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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
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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.
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Meanwhile, the detailed mechanisms of E-S PPD on crude oil are further investigated.
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2. EXPERIMENTAL SECTION
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2.1 Materials.
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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.
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Crude oil, wax and -35# diesel were supplied by Daqing Oilfield and Sinopec, China. (The
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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.
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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
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deionized water. Then, nano-SiO2 was dispersed into the mixed solution with stirring and
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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
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unreacted HK570 were completely removed (pH =7).[14-15] The organic modified nano-SiO2 was
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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
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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.
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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
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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.
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2.3 Properties Characterization.
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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
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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.
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2.3.3. Polarized optical microscopy.
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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
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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
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recorded. 5
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3. RESULTS AND DISCUSSION.
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3.1 Characterization Analysis.
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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
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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
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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.
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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)
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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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179 180
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)
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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)
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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.
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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
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to 7°C and 22°C, respectively. The depression rate reached 79.4% and 42%. E-S PPD can also
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reduce the viscosity of both oils effectively, while the trends of the two oils were different which
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can be attributed to the existence of asphaltene and resin in crude oil. The different phenomenon
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also appeared in the waxy crystal morphology.
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AUTHOR INFORMATION
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Corresponding Authors
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Guoliang Mao
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Northeast Petroleum University, 163000, China.
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Tel.: 15846160383. Fax.: 0459-6504163
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E-mail:
[email protected] 288
Notes
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The authors declare no competing financial interest. ACKNOWLEDGEMENTS
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This work was supported by the National Natural Science Foundation of China [51534004,
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U1362110] and Northeast Petroleum University (No. JYCX_CX03_2018).
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