Detailed Study of Low-Temperature Oxidation of an Alaska Heavy Oil

Feb 18, 2012 - ABSTRACT: In this work, low-temperature oxidation (LTO) of a heavy oil sample from Alaska has been investigated. Six isothermal and one...
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Detailed Study of Low-Temperature Oxidation of an Alaska Heavy Oil Zeinab Khansari, Ian D. Gates, and Nader Mahinpey* Department of Chemical and Petroleum Engineering, Schulich School of Engineering, The University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada ABSTRACT: In this work, low-temperature oxidation (LTO) of a heavy oil sample from Alaska has been investigated. Six isothermal and one non-isothermal experimental runs were conducted between 100 and 350 °C, where LTO dominates. The combustion data obtained from a thermogravimetric analyzer (TGA) was analyzed, and a comparison has been made between the kinetic parameters (reaction order, rate constant, and activation energy) from a first-order, general reaction rate and Segal and Fatu’s approach. The temperature scan of the oxidation process revealed that there were four temperature intervals during which different modes dominated the LTO process. The first temperature interval, ranging from 100 to 150 °C, and the third interval, ranging from 200 to 250 °C, both had overall reactions that were endothermic. However, in the second zone from 150 and 200 °C and the fourth subzone from 250 to 350 °C, exothermic reactions were dominant. The peak LTO rate occurred during the fourth interval, from 250 to 350 °C, and exhibits a decreasing rate versus temperature: the greater the temperature, the lower the reaction rate. The results obtained from the isothermal runs reveal that the reaction rate constant, activation energy, and pre-exponential factor for each temperature (determined from the first-order rate model) are all higher than those measures obtained in the general reaction rate model. The results from the non-isothermal runs showed that the values of the reaction rate constant, activation energy, and pre-exponential factor obtained from the approach by Segal and Fatu are similar to those calculated from the general reaction rate model. This was particularly true for the LTO peak temperature. The analysis of the results yields the mean activation energy for the LTO peak equal to 1130.2 cal/mol for isothermal experiments.

1. INTRODUCTION In in situ combustion (ISC) oil recovery processes, air or oxygen-enriched air is injected into the oil formation. Some fraction of the oil within the formation combusts, producing both heat and gas. If the viscosity of the oil is sufficiently low, the gases generated from combustion displace oil to the production wells. If the viscosity is too high, the oil must first be heated (which lowers its viscosity), and once the oil is sufficiently mobile, it is then displaced to the production wells. ISC recovery processes for the production of heavy oil (in situ viscosities between hundreds to a few tens of thousands of centipoise) and bitumen (in situ viscosities in the hundreds of thousands to millions of centipoise) have several potential benefits. First, energy generation is performed within the oilbearing formation. This means that the heat losses often suffered by traditional steam-based recovery processes (efficiency of the steam generator, from surface lines and from injection wellbores) are avoided. Second, greenhouse gases generated from surface steam generation are not fully emitted to the surface. An analysis of ISC recovery processes suggests that a significant fraction of carbon dioxide is generated in ISC. The carbon dioxide can be sequestered in the formation.12 The third benefit of ISC recovery processes is that the temperatures achieved are often significantly higher than those achieved from steam-based recovery processes that enable the potential upgrading of the oil within the formation. Thus, the quality of the oil produced from the reservoir is improved. In the reservoir, there are three major reactive zones in the combustion process: (1) low-temperature oxidation (LTO), (2) fuel deposition [FD, sometimes referred to as thermal cracking (TC)], and (3) high-temperature oxidation (HTO). © 2012 American Chemical Society

These zones move with the combustion front as it propagates within the reservoir. The LTO and FD zones are important because they both provide fuel (coke) for HTO, the main energy-generating reactions in the process. FD reactions generate coke and crack the heavy oil components into lower molecular-weight compounds. These processes decrease the residual oil viscosity. LTO generates partially oxygenated oil compounds that can be more viscous than the original oil. Although the mechanism is not fully understood, analysis of LTO products reveal that LTO causes the conversion of lowmolecular-weight components into high-molecular-weight components.1,2 This process increases the density of the oil phase. The LTO also increases the asphaltene content and decreases the aromatic and resin fraction of the oil.3 To ensure an efficient and effective ISC process design, the impact of LTO must be considered to ascertain that it does not impair the performance of an ISC process. The first attempt to use thermogravimetric data to understand the kinetics of combustion was performed by Tameda in 1959. He conducted differential thermal analysis (DTA) of a mixture of crude oil and sand and observed two distinct peaks, one for LTO and the other for HTO.4 Burger and Sahuquet showed that LTO products include oxygenated hydrocarbons, such as alcohols, aldehydes, ketones, hydroperoxides, and carboxylic acid.5 They compared the heat of combustion for different bonds and different LTO products that released during the ISC process. In 1977, Bae investigated the thermal behavior of different oil samples and found that Received: November 21, 2011 Revised: February 17, 2012 Published: February 18, 2012 1592

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each sample could be classified according to their oxidation behavior.6 Nickle et al. studied the effect of the oxygen partial pressure on the oxidation behavior using pressurized differential scanning calorimetry (PDSC). They found that, at high oxygen partial pressure, the LTO and HTO peaks are sharper and they shift to a lower temperature.7 Yoshiki and Philips and Belkharchouche et al., in a series of experiments, showed that, the higher the total pressure, the greater the overall exothermicity of oxidation.8,9 Vossoughi et al. also used thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) data to develop a kinetic model for ISC. Their model was able to predict the overall rate of combustion from HTO for two combustion tube experimental runs.10 Ambalae et al. used a TGA to estimate combustion kinetic parameters for Neilburg crude oil. They applied different temperatures to determine the order of the reaction. Their results showed that, the higher the temperature, the larger the order of the reaction.11 Yang and Gates matched temperature profiles and gas and liquid production data obtained from a combustion tube experiment using Athabasca bitumen to establish kinetic parameters of LTO, FD, and HTO. They showed that the model could be used to predict temperature profiles and production volumes from four other combustion tube experiments.12 In 2009, Murugan et al. used TGA data to obtain the kinetic parameter for the combustion of Fosterton oil using an Arrhenius model. They demnstrated that the order of the reaction at different temperatures approaches unity under isothermal conditions.13 Freitag and Verkoczy investigated LTO reactions of saturates, aromatics, resins, and asphaltenes (SARA) fractions of two different oil samples from the Neilburg field in Lloydminster, Saskatchewan, Canada, and Wolf Lake in the Cold Lake region of Alberta, Canada, mixed with reservoir sand. They found that the order of the reaction, with respect to the oxygen concentration, varied from 0.5 to 1 as the temperature increased.14 This work used Alaska heavy oil to study the thermal behavior and kinetic parameters for LTO using thermogravimetric analysis. LTO plays a very important role in the sustainability of ISC processes because it provides fuel for HTO. There are many complex reactions that occur in this region, and the exact role of LTO is not fully understood. The effort of this work is to investigate and disclose what takes place during LTO. Three kinetic approaches, including the first-order reaction rate model, the general reaction rate, and the approach by Segal and Fatu, have been used to investigate the LTO behavior and evaluate the kinetic parameters.

Table 1. Clean Oil Sample Properties density (g/mL) at 15 °C at 25 °C American Petroleum Institute (API) gravity (deg) viscosity (mPa s) at 25 °C at 40 °C SARA analysis (wt %) saturates aromatics resins asphaltenes

0.9509 0.9442 18.36 241 104.5 32.30 38.25 21.60 6.04

Table 2. Standard Industrial Quart Sand Composition (Granusil, Silica Fillers, Edmonton Distribution Center) compound

wt %

silicon dioxide iron oxide aluminum oxide calcium oxide titanium dioxide magnesium oxide potassium oxide sodium oxide

90.484 0.095 5.451 0.358 0.016 0.21 2.536 0.714

2.1. Ramped Temperature Oxidation Experiments. To evaluate the thermal behavior of the oil during combustion, roughly 50 mg of the oil−sand mixture was put in a platinum crucible that was then placed in the TGA. The sample was exposed to an atmospheric air, and the temperature was ramped from room temperature (roughly equal to 25 °C) to 800 °C at 10 °C/min. After 800 °C was achieved, the sample was maintained at this temperature for about 30 min to ensure all oxidation reactions were completed. 2.2. Isothermal Oxidation Experiments. Isothermal experiments were performed to study LTO kinetics and the thermal behavior of the Alaska oil at six temperatures ranging from 100 to 350 °C at atmospheric pressure. First, the sample was placed in the TGA, and the temperature was raised at 80 °C/min in a nitrogen atmosphere. This was continued until the target temperature was reached. At that point, the gas stream was switched to air.

3. MODELING The general reaction rate for the change of the sample weight is given by15 dα /dt = k(1 − α)n

(1)

where α is the fractional mass change of the sample (w0/wt)/ (w0 − w∞), with w0 being the initial mass of the sample, wt being the mass of the sample at time t, and w∞ being the final mass of the sample, k is the overall reaction rate constant, and n is the order of the reaction. For example, for a first-order system, the rate of change of the fractional change of mass is given by

2. MATERIALS AND METHODS Table 1 lists the key characteristics of the Alaska heavy oil. Prior to the combustion experiments, the oil was separated from the sand to eliminate any catalytic effects arising from metals and salt in the oxidation reactions that might exist. Standard quartz sand (Sil Industrial Mineral, Inc.) was mixed with the cleaned oil (mass ratio of 1:4 oil/sand) for the oxidation experiments. The composition of the sand is listed in Table 2. The TGA (Perkin-Elmer model TGA 7 with TAC 7/DX controller) used in each experiment was initially purged with nitrogen (20 mL/min flow rate) for about 70 min. To start the oxidation reactions, in all experiments, air flowing at 10 mL/min was supplied to the TGA. The weight loss or derivative weight loss of the oil sand sample versus time (temperature) was recorded by the thermal analyzer control and data acquisition system. All experiments were performed twice to ensure reproducibility of the results. On the basis of the error analysis, the source of error in estimating the kinetic parameters is less than ±5%.

dα /dt = k(1 − α)

(2)

The reaction rate is represented by the Arrhenius equation k = Ae−E / RT

(3)

where A is the pre-exponential factor, E is the activation energy, R is the universal gas constant, and T is the absolute temperature. 1593

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Figure 1. Ramped temperature oxidation experiment: (a) thermal behavior of the oil sand mixture during oxidation and (b) magnified derivative weight in the LTO region.

4. RESULTS AND DISCUSSION The ramped temperature scan of a sample of the mixed oil and sand is shown in Figure 1a. These curves reveal that there are several temperature zones that occur during the combustion process. Figure 1b is the magnified curve of Figure 1a (derivative weight versus temperature) for the LTO region. The temperature zones and peak rates have been listed in Table 3. On the basis of the results, the LTO region lies between about 80 and 380 °C. Figure 2 represents the TGA outcome of isothermal runs at different temperatures during the interval. During the LTO process, the oil is a lumped reactant: it reacts with the oxygen in the air.

Segal and Fatu proposed a model to predict kinematic parameters from TGA/DTG data for non-isothermal reactions.16 To use this method, isokinetic conditions must exist during the experiment. According to the DTG curve displayed in Figure 1a, both 320 and 360 °C have the same rate of reaction.17 In this model, the rate of change of the fractional mass change of the sample is given by dα /dt = A e−E / RT (1 − α)n

(4)

At isokinetic points, dα/dt is equal to a constant, K, and thus A e−E / RT (1 − α)n = K

oil + oxygen→LTO products

(5)

Here, the data obtained from the isothermal LTO experiments were analyzed using both first-order reaction kinetic and general rate reaction models to provide estimates of the value of the kinetic parameters. The ramped temperature oxidation data obtained from the TGA/DTG curves were processed using the general reaction rate equation and the approach by Segal and Fatu.

Table 3. Temperature Intervals and Peak Temperature of Different Zones

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reaction region

temperature interval (°C)

peak temperature (°C)

LTO TC HTO

80−380 380−490 490−570

335 460 530

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Figure 2. Weight loss versus time for isothermal reactions in the temperature interval of LTO.

Initially, oxygen molecules reach the fresh untouched surface of the oil sand mixture, and the LTO process initiates. As LTO progresses, a layer of products accumulates at the edges of the sample and the rate-limiting step shifts from being reactioncontrolled to diffusion-controlled. In other words, the ratelimiting step is oxygen transfer through the product layer at the edge of the sample. As the extent of LTO increases, a thicker layer of product is formed. As a result, it is difficult for the molecules of oxygen to diffuse to the surface of the oil. In the next region, FD, the produced coke is deposited as a fuel to be consumed in the HTO region. In this study, the analysis of the reaction-controlled part is of prime interest. Figure 2 shows that the first-order kinetic model is not an appropriate method for LTO, and its application here serves to provide the deviations between this model and other methods. 4.1. Analysis of Isothermal Experimental Data. For the isothermal experiments, the order of the reaction and log(k) are equal to the slope and intercept of the log(dα/dt) versus log(1 − α) plot. The results of the analysis completed using the general reaction rate equation and the first-order reaction rate equation are both listed in Table 4. The reaction rate constants for both models follow the same trend, increasing from 100 to 150 °C, decreasing from 150 to 200 °C, again increasing from 200 to 250 °C, and finally, decreasing from 250 to 350 °C. The results indicate that, in the first and third intervals, the reaction is enhanced because of the produced heat of the oxidation reaction, whereas in the second and forth intervals, the reaction rate decreases with an increase in the temperature because of the presence of excess heat from the exothermic oxidation reaction. The results are consistent with the activation energies determined over the four intervals: a plot of log(k) versus the inverse of the absolute temperature provides the value of the activation energy. According to the data represented in Table 4, Figure 3 shows the activation energy curve calculated from the general reaction rate model, whereas Figure 4 presents the results obtained from the first-order reaction rate model. The results reveal that both curves follow the same pattern and are consistent with TGA/DTG data. On the basis of the data in Figures 3 and 4 in the temperature interval between 100 and 150 °C, the rate of reaction increases with a rising temperature. From 150 to 200 °C, the reaction rate decreases as the temperature rises. Again, from 200 to 250 °C, an increase of the temperature enhances the reaction rate, whereas between 250 and 350 °C, an increase of the temperature lowers the reaction rate. The pre-exponential factors and activation energies

Table 4. Order of the Reaction and Reaction Rate Constant for General and First-Order Reaction Rate Models temperature (°C)

general dα/dt = k(1 − α)n

100

150

200

250

300

350

n = 0.3 k = 0.4 R2 = 0.98 n = 0.5 k = 0.4 R2 = 0.97 n = 0.4 k = 0.4 R2 = 0.98 n = 0.4 k = 0.4 R2 = 0.99 n = 0.2 k = 0.4 R2 = 0.99 n = 0.4 k = 0.4 R2 = 0.90

first-order dα/dt = k(1 − α) k = 0.2 R2 = 0.95 k = 0.3 R2 = 0.99 k = 0.3 R2 = 0.94 k = 0.3 R2 = 0.98 k = 0.3 R2 = 0.946 k = 0.2 R2 = 0.85

extracted from the data plotted in Figures 3 and 4 are listed in Tables 5 and 6.

Figure 3. log(k) versus the inverse of the temperature for the general reaction rate equation. 1595

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are closer to the values obtained from the general rate equation model rather than the first-order reaction rate kinetic model. The activation energies calculated for the ramped temperature experiments are much lower than those for the isothermal experiments. This may be partially attributed to the ramped temperature trajectory, which allows for a sequential buildup of partially oxidized hydrocarbons, e.g., carboxylic acids, that can partly catalyze subsequent reactions as the temperature rises. On the basis of the results listed in Table 4 and plotted in Figures 3 and 4, four different subzones are detected in the LTO region. They might be connected to the LTO products. According to the heat of formation values18−20 for the main products of LTO, in the first region from 100 to 150 °C, production of carboxylic acids might be the major reaction. This releases significant heat. In the second temperature interval from 150 to 200 °C, alcohols, particularly alcohols with endothermic formation reactions, can be formed consuming the produced heat from the previous interval. The third region from 200 to 250 °C may represent ketone formation, which mainly generates heat. Finally, the fourth subzone from 250 to 350 °C may yield the aldehyde and hydroperoxide production. These reactions have a competitive nature, and all of them are presented in each subzone. A further investigation is needed to characterize each subzone more precisely.

Figure 4. log(k) versus the inverse of the temperature for the firstorder reaction model.

Table 5. Pre-exponential Factor and Activation Energy for the General Reaction Rate Model temperature interval (°C)

pre-exponential factor, A

activation energy, E (cal/mol)

100−150 150−200 200−250 250−350

1.2 0.3 1.5 0.1

808.3 238.2 406.7 1130.2

5. CONCLUSION LTO of Alaska heavy oil has been studied using isothermal and ramped temperature thermogravimetry. The data obtained from the isothermal experiments were analyzed using first-order and general reaction rate models to find kinetic parameters. The first-order reaction kinetic model gave higher values for the activation energy and pre-exponential factor in comparison to values obtained from the general reaction rate model. The results of the analysis of the ramped temperature experiment demonstrate that a LTO region occurs between 80 and 380 °C. This temperature range can be subdivided into four distinct temperature intervals. The peak reaction rate lies in the 250− 350 °C temperature interval, where an increase of the temperature reduces the overall reaction rate. The adverse rate−temperature relationship in this temperature interval can potentially limit LTO, which can be advantageous because it limits the generation of higher viscosity oxygenated hydrocarbons. It is also harmful because it limits fuel deposition for HTO for ISC processes. The data from the ramped temperature experiments analyzed using the approach by Segal and Fatu reveal that the obtained order of the reactions, kinetic rate constants, and pre-exponential factors are close to the values calculated by the general reaction rate model. The activation energy for a non-isothermal run is much lower than that for the isothermal experiment.

Table 6. Pre-exponential Factor and Activation Energy for the First-Order Reaction Rate Model temperature interval (°C)

pre-exponential factor, A

activation energy, E (cal/mol)

100−150 150−200 200−250 250−350

1.4 0.1 2.6 0.04

1262.7 1080.1 2134.0 2024.7

4.2. Analysis of Ramped Temperature Experimental Data. As described above, to obtain the activation energy and order of the reaction using the approach by Segal and Fatu for the ramped temperature experiment, isokinetic points are required. According to Figure 1, there is no isokinetic point in the interval from 100 to 200 °C. An examination of the data reveals that there are isokinetic points at 225 and 285 °C and at 320 and 360 °C. The results of the calculation according to the approach by Segal and Fatu are listed in Tables 7 and 8. The calculated Table 7. Order of the Reaction and Activation Energy Based on the Approach by Segal and Fatu temperature interval (°C)

order of the reaction, n

activation energy, E (cal/mol)

225−285 320−360

0.3 0.4

71.6 123.9

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (403) 210-6503. Fax: (403) 284-4852. E-mail: [email protected].

values for the order of the reaction, pre-exponential factor, and reaction rate constant based on the method by Segal and Fatu

Notes

Table 8. Rate Constant and Pre-exponential Factor for the Non-isothermal Experiment temperature (°C)

rate constant, k

pre-exponential factor

225 320

0.3 0.4

0.4 0.4

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their appreciation for the financial support from the Business-Led Networks Centres of Excellence (NCE) of Canada, the Sustainable Technologies for Energy Production 1596

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Systems (STEPS), and the Petroleum Technology Research Centre (PTRC).

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REFERENCES

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