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Preparation and Evaluation of Polymethyl MethacrylateGraphene Oxide Nano-Hybrid Polymers as Pour Point Depressants and Flow Improvers for Waxy Crude Oil Ahmad Mohamad Alsabagh, Mohamed A. Betiha, Doaa I Osman, Ahmed I. Hashim, Mohamed M. El-Sukkary, and Tahany Mahmoud Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01105 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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Preparation and Evaluation of Polymethyl Methacrylate-Graphene Oxide Nano-Hybrid Polymers as Pour Point Depressants and Flow Improvers for Waxy Crude Oil A. M. Al-Sabagha, M. A. Betihaa*, D.I. Osmana, A. I. Hashimb, M. M. El-Sukkarya, Tahany Mahmouda*

a

Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt

b

Chemistry Department, Faculty of Science, Ain Shams University, Egypt

*Corresponding authors. Fax: +20 222747433 E-mail addresses: [email protected] : [email protected]

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Abstract A diversity of solution has been used and utilized in order to minimize cold flow properties problems caused by paraffins crystallization during production and/or transportation of waxy crude oils. Herein, a novel series of nano-hybrids of polymethyl methacrylate-graphene oxide (PMMA-GO) as pour point depressants was prepared successfully by dispersing the inorganic nano-sheets of GO on the organic, PMMA, matrix via in situ free radical polymerization. The prepared PMMA-GO nano-hybrids were characterized by FT-IR and Raman spectroscopy, XRD, RHTEM, SEM and GPC chromatography. The thermal stability of PMMA-GO nano-hybrids was also studied by both TGA and DSC. The results showed that the pour point and the apparent viscosity of waxy crude oil were reduced significantly upon adding of PMMA-GO nano-hybrid, and the long-term stability of the PMMA-GO nano-hybrid was superior to that of a conventional PMMA material. The significant effect of this PMMA-GO nano-hybrid as pour point depressant and flow improver on waxy crude oil problem is carrying new technology to minimization the permeability during the charge and transportation process. The effect of oil soluble PMMA-GO nano-hybrid on the pour point/rheological properties is discussed, and a beneficiation mechanism is suggested. Keywords: nanohybrid; Graphene; flow improver; Pour point depressant; Rheological behavior; crude oil.

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Introduction Graphene has two dimensional (2D) hexagonal lattices of sp2 hybridized carbon atoms and has got great attention due to its exceptional properties such as mechanical, optical and thermal properties [1-3]. These exceptional material characteristics, resulted from single atom packed in the hexagonal crystal lattice, make graphene a promising candidate for a wide variety of technological application area likes energy storage, electronics, and optoelectronics [4]. Nevertheless, graphene showed poor solubility in both aqueous and non-aqueous solution, which obstacles to its benefits. One of the possible ways to solve this issue is the chemical modification of graphene with some oxidants [5-8]. The graphene oxide (GO) having different function groups, such as hydroxyl (-OH) groups, carbonyl (-C=O) groups, epoxy groups (-COC) and carboxylic groups (-COOH), which enable a wide range of graphene oxide functionalization with covalent and non-covalent bonds. Moreover, the oxygen or organic functionalization leads to isolation of nano-sheets of sp2-carbon domains by sp3-carbon carbon and enhance its dispersibility in different matrices depending on the nature of functionalization types [9-10]. Polymers are of importance for large materials in today's society because it is cheap and easy to manufacture. However, the physical properties of these polymers are one of the most important challenges for the expansion their use in various fields. Integrating nano-graphenic sheets into the polymer materials improve the physical properties of polymers, and their considerable impact is reviewed by Xu et al. [11]. The major variation in the final polymer characteristics was conditional with the type of graphene oxide modifiers, however, the chemical bond between GO and organic

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modifier appears to be better in GO dispersion, and similar results observed for montmorillonite modification and metalo-surfactant materials [12-16]. The extraction and transportation of waxy crude oils from oil fields is complex and expensive and relying on their chemical composition and weather conditions, and among other. The overwhelming preponderance of crude oil composition and their petroleum products contain paraffin, which is a mixture of normal hydrocarbons. Paraffin wax of carbon chain from 20 to 40 represents a considerable proportion of crude oil called wax. Paraffins or wax crystals are grown with lowering temperature, forming crystalline networks deteriorate the flow properties of the oil [17-18]. This result in lower production and maintenance lines from sediment deposits, increasing the cost of transportation. Consequently, during the processing of waxy crude, it is basically holed the crude oil at a temperature more than its original PPD. There are varieties of polymer and copolymer that employed as PPDs to change the crystallization behavior of the paraffin crude oil [19]. Along with the common mechanism theories; nucleation, cocrystallization, adsorption and enhanced solubility wax are acknowledged [20-24]. The wax deposition inhibitors of paraffinic crude oil are often polymer with definite characterization, in which the hydrocarbon chains are comparable to waxy moieties of crude oil, and the polymeric polar moiety is responsible for modification of wax crystals that is necessary to the damped crystal in the aggregation stage. Earlier studies have shown that nano particles dispersed in different matrixes have a noticeable impact on the degree of crystallization and heat distortion temperature, and the physical properties these matrixes are greatly affected by their size geometrical shape, interaction nature, and dispersion degree.

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Afore, studies have shown that nanoparticles can greatly affect the heat distortion and crystallization temperature, and the final size of polymer nanocomposites. In addition, both geometry and dispersion of nanoparticle or sheets in the polymer as well as the type of interaction (covalent or physical) with polymer matrixes defined the physical properties of the polymer nano-hybrids [25-28]. Fu et al, [29] studied the effect of microencapsulated (prepared from n-octadecane, triton X-100, melamine and formaldehyde at pH of 11) on crystallization behavior of n-dodecane, and the obtained data proved that the microcapsule weakened the interaction between two neighbors and this results may indicate flow property of n-alkane [17]. Based on that, the polymer nano-hybrid as PPD was synthesized and successfully applied in the waxy crude oil field. In this work, we aimed to prepare a novel series of PMMA-GO nanohybrid via in situ free radical polymerization and evaluate the prepared polymer nano-hybrid as pour point depressants and flow improvers for waxy crude oil. Our attention should be extended to make a complete characterization of the nano-hybrid polymers and predict a mechanism for their role in this area. 2. Experimental 2.1 Materials All chemicals including methyl methacrylate (MMA, 99%), acrylonitrile (≥99%), benzoyl peroxide (BOP, ≥70%), hydrochloric acid (36.5%), sulfuric acid (99.999%), 1,4-dioxane (99.8%, sodium chloride (≥99% %), calcium hydride (95%) and potassium permanganate (≥99% %) were obtained from Sigma-Aldrich. The solvents including benzene, acetonitrile, methanol and ethanol were purchased from Adwic Chemicals Co. (Egypt). The crude oil was supplied from Qarun Petroleum Company,

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North Qarun field, without any treatment, and its composition and physico-chemical properties are revealed in Table 1. 2.2 Synthesis of graphene oxide (GO) Graphene oxide, a pseudo 2D dimension solid containing deferent oxygenated groups in bulk appearance, was synthesized from graphite powder by chemical oxidation with KMnO4 in conc. sulfuric acid in accordance with to modified Hummers’ method [7]. In 250 ml round flask equipped with magnetic stirring, 5g of graphite, 2.5g NaNO3 and 110 ml H2SO4 at 0 °C were added. Then, 15g of KMnO4 (the addition rate must be controlled to avoid overheating). After about 30 min, the temperature is gradually increased to 35 °C, and the mixture became more thickener by the time. By the complete of 30 minutes, the obtained paste material was slowly added to stirred 1 liter of distilled water, causing violent effervescence and the temperature is adjusted at 98 °C for 15 min. An additional 500 ml of warm distilled water and 100 ml of H2O2 (30%) is added to facilitate removal of remaining KMnO4. The obtained bright yellow suspension is filtered off, washed several times with warm water until the pH of the filtrate is of 6. Finally, the obtained paste is dried under vacuum at 70 °C for 2 days. The GO is highly dispersed in distilled water due to the presence of different hydrophilic groups likes –OH, COC and –COOH groups at edges and basal plane of graphene sheets (Fig.1.). 2.3. Synthesis of Polymerizable Graphene oxide (vinyl-GO) In 150 ml round flask equipped with magnetic stirring bar, GO (1.46 g) was suspended in a water/dioxin solvent mixture (75/25: v/v; 50 ml) and the suspension was sonicated for 30 s to ensure platelet dispersion after acidification with HCl ([HCl] = 0.5) [30]. The acrylonitrile monomer (50 mL) was added to the suspension of GO material

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under vigorously stirred at ambient temperature for 3 h, treated with 5%NaCl and to facilitate platelet aggregation at the water/organic interface and the resulting floating layer (functionalized platelets) were cut off by filtration, washed several time with deionized water-dioxane (90/10: v/v) to remove HCl and the unreacted acrylonitrile. Fig. 2 shows the reaction between GO and acrylonitrile monomer (according to Pinner Reaction). Firstly, nitrile group reacted with alcoholic group GO in presence HCl, forming imino ether hydrochloride, which hydrolyzes in water and formation of ester bond between GO-OH and carbon atom attached to an unsaturated group associated with NH3 release. 2.4 Synthesis of poly(methyl methacrylate)-graphene oxide (PMMA-GO) nanohybrids In a typical procedure, different weight of vinyl-GO (0.01, 0.03, 0.05 and 0.1 g) was added individually under sonication to 100 ml round flask containing dry toluene (15 ml) and the mixture kept for 30 min at 5 °C under N2-atomsphere. A calculated weight of MMA (diluted in toluene; 40 wt %) that gives 0.1%, 0.3%, 0.5% and 1.0%GO to PMMA was added to dispersed GO mixture, and the mixture was further sonicated for 5 min to obtain high dispersed solution at 5 °C under N2-flow. Finally, BOP (0.03 g dissolved in 1 ml toluene) was injected using electronic pipette under stirring after the temperature reached 75 °C and the mixture was maintained at 75 °C for 15 min, and then decreased to 55 °C for additional 6 h. The reaction mixture was thereafter added to 45 ml of methanol to obtained the precipitated the PMMA-GO nanohybrid products. The resulting polymer was then dissolved in toluene and casted in petri-dish, dried at 60 °C to get PMMA-GO films. After complete drying, the obtained polymer nanohybrids

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were of 89%, 86%, 84% and 78% for PMMA-(0.1%, 0.3%, 0.5% and 1%)GO, respectively. 2.5 Characterization tools of the prepared PMMA-GO nano-hybrid The FTIR spectra were investigated by a Nicolet Is-10 instrument in the range of 4000–400 cm-1 using Thermo Fisher scientific spectrometer to recognize molecular functionality and structures of the prepared nano-hybrid materials. Raman spectra measurement were carried out to confirm the result of IR on Dispersive Raman Microscope (Senterra, Bruker) laser; 542 nm and power of 10 mw to supply a fingerprint of modified graphite by which structure can be recognized. The X-ray diffractograms were carried out with a XPERT X-ray differactmeters equipped with Cukα radiation (λ= 0.1542 nm) and X-ray radiation (X-ray generator current and voltage set at 40 mA and 45 KV). The TGA/DSC analysis was measured by simultaneous thermal analyzer, TA SDT Q600 V20.5 Build 15. About 6-8 mg were placed in aluminum pan and the analysis is carried out at temperature regime of 35 to 500 °C with heating rate of 10 °C under N2-atomsphere (50 ml min-1). The morphology of polymer nano-hybrid was examined from high-resolution transmission electron (HRTEM), JEOL JEM-2100 operated at 200 KV and field emission scanning electron microscopy (FESEM) was investigated from JEOL 5410 instrument. Gel permeation chromatography (GPC; model waters 515/2410) is used to determine the molecular weight and number, and polydispersity indices (Mw, Mn and Mw/Mn), for PMMA and PMMA-GO nano-hybrids. The dissolved sample in THF is passed across filter paper (pore size of 0.45 μm) and column (Styragel) calibrated using standard (polystyrene; Shodex) is used at constant temperature (40 °C) and flow rate (1 ml min-1). 2.5.1. Crude oil characterization

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At first, API gravity, pour point and water content of waxy crude oil were obtained by standard ASTM/IP methods, while bottom sediments and water were recognized by a centrifugal method (BS&W; ASTM D 96). The classification of the crude oil is identified at 60 °F and density of the crude oil was measured using a pycnometer [31, 32]. PPD measurement of the crude oil is obtained by a standard test method (ASTM standard D97) [33]. The device consists of ethylene glycol bath, test jar and jacket equipped with a digital thermometer. Been getting PPD of the crude oil, the sample was heated to 50 °C and the temperature cooled down to room temperature, then sample transferred to cooling bath and pour point data obtained at an interval of 3 °C. Upon the flow stopped after the weighting test jar horizontally, the recorded temperature is the pour point [34]. To obtain the content of wax, the crude oil (2 g) was suspended in normal hexane with the addition of few drop of conc. H2SO4 and stirred for 30 min and settled overnight. The oil layer was extracted using methylene chloride, and the remaining was cooled to −30 °C for one day. The solid was filtered off using Whatman filter (No. 934), and the product is weighted after solvent removal [35]. The flow curve can be analyzed using the Herschel-Bulkley equation, τ = τB + k Dm, where τ is the shear stress; τB is the dynamic yield stress ―Bingham yield value‖; k is the consistency index; m is the shear thinning index and D is the shear rate. The Bingham yield value (τB) can be obtained from the intercept of the graph relation between shear rate and shear stress. On the other hand, the linear line from the relation between shear rate and viscosity give us the dynamic ―apparent‖ viscosity. The flow curve is analyzed using the Herschel-Bulkley equation, τ = τB + k Dm, where τ is the shear stress; τB is the dynamic yield stress ―Bingham yield value‖; k is the consistency index; m is the shear thinning index and D is the shear rate.

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Rheological measurements (dynamic viscosity) were collected using the Rheometer programmable UV-III (Brookfield). The pristine polymer or polymer nanohybrid was dissolved in xylene (10 wt% in xylene), and this solution was injected into the crude oil in the regime of 500-2500 and 500-1500 ppm for PPD and viscosity measurements. The temperature at which the measurement is carried out is of 25, 35 and 45 °C. The sample containing additive is called blended while the pristine crude oil is called unblended. The aging test was carried out by adding the optimum ratio of PMMA or PMMA-GO to the crude oil in different jars, and the rheological measurement was collected after certain duration time to determine the stability of the crude oil. 3. Results and Discussion 3.1 Chemical and physical characterizations Fig. 3 shows the FTIR spectra of graphite, GO and vinyl-GO. Graphite shows very weak bands resulted from the constraint molecular motion of its groups involved in its compact structure. However, GO displays strong absorption frequency bands after graphite oxidation. The broad absorption band with a maximum at 3398 cm-1 can be attributed to the hydroxyl group stretching and the bands at 1737 cm-1 and 1288 cm-1 are assigned to the stretching vibration of –C=O and –C-O-C-, respectively. The vibration frequency band at 1635 cm-1 is assigned to the skeletal vibration band of un-oxidized graphene (–C C–) or may be result from the stretching deformation vibration of H–OH adsorbed molecule and the band at 1015 cm-1 are related to the epoxy stretching vibration mode of the alkoxy groups. The successful Pinner reaction for grafting vinyl group on graphene oxide was GO manifested by the intensity diminished or suppression of different oxygen

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containing groups except the band at 1737 cm-1 resulted from the formation of ester bond between GO and the vinyl group of the acrylonitrile. The bands at 3398 cm−1 and 1172 cm−1 are decreased due to consumption of hydroxyl groups and phenolic O-H stretching mode. Moreover, the bands at 3398 cm−1 and 1737 cm−1 showed blue shift by 8 and 5 cm−1 due to a hydrogen bond between formed ester and carboxylic groups. Unfortunately, almost all FTIR band frequencies characterizing of the vinyl containing molecules precisely agree to some extent with the frequency bands of multiple carboncarbon bonds. Indeed, the suitable vibration frequency for evaluation vinyl and phenyl group is at 1405 cm-1 (Figure 3c) and bands at 889 and 855 cm−1 (Figure 3b), where the deformation vibration band of C-H is assigned to the vinyl group and the two bands whose located at 889 and 855 cm−1 related to the phenyl group. The later two bands may be due to phenol contains two phenyl rings, and these bands are completely diminished while the band at 1405 cm−1 became clear, indicating grafting of the vinyl group into GO. Fig. 3d shows the FTIR spectrum of PMMA-1%GO nano-hybrid, two absorption peaks at 2930 and 1720 cm−1 appeared and this can be associated with C—H stretching and C=O stretching respectively. The frequency bands located at 2992, 2967, 1731, 1450 and 1179 cm-1 are the characterization bands of PMMA. Fig.4a. shows the Raman spectra of graphite, GO and vinyl-GO materials. It is well known that the D and G peaks are found in all polyaromatic hydrocarbons [36]. The G peak is attributed to the –C=C- stretching bonds in both aromatic and pendent chains, while the D peak is assigned to the breathing mode of sp2 atoms in the rings only [37]. Generally, the G peak and D peak showed Raman spectra at around 1580 and 1365 cm−1, respectively, and the additional Raman peaks at 1621 cm-1 (D' mode), 2681 cm-1 (2D mode) and 2949 cm-1 (D + G modes). Furthermore, the 2D band feature can

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differentiate between graphene and graphite, where this band is more intense than G band in graphite. Moreover, Raman bands at 2692 cm−1 and 1578 cm−1 due to 2D band and G band, while the band at 1350 cm−1 is due to disordered carbon. As per the spectrum of GO, there is a considerable change in the intensity and shape of 2D band and the G band after graphite oxidation, where the band at 2692 cm−1 is became broadness and undergoes blue shifted by ~15 cm−1. This behavior could be attributed to the considerable reduction in the size of the in-plane sp2 domains due to oxidation. Comparing Raman spectra of GO with vinyl-GO, subtle rather than drastic changes were observed. The band at 1358 cm−1 and 1609 cm−1 for GO shifted downward to 1350 cm−1 and 1602 cm−1 associated with intensity enhancement, which might be caused by the increasing of layers density per volume. As shown in Fig.2 the unsaturated organic are covalently attached to GO through ester-linkage that may be weak the physical interaction between two stacked monolayer sheets. Furthermore, the size of crystallinity (La) of GO and nano-hybrids were estimated by the following equation [38]:

La (nm) = 2.4  10-10 (nm-3 )  λ4 (nm4 ) × (IG /ID )-1 where λ is the wavelength used for the Raman measurement, and I D and I G are the Raman intensity of D and G peaks. The calculated values are of 6.1 nm, 19.33 nm and 19.86 nm for graphite, GO and vinyl-GO, respectively, which suggest that the crystallite size of GO is improved after performing Pinner reaction. Fig.4b displays the Raman spectrum of PMMA-GO nano-hybrid. An absorption peak appears at 2951 cm-1 is due to the stretching vibration of CH2 and CH3 moieties. The Raman peaks in range of 1720-1735 cm-1 (carbonyl

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group, stretching), 1450 cm-1 (-CH, bending), 1271-970 cm-1 (ether group, stretching) and 812 cm-1 (CH3, rocking) are well observed. The XRD patterns of GO, vinyl-GO, PMMA and PMMA-1%GO is shown in Fig. 5. The Strong peak is observed at 26.7° for graphite material, while this peak is shifted to lower angle after modification. According to Bragg's law (nλ = 2dsinθ), the graphite and GO layers have d-spacing of 0.333 nm and 0.823 nm, due to the stretching of sonication process and the presence of oxygen functional groups in interlayer spacing [39-41]. After grafting organic moieties the d-spacing in more enlarged and became 1.29 nm, confirming attachment of vinyl ester moieties at the edge of graphene oxide. To establish the degree exfoliation and quality of GO dispersion in PMMA, a FESEM was used and shown in Fig. 6. As seen in Fig. 6, the vinyl-GO in PMMA1%GO nanohybrid is well dispersed and showed high dense packed distribution. The vinyl-GO platelets less than 3 µm are observed, confirming that the polymerization of MMA monomer with vinyl-GO prevents agglomeration of GO sheets‖. The HRTEM image of PMMA-1%GO nanohybrid is displayed in Fig. 7. The nanohybrid agglomerate size is around 18 nm, where the lateral size is about 2 nm and the white color between lateral sizes is about 6 nm, reflecting exfoliated graphene sheets in PMMA. To study the thermal and degradation stability of PMMA, GO and PMMA-GO nano-hybrids, a TGA analysis was employed and displayed in Fig. 8. The weight loss of vinyl-GO occurs due to the loss of adsorbed water before 110 °C and decomposition of labile oxygen functional-containing groups between 170 and 220 °C [42], while the weight loss in the temperature regime of 248 to 500°C is due to the destruction of organic moieties. Generally, thermal degradation of PMMA-GO nano-hybrids is higher

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than pristine PMMA and mainly takes place in the temperature regime of 180–425 °C and the remnants are increased with increasing GO material in the samples. The residual weight after pyrolysis at 500 °C was of 0.13%, 0.86%, 1.23% and 2.1% for PMMA0.1%-GO, PMMA-0.3%-GO, PMMA-0.5%-GO, and PMMA-1%-GO, respectively. Generally, the PMMA-GO nano-hybrids shows weight loss in three main steps and a similar results were reported elsewhere [43]; (1) the first step takes place in temperature regime of 180 to 239 °C attributed to head-head linkages (H–H) the scission, (2) the next degradation that ascribed to the endcapped olefinated chain scissions caused by disproportionation termination that commonly occurs at temperature of 240 to 286 °C, (3) the last degradation comes from 310 to 412 °C comes from PMMA backbone scission [44] or homolytic scission followed by β-scission [45]. The broad range for the last step thermal degradation rate of PMMA-GO resulted from the restricted effect of GO layers for diffusion of cracked gaseous material outputs throughout the exfoliated graphene layers in nano-hybrid samples. Over ≈430 °C, all curves become flat due to no further degradation of carbonaceous material. It is well known that the traditional radical–radical termination (such as disproportionation or combination) is common in free radical polymerization. However, the addition of modified graphene is decline chain-capped head to head and unsaturation that are weak thermally stable links. It is clear from degradation at Td10 (10 % degradation occurs at certain temperature) that the Td10 occurs at 195 °C, 198 °C, 203 °C and 235 °C, taking into consideration the Mw of different PMMA-GO nano-hybrids. The Mw is of 153.3 x103, 191.0 x103, 198.5 x103 and 204.6 x103 g mol−1 for PMMA-0.1%GO, PMMA0.3%GO, PMMA-0.5%GO, and PMMA-1%GO, respectively (Table 2).

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The DSC curves of PMMA-0.1%GO, PMMA-0.3%GO PMMA-0.5%GO and PMMA-1% GO nano-hybrid is shown in Fig. 9. It is clear that the glass transition temperature (Tg) is not sharply shown for all nano-hybrids, may be due to restricts the mobility of the polymer chains by layers of graphene oxide, indicating strong uniform links between GO and PMMA chains. Nevertheless, the peaks at a temperature above 150 °C are attributed to thermal decomposition of PMMA-GO nano-hybrids. The broad weak peak around 182 °C and 187°C for PMMA-0.1&0.3%GO are attributed to cleavage of the thermally less stable head-head linkages that is weaker than backbone C-C bonds due to large steric hindrance. The broadness of this peak may be come from fewer head to head scissions. The 2nd and 3rd peaks at ≈300 °C and 385 °C are attributed to the scission of unsaturated chain-ends and destruction of polymer backbone. The molecular weight distribution of the PMMA-GO nanohybrids was measured and the results are listed in Table 2. It was found that Mw is increased with increasing vinyl-GO ratio, may be due to the gel effect resulted from the dispersion of graphene oxide sheets. The dispersion of vinyl-GO in MMA resulted in semi viscous mixture, which increases the gel effect. Increasing mixture gelation causes decreasing in termination step and to increasing the polymerization rate and consequently the molecular weight. 3.2. Effect of PMMA-GO nano-hybrids on pour point of the waxy crude oil Depression in the pour point is due to the wax crystal modification. The PMMAGO causes a dramatic reduction in pour point of the crude oil by preventing wax deposition. As seen from Fig. 10 and Table 3, the PMMA-GO reduced the pour point of the crude oil from 27 °C to 6 °C (PP  21 °C) . Oxygen-containing groups on GO may participate in the role of inhibition of the wax growth by surface adsorption

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mechanism. The adsorption of nano-hybrid on the surface of wax molecules makes the wax nuclei inactive, and further growth is prevented. Consequently, the crystal of wax molecules occurs in fine small size or in high dispersion in the crude oil, and therefore the net like structure, which is necessary for solidification is inhibited. Recently, Khidr et al, [46] stated that the adsorption of PPD molecules on different crystal faces render the regular growth of wax crystals and decreasing the overlapping between wax nuclei. Moreover, the presence of well defined GO lattice in nanohybrid adsorbed on waxy crude oil may be cause shape change of wax crystal. Au reported [47] that the crystal phase transition of wax crystal from the thin plate like structure to spherulitic structure accompanied with a reduction in pour point. The PMMA-1%GO exhibited high depression in pour point due to presence of high nano-GO sheets in PMMA than other nano-hybrid, which gives some polarity of PMMA and increases the interface between wax molecules, preventing the formation of a wax crystal in early stage process. By decreasing the temperature, the rate of crystallization increase and more short chain paraffins start to inlay holes between the separated wax clusters and PMMA-GO-paraffin agglomerates, as described in the suggested mechanism (Fig. 11). 3.3. Effect nano-hybrid on rheological behavior of waxy crude oil The rheological behaviors of NQ7 crude oil at a temperature under and above the pour point (25, 35 and 40°C) and a dose of PMMA-GO nano-hybrids ranged from 500 to 2500 ppm, and compared with unblended NQ7 were listed in Table 4. Fig. 12,13&14 demonstrated the shear stress vs shear rate while Fig. 15,16&17 displayed the viscosity vs. shear rate for blended and unblended crude oil with PMMA-1%GO nano-hybrid at 25, 35 and 40°C. The obtained data implies that the unblended. The data

- 16 -


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Energy & Fuels

reveals that the unblended crude oil follows non-Newtonian behavior. By inspection the data in Table 5, it was found that the values of apparent viscosity (ηapp) of the NQ7 crude oil were of 56.8, 40.1 and 26.7 cp and the yield stress (τB) values were of 5.7, 3.3 and 2.8 D/cm2 at temperatures of 25, 35 and 45 °C, respectively. A small decrease in the ηapp was obtained by adding PMMA, where ηapp values were of 52.9, 37.9 and 24.9 cp accompanied with Bingham yield stress (τB) values were of 4.3, 2.9 and 2.3 D/cm2 at temperatures 25, 35 and 45 °C, respectively. A great significant depression in ηapp and τB

values

was

observed

by

blending

the

waxy

crude

oil

with

the

PMMA-0.1,0.3,0.5&1%GO nano-hybrid. The maximum decrease in ηapp and τB was achieved by PMMA-1%GO at 500 ppm as shown in Table 5, whereas the ηapp values were of 10.11, 7.05 and 3.0 cp, while the τB values were of 1.6, 0.95 and 0.29 D/cm2 at temperatures 25, 35 and 45 °C, respectively. The enhancement of rheological properties by the PMMA-GO nano-hybrid is recorded for the first time in petroleum application, and the suggested mechanism is shown in Fig.11. The GO in nano-hybrid acts heterogeneous poisoning nucleation sites for paraffin chains and modified wax crystals, resulting in high dispersed wax crystals beyond the original pour point of NQ7 crude oil. The synergistic of polymer nano-hybrids may be that the PMMA component could slightly modify both size and crystal shape of the wax molecule via co-crystallization of polymer and paraffin chains, while the oxygen containing in nano-GO sheets charged wax crystals, which led to electrostatic repulsion between wax crystals. The high flowability of waxy crude oil is gained as resulted of the good solubility of PMMA-GO, leading to a great number of nano-sheets that effectively rendered the wax network formation and thus the solubility of waxy crude oil is enhanced in its low chain paraffins.

- 17 -


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The dynamic viscosity decreased with the increasing all PMMA-GO nanohybrid dosage as shown in Table 4. The maximum decrease in the viscosity was obtained by PMMA-1%GO at 1500 ppm. Furthermore, the increase in additive concentration was accompanied by an inconsiderable reduction in the viscosity Table 5. For instance, when shear rate value of 72.6 (1/sec) was employed at 25 °C, the PMMA-1%GO showed a reduction in viscosity by 82.2% (from 56.79 cp vs 10.11 cp), while that upon using pristine PMMA was of 6.9 % (from 56.79 cp vs. 52.85 cp). 3. 5. Aging test Fig. 18 illustrates the influence the time pass of polymer nano-hybrid (aging) on yield stress of the crude oil. The PMMA-1%GO reduced yield stress by 94.9% to 0.29 Pa, while pristine PMMA showed a reduction of 59.1% (2.33 Pa). Furthermore, longterm stability of PMMA-1%GO noticed is excellent. The yield stress values were nearly constant even after reheating the blended crude oil for four times, however, the blended crude oil with pristine PMMA showed an increase to some extent. These results may be due to the polymer nano-hybrids have high mechanical and electrostatic stability in the hydrocarbon system in comparing to the pristine polymers, which suffered from a complete deformation of rheological properties after 60 days only. Conclusion 

The nano-hybrid polymers were successfully prepared, well characterized and applied in improving the pour point and flowability of wax crude oil.



The action mechanism of polymer nano-hybrids might be that the PMMA could be adsorbed and be co-recrystaized with paraffin chains, modifying the size and crystal shape of wax crystal, while the oxygen containing of GO nano-sheets acted as heterogeneous nucleation sites and charged wax crystals causing

- 18 -


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Energy & Fuels

electrostatic repulsion between wax crystals that hindered formation of wax network and increase wax solubility leading improving flowability of wax crude oil. References 1.

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Ferreira, C. I.; Dal Castel, C.; Oviedo, M. A. S.; Mauler, R. S., Isothermal and

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polymer melt with a nanoscopic particle. Macromolecules 2002, 35, (11), 4481-4492. 27.

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CaCO3 nanoparticles shape on thermal and crystallization behavior of isotactic polypropylene based nanocomposites. Journal of thermal analysis and calorimetry 2005, 80, (1), 131-136. 28.

Avella, M.; Cosco, S.; Di Lorenzo, M.; Di Pace, E.; Errico, M., Influence of

CaCO3 nanoparticles shape on thermal and crystallization behavior of isotactic polypropylene based nanocomposites. Journal of thermal analysis and calorimetry 2005, 80, (1), 131-136. 29.

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alkane mixtures: stabilization of a new rotator phase by enhanced surface freezing and weakened intermolecular interactions. Physical Chemistry Chemical Physics 2011, 13, (33), 15031-15036. 30.

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reactions, mechanisms, and structure. Wiley: New York, 2001. 31.

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high-wax crude oils. Resins, asphaltenes, paraffin waxes. Chemistry and technology of fuels and oils 2005, 41, (3), 236-240. 32.

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structured polystyrene microspheres and their electrorheological characteristics under applied electric field. Journal of Materials Chemistry 2011, 21, (19), 6916-6921. 43.

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Page 25 of 44

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Energy & Fuels

Table 1. The Physicochemical properties of NQ-7 crude oil Experiment

Method

Result

API gravity @ 60 °F

ASTM D-4052

40.87

Specific Gravity@60/60F

ASTM D-4052

0.8209

Kinematic viscosity, cSt, @ 40o C

ASTM D-445

35.05

IP-143

2.56

UOP-64

12.30

Pour point, °C

ASTM D-97

27

Water content, vol. %

ASTM D-95

0.4

BS & W, vol. %

ASTM D-96

0.2

Total Paraffin Content, wt. %

ASTM D2887 (GLC)

29.9

Average Carbon Number (n)

IP 372 / 85 (GLC)

18.34

Asphaltene content, wt.% Wax content, wt. %

Table 2. Molecular weight distribution of PMMA and GO-PMMA nanohybrids Samples

Mn

Mw x103

Mp

Poly dispersity

PMMA

63149

206.3

191663

3.266

PMMA-0.1%GO

60846

153.3

155307

2.519

PMMA-0.3%GO

73738

191.0

213393

2.591

PMMA-0.5%GO

74583

198.5

199613

2.661

72764

204.6

215617

2.811

PMMA-1%GO

- 25 -


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 44

Table 3. Pour point of treated crude oil with PMMA and PMMA-GO nanohybrids Pour point temperatures (°C) at different Conc. (ppm) Samples 500

1000

1500

2000

2500

PMMA

27

27

27

27

27

PMMA-0.1%GO

24

24

21

18

15

PMMA-0.3%GO

21

18

15

15

12

PMMA-0.5%GO

18

15

12

12

9

PMMA-1%GO

15

12

9

9

6

Table 4: Viscosity of treated crude oil with PMMA and GO-PMMA nanohybrids polymers at different temperatures (25, 35 and 45 °C) and different concentrations. Sample Name Temp (°C) Viscosity (cp) 500 ppm

1000 ppm

1500 ppm

25

52.85

49.33

46.91

35

37.91

35.11

34.20

45

24.98

23.00

20.94

PMMA-

25

37.10

35.51

30.10

0.1%GO

35

29.98

27.09

23.81

45

21.92

16.95

14.50

PMMA-

25

35.65

31.44

29.53

0.3%GO

35

23.70

20.12

18.00

45

20.42

15.30

13.65

PMMA-

25

27.61

24.23

19.96

0.5%GO

35

20.92

17.09

12.69

45

15.03

10.98

9.04

25

10.11

7.64

6.00

35

7.05

5.04

3.98

45

3.00

2.24

1.75

PMMA

PMMA-1%GO

- 26 -


Page 27 of 44

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Energy & Fuels

Table 5: Rheological parameters of untreated and treated crude oil with PMMA andGO-PMMA nanohybrid polymers at different temperatures (25, 35 and 45 °C) at 500 ppm. Sample

Blank

PMMA

PMMA-0.1%GO

PMMA-0.3%GO

PMMA-0.5%GO

PMMA-1%GO

Temp. (°C) 25

Apparent Viscosity (cp) 56.79

Yield Stress (D/Cm2) 5.7

35

40.12

3.29

45

26.70

2.78

25

52.85

4.30

35

37.91

2.86

45

24.98

2.33

25

37.10

3.00

35

29.98

2.59

45

21.92

2.03

25

35.65

2.31

35

23.70

2.04

45

20.42

1.40

25

27.61

2.05

35

20.92

1.80

45

15.03

1.18

25

10.11

1.60

35

7.05

0.95

45

3.00

0.29

- 27 -


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1. Schematic diagram of preparation of graphene oxide (GO)

- 28 -


Page 28 of 44

Page 29 of 44

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Energy & Fuels

Fig.2. Schematic diagram for the preparation of polymethyl methacrylate-graphene oxide (PMMA/GO) nanohybrid

- 29 -


Energy & Fuels

d

T(%)

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 30 of 44

c b

a

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm−1)

Fig. 3. FT-IR spectra of (a) graphite, (b) materials

- 30 -

GO, (c) vinyl-GO and PMMA-1%GO


Page 31 of 44

PMM-1%GO

a

b

0

Intensity

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

1000

2000

3000

4000

GO Viny-GO Graphite

500

1000

1500

2000

2500

3000

Raman shift (cm−1)

Fig. 4. Raman spectra of (a) graphite, GO and vinyl-GO, and (b) PMMA-1%GO materials

- 31 -


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a u)

Energy & Fuels

Page 32 of 44

PMMA-1%GO

Vinyl-GO

GO

PMMA Graphite 0

10

20

30

40

50

2q Fig. 5. XRD of graphite, GO, vinyl-GO, PMMA and PMMA-1%GO materials

- 32 -


60

Page 33 of 44

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Energy & Fuels

Fig. 6. FE-SEM micrographs of PMMA-1%GO nano-hybrid at different magnification

Fig. 7. HRTEM micrographs of PMMA-1%GO nano-hybrid at different magnification

- 33 -


Energy & Fuels

110 100 90 80 70

Weight (%)

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 34 of 44

PMMA GO PMMA-0.1%GO PMMA-0.3%GO PMMA-0.5%GO PMMA-1%GO

60 50 40 30 20 10 0 0

100

200

300

400

500

Temperature (°C)

Fig. 8. TGA curves of vinyl-GO, PMMA, PMMA-0.1%GO, PMMA-0.3%GO, PMMA- 0.5%GO and PMMA-1%GO nanohybrids

- 34 -


Page 35 of 44

(4)

(3) Heat Flow (W/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

Energy & Fuels

(2) (1) PMMA-0.1%GO (1)

(2) PMMA-0.3%GO (3) PMMA-0.5%GO (4) PMMA-1%GO

Endothermi c

50

150

250

350

450

Temperature (°C)

Fig. 9. DSC curves of functionalized GO, PMMA-0.1%GO, PMMA-0.3%GO, PMMA0.5%GO, and PMMA-1%GO nano-hybrids

- 35 -


Energy & Fuels

PMMA PMMA-0.3%GO PMMA-1%GO

30

Temprature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 44

PMMA-0.1%GO PMMA-0.5%GO

20

10

0 500

1000

1500

2000

2500

Concentration (ppm)

Fig. 10. Relation between pour point of blended crude oil with PMMA and PMMA nanohybrids

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Page 37 of 44

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Energy & Fuels

Fig. 11. The suggested mechanism for enhancement of pour point by using the PMMAGO nanohybrids

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Energy & Fuels

31

Blank

28

PMMA 25 PMMA-1%GO 22 shear stress (D/cm2)

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 38 of 44

19

16

13

10

7

4 70

80

90

100

110

120

130

140

150

160

Shear rate (1/Sec) Fig. 12. Relation between shear rate and shear stress for unblended and blended crude oil (500 ppm) using PMMA and PMMA-1%GO at T= 25 °C

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Page 39 of 44

30

Blank 25 PMMA 1% GO-PMMA 20 shear stress (D/cm2)

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Energy & Fuels

15

10

5

0 40

50

60

70

80

90

100

110

120

130

Shear rate (1/Sec)

Fig. 13. Relation between shear rate and shear stress for unblended and blended crude oil (500 ppm) using PMMA and PMMA-1%GO at T= 35 °C

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Energy & Fuels

20 Blank PMMA 15

shear stress (D/cm2)

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 40 of 44

1% GO-PMMA

10

5

0 40

50

60

70

80

90

100

110

120

130

Shear rate (1/Sec)

Fig. 14. Relation between shear rate and shear stress for unblended and blended crude oil (500 ppm) using PMMA and PMMA-1%GO at T= 45 °C

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Page 41 of 44

60

50 Blank 40 Viscosity (cp)

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

PMMA 1%GO-PMMA

30

20

10

0 60

70

80

90

100 110 120 130 140 150 160 Shear rate (1/Sec)

Fig. 15. Relation between shear rate and viscosity for unblended and blended crude oil (500 ppm) using PMMA and PMMA-1% GO at T= 25 °C

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Energy & Fuels

40

30 Viscosity (cp)

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 42 of 44

Blank PMMA PMMA-1%GO

20

10

0 40

60

80

100

120

140

Shear rate (1/Sec)

Fig. 16. Relation between shear rate and viscosity for unblended and blended crude oil (500 ppm) using PMMA and PMMA-1% GO at T= 35 °C.

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Page 43 of 44

30

25

Blank

20 Viscosity (cp)

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

PMMA PMMA-1% GO

15

10

5

0 40

50

60

70

80

90

100

110

120

130

Shear rate (1/Sec)

Fig. 17. Relation between shear rate and viscosity for unblended and blended crude oil (500 ppm) using PMMA and 1% GO-PMMA at T= 45 °C.

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Energy & Fuels

100

Blank

PMMA

1% GO-PMMA

80

Viscosity (cp)

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 44 of 44

60

40

20

0 0

5

10

15 20 Time (days)

25

30

45

60

Fig. 18. Viscosities versus time of unblended and blended crude oil (500 ppm) using PMMA and PMMA-1% GO polymers at T= 25 °C

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Preparation and Evaluation of Poly(methyl ... - ACS Publications

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