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Fossil Fuels
Removing water from diesel fuel: Understanding the impact of droplet size on dynamic interfacial tension of water-in-fuel emulsions Shweta Narayan, Davis B. Moravec, Brad G. Hauser, Andrew J. Dallas, and Cari S. Dutcher Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00502 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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Energy & Fuels
Removing water from diesel fuel: Understanding the impact of droplet size on dynamic interfacial tension of water-in-fuel emulsions
Author Affiliations: Shweta Narayan1, Davis B. Moravec2, Brad G. Hauser2, Andrew J. Dallas2, Cari S. Dutcher1 1
Department of Mechanical Engineering, University of Minnesota – Twin Cities, 111 Church Street SE,
Minneapolis, MN 55455 2
Donaldson Company, 1400 W 94th Street, Bloomington, MN 55431
Corresponding Author: Cari S. Dutcher Office: ME 111, Department of Mechanical Engineering, University of Minnesota – Twin Cities, 111 Church Street SE, Minneapolis, MN 55455 Phone: 612-624-0428 Email:
[email protected] ORCID IDs: Shweta Narayan: 0000-0003-1829-365X Davis B. Moravec: 0000-001-6763-8779 Brad G. Hauser: 0000-0002-4875-7630 Andrew J. Dallas: 0000-0002-6836-7891 Cari S. Dutcher: 0000-0003-4325-9197
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Abstract Small amounts of water can enter diesel fuel during usage, causing major damage and failure of engine parts. Water is dispersed in fuel as droplets stabilized by the presence of surface-active compounds in the original fuel mixture as well as in fuel additives, including lubricity improvers and deposit control agents. Additives partition to the fuel-water interface and lower the interfacial tension (IFT), decreasing the ability to coalesce and separate water from fuel. The ability to capture and coalesce emulsion droplets by standard coalescing filters depends on dynamic IFT, conventionally measured for large millimeter-sized drops or planar interfaces. In this work, a microfluidic platform is employed to generate a monodisperse stream of small micrometer-sized water droplets in model fuel and ultra-low sulphur diesel, mimicking the size of droplets in actual fuel-water emulsions. The deformation of hundreds of droplets is tracked at high speed through twenty-six geometric contractions, to find time-dependent IFT. It is found that the timescale associated with decrease of IFT is orders of magnitude smaller in micrometer-sized droplets compared to millimeter-sized drops from pendant drop experiments, due to smaller diffusion timescale in smaller droplets. This finding suggests that in real emulsion processing conditions such as fuel filtration, the residence time of droplets from the point of formation to filtration is such that IFT has already decreased to the equilibrium value. This work results in clear implications that standardized tests used by industry for qualifying diesel fuels must be reconsidered to account for droplet size, to enable design of efficient fuel filtration systems.
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Energy & Fuels
1. Introduction Diesel-powered engines are ubiquitous, particularly for use in heavy equipment, military vehicles and even commercial automobiles, due to greater fuel efficiency than gasoline engines. However, the detrimental effects of high sulphur levels in diesel fuels, such as increased PM2.5 generation1 have led to severe restrictions on sulphur content in commercially available diesel fuels. ‘Diesel fuel’ is any fluid that meets the ASTM D975 standard.2 Since 2006, Europe and North America in particular are promoting the use of ultra-low sulphur diesel (ULSD). ULSD is produced by deep desulphurization3 of diesel fuel, and has significantly lower lubricity.4 In recent years, petro-diesel blends with significant biodiesel content have also gained popularity.5 Increasing demand for diesel-powered engines has led to the introduction of a range of additives into diesel fuel to boost performance and ensure longer component lifetimes in diesel engines. Primarily, diesel fuel additives were introduced to circumvent cold-start ignition issues6, followed by cetane number improvers7, lubricity enhancers, particularly for ULSD8, and deposit control additives such as polyisobutylene succinimides (PIBSI).9 Water can enter diesel fuel at any stage during transportation, storage, and actual use in an engine. This entrained water can have damaging effects on parts of an engine, such as fouling due to microbial growth, corrosion and rust of pipelines, and potentially lead to failure of common rail fuel injection components.10 Consequently, it becomes essential to separate entrained water from diesel fuel to ensure smooth functioning of the system. However, entrained water can be notoriously difficult to separate, particularly if it has been emulsified to form micron-sized droplets in a continuous phase of fuel. For example, fuel pumps can cause shear-induced breakage of large water drops, and the issue is further exacerbated by the presence of additives in the diesel fuel, leading to formation of highly stable water-infuel emulsions with droplet sizes ranging from 10-150 µm.11,12 Fibrous filter media in coalescing filters, such as the one shown in Figure 1a, are typically employed to trap and coalesce water droplets13–15, following which larger drops can subsequently be removed from the system by gravitational settling.16
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The properties of the filter media, such as fiber wettability, are known to significantly impact the efficiency of the filtration process. For example, both superhydrophobic and superhydrophilic fibers have been tested for various liquid-liquid filtration applications.15,17–19 Additionally, electrostatic20 and acoustic techniques21,22 can be used to enhance separation performance. In addition to filter media and filtration techniques themselves, the properties of the biphasic system as well as the two liquids being separated are key considerations in design of an efficient liquid-liquid separation system. Interfacial tension between the fuel and water phases is an important property impacting fuel-water separation efficiency. Fuel additives can adversely affect separation performance by lowering the interfacial tension between fuel and water. Interfacial tension between two immiscible liquid phases is defined thermodynamically as the change in free energy that is associated with creation of a unit area at the interface.23 Driven by cohesive forces between the molecules in the two phases, interfacial tension plays a pivotal role in determining the stability of foams and emulsion systems.24 A lower interfacial tension results in a lower thermodynamic driving force for coalescence, and a concomitant increase in emulsion stability. Therefore, the interfacial tension between fuel and water will depend on the fuel and additive chemistry and additive concentration. Moreover, diesel fuel containing surface-active additives will exhibit time-dependent or dynamic interfacial tension with water as the additives adsorb to the interface; hence the kinetics and timescales of additive transport to the fuel-water interface become important to filtration performance. The ASTM D975 standard2 specifies requirements for density, viscosity and other bulk properties for diesel fuels, but does not specify interfacial properties, meaning that the interfacial properties between fuel and water can vary greatly based on the additives and fuel components. Surface or interfacial tension can be measured directly as force per unit length using a Du Noüy ring25 or Wilhelmy balance24,26 or indirectly using geometric methods such as pendant drop, oscillating bubble27,28, spinning drop29,30, capillary rise31 or oscillating jet.24,32 An extensive body of literature exists33– 36
, discussing dynamic interfacial tension measurements using pendant drop tensiometry.37 Pendant drop is
an indirect measurement technique utilizing the Laplace pressure differential across a curved interface given
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Energy & Fuels
by ∆𝑃 = (
% &'
+
% &)
)𝛾, where 𝛾 is the interfacial tension, 𝑅% and 𝑅- are the principal radii of curvature, and
Δ𝑃 is the pressure differential between the concave and convex sides of a drop.24 The drops used in pendant drop experiments are typically millimeter-sized as shown in Figure 1b (top), and they tend to deviate slightly from a spherical shape owing to the balance between capillary forces and gravity. During interfacial tension measurements, when a fresh interface is formed in a solution containing a surfactant, the processes of adsorption/desorption and diffusion from the bulk lead to population of the interface by the surface-active species.38 Driven entropically, surfactant molecules will adsorb to the fresh interface, and this leads to depletion of surfactant from the bulk solution adjacent to the interface, particularly if the interfacial area is large or if convection is absent. Consequently, surfactant molecules from the bulk, driven by the concentration gradient, will diffuse towards the depleted region. These simultaneous processes drive towards a state where the surface and bulk chemical potentials are in equilibrium. When the surface or interfacial tension has stabilized to a constant value, this is typically referred to as the equilibrium value.38 If the timescale for diffusion of surfactant from the bulk is significantly slower than the adsorption/desorption timescale, the surfactant dynamics are ‘diffusionlimited’, whereas if the adsorption/desorption timescales are relatively slow, the dynamics are said to be ‘kinetic-limited’.39 This study is motivated by droplet size-dependent kinetics observed in pendant drop experiments with water droplets formed in a continuous phase of fuels containing additives. Figure 1c shows dynamic interfacial tension measurements for ‘model fuel’ (methyl laurate + 1-decene 4:1 v/v) containing the deposit control additive PIBSI at a standard treatment dose (for fuel-water separation testing) as the continuous phase, with water as the drop phase, conducted using a pendant drop apparatus. The interfacial tension is found to reduce from ~25 mN/m (model fuel without surfactant) to the equilibrium IFT of ~5 mN/m in about 10 minutes for a ~2mm droplet. However, as shown in Figure 1c, the time required to reach IFT of 15 mN/m is smaller at smaller droplet sizes, suggesting that IFT equilibrates faster for smaller droplets.
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At any given surfactant concentration, a micron-sized interface (as opposed to a large pendant drop) has access to a greater number of surfactant molecules per unit area. This means that the time required for diffusion from the bulk to the interface due to the concentration gradient is greatly reduced, since surfactant molecules must no longer travel a large distance through the bulk to populate the interface, thereby increasing the mass transfer rate or flux to the spherical interface. This observed decrease in IFT equilibration time with a decrease in droplet size is in agreement with previous work on surfactant diffusion to spherical interfaces, which posits that the surfactant transport mechanism shifts from diffusion-limited to kinetic-limited as the droplet radius is decreased to some critical value.38,39 Previous work by Alvarez38 and Jin et al.39 discusses the spherical depletion depth (characteristic length scale) and the characteristic time scale for diffusion to a spherical interface, which varies directly as the radius (𝑎0 ) of the spherical interface. The characteristic timescale for diffusion is given by 𝜏23 =
567898: ;< 38 , 2=
where ℎ?@;A;B is the
depletion depth for a planar interface (ratio of the equilibrium surface concentration to the bulk surfactant concentration) and 𝐷D is the diffusivity of the surfactant molecule. Therefore, as the radius of the droplet decreases (or curvature increases), the characteristic timescale for diffusion decreases i.e. smaller droplets will reach equilibrium sooner. At some critical radius, the surfactant transport mechanism would shift to being kinetic-limited. Droplets in typical emulsion systems are micron-sized, and since size of the droplet governs surfactant mass transfer to the interface39, methods such as pendant drop tensiometry which employ millimeter-sized drops may not be representative of real emulsion systems. Therefore, it is preferable to employ techniques for dynamic IFT measurement where the length scales (and hence the timescales for surfactant transport) are comparable to real emulsion systems i.e. on the order of microns. A novel ‘micro-tensiometer’ developed recently by Alvarez et al.40–43 measures dynamic interfacial tension for systems containing surfactants or polymer-grafted nanoparticles by employing micron-sized droplets formed at the end of a capillary or needle in a custom-built flow cell to access the kinetic-limited regime of surfactant transport. Alternately, microfluidic devices offer a versatile platform to generate monodisperse emulsions using two fluid phases, known as the ‘continuous’ and ‘dispersed’ phases, using either 6 ACS Paragon Plus Environment
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cross-flowing or co-flowing streams.44–46 Once formed, these droplets can be manipulated using external electric, magnetic or optical fields or internal hydrodynamic fields.47 This study focuses on using a microfluidic tensiometer with contraction geometries to deform droplets as shown in Figure 1b (bottom), based on a design by Hudson et al.48–50, which has previously been applied for measuring both static as well as dynamic interfacial tension using aqueous solutions in the dispersed phase and oils in the continuous phase.48–52 Additionally, this microfluidic tensiometer design has been applied towards temperaturedependent interfacial tension measurements, between silicone oil or mineral oil and water, wherein on-chip temperature control elements were integrated into the device design.53 While previous microfluidic work has demonstrated the measurement of IFT between various oils (continuous) and aqueous (dispersed) phases, the current work applies microfluidic tensiometry to diesel fuel-water systems. In addition to static IFT measurements at a single tensiometer, interfacial tension is also measured at various points along the microfluidic device, yielding time-dependent or dynamic interfacial tension measurements for fuel-water emulsions with micron-sized droplets. Further, the surfactants in this microfluidic system are in the continuous phase, which contrasts with previous mass transfer studies using microfluidic tensiometry50,52 where the surfactants are in the dispersed phase. The goal of this work is to measure the time required for the IFT to decrease to its equilibrium value when droplets are in the size range of 75-90 µm, which are more relevant to fuel-water separation testing and real-world applications, instead of a few millimeters (as in a pendant drop experiment). Three different commonly used hydrocarbon mixtures are investigated in this study. The first is a model fuel, which is a 1:4 v/v mixture of 1-decene and methyl laurate, which can be used as a surrogate for fuel-water separation testing. The second is an ultra-low sulphur diesel (ULSD), which is used commercially, and may have a mixture of surface-active additives including biodiesel (which enhances lubricity) or PIBSI. The third is a clay-treated ULSD, which is treated to remove surface-active species in accordance with SAE J148854. The surfactants used in this work include PIBSI, which is added in a controlled standard dose to model fuel, and mono-olein (commonly used for fuel-water separation testing), introduced at a 7 ACS Paragon Plus Environment
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concentration of 0.01 mg/mL to clay-treated ULSD. Here, we measure the time required for the IFT to reduce to its equilibrium value when droplets are in the size range of 75-90 µm, relevant to fuel-water separation testing and real-world applications. In this size range, it is expected that the IFT profile may be markedly different from that of millimeter-sized droplets in pendant drop experiments, in part due to a change in the length scale for surfactant transport to the interface.
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2. Experimental
2.1 Materials All solvents were received from commercial sources and used without additional purification. 1oleoyl-rac-glycerol (mono-olein) was received from Sigma-Aldrich (>99% purity). Polyisobutylene succinimide fuel additive (PIBSI) was received from a commercial supplier. Ultra-pure water used in pendant drop measurements was generated from a Millipore Milli-Q Advantage A10 system. HPLC water (Fisher Scientific) was used in microfluidic experiments. A mixture of 1-decene and methyl laurate (1:4 v/v) was prepared and is referred to as a ‘model fuel’. Ultra-low sulphur diesel (ULSD) was purchased from Chevron (Diesel emission certification fuel). Fuel additives and polar components were removed according to the SAE J1488 standard54 for emulsified water/fuel separation test procedure (Clay-treated ULSD). A stock solution of model fuel containing PIBSI (100x standard dose) was prepared by dissolving 3.75 grams of PIBSI in 500 mL of model fuel. The stock solution is diluted 1:100 through serial dilutions to prepare a standard dose of PIBSI in model fuel. The densities of the fuel samples were determined using a Krüss K11 Tensiometer, by measuring the displacement of the fluid by a probe of known density. Viscosities of the diesel fuels were measured using the AR-G2 rotational rheometer (TA Instruments) with a cup and bob geometry at 25°C. The densities and viscosities of all the fuel samples employed in this work are listed in Table 1.
2.2 Pendant drop tensiometry Pendant drop measurements were acquired using a Kyowa Dropmaster DM-701 Contact Angle Meter using standard protocol.37 In brief, a needle was submersed in the hydrocarbon fluid and a water drop was formed at the needle tip. A fresh interface was prepared prior to each measurement. After forming a drop, time-dependent interfacial tension measurements were acquired, with the data being analyzed using a Young-Laplace fit.
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2.3 Microfluidic tensiometry The microfluidic device for interfacial tension measurements is fabricated using standard softlithography techniques as described extensively in the literature.55–57 Briefly, a silicon wafer is cleaned using a piranha solution and spin coated with a negative photoresist (SU-8 2050, Microchem). The masks containing the device patterns are designed using a 2D CAD software (Draftsight) and printed on transparencies with a resolution of 20,000 dpi (CAD/Art Services, Inc.). After a pre-exposure bake, the photoresist is exposed to UV light using a mask aligner (Karl Süss) and developed to form an SU-8 mold of the device design. Following this step, poly(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning Corporation) is poured on the mold and baked for at least 4 hours. After baking, holes are punched into the PDMS using a 1.25 mm OD biopsy punch (World Precision Instruments) to create inlets for the tubing. The PDMS device and a glass slide or cover slip are plasma treated (Harrick Plasma) and sealed to form an irreversible bond. The device is baked for at least 2 hours after plasma treatment to make the channels hydrophobic, since water is the dispersed phase and hence should not adhere to the channel walls. The droplet size in the microfluidic flow-focusing device can be controlled by varying the ratio of dispersed phase pressure to continuous phase pressure, controlled by pressure regulating solenoid valves (Marsh Bellofram Type 3110 Analog circuit card regulators). Droplet speed and spacing is controlled by a sheath flow of fuel, seen in Figure 2a. The required pressure-to-voltage signals are transmitted to the regulators through a National Instruments Analog Input/Output device with a cDAQ-9174 chassis, NI 9264 AO module and NI 9201 AI module. The regulators exert the input pressures on a headspace of air in microfluidic reservoirs (Elveflow) containing the fluids to be delivered to the microfluidic device using polyethylene tubing (BD Intramedic PE tubing, 1.52 mm OD, 0.86 mm ID) and fittings (IDEX). The device is mounted on an inverted microscope stage (Olympus IX73) and imaged using a 10X objective (Olympus) and a high-speed camera (Photron Mini UX100). The planar microfluidic device used for interfacial tension measurements (Figure 2a) consists of a flow focusing geometry for droplet formation44–46, where the continuous phase (hydrocarbon fuel) exerts a
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viscous force driving the breakup of the dispersed phase thread (water) into discrete droplets. The interfacial tension between the fluids resists breakup of the dispersed phase thread, hence systems with larger values of interfacial tension tend to form larger droplets. After drop formation, a sheath flow directs the droplets into a series of contraction-expansion geometries following the design by Hudson et al.48, which deform the water droplets in an extensional flow field. Undeformed water droplets are shown in Figure 2b, and the deformed ellipsoidal shape of these droplets in the extensional flow field induced by the contraction geometry is indicated in Figure 2c. The bottom panels of Figures 2b,c show the corresponding analyzed images of the droplets, with the relevant dimensions being the undeformed droplet diameter and the lengths of the major and minor axes. It is important to note that the microfluidic contraction geometry induces both shear as well as extension, due to confinement by the walls.58 However, if the droplet sizes are small enough (
