Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
Ethylene-Vinyl Acetate Copolymer (EVA) and Resin-stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils: 1. Effect of Wax Content and the Synergistic Mechanism Bo Yao, Chuanxian Li, Fei Yang, Xiaoping Zhang, Zhonghua Mu, Guangyu Sun, and Yansong Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03657 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Ethylene-Vinyl Acetate Copolymer (EVA) and Resin-stabilized Asphaltenes
2
Synergistically Improve the Flow Behavior of Model Waxy Oils: 1. Effect of
3
Wax Content and the Synergistic Mechanism
4
Bo Yao a, b, Chuanxian Li a, b, Fei Yang a, b, *, Xiaoping Zhang a, b, Zhonghua Mu a, b, Guangyu Sun a, b and Yansong Zhao c
5
a
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266580, PR China
6
b
Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao, Shandong 266580, PR
7
China
8
c
9
Norway University of Applied Sciences, Inndalsveien 28, 5063 Bergen, Norway
Department of Biomedical Laboratory Sciences and Chemical Engineering, Faculty of Engineering and Science, Western
10
Abstract: Both polymeric pour point depressants (PPDs) and asphaltenes can improve the flowability of
11
waxy oils. However, the effect of polymeric PPDs together with asphaltenes on the flowability of waxy oils
12
is not clear. In this paper, the synergistic effect of EVA PPD (100 ppm) and resin-stabilized asphaltenes (0.75
13
wt%) on the flow behavior of model waxy oils (10~20 wt% wax content) was investigated through
14
rheological test, DSC analysis, microscopic observation and asphaltenes precipitation test. The results
15
showed that the asphaltenes disperse well in the xylene/mineral oil solvent as small aggregates (around 550
16
nm) with the aid of resins. The EVA or asphaltenes alone moderately improve the flow behavior of waxy oils
17
by changing the wax crystals’ morphology from long needle-like to large radical pattern or fine particles,
18
respectively. The wax precipitation temperatures(WPTs) of waxy oils are also slightly decreased by adding
19
EVA or asphaltenes, meaning that the co-crystallization effect between the additives and waxes is dominant.
20
The EVA together with asphaltenes cannot further decrease the WPT, but can dramatically decreases the
21
pour point, gelation point, G′, G″ and apparent viscosity of waxy oils, indicating that a synergistic effect
22
exists between EVA and asphaltenes. The synergistic effect deteriorates with increasing the wax content of
23
waxy oils. The EVA molecules can adsorb on the surface of asphaltene aggregates, thus inhibiting the ACS Paragon Plus Environment
1
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 31
24
asphaltenes precipitation and forming the EVA/asphaltenes composite particles. The formed composite
25
particles can act as wax crystallizing templates and then greatly change the wax crystals’ morphology into
26
large, compact and spherical-like wax crystal flocs, thus dramatically improving the waxy oil flow behavior.
27
This work enriches the theory of micro/nano composite PPDs, which is helpful for developing new PPDs
28
with high efficiency.
29
1. Introduction
30
Waxy crude oil with the wax content ≥ 10 wt% is an important fossil resource. The waxes in crude oil are
31
usually denoted as the high molecular weight n-alkanes (paraffin waxes) with carbon number ranging from
32
C18 to C40 [1]. The solubility of paraffin waxes decreases quickly with temperature drop; when the waxy
33
crude oil temperature is below the wax appearance temperature (WAT), paraffin waxes crystallize out of the
34
oil and start to build the three-dimensional network structure. Due to the irregular morphology of the
35
crystals (plate-like or needle-like), a small amount of the precipitated wax crystals (around 1 wt%) is enough
36
to build a stable network structure, resulting in gelling of the waxy crude oil [2-4]. The gelling significantly
37
increases the oil viscosity and changes the rheological behavior to non-Newtonian, which causes huge
38
economic losses in the production, pipeline transportation and storage processes of waxy crude oil. In
39
addition, the gelling brings huge problems in the pipeline shutdown/restart processes. In such a case, the gel
40
structure formed in waxy crude oil pipeline during shutdown makes the restart operation of the pipeline
41
more difficult [5].
42
In crude oil industry, a small amount of polymeric pour point depressants (PPDs) is often added into waxy
43
crude oil to improve the flow behavior of the oil [6]. The molecular structure of polymeric PPDs normally
44
contains both the non-polar moieties and the polar moieties [6]: the non-polar moieties are long alkyl chains
45
with carbon number ≥ 18, which could participate in the precipitation process of paraffin waxes; the polar
46
moieties such as ester, maleic anhydride (MA) and vinyl acetate (VA) groups, can interfere the growth of ACS Paragon Plus Environment
2
Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
47
wax crystals. According to the position of the long alkyl chains in the molecular structure, polymeric PPDs
48
could be classified into two types: the linear copolymers with the long alkyl chains located in the backbone
49
(such as the ethylene-vinyl acetate copolymer (EVA) [7-9]) and the comb-like copolymers with the long
50
alkyl chains located in the side chain (such as the polyacrylates [10-12]). To guide the application of
51
polymeric PPDs in crude oil pipelines, the effects of the molecular structure of PPDs [13-15], wax
52
composition and content [16, 17], thermal/shear histories [6, 18] on the efficiency of the polymeric PPDs
53
have been widely studied; the mechanisms of polymeric PPDs were also well discovered [6-18]: the
54
polymeric PPDs could participate in the precipitation process of wax molecules and then change the
55
morphology of precipitated wax crystals, thus improving the flow behavior of waxy crude oil.
56
Most of the crude oil contains a certain amount of asphaltenes, which are the heaviest and most polar
57
portion of crude oils [19, 20]. Asphaltenes are often composed of condensed polyaromatic rings containing
58
aliphatic and naphthenic side chains and sulfur, oxygen, nitrogen as heteroelements or functional groups.
59
Metals such as vanadium and nickel are also present in this fraction as part of porphyrinic or nonporphyrinic
60
groups [19, 20]. Asphaltene molecules often exist in the crude oil as asphaltene aggregates due to the strong
61
self-association propensity of the molecules, and the aggregate size could range from nano- to micro-meter
62
[21, 22]. During the last two decades, the effects of asphaltenes on the flow behavior of model waxy oil have
63
been widely studied. The asphaltenes were first extracted from real crude oil by n-pentane/n-heptane
64
precipitation, and then the extracted asphaltenes were re-dispersed in waxy oil with the aid of heating and
65
agitation [23-28]. The results showed that [23-28]: (a) the asphaltenes disperse in oil phase as asphaltene
66
aggregates; (b) a small dosage of asphaltenes (around 0.1 wt%) can greatly modify the morphology of
67
precipitated wax crystals, thus improving the flow behavior of waxy oil; (c) the effect of asphaltenes on the
68
WAT of waxy oil is controversial: some works show the WAT decreases after adding asphaltenes, but some
69
other works get the opposite result; (d) re-dispersion of asphaltenes in waxy oil is difficult due to the lack of ACS Paragon Plus Environment
3
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
70
resins, and the dispersion state of asphaltenes in waxy oil is quite different from that in real crude oil.
71
Therefore, a more reasonable method should be proposed to make sure the similar dispersion state of
72
asphaltenes in model waxy oil and real crude oil.
73
Obviously, both the polymeric PPDs and asphaltenes could improve the flow behavior of waxy oil.
74
However, the effect of polymeric PPDs together with asphaltenes on the flow behavior of waxy oil is unclear.
75
Recently, several researchers stated that the performance of polymeric PPDs is influenced by asphaltenes.
76
Zhang et al [29, 30] synthesized a series of terpolymer PPDs with the octadecyl acrylate, MA and VA as the
77
reacting monomers and found that the terpolymer PPDs have good pour point depressing efficiency on waxy
78
oils. They deduced that the terpolymer PPDs could interact with the resins and asphaltenes and then form
79
asphaltene-PPD-resin agglomerates. The new agglomerates could change the crystallization process of wax
80
molecules, thus obviously improving the flowability of waxy oils. Guo et al [13, 31] synthesized comb-like
81
maleic alkylamide-α-octadecene copolymers (MACs) and its derivatives, and evaluated the efficiency of the
82
copolymers as PPDs for waxy oils. They found that the MACs with an appropriate ratio of polar/nonpolar
83
group or with small aromatic pendants possess the best effect on the flowability of waxy oils. They supposed
84
that the MACs could adsorb on the surface of asphaltenes and then form assembly model of MACs and
85
asphaltenes, which contributed to the improvement of cold flowability of waxy oils. However, the works
86
mentioned above are preliminary and do not provide convincible evidences upon the interactions between
87
the PPDs and asphaltenes. An in-depth understanding of the synergistic effect of polymeric PPDs and
88
asphaltenes on the flow behavior of waxy oils is urgent because it not only enriches the polymeric PPDs’
89
theory but also favors the development of new PPDs with high efficiency.
90
Linear EVA copolymer is a kind of effective polymeric PPDs and has been widely studied as model PPD
91
due to its explicit molecular structure. In this paper, the synergistic effect of EVA (100 ppm) and
92
resin-stabilized asphaltenes (0.75 wt%) on the microstructure and flow behavior of model waxy oils with ACS Paragon Plus Environment
4
Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
93
different wax content (10~20 wt%) was investigated through rheological test, differential scanning
94
calorimetric (DSC) analysis, microscopic observation and asphaltenes precipitation test. The results showed
95
that the EVA together with asphaltenes can synergistically modify the precipitated wax crystals’ morphology,
96
thus dramatically improving the flow behavior of the waxy oils. The synergistic mechanism was also well
97
discovered base on the interaction between the EVA and asphaltene aggregates.
98
2. Experimental
99
2.1 Materials
100
The mineral oil, n-pentane, xylene and EVA PPD were purchased from Sigma-Aldrich Co., Ltd. The
101
mineral oil is mainly composed of isoalkanes with the carbon number ranging from C16 to C26 and contains
102
little paraffin waxes (see Figure S1 in the support information file). The solid wax used here is the same as
103
that mentioned in a former published paper [32], which is a mixture of two paraffin waxes with different
104
melting point ranges. As seen in Figure S2 in the support information file, the solid wax has a wide carbon
105
number distribution (C19~C50), which is similar to the carbon number distribution of waxes in the real crude
106
oil. The VA content and the melting index of the EVA PPD are 28 wt% and 6, respectively. According to the
107
melting index, the average molecular weight of EVA copolymer is calculated as 20000 [33]. The dosage of
108
EVA in the subsequent model waxy oils was fixed at 100 ppm.
109
The model waxy oils were prepared by dissolving a certain amount of solid wax in the xylene/mineral oil
110
solvent. The xylene content in waxy oils was fixed at 20 wt%, while the solid wax content was fixed at
111
10~20 wt%. The rest part of the waxy oils was the mineral oil. In order to keep the original dispersion state
112
of asphaltenes in crude oil, a small amount of Tahe heavy oil was directly added into the waxy oils. As seen
113
in Figure S3 and Table 1 in the support information file, the heavy oil has little paraffin wax and the contents
114
of n-petane precipitated asphaltenses (C5-asphaltenes) and resins are 29.8 wt% and 5.1 wt%, respectively.
115
The final content of C5-asphaltenes in the waxy oils was fixed at 0.75 wt%, that is, the content of the heavy ACS Paragon Plus Environment
5
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 31
116
oil in the waxy oils should be around 2.5 wt%. The dispersion state of the asphaltenes in the xylene/mineral
117
oil solvent (the mass ratio of xylene/mineral oil is same as that of the 10 wt% waxy oil) is shown in Figure
118
S4 of the support information file. Obviously, the asphaltenes stabilized by resins disperse well in the
119
xylene/mineral oil solvent as asphaltene aggregates, which have a wide particle size distribution (113
120
nm~1.45 µm) and an average particle sizes around 550 nm.
121
2.2 Methods
122
2.2.1 Pour point tests
123
The pour points of the undoped/doped waxy oils were measured on the basis of the method given in the
124
Chinese Standard SY/T 0541-2009 [34].
125
2.2.2 Rheological tests
126
All the rheological tests were conducted by using an AR-G2 Rheometer (TA instrument Co., USA)
127
equipped with a coaxial cylinder system (a standard cup having a diameter of 30 mm, configured with a DIN
128
Rotor having a diameter of 28 mm). A plastic cover was placed over the measuring cell to minimize
129
evaporation. Before loaded into the rheometer for testing, the undoped/doped waxy oils were preheated at
130
60 °C for 20 min in sealed glass bottles. The cooling rate of each rheological test was fixed at 0.5 °C/min.
131
Structural development of the undoped/doped waxy oils under static cooling condition. The
132
undoped/doped waxy oils were cooled from 60 °C to 15 °C under the oscillation mode, during which the
133
elastic modulus G′, viscous modulus G″ and loss angle δ were monitored. The gelation point, at which the G′
134
equals G″ or δ equals 45°, was also obtained. The oscillatory amplitude was fixed at 0.0005, which was so
135
small that the formation of the waxy oil gel would not be disturbed (that is, static cooling condition) [32,34].
136
The oscillatory frequency was fixed at 1 Hz.
137
Structural development of the undoped/doped waxy oils under dynamic cooling condition. A constant
138
shear rate (10 s-1) was imposed on the undoped/doped waxy oils during the cooling process from 60 °C to ACS Paragon Plus Environment
6
Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
139
Energy & Fuels
15 °C. The variation of the oil viscosity/apparent viscosity with temperature drop was monitored.
140
2.2.3 Crystallization exothermic characteristics tests
141
By using a DSC 821e calorimeter (Mettler-Toledo Co., Switzerland), the exothermic characteristics of
142
undoped/doped waxy oils were analyzed. The temperature scanning range was fixed at 85~-20°C and the
143
cooling rate was fixed at 2, 5, 7 and 10 °C/min, respectively. Based on the exothermic DSC curves, the wax
144
precipitation temperature (WPT) and amount of precipitated wax crystals at different temperatures were also
145
calculated and recorded.
146
2.2.4 Microstructure observation tests
147
By using a high-resolution OLYMPUS BX51 microscope (Olympus Co., Japan) fitted with an automatic
148
thermal stage, the microstructure of the undoped/doped waxy oil was observed under both the normal optical
149
and the polarized optical conditions. After being preheated at 60 °C for 20 mins, one droplet of the oil sample
150
was transferred to a glass slide covered by a coverslip. The loaded oil sample was cooled statically from 60 °C
151
to 30 °C and 20 °C on the thermal stage under a fixed cooling rate of 0.5 °C/min. The microstructure of the oil
152
samples at 30 °C and 20 °C was observed and photographed.
153
2.2.5 Asphaltenes precipitation test
154
A certain amount of Tahe heavy oil was first dispersed in the mineral oil/xylene solution (with the mass
155
ratio of mineral oil/xylene at 4:1) to obtain the mixed solution containing 0.75 wt% asphaltenes (with the
156
mass of the mixed solution as W ). Then the effect of EVA PPD on the stability of asphaltenes in the mixed
157
solution was investigated through n-pentane precipitation. The dosage of EVA was fixed at 100 ppm, while
158
the volume ratio of the mixed solution/n-pentane was fixed at 1:1, 1:3, 1:5, 1:10 and 1:20, respectively. The
159
precipitated sediments (that is, the asphaltenes) were high-speed centrifuged, vacuum dried and weighted as
160
W1 . Therefore, the precipitated asphaltenes percentage f ASP could be calculated as f ASP =
161
addition, the morphology of the dried asphaltenes was tested by Zeiss Merlin SEM (Zeiss Co., German).
W1 ×100% . In W
ACS Paragon Plus Environment
7
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
162
3. Results and discussions
163
3.1 Effect of EVA together with asphaltenes on the pour point of waxy oils
Page 8 of 31
164
To eliminate the influence of deasphaltened oil on the flow behavior of waxy oils, the pour point of the
165
waxy oils doped with deasphaltened oil was first measured here. The deasphaltened oil was prepared by
166
removing the asphaltenes from Tahe heavy oil using excess n-pentane. The content of deasphaltened oil in
167
the waxy oils was fixed at 1.75 wt%. As seen in Table S2 of support information file, the addition of
168
deasphaltened oil slightly decreases the pour point of 10 wt% waxy oil from 36 °C to 35 °C. For 15 wt% and
169
20 wt% waxy oils, adding deasphlatened oil does not affect the pour point of the waxy oils. Obviously,
170
adding 1.75 wt% deasphaltened oil has little influence on the flow behavior of waxy oils, which is in
171
agreement with the previously reported works [29, 35].
172
The pour points of the waxy oils undoped/doped with EVA, asphaltenes and EVA/asphaltenes are
173
illustrated in Table 1. For 10 wt% waxy oil, the original pour point is 36 °C. Adding 100 ppm EVA depresses
174
the pour point to 31 °C; adding 0.75 wt% asphaltenes depresses the pour point to 30 °C; adding EVA
175
together with asphaltenes can suppress the pour point to < -10 °C. For 15 wt% waxy oil, the original pour
176
point increases to 40 °C, and the pour point depressing ability of neat EVA (37 °C) or neat asphaltenes
177
(36 °C) is slightly inhibited. However, adding EVA together with asphaltenes can still suppress the pour
178
point of 15 wt% waxy oil to < -10 °C. For 20 wt% waxy oil, the original pour point is the highest (42 °C),
179
and the pour point depressing ability of neat EVA (41 °C) or neat asphaltenes (39 °C) is further inhibited.
180
Nevertheless, the EVA together with asphaltenes still exhibit a much better performance in suppressing the
181
pour point of 20 wt% waxy oil to 30 °C. It could be concluded here that the EVA together with asphaltenes
182
can synergistically depress the pour point of waxy oils, and the synergistic effect deteriorates with increasing
183
the wax content of waxy oils.
184
3.2 Effect of EVA together with asphaltenes on the structural development of waxy oils ACS Paragon Plus Environment
8
Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
185
3.2.1 Structural development of waxy oils under static cooling condition
186
The viscoelastic test under oscillation mode is an efficient way to investigate the sol-gel transition process
187
of waxy crude oil under static cooling condition [4, 32, 34, 36]. Figure 1 shows the viscoelastic development
188
of undoped/doped 10 wt% waxy oils with temperature drop. For the pure waxy oil (Figure 1a), although
189
both G′ and G″ are very small at temperatures higher than or around the WPT, G″ (around 10-2 Pa) is quite
190
higher than G′ (around 10-3 Pa), resulting in the value of δ approaching 90° and a pure viscous fluid behavior
191
of the oil. With the decrease of oil temperature, both G′ and G″ increase quickly due to the continuous
192
precipitation of wax crystals, and the rising speed of G′ is much larger than that of G″, finally leading to a
193
cross of the G′ and G″ at the gelation point of the oil (37 °C). After that, G′ and G″ increase continuously
194
with the further decrease of oil temperature and the value of G′ is always higher than G″, indicating that
195
elastic response of the oil is dominant below the gelation point. Adding 100 ppm EVA PPD or 0.75 wt%
196
asphaltenes decreases the gelation point to 25.3 °C and 30.9 °C, respectively, indicating that the waxy oil
197
gelation process is retarded after the addition of EVA or asphaltenes. Meanwhile, the values of G′ and G″ are
198
also decreased by adding the EVA or asphaltenes: at the test temperature 15 °C, the G′ and G″ of the
199
undoped waxy oil are 2.011×105 Pa and 1.207×104 Pa, respectively, confirming a strong gel structure of
200
undoped waxy oil; the G′ and G″ decrease to 3.198×104 Pa and 5.922×103 Pa after adding neat EVA and
201
decrease to 5.068×104 Pa and 1.105×104 Pa after adding neat asphaltenes. For the waxy oil doped with
202
EVA/asphaltenes, during the entire cooling process (55~15°C), the value of G″ presents a very slow
203
increasing trend with the decrease of temperatures (always lower than 10-1 Pa), while the value of G′ is
204
around 10-3~10-2 Pa without any dramatic increase. At the test temperature 15 °C, the G′ and G″ of the waxy
205
oil doped with EVA/asphaltenes are only 2.240×10-3 Pa and 6.391×10-2 Pa, respectively. This indicates that
206
the waxy oil doped with EVA/asphaltenes behaves as a viscous fluid under the experimental condition.
207
The viscoelastic development of undoped/doped 15 wt% waxy oil with temperature drop is displayed in ACS Paragon Plus Environment
9
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 31
208
Figure 2. The gelation point of the undoped waxy oil increases to 40.4 °C due to the increase of wax content.
209
The G′ and G″ of the undoped waxy oil at 25 °C are 1.014×105 Pa and 2.104×104 Pa, respectively. Adding
210
neat EVA or asphaltenes decreases the gelation point to 30.9 °C and 37.6 °C, respectively. The G′ and G″ at
211
25 °C decrease to 8.370×102 Pa and 2.490×102 Pa after adding neat EVA and decrease to 2.980×104 Pa and
212
1.199×104 Pa after adding neat asphaltenes. Similar to the 10 wt% waxy oil, the 15 wt% waxy oil doped
213
with EVA/asphaltenes has very small values of G′ (around 7.000×10-3 Pa) and G″ (around 4.000×10-2 Pa),
214
and behaves as a viscous fluid under the experimental condition.
215
When the wax content increases to 20 wt% (see Figure 3), the performance of the additives is greatly
216
inhibited. The gelation point of the undoped waxy oil increases to 43.3 °C. The G′ and G″ of the undoped
217
waxy oil at 30 °C are 2.430×105 Pa and 3.612×104 Pa, respectively. Adding neat EVA or asphaltenes could
218
only decrease the gelation point to 39.9 °C and 40.2 °C, respectively. The G′ and G″ at 30 °C decrease
219
slightly to 2.361×105 Pa and 2.975×104 Pa after adding neat EVA but increase slightly to 3.412×105 Pa and
220
3.740×104 Pa after adding neat asphaltenes. Adding EVA/asphaltenes still takes synergistic effect and
221
decreases the G′ and G″ at 30 °C to 2.758×102 Pa and 3.590×101 Pa, respectively; but the viscoelastic curve
222
starts to exhibit the gelation point at 38.4 °C.
223
Based on the results mentioned above, it could be concluded that the EVA together with asphaltenes can
224
synergistically improve the viscoelastic properties of waxy oils, and the synergistic effect deteriorates with
225
increasing the wax content of waxy oils.
226
3.2.2 Structural development of waxy oils under dynamic cooling condition
227
Figure 4a displays the viscosity/apparent viscosity increase of 10 wt% undoped/doped waxy oils with
228
decreasing temperature measured at 10 s-1. At temperatures above the WPT, the viscosities of the
229
undoped/doped waxy oils exhibit a strict linear dependence with the decreasing temperatures (in the
230
semi-log coordinate), and the additives show no viscosity reducing performances but slightly increase the ACS Paragon Plus Environment
10
Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
231
viscosity of the oil. When the temperature decreases to below WPT, the apparent viscosity of undoped waxy
232
oil first increases sharply. After adding the neat asphaltenes, the sharp increase of the apparent viscosity
233
shifts to a lower temperature, verifying the rheological improvement after the asphaltenes beneficiation.
234
However, the apparent viscosity of the waxy oil doped with neat asphaltenes exhibits a quick increase trend,
235
and finally, grows over that of undoped waxy oil at 29.5 °C. For the waxy oil doped with neat EVA, the
236
apparent viscosity development below WPT is a slowly increasing process. At temperatures above 16 °C,
237
EVA reduces the apparent viscosity of waxy oil effectively, but below 16 °C, the apparent viscosity of waxy
238
oil doped with EVA even exceeds that of undoped waxy oil. The waxy oil doped with EVA/asphaltenes
239
displays no sharp apparent viscosity increase during the entire cooling process (60~15 °C) and the apparent
240
viscosity at 15 °C is only 8.07 mPa·s, which is orders of magnitude smaller than the other oil samples.
241
As seen in Figure 4b, for 15 wt% waxy oils, the viscosity reducing efficiency of the neat EVA and neat
242
asphaltenes has deteriorated a lot, while adding EVA together with asphaltenes still takes a good effect in
243
reducing the apparent viscosity of waxy oil. When the wax content increases to 20 wt%, adding the
244
individual additives (EVA or asphaltenes) nearly exhibits no viscosity reducing performance. Adding EVA
245
together with asphaltenes can still effectively decrease the apparent viscosity of 20 wt% waxy oil, but its
246
efficiency has been greatly hindered.
247
Obviously, add EVA together with asphaltenes can synergistically decrease the apparent viscosity of waxy
248
oils under dynamic cooling, and the synergistic effect deteriorates with increasing the wax content of waxy
249
oils.
250
3.3 Crystallization exothermic characteristics of the undoped/doped waxy oils
251
The DSC curves at cooling rate of 10 °C/min and the precipitated wax crystals’ amount of the
252
undoped/doped waxy oil are exhibited in Figure S5 and S6 of support information file. It is clear that adding
253
different additives takes prominent effect in decreasing the WPT of waxy oil, meaning that the ACS Paragon Plus Environment
11
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 31
254
co-crystallization effect between the additives and waxes is dominant. The precipitated wax crystals’ amount
255
at low temperatures changes little, indicating that adding different additives has little influence on the final
256
amount of the precipitated wax crystals.
257
It is well known that the WPT of waxy oil measured by DSC instrument is closely related to the applied
258
cooling rate. The DSC curves of the undoped/doped waxy oils at different cooling rates (2 °C/min, 5 °C/min,
259
7 °C/min and 10 °C/min) were measured, and the WPTs corresponding with different cooling rates were
260
obtained and shown in Figure 5 and Table 2. Obviously, the WPTs of the undoped/doped waxy oils decrease
261
linearly with the increase of cooling rate corresponding to the results in previous works [37-40], which could
262
be explained by the combination effects of wax crystals’ nucleation and growth, and the thermal lag between
263
sample and DSC platform [41,42].
264
Based on Figure 5, the WPTs of the undoped/doped waxy oils at the cooling rate of 0.5 °C/min were
265
calculated and shown in Table 2. At fixed wax content, adding 100 ppm EVA can slightly decrease the WPT
266
of waxy oil. On the one hand, EVA molecules can co-crystallize with the wax molecules, promoting the
267
effective solubility of wax molecules in the oil phase. On the other, the incoming of impurities (VA groups)
268
increase the interfacial tension between the contaminated wax crystals and liquid phase, effectively
269
increasing the critical nucleation radius as well as the nucleation potential barrier of wax crystals, thereby
270
inhibiting the wax precipitation [7,34]. The WPT of waxy oil is decreased further after adding 0.75 wt%
271
asphaltenes. Asphaltenes can provide large amount of nucleation sites for the crystallization of wax
272
molecules (see Figure 8), which should increase the WPT of waxy oil. However, the resin-stabilized
273
asphaltenes aggregates are often surrounded by large amounts of aliphatic lateral chains, which can
274
co-crystallize with wax molecules and then decrease the WPT of the oil. We consider that the
275
co-crystallization effect dominates over the nucleation effect, thus causing the decrease of the WPT after
276
adding 0.75 wt% asphatlenes. Adding EVA together with asphaltenes cannot further decrease the WPT of the ACS Paragon Plus Environment
12
Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
277
Energy & Fuels
waxy oils.
278
Based on the discussions mentioned above, it could be concluded that the co-crystallization effect
279
between the additives and waxes is dominant and results in the decrease of WPT, but adding different
280
additives has little influence on the precipitated wax crystals amount at low temperatures.
281
3.4 Microstructures of the undoped/doped waxy oils
282
Microstructure images of the undoped 10 wt% waxy oil at 30 °C (Figure 6a and b) show that the
283
precipitated wax crystals are long needle-like with high aspect ratios. This kind of wax crystal is easier to
284
form a continuous network structure, thus causing high pour point/gelation point of the oil. When the oil
285
temperature drops to 20 °C (Figure 6c and d), the precipitated wax crystals become longer with larger
286
amount, resulting in the formation of a strong three-dimensional network structure.
287
As illustrated in Figure 7a and b, adding neat EVA greatly modifies the microstructures of the precipitated
288
wax crystals at 30 °C. Compared to the undoped waxy oil, the morphology of single wax crystal is still
289
needle-like but much shorter in length, and the precipitated needle-like crystals tend to aggregate into loose
290
radical pattern wax flocs, which favor the flow improvement of the oil. When the oil temperature drops to
291
20 °C (Figure 7c and d), the radical pattern wax flocs become larger in size and more obvious, indicating
292
that more waxes precipitate from the oil.
293
As seen in Figure 8a and b, the precipitated wax crystals at 30 °C become fine particles with relatively
294
large amount after adding 0.75 wt% asphaltenes, meaning that the asphaltenes act as the wax nucleates. The
295
fine wax particles are helpful for the initial flow improvement of the oil. When the oil temperature drops to
296
20 °C (Figure 8c and d), the precipitated wax crystal size is still small but with very large amount, which is
297
adverse for the flow improvement of the oil.
298
As shown in Figure 9a and b, the precipitated wax crystals at 30 °C begin to aggregate into large, compact
299
and spherical-like wax crystal flocs after adding EVA/asphaltenes. With the decreasing of temperatures ACS Paragon Plus Environment
13
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 31
300
(Figure 9c and d), the sizes of wax crystal flocs grow continuously and the morphology of the wax crystal
301
flocs is still compact and spherical-like. This kind of wax crystal flocs is very difficult to build a continuous
302
network structure, thus dramatically improving the oil flowability.
303
3.5 Asphaltenes precipitation test
304
SEM images of the precipitated asphaltenes without and with the addition of EVA are present in Figure
305
10a and b. It is clear that adding EVA greatly decreases the size of the precipitated asphaltene aggregates,
306
which indicates that the EVA molecules adsorb on the asphaltene aggregates and inhibit the agglomeration
307
of the asphaltene aggregates. The percent of precipitated asphaltenes f ASP at different volume ratios of
308
mixed solution and n-pentane is shown in Figure 10c. The f ASP first increases quickly with the increase of
309
n-pentane ratio, and then reaches nearly saturated when n-pentane ratio is high. Meanwhile, at each
310
n-pentane ratio, the value of f ASP decreases obviously after adding EVA, which also means that the EVA
311
molecules adsorb on the asphaltene aggregates and act as asphaltenes dispersant. As reported in previous
312
works [43,44], EVA is a macro-surfactant whose structure contains both polar and nonpolar moieties, which
313
can act as asphaltene dispersant to adsorb onto the asphaltene flocs, thus forming the EVA/asphaltenes
314
composite particles.
315
3.6 Synergistic mechanism of EVA and asphaltenes on improving the flow behavior of waxy oils
316
Recently, introducing inorganic nano/micro particles, such as silica [45,46], clay [12,47] and graphene
317
oxide [48], into the polymeric PPDs’ matrix to improve the performance of the PPDs has become a hot
318
research spot. Yang et al [12,34,47,49] detailedly investigated the effects of different nano/micro particles on
319
the performance of the polymeric PPDs (such as EVA and POA). They found that the polymeric PPDs and
320
inorganic particles can form nano/micro composite particles in oil phase, which could act nucleation
321
templates of wax crystals and then further modify the morphology of precipitated wax crystals. Therefore,
322
the rheology of waxy crude oil is further improved after the addition of nano/micro composite PPDs. ACS Paragon Plus Environment
14
Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
323
As illustrated in section 3.5, the EVA PPD can adsorb on the asphaltene aggregates to form the
324
EVA/asphaltenes composite particles. Like the nano/micro composite PPDs, we consider that the
325
EVA/asphaltenes composite particles can also effectively act as the heterogeneous nucleation templates for
326
wax molecules to precipitate, thus forming large, compact and spherical-like wax crystal flocs (see Figure
327
11). On the one hand, the larger size of the wax flocs with regular morphologies greatly reduces the wax
328
crystal/oil interface area and then weakens the interactions among the precipitated wax crystals; on the other,
329
the compact microstructure of the wax flocs enables the wax flocs to occlude less liquid oils. Both of two
330
sides dramatically improve the flow behavior of the oil. The synergistic mechanism of EVA together with
331
asphaltenes on the waxy oil flow behavior not only enriches the theory of nano/micro composite PPDs, but
332
also helps for developing new PPDs with high efficiency.
333
4. Conclusions
334
In this paper, the synergistic effect of EVA PPD (100 ppm) and resin-stabilized asphaltenes (0.75 wt%) on
335
the flow behavior of model waxy oils (10~20 wt% wax content) was studied detailed by using through
336
rheological test, DSC analysis, microscopic observation and asphaltenes precipitation test. The following
337
conclusions are drawn:
338
(a) With the aid of resins, the asphaltenes stabilized by resins disperse well in the xylene/mineral oil
339
solvent as asphaltene aggregates, which have a wide particle size distribution (113 nm~1.45 µm) and an
340
average particle sizes around 550 nm.
341
(b) Adding the EVA or asphaltenes alone moderately improve the flow behavior of waxy oils by changing
342
the wax crystals’ morphology from long needle-like into large radical pattern wax crystal flocs or fine wax
343
crystal particles, respectively. The WPTs of waxy oils are also slightly decreased by adding EVA or
344
asphaltene alone.
345
(c) Adding the EVA together with asphaltene cannot further decrease the WPT, but can synergistically ACS Paragon Plus Environment
15
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 31
346
decreases the pour point, gelation point, G′, G″ and apparent viscosity of waxy oils. Meanwhile, the
347
synergistic effect deteriorates with increasing the wax content of waxy oils.
348
(d) Asphaltenes precipitation test shows that the EVA molecules can adsorb on the surface of asphaltene
349
aggregates, thus inhibiting the asphaltenes precipitation and forming the EVA/asphaltenes composite
350
particles. The formed composite particles can act as wax crystallizing templates and then greatly change the
351
wax crystals’ morphology into large, compact and spherical-like wax crystal flocs, thus outstandingly
352
improving the waxy oil flow behavior. The synergistic mechanism of EVA together with asphaltenes on the
353
waxy oil flow behavior not only enriches the theory of nano/micro composite PPDs, but also helps for
354
developing new PPDs with high efficiency.
355
Acknowledgement
356
This work was financially supported by National Natural Science Foundation of China (51774311),
357
Natural Science Foundation of Shandong Province of China (ZR2017MEE022), Key Research Project of
358
Shandong Province of China (GG201703230122), and the Fundamental Research Funds for the Central
359
Universities-China.
360
References
361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377
[1] H.P. Rønningsen, B. Bjoerndal, A.B. Hansen, W.B. Pedersen, Wax precipitation from North Sea crude oils: 1. Crystallization and dissolution temperatures, and Newtonian and non-Newtonian flow properties, Energy Fuels 5 (1991) 895-908. [2] M. Kané, M. Djabourov, J.-L. Volle, Rheology and structure of waxy crude oils in quiescent and under shearing conditions, Fuel 83 (2004) 1591-1605. [3] R.F.G. Visintin, R. Lapasin, E. Vignati, P. D'Antona, T.P. Lockhart. Rheological Behavior and Structural Interpretation of Waxy Crude Oil Gels, Langmuir 21 (2005) 6240-6249. [4] F. Yang, C. Li, D. Wang, Studies on the Structural Characteristics of Gelled Waxy Crude Oils Based on Scaling Model, Energy Fuels 27 (2013) 1307-1313. [5] C. Chang, Q.D. Nguyen, H.P. Rønningsen, Isothermal start-up of pipeline transporting waxy crude oil, J. Non-Newton. Fluid, 87 (1999) 127-154. [6] F. Yang, Y. Zhao, J. Sjöblom, C. Li, K.G. Paso, Polymeric Wax Inhibitors and Pour Point Depressants for Waxy Crude Oils: A Critical Review, J. Dispersion Sci. Technol. 36 (2015) 213-225. [7] A.L.C. Machado, E. FLucas, G. González, Poly(ethylene-co-vinyl acetate) (EVA) as wax inhibitor of a Brazilian crude oil: oil viscosity, pour point and phase behavior of organic solutions, J. Petrol. Sci. Eng. 32 (2001) 159-165. [8] C. Wu, J.-L. Zhang, W. Li, N. Wu, Molecular dynamics simulation guiding the improvement of EVA-type pour point depressant, Fuel 84 (2005) 2039-2047.
ACS Paragon Plus Environment
16
Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424
Energy & Fuels
[9] Y. Ren, Z. Chen, H. Du, L. Fang, X. Zhang, Preparation and Evaluation of Modified Ethylene–Vinyl Acetate Copolymer as Pour Point Depressant and Flow Improver for Jianghan Crude Oil, Ind. Eng. Chem. Res. 56 (2017) 11161-11166. [10] X. Ding, G. Qi, S. Yang, Thermodynamic analysis for the interaction of polyacrylate with wax in heptane, Polymer 40 (1999) 4139-4122. [11] R.A. Soldi, A.R.S. Oliveira, R.V. Barbosa, M.A.F. César-Oliveira, Polymethacrylates: Pour point depressants in diesel oil, Eur. Polym. J. 43 (2007) 3671-3678. [12] B. Yao, C. Li, F. Yang, J. Sjöblom, J. Norrman, K. Paso, Z. Xiao, Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate, Fuel 166 (2016) 96-105. [13] J. Xu, S. Xing, H. Qian, S. Chen, X. Wei, R. Zhang, L. Li, X. Guo, Effect of polar/nonpolar groups in comb-type copolymers on cold flowability and paraffin crystallization of waxy oils, Fuel 103 (2013) 600-605. [14] F. Yang, Z. Xiao, B. Yao, C. Li, L. Wang, X. Shi, G. Sun, K. Yan, Influences of different functional groups on the performance of polyoctadecyl acrylate pour point depressant, Petrol. Sci. Technol. 34 (2016) 1712-1719. [15] B. Yao, L. Wang, F. Yang, C. Li, Y. Zhao, Effect of Vinyl-Acetate Moiety Molar Fraction on the Performance of Poly(Octadecyl Acrylate-Vinyl Acetate) Pour Point Depressants: Experiments and Mesoscopic Dynamics Simulation, Energy Fuels 31 (2017) 448-457. [16] S. Yi, J. Zhang, Relationship between Waxy Crude Oil Composition and Change in the Morphology and Structure of Wax Crystals Induced by Pour-Point-Depressant Beneficiation, Energy Fuels 25 (2011) 1686-1696. [17] L.M.S.L. Oliveira, R.C.P. Nunes, I.C. Melo, Y.L.L. Ribeiro, L.G. Reis, J.C.M. Dias, R.C.L. Guimarães, E.F. Lucas, Evaluation of the correlation between wax type and structure/behavior of the pour point depressant, Fuel Process. Technol. 149 (2016) 268-274. [18] S. Yi, J. Zhang, Shear-Induced Change in Morphology of Wax Crystals and Flow Properties of Waxy Crudes Modified with the Pour-Point Depressant, Energy Fuels 25 (2011) 5660-5671. [19] J. Ancheyta, G. Centeno, F. Trejo, G. Marroquín, J.A. García, E. Tenorio, A. Torres, Extraction and Characterization of Asphaltenes from Different Crude Oils and Solvents, Energy Fuels 16 (2002) 1121-1127. [20] F. Yang, C. Li, S. Yang, Q. Zhang, J. Xu, Effect of dodecyl benzene sulfonic acid (DBSA) and lauric amine (LA) on the associating state and rheology of heavy oils, J. Petrol. Sci. Eng. 124 (2014) 19-26. [21] G. Porte, H. Zhou, V. Lazzeri, Reversible Description of Asphaltene Colloidal Association and Precipitation, Langmuir 19 (2003) 40-47. [22] J. Eyssautier, P. Levitz, D. Espinat, J. Jestin, J. Gummel, I. Grillo, L. Barre, Insight into asphaltene nanoaggregate structure inferred by small angle neutron and X-ray scattering, J. Phys. Chem. B. 115 (2011) 6827-6837. [23] R. Venkatesan, J.-A. Östlund, H. Chawla, P. Wattana, M. Nydén, H.S. Fogler, The Effect of Asphaltenes on the Gelation of Waxy Oils, Energy Fuels 17 (2003) 1630-1640. [24] J.F. Tinsley, J.P. Jahnke, H.D. Dettman, R.K. Prud’home, Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology, Energy Fuels 23 (2009) 2056-2064. [25] K. Oh, M. Deo, Characteristics of Wax Gel Formation in the Presence of Asphaltenes, Energy Fuels 23 (2009) 1289-1293. [26] K.G. Paso. Comprehensive treatise on shut-in and restart of waxy oil pipelines. J. Disper. Sci. Technol. 2014, 35(8): 1060-1085. [27] C. Li, J. Cai, F. yang, Y. Zhang, F. Bai, Y. Ma, B. Yao, Effect of asphaltenes on the stratification phenomenon of wax-oil gel deposits formed in a new cylindrical Couette device, J. Petrol. Sci. Eng. 140 (2016) 73-84. [28] F. Yang, J. Cai, L. Cheng, C. Li, Z. Ji, B. Yao, Y. Zhao, G. Zhang, Development of Asphaltenes-Triggered Two-Layer Waxy Oil Gel Deposit under Laminar Flow: An Experimental Study, Energy Fuels 30 (2016) 9922-9932. [29] L. Fang, X. Zhang, J. Ma, B. Zhang, Investigation into a Pour Point Depressant for Shengli Crude Oil, Ind. Eng. Chem. Res. 51 (2012) 11605-11612. [30] T. Liu, L. Fang, X. Liu, X. Zhang, Preparation of a kind of reactive pour point depressant and its action mechanism, Fuel 143 (2015) 448-454.
ACS Paragon Plus Environment
17
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467
Page 18 of 31
[31] J. Xu, H. Jiang, T. Li, X. Wei, T. Wang, J. Huang, W. Wang, A.L. Smith, J. Wang, R. Zhang, Y. Xu, L. Li, R.K. Prud’homme, X. Guo, Effect of Comb-type Copolymers with Various Pendants on Flow Ability of Heavy Crude Oil, Ind. Eng. Chem. Res. 54 (2015) 5204-5212. [32] B. Yao, C. Li, F. Yang, Z. Mu, X. Zhang, G. Sun, Effect of oil dispersible polymethylsilsesquioxane microspheres on the formation and breakage of model waxy oil gels, Fuel 209 (2017) 424-433. [33] S. Bistac, P. Kunemann, J. Schultz, Crystalline modifications of ethylene-vinyl acetate copolymers induced by a tensile drawing: effect of the molecular weight,. Polymer 39 (1998) 4875-4881. [34] F. Yang, B. Yao, C. Li, X. Shi, G. Sun, X. Ma, Performance improvement of the ethylene-vinyl acetate copolymer (EVA) pour point depressant by small dosages of the polymethylsilsesquioxane (PMSQ) microsphere: An experimental study, Fuel 207 (2017) 204-213. [35] D. Chanda, A. Sarmah, A. Borthakur, K.V. Rao, B. Subrahmanyam, H.C. Das, Combined effect of asphaltenes and flow improvers on the rheological behaviour of Indian waxy crude oil, Fuel 77 (1998) 1163-1167. [36] F. Yang, B. Yao, C. Li, G. Sun, X. Ma, Oil dispersible polymethylsilsesquioxane (PMSQ) microspheres improve the flow behavior of waxy crude oil through spacial hindrance effect, Fuel 199 (2017) 4-13. [37] X. Guo, B.A. Pethica, J.S. Huang, D.H. Adamson, R.K. Prud'homme, Effect of Cooling Rate on Crystallization of Model Waxy Oils with Microcrystalline Poly(ethylene butene), Energy Fuels 20 (2006) 250-256. [38] A.S. Kasumu, S. Arumugam, A.K. Mehrotra, Effect of cooling rate on the wax precipitation temperature of “waxy” mixtures, Fuel 103 (2013) 1144-1147. [39] A. Japper-Jaafar, P.T. Bhaskoro, Z.S. Mior, A new perspective on the measurements of wax appearance temperature: Comparison between DSC, thermomicroscopy and rheometry and the cooling rate effects, J. Pet. Sci. Eng. 147 (2016) 672-681. [40] D.E.V. Andrade, M.A. Marcelino Neto, C.O.R. Negrão, The importance of supersaturation on determining the solid-liquid equilibrium temperature of waxy oils, Fuel 206 (2017) 516-523. [41] E.B. Sirota, Supercooling and transient phase induced nucleation in n-alkane solutions, J. Chem. Phys. 112 (2000) 492-500. [42] F.L. Paiva, V.M.A. Calado, F.H. Marchesini, On the use of Modulated Temperature Differential Scanning Calorimetry to assess wax crystallization in crude oils, Fuel 202 (2017) 216-226. [43] S.M. Hashmi, L.A. Quintiliano, A. Firoozabadi, Polymeric dispersants delay sedimentation in colloidal asphaltene suspensions, Langmuir 26 (2010) 8021-8029. [44] E. Lowry, M. Sedghi, L. Goual, Polymers for asphaltene dispersion: Interaction mechanisms and molecular design considerations, J. Mol. Liq. 230 (2017) 589-599. [45] Z. Sun, G. Jing, Z. Tu, Effect of modified nano-silica/EVA on flow behavior and wax crystallization of model oils with different wax contents, J. Dispersion Sci. Technol. (2017) doi: 10.1080/01932691.2017.1295869. [46] F. Yang, K. Paso, J. Norrman, C. Li, H. Oschmann, J. Sjöblom, Hydrophilic nanoparticles facilitate wax inhibition, Energy Fuels 29 (2015) 1368-1374. [47] B. Yao, C. Li, F. Yang, Y. Zhang, Z. Xiao, G. Sun, Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant, Fuel 184 (2016) 544-554. [48] A.M. Al-Sabagh, M.A. Betiha, D.I. Osman, A.I. Hashim, M.M. EI-Sukkary, T. Mahmoud, Preparation and evaluation of poly (methyl methacrylate)-graphene oxide nanohybrid polymers as pour point depressants and flow improvers for waxy crude oil, Energy Fuels 30 (2016) 7610-7621. [49] B. Yao, C. Li, X. Zhang, F. Yang, G. Sun, Y. Zhao, Performance Improvement of the Ethylene-Vinyl Acetate Copolymer (EVA) Pour Point Depressant by Small Dosage of the Amino-functionalized Polymethylsilsesquioxane (PAMSQ) Microsphere, Fuel under review.
468
ACS Paragon Plus Environment
18
Page 19 of 31
469
Figures
470 471 5
10
5
10
80
4
10
4
10
10 G' G'' / Pa
1
10
GP: 37.0 °C
G' G'' δ
0
10
-1
10
40 20
2
60
doped with EVA GP: 25.8 °C
10
1
10
δ/°
10% undoped waxy oil
G' G'' / Pa
60
2
10
80
3
10
3
δ/°
40
G' G'' δ
0
10
-1
10
20
-2
10
-2
10
0
-3
10
0
-3
10
20
30 40 Temperature / °C
472
50
20
(a)
30 40 Temperature / °C
50
(b)
4
10
80
G' G '' / Pa
1
10
G' G'' δ
0
10
-1
10
60
40
-1
10
GP: N/A
20
10
40
-2
20
10
0
-3
10
20
473 474 475
Figure 1
60
10
-3
-2
G' G'' δ
δ/°
GP: 30.9 °C
doped with EVA/asphaltenes G' G'' / Pa
doped with asphaltenes
2
10
80
0
10
3
10
δ/°
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
30 40 Temperature / °C
50
(c)
20
30 40 Temperature / °C
50 (d)
Viscoelasticity development during cooling of the 10 wt% waxy oil undoped/doped with EVA, asphaltene and EVA/asphaltene.
476
ACS Paragon Plus Environment
19
Energy & Fuels
477 478 4
10
5
10
80
3
10
4
10
3
10
GP: 40.4 °C
1
10
40 G' G'' δ
0
10
-1
10
20
G' G'' / Pa
10
10
60
15% undoped waxy oil
2
1
doped with EVA
10
GP: 30.9 °C
0
10
G' G'' δ
40
-1
10
-2
-2
10
-3
10
10
20
-3
10
30
35
479
40 45 Temperature / °C
50
0
55
60
δ/°
G' G'' / Pa
80
2
δ/°
30
35
(a)
40 45 50 Temperature / °C
55 (b)
5
10
4
80
10
80
10
0
10
doped with asphaltenes
1
10
GP: 37.6 °C
0
10
40
G' G'' δ
-1
10
-2
10
40 20
-3
0
10
-4
30
482
60
20
10
Figure 2
G' G'' δ
-2
-3
480
-1
10
doped with EVA/asphaltenes GP: N/A
10
10
481
60
δ/°
G' G'' / Pa
2
10
G' G'' / Pa
3
δ/°
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 31
35
40 45 50 Temperature / °C
30
55 (c)
35
40 45 50 Temperature / °C
55 (d)
Viscoelasticity development during cooling of the 15 wt% waxy oil undoped/doped with EVA, asphaltene and EVA/asphaltene.
483 484
ACS Paragon Plus Environment
20
Page 21 of 31
485 486 6
10
5
10
5
10
10
3
10
3
1
10
60
20% undoped waxy oil GP: 43.3 °C
40
G' G'' δ
0
10
-1
10
2
10
G' G'' / Pa
2
60
doped with EVA GP: 39.9 °C
1
10
-1
10 20
-2
10
40
G' G'' δ
0
10
δ/°
G' G'' / Pa
10 10
80
4
80
4
10
δ/°
20
-2
10
-3
10
-3
-4
10
30
35
487
40 45 Temperature / °C
50
10
0
30
55 (a)
35
40 45 50 Temperature / °C
55
0 (b)
5
10
2
10
3
1
G ' G '' / Pa
2
10
1
10
G' G'' δ
0
10
-1
60 40
10
-2
10
-4
488 489 490
Figure 3
30
35
40 45 Temperature / °C
50
55
doped with EVA/asphaltenes 60 GP: 38.4 °C
0
10
10
0
10
40
G' G'' δ
-1
10
20
-3
10
10
δ/°
doped with asphaltenes GP: 40.2 °C
G' G'' / Pa
10
10
80
10
80
4
δ/°
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
20
-2
-3
0 30
35
(c)
40 45 50 Temperature / °C
55 (d)
Viscoelasticity development during cooling of the 20 wt% waxy oil undoped/doped with EVA, asphaltene and EVA/asphaltene.
491 492
ACS Paragon Plus Environment
21
Energy & Fuels
493 494 495 0
10 wt% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
-1
10
-2
10
40
10
20
496 0
15 wt% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
10
-1
10
-2
10
10
497
20
30 40 50 Temperature / °C
45
50
30 40 50 Temperature / °C
60 (b)
55
60
60 (a) 20 wt% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
0
10 Apparent viscosity / Pas
Apparent viscosity / Pas
10
Apparent viscosity / Pas
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 22 of 31
-1
10
-2
10
10
20
30 40 50 Temperature / °C
60 (c)
498
Figure 4
Apparent viscosity-temperature curves under dynamic cooling of the waxy oils undoped/doped
499
with EVA, asphaltene and EVA/asphaltene. (a: 10 wt% waxy oil; b: 15 wt% waxy oil; c: 20 wt% waxy oil)
500
ACS Paragon Plus Environment
22
Page 23 of 31
wax precipitation point / °C
36
35
34 10% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
33
2
4
6 8 -1 VT / °Cmin
501
37 36 15% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
35 34
502
504
wax precipitation point / °C
38
2
503
10
(a)
42
39 wax precipitation point / °C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 5
4
6 8 -1 VT / °Cmin
41 40 39 38 37 36
10
20% undoped waxy oil doped with EVA doped with asphaltenes doped with EVA/asphaltenes
2
(b)
4
6 -1 VT / °Cmin
8
10
(c)
WPTs at different cooling rates of the waxy oil undoped/doped with EVA, asphaltene and EVA/asphaltene. (a: 10 wt% waxy oil; b: 15 wt% waxy oil; c: 20 wt% waxy oil)
505
ACS Paragon Plus Environment
23
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 31
506 507 508
509
510 511 512
Figure 6
Microstructure of the 10 wt% undoped waxy oil (a: 30 °C, normal optical; b: 30 °C, polarized optical;c: 20 °C, normal optical;d: 20 °C, polarized optical).
513
ACS Paragon Plus Environment
24
Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
514 515 516 517
518
519 520 521
Figure 7.
Microstructure of the 10 wt% waxy oil doped with EVA (a: 30 °C, normal optical; b: 30 °C, polarized optical;c: 20 °C, normal optical;d: 20 °C, polarized optical).
522
ACS Paragon Plus Environment
25
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 26 of 31
523 524 525 526 527
528
529 530 531
Figure 8.
Microstructure of the 10 wt% waxy oil doped with asphaltenes (a: 30 °C, normal optical; b: 30 °C, polarized optical;c: 20 °C, normal optical;d: 20 °C, polarized optical).
532
ACS Paragon Plus Environment
26
Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
533 534 535 536 537
538
539 540 541
Figure 9.
Microstructure of the 10 wt% waxy oil doped with EVA/asphaltenes (a: 30 °C, normal optical; b: 30 °C, polarized optical;c: 20 °C, normal optical;d: 20 °C, polarized optical).
542
ACS Paragon Plus Environment
27
Energy & Fuels
543 544 545
546 Percentage of precipitated asphaltenes / wt%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 28 of 31
547 548 549
Figure 10.
7.5
7.46
7.36
7.51 7.15
7.0 6.87
6.85
6.5
6.39 6.34
without EVA with EVA
6.0 5.5 5.0
5.65 5.52
1:3 1:20 1:1 1:5 1:10 Volume ratio of mixed solution/n-pentane (c)
The precipitated asphaltenes without (a) and with (b) the addition of EVA; the percentage of
precipitated asphaltenes at different volume ratios of mixed solution and n-pentane (c).
550
ACS Paragon Plus Environment
28
Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
551
552 553 554
Figure 11
Schematic illustration of the synergistic mechanism of EVA and resin stabilized asphaltenes for improving the flow behavior of waxy oil.
555
ACS Paragon Plus Environment
29
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
556
Page 30 of 31
Tables
557 558
Table 1.
The pour points of the waxy oils undoped/doped with EVA, asphaltenes and EVA/asphaltenes. Waxy oil type
Dosage
Pour point / °C
undoped
36
+100 ppm EVA
31
+0.75 wt%asphaltenes
30
+100 ppm EVA/0.75 wt% asphaltenes
