Catalytic Effects of Montmorillonite on Coke Formation during Thermal

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Fossil Fuels

Catalytic effects of montmorillonite on coke formation during thermal conversion of heavy oil Ruonan Zheng, Jingjun Pan, Lijuan Chen, Junshi Tang, Dong Liu, Qiang Song, Long Chen, and Qiang Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01157 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Catalytic effects of montmorillonite on coke formation during thermal conversion of heavy oil Ruonan Zheng†, Jingjun Pan‡, Lijuan Chen‡, Junshi Tang§, Dong Liu†, Qiang Song†*, Long Chen‡, Qiang Yao† †

Key Laboratory of Thermal Science and Power Engineering of Ministry of Education,

Department of Energy and Power Engineering, Tsinghua University, 100084 Beijing, China ‡

Engineering and Technology Research Institute of Xinjiang Oilfield Company, Karamay, China §

Research Institute of Petroleum Exploration & Development, 100083 Beijing, China

ABSTRACT: In situ combustion is an enhanced method to recover heavy oil. The formation and oxidation of coke is crucial to promote the combustion front. Heavy oil from China and montmorillonite, a major type of clay, were used as samples in this study. The thermogravimetric analyzer (TGA) was applied to temperature-programmed oxidation/pyrolysis experiments to study the effect of montmorillonite on the thermal conversion characteristics of heavy oil. A fixed-bed reactor was then used to obtain coke and study the effect of montmorillonite on coke properties.

The

characteristic

temperatures

of

thermal

conversion

decreased

with

montmorillonite in the oxidizing atmosphere while remained unaffected in the pyrolysis atmosphere. The fuel deposition increased in both atmospheres because of montmorillonite’s

ACS Paragon Plus Environment

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strong adsorption. In the oxidizing atmosphere, the presence of montmorillonite obviously promoted the progress of coke formation and increased coke yield. The content of O was increased, and the contents of C and H were decreased in coke. The oxidation activity of coke was improved, while trailing occurred due to the blocking in the pore structures of montmorillonite skeleton. In the pyrolysis atmosphere, the presence of montmorillonite did not influence the coke formation temperature but increased coke yield. The content of C was increased, and the contents of H and O were decreased in coke. Coke oxidation activity was reduced with the more serious trailing phenomenon. Montmorillonite significantly affected coke formation through its strong adsorption to polar components in heavy oil, obvious catalysis on dehydrogenation as an acid catalyst in the pyrolysis atmosphere while enhanced the oxygenation due to its large surface area and catalyzed polycondensation in the oxidizing atmosphere.

1. INTRODUCTION In situ combustion (ISC) is a highly efficient and widely applicable method to recover heavy oil.1,2 The process injects air and sets fire to the oil layer. With the increase in temperature, the viscosity of crude oil decreases, the light ends become distilled, and the heavy ends translate to solid fuel, which is called coke. The combustion of coke provides heat with no external heat.3,4 This process occurs in the porous media underground; although the content of quartz is high in this region, the content of clay minerals, including montmorillonite, illite, kaolinite, and chlorite, accounts for 10–30%.5 These clay minerals present large surface area, which means a strong adsorption of organic matter; they are also natural solid acids and exhibit catalytic activity.6-8


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Clay minerals may influence the thermal conversion of heavy oil. The effect of clay minerals on the thermal conversion of heavy oil has been studied using TG/DSC to conduct the kinetic analysis of the process. Kok et al.9-11 studied the effect of three clays with different compositions in the oxidizing atmosphere and found that the clays do not change the two main distinct reaction regions, namely, the low temperature oxidation (LTO) and the high temperature oxidation (HTO) regions. The three clays show obvious catalysis in both regions. The demarcation point in the LTO and HTO regions is in advance, and the peak temperature in the HTO region is lower than the sample without clays. The activation energies in the LTO and HTO regions decrease, but the catalytic activities of the three clays differ. This difference is closely related to their chemical constituents. Vossoughi5 studied the influence of kaolinite, a major type of clay. The activation energy in the HTO region is 177.2 kJ/mol when heavy oil is mixed with sand (80%/20%) and declines to 75.4 kJ/mol when the sand is replaced by kaolinite. The author also applied DSC to observe the change in heat flow with kaolinite, and the major effect is the shift of energy release from the high-temperature range to the low-temperature range. Studies focusing on the influence of other major types of clay on the thermal conversion of heavy oil are lacking. However, in the study of light oil oxidation, four types of clay are used.12 Among the four clays, montmorillonite shows the highest catalytic activity with the lowest activation energy, and it is followed by illite, chlorite and kaolinite with the lowest catalytic activity. Clay minerals affect not only the dynamic characteristics of heavy oil transformation but also the reaction products. Kok10 concluded that the presence of clays increases the fuel


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deposition in the oxidizing atmosphere after studying the mass loss of TG curves. Bousaid and Ramey13 applied a fixed-bed reactor to obtain coke in the pyrolysis atmosphere and found that the fuel deposition increases by 112.5% and 88.9% with 20% clay at 800 °F and 900 °F, respectively, compared with pure sand. Pan14 reported that clays easily absorb the polar components in crude oil, such as resin and asphaltene, which are major fractions to form coke. The mobility of polar components decreases when absorbed by clays; with the catalysis of clays, the coke yield increases, and the characteristics of coke may change. Different clays may show different absorption and catalysis, thereby indicating different effects on the coke yield and characteristics. Vossoughi5 found that the mass loss of heavy oil is lower when mixed with kaolinite compared to the oil mixed with sand in the pyrolysis atmosphere, which means more coke remains. Tannenbuam15-17 studied the pyrolysis products of kerogen with and without clays. In the presence of montmorillonite and illite, a considerable adsorption of the generated bitumen, which is composed of polar compounds and asphaltenes, is observed, and this bitumen irreversibly adsorbed on the surface of clays then cracks to yield insoluble pyrobitumen during heating. Montmorillonite also shows more pronounced adsorptive and catalytic effects than illite. The above studies carried out under atmospheric pressure show that clays affect the dynamic characteristics of heavy oil transformation by decreasing the activation energy and the reaction products by increasing the fuel deposition. 5,9-12 In the presence of clays, the decrease in the activation energy of heavy oil transformation and the increase in the fuel deposition are also observed under high pressure.13,18-20 The effects of the clays obtained under atmospheric pressure are consistent with the results under high pressure. The formation of coke connects the low


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temperature reaction and high temperature oxidation during the process of ISC, which depends on the atmosphere and the effect of clays during the low temperature region. Meanwhile, the characteristics of the coke formed determine the properties of the HTO region. The yield and oxidative activity of coke are crucial to the formation and promotion of the combustion front in the ISC process. In the numerical simulation of the ISC process, the equations describing the formation and oxidation of coke are the important foundation of the mathematical modeling, which directly affects the accuracy of the simulation.21-23 However, the effects of clays on the formation characteristics and oxidation properties of coke are not considered in the present reaction models. The application of the models to different oilfields may therefore be restricted. Clays exhibit dissimilar activities owing to their complex compositions. Understanding the effect of main clay minerals is helpful for regulating specifically the parameters of ISC. Montmorillonite is a common clay mineral in oil reservoirs and identified as a typical acid catalyst with large specific surface area including outer and interlayer surface and shows strong adsorption to organic matters.24,25 But its effect on the coke formation during the thermal conversion of heavy oil is not clear yet. The current research first conducted the temperature-programmed experiments with a TGA to study the effect of montmorillonite on the thermal conversion characteristics of heavy oil. In view of the importance and complexity of coke formation

22, 26-28

, a fixed-bed reactor was then used to obtain the coke formed at different

temperatures in the oxidizing and pyrolysis atmospheres. The yield and physical and chemical properties of coke were studied to reveal the effect of montmorillonite on the formation characteristics and oxidation properties of coke during the thermal conversion of heavy oil.


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2. EXPERIMENTAL 2.1. Samples. The heavy oil from an oilfield in China was used in this study. The viscosity of the oil was 1878 mPa⋅s (BROOKFIELD DV2T) at 50 °C under atmospheric pressure, and the °API was 18. Water and impurities were removed from the oil sample according to the Chinese national standards SY/T 6316 and SY/T 6520. Water content was less than 0.5% after processing. SARA fractions were separated from the oil sample according to the Chinese national standards NBSHT 0509-2010.29 This method used n-heptane as the solvent for separating asphaltenes. The content of SARA is shown in Table 1. The montmorillonite (denoted by “M” in the following figures) was from Aladdin, Shanghai, China, and consisted of 62.24% SiO2, 29.24% Al2O3, 3.33% Fe2O3, and 2.7% MgO (SHIMADZU XRF−1800). It was analyzed by the X-ray diffraction (D/max-rB, Japan) to verify its mineral structure. The inert silica was from Donghai Mineral, Jiangsu, China, and consisted of 99.7% SiO2. The mass fractions of heavy oil, montmorillonite and silica were calculated according to the oil saturation, porosity of sand and the content of montmorillonite in the core. The test sample was a mixture of 10 wt% oil, 27 wt% montmorillonite, and 63 wt% silica and compared with a mixture of 10 wt% oil and 90 wt% silica. Table 1. SARA Composition of Heavy Oil Sample (wt %) Saturate

41.6

Aromatic

19.1

Resin

37.2

Asphaltene

2.1


6

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2.2. Experimental system. The thermogravimetric analyzer (Mettler Toledo TGA/DSC 3+) was used to study the effect of montmorillonite on the thermal conversion characteristics of heavy oil. The samples were approximately 12–15 mg with a gas flow of 50 ml/min at atmospheric pressure. The oxygen in the air injected into the reservoir is mainly consumed in the combustion zone during the ISC process. Therefore, the coking zone in the downstream is in an atmosphere with low oxygen concentration or inertia. Therefore, 5% O2 with Ar as a balance was used in the oxidizing experiments to simulate the atmosphere of low oxygen concentration during the conversion of heavy oil into coke, in which the temperature ranged from 50 °C to 850 °C with a heating rate of 6 K/min to ensure the burnout of the coke. Ar was used in the pyrolysis experiments to simulate the inertia atmosphere, in which the temperature ranged from 50 °C to 600 °C with the same heating rate to ensure no weight loss. A fixed-bed reactor was applied to obtain the coke formed during the thermal conversion of heavy oil. The experimental setup is shown in Figure 1. The reactor consisted of a quartz inner tube (external diameter of 25 mm) and a quartz outer tube (internal diameter of 40 mm). The inner tube was connected with a removable quartz basket (external diameter of 34 mm and height of 45 mm) above with a scrub connection. The quartz basket located in the constant-temperature zone was equipped with a quartz filter at the bottom to place 1.5–1.8 g of samples. The gas flow of 500 ml/min was purged from the top of the quartz outer tube, passed through the samples, flowed out from the quartz inner tube, and finally entered the infrared detection system (FTIR, Nicolet 6700, U.S.A.) after condensation. Thermocouple 1 was used to measure the temperature in the furnace to realize ramped temperature, and the temperature of the


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sample was measured by thermocouple 2. All the experiments were conducted in atmospheric pressure. Ar was used in the pyrolysis experiments, and 5% O2 with Ar as a balance was used in the

oxidizing

experiments.

The

fixed-bed

reactor

heated

the

samples

to

250 °C/300 °C/350 °C/400 °C in the oxidizing atmosphere and to 400 °C/500 °C/600 °C in the pyrolysis atmosphere with a heating rate of 6 K/min from room temperature. When the samples were heated to the programmed temperature, the furnace was opened immediately with 1.5 L/min Ar purging into the tube to quickly cool the remaining solids. The samples collected from the tube were then dissolved with toluene for 6 h and filtered. This process was repeated several times until the upper liquid was clear in the filtration. The solid residue was the coke attached to the minerals. The physical and chemical properties of this coke were characterized after drying. Given that montmorillonite would dehydrate during the heating process, the TGA and fixed-bed experiments were carried out separately for montmorillonite under the same conditions.

Mass flow controller

Quartz filter

Quartz outer tube

Removable quartz basket Quartz frit Computer

Electric furnace Quartz inner tube

Plug FTIR

Thermocouple 1 Ar

O2 Thermocouple 2 Alcohol-cold trap Temperature controller

Figure 1. Schematic of the fixed-bed reactor system


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2.3. Methods. 2.3.1. Thermogravimetric analysis. The thermal conversion characteristics of heavy oil were described by the conversions and relative reaction rates varying with temperature. The conversion was defined as the proportion of oil weight loss to the total oil weight, and the relative reaction rate was defined as the derivative of conversion with respect to time. In the experiments of oil mixed with montmorillonite and silica, oil and montmorillonite lost their weight in the heating process. The weight loss of oil was calculated by the subtraction between the total weight loss and the weight loss of montmorillonite separately in the same temperature program. The conversion and reaction rates were obtained using Eqs. 1 and 2.

α= r=

∆moil+mont − ∆mmont , moil

(1)

dα , dt

(2)

where α is the conversion; ∆moil+mont is the weight loss of test samples including oil, silica, and montmorillonite; ∆mmont is the weight loss of montmorillonite separately; moil is the initial mass of oil; r is the reaction rate; and t is the time. In the oxidizing atmosphere, the reaction in the HTO region corresponds to the oxidation of the deposited fuel. Therefore the fuel deposition is calculated by the difference between the conversion of the demarcation point of the LTO and HTO regions and the final conversion. In the pyrolysis atmosphere, the remained solid after temperature-programmed pyrolysis is the deposited fuel and therefore the fuel deposition is calculated by the difference between the final conversion and 1.


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2.3.2. Sample characterization of fixed-bed experiments. The fixed-bed experiments were used to obtain coke samples. The coke characterization included elemental composition, surface functional groups, specific surface area, pore structures, and oxidation activity. The elemental composition of coke was measured with an elemental analyzer (Eurovector EA3000, Italy). The coke formed with oil, silica and montmorillonite was partly attached to the surface of montmorillonite, which would dehydrate during the elemental analysis. Thus, montmorillonite prepared in the same condition as the oil sample was measured separately using an elemental analyzer. The elemental composition of coke was obtained using the subtraction between the data from the solid residue formed with the oil, silica and montmorillonite and the data from the montmorillonite (Eq. 4). The relative standard deviation of C was less than 3%, while the relative standard deviation of H/O/N was less than 10%. Based on the weight change of the samples before and after the fixed-bed experiments and the results of elemental analysis, the coke yield could be calculated using Eq. 3. The relative error was less than 3%.

γ=

xm2 ×100% , ym1

(3)

where γ is the coke yield, m2 is the solid residue weight after the toluene dissolution, x is the coke content in the solid residue, m1 is the sample weight before the fixed-bed experiment, and y is the initial oil proportion (obtained from the data of TGA). For the coke formed with oil and silica, x was equal to the sum of C/H/O/N in the elemental analysis. For the coke formed with oil, silica


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and montmorillonite, x was calculated using Eq. 4.

coil+mont − (1 − x) ⋅ cmont = 1, x C,H,O,N

∑

(4)

where coil+mont is the elemental contents of the solid residue formed with oil, silica and montmorillonite, and cmont is the elemental contents of the montmorillonite. The result of the subtraction was considered as the elemental contents of the coke. If the coke was made up of C/H/O/N, then the sum should be 1. When x was calculated, the contents of C/H/O/N in the coke were easily obtained using Eq. 4. The surface functional groups of coke were determined using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS, Nicolet 6700, U.S.A.). The samples were mixed with KBr. When measuring the coke formed with oil, silica and montmorillonite, the mixture of montmorillonite and silica formed in the same heating process, and KBr was used as background. When measuring the coke formed with oil and silica, the mixture of silica and KBr was used as background. Specific surface area and pore structures were measured by a Surface Area and Porosity Analyzer (Micrometrics ASAP 2020, U.S.A.). The cokes formed in the oxidation and pyrolysis atmospheres were represented by the coke formed at 400 °C and 600 °C, respectively. The test sample of coke formed with oil, silica and montmorillonite was attached to the mineral skeleton, which meant that the results of the measurement did not directly describe the coke itself. Therefore, a mixture of silica and montmorillonite prepared at the same temperature was measured as the blank sample of the mineral skeleton. By comparing the specific surface area


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and pore structures between the blank sample and the test sample, the information of coke itself could be obtained. The coke formed with oil and silica was handled in the same way except that there was no change in silica with the increase in temperature. The fixed-bed reactor was utilized to study the oxidation activity of coke. The coke formed at 400 °C in the oxidizing atmosphere and that formed at 600 °C in the pyrolysis atmosphere were used as samples. The reactant gas was 5% O2 with Ar as a balance, and the reactor was up to 800 °C to achieve the complete combustion with a heating rate of 6 K/min. The product gas of the coke oxidation was measured using Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, U.S.A.). The results showed that the main product gases were CO and CO2 with a small amount of water. The uncertainty of FTIR was less than 3%. The release rate of COx was used to characterize the reaction progress, and the relative reaction rate of the coke oxidation was calculated in accordance with Eq. 5.

r =

c co x

∫

tf t0

c co x d t

,

(5)

where t0 is the initial time, tf is the final time, and cCOx is the real-time concentration of COx.

3. RESULTS AND DISCUSSION 3.1. Effect of montmorillonite on the thermal conversion characteristics of heavy oil and coke yield 3.1.1. Thermal conversion characteristics


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1.2

oil and silica oil, silica and M

5.0E-4 1.0

4.0E-4

r / (1/s)

0.8

α

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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0.6

2.0E-4

0.4 oil and silica oil, silica and M

0.2 0.0

3.0E-4

0

200

400

600

800

1.0E-4 0.0

0

200

400

600

800

T / °C

T / °C

(a)

(b)

Figure 2. Conversions (a) and relative reaction rates (b) of heavy oil in the temperature-programmed oxidation with and without montmorillonite (5% O2, 6 K/min) TGA was used to study the thermal conversion characteristics of heavy oil. The difference in the conversions and relative reaction rates with and without montmorillonite in the oxidizing atmosphere is shown in Figure 2. From Figure 2(a), it can be seen that the conversion was found to differ in some temperature regions. The oil, silica and montmorillonite showed firstly lower, then higher and finally lower conversion than the oil and silica. Figure 2(b) shows that the presence of montmorillonite did not change the bimodal distribution of relative reaction rate. There existed a shoulder peak at 280 °C and a broad peak at 330−350 °C in the LTO region in which the low temperature oxidation mainly occurred for the oil and silica. . However, there was a broad peak at 280−300 °C and a shoulder peak at 335 °C in the LTO region, which meant that the main occurrence temperature of the low temperature oxidation advanced from 330−350 °C to 280−300 °C in the presence of montmorillonite. The peak temperature in the HTO region advanced from 524 °C to 483 °C, and the demarcation point of the two regions moved from


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425 °C (conversion of 0.715) to 400 °C (conversion of 0.603). The relative reaction rate decreased in the LTO region but obviously increased in the HTO region. Thereafter, trailing occurred at 537 °C with montmorillonite. The lower peak temperature in the LTO region may be related to the large surface area of montmorillonite. It decreased the thickness of the oil layer and enhanced oxygen transfer in the oil layer. This helped to shorten the time required for low temperature oxidation and therefore the peak temperature in the LTO region advanced. The effects of surface area on crude oil oxidation have been reported by Drici and Vossoughi.30 Their results of DSC show that the addition of the solid surface to the crude oil causes a shift of heat from a high to a low temperature range, which is consistent with the results of TGA in this paper. The decrease in the relative reaction rates in the LTO region with montmorillonite showed that the weight loss of oil in the LTO region decreased because of the strong adsorption of montmorillonite. Therefore, the fuel deposition increased from 28.5% to 39.7% and more deposited fuel participated in the HTO in the presence of montmorillonite. The lower peak temperature in the HTO region indicated an improved oxidation activity of the fuel in the presence of montmorillonite. The trailing was probably attributed to the coke deposited in the interlayer of montmorillonite,31 which would be discussed below.


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6.0E-4

1.0 oil and silica oil, silica and M

0.8

oil and silica oil, silica and M

5.0E-4

r / (1/s)

4.0E-4 0.6

α

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

0.4

2.0E-4

0.2 0.0

3.0E-4

1.0E-4 0.0 0

200

T / °C

400

600

0

(a)

200

T / °C

400

600

(b)

Figure 3. Conversions (a) and relative reaction rates (b) of heavy oil in the temperature-programmed pyrolysis with and without montmorillonite (Ar, 6 K/min)

The difference in the conversions and relative reaction rates with and without montmorillonite in the pyrolysis atmosphere is shown in Figure 3. From Figure 3(a), it can be seen that the conversion curves were found to be basically the same before 400 °C and afterwards the conversion in the presence of montmorillonite was lower. The final conversion changed from 0.922 to 0.899 after mixing montmorillonite, which indicated the obvious increase in fuel deposition from 7.8% to 10.1%. Figure 3(b) shows that the relative reaction rates of oil and silica presented a bimodal distribution. The first peak area corresponded to the evaporation of the light ends in the low temperatures, and the second peak area corresponded to the cracking of the polar components in the high temperatures.29 The peak temperatures were 267 °C and 442 °C respectively. The demarcation point of the two regions was at 355 °C. The peak temperatures did not change in the presence of montmorillonite, and the evaporation rate in the low temperature region was nearly the same, while the cracking rate in the high temperature


15

Energy & Fuels

region decreased, especially in the peak. This condition increased the fuel deposition. A comparison of the effect of montmorillonite in the oxidizing and pyrolysis atmospheres showed that the fuel deposition increased in the two atmospheres. Montmorillonite’s strong adsorption to polar components could contribute to the increase in fuel deposition. However, the peak temperatures in the LTO and HTO regions decreased with montmorillonite in the oxidizing atmosphere while the characteristic temperatures of thermal conversion were not affected in the pyrolysis atmosphere. This should be related to the oxygen transfer enhanced by montmorillonite in the oxidizing atmosphere while there was no such process in the pyrolysis atmosphere.

3.1.2. Coke yield oil and silica-oxidation oil, silica and M-oxidation oil and silica-pyrolysis oil, silica and M-pyrolysis

50 40

γ/%

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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30 20 10 0

250 300 350 400 T / °C

500

600

Figure 4. Effect of montmorillonite on the coke yield at different temperatures in the thermal conversion process of heavy oil

The thermogravimetric analysis data showed that the demarcation point in the LTO and HTO regions was around 400 °C in the oxidation process. In studying the effect of


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montmorillonite on the coke yield, the fixed-bed reactor was used to heat the samples to 250 °C/300 °C/350 °C/400 °C in the oxidizing atmosphere. The resulting coke yields (toluene insoluble) are shown in Figure 4. The demarcation point of evaporation and cracking was 355 °C, and the samples in the fixed-bed reactor were heated to 400 °C/500 °C/600 °C in the pyrolysis atmosphere. No coke was formed at 400 °C. Thus, the results for 500 °C/600 °C are shown in Figure 4. In the oxidizing atmosphere, no coke was formed at 250 °C for the samples of oil and silica. The coke yield increased from 17.84% to 33.36% when the temperature increased from 300 °C to 350 °C, and the solid residue was coke and minerals without toluene soluble at 350 °C. From 350 °C to 400 °C, the coke yield decreased from 33.36% to 29.31%. The change in the coke yield with temperature showed that the temperature range of 300–350 °C was the main formation stage of coke. For the samples of oil, silica and montmorillonite, the coke was formed at 250 °C, and the coke yield increased from 11.06% to 48.46% when the temperature was from 250 °C to 300 °C. The solid residue was coke and minerals without toluene soluble at 300 °C, and the coke yield decreased from 48.46% to 36.51% when the temperature was from 300 °C to 400 °C. The change in coke yield with temperature showed that the temperature range of 250–300 °C was the main formation stage of coke. The presence of montmorillonite promoted the coke formation process and increased the coke yield. This finding was consistent with the results of TGA. The heavy oil transforms to coke by oxygenation to form the oxygen−containing functional groups and subsequent polycondensation. When the temperature further increases, deep


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carbonization occurs by dealkylation and decarbonylation. The formation of coke advanced to 250 °C in the presence of montmorillonite. This phenomenon indicated that montmorillonite catalyzed polycondensation of the oxygen−containing functional groups which were strongly adsorbed on the surface. The maximum coke yield occurred at lower temperature (300 °C) with montmorillonite, which could be owing to enhanced oxygen transfer in the thinner oil layer. Oxygenation was promoted by more adequate oxygen supply in the oil layer, which resulted in that coke formation from heavy oil was completed at lower temperature. The coke yield was higher in the entire low temperature range in the presence of montmorillonite, because of its strong adsorption of polar components. In the pyrolysis atmosphere, the solid residue of the samples with and without montmorillonite was coke and minerals without toluene soluble at 500 °C. The coke yield decreased slightly when the temperature increased from 500 °C to 600 °C because of the cracking reaction. The coke yield increased from 7.86% to 10.44% at 500 °C and from 6.82% to 10.21% at 600 °C with montmorillonite. Montmorillonite showed no effect on the formation temperature of coke, but increased the coke yield obviously. This could be related to its strong adsorption of polar components.

3.2. Effect of montmorillonite on the physical and chemical properties of the coke formed in the oxidizing atmosphere 3.2.1. Elemental composition Table 2．．Elemental analysis of the coke formed in the oxidizing atmosphere (wt%)


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Energy & Fuels

a

Coke−

Coke−

Coke−

Coke−oil

Coke−oil

Coke−oil

Heavy

oila at

oila at

oila at

and Mb at

and Mb at

and Mb at

oil

300 °C

350 °C

400 °C

300 °C

350 °C

400 °C

C

84.16

83.21

82.73

73.77

79.48

78.20

65.33

H

11.81

9.89

7.93

4.11

8.09

5.19

2.25

O

3.12

4.92

7.46

19.84

11.03

14.95

30.83

Coke formed with oil and silica; bCoke formed with oil, silica, and montmorillonite The elemental contents of the heavy oil and coke formed at different temperatures in the

oxidizing atmosphere are shown in Table 2. With the increase in temperature, the heavy oil transformed to coke, and the contents of C and H decreased, whereas the content of O increased. For the two types of coke, the increase in the content of O was significant when the temperature was from 350 °C to 400 °C, which was related to acceleration of dealkylation in this temperature range. The contents of C and H were lower and the content of O was larger for the coke formed with oil, silica, and montmorillonite than those of the coke formed with oil and silica at the same temperatures. In the oxidizing atmosphere, the heavy oil transforms to coke by oxygenation to form the oxygen−containing functional groups and subsequent polycondensation. The enhanced oxygen transfer in the oil layer promoted oxygenation and contributed to the higher content of O and the lower contents of C and H in coke.

3.2.2. DRIFTS analysis


19

Energy & Fuels

(f) (e)

Absorbance / (a.u.)

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 20 of 35

(d) (c)

(b)

3071 2925 3200-3500

2851

1753 1710 1588 1461

(a)

4000

3500

3000 2500 2000 Wavenumber / (cm-1)

1500

1000

Figure 5. DRIFTS analysis of the coke formed at different temperatures in the oxidizing atmosphere: (a) coke formed with oil and silica at 300 °C; (b) coke formed with oil and silica at 350 °C; (c) coke formed with oil and silica at 400 °C; (d) coke formed with oil, silica, and montmorillonite at 300 °C; (e) coke formed with oil, silica, and montmorillonite at 350 °C; and (f) coke formed with oil, silica, and montmorillonite at 400 °C.

Figure 5 shows the DRIFTS analysis of the coke formed at different temperatures in the oxidizing atmosphere. The broad peaks between 3200 and 3500 cm−1 corresponded to the stretch vibrations of hydrogen-bonded OH groups. The peak at 3071 cm−1 was assumed to be the C–H vibration in the aromatic rings, and the peaks at 2925 and 2851 cm−1 were assumed to be the C– H stretch vibration in the alkanes. The peaks at 1710 and 1753 cm−1 corresponded to the C=O groups, and the peak at 1588 cm−1 corresponded to the C–C groups in the aromatic rings. The peak at 1461 cm−1 represented the bending vibration of C–H groups in the alkanes.32-34 With the increase in temperature, the C–H groups decreased and the C=O groups increased for both cokes


20

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formed with and without montmorillonite. From 350 °C to 400 °C, the C–H groups vanished, which indicated the acceleration of dealkylation in this temperature range. Compared with those for the coke formed with oil and silica at the same temperatures, the C=O groups were stronger, the C–H groups were weaker, and the OH groups were smaller for the coke formed with montmorillonite. The C–H groups of oil underwent oxygenation first to form the OH groups and then polycondensed to form the C=O groups.35 Therefore, the presence of montmorillonite promoted the oxidation of the C–H groups and the polycondensation of the OH groups in oil to form more C=O groups.

3.2.3. Specific surface area and pore structures

1 0.1 1E-2 1E-3 1E-4 1E-5

1

5

10

D / nm

50

100

(a) Surface area distribution of coke

0.1

dV/dD / (cm3.g−1.nm−1)

silica silica and M a coke − oil and silica b coke − oil, silica and M

10

dS/dD / (m2.g−1.nm−1)

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

0.01 1E-3 1E-4 1E-5 1E-6 1

5

10

50

100

D / nm

(b) Pore size distribution of coke

Figure 6. (a) Surface area and (b) pore size distribution of the coke formed in the oxidizing atmosphere and the minerals. (aCoke formed with oil and silica; bCoke formed with oil, silica, and montmorillonite; D is pore diameter, and S and V are pore surface area and volume of the unit mass, respectively).

The results of BET show that the specific surface area of coke formed of oil and silica was


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Page 22 of 35

20.01 m2/g and that of coke formed of oil, silica and montmorillonite was 28.16 m2/g in the oxidizing atmosphere. The specific surface areas of silica alone and silica mixed with montmorillonite at 400 °C were 2.79 and 16.80 m2/g, respectively, which indicated that for the two types of coke, the pore structure formed by the coke itself occupied an important proportion in the specific surface area. The distribution of the pore structure of cokes and mineral skeletons is shown in Figure 6. For the two types of coke formed with and without montmorillonite, the pore area and pore volume showed an obvious peak at 1–5 nm. The coke formed with oil and silica showed a larger pore area and pore volume than silica at 1–10 nm, and the pores formed by the coke itself were concentrated at 1–5 nm. However, the pore area and pore volume of the coke formed with oil, silica, and montmorillonite were larger than those of the silica mixed with montmorillonite at 1– 5 nm whereas smaller after 5 nm. These findings indicated that the coke formed a large number of pores at 1–5 nm, and meanwhile, it blocked some mesoporous of the silica mixed with montmorillonite skeleton, which could belong to the interlayer structure of montmorillonite. The coke deposited at the interlayer of montmorillonite could be difficult to contact with oxygen and showed poor reactivity owing to blocking, which caused the trailing in the HTO region.

3.2.4. Oxidation activity The oxidative characteristic curves of coke were obtained by the temperature-programmed experiments in the fixed-bed FTIR system. The oxidation activity of the coke formed with and without montmorillonite in the oxidizing atmosphere is shown in Figure 7.


22

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coke formed of oil and silica coke formed of oil, silica and M

8.0E-4 6.0E-4

r / (1/s)

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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4.0E-4 2.0E-4 0.0

0

100

200

300

400

T / °C

500

600

700

800

Figure 7. Oxidation activity of the coke formed in the oxidizing atmosphere

The relative reaction rate of coke oxidation showed a unimodal distribution with the increase in temperature. The peak of the relative reaction rate was at 537.9 °C for the coke formed with oil and silica, and moved to 491.6 °C for the coke formed with oil, silica, and montmorillonite. The coke formed with montmorillonite in the oxidizing atmosphere showed an improved activity and could stably combust at low temperature. This condition was conducive to the establishment and advancement of the combustion front. The reaction rate of the coke formed with montmorillonite decreased slowly after 550 °C, which was consistent with the trailing in the HTO region of TGA experiment.

3.3. Effect of montmorillonite on the physical and chemical properties of the coke formed in the pyrolysis atmosphere 3.3.1. Elemental composition Table 3. Elemental analysis of the coke formed in the pyrolysis atmosphere (wt%)


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Coke−oila

Coke−oila

Coke−oil and Mb

Coke−oil and Mb

at 500 °C

at 600 °C

at 500 °C

at 600 °C

Heavy oil

C

84.16

84.46

86.29

93.26

96.88

H

11.81

7.96

7.51

1.60

/

O

3.12

4.11

2.78

/

/

a

Coke formed with oil and silica; bCoke formed with oil, silica, and montmorillonite The elemental contents of the heavy oil and coke formed at different temperatures in the

pyrolysis atmosphere are shown in Table 3. With the increase in temperature from 500 °C to 600 °C, the content of C increased slightly, and the contents of H and O decreased slightly for the coke formed with oil and silica. The reaction occurring in this temperature range was mainly cracking of coke. The fracture of alkyl branch chains released a large number of small molecule organic gases. Thus, the content of H decreased, whereas the content of C increased. The content of C was higher, and the content of H was lower for the coke formed with montmorillonite than for the coke formed without montmorillonite at the same temperature. The H/C (mole ratio) of coke formed with oil and silica at 500 °C was 1.04, which implied that dehydrogenation did not occur yet. Dehydrogenation usually begins at 600 °C.32 The H/C (mole ratio) of coke formed with oil, silica, and montmorillonite at 500 °C was 0.21. Montmorillonite probably catalyzed the dehydrogenation of coke to occur earlier as a typical acid catalyst,24 which caused higher content of C of the coke formed with oil, silica and montmorillonite than those of the coke formed with oil and silica. The decrease in the content of O in the presence of montmorillonite might be caused by its catalysis on decarbonylation, which needed to be further studied.


24

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3.3.2．．DRIFTS analysis

(d)

Absorbance / (a.u.)

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

1569

(c) (b)

3051

2955 2914

1563

(a)

3500

3000

2500 2000 1500 -1 Wavenumber / (cm )

1000

Figure 8. DRIFTS analysis of the coke formed at different temperatures in the pyrolysis atmosphere: (a) coke formed with oil and silica at 500 °C; (b) coke formed with oil and silica at 600 °C; (c) coke formed with oil, silica, and montmorillonite at 500 °C; and (d) coke of oil, silica, and montmorillonite at 600 °C.

Figure 8 shows the DRIFTS analysis of the coke formed at different temperatures in the pyrolysis atmosphere. The types of functional groups of the coke formed with and without montmorillonite were the same, including the C–H groups in the aromatic rings with the peak at 3051 cm−1, the C–H groups in the alkanes with the peaks at 2955 and 2914 cm−1, and the C–C groups in the aromatic rings with the peaks at 1563 and 1569 cm−1. Compared with those for the coke formed with oil and silica at the same temperatures, the C–C groups were stronger and the C–H groups were weaker for the coke formed with oil, silica, and montmorillonite. This finding


25

Energy & Fuels

was consistent with the higher contents of C and lower contents of H for the coke formed with montmorillonite than those for the coke formed without montmorillonite in the elemental analysis.

3.3.3．．Specific surface area and pore structures 1

1E-3

0.1

dV/dD / (cm3.g−1.nm−1)

dS/dD / (m2.g−1.nm−1)

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 26 of 35

0.01 1E-3 silica silica and M a coke − oil and silica b coke − oil, silica and M

1E-4 1E-5 1

5

50

10

100

1E-4

1E-5

1E-6

5

1

(a) Surface area distribution of coke

50

10

100

D / nm

D / nm

(b) Pore size distribution of coke

Figure 9. (a) Surface area and (b) pore size distribution of the coke formed in the pyrolysis atmosphere and the minerals. (aCoke formed with oil and silica; bCoke formed with oil, silica, and montmorillonite)

The results of BET show that the specific surface area of coke formed of oil and silica was 7.81 m2/g and that of coke formed of oil, silica and montmorillonite was 9.60 m2/g in the pyrolysis atmosphere. The specific surface areas of silica alone and silica mixed with montmorillonite at 600 °C were 2.79 and 12.78 m2/g (smaller than the value of montmorillonite at 400 °C in section 3.2.3 because of the structural change), respectively. The coke formed of oil and silica showed larger specific surface area than silica, while the coke formed of oil, silica and montmorillonite showed smaller specific surface area than silica and montmorillonite.


26

Page 27 of 35

The distribution of pore structures of the coke and mineral skeleton is shown in Figure 9. For the two types of coke formed with and without montmorillonite, the pore area and pore volume showed a peak at 1–5 nm. The coke formed with oil and silica showed larger pore area and pore volume than silica at 1–10 nm, which indicated that the coke formed the pore structures by itself and that the pores were concentrated at 1–5 nm. However, the coke formed with oil, silica, and montmorillonite showed smaller pore area and pore volume than the silica mixed with montmorillonite in the entire range. This result indicated that the coke was partly attached to the interlayer structure of montmorillonite to block the pore structures. Meanwhile, the coke did not form the pore structures at 1–5 nm by itself as the coke formed in the oxidizing atmosphere because the yield of coke formed in the pyrolysis atmosphere was less. This condition could cause more obvious trailing in the high temperature oxidation region than the coke formed with montmorillonite in the oxidizing atmosphere.

3.3.4．．Oxidation activity 1.0E-3 coke formed of oil and silica coke formed of oil, silica and M

8.0E-4 6.0E-4

r / (1/s)

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

4.0E-4 2.0E-4 0.0

0

100 200 300 400 500 600 700 800

T / °C

Figure 10. Oxidation activity of the coke formed in the pyrolysis atmosphere


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Page 28 of 35

The oxidation activity of the coke formed with and without montmorillonite in the pyrolysis atmosphere is shown in Figure 10. The first peak of reaction rate for the coke formed with and without montmorillonite was nearly the same with the former at 567 °C and the latter at 571 °C. However, the reaction rate in the peak of the coke formed with montmorillonite was lower than that of the coke formed without montmorillonite. The second peak of the reaction rate for the coke formed with montmorillonite was at 681 °C, which was similar to the trailing in the oxidation of coke formed with montmorillonite in the oxidizing atmosphere but was more obvious. This finding could be attributed to the distribution of coke on montmorillonite. Montmorillonite had outer and interlayer surfaces. The coke deposited on the outer surface easily contacted with oxygen, whereas the coke deposited on the interlayer surface exhibited difficulty in contacting with oxygen and showed poor reactivity. This result was implied by the analysis of the pore structures. The coke formed with montmorillonite in the oxidizing atmosphere showed better reactivity than the coke formed without montmorillonite. This finding was totally different in the pyrolysis atmosphere. The difference in the oxidation activity of coke could be caused by the difference in montmorillonite’s effects on coke properties.

4. CONCLUSION In this study, the effects of montmorillonite on the thermal conversion characteristics of heavy oil, formation characteristics and properties of coke in the oxidation and pyrolysis atmospheres were studied. The main conclusions can be summarized as follows. In the oxidizing atmosphere, the presence of montmorillonite decreased the peak


28

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Energy & Fuels

temperatures in the LTO and HTO regions. The formation temperature of coke was significantly advanced. The maximum coke yield occurred at lower temperature with montmorillonite. The O content of the coke increased and the coke formed pore structures at 1-5 nm. The oxidation activity of the coke formed with montmorillonite got better because of the change in coke properties. However, trailing occurred in the end owing to the blocking. In the pyrolysis atmosphere, the presence of montmorillonite did not affect the characteristic temperatures of the thermal conversion and the formation temperature of coke.

The C content

of the coke increased and the coke did not form pore structures at 1-5 nm. The coke formed with montmorillonite did not show better oxidation activity and burned out at higher temperature owing to serious trailing. The fuel deposition and the coke yield increased with montmorillonite in both atmospheres. Montmorillonite significantly affected coke formation through its strong adsorption to polar components in heavy oil, obvious catalysis on dehydrogenation as an acid catalyst in the pyrolysis atmosphere while enhanced the oxygenation due to its large surface area and catalyzed polycondensation in the oxidizing atmosphere. The results could help to give some qualitative guidance in the ISC process applied in the reservoir with the existence of montmorillonite.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +86 10 62781740


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Page 30 of 35

ORCID

Qiang Song: 0000-0002-5484-3594

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the fund from Xinjiang Oilfield Company, China.

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Catalytic Effects of Montmorillonite on Coke Formation during Thermal

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