Effect Evaluation and Mechanism Analysis of Novel Nano-hybrid Pour

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Effect Evaluation and Mechanism Analysis of Novel Nano-hybrid Pour Point Depressant on Facilitating Flow Properties of Crude Oil Na Li, Guoliang Mao, and Yang Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02371 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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Effect Evaluation and Mechanism Analysis

1

of Novel Nano-hybrid Pour Point Depressant on

2

Facilitating Flow Properties of Crude Oil

3

Na Li, GuoLiang Mao∗, Yang Liu∗

4 5

ABSTRACT

6

Novel nano-hybrid pour point depressants(PPD) were prepared by combining ethylene-vinyl

7

acetate (EVA) copolymers with nano-SiO2 particles, a series of experiments were carried out to

8

test the effects of EVA/nano-SiO2 (E-S) hybrid PPDs on model waxy oil containing 25wt% wax

9

and crude oil, respectively. The results showed that optimal PPD performance was attained at the

10

concentration of 0.08 wt% which can suppress the formation of gelling effectively and improve

11

the fluidity of both oils. The presence of E-S hybrid PPD effectively modulated the crystal

12

morphology to the orientation which was conducive to reducing gelling temperature and inhibiting

13

the formation of network structure. Meanwhile, PPD adsorbed on the surface of asphaltene and

14

resin can prohibit the self-association of asphaltene and resin which further improved the fluidity

15

of crude oil.

16

1. INTRODUCTION

17

The cost of crude oil is one of the crucial factors that affect the economy all over the world

18

since crude oil is a dominant source of energy resources and petrochemical products. The wax

19

appearance temperature is a key factor affecting the fluidity of waxy oil. The continuously

20

precipitated wax forms net-like or cage-like structure which worsens the fluidity of crude oil. So

21

the precipitated wax would further increase the costs and risks of oil transportation.[1-3] In order to

∗

GuoLiang Mao: College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, China, E-mail address:[email protected]. ∗ Yang Liu: College of Petroleum Engineering, Northeast Petroleum University, Daqing, China, E-mail address:[email protected].

1


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solve these problems, pour point depressants (PPDs) were developed and proved to be a cheap but

23

effective means.[4-6]

24

Ethylene-vinyl acetate (EVA) PPD has been widely used in crude oil transportation. EVA is

25

made of both polar and nonpolar monomers, the nonpolar groups of EVA can increase the

26

solubility of PPD in the oil and lead to the co-crystal with wax, and polar groups are a crucial

27

element in modulating the morphology of wax crystals which can inhibit the formation of network

28

structure. Meanwhile, the effect of PPD is also determined by the properties of targeted crude oils,

29

which limit the applications of EVA PPDs.[4, 7]

30

Up to now, new materials derived from nanotechnology have been used in the petroleum

31

industry.[8] A lot of researchers combine conventional PPDs with the nanoparticles to improve the

32

performance and widen the applications of PPDs.[1-2,

33

polymethyl methacrylate nanohybrid PPD, the results showed that the property of the new

34

nanohybrid PPD had an advantage over the conventional ones and reduced the viscosity of crude

35

oil by 80%.[11] Zhao et al. prepared the new PPDs which used organically modified nano-clay

36

covered by polymer, the experimental results showed that nano-hybrid PPDs could effectively

37

change the morphology of wax crystals and improve the low temperature fluidity.[2] Although

38

nanohybrid PPD has been prepared, the interaction mechanism of nano-hybrid PPD and crude oil

39

needs to be further explored.

9-10]

Alsabagh et al. prepared a new

40

As one of the most widely used, cost-effective and environmentally friendly nanomaterials,

41

much attention has been drawn to nano-SiO2 in materials research fields.[12-13] This article explores

42

the applications for nano-hybrid PPD for crude oil which combines the EVA and nano-SiO2. The

43

influences of wax, asphaltene and resin on the macroscopical rheology related properties and 2


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microscopic crystallization behavior with EVA/nano-SiO2 (E-S) PPD are also discussed.

45

Meanwhile, the detailed mechanisms of E-S PPD on crude oil are further investigated.

46

2. EXPERIMENTAL SECTION

47

2.1 Materials.

48

Dodecane, unmodified nano-SiO2 (The average particle size is 30 nm.), toluene, ethanol, and

49

silane coupling agent HK570 were purchased from Shanghai Macklin Biochemical Co., Ltd.

50

Crude oil, wax and -35# diesel were supplied by Daqing Oilfield and Sinopec, China. (The

51

physical characteristics of oils are shown in Table 1.) EVAs were purchased from Heinz industrail

52

company, China.

53

Table 1. Physical characteristics of oils Specification

Value

Pour point, °C

38 (crude oil） 34 (model oil)

Asphaltene, wt%

0.98

Resin, wt%

8.2

Wax content, wt%

54

25

The molecular mass of EVAs with different VA content is shown in Table 2.

55

Table 2. Molecular mass of EVAs VA content (wt%)

Mn

Mw

Mw/ Mn

28

34897

73572

2.11

33

35379

72577

2.05

38

59026

174543

2.96

56

2.2 Specimen Preparation.

57

2.2.1 Organic modification of hydrophilic nano-SiO2.

58

Silane coupling agent HK570 was used to organically modify the hydrophilic nano-SiO2. The

59

HK570 with weak acidity was added into mixed solution with the 1:1 ratio of ethanol and

60

deionized water. Then, nano-SiO2 was dispersed into the mixed solution with stirring and

61

ultrasonic wave at 80°C for 2 h. Then, vacuum filtration was used to remove the liquid, and the 3


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solid product was washed repeatedly with the solution of deionized water and ethanol until the

63

unreacted HK570 were completely removed (pH =7).[14-15] The organic modified nano-SiO2 was

64

obtained after drying.

65

The structure and composition of modified nano-SiO2 were analyzed by Fourier transforms

66

infrared (FT-IR) spectrometer (TENSOR27, Bruker, Germany) and X-ray diffraction (XRD)

67

（ Rigaku, Japan). The dispersion evaluation of nano-SiO2 was directly evaluated by the

68

dispersion status of nano-SiO2 particles in water and dodecane. The modification degree of

69

nano-SiO2 was measured by thermogravimetric analysis (TGA) (STA 449 F5 Jupiter instrument,

70

Germany) under a nitrogen atmosphere.[16]

71

First step: KH570 hydrolysis reaction H3 CO H 3CO

HO

O 2Si OH + HO

75

H2 O O O C C CH2 CH 3

HO

O Si CH 2 3 O C C CH2 CH 3

HO HO

Second step: condensation reaction OH

74

CH 2 3

H 3CO

72 73

Si

OH

Si

HO

O CH 2 3 O C C CH 2 CH3

O O2 Si

O

Si

O

O CH 2 3 O C C CH2 + H O 2 CH 3

2.2.2 Preparation of nano-hybrid PPD.

76

First, EVA was dissolved in toluene. The nano-hybrid PPD was prepared by dispersing a

77

certain amount of modified nano-SiO2 into the EVA solution (the mass ratio between modified

78

nano-SiO2 and EVA was 1:1) through ultrasonic treatment to avoid the agglomeration of

79

nano-SiO2 and form even solution. Then, toluene solvent was removed by stirring and heating to

80

obtain the targeting E-S PPD.[17] The structure and composition of the obtained E-S PPD were

81

analyzed by X-ray diffraction and FT-IR spectrometry. The dispersion evaluation of E-S PPD was

82

carried out with dodecane and water.

83

2.2.3 Preparation of model oil. 4


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The preparation of model oil with 25 wt% was mixed the wax and -35# diesel together at

85

70°C and continuously stirred until it became an even solution.

86

2.3 Properties Characterization.

87

2.3.1. Pour point test.

88

A temperature ramp test was carried out to test the effect of PPD on the pour point of

89

model/crude oil.[18] The preheated oil samples were putted into test tubes, and then preserved at

90

50°C until the temperature inside and outside of the tubes was constant. Then the oil samples were

91

cooled at a constant rate of 1°C in thermostatic bath and tilted by 45 degrees until the oil level

92

cannot restore in 1 min. The temperature was recorded as the pour point of oil sample. The

93

experiment was repeated three times for each sample, and the average value of the three times was

94

designated as the pour point.

95

2.3.2. Rheological measurements.

96

The Brookfield rotational rheometer (DV-II+Pro) was used to measure the viscosity of

97

model/crude oil. Model oil and crude oil were reheated and stirred for 2 h at 70°C and 80°C before

98

being measured to remove thermal and shearing history. The apparent viscosity corresponding to

99

the temperature was recorded at constant cooling rate of 1.0 °C/min and shearing rate of 1.5r/min.

100

2.3.3. Polarized optical microscopy.

101

The effect of PPD on the morphology microstructure of wax crystal was explored with the

102

polarized optical microscopic (POM) (XFP600c) fitted with a thermal stage. The oil was

103

preheated and then maintained at 50°C. Then oil sample carried by a glass slide was placed on the

104

thermal stage and gradually cooled to 20°C. The morphology microstructure of wax crystal was

105

recorded. 5


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3. RESULTS AND DISCUSSION.

107

3.1 Characterization Analysis.

108

3.1.1. FT-IR spectra analysis of nano-SiO2.

109

FT-IR spectra of modified and unmodified nano-SiO2 are shown in Figure 1. The Si-O-Si

110

bending vibration absorption peak, symmetric stretching vibration absorption peak and

111

antisymmetric stretching vibration absorption peak of SiO2 were detected at 470 cm-1, 794 cm-1

112

and 1100 cm-1 in both curves a and b. while, the new C=O stretching vibration absorption peak,

113

CH3 and CH2 asymmetric stretching vibration absorption peak were detected at 1727, 2854, 2926

114

cm-1 in curve a which confirm that the KH-570 had been successfully grafted to the surface of

115

nano-SiO2.

116 117 118

Figure 1. FT-IR spectra of (a) modified and (b) unmodified nano-SiO2

3.1.2. TGA of nano-SiO2.

119

The curves representing the extent of organic modification of nano-SiO2 are presented in

120

Figure 2. Below 230 °C, the weight loss of both modified and unmodified nano-SiO2 is caused by

121

the thermal decomposition of water molecules adsorbed on the surface of nano-SiO2 and Si-OH.

122

While, the weight loss of modified nano-SiO2 is less than the unmodified nano-SiO2 which is

123

caused by the reduction of -OH on the surface of modified nano-SiO2. The 0.12% weight loss 6


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happens to both modified and unmodified nano-SiO2 at 230 °C. From 230 °C to 580 °C, the

125

thermal decomposition of the water between Si-OH bonds is occurred in unmodified nano-SiO2

126

which leads to the weight loss. Since the modified nano-SiO2 has been sufficiently washed, the

127

effect of physical adsorption is wholly excluded. So the thermal decomposition of organic

128

functional groups is occurred in modified nano-SiO2 which leads to the weight loss. The entire

129

weight loss of modified nano-SiO2 is 3.4% of which organic groups account for 1.59%.

130 131 132

Figure 2. TGA of (a) unmodified and (b) modified nano-SiO2

3.1.3. XRD patterns of nano-SiO2 and E-S PPD.

133

The XRD patterns of unmodified and modified nano-SiO2 are presented in Figure 3.

134

According to the Figure 3, the diffraction peaks are in a bread-like shape of both unmodified and

135

modified nano-SiO2 which indicates the structure of nano-SiO2 is amorphous. Compared with the

136

XRD of unmodified nano-SiO2, the height of diffraction peak of organic one slightly reduces. It

137

indicates that the content of pure nano-SiO2 in the organically modified samples is reduced, the

138

nano-SiO2 and KH570 have formed cross-linked. By contrast, the diffraction peak of E-S PPD is

139

higher and sharper which indicates that the particles change towards crystal, as the nano-SiO2 had

140

combined with EVA.

7


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Figure 3. XRD patterns of unmodified, modified nano-SiO2 and E-S PPD.

3.1.4. Dispersion evaluation of nano-SiO2 and E-S PPD.

144

In order to simplify the test and get more explicit comparisons, water and dodecane (instead

145

of -35# diesel) were used to explore the dispersion of 0.2 wt% unmodified and modified

146

nano-SiO2. As shown in Figure 4(a), the unmodified nano-SiO2 sediments entirely to the bottom of

147

the bottle in dodecane and forms homogeneous solution in water which complies with its

148

hydrophilicity. In Figure 4(b), modified nano-SiO2 forms flocculent precipitation in water and the

149

particles stick to the wall of the bottle, while it disperses well in dodecane which complies with its

150

lipophilicity.

151

The dispersion of 0.2 wt% E-S PPD was also evaluated in Figure 4(c). E-S PPD remarkably

152

sediments and sticks to the wall of the bottle in water but forms homogeneous solution in

153

dodecane which complies with the lipophilicity. The clear phenomenon shows the good

154

compatibility between the nano-SiO2 and EVA, and good lipophilicity of E-S PPD.

155 156

D

W

D

(a)

W

(b)

8


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D

157 158 159 160

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W

(c)

Figure 4. Dispersion of (a) unmodified nano-SiO2, (b) modified nano-SiO2, (c) E-S PPD (W represents water, D represents dodecane).

161

3.2 Effect of E-S PPDs on Rheology and Crystallization Related Properties of Model/Crude

162

oil.

163

3.2.1. Pour point.

164

Compared to the pour points of oils undoped with PPDs, the pour points of both oils doped

165

with PPDs dropped significantly in Figure 5. By contrast (a) and (b), (c) and (d) in Figure 5, the

166

depression performance of E-S PPDs is much better than that of pure EVA PPDs. As shown in

167

Figure 5(d), the adaptability of both oils to the different PPDs is consistent, and the depression

168

performance of E(33)-S PPD is superior to the other PPDs with the optimal concentration. By

169

adding E(33)-S PPD with the concentration of 0.08 wt% to the samples, the pour points of model

170

oil and crude oil dropped from 34°C and 38°C to 7°C and 22°C, respectively. The depression rates

171

were 79.4% and 42%. As shown in Figure 5, the E-S PPDs give the outstanding depression

172

performance, while the VA content is not the higher, the better. From the perspective of EVA

173

molecular mass (see Table 2), the molecular mass of EVA(38) is higher than the other kinds of

174

EVAs, which is adverse to the solubility of PPD in oil and affects its depression performance. The

175

depression performance of E-S PPD on model oil is more evident than that for crude oil. It can be

176

explained as follows: As temperature drops, amino and hydroxyl in asphaltene and resin can make

177

substantial use of intermolecular force to cross-link forming self-association and enhance

178

difficulty to proton transfer which accelerate the formation of gelling.[19] 9


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181 182

183 184 185 186 187 188

The above results show that E-S PPDs give the outstanding depression performance on both oils.

(a)

(b)

(c)

(d)

Figure 5. Pour points of crude oil (C) and model oil (M) doped with different PPDs at different concentrations. (a) M doped with EVA PPDs, (b) M doped with E-S PPDs, (c) C doped with EVA PPDs, (d) C doped with E-S PPDs.

3.2.2. Viscosity-temperature curve.

189

The viscosity-temperature relationship curves of model/crude oil were measured. As shown

190

in Figure 6, with the viscosity of model oil increases slowly as the decrease of temperature in a

191

narrow temperature range. By contrast, the viscosity of crude oil increases faster and higher than

192

that of the model oil. Moreover, from their variation tendency of viscosity, both of them almost

193

increase linearly with the decrease of temperature. The viscosity of model oil increases

194

dramatically around 40°C (see 6(a)), the reasons for this phenomenon may the wax crystals

195

precipitate continuously, and interlock to impeding the flow of oil, the state of model oil converts 10


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196

from sol to gel. As shown in Figure 6(b), there is no dramatic increase. The difference in the

197

phenomenon is attributed to asphaltene which serves as a natural flow improver in inhibiting the

198

abrupt precipitation of wax.[17] Since asphaltene and resin are large heterogeneous molecules, as

199

the temperature further drops, asphaltene and resin may associate to form colloidal particles and

200

interlock each other which promote the rise of viscosity. The viscosity of crude oil with E(28)-S

201

PPD and E(33)-S PPD is similar, by contrast, the effect of E(38)-S PPD is a little weak. The

202

phenomenon might be explained that the excessive VA content is adverse to co-crystallization of

203

PPD molecules and wax molecules, meanwhile, the big molecular weight of E(38)-S leads to its

204

poor solubility in oil and cross-link between molecules to the disadvantage of reducing viscosity

205

of oil.

206 207

The above results show that E-S PPDs give the outstanding effect on improving the fluidity of both oils.

208 209 210 211

(a)

(b)

Figure 6. Viscosity-temperature curves of M and C

3.2.3. Microscopic study.

212

The images of wax crystal of model oil are listed in Figure 7. The wax crystals of pure model

213

oil are net-like and interlock each other (see Figure 7(a)). We notice that the wax crystals

214

morphology doped with EVA(33) change from originally net-like to thickly needle-like (see

11


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Figure 7(b)). Compared with Figure 7(a), after addition of different kinds of E-S PPDs, the wax

216

crystal morphology (see Figure 7(c-e)) changes obviously. The wax crystal morphology doped

217

with E(38)-S PPD changes from net-like to sheet-like (see Figure 7(c)). The wax crystal

218

morphology doped with E(28)-S PPD changes from net-like to rod-like (see Figure 7(d)). The wax

219

crystal morphology doped with E(33)-S PPD changes from net-like to more compact and

220

slenderly rod-like (see Figure 7(e)). The changes of wax crystal morphology might be explained

221

as follows: The E-S PPDs serve as crystalline nucleus which changes the precipitating behavior of

222

wax crystals. The suitable content of VA groups contributes to the reduction of solid–liquid

223

interfacial area between oil and wax crystal.

50µm

224 225

(a)

50µm

50µm

226 227

(b)

(c)

12


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50µm

50µm

228 229 230 231

Figure 7. Wax crystal images of M at 20°C. (a) Pure M, M doped with (b) EVA(33), (c) E(38)-S, (d) E(28)-S, (e) E(33)-S

232

The wax crystal change images of crude oil are listed in Figure 8. The wax crystal of pure

233

crude oil is small and highly abundant (see Figure 8(a)). We notice that the wax crystals of crude

234

oil doped with EVA(33) become smaller than that of pure crude oil (see Figure 8(b)). While, after

235

addition of different kinds of E-S PPDs, compared with Figure 8(a), the wax crystal morphology

236

changes obviously. After addition of E(38)-S PPD, the amount of precipitated wax crystals

237

reduces significantly (see Figure 8(c)). After addition of E(28)-S PPDs, the wax crystal aggregates

238

into large flocs, the size is much bigger than that in Figure 8(a-c) (see Figure 8(d)). As shown in

239

Figure 8(e), the addition of E(33)-S PPDs further facilitates the aggregation of wax crystal into

240

more regular and compact large flocs, the solid-liquid interfacial area obviously narrow, and the

241

amount of wax crystal reduces sharply.

(d)

(e)

50µm

242 243

(a)

13


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50µm

244 245

50µm

(b)

(c)

50µm

50µm

246 247 248 249

(d)

(e)

Figure 8. Wax crystal images of C at 20°C. (a) Pure C, C doped with (b) EVA(33), (c) E(38)-S, (d) E(28)-S, (e) E(33)-S

250

The microscopic images of both oils show that the morphology of wax crystals and

251

crystallization behavior are changed with PPDs, among which the effect of E(33)-S PPD is the

252

best. The phenomenon can be explained as follows: The content of VA group and EVA molecular

253

mass affect the performance of the PPDs, the appropriate VA content and EVA molecular mass are

254

more conducive to the co-crystallization of wax molecules and PPD molecules. E-S PPDs provide

255

more nucleation sited for wax molecules precipitating which can modulate waxy crystal

256

morphology. The morphology is conducive to reducing the content of liquid oil occluded in wax

257

crystal and reduces the contact among wax crystals, thus inhibiting the interlock. Meanwhile, by

258

means of high surface energy of nano-SiO2, the change of the waxy crystal morphology to more

259

compact structure narrows the interfacial area of wax crystal, thus makes surface energy maintain

260

a stable energy system in oil.

14


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261

The inconsistency of the wax crystal morphology of both oils might be attributed to the

262

existence of asphaltene and resin. Asphaltene and resin are high-molecular compounds which

263

easily aggregate and form colloidal particles in low temperature. Waxes adhered on the colloidal

264

particles and make the aggregation further developed which strongly affect the crystallization of

265

wax.[19-27] The E-S PPD molecules acting as small crystalline nucleus can be adsorbed on,

266

co-crystal with wax and effectively disperse the aggregation to inhibit the gel-forming.[3,19] And

267

the wax crystal morphology is modulated to the orientation which is adverse to the overlap of wax

268

crystals and the formation of net structure, thus the fluidity of oil can be further improved. As

269

shown in Figure 9, the forming process of E-S PPD and the mechanism analysis of E-S PPD on

270

model/crude oil are intuitively presented.

271 272 273

Figure 9. Mechanism analysis of E-S PPD on oil

CONCLUSION

274

Novel E-S PPDs were prepared and their effects were tested with model oil and crude oil,

275

respectively. The experimental results showed E-S PPD effectively suppressed the gelling process

276

and improved the fluidity of both oils. 0.08 wt% E(33)-S PPD gave the best depressing 15


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performance which made the pour points of model oil and crude oil dropped from 34°C and 38°C

278

to 7°C and 22°C, respectively. The depression rate reached 79.4% and 42%. E-S PPD can also

279

reduce the viscosity of both oils effectively, while the trends of the two oils were different which

280

can be attributed to the existence of asphaltene and resin in crude oil. The different phenomenon

281

also appeared in the waxy crystal morphology.

282

AUTHOR INFORMATION

283

Corresponding Authors

284

Guoliang Mao

285

Northeast Petroleum University, 163000, China.

286

Tel.: 15846160383. Fax.: 0459-6504163

287

E-mail: [email protected]

288

Notes

289 290

The authors declare no competing financial interest. ACKNOWLEDGEMENTS

291

This work was supported by the National Natural Science Foundation of China [51534004,

292

U1362110] and Northeast Petroleum University (No. JYCX_CX03_2018).

293

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Effect Evaluation and Mechanism Analysis of Novel Nano-hybrid Pour

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