Liquid–Liquid Flow Pattern Visualization and Mapping in a Millimetric

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Liquid−Liquid Flow Pattern Visualization and Mapping in a Millimetric Size Coiled Tube Enrique A. Loṕ ez-Guajardo,*,† Gabriela M. Garza-Cantú,† Andre ́ Marques-Camarena,† Enrique Ortiz-Nadal,† Krishna D.P. Nigam,†,‡ and Alejandro Montesinos-Castellanos*,† †

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Escuela de Ingeniería y Ciencias, Tecnologico de Monterrey, Campus Monterrey Avenida. Eugenio Garza Sada 2501 Sur, Monterrey, Nuevo León 64849, México ‡ Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, Delhi 110016, India ABSTRACT: Three different liquid−liquid systems were experimentally studied for flow pattern visualization in a helically coiled geometry. For the sunflower oil−water system, interfacial forces overcome inertial forces leading to slug flow patterns, contrary to the biodiesel−glycerol system in which inertial forces dominate over interfacial forces resulting in parallel-like flow patterns. For the sunflower oil−methanol system, a transition zone from slug flow patterns to parallel flow patterns was observed. A Weber−Ohnesourge flow map (WOFM) was developed for a wide range of interfacial tension, viscosities, and flow conditions, which predicts different flow patterns. This flow map is an important tool for the design of liquid−liquid processing equipment, which depends on the device geometry. The proposed WOFM was used to predict the flow patterns in a reactive liquid−liquid system, showing good agreement with the reactor performance and the flow map predictions.

1. INTRODUCTION The interaction between liquid−liquid phases is a crucial factor in the design and performance of many unit operations in process engineering. Typically, these types of systems exhibit mass and heat transfer limitations due to the small contact area of the liquid−liquid interface,1,2 which leads to a decrease in the overall performance of the equipment. Technologies such as reactive distillation, catalytic-packed beds, sono-chemical reactors, oscillatory-flow reactors, and microdevices3−9 were developed to overcome heat and mass transfer limitations. Microdevices have been widely explored due to their high surface-to-volume ratio and minimal transfer paths,1,10,11 which result in the enhancement of mass and heat transfer coefficients, better quality control, precise temperature control, and lower health risks due to operation compared to conventional equipment.12,13 For instance, it has been reported that microdevices such as herringbone microchannels could achieve Nusselt numbers of 32.2 (at Re = 350)14 and Sherwood numbers of 40 (at Pe = 1 × 105),15 which is a sevenfold increase compared to those obtained in a conventional straight channel under laminar flow. Moreover, coiled flow inverters (CFIs), which consist of helically coiled tubes with equally spaced 90° bends, take advantage of microfluidics for liquid−liquid systems while adding the contribution of centrifugal force and flow inversion.16 As the CFI is a coiled tube, the unbalanced centrifugal force acts perpendicular to the flow direction, leading to the formation of a secondary flow in radial direction © XXXX American Chemical Society

(formation of two symmetrical vortices) known as Dean flow.17 The inclusion of equally spaced 90° bends in a coiled geometry further enhances mass transfer in the system due to the inversion of the velocity profile. CFIs have been shown to narrow the residence time distribution (RTD) compared to other geometries,16,18 and their application in multiphasic flows has been explored in recent years.19−24 The performance of microdevices and CFIs for liquid−liquid processing highly depends on the flow patterns formed within the system.2,25 For instance, for a liquid−liquid reactive system in a microreactor, having slug flow as the main flow pattern increased the specific interfacial area available for mass transfer by 131% compared to that obtained through parallel flow. This results in a 1.73-fold increase in product conversion.26 As mentioned above, several flow patterns have been observed in straight-tube microdevices, of which drop, slug, and parallel flow are among the most common. However, there is limited information available for liquid−liquid systems in coiled geometries, specifically in CFIs.1,19,20,25,27 For millisized curved tubes, how the phases are transported (stability and location of the liquid−liquid interface, flow inversion, and secondary flow formation) could depend on the buoyancy force and the centrifugal force acting in the system. However, Received: Revised: Accepted: Published: A

October 29, 2018 December 21, 2018 December 24, 2018 December 24, 2018 DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Flow Patterns for Different Liquid−Liquid Systems authors

geometry

l/l system

Sarkar et al 2012.2

serpentine μ-channel

water−succinic acid− butanol

Yagodnitsyna et al.25

T-junction, straight μ-channel

kerosene−water

char. length

total flow rate (mL/min)

organic:aqueous phase ratio

We no. (organic)a

We no. (aqueous)a

272 μm

0.6 1.4

1:2 6:1

4 × 10−1 14.4

1.88 4.7 × 10−1

2.4

1:1.4

10

23.1

0.172

0.17:0.00217

1.1 × 10−2

2.4 × 10−6

∼0.018 ∼1.71 0.5 4

0.017:0.00088 1.7:0.0072 4:1 4:1

1.1 × 10−4 1.1 2 × 10−2 1.3

4 × 10−7 2.6 × 10−5 1 × 10−3 6.7 × 10−2

3 3 3 4 12 12 0.4 4 6

2:1 1:1 1:2 2:2 2:10 10:2 1:1 1:1 1:1

1.9 × 10−1 1 × 10−1 4.6 × 10−2 2.7 × 10−3 2.7 × 10−3 6.7 × 10−2 2.9 × 10−4 2.9 × 10−2 6.5 × 10−2

4.9 × 10−2 1.1 × 10−1 2 × 10−1 2.9 × 10−3 7.3 × 10−2 2.9 × 10−3 3.9 × 10−4 3.9 × 10−2 8.8 × 10−2

267 μm

López-Guajardo et al.26

T-junction, straight μ-tube

vegetable oil− methanol

710 μm

Kutup et al.20

T-junction, μ-CFI

n-butyl acetate− acetone−water

1 mm

Vural Gürsel et al.19

X-junction, milli-CFI

toluene−water

3.2 mm

Zhang et.al.22

T-junction, milli-CFI

kerosene−water

1 mm

flow pattern obsd slug flow slug-drop flow annular flow parallel flow slug flow drop flow slug flow parallel flow slug flow slug flow slug flow drop flow slug flow drop flow slug flow slug flow slug flow

a

Estimated Weber numbers from the data reported in the literature.

the formation of each of these flow patterns will mainly depend on the interaction of inertial, viscous, and interfacial forces at the inlet of the system, particularly in pressure-driven flows. For drop flow in microchannels, higher interfacial forces overshadow inertial forces, leading to a breaking up of the interface and forming a drop of aqueous phase dispersed in a continuous organic phase.2 Slug flow is characterized by the formation of a segment or plug of continuous phase followed by an organic phase segment without presenting coalescence between phases. However, a thin film of aqueous phase surrounding the organic phase could form.28 During slug flow, interfacial forces still dominate over inertial forces, specifically due to the low velocity of the continuous and dispersed phase, while maintaining a determined volumetric ratio.29 Furthermore, internal circulations take place within the slug flow pattern, promoting the renewal of the interface and enhancing mass transfer in the system.26,30 In parallel flow, as the total flow rate increases, inertial and viscous forces dominate over interfacial forces, resulting in an enlargement of the interface and leading to the constant flow of aqueous phase parallel to the organic phase throughout the system.27 Table 1 summarizes the observed flow patterns achieved for different hydrodynamic conditions reported in the literature for microchannels and CFIs. Recent studies have explored the use of CFIs for liquid− liquid reaction21 and extraction,19,20,22 taking advantage of the formation of different flow patterns (specifically slug flow) in combination with the contribution of radial mixing and the inversion of velocity profiles at the equally spaced 90° bends. For instance, Vural Gürsel et al.,19 while using a milli-CFI for the extraction of acetone in water into toluene, obtained a 94% extraction efficiency while operating in slug flow at 50 mL/min (Table 1). However, for a straight tube (at the same flow rate), extraction efficiency drops to approximately 75% due to a shift from slug flow to parallel interface. This could be explained by the absence of secondary flows (Dean flow and internal circulations) and flow inversion in the straight tube. Similarly,

Zhang et al.,22 while operating in slug flow regime (Table 1) in a micro-CFI, reported a 4.5-fold increase in mass transfer coefficient with respect to those obtained in a batch system for the extraction of Co and Ni. López-Guajardo et al.21 reported an 18% and 16% increase in oil conversion compared to those obtained in straight-tube reactors and helical-coiled reactors, respectively, for biodiesel production under the same hydrodynamic conditions. The authors conclude that a combination of different phenomena could be affecting the reactor performance, such as (a) the type of flow pattern, (b) the contribution of Dean flow, and (c) the effect of flow inversion. It is noteworthy that, under parallel flow the contribution of Dean flow is overshadowed by the reduction in contact time, as will be discussed in the following sections. As mentioned above, the formation of the flow patterns observed in the systems reported in Table 1 will depend on the interaction of different forces, mainly at the mixing element or entrance region. This interaction could be quantified by different dimensionless numbers such as Weber number (ratio of inertial forces and interfacial forces), Reynolds number (ratio of inertial and viscous forces), capillary number (ratio of viscous and interfacial forces), and Ohnesourge number (ratio of viscous forces to interfacial and inertial forces). Some efforts have been made in the literature to characterize and map the different flow patterns in terms of the flow rates of the aqueous and organic phases.2,19,29,31 Nonetheless, an appropriate mapping of flow patterns should account for the interaction of all acting forces in terms of dimensionless numbers. This dimensionless flow map could be used as an important tool for the design and control of liquid−liquid systems in micro- and millidevices. Similarly, the vast majority of flow maps reported in the literature studied a micro T- or Y-junction connected to a straight-tube/channel without any geometrical perturbations, for a given liquid−liquid system that could lead to disturbances in the stability of a flow pattern.25,27,32−35 The use of these reported flow maps is limited for tubes/channels with similar geometrical configuration; thus, more data is required with B

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research different types of geometries to increase the database of flow maps in the literature. Since helical coiled tubes, in particular CFIs, have become more important for liquid−liquid system processing, there is a need to map the different flow patterns that could be present within the system in terms of the hydrodynamic conditions (flow rates and volumetric ratios). Furthermore, there are limited studies for specific liquid−liquid systems for helical coiled tubes reported in the literature that cover a wide range of process parameters such as viscosity, density, and interfacial tension, which are necessary to establish some design criteria for liquid−liquid processing systems. This paper explores the use of a glass CFI for the flow visualization and flow mapping of different liquid−liquid systems: (a) biodiesel−glycerol, (b) sunflower oil−methanol, and (c) sunflower oil−water. Experiments were carried out at different flow ratios and flow rates. Different flow patterns were mapped and analyzed in terms of dimensional numbers (as described in the following section), which may be used as a design tool to obtain the most suitable conditions for a specific flow pattern in a given liquid−liquid helically coiled processing equipment.

Table 2. Physical Properties of Different Liquid−Liquid Systems @ 25°C system

phase

phase

1

aqueous

glycerol + methanol biodiesel methanol sunflower oil water sunflower oil

2 3

organic aqueous organic aqueous organic

ρ (kg/m3) 1055 870 787 910 996 910

μ (Pa-s) 0.046 0.0059 0.00068 0.057 0.001 0.057

γ (mN/m) 0.639

3.99 23.9140

estimated using the Girifalco and Good derivation,38 while data for systems 1 and 3 were obtained from those reported by Abeynaike et al.39 and Fisher et al.,40 respectively.

2. MATERIALS AND METHODS 2.1. Liquid−Liquid Systems and Properties. Three different liquid−liquid systems were tested for flow visualization experiments: (1) biodiesel−glycerol, (2) sunflower oil− methanol, and (3) sunflower oil−water. Edible sunflower oil was provided by a local food supplier, while methanol (J.T. Baker, 99% purity, analytical grade) and deionized water were purchased from CTR Scientific and Laboratorios Monterrey, respectively. The fatty acid composition of the sunflower oil used (% wt) is the following: 27.53% linoleic (C18:2), 59.78% oleic (C18:1), 4.68% palmitic (C16:0), 2.61% stearic (C18:0), and 5.4% others. Biodiesel and glycerol were obtained by carrying out the transesterification reaction in a batch reactor following the methodology proposed in ref 21. The liquid− liquid systems used in this study were chosen because they could be representative of typical liquid−liquid reactive systems spanning a wide range of interfacial tensions (0.6− 23.91 mN/m) and viscosities (6.8 × 10−4−4.6 × 10−1 Pa·s), since most of the liquid−liquid systems used for flow mapping are related to liquid−liquid extraction within a range of interfacial tension.19,32,36 For the flow of immiscible fluids through the coiled tubes, the most important parameters to consider are the properties of the fluids used and the wettability of the surface. However, a hydrophilic surface (borosilicate glass without any surface treatment) was used for all experiments carried out to construct a flow map which can be used to predict the flow behavior obtained in a stainless steel tube (hydrophilic surface) reported in ref 21. It is worth mentioning that the contact angle of a glass surface and stainless steel surface for crude glycerol is below 50°; therefore, both fall within the hydrophilic behavior.37 Additionally, it has been reported in the literature34,37 that the contact angle for similar systems (oil, water, and crude glycerol) in a glass tube are 3°, 38°, and 37°, respectively. The physical properties for each system are reported in Table 2. The viscosity of each liquid was measured using an Oswald viscometer (KIMAX 200), using water as a reference liquid. Standard error of the measurements falls within 2%. Density was measured using a 25 mL pycnometer (BRAND, Germany). The interfacial tension of system 2 was

Figure 1. Experimental setup for flow visualization in a glass CFI.

2.2. Experimental Setup. A schematic diagram of the experimental setup is presented in Figure 1. Flow visualization experiments were carried out in a borosilicate glass CFI (manufactured by a local labware shop), which consisted of a helical coiled tube with equally spaced 90° bends along the tube length, with an internal diameter (di) of 3 mm and a curvature diameter (dc) of 20 mm, achieving a curvature ratio (λ) of 6.7 with an average axial pitch (space between each turn) of 12 mm. This curvature ratio was selected to obtain a maximum benefit from centrifugal force in the coiled geometry (values of λ ≤ 10 are shown to maximize the effect of Dean flow, while higher values diminish the effect of centrifugal force and the system behaves as a straight tube41). A 90° bend was placed after every five turns of equal tube length (CFI arm) until the total length of the tube was reached. The CFI used has a total of 20 turns, equivalent to 4 arms and 3 inversions (90° bends). This configuration was selected based on values reported in the literature19,21,42,43 that ensure near plug flow behavior and maximize centrifugal force and flow development within four turns. Each phase of the different systems was fed into a vertically mounted CFI through a glass 1/4″ do (outer diameter) Tjunction by using two positive displacement pumps (Watson Marlow 120s). A straight glass tube with a 150 mm length and 3 mm di (same as the CFI) was used as the entrance region C

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research between the T-junction and CFI to ensure flow development and avoid entrance effects in the coiled tube. The T-junction and entrance region were not considered in the final flow mapping for the above-mentioned reasons. Photographs were taken using a camera with a highdefinition sensor (Sony α a5000 with a Sony lens SELP165016-50 mm) connected to a computer for video and photograph postprocessing. 2.3. Flow Visualization Experiments. Experiments were carried out by pumping each phase of the above-mentioned liquid−liquid systems through the CFI at different flow velocity ratios. The first set of visualization experiments were obtained by fixing the flow rate of organic phase (starting at 1 mL/min) and varying the flow rate of aqueous phase from 1 to 7 mL/min and from 9 to 11 mL/min with incremental steps of 1 mL/min, and from 15 to 20 mL/min with incremental steps of 5 mL/min (yielding a total of 12 flow rates). Then, for the second set, organic phase flow rate was fixed at 2 mL/min while aqueous phase flow rate was varied at the same flow rates as described above. This process was repeated until a flow rate of 20 mL/min of organic phase was achieved, resulting in the formation of a 12 × 12 matrix of different flow rates tested. Each positive displacement pump was calibrated to obtain the smallest deviation between the combined flow rates of the pumps. Photographs and videos were taken at (1) the vertically oriented coiled arm (50 hydraulic diameters away from the Tjunction to avoid entrance effects), (2) at the first 90° bend, and (3) with the complete CFI in frame to ensure consistency of the flow pattern throughout the system. The data presented in the flow pattern maps correspond to those observed in the first coiled arm. It is necessary to clarify that flow patterns are formed in the T-junction and the stability of the formed flow pattern could depend on the geometry of the system. That is, the formation of each flow pattern will depend on the balance of each contributing force at the T-junction (function of flow ratio, flow velocity of each phase, and properties of each fluid), and the stability of the flow pattern will be a function of the geometrical perturbations in the system. To obtain higher contrast and better visualization of each flow pattern in the CFI, fluorescein dye was added to each aqueous phase system while using a blacklight source. A total of 144 flow visualization experiments were carried out for each liquid−liquid system, resulting in the analysis of 5390 photos and 432 videos (approximately 1798 photos and 144 videos per liquid−liquid system). Image processing and analysis was made by using ImageJ software. Photographs taken for slug length quantification were analyzed at the same orientation plane and relative position of the coil to minimize uncertainties in image analysis (such as the influence of the glass refraction relative to the angle of the camera). Moreover, a color threshold was used to highlight the slugs analyzed, and their contours were defined. The uncertainty of the measurements for slugs are below 0.4 mm corresponding to approximately 27 pixels with a 95% confidence level. 2.4. Dimensional Analysis. The formation of each flow pattern observed in the CFI will depend on different process parameters; therefore, a dimensionless analysis is required. This analysis will provide insight on the interactions between competing forces in each system tested. The functional relationship between the variables of the systems will be in the form of:

f (q1 , q2 , ..., qn) = 0

(1)

where q1, q2, ..., qn are the variables involved in the process. For this study, the variables (with their respective dimension) affecting the flow pattern are density (ρ, dimension: ML−3), viscosity (μ, dimension: ML−1 T−1), interfacial tension (γ, dimension: MT−2), velocity (v, dimension: LT−1), and internal diameter of the tube (di, dimension: L). Applying the Buckingham pi theorem for the variables acting in the system, the functional relationship can be determined in terms of the following dimensionless numbers: ij d v 2ρ yz μ = φjjj i zzz → Oh = φ(We) j γ z ργdi k {

(2)

In eq 2, Oh is the Ohnesourge number which describes the ratio between viscous forces with inertia and interfacial forces, while We denotes the Weber number describing the ratio between inertia and interfacial forces. The first part of the flow patterns analysis will only consider the Weber number for each phase in the different liquid−liquid systems in order to obtain the ranges in which each flow pattern is formed in the system. Moreover, to obtain a flow map independent of the fluid viscosity and interfacial tension, the product of the Ohnesourge and Weber numbers was proposed as the function that described the obtained functional relationship. Centrifugal force will be acting on the phases of each system due to the curvature of the coil in combination with inertial and viscous forces. This interaction will be analyzed in terms of the balance between of these three forces as described by the Dean number (eq 3). De =

ρvdi μ

1 = λ

inertial force × centrifugal force viscous force (3)

In eq 3, λ denotes the curvature ratio. This number offers insight into the intensity of a secondary flow in radial direction (Dean flow).

3. RESULTS AND DISCUSSION Flow visualization experiments were carried out at different organic/aqueous ratios and flow rates for each system, in which different flow patterns were observed for each liquid−liquid system within the range of flow rates tested. The formation of these observed flow patterns will be a function of two competing forces within the system: (1) inertial forces (Finertial ∼ ρv2) and (2) interfacial forces (Finterfacial ∼ γ/di). Thus, Weber number may be used as a parameter to classify and obtain the ranges in which each force contributes to the formation of a specific flow pattern, as suggested by Zhao et al.27 For this study, flow maps were constructed as a function of Weber number of the organic phase (Weorg) vs Weber number of the aqueous phase (Weaq). Weber numbers were obtained as a function of the superficial velocity of each phase at the inlet of the T-junction, as suggested in the literature, due to the absence of actual measurements of phase velocity.25,27 A representative photograph of each observed flow pattern for the systems tested is shown in Figure 2A−F, while the Weber number flow maps for each system are shown in Figure 3A−C (boundaries between each flow pattern observed are represented with lines). D

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. Different flow patterns observed in CFI: (A) parallel flow (PF, biodiesel−glycerol: Webio = 1.83, Wegly= 3.78); (B) parallel-wavy flow (PWF, biodiesel−glycerol: Webio = 3.84, Wegly = 0.153); (C) slug flow (SF, sunflower oil−water: Weoil = 0.029, Wewt = 0.0015); (D) transitional-parallel flow (T-PF, sunflower oil−methanol: Weoil = 0.06, Wemet = 0.024); (E) slug-drop flow (SDF, sunflower oil−water: Weoil = 0.0018, Wewt = 0.09), and (F) parallel-slug flow (PSF, sunflower oil−methanol: Weoil = 1.07, Wemet = 0.33). Color version of the figure can be found in the online version of the document.

3.1. Biodiesel−Glycerol Visualization. For the biodiesel−glycerol system, inertial forces govern the flow pattern observed in the helical coil due to the low interfacial tension between the two liquids (as seen in Table 2). Two distinctive flow patterns were observed in the coiled arm: parallel flow (PF, Figure 2A) and parallel-wavy flow (PWF, Figure 2B). PF visualized is characterized by two well-defined phases with a smooth interface, as seen in Figure 2A. It can be seen from Figure 3A that PF is the flow pattern with the most occurrences in this liquid−liquid system (1.95 × 10−2 < Webio < 1.83 and 1.6 × 10−2 < Wegly < 6.54). From Figure 4, it may be noted that the glycerol phase tends to move toward the bottom position of the two liquid layers. For instance, glycerol flowing upward near the outer wall of the coil tube wrings down toward the inner wall of the tube due to its higher density compared to biodiesel, causing a perturbation of the interface. The same behavior was observed for glycerol flowing downward as it wrings down from the inner wall to the outer wall. This behavior was only observed on the horizontal coiled arms of the CFI, as the phases could experience both (1) a buoyancy force in which the higher density fluid (glycerol) tries to stay at the bottom position of the two phases on its way up and down the horizontal coil and (2) centrifugal force resulting in the glycerol phase swirling from the outer wall toward inner wall on its way up the coil. However, this phenomenon does not occur on the vertical coiled arm since both phases experience a change in the flow

Figure 3. Weber flow maps for the liq−liq systems tested. Flow maps are ordered from lowest interfacial tension to highest. Boundaries between the flow patterns observed are marked with red lines.

plane; that is, in the horizontal coil the buoyancy force and centrifugal force change as a function of the angle along the axis, while in the vertical oriented coil this angle remains constant. This results in the glycerol phase not experiencing a change in vertical direction, maintaining its bottom position throughout the vertical coiled arm (as seen in Figure 4). PWF (Figure 2B) was observed at Weber numbers in the range of 3.8 < Webio < 6.8 and 1.6 × 10−2 < Wegly < 2.6 × 10−1 (Figure 3A). Similar to PF, two well-defined phases were observed. However, this interface oscillates while it flows upward toward the top position of the horizontal coiled arm. A similar flow pattern has been observed by Yagodnitsyna et al.,25 in which the interface of the denser fluid (water) periodically E

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

T-PF (Figure 2D, approximately 3 × 10−2 < Weoil < 6 × 10−1 and 1.31 × 10−2 < Wemet < 1.2 × 10−1) could be obtained as the result of two main factors: (1) flow instabilities with the increase of flow velocity (mainly aqueous phase velocity at lower organic phase flow velocity); (2) flow instabilities due to the curvature of the coil. In this flow pattern, an unstable interface is formed, in which slugs are released. Depending on the total flow rate of the system, this unstable interface is observed at certain lengths of the coiled arm and CFI. For instance, Figure 2D shows that dragging of the interface takes place until the fifth turn of the coiled arm, in which slugs consistently separate from the interface. A similar flow pattern was reported by Kashid et al.31 in microchannels. The data plotted in Figure 3B (categorized as T-PF) correspond to those flow rates and ratios in which the dragging of the interface and separation of the slugs took place within the first coiled arm. The data points plotted are the ones in which the interface is at a constant length of the coiled arm and slugs begin to separate from the interface. Nevertheless, this should be considered as a transition region and not a stable flow pattern; data plotted are used only to distinguish this region from the stable flow patterns. PF (Figure 2B) for the sunflower oil−methanol system could be described similarly to what was observed in the biodiesel−glycerol system. This flow pattern is positioned in the upper region of the flow map (Figure 3B), where interfacial forces dominate the flow pattern formation. Methanol (in the horizontal coiled arm) moves toward the top position of both phases, whether it is on the outer wall or inner wall of the coil, since methanol has lower density than sunflower−oil; that is, as the methanol flows through the coiled tube, the interface will shift position from the outer wall of the coil toward the inner wall to keep the methanol interface on the top side of both phases (similar behavior was described for the biodiesel− glycerol system, Figure 4). Moreover, the lower viscosity of methanol combined with flow rates above 9 mL/min makes the methanol phase shift from the outer wall of the coil to the inner wall at the 90° bend (methanol is always the top side of both phases, Figure 5), making evident the effect of flow inversion in the CFI due to 90° bends. This inversion could increase mass transfer performance despite the characteristic reduced specific surface area of this flow pattern.19,21

Figure 4. Change of direction of biodiesel−glycerol interface at the horizontal coiled arm. Glycerol wrings out from the outer wall to the inner wall. Color version of the figure can be found in the online version of the document.

waves throughout a rectangular microchannel. However, the mechanisms of formation of this flow pattern described by Yagodnitsyna et al.25 and the present study are different. For instance, the oscillations of the interface for the horizontal coiled tube could be a consequence of flow instabilities caused by a buoyancy force resulting in a change of position in the interface at the horizontal flow plane in combination with centrifugal force and inertial force (mainly for the higher biodiesel flow rates in which this flow pattern appears). In contrast, for the microchannel in Yagodnitsyna et al.25 study, it is reported that a Kelvin−Helmholtz instability could be causing the interface to oscillate. It was observed that, by increasing the total flow rate from 14 to 20 m L/min, the frequency of the oscillations also increases (Wegly was varied from 2.6 × 10−1 to 6.4, increase in the glycerol/biodiesel ratio) until a smooth interface is observed, thus shifting toward a PF pattern. This last behavior coincides with those reported by Kashid et al.31 in which a better stability of the interface (PF) is observed as flow velocity increases. 3.2. Sunflower Oil−Methanol Flow Patterns. Compared to the biodiesel−glycerol system, the sunflower oil− methanol system has an interfacial tension 1 order of magnitude higher (Table 2), which increases the influence of interfacial forces in the system, leading to a new set of flow patterns obtained within the CFI. For this system, three stable flow patterns were visualized at different flow rates and ratios: slug flow (SF), parallel flow (PF), and parallel-slug low (PSF); and a transition region was observed (transitional-parallel flow, T-PF). SF (Figure 2C) is characterized by a highly organized and consistent formation of methanol slugs (with a convex interface) dispersed in a continuous sunflower oil phase (concave interface). A minimum length of the coil internal diameter is required for the slugs to be considered SF (Lslug ≥ di). The smallest methanol slug observed had an average length of 7 mm (at O/A ratio of 20:1) while the largest methanol slug was found to have an average length of 35.2 mm before entering a transition region. It can be seen from Figure 3B that slug flow patterns are observed at the bottom region of the flow map, where interfacial forces dominate over inertial forces (corresponding to Wemet in the range of 2.11 × 10−3 to 2.4 × 10−2). With an increase in the total flow rate, SF starts shifting toward PF, passing through a transition region (middle region of Figure 3B) in which a transitional flow pattern is observed.

Figure 5. Effect of flow inversions in a parallel-wavy flow pattern for the sunflower oil−methanol system. Color version of the figure can be found in the online version of the document. F

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Changing the total flow rate from 17 to 40 mL/min leads to the formation of PSF (Figure 2F) throughout the system. This flow pattern is observed between 6 × 10−1 < Weoil < 1.1 and 7.9 × 10−2 < Wemet < 5.7 × 10−1 (right corner of Figure 3B). For this flow pattern, inertial forces may be competing with interfacial forces, leading to a formation of oil slugs and methanol slugs. The presence of a thin film of methanol surrounding the oil slug was observed at oil flow rates higher than 11 mL/min and methanol flow rates higher than 7 mL/ min, where a phase inversion takes place (methanol becomes the continuous phase and oil the dispersed phase, Figure 2F). However, when the total flow was higher than 20 mL/min, a dragging of the thin methanol film surrounding the oil slug took place, forming a parallel interface in the oil phase. This dragging effect could be a consequence of the inertial forces within the system in combination with the wettability of methanol and phase inversion. Another possible explanation for the formation of this flow pattern is the difference between local velocities mainly at the inner and outer walls of the coil; that is, a higher velocity could be expected in the outer wall of the coil, compared to the inner wall. This could result in higher local Weber numbers, and thus, a dragging of the interface may take place. However, further studies are required to fully determine the mechanism of formation of this specific flow pattern. 3.3. Sunflower Oil−Water Flow Patterns. Interfacial forces for this system are 2 orders of magnitude higher than those experienced by the biodiesel−glycerol system; thus, the flow patterns observed are mainly controlled by the interfacial tension of the sunflower oil−water. Two flow patterns were observed for this liquid−liquid system: (1) slug-drop flow (SDF, Figure 2E) and (2) slug flow (SF, Figure 2C). As expected, for the range of rates tested, only sluglike flow patterns were observed throughout the coiled tube. SDF was observed at 5.1 × 10−4 < Weoil < 8.2 × 10−3 and 1 × 10−2 < Wewt < 1.5 × 10−1 (left corner of Figure 3C) and is characterized by oil slugs and drops of irregular size dispersed in a continuous water phase. If the length of the slug is below the internal diameter of the tube (Ldrop < di), it is considered as a droplet. The length of the droplets observed in the system ranges from 0.54 to 2.17 mm. The formation of oil drops could be a consequence of flow instabilities at the entrance region (T-junction + straight tube), where slugs are forming and the water slugs “trap” some of the oil phase. Moreover, this flow pattern occurs when the dispersed phase flow velocity is lower than the continuous phase. SDF was consistently observed throughout the coiled arm, and the drops formed did not coalesce with other drops or slugs throughout CFI. SF for this system was mainly observed at 5.1 × 10−4 < Weoil < 4.9 × 10−3; 3.7 × 10−4 < Wewt < 1.5 × 10−2 and 1.3 × 10−2 < Weoil < 1.8 × 10−1; 3.7 × 10−4 < Wewt < 1.5 × 10−1 as seen in Figure 3C. This flow pattern could be described as similar to the SF in system 2 (sunflower oil−methanol). However, a more evident phase inversion was observed for this particular system. Phase inversion took place when the continuous phase became a dispersed phase. For instance, at flow rates below 6 mL/min of organic phase (with its respective O/A ratios), the sunflower oil is dispersed in a continuous water phase. This is noted by the shape of the slug interface (as seen in Figure 6A, B); that is, oil slugs had a convex interface while water slugs a concave interface (Figure 6A. However, for organic flow rates above 6 mL/min a phase inversion took place, having the sunflower oil as a continuous phase (with a concave interface)

Figure 6. Phase inversion in sunflower oil−water system. (A) Oil slugs (convex interface) are dispersed in a continuous water phase (concave interface). (B) Water slugs (convex interface) are dispersed in a continuous oil phase (concave interface). Color version of the figure can be found in the online version of the document.

and water as a dispersed phase (with a convex interface), as shown in Figure 6B. The shapes of the interface that distinguish a continuous phase and a dispersed phase were also observed by Gürsel et al.19 in a toluene−water system and butyl acetate−water system. This phase inversion was not observed under SF for the sunflower oil−methanol system (having methanol slugs dispersed in a continuous oil slug, which is attributable to the lower viscosity of methanol) as it shifts toward PF, in agreement with the observations of Sarkar et al.2 However, as the total flow rate of sunflower oil− methanol increases to 22 mL/min (with an O/A ∼ 2:1) this phase inversion took place for the oil−methanol system under the PSF pattern. Moreover, water slug length was measured at different organic to aqueous flow ratios (O/A) while maintaining the oil flow rate at 8 mL/min. For each O/A ratio, a number of water slugs were measured to obtain their average length. Figure 7

Figure 7. Effect of organic/aqueous flow ratio in water slug length.

shows the change in average slug length with an increase in O/A ratio. Results reported do not exceed a 5.2% in standard deviation. A change of O/A ratio from 0.4 to 1.3 results in a 51% reduction of the water slug length, thus increasing the length of the oil slug. It can be noted from Figure 7 that for O/A in the range of 1 and 2, water slug length remained constant until a further increase in O/A took place, reducing G

DOI: 10.1021/acs.iecr.8b05315 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

drops, and parallel interface could be obtained, and (III) where parallel-like flows (PLF) were observed. Region I (5 × 10−4 < Weorg < 1; 3 × 10−4 < Weaq < 1.1 × 10−2) could be classified as the stable region where interfacial forces overcome inertial forces, controlling the production of sluglike flow patterns. In this region, slugs of sunflower oil, methanol, and water could be obtained depending on the ratio of aqueous phase to organic phase. As was expected, no flow patterns of biodiesel and glycerol were observed within this region. In the case of mass transfer limited reactions, this region becomes relevant due to the enhancement in mass transfer related to liquid−liquid slugs. Region II (3 × 10−3 < Weorg < 1.8; 1.9 × 10−2 < Weaq < 1.8 × 10−1) or transition region is present when flow instabilities start to take place with a change in aqueous phase flow rate, resulting in a deformation of the interface. In this region, inertial forces start to become more relevant and compete with interfacial forces; thus, within this region sluglike flow patterns (SLF, for systems 2 and 3) and parallel-like flow patterns (PLF, for all systems) could be observed. Moreover, the T-PF pattern (observed at the middle point of the interfacial tensions tested) falls within this region, making it evident that a change in flow pattern takes place. Region III (3 × 10−3 < Weorg < 6.8; 3.3 × 10−1 < Weaq < 6.54) is where a further increase in overall flow rate induces a dragging effect on the interface between liquids, forming parallel-like flow patterns in the coiled tube. In this region, inertial forces completely overcome interfacial forces, causing a deformation of the sluglike flow patterns into two continuous laminae of both phases. Depending on the density and viscosity of each phase and its relative position in the CFI, some instabilities could take place in the interface, such as the parallel-wavy flow or the movement of the interface from one side of the wall to the other as was observed in the biodiesel− glycerol and sunflower oil−methanol systems (Figures 4-5). It is worth noting in Figure 8 that the ranges of the regions presented in this Weber flow map are particular to each liquid−liquid system tested (explaining the boundaries between the mapped flow patterns). However, there are some problems with the use of this flow map since viscous forces are not taken into account. For instance, in Region II, not only are the transition flow patterns observed, but also slug flow patterns and parallel flow patterns overlap, showing similarities between the systems. Thus, a new parameter is needed to quantify the effect of viscous forces, inertial forces, and interfacial forces. From the dimensionless analysis in section 2, a functional relationship between the Weber number and Ohnesourge number is obtained. This functional relationship could take the form of the product of these two numbers (We·Oh) as proposed by Yagodnitsyna et al.25 We·Oh was used as a parameter for the construction of a flow map which is independent of the properties of the immiscible Newtonian fluids used and includes the effect of viscous forces in the system. Figure 9 shows the We·Oh flow map (WOFM) for a helical coiled tube constructed from the systems tested in this work. In this WOFM all slug flow and slug-drop flow are included within the term “SLF”, and all variations of parallel flows are included in the term “PLF” to have a clearer view of the flow map. It can be seen from this WOFM that the boundaries between the regions are more defined, making clear the effect of considering viscous forces in the system and validating the parameter proposed by Yagodnitsyna et al.25 Similar to the

water slug length from 23.5 to 14.6 mm. This behavior was also observed for oil slugs dispersed in the water phase. In Figure 7 it may be observed that a phase inversion could take place with further reduction of O/A (