Thermal Analysis and Combustion Kinetic of Heavy Oils and Their

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Thermal Analysis and Combustion Kinetic of Heavy Oils and Its Asphaltene and Maltene Fractions Using Accelerated Rate Calorimetry Ronaldo Gonçalves dos Santos, Janeth Alina Vidal Vargas, and Osvair Vidal Trevisan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5007353 • Publication Date (Web): 17 Oct 2014 Downloaded from http://pubs.acs.org on October 21, 2014

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Thermal Analysis and Combustion Kinetic of Heavy Oils and Their Asphaltene and Maltene Fractions Using Accelerating Rate Calorimetry Ronaldo G. dos Santos1*, Janeth A. Vidal Vargas2 and Osvair V. Trevisan2 1

Centre for Petroleum Studies, State University of Campinas, São Paulo, Brazil, 2Department of

Petroleum Engineering, Faculty of Mechanical Engineering, State University of Campinas, São Paulo, Brazil

TITLE RUNNING HEAD: Thermal Analysis and Combustion Kinetic of Heavy Oils. * Corresponding author Current affiliation: Department of Chemical Engineering, Centro Universitário da FEI, Avenida Humberto de Alencar Castelo Branco, 3972, 09850-901 São Bernardo do Campo, São Paulo, Brazil Email: [email protected] Phone: +55 11 4353 2900 Ext.2235 Fax: +55 11 4109 5994

ACS Paragon Plus Environment

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ABSTRACT

This work presents an experimental study on the thermal and kinetic parameters of the oxidation reaction of heavy crude oils to support the in-situ combustion oil recovery technique. An accelerated rate calorimeter was used to investigate the thermal behavior of two Brazilian heavy oils in an open system under constant pressure and continuous heating up to 550 °C. The goal was to identify and characterize the several oxidation regions and to obtain the activation energy and the reaction order. The temperature-time curves showed three clearly distinct regions. During the heating, auto-ignition was found to have started at 180-220 °C. Then, the reaction curves exhibit a strong exothermic behavior up to 250-350 °C, which represents the low temperature oxidation. Asphaltenes were found to play a crucial role in the kinetic behavior of the crude oils. The results show that oxygen-addition reactions are dominant from 200 to 300 °C and that bond-scission reactions are dominant above 350 °C. Though the presence of sand and clay improves the contact between the oil and oxygen and accelerates both of these reactions, it has a greater impact on the oxygen-addition reactions. The results provide helpful data for the screening of crude oils as candidates for recovery by in-situ combustion.

KEYWORDS: Heavy oil, oxidation, combustion reaction, calorimetry, maltenes, asphaltenes.


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1. INTRODUCTION

In-situ combustion (ISC) is a thermal method that has been successfully applied in several oilfields as an enhanced recovery method. In ISC, the continuous injection of a gas creates a combustion front that propagates itself through the reservoir. Air has been the most frequently used injection gas for the production of viscous oils. The heat produced by the ISC process reduces the oil viscosity and consequently improves the oil displacement. Under controlled conditions, the oil that is consumed in the formation of the combustion front represents only a minor fraction of the oil in the reservoir. The relatively major amount of oil that can displaced and eventually refined is of great economic interest. The non-displaced or residual oil undergoes changes in its physical and chemical structure and yields a solid or semi-solid material that becomes a consumable at the combustion front.

The ISC performance as a successful enhanced oil recovery (EOR) method depends on an understanding of the thermal and kinetic aspects involved in the oil combustion reactions. Typically, these aspects are investigated through the application of TGA, DSC and DTGA techniques, which are suitable for studying the low temperature oxidation (LTO) and high temperature oxidation (HTO) reactions involved. However, almost all of these methods fail when addressing the temperature interval between the LTO and HTO, which is called the fuel deposition interval (FD). Additionally, the restraint of a constant heat flow during the tests poses a strict limitation for the use of these methods 1,2. Accelerated Rate Calorimetry (ARC) has been proposed for adiabatically studying reaction kinetics.1


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Yannimaras et al. 3 presented an investigation into the application of ARC to study the kinetic oxidation of light oil under high pressures. Thereafter, several works have been dedicated to studying the thermal and kinetic behavior of crude oils, aiming to obtain the parameters of the reactions (constant of reaction, activation energy and order of reaction) and identify the different regions of the oxidation reactions. These studies have utilized customized systems that are capable of operation at temperatures up to 600 °C and under pressures up to 4 MPa 4–6.

For ISC application, solid materials may be mixed with the oil to create a better representation of the reservoir composition. Greaves et al. 7 investigated the thermal behavior of the oxidation reactions for four different light oils in the presence of rock and water from the reservoir in a closed-system ARC. They identified the presence of LTO reactions in the approximate temperature range of 250-300 °C, followed by HTO reactions, until a temperature of 500 °C or higher was reached. Greaves and Bentaher 8 studied three oils at reservoir conditions with the presence of water and rock. The authors found that the oils present continuous exothermic behavior with transition reactions between the LTO and HTO reactions; they also calculated the kinetic parameters of each reaction type.

An important concern of ISC applications is the lowest temperature that is sufficient to initiate a spontaneous ignition. Price et al. 9 used ARC to investigate the self-ignition temperature (SIT) in a typical reservoir pressure (8.96 MPa) and determine the kinetic parameters of the oxidation oil, both with and without the presence of reservoir rock. The kinetic parameters of the combustion reactions depend on the process variables at the conditions at which they occur. The effects of the process conditions on the reaction kinetic parameters was evaluated by Greaves et al. 10, who


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investigated the use of partial pressure and changes in the oxygen flow rate in studies with a combustion tube. The authors found the reaction stoichiometry to be independent of all of the studied factors. However, the combustion time and the oxygen consumption were, in some extension, dependent on these factors. Additionally, Bagci 11 reported the effects of pressure on oil volatility and how the former affects the reaction rate.

Kok et al. 12 evaluated the reaction combustion of unconventional crude oils and their SARA fractions using thermogravimety (TG/DTG). Asphaltenes from crude oils presented the highest activation energy. Saturates presented the lowest values to the activation energy are thus prone to be the starter of the reactions at every reaction interval (LTO, FD and HTO). These observations are in agreement with the results of the study by Freitag and Verkoczy13, who used PDSC. The process variables affecting the reaction kinetics have also been investigated using conventional batch reactors and accelerate rate calorimetry 14,15. ARC was able to successfully resolve the LTO, FD and HTO reactions and successfully identify minor sub-reactions occurring within each reaction interval 15, which is a meaningful contribution because it can allow for a single model representation of a series of parallel and consecutive reactions occurring during the oil combustion. Sub-reactions were also identified using TGA for LTO reactions 16; however, that technique is not suitable for investigating oil oxidation at higher temperatures.

Several research groups have used ARC to study the thermal behavior of crude oils and their mixtures with oilfield materials, such as rock, water, and brine, because of its reliable performance. The accelerating rate calorimeter has the edge over other equipment because of its ability to allow for adiabatic conditions, because it was designed to test large samples, because of


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its ability to work under high pressures, and because of its ability to allow testing with an open flow over the sample1. The calorimeter is able to provide information about kinetics parameters over a wide range of temperatures (0 to 550 °C) and pressures (0 to 200 bar) under controlled air injection flows. Figure 1 shows a schematic illustration of the AR calorimeter.

The system consists of a furnace equipped with high accuracy heaters and temperature sensors, and the sample holder is located inside the furnace. Reactive tests were planned to provide the Arrhenius activation energy, pre-exponential factors, and the reaction order, as well as the autoignition temperature and the exothermic extension of the reactions. The system allows for the evaluation of the combustion reaction under reservoir conditions.

The present work presents an experimental study on the thermal and kinetic parameters of the oxidation reactions of Brazilian heavy oil and its maltene (n-pentane soluble, termed C5S) and asphaltene (n-pentane insoluble, termed C5I) fractions using ARC. The primary goal is to obtain the experimental parameters of the combustion reaction kinetics and the thermal behavior of the different crude oils under a heat-wait-seek mode. Tests were set to identify and characterize several oxidation regions and obtain the auto-ignition conditions as well as the heat that was created during the reactions. The main contribution of this work is to show the influence of crude oil fractions, specifically asphaltenes and maltenes, on the global combustion reactions of heavy oils. In addition, the study shows that the reaction regions (LTO, FD and HTO) can be subdivided by minor reactions when the major reaction occurring in the region cannot be represented by a single reaction.


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2. EXPERIMENTAL SECTION

2.1. Materials

The crude oils evaluated in the study are Brazilian heavy oils (BHO) whose main properties are presented in Table 1. Molecular weights (MW) were obtained from fingerprint gaschromatographic analysis. Viscosity was obtained using a Cambridge ViscoPro viscometer. Water content was obtained via Karl-Fischer titration. MW, water content and asphaltene content were not measured for BHO-2. Figure 2 shows the distribution of the n-alkanes of the heavy oil used in the combustion tests. Sand and clay were mixed with the BHO to simulate reaction conditions similar to those of the oil reservoir. The properties of these minerals are presented in Table 2.

Combustion reactions were also carried out using the maltene and asphaltene oil fractions extracted from BHO. Asphaltenes were isolated from the oil by precipitation with low molecularweight alkanes and burned under the same conditions used for BHO. Briefly, the asphaltene extraction method can be described by the addition of n-pentane into the crude oil at a 40:1 alkane/oil ratio (mL/g), followed by a period of agitation that occurred overnight and then by filtration of the solid content. The solid precipitate was washed out with hot n-pentane and then weighed. The asphaltene amount is expressed as a percentage of the original oil mass.

2.2. Experimental Set-up

An Accelerated Rate Calorimeter by THT (Betchley, England) was used to evaluate the thermal


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parameters involved in the combustion reactions of the crude oil. The equipment is composed of a gas injection system, a calorimeter chamber, a gas gathering system and the monitoring and analysis system (Figure 1). The gas injection system injects the reactant gas into the calorimeter and promotes the combustion reaction. Gases leaving the calorimeter are partially converted to liquid in the condenser. Non-converted gases are measured at the flow meter, which is set to keep the gas outflow constant.

Attached to the equipment ports is a small, sphere-shaped sample holder, termed bomb, which holds the oil sample and is connected to the gas system to receive the injection of the reactant gas. The ARC bomb is a 2.5-cm-diameter sphere that is composed of Titanium or Hastelloy C with a heat capacity of 0.52 kJ.kg.K-1 or 0.42 kJ.kg.K-1, respectively. After starting the gas injection, the ARC executes temperature scanning under quasi-adiabatic conditions at the designated time intervals.

Standard operation of the ARC is in the heat-wait-seek (HWS) mode. In this mode, the experiment starts by setting the initial temperature of the test and follows with a sequential heating in predetermined temperature steps. Each step-raise of the temperature is followed by a waiting period to achieve the (adiabatic) thermal equilibrium between the bomb and the calorimeter jacket. The seek period starts once thermal equilibrium is reached. In the seek mode, the ARC system performs calculations of the temperature changes over time, and the temperature rate is continually recorded. If the temperature rate is lower than the sensitivity selected for the test (typically 0.02 °C/min), a new heating step is performed and a new HSW is initiated. If the temperature rate is higher than the sensitivity, the exothermal mode is triggered, i.e., the


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occurrence of a self-heating period, which is generally associated with reactions. When the reaction begins, the mechanism automatically switches to the exotherm mode, and the self-heating rate is recorded. The test is finished if the temperature reaches the set final temperature (TF) for the experiment. Figure 3 summarizes the algorithm of the HWS mode.

The experimental set up aimed to evaluate the oxidation reactions of crude oils occurring between 50 °C and 550 °C in both closed and open flow systems. In all of the tests, the temperature steps were 15 °C, the wait time was 15 min and the sensitivity of the temperature rate was 0.02 °C/min. Before the tests with oil samples, calibration and drift tests were performed to ensure an accurate performance of the system. The sample holders were made of titanium, hastelloy or stainless steel.

The ARC performance was evaluated by calibration tests using an empty bomb under the calibration mode. Calibration data are checked by drift tests carried out at the same conditions of the tests. Di- tertiary Butyl Peroxide 20% in toluene (DTBP) was used as a primary standard for evaluating the ARC performance. Figure 4 presents the temperature-time and pressure-time curves for 6.4 g of DTBP under closed conditions. The temperature curve shows an initial zone of heatwait-seek mode up to approximately 280 minutes, followed by an exothermic zone up to 605 min, and then a heat-wait-seek mode again after the reaction is finished. A corresponding behavior appears on the pressure curve.

The onset temperature for DTBP auto-ignition was 102 °C, which can be observed more explicitly in Figure 5. Before this temperature, no exothermic reaction was detected. Additionally, a theoretical model was adjusted to the experimental data, as observed in Figure 6. Literature17


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indicates the onset temperature for DTBP as 111 °C, which represents an experimental error of 8% with the observed temperature.

The experimental conditions for the oxidation reactions occurring because of the contact between oil and air are described in Table 3. High purity air (Alphagaz, N50; 99.999%) was used in the oil oxidation tests. The air flow rate was set to keep the pressure constant during the test. Sand and clay were mixed with the oil according to the compositions described in Table 3 to simulate reservoir conditions. Oil oxidation processes were characterized by temperature - time (T-t) and temperature rate - temperature (TR-T) curves.

A thermal analysis of DTBP provides the standard procedure for every sample analysis. Reactions involving crude oils and their fractions were evaluated in the present work by following the procedure described. Data analysis of the temperature-time data was performed using the ARC Plus software, which is the analysis software provided by THT for its accelerating rate calorimeter, to obtain the thermal and kinetic parameters of the oil oxidation reactions. The calculation procedure was previously described early15. The reaction rate constant (k) was obtained by applying the Arrhenius Equation:

k = A⋅e

−

Ea R⋅T

(1)

where k is the rate constant, Ea is the energy activation, A is the frequency factor, T is the temperature, and R is the universal gas constant. The crude oil was considered a single reactant


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during its oxidation process because separating all of the oil components is unfeasible. The reaction rate for an n-order reaction as a function of concentration may be expressed as

dC = −k ⋅ C n dt

(2)

Under adiabatic conditions, the concentration changes during the reaction can be represented by

C=

T f −T ∆Tad

⋅ Co

(3)

where Tf is the temperature at the end of reaction, T is the temperature at any time, ∆Tad is the adiabatic temperature rise and Co is the initial concentration. Differentiating Equation (3) with respect to temperature and inserting it into Equation (2) yields

n

T −T  dT  ⋅ ∆Tad = k ⋅ Con −1 ⋅  f dt ∆ T ad  

(4)

Re-arranging this equation yields

dT dt k ⋅ Con −1 = n  Tf − T    ⋅ ∆Tad  ∆Tad 

(5)


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The product k.Con-1 is defined as the pseudo zero-order rate constant, k*. Next, k* can be expressed as

dT dt k* = n  Tf − T    ⋅ ∆Tad ∆ T ad  

(6)

By applying the Arrhenius equation for the pseudo zero-order rate constant, Equation 7 is obtained:

(

)

ln k * = ln ACon−1 −

Ea 1 ⋅ R T

(7)

where k* is the pseudo zero-order rate constant and Co is the initial concentration of the sample. The calculation procedure after Equation (7) consists of plotting ln(k*) versus the reciprocal of the temperature for different values of n. When the plotted points become a straight line, the value of n is the correct value. The slope of the straight line is used to calculate both the activation energy and the constant ACon-1.

3. RESULTS AND DISCUSSION

3.1. Self-ignition and thermal analysis

Figure 7 presents the curves for the exothermic releasing of heat for the BHO-1 and BHO-2


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oxidation reactions at 8 bar. The exothermic releasing of heat for BHO-1 at 20 bar is also presented in Figure 7. The exothermic profile was described by the temperature-time curve. For the test with BHO-1 at 8 bar, the system was operated on the heat-wait-seek mode from the starting temperature (50 °C) up to 200 °C.

At this point, the operation mode changed to the exothermic mode because a heat release higher than the sensitivity of the equipment was detected. Heat was continuously released up to 230 °C and, later, from 320 °C to 550 °C. Abrupt increases in the temperature rate were observed at 270 °C and 320 °C. The temperature increased because of the heat release at a relatively low rate between 200 and 270 °C, which is most likely related to the type of reaction started in this temperature interval. The distinguishing aspect between the different regions of reactions is in agreement with previous work.3

A sample of a different heavy oil, termed BHO-2, was tested to check the effects of the variation of the oil composition on the oxidation kinetics, as expressed by the temperature profile curve in Figure 7. The system operated on the heat-wait-seek mode until the temperature reached 190 °C, from which the exotherm starts. However, a discontinuity on the latter mode was detected at 200 °C. After this temperature, a continuous heat release proceeded until approximately 275 °C. Because the exothermic heat was not sufficient to sustain the continuous temperature increase, the reaction was attributed to the class of the low temperature oxidation reactions. This fact is related to a lack of the specific hydrocarbon species present in the crude oil that causes this exothermic discontinuity. It should be stressed that the discontinuity in the exothermic release may represent the existence of a barrier to using ISC as a recovery method for BHO-2. The in-between


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temperature interval was assigned as the medium temperature oxidation category, termed also as the fuel deposition. Then, at 330 °C, an abrupt increase in the temperature rate was observed, which is characteristic of the high temperature oxidation class of reactions.

The difference observed between the thermal behaviors presented for BHO-1 and BHO-2 in Figure 7 is attributed to the difference in the relative composition of the two hydrocarbons. Oxidation reactions should primarily occur by chain branching in the liquid phase. Figure 7 shows the temperature profile for BHO-1 at 20 bar to evaluate the pressure effect on the exothermal releasing of heat. The curve shows a continuous heat release from the onset temperature until the end temperature. For the conditions of the test, the onset temperature was reduced to 180 °C. The temperature increasing rate is slow up to 295 °C, and it occasionally passes with very short interruptions, which is typical of poor heat release processes. The temperature rates for the high temperature oxidation reactions are higher than those obtained for 8 bar.

Figure 8 shows the exothermic temperature profile for the oxidation reactions of BHO-1 with and without the addition of sand at 20 bar. Because the temperature rate is given by the tangent on the curve at a specific time, the temperature profile in Figure 8 shows that the presence of sand accelerates the low temperature oxidation reaction and expands the fuel deposition region, implying a high temperature rate in the HTO region. In the same way, the high temperature reactions occur very fast. The regions of the low temperature reactions and fuel deposition are enlarged if sand is placed together with the oil, as observed in the comparison of the curve in Figure 8 and the corresponding BHO-1 curve in Figure 7.


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When sand is replaced by clay, the low temperature and fuel deposition regions are compressed, and the high temperature oxidation reactions occur more slowly (see the BHO-1/clay curve in Figure 9). These observations are in agreement with the data in the literature. 4 This finding is most likely due to the larger reaction rate of the low temperature oxidation and fuel deposition reaction regions experienced when clay was present in the mixture. Clay is composed of very fine grains that confer a very high surface area and thus greatly increase the contact between the oil and oxygen, which, consequently, accelerates the reactions. This sequence of reactions results in more products to be fed into and spent by the high temperature oxidation.

The Temperature - time curves for C5I and maltenes extracted from BHO-1 are presented in Figure 9. These curves describe the exothermic heat release and exhibit similar behavior to the BHO-1/Clay curves, particularly in the LTO region. The initial exothermic temperature is nearly unchanged for these systems. However, at higher temperatures, the BHO-1/clay exothermic temperature profile, which describes the oil oxidation process, is more similar to the exothermic profile displayed by the maltenes.

3.2. Kinetics of reactions

The kinetics of the combustion reactions of the heavy oils was evaluated for the oil composition (asphaltenes and maltenes) and the presence of a solid phase (sand and clay) that mimics the unconsolidated rock of the reservoir. The kinetic parameters of the oil combustion reactions were obtained from the ARC measurements, and the results are presented by the temperature intervals of the exothermic reactions.


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Kinetic parameters for LTO

The effects of the oil composition and the reaction pressure on the exothermic temperature intervals and the order of reaction for the low temperature oxidation were investigated for BHO-1 and BHO-2 at 8 and 20 bar. The results are presented in Table 4. Previous data18 have highlighted the effects of the API gravity of crude oil on the kinetic properties.

The self-ignition temperature (SIT) represents the temperature at which spontaneous combustion starts. The self-ignition temperature was found to be dependent on the oil composition and on the pressure. SIT is reduced from 231.0 to 187.5 °C when the pressure is increased from 8 to 20 bar. Because SIT represents the temperature at which the spontaneous oxidation of the more reactive compounds of the oil starts, the increase in pressure thus reduces the energetic barrier (activation energy) to begin the reaction. The increase in pressure also amplifies the exothermic temperature interval. The temperature range changes from 39.1 °C at 8 bar to 108.6 °C at 20 bar. At 20 bar, the LTO reactions took more time to complete and consumed a higher quantity of reactants, as observed by the fact that the final temperature was increased to 25.6 °C. However, the number of LTO sub-reactions was reduced from 6 to 4 with the increase in pressure. This means that at 20 bar, as few as 4 sub-reactions can sufficiently represent the kinetic behavior of the whole oil in the LTO region. An important aspect to be emphasized is the discontinuity in the temperature intervals for the combustion reaction at higher temperatures and 20 bar. The global behavior of the combustion sub-reactions at 20 bar is composed of four separate reactions that are nearly independent of each other, whereas at 8 bar, the combustion sub-reactions behave as


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interconnected reactions in which the starting temperature of one sub-reaction coincides with the ending temperature of the preceding sub-reaction. Finally, it is important to stress the increase in the dependence of the pseudo zero-order rate constant for LTO reaction kinetics in relation to the temperature at 20 bar, which is illustrated by the order of reaction applied to Equation (6).

The effect of the oil composition was evaluated using the combustion data generated with two different heavy oils (BHO-1 and BHO-2) at 8 bar. The ARC combustion of both BHO-1 and BHO-2 produces 6 LTO sub-reactions. The self-ignition temperature for BHO-1 is higher than that for BHO-2. However, BHO-2 presents a wider exothermic temperature interval. The temperature range for BHO-2 was 97.2 °C, which is 2.5 times wider than the temperature range for BHO-1. In addition to the thermal parameters, the kinetic parameters were found to be dependent on both the pressure and the oil composition. At 8 bar, the activation energy (Ea) for the combustion sub-reactions was found to be in the range of 2.38 to 11.15 102 kJ.mol-1 and of 0.68 to 2.78 102 kJ.mol-1, respectively, for BHO-1 and BHO-2. A higher Ea indicates that a higher temperature is necessary to start the reaction. This observation is in agreement with the SIT values at 8 bar presented in Table 4. The activation energy values for BHO-1 displayed at 20 bar ranged from 1.85 to 12.93 102 kJ.mol-1. The higher amplitude of the exothermic temperature interval at 20 bar results from the changes in the activation energy caused by the pressure increase.

The kinetic parameters for the LTO reactions were also investigated concerning the presence of a solid phase. The BHO-1 was mixed with finely granulated sand and clay at 1:3 mass ratios to mimic reservoir conditions. Table 5 shows the exothermic temperature intervals and the order of reaction obtained in the tests with sand and clay at 20 bar. In the presence of sand and clay, the


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LTO reactions are almost continuous. In both cases, the SIT was reduced from 187.1 °C (without oil) to 171 ± 0.6 °C (with sand or clay), and the exothermic temperature interval grew to 114.3 °C and 129.7 °C, respectively, for the sand and clay tests. The exothermic ∆T was 108.6 °C in the absence of sand and clay. The final temperature of the reaction was reduced by the addition of sand and increased by the addition of clay. The number of LTO sub-reactions was reduced to 3 by the clay addition, but the presence of sand did not alter the sub-reaction numbers. Further, the addition of clay promotes a greater decrease in the order of reaction, and it renders each subreaction less dependent on the burning product concentration. The temperature intervals for the oil oxidation obtained via differential scanning calorimetry (DSC) have shown wider reaction intervals for LTO and HTO19. It is important to note that DSC allows for studies at temperatures higher than 800 °C.

The activation energy was found to be in the range of 0.28 to 1.72 102 kJ.mol-1 for BHO/clay and of 0.71 to 4.74 102 kJ.mol-1 for BHO/sand, which represents an activation energy reduction of 8587% and 62-63%, respectively, for the additions of clay and sand. Kok (2011)18 reported activation energy values between 8.2 and 59.3 kJ.mol-1 for low temperature oxidation and between 22.8 and 101.1 kJ.mol-1 for high temperature oxidation in crude oil tests carried out in a limestone matrix. Nevertheless, the activation energy for crude oil can reach values higher than 320 kJ.mol-1, which agrees with the results of studies in the limestone matrix using differential scanning calorimetry20. The most important aspect to stress is the high surface area that results from the small particle sizes of clay (Table 2). Under these mixture conditions, oil covers the particle surface, forming a surface layer that allows the oil be contacted easily by the injected air.


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Consequently, the combustion process is more effective.

To understand the effects of the molecular classes present in the oil on the overall combustion reaction, BHO-1 was fractionated into two main fractions, asphaltene and maltene, as described in Section 2.1. Both fractions of asphaltene and maltene were mixed with clay at 1:3 ratios and burned according the procedure stated in Section 2.2. The exothermic ∆T and order of reaction data are presented in Table 6. Compared with the results of similar tests for BHO-1/clay at the same pressure (20 bar), the data show that the maltene fraction combustion yields parameters that are similar to those of the whole oil. This finding means that the LTO reactions are composed essentially of the maltene oxidation reactions and that asphaltenes must be present in the LTO residue, which is used as feed in the next stage of the combustion process (known as the Fuel Deposition reactions, FD).

However, two points should be stressed. First, a small amount of the asphaltene removed from the oil must be burned individually by oxygen addition. Second, the activation energy obtained for the maltene oxidation was approximately the same as that of the whole oil oxidation under the same conditions, whereas the activation energy of the asphaltene oxidation was slightly greater than that of the whole oil (the activation energy was found to be in the range of 0.72 to 4.85 102 kJ.mol-1 for asphaltene/clay and 0.63 to 2.68 102kJ.mol-1 for maltene/clay). These observations lead to the conclusion that the maltene fraction is most likely responsible for the self-ignition of the oil. In DSC studies, the data from Kok and Gul (2013)19,20 agree that heavier oil fractions release more heat during the combustion. Additionally, these data correspond to changes in the activation


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energy with the degree of conversion and serve as an indicator of the parallel and complex reaction mechanisms.

Kinetic parameters for MTO

Medium temperature oxidation reactions, termed FD reactions, were analyzed similarly to the LTO reactions. The evaluation of the composition and pressure effects on the exothermic ∆T and order of reaction data are displayed in Table 7. FD involves successive and consecutive highcomplexity reactions, primarily cracking and pyrolysis, which promote a great number of subreactions.

Table 7 shows that the FD reactions are also affected by both the composition and pressure. However, in contrast to LTO reactions, the pressure increase from 8 to 20 bar produces a SIT increase, a decrease of the exothermic ∆T and a slight reduction in the number of sub-reactions. Additionally, the SIT for the FD reactions was higher for BHO-2 than BHO-1 at 8 bar. The order of reaction was higher than that for LTO, indicating that the reactions are generally more concentration dependent in the FD region. At 8 bar, the activation energy ranged from 1.59 to 5.61 102 kJ.mol-1 for BHO-1 and from 1.01 to 5.04 102 kJ.mol-1 for BHO-2. The pressure change from 8 to 20 bar changed the BHO-1 activation energy to the range of 0.50 to 12.43 102 kJ.mol-1.

The results obtained for BHO-1 oxidation at 20 bar in the presence of sand and clay are displayed in Table 8. The addition of sand and clay to BHO-1 changes both the SIT and the final


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temperature to higher values. The number of sub-reactions was reduced from 14 (without sand and clay) to 12 (with sand) and 2 (with clay). For the oxidation reactions using sand, the activation energy was in the range of 1.03 to 11.21 102 kJ.mol-1, similar to that found in the absence of solids. However, the presence of clay drastically reduced the activation energy from the range of 0.50 - 12.43 102 kJ.mol-1 to 0.30 102 kJ.mol-1 and 1.56 102 kJ.mol-1, respectively, for the first and second reaction.

In the FD region, the oxidation of the asphaltene and maltene fractions extracted from BHO-1 was described by two and three single reactions, respectively. The kinetic parameters for asphaltenes and maltenes are similar to those for BHO-1. Table 9 presents the results for asphaltene and maltene combustion in the presence of clay in the FD region. Previous data from BHO-1 was also added to Table 9 for comparison. The number of sub-reactions and the exothermic temperature interval obtained for asphaltene are similar to those for the whole oil. The orders of reaction for the whole oil are similar to those obtained for the maltenes. However, maltenes present a shorter exothermic temperature interval and a greater number of reactions with regards to the whole oil. Asphaltene has the lowest activation energy, and it must burn early in the process to yield the heat that will sustain the total reaction series.

The global performance of the FD reactions seems to be controlled by maltenes. For the FD region, higher activation energy values were obtained in the sub-reactions occurring in higher temperatures. This observation is in agreement with the global behavior reported for high temperature oxidation21.


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Kinetic parameters for HTO

An analysis of the high temperature oxidation reactions for BHO-1 and BHO-2 shows continuous intervals for the exothermic temperature, as illustrated in Table 10. HTO reactions were found to be nearly independent of pressure and oil composition. Both the self-ignition and final reaction temperatures changed by ± 0.5 °C when BHO-1 was replaced by BHO-2. A total of 6 subreactions were identified for BHO-1, and 5 were identified for BHO-2. The order of reaction for the initial sub-reactions for BHO-1 was higher than that for BHO-2. At 8 bar, the activation energy for BHO-2 (1.46 - 9.84 102 kJ.mol-1) was slightly higher than that for BHO-1 (1.02 - 5.64 102 kJ.mol-1). If the reaction pressure was changed from 8 bar to 20 bar, the BHO-1 activation energy was increased in the range of 2.04 to 16.86 102 kJ.mol-1. The Ea increase was accompanied by an increase of the sub-reaction numbers (from 5 to 6) and a decrease of the initial sub-reaction order (from 0.80 to 0.50).

For BHO-1 combustion at 20 bar in the presence of sand and clay, the HTO region was modeled by two sub-reactions. The results are presented in Table 11. With sand, the second reaction presented a very high order of reaction (n = 1.5), and its exothermic temperature interval was only 3.5 °C. The first BHO-1/sand oxidation reaction presented an activation energy of 1.42 102 kJ.mol1

, whereas for BHO-1/clay, the values were 1.55 102 kJ.mol-1 and 2.77 102 kJ.mol-1, respectively,

for the first and second sub-reactions. This finding indicates a dramatic reduction in relation to Ea obtained in the absence of a solid phase.

The high temperature oxidation of asphaltenes from BHO-1 with clay at 20 bar was represented by


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one single reaction with an order of reaction of 0.50 and an activation energy of 0.99 102 kJ.mol-1. The maltene reaction was described by six individual sub-reactions, and it possessed higher activation energies (from 2.08 to 17.88 102 kJ.mol-1).

4. CONCLUSIONS

The data obtained from the oxidation reactions for two Brazilian heavy crude oils show that accelerated rate calorimetry is an adequate technique for resolving the three main regions of the oxidation reactions for the crude oil. The results show that the oxygen-addition reactions are dominant from 200 to 300 °C, whereas bond-scission reactions dominate above 350 °C. Each reaction region delineated by the oxidation temperature interval should be represented by a series of sub-reactions that occur simultaneously and successively, producing overlapping reactions. Each oxidation temperature interval was divided into sub-reactions, which allows for the opportunity to obtain a more accurate model to describe the reaction kinetics for the whole oil. Both pressure and oil composition affect the number and properties of the sub-reactions. An increase in pressure tends to reduce the number of sub-reactions and the self-ignition temperature. The order of reaction was generally found to be lower than 1.0. Maltene had the highest effect on the activation energy of the oil, primarily in the FD reactions.

The kinetic parameters for the LTO and HTO reactions were almost always dependent on the reaction pressure and the oil composition. Asphaltenes were supposed to be the main feed to start the FD reactions, and they were almost entirely expended by the HTO reactions. The presence of


23

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sand and clay improved the contact between the oil and oxygen and accelerated the reactions, thereby reducing the activation energy. This improvement affected the oxygen-addition reactions more strongly than the bond-scission reactions, allowing for the low temperature reactions to be better resolved.


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REFERENCES

(1)

Sarathi, P. In-Situ Combustion Handbook Principles and Practices. United States

Department of Energy, National Petroleum Technology Office, Report DOE/PC/91008-0374, OSTI ID 3175, 1999; pp 424. (2)

Kök, M. V. J. Therm Anal Calorim. 2009, 97, 403–407.

(3)

Yannimaras, D. V.; Sufi, A. H.; Fassihi, M. R. Sixth European Symposium on IOR,

Stavanger, Norway, 1991, pp 55-64. (4)

Yannimaras, D. V.; Tiffin, D. L. SPE Reservoir Eng. 1995, 10, 36-39.

(5)

Sarma, H. K.; Yazawa, N.; Moore, R. G.; Metha, S. A.; Okazawa, N. E.; Ferguson, H.;

Ursenbach, M. G. J. Can Petrol Tech. 2002, 41. (6)

Li, J., Metha, S. A., Moore, R. G., Ursenbach, M. G., Zalewski, E., Van Fraassen, K. J

Can Petrol Techn. 2008, 47. (7)

Greaves, M., Osindero, A., Rathbone, R. R. Trans IChemE. 2000, 78, 715-720.

(8)

Greaves, M., Bentaher, A. H. J Can Petrol Technol. 2007, 46, 16-19.

(9)

Price D., Razzaghi S., Kharrat R., Rashtchian D., Vossoughi S. Chem. Eng. Comm. 2009,

196, 643–657. (10)

Greaves, M., Dudley, J. W. O., Field, R. W. SPE 19463, 1989.

(11)

Bagci S. Energy Source. 2005, 27, 887-898.

(12)

Kok, M. V., Karacan, O., Pamir, R. Energy & Fuels. 1998, 1-3, 580–588.

(13)

Freitag, N. P., Verkoczy, B. J Can Petrol Technol. 2005, 44, 54-61.

(14)

Chen, Z., Wang, L., Tang, L., Huang, A. Advances in Petroleum Exploration and


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Development. 4 (2), 2012, pp. 58-62. (15)

Vidal Vargas, J. A, Santos, R. G., Trevisan, O. V. J. Therm. Anal. Calorim. 2013, 113,

897-908. (16)

Khansari, Z., Kapadia, P., Mahinpey, N., Gates, I. D. Energy. 2014, 64, 419-428.

(17)

ARC operator’s manual. Thermal Hazard Technology. England, 2009.

(18)

Kok, M. V., J. Therm. Anal. Calorim. 2011, 105, 411-414.

(19)

Kok, M. V., Gul, K. G., J. Therm. Anal. Calorim. 2013, 114, 269-275.

(20)

Kok, M. V., Gul, K. G., Thermochimica Acta. 2013, 569, 66-70.

(21)

Kok, M. V., Fuel Processing Techonology. 2012, 96, 123-127.


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TABLES

Table 1. Properties of Brazilian Heavy Oils BHO-1 and BHO-2. Property

BHO-1

BHO-2

328

285

0.98

0.96

API gravity ( API)

12

16

Heat Capacity (kJ/kg.K)

1.6

1.6

14,500

2,021

10.3

0.09

Molecular weight (g/mol)

a

Specific gravity (g/mL) o

o

Viscosity at 30 C Water content (Wt.%)

Asphaltene C5I content (Wt.%) 19.4 21.0 Based on the fingerprint chromatographic analysis of the crude oil. b Viscosity at 30oC c Viscosity at 51oC a

Table 2. Properties of sand and clay mixed to oil. Property Origin Composition Grain size (µm) 1 From Brazil

Sand Jundu Mining 1 Quartz 210 - 250

Clay BrasilMinas 1 Aluminum silicate < 45

Table 3. Experimental conditions of the ARC test. Sample BHO-1 BHO-1 BHO-2 BHO-1/Sand 1:3 BHO-1 /Clay 1:3 C5I/Clay 1:3

Sample mass (g) 2.2 2.4 2.45 2.19:6.05 1.81:6.07 1.93:6.04

Pressure (bar) 20.59 7.9 8.27 20.41 20.36 20.21

Flow rate (ml.min-1) 45 45 40 60 50 45


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Table 4. Kinetic parameter for LTO reactions for BHO-1 and BHO-2 at 8 bar and 20 bar. BHO-1 P = 8 bar

Temperature range/°C 231.0-241.0 241.0-246.1 246.1-251.1 251.1-258.1 258.1-266.1 266.1-270.1

BHO-2 P = 8 bar

P = 20 bar

n 0.25 0.35 0.70 0.85 1.20 0.40

Temperature range/°C 187.1-212.5 230.5-244.6 260.5-263.5 290.6-295.7 -

n 1.00 0.85 0.40 1.00 -

Temperature range/°C 202.0-256.2 256.2-268.2 268.2-273.2 273.2-279.2 279.2-286.2 286.2-299.2

n 0.50 0.35 0.50 0.55 0.40 0.55

Table 5. Kinetic parameters for LTO reactions for BHO-1-solid phase mixture at 20 bar. BHO-1/Sand 1:3 Temperature n range/°C 170.9-198.9 0.20 198.9-209.0 0.35 209.0-227.0 0.65 227.0-265.1 0.65 265.1-280.2 0.50 280.2-285.2 0.65

BHO-1/Clay 1:3 Temperature n range/°C 170.9-198.9 0.20 198.9-209.0 0.35 209.0-227.0 0.65 227.0-265.1 0.65 265.1-280.2 0.50 280.2-285.2 0.65

Table 6. Kinetic parameters for LTO reactions for asphaltene and maltene fractions from BHO-1 in presence of clay at 20 bar. Asphaltene/Clay 1:3 Temperature n range/°C 182.1-205.2 0.25 205.2-213.3 0.40 213.3-240.7 0.35 240.7-256.0 0.55 256.0-302.6 0.60

Maltene/Clay 1:3 Temperature n range/°C 171.9-186.2 0.10 186.2-241.6 0.35 241.6-303.8 0.25 -


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Table 7. Kinetic parameter for FD reactions for BHO-1 and BHO-2 at 8 bar and 20 bar. BHO-1 P = 8 bar

Temperature range/°C 272.4-276.7 291.4-305.2 305.2-311.1 311.1-323.2 323.2-351.7 351.7-362.7 362.7-372.8 372.8-379.8 379.8-398.9 398.9-416.0 416.0-425.1 425.1-431.1 431.1-434.1 434.1-440.2 440.2-452.2

BHO-2 P = 8 bar

P = 20 bar

n 1.03 0.65 0.50 0.75 1.45 0.45 0.45 1.50 0.45 0.35 0.30 0.20 1.00 1.30 0.25

Temperature range/°C 296.7-308.0 308.0-316.4 316.4-323.4 323.4-337.8 337.8-351.3 351.3-369.3 369.3-376.3 376.3-385.8 397.7-402.9 402.9-414.0 414.0-418.3 418.3-436.0 436.0-449.0 449.0-456.3 -

n 1.50 1.20 0.35 0.60 0.60 0.50 0.05 0.05 0.40 0.55 0.55 0.80 0.70 0.50 -

Temperature range/°C 299.2-302.3 303.4-310.6 310.6-324.8 324.8-345.1 345.1-351.1 351.1-362.2 362.2-369.2 369.2-377.3 377.3-384.3 384.3-404.5 404.5-414.5 414.5-430.6 430.6-444.7 444,7-451,7 -

n 0 1.50 0.35 0.35 0.60 0.40 0.45 0.45 0.10 0.30 0.35 0.25 0.35 0.30 -


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Table 8. Kinetic parameters for FD reactions for BHO-1-solid phase mixture at 20 bar. BHO-1/Sand 1:3 Temperature range/°C 317.2-323.3 323.3-330.4 330.4-340.9 340.9-348.1 348.1-353.2 353.2-361.2 361.2-369.0 376.0-387.3 387.3-427.8 429.5-441.0 441.0-471.9 471.9-483.4

n 1.50 1.50 1.50 0.65 0.25 1.10 0.95 0.15 0.70 0.30 0.35 1.50

BHO-1/Clay 1:3 Temperature range/°C 301.3-438.9 438.9-472.3 -

n 0.45 0.50 -

Table 9. Kinetic parameters for FD reactions for asphaltene and maltene fractions from BHO-1 in presence of clay at 20 bar. Oil BHO-1 Asphaltene Maltene

Temperature range / °C 301.3-438.9 438.9-472.3 302.6-319.7 319.7-485.5 303.8-332.8 332.8-377.3 377.3-418.6

n

Ea / 102 kJ.mol-1

0.45 0.50 0.20 0.40 0.45 0.45 0.55

0.30 1.56 0.20 0.40 0.91 1.15 1.32


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Table 10. Kinetic parameters for LTO reactions for BHO-1 and BHO-2 at 8 bar and 20 bar. BHO-1 P = 8 bar

Temperature range/°C 452.2-459.2 459.2-465.3 465.3-496.4 496.4-537.7 537.7-545.7 545.7-549.8 -

BHO-2 P = 8 bar

P = 20 bar

n 0.80 0.50 0.15 0.15 0.15 0.15 -

Temperature range/°C 456.3-467.9 467.9-481.4 481.4-504.2 504.2-515.7 515.7-527.5 527.5-538.5 538.5-550.3

n 0.50 0.45 0.25 1.50 0.45 0.15 0.15

Temperature range/°C 451.7-484.9 484.9-497.0 497.0-511.1 511.1-535.2 535.2-549.3 -

n 0.25 0.25 0.15 0.20 0.15 -

Table 11. Kinetic parameters for HTO reactions for BHO-1-solid phase mixture at 20 bar. BHO-1/Sand 1:3 Temperature n range/°C 483.4-547.4 0.50 547.4-550.9 1.50

BHO-1/Clay 1:3 Temperature n range/°C 472.3-523.3 0.50 523.3-549.7 0.50

Table 12. Kinetic parameters for HTO reactions for asphaltene and maltene fractions from BHO-1 in presence of clay at 20 bar. Asphaltene/Clay 1:3 Temperature range/°C 485.5-549.6 -

n 0.50 -

Maltene/Clay 1:3 Temperature range/°C 418.6-425.7 425.7-444.2 444.2-463.6 463.6-485.2 485.2-525.3 525.3-551.0

n 1.50 1.10 1.30 1.50 0.35 0.45


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FIGURES

(a)

(b)

Figure 1. Schematic representation for the accelerating rate calorimeter: (a) Detailed description for the calorimeter, including thermocouple, transducer, air inlet and outlet, heaters and sample holder (bomb) and (b) Full representation for the combustion system.


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(a)

90 80

(b)

70

Concentration (% molar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60 50 40 30 20 10 0 C9

C10

C11

C12

C13

C14

C15

C16

C17

C18

C19

C20+

Component (n-Alkane)

Figure 2. Normalized distribution of n-alkanes for BHO obtained from gas chromatograph: (a) Fingerprint analysis for BHO-1 and (b) C9 to C19 and C20+ composition for BHO-2.


33

Energy & Fuels

Figure 3. Simplified diagram of the heat-wait-seek method.

18 Temperature Pressure

250

16 14

200 12 10

150

8 6

100

Pressure(bar)

Temperature(°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 50

2 0

0 0

200

400

600

800

1000

Time (min)

Figure 4. Temperature-Time and Pressure-Time curves for Di- tertiary Butyl Peroxide 20% in toluene (DTBP).


34

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110

Onset

100

Temperature(°C)

90 80 70 60 50 40 30 20 10 0

100

200

300

400

500

600

Time(min)

Figure 5. Onset temperature for auto-ignition of Di- tertiary Butyl Peroxide 20% in toluene (DTBP).

1000

100

Temperature rate (ºC/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

1

0,1

0,01 100

120

140

160

180

200

Time (min) Temperature rate

Temperature rate model

Figure 6. Exothermic rate increase versus time for Di- tertiary Butyl Peroxide 20% in toluene (DTBP). Red line is the kinetic model from Equation (4).


35

Energy & Fuels

600 550

Temperature (ºC)

500 450 400 350 300 250 200 150 400

600

800

1000

1200

1400

1600

1800

2000

Time (min) BHO-1 at 8 bar

BHO-2 at 8 bar

BHO-1 at 20 bar

Figure 7. Exothermic temperature profile for oxidation reaction for different crude oils and different pressures.

600

500

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

300

200

100 500

1000

1500

2000

2500

Time (min) BHO-1 at 20 bar

BHO-1/Sand at 20 bar

Figure 8. Exothermic temperature profile for BHO-1/Sand at 20 bar.


36

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600 550 500

Temperature (ºC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

450 400 350 300 250 200 150 400

600

800

1000

1200

1400

Time (min) BHO-1/Clay

C5I/Clay

Maltenes/Clay

Figure 9 Exothermic temperature profile for BHO-1 and its maltene and asphaltene mixed with Clay at 20 bar.


37

Thermal Analysis and Combustion Kinetic of Heavy Oils and Their

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