Combustion Reaction Kinetics Studies of Turkish Crude Oils

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Energy & Fuels 2004, 18, 1472-1481

Combustion Reaction Kinetics Studies of Turkish Crude Oils Suat Bagcı and Mustafa Versan Kok* Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531 Ankara, Turkey Received January 22, 2004. Revised Manuscript Received June 2, 2004

In this research, sixteen experiments were conducted to study the combustion reaction kinetics of Turkish crude oils (Batı Raman, Ç amurlu, Raman, Adıyaman, Garzan, Karakus¸, and Beykan) in a limestone medium. A mixture of the limestone and the crude oil was subjected to a controlled heating program under a constant flow rate of air. The produced gas was analyzed for its oxygen and carbon oxides contents. Although the molar CO2/CO ratios vary during low-temperature oxidation (LTO), for fuel deposition (FD) and high-temperature oxidation (HTO), these values can be indicated at different temperatures. A decrease in the atomic H/C ratio with an increase in temperature was observed for all runs. The activation energies of the Ç amurlu, Batı Raman, and Raman crude oils for both fuel deposition and high-temperature oxidation reactions were observed to be similar. For medium-gravity oils (Adıyaman and Garzan), the activation energies for the high-temperature oxidation reaction is higher than that for the fuel deposition reaction. For the LTO reaction, the activation energies are almost twice those of FD and HTO reactions for each crude oil. The activation energies are almost independent of the gravity of the oil used. The Arrhenius constant is not affected by the API gravity of the oils.

Introduction In situ combustion is a technique that is suitable for the recovery of oil from medium to heavy crude-oil reservoirs. In this process, crude oil is ignited at the well bore and a combustion front is generated via a continuous injection of air into the reservoir. The combustion front is sustained as long as enough coke is produced by the cracking of crude oil to be consumed as fuel. The major constraint limiting the applicability of in situ combustion is the amount of fuel formed on the reservoir matrix ahead of the combustion zone. If insufficient fuel were deposited, as can be the case for light oils, the combustion front would not be self-sustaining and will be quickly extinguished. In contrast, heavy crude can produce too much fuel, which results in either a slow advance of the combustion front or incomplete combustion of the fuel. Forward in situ combustion is described by a simple chain reaction that consists of two competitive steps: fuel deposition (FD) and high-temperature oxidation (HTO).1-3 A third reaction (low-temperature oxidation (LTO)) may occur, if oxygen is present downstream from the combustion front. FD is the process of leaving fuel on the reservoir matrix. The amount of fuel deposited, or fuel concentration, is an important parameter in in situ combustion project design. A high fuel concentration will reduce the combustion front velocity and

increase air requirements, which will result in higher air compression costs. On the other hand, if the fuel concentration is too low, combustion heat generated may be insufficient to propagate a self-sustaining combustion. The hydrocarbon fuel deposited at the combustion zone reacts with injected oxygen to generate heat for the combustion process. LTO of crude oil occurs at temperatures less than ∼345 °C (650 °F). The reaction between fuel and oxygen in an in situ combustion process is a heterogeneous flow reaction. To sustain the combustion, the injected oxidant gas must be passed through the burning zone. In the combustion zone, exothermic heterogeneous reactions occur between oxygen in the gas phase and the heavy residue of oil that is deposited on the rock matrix at lower temperatures. The reaction rate is often assumed to be first order, with respect to fuel concentration.4 Burger and Sahuquet5 considered the complete and incomplete combustion reactions of hydrocarbon to carbon dioxide (CO2) and carbon monoxide (CO) in LTO and HTO reactions. Dabbous and Fulton2 used isothermal integral reactor data to evaluate the kinetics of the LTO of crude oil. Thomas et al.3 characterized the overall forward combustion process via a simple two-step chain reaction, namely fuel laydown and fuel burnoff. Hughes et al.6 used a combustion cell, which was operated at atmospheric pressure, to study the effect of the sand surface area and oxygen partial pressure on the combustion of

(1) Bousaid, I. S.; Ramey, H. J., Jr. Soc. Pet. Eng. J. 1968, 137148. (2) Dabbous, M. K.; Fulton, P. F. J. Can. Pet. Technol. 1974, 2023. (3) Thomas, G. W.; Buthod, A. P.; Allag, O. Report No. BETC-18201, U.S. Department of Energy, 1979.

(4) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J., Jr. Presented at the 55th Annual Fall Technical Conference and Exhibition of SPE, Dallas, TX, 1980, Paper No. SPE 9454. (5) Burger, J. G.; Sahuquet, B. C. Soc. Pet. Eng. J. 1972, 410-422. (6) Hughes, R.; Kamath, V. M.; Price, D. Chem. Eng. Res. Des. 1987, 65, 23-28.

10.1021/ef040014g CCC: $27.50 © 2004 American Chemical Society Published on Web 08/04/2004

Combustion Reaction Kinetics of Turkish Crude Oils

a sample of Marguerite Lake heavy crude oil. The apparent activation energy of the high-temperature oxidation reaction was observed to decrease with increasing surface area of the substrate and was also affected by the partial pressure of oxygen over the range used. Another interesting feature was that the fraction of oxygen utilized increased as the partial pressure of oxygen decreased, which indicates that most of the oxygen was not consumed in the reaction and appeared in the exit gas. In 1990, Dubdub et al.7 used a small packed differential flow reactor to study the combustion reaction kinetics of Athabasca tar sand. They investigated the effect of oxygen partial pressure on the reaction kinetics parameters. The importance of increased partial pressure of oxygen to give better use of the fuel that was laid down in the combustion process is related to the virtual completion of LTO reactions at 470 °C with high concentrations of oxygen (g30%-40%). At lower oxygen concentrations, LTO reactions seem to persist into the high-temperature combustion region. Increased oxygen concentrations also produced a decrease in the activation energy for the high-temperature combustion reaction. Belgrave et al.8 described a unified pseudo-mechanistic reaction model for mathematical modeling of the in situ combustion of Athabasca bitumen. The model represented a consolidation of individual experimental kinetic studies on thermal cracking and LTO of Athabasca bitumen. The formulation was comprehensive in that it allowed bitumen to undergo density and viscosity increases, as well as reduced reactivity to oxidation, with increased oxidation extent. Ren et al.9 studied the oxidation kinetics of light crude oils; the investigations were made at typical reservoir temperatures (90-140 °C) and high pressures. The oxygen consumption rate was measured from the reduction in the oxygen partial pressure, using a small batch reactor. Measured pressure data for different crude oils were used to establish a simple reaction rate model of acceptable accuracy for reservoir simulation. At the relatively low reservoir temperature of the experiments, the main gaseous product from the reaction was CO2. Abu-Khamsin10 investigated the LTO of four Arabian crudes as well as blends of naphtha with a superlight crude, using differential thermal analysis (DTA). The mass fraction of in-saturates in the reactants varied between 0.2 and 0.9. All reactants showed LTO peaks at 230-264 °C; heat flow at the peak, however, varied widely. The data revealed a clear increase in LTOgenerated heat as the reactant content of un-saturates increased. The lightest crude, with 51.1 °API gravity and un-saturates fraction of 0.2, showed the least LTO reactivity. Therefore, it is concluded that the unsaturates content of a crude is an influential factor in its LTO tendency and, thus, its potential for spontaneous ignition and other enhanced-recovery techniques that rely on LTO. Al-Saffar et al.11 investigated the oxidation behavior of a North Sea light crude oil and (7) Dubdub, I.; Hughes, R.; Price, D. Chem. Eng. Res. Des. 1990, 68, 342-349. (8) Belgrave, J. D. M.; Moore, R. G.; Ursenbach, M. G.; Bennion, D. W. SPE Adv. Technol. Ser. 1990, 1, 98-107. (9) Ren, S. R.; Greaves, M.; Rathbone, R. R. Chem. Eng. Res. Des. 1999, 77, 385-394. (10) Abu-Khamsin, S. A. Pet. Sci. Technol. 2003, 21, 1065-1075. (11) Al-Saffar, H. B.; Hasanin, H.; Price, D.; Hughes, R. Chem. Eng. Res. Des. 2001, 15, 182-188.

Energy & Fuels, Vol. 18, No. 5, 2004 1473

its individual SARA fractions (saturates, aromatics, resins, and asphaltenes) in the presence of consolidated cores. The data from these experiments may be used to assist the development of a comprehensive simulator for thermal oil recovery. Another purpose of the overall work is to replace the conventional two pseudo-component representation of crude oil (heavy and light) by a SARA representation with its accompanying interaction and interconversion reactions, together with the conventional oxidation reactions. This approach should provide a more realistic representation of the oil oxidation process. Kissler and Shallcross12 studied the oxidation kinetics of a light Australian crude oil, using an evolved gas analysis technique. The effluent gas was continually analyzed for its oxygen, CO, and CO2 contents. The oxidation behavior was determined to be substantially different from that previously observed for heavier crudes. An improved mathematical model is proposed that allows the oxidation kinetics of the oil to be modeled by considering three competing and overlapping classes of reactions. The model allows the kinetic parameters for each class of reaction to be estimated. The effects on the kinetic parameters of variables such as pressure, sand grain size, and CO2 content of the injection gas were also investigated experimentally. Sonibare et al.13 studied the thermal behavior of the Nigerian oil sand bitumen in an oxidizing environment using nonisothermal thermogravimetric analysis (TGA) and DTA. The kinetics of the reactions was also determined using an Arrhenius plot method. Three regions of weight lossscorresponding to LTO, FD, and HTOs were identified. Increasing the heating rate caused a shift in the reaction regions and peak temperatures to higher temperatures. No effect of gas flow rate was observed on the reactions. The oil sands have lower peak temperatures and activation energies, compared to their corresponding bitumen extracts, suggesting a catalytic effect of sand on the reactions. DTA revealed the exothermic nature of the reactions. The exothermicity increased as the heating rate increased. Kok and coworkers14-19 analyzed the pyrolysis and combustion properties of crude oils via differential scanning calorimetry-thermogravimetry/differential thermal gravimetry (DSC-TG/DTG) and partial differential scanning calorimetry (PDSC). They characterized the pyrolysis and combustion properties of crude oils. When they were combusted in air, three different reaction regions were identified: LTO, FD, and HTO. Crude oil pyrolysis indicated two main temperature ranges where loss of mass was observed. The first region between ambient and 400 °C; this region was termed the distillation region. The second region, at 400-600 °C, was termed visbreaking and thermal breaking. A computer program was developed to process the data automatically, to (12) Kisler, J. P.; Shallcross, D. C. Chem. Eng. Res. Des. 1997, 75, 392-400. (13) Sonibare, O. O.; Egashira, R.; Adedosu, T. A. Thermochim. Acta 2003, 405, 195-205. (14) Kok, M. V.; Okandan, E. Fuel 1992, 71, 1499-1503. (15) Kok, M. V. Thermochim. Acta 1993, 214, 315-327. (16) Kok, M. V.; Sztatisz, J.; Pokol, G. Energy Fuels 1997, 11, 11371142. (17) Kok, M. V.; Karacan, O. J. Therm. Anal. Calorim. 1998, 52, 781-788. (18) Kok, M. V.; Iscan, A. G. J. Therm. Anal. Calorim. 2001, 64, 1311-1318. (19) Kok, M. V.; Keskin, C. Thermochim. Acta 2001, 369, 143-147.

1474 Energy & Fuels, Vol. 18, No. 5, 2004

Bagcı and Kok

Figure 1. Schematic diagram of the reaction kinetics apparatus. Legend is as follows: 1, high-pressure air cylinder; 2, highpressure nitrogen cylinder; 3, rotameter; 4, reaction kinetics cell; 5, digital temperature readout; 6, temperature programmer control unit; 7, continuous gas analyzer; and 8, wet test meter.

estimate the reaction parameters of the crude oils studied. Bagˇcı and Kok20 performed dry and wet forward combustion experiments for Turkish heavy oil reservoirs (Raman, Adiyaman, Camurlu, and Batı Kozluca), under different experimental conditions. In the experiments, a vertical tube was packed with crushed limestone and saturated with crude oil and water. The peak temperatures were observed to be higher when stabilized combustion was achieved and decreased as the combustion front approached the outlet end of the tube. Bagˇcı and Aybak21 developed a three-dimensional (3D) physical model to investigate the combustion override splitproduction horizontal well (COSH) process. A total of eight experimentsssix of dry combustion and two of wet combustionswere conducted to investigate the process parameters. Combustion peak temperatures up to 450 °C were recorded, with oil recoveries exceeding 40% of the original oil in place for the single vertical producer well case. The recovery using a horizontal producer placed at the base of the model was 65% of the original oil in place. Bagˇcı22 conducted dry and wet combustion runs, using three different crude oils. The thickness of the combustion zone decreased as the °API gravity of crude oils increased. With water injection, the thickness of the combustion zone decreased, as expected in a wet combustion temperature profile. Temperature and oxygen concentration data were used to evaluate the commonly used combustion reaction rate expression. The experimentally determined relationship between oxygen, combustion temperature, and reaction rate could not be represented by the commonly used firstorder, Arrhenius-type reaction rate expression. This research has been conducted to study the reaction kinetics parameters of different °API gravity crude oils produced in Turkey in a limestone medium. Experimental Section The crude oil samples used throughout the research were from the seven different crude oil reservoirs in Turkey. They were from the Batı Raman, Ç amurlu, Raman, Adıyaman, Garzan, Karakus¸ , and Beykan fields. The properties of these crude oils are given in Table 1. The premixing method has (20) Bagcı, S.; Kok, M. V. Fuel Process. Technol. 2001, 74, 65-79. (21) Bagcı, S.; Aybak, T. J. Can. Pet. Technol. 2000, 39, 42-50. (22) Bagcı, S. Energy Fuels 1998, 12, 1153-1160.

Table 1. Properties of Crude Oils crude oil

API gravity (°API)

viscosity at 20 °C (cp)

Batı Raman Ç amurlu Raman Adıyaman Garzan Karakus¸ Beykan

12 12 18 26 28 29 32

52 000 64 000 2260 64 37 87 12

Table 2. Properties of Packing Data property

value

matrix type porosity permeability oil saturation

crushed limestone 38% 10 darcy 15%

been used to prepare sand pack mixtures for the runs. A mixture of 20 g of crushed limestone and oil was packed in a reaction kinetics cell. An oil saturation of ∼15% was chosen, assuming that the average oil saturation in the combustion zone is the oil saturation responsible for the coke deposition and burning. The properties of packing data are given in Table 2. In this research, a reaction kinetics cell was used to conduct the experiments. The reaction kinetics cell is constructed from a stainless-steel tube, and its dimensions are as follows: length, 130 mm; outside diameter, 34 mm; and wall thickness, 1.1 mm. Two iron-constantan thermocouples are inserted into the reaction kinetics cell: one of them is used to measure the temperatures at the center of the reaction cell. Another thermocouple is used to control and measure the temperature of the heater. The cell was equipped with a furnace, temperature programmer, air and nitrogen flow rate controller, digital temperature indicator, pressure gauges, and continuous gas analysis equipment. A simplified schematic diagram of the reaction kinetics cell and the auxiliary equipment is shown in Figure 1. The reaction kinetics cell was placed concentrically in a cylindrical tube surrounded by electrical heating coils. The center thermocouple was inserted into the cell to measure the temperature in the center of the cell. Another thermocouple that was welded on the tube within electrical heating coils was used to control the heating temperature, which was given by a temperature programmer. A variable temperature program was applied to the reaction kinetics cell, with increasing temperature at a constant heating rate, to determine the reaction rate data. During the experiments, the initial temperature of the reaction cell was set to ∼20 °C; the cell temperature then was increased to 100 °C at a rate of 5 °C/ min rate and held at that temperature for 0.5 h at 100 °C, to



Figure 2. Gas compositions and temperature as a function of time for Run No. CAM01. Table 3. Experimental Conditions Run No.

API gravity (°API)

air injection pressure (kPa)

air injection rate (L/min)

BR01 BR02 BR03 BR04 CA01 CA02 RA01 RA02 AD01 AD02 GA01 GA02 KR01 KR02 BY01 BY02

12 12 12 12 12 12 18 18 26 26 28 28 29 29 32 32

35 70 172 345 172 345 172 345 172 345 172 345 172 345 172 345

1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5

reach thermal equilibrium in the reaction cell. The reaction cell temperature then was increased at a constant rate of 1 °C/min after this point throughout the heating of the cell, and air (21% oxygen, 79% nitrogen) was injected into the cell at a constant rate of 1.5 L/min. The cell was continuously heated to 500-600 °C, using a heating rate of 1 °C/min. During the experiments, the center temperatures in the cell and O2, CO2, and CO gases were analyzed and recorded from a continuous gas analyzer. This recording procedure continued until no carbon oxide gases were observed in the gas produced from the reaction kinetics cell. To check the reproducibility of the experimental data, two standard runs for the oils were performed under the same experimental conditions, and the results were identical.

Results and Discussion A total of sixteen combustion reaction kinetics runs were performed, using seven different crude oils. Table 3 gives the experimental conditions for each of the runs. Throughout the reaction kinetics runs, the initial objective was to analyze the gases produced from the combustion reaction during heating to a temperature range of 600 °C. Produced gas compositions and tem-

perature versus time curves are shown in Figures 2-5. They represent the produced CO2, CO, and consumed oxygen and the temperature (given in Kelvins) of the pack, as a function of time from the beginning of air injection. Two apparent peaks existed for consumed oxygen, CO2, and CO at different temperatures and pressures for the runs. The first peak represents the low temperature oxidation (LTO), whereas the second peak represents the high-temperature oxidation (HTO) or fuel combustion. Between these two successive peaks, there is an interval, which shows the fuel deposition as the temperature is increased. During this reaction, the crude oil is coked and deposited on the solid matrix as fuel during the in situ combustion process. At low temperatures, some oxygen is consumed to produce carbon oxides as CO2 and CO, which is a smaller amount than the oxygen consumed, indicating that some oxygen is consumed in other reactions. At high temperatures, almost all of the oxygen is consumed to produce CO2 and CO, which indicates complete combustion. These two peaks are observed in the standard runs, with both crude oils at pressures of 172 and 345 kPa. At higher temperatures, the oxygen consumed is stoichiometrically equivalent to the carbon oxides produced. However, at lower temperatures, the oxygen consumption is greater than that accounted for in the carbon oxides. This suggests that LTO and/or the cracking reaction are occurring in this lower-temperature region. The behavior of the lighter Beykan crude oil is quite different, as shown in Figures 4 and 5. Distinct peaks of almost-equal magnitude now are evident in both the LTO region and the HTO region. The increased oxygen consumption and greater production of CO2 and CO at lower temperatures for the lighter crude indicates a higher reactivity of this oil with oxygen at lower temperatures, as might be expected. Another feature of the combustion of the Beykan crude is that the concentration of CO2 is now much greater than that of CO in both peaks. Thus, although in this case, the total fuel deposited and/or burnt is low, compared to that for the


Bagcı and Kok

Figure 3. Gas compositions and temperature as a function of time for Run No. CAM02.

Figure 4. Gas composition and temperature as a function of time for Run No. BEY01.

heavy crude, almost all of the oxygen consumed is accounted for, particularly in the HTO region. This suggests less cracking than with the heavy crude. Throughout the runs, the air injection rate was held constant. As a result of the reaction of oxygen from air with the crude oil, the rate of produced gases varied. In HTO, the amount of oxygen consumed is comparable to the amount of produced carbon oxides. In Figure 6, oxygen consumption curves are shown for different °API gravity crude oils. The increased oxygen consumption and greater production of CO2 and CO at lower temperatures for the heavier crude indicate a higher reactivity of these oils with oxygen at lower tempera-

tures, as might be expected. Another feature of the combustion of the heavy crude is that the concentration of CO2 is now much greater than that of CO in both peaks (Figures 7 and 8). Thus, although in this case, the total fuel deposited and/or burnt is low, compared to that for the heavy crude, almost all of the oxygen consumed is accounted for, particularly in the HTO region. This suggests that less cracking has occurred during fuel deposition and combustion than with the heavy crude. To investigate the fuel combustion reactions, the molar CO2/CO ratios were calculated for each run. When oxygen in air reacted with crude oil, CO2 and CO are



Figure 5. Gas composition and temperature as a function of time for Run No. BEY02.

Figure 6. Oxygen consumption, as a function of time for different API gravity oils.

produced as the primary products. Furthermore, the CO2 composition of the exit gases increased as the temperature of the reaction kinetics cell increased. The values of molar CO2/CO ratios for fuel combustion reactions are given Table 4. The molar CO2/CO ratios of carbon oxides produced from reaction of crude oils at different temperatures are shown in Figure 9, when oxygen in air reacted with crude oil, CO2 and CO are produced as the primary products. The molar CO2/CO values for the light crude oils were less than the molar CO2/CO values of high API gravity crude oils. The atomic H/C ratio of the reacting fuel was calculated for each combustion reaction. The atomic H/C ratio of the

fuel consumed was calculated from the analysis of produced gases for each run, as a function of temperature. These calculations were based on the assumption that all the oxygen not observed in the exit gas was reacted to form water. A general decrease in the atomic H/C ratio with an increase in temperature was observed for all runs. According to these results, the atomic H/C ratio of fuel decreased as the temperature increased. Kinetic Analysis A kinetic model developed by Weijdema23 and adapted to the reaction kinetics studies by Fassihi et al.24 was


Bagcı and Kok

Figure 7. Combustion peak temperature, as a function of time for different API gravity oils.

Figure 8. CO2 + 1/2CO, as a function of time for different API gravity oils.

used for analysis of nonisothermal runs of this study. The rate of oxygen consumption per unit volume is

oil. The rate of oxygen consumption at any time can be obtained by combining eqs 1 and 2:

∆γ E m n q C exp ) ArPO 2 f AL RT

dCf E ∆γ m n C exp ) -R q ) ArPO f 2 AL RT dt

(

)

(1)

This is also equal to the rate of decrease of oil saturations:

dCf ∆γ ) -R q AL dt

(2)

in which R is the proportionality factor, equal to the amount of oxygen (in moles) that reacts with 1 g of the (23) Weijdema, J. Report from Koninklijhe/Shell, Exploratie En Produktic Laboratorium, Rijswijk, The Netherlands, 1968.

(

)

(3)

Integration between time t and t ) ∞ yields

RCf(t) )

∆γ ∫t∞ (qAL ) dt′

(4)

where Cf ) 0 at t ) ∞. From eq 3, we obtain

∆γ 1 Cf n(t) ) q AL ArPm Cn exp[-E/(RT)] O2 f

( )

(5)



Figure 9. Molar CO2/CO ratio, as a function of temperature for different API gravity oils. Table 4. Molar CO2/CO and Activation Energies of Crude Oils

Run No. BR01 BR02 BR03 BR04 CA01 CA02 RA01 RA02 AD01 AD02 GA01 GA02 KR01 KR02 BY01 BY02

API gravity (°API)

air injection pressure (kPa)

molar CO2/CO

activation energy (J/g-mol)

12 12 12 12 12 12 18 18 26 26 28 28 29 29 32 32

35 70 172 345 172 345 172 345 172 345 172 345 172 345 172 345

3.64 3.71 4.11 4.26 4.60 6.40 5.00 7.50 4.34 6.20 3.80 6.30 3.21 2.75 3.22 2.06

81.112 84.679 87.687 88.922 80.797 89.586 59.477 70.337 66.221 70.087 65.822 66.503 85.200 88.800 107.900 113.800

If we substitute eq 5 in eq 4, we obtain

∆γ [

∫t ∆γ dt′] ∞

n

(

) β′ exp -

E RT

)

(6)

where m

q n-1ArPO2 β′ ) AL Rn

( )

(7)

Values for the left-hand side of eq 6 can be found by graphical integration of the curve ∆γ ) f(t). The logarithm of the left-hand side of eq 6 then can be graphed vs 1/T to obtain -E/(2.303R) as the slope and log β′ as the intercept. To find the values of the lefthand side of eq 6, the graphical integration can be applied to the curve W ) f(t). The trapezoidal rule was (24) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J., Jr. SPEJ, Soc. Pet. Eng. J. 1984, 24, 399-416. (25) Lewis, W. K.; Gilliand, E. R.; Paxton, R. R. Ind. Eng. Chem. 1954, 1327-1331.

Table 5. Arrhenius Constant (Ar) and Reaction Order (m) Values of Crude Oils crude oil

°API gravity

Arrhenius constant, Ar (L/min)

reaction order, m

Batı Raman Ç amurlu Raman Adıyaman Garzan Karakus¸ Beykan

12 12 18 26 28 29 32

44.9 12.65 1037 437 000 6.41 45.8 0.048

0.65 0.39 0.22 0.55 0.49 1.01 3.94

applied to the area under this curve to calculate the relative reaction rate. In this model, the temperature linearly increases with time, and by proper graphing of the variables, a semilogarithmic straight line should result. The relative reaction rate was calculated and is graphed. At lower temperatures, a departure from the straight line is observed. At high temperatures, the amount of carbon oxides formed closely matches the amount of oxygen consumed; however, at medium temperature, the oxygen consumed is greater than the carbon oxides formed. At low temperatures, oxygen is consumed with no carbon oxide formation. Plots of the relative reaction rate versus the inverse of temperature for all crude oils are shown in Figure 10. Straight lines of the same slope were drawn in the Arrhenius plots shown in Figure 11. From this figure, a straight line was obtained for the fuel combustion region. Activation energy values for the fuel combustion region were determined from the slope of that line and are tabulated in Table 4. The activation energies of the Ç amurlu, Batı Raman, and Raman crude oils for both FD and HTO reactions were observed to be similar. For lighter oils (Karakus¸ and Beykan) the activation energies for the HTO reaction are higher than that of the FD reaction. For medium gravity oils (Adıyaman and Garzan), the activation energies for the HTO reaction is higher than that for the FD reaction. The same trends were also observed in the research performed by thermal analysis experiments with the same crude oils.15,16 Using the log-log plot of the true


Bagcı and Kok

Figure 10. Relative reaction rate, as a function of inverse temperature for different API gravity oils.

Figure 11. Arrhenius plot of the high-temperature oxidation (HTO) region for all crude oils. Table 6. Kinetic Parameters of the Crude Oils from Literature author

crude oil

bed type

Arrhenius constant (Pa-1 s-1)

activation energy (J/g-mol)

pressure (kPa)

m

n

Lewis et al.25 Bousaid and Ramey1 Bousaid and Ramey1 Bousaid and Ramey1 Dabbous and Fulton2 Weijdema23 Fassihi et al.4,24 present work present work present work present work present work present work present work

metallurgical coke precoked (22 °API) 14 °API 14 °API 20 °API n.a. 18.5 °API Batı Raman (12 °API) Ç amurlu (12 °API) Raman (18 °API) Adıyaman (26 °API) Garzan (28 °API) Karakus¸ (29 °API) Beykan (32 °API)

fluidized Berea sand 80% Berea sand, 20% clay Berea sand Berea sand sand pack sand pack limestone limestone limestone limestone limestone limestone limestone

7.4 × 10-2 1.38 × 10-3 2.43 × 10-4 2.37 × 10-3 1.38 × 10-3 n.a. 6 × 10-5 4.49 × 101 1.265 × 101 1.037 × 103 4.37 × 105 6.41 4.58 × 101 4.8 × 10-2

121 400 59 800 48 400 61 900 58 900 125 500 135 000 88 922 89 586 70 337 70 087 66 503 88 800 113 800

100 200 500 300 300 4000 54 345 345 345 345 345 345 345

1 1 1 1 1 n.a. 0.66 0.65 0.39 0.22 0.55 0.49 1.01 3.94

1 2 1 1 2 n.a. 1 1 1 1 1 1 1 1

intercept, with respect to the partial pressure of oxygen, both the Arrhenius constant (Ar) and the reaction order

with respect to the oxygen partial pressure (m) can be found. The intercept in this graph is Ar and the slope of


it gives m. Using the same procedure, Ar and m were evaluated for each of the runs (Table 5). Although the reaction order with respect to the fuel concentration (n) does not change, the reaction order with respect to the oxygen partial pressure (m) varies, as given in the literature (Table 6). The Arrhenius constant Ar is different for all oils during different reactions. It is observed that the operating pressure increased the Ar value for all crude oils. Conclusions A total of sixteen experiments were performed, as well as a reaction kinetics analysis of seven crude oils. The following conclusions were drawn: (1) A series of reactions were observed during the oxidation of crude oil in porous media by air injection as (i) low-temperature oxidation (LTO) reactions, (ii) fuel deposition (FD) reactions, and (iii) high-temperature oxidation (HTO) reactions. (2) Although the molar CO2/CO ratios vary during LTO, those observed during FD and HTO can be indicated at different temperatures.


(3) The atomic H/C ratio decreased as the temperature for each crude oil increased, and the following relationship was observed: ln(H/C) ) a - bT. (4) The activation energies of the Ç amurlu, Batı Raman, and Raman crude oils for both FD and HTO reactions were observed to be similar. For lighter oils (Karakus¸ and Beykan), the activation energies for the HTO reaction is higher than that for the FD reaction. For medium gravity oils (Adıyaman and Garzan), the activation energies for the HTO reaction is higher than that for the FD reaction. For the LTO reaction, the activation energies are almost twice that of the FD and HTO reactions for each crude oil. The activation energies are almost independent of the gravity of the oil used. The Arrhenius constant is not affected by the API gravity of the oils. (5) The reaction order (m) in the Arrhenius equation varied, with respect to the oxygen partial pressure, and m < 1 for all the oils except for the Beykan crude oil. The Arrhenius constant (Ar) is not affected by the API gravity of the crude oils. EF040014G

Combustion Reaction Kinetics Studies of Turkish Crude Oils

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