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Continuous Droplet-based Liquid-Liquid Extraction of Phenol from Oil a

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Dhiman Das , Suhanya Duraiswamy , Zhou Yi , Vincent Chan & Chun Yang a

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore b

School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Accepted author version posted online: 15 Jan 2015.

Click for updates To cite this article: Dhiman Das, Suhanya Duraiswamy, Zhou Yi, Vincent Chan & Chun Yang (2015): Continuous Droplet-based Liquid-Liquid Extraction of Phenol from Oil, Separation Science and Technology, DOI: 10.1080/01496395.2014.978466 To link to this article: http://dx.doi.org/10.1080/01496395.2014.978466

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Continuous Droplet-based Liquid-Liquid Extraction of Phenol from Oil Dhiman Das*1 ,Suhanya Duraiswamy2, Zhou Yi2,Vincent Chan1 and Chun Yang2 1

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School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore

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School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

Multiphase droplet-based and continuous liquid-liquid extraction (LLE) of phenol from oil is

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studied in a low width-to-height aspect ratio polymethylmethacrylate (PMMA)-based microchannel reactor. The extraction efficiency of phenol from the dispersed phase consisting of

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silicone oil to the external continuous water phase is studied as a function of the flow rate ratio of the two immiscible phases. This paper demonstrates that for enhancing the extraction efficiency

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of phenol from silicone oil to water in droplet-based microfluidic reactors; a low flow rate ratio of the external continuous phase, Qc to dispersed oil phase, Qd is preferable. Keywords droplets, microfluidics, phenol, continuous phase, dispersed phase

INTRODUCTION

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Various kinds of droplet-based microfluidic devices have been developed for a wide spectrum

of applications in the past decade. Several fundamental and application oriented works have been published which investigated liquid-liquid extraction (LLE) between two immiscible phases in droplet-based microreactors

[1-4]

using fluorescence microscopy. Among other areas of

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applications of droplet-based microfluidics, examples include laminar cell patterning[5], cellbased assays[6, 7] and enzymatic activity measurements[8-10], screening of engineered proteins[11], protein crystallization[12] and reaction catalysts[13] to name a few.

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In the context of this work, LLE has been done traditionally, using batch reactors [14, 15] where

final concentration[3] of the analyte in the two phases depends on its partition coefficient which is

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a measure of the relative solubility of the analyte between the two phases. Such batch reactors offer limited a throughput as the samples can only be processed in batches. Today, with the [16]

in microchannels is possible which

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advent of microfluidics; continuous flow extraction

increases the throughput of the extraction process. Using alternately spaced immiscible phases in

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the form of droplets encapsulated in an external phase within microchannels also greatly shortens

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the molecular diffusion distance and increases the interfacial area of contact per unit volume between the liquids thereby increasing the mass transfer rate. These advantages inspired this work for performing droplet- based liquid-liquid extraction of a target analyte molecule, namely phenol, from droplets of complex organic solvents into an external more aqueous medium, namely water.

Phenol, a polycyclic aromatic hydrocarbon (PAH) is selected as our target analyte species

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the two immiscible organic and aqueous phases are separated by gravity or by centrifuge. The

because PAHs, being lipophilic in nature, are widespread organic pollutants and often linked to oil spills[17]. To simulate the complexity of real-time samples, we solubilize the phenol molecules

in silicone oil[18], a multi-component mixture of linear polymers. However, in our case; for extracting phenol, we used a polymethylmethacrylate (PMMA)-based low width (W)-to-height

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(H) aspect ratio microchannel reactor instead of PDMS as PDMS swells in the presence of organic solvents like phenol due to cross-linking with the microchannel walls. Continuous liquid-liquid extraction of phenolic compounds from oil has potential from the standpoint of

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[19]

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applications as a high throughput sample preparation method

environmental analyses from the field are carried out on complex samples such as oil spills and

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organic wastes; hence, cannot be analyzed directly with most common analytical instrumentation equipments such as gas chromatography (GC), high-performance liquid chromatography

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(HPLC), capillary electrophoresis (CE) and atomic absorption spectroscopy (AAS). Therefore, a sample preparation method must be implemented for prior separation of the target analyte

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species from these complex solvents.

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The significance of this work is that in a scenario where we have to extract target analytes from a large volume of complex solvents such as silicone oil, the use of a continuous flow droplet based reactor is ideal. In this regard, the conclusion of this work becomes beneficial as we relate the flow rate ratios of the two immiscible phases to the extraction efficiency of the analyte species and conclude that a lower flow rate ratio of continuous to dispersed phase (Qc /Qd) is to be preferred for the maximum extraction efficiency of the analyte species from

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extracting the ionic species in an organic phase. Sample preparation is a necessary step as most

dispersed to continuous phase. In this report, we employ a cross flow junction [please see Fig.1] for emulsifying silicone oil containing phenol into discrete droplets and DI water is used as a continuous phase. We observe that the emulsification of the oil phase into droplets consists of three stages after the oil phase stations itself at the cross-flow junction (which may be regarded

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as the start-up point). The three stages in the chronological order are: the filling stage, the pinchoff stage and the lagging stage which are shown in Fig.1. The physical forces involved in these three stages and the time duration of each of these stages significantly impacts the mass transfer

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section. After the droplet formation stages, internal circulations

within the discrete oil

droplets enhances the interfacial mass transfer rate of phenol from the oil droplets to the external

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water phase and the mass transfer equilibrium between the two phases is achieved rapidly. Subsequently, the external water phase gets saturated with phenolate ions. Thereafter, the oil

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droplet is drained out at the outlet bifurcation as is shown in Fig.3 and the external water phase containing phenolate ions is injected into HPLC for

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quantifying the extraction efficiency from oil to the water phase. It needs to be noted that only

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the water phase is to be injected separately into the reverse phase C18 column which we use for phenol detection so the separation of the oil droplets is necessary. The droplet enters the shorter and wider microchannel following the flow-pressure relation in a pressure- driven channel which is simplified as Q = ∆p / R H

where Q is the flow rate, RH is the hydraulic resistance and ∆p is

the pressure drop. Considering that flow rate is inversely proportional to the resistance along the arms, we can calculate the relative resistance at the bifurcation [22]. Due to an increased channel length, there is a higher resistance in the longer arm at the outlet and also, there is a net Laplace

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[1, 20, 21]

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of phenol from oil to water. This is explained in detail later in the Results and Discussions

pressure drop along the droplet interface due to the sudden expansion from 170 µm to 750 µm due to the forces of surface tension. This ensures that all the droplets are drained into the shorter arm. We will explain the fabrication of the device, consumables used and the experimental set-up in the next section.

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EXPERIMENTAL Initially, a PMMA acrylic sheet of thickness 1 mm is being cut through to obtain the

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microchannels of minimum thickness (170 µm) using CO2 laser (Universal M-300 Laser

bottom. The PMMA sheets after laser cut are treated with oxygen plasma using Harrick scientific

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plasma cleaner (PDC-32G) at a maximum power setting of 100 W for a duration of 5 minutes for hydrophilization. This is followed by rinsing with 2-propanol, dried with nitrogen to thoroughly

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cleanse the surface of all dust particles and then the three layers are bonded together via hot embossing. Contact angle measurements of DI water on the PMMA were done using FTA

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200.The average value of contact angle of DI water on a freshly cut PMMA surface is 71o. This value reduces to 5o-15o after hydrophilization. The dispersed phases, that we used, are two

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samples of silicone oil with viscosity as follows: 5 and 50 cSt (Sigma, Singapore). First, the silicone oil (which is our dispersed phase) is spiked with 5 mg/ml of phenol (Sigma, Singapore). The dispersed phase is then emulsified in an aqueous sodium dodecyl sulphate (SDS) (Sigma, Singapore) solution at the critical micelle concentration (CMC) value, which makes up our continuous phase. . The droplet formation cycle at the cross- flow junction of the microchip is observed with a Carl Zeiss microscope fitted with a Zeiss lens (CP-Achromat 5x/0.12) and the

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Platform). This PMMA plate is flanked by two other cut PMMA layers: at the top and the

droplet lengths were obtained using the Phantom Miro M310 digital high-speed camera by comparing it with the channel width (W). Finally, 0.5 ml of the continuous phase is collected from the longer microchannel of the outlet bifurcation for all the flow rate ratios, which is then

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injected into HPLC-UV for phenol detection. A reverse phase column, Zorbax PAH Eclipse Plus and a mobile phase of HPLC grade methanol: DI water is used for detecting the phenolate ions.

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RESULTS AND DISCUSSIONS

the silicone oil (dispersed phase) to water (continuous phase) and the flow rate ratio of

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continuous, Qc to dispersed phase, Qd for optimizing the liquid-liquid extraction rate. The mass

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transfer rate of phenolate ions from oil phase to the water phase may be represented by the extraction efficiency (I/Io) which is the ratio of I, the absorption peak intensity of phenolate ions

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per unit volume in the DI water (continuous phase) present in the 0.5 ml which is collected from the outlet of the microreactor [Fig.1] for all the flow rate ratios and Io is the absorption peak

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intensity of phenolate ions per unit volume in the DI water assuming there is 100% mass transfer from oil droplets to the external water phase. As phenol is soluble in water upto 0.5 M[23]; in our experiments, all phenol molecules will transfer from oil phase to the water phase as per the partition coefficient driven oil and water LLE due to the convective-diffusive force.

For

calculating Io, we prepare samples containing 5 mg/20 ml, 5 mg/22.5 ml, 5 mg/25ml and so on upto 5 mg/32.5 ml of phenol in water which in proportion to the oil and water volumetric flow

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In this report, we studied the relationship between the mass transfer rate of phenolate ions from

rate ratio in our experiments. This is done because as we have increased the continuous phase flow rate, Qc; keeping the dispersed phase flow rate, Qd constant in our study; the concentration

of the phenol molecules in water also gets diluted. Moreover, it was observed via HPLC that all the phenol molecules get transferred from oil to the water phase when the mixture was shaken vigorously.

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To increase the extraction efficiency of phenol from oil droplets to water, we increase the flow rate of the continuous phase to obtain smaller sized droplets[24]. However, producing smaller droplets require a higher flow rate ratio which reduces the residence time of the droplets. So, we

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used different reactors with increasingly longer microchannels so as to keep the residence time

reduction in the size of the oil droplets with increasing flow rate ratio keeping the dispersed oil

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flow rate constant. The smaller sized droplets increase the interfacial area of contact per unit volume between the dispersed and the external continuous water phase which will, in turn,

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increase the mass transfer rate between the two immiscible phases. The relationship between the interfacial area and the moles of ions transferred per unit time and area may be represented by a

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mass transfer equation, as per Fick’s law, given below:

Ai [ KCint − Cext (t )] δ t

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J=

where J is the number of moles transferred per unit time and per unit area, Ai is the interfacial area of contact between the two phases and is actually the surface area of the droplets here, t is the residence time of the droplets in the microreactor, δ is the thickness of the oil-water interface which acts as a membrane and a resistor for mass transport; K is the partition coefficient of the molecules, Cint and Cext denote the concentration of molecules in the interior and outside the

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constant for all flow rate ratios which is 2.1 seconds in our experiments. Fig.6(a) shows the

droplet, respectively. Increasing the continuous phase velocity also reduces the thickness of the interface[19] .Hence, increasing the flow rate ratio should increase the mass transfer rate between the two phases as per the transient mass transfer equation. We observed that even with larger interfacial area per unit volume of oil due to smaller droplets and an equal residence time, the

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best extraction efficiency occurred at lower flow rate ratios (Qc/Qd) [Fig6.(b)]. But it is not so, i.e; the extraction efficiency reduces with increase in the flow rate ratio. Hence, it was concluded that there are two different diffusion regimes: during the droplet formation stage and after the

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formation of discrete oil droplets; and the duration of a particular droplet formation sub-stage

microreactor. Because in the droplet formation (filling) stage; as the flow rate, Qd of the

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emerging oil phase is much slower than that of the continuous phase, achieving convective mass transfer equilibrium takes longer. But, after the formation of discrete oil droplets, these droplets

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travel through the channel at approximately the same velocity of the continuous phase[24], which makes the molecular convective-diffusive mass transfer rate much faster. In other words, mass

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balance equilibrium is achieved much slower in the droplet formation stage which makes it the rate determining step. The duration of this droplet formation (filling stage) however reduces with

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the increase of the flow rate ratio of continuous to dispersed phase (Qc/Qd). Next, we will explain the different stages of droplet formation cycle at the cross-flow junction [Fig.1] and thereafter, relate the duration of each of these stages with the flow rate ratio(Qc/Qd). The oil droplet formation cycle may be divided into three identifiable sub-stages: the filling stage, the necking stage and the lagging stage [Fig.1]. The droplet formation at the cross flow junction is due to the presence of two competitive physical forces: the interfacial tension force

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(filling stage) plays a more critical role than the residence time of the droplets in the

which resists the pinching of the dispersed stream into droplets and the viscous shear stress which exerted by the external continuous phase. A schematic illustration of these two forces at the cross flow junction is shown in Fig. 4. At the cross flow junction, initially, the oil stream stations itself which may be referred to as the start-up point. The filling period begins wherein

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the oil phase extends into the main channel beyond the start-up point [Fig.1]. The distance that the oil stream traverses beyond this point determines the length, L of the droplet. The filling stage [Fig.1] continues as long as the oil stream grows until the shear stress is exceeded by the

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interfacial tension acting at the oil-water interface, after which the dispersed phase snaps in the

lagging stage [Fig.1] where the oil stream retracts back to its original position, the start-up point.

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Again, the entire cycle is repeated.

We report the effect of the flow rate ratio(Qc/Qd) on the duration of the three sub-stages. The

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Phantom Camera Control (PCC 2.14b) software was used to analyze the videos in order to determine the exact duration occupied by the three stages. The results are displayed in Fig. 5. for

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5 cSt silicone oil. It was observed that as the flow rate of the continuous phase increases, keeping

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the dispersed (oil) phase flow rate constant, the fraction of time occupied by the filling stage progressively decreases. Now, the droplet formation regime in immiscible fluids occurs because of the competition between viscous shear and interfacial tension forces at the oil-water interface which is represented by the Capillary number, Ca=µV/γ[24]. Here µ is the kinematic viscosity and V is the characteristic mean velocity of the continuous phase and γ is the interfacial tension between the two phases[24]. µV is regarded as the viscous shear force component of the Capillary Number. This phenomenon with progressively increased and decreased duration of filling and

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form of discrete droplets. This may be regarded as the necking stage [Fig.1] followed by the

lagging stage respectively, as shown in the Fig. 5; can be explained from the fact that as the continuous phase velocity is increased; the viscous shear becomes more dominant compared to the interfacial tension forces. The necking stage, which involves pinching-off of the oil stream, is of the order of a few milliseconds. Hence is not visible in the Fig. 5.

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This occurs because there are two different diffusion regimes: during the droplet formation stage and after the formation of oil droplets. For colloidal suspensions (such as phenol molecules in silicone oil) in a pressure driven flow, the dispersion of solutes occur in the axial direction

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across the streamlines, which is analogous to the Taylor’s dispersion[25]. This accounts for a

•

Do ( z ) = DW + a 2 γ

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phase given as:

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where Dw is obtained from the Wilke-Chang equation[26] and a is the particle radius. The •

shearing rate γ , because of the laminar pressure-driven axial dispersion , is given as[25]: ∂u ( r ) ∂[2u (1 − ( r / R ) 2 )] = = −4ur / R 2 ∂r ∂r

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•

γ=

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Hence, the shearing by the microchannel walls is minimum at the channel centreline which causes the ions to emerge out of the droplet through the tip. In the droplet formation (filling stage), there is a parabolic velocity profile along the oil-water interface with the portion of the oil stream closer to the microchannel walls, lagging behind as shown in Fig.7(b) which implies that the ions closest to the r-axis will have the maximum convective diffusion in the axial direction [Fig. 7(d, e, f)]. Also, the straight streamlines within the elongated oil tip come in phase with the

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cross-streamline diffusivity because of the shearing effect by the microchannel walls, in the oil

axial dispersion of colloids. In other words, complete dispersion of ions will take place from the oil tip to the continuous phase. Following this, comes the diffusion regime from individual oil droplets. Different from freely suspended droplets, which are spherical[27]; due to geometric

confinement in the microchannels, uniaxial stretching of the droplets takes place resulting in a

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flattened disk-like shape[24] of droplets. As we have adopted an aspect ratio of W