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Thermal tolerance and Compatibility of NaOH-Poly(vinyl alcohol) in Bitumen Emulsification for Improved Flow properties Olalekan Saheed Alade, Kyuro Sasaki, Adeniyi Sunday Ogunlaja, Yuichi Sugai, Bayonile Ademodi, Junpei Kumasaka, Masanori Nakano, and Ryo Ueda Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02060 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016
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Energy & Fuels
Thermal tolerance and Compatibility of NaOHPoly(vinyl alcohol) in Bitumen Emulsification for Improved Flow properties Olalekan S. Alade*1,2, Kyuro Sasaki1, Adeniyi S. Ogunlaja3, Yuichi Sugai1, Bayonile Ademodi2, Junpei Kumasaka1, Masanori Nakano4, Ryo Ueda4 1
Resources Production and Safety Engineering Laboratory, Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan 2 Petroleum and Petrochemical Engineering Laboratory, Department of Chemical Engineering, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria 3Department of Chemistry, Nelson Mandela Metropolitan University, Port-Elizabeth, South Africa 4 Research Center, Japan Petroleum Exploration, CO., Ltd., Chiba, Japan.
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ABSTRACT: Poly(vinyl alcohol) (PVA) is often used to reduce the viscosity of bitumen.
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Although some studies have investigated the effects of the emulsification conditions, including
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aqueous phase salinity, on the performance of PVA, the tolerance to thermal degradation and
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compatibility with other additives, including alkaline solutions, have not been studied. Therefore,
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to increase the applicability of this technique, batch emulsification experiments were performed
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to investigate the thermal degradation tolerance and compatibility with ethanol-NaOH solution.
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Bitumen was emulsified using the heated samples of PVA. Different bitumen samples were also
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emulsified using the mixture of PVA and NaOH in the presence of ethanol. Evaluation of the
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particle sizes of the emulsions confirmed that PVA could be reliably applied between ambient
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temperature and 200 °C. In addition, evaluation of the apparent viscosity and stability of each
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bitumen emulsion produced showed that the mixture of PVA and ethanol-NaOH had a
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synergistic effect on formation of stable oil-in-water emulsions with increased reduction in the
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viscosity. With the decrease in viscosity, a large decrease in the power required for pumping of
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the fluid was observed in flow simulation.
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1. INTRODUCTION
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Increasing demands for light crude oil have triggered concerns about a future energy
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crisis, and this has led to development of alternative fuel sources such as heavy oil, extra-heavy
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oil, and bitumen. However, heavy oil has high viscosity, which leads to poor mobility and
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relatively low recovery efficiency; this represents a major hurdle to efficient production,
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transportation, and processing of heavy oil.1 In response to this challenge and expectations for
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low fuel prices, further efforts must be devoted to cost effective methods to reduce the viscosity
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of heavy oil and improve production outcomes.
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Recently, hydrophilic polymeric surfactants, such as poly(vinyl alcohol) (PVA), have
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been introduced to reduce the viscosity of heavy oil by dispersion or emulsification, which
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should facilitate production and transportation.2-6 PVA is a water-soluble polymer with the
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chemical formula CH2CH(OH)]n that contains carbon, hydrogen, oxygen and hydroxyl groups.7,8
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It has the ability to lower interfacial tension and function as a non-ionic surfactant, is tolerant to
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high concentrations of electrolytes (particularly acids), and is compatible with other chemicals
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such as wax crystal modifiers, pour point depressants, and corrosion inhibitors.3, 9–11 A recently
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reported study on computationally modelled PVA demonstrated excellent wettability, an
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important surface active property (due to the –OH functionalities on PVA), hence allowing
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neighboring molecules to interact.12 The colour-mapped electrostatic potential for PVA (Figure
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1), based on its electron density distribution confirmed the red end of the spectrum as possessing
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the highest stability for a positive test charge (more favourable to interactions). PVA also
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possesses a high HOMO-LUMO energy gaps which are indicative of hard molecules.12
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In addition to the salinity, the reservoir temperature is a crucial factor in determining the
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success of chemicals such as PVA in enhanced oil recovery (EOR). However, polymeric
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materials generally undergo thermal degradation when heated, and this could affect the
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successful application of PVA in EOR processes, such as surfactant flooding or surfactant-
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assisted hot water extraction, and other aspects of the bitumen emulsification process that are
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conducted at high temperature. Earlier studies have confirmed that PVA is tolerant to salinity in
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heavy oil emulsification,2,
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solution on its performance in bitumen emulsification.
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but there have been no reports on the effect of preheating of the
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To improve mobility in EOR and pipeline transportation, reduction of the viscosity of
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heavy oil by emulsification is an important process. In earlier studies, mixed surfactant systems
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have shown greater efficiency than single surfactants for reducing viscosity.13-16 The use of an
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aqueous solution of PVA to form an oil-in-water emulsion to reduce the viscosity of heavy oil
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has been reported previously.2-4 However, this process is reportedly difficult to apply to some
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types of extra-heavy oil (viscosity = 200, 000 mPa s to 1.4 million mPa s at 25 °C).4
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The aim of the present study was to investigate the thermal tolerance of PVA solutions
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with regards to preheating of the solution, and the effects on emulsification. The results could be
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used to evaluate potential applications of PVA in EOR processes conducted at high temperature,
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such as surfactant-assisted hot water extraction. In addition, the potential synergistic effects of
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PVA solution in combination with ethanol-NaOH solution were investigated to overcome
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difficulties encountered in the emulsification of highly viscous extra-heavy oil.
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2. EXPERIMENTAL SECTION
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2.1 Materials
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Bitumen samples were collected from an uncapped oil well located at Ondo State, Nigeria (oil N)
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and an oil field in Alberta, Canada (oil C). Partially hydrolyzed PVA solutions (1.0% mass
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fraction in water; PVA 205 and PVA 235) were obtained from Kuraray Co., Ltd. (Kurashiki,
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Japan). These surfactant solutions were diluted with water to prepare stock solutions (0.5% mass
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fraction) for use in the experiments. Analytical grade NaOH and 99.5% ethanol were purchased
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from Junsei Chemical Co., Ltd. (Tokyo, Japan). The properties of the bitumen and PVA samples
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are detailed in Table 1.
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2.2 Methodology
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2.2.1 Thermal stability of PVA samples
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The thermal resistance of each polymeric surfactant (PVA 205 and PVA 235) was tested
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by heating the sample in an oven (OFW-300B, Ettas, AS-One, Japan) at different temperatures
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(Tt = 50 to 270 °C) under nitrogen (1 MPa) for 1 h (Figure 2). Then, the samples were cooled by
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placing in the open space at normal room temperature before used in emulsification of bitumen
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(oil C). Batch emulsification was performed in a microreactor with a volume of 50 mL under
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controlled high temperature and pressure conditions (MMJ-50, OM Labtech, Tochigi, Japan)
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with an anchor impeller for agitation (Figure 3). In a batch emulsification process, 25 mL of the
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sample with a bitumen:water ratio of 1:1 (CB = 50% and CW = 50%) was mixed at a formation
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temperature (Tf) of 40 °C and an agitation rate (Ar) of 500 rpm for a mixing time (tm) of 10 min.
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The emulsification experiments were numbered according to the temperature the sample was
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heated at (i.e. E50–E270). A reference experiment (Eref) was performed using a PVA sample that
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was not heated.
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To investigate the degradation of PVA because of heating, the optical properties (color
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intensities) of the heated samples were tested using an UV-Vis spectrophotometer (UV-2450,
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Shimadzu, Kyoto, Japan) at 250 nm. The unheated PVA sample was used as a reference.
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2.2.2 Comparative experiment and compatibility with NaOH solution.
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Batch experiments were performed to compare the performance of solutions of PVA and other
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additives for emulsification of the bitumen samples (oil C and N). The emulsification
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experiments were conducted in the microreactor as described in Section 2.2.1, with a reaction
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volume (Vr) of 25 mL, a bitumen mass fraction (CB) and a water mass fraction (Cw) of 50%
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(Table 2). The performance of each solution was evaluated by measuring the particle size, kinetic
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stability, and apparent viscosity of the emulsion as described in Sections 2.2.3–2.2.5.
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2.2.3 Particle size analysis
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The emulsion particle size was measured using optical/video microscopy with a light microscope
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(objective lenses 40/0.65 and 160/0.17) interfaced with a computer. Emulsion samples were
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collected immediately after formation in the microreactor. For each sample, a drop was carefully
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put on a microscope slide (76 mm × 26 mm, 0.3 mm–1.0 mm; Matsunami Glass, Osaka, Japan)
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and a cover slip (22 mm × 22 mm, 0.12 mm– 0.17 mm thick; Matsunami Glass, Osaka, Japan)
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was placed over it immediately. Then, video and still images of the emulsion particles were
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recorded. The particle sizes were determined using ImageJTM software following the image
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enhancement procedure recommended by Moradi et al.17 The average diameters of the particles
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in the emulsions were estimated using the mean volume diameter (dv), which is given by Eq. 1:
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dv = ∑( )
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where ni is number of the droplets counted as ith diameter (di) of droplet (µm).
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The particle size distribution was expressed as the log-normal probability density function f(x)
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(Eq. 2):
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( ) =
∑( )
,
√
1
(())
2
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where x is the variable, σ is the shape parameter (and is the standard deviation of the log of the
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distribution), is the mean of the log of the distribution.
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Moreover, it should be noted that covering the drop of emulsion with a cover slip as described
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above leads to particle diameters that are probably larger than actual ones. Therefore, the
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calculated average diameters of emulsion particles represents relative but not absolute values.
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2.2.4 Evaluation of kinetic stability by the jar test
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The stability of each emulsion to settling was investigated using the jar test. About 8 mL
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of a fresh emulsion was transferred from the microreactor into a graduated 10 mL Pyrex glass
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test tube (with cap), and immediately covered to exclude air. Phase separation was observed by
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the naked eye in real time with the aid of a light source (LA-150TX, Hayashi, Tokyo, Japan)
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when necessary. The kinetic stability was determined as the change in the position of the phase
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boundary (volume of water separated) with time as the bitumen particles settled at a storage
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temperature (Ts) of 25 °C. The percentage stability (W) of each emulsion was estimated using the
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following equation (Eq. 3):
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=1−
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where Wf and Wi are volume fraction of water separated from the emulsion as free water and the
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initial volume fraction of water in the emulsion, respectively.
3
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During storage, settling of the bitumen particles occurred, and the turbidity of the upper
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layer of the emulsion (aqueous phase) decreased. Thus, particle loss, which corresponded to
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separation of the emulsion, was monitored by taking samples from a constant height in the upper
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layer (80% of the settled emulsion height). The particle size was analyzed as described in Section
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2.2.3.
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2.2.5 Viscosity test
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The viscosities of the original bitumen samples were measured at atmospheric pressure and a
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temperature between 85 °C and 150 °C using a Viscopro2000 Cambridge viscometer (Model
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SPL372, PAC, Houston, TX). The rheological properties of the emulsions were studied at
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emulsification temperatures (Te) between 40 and 100 °C with a shear rate of 0.14-132 s-1 using a
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viscometer (Brookfield DV-I) equipped with a programmable temperature controller (Model
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106, Brookfield AMETEK, Inc., Middleboro, MA). Spindle number SC4-18 (shear rate: 0.66-
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132 s-1) or SC4-34 (shear rate: 0.14 – 28 s-1) was used depending on the viscosity. For analysis,
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either 6.8 mL or 9.4 mL of a fresh emulsion sample was transferred from the microreactor into
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the sample chamber in the thermocontainer. The samples were left to equilibrate to the desired
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Te. The equilibration time was typically between 1 and 3 min. During this time, the emulsion was
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continuously sheared at the lowest rate of 0.14 s–1 (or 0.66 s-1) to prevent separation. The
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viscometer was regularly calibrated with the appropriate standard fluids before use. The
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percentage error was typically in the range ±0.2–1.4%. The rheological properties of the fluids
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were evaluated as described elsewhere.2
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2.2.6 Calculation of the power required for pumping
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Oil-in-water emulsions were formed with the two bitumen samples (C and N) under the
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conditions described in Sections 2.2.1 and 2.2.2, by dispersing the bitumen (CB = 50 and 70%) in
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aqueous solutions containing PVA235, ethanol-NaOH, and a mixture of PVA235 and ethanol-
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NaOH (CW = 50 and 30%). The power required for pumping (Qt) was calculated over a range of
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flow temperatures (Tfw) to compare the contributions of the additives to flow to the effect of
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changing the temperature. Emulsion composition and experimental specification are detailed in
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Table 3.
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For this purpose, the power required for pumping 5000 barrels per day of the original
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bitumen samples and the emulsions were simulated at different flow temperatures through a
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stainless steel pipe (ANSI grade) over 16 km using a rotary pump with mechanical efficiency of
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0.85. A steady state incompressible and isothermal flow of the fluid was considered. It was
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assumed that the emulsions were stable and flowed in a single phase.
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The Reynolds number (Re) is calculated for a Newtonian fluid in a smooth pipe using Eq.
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4. The Hagen–Poiseuille equation (Eq. 5) relates the pressure drop ∆P to the friction factor f.
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This can be determined from Eq. 6 for laminar flow (Re ≤ 2100). The friction factor can be
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calculated for both smooth and rough pipes with turbulent flow (Re > 4000) using the Churchill
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equation (Eq. 7).
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=
!"#
4
177
=
∆&#
5
178
=
$
!" '
(
6
)*
= −4./0 1
2.45
4 2.9
+ 7) 8 :
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+,
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where v is the flow velocity, µ is the dynamic viscosity, ρ is the fluid density, D is the pipe
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diameter and L is the pipe length.
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For non-Newtonian fluids, which obey the power law, the flow rate (;< ), Reynolds number, and
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the frictional factor with turbulent flow are given by Eqs. 8, 9, and 10, respectively.
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∆& @
;< = = 7>'8 7AB 8
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=
#
?
186
+,
=
7
*
7
?C@ 8 @
8
# @ " D@ !
9
>E@D?
+,F
+ 8.2 7 B8 + 1.77ln 7
B
8
10
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where n is the flow behaviour index, and k is the flow consistency index.
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The power required for pumping the fluid (Qw) is related to the total pressure drop and fluid flow
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rate (for the pump mechanical efficiency η) as shown in Eq. 11.
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;L =
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3. RESULTS AND DISCUSSION
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3.1 Thermal tolerance inspection
∆&M
