Hydrophilic Nanoparticles Facilitate Wax Inhibition - Energy & Fuels

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Hydrophilic Nanoparticles Facilitate Wax Inhibition Fei Yang,† Kristofer Paso,*,‡ Jens Norrman,‡ Chuanxian Li,† Hans Oschmann,§ and Johan Sjöblom‡ †

College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway § Nalco Champion, Langestraat 169, 7491 AE Delden, Netherlands ‡

ABSTRACT: A new class of hybrid pour point depressants (PPDs) are developed on the basis of poly(octadecyl acrylate) (POA) functionality and POA/nanosilica hybrid particles. Activity performance is demonstrated using a model waxy oil system consisting of 10 wt % macrocrystalline wax dissolved in dodecane, effectively emulating the essential characteristics of waxy petroleum fluids. Differential scanning calorimetry (DSC) evidence confirms that POA molecules enhance the solubility of wax in the continuous oil phase, reducing the wax appearance temperature. The presence of POA also serves effectively to modulate the crystal morphology to a more regular spherical-like shape, instead of the disc-like morphologies common to pristine paraffin wax crystals, affecting reduced gelation temperatures in accordance with percolation theory predictions. In addition, rheometric yield stresses decrease with increasing dosage rates of POA. A solvent-blending protocol is followed to subsequently prepare POA/nanosilica hybrid particles. Optimal PPD performance of the hybrid particle system is attained at a dosage rate of 100 ppm. At dosages higher than the optimal dose, the gel strength increases in an analogous manner to the directionality of the Einstein equation for viscosity. The POA/nanosilica hybrid particle system provides spherical templates for wax precipitation, resulting in a compact precipitate structure, which suppresses gelation and improves the flowability of the model waxy oil by several orders of magnitude. DSC data confirm that a vast majority of the POA molecules become solubilized in the continuous oil phase upon dispersion of the hybrid nanoparticle system. As such, the free POA molecules enhance wax solubility in the continuous oil phase. Hydrophobic nanoparticles retain a more robust ability to modulate waxy oil rheology at low-dosage rates, as compared to purely polymeric functionality. The primary mechanism of hybrid particle PPDs involves heterogeneous nucleation activity. The hybrid particles effectively provide solid−liquid interface sites as wax precipitation templates, which result in spherical-like spherical wax morphologies. The compact morphologies hinder and suppress the percolation process necessary to form a volume-spanning network of wax crystals. As such, the hybrid nanoparticles constitute effective and economic PPD additives and may serve as the basis for next-generation environmentally friendly wax inhibition agents, by reducing the amount of additives needed.

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INTRODUCTION Crude oil often contains substantial amounts of paraffin wax, which constitutes the saturated aliphatic fluid fraction.1 When the crude oil temperature falls below the wax appearance temperature (WAT), the paraffin wax precipitates in an orthorhombic unit cell as needle- or plate-like crystals.2 Because of large aspect ratios, precipitated wax crystals often form a “house of cards” network structure at very low precipitated amounts (∼1 wt %),3 thus imparting high pour point, high viscosity, high yield stress, and non-Newtonian flow behavior to waxy crude oils. For pipe transport of waxy crude oil and gas condensate fluids, precipitation of paraffin wax from the continuous oil phase not only increases viscosity and associated pressure losses but also endangers restart of shut-in pipelines.4 Effective management of paraffin wax precipitation, gelation, and deposition is a large challenge for flow assurance, risk abatement, integrity management, and emergency intervention planning. Standard industrial practice for improving flowability is to dose the waxy crude oil with an oil-soluble pour point depressant (PPD). PPDs are highly interfacially active polymers, which serve to modulate crystal morphology as well as intercrystal interactions. Various types of polymeric additives have been developed and used as PPDs.5 The most widely used types are linear copolymerized ethylene, such as © 2015 American Chemical Society

poly(ethylene−vinyl acetate) (EVA), and comb polymers, such as polyacrylate. Effects of alkyl side chain length,6,7 polar group type8,9 and content,10,11 molecular weight,12,13 and oil-phase composition14,15 have been widely studied. PPD activity mechanisms include crystal habit alteration and entropic repulsion.5 Interactions between polymeric PPDs and asphaltene molecules also influence the efficiency of PPDs.16 With recent progress in nanotechnology, many nanomaterials have been prepared and applied in industry. Research and development efforts17−20 have resulted in new polymer/ inorganic nanocomposites or nanohybrids. With introduction of inorganic nanoparticles into polymer matrixes, resultant properties (ie. mechanical, thermal, magnetic, and electrical) are modulated. In the field of the crude oil industry, nanomaterials have been used as lubricants,21 catalysts,22 oil recovery agents,23 and paving asphalt improvers.24 Recently, Wang et al.25 developed nanohybrid PPDs and compared performance efficiencies between nanohybrid PPDs and traditional EVA PPDs. Nanohybrid PPDs exhibit superior pour point depression than EVAs. Microscopic images of precipitated wax crystals demonstrate that nanohybrid PPDs Received: October 23, 2014 Revised: February 23, 2015 Published: March 2, 2015 1368

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Energy & Fuels result in smaller and more dispersed wax crystals, improving the rheology of waxy crude oil, confirming a synergistic effect of hybrid EVA nanoparticles. The previous art corroborates multiple PPD activity mechanisms. Possible mechanisms for these non-hybrid PPDs include nucleation (functioning as additional dispersed nucleation sites, resulting in the formation of a larger number of smaller crystals), adsorption (adsorbing on the surface of wax crystals, hindering crystal growth), and co-crystallization (incorporating into the forming wax crystals, thereby hindering or changing the crystal growth). In the current work, nanohybrid particles composed of nanosilica and poly(octadecyl acrylate) (POA) are prepared via a solvent-blending protocol. Effects of POA and hybrid particles on the gelation point and yield stress of a model fluid system are investigated. Differential scanning calorimetry (DSC) measurements and microscopic observation of the treated and untreated systems are performed. Hybrid systems are shown to retain a robust ability to impart fluid flowability at low dosages (e.g., 100 ppm). The hybrid particles appear to modulate the crystal morphology, resulting in more spherical and compact shapes, serving to reduce the gelation point and rheological yield stress.

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Figure 1. Fourier transform infrared (FTIR) curves of octadecyl acrylate and POA.

EXPERIMENTAL SECTION

Chemical agents used in this work were purchased from Sigma-Aldrich Co. The mass fraction purity of dodecane, toluene, ethanol, octadecyl acrylate, and 2,2′-azobis(2-methylpropionitrile) (AIBN) were all ≥99 wt %. Macrocrystalline paraffin wax used (Sasolwax 5405, primarily composed of linear paraffin components) was supplied by Sasol Wax Co. Nanosilica used was hydrophilic fumed silica with a specific surface area of 200 m2/g (Aerosil 200, Evonik Industries Co.). Model waxy oil was used for the experiments, prepared by dissolving 10 wt % Sasolwax 5405 in dodecane. The model fluid WAT (38.7 °C) is not close to the gelation point (24.7 °C), and the model fluid also retains the essential compositional characteristics of waxy petroleum fluids and has successfully been used in previous work on waxy petroleum.26−29 This model system was used to obtain more insight into the effect of the nanohybrid particles on the waxy oil without having to try to include the effect of native surface-active species of crude oil (such as asphaltenes and resins). The carbon number distribution of the wax used, as obtained from the high-temperature gas chromatography (HTGC), is given in ref 29. The carbon number distribution of linear alkane components ranges from n-C20 to at least n-C44, with a maximum at n-C26. A long “tail” in the distribution exists from n-C34 to n-C44, imparting large disparities in “apparent WAT” values obtained by isothermal versus non-isothermal techniques. The carbon number distribution of branched and cyclic components ranges from i-C20/cC20 to i-C41/c-C41, as categorized on a boiling point fractionation basis. POA was synthesized by solvent free-radical polymerization of octadecyl acrylate under a nitrogen atmosphere with constant stirring. The polymerization was conducted in toluene solution (30 wt %) at 70 °C for 8 h using AIBN as an initiator. The polymer was precipitated in excess ethanol, filtered, and dried under vacuum at 60 °C, with the final state of POA being a white solid. Figure 1 shows infrared (IR) curves of octadecyl acrylate and POA used in this work. For octadecyl acrylate, CC absorption peaks appear at 810, 900, and 1630. For POA, however, CC absorption peaks disappear, confirming successful POA synthesis. The mean molecular weight of POA as assessed by gel permeation chromatography (GPC) was ∼18 000 Da. POA/nanosilica hybrid particles were prepared using a solventblending protocol. A specified amount of POA was first dissolved in toluene; subsequently, an identical amount of nanosilica was dispersed in the POA/toluene solution though ultrasonic treatment and vigorous stirring. The solution was carefully evaporated at 70 °C under continuous stirring to remove toluene. Finally, dried POA/nanosilica hybrid material was ground into small particles. The POA/nanosilica mass ratio was maintained at 1:1. As shown in Figure 2a, the original

Figure 2. Dispersion state of 0.1 wt % nanosilica and 0.1 wt % hybrid particles in dodecane after 1 h: (a) nanosilica dispersion and (b) hybrid particle dispersion and (c) microscopic image of hybrid particles in dodecane. nanosilica particles do not disperse in dodecane and sediment quickly to the bottom of the vial. No evident sedimentation is observed after 1 h for the 0.1 wt % hybrid particle-in-dodecane solution (Figure 2b). The microscopic image (Figure 2c) shows that hybrid particles dispersed in dodecane retain a spherical-like morphology and have a particle size of ∼10 μm. Rheology of the 10 wt % model waxy oil system was assessed using a Physica 301 rheometer (Anton Paar Co., Germany) equipped with a 1369

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Energy & Fuels 2° cone-and-plate geometry with a diameter of 4 cm. The sample temperature is Peltier-controlled, and a plastic cover is placed over the cell to minimize evaporation. The gelation point was assessed as follows: the oil sample was first maintained at 60 °C for 5 min and cooled quiescently to 40 °C at a rate of 20 °C/min. Subsequently, the oil sample was cooled at 0.5 °C/ min and sheared under an imposed shear stress of 0.01 Pa. The variation in shear rate with the temperature was monitored; the gelation point is ascribed to a break in the shearing curve. Yield stress was assessed as follows: the oil sample was first maintained at 60 °C for 5 min and cooled quiescently to 40 °C at a rate of 20 °C/min. Subsequently, the oil sample was quiescently cooled at a rate of 0.5 °C/min to 18 °C; the temperature was maintained at 18 °C for 10 min under quiescent conditions. Finally, flow curves were measured with a shearing protocol ranging from 0.001 to 10 s−1. The yield stress was obtained from the flow curves. The reproducibility of the different rheology experiments was in general very good (data not shown). Exothermic characteristics of the model fluid system, in untreated and treated states, were investigated using a Q2000 differential scanning calorimeter (TA Instruments). The temperature scanning range was between −10 and 60 °C; a fixed cooling rate of 5 °C/min was employed during the crystallization process. The DSC curves provide a measure of the crystallization onset temperature, which is evidenced by a sharp increase in the exothermic heat flux signal. Wax crystal morphology was investigated using a light microscope TE 2000-S (Nikon Eclipse, Japan) in phase contrast and crosspolarized mode. Treated and untreated model fluid samples were initially heated to 60 °C for 5 min, and then one droplet was transferred to a glass slide covered by a coverslip. The samples were viewed with the microscope at a cooling rate of 5 °C/min in the temperature range from 60 to 40 °C and then cooled a rate of 0.5 °C/ min in the temperature range from 40 to 10 °C. The oil samples were maintained at 10 °C for 5 min, and then images of the wax crystal structure were recorded manually.

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RESULTS AND DISCUSSION The effect of POA on the gelation point is illustrated in Figure 3a. The shear rate initially decreases upon temperature reduction in accordance with an Arrhenius relation. At the wax precipitation onset (see Table 1), the shear rate deviates from the Arrhenius trend. A sharp break in the curve is observed at the gelation point. Below the gelation point, creeping flow continues. The gelation point of the untreated model fluid system is 24.7 °C. The gelation point decreases gradually with an increasing POA concentration. The gelation point is 23.2 °C at 50 ppm, 22.6 °C at 100 ppm, 22.1 °C at 200 ppm, and 21.2 °C at 500 ppm. POA molecules are present at the wax crystal interface, altering the crystallization habit and affording entropic repulsion between the wax crystals. As observed in Figure 3b, the hybrid particle system retains a stronger ability to decrease the gelation point of the representative model fluid system at 100 ppm. The gelation point is 21.7 °C at 50 ppm, 18.7 °C at 100 ppm, 19.5 °C at 200 ppm, and 21.0 °C at 500 ppm. A minimum value of the gelation point is achieved at 100 ppm of hybrid particles. A further increase in the hybrid particle concentration causes the gelation point to increase. At concentrations of hybrid nanosilica particles higher than this optimum, the additional particles added to the solution work as additional suspended particles, thus strengthening the gel structure (in accordance with the Einstein equation). At low concentrations of nanohybrid particles, the dispersed particles modulate the wax crystallization in such a way that the gelation point is lowered. At the same time, adding solid particles to the suspension adds additional particles to potentially form a network. At some

Figure 3. Effect of (a) POA and (b) hybrid particles on the gelation point of the model waxy oil.

point, these two opposing forces come to a minimum, such that, at and below this minimum, the change in wax crystallization has a greater effect on the gelation point than the additional volume of added particles, but above this optimal concentration, the additional volume of solid particles instead counteracts this effect and the gelation point rises again. A possible complication could be if the attached POA detaches and solubilizes in the bulk phase, leaving behind naked silica particles. These naked or partially naked silica particles could then become incorporated within the growing paraffin wax crystal network, further adding to the effect described above. Figure 4 illustrates the yield stress of the treated and untreated model fluid systems. In the untreated state, the yield stress is ∼100 Pa at a temperature of 18 °C. When 100 ppm of POA is added, the yield stress is reduced to ∼1 Pa. The traditional additive affords 2 orders of magnitude reduction in gel strength. In comparison, after the addition of 100 ppm of the hybrid particle system, the yield stress is reduced to 0.2 Pa. The nanohybrid system provides nearly 3 orders of magnitude reduction in gel strength. It is evident that the addition of POA and hybrid particles greatly decreases the strength of the wax− oil gel. The hybrid particles maintain a robust ability to reduce the yield stress. DSC curves obtained with the model waxy fluid system are shown in Figure 5. Upon temperature reduction, the heat flow 1370

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Table 1. Wax Precipitation Onset (WPO, by DSC at 5 °C/min) and Gelation Point of 10 wt % Model Waxy Oil at Various POA/Hybrid Particle Concentrationsa concentration (ppm) WPO gelation point a

0 POA hybrids POA hybrids

25.6 25.6 24.7 24.7

50 °C °C °C °C

24.1 24.5 23.2 21.7

°C °C °C °C

100 23.6 24.0 22.6 18.7

°C °C °C °C

200 23.0 23.6 22.1 19.5

°C °C °C °C

500 22.1 23.1 21.2 21.0

°C °C °C °C

For reference, the system WAT is 38.7 °C.

reduction is induced solely by POA; additional silica particles do not reduce the precipitation onset point. The WAT of Salsol wax 5405 in dodecane was further investigated by visual observation. This was performed by placing a sealed sample vial with known wax content at a temperature at least 25 °C above the WAT. This sample was then placed in a water bath (with internal circulation) at a lower temperature and maintained at isothermal conditions for 24 h. After this time, the sample was compared to a clear sample (dodecane) to see if any wax had precipitated. This was repeated (with a precision of 0.1 °C) until the temperature at which the sample became turbid was narrowed down and determined within 0.1 °C, establishing the WAT. The results are shown in Figure 6. The WAT by this method of 10 wt % Figure 4. Effect of POA and hybrid particles on the yield stress of the model fluid at 18 °C.

Figure 6. Solubility of Sasol wax 5405 in dodecane as a function of the temperature, as determined by visual observation, establishing the WAT. The symbol “x” denotes the mole fraction of paraffin wax in the system. Figure 5. DSC curves of 10 wt % model waxy oil undoped/doped with POA and hybrid particles.

Sasol 5405 wax in dodecane is 38.7 °C and is governed by the high-molecular-weight “tail” of paraffins in wax composition. The difference between the high turbidity observation and the lower WAT determined by DSC illustrates the strong impact of nucleation kinetics for this system and can be explained by the high cooling rate of the DSC experiment in conjunction with the high-molecular-weight tail of n-alkanes in the wax composition, which are subject to highly nonlinear nucleation kinetics. Figure 7 shows microscopic images of the untreated and treated model waxy oil system at a temperature of 10 °C. Because small wax crystals (see panels a1 and a2 of Figure 7) in the pristine waxy oil are not easily observed under polarized light, the image is presented in phase contrast mode. Small crystals exhibit large relative surface areas and disc-like morphologies, which readily form network structures, resulting in a physical colloidal gel. When a small amount of POA (100 ppm; see Figure 7b) is added to the waxy model oil, the crystal

signal decreases initially and then increases sharply with wax precipitation. The temperature at which the heat flow signal deviates from the liquid state heat capacity trend establishes the WAT. The observed wax precipitation onset of the untreated system is 25.6 °C. When 100 ppm of POA is added to the model fluid system, the observed wax precipitation onset of the waxy oil decreases to 23.6 °C, corroborating a small solubility enhancement with the presence of POA. When 100 ppm of hybrid particles are added to the model fluid system, the wax precipitation onset is 24.0 °C. Hence, the hybrid particles do not provide the same solubilization efficacy as pure POA. Moreover, the precipitation onset at 50 ppm of POA is nearly the same as the onset at 100 ppm of hybrid particles (composed by half POA and half silica particles). A similar comparison may be drawn at 100 ppm of POA and 200 ppm of hybrid particles, indicating that the wax precipitation point 1371

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Figure 7. Microscopic images of 10 wt % model waxy oil undoped/doped with POA and hybrid particles at 10 °C. Without polarized light: (a1 and a2) non-added. With polarized light: (b) 100 ppm of POA, (C) 500 ppm of POA, and (d) 100 ppm of hybrid particles.

in the continuous oil phase, reducing the WAT. In addition, POA effectively modulates the crystal morphology to a more regular spherical-like shape. The data are consistent with overall percolation theory predictions of lower gelation temperatures for lower aspect ratio morphologies. Assuming free crystal rotation, the theoretical percolation threshold φg for ellipsoidal discs is φg = 0.295/α, where α denotes the aspect ratio. Hence, reduced crystal aspect ratios affected by added chemicals directly increase the solid fraction necessary for formation of a volume-spanning solid-phase network. Hybrid particles retain a more robust ability to modulate waxy oil rheology at 100 ppm, as compared to traditional PPD functionality. According to the DSC results, hybrid particles induces the same degree of solubilization as the inherent polymer content within the nanohybrid. However, the primary function of hybrid particle PPDs is to provide large amounts of nucleation sites with interfaces that provide regular (spherical-like) templates upon which wax molecules can precipitate. These precipitated wax crystals have a more regular shape (spherical-like) and a compact structure, which hinders the percolation process necessary to form a volume-spanning network of wax crystals,

size increases and the crystal morphology likely becomes more compact. The gelation point is reduced to 22.6 °C, and the yield stress is reduced to 1 Pa. As the POA concentration increases from 100 to 500 ppm, the morphology of precipitated wax crystals becomes even more spherical-like and the size of the particles increases to ∼20 μm in diameter (see Figure 7c). The more regular morphology (spherical-like) interferes with the gelation process, thus further decreasing the gelation point (21.2 °C) of the waxy oil. Upon addition of 100 ppm of hybrid nanosilica particles, the wax crystals have a size of ∼20 μm and a regular shape (spherical-like) with a compact structure (see Figure 7d). The regular morphology serves to reduce the volume fraction of occluded oil and increase the precipitated solid fraction required for percolation conditions, thereby suppressing the overall gelation process. The suppressed gelation process in the presence of hybrid nanoparticles is corroborated by a measured gelation temperature of 18.7 °C. POA suppresses the gelation process and improves the flowability of the model waxy oil by several mechanisms. DSC data confirms that POA molecules enhance the solubility of wax 1372

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basis for next-generation environmentally friendly wax inhibitor additive agents. Further investigation into how these nanohybrid systems behave under realistic production conditions and, more importantly, with real waxy crude oils together with all of the different components in addition to the waxes is an important step forward, and such investigations are currently underway.

as shown in Figure 8. The presence of a hybrid nanoparticle additive results in free wax crystals of a modified morphology,

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Figure 8. Schematic of the morphological modulation mechanism.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

providing a mechanistic impact of morphological modulation on PPD performance.

The authors declare no competing financial interest.

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CONCLUSION POA and corresponding nanosilica hybrid particles are prepared and demonstrated as effective PPD additive agents, with potential for use in industry. Activity performance is demonstrated using model waxy oil consisting of 10 wt % macrocrystalline wax dissolved in dodecane, which emulates the essential characteristics of waxy petroleum fluids. The hybrid particles affect a reduction in gelation point as well as yield stress of the model wax−oil gels. An optimal performance is obtained at a dosage of 100 ppm for the nanohybrid system. Beyond this optimal dosage, additional particles result in increased gel strength, analogous to the directionality of the Einstein equation for viscosity. DSC solubility studies indicate that solely the polymer content of the hybrid system governs the effective increase in wax solubility. Hence, a majority of the polymer likely becomes solubilized in the continuous oil phase upon dispersion. The comb-like POA appears to favor the formation of island defects on the wax surface, which have weak interactions with the surrounding crystal and, therefore, act as impurity sites for blocking growth steps.30,31 The silica particles act as anchoring points, with many POA molecules bound to the particle surface, creating a localized higher concentration of POA. Therefore, the wax molecules co-crystallize with POA on the particle, leading to the formation of large and compact wax crystals. The compact structure and large size of wax crystals are adverse to the formation of network structures, thus modulating the rheology of the waxy oil further. The fact that there is an optimum dosage and that particle concentrations above 100 ppm do not increase the effect can possibly be explained by the fact that not all silica particles are fully covered in POA (dark patches in Figure 7d are silica particles). These silica particles would then act as additional suspended particles and increase the viscosity in accordance with the Einstein equation. This could possibly be avoided in further work by increasing the binding of POA to the underlying substrate, by either pretreating the silica or chemical grafting of POA to the silica, thus avoiding the neat silica particles in the solution. The more relevant activity mechanism of the nanohybrid PPDs is the impact upon crystal morphology. The hybrid particles provide heterogeneous nucleation sites for paraffin wax crystallization, causing the formed wax crystals thus attained to have a modified morphology characterized by compact, spherical-like structures, which hinder and suppress the gelation process. The low dosage of these systems is promising and indicates that they are a good candidate as the

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (51204202) and the Natural Science Foundation of Shandong Province of China (ZR2012EEQ002).

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