- Email: [email protected]
nano-montmorillonite pour-point depressant on improving the flow properties of model oil
Colloids and Surfaces A 555 (2018) 296–303
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Effect evaluation of ethylene vinyl acetate/nano-montmorillonite pour-point depressant on improving the flow properties of model oil ⁎
Na Lia, GuoLiang Maoa, , Wei Wua, Yang Liub, a b
T
⁎
College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, China College of Petroleum Engineering, Northeast Petroleum University, Daqing, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Action mechanism Ethylene vinyl acetate Flow properties Nanocomposite pour point depressant Wax crystal
A series of novel nano-hybrid pour-point depressants (PPDs) which combined nano-montmorillonite (MMT) and ethylene vinyl acetate (EVA) copolymers were prepared. Compared with pure EVA PPDs with identical content, the effects of EVA/nano-MMT PPDs on the cold flow properties of 25 wt% wax model oil were evaluated. Polarized optical microscopy was used to study the effect of the EVA/nano-MMT PPDs on the crystallization behavior and crystal morphology of the model oil at low temperature. The results indicated that the vinyl acetate content of EVAs has a significant impact on the performance of EVA/nano-MMT PPDs, and appropriate dosage of the EVA/nano-MMT PPDs can be more effective than EVA PPDs in modifying the wax crystal morphology, preventing their aggregation and improving the fluidity of model oil at low temperatures.
1. Introduction Crude oil is a complex mixture of saturated hydrocarbons, aromatics, wax, asphaltene and resin. Crude oil with a high content of wax
⁎
often exhibits a high wax apparent temperature (WAT) [1]. Below this temperature, wax molecules begin to precipitate, overlap and interlock to form a three-dimensional network structure in the oil phase, which traps the molecules of liquid hydrocarbon and reduces the mobility of
Corresponding authors. E-mail addresses: [email protected] (G. Mao), [email protected] (Y. Liu).
https://doi.org/10.1016/j.colsurfa.2018.06.065 Received 3 April 2018; Received in revised form 24 June 2018; Accepted 24 June 2018 Available online 30 June 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
the crude oil. The temperature below which the crude oil loses its flow characteristics is defined as the crude oil’s pour point. The precipitation and deposit of wax crystals, at low temperature, cause severe problems in oil production, storage, and transportation [2–5]. In order to overcome those problems, both physical and chemical methods have been developed so far. Heating transportation technology is a simple and effective physical technology, which consumes a great deal of energy, resulting in the sharp increase of the cost. On the contrary, chemical approaches, particularly the addition of polymeric pour-point depressant is usually cost-effective and timesaving. It was commonly accepted that PPD can effectively change the structure and morphology of wax crystals and prohibit the formation of rigid three-dimensional networks at low temperature [6]. Conventional PPDs are homo- or co-polymers of various monomers. Among them, EVA copolymers are most widely used. Nonpolar ethylene chain and polar vinyl acetate (VA) chain, act as crystalline phase and non-crystalline phase, respectively. Ethylene chain can absorb or cocrystallize with wax molecules. The introduction of vinyl acetate chains can change the direction of wax crystals’ growth [1,7–9]. In recent years, with the rapid development of nanotechnology, nano-hybrid materials have also been applied in crude oil production and transportation. Nano-MMT, as a layer-lattice-silica-aluminate clay mineral with negative charges, has been introduced into polymer matrices PPDs to form polymer/ nano-MMT PPDs [10]. Because of the unique high adsorption affinity of nanoparticles [11–15]. In order to get clearer results, model oil with 25 wt% of paraffin was used in testing the rheological beneficiation of the EVA/ nano-MMT PPDs. The effect of VA content and nano-MMT on the performance of EVA/ nano-MMT PPDs was evaluated by the experimental results. Polarized optical microscopy was used to observe the effect of EVA/ nano-MMT PPDs on the morphology and crystallization behavior of model oil. The specific action mechanism of EVA/ nano-MMT PPD on model oil was also presented.
Fig. 1. FTIR spectra of (a) unmodified nano-MMT, (b) OTAC and (c) organic nano-MMT.
2. Experimental section 2.1. Materials Dodecane (AR), toluene (AR), octadecyl trimethyl ammonium chloride (OTAC), and unmodified nano- montmorillonite (MMT) (average particle size 25 nm) were purchased from Macklin, China. EVAs (The content of vinyl acetate included in EVA is respectively 28%, 33%, 38%.) were purchased from Heinz technical company, China. -35# diesel and paraffin were supplied by Daqing Oilfield, Sinopec, China.
Fig. 2. TGA of (a) unmodified nano-MMT, (b) organic nano-MMT and (c) OTAC.
2.2. Sample preparation 2.2.1. Modification of nano-MMT Hydrophilic nano-MMT was modified with 0.75 times of OTAC via cationic exchange. A defined amount of unmodified nano-MMT was first dispersed and stirred until the nano-MMT was evenly swelled into deionized water at 80 °C. Subsequently, OTAC was added into the dispersion under ultrasonication and the mixture was stirred vigorously at 70 °C for 5 h. After the reaction, the white products were separated by centrifugation and washed several times with deionized water (pH = 7) until Cl− could not be detected, which indicated the complete removal of free OTAC. Modified nano-MMT was obtained after the product was dried at 90 °C for 12 h and subsequently ground [16,17]. Fig. 3. X-ray powder diffraction patterns of unmodified nano-MMT, organic nano- MMT and EVA/nano-MMT PPD.
2.2.2. Preparation of nano-hybrid PPD A certain amount of polymeric PPDs was dissolved in toluene, subsequently, a certain proportion of modified nano-MMT (with the nano-MMT/ EVAs mass ratio of 1:3) was added into the polymeric PPD/ toluene solution, stirred with ultrasonicator at room temperature until organic nano-MMT was evenly dispersed in toluene, and then heated to
80 °C and kept for 2 h with stirring. The solution was continuously stirred and evaporated to remove toluene at 120 °C [18]. Finally, the nano-hybrid PPDs were obtained after the removal of the solvent.
297
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 4. (a) Unmodified nano-MMT, (b) organic nano-MMT, (c) EVA/nano-MMT dispersed in dodecane and water (W represents water, D represents dodecane).
measured at the cooling rate of 1.0 °C/min with a shear rate of 1.5 r/ min.
2.2.3. Preparation of model oil Model oil containing 25 wt % of paraffin was prepared by adding paraffin into −35# diesel, the mixture was then stirred continuously at 70 °C until the paraffin was completely dissolved.
2.3.2.3. Polarized optical microscopy (POM). Microscopic observation is an effective method to describe the microstructure of colloidal dispersions or gels. A polarizable microscopic XFP600c fitted with a ker3100-08S thermal stage was used to observe the wax crystal morphology of the model oil undoped/doped with PPDs. The oil sample was pre-heated to 70 °C and kept at that temperature for 30 min, a small drop of the sample was dropped onto a glass slide, covered with a coverslip and cooled from 50 °C to 20 °C in the stage at a fixed cooling rate of 0.5 °C/min. The morphology of wax crystals was then observed and photographed.
2.3. Characterization 2.3.1. Nano-MMT properties characterization The unmodified and organic nano-MMT samples were characterized with fourier transform infrared (FT-IR) spectrometry (TENSOR27, Bruker, Germany), X-ray diffraction (XRD) (Rigaku, Japan) and thermogravimetric analysis (TGA) (STA 449 F5 Jupiter instrument, Germany). A clear and intuitive simulation experiment was used to evaluate the dispersion of nano-MMT and EVA/nano-MMT PPDs in dodecane and water.
3. Results and discussion 3.1. Characterization analysis
2.3.2. Model oil properties characterization 2.3.2.1. Pour point test. According to GB510-83, a test tube containing a model oil sample was immersed vertically in a water bath of 50 ± 1 ℃ until the sample reached that temperature [19]. Then the test tube was cooled in a thermostatic bath, taken out and tilted by 45 degrees for 1 min to observe the flowability of the sample after each 1 °C’s temperature drop. The highest temperature at which the sample could not flow was designated as the pour point. For each sample, the tests were repeated for three times and the mean temperature was regarded as the pour point.
3.1.1. Fourier transform infrared (FT-IR) analysis of nano-MMT The FT-IR spectra of OTAC, unmodified and organic nano-MMT are presented in Fig. 1. Both unmodified and organic nano-MMT spectrum display SieOeSi symmetric stretching vibration peak at 792 cm−1, SieOeSi antisymmetric stretching vibration peak at 1016 cm−1, and ReOeH stretching vibration peak at 3491–3632 cm−1. Both OTAC and organic nano-MMT spectrum display the characteristic absorption peaks at 2917 and 2847 cm-1 which can be assigned to C–H stretching, representing the presence of −CH2 and −CH3 groups of quaternary ammonium [20,21]. It is proved that nano-MMT had been successfully organically modified through cationic modification.
2.3.2.2. Rheological measurements. Rheological measurements of model oil doped/undoped with PPDs were carried out with a Brookfield rotational rheometer (DV-II + Pro) equipped with a heating/cooling system. In order to remove the heat-history and shear-history, spare model oil was initially heated to 70 °C and stirred for at least 2 h, then the model oil samples undoped/doped with PPDs were kept isothermally at 65 °C for 30 min. The apparent viscosity was
3.1.2. Thermo gravimetric analysis (TGA) of nano-MMT Organic nano-MMT was also tested with TGA (shown in Fig. 2) to quantify the extent of organic modification. At the temperature below 220℃, weight loss happens to both organic nano-MMT and unmodified 298
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 5. Pour Point of model oil (M) doped with PPDs with different concentrations. (a) EVA(28) and EVA(28)/nano-MMT, (b) EVA(33) and EVA(33)/nano-MMT, (c) EVA(38) and EVA(38)/nano-MMT.
peaks indicate the successful intercalation of OTAC molecules, which increase the layer spacing and reduce the interaction between the layers of nano-MMT to some extent. The X-ray powder diffraction pattern of EVA/nano-MMT is also presented in Fig. 3. Compared with the organic nano-MMT, the 2θ of EVA/nano-MMT decreases and the d001 spacing reaches 2.10 nm and d002 spacing reaches 1.32 nm, which prove the formation of EVA/nanoMMT nanocomposite.
nano-MMT samples, which is attributed to the escape of water molecules adsorbed on the surface or between sheets of nano-MMT. From 220 ℃ to 450 ℃, the weight loss of organic nano-MMT is caused by the decomposition of the organic molecular chains, on the contrary, no weight loss is observed for the unmodified nano-MMT in this temperature scope. The weight loss of organic nano-MMT occurred in the range of 450℃∼700℃ is attributed to the collapse of nano-MMT sheets. Since the organic nano-MMT and OTAC begin to decompose around 220 ℃, their termination temperatures are obviously different. This phenomenon might be explained as follows: The organic molecular chains have inserted between layers or on the surface of nano-MMT, the mobility of organic molecular is limited and the poor airflow between sheets further inhibits the decomposition of organic molecular chains, leading to the weight loss of organic nano-MMT in the temperature range from 220 ℃ to 700 ℃ [22,23]. The weight loss of modified nanoMMT is approximately 32.35% over the entire temperature range and the organic compounds take 24.98% of the weight of the modified nano-MMT.
3.1.4. Dispersion evaluation of nano-MMT and EVA/nano-MMT PPD Dispersion experiments were carried out to evaluate the dispersion of unmodified, organic nano-MMT and EVA/nano-MMT. In order to obtain clearer image, water was used to test the hydrophilic, while dodecane, instead of −35# diesel, was used to test the oleophilic. Nano-MMT with 0.2 wt% dosage was stirred with ultrasonicator at room temperature until nano-MMT was evenly dispersed in water/ dodecane and then placed quiescently at room temperature for 1 h before observation. As shown in Fig. 4(a), the existence of turbidity is in compliance with the hydrophilic nature of unmodified nano-MMT. In Fig. 4(b), organic nano-MMT disperses well in dodecane but subsides in water, which is in accordance with the oleophilic nature. In Fig. 4(c), organic EVA/nano-MMT with 0.2 wt% dosage is well dispersed and forms a homogeneous solution in dodecane but layers in water which is in line with the oleophilic nature. The clear images and intuitive phenomenon show the good compatibility between the nanoMMT and EVA, and the good lipophilicity of EVA/nano-MMT.
3.1.3. X-ray diffraction (XRD) of nano-MMT and EVA/nano-MMT PPD The X-ray powder diffraction patterns of organic and unmodified nano-MMT are presented in Fig. 3. The Bragg equation 2dsinθ=nλ is used to calculate d spacing values. Compared with the unmodified nano-MMT, the d001 spacing of organic nano-clay increases from 1.51 nm to 1.96 nm and d002 peak begins to appear at 1.24 nm. Both 299
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 6. Viscosity of M undoped/doped with PPDs. (a) EVA (28) and EVA(28)/nano-MMT, (b) EVA(33) and EVA(33)/nano-MMT, (c) EVA(38) and EVA(38)/nanoMMT, (d) EVA(38)/nano-MMT, EVA(33)/nano-MMT and EVA(28)/nano-MMT.
between EVA and nano-MMT which leads to the re-desorption of the adsorbed EVA molecules. EVA/nano-MMT PPDs containing different kinds of EVAs exhibit different performance to the model oil and present the following performance order: EVA(38)/nano-MMT < EVA(28)/nano-MMT < EVA(33)/ nano-MMT.
3.2. Effect evaluation of nano-mmt PPD on model oil 3.2.1. Pour point As shown in Fig. 5, the effect of the three EVA/nano-MMT PPDs on the cold flow properties of model oil is studied. With the increase of PPD dosages, the pour points of model oil decrease first and then increase. Compared with pure EVA, the depression performance of organic nano-clay on model oil is weak. Compared with pure EVA(28) and EVA(38), the EVA(33) exhibits the best depression performance on model oil. Compared with EVA(33), the EVA(33)/nano-MMT PPD exhibits better depression performance with the same dosage. EVA(33)/ nano-MMT combines the advantages of EVA(33) and nano-MMT which are conducive to the dispersion and inhibiting overlap of wax crystals. The best pour point depressing performance is obtained with 0.08 wt% dosage of EVA(33)/nano-MMT, in which the pour point drops from 34 °C to 2 °C. However, EVA(38)/nano-MMT PPD is less effective than pure EVA(38). Some possible reasons might explain the phenomenon: EVA molecules interact with the organic nano-MMT and are adsorbed on the surface of particles. On the one hand, nano-MMT combines with EVA(38) molecules, when the added amount of nano-MMT reaches or exceeds the saturation value, agglomeration would happen among nano-MMT, the unbalanced interaction between nano-MMT and EVA reduces the depression performance of EVA/nano-MMT PPD on model oil. On the other hand, with the increase of polar group, the compatibility between nano-MMT and EVA slightly improves, an excessive content of the polar groups in EVA affects the bonding property
3.2.2. Rheological properties The viscosity of the model oil is plotted as the function of temperature in Fig. 6. Compared with EVA, there is no obvious effect of organic nano-clay on the viscosity reduction of model oil. With the same dose, EVA/nano-MMT PPDs exhibit better viscosity reduction performance than pure EVA. The effect of the EVA/nano-MMT PPDs might be explained as follows: First, the EVA molecules contain ethylene segments and vinyl acetate segments. Ethylene segments containing nonpolar groups act as crystalline phase, vinyl acetate (VA) segments containing polar groups act as non-crystalline phase. The nonpolar groups can incorporate with wax via adsorption or co-crystallization, the polar groups may modulate the morphology of wax crystals, interrupt the wax crystals growth and inhibit the overlap of wax crystals. Secondly, by dispersing nano-MMT into the polymer phase which changes the matrix of polymers, the compatibility between the nanoparticles and EVA affects the performance of PPDs. Since nanoMMT can provide more crystal nucleus and effectively disperse wax crystals. Combined with the advantages of EVA, EVA/nano-MMT PPD can effectively inhibit the formation of three-dimensional network structure in the oil phase and delay the conversion from sol to gel at 300
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 7. POM images of M samples undoped and doped with PPD at 20 ℃. (a) Undoped with PPD, doped with (b) nano-MMT, (c) EVA (28), (d) EVA(28)/nano-MM, (e) EVA (33), (f) EVA(33)/nano-MMT, (g) EVA (38), (h) EVA(38)/nano-MMT.
lower temperature. Compared with the viscosity of model oil with EVA (38), the one with EVA (38)/nano-MMT PPD has no outstanding advantage on improving flow behavior (see Fig. 6(c)).
doped with the optimized dosages of PPDs are shown in Fig. 7. Wax crystal reflects polarized light and exhibits bright white images, meanwhile, the solvent forms the black background. The variation in the morphology and quantity of the wax crystals were recorded in Fig. 7. As shown in Fig. 7(a), the wax crystal morphology of the pure model oil is large and feather-like. By comparing the images a, the wax
3.2.3. Microscopic study Polarized optical microscopic images of the model oil undoped/ 301
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 8. Diagram description of the interaction between EVA/nano-MMT PPD and wax.
solid–liquid interfacial area between oil and wax crystal and reducing the amount of oil occluded in the wax crystal. So the EVA(33)/nanoMMT exhibits the best effect. Under the combined action of EVA and nano-MMT, the heterogeneous nucleation effect is remarkably reinforced as presented in Fig. 8.
crystal morphology of model oil doped with pure organic nano-MMT in Fig. 7(b) is still large and feather-like which is obvious that pure organic nano-MMT brought little influence on modifying wax crystals. In the images (c–h) of Fig. 7, it can be seen that obvious changes occurred in the wax crystal morphology of model oil doped with PPDs. By comparing the images c, e, and g, we notice that EVAs(28/33/38) change the wax crystal morphology of all samples from feather-like to needlelike. This phenomenon might be explained as follows: When the content of VA is relatively low, the waxy particles forms by the co-crystallization between polymer molecules, and wax molecules are highly solvated, and the interattraction between particles is weak, which leads to poor aggregation between wax particles. If the content of VA is relatively high, the effective content of non-polar groups is relatively low, resulting in the decrease of the co-crystallization ability between pour point depressant and wax molecules. So the EVA(33) exhibits better effect than the others. The same trend is observed in the experiments using EVA/nanoMMT PPDs. The EVA/nano-MMT PPDs can control the size and morphology of the wax crystals. While compared with the wax crystal morphology of the model oil doped with EVA(33), the wax crystal morphology with EVA(33)/nano-MMT is obviously different, it appears to be rod-like with smaller size and more compact structure. The phenomenon might be explained as follows: The wax crystal grows with EVA/nano-MMT PPD which acts as smaller crystalline nucleus precipitating earlier than wax. The new crystalline nucleus changes the precipitating habit of wax which is adverse to the formation of network structure through the overlap and interlock of wax crystals, but favors the formation of small wax crystals dispersed well in the oil phase. The optimum content of polar groups is conducive to reducing the
4. Conclusion Novel nano-hybrid pour point depressants (PPDs) were prepared by dispersing the organic nano-MMT in EVA PPD matrix and used in reducing the pour point and viscosity of the model oil. At the same dosage, EVA/nano-MMT PPDs showed better performance than their pure EVA analogs. The best pour point depressing performance of the model oil was obtained with 0.08 wt% of EVA(33)/nano-MMT PPD, in which the pour point dropped from 34 °C to 2 °C. The vinyl acetate content of EVAs had a significant impact on the performance of EVA/nano-MMT. Compared with EVA(28) and EVA(38), the EVA(33) exhibited better performance in modifying wax crystal morphology and improving the fluidity of model oil. The EVA/nano-MMT PPD can effectively change the morphology of wax crystals from fluffy feather-like to compact rodlike accompanied by the sharp decrease of the crystals’ size. Acknowledgment This work was supported by the National Natural Science Foundation of China [51534004, U1362110] and Northeast Petroleum University (No. JYCX_CX03_2018). 302
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
point depressant on flow properties of waxy crude oil, Fuel 167 (2016) 40–48. [13] Z.N. Sun, G.L. Jing, Z.Y. Tu, Effect of modified nano-silica/EVA on flow behavior and wax crystallization of model oils with different wax content, J. Dispers. Sci. Technol. 39 (2018) 71–76. [14] B. Yao, C.X. Li, F. Yang, J. Sjöblom, Y. Zhang, J. Norrman, K. Paso, Z.Q. Xiao, Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate, Fuel 166 (2016) 96–105. [15] A.M. Alsabagh, M.A. Betiha, D.I. Osman, A.I. Hashim, M.M. Elsukkary, T. Mahmoud, A new covalent strategy for functionalized montmorillonite–poly (methyl methacrylate) for improving the flowability of crude oil, RSC Adv. 6 (2016) 109460–109472. [16] S.M. Liu, B.B. Yan, P. Wang, Studied on preparation montmorillonite modified by CTAMB and adsorption performance on nitrobenzene, Adv. Mater. Res. 557–559 (2012) 996–1004. [17] S.I.S. Shahabadi, H. Garmabi, Response surface analysis of structural, mechanical, and permeability properties of polyethylene/Na+−montmorillonite composites, prepared by slurry-fed melt intercalation, Exp. Polym. Lett. 6 (2012) 657–671. [18] Y. Xue, Z.C. Zhao, G.W. Xu, X. Lian, C. Yang, W.N. Zhao, P. Ma, H.L. Lin, S. Han, Effect of poly-alpha-olefin pour point depressant on cold flow properties of waste cooking oil biodiesel blends, Fuel 184 (2016) 110–117. [19] Petroleum products-determination of solidification point. GB510-83, China (1983). [20] G.B. Huang, J.R. Gao, Y.J. Li, L. Han, Functionalizing nano-montmorillonites by modified with intumescent flame retardant: preparation and application in polyurethane, Polym. Degrad. Stabil. 95 (2010) 245–253. [21] M.C. Costache, M.J. Heidecker, E. Manias, G. Camino, A. Frache, The influence of carbon nanotubes, organically modified montmorillonites and layered double hydroxides on the thermal degradation and fire retardancy of polyethylene, ethylene–vinyl acetate copolymer and polystyrene, Polymer 48 (2007) 6532–6545. [22] V.L.P. Soares, R.S.V. Nascimento, V.J. Menezes, L. Batista, TG characterization of organically modified montmorillonite, J. Therm. Anal. Calorim. 75 (2004) 671–676. [23] W. Wei, Z.M. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermal degradation chemistry of alkyl quaternary ammonium montmorillonite, Chem. Mater. 13 (2001) 2979–2990.
References [1] A.L.C. Machado, E.F. Lucas, 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. Pet. Sci. Eng. 32 (2001) 159–165. [2] B. Yao, C.X. Li, F. Yang, Y. Zhang, Z.Q. Xiao, G.Y. Sun, Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant, Fuel 184 (2016) 544–554. [3] Z.C. Zhao, Y. Xue, G.W. Xu, J.W. Zhou, X. Lian, P. Liu, Effect of the nano-hybrid pour point depressants on the cold flow properties of diesel fuel, Fuel 193 (2017) 65–71. [4] T.J. Behbahani, A. Dahaghin, K. Kashefi, Effect of solvent on rheological behavior of Iranian waxy crude oil, Petrol. Sci. Technol. 29 (2011) 933–941. [5] C. Jiang, K. Zhao, C. Fu, L.Z. Xiao, Characterization of morphology and structure of wax crystals in waxy crude oils by terahertz time-domain spectroscopy, Energy Fuels 31 (2017) 1416–1421. [6] A. Radulescu, D. Schwahn, J. Stellbrink, E. Kentzinger, A.M. Heiderich, D. Richter, Wax crystallization from solution in hierarchical morphology templated by random poly(ethylene-co-butene) self-assemblies, Macromolecules 39 (2006) 6142–6151. [7] F. Yang, Z.Q. Xiao, B. Yao, C.X. Li, L. Wang, X. Shi, G.Y. Sun, K.Y. Yan, Influences of different functional groups on the performance of polyoctadecyl acrylate pour point depressant, Liq. Fuels Technol. 34 (2016) 1712–1719. [8] J.B. Taraneh, G. Rahmatollah, A. Hassan, D. Alireza, Effect of wax inhibitors on pour point and rheological properties of Iranian waxy crude oil, Fuel Process. Technol. 89 (2008) 973–977. [9] F. Yang, K. Paso, J. Norrman, C.X. Li, H. Oschmann, J. Sjöblom, Hydrophilic nanoparticles facilitate wax inhibition, Energy Fuels 29 (2015) 1368–1374. [10] H.M. Wang, M.H. Mei, Y.C. Jiang, Q.S. Li, X.H. Zhang, S.K. Wu, A study on the preparation of polymer/montmorillonite nano-composite materials by photo-polymerization, Photogr. Sci. Photochem. 51 (2001) 7–11. [11] G.L. Jing, Z.N. Sun, Z.Y. Tu, X.D. Bian, Y. Liang, Influence of different vinyl acetate content on the properties of EVA/modified nano-SiO2 composite PPD, Energy Fuels 31 (2017) 5854–5859. [12] C.Z. He, Y.F. Ding, J. Chen, F. Wang, C. Gao, Influence of the nano-hybrid pour
303
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Effect evaluation of ethylene vinyl acetate/nano-montmorillonite pour-point depressant on improving the flow properties of model oil ⁎
Na Lia, GuoLiang Maoa, , Wei Wua, Yang Liub, a b
T
⁎
College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, China College of Petroleum Engineering, Northeast Petroleum University, Daqing, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Action mechanism Ethylene vinyl acetate Flow properties Nanocomposite pour point depressant Wax crystal
A series of novel nano-hybrid pour-point depressants (PPDs) which combined nano-montmorillonite (MMT) and ethylene vinyl acetate (EVA) copolymers were prepared. Compared with pure EVA PPDs with identical content, the effects of EVA/nano-MMT PPDs on the cold flow properties of 25 wt% wax model oil were evaluated. Polarized optical microscopy was used to study the effect of the EVA/nano-MMT PPDs on the crystallization behavior and crystal morphology of the model oil at low temperature. The results indicated that the vinyl acetate content of EVAs has a significant impact on the performance of EVA/nano-MMT PPDs, and appropriate dosage of the EVA/nano-MMT PPDs can be more effective than EVA PPDs in modifying the wax crystal morphology, preventing their aggregation and improving the fluidity of model oil at low temperatures.
1. Introduction Crude oil is a complex mixture of saturated hydrocarbons, aromatics, wax, asphaltene and resin. Crude oil with a high content of wax
⁎
often exhibits a high wax apparent temperature (WAT) [1]. Below this temperature, wax molecules begin to precipitate, overlap and interlock to form a three-dimensional network structure in the oil phase, which traps the molecules of liquid hydrocarbon and reduces the mobility of
Corresponding authors. E-mail addresses: [email protected] (G. Mao), [email protected] (Y. Liu).
https://doi.org/10.1016/j.colsurfa.2018.06.065 Received 3 April 2018; Received in revised form 24 June 2018; Accepted 24 June 2018 Available online 30 June 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
the crude oil. The temperature below which the crude oil loses its flow characteristics is defined as the crude oil’s pour point. The precipitation and deposit of wax crystals, at low temperature, cause severe problems in oil production, storage, and transportation [2–5]. In order to overcome those problems, both physical and chemical methods have been developed so far. Heating transportation technology is a simple and effective physical technology, which consumes a great deal of energy, resulting in the sharp increase of the cost. On the contrary, chemical approaches, particularly the addition of polymeric pour-point depressant is usually cost-effective and timesaving. It was commonly accepted that PPD can effectively change the structure and morphology of wax crystals and prohibit the formation of rigid three-dimensional networks at low temperature [6]. Conventional PPDs are homo- or co-polymers of various monomers. Among them, EVA copolymers are most widely used. Nonpolar ethylene chain and polar vinyl acetate (VA) chain, act as crystalline phase and non-crystalline phase, respectively. Ethylene chain can absorb or cocrystallize with wax molecules. The introduction of vinyl acetate chains can change the direction of wax crystals’ growth [1,7–9]. In recent years, with the rapid development of nanotechnology, nano-hybrid materials have also been applied in crude oil production and transportation. Nano-MMT, as a layer-lattice-silica-aluminate clay mineral with negative charges, has been introduced into polymer matrices PPDs to form polymer/ nano-MMT PPDs [10]. Because of the unique high adsorption affinity of nanoparticles [11–15]. In order to get clearer results, model oil with 25 wt% of paraffin was used in testing the rheological beneficiation of the EVA/ nano-MMT PPDs. The effect of VA content and nano-MMT on the performance of EVA/ nano-MMT PPDs was evaluated by the experimental results. Polarized optical microscopy was used to observe the effect of EVA/ nano-MMT PPDs on the morphology and crystallization behavior of model oil. The specific action mechanism of EVA/ nano-MMT PPD on model oil was also presented.
Fig. 1. FTIR spectra of (a) unmodified nano-MMT, (b) OTAC and (c) organic nano-MMT.
2. Experimental section 2.1. Materials Dodecane (AR), toluene (AR), octadecyl trimethyl ammonium chloride (OTAC), and unmodified nano- montmorillonite (MMT) (average particle size 25 nm) were purchased from Macklin, China. EVAs (The content of vinyl acetate included in EVA is respectively 28%, 33%, 38%.) were purchased from Heinz technical company, China. -35# diesel and paraffin were supplied by Daqing Oilfield, Sinopec, China.
Fig. 2. TGA of (a) unmodified nano-MMT, (b) organic nano-MMT and (c) OTAC.
2.2. Sample preparation 2.2.1. Modification of nano-MMT Hydrophilic nano-MMT was modified with 0.75 times of OTAC via cationic exchange. A defined amount of unmodified nano-MMT was first dispersed and stirred until the nano-MMT was evenly swelled into deionized water at 80 °C. Subsequently, OTAC was added into the dispersion under ultrasonication and the mixture was stirred vigorously at 70 °C for 5 h. After the reaction, the white products were separated by centrifugation and washed several times with deionized water (pH = 7) until Cl− could not be detected, which indicated the complete removal of free OTAC. Modified nano-MMT was obtained after the product was dried at 90 °C for 12 h and subsequently ground [16,17]. Fig. 3. X-ray powder diffraction patterns of unmodified nano-MMT, organic nano- MMT and EVA/nano-MMT PPD.
2.2.2. Preparation of nano-hybrid PPD A certain amount of polymeric PPDs was dissolved in toluene, subsequently, a certain proportion of modified nano-MMT (with the nano-MMT/ EVAs mass ratio of 1:3) was added into the polymeric PPD/ toluene solution, stirred with ultrasonicator at room temperature until organic nano-MMT was evenly dispersed in toluene, and then heated to
80 °C and kept for 2 h with stirring. The solution was continuously stirred and evaporated to remove toluene at 120 °C [18]. Finally, the nano-hybrid PPDs were obtained after the removal of the solvent.
297
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 4. (a) Unmodified nano-MMT, (b) organic nano-MMT, (c) EVA/nano-MMT dispersed in dodecane and water (W represents water, D represents dodecane).
measured at the cooling rate of 1.0 °C/min with a shear rate of 1.5 r/ min.
2.2.3. Preparation of model oil Model oil containing 25 wt % of paraffin was prepared by adding paraffin into −35# diesel, the mixture was then stirred continuously at 70 °C until the paraffin was completely dissolved.
2.3.2.3. Polarized optical microscopy (POM). Microscopic observation is an effective method to describe the microstructure of colloidal dispersions or gels. A polarizable microscopic XFP600c fitted with a ker3100-08S thermal stage was used to observe the wax crystal morphology of the model oil undoped/doped with PPDs. The oil sample was pre-heated to 70 °C and kept at that temperature for 30 min, a small drop of the sample was dropped onto a glass slide, covered with a coverslip and cooled from 50 °C to 20 °C in the stage at a fixed cooling rate of 0.5 °C/min. The morphology of wax crystals was then observed and photographed.
2.3. Characterization 2.3.1. Nano-MMT properties characterization The unmodified and organic nano-MMT samples were characterized with fourier transform infrared (FT-IR) spectrometry (TENSOR27, Bruker, Germany), X-ray diffraction (XRD) (Rigaku, Japan) and thermogravimetric analysis (TGA) (STA 449 F5 Jupiter instrument, Germany). A clear and intuitive simulation experiment was used to evaluate the dispersion of nano-MMT and EVA/nano-MMT PPDs in dodecane and water.
3. Results and discussion 3.1. Characterization analysis
2.3.2. Model oil properties characterization 2.3.2.1. Pour point test. According to GB510-83, a test tube containing a model oil sample was immersed vertically in a water bath of 50 ± 1 ℃ until the sample reached that temperature [19]. Then the test tube was cooled in a thermostatic bath, taken out and tilted by 45 degrees for 1 min to observe the flowability of the sample after each 1 °C’s temperature drop. The highest temperature at which the sample could not flow was designated as the pour point. For each sample, the tests were repeated for three times and the mean temperature was regarded as the pour point.
3.1.1. Fourier transform infrared (FT-IR) analysis of nano-MMT The FT-IR spectra of OTAC, unmodified and organic nano-MMT are presented in Fig. 1. Both unmodified and organic nano-MMT spectrum display SieOeSi symmetric stretching vibration peak at 792 cm−1, SieOeSi antisymmetric stretching vibration peak at 1016 cm−1, and ReOeH stretching vibration peak at 3491–3632 cm−1. Both OTAC and organic nano-MMT spectrum display the characteristic absorption peaks at 2917 and 2847 cm-1 which can be assigned to C–H stretching, representing the presence of −CH2 and −CH3 groups of quaternary ammonium [20,21]. It is proved that nano-MMT had been successfully organically modified through cationic modification.
2.3.2.2. Rheological measurements. Rheological measurements of model oil doped/undoped with PPDs were carried out with a Brookfield rotational rheometer (DV-II + Pro) equipped with a heating/cooling system. In order to remove the heat-history and shear-history, spare model oil was initially heated to 70 °C and stirred for at least 2 h, then the model oil samples undoped/doped with PPDs were kept isothermally at 65 °C for 30 min. The apparent viscosity was
3.1.2. Thermo gravimetric analysis (TGA) of nano-MMT Organic nano-MMT was also tested with TGA (shown in Fig. 2) to quantify the extent of organic modification. At the temperature below 220℃, weight loss happens to both organic nano-MMT and unmodified 298
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 5. Pour Point of model oil (M) doped with PPDs with different concentrations. (a) EVA(28) and EVA(28)/nano-MMT, (b) EVA(33) and EVA(33)/nano-MMT, (c) EVA(38) and EVA(38)/nano-MMT.
peaks indicate the successful intercalation of OTAC molecules, which increase the layer spacing and reduce the interaction between the layers of nano-MMT to some extent. The X-ray powder diffraction pattern of EVA/nano-MMT is also presented in Fig. 3. Compared with the organic nano-MMT, the 2θ of EVA/nano-MMT decreases and the d001 spacing reaches 2.10 nm and d002 spacing reaches 1.32 nm, which prove the formation of EVA/nanoMMT nanocomposite.
nano-MMT samples, which is attributed to the escape of water molecules adsorbed on the surface or between sheets of nano-MMT. From 220 ℃ to 450 ℃, the weight loss of organic nano-MMT is caused by the decomposition of the organic molecular chains, on the contrary, no weight loss is observed for the unmodified nano-MMT in this temperature scope. The weight loss of organic nano-MMT occurred in the range of 450℃∼700℃ is attributed to the collapse of nano-MMT sheets. Since the organic nano-MMT and OTAC begin to decompose around 220 ℃, their termination temperatures are obviously different. This phenomenon might be explained as follows: The organic molecular chains have inserted between layers or on the surface of nano-MMT, the mobility of organic molecular is limited and the poor airflow between sheets further inhibits the decomposition of organic molecular chains, leading to the weight loss of organic nano-MMT in the temperature range from 220 ℃ to 700 ℃ [22,23]. The weight loss of modified nanoMMT is approximately 32.35% over the entire temperature range and the organic compounds take 24.98% of the weight of the modified nano-MMT.
3.1.4. Dispersion evaluation of nano-MMT and EVA/nano-MMT PPD Dispersion experiments were carried out to evaluate the dispersion of unmodified, organic nano-MMT and EVA/nano-MMT. In order to obtain clearer image, water was used to test the hydrophilic, while dodecane, instead of −35# diesel, was used to test the oleophilic. Nano-MMT with 0.2 wt% dosage was stirred with ultrasonicator at room temperature until nano-MMT was evenly dispersed in water/ dodecane and then placed quiescently at room temperature for 1 h before observation. As shown in Fig. 4(a), the existence of turbidity is in compliance with the hydrophilic nature of unmodified nano-MMT. In Fig. 4(b), organic nano-MMT disperses well in dodecane but subsides in water, which is in accordance with the oleophilic nature. In Fig. 4(c), organic EVA/nano-MMT with 0.2 wt% dosage is well dispersed and forms a homogeneous solution in dodecane but layers in water which is in line with the oleophilic nature. The clear images and intuitive phenomenon show the good compatibility between the nanoMMT and EVA, and the good lipophilicity of EVA/nano-MMT.
3.1.3. X-ray diffraction (XRD) of nano-MMT and EVA/nano-MMT PPD The X-ray powder diffraction patterns of organic and unmodified nano-MMT are presented in Fig. 3. The Bragg equation 2dsinθ=nλ is used to calculate d spacing values. Compared with the unmodified nano-MMT, the d001 spacing of organic nano-clay increases from 1.51 nm to 1.96 nm and d002 peak begins to appear at 1.24 nm. Both 299
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 6. Viscosity of M undoped/doped with PPDs. (a) EVA (28) and EVA(28)/nano-MMT, (b) EVA(33) and EVA(33)/nano-MMT, (c) EVA(38) and EVA(38)/nanoMMT, (d) EVA(38)/nano-MMT, EVA(33)/nano-MMT and EVA(28)/nano-MMT.
between EVA and nano-MMT which leads to the re-desorption of the adsorbed EVA molecules. EVA/nano-MMT PPDs containing different kinds of EVAs exhibit different performance to the model oil and present the following performance order: EVA(38)/nano-MMT < EVA(28)/nano-MMT < EVA(33)/ nano-MMT.
3.2. Effect evaluation of nano-mmt PPD on model oil 3.2.1. Pour point As shown in Fig. 5, the effect of the three EVA/nano-MMT PPDs on the cold flow properties of model oil is studied. With the increase of PPD dosages, the pour points of model oil decrease first and then increase. Compared with pure EVA, the depression performance of organic nano-clay on model oil is weak. Compared with pure EVA(28) and EVA(38), the EVA(33) exhibits the best depression performance on model oil. Compared with EVA(33), the EVA(33)/nano-MMT PPD exhibits better depression performance with the same dosage. EVA(33)/ nano-MMT combines the advantages of EVA(33) and nano-MMT which are conducive to the dispersion and inhibiting overlap of wax crystals. The best pour point depressing performance is obtained with 0.08 wt% dosage of EVA(33)/nano-MMT, in which the pour point drops from 34 °C to 2 °C. However, EVA(38)/nano-MMT PPD is less effective than pure EVA(38). Some possible reasons might explain the phenomenon: EVA molecules interact with the organic nano-MMT and are adsorbed on the surface of particles. On the one hand, nano-MMT combines with EVA(38) molecules, when the added amount of nano-MMT reaches or exceeds the saturation value, agglomeration would happen among nano-MMT, the unbalanced interaction between nano-MMT and EVA reduces the depression performance of EVA/nano-MMT PPD on model oil. On the other hand, with the increase of polar group, the compatibility between nano-MMT and EVA slightly improves, an excessive content of the polar groups in EVA affects the bonding property
3.2.2. Rheological properties The viscosity of the model oil is plotted as the function of temperature in Fig. 6. Compared with EVA, there is no obvious effect of organic nano-clay on the viscosity reduction of model oil. With the same dose, EVA/nano-MMT PPDs exhibit better viscosity reduction performance than pure EVA. The effect of the EVA/nano-MMT PPDs might be explained as follows: First, the EVA molecules contain ethylene segments and vinyl acetate segments. Ethylene segments containing nonpolar groups act as crystalline phase, vinyl acetate (VA) segments containing polar groups act as non-crystalline phase. The nonpolar groups can incorporate with wax via adsorption or co-crystallization, the polar groups may modulate the morphology of wax crystals, interrupt the wax crystals growth and inhibit the overlap of wax crystals. Secondly, by dispersing nano-MMT into the polymer phase which changes the matrix of polymers, the compatibility between the nanoparticles and EVA affects the performance of PPDs. Since nanoMMT can provide more crystal nucleus and effectively disperse wax crystals. Combined with the advantages of EVA, EVA/nano-MMT PPD can effectively inhibit the formation of three-dimensional network structure in the oil phase and delay the conversion from sol to gel at 300
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 7. POM images of M samples undoped and doped with PPD at 20 ℃. (a) Undoped with PPD, doped with (b) nano-MMT, (c) EVA (28), (d) EVA(28)/nano-MM, (e) EVA (33), (f) EVA(33)/nano-MMT, (g) EVA (38), (h) EVA(38)/nano-MMT.
lower temperature. Compared with the viscosity of model oil with EVA (38), the one with EVA (38)/nano-MMT PPD has no outstanding advantage on improving flow behavior (see Fig. 6(c)).
doped with the optimized dosages of PPDs are shown in Fig. 7. Wax crystal reflects polarized light and exhibits bright white images, meanwhile, the solvent forms the black background. The variation in the morphology and quantity of the wax crystals were recorded in Fig. 7. As shown in Fig. 7(a), the wax crystal morphology of the pure model oil is large and feather-like. By comparing the images a, the wax
3.2.3. Microscopic study Polarized optical microscopic images of the model oil undoped/ 301
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
Fig. 8. Diagram description of the interaction between EVA/nano-MMT PPD and wax.
solid–liquid interfacial area between oil and wax crystal and reducing the amount of oil occluded in the wax crystal. So the EVA(33)/nanoMMT exhibits the best effect. Under the combined action of EVA and nano-MMT, the heterogeneous nucleation effect is remarkably reinforced as presented in Fig. 8.
crystal morphology of model oil doped with pure organic nano-MMT in Fig. 7(b) is still large and feather-like which is obvious that pure organic nano-MMT brought little influence on modifying wax crystals. In the images (c–h) of Fig. 7, it can be seen that obvious changes occurred in the wax crystal morphology of model oil doped with PPDs. By comparing the images c, e, and g, we notice that EVAs(28/33/38) change the wax crystal morphology of all samples from feather-like to needlelike. This phenomenon might be explained as follows: When the content of VA is relatively low, the waxy particles forms by the co-crystallization between polymer molecules, and wax molecules are highly solvated, and the interattraction between particles is weak, which leads to poor aggregation between wax particles. If the content of VA is relatively high, the effective content of non-polar groups is relatively low, resulting in the decrease of the co-crystallization ability between pour point depressant and wax molecules. So the EVA(33) exhibits better effect than the others. The same trend is observed in the experiments using EVA/nanoMMT PPDs. The EVA/nano-MMT PPDs can control the size and morphology of the wax crystals. While compared with the wax crystal morphology of the model oil doped with EVA(33), the wax crystal morphology with EVA(33)/nano-MMT is obviously different, it appears to be rod-like with smaller size and more compact structure. The phenomenon might be explained as follows: The wax crystal grows with EVA/nano-MMT PPD which acts as smaller crystalline nucleus precipitating earlier than wax. The new crystalline nucleus changes the precipitating habit of wax which is adverse to the formation of network structure through the overlap and interlock of wax crystals, but favors the formation of small wax crystals dispersed well in the oil phase. The optimum content of polar groups is conducive to reducing the
4. Conclusion Novel nano-hybrid pour point depressants (PPDs) were prepared by dispersing the organic nano-MMT in EVA PPD matrix and used in reducing the pour point and viscosity of the model oil. At the same dosage, EVA/nano-MMT PPDs showed better performance than their pure EVA analogs. The best pour point depressing performance of the model oil was obtained with 0.08 wt% of EVA(33)/nano-MMT PPD, in which the pour point dropped from 34 °C to 2 °C. The vinyl acetate content of EVAs had a significant impact on the performance of EVA/nano-MMT. Compared with EVA(28) and EVA(38), the EVA(33) exhibited better performance in modifying wax crystal morphology and improving the fluidity of model oil. The EVA/nano-MMT PPD can effectively change the morphology of wax crystals from fluffy feather-like to compact rodlike accompanied by the sharp decrease of the crystals’ size. Acknowledgment This work was supported by the National Natural Science Foundation of China [51534004, U1362110] and Northeast Petroleum University (No. JYCX_CX03_2018). 302
Colloids and Surfaces A 555 (2018) 296–303
N. Li et al.
point depressant on flow properties of waxy crude oil, Fuel 167 (2016) 40–48. [13] Z.N. Sun, G.L. Jing, Z.Y. Tu, Effect of modified nano-silica/EVA on flow behavior and wax crystallization of model oils with different wax content, J. Dispers. Sci. Technol. 39 (2018) 71–76. [14] B. Yao, C.X. Li, F. Yang, J. Sjöblom, Y. Zhang, J. Norrman, K. Paso, Z.Q. Xiao, Organically modified nano-clay facilitates pour point depressing activity of polyoctadecylacrylate, Fuel 166 (2016) 96–105. [15] A.M. Alsabagh, M.A. Betiha, D.I. Osman, A.I. Hashim, M.M. Elsukkary, T. Mahmoud, A new covalent strategy for functionalized montmorillonite–poly (methyl methacrylate) for improving the flowability of crude oil, RSC Adv. 6 (2016) 109460–109472. [16] S.M. Liu, B.B. Yan, P. Wang, Studied on preparation montmorillonite modified by CTAMB and adsorption performance on nitrobenzene, Adv. Mater. Res. 557–559 (2012) 996–1004. [17] S.I.S. Shahabadi, H. Garmabi, Response surface analysis of structural, mechanical, and permeability properties of polyethylene/Na+−montmorillonite composites, prepared by slurry-fed melt intercalation, Exp. Polym. Lett. 6 (2012) 657–671. [18] Y. Xue, Z.C. Zhao, G.W. Xu, X. Lian, C. Yang, W.N. Zhao, P. Ma, H.L. Lin, S. Han, Effect of poly-alpha-olefin pour point depressant on cold flow properties of waste cooking oil biodiesel blends, Fuel 184 (2016) 110–117. [19] Petroleum products-determination of solidification point. GB510-83, China (1983). [20] G.B. Huang, J.R. Gao, Y.J. Li, L. Han, Functionalizing nano-montmorillonites by modified with intumescent flame retardant: preparation and application in polyurethane, Polym. Degrad. Stabil. 95 (2010) 245–253. [21] M.C. Costache, M.J. Heidecker, E. Manias, G. Camino, A. Frache, The influence of carbon nanotubes, organically modified montmorillonites and layered double hydroxides on the thermal degradation and fire retardancy of polyethylene, ethylene–vinyl acetate copolymer and polystyrene, Polymer 48 (2007) 6532–6545. [22] V.L.P. Soares, R.S.V. Nascimento, V.J. Menezes, L. Batista, TG characterization of organically modified montmorillonite, J. Therm. Anal. Calorim. 75 (2004) 671–676. [23] W. Wei, Z.M. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermal degradation chemistry of alkyl quaternary ammonium montmorillonite, Chem. Mater. 13 (2001) 2979–2990.
References [1] A.L.C. Machado, E.F. Lucas, 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. Pet. Sci. Eng. 32 (2001) 159–165. [2] B. Yao, C.X. Li, F. Yang, Y. Zhang, Z.Q. Xiao, G.Y. Sun, Structural properties of gelled Changqing waxy crude oil benefitted with nanocomposite pour point depressant, Fuel 184 (2016) 544–554. [3] Z.C. Zhao, Y. Xue, G.W. Xu, J.W. Zhou, X. Lian, P. Liu, Effect of the nano-hybrid pour point depressants on the cold flow properties of diesel fuel, Fuel 193 (2017) 65–71. [4] T.J. Behbahani, A. Dahaghin, K. Kashefi, Effect of solvent on rheological behavior of Iranian waxy crude oil, Petrol. Sci. Technol. 29 (2011) 933–941. [5] C. Jiang, K. Zhao, C. Fu, L.Z. Xiao, Characterization of morphology and structure of wax crystals in waxy crude oils by terahertz time-domain spectroscopy, Energy Fuels 31 (2017) 1416–1421. [6] A. Radulescu, D. Schwahn, J. Stellbrink, E. Kentzinger, A.M. Heiderich, D. Richter, Wax crystallization from solution in hierarchical morphology templated by random poly(ethylene-co-butene) self-assemblies, Macromolecules 39 (2006) 6142–6151. [7] F. Yang, Z.Q. Xiao, B. Yao, C.X. Li, L. Wang, X. Shi, G.Y. Sun, K.Y. Yan, Influences of different functional groups on the performance of polyoctadecyl acrylate pour point depressant, Liq. Fuels Technol. 34 (2016) 1712–1719. [8] J.B. Taraneh, G. Rahmatollah, A. Hassan, D. Alireza, Effect of wax inhibitors on pour point and rheological properties of Iranian waxy crude oil, Fuel Process. Technol. 89 (2008) 973–977. [9] F. Yang, K. Paso, J. Norrman, C.X. Li, H. Oschmann, J. Sjöblom, Hydrophilic nanoparticles facilitate wax inhibition, Energy Fuels 29 (2015) 1368–1374. [10] H.M. Wang, M.H. Mei, Y.C. Jiang, Q.S. Li, X.H. Zhang, S.K. Wu, A study on the preparation of polymer/montmorillonite nano-composite materials by photo-polymerization, Photogr. Sci. Photochem. 51 (2001) 7–11. [11] G.L. Jing, Z.N. Sun, Z.Y. Tu, X.D. Bian, Y. Liang, Influence of different vinyl acetate content on the properties of EVA/modified nano-SiO2 composite PPD, Energy Fuels 31 (2017) 5854–5859. [12] C.Z. He, Y.F. Ding, J. Chen, F. Wang, C. Gao, Influence of the nano-hybrid pour
303