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In situ Raman imaging combined with computational fluid dynamics for measuring concentration profiles during mixing processes
Chemical Engineering Journal 179 (2012) 338–348
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
In situ Raman imaging combined with computational fluid dynamics for measuring concentration profiles during mixing processes Günter Rinke a,∗ , Achim Wenka a , Karsten Roetmann b , Hainer Wackerbarth b a b
Karlsruhe Institute of Technology (KIT), Institute for Micro Process Engineering, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Laser-Laboratorium Göttingen, Photonic Sensor Technologies, Hans-Adolf-Krebs-Weg 1, D-37077 Göttingen, Germany
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 4 November 2011 Accepted 5 November 2011 Keywords: Raman imaging Light sheets Micro mixer CFD
a b s t r a c t Micro structured components for process engineering have gained increasing importance in chemical, pharmaceutical and life sciences applications. Understanding of mixing processes in those devices is of fundamental interest for their performance. Raman imaging with light sheets is a powerful tool to visualize concentration profiles. For the first time planar Raman imaging is combined with computational fluid dynamic (CFD) calculations to analyze the concentration of two components at the outlet of a micro mixer. The mixing process was monitored within an attached quartz cuvette. The laser light sheet had a thickness of 0.5 mm and a width of 10 mm matching the inner width of the cuvette. In this investigation ethanol and water were mixed. We present measured concentration maps within 10 mm × 10 mm using single shot laser pulses (6 ns). The lateral resolution is only determined by the pixel size of the CCD camera. Standard deviations and averaged data of the concentration profiles are compared with CFD calculations. The computations were done for a laminar flow at constant temperature under non-steadystate conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1.1. In situ methods Production and combustion processes as well as many analytical detection methods require reactions and thus mixing of the involved chemicals. In the dimensions of micro fluidics, mixing processes are often based on diffusion. This is due to the small system sizes and slow flow velocities within each micro channel. Optimization of micro mixers and the understanding of the fluid flows are objectives of computational fluid dynamics (CFD) calculations which require high-quality experimental data sets for validation of their numerical approaches. It is reported [1] that chemical reactions can be monitored with infrared spectroscopy in 10 m deep micro channels sandwiched between two infrared transmitting CaF2 discs. Antes et al. [2] examined strong exothermic nitration reactions in micro reactors made of silicon, measuring the transmission of a micro channel by online FT-IR microscopy. In a similar way Keoschkerjan et al. [3] characterized micro reactors using IR reflection spectroscopy. The mixing process within thin silicon micro mixers was examined with infrared spectroscopy and visual observation with a CCD camera [4]. Though infrared spectroscopy can measure many
∗ Corresponding author. Tel.: +49 721 608 23556; fax: +49 721 608 23186. E-mail address: [email protected] (G. Rinke). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.11.016
chemical compounds selectively, it is often limited by strong absorption, especially in aqueous media. Therefore liquids within micro channels can only be monitored when the channel depths are very small [3,4]. Absorption of aqueous media is not a problem in Raman spectroscopy, which is also selective for individual chemical compounds; the Raman spectrum provides a fingerprint of the molecules, enabling identification of the substances. Moreover, the intensities of the Raman bands are proportional to the concentration of the species, thus allowing quantification. A disadvantage of Raman spectroscopy is its sensitivity which is some orders of magnitude lower compared to IR spectroscopy. Therefore measurable concentrations should lie in the region of 0.1% and above. This is hardly ever a disadvantage in micro chemical engineering where often educts with high concentrations are used. Salmon et al. [5] used a Raman microscope to observe the mixing processes of chloroform and methylene chloride within a commercially available micro device. Rinke et al. [6] used a Raman microscope to monitor a chemical reaction within a home-made micro reactor. Park et al. [7] applied both, confocal fluorescence spectroscopy and Raman spectroscopy, to monitor the mixing processes of ethanol and isopropanol in a micro mixer. Sarrazin et al. [8] used a commercial Raman microscope to monitor the concentration profiles during isotopic exchange reaction between D2 O and H2 O within a micro mixer consisting of PDMS. Some more applications emerge in the literature [9–12]. However, these approaches only used the spectral information or their temporal evolutions.
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Research is done combining Raman spectroscopy and CFD, e.g., Sung et al. [13] measured Raman spectra in flames and Luo et al. [14] compared these results with CFD. However, the Raman measurements were done by focussing the laser light to a spot and not to an image which is more complicated. Egawa et al. [15] used Raman spectroscopy and imaging to analyze a micro mixer. However, Raman spectroscopy was used with a spot focus only and imaging was done with fluorescence which cannot be selective in general. Dealing with two-dimensional visualization of the concentrations during micro mixing laser induced fluorescence (LIF) is applied. Being sensitive and rapid, low concentrations can be measured quickly. A disadvantage of this technique is that only few substances have a native fluorescence which can be used for detection. Moreover, quenching processes delimit quantitative analysis and chemical selectivity is worse in most cases. Hence, only a few chemical reactions or pure mixing can be observed with LiF, for example Hoffmann et al. [16] used a confocal scanning system to examine the 3D distribution of the concentrations of a pure mixing process within T-shaped micro mixers. To our knowledge, two-dimensional visualizing of a micro flow analysis by using Raman scattering has performed so far only by Roetmann et al. [10]. They developed planar spontaneous Raman scattering or short Raman imaging to monitor the spatial distribution of concentration fields for different components. The setup was used to measure the concentration distribution in the volume of the mixing path of a special mixer. Therefore the volume is divided in several planes and these are scanned by light sheets. In this paper, detailed concentration distributions during the mixing process of ethanol and water recorded by planar spontaneous Raman scattering are presented. The Raman imaging set up developed by Roetmann et al. [10] is modified to fulfill the requirements on the data for comparison with the CFD calculations which are discussed in the corresponding paragraphs. Recent comprehensive experimental data are compared with extensive numerical simulations (CFD) for the first time.
1.2. Micro mixers The use of micro structured components for process engineering has gained increasing importance in chemical, pharmaceutical and life sciences applications during the last years [17–20]. Small devices – reactors, heat exchangers, static mixers and other process components – can be fabricated in configurations scaled in millimeters and embedded in micrometer-sized channels. Due to large specific surface areas, devices with these small dimensions provide more efficient mass and heat transfer. This can result in greater selectivity and higher yield for chemical reactions [19,21]. In the last years, the development of countless techniques to accelerate and improve mixing was promoted. A review about different available types of micro mixers outlines what is possible today [22,23]. At the Karlsruhe Institute of Technology (KIT) micro structured devices can be made where widths and depths of the micro channels lie in the range of 0.1–0.2 mm. Examples are cross or counter flow micro heat exchangers, electrically powered micro heat exchangers, micro mixers as well as micro structured reactors for chemical reactions with liquid or gaseous media [24]. These components are pressure resistant up to several hundred bars and have high heat transfer coefficients. As one example a cross flow micro heat exchanger with an outer volume of 1 × 10−6 m−3 and a heat transfer surface area of 0.0045 m−3 allows a heat transfer of 10 kW between two passages at a throughput of 1000 kg/h per passage at 3.5 bar pressure drop. In this case, cold water of 10 ◦ C at one inlet was warmed up to 18 ◦ C at the outlet whereas hot water
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of 95 ◦ C input temperature was cooled down to 85 ◦ C at the outlet. The heat transfer coefficient is 30 kW m−2 K−1 [24]. The production of these microstructured devices is based on mechanical micro machining of metal foils. Micromechanical processes are for instance precision turning, precision milling and micro etching. The following materials have been used: stainless steel as standard, hastelloy, aluminium alloys and different metals like silver, palladium, rhodium and copper. The foils are joined by thermal diffusion bonding at high temperature. The diffusion bonded body is then welded in a standardized housing. These micro heat exchangers and micro reactors have already found industrial applications. Areas of applications are for example chemical production [24,25] and biodiesel production [26]. Fig. 1(left part) shows a complete micro mixer made from stainless steel with two inlets and one outlet where all metal foils are diffusion bonded. The principle of construction of this micro mixer is shown in the right part of Fig. 1. It consists of several plates, typically 200 m thick, which contain grooves at an angle of 45◦ . These micro mixers usually contain 120 micro channels in total, each with a width of 100 m and a depth of 100 m. Mixing is achieved by fast diffusion at the outlet channels which are separated 200 m in height.
2. Experimental 2.1. General remarks The measurements presented here were carried out with water and ethanol for a first demonstration. Ethanol as an organic solvent has an intense characteristic band in the Raman spectrum at 2900 cm−1 (CH-band). Moreover, water and ethanol show broad Raman OH-bands at approximately 3300 cm−1 . The measurement of water and ethanol can disturb the laser radiation within the cuvette because the refractive indices of water (1.3330) and ethanol (1.361) are different. Therefore the refractive index of water should be raised to the value of ethanol by addition of other compounds to water. Some conditions must be fulfilled: the additional component must not fluoresce and does not show a strong absorption spectrum or an overlapping Raman spectrum. Furthermore, this compound must not be reactive and must be soluble. Small inorganic compounds are suitable, e.g., CaCl2 which dissociates into single ions without emitting a Raman spectrum. We chose 12.1 mass% CaCl2 in water giving a refractive index of 1.362 and so resulting in a good refractive index matching with ethanol.
2.2. Experimental setup 2.2.1. Chemicals and fluidics We used bi-distilled water and removed dissolved oxygen with an ultrasonic instrument. Ethanol was “p.a.” grade from Merck Chemical Germany and CaCl2 from VWR International. The liquid flow inside the micro channels is driven by air pressure in a reservoir and controlled by precision valves. Fig. 2 shows the fluidic setup schematically. The liquids are filled into the reservoirs via a 2-way valve. Then pressured air is applied via manifold valves. The constant gas pressure guarantees a stable and pulsation free flow. The throughputs are determined by needle valves and coarsely measured by a float-type flow meter and more precisely measured by the height difference of the liquid from the beginning to the end of a measurement and the corresponding time. The accuracy is mainly determined by the precision of the height measurement and amounts to ±10%. Degassing components are used to eliminate gas bubbles.
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Fig. 1. Standard micro mixer of stainless steel with bonded foils (left), schematical presentation of the foil stack (right), construction of the mixer used here (below).
2.2.2. Optics The experimental system is shown in Fig. 3. Compared to the set up used in the studies of Roetmann et al. [10], a reference branch and special optics for light sheet formation are added. A similar set up will be described by Wellhausen, Rinke and Wackerbarth [unpublished results]. The latter is a special optical design for the generation of a homogeneous sheet. Here, the radiation of a pulsed
Nd:YAG laser at 532 nm (6 ns pulse duration, 10 Hz repetition rate) is divided into two parts by a beam splitter, a measuring branch (40%) and a reference branch (60%). This reference branch was used to correct time dependent fluctuations of the laser intensity. After dividing all intensity values of the CCD camera in the measuring path through the average values of the reference camera, the residual pulse to pulse fluctuations amounted ±1%.
Fig. 2. Fluidic system schematically, containing reservoirs for water and ethanol, which are delivered to the micro mixer by air pressure.
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Fig. 3. Experimental Raman system setup with light sheets. The flow direction within the micro mixer is perpendicular to this picture, see Fig. 5.
The Raman filter is a central element for these measurements. It has to overlap the Raman spectrum of the samples to be observed. Fig. 4 shows the transmission of the filter as well as the Raman spectra of liquid ethanol and water. Especially the CH– and the OHbands of ethanol and water are interesting, if mixtures of these fluids are to be observed. The intense CH-band of ethanol nearly fits the range of the filter. The spectra of both fluids show broad OH-bands around 650 nm, but the one of ethanol is much weaker. A small contribution from the OH-band of water is within the area of the filter. This applies also to mixtures consisting of both components. This light can be removed easily by a black- and white-image correction. The black-image contains the intensities measured of pure water and the white-image is measured by using pure ethanol. The intensity of the black-image is subtracted from the measured intensity and subsequently divided by the difference of the whiteand black-image intensities. This results in a standardized intensity
Fig. 4. Transmission of the Raman filter (red), Raman spectra of liquid ethanol (black) and water (blue). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
between zero and one which is equivalent to the particle number concentration. The laser pulse duration of approximately 6 ns gives the exposure time; the acquisition rate is limited by the readout frequency of the CCD camera and amounted to 2 Hz. In this case this is much to slow to measure the time resolved processes. At each measurement position 100 pictures were recorded during 50 s and every picture contains 5 laser pulses every 100 ms. The laser light sheet had a height of 10 mm and a thickness of 0.5 mm. Therefore the Raman intensity is averaged over a depth of 0.5 mm. A calibration procedure was employed to convert intensity values into concentrations. 2.2.3. Micro mixers The micro mixer used for these experiments should contain much larger micro channels because the CFD calculations would be faster. Furthermore, this micro mixer should have a nearly ideal rectangular cross section, especially at the outlet, to avoid distortion of the flow profile. Otherwise, experiment and simulation might differ. For this reason in this case we did not use diffusion bonding, where slight deformations of the micro channels are possible. Instead, we clamped all plates. Fig. 1(lower part) shows the applied geometry in detail. This micro mixer was made from an aluminium alloy (AlMg3 , material code 3.3535) and consisted of 4 plates. In each of these 2 mm thick plates, 4 micro channels were milled with 0.9 mm width, 1.0 mm depth and 14.1 mm length, which were separated 0.45 mm. Because of the angle of 45◦ the outlet cross section of each micro channel was 1.27 mm wide and 1.0 mm high. The mixer had 16 micro channels in total, 8 per passage. This stack was arranged in such a way that two neighboring plates led fluids of the two passages. The stack of these micro structured foils was clamped on one side to an adapter with fluid connections (Fig. 5, right part). Above the stack with the 16 micro channel outlets a rectangular optical tube with x-coordinate in flow direction was attached by a thin PTFE foil. This cell is based on a standard cuvette from Hellma and made of fused silica, but with open ends. It was 40 mm long, has an inner cross section of 10 mm × 10 mm and an outer cross section of 12.5 mm × 12.5 mm. At the top of this cell a further adapter with a PTFE foil and a fluid connection was clamped. The left part of Fig. 5 shows the arrangement of the 16 outlet channels, red indicates ethanol and blue water. The coordinate system is shown, too.
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Fig. 5. Micro mixer attached to a cuvette and the coordinate system used here. The x-coordinate is perpendicular to this picture and corresponds to the flow direction. The zoom shows the scheme of the outlets channels below the cuvette with an inner width of 10 mm. The transparent rectangular band shall illustrate the laser light sheet.
3. Results 3.1. Measurement of concentration fields All measurements are based on the coordinate system shown in Fig. 5. The laser sheet selected a plane cutting through the inlets of ethanol, water or in between. We used two flow rates, 0.6 l/h and 6 l/h per fluid. This corresponds to linear velocities of 0.23 m s−1 and a Reynolds number of 22 at the outlet of each micro channel at 0.6 l/h per fluid and 2.3 m s−1 and 220, respectively. Fig. 6 shows the mean values of 100 pictures of measurements at low throughput (0.6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The distribution of ethanol is clearly
visible, especially when the laser sheet is positioned at the outlets of the ethanol micro channels. The intensity values within the filaments of ethanol are near one as expected. The filaments are nearly vertical. Between the filaments, ethanol is mixed with water in a lower concentration. Above 20 mm height ethanol and water are mixed and no filaments are seen in these averaged pictures. However, if single shots are viewed, inhomogenities still can be detected (see below). Interesting is the black layer rising from the left to the right at the bottom in Fig. 6. This is not an artifact, but corresponds to a nearly 100% concentration of water. Furthermore, we first expected that at laser sheet position Z = 4 mm black filaments for water would arise similar to the ethanol filaments at position Z = 2 mm. We will discuss both effects later.
Fig. 6. Measured concentration image of ethanol–water-mixtures, slow throughput (1.2 l/h total), mean values, z-planes: 1, 2, 3, 4 mm.
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Fig. 7. Image of the measured variance of the concentration of ethanol–water-mixtures, slow throughput (1.2 l/h total), z-planes: 1, 2, 3, 4 mm.
Fig. 7 shows the variance of the 100 pictures from Fig. 6 at low throughput (0.6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The fluctuations are large for the ethanol filaments. Fig. 8 shows the mean values of 100 pictures of measurements at fast throughput (6 l/h per component) for 4 Z-positions. The distribution of ethanol is clearly visible, especially of course when the
laser sheet is positioned at the outlets of the ethanol micro channels. The intensity values within the ethanol filaments are near one as expected. The filaments now have an angle of 45◦ , which is the result of the micro channels cut at 45◦ (Fig. 1) and the higher velocity. Above 20 mm height ethanol and water are mixed again and no filaments are seen in theses averaged pictures. However, viewing single shots, inhomogenities still can be detected (see below).
Fig. 8. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), mean values, z-planes: 1, 2, 3, 4 mm.
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Fig. 9. Image of the measured variance of the measured concentration of ethanol–water-mixtures, fast throughput (12 l/h total), z-planes: 1, 2, 3, 4 mm.
The black layer at the bottom observed at lower throughputs (Fig. 6) is absent here. Instead, black filaments (pure water) are observed. The reason is the higher momentum of the water jet in this case. At lower velocities this momentum is not strong enough to compete with gravity which leads to the usual distribution of densities (1100 kg m−3 for water-salt-solution and 790 kg m−3 for ethanol). Looking at laser sheet position Z = 3 mm ethanol filaments at a height of approximately X = 3 mm can be seen. These ethanol
streams come from the plane Z = 2 mm where they vanish at a height of X = 3 mm. Fig. 9 shows the variance of 100 pictures from Fig. 8 at high throughput (6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The fluctuations are large for the ethanol filaments. A further interesting effect is that there are two different areas within the cuvette (Fig. 8): below X = 15 mm (areas X1 and X2) filaments are seen and mixing is not complete. Above X = 15 mm
Fig. 10. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), area X2Z4, averaged values (left) and shingle shot (right).
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Fig. 11. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), area X4Z2, averaged values (left) and single shot (right).
no filaments are seen in these averaged pictures. To understand this behavior in more detail, it is necessary to compare averaged pictures with single shot pictures. Fig. 10 shows the area X2Z4 as averaged picture (left part) and as a single shot (right part). Fluctuations of ethanol and water can still be seen with a single shot. Further single shot measurements show that filaments of ethanol at first flow at an angle of 45◦ to the left vertical glass wall and later nearly flow vertically. However, the water filaments firstly flow at an angle of 45◦ to the right vertical glass wall, then rotate and flow in the direction of the ethanol filaments. This upper interface at a height of 15 mm may be a result of the higher density of water. The height at which the left water filament (Fig. 8, bottom right) hits the glass wall can be calculated by geometrical considerations to 9 mm. This is below the transition zone at X = 15 mm. The upper areas in Fig. 8 (X3–X4) suggest that mixing is complete. However, these pictures are mean values of 100 measurements. Fig. 11 compares the highest region in the cuvette (X4Z2), averaged pictures on the left side and a single shot on the right side. Although the averaged picture shows a nearly homogeneous concentration distribution, the single shot shows filaments indicating that mixing is not complete.
In these equations is the density, v is the velocity vector, is the tension tensor, g is the gravitation vector, Yi is the mass fraction and Ji is the diffusion flux of species i. The computations were done for a laminar flow at constant temperature under non-steady-state conditions. In the simulations two fluids were modeled, ethanol with a density of 789 kg/m3 and water with a density of 998 kg/m3 . In a computational cell the mixed density was calculated by using the volume fraction of ethanol. The quartz cuvette as mixing chamber was modeled as shown in Fig. 12. For its discretisation 500,000 cubical elements in total were used. Each had a side length of 0.2 mm. The complete mass flow of one passage/section was distributed uniformly over the eight channels and a plug-flow velocity profile was set as boundary condition at each inlet. During the flow within each micro channel a parabolic velocity profile will develop.
3.2. CFD calculations In order to compare the experimental results, computational fluid dynamics (CFD) was applied. We used the CFD program FLUENT/ANSYS (version 12.1.4) to solve the following equations [27,28]: Continuity equation (conservation of mass): ∂ + ∇ · (v) = 0 ∂t Navier–Stokes equation (conservation of momentum): ∂ (v) + ∇ · (vv) = −∇ p + ∇ · ( ) + g ∂t Transport equation for species: ∂ (Yi ) + ∇ · (vYi ) = −∇ · Ji ∂t
Fig. 12. CFD-model of the mixing and observation tube (10 mm × 10 mm × 40 mm) represented by 500,000 cubical elements with a side length of 0.2 mm.
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Fig. 13. CFD-simulation of the concentration map of ethanol–water-mixtures, slow throughput, z-planes: 1, 2, 3, 4 mm.
The time step size chosen was 10 ms for the low flow-rate and 1 ms for the high flow-rate. The mass flux chosen for the low flowrate was 0.49 kg/h (0.62 l/h) for ethanol and 0.52 kg/h (0.52 l/h) for water and for the high flow-rate 4.64 kg/h (5.9 l/h) and 6.47 kg/h (6.5 l/h), respectively. These mass fluxes were the mean values from all measurements. The SIMPLE scheme was used for the pressure velocity coupling. The spatial discretisation of the pressure was
done using the standard method of FLUENT and the variables for momentum and species were discretised by the QUICK scheme. Fig. 13 shows the results of the numerical computations for the lower flow rate. These pictures correspond to the measurements shown in Fig. 6. However, it is a snap shot (10 ms time step) without averaging. The ethanol filaments rising nearly vertically are seen at position Z = 2 mm and vanishing at a height of 20 mm. The
Fig. 14. CFD-simulation of the concentration map of ethanol–water-mixtures, fast throughput, z-planes: 1, 2, 3, 4 mm.
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black layer of water is seen, too. If gravitation is not considered in the Navier–Stokes equations, this layer at the bottom is absent and water filaments could be seen. The videos (as supplemental files for Z = 2 mm and Z = 4 mm) of this simulation show some time dependent fluctuations of concentrations, even at a height of 20 mm. This is seen in this snap shot, too. Fig. 14 visualizes the results of the numerical computations for the higher flow rate. These pictures correspond to the measurements shown in Fig. 8. However, it is a snap shot (1 ms time step) without averaging. The filaments have an angle of 45◦ as observed by our laser Raman measurements. Above 20 mm height no straight filaments are seen, but an incomplete mixing. The black layer at the bottom observed at lower throughputs (Fig. 6 and Fig. 13) is absent here. Instead, black filaments (pure water) are observed as in the experiments (Fig. 8). These simulations further confirm the observed two different areas within the cuvette (Fig. 8). A video (as supplemental files for Z = 2 mm and Z = 4 mm) of Fig. 14 more clearly shows the vortex of water in the lower right part of the cuvette. The time dependent variations in the concentrations observed in the single shot measurements (Figs. 10 and 11) qualitatively correspond to these simulations. A direct quantitative comparison between experiment and simulation is not possible because the mixing process is instationary. The video of Fig. 14 gives a more detailed view of the nonstationary behavior of concentrations within the cuvette, even at the top of the cuvette. It should be mentioned that normally these micro mixers contain much more micro channels and these are operated at higher throughputs. In these cases the mixing behavior is much better, of course. Investigations of our standard micro mixer show that mixing is good when linear velocity at the outlet of each micro channel is larger than 2 m s−1 [29]. In this work one important aim was a comparison between measurements and CFD. A CFD simulation with smaller and much more micro channels would be difficult and would take a long computation time. Therefore we used larger micro channels than usual and we only reached a linear velocity in each micro channel of 0.023 m s−1 at the low flow rate and 0.23 m s−1 at the high flow rate.
4. Conclusion and outlook We have shown that pulsed Raman imaging can be used to measure concentration profiles during a mixing process. In this case the mixing behavior of ethanol and water at the outlet of a micro mixer was measured. A special optics for light sheet formation is applied to generate planes for monitoring the mixing path above the outlet of the micro mixer. The planes of the laser light sheet are put together so that the concentrations can be mapped in three dimensions. With the optics used here, a light sheet with 10 mm × 10 mm can be observed, the lateral resolution being determined by the pixels of the CCD camera. The depth resolution amounts 500 m. One light sheet is recorded in 6 ns. The frame rate is determined by the readout time of the CCD camera (0.5 s) and not by the laser. With this technique it was possible to monitor ethanol filaments within a plane of 10 mm × 10 mm with one laser shot (6 ns). The temporal progress was measured with a rate of 2 Hz. The experimental results agree well with CFD calculations. The computations were done for a laminar flow at constant temperature under nonsteady-state conditions. The spatial discretisation of the pressure was done using the standard method of FLUENT and the variables for momentum and species were discretised by the QUICK scheme. This method can be applied not only to mixing processes, but for monitoring chemical reactions, too. This is an advantage in comparison with laser induced fluorescence, where only mixing
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processes with fluorescent substances can be monitored or very special cases where a product shows fluorescence. The optical setup may be improved concerning time resolution when a more powerful laser and a faster CCD camera would be used. Without loss of geometrical resolution (pixel numbers) and signal to noise ratio, a commercially available expensive EMCCD camera with a frame rate of 35 fps instead of 2 Hz could be used. This would lead to a time resolution of 30 ms. If a laser with 35 Hz and 200 mJ pulse energy would be used, there would be no decrease in performance. If geometrical resolution can be reduced, binning of CCD pixels is possible, which would decrease the readout time even lower. In this case the limiting time resolution (10 ms) would be the laser repetition rate. This would be the best time-resolution without degrading performance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2011.11.016. Please note that the videos use codecs which not all players accept. The VLC player should work. References [1] P. Hinsmann, et al., Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution, Lab on a Chip 1 (2001) 16–21. [2] J. Antes, D. Boskovic, H. Krause, S. Loebbecke, N. Lutz, T. Tuercke, W. Schweikert, Analysis and improvement of strong exothermic nitrations in microreactors, Chemical Engineering Research and Design 81 (2003) 760–765. [3] R. Keoschkerjan, M. Richter, D. Boskovic, F. Schnürer, S. Löbbecke, Novel multifunctional microreaction unit for chemical engineering, Chemical Engineering Journal 101 (2004) 469–475. [4] T.M. Floyd, M.A. Schmidt, K.F. Jensen, Silicon micromixers with infrared detection for studies of liquid-phase reactions, Industrial & Engineering Chemistry Research 44 (2005) 2351–2358. [5] J.B. Salmon, A. Ajdari, P. Tabeling, L. Servant, D. Talaga, M. Joanicot, In situ Raman imaging of interdiffusion in a microchannel, Applied Physics Letters 86 (2005) 094106. [6] G. Rinke, A. Ewinger, S. Kerschbaum, M. Rinke, In situ Raman spectroscopy to monitor the hydrolysis of acetal in microreactors, Microfluidics and Nanofluidics 10 (2011) 145–153. [7] T. Park, M. Lee, J. Choo, Y.S. Kim, E.K. Lee, D.J. Kim, S.H. Lee, Analysis of passive mixing behavior in a poly(dimethylsiloxane) microfluidic channel using confocal fluorescence and Raman microscopy, Applied Spectroscopy 58 (2004) 1172–1179. [8] F. Sarrazin, J.-B. Salmon, Chemical reaction imaging within microfluidic devices using confocal Raman spectroscopy: the case of water and deuterium oxide as a model system, Analytical Chemistry 80 (2008) 1689–1695. [9] A. Ewinger, G. Rinke, S. Kerschbaum, M. Rinke, K. Schubert, Ramanspectroscopy for measuring chemical reactions in micro reactors, in: Proceedings of the 6th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2008 (PART A), 2008, pp. 747–748. [10] K. Roetmann, W. Schmunk, C. Garbe, V. Beushausen, Micro-flow analysis by molecular tagging velocymetry and planar Raman-scatttering, Experiments in Fluids 44 (2008) 419–430. [11] M. Lee, J.P. Lee, H. Rhee, J. Choo, Y.G. Chai, E.K. Lee, Applicability of laserinduced Raman microscopy for in situ monitoring of imine formation in a glass microfluidic chip, Journal of Raman Spectroscopy 34 (2003) 737–742. [12] S.A. Leung, R.F. Winkle, R.C.R. Wootton, A.J. deMello, A method for rapid reaction optimisation in continuous-flow microfluidic reactors using online Raman spectroscopic detection, Analyst 130 (2004) 46–51. [13] C.J. Sung, J.B. Liu, C.K. Law, Structural response of counterflow diffusion flames to strain rate variations, Combustion and Flame 102 (1995) 481–492. [14] C. Luo, B. Moghtaderi, E. Kennedy, B. Dlugogorski, Three-dimensional numerical study on flames, Chemical Product and Process Modeling 4 (2009), art. no. 10. [15] T. Egawa, J.L. Durand, E.Y. Hayden, D.L. Rousseau, D.-R. Yeh, Design and evaluation of a passive alcove-based microfluidic mixer, Analytical Chemistry 81 (2009) 1622–1627. [16] M. Hoffmann, M. Schlüter, N. Räbiger, Experimental investigation of liquid–liquid mixing in T-shaped micro-mixers using -LIF and -PIV, Chemical Engineering Science 61 (2006) 2968–2976. [17] V. Hessel, A. Renken, J.C. Schouten, J. Yoshida, Micro Process Engineering: A Comprehensive Handbook, Wiley–VCH, Weinheim, 2009. [18] T. Wirth, Microreactors in Organic Synthesis and Catalysis, Wiley–VCH, Weinheim, 2008. [19] P. Watts, C. Wiles, Recent advances in synthetic micro reaction technology, Chemical Communications 5 (2007) 443–467.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
In situ Raman imaging combined with computational fluid dynamics for measuring concentration profiles during mixing processes Günter Rinke a,∗ , Achim Wenka a , Karsten Roetmann b , Hainer Wackerbarth b a b
Karlsruhe Institute of Technology (KIT), Institute for Micro Process Engineering, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Laser-Laboratorium Göttingen, Photonic Sensor Technologies, Hans-Adolf-Krebs-Weg 1, D-37077 Göttingen, Germany
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 4 November 2011 Accepted 5 November 2011 Keywords: Raman imaging Light sheets Micro mixer CFD
a b s t r a c t Micro structured components for process engineering have gained increasing importance in chemical, pharmaceutical and life sciences applications. Understanding of mixing processes in those devices is of fundamental interest for their performance. Raman imaging with light sheets is a powerful tool to visualize concentration profiles. For the first time planar Raman imaging is combined with computational fluid dynamic (CFD) calculations to analyze the concentration of two components at the outlet of a micro mixer. The mixing process was monitored within an attached quartz cuvette. The laser light sheet had a thickness of 0.5 mm and a width of 10 mm matching the inner width of the cuvette. In this investigation ethanol and water were mixed. We present measured concentration maps within 10 mm × 10 mm using single shot laser pulses (6 ns). The lateral resolution is only determined by the pixel size of the CCD camera. Standard deviations and averaged data of the concentration profiles are compared with CFD calculations. The computations were done for a laminar flow at constant temperature under non-steadystate conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1.1. In situ methods Production and combustion processes as well as many analytical detection methods require reactions and thus mixing of the involved chemicals. In the dimensions of micro fluidics, mixing processes are often based on diffusion. This is due to the small system sizes and slow flow velocities within each micro channel. Optimization of micro mixers and the understanding of the fluid flows are objectives of computational fluid dynamics (CFD) calculations which require high-quality experimental data sets for validation of their numerical approaches. It is reported [1] that chemical reactions can be monitored with infrared spectroscopy in 10 m deep micro channels sandwiched between two infrared transmitting CaF2 discs. Antes et al. [2] examined strong exothermic nitration reactions in micro reactors made of silicon, measuring the transmission of a micro channel by online FT-IR microscopy. In a similar way Keoschkerjan et al. [3] characterized micro reactors using IR reflection spectroscopy. The mixing process within thin silicon micro mixers was examined with infrared spectroscopy and visual observation with a CCD camera [4]. Though infrared spectroscopy can measure many
∗ Corresponding author. Tel.: +49 721 608 23556; fax: +49 721 608 23186. E-mail address: [email protected] (G. Rinke). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.11.016
chemical compounds selectively, it is often limited by strong absorption, especially in aqueous media. Therefore liquids within micro channels can only be monitored when the channel depths are very small [3,4]. Absorption of aqueous media is not a problem in Raman spectroscopy, which is also selective for individual chemical compounds; the Raman spectrum provides a fingerprint of the molecules, enabling identification of the substances. Moreover, the intensities of the Raman bands are proportional to the concentration of the species, thus allowing quantification. A disadvantage of Raman spectroscopy is its sensitivity which is some orders of magnitude lower compared to IR spectroscopy. Therefore measurable concentrations should lie in the region of 0.1% and above. This is hardly ever a disadvantage in micro chemical engineering where often educts with high concentrations are used. Salmon et al. [5] used a Raman microscope to observe the mixing processes of chloroform and methylene chloride within a commercially available micro device. Rinke et al. [6] used a Raman microscope to monitor a chemical reaction within a home-made micro reactor. Park et al. [7] applied both, confocal fluorescence spectroscopy and Raman spectroscopy, to monitor the mixing processes of ethanol and isopropanol in a micro mixer. Sarrazin et al. [8] used a commercial Raman microscope to monitor the concentration profiles during isotopic exchange reaction between D2 O and H2 O within a micro mixer consisting of PDMS. Some more applications emerge in the literature [9–12]. However, these approaches only used the spectral information or their temporal evolutions.
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Research is done combining Raman spectroscopy and CFD, e.g., Sung et al. [13] measured Raman spectra in flames and Luo et al. [14] compared these results with CFD. However, the Raman measurements were done by focussing the laser light to a spot and not to an image which is more complicated. Egawa et al. [15] used Raman spectroscopy and imaging to analyze a micro mixer. However, Raman spectroscopy was used with a spot focus only and imaging was done with fluorescence which cannot be selective in general. Dealing with two-dimensional visualization of the concentrations during micro mixing laser induced fluorescence (LIF) is applied. Being sensitive and rapid, low concentrations can be measured quickly. A disadvantage of this technique is that only few substances have a native fluorescence which can be used for detection. Moreover, quenching processes delimit quantitative analysis and chemical selectivity is worse in most cases. Hence, only a few chemical reactions or pure mixing can be observed with LiF, for example Hoffmann et al. [16] used a confocal scanning system to examine the 3D distribution of the concentrations of a pure mixing process within T-shaped micro mixers. To our knowledge, two-dimensional visualizing of a micro flow analysis by using Raman scattering has performed so far only by Roetmann et al. [10]. They developed planar spontaneous Raman scattering or short Raman imaging to monitor the spatial distribution of concentration fields for different components. The setup was used to measure the concentration distribution in the volume of the mixing path of a special mixer. Therefore the volume is divided in several planes and these are scanned by light sheets. In this paper, detailed concentration distributions during the mixing process of ethanol and water recorded by planar spontaneous Raman scattering are presented. The Raman imaging set up developed by Roetmann et al. [10] is modified to fulfill the requirements on the data for comparison with the CFD calculations which are discussed in the corresponding paragraphs. Recent comprehensive experimental data are compared with extensive numerical simulations (CFD) for the first time.
1.2. Micro mixers The use of micro structured components for process engineering has gained increasing importance in chemical, pharmaceutical and life sciences applications during the last years [17–20]. Small devices – reactors, heat exchangers, static mixers and other process components – can be fabricated in configurations scaled in millimeters and embedded in micrometer-sized channels. Due to large specific surface areas, devices with these small dimensions provide more efficient mass and heat transfer. This can result in greater selectivity and higher yield for chemical reactions [19,21]. In the last years, the development of countless techniques to accelerate and improve mixing was promoted. A review about different available types of micro mixers outlines what is possible today [22,23]. At the Karlsruhe Institute of Technology (KIT) micro structured devices can be made where widths and depths of the micro channels lie in the range of 0.1–0.2 mm. Examples are cross or counter flow micro heat exchangers, electrically powered micro heat exchangers, micro mixers as well as micro structured reactors for chemical reactions with liquid or gaseous media [24]. These components are pressure resistant up to several hundred bars and have high heat transfer coefficients. As one example a cross flow micro heat exchanger with an outer volume of 1 × 10−6 m−3 and a heat transfer surface area of 0.0045 m−3 allows a heat transfer of 10 kW between two passages at a throughput of 1000 kg/h per passage at 3.5 bar pressure drop. In this case, cold water of 10 ◦ C at one inlet was warmed up to 18 ◦ C at the outlet whereas hot water
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of 95 ◦ C input temperature was cooled down to 85 ◦ C at the outlet. The heat transfer coefficient is 30 kW m−2 K−1 [24]. The production of these microstructured devices is based on mechanical micro machining of metal foils. Micromechanical processes are for instance precision turning, precision milling and micro etching. The following materials have been used: stainless steel as standard, hastelloy, aluminium alloys and different metals like silver, palladium, rhodium and copper. The foils are joined by thermal diffusion bonding at high temperature. The diffusion bonded body is then welded in a standardized housing. These micro heat exchangers and micro reactors have already found industrial applications. Areas of applications are for example chemical production [24,25] and biodiesel production [26]. Fig. 1(left part) shows a complete micro mixer made from stainless steel with two inlets and one outlet where all metal foils are diffusion bonded. The principle of construction of this micro mixer is shown in the right part of Fig. 1. It consists of several plates, typically 200 m thick, which contain grooves at an angle of 45◦ . These micro mixers usually contain 120 micro channels in total, each with a width of 100 m and a depth of 100 m. Mixing is achieved by fast diffusion at the outlet channels which are separated 200 m in height.
2. Experimental 2.1. General remarks The measurements presented here were carried out with water and ethanol for a first demonstration. Ethanol as an organic solvent has an intense characteristic band in the Raman spectrum at 2900 cm−1 (CH-band). Moreover, water and ethanol show broad Raman OH-bands at approximately 3300 cm−1 . The measurement of water and ethanol can disturb the laser radiation within the cuvette because the refractive indices of water (1.3330) and ethanol (1.361) are different. Therefore the refractive index of water should be raised to the value of ethanol by addition of other compounds to water. Some conditions must be fulfilled: the additional component must not fluoresce and does not show a strong absorption spectrum or an overlapping Raman spectrum. Furthermore, this compound must not be reactive and must be soluble. Small inorganic compounds are suitable, e.g., CaCl2 which dissociates into single ions without emitting a Raman spectrum. We chose 12.1 mass% CaCl2 in water giving a refractive index of 1.362 and so resulting in a good refractive index matching with ethanol.
2.2. Experimental setup 2.2.1. Chemicals and fluidics We used bi-distilled water and removed dissolved oxygen with an ultrasonic instrument. Ethanol was “p.a.” grade from Merck Chemical Germany and CaCl2 from VWR International. The liquid flow inside the micro channels is driven by air pressure in a reservoir and controlled by precision valves. Fig. 2 shows the fluidic setup schematically. The liquids are filled into the reservoirs via a 2-way valve. Then pressured air is applied via manifold valves. The constant gas pressure guarantees a stable and pulsation free flow. The throughputs are determined by needle valves and coarsely measured by a float-type flow meter and more precisely measured by the height difference of the liquid from the beginning to the end of a measurement and the corresponding time. The accuracy is mainly determined by the precision of the height measurement and amounts to ±10%. Degassing components are used to eliminate gas bubbles.
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Fig. 1. Standard micro mixer of stainless steel with bonded foils (left), schematical presentation of the foil stack (right), construction of the mixer used here (below).
2.2.2. Optics The experimental system is shown in Fig. 3. Compared to the set up used in the studies of Roetmann et al. [10], a reference branch and special optics for light sheet formation are added. A similar set up will be described by Wellhausen, Rinke and Wackerbarth [unpublished results]. The latter is a special optical design for the generation of a homogeneous sheet. Here, the radiation of a pulsed
Nd:YAG laser at 532 nm (6 ns pulse duration, 10 Hz repetition rate) is divided into two parts by a beam splitter, a measuring branch (40%) and a reference branch (60%). This reference branch was used to correct time dependent fluctuations of the laser intensity. After dividing all intensity values of the CCD camera in the measuring path through the average values of the reference camera, the residual pulse to pulse fluctuations amounted ±1%.
Fig. 2. Fluidic system schematically, containing reservoirs for water and ethanol, which are delivered to the micro mixer by air pressure.
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Fig. 3. Experimental Raman system setup with light sheets. The flow direction within the micro mixer is perpendicular to this picture, see Fig. 5.
The Raman filter is a central element for these measurements. It has to overlap the Raman spectrum of the samples to be observed. Fig. 4 shows the transmission of the filter as well as the Raman spectra of liquid ethanol and water. Especially the CH– and the OHbands of ethanol and water are interesting, if mixtures of these fluids are to be observed. The intense CH-band of ethanol nearly fits the range of the filter. The spectra of both fluids show broad OH-bands around 650 nm, but the one of ethanol is much weaker. A small contribution from the OH-band of water is within the area of the filter. This applies also to mixtures consisting of both components. This light can be removed easily by a black- and white-image correction. The black-image contains the intensities measured of pure water and the white-image is measured by using pure ethanol. The intensity of the black-image is subtracted from the measured intensity and subsequently divided by the difference of the whiteand black-image intensities. This results in a standardized intensity
Fig. 4. Transmission of the Raman filter (red), Raman spectra of liquid ethanol (black) and water (blue). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
between zero and one which is equivalent to the particle number concentration. The laser pulse duration of approximately 6 ns gives the exposure time; the acquisition rate is limited by the readout frequency of the CCD camera and amounted to 2 Hz. In this case this is much to slow to measure the time resolved processes. At each measurement position 100 pictures were recorded during 50 s and every picture contains 5 laser pulses every 100 ms. The laser light sheet had a height of 10 mm and a thickness of 0.5 mm. Therefore the Raman intensity is averaged over a depth of 0.5 mm. A calibration procedure was employed to convert intensity values into concentrations. 2.2.3. Micro mixers The micro mixer used for these experiments should contain much larger micro channels because the CFD calculations would be faster. Furthermore, this micro mixer should have a nearly ideal rectangular cross section, especially at the outlet, to avoid distortion of the flow profile. Otherwise, experiment and simulation might differ. For this reason in this case we did not use diffusion bonding, where slight deformations of the micro channels are possible. Instead, we clamped all plates. Fig. 1(lower part) shows the applied geometry in detail. This micro mixer was made from an aluminium alloy (AlMg3 , material code 3.3535) and consisted of 4 plates. In each of these 2 mm thick plates, 4 micro channels were milled with 0.9 mm width, 1.0 mm depth and 14.1 mm length, which were separated 0.45 mm. Because of the angle of 45◦ the outlet cross section of each micro channel was 1.27 mm wide and 1.0 mm high. The mixer had 16 micro channels in total, 8 per passage. This stack was arranged in such a way that two neighboring plates led fluids of the two passages. The stack of these micro structured foils was clamped on one side to an adapter with fluid connections (Fig. 5, right part). Above the stack with the 16 micro channel outlets a rectangular optical tube with x-coordinate in flow direction was attached by a thin PTFE foil. This cell is based on a standard cuvette from Hellma and made of fused silica, but with open ends. It was 40 mm long, has an inner cross section of 10 mm × 10 mm and an outer cross section of 12.5 mm × 12.5 mm. At the top of this cell a further adapter with a PTFE foil and a fluid connection was clamped. The left part of Fig. 5 shows the arrangement of the 16 outlet channels, red indicates ethanol and blue water. The coordinate system is shown, too.
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Fig. 5. Micro mixer attached to a cuvette and the coordinate system used here. The x-coordinate is perpendicular to this picture and corresponds to the flow direction. The zoom shows the scheme of the outlets channels below the cuvette with an inner width of 10 mm. The transparent rectangular band shall illustrate the laser light sheet.
3. Results 3.1. Measurement of concentration fields All measurements are based on the coordinate system shown in Fig. 5. The laser sheet selected a plane cutting through the inlets of ethanol, water or in between. We used two flow rates, 0.6 l/h and 6 l/h per fluid. This corresponds to linear velocities of 0.23 m s−1 and a Reynolds number of 22 at the outlet of each micro channel at 0.6 l/h per fluid and 2.3 m s−1 and 220, respectively. Fig. 6 shows the mean values of 100 pictures of measurements at low throughput (0.6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The distribution of ethanol is clearly
visible, especially when the laser sheet is positioned at the outlets of the ethanol micro channels. The intensity values within the filaments of ethanol are near one as expected. The filaments are nearly vertical. Between the filaments, ethanol is mixed with water in a lower concentration. Above 20 mm height ethanol and water are mixed and no filaments are seen in these averaged pictures. However, if single shots are viewed, inhomogenities still can be detected (see below). Interesting is the black layer rising from the left to the right at the bottom in Fig. 6. This is not an artifact, but corresponds to a nearly 100% concentration of water. Furthermore, we first expected that at laser sheet position Z = 4 mm black filaments for water would arise similar to the ethanol filaments at position Z = 2 mm. We will discuss both effects later.
Fig. 6. Measured concentration image of ethanol–water-mixtures, slow throughput (1.2 l/h total), mean values, z-planes: 1, 2, 3, 4 mm.
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Fig. 7. Image of the measured variance of the concentration of ethanol–water-mixtures, slow throughput (1.2 l/h total), z-planes: 1, 2, 3, 4 mm.
Fig. 7 shows the variance of the 100 pictures from Fig. 6 at low throughput (0.6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The fluctuations are large for the ethanol filaments. Fig. 8 shows the mean values of 100 pictures of measurements at fast throughput (6 l/h per component) for 4 Z-positions. The distribution of ethanol is clearly visible, especially of course when the
laser sheet is positioned at the outlets of the ethanol micro channels. The intensity values within the ethanol filaments are near one as expected. The filaments now have an angle of 45◦ , which is the result of the micro channels cut at 45◦ (Fig. 1) and the higher velocity. Above 20 mm height ethanol and water are mixed again and no filaments are seen in theses averaged pictures. However, viewing single shots, inhomogenities still can be detected (see below).
Fig. 8. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), mean values, z-planes: 1, 2, 3, 4 mm.
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Fig. 9. Image of the measured variance of the measured concentration of ethanol–water-mixtures, fast throughput (12 l/h total), z-planes: 1, 2, 3, 4 mm.
The black layer at the bottom observed at lower throughputs (Fig. 6) is absent here. Instead, black filaments (pure water) are observed. The reason is the higher momentum of the water jet in this case. At lower velocities this momentum is not strong enough to compete with gravity which leads to the usual distribution of densities (1100 kg m−3 for water-salt-solution and 790 kg m−3 for ethanol). Looking at laser sheet position Z = 3 mm ethanol filaments at a height of approximately X = 3 mm can be seen. These ethanol
streams come from the plane Z = 2 mm where they vanish at a height of X = 3 mm. Fig. 9 shows the variance of 100 pictures from Fig. 8 at high throughput (6 l/h per component) for 4 X-positions, each 10 mm high, and 4 Z-positions. The fluctuations are large for the ethanol filaments. A further interesting effect is that there are two different areas within the cuvette (Fig. 8): below X = 15 mm (areas X1 and X2) filaments are seen and mixing is not complete. Above X = 15 mm
Fig. 10. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), area X2Z4, averaged values (left) and shingle shot (right).
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Fig. 11. Measured concentration image of ethanol–water-mixtures, fast throughput (12 l/h total), area X4Z2, averaged values (left) and single shot (right).
no filaments are seen in these averaged pictures. To understand this behavior in more detail, it is necessary to compare averaged pictures with single shot pictures. Fig. 10 shows the area X2Z4 as averaged picture (left part) and as a single shot (right part). Fluctuations of ethanol and water can still be seen with a single shot. Further single shot measurements show that filaments of ethanol at first flow at an angle of 45◦ to the left vertical glass wall and later nearly flow vertically. However, the water filaments firstly flow at an angle of 45◦ to the right vertical glass wall, then rotate and flow in the direction of the ethanol filaments. This upper interface at a height of 15 mm may be a result of the higher density of water. The height at which the left water filament (Fig. 8, bottom right) hits the glass wall can be calculated by geometrical considerations to 9 mm. This is below the transition zone at X = 15 mm. The upper areas in Fig. 8 (X3–X4) suggest that mixing is complete. However, these pictures are mean values of 100 measurements. Fig. 11 compares the highest region in the cuvette (X4Z2), averaged pictures on the left side and a single shot on the right side. Although the averaged picture shows a nearly homogeneous concentration distribution, the single shot shows filaments indicating that mixing is not complete.
In these equations is the density, v is the velocity vector, is the tension tensor, g is the gravitation vector, Yi is the mass fraction and Ji is the diffusion flux of species i. The computations were done for a laminar flow at constant temperature under non-steady-state conditions. In the simulations two fluids were modeled, ethanol with a density of 789 kg/m3 and water with a density of 998 kg/m3 . In a computational cell the mixed density was calculated by using the volume fraction of ethanol. The quartz cuvette as mixing chamber was modeled as shown in Fig. 12. For its discretisation 500,000 cubical elements in total were used. Each had a side length of 0.2 mm. The complete mass flow of one passage/section was distributed uniformly over the eight channels and a plug-flow velocity profile was set as boundary condition at each inlet. During the flow within each micro channel a parabolic velocity profile will develop.
3.2. CFD calculations In order to compare the experimental results, computational fluid dynamics (CFD) was applied. We used the CFD program FLUENT/ANSYS (version 12.1.4) to solve the following equations [27,28]: Continuity equation (conservation of mass): ∂ + ∇ · (v) = 0 ∂t Navier–Stokes equation (conservation of momentum): ∂ (v) + ∇ · (vv) = −∇ p + ∇ · ( ) + g ∂t Transport equation for species: ∂ (Yi ) + ∇ · (vYi ) = −∇ · Ji ∂t
Fig. 12. CFD-model of the mixing and observation tube (10 mm × 10 mm × 40 mm) represented by 500,000 cubical elements with a side length of 0.2 mm.
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Fig. 13. CFD-simulation of the concentration map of ethanol–water-mixtures, slow throughput, z-planes: 1, 2, 3, 4 mm.
The time step size chosen was 10 ms for the low flow-rate and 1 ms for the high flow-rate. The mass flux chosen for the low flowrate was 0.49 kg/h (0.62 l/h) for ethanol and 0.52 kg/h (0.52 l/h) for water and for the high flow-rate 4.64 kg/h (5.9 l/h) and 6.47 kg/h (6.5 l/h), respectively. These mass fluxes were the mean values from all measurements. The SIMPLE scheme was used for the pressure velocity coupling. The spatial discretisation of the pressure was
done using the standard method of FLUENT and the variables for momentum and species were discretised by the QUICK scheme. Fig. 13 shows the results of the numerical computations for the lower flow rate. These pictures correspond to the measurements shown in Fig. 6. However, it is a snap shot (10 ms time step) without averaging. The ethanol filaments rising nearly vertically are seen at position Z = 2 mm and vanishing at a height of 20 mm. The
Fig. 14. CFD-simulation of the concentration map of ethanol–water-mixtures, fast throughput, z-planes: 1, 2, 3, 4 mm.
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black layer of water is seen, too. If gravitation is not considered in the Navier–Stokes equations, this layer at the bottom is absent and water filaments could be seen. The videos (as supplemental files for Z = 2 mm and Z = 4 mm) of this simulation show some time dependent fluctuations of concentrations, even at a height of 20 mm. This is seen in this snap shot, too. Fig. 14 visualizes the results of the numerical computations for the higher flow rate. These pictures correspond to the measurements shown in Fig. 8. However, it is a snap shot (1 ms time step) without averaging. The filaments have an angle of 45◦ as observed by our laser Raman measurements. Above 20 mm height no straight filaments are seen, but an incomplete mixing. The black layer at the bottom observed at lower throughputs (Fig. 6 and Fig. 13) is absent here. Instead, black filaments (pure water) are observed as in the experiments (Fig. 8). These simulations further confirm the observed two different areas within the cuvette (Fig. 8). A video (as supplemental files for Z = 2 mm and Z = 4 mm) of Fig. 14 more clearly shows the vortex of water in the lower right part of the cuvette. The time dependent variations in the concentrations observed in the single shot measurements (Figs. 10 and 11) qualitatively correspond to these simulations. A direct quantitative comparison between experiment and simulation is not possible because the mixing process is instationary. The video of Fig. 14 gives a more detailed view of the nonstationary behavior of concentrations within the cuvette, even at the top of the cuvette. It should be mentioned that normally these micro mixers contain much more micro channels and these are operated at higher throughputs. In these cases the mixing behavior is much better, of course. Investigations of our standard micro mixer show that mixing is good when linear velocity at the outlet of each micro channel is larger than 2 m s−1 [29]. In this work one important aim was a comparison between measurements and CFD. A CFD simulation with smaller and much more micro channels would be difficult and would take a long computation time. Therefore we used larger micro channels than usual and we only reached a linear velocity in each micro channel of 0.023 m s−1 at the low flow rate and 0.23 m s−1 at the high flow rate.
4. Conclusion and outlook We have shown that pulsed Raman imaging can be used to measure concentration profiles during a mixing process. In this case the mixing behavior of ethanol and water at the outlet of a micro mixer was measured. A special optics for light sheet formation is applied to generate planes for monitoring the mixing path above the outlet of the micro mixer. The planes of the laser light sheet are put together so that the concentrations can be mapped in three dimensions. With the optics used here, a light sheet with 10 mm × 10 mm can be observed, the lateral resolution being determined by the pixels of the CCD camera. The depth resolution amounts 500 m. One light sheet is recorded in 6 ns. The frame rate is determined by the readout time of the CCD camera (0.5 s) and not by the laser. With this technique it was possible to monitor ethanol filaments within a plane of 10 mm × 10 mm with one laser shot (6 ns). The temporal progress was measured with a rate of 2 Hz. The experimental results agree well with CFD calculations. The computations were done for a laminar flow at constant temperature under nonsteady-state conditions. The spatial discretisation of the pressure was done using the standard method of FLUENT and the variables for momentum and species were discretised by the QUICK scheme. This method can be applied not only to mixing processes, but for monitoring chemical reactions, too. This is an advantage in comparison with laser induced fluorescence, where only mixing
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processes with fluorescent substances can be monitored or very special cases where a product shows fluorescence. The optical setup may be improved concerning time resolution when a more powerful laser and a faster CCD camera would be used. Without loss of geometrical resolution (pixel numbers) and signal to noise ratio, a commercially available expensive EMCCD camera with a frame rate of 35 fps instead of 2 Hz could be used. This would lead to a time resolution of 30 ms. If a laser with 35 Hz and 200 mJ pulse energy would be used, there would be no decrease in performance. If geometrical resolution can be reduced, binning of CCD pixels is possible, which would decrease the readout time even lower. In this case the limiting time resolution (10 ms) would be the laser repetition rate. This would be the best time-resolution without degrading performance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2011.11.016. Please note that the videos use codecs which not all players accept. The VLC player should work. References [1] P. Hinsmann, et al., Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution, Lab on a Chip 1 (2001) 16–21. [2] J. Antes, D. Boskovic, H. Krause, S. Loebbecke, N. Lutz, T. Tuercke, W. Schweikert, Analysis and improvement of strong exothermic nitrations in microreactors, Chemical Engineering Research and Design 81 (2003) 760–765. [3] R. Keoschkerjan, M. Richter, D. Boskovic, F. Schnürer, S. Löbbecke, Novel multifunctional microreaction unit for chemical engineering, Chemical Engineering Journal 101 (2004) 469–475. [4] T.M. Floyd, M.A. Schmidt, K.F. Jensen, Silicon micromixers with infrared detection for studies of liquid-phase reactions, Industrial & Engineering Chemistry Research 44 (2005) 2351–2358. [5] J.B. Salmon, A. Ajdari, P. Tabeling, L. Servant, D. Talaga, M. Joanicot, In situ Raman imaging of interdiffusion in a microchannel, Applied Physics Letters 86 (2005) 094106. [6] G. Rinke, A. Ewinger, S. Kerschbaum, M. Rinke, In situ Raman spectroscopy to monitor the hydrolysis of acetal in microreactors, Microfluidics and Nanofluidics 10 (2011) 145–153. [7] T. Park, M. Lee, J. Choo, Y.S. Kim, E.K. Lee, D.J. Kim, S.H. Lee, Analysis of passive mixing behavior in a poly(dimethylsiloxane) microfluidic channel using confocal fluorescence and Raman microscopy, Applied Spectroscopy 58 (2004) 1172–1179. [8] F. Sarrazin, J.-B. Salmon, Chemical reaction imaging within microfluidic devices using confocal Raman spectroscopy: the case of water and deuterium oxide as a model system, Analytical Chemistry 80 (2008) 1689–1695. [9] A. Ewinger, G. Rinke, S. Kerschbaum, M. Rinke, K. Schubert, Ramanspectroscopy for measuring chemical reactions in micro reactors, in: Proceedings of the 6th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2008 (PART A), 2008, pp. 747–748. [10] K. Roetmann, W. Schmunk, C. Garbe, V. Beushausen, Micro-flow analysis by molecular tagging velocymetry and planar Raman-scatttering, Experiments in Fluids 44 (2008) 419–430. [11] M. Lee, J.P. Lee, H. Rhee, J. Choo, Y.G. Chai, E.K. Lee, Applicability of laserinduced Raman microscopy for in situ monitoring of imine formation in a glass microfluidic chip, Journal of Raman Spectroscopy 34 (2003) 737–742. [12] S.A. Leung, R.F. Winkle, R.C.R. Wootton, A.J. deMello, A method for rapid reaction optimisation in continuous-flow microfluidic reactors using online Raman spectroscopic detection, Analyst 130 (2004) 46–51. [13] C.J. Sung, J.B. Liu, C.K. Law, Structural response of counterflow diffusion flames to strain rate variations, Combustion and Flame 102 (1995) 481–492. [14] C. Luo, B. Moghtaderi, E. Kennedy, B. Dlugogorski, Three-dimensional numerical study on flames, Chemical Product and Process Modeling 4 (2009), art. no. 10. [15] T. Egawa, J.L. Durand, E.Y. Hayden, D.L. Rousseau, D.-R. Yeh, Design and evaluation of a passive alcove-based microfluidic mixer, Analytical Chemistry 81 (2009) 1622–1627. [16] M. Hoffmann, M. Schlüter, N. Räbiger, Experimental investigation of liquid–liquid mixing in T-shaped micro-mixers using -LIF and -PIV, Chemical Engineering Science 61 (2006) 2968–2976. [17] V. Hessel, A. Renken, J.C. Schouten, J. Yoshida, Micro Process Engineering: A Comprehensive Handbook, Wiley–VCH, Weinheim, 2009. [18] T. Wirth, Microreactors in Organic Synthesis and Catalysis, Wiley–VCH, Weinheim, 2008. [19] P. Watts, C. Wiles, Recent advances in synthetic micro reaction technology, Chemical Communications 5 (2007) 443–467.
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