The Effect of Waxes Addition on Rheological

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Mar 19, 2012 - Microcrystalline or amorphous waxes have a high amount of isoparaffins and naphthenes within the range C30–C60 (Coto et al., 2008).

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The Effect of Waxes Addition on Rheological Properties of O/W Concentrated Model Emulsions a

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M. Migliori , D. Gabriele , F. R. Lupi & B. De Cindio

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Department of Engineering Modelling, University of Calabria, Rende, Italy Available online: 19 Mar 2012

To cite this article: M. Migliori, D. Gabriele, F. R. Lupi & B. De Cindio (2012): The Effect of Waxes Addition on Rheological Properties of O/W Concentrated Model Emulsions, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 34:9, 851-857 To link to this article: http://dx.doi.org/10.1080/15567031003627963

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Energy Sources, Part A, 34:851–857, 2012 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567031003627963

The Effect of Waxes Addition on Rheological Properties of O/W Concentrated Model Emulsions M. MIGLIORI,1 D. GABRIELE,1 F. R. LUPI,1 and B. DE CINDIO1

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Department of Engineering Modelling, University of Calabria, Rende, Italy Abstract As heavy oil contains different impurities that increase the viscosity, pipeline transportation becomes economically disadvantageous. Oil-in-water emulsification, adopted as a cost reduction transportation technique, may take advantage of the presence of natural components of heavy crudes, asphaltenes, and resins as emulsifiers and waxes as stabilizers, decreasing the viscosity with respect to the pure-oil one. This article aims at investigating the effect of waxes addition on the viscosity of model oil-in-water emulsions produced in bench-scale, using two different emulsification techniques. Results showed that paraffin accelerates the emulsification completion during an in-flow test and also emulsion viscosity was lowered by waxes addition. Keywords O/W emulsions, rheology, waxes, viscosity

1. Introduction During upstream of conventional heavy oils, emulsification occurs when crude oil is mixed with water already present or injected in the reservoir, at well bore, in pipelines, and at surface facilities. If the dispersed phase of the emulsion is water, the system viscosity increases with respect to the oil one and this may cause high pressure drops throughout the pipeline (Al-Sahhaf et al., 2009). On the contrary, emulsification of the oily phase in water is a reliable method to decrease the fluid viscosity for crude oil transportation in pipelines (Saniere et al., 2004). Concerning the crude oil composition, there are many components naturally present in the raw crude oil that can act as emulsifiers or stabilizer materials in different cases. In fact, emulsion characteristics (stability formation mechanism, drop size distribution) can be significantly affected by the presence of asphaltenes, resins, waxes, and inorganic solids. Asphaltenes are present in crude oil mixtures in the form of colloidal dispersed particles, soluble in toluene and precipitate in the presence of aliphatic solvents (Hannisdal et al., 2007). During emulsion formation, asphaltenes interact with resins, forming an interfacial film surrounding droplets that are then protected against coalescence. Resins are polar and non volatile compounds; soluble in n-pentane, n-heptane, and aromatic solvents; and insoluble in methanol and propanol. Waxes are natural components of high viscous crude oil, and their deposition at the pipe wall, under certain conditions during crude transportation, can cause serious damage to conventional pipelines (Burger et al., Address correspondence to Dr. Massimo Migliori, Department of Engineering and Modelling, University of Calabria, Via P. Bucci Cubo 39c, Rende 87036, Italy. E-mail: [email protected]

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1981). This class of compounds is solid at room temperature and it can be divided into microcrystalline or macrocrystalline (Thanh et al., 1999). Macrocrystalline waxes are mainly n-alkanes with chain length within the range C20–C50. Microcrystalline or amorphous waxes have a high amount of isoparaffins and naphthenes within the range C30–C60 (Coto et al., 2008). The pentane-soluble waxes contain predominantly n-alkanes (C40) and are extremely poor soluble. This fact is mainly responsible for the deposition phenomena. In the case of emulsions formation, waxes can co-adsorb at the oil/water interface improving the film stability. At low temperature, wax crystals (>1% w/w) may promote droplets coalescence as they break the interface protruding into the disperse phase drop. On the contrary, at higher temperatures, it was observed that the droplet size decreases with wax concentration. In fact, Li et al. (2009) showed that waxes are liquid-like during the high temperature emulsification process, but during the cooling phase, wax crystals appear around the oil/water interface stabilizing the droplet. The aim of this work is to investigate the effect of n-paraffins (C40) addition into the oily phase onto rheological properties of a model oil-in-water (O/W) concentrated emulsion. Samples containing different amounts of waxes are produced both in batch and in flow by using a lab-scale loop plant and viscosity measurement are performed. The lab-scale plant allows the samples to be collected at different emulsification times and this allows the effect of waxes on the emulsion formation to be investigated.

2. Materials and Methods 2.1.

Materials

O/W emulsions were prepared using commercial products and tap water. Paraffin oil was purchased from Total (Finavestan A360B, Paris, France) and waxes were paraffin cut (Carlo Erba, Reagenti, Italy) ranging from C-20 to C-40. Emulsifier was hydrophilic nonionic Tween 60 (polyoxyethilenesorbitan monostereate, HLB 14.9). Volume ratio of oil and water was fixed at 1.5 and, keeping a constant emulsifier quantity (2%), waxes content was varied (0.5 and 1%). Both emulsifier and wax are weight percentage referred to the oil amount. Table 1 shows emulsions composition and sample ID as used in the rest of the article: Bx indicates batch prepared emulsions and Ex labels in-flow prepared ones.

Table 1 Samples composition and labels Sample ID

Paraffin oil, % v/v

Water, % v/v

Wax, % w/w

Emulsification method

B1 B2 B3 E1 E2 E3

60 60 60 60 60 60

40 40 40 40 40 40

0 0.5 1 0 0.5 1

Batch Batch Batch Flow Flow Flow

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In addition, in order to clarify the effect of waxes addition onto oil viscosity, wax-in-oil solutions were tested for a wax content of 0.5 and 1%. 2.2.

Batch Emulsification

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Both water and oil were separately heated up to 70ı C in two cylindrical beakers (volume 600 ml). The hydrophilic emulsifier was gently added to water and waxes to oil. The resulting solutions were kept under stirring for 5 min. Homogenization of the emulsion was then performed by using a commercial blender (Minipimer Braun MR 404 Plus, Kronberg, Germany) having a nominal power of 300 W for 10 min. During heating and stirring, water evaporation was prevented by placing a lid over the beaker. After emulsification completion, rheological characterization was performed after 3 h rest at room temperature (Samples B1–3). 2.3.

In-flow Emulsification

Flow emulsification on laboratory scale was performed at room temperature using the experimental home-made system described elsewhere (Gabriele et al., 2010). In order to follow the viscosity evolution during the flow process, sampling was performed over time assuming zero-reference time as the end of the start-up procedure (when fluids were fed separately as a pre-mixing step). 2.4.

Rheological Characterization

Flow curves were performed using a controlled strain rheometer ARES-RFS (TA Instrument, New Castle, DE) equipped with a Couette cylinder geometry (internal bob diameter 32 mm, gap 1 mm). Viscosity as a function of shear rate was measured at 25ı C, controlling the temperature by using a thermostatic bath (Julabo, Allentown, PA). The shear rate range was 1–100 s 1 and a preliminary “step shear rate” test at 1 s 1 was performed to evaluate the time needed to reach the steady state before measurement. For all samples, a time of 10 s was found to be sufficient to reach the time independent viscosity value and steady shear viscosity was averaged over 10 s sampling time. All tests were done in triplicate and flow curves were preceded by a pre-shear step (500 s 1 for 180 s) in order tp homogenize the sample before viscosity measurement.

3. Results and Discussion 3.1.

Oil and Wax Mixture

In Table 2, the viscosity of wax-in-oil mixtures at 25ı C are presented. It clearly appears that the addition of a small amount of wax does not affect the viscosity of the oily phase. This aspect should be taken into account when treating the emulsification process of waxy oil, because drop size distribution may be affected by changes in the disperse phase viscosity, in any way induced. 3.2.

Batch Produced Emulsions

Batch produced emulsions viscosity showed an apparent Newtonian behavior (Gabriele et al., 2010) and Table 3 reports viscosity values averaged throughout the investigated

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Table 2 Oil–wax mixtures viscosity at 25ı C Wax content, % w/w

Viscosity, mPas

0 0.5 1

155 ˙ 3 154 ˙ 3 154 ˙ 1

Table 3 Apparent Newtonian viscosity of batch produced emulsions Sample ID

Viscosity, mPas

B1 B2 B3

28.0 ˙ 0.5 34.6 ˙ 1.3 34.9 ˙ 1.3

shear rate range at 25ıC. A comparison with Table 2 shows that the emulsification process reduces the viscosity at about one-fifth of the waxy oil value. In addition, it is noteworthy that viscosity increases by adding 0.5% of wax, with respect to the pureoil/water emulsion, while no further effect is observed when the wax content is increased up to 1%. 3.3.

In-flow Produced Emulsions

Table 4 reports sampling time for all the produced emulsion and the Px ID is used throughout the text. It is important to clarify that the last sample (P4) is always collected after the emulsification process completion, i.e., when emulsion showed apparent Newtonian behavior and no further viscosity change was observed when increasing process time. Figures 1–3 show viscosity evolution over time for samples E1, E2, and E3, respectively. For reader convenience, in the same plot is also reported the value of the correspondent

Table 4 Sampling time in minutes during in-flow emulsification Sample ID

E1

E2

E3

P1 P2 P3 P4

0 11 65 151

0 15 54 134

0 16 57 132

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Figure 1. Flow curves for test E1 compared to paraffin oil and batch test.

batch prepared emulsion as well as the viscosity value of pure emulsion phases, water, and (waxy) oil. All the investigated samples showed a decrease of viscosity when increasing the residence time in the emulsification loop. As for the viscosity data at low emulsification time (P1 and P2), the sample without wax exhibits an apparent Newtonian behavior straightaway (Figure 1), indicating a fully developed emulsification process. On the contrary, samples prepared with waxy oil (Figures 2 and 3) exhibit a decreasing-increasing viscosity path in the early stage of the emulsification process. This phenomenon indicates a poor material stability during measurements due to flow-induced drop size distribution change of the disperse phase probably due to the destabilizing effect of wax crystals

Figure 2. Flow curves for test E2 compared to 0.5% waxy oil and batch test.

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Figure 3. Flow curves for test E3 compared to 1% waxy oil and batch test.

formed at room temperature (Li et al., 2009). On the contrary, when considering the time of emulsification completion (P4 data in Table 4), it clearly appears that the addition of waxes positively acts, as the time requested to achieve a shear- and time-independent viscosity value (assumed as indication of the emulsification process end) is significantly reduced. In fact, the addition of wax reduces by about 20 min the completion time (from 150 min of E1 sample) and this effect does not depend on the wax amount in the investigated range. Another significant effect on the wax addition is observed when evaluating the ratio  of the apparent Newtonian viscosity  of batch and long time in-flow process (P4). D

Ex jP 4 : Bx

(1)

The data of Table 5 shows that  decreases when increasing the wax content in the oily phase, indicating that the in-flow process ends with a final viscosity significantly lower than the batch step one. In fact, the viscosity reduction can be attributed to the increase of the emulsion polydispersity that is increased during in-flow process. If the emulsions are prepared at low temperature, as in the case of in-flow samples, at the added waxes concentrations (1 w/w%), few crystals are present in the oil phase that Table 5 Final viscosity ratio of batch and in-flow emulsification processes Wax content, % w/w

, [ ]

0 0.5 1

0.85 0.51 0.42

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cannot significantly reduce the oil droplets size acting as Pickering stabilizers, giving low viscous systems (Li et al., 2009). For emulsions produced at higher temperatures (the batch ones), wax melting and adsorption on oil/water interface is favored by the heat produced during the emulsification process. This can produce smaller droplets leading to more viscous emulsions. In addition, it is worthy to notice that the phenomenon becomes more relevant as the wax amount is increased.

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4. Conclusions In this article, the effect of waxes addition on the viscosity of concentrated O/W emulsion is presented. Model emulsions O/W were prepared using two different emulsification methods, batch and in-flow loop, and the effect of different amounts of waxes in the oily phase was investigated. Samples of O/W emulsion prepared in batch all exhibited a constant viscosity versus shear rate behavior, while during in-flow emulsification, timedependent behavior of viscosity was observed. This effect was found for the samples containing wax at low process time. Therefore, it was concluded that, in the early stage of the emulsification, waxes seem to reduce the system stability under shear. As for the emulsification completion, it was observed that the wax addition reduces the time requested to reach a constant apparent Newtonian behavior. This evidence did not depend on the amount, but only on the presence, of waxes. When comparing long-term viscosity of the two emulsification methods, it was found that addition of wax promoted the viscosity drop of the emulsion prepared using the in-flow technique, with respect to the batch one. This effect was more evident when increasing the wax amount and can be attributed to a variation of the emulsion polidispersity induced by the different thermal history of the two preparation methods.

References Al-Sahhaf, T. A., Fahim, M. A., and Elsharkawy, A. M. 2009. Effect of inorganic solids, wax to asphaltene ratio, and water cut on the stability of water-in-crude oil emulsions. J. Dispers. Sci. Tech. 30:597–604. Burger, E. D., Perkins, T. K., and Striegler, J. H. 1981. Studies of wax deposition in the trans Alaska pipeline. J. Petrol. Tech. 33:1075–1086. Coto, B., Martos, C., Peña, J. L., Espada, J. J., and Robustillo, M. D. 2008. A new method for the determination of wax precipitation from non-diluted crude oils by fractional precipitation. Fuel 87:2090–2094. Gabriele, D., Migliori, M., Lupi, F. R., and de Cindio, B. 2010. Rheological study of O/W concentrated model emulsions for heavy crude oil transportation. Energy Sources Part A 33:72–79. Hannisdal, A., Orr, R., and Sjöblom, J. 2007. Viscoelastic properties of crude oil components at oil-water interfaces. 1. The effect of dilution. J. Dispers. Sci. Tech. 28:81–93. Li, C., Liu, Q., Mei, Z., Wang, J., Xu, J., and Sun, D. 2009. Pickering emulsions stabilized by paraffin wax and laponite clay particles. J. Colloid Interface Sci. 336:314–321. Saniere, A., Hénaut, I., and Argillier, J. F. 2004. Pipeline transportation of heavy oils, a strategic, economic and technological challenge. Oil & Gas Sci. Tech. Rev. IFP 59:455–465. Thanh, N. X., Hsieh, M., and Philp, R. P. 1999. Waxes and asphaltenes in crude oils. Organic Geochem. 30:119–132.

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