Article pubs.acs.org/EF
Structural Characteristics of Asphaltenes Derived from Condensation of Maltenes in Supercritical Water Dao-Qi Zhu, Qing-Kun Liu, Xue-Cai Tan, Jing-Yi Yang, Pei-Qing Yuan,* Zhen-Min Cheng, and Wei-Kang Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *
ABSTRACT: Asphaltenes derived from the condensation of maltenes under high-pressure N2 and supercritical water (SCW) environments were characterized with various approaches. The reaction kinetics of the condensation of asphaltenes under hydrothermal environments were also measured. By improving the diffusivity of hydrocarbon species and the isolation of aromatic carbon radicals from hydrogen donors in the SCW phase, the dealkylation of alkyl substitutes and the condensation of aromatic rings involved in the condensation of maltenes are promoted. Consequently, asphaltenes formed in the SCW phase (AsSCW) have a higher aromaticity and lower alkyl and naphthenic fractions than asphaltenes formed in the oil phase (As-oil). Nevertheless, As-SCW and As-oil both present a cokelike supermolecular structure with the stacking of polycyclic aromatic rings. Benefitting from the highly fused aromatic molecular structure and the cokelike supermolecular structure, the condensation of As-SCW to coke occurs readily under hydrothermal environments, significantly faster than that of As-oil.
1. INTRODUCTION Petroleum is formed by the decay of animals and plants in a reducing environment in sedimentary rocks. With the variation of biomass sources and deposition conditions, a huge difference in the properties of the asphaltenes contained in petroleum can be observed. The term “asphaltenes” was defined first by J. B. Boussingault in 1837, as the residue of the distillation of bitumen, which is insoluble in alcohol but soluble in turpentine. The definition in use todaythat is, oil fractions insoluble in nalkanes but soluble in tolueneis similar. The presence of asphaltenes is extremely unfavorable for the transportation, as well as thermal and hydrothermal processing, of crude oil and heavy oil.1−3 The properties of asphaltenes are determined simultaneously by their molecular and supermolecular structures. An asphaltene molecule usually contains units consisting of polycyclic aromatic, naphthenic, and heterocyclic rings. By the interlinkage of alkyl substitutes, asphaltene molecules may present an island or archipelago architecture.4 With a micelle supermolecular structure in which polycyclic aromatic rings are stacked with the face-to-face arrangement, asphaltenes are nanocolloidally suspended in crude oil and heavy oil.5 The stability of the micelle supermolecular structure is primarily dependent on the aromaticity of the hydrocarbon surroundings.6 In recent years, there has been an increasing interest in the upgrading of heavy oil in environmentally benign supercritical water (SCW, with a critical temperature (Tc) of 647 K and a critical pressure (Pc) of 22.1 MPa). As an approximately nonpolar solvent for organics, an effective acid/base catalyst, and even a hydrogen donor, the pyrolysis, desulfurization, and denitrification of heavy oil in SCW were supposed to be accomplished simultaneously.7−12 Although a considerable number of related studies have been performed, still there is © 2015 American Chemical Society
a growing dispute on the essential issue, that is, whether coking is suppressed during the upgrading of heavy oil under SCW environments.13−16 The introduction of SCW into the upgrading of heavy oil makes it possible that the reaction could be run in an oil/water two-phase structure or a pseudo-single-phase structure.17−19 Under the two-phase structure, maltenes in heavy oil are extracted substantially into the SCW phase and a limited amount of water is solubilized into the oil phase.13,20 Therefore, the upgrading of heavy oil, which is composed of the condensation of maltenes to asphaltenes and the subsequent condensation of asphaltenes to coke, occurs not only in the oil phase but also in the SCW phase.21 With the aid of a high water density and a high water:oil ratio, the upgrading that occurs in the SCW phase may have a dominant role on the reaction behavior of heavy oil under SCW environments.22 When run in the pseudo-single-phase structure, the upgrading of heavy oil occurs exclusively in the SCW phase. It is noteworthy that the initial content of asphaltenes in heavy oil usually is As-oil > As-SCW appears to be related to the molecular weight of asphaltene molecules as listed in Table 2. The extension of condensation time results in a decreasing average size of asphaltene crystallites in As-oil, but has no essential influence on the structural parameters of asphaltene crystallites in As-SCW. For As-SCW, the height of asphaltene crystallites (Lc) varies around 14.7 Å, and the diameter of asphaltene crystallites (La) ranges from 5.4 Å to 7.6 Å. Meanwhile, the corresponding value of Lc of As-oil decreases significantly from 29.1 Å to 22.0 Å. Nevertheless, the average number of aromatic sheets (ns) in asphaltene crystallites of As-oil, ranging from 7.1 to 9.2, is higher than that of As-SCW (∼5.0). Note that the average interlayer distance between polycyclic aromatic rings (d002) in As-oil and As-SCW varies at ∼3.60 Å, which is significantly larger than the corresponding value of 3.35 Å in graphite.33 Therefore, despite of the occurrence of dealkylation and dehydrogenation during the condensation of maltenes, the remaining alkyl substitutes and naphthenic rings in asphaltenes, even in a small amount, may interfere with the ordered stacking of aromatic rings. 3.3. Condensation of Asphaltenes under Hydrothermal Environments. At a fixed water density of 0.25 g/ cm3 and temperatures ranging from 603 K to 663 K, the reaction kinetics of the condensation of asphaltenes under hydrothermal environments were measured. Initially, it was assumed that the condensation of asphaltenes started at the moment the autoclave reached the preset temperature. However, such an assumption was found to be inapplicable at higher temperatures, especially for the condensation of AsSCW. For the condensation of As-SCW at a temperature of 663 K, a high conversion of maltenes (>60.0 wt %) was observed at the assumed starting time. Only when the reaction temperature was decreased to 643 K or lower can the reaction kinetics be measured without significant errors that result from the preheating stage. At that time, the thermodynamic state of water is located in the liquid−vapor two-phase region. The product distributions of the condensation of As-raw, Asoil, and As-SCW, measured at two temperatures of 623 K and 643 K, are illustrated in Figures 4 and 5. As-oil and As-SCW were prepared by the condensation of maltenes under N2 and SCW environments at a reaction time of 60 min.
still results in a high aromaticity of both C and H atoms in AsSCW. 3.2.4. XRD Analysis. The supermolecular structure of As-oil and As-SCW was characterized by XRD method, with the typical results illustrated in Figure 3. For comparison, the XRD spectrum of As-raw is also attached.
Figure 3. XRD pattern of asphaltenes derived from condensation of maltenes under N2 and SCW environments; condensation time = 60 min.
Compared with the XRD spectra of coke reported, it can be confirmed that As-oil, As-raw, and As-SCW all have a cokelike supermolecular structure with the face-to-face stacking of polycyclic aromatic rings.30,31 On the XRD spectrum of As-raw, three broadened and superimposed diffraction peaks at the 2θ values of 17.1°, 25.2°, and 42.4°, designated as the Gamma, 002, and 100 peaks, are observed. The Gamma peak, which corresponds to the ordered stacking of alkyl and naphthenic fractions in asphaltene crystallites, is common on the XRD spectra of oil-based asphaltenes.32 The 002 peak is related to the ordered stacking of polycyclic aromatic rings in asphaltene crystallites. Regardless of the condensation of maltenes applied in the oil or SCW phase, only a relatively sharper 002 peak at the 2θ value of 25.2° and a broadened peak at the 2θ value of 42.4° can be found on the XRD spectra of As-oil and As-SCW. The Gamma peak appearing on the XRD spectrum of As-raw is overlapped by the 002 peak nearby. Based on the Bragg equation and the Scherrer function, the parameters of the supermolecular structure of asphaltene crystallites were calculated, with the results listed in Table 6.32 The definition of symbols involved is explained in the Notation section. Table 6. Structural Parameters of Asphaltene Crystallites Derived from Condensation of Maltenes under N2 and SCW Environments condensation time (min)
60 180 60 180
d002 (Å) As-raw 3.54 As-oil 3.56 3.60 As-SCW 3.63 3.64
Lc (Å)
La (Å)
ns
30.4
10.8
9.6
29.1 22.0
10.9 9.5
9.2 7.1
14.7 14.7
5.4 7.6
5.0 5.0
Figure 4. Product distribution of condensation of asphaltenes under hydrothermal environments; reaction temperature = 623 K. 7811
DOI: 10.1021/acs.energyfuels.5b01664 Energy Fuels 2015, 29, 7807−7815
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SCW varies with reaction temperature, to varying degrees. For the condensation of As-SCW, the equilibrium yields of coke at temperatures of 623 K and 643 K are 67.0 wt % and 76.0 wt %, respectively. When the condensation of asphaltenes was terminated at a reaction time of 90 min, the yield of maltenes could be ranked in the following order: As-raw > As-oil > AsSCW. According to the data listed in Table 5, one may propose that the yield of maltenes should have dependence on the fractions of alkyl substitutes and naphthenic rings in asphaltenes.
4. DISCUSSION Two types of asphaltenes were prepared by the condensation of maltenes under different solvent environments. The condensation under SCW environments occurs simultaneously in the oil and SCW phases. A previous study with the same operating conditions has confirmed that condensation in the dense SCW phase presents an overwhelming influence on the reaction in the oil phase.34 Since the condensation under N2 environments occurs primarily in the oil phase, As-SCW and As-oil prepared approximately reflect the characteristics of the condensation of maltenes in the SCW phase and in the oil phase, respectively. 4.1. Effect of SCW on Structural Characteristics of Asphaltenes. Although SCW can participate in the decomposition of alkyl sulfides contained in maltenes, it is virtually an inert reaction medium to hydrocarbon radicals involved in reactions.12,35−37 Accordingly, the condensation of maltenes to asphaltenes in the conventional oil phase and in the novel SCW phase should follow similar free-radical mechanisms. Some key elementary reactions involved in the dealkylation of alkyl substitutes of polycyclic aromatics and the condensation of polycyclic aromatics are schematically illustrated in Figure 6. The dealkylation of alkyl substitutes is composed of elementary radical reactions as follows: the formation of primary carbon radicals through the C−C cleavage of alkyl substitutes; the formation of secondary and tertiary carbon radicals through the H-abstraction of primary carbon radicals
Figure 5. Product distribution of condensation of asphaltenes under hydrothermal environments; reaction temperature = 643 K.
At temperatures of 623 K and 643 K, the conversion rate of asphaltenes ranks as follows: As-SCW > As-oil > As-raw. The difference in condensation rate among three asphaltenes is remarkable at the initial reaction stage but is reduced at the latter reaction stage. Take the condensation at the temperature of 623 K as an example. After only 5 min of reaction, the conversions of As-SCW, As-oil, and As-raw have already reached 63.0, 43.0, and 11.0 wt %, respectively. However, at a reaction time of 60 min, the fraction of asphaltenes in the entire product varies between 2.0 wt % and 5.0 wt %. Under hydrothermal environments applied, eventually asphaltenes are transformed completely to coke, maltenes, and gas. Once the yield of coke reaches its equilibrium value (ranging from 60.0 wt % to 75.0 wt %), only a slow transformation from maltenes to gas can be observed, along with the further extension of reaction time. The equilibrium yield of coke obtained in the condensation of As-oil and As-
Figure 6. Schematic mechanisms of dealkylation and condensation during the condensation of maltenes to asphaltenes. 7812
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unique molecular and supermolecular structures of As-SCW play vital roles in its condensation activity. The condensation of asphaltenes to coke is similar in mechanism to the condensation of maltenes to asphaltenes. Comparatively speaking, the former relies more on the condensation of polycyclic aromatic rings. Not only is the average number of alkyl substitutes in As-SCW less than that in As-oil, but also the average length of alkyl substitutes in AsSCW is shorter than that in As-oil. Therefore, when the condensation of As-SCW occurs, the possibility of the saturation of aromatic carbon radicals through the Habstraction from alkyl substitutes nearby is reduced. By the π−π interaction between polycyclic aromatic rings, As-raw, As-oil, and As-SCW all present a cokelike supermolecular structure. With the further two-dimensional (2D) extension and three-dimensional (3D) stacking of aromatic segments, asphaltenes can be readily transformed to coke during which a typical characteristic of autocatalysis can be observed.34 The coke introduced in the feedstock and formed during condensation may act as coking templates to accelerate the transformation of asphaltenes to coke. As the data illustrated in Figures 4 and 5 show, under hydrothermal environments, the yields of coke in the condensation of As-raw vary along sigmoidal curves, suggesting the dependence of condensation on autocatalysis. However, with a highly fused molecular structure and a cokelike supermolecular structure, the dependence of the condensation of As-SCW on autocatalysis is greatly reduced. Under the same hydrothermal environments, the yields of coke in the condensation of AsSCW only vary along convex upward curves.
from alkyl substitutes of polycyclic aromatic rings; and the departure of low alkenes through the β-scission of secondary and tertiary carbon radicals. The condensation of polycyclic aromatics consists of the formation of aromatic carbon radicals through direct dehydrogenation from aromatic rings and the following coupling of aromatic carbon radicals. It is noteworthy that the H-abstraction of primary carbon radicals from alkyl substitutes is a bimolecular type reaction with lower activation energies ranging from 30 kJ/mol to 60 kJ/mol.38−42 In addition, the coupling of aromatic carbon radicals, which also is a bimolecular-type reaction, can be accomplished with no reaction barrier. When the condensation of maltenes is transferred from the oil phase into the SCW phase, the diffusivity of hydrocarbon species is drastically improved. Based on the IAPWS-95 formulation, the dynamic viscosity of the SCW phase under the hydrothermal environments applied varies between 10−4 Pa s and 10−5 Pa s. Over the same temperature range, the dynamic viscosity of the oil phase varies at ∼10−3 Pa s, equivalent to that of water under ambient conditions.43 For a fast bimolecular reaction occurring in a solvent, the diffusion limited rate constant is conversely proportional to the viscosity of the solvent.44 Provided that the bimolecular reactions involved in the condensation of maltenes in both SCW and oil phases are limited by diffusion, the diffusion limited rate constant in the SCW phase should be approximately larger than that in the oil phase, by 1 or 2 orders of magnitude. With an extremely high reaction barrier, aromatic carbon radicals are formed through the direct dehydrogenation from aromatic rings. If there are any possible hydrogen donors nearby, they can be readily capped through H-abstraction. For the condensation of maltenes in the oil phase, the alkyl and naphthenic fractions contained in maltenes become the potential hydrogen donors for the saturation of aromatic carbon radicals. When the condensation of maltenes is run in the SCW phase, the presence of water molecules may isolate aromatic carbon radicals from the contact with other maltene molecules dissolved in SCW. Therefore, the coupling of aromatic carbon radicals, which is essential to the condensation of polycyclic aromatics, is guaranteed. Also, the further growth of asphaltenes through the combination of asphaltenes of lower molecular scale could be suppressed simultaneously by the steric hindrance resulting from solvent molecules. Accordingly, the molecular weight of As-SCW is significantly lower than that of As-oil. Benefitting from the improved diffusivity of hydrocarbon species and the isolation of aromatic carbon radicals from hydrogen donors, the condensation of maltenes to asphaltenes in the SCW phase is promoted. At the molecular level, the formed As-SCW has a higher H and C aromaticity than As-oil does. Also, the fractions of alkyl substitutes and cycloparaffins contained in As-SCW are lower than those contained in As-oil. Such a result is in accordance with the phenomena observed by Morimoto et al.14 However, As-oil and As-SCW both have a cokelike supermolecular structure, because of the stacking of polycyclic aromatic rings. 4.2. Effect of Structural Characteristics on Condensation Behavior of Asphaltenes. According to the pyrolysis of heavy oil in the oil and SCW phases, the condensation of AsSCW in the SCW phase is much faster than that of As-oil in the oil phase.22 Using the data presented in Figures 4 and 5, even in the liquid−vapor two-phase region, the condensation of AsSCW is still faster than that of As-oil. One may propose that the
5. CONCLUSIONS The condensation of maltenes under high-pressure N 2 environments and SCW environments was applied to obtain asphaltenes formed in the conventional oil and novel SCW phases. Because of the improved diffusivity in the SCW phase, the bimolecular H-abstraction reactions involved in dealkylation are promoted. In the meantime, the coupling of aromatic carbon radicals is guaranteed by the isolation of radicals from potential hydrogen donors. Therefore, As-SCW formed in the SCW phase has lower alkyl and naphthenic fractions than As-oil formed in the oil phase does. Also, the formation of As-SCW is faster than the formation of As-oil. Besides, the steric hindrance resulted from the presence of the SCW solvent may prevent the contact between asphaltenes of lower molecular scale, by which the molecular weight of AsSCW is lower than that of As-oil. According to XRD characterization, both As-SCW and AS-oil have a cokelike supermolecular structure in which polycyclic aromatic rings are arranged with face-to-face stacking. The presence of alkyl and naphthentic fractions, even in a small amount, interferes with the ordered stacking of polycyclic aromatic sheets. At a water density of 0.25 g/cm3 and temperatures of >623 K, As-oil and As-SCW, under hydrothermal environments, are transformed completely to coke, maltenes, and gas within a limited reaction time of 90 min. With a highly fused aromatic molecular structure, the condensation of As-SCW to coke is significantly faster than that of As-oil to coke. 7813
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01664. Results from additional experiments on the condensation of maltenes in supercritical water, run at a higher temperature, showing exactly the same tendency as the results presented in the manuscript (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 2164253529. Fax: +86 2164253528. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Open Project of State Key Laboratory of Chemical Engineering (No. SKL-ChE-13C02) and the National Natural Science Foundation of China (Grant No. 21376075).
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NOTATION CAli = nonaromatic carbon atoms CAro = aromatic carbon atoms d002 = interlayer distance of aromatic sheets; d002 = λ/(2 sin θ002), Å HAro = aromatic hydrogens Hα = aliphatic hydrogens on Cα to aromatic rings Hβ = aliphatic hydrogens on Cβ and CH2, CH beyond Cβ to aromatic rings Hγ = aliphatic hydrogens on Cγ and CH3 beyond Cγ to aromatic rings Lc = average height of asphaltene crystallites; Lc = 0.9λ/(ω cos θ002), Å La = average diameter of asphaltene crystallites; La = 1.84λ/ (ω cos θ100), Å mi = weight of the collected condensation product, g MB = mass balance of each experimental run Mn = number-average molecular weight, Da n = average number of carbon atoms per alkyl substituent ns = average number of polycyclic aromatic sheet per crystallite; ns = Lc/d002 + 1 Yi = weight fraction of the collected condensation product r = number of naphthene rings per substituent RA = average number of aromatic rings per average molecule RN = average number of naphthenic rings per average molecule RS = average number of alkyl substituents per average molecule
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