Pressure Effect on the Rheological Behavior of Waxy Crude Oil with

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Materials and Interfaces

Pressure Effect on the Rheological Behavior of Waxy Crude Oil with Comb-type Copolymers Bearing Azobenzene Pendant Tao Li, Tongshuai Wang, Jun Xu, Run Zou, Zhongye Si, Julian Becker, Li Li, Martien Abraham Cohen Stuart, Robert K Prud'homme, and Xuhong Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05217 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Industrial & Engineering Chemistry Research

Pressure Effect on the Rheological Behavior of Waxy Crude Oil with Comb-type Copolymers Bearing Azobenzene Pendant

Tao Li1, Tongshuai Wang2, Jun Xu1*, Run Zou1, Zhongye Si1, Julian Becker3, Li Li1, Martien A. Cohen Stuart1, Robert K. Prud’homme4, Xuhong Guo1,5*

1

State Key Laboratory of Chemical Engineering, East China University of Science

and Technology, Shanghai 200237, China 2

Department of Chemical Engineering, University of Illinois at Chicago, Chicago,

Illinois 60607, USA 3

Department of Process Engineering, Nuremberg Institute of Technology, Nuremberg

90489, Germany 4

Department of Chemical Engineering and Princeton Materials Institute, Princeton

University, Princeton, New Jersey 08544, USA 5

Engineering Research Center of Materials Chemical Engineering of Xinjiang

Bingtuan, Shihezi University, Xinjiang 832000, China

*To whom correspondence should be addressed. E-mail: [email protected] (Jun Xu) or [email protected] (Xuhong Guo) 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this work, a pressure responsive poly(α-octadecene -co- maleic acid azobenzene amide) (Azo-MAC) was synthesized and able to enhance the flowability of waxy crude oil with asphaltenes significantly under high pressure. High-pressure UV-Vis spectrometer was used to characterize its pressure response under pressures. The onset temperature and enthalpy during wax crystallization of Liaohe waxy crude oil under various pressures were determined by using high-pressure differential scanning calorimeter. The viscosity and yield stress were measured by high-pressure rheometer. Based on our experimental data, a mechanism was propounded that the conformation transformation from Cis to Trans of azobenzene groups in Azo-MAC at enhanced pressure may destroy the assembly of asphaltenes, disturb the crystallization of paraffins and thus improve the flowability of oils. Azo-MAC should be an ideal additive for exploitation and transportation of oils under high pressure.

Keywords: Waxy crude oil, flowability, pressure responsive polymer, wax crystallization.

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1. Introduction Long chain paraffins in oils begin to deposit as wax crystals when the temperature reduces under the wax appearance temperature (WAT).1 The formation of wax crystals may give rise to either a reduction of oil flow or a clogging of pipelines. Both scenarios cause the pumping system to work at higher pressure.2 In fact, crude oils are usually stored in the reservoir or transported in pipelines under high pressures. In the case of deep oil spills, the surrounding pressure is of even greater concern.3-8 Due to the limitations of current measuring methods, few work on the flowability of oils at high-pressure is reported.9, 10 As reported in a literature, under high pressure, the oils show persistence and the moduli at rest recover rapidly.11 It reveals that high-pressure not only results in the high viscosity of oils, but leads to typical non-Newtonian behaviors. So far, the cold flowability of oils and the efficacy of chemical additives have been intensively researched at atmospheric pressure, including the rheological properties and the crystallization behaviors of waxes.12-15 However, under high pressure the effect of polymeric flow modifiers on the flowability of oil was rarely investigated. Under normal pressure, polymeric additives, acting as pour-point depressants (PPDs) or wax modifierscan incorporate themselves with crystallized wax, disturb the wax formation process by preventing the interconnecting of crystalized wax into large agglomerates.16-19 Various kinds of polymers have been applied to modify the cold flowability of oils, such as the copolymer of ethylene and vinyl acetate (EVA),20, 21 copolymer

of

ethylene

and

butene

(PE-PEB),6,12,17

3


the

derivatives

of


poly(styrene-maleic anhydride alkyl esters), as well as poly(α-olefins-co-maleic anhydride) or poly(α-olefins-co-acrylic acid) modified by long-chain alcohols or amines.13, 14 It is found that the appropriate pendants in molecular structure of these polymers, are very important to improve the rheological property of oils.22 Copolymer which possesses aniline pendant is more efficient to modify the rheological properties of oils than that with 1-naphthylamine pendant due to a lower steric hindrance with backbone.23 If soft aliphatic spacer is introduced between the backbone and phenyl pendant, the steric hindrance will be impaired. The polymer with a long aliphatic spacer reduces the viscosity and yield stress more than that with a short one. This can be explained with the phenyl pendant in the former one, which can better assemble with asphaltene molecule and disperses the waxes in oils. However, when the pressure is increased, the molecules movement of the most copolymers would be inhibited leading to diminish the interaction with wax crystals or asphaltenes. Therefore, it is important to investigate on the pressure effect on the performance of the additives in oils. Azobenzene and its derivatives are some of the most typical representative of photo-sensitive groups because of the invertible photoisomerization.24 Additionally, mechanical force can also induce conformation shift between Cis and Trans of azobenzene and its derivatives.25-27 If azobenzene is introduced as a pendant of copolymer addictive, when environmental pressure changes, it will change the assembly behavior between copolymer and asphaltene as well as paraffin crystals in oils. 4


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In this study, a pressure responsive polymer, poly(α-olefin - maleic acid azobenzene amide), was synthesized. The flowability of Liaohe waxy oil before and after adding polymeric flow improvers under various pressures was investigated with the help of rheometer equipped with high pressure parts (HP-rheometer). High-pressure differential scanning calorimetry (HP-DSC) was employed to explore the influence of polymers on the crystallization behaviors of Liaohe waxy oils. In order to study the conformation shift of the azobenzene group under pressure environment, high-pressure UV-Vis spectrometer was also employed. Based on those measurements, a feasible mechanism was put forward to reveal the effect of azo group on the interaction between copolymers with the asphaltenes and paraffin crystals in waxy oils under pressures.

2. Experimental 2.1 Materials Decane (anhydrous, 99%), anhydride maleic (99%), α-octadecene (90%), benzoyl

peroxide

(99%)

and

o-xylene

(98%),

aniline

(97%),

1-methylnaphthalene(97%) were bought from Acros Company and used without father purification. Aminoazobenzene (98%) and chloroform-d (99%) were purchased from J&K Scientific Company as the solvent for the 1H NMR measurements. Toluene (99%) and methanol (99%) were bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. Crude Oil sample is provided by Petroleum Liaohe Oilfield Company 5



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( under 70 °C, density: 0.8676 g/cm3, freezing point: 54 °C, viscosity: 7.2 mPa.s). The contents of paraffins and asphaltenes in this oil are ca. 39 wt% and 16 wt%, respectively. The water content is 0.5 wt%. The metal content is listed in the Table S1. The pour point and WAT are 50.2 and 52.6 ℃, respectively. No sediment has been detected in the crude oil.

2.2 Synthesis of copolymers Maleic anhydride copolymers with azobenzene pendant were synthesized by radical polymerization similar to our previous work (Figure 1).28,

29

At first, the

copolymerization of a-olefin and maleic anhydride was performed at 120 C in nitrogen for 1.5 hours, with 0.1 wt% of dibenzoyl superoxide (initiator) and o-xylene(solvent). Then, aminoazobenzene was introduced in the state of reflux for 15 hours. For purification, obtained copolymer precipitated in methanol and following filtration. The final product poly(α-olefin -co- maleic acid aminoazobenzene amide) (Azo-MAC) was acquired after vacuum drying for 12 hours. For comparison, two copolymers, maleic anhydride copolymer with phenyl pendant (AMAC) were synthesized according to the method that we previously reported (Figure 1b).[17,23]

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Figure 1. Chemical structures of copolymers. (a) copolymer ofα-olefin and maleic anhydride; (b) poly(α-olefin -co- maleic anhydride aniline) (AMAC); (c) poly(α-olefin -co- maleic anhydride aminoazobenzene) (Azo-MAC).

2.3 Characterization of Azo-MAC The synthesized Azo-MACs were dissolved in deuterated chloroform solution and 1H NMR spectra was obtained by using Bruker Advance 500 spectrometer. FTIR spectroscopy (Tensor 27 from Bruker Optics, Stockholm, Sweden) was employed to analyze the structure of Azo-MAC copolymer. To correct the baseline of IR spectra, straight lines were drawn from 4000 to 1800 cm-1 and from 1800 to 610 cm-1. The copolymers molecular weights were measured by GPC (Waters 1525; Mobile phase: Tetrahydrofuran; Flow rate of 1.0 mL/min). The calibration curve of GPC is operated by poly(methyl methacrylate) (PMMA) standards.

2.4 High-pressure UV-Vis spectroscopy 7



UV-Vis spectra were collected by UV-2550 UV−vis spectrometer (Shimadzu), equipped with a high-pressure accessory which contains a quartz sample cell with 1 cm optical path. Before each measurement, the sample needs to be placed into the cell and repeated addition of nitrogen at least three times (Figure S1). All the measurements were performed at room temperature.

2.5 Rheological measurements The rheological properties were determined by the MCR501 rheometer (Anton-Paar Physical, Austria) with a 20 mm diameter parallel-plate. A magnetic coupling connecting to the torque measuring system drives the geometry in the pressure cell. The booster pump from Shenzhen Telide Fluid Transmission System Co., Ltd. was employed to increase the pressure (Figure 2). Compressed air supported the driving force for elevate the pressure of the nitrogen in the measuring cell and the air bearing of rheometer, which was produced by the OF302-25M oil-less rocking piston air compressor from Junair (America). The viscosity measurements under UV-Vis irradiation were operated by the MCR302 rheometer (Anton-Paar Physical, Austria) with a 25 mm diameter parallel-plate. The UV light source was driven by OmniCure Series 1500 which contains two kinds of optical filter (365 nm and 500 nm).

8


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Figure 2. Scheme of the high-pressure rheological measurement system.

Under specified pressure, at a constant shear rate of 10 s-1, the viscosity change of oil with temperature was determined by cooling from 70 °C to 45 °C in 1 ℃/min at a same shear rate (10 s-1) . Oil shows Newtonian behaviors at the temperature over WAT. Heavy waxes will crystallize at the temperature lower than WAT, and the oils rheological behavior is non-Newtonian. The correlation between temperature and viscosity above the WAT is described by:30 -

Ea

η  A  e RT

(1)

where A is a constant. Ea is activation energy, which represents the friction between fluid molecules. R calls the universal gas constant which equals 8.314 J/mol·K. During extraction and pipeline transportation, when the crude oil were cooled down and stopped flowing, it is necessary to restart the flow with a minimum differential pressure. At restart time, it is vital to know the distribution of the pressure 9



and yield stress along the pipe.31, 32 During the yield stress measurements, oils were warmed to 80 oC hold for 30 minutes and finally cooled down to 50 oC with 10 o

C/min under specified pressure. When the temperature is invariant, tests were

performed with shear stress increasing logarithmically in the range of 0.01 Pa to 1000 Pa. Average data of three tests were reported.

2.6 High-pressure differential scanning calorimeter (HP-DSC) A differential scanning calorimeter (DSC) (TA Q2000, TA Instruments, USA) installed with a pressure cell was employed to measure the crystallization of the waxy component in crude oil. A cooling system, which consists of a homemade cooling coil and a low temperature cooling circulating pump (Yuhua instrument co., lid, China), was employed to realize a linear cooling rate for the pressure cell. Samples measured for HP-DSC are similar to those were measured for rheological tests.

3. Results and Discussion 3.1 Characterization of copolymer 1

H NMR spectra of Azo-MAC is shown in Figure 3. The peaks of the protons in

CH3, CH2 and CH in α-octadecene monomer unit appear around 0.9, 1.5 and 2.0 ppm. Peaks at the range of 6.7~8.2 ppm are assigned to the azobenzene protons. Amidation ratio (f) is defined as the amine group number which react with each maleic anhydride unit. It was calculated by integrating peaks areas of proton of benzyl (Aazobenzene) and 10


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methyl groups (ACH3) and listed in Table 1. Azo-MAC2.0 and AMAC2.0 which have close amidation ratios were mainly used in this work and abbreviated as Azo-MAC and AMAC if there is no specific mention.

Figure 3. Structure and 1H NMR spectrum of Azo-MAC copolymer.

Table 1. Feeding rate of monomers and f of Azo-MAC and AMAC Feeding Rate of Maleic f

Copolymer Anhydride/Amine Azo-MAC1.0

1:1

0.38

Azo-MAC2.0

1:2

0.73

AMAC1.0

1:1

0.58

AMAC2.0

1:2

0.83

On the FTIR spectrum in Figure 4, quite a few vibration bands can be found at 3380 cm-1 (-NH stretch), 2852 and 2923 cm-1 (-CH stretch), 1710 cm-1(C=O stretch), 11



1599 cm-1 (-N=N-) and 1466 cm-1 (-CH2- bending). The peak at 1407 cm-1 reflects the C–C stretching in the phenyl rings. The results of the FTIR coincide with those of the 1

H NMR spectrum, which indicates that Azo-MAC copolymer was synthesized

successfully.

Figure 4. FTIR spectrum of Azo-MAC copolymer.

The molecular weights of Azo-MAC and AMAC copolymers were determined by GPC (Table S2). The weight average molecular weights (Mw) of Azo-MAC and AMAC are 9423 and 7991 g/mol, respectively.

3.2 Effect of pressure on viscosity and WAT In Figure 5, crude oil viscosity with and without appearance of copolymers increased upon increasing pressure from atmospheric one to 10 MPa. Both AMAC and Azo-MAC reduced the oil viscosity under normal and enhanced pressures, while Azo-MAC performed more effectively on reducing the viscosity of crude oil than 12


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AMAC. WAT is the temperature at which the change in viscosity deviates from linearity during reducing temperature.33, 34 The influence of hydrostatic pressure on WAT of oils is evaluated by comparing viscosity of crude oil under various pressures (0.1, 6 and 10 MPa) (Figure 5a). At atmospheric pressure, the WAT of crude oil is around 56.2 oC, while increased to 56.6 oC under 6 MPa and further increased to 57.0 oC under 10 MPa, indicating that the WAT is increased with the increase in pressure. The ramp rate of WAT upon changing pressure is approximately 0.1 oC/MPa. It seems that the packing regularity of paraffin chains which form the crystalline region is slightly enhanced when pressure is increased, and thus enhances the WAT. The melting temperature (Tm) of n-paraffins consist of C12 to C28 and pressure (P) are in the direct ratio, and dTm/dP is ranging from 0.022 to 0.26 oC/MPa, order of our result (0.1 oC/MPa).

13


35-37

which is in the same


Figure 5. Viscosity-temperature relationship of oils before and after adding copolymers under normal pressure, 6 MPa and 10 MPa. (a) Crude Oil, (b) Crude Oil + 0.5 wt% AMAC, (c) Crude Oil + 0.5 wt% Azo-MAC.

The influence of hydrostatic pressure on WAT of oils in the after adding AMAC and Azo-MAC with the concentration of 0.5 wt% was evaluated under different pressure (Figure 5b and c). The WAT of the oil before and after adding AMAC and Azo-MAC were compared and listed in Table S3. It shows that AMAC is capable of reducing the WAT by 1.2 oC at normal pressure, but the degree of reduction reduced with the increase of pressure. At 10 MPa, the degree of reduction is the half of that at normal pressure. Interestingly, Azo-MAC reduced the WAT by 2.9 oC at normal pressure. With the increase of pressure, the degree of reduction of WAT enhanced to 3.1 oC at 6 MPa and further increased to 3.3 oC at 10 MPa. As we know, the phenyl ring in AMAC has no conformation transformation when pressure is enhanced.28,

29

But azobenzene group in Azo-MAC shows a

transformation between Trans and Cis by changing pressure.27 Under low pressures, the two phenyl rings in the azobenzene group are in Trans conformation and locate in a plane. At enhanced pressure, azobenzene group can transform to Cis conformation, where the two phenyl rings will not be in a plane and form an angle. The conformation change of azobenzene group in Azo-MAC may disturb the assembly of asphaltene molecules and waxy crystallization in oils, and thus reduce the viscosity 14


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and WAT of waxy crude oil more effectively than that of the AMAC.

3.3 Effect of pressure on yield stress In Figure 6, it is found that the yield stresses of crude oils with and without addition of copolymers increased monotonically upon increasing pressure. The reduction of yield stress by AMAC and Azo-MAC are listed in Table 2. Although both copolymers reduced the oil yield stress dramatically, the performance of Azo-MAC is much better than that of AMAC, especially at higher pressures. In Figure 6, the growth rate of yield stress with pressure is smaller for Azo-MAC than for AMAC, which implies that at enhanced pressure Azo-MAC becomes more effective in reducing the yield stress of oils compared to AMAC.

Figure 6. Yield stress of oils before and after adding Azo-MAC and AMAC under various pressures at 50 oC. 15



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Table 2. Yield stress reduction of oils after adding AMAC and Azo-MAC copolymers under various pressures Pressure

Reduction of yield stress (Pa)

Reduction percentage (%)

(MPa)

AMAC

Azo-MAC

AMAC

Azo-MAC

0.1

102 ±3

121 ±3

69

83

2

201 ±11

201 ±11

70

90

4

165 ±16

312 ±16

47

89

6

382 ±18

714 ±9

50

93

8

664 ±21

1262 ±17

50

95

10

776 ±17

1477 ±11

50

95

This observation consists with the finding in WAT measurements, which seems to confirm that the Trans conformation of azobenzene unit in Azo-MAC is changed to Cis one under high pressure helps to disturb the assembly of asphaltenes and wax crystals parking and to modify the flowability of oils.

3.4 Effect of pressure on thermodynamic behavior The thermodynamic behavior of oils was observed by DSC (Figure 7). DSC curves of the oils during cooling before and after adding copolymers were similar, while the onset temperature Tonset and enthalpy of wax crystallization of crude oil (large peak around 45 C) reduced upon increasing pressure. Compared to crude oil, the decreases in Tonset and enthalpy in the present of AMAC and Azo-MAC were listed in Table S4. Obviously, the efficiency of 16


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Azo-MAC in reduction of both Tonset and enthalpy is higher than that of AMAC. Again, Azo-MAC exhibited a better performance than AMAC here.

Figure 7. DSC thermograms of oils before and after adding polymers. (a) Crude oil, (b) Crude oil + 0.5 wt% AMAC, (c) Crude oil + 0.5 wt% Azo-MAC.

3.5 Possible Mechanism In order to understand the rheology of waxy oils after adding Azo-MAC at enhanced pressure, a possible mechanism was proposed as shown in Figure 8. Based on the molecular simulation, the interaction energies (Eint) between the model asphaltene molecule (Figure S2a) and trans-azobenzene (Figure S2b) and cis-azobenzene (Figure S2c) are -15.4 and -16.1 Kcal/mol, respectively, which are higher than that with phenyl ring (-9.6 Kcal/mol). It implies that the azobenzene groups prefer to assemble with asphaltene molecules spontaneously compared to 17



phenyl groups, and azobenzene group in Cis conformation has a higher adsorption strength than that in Trans conformation.

Figure 8. The possible mechanism to modify the flowability of waxy oils by Azo-MAC at enhanced pressure.

The azobenzene group in Azo-MAC is capable of transforming its conformation reversibly between Trans (at low pressure) and Cis (at high pressure) by changing pressure.27 Azobenzene group in Cis conformation endows itself a higher thickness than that in Trans conformation (Figure 8). The thickness of azobenzene is increased to 3.06 Å if one of phenyl rings is taken as a reference plane. This thickness enhancement of azobenzene group enlarges the distance between asphaltene molecules if they are assembled with asphaltenes by - stacking. As a result, the dispersion of asphaltenes will be improved, and the crystallization of long chain 18


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paraffins also be disturbed because the dispersed asphaltenes can act as natural wax crystallization inhibitors. The reduced amount and regularity of wax crystals lead to the reduction in Tonset and enthalpy as determined by DSC. Both the improved dispersion of asphaltenes and depressed wax crystallization help to modify flowability of oils as determined by rheology.

Figure 9. UV-vis adsorption of Azo-MAC toluene solution (100 mg/ml) as a function of pressure.

To confirm the conformation change of Azo-MAC with pressure, a high-pressure UV-vis spectroscopy was applied to investigate the isomerization of azobenzene moieties changing with the pressure. As shown in Figure 9, upon increasing pressure from 0.01 to 5 MPa, the adsorption at 346 nm is remarkably 19



weakened and the adsorption at 442 nm slightly enhanced, which are ascribed to the p–p* and n–p* transitions of azobenzene group, corresponding to the conformation changing from Trans to Cis.38 It indicates that the azobenzene moieties of Azo-MAC show pressure responsive whose conformation can transform between Trans and Cis by changing pressure. To further confirm the proposed mechanism on the correlation between Azo-MAC conformation change and the flowability of waxy oils, viscosities of oils with polymeric additives after UV irradiation with two different wavelengths were compared (Figure 10).

Figure 10. (a) Relationship between viscosity and temperature for crude oil before and after adding 0.5 wt% of AMAC, and Azo-MAC under two kind of UV light irradiation (dark, 500 nm and 365 nm). (b) The compare of the WAT value under different condition.

In Figure 10, oil viscosity after adding 0.5 wt% Azo-MAC shows a significant 20


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decrease after exposed in UV light with 365 nm. The azobenzene group can partly transform its conformation from Trans to Cis under 365 nm UV light, while the conformation is not changed after irradiation of UV light with 500 nm (Figure 9). Indeed, flowability of waxy oils was modified if the conformation of azobenzene groups in Azo-MAC partially transformed from Trans to Cis. However, in the case of AMAC no change in viscosity was observed (Figure 10).

4. Conclusion A pressure responsive copolymer with azobenzene pendant (Azo-MAC) was designed and prepared based on copolymer of α-olefin and maleic anhydride. The chemical structure of this copolymer is characterized and confirmed by 1H NMR and FTIR. By using high-pressure UV-vis spectroscopy, Azo-MAC exhibited a conformation transformation from Trans to Cis when pressure was increased. As determined by high-pressure differential scanning calorimeter, the onset temperature and enthalpy during wax crystallization of Liaohe waxy crude oil reduced upon increasing pressure. The viscosity and yield stress of crude oil increased monotonically upon increasing pressure, while decreased significantly by addition of Azo-MAC. The performance of Azo-MAC is much better than that of AMAC, especially at enhanced pressures. Based on the experimental results, the possible mechanism is that the conformation transformation from Trans and Cis of azobenzene groups in Azo-MAC by increasing pressure leads to destruction of asphaltene 21



assembly and thus the improvement of the asphaltene dispersion and the suppression of wax crystallization. Compared to AMAC without pressure sensitive conformation transformation, Azo-MAC is able to enhance the flowability of oils under enhanced pressure and is ideal candidate for oil recovery and transportation in high pressure environment.

5. Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

6. Acknowledgement We thank the financial support by National Natural Science and Foundation of China (21476143, 51003028), 111 Project Grant (B08021), PetroChina Innovation Foundation (2016D-5007-0211) and the Open Project of State Key Laboratory of Shihezi University (2016BTRC004). The authors also thank Petrochina Liaohe Oilfield Company for affording oil samples.

Staffs of High Performance Computing

Center (HPCC) of East China University of Science and Technology are greatly acknowledged.

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Pressure Effect on the Rheological Behavior of Waxy Crude Oil with

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