Thermal Characteristics and Combustion Kinetics Analysis of Heavy

Article pubs.acs.org/IECR

Thermal Characteristics and Combustion Kinetics Analysis of Heavy Crude Oil Catalyzed by Metallic Additives Wan-Fen Pu,* Peng-Gang Liu,* Yi-Bo Li, Fa-Yang Jin, and Zhe-Zhi Liu State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China ABSTRACT: This work reports the investigation of the catalytic effect of metallic additives on heavy oil oxidation during the air injection process through thermogravimetric testing. The results indicate that the crude oil and oil mixed with metallic additives, combusted in an air atmosphere, exhibit three different types of reactions defined as low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO). Because of differences in individual catalytic activity and specific surface area effects, metallic additives exhibit varied catalytic effects on heavy oil oxidation. Meanwhile, the difference of activation energy reduction between LTO and HTO is analyzed in depth to present new insights into the catalytic effect of metallic additives on asphaltene and mechanistic understanding in terms of microscopic molecular structure. Through the combined analysis of thermal behavior and combustion kinetics, CuCl2 is found to be an excellent catalyst for upgrading the performance of an air injection project through positively influencing the oxidation reactions of Tahe heavy crude oil.

1. INTRODUCTION Recent decades have seen a remarkable increase of demand for crude oil. However, the supply from mature reservoirs is limited or even declining because most of the conventional light and medium oilfields have gradually entered into the high water cut period. As a result, more and more attention is focused on the heavy oil resources of huge reserves. 1 However, the composition complexity, high viscosity, and correspondingly poor flowing property of heavy crude oils make economical and efficient production extremely difficult.2 Among a variety of enhanced oil recovery technologies, such as thermal recovery (steam flooding, cyclic steam stimulation, steam-assisted gravity drainage), chemical recovery, microbial recovery, etc., the application of in situ combustion (ISC) process has been proven to be a promising strategy for the exploitation of heavy oil reservoirs. A combustion front is generated within the oilbearing reservoir through continuously injecting high-pressure air. Violent heat release and the resulting temperature increase significantly lower heavy oil viscosity. The activated oil is driven forward with improved mobility by miscible and/or immiscible flue gas flooding coupled with thermal effects. Commercial thermal analyzers such as thermogravimetric/ differential thermogravimetric (TG/DTG) analysis, differential thermal analysis (DTA), and differential scanning calorimetry (DSC) have been widely applied in the research of thermal characteristics and combustion kinetics of oil oxidation because of their significant economic benefits and relatively easy operation. Kök et al.3−6 took advantage of TG/DTG, DTA, and DSC to study oxidation behaviors of different °API gravity oils and identified three totally different reactions (lowtemperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO)). The heat values and kinetics had also been determined by DSC and TG data. The activation energy calculated for HTO was higher than that for LTO, and higher asphaltene content could lead to the increase of activation energy value of oil. Fan et al.7 investigated the lowtemperature oxidation characteristics of heavy oil via TG/DSC © 2015 American Chemical Society

and pressure differential scanning calorimeter (PDSC); a violent exothermic result was detected when the oxygen partial pressure increased. The chemical nature of each SARA fraction is an important factor affecting the oxidation behavior of the whole oil. In general, oil with more heavy components releases more heat during the heating process and holds higher apparent activation energy.8−10 Meanwhile, the addition of clay minerals has non-negligible impact on oxidation process due to their catalytic properties and surface area effects.11,12 So far, much work has also been done regarding the influence of metallic additives on crude oil. Abuhesa et al.13 carried out a comparative experiment to analyze the effect of catalyst on heavy oil recovery, and the results reflected that the combustion process was promoted and produced oil was upgraded when the catalyst was introduced. Kök and Bagci14,15 applied thermal analysis techniques to the combustion of crude oils and detected that temperature intervals of characteristic reactions and corresponding activation energy were changed considerably when metal oxides were added. Wang et al.16 performed low-temperature oxidation experiments to investigate the effects of different organometallic additives on crude oil oxidation behavior. O2 consumption capability of oil was enhanced, and more heavy components were generated in relative to blank experiment. Rezaei et al.17 employed simultaneous TG and DSC tests to characterize thermocatalytic effect of nanoparticles on light oil oxidation and cracking reaction. Results showed that activation energy experienced a significant decline in HTO stages, indicating the powerful catalytic capability of nanoparticles. The catalytic oxidation of light crude oil has also been investigated by Li et al.18 using TG and DSC. Their results suggested that metallic additives had Received: Revised: Accepted: Published: 11525

August 10, 2015 November 3, 2015 November 3, 2015 November 12, 2015 DOI: 10.1021/acs.iecr.5b02937 Ind. Eng. Chem. Res. 2015, 54, 11525−11533

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Industrial & Engineering Chemistry Research Table 1. Specific Parameters of Tahe Crude Oil Properties physical property

SARA analysis (wt %)

elemental analysis (wt %)

density(g/cm3)

viscosity(mpa s)

°API

saturate + aromatic

resin

asphaltene

C

H

N

S

0.941

2781.5

18.9

47.91

43.15

8.94

81.27

12.32

1.97

1.18

and metallic additives (∼25 mg) (mass ratio 9:1) in atmospheric pressure from 25 to 800 °C. TG/DTG measures the mass change, whereas DTA monitors the differential heat flow of the samples during the whole heating process in a controlled and oxidizing atmosphere. The procedures involve placing samples, setting the air flow rate (constant 50 mL min−1) and heating rate (constant 10 °C min−1), and then commencing the experiment. Prior to the test, thermal analysis systems are calibrated with calcium oxalate monohydrate for temperature readings, and silver is used to correct for buoyancy effects. All the runs are performed once again to guarantee the accuracy of experimental data. 2.3. Kinetic Theory. A nonisothermal kinetic study of oil combustion is extremely complicated because of the existence of multitudinous constituents and their parallel, competitive, and consecutive chemical and physical reactions. Large numbers of mathematical models have been deduced by different assumptions to obtain kinetic parameters such as activation energy (E), Arrhenius constant (Ar), and reaction order (n).19−22 The general formula adopted for kinetic calculation of the data is as follows:

obvious influence on the oxidation reactivity as well as exothermic heat. Despite ISC exhibiting a number of distinct advantages, there are still some complexities and uncertainties which limit its widespread application in the field. Combustion front stability and rapid propagation are decisive factors in judging the success of an ISC process. However, high-temperature gradient in combustion zones makes this process vulnerable to sharp heat loss, and the heat loss is more dramatic especially in high water cut reservoirs. As a result, the surrounding temperatures may be reduced considerably, leading to a failed self-sustaining combustion front as well as deteriorated ISC performance. Meanwhile, as is often the case in heavy oil reservoirs, there is excessive fuel deposited in the reaction zone, resulting in both incomplete combustion of the fuel and slow advance of the combustion front. To overcome these problems and accelerate the reaction process, catalytic ISC has received considerable interest. It has been proven that crude oil oxidation and cracking reaction can be catalyzed by metallic additives. Besides the catalytic properties, metallic additives can also promote oxidation indirectly through breaking down the antioxidant materials naturally present in most crude oil. However, there are few literature contributions available regarding the catalytic investigation of metallic additives on heavy crude oil oxidation based on TG-DTA testing. In addition, most published research focuses on the oil mixed with rock matrix and metallic additives or clay to simulate real reservoir conditions, failing to reflect the catalytic effect of a single factor on specific oil. Thus, this study has a unique character regarding the use of catalyst for the application of heavy oil reservoir air injection projects. In this work, we conducted catalytic investigation of metallic additive on heavy oil through TG-DTA testing. The main objective of this contribution is not only to explore the thermal behavior and estimate kinetic parameter of Tahe heavy oil in the presence and absence of metallic additives, but also to provide new insight into the oil oxidation mechanism and catalytic mechanism. Significant guidance is expected for preparing efficient catalyst for field application as well as direction for the next research steps.

dα 1 = k(T ) f (α) dT β

(1)

The k characterized by temperature dependence is reaction rate and obeys the Arrhenius law:23

⎛ E ⎞ ⎟ k = A exp⎜ − ⎝ RT ⎠

(2)

By rearranging eqs 1 and 2, we obtain dα A ⎛ E ⎞⎟ = exp⎜ − f (α ) ⎝ RT ⎠ β dT

(3)

The terms in eq 3 have been described in detail elsewhere.24 Through various transformations of eq 3, many differentialbased computational models are established to obtain the kinetic parameters. In this research, the classical Arrhenius method25 is employed to deal with the TG/DTG data of crude oil and oil−metallic additives. The equation is expressed as

2. EXPERIMENTAL SECTION 2.1. Materials. The heavy crude oil employed in this experiment was provided by Tahe oilfield (Tarim Basin, China). The natural formation temperature is about 107 °C, and the reservoir pressure is 47.6 MPa. The density and viscosity of crude oil at atmospheric pressure are 0.941 g/cm3 at 25 °C and 2781 mPa s at 50 °C, respectively. The specific parameters of crude oil properties are tabulated in Table 1. Four metallic additives, i.e., ZnSO4, CuCl2, FeCl2, and AlCl3· 6H2O, were chosen to carry out the thermogravimetric testing and were purchased commercially from Chengdu Kelong Chemical Reagent Co., Ltd. (Sichuan province, China) with an effective content of more than 98.0%. 2.2. Nonisothermal Thermogravimetric Tests. The NETZSCH STA 409 PC/PG (NETZSCH, Ltd., German) with a TG/DTA module was applied to evaluate the thermal characteristics of crude oil (∼25 mg) and the mixtures of oil

dW = kW n dt

(4)

⎛ E ⎞ ⎟ k = A r exp⎜ − ⎝ RT ⎠

(5)

The reaction progress is independent of the O2 concentration and can be described using first-order kinetics:26 ⎛ E ⎞ dW ⎟W = A r exp⎜ − ⎝ RT ⎠ dt

(6)

⎛ E ⎞ dW /dt ⎟ = A r exp⎜ − ⎝ RT ⎠ W

(7)

Next, we take the logarithm of both sides of eq 7 11526

DOI: 10.1021/acs.iecr.5b02937 Ind. Eng. Chem. Res. 2015, 54, 11525−11533

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Industrial & Engineering Chemistry Research

Figure 1. TG/DTG curves of Tahe heavy oil in atmospheric environment.

Figure 2. Mass loss curves of crude oil and oil−metallic additives during heating process.

⎛ dW /dt ⎞ E ⎟ = log A − log⎜ r ⎝ W ⎠ 2.303RT

temperature oxidation are weak, the oxygen atom is chemically bonded into hydrocarbon compounds to generate hyperoxide such as ketones, aldehydes, alcohols, and so on. These unstable intermediates keep reacting with oxygen, bringing the whole reaction to the second step: chain scission reaction. The chains with oxygen-containing functional groups are cleaved to carbon oxides, water, and hydrocarbons of shorter chains. Addition oxygen reaction and chain scission reaction play a dominant role in LTO stage. It can also be observed from Figure 1 that the mass loss trend of heavy oil is not linear, with a final drop of 49.51% mass at 407 °C. It is deduced that the volatile matter and free moisture are evaporated from the crude oil, light hydrocarbons are burned, and a small amount of fuel is formed because of increasing temperature, which are responsible for the mass loss. The second reaction interval with a distinct peak valley in the DTG curve is called fuel deposition, which takes place between 407 and 504 °C. It is reported by Pu et al. that FD is related to negative temperature coefficient behavior in gas-phase hydrocarbon oxidation, and the oxidation of hydrocarbons at relatively low temperatures is a complex process involving degenerate free-radical chain branching reactions.27 Meanwhile, FD is believed to be a transition region where the importance of a low-temperature mechanism diminishes before the higher-

(8)

A linear fit line can be obtained when plotting log(dW/dt/ W) versus 1/T, the slope (−E/2.303R) and intercept (log Ar) of which can be used to calculated the values of activation energy and Arrhenius constant, respectively.

3. RESULTS AND DISCUSSION 3.1. TG/DTG Test of Tahe Heavy Oil. The resulting TG/ DTG curves in Figure 1 show that the heavy oil experiences three major temperature ranges (LTO, FD, and HTO) divided by reaction mechanism during the whole heating process. The oil vanishes quickly in the oxidizing and heating environment from 25 to 800 °C, and only 0.14% of it remained at the end because of evaporation and numerous complicated oxidation and pyrolysis reactions. The first reaction region occurs in the range of 25−407 °C and is called low-temperature oxidation. It is characterized by apparent mass loss, low peak temperature, and small amounts of carbon oxides produced. Various reactions happen within this region and fall into two categories according to the oxidation proceeding order. The first step is oxygen addition reaction. Because the reactions in the early stage of low11527


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Industrial & Engineering Chemistry Research Table 2. Reaction Region, Peak Temperature, and Mass Loss for Samples Tested low-temperature oxidation (LTO) sample oil oil oil oil oil

+ + + +

ZnSO4 CuCl2 FeCl2 AlCl3·6H2O

region (°C)

peak temp. (°C)

mass loss (%)

25−407 25−401 25−365 25−367 25−389

369 347 311 246 324

49.51 47.69 36.82 33.69 48.63

fuel deposition (FD)

high-temperature oxidation (HTO)

region (°C)

mass loss (%)

region (°C)

peak temp. (°C)

mass loss (%)

burn-out temp. (°C)

407−504 401−508 365−489 367−499 389−501

25.24 24.72 30.26 42.46 24.57

504−760 508−760 489−705 499−710 501−700

563 552 510 559 547

25.11 27.31 32.58 23.81 26.64

763.5 761.9 708.6 713.4 704.2

Figure 3. DTG and DTA profiles of oil samples under atmospheric pressure.

process, because of sufficient heat supply from the increasing temperature, the fuel undergoes a series of drastic and complex combustion exothermic reactions, resulting in another significant mass drop of reactant as shown in Figure 1. Because of the higher reactivity of cokelike materials, high-temperature oxidation serves as the major exothermic stage providing the most heat and flue gases flooding needed for enhancing oil recovery. 3.2. Comparative Analysis of Thermal Behavior for Oil-Only and Oil−Metallic Additives. 3.2.1. Analysis of Reaction Regions. The TG curves of the oil-only and oil mixed with four metallic additives commonly found in reservoirs is shown in Figure 2, which is provided to compare the thermal behaviors between heavy oil and oil mixtures. During the heating process, the samples lose mass continuously and the different catalytic effects of individual metallic additives make

temperature mechanism becomes important. According to Wilk et al.,28 the conversion of reaction mechanism and negative temperature coefficient result from the competition between reactions involving the addition of O2 to the alkyl radical forming the alkylperoxy radical and reactions involving the abstraction of hydrogen from the alkyl radical by O2 to form the conjugate alkene and the hydroperoxyl radical. Because the fuel is generated via the conversion of heavier components into a carbonaceous deposit with lower hydrogen content during the FD stage, FD exerts a decisive effect on HTO. In addition, on the basis of Rice−Kossiakoff theory of cracking,29 other parts of the oil components can be cracked into products such as olefin as the base of the fuel to be deposited because of the intensive heat release. The final region for violent fuel combustion occurring from 504 to 760 °C is called high-temperature oxidation. During this 11528


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simultaneously at the cracking reaction stage with different intensities. These products are generally characterized by higher viscosity and density and lower volatility relative to that of the original oil, which tend to further react and be transformed to fuel in the FD stage. The DTG curves in Figure 3 indicate that all the metallic additives lowered the ending temperature and the mass loss rate, which is in favor of decreasing the consumption of hydrocarbon components during the LTO process. Corresponding peak temperature where the maximum decomposition rate occurs is also decreased. Because of the surface area effect of metallic additives, some of light hydrocarbons in oil-metallic additive mixtures will be absorbed by metallic additives and the evaporation rate will be reduced. As a result, the amount of mass loss of oil-metallic additive mixtures in LTO region is lower than that for oil-only. According to the DTA curves, some of the metallic added samples, such as ZnSO4 and CuCl2, achieve heat release rate during the LTO process higher than that of oil-only. For heavy crude oil, the higher exothermic quality promotes the hydrocarbon cracking process and more short chain hydrocarbon is generated,29,33 which provides sufficient deposited fuel for the HTO reaction. Unlike other mixtures, an interesting phenomenon is detected for the sample mixed with AlCl3· 6H2O. Aluminum has the ability to catalyze both thermal and acid-activated types of cracking processes in the LTO region, generating higher amounts of light components.33 As a result, almost 48.6% of the mass of the added AlCl3·6H2O sample vanishes by the end of the LTO stage, although the temperature range is shortened, which is more than those of other mixtures. There are two distinct peak rates on the DTG curve of the sample mixed with AlCl3·6H2O appearing at 210.7 and 323.8 °C, respectively. The temperature range between 25 and 210.7 °C with a rapid mass loss rate is probably due to evaporation behavior rather than cracking reaction. Distillation and evaporation need to absorb heat, which may be the reason why the sample is slightly endothermic in the first stage of LTO while exothermic as other samples in the second stage. Another reason may be that AlCl3·6H2O starts to sublimate at 178 °C, leading to a distinct peak valley at 210.7 °C in the DTG curve and endothermic behavior in the DTA curve. There are many literature contributions about three similar LTO, FD, and HTO regions observed during the whole crude oil oxidation process.6,30,34 However, in those studies, the FD representing a shift of chemical mechanism from lowtemperature oxidation to high-temperature oxidation is not evident in the thermogravimetric curves, and it is difficult to obtain kinetic parameters for the FD from corresponding fitting curves. In this study, not only are obvious FD stages in TG/ DTG curves observed, but also the kinetic parameters are obtained with high-quality linear regression (the kinetic analyses will be presented in a later section). The various chemical compositions among crude oils of different origin have decisive impact on the deposition rate and final amount of fuel. The additives such as clay minerals, metallic salts, and nanoparticles can be another important factor affecting the FD behavior. In this study, it is observed from DTG curves that the metallic additives extend the reaction interval and generate more deposited fuel by the end of the FD stage (except FeCl2), suggesting that in the presence of metallic additives, a stable and self-sustaining combustion is made possible. In the HTO region, with higher surrounding temperatures, these cokelike deposits are involved in violent and heterogeneous reactions which release a huge amount of heat and cause

the TG curves present various trends. The data of reaction intervals, corresponding peak temperatures, and mass loss are given in Table 2. It is obvious that the range of LTO process and corresponding peak temperature are lowered in the presence of oil−metallic additives compared with those for oil-only. Because LTO reaction is finished at a lower temperature, a huge amount of hydrocarbon components are able to be deposited as fuel before they distillate and evaporate. Taking CuCl2 and FeCl2 as examples, because of the lower temperature range of LTO processes, the lost mass of these samples are apparently less than those of the other samples. In addition, it is observed that oil−metallic additives exhibit a wider FD temperature region relative to oil-only (407−504 °C), indicating that metallic additives can prolong the fuel deposition process. A similar conclusion has been drawn by Jia et al. that the appearance of cuttings in crude oil led to more deposited fuel.30 The strong surface area effect of metallic additives is the main reason for broadened FD range.31 When heavy crude oil reacts in air without a matrix during the thermogravimetric test, fuel deposition occurs at the crucible surface. However, there should be an apparently increased amount of deposited fuel on the matrix when solid metallic additives with sufficient surface are added. The catalytic properties of metallic additives can also influence the FD stage. Regarding the HTO stage, it is found that the introduced metallic additives cause a distinct shift of high-temperature range to a lower one. High-temperature oxidation is the main combustion region which contributes the most to air injection process with violent exothermic reactions and plenty of flue gases. The lowered temperature region of HTO means less energy is needed for the commencement of HTO, and the transition of reaction stage from FD to HTO becomes easier. Accordingly, the burn-out temperature indicating the combustion is finished is also lowered for oil−metallic additives relative to the oil-only. Compared with crude oil, the samples mixed with ZnSO4, CuCl2, and AlCl3·6H2O are able to generate more fuel after LTO and FD processes combusted in the HTO stage. The only exception is the oil mixed with FeCl2. The mass loss of the sample with FeCl2 in the LTO stage is less than twothirds of that of the oil-only sample, and the range of the FD is extended because of lower end temperature of LTO. Because of the inadequate distillation and volatilization before the end of LTO, the sample maintains a relatively high mass loss rate during the FD. Consequently, the total amount of mass loss during the LTO and FD processes is higher than that of the rest. Owing to the lowered HTO region and the increased amount of fuel deposited, it favors the combustion reaction and huge amounts of heat are released to maintain a stable and continuous combustion front during the catalytic ISC process. 3.2.2. Analyais of Catalytic Effects of Metallic Additives. Figure 3 shows the contrast DTG and DTA profiles of crude oil thermal behavior in the presence and absence of metallic additives. From the resulting curves, all samples experience the same reaction modes (LTO, FD, and HTO) in varied degrees. As reported by Jia et al.,32 the LTO reaction can further be divided into two stages according to the temperature range of occurrence. In the first stage, mass loss of samples results from distillation and evaporation of volatile hydrocarbon components and free moisture. Then, plenty of oxygen bonds into the liquid oil chemically during the second stage. Mainly the branched chains with weak bond energy in hydrocarbon molecule structure are fractured and oxidized to generate peroxides. O2 addition and chain-breaking reactions happen 11529


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to be the apparent value reflecting different complicated reactions and oxidation chemistry. The values of activation energy for crude oil alone in LTO and HTO are 19.61 and 116.03 kJ·mol−1, respectively, while varying within the range of 13.78−17.76 kJ·mol−1 during the LTO process and within the range of 40.65−105.58 kJ·mol−1 during the HTO process when metallic additives are added. The kinetic data indicate that the oil−metallic additives have lower activation energy values. This is attributed to the fact that the catalytic effect of metallic additives lowers the energy needed for commencing the reactions. In addition, they also can promote oxidation indirectly via breaking down the antioxidant materials naturally present. On the other hand, the various kinetic parameters in Table 3 indicate that different metallic additives have different effects on each oxidation region of samples. At the HTO stage, the sample with added ZnSO4 achieves relatively low activation energy. Any decrease in the activation energy means lower energy is required to overcome the starting of the reaction, which suggests that the addition of ZnSO4 makes it easier to initiate the HTO reaction. For FeCl2, the activation energy of FD is as high as 100.46 kJ·mol−1. Therefore, a higher value means it is more difficult to react at the same temperature region and less fuel is generated, which is not conducive to maintaining a stable combustion front. For AlCl3·6H2O, because of the particular double catalytic influence of aluminum, it causes the LTO reactions to occur more rapidly with the lowest activation energy relative to other additives, which suggests that AlCl3·6H2O has a good ability to catalyze the oxidation reaction in the LTO process. However, activation energies of FD and HTO are so high that it needs a huge amount of energy to decompose molecules. Consequently, there is less fuel deposited and it is harder to commence the HTO reaction. This may be the reason why the sample with added AlCl3·6H2O exhibits a shorter exothermic interval and a relatively low heat release rate in the DTA curve during the HTO stage, compared with that of other samples. For CuCl2, the activation energies of FD and HTO decrease abruptly down to 67.29 kJ·mol−1 and 40.65 kJ·mol−1, respectively, while other mixtures keep relatively higher values. The activation energy of LTO is also low enough. The reason may be that the presence of CuCl2 not only lowers the energy required for commencing the reactions but also provides extra paths that cause the reactions to be accelerated, presenting an amazing influence in terms of oil oxidation. Meanwhile, throughout the process of oxidation, the exothermic quality is significantly stronger than crude oil alone and other mixtures. Hence, CuCl2 has an excellent catalytic capacity for better air injection performance through positively influencing the thermal behavior and kinetics of the combustion reaction. 3.3.2. Specific Surface Area Effect Analysis of Metallic Additives. Because a reaction can be positively catalyzed with a lower Arrhenius constant even though the activation energy is increased, the combined effects of Arrhenius constant and activation energy must be taken into consideration. It is also observed from Table 3 that the addition of metallic additives leads to the decrease of Arrhenius constant to various degrees in all reaction regions. The result obtained is perfectly consistent with similar research in the available literature. Kök and Iscan35 employed thermal techniques to specialize in the influence of metal chloride compounds on oil oxidation. When lower ratios of additives are added, surface reactions play a dominant role while the catalyst does not affect the reactions much. In their another study,11 increased clay content also leads

rapid mass loss to produce carbon oxides and water. With the addition of metallic additives, it is observed from the DTG curves that the HTO reaction intervals are shifted to lower temperatures and corresponding peak temperatures are decreased. As for the DTA curves, one drastic exothermic region is detected for all samples, which contributes most of the heat release during the whole oxidation process. Sufficient combustion with high heat release rate is possible for samples mixed with ZnSO4 because of the high amount of fuel deposited; thus, ZnSO4 has positive catalytic effect on the HTO reaction. However, although the sample with added AlCl3· 6H2O starts the HTO process with a little more remaining amount than the oil-only sample, in the HTO process, the exothermic interval is shorter and the heat release rate stays in a relative low level all the time, which can deteriorate the combustion front and make it difficult to sustain. For FeCl2, because of the low remaining amount of fuel and weak heat stability, it does not exhibit a positive effect on combustion. Among these metallic additives, CuCl2 has a distinguished influence on oil exothermic behavior. In the HTO stage, both the peak heat flow rate (as high as 4.46 uV mg−1) and the average heat flow rate are higher than others. The increase of the heat released can be attributed to the increase in remaining amount of fuel as high as 32.58% after FD. The excellent exothermic effects can benefit some additional reactions such as hydrocarbon pyrolysis. Meanwhile, the viscous oil can be activated to flow easily and high displacement efficiency can be achieved because of the violent heat emission. Thus, it can be explained that CuCl2 has an excellent catalytic effect on Tahe heavy crude oil during the high-pressure air injection process. 3.3. Kinetic Analysis. 3.3.1. Catalytic Activity Analysis of Metallic Additives. The kinetic parameters of crude oil alone and oil−metallic mixtures in LTO, FD, and HTO regions are obtained, respectively, by Arrhenius method on the basis of TG/DTG curves (Figures 4 and 5), and the experimental

Figure 4. Kinetic parameters of oil-only determined by Arrhenius method.

results are summarized in Table 3. All Arrhenius plots are found to be performed well in all regions with high correlation coefficients, which illustrates that this model is sufficient to calculate the values of E and Ar with high accuracy. From Table 3 it is found that the activation energy requirements of crude oil during LTO, FD, and HTO regions show significant differences. Generally speaking, the activation energy required is positively correlated with surrounding temperature during the heating process. The kinetic parameter acquired is considered 11530


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Figure 5. Kinetic parameters of oil−metallic additives determined by Arrhenius method.

Table 3. Activation Energy (E) and Arrhenius Constant (Ar) Calculated for Oil-Only and Oil−Metallic Additives activation energy (kJ·mol−1)

slope of fitted line sample oil oil oil oil oil

+ + + +

ZnSO4 CuCl2 FeCl2 AlCl3·6H2O

Arrhenius const. (min−1)

LTO

FD

HTO

LTO

FD

HTO

LTO

−1024.03 −927.31 −728.04 −853.06 −719.45

−5756.18 −4857.42 −3514.26 −5246.53 −5460.48

−6059.86 −2916.91 −2123.16 −3274.94 −5513.92

19.61 17.76 13.94 16.33 13.78

110.21 93.01 67.29 100.46 104.55

116.03 55.85 40.65 62.71 105.58

1.82 1.20 0.49 0.87 0.58

FD 1.04 5.12 6.16 2.57 4.68

× × × × ×

HTO 107 105 103 106 106

6.16 × 105 190.55 23.44 776.25 3.16 × 105

heterogeneous catalysis, the fuels are more likely to react with air, and as a result, the HTO reactions are accelerated. 3.3.3. Mechanism Recognition on Changes in the Activation Energy. From Table 3, it is not difficult to find that the activation energies of all samples in the HTO stage are much higher than those in the LTO stage. The reason may be that, among SARA fractions, the contents of ash and volatile materials are lower in asphaltene while the value of fixed carbon is higher relative to the others. The molecule is so heavy and resistant that almost all of the mass is lost in the HTO stage while little is lost in the LTO stage.36,37 In contrast, the saturate vanishes at high amounts in the LTO process. As a result, the HTO needs more energy than LTO to activate the reaction. It is also observed from Table 3 that metallic additives cause a greater activation energy reduction in HTO than that in LTO, which coincides with the findings of the studies by Rezaei et al.17 and Li et al.18 in the presence of additives. There are large amounts of asphaltene in low API gravity crude oil, and the LTO process can also lead to the conversion of resin to asphaltene;38 the oxidation behavior of heavy oil in the HTO stage is more related to the oxidation behavior of asphaltene fraction.36 Because the pyrolysis of asphaltenes is considered to

to a decrease in activation energy values during the thermal kinetic research on the target heavy oil, which indicates that large surface area of clay minerals influence the relative size of the Arrhenius constant value. Generally, it is believed that an additive’s catalytic activity lowers the activation energies, while it is the surface area effect that changes the Arrhenius constant value of all three major reactions. The strong specific surface effect of metal additive on heavy oil oxidation is believed to be influenced by two factors. On the one hand, the metallic additives have a positive impact on the FD stage. These oil compounds ,which are preferable to undergo evaporation and cracking reaction in liquid oil, are now being deposited on the increased available matrix surface. Consequently, an extended transition is observed in all oil−metallic additives Arrhenius fitting lines, as shown in DTG curves with broadened temperature interval for the FD stage. Large amounts of fuel are deposited because of the surface effect of metallic additives, which is beneficial to HTO with adequate fuel supply. On the other hand, metallic additives are composed of very fine particles that possess very high surface area and thus greatly increase the contact between the oil and oxygen. Because of the 11531


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exothermic qualtiy during the whole process, indicating that CuCl2 has an excellent catalytic capability to upgrade the performance of air injection process through positively influencing Tahe oil oxidation. Because of the particular double catalytic influence, the sample with added AlCl3·6H2O drops a larger amount of mass than others in the LTO stage. However, the catalytic effect is not obvious in the other two stages. The sample mixed with FeCl2 achieves the least fuel deposited because of high activation energy in FD stage. For ZnSO4, because of the high remaining amount and low activation energy, it exhibits a good catalytic influence on the HTO process. (5) The catalytic cracking mechanism of metallic additives on asphaltene in HTO stage has been analyzed in terms of microscopic molecular structure, and probably this is the reason for greater activation energy reduction in HTO than in LTO. Fully understanding the oxidation mechanism and catalytic mechanism in our study is extremely difficult because of a multitude of complex and consecutive physical and chemical reactions. The direct evidence of catalytic effects of metallic additives on heavy oil oxidation at molecular scale is unavailable and yet to be further investigated.

be depolymerization in parallel with thermal decomposition of functional groups,39 the reason for greater activation energy reduction at higher temperatures may be that the metallic additives provide additional pathways for the reactions. In terms of microscopic molecular structure, asphaltene consists mainly of polycyclic aromatic hydrocarbons connected with cycloalkanes and various branched chains as well as heteroatoms. There are a multitude of C−R (R = S, N, O, etc.) bonds with different energies. The metallic additives can lower the bond energy by attacking the heteroatoms in asphaltene and make these weakened bonds fracture easily in high temperature, leading to the tight macromolecular ring system being depolymerized to chains of different sizes.40 Because of the increase of light components and their very low energy requirement, they can be treated as the readily available fraction to touch off the combustion of whole oil. In contrast, because there is already a large amount of light components in the LTO stage, the metallic additives just cause a slight activation energy reduction. Fully understanding the oxidation mechanism and catalytic mechanism in our study is extremely difficult because of a series of complex and consecutive physical and chemical reactions. The direct evidence of catalytic effects of metallic additives on heavy oil oxidation at molecular scale is still unavailable and yet to be further investigated. From the results of quantitative comparison (Table 3), it could be pointed out that the type of metallic additives determines the catalytic efficiency. However, any increase in additive concentration can also increase the amount of fuel formed and deposition rate because of the increased availability of surface area. In addition, the chemical composition of crude oil plays a non-negligible role in the catalytic process of metallic additives. Therefore, it is of great value to determine the optimal type of catalyst and corresponding concentration for particular oil oxidation. Assessing the impact of these factors (especially asphaltene content) on catalytic efficiency of metallic additives should be included in subsequent study.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC) (51404202). The special fund of China’s Central Government for the Development of Local Colleges and Universities, the project of National First-Level Discipline in Oil and Gas Engineering, is also greatly appreciated.

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4. CONCLUSION In this work, TG and DTA experiments have been performed to investigate catalytic effects of metallic additives on heavy oil thermal behaviors and combustion kinetics, and the conclusions drawn are as follows: (1) In combustion with air, three major reaction stages are recognized based on the temperature range of occurrence in DTG and DTA curves for all tested samples: low-temperature oxidation, fuel deposition, and high-temperature oxidation. (2) All the metallic additives shorten the LTO range and lower the corresponding peak temperature. Conversely, the FD regions are extended with more fuel deposited before the HTO, which contributes greatly to the stability and sustainability of the combustion front. Not only are clear FD stages in DTG/ DTA curves detected in this test, but also the kinetic parameters are obtained from the Arrhenius fitting lines with high correlation coefficients. (3) Kinetic analysis indicates that metallic additives possess the ability to reduce the energetic barrier to commence the reactions at relatively low temperature and accelerate the process, and they have a more pronounced effect on the HTO activation energy. Furthermore, the strong specific surface effect of the metallic additives has also been taken into consideration. (4) The sample mixed with CuCl2 holds the lowest activation energy values in FD and HTO regions and attains huge

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