Article pubs.acs.org/Langmuir
Extraction of Oil from an Aqueous Emulsion by Coupling Thermal Swing with a Capillary Pump Saheli Biswas,† Hugo Caram,† Ramesh Gupta,§,‡ and Manoj K. Chaudhury*,† †
Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem Pennsylvania 18015 United States ExxonMobil Research and Engineering, Clinton, New Jersey 08801, United States
‡
S Supporting Information *
ABSTRACT: Separation of oil from water is an area of increasing interest because of the ever-increasing emphasis on reducing discharge of oily wastewater streams and for managing accidental oil spills. While several methods to separate oil from water are available, the current methods often require elaborate processing steps and/or have low extraction rates. Here, we report two simple and potentially inexpensive methods of separating oil from aqueous emulsions. The first method employs hydrophobized glass wool in a pressure-driven capillary pump, while the second method employs novel zeolite pellets the exterior surface of which is hydrophobic. These pellets selectively absorb oil from an aqueous emulsion, which can subsequently be recovered using thermal swing with hot fluid at a temperature far below the boiling point of the oil. Separation of oil with a very high yield (ca. 97%) appears possible using a combination of the two methods.
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INTRODUCTION Demulsification, i.e., breaking of an oil/water emulsion into its components, is of importance in chemical and petroleum process industries. In recent years, oil−water separation has become increasingly important due to stricter regulations on discharge of oily wastewater and for managing oil spill accidents. Several techniques are currently available for the removal of oil from aqueous emulsions. These include thermal,1 mechanical,2 chemical,3 and electrical methods4 among which the chemical methods,3 i.e., the addition of demulsifying agents to the emulsion, are the most ubiquitous. Several intriguing methods5−51 have also been developed in recent years that allow oil− water separation by utilizing unique wetting characteristics of certain materials, such as kapok (a plant fiber),5 activated carbon,6,7 hydrophobic aerogels8−10 and cross-linked polymers11−15 that selectively absorb oil from water. The new classes of materials include magnetic foams,16 specially fabricated films,17 carbon nanotubes,18,19 and meshes coated with hydrogel,20−22 attapulgite,23 zeolites,24 polycarbonate,25 and other materials.26−31 Hygro-responsive membranes,32−39 which exhibit superhydrophilicity in air and superoleophobicity under water, have also been used. Membranes coated with cellulose,40 polyacrylonitrile,41 poly(ether imide)/polyvinylpyrrolidone42 and other novel materials43−48 have been used successfully. Several textiles49−51 have received significant attention for their high oil absorption capacity, mechanical flexibility, low density, ability to withstand harsh conditions and for their low cost. It has been pointed out in the literature that, in many of the above gravity31−34 driven oil separation processes, the flow rates are not easily regulated. This is because the window of operation has to © XXXX American Chemical Society
be such that the gravitational force must not exceed the intrusion (or Laplace) pressure20,23,25,33,52,53 of one of the components in the porous medium. One objective of the present work is to develop separation methods in which the rate of separation can be easily controlled. One of these methods uses hydrophobized glass wool that selectively absorbs oil from an oil−water emulsion. When an external shear force is applied by passing compressed air, the selectively absorbed oil migrates along the glass fibers that is subsequently collected through a conduit at a rate depending upon the applied force. Our second separation method uses porous hydrophobic zeolite pellets in a packed absorption column54 where the selectively absorbed oil is subsequently recovered by heating of the pellets with a hot fluid. A two-step separation process that uses hydrophobic glass wool in conjunction with the thermal swing induced extraction of the oil from a zeolite column is capable of achieving very high oil yields (ca. 97%).
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PREAMBLE Transportation of liquids from one reservoir to another via capillary action in narrow spaces of a porous medium in conjunction with gravity is well-known.55 However, this method does not usually offer much latitude in controlling the flow rate of the liquid being transported. However, an excellent control over the flow rate can be achieved if an external shear force is applied Received: August 7, 2016 Revised: September 13, 2016
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Figure 1. A bundle of hydrophobized glass fibers is immersed in a pool of liquid in the present of a positive air pressure. (A) With a nonwetting liquid, there is no flow along the walls of the fibers. (B) When the liquid wets the fibers, it rises through their interstitial spaces and is dragged along further by the inflowing air. The rising stream of the liquid breaks up into small droplets by Rayleigh instability. (C) Experimental verification of scheme A, in which the liquid (dodecane) streams out of the container and undergoes Rayleigh instability. (D) Experimental verification of scheme B, in which an oil (dodecane) is extracted from an aqueous emulsion.
Figure 2. Protocol I. (A) Hydrophobic glass wool is plugged into a polypropylene tube with its tail spreading out like a feather. (B) When immersed in an oil/water emulsion, the hydrophobic glass fibers are wetted by only oil droplets, which are then dragged along their length by the outflowing air. Protocol II. (C) Oil adheres to a hydrophobized zeolite from an oil/water emulsion, which then penetrates its pores. Upon heating, the absorbed oil is extracted in vapor form, which is condensed and collected.
liquid occurs along the fiber walls. On the other hand, if the liquid wets the fibers and rises somewhat through their interstitial spaces, air drags the liquid along the fibers, thus transporting it out of the container (Movie 1). The discriminating factor here is the wettability that may be controlled by appropriate surface modification of the glass fibers. When the liquid is an oil/water emulsion, oil selectively adheres to the hydrophobized glass fibers that exits the container by flowing over the external surfaces of the fibers in the form of thin liquid streams, which eventually undergo Rayleigh instability (Movie 2). At the end, the oil is transported in the form of small droplets. The thermodynamic criterion for the oil drops to adhere to the surface of hydrophobic fibers is that the free energy of adhesion
to the liquid that flows against gravity. This external force driven flow is feasible if a liquid partially wets the fiber (i.e., the contact angle is less than 90°) (Figure 1). The liquid being transported undergoes the well-known Rayleigh instability, and the ensuing droplets clinging on to the fiber can then be transported faster using the shear induced drag force exerted by the flowing gas.
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PRINCIPLE OF CAPILLARY PUMP The basic principle of the capillary pump can be illustrated with the help of a bundle of a few hydrophobized glass fibers that are immersed in the liquid in the presence of a positive flow of air as depicted in Figure 1. If the contact angle of the liquid on the glass fibers is such that it undergoes a capillary depression, no flow of B
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Figure 3. (A) Hydrophobic glass wool plugged into a small tube was inserted through the stem of a round-bottom glass flask. The flask was partially filled with ethanol−water mixture and pressurized with air. Liquid transported through the glass wool and the plastic tube was collected in a graduated cylinder. (B) Variation of the flow rates Q of ethanol−water mixtures in terms of their surface tensions γ. These experiments were carried out at an air pressure of 9 psig. Inset shows how water beads up on hydrophobic glass wool fibers when water is sprayed upon it. (C) Variation of the hydrostatic head (h) of water−ethanol solutions as a function of its surface tension. Inset shows water droplets deposited on the hydrophobic zeolite. (D) The video captured images of the liquid column before it intrudes the zeolite bed at different volume percent of water in the solution (marked below each image).
be negative,56 which will be the case when the following inequality is satisfied: γsw − γso < γow
method in conjunction with another novel method, namely, a thermal swing induced demulsification, improves the overall separation efficiency particularly when the emulsion is stabilized using a surfactant.
(1)
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Where, γsw, γso and γow are the fiber-water, fiber−oil, and oil− water interfacial tensions. γsw and γso can be expressed in terms of the Young’s equations:
γw cos θsw = γs − γsw
(2)
γo cos θso = γs − γso
(3)
THERMAL SWING INDUCED EXTRACTION OF OIL FROM A POROUS BED Thermal swing57 as used here takes advantage of the absorption of oil in a porous medium at a low temperature followed by its extraction using a two-phase flow assisted evaporation of the oil at a higher temperature. In order to accomplish this objective, we used porous zeolite, the surface of which was rendered hydrophobic while its internal pores remained unmodified. When an oil−water emulsion passes through a bed filled with such surface modified zeolite, oil penetrates inside their pores while water flows through the interstitial spaces of the particles. In order to extract the absorbed oil, the bed is moderately heated with another immiscible fluid. We describe below the details of operation of both the capillary pump and the thermal swing induced extraction of oil from an aqueous emulsion. However, before elucidating these processes in details, we first describe briefly the methods used to modify the surfaces of both the glass wool and the zeolite and how they were characterized (see also the methods section).
where γs and γo are the surface tensions of water and oil, respectively, and γs is the surface energy of the fiber. Substituting eqs 2 and 3 in eq 1 yields γo cosso −γw cos θsw < γow
(4)
A fair assessment of whether the oil droplets would adhere to a surface can be made by examining the above inequality (eq 4). For example, the contact angle of water on a typical perfluorinated surface is 112°, whereas the contact angles for octane, decane, dodecane, tetradecane and hexadecane on the same surface vary from 63° to 78°. Since the interfacial tension of an alkane with water is 51 mN/m, it is easy to show that the inequality condition (eq 4) is satisfied with all the above alkanes. Naturally, thus, the oil droplets are expected to deposit and coalesce on the surface of the fluorosilane treated glass fiber that are then transported along its surface in the form of thin streams and droplets while the water remains behind in the reservoir. Based on the above depiction of oil−water separation, we can surmise that the overall efficiency of separation would be enhanced by increasing the area of contact of oil and the fibers, which can be achieved using an agglomeration of a myriad of fine fibers, such as hydrophobized glass wool (Figure 2) plugged inside a plastic tube. In the presence of an external pressure, the oil droplets deposited on the glass fibers would thus migrate along its surface and eventually exit to another reservoir (Figure 2) through the plastic conduit. We name this device a capillary pump that constitutes the first protocol for separating oil from water. Although this method of separation is quite versatile, this
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CHEMICAL MODIFICATION AND CHARACTERIZATION OF GLASS WOOL AND ZEOLITE Glass Wool. Glass wool made of Pyrex glass fibers with a nominal diameter of ∼2.86 μm was hydrophobized by reacting it with 1H, 1H, 2H, 2H perfluorooctyltrichlorosilane from a liquid phase (see Materials and Methods). The hydrophobicity of the glass wool was evident in that the water droplets sprayed on it beaded up upon making contact with the individual fibers (inset of Figure 3B). Its surface energy could be quantified by studying its wettability toward various mixtures of ethanol and water, exhibiting an appreciable range of surface tension.58 In particular, one end of a polypropylene tube, partially plugged with glass wool (Figure 2A), was inserted into a round bottomed glass flask C
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Figure 4. (A) Schematic shows the experimental set up used for measuring the rise of the temperature of the suspension resulting from the heat of immersion of treated and untreated zeolites (type 13X) in various liquids. (B) A plot of the heat of immersion per unit mass of zeolite (Δh) in different liquids as a function of time t for both untreated (Z) and hydrophobic zeolite (Z (H)). Solid lines passing through the experimental data points serve as guide to the eyes. Inset shows how (Δh) varies as a function of t when treated zeolite particles are immersed in water, octane, and octane/water emulsion.
hydrostatic pressure needed for a liquid to flow through it. In this method, a hydrophobic glass column was first partially packed (∼9 cm high) with the zeolite beads the head space of which was gently filled by water and the mixtures of water and methanol with the aid of a syringe pump that allowed dropwise entrance of the liquid in the column. As the liquid head accumulated above the zeolite bed, the gradual rise of the air−liquid meniscus was video-recorded until the liquid intruded into it. This hydrostatic pressure head decreased with the surface tension of the liquid, as the intrusion pressure decreased owing to the increasing wettability of the particles. From an extrapolation of this hydrostatic pressure head versus the surface tension of the liquid, it is observed that when the surface tension of the liquid is less than ca. 28 mN/m, it spontaneously penetrated the bed without exhibiting any accumulation above it. A zero intrusion pressure suggests that the intrinsic contact angle of the liquid in the pore space is equal to 90°, suggesting that γsl ≈ 28 mN/m, or γs ∼ 10 mN/m (eq 5), which is indeed the same as that obtained with the hydrophobic glass wool. Additional Characterization of Hydrophobic Zeolite. While the experiments as described above provide an estimate of the surface free energy of the treated zeolite particles, they do not say anything about the penetration of liquid into their pores. With the anticipation that penetration of a liquid into the pores would lead to the evolution of heat due to its interaction with the high energy sites spanning a large pore surface area, we carried out the following heat of immersion measurement.61 Specifically, a few zeolite particles were immersed in a small amount of such liquids as water, ethanol, octane and octane/water emulsion in an insulated vial, the temperature of which was measured with a thermocouple. As expected, the increase of temperature was markedly higher with the H-bonding water than with the nonpolar octane in general, which can be converted to the evolution of heat by multiplying it with the net thermal mass of the liquid and the zeolite. As shown in Figure 4, heat of immersion of the untreated zeolite in water is larger than the treated zeolite, while they are about the same for octane. For treated zeolite immersed in both
(Figure 3A), while its other end was inserted into a graduated cylinder (Figure 3A). After partially filling the flask with ethanol− water solution, a positive pressure (9 psig) was generated inside it with air that sheared the liquid along the glass wool fibers. The liquid was eventually collected in the cylinder, the rate of which strongly depended upon the surface tension of the ethanol− water mixture. When the concentration of ethanol in water was below ∼45%, no flow of the solution was observed. This corresponds to the condition of liquid−solid contact angle of 90° implying that the solid−liquid interfacial free energy (γsl) is same as the solid surface free energy (γs). If we now assume that the surface treated glass wool interacts with the liquid mainly via dispersion interaction, and considering the fact that the dispersion component of the surface tension of alcohol is nearly the same as that of water (22 mN/m), we can estimate the surface free energy of the solid using the Good−Girifalco− Fowkes equation:56 γs = γsl ≈ γs + γl − 2 γsγld
(5)
γdl
where is the dispersion component of the surface tension of the liquid. Using the values of γdl and γl as 22 and 28 mN/m (corresponding to the solution that demarcates flow from noflow) we obtain γs ∼ 10 mN/m, which is typically the value of the surface free energy of a surface composed of primarily the −CF3 group.59 Surface Characterization of Zeolite. The strong hydrophobicity of the zeolite particles (average diameter ∼2.5 mm) that were hydrophobized by reacting them with 1H, 1H, 2H, 2H perfluorooctyltrichlorosilane (see Materials and Methods) became evident in that the small water drops beaded up on them with a contact angle of 141° (±2°) (see also Supporting Information). When these particles were gently placed on the surfaces of the ethanol−water solutions, they kept floating as long as the volume percent of water in the solution was more than 50%. From these observations, we infer a critical surface tension (∼27 mN/m) of floatation of the hydrophobic zeolite particle.60 A somewhat more direct estimation of the wettability of the particles in a packed bed was made by measuring the D
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Figure 5. Different parts of the set up: (A) round-bottom glass flask containing an emulsion being vigorously agitated with a magnetic stirrer (B). A pressure gauge (C) was connected to the tube through which air was purged into the flask. The capillary pump (D) was connected to another glass flask (E) and then a graduated measuring flask (F) for collecting oil. A small condenser (G) salvaged some of the oil vapor escaping with the flowing air. The condenser outlet was connected to a flow meter (H). (B) At the end of 2 h, the turbid emulsion became clear and clean oil (here it is tetradecane) was collected in the measuring flask.
Extraction of Oil from an Aqueous Emulsion Using Capillary Pump. We demonstrate the principle of the capillary pump by extracting oil selectively from an oil−water mixture that was emulsified by vigorous stirring. In this turbid emulsion, the hydrophobic glass wool with its one end plugged inside a polypropylene tube was immersed, while the other end of the tube was placed inside a graduated cylinder (Figure 5). While the emulsion was being continuously stirred, a positive pressure was generated in the flask using clean air that resulted in the extraction of the oil by the glass wool and its eventual transport through the plastic conduit into the cylinder. As the oil was extracted, the turbid emulsion (Figure 5A) continued to become transparent (Figure 5B). The flow rate of each alkane (octane, decane, dodecane, tetradecane, and hexadecane) was obtained by measuring the volume collected in the graduated cylinder as a function of time, which increased with the applied air pressure linearly at first, but tended to saturate at a higher pressure. As a first approximation, the flow rate of each alkane can be modeled according to the well-known Darcy’s law:62-
the liquids, there is a notable induction time, following which heat evolves. We feel that the differences in this induction time observed with both water and octane for treated and untreated zeolite is due to a kinetic barrier brought about by the surface treatment. On the other hand, when a liquid composed of small molecules such as ethanol is used, the difference of this induction time is small in both the treated and untreated zeolites coupled with the fact that the evolution of heat is similar in both cases. These observations suggest that the internal area of the porous zeolite is not substantially reduced by reacting with the perfluoroalkylsilane during the surface modification process. The previous thermodynamic argument (eq 4) suggests that a typical alkane would adhere selectively to perfluoroalkylsilane treated zeolite from an aqueous suspension, following which it should penetrate into its pore space. When that happens, the filled pores should thwart any significant penetration of water into it. We may thus envisage that when the surface-treated zeolite is immersed in an oil−water emulsion, the evolution of heat would be similar to that of pure alkane, which is indeed borne out in the experimental observation (inset of Figure 4B). The fact that the oil absorption capacity of the zeolite bead is not altered after it is silanized can also be demonstrated by the following simple experiment. That is, some of the untreated and the treated zeolite (13X) particles (5 g each) were immersed in octane overnight. The following day, the octane was removed by filtration, and any excess oil adhering to the outer surfaces of the particles were botted out. From the changes of the weights of the oil soaked particles, and using the known density of octane, the oil absorption capacities were found to be 0.493 and 0.499 L per 1 kg of the treated and the untreated particles, respectively. This observation suggests that the oil absorption capacity of zeolite is not altered significantly following the silanization process.
⎛ ΔP ⎞ μQ ⎜ ⎟ = − ⎝ L ⎠ κA
(6)
where Q is the volumetric flow rate (m /s), κ (m ) is the intrinsic permeability of the medium, A (m2) is the cross-sectional area to flow, ΔP (MPa) is the total pressure drop L (m) is the length over which the pressure drop occurs; ρ (kg/m3) and μ (Pa·s) are the density and the viscosity of the alkane, respectively. This Darcy’s law is, however, rigorously applicable for singlephase laminar flow. In our case, with two-phase flow, as the liquid stream breaks up into droplets, its resistance to the motion along the fiber depends upon the slope of the liquid surface at the three phase contact line63 as well. We thus use a slightly modified Darcy’s law: 3
E
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Figure 6. (A) Variation of flow rates of alkane (Q) as a function of ΔP/L in a capillary pump. (B) A plot of Qμ/A tan θ as a function of ΔP/L reveals that in the low pressure region Q increases almost linearly with ΔP; however, in the high pressure regime, the curve tends to flattens out. Inset shows that the modified air flow rate Qμ/A varies linearly with pressure drop for each of the test alkanes in both high and low pressure regimes. (C) Table shows measured (ρm) and actual (ρa) densities of the alkanes collected using capillary pump. (D) Efficiency (εoil) of separation for various alkanes using capillary pump.
Figure 7. A glass column (A) was tightly packed with treated zeolite particles. A transient emulsion, stabilized by vigorous mechanical stirring (B) was pumped through the column from a flask (C) using a constant volume displacement pump (D) connected to a flexible tube. Before entering the column, the tube was connected to a glass bifurcator (E) that was connected to another flask (F) containing pure water that could be heated separately. The outlet of the column could also be switched between the flasks containing the emulsion and a condenser (G). The condenser was connected to two round bottomed glass flasks (H) and (I) in series for collecting the condensate. Three thermocouples (T1, T2, and T3) connected to a digital meter read the temperatures at the bottom, middle, and topmost parts of the column. The emulsion was maintained at a temperature of 10 °C by circulating a mixture of cold water and polyethylene glycol in the jacketed flask. Dry air at a controlled pressure (J) was purged through the top of the column as soon as the emulsion began to flow. During the desorption phase, hot distilled water and/or air were passed through the column. Desorbed gaseous alkane was carried along with the outflowing air through the bottom of the column to the condenser (G) where it was collected as liquid in the flask (I). Flask (I) was connected to another condenser (K) to condense part of the escaping alkane vapor, the rest of which was fed back to flask (C) through a silicone tube (L) connecting C and K.
⎛ ΔP ⎞ μQ ⎜ ⎟ = − ⎝ L ⎠ κA tan θ
wool. Plots of Qμ/A tan θ as a function of ΔP/L for the different alkanes (Figure 6) shows a nice clustering of all the data about a straight line, which, however, diverge in the high pressure regime. The airflow rate, however, increases linearly with ΔP/L in both high and low pressure regimes (inset of Figure 6B). The nonlinear deviation of the liquid flow at high pressure is likely due to inertial effects that can be accounted for with a Forchheimer64 correction to the Darcy’s equation. We refrained from such
(7)
where θ is the contact angle subtended by the alkane on the treated glass wool. While it was not straightforward to measure these angles of the liquids on the fine glass wool fiber, we used their values on a glass slide that was silanized under the same conditions and following the same procedure as that for the glass F
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Figure 8. (A) Sequence of octane absorption by hydrophobic zeolites (type 4A) in an absorption column. In the first figure, A*, B* and C* refer to untreated zeolite, glass beads, and silanized zeolite, respectively. Both the untreated and the treated zeolite become brown once the emulsion is passed through the column. (B) Sequence of octane desorption by the zeolites. The treated zeolites become completely white, while the untreated ones at the top of the column remain brown. (C) Variations of the temperature of the top (T3), middle (T2) and bottom (T1) portions of the absorption column as a function of time. (D) Desorption temperatures (Td) and boiling points (Tb) of the alkanes used in our experiments.
that resulted in a well irrigation of the emulsion in the packed bed. Under this condition, alkane adsorbed in the zeolite pore that was evident from the fact that the entire bed became dark brown in about 2 min (Figure 8A and Movie 3), following which, hot water (75 °C) was passed through the bed accompanied by air. As the alkane evaporated via equilibration with the flowing air, the brown zeolite particles turned completely white (Figure 8B and Movie 4) in about 3 min, indicating that the alkane exited the pores. The recovered alkane vapor and liquid water flowed down the column, which were then split so that water was collected in one reservoir, while the alkane vapor was passed through a condenser and collected to a different flask. A small amount of condensed water collected in this flask could be removed easily using a capillary pump of the type described in a preceding section. This absorption−desorption cycle can be repeated until there is no trace of absorption of alkane by the zeolite bed from the emulsion. One point to be mentioned here is that the untreated zeolites (Figure 8B) present at the top of the column as an indicator remained brown, while the treated zeolites turned white at the end of the desorption phase. As these hydrophilic zeolites cannot absorb an alkane selectively from water, their pores get filled with water and alkane indiscriminately that require prolonged heating accompanied by the flow of dry air to remove the internal water. Thus, the demulsification process that is possible with a dual zone zeolite (hydrophobic surface, but unmodified oleophilic interior) is not feasible with the untreated zeolites. The bed temperatures at which the alkanes begin to desorb (Td) are significantly lower than their boiling points (Tb) (Figure 8D). Thus, the desorption of the alkane is not due to boiling; rather it is governed by the equilibration of the vapor from the liquid trapped in the inner space of the zeolites with the flowing air. A plot of the temperatures of the column at three locations, top (T3), middle (T2), and bottom (T1) (Figure 8C) show that they all increase and reach more or less the same value, although a slight gradient was noticeable, i.e., T3 < T2 < T1. As the hot water and air flow down the bed, evaporation of absorbed alkane begins
detailed analysis at present as all the subsequent experiments of oil−water separation were performed in the region where the flow rate varies linearly with the applied pressure. Density of each alkane collected after separation was measured at a temperature of 23 °C, which was indeed close to that of the pure alkane (table of Figure 6C). We also calculated the separation efficiency εoil of the process using εoil = (Vc/V ) × 100
(8)
where Vc is the volume of oil collected after separation and V is the volume initially added to the emulsion. The data summarized in Figure 6D show that εoil is in the range of 84% to 88%. While this efficiency is excellent, it is possible, in principle, to improve it further. The way we assembled the apparatus, some of the oil escaped the collection flask along with the flowing air. Even though a condenser was used to collect some of the alkane from the flowing vapor, not all the oil could be salvaged as some vapor escaped to the ambient. In the later version of the apparatus, we created a loop in which this vapor is recycled back to the source that resulted in much improved efficiency (see below, Figure 7). Extraction of Oil from Aqueous Emulsion Using Thermal Swing Absorption in Hydrophobic Zeolite. We now describe the method used to extract oil from an aqueous emulsion using a thermal swing enabled absorption and evaporation of oil using hydrophobic zeolite. This can be accomplished with a column filled with surface modified zeolite, which can be either type 4A and/or 13X variety. In these experiments, there was no substantial advantage of one over the other type of zeolite except that 4A is a good indicator of oil absorption based on the change of color. As oil penetrates the pore of 4A type zeolite, it changes color from white to dark brown more prominently than 13X, which upon extraction of oil becomes white again. The first set of experiments were carried out with a column filled with hydrophobic zeolite 4A, through which a mechanically stabilized alkane−water emulsion (precooled to 10 °C) was passed along with a concurrent flow of clean air (Figure 7). The flow rates of the water and air were adjusted to the ratio of 3:4 G
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Figure 9. (A) A small amount of hydrophobic glass wool that was stuffed in between two parallel glass plates and placed vertically in a beaker containing oil/water emulsion. Upon application of a suction pressure at the other end of the glass slides, the emulsion is expected to climb upward. The expectation that the oil droplets would adhere to the individual fibers is verified in experiment (B). (C) Sequences of the absorption of dodecane in hydrophobic zeolite particles from a surfactant stabilized aqueous emulsion. A* and B* refer to zeolites of types 4A and 13X, respectively. (D) Sequences of dodecane being removed by the zeolites as evidenced by their color changes from brown to white. (E) Variations of the temperatures of the top (T3), middle (T2), and bottom (T1) portions of the absorption column are plotted as a function of time. (F) Three round-bottom glass flasks (a, c, and d) were connected in series via two capillary pumps (b and e). Flask (a) containing residual the emulsion was purged with dry air. The extracted oil was collected in flask (d) as a clear liquid.
from the top of the column, which then proceeds downward, as expected. While the extracted oil was quite pure as evident from the measurements of their densities, there was one problem associated with this technique in that after about 70−80% of the oil was extracted following several passages of the emulsion through the zeolite bed, some water began to penetrate the pores of the zeolite that hindered its selective absorption of oil from the emulsion. At this point, the bed can be regenerated by heating it with hot air that brings it back to the original operating condition. Alternately, the heating of the bed can be carried out with only hot air. As the replacement of the step involving hot water with air compromises efficient heat transfer throughout the bed, extra heat needs to be supplied externally to carry out the operation. This could be accomplished by heating the entire column with a heating pad as discussed below. At this point, we illustrate the highly efficient method of extracting oil from a surfactant and agitation stabilized oil−water emulsion, in which the thermal swing induced extraction is carried out in conjunction with a capillary pump (see below). Extraction of Oil from an Aqueous Emulsion Using Thermal Swing and Capillary Pumping. An emulsion was prepared using dodecane and water with a small amount of sodium dodecyl sulfate under vigorous agitation. An approximate estimation of the size of the oil droplet in such an emulsion and its ability to adhere to a hydrophobic glass wool was made by passing the emulsion between two parallel glass plates in which small amount of the glass wool was already embedded (Figure 9). When the emulsion was pulled in between the glass plates with an externally applied suction pressure, small oil droplets gradually deposited on the individual fibers of the glass wool, the average size of which was about 10 μm. This emulsion, precooled to 10 °C, was pumped into a column loaded with hydrophobized 13X type zeolite with a concurrent flow of air. As the 13X zeolite absorbed the oil, it did not turn as brownish as did the 4A type zeolite. In order to follow the absorption of oil in the zeolite,
small amounts of hydrophobic 4A type zeolite were placed above and below the 13X type zeolite bed, the change of color of which indicated the onset of absorption and extraction of the oil from the 13X zeolite bed in a single cycle (Figure 9) of thermal swing. The headspace of the rest of the column was filled with glass beads (diameter 2.4 mm, McMaster Carr). Following absorption of oil by the 13X type zeolite, pure air was forced downward by supplying heat to the column externally with heating pads, simultaneously. As the alkane evaporated from the zeolite via equilibration with the air, those present at the top of the column turned white (Figure 8B). The recovered alkane vapor that flowed downward was condensed and collected in a flask as liquid. Any uncondensed alkane vapor was recycled back to the source where it was further condensed that ensured minimal loss of the alkane in the ambient. Temperatures in the column bed were again monitored by inserting thermocouples at three different locations in the bed (Figure 9). After 10 absorption− desorption cycles, oil could be recovered with a separation efficiency (ε1) of 80%, after which the residual oil could not be separated efficiently using this method. The residual emulsion was then subjected to a two-stage capillary pump separation technique that recovered oil from that remaining in the emulsion with a separation efficiency (ε2) of 85%. On the basis of the initial loading of oil in the emulsion, the total efficiency (ε1 + ε2 − ε1ε2/ 100) of oil extraction was 97%. Summarizing Comments. Two facile techniques have been described here for effective separation of oil from an aqueous emulsion. The first method employs low surface energy hydrophobic glass wool that selectively absorbs oil from water, which can be transported out of the source by an aerodynamic drag. This capillary pump guarantees a fast and efficient way to remove oil from an emulsion. In our effort to characterize the surface property of the glass wool, we also developed a new method of estimating the surface energy of such a disordered medium simply by noting the surface tension of the liquid that H
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mixtures of fluoro- and hydrido-functional silanes, could also be explored in future. While more research would be needed to compare and contrast the methods described here with the conventional separation technologies, these could find specific applications where thermal stability or very close boiling points of the components precludes separation by distillation, or when their close densities preclude separation by gravity, cyclones, or centrifugation. It is also tempting to consider that the thermal swing method could be used in analytical separation of components, as the extraction temperature of each would depend upon the airflow rate and the operating temperature both of which can be easily modulated. In order to make such progresses, detailed knowledge of the vapor−liquid equilibrium in the specific flow conditions and the related (mass, momentum and heat) transport processes need to be developed, which are the subjects of ongoing research in our group.
demarcates the condition of its flow and no-flow when attempt is made to move the liquid against gravity. The method of estimating surface energy contrasts the wellknown and well-established method of Zisman. Zisman’s method of estimating surface free energy (rather the critical surface tension of wetting) of a solid surface is based upon a putative liquid that would just spread on the solid as a thin continuous film. This leads to the conditionγl = γs − γsl , which for rough surface is modified by the Wenzel roughness parameter (r) as γl = r(γs − γsl). The method described here is, in principle, independent of roughness. A more refined approach would, however, need to be considered in order to account for hysteresis, which will be the subject of a future study. The second method used a thermal swing induced extraction of oil from an aqueous medium by coupling it with a two-phase flow of air and the emulsion through a porous bed of hydrophobic zeolite. Here, in connection with the characterization of the zeolite particles, we showed that the measurement of heat of immersion is ideal in characterizing these particles. While the measurement of the heat of immersion is classical,61 the dual properties of the zeolite, in that its surface is hydrophobic but the inner pore is oleophilic lead to certain nontrivial results, especially with an oil−water emulsion. Based on these measurements, we inferred that the surface hydrophobicity of the zeolite creates a kinetic barrier to the penetration of the liquid into its inner pore space that also discriminates oil from water when it is dispersed in an aqueous emulsion. With the well-irrigated flows of the emulsion throughout the column, the oil is preferentially absorbed by the zeolite that can be extracted by swinging the bed to a higher temperature. The remarkable observation here is that the oil can be extracted from the zeolite at a temperature much lower than its boiling point. The thermal swing method could also be operated at a faster rate than the capillary pump. The combination of above two methods, however, garnered a highly efficient (97%) separation of oil from an aqueous emulsion even when it is stabilized by a surfactant and strong agitation. Both the surface modified glass wool and the zeolite can be used repeatedly (for at least 40 to 50 cycles) and/or be regenerated when they become wet by passing purified air or nitrogen over them. Thus, we believe that these technologies could potentially be scaled up for effective low-cost oil/water separation at an industrial scale. Several possibilities to extend the studies reported here present themselves. An obvious direction for future research would be to study the effect of temperature in the oil−water separation process. Our preliminary studies indicate that temperature could have a profound effect on the separation oil using a hydrophobic glass wool (Supporting Information). It would be prudent to consider any possible role played by the temperature gradient that might be set up along the length of the plugged wool, thereby giving rise to a Marangoni force, which could enhance the effect produced by the air drag alone. Another possible direction would be explore other methods for modifying the surface of glass wool and the zeolite beads beyond what used in the current work. An interesting recent paper65 used a hydridosilane in the preparation of a hydrophobic sponge for extraction of oil from water. In the current paper, we used a fluorosilane as opposed to a hydridosilane because the former provides a larger driving force for the deposition of the oil droplets owing to its very low surface energy. However, in order to identify the full parameter space for the oil−water separation, surface modification using a hydridosilane, and/or various
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MATERIALS AND METHODS Materials. Zeolite particles of types 4A (free diameter of 4.2 Å) and 13X (free diameter of 7−7.7 Å, Chem ID 63231-69-6) were obtained from Davison Chemical and Exxon Mobil, respectively. Pyrex fiber glass wool was procured from SigmaAldrich (8 μm pore size, Borosilicate glass; Z25589-0). 1H, 1H, 2H, 2H perfluorooctyltrichlorosilane (97%) was purchased from Lancaster Synthesis, Inc. Heptane was obtained from Fischer Chemicals and all other test alkanes (octane, decane, dodecane, tetradecane, and hexadecane) from Sigma-Aldrich. High purity distilled water (surface tension ∼ 73 mN/m) supplied by a steam-distillation unit (Distiller warehouse) was used for all the experiments. All contact angle measurements were performed with a home-built equipment. Oil Flowing through Capillaries Undergoes Rayleigh Instability. Five end-sealed glass capillaries (diameter ∼ 1.23 mm) were bundled tightly and inserted through the mouth of a glass flask containing 25 mL of octane. The flask was purged with clean dry air (3.5 psig) generated from a FT-IR purge gas generator and tilted such that the capillary bundle just touched the liquid inside. Octane rising through the interstitial spaces between the capillaries was video-recorded using a variable focal length microscope (Infinity) attached to a camera (Hitachi). The same experiment was repeated with an octane/water emulsion (1:2 v/v) with air being purged at a pressure of 4 psig. Surface Treatment of Glass Wool. As received glass wool (8 g) was heated to a temperature of 180 °C in a hot air oven for 15 h in order to remove the entrapped moisture and contamination. Following this treatment, it was cooled down to room temperature and immersed in a solution of 300 mL of chloroform (GRACS 99.8% pure) containing four drops of 1H, 1H, 2H, 2H perfluorooctyltrichlorosilane. After 24 h, the solution was decanted and the wool was cleaned thoroughly in pure chloroform, following which it was heated at 60 °C in hot air oven for 3 h. Surface Treatment of Zeolite Particles. Fresh zeolite particles were heated to a temperature of 180 °C in a hot air oven for 15 h in order to remove the entrapped moisture and contamination. The particles were then transferred to a clean round bottomed glass flask attached to a rotavapor, and heated to a temperature of 250 °C using two high power heat guns. Temperature of the zeolites were monitored using a k-type thermocouple (Omega Engineering, Inc.) connected to a digital thermometer (Thermolyn obtained from Mirak). Another round bottomed glass flask containing some 1H, 1H, 2H, 2H
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DOI: 10.1021/acs.langmuir.6b02938 Langmuir XXXX, XXX, XXX−XXX
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Oil/Water Separation Using Hydrophobic Glass Wool. A round-bottom glass flask, placed atop a magnetic stirrer (Corning stirrer PC-353), was firmly clamped and tilted to an angle of 40° from the horizontal. Next, some hydrophobic glass wool plugged into a polypropylene tube (inner diameter ∼0.6 cm) up to a length of 6.4 cm was inserted through the mouth of the flask, which was then sealed tightly with a rubber stopper (Septa, white rubber obtained from Aldrich Chemical Co., Inc.). The other end of the tube was inserted through the mouth of another round bottomed flask, which was also sealed the same way. A graduated measuring cylinder and a small condenser were tightly attached to the lower and upper stems of the flask, respectively. A flow meter (Aalborg Instruments FM-092-04) was next connected to the upper end of the condenser. Following this, a transient emulsion containing 30 mL of alkane and100 mL of DI water was added to the wool-stuffed glass flask and stirred vigorously using a magnetic stir bar (obtained from McMaster Carr). Dry air was purged into the flask at a controlled flow rate for pressure varying from 0.5 to 9 psi (U.S. Gauge Co.), and oil absorbed by the wool gradually started collecting in the measuring cylinder. Time needed to collect a certain volume of liquid was recorded as a function of air pressure for all the test alkanes. Oil/Water Separation Using Hydrophobized Zeolites. A glass column (obtained from ACE glassware; inner diameter 1.7 cm, outer diameter 2.2 cm and length 34 cm) was firmly clamped to keep it vertical. The emulsion was pumped to this column from a 1000 mL flask containing the using a constant volume displacement pump (Masterflex; model 77201-60) that periodically compressed and relaxed a flexible tube (Inner diameter 3.37 mm, outer diameter 6.43 mm, obtained from Masterflex). Before entering the column, the tube was connected to a glass bifurcator that was connected to another 1000 mL flask containing pure water that could be heated separately. This was done in order to control the flow of water and emulsion alternatively through the column. The outlet of the column could also be switched between the flasks containing the emulsion and a condenser. The condenser was connected to two round bottomed glass flasks in series for collecting the condensate. The alkane vapor condensed in the second flask, which is a capillary pump that removes any condensed water. Any uncondensed oil vapor was looped back to the main flask containing the emulsion through a tube (outer diameter 8.25 mm). Three thermocouples (k-type) from a digital thermometer (acquired from Fluke; model 2176 A) were then inserted into the column through the outlet tube and placed at heights of 5 cm (bottom), 10.3 cm (middle) and 16.7 cm (top). In the next step, 26 g of treated zeolite particles were tightly packed inside the column with glass beads (27 g) both at the top and at the bottom of the packed bed to ensure uniform flow distribution. The total bed height was 22.5 cm of which the zeolite particles occupied a height of 14 cm. A small zone of untreated particles was kept inside the column, occupying a height of 2 cm and separated from the treated zeolite by a layer of glass beads. Following this, 60 mL of octane-in-water emulsion (1:2 v/v) was poured into the glass flask, injected from the top of the glass column using the constant volume displacement pump at a flow rate of 0.0022 m3/h and recycled back to the same flask from the bottom of the column. A mechanical stirrer operated by a motor (Universal Electric Company) was used inside the flask for producing a uniform emulsion. The emulsion was maintained at a temperature of 10 °C by circulating a mixture of cold water and polyethylene glycol (antifreeze) in the jacketed flask. High
perfluorooctyltrichlorosilane was maintained at a temperature of 40 °C by constant heating, and purged continuously with ultra high purity nitrogen (0.004 m3/h) to transport the silane vapors to the flask containing the zeolite particles through a glass pipet connecting the two flasks (Figure 10). This process was carried
Figure 10. (A) A round-bottom glass flask containing Perfluorooctylsilane was placed inside a heating muffle (B) and purged with pure nitrogen, the flow of which was regulated using a flow meter (C). Silane vapors were transported to another glass flask (D) containing zeolite particles (Z) and attached to a rotavapor (E). The particles were constantly heated using heat guns (F) and (G), and the temperature was displayed in a digital thermometer (H).
out for 6 h, with an uninterrupted heating of the particles to 250 °C. This ensured good bonding66 between the silane and the active sites of the outer surface of the zeolite. Characterization of Glass Wool. A polypropylene tube (inner diameter ∼0.6 cm) plugged with the hydrophobic glass wool up to a length of 4.4 cm (Figure 2A) was inserted through the stem of a round bottomed glass flask that was clamped to a stand at an angle of 40°. The other end of the tube was inserted into a graduated measuring cylinder. The flask was then filled with 75 mL of ethanol−water mixture (v/v 1:4) such that the liquid level was somewhat below the mouth of the tube inside, and purged with dry air (9 psig) from an FT-IR purge gas generator (Whatmann). As the liquid continued to collect in the cylinder, the rate of collection was estimated from the volume accumulated within a given time window. The same procedure was repeated for ethanol−water mixtures of different compositions. Characterization of Zeolite. A glass vial insulated with glass wool (7 g) was filled with a weighed amount of hydrophobic zeolite particles of type 13X and placed inside a glass beaker. 1 mL of a given liquid was then injected into the vial using a glass syringe, following which the upper space of the vial was quickly plugged with hydrophobized glass wool (0.42 g) to prevent heat loss. Change in temperature was recorded by a digital thermometer (Fluke Digital Thermometer 2176-A). While repeating the same experiment with octane-water emulsion, 1 mL of a freshly prepared emulsion was injected into a glass vial, weighed and stirred vigorously atop a magnetic stirrer. The thermocouple was positioned inside the vial and a weighed amount of hydrophobic zeolite particles of type 13X was added to it while constantly stirring the emulsion inside. The mouth of the vial was plugged with treated glass wool just like before. J
DOI: 10.1021/acs.langmuir.6b02938 Langmuir XXXX, XXX, XXX−XXX
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Langmuir purity dry air (25 °C and 0% RH) was purged through the top of the column as soon as the emulsion began to flow at a rate of 0.0015 m3/h. During the desorption phase, hot distilled water (70 °C) was injected from the top of the column through the same displacement pump at the same flow rate and recycled back. This was accompanied by a simultaneous flow of air (25 °C) through the column at a flow rate of 0.003 m3/h. Desorbed gaseous alkane was carried along with the outflowing air through the bottom of the column to a condenser maintained at a temperature of 10 °C, where it condensed and was collected as pure alkane. When the surfactant and agitation stabilized emulsion was used, the same glass column was filled up with silanized zeolite of type 13X (28 g) up to a height of 15.6 cm. At the top and the bottom of the zeolite bed, glass beads (27 g) were placed at heights of 2.5 and 7.5 cm, respectively. On top and bottom of the treated 13X type zeolite small layers (1 cm) of treated 4A type zeolite were also placed for facile change of color indicating absorption and desorption of alkane by the zeolite. The column was wrapped externally with heating pad except a small portion at the top and at the bottom through which the zeolites could be observed. All the other connections of the set up were kept the same as before. Next a surfactant stabilized emulsion was prepared by stirring 100 mL distilled water, 50 mL dodecane and 0.5 g surfactant (sodium dodecyl sulfate) in a round-bottom glass flask for 20 min and then keeping it overnight undisturbed to allow the surfactant particles to dissolve well. It was poured into the emulsion flask and the experiment was carried out following the same sequences as narrated before. The only difference was that at the end of each absorption cycle, 22 mL of distilled water was passed through the bed to wash off any surfactant trapped in the column. This water was also collected in the same emulsion flask in order to avoid any loss of alkane entrapped in the bed. Following this, dry air was purged through the column to blow off all the water entrapped in the column. This marked the end of the absorption cycle following which the air flow rate was adjusted to 0.003 m3/h and the tube connected to the outlet of the column was switched from the emulsion flask to the condenser. This was iterated until there was no more absorption of alkane in the column. The residual emulsion was then subjected to a two-stage capillary pump separation by connecting two capillary pumps in series for the sake of improved separation of oil from the emulsion (Figure 9). In order to ensure accuracy of the measurement of the alkane collected from experiment, the treated glass wool that was used as our capillary pump was presoaked with the desired alkane when the operation began.
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Video showing a bundle of end-sealed hydrophobized glass capillaries immersed in hexadecane/water emulsion in the presence of a positive flow of air (AVI) Video showing agitation-stabilized octane/water emulsion being passed through a packed bed of dual zone 4A type zeolites (AVI) Video showing oil extracted from an oil-saturated hydrophobic zeolite bed (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. § R.G. is retired.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02938. Floatation of hydrophobic zeolite particles on the surface of various mixtures of water and ethanol, ballistic deposition of water droplets on zeolite, and effect of temperature on the extraction of oil from transient oil water emulsion (PDF) Video showing a bundle of end-sealed hydrophobized glass fibers immersed in hexadecane in the presence of a positive flow of air (AVI) K
DOI: 10.1021/acs.langmuir.6b02938 Langmuir XXXX, XXX, XXX−XXX
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