Single Particle Asphaltene Pyrolysis in a Drop-Tube Furnace - Energy

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Single particle asphaltene pyrolysis in a drop-tube furnace Satarupa Dhir, Nirlipt Mahapatra, Vinoj Kurian, Mehdi Alipour, and Rajender Gupta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01195 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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Single particle asphaltene pyrolysis in a drop-tube furnace Satarupa Dhir, Nirlipt Mahapatra, Vinoj Kurian, Mehdi Alipour, Rajender Gupta* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada ABSTRACT: Oil sands in the Athabasca and Cold Lake regions of Northern Alberta form Canada’s primary source of energy reserves. Asphaltenes, a significant part of bitumen, are often considered to be the least valuable component of crude oil due to various factors such as difficulty in transporting and processing. Very few studies have been carried out on the pyrolysis of asphaltenes in entrained flow conditions. Single particle investigations are useful since they are conducted in a well-controlled environment allowing for the elimination of complexities arising from particle-particle interactions. Char is an intermediate of the gasification process and the structural, morphological behavior of char plays a pivotal role in determining the rate of gasification. In this work, the pyrolysis of pulverized asphaltene feedstock was carried out in a tube furnace maintained at atmospheric pressure. The effects of furnace temperature and particle size on char formation and char characteristics were investigated. Chars obtained from larger particles (1.7 mm to 850 µm) exhibited morphology similar to that of asphaltene particles while the pyrolysis of particles ranging from 250-425 µm at higher temperatures resulted in char with less volatile matter remaining. Scanning electron microscope (SEM) and cross-sectional images 1 ACS Paragon Plus Environment

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of char particles indicated the formation of cenospheres and the fragmentation of char particles at higher pyrolysis temperatures. High pyrolysis temperatures also led to a loss of active sites, an increase in alkene content, and aromatic condensation. Inductively coupled plasma mass spectroscopy (ICP-MS) and X-ray fluorescence (XRF) investigations validated the retention of K and Na along with heavy elements such as V, Ni and Cu in char at temperatures above 700 ºC. Ultimately, the combustion reactivities of char were obtained at 700 ºC, 800 ºC, and 900 ºC for particles of 425-850 µm, 355-425 µm, and 250-355 µm, respectively, and were compared using the Flynn Wall Ozawa method. Keywords: Asphaltene, Pyrolysis, SEM/ICP-MS/XRF/FTIR, Combustion reactivity 1. INTRODUCTION The increase in global energy demands calls for an increase in all forms of conventional (oil, coal, natural gas, nuclear) or unconventional (oil sands, oil shale, biomass) energy. According to an IEA report, the global demand will rise by 33% in 2035 with China and India as the major consumers1. With demand rising worldwide and declining conventional resources there will be a need to secure the supply of energy through unconventional resources. Canada has the third largest reserve of oil in the world after Venezuela and Saudi Arabia. Most of the reserves are in form of unconventional oil, i.e., oil sands. Of the 173 billion barrels of oil that can be recovered economically, about 137 billion barrels are concentrated in oil sands. The depletion of light petroleum has forced the petroleum refining industry to explore opportunities in heavy or extra heavy oil2. However, heavy crude oils have a low yield of lighter fractions and a high yield of bottom residue. Albertan oil sands bitumen has a by weight asphaltene content of 16-25%3. Asphaltenes are the heaviest fraction of bitumen. They are insoluble in n-paraffins (such as n-

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pentane and n-heptane) but soluble in toluene and usually have a high content of heteroatom and inorganic4. Asphaltene is comprised of aromatic rings with alkyl bridges of about 30 carbon atoms, ketones, phenols, and carboxylic acids. Heteroatoms such as nitrogen are present in the pyrrole and pyridine form while sulfur is prevalent as benzothiopene rings. Basic nitrogen structures such as quinolone and acridine in asphaltenes are stronger inhibitors of hydrotreating (HDT) reactions than normal carbazole and indole structures5. Transition metals such as nickel and vanadium are present in a complex porphyrinic structure with nitrogen6. Asphaltene is considered a concern in petroleum operations such as transportation, refining, wax crystallization, and emulsification/de-emulsification. Changes in the temperature, pressure, or composition of crude oil lead to precipitation and the deposition of asphaltenes during production and transportation through pipelines. During processing, asphaltenes can deposit on the catalyst or accumulate gradually on equipment, reducing efficiency and causing operational hazards7. Therefore, it is necessary to separate the asphaltenes when extracting oil from bitumen. The complex nature of pyrolysis and gasification processes in a reactor or gasifier makes it tedious to comprehend burning behavior or a volatile emission strategy. A single particle investigation is a realistic and well-defined method for any fundamental study. Single particle investigations are conducted in a well-controlled environment, allowing the elimination of particle-particle interactions. The effects of variation in parameters such as composition, ash and moisture content, particle size, and temperature, are suitable for studies in such investigations. The analysis of a single particle has been well applied to coal, biomass, and heavy oil combustion and pyrolysis. Moszkowicz8 studied cenosphere formation during fast pyrolysis of heavy fuel in a drop-tube furnace. Cenospheres are hollow spheres generated when volatiles have 3 ACS Paragon Plus Environment

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been evaporated out during the pyrolysis stage. In experiments, the feeding stream, comprised of single droplets of heavy fuel in a crystal glass tube, was maintained at 1000 ºC. Particle size varied from 140 µm to 200 µm and the atmosphere was kept inert. Cenospheres formed were collected using a 40-µm mesh bag filter and analyzed under SEM. Moszkowicz et al observed that the cenospheres’ formation ratio was independent of varying droplet size, residence time, and high pyrolysis temperature. Cenospheres have honeycomb morphology with diameters ranging between 50 and 600 µm. A high furnace temperature and lower particles led to complete pyrolysis with less coking. However a thin layer of solids accumulated on the surface of cenospheres, resulting in an increase in diameter. Volatile combustion during the initial stage of pyrolysis gave rise to cracking reactions. This intensified the solids formation but was a preamble to the volatile release8. In early 1984, Behar et al. used pyrolysis chromatography to study the characterization of various asphaltenes. Pyrolysis yields at 550 ºC confirm the preconceived structures of asphaltenes: peri-condensed and kata-condensed. The products of asphaltene pyrolysis have an n-alkanes distribution in the range of about C30. Behar et al also concluded that hydrocarbons and asphaltenes of oil shared a common origin based on the distribution of n-alkanes9. Zhao et al. investigated carbonization in the pyrolysis of asphaltene derived from Athabasca bitumen. They reported an increase in the coke and gas yield as the temperature of the bath reactor increased from 430-550 ºC, and a decrease in the liquid yield. According to their observation, the carbonization followed complex mechanisms involving the cyclization of alkyl chains, dehydrogenation, aromatization, condensation, and peri-condensation of aromatic rings10. Additionally, the char structure formed because the intermediate pyrolysis product played a significant role during char gasification. The presence and distribution of pores on char determine the diffusion of reactant gases on the char surface, which is a rate-limiting


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step. Extensive loss of volatiles results in highly porous chars, which are expected to have early burnout during combustion. Macropores (pore diameter >5 µm) and mesopores (2 nm < pore diameter < 5 nm) have marked effects on char reactivities during gasification. Porous and hollow chars have higher chances of fragmentation and enhanced gasification rates11. Fragmentation also affects the particle size distribution of final ash because it increases the generation of finer ash12. The char structures are classified as: Group A cenospheres with a thin wall, group B cenospheres with a thick wall, and group C cenospheres with ribs (honeycomb structure)12. As the pyrolysis temperature increases, thin-walled chars with larger central pores and voids are produced. External factors such as the heating rate, temperature, residence time, and pressure are pivotal in influencing the nature of complex pyrolysis reactions as well as char oxidation. It becomes necessary to understand the products of pyrolysis for process optimization13. Similarly, the combustion reactivity of chars formed at the commencement of gasification in a reactor can be a critical process parameter14. Massive loss of active sites, the drop in hydrogen content, and the progressive ordering of char structures with increased pyrolysis temperature are responsible for the reduced combustion reactivity of bituminous and semi-anthracite coals. The activation energy for combustion reactions is calculated to be in the range of 64-139 kJ/mol for original lignite samples, while it is in range of 77-144 kJ/mol for demineralized samples15. The increase in activation energy of low-grade coals indicates that the mineral matter content influenced the reactivity of the lignite coals. Other recent study by Ambalae et al. calculated the combustion activation energy of Neilburg oil asphaltenes as 117 kJ/mol, using a classical Arrhenius model with the help of a thermogravimetric analyzer16. Combustion and pyrolysis parameters were investigated via thermogravimetric analysis (TGA). Of the various methods to analyze available 5 ACS Paragon Plus Environment

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kinetic data, the most popular are model fitting and model free (isoconversional). Model fitting methods require choosing appropriate reaction models, based on supplementary information such as morphological studies. The isoconversional method helps to forgo limitations faced by model fitting methods, as it can estimate apparent activation energies without a detailed model for the reaction path17. They are based on the assumption that the same products are produced regardless of the heating rates in the TGA. Apparent activation energies are obtained at progressive degrees of conversion by conducting collective experiments at different heating rates in TGA18. The common method for activation energy calculation is based on plotting the heating rate (β) with the temperature at particular conversions. This type of isoconversional technique is called the Flynn Wall Ozawa (FWO) method. In this method, the governing relation for kinetic analysis of any solid-state decomposition depends on both the conversion function and temperature. The FWO method is based on the following expression18:

݈݊ߚ௜ = ݈݊

‫ܣ‬ఈ ‫ܧ‬ఈ ‫ܧ‬ఈ − 5.331 − 1.052 ܴ݃(ߙ) ܴܶఈ௜

(1)

where βi represents the heating rates from TGA experiments, R is the gas constant (8.314 JK1

mol-1), g(α) is a constant at a given value of conversion, Eα is the activation energy calculated

from the slope of the above equation, subscripts i and α denote given values of heating rates and conversion respectively, and Aα is the frequency factor of the rate equation determined from the model fitting. The investigation of single asphaltene particle behavior during pyrolysis and partial oxidation is the major objective of this work. The first segment of work necessitated the design of a laboratory-scale drop-tube furnace to carry out entrained flow conditions. This fundamental


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study on single particles supplements the previous work on asphaltene pyrolysis in the drop-tube furnace 19, 20. The objectives for the given study are to understand:

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The effect of operating parameters on the morphological and structural changes of char.

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The behavior of inorganic matter in char during pyrolysis.

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The combustion reactivity of char formed during pyrolysis.

2. EXPERIMENTAL DETAILS 2.1 Experimental setup Figure 1 shows the atmospheric entrained flow gasifier used for pyrolysis and partial oxidation studies. The setup consisted of small Thermolyne-type 21100 furnaces. The overall furnace unit consisted of a control unit, a tubular heating chamber, and a pyrometer. The heating chamber was 47 cm long and 40 cm wide. It consisted of an Al-Cr-Fe coil embedded in the rigid refractory material, which was ceramically insulated. The furnace temperature was controlled by an input controller, which compensated for fluctuations in line voltage and ambient temperature change. The temperature was measured using a K-type thermocouple. The maximum working temperature was 1093 ºC when used intermittently and 982 ºC for continuous use (more than 3 hrs). The furnace was used to heat a quartz tube that was 60 cm long with an inner diameter of 5 cm. The exposed parts of the quartz tube were insulated with K-wool and Aluminum-Silica Fiberfrax material to prevent heat loss. A double valve feeder was used to feed asphaltene particles one at a time. A primary flow of N2 was used in pyrolysis experiments to entrain the feed particles. Gas flow rates were adjusted using Cole-Parmer flow meters. The top part of the quartz tube was closed using a flange with two ports: one for feeding and other for gas flow. The lower part of the tube had a quarter inch outlet to remove the volatiles that formed. Volatiles 7 ACS Paragon Plus Environment

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formed during the experiments moved to an activated carbon filter where heavy tar and particulate matter were removed. Then volatiles were further passed into a water scrubber where acid gases such as H2S were removed. A Rocker 300 series pump was used at the end of the scrubbing flask to maintain a unidirectional flow of volatiles. It also helped to control the pressure inside the quartz tube at 1 atm. A 45-degree inclined mirror was placed at the bottom of the quartz tube to view the reaction occurring inside. 2.2 Char preparation Athabasca asphaltene samples obtained from a de-asphalting unit in Fort McMurray were solid and black in color with shiny surface character19. Solid feeding of asphaltene is preferred in the atmospheric drop-tube furnace. For this, the solid samples were manually ground using mortar and pestle and sieved to obtain specific cut fractions. Six cut sizes were obtained. These covered large particles (1.4-1.7 mm, 1.0 -1.4 mm, 850 µm-1 mm), intermediate particles (425-850 µm, 355-425 µm), and smaller particles (250-355 µm). The furnace was operated at 600 ºC, 700 ºC, 800 ºC, and 900ºC for pyrolysis experiments. The furnace was heated at a rate of 15 ºC/min the required temperature, which was then maintained for the experiments. A flow rate of N2 (99.9998% purity) was maintained at 5 liters per minute for better entrainment of the feed particles. After each experiment, pyrolyzed char samples were collected from the bottom of the quartz tube. Tetrahydrofuran (THF) or toluene was used to clean the tube after each experiment to prevent the samples from being contaminated by previous tars adhering to lower cooler zones in the tube. 2.3 Char characterization


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The surface morphology of the char was studied using a Zeiss Evo MA 15 LaB6 filament scanning electron microscope. Secondary electron images were obtained using an EverhartThornley detector. A few micrograms of char samples were placed on the carbon tape, which was glued to the SEM stub. The stub was then air blown to remove any loosely clinging particles. Subsequently, the samples were carbon coated in Leica EMSCDE005 to improve imaging. Carbon coating inhibits charging and improves the secondary electron signal’s ability to give detailed topography. For cross-sectional analysis, a few particles of char were dispersed in three parts of West System 105 epoxy resin and one part of West System 205 hardener. The exact proportion, as mentioned in the manual, was put into plastic container and mixed with care to prevent air bubbles from forming. Then it was poured into molding caps with the char samples placed at bottom. The mixture was allowed to set overnight for proper curing. All of the specimens were ground at 100 RPM (rotation per minute) in running water with SiC grit paper with 320, 400, 600, 800, 1200 grits. This was followed by polishing using a 1-micron water soluble polycrystalline diamond suspension. Fourier transform infrared spectroscopy (FTIR) is a high speed, sensitive tool used to provide insight into functional groups in chars. It works on the principle that infrared radiation is absorbed from molecular stretching and the bending vibration of characteristic functional groups. FTIR spectra were recorded using an MB3000-PH spectrometer in transmittance mode. Each result was obtained after the accumulation of 120 spectral scans with a resolution of 4 cm-1 in the spectrum range of 4000-400 cm-1. The range below 600 cm-1 was omitted due to high noise. For analysis, about 1 mg of finely ground asphaltene and char samples was placed under the probe. The semi-quantitative method was used to compare the presence and absence of certain bonds with varying temperatures and particle sizes. 9 ACS Paragon Plus Environment

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The high carbon content in char samples makes it difficult to detect inorganic matter using techniques including EDX, XRF, or ICP-MS. Dry ashing was the preferred method to eliminate organic content in the char. The char obtained from pyrolysis was heated in air using a muffle furnace at 450 ºC for 6 hrs until no further change in mass was observed. Ash obtained from the muffle furnace was used for SEM-EDX and ICP-MS analysis. The low temperature was chosen for ashing so that the Na and K content would not change after the sodium and potassiumbearing species evaporated. EDX analysis was carried out in a similar fashion as SEM, using the same instrument that generated results with a Peltiercooled 10 mm2 Bruker Quantax 200 Silicon drift detector with 123 eV resolutions. For ICP-MS about 20 mg of sample was used in a Thermo Scientific iCAP Q instrument. Furthermore, 2 mm collimators were used for XRF bulk analysis with an Ametek Edax Orbis PC Energy Dispersive Instrument. The char combustion experiments were performed using a TA instruments SDT Q600 thermogravimetric analyzer. To maintain combustion conditions, high purity air at 50 ml/min was used as carrier gas. About 5-6 mg of char samples obtained from pyrolysis of asphaltene size fractions 425-850 µm, 355-425 µm, and 250-355 µm at 700 ºC and 800 ºC were placed in the alumina crucible. For all cases, five different heating rates (2.5, 5, 7.5, 10, and 12 ºC/min were considered. The weight loss curve was used for an activation energy calculation using the isoconversional technique. A similar heating rate and nitrogen flow rate were maintained for the asphaltene pyrolysis kinetics calculation. 3. RESULTS AND DISCUSSION 3.1 Char yield and composition from pyrolysis experiments


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The yield of char from pyrolysis experiments was calculated from the overall mass balance. The char yield at 600 ºC was high for particles that were 850 µm-1.7 mm, as shown in Figure 2(a). When the furnace temperature increased, the yield decreased. This indicated that larger particles needed longer residence for a complete reaction. Hence, for particles smaller than 850 µm, all experiments were carried out at temperatures of 900 ºC and those at 600 ºC were neglected. For the smallest particles, the char yield reduced significantly to 1.08% at 900 ºC. Figure 2(b) shows that at 900 ºC, the yields were similar for particles that were 355-425 µm and 250-355 µm. The decrease in char yield with an increase in temperature could be related to the primary or secondary decomposition of the char residue21. 3.2 Functional groups in char The effect of temperature on the structure of char was studied by varying the particle size from 1-1.4 mm, 425-850 µm, and 250-355 µm. Figure 3-5 illustrates the FTIR spectra of asphaltene and char obtained for various sizes at different pyrolysis temperatures. All the variations in structure can be categorized as follows: 3.2.1 Changes in Aliphatic structures: The stable C-H stretching in 3000-2840 cm-1, the C-H ending vibration at 1473 cm-1 and 1375 cm-1, along with the methylene rocking at 720 cm-1, indicated the presence of a straight chain alkane of seven or more carbon atoms in asphaltene as well as the alkane’s char at all temperatures. Chars obtained from 1-1.4 mm asphaltenes at 700 ºC had similar absorption arising for C-H stretching and bending in alkanes. However the intensity of the alkane band decreased with the pyrolysis temperature. Moderate to weak C=C stretching of unconjugated alkenes along with a strong absorption of =CH2 wagging could be observed at 1667-1640 cm-1 and 840 cm-1 11 ACS Paragon Plus Environment

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respectively. The alkene presence decreased for char obtained from 425-850 µm asphaltenes at 700 ºC, 800 ºC, and 900 ºC and for char obtained from 250-355 µm asphaltenes at 700 ºC. But the intensity increased slightly for char obtained from 250-355 µm asphaltenes at 800 ºC and 900 ºC. 3.2.2 Changes in Aromatic structures: The most characteristic absorption bands resulting from the C-H out-of-plane bending occurred between 870-675 cm-1. For chars from 1-1.4 mm asphaltene, there was not much change in this band with temperature. However, this absorption band drastically decreased in intensity for chars from 425-850 µm at pyrolysis temperatures between 700-900 ºC. The same trend was observed withthe absorption band of chars from 250-355 µm at 700 ºC. This indicated that large aromatic rings formed due to aromatic condensation that decreased with pyrolysis temperature. For chars from 250-355 µm asphaltenes, the region between 873-735 cm-1 increased as the pyrolysis temperature increased from 800 to 900 ºC because in the rapid release of volatiles, the free H radical stabilised. At high temperatures, there might be an increase in the degree of aromatic condensation, and in the formation of larger aromatic nuclei22 . 3.2.3 Changes in Oxygen containing structures: The C=O stretching absorption band between 1870-1540 cm-1 is an indicator for ketones, aldehydes, and carboxylic acids. Moderate absorption between 1300-1100 cm-1 is a result of CC-C stretching and C-C(=O)-C bending, and indicates the presence of ketones. Oxygen is also distributed in the form of aldehyde, which was evident from the aldehydic C-H stretching at 2830-2695 cm-1 with the first overtone of C-H bending near 1370 cm-1. The characteristic C-OC stretching vibration at 1085 cm-1 was seen for chars obtained from 1-1.4 mm and 425-850 µm 12 ACS Paragon Plus Environment

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asphaltenes at 800 ºC. At temperatures above 800 ºC, the ether bond was unstable22. The absence of a band at a higher wavenumber such as 3000 cm-1 implies the absence of O-H and N-H functional groups. This ruled out the possibility of oxygen occurring in the form of a carboxylic acid or an alcohol. With an increase in temperature, the C=O and C-O bond was destroyed. The absorption spectrum showed that C=C and C=O overlap with one another. The loss of C=O and C-O bonds made the C=C symmetrical and hence the infrared (IR)spectra became inactive. Decarboxylation and decarbonylation reactions at higher temperatures lead to crosslinking, forming more stable C=C structures and alkyl-aryl C-C forms. However, the rapid release of volatiles at high temepratures resulted in loss of active sites, indicating lower combustion and gasification rates22, 23

.

3.2.4 Changes in Sulfur-containing structures: S-H stretching vibrations between 2600-2550 cm-1 were absent and mostly obscured by strong carbonyl absorption. But sulfur was present in asphaltene and char in the form of C=S (thiocarbonyl), which was evident between 1250-1020 cm-1. This region also had C-O and C-N stretching, so considerable interaction was expected between these vibrations in a single molecule. The presence of sulfur decreased with pyrolysis temperature, but was more perceptible in case of char obtained from 250-355 µm asphaltenes. 3.2.5 Changes in Nitrogen containing structures: The absorption spectra between 1576-1429 cm-1 were mostly weak and demonstrated the presence of a polar N=N bond in asphaltene particles and char. There was no significant change


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in this region with pyrolysis temperature. Nitrogen was also distributed in the form of nitriles, as demonstrated by the 2300-2210 cm-1 absorption stretch. The presence of nitrile groups in asphaltene or chars from 1-1.4 mm asphaltenes was not noticeable. However, when the temperature was above 700 ºC for chars between 425-850 µm and 250-355 µm asphaltenes, the number of nitrile groups present increased. The appearance of nitrile groups at a high temperature indicated a rupture of complex porphyrnic structures. The absence of an O-H functional group in the char also prevented the nitrogen in the rings from converting into an N-H forming structure24. Hence the possibility of a C≡N structure increased. The presence of organically bound nitrogen had an adverse effect on the performance of the catalyst in the fluid catalytic cracker units due to ability of attacking the active sites in any catalyst used for cracking. It is worth noting that the presence of asphaltene was negligible in most of the polar groups; the high polarity is attributed to existence of porphyrins and their complexes with transition metal ions such as vanadium and nickel25. 3.3 Inorganic matter in char The ash content for char between 425-850 µm and 355-425 µm at pyrolysis temperatures between 700 ºC and 900 ºC was estimated via a dry ashing technique using a muffle furnace (Table 1). In contrast to the char yield, the ash content increased significantly with pyrolysis temperature and a decrease in particle size26. It was also noted that the feedstock had lower ash content than that of the corresponding char. As confirmed by FTIR, during pyrolysis the volatile release led to structural transformations such as the collapse of weakly bound organic structures. The complex organically bound metal ions transformed to free ions and with ashing conditions formed heavy metal oxides leading to increased ash content with pyrolysis temperatures. It was noted that 80-90% of the metals in crude were concentrated in its asphaltenes with 25-35% in 14 ACS Paragon Plus Environment

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porphyrin structures and the remaining percentage as organically bound27. Weak bonds of organically associated metals such as V and Ni caused an easy release at a given pyrolysis temperature. The presence of inorganic matter in ash/char was essential for their catalytic effect and influence in increased reactivity28. The bulk EDX of the ash samples gave semi-quantitative information about the concentration of inorganic matter. Figure 6 shows the area of the ash sample being studied. EDX analysis showed a uniform distribution of transition metals such as V and Ni under all conditions. An examination of the ash structure in Figure 6 revealed a fused-like state, which was mostly due to a combination of Al and Si with alkali metals such as Na or K. The existence of Al and Si might be due to a trace quantity of clay or sand in the original asphaltene sample or contamination from the insulating Fiberfrax material. Table 2 shows the elemental composition obtained from the EDX analysis. The accuracy of the EDX analysis was questionable, as it gave an elemental analysis for a given area under study. A larger part of the sample and elements remained undetected. Hence, ICP-MS analysis was carried out, as it was not only sensitive to elements lighter than Al (such as Be, Li, and B), but also detected light elements in the presence of heavier ones as shown in Figure 7 (a) and (b). Metals including Mn, Ti, Mg, and Cu were also detected using ICP-MS. The presence of K and Na was also evident at high pyrolysis temperatures (900 ºC), even when the volatilization of alkali salts might occur at low temperatures. Alkali metals present in asphaltenes can be classified as organically bound, inorganically bound, complex inorganically bound (like Na2Si2O5) and stable (present as alumina-silicates) 29. Below 700 ºC, the release of K was mostly due to the decomposition of organically associated K, while above 800 ºC stable K-silicates might vaporize. Na might also follow a release mechanism similar to that of K.


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The low ash content in asphaltene and char obtained from the pyrolysis of asphaltene made the data collected using ICP-MS inaccurate. The maximum standard error of the ICP-MS results was calculated to be 36%. For that reason, it is recommended that results obtained from ICP-MS should be used only for qualitative purposes, to determine the range of elements present. Hence, XRF analysis was carried out to lead to a better understanding of major inorganic matter content in char obtained from the pyrolysis of asphaltene. The standard error for XRF analysis varied from 2-19%. Al and Fe were contaminants and their presence should not be compared. Most of the inorganic matter in both the asphaltene and char came from V and Ni. A significant fraction of phosphorus (P) and sulfur (S) indicated that the metals were present in the form of phosphates and sulfides. K, Ca, Ti, and Zr were also present in char at pyrolysis temperatures of 900 ºC. Some of the Si might be a result of contamination from the Fiberfrax and K-wool insulation. For both the particle sizes, inorganic matter including K, Ca, V, Ni, P, and Zr decreases with pyrolysis temperature (Figure 8(a), (b)). The S content increases with temperature for 425-850 µm asphaltenes, a trend corroborated by FTIR analysis. With particles of 355-425 µm, the inorganic content in char was lower than that of particles ranging from 425-850 µm. This could be attributed to the higher heating rate in small particles, thereby better metal release into gaseous phase. 3.4 Morphological changes in char The morphology of char collected for a range of 850 µm-1.7 mm at all pyrolysis temperatures had a structure similar to that of the parent asphaltene (Figure 9). However at 900 ºC for 425-850 µm, the chars turned irregular and flaky. As the size was reduced to below 425 µm, notable


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changes were recorded at different pyrolysis temperatures. At 700 ºC for 355-425 µm, the chars formed solid spheres with thick boundaries. At 800 ºC, the chars obtained were light and porous with a central void and thin wall. At 900 ºC, the chars formed “Group C” cenospheres with honeycomb structures. At this condition there was a prominent increase in a bubble formation (Figure 10). At 700 ºC for a feed of 250-355 µm, the cross-sectional analysis indicated that bubbles formed, along with a thick wall, whereas at 800 ºC, the chars developed a round smooth structure with single central void, as shown in Figure 11. Such structures belonged to “Group A” cenospheres30. The cross-sectional SEM exhibited numerous macropores with irregular pore boundaries. Small pores seemed to be embedded in the walls separating the larger cavities. Using ImageJ, the wall was estimated to be less than 5 µm thick. The existence of bubble indicated that some volatile matters were still trapped in the given char. At 900 ºC, the cenospheres developed a rugged exterior with increased macropores, as evident in Figure 12. This was probably due to the rapid release of volatiles. Some of these smaller macropores coalesced to form larger macropores. No bubbles were observed, suggesting that most of the volatile species had been liberated. 3.5 Pyrolysis kinetics of asphaltene and combustion kinetics of char The calculations of combustion reactivities were considered complex because oxygen brought about additional reactions, such as gas phase reactions between volatiles and oxygen, and a heterogeneous combustion of remaining char formed31. The weight loss curve (TG) for char between 425-850 µm obtained at 700 ºC from DTF indicated two stages at all heating rates (Figure 13 a). The initiation of devolatilization and combustion of volatiles (stage A) remaining 17 ACS Paragon Plus Environment

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in the char took place between 330-390 ºC, while the combustion of fixed carbon (stage B) in the char occurred between 440-470 ºC. The burnout temperature for this case ranged from 490-550 ºC. The peak temperature for maximum evolution was indicated with the help of a DTG curve (Figure 15 a). The peak temperature for gas phase reactions varied from 430-450 ºC while that of char gas reactions occurred between 450-530 ºC. The TG curves and peak temperatures in DTG shifted towards a higher temperature. During high heating rates, more instantaneous energy was provided to the system for short periods/reaction times: hence, a high temperature was required for decomposition, while for slow heating rates; a longer time provided better heat transfer, preventing any temperature gradient inside the sample. The weight loss curve (TG) for 425-850 µm of char obtained at 800 ºC from DTF suggested a single stage at all heating rates (Figure 13 b). The weight loss curve (TG) for 250-355 µm of char obtained at 700ºC from DTF indicated two stages at higher heating rates and a single stage for 2.5-5 ºC/min (Figure 14 a). This could be associated with the low volatile content that remained after pyrolysis. Moreover, high heating rates encouraged the simultaneous evolution and ignition of volatiles, while low heating rates had evolved prior to ignition. For chars collected at 800 ºC, the TG curve again revealed a single stage of decomposition (Figure 14 b). Similar behavior was observed in the case of 355-425 µm of char obtained at 700 ºC and 800 ºC. The results obtained from TGA were elaborated to calculate kinetic parameters using the model free method. Activation energy (Ea) served as an indicator to compare the reactivity between chars obtained under different pyrolysis conditions. The FWO method was used to obtain apparent Ea at different conversion levels without assuming the actual reaction model. This was done by plotting individual conversions measured via the natural logarithm of the 18 ACS Paragon Plus Environment

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heating rate (β) versus 1000/T. The plots were linearly fitted to obtain the slope, which was equated with -1.052×Ea/R, as stated in Equation 1, to obtain the activation energy32. The square of the correlation coefficient for linear fitting varied from 0.9586-0.9997. Table 3 shows the temperature range for the two stages and the peak value for all conditions. It was observed that the apparent activation energies calculated by the FWO method were not similar for all conversions, indicating a complex multi-step mechanism for the combustion process. There was a stark difference between Stage A and Stage B of the combustion process. The conversion values in Stage A varied from 0.05-0.3 while those in Stage B ranged from 0.40.9. It could be suggested that the combustion of volatiles and the combustion of fixed carbon in char used separate solid-state reaction mechanisms. To obtain a single value for each stage, an average of activation energy was considered for the individual stages. With an increase in the pyrolysis temperature, the activation energy for both the stages decreased when the particle was 425-850 µm and 355-425 µm. This indicated that combustion reactivity increased with pyrolysis temperature for the given size range. The morphology of chars obtained also suggested an increase in macropores. However, the trend reversed when the particle size was reduced to 250-355 µm. The reduction in char combustion reactivity with an increase in the pyrolysis temperature can be explained by the massive loss in active sites (observed from FTIR). The decreasing reactivity might also be associated with the progressive ordering of the char at high pyrolysis temperatures, leading to a loss of micropores sites19. Table 4 shows the average activation energy at different stages for all samples. To obtain the Ea of asphaltene pyrolysis, a calculation procedure was followed that was similar to the procedure used in the combustion experiments. The TG curve (Figure 18) indicated


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a maximum conversion between 350-470 ºC. The conversion for the region varied from 0.05-0.5. The average value for the FWO method was calculated to be 181.13 kJ/mol. The activation energy for the pyrolysis of oil sand asphaltenes obtained was comparable to those obtained by Karacan et al. for asphaltenes of the B. Raman oil fields (157.75 kJ/mol) and by Ancheyta et al. (163-209 kJ/mol)33,34. The average value of Ea obtained for pyrolysis was in the same range as Stage A for the char combustion experiments. However, Stage B of the char combustion experiments had lower Ea indicating that the combustion rate was faster than the pyrolysis rate. 4. CONCLUSIONS The following conclusions were drawn from the pyrolysis and partial oxidation experiments using the drop-tube furnace:

•

A high pyrolysis temperature and smaller particles leads to a decrease in char yield. Particles smaller than 850 µm had a considerable impact on the char yield at all furnace temperatures.

•

Both particle size and pyrolysis temperature influenced the structure of char.

•

Oxygen distributed in form of aldehydes, ketones and ether decreased with pyrolysis temperature. Sulfur present in the form of thiocarbonyl also decreased with an increase in the pyrolysis temperature. Nitrogen present in asphaltene rearranged as C≡N and appeared only when the particle size was below 850 µm.

•

Chars in the form of honeycombs and ribbed cenospheres were formed at temperatures above 800 ºC for particles of 355-425 µm and 250-355 µm. Cross-sectional SEM indicated fragmentation for particles smaller than 355 µm at a pyrolysis temperature of 900 ºC. 20 ACS Paragon Plus Environment

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•

V and Ni made up the majority ash constituents obtained from char during pyrolysis. During ashing conditions, most of the inorganic matter present converted into respective oxides. The retention of K and Na at high pyrolysis temperature of 700 ºC and 900 ºC suggested the existence of stable silicates or clay. However, under reducing conditions and with increased pyrolysis temperatures, some alkali metals were released.

•

An extensive loss of active sites (C=O, C=S) and structural ordering at high devolatilization temperatures for 250-355 µm resulted in high Ea and low reactivity. However, due to presence of larger macropores, the combustion reactivity of char increased with pyrolysis temperature for particles of 355-850 µm.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel.: 780.492.6861. Fax: 780.492.2881 Acknowledgements The authors acknowledge the financial support provided by Nexen Energy ULC., Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5MPT), Helmholtz Alberta Initiative (HAI) and Natural Sciences and Engineering Research Council of Canada (NSERC). References [1] U. S. Energy Administration, International Energy Outlook 2013 (IEO2013), Washington, DC, 2013. [2] Brons, G.; Yu, J. M. Energy Fuel 1995, 9, 641–647. [3] Strausz, O.P. Symposium on oil sand and oil shale. 1977, Spring (Montreal), 22(3).


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[4] Rana, M. S.; Ancheyta, J.; Maity, S. K.; Rayo, P. Petrol. Sci. Tech. 2007, 25, 201–214. [5] Mitra-Kirtley, S.; Mullins, O. C.; Elp, J. V.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252–258. [6] Demirbas, A. Petrol. Sci. Tech. 2002, 20, 485–495. [7] Mohammed, I.; Okoro, L. Eur. Chem. Bulletin 2013, 2, 393–396. [8] Moszkowicz, P.; Witzel, L. Chem. Eng. Sci. 1996, 51, 4075–4086. [9] Behar, F.; Pelet, R. J. Anal. Appl. Pyrolysis 1984, 7, 121–135. [10] Zhao, Y.; Wei, F.; Yu, Y. J. Petrol. Sci. Eng. 2010, 74, 20–25. [11] Zygourakis, K. Energy Fuel 1993, 7, 33–41. [12] Wu, H.; Bryant, G.; Wall, T. F. Energy Fuel 2000, 14, 745–750. [13] Slopiecka, K.; Bartocci, P.; Fantozzi, F. Appl. Energy 2012, 97, 491–497. [14] Cai, H.; Guell, A. J.; Chatzakis, I. N.; Lim, J. Y.; Dugwell, R.; Kandiyoti, R. Fuel 1996, 75, 15–24. [15] Acma H. H; Mericboyu E.A; Kucukbayrak S. Energy Conversion & Management 2001, 42, 11–20. [16] Ambalae A; Mahinpey N; Freitag N. Energy & Fuels 2006, 20, 560–565. [17] Khawan, A.; Flanagan, R. D. J. Phys. Chem. B 2006, 110, 17315–17328. [18] Aboulkas, A.; El Harfi, K. Oil Shale 2008, 25, 426–443. [19] Mahapatra, N.; Kurian, V.; Wang, B.; Martens, F.; Gupta, R. Fuel 2015, 152, 29-37. [20] Kurian, V.; Mahapatra, N.; Wang, B.; Alipour, M.; Martens, F.; Gupta, R. Energy Fuel 2015, 29, 6823-6831. [21] Onay, O. Fuel Process. Technol. 2007, 88, 523–531.


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[22] Meng, F.; Yu, J.; Tahmasebi, A.; Han, Y.; Zhao, H.; Lucas, J.; Wall, T. F. Energy Fuel 2014, 28, 275–284. [23] Olukcu, N.; Yanik, J.; Saglam, M.; Yuksel, M. J. Anal. Appl. Pyrolysis 2002, 64, 29–41. [24] Zhu, L. C. Conversion of coal-N and coal-S during pyrolysis, gasification and combustion, in Advances in the science of Victorian brown coal, Elsevier, 2004. [25] Bukka, K.; Miller, J. D.; Obladt, A. G. Energy Fuel 1991, 5, 333–340. [26] Al-Wabel, M.; Al-Omran, A.; El-Naggar, A.; Ma, N.; Usman, A. R. A. Bioresour. Technol. 2013, 131, 374–379. [27] Gary, J.; Handwerk, G. E.; Kaiser, M. J. Petroleum refining technology and economics, 5th ed. Taylor and Francis group, 2007. [28] Jayanti, S.; Maheswaran, K.; Saravanan, K. Appl. Math. Model. 2007, 31, 934–953. [29] Wei, X.; Huang, J.; Liu, T.; Fang, Y.; Wang, Y. Energy Fuel 2008, 22, 1840–1844. [30] Yu, J.; Lucas, J.; Wall, T. F. Prog. Energy Combust. Sci. 2007, 33, 135–170. [31] Gil, M. V.; Casal, M. D.; Pevida, C.; Pis, J. J.; Rubeira, F. Bioresour. Technol. 2010, 101, 5601–5608. [32] Slopiecka, K.; Bartocci, P.; Fantozzi, F. Appl. Energy 2012, 97, 491–497. [33] Karacan, O ; Kok V M. Energy & Fuels 1997, 11, 385–391. [34] Ancheyta J; Trejo F; Rana M. S. Asphaltenes Chemical Transformation during hydroprocessing of heavy oils, CRC Press Taylor and Francis group, 2009


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Figure 1. Schematic representation of drop tube furnace used for pyrolysis experiments


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a)

b) 1.4-1.7mm 1-1.4mm 850µm-1mm

70 60 50 40 30

425-850µm 355-425µm 250-355µm

20

Y ield (% )

Y ield (% )

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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10

20 10 0

0 600C

700C

800C

700C

o

Pyrolysis Temperature (oC)

Pyrolysis Temperature ( C)

800C

900C

Figure 2. Char yield at varying furnace temperatures: a) High size particles b) Low size particles


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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

Figure 3. FTIR spectra of asphaltenes and char from 1-1.4 mm particle size


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Figure 4. FTIR spectra of asphaltenes and char from 425-850 µm particle size


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Figure 5. FTIR spectra of asphaltenes and char from 250-355 µm particle size


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a

b

c

d

Figure 6. SEM of ash obtained; a-b: 425-850µm size at 700 ºC and 900 ºC pyrolysis temperature, c-d: 355-425 µm size at 700 ºC and 900 ºC pyrolysis temperature respectively


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a)

Inorganic matter (ppm)

14000

700oC 900oC

12000 10000 8000 6000 4000 2000 0

V Ni P Ca Ti Mn Na K Mg Cu Zr

Elements b)

Inorganic matter (ppm)

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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700oC 900oC

10000 8000 6000 4000 2000 0

V Ni P Ca Ti Mn Na K Mg Cu Zr

Elements Figure 7. Metal content: a) 425-850 µm size, b) 355-425 µm size char ash from ICP-MS


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a)

Inorganic matter (wt%)

60 Asphaltenes 700oC 900oC

50 40 30 20 10 0

K Ca Ti V Fe Ni Zr Si

P

S Al

Elements

b)

60

Inorganic matter (wt%)

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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Asphaltenes 700oC 900oC

50 40 30 20 10 0

K Ca Ti V Fe Ni Zr Si

P

S Al

Elements Figure 8. Metal content: a) 425-850 µm size, b) 355-425 µm size char ash from XRF


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Figure 9. SEM of: a) Raw asphaltenes, b)-c) char from 1.4-1.7 mm at 600 ºC and 900 ºC, d)-f) char from 425-850 µm at 600 ºC, 800 ºC and 900 ºC


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Figure 10. a) SEM of char from 355-425 µm size obtained at 900 ºC b) cross-sectional view


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Figure 11. a) SEM of char from 250-355 µm size obtained at 800 ºC, b) cross-section, c) bubble formation, d) details on wall


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Figure 12. a) SEM of char from 250-355 µm size obtained at 900 ºC, b) cross-section, c) fragmentation d) fragmentation details


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a)

b) 12C/min 10C/min 7.5C/min 5C/min 2.5C/min

(370, 96.79) (350, 97.9)

(440, 64.03)

(480, 58.75)

50

(490, 2.225)

(330, 99.52)

50

(550, 2.225)

(550, 1.283)

(460, 3.552)

0

12C/min 10C/min 7.5C/min 5C/min 2.5C/min

(360, 97.84)

100

W eight (% )

100

W eight (% )

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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0 200

400

600

800

200

400

600 o

o

Temperature ( C)

Temperature ( C)

Figure 13. TG weight loss curve for 425-850 µm size char obtained: a) 700 ºC, b) 800 ºC for five heating rates


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a)

b) 12C/min 10C/min 7.5C/min 5C/min 2.5C/min

(400, 94.83) (310, 98.85)

100

W eight (% )

100

W eight (% )

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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(470, 69.17)

50

(570, 3.415)

(480, 0)

0 200

400

(360, 94.57) (320, 98.9)

12C/min 10C/min 7.5C/min 5C/min 2.5C/min

50

(470, 0.9365)

0

600

200

o

Temperature ( C)

400

(550, 2.207)

Temperature (oC)

600

Figure 14. TG weight loss curve for 250-355 µm size char obtained: a) 700 ºC, b) 800 ºC for five heating rates


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b) 480

18

20 2.5C/min 5C/min 7.5C/min 10C/min 12C/min

531

15

10

453

438

5

D e riv . W e ig h t (% /°C )

a)

D e riv . W eig h t (% /°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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16 14 12

2.5C/min 5C/min 7.5C/min 10C/min 12C/min

10 8 6

436

4 2

0 200

427

300

400

0 500

600

200

700

400

600

Temperature (°C)

Temperature (°C)

Figure 15. DTG curves for 425-850 µm size char obtained at a) 700 ºC and b) 800 ºC for five heating rates


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α=0.1 α=0.2 α=0.3 R2= 0.9731, slope = -29.21 R2= 0.9774, slope = -22.22 R2= 0.9888, slope = -26.92

2.4

2.0

Ln ß

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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1.6

1.2

0.8 1.36

1.40

1.44

1.48

1.52

1000/T (K-1)

Figure 16. FWO plots for stage A conversion of 425-850 µm asphaltenes size pyrolyzed at 700 ºC in furnace


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α=0.4 α=0.5 α=0.8 α=0.9 R2 = 0.9565, slope = -19.74 R2 = 0.8919, slope = -26.66 R2 = 0.9934, slope = -15.07 R2 = 0.9923, slope = -14.76

2.4

2.0

Ln ß

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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1.6

1.2

0.8 1.24

1.28

1.32

1.36

1.40

1000/T (K-1) Figure 17. FWO plots for stage B conversion of 425-850 µm asphaltenes size pyrolyzed at 800 ºC in furnace


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100 2.5C/min 5C/min 7.5C/min 10C/min 12C/min

90 80

Weight %

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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70 60 50 40 30 20 0

200

400

600

800

o

Temperature ( C)

Figure 18. Weight loss curve for asphaltenes in N2 atmosphere


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Table 1. Ash content in char

Ash content (%) Particle size

700 ºC

900 ºC

425-850 µm

0.20

0.56

355-425 µm

0.30

0.97


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Table 2. EDX elemental data for chars obtained

425-850 µm Elements (wt. %)

700 ºC

355-425 µm

900 ºC

700 ºC

900 ºC

1

2

1

2

1

2

1

2

C

45.6

42.8

55.7

55.9

54.8

57.4

62.8

68.6

O

35.3

35.0

30.6

27.2

31.5

30.0

27.2

23.9

Al

1.09

1.56

-

-

0.89

0.49

-

-

Si

1.04

1.56

0.35

0.49

0.82

0.59

-

-

S

0.37

0.44

-

-

0.24

0.27

-

-

K

0.16

-

-

-

-

-

-

-

Ca

1.03

0.27

-

-

0.24

-

-

-

V

11.4

13.1

9.95

11.7

8.17

7.96

6.69

5.26

Fe

0.33

0

-

-

-

-

-

-

Ni

4.43

5.34

3.40

4.69

3.34

3.34

3.33

2.25


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Table 3. Temperature range and peak temperature for all samples

Temperature

Peak

Range (ºC)

Temperature

Pyrolysis

Heating

Particle

Temp

rate

size

(ºC)

(ºC/min)

Stage A

Stage B

2.5

362-438

438-492

427.3

474.2

5

359-449

449-521

440.8

498.7

7.5

352-464

464-540

447.5

516.2

10

371-473

473-556

449.7

527.5

12

365-477

477-574

453.7

531.2

425-850

700

µm

425-850

800

µm

355-425 µm

700

(ºC) Stage A Stage B

2.5

326-475

435.9

5

329-504

454.9

7.5

340-523

472.5

10

344-541

477.6

12

352-547

480

2.5

308-441

441-483

435.8

466.5

5

363-462

462-516

457.4

481.2

7.5

370-474

474-539

462.2

496.6

10

385-483

483-555

441.4

506.9

12

396-477

477-584

450.0

527.4

2.5

309-494


436.3

Page 45 of 46

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

355-425

800

µm

250-355

700

µm

250-355 µm

800

5

311-512

460

7.5

333-543

462.2

10

336-561

447.4

12

348-565

481.2

2.5

311-441

441-484

437.5

465.6

5

348-468

348-468

454.7

489.1

7.5

377-478

478-530

467.7

494

10

371-479

479-547

476.2

503.7

12

387-474

474-574

452.5

526.2

2.5

319-474

431.2

5

333-496

448.5

7.5

346-518

458.3

10

348-540

475

12

358-547

475.7


Energy & Fuels

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

Page 46 of 46

Table 4. Activation energy of all samples

Particle Size

Pyrolysis Temp

Activation Energy (kJ/mol)

(ºC)

Stage A

Stage B

425-850µm

700

206.4

143.7

425-850µm

800

-

110.1

355-425µm

700

136.0

99.2

355-425µm

800

-

96.7

250-355µm

700

120.4

99.6

250-355µm

800

-

118.1


Single Particle Asphaltene Pyrolysis in a Drop-Tube Furnace - Energy

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