Characterization of Water-in-Oil Emulsion and Upgrading of Asphalt

Jan 25, 2017 - Yang, Guo, Cheng, Wu, Zhang, Zhang, Yang, and Zhang. 2017 31 (2), pp 1159–1173. Abstract: The recovery of heavy oil from ultradeep we...
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Characterization of Water-in-Oil Emulsion and Upgrading of Asphalt with Supercritical Water Treatment Di Shu, Yong Chi,* Jieli Liu, and Qunxing Huang State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The products of asphalt upgrading in supercritical water (SCW) forms a mixture of cracked oil, water, and fine coke particles; this mixture tends to be emulsified, which introduces direct challenges to emulsified water removal. In this study, the asphalt upgrading in SCW was performed at 390−450 °C and 22.4−27.2 MPa for 40 min. A differential scanning calorimetry analysis between −60 and 50 °C was conducted to evaluate the water content and size distribution in the water-in-oil emulsions. The mechanical centrifugation and chemical demulsification were performed to test the water removal efficiency and stability of the emulsions. The results of properties and emulsification characteristics of the oil products indicate that the transition temperature of asphalt upgrading was between 400 and 410 °C. At temperatures lower than the transition temperature, the oil products were similar to the feed asphalt in terms of high viscosity, carbon residue, and asphaltene content. In the emulsification process, the contact between water and oil products was limited by the semisolid state of the oil products, which decreased the water content and size of the droplets in the emulsions. At temperatures higher than the transition temperature, the viscosities of the oil products were below 1.2 Pa·s, and a large number of water droplets were observed clearly in the fluid emulsions. The emulsified water was partially removed by centrifugation; however, the content of fine water droplets with diameters less than 5 μm was still greater than 14% in the centrifuged emulsions. Furthermore, the addition of chemical demulsifier was more effective on demulsification than centrifugation. When the temperature was higher than 430 °C, the water content of the demulsified emulsion was less than 2.5%. On the basis of comprehensive consideration of maltene yield, oil property, and water removal of water-in-oil emulsions, the temperature of 430−440 °C is the most beneficial condition for asphalt upgrading.

1. INTRODUCTION Asphalt is a sticky, black, highly viscous, and carefully refined residue from the solvent-deasphalting process of vacuum or atmospheric residue.1 In China, the annual output of asphalt is approximately 20 million tons, which accounts for 3−4 wt % of crude oil consumption. Asphalt is mainly used as cementitious material for road construction or bituminous waterproofing products instead of further refining.2 However, as a heavy oil resource, asphalt can be upgraded to satisfy the increasing demand for light oil. Because of the high concentrations of asphaltene, heteroatoms (N, S), and metal elements (Ni, V) in asphalt, the conventional conversion approaches (carbonrejection and hydrogen-addition) become very difficult, costly, or even inapplicable.3 Over the past decade, as an effective method of viscosity breaking and coke suppression, supercritical water (SCW) treatment in asphalt upgrading has attracted much attention.4−8 SCW exhibits a potential application as an effective reaction medium for the environmentally benign conversion of various heavy oil resources such as bitumen, oil sand, oil shale, and vacuum residue.9−13 It possesses the excellent dissolution behavior of light hydrocarbons14−16 and effective extraction of the asphaltene core from oil-rich phases to water-rich phases.17 Morimoto et al.18 found that the high dispersion effect on heavy fractions caused the intramolecular dehydrogenation of the heavier component and prevented recombination reactions during SCW treatment, which led to a greater oil yield and a lower coke yield compared to treatment without SCW. When © 2017 American Chemical Society

the hydrogenation process is performed in an SCW medium, the hydrogen donor faces little mass-transfer resistance from heavy oil because of the evanescent phase boundaries.19 Vilcaez et al.9 observed that heavy oils did not form a homogeneous phase in SCW at 2.5:1 water/bitumen ratio using autoclave reactors with viewing ports, while near complete solubilization of bitumen in water was viewed at 420 °C after 30 min of reaction time at a 10:1 water/bitumen ratio. In this research, a high water:asphalt ratio of 16:1 (g/g) without stirring was performed to improve the solubility of asphalt in SCW and to provide a more homogeneous environment, which can enhance the involvement of SCW in the cracking of heavy hydrocarbons. As a new application on upgrading heavy oil using SCW, investigations have been increasingly focused on optimizing the reaction process, such as hydrogen addition,20,21 extraction,22,23 transport properties of SCW and oil,24−26 coke suppression,3,18 catalytic cracking,27,28 and desulfurization.13,29 However, the separation of the oil product, water, and fine coke particles is lacking attention after the upgrade in SCW. After flocculation and maturing, the mixed products become a water-in-oil emulsion. Water and cokes in the emulsions must be removed to satisfy pipeline and downstream process specifications.30 However, the stability and components of the formed Received: November 23, 2016 Revised: January 15, 2017 Published: January 25, 2017 1468

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Energy & Fuels Table 1. Properties of Raw Asphalt proximate analysis (wt %) ash

volatile matter

0.06

fixed carbon

ultimate analysis (wt %) density (g/cm3)

83.28 16.66 1.038 weighted average molecular weight (Mw)

viscosity (Pa·s)

C

H

264.7

83.24

8.90

O

S

1.56 5.31 SARA (wt %) (dry)

N

H/C atomic ratio

0.99

1.28

raw asphalt

asphaltene

maltene

saturates

aromatics

resins

asphaltenes

2297

4760

1597

12.5

38.0

27.9

21.6

demulsifier product based on poly(ethylene oxide)/ poly(propylene oxide) block copolymers made by Jiangsu Haian Petroleum, Inc. 2.2. Upgrading Reactions in SCW. The upgrading reactions in SCW were performed using a 500 mL Hastelloy C276 autoclave purchased from Parr Instruments. A 5.0 g sample of asphalt and 80.0 g of deionized water (6.25 wt % of asphalt mass fraction) were loaded in the batch reactor. After the reactor vessel was sealed, the entire system was purged with nitrogen for 3 min, and an initial reactor pressure of 0.2 MPa was applied. Then, the reactor was heated at a rate of 15 °C/ min in an electric furnace for approximately half an hour to reach the reaction temperature. After 40 min of retention time at a certain reaction temperature with ±1 °C of fluctuation, the heating furnace was removed, and the cooling water flowed through the solenoid valve into the coil to cool the reactor to room temperature. 2.3. Emulsion Analysis. The separation procedure and subsequent analytical methods are shown in Figure 1. In the whole emulsion

emulsions are decisively affected by the upgraded products, which presents direct challenges to the separation. In addition to the upgrading with SCW, water-in-oil emulsions can be produced in other crude oil processes with water, such as the bitumen froth in the oil sand mining process31 and petroleum sludge in the oil field.32 The water-inoil emulsion is essentially stabilized by surfactants such as oilsoluble organic acids, fine coke particles,33 or asphaltenes.34 The asphaltene cores are connected to one another by alicyclic groups and alkyl chains,35 and the alkyl side chains enhance the interfacial activity of asphaltenes.36 The accumulation of asphaltenes at the oil−water interface results in the formation of a rigid film, which acts as a barrier to droplet coalescence.37 Ali et al.38 investigated the effect of asphaltenes and resins on the stabilization of water-in-oil emulsions obtained from well heads of different oil fields. They found that the oil fractions in emulsions with high resin/asphaltene (r/a) ratios were separated easily. In addition, the resins increase the solubility of asphaltenes in the oil phase, which minimizes the asphaltene interaction with water droplets.39 Simon et al.40 found that the interfacial tension between water and asphaltene in xylene (1.0 g/L) ranged in 20−27.5 mN/m, showing a slight decrease with the pH increasing from 3 to 6. Feng et al.41 studied the effect of demulsifier dosage on interfacial tension of the water−naphthadiluted bitumen interface. It was found that a higher oil/water interfacial tension would increase the force required to remove the interfacial particles or increase the difficulty of demulsification. The solvent (naphtha or paraffin) extraction, even with the addition of chemical demulsifiers, is commonly used for emulsion treatment.42,43 After the SCW treatment, the stability of the water-in-oil emulsion is mainly controlled by the upgrading process through the interface behavior of cracked oil products. The emulsification behavior and upgrading reactions should be linked to predict or control the stability and water content of the emulsions. To the best of our knowledge, a study on the emulsification behavior after upgrading in SCW has not been reported. In this paper, the property of extracted oil products and the characterization of water-in-oil emulsions after asphalt upgrading in SCW at 390−450 °C were investigated to study the effect of the upgrading reaction on emulsification. The water content and size distribution of the water-in-oil emulsions were evaluated using a differential scanning calorimetry (DSC) analysis. Furthermore, chemical demulsification and mechanical centrifugation were conducted to examine the water removal efficiency and stability of the water-in-oil emulsions.

Figure 1. Procedure to recover the products and analytical methods.

analysis, organic solvent was not used to recover the products. The emulsion was formed as a mixture of oil products, water, and coke in the reactor after the upgrading and cooling process. Agitation was avoided as much as possible to keep the original emulsion during the recovery process. After the products were transferred into a funnel and allowed to stand for 6 h, the mixture was separated into an oil-rich phase and a water-rich phase. The emulsions obtained without chemical demulsification or centrifugation were identified as nontreated (NT) emulsions. The microstructure of the oil- or water-rich phase was observed using an optical microscope (Olympus BX53) equipped with a charge-coupled device (CCD). The water content and droplet size distribution in the oil-rich phase were obtained from a DSC analysis. The oil-rich phase was submitted to a regular heating and cooling cycle in a temperature range of 50 to −60 °C. A Netzsch DSC 200F3 thermo-analyzer with nitrogen carrier gas at a flow rate of 60 mL/min was used to detect the absorbed or released heat flux during the phase change of the water-in-oil emulsions. A hermetically sealed aluminum crucible that contained 9 mg of the sample and a sealed empty crucible as the reference cell were subjected to the analysis. All heating and cooling rates were set to 5 °C/min. The samples were heated to 50 °C, where they were held for 5 min to enable better contact with the crucible. Then, the sample was cooled with liquid nitrogen coolant to −60 °C and reheated to 20 °C. The water content w (wt %) of the water-in-oil emulsion was calculated using eq 1.

2. EXPERIMENTAL SECTION 2.1. Asphalt. Asphalt, which was used as feedstock, was produced from the vacuum residue using the solvent (propane) deasphalting process (China Sinopec Maoming Co., Ltd.). Asphalt is a solid at room temperature and is completely soluble in toluene. The properties of raw asphalt are shown in Table 1. Demulsifier is a formulated 1469

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Energy & Fuels Table 2. Yield Distribution after Upgrading in SCW yield (wt %) maltene temperature (°C) feed 390 400 410 420 430 440 450

w=

pressure (Mpa)

coke

asphaltene

resin

aromatic

saturate

coke + oil

22.39 22.92 23.75 24.45 25.32 26.02 27.21

0 0.8 1.0 10.2 18.5 19.5 17.7 17.7

21.6 19.6 19.6 9.3 4.3 3.0 1.7 1.1

27.9 24.9 24.6 16.0 14.0 11.8 11.4 9.2

38.0 35.0 33.7 31.8 27.4 24.9 24.3 20.2

12.5 16.9 17.4 19.7 17.2 15.3 13.9 8.3

100.0 97.2 96.3 87.0 81.4 74.5 69.0 56.5

qw Hf

a gel permeation chromatography instrument (GPC, Waters 1525/ 2414) with μStyragel HR1 columns (7.8 mm Ø × 300 mm, exclusion limit of 5000). The elemental composition was detected using a CHN analyzer (Elementar vario MAX cube, Germany). The thermogravimetric (TG) analysis of the oil products was conducted with a TG instrument (TGA/SDTA851, Swartzer Land) under nitrogen atmosphere. The temperature increased at a rate of 15 °C/min from room temperature to 800 °C. The total organic carbon (TOC) in aqueous phase was measured on a COD/TOC Multiparameter water quality detector (ET99731, Lovibond); the analytical principle of this instrument complied with U.S. EPA Standard 415.3 and was based on the carbon oxidation method to measure the TOC. The dynamic viscosity of two types of oil products was tested by two instruments with different range and precision. The fluid oil product with viscosity below 10 Pa·s was directly placed on a parallelplate viscometer (ARES-G2, TA Instruments) with a diameter of 20 mm. The semisolid oil product with viscosity above 50 Pa·s was first heated to 110 °C to melt and was subsequently poured into the coaxial cylinder of the rotary viscometer (HAAKE, VT550), which was equipped with immersion sensors (SV-2). When the temperature was stable at 60 °C, the viscosity was measured at a constant shearing rate of 60 rad/s.

(1)

where qw (J/g) is the specific heat absorbed during melting, and the fusion enthalpy (Hf) was set as 233 J/g. The results of the water content are based on the emulsion mass. All DSC analyses were repeated twice for accuracy and repeatability. The droplet diameters were described based on the crystallization temperature, as shown in eq 2.

⎛ − 35 ⎞ Di = exp⎜ ⎟ ⎝ Ti + 10 ⎠

(2)

where the droplet diameter (D) was determined from the droplet freezing temperature (T), which can be obtained from the DSC analysis at a known cooling rate. Because the water droplet size is known, the water size distribution can be calculated using the sphere model. The details of the description of water droplets in the DSC analysis were presented in a previous work.44 2.4. Centrifugation of the Water-in-Oil Emulsion. To remove water using mechanical centrifugation, 2.5 g of nontreated water-in-oil emulsion was placed in a 25 mL centrifugal tube and centrifuged at 3000 rpm using a Thermo Scientific ST40 centrifuge for 10 min. Then, the bottom oil layer was collected. The emulsions obtained after centrifugation were labeled as C-emulsions. 2.5. Chemical Demulsification of the Water-in-Oil Emulsion. In a demulsification test, 2.5 mg of demulsifier was injected into 2.5 g of nontreated water-in-oil emulsion. The emulsion was shaken for 5 min on a reciprocating mechanical shaker at 200 oscillation/min, followed by gravity settling at 45 °C for 90 min. The emulsions obtained with demulsifier addition were labeled as D-emulsions. 2.6. Oil Product Analysis. Considering the consumption of products in the emulsion analysis and the lack of addition of organic solvent, it was difficult to collect all of the oil products after the emulsion analysis. To obtain accurate yields, all SCW experiments were repeated. After the upgrading process, 150 mL of hot toluene was used to extract the oil products. The coke was separated as a tolueneinsoluble fraction under vacuum filtration through a 0.22 μm pore diameter filter. A separating funnel was used to separate the water phase from the toluene solution. The interlayer froth of toluene and water was flowed through an anhydrous sodium sulfate column to remove water. Then, the oil products (toluene-soluble fraction) were recovered using a rotary evaporator at 50 °C and 6000 Pa. The maltene (Heptane-soluble fraction) and asphaltene (Heptaneinsoluble and toluene-soluble fraction) in the oil products were extracted using the method described in ASTM Standard D 6560. For the SARA analysis, the maltene was further divided into saturates, aromatics, and resins by washing with different solvents in an open glass column, which was placed using neutral alumina and silica gel, based on ASTM Standard D 2007.45 The yields of oil products and coke were evaluated as percentages based on the weight of products and feedstock. The weight-average molecular weight (Mw) and molecular weight (MW) distribution were analyzed at 40 °C with tetrahydrofuran using

3. RESULTS AND DISCUSSION 3.1. Yield Distribution of Asphalt Upgrading in SCW. Table 2 reports the yield distribution of the products after the SCW treatment. First, the products were separated into gas, oil products (toluene-soluble fraction), and coke (tolueneinsoluble fraction). The reproducibility error in the total yield of coke and oil was less than 3%. The toluene-soluble fraction was further divided into asphaltenes and maltenes (resins, aromatics, and saturates). As observed, the coke yields increased with the increase in reaction temperature, whereas the oil product yields rapidly decreased. Obviously, the cracking behavior at 390 or 400 °C was different from that at 410−450 °C. In the low-temperature region (at 390 or 400 °C), the yields of all types of products did not significantly change compared to that of the feed, and the conversion of asphaltenes and maltenes was weak. However, the upgrading reaction of asphalt proceeded intensely when the temperature was higher than 400 °C. After cracking in SCW, the combined yield of coke and asphaltene ranged from 18.8% to 22.8%, which was approximately the content of asphaltene in the feed asphalt. Similar results were observed in bitumen upgrading with SCW treatment at 360−420 °C,9 indicating that the asphaltenes were not only upgraded to maltenes but also converted to undesirable coke. When the reaction temperature was higher than 430 °C, the conversion of asphaltenes was almost complete. The maximum yield of coke was 19.5% at 430 °C. Furthermore, when the reaction temperature increased from 390 to 450 °C, the total yield of coke and oil decreased from 1470

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Energy & Fuels 97.2% to 56.5%, and the maltene yield decreased from 76.8% to 37.7%. The increased mass loss of recovered products mainly resulted from the generation of gaseous products or low boiling-point compounds, which were lost during the recovery process. 3.2. Oil Product Analysis after the Asphalt Upgrading in SCW. In the asphalt-cracking process, the coke formation has a positive effect on the H/C ratio as a carbon-rejecting process. Moreover, the H/C ratio in oil products is affected by the gas yield and composition, and the generation of hydrogen and hydrocarbon gases removed the fractions with high H/C ratios. The composition of gaseous products is shown in the Supporting Information. As shown in Figure 2, the H/C ratio

Figure 3. GPC chromatograms of asphaltenes after the cracking in SCW.

Figure 4 shows the TG analysis of the toluene-soluble fractions, and the DTG plot is present in the Supporting

Figure 2. Molecular weight (Mw) and H/C ratio of oil products after the cracking in SCW.

reached a maximum value of 1.44 at 430 °C, and the coke yield reached the maximum value at an identical reaction temperature. When the temperature was higher than 430 °C, the generation of gases and the decline of coke yield led to the decrease of H/C ratio in oil products. In the GPC process, the measured value was a combination of molecular weight and molecular aggregation, as well as a monomer.20,46 The Mw of asphaltenes, maltenes, and toluenesoluble fractions decreased with the increasing reaction temperature, as shown in Figure 2. In addition, the numberaverage molecular weight is shown in the Supporting Information. The decreasing rate of Mw became slow in the high-temperature region. At 400 °C, the Mw of asphaltenes and maltenes decreased by 33.8% and 57.5%, respectively. However, compared to the feed asphalt, the yields of asphaltenes and maltenes decreased by only 9.3% and 3.4%, respectively. The cracking of asphaltene molecules in the upgrading process or the disassembly of aggregates decreased the molecular weight measured by the GPC process, whereas most of the cracked structures were still classified as asphaltene based on the solubility property. The GPC chromatograms of asphaltenes are presented in Figure 3. The MW distributions of asphaltenes obtained at 390 °C are notably similar to those of the feed asphalt, particularly with a MW higher than 5000. The upgrading reaction of asphaltenes at 390 °C was weak. Then, the structures with MW > 2000 were almost broken, and the MW distributions remained stable when the reaction temperature was higher than 430 °C.

Figure 4. TG analysis of the toluene-soluble fractions obtained after the cracking in SCW.

Information. The weight loss rate of the oil products increased with increasing reaction temperature. Furthermore, the TG curves of the weight loss rate and mass residue were not distinguishable when the reaction temperature was higher than 420 °C. At a TG analysis temperature of 350 °C, the weight loss of feed asphalt was only 3%, whereas the weight loss of oil products obtained at the upgrading temperature of 450 °C was greater than 50%. When the TG analysis temperature was higher than 550 °C, the weight was stable, and the proportion of residue was defined as the carbon residue (according to ASTM Standard D 4530). It is noteworthy that the carbon residue in the oil products obtained at 390 and 400 °C was 21.2%, which is higher than that in the feed asphalt. The carbon residue decreased with the increasing reaction temperature and reached the lowest value of 5.3% at 450 °C. 1471

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Energy & Fuels At 60 °C, the feed asphalt remained in a semisolid state, with a high dynamic viscosity of 264.7 Pa·s. The viscosities of the toluene-soluble fractions after the SCW treatment are shown in Figure 5. The oil products obtained at 390 and 400 °C

remained in a semisolid state similar to that of the feed asphalt, and their viscosities were both higher than 100 Pa·s. When the temperature was higher than 400 °C, the oil products became fluid, and the viscosities were less than 1.2 Pa·s and even 0.205 Pa·s at 450 °C. Consequently, the viscosity breaking of the oil products was efficient at a high upgrading temperature. 3.3. Nontreated Emulsion Analysis. After the upgrading in SCW, the formed emulsions in the oil-rich phase were observable using a microscope, as shown in Figure 6. In the oilrich phase, the background color mainly represents the maltenes. The dispersed and emulsified water droplets were covered with a black and rigid film. Moreover, the formed coke particles or aggregated asphaltenes were concomitant in the water-in-oil emulsions. At 390 or 400 °C, there were few water droplets in the oil-rich phase. The relative contents of asphaltenes exceeded 20% in the oil products obtained at 390 or 400 °C. As previously reported, asphaltenes are considered natural surfactants with interfacial properties to stabilize the emulsion.47 To explain the few water droplets in the emulsion with high concentration of asphaltenes, the viscosity and solubility in the oil products should be considered. As discussed in section 3.2, the oil products obtained at 390 and 400 °C remain in a semisolid state with a viscosity above 100 Pa·s; hence, asphaltenes with poor fluidity in the oil products have fewer chances to adhere to water. In addition, the resin/ asphaltene ratios in Table 3 indicate the solubility of asphaltene in the oil products. The oil phase with a low r/a ratio usually

Figure 5. Dynamic viscosity of the toluene-soluble fractions obtained after the cracking in SCW.

Figure 6. Microstructure of the (a) oil-rich phase and (b) water-rich phase prepared without demulsification. 1472

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Energy & Fuels Table 3. Characteristics of the Oil Products and Water-in-Oil Emulsions

a

reaction temperature (°C)

feed

390

400

410

420

430

440

450

viscosity of oil product (Pa·s) resin/asphaltene (r/a) ratioa asphaltene/maltene (a/m) ratiob water content of NT-emulsion (wt %) water content of C-emulsion (wt %) water content of D-emulsion (wt %) Dmaxc of NT-emulsion (μm) Dmax of C-emulsion (μm) Dmax of D-emulsion (μm)

264.7 1.29 0.276

201.1 1.27 0.255 0.1 0.1 0.8 3.7 2.4 3.4

104.3 1.26 0.259 6.3 4.5 6.1 6.6 3.4 7.2

1.138 1.72 0.138 42.4 15.4 19.8 7.2 13.9 16.7

0.515 3.26 0.073 36.5 20.6 12.2 9.5 17.6 21.9

0.380 3.93 0.058 39.2 20.4 7.0 10.4 19.0 17.0

0.231 6.71 0.034 32.5 20.9 2.5 18.0 21.1 17.7

0.205 8.36 0.029 10.9 3.8 1.4 41.9 64.8 17.9

“r/a” is the resin/asphaltene ratio. b“a/m” is the asphaltene/maltene ratio. cMaximum diameter of water droplets in water-in-oil emulsions.

The DSC thermograms of the noncentrifuged emulsions are partially presented in Figure 8 to better distinguish the

has poor solubility of asphaltene, and the emulsified oil−water interface is difficult to migrate or coalesce. In the water-rich phase, the dispersed and emulsified oil droplets were observed only at 440 and 450 °C, as shown in Figure 6. During the microscope observation, the oil droplets moved with Brownian motion. In the water-rich phase that was prepared below 440 °C, the droplets or Brownian motion were not observed through the optical microscope. The emulsified oil at 440 °C was dispersed as single droplets in the water phase; however, at 450 °C, the oil droplets partially clustered. Figure 7 shows that the pH of the water phase before filtration

Figure 8. DSC thermogram of the nontreated (NT) emulsions with a cooling−heating rate of 5 °C/min.

differences of the emulsions prepared at various reaction temperatures. In the DSC process, water droplets were crystallized in the cooling process from 50 °C to −60 °C; then, the crystallized water was heated for melting. According to a previous work,44 the only Gaussian-shaped peak in the melting process was selected to deduce the water content of emulsion. The asymmetrical exothermic peaks in the crystallization process are considered the product of several individual Gaussian-shaped peaks, each of which reflects water droplets with a similar size, which can provide information about the droplet size distribution. In conclusion, the higher freezing temperature represents larger water droplets. As shown in Figure 8, the NT-emulsion prepared at 390 °C showed small peaks in the DSC process, which is similar to the pure oil product with the DSC behavior. When the temperature was higher than 400 °C, the melting peak intensity decreased with the increasing temperature, whereas the initial freezing temperature increased. In the crystallization process, a stable and distinguishing peak always appeared at approximately −37 °C, which indicates that all of the fine droplets with diameters less than 4 μm may be solidified between −35 and −39 °C. The water content and maximum diameter, which were deduced from the DSC analysis, are shown in Table 3. In the NTemulsion, the maximum diameter of the water droplets

Figure 7. TOC content and pH value of the filtered water phase and the pH value of the water-rich phase with dispersed oil.

ranged from 2.6 to 3.4. After filtration, the pH value increased with the removal of the acidic dispersed oil, particularly at high upgrading temperatures. The highest water-soluble TOC content was found at 400 °C, a value of 756 mg/L. The alicyclic groups that were connected to the asphaltene core were probably broken and dissolved in the water at the relatively lower temperature, which also resulted in the decline of H/C ratio at 400 °C in Figure 2. Moreover, the water-soluble TOC content decreased markedly when the upgrading temperature was higher than 430 °C. The dispersed or soluble alicyclic acids provided the acidic environment of the water-rich phase. Subsequently, the alicyclic groups in the water were gathered by the formed oil droplets at high temperature, which sharply decreased the water-soluble TOC content. To evaluate the water content and droplet size distribution in the water-in-oil emulsions, the DSC analysis was conducted. 1473

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Energy & Fuels

Figure 9. DSC thermograms of the (NT) nontreated, (C) centrifuged and (D) demulsified emulsions with a cooling/heating rate of 5 °C/min.

increased from 3.7 to 41.9 μm when the reaction temperature increased from 390 to 450 °C, indicating that the water droplets aggregated and coalesced to form larger droplets. 3.4. Comparison of Demulsification Methods. Figure 9 shows the comparison of the nontreated, centrifuged, and demulsified emulsions obtained at 390, 410, 430, and 450 °C in the DSC analysis; moreover, the DSC thermographs of emulsions obtained at 400, 420, and 440 °C are presented in the Supporting Information. After centrifugation, the melting peak intensity decreased, which indicates the water removal from the water-in-oil emulsions. In particular, most crystallization peaks at high freezing temperature (higher than −35 °C) disappeared, but the initial freezing temperature increased. The intensity of crystallization peaks between −44 and −35 °C decreased in the order NT-emulsion > C-emulsion > Demulsion. It is indicated that the chemical demulsification was more effective than centrifugation in water removal of small droplets. After chemical demulsification, the crystallization peaks at −22.4 °C were enhanced significantly at 410 °C. When the temperature was higher than 400 °C, the initial freezing temperature in D-emulsions ranged from −21.3 to −22.4 °C, which was more stable than the NT- and C-emulsions. The water content and maximum diameter of the water droplets in NT-, C-, and D-emulsions are shown in Table 3. The r/a ratio increased from 1.26 to 8.36, which enhanced the solubility of asphaltene in the oil product. The rigid asphaltene film at the oil−water interface became thin and could be replaced by a continuous open network of resins and demulsifiers,36 which led to a high coalescence probability of water droplets during the flocculation stage in the emulsions

with high r/a ratio. As shown in Figure 10, the maximum diameter of water droplets increased with the increase of r/a ratio in NT- and C-emulsions. When the temperature was higher than 400 °C, the maximum diameter in the C-emulsion was greater than that in the NT-emulsion, indicating that the coalescence of droplets was accelerated in the centrifugation process and eventually formed a bulk aqueous phase. The

Figure 10. Maximum diameter as a function of the r/a ratio for the nontreated (NT), centrifuged (C), and demulsified (D) emulsions obtained at various upgrading temperatures. 1474

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Energy & Fuels

Figure 11. Microstructure of the centrifuged (C) and demulsified (D) emulsions.

coalesced large droplets were much easier to remove than were the fine and emulsified water droplets. On the other hand, the maximum diameter in D-emulsions ranged from 16.7 to 21.9 μm at 410−450 °C. The water droplets with diameter greater than 21.9 μm might have been removed into a bulk aqueous phase in the presence of a demulsifier. The microstructures of the C- and D-emulsions are shown in Figure 11. The number of observed water droplets decreased after centrifugation or chemical demulsification. In C-450 emulsion, the water droplets with diameter below 10 μm were much less than the C-410 or C-430 emulsions, whereas a large droplet with diameter of ∼25 μm was observed to adhere into the coke particles. Compared to NT-410 and C-410 emulsions, the water droplets with diameter of ∼15 μm appeared in the D410 emulsion. The droplet−droplet coalescence was enhanced by the chemical demulsifier. When the temperature was higher than 420 °C, the large droplets (>10 μm) in D-emulsions were not found by optical microscopy. The observed microstructures of water-in-oil emulsions are consistent with the DSC analysis in Table 3. The water content and water droplet size distribution of the NT-, C-, and D-emulsions are shown in Figure 12. At an upgrading temperature of 390 or 400 °C, the viscosity was the main factor that affected the water content of the emulsions. The oil products with high viscosity, poor fluidity, and few coke particles had a water content below 6.3%. When the upgrading temperature was higher than 400 °C, the fluidity of the oil products was not the main problem in the emulsification. The water content of the emulsions decreased with increasing reaction temperature, but the proportion of droplets with diameter greater than 5 μm increased. In the centrifugation process, the removal of water droplets was limited. At 410−440 °C, the contents of centrifuged water droplets with diameter below 5 μm remained at 14−17%. The mechanical centrifugation did not sufficiently remove the fine water droplets with diameters less than 5 μm. After SCW upgrading, the oil-rich phase maintained an emulsion state, despite gravity settling or centrifugation.

Figure 12. Water content and water droplet size distribution of the nontreated (NT-), centrifuged (C-), and demulsified (D-) emulsions after upgrade in SCW.

After demulsification with chemical demulsifier addition, the water content reduced sharply in comparison to NT-emulsion at 410−450 °C. The water content in the D-440 emulsion was as low as 2.5%, indicating that 92.3% of water in the NT-440 emulsion was removed. Obviously, the water content in Demulsions was much lower than that in C-emulsions when then upgrading temperature was higher than 410 °C. The demulsifier was adsorbed to the oil layer between water droplets and then provokes the emulsion breakup and coalescence of water droplets.48,49 In particular, a large amount of water droplets with diameter greater than 10 μm appeared in D-410 and D-420 emulsions, while those droplets (D > 10 μm) were not observed in NT-emulsions at 410 and 420 °C. Some coalesced droplets were still trapped in the emulsions at 410 1475

DOI: 10.1021/acs.energyfuels.6b03126 Energy Fuels 2017, 31, 1468−1477

Article

Energy & Fuels and 420 °C. In addition, the total water content in D-emulsions decreased with increasing temperature, which might correspond to the decreased a/m ratio, as shown in Table 3. The chemical demulsifier was much easier to replace the asphaltene film in the emulsion with lower a/m ratio, resulting in the increased water removal efficiency in the D-emulsions. 3.5. Overall Effect of the Upgrading Reaction on Emulsification. The emulsification characteristics were mainly controlled by the properties of the upgraded oil products. At low upgrading temperature (at or below 400 °C), the formed coke was negligible, and over 95% of the products were toluene-soluble fractions (Table 2). The viscosities of the toluene-soluble products were higher than 100 Pa·s, and the oil products remained in a semisolid state at ambient temperature. The asphaltene yield was close to that in the feed, whereas the molecular weight decreased rapidly (Figure 2), which indicates that the cracking reaction might decrease the number of fused rings in one molecule at low temperature. Despite the high concentration and interfacial activity of asphaltenes, the poor fluidity of the oil product prevented its contact at the water−oil interface in the emulsification process. At a low upgrading temperature, the high viscosity of the oil products is the main factor in the emulsification. When the viscosity was lower than 1.2 Pa·s, the fluidity of the oil products had limited effect on the emulsification. When the upgrading temperature was higher than 400 °C, the intense conversion of asphaltenes and maltenes led to high yields of coke and gases. The relative content of asphaltene in the oil products decreased with increasing temperature, whereas the solubility of asphaltene in the oil products increased. As the main strong surfactant, the cracked asphaltenes stabilized the water-in-oil emulsion by cohering on the water droplets. However, the excellent solubility of asphaltene in the oil phase with high r/a ratio promoted the coalescence of water droplets. Hence, the water content of small droplets (49%), H/C ratio (>1.4), MW (