Hydrodynamics, Pressure Drop, and Residence Time Distribution

Ind. Eng. Chem. Res. 2009, 48, 8193–8204

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Two-Phase Flow in Minichannels: Hydrodynamics, Pressure Drop, and Residence Time Distribution† A. A. Kulkarni* and V. S. Kalyani CEPD, National Chemical Laboratory, Punes411 008, India

Two-phase flow in mini-channels (1 mm × 1.5 mm × 430 mm and 1.5 mm × 0.5 mm × 430 mm serpentine channel geometry) made in different materials (SS 316, PMMA and Teflon) was studied at different flow rate ratio (0.66, 1.0 and 1.56) of the two immiscible fluids. A dual syringe pump was used to pump the fluids (air-water and water-kerosene) through the channels. For characterization of the two phase flow, experiments were carried out to measure the slug size distribution and relevant hydrodynamic properties, pressure drop across a single serpentine unit (i.e., one sinusoidal unit that includes two 180° return bends connected by a straight portion of 20 mm) and also the residence time distribution of water. In all the cases, the effect of material of fabrication on the hydrodynamics was significant. Apart from flow rates and flow rate ratio of the two fluids, the slug size distribution was seen to have a strong effect of the channel orientation (vertical, horizontal) and also the flow direction (up-flow and down-flow). In the RTD studies, the response curve observed at the end of the channel was significantly different than the published literature. The reasons for such observations are discussed in detail. 1. Introduction The miniaturized process devices or microreactors are now widely known to offer better performance in terms of fast mixing, narrow residence time when compared with the conventional batch reactors or CSTRs, higher overall interfacial areas and also the transport coefficients, needs small amounts of inventories for process development purpose, and better control on the product quality. These devices are used either as a component in a microplant1,2 or as an integrated device that can work stand-alone for serving a specific application.3-6 A wide range of single phase and multiphase catalytic and noncatalytic processes are shown to perform better when carried out in microreactors.7-15 Apart from lab-scale reactions and processes, recently there are some examples where microreactors are used at pilot-plant scale and also for commercial production.16-19 However there is a large gap in the knowledge about the extension of laboratory-scale inventions using microreactors and their use for commercial production, one of the reasons being the inadequate understanding about the transport phenomena and the role of different design and operation parameters on the performance of the device. Broadly, on the basis of geometry, the microreactors are classified as posted microreactors20-22 and microchannels reactors.23,24 Among the two, the latter configuration, that is, discretely distributed parallel microchannels, is favored, and it can have many different configurations (parallel straight channels, serpentine channels for enhancing mixing due to induction † The design of multiphase chemical reactors is one of the extensively contributed and practiced research areas by Professor J. B. Joshi. In his career as a researcher and as a teacher, spanning the last three decades, he has successfully demonstrated the importance of understanding the hydrodynamics at different scales (length and time) and its rationale toward the design of industrial reactors. This paper, although addressing length scales a few hundred times smaller than the industrial reactors he has designed, makes a humble attempt to understand the hydrodynamics of two-phase flow in a mini-channel reactor. We dedicate this work to Professor Joshi on completing 60 years. * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +91-20-25902153. Fax: +91-2025902621.

of secondary vortices, channels with varying cross-sectional areas, and also the depth to develop pressure variations and instability in the flow along the length, etc.) and geometries with different cross-sectional shapes (square, rectangle, cylindrical, a rectangular channel with bottom cylindrical shape, triangular channels, etc.). The microchannel reactors also have different forms like falling film microreactor and micro-fixedbed reactor. A large amount of literature can be seen on the observations of two or three phase flow under these different situations in different contexts. In addition to the straight microchannels/tubes, the serpentine channel-type configuration is widely used for carrying out twophase and three-phase catalytic and noncatalytic reactions in miniaturized environment. In many cases, within the available geometric space, to increase the flow length or avail longer residence time for given flow rates, the practice has been to connect the straight channels by curved portions of identical width and depth. Some information on the role of these kinds of serpentine channels on enhancing mixing, creating secondary flows for single phase flows, and on the separation of particles using a half serpentine unit, etc. is available in the literature. For the case of two-phase flows in microchannels with slightly different geometry, although RTD studies are reported for gas-liquid flows (400 µm wide and 115 µm deep),28 not much information is available in the literature. One of the advantages of such a configuration is that it helps to utilize most of the available surface area on the microreactor plate for flow fluids, thereby yielding a longer residence time. A summary of a few of such configurations from the literature is given in Table 1. Not every reaction that has been reported in the literature and which works much better in a microreactor-based system than the conventional batch mode has been extended to large scale production. Most of the reactions of commercial interest have components that are stable in different phases and/or have multiple steps. For the category of systems involving multiple phases, the knowledge about the interfacial transport phenomena and the hydrodynamics of individual phases in the system is one of the prerequisites for the design or proper utilization of a microreactor. Also, the fluid-fluid-solid interface and the

10.1021/ie801937x CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 17, 2009

Table 1. Typical Examples of Serpentine Channel Geometry Used for Microfluidics and Microreaction Studies authors

channel geometry and dimensions (µm)

Gervais and Jensen25 Kralj et al.26 Gu¨nther et al.27 Trachsel et al.28

flat geometry 50 × 500 50 × 50 × 200 4000 × 1500 400 × 115

Song et al.29

60 × 30 × 60

Chen et al. Lu et al.31

30

100 × 200 500 × 250

flow rate (Q, µL/min)

system (gas-liquid, liquid-liquid) liquid-liquid liquid-liquid (hexane and water) gas-liquid (fluorescent dye) gas-liquid, liquid-liquid (rhodamine-B in ethanol) gas-liquid, liquid-liquid (surfactant span 80 and mixture of hydrocarbon) liquid-liquid protein (thaumatin) and precipitant (KNaC4H4O6) solutions liquid-liquid (benzophenone isopropanol)

three-phase contact angle have a strong effect on the hydrodynamics of the flow in the microchannel. This paper aims at understanding hydrodynamics of two-phase flow in serpentine miniaturized channel reactors fabricated in different materials. Here we bring out a few interesting experimental observations on the effect of inlet flow rates for two immiscible fluids on the nature of flow, pressure drop and residence time distribution in the serpentine minichannels made in three different materials at relatively low flow rates which achieve the gas-liquid slug flow. The channel sizes that are studied here are in the millimeter range. The manuscript is organized as follows: After the Introduction, we discuss the experimental details, the method of data acquisition, and analysis. The experimental observations are discussed in the section of Results and Discussions. This is followed by the implications of these observations on guidelines toward further work and conclusions.

objectives/measured parameters

5 10-2000 at 85 °C 10 30-70

mapping of flow regimes liquid-liquid extraction RTD RTD

45

mixing, organic synthesis

1.8 and 13.6 at 18 °C

protein analysis effect of mixing on nucleation of protein photochemical reaction

2-12

the two fluids were chosen independently. The flow rates (denoted hereafter as QW and QK for water and kerosene system, respectively) were varied in the range of 0.25-7 mL/min. For the case of water-kerosene system, in most of the cases, the experiments were carried out at three flow rate ratios Qw/Qk ) 0.64, 1.0, and 1.56. The effect of flow rate ratio on the hydrodynamics was studied as in many reactions, viz. nitration of insoluble aromatics, although the mole ratio of the immiscible reactants can be stoichiometric, the volumetric flow rate ratios are significantly different. The two fluids were brought into the

2. Experimental Section The experiments were carried out for the measurement of hydrodynamic properties and the residence time distribution for the case of two-phase flow (air-water and water-kerosene) in the serpentine minichannels. The channels were machined in three different materials, SS316, polymethylmethacrylate (PMMA), and Teflon. The schematic of the machined plate with necessary dimensions is shown in Figure 1. For most of the plates, the channel dimensions are 1 mm (width) × 1.5 mm (depth) × 430 mm (length), with 20 straight channels of 20 mm length connected in sequence with a curved 180° return bend. One more plate with dimensions of 1.5 mm (width) × 0.5 mm (depth) × 430 mm (length) was fabricated in PMMA with an objective to see the effect of change in the flow area on the hydrodynamics. The machining of three different plates (shown in Figure 2) was done using the conventional micromachining tools. We have purposely chosen to be on the higher side of the channel dimension because (i) minichannels are easy to fabricate, (ii) sufficiently long minichannels with 180° return bends provide necessary mixing length even if the channel sizes (>500 µm) are not very small, (iii) rapid prototyping or fabrication of the channels in this range are easy and affordable, (iv) cleaning of channels is relatively easy, and finally (v) although the heat transfer area per unit volume of fluid gets reduced due to relatively larger channel sizes, still the specific heat transfer area is about 9000 m2/m3. 2.1. Experimental Setup. The experimental setup consisted of a micromachined plate covered with a PMMA plate to visually observe the dispersed phase flow and distribution. It was assumed that the top cover plate would not change the pertinent hydrodynamics from the channels made in different materials. The fluids were pumped at constant flow rate using a dual syringe pump (Boading Longer, China). For achieving the different flow rate ratios of the two fluids, the syringes for

Figure 1. Schematic of the serpentine mini-channels used for micromachining. For the channel sizes with higher width (1.5 mm) and smaller depth (0.5 mm) the schematic (including the length of channel) remains the same as shown above. The dark filled circle indicates the tracer injection point, while the two open circles along the center line of the plate indicate the location of the pressure drop ports.

Figure 2. Photographs of the 1 mm × 1.5 mm × 430 mm minichannels in (A) SS, (B) PMMA, (C) Teflon, and (D) 1.5 mm × 0.5 mm × 430 mm minichannel in PMMA plate.


machined plate by mixing them using T shape geometry (1 mm i.d.), positioned just above the channel. Hydrodynamics of Two-Phase Flow. The flow visualization for the measurement of slug sizes and slug velocity was carried out using three different cameras. For low flow rates 0.5-3.0 mL/min, a digital camera (Sony Inc., 8 MPx) was used, while for relatively high flow rates a Red Lake CMOS camera (having maximum image acquisition rate of 2000 fps) was used and images were captured at 200 frames per second and a few image sequences were also taken using a Photron camera (FastCam SA5) at an acquisition rate of 10000 fps. The acquired images for gas-liquid and liquid-liquid flows were analyzed for the measurement of slug sizes and velocity for different flow rates using ImagePro Plus 5.1. In all the cases, the visibility of the object was enhanced using front illumination (for SS316 and Teflon plate) as well as back illumination (for PMMA plates). Typical images obtained for PMMA plate are shown in Figure 2D. The flow visualization in Teflon plate was not very clear (even after using different color combinations for the two fluids) and hence we could not quantify the slug size distribution on Teflon plate. Residence Time Distribution. For the liquid phase RTD measurements, the tracer injection method was used and the variation in the liquid conductivity at the outlet of the microreactor was monitored. A tracer in the form of a pulse (20 µL pulse of salt solution NaCl, 20 g in 100 mL distilled water) was injected in the flow after the two phases enter the serpentine flow domain. The tracer injection port was located at 8 mm downstream of the inlet for the mini-channel. The injection time was maintained at 0.05 s using a pneumatic drive connected to a micropipet. The top cover plate used for the flow visualization experiments and for the measurement of RTD was the same. The change in the concentration of salt solution at the outlet of the serpentine channel plate was monitored using the two parallel copper wires (0.2 mm diameter) placed at a distance of 1 mm from the outlet. Care was taken that the wires do not touch each other physically and only the continuously flowing liquid film between the wires would give the conductivity values with time. The data were acquired using a microcomputer via a 16bit PCMCIA A/D converter card at a sampling frequency of 100 Hz. The acquisition time was varied for case-to-case depending upon the complete exit of the salt solution from the plate. The data were then processed separately to study the RTD. Pressure Drop. The two-phase flow behavior in the serpentine minichannels certainly affects the pressure drop across the channels. The information on the pressure drop across the channel is important to realize the flow transition and the extent of energy input needed for achieving specific type of flow in the channels. Since each of the machined plates consisted of 20 straight channels connected in sequence (Figure 1) by a curved channel without any change in the width and depth, every single serpentine section (i.e., one sinusoidal unit) is expected to offer identical pressure drop. Hence an arrangement was made to measure the pressure drop over one serpentine unit that consists of two 180° return bends connected by a straight portion of 20 mm. The total pressure drop across the entire channel can be obtained by multiplying the pressure drop per unit by the number of units (provided all the channels behave in similar manner). Two SS316 tubes with inner diameter equal to the channel size (1 mm) were inserted from the top cover plate such that the two tubes touch the open surface of the channels. These two tubes were then connected to a differential digital micromanometer (AZ Instruments, Taiwan with measurement accuracy of (0.3% of the measurement range) using two inflexible

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polymeric tubes having identical length. The two connecting tubes were filled with water to maintain continuity in the medium for transferring the pressure variations in the domain. The manometer readings were set to zero before starting the measurements at all the flow rates. The differential pressure across two points was measured and the steady state values were recorded. The extent of variation in the data was having a standard deviation less than (3% and its implications are discussed elsewhere. The acquired images where subjected to analysis using ImagePro. The image-by-image analysis was carried out for the measurement of flow velocity of the individual slugs. The slug size distribution for every experiment was obtained by analyzing many randomly chosen images in the entire sequence, where the choice of the image was based on nonrepetition of any slug between two images. Thus, an image was chosen only after all the slugs from a previous image have flown over the plate, thereby avoiding the repetition of slugs. The average velocity of a slug was estimated by tracking their movement in time over the entire plate. 3. Results and Discussions A large amount of literature can be seen on the two-phase flow regimes in mini- and microchannels, and it is evident that there are several factors that govern these flow regimes. In many cases, in order to maximize the use of the available geometric space, the flow length is increased by connecting several parallel straight channels by curved portions of identical width and depth (or analogous ways mentioned in Table 1). A few investigations on the role of these kinds of serpentine channels in enhancing mixing, creating secondary flows for single-pahse flows, separation of particles using one-half of a serpentine unit by inducing the lift force, etc., are reported in the literature. For one such geometry, although RTD studies are reported for gas-liquid flows,28 not much information that can be used as a design guideline is available in the literature. Moreover, no specific analysis is available on the effect of minichannel material on the two-phase hydrodynamics. In the rest of this section, we bring out the experimental observations on the properties of the dispersed phase, pressure drop, and the RTD studied in minichannels. Since some information on the gas-liquid twophase flow in 180° return bends is available in the literature,32 here we have given more emphasis on the analysis of liquidliquid flow. 3.1. Slug Size Distribution and Phase Hold-up. Information about the slug size distribution is important as it helps to estimate the interfacial area, relative slug velocity, circulation in the continuous phase slugs, and hence the interfacial mass transfer coefficient. The slug size distributions in the minichannels made in the PMMA plate were obtained from the images and are shown in Figure 3, top row. The inlet flow rate and flow rate ratio of the two immiscible fluids were having a strong effect on the nature of slug size distribution. At Qw/Qk * 1, although many small slugs existed in the channel, a few slugs having much longer length were also seen. This situation was rarely observed for equal volumetric flow rates of two fluids (Qw/Qk ) 1). Also, due to the curvature induced coalescence near the 180° bend, the dispersed phase slugs were seen to spend relatively longer time. For Qw/Qk > 1, a large difference was seen in the slug sizes observed at higher flow rates and lower flow rates. For the lowest flow rate under consideration, the mean slug size of the dispersed phase (kerosene) was almost 4 times that of the slug sizes observed at higher flow rates.

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Figure 3. Slug size distribution observed on the serpentine channel: (Top row) PMMA plate, (middle row) SS316 plate, (bottom row) PMMA plate with 1.5 mm channel size. The legends indicate the water flow rate Qw in (mL/min). Qw/Qk indicates the water to kerosene flow rate ratio.

For the case two-phase flow in channels in SS 316 plate (Figure 3, middle row), the distribution was Gaussian at all flow rates under consideration and also for the different flow rate ratios. Interestingly, the range of slug sizes of the dispersed phase (kerosene) was almost identical. For a value of Qw/Qk < 1, the dispersed phase slug size was seen to be inversely proportional to the total flow rate. For Qw/Qk ) 1, the average dispersed phase slug size was seen to increase continuously with increase in the flow rates. This observation was seen to be consistent for the case of Qw/Qk > 1. In the minichannels of relatively larger aspect ratio (1.5 mm × 0.5 mm) machined in a PMMA plate, no specific trend in the slug size distribution was observed (Figure 3, bottom row). Large as well as small slugs were observed at all the flow rates and flow rate ratios. The visual observations showed that at higher flow rates, the slug sizes were seen to change along the channel length due to compression and the pressure recovery in the bend region, thereby yielding significantly different

dispersed phase slug sizes toward the exit. At higher flow rates, in the curved portion of the channel, the two phases were seen to flow by forming two different layers thereby showing slugs only in the straight portion. Importantly, channeling of fluid was prominent thereby clearly indicating the possibility of lower internal circulation in the slugs and thus would affect the interfacial mass transfer. This observation clearly indicates that higher width and lower depth (or vice versa) channel dimensions do not yield a clear Taylor flow with very well separated dispersed phase slugs. From the analysis of the gas-liquid flow in identical channels (1 mm × 1.5 mm, with air-water as the model system), the liquid slug size distribution was seen to be skewed at lower liquid slug sizes (Figure 4A). In all the experiments, the flow rate for both the phases was kept identical. An increase in the flow rate was seen to reduce the liquid slug sizes. Also, at very low flow rates ( 180. For the flow rates below 2 mL/min, the gas slugs (for air-water system) were longer than the kerosene slugs (kerosene-water system). This is mainly due to the interfacial tension for the individual systems. The snapshots of the acrylic plate (1 mm width and 1.5 mm width) for water-kerosene system at different flow rates and flow ratios are shown in Appendix 1. These images depict the effect of flow rate and flow ratio on the slug properties. The average slug lengths for the case of water-kerosene system (Figure 5A,B,C) showed a strong effect of the channel material. For SS plate, the average slug size of the dispersed phase was almost independent of the flow rate ratio Qw/Qk. For the flow rate ratios under consideration, water was always the continuous phase while kerosene was the dispersed phase. At higher Qw/Qk, the water slug sizes were seen to increase continuously. For the case of PMMA plate with similar channel size, the average slug size of the dispersed phase (kerosene) decreased continuously with increase in the water velocity. This observation was consistent at all the flow rate ratios. This effect was prominent at higher values of Qw/Qk. For the PMMA plate with channels of 1.5 mm width, the dispersed phase slug sizes varied in a narrow range with increasing slug velocities. For identical flow rates and flow rate ratio values, the mini-channel in SS316 yielded smaller slugs than the similar channels in PMMA plate. Thus, the wetting characteristics of the fluid-solid system strongly affect the slug sizes and hence also the effective interfacial area available for interfacial mass transfer. The variation in the number of slugs of the dispersed phase (and hence also the continuous phase) formed for all these cases is shown in Figure 5D. In the mini-channels of 1 mm × 1.5 mm size, the number of dispersed phase slugs observed on the plate was seen to go through a maximum with increasing total liquid flow rate. The number of slugs that are seen on the PMMA plate is always higher than that of on the SS plate. However for the minichannel of 1.5 mm × 0.5 mm in PMMA, the number of slugs at any instant was seen to increase with increasing liquid flow rate but it was much smaller than the minichannels of 1 mm × 1.5 mm size. The relation between the slug length and slug velocity (not shown here) on SS plate was positive while it was negative for the channels in PMMA plate. The later observation strongly depends on the slug velocity as well as value of Qw/Qk. For the case of PMMA plate with larger width channels, no specific trends were seen, except at Qw/Qk ) 1.56, where the slug size had a weak negative dependence on the slug velocity. The dispersed phase fraction (∈D) in the serpentine channels was estimated using the images acquired at different flow rates. The observations from three different plates are shown in Figure 5E-G. For the case of SS316 plate, the ∈D was seen to go through a maximum at all values of Qw/Qk while for channels in PMMA plate, ∈D decreased continuously with increasing water flow rates. The trends were similar at all the flow rate ratios, except that, the values of ∈D did not change much for Qw/Qk g 1. At Qw/Qk ) 1, increase in the total flow rate leads to larger slugs due to on-plate coalescence, thereby decreasing the total voidage and also the interfacial area available for mass transfer between the two phases. In the channels with relatively larger width, the value of ∈D did not show any specific dependence on Qw. For very low Qw/Qk, the flow was seen to undergo a transition or regime change thereby yielding higher dispersed phase hold-up at higher flow rates. At higher Qw/Qk

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Figure 5. Hydrodynamics of the two phase flow in serpentine channels: (A,B,C) average slug length, (D) slug number, (E,F,G) dispersed phase fraction, (H) Vsk/D vs Vsk, (I) variation in the slip ratio with superficial velocity of dispersed phase. For Figure 5D,H,I, the nomenclature for the legends is as follows: The first number shows the flow rate ratio Qw/Qk followed by plate material (Acr for PMMA plate, SS for SS316 plate), which is then followed by a number indicating the channel width in mm.

similar observations were made, where ∈D was seen to go through a minimum with increasing flow rates. Interestingly, at all the times, the value of ∈D showed a positive dependence on Qw/Qk, thereby clearly indicating that higher flow rate ratio yields smaller slugs. However the limiting value of this can be known only after a wide range of flow rate ratios is studied and it will be discussed separately. For the two-phase flow systems, it is known that depending upon the design of the device (dimensions, method of dispersed phase injection, etc.) and the system (fluid-fluid), the fractional dispersed phase hold-up can have a strong relationship with the superficial inlet velocities. To check such a possibility, the Drift flux model by Zuber and Findley33 was applied considering the cocurrent flow of two fluids. VSW VSK ) C0(VSK) + C1 ∈D ∈C

(1)

Figure 5H shows the different trends of (VSK/∈D) - (VSW/ ∈C) with VSK for different flow rates and flow rate ratios. Conventionally, the intercept of the plot corresponds to the average slip velocity of the dispersed phase. The plot showed that depending upon the minichannel material, and the flow rate ratio, different trends could be seen. The nature of these trends is similar to the different situations that can be seen for the dispersions in relatively large-scale bubble column reactors. In most of the cases, the intercept on the ordinate was in the range of 0.002-0.016 m/s, which was close to the values of dispersed phase velocity measured from the image analysis. One of the ways to characterize the deterministic behavior of the flow is to observe the variation in the slip ratio, S ) VSK/VSW ) (VSK/ ∈D)/(VSW/∈C) with the dispersed flow velocity. The slip ratio is a measure of the relative velocity between the dispersed and the continuous phase and its value greater than 1 usually indicates the irregular flow behavior (on the basis of the flow


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Figure 6. Pressure drop per unit serpentine of the serpentine mini-channel: (A) PMMA plate with 1 mm × 1.5 mm × 430 mm channel, (B) Teflon plate with 1 mm × 1.5 mm × 430 mm channel, (C) SS316 plate with 1 mm × 1.5 mm × 430 mm channel, (D) plate with 1.5 mm × 0.5 mm × 430 mm channel, (E) SS 316 plate with 1 mm × 1.5 mm × 430 mm channel, (F) Parity between the experimentally measured pressure drop and the estimations based on the correlation for return bend with modified friction factor for the geometry under consideration. The accuracy of pressure loss measurement is (5% of the measurement range (5 PSI). Dotted line indicates the (20% deviation.

in straight capillaries). In our case, the slip ratio (Figure 5I) was a strong function of the channel geometry, the material, and the flow rate ratio. For the channel in SS plate the slip ratio was always higher than 1 while for the other two channel sizes in PMMA, the nature of the flow showed different behavior. 3.2. Pressure Drop Across a Single Serpentine Unit. The pressure drop over one unit serpentine channel was measured as discussed in Section 2.1. For the serpentine channels on PMMA plate, the pressure drop (∆p) was seen to increase with increasing flow Re. Re is defined on the basis of the hydraulic diameter of the channel and the total superficial velocity, average density, and viscosity of the fluids estimated using the flow ratios. For the pure liquids (single phase flows) the value of ∆p was lower for water and higher for kerosene. For the twophase flow with Qw/Qk ) 1, the ∆p values were between the pressure drop for two pure fluids. For Qw/Qk * 1, the ∆p was always higher than that for the single phase flow. Apart from the wetting characteristics of the fluids, the three phase contact angle and the emerging slug size distribution has a significant contribution to this enhancement in the pressure drop. For the serpentine channels on Teflon plate, the observation of higher ∆p for kerosene was similar as seen for the PMMA plate. At Qw/Qk ) 1 pressure drop was lower than the single phase flows. With small changes in the flow rate ratio, the values of ∆p were seen to increase significantly, thereby indicating very different flow regime in the channels. At lower flow rate ratio, ∆p was seen to increase with increasing total flow rate. This increment was rather large for the case of Teflon plate and the PMMA channels of 1.5 mm width but not very

significant for the PMMA channel with 1 mm width. The later observation was consistent even for higher flow rate ratios. On the SS316 plate, for pure liquids, the pressure drop was seen to increase continuously with increase in the flow rate. The pressure drop data for air-water and air-kerosene systems increased continuously with the flow rates (QW/QK ) 1) and in both the cases the ∆p values were almost identical indicating the dominance of three phase contact angle and density variation of the two fluids at the interface rather than only the surface wettability of both the fluids. On estimating the pressure drop per unit serpentine section (spanning 4.6 cm) using Hagen-Poiseuille equation (sheet with ]) except for minichannels of 1.5 mm × 0.5 mm on PMMA plate, a significant deviation was seen between the experimentally measured and the estimated pressure drop values. One of the main reasons for this deviation was the fact that, unlike in a straight tube, in a horizontal serpentine channel with several connected return bends of 180°, while passing through a single return bend, the flow experiences an upstream section having geometry of a straight channel followed by a curved section that induces secondary vortices in the flow followed by a straight section. As a result, the pressure drop is always higher than that of a straight tube. In view of this, the experimental data was compared with the correlations for pressure drop for return bends.34 Figure 6F shows the parity between the predicted and the experimentally measured pressure drop in PMMA channel (of 1 mm × 1.5 mm × 430 mm). The deviations are mainly due to the difference in the cross-section of the channels for which the correlation was proposed.

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Figure 7. Typical slug distribution seen on the SS316 plate for the vertical orientation of the plate. The flow is from bottom to top: (A) 6 mL/min, (B) 10 mL/min, (C) 14 mL/min; (D) slug size distribution observed for the three flow rates; (E) effect of plate orientation on pressure drop.

Effect of Plate Orientation. The experimental observations reported so far are from the case, where the microchannel machined plate was kept horizontal. For the system where the fluid or the slugs have to flow in a direction in an altogether different plane, along with the viscous, momentum and surface tension forces, gravitational force also would play an important role in possibly varying the flow pattern along the microchannel. The experimental observations are shown in Figure 7. Interestingly, for flow rate above Qw ) Qk ) 2 mL/min the slug behavior was seen to be significantly different for different flow rates and hence here we bring out observations for Qw ) Qk ) 3 mL/min, 5 mL/min, and 7 mL/min. For each of these flow rates, the dispersed phase slugs were seen to spend more time on the curvature region, thereby possibly leading to slug coalescence at lower flow rates and slug break-up at higher flow rates. At 3 mL/min flow rate, the slugs were seen to appear discretely and unequally spaced around the channels. This was further seen to become more regular at 5 mL/min and again at 7 mL/min; the dispersed phase slugs were seen to break at the curved portion of the serpentine channels thereby continuously resulting into smaller sized drops. As a result of which, at higher flow rates, the interfacial area available for mass transfer was seen to increase along the length of the channel. The effect of channel orientation on the pressure drop for a single serpentine unit at different flow rates (Qw/Qk ) 1) for the channels in SS316 plate are shown in Figure 7E. The pressure drop at all the flow rates was seen to decrease in the order of upward flow (vertical orientation of plate) followed by the downward flow (vertical orientation of plate) followed by the horizontal orientation. Thus, at higher flow rates although the horizontal orientation offers lowest pressure drop, the upward flow was seen to yield better dispersion by breaking of slugs at the turning points. Thus, depending upon the need of specific slug sizes, the channel orientation can also be used as an operating parameter to yield the desired flow. One of the reasons for the upward flow

configuration that yields higher pressure drop was due to the presence of a decelerating region before the return bend followed by the bend region that induces a circulatory motion and a recovery region depending upon the flow rates. The experimentally measured pressure drop data was further used for the estimation of Fanning friction factor f (using f ) (∆PDeq)/(2LFVST2), where Deq is the equivalent diameter of the channel, L is the length over which the pressure drop was measured, F is the fluid density, and VST is the fluid velocity)35,36 for the various flow compositions flowing through the different minichannel plates. For the case of two-phase flows, the dispersion density was estimated on the basis of the inlet flow rate ratio of individual phases. Similar to the plots of pressure drop (Figure 6), the friction factor variation was having similar trend for all the channel materials for all the compositions of the two fluids. The predicted friction factor based on the formulation by Shah and London37 f ) 22.4/Re (for single phase flow in straight channels) was seen to show the trends similar to that of the experimental data, however the predicted values were significantly lower than the data obtained from the experiments. One of the reasons can be the presence of the 180° bend that connects the two straight sections, which develops secondary circulatory flow in the curved section.38 Even for the straight channels (0.8 µm deep by 100-µm wide channel) Pfahler et al.39 found friction factors in laminar flow of n-propanol to be greater than theoretical values. For the case of channels in Teflon plate, the trends for single phase flow with pure fluids and the two phase flow were different, thereby indicating that the friction factor dependence on Re should have a dependence on the three phase contact angle or wettability of fluids (Figure 8). This also indicates that for the case of microreactors made in glass, silicon, different metals, and alloys, Teflon, etc., the wettability property of the fluid-solid combination is significantly important as the pressure drop or the friction factor can not be predicted a priori using conventional relations. Within


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Figure 8. Friction factor vs Re (logslog) plot for the serpentine channels. The friction factor is estimated on the basis of the pressure drop data from Figure 6 and 7.

the experimental conditions, the friction constant f · Re was seen to decrease with increasing Re. 3.3. Residence Time Distribution (RTD). The measurement of RTD is an important design parameter to evaluate the nonideality of flow in any process equipment. Once the RTD information is obtained, chemical reactor behavior can be predicted if kinetic data for the reaction are available. A few reports on the measurement of RTD in microchannels discuss the method of measurement,22,28,40,41 analysis of data, effect of presence of a second phase on the dispersion characteristics42 and theoretical analysis to predict the dispersion coefficient in different geometries.43-45 In the present case, the RTD data obtained for different flow rates was subjected for the estimation of mean residence time distribution, its statistical variants, and the dispersion coefficient. We follow the methodology described by Levenspiel46 to obtain RTD curves from the discrete pulseresponse measurements. As mentioned by Trachsel et al.,28 for the geometry of serpentine channels, predominantly the liquid menisci in the corners of the rectangular microchannels provide a mechanism for intermixing of fluid elements in different slugs, as does the merging of slugs in regions of the microfluidic device which consist of channel bends and sharp corners. As a result of connecting slugs with some extent of local back flow through the liquid films, the pulse response is expected to get wider. In this case of two phase flows in capillaries, the pulse response at the outlet is a function of (i) slug size distribution, (ii) dispersed phase fraction, (iii) flow rates and flow rate ratio of the dispersed and continuous phases, (iv) the three phase contact angle, which depends upon the fluid properties and the solid material, (v) film thickness of the continuous phase, (vi) flow regime, and (vii) circulation velocity in the liquid slugs. To experimentally understand this dependence, here we present our observations. The tracer injection time was significantly smaller than the mean residence time of the fluid elements in the channel (the ratio of slug passage time at the point of injection to the tracer injection time varied in the range of 0.9 to 4.2 indicating the existence of different possibilities). A typical signal obtained at the channel outlet for the water, water-kerosene, and air-water flows through the channel made in the Teflon plate are shown in Figure 9. In all the experiments we aim at measuring the RTD for water which is the continuous phase. The concentration curve for the single-phase flow differs significantly than that in the case of the two-phase flow. The

Figure 9. RTD in minichannel reactors. (A) Tracer response curve in Teflon mini-channels; (B) variation of estimated dispersion coefficient and vessel dispersion number with fluid velocity. Filled symbols indicate the dispersion coefficient and open symbols show the vessel dispersion number.

reasons for this kind of an outlet RTD signal are (i) the tracer was injected at a distance of 2 mm from the point where the two phase flow is generated in the channel and (ii) the tracer was miscible only in water and hence the injected tracer would itself get mixed in the liquid slug(s) depending upon the slug size and velocity at the point of injection. Thus, if the tracer gets dispersed in a single liquid slug, it can get distributed in a few neighboring slugs due to the mass transfer of tracer through the connecting liquid film. And the number of slugs in which it can get dispersed will strongly depend on the flow velocity, the number of turns in the channel length, and the local hydrodynamics at the curved sections. If the tracer injection time is higher than the liquid slug passage time at the injection port, the tracer would get mixed in different liquid slugs thereby carrying discontinuities in the local concentration right from the beginning of the channel. In all our experiments, we had maintained a tracer injection time of 0.05 s and it is possible that for low flow rates, since the slug velocity is low it would spend relatively more time at the tracer injection point than the slugs at higher flow rate. This may thus get the tracer dispersed in a single slug, while at higher flow rates, many small sized slugs are formed and it is always possible that, if the slug

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Figure 10

passage time is lower than the injection time, the tracer would get dispersed in more than one slug of the continuous phase. Importantly, in the later situation, although the tracer injection time is the same and the amount of tracer that is injected in the slugs still remains as a sharp pulse, some amount of the tracer gets mixed with the liquid in the slug form and the remaining mixes in the liquid film that connects the two slugs. The nature of the outlet response curve for the two-phase flow shown in Figure 9A clearly indicates the discontinuities in the signal. The discontinuities do not reach zero values but some finite concentration is still detected as some amount of tracer is carried along with the liquid film. For the case of air-water system, the discontinuities even reach to zero in many cases due to the poor wetting characteristics. The nature of the signal with discontinuities was seen to have an effect of the flow rates, flow rate ratio of the two phases as well as the material of minichannel, which largely governed the slug size distribution and the continuous phase film thickness in the

channel. One of the main reasons that such kind of behavior is not observed for the two phase flow studied by Trachsel et al.28 is that their channel design included the curved portions connected with very small straight length section (much smaller than the radius of curvature). This would slightly change the flow pattern in the curved portions, which do not experience significant changes as in our system, where curved portions are connected by relatively much longer straight sections. As a result, they would see a well-characterized plug flow in the system which may not be the case in our system where the dispersion of tracer from slug to slug through the film (and the film thickness may change in the curved sections) is possible. Despite this difference in the characteristic flow, we have followed the data analysis method and the estimation procedure for different parameters as given in Trachsel et al.32 The results for a typical experiment in the channels in SS316 are shown in Figure 9B. For the air-water case, the dispersion coefficient was seen to increase continuously, indicating that the system


behaves more like a mixed flow system than a plug flow system. The extent of increase, as well as the value of D, for the air-water system was much higher than the warer-kerosene system, and this can be attributed to the nature of the slug sizes, film thickness, and slip velocity, which are primarily affected by the three-pahse contact angle. Thus, a reduced surface tension gradient in the three-pahse contact portion helps to retain better slug-to-slug mass transfer, thereby yielding a system that is more similar to plug flow. Similarly, the vessel dispersion number D/(VSTLT) showed different trends with increasing fluid velocities. These observations indicate that the nature of mixing in the slug flow systems is strongly dependent on the properties of fluids, channel material flow velocities, and the flow ratio. Thus, preliminary experiments that characterize the slug flow for the actual fluids would always help to decide the suitable operating conditions and also to estimate the transport coefficients, to some extent. Although this approach would predict some overall behavior neglecting the discontinuities in the signal, we do not feel that this is an appropriate approach for quantifying the residence time distribution in the slug flow system, where individual slugs of the continuous phase are connected to each other via the film and the transport of tracer and its contribution to the residence time (in laminar boundary layer flow) in the film need different treatment. The most realistic model for such situations would be a combination of two-plug flow reactors in series, with one having a feed forward recycle and the other with a feed backward recycle. A detailed analysis (c-curve with periodic or a-periodic discontinuities) of such a system is under investigation. 4. Conclusions Two-phase flow in minichannels (1 mm × 1.5 mm × 430 mm and 1.5 mm × 0.5 mm × 430 mm) made in different materials was studied at different flow rate ratios (0.66, 1.0, and 1.56) of the two immiscible fluids. Experiments were carried out to measure the dispersed phase properties, pressure drop, and the residence time distribution. The effect of material of fabrication on the hydrodynamics was significant in all the cases. (i) The channel aspect ratio had a strong effect on the slug sizes and other hydrodynamic properties. The slug size distribution observed in the minichannels in SS 316 plate was Gaussian at all flow rates and flow rate ratios under consideration. In the channel in PMMA the nature of distribution strongly relied on the inlet flow rate ratio of the two immiscible fluids. Larger aspect ratio channels in PMMA showed no specific trend in the slug size distribution. (ii) For the case of SS316 plate, the dispersed phase hold-up was seen to go through a maximum while for a PMMA plate, it decreased continuously with increasing water flow rate. (iii) The slip ratio for the liquid-liquid flow on SS plate was always greater than unity indicating the flow to be nonuniform or irregular, while the PMMA plates showed exactly opposite behavior. (iv) The pressure drop and frictional characteristics of the flow were seen to differ from the literature on straight channels. The estimated pressure drop using the correlations for 180° bends were seen to deviate from the experimental results to some extent indicating that pressure drop was a strong function of the wetting characteristics of the material and the channel geometry. (v) The nature of response signal for the analysis of RTD from the two phase flow was significantly different from the single phase flow. Although the dispersion characteristics were

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estimated using the known method, the approach was seen to be unsuitable for the response curves obtained here. More detailed work on developing suitable theory for the analysis of RTD signals with discontinuities is in progress. Acknowledgment The authors acknowledge the financial support from the Consortium on Microreaction Technology (www.ncl-india.org/ cmr/) and the Centre of Excellence in Microreaction Technology of the National Chemical Laboratory, Pune. The authors also thank Tesscorn India Pvt. Ltd. for providing Photron (FastCam SA5) for imaging at very high frame rates. Appendix Figure 10 shows typical snapshots of the liquid-liquid twophase flow for different flow rates and flow ratios in (A) acrylic plate with 1 mm width × 1.5 mm depth (rows 1-3) and (B) acrylic plate with 1.5 mm width × 0.5 mm depth (rows 4-6). Nomenclature C0, C1 ) constants in the drift flux model (eq 1) Ca ) capillary number (unitless) D ) dispersion coefficient (m2/s) Deq ) equivalent diameter of the channel (m) f ) friction factor (unitless) L ) length over which the pressure drop was measured (m) LT ) total channel length (m) Q ) flow rate at the inlet (m3/s) Re ) reynolds number (unitless) VS ) superficial fluid velocity (m/s) VSK, VSW, VST ) superficial velocity of kerosene, water, and overall VS F ) fluid density (kg/m3) δP ) pressure drop (Pa) ∈ ) fractional hold-up (unitless) Subscripts C ) continuous phase D ) dispersed phase G ) gas phase K ) kerosene T ) total W ) water

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ReceiVed for reView December 16, 2008 ReVised manuscript receiVed May 9, 2009 Accepted May 13, 2009 IE801937X

Hydrodynamics, Pressure Drop, and Residence Time Distribution

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