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Ethylene-Vinyl Acetate Copolymer (EVA) and Resin-stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 3. Effect of Vinyl Acetate Content Bo Yao, Chuanxian Li, Zhonghua Mu, Xiaoping Zhang, Fei Yang, Guangyu Sun, and Yansong Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01937 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
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Ethylene-Vinyl Acetate Copolymer (EVA) and Resin-stabilized Asphaltenes
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Synergistically Improve the Flow Behavior of Model Waxy Oils. 3. Effect of
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Vinyl Acetate Content
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Bo Yao a, b, Chuanxian Li a, b, Zhonghua Mu
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Yansong Zhao c
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a
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China
8
b
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266580, PR China
a, b
, Xiaoping Zhang a, b, Fei Yang a, b, *, Guangyu Sun a, b, and
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266580, PR
Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao, Shandong
10
c
11
Science, Western Norway University of Applied Sciences, Inndalsveien 28, 5063 Bergen, Norway
Department of Biomedical Laboratory Sciences and Chemical Engineering, Faculty of Engineering and
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Abstract: In last two published papers, the influences of wax and asphaltene content on the synergistic
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performance of ethylene-vinyl acetate (EVA) copolymer together with resin-stabilized asphaltenes on the
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flow behavior improving of model waxy oil have been systematically investigated and the relevant
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mechanism has been proposed. Here, the effects of vinyl acetate (VA) content (12~40 wt%) on the
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synergistic performance between EVA and asphaltenes is continuously studied to develop and complete the
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synergistic theories. Results show that different VA contents slightly influence on the rheological properties
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of model waxy oils doped with neat EVA, but play a significant role in the flow behavior improvement of
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the oil doped with EVA and asphaltenes. EVA with moderate VA content (28 wt%) possesses the best flow
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improving efficiency among the neat EVA PPDs, but associated with asphaltenes, EVA with a higher VA
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content (33 wt%) does the best. According to the DSC tests, when the VA content is low or moderate (12~33
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wt%), the wax precipitation temperature (WPT) of the waxy oil is found to be decreased after adding neat
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EVA, while the phenomenon for the EVA with too high VA content (40 wt%) is the opposite. WPT of oil ACS Paragon Plus Environment
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would not be further suppressed by EVA/asphaltenes, but EVA/asphaltenes can facilitate the crystallization
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of paraffin waxes and accelerates the precipitation process of wax crystals below WPT. Increasing the
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polarity of EVA (12~33wt%) can strengthen the polar interaction between EVA and asphaltenes in oil phase,
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promoting EVA molecules to adsorb onto the asphaltene aggregates to form the EVA/asphaltenes composite
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particles. The composite particles favor the formation of the large, compact, and spherical wax flocs to
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release the more liquid oil phase, reduce the solid-liquid interfacial areas and weaken the interactions
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between wax crystals, therefore facilitating the outstandingly rheological improving of waxy oils. As VA
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content increases to much higher level (40 wt%), however, the rigidity of EVA molecules is high, which is
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adverse for the well oil-dispersing ability of EVA and the corresponding interactions with wax molecules.
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Meanwhile, the high polar EVA disperses the wax crystals into smaller sizes. Both of these two sides enlarge
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the solid-liquid interfacial area and strengthen the interactions between wax crystals, making them easier to
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build up a continuous wax crystal’s network structure and leading to the performance deterioration of the
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EVA together with asphaltenes. This conclusion that the modest increase of PPD’s polarity facilitates the
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improving efficiency between PPDs and asphaltenes gives another powerful proof to the correctness of
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EVA/asphaltenes composite particles mechanism.
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Keywords: EVA; asphaltene aggregates; waxy oil; flow behavior; synergistic effect
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1. Introduction
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When the temperature of waxy crude oil falls below its wax precipitation temperature (WPT), the paraffin
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waxes contained in the crude oil would precipitate continuously as wax crystals due to the super-saturation1.
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The formed wax crystals are usually with large aspect ratio and strong non-polarity, which would easily
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build up the three dimensional network structure and quickly occlude the flowable liquid oil phase, thus
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resulting in the gelation of waxy crude oil and shutdown of pipelines2. In industrial applications, a
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well-recognized solution for this problem is adding small dosage of polymers with special chemical ACS Paragon Plus Environment
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structures and properties, which are often identified as polymeric pour point depressant (PPD), to suppress
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the gelation temperature and weaken the gel structure of oil3. Generally, the chemical structure of polymeric
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PPDs is made of the nonpolar and the polar moieties that can take distinct effects in interacting with wax
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molecules during the crystallization process.
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Ethylene-vinyl acetate copolymer (EVA) is a typical kind of linear copolymers, which has been widely
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employed as an effective PPD for waxy crude oils4, 5. The ethylene and vinyl acetate (VA) of EVA belong to
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the nonpolar and polar moiety, respectively. The molecular rigidity and chain movement of EVA are
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determined by the polar moiety content, which dominates the crystallization properties and dispersing ability
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in oil phase of EVA6, 7. Therefore, the VA content of EVA exerts an important influence on its pour point
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depressing performance.
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To develop the polymeric PPDs with higher rheological improving efficiency for waxy crude oils, various
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types of inorganic or organically modified nano/micro particles have been blended into the PPD matrixes to
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develop
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silica/polyoctadecylacrylate (POA) and organically modified nano-clay/POA composite PPDs, and then
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studied their influences on the flow behavior of waxy crude oil. It was found that the composite PPDs could
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take effect as the heterogeneous nucleus to change the morphologies of wax crystals largely, and therefore
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significantly improving the oil rheological characteristics. Furthermore, they reported that the organic
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modification degree of inorganic nanoparticles played a vital role in the PPD efficiency enhancement. After
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that, they developed the polymethylsilsesquioxane (PMSQ) microspheres and amino-functionalized PMSQ
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microspheres/EVA composite PPDs, and found that the composite PPDs dispersed in oil phase as composite
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particles and could generate the entirely different microstructures of wax crystal, thus enhancing the
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polymeric PPDs’ efficiency11, 12. Some other researchers sought and prepared the composite PPDs based on
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the nano-/micro-substrates such as graphene oxide13, 14, attapulgite15, fullerene16, montmorillonite17and so on,
the
nano-/micro-composite
PPDs.
Recently,
Yang
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al8-10
first
synthesized
the
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and also reached the similar conclusion. Obviously, study on the composite PPDs for waxy crude oils has
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become a hotspot.
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Asphaltene is defined as the fraction of a crude oil that is insoluble in light saturated oils such as pentane,
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and comprises the heaviest and most polar components of the oil18, 19. Generally, asphaltenes often disperse
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as associated aggregates in oil phase with the aid of resins, and can act as the natural PPDs to improve the
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rheological properties of waxy oils at low temperatures20, 21. In the previous works, considering the effects of
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wax and asphaltenes contents, Yang et al22, 23 found that a remarkable synergistic performance in improving
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the rheological properties exists between EVA and asphaltenes. They concluded that: EVA molecules could
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adsorb on the resin-stabilized asphaltene aggregates to form the composite particles to change the wax
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crystals’ morphologies, hence dramatically enhancing the flowability of oil.
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This work is the third part of the continuous studies22, 23. Here, the effect of EVA (100 ppm) containing
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different VA contents (12~40 wt%) together with asphaltenes (0.3 wt%) on the flow behavior of waxy oil
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was studied. Firstly, the rheological tests were applied to evaluate the influences of VA content in EVA on
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the flowability of waxy oil. Then, the crystallization exothermic and microscopic properties of the oils
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doped with EVA or EVA/asphaltenes were monitored by DSC and microscopic observation tests. Finally, the
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mechanism of VA content influencing the performance of EVA and asphaltenes was detailed discussed.
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2. Experimental
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2.1 Materials
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All the chemicals used here were purchased from Sigma-Aldrich Co., Ltd. With the aid of slight heat and
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stirring, a settled amount of solid wax was dissolved into the xylene/mineral oil solvent to obtain the model
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waxy oil. Figure S1 of the support information illustrates that the mineral oil mainly contains isoalkanes
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with few macro-paraffin waxes. The solid wax is composed of two macro-paraffin waxes with different
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carbon number distributions (consistent with the previously published paper22). Therefore, the solid wax ACS Paragon Plus Environment
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possesses a wide carbon number distribution (C19~C50), quite similar to that of the real waxy crude oil
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(Figure S2 of the support information). In the model waxy oil, the concentration of solid wax, xylene, and
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mineral oil was set at 10, 20 and 70 wt%, respectively.
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The VA content of the six EVA copolymers used here is 12, 18, 25, 28, 33 and 40 wt%, respectively; while
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the melt indexes of the EVA copolymers are similar (around 6), thus excluding the influences of the EVA
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molecular weight. The six EVA copolymers are abbreviated as EVA12, EVA18, EVA25, EVA28, EVA33 and
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EVA40, respectively. EVA was added into the model waxy oils by 100 ppm. The VA moieties in the EVA
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structure can disturb the normal crystallization process of the ethylene moieties, therefore the crystallization
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exothermic curves of the EVA copolymers displayed in Figure 1 show that increasing the VA content results
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in the decrease of both the onset crystallizing temperature and the degree of crystallinity due to the
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enhancement of EVA polarity.
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The composition of Tahe heavy oil is shown in the support information. As reported in the previous
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article22, small dosage of the deasphaltened oil has no apparent influences on the flowability of the model
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waxy oil. For purpose of keeping the original solvated state of asphaltenes, Tahe heavy oil was directly
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added into the model waxy oil, and the asphaltenes dosage was fixed at 0.3 wt%.
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2.2 Method
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2.2.1 Pour point test
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Following the method in Chinese Standard SY/T 0541-20099, the pour point for each oil sample was
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evaluated and the thermal treat temperature was 60 °C.
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2.2.2 Rheological test
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All the rheological tests were conducted on the rheometer platform (AR-G2, TA instrument Co., USA).
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The detailed parameter could be seen in the previous paper22. Structural development of the waxy oils under static cooling. During cooling from 60 °C to 15 °C, the oil ACS Paragon Plus Environment
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sample was given a small oscillatory amplitude (0.0005)
, which would not affect the wax crystal
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network buildup, and the oscillatory frequency was chosen as 1 Hz. The viscoelastic parameters were
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recorded during cooling. Gelation point of oil was found where the G′ equals to G″ and δ equals to 45°.
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Structural properties of the waxy oils after static cooling. All the oil samples were cooled quiescently to
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15 °C and kept at 15 °C for 40 mins. Shear rate ramping from 0 to 1 s-1 in 5 minutes isothermally was used
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to test the yield behavior of oil, during which the relationship of shear stress/shear strain was monitored.
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Shear rate ramping from 5 to 200 s-1 in 10 minutes isothermally was used to test the flow curve of oil, during
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which the relationship of apparent viscosity/shear rate was monitored.
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Structural development of the waxy oils under dynamic cooling. All the oil samples were given a constant
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shear rate (10 s-1) during cooling from 60 °C to15 °C, where the apparent viscosity-temperature was
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recorded.
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2.2.3 Crystallization exothermic properties test
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A DSC 821e calorimeter (Mettler-Toledo Co., Switzerland) was applied to evaluate the crystallization
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exothermic properties of the undoped/doped waxy oils. Each oil sample (6-9 mg) was sealed in an aluminum
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crucible and maintained at 85 °C for 10 mins. Then, the cooling step (85~-20°C) was conducted under the
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cooling rate of 10 °C/min. The heat flow values with decreasing temperatures were recorded during cooling.
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2.2.4 Microscopic observation test
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The microstructures of the undoped/doped waxy oils were observed on a polarized microscope (BX51
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Olympus, Japan) equipped with a precisely thermal stage. Theoil sample was first one-dropped on the slide
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and then covered by the coverslip. Then, the sample was statically cooled to the experimental temperature at a
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settled cooling rate of 0.5 °C/min. The microscopic properties of the precipitated wax crystals were observed
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and photographed at the experimental temperature.
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3. Results and discussions ACS Paragon Plus Environment
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3.1 Pour point of undoped/doped waxy oils
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As seen in Table 1, with neither EVA nor asphaltenes, the pour point of original oil sample is 36 °C. The
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addition of neat EVA can effectively depress the pour point of the oil, and the pour point depressing
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efficiency first improves and then deteriorates with increasing the VA content. The best pour point
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depressing efficiency is found at adding 100 ppm EVA28, which can decrease the pour point of the oil by
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6 °C. The neat asphaltenes can only depress the pour point of the oil by 2 °C, while adding EVA/asphaltenes
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is able to dramatically decrease the pour point of the oil. The VA content of EVA has a pronounced influence
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on the synergistic efficiency of the EVA/asphaltenes. For example, when the VA content is relatively low,
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EVA12/asphaltenes and EVA18/asphaltenes can only decrease the pour point of the oil to 18 °C and 16 °C,
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respectively. At moderate VA content, EVA25/asphaltenes sharply drops the pour point down to 2 °C. The
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largest pour point depression is found at adding EVA28/asphaltenes, and the corresponding pour point is
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-5 °C. As the VA content increases to 33 wt% and 40 wt%, the pour points of the doped oils recover to 6 °C
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and 22 °C, respectively. Obviously, EVA and asphaltenes can suppress the pour point of the model waxy oil
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synergistically; the synergistic efficiency improves initially and then deteriorates with the VA content further
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increase of EVA.
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3.2 Rheological properties
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3.2.1 Structural development of waxy oils under static cooling condition
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The elasticity modulus (G′) denotes the elasticity response of a viscoelastic body, which is often employed
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to represent the strength of a waxy oil gel24, 25. The development of G′ for the oil samples with the
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decreasing temperatures is illustrated in Figure 2. As seen in Figure 2a, when the oil temperature is high, no
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wax crystals precipitate from waxy oil and the G′ is very small (around 10-4 Pa). With the oil temperature
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declining, the G′ of undoped waxy oil first starts to rise up at a relatively high temperature and the rising rate
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is quite fast, implying the easy and quick formation of wax crystal network structure. The increase of G′ of ACS Paragon Plus Environment
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the waxy oils adding EVA is delayed to a lower temperature, indicating that the buildup of wax crystal
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network structure is inhibited. When the VA content of EVA increases from 12 wt% to 28 wt%, the
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inhibiting efficiency of EVA is improved. However, the further increase of VA content from 33 wt% to 40 wt%
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weakens the inhibiting efficiency. Obviously, EVA28 displays the highest efficiency in inhibiting the wax
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crystal network formation of waxy oil. In Figure 2b, adding 0.3 wt% asphaltenes cannot evidently prevent
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the formation of wax crystal network, while adding EVA/asphaltenes can outstandingly inhibit the wax
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crystal network buildup and the VA content of EVA has a great impact on the inhibiting efficiency. As the VA
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content increases from 12 wt% to 33 wt%, G′ value of the waxy oil decreases obviously, indicating that
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increasing the VA content improves the inhibiting efficiency of EVA/asphaltenes. At the highest VA content
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(40 wt%), however, the inhibiting efficiency of EVA/asphaltenes is weakened a lot. Clearly, the
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EVA33/asphaltenes has the best performance to inhibit the wax crystal network buildup of the waxy oil.
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Gelation point is the sol-to-gel critical transition temperature of waxy oil. Table 1 lists the gelation points
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of the undoped/doped waxy oils. It is evident that the gelation point of the waxy oil presents no strict
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correlation with its pour point. For example, the gelation point of the undoped waxy oil (37.0 °C) is 1 °C
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higher than its pour point (36 °C), while the gelation points of the waxy oil doped with EVA25, EVA28,
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EVA33 and EVA40 are all lower than their pour points. However, the variation trend of gelation point with
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the VA content of EVA is corresponding to the variation trend of pour point. Apparently, EVA28 displays the
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best gelation point depressing performance among the six EVA copolymers. Adding EVA/asphaltenes can
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depress the gelation point of the oil further and the VA content of EVA has a great impact on the depressing
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efficiency. After adding EVA12, EVA18, EVA25, EVA28 or EVA33 together with asphaltenes, the gelation
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point is depressed to 24.0, 20.4, 18.8, 14.4 and 13.3 °C, respectively. However, after adding
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EVA40/asphaltenes, the gelation point recovers to 32.6 °C. It is clear that the EVA33/asphaltenes exhibits
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the best efficiency in reducing the oil gelation point. ACS Paragon Plus Environment
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3.2.2 Structural properties of waxy oils after static cooling process
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The transient apparent viscosity-shear rate curves of the undoped/doped waxy oils at 15 °C are displayed
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in Figure 3a and b. As seen in Figure 3a, adding 100 ppm EVA greatly reduces the oil’s apparent viscosities,
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and the viscosity reducing rate increases initially and decreases with the VA content increasing further. For
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instance, at the fixed shear rate of 50 s-1, the transient apparent viscosity of the undoped waxy oil is
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1650mPa·s; the addition of EVA12, EVA18, EVA25 and EVA28 reduces it to 1283, 1309, 490 and 415
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mPa·s, respectively. As the VA content further increases to 33 wt% and 40 wt%, the transient apparent
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viscosity of the waxy oil recovers to 577 and 818 mPa·s, respectively. As seen in Figure 3b, the neat
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asphaltenes can slightly decrease the oil viscosity and the transient apparent viscosity of the waxy oil at 50
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s-1 is 1590mPa·s. In contrast with the neat EVA or the neat asphaltenes, more prominent viscosity reducing
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performance is observed after adding EVA together with asphaltenes. Adding EVA12, EVA18, EVA25,
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EVA28 or EVA33 with asphaltenes is able to reduce the transient apparent viscosity at 50 s-1 to 49.16, 20.42,
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18.57, 17.55 and 16.60mPa·s, respectively. However, EVA40/asphaltenes raises the transient apparent
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viscosities of oil back to 54.54 mPa·s. Clearly, EVA33/asphaltenes possesses the best synergistic
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performance in reducing the apparent viscosity of waxy oil.
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The yield stress of crude oil sample at a certain temperature represents the structural strength of wax
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crystal gel, which is directly related to the restart problem of waxy oil pipeline. The yield behavior of the
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undoped/doped waxy oils at 15 °C is shown in Figure 3c and d. As seen in Figure 3c, the yield stress of the
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undoped waxy oil at 15 °C is extremely high (1519 Pa), indicating a strong gel structure of the undoped
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waxy oil. After adding EVA12, EVA18, EVA25, EVA28, EVA33 and EVA40, the yield stress of the waxy oil
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is reduced to 816.1, 110.2, 56.24, 50.34, 359.9 and 879.5 Pa, respectively. The neat asphaltenes reduces the
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yield stress to 293.7 Pa, while adding EVA with asphaltenes dramatically suppresses the yield stress of the
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waxy oil at 15 °C. The yield stress of the waxy oil doped with EVA 12, EVA18, EVA25, EVA28 or EVA33 ACS Paragon Plus Environment
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together with asphaltenes falls down to 3.067, 0.661, 0.399, 0.302 and 0.114 Pa, respectively. However, the
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yield stress recovers to 4.310 Pa after adding EVA40/asphaltenes.
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3.2.2 Structural development of waxy oils under dynamic cooling condition
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Figure 4a and b illustrate the apparent viscosity development of oil samples with the decreasing
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temperatures under dynamics cooling process. For the undoped oil sample, the apparent viscosity increases
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slowly with the decreasing temperatures higher than WPT, and rises up dramatically just below the WPT
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(around 35.5 °C, see Figure 5a). After that, the apparent viscosity displays a second slow increase with a
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larger slope (in the semi-logarithmic coordinate). Adding EVA with different VA contents cannot evidently
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influence the flowability of oil above WPT, but can energetically decrease the apparent viscosity below WPT.
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The viscosity reducing efficiency of EVA increases with the VA content increasing from 12 wt% to 28 wt%,
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and the further increase of VA content is adverse for the viscosity reducing.
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Adding 0.3 wt% asphaltenes can only depress the apparent viscosities of oil sample in the temperature
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region just below WPT; the further temperature decreasing results in that the apparent viscosity increases
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rapidly and exceeds the undoped oil, which has been discussed in a former published paper10. The apparent
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viscosity development of the oil with the addition of EVA/asphaltenes displays a gradually and slowly
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increasing trend with no sharp viscosity increase, meaning that EVA/asphaltenes dramatically improves the
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flowability of the waxy oil below WPT. The transient viscosities at 15 °C for the waxy oil doped with
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EVA12, EVA18, EVA25, EVA28, EVA33 and EVA40 together with asphaltenes are 99.7, 17.2, 16.1, 17.3,
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15.7 and 41.9 mPa·s, respectively, which are orders of magnitude lower than those doped with neat EVA or
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asphaltenes.
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Based on the discussion mentioned in 3.2.1~3.2.2, conclusively, adding EVA/asphaltenes is able to
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synergistically enhance the flowability of the waxy oil. The synergistic efficiency increases initially and then
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decreases with the increase of VA content. At the VA content of 28~33 wt%, the EVA/asphaltenes shows the ACS Paragon Plus Environment
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highest synergistic efficiency.
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3.3 Crystallization exothermic characteristics
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The exothermic curves of the waxy oils samples are shown in Figure 5 and the corresponding WPTs of
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oils are listed in Table 2. As seen in Figure 5a, adding EVA with different VA contents obviously affects the
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WPT of the waxy oil. For example, the WPT of the undoped oil is 34.8 °C, and adding neat EVA12, EVA18,
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EVA25, EVA28 and EVA33 decreases the WPT of the waxy oil to 34.7, 33.9, 33.3, 32.8 and 33.4 °C,
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respectively. On the one hand, the non-polar groups of EVA PPDs can co-crystallize with wax molecules in
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the crystallization process, which promotes the dissolution of waxes at a certain temperature; on the other,
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the polar groups induced by EVA effectively raises the solid-liquid interfacial tension, restraining the
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formation of stable wax crystal nucleus and delaying the precipitation of paraffin waxes. These two aspects
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are determined by the ratio of the nonpolar and polar groups in EVA, and therefore, EVA28 is found to
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decrease the WPT of oil to the lowest. Due to the bad dispersion state and poor co-crystallization ability of
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EVA40 (with the highest VA content), adding EVA40 raises the WPT of the waxy oil up to 36.1 °C. As seen
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in Figure 5b, adding neat asphaltenes (0.3 wt%) can decrease the WPT of the waxy oil to 32.9 °C. The
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aliphatic side chains in the asphaltenes structure make the co-crystallization efficiency of asphaltenes more
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obvious than the nucleation efficiency, thus reducing the WPT of waxy oil sample. Adding EVA12, EVA18,
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EVA25, EVA28, EVA33 or EVA40 together with asphaltenes increases the WPT of the oil to 34.9°C, 34.2°C,
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33.5°C, 33.4°C, 34.8°C and 35.9 °C, respectively, indicating that the nucleation efficiency becomes more
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obvious.
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In Figure S2 of support information, the mixed paraffin waxes of the waxy oil have two carbon number
251
distributing peaks at around C26-C31 and C37-C38, meaning that the exothermic curve of the waxy oil may
252
have two prominent exothermic peaks during cooling. Accordingly, for the undoped waxy oil, the heat flow
253
starts to rise up below the WPT (34.8 °C) and quickly forms a small exothermic peak (recognized as the first ACS Paragon Plus Environment
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Page 12 of 28
254
exothermic peak). After that, with the decrease of oil temperature, the heat flow continues to increase and
255
generates a wide exothermic peak (recognized as the second exothermic peak). After adding EVA, the width
256
and height of the first exothermic peak increase a lot but the second exothermic peak turns to be less
257
prominent. This indicates that, although EVA can delay the precipitation of paraffin waxes, EVA cannot
258
prevent the precipitation of paraffin waxes completely. Once the paraffin waxes precipitate from the oil,
259
adding EVA effectively increases the crystallization rate of waxes. After adding EVA together with
260
asphaltenes, the first exothermic peak becomes much sharper, indicating that EVA together with asphaltenes
261
can provide more nucleation cites and crystalizable moieties to promote the aggregation and crystallization
262
of wax molecules, thus further accelerating the precipitation process of wax crystals.
263
From the above discussion, it can be concluded that adding neat EVA or asphaltenes can decrease the
264
WPT of the waxy oil, and the EVA with moderate VA content has the best WPT depressing ability. Adding
265
EVA/asphaltenes cannot further reduce the WPT of oil, but slightly increase the WPT. Adding
266
EVA/asphaltenes facilitates the crystallization of paraffin waxes and accelerates the precipitation process of
267
wax crystals below WPT.
268
3.4 Wax crystal microstructure
269
Figure 6 displays the wax crystal microstructure of the waxy oils undoped/doped with neat EVA at 20 °C.
270
In the undoped oil sample, the formed wax crystals are in long and thin needle-like morphologies and
271
arrange in a disordered state. Adding EVA12 cannot change the morphology of the precipitated wax crystals
272
a lot, but an obvious growth in the thickness and aggregation of wax crystals is found after adding EVA18.
273
When the VA content increases to 25 wt%, the radial wax flocs with larger size and higher aggregation
274
degree start to appear in the oil phase, but the whole wax crystal structure is loose with small needle-like
275
wax crystals existing. After adding EVA28, the size of the radial wax flocs is slightly reduced but the
276
structural compactness is enhanced a lot. The further increase of VA content to 33 wt% generates the ACS Paragon Plus Environment
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continuous decline in the size of wax flocs. After adding EVA40, the wax flocs tend to grow into much
278
smaller sizes with lower aggregation degree and looser structure.
279
As seen in Figure 7, the precipitated wax crystals in the waxy oil after adding neat asphaltenes keep the
280
needle-like morphology but the length is far shorter. The addition of EVA12/asphaltenes generates the
281
formation of large and bright wax flocs, but the surroundings of the wax flocs presents radial morphologies
282
and the flocs structure is loose and porous. The size of the wax flocs in the waxy oil doped with
283
EVA18/asphaltenes continues to grow, and the surrounding radial shape becomes more obvious. After
284
adding EVA25/asphaltenes, the precipitated wax flocs are more aggregated and compact, and the
285
surrounding radial shape of wax flocs disappears. The size and compactness of the wax flocs continue to
286
grow after the addition of EVA28/asphaltenes. The wax flocs of the oil adding EVA33/asphaltenes are
287
slightly decreased in size but become more spherical-like. Adding EVA40/asphaltenes disperses the
288
precipitated wax crystals into much smaller size with larger amount.
289
3.5 Effect of VA content on the precipitating properties of asphaltenes
290
The process of the asphaltenes precipitation test can be seen in the reference22. The effect of VA content
291
on the asphaltenes precipitating properties is shown in Figure 8. The percentage of the precipitated
292
asphaltenes is decreased after adding EVA, indicating that EVA can adsorb onto the asphaltene aggregates
293
and act as asphaltenes dispersant to inhibit the precipitation of the asphaltenes. Therefore, it can be deduced
294
that EVA molecules can dissolved in the oil phase and concentrate on the asphaltene aggregates in oil phase,
295
forming the EVA/asphaltenes composite particles.
296
In addition, increasing the polarity of EVA can strengthen the polar interaction between EVA and
297
asphaltenes in oil phase. Therefore, the inhibition efficiency of EVA increases with increasing the VA
298
content. Nevertheless, at the highest VA content (40 wt%), the rigidity of EVA molecules is high and the
299
dispersion ability of EVA in oil phase deteriorates a lot, which lead to the decrease of the inhibiting ACS Paragon Plus Environment
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300
efficiency. Accordingly, the best inhibition performance is found at 33 wt% VA content (EVA33).
301
3.6 Mechanism discussion
Page 14 of 28
302
In the absence of asphaltenes, EVA can improve the flow behavior of the model waxy oil below WPT and
303
the improving efficiency is closely related to the VA content of EVA. When the VA content is low (e.g. 12 wt%
304
and 18 wt%), EVA cannot exert an apparent influences on the wax crystal microstructure, and the
305
precipitated wax crystals still present needle-like shapes and easily form the wax crystal network structure,
306
resulting in the poor performance of EVA. When the VA content is at moderate level (e.g. 25wt%~33wt%),
307
the precipitated wax crystals contaminated by EVA start to assemble and form the radially patterned wax
308
flocs through the polar attractions, and adding EVA28 enhances the compactness of wax crystals structure,
309
resulting in its best rheological improving performance. EVA40, owning the highest polarity, on the one
310
hand, strengthens the rigidity of molecular chains and worsens the dispersibility in oil phase, both of which
311
are adverse for the interactions with wax molecules. On the other, a large amount of polar moieties disperses
312
the wax flocs into smallest sizes with lower aggregation degree and looser structure. These two sides cause
313
the worst rheological improving performance of EVA40. Furthermore, for all the oils doped with neat EVA,
314
at relatively low precipitated wax crystal amount, the radially patterned wax floc morphology reduces the
315
solid-liquid interfacial areas and is adverse for the buildup of the gel structure, thus favoring the flow
316
improvement of oil. Nevertheless, when the precipitated wax crystals accumulate to an extent with the
317
decreasing of temperatures, the rheological properties of the oil deteriorate quickly (see Figure 4).
318
In the presence of asphaltenes, EVA can adsorb on the asphaltene aggregates to form the EVA/asphaltenes
319
composite particles, and the composite particles can act as the heterogeneous nucleation templates for wax
320
molecules to precipitate, thus forming the large, compact, and spherical wax flocs. The compact structure of
321
wax flocs releases more liquid oil phase, and the large and regular spherical wax flocs reduce the
322
solid-liquid interfacial area effectively and weaken the interactions between wax crystals. These two sides ACS Paragon Plus Environment
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323
hinder the construction of continuous wax crystal network structure and favor the outstandingly flow
324
behavior improving of waxy oils. When the VA content is low (e.g. 12 wt% and 18 wt%), the formed wax
325
flocs are in loose structure and porous state, and also surrounded by needle-like wax crystals, which is
326
against to the flow improving of oil. As the VA content rises up from 25 wt% to 33 wt%, the wax flocs
327
become increasingly regular, compact and spherical-like, which is consistent with the gradual flowability
328
enhancement of the oil. The highest VA content (40 wt%) disperses the wax flocs into smaller sizes. The
329
enlarged solid-liquid interfacial area and strengthened interactions between wax crystals facilitates the
330
formation of the wax crystal continuous network structure, leading to the performance deterioration of the
331
EVA40/asphaltenes.
332
Above all, EVA28 possesses the best flow improving efficiency among the neat EVA PPDs, but associated
333
with asphaltenes, EVA33 does the best. This indicates that the modest increase of PPD’s polarity facilitates
334
the synergistic interactions between polymeric PPDs and asphaltenes. This conclusion gives another
335
powerful proof to the correctness of EVA/asphaltenes composite particles mechanism.
336
4. Conclusions
337
In this article, the influences of VA content on the synergistic performance of EVA PPDs and
338
resin-stabilized asphaltenes were detailed investigated and the conclusions are drawn as follows:(a)
339
increasing the VA content strengthens the rigidity of the EVA molecules, resulting in the decrease of both the
340
onset crystallizing temperature and the degree of crystallinity due to the enhancement of EVA polarity. (b)
341
Different VA contents slightly influence on the rheological parameters such as pour point, viscoelasticity,
342
viscosity and yielding behavior of the model waxy oils doped with neat EVA, but play a significant role in
343
the flow ability enhancement of the oil adding EVA/asphaltenes. EVA with moderate VA content (28 wt%)
344
possesses the best flow improving efficiency among the neat EVA PPDs, but associated with asphaltenes,
345
EVA with a higher VA content (33 wt%) does the best. (c) DSC tests show that adding 100 ppm ACS Paragon Plus Environment
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Page 16 of 28
346
EVA12~EVA33 or 0.3 wt% asphaltenes can decrease the WPTs of oils by 0.1~2.0 °C, and EVA28 is found
347
to decrease it to the lowest. Due to the bad dispersion state and poor co-crystallization ability of EVA40,
348
adding 100 ppm EVA40 raises the WPT up by 1.3 °C. EVA/asphaltenes cannot decrease the waxy oil’s WPT
349
further, but facilitates the crystallization of paraffin waxes and accelerates the precipitation process of wax
350
crystals below WPT. (d) For the waxy oils adding neat EVA, the precipitated wax crystals assemble into the
351
radical-like wax flocs, and the compactness of the wax flocs increases initially and then decreases with the
352
VA content increasing. Adding EVA/asphaltenes leads to the formation of larger, more compact and
353
spherical-like wax flocs, and the size of wax flocs increases initially and then decreases with the VA content
354
increasing. The wax flocs appear to be the most spherical-like after adding EVA33 together with asphaltenes.
355
(e) Increasing the polarity of EVA can strengthen the polar interaction between EVA and asphaltenes in oil
356
phase, thus inhibiting the asphaltenes precipitation, but at the highest VA content (40 wt%), the rigidity of
357
EVA molecules is high and the dispersion ability of EVA in oil phase deteriorates a lot, which leads to the
358
decrease of the inhibiting efficiency. (f) In the presence of asphaltenes, EVA can adsorb on the asphaltene
359
aggregates to form the EVA/asphaltenes composite particles. When the VA content is low, EVA cannot exert
360
an apparent influences on the wax crystal microstructure, resulting in the poor performance of EVA. At the
361
moderate VA content, the wax flocs become increasingly regular, compact and spherical-like, which releases
362
more liquid oil phase, reduces the solid-liquid interfacial area and weakens the interactions between wax
363
crystals, thus favoring the outstandingly flow behavior improving of waxy oils. As the VA content further
364
increases (40 wt%), the wax flocs were dispersed into smaller sizes, facilitating forming the continuous
365
network structure. The conclusion that the modest increase of PPD’s polarity facilitates the synergistic
366
interactions between PPDs and asphaltenes gives another powerful proof to the correctness of
367
EVA/asphaltenes composite particles mechanism.
368
Acknowledgement ACS Paragon Plus Environment
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369
This research was funded by National Natural Science Foundation of China (51774311), Natural Science
370
Foundation of Shandong Province of China (ZR2017MEE022), and Key Research Project of Shandong
371
Province of China (2017GSF216003).
372
References
373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410
1. Fang, L.; Zhang, X.; Ma, J.; Zhang, B., Investigation into a Pour Point Depressant for Shengli Crude Oil. Ind. Eng. Chem. Res.2012, 51, (36), 11605-11612. 2. Yang, F.; Li, C.; Li, C.; Wang, D., Scaling of Structural Characteristics of Gelled Model Waxy Oils. Energy Fuels 2013, 27, (7), 3718-3724. 3. Yang, F.; Zhao, Y.; Sjöblom, J.; Li, C.; Paso, K. G., Polymeric Wax Inhibitors and Pour Point Depressants for Waxy Crude Oils: A Critical Review. J. Dispersion Sci. Technol.2014, 36, (2), 213-225. 4. Wu, C.; Zhang, J.; Li, W.; Wu, N., Molecular dynamics simulation guiding the improvement of EVA-type pour point depressant. Fuel 2005, 84, (16), 2039-2047. 5. Ashbaugh, H. S.; Guo, X.; Schwahn, D.; Prud'homme, R. K.; Richter, D.; Fetters, L. J., Interaction of Paraffin Wax Gels with Ethylene/Vinyl Acetate Co-polymers. Energy Fuels 2005, 19, (1), 138-144. 6. Zhang, W. a.; Chen, D.; Zhao, Q.; Fang, Y. e., Effects of different kinds of clay and different vinyl acetate content on the morphology and properties of EVA/clay nanocomposites. Polymer 2003, 44, (26), 7953-7961. 7.Osman, A. F.; Abdul Hamid, A. R.; Rakibuddin, M.; Khung Weng, G.; Ananthakrishnan, R.; Ghani, S. A.; Mustafa, Z., Hybrid silicate nanofillers: Impact on morphology and performance of EVA copolymer uponin vitrophysiological fluid exposure. J. Appl. Polym. Sci. 2017, 134, (12). 8. Yang, F.; Paso, K.; Norrman, J.; Li, C.; Oschmann, H.; Sjöblom, J., Hydrophilic Nanoparticles Facilitate Wax Inhibition. Energy Fuels 2015, 29, (3), 1368-1374. 9. Yao, B.; Li, C.; Yang, F.; Sjöblom, J.; Zhang, Y.; Norrman, J.; Paso, K.; Xiao, Z., Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate. Fuel 2016, 166, 96-105. 10. Yao, B.; Li, C.; Yang, F.; Zhang, Y.; Xiao, Z.; Sun, G., Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant. Fuel 2016, 184, 544-554. 11. Yao, B.; Li, C.; Zhang, X.; Yang, F.; Sun, G.; Zhao, Y., Performance improvement of the ethylene-vinyl acetate copolymer (EVA) pour point depressant by small dosage of the amino-functionalized polymethylsilsesquioxane (PAMSQ) microsphere. Fuel 2018, 220, 167-176. 12. Yang, F.; Yao, B.; Li, C.; Shi, X.; Sun, G.; Ma, X., Performance improvement of the ethylene-vinyl acetate copolymer (EVA) pour point depressant by small dosages of the polymethylsilsesquioxane (PMSQ) microsphere: An experimental study. Fuel 2017, 207, 204-213. 13. Zhao, Z.; Yan, S.; Lian, J.; Chang, W.; Xue, Y.; He, Z.; Bi, D.; Han, S., A new kind of nanohybrid poly(tetradecyl methyl-acrylate)-graphene oxide as pour point depressant to evaluate the cold flow properties and exhaust gas emissions of diesel fuels. Fuel 2018, 216, 818-825. 14. Al-Sabagh, A. M.; Betiha, M. A.; Osman, D. I.; Hashim, A. I.; El-Sukkary, M. M.; Mahmoud, T., Preparation and Evaluation of Poly(methyl methacrylate)-Graphene Oxide Nanohybrid Polymers as Pour Point Depressants and Flow Improvers for Waxy Crude Oil. Energy Fuels 2016, 30, (9), 7610-7621. 15. Tu, Z.; Jing, G.; Sun, Z.; Zhen, Z.; Li, W., Effect of nanocomposite of attapulgite/EVA on flow behavior and wax crystallization of model oil. J. Dispersion Sci. Technol.2017, 1-5. 16. Jafari Behbahani, T.; Beigi, A. A. M.; Taheri, Z.; Ghanbari, B., The effect of amino [60] fullerene derivatives on pour point and rheological properties of waxy crude oil. J. Mol. Liq.2015, 211, 308-314. ACS Paragon Plus Environment
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411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434
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17. Al-Sabagh, A. M.; Betiha, M. A.; Osman, D. I.; Hashim, A. I.; El-Sukkary, M. M.; Mahmoud, T., A new covalent strategy for functionalized montmorillonite–poly(methyl methacrylate) for improving the flowability of crude oil. RSC Adv.2016, 6, (111), 109460-109472. 18. Molina V, D.; Ariza León, E.; Chaves-Guerrero, A., Understanding the Effect of Chemical Structure of Asphaltenes on Wax Crystallization of Crude Oils from Colorado Oil Field. Energy Fuels 2017, 31, (9), 8997-9005. 19. Rogel, E.; Ovalles, C.; Vien, J.; Moir, M., Asphaltene characterization of paraffinic crude oils. Fuel 2016, 178, 71-76. 20. Oh, K.; Deo, M., Characteristics of Wax Gel Formation in the Presence of Asphaltenes. Energy Fuels 2009, 23, (3), 1289-1293. 21. Tinsley, J. F.; Jahnke, J. P.; Dettman, H. D.; Prud’home, R. K., Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology. Energy Fuels 2009, 23, (4), 2056-2064. 22. Yao, B.; Li, C.; Yang, F.; Zhang, X.; Mu, Z.; Sun, G.; Zhao, Y., Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 1. Effect of Wax Content and the Synergistic Mechanism. Energy Fuels 2018, 32, (2), 1567-1578. 23. Yao, B.; Li, C.; Yang, F.; Zhang, X.; Mu, Z.; Sun, G.; Liu, G.; Zhao, Y., Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 2. Effect of Asphaltene Content. Energy Fuels 2018, 32, (5), 5834-5845. 24. da Silva, J. A. L.; Coutinho, J. A. P., Dynamic rheological analysis of the gelation behaviour of waxy crude oils. Rheol. Acta2004, 43, (5), 433-441. 25. Yang, F.; Li, C.; Wang, D., Studies on the Structural Characteristics of Gelled Waxy Crude Oils Based on Scaling Model. Energy Fuels 2013, 27, (3), 1307-1313.
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435
Figures
436 437 438
-1
1.0
Heat flow per mass / mWmg
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
EVA12 EVA18 EVA25 EVA28 EVA33 EVA40
0.8 0.6 0.4 0.2 0.0 20
60
80
100
Temperature / °C
439 440
40
Figure 1
Crystallization exothermic curves of the EVA copolymers with different VA contents.
441
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442 443 444 300 undoped doped with EVA12 doped with EVA18 doped with EVA25 doped with EVA28 doped with EVA33 doped with EVA40
15000
asphaltenes EVA12/asphaltenes EVA18/asphaltenes EVA25/asphaltenes EVA28/asphaltenes EVA33/asphaltenes EVA40/asphaltenes
250 200
G'
10000
G'
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 28
150 100
5000 50
0
0
25
30
35
Temperature / °C
445
15
40
(a)
20
25
30
Temperature / °C
35
40
(b)
446
Figure 2
Elasticity modulus (G′) development of the undoped/doped waxy oils with the decreasing
447
temperatures (a: undoped/doped with neat EVA; b: doped with neat asphaltenes or EVA/asphaltenes).
448
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Page 21 of 28
449 450 undoped doped with EVA12 doped with EVA18 doped with EVA25 doped with EVA28 doped with EVA33 doped with EVA40
Apparent viscosity / Pas
4
2
1 0.8 0.6 0.4
0.2
0
50
100
-1
150
Shear rate / s
451
4 2
Apparent viscosity / Pas
6
200
1 0.8 0.6 0.4
0.04
0.03
0.2 0.02 5
0.1 0.08 0.06 0.04
10
15
20
25
30
Asphaltenes EVA12/asphaltenes EVA18/asphaltenes EVA25/asphaltenes EVA28/asphaltenes EVA33/asphaltenes EVA40/asphaltenes
0.02 0.01
0
50
(a)
100 150 -1 Shear rate / s
200
(b)
3
10
2
10
Asphaltenes EVA12/asphaltenes EVA18/asphaltenes EVA25/asphaltenes EVA28/asphaltenes EVA33/asphaltenes EVA40/asphaltenes
2
Shear stress / Pa
10 Shear stress / Pa
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
10
Yield stress: undoped doped with EVA12 doped with EVA18 doped with EVA25 doped with EVA28 doped with EVA33 doped with EVA40
0
10
-1
10
-2
10
1519 Pa 816.1 Pa 110.2 Pa 56.24 Pa 50.34 Pa 359.9 Pa 879.5 Pa
1
10
0
Yield stress: 293.7 Pa 3.067 Pa 0.661 Pa 0.399 Pa 0.302 Pa 0.114 Pa 4.310 Pa
10
-1
10
-3
10
-2
0
452 453 454
Figure 3
500
1000 Shear strain
1500
2000
10
0
(c)
500
1000 Shear strain
1500
2000
(d)
Flow curves (a/b) and yield behavior (c/d) of the undoped/doped waxy oils at 3°C (a and c:
undoped/doped with neat EVA; b and d: doped with neat asphaltenes or EVA/asphaltenes).
455
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Energy & Fuels
456 0
Apparent viscosity / Pas
10
undoped doped with EVA12 doped with EVA18 doped with EVA25 doped with EVA28 doped with EVA33 doped with EVA40
-1
10
-2
10
15
20
25 30 35 Temperature / °C
457
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 28
0.016
10
0.014
0.012
0.01
15
459 460
Figure 4
16
17
18
19
20
-2
10
15
458
45
(a) asphaltenes EVA12/asphaltenes EVA18/asphaltenes EVA25/asphaltenes EVA28/asphaltenes EVA33/asphaltenes EVA40/asphaltenes
0.018
-1
40
20
25 30 35 Temperature / °C
40
45
(b)
Apparent viscosity-temperature curves of the undoped/doped waxy oils (a: undoped/doped with neat EVA; b: doped with neat asphaltenes or EVA/asphaltenes).
461
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Energy & Fuels
462 463
464
465 466
Figure 5
Crystallization exothermic curves of the undoped/doped waxy oils (a: undoped/doped with neat
467
EVA; b: doped with neat asphaltenes or EVA/asphaltenes). (The corresponding WPTs of oils are shown in
468
Table 2)
469
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470 471
472
473
474 475
Figure 6
Polarized-light microscopic images of the waxy oils undoped/doped with neat EVA at 20 °C.
476 477
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478
479
480 481 482
Figure 7
Polarized-light microscopic images of the waxy oils doped with neat asphaltenes or EVA/asphaltenes at 20 °C.
483 484
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Energy & Fuels
485 486 487 488 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 26 of 28
489 490
Figure 8
3.2 3.07
2.8 2.64
2.66 2.61 2.48
2.45
2.4
2.41
no EVA EVA12 EVA18 EVA25 EVA28 EVA33 EVA40
Effect of VA content on the asphaltenes precipitating properties.
491
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492
Energy & Fuels
Tables
493 494 495 496 497
Table 1
Effect of VA content on the pour points and gelation points of the undoped/doped waxy oils. Asphaltenes content
498
No EVA EVA12 EVA18 EVA25 EVA28
EVA33
EVA40
Pour point
0
36
35
34
31
30
31
34
/ °C
0.3 wt%
34
18
16
2
-5
6
22
Gelation point
0
37.0
33.8
34.0
29.4
25.8
30.2
32.0
/ °C
0.3 wt%
33.0
24.0
20.4
18.8
14.4
13.5
32.6
Note: the dosage of EVA was fixed at 100 ppm.
499
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500 501 502 503 504 505 506
Table 2Effect of VA content on the wax precipitation temperatures (WPTs) of the undoped/doped waxy oils. Asphaltenes content
No EVA EVA12 EVA18 EVA25 EVA28
EVA33
EVA40
0
34.8
34.7
33.9
33.3
32.8
33.4
36.1
0.3 wt%
32.9
34.9
34.2
33.5
33.4
34.8
35.9
507 508 509 510 511
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