Article pubs.acs.org/EF
Investigation on Asphaltene Structures during Venezuela Heavy Oil Hydrocracking under Various Hydrogen Pressures Dong Liu,*,† Zhongtao Li,† Yue Fu,‡ Yinghao Zhang,§ Peng Gao,† Caili Dai,*,† and Kaiyuan Zheng† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, People’s Republic of China China National Offshore Oil Corporation, Beijing 100010, People’s Republic of China § Mechanical and Electrical Engineering Institute, Beijing Institute of Technology, Beijing 100010, People’s Republic of China ‡
ABSTRACT: Venezuela heavy oil under various hydrogen pressures has been hydrocracked to investigate the variation of asphaltene components during reaction. Asphaltenes have been isolated from the product and analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR). The experimental data revealed that the interlamellar spacing and interchain spacing of the asphaltenes increased while the layer diameter decreased with the hydrogen pressure increasing. At the same time, the amount of aromatic carbon and alkyl carbon of the asphaltenes decreased gradually and the amount of naphthenic carbon increased. As the hydrogen pressure increased, the substitution ratio and the condensation degree parameter (HAU/CA) of the aromatic system in the periphery increased gradually and the replacement index and peri-position condensation index of asphaltenes decreased obviously.
1. INTRODUCTION
Herein, asphaltenes of a suspended-bed hydrocracking product in different hydrogen pressures have been analyzed by NMR, XRD, etc. The variations of structure composition and parameters in asphaltenes have been described, and the mechanism of asphaltenes in the process of heavy oil hydrogenation in the presence of hydrogen pressure has also been discussed.
As one of the most complicated components in residue oil, asphaltene is composed of various aromatic hydrocarbons with a high content of heterocyclic compounds and a large molecular weight.1,2 The micelle core structure of oil colloid plays an important role in keeping the stability. Some increasing attention has been focused on detecting the structure of the asphaltenes during hydrogenation through size-exclusion chromatography (SEC), small-angle neutron scattering (SANS),3−5 1H nuclear magnetic resonance (NMR), elemental analysis, and average relative molecule weight.6−8 Although many significant conclusions have been obtained, the role of hydrogen pressure during hydrocracking, structural changes of components, and reaction mechanism during residue hydrogenation have not been studied thoroughly and still need to be further investigated. With the progress of characterization methods, the structure of asphaltenes has been well-studied. Modern analytical instruments have been employed to readily detect routes that can greatly improve the current research. The most useful physical analytical instruments include NMR, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), etc.,9−12 especially NMR, which has been considered as an important way for heavy oil structural analysis. 1H NMR has been widely used in the identification of aromatic protons and saturated protons. It can also provide more details about the hydrogen chemical environments for predicting distribution of hydrogen types of compounds. 13C NMR can detect different types of carbon directly. However, thousands of cyclic scans are essential to obtain a reasonable spectrogram because of the very low abundance of 13C in nature. X-ray is also applied to the study of asphaltene structure very frequently. Yen and coworkers13−15 obtained many parameters, such as layer diameter, interlamellar spacing, interchain spacing, and number of layers, from the X-ray signal in different angles. © 2013 American Chemical Society
2. EXPERIMENTAL SECTION The Venezuelan heavy oil was used as raw materials. The Venezuelan heavy oil hydrocracking was processed in a 0.5 L reactor for 1 h with 200 μg g−1 of oil-soluble molybdenum catalyst (calculated by the iron naphthenate) under different initial hydrogen pressures (6.0−10.0 MPa) at 430 °C. The C7 asphaltenes was separated from the product for further analysis. Through traditional separation methods,16,17 four components (saturate, aromatic, colloid, and asphaltenes) of residue oil have been separated. Elemental analysis was performed using the Varil EL-3 elemental analyzer to define the contents of C, H, S, and N. The asphaltenes was dissolved in toluene to measure the molecular weight by a Knauwer molecular weight apparatus and calculated by the method of vapor pressure osmometry (VPO). XRD was analyzed by the D/MAX-IIIA X-ray diffractometer using a Cu target as the X-ray source. XRD spectrograms of asphaltenes (products at different initial hydrogen pressures) were calculated for monitoring the variations of asphaltene lamellar structures. A Bruker Avance DMX500-type superconducting NMR spectrometer was used to determinate the 1H and 13C NMR spectra of asphaltenes. CDCl3 was used as the solvent, and tetramethylsilane (TMSi) was used as the internal standard. The resonance frequency was 500 MHz. Received: March 12, 2013 Revised: June 6, 2013 Published: June 20, 2013 3692
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3. RESULTS AND DISCUSSION 3.1. Element and Component Analyses of the Atmospheric Residue in the Hydrocracking Product under Different Hydrogen Pressures. Venezuelan heavy oil was hydrocracked with various reaction pressures and 200 μg g−1 of oil-soluble molybdenum catalyst at 430 °C. The atmospheric residue has been analyzed, and the results are shown in Table 1.
radical increased with the increase of the hydrogen initial pressure, which hydrocracked the alkyl side chain of asphaltenes. When the pressure increased higher, part of the aromatic system would be hydrogenated and cracked, resulting in the decrease of the asphaltene molecular weight. 3.2. XRD Analysis of Asphaltenes in the Reaction Product under Different Hydrogen Initial Pressures. The asphaltenes of the cracking product at different hydrogen pressures were analyzed by an X-ray diffractometer. Many structural parameters, such as layer diameter (La), interlamellar spacing (dm), interchain spacing (dr), cell height (Lc), and number of piles have been calculated. The XRD spectrogram of AR asphaltenes of the product at the initial hydrogen pressure of 10 MPa is shown in Figure 1.
Table 1. Four-Component Analysis of the Atmospheric Residue initial hydrogen pressure (MPa)
6
8
10
saturate (%) aromatic (%) resin (%) asphaltene (%)
34.83 39.84 11.32 12.43
40.02 39.08 12.90 9.958
42.56 38.28 13.06 6.103
As shown in Table 1, the content of the saturate component increases with the hydrogen pressure increasing, revealing that the hydrocracking reaction and side-chain cracking reaction took place. The aromatic content slightly changed because of the dynamic equilibrium between composition and decomposition at high hydrogen pressure and temperature. Because two reactions cooperated simultaneously with each other, the aromatic content was kept at the same level during hydrocracking. The asphaltene content decreased dramatically because the high initial hydrogen pressure constrained the condensation of the aromatic with resin. More hydrogen radicals were formed under high initial hydrogen pressure, and these radicals neutralized the aromatic radicals that generated during cracking. Therefore, the resin content slightly increased, and the asphaltene content decreased. Furthermore, a large number of hydrogen radicals also constrained the cross-linking of macromolecular radicals to prevent the generation of coke. Venezuelan heavy oil was hydrocracked under different hydrogen pressures at 430 °C. The results of elemental analysis of asphaltenes in the product are tabulated in Table 2.
Figure 1. XRD spectrum of asphaltenes in the product at 10 MPa.
From the XRD spectrogram, Bragg angle 2θ is 42.48°, λ = 1.5406 nm, and the other structural parameters are calculated as follows: interlamellar spacing
Table 2. Elemental Composition of Asphaltenes in the Reaction Products under Different Hydrogen Pressures initial hydrogen pressure (MPa)
C (wt %)
H (wt %)
S (wt %)
N (wt %)
molecular weight
6 8 10
85.11 84.09 83.41
8.36 9.73 9.88
3.48 2.94 2.54
1.91 1.84 1.59
3438.9 3356.2 3201.6
dm = λ/2 sin θ
interchain spacing dr = 5λ /8 sin θ
layer diameter La = 1.84λ /ω cos θ = 0.92/B1/2
cell height As seen in Table 2, the content of carbon in the asphaltenes gradually decreased with an increasing of initial hydrogen pressure, while the content of hydrogen increased. Under a low initial hydrogen pressure, the macromolecule radicals were dramatically correlated with each other, which led to the increasing content of carbon in asphaltenes. Thus, secondary asphaltenes generated easily and tended to coke. A higher initial hydrogen pressure played a positive role on hydrogenolysis reaction, such as hydrodesulfurization and hydrodenitrogeneration. As a result, the C, S, and N contents in asphaltenes decreased under higher hydrogen pressure. At the same time, it can be seen that the asphaltene molecule weight of atmospheric residue (AR) in product decreased obviously with the increase of the initial hydrogen pressure. The reason would be that the concentration of the hydrogen
Lc = 0.9λ /ω cos θ = 0.45B
layer number Me = (Lc/dm) + 1
average ring number at any orientation
Ra = La/2.667 where θ is the diffracion angle, λ is the X-ray optical wavelength, ω is the peak width, B1/2 is the full width at half maximum, and 2.667 Å is the width of a single aromatic nucleus. The crystal parameters of asphaltenes in the product under different hydrogen pressures can be calculated by a similar method. The results are shown in Table 3. 3693
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dominated, which gave rise to the increase of asphaltene sheet diameter. When the initial hydrogen pressure increased, the macromolecule radicals of asphaltenes were terminated by a large number of hydrogen radicals. Thus, condensation between radicals slowed, and the hydrogenolysis reaction could occur in the cata-condensation carbon and peripheral carbon of the aromatic ring system, resulting in a decrease of the asphaltene layer diameter. The layer number and the number of rings of asphaltenes decreased with the increase of the initial hydrogen pressure. At a lower hydrogen pressure, the condensation reaction of aromatic radicals took place dramatically, which increased the condensation rate among different layers of asphaltenes. When the hydrogen pressure increased, this condensation reaction would be weakened; therefore, the layer number and number of rings decreased. 3.3. 1H NMR Analysis of Asphaltenes under Different Hydrogen Initial Pressures. According to the 1H NMR data of asphaltenes from the product under different hydrogen pressures, the average structural parameters of asphaltenes were calculated in terms of the Browen−Landner method.18,19 The asphaltene 1H NMR spectrum of the hydrocracked product under a 10 MPa hydrogen atmosphere is shown in Figure 2. The intensities of various protons were calculated by peak integration. According to the Browen−Landner method, the
Table 3. Crystalline Parameters of Asphaltenes at Different Hydrogen Pressures initial hydrogen pressure (MPa)
6
8
10
interlamellar spacing (Å) interchain spacing (Å) layer diameter (Å) cell height (Å) number of piles average ring number at any orientation
5.618 7.022 13.333 6.522 3.373 4.999
6.116 7.645 12.432 6.081 3.033 4.662
6.543 8.179 11.5 5.625 2.758 4.312
The variation tendency of interlamellar spacing, interchain spacing, and chain diameter of asphaltenes of the hydrocracking product at different hydrogen pressures was listed in Table 3. The experimental data revealed that interlamellar and interchain space of the product asphaltenes increased and the chain diameter decreased with the increase of the hydrogen pressure, demonstrating that a higher hydrogen pressure weakened the interaction between asphaltene layers and relaxed the layer-by-layer structure. The aliphatic chains could be cracked in the hydrocracking. Thus, the interlayer space and curvature of the layers became smaller, which led to an increase of the distance between the aliphatic chains. When the hydrogen pressure was lower, the condensation reaction between macromolecule radicals of asphaltenes became
Figure 2. 1H NMR of asphaltenes from the product under 10 MPa. 3694
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during which radical generation and connection reacted simultaneously. Therefore, the higher initial hydrogen pressure would prevent the linking of the macromolecule radical from forming a polyaromatic system. Thus, the aromatic carbon rate, total ring number, aromatic carbon number, and aromatic ring number decreased significantly. Moreover, a high initial hydrogen pressure could accelerate the hydrogenolysis reaction, and some periphery aromatic rings hydrogenated and saturated into a naphthenic ring, which led to the increase of the naphthenic ring number. The decrease of the total carbon number of asphaltenes may be ascribed to the main reaction of the hydrogenolysis reaction under a higher initial hydrogen pressure, which could lead to weakening of the interlayer interactions of the asphaltenes and loosing of asphaltene layers from each other. Therefore, some chains of the aromatic ring system would be broken, and even some smaller laminas in the hydrogenation process might be separated, giving rise to a decrease of the total carbon number of asphaltenes. From Table 4, the substituted rate of the aromatic protons gradually increased with the increase of the reactive hydrogen pressure, which was caused by the hydrogenation of periphery aromatic rings of asphaltenes. The higher initial hydrogen pressure played a positive role on the hydrogenation of the aromatic ring, and some kinetic studies indicated that the ringopening reaction of the naphthenic ring could be inhibited under the high initial hydrogen pressure.20,21 When the rate of hydrogenation was higher than that of naphthenic ring opening, the substitution rate of aromatic protons would increase and the number of naphthenic ring also increased. This was also the reason for the increase of the naphthenic ring and number of naphthenic carbons. Under different hydrogen pressures, the
types of carbon atoms of asphaltenes in the product under different hydrogen pressures are shown in Table 4. Table 4. Average Structural Parameters of Asphaltenes from the Product under Different Hydrogen Pressures symbol CT fA CA σ HAU/ CA RT RA RN CS CN Cp fN fP
name
6 MPa
8 MPa
10 MPa
total carbon aromatic rate aromatic carbon replacement rate of periphery hydrogen in the aromatic ring system condensation degree parameter of the aromatic ring system total ring number aromatic ring number naphthenic ring number saturated carbon number cycloalkyl carbon number paraffin carbon number naphthenic carbon rate paraffin carbon rate
243.9 0.5624 137.2 0.3328
235.2 0.5089 119.7 0.3574
222.5 0.4845 74.44 0.3822
0.3903
0.5257
0.6417
53.15 44.39 8.756 106.8 26.27 80.51 0.1077 0.3301
51.38 38.56 12.82 114.72 38.45 77.05 0.1635 0.3276
38.79 23.48 15.31 115.5 45.93 68.78 0.2064 0.3091
As shown in Table 4, with the initial hydrogen pressure increasing, the variation of average structural parameters of asphaltenes in the product abode by the following rules: the total carbon number, total ring number, aromatic carbon number, and aromatic ring number decreased with the great decrease of the aromatic carbon rate, while the naphthenic ring number increased. For the heavy oil suspension-bed hydrocracking, most of the thermal reactions were radical reactions,
Figure 3. 13C NMR of asphaltenes from the product at 10 MPa. 3695
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condensation degree parameter HAU/CA of products increased with the increase of the hydrogen pressure, meaning a smaller condensation degree of the aromatic ring. 3.4. 13C NMR Analysis of Asphaltenes in Hydrocracked Venezuelan Heavy Oil. The ratio of different types of carbon atoms in asphaltenes was calculated from the 13C NMR spectrum, and the results are shown in Figure 3 and Table 5.
saturated carbon in asphaltenes. While the condensation reaction led to more aromatic compounds, the content of aromatic carbon and endocyclic carbon in the product increased. However, the concentration of hydrogen radicals increased with the initial hydrogen pressure increasing, generating more cycloalkane rings by the hydrogenation reaction of the aromatic ring. The restraint of cracking resulted in the decrease of side chains breaking and the increase of saturated carbon. The higher initial hydrogen pressure terminated radical chain transfer by the linking of hydrogen radicals and macromolecule radicals, causing the decrease of aromatic carbon and inner carbon of the aromatic ring system. As shown in Table 5, the number of peripheral aliphatic substituted carbons was gradually reduced with the increase of hydrogen pressure. This may be assigned to the decrease of the generation of the hydrogen radical with the reduction of the initial hydrogen pressure. Therefore, aromatic ring radicals and aliphatic chain radicals were ready to cross-link with each other. However, when the hydrogen pressure increased, the concentration of hydrogen radicals in the reaction system became higher and the hydrogenolysis reaction was strengthened; therefore, the peripheral aliphatic substituted carbons in the aromatic ring decreased. At the same time, secondary asphaltenes was also generated because of the condensation reaction of the aromatic ring radical, resulting in the increase of the unsubstituted peripheral carbon of the aromatic ring system in asphaltenes. The content of carbonyl carbon obviously reduced with the increase of the initial hydrogen pressure. At the low initial hydrogen pressure, the hydroxyl was generated by hydrogenation of carbonyl, while at the high initial hydrogen pressure, the hydroxyl would react further. The infrared spectra of asphaltenes (Figure 4), which were generated under a pressure of 5 MPa (red line) and 10 MPa (blue line), revealed that there was a 3300 cm−1 absorption peak in the 5 MPa asphaltenes, while this absorption peak was weakened for the 10 MPa asphaltene IR spectra, indicating that the content of
Table 5. Distribution of the Different Types of Carbon Atoms in Asphaltenes type
aromatic carbon
substituted carbon
carbonyl carbon peripheral carbon of the aromatic system substituted by an aliphatic chain peripheral carbon of the aromatic system unsubstituted by an aliphatic chain inner carbon of the aromatic ring system aromatic carbon methyl methylene tertiary carbon saturated carbon
6 MPa (%)
8 MPa (%)
10 MPa (%)
0.4072 14.49
0.2818 12.19
0.0621 10.79
24.26
25.32
26.51
16.87
13.50
11.90
55.63 7.965 32.63 3.392 43.96
51.02 10.10 35.35 3.255 48.70
49.21 13.25 37.34 0.1449 50.73
From Table 5, it can be found that the aromatic carbon and endocyclic carbon in asphaltenes gradually reduced with the increase of hydrogen pressure, while the content of saturated carbon significantly increased. The reason would be that the main reaction of Venezuelan heavy oil in the slurry-bed hydrocracking contains a thermal reaction, a cracking reaction, and a radical condensation reaction. The side chains of the aliphatic were broken by cracking under low hydrogen pressure and high temperature, leading to the obvious decrease of
Figure 4. Infrared spectra of asphaltenes of the product at 5 and 10 MPa. 3696
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Notes
carbonyl carbons in produced asphaltenes gradually decreased with the improvement of the hydrogen pressure. The saturated carbon of asphaltenes was substantially free of quaternary carbon and also with a relatively small amount of tertiary carbon. As shown in Table 5, the content of the methyl and methylene increased and the tertiary carbon content significantly decreased with the increase of the initial hydrogen pressure. It may be due to the more active and reactive βhydrogen on the side chain of the aromatic ring under high hydrogen pressure. After cracking, the methyl still connected with the aromatic ring, which was attributed to the increase of the methyl with the increase of the pressure. The increasing content of methylene was primarily ascribed to the generation of the cycloalkyl ring during the aromatic ring hydrogenation under a high initial hydrogen pressure. The 13C NMR was used to characterize the structure of asphaltenes by Gauthie and co-workers,22 and the pericondensation index and replacement index were employed to characterize the changes of the structure of asphaltenes. The substituted index and the condensation index of raw material and products under different hydrogen partial pressures were calculated. As shown in Table 6, the substituted index and the pericondensation index decreased dramatically with the increase of
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Natural Science Foundation of China (21176259) and the award from the Foundation for Excellent Young and MiddleAged Scientist of Shandong Province, China (BS2010NJ024).
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Table 6. Substitution Index and Condensation Index of Asphaltenes in the Reaction Product at Different Hydrogen Pressures type
6 MPa
8 MPa
10 MPa
replacement index peri-condensation index
0.3738 3.312
0.3249 2.686
0.2892 2.405
the hydrogen pressure. This may be ascribed to saturation of the aromatic rings in asphaltenes by hydrogenation at a high initial hydrogen pressure during the generation of the naphthenic ring. At a high temperature and high hydrogen pressure, the following ring opening and side chain cleavage of the cycloalkyl ring reactions could occur; therefore, the substituted index showed a reducing trend with the increase of the hydrogen pressure. While the aromatic rings at the edge of the conjugated system were saturated by hydrogenation and finally broken, the original internal peri-condensed melting carbon would become the peripheral carbon and the pericondensation index declined obviously.
4. CONCLUSION Venezuelan heavy oil has been hydrocracked under various initial hydrogen pressures to investigate the influencing factors and mechanism during the reaction. Experimental data revealed that the interlamellar space and interchain space of asphaltenes increased with the increase of the pressure, while the diameter of each layer decreased. While the aromatic carbon and alkyl carbon in asphaltenes decreased gradually, the naphthenic carbon increased. The aromatic carbon and aliphatic substituted aromatic carbon decreased obviously. Finally, the replacement index and peri-position condensation index of asphaltenes decreased with the increase of the initial hydrogen pressure.
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