Article pubs.acs.org/EF
Role of Hydrogen Pressure in Slurry-Phase Hydrocracking of Venezuela Heavy Oil Hui Du,† Dong Liu,*,† Hua Liu,‡ Peng Gao,§ Renqing Lv,∥ Ming Li,† Bin Lou,† and Yuanxi Yang† †
State Key Laboratory of Heavy Oil Processing, ‡School of Geosciences, and ∥College of Science, China University of Petroleum, Qingdao, Shandong 266555, People’s Republic of China § National Institute of Clean-and-Low-Carbon Energy, Beijing 102209, People’s Republic of China ABSTRACT: The composition, structure, and colloidal stability of hydrocracked products at various hydrogen pressures were analyzed to investigate the role of hydrogen pressure in slurry-phase hydrocracking of Venezuela heavy oil. Experimental data showed that the formation of gas, naphtha, and coke was suppressed by a high hydrogen pressure during the slurry-phase hydrocracking. In addition, the maximum of light oil per coke ratio was observed at 8 MPa of hydrogen. The desulfurization and denitrogenation were promoted by the increase of hydrogen pressure. However, the influence of hydrogen pressure on the desulfurization rate was reduced at higher pressure, while the influence on the denitrogenation rate was still obvious. With the increase of hydrogen pressure, the Conradson carbon residue (CCR) of the vacuum residue (VR) product decreased. Meanwhile, the coking inducing period of the atmospheric residue (AR) product prolonged, which means that the colloidal stability of the AR product was enhanced by a high hydrogen pressure. Structural parameters of asphaltene were studied according to the reference of the Fourier transform infrared (FTIR) spectroscopy method of kerogen structure studying. The cracking and hydrogenation saturation of asphaltene were promoted by a high hydrogen pressure, which lead to the increase of the A factor (infrared absorption intensity ratio of saturated aliphatic carbon and aromatic carbon) and nCH2/nCH3 of asphaltene products as well as the decrease of the Y factor (condensation index of asphaltene) and Z factor (discriminative index of the side-chain fracture situation of asphaltene).
1. INTRODUCTION Slurry-phase hydrocracking is a complicated process, which could process inferior feedstock oils under the action of hydrogen and dispersed catalyst.1−5 Most reactions during slurry-phase hydrocracking could be considered as thermal cracking and hydrocracking reactions. Hydrogen is mainly used to reduce the coke yield by inhibiting the condensation of macromolecular free radicals.6,7 Moreover, the hydrogen pressure plays a significant role in the composition of hydrocracked products.8−10 Heavy oil is a colloidal system that is composed of saturates and aromatics (dispersion medium), resin (solvation layer), and asphaltene (dispersed phase). The fractions in heavy oil are mixed together and interacted with other fractions to achieve a dynamic equilibrium between different phases.11,12 Because of the high temperature and hydrogenation reaction in the hydrocracking process, the structure and composition of asphaltene would be changed. For example, asphaltene can be converted to resin and coke through cracking and condensation reaction, respectively. Then, the colloidal structure of hydrogenated products would be changed. Bartholdy et al. reported that the stability of the residue colloid system was broke by high temperature.13 Zhang et al. employed mass fraction normalized conductivity to detect the precipitation of asphaltene, and the colloid stability of hydrotreated products was investigated when the hydrocracking reactions were carried out over different supported catalysts.14 However, to our knowledge, the effect of hydrogen pressure on the colloidal stability of the atmospheric residue (AR) product with the presence of dispersed catalysts has not been reported. © 2015 American Chemical Society
Asphaltene is the most complex component in residue oil, which is composed of high-molecular-weight polycyclic aromatics and a high content of heterocycles.15,16 As the micelle core of the residue oil colloid system, asphaltene plays an important role in the colloidal stability of the residue. The structure of asphaltene would be changed in the hydrocracking process, which results in the changes of asphaltene properties and residue colloidal stability. Ancheyta et al. discussed the changes of asphaltene at different temperatures and found that the aromaticity, average chain length, substitution, and average ring number of aromatic systems were changed during the hydrogenation reaction.17 Sun et al. investigated the asphaltene structure and composition changes of the hydrotreated residue by proton nuclear magnetic resonance (1H NMR), elemental analysis, molecular weight determination, etc.18 A systematic study of the infrared (IR) spectra application in kerogen research has been reported.19 Thus, Fourier transform infrared (FTIR) spectroscopy would be an effective method to characterize the functional group of asphaltene because of the similar structural characters between asphaltene and kerogen. Hence, the distribution and composition of hydrocracked products at different hydrogen pressures were determined in this study. The coking inducing period and colloidal stability of AR products were discussed by the mass fraction normalized conductivity method. Besides, the structural parameters of Received: November 3, 2014 Revised: January 26, 2015 Published: March 11, 2015 2104
DOI: 10.1021/ef502457p Energy Fuels 2015, 29, 2104−2110
Article
Energy & Fuels Table 1. Composition and Properties of Venezuelan Heavy Oil elemental composition (wt %) ρ20 (g cm−3)
ν (mm2 s−1)
C
H
12.0
85.06
11.17
92.4 (80 °C) SARA (wt %)
0.9664
a
carbon residue (wt %)
S
N
2.54 metal (μg g−1)a
0.43
saturates
aromatics
resins
n-C7 asphaltenes
Ni
V
Fe
41.92
34.99
17.32
5.77
34.0
196
6.7
Micrograms of metal mass in 1 g of Venezuelan heavy oil. analyzer, and then the mixture was kept at 35 ± 0.1 °C. After 20 min, the electrical conductivity (κ) of the mixture was obtained from an impedance spectroscopic analyzer. Then, the mass fraction normalized conductivity (Λ) was calculated by the following equation:
asphaltene products were studied using the reference of the FTIR spectroscopy studying method of the kerogen structure.
2. EXPERIMENTAL SECTION
κ = Λ(1 + ν1ρ1 /m0)
2.1. Materials. Venezuelan heavy oil was taken as feedstock oil, and molybdenum naphthenate (purity of 99% and molybdenum content of 6.7 wt %, Aladdin Industrial, Inc.) was applied as the dispersed catalyst for slurry-phase hydrocracking. The composition and properties of Venezuelan heavy oil are shown in Table 1.20 2.2. Hydrocracking of Venezuelan Heavy Oil. Slurry-phase hydrocracking of Venezuelan heavy oil was carried out in a 500 mL batch-type autoclave. A total of 200 g of Venezuelan heavy oil and 200 μg g−1 of molybdenum naphthenate (calculated by the molybdenum content) were loaded into the autoclave, which was charged with different pressures (4−10 MPa) of hydrogen at room temperature. After 1.0 h of reaction at 430 °C with the stirring speed of 500 rpm, the autoclave was cooled by water to cease the reaction. Naphtha (350 °C) were separated from autoclave contents by atmospheric distillation. Furthermore, vacuum gas oil (VGO, 350−500 °C) and distilled bottom (>500 °C) were separated from AR by vacuum distillation. Vacuum residue (VR) was the toluene-soluble fraction of distilled bottom, while the tolueneinsoluble fraction was coke. The yields of products were calculated by the following equations:
where m0 is the mass of the AR product (g), ν1 is the volume of nheptane (mL), and ρ1 is the density of n-heptane (0.685 g cm−3).21 Using the mass ratio of n-heptane to AR product as the x axis and Λ as the y coordinate, the linear chart was constructed. The mass ratio of n-heptane to AR product, which could be calculated at maximum Λ, was determined as the CSP of the AR product. 2.6. Characterization of the Asphaltene Product. Asphaltene was separated from the AR product by the method in the literature.22 A Nicolet Magna-750 FTIR spectrometer was used to determinate the spectra of asphaltene. The spectra was an average of 32 scans with a resolution of 2 cm−1 in the region of 4000−500 cm−1.
3. RESULTS AND DISCUSSION 3.1. Distribution and Composition of Hydrocracked Products. In the slurry-phase hydrocracking of heavy oil, a hydrocarbon molecule could be transformed into a small molecule and coke through thermal cracking and thermal condensation, respectively. Because of the presence of the dispersed catalyst, the opportunity of hydrogen molecule participation in the hydrocracking reaction increases, which could have a significant influence on the ideal products (naphtha and diesel) and non-ideal product (coke). A new parameter, light oil (naphtha and diesel) per coke ratio, was introduced to characterize the hydrogenation effect at different hydrogen pressures. The light oil per coke ratio was calculated by the following equation:
yield of product (wt%) = product weight/weight of feedstock × 100 yield of gas (wt%) = (weight of feedstock − weight of autoclave content) /weight of feedstock × 100 A Varil EL-III elemental analyzer with a thermal conductive detector (TCD) was used to determinate the sulfur and nitrogen contents in autoclave content and each fraction. The removal ratios of sulfur and nitrogen were calculated by the following equation:
light oil per coke ratio (%) = (naphtha weight + diesel weight)/coke weight × 100
As Table 2 lists, after the hydrocracking reactions at all of the different hydrogen pressures, the contents of diesel increased,
removal ratio (%) = (content in feedstock − content in autoclave content) /content in feedstock × 100
Table 2. Product Distribution of Venezuelan Heavy Oil Slurry-Phase Hydrocracking at 430 °C for 1.0 h under Different Hydrogen Pressures
2.3. Conradson Carbon Residue (CCR) of the VR Product. The CCR of VR products was determined by the ASTM D189 method. 2.4. Coking Inducing Period of the AR Product. A total of 10− 15 g of AR product was loaded into a 25 mL micro autoclave, which was charged with 1.0 MPa of nitrogen. The micro autoclave was preheated to 150 °C in an oven and then heated to 400 °C in a tin bath for 30, 60, 90, and 120 min. After the reaction, the micro autoclave was put into cold water to chill. The coke product was separated by high-speed centrifugation and washed by toluene. 2.5. Colloidal Stability Parameter (CSP) of the AR Product. The CSP of the AR product was measured by a HP4194A frequency and phase impedance spectroscopic analyzer, which was equipped with a homemade electrode. A well-mixed n-heptane/AR mixture was loaded into the measuring container of an Agilent 4263B LCR
Venezuelan heavy oil gas (wt %) naphtha (wt %) diesel (wt %) VGO (wt %) VR (wt %) coke (wt %) light oil (wt %) light oil per coke ratio (%) 2105
6.19 33.12 60.69
4 MPa
6 MPa
8 MPa
10 MPa
16.26 11.38 24.21 24.82 21.80 1.53 35.59 23.26
15.19 10.70 24.15 26.21 22.42 1.33 34.85 26.20
14.42 8.86 25.85 27.63 22.43 0.81 34.71 42.85
13.95 8.64 23.83 30.73 22.05 0.80 32.47 40.59
DOI: 10.1021/ef502457p Energy Fuels 2015, 29, 2104−2110
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Energy & Fuels while the contents of VGO and VR decreased. The yields of gas, naphtha, and coke decreased gradually with the increase of hydrogen pressure. In the slurry-phase hydrocracking process, the macromolecules converted to smaller molecules by hydrogenolysis and side-chain breaking.23 However, hydrogen free radicals were created on a dispersed catalyst, which could quench with macromolecular free radicals.24 Under a high hydrogen pressure, more hydrogen molecules participated in slurry-phase hydrocracking of Venezuelan heavy oil. Thus, the serious thermal cracking and condensation of hydrocarbon would be further suppressed by a high hydrogen pressure, which results in the decreasing yields of gas, naphtha, and coke. It is similar to the conclusion obtained by Aimoto et al., which was based on the hydrocracking of AR over a nickel catalyst.25 The light oil per coke ratio increased first and then decreased slightly with the increase of hydrogen pressure, and the maximum was reached at 8 MPa. However, the total yield of light oil and VGO reached the maximum at 10 MPa. As demonstrated in Figure 1, both desulfurization and denitrogenation rates increased with the increase of hydrogen
Table 3. Content of Sulfur in Each Fraction of Venezuelan Heavy Oil Slurry-Phase Hydrocracking at 430 °C for 1.0 h under Different Hydrogen Pressures
a
hydrogen pressure (MPa)
light oil (μg g−1)a
VGO (μg g−1)a
VR (μg g−1)a
4 6 8 10
87.7 92.0 86.1 68.8
214.2 192.4 187.5 177.1
345.7 305.4 282.7 267.9
Micrograms of sulfur mass in 1 g of fraction.
Table 4. Content of Nitrogen in Each Fraction of Venezuelan Heavy Oil Slurry-Phase Hydrocracking at 430 °C for 1.0 h under Different Hydrogen Pressures
a
hydrogen pressure (MPa)
light oil (μg g−1)a
VGO (μg g−1)a
VR (μg g−1)a
4 6 8 10
8.88 8.32 8.60 8.30
23.9 22.1 20.4 17.0
135 120 110 101
Micrograms of nitrogen mass in 1 g of fraction.
of the nitrogen heterocyclic compound and hydrogenolysis of carbon−nitrogen bonds. 3.2. CCR of the VR Product. CCR is an important parameter to characterize heavy oils and their products. The CCR of heavy oil mainly depends upon the contents of asphaltenes as well as part of resins and polyaromatics.30,31 As demonstrated in Figure 2, the CCR of the VR product
Figure 1. Desulfurization and denitrogenation rates at different hydrogen pressures.
pressure. Furthermore, the desulfurization rate increased rapidly first and then decreased with the increase of hydrogen pressure, while the denitrogenation rate grew rapidly all of the time. The different effects of hydrogen pressure on desulfurization and denitrogenation rates could be ascribed to different mechanisms. The sulfocompound could be converted to corresponding hydrocarbon and H2S directly via the breaking of carbon−sulfur bonds. Besides, the sulfur of thiophene can be removed at a certain hydrogen pressure.26,27 In the hydrodenitrogenation progress, most nitrogen heterocyclic compounds are pyridine and pyrrole derivatives, which should be transformed into a C−N single bond by the saturation at first, and then, the hydrogenolysis occurs.28,29 Therefore, the hydrogen pressure plays a significant role in denitrogenation, and a high pressure contributes to the hydrogenation saturation and hydrogenolysis of carbon− nitrogen bonds. The results in Tables 3 and 4 revealed that the sulfur in VGO and VR decreased with the increase of hydrogen pressure; however, the sulfur content of light oil decreased only at a high hydrogen pressure. In addition, nitrogen in VGO and VR decreased with the increase of the hydrogen pressure, while the nitrogen content in light oil remained essentially constant. The high hydrogen pressure promotes the hydrogenation saturation
Figure 2. CCR of VR products from slurry-phase hydrocracking at 430 °C for 1.0 h under different hydrogen pressures.
decreased with the increase of hydrogen pressure. The reason would be the decrease of the asphaltene content and the increase of the H/C ratio. During the slurry-phase hydrocracking of Venezuelan heavy oil, asphaltenes translate into resin or other smaller molecules through cracking and hydrogenation pathways, and the translation is promoted by high hydrogen pressure.32 Moreover, asphaltene formation through the condensation of hydrocarbon free radicals is inhibited by the quenching with hydrogen free radicals. It is approved that the H/C ratio (or hydrogen content) of heavy oil has linear relationships with CCR, and a higher H/C ratio (or hydrogen content) corresponds to a smaller CCR.33,34 It has been known that the H/C ratio of heavy oil would be 2106
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before the precipitation of asphaltene, therefore increasing electric conductivity just because of the decline of viscosity. However, the precipitation of asphaltene occurs with the increase of n-heptane, which gives rise to the obvious decline of the conductive particle, and then the mass fraction normalized conductivity decreased. Thus, the maximal value of mass fraction normalized conductivity is considered to be the sign of asphaltene precipitation.37 As shown in Figure 4, when the mass fraction normalized conductivity reaches its maximum, the corresponding m(n-heptane)/m(residue) is determined as the CSP of the AR product.
increased through the hydrocracking process. Thus, the CCR of VR products was lower than that of feedstock oil. 3.3. Colloidal Stability of the AR Product. The AR products at different hydrogen pressures were reacted at 400 °C under 1.0 MPa of nitrogen. As shown in Table 5, the coke yield Table 5. Coke Yield of Thermal Cracking of AR Products from Slurry-Phase Hydrocracking at 430 °C for 1.0 h under Different Hydrogen Pressures coke (wt %)
a
hydrogen pressure (MPa)
30 min
60 min
90 min
120 min
4 6 8 10
1.163 0.8171 0.5632 0.2538
a 2.374 0.8017 0.5612
a 2.755 1.704 0.7711
a 3.353 1.912 1.5504
No data.
from the AR product increased with the increase of the reaction time; however, the coke yield of the AR product (4 MPa) has already surpassed 1.0 wt % when the reaction time was 30 min. The coking inducing period is defined as the time of 1.0 wt % coke generation. In that case, the coking inducing period of the AR product at 4 MPa is less than 30 min. Then, the coking inducing periods are marked in Figure 3. The coking inducing period of the AR product prolonged with Figure 4. Mass fraction normalized conductivity of the AR product from slurry-phase hydrocracking at 430 °C for 1.0 h under 8 MPa of hydrogen.
As demonstrated in Figure 5, when the hydrogen pressure increased from 4 to 8 MPa, the CSP of the AR product
Figure 3. Coking inducing periods of AR products from slurry-phase hydrocracking at 430 °C for 1.0 h under different hydrogen pressures.
the hydrogen pressure, increasing from 6 to 10 MPa, which was 34.5, 66.8, and 101.4 min. The prolonging coking inducing period reveals that the colloidal stability of the AR product is improved by a high hydrogen pressure. The electrical conductivity method is widely used to study the asphaltene precipitation of residue at present.35,36 In the residue colloidal system, asphaltene is the dispersed phase, which could be regarded as the conductive particle. The size of the colloidal particle enlarges with the association of asphaltene molecules, which causes the decline of the conductivity of the colloidal particle, accompanied by the decrease of conductivity of the colloidal system. However, if the dissociation of asphaltene happens, the size of the colloidal particle becomes small and the conductivity of the colloidal system rises. The viscosity of the sample decreases, and the mass fraction normalized conductivity increases, as the residue sample is gradually diluted by n-heptane. However, the dispersed state of the residue colloidal system does not change significantly
Figure 5. CSPs of AR products from slurry-phase hydrocracking at 430 °C for 1.0 h under different hydrogen pressures.
increased significantly. It indicates that the growth of initial hydrogen pressure is beneficial to the asphaltene stability within the low range of hydrogen pressure. However, the influence of hydrogen pressure on the CSP improvement is weakened under high pressure. The colloidal system of heavy oil is composed of saturates and aromatics (dispersion medium), resins (solvation layer), and asphaltenes (dispersed phase).38 The thermal effect is the major affecting factor that breaks the colloidal stability of heavy oil. On the one hand, the solubility of resin in the dispersion medium and the thermal motion of asphaltene are enhanced by high temperature; thus, the asphaltene which lost the protection of resin would be aggregated. On the other hand, the distribution of phases 2107
DOI: 10.1021/ef502457p Energy Fuels 2015, 29, 2104−2110
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Energy & Fuels Y = I1600 cm−1/I1460 cm−1
becomes discontinuous in heavy oil because of the increase of light and heavy fractions during the thermal process, and then the consistency of significantly different solubility parameter fractions decreases because of the loss of resin, which results in the phase separation according to the least interfacial energy axiom.39 The latter is restrained by hydrogen in the hydrocracking process of heavy oil because, as previously shown, the serious cracking and condensation of the macromolecule were inhibited by hydrogen free radicals. With the increase of hydrogen pressure, the coking inhibition of hydrogen increases because of the improvement of colloidal stability. Although the high hydrogen pressure contributes to the improvement of product stability, the improvement effect is not remarkable with continuous growth of hydrogen pressure. Besides, excessive hydrogen pressure increases the cost of equipment and operation. 3.4. Structural Parameters of Asphaltene Products. As demonstrated in Figure 6, aliphatic methyl and methylene
Z = I1380 cm−1/I720 cm−1 B = I1460 cm−1/I1380 cm−1 nCH2 /nCH3 = 2.93B − 3.70
where the A factor represents the IR absorption intensity ratio of saturated aliphatic carbon and aromatic carbon in asphaltene, the Y factor is the condensation index of asphaltene, the Z factor can be used as the discriminative index of the side-chain fracture situation of asphaltene, and the B factor can be used to calculate the ratio of methylene and methyl (nCH2/nCH3) of asphaltene. As listed in Table 6, the A factors of asphaltene products are smaller than that of feedstock. The reason may be that the Table 6. IR Structural Parameters of Asphaltene Products from Slurry-Phase Hydrocracking at 430 °C for 1.0 h under Different Hydrogen Pressures feedstock 4 MPa 6 MPa 8 MPa 10 MPa
A
Y
Z
B
nCH2/nCH3
0.8664 0.6299 0.6513 0.7139 0.7214
0.4962 1.4758 1.1306 0.8901 0.8511
3.8325 2.0607 1.8032 1.6464 1.5206
2.4335 2.3860 2.4723 2.4967 2.5455
6.7600 6.6210 7.0046 6.9454 7.0883
breaking of long side chains is promoted by a serious freeradical cracking reaction, and then the rate of aromatic carbon increases. Besides, the A factor is gently increased with the hydrogen pressure increasing. The aromatic rings at the edge of the condensed nucleus convert to naphthenic rings by hydrogenation saturation, and the naphthenic rings may convert to side chains by further cracking. Thus, the rate of aromatic carbon decreases, while the rate of saturated aliphatic carbon increases. The Y factors of asphaltene products are considerably larger than that of feedstock, which decreases with the hydrogen pressure increasing. Slurry-phase hydrocracking is processed mainly through free-radical cracking and condensation reaction. The aromatic systems in asphaltene are grown by the condensation reaction, which leads to more compacted aromatic rings. However, the condensation reaction of free radicals is inhibited significantly by hydrogen in the reaction system, and the greater aromatic molecule generation is decreased. The Z factor decreases with the increasing hydrogen pressure. Z factors of asphaltene products are smaller than that of feedstock. The intrinsic side chains of asphaltene are broken during the slurry-phase hydrocracking process; meanwhile, a part of aromatic rings of the condensed nucleus convert to side chains by hydrogenation saturation and cracking. The B factor and nCH2/nCH3 grow slightly as the hydrogen pressure increases. The reason is similar to the change of the A factor: the free-radical reaction of the aliphatic side chain is inhibited by hydrogen free radical; meanwhile, the naphthenic ring is created by hydrogenation saturation of the aromatic ring, accompanied by generation of methylene.
Figure 6. IR spectrum of the asphaltene product from slurry-phase hydrocracking at 430 °C for 1.0 h under 6 MPa of hydrogen.
(CH3 + CH2) have three characteristic absorption peaks arising from the C−H stretching vibration around 2850−2960 cm−1 and asymmetric δC−H around 1460 cm−1. The peak around 1380 cm−1 is assigned to the symmetric δC−H of methyl. The peak around 720 cm−1 is assigned to the rocking vibration of linear alkyl containing four or more CH2 groups. Another strong absorption peak around 1600 cm−1 is assigned to the stretching vibration of the conjugated double bond (CC) in aromatics. The stretching vibration of carbonyl (CO) appears at 1710 cm−1. The relative content of the three main components, including saturated aliphatic carbon, aromatic carbon, and oxygen-containing group, is expressed by the relative intensity of the corresponding absorption peaks. Because of the non-uniformity of the preparing sample, it is meaningless to compare the transmittance difference of a peak. Some parameters of kerogen can also be referenced in the structural characterization of asphaltene.40,41 The atomic composition and bonding of asphaltene can identify the intensity and location of corresponding characteristic absorption peaks. Therefore, five parameters are used to characterize the structural changes of asphaltenes, which are calculated by the following equations:
4. CONCLUSION Herein, the influences of hydrogen pressure on slurry-phase hydrocracked products have been studied. The formation of
A = (I2930 cm−1 + I2860 cm−1)/(I2930 cm−1 + I2860 cm−1 + I1630 cm−1) 2108
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gas, naphtha, and coke were suppressed by a high hydrogen pressure during the slurry-phase hydrocracking. In addition, the maximum of light oil per coke ratio was observed at 8 MPa of hydrogen. Even through desulfurization and denitrogenation were promoted under a higher hydrogen pressure, the influence of hydrogen pressure on the desulfurization rate was not changed so obviously as that on the denitrogenation rate. CCR of VR products decreased with the increase of hydrogen pressure. Meanwhile, the coking inducing period of the AR product was prolonged, and its colloidal structure became more stable. The cracking and hydrogenation saturation of asphaltene were promoted by high hydrogen pressure, which lead to the increase of the A factor and nCH2/nCH3 of asphaltene products as well as the decrease of Y and Z factors.
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Corresponding Author
*Telephone/Fax: +86-0532-86984629. E-mail: ldongupc@vip. sina.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21176259).
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REFERENCES
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