A pH-Sensitive Laser-Induced Fluorescence Technique To Monitor

Article pubs.acs.org/IECR

A pH-Sensitive Laser-Induced Fluorescence Technique To Monitor Mass Transfer in Multiphase Flows in Microfluidic Devices Simon Kuhn and Klavs F. Jensen* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: We present a pH-sensitive laser-induced fluorescence (LIF) technique to investigate mass transfer in reactive flows. As a fluorescent dye, we used 5-(and-6)-carboxy SNARF-1, which, when excited with a pulsed Nd:YAG laser at 532 nm, provides good sensitivity in the range 4 ≤ pH ≤ 12. For validation, we first applied the dye to single-phase reactive flows by investigating the neutralization of sodium hydroxide with hydrochloric acid. Comparison to the classical passive mixing case showed that this dye was able to capture the reaction progress and to quantify the mass transport. Next, we investigated the absorption of CO2 in an alkaline solution using gas−liquid flow and found that the LIF technique is able to quantify the local mass-transfer rate in microfluidic systems. Results for different microchannel geometries highlight the strong connection between local mass transfer and secondary flow structures in gas−liquid Taylor flow.

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INTRODUCTION The investigation and characterization of mass-transfer phenomena in single- and two-phase flow is important in understanding transport mechanisms influencing chemical reactions in microscale devices, where no turbulent actions promote mixing and mass transfer. The complexity of the involved transport mechanism is increased by the transition from nonreactive mixing processes to reactive scalar mixing introduced by a chemical reaction. By combining particle image velocimetry (PIV) to assess the fluid velocity field and laserinduced fluorescence (LIF) with pH-dependent dyes, it is possible to track the front of the ongoing chemical reaction and its connection to convective transport. In their comprehensive study, Coppeta and Rogers1 investigated different dyes and identified several candidates that are excitable with an Ar-ion laser (at either λex = 488 nm or λex = 514 nm) and exhibit a dependency of their emitted light on pH in the range of pH 6−9. With such a dye, it becomes feasible to investigate the neutralization between an acid and a base. An investigation of the neutralization of acetic acid with ammonium hydroxide in a channel flow was reported2,3 in studies employing point-measurement techniques [laser Doppler velocimetry (LDV) and LIF] to record the velocity and the concentration of acetic acid, respectively. In that particular experimental setup, mixing of the two reacting streams was achieved using grid-generated turbulence. Furthermore, the effects of unstable thermal stratification (i.e., buoyancy effects) and mean shear on a chemical reaction in grid turbulence were also investigated.4 The main findings were that buoyancy-induced motions promoted the mixing processes and that the reaction was much more efficient compared to that under classical shear flows. The effect of a first-order chemical reaction on mass transfer in channel flow was also investigated by numerical simulations.5 In all of these studies, turbulent actions promoted mixing and the chemical reaction, but reactive mixing in microchannels is dominated by diffusion. An understanding of mass transfer in these systems, in © 2012 American Chemical Society

particular multiphase systems, gained through direct measurements of mass-transfer and velocity fields would be useful for a wide range of biological and chemical applications.6 For gas−liquid two-phase flow, the absorption of CO2 in an alkaline solution has served as a model reaction in many studies to characterize microchemical systems, because it can be tracked by measuring the change in pH of the aqueous solution throughout the microchannel.7−9 The aim of these and other studies10−12 has been to provide engineering correlations for the overall mass-transfer coefficient kLa. However, these correlations do not capture microscopic effects and rely purely on the observed mass transfer of the entire microreactor setup. To gain more insight into the local mass-transfer phenomena, an experimental investigation of these local effects is desirable. Therefore, we propose laser-induced fluorescence (LIF) as a tool for studying scalar quantities altered by a chemical reaction and, thus, for characterizing mass transfer. LIF is a noninvasive technique for measuring changes in the spatial distributions of scalar variables,13,14 including concentration fields of tracer dyes15−19 and one- and two-color LIF to study temperature fields.20−24 We chose the absorption of CO2 in an alkaline solution as a model reaction for demonstrating the application of pH−LIF to quantify mass transfer. In a recent study, the visualization of gas−liquid mass transfer using pH-sensitive LIF was reported.25 This study addressed rising bubbles of CO2 in a water tank with naphthofluorescein as a pH-sensitive fluorescent dye. However, this choice limited the measurable pH range to values greater than 6 with a high sensitivity around the neutral point (pH 7). Because the equilibrium concentration of absorbed CO2 in water at ambient conditions (p = 1 bar, T = 298 K) Received: Revised: Accepted: Published: 8999

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of the microchannel were 0.4 × 0.4 mm2. The microreactors were connected to inlet and outlet fluid tubes by mounting them with a compression stainless steel device.29 We performed both single-phase and gas−liquid two-phase flow experiments. The liquids were delivered by syringe pumps (Harvard Apparatus HP 2000), and for gas−liquid flows, the gas was provided by a mass flow controller (Sierra Flowbox). The single-phase experiments were run as a first step to validate the pH−LIF technique by investigating the neutralization of aqueous sodium hydroxide and hydrochloric acid solutions. The two solutions (sodium hydroxide at pH 12 and hydrochloric acid at pH 4.5) were fed separately into the microreactor at equal flow rates of 25 μL/min. 5-(and-6)Carboxy SNARF-1 was added to both streams (c = 1.4 × 10−5 mol/L). The spatial variation in pH was measured at different positions downstream of the mixing section. In addition, we repeated the same experiment with two streams of deionized water with Rhodamine 6G added as a tracer dye (c0 = 1.2 × 10−5 mol/L) to one of the streams. Thus, we were able to investigate single-phase reactive and nonreactive scalar mixing. As a second step, we applied the pH−LIF technique to twophase flow measuring the absorption of CO2 in an aqueous sodium hydroxide (pH 12) solution to study interfacial mass transfer. For these measurements, we added 1.4 × 10−5 mol/L 5-(and-6)-carboxy SNARF-1 to the liquid phase. In addition, we used microscale particle image velocimetry (PIV) to assess the fluid velocity of the continuous phase.30,31 The pH−LIF and PIV measurements were performed using an inverted fluorescence microscope (Zeiss Axiovert 200). We used a 10× microscope objective with a numerical aperture of 0.30, which resulted in a field of view (FOV) of 0.76 × 0.58 mm2. Furthermore, the microscope was equipped with a 605/70 nm bandpass filter for the emitted light. The light source for pH− LIF and PIV was provided by a frequency-doubled Nd:YAG laser (BigSky Ultra CFR, 30 mJ, 532 nm) coupled into the microscope using a dichroic mirror. The images were recorded using a dual-frame CCD camera (PCO Sensicam QE, 1376 × 1024 pixels2, 8 bit). As seeding material for the PIV measurements, Nile Red-coated spheres with a mean diameter of 1 μm were used (Invitrogen). We considered an ensemble size of 100 images acquired at 4 Hz, and in the postprocessing, the PIV data were correlated on 32 × 32 pixels2 with an overlap of 50% and then filtered for spurious vectors. For the calculation of the interfacial area and the holdup of the twophase flow, images of the entire microreactor were obtained with a Nikon D200 digital camera and were subsequently processed using ImageJ.32 Excitation and emission spectra of 5(and-6)-carboxy SNARF-1 were measured using a SPECTRAmax GEMINI XS Dual-Scanning Microplate Spectrofluorometer (Molecular Devices). Fluorescent Dye Characterization. We used 5-(and-6)carboxy SNARF-1 (Invitrogen) as a pH-sensitive fluorescent dye that was excited using a pulsed Nd:YAG laser at a wavelength of 532 nm (Figure 3, top). At this wavelength, the absorbance of 5-(and-6)-carboxy SNARF-1 is nearly independent of the pH value, whereas the emission increases with increasing pH (Figure 3, bottom). To obtain the calibration function, the emitted light intensity was recorded as buffer solutions ranging from pH 4 to pH 12 were flowed through the microchannel. For the calibration, we considered an ensemble of 20 images recorded at 4 Hz to account for pulse-to-pulse variations of the laser and averaged the recorded light information over an area of interest (AOI) of 8 × 8 pixels2.

corresponds to pH ≈ 5, no quantitative data could be presented, as many fluid regions were found to have pH < 6. In the present study, we introduce a quantitative pH−LIF technique using 5-(and-6)-carboxy SNARF-1 as the fluorescent dye. This dye has its main application in biological systems, for example, to estimate intracellular pH.26 To our knowledge, it has not been applied to microchemical systems or quantitative mass-transfer studies. As a first step, we validated the technique by addressing the single-phase neutralization of aqueous solutions of sodium hydroxide with hydrochloric acid, and we then investigated the local and global mass-transfer phenomena for the absorption of CO2 in an alkaline solution in microchannels of different geometries. The results highlight the connection between local mass transfer and secondary flow structures in gas−liquid Taylor flow.

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EXPERIMENTAL METHODS Flow Description. Figure 1 depicts the Y-shaped microchannel used for the single-phase passive/reactive mixing

Figure 1. Photograph of the Y-shaped microchannel used for the single-phase flow experiments.

Figure 2. Photographs of the spiral- (left) and meandering- (right) channel microreactor used for the two-phase flow experiments study. The enlargement on the right shows the details of the meanderingchannel structure.

experiments to validate the pH−LIF technique, and Figure 2 shows the two different microreactors used in the gas−liquid Taylor flow experiments. The first reactor (Figure 2, left) features a small meandering mixing section (V = 20 μL) followed by a spiral channel (V = 220 μL), whereas the second reactor (Figure 2, right) consists solely of meandering channels (V = 240 μL). The third inlet of the reactor in Figure 2 (left) serves as an on-chip quench and was inactive for the present study. These devices were fabricated from a double-polished silicon wafer and a Pyrex wafer (diameter, 150 mm; thickness, 650 μm). The fabrication process consisted of several photolithography steps, deep reactive-ion etching of silicon, and the growth of silicon nitride (0.5 μm).27,28 Anodic bonding of the Pyrex top to the silicon wafer resulted in the formation of an optical accessible microreactor channel. The cross sections 9000

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concentration field was measured at five positions downstream of the mixing section (x/dH = 4, 16, 28, 40, and 280, where dH = 400 μm is the hydraulic diameter); see Figure 5. The

Figure 5. Schematic representation of the Y-shaped microchannel and the inlet streams used in the single-phase comparison between passive (top) and reactive (bottom) mixing. The dashed lines indicate the positions where the profiles were extracted.

Figure 3. Absorption (top) and emission (bottom) spectra of 5-(and6)-carboxy SNARF-1 excited at λex = 532 nm (vertical line).

Figure 6. Concentration profiles (c/c0) at five different positions downstream of the mixing section for the nonreactive scalar mixing of two deionized water streams.

extracted concentration profile c/c0 shows a steep concentration gradient across the microchannel at x/dH = 4 (Figure 6) that slowly decreased farther downstream due to diffusive actions and, after 280 hydraulic diameters, became nearly flat (i.e., the mixing process was completed). This is the classical onedimensional diffusion process as described by Fick’s second law

Figure 4. Emitted light intensity as a function of the pH value of various buffer solutions.

The resulting curve shown in Figure 4 indicates that the best pH sensitivity was around the neutral point (pH 7) and the measurable pH range was 4 ≤ pH ≤ 12, because the intensity signal leveled off at higher and lower pH values. During postprocessing, this curve was fitted with a fifth-order polynomial to map the recorded light intensity to the pH. To account for light sheet inhomogeneities, this calibration was applied to each individual AOI of the recorded image.

∂c ∂ 2c =D 2 ∂t ∂y

(1)

where D denotes the diffusion coefficient. A solution for Fick’s second law in one dimension with the initial conditions ⎧ c0 , y ≥ 0 c(t = 0, y) = ⎨ ⎩ 0, y < 0

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RESULTS Validation of pH−LIF Technique in Single-Phase Flows. As a first step, we considered nonreactive mixing in the Y-shaped microchannel by injecting two streams of deionized water, one doped with Rhodamine 6G. The resulting

(2)

is given by c(t , y) = 9001

2 c0 e y /4Dt 4πDt

(3)

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Local Mass Transfer in Gas−Liquid Flow. Having validated the pH−LIF technique in single-phase flows, we considered gas−liquid two-phase flow by addressing the absorption of CO2 in an alkaline solution. The starting sodium hydroxide solution had a pH of 12 and included 1.4 × 10−5 mol/L 5-(and-6)-carboxy SNARF-1 as a pH-sensitive dye. After establishing the gas−liquid segmented flow, we measured the two-dimensional pH field surrounding the CO2 bubbles (Figure 8). The gradient in the pH field between the CO2 bubble and

With the corresponding concentration gradient centered at the position y = 0 ∂c ∂y

=− t ,y=0

c0 4πDt

(4)

where the time t is determined by the downstream position x/ dH and the mean bulk velocity. Thus, the diffusion coefficient D could be extracted from the measured concentration profiles, producing D = 7.8 × 10−10 m2/s. This value was extracted at the streamwise position x/dH = 4, and we observed no variation between positions 1 to 4. Furthermore, this value of the diffusion coefficient of Rhodamine 6G is in agreement with the literature, where values on the order of 3 × 10−10 m2/s (exact value depending on the measurement technique) have been reported.33 Thus, the optical setup and the microreactor are capable of providing quantitative LIF data. Next, we addressed the neutralization of sodium hydroxide and hydrochloric acid. The starting pH values of the two aqueous solutions were set to pH 12 and pH 4.5, respectively, and thus, the whole measurable range of the dye was explored. Profiles of the pH were extracted at identical positions downstream of the mixing section (Figure 7). Similarly to the

Figure 8. Contours of pH at the back (left) and front (right) of a CO2 bubble.

the bulk sodium hydroxide solution was fully resolved. The lowest pH value closest to the CO2 bubble was 4.75, corresponding to the equilibrium concentration of absorbed CO2 at these conditions. Moreover, the pH field was not uniformly distributed around the bubble, indicating a preferential direction of mass transfer toward the caps of the CO2 bubble. Furthermore, these pronounced regions appeared to be symmetrical with respect to the centerline of the microchannel, suggesting a relationship between the interfacial mass transfer and the liquid-phase velocity field. To confirm this finding we applied microscale PIV to characterize the velocity field in the sodium hydroxide solution adjacent to the CO2 bubble (Figure 9). After subtraction of the Figure 7. pH profiles at five different positions downstream of the mixing section for the reactive scalar mixing of sodium hydroxide and hydrochloric acid.

nonreactive mixing case, a steep pH gradient across the microchannel was observed that flattened out as the neutralization reaction progressed. Following the same approach as outlined above, after converting the pH to a proton concentration, we estimated that apparent diffusion coefficient as Dapp = 7.4 × 10−3 m2/s. This observed value is orders of magnitude larger than that obtained in the nonreactive case, which is partly due to the increased mobility of the proton, but is mainly attributable to the decrease in proton concentration caused by the neutralization reaction. This results in much steeper pH gradients, which are reflected in the large apparent diffusion coefficient. Thus, the pHsensitive LIF method is indeed capable of accurately measuring scalar transport in reactive flows. To further elucidate the reactive mixing process in the Y-shaped microchannel, we applied a computational fluid dynamics (CFD) model that accounts for convective transport, species diffusion, and reaction; details are provided in the Supporting Information. We simulated both the passive and reactive mixing cases and obtained good agreement between the experimental and numerical results for both.

Figure 9. Secondary flow structures in the wake (left) and in front (right) of a CO2 bubble.

average flow velocity of the liquid phase (bulk velocity), the secondary flow structures in the liquid phase were revealed as internal recirculations induced by the two-phase slip.34 The measured two-dimensional velocity fields showed that these zones of recirculation were at the same location as the regions of increased mass transfer indicated by the pH field. Thus, the pH−LIF technique shows that the interfacial mass transfer is related to the secondary flow structures. Furthermore, we were able to quantify the local mass-transfer coefficient kLa according to the expression8 9002

dx.doi.org/10.1021/ie300978n | Ind. Eng. Chem. Res. 2012, 51, 8999−9006

Industrial & Engineering Chemistry Research kLa =

in ⎞ jL ⎛ c eq − cCO 2 ⎟ ln⎜⎜ eq out ⎟ L ⎝ c − cCO 2⎠

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ε̇G = (5)

(6)

HCO3− + OH− ↔ CO32 − + H 2O

(7)

(9)

and the two-phase holdup εG

where jL denotes the superficial liquid velocity, L is the position in the reactor, ceq is the equilibrium concentration, cinCO2 is the CO2 concentration at the inlet (which is zero for our out conditions), and cCO is the CO2 concentration at the 2 measurement location. The absorption of CO2 in an alkaline solution follows two different reaction steps depending on the local pH35 CO2L + OH− ↔ HCO3−

VĠ VĠ + VL̇

εG =

VG VG + VL

(10)

where Vi (i = G, L) denotes the volume occupied by each phase. Usually, the volume transport fraction does not equal the holdup, and rather, the two quantities are related by the twophase slip s as s=

In an alkaline solution, both reactions occur very rapidly, and reaction 7 can be regarded as instantaneous (the equilibrium has been reported in the literature35). After the physical absorption of gaseous CO2 into the liquid phase (which represents the rate-limiting step), COL2 is available to initiate reaction 6, with the generated bicarbonate ion serving as educt for reaction 7. Hence, the decrease of hydroxide ions (and consequently the decrease of pH) corresponds to the amount of absorbed CO2. The contours of kLa reveal the same structure and connection to the recirculative motions as already observed for the pH (Figure 10). In the vicinity of the bubble, a kLa value

wG ε ̇ (1 − εG) = G wL εG(1 − εĠ )

(11)

where wi (i = G, L) denotes the local phase velocity. We ran two different sets of experiments, the first at a constant mean residence time and varying gas volume transport fractions and the second at constant gas volume transport fractions and varying residence times, enabling us to systematically study the effects of both variables. Figure 11 depicts the

Figure 11. Overall mass-transfer coefficient kLa and interfacial area a at a constant residence time of τ = 10 s and varying gas volume transport fraction ε̇G for the spiral microreactor. Figure 10. Contours of the local mass-transfer coefficient kLa at the back (left) and the front (right) of a CO2 bubble.

overall mass-transfer coefficient kLa and interfacial area a at a constant residence time of τ = 10 s and gas volume transport fractions ε̇G from 0.35 to 0.70 for the spiral microreactor. It can be observed that the interfacial area increased with increasing volume transport fraction, which reflects the increase in bubble length (from 0.44 to 1.62 mm). Consequently, the overall mass-transfer coefficient also increased with increasing volume transport phase fraction. However, as is evident from the difference in slopes, this increase cannot be attributed solely to the increasing interfacial area. The local mass-transfer and velocity field measurements (Figures 9 and 10) indicate that the strength of the internal recirculation in the aqueous phase also influences mass transfer, which is confirmed by the increase of the two-phase slip s from 1.07 to 1.16. Figure 12 depicts the overall mass-transfer coefficient kLa at varying residence times τ and gas volume transport fractions ε̇G ranging from 0.40 to 0.80 for the spiral microreactor (open symbols) and ε̇G = 0.80 for the meandering microreactor (closed symbols). A trend of increasing mass-transfer coefficient with increasing volume transport fraction at identical residence times can again be observed. Furthermore, kLa increased with decreasing residence time (i.e., increasing flow rates). It is interesting to compare the mass-transfer perform-

between 0.01 and 0.02 1/s was computed that increased with decreasing distance to the bubble interface. These contours conclusively demonstrate that the quantification of the masstransfer coefficient using the newly developed technique is feasible. Influence of Microreactor Geometry. To quantify the influence of the microreactor geometry on the overall masstransfer coefficient, we measured kLa values over a range of residence times and gas-phase holdup. The mean residence time τ is given by τ=

VR VĠ + VL̇

(8)

where V̇ i represents the volumetric flow rate of each phase (i = G and L for the gas and liquid phases, respectively) and VR denotes the reactor volume. The phase distribution of the resulting two-phase flow is characterized by the volume transport fraction ε̇G (defined for the gas phase) 9003

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small thickness, the film is easily saturated at these residence times, which means that increasing the interfacial area by increasing the bubble length does not contribute to the overall mass transfer; on the contrary, the bubble length should be kept small. Our experimental results and the derived conclusions are in line with a numerical study on mass transfer in Taylor flow.36 Mass-Transfer Correlations. We used the generated data to test the predictive capability of different mass-transfer correlations available in the literature. Therefore, we selected the correlations of Bercic and Pintar10 kLa = 0.111 Figure 12. Overall mass-transfer coefficient kLa at varying residence times τ and gas volume transport fractions ε̇G for the spiral (open symbols) and meandering (closed symbols) microreactors.

(uG + uL)1.19 [(1 − εG)(LG + L L)]0.57

van Baten and Krishna kLa =

ance between the two microchannel designs: At identical volume transport fractions and in the same range of residence times, the spiral design with the short meandering mixing section shows increased mass-transfer coefficients compared to the reactor consisting entirely of meandering microchannels. This reduced performance of the meandering design is even more surprising because we also observed an increase in interfacial area resulting from longer bubbles formed by the gas−liquid contactor. However, as indicated above, the twophase slip plays a major role in interfacial mass transfer, and as shown in Figure 13, we observed a significant difference in this quantity.

4 2 2 LG + L L π

Vandu et al.

(12)

37

DuG dH

(13)

38

kLa = C1

and Yue et al.

DuG 1 LG + L L dH

with

C1 = 3

(14)

8

ShLadH = 0.084ReG 0.213ReL 0.937Sc L 0.5

(15)

In eqs 12−14, dH denotes the hydraulic diameter, ui is the superficial velocity of each phase (i = G, L), LG denotes the bubble length, and LL denotes the continuous liquid slug length. Equation 15 depends solely on dimensionless numbers, which are the Sherwood number ShL =

kLdH D

(16)

with the mass-transfer coefficient kL and the diffusivity D of the mass-transfer component; the gas- and liquid-phase Reynolds numbers

Rei =

uidH νi

(17)

with the kinematic viscosity of each phase νi; and the Schmidt number, which is given by the ratio of the kinematic viscosity of the liquid phase νL to the diffusivity D of the mass-transfer component ν Sc L = L (18) D

Figure 13. Two-phase slip at a gas volume transport fraction of ε̇G = 0.8 as a function of residence time τ for the spiral (open symbols) and meandering (closed symbols) microreactors.

A comparison between the experimentally observed masstransfer coefficients and the predictions of these correlations is presented in Figure 14. All of the correlations failed to predict the kLa value in the regime of low volume transport fraction ε̇G (points on the left side of Figure 14). However, it should also be noted that it is not feasible to operate systems in this regime because of the very low observed mass-transfer coefficients (see Figure 11). At volume transport fractions above ε̇G = 0.6, good agreement between the experimentally obtained mass-transfer coefficients and the values predicted using the correlations of Bercic and Pintar 10 and Yue at al.8 was observed. From an engineering standpoint, it is preferable to use the correlation of Yue et al.8 (eq 15) for mass-transfer predictions, because it is based solely on dimensionless groups defined by the fluid

At comparable residence times, the two-phase slip in the spiral design was found to always be higher. This effect outweighed the difference in interfacial area for the overall mass-transfer coefficient. After passing the mixing section in the spiral design, the two-phase flow was able to develop undisturbed and establish itself similarly to the case of a straight capillary. In contrast, in the meandering design, the constant change in flow direction hindered this development, resulting in lower two-phase slip. These results conclusively highlight the importance of slip as a determining parameter for interfacial mass transfer, which, in turn, also implies that the contribution to mass transfer from the bubble caps is larger than that from the film surrounding the bubble. Because of its 9004

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good pH sensitivity in the range between 4 and 12, but especially around the neutral point (pH 7). For single-phase flow, the system is capable of capturing the differences in nonreactive and reactive mixing. The diffusion coefficient extracted from the measured concentration profiles for the nonreactive case was in good agreement with literature values. The same procedure resulted in a larger apparent diffusion coefficient for the reactive mixing case, demonstrating that the pH-sensitive LIF technique is capable of also capturing the reduction in proton concentration due to the neutralization reaction. Moreover, the pH−LIF technique was able to resolve the mass transfer around a CO2 bubble. Thus, the developed technique could serve as a useful tool for quantitatively measuring mass transfer in microfluidic systems. Microscale PIV measurements revealed that regions of enhanced interfacial mass transfer were connected to secondary flow structures in the liquid phase. We investigated the influence of the phase distribution and residence time on the mass-transfer coefficients. These results suggested a strong relationship between the strength of the recirculation zone in the continuous phase and the observed mass transfer, because the kLa values increased with decreasing residence time and increasing gas volume transport fraction. The influence of the microchannel geometry on the overall mass transfer was characterized by comparing a spiral-channel design to a meandering-channel design. The spiral design showed increased mass transfer, which was explained by the increased slip for this particular geometry in which the two-phase flow is able to develop undisturbed. Comparison of the experimentally obtained mass-transfer coefficients with predictive correlations available in the literature resulted in large deviations for low volume transport fractions and partial agreement for high volume transport fractions. Therefore, we presented an adapted formulation of a correlation based on dimensionless numbers (ShL, ReG, ReL, ShL) that was able to reproduce the measured data. The results obtained in this study indicate that the strength of the internal recirculation in the continuous phase and, thus, the mass transfer through bubble caps make a major contribution to the overall mass transfer. As a consequence, a recommendation for the design of gas−liquid contactors would be to optimize them toward the generation of bubbles with small axial dimensions.

Figure 14. Comparison of experimentally observed mass-transfer coefficients kLa with predictions from correlations.8,10,37,38 The solid line represents the parity line.

properties and the desired operating conditions. Thus, we constructed a custom formulation based on this correlation with the form ShLadH = C1 + aReGbReL cReL 0.5

(19)

with C1 = 20.8 a = 0.07825 b = 1.779 c = −0.1124

Figure 15 presents the predictive performance of this custom correlation. The largest deviation was again found for very low

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ASSOCIATED CONTENT

S Supporting Information *

Details on CFD modeling of passive and reactive mixing in the Y-shaped reactor. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 15. Comparison of experimentally observed mass-transfer coefficients kLa with the original correlations of Bercic and Pintar10 and Yue at al.8 and the custom correlation presented in eq 19. The solid line represents the parity line.

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volume transport fractions ε̇G, but above ε̇G = 0.6, good agreement between the experimentally measured mass-transfer coefficients and the predicted values using eq 19 can be observed.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Mass transfer in reactive single- and multiphase microfluidic system has been quantified by a pH-sensitive LIF technique using 5-(and-6)-carboxy SNARF-1 as the fluorescent dye with a Nd:YAG laser as the excitation source. The method provides

ACKNOWLEDGMENTS This work has been funded by the Novartis-MIT Center for Continuous Manufacturing. S.K. acknowledges funding from the Swiss National Science Foundation (SNF). 9005

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dx.doi.org/10.1021/ie300978n | Ind. Eng. Chem. Res. 2012, 51, 8999−9006