Article pubs.acs.org/EF
Influence of Different Vinyl Acetate Contents on the Properties of the Copolymer of Ethylene and Vinyl Acetate/Modified Nano-SiO2 Composite Pour-Point Depressant Guolin Jing,*,† Zhengnan Sun,† Ziyi Tu,† Xudong Bian,† and Yu Liang‡ †
Provincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjiang 163318, People’s Republic of China ‡ Daqing Petrochemical Research Center, China National Petroleum Corporation, Daqing, Heilongjiang 163318, People’s Republic of China ABSTRACT: Nanohybrid pour-point depressants (PPDs) were prepared by organically modified nano-SiO2 in copolymers of ethylene and vinyl acetate (EVA) with various vinyl acetate contents. The effects of nanohybrid PPDs on the cold-flow properties of 25 wt % wax model oil were evaluated. The crystallization behavior and crystal morphology of the model oil at low temperatures were also observed using polarized optical microscopy. The results indicated that the co-crystallization and dispersion effects of EVA on wax crystals are the key factors in determining the heterogeneous nucleation effect of the nanohybrid PPDs. The vinyl acetate content in EVA strongly affects the compatibility between the modified nanosilica and EVA. As a result, the selection of an appropriate EVA according to the oils used and the compatibility factor is important. In this work, EVA (VA = 32%)/modified nano-SiO2 composite PPD [N-SiPPD(32)] provided the best cold-flow improvement for the model oil when doped at 500 ppm. The pour point of the model oil was reduced from 34 to 5 °C, which resulted in a regular, diamondshaped, and uniform arrangement of wax morphologies.
1. INTRODUCTION High-wax crude oil with a high pour point and low-temperature fluidity greatly complicates oil extraction and transportation.1,2 Polymeric pour-point depressants (PPDs) are added in the industry for the wide transmission of wax crude oil.3,4 The growth habits of wax crystals in crude oil can be substantially modified by adding PPDs, which can cause the wax crystals to form a three-dimensional network, resulting in a reduced pour point of the crude oil.5,6 Because of the complexity of the crude oil composition and the specificity of the polymeric PPD, several different types of PPDs for crude oils have been developed.7,8 Among them, a copolymer of ethylene and vinyl acetate (EVA) that features excellent adaptability and pourpoint depression properties has been widely used in the pipeline transportation of crude oil. The EVA molecule is divided into segments by polar groups, with the segments being composed of nonpolar carbon chains. The nonpolar parts of the EVA molecule have a good affinity to the adjacent alkane waxy molecules, and the polar parts can increase the repulsion effect for the deposition of the alkane waxy molecules. A number of studies9−11 have shown that the acetate percentage of EVA molecules is an important factor governing the performance properties of PPDs. The specific value should be determined according to the oils that are used. Recently, nanohybrid materials have been developed as a new type of PPD for the petroleum industry;12−16 these materials can effectively reduce the pour point and viscosity of crude oil. Zhang et al.17,18 prepared a nanohybrid PPD with a long effect time, which can improve the anti-reheating and antishearing properties of the PPD. Yang et al.19,20 prepared a poly(octadecyl acrylate) (POA)/nanosilica hybrid particle as a new class of hybrid PPD; the resulting nanohybrid PPD can © XXXX American Chemical Society
provide spherical templates for wax precipitation and cause wax crystals to form a compact precipitate structure, which suppresses gelation and improves the flowability of the model waxy oil by several orders of magnitude. Notably, the POA coverage on the surface of the nanoparticles is an important factor affecting the performance properties of hybrid PPDs.21 Nanoparticles with a low POA coverage exhibited almost no effect on the strength of the formed wax gel, whereas nanoparticles with full POA coverage substantially lowered the wax gel strength; in addition, nanoparticles with more than full POA coverage further lowered the strength of the formed wax gel. The wax appearance temperature was also lowered by the nanoparticles, and using nanoparticles with greater than 100% POA coverage resulted in a limited or no additional effect. In the current work, nanohybrid PPDs were prepared using organically modified nano-SiO2 in EVA with various vinyl acetate contents. The effects of various PPDs on the cold-flow properties of 25 wt % wax model oil were comparatively studied. Finally, the crystal morphology and crystallization behavior of model oil with treated PPDs were observed using polarized optical microscopy. The results demonstrate commercial viability of the nanotemplates.
2. EXPERIMENTAL SECTION 2.1. Materials. Deionized water, toluene [analytical reagent (AR)], stearic acid (AR), ethanol, EVA [vinyl acetate (VA) = 28%], EVA (VA = 32%), EVA (VA = 40%), γ-aminopropyltriethoxysilane (silane Received: January 18, 2017 Revised: April 25, 2017 Published: April 27, 2017 A
DOI: 10.1021/acs.energyfuels.7b00189 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
2.3.3. Effect of PPDs on Cold-Flow Properties of Model Oil. 2.3.3.1. Pour-Point Tests. Pour points of the model oils, both undoped and doped with PPDs, were determined according to the China Petroleum and Natural Gas Industry Standard SY/T 0541-94. A model oil sample was initially kept in a water bath at 60 °C for at least 1 h. The sample was then cooled quiescently at a rate of 1 °C/min. When the temperature approached 50 °C, the test tube was removed to observe the viscosity of the oil for each additional 2 °C drop. As the sample was cooled close to the expected pour point, observations were taken at each 1 °C. The temperature at which the sample did not flow, even when the test tube was held horizontal for 5 s, was recorded as the pour point. 2.3.3.2. Rheological Measurements. The rheological measurements of the model oil and PPD-doped oils were performed on a Brookfield rotational rheometer (DV-II+Pro) equipped with a heating/cooling system. To improve the repeatability of the rheological measurements, the model oils undoped/doped with PPDs were initially heated to 60 °C for at least 1 h and then loaded on the rheometer to start the tests. Afterward, shearing was carried out for 5 min at 60 °C at a constant shear rate of 20 s−1; the test conditions were then changed, where the temperature was decreased from 60 to 20 °C at a cooling rate 0.5 °C/min, for determination of the viscosity−temperature curve. 2.3.3.3. Polarized Optical Microscopy (POM). A cross-polarized light microscope (ZEISS AXIO, Germany) equipped with an IMc 5type camera was used to determine the wax crystal morphology of the model oil undoped/doped with PPDs. Samples were initially heated to 60 °C and were then cooled to 15 °C at a cooling rate of 0.5 °C/min for observation.
coupling agent KH550), and nano-SiO2 (average primary particle size of 30 nm) were acquired from Aladdin. 2.2. Sample Preparation. 2.2.1. Preparation of Model Oil. Model oil with a 25 wt % content of paraffins was prepared with crude oil from the Daqing oilfield. Model oil containing 25 wt % wax was prepared by dissolving paraffins in −35 diesel oil and then stirring at 70 °C for 2 h. 2.2.2. Preparation of Modified Nano-SiO2. Equimolar amounts of succinic anhydride and KH550 were evenly dispersed in an ethanol solution (ethanol/water = 9:1) and stirred at room temperature for 1.5 h. The ratio of KH550 was 5 wt % SiO2. Then, SiO2 nanoparticles were added to the solution under ultrasonication and stirred at 75 °C for 3 h. After the reaction, the mixture was separated by centrifugation and the product was washed with deionized water. Modified nanosilica was obtained after the product was dried under vacuum and subsequently ground. The general steps for the preparation of organic SiO2 are as follows:
3. RESULTS AND DISCUSSION 3.1. Characteristics of Modified Nano-SiO2. 3.1.1. FTIR Analysis. The FTIR spectra of KH550, SiO2, and modified SiO2 are shown in Figure 1. As shown in spectrum a of Figure 1,
2.2.3. Preparation of EVA/Modified Nano-SiO2 Composite PPD. Certain quantities of EVA and modified nano-SiO2 (where the mass ratio between EVA and modified nano-SiO2 is 1:1) were added to toluene and stirred adequately with ultrasonication until they were evenly dispersed in toluene. The resulting mixture was heated to 80 °C for 3 h with stirring. The solution was carefully evaporated by distillation under vacuum and continuous stirring to remove toluene. Finally, the EVA/modified nano-SiO2 composite PPD was prepared. The EVA (VA = 28%)/modified nano-SiO2 composite PPD is denoted by N-SiPPD(28); the EVA (VA = 32%)/modified nano-SiO2 composite PPD is denoted by N-SiPPD(32); and the EVA (VA = 40%)/modified nano-SiO2 composite PPD is denoted by NSiPPD(40). 2.3. Characterization. 2.3.1. Nanosilica Surface Characterization. 2.3.1.1. Fourier Transform Infrared (FTIR). FTIR spectra were obtained using a FTIR spectrometer (TENSOR27, Bruker, Germany) to characterize the compositions of the original and modified nanosilica. The samples were pressed into disks with KBr for analysis. 2.3.1.2. Thermogravimetric analysis (TGA). TGA was carried out using a TG/DTA apparatus (Diamond, Japan) at a heating rate of 10 °C/min under a flowing nitrogen atmosphere. 2.3.2. Structure and Morphology of Nano-SiO2 Composite PPD. 2.3.2.1. Adsorption Experiments. Adsorption experiments were conducted using a Q-sense E4 quartz crystal microbalance (QCM) with simultaneous frequency and dissipation monitoring and mounted with a sensor crystal coated with a top layer of silica to determine the amount of EVA adsorbed onto the silica surface. In a typical experiment, a stable baseline of the toluene solvent was established before dissolved EVA was injected in the same solvent at a concentration of 2.5 wt %. The EVA (VA = 32%) solution was freshly prepared before injection. After the frequency shift stabilized on the QCM, pure solvent was washed over the crystal to ensure that the material was properly adsorbed. The QCM experiment was carried out at 25 °C. 2.3.2.2. Scanning Electron Microscopy (SEM). The morphology of the EVA/modified nano-SiO2 composite PPDs was studied by SEM using SIGMA (Zeiss, Germany).
Figure 1. FTIR spectra of (a) KH550, (b) SiO2, and (c) modified SiO2.
−NH2 groups were confirmed by the N−H bending vibration peak and the out-of-plane bending absorption peak at 785 and 1575 cm−1, respectively; both of these peaks are characteristic absorption peaks of the KH550 silane coupling agent. As shown in spectra b and c of Figure 1, Si−O−Si bending and stretching vibrations of the silanol groups were detected at 469, 800, and 1096 cm−1, which are the characteristic absorption peaks of SiO2. As shown in spectrum c of Figure 1, new adsorption peaks at 2855 and 2926 cm−1 were assigned to C−H stretching vibrations, confirming the presence of −CH 2 groups. Furthermore, a new adsorption peak at approximately 1379 cm−1 was assigned as a CO−NH absorption band, suggesting B
DOI: 10.1021/acs.energyfuels.7b00189 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels that the amino organic groups were introduced onto the surface of SiO2. 3.1.2. TGA. The TGA curves of original nano-SiO2 and modified nano-SiO2 are shown in Figure 2, from which an
Figure 3. QCM-D of the EVA adsorption on the silica surface from toluene. The EVA (2.5 wt %) flow started at 600 s, whereas the flow of pure solvent started at 3100 s. The frequency is shown as the black curve, and the dissipation is shown in blue.
Figure 2. TGA of (a) original nano-SiO2 and (b) modified nano-SiO2.
approximate number of functional groups on the silica surface were confirmed.22 The weight loss of original nano-SiO2 was approximately 4.53% over the entire temperature range; this weight loss is primarily attributed to the loss of water molecules trapped in the nanoparticles. We can see from curve b of Figure 2 that there are three stages of weight loss. The first stage of weight loss, observed in the range of 30−140 °C, is attributed to the thermal decomposition of water molecules trapped in the nanoparticles. The second stage of weight loss, observed in the range of 280−450 °C, is attributed to the thermal decomposition of the organic molecular chain on the surface of SiO2. The third stage of weight loss, observed in the range of 450−800 °C, is attributed to the thermal decomposition of the Si−O−Si bond, which is formed by the condensation reaction. These last two stages of weight loss indicate that functional groups are present on the silica surface. The weight loss of modified nanosilica was approximately 9.67% over the entire temperature range; an approximate 5.14% mass loss was calculated for the functional groups on the silica surface. 3.2. Structure and Morphology of Nano-SiO2 Composite PPDs. 3.2.1. Adsorption Experiments. A study of the adsorption of EVA onto silica was performed using the quartz crystal microbalance with dissipation monitoring (QCM-D), with adsorption onto a silica surface from toluene. Figure 3 shows the adsorption of EVA in toluene onto a silica surface. The frequency shift in the QCM indicated a slow adsorption onto the silica surface when the EVA solution passed over the silica surface, which gradually leveled out. In the case of toluene, a slight loss of material was observed when the pure solvent was passed over the surface, indicating that some loosely adsorbed material was present on the sensor, and the dissipation was low (
