Visbreaking of Heavy Oil in Supercritical Benzene - Energy & Fuels

Jan 23, 2019 - ... the efficiencies of the initiation and propagation of visbreaking network, by which the reaction could be run in the desired tandem...
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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Visbreaking of Heavy Oil in Supercritical Benzene Xue-Qin Liu,† Hao Qu,† Jing-Yi Yang,‡ Pei-Qing Yuan,*,† and Wei-Kang Yuan† †

State Key Laboratory of Chemical Engineering and ‡Research Institute of Petroleum Processing, East China University of Science and Technology, Shanghai 200237, China

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S Supporting Information *

ABSTRACT: The visbreaking of heavy oil in supercritical benzene (SCbenzene) was investigated. By introducing SCbenzene, the visbreaking originally occurring in the oil phase is transferred into SCbenzene. The superior diffusivity in SCbenzene improves the efficiencies of the initiation and propagation of visbreaking network, by which the reaction could be run in the desired tandem structure. By mitigating diffusion limitation to reaction kinetics, the cracking of alkyl substitutes of aromatics vital to viscosity reduction is accelerated. Being the secondary reaction of the cracking, condensation could be terminated promptly at the shortened reaction time necessary for visbreaking. A comparison between the visbreaking in SCbenzene and supercritical water (SCH2O) confirms the effectiveness of improving diffusion for the optimization on heavy oil visbreaking. Nevertheless, the optimal operating conditions involved must be determined experimentally because of the complicated interaction between phase structure and reaction kinetics. The visbreaking in SCbenzene shows the advantages of milder operating conditions and better product stability over the visbreaking in SCH2O.

1. INTRODUCTION China consumed about 560 million tons of crude oil in 2018, 70% of which was imported from overseas. In terms of energy security, even inferior heavy crude oils of high viscosity, such as Canadian oil sand bitumen and Venezuela ultraheavy crude oil, are considered as potential oil resources. At present, these heavy crude oils diluted with a large amount of solvents are transported to China by ocean-going tankers. To improve the transport properties of heavy crude oil, it is highly desirable to develop an effective and economical viscosity reduction technique. Visbreaking involving the cracking of alkyl substitutes of aromatics and the condensation of aromatics is one of the important thermal processing techniques for heavy oil, lowering the viscosity of products and increasing the proportion of middle distillates.1−4 Two typical techniques, that is, coil visbreaking and soaker visbreaking, exist in refineries. A high reaction temperature of 743 K is adopted in the coil visbreaking so as to intensify the cracking, determining the efficiency of viscosity reduction. By quenching the product immediately after exiting furnace tubes, the condensation leading to the formation of heavier fractions is supposed to be suppressed. Unfortunately, high asphaltene and olefin contents can always be found in the coil visbreaking product. Currently, the soaker visbreaking running at a relatively lower reaction temperature between 693 and 723 K is more popular. Still, the quality of the product obtained is far from satisfactory. One may notice that the reaction temperature adopted in either coil visbreaking or soaker visbreaking is well above the threshold temperature of the thermal cracking of oil fractions around 623 K. A question, why the visbreaking techniques available have to be operated at high temperatures without exception, thus arises. The answer lies in the fact that the dynamic viscosity of heavy oil at typical visbreaking temperatures maintains at the order of magnitude of 10−3 Pa·s.5 As a © XXXX American Chemical Society

result, diffusion becomes the rate-determining step of both cracking and condensation, resulting in an extremely poor viscosity reduction efficiency especially at low temperatures. Because the apparent activation energy of condensation is larger than that of cracking, a high visbreaking temperature not only intensifies the viscosity reduction but also significantly promotes the undesired formation of heavier fractions.6−8 To optimize visbreaking, an improvement on the diffusion environment of heavy oil fractions is expected. The physicochemical properties of fluids at supercritical state can change drastically. Notably, water and methanol are typical polar solvents under ambient conditions but nonpolar solvents in their supercritical region. Inspired by using supercritical fluids (SCFs) as H-donors or solvents, supercritical water (SCH2O, Tc = 647 K, Pc = 22.1 MPa) and supercritical methanol (SCMeOH, Tc = 512 K, Pc = 8.1 MPa) have been introduced into the pyrolysis of heavy oil, a process sharing the same free-radical reaction mechanism with visbreaking.9−11 According to the phase analysis by Bai, Xin, and Qu et al., the mixture of heavy oil and SCH2O may form a pseudo single-phase structure by which SCH2O becomes the continuous phase of the reaction system.12−14 The existence of the pseudo single-phase structure was confirmed by Yan et al. who performed the pyrolysis of heavy oil under SCMeOH environment.15 Benefiting from the low viscosity of SCFs at the order of magnitude of 10−5 Pa·s, the limitation of diffusion to the pyrolysis of heavy oil occurring in SCFs is partly relieved. By reaction kinetics analysis, Liu and Tan et al. found that the cracking and condensation are both accelerated when the pyrolysis is transferred into SCH2O.16,17 Recently, the visbreaking of heavy oil under SCH2O environment was attempted by the authors.18 The cracking Received: November 27, 2018 Revised: January 20, 2019 Published: January 23, 2019 A

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Properties of Raw Heavy Oil Used in Visbreaking SARA fractions (wt %) H/C ratio 1.54 C residue (wt %)

μ (Pa·s)a 1.78 CS (%)

18.6

70.9

Mn (Da)

CSI

saturates

aromatics

resins

52.4

asphaltenes

825 CA (%)

0.27 17.5 olefin content (mol %)

RA

RN

25.1 RS

5.0 n

29.1

0

5.1

0.8

4.0

10.7

a

Viscosity measured at 393 K.

and the condensation involved in pyrolysis/visbreaking were suggested to be in a tandem structure at moderate temperatures and in a parallel structure at high temperatures. The presence of the tandem structure makes it possible that the visbreaking could be accomplished without excessively sacrificing the quality of liquid products. It is noteworthy that the visbreaking under SCH2O environment is operated at the pressure approaching 30 MPa. Despite the simultaneous improvement on visbreaking efficiency and product quality, the extremely severe conditions adopted make the industrialization of the visbreaking under SCH2O environment difficult, technically and economically. Benzene, having a critical temperature of 562 K and a critical pressure of 4.9 MPa, is stable at the temperatures applied in visbreaking.19−22 According to the good miscibility between heavy oil and benzene under ambient condition, it can be proposed that the mixture of heavy oil and supercritical benzene (SCbenzene) should exist in the single-phase structure. By transferring the visbreaking of heavy oil into SCbenzene, hopefully, the advantages of the visbreaking under SCH2O environment could be retained. In addition, the process will be run under much milder operating conditions. So far, no work concerning the visbreaking of heavy oil in other SCFs, such as SCbenzene, was reported in the literature. Also, the suggestion about the varied structure of cracking and condensation at increasing temperatures needs verification. Hereby, the visbreaking of heavy oil in the presence of benzene was experimentally surveyed in this work. By SC analyzing the composition and the average molecular structure parameters of the liquid visbreaking products obtained under a typical SCbenzene environment, the mechanism responsible for visbreaking was determined first. Then, the visbreaking was applied at various benzene-to-oil ratios, benzene densities, and reaction temperatures to achieve a comprehensive understanding of the visbreaking behavior under SCbenzene environment. On the basis of a comparison with the visbreaking under N2 and SCH2O environments, the possible roles of the introduction of SCFs in the visbreaking of heavy oil were discussed from the aspects of phase structure and diffusivity.

Table 2. Details of Visbreaking Experiments under SCBenzene Environment solvent density (g/cm3)

T (K)

solvent-tooil ratio (wt)

633

0.20−0.35

2:1

633

0.30

1.5:1−3:1

613− 663

0.30

3:1

reactant and solvent loaded 10−17.5 g of oil and 20−35 g of benzene 10−15 g of oil and 30 g of benzene 10 g of oil and 30 g of benzene

reaction pressure (MPa) 7.5−11.5 10.0 8.5−12.0

sealed autoclave was heated from the ambient temperature to a scheduled reaction temperature at a slope of 15 K/min. The moment at which the temperature reached the scheduled value was considered as the beginning of visbreaking. The visbreaking lasting a given reaction time was terminated by subjecting the reactor to forced air cooling. During preheating and visbreaking stages, a stirring rate was kept at 800 rpm. 2.2. Analytical Procedures. After visbreaking, the gas product was collected over water first. Then, the autoclave was washed thoroughly with trichloromethane. The mixture obtained was subjected to filtration through the filter paper with an average pore size of 15 to 20 μm to remove coke. After that, trichloromethane and benzene were separated sequentially from liquid products by rotary evaporation at the temperatures of 303 and 318 K. The remaining liquid product was separated into SARA fractions, that is, saturates, aromatics, resins, and asphaltenes, on the basis of the standard of Chinese Oil & Gas SY/T 5119-2016. The mass balance (MB) of each experimental run was determined by MB =

∑ mi × 100% mraw

(1)

where mi represents the weight of SARA fractions or coke collected. mraw is the weight of the heavy oil loaded. Only the experiment with the MB better than 98% was accepted for subsequent analyses. The yields of SARA fractions and coke were evaluated by the following equation, whereas the yield of gas was determined by the normalization of product composition. Yi =

2. EXPERIMENTAL SECTION

mi × 100% mraw

(2)

To evaluate the stability of the asphaltenes contained in liquid products, the colloidal stability index (CSI) was calculated by23

2.1. Apparatus and Reaction Runs. To compare with the visbreaking under SCH2O environment, Changling vacuum residuum used in previous work was adopted as the raw heavy oil.18 Some basic properties of the heavy oil are listed in Table 1, and the symbols used in the table are explained in the Nomenclature. Details relating to operating parameters as well as the loadings of heavy oil and benzene are listed in Table 2. All the experiments were run in a Parr 4598 HPHT autoclave with a capacity of 0.1 L. A typical procedure of the visbreaking under SCbenzene environment is described as follows. First, certain amounts of heavy oil and benzene were loaded into the autoclave, followed by purging with N2 of high purity (>99.99 vol %) at ambient pressure. After that, the

CSI =

msaturates + masphaltenes maromatics + mresins

(3)

where msaturates and masphaltenes are the weights of saturates and asphaltenes contained in liquid products. maromatics and mresins are the corresponding values of aromatics and resins. The H atoms relating to olefins can be distinguished by their chemical shift on 1H NMR spectra. According to the literature data listed in Table S1 in Supporting Information, the olefin content (mol %) of liquid products was calculated by24,25 B

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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Table 3. Dynamic Viscosity and Average Structural Parameters of Liquid Visbreaking Products Obtained at Extending Reaction Timea t (min)

μ (Pa·s)

Ycoke (wt %)

H/C ratio

Mn (Da)

CS (%)

CA (%)

RA

RN

RS

n

Rs × n

15 30 45 60

0.78 0.63 0.41 0.31

0 0 0 0.5

1.54 1.53 1.53 1.53

797 682 630 541

66.8 66.7 66.5 66.1

33.2 33.3 33.5 33.9

4.0 4.0 3.7 3.3

0.8 0.8 0.8 0.7

4.0 3.8 3.6 3.4

8.5 8.5 8.2 8.0

34.0 32.3 29.5 27.2

a

Temperature of 633 K, benzene-to-oil ratio of 3:1 (wt), and benzene density of 0.30 g/cm3. olefin content =

∑

A i WSM × 4 × 100% A SWMSNi

of alkyl substitutes of aromatics, decreases from its initial value of 42.8 to 27.2. Because of the cleavage of alkyl substitutes connecting aromatic segments, the average number of aromatic rings per average molecule of liquid products (RA) decreases from 5.1 to 3.3. Apart from cracking, condensation characterized by the formation of asphaltenes and coke appears along with the proceeding of visbreaking. At a reaction time of 60 min, the fraction of aromatic carbon atoms in the total carbon atoms contained in liquid products (CA) increases from 29.1 to 33.9%. The competition between cracking and condensation can be identified by the product distribution analysis where results are illustrated in Figure 1.

(4)

where AS and Ai are the peak areas of the H atoms of dichloroethane added as an internal standard and the specific type of H contained in liquid products on the 1H NMR spectra. WS and W are the weights of dichloroethane and liquid products. MS and M are the molecular weights of dichloroethane and liquid products. Ni is the number of H atoms of the specific type of H contained in liquid products. The dynamic viscosity of oil samples including liquid products and the raw heavy oil was determined on an advanced rheology expand system (TA Instruments). The H/C ratio of oil samples was obtained on a vario EL III elemental analyzer. The number average molecular weight (Mn) of oil samples was characterized on a KNAUER K7000 vapor pressure osmometer based on the standard of Chinese Petrochemical SH/T 0583-94. The NMR spectrum of oil samples was analyzed on a Bruker AVANCE 400 MHz NMR spectrometer. The boiling point distribution of oil samples was measured on an Agilent 7890 simulated distillation gas chromatography based on the standard of ASTM D2887.

3. RESULTS AND DISCUSSION 3.1. Visbreaking Mechanism under SCBenzene Environment. Under a typical SCbenzene environment, that is, the temperature of 633 K, benzene-to-oil ratio of 3:1 (wt), and benzene density of 0.30 g/cm3, the visbreaking of heavy oil was performed. Condensation is highly undesired in visbreaking, so the experiment with the coke yield less than 1.0 wt % was accepted here as a valid operation. The liquid visbreaking products were sampled at extending reaction time. The dynamic viscosity of liquid products measured at a temperature of 353 K and the average molecular structure parameters of liquid products calculated by the modified Brown−Ladner (B−L) method are listed in Table 3.26−28 Detailed equations involved in the B−L method are listed in Table S2. Along with the extension of reaction time from 15 to 45 min, the viscosity of the liquid visbreaking product decreases monotonically from 0.78 to 0.41 Pa·s without the formation of coke. Although an even lower product viscosity of 0.31 Pa·s could be obtained at the reaction time of 60 min, coke at a yield of 0.5 wt % appears in the product. A significant decrease in product viscosity suggests the change of the chemical structure of liquid products. In delayed coking, the H/C ratio of liquid products could be effectively adjusted through coke formation. During the visbreaking in SCbenzene, coke formation is strictly restricted. Besides, the production of gas was found to be negligible. Consequently, the H/C ratio of liquid products is nearly the same with that of the raw heavy oil. Nevertheless, the degradation of heavy fractions reflected by the decreasing average molecular weight of liquid products from 825 to 541 Da could be observed. It is evident that the simultaneous decrease in the viscosity and molecular weight of liquid products results primarily from the cracking of alkyl substitutes of aromatics. The product of Rs and n, which indicates the scale

Figure 1. Variation of SARA fractions, coke, and gas during visbreaking in SCbenzene; a temperature of 633 K, a benzene-to-oil ratio of 3:1 (wt), and a benzene density of 0.30 g/cm3.

A consistent tendency, that is, the decrease in the fractions of aromatics and resins and the increase in the fractions of saturates and asphaltenes, could be observed at the reaction time ranging from 15 to 45 min. The formation of asphaltenes shows the sequential condensation of aromatics and resins. Meanwhile, the simultaneous formation of saturates confirms the degradation of heavy fractions through the cracking of alkyl substitutes, by which the viscosity reduction of liquid products is realized. At the early stage of visbreaking, the cracking and the condensation are comparable because the variation of saturate fraction is approximately equivalent to that of asphaltene fraction. With the extension of reaction time to 60 min, the fraction of asphaltenes increases continuously but that of saturates decreases slightly. The appearance of coke at that time signifies that condensation rather than cracking becomes the dominant reaction at the later stage of visbreaking. It was reported that the coke existing in the thermal treatment of oil fractions could be a template for the further adsorption and condensation of aromatics, which makes the formation of coke possesses the characteristic of C

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Performance of visbreaking of heavy oil in SCbenzene at various benzene-to-oil ratios (left) or various benzene densities (right); a temperature of 633 K; coke yield is labeled by the symbol if coke appears.

Figure 3. Performance of visbreaking of heavy oil in SCbenzene at various reaction temperatures; a benzene-to-oil ratio of 3:1 (wt) and a benzene density of 0.30 g/cm3; coke yield is labeled by the symbol if coke appears.

autocatalysis.17 Once a small amount of coke appears in the visbreaking system, its yield varying along sigmoid curves may readily exceed the limitation of 1 wt %. 3.2. Visbreaking Performance under Various SCBenzene Environments. After the determination of the visbreaking mechanism of heavy oil under SCbenzene environment, the influences of benzene density, benzene-to-oil ratio, and reaction temperature on the performance of visbreaking were examined. The results are described briefly as follows. 3.2.1. Visbreaking at Various SCBenzene-to-Oil Ratios or SCBenzene Densities. At a fixed temperature of 633 K and a benzene density of 0.30 g/cm3, the visbreaking at increasing benzene-to-oil ratios from 1.5:1 to 3:1 (wt) was examined. Also, at a fixed temperature of 633 K and a benzene-to-oil ratio

of 2:1 (wt), the visbreaking at increasing benzene densities from 0.20 to 0.35 g/cm3 was surveyed. The corresponding results are illustrated in Figure 2. The visbreaking at increasing benzene densities is much similar in reaction behavior to the visbreaking at decreasing benzene-to-oil ratios. Take the visbreaking at various benzene densities as an example. At the benzene densities of 0.20 and 0.25 g/cm3, the dynamic viscosity of liquid products decreases with the extension of reaction time, and coke appears at a reaction time of 45 min. On the whole, the efficiency of viscosity reduction at a benzene density of 0.25 g/cm3 is better than that at a benzene density of 0.20 g/cm3. With the increase in benzene density to 0.30 g/cm3, the visbreaking performance can be improved further. In the meantime, the formation of D

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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At a temperature of 613 K, it takes 120 min to obtain the liquid product with a satisfactory dynamic viscosity of 0.4 Pa·s. However, the simultaneous occurrence of condensation leads to the production of asphaltenes, coke, and gas in large quantities. With the increase in reaction temperature, the reaction time required for cracking is drastically shortened. The visbreaking at a temperature of 663 K is virtually accomplished during the preheating to the scheduled reaction temperature. At that time, the sequential condensation of aromatics and resins to asphaltenes and coke is suppressed effectively. However, coke at a yield above 1 wt % could always be observed when the visbreaking at the temperature exceeding 673 K was attempted subsequently. Regardless of the variation of benzene density or benzene-to-heavy oil ratio, 673 K was found to be the upper temperature limit for the valid visbreaking operation in SCbenzene. 3.3. Optimal Conditions for Visbreaking under SCBenzene Environment. By the data presented in Figures 2 and 3, viscosity reduction could be accelerated by increasing oil fraction concentration or visbreaking temperature. Typical visbreaking performance based on two different approaches is listed in Table 4. The experiments involved both were terminated at the moment the preheating temperature reached the scheduled visbreaking temperature. By decreasing the benzene-to-oil ratio to 1.5:1 (wt), the liquid product with a dynamic viscosity of 0.18 Pa·s was obtained. A high oil fraction concentration reduces the operating cost of solvent separation and solvent circulation, intensifying the processing capability of the reactor. Unfortunately, the fraction of asphaltenes contained in the liquid product is doubled by the sensitive response of condensation to the increase in oil fraction concentration. What is worse, the appearance of coke at a yield of 0.1 wt % indicates a high risk of coke deposition on the wall of the reactor. On the contrary, the visbreaking at a lower oil fraction concentration may surely increase the operating cost of the visbreaking under SCbenzene environment. Nevertheless, with a moderate increase in reaction temperature not only the viscosity reduction is accelerated but also the condensation is suppressed effectively. After the visbreaking at a temperature of 663 K and a benzeneto-oil ratio of 3:1 (wt), the product viscosity decreases to 0.26 Pa·s, and the fraction of asphaltenes in the liquid product only increases slightly by 2.2 wt %. The suppression of condensation makes the visbreaking operated in the continuous mode stable and flexible. To balance the technical and economic requirements for industrialization, it is better to run the visbreaking under SCbenzene environment at an appropriate oil fraction concentration and a moderate temperature of no more than 673 K. 3.4. Visbreaking Performance under Various Medium Environments. It was reported that an optimal viscosity reduction in the visbreaking under SCH2O environment could be obtained at a temperature of 683 K, a water density of 0.25 g/cm3, and a water-to-oil ratio of 3:1 (wt).18 Besides, the

coke comes earlier at a reaction time of 30 min. At an even higher benzene density of 0.35 g/cm3, the liquid product with a satisfactory dynamic viscosity as low as 0.24 Pa·s together with the coke at a yield of 0.1 wt % could be obtained at a reaction time of 15 min. An increase in benzene density at the fixed benzene-to-oil ratio or a decrease in benzene-to-oil ratio at the fixed benzene density results in a higher concentration of oil fractions in the reaction system, by which an accelerated viscosity reduction could be observed. From the view point of chemical reaction engineering, the condensation participated by multiple molecules should be sensitive to the variation of oil fraction concentration. As the data presented in Figure 2 show, the reaction time for coke formation is shortened rapidly at increasing oil fraction concentration. 3.2.2. Visbreaking at Various Reaction Temperatures. At a benzene density of 0.30 g/cm3 and a benzene-to-oil ratio of 3:1 (wt), the visbreaking performance at increasing temperatures from 613 to 663 K is illustrated in Figure 3. Preliminary experiments under N2 environment suggested that, after 6 h of visbreaking at the threshold temperature for the C−C cleavage of 623 K, still it is impossible to measure the dynamic viscosity of liquid products at 353 K. By contrast, the visbreaking under SCbenzene environment proceeds effectively even at a temperature of 613 K. After 15 min of visbreaking, the dynamic viscosities of liquid products obtained at the temperatures of 613, 633, 653, and 663 K are 1.64, 0.77, 0.36, and 0.20 Pa·s, respectively. In terms of the accelerated coke formation at increasing temperatures, both cracking and condensation are promoted at high temperatures. The distribution of the products obtained at various temperatures with the liquid product viscosity ranging from 0.25 to 0.40 Pa·s is illustrated in Figure 4.

Figure 4. Variation of SARA fractions, coke, and gas of visbreaking products obtained at different temperatures with similar liquid viscosity; a benzene-to-oil ratio of 3:1 (wt) and a benzene density of 0.30 g/cm3.

Table 4. Visbreaking Performance under Temperature

SCBenzene

Environment at a High Oil Fraction Concentration or a High Reaction

operating conditions

SARA fractions (wt %)

T (K)

benzene density (g/cm )

benzene-to-oil ratio (wt)

μ (Pa·s)

Ycoke (wt %)

saturates

aromatics

resins

asphaltenes

633 663

0.30 0.30

1.5:1 3:1

0.18 0.26

0.1 0

18.4 19.9

51.9 50.7

19.5 22.2

10.3 7.2

3

E

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Energy & Fuels Table 5. Properties of Liquid Visbreaking Products Obtained under Various Medium Environments SARA fractions (wt %) reaction media

solvent density (g/cm3)/solvent-to-oil ratio (wt)

T (K)

t (min)

μ (Pa·s)

SCbenzene

0.30/3:1 0.25/3:1

663 683 693

0a 0a 30

0.26 0.31 0.27

SCH2O N2

saturates aromatics 19.9 20.7 24.3

50.7 53.2 46.5

resins

asphaltenes

CSI

olefin content (mol %)

22.2 18.2 14.0

7.2 7.9 15.2

0.37 0.40 0.65

6.3 27.5 50.4

a

The reaction was terminated immediately after the preheating temperature reached the scheduled value.

visbreaking under N2 environment was intensified by increasing temperature from 663 to 693 K. The visbreaking performance under optimized SCbenzene environment, that is, a temperature of 663 K, a benzene density of 0.30 g/cm3, and a benzene-to-oil ratio of 3:1 (wt), was then compared with the visbreaking under optimized SCH2O and N2 environments, with the results listed in Table 5. To evaluate product stability, the corresponding values of CSI and olefin content were also calculated. For the visbreaking under N2 environment, a low product viscosity of 0.27 Pa·s could be obtained after 30 min of reaction at a temperature of 693 K. Apparently, the viscosity reduction is at the cost of condensation in large quantities because the content of asphaltenes increases drastically to 15.2 wt %. It is generally acknowledged that asphaltenes dispersed in heavy oil are in a colloid structure in which asphaltene micelles are stabilized by aromatic surroundings.29,30 Asphaltene micelles are unstable at the CSI larger than 0.9, metastable at the CSI between 0.7 and 0.9, and stable at the CSI lower than 0.7.23 The CSI of the visbreaking product obtained under N2 environment can be as high as 0.65, far above its initial value of 0.29 and close to the metastable range. With the combination of the olefin content up to 50.4 mol %, sedimentation is likely to occur during the storage and transportation of visbreaking products. After the introduction of SCbenzene or SCH2O into the visbreaking of heavy oil, the viscosity reduction is drastically accelerated. Also, condensation is effectively suppressed because the increase in the asphaltene content of the liquid products obtained under both SCF environments does not exceed 3.0 wt %. In terms of the CSI values of 0.37 and 0.40, the asphaltenes contained in the products remain stable. It is noteworthy that the olefin content of the product obtained under SCW environment is 27.5 mol %, about one-half that of the product obtained under N2 environment. Owing to a moderate temperature of 663 K applied, the olefin content of the product obtained under SCbenzene environment decreases further to 6.3 mol %. For the visbreaking run under various medium environments, the product obtained under SCbenzene environment therefore possesses the best stability. To further examine the property of the liquid products obtained under optimized SCF environments, simulated distillation GC analysis was performed, and the results are illustrated in Figure 5. Oil fractions are divided into gasoline, diesel, vacuum gas oil (VGO), and vacuum residua (VR) based on the boiling point range as follows: initial boiling point (IBP) to 453 K, 453 to 623 K, 623 to 773 K, and 773 K to final boiling point. The fraction of VR contained in the raw heavy oil is 99.0 wt %. When the visbreaking was run under SCbenzene environment, the IBP of the visbreaking product decreases significantly from its initial value of 752.3 to 429.2 K. Accordingly, the fractions of middle distillates, that is, gasoline, diesel, and VGO increase

Figure 5. Distribution of boiling point of raw heavy oil and liquid visbreaking products obtained under optimized SCbenzene or SCH2O environment.

by 3.8, 5.3, and 11.0 wt %, respectively. Similarly, a lowered IBP to 434.6 K can be observed for the product obtained under SCH2O environment. In the meantime, the fractions of gasoline, diesel, and VGO increase by 1.9, 9.1, and 18.0 wt %, respectively. The increase in middle distillate fraction and the decrease in VR fraction confirm the degradation of heavy fractions through the cracking of alkyl substitutes of aromatic segments, in accordance with the results presented in Table 3. The IBP of liquid products is far higher than the boiling point of benzene by about 70 K, so a clear separation of benzene from liquid products can be realized by distillation. 3.5. Roles of SCFs in Visbreaking of Heavy Oil. With the introduction of two typical but different SCFs, SCbenzene and SCH2O, into the visbreaking of heavy oil, an improvement on visbreaking performance was observed. Both SCbenzene and SCH2O can be considered approximately as inert reaction media for visbreaking.19−22,31−33 Therefore, the various phase structures resulting from the introduction of SCFs should play a vital role on the improved visbreaking performance. For the mixture of heavy oil and SCbenzene, a single-phase structure can always be maintained despite the difference in the thermodynamic state of benzene. As for the mixture of heavy oil and SCH2O following the type IIIb or type II phase behavior defined by van Konynenburg and Scottis, its phase structure is determined simultaneously by the thermodynamic state of water and the operating condition applied.34−38 A condensed oil-in-water emulsion structure could be formed at a low water density, a low water-to-oil ratio, or a low temperature.14,18 At that time, light fractions are extracted into SCH2O, leaving heavy fractions concentrated in oil droplets. Driven by the attractive electrostatic interaction between oil F

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

transition of the visbreaking from the tandem structure to the parallel structure. The visbreaking under SCbenzene has the advantages of milder operating pressure around 10 MPa and better product stability in terms of olefin content over the visbreaking under SCH2O environment. Nevertheless, the visbreaking under SCH2O environment is more economical in energy consumption. The separation of water from liquid products can be achieved simply by decantation at low temperatures, whereas the separation of benzene from liquid products relies on distillation. Whether the specific SCF technique has the feasibility of industrialization depends on a comprehensive evaluation on operating cost, operating stability, and the added value of products.

fractions and SCH2O, the majority of heavy oil fractions tend to dissolve into SCH2O along with the increase in water density or temperature. The heaviest asphaltenes, however, are retained in the oil droplets of reduced size, eventually dispersed in 13 SCH2O in the form of coke-like assembly. By the formation of the single-phase for the heavy oil/SCbenzene system or the pseudo single-phase for the heavy oil/SCH2O system, the visbreaking is transferred from the viscous oil phase into SCFs. Benefiting from the superior diffusivity in SCFs, the recombination of alkyl carbon radicals resulting from the homolytic cleavage of α and β carbons with an intrinsic activation energy of 317 kJ/mol is effectively prevented, ensuring the initiation efficiency of the cracking.39 Meanwhile, the bimolecular H-abstraction vital to the propagation of the cracking is accelerated because the apparent reaction rate of a diffusion limited reaction is inversely proportional to the viscosity of the reaction system.40,41 Accordingly, the visbreaking of heavy oil in SCFs could be accomplished at the temperature much lower than that applied in the traditional visbreaking techniques, by which the reaction is operated in the tandem structure suggested. Figure 6 illustrates

4. CONCLUSIONS With the introduction of SCbenzene as a solvent, the visbreaking of heavy oil is transferred from the viscous oil phase into SCbenzene. Because of superior diffusivity in SCbenzene, the visbreaking with improved efficiencies of initiation and propagation could be run at relatively lower temperatures, on the basis of which the visbreaking shows the desired tandem structure. By mitigating the limitation of diffusion to cracking, the viscosity reduction is accelerated. Being the secondary reaction of cracking, the sequential condensation to asphaltenes and coke could be terminated promptly at the shortened visbreaking time, guaranteeing the quality of products. By increasing reaction temperature not exceeding the critical value for the transition to the parallel structure, 673 K observed in this work, both the viscosity reduction efficiency and the suppression to condensation could be further improved. The visbreaking under SCbenzene environment maintains the single-phase structure regardless of the thermodynamic state of benzene. By contrast, the visbreaking under SCH2O environment is run in the condensed emulsion or pseudo single-phase structure, depending on the operating conditions applied. The visbreaking transferred into SCFs, either SCbenzene or SCH2O, responds similarly to the variation of oil fraction concentration and reaction temperature. Both accelerated viscosity reduction and suppressed condensation could be obtained by optimizing reaction conditions. Compared with the visbreaking under SCH2O environment, the visbreaking under SCbenzene is characterized by the better product quality, milder operating condition but higher energy consumption for solvent/product separation.

Figure 6. Schematic illustration of the effect of diffusion on the reaction rates of cracking and condensation in a tandem or parallel structure.

schematically the effect of diffusion on the reaction rate of cracking and condensation in the tandem or parallel structure. With the intensification of the foregoing cracking by mitigating diffusion limitation, the secondary condensation in the tandem structure could be terminated promptly at the shortened reaction time necessary for viscosity reduction. By contrast, a high reaction temperature has to be adopted in the traditional visbreaking techniques to offset the diffusion limitation to reaction kinetics. Consequently, condensation with a large apparent activation energy plays an overwhelming role on the visbreaking in the parallel structure. For the visbreaking operated in the tandem structure, the cracking free of diffusion limitation responds sensitively to the increase in reaction temperature. In the meantime, the response of the secondary condensation to temperature increase lags behind that of cracking. According to the data presented in Figures 3 and 4, both the viscosity reduction efficiency and the product stability can be improved by increasing reaction temperature over a reasonable range. However, the transition to the unfavorable parallel structure occurs eventually with the further increase in temperature. By this consideration, it is recommended that the visbreaking of heavy oil under SCbenzene environment be operated at the temperature no more than 673 K, a critical value for the

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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.8b04136.

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Chemical shift of H atoms relating to olefins contained in oil samples and equations adopted in modified B−L method for determining average molecular structure parameters of raw heavy oil or liquid products (PDF)

AUTHOR INFORMATION

Corresponding Author

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

DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX

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Jing-Yi Yang: 0000-0002-4694-7860 Pei-Qing Yuan: 0000-0003-0797-963X 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 (Grant No. 21676084) and the project from SINOPEC (Grant No. ST18011-3-2).

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NOMENCLATURE CA = fraction of aromatic C atoms in total C atoms contained in oil samples, % CS = fraction of non-aromatic 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 Yi = weigh fraction of visbreaking product collected 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 t = visbreaking time, min T = reaction temperature, K

Greek Letters

μ = dynamic viscosity of oil samples, Pa·s

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DOI: 10.1021/acs.energyfuels.8b04136 Energy Fuels XXXX, XXX, XXX−XXX