Article pubs.acs.org/EF
Kinetics of Low-Temperature Oxidation of Light Crude Oil R. B. Zhao,*,† Y. G. Wei,† Z. M. Wang,‡ W. Yan,† H. J. Yang,‡ and S. J. Liu† †
Ministry of Education Key Laboratory of Petroleum Engineering, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ PetroChina Company, Limited, Xicheng, Beijing 100007, People’s Republic of China ABSTRACT: The in situ combustion (ISC) process has drawn a lot of attention in the field of heavy oils. However, in the case of a light crude oil reservoir, in which low-temperature oxidation (LTO) is dominant, it is still less well-understood, especially for its reaction mechanism. In this paper, ramped temperature oxidation (RTO) experiments with different temperature intervals are used to investigate the oxidation reaction behaviors on various distillation pseudo-components from Dagang light crude oil. Both RTO and isothermal experiments are conducted on the whole crude oil and the sand mixture to obtain the LTO kinetic behaviors. The results indicate that oxygen addition reaction of the crude oil occurs to a great extent in the low-temperature region of 120−200 °C. Because the LTO reaction incorporates an oxygen atom into petroleum molecules rather than forming high-temperature oxidation (HTO) products (i.e., CO2, CO, and H2O), CO2 production is minor during the LTO process. The acid number of the crude oil increases with an increasing reaction time and temperature during the LTO as a result of the formation of organic acids. Two pseudo-component distillates were subjected to major oxygen additions as evidenced by oxygen uptake and increases of the acid numbers of oxidation products. The apparent activation energy (Ea) of the crude oil that derived from the results of RTO tests (at different temperature ranges) accompanied by the isoconversional method present the Ea values varying from 160 to 350 kJ/mol as the temperature changes from 205 to 230 °C. The Ea value obtained through the isothermal experiment shows a decreasing trend from 200 to 33 kJ/mol as the temperature increases from 148 to 235 °C.
1. INTRODUCTION Air injection has been used to enhance oil recovery for over 50 years. Air or oxygen-enriched air is injected into heavy oil reservoirs and usually propagates an in situ combustion (ISC) front once the formation is successfully ignited. A significant amount of heat is generated in situ that vaporizes the reservoir liquids. Heat generation is considered as a major driving force for production of viscous oils. As another potential candidate, air can be injected into oil reservoirs containing lighter crude oil,1−8 especially for water-sensitive reservoirs or if water is not sufficient.9 As a result of the large mobility of light oil, some researchers believed that the heat generated during oil oxidation is not essential to the recovery process.9 Oxygen in the injected air reacts in situ with a fraction of oil to produce CO2, and in addition, the resulting flue gas mixture, which is primarily CO2 and N2, provides the mobilizing force to the oil, sweeping it to the production well. Considering the cost and availability, air is the best candidate not only for the heavy oil recovery but also for the light oil reservoir if oxygen in the effluent gas is completely consumed. Mechanism studies of low-temperature oxidation (LTO) have been conducted experimentally to achieve a better understanding of this enhanced oil recovery (EOR) process. Experimental results indicate that LTO increases the asphaltene content and decreases the aromatic and resin fractions of crude oil.9 Burger10 presented the results that LTO has a significant influence on the oil properties, such as molecule weight and viscosity of the oil. Analyses of those products reveal that LTO causes conversion of low-molecular-weight compounds into high-molecular-weight compounds.11−13 Other researchers,14 however, believed that the influence becomes less if the oil [31° American Petroleum Institute (API) gravity] is lighter enough. © XXXX American Chemical Society
In addition to reaction products of the LTO process, the reaction kinetics using different methods were also investigated. Khansari et al.15 conducted six isothermal and one nonisothermal experimental run between 100 and 350 °C using the Alaska heavy oil (18.36° API gravity) mixed with standard quartz sand. In total, four temperature intervals, which represented different reaction modes during the LTO process, were found. Freitag and Verkoczy16 investigated the LTO reactions with saturate, aromatic, resin, and asphaltene (SARA) fractions extracted from two crude oils (with different API gravity values). The results showed that different SARA compositions had different reaction rates and only saturate content changes took a relatively great influence on LTO rates. Similar experiments were conducted by Dabbous and Fulton17 with whole oils and crushed Berea sand over the temperature range of 121−246 °C. The activation energy is 72 000 J/mol. Results of Ea measurements published in the literature18−21 under the low-temperature region using different API gravity oil samples are consistent with the values ranging from 60 000 to 80 000 J/mol. A major concern for air injection into oil reservoirs is to control the oxygen concentration from production wells to a safe level. Typically, light oil reservoirs have characteristics of relatively high temperature (80−140 °C) and high pressure. Some crude oil components are also readily reactive to oxygen when the contact time is long enough. Hence, it is necessary to gain a better understanding of oxidation reaction kinetics at reservoir temperature levels under conditions of air injection to Received: December 3, 2015 Revised: February 18, 2016
A
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Carbon Number Distribution and Relative Peak Area (%) of GC Analysis from the Light Crude Oil
a
carbon number
content (%)
carbon number
content (%)
carbon number
content (%)
others
C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22
2.71 2.81 2.63 2.78 3.27 4.30 3.74 4.30 3.92 4.18 4.35 4.14 4.67 4.74 4.61
C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 C34 C35 C36 C37
4.82 4.19 4.61 3.58 3.74 2.86 3.01 2.44 2.81 1.73 0.93 0.77 0.77 0.23 0.24
C38 C39 C40 C41 C42 C43 C44 C45 total Pr Ph
0.28 0.17 0.08
max peak C21−/C22+ C21 + 22/C28 + 29 Pr/Pha Pr/n-C17 Ph/n-C18
C23 1.26 1.59 1.16 0.72 0.60
100.00 3.01 2.60
Pr/Ph is the ratio of pristane and phytane.
Figure 1. Gas chromatograph spectrum of whole oil of the light crude oil (the graph in the top left corner shows the fraction distribution with a carbon number lower than 8).
maintain the safety of this process. Previous report22 shows that pseudo-components with different boiling ranges have different fuel availabilities and different reaction kinetics (i.e., Ea). For heavy oil oxidation, content of asphaltenes are proven to be a favorable factor for coke formation and can even be used to predict whether a successful ISC happens or not. For light crude oil oxidation, however, kinetics of fuel formation in the reservoir condition is not well understood. Therefore, how to elucidate the evolution of the light oil under LTO conditions is one of the core issues on the prediction of the ISC process. Hence, the objectives of our studies on pseudo-components are to promote a better understanding of the reaction mechanisms and try to provide more parameters used in the simulation of the air injection process.
Metrohm, Swiss), and an ultrasonic water bath (VWR International, Radnor, PA). Detailed information about the RTO kinetics cell and its operation procedure refers to the literature.22 About 40 kg of light crude oil and produced sand were collected from the Dagang field, Tianjin, China. Viscosity of this oil is around 32.0 mPa s at 25 °C. The dewatering process is implemented before experiments. Isopropanol [analytical reagent (AR)] and toluene (AR) were purchased from Merck. Standard isopropanol solution containing 0.1 M KOH was purchased from Aldrich, St. Louis, MO. “Cooked” sand (Monterey beach sand) was preheated at 600 °C for 24 h to passivate any clays and remove organic material. Table 1 is the collection of components of Dagang light crude oil with a carbon number greater than 8. These components are around 86.04% (w/w), and the rest of the components (lighter hydrocarbons with a carbon number less than 8) is 13.96% (w/w), which is in favor of evaporation other than oxidation and has a small influence on the acid number variation. 2.2. Properties of Dagang Light Crude Oil and Pseudocomponent Preparation. Gas chromatography (GC) analysis using ASTM D3415-98 is carried out to characterize the carbon number distribution. Two GC columns are used for lighter and heavier components. Figure 1 shows that a carbon number greater than C8 is predominant and compounds are grouped according to carbon number as presented in Table 1. Notably, the amounts of carbon
2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. The apparatus used in this work includes an electronic balance (Mettler PE 160, with a resolution 0.0001 g), a set of true boiling point distillations (Petroleum Analytical Instrument Company, Dalian, China), a ramped temperature oxidation (RTO) kinetics cell experimental apparatus (for more details, refer to ref 23), an automatic potentiometric titrator (916 Ti-Touch, B
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels numbers from C8 to C31 are almost equal. C31 and greater carbons with the contents declining dramatically are consistent with the definition of the light crude oil. Analysis results for components < C8 are shown in the top left corner of Figure 1. The SARA fractionation was also conducted. The SARA analysis showed that the contents of saturates, aromatics, and resins are 65.64, 19.92, and 13.74% (w/w), respectively, and the asphaltene content is only 0.71% (w/w). Clearly, this crude oil is mainly composed of saturates (paraffins, linear, branched, and cycled saturated hydrocarbons). To obtain detailed information on the LTO reaction, pseudocomponents with different boiling points are used to perform the kinetics cell experiment, respectively. These pseudo-components are collected through a true boiling point distillation experiment, and the fraction distribution of Dagang light crude oil is listed in Table 3. 2.3. Acid Number Measurement. To evaluate the oxidation behavior of the light crude oil and pseudo-components with different boiling points, 1.5 g of oil sample is thoroughly mixed with 50.0 g of produced sand collected from the Dagang oil field. Sand is first dried for 24 h at 80 °C before use. The mixture is put into the kinetics cell with 5.0 g of cooked sand (pretreated at 600 °C for at least 8 h) in the bottom of the cell. The kinetics cell is sealed and placed in the furnace. The sample is heated at a certain temperature for different times. Air is injected into the cell once the heating process begins. Acid numbers of the crude oil and pseudo-components before and after heating are measured. The mixed solution (standard solution) is prepared by mixing distilled water, isopropanol, and toluene with a ratio of 6:494:500. Unheated samples (0.5 g each) are diluted with 40 mL of standard solution. The acid number measurement is carried out after the oil is completely dissolved. For heated hydrocarbons, 20.0 g of heated oil and sand samples collected from the kinetics cell are placed in a conical flask with 20 mL of standard solution. The flask is sealed and ultrasonicated in an ultrasonic bath for 12 h. Sand and clay are removed by filtering and rinsed 3 or 4 times with the mixed solution, and the effluent is combined with the original solution to obtain a total volume of 40 mL, mixed well, and measured. 2.4. RTO Experiment. To evaluate the LTO behavior of the light crude oil and pseudo-components, the same sample preparation processes are followed in the procedure in section 2.3. A prescribed constant heating rate is programmed into the furnace controller, and the temperature is ramped from 25 to 230 °C. The sample is exposed to air continuously during the heating. Heating stops when the temperature reaches the maximum. At least three different heating rates are used to allow for data analyses with the isoconversional method to calculate the activation energy. RTO experiments with the temperature ranging from 25 to 550 °C are also conducted using the light crude oil. A total of 0.5 g of oil sample is fully mixed with 20.0 g of produced sand. Sand is dried for 24 h at 80 °C before use. The mixture is transferred into the kinetics cell with 5.0 g of cooked sand (pretreated at 600 °C for at least 8 h) in the bottom of the cell and 5.0 g covering. At least three different heating rates are used. 2.5. Isothermal Oxidation Experiment. Isothermal experiments are also performed to study LTO kinetics of the light crude oil for temperatures in the range of 160−430 °C under the pressure of 0.5 MPa. The sample is first placed into the kinetics cell and then heated at a rate of 40 °C/min. Once the furnace approaches the target temperature, the temperature will be held until the concentration of CO2 and O2 becomes steady and constant.
Table 2. Acid Number Analysis of the Light Crude Oil after LTO with Cooked Sand and Produced Sand acid number (mg of KOH/g) test
temperature range (°C)
duration time (h)
cooked sand
produced sand
1 2 3 4 5 6 7
22−80 27−100 30−120 27−140 25−160 25−180 26−200
8 8 8 8 8 8 8
0.93 1.29 1.23 2.37 2.70 4.71 6.75
1.24 1.50 1.60 2.19 3.85 3.69 7.29
Figure 2. Relationship of the acid number of the light crude oil with the oxidation reaction happened in different temperature ranges and duration times.
reaction rate decreases as the temperature becomes lower. Researchers24 reported that the acid number of the crude oil first increased as a result of oxidation at temperatures below 200 °C and then declined at temperatures above 200 °C as a result of thermal degradation of acidic groups. However, different acid number changes were also reported.25,26 The acid number decreases were considered to be caused by the increase of decarboxylation. Table 2 and Figure 2 summarize results from the acid number analyses of the heated whole crude oil. Acid numbers of Dagang light are sensitive to temperature changes and increase with the increase of temperatures (with a constant heating period of 8 h). The experiments were conducted using cooked sand and also repeated with produced sand as well. Both results show the same trend. Curves plotted in Figure 2 indicate that, for the same setting temperature, oils for different heating times also result in different acid numbers; the values and heating time increase are linearly correlated. These results indicate that the acid number increases with the increase of the temperature and reaction time. We take this as evidence that oxygen addition reactions are generating, at least in part, organic acids (e.g., carboxylic acids).27 The original acid number of the whole crude oil, 0.5 mg of KOH/g, increased 4.2 times to 2.59 mg of KOH/g after 40 h of reaction with air (in the range of 30−80 °C). The value is varied and linearly increased as the reaction time increases from 8 to 40 h. When the target temperature is set at different values, such as 100, 120, and 140 °C, a higher acid number is obtained, forming a greater slope of the curve once it is plotted with the reaction time. As for a constant heating time span of 8 h, the sample
3. RESULTS AND DISCUSSION 3.1. Acid Number Variation. Two different kinds of sand samples were used and mixed with the light crude oil to investigate the influence of the LTO process on the acid number variation. One is produced sand produced from the reservoir, and the other is cooked sand collected from Monterey, CA. When considering the LTO, the reaction normally occurs with the temperature no higher than 350 °C.14 Oxygen addition is considered as the major reaction. The C
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Relationships of the (a) acid number and oxygen consumption and (b) acid number with reaction time (22−80 °C).
Table 3. Acid Number Analysis of the Pseudo-component of the Light Crude Oil after LTO test
boiling range of pseudo-component (°C)
weight percent (%, w/w)
temperature range (°C)
duration time (h)
acid number (mg of KOH/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
35−43.5 43.5−70.0 70.0−97.3 97.3−98.0 138−200 138−200 138−200 200−260 330+ 330+ 330+ 330+ 330+ 330+
0.734 2.841 6.075 0.653 3.152
25−160 25−160 25−160 25−160 30−80 30−80 30−160 30−160 28−80 26−80 26−80 28−120 26−140 44−160
8 8 8 8 16 24 48 16 8 16 48 8 8 8
0.15 0.15 0.15 0.15 2.64 3.11 5.04 0.12 4.31 4.26 6.93 4.31 4.26 6.93
9.782 72.948
Figure 4. RTO results for the pseudo-component of the fraction of the boiling range of 138−200 °C. Produced gas concentration variation with time.
Figure 5. RTO results for the pseudo-component fraction of the boiling range of 330+ °C. Produced gas concentration variation with time.
exposed to a higher temperature displays the oil characteristics with even greater acid numbers. As shown in Figure 2, acid numbers rise from around 1.0 to 7.29 mg of KOH/g as the temperature varies from 80 to 200 °C. Hence, the acid number of production is more sensitive to the temperature as the temperature increases, and the reaction time impacts the acid number of the sample to a much greater extent. These results indicate that oxygen consumption becomes greater as the temperature and reaction time increase. Panels a and b of Figure 3 plot correlations of the amount of oxygen consumed versus the resulting acid number and the
resulting acid number versus reaction time, respectively. A positive linear correlation is drawn for moles of oxygen consumption and acid number. Linear regression gives y = 0.022x − 0.017 (1) where x is the acid number. The exposure time of the crude oil versus oxygen helps to explain the scatter in Figure 3a. As the temperature varies from 22 to 80 °C, the acid number also linearly increases with the reaction time (Figure 3b). D
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 9. Temperature and gas concentration variation of the crude oil, in the high-temperature range.
Figure 6. Produced gas concentration history during the LTO process.
boiling ranges (shown in Table 3). The acid number for components with the boiling point lower than 137 °C and the boiling range of 200−260 °C have the same value of 0.15 mg of KOH/g and can be treated as a non-sensitive component to the LTO process. In comparison to other pseudo-components, components with the boiling range of 138−200 °C and higher than 330 °C are sensitive to the oxygen addition and the acid number is increased with the increase of the temperature. Figure 4 presents the RTO results of the pseudo-component with the boiling range from 138 to 200 °C. The temperature is ramped from 25 to 180 °C with the heating rate of 0.323 °C/ min. The oxygen consumption and carbon dioxide curves show that, as the temperature increased, the oxygen addition occurred, with a minor cracking reaction. That is, although oxygen was consumed, very little carbon oxide production measured indicated uptake of oxygen by the crude oil components. Figure 4 also indicates that oxygen consumption occurred when the temperature approached 120−140 °C. Figure 5 shows the RTO results of the pseudo-component with the boiling range greater than 330 °C. Three different temperatures (i.e., 120, 140, and 160 °C, respectively) are set with different heating rates (i.e., 0.200, 0.242, and 0.283 °C/ min, respectively) to obtain different heating effects. The results indicate that concentrations of oxygen consumption are greater at a higher temperature region. In comparison to the 138−200 °C fraction, oxygen addition occurs at even lower temperatures. Oxygen consumption begins at roughly 80 °C in this case. Substantial oxygen consumption with very little carbon oxide production is observed at around the temperature of 110−120 °C. When the temperature increases from 130 to 150 °C, the peak of oxygen consumption is increased from 0.06 to 0.09% (v/v), and this value approaches 0.2% when the temperature goes to 180 °C (Figure 5). The measurement accuracy of the gas analyzer is 0.01%. In these studies, gas analysis experiments are conducted to validate acid number measurement results and try to exhibit the evolution trend of this kind of LTO reaction behavior. Furthermore, the measurement errors can be minimized through proper calibration before running an experiment. In comparison to the other pseudo-component fractions (the boiling range from 70 to 300 °C), the 330+ °C fraction experienced the greatest amount of oxygen addition reaction and, at the same time, its acid number is the greatest (seen in Table 3). Clearly, the fractions of 138−200 and 330+ °C
Figure 7. Consistency analysis of light crude oil.
Figure 8. Isoconversional fingerprint of the light crude oil versus temperature at the low-temperature range.
Because the crude oil is a heterogeneous mixture containing thousands of hydrocarbons and heteroatomic (i.e., O, N, and S) compounds with different molecular weights, chemical structures, and functional groups, particular reactions that correlate to their chemical characteristics, such as oxygen addition, could occur. To figure out the influence of compounds on the acid number variation, experiments were conducted using pseudo-components collected from different E
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 10. Isoconversional fingerprint of the light crude oil with (left) conversion and (right) temperature.
sharply decrease, which eventually make the reaction rate lower than that of heavier components. Hence, for the whole crude oil, oxygen selectively reacts with heavy components and partial oxidized heavy components contribute significantly to the acid number of the sample. 3.2. RTO Kinetics. As for a LTO reaction with a single step, the kinetics can be described as follows: light oil + O2 → oxygenated oil + CO2 + H 2O
For given temperature and pressure conditions, a reaction stoichiometry equation can be proposed. If we eliminate the evaporation effects, the conversion of the reactant (O2) to the product (oxygenated oil) for the above reaction equation can be presented as t
α (t ) =
Figure 11. Isoconversional fingerprint of the light crude oil.
∫0 (CO02 − CO2) dt ∞
∫0 (CO02 − CO2) dt
(2)
where C0O2 is initial concentration of oxygen and CO2 is the oxygen concentration at time t. The RTO experimental results combining with the isoconversional method are used to obtain the activation energy. For the LTO reaction, the reaction rate can be written as ⎛ −E ⎞ dα ⎟ f (α ) = A exp⎜ ⎝ RT ⎠ dT
(3)
where α is the conversion. Using the method of isoconversional analysis by Friendman,28 the apparent activation energy for the light oil is calculated on the basis of the RTO experimental data. Figure 6 shows the temperature and oxygen consumption versus time during the whole heating period of 20−225 °C. Each oxygen concentration curve with only one peak corresponds to each RTO process. That is to say, fuel or oxygenated hydrocarbons, which are deposited from the crude oil at this temperature interval, cannot burn at all in this temperature region. To check the rationality of the RTO experiment or the reliability of activation energy calculations, consistency analysis is a good assessment method.29 We plot ln(d[C]/dt) against −1/T for a consistency check (see Figure 7). The green solid lines connect dots of the same conversion C of different heat rates. The linearity of each line for a particular conversion not only correlate the activation energy (equal to the slope) but also reflect the consistency of those RTO measurements. The poorer the linearites of the lines, the less reliable the activation
Figure 12. Temperature and gas concentration change with time during the isothermal experiment.
undergo the greatest oxygen addition in the low-temperature region. In summary, lighter components with shorter chain organic carbons, which normally have a higher energy barrier, need more energy than longer chain organic carbons after the oxygen incorporation process is completed. Another influential factor that needs to be considered is that lighter components are in favor of evaporation other than staying on the surface of sand, making the contact frequencies of oxygen and light crude oil F
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 4. Calculated Activation Energy of the Sample 3 temperature range (°C)
average temperature (°C)
duration (s)
100.0−113.3 120.7−127.9 137.9−150.9 150.9−181.0 181.0−214.0 214.0−242.0 242.0−273.0 273.0−382.9 382.9−408.4 408.0−600.0
107.4 127.2 148.6 172.8 201.4 235.0 265.3 295.1 337.5
10000 2551 1548 1837 1908 1653 1708 2118 15494
oxygen addition (mol) 0 0 3.60 5.90 4.68 4.07 6.59 5.77 8.90 1.49
total mole number
reaction constant, k
Ea (J/mol) (count on O2)
0.2735 × × × × × × × ×
10−5 10−4 10−3 10−2 10−2 10−2 10−2 10−2
energy measurements. Figure 7 shows the consistency analysis for the data in Figure 6. The results from these measurements of three different heating rates are consistent with each other, and clearly, the Ea values are reliable. Figure 8 shows the apparent activation energy versus temperature. During the LTO and cracking process as the temperature increases from 200 to 225 °C, the Ea shows first a continuous decrease (200−213 °C) in the beginning and then an increase (213−225 °C), which may be attributable to evaporation of light components. Additional experiments over a wider temperature region (from room temperature to 500 °C) were conducted to confirm the reasonability of the Ea results in the lowtemperature region. Panels a and b of Figure 9 show the temperature and CO2 production versus time, respectively. Changes of Ea values with conversion (left) and temperature (right) are plotted in Figure 10. To obtain the whole information on the reaction process, first, we obtain the 100% conversion by complete combustion of the oil. The total amount of oxygen consumption for the LTO reaction can be obtained through the curve of the oxygen concentration versus time, and then the ratio of conversion for LTO and hightemperature oxidation (HTO) regions can be calculated. In these reaction processes, the ratio is around 22%. The LTO and HTO regions are clearly observed in Figure 11 and marked in scatters of blue and red, respectively. In the LTO period of the process with the temperature changing from 20 to 500 °C, the Ea varies in the range of 200−440 kJ/mol as the temperature increases from 260 to 310 °C. During the whole LTO process (20−230 °C), the activation energy varies in the range of 180− 390 kJ/mol as the temperature rises from 200 to 225 °C, indicating that the results are very similar and consistent with each other. Plotted against conversion, the activation energy is fairly constant over the conversions from 0.1 to 0.2. The corresponding reaction temperatures vary from 220 to 230 °C. Combined with the above results of acid numbers, the activation energy decreasing slightly as the temperature increases from 200 to 214 °C indicates that faster oxygen addition reactions occur at the same time. 3.3. Isothermal Kinetics. In addition to the RTO tests, we undertook isothermal tests to measure and confirm LTO kinetics. Figure 12 shows the temperature and effluent gas concentration history. Oxygen addition starts once the temperature reaches roughly 150 °C. The two gas concentration curves of CO2 and O2 differ because of different reaction mechanisms. For chemical reactions occurring with very low reaction rates in the low-temperature region, we assume that a kinetic equation below is appropriate
8.50 1.17 8.97 9.00 1.41 9.96 2.10
× × × × × × ×
10−8 10−8 10−8 10−8 10−8 10−8 10−8
1.69 1.47 1.38 3.37 9.60 6.24
× × × × × ×
105 105 105 104 105 105
r ̅ = dα /dt = k(1 − α)n
(4)
k = Ae−E / RT
(5)
where r ̅ is the average reaction rate and α is the conversion given in eq 3. Assuming that LTO occurs via a one-step reaction, n is 1, and f(α) has the same reaction mechanism as the temperature varies from 150 to 340 °C, we have ln(k1̅ ) E⎛ 1 1⎞ = ⎜ − ⎟ ln(k ̅2) R ⎝ T2̅ T1̅ ⎠
(6)
The results of this activation energy calculation are listed in Table 4. The value of Ea shows a decreasing trend in the temperature range from 150 to 340 °C. The LTO reaction begins with the greatest Ea at around 170 kJ/mol once the temperature approaches 150 °C, indicating the lowest reaction rate. Then, the activation energy is decreased with the increase of the temperature. The Ea reaches the lowest value when the temperature approaches 235 °C.
4. CONCLUSION Various pseudo-components that were extracted from Dagang light crude oil are used to conduct RTO experiments to obtain a better understanding of LTO mechanisms. The oil contains a large proportion of saturates with a relatively small proportion of long-chain hydrocarbons (carbon numbers greater than 31). By analysis of oxygen consumption of the effluent gases and acid number measurements, the increase of the acid number correlates to the increase of oxygen consumption. The reasonable explanation is the formation of carboxylic acid groups in the oil molecules. Different pseudo-components possess different oxidation reactivities. Distillates with boiling ranges of 138−200 and 300+ °C show much greater reactivities than others. Significant acid number variations for both distillates are clearly observed during the LTO process. The Ea values of crude oil derived from the isothermal and RTO experiments are very close, which are consistent with published literature. Meanwhile, RTO experiments accompanying the isoconversional method are used to examine the linearity of the LTO reaction. Activation energies calculated for two temperature intervals (22−220 and 22−550 °C) are compared to validate the measurement results. These results present a successful case to examine the evaluation methods for rationality of the Ea calculation proposed in the literature. Fuels or oxidized hydrocarbons that contain many polar compounds and coke are converted from the LTO process. They can burn when the temperature is high enough. Other factors, however, such as contact time and sweep efficiency as G
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
(16) Freitag, N. P.; Verkoczy, B. Low temperature oxidation of oil in terms of SARA fractions: Why simple reaction models do not work. J. Can. Petrol. Technol. 2005, 44 (03), 54−61. (17) Dabbous, M. K.; Fulton, P. F. Low-temperature-oxidation reaction kinetics and Effect on the In-Situ Combustion Process. SPEJ, Soc. Pet. Eng. J. 1974, 14 (3), 253−262. (18) Phillips, C. R.; Hsieh, I. C. Oxidation reaction kinetics of Athabasca bitumen. Fuel 1985, 64 (7), 985−989. (19) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J., Jr. Reaction kinetics of in-situ combustion: Part 2Modeling. SPEJ, Soc. Pet. Eng. J. 1984, 24 (04), 408−416. (20) Millour, J. P.; Moore, R. G.; Bennion, D. W.; Ursenbach, M. G.; Gie, D. N. An expanded compositional model for low temperature oxidation of Athabasca bitumen. J. Can. Petrol. Technol. 1987, 26 (03), 24−32. (21) Mamora, D. D. New findings in low-temperature oxidation of crude oil. Proceedings of the SPE Asia Pacific Oil and Gas Conference; Kuala Lumpur, Malaysia, March 20−22, 1995; DOI: 10.2118/29324MS. (22) Zhao, R. B.; Chen, Y. X.; Huan, R. P.; Castanier, L. M.; Kovscek, A. R. An experimental investigation of the in-situ combustion behavior of Karamay crude oil. J. Pet. Sci. Eng. 2015, 127 (3), 82−92. (23) Cinar, M.; Castanier, L. M.; Kovscek, A. R. Isoconversional Kinetic Analysis of the Combustion of Heavy Hydrocarbons. Energy Fuels 2009, 23 (8), 4003−4015. (24) Parker, R. J.; Chung, E. S. N. Acid numbers of Saskatchewan heavy oils. J. Can. Pet. Technol. 1986, 25 (4), 72−75. (25) Zhang, P.; Austad, T. The relative effects of acid number and temperature on chalk wettability. Proceedings of the SPE International Symposium on Oilfield Chemistry; Houston, TX, Feb 2−4, 2005; DOI: 10.2118/92999-MS. (26) Bi, Y. T.; Wang, G.; Shi, Q.; Xu, C. M.; Gao, J. S. Compositional changes during hydrodeoxygenation of biomass pyrolysis oil. Energy Fuels 2014, 28 (4), 2571−2580. (27) Zhao, R. B.; Xia, X. T.; Luo, W. W.; Shi, Y. L.; Diao, C. J. Alteration of heavy oil properties under in situ combustion: A field study. Energy Fuels 2015, 29 (10), 6839−6848. (28) Friedman, H. L. Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic. J. Polym. Sci., Part C: Polym. Symp. 1964, 6 (1), 183−195. (29) Chen, B.; Castanier, L. M.; Kovscek, A. R. Consistency measures for isoconversional interpretation of in-situ combustion reaction kinetics. Energy Fuels 2014, 28 (2), 868−876.
well as the amount of heat produced in the LTO process, also play key roles in the transfer from LTO to a combustion process. Therefore, it is even harder to predict an evolution of LTO for light oil than that of the ISC process for heavy oil.
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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 authors thank Dr. Guixue Song for helpful discussion. The work is supported by the National Science and Technology Major Project (2016ZX05012). The project of the National Natural Science Foundation of China (Award 51274217) is also appreciated.
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REFERENCES
(1) Shah, A.; Modi, K. Feasibility of high pressure air injection in heterogeneous light oil reservoir by thermal simulation. Proceedings of the SPE EOR Conference at Oil and Gas West Asia; Muscat, Oman, April 16−18, 2012; DOI: 10.2118/152978-MS. (2) Fassihi, M. R.; Yannimaras, D. V.; Kumar, V. K. Estimation of recovery factor in light oil air injection projects. SPE Reservoir Eng. 1997, 12 (3), 173−178. (3) Sakthikumar, S.; Madaoui, K.; Chastang, J. An investigation of the feasibility of air injection into a water flooded light oil reservoir. Proceedings of the Middle East Oil Show; Bahrain, March 11−14, 1995; DOI: 10.2118/29806-MS. (4) Yannimaras, D. V.; Tiffin, D. L. Screening of oils for in situ combustion at reservoir conditions by accelerating rate calorimetry. SPE Reservoir Eng. 1995, 10 (1), 36−39. (5) Germain, P.; Geyelin, J. L. Air injection into a light oil reservoirs: The Horse Creek project. Proceedings of the Middle East Oil Show and Conference; Bahrain, March 15−18, 1997; DOI: 10.2118/37782-MS. (6) Watts, B. B.; Hall, T. F.; Petri, D. J. The Horse Creek air injection project: An overview. Proceedings of the SPE Rocky Mountain Regional Meeting; Casper, WY, May 18−21, 1997; DOI: 10.2118/38359-MS. (7) Niu, B. L.; Ren, S. R.; Liu, Y. H.; Wang, D. Z.; Tang, L. Z.; Chen, B. L. Low-temperature oxidation of oil components in an air injection process for improved oil recovery. Energy Fuels 2011, 25 (10), 4299− 4304. (8) Chen, Z. Y.; Wang, L.; Duan, Q.; Zhang, L.; Ren, S. R. Highpressure air injection for improved oil recovery: Low-temperature oxidation models and thermal effect. Energy Fuels 2013, 27 (2), 780− 786. (9) Ren, S. R.; Greaves, M.; Rathbone, R. R. Oxidation kinetics of North Sea light crude oils at reservoir temperature. Chem. Eng. Res. Des. 1999, 77 (5), 385−394. (10) Burger, J. G. Chemical aspect of in situ combustion heat of combustion and kinetics. SPEJ, Soc. Pet. Eng. J. 1972, 12 (5), 410−422. (11) Kapadia, P. R.; Kallos, M. S.; Gates, I. D. A review of pyrolysis, aquathermolysis, and oxidation of Athabasca bitumen. Fuel Process. Technol. 2015, 131 (3), 270−289. (12) Babu, D. R.; Cormack, D. E. Effect of oxidation on the viscosity of Athabasca bitumen. Can. J. Chem. Eng. 1984, 62 (4), 562−564. (13) Fassihi, M. R.; Meyers, K. O.; Baslie, P. F. Low-temperature oxidation of viscous crude oils. SPE Reservoir Eng. 1990, 5 (4), 609− 616. (14) Morton, F.; Bell, R. T. T. The low temperature liquid phase oxidation of hydrocarbons: A literature survey. J. Inst. Petrol. 1958, 44 (417), 260−272. (15) Khansari, Z.; Gates, I. D.; Mahinpey, N. Detailed study of lowtemperature oxidation of an Alaska heavy oil. Energy Fuels 2012, 26 (3), 1592−1597. H
DOI: 10.1021/acs.energyfuels.5b02832 Energy Fuels XXXX, XXX, XXX−XXX
