasing temperature beyond Tmax, the condensation of polycyclic aromatics and dehydroaromatization of naphthenic rings gradually dominated the thermal cracking process; thus, the dw/dt for oil sand bitumen and SARA fractions all displayed the decreasing trend. 3.3. FTIR Analysis of Gaseous Products. The FTIR spectra of evolved gaseous products in two different stages, i.e., the volatilization stage and main reaction stage, in the thermal cracking process of oil sand bitumen and its SARA fractions are shown in Figure 3(a) and 3(b), respectively. The FTIR spectra

temperature range. A possible reason to explain this observation was that the alkyl free radicals in resins produced by the intense cracking of alkyl chain structures were more than those in oil sand bitumen and aromatics on the basis of the order of resins > oil sand bitumen > aromatics for f P. Moreover, these alkyl free radicals could not only seize the active hydrogen atoms located at naphthenic rings to promote the ring-opening of naphthenic rings which was normally difficult to take place, but could also be further broken to form small molecule products and new alkyl free radicals.42 Besides, as shown in Figure 2(b)-1, the dw/dt of oil sand bitumen and aromatics was exceeded by that of resins at 462 and 475 °C, respectively. Meanwhile, the (dw/dt)max for resins was larger than that of oil sand bitumen and aromatics. These observations might be revealed by the fact that the free radical chain reactions were, to some extent, inhibited by the combination of these free radicals with other free radicals, especially the H free radicals donated from naphthenic rings and aromatic rings.43,50 Nevertheless, it might be suggested that the H free radicals mainly G


Article

Energy & Fuels

1606 cm−1 shown in Figure 3(b)-2, which was assigned to the stretching vibration of the CC bond in the aromatic ring skeleton, could also recognize the existence of light aromatics. Moreover, the monosubstituted aromatic hydrocarbons could be further indentified by the characteristic absorption peaks at 730 and 690 cm−1. Besides, the weak adsorption peak at 1267 cm−1 indicated the releasing of hydrogen sulfide in this stage, which was mainly attributed to the thermal decomposition of thioethers with weak thermal stability, such aryl sulfides, cyclic sulfides, and alkyl sulfides.43 3.4. Evolution Characteristic of Gaseous Products. As is know from the literature,54 it can be concluded from the Lambert−Beer law widely applied that there is a linear relationship between the absorbance of a typical functional group and the concentration of the gaseous product containing this functional group. Thus, the relative concentrations of the gaseous products evolved during the thermal cracking process of oil sand bitumen and its SARA fractions can be represented by the absorbance of the typical functional group corresponding to this gaseous product. The variations of the absorbance of several evolved gaseous products, such as CO2, CO, methane, ethylene, and light aromatics, as a function of temperature are shown in Figure 4(a) to 4(e) to study the formation mechanism of the evolved gaseous products. The evolution of CO2 and CO as the main oxygencontaining gaseous products was the most essential deoxygenation pathway during the thermal cracking process of oil sand bitumen and its fractions. The releasing of CO2 could be observed in the whole thermal cracking process for all samples, but the evolution behaviors of CO2 for these samples were different between each other. For instance, the evolution curves of CO2 for saturatthees and aromatics shown in Figure 4(a) both mainly displayed two peaks, while three peaks both existed in that for resins and asphaltenes. The differences in the number of peaks and temperature range corresponding to the main evolving interval among all samples might be mainly attributed to the differences in the kind and quantity of oxygen-containing functional groups. The formation of CO2 at low temperature was mainly attributed to the decomposition of carboxyl and ester groups with weak thermal stability, while CO2 released in the high temperature interval was generated from more stable ethers, ketones, and oxygen-containing heterocycles.23,43,55 Besides, it was found that the emission of CO2 for oil sand bitumen and its fractions, except asphaltenes, mainly occurred in the temperature range 100−350 °C, whereas the temperature at which the CO2 was emitted abundantly for asphaltenes ranged from 350 to 650 °C. The largest emission amount of CO2 represented by the largest absorbance of CO2 was ranked as follows: asphaltenes > resins > oil sand bitumen > saturates > aromatics, which was in agreement with the ranked order of oxygen content in oil sand bitumen and its SARA fractions as seen in Table 3. Figure 4(b) showed that the temperature ranges at which the evolution of CO occurred for oil sand bitumen and its SARA fractions were similar and in the range of about 400−800 °C. Two peaks in this temperature range can be observed in the evolution curve of CO for oil sand bitumen and its fractions. The first evolving peak of CO was ascribed to cracking and reforming of oxygen-containing groups, such as carbonyl and ether groups, especially the cleavage of ether bridges linking with the aromatic ring system,56 while the second increasing stage of CO might be a result of the Boudouard reaction, whose process is that the nascent coke exposed to freshly CO2 formed

at two representative temperatures corresponding to these two different stages, respectively, for oil sand bitumen and its fractions were chosen to investigate the distribution of the evolved gaseous products during the thermal cracking process. The main evolved gases and functional groups of gaseous products were recognized by the FTIR absorption bands of typical functional groups as reported in the literature.23,51−53 The spectra of evolved gaseous products at the temperatures 276 °C, 277 °C, 276 °C, 278 °C, and 276 °C in the volatilization stage for oil sand bitumen, saturates, aromatics, resins, and asphaltenes, respectively, were shown in Figure 3(a). It was observed that the distributions of the absorption bands in the FTIR spectra of gaseous the products released from oil sand bitumen and its fractions were different between each other. The significant absorption bands at around 2359 and 669 cm−1, which were observed in the FTIR spectra of gaseous products of all samples in this stage, indicated the formation of CO2, which was attributed to the decarboxylation reaction. The weak absorption bands at around 2172 and 2117 cm−1, which were the characteristic peaks of CO, were only observed in the FTIR spectra of saturates and aromatics. The existence of aliphatic hydrocarbons was evidenced by the absorption bands of 2958 cm−1, 2957 cm−1, 1457 cm−1, and 1376 cm−1. The characteristic adsorption bands of aliphatic hydrocarbons for saturates were obviously observed, while that for oil sand bitumen and other fractions were much weaker compared with saturates. The absorbances of the absorption bands of aliphatic hydrocarbons were ranked in this order: saturates > oil sand bitumen > aromatics > resins> asphaltenes, of which that of asphaltenes was almost zero. More gaseous products were evolved during the main reaction stage of thermal cracking process of oil sand bitumen and its SARA fractions, as shown in Figure 3(b), which exhibited the FTIR spectra of gaseous products released near Tmax. However, because the evolving of gaseous products at this temperature for saturates was mainly ascribed to the distillation of light components, the FTIR spectrum of gaseous products for saturates at 443 °C was selected to analyze the composition of gaseous products which were mainly produced from the cracking reaction of saturates. The spectra in this stage showed that the distributions of the absorption bands of the typical function groups of gaseous products for oil sand bitumen and its fractions were similar, but the intensities of the absorbances for a specific absorption band were different between each other. The characteristic adsorption peaks of CO2, CO, and light aliphatic hydrocarbons could also be observed in these spectra of this stage. Moreover, the absorbance of the absorption peaks corresponding to aliphatic hydrocarbons became larger than that in the distillation stage, which was ascribed to the rupture of C−C bonds in chain aliphatic hydrocarbons, the alkyl side chain located in cycloalkanes, and aromatics. The narrow adsorption bands at around 3014 and 1303 cm−1 indicated the presence of methane.23 Meanwhile, the evolution of aliphatic olefins whose structural formula is R−CHCH2 could be identified by the adsorption bands at 3081 cm−1, 1647 cm−1, 990 cm−1, and 911 cm−1, and the characteristic bands of 950 and 1647 cm−1 showed the releasing of ethylene as a specific species of olefins. As shown in Figure 3(b)-1, displaying the spectra at the wavenumber range from 3030 to 3090 cm−1 for oil sand bitumen and its SARA fractions, the absorption bands at 3063 and 3034 cm−1 were attributed to the stretching vibration of the C−H bond located in the aromatic ring of evolved light aromatics. The absorption peak at around H


Article

Energy & Fuels

Figure 4. Evolution curves of several gaseous products during the thermal cracking process of oil sand bitumen and its SARA fractions at 50 K/min.

The evolution profiles of ethylene released from oil sand bitumen and its fractions in Figure 4(d) showed that two peaks (i.e., a sharp one and a shoulder one) existed in that of oil sand bitumen, aromatics, and resins, while only one peak could be observed in that of saturates and asphaltenes. The temperature ranges in which ethylene were evolved from oil sand bitumen and its aromatic, resins, and asphaltenes fraction were similar, and all from 400 to 800 °C, while that for saturates ranged from 350 to 800 °C, which was the broadest compared to the others. The formation of ethylene for oil sand bitumen and its SARA fractions mainly occurred at the temperature range from it own initial temperature to 550 °C, and the temperature corresponding to the maximum emission peak of ethylene was at 492 °C, 526 °C, 501 °C, 501 °C, and 494 °C for oil sand bitumen and its SARA fraction, respectively. The first peak was generated from the fact that the penultimate β C−C bond in alkyl free radicals whose carbon atom had an unpaired electron located at one end of the alkyl free radical was ruptured based on the free radical mechanism. These radicals were formed via the fragmentation of aliphatic hydrocarbons (especially the n-alkanes) and alkyl side-chains attached to naphthenic rings and aromatic rings.

during the thermal cracking process occurring for the gasification reaction of carbon in coke to transform into CO as indicated by the concomitant rapid rate decay of the CO2 emission amount.29,57 The releasing curves of methane versus temperature for oil sand bitumen and its SARA fractions were shown in Figure 4(c). Except saturates, the variations of the absorbance of methane with temperature for oil sand bitumen and its other fractions were similar, and all exhibited two peaks which consisted of a sharp peak with the largest absorbance at the temperature approaching the Tmax, and a shoulder peak at a wider temperature range of about 550−800 °C. The formation of methane at the temperature range corresponding to the first peak mainly resulted from the cracking of methyl (−CH3−) and methylene (−CH2−) located at aliphatic hydrocarbons and alkyl side chains attaching to aromatic rings and naphthenic rings. Moreover, the demethylation of the methoxyl groups could also, to some extent, contribute to the methane evolution. In addition, the evolving of methane in the higher temperature range might be attributed to the cleavage of aryl-methyl groups with strong bond energy. I


Article

Energy & Fuels

Figure 5. Curves of dα/dt versus temperature for oil sand bitumen and its fractions at different heating rates.

In addition, the cleavage of the C−C bond in macromolecular olefins in samples or products generated from the thermal cracking process of samples could also contribute to a little emission of ethylene.43,58 As the temperature further increased, the emission amount of ethylene for oil sand bitumen, aromatics, and resins continually increased to form a shoulder peak. This observation might be attributed to not only cleavage of the β C−C bond in the alkyl radicals which were formed by the breakage of the shorter alkyl side chains and the dealkylation of the aromatic ring or naphthenic rings (i.e., the rupture of Caromatic−Cα or Cnaphthenic−Cα), but also cleavage of the β C−C bond of olefins generated from the ring-opening of naphthenic hydrocarbons.59 The evolution curves of light aromatics for oil sand bitumen and its fractions shown in Figure 4(e) all mainly exhibited two emission peaks, and the temperatures corresponding to the main emission peak were near 570 °C for saturates and 490 °C for oil sand bitumen and its other fractions. The absorbance of light aromatics evolved from saturates corresponding to the peak in the high temperature interval from 450 to 750 °C was larger than that in the relative low temperature interval from 200 to 450 °C. Nevertheless, this trend for oil sand bitumen and its other fractions was opposite to that for saturates. This could be attributed to the fact that aromatic ring structures with

only one ring in saturates shown in Table 4 were likely to form the monocyclic aromatic hydrocarbons with low boiling point by the cleavage of alkyl side chains located in aromatic rings in the relatively low temperature interval, while the aromatic hydrocarbon released from saturates in the high temperature interval was mainly attributed to the dehydro-aromatization of naphthenic rings. Based on the fact that RN for saturates was about two, and f N was obviously larger than fA, the emission amount of light aromatics for saturates at high temperature was larger than that at relative low temperature. However, the condensation degree of the aromatic structure and RN and RA for the average molecular structure of oil sand bitumen, aromatics, resins, and asphaltenes were obviously larger than those of saturates, meaning that, compared to saturates, higher temperature was required to grow the cracking depth of the aromatic ring structure for oil sand bitumen and its aromatics, resins, and asphaltenes. Thus, the evolving of light aromatics was accelerated. Because the aromatic structure mainly occurred with dehydrogenation of the naphthenic ring and condensation of the aromatic ring to further form coke in the high temperature interval from 550 to 750 °C, the emission amount of the light aromatics in the relatively low temperature interval from 400 to 550 °C was obviously larger than that in the high J


Article

Energy & Fuels

Figure 6. Fitting straights of ln(dα/dt) versus 1/T of oil sand bitumen and its SARA fractions.

temperature to reach the balance of the temperature of samples during the thermal cracking process.63 The values of dα/dt and temperature corresponding to the specific conversion rate ranging from 0.1 to 0.9 at different heating rates (10, 20, 50, 80, 120, 200, 500, and 800 K/min) were taken with the Friedman procedure described by the eq 4 to calculate the activation energy E and pre-exponential factors A. At the specific conversion rate ranging from 0.1 to 0.9, fitting straights of ln(dα/dt) versus 1/T for oil sand bitumen and its SARA fractions are shown in Figure 6. The activation energy E, the pre-exponential factor A, and the correlation coefficients R2 of oil sand bitumen and its SARA fractions are listed in Table 6. It was known from Figure 6 and Table 6 that linear fitting results of ln(dα/dt) versus 1/T for oil sand bitumen and its SARA fractions showed good correlation coefficients R2, which were all larger than 0.96; especially, those of the saturates all exceeded 0.99 in the whole conversion rate regions. The relationships between activation energy and conversion rates for oil sand bitumen and its SARA fractions were revealed in Figure 7. With the conversion rate increasing, the activation energy of saturates increased gradually, while that of oil sand bitumen and other fractions increased first and then decreased. For oil sand bitumen, saturates, aromatics, resins,

temperature interval for oil sand bitumen and its aromatics, resins, and asphaltenes. 3.5. Kinetics of Oil Sand Bitumen and Its SARA Fractions. As was shown in Figure 5, the variations of thermal cracking reaction rate (dα/dt) with temperature for oil sand bitumen and its SARA fractions at different heating rates were similar, and all presented a trend of increasing first and then decreasing with the increasing temperature. In addition, as the heating rate increased, the maximum values of dα/dt for oil sand bitumen and its SARA fractions gradually increased. The reason for this was that the larger instantaneous thermal energy produced at higher heating rate promoted the cleavage of the chemical bond (especially the stable chemical bond) to enhance the reaction depth of thermal cracking, and facilitate efficient transfer of thermal cracking products.60,61 Moreover, the temperature corresponding to the maximum value of dα/dt all also increased gradually with the increasing heating rates, which was ascribed to the heat transfer limitations. The further explanation for this was that, in the same temperature range, the retention time at the higher heating rate was much shorter than that at the lower heating rate.62 Therefore, the heat transfer between the surroundings and the interiors of the samples at higher heating rate was not effective so as to require higher K


Article

0.977 0.971 0.961 0.965 0.972 0.973 0.970 0.975 0.980 0.991 0.993 0.993 0.994 0.997 0.996 0.989 0.992 10 1017 1018 1018 1018 1018 1018 1018 1018 1018 1017 1016 1017 1017 1016 1015 1015 223.28 248.32 253.48 256.89 261.58 265.86 263.14 266.50 264.46 261.91 252.45 242.63 244.69 247.27 244.83 228.06 235.22 × × × × × × × × × × × × × × × × ×

10 1014 1015 1016 1017 1017 1017 1017 1017 1017 1017 1016 1016 1016 1016 1015 1015

0.983 0.982 0.987 0.987 0.988 0.982 0.985 0.982 0.984 0.983 0.987 0.989 0.993 0.994 0.996 0.994 0.989

1.09 5.44 1.03 1.50 2.86 5.16 3.12 4.90 3.37 2.21 4.81 9.51 1.11 1.37 7.33 4.32 7.93

× × × × × × × × × × × × × × × × ×

16

A, min−1 E, kJ/mol

2.30 2.85 1.92 2.14 1.17 2.00 5.60 5.97 6.85 3.70 2.20 8.14 5.41 2.97 1.47 4.91 1.06 159.56 203.37 216.03 231.40 242.58 246.33 253.10 253.81 255.05 251.49 248.77 243.02 240.98 237.88 234.18 228.22 219.71 0.986 0.989 0.984 0.973 0.971 0.971 0.974 0.980 0.982 0.989 0.992 0.994 0.994 0.996 0.997 0.996 0.998 10 107 107 109 1010 1012 1013 1014 1015 1015 1015 1015 1015 1015 1014 1013 1012

× × × × × × × × × × × × × × × × × 2.56 2.59 9.77 1.22 4.15 1.49 2.16 3.74 1.54 6.50 6.31 4.72 2.72 1.55 2.86 3.60 6.30 85.50 99.78 109.32 125.25 147.01 169.71 186.82 204.97 214.40 223.94 224.27 222.81 219.72 216.65 206.24 193.39 182.68 0.997 0.996 0.995 0.994 0.995 0.995 0.996 0.997 0.998 0.998 0.998 0.997 0.997 0.997 0.998 0.999 0.999 × × × × × × × × × × × × × × × × × 6.06 3.86 5.79 7.79 1.05 1.68 2.12 3.11 3.46 4.40 4.44 6.45 8.18 1.43 4.43 1.22 4.92 77.29 76.57 79.35 81.63 84.07 87.28 89.42 92.30 93.81 95.96 96.81 99.73 101.91 106.06 113.82 121.10 131.08 0.961 0.977 0.980 0.971 0.970 0.985 0.985 0.983 0.981 0.983 0.986 0.989 0.990 0.992 0.991 0.988 0.981 10 107 108 109 1010 1013 1014 1015 1015 1015 1015 1015 1014 1014 1014 1013 1012

A, min E, kJ/mol R

93.74 101.03 112.59 127.07 150.51 183.09 200.63 211.33 214.36 214.44 215.99 214.45 211.70 207.14 201.19 195.059 186.24 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90

1.93 4.15 2.10 1.83 7.95 1.54 2.30 1.10 1.58 1.44 1.70 1.23 7.42 3.35 1.22 4.20 9.09

× × × × × × × × × × × × × × × × ×

7

E kJ/mol α

A, min

10 106 106 106 107 107 107 107 107 107 107 107 107 108 108 109 109

R A, min E, kJ/mol R A, min E, kJ/mol

6

R

and asphaltenes, the range of activation energy was 93.74− 215.99 kJ/mol, 76.57−131.08 kJ/mol, 85.50−224.27 kJ/mol, 159.56−255.05 kJ/mol, and 223.28−266.5 kJ/mol at the conversion rate range from 0.1 to 0.9, respectively. In addition, the conversion rate corresponding to the maximum activation energy was 0.6, 0.6, 0.5, and 0.45 for oil sand bitumen, aromatics, resins, and asphaltenes, respectively. During the thermal cracking process, the activation energy of asphaltenes was larger than that of oil sand bitumen and other fractions at the same conversion rate, following the order resins > aromatics ≈ oil sand bitumen > saturates. Because the activation energy represented the minimum energy requirement for the reaction to occur,22 differences in activation energy between each other at the same conversion rate to some extent reflected the difficulty of the thermal cracking process. These differences can be attributed to differences in the structure and composition among oil sand bitumen and its fractions. It was observed from the plots of activation energy as a function of conversion rate for saturates in Figure 7 that the increasing trend of activation energy with conversion rate in the range 0.1−0.7 was relatively weaker than that for the conversion rate in the range 0.7−0.9. At the low conversion rate (from 0.1 to 0.7), the reaction mainly contained the volatilization of low-boiling components, and cleavage of weak chemical bonds. However, at the higher conversion rate (from 0.7 to 0.9), the thermal cracking process of saturates required higher energy to promote the cleavage of shorter alkyl side chains originated from the cracking of long side chains, the ring opening reaction, and dehydrogenation of naphthenic rings. Thus, these reactions could be responsible for the significant increase of activation energy for saturates. Because the average structure of saturates is mainly composed of aliphatic chains and a naphthenic structure with only two rings shown in Table 4, the condensation reactions of polyaromatics to form coke are almost not occurring so as to not require much higher energy compared with the oil sand bitumen and other fractions. The activation energy of aromatics was lower than that of resins and asphaltenes within the conversion rates of 0.1−0.9, which was ascribed to the relatively simpler molecular structure of aromatics than that of resins and asphaltenes, evidenced by the lower molecular weight, with the larger HAU/CA meaning a lower condensation degree and a lower RT for aromatics. The value of the activation energy of asphaltenes was larger than that of resins at the same conversional rate. However, it was worth noting that the difference in the values of activation energy for resins and asphaltenes at the same conversion rate

6

−1

Resins

2 −1

Aomatics

2

−1

Saturates

2 −1

Oil sand bitumen

Table 6. Thermal Cracking Kinetic Parameters of Oil Sand Bitumen and Its SARA Fractions Calculated by the Friedman Procedure

Figure 7. Activation energy as a function of conversion rates of oil sand bitumen and its SARA fractions.

11

2

Asphaltenes

R2

Energy & Fuels

L


Article

Energy & Fuels

The compositions of gaseous products produced in the main reaction stage of oil sand bitumen and its fractions were similar to each other. In addition to CO2 and CO, more species of gaseous products were detected, such as CH4, C2H4, other light aliphatic hydrocarbons and olefins, light aromatics, and hydrogen sulfide. (4) All curves of the absorbance of several typical gaseous products (CO2, CO, CH4, C2H4, and light aromatics) with temperature exhibited multimodal evolution peaks. The evolving behaviors of CO2 and CO with temperature for oil sand bitumen and its SARA fractions were both different between each other. However, except saturates, the evolving behaviors of CH4, C2H4, and light aromatics with temperature for oil sand bitumen and other fractions were similar, and the temperature corresponding to the major evolving peak got close to Tmax. In addition, the release behaviors of typical gaseous products for oil sand bitumen and its SARA fractions were different due to the differences in composition and structure between each other. (5) The correlation coefficients R2 at each conversion rate ranged from 0.1 to 0.9 for oil sand bitumen, and its fractions were all greater than 0.96; especially, that of the saturates exceeded 0.99 in the whole conversion rate region. The ranges of activation energy for oil sand bitumen, saturates, aromatics, resins, and asphaltenes were 93.74−215.99 kJ/mol, 76.57−131.08 kJ/mol, 85.50−224.27 kJ/mol, 159.56−255.05 kJ/mol, and 223.28−266.5 kJ/mol in the conversion rate range from 0.1 to 0.9, respectively. The variation of activation energy with the conversion rate (α) for oil sand bitumen was influenced by the interaction between SARA fractions occurring during the thermal cracking process of oil sand bitumen.

gradually decreased with the conversion rate increasing. This was attributed to the fact that the length and the number of the alkyl chains contained in the average molecular structure of resins were longer and more compared to those of asphaltenes, respectively, which was concluded by the comparison of the average structural parameters between resins and asphaltenes. Thus, resins had easier occurrance of breakage of side chains at lower conversion rate with relatively lower activation energy. However, with the depth of thermal cracking reaction increasing, the free radicals formed from the rupture of alkyl chains facilitate dehydro-aromatization of naphthenic rings and the condensation of aromatic rings in resins to form compounds with more condensed structure, e.g. asphaltenes. These asphaltenes formed from resins further condensed to generate the coke with higher activation energy are similar to the coke formation reaction of asphaltenes derived from oil sand bitumen, which is responsible for deducing the difference of the value of activation energy between resins and asphaltenes at high conversion rate. The variation of the activation energy of oil sand bitumen with the conversion rate in the range 0.1−0.9 was similar to that of aromatics in range, and the activation energy for oil sand bitumen in the whole thermal cracking process fell in between SARA fractions. This observation might also be attributed to the interaction between SARA fractions during the thermal cracking process of oil sand bitumen, rather than the simple weight stacking of thermal cracking behavior of each fraction.

4. CONCLUSION The thermal cracking behaviors, online analysis of evolved gaseous products, and kinetics for oil sand bitumen and its SARA fractions (saturates, aromatics, resins, asphaltenes) were studied using TG−FTIR in this paper. According to the above analysis, the following conclusions can be obtained: (1) On the basis of the FTIR analysis, the distribution of functional groups in oil sand bitumen was similar to that of asphaltenes and resins. Moreover, the species and amounts of the polar functional groups of these samples above were both more than those of saturates and aromatics. Combining the average structural parameters, saturates were seen as the constituent with the simplest average molecular structure compared to the others, mainly consisting of alkyl chains and a naphthenic ring structure, while asphaltenes were the constituent with the highest condensation degree and the most aromatic ring structures compared to others in oil sand bitumen. (2) The thermal cracking process of oil sand bitumen and its fractions mainly consisted of two stages, including the volatilization stage and the main reaction stage, i.e. the main weight loss stage. The largest weighted coke yield for asphaltenes compared with that of others indicated that asphaltenes were the main contributor to coke formation of oil sand bitumen. It was suggested that the interactive effect between SARA fractions existed during the thermal cracking process of oil sand bitumen through the comparison between the coke yield of oil sand bitumen and the sum of weighted coke yields of SARA fractions. (3) Light aliphatic hydrocarbons and CO2 were released during the distillation stage of oil sand bitumen and its fractions, in which the weak absorption band of CO could only be observed in the spectra of saturates and aromatics.

■

AUTHOR INFORMATION

Corresponding Authors

*(Y. Tian). Tel.: +86-532-86981711; E-mail address: [email protected]. *(Y. Qiao). Tel.: +86-532-86981711; E-mail address: [email protected]. ORCID

Junhui Hao: 0000-0001-7058-4804 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Natural Science Foundation of China (21576293 and 21576294), the Natural Science Foundation of Shandong Province (BS2015NJ006), and the Fundamental Research Funds for the Central Universities (15CX02020A and 15CX05044A).

■

M

NOMENCLATURE E = activation energy (kJ/mol) A = pre-exponential factor (min−1) f(α) = differential form of reaction mechanism function of α α = reaction conversional rate at temperature T or time t m0 = the initial mass of the sample mt = the mass of the sample at time t m∞ = the final mass of the sample R = universal gas constant (8.3145 J/(mol·K)) DOI: 10.1021/acs.energyfuels.6b02598 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels dα/dt = thermal cracking reaction rate (min−1) β = heating rate (K/min) M = average molecular weight CT = total number of carbons in sample fA = fraction of aromatic carbon f N = fraction of naphthenic carbon f P = fraction of paraffinic carbon RT = number of total rings RA = number of aromatic rings RN = number of naphthenic rings n = number of structure unit sheets usw = unit sheet weight RT* = number of total rings in one structure unit sheet RA* = number of aromatic rings in one structure unit sheet RN* = number of naphthenic rings in one structure unit sheet HAU/CA = condensation degree parameter of the aromatic ring system σ = the degree of substituent of peripheral hydrogen in the aromatic ring system Ti = the initial temperature of the intense weight loss temperature interval (°C) Tf = the final temperature of the intense weight loss temperature interval (°C) Tmax = the temperature at the maximum weight loss rate (°C) dw/dt = the absolute value of the weight loss rate (%/min) (dw/dt)max = the absolute value of the maximum weight loss rate (%/min) αM = conversion rate at the maximum weight loss rate (%)

■

(12) Pakdel, H.; Roy, C. Recovery of Bitumen by Vacuum Pyrolysis of Alberta Tar Sands. Energy Fuels 2003, 17 (5), 1145−1152. (13) Meng, M.; Hu, H.; Zhang, Q.; Li, X.; Wu, B. Pyrolysis Behaviors of Tumuji Oil Sand by Thermogravimetry (TG) and in a Fixed Bed Reactor. Energy Fuels 2007, 21 (4), 2245−2249. (14) Park, Y. C.; Paek, J. Y.; Bae, D. H.; Shun, D. Study of Pyrolysis Kinetics of Alberta Oil Sand by Thermogravimetric Analysis. Korean J. Chem. Eng. 2009, 26 (6), 1608−1612. (15) Ma, Y.; Li, S. Study of the Characteristics and Kinetics of Oil Sand Pyrolysis. Energy Fuels 2010, 24 (3), 1844−1847. (16) Jia, C. X.; Wang, Q.; Ge, J. X.; Xu, X. F. Pyrolysis and combustion model of oil sand from non-isothermal thermogravimetric analysis data. J. Therm. Anal. Calorim. 2014, 116 (2), 1073−1081. (17) Liu, P. F.; Zhu, M. M.; Zhang, Z. Z.; Zhang, D. K. Pyrolysis of an Indonesian Oil Sand in a Thermogravimetric Analyser and a FixedBed Reactor. J. Anal. Appl. Pyrolysis 2016, 117, 191−198. (18) Strausz, O. P.; Jha, K. N.; Montgomery, D. S. Chemical composition of gases in Athabasca bitumen and in low-temperature thermolysis of oil sand, asphaltene and maltene. Fuel 1977, 56 (2), 114−120. (19) Fletcher, J. V.; Deo, M. D.; Hanson, F. V. Fluidized bed pyrolysis of a Uinta Basin oil sand. Fuel 1995, 74 (3), 311−316. (20) Eshraghian, A.; Husein, M. M. Thermal cracking of Athabasca VR and bitumen and their maltene fraction in a closed reactor system. Fuel 2017, 190, 396−408. (21) Yan, J. W.; Jiang, X. M.; Han, X. X.; Liu, J. G. A TG−FTIR investigation to the catalytic effect of mineral matrix in oil shale on the pyrolysis and combustion of kerogen. Fuel 2013, 104 (2), 307−317. (22) Ma, Z. Q.; Chen, D. Y.; Gu, J.; Bao, B. F.; Zhang, Q. S. Determination of pyrolysis characteristics and kinetics of palm kernel shell using TGA-FTIR and model-free integral methods. Energy Convers. Manage. 2015, 89, 251−259. (23) Jia, C. X.; Wang, Z. C.; Liu, H. P.; Bai, J. R.; Chi, M. S.; Wang, Q. Pyrolysis behavior of Indonesia oil sand by TG−FTIR and in a fixed bed reactor. J. Anal. Appl. Pyrolysis 2015, 114, 250−255. (24) Duan, L.; Zhao, C.; Zhou, W.; Qu, C.; Chen, X. Investigation on Coal Pyrolysis in CO2 Atmosphere. Energy Fuels 2009, 23 (23), 3826− 3830. (25) Zhao, Y. P.; Hu, H. Q.; Jin, L. J.; He, X. F.; Zhu, S. W. Pyrolysis Behavior of Macerals from Weakly Reductive Coals. Energy Fuels 2010, 24 (12), 6314−6320. (26) Yang, J. B.; Cai, N. S. A TG−FTIR study on catalytic pyrolysis of coal. J. Fuel Chem. Technol. 2006, 34 (6), 650−654. (27) Chen, D. Y.; Liu, D.; Zhang, H. R.; Chen, Y.; Li, Q. Bamboo pyrolysis using TG−FTIR and a lab-scale reactor: Analysis of pyrolysis behavior, product properties, and carbon and energy yields. Fuel 2015, 148, 79−86. (28) Tao, L.; Zhao, G. B.; Qian, J.; Qin, Y. k TG−FTIR characterization of pyrolysis of waste mixtures of paint and tar slag. J. Hazard. Mater. 2010, 175 (1−3), 754−61. (29) Scaccia, S. TG−FTIR and kinetics of devolatilization of Sulcis coal. J. Anal. Appl. Pyrolysis 2013, 104 (104), 95−102. (30) Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N. ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data. Thermochim. Acta 2011, 520 (1−2), 1−19. (31) Liu, P. F.; Zhu, M. M.; Zhang, Z. Z.; Wan, W. C.; Yani, S.; Zhang, D. K. Thermogravimetric Studies of Characteristics and Kinetics of Pyrolysis of Buton Oil Sand. Energy Procedia 2014, 61, 2741−2744. (32) Schucker, R. C. Thermogravimetric determination of the coking kinetics of Arab heavy vacuum residuum. Ind. Eng. Chem. Process Des. Dev. 1983, 22 (4), 615−619. (33) Alvarez, E.; Marroquín, G.; Trejo, F.; Centeno, G.; Ancheyta, J.; Díaz, J. A. I. Pyrolysis kinetics of atmospheric residue and its SARA fractions. Fuel 2011, 90 (12), 3602−3607. (34) Li, S. Y.; Wang, J. Q.; Tan, H. P.; Wu, Z. L. Study of extraction and pyrolysis of Chinese oil sands. Fuel 1995, 74 (8), 1191−1193.

REFERENCES

(1) Corma, A.; Díaz-Cabañas, M. J.; Martínez-Triguero, J.; Rey, F.; Rius, J. A large-cavity zeolite with wide pore windows and potential as an oil refining catalyst. Nature 2002, 418 (6897), 514−517. (2) Zhang, A. G.; Gao, J. S.; Wang, G.; Xu, C. M.; Lan, X. Y.; Ning, G. Q.; Liang, Y. M. Reaction Performance and Chemical Structure Changes of Oil Sand Bitumen during the Fluid Thermal Process. Energy Fuels 2011, 25 (8), 3615−3623. (3) Wang, T.; Zhang, C.; Zhao, R. Y.; Zhu, C. J.; Yang, C. H.; Liu, C. G. Solvent Extraction of Bitumen from Oil sand. Energy Fuels 2014, 28 (4), 2297−2304. (4) Zhang, K. H.; Bin, Y. F. Study on the process conditions of extracting bitumen from Indonesia oil sand with organic solvent. Pet. Process. Petrochem. 2008, 39 (6), 63−66. (5) Schramm, L. L.; Morrison, C.; Stasiuk, E. N. Some effects of chemical additions to nascent primary froth from the hot water flotation of bitumen from Athabasca oil sand. Fuel Process. Technol. 1998, 56 (3), 243−261. (6) La, H.; Guigard, S. E. Extraction of hydrocarbons from Athabasca oil sand slurry using supercritical carbon dioxide. J. Supercrit. Fluids 2015, 100, 146−154. (7) Zhao, D. Z.; Sun, W. W.; Sun, M. Z. The Separating of Inner Mongolian Oil Sand with Ultrasound. Pet. Sci. Technol. 2011, 29 (24), 2530−2535. (8) Fletcher, J. V.; Deo, M. D.; Hanson, F. V. Fluidized bed pyrolysis of a Uinta Basin oil sand. Fuel 1995, 74 (3), 311−316. (9) Morimoto, M.; Sugimoto, Y.; Sato, S.; Takanohashi, T. Comparison of Thermal Cracking Processes for Athabasca Oil Sand Bitumen: Relationship between Conversion and Yield. Energy Fuels 2014, 28 (10), 6322−6325. (10) Murugan, P.; Mani, T.; Mahinpey, N.; Dong, M. Z. Pyrolysis kinetics of Athabasca bitumen using a TGA under the influence of reservoir sand. Can. J. Chem. Eng. 2012, 90 (2), 315−319. (11) Khraisha, Y. H. Study of extraction and pyrolysis of Jordan tar sand. Int. J. Energy Res. 1999, 23 (10), 833−839. N


Article

Energy & Fuels (35) Ma, Y.; Li, S. Y. The pyrolysis, extraction and kinetics of Buton oil sand bitumen. Fuel Process. Technol. 2012, 100, 11−15. (36) Hu, J.; Xiao, R.; Shen, D. K.; Zhang, H. Y. Structure analysis of lignin residue from black liquor and its thermal performance in thermogravimetric-Fourier transform infrared spectroscopy. Bioresour. Technol. 2013, 128c (1), 633−639. (37) He, L.; Li, X. G.; Wu, G. Z.; Lin, F.; Sui, H. Distribution of Saturates, Aromatics, Resins, and Asphaltenes Fractions in the Bituminous Layer of Athabasca Oil Sands. Energy Fuels 2013, 27 (8), 4677−4683. (38) Yoon, S.; Bhatt, S. D.; Lee, W.; Lee, H. Y.; Jeong, S. Y.; Baeg, J. O.; Lee, C. W. Separation and characterization of bitumen from Athabasca oil sand. Korean J. Chem. Eng. 2009, 26 (1), 64−71. (39) Wang, Z. C.; Ge, Y.; Shui, H. F.; Ren, S. B.; Pan, C. X.; Kang, S. G.; Lei, Z. P.; Zhao, Z. J.; Hu, J. C. Molecular structure and size of asphaltene and preasphaltene from direct coal liquefaction. Fuel Process. Technol. 2015, 137, 305−311. (40) Liang, W. J.; Que, G. H.; Chen, Y. Z. Chemical composition and structure of vacuum residues of Chinese crudesII·average structure of vacuum residues and their fractions. Acta Pet. Sin. (Pet. Process. Sect.) 1991, 7 (4), 1−11. (41) Wang, Q.; Jia, C. X.; Ge, J. X.; Guo, W. X. 1H NMR and 13C NMR Studies of Oil from Pyrolysis of Indonesian Oil Sands. Energy Fuels 2016, 30, 2478−2491. (42) Liang, W. J. Heavy Oil Chemistry; China University of Petroleum Press: Dongying, Shandong, China, 2000. (43) Liang, W. J.; Que, G. H.; Liu, C. G.; Yang, Q. S. Petroleum chemistry, 2nd ed.; China University of Petroleum Press: Dongying, Shandong, China, 2009. (44) Kapadia, P. R.; Kallos, M. S.; Gates, I. D. A review of pyrolysis, aquathermolysis, and oxidation of Athabasca bitumen. Fuel Process. Technol. 2015, 131, 270−289. (45) Yang, J. T.; Chen, J. R.; Sun, Z. C.; Liu, S. Q.; Zhang, R. D.; Fan, Y. H. Kinetic investigation of thermolysis for four subfraction from Shengli Vacuum Residue. J. Univ. Pet. 1992, 16 (4), 61−65. (46) Deng, W. A.; Que, G. H. Thermal reaction characteristics of ShengLi vacuum residue constituents. J. China Univ. Pet.(Ed. Nat. Sci.) 1997, 21 (6), 67−70. (47) Karacan, O.; Kok, M. V. Pyrolysis Analysis of Crude Oils and Their Fractions. Energy Fuels 1997, 11 (2), 385−391. (48) Trejo, F.; Rana, M. S.; Ancheyta, J. Thermogravimetric determination of coke from asphaltenes, resins and sediments and coking kinetics of heavy crude asphaltenes. Catal. Today 2010, 150 (3), 272−278. (49) Dong, X. G.; Lei, Q. F.; Fang, W. J.; Yu, Q. S. Thermogravimetric analysis of petroleum asphaltenes along with estimation of average chemical structure by nuclear magnetic resonance spectroscopy. Thermochim. Acta 2005, 427 (1−2), 149− 153. (50) Rahimi, P.; Gentzis, T.; Kubo, J.; Fairbridge, C.; Khulbe, C. Coking propensity of Athabasca bitumen vacuum bottoms in the presence of H-donors−formation and dissolution of mesophase from a hydrotreated petroleum stream (H-donor). Fuel Process. Technol. 1999, 60 (2), 157−170. (51) Li, B. S.; Lv, W.; Zhang, Q.; Wang, T. J.; Ma, L. L. Pyrolysis and Catalytic Pyrolysis of Industrial Lignins by TG−FTIR: Kinetics and Products. J. Anal. Appl. Pyrolysis 2014, 108 (7), 295−300. (52) Kaljuvee, T.; Pelt, J.; Radin, M. TG−FTIR study of gaseous compounds evolved at thermooxidation of oil shale. J. Therm. Anal. Calorim. 2004, 78 (2), 399−414. (53) Nola, G. D.; Jong, W. D.; Spliethoff, H. TG−FTIR characterization of coal and biomass single fuels and blends under slow heating rate conditions: Partitioning of the fuel-bound nitrogen. Fuel Process. Technol. 2010, 91 (1), 103−115. (54) Liu, Q.; Wang, S. R.; Zheng, Y.; Luo, Z. Y.; Cen, K. F. Mechanism study of wood lignin pyrolysis by using TG−FTIR analysis. J. Anal. Appl. Pyrolysis 2008, 82 (1), 170−177.

(55) Wang, S. R.; Ru, B.; Lin, H. Z.; Sun, W. X.; Luo, Z. Y. Pyrolysis behaviors of four lignin polymers isolated from the same pine wood. Bioresour. Technol. 2015, 182, 120−127. (56) Fu, P.; Hu, S.; Xiang, J.; Li, P. S.; Huang, D.; Jiang, L.; Zhang, A. C.; Zhang, J. Y. FTIR study of pyrolysis products evolving from typical agricultural residues. J. Anal. Appl. Pyrolysis 2010, 88 (2), 117−123. (57) Ritchie, R. G. S.; Roche, R. S.; Steedman, W. Non-isothermal programmed pyrolysis studies of oil sand bitumens and bitumen fractions: 1. Athabasca asphaltene. Fuel 1985, 64 (3), 391−399. (58) Hunter, K. C.; East, A. L. L. Properties of C−C bonds in nalkanes: relevance to cracking mechanisms. J. Phys. Chem. A 2002, 106 (7), 1346−1356. (59) Gray, M. R.; Mccaffrey, W. C. Role of Chain Reactions and Olefin Formation in Cracking, Hydroconversion, and Coking of Petroleum and Bitumen Fractions. Energy Fuels 2002, 16 (3), 756− 766. (60) Quan, C.; Li, A.; Gao, N. Thermogravimetric analysis and kinetic study on large particles of printed circuit board wastes. Waste Manage. 2009, 29 (8), 2353−2360. (61) Slopiecka, K.; Bartocci, P.; Fantozzi, F. Thermogravimetric analysis and kinetic study of poplar wood pyrolysis. Appl. Energy 2012, 97, 491−497. (62) Al-Otoom, A.; Al-Harahsheh, M.; Allawzi, M.; Kingman, S.; Robinson, J.; Al-Harahsheh, A.; Saeid, A. Physical and thermal properties of Jordanian tar sand. Fuel Process. Technol. 2013, 106 (2), 174−180. (63) Chen, D.; Zhou, J.; Zhang, Q. Effects of heating rate on slow pyrolysis behavior, kinetic parameters and products properties of moso bamboo. Bioresour. Technol. 2014, 169 (5), 313−319.

O


Thermal Cracking Characteristics and Kinetics of Oil Sand Bitumen

Recommend Documents