Demetalization of Heavy Oil Based on the Preferential Self-assembly

Mar 14, 2017 - of subcritical water or supercritical water (SCW) was experimentally ... in SCW, the coke-like self-assembly of metal-containing heavy ...
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Demetalization of Heavy Oil Based on the Preferential Self-assembly of Heavy Aromatics in Supercritical Water Kai Wang,† Liu-Yi Bao,‡ Yu Xing,‡ Pei-Qing Yuan,*,† Zhen-Min Cheng,† and Wei-Kang Yuan† †

State Key Laboratory of Chemical Engineering and ‡School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Demetalization of heavy oil through pyrolysis in the presence of subcritical water or supercritical water (SCW) was experimentally investigated. At a high water-to-oil ratio and high water density, the occurrence of pyrolysis can be transferred to the SCW phase. Driven by the π−π attractive interaction between aromatic sheets and the superb diffusivity in SCW, the coke-like self-assembly of metal-containing heavy aromatics occurs spontaneously and rapidly. The self-assembly behavior of aromatics in SCW depends not only on the thermodynamic state of SCW but also on the average scale of aromatics. With the aid of self-assembly in dense SCW, the condensation of metal-containing heavy aromatics, distributed mainly in a vacuum residue, to coke is significantly accelerated, by which the rate of demetalization is improved simultaneously. Owing to the preferential selfassembly of metal-rich heavy aromatics, an increasing yield of liquid products can also be obtained under an optimized SCW environment. widespread attention in the field of oil processing since the 1990s. The initial motivation to apply the upgrading of heavy oil in SCW is that SCW was considered to be a direct hydrogen donor for pyrolysis.8−10 Unexpectedly, theoretical calculation based on a quantum mechanism indicated that the hydrogen abstraction between hydrocarbon radicals and water has a large positive Gibbs energy change.11 Water even in the supercritical state thus can hardly be a direct hydrogen donor for the upgrading of heavy oil. Nevertheless, SCW may donate hydrogen atoms indirectly by participating in a series of reactions as follows: the partial oxidation of hydrocarbons followed by water−gas shift reaction (WGSR) and the reforming of hydrocarbons.12−14 In order to accelerate the reactions involved, heterogeneous catalysis usually was adopted. During the extensive studies on the upgrading of heavy oil under a SCW environment, it was observed that the removal of heteroatoms could be partly accomplished at the same time. By density functional theory based calculation and experimental characterization, Green et al. found that SCW, acting as a reactant and a catalyst, takes part in the decomposition of sulfur-containing compounds, such as thioether and mercaptan.15,16 Adschiri and Yuan reported that, with the aid of the partial oxidation of hydrocarbons and WGSR, dibenzothiophene and nitrogen-containing hydrocarbons could be effectively removed through in situ hydrogenation over a

1. INTRODUCTION With the rising demands for fuel oil but the declining reserves of light crude oil, a comprehensive utilization of heavy oil becomes vital to global energy security. Currently, the processing of heavy oil is accomplished primarily by integrated technologies, such as solvent deasphalting/fluidized catalytic cracking (FCC) and fluidized-bed hydrogenation/FCC.1 Because of the high fractions of nickel, vanadium, sulfur, nitrogen, and carbon residue contained, now it is increasingly difficult for refineries to deal with heavy oil of deteriorating quality through a traditional “hydrogen addition” or “carbon rejection” treatment. Demetalization, referred to specifically for the removal of nickel and vanadium, is always the first priority in the processing of heavy oil. The deposition of nickel and vanadium on FCC catalysts promotes excessive coke and gas production because of their inherent dehydrogenation activity.2,3 Besides, the accumulation of these metals on CoMoS and NiMoS hydrogenation catalysts results in the substitution and decomposition of active components.4,5 Various technologies for demetalization, physically or chemically, have been developed.6 For the typical physical process of propane deasphalting, a large amount of organic solvent has to be cycled within the system, together with the loss of heavy oil up to 30 wt %.7 Hydrodemetalization, which is supposed to be an atom economic process for the upgrading of heavy oil, is also restricted by the content of metals contained in the feedstock, even the processing is applied in a fluidized bed with catalyst circulation. Being an environmentally benign solvent with superb solubility and diffusivity for organics, supercritical water (SCW; Tc = 647 K and Pc = 22.1 MPa) has received © XXXX American Chemical Society

Special Issue: Tapio Salmi Festschrift Received: Revised: Accepted: Published: A

January 9, 2017 March 10, 2017 March 14, 2017 March 14, 2017 DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Properties of Raw Heavy Oil non-VR

VR

H/C ratio

carbon residue (wt %)

S (wt %)

N (wt %)

fraction (wt %)

Ni (ppm)

V (ppm)

fraction (wt %)

Ni (ppm)

V (ppm)

1.10

18.64

1.24

0.62

15.0

3.8

2.4

85.0

58.4

31.5

The autoclave equipped with flat paddles is made of SS 316 L stainless steel. The raw heavy oil, whose properties are listed in Table 1, was obtained from Sinopec Shanghai Petrochemical Company Limited. Non-VR denoted in the table is defined as the oil fraction with a boiling point lower than 833 K. According to preliminary thermogravimetric analysis, the raw heavy oil starts cracking at a temperature of around 623 K. A typical procedure for demetalization under a hydrothermal environment is as follows. First, a certain amount of water and heavy oil were loaded into the autoclave, followed by a purge with nitrogen of high purity (>99.99 vol %). Then the sealed reactor was heated from ambient temperature to 653 K at a slope of 15 K/min. The reaction lasting 15−960 min finally was terminated by subjecting the reactor to forced-air cooling. During the preheating and demetalization stages, the stirring rate was always kept at 800 rpm. By the difference in the water density applied, the reaction pressure varied from 19.0 to 25 MPa. As for demetalization under a nitrogen environment, only 30 g of heavy oil was loaded into the reactor. After purging, nitrogen at a pressure of 9.0 MPa was charged into the reactor. Subsequent operations were the same as those of demetalization under a hydrothermal environment. At a temperature of 653 K, the reaction lasting 4−25 h was run at pressures varying from 22.0 to 23.0 MPa. The experiments mentioned above might be repeated up to eight times in order to satisfy the requirement for vacuum distillation analysis. Only the average values of the product distribution and the metal content of the liquid products were presented in the following figures and tables. 2.2. Analytical Procedures. After demetalization, the gas product was collected over water first, followed by a thorough washing of the reactor with dichloromethane. Through filtration and Soxhlet extraction, the solid and liquid mixtures collected were separated sequentially into coke and liquid products. Following the standard of GB/T 9168-1997, the liquid product then was divided into non-VR and VR in a batch vacuum distillation apparatus operated at a pressure of 100 Pa and a cutting temperature of 593 K. The weight fraction of the components, that is, non-VR, VR, coke, and gas, in the product was evaluated by

NiMoS or CoMoS catalyst.17−19 When running the cracking of a vacuum residue (VR) in the presence of SCW, Cheng et al. detected the simultaneous reduction of sulfur, nickel, and vanadium by 53.9, 83.0, and 86.0% under optimal operating conditions.20 In heavy oil, vanadium mostly in a valence of 4+ and nickel exclusively in a valence of 2+ are chelated with porphyrins consisting of four pyrrolic subunits linked on opposing sides at α positions through four methine (CH) bridges.21,22 Different from the broad distribution of nitrogen and sulfur, porphyrins chelating with nickel and vanadium are conjugated with condensed aromatics and distributed narrowly in VR with a boiling point higher than 833 K.23−27 The heavier the oil fractions are, the higher the contents of nickel and vanadium. On the basis of reaction kinetic analysis, Liu et al. observed that in the SCW phase the condensation of asphaltenes is significantly faster than that of maltenes.28,29 The following molecular dynamics (MD) simulation suggests that the selectively accelerated condensation of asphaltenes in SCW could be ascribed to the rapid and spontaneous coke-like selfassembly of asphaltenes stimulated by the π−π interaction between aromatic sheets.30 In terms of the reaction kinetic analysis and MD simulation results, one may reasonably propose that aromatics with increasing boiling point have a differential self-assembly tendency in SCW. Through the benefit of preferential selfassembly and further condensation of metal-rich heavy aromatics, there is a good chance that demetalization of heavy oil through pyrolysis can proceed with efficiency, together with an improved yield of the liquid products. The economy of running demetalization under a SCW environment is determined simultaneously by the quality and quantity of the liquid products, operating cost, and price of heavy oil. Although demetalization of heavy oil under a SCW environment has potential applications in industry, unfortunately no relevant research based on the selective self-assembly of aromatics has been reported in the literature so far. Hereby, demetalization of heavy oil through pyrolysis under a subcritical water (sub-CW) or SCW environment was investigated. By a comparison between pyrolysis under highpressure nitrogen and SCW environments, the mechanism involved in demetalization was determined first. Then, demetalization was applied under varied hydrothermal environments so as to survey the differential self-assembly of aromatics in SCW. On the basis of experimental and characterization results, the effect of the introduction of the hydrothermal environment on demetalization was discussed further.

Yi =

mi ∑ mi

(1)

where mi represents the weight of any component collected. The mass balance of components in experimental runs was determined by

2. EXPERIMENTAL SECTION 2.1. Apparatus and Reaction Runs. Demetalization of heavy oil under a hydrothermal environment was applied at a fixed temperature of 653 K, with the water densities ranging from 0.1 to 0.30 g/cm3 and water-to-oil ratios from 2:1 to 6:1 (volume at ambient temperature). For comparison purposes, the reaction was also performed under a high-pressure nitrogen environment at the same temperature. All experiments were run in a Parr 4575A-HPHT autoclave with a capacity of 0.5 L.

MB =

∑ mi mraw

(2)

where mraw is the weight of the heavy oil loaded. Normally, the value of MB varies between 0.92 and 0.95. The poor mass balance, observed at the high conversion of heavy oil, resulted primarily from the loss of light oil products during rotary evaporation to remove solvent from the liquid mixtures collected. B

DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research The mass balance of metals in experimental runs was calculated by MBm =

∑ miCi , m mraw Craw, m

(3)

where Ci,m is the content of nickel or vanadium contained in non-VR, VR, or coke and Craw,m is the content of nickel or vanadium contained in the raw heavy oil. The value of MBm varies between 0.91 and 1.05. A slightly higher MBm exceeding 1.0 can be ascribed to the analytical error of the metals contained in oil fractions or coke. The contents of nickel and vanadium contained in oil samples and coke were determined on an atomic emission spectrometer (Agilent ICP-OES 725ES), while the contents of sulfur and nitrogen were analyzed by a nitrogen−sulfur analyzer (AnteR 900Ns). The number-average molecular weight (Mn) values of the oil samples were measured on a Knauer K7000 vapor-pressure osmometer following the Industrial Standard of Chinese Petrochemical SH/T 0583-94. The H/C ratios and NMR spectra of the oil samples were obtained on a Vario EL III elemental analyzer and a Bruker AVANCE 500 MHz NMR spectrometer, respectively. The content of the carbon residue of the oil samples was characterized based on the Industrial Standard of Chinese Petrochemical SH/T 0170-1992.

Figure 2. Contents of nickel and vanadium in liquid products obtained under a nitrogen or SCW environment (SCW environment: a temperature of 653 K, a water density of 0.30 g/cm3, and a water-to-oil ratio of 4:1).

of aromatics to coke has the characteristic of autocatalysis.29 As for non-VR, its fraction in the product increases gradually from the initial value of 15.0 wt % to a maximum value of 67.0 wt % at a reaction time of 14 h, then decreasing until termination of the reaction. Once pyrolysis is transferred into a SCW environment, the reaction run at a water density of 0.30 g/cm3 and a water-to-oil ratio of 4:1 is accelerated substantially. As the data presented in Figure 1 show, the drastically increasing yields of coke and gas together with the rapidly decreasing yield of VR occur simultaneously. Similar to the reaction under a nitrogen environment, a maximum non-VR fraction of around 68.0 wt % can also be observed. Nevertheless, the appearance of the maximum fraction of non-VR occurs at a reaction time of 2 h. After 4 h of reaction, no more significant change in the product distribution could be found. The further extension of the reaction time only results in an increase in the yield of gas and a decrease in the yield of non-VR to a limited extent. For the convenience of evaluating the demetalization efficiency, the rate of demetalization is defined here as the reduction in the metal content of the liquid products per unit of reaction time. A comparison of the data presented in Figures 1 and 2 shows a clear relationship between the rate of coke formation and the rate of demetalization. For pyrolysis under a nitrogen environment, a retarded rate of coke formation and a poor rate of demetalization can be observed at the same time. When pyrolysis is run under a SCW environment, both of them are accelerated. Demetalization therefore should be accomplished by the condensation of metal-containing heavy aromatics to coke, which is confirmed by evaluation of the mass balance of metals, whose typical results are listed in Table 2. Porphyrins chelating with nickel and vanadium are stable in SCW up to 673 K.21,31 During pyrolysis in SCW at a temperature of 653 K, the metals dissolved in water were found to be below the limits of detection. The total metal content of the non-VR products may approach zero, but that of the VR products can be several times its initial value of 89.8 ppm. That is to say, nickel and vanadium are unevenly distributed in VR. Accordingly, the average metal content of the VR products in the middle of pyrolysis should be determined by the

3. RESULTS AND DISCUSSION 3.1. Demetalization Mechanism under a Hydrothermal Environment. To understand the demetalization mechanism involved in pyrolysis of heavy oil under a severe hydrothermal environment, at a temperature of 653 K, demetalization in the presence of high-pressure nitrogen or SCW was compared first with the results illustrated in Figures 1 and 2.

Figure 1. Product distribution of the demetalization of heavy oil under a nitrogen or SCW environment (SCW environment: a temperature of 653 K, a water density of 0.30 g/cm3, and a water-to-oil ratio of 4:1).

Pyrolysis under a nitrogen environment occurs mainly in the oil phase. The reaction consisting of the transformation among non-VR, VR, gas, and coke is so slow that the product distribution varies steadily even at a reaction time of 25 h. Along with extension of the reaction time, the yield of VR decreases monotonically. Meanwhile, the yields of coke and gas increase along sigmoid curves, suggesting that the condensation C

DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Mass Balance of Metals during Pyrolysis under a SCW Environmenta total metal content (ppm)/yield (wt %) reaction time (h)

non-VR

VR

coke

mass balance

1.5 4.0

2.6/67.0 0.2/54.6

167.7/20.4 57.8/2.9

645/6.3 370/19.3

0.98 0.94

a

A temperature of 653 K, a water density of 0.30 g/cm3, and a waterto-oil ratio of 4:1 (by volume)

competition between the decomposition of lighter VR to nonVR and the condensation of heavier VR to coke. The condensation of heavier aromatics results in the formation of coke whose metal content can be extremely high, especially at lower yields of coke. A mass balance of metals of over 0.90 indicates that demetalization under a SCW environment is accomplished by condensation to coke, which is the same as the reaction under a nitrogen environment. It is worth noting that the major difference between pyrolysis under nitrogen and SCW environments is the occurrence of the self-assembly of heavy aromatics in SCW. The coke-like self-assembly thus has a positive influence on the accelerated condensation of metalcontaining heavy aromatics to coke. 3.2. Demetalization under Various Hydrothermal Environments. The self-assembly of aromatics is driven by the π−π attractive interaction between aromatic sheets.32,33 Aromatics contained in heavy oil are widely different in scale and structure, while the properties of SCW change substantially with the thermodynamic state. There is the possibility that the self-assembly of metal-rich heavy aromatics could be selectively accelerated under an optimized hydrothermal environment. By this consideration, demetalization under various hydrothermal environments, water-to-oil ratios from 2:1 to 6:1, and water densities from 0.10 to 0.30 g/cm3 was surveyed at a fixed temperature of 653 K. Demetalization at Various Water-to-Oil Ratios. At a fixed water density of 0.30 g/cm3, the effect of the water-to-oil ratio on demetalization was examined, with the results illustrated in Figures 3 and 4.

Figure 4. Metal content of liquid products versus yield of liquid products for demetalization under a SCW environment at various water-to-oil ratios (a temperature of 653 K and a water density of 0.30 g/cm3).

Compared with pyrolysis under a nitrogen environment, the introduction of dense SCW, even at a low water-to-oil ratio of 2:1, may accelerate pyrolysis of heavy oil. In the meantime, the rate of demetalization is improved synchronously. When the water-to-oil ratio is increased to 4:1, a significantly accelerated pyrolysis rate together with a drastically improved demetalization rate is observed. As the data presented in Figure 3 show, the removal rate of metals contained in the liquid products reaches 77.1% in 1 h, less than half of the time needed at a water-to-oil ratio of 2:1. With a further increase in the water-tooil ratio to 6:1, a rapid decline in the metal content of the liquid products occurs even at the initial reaction stage. At that time, the removal of metals varies along a concave downward curve rather than the inverted sigmoid curves at lower water-to-oil ratios. At water-to-oil ratios of 4:1 and 6:1, the yield of liquid products with the same metal content could be higher than that obtained at a water-to-oil ratio of 2:1. Such a difference is more pronounced at yields of the liquid products between 70.0 and 95.0 wt %, reducing gradually at a yield of the liquid products of lower than 60.0 wt %. Upon approaching equilibrium, the demetalization behavior at various water-to-oil ratios becomes identical, although pyrolysis may be proceeded with widely different reaction rates. Demetalization at Various Water Densities. From the data presented in Figure 3, it can be theorized that a high water-tooil ratio may accelerate demetalization in SCW. Nevertheless, the processing capacity of the reactor is seriously lowered, which goes against the economics of its industrial application. At a moderate water-to-oil ratio of 4:1, demetalization under a hydrothermal environment was examined at water densities from 0.10 to 0.30 g/cm3, with the results illustrated in Figures 5 and 6. At a water density of 0.10 g/cm3, no observable advantage in the demetalization rate over pyrolysis under a nitrogen environment is observed. With an increase in the water density to 0.20 g/cm3, pyrolysis is accelerated to a certain extent but is still far from satisfaction. Such a situation changes dramatically when the water density is increased to 0.25 g/cm3. Only in 3.0 h, the total metal content of liquid products has already been decreased to an acceptable value of 5.0 ppm. At an even higher

Figure 3. Effect of the water-to-oil ratio on demetalization rate under a SCW environment (a temperature of 653 K and a water density of 0.30 g/cm3). D

DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

the subsequent condensation to coke. Because pyrolysis was run at a fixed temperature, one may propose that the intricate demetalization behavior, as illustrated in Figures 3−6, resulted from the differential self-assembly of aromatics in SCW. To validate this proposal, the metal content of coke collected under nitrogen and SCW environments was analyzed, with typical results illustrated in Figure 7.

Figure 5. Effect of a water density on the rate of demetalization under a SCW environment (a temperature of 653 K and a water-to-oil ratio of 4:1).

Figure 7. Total metal content of coke versus yield of coke obtained under a nitrogen or SCW environment.

No matter whether demetalization is run under a nitrogen or SCW environment, the metal content of coke collected at an early reaction stage is far higher than that of raw heavy oil without exception. Among the oil fractions contained in heavy oil, condensation of the metal-rich heavy aromatics therefore should occur with priority. At a yield of coke of around 6.0 wt %, the metal content of the coke collected under a nitrogen or SCW environment at a water density of 0.20 g/cm3 varies around 600 ppm. The corresponding value increases drastically to 794 ppm at a SCW density of 0.25 g/cm3 but decreases to 645 ppm at a water density of 0.30 g/cm3. In other words, the thermodynamic state of water plays an important role in the self-assembly of aromatics in SCW. At a water density of 0.30 g/cm3, the self-assembly of both heavy and light aromatics is promoted, leading to a simultaneous reduction in the metal content of coke and the yield of liquid products. Apparently, the self-assembly of heavy aromatics is preferentially promoted at a SCW density of 0.25 g/cm3. Owing to the formation of coke enrichment in metals, the yield of liquid products with the same metal content is higher than that obtained under nitrogen and other hydrothermal environments. The differential self-assembly of aromatics may surely have an influence on the constitution of the resulting liquid products. On the basis of the H/C ratio, NMR, and vapor-pressure osmometry characterizations, the average molecular structure of liquid products was evaluated in terms of the Brown−Ladner equation.34 At increasing yields of coke, the average number of aromatic rings contained in liquid products was calculated, with typical results illustrated in Figure 8. The average number of aromatic rings contained in liquid products decreases monotonically along with the formation of coke. As a result of the promoted self-assembly of heavy aromatics in dense SCW, the average scale of aromatics contained in the liquid products collected at SCW densities of 0.25 g/cm3 or higher is smaller than that of the liquid products

Figure 6. Metal content of liquid products versus yield of liquid products for demetalization under a SCW environment at various water densities (a temperature of 653 K and a water-to-oil ratio of 4:1).

water density of 0.30 g/cm3, metals can hardly be detected in the liquid products at the reaction time of 4 h. According to the data presented in Figure 6, at a water density of 0.25 g/cm3, the yield of liquid products with the same metal content is always higher than that obtained at a water density of 0.20 g/cm3. Once reactions at two water densities are initiated, a steady difference in the yield of liquid products, up to 10.0 wt %, is established. With an increase in the water density to 0.30 g/cm3, the yield of liquid products with the same metal content declines unexpectedly. At the later pyrolysis stage, the demetalization behavior at three water densities tends to be identical, similar to demetalization at various water-to-oil ratios. 3.3. Differential Self-assembly of Aromatics in SCW. Through pyrolysis under a SCW environment, nickel and vanadium contained in heavy oil are removed with efficiency. A linear correlation coefficient above 0.90 between the removal rates of nickel and vanadium suggests that demetalization of these metals in SCW follows the same mechanism as follows: the coke-like self-assembly of metal-containing aromatics and E

DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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phase structure is observed at water-to-oil ratios of 4:1 or higher at a fixed water density of 0.30 g/cm3. When the waterto-oil ratio is fixed at 4:1, the transition occurs at water densities of 0.25 g/cm3 or higher. To guarantee the appearance of the favorable pseudo-one-phase structure, both a high water-to-oil ratio and a high water density are thus necessary. When transferred into the SCW phase, demetalization of heavy oil responds differently with respect to a further variation of the water-to-oil ratio or water density. At a water density of 0.30 g/cm3, demetalization can still be accelerated with an increase in the water-to-oil ratio from 4:1 to 6:1. However, there is no substantial difference in the yield of liquid products at two water-to-oil ratios. It should be noted that the processing capacity of the reactor declines with an increase in the water-tooil ratio because of the decreasing loading of heavy oil in each run. By contrast, at a water-to-oil ratio of 4:1, an increase in the water density from 0.25 to 0.30 g/cm3 only leads to an undesired decrease in the yield of liquid products, having no further contribution to the improvement of the demetalization rate. To get a compromise among the rate of demetalization, the yield of liquid products, and the economics of the process, a moderate water-to-oil ratio and an optimized water density, 4:1 and 0.25 g/cm3 observed in this work, respectively, exist for demetalization of heavy oil under a SCW environment.

Figure 8. Average number of aromatic rings of liquid products versus yield of coke obtained under a nitrogen or SCW environment.

collected under a nitrogen or SCW environment at a water density of 0.20 g/cm3. At a yield of coke of around 6.0 wt %, a minimum average number of aromatic rings of 5.1 is observed at a SCW density of 0.25 g/cm3, in accordance with the preferential condensation of metal-rich heavy aromatics, as illustrated in Figure 7. Regardless of the difference in reaction media, eventually the average number of aromatic rings contained in liquid products varies around 2. Meanwhile, the metal content of coke fluctuates around 350 ppm. A comparison of the demetalization at the early and later pyrolysis stages suggests that the selfassembly of aromatics in SCW depends not only on the thermodynamic state of water but also on the average scale of aromatics. The influence of the presence of dense SCW on the self-assembly is remarkable for aromatics of larger scale but becomes marginal for aromatics of smaller scale. 3.4. Effect of the Hydrothermal Environment on Demetalization. The data presented in Figures 1−6 show that demetalization of heavy oil through pyrolysis in SCW can be accomplished with efficiency. What is more, an improved yield of liquid products can be obtained under a favorable SCW environment. In addition to the reaction temperature and residence time, demetalization under a SCW environment now can be optimized with extra parameters, that is, the thermodynamic state of water. According to the definition of van Konynenburg and Scott, the heavy oil/water system follows type IIIb or II phase behavior.35,36 Consequently, pyrolysis of heavy oil in the presence of sub-CW or SCW can be run in the partially miscible two-phase or pseudo-one-phase structure. For pyrolysis in the former structure, the reaction occurs mainly in the oil-rich phase where heavy aromatics are highly concentrated. Even though the self-assembly of heavy aromatics is thermodynamically feasible, it will be seriously retarded in the viscous oil-rich phase. As for pyrolysis in the pseudo-one-phase structure, pyrolysis occurs in the water phase whose viscosity is much lower than that of the traditional oil phase by at least 2 orders of magnitude.37 With the aid of excellent diffusivity in SCW, the self-assembly and the following condensation of aromatics are promoted. The transition of the phase structure, characterized by the drastically accelerated pyrolysis rate, can be realized by an increase in the water density and water-to-oil ratio under vigorous agitation. In this work, the transition of the

4. CONCLUSIONS The introduction of a SCW environment at a high water-to-oil ratio and a high water density makes it possible that pyrolysis of heavy oil is transferred from the traditional oil phase into the novel SCW phase. Nevertheless, demetalization through pyrolysis in the oil or SCW phase is accomplished by the same mechanism, that is, the condensation of metal-containing heavy aromatics to coke. The coke-like self-assembly of heavy aromatics in SCW is found to be essential to the accelerated pyrolysis and demetalization under a SCW environment. Whether the presence of SCW has an influence on the selfassembly of aromatics is determined simultaneously by the thermodynamic state of water and the average scale of aromatics. Consequently, the differential self-assembly of aromatics in the presence of dense SCW is remarkable at the early pyrolysis stage but becomes marginal at the later pyrolysis stage. Through the benefit of preferential self-assembly of metal-rich heavy aromatics under an optimized SCW environment, an increasing yield of liquid products can be obtained, together with the satisfactory rate of demetalization.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +862164253529. Fax: +862164253528. E-mail: [email protected](P.Q.Y.). ORCID

Pei-Qing Yuan: 0000-0003-0797-963X Zhen-Min Cheng: 0000-0001-7467-7135 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21376075 and 21676084). F

DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.7b00102 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX