Combined treatment of electrical and ethylene-vinyl acetate copolymer (EVA) to improve the cold flowability of waxy crude oils

Fuel 267 (2020) 117161

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Full Length Article

Combined treatment of electrical and ethylene-vinyl acetate copolymer (EVA) to improve the cold flowability of waxy crude oils

T

Yiwei Xiea,1, Jinjun Zhanga, , Chenbo Maa,b,1, Chaohui Chena, Qian Huanga, Zixin Lia,c, Yifei Dinga, Hongying Lia, Shanpeng Hana ⁎

a

National Engineering Laboratory for Pipeline Safety/MOE Key Laboratory of Petroleum Engineering/Beijing Key Laboratory of Urban Oil & Gas Distribution Technology/ China University of Petroleum, Beijing 102249, China b Process Section, Engineering Research & Design Department, CNOOC Research Institute Co., Ltd., Beijing 100028, China c PetroChina Beijing Gas Pipeline Co., Ltd., Beijing 100101, China

ARTICLE INFO

ABSTRACT

Keywords: Waxy crude oil Combined treatment Electric field EVA Cold flowability

Flow assurance issues related to wax precipitation, are one of the most challenging problems in the pipeline transportation of waxy crude oils. Flowability of waxy oils can be improved by suitable pour point depressants (PPDs). Besides chemical treatments, exposing waxy oils to high-voltage electric fields can also significantly improve their cold flowability. However, the combined effect of electric fields and PPD on the flowability of waxy oils is not yet clear. In this investigation, the effect of the combined electrical and EVA treatment on the flow properties of a waxy crude oil was studied. It is found that the combined treatment can lead to more significant reductions in the viscosity, the yield stress and the thixotropy of waxy oils compared to oils treated solely by electric field or EVA. However, the overall improvement in the flowability achieved by the joint treatment is lower than the sum of improvements achieved by the electric field and EVA. The combined treatment outperforms the individual treatments for hours under a low shear of 10 s−1. However, the performance cannot be preserved even for minutes under a high shear of 1200 s−1. Reheating has a negative impact on the electrical treatment, but an intricate impact on the EVA treatment and the combined treatment. The electrical- and chemical performance both undermine upon reheating, but chemical performance can be partially recovered when reheating temperature is above the wax disappearance temperature. Microscopic observation showed that the combined treatment leads to larger size and broader size distribution of wax particles.

1. Introduction

To mitigate wax-related flow assurance challenges, pour point depressants (PPDs) are commonly used to improve the flowability of waxy crude oils. PPD addition can suppress the pour point, reduce the viscosity and lower the yield stress of waxy crude oils [6,7]. Multiple types of polymeric PPDs have been developed over the past decades, among which Ethylene-Vinyl Acetate copolymer (EVA) has been widely applied [8–15]. It is well established that the wax-controlling performance of EVA in a particular oil depends on many factors, including dosage [8,9], molecular weight [10,11] and vinyl acetate (VA) content of the EVA [10,11], thermal and shear histories of the oil [12–15], etc. Yang et al. [8] found that the wax-controlling performance of EVA improves with increasing dosage and then kept nearly unchanged at the concentrations of 100–300 ppm. Machado et al [10] evaluated the pour

Waxy crude oil is an important type of fossil fuel resource. At high temperatures, waxy crude oils exist in one homogeneous liquid state, which is readily transportable. However, when the temperature of waxy oils drops to a critical value, also known as the wax appearance temperature (WAT), paraffin wax precipitates to form crystals and the extent of precipitation continues to increase with decreasing temperature. The precipitated paraffin wax particles can interconnect and form a spongy network, deteriorating the flowability of waxy crude oils [1,2]. With a sufficient amount of precipitated wax crystals in the oil, the oil can gel and cease to flow, causing costly flow assurance challenges during oil production and transportation [3–5].

Corresponding author at: China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Beijing 102249, China. E-mail address: [email protected] (J. Zhang). 1 Yiwei Xie and Chenbo Ma contributed equally to this work. ⁎

https://doi.org/10.1016/j.fuel.2020.117161 Received 13 August 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 Available online 31 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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point depression performance of EVA samples with molecular weights ranging from 0.2 × 104 g/mol to 1.9 × 104 g/mol. The EVA chemistry with an average molecular weight of 1.0 × 104 g/mol was found to give the best pour point depression performance. This finding was confirmed in the study by Jiang et al. [11] from which it was discovered that the EVA chemistries with a molecular weight of approximately 1.2 × 104 g/mol outperform the chemistries with molecular weights as low as 0.4 × 104 and as high as 1.8 × 104 g/mol. Machado et al. [10] also investigated the pour point depression performance of EVA with different vinyl acetate (VA) contents. Among EVAs with a VA content of 20, 30, 40, and 80 wt%, the chemistry with a VA content of 30 wt% provides the best performance. This optimal VA content is consistent with the recommended range of VA content, 23–35%, identified by Jiang et al. [11]. Many laboratory and field-scale investigations [12–22] have reported that the effectiveness of EVA treatments significantly depends on the shear and thermal histories of the oil. El-Gamal et al. [14,15] found that the rheology of the doped Umbarka waxy crude is shear-rate dependent. Zhao et al. [16] found that within the temperature range where wax precipitates rapidly, high shear rates weaken the performance of PPDs. Reznik et al [17] studied the long-term effect of shear on PPD-treated crude oil by imposing a sustained shear for a prolonged period, and it was found that the viscosity of the doped oil increases by about 3–5 mPa·s over a duration of 1 h. Zhang et al. [13,18] modeled the effect of shearing on the rheology of waxy crude oil based on entropy generation from viscous flows. They established correlations between the rheological parameters, such as gel points [13,19] and viscosities [20] of the sheared PPD-treated crude oils, and the entropy generation. Many studies [16,23–26] show that when oils beneficiated with PPD are reheated to a certain temperature range, the effectiveness of PPD may greatly degrade, causing the flowability of doped oils to become worse significantly. Recently, Tao et al. [27–31] developed a novel method to reduce the viscosity of waxy oils via application of high-voltage direct current (DC) electric field. They found that by imposing an electric field on the oil for seconds, the viscosity of the oil can be reduced by as much as 80% at temperatures near the pour point. Ma et al. [32] systematically studied the effect of the field strength and the treatment temperature on the performance of the electrical treatments. Lower treatment temperatures and higher field strengths lead to better performance of electrical treatments. In addition, Ma et al. [33] found that electrical treatment is also effective for waxy oil under static conditions. However, the performance of electrical treatment of waxy oils undermines in the presence of Asphaltenes [33]. Li et al. [34] discovered that electrical treatment can also alter the rheology of gelled waxy crude oil. When flowing waxy oil was electrically treated below WAT, decreasing dynamic modulus, reducing yield stress, weakening elastically domination and thixotropy can be observed, although the WAT and gelation temperature of the untreated waxy oil remain unchanged post-treatment. Key findings from previous research on electrical treatments of waxy oils can be summarized as follows: electrical treatment can effectively reduce the viscosity and the yield stress of waxy oil below the WAT regardless it’s flowing or static while the electrical treatment has almost no impact on the WAT and gelation temperature of oils [34]. The objective of this work is to investigate the influence of combined electric and EVA treatment on the flow behavior of waxy crude oils, characterized by their corresponding viscosities, gelation temperatures, yield stresses and thixotropy. In addition, the impacts of the operational parameters, including the reheating procedure and shearing conditions on the performance of the treatments was investigated. Finally, the morphology of wax particles after the combined electrical and EVA treatment was observed under the microscope and possible functioning mechanisms of the combined treatment was discussed based on the microscopic observation.

Table 1 Physical properties and corresponding test methods of the waxy crude oil. Parameter 3

Density at 20 °C (kg/m ) Pour point (°C) WAT (°C) WDT (°C) Wax content (wt%) Asphaltenes (wt%) Resins (wt%) Electrical conductivity at 20 °C (nS/m) Relative permittivity at 20 °C, 1 kHz

Value

Test Method

867.9 24 36.1 44.7 9.94 1.56 8.68 19.2 2.36

ISO 3675-1998 ASTM D5853-11 SY/T 0545-2012 DSC SY/T 0545-2012 ASTM D4124-09 ASTM D4124-09 ISO 6297-1997 ASTM D924-15

2. Experimental section 2.1. Material 2.1.1. Crude oil In this work, a waxy crude oil produced from China was used as the test fluid. The crude oil was first preheated to 80 °C in a tightly sealed flask under constant stirring for 2 h to eliminate any pre-existing shear and heat history. The heated and homogenized oil sample was then naturally cooled down and maintained at room temperature for 48 h [32]. The physical properties of the waxy crude oil and the experimental methods used to acquire these properties are listed in Table 1. The WAT and wax disappearance temperature (WDT) of the waxy oil were tested by differential scanning calorimetry (DSC) [35,36]. The wax precipitation curve of the waxy crude oil is presented in Fig. 1. 2.1.2. EVA The EVA used in this study is a commercial product purchased from DuPont, USA, with a VA content of 40% and a melting index [37] of 52. 2.2. Apparatus and methods We used “No-T”, “Elec-T”, “EVA-T” and “Comb-T” to refer to the untreated oil sample, the sample solely treated with electric field, the sample solely treated with EVA and the sample treated with both the electric field and EVA, respectively.

Cumulative precipatated wax (wt%)

2.2.1. EVA treatment 100 ppm of EVA was added to the waxy crude oil at room temperature. The oil sample was then heated to 50 °C (above the WDT) under constant stirring in a tightly sealed flask and kept isothermal for

10 8

6 4 2 0

-20

-10

0

10 20 Temperature ( )

30

40

Fig. 1. Wax precipitation curve of the waxy crude oil determined by DSC measurement. 2

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[42,43]. The shear rate was held at 1 s−1 for 30 min and held at the other values for 15 min.

30 min to ensure that EVA can fully interact with the oil. After that, the oil sample was cooled down to 25 °C (below the WAT) at a cooling rate of 0.5 °C/min and held for 10 min.

2.2.5. Stability of treatments effect The performance of the treatments may be affected by the temperature variation and shearing experienced by the treated oil during pipeline transportation. To understand the robustness of the treatment programs, the viscosity-reducing effect of the electrical, EVA and combined treatments were studied with treated oils underwent reheating, shearing with a relatively low shear rate (10 s−1) for hours, and shearing with a high shear rate (1200 s−1) for seconds.

2.2.2. Electrical treatment The high-voltage DC electric field was applied to waxy crude oils under static condition. The apparatus and operational procedure used to apply the electrical treatment can be found in the previous work, and thus are not elaborated in this manuscript [33]. Several key steps are recapitulated below. Waxy crude oils at 50 °C were loaded into the preheated sample holder and kept sealed for 10 min. The loaded sample was then cooled down to 25 °C at a rate of 0.5 °C/min and held at the final temperature for 10 min. Once the sample was fully equilibrated at 25 °C, a voltage of the magnitude of 24 kV was imposed across the oil for 120 s, generating an electric field with the strength of 0.8 kV/mm across the sample. An electric field with this strength is expected to cause significant changes in the rheological properties of the oil, beyond which electrical breakdown was observed to happen [34]. After the electrical treatment, the treated oil was quickly transferred to the rheometer (within 20 s) for the subsequent rheological tests.

2.2.5.1. Performance test following low shearing and high shearing. The desired treatments were first applied on the waxy crude oil at 25 °C. The treated oil was quickly loaded into the rheometer and held isothermally for 1, 2, 4, 8, 16, 24, 32 h, respectively. During the isothermal holding step, the oils was constantly sheared at a shear rate of 10 s−1. Upon completion of the isothermal shearing step, the oil was cooled further down to 23 °C at a cooling rate of 0.5 °C/min and held at the final temperature for 10 min. The viscosity of the treated and sheared oil was examined with a shear rate of 10 s−1. To assess the effect of high shearing on the performance of the treatment, the shear rate was increased from 10 s−1 to 1200 s−1 during the isothermal holding step and the duration of shearing was reduced from several hours to 100 s, while the other steps were unchanged as compared with the test performed under low shearing.

2.2.3. EVA-electric field combined treatment Crude oils were first doped with 100 ppm of EVA at room temperature and then heated to 50 °C under constant stirring and kept isothermal for 30 min in the sealed flask. Following the procedures listed in Section 2.2.2, the beneficiated oils were then loaded into the preheated sample holder before the electrical treatment. Finally, the oil sample was exposed to the electric field with the strength for 120 s.

2.2.5.2. Performance test following reheating. The desired treatments were first applied on the waxy crude oil at 25 °C. The treated oil was quickly loaded into the rheometer, followed by reheating to 30, 35, 40, 45 and 50 °C, respectively. After holding for 10 min at the final temperature, the oil sample was cooled down to 25 °C at a rate of 0.5 °C/min and held at that temperature for 10 min. The viscosity of the treated sample underwent reheating was then measured at a shear rate of 10 s−1.

2.2.4. Rheological tests The viscosity, gelation temperature and thixotropy of waxy oils were examined by a stress-controlled HAAKE RS150H rheometer. A Z41Ti coaxial cylinder geometry was used in the tests. The yield stresses of waxy oils were determined by an Anton Paar Rheolab QC rheometer equipped with a paddle rotor system, which can prevent slippage between the geometry and the sample and improve the reliability of the yield stress measurements [38,39]. To avoid the influence of different thermal and shear histories of oils on rheological test results, we strictly controlled the experimental procedures of oil treated by all methods. So prior to a test, the untreated oil sample and the sample solely treated with EVA were loaded into the preheated electrical treatment apparatus without applying electric field and kept for 120 s. Each experiment was repeated 2–3 times and the average value was reported.

2.2.6. Microscopic observation The morphology of waxy crystals was observed by a Nikon OPTIPHOT2-POL polarized microscope. The procedure reported in previous studies [32,33] was used to prepare waxy crude oil samples for microscopic observation. The temperature of the specimen was controlled by a thermal stage. With each specimen, 15 high-resolution images were captured by a CCD camera. The microscopic characteristics of the wax crystals, including the average perimeter, aspect ratio and boundary fractal dimension were extracted from the micrographs using the ImageJ software [32].

2.2.4.1. Viscosity. The viscosity was measured at 25 °C and under five shear rates: 10, 20, 50, 100, 150 s−1. At each shear rate, the sample was continuously sheared for 5 min and the viscosity data was acquired during the shearing process.

3. Results and discussion 3.1. Viscosity

2.2.4.2. Gelation temperature. The oil samples were cooled from 25 °C to 20 °C at a cooling rate of 0.5 °C /min in the presence of a small amplitude oscillatory shear (SAOS) stress of a magnitude of 0.1 Pa and with a frequency of 1.0 Hz. The gelation temperature of oils was determined as the temperature at which the storage modulus was equal to the loss modulus [40].

The waxy oil was treated with an electric field of 0.8 kV/mm, 100 ppm of EVA and jointly by the electric field and EVA. The performance of the treatments was evaluated based on the comparison between the viscosity of the treated oil and that of the untreated oil, as shown in Fig. 2. As can be seen in Fig. 2, compared to the untreated oil, the treated oils present lower viscosities on the entire shear rate range investigated. For a given shear rate, the viscosity of the untreated oil is the highest, followed by the electrically treated and EVA-treated oils, and the oil treated jointly by the electric field and EVA PPD is the lowest. For example, at a shear rate of 10 s−1, the viscosity of the untreated oil was 105.40 mPa·s. When the oil was exposed to an electric field, its viscosity was reduced by 37.1% to 66.27 mPa·s. EVA treatment caused a similar level of viscosity reduction, i.e., 40.9%. Under the experimental conditions of this work, the effect of EVA treatment on reducing viscosity is

2.2.4.3. Yield behavior. A waxy gel was first formed by cooling the oil down to 20 °C and holding at this final temperature for 90 min. A linearly increasing shear stress was then imposed on the waxy gel to measure its yield stress. The rate of shear stress loading was set at 1 Pa/min [41]. 2.2.4.4. Thixotropy. A waxy gel was first formed following the same cooling protocol as was used in the yielding tests. Six constant shear rates (1, 2, 4, 8, 16, 32 s−1) were sequentially applied on the waxy gel and the time-evolution of the corresponding shear stress was recorded 3

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Viscosity (mPa·s)

140

different treatments on the gelation temperature and the viscoelastic characteristics of the oil. It is seen from Fig. 3 that electrical treatment has a negligible effect on the gelation temperature of the oil, which is consistent with the conclusion from the previous work [34]. However, the electrical treatment appears to affect the storage and loss moduli of the waxy oil. The storage modulus at 20 °C decreases from 1920 Pa to 1027 Pa and the loss modulus decreases from 276 Pa to 175 Pa as results of the electrical treatment. EVA treatment causes the gelation temperature to decrease by 2.8 °C. In addition, the storage and loss moduli of the EVA treated oil are 183 Pa and 32 Pa, respectively. The storage and loss moduli of the oil (at 20 °C) treated by combined method with both the electric field and EVA are 97 Pa and 20 Pa, respectively. Notably, the decreased magnitude of storage modulus and loss modulus of combinedly-treated oil is smaller than the sum of that of electrically-treated oil and EVA-treated oil. These phenomena are in consistent with our finding in section 3.1, which means that the combination of electric field and EVA is more like an improvement for electrical treatment or EVA treatment alone, rather than their synergistic effect.

No-T Elec-T

105

EVA-T

Comb-T 70

35

0

0

40

80

Shear rate

120

160

(s-1)

Fig. 2. The viscosity of oils at 25 °C treated under different conditions.

better than that of electrical treatment. The oil treated with both the electric field and EVA underwent a viscosity reduction by 59.1% and the absolute viscosity after treatment is 43.97 mPa·s. It is noticed that an additional viscosity reduction of 22.0% and 18.2% can be achieved with the combined treatment in comparison with the electrical and EVA treatment, respectively. However, the effect of combined treatment is lower than addition of the effect of electrical treatment and the effect of EVA treatment. For example, at the shear rate of 10 s−1, the viscosity reduction produced by combined treatment is 59.1%, which is 18.9% lower than that of single electrical treatment plus single EVA treatment. It is also observed that the viscosity reduction achieved by different treatments universally decreases as the shear rate increases, which is consistent with the previous work [32,33]. The shear thinning characteristics of the untreated and treated oils were also fitted with the power-law model, shown in Equation (1):

=K

3.3. Yielding behavior The yielding behavior of the untreated and treated oils was studied by the stress-loading method with a constant shear stress ramp rate of 1 Pa/min. The corresponding evolution of the strain was recorded during the stress-loading process and is shown in Fig. 4. As can be seen from Fig. 4, upon loading of a shear stress, the strain of the sample started to increase slowly at the initial stage, indicating that the sample underwent creep deformation. As the shear stress increased, the deformation of gelled oil increase. When the deformation accumulates to a certain level, the gel structure fractured and yielded catastrophically, characterized by a rapid increase of the strain. The strain and stress at which the oil samples yielded are defined as the yield strain and yield stress respectively [44,45]. It is seen from Fig. 4 that the yield stress of the untreated oil is 291.1 Pa. With the electrical treatment, the yield stress of the waxy crude oil is reduced to 226.2 Pa, corresponding to a 22.3% reduction in comparison with the yield stress of the untreated oil. EVA addition lowers the yield stress of the waxy oil to 151.8 Pa, corresponding to a reduction of 47.7%. In terms of reducing the yield stress of crude oil, EVA treatment also exhibited better effects than electrical treatment in which the electric field strength has reached the maximum acceptable value of the device. The yield stress of the oil lowered to less than half of the original value after the combined treatment with the electric field and EVA. The yield stress of the oil treated by both means is 104.1 Pa and the reduction in the yield stress is as high as 64.3%. It is shown that the combined treatment with both the electric field and EVA is more effective to reduce the yield stress of gelled oil than the electrical and EVA treatment alone. As for the yield strain, the values of oils with or without various treatments lie in the range of 0.42 to 0.61.

(1)

n

−1

where τ is the shear stress of fluid, mPa; is the shear rate of fluid, s ; n is the flow behavior index, the deviation of n from 1 indicates the deviation of the fluid properties to a Newtonian fluid; K is the consistency coefficient, mPa·sn. The corresponding rheological equations under different conditions were displayed in Table 2. Table 2 shows that treated oils in general present higher flow indices, compared with the untreated oil. The untreated oil presents a flow index of 0.7719, suggesting distinct shear thinning characteristics. The flow index of the oil, n, increases to 0.8607 after electrical treatment. An even higher flow index of 0.8894 can be obtained with the EVA treatment. With the combined electrical and EVA treatment, the flow index n rises to 0.9672, suggesting that the treated oil resembles a Newtonian fluid. 3.2. Gelation temperature

3.4. Thixotropic behaviors

To investigate the effect of the electrical, EVA and combined treatments on the gelation temperature of the waxy crude oil, the gelation temperature, storage modulus G′ and loss modulus G″ were recorded during SAOS measurements. Fig. 3 presents the effects of

Fig. 5 presents the thixotropic behavior of the untreated and treated oils characterized at 20 °C and at different shear rates. As is shown in Fig. 5, at each shear rate, the corresponding shear stress decreases as time elapses, exemplifying the thixotropic characteristics of the waxy gels. The shear stress of the oil treated with both the electric field and EVA is the lowest among all samples studied. The viscosity of both the oils treated with the electric field and EVA alone is lower than that of the treated oil, but is higher than that of the oil submitted for the combined treatment. Furthermore, the viscoplastic model proposed by Teng et al. [46] shown in Equation (2) was employed to quantitatively analyze the thixotropic characteristics shown in Fig. 5.

Table 2 Rheological equations under different conditions. Oil sample No-T

Elec-T

EVA-T

Comb-T

Rheological equation

= 180.71

0.7719

= 92.90

0.8607

= 80.44

0.8894

= 46.63

0.9672

4

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Fig. 3. Gelation temperatures of oils with different methods, (a) No treatment, (b) Electrical treatment, (c) EVA treatment, (d) Combined treatment.

where τ is the shear stress of fluid, Pa; τy is the yield stress for the fully developed structure, Pa; is the scalar structural parameter; K is a completely unstructured consistency, Pa·sn; ΔK is a structure-parameterdependent consistency, Pa·sn; is the shear rate of fluid, s−1; n1 is a kinetic index; γ is the total shear strain; a is a rate constant for structure buildup, s−1; b is a rate constant for structure breakdown, Pa-m·sm-1; and c as well as m are positive dimensionless material parameters. The parameters in Teng’s model [46] associated with different samples, i.e., “No-T”, “Elec-T”, “EVA-T” and “Comb-T” were obtained by fitted based on the experimental measurements and are listed in Table 3. In general, treated oils present lower τy, ΔK and K1 values compared to the untreated oil, suggesting that the solid wax matrices formed under the influence of various treatments are weaker than the one formed in absence of treatments [46]. Fig. 4. Creep and yielding of oils at 20 °C with different treatments.

No-T Elec-T EVA-T Comb-T fitted (colorful line)

30

24

16

20 10

8

0

0

0

25

50 Time (min)

75

100

s-1

Shear stress (Pa)

40

It has been shown in this study that electrical treatment, EVA treatment and the combined treatment can reduce the viscosity of the waxy oil and weaken the strength of the waxy gel. It is of interest to perform additional tests to understand how the performance of different treatment programs evolve as a function of time in the presence of continuous shearing and temperature variations. Such assessments can shed light on the robustness of the treatments and their applicability in the presence of relatively harsh shearing and temperature fluctuations, which are commonly encountered in industrial settings.

32

Shear rate

50

3.5. Stability of the treatment effect

3.5.1. Stability under low shear This subsection is dedicated to understanding the time-evolution of the flowability of treated oil under the simulated flow condition in the presence of a persistent shear. After shearing at a shear rate of 10 s−1 and a temperature of 25 °C for 1, 2, 4, 8, 16, 24 and 32 h respectively, the treated oil was cooled to 23 °C, at which its viscosity was measured. The viscosity of the oil samples exposed to different amounts of shearing is shown in Fig. 6. As can be seen from Fig. 6, the viscosity of the untreated oil is determined to be approximately 150 mPa·s at 23 °C and it appears that

Fig. 5. Thixotropic behaviors of oils at 20 °C with different treatments.

d dt

=

y

=

1 1+ c

+ (K + [a (1

K) )

b

n1 m]

(2) 5

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Table 3 The parameters of oils at 20 °C fitted by Eq. (2). Oil sample

τy (Pa)

ΔK (Pa·sn1 )

K (Pa·sn1 )

n

No-T Elec-T EVA-T Comb-T

20.97 19.96 9.37 6.82

14.032 7.508 7.442 4.795

1.148 0.962 0.318 0.305

0.5487 0.6301 0.6730 0.7223

n i=1

*ADD means average absolute deviations, ADD =

yimeasured yifitted yimeasured

1

c (–)

a (s−1)

b (Pa−m·sm−1)

m (–)

ADD (%)

0.4301 0.2716 0.2505 0.2036

0.0101 0.0062 0.0052 0.0033

0.0222 0.0117 0.0093 0.0059

0.512 0.504 0.414 0.457

1.4 1.9 1.5 0.9

× 100\% where n is the total number of measured data points; yimeasured is the experimental

result of the data point; yifitted is the fitting result of the data point.

200

150

Viscosity at 10s-1 (mPa·s)

Viscosity at 10 s-1 (mPa·s)

175

125 100

No-T Elec-T EVA-T Comb-T

75 50

0

8

16

24

32

40

After high shear

150

100

50

0

Duration of Low shearing (h)

Before high shear

No-T

Elec-T

EVA-T

Comb-T

Fig. 6. The viscosity variation with time elapse for oils with different treatments.

Fig. 7. The viscosity of oils with different treatments, before and after high shear at 23 °C.

shearing prior to the viscosity characterization has negligible effect on the measured viscosity. On the contrary, the viscosity reduction can be observed immediately after the application of the treatment, whereas shearing tends to cause the viscosity reduction to diminish as time elapses. The effect of the electrical treatment diminishes almost completely after a shearing period of 24 h. The performance of the EVA treatment is also observed to deteriorate rapidly at the initial stage of the shearing period while the reduction did not continue to occur and the performance of the EVA treatment can be partially preserved. The combined treatment showed more satisfying robustness at the initial stage of the shearing period in comparison with the EVA treatment. As time elapses, the effect of the electric field in the combined treatment diminishes completely while the chemical component of the combined treatment continues to be effective. As a result, it is determined that the combined treatment presents the best robustness in the presence of a lasting low shear.

similarly, which is 37.83 mPa·s higher than the viscosity of the oil before high shearing. For the oil treated by both the electric field and EVA addition, an increase in the viscosity by 42.68 mPa·s can be observed after shearing. This phenomenon means that high shear can severely damage the beneficiation effect of EVA on the waxy crude oil and leads to a poorer flowability. 3.5.3. Stability in the presence of reheating Heating is commonly implemented during the transportation of waxy oils over a long distance. In this section, the influence of heating on the flowability of oils with different treatments was examined. The oil samples were pre-treated at 25 °C, followed by heating to different temperatures, i.e., 30, 35, 40, 45, 50 °C, and finally cooled down to 25 °C at which their viscosities were measured. The influence of reheating on the viscosity of oils treated by different methods is shown in Fig. 8. It can be seen from Fig. 8 that reheating has a negligible effect on the viscosity of the untreated oil. The viscosity of the electrically treated oil continuously increases with increasing reheating temperature. A fraction of the treated solid wax particles dissolves during reheating and precipitates as untreated solid particles when the temperature drops back to 25 °C, causing the overall performance of the electrical treatment to diminish. Moreover, the performance of the electrical treatment is lost entirely when the oil sample is reheated to above the WDT of 45 °C. This phenomenon shows that electrical treatment does not change the chemical components of crude oil and thus is friendly to the refinery process. On the contrary, the viscosity of the EVA-treated oil first increases then decreases as the reheating temperature increases. This phenomenon is in accordance with the knowledge acquired from previous field tests and lab experiments [16,23–26]. This is because the PPD can fully function only when waxy oils are heated to temperatures high enough

3.5.2. Stability under high shear Transient high shear on the oil can be encountered in a scenario where the oil flows through a centrifugal pump. To understand the effect of transient high shear on the performance of treatment programs, the oil samples at 25 °C were isothermally sheared at a shear rate of 1200 s−1 for 100 s and subsequently cooled down to 23 °C, at which the viscosities of the oils were measured. The effects of shearing on the viscosities of the treated and untreated oils are shown in Fig. 7. The viscosity of the untreated oil is unaffected by shearing. Only a minor reduction in the viscosity was observed with the electrically treated oil in the presence of electric treatment. High shearing behaves as a negative influence that greatly undermines the effectiveness of the EVA treatment and the combined treatment. The viscosity of the EVAtreated oil rises from 70.52 mPa·s to 108.35 mPa·s after shearing and 6

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120

recovered with reheating to a temperature that is significantly higher than the WDT.

100

3.6. Microscopy

Viscosity at 10s-1 (mPa·s)

Y. Xie, et al.

Previous work suggests that the flowability of waxy oils might correlate with the microscopic characteristics of wax crystals [32–34]. In this section, we conducted microscopic observation of wax crystals formed in the absence and presence of treatments to provide mechanistic insights to treatment performance from microscopic aspects. Fig. 9 shows the micrographs of the untreated waxy crude oil and those submitted to different treatments. All micrographs were acquired at 25 °C. It is seen from Fig. 9 that the wax particles of waxy oils in the presence of treatments appear to be larger in size, compared to the particles formed in the absence of treatments. This qualitative observation is corroborated by the microscopic structural parameters of the wax crystals determined based on the micrographs, shown in Table 4. The size distributions of wax crystals of oils under different conditions were also obtained via image analysis and were shown in Fig. 10. It is seen from Fig. 10 that compared to the wax particles formed from an untreated oil, the particles formed from treated oils present larger mean sizes and broader particle size distributions. In addition, the combined treatment generates the broadest particle size distribution of wax crystals. Application of suspension theories with the broad wax particle size distribution observed with the combined treatment can explain its superior viscosity-reducing performance. According to suspension theories, a particulate suspension with a broad particle size distribution generally has a low viscosity, because the small particles can reside in the voids between large particles [32,47–50].

80

60

40

25

30

No-T

Elec-T

EVA-T

Comb-T WDT

35

40

45

50

Reheating Temperature ( C) Fig. 8. The viscosity of oils at 25 °C after reheating.

to dissolve all wax crystals and thus the PPD can fully co-crystallize with the wax particles [24,25]. However, at heating temperatures lower than the WDT, only partial wax crystals can dissolve, which may deteriorate the effect of PPD and even lead to a worse flowability of oil. The performance of the combined treated also undermines with increasing reheating temperature when the reheating temperature is below the WDT. This trend is consistent with the trends observed with electrical- and EVA-treated oils. When the oil is reheated above the WDT, the viscosity reduction due to EVA addition in the combined treatment can be recovered while the viscosity reduction induced by the electric field continues to diminish. As a result, the overall viscosity reduction by the combined treatment approaches that achieved by the chemical treatment as the reheating temperature increases. As can be seen from the analysis presented in this section, reheating in general undermines the performance of the electrical, EVA and combined treatments. In the extreme case, the performance of the electrical treatment can be lost entirely when the oil is reheated to above the WDT. The performance of the EVA treatment also undermines with reheating but the majority of the performance can be

4. Conclusions The influence of the combined treatment of electric field and Ethylene-Vinyl Acetate Copolymer (EVA) on the cold flowability of waxy crude oil was investigated. Compared to electrical treatment and EVA treatment alone, the combined treatment is more effective in

Fig. 9. Micrographs for (a) untreated oil, (b) electrically-treated oil, (c) EVA-treated oil, (d) the oil treated by both electric field and EVA. 7

Fuel 267 (2020) 117161

Y. Xie, et al.

curation, Writing - original draft. Jinjun Zhang: Supervision, Visualization, Funding acquisition, Project administration. Chenbo Ma: Writing - review & editing. Chaohui Chen: Validation, Writing - review & editing. Qian Huang: Validation, Writing - review & editing. Zixin Li: Validation. Yifei Ding: Validation. Hongying Li: Supervision, Writing - review & editing. Shanpeng Han: Supervision.

Table 4 Microscopic parameters of wax crystals for oils with different treatments. Oil

No-T Elec-T EVA-T Comb-T

Perimeter (μm)

Aspect ratio

Average

Standard deviation

Average

Standard deviation

2.09 2.42 2.91 3.41

2.45 2.78 3.05 3.26

1.8 2.0 2.2 2.3

0.82 0.88 0.87 0.95

Boundary box fractal dimension

1.11 1.20 1.23 1.26

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51534007 and No. 51134006). The authors would like to acknowledge Dr. Sheng Zheng from TOTAL E&P for his suggestions on the writing of the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117161. References [1] Oliveira MCKD, Teixeira A, Vieira LC, et al. Flow assurance study for waxy crude oils. Energy Fuels 2012;26(5):2688–95. [2] Liu H, Zhang J, Lu Y. Yielding characterization of waxy gels by energy dissipation. Rheol Acta 2018;57(6–7):473–80. [3] Bai C, Zhang J. Effect of carbon number distribution of wax on the yield stress of waxy oil gels. Ind Eng Chem Res 2013;52(7):2732–9. [4] Visintin RFG, Lapasin R, Vignati E, et al. Rheological behavior and structural interpretation of waxy crude oil gels. Langmuir 2005;21(14):6240–9. [5] Li H, Zhang J. A generalized model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax. Fuel 2003;82(11):1387–97. [6] Taraneh JB, Rahmatollah G, Hassan A, et al. Effect of wax inhibitors on pour point and rheological properties of Iranian waxy crude oil. Fuel Process Technol 2008;89(10):973–7. [7] Yi S, Zhang J. Relationship between waxy crude oil composition and change in the morphology and structure of wax crystals induced by pour-point-depressant beneficiation. Energy Fuels 2011;25(4):1686–96. [8] Yang F, Li C, Lin M. Depressive effects evaluation of ethylene-vinyl acetate copolymer on waxy crude oils. J China Univ Petrol 2009;33(5):108–13. (In Chinese). [9] Ashbaugh HS, Guo X, Schwahn D, et al. Interaction of paraffin wax gels with ethylene/vinyl acetate co-polymers. Energy Fuels 2005;19(1):138–44. [10] Machado ALC, Lucas EF. Poly (ethylene-co-vinyl acetate) (EVA) copolymers as modifiers of oil wax crystallization. Pet Sci Technol 1999;17(9–10):1029–41. [11] Jiang Q, Yue G, Song Z. Relation between structure of ethene-vinylacetate copolymers and their pour point depression. J Southwest Petroleum Inst 2006;28(2):71–4. (In Chinese). [12] Zhang J, Liu X. Some advances in crude oil rheology and its application. J Central South Univ Technol 2008;15(1 Supplement):288–92. [13] Zhang J, Zhang F, Huang Q, et al. Experimental simulation of effect of shear on rheological properties of beneficiated waxy crude oils. J Central South Univ Technol 2007;14(1):108–11. [14] El-Gamal IM, Gad EAM. Low temperature rheological behavior of Umbarka waxy crude and influence of flow improver. Colloids Surf, A 1998;131(1):181–91. [15] El-Gamal IM. Combined effects of shear and flow improvers: the optimum solution for handling waxy crudes below pour point. Colloids Surf, A 1998;135(135):283–91. [16] Zhao X, Liu S, She Q, et al. Research into the effects of pour-point depressant on different wax-contents crude oils. Adv Mater Res 2011;236–238:804–7. [17] Reznik YM, Lemeulle C, Chardes T, et al. Combined effect of asphaltenes and flow improvers on the rheological behaviour of Indian waxy crude oil. Fuel 1998;77(11):1163–7. [18] Zhang J, Yu B, Li H, et al. Advances in rheology and flow assurance studies of waxy crude. Pet Sci 2013;10(4):538–47. [19] Yi S, Zhang J. Shear-induced change in morphology of wax crystals and flow properties of waxy crudes modified with the pour-point depressant. Energy Fuels 2011;25(12):5660–71. [20] Li Y, Zhang J. Prediction of viscosity variation for waxy crude oils beneficiated by pour point depressants during pipelining. Liquid Fuels Technol 2005;23(7–8):16. [21] Cao D, Meng Y, Zu H, et al. The study of new pour point depressant and application in ZhongLuo pipeline to transport different crude oils. Oil Gas Storage Transp

Fig. 10. Wax particle size distribution of the oils under different treatments.

improving the cold flowability of waxy crude oil by reducing the viscosity, weakening waxy gels and depressing thixotropy. A viscosity reduction of as high as 59.1% can be achieved at a shear rate of 10 s−1 and 25 °C with the combined treatment. This viscosity reduction is 22.0% and 18.2% higher than that achieved with the electrical and EVA treatments alone. At a temperature of 20 °C, the treated waxy gel has a yield stress of 104.1 Pa, lower than the yield stresses of electricallytreated and EVA-treated oils. In the temperature range studied, both storage and loss moduli of the oils treated by both the electric field and chemical are smaller than those of the oils treated by the electric field or EVA. However, the overall improvement in the flowability achieved by the combined treatment is lower than the sum of the improvements achieved by the electric field and EVA. The robustness of the treatment programs is also studied. It was discovered that in general, both shearing and reheating causes the performance of all the treatments to deteriorate. Furthermore, the viscosity-reducing effect of the electric field on the waxy oil can completely diminish after a prolonged period of shearing or reheating to a temperature above the WDT. On the contrary, the performance of the chemical, although diminishes as the reheating temperature increases, can be partially recovered when the oil is reheated to above the WDT. The combined treatment does not provide significant improvement in the overall robustness against shearing and reheating, but can outperform the electric and EVA treatments alone in the presence of a low shear within a short shearing period. Microscopic analysis shows that the size distribution of wax crystals broadens and shifts towards a larger average particle size in the presence of treatments. The size distribution of the waxy particles of waxy oils in the presence of the combined treatment is the broadest, which is partially responsible for the most significant flowability improvement achieved by the combined treatment, according to suspension theories. Credit authorship contribution statement Yiwei Xie: Conceptualization, Validation, Investigation, Data 8

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