Catalytic Effect of Transition Metallic Additives on the Light Oil Low

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Catalytic Effect of Transition Metallic Additives on the Light Oil LowTemperature Oxidation Reaction Jiexiang Wang, Tengfei Wang,* Chuanming Feng, Changhua Yang, Zheng Chen, and Guochen Lu Institute of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: Increasing the reaction rate between crude oil and oxygen (oxygen consumption rate) is an effective method for improving the safety of air flooding. The catalytic effect of the transition metallic additives copper chloride, manganese acetate, cobalt chloride, and nickel chloride on the light oil low-temperature oxidation (LTO) was determined through static oxidation experiments. Additionally, the influence of temperature, pressure, and reaction time on the catalytic effect of the additives was investigated. The changes of the oil characteristics due to low-temperature oxidation (LTO) and catalytic low-temperature oxidation (CLTO) were investigated through SARA composition tests, elements analysis, n-alkanes components analysis, and infrared spectrum analysis. In addition, the influence of additives on the kinetic parameters of LTO was also researched. The results showed that the transition metallic additives significantly improved the oxygen consumption rate of light oil LTO, notably the copper chloride. For three oils from different reservoirs, the oxygen consumption rates increased to above 2.2 times the original after the addition of copper chloride at 70 °C and 16 MPa. Besides, the synergistic effect between reaction temperature and catalyst was significant in promoting oxygen consumption. When the reaction temperature reached 130 °C, the oxygen content after reaction reduced to 2.67% from 9.18% after the addition of copper chloride. The oxygen in air reacted with saturate and aromatics, generating resin and asphaltene. The distribution of n-alkanes was shifted to higher molecular weight components during LTO, and the addition of catalyst could enhance the changing trend. In addition, new oxygen-containing groups were generated during LTO and CLTO. The order of the oxygen partial pressure and the activation energy of LTO were all reduced due to the addition of catalyst. This study can provide guidelines to improve the safety and application of air flooding technology.

1. INTRODUCTION Gas flooding has been a major enhanced oil recovery (EOR) technology for decades, especially when applied to develop lowpermeability reservoirs, for its low injection pressure and high oil recovery efficiency.1−8 However, because of the gas source shortages and high cost, the application of carbon dioxide flooding, nature gas flooding, and nitrogen flooding subjects a certain restriction. The air supply has no geographical restriction, and the only cost needed is the air compression expenses. Therefore, air flooding is receiving increased attention among the gas flooding technologies.1,9,10 Field tests of air flooding have been conducted successfully in many light oil reservoirs such as MPHU, West Hackberry, Coral Creek, Ekofisk, Buffalo and Horse Creek oilfields.11−22 However, safety remains a major obstacle for its large-scale application because of the oxygen in the air. The oxygen injected into the reservoir must be completely consumed before it breaks through to the production wells to ensure the safety of air flooding. Two methods can be used to improve the oxygen consumption: (1) increase the well spacing between the injection wells and production wells to prolong the oxygen residence time in the reservoir, and (2) increase the reaction rate between the oxygen and crude oil to enhance the oxygen consumption. The commonly used method at present is increasing the well spacing. However, for reservoirs that have been developed, the well spacing has been fixed, and the former method is restricted for the substantial amount of work and cost required to adjust the well pattern. In contrast, it is an effective and easily implemented method to improve the air flooding safety by enhancing the oxygen consumption. © XXXX American Chemical Society

Therefore, catalytic low-temperature oxidation (CLTO) technology should be studied to accelerate the oxygen consumption during air flooding. As previously reported,23−26 low-temperature oxidation (LTO) is a major reaction mode when oxygen reacts with crude oil in porous media, typically occurring at temperatures below 350 °C. LTO reactions are heterogeneous (gas/liquid) and produce no or low levels of carbon oxides. Some of the products of LTO are alcohols, aldehydes, ketones, acids, and peracids.27 In this study, the LTO temperature researched is lower than 150 °C, and the aim of the air injection in light oil reservoirs is not to generate heat and promote EOR by thermal effects (in situ combustion), rather, to create a gas drive by generation of flue gas in situ.10 The purpose of this research of applying metallic additives is to enhance the oxygen consumption rate and improve the safety of air flooding. Currently, studies covering crude oil catalytic oxidation technologies mainly focus on the catalytic high-temperature oxidation (in situ combustion) technology (CHTO). Studies of catalytic low-temperature oxidation (CLTO) technologies applied on light oil at reservoir temperatures and pressures have seldom been reported in the published literature. The procedures and results of the previous studies23,28−35 of crude oil catalytic oxidation technology are summarized in Table 1. Received: October 23, 2014 Revised: May 24, 2015

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Californian (18.5°API) and Venezuelan (10.5°API) oils

heavy oil from Gulf of Mexico (12.5°API)

combustion cell experiments

combustion tube experiments

combustion cell experiments

combustion tube experiments

differential scanning calorimetry (DSC) and therrnogravimetric analysis (TGA)

combustion tube experiments and ramped temperature oxidation tests

Shallcross et al.23

Ramirez-Garnica et al.29,30

Fassihi et al.31,32

Castanier et al.33

Drici and Vossoughi34

He et al.35

crude oil (API gravity)

B

Fe(NO3)3

Temperature was increased to 600 °C at a rate of 10 °C/h at atmospheric pressure.

Tube experiments: ignition temp, 400 °C; pressure, 690 kPa. RTO tests: temp, increased to 470 °C at a rate of 60 °C/h at pressures of 690, 550, or 310 kPa

crude oil from the Iola field (19.8°API)

heavy (12°API) and light (34°API) oil from Cymric

heavy metal oxides such as titanium, ferric, nickel, cupric, vanadium, and chromium oxides

ignition temp, 315 °C; pressure, 690 kPa

Temperature was increased to 450 °C at a rate of 55 °C/h at pressures of 690, 550, or 138 kPa

ignition temp, 300 °C; pressures, 2070 kPa

FeCl2, SnCl2, CuSO4, ZnCl2, MgCl2, K2Cr2O7, Al2Cl3, MnCl2, Ni(NO3)2, and CdSO4 Additives included Mo, Co, Ni, and Fe, and the anionic part of the salt was acetylacetonate or alkylhexanoate.. Additives included Cu, Ni, Va, and Fe, but the anionic part of the salt was not reported. Additives included Fe, Sn, and Zn, but the anionic part of the salt was not reported.

Temperature was increased to 450 °C at a rate of 50 °C/h at pressures of 280 or 550 kPa

additives CuCl, FeCl3, and MgCl2

exptl condition (temp/pressure) The cell was continuously heated to 500−600 °C with a 1 °C/min heating rate at pressures of 172 or 345 kPa.

Huntington Beach oil (22°API), Hamaca oil (10°API), Cymric heavy oil (12°API), and Cymric light oil (34°API)

San Ardo oil (11.2°API), Venezuela oil (9.5°API), Huntington Beach oil (18.5°API), and Lynch Canyon oil (10°API)

Karakus (29°API) and Beykan (32°API) crude oils from Turkish oil fields

exptl procedure

combustion reaction kinetics experiments

researchers

Bagci and Celebioglu28

Table 1. Summary of Previous Studies Covering Crude Oil Catalytic Oxidation Technologies results

The combustion efficiency and front velocities could all be improved by the metallic additives, and the H/C ratio of the fuel, heat of combustion, air requirements, and density of the crude produced changed after the addition of metallic salts. In addition, the combustion of light oil would also be improved by metallic additives. The effect of titanium oxide was similar to that of silica and alumina. Vanadium, nickel, and ferric oxides behaved similarly in enhancing the endothermic reactions. The effect of a small amount of metal oxide was weak in the presence of a large surface area such as with silica. The combustion reaction catalytic mechanism was cation exchange of metallic salts with clay to create activated sites that enhance the combustion reactions between oil and oxygen. The combustion of light oil could also be improved by metallic additives.

The additives could lower the activation energy of the combustion reaction and lower the temperature at which the combustion reaction occurred under the identical reservoir conditions.

The addition of metallic additives could accelerate the propagation velocity of the combustion front, improve combustion efficiency, and increase oil production.

The catalyst type and concentration influenced the kinetic parameters (reaction order and activation energy) of high-temperature oxidation. Iron and tin salts could enhance fuel formation, whereas copper, nickel, and cadmium salts displayed no significant effects.

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6H2O), manganese acetate (Mn(CH3COO)2·4H2O), and nickel chloride (NiCl2·6H2O). Aqueous metallic salt solutions were prepared using distilled water. The premixing method was used to prepare the reactant mixtures for the runs. A 50 g amount of the reactant mixture containing 45 g of light oil and 5 g of metallic salt solution was used as the reactant. After being filled with oil and additives, the oxidation reactor was heated to the experimental temperature by the oil bath and retained for a half-hour to reach thermal equilibrium in the reactor. Highpressure air (21% oxygen, 79% nitrogen) was then injected into the reactor. The pressure in the reactor was recorded during the experiments, and the changes in the oil and gas composition were analyzed at the end of the runs. To investigate the influence of the additives on the properties of crude oil, the changes of oil elements, SARA composition, n-alkanes components, and oxygen-containing functional groups before and after LTO were all analyzed. SARA is an acronym for a group of analytical procedures that separate oil, mainly by liquid chromatography, into fractions called saturates, aromatics, resins, and asphaltenes.44 SARA fractions have been used to represent the chemistry of crude oil oxidation reactions in several previous investigations.45−48 In this study, the oil SARA composition tests were conducted according to NB/SH/T 0509-2010. The devices used for the oil elements analysis, n-alkanes components analysis, and functional groups analysis were elemental analyzer (elementar vario EL), HP 6890 gas chromatograph and Fourier transform infrared spectrometer (TENSOR 27), respectively. 2.2. Experimental Conditions. The catalytic performance of the four additives at 70 °C and 16 MPa was investigated taking the oxygen consumption rate as the main evaluation index. The influence of the additive mass fraction and oil type on the catalytic effect was also researched under the identical condition. In addition, the influencing factors, such as the reaction temperature and pressure, were also investigated at 30−130 °C and 12−20 MPa. Multitube experiments were conducted to determine the influence of reaction time; the reactors were placed into the ice water immediately at the end of the runs to interrupt the LTO reaction.

As shown in Table 1, the catalytic effect of different metallic additives varied, and the catalyst type and concentration displayed a substantial influence on the performance of the reaction and kinetic parameters. However, when properly selected, the metallic additives displayed a positive effect on the combustion efficiency and oil production. In addition, the combustion of light oil could also be improved by metallic additives. The catalytic effect of additives was mainly dependent on the metallic parts of the salts used and was minimally influenced by the anions. For example, the ferric salts used in refs 19, 20, 23, and24 could all enhance the combustion reaction, whereas the anionic parts of the salts were different. Besides, Akkutlu et al.36,37 and Adagulu et al.38,39 investigated the front performance in the presence of catalytic agents under reservoir conditions using an analytical approach. The proposed model described the front propagation in a homogeneous porous medium. The front involved the coherent propagation of low-temperature (fuel-generating) and high-temperature (fuel-burning) reaction regions under the influence of reservoir heat losses. The catalytic agents were implicitly introduced to the model in terms of their dual effects: (1) increase hydrocarbon deposition ahead of the front, and (2) modify the kinetics of oxidation reactions inside the front. The results showed that the improvement of combustion was mainly because of the increased specific sand grain surface area on the hydrocarbon deposition ahead. In view of the good catalytic performance of metallic additives, 40−43 the catalytic effect of copper chloride, manganese acetate, cobalt chloride, and nickel chloride on the light oil LTO under reservoir conditions was investigated in this study. Additionally, the influence of temperature, pressure, additive mass fraction and reaction time on the catalytic effect was determined. The changes in oil characteristics due to LTO and CLTO were investigated based on SARA composition tests, elements analysis, n-alkanes components analysis, and infrared spectrum analysis.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance of Metallic Additives. The catalytic effect of the four additives on Dong69-57 crude oil LTO at 70 °C and 16 MPa are shown in Table 2. The oxygen consumption rate was calculated by material balance method. The amount of oxygen consumed during LTO could be got from the changes in gas pressure and composition before and after the reaction. The compressibility factor was calculated with the Key mixing rule and Peng−Robinson (P-R) equation.49 The gas composition changed significantly after LTO: the oxygen content reduced obviously, whereas only a minimal quantity of carbon oxides and small hydrocarbons were generated. The addition of metallic additives could enhance the changes in composition, notably for the oxygen component. The additives used increased the oxygen consumption, especially the copper chloride, which increased the oxygen consumption rate by 1.7 times. The decreasing order of the effect of the additives is the following: copper chloride > nickel chloride > cobalt chloride > manganese acetate. LTO reaction consists of two reaction modes: oxygen addition reaction and bond scission reaction.50 Oxygenated compounds which tend to further react and polymerize with each other to form heavier compounds can be generated during the oxygen addition reaction. Carbon oxides and low carbon number hydrocarbons will be generated during the continuous bond scission reaction.

2. EXPERIMENTAL SECTION 2.1. Experimental Device and Procedure. A crude oil static oxidation device was used to perform the experiments. The device was equipped with a stainless steel reactor with a Teflon bushing (100 mL volume), an oil bath, a high-pressure air cylinder, a pressure gauge, a preheater, a 6-way valve, a pressure reducing valve and a gas chromatograph. The schematic of the device is shown in Figure 1. The light oils used for the runs were from three different oil reservoirs in China: Dong69-57 and An83 crude oils from the Dongzhi and Ansai reservoirs in the Changqing oilfield, respectively, and SW1014 crude oil from the Northeast Petroleum Bureau. The API gravity of the oils were 35, 33, and 30°API, respectively. The metallic additives used were copper chloride (CuCl2·2H2O), cobalt chloride (CoCl2·

Figure 1. Schematic of the crude oil static oxidation device. C

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Energy & Fuels Table 2. Catalytic Performance of Metallic Additivesa exptl condition crude crude crude crude crude crude

oil oil oil oil oil oil

only + H2O + Mn(Ac)2 + CoCl2 + NiCl2 + CuCl2

gas composn after reacn, % (O2/CO2/CO/CH4/N2)

increment of SARA components after reacn, % (saturate/aromatics/resin/asphaltene)

oxygen consumption rate, mol of O2/(d·m3 [oil])

19.47/0.07/0/0.13/80.26 19.50/0.07/0/0.11/80.24 19.21/0.09/0/0.15/80.43 17.84/0.11/0.01/0.26/81.61 17.71/0.11/0.01/0.24/81.74 15.04/0.14/0.02/0.37/84.21

−3.54/−2.02/3.53/2.03 −3.18/−2.02/3.51/1.70 −3.89/−2.21/4.11/1.99 −4.61/−2.91/4.62/2.80 −4.65/−2.97/4.81/2.81 −6.39/−3.48/5.54/2.92

47.29 50.08 54.50 80.97 84.87 127.16

Reaction condition: 70 °C and 16 MPa. SARA composition (saturate/aromatics/resin/asphaltene) of Dong69-57 crude oil before LTO: 70.91, 16.07, 9.78, and 3.24. Additives mass fraction: 10% of the metallic salt solution.

a

Figure 2. Simplified LTO reaction scheme.

The simplified mechanism by which carbon oxides are generated when a hydrocarbon molecule undergoes LTO can be represented by the reaction scheme shown in Figure 2.51 The reaction pathways ①, ②, ③, and ④ are all oxygen addition reactions, and H2O and oxygenated compounds such as RCH2OH, R-CHO, and R-CO3H, etc., are generated; ⑤ and ⑥ are bond scission reactions, and the products are CO2, CO, R•, and RO• etc. The radicals R•, RO•, and HO• continue to react with oxygen and other hydrocarbons in the oil. Under mild conditions, the main reaction pathways are ①, ②, ③, and ④, while ⑤ and ⑥ will occur frequently when reaction temperature is high enough. The reaction temperature researched (70 °C) is quite mild, so the main reactions that occurred are oxygen addition reactions, and little carbon oxides can be produced. Therefore, both reaction modes exist but the oxygen addition reaction dominated the bond scission reaction during LTO of Dong69-57 light oil at 16 MPa and 70 °C. Similar results were obtained by Jia et al. and Gutierrez et al.52,53 After the addition of additives, the oxygen content reduced significantly while the content of carbon oxides and small hydrocarbons increased slightly, inferring that the catalytic effect of additives on the oxygen addition reaction was noted but limited for the bond scission reaction. The oil SARA composition altered after LTO: a decrease in saturate and aromatics content was noted, accompanied by an increase in resin and asphaltene content. After the addition of

additives, the changes in the SARA composition became more significant. Higher oxygen consumption levels during LTO caused a more significant change in the oil SARA composition. After LTO with copper chloride, a black toluene insoluble powdery solid was found in the oil sample. After analysis, this insoluble substance was determined to be coke. The generation of coke was caused by the limited asphaltene solubility of crude oil; the excessive asphaltene generated during LTO would precipitate out and be left as toluene insoluble solid.54 The addition of copper chloride increased the quantity of asphaltene generated and resulted in more coke precipitate. The coke content reached 1.4% after CLTO with copper chloride at 70 °C and 16 MPa. 3.2. Influence of the Catalyst Mass Fraction. In view of the obvious improvements to the oxygen consumption rate, copper chloride was selected as the catalyst of light oil LTO in the follow-up studies. Five runs were conducted to research the influence of the catalyst mass fraction on the catalytic effect. The catalyst mass fractions were 2, 3, 6, 8, and 10%, and the results are shown in Figure 3. As shown in Figure 3, the oxygen consumption rate increased with increasing catalyst mass fraction, but was steady when the mass fraction exceeded 8%, so the optimal value of the mass fraction of copper chloride was 8%. In view of the amount of crude oil and catalyst solution (45 and 5 g) used in the D

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carbon and hydrogen elements decreased in oil converted to carbon oxides, small hydrocarbons, and H2O. For the reaction system of LTO with pure oil, the oxygen element decreased in the gas phase after LTO was 235.77 mg, the increment of oxygen element in the oil was 200 mg, and the exceeded oxygen element existed in the H2O generated. Therefore, the hydrogen element converted to H2O was 3.97 mg, while the hydrogen element decreased in the oil was 5 mg, so the hydrogen element increased in gas phase was only 1.03 mg. The result of minimal generation of small hydrocarbons was confirmed.A similar result could be reached in the reaction system of CLTO. According to the results of the gas composition tests, oil SARA analysis and elements analysis, the oxygen in air decreased while the oxygen element of oil increased after LTO. The content of saturate and aromatics decreased, but the resin and asphaltene content increased. The oxygen element was mainly contained in resin and asphaltene, but not in saturate and aromatics. Therefore, the oxygen in air would react with the saturate and aromatics and generate resin and asphaltene during LTO. The addition of copper chloride promoted the reactions. Generally, the oxidation of oil and increase in resin and asphaltene content during LTO/CLTO have some disadvantages such as increasing the oil viscosity. However, this influence on the property of oil produced from light oil reservoirs developed by air flooding is minor, because during the air flooding process, the oil produced, prior to gas breakthrough, is unreacted virgin oil. Behind the gas-flooding front (flue gas), the oxygen reacts mainly with the residual oil (oil that can not be produced) and will be consumed; thus, the effect of LTO on the produced oil would be minor.55 So during air flooding, although the catalyst will enhance the oxidation degree of the residual oil when improving the oxygen consumption, the advantages of catalyst addition far outweigh its disadvantages. 3.4. n-Alkanes Components Analysis. Figure 4 shows the n-alkanes distribution of the oil samples before and after LTO. The distribution of n-alkanes researched was in the range of C9−C30, and the most abundant n-alkane was C15 for all of the oil samples tested. The oil after LTO had a higher content of n-alkanes with a carbon number > 23 and a lower content in the range of C9−C22 compared to the oil before LTO. The increase of temperature and addition of catalyst enhanced the

Figure 3. Influence of catalyst mass fraction on the catalytic effect (70 °C, 16 MPa, Dong69-57 crude oil).

experiments, the recommended dosage of catalyst was 0.88 wt % the crude oil. 3.3. Oil Elements Analysis. The influence of the catalyst on the changes of oil elements (C, H, N, and O) after LTO was researched by elements analysis. The results are shown in Table 3. Table 3. Elements Analysis of Crude Oil before and after LTOa oil sample

C, %

H, %

N, %

O, %

oil before LTO oil after LTO oil after CLTO

83.15 83.12 83.06

11.18 11.17 11.12

0.26 0.27 0.29

0.32 0.72 1.31

Reaction condition: 70 °C and 16 MPa. Crude oil: Dong69-57. Additive: copper chloride.

a

The content of carbon, hydrogen, and nitrogen elements changed slightly after LTO, but the oxygen element displayed a significant change, notably when the catalyst was added. The results confirmed that some oxygenates were generated during LTO, and the addition of copper chloride promoted the generation. At 16 MPa and 70 °C, the main reaction mode of LTO was oxygen addition reaction. Most of the oxygen that decreased in air existed in the oxygenates and H2O generated, and only a minimal amount existed in carbon oxides. The small amount of

Figure 4. N-Alkanes distribution of the test oils (LTO condition, 16 MPa; crude oil, Dong69-57; additive, copper chloride). E

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generated more coke. The coke content was 3.8% when the reaction temperature was 130 °C. 3.7. Influence of Reaction Time. The influence of reaction time on the oxygen consumption of LTO is shown in Figure 8. The oxygen consumption rate decreased rapidly with increases in reaction time, and the addition of catalyst intensified the decreasing trend. The oxygen consumption rate of LTO at 10 h was 0.27 times the rate at 1 h, whereas the consumption rate of CLTO at 10 h was only 0.14 times the rate at 1 h. The reason for this phenomenon was the reduction in the reactant concentration. The addition of catalyst accelerated the reduction of the reactant concentration; therefore, the decreasing trend in the oxygen consumption rate of CLTO was more pronounced. However, after the addition of catalyst, the oxygen consumption rates at all reaction times increased. For example, the oxygen consumption rate ratio of CLTO and LTO (K(CLTO)/K(LTO)) was 4.90 and 2.60, respectively, when the reaction time was 1 and 10 h. 3.8. Infrared Spectrum Analysis. The infrared spectra of the crude oil before and after LTO are shown in Figure 9. Compared with the unreacted crude oil, two new characteristic peaks appeared in the infrared spectrum of crude oil after LTO at wave numbers 3441 and 1700 cm−1, representing -OH and -CO, respectively. A third new characteristic peak was noted in the infrared spectrum of crude oil after CLTO at 1032 cm−1, representing -C−O−C-. The changes of the crude oil functional group indicated that the alcohols, ketones, aldehydes, and carboxylic acids were generated during LTO, and the addition of copper chloride could affect the LTO reaction pathway. 3.9. Kinetic Parameters Analysis. Two possible catalytic mechanisms are available for crude oil oxidation reaction: the catalyst can reduce the oxidation reaction activation energy or the catalyst can destroy the antioxidants naturally present in crude oil.34,57−60 The LTO catalytic mechanism can be confirmed with Arrhenius’ law by analyzing the changes of kinetic parameters of LTO before and after adding catalyst.

changing trend. This increase of high-molecular-weight compounds with increasing oxidation agreed with the results observed by Gui et al.56 The reason for this phenomenon was that the oxidized n-alkanes resulting from oxygen addition reactions would further react and polymerize with each other to form heavier compounds.50 3.5. Influence of the Oil Type. To determine the influence of the oil type on the catalytic effect, another two runs were conducted taking copper chloride as catalyst using An83 and SW10-14 light oils drawn from different reservoirs in China. As shown in Figure 5, copper chloride had a good catalytic effect

Figure 5. Influence of the oil type on the catalytic effect (70 °C, 16 MPa).

on both oils. After the addition of catalyst, the oxygen consumption rate of SW10-14 and An83 increased to 2.24 and 2.35 times of the original, respectively. According to the results, copper chloride had a good catalytic effect on LTO of crude oils researched. However, additional runs should be conducted with oils from different reservoirs to confirm the universality of catalyst in future researches. 3.6. Influence of Temperature and Pressure. The influence of temperature and pressure on the catalytic effect of copper chloride is shown in Figures 6 and 7. The K(CLTO)/ K(LTO) used in Figure 6 means the oxygen consumption rate ratio of CLTO and LTO. The oxygen consumption rate increased with increasing temperature and pressure, while the influence of temperature was more significant. After the addition of copper chloride, the oxygen consumption rate increased under the identical conditions. In addition, the catalytic effect of copper chloride was significant within the range of the investigated temperatures and pressures. The temperature and catalyst had a synergistic effect on oxygen consumption, and higher temperatures displayed a more pronounced synergistic effect. When the reaction temperature reached 130 °C, the oxygen content of the gas sample after CLTO reduced to 2.67%, much lower than the 9.18% without catalyst. As shown in Figure 7, the oil SARA composition was influenced by the reaction temperature and pressure. With increases in temperature, the content of saturate and aromatics after LTO decreased gradually, while the resin and asphaltene content increased simultaneously. The influence of pressure on the changes in the SARA composition displayed the identical trend. After the addition of copper chloride, the changes in the oil SARA composition became more significant. The coke was noted in all oil samples at the end of CLTO (the reaction temperature exceeded 70 °C), and higher temperatures

dc(O2 ) = f e−E / RT px m [oil]n dt

(1)

where px is the oxygen partial pressure (kPa), E is the activation energy (J/mol), f is the frequency factor [(d·kPa)−1], m and n are the reaction orders, R is the universal gas constant [8.31447 J/(mol·K)], [oil] is the reactant concentration (mol/m3), and T is the absolute temperature (K). Because of the excessive amount of oil relative to oxygen, the reaction rate is assumed to be independent of the crude oil concentration, so the reaction order n can be supposed to be 0.41 Formula 1 can then be simplified as follows:

dc(O2 ) = f e−E / RT px m dt

(2)

To obtain the order of the oxygen partial pressure, the logarithms on both sides of the equation are computed: ⎛ dc(O2 ) ⎞ E ln⎜ + m ln px ⎟ = ln f − ⎝ dt ⎠ RT

(3)

According to formula 3, it is a liner relationship between ln(dc(O2)/dt) and ln pxwhen the temperature is fixed, and the slope is the order of the oxygen partial pressure, m. According to the results of LTO runs conducted under different pressures at 70 °C, Figure 10 is obtained. F

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Figure 6. Influence of (a) temperature (16 MPa) and (b) pressure (70 °C) on LTO (Dong69-57 crude oil; copper chloride)and (c) synergistic effect of the temperature and catalyst at 16 MPa (initial oxygen content, 21%).

The order of the oxygen partial pressure m could be changed due to the addition of catalyst. For LTO of pure oil, m was 1.28, but it was reduced to 0.49 after the addition of catalyst. The reduction of m indicated that the influence of the oxygen partial pressure on the oxygen consumption rate was reduced after the catalyst was added. The reported orders of reaction with respect to oxygen have varied widely from 0 to 2.44 When the pressure is fixed, a linear regression between 1/T and ln(dc(O2)/dt) in formula 3 can be used to obtain the kinetic parameters. The Arrhenius plot in the presence and absence of catalyst at 16 MPa is shown in Figure 11. The logarithm of the oxygen consumption rate had a good linear relationship with 1/T in Figure 11. The kinetic parameters of the LTO reaction before and after adding catalyst was determined by corresponding to the trend line to formula 3. The reaction activation energy reduced to 41055 J/

mol from 50981 J/mol after the addition of copper chloride. The activation energy values obtained here had the identical magnitude order with those reported for other light oils.1,35 The application of catalyst could reduce the activation energy of LTO, and then the oxygen consumption capacity of light oil was improved significantly. In summary, the addition of transition metallic additives can reduce the activation energy of LTO and increase the oxygen consumption rate. However, the exact catalytic mechanism is not clear, and related studies have seldom been reported in the published literature. The exact catalytic mechanism is complex because of the diverse composition of crude oil. But the main mechanism may be drawn on the basis of molecular interactions of metallic ions with oil components. The metallic ion−oil component interactions show the formation of bonds on aromatic rings G

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Figure 7. (a) Influence of temperature (16 MPa) and (b) pressure (70 °C) on SARA composition (Dong69-57 oil; copper chloride).

Figure 8. Influence of the reaction time on LTO (70 °C and 16 MPa, Dong69-57 oil).

4. CONCLUSION The catalytic effect of transition metallic additives on the LTO of light oils is investigated based on static oxidation experiments. The following conclusions are drawn from this study. (1) Additives can increase the oxygen consumption rate and improve the safety of air flooding significantly. The catalytic effect series of the additives researched reads copper chloride > nickel chloride > cobalt chloride > manganese acetate. For three oils from different reservoirs, the oxygen consumption rates all increase to above 2.2 times the original after addition of copper chloride at 70 °C and 16 MPa. (2) Both reaction modes of LTO exist under reservoir conditions, but the oxygen

or directly on heteroatoms and the formation of metalcomponent complexes, resulting in a decrease of C−C, C−N, and C−S bond energies.61,62 This suggests the possible promotion of LTO reactions due to the decrease of activation energy caused by metal-component bonding interactions. In addition, the oxygen molecule, which is small in size, has a certain oxidation activity and can be adsorbed on the metalcomponent complex metallic site, weakening or activating the O−O bonds.61 Then, it is easier for the oxygen to react with the crude oil components. From a macroperspective, this process promotes the oxygen consumption rate and improves the safety of air flooding significantly. H

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Figure 9. Infrared spectrum analysis of the oil (a) before and (b) after LTO (LTO condition, 70 °C and 16 MPa; crude oil, Dong69-57; additive, copper chloride) and (c) after CLTO.

addition reaction dominates the bond scission reaction. In comparison, the catalytic effect of metallic additives on the oxygen addition reaction is pronounced, but limited for the bond scission reaction. (3) The oxygen in air will react with the saturate and aromatics and generate resin and asphaltene, the distribution of n-alkanes is shifted to higher molecular weight components during LTO, and the addition of catalyst can enhance the changing trend. (4) The oxygen consumption rate increases with increasing temperatures and pressures. The

temperature and catalyst have a synergistic effect on the oxygen consumption of LTO, and higher temperatures produce more significant synergistic effects. (5) The addition of copper chloride can affect the LTO reaction pathway, and new oxygencontaining groups can be generated during LTO and CLTO. (6) The order of the oxygen partial pressure reduces to 0.49 from 1.28, and the reaction activation energy reduces to 41055 J/mol from 50981 J/mol after the addition of copper chloride. I

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Figure 10. Oxygen consumption rate versus the oxygen partial pressure at 70 °C.

Figure 11. Arrhenius plot in the presence and absence of catalyst at 16 MPa.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support received from the “Fundamental Research Funds for the Central Universities” is acknowledged gratefully.

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REFERENCES

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DOI: 10.1021/ef5023913 Energy Fuels XXXX, XXX, XXX−XXX