Electrospun Superhydrophobic Poly(vinylidene fluoride-co

Apr 10, 2013 - *Telephone: 330-972-7943. ... an electrospun layer of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) fibers for the separa...
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Electrospun Superhydrophobic Poly(vinylidene fluoride-cohexafluoropropylene) Fibrous Membranes for the Separation of Dispersed Water from Ultralow Sulfur Diesel Sarfaraz U. Patel, Shagufta U. Patel, and George G. Chase* Department of Chemical and Biomolecular Engineering, University of Akron, Akron, Ohio 44325-3906, United States ABSTRACT: One of the major causes of diesel engine maintenance problems is water contamination in fuels. This paper discusses fabrication, characterization, and testing of non-woven glass fiber mats augmented with an electrospun layer of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) fibers for the separation of secondary water dispersions from ultralow sulfur diesel. The PVDF-HFP fibers had an average fiber diameter of 334 nm. All PVDF-HFP layers had water contact angles greater than 150° with relatively low hysteresis values, making them superhydrophobic. Water separation experiments showed that glass fiber media augmented with electrospun PVDF-HFP fiber mats significantly improved water separation, with water removal efficiencies reaching 99%, as compared to glass fiber media without the PVDF-HFP layer.

1. INTRODUCTION Recent advances in diesel engine design have improved engine performance and have reduced exhaust pollutants. Emissions of pollutants from diesel engines are regulated because of their harmful effects on human health, especially cardiovascular and respiratory problems.1−5 Common combustion-related pollutants include particulate matter (soot), oxides of nitrogen, carbon monoxide, and incompletely combusted hydrocarbons. Some of the most attractive methods to reduce particulate matter and nitrogen oxides emissions include high-pressure injection, turbocharging, exhaust after treatments, and the use of fuel additives.6−8 Commercially available ultralow sulfur diesel (ULSD) is produced through hydrodesulfurization to remove sulfur as well as other heteroatoms and conjugated aromatic compounds from the petroleum fraction. As a result of this refining step, ULSD loses its inherent lubricity.9 To keep the lubricity of ULSD normal, synthetic lubricant additives are added. The additives increase the surfactancy of the fuel, reduce the interfacial tension (IFT) of fuel, and decrease the stable drop sizes of water in fuel. As a whole, the production of ULSD makes water dispersions in the fuel more stable and more difficult to separate from the fuel.10 Water contamination in fuels comes from several sources but mainly via precipitation, condensation of atmospheric moisture, and adsorption from humid air. The effects of water in diesel fuel are characterized by corrosion of fuel system parts, plugging of filters and orifices, and in some cases failure of fuel injection equipment.11 Magiera and Blass reported that hydrophilic glass and stainless-steel fiber media have good separation efficiency but hydrophobic media made of Teflon fibers had poor separation performance.12 Shin and Chase used hydrophobic polystyrene fibers to separate water from oil and found that the addition of sub-micrometer-sized polystyrene fibers showed improvement in capture efficiency without significantly increasing the pressure drop.13 This paper describes the fabrication and tests of a superhydrophobic (oleophilic) material for use in filter media or © 2013 American Chemical Society

membranes for the separation of water from ULSD. The membranes were fabricated by electrospinning, and their water separation performances were studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with a molecular weight of 450 000 (Arkema, Inc., King of Prussia, PA) was dissolved in acetone (Sigma Aldrich, St. Louis, MO) at room temperature to form electrospinning solutions with polymer concentrations of 10% by weight. Mild stirring was used to make all electrospinning solutions. 2.2. Electrospinning. Figure 1 shows a diagram of the electrospinning setup used in this work. The polymer solution was fed from a 5

Figure 1. Electrospinning setup schematic. mL syringe equipped with a 21-gauge needle via a syringe pump (SP220i, World Precision Instruments, Sarasota, FL) at a feed rate of 15 mL/h. A high-voltage power supply (Gamma High Voltage Research, Ormond Beach, FL) was used to generate a potential difference of 30 kV between the needle and a grounded aluminum foil sheet positioned at a distance of 15 cm from the needle tip. A glass fiber mat to serve as a Received: January 12, 2013 Revised: April 10, 2013 Published: April 10, 2013 2458

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Table 1. Electrospinning Conditions for Synthesizing the PVDF-HFP Fiber Mats PVDF-HFP concentration (wt %)

solvent

voltage (kV)

tip−collector distance (cm)

flow rate (mL/h)

fiber diameter range (nm)

log normal mean diameter (nm)

log normal standard deviation

10

acetone

30

15

15

123−760

334

0.414

steadily extracted from an 8 μL water drop to observe the receding contact angle. The porosity of a filter layer, by definition, is related to the volume of the fibers and the volume of the layer by the relation

support structure (Hollingsworth and Vose Co., East Walpole, MA, with an average fiber diameter of 6 μm) was placed on a grounded aluminum foil, and the PVDF-HFP fibers were electrospun onto the glass fiber mat. The electrospinning time was varied to obtain membranes with different porosity values. 2.3. Characterization. The morphologies of the electrospun fibers were evaluated using field emission scanning electron microscopy (FESEM, JSM-7401F, JEOLUSA, Peabody, MA). The samples were silver-coated for 25 s to minimize charging effects. Fiber diameters were measured from the FESEM images via ImageJ analysis software (http:// rsbweb.nih.gov/ij/index.html). Fiber diameters were calculated using a length-weighted approach. 14 To determine the fiber diameter distribution, more than 100 fibers were measured. Details of the electrospinning experiment are given in Table 1. The PVDF-HFP fiber size distribution is shown in Figure 2.

εlayer = 1 −

Vfibers Vlayer

(1)

One of the simplest ways to determine the volume of the fibers is to weigh the layer in air and divide by the intrinsic density of the fiber material. If the intrinsic density of the material is not certain or the filter is a mixture of different materials, then other methods, such as gas expansion,15 are needed. The gas-expansion method, using a custom-made pycnometer, was used to measure the porosities of the glass fiber mat alone and the composite of glass fiber mat + PVDF-HFP fiber layer. The electrospinning times for fiber generation were adjusted, so that composite membranes with three different porosity values were obtained and their water separation performances were compared. The porosity of the composite filter membrane is denoted as εc, while the porosity of just glass fiber or PVDF-HFP fiber layer is denoted as ε in Table 2. The pycnometer was not sensitive enough to measure the porosity of the PVDF-HFP mat alone. The porosities of the electrospun fiber layers were determined by back-calculating from porosity measurements of the glass fiber media alone and glass fiber media augmented with the electrospun fiber layer. The performance measure of a filter medium has several equivalent definitions, such as filtration index,16 figure of merit,17 or quality factor (QF).18 A higher quality factor indicates better filter performance. The quality factor is defined as

Figure 2. PVDF-HFP electrospun fiber diameter distribution. QF = The wettabilities of the membranes were characterized by water contact angle and contact angle hysteresis in air. Water contact angles of the membranes submerged in ULSD were also determined. Water drops of 5 μL volumes were placed on the fiber mats and imaged using a drop shape analyzer (DSA20E, Krüss GmbH, Germany). For contact angles of water drops submerged in ULSD, a sample of the PVDF-HFP fiber mat attached to the glass fiber mat was placed in a spectrophotometric cell, and the cell was filled with ULSD. A 5 μL water drop was placed by a syringe onto the PVDF-HFP fiber layer surface, and the water contact angle in ULSD was observed. At least five independent drops were observed, and the contact angles were averaged to determine the contact angle and contact angle hysteresis values of the electrospun fibrous surfaces. Contact angle hysteresis is defined as the difference between an advancing and a receding contact angle. The advancing contact angle was observed by placing a 3 μL drop on a mat surface and steadily injecting water by the syringe until a drop volume of 8 μL was obtained. Similarly, water was

⎛C ⎞ − ln⎜ Cdownstream ⎟ ⎝ upstream ⎠ (2)

ΔP

where Cdownstream and Cupstream are the outlet (downstream) and inlet (upstream) dispersed water mass concentrations, respectively, and ΔP is the value of the pressure drop across the filter membrane. The separation efficiency is given by

η=

Cupstream − Cdownstream Cupstream

(3)

The upstream and downstream concentrations were calculated by the formula π C = ∑ Ni di 3ρwater (4) 6 where C is the water concentration, Ni is the number of water droplets, di is the water droplet diameter, and ρwater is the density of water. A schematic of the filtration experimental apparatus is shown in Figure 3. Water−ULSD separation experiments were performed using

Table 2. Properties of Filter Membranes filter ID

fiber mat surface PVDF-HFP or glass fibers

porosity of composite membrane (εc)

porosity of PVDF-HFP fiber or glass fiber layer (ε)

WCA in air (deg)

G

glass fiber

0.96 ± 0.02

0.96 ± 0.02

9±2

A1 A2 A3

PVDF-HFPa PVDF-HFPa PVDF-HFPa

0.94 ± 0.08 0.86 ± 0.08 0.82 ± 0.09

0.73 ± 0.11 0.48 ± 0.11 0.22 ± 0.13

170.3 ± 2 167.2 ± 3 159.7 ± 4

hysteresis (deg) water spreads 6 8 9

WCA in ULSD (deg) water spreads 162.4 ± 3 160.3 ± 3 155.5 ± 3

Composite membrane of PVDF-HFP fiber layer on a glass fiber mat as a structural support; the PVDF-HFP layer surface faces the incoming flow stream. a

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Figure 4. (a) Water drop on a PVDF-HFP fiber membrane immersed in ULSD in an optical cell and (b) close-up view of the water drop submerged in ULSD on the PVDF-HFP fiber membrane.

Figure 3. ULSD water separation experiment schematic.

the electrospun mats, are commonly affected by the solution properties, temperature, applied voltage, solvent type, and concentration of polymer.23,24 The FESEM image in Figure 5

deionized water droplets dispersed in ULSD. ULSD and water (0.2% water by volume of ULSD) were mixed by an impeller for 10 min and then pumped by a 3500 rpm centrifugal fuel transfer pump (Airtex ATXE3309 GIC07D02) in a recycle flow loop to generate the fine droplet dispersion. A pipe tee and valve were positioned in the recycle line, so that part of the flow could be recycled and part of the flow could be directed to the filter. Sample points were positioned upstream and downstream a short distance from the filter holder to obtain fluid samples for measuring water droplet size distributions. The size distributions of water droplets in the flow streams were measured using a particle counter (780 PALSParticle Sizing Systems, Port Richey, FL, with a sensor range of 0.5−500 μm). The pressure drop was measured with an electronic pressure gauge with pressure taps similarly positioned upstream and downstream of the filter holder. The pressure drop and particle size distributions were measured at 10 min intervals. The filter membranes were cut to size 10.2 × 10.2 cm and placed on a supporting wire mesh inside a Plexiglas filter holder. Water, being denser than ULSD, tended to settle at the bottom of the supply tank and filter holder. The impeller agitation and the recycle stream helped to maintain a stable dispersion concentration and drop distribution during the course of experiment. The flow rates of the dispersion challenged to the membrane were varied as 210, 420, and 630 mL/min corresponding to filter face velocities of 2.0, 4.0, and 6.0 cm/ min, respectively. The recycle part of the flow stream had flow rates ranging from about 2000 to 1600 mL/min depending upon the flow rate challenged to the filter membrane.

3. RESULTS AND DISCUSSION All of the fiber membranes were characterized for their hydrophobicity by measuring water contact angle (WCA) and contact angle hysteresis. The measured WCA and hysteresis for the fiber mats are listed in Table 2. It was observed that the water contact angles decreased with an increase in the amount of PVDF-HFP fibers. Contact angle hysteresis increased with an increase in the amount of PVDF-HFP fibers. An increase in the mass of PVDF-HFP fibers may cause wetting behavior to transition from a Cassie−Baxter state to a Wenzel state.19 As more fibers are added, the amount of void space/air becomes covered by fibers; therefore, the contact area of the water drop with air decreases, decreasing the contact angle values. All of the PVDF-HFP membranes were superhydrophobic with contact angles greater than 150°. For solid flat surfaces, the contact angle hysteresis mainly reflects roughness and chemical heterogeneity of the surface. It can also tell about the interaction between the probe liquid and the surface.20−22 For the fiber mats, the relatively low hysteresis values (less than 10°) means that the membranes had fairly uniform wetting properties. Images of a water drop in contact with the PVDF-HFP fiber mat submerged in ULSD are shown in Figure 4. Electrospinning solution properties and the surrounding environment conditions affect the morphology of electrospun fibers and their spinnability. The occurrence of morphologies, such as beads, bead-on-string structures, and fiber structures in

Figure 5. FESEM image of PVDF-HFP fibers and some beads formed on the fibers during electrospinning.

shows that the electrospun fiber mats used in this work had beads. The number of beads per unit area was fairly low; therefore, the beads were ignored in fiber diameter distribution calculations. The beads add to the mat roughness, which contributes to the superhydrophobic nature of the electrospun fiber mat and can reduce the pore sizes.22,25 Smaller pores also require a greater fluid pressure differential across the filter to push a drop through the pore. All of the membranes were constructed in triplicate, and the experimental results show steady-state values averaged over the three samples. The error bars in the plots and the error ranges in the tables indicate one standard deviation of the three averaged points. In Figure 6 the separation efficiencies are plotted for the membrane samples at different face velocities. Experiments were performed with a glass fiber membrane without a PVDF-HFP fiber layer to determine the efficiency of the glass fiber membrane by itself. All of the filters with PVDF-HFP fiber mats showed improved efficiency compared to the glass fiber mat by itself. The highest separation efficiencies were observed for composite membranes A2 and A3. This can be attributed to the blocking of the penetration of water drops by the superhydrophobic PVDF-HFP fiber layer. The composite membrane A1 at face velocity of 6 cm/min showed relatively 2460

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Figure 6. Separation efficiencies of filter media with different PVDFHFP fiber layers tested at different filtration face velocities. The plotted values are the averages of three experiments, and the error bars represent the standard deviation between the three measurements.

Figure 8. Quality factors of the filter media with different PVDF-HFP fiber layers experimentally tested at different filtration face velocities. The plotted values are the averages of three experiments, and the error bars represent the standard deviations between the three measurements.

low separation efficiency compared to the other membranes. At higher face velocities, the PVDF-HFP fiber layers may deform to increase the pore size and allow for the dispersion to pass through the membrane, thus decreasing the separation efficiency. The PVDF-HFP layers with lower porosity had greater amounts of fibers that may be equivalent to stacking multiple independent PVDF-HFP layers. Multiple layers of PVDF-HFP fibers may improve the separation efficiencies at higher face velocities. Media constructed of multiple independent PVDF-HFP layers were not evaluated in this work but are a topic of future work. Glass fiber media augmented with PVDF-HFP fibers improved water separation significantly. However, the pressure drop increased with the decrease in porosity, as shown in Figure 7. Separation efficiency and pressure drop data were applied in eq

The RQF plotted in Figure 9 shows the composite medium A3 at 6 cm/min face velocity had the greatest overall improvement

Figure 9. Relative quality factors of the filter media with different PVDFHFP fiber layers experimentally tested at different filtration face velocities. The plotted values are the averages of three experiments, and the error bars represent the standard deviation between the three measurements.

over the corresponding glass fiber medium. The cause of the greatest improvement by the medium A3 is a combination of improved capture efficiency and the pressure drop not rising as much as the other filters, possibly because of the liquid holdup in the filter not blocking the pores. This is a topic of current and future research. The droplet distribution for the upstream side of the filter membrane followed a bell-shaped curve, as shown in Figure 10. The inlet droplet distribution was between 0.5 and 50 μm, with an average drop size of about 18 μm. It was observed that on the downstream side of the filter media, many more smaller sized water drops were detected than were entering the filter. This is probably due to some of the larger drops breaking into smaller drops during the separation, and these smaller drops were observed on the downstream side of filter media. The efficiency of the separation is based on the total mass of water that is separated, and the mass increase of the small drops on the downstream side is relatively small compared to the total water challenging the filter. Figures 11−13 show that, as the amount of PVDF-HFP fibers increased and the porosity of membranes decreased, the drop removal efficiency and the minimum size of the drop removed improved significantly. The separation efficiencies were calcu-

Figure 7. Measured pressure drops across filter media with different PVDF-HFP fiber layers experimentally tested at different filtration face velocities. The plotted values are the averages of three experiments, and the error bars represent the standard deviation between the three measurements.

2 to calculate the quality factor (QF) plotted in Figure 8. It was observed that the electrospun PVDF-HFP membranes with lowest porosity values had highest quality factor values. The improvement in quality factor was normalized by dividing the quality factor of the test sample by the quality factor of the control sample (the glass fiber mat with no PVDF-HFP fiber layer), at the same flow rate, to obtain the relative quality factor (RQF) for each flow rate, defined by RQF =

QF QFcontrol

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Figure 13. Typical separation efficiency per drop size at a face velocity of 6 cm/min.

Figure 10. Typical water droplet distributions at a face velocity of 4 cm/ min for filter samples G and A2. Similar distributions were observed for all of the experiments.

reusability, one composite membrane A1 was tested, removed from the filter holder, air-dried for 3 days, and then tested again. In this way, the same membrane was tested a total of 4 times, with drying between each experiment. The filter was tested at a face velocity of 2 cm/min. It was observed that separation efficiency declined from the initial unused filter from 97 to 95%. After the first reuse of the membrane, the pressure drop increased by about 25% and became steady in subsequent repeated experiments. These data are shown in Figure 14. This indicates that the membranes can be reused. A more complete reusability study is needed to understand the limitations of reusing the filters. Figure 11. Tyipical separation efficiency per drop size at a face velocity of 2 cm/min.

Figure 14. Separation efficiency and pressure drop of one filter media sample A1 reused 3 times. A fresh A1 filter was experimentally tested, removed from the holder, air-dried at room temperature for 3 days, tested again, and dried again for a total of four tests. The experiments were conducted at a face velocity of 2 cm/min. Figure 12. Typical separation efficiency per drop size at a face velocity of 4 cm/min.

Nazzal and Wiesner determined a critical pressure required to push a drop of liquid through an idealized circular pore in a porous membrane.26 The critical pressure is given by

lated and plotted only for the drop sizes that showed a decrease in the number of drops between the upstream and downstream. Where the number of drops increased because of drop breakup, the formula in eq 3 calculates negative efficiencies. The negative values occurred for drops smaller than the minimum size shown in the plotted data and are not included in the plots. The filter media were not effective for drops smaller than the minimum size shown in these plots. Reusability of the filter membranes was demonstrated by drying the membrane and reusing it in the filtration experiments in similar conditions as the fresh membranes. The results are promising and suggest that a more thorough study of reusability in future work should be conducted. To demonstrate the

Pcrit

⎡ ⎫1/3⎤ ⎧ ⎢ ⎪ ⎥ ⎪ ⎪ ⎪ ⎥ 2Γo/w cos θ ⎢ 2 + 3 cos θ − cos3 θ ⎬ ⎥ ⎨ = − 1 3 ⎢ rpore ⎪ ⎛ rdrop ⎞ ⎪ 3 3 ⎢ ⎪ 4⎜⎝ rpore ⎟⎠ cos θ − (2 − 3 sin θ + sin θ) ⎪ ⎥ ⎩ ⎣ ⎭ ⎦

(6)

where Γo/w is the IFT between the two immiscible liquids used, θ is the contact angle of water on the surface of the membrane, rpore is the pore diameter of the membrane, and rdrop is the average drop size of the liquid. It is clear from eq 6 that the critical pressure values are directly proportional to the cosine of the contact angle and the IFT of the system. In all experiments, the IFT of ULSD−water was constant (23 mN/m); therefore, the 2462

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Table 3. Comparison of the Experimental Pressure Drop Across the Filters and the Calculated Critical Pressure Drop for Forcing an Average Size Drop through the Pores of the Membranes filter ID

filter sample

porosity of composite membrane (εc)a

porosity of PVDF-HFP fiber (ε)

WCA in ULSD (deg)

Pcrit (kPa)b

Pexpt (kPa)b

G A1 A2 A3

glass fiber only PVDF-HFPc PVDF-HFPc PVDF-HFPc

0.96 ± 0.02 0.94 ± 0.02 0.86 ± 0.08 0.82 ± 0.09

NA 0.73 ± 0.11 0.48 ± 0.11 0.22 ± 0.13

9 162.4 160.3 155.5

NA 17−70 17−69 16−67

1.73 2.86 3.78 3.15

Porosity of the glass fiber mat alone or the composite membrane (glassfiber layer + PVDF-HFP fiber layer) as measured by the gas-expansion method. bPcrit is calculated using eq 6, while Pexpt is the highest pressure drop among all face velocities observed during the experiments. Equation 6 is not applicable when rdrop/rpore < 1 or when θ is small (wetting material); hence, Pcrit is not calculated for the glass fibers. cThe composite membrane is made by electrospinning the PVDF-HFP fiber layer on a glass fiber mat. The glass fiber mat acts as a structural support; the PVDF-HFP layer surface faces the incoming flow stream. a

Table 4. Measured IFT Values Upstream and Downstream of the Filter Membrane in the Reusability Tests time (min)

IFT upstream (mN/m)a

IFT downstream fresh (mN/m)

IFT downstream reuse 1 (mN/m)

IFT downstream reuse 2 (mN/m)

IFT downstream reuse 3 (mN/m)

10 30 40 50 average

22.54 ± 0.20 22.89 ± 0.56 22.74 ± 0.87 21.97 ± 0.64 22.54 ± 0.40

22.47 ± 0.16 22.34 ± 0.76 21.98 ± 0.22 22.49 ± 0.26 22.32 ± 0.24

21.20 ± 0.22 21.78 ± 0.46 21.89 ± 0.36 21.77 ± 0.92 21.66 ± 0.31

22.30 ± 0.21 22.43 ± 0.23 22.15 ± 0.28 22.70 ± 0.61 22.40 ± 0.23

21.09 ± 0.14 21.64 ± 0.20 21.33 ± 0.23 22.09 ± 0.34 21.54 ± 0.43

The upstream values are those for the fresh (unused) filter sample. Similar values of approximately 22 mN/m were obtained for the upstream fluid for all of the reuse tests. a

of the membrane, rolled down and collected at the bottom of the upstream side of the membrane. This can be seen in Figure 15.

critical pressure drop required to push the drop of water through membrane increased by increasing the water contact angle values (θ) with the superhydrophobic membranes. Pore sizes of the PVDF-HFP fibers and glass fibers were estimated using FESEM images. The vacant spaces seen in images were assumed to be circular. The area of those spaces was equated to the area of a circle, and pore diameter values were calculated. The pore diameter values ranged between 0.6 and 2.6 μm for PVDF-HFP and between 17 and 74 μm for a glass fiber membrane. A thin layer of fibers was used to determine the pore diameters. Because it was difficult to estimate the pore sizes for thick layers of the PVDF-HFP membranes, these pore sizes were used for a range of critical pressures of all filter membranes to obtain a conservative estimate. The calculated range and experimentally observed pressure value for PVDF-HFP membrane filters are listed in Table 3. In the case of the glass fiber membranes, the water spread on glass fibers immersed in ULSD; therefore, the critical pressure was not calculated. Because the calculated critical pressure required for pushing a drop of water through the PVDF-HFP fiber layer is much higher than the available pressure drop in the experiments, the water drops of average size and larger were expected to not penetrate the PVDFHFP fiber layers. However, the drops at the small size of the drop size distribution could have critical pressures less than the pressure drop across the membrane. Smaller drops were expected to penetrate and pass through the membrane. IFTs of the ULSD with water were measured periodically upstream and downstream during all experiments. In all of the experiments, the upstream and downstream values were essentially unchanged (about 22 mN/m), indicating that any lubricity agents (surfactants) that may have been added to the ULSD are not significantly removed by the membrane. This is a topic of ongoing research. The measured IFT values upstream and downstream of the filter membrane in the reusability tests discussed above are shown in Table 4. When the concentration of water drops on the surface of the mat was large enough, the drops coalesced into larger drops. The larger drops, with the help of gravity and the vertical orientation

Figure 15. (a) Water collected at the bottom of upstream side of the filter holder and (b) close-up view of the droplet layer at the bottom of the filter holder.

Zhang et al. found that a filter made up of multiple thin layers of polyacrylonitrile fibers had much higher quality factor values than a single layer of fiber mat.27 They concluded that multiple layers of fiber have better thickness uniformity because of stacking compensation that helps in improving quality factor of filter media. Leung et al. observed that, with an increased amount of polyethylene oxide fibers in the membrane, the pressure drop and filtration efficiency values increased.28 They also noted that 2463

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filtration efficiency decreased with an increase in face velocity. These results are consistent with the results here with PVDFHFP fiber layers. The highest pressure drop and separation efficiency values were observed for membranes with lowest porosity values. A combination of superhydrophobicity and relatively small pore diameters of the membranes enabled the effective separation of dispersed water drops from ULSD. In summary, the separation efficiency and pressure drop across the filter membrane increased with an increase in the fiber amount or a decrease in the porosity of the membrane filter. It was seen that the PVDF-HFP membrane filter with a porosity value of 0.22 had more than 6 times improvement in quality factor compared to glass fiber media without the PVDF-HFP fiber layer.

(10) Yang, C.; Larsen, S.; Wagner, S. Understanding emulsified water filtration from diesel fuels. Proceedings of the 8th International Filtration Conference; San Antonio, TX, Jan 16−18, 2007; IFC08-001. (11) Stanfel, C. Filtr. Sep. 2009, 3, 22−25. (12) Magiera, R.; Blass, E. Filtr. Sep. 1997, 4, 369−376. (13) Shin, C.; Chase, G. G. AIChE J. 2004, 2, 343−350. (14) Varabhas, J. S.; Chase, G. G.; Reneker, D. H. Polymer 2008, 19, 4226−4229. (15) Nakao, S. J. Membr. Sci. 1994, 1−2, 131−165. (16) Juvinal, R. A.; Kessir, R. W.; Steindler, M. J. Argonne Natl. Lab. 1970, 47. (17) Wang, J.; Kim, S. C.; Pui, D. Y. H. Aerosol Sci. Technol. 2008, 9, 722−728. (18) Brown, R. C. Air Filtration: An Integrated Approach to the Theory and Applications of Fibrous Filters; Pergamon Press: Oxford, NY, 1993. (19) Liu, G.; Fu, L.; Rode, A. V.; Craig, V. S. J. Langmuir 2011, 6, 2595−2600. (20) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 24, 1857−1860. (21) Li, W.; Fang, G.; Li, Y.; Qiao, G. J. Phys. Chem. B 2008, 24, 7234− 7243. (22) Ma, M.; Hill, R. M.; Rutledge, G. C. J. Adhes. Sci. Technol. 2008, 15, 1799−1817. (23) Doshi, J.; Reneker, D. H. J. Electrost. 1995, 2−3, 151−160. (24) Reneker, D. H.; Chun, I. Nanotechnology 1997, 3, 216−223. (25) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 546−551. (26) Nazzal, F. F.; Wiesner, M. R. Water Environ. Res. 1996, 7, 1187− 1190. (27) Zhang, Q.; Welch, J.; Park, H.; Wu, C.; Sigmund, W.; Marijnissen, J. C. M. J. Aerosol Sci. 2010, 2, 230−236. (28) Leung, W. W.; Hung, C.; Yuen, P. Sep. Purif. Technol. 2010, 1, 30− 37.

4. CONCLUSION In this work, superhydrophobic PVDF-HFP fiber layers were generated by electrospinning and applied to augment glass fiber filter media. The mean fiber diameter of PVDF-HFP fibers was 334 nm. The augmented filter media were effective in the separation of dispersed water drops from ULSD. The porosities of the PVDF-HFP fiber layers were varied by electrospinning different amounts of fiber on the glass fiber media. The face velocities of flow through the filters were varied in the experiments. All of the filter membranes showed higher separation efficiency with a decrease in porosity. Separation efficiency decreased with an increase in face velocity. The pressure drop across the filter increased with a decrease in PVDFHFP fiber layer porosity.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 330-972-7943. Fax: 330-972-5856. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Cummins Filtration. The authors also thank Arkema, Inc. for providing the polymer used in this study.

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REFERENCES

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dx.doi.org/10.1021/ef400248c | Energy Fuels 2013, 27, 2458−2464