Visbreaking of Heavy Oil under Supercritical Water Environment

Dec 26, 2017 - An increase in the fraction of saturates can be ascribed to the dealkylation of aromatic rings distributed in aromatics, resins, and as...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Visbreaking of Heavy Oil under Supercritical Water Environment Jun Liu,† Yu Xing,‡ Yi-Xiao Chen,‡ 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 S Supporting Information *

ABSTRACT: The visbreaking of heavy oil under high-pressure N2 or supercritical water (SCW) environment was experimentally investigated. Despite the difference in the reaction media, the visbreaking follows the same mechanism, that is, dealkylation and condensation of aromatics. The presence of SCW makes it possible that the visbreaking of heavy oil is transferred to the SCW phase with superior diffusivity by which the visbreaking tends to be controlled by intrinsic reaction kinetics rather than by diffusion. Accordingly, dealkylation occurring in the SCW phase, which is vital to the viscosity reduction of heavy oil, responds sensitively to the increase in reaction temperature. Being the secondary reaction of dealkylation at moderate temperatures, condensation is effectively suppressed with reduced reaction time required for dealkylation. By the introduction of SCW and the adoption of an appropriate reaction temperature, the visbreaking efficiency could be drastically improved together with guaranteed stability of visbreaking products.

1. INTRODUCTION Worlwide crude oil reserves are mostly natural bitumen with an American Petroleum Institute (API) gravity less than 20°. Because of the deteriorating quality of crude oil, an increasing amount of vacuum residue now being obtained in refineries. Prior to the transportation, storage, or downstream processing of highly viscous heavy oil, like bitumen and vacuum residue, treatment of viscosity reduction usually has to be performed. Noncatalytic visbreaking based on free radical thermal cracking, having no specific requirement for the quality of heavy oil, is still a dominant processing technology for the viscosity reduction of heavy oil in the world.1−3 Visbreaking of heavy oil is initiated by the C−C cleavage of alkyl substitutes of aromatics, a process driven by entropy change at high temperature. Essentially, there is no difference in the reaction mechanism between typical thermal cracking operations, such as visbreaking and delayed coking. These processes are composed of various free radical reactions related to dealkylation and condensation.4 For visbreaking products to be obtained with both low viscosity and good stability, the visbreaking of heavy oil is supposed to be accomplished with promoted dealkylation but suppressed condensation. The performance of traditional visbreaking technologies, adjusted mainly by the variation of reaction temperature and residence time, is far from satisfactory. Because of the high viscosity of heavy oil, the alkyl C radicals resulting from C−C cleavage can be readily quenched by in situ coupling. Limited by poor initiation efficiency, an extremely long reaction time is needed for the visbreaking at low temperature. It should be noted that the apparent activation energy of condensation is higher than that of dealkylation. As a result, the stability of the visbreaking product obtained at a high temperature declines because of accelerated condensation. © XXXX American Chemical Society

Since the 1990s, the upgrading of heavy oil in the presence of supercritical water (SCW, Tc = 647 K, Pc = 22.1 MPa), based mostly on thermal cracking, has attracted increasing attention in the field of oil processing.5−8 Early studies believed that SCW could be a H-donor for upgrading through the Habstraction between hydrocarbon radicals and water.9−11 The quantum mechanism-based calculation, however, indicates that the direct H-abstraction of hydrocarbon radicals from SCW is thermodynamically infeasible.12 Nevertheless, SCW may donate H atoms indirectly by participating in the reforming of hydrocarbons and the water−gas shift reaction. For the upgrading of heavy oil in SCW applied commonly at temperatures between 653 and 723 K, the reforming of aliphatics rather than aromatics is responsible for the in situ H production.13−15 Because the reforming of smaller aliphatic species is thermodynamically and kinetically more feasible, the production of H2 accompanied by the appearance of CO and CO2 is promoted along with the procession of the upgrading. Consequently, massive H2 production could be observed at the middle or later reaction stage.16 Whether the in situ H production at that time may effectively suppress condensation is highly controversial.7,13,17,18 Except for the few research studies adopting an extremely high water-to-oil ratio or high reaction temperature, SCW has generally been considered an inert reaction medium for the upgrading of heavy oil based primarily on the free radical mechanism. Recent studies suggested that it is the variation of phase structure that has a profound influence on the reaction kinetics Received: Revised: Accepted: Published: A

September 26, 2017 December 6, 2017 December 26, 2017 December 26, 2017 DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research and product distribution of the cracking of heavy oil in the presence of SCW. Owing to the dissolution of maltenes in nearly nonpolar SCW, the heavy oil/SCW mixture may follow type IIIb or type II phase behavior defined by van Konynenburg and Scott.19,20 By the differences in hydrothermal environment and operating condition applied, the water/oil partially miscible two-phase structure, oil-in-water emulsion phase structure, and pseudo single-phase structure were proposed in the literature.18,21,22 Among the phase structures mentioned above, the pseudo single-phase structure in which the cracking of heavy oil is transferred from the traditional oil phase into the novel SCW phase undoubtedly is the most attractive. According to reaction kinetics analysis, the dealkylation of aromatics occurring in the SCW phase is much faster than that occurring in the oil phase, which might be intuitively ascribed to the improved diffusion in SCW.23−25 However, driven by the spontaneous cokelike self-assembly of heavy aromatics in SCW, the condensation of aromatics is also accelerated.26 Fortunately, at moderate temperature, the formation of aromatic C radicals, which are vital to condensation, relies on the H-abstraction of alkyl C radicals from aromatic rings.27,28 The subsequent addition of low alkenes with aromatic C radicals is one of the key steps of the two-dimensional extension of polycyclic centers.29−31 Because alkyl C radicals and low alkenes are both intermediates of dealkylation, condensation at that time can be considered as the secondary reaction of dealkylation. From a reaction engineering point of view, a secondary reaction could be suppressed by optimizing the reaction time needed for the foregoing reaction. Moreover, Kida et al. found that SCW might participate in the decomposition of S-containing alkyl substitutes of aromatics.32 Chen et al. has confirmed that the αolefin terminal of the alkyl substitutes of aromatics could be spontaneously protonated by hydronium ions resulting from the autoionization of SCW followed by carbonium mechanismbased β-scission.33 The dealkylation of aromatics in SCW thus additionally benefits from the participation of SCW as a reactant or catalyst. By the considerations mentioned above, there is a good chance that accelerated dealkylation but suppressed condensation could be obtained during the visbreaking of heavy oil in SCW. Previous related research used to emphasize the upgrading characterized by condensation. To date, visbreaking featured by dealkylation in SCW has seldom been addressed in academia. Hereby, in the presence of high-pressure N2 or SCW, the visbreaking of a vacuum residue sample used as the representative of heavy oil was experimentally surveyed. By visbreaking under a N2 environment, the crucial role of dealkylation involved in the viscosity reduction of heavy oil was confirmed first. Then, the visbreaking under the SCW environment was examined at varied water-to-oil ratios, water densities, and reaction temperatures followed by a detailed characterization of the average molecular structure of liquid visbreaking products. After that, a saturates-aromatics-resinsasphaltenes (SARA) distribution analysis was performed so as to evaluate the stability of the visbreaking products collected under a N2 or SCW environment. On the basis of the results observed, the possible influence of the introduction of SCW on the visbreaking of heavy oil was discussed further.

Table 1. Basic Properties of Raw Heavy Oil H/C 1.54 C residue (wt %) 18.6 a

μ (Pa.s)a

Mn (Da)

Sat (wt %)

Aro (wt %)

resin (wt %)

As (wt %)

1.78 CS (%)

831 CA (%) 29.1

17.5 RA

52.5 RN

25.2 RS

5.0 n

5.1

0.8

4.0

10.2

70.9

Viscosity measured at 393 K.

simulate the visbreaking of heavy oil. Definitions of the symbols used are explained in the Nomenclature. According to thermogravimetric analysis, the heavy oil starts cracking at a temperature of around 623 K. The heavy oil is stagnant at ambient temperature. Only at a temperature of 393 K or higher can its viscosity be measured. All visbreaking experiments were applied in a Parr HPHT-4598 autoclave with a capacity of 0.1 L. The reactor equipped with flat paddles was made of SS316 L stainless steel. The visbreaking of heavy oil was applied under a highpressure N2 or SCW environment. Previous studies indicated that a water density of 0.25 g/cm3 and a water-to-oil ratio of 3:1 (vol) are sensitive thresholds for the transition of the phase structure of the heavy oil/SCW mixture.25,34 Moreover, an unavoidable rapid condensation of heavy aromatics in SCW usually occurs at a temperature of around 693 K.35 Several series of experiments covering water densities from 0.20 to 0.30 g/cm3, water-to-oil ratios from 2:1 to 3:1 (volume at ambient condition), and reaction temperatures from 663 to 693 K were therefore designed so as to examine the involvement of SCW in visbreaking. Details of the experimental runs are listed in Table S1. A typical procedure for the visbreaking under the SCW environment is as follows. Certain amounts of heavy oil and water were loaded into the autoclave, followed by purging with N2 of high purity. Then, the sealed autoclave was heated from the ambient temperature to a preset reaction temperature at a slope of 15 K/min. The reaction time was measured when the reaction temperature reached the preset value. No matter the preheating or visbreaking stage, the stirring rate was always kept at 800 rpm. Finally, the reaction was terminated by subjecting the mixture to an autoclave with forced air cooling. As a result of the condensation of a small amount of water in the dead volume at the upper part of the autoclave, the measured reaction pressures of the visbreaking under the SCW environment were always slightly lower than the theoretical values. For the visbreaking under a N2 environment, a certain amount of heavy oil was first loaded into the autoclave. After purging with N2 of high purity, N2 at pressures varying from 7.5 to 8.0 MPa was charged into the reactor. The following operating procedures were just the same as those under the SCW environment. Analytical Procedures. After visbreaking, gas product was collected over water first. The autoclave was then washed thoroughly with toluene followed by the separation of the solid and liquid mixture collected into coke and liquid products by filtration. On the basis of the Industrial Standard of Chinese Petrochemical NB/SH/T 0509-2010 described elsewhere, the liquid product was separated into saturates (Sat), aromatics (Aro), resins, and asphaltenes (As).36 The yield of the component except gas in the product was evaluated by

2. EXPERIMENTAL SECTION Apparatus and Reaction Runs. A vacuum residue sample, whose basic properties are listed in Table 1, was adopted to B

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Dynamic viscosity of liquid visbreaking products obtained under a N2 environment.

Yi =

mi × 100% mraw

of coke at a yield of 0.5 wt % appears at the reaction time of 120 min. The yield of coke remains nearly unchanged in the following 30 min of reaction but increases suddenly to 2.1 wt % at a reaction time of 180 min. During visbreaking, the production of gas can always be detected even without the appearance of coke. With the increase in reaction temperature to 693 K, the visbreaking seems to be accelerated. The yield of coke is zero at a reaction time of 30 min but increases rapidly to 3.6 wt % at a reaction time of 45 min. Because of the production of gas, the yield of liquid product decreases to 92.3 wt % synchronously. The purpose of visbreaking is widely different from that of the upgrading relying on “carbon rejection”; thus, an effective visbreaking operation is defined here as the cracking with the yield of coke less than 1.0 wt %. Following this criterion, the dynamic viscosity of valid liquid visbreaking products, measured at 353 K, is illustrated in Figure 1 with the corresponding coke yield labeled next to the symbol. The raw heavy oil used is highly viscous. By thermal cracking, the viscosity of liquid products is significantly reduced. Furthermore, an increase in reaction temperature facilitates the improvement of visbreaking efficiency. At a temperature of 663 K, the viscosity of the visbreaking product decreases monotonically along with the extension of reaction time, reaching 0.32 Pa.s at the reaction time of 150 min. At a temperature of 693 K, only in 30 min the viscosity of the liquid product has already decreased to 0.27 Pa.s. 3.2. Mechanism of Visbreaking of Heavy Oil. For understanding the mechanism of the visbreaking in the traditional oil phase, the properties of the liquid products obtained under a N2 environment were characterized followed by a calculation on their average molecular structure parameters. The results are listed in Table S2. The H/C ratio of liquid products, decreasing with the extension of reaction time, is lower than that of the raw heavy oil. Even after the appearance of a small quantity of coke, such a tendency remains unchanged. According to GC analysis, the gas produced in the process of visbreaking consists primarily of H2, low alkanes, and low alkenes. For the visbreaking run at 663 K, the gas product collected at a coke yield of 0.6 wt % contains

(1)

where mi represents the weight of the coke, saturates, aromatics, resins, and asphaltenes collected. The yield of gas was determined by the normalization of product composition. The average molecular structure parameters of liquid visbreaking products and raw heavy oil were calculated in terms of the modified Brown−Ladner (BL) method.37−39 Following the definition in the BL method, the removal rate of alkyl substitutes of aromatics during visbreaking was evaluated by R% =

R S,raw ·nraw − R S·n R S,raw ·nraw

× 100% (2)

where RS,raw and nraw are the average number of alkyl substituents per average raw heavy oil molecule and the average number of carbon atoms per alkyl substituent. RS and n are the corresponding values of liquid visbreaking products. The composition of gas product was analyzed on a gas chromatograph GC 2060 equipped with an HP Plot-Q column. The dynamic viscosity of oil samples, liquid visbreaking products, or raw heavy oil was determined on an Advanced Rheology Expand System (TA Instruments, ARES). The H/C ratio of oil samples was measured on a Vario EL III element analyzer. The number average molecular weight (Mn) of oil samples was obtained on a KNAUER K7000 vapor pressure osmometer (VPO) based on the Industrial Standard of Chinese Petrochemical SH/T 0583-94. The nuclear magnetic resonance (NMR) spectrum of oil samples was analyzed on a Bruker Model AVANCE 400 MHz NMR spectrometer during which CDCl3 and TMS were used as the solvent and the internal standard, respectively, for calibrating the chemical shift. The classification of protons and C atoms in oil samples can be seen elsewhere.40,41

3. RESULTS AND DISCUSSION 3.1. Visbreaking under a N2 Environment. The visbreaking of heavy oil under a high-pressure N2 environment was examined first. At a temperature of 663 K, a small amount C

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Schematic illustration of dealkylation and condensation involved in visbreaking of heavy oil at moderate temperature.

Figure 3. Effect of water-to-oil ratio on visbreaking of heavy oil under the SCW environment at 663 K with a water density of 0.25 g/cm3.

that it is dealkylation that plays a decisive role in the viscosity reduction of visbreaking products. However, apart from dealkylation, condensation that results in potential instability of visbreaking products occurs at the same time. Accompanied by the reducing RA, the average fraction of aromatic C atoms in the total C atoms contained in liquid products (CA) increases from its initial value of 29.1% to 36.5%. On the basis of the reaction kinetic data of the free radical reactions involved in the thermal cracking of heavy oil, the visbreaking mechanism schematically illustrated in Figure 2 can be proposed.27−31 As mentioned before, the visbreaking of heavy oil at moderate temperature is initiated by the C−C cleavage of alkyl substitutes of aromatics with an activation energy of ∼330 kJ/mol. The alkyl C radicals formed could be saturated by the H-abstraction from other alkyl substitutes with activation energies ranging from 30 to 60 kJ/mol or be shortened by βscission to produce low alkenes. The dealkylation of aromatics,

H2, CH4, C2H4, and C2H6 at concentrations of 7.6, 31.7, 29.3, and 15.7 vol %, respectively. The production of H-rich gaseous species should be responsible for the decreasing H/C ratio of the liquid visbreaking product. As visbreaking proceeds, the values of RS and n both decrease, indicating the occurrence of dealkylation. Taking the liquid product obtained at 663 K and reaction time of 150 min as an example, the average scale of alkyl substituents, that is, the product of RS and n, is merely 54.8% that of the raw heavy oil. By the cleavage of alkyl substitutes connecting polycyclic centers, the average number of aromatic rings per average liquid product molecule (RA) decreases from its initial value of 5.1 to 2.6 together with reduction of the average molecular weight from 831 to 491 Da. By contrast, the average number of naphthenic rings per average liquid product molecule (RN) decreases slightly from 0.8 to 0.7, which means the naphthenic structure maintains rather stable during visbreaking. Comparing the data presented in Figure 1 and Table S2, one may confirm D

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 4. Effect of water density on visbreaking of heavy oil under the SCW environment at 663 K and a water-to-oil ratio of 3:1 (vol).

At a water-to-oil ratio of 2:1 (vol), the viscosity of the liquid product obtained at a reaction time of 30 min can be measured at 353 K but is far higher than that of the product obtained under the N2 environment. By the nearly unchanged average scale of alkyl substitutes, one may confirm further that the viscosity of heavy oil or visbreaking products is closely related to the topological structure of alkyl substitutes of aromatics. With the extension of reaction time to 45 min, coke at a yield of 1.3 wt % appears abruptly in the reaction system. At a water-tooil ratio of 3:1 (vol), however, the visbreaking performance is improved substantially. The viscosity of the liquid product is 0.62 Pa.s at a reaction time of 15 min and decreases continuously to 0.36 Pa.s at a reaction time of 45 min. In the meantime, the average scale of alkyl substitutes decreases together with a synchronous reduction of the molecular weight of liquid products to 474 Da. At 663 K and a water-to-oil ratio of 3:1 (vol), the visbreaking performance at various water densities from 0.20 to 0.30 g/cm3 is illustrated in Figure 4. The corresponding data of the visbreaking under the N2 environment are also attached. After 30 min of visbreaking at a water density of 0.20 g/cm3, the viscosity of the liquid product decreases to 0.91 Pa.s, slightly higher than that of the liquid product obtained under the N2 environment at the same reaction time. Along with the extension of reaction time to 45 min, the premature appearance of coke at a yield of 1.1 wt % could be observed, which is similar to the visbreaking at the low water-to-oil ratio of 2:1 (vol). At an increasing water density of 0.25 g/cm3 or higher, the visbreaking efficiency is improved drastically. In only 30 min, the viscosity of the liquid product obtained at a water density of 0.30 g/cm3 has already decreased to 0.11 Pa.s. Such a value is even lower than that of the product obtained under the N2 environment at 693 K. Despite variations in the water-to-oil ratio or water density, the naphthenic structure contained in the liquid products is rather stable during the visbreaking under the SCW environment. Furthermore, the values of H/C ratio and RA of liquid products both decrease. In light of the data listed in Tables S2 and S3, the visbreaking under the N2 and SCW environments should follow the same mechanism. Nevertheless, condensation is suppressed by the presence of SCW because the value of CA

responsible for the viscosity reduction of heavy oil, is accomplished through the cleavage and shortening of alkyl substitutes. With the procession of dealkylation, active methyl radicals are released into the reaction system. Through the Habstraction between methyl radicals and aromatic rings with an activation energy of ∼65 kJ/mol, aromatic C radicals vital to condensation are produced. The extension of polycyclic centers could be realized via the addition of low alkenes on aromatic C radicals, cyclization, and dehydrogenation in series. By the fact that the formation of methyl radicals and low alkenes both rely on the precedent occurrence of dealkylation, condensation becomes the secondary reaction of dealkylation. With the increase in reaction temperature, the probability of the formation of aromatic C radicals through the direct dehydrogenation from aromatic rings with an activation energy of 474 kJ/mol is increased. Because of the Arrhenius effect, dealkylation and condensation tend to run in parallel rather than in series at that time. Eventually, condensation should dominate the reaction kinetics of visbreaking at a high enough temperature. From a reaction engineering point of view, condensation could be suppressed by optimizing the reaction time required for foregoing dealkylation. Compared the visbreaking at 663 K for 150 min with the visbreaking at 693 K for 30 min, the liquid products obtained have a similar viscosity and approximately the same average scale of alkyl substitutes. The value of CA of the liquid product obtained at the high temperature is lower than that of the liquid product obtained at the low temperature by 3%, suggesting that condensation is partially suppressed with reduced visbreaking time. 3.3. Visbreaking under the SCW Environment. After the confirmation of visbreaking mechanism in the oil phase, the visbreaking of heavy oil under various SCW environments was surveyed with the results illustrated in Figures 3−5 and Table S3. Visbreaking at Various Water-to-Oil Ratios and Water Densities. At 663 K and a water density of 0.25 g/cm3, the visbreaking performance at various water-to-oil ratios from 2:1 to 3:1 (vol) is illustrated in Figure 3. For comparison, the corresponding data of the visbreaking under the N2 environment are attached. E

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Effect of reaction temperature on visbreaking of heavy oil under the SCW environment at a water density of 0.25 g/cm3 and water-to-oil ratio of 3:1 (vol).

low temperature or as the parallel reaction of dealkylation at high temperature, is virtually inevitable during the visbreaking in the oil phase. Asphaltenes are nanocolloidally suspended in heavy oil with a micelle supramolecular structure whose stability is sensitive to the aromaticity of its surroundings.42−44 A decrease in the fraction of aromatics might lead to the destruction of the micelle structure. In the raw heavy oil, the weight ratio of asphaltenes to aromatics (As/Aro) is 0.1. The corresponding value of the visbreaking product obtained under the N2 environment increases drastically to 0.37 at 663 K and 0.33 at 693 K, indicating an increasing risk of asphaltene aggregation. In the presence of SCW, the SARA distribution of the liquid product obtained at 663 K varies similarly to that of the liquid product obtained under the N2 environment. However, the fraction of asphaltenes on the whole is lower than that obtained under the N2 environment by 2−4 wt %. Furthermore, the ratio of As/Aro of the visbreaking products is maintained around a relatively lower value of 0.28. With the increase in reaction temperature, not only is the visbreaking efficiency as shown in Figure 5 improved but also the group composition of the liquid product varies differently. For the liquid products obtained at a fixed water density of 0.25 g/cm3 and water-to-oil ratio of 3:1 (vol), a decreasing asphaltene fraction from 13.5 to 7.9 wt % and an increasing aromatic fraction from 46.6 to 53.2 wt % can be observed upon increasing the temperature from 663 to 683 K. Accordingly, an acceptable As/Aro value as low as 0.15 is obtained at 683 K. 3.5. Roles of SCW in Visbreaking of Heavy Oil. By the introduction of SCW, the reaction kinetics of the visbreaking of heavy oil are changed. The visbreaking efficiency and the SARA distribution of visbreaking products depend simultaneously on the water-to-oil ratio, water density, and reaction temperature adopted. For the roles of SCW in visbreaking of heavy oil to be determined, an analysis of the gas products collected under typical operating conditions was performed with the results listed in Table S5.

of the product obtained under the SCW environment is systematically lower than that of the product obtained under the N2 environment by ∼3%. Visbreaking at Various Temperatures. At a water density of 0.25 g/cm3 and water-to-oil ratio of 3:1 (vol), the visbreaking under the SCW environment was performed at various reaction temperatures from 663 to 693 K. Limited by the batch operation mode of visbreaking, a preheating time lasting 30−35 min is needed. For the visbreaking at 693 K, coke at a yield of 2.0 wt % appears the moment the reaction temperature reaches the preset value. Consequently, only the experimental results observed at 663−683 K are illustrated in Figure 5. After 45 min of cracking at 663 K, the viscosity of the liquid product is decreased to 0.36 Pa.s together with the reduced scale of alkyl substitutes and molecular weight. With the increase in reaction temperature, the reaction time required to achieve a similar visbreaking performance is reduced obviously. The dealkylation at 683 K is actually accomplished in the middle of the preheating from 663 to 683 K lasting a few minutes by which point a satisfactory product viscosity as low as 0.31 Pa.s is observed. More importantly, the value of CA of the liquid product obtained at 683 K is merely 28.3%. 3.4. Stability of Products Obtained under the N2 or SCW Environment. Considering that the stability of visbreaking products is associated with their group composition, the SARA analysis of the liquid product with the lowest viscosity in different series of experiments was applied. The results are listed in Table S4. After visbreaking under the N2 environment, the fractions of aromatics and resins in liquid product decrease by 7 and 10%, respectively. In the meantime, the fractions of saturates and asphaltenes increase roughly by the same amount. An increase in the fraction of saturates can be ascribed to the dealkylation of aromatic rings distributed in aromatics, resins, and asphaltenes, whereas the sequential condensation of aromatics and resins is responsible for the increase in the fraction of asphaltenes. A significantly increasing asphaltene fraction by 10−11 wt % is in accordance with the increasing CA of liquid products. Condensation, as the secondary reaction of dealkylation at F

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 6. Schematic illustration of phase structures involved in visbreaking of heavy oil under the SCW environment.

kylation.32,33 A drastically improved visbreaking efficiency as shown in Figure 5 is thus observed. For the suppression of condensation in SCW to be shown intuitively, the fraction of asphaltenes vs the removal rate of alkyl substitutes was compared between the visbreaking products obtained under the N2 and SCW environments with the results illustrated in Figure 7. For the visbreaking under the

For a valid visbreaking operation with the yield of coke less than 1.0 wt %, the yield of gas is approximately the same as that of coke. Different from the condensation-based upgrading in SCW, the contents of CO2 and CO in the gas products collected in the visbreaking in SCW are below the limits of detection. As a result, the possible H-donation to visbreaking through the reforming of aliphatics in SCW is excluded under the operating conditions applied. Lighter oil fractions, like saturates and aromatics, dissolve readily into SCW, so a partially miscible water/oil two-phase structure as schematically illustrated in Figure 6 is developed after the introduction of SCW into the visbreaking of heavy oil. With the further involvement of vigorous agitation, an oil-inwater emulsion structure or a pseudo single-phase structure could be formed according to the water-to-oil ratio and water density applied.18,21,22 At a lower water density or lower water-to-oil ratio, the heavy oil/SCW mixture exists in the form of oil/water two-phase or oil-in-water emulsion structure. Because of the extraction of lighter oil fractions into the SCW phase, heavier fractions of resins and asphaltenes are highly concentrated in the oil phase and oil droplets. Consequently, the aggregation of asphaltenes and even part of resins occurs prematurely before the effective occurrence of dealkylation. As shown in the data presented in Figures 3 and 4, the visbreaking performance under the SCW environment at a water-to-oil ratio of 2:1 (vol) or water density of 0.20 g/cm3 is inferior to that of the visbreaking under the N2 environment. With the simultaneous increase in the water-to-oil ratio and water density, the phase structure of the heavy oil/SCW mixture evolves to the pseudo single-phase structure. The nonpolar fractions of saturates and aromatics as well as resins of weak polarity are extracted into the SCW phase, leaving asphaltenes in the form of nanoscale cokelike aggregates highly dispersed. Although the viscosity of the SCW phase might increase to various degrees after the dissolution of hydrocarbons, it should still be far lower than that of the original oil phase. For a reaction occurring in a solvent, the diffusion limited rate constant kd is directly proportional to the diffusivity of the solute but inversely proportional to the viscosity of the solvent.45 The visbreaking transferred into the SCW phase tends to be controlled by intrinsic reaction kinetics rather than by diffusion. Accordingly, the dealkylation responds rapidly to the increase in reaction temperature. In addition, the involvement of SCW in the decomposition of S-containing alkyl substitutes and in the carbonium mechanism-based βscission of alkyl substitutes contributes partly to deal-

Figure 7. Fraction of asphaltenes vs removal rate of alkyl substitutes of liquid visbreaking products obtained in oil or SCW phase.

N2 environment, the dealkylation occurring in the oil phase is always accompanied by significant condensation. Along with the procession of the removal of alkyl substitutes, the fraction of asphaltenes contained in visbreaking products increases steadily up to 16.3 wt %. As for the visbreaking transferred into the SCW phase, at the same removal rate of alkyl substitutes, the fractions of asphaltenes contained in the products are systematically lower than those of the products obtained in the oil phase. Such a difference becomes pronounced at increasing water density and reaction temperature, by which an effective suppression of the secondary condensation in SCW can be guaranteed.

4. CONCLUSIONS Under the N2 environment, the visbreaking of heavy oil occurring in the oil phase is composed of the dealkylation and G

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research condensation of aromatics. The former is responsible for the viscosity reduction of visbreaking products, and the latter results in the instability of visbreaking products. Constrained by the poor diffusivity in the viscous oil phase, the visbreaking under the N2 environment is seriously retarded together with the deteriorating stability of visbreaking products. After the introduction of SCW, the visbreaking still follows the same reaction mechanism. Nevertheless, under vigorous agitation, the introduction of SCW at a water density of 0.25 g/cm3 or higher and a water-to-oil ratio of 3:1 (vol) makes it possible that the visbreaking of heavy oil is transferred into the SCW phase. Benefiting from improved diffusion in SCW, the dealkylation of aromatics responds sensitively to the increase in reaction temperature by which a rapid accomplishment of dealkylation and viscosity reduction could be observed at 683 K. Owing to the drastically reduced reaction time for dealkylation, condensation, which is the secondary reaction of dealkylation at moderate temperature, is effectively suppressed. Therefore, the visbreaking in the novel SCW phase has the advantages of accelerated dealkylation and suppressed condensation over those of the visbreaking in the traditional oil phase.

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GREEK LETTERS μ = dynamic viscosity of oil samples, Pa.s ρw = water density in reaction system, g/cm3 REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04024. Details of visbreaking runs , properties and average molecular structure parameters of the liquid visbreaking products, SARA distribution of visbreaking products, and volume fractions of species contained in gas products under the N2 or SCW environments (PDF)

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RN = average number of naphthenic rings per average molecule RS = average number of alkyl substituents per average molecule RS,raw = average number of alkyl substituents per average raw heavy oil molecule t = visbreaking time, min T = visbreaking temperature, K w/o = water-to-oil ratio in volume at ambient temperature

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64253529. Fax: +86 21 64253528. E-mail: [email protected]. 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).

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NOMENCLATURE CA = fraction of aromatic C atoms in total C atoms contained in oil samples CS = fraction of nonaromatic C atoms in total C atoms contained in oil samples mi = weight of visbreaking product collected, g mraw = weight of raw heavy oil loaded, g Mn = number average molecular weight of oil samples, Da n = average number of carbon atoms per alkyl substituent nraw = average number of carbon atoms per alkyl substituent of raw heavy oil molecule Yi = weight fraction of visbreaking product collected RA = average number of aromatic rings per average molecule H

DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.7b04024 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX