Pyrolysis Kinetics of Heavy Oil Asphaltenes under Steam Atmosphere

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Pyrolysis kinetics of heavy oil asphaltenes under steam atmosphere at different pressures Alexandra Boytsova, Natalia Kondrasheva, and Jorge Ancheyta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02716 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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Pyrolysis kinetics of heavy oil asphaltenes under steam atmosphere at different pressures Alexandra Boytsovaa,*, Natalia Kondrashevaa, Jorge Ancheytab a

Saint Petersburg Mining University, 21 line of Vasilievsky island 2, 199106 Saint Petersburg, Russia, Email: [email protected] b

Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, México City 07730, México Abstract

Thermogravimetric analysis was used to study pressure effect on the activation energy during asphaltene gasification. The experiments were carried out under steam atmosphere at different pressure (1-80 bar) and temperature (100-900°C). The measured values of the total mass loss of asphaltenes are pressure-dependent. They increase with rising pressure. Kinetic parameters were determined using a first-order kinetic model and integral method with thermogravimetric analysis data. The activation energy was found to vary from 189.6 to 130.4 kJ/mol and frequency factor from 4.1X1010 to 1.2X106 min−1. A decrease of both parameters was observed with an increasing pressure. Coke produced during the gasification is obviously characterized by bigger pore size and weaker mechanical strength as the pressure increases from 1 to 80 bar. The structure of the produced coke becomes more crumbly with raising pressure. The formation of spherical carbon particles with a radius around 5 μm was observed at high pressure (20-80 bar). The elemental composition of these particles is roughly equal, C (≈97%), S (≈2%) and O (≈1%). Keywords: thermogravimetric analysis, scanning electron microscope (SEM), asphaltenes, steam, kinetic parameters, pressure, cenospheres 1. Introduction A great variety of complex compounds from low to high molecular weight are present in petroleum. At reservoir conditions, most of these compounds are maintained soluble in the fluid, however at some stage during processing of crude oil (upstream, transportation, and 1 ACS Paragon Plus Environment

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downstream) they may precipitate as asphaltenes. Particularly, heavy crude oil contains significant amount of asphaltenes [1]. It is well-known that asphaltenes are composed by various sheets of systems with: condensed polynuclear and heterocyclic aromatic, alkyl side chains, naphthenic rings, heteroatoms (sulfur and nitrogen), and other elements [2]. It is also widely accepted that the main responsible for formation of coke and deactivation of catalyst during hydroprocessing operations are asphaltenes [3-5]. It results then necessary to understand the structure of asphaltene to optimize the usage of heavy oil. Significant progress has been achieved so far for better understanding of the physical properties and structure of asphaltenes. Some of the representative reports are: molecular weight of asphaltenes by osmometry was determined by Moschopedis et al. [6], comparisons of asphaltenes macrostructures mixed with toluene were reported by Storm et al. [7], several types of asphaltenes structures rather than a definitive molecular structure was concluded by Speight et al. [8], detailed characterization of asphaltenes (XRD, SEC, NMR, MALDI, LDMS) was carried out by Trejo et al. [9], the interaction enthalpy between asphaltenes and resins was reported by Aguiar and Mansur [10] by microcalorimetry, comparison of the nature of asphaltene from cleaning of submersible pumps and asphaltene from the crude oil with heptane was reported by Rogel et al. [11], who concluded that asphaltenes deposits are less soluble and more aromatic than those coming from the crude oil. Nowadays, not only the structure of asphaltenes, but also the study of kinetics of decomposition of asphaltenes had received significant interest. Recently, TGA (thermogravimetric analysis) and DSC (differential scanning calorimetry) are gaining more attention for the characterization of the asphaltene behavior under different conditions. These are appropriate methods for the characterization of thermal behavior of highmolecular oil fractions at elevated temperatures. The pioneer work for the use of TGA to understand the oil combustion kinetics was reported by Tadema [12]. Since then, different authors have applied these characterization method to oil oxidation and pyrolysis.

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Marafi et al. [13] reviewed the kinetics and modeling of hydroprocessing of petroleum residue and analyzed the differences in the reported kinetic models and values of activation energy of asphaltene conversion at different reaction conditions. Wiehe [14] reported a reaction rate coefficient of first-order of 0.026 min−1 for the thermal cracking of asphaltenes from Cold Lake crude oil at 400°C. Zhao et al. [15] calculated the activation energy for cracking of Athabasca asphaltenes (176 kJ/mol) in the range of350-430°C. Zhao and Ying [16] compared the reactivities of asphaltenes in catalytic hydrocracking and thermal cracking at 430°C and determined the kinetics of these reactions. Trejo et al. [17] compared data for asphaltenes, resins and sediments. They reported that sediments from hydrotreated oil exhibit are more prone to form coke as compared with asphaltenes and resins. They calculated the reaction rate coefficients for asphaltene cracking and reported activation energy ranged from 116.8 to 209.6 kJ/mol. Kok et al. [18] studied by TGA Bati Raman and Garzan crude oils and their fractions. They found the pyrolysis mechanism to be dependent on the chemical nature of the constituents. Also, that each fraction of the crude oil follows a reaction mechanisms independent of the other fractions. In addition, Kok at al. [19] observed that the cracking activation energy is function of on asphaltene content and increases as crude oil become heavier. However, there is no experimental information in the literature about asphaltene behavior during pyrolysis and gasification under different pressures. The main objective of this work is to study the decomposition of asphaltenes obtained from heavy crude oil during gasification under steam atmosphere at different pressures (from 1 to 80 bar). It should be noted that pyrolysis is the first step of gasification, thus experiments were carried out in steam condition. Pre-exponential factor and activation energy were obtained in dependence on pressure. The obtained results will provide a helpful contribution to the optimal utilization of heavy oil and asphaltenes, and also for understanding the coke structure. 2. Experimental 2.1. Preparation of asphaltenes 3 ACS Paragon Plus Environment

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Yarega heavy crude oil (Table 1) was used as feedstock to precipitate asphaltenes with n-heptane with a ratio of crude-to-solvent of 1:40 (weight/volume). The mixture of crude oil and n-heptane was settled for 16 hours in the dark to allow asphaltenes to precipitate. The obtained mixture was filtered and dried. The precipitated samples were washed with 150 mL of n-heptane. To obtain asphaltenes free of resins, Soxhlet apparatus was used for 6 hours under n-C7 reflux. A black colored liquid, composed by composes of n-heptane and solved resins, was obtained after Soxhlet extraction. The density of asphaltenes was calculated according to the following equation [20].

ρa =

ωa 1

ρo

−

1 − ωa

ρm

(1)

where ρa is the density of asphaltenes in kg/m3; ωa is the weight fraction of asphaltenes; ρo is the density of oil in kg/m3, and ρm is the density of maltenes (deasphaltized oil) in kg/m3. Density values are also reported in Table 1.

2.2. Thermogravimetric analysis Kinetic data were determined using a DMT-High Pressure Thermogravimetric analyzer. Steam was employed as purge gas at a constant rate of 0.8-7.2 mL/min, depending on the used pressure. The temperature ranged from 100 to 900°C and pressure from 1 to 80 bar. Gasification was carried out non-isothermally using a heating rate of 20°C/min. Sample mass between 0.15 and 0.17 mg was used.

2.3. SEM and EDX analysis For characterization of the differences in structure of the asphaltenes obtained after the gasification process, scanning electron microscope Quanta 250 FEG from FEI (with warm field emission gun) was used. EDX analysis at high voltage of 20 kV was used in order to determine the elemental composition of asphaltenes.

3. Kinetic and thermodynamic analysis of hydrocarbon cracking 4 ACS Paragon Plus Environment

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The mathematical procedure used for determination of kinetics parameters from TGA was described elsewhere [21-27]. Assuming first-order kinetics for the pyrolysis experiments, the following well-known reaction rate expression can be derived: (2)

where x represents the conversion of asphaltene and k the reaction rate coefficient, which is related to temperature by the Arrhenius expression: (3)

where k0 is the pre-exponential factor or frequency factor in s-1, T is the temperature in K, R is the universal gas constant (8.31451J/K·mol), and Ea is the activation energy in J/mol. Using logarithms, Eq. 3 is transformed to: (4)

Plotting ln k against 1/T gives a straight-line. The slope is equal to –Ea/R, from the activation energy can be calculated. From the intercept of the ordinate axis, k0 can be determined. Conversion degree is calculated with the following equation: (5)

where W0 is the initial mass, Wt is the mass at time t, and Wf is the final mass of the sample. Reaction rate coefficient is determined by:

∆xt k= ∆t ⋅ (1 − xt )

(6)


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where ∆xt is the difference between two neighboring experimental points at time t. With this method information about the kinetic parameters for the gasification of asphaltenes obtained from heavy oil can be determined.

4. Results and discussion The study of thermal analysis may contribute to the better understanding of the mechanism of the gasification of oil asphaltenes under different pressures. Therefore, the thermal behavior of asphaltenes can be used in a wide range of reaction conditions with high accuracy. That is why in this study, an effort has been made to evaluate the effect of pressure on the gasification rates and kinetic parameters for asphaltenes obtained from heavy oil.

4.1. TGA Figure 1 shows the weight loss curves for asphaltenes at different pressures in the temperature range up to 900°C. Asphaltenes gasification studies indicate that their conversion is a one-stage process. Nevertheless, the mechanism of asphaltenes thermal decomposition consists of various parallel complex reactions. At temperature below 389°C at 1 bar, 464°C at 10 bar, 476°C at 20 bar, 524°C at 50 bar and 536°C at 80 bar, the sample mass is nearly constant. This could indicate that the structure remains almost intact. After that, strong mass loss was observed which could be attributed to the gasification of the samples. This main mass loss step or pyrolysis step occurs at temperatures of about 560°C at 1 bar, 583°C at 10 bar, 593°C at 20 bar, 631°C at 50 bar and 640°C at 80 bar. It is assumed the destruction of intermolecular associations and weaker chemical bonds, e.g. sulfur bridges, in this stage [17]. During the decomposition of asphaltenes several volatile compounds (gases, oils and resins) were released and coke is produced as solid residue. TGA experiments show that a less pronounced mass loss is still occurred after the end of the main loss step (gasification step). In this stage asphaltenes form coke as final residue. The maximum rate of weight loss occurs at around 475°C at a pressure of 1 bar, 524°C at 10 bar, 535°C at 20 bar, 578°C at 50 bar and 588°C at 80 bar. The release of volatiles at higher pressures 6 ACS Paragon Plus Environment

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seems to be accompanied by multiple parallel and consecutive reactions. This assumption is supported by data shown in Table 2: higher pressure results in shorter temperature range for the main weight loss step. The weight losses during asphaltenes pyrolysis carried out at different pressure are also presented in Table 2. The temperature at which the maximum rate of decomposition occurred is indicated. The data indicate that the extent of coke formation increases with rising pressure. Coke yield ranges from 30% at 1 bar to 65% at 80 bar. The difference between the lowest mass loss at 1 bar and the highest one at 80 bar is 35%. It should be noted that the char has not been totally gasified due to low temperature and available reaction time, according to the reaction data. In previous studies it was found that asphaltenes adsorb at the interface of oil and water with following intensification of solid and strong structure formation [28-30]. As mentioned earlier, asphaltenes produce gaseous, liquid and solid components during gasification. It could be assumed that during gasification liquid products react with steam, and products adsorb on solid interface with following formation of strong structure. Table 2 also shows the initial and final temperature attributed to the main mass loss step. It was observed that the main stage of weight loss starts at higher temperature with increasing pressure, but temperature range has another behaviour. The higher pressure the lower temperature range. Temperature range varied from 171°C to 104°C at 1 bar and 80 bar, respectively.

4.2. Kinetic parameters During pyrolysis and gasification many competing complex processes contribute to thermal curves. At the beginning, release of components with low molecular weight occurs, but when the temperature is raised, in addition to the increased volatilization rate due to progressive evaporation of bigger molecules, production of volatile fragments via cracking of these molecules and formation of coke may take place [28, 29]. With the data of TGA and using Eq. 4, various plots were prepared, whereby activation energies were calculated from the best-fit lines. 7 ACS Paragon Plus Environment

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It was observed a good fit so that the assumption of first-order is acceptable (correlation coefficients of 0.866-0.971) over the temperature range used (Figure 2). There are differences in kinetic parameters at each pressure, representing the stage of decomposition during the pyrolysis reaction for the asphaltenes obtained from heavy oil (Table 3). The influence of pressure on the activation energy is clearly observed. A decrease in the apparent activation energy when pressure is increased was found: 189.6 kJ/mol at 1 bar to 130.4 kJ/mol at 80 bar. Although not totally comparable due to other environment during TGA experiment, the obtained results of activation energy are within the range of those reported in the literature: 116.8-209.6 KJ/mol [15,17]. It indicates that coke becomes easy to be broken and requires lower energy as pressure increases, which can be assigned to the occurrence of faster rates of heat transfer at different pressures and kinetics of asphaltene reactions, that results in the decomposition of heavy hydrocarbons (high molecular weight) to smaller fractions, characterized by low activation energy. Also, the values of the pre-exponential factors have a significant dependency on the pressure. The frequency factors change from 4.1X1010 min−1 at 1 bar to 1.2X106 min−1 at 80 bar. The temperature range of gasification depends on pressure: the higher pressure, the higher temperature range. Figure 3 shows structure of original asphaltenes, which have different size and shape.

4.3 SEM and EDX Figure 4 shows SEM photos of coke obtained under pressures from 1 to 80 bar. It should be noted that the size of pores in asphaltene particles become bigger and coke has more obvious cracking structure as a result of pyrolysis and gasification from 1 to 80 bar. It could be seen that obtained coke has two types of particle morphologies: 1) big hollow, crumbly and solid particles, and 2) small dense and glassy spheres. The structures of solid particles of coke become more crumbly with increased pressure. The amount of sphere particles becomes higher with increasing pressure.


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Figure 5 shows that under high pressure (20-80 bar) some particles present in spherical shapes with about 5 μm size. The higher the pressure the higher the amount of these spheric particles. Toussaint and Perot [31] investigated these sphere particles obtained from pyrolysis and called them “cenospheres”. Witzel et al. [32] determined that cenospheres formation ratios lie in a narrow range from 5 to 13%, depending on raw oil material. Diameters range between 50 and 600 μm. The formation mechanism of sphere particles was described earlier [32-36]. The sample is injected to the reactor. Heat provokes vaporization of the released fraction, which ultimately ignites at some distance from the droplet’s surface. The temperature increase leads to cracking reaction and local solid formations. These formations can accumulate on the surface of the droplet and form a film. Formation of cenospheres from polymerization reaction of heavy components in the oil has been suggested previously [37]. Table 4 and Figure 6 compare chemical analysis of cenospheres and the surface of crumbly coke samples. It is interesting to know that elemental compositions of coke surface and cenospheres are quite similar for obtained coke under different pressure. In general the cenospheres content higher amount of major elements. It consists of carbon (≈97%), sulfur (≈1%) and oxygen (≈2%). Surface of coke has other different compounds: Al, Ca, Fe, K, Mg, Na, Ni, Si, V. It means that inorganic material present in original asphaltenes is not contained in the cenospheres. The reason for this difference in element content has not been investigated. Approximately the same results reported by Moszkowicz and Witzel [33] for cenospheres obtained from pyrolysis of heavy fuel oil. They found that these sphere particles consist of 90% of carbon, 5-6% of sulfur, 3-4% of hydrogen, 0.1% of metals.

Conclusions Nonisothermal gasification of heavy oil asphaltene was studied based on TG analysis. It should be noted that information about pyrolysis and gasification of heavy oil asphaltenes in steam


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atmosphere at different pressure is not found in the literature. Asphaltenes produced coke in the range of 30-65% coke, depending on pressure. Both pre-exponential factor and activation energy for asphaltene pyrolysis decrease with increased pressure. The assumption of first-order kinetics was validated, and the values of activation energy decrease from 189.6 kJ/mol at 1 bar to 130.4 kJ/mol at 80 bar, while frequency factors change from 4.1X1010 min−1 at 1 bar to 1.2X106 min−1 at 80 bar. The temperature range of pyrolysis process increases with the pressure increase. The pore size becomes bigger and coke has more obvious cracking structure as a result of thermodestructive process from 1 to 80 bar. The structure of coke becomes more crumbly with the increase of pressure. Under high pressure (20-80 bar) some carbon particles show spherical shapes with about 5 μm, which are called “cenospheres”. The elemental composition of cenospheres consists of C (≈97%), S (≈2%) and O (≈1%) and any inorganic material present in asphaltenes was not found.

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Table 1. Properties of the Yarega heavy crude oil and its asphaltenes

Parameters

Heavy crude oil (HCO)

Asphaltenes

Density at 20°C, kg/m3

939.8

1009.19

API gravity

19

9

Saturates, wt. %

16

-

Aromatics, wt. %

35

-

Resins, wt. %

32

-

Asphaltenes, wt. %

17

-


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Table 2. Coke yield for different pressure rates during pyrolysis

Initial and final Pressure,

Coke yield,

temperature attributed

Temperature of

T of maximum mass

bar

wt.%

to the main weight loss

weight loss, °C

loss rate (Tmax), °C

stage, °C 1

30

389-560

171

475

10

42

464-583

119

524

20

44

476-593

117

535

50

55

524-631

107

578

80

65

536-640

104

588


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Table 3. Activation energy and pre-exponential factor of asphaltene pyrolysis

Pressure,

Temperature

Temperature

Ea

2

k0

r

(min-1)

bar

range, (°C)

difference, (°C)

(kJ/mol)

1

390-522

132

189.6

0.925

4.1·1010

10

396-531

135

178.5

0.971

3.1·109

20

400-540

140

171.2

0.971

8,5·108

50

416-557

141

151.6

0.937

7.9·108

80

480-623

143

130.4

0.866

1.2·106


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Table 4. Elemenlal composition of coke yield surface and cenospheres

Element

Asphaltene

Coke surface

Coke cenospheres

C

93.2

91.1

97.1

O

4.1

1.6

2.1

S

1.4

2.8

0.8

Mg

0.1

0.1

-

Al

0.2

0.5

-

Si

0.3

1.2

-

Na

0.1

0.2

-

K

0.1

0.3

-

Ca

0.1

0.2

-

V

0.1

0.3

-

Fe

0.2

1.4

-

Ni

0.1

0.3

-


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Figure Captions Figure 1. Weight loss curves for thermal decomposition of asphaltenes (under steam atmosphere) under pressures from 1 to 80 bar Figure 2. Arrhenius plot for the main region of the asphaltene pyrolysis Figure 3. Structure of asphaltenes from Yarega crude oil Figure 4. Coke under different pressure Figure 5. Sphere particles obtained under high pressure Figure 6. Elements present in a) coke surface, b) cenospheres


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Figure 1


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Figure 2


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Figure 3


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P

100x

500x

Page 22 of 24

1000x

1

10

20

50

80

Figure 4


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20 bar

50 bar

80 bar

Figure 5


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Page 24 of 24

a)

b)

Figure 6


Pyrolysis Kinetics of Heavy Oil Asphaltenes under Steam Atmosphere

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