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3D investigations of microscale mixing in helically coiled capillaries

3D investigations of microscale mixing in helically coiled capillaries In capillary reactors, improving radial mixing and narrowing the residence time distribution is of great importance for high selectivity and reaction performance. A well-known approach is inducing secondary flow patterns by coiling the capillary around a cylinder. To increase understanding of transport phenomena in helically coiled capillaries non-invasive 3D imaging approaches are required. In this perspective paper, we introduce X-ray-based micro-computed tomography for the investigation of dispersion of iodide in a helically coiled tube. The methodology presented here allows for the direct evaluation of radial concentration fields. By varying Dean number Dn and modified torsion parameter T , the effect of torsion and curvature on the radial concentration profile can be identified. Detailed knowledge of local radial mixing in helically coiled capillaries will help the precise prediction of reaction progress and selectivity. . . . . . Keywords Helically coiled capillary Micro-computed tomography Dean flow Radial mixing Capillary flow reactor Local concentration fields Introduction centrifugal force is imposed on the parabolic flow profile lead- ing to a secondary flow pattern. Therefore, the laminar flow In the context of process intensification in the chemical indus- profile is superimposed with two counter-rotating vortices try, performing processes in microfluidic devices is advanta- normal to the main flow direction, the Dean vortices, which geous, especially for highly exothermic reactions, highly spe- were first described by Dean [10, 11]. In a helically coiled cialized products, or small production quantities. Continuous tube (HCT) the two main parameters that affect the secondary processes can be realized via laminar flow in simple straight flow pattern are curvature and torsion, represented by the non- capillaries. However, the flow profile is parabolic and radial dimensional Dean number Dn (Eq. (1)) [12–15] and the non- mixing is solely governed by molecular diffusion [1]. Radial dimensional modified torsion parameter T (Eq. (2)) [15]. In mixing can be improved by coiling the capillary. This ap- Eqs. (1)and (2), Re is the Reynolds number, d and d are the i c proach is widely applied in the field of flow chemistry, as inner diameter of the capillary and the coil diameter, and p is for the continuous production of nano-materials [2–6]or in the pitch, see Fig. 3. viral inactivation reactors [7–9]. By coiling the capillary, a sffiffiffiffiffi Dn ¼ Re  ð1Þ Highlights d • Dispersion of iodide in water due to Dean vortices is investigated in a Re    d helically coiled tube. c T ¼ ð2Þ • X-ray based micro-computed tomography is applied for the visualiza- tion of cross-sectional concentration fields. • Increase in Dean number causes the formation of characteristic concen- Increasing curvature, respectivelyDnnumber, increases the tration field normal to the main flow direction. intensity of Dean flow [16] and leads to improved radial mixing [12], a narrower residence time distribution (RTD) * Julia Schuler [17], and stabilization of the flow against turbulence [13, 18, julia.schuler@tu-dortmund.de 19]. Increasing torsion, which means decreasing the modified torsion parameter T , leads to destabilization of the flow Laboratory of Equipment Design, Department of Biochemical and against turbulences, reduction of the symmetry of the vortices, Chemical Engineering, TU Dortmund University, 44227 Dortmund, Germany and broadening of the RTD [18–20]. 218 J Flow Chem (2021) 11:217–222 Investigating transport phenomena in helically coiled tubes Re is realized by increasing the total volume flow rate V . is still of great interest [7, 8, 21–23]. Especially non-invasive Increasing Re simultaneously increases Dn and T .For the imaging approaches promise great insights and are crucial for lower volume flow rate tested (V ¼ 1:5mL min , Re ¼ 25, * − the validation of numerical simulations [24]. Flow profiles Dn ¼ 6, T =476), I is transported upwards almost uniformly were already visualized in curved tubes using magnetic reso- when following the main flow direction. Dean flow does not nance imaging (MRI) [14, 25], digital holographic particle seem to be pronounced sufficiently to disturb the natural evo- tracking velocimetry [12], and 2D and 3D particle image lution of the concentration field due to diffusion. When increas- velocimetry (PIV) [24, 26–29]. In this perspective paper, X- 1 ing the total volume flow rate to V ¼ 7mL min (Re ¼ 118, ray micro-computed tomography (X-µCT) is applied to visu- * Dn ¼ 28, and T =2222) the concentration field significantly alize 3D concentration profiles of iodide (I )in an HCT. X- differs from the concentration field obtained for lower Re, Dn, µCT is non-invasive and offers high spatial resolutions with- and T . At an angular position of 360°, the concentration field out requiring optical access [30, 31]. Even though X-ray based shows the effect of the Dean vortices. The upper part of the tube computed tomography has already been successfully applied cross-section predominantly contains pure water, but some I is for the characterization of processes on a larger scale, among entrained into the iodide-poor region of the cross-section, locat- others [32–37], studies concerning its extension to the mini- or ed at the tube wall. Furthermore, a higher I concentration can microscale are limited [31] and only very few studies concern be found in the direction of the centrifugal force F .The same the X-ray based investigation of mass transfer related prob- can be recognized inversely in the lower part of the tube cross- lems [38–40]. Nonetheless, the proposed methodology offers section for the iodide-rich phase and water. This characteristic great potential in gaining deeper knowledge about local radial concentration field is even pronounced stronger for an angular mixing in HCTs, which is crucial for controlling reactions and position of 540°. In total, better radial mixing can be observed their selectivity. 1 1 for V ¼ 7mL min than for V ¼ 1:5mL min from the 2 1 reconstructed cross-sectional views. This is especially true be- cause the residence time for higher V in the shown number of turns is significantly reduced. Figure 2 gives a qualitative im- Results and discussion pression about the separation area between the iodide rich and the iodide poor region for p along one turn of the helix starting Figure 1 shows resulting projection images and cross- at an angular position of 540°. For lower Re, the total fluid sectional views of the FEP tube and the concentration field stream is clearly divided into the iodide rich region at the bot- of I at angular positions of 0°, 360°, and 540° for different tom and the iodide poor region at the top, both are clearly total volume flow rates and pitches. The angular positions are distinguished from each other by a nearly horizontal separation also marked in the projection images. In the cross-sectional area. For the higher Re, a distinction can be made between the slices, bright voxels indicate a high concentration of I and separation area between the iodide rich region and a region dark voxels indicate pure water. containing an intermediate concentration of iodide at the bot- It is visible from the cross-sectional voxel slices that the total tom (orange) and the separation area between the iodide poor liquid flow is divided into an iodide-rich and an iodide-poor region and the intermediate region at the top (blue). The differ- region that are separated by a clear separation line. For an angular ence between the appearances of the separation areas for differ- position of 0°, this separation line is horizontal for the lower ent Re is contributed by the fact that Dean vortices are pro- volume flow rate, while it is inclined to the outside of the helix nounced for higher Re. for the higher flow rate. The difference in the concentration fields The sole increase of the total volume flow rate is insuffi- resulting fromV andV islikelytobecausedbysettlingeffects, 1 2 cient to distinguish between the influence of Dn and T on the as the KI solution (15w% KI, ρ  1173kg m ) is known to resulting local concentration fields, as Dn and T both depend have a higher density (Δρ  0.18) than pure water. The rel on Re. Different pitches, p ¼ 4:8mm and p ¼ 9mm are 1 2 residence time between the T-junction and the reference compared for the same total volume flow rates. point of the helix (see Fig. 3) is significantly lower for 1 1 V ¼ 1:5mL min and V ¼ 7mL min to isolate the 1 2 the higher total volume flow rate V . Here, the concen- effect of T .For both Re tested, the cross-sectional con- tration profile at an angular position of 0° is still affect- centration fields for p are very similar to the concentration ed strongly by the first contact of the KI-rich liquid and 1 fields obtained with p : However, at the higher volume the KI-poor liquid in the T-junction and the redirecting flow rate ( V ¼ 7mL min ) the concentration fields at of the contacted liquids in the feed section. Between 0° 2 360° and 540° show slightly better mixing for p than for and 360° the separation line becomes horizontal. p with slightly more symmetrical concentration fields for In the following, the effect of Re number on the Dean vor- the lower pitch. Additionally, it must be considered that for tices, and hence the resulting radial concentration fields, is con- a higher pitch the total residence time within one turn is sidered forp ¼ 4:8mm. For a given helix geometry, increasing 1 J Flow Chem (2021) 11:217–222 219 Fig. 1 Left: Projection images (1944 × 1382) pixels for HCT with p ¼ 4:8mm and p ¼ 9m 1 2 m. The orange lines indicate the main flow direction. Right: Cross- sectional views (180 × 180) voxels of capillaries with radial concentration field. Inside the tube, bright voxels indicate I -rich liquid, darker voxels indicate pure water. The red arrows indicate the direction of gravity g and centrif- ugal force F longer, as is the tube distance. This supports the hypothesis apparently low, the strong dependence of Re on the con- that higher torsion from a larger pitch leads to reduced centration fields described previously originates from the effect of curvature (Dn ). mixing. As the effect of T on the concentration fields is Fig. 2 Qualitative visualization of the separation area between iodide poor and iodide rich regions for different Dean numbers and p extracted from the reconstructed CT-volume. The vertical line is the rotation axis of the helix. The size of the reconstructed voxel slices is (180 × 180) voxels 220 J Flow Chem (2021) 11:217–222 It can be concluded that both, torsion and curvature, affect the radial mixing in an HCT, as already reported in the liter- ature [12, 13, 16–20]. Increasing curvature, represented by the Dean number, significantly increases radial mixing as the con- centration field is affected by the secondary Dean flow pat- tern. Increasing torsion (decreasing T )fora fixed Dn number only slightly reduces the effect of Dean flow on radial mixing. Experimental The main objective of this perspective paper is to generate a defined radial concentration profile of an X-ray contrast agent in demineralized water whose disturbance can be observed Fig. 3 a Schematic of the experimental set-up. Pure water and pure water due to the effect of Dean vortices. Potassium iodide is used enriched with potassium iodide (KI) are pumped into the rotating helical- ly coiled tube (HCT) that is mounted into the computed tomography as the contrast agent, which, when dissolved in water, forms + − + − scanner (Skyscan 1275). The total volume flow rate is measured at the K and I ions. Both, K and I , attenuate X-rays to a signif- outlet gravimetrically. b Photograph of HCT and schematic of the icantly higher level than pure water. Thereby, X-ray attenua- contacting device (T-junction). Pure water is fed from the side inlet and − + tion by I is higher than X-ray attenuation by K ,such that KI-rich water from the bottom inlet only the diffusion and dispersion of I can be registered in X- ray imaging [39]. The pure water and water containing 15 w% reconstruction software NRecon (Bruker, Billerica, MA). The KI are contacted in a T-junction (IDEX Health & Science, 3D data set consists of a stack of cross-sectional slices, each of IDEX Corporation, Northbrook, IL) that is connected to a which is composed of voxels, the 3D equivalent of pixels. The helically coiled tube (HCT). The HCT consists out of a total size of the 3D image is (1944 × 1944 × 1382) voxels with Fluorinated Ethylene Propylene (FEP) tube (d ¼ 1:58mm, i voxel size = 18μm  18μm  18μm. The reconstructed CT- d ¼ 3:2mm) that is coiled around a polylactide (PLA) sup- o volume is used to extract the qualitative separation areas be- port structure with a coil diameter (d ¼ 28:8mm). To test the c tween iodide rich, intermediate, and iodide poor region of the effect of torsion, the pitch of the HCT is varied (p ¼ 4:8mm, 1 total fluid stream. p ¼ 9mm). The HCT is mounted into a micro-computed to- mography scanner (Bruker Skyscan 1275, RJL Micro & Analytic GmbH, Karlsdorf Neuthart, Germany) that is equipped with tubes for liquid supply and removal. The KI-rich mixture is Summary and outlook pumped using a syringe pump (LAMBDA VIT-FIT by LAMBDA Instruments GmbH, Baar, Switzerland) and pure wa- In this perspective paper, micro-computed tomography was ter is pumped using a high-pressure dosing pump (BlueShadow successfully applied for the visualization of the concentration Pump 40P, KNAUER Wissenschaftliche Geräte GmbH, Berlin, field of iodide in helically coiled tubes (HCT). The effect of Germany). Both pumps are placed outside the CT, see Fig. 3. curvature and torsion on radial mixing was studied by varying Two different volume flow rates are tested, V ¼ 1:5mLmin the total volume flow rate and the pitch. It was found that and V ¼ 7mL min , resulting in Reynolds numbers of Re increasingDnnumber leads to the formation of a characteristic 2 1 ¼ 25 and Re ¼ 118, Dean numbers of Dn ¼ 6and Dn ¼ 28, concentration profile and enhanced radial mixing. At the same 2 1 2 and modified torsion numbers 254 < T∗ < 2222. The mean time, increasing the pitch reduces radial mixing due to Dean dynamic viscosity (η ¼ 0:86mPa s) for the calculation of Re is vortices only slightly. The presented methodology offers high estimatedaccordingtodatabyGoldsacketal. [41]. The vol- potential and will therefore be systematically extended. The ume flow rate ratio of pure water and KI-rich water is current investigations will be enriched with further post- 1. The total flow is monitored gravimetrically during the processing of reconstructed voxel slices to obtain concentra- experiments. Scanning voltage is 43kV, the current is 1 tion profiles and robust quantitative statements about mixing 90μA, and exposure time is 30ms.The size of the quality. Additionally, more flow conditions will be tested and resulting X-ray projection images is (1944 × 1382) the effect of different liquid densities on radial mixing must be pixels with pixel size = 18μm  18μm. addressed in more detail. The proposed method will not only During image acquisition, X-ray projection images are ac- help the fundamental understanding of dispersion in HCT but quired for different angular positions. The resulting set of also in diverse forms of passive microreactors. It can even be projection images is reconstructed to a 3D dataset using the extended to the investigation of certain reactions, such as J Flow Chem (2021) 11:217–222 221 temperature synthesis. React Chem Eng 5:1474–1483. https://doi. precipitation reactions. Finally, concentration fields obtained org/10.1039/D0RE00078G with the presented methodology will help in validating numer- 4. Epps RW, Volk AA, Abdel-Latif K et al (2020) An automated flow ical simulations that aim to simulate and optimize chemistry platform to decouple mixing and reaction times. React microreactors with internal convective mixing. Chem Eng 5:1212–1217. https://doi.org/10.1039/D0RE00129E 5. Besenhard MO, LaGrow AP, Hodzic A et al (2020) Co- precipitation synthesis of stable iron oxide nanoparticles with NaOH: New insights and continuous production via flow chemis- Notation d , coil diameter [m];d , inner tube diameter [m]; d , outer c i o try. Chem Eng J 399:125740. https://doi.org/10.1016/j.cej.2020. tube diameter [m]; Δρ , relative density difference between KI solu- rel − 2 tion and water [-]; F , centrfugal force [N]; g , gravity constant [m s ]; 6. Epps RW, Bowen MS, Volk AA et al (2020) Artificial chemist: an − 3 Dn , Dean number [-];p , pitch [m];ρ , density of KI solution [kg m ]; s autonomous quantum dot synthesis bot. Adv Mater 32:e2001626. Re, Reynolds number [-];T , modfied torsion parameter [-];V , volume https://doi.org/10.1002/adma.202001626 3 − 1 flow rate [m s ] 7. Amarikwa L, Orozco R, Brown M et al (2019) Impact of dean vortices on the integrity testing of a continuous viral inactivation reactor. Biotechnol J 14:e1700726. https://doi.org/10.1002/biot. Acknowledgements We would like to thank RJL Micro & Analytic, Karlsdorf‐Neuthard, Germany, for technical support and training. 8. Brown MR, Orozco R, Coffman J (2020) Leveraging flow mechan- Furthermore, we would like to acknowledge C. Schrömges (Laboratory ics to determine critical process and scaling parameters in a contin- of Equipment Design, TU Dortmund University, Germany) for technical uous viral inactivation reactor. 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Int J Heat Fluid Flow 68: Publisher’snote Springer Nature remains neutral with regard to jurisdic- 189–202. https://doi.org/10.1016/j.ijheatfluidflow.2017.10.011 tional claims in published maps and institutional affiliations. 29. Kalpakli Vester A, Sattarzadeh SS, Örlü R (2016) Combined hot- wire and PIV measurements of a swirling turbulent flow at the exit of a 90° pipe bend. J Vis 19:261–273. https://doi.org/10.1007/ s12650-015-0310-1 Julia Schuler studied Mechanical Engineering at the Paderborn 30. Schuler J, Kockmann N (2020) Micro-computed tomography for University. In July 2018, she started her work as a research assistant at the investigation of stationary liquid–liquid and liquid–gas inter- the Laboratory of Equipment Design, TU Dortmund University, faces in capillaries. AIChE J 66. https://doi.org/10.1002/aic.16890 Germany. Her focus is on the experimental investigation of single- and 31. 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In April 2011, Norbert ces.2019.03.059 Kockmann was appointed as full professor for Equipment Design at TU 35. Lau YM, Möller F, Hampel U et al (2018) Ultrafast X-ray tomo- Dortmund University, Germany. His research interests are on small-scale graphic imaging of multiphase flow in bubble columns – Part 2: device for continuous chemical processes, modular design, and process Characterisation of bubbles in the dense regime. Int J Multiphase intensification. His work includes fundamental investigations of small- Flow 104:272–285. https://doi.org/10.1016/j.ijmultiphaseflow. scale multiphase flow, modelling and simulation accompanied by modern 2018.02.009 sensing technology and machine-learning methods. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Flow Chemistry Springer Journals

3D investigations of microscale mixing in helically coiled capillaries

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Abstract

In capillary reactors, improving radial mixing and narrowing the residence time distribution is of great importance for high selectivity and reaction performance. A well-known approach is inducing secondary flow patterns by coiling the capillary around a cylinder. To increase understanding of transport phenomena in helically coiled capillaries non-invasive 3D imaging approaches are required. In this perspective paper, we introduce X-ray-based micro-computed tomography for the investigation of dispersion of iodide in a helically coiled tube. The methodology presented here allows for the direct evaluation of radial concentration fields. By varying Dean number Dn and modified torsion parameter T , the effect of torsion and curvature on the radial concentration profile can be identified. Detailed knowledge of local radial mixing in helically coiled capillaries will help the precise prediction of reaction progress and selectivity. . . . . . Keywords Helically coiled capillary Micro-computed tomography Dean flow Radial mixing Capillary flow reactor Local concentration fields Introduction centrifugal force is imposed on the parabolic flow profile lead- ing to a secondary flow pattern. Therefore, the laminar flow In the context of process intensification in the chemical indus- profile is superimposed with two counter-rotating vortices try, performing processes in microfluidic devices is advanta- normal to the main flow direction, the Dean vortices, which geous, especially for highly exothermic reactions, highly spe- were first described by Dean [10, 11]. In a helically coiled cialized products, or small production quantities. Continuous tube (HCT) the two main parameters that affect the secondary processes can be realized via laminar flow in simple straight flow pattern are curvature and torsion, represented by the non- capillaries. However, the flow profile is parabolic and radial dimensional Dean number Dn (Eq. (1)) [12–15] and the non- mixing is solely governed by molecular diffusion [1]. Radial dimensional modified torsion parameter T (Eq. (2)) [15]. In mixing can be improved by coiling the capillary. This ap- Eqs. (1)and (2), Re is the Reynolds number, d and d are the i c proach is widely applied in the field of flow chemistry, as inner diameter of the capillary and the coil diameter, and p is for the continuous production of nano-materials [2–6]or in the pitch, see Fig. 3. viral inactivation reactors [7–9]. By coiling the capillary, a sffiffiffiffiffi Dn ¼ Re  ð1Þ Highlights d • Dispersion of iodide in water due to Dean vortices is investigated in a Re    d helically coiled tube. c T ¼ ð2Þ • X-ray based micro-computed tomography is applied for the visualiza- tion of cross-sectional concentration fields. • Increase in Dean number causes the formation of characteristic concen- Increasing curvature, respectivelyDnnumber, increases the tration field normal to the main flow direction. intensity of Dean flow [16] and leads to improved radial mixing [12], a narrower residence time distribution (RTD) * Julia Schuler [17], and stabilization of the flow against turbulence [13, 18, julia.schuler@tu-dortmund.de 19]. Increasing torsion, which means decreasing the modified torsion parameter T , leads to destabilization of the flow Laboratory of Equipment Design, Department of Biochemical and against turbulences, reduction of the symmetry of the vortices, Chemical Engineering, TU Dortmund University, 44227 Dortmund, Germany and broadening of the RTD [18–20]. 218 J Flow Chem (2021) 11:217–222 Investigating transport phenomena in helically coiled tubes Re is realized by increasing the total volume flow rate V . is still of great interest [7, 8, 21–23]. Especially non-invasive Increasing Re simultaneously increases Dn and T .For the imaging approaches promise great insights and are crucial for lower volume flow rate tested (V ¼ 1:5mL min , Re ¼ 25, * − the validation of numerical simulations [24]. Flow profiles Dn ¼ 6, T =476), I is transported upwards almost uniformly were already visualized in curved tubes using magnetic reso- when following the main flow direction. Dean flow does not nance imaging (MRI) [14, 25], digital holographic particle seem to be pronounced sufficiently to disturb the natural evo- tracking velocimetry [12], and 2D and 3D particle image lution of the concentration field due to diffusion. When increas- velocimetry (PIV) [24, 26–29]. In this perspective paper, X- 1 ing the total volume flow rate to V ¼ 7mL min (Re ¼ 118, ray micro-computed tomography (X-µCT) is applied to visu- * Dn ¼ 28, and T =2222) the concentration field significantly alize 3D concentration profiles of iodide (I )in an HCT. X- differs from the concentration field obtained for lower Re, Dn, µCT is non-invasive and offers high spatial resolutions with- and T . At an angular position of 360°, the concentration field out requiring optical access [30, 31]. Even though X-ray based shows the effect of the Dean vortices. The upper part of the tube computed tomography has already been successfully applied cross-section predominantly contains pure water, but some I is for the characterization of processes on a larger scale, among entrained into the iodide-poor region of the cross-section, locat- others [32–37], studies concerning its extension to the mini- or ed at the tube wall. Furthermore, a higher I concentration can microscale are limited [31] and only very few studies concern be found in the direction of the centrifugal force F .The same the X-ray based investigation of mass transfer related prob- can be recognized inversely in the lower part of the tube cross- lems [38–40]. Nonetheless, the proposed methodology offers section for the iodide-rich phase and water. This characteristic great potential in gaining deeper knowledge about local radial concentration field is even pronounced stronger for an angular mixing in HCTs, which is crucial for controlling reactions and position of 540°. In total, better radial mixing can be observed their selectivity. 1 1 for V ¼ 7mL min than for V ¼ 1:5mL min from the 2 1 reconstructed cross-sectional views. This is especially true be- cause the residence time for higher V in the shown number of turns is significantly reduced. Figure 2 gives a qualitative im- Results and discussion pression about the separation area between the iodide rich and the iodide poor region for p along one turn of the helix starting Figure 1 shows resulting projection images and cross- at an angular position of 540°. For lower Re, the total fluid sectional views of the FEP tube and the concentration field stream is clearly divided into the iodide rich region at the bot- of I at angular positions of 0°, 360°, and 540° for different tom and the iodide poor region at the top, both are clearly total volume flow rates and pitches. The angular positions are distinguished from each other by a nearly horizontal separation also marked in the projection images. In the cross-sectional area. For the higher Re, a distinction can be made between the slices, bright voxels indicate a high concentration of I and separation area between the iodide rich region and a region dark voxels indicate pure water. containing an intermediate concentration of iodide at the bot- It is visible from the cross-sectional voxel slices that the total tom (orange) and the separation area between the iodide poor liquid flow is divided into an iodide-rich and an iodide-poor region and the intermediate region at the top (blue). The differ- region that are separated by a clear separation line. For an angular ence between the appearances of the separation areas for differ- position of 0°, this separation line is horizontal for the lower ent Re is contributed by the fact that Dean vortices are pro- volume flow rate, while it is inclined to the outside of the helix nounced for higher Re. for the higher flow rate. The difference in the concentration fields The sole increase of the total volume flow rate is insuffi- resulting fromV andV islikelytobecausedbysettlingeffects, 1 2 cient to distinguish between the influence of Dn and T on the as the KI solution (15w% KI, ρ  1173kg m ) is known to resulting local concentration fields, as Dn and T both depend have a higher density (Δρ  0.18) than pure water. The rel on Re. Different pitches, p ¼ 4:8mm and p ¼ 9mm are 1 2 residence time between the T-junction and the reference compared for the same total volume flow rates. point of the helix (see Fig. 3) is significantly lower for 1 1 V ¼ 1:5mL min and V ¼ 7mL min to isolate the 1 2 the higher total volume flow rate V . Here, the concen- effect of T .For both Re tested, the cross-sectional con- tration profile at an angular position of 0° is still affect- centration fields for p are very similar to the concentration ed strongly by the first contact of the KI-rich liquid and 1 fields obtained with p : However, at the higher volume the KI-poor liquid in the T-junction and the redirecting flow rate ( V ¼ 7mL min ) the concentration fields at of the contacted liquids in the feed section. Between 0° 2 360° and 540° show slightly better mixing for p than for and 360° the separation line becomes horizontal. p with slightly more symmetrical concentration fields for In the following, the effect of Re number on the Dean vor- the lower pitch. Additionally, it must be considered that for tices, and hence the resulting radial concentration fields, is con- a higher pitch the total residence time within one turn is sidered forp ¼ 4:8mm. For a given helix geometry, increasing 1 J Flow Chem (2021) 11:217–222 219 Fig. 1 Left: Projection images (1944 × 1382) pixels for HCT with p ¼ 4:8mm and p ¼ 9m 1 2 m. The orange lines indicate the main flow direction. Right: Cross- sectional views (180 × 180) voxels of capillaries with radial concentration field. Inside the tube, bright voxels indicate I -rich liquid, darker voxels indicate pure water. The red arrows indicate the direction of gravity g and centrif- ugal force F longer, as is the tube distance. This supports the hypothesis apparently low, the strong dependence of Re on the con- that higher torsion from a larger pitch leads to reduced centration fields described previously originates from the effect of curvature (Dn ). mixing. As the effect of T on the concentration fields is Fig. 2 Qualitative visualization of the separation area between iodide poor and iodide rich regions for different Dean numbers and p extracted from the reconstructed CT-volume. The vertical line is the rotation axis of the helix. The size of the reconstructed voxel slices is (180 × 180) voxels 220 J Flow Chem (2021) 11:217–222 It can be concluded that both, torsion and curvature, affect the radial mixing in an HCT, as already reported in the liter- ature [12, 13, 16–20]. Increasing curvature, represented by the Dean number, significantly increases radial mixing as the con- centration field is affected by the secondary Dean flow pat- tern. Increasing torsion (decreasing T )fora fixed Dn number only slightly reduces the effect of Dean flow on radial mixing. Experimental The main objective of this perspective paper is to generate a defined radial concentration profile of an X-ray contrast agent in demineralized water whose disturbance can be observed Fig. 3 a Schematic of the experimental set-up. Pure water and pure water due to the effect of Dean vortices. Potassium iodide is used enriched with potassium iodide (KI) are pumped into the rotating helical- ly coiled tube (HCT) that is mounted into the computed tomography as the contrast agent, which, when dissolved in water, forms + − + − scanner (Skyscan 1275). The total volume flow rate is measured at the K and I ions. Both, K and I , attenuate X-rays to a signif- outlet gravimetrically. b Photograph of HCT and schematic of the icantly higher level than pure water. Thereby, X-ray attenua- contacting device (T-junction). Pure water is fed from the side inlet and − + tion by I is higher than X-ray attenuation by K ,such that KI-rich water from the bottom inlet only the diffusion and dispersion of I can be registered in X- ray imaging [39]. The pure water and water containing 15 w% reconstruction software NRecon (Bruker, Billerica, MA). The KI are contacted in a T-junction (IDEX Health & Science, 3D data set consists of a stack of cross-sectional slices, each of IDEX Corporation, Northbrook, IL) that is connected to a which is composed of voxels, the 3D equivalent of pixels. The helically coiled tube (HCT). The HCT consists out of a total size of the 3D image is (1944 × 1944 × 1382) voxels with Fluorinated Ethylene Propylene (FEP) tube (d ¼ 1:58mm, i voxel size = 18μm  18μm  18μm. The reconstructed CT- d ¼ 3:2mm) that is coiled around a polylactide (PLA) sup- o volume is used to extract the qualitative separation areas be- port structure with a coil diameter (d ¼ 28:8mm). To test the c tween iodide rich, intermediate, and iodide poor region of the effect of torsion, the pitch of the HCT is varied (p ¼ 4:8mm, 1 total fluid stream. p ¼ 9mm). The HCT is mounted into a micro-computed to- mography scanner (Bruker Skyscan 1275, RJL Micro & Analytic GmbH, Karlsdorf Neuthart, Germany) that is equipped with tubes for liquid supply and removal. The KI-rich mixture is Summary and outlook pumped using a syringe pump (LAMBDA VIT-FIT by LAMBDA Instruments GmbH, Baar, Switzerland) and pure wa- In this perspective paper, micro-computed tomography was ter is pumped using a high-pressure dosing pump (BlueShadow successfully applied for the visualization of the concentration Pump 40P, KNAUER Wissenschaftliche Geräte GmbH, Berlin, field of iodide in helically coiled tubes (HCT). The effect of Germany). Both pumps are placed outside the CT, see Fig. 3. curvature and torsion on radial mixing was studied by varying Two different volume flow rates are tested, V ¼ 1:5mLmin the total volume flow rate and the pitch. It was found that and V ¼ 7mL min , resulting in Reynolds numbers of Re increasingDnnumber leads to the formation of a characteristic 2 1 ¼ 25 and Re ¼ 118, Dean numbers of Dn ¼ 6and Dn ¼ 28, concentration profile and enhanced radial mixing. At the same 2 1 2 and modified torsion numbers 254 < T∗ < 2222. The mean time, increasing the pitch reduces radial mixing due to Dean dynamic viscosity (η ¼ 0:86mPa s) for the calculation of Re is vortices only slightly. The presented methodology offers high estimatedaccordingtodatabyGoldsacketal. [41]. The vol- potential and will therefore be systematically extended. The ume flow rate ratio of pure water and KI-rich water is current investigations will be enriched with further post- 1. The total flow is monitored gravimetrically during the processing of reconstructed voxel slices to obtain concentra- experiments. Scanning voltage is 43kV, the current is 1 tion profiles and robust quantitative statements about mixing 90μA, and exposure time is 30ms.The size of the quality. Additionally, more flow conditions will be tested and resulting X-ray projection images is (1944 × 1382) the effect of different liquid densities on radial mixing must be pixels with pixel size = 18μm  18μm. addressed in more detail. The proposed method will not only During image acquisition, X-ray projection images are ac- help the fundamental understanding of dispersion in HCT but quired for different angular positions. The resulting set of also in diverse forms of passive microreactors. It can even be projection images is reconstructed to a 3D dataset using the extended to the investigation of certain reactions, such as J Flow Chem (2021) 11:217–222 221 temperature synthesis. React Chem Eng 5:1474–1483. https://doi. precipitation reactions. Finally, concentration fields obtained org/10.1039/D0RE00078G with the presented methodology will help in validating numer- 4. Epps RW, Volk AA, Abdel-Latif K et al (2020) An automated flow ical simulations that aim to simulate and optimize chemistry platform to decouple mixing and reaction times. React microreactors with internal convective mixing. Chem Eng 5:1212–1217. https://doi.org/10.1039/D0RE00129E 5. Besenhard MO, LaGrow AP, Hodzic A et al (2020) Co- precipitation synthesis of stable iron oxide nanoparticles with NaOH: New insights and continuous production via flow chemis- Notation d , coil diameter [m];d , inner tube diameter [m]; d , outer c i o try. Chem Eng J 399:125740. https://doi.org/10.1016/j.cej.2020. tube diameter [m]; Δρ , relative density difference between KI solu- rel − 2 tion and water [-]; F , centrfugal force [N]; g , gravity constant [m s ]; 6. 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Int J Heat Fluid Flow 68: Publisher’snote Springer Nature remains neutral with regard to jurisdic- 189–202. https://doi.org/10.1016/j.ijheatfluidflow.2017.10.011 tional claims in published maps and institutional affiliations. 29. Kalpakli Vester A, Sattarzadeh SS, Örlü R (2016) Combined hot- wire and PIV measurements of a swirling turbulent flow at the exit of a 90° pipe bend. J Vis 19:261–273. https://doi.org/10.1007/ s12650-015-0310-1 Julia Schuler studied Mechanical Engineering at the Paderborn 30. Schuler J, Kockmann N (2020) Micro-computed tomography for University. In July 2018, she started her work as a research assistant at the investigation of stationary liquid–liquid and liquid–gas inter- the Laboratory of Equipment Design, TU Dortmund University, faces in capillaries. AIChE J 66. https://doi.org/10.1002/aic.16890 Germany. Her focus is on the experimental investigation of single- and 31. 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Bieberle M, Barthel F, Hampel U (2012) Ultrafast X-ray computed Norbert Kockmann studied mechanical engineering at the Technical tomography for the analysis of gas–solid fluidized beds. Chem Eng University of Munich and completed his Dr.-Ing. in process engineering J189–190:356–363. https://doi.org/10.1016/j.cej.2012.02.028 at the University in Bremen. After 5 years in chemical industry as project 34. Große Daldrup A, Crine M, Marchot P et al (2019) An approach to manager he joined Freiburg University, IMTEK in 2001 as group leader separation efficiency modelling of structured packings based on X- for micro process engineering. In 2007, Dr. Kockmann joined Lonza Ltd., ray tomography measurements: Application to aqueous viscous Visp, Switzerland, as senior scientist responsible for continuous flow systems. Chem Eng Sci 204:310–319. https://doi.org/10.1016/j. processes and microreactor technology. In April 2011, Norbert ces.2019.03.059 Kockmann was appointed as full professor for Equipment Design at TU 35. Lau YM, Möller F, Hampel U et al (2018) Ultrafast X-ray tomo- Dortmund University, Germany. His research interests are on small-scale graphic imaging of multiphase flow in bubble columns – Part 2: device for continuous chemical processes, modular design, and process Characterisation of bubbles in the dense regime. Int J Multiphase intensification. His work includes fundamental investigations of small- Flow 104:272–285. https://doi.org/10.1016/j.ijmultiphaseflow. scale multiphase flow, modelling and simulation accompanied by modern 2018.02.009 sensing technology and machine-learning methods.

Journal

Journal of Flow ChemistrySpringer Journals

Published: Sep 1, 2021

Keywords: Helically coiled capillary; Micro-computed tomography; Dean flow; Radial mixing; Capillary flow reactor; Local concentration fields

References