Submicron carbon-based hybrid nano-pour-point depressant with outstanding pour point depressant and excellent viscosity depressant

2.2.1 Synthesis of basic pour point depressant

Preparation of octadecyl acrylate: put quantitative octadecanol, solvent xylene and polymer inhibitor hydroquinone into three flasks with thermometers, condenser tubes and electric mixers. Dissolve octadecanol completely by stirring and heating to 55 °C. Then 25 ml acrylic acid and tert-butyl benzoyl peroxide were rapidly added to the flask, the stirring rate was adjusted to about 400 r/min, and the temperature was slowly increased to 100 °C. After a period of heat preservation, the temperature gradually increased to 100–130 °C, and the stirring reaction was increased to 550 r/min. The heating reflux was about 5 h, and the reaction stopped when the water quantity of the separator below the condenser reached the theoretical value. After the reaction, xylene and excess acrylic acid in the reaction product were extracted by vacuum distillation immediately. Octadecyl acrylate was obtained by vacuum drying in a constant temperature drying oven at 40 °C for 24 h.

Preparation of poly-α-olefin-carbon acrylate pour point depressant (PAA18): The flask containing octadecyl acrylate, a-octadecyl olefin and solvent xylene was placed in an oil bath, and the high-speed agitator and nitrogen pipe were installed. A certain amount of azobisisobutyronitrile was used as initiator for free radical polymerization. Replacing the air in the flask with nitrogen purge and slowly dropped the initiator azobisisobutyronitrile solution when heated to 70 °C by high speed stirring. When the temperature was slowly raised to 85 °C −100 °C, the stirring rate was kept at 500 r/min, the polymerization was finished after 6 h, and the reactant was poured out. When the solution was cooled to room temperature, it was poured into the beaker, and 50 ml methanol was added to purify the polymer. The precipitate was taken out and dried in a constant temperature drying oven at 40 °C for 24 h in vacuum to obtain poly-α-olefin-carbon acrylate pour point depressant (PAA18). Synthesis route of PAA18 is shown in Fig. 1.

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Fig. 1

Synthesis route of poly-α-olefin-high-carbon acrylate pour point depressant.

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2.2.2 Pretreatment of carbon materials

(1) Preparation of graphene oxide.

The preparation process of graphene (GR) treated by Hummer method is shown in S1.

(2) Preparation of carbon nanospheres.

Carbon nanospheres were prepared by mature glucose hydrolysis technology, and the preparation process is shown in S2.

(3) Preparation of oxidized carbon nanotubes.

The oxidation process of S3 carbon nanotubes is shown in S3.

2.2.3 Synthesis of carbon-based hybrid nano-depressant

Different amounts of graphene oxide (GO), carbon nanospheres and carbon nanotubes (O-CNTs) were added to 40 ml xylene, and the suspension was ultrasonically treated at low temperature (less than 5 °C) for 30 min to make it uniformly dispersed. Then PAA18 (80 wt%) was added into the uniform dispersions of the three carbon materials to form the mixed solutions of 0.5 %, 1 %, and 1.5 % poly-α-olefins-high carbon acrylate and carbon materials. The mixed solution was poured into the reactor and purged with nitrogen for 10 min. After exhausting the air in the reactor, it was sealed and placed in a muffle furnace. The reaction was held at 100 °C for 7 h. After the reaction was cooled to room temperature, 40 ml methanol was added to initiate precipitation, and the precipitation product was dried in an electrothermal constant temperature drying oven at 80 °C for 12 h to obtain graphene oxide composite pour point depressant, carbon nanospheres composite pour point depressant and carbon nanotubes composite pour point depressant. The preparation process is shown in Fig. 2.

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Fig. 2

Synthesis of carbon-based hybrid nano-depressant.

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3.1.1 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis

The size of graphene oxide, carbon nanospheres and carbon nanotubes, and the morphology of graphene oxide composite pour point depressant (PAA18-1 % GO) with 1 % carbon content, carbon nanospheres composite pour point depressant (PAA18-1 % Cna) with 1 % carbon content and carbon nanotubes composite pour point depressant (PAA18-1 % OCNTs) with 1 % carbon content were tested by SEM and TEM. The results are shown in Fig. 3.

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Fig. 3

Morphology of Carbon Materials and Composites (a) GO (b) Cna (c) OCNTs (d) PAA18-GO (e) PAA18-Cna (f) PAA18-OCNTs.

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Fig. 2 (a) shows that graphene oxide is irregular and thin layered structure with different sizes. There are many folds in the single layer, and the edges are smooth. Layers overlap with each other. Organic molecules exist between layers through electrostatic interaction. Interestingly, as shown in Fig. 2(b), it is found that the layered structure of graphene has been combined with the poly-α-olefins-co-glycol acrylate pour point depressant, showing a thick layered structure as a whole. The layers adhere to each other, showing irregular morphology and some flake and block structures attached to the layered structure, indicating that the graphene oxide aggregates have been densely dispersed on the skeleton of the polymer. Fig. 2(c) shows that the carbon nanospheres have uniform and regular spherical structure, smooth edge and no pit-like defects. From 2(d), it can be observed that the composite materials are in irregular spherical arrangement, and the particle size is larger than that of the carbon nanospheres. We are surprised to find that there are uniform block attachments on the surface of the carbon nanospheres (as shown in Fig. 2(d) embedding figure), which are the free radical polymerization products between Cna and PAA18. Fig. 2(e) shows that the oxidized carbon nanotubes (OCNTs) are cylindrical structures, showing a transparent tubular structure as a whole. The appearance is round and smooth, and they are intertwined with each other. Through Fig. 2(f) and the embedded partial amplification diagram, we can clearly see that poly-α-olefins-acrylic acid high-carbon alcohol ester pour point depressant (PAA18) is free radical polymerized on the oxidized carbon nanotubes.

Through morphology analysis, we can preliminarily see that the three kinds of nanocomposite pour point depressants have been successfully prepared, and the structure and morphology of the nanocomposite pour point depressants retain the structure of raw materials graphene oxide (GO), carbon nanospheres (Cna) and carbon nanotubes oxide (OCNTs), and there is a big difference at the same time. It can be seen that different structural appearance of particles may represent different physical and chemical properties. The geometric shape and dispersion state of nanoparticles in the polymer matrix and the interaction between particles and polymer chains determine the structure and properties of nanocomposites. Solvothermal method can not only change the size of the crystal, but also change its apparent morphology (Venugopal et al., 2012; Du et al., 2010).

3.1.2 Fourier transform infrared spectroscopy (FT-IR) a nalysis

The functional groups of the basic pour point depressant, graphene oxide, carbon nanospheres, carbon nanotubes and graphene oxide composite pour point depressant (PAA18-1 % GO), carbon nanospheres composite pour point depressant (PAA18- 1 % Cna) and carbon nanotubes composite pour point depressant (PAA18-1 % OCNTs) with carbon content of 1 % were tested by Fourier infrared spectrometer, and the synthesis results of the composite pour point depressant were further discussed as shown in Fig. 4.

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Fig. 4

FI-IR of Carbon Materials and Composites.

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It can be observed from Fig. 4 that PAA18 had no double bond stretching vibration absorption peak, indicating that the pour point depressant had become polymer from monomer. completed the transformation from monomer to polymer. Among them, the non-cyclic ester carbonyl stretching vibration absorption peak at 1727 cm⁻¹, the $-\left[C{H}_2\right]{-}_n(n⩾4)$ in-plane rolling vibration absorption peak at 716 cm⁻¹, the in-plane bending vibration absorption peak at 1163 cm⁻¹, and the stretching vibration absorption peaks of C—H bond at 2922 cm⁻¹ and 2854 cm⁻¹ were all the characteristic peaks of the synthesized basic pour point depressant. And we found that ester carbonyl absorption peak and C—H bond absorption peak was more obvious, which was due to its large dipole moment, resulting in strong absorption. We found that in addition to the absorption peaks caused by C⚌C stretching vibration at 1618 cm⁻¹ and graphene characteristic absorption peaks caused by carbon skeleton vibration at 1388 cm⁻¹, there were also strong characteristic absorption peaks of 1713 cm⁻¹ carbonyl group, O—H in-plane deformation stretching vibration absorption peaks at 1218 cm⁻¹, and epoxy C—O characteristic stretching vibration absorption peaks at 1058 cm⁻¹ and 1000 cm⁻¹ in the characteristic curve of GO. This was because many characteristic oxygen-containing groups appeard in the oxidation process of graphene, such as –COOH, –OH, -C⚌O, etc. (Fujii et al., 2020). At the same time, we can observe the characteristic peaks of the infrared curve of carbon spheres were as follows: the stretching vibration broadening peak of hydroxyl at 3411 cm⁻¹, the stretching vibration peak of C⚌O in aromatic ring at 1700 cm⁻¹, the in-plane stretching vibration peak of C⚌C in aromatic ring at 1605 cm⁻¹, the bending vibration peak of aromatic hydroxyl at 1028 cm⁻¹, and the out-of-plane bending vibration peak of aromatic hydrocarbon C—H at 784 cm⁻¹. The key to the synthesis of carbon spheres was the emergence of aromatic rings, which indicated that glucose undergoes hydrolysis and aromatization in the hydrothermal process (Wang and Liu, 2014). Moreover, we found the stretching vibration peak of -C⚌C- at 1604 cm⁻¹, the stretching vibration characteristic broad peak of hydroxyl at 3465 cm⁻¹, and the bending vibration absorption peak of ester base at 1163 cm⁻¹. This was because there were many characteristic oxygen-containing groups in the oxidation process of CNTs. The experimental results showed that the curve trend of PAA18 did not change significantly after the addition of carbon materials. This was because the addition of carbon materials was small, and the composite pour point depressant was based on PAA18. We found that the infrared curve of the composite pour point depressant included not only the characteristic strong peaks of the basic pour point depressant, but also the characteristic strong peaks of the carbon nanospheres, indicating that the three carbon-based hybrid nano-pour-point depressants were successfully prepared.

3.1.3 X-ray diffraction (XRD) analysis

The crystallinity of basic pour point depressant PAA18, carbon materials and carbon-based hybrid nano-depressant with 1 % carbon content were tested and analyzed by X-ray diffractometer, and the synthesis of composite pour point depressant was further discussed. The XRD spectrum is shown in Fig. 5.

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Fig. 5

XRD patterns of composites.

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At present, it is recognized that the strong diffraction peaks of graphene (GR) and carbon nanotubes (CNTs) should be around 23° and 26.4°. However, the diffraction peaks of GO and OCNTs measured in this study shift to the left, which were located at 12.43° and 25.89°, respectively. It was found that the cell parameters of the two composites increased and the crystal plane spacing d increased through the calculation of Braggs equation (Li et al., 2013). This was due to the adhesion of hydroxyl, carbonyl and epoxy groups on the surface and edge of CNTs, which can prove that graphene and CNTs have been completely oxidized. It was also observed from Fig. 4 that the characteristic peaks of PAA18 at 8.59° and 39.78° and the characteristic peaks of GO at 42.36° could correspond to the diffraction peaks of PAA18-1 % GO, respectively. At the same time, the characteristic peak of 12.43° corresponding to GO disappeared in the composite, while the diffraction peak of PAA18 at 22.36° disappeared and shifted to the left, about 0.81°, and a strong and wide characteristic peak appeared at 21.55°. A prominent hump appeared in the carbon nanospheres, and the diffraction peaks of PAA18-1 % Cna correspond to PAA18 and Cna at 2θ = 8.59°and 21.51°, respectively. It was found from Fig. 4 that the characteristic absorption peaks of OCNTs with different intensities appeared at 25.89°, 42.38°, 44.44°and 51.61°, mainly corresponding to the (0 0 2), (0 1 3), (0 1 3) and (0 1 6) crystal planes of the carbon nanotube structure, which can be found in the characteristic peaks of PAA18-1 % OCNTs, and the diffraction peak of PAA18-1 % OCNTs contains the diffraction peak of PAA18 at 8.59°. In this study, the broadening, moving and strengthening of the diffraction peaks were due to the preferential growth orientation of the carbon material due to its high surface activity during the free radical polymerization of carbon material and PAA18, which made the polymer adhere to the carbon material to form a coating process without damaging the crystal structure. At the same time, it was found through calculation that the spacing of PAA18-1 % GO, PAA18-1 % Cna and PAA18-1 % OCNTs after polymer grafting was further increased, which confirmed that the three carbon materials experienced the recrystallization growth process in the solvothermal preparation of composites, and the adhesion of PAA18 groups on the surface and edge of carbon materials. From the XRD curve diffraction peak shape of Fig. 5, the peak width and sharp, indicating that PAA18-1 % GO, PAA18-1 % Cna and PAA18-1 % OCNTs composites were successfully prepared, and had the characteristics of small grain, good crystallinity and high crystalline phase content.

3.2.1 Effect of pour point depressant on wax-containing simulated oil pour point

The effects of basic pour point depressant PAA18 and carbon-based hybrid nano-pour-point depressants with different carbon contents on wax-containing simulated oil pour point were mainly studied, and the results are shown in Fig. 6.

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Fig. 6

Pour point of simulated oil containing wax.

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Fig. 6 and Table 2 showed that the pour point of the blank wax-containing simulated oil is 30 °C, and the pour point of the simulated oil decreased gradually with the increased of the dosage of the pour point depressant, and the trend was the same. It showed that the low temperature fluidity of the oil can be improved by PAA18 basic pour point depressant and carbon-based hybrid nano-pour-point depressant with different carbon content, and the dosage of the pour point depressant can significantly affect the pour point depressant effect. From Fig. 5, it can be found that adding 1250 ppm PAA18 can reduce the pour point of wax-containing simulated oil from 30 °C to 19 °C, and the decrease was 11 °C. Among the graphene oxide composite pour point depressants, PAA18-1 % GO had the best pour point depressant effect at the dosage of 1250 ppm, which can reduce the pour point of wax-containing simulated oil to 15 °C and further reduced 4 °C on the basis of PAA18 pour point depressant. In the carbon nanospheres composite pour point depressant, when the dosage was 1000 ppm and 1250 ppm, PAA18-0.5 % Cna can make wax simulated oil pour point dropped 13 °C, 2 °C more than PAA18. In the composite pour point depressant of oxidized carbon nanotubes, we find that the pour point of simulated oil containing wax can be reduced by 16 °C at the dosage of 1250 ppm and 5 °C on the basis of PAA18 pour point depressant, which improved the low temperature fluidity of waxy crude oil best. When the dosage is less than 1000 ppm, GO provides nucleation sites on the surface of wax molecules and produces adsorption. The wax crystal is adsorbed on the nanoparticles and maintains a dense morphology, so it has good pour point depressant effect. However, when the dosage is more than 1000 ppm, it may produce self-gelation phenomenon, which makes the solubility of some pour point depressants worse, thus affecting the pour point depressant effect of GO composite pour point depressant. Different pour point depressants have different pour point depressant effects and the optimal dosage is also different. At the same time, the solubility of carbon nanotube composite pour point depressant may be better than that of graphene oxide composite pour point depressant, so the above phenomenon occurs. It was worth noting, however, that when the dosage reached 1500 ppm, there was a certain rebound in the pour point of the simulated oil containing wax, which may be due to the excess dosage of the pour point depressant, which increased the volume of the pour point depressant molecule or carbon material and leaded to self-aggregation, making it partially ineffective or difficult to dissolve in the oil, thereby affecting the improvement of the low temperature fluidity of crude oil. In short, the addition of three kinds of carbon materials can improve the pour point depressant effect to a certain extent, and have a good synergistic effect with the traditional polymer pour point depressant (PAA18). The synergistic effect of carbon nanotubes composite pour point depressant is better than that of graphene oxide nanocomposite pour point depressant and carbon nanospheres composite pour point depressant.

3.2.2 Effect of pour point depressant on rheological properties of wax-containing simulated oil

In order to explore the effect of carbon-based nanohybrid pour point depressant on the rheological properties of oil products, the flow behavior of wax-containing simulated oil treated with carbon-based nanohybrid pour point depressants with different carbon contents in the range of 0–500 s⁻¹ shear rate was studied under the optimal dosage of 1250 ppm at 30 °C. The results are shown in Fig. 7.

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Fig. 7

Rheological Properties of Carbon Materials and Composites.

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It can be observed in Fig. 6 that the viscosity trend of wax-containing simulated oil decreased with the increase of shear rate, and the viscosity reduction rate of simulated oil showed the shear dilution behavior of non-Newtonian fluid from large to small. The results showed that when the shear rate reached a certain value, the viscosity changed little and the trend tended to be stable. It can be seen from the flow chart 7 (a) that when the shear rate reached 400 s⁻¹, the viscosity of the oil tended to be stable. PAA18 basic pour point depressant can reduce the viscosity of wax-containing simulated oil from 31.8 mPa·s to 7.7 mPa·s, and the viscosity reduction rate was 75.8 %. The viscosity of simulated waxy oil treated with PAA18-0.5 % GO, PAA18-1 % GO and PAA18 − 1.5 % GO composite pour point depressants decreased from 31.8 mPa·s to 4.1 mPa·s, 2.8 mPa·s and 4.7 mPa·s, respectively, and the viscosity reduction rates were 87.1 %, 91.2 % and 85.2 %, respectively. From Fig. 7(b), it can be observed that the viscosity of simulated waxy oil treated with PAA18-0.5 % Cna, PAA18-1 % Cna and PAA18-1.5 % Cna pour point depressants was 5.7 mPa·s, 6.9 mPa·s and 7.2 mPa·s, and the viscosity reduction rates were 82.1 %, 78.3 % and 77.4 %. Fig. 7(c) showed that PAA18-0.5 % CNTs, PAA18-1 % CNTs and PAA18-1.5 % CNTs pour point depressants can reduce the viscosity of wax-containing simulated oil to 4.3 mPa·s, 2.5 mPa·s and 3.7 mPa·s, and the viscosity reduction rate was 86.5 %, 92.1 % and 88.4 %. Among them, PAA18-1 % CNTs pour point depressant had the largest reduction in crude oil viscosity, and the effect of improving the low temperature fluidity of waxy simulated oil was the best, which increased the viscosity reduction efficiency by 16.3 %. This showed that carbon-based nanohybrid pour point depressants with different carbon content can improve the low temperature fluidity of oil products, and had high sensitivity to simulated oil, and achieved good viscosity reduction effect. Three kinds of carbon-based hybrid nanoparticles have high surface energy and become the ‘new’ nucleation site of wax crystal, and adsorb on the new crystal nucleus to form a relatively narrow and dense morphology, thereby reducing the interface area and reducing the surface energy, thus forming a larger gap between the disconnected structure, releasing liquid oil wrapped by wax crystal. Therefore, the viscosity of the oil drop. In summary, in terms of improving the rheological properties of oil products, the order of viscosity reduction effect is as follows: carbon nanotubes composite pour point depressant > graphene oxide composite pour point depressant > carbon nanospheres composite pour point depressant > PAA18 basic pour point depressant.

3.2.3 Effect of pour point depressant on wax crystal morphology of wax-containing simulated oil

In order to explore the mechanism between graphene oxide nano-depressant and oil more comprehensively, the effect of carbon-based nano-hybrid pour point depressants with different carbon content on wax crystal morphology of wax-containing simulated oil was mainly studied. Polarized microscope was used to observe the wax crystal morphology of different composite pour point depressants. The results are shown in Fig. 8.

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Fig. 8

Optical micrographs of simulated oil containing wax (a) and (b)-blank wax-containing simulation oil, (c)-PAA18, (d)-PAA18-0.5 % GO, (e) -PAA18-1 % GO, (f) -PAA18-1.5 % GO, (g)-PAA18-0.5 % Cna, (h)-PAA18-1 % Cna, (i) -PAA18-1.5 % Cna, (j)-PAA18-1.5 % Cna, (k) -PAA18-0.5 % OCNTs, (l) -PAA18-0.5 % OCNTs.

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As shown in Fig. 8 (a) and (b), it can be observed that the wax wax-containing simulated oil without pour point depressant grew irregularly in large slices. The interval of a single wax crystal was large, and it overlapped and intersected each other to form a relatively stable three-dimensional network structure, which caused the liquid oil to be wrapped in it, so that the simulated oil lost its low-temperature fluidity and obtained a pour point. With the addition of pour point depressant, we found that the number, structure and size of wax crystals changed. As shown in Fig. 8 (c), when the dosage of PAA18 reached 1250 ppm, the three-dimensional network structure gradually became loose, the number of wax crystals increased, and the wax crystals decreased significantly. Among them, the small wax crystals gradually formed a number of wax crystal aggregates, which were tree-like. This structure reduced the density of wax crystal aggregates and increased the distance, and released the liquid simulated oil trapped in the stacking of large wax crystals, which weakened the ability of wax crystals to form three-dimensional network structure, and then changed the low temperature fluidity of oil products, thereby reducing the oil pour point. Moreover, we were surprised to find that the wax crystal size of the simulated oil containing wax added with three kinds of carbon-based hybrid nano-composite pour point depressants (carbon content of 0.5 %, 1 %, 1.5 %) was larger and more dispersed than that of the basic pour point depressant PAA18, and wax crystals were surrounded by black dots. This may be because some wax crystals were co-crystallized through traditional polymer pour point depressants, and others were wax crystals. Because carbon materials provided nucleation sites on the surface of wax molecules to produce adsorption, wax crystals were adsorbed on nanoparticles and maintained a dense morphology, and moved with its movement, thereby preventing the growth of crystal nuclei. This also leaded to good dispersion of wax crystals, thus hindering the formation of three-dimensional network structure. However, we found that the wax crystals in the wax-containing simulated oil increased and aggregated after the addition of PAA18-1.5 % GO, PAA18-1 % Cna, PAA18-1.5 % Cna, and PAA18-1.5 % OCNTs. This may be because the excessive addition of carbon materials led to the low free radical polymerization degree of carbon materials and polymers, which led to the poor pour point depressant effect. In summary, the three carbon-based hybrid nano-composite pour point depressants can change the size, number and dispersion of wax crystals to a certain extent. Among them, the wax crystal morphology is best improved by PAA18-1.5 % GO, PAA18-1 % Cna, PAA18-1.5 % Cna and PAA18-1 % OCNTs composite pour point depressants.