Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert

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Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert Particle Tracking, Modular Design, and Production of Uniform CaCO Particles 3

Safa Kutup Kurt, Mohd Akhtar, Krishna D. P. Nigam, and Norbert Kockmann Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02240 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Industrial & Engineering Chemistry Research

Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert Particle Tracking, Modular Design, and Production of Uniform CaCO3 Particles Safa Kutup Kurt1*, Mohd Akhtar1, Krishna D. P. Nigam2, Norbert Kockmann1 1

TU Dortmund University, Biochemical and Chemical Engineering, Equipment Design, Emil-Figge-Str. 68, D-44227

Dortmund, Germany 2

Indian Institute of Technology, Department of Chemical Engineering, Hauz Khas, New Delhi-110016, India

*corresponding author: Safa Kutup Kurt, Tel. +49 (0) 231 7558067, E-mail: [email protected]

Abstract Multiphase flow profile inside a helically coiled tubular device (HCTD) was observed by using a high-speed camera. Gas-liquid slug flow observations revealed that the Taylor vortices are influenced by the secondary flow due to the centrifugal force acting perpendicular to the flow direction. Hence, mixing inside the liquid slug is enhanced by the combination of Dean and Taylor vortices in HCTD. Modular design of a specific type of HCTD, i.e., Coiled Flow Inverter (CFI) is elucidated by the representation of a new design space diagram. Continuous precipitation of calcium carbonate (𝐶𝑎𝐶𝑂3) was investigated for modular CFI made of polyvinyl chloride (PVC) tubes (𝑑𝑖 = 3.2 mm) with slug flow patterns. 𝐶𝑎𝐶𝑂3 was continuously precipitated along CFI with a conversion of ca. 90 %. CFI provided a narrower particle size distribution with median particle diameters around 28 µm and more uniform morphology in comparison to a batch reactor. Keywords: Coiled flow inverter; Modular design; Slug flow; Dean vortices; Gas-liquid reaction; Continuous reactive precipitation

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1 Introduction Curved microchannels and capillaries provide enhanced heat and mass transport due to their controllable flow patterns and high surface to volume ratio. Thus, researchers have long been investigated these devices for process intensification (PI)

1–6

. Planar designs with rectangular

channel cross section are commonly utilized for two-phase flow applications with different flow patterns. Slug flow (segmented or Taylor flow) is one of the most investigated flow pattern in microreactors as it provides a wide operating window with a well-defined and stable flow profile 7– 20

. Several experimental and numerical studies are available for the characterization of the slug

flow profile in straight horizontal, inclined, and vertical tubes either with circular or rectangular cross section

21–24

. Furthermore, the heat and mass transfer characterization along with mixing

behavior of a gas-liquid (G-L) slug flow were also investigated in microchannels 25–30. These studies revealed that the enhanced heat and mass transfer can be achieved by utilizing G-L slug flow in microfluidics with a slight increase in pressure drop. This contribution on the transport phenomena is addressed to the internal circulations (Taylor vortices) within the liquid slug. Taylor vortices are represented for a straight capillary with circular cross section indicating the vortex regions 1 and 2 in Figure 1. In Taylor flow a liquid wall film is obtained around the gas phase increasing the interfacial area. However, the mass transfer is still diffusion dominant along the vortex region 2 to the vortex region 1. A stagnant circle including the vortex regions 1 and 2 is represented at the cross section of the straight capillary in Figure 1. This stagnant circle indicates that the mixing in a straight capillary is solely controlled by diffusion. Thus, the perturbation of these vortex regions is required for the further enhancement of the mass transfer.


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flow direction A

A 1

2

A´

channel wall

mean velocity gas phase dispersed phase (d)

A´

stagnant circle

liquid phase continuous phase (c)

Figure 1: left: Longitudinal view of a gas-liquid slug flow pattern in a straight capillary with circular cross section, where the gas phase and the liquid phase are represented as the dispersed phase (d) and the continuous phase (c), respectively. The direction of the black arrows indicates the mass transfer of the gas phase to the liquid bulk. right: Cross-sectional view of a liquid slug with the stagnant circle, on which the mixing is solely controlled by diffusion; adapted with the permission from [Kurt, S. K.; Warnebold, F.; Nigam, K. D.; Kockmann, N. Gas-liquid reaction and mass transfer in microstructured coiled flow inverter. Chemical Engineering Science 2017, DOI: 10.1016/j.ces.2017.01.017]. copyright [2017/Elsevier] [Elsevier/Elsevier].

Günther et al. 28 optically characterized the G-L slug flow in a planar microchannel via micro particle image velocimetry (µPIV) and fluorescence microscopy techniques at low superficial velocities that are relevant for the chemical reactions with the residence times up to several minutes. They investigated the vortex regions of the liquid slug in straight channels (Figure 2 design 1) and in meandering (curved) channel design (Figure 2 design 2) with rectangular cross section (𝑤 = 400 µm, depth = 150 µm, 𝑑ℎ = 220 µm). They observed miscible liquid-liquid (L-L) mixing in GL slug flow. They indicated that the channel length, which is required to homogenize the concentration fields inside the liquid slug, decreases for the meandering channel design in comparison to the straight channel design due to the asymmetric liquid velocity fields with respect 3 ACS Paragon Plus Environment


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to the channel centerline. These asymmetric streamline contours are resulted from the centrifugal force (𝐹𝑐 ) acting perpendicular to the flow direction (Figure 2). Their investigations revealed that curved channel geometries contribute to maintain the homogeneity conditions inside the liquid slugs in planar microreactors.

Design 1

a)

Design 2

b)

flow direction

G

flow direction

G

G

G 200 µm

𝐹𝑐 Figure 2: Representative streamline contours that are resulted from the µPIV measurements of a gas-liquid slug flow a) at the straight channel design, b) at the meandering (curved)channel design (𝒘 = 400 µm, depth = 150 µm, 𝒅𝒉 = 220 µm) with the curvature radius of 600 µm; adapted with the permission from [Günther, A.; Khan, S. A.; Thalmann, M.; Trachsel, F.; Jensen, K. F. Transport and reaction in microscale segmented gas-liquid flow. Lab Chip 2004, 4, 278]. copyright [2004/Royal Society of Chemistry] [Royal Society of Chemistry/Royal Society of Chemistry].

Besides planar channel geometries, tubular devices with circular cross-sections have recently been utilized to fabricate the microreactors in three dimensional (3D) geometries providing enhanced heat and mass transfer. Helically coiled tubular devices (HCTD) induce secondary flow 4 ACS Paragon Plus Environment

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patterns (Dean vortices) that are generated by the centrifugal forces. These vortices enhance the radial mixing and thus provide a narrow residence time distribution (RTD) 31. A specific type of HCTD, i.e. Coiled Flow Inverter (CFI) includes 90° bends rotating (inverting) the Dean vortices by 90° (Figure 3). This results in moving the stagnant zones on the tube cross section into the central stream of the Dean vortex pair, which enhances the heat transfer and the mixing 32–35. Additionally, this further reduces the axial mixing and thus narrowing the RTD in terms of single phase flow applications 35–39. CFI has also been fabricated by using micro and milli scale tubes for PI 37,38. Different tube materials such as metal, glass (Figure 3), and polymers can be utilized for the fabrication of micro and milli scale CFI for the processes on laboratory (lab), pilot, and production scales. A coiled flow inverter can also be utilized for two phase flow applications. The L-L slug flow mass transfer characterization of the CFI revealed that the mass transfer limitations inside the liquid slugs are eliminated due to the formation of Dean vortices and changing their directions with 90° bends 40–42

. Even though various empirical and numerical studies are available for the gas-liquid (G-L)

flow in helically coiled tubes (HCT) 43–46 there is only a single experimental study regarding to GL flow in CFI. Vashisth and Nigam 47 investigated the liquid phase RTD of CFI in a G-L slug flow. They revealed that the reduction of the axial dispersion number is nearly 2.6 times in CFI with 15 bends as compared to a HCT (or straight helix) under identical process conditions. This opens up an interesting research field for the characterization of the CFI in multiphase flow applications, where complex chemical reactions take place and close-to-ideal-plug-flow reactor behavior, i.e. narrow RTD is required. For instance, segmented flow (slug or Taylor flow) tubular reactors (SFTR) have already been utilized in the continuous precipitation of calcium carbonate, barium titanate, and zinc oxide instead of the conventional batch reactors to overcome the scale-up problems that are linked with mixing, homogeneity of the species, and heat transfer 48–50. Vacassy 5 ACS Paragon Plus Environment


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et al. 51 investigated the precipitated calcium carbonate (PCC) synthesis in a SFTR that was made of PTFE tubes (𝑑𝑖 = 4 mm; 𝐿 = 10 m) in order to provide better mixing and homogeneity conditions during the reaction along with enhanced heat transfer. The PCC was produced at a feed flow rate of 1.67 mL∙min-1 by mixing a solution containing the 𝐶𝑂3−2 anions and the Ca+2 cations. Ammonium carbonate ((𝑁𝐻4 )2 𝐶𝑂3 ) and calcium chloride (𝐶𝑎𝐶𝑙2) were utilized as the anion and the cation source, respectively. The reactants were injected to the system simultaneously with the injection of air by a Y-mixer and/or cross-mixer. G-L segmentation of the reactants was achieved by the existence of the air preventing the back mixing and, thus, assuring narrow RTD. However, they observed scaling (fouling) of calcium carbonate particles on the wall material (PTFE tubes). They indicated that segmentation of the flow by air contributed to flushing out the PCC particles. Their investigations with statistical design of experiments revealed that the control of the PCC morphology is possible with a narrow PSD. The study of Vacassy et al.

51

was promising for the

continuous production of calcium carbonate; however, it was only achievable at very low flow rates in the range of 1.67 - 11.7 mL∙min-1 in presence of scaling. Thus, Aimable et al. developed the study of Vacassy et al.

51

48

further

for the continuous production of PCC by eliminating the

issues arisen from the scaling. In their experimental setup, the reaction mixture was segmented by an immiscible organic solvent, i.e., dodecane. L-L slug flow patterns were achieved along the tube and the scaling was avoided by the dodecane forming a thin film at the wall. However, they required a separation unit for the immiscible phase in case of the carbonation in L-L segmentation route. Additionally, the mixing enhancement within the liquid slugs is limited in SFTR due to the poorly mixed regions as it is explained in Section 3. The further intensification of the mixing in continuous production of PCC can be achieved by using a CFI with G-L slug flow patterns. This can also pave the way for the integration of CFI into multiphase flow applications.


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(a)

(b)

Lc p outlet

2 cm cross-sectional velocity contour at the inlet of the straight pipe

dc dct

outlet

di inlet

cross-sectional velocity contour and streamlines at the second turn of the second coil

inlet

cross-sectional velocity contour and streamlines at the second turn of the first coil

Figure 3: a) Design parameters of a frame-wise CFI configuration and the cross-sectional velocity contours with streamlines at the inlet and after 90° bend, where 𝒅𝒊 is the inner tube diameter, 𝒑 is the pitch distance between two turns, 𝒅𝒄𝒕 is the coil tube diameter that can be used as supporting structure, 𝒅𝒄 is the coil diameter, and 𝑳𝒄 is the length of a single coil. The cross-sectional velocity contours and the streamlines for single phase flow are taken from 52. b) Fabricated frame-wise CFI design made of borosilicate glass tubes with 𝒅𝒊 = 3 mm (outer diameter, 𝒅𝒐 = 6 mm), where the white arrows indicate the flow direction at the inlet and the outlet of the CFI; adapted with the permission from [Kurt, S. K.; Warnebold, F.; Nigam, K. D.; Kockmann, N. Gas-liquid reaction and mass transfer in microstructured coiled flow inverter. Chemical Engineering Science 2017, DOI: 10.1016/j.ces.2017.01.017]. copyright [2017/Elsevier] [Elsevier/Elsevier].

In this work, the path of inert particles in G-L slug flow within a HCT (3D structure) is optically investigated for the first time in literature proving the existence of secondary flow. Afterwards, modular design of a CFI is presented providing variable residence time with various tube and coil diameter as well as number of helices. A new design space diagram (DSD) for CFI is introduced indicating the degree of freedom in choosing the geometrical design parameters of the modular CFI reactor for lab scale applications. The modular CFI reactor is investigated for 7 ACS Paragon Plus Environment


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continuous production of PCC with G-L slug flow patterns in comparison to conventional batch reactors. Slug flow profile is generated by using a Y-mixer, at which the gaseous 𝐶𝑂2/synthetic air mixture and the saturated calcium hydroxide solution are contacted at ambient conditions prior to the CFI inlet. The variation in residence time and 𝐶𝑂2 content of the gaseous mixture on conversion, particle size distribution (PSD), and particle morphology are investigated with the modular CFI reactor. The morphology and the PSD of PCC particles produced with the modular CFI are compared with the results of a batch reactor, which provides similar conversion values under identical process conditions. In conclusion, some major results and future research direction are presented.

2 Design of a Modular Coiled Flow Inverter via 3D Design Space Diagram Single phase flow RTD characterization of a CFI has been intensively investigated by various researchers 35–39. Klutz et al. 37 proposed a design space diagram (DSD) to select the geometrical design parameters of a CFI reactor, which allows for designing a CFI with close-to-plug-flow reactor behavior 53 for a given residence time and volumetric flow rate. Afterwards, Kurt et al.

38

investigated the influence of the geometrical design parameters along with the new CFI configurations (Figure 4), i.e. zigzag and step-wise on RTD curves. The resulting RTD curves showed similar behavior in terms of narrowness in comparison to the conventional frame-wise CFI configuration. They concluded that one of these configurations can be designed by considering the space requirements of the different applications, once the geometrical parameters (Figure 3) and the resulting dimensionless numbers are appropriately chosen according to their investigations with the single-phase flow.


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Figure 4: Different CFI configurations made of polytetrafluoroethylene (PTFE) tubes and coiled on silicon tubes with an outer diameter (𝑑𝑜 ) of 18 mm a) stepwise structure (PTFE, 𝑑𝑖 = 1 mm), b) zigzag structure (PTFE, 𝑑𝑖 = 1 mm), c) frame-wise structure (PTFE, 𝑑𝑖 = 0.5 mm); adapted with the permission from [Kurt, S. K.; Gelhausen, M. G.; Kockmann, N. Axial Dispersion and Heat Transfer in a Milli/Microstructured Coiled Flow Inverter for Narrow Residence Time Distribution at Laminar Flow. Chem. Eng. Technol. 2015, 38, 1122–1130]. copyright [2015/ John Wiley and Sons] [John Wiley and Sons/ John Wiley and Sons].

The dimensionless numbers, i.e. Dean number (𝐷𝑛) and modified Torsion parameter (𝑇 ∗ ) can be calculated for single phase flow applications according to the equations 1 and 2, respectively. 1

𝑑

𝐷𝑛 = 𝑅𝑒 ∙ √𝜆 = 𝑅𝑒 ∙ √𝑑 𝑖

(1)

𝑐

𝑇 ∗ = 𝑅𝑒

𝜋∙𝑑𝑐

(2)

𝑝

with

𝑅𝑒 =

𝜆=

̅∙𝜌∙𝑑𝑖 𝑢

(3)

𝜂

𝑑𝑐

(4)

𝑑𝑖

𝑑𝑐 = 𝑑𝑖 + 2𝑠 + 𝑑𝑐𝑡

(5)



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Here 𝑢̅, 𝜌, 𝜂, and 𝑠 indicate the average velocity, fluid density, the dynamic viscosity of the fluid, and the wall thickness of the tube that is used to fabricate the CFI, respectively. The Dean number characterizes the flow phenomenon in curved geometries and it was firstly described by Dean

54,55

for the motion of fluid in curved channels. The intensity of Dean vortices can be

described by Dean number that takes into account of curvature ratio (𝜆) and Reynolds number (𝑅𝑒). It was studied that an increase in Dean number up to 3.0 narrows the RTD curve in CFI, however, Dean number greater than 3.0 does not further narrow the RTD curve 36. Thus, CFI design must be proper to result in a minimum value for 𝐷𝑛 as 3.0 if a narrow RTD behavior is desired. Moreover, main fluid motion and secondary flow persist in three dimensions in CFI and thus the effect of torsion caused by a finite pitch determines the formation of Dean vortices, too. Therefore, this parameter has to be considered for the design of a CFI in order to maintain an enhanced radial mixing. Experimental investigations revealed that 𝑇 ∗ , which is calculated by Eq. (2), must be greater than 1000 in order to minimize the influence of torsion on the secondary flow, and thus, narrower RTD can be achieved 38. In the content of this study the modular concept is incorporated with an objective to provide flexible design of a CFI that provides a close-to-plug-flow reactor behavior for the lab, pilot, and production scale applications. The flexibility term takes into account of a degree of freedom to choose the geometrical parameters of the CFI allowing variable residence times at constant volumetric flow rates (𝑉̇ ). Additionally, it is also considered to assemble the different configurations of the CFI (Figure 4) via a simple modification in the construction. Furthermore, the dimensionless numbers (𝐷𝑛 and 𝑇 ∗ ) can be manipulated by varying the coil tube diameter (𝑑𝑐𝑡 ) and the pitch (𝑝) independent of Reynolds number. Adjustment of 𝑝 to a certain value provides a degree of freedom to choose 𝑑𝑐𝑡 according to the purpose of an application. This flexible design parameter of the CFI was kept constant up to now during the fabrication 36–38. However, in present 10 ACS Paragon Plus Environment

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study a modular CFI design is subjected to be fabricated by using variable 𝑑𝑐𝑡 , and thus, the intensity of the Dean vortices can be manipulated influencing the heat and mass transfer at constant 𝑅𝑒. Klutz et al. 37 introduced a two dimensional DSD for the representation of the promising CFI configurations that provide enhanced radial mixing at a specific volumetric flow rate (𝑉̇ ). The drawback of the proposed DSD was the necessity of a new diagram to find out available CFI configurations in case of a change in volumetric flow rate. Therefore, a three dimensional (3D) design space diagram is presented here by considering the different volumetric flow rates that can be required for an application in lab scale. Thus, the volumetric flow rate is kept in the range of 110 mL min-1, which is suitable for lab scale applications. Please note that the aim of the represented 3D DSD is not extracting the optimal operating conditions from the diagram, rather elucidating the modular design concept. Thus, it presents the flexibility of the equipment in terms of residence time and volumetric flow rate of a process in one diagram. If these process parameters are unknown for the lab-scale investigations of a process such as continuous precipitation of calcium carbonate, the equipment should be flexible enough to be modified with a simple manipulation. The diagram is generated in MATLAB® by using the geometrical parameters of a CFI (Figure 3) by taking the dimensionless numbers 𝐷𝑛 and 𝑇 ∗ and water at 40° C into account. 𝐷𝑛 and 𝑇 ∗ are calculated for different arrangements of 𝑑𝑖 and 𝑑𝑐𝑡 at the volumetric flow rate range. The minimum 𝑝 is considered for the calculation of 𝑇 ∗ with the tube wall thicknesses (s), which are determined by the regression equation, based on the data of the commercially available tubes. As it can be seen from Figure 5, dark grey and turquois surfaces stand for 𝐷𝑛 and 𝑇 ∗ with the values of 3.0 and 1000, respectively. A feasible region above the turquois surface and under the dark grey surface is indicated for the different arrangements of 𝑑𝑖 and 𝑑𝑐𝑡 with respect to 𝑉̇ as the values of 𝐷𝑛 and 𝑇 ∗ are greater than 3.0 and 1000, respectively. Thus, this feasible region provides to design a modular 11 ACS Paragon Plus Environment


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CFI ensuring the enhanced radial mixing with different arrangements of 𝑑𝑖 and 𝑑𝑐𝑡 on lab scale applications. For instance, a CFI design can be fabricated by coiling commercially available tubes with inner diameter of 2.4 and 3.2 mm (orange dots in Figure 5) on the available coil tube diameters of 25 and 45 mm (purple and yellow planes in Figure 5, respectively) for a specific application at 7 mL min-1 (green dots). These suitable arrangements are in the feasible region, and thus, provide enhanced radial mixing. If the volumetric flow rate of 3 mL min-1 should be applied for the same application, the combination with tube of 3.2 mm inner diameter and the coil tube of 25 mm diameter is outside of the recommended region (red dot in Figure 5). However, the feasible region can be reached by increasing the 𝑑𝑐𝑡 from 25 mm to 45 mm. Therefore, the modular CFI can necessarily satisfy the requirements for the enhanced radial mixing with a simple modification in its configuration, e.g. by changing the 𝑑𝑐𝑡 at constant 𝑑𝑖 and 𝑉̇ . Additionally, the narrowest RTD of the modular CFI can be provided by achieving the maximum number of 90° bends (𝑛𝑏𝑒𝑛𝑑𝑠 ) with the tube length assuring the required residence time at a given 𝑉̇ . Here, at least 5 number of turns per coil (𝑛𝑡𝑢𝑟𝑛𝑠 ) should be arranged 36,38. Furthermore, using different 𝑑𝑐𝑡 arrangements in a CFI module allows for compensating the changes in the radial mixing in case of an application, where the physical properties of the fluid changes during the process. In this work, a modular CFI reactor is fabricated by choosing the 𝑑𝑖 and 𝑑𝑐𝑡 as 1.6 mm, 2.4 mm, 3.2 mm and 25 mm, 35 mm, 45 mm, respectively from the DSD that is represented in Figure 5. The modular CFI design should ensure a wide range of the residence time for a specific 𝑉̇ that can be used for the investigations on the continuous precipitation of the calcium carbonate as it is represented in Section 5.2.


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𝐷𝑛 = 3 𝑇 ∗ = 1000

feasible region

𝑑𝑐𝑡 [mm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑑𝑐𝑡 = 45

𝑑𝑐𝑡 = 25

Axes

available inner tube diameters

suitable arrangement

desired volumetric flow rates

unsuitable arrangement

Figure 5: Design space diagram (DSD) of the modular CFI configuration ensuring the enhanced radial mixing for a close-to-plug-flow reactor behavior on lab scale applications (water at 40 °C as working fluid with 𝛒 = 992.4 kg∙m-3 and 𝛈 = 0.653 mPa∙s).

3 Slug Flow Mixing Behavior in a Coiled Flow Inverter Two-phase flow characteristics of a coiled flow inverter has recently been investigated for L-L slug flow patterns by the researchers

40–42

. In these studies, it was concluded that the CFI offers

enhanced L-L mass transfer in comparison to a straight tube and a HCT due to the formation of secondary flow patterns and their alternating 90° inversions. Furthermore, G-L mass transfer performance of a straight tube, HCT, and CFI in case of an oxidation reaction has recently been 13 ACS Paragon Plus Environment


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investigated with slug flow patterns in our group 56. The results experimentally prevailed that the mixing performance within the liquid slug is enhanced in CFI due to the formation of Dean vortices and their 90° inversions. Thus, CFI provided up to 14 % higher conversion in comparison to straight tube and HCT under identical process conditions. A novel explanation was also presented in order to distinguish the mixing behavior of a CFI from a straight tube and a HCT. Figure 6 represents the representative drawings, which were used as hypothesis for the enhanced mixing performance in our previous work 56, in order to elucidate the mixing behavior of these devices in detail. In Figure 6, the longitudinal and cross-sectional views of G-L slug flow profiles are represented. Taylor and Dean vortices are presented for the liquid slugs (continuous phase) according to the knowledge gained from the investigations that are available in literature

13,28,57

.

For straight tube, Taylor vortices are symmetrically represented with respect to the tube centerline at longitudinal view. The red points inside the Taylor vortices indicate the stagnant zones (areas), which are also represented on the stagnant circle at cross-sectional view, prevent the enhanced mixing along the tube (cf. Figure 1 with vortex regions 1 and 2). At these stagnant areas, or more correctly, on the stagnant circle at the cross-sectional view, mixing is controlled solely by diffusion, which is quite slow in liquid phase flow. To overcome the poor mixing inside the liquid slug, the perturbation of the stagnant zones is required along the tube. The perturbation can be achieved by coiling the straight tube on an axis forming a helically coiled tube (HCT). As a result, enhanced mixing can be achieved inside the liquid slug due to the centrifugal force (𝐹𝐶 ) acting perpendicular to the flow direction. The asymmetric Taylor vortices with respect to the tube centerline represent the perturbation of the stagnant red points, which exist inside the straight tube, at the longitudinal view of the G-L slug flow in HCT. This perturbation can also be seen at the cross-sectional view of the flow profile in HCT. The secondary flow profile (Dean vortices), which is induced by the 14 ACS Paragon Plus Environment

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centrifugal force, is represented at the cross-sectional view with two symmetrical vortices. The black points inside those vortices represent the stagnant zones limiting the enhanced mixing. Nevertheless, a stagnant zone is still present in the helical slug flow. Thus, the combination of Taylor and Dean vortices is not sufficient enough to eliminate the stagnant points for the efficient mixing in a HCT. This was proved with the results of the experimental work in our previous studies 40,56

. Coiled flow inverter can further enhance the mixing by changing the flow direction with 90°

bends achieving the perturbation of the stagnant points inside the Dean vortices. In Figure 6, the longitudinal and cross-sectional views of the slug flow profile are represented for the first coil of the CFI that are the same as for HCT. The black points indicate the stagnant points inside the Dean vortices at the first coil of the CFI. At the second coil, the green points indicate the centers of the perturbed Taylor vortices (cf. at the first coil with the red points). The cross-sectional flow profile represents the Dean vortices before and after the 90° bend with dashed and solid lines, respectively at the second coil. After the 90° bend (at the second coil) the black points are perturbed by the centrifugal force, which has changed its direction by bending. Furthermore, new Dean vortices (solid line) are generated perpendicular to the previous vortices (dashed line). The new stagnant points inside the new Dean vortices are represented with blue points at the second coil. It must be noted that the perturbation of these points is also achieved by changing the flow direction with another 90° bend along the CFI. Therefore, CFI can provide an enhanced radial mixing, i.e. narrow residence time distribution in comparison to a HCT and straight tube with G-L slug flow pattern. This also allows for achieving more homogenous conditions inside the liquid slugs, and thus, further intensification of the process at a certain flow rate and a residence time. Thus, the continuous precipitation of the calcium carbonate with G-L reaction is more favorable to be investigated in CFI rather than a straight tube and a HCT. 15 ACS Paragon Plus Environment


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longitudinal view

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cross-sectional view

a) Straight tube

A flow direction

A

flow direction

inlet

outlet

A´

mean velocity

stagnant circle A´

b) Helically coiled tube (HCT)

mean velocity outlet

flow direction

flow direction

FC

FC

B

mean velocity

C

B´

inlet flow direction B

B´

c) Coiled flow inverter (CFI)

outlet

2nd coil

FC

1st coil

flow direction

FC

flow direction

flow direction

C´

inlet

C

D C´

flow direction

D

D´

mean velocity

D´

FC hydrophilic tube wall

gas phase

aqueous phase

FC

Figure 6: Gas-liquid slug flow mixing behavior in a) straight pipe b) helically coiled tube c) coiled flow inverter; adapted with the permission from [Kurt, S. K.; Warnebold, F.; Nigam, K. D.; Kockmann, N. Gas16 ACS Paragon Plus Environment

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liquid reaction and mass transfer in microstructured coiled flow inverter. Chemical Engineering Science 2017, DOI: 10.1016/j.ces.2017.01.017]. copyright [2017/Elsevier] [Elsevier/Elsevier].

4 Materials and Methods 4.1 Particle Tracking in Gas-Liquid Slug Flow Profile with Inert Particles In this work, the particle tracking was observed for G-L slug flow in a HCT (𝑑𝑖 = 3 mm, 𝑑𝑐 = 31 mm, 𝑝 ≅ 9 mm) with a circular cross section that is made of the borosilicate glass (from the Workshop of Biochemical and Chemical Engineering Department, TU Dortmund University) for the first time according to our knowledge. Currently it is not technically possible to track the particles in CFI, where flow inversions occur due to the 90° bends. This is because of the complex three-dimensional (3D) structure, which limits the optical observations. Thus, the particle movement was observed in HCT, which represents the main flow motion in a CFI. In our group, we are willing to elucidate the particle movement at the flow inversion parts in near future with experimental studies that will be supported by numerical simulation results. A Y-mixer, which includes a 60° connecting angle for the inlets (𝑑𝑖 = 3 mm, 𝐿 = 60 mm) and a straight pipe as an outlet (𝑑𝑖 = 3 mm, 𝐿 = 60 mm), was made of borosilicate glass and welded to the inlet of the HCT for the slug flow generation. Afterwards it was immersed in a water bath that is made of polymethylmethacrylate (PMMA) for better optical observation under the exposure of LED lamp (Kaiser Fototechnik GmbH & Co. KG). A high-speed camera (Xtra Motion NR4 from Imaging Solutions GmbH, Germany) and a macro zoom objective (CBC Europe GmbH, Germany) were used for the acquisition of the videos, which were recorded to the computer via IDT Motion Studio software. Butyl acetate (99.5 %, Merck KGaA, Germany) and synthetic air (80 Vol. % N2 and 20 Vol. % O2, Messer Group GmbH, Germany) were introduced as the liquid phase and gas phase, respectively. The liquid phase was injected by using a syringe pump (SyrDos2, HiTec Zang 17 ACS Paragon Plus Environment


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GmbH, Germany), while the gas phase was directly injected from the storage cylinder of the synthetic air via a rotameter. Polyethylene particles (from the Laboratory of Solids Process Engineering, TU Dortmund University) with a median diameter of (𝑑50,2 , which is also indicated as Ds50 in literature) 10.07 µm and the density of (𝜌) 960 kg∙m-3 were used for optical observations. Approximately 2 mL of 𝑛-butyl acetate with the particles of 5-10 weight percent (wt %) were promptly injected through a T-junction (P-713-PEEK, IDEX Europe GmbH, Germany) prior to the liquid inlet of the Y-mixer. The complete experimental setup is presented in Figure 7.

outlet of the helically coiled tube

water bath

Y-mixer

high speed camera

helically coiled tube air inlet butyl acetate inlet 𝑛-butyl acetate

particles

syringe for the particle injection

T-mixer Figure 7: Experimental setup for the particle tracking. 18 ACS Paragon Plus Environment

macro zoom objective T-mixer

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4.2 Fabrication of a Modular Coiled Flow Inverter (CFI) Coil tube diameters were considered as straight cylindrical rods that are made of PVC (from the Workshop of Biochemical and Chemical Engineering Department, TU Dortmund University) with the outer diameters of 25 mm, 35 mm, and 45 mm and the length (𝐿𝑐𝑡 ) of 60 mm for the fabrication of the modular CFI. A simple 90° connector that was made of polyoxymethylene (POM, from the Workshop of Biochemical and Chemical Engineering Department, TU Dortmund University) to connect the coil tube diameters via screwing. This connector provides a degree of freedom to design the CFI in zigzag and frame-wise configurations. PVC tubes with inner diameters of 1.6 mm, 2.4 mm, and 3.2 mm are coiled on the coil tube diameters allowing at least 5 turns (𝑛𝑡𝑢𝑟𝑛𝑠 ) per coil. Figure 8 represents the required elements of the modular CFI. Furthermore, assembling steps of the elements in order to fabricate a complete modular CFI setup in frame-wise configuration is shown in Figure 9. In step 1 all the elements of a single frame-wise CFI are represented. In step 2 the screws are inserted to the 90°connectors. Afterwards, until step 6 the coil tube diameters are fixed to the 90° connectors to assemble a frame-wise CFI configuration by screwing. Please note that the zigzag CFI configuration can also be assembled by using the same 90° connectors. A plate (PVC, 200x200x20 mm) is used to insert the frames of the modular CFI by using stainless steel threaded rods (𝐿𝑟𝑜𝑑 = 250 mm; 𝑑𝑟𝑜𝑑 = 8 mm) maintaining a stable structure. The distance between the different frames can be adjusted by using screw nuts, which allows coiling the tubes with different outer diameters.



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(a)

(b)

(c)

2 cm

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2 cm

2 cm

(d) 3 cm

(e)

3 cm

(f)

Figure 8: Coil tube diameters (𝒅𝒄𝒕 ) for the modular CFI design; a) with 𝒅𝒄𝒕 = 25 mm; 𝑳𝒄𝒕 = 60 mm; b) with 𝒅𝒄𝒕 = 35 mm; 𝑳𝒄𝒕 = 60 mm; c) with 𝐝𝐜𝐭 = 45 mm; 𝐋𝐜𝐭 = 60 mm and 90° connectors; d) without connecting a 𝒅𝒄𝒕 ; e) with connecting a 𝒅𝒄𝒕 = 35 mm; f) a single element of the CFI module, where the tube with inner diameter of 2.4 mm is coiled on a 𝒅𝒄𝒕 = 35 mm.

10 cm

step 1

step 2

step 3

step 4

step 5

step 6

Figure 9: Drawings of the assembling steps of the modular CFI via SolidWorks 3D-CAD software program (on the left) and photograph of the assembled frame-wise configuration of the modular CFI with different coil tube diameters (from bottom to top: 2 frames with 𝑑𝑐𝑡 = 45 mm, 1 frame with 𝑑𝑐𝑡 = 35 mm, and 1 frame with 𝑑𝑐𝑡 = 25 mm) after the fabrication (on the right).

4.3 Continuous production of calcium carbonate The limestone (calcium carbonate, 𝐶𝑎𝐶𝑂3), which is the most common calcium compound, is found on the earth with the purities above 90% 58. However, the limestone in its natural state is not 20 ACS Paragon Plus Environment

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suitable for many applications due to impurities. Furthermore, the difficulties in reducing the size of the particle to desired scale also limits the utilization of the natural limestone as filling material for plastic, rubber, paper etc. 59. One common method to recover the calcium carbonate from the calcium hydroxide (Ca(OH)2) slurry (milk of lime) is the carbonation process. The reaction steps of the carbonation process are given below 60. Step 1: Dissolution of 𝐶𝑎(𝑂𝐻)2 (pH ≅ 12.5) 𝐶𝑎(𝑂𝐻)2 (𝑠) + 𝐻2 𝑂(𝑙) → 𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) Step 2: Carbonation reaction (pH ≥ 8.6) 𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) + 𝐶𝑂2 (𝑔) → 𝐶𝑎𝐶𝑂3 (𝑠) + 𝐻2 𝑂(𝑙) Step 3: Hydrolysis of the PCC (pH  8.6) 𝐶𝑎𝐶𝑂3 (𝑠) + 𝐶𝑂2 (𝑔) + 𝐻2 𝑂(𝑙) → 𝐶𝑎(𝐻𝐶𝑂3 )2 (𝑎𝑞) In industry, this process is conducted in a batch reactor, in which the 𝐶𝑎(𝑂𝐻)2 is present as suspension with a solid content of 20 to 30 % by weight at isothermal conditions (𝑇 = 30-80 °C). The 𝐶𝑂2 gas (≅ 20 Vol. %) is injected from the bottom part of the reactor achieving a better bubble size distribution (BSD) via the rotation of a stirrer. The reaction can be monitored by continuous measurement of the pH and conductivity. However, the solubility of calcium ions (𝐶𝑎+2) is significantly dependent on the pH value, and thus, the carbonation reaction is promoted by pH values greater than 8.6

61

. The pH values lower than 8.6 results in the dissolution of 𝐶𝑎𝐶𝑂3

promoting the formation of the calcium bicarbonate 𝐶𝑎(𝐻𝐶𝑂3 )2 as it is given in reaction step 3. Three different morphologies of the PCC are available, i.e. calcite, vaterite and aragonite. The



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crystal form depends on the different factors and their interactions such as the ratio of the calcium ions (𝐶𝑎+2) to the carbonate ions (𝐶𝑂3−2), pH, temperature, and the intensity of the mixing 62. To investigate the continuous precipitation of the calcium carbonate with a modular CFI, the 𝐶𝑎(𝑂𝐻)2 (VWR International GmbH, Germany), which is dissolved in deionized water at normal temperature and the pressure (NTP, i.e. 20 °C, 1 bar) achieving a saturated mother liquor solution, was utilized as liquid phase. The gaseous 𝐶𝑂2/air (Messer Group GmbH, Germany) mixture was injected to the system as gas phase. The complete experimental setup is presented in Figure 10. A stirred reactor (𝑉𝑅 = 400 mL) with a jacket was used for the preparation of saturated 𝐶𝑎(𝑂𝐻)2 solution at NTP. 𝐶𝑎(𝑂𝐻)2 was dissolved in the presence of 𝑁2 (Messer Group GmbH, Germany) while stirring at 300 rpm in order to avoid contacting mother liquor solution with atmospheric air (no contact with 𝐶𝑂2). Vacuum filtration was directly applied for the saturated 𝐶𝑎(𝑂𝐻)2 solution, where the filtration unit was connected to the stirred reactor (closed system) at the bottom. 𝑁2 was still injected to the stirred reactor during the filtration. The impurities are subjected to be separated from the mother liquor solution. A gear pump (Ismatec® Cole-Parmer, GmbH, Germany) was used for liquid injection. Volumetric flow rate of the liquid phase was arranged by the calibrated gear pump. Volumetric gas flow rate of air and 𝐶𝑂2 were adjusted via rotameters (Aalborg Instruments & Controls Inc., USA) that were followed by the temperature (RÖSSEL-Messtechnik GmbH, Germany) and the pressure (Afriso-Euro-Index GmbH, Germany) sensors in order to determine the molar concentration of the gases that were injected into the CFI. The temperature and pressure measurements were recorded in LabVIEW (National Instruments Germany GmbH, Germany). Both gases were mixed by using a T-mixer prior to the inlet of the CFI. The slug flow patterns were generated via a Y-Mixer (made of borosilicate glass with di = 3 mm) that was connected to the inlet of the CFI. All the units were inserted into a thermostat and the reaction was run at 35°C. Heat exchangers were connected prior to the inlet of the Y-mixer to maintain the reaction at 35°C for all 22 ACS Paragon Plus Environment

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precipitation experiments. Reactor outlet was collected into a vessel that was continuously stirred with a magnet-fish. Injection of 𝑁2 was applied to avoid further contact of air with the reactor outlet stream. In order to check whether the dissolved 𝐶𝑂2 reacts with the 𝐶𝑎(𝑂𝐻)2 after the CFI reactor, several samples from the reactor outlet were collected and kept for 3 days. The pH and the conductivity of the samples were measured each day and the conversion of the samples was determined by titration method that is described in Section 4.4.1. The results of these samples were compared with the measurements of a sample that was taken right after the reaction. No significant difference was observed between the measurements, as it indicates that dissolved 𝐶𝑂2 does not significantly react after the reaction. This also proves that injecting 𝑁2 into the collecting vessel prevents the further reaction after the CFI. Suspension samples with a volume of ca. 50 mL and 10 mL were collected for the conversion and the microscopic image analyses, respectively. Suspension with 𝐶𝑎𝐶𝑂3 particles and unreacted 𝐶𝑎(𝑂𝐻)2 was filtrated and washed out with deionized water in presence of 𝑁2 to flush out the unreacted 𝐶𝑎(𝑂𝐻)2. The PCC particles were dried for 24 hours at a temperature of 60 °C in an oven (Memmert GmbH, Germany). The downstream analyses for the characterization of the PCC particles are explained in the following section.



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pressure and temperature sensors

shut-off valves

air and CO2 flow meters

stirred reactor

back pressure valves

T-mixer for gases vacuum filtration

heat exchangers

gear pump thermostats coiled flow inverter

Figure 10: Experimental setup for the continuous precipitation of calcium carbonate.

4.4 Downstream Analyses Titration method is used to determine the unreacted 𝐶𝑎(𝑂𝐻)2 that is present in the solution. This method is standardized and registered under ‘DIN EN 12485:2010-08 (E)’. According to this method, 𝐶𝑎(𝑂𝐻)2 is mixed with sugar solution, which forms calcium sucrate. This is then determined by titrating with 𝐻𝐶𝑙 solution (1 M) using the phenolphthalein as color indicator. The solution is prepared by dissolving 0.5 g of phenolphthalein into 50 mL of ethanol and then diluted to 100 mL with deionized water. 2 – 3 drops of the indicator solution is used for each titration analysis. The maximum error that is resulted from the titration analyses was determined prior to the calcium carbonate precipitation experiments. The results revealed that the maximum conversion error was less than 1.5 % with respect to the average value. 24 ACS Paragon Plus Environment

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Approximately 15 μL of the product suspension is placed in between two microscopic slides and the images were taken by using a digital camera (Nikon D7000) that is connected to a microscope (Bresser GmbG, Germany). Images of various section of the slide are taken to cover the maximum range of the produced particles. Each image is analyzed for PSD by using ImageJ software (Java 1.6.0_20). Maximum Feret diameter (𝑑Feret,max) is calculated by using the surface area of a particle that is analyzed by ImageJ. It corresponds to maximum visible 2D diameter. The morphology is examined with Scanning Electron Microscopy (SEM) by using a JSM6610LA supplied by JEOL. SEM analyses are conducted for the dried samples of PCC by Lhoist Recherche et Développement S.A., Belgium. The pH and the conductivity are measured by using the electrodes (Consort bvba, Belgium) before and after the reaction. Monitoring these values provide the information related to the conversion.

5 Results and Discussion 5.1 Particle Tracking in Gas-Liquid Slug Flow Profile with Inert Particles In this work, the summation of the superficial gas velocity (𝑢̅𝑆𝐺 ) and the superficial liquid velocity (𝑢̅𝑆𝐿 ) is used to calculate the two-phase flow (G-L) Reynolds number (𝑅𝑒𝑇𝑃 ) and Dean number (𝐷𝑛𝑇𝑃 ) with respect to the physical properties of the liquid phase at the NTP conditions for G-L slug flow patterns. Thus, the Eq. (1) and (3) can be rewritten as following: 𝑅𝑒𝑇𝑃 =

𝐷𝑛𝑇𝑃 =

̅𝑆𝐿 +𝑢 ̅𝑆𝐺 )∙𝜌𝐿 ∙𝑑𝑖 (𝑢

(6)

𝜂𝐿

̅𝑆𝐿 +𝑢 ̅𝑆𝐺 )∙𝜌𝐿 ∙𝑑𝑖 (𝑢 𝜂𝐿

1

∙ √𝜆

(7)



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In order to characterize the G-L slug flow behavior along with the effect of Dean vortices, total volumetric flow rate of the phases was adjusted in the range of 8 – 20 mL∙min-1. As it is described in Section 4.1, the particles were introduced within 𝑛-butyl acetate (BuAc). The pictures were taken at the same position of the HCT, i.e. the angle of the first coil from the inlet was in the range of 180° - 270°. Several particles were detected from the taken pictures at different starting radial positions. Only their radial movement was tracked in different time intervals. The inner wall and the outer wall of the tube cross sections were indicated as -0.5 and +0.5, respectively (𝑑𝑖 = 3 mm). The tube centerline is shown with black dashed curve. The images were analyzed by using ImageJ software and GIMP 2 image manipulation program. As it can be seen from Figure 11 (𝑅𝑒𝑇𝑃 = 67, 𝐷𝑛𝑇𝑃 = 21), the particle with the starting position of -0.12 (position #1, 𝑡 = 0 s) shifts to the outer wall along the flow direction due to the centrifugal (𝐹𝑐 ) force acting perpendicular to the flow direction. The radial movement of a particle can be qualitatively used to calculate the radial velocity of the particle at different 2D positions (position #1 - #6). Figure 12 represents four different particles that are located at four different radial positions at the beginning of the observation. For instance, the radial position and the radial velocity of the particle that is tracked in Figure 11 are indicated by the corresponding position numbers in Figure 12. The particles, which are located at the inner half of the tube cross section with the radial positions in the range of -0.5 – 0, tends to move to the outer wall due to the centrifugal force. Thus, close to the outer wall of the HCT more particles are located in comparison to the inner wall, which can be seen from the high intensity of the grey color (shading) at the outer wall in Figure 11. Moreover, a particle with a radial starting position of 0.15 tends to shift to the outer wall up to the dimensionless time of 0.5, and then tends to move to the center of the tube. This movement can be analogously explained by the streamlines of the fluid elements that are shifted to the outer wall and


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then divided into two vortices (Dean vortices) in case of a slug flow in HCTs as it is represented in Figure 6.

200 fps

200 fps

200 fps

flow direction

flow direction

flow direction -0.5

-0.5

-0.5 +0.5

+0.5

+0.5

1 mm

1 mm

1 mm

Position #1 t0 = 0 s

Position #2 t1 = 0.05 s

200 fps

200 fps

200 fps

flow direction

flow direction -0.5

-0.5 +0.5

1 mm Position #4 t4 = 0.155 s


flow direction -0.5

+0.5

+0.5

1 mm

1 mm Position #5 t5 = 0.22 s


Figure 11: High speed camera images (200 fps) for analyzing the radial position of a single particle (inside the red circles) with respect to time (𝒕𝒐 - 𝒕𝟔 ). The inner wall and the outer wall of the tube are indicated with black solid lines (𝒅𝒊 = 3 mm). 𝑭𝒄 indicates the direction of the centrifugal force that is perpendicular to the



flow direction. The total volumetric flow rate is 8 mL∙min-1 (corresponding two-phase flow Reynolds number (𝑹𝒆𝑻𝑷 = 𝟔𝟕) and Dean number (𝑫𝒏𝑻𝑷 = 𝟐𝟏).

Radial velocities of the particles are represented in Figure 12 with respect to the dimensionless time (𝑡/𝑡𝑚𝑎𝑥 ) for different Dean numbers in order to understand the influence of Dean vortices on the radial mixing in case of G-L slug flow. The maximum radial velocity is achieved as -8.9 mm∙s1

for 𝐷𝑛𝑇𝑃 = 52. However, the maximum radial velocity is around 5.9 mm∙s-1 in case of a total

volumetric flow rate of 8 mL∙min-1, which corresponds to a 𝐷𝑛𝑇𝑃 of 21. As it was mentioned in Section 2, the intensity of secondary flow (Dean vortices) in curved geometries can be described by the Dean number, which takes into account of viscous and centrifugal forces in case of single phase flow

54,55

. Thus, increase in centrifugal force at higher velocities enhances the intensity of

Dean vortices. Similar to this, it can be concluded that the intensity of Dean vortices are dependent on the 𝐷𝑛𝑇𝑃 in case of G-L slug flow patterns according to the qualitative results represented in Figure 12. (a)

(b)

0.5

20

𝑑𝑐𝑡 = 25 mm, 𝑑𝑖 = 3 mm, 𝑠 = 1.5 mm

0.4

𝑑𝑐𝑡 = 25 mm, 𝑑𝑖 = 3 mm, 𝑠 = 1.5 mm

0.2

#5

0.1

#6

#4

0

#3 -0.1

#1

-0.2

#2

𝑉̇ = 8 mL∙min-1 (Air/BuAc = 3) 𝑅𝑒𝑇𝑃 = 67; 𝐷𝑛 𝑇𝑃 = 21 𝑉̇𝑡𝑜𝑡𝑎 = 20 mL∙min-1 (Air/BuAc = 4) 20ml_min_R16_4_PIV_Air_or 𝑅𝑒𝑇𝑃 = 168; 𝐷𝑛 𝑇𝑃 = 52 𝑉̇𝑡𝑜𝑡𝑎 = 20 mL∙min-1 (Air/BuAc = 4) 20ml_min_R16_4_PIV_Air_or_2 𝑅𝑒𝑇𝑃 = 168; 𝐷𝑛 𝑇𝑃 = 52 8mL_min_R6_2_PIV_Air_Or_2.Partikel 𝑡𝑜𝑡𝑎

-0.3 -0.4 -0.5 0

𝑉̇ = 8 mL∙min-1 (Air/BuAc = 3) 𝑅𝑒𝑇𝑃 = 67; 𝐷𝑛 𝑇𝑃 = 21

8mL_min_R6_2_PIV_Air_Or 𝑡𝑜𝑡𝑎

0.2

0.4

0.6

0.8

radiale Geschwindigkeit [mm∙s-1] Radial velocity[mm/s]

15

0.3

Radial position [-] radiale Position [-]

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10

#2

5

#4

#1

#5

0

#6

𝑉̇ =8 (Air/BuAc = 3) 𝑅𝑒𝑇𝑃 = 67; 𝐷𝑛 𝑇𝑃 = 21 ̇ 8mL_min_R6_2_PIV_Air_Or_2.Partikel 𝑉𝑡𝑜𝑡𝑎 = 8 mL∙min-1 (Air/BuAc = 3) 𝑅𝑒𝑇𝑃 = 67; 𝐷𝑛 𝑇𝑃 = 21 ̇ 𝑉𝑡𝑜𝑡𝑎 = 20 mL∙min-1 (Air/BuAc = 4) 20ml_min_R16_4_PIV_Air_or 𝑅𝑒𝑇𝑃 = 168; 𝐷𝑛 𝑇𝑃 = 52 20ml_min_R16_4_PIV_Air_or_2 𝑉̇𝑡𝑜𝑡𝑎 = 20 mL∙min-1 (Air/BuAc = 4) 𝑅𝑒𝑇𝑃 = 168; 𝐷𝑛 𝑇𝑃 = 52 mL∙min-1

8mL_min_R6_2_PIV_Air_Or 𝑡𝑜𝑡𝑎

-5

-10 -15 -20 0

1

#3

0.2

𝑡 𝑡𝑚𝑎𝑥 [-]

0.4

0.6

0.8

1

𝑡 𝑡𝑚𝑎𝑥 [-]

t/t_max [-]

Figure 12: a) Radial positions of four different particles for two different total volumetric flow rates with respect to dimensionless time (𝐭/𝐭 𝐦𝐚𝐱) b) Radial velocity of the particles for different Dean numbers with respect to dimensionless time (𝐭/𝐭 𝐦𝐚𝐱).


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5.2 Variable Residence Time via a Modular Coiled Flow Inverter The fabrication of a modular CFI is explicitly described in in Section 4.2. The modular CFI provides a wide range of residence time with respect to the volumetric flow rate of an application. Since the precipitation of calcium carbonate is firstly investigated in a CFI with G-L slug flow pattern, no experimental data was available regarding to the necessary residence time for the continuous precipitation experiments. An inner tube diameter of 2.4 mm was selected (Figure 8) as a starting point. The residence time ranges of the different modules are presented as an example in Figure 13. The minimum residence time of a single element, i.e. for a single coil with 5 turns is 3 s for the volumetric flow rate of 100 mL∙min-1 with a 𝑑𝑐𝑡 of 25 mm. One should consider that a single coil does not consist of a 90° bend that is required for the CFI. Thus, at least 2 coils should be considered for a CFI, and thus, the minimum residence time is actually 6 s. Furthermore, the maximum residence time can be achieved as 136 min by using a 𝑑𝑐𝑡 of 45 mm with 16 coils at 1 mL∙min-1. However, this setup does not satisfy the minimum value of 𝑇 ∗ (≥ 1000) as it is represented in Figure 5. Therefore, while selecting a residence time unit for the PCC experiments, these constraints have to be considered. It has to be noted that Figure 13 represents the residence time range only for a single coil and 16 coils (i.e. 15 bends) with a tube diameter of 2.4 mm and coil tube diameters of 25, 35, and 45 mm. The fabricated modular CFI can be configured by using different combinations of inner tube diameters and coil tube diameters as explained in Section 4.2. Therefore, a wide range of residence time can be achieved for the investigation of continuous precipitation process.



10000

𝑑𝑖 = 2.4 mm, 𝑠 = 0.8 mm

1000

Residence time [s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

10

dct=25mm; module1 𝑑𝑐𝑡 = 251mm;

coil

𝑑𝑐𝑡 = 351mm; 1 dct=35mm; module

coil


coil


coil


coil


coil

1 1

10

Volumetric flow rate

100

[mL∙min-1]

Figure 13: Residence time ranges of the modular CFI design for a single element and 16 coil with 𝑑𝑖 = 2.4 mm and 𝑑𝑐𝑡 = 25 mm, 35 mm and 45 mm for the volumetric flow rates in the range of 1 - 100 mL∙min-1.

5.3 Continuous Precipitation of Calcium Carbonate with Modular Coiled Flow Inverter G-L slug flow patterns of different flow rates were observed in case of a precipitation reaction at 35 °C in order to determine the flow map. Figure 14 presents the different flow patterns that were observed for different flow rates in case of using a prototype CFI reactor (𝑑𝑖 = 3 mm) that is made of borosilicate glass with an inner volume of 14 mL. Please note that the volumetric flow rate of 𝐶𝑂2 (V̇𝐶𝑂2 ) was kept as 3.33 mL∙min-1. The observations revealed that the flow was pulsating (fluctuating back and forth) for several data points in flow map (Figure 14) and thus those points were indicated as unstable slug flow. At these points, the slug size was not uniform due to 30 ACS Paragon Plus Environment

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the disturbances arisen from pulsating flow. However, uniform slug size was achieved for the data points, which are indicated as slug flow in Figure 14. An operating point was selected for the volumetric flow rate of the mother liquor solution (V̇ML ) and the total volumetric gas flow rate (V̇g ) as 46 mL∙min-1 and 50 mL∙min-1, respectively. This operating condition was used for the investigations that are explained in the following sections. Please note that the residence time for CFI was calculated by using the summation of the inlet flow rates.

Unstable slug flow

Slug flow

Operating point

80

Volumetric flow rate of the mother liquor solution (𝑉̇ 𝐿 ) [mL∙min-1] Mother liquor flow rate [mL/min]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 operating point 𝑉𝑔̇ = 50 mL∙min-1 𝑉̇ 𝐿 = 46 mL∙min-1

50 40 30 20 20

25

30 35 40 45 50 55 Total gas flow rate (Normal Condition) [mL/min] ̇ Total volumetric gas flow rate (𝑉𝑔 at NTP) [mL∙min-1]

60

65

Figure 14: Flow map of the G-L slug flow patterns in case of the precipitation reaction.

5.3.1 Influence of 𝑪𝑶𝟐 Content in Gas Feed on the Precipitation The modular CFI (PVC, 𝑑𝑖 = 3.2 mm, 𝑑𝑐𝑡 = 25 mm) that consists of 4 coils and 3 bends was used for the investigation of the effect of 𝐶𝑂2 content in gas feed. The volumetric gas flow rate ratio (𝑅), which is defined as the ratio of 𝑉̇𝐶𝑂2 to the 𝑉𝑔̇ , was set to 20 % and 30 % by varying the 𝑉̇𝐶𝑂2 at constant 𝑉𝑔̇ (50 mL∙min-1). Initial pH and conductivity (𝜎) measurements of the mother 31 ACS Paragon Plus Environment


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liquor solution were recorded prior to the reaction. The molar concentration of the mother liquor solution was also determined by titration analyses. Reaction outlet stream was collected into a stirred vessel in the presence of 𝑁2 injection preventing the further reaction. Samples were taken from the vessel in order to measure pH, conductivity and to calculate the conversion (𝑋) after the reaction. The measurements for these parameters are given in Table 1. The increase in 𝑅 from 20 % to 30 % results in higher conversion values with an increase from 58 % to 81 %, respectively. Table 1: The measurements of the pH, the conductivity (𝝈), and the conversion (𝐗) for the volumetric gas flow rate ratio (𝑹) of 20 % and 30 % with a CFI reactor, which consists of 3 bends and provides a residence time (𝝉) of ca. 21 s (please note that the terms before and after indicating the values that are measured before and after the reaction).

𝑅

pH [-]

σ [mS∙cm-1]

τ

X

[%]

(before / after)

(before / after)

[s]

[-]

CFI (3 bends)

30

11.97 / 9.91

7.55 / 2.15

≅21

≅81

CFI (3 bends)

20

12.15 / 11.42

7.03 / 4.21

≅21

≅58

Reactor

The decrease in pH and conductivity in comparison to before and after the reaction was also increased proportional to the conversion. These values revealed that the increase in the molar concentration of 𝐶𝑂2 promotes the carbonation reaction. As it was explained in Section 4.3, precipitation of calcium carbonate is commercially done in a batch reactor

60

. In a batch reactor,

dissolved 𝐶𝑎(𝑂𝐻)2 reacts with 𝐶𝑂2 that is available as 20 vol. % in air mixture. If the concentration of 𝐶𝑂2 is increased in the reactor, it promotes the carbonation reaction and thus 𝐶𝑎(𝑂𝐻)2 is further consumed. As a result of this, pH value of the reaction medium decreases. The similar trends are achieved in continuous mode operation with CFI. Furthermore, the PSD curves of the suspended PCC particles were determined by using microscopic image analyses method as 32 ACS Paragon Plus Environment

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it is described in Section 4.4.2. Figure 15 represents the PSD curves of the different PCC particles in suspension for the experiments with the 𝑅 of 20 % and 30 %.

CFI - 3 bends - 𝑹 = 20 % 𝑑50,2 = 18.43 µm; 𝑝𝑎𝑛 = 1.218

CFI - 3 bends - 𝑹= 30 % 𝑑50,2 = 22.29 µm; 𝑝𝑎𝑛 = 1.014

100

rhombohedral calcite

90 80

[%]

70 60 50

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

10 µm

20

𝑝𝑎𝑛 =

10

(𝑑

0,2

𝑑10,2 )

𝑑50,2

0 0

10

20

30

40

50

60

70

80

𝑑Feret,max [µm] Figure 15: PSD analyses of the PCC particles in suspension for the experiments with the volumetric gas flow rate ratio (𝑹) of 20 % and 30 % in a CFI reactor, which consists of 3 bends and provides a residence time (𝛕) of ca. 21 s. The SEM image is obtained from the dried sample of PCC (drying at 60 °C, 24 h) that is produced by using 𝑹 of 30 %.

The median particle diameter (𝑑50,2 ) was determined from the curves as 22.9 µm and 18.43 µm for V̇R of 30 % and 20 %, respectively. The decrease in 𝑑50,2 can be explained by the lower 𝐶𝑂2 concentration. As the reaction takes place with the limited 𝐶𝑂2 content (𝑅) of 20 %, the particle growth along the tube length is limited after the nucleation, and thus, the particles were produced with smaller 𝑑50,2 . However, more uniform particles (cf. span values in Figure 15) were obtained in case of higher 𝐶𝑂2 content (𝑅 = 30%). This behavior cannot be precisely explained as several factors such as the supersaturation rate, homogeneity condition within the liquid slug, and high 33 ACS Paragon Plus Environment


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conversion values might influence the uniformity and morphology

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62

. This will be further

investigated in our future works. Besides this, the SEM image in Figure 15 shows that the morphology of the PCC particle is rhombohedral calcite. This morphology is more pronounced for the carbonation reaction at medium temperatures (30 °C) with the low content of the 𝐶𝑎(𝑂𝐻)2 solution

62

. Thus, the rhombohedral calcite morphology results from the reaction of the low

𝐶𝑎(𝑂𝐻)2 content at a reaction temperature of 35 °C. 5.3.2 Influence of Residence Time on the Precipitation The performance of a modular CFI (PVC, 𝑑𝑖 = 3.2 mm, 𝑑𝑐𝑡 = 25 mm) that consists of 4 coils and 3 bends was compared with a CFI consisting of 8 coils and 7 bends. These setups provide different residence times with approximately 21 s and 42 s, respectively. The volumetric flow rate ratio of the gases (𝑅) was set to 30 % and the measurements of the parameters are tabulated in Table 2. The increase in residence time from 21 s to 42 s results in the higher conversion values with an increase from 81 % to 87 %, respectively. The decrease in pH and conductivity in comparison to before and after the reaction was also affected with respect to the conversion. These values revealed that the variation of the residence time can be easily achieved by using a modular CFI reactor in order to manipulate the conversion of the reaction. Furthermore, the increase in residence time results in producing PCC particles with larger 𝑑50,2 (28.38 µm) values (Figure 16). According to the span index values of 1.014 and 1.059 for the CFI setups with 3 bends and 7 bends, respectively, it can be concluded that the uniformity of the particles are similar. However, SEM images (Figure 16) show that more uniform rhombohedral calcite morphology was achieved by using a CFI reactor with 7 bends in comparison to 3 bends. This can be explained by the decrease in axial dispersion number, i.e. narrower RTD was obtained by using 7 bends in comparison to 3 bends. This confirms the experimental study of Vashisth and Nigam 47, in which the liquid phase 34 ACS Paragon Plus Environment

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RTD was characterized in CFI with G-L flow patterns. They concluded that the increase in number of bends narrows the RTD of the liquid phase. Table 2: The measurements of the pH, the conductivity (𝝈), and the conversion (𝑿) for the volumetric flow rate ratio (𝑹) 30 % with a CFI reactor, which consists of 3 bends and 7 bends providing a residence time (𝛕) of ca. 21 s and 42 s, respectively (please note that the terms before and after indicating the values that are measured before and after the reaction).

𝑅

pH [-]

𝜎 [mS∙cm-1]

𝜏

𝑋

[%]

(before / after)

(before / after)

[s]

[-]

CFI (3 bends)

30

11.97 / 9.91

7.55 / 2.15

≅21

≅81

CFI (7 bends)

30

11.98 / 9.10

7.48 / 1.5

≅42

≅87

Reactor



CFI - 3 bends (𝝉 ≅ 21 s) 𝑅 = 30 % 𝒅 ,𝟐 = 22.29 µm; 𝑝𝑎𝑛 = 1.014

CFI - 7 bends (𝝉 ≅ 42 s) - 𝑅 = 30 % 𝒅 ,𝟐 = 28.38 µm; 𝑝𝑎𝑛 = 1.059

rhombohedral calcite

100 90

80 70

[%] [%] 2Q2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 49

60 10 µm

50 40 𝑝𝑎𝑛 =

30

(𝑑

0,2

𝑑10,2 )

𝑑50,2

20 10 0

0

10

20

30 40 50 Feret [µm] 𝑑 diameter [µm]

60

70

10 µm

Feret,max

Figure 16: PSD analyses of the PCC particles in suspension for the experiments with the volumetric gas flow rate ratio (𝑹) 30 % in CFI reactors, which consist of 3 bends and 7 bends providing a residence time (𝝉) of ca. 21 s and 42 s, respectively. The SEM images are obtained from the dried samples of the PCC (drying at 60 °C, 24 h).

5.3.3 Benchmarking of the Modular Coiled Flow Inverter with a Batch Reactor The performance of the modular CFI (PVC, 𝑑𝑖 = 3.2 mm, 𝑑𝑐𝑡 = 25 mm) that consists of 8 coils and 7 bends was compared with a batch reactor (𝑉𝑅 = 400 mL). The batch rector was filled with 350 mL of the mother liquor solution in presence of 𝑁2 injection. The pH and conductivity was monitored by inserting the electrodes into the reactor (from top). A glass capillary (borosilicate, 𝑑𝑖 = 3 mm) was inserted close to the stirrer (stainless steel, 4 blade axial stirrer) from the top of the reactor for the injection of 𝐶𝑂2 during the carbonation reaction. The stirrer speed was set to 600 rpm providing better bubble size distribution (BSD) for the 𝐶𝑂2. Please note that these parameters are not the optimum ones that are required for the PCC production in batch reactor. The 36 ACS Paragon Plus Environment

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optimization of these parameters requires several experimental runs, but it was not the main focus of this study. Similarly, a moderate operating point was chosen from the flow map that is presented in Figure 14 for CFI. Thus, one should keep in mind that none of the reactors was optimized during the experimental work. However, a comparison can still be done between the performance of the batch reactor and CFI for identical process conditions, at which the similar conversion values are achieved. Therefore, the volumetric gas flow rate ratio (𝑅) was set to 30 % and the carbonation was proceeded until the pH value reaches close to the pH value of the carbonation reaction that is achieved by the modular CFI. This value is 9.10 as it is given in Table 2. Carbonation time of 15 min was required to reach a pH value of 8.57 in batch reactor. Space time (𝜏𝑠 ) of the batch reactor with the holdup volume of 350 mL is compared with the time that is spent to collect 350 mL solution from the outlet stream of the modular CFI, in which the volumetric flow rate of the mother liquor solution (𝑉̇ 𝐿 ) was set to 46 mL∙min-1. Please note that the space time in a batch reactor is defined as the time that is necessary to process one reactor volume of fluid, which corresponds to 350 mL for the batch reactor used in this work. Table 3 shows the values of the parameters that are obtained from the batch reactor and the modular CFI with 7 bends. The conversion values were obtained as 89 % and 87 % in batch reactor and CFI, respectively. The pH values, which is measured for monitoring the reaction progress, after the carbonation reaction were found to be closer for both reactors. The carbonation reaction of 15 min corresponds to a space time value of 900 s for the batch reactor with a holdup volume of 350 mL, whereas the volume of 350 mL solution can be collected in approximately 456 s by using a CFI reactor providing a residence time of 42 s with a 𝑉̇

𝐿

of 46 mL∙min-1.



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Table 3: The measurements of the pH, the conductivity (𝝈), and the conversion (𝑿) for the volumetric gas flow rate ratio (𝑹) 30 % with the batch reactor and the modular CFI reactor, which consists of and 7 bends providing a residence time (𝝉) of ca. 42 s (𝑽̇𝑴𝑳 = 46 mL∙min-1). The time of the carbonation reaction (space time, 𝝉𝒔 ) is 15 min in batch reactor with a holdup volume of 350 mL (please note that the terms before and after indicating the values that are measured before and after the reaction).

𝑅

pH [-]

𝜎 [mS∙cm-1]

𝝉𝒔

𝑋

[%]

(before / after)

(before / after)

[s]

[-]

Batch

30

12.05 / 8.57

7.08 / 0.637

≅900

≅89

CFI (7 bends)

30

11.98 / 9.10

7.48 / 1.5

≅456

≅87

Reactor

PSD curves are presented with SEM images in Figure 17. The modular CFI reactor provides a smaller 𝑑50,2 value (28.38 µm) in comparison to the 𝑑50,2 value of the batch reactor (36.96 µm). In addition to this, a narrower PSD curve is obtained in CFI (cf. span values in Figure 17). This proves that better homogeneity conditions can be achieved for the carbonation reaction in CFI in case of G-L slug flow patterns.


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batch reactor; 𝑅 = 30 % 𝒅 ,𝟐 = 36.96 µm; 𝑝𝑎𝑛 = 1.624

CFI - 7 bends (𝝉 ≅ 42 s) - 𝑅 = 30 % 𝒅 ,𝟐 = 28.38 µm; 𝑝𝑎𝑛 = 1.059

100 90 80

[%]

70

2

Q2 [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

10 µm

50 40 30 20

𝑝𝑎𝑛 =

10

(𝑑

0,2

𝑑10,2 )

𝑑50,2

0

10 µm 0

25

50

75

100 125 150 175 200 225 250

𝑑Feret,max [µm] Feret diameter [µm]

Figure 17: PSD analyses of the PCC particles in suspension for the experiments with the volumetric gas flow rate ratio of (𝐑) 30 % in a batch reactor and a modular CFI reactor, which consists of and 7 bends providing a residence time (𝝉) of ca. 42 s (𝑽̇𝑴𝑳 = 46 mL∙min-1). The time of the carbonation reaction (space time, 𝝉𝒔 ) is 15 min in batch reactor.

The enhanced radial mixing inside the liquid slugs allows to control the nucleation and particle growth with respect to the uniform bubble size distribution (BSD). Additionally, the controlled consumption of the 𝐶𝑂2 bubbles along the CFI reactor prevents the occurrence of different nucleation zones with respect to the different relative supersaturation values

63

inside the liquid

slug. In contrast to this, a broad BSD along with different retention times of the bubbles in a batch reactor result in different nucleation zones, and thus, an uncontrolled (or random) particle growth. As it can be seen from the SEM pictures in Figure 17, the particle morphologies are found to be rhombohedral calcite. However, more uniform calcite morphology is obtained by using a CFI rector in comparison to the batch reactor, which results in clustered rhombohedral calcite particles. 39 ACS Paragon Plus Environment


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It should be indicated that fouling of the PVC tubes was observed for operation times over 30 min. However, several experiments were also conducted by using fluorinated ethylene propylene (FEP) tubes in the content of this work. The production of PCC was studied by using FEP tubes, but less fouling was observed for 30 min of operation. Therefore, FEP tubes will be used in our future work. The effect of the different process parameters on PCC process is going to be investigated by conducting several DoE for better interpretation of the results.

6 Conclusion In this work, gas-liquid (G-L) slug flow mixing behavior of a coiled flow inverter (CFI) was presented with a novel explanation for better mixing in comparison to a straight tube and a helically coiled tube (HCT). It was shown that the CFI can provide enhanced radial mixing due to the secondary flow profiles and their inversion with 90° bends in case of G-L slug flow patterns. The three main topics were studied the first time in this work, i.e. the particle tracking in a G-L slug flow within a HCT, the design and implementation of a modular CFI reactor, and the continuous precipitation of the calcium carbonate. Particle tracking method was investigated by a HCT with circular cross section made of borosilicate glass with an inner diameter of (𝑑𝑖 ) 3 mm. G-L slug flow was achieved by using 𝑛-butyl acetate and synthetic air via a Y-mixer (borosilicate glass, 𝑑𝑖 = 3 mm). Inert polyethylene (PE) particles were injected with 𝑛-butyl acetate and the pictures were taken by using a high speed camera. The analyses of the particle tracking experiments revealed that radial mixing within the liquid slugs is enhanced due to secondary flow profile of combined Dean and Taylor vortices in GL slug flow. A comprehensive way to design a modular CFI was given providing a wide range of residence time displayed in a three-dimensional design space diagram (DSD) for the different volumetric 40 ACS Paragon Plus Environment

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flow rates. The DSD simplifies the selection of the geometrical design parameters of a CFI ensuring enhanced radial mixing by considering the process conditions and physical properties of the fluids. In the last part of this work a modular CFI design was selected from the DSD for the investigations in the continuous precipitation of the calcium carbonate in case of a G-L reaction. The analyses on the precipitated calcium carbonate (PCC) particles revealed that CFI provides narrower particle size distribution (median particle diameters around 28 µm) and more uniform morphology (rhombohedral calcite) in comparison to a batch reactor. Additionally, it was found that the median particle size diameter of the produced PCCs can be simply varied by adjusting the residence time with the modular CFI. Finally, a modular, continuously operated tubular reactor concept, i.e. CFI with a robust and an easy fabrication was proposed in the content of this work for the multiphase reaction systems. In future work, the modular CFI will be further investigated to achieve the production of uniform nano particles with larger throughputs in continuous mode operation.

Acknowledgements The authors would like to thank to Mr. Daniel Lenz and Mr. Florian Schilling for their contributions in this project at the Laboratory of Equipment Design, TU Dortmund University. Additionally, the authors would like to thank to Mr. Thierry Chopin, Mr. Guillaume Criniére, Mr. Robert Gaertner, and Mr. Jean-Marc Vanderaa from Lhoist Recherche et Développement S.A. for providing the SEM images and their support in analysis.

Nomenclature dc

coil diameter [m]

dct

coil tube diameter [m]



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dFeret,max

maximum visible 2D diameter from image analysis [µm]

d50,2 (Ds50)

median diameter [µm]

dh

hydraulic diameter [m]

di

inner tube diameter [m]

do

outer tube diameter [m]

Dn

Dean number [-]

drod

threaded rod diameter [m]

L

tube length [m]

Lc

length of a single coil [m]

Lrod

length of a threaded rod [m]

nbends

number of bends [-]

ntunrs

number of turns [-]

p

pitch distance [m]

R

the ratio of V̇CO2 to V̇g [-]

Re

Reynolds number [-]

u

average velocity [m s-1]

𝑢̅𝑆𝐺

average superficial gas velocity [m s-1]

𝑢̅𝑆𝐿

average superficial liquid velocity [m s-1]

V

volumetric flow rate [mL min-1]

𝑉̇𝐶𝑂2

volumetric flow rate of 𝐶𝑂2 [mL∙min-1]

𝑉𝑔̇

total volumetric gas flow rate [mL∙min-1]

𝑉̇ s

𝐿

volumetric flow rate of the mother liquor solution [mL∙min-1] wall thickness [m] 42 ACS Paragon Plus Environment

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ti

time of the ith image [s]

tmax

time of the last image [s]

𝑇∗

modified Torsion parameter [-]

𝑤

width [m]

X

conversion [-]

Greek Symbols 

dynamic viscosity [Pa s]

  (d c / d i )

curvature ratio [-]



density [kg m-3]

σ

conductivity [mS∙cm-1]



average residence time [s]

Subscripts TP

two phase

s

space time

Abbreviations aq

aqueous

BSD

bubble size distribution

BuAc

𝑛-butyl acetate

c

continuous

d

dispersed

CFI

coiled flow inverter

DSD

design space diagram 43 ACS Paragon Plus Environment


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FEP

fluorinated ethylene propylene

fps

frame per second

G-L

gas-liquid

HCT

helically coiled tube

HCTD

helically coiled tubular device

HCTR

helically coiled tubular reactor

L-L

liquid-liquid

lab

laboratory

NTP

normal pressure and temperature

PCC

precipitated calcium carbonate

PEEK

polyether ether ketone

PMMA

polymethylmethacrylate

PI

process intensification

POM

polyoxymethylene

PSD

particle size distribution

PTFE

polytetrafluoroethylene

PVC

polyvinyl chloride

R&D

research and development

RT

residence time

RTD

residence time distribution

SFTR

segmented flow tubular reactor


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References (1) Kockmann, N.; Holbach, A. Microchannel Device for Droplet Generation, Mixing, and Phase Separation for Continuous Counter-Current Flow Extraction. ASME-ICNMM 2013, V001T03A011. (2) Holbach, A.; Kockmann, N. Counter-current arrangement of microfluidic liquid-liquid droplet flow contactors. Green Process. Synth 2013, 2, 157–167. (3) Holvey, C. P.; Roberge, D. M.; Gottsponer, M.; Kockmann, N.; Macchi, A. Pressure drop and mixing in single phase microreactors: Simplified designs of micromixers. Chem. Eng. Process. Process Intensif. 2011, 50, 1069–1075. (4) Kockmann, N.; Engler, M.; Haller, D.; Woias, P. Fluid Dynamics and Transfer Processes in Bended Microchannels. Heat Transfer Eng. 2005, 26, 71–78. (5) Zhao, C.-X.; Middelberg, A. P.J. Two-phase microfluidic flows. Chem. Eng. Sci. 2011, 66, 1394–1411. (6) Tsaoulidis, D. A. Studies of intensified small-scale processes for liquid-liquid separations in spent nuclear fuel reprocessing. Doctoral Thesis, University College London, UK. (7) Dessimoz, A.-L.; Cavin, L.; Renken, A.; Kiwi-Minsker, L. Liquid-liquid two-phase flow patterns and mass transfer characteristics in rectangular glass microreactors. Chem. Eng. Sci. 2008, 63, 4035–4044. (8) Biswas, K. G.; Das, G.; Ray, S.; Basu, J. K. Mass transfer characteristics of liquid-liquid flow in small diameter conduits. Chem. Eng. Sci. 2015, 122, 652–661. (9) Di Miceli Raimondi, N.; Prat, L.; Gourdon, C.; Tasselli, J. Experiments of mass transfer with liquid– liquid slug flow in square microchannels. Chem. Eng. Sci. 2014, 105, 169–178. (10) Burlage, K.; Gerhardy, C.; Praefke, H.; Liauw, M. A.; Schomburg, W. K. Slug length monitoring in liquid-liquid Taylor-flow integrated in a novel PVDF micro-channel. Chem. Eng. J. 2013, 227, 111–115. (11) Darekar, M.; Sen, N.; Singh, K. K.; Mukhopadhyay, S.; Shenoy, K. T.; Ghosh, S. K. Liquid-liquid extraction in microchannels with Zinc–D2EHPA system. Hydrometallurgy 2014, 144-145, 54–62. (12) Kashid, M. N.; Kiwi-Minsker, L. Microstructured reactors for multiphase reactions: state of the art. Ind. Eng. Chem. Res. 2009, 48, 6465–6485. (13) Kashid, M. N.; Renken, A.; Kiwi-Minsker, L. Gas-liquid and liquid-liquid mass transfer in microstructured reactors. Chem. Eng. Sci. 2011, 66, 3876–3897. (14) Ghaini, A.; Kashid, M. N.; Agar, D. W. Effective interfacial area for mass transfer in the liquid-liquid slug flow capillary microreactors. Chem. Eng. Process. Process Intensif. 2010, 49, 358–366. (15) Kashid, M. N. Experimental and modelling studies on liquid-liquid slug flow capillary microreactors. Dissertation, TU Dortmund University, Dortmund, 2008. (16) Kashid, M. N.; Gerlach, I.; Goetz, S.; Franzke, J.; Acker, J. F.; Platte, F.; Agar, D. W.; Turek, S. Internal Circulation within the Liquid Slugs of a Liquid−Liquid Slug-Flow Capillary Microreactor. Ind. Eng. Chem. Res. 2005, 44, 5003–5010. (17) Kashid, M. N.; Harshe, Y. M.; Agar, D. W. Liquid-liquid slug flow in a capillary: an alternative to suspended drop or film contactors. Ind. Eng. Chem. Res. 2007, 46, 8420–8430. (18) Kashid, M. N.; Agar, D. W. Hydrodynamics of liquid-liquid slug flow capillary microreactor: flow regimes, slug size and pressure drop. Chem. Eng. J. 2007, 131, 1–13. (19) Ahmed, B.; Barrow, D.; Wirth, T. Enhancement of reaction rates by segmented fluid flow in capillary scale reactors. Adv. Synth. Catal. 2006, 348, 1043–1048. (20) Kreutzer, M. T.; Kapteijn, F.; Moulijn, J. A.; Kleijn, C. R.; Heiszwolf, J. J. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. AIChE J. 2005, 51, 2428–2440. 45 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 49

(21) Abdul-Majeed, G. H. Liquid slug holdup in horizontal and slightly inclined two-phase slug flow. J. Pet. Sci. Eng. 2000, 27, 27–32. (22) Wang, C.-C.; Youn Chen, I.; Shyu, H.-J. Frictional performance of R-22 and R-410A inside a 5.0 mm wavy diameter tube. Int. J. Heat Mass Transfer 2003, 46, 755–760. (23) Gupta, R.; Fletcher, D. F.; Haynes, B. S. On the CFD modelling of Taylor flow in microchannels. Chem. Eng. Sci. 2009, 64, 2941–2950. (24) Laborie, S.; Cabassud, C.; Durand-Bourlier, L.; Lainé, J. M. Characterisation of gas–liquid two-phase flow inside capillaries. Chem. Eng. Sci. 1999, 54, 5723–5735. (25) Mehdizadeh, A.; Sherif, S. A.; Lear, W. E. Numerical simulation of thermofluid characteristics of twophase slug flow in microchannels. Int. J. Heat Mass Transfer 2011, 54, 3457–3465. (26) Kastens, S.; Hosoda, S.; Schlüter, M.; Tomiyama, A. Mass transfer from single Taylor bubbles in minichannels. Chem. Eng. Technol. 2015, 38, 1925–1932. (27) Günther, A.; Jhunjhunwala, M.; Thalmann, M.; Schmidt, M. A.; Jensen, K. F. Micromixing of miscible liquids in segmented gas-liquid flow. Langmuir 2005, 21, 1547–1555. (28) Günther, A.; Khan, S. A.; Thalmann, M.; Trachsel, F.; Jensen, K. F. Transport and reaction in microscale segmented gas-liquid flow. Lab Chip 2004, 4, 278. (29) Kreutzer, M. T.; Bakker, J. J. W.; Kapteijn, F.; Moulijn, J. A.; Verheijen, P. J. T. Scaling-up multiphase monolith reactors: linking residence time distribution and feed maldistribution. Ind. Eng. Chem. Res. 2005, 44, 4898–4913. (30) Kreutzer, M. T.; Kapteijn, F.; Moulijn, J. A.; Heiszwolf, J. J. Multiphase monolith reactors: chemical reaction engineering of segmented flow in microchannels. Chem. Eng. Sci. 2005, 60, 5895–5916. (31) Vashisth, S.; Kumar, V.; Nigam, K. D. P. A review on the potential applications of curved geometries in process industry. Ind. Eng. Chem. Res. 2008, 47, 3291–3337. (32) Singh, J.; Kockmann, N.; Nigam, K. D. P. Novel three-dimensional microfluidic device for process intensification. Chem. Eng. Process. Process Intensif. 2014, 86, 78–89. (33) Mandal, M. M.; Kumar, V.; Nigam, K. D. P. Augmentation of heat transfer performance in coiled flow inverter vis-à-vis conventional heat exchanger. Chem. Eng. Sci. 2010, 65, 999–1007. (34) Kumar, V.; Mridha, M.; Gupta, A. K.; Nigam, K. D. P. Coiled flow inverter as a heat exchanger. Chem. Eng. Sci. 2007, 62, 2386–2396. (35) Nigam, K. D. P. Baffle and tube for a heat exchanger, Patent number: US 7,337,835 B2. (36) Saxena, A. K.; Nigam, K. D. P. Coiled configuration for flow inversion and its effect on residence time distribution. AIChE J. 1984, 30, 363–368. (37) Klutz, S.; Kurt, S. K.; Lobedann, M.; Kockmann, N. Narrow residence time distribution in tubular reactor concept for Reynolds number range of 10–100. Chem. Eng. Res. Des. 2015, 95, 22–33. (38) Kurt, S. K.; Gelhausen, M. G.; Kockmann, N. Axial dispersion and heat transfer in a milli/microstructured coiled flow inverter for narrow residence time distribution at laminar flow. Chem. Eng. Technol. 2015, 38, 1122–1130. (39) Lobedann, M.; Klutz, S.; Kurt, S. K. Device and method for continuous virus inactivation, Patent number: WO 2015/135844 A1. (40) Kurt, S. K.; Vural Gürsel, I.; Hessel, V.; Nigam, K. D.P.; Kockmann, N. Liquid–liquid extraction system with microstructured coiled flow inverter and other capillary setups for single-stage extraction applications. Chem. Eng. J. 2016, 284, 764–777. 46 ACS Paragon Plus Environment

Page 47 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Kurt, S. K.; Nigam, K. D. P.; Kockmann, N. Two-Phase Flow and Mass Transfer in Helical Capillary Flow Reactors With Alternating Bends. In ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels collocated with the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, Monday 2015; pp V001T03A014. (42) Vural Gürsel, I.; Kurt, S. K.; Aalders, J.; Wang, Q.; Noël, T.; Nigam, K. D.P.; Kockmann, N.; Hessel, V. Utilization of milli-scale coiled flow inverter in combination with phase separator for continuous flow liquid–liquid extraction processes. Chem. Eng. J. 2016, 283, 855–868. (43) Akagawa, K.; Sakaguchi, T.; Ueda, M. Study On a Gas-Liquid Two-Phase Flow in Helically Coiled Tubes. JSME 1971, 14, 564–571. (44) Wang, C.-C.; Chen, I. Y.; Yang, Y.-W.; Chang, Y.-J. Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend. Int. J. Heat Mass Transfer 2003, 46, 2975–2981. (45) Vashisth, S.; Nigam, K. D. P. Prediction of flow profiles and interfacial phenomena for two-phase flow in coiled tubes. Chem. Eng. Process. Process Intensif. 2009, 48, 452–463. (46) Kumar, V.; Vashisth, S.; Hoarau, Y.; Nigam, K. D. P. Slug flow in curved microreactors: hydrodynamic study. Chem. Eng. Sci. 2007, 62, 7494–7504. (47) Vashisth, S.; Nigam, K. D. P. Liquid-phase residence time distribution for two-phase flow in coiled flow inverter. Ind. Eng. Chem. Res. 2008, 47, 3630–3638. (48) Aimable, A.; Jongen, N.; Testino, A.; Donnet, M.; Lemaître, J.; Hofmann, H.; Bowen, P. Precipitation of nanosized and nanostructured powders: process intensification and scale-out using a segmented flow tubular reactor (SFTR). Chem. Eng. Technol. 2011, 34, 344–352. (49) Köhler, J. M.; Li, S.; Knauer, A. Why is micro segmented flow particularly promising for the synthesis of nanomaterials? Chem. Eng. Technol. 2013, 36, 887–899. (50) Köhler, J. M.; Cahill, B. P. Micro-Segmented Flow: Applications in Chemistry and Biology; Springer: Berlin, Heidelberg, 2014. (51) Vacassy, R.; Lemaître, J.; Hofmann, H.; Gerlings, J. H. Calcium carbonate precipitation using new segmented flow tubular reactor. AIChE J. 2000, 46, 1241–1252. (52) Farook, A. A. CFD Simulation of Single-phase Fluid Flow in Milli/Microstructured Helically Coiled Tubular Reactors. M.Sc. Thesis, TU Dortmund University, Dortmund, 2014. (53) Levenspiel, O. Chemical reaction engineering, 3rd ed; Wiley: New York, 1999. (54) Dean, W. R. XVI. Note on the motion of fluid in a curved pipe. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1927, 4, 208–223. (55) Dean, W. R. LXXII. The stream-line motion of fluid in a curved pipe (second paper). The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1928, 5, 673–695. (56) Kurt, S. K.; Warnebold, F.; Nigam, K. D.P.; Kockmann, N. Gas-liquid reaction and mass transfer in microstructured coiled flow inverter. Chem. Eng. Sci. 2017, DOI: 10.1016/j.ces.2017.01.017. (57) Malsch, D.; Kielpinski, M.; Merthan, R.; Albert, J.; Mayer, G.; Köhler, J. M.; Süße, H.; Stahl, M.; Henkel, T. μPIV-analysis of Taylor flow in micro channels. Chem. Eng. J. 2008, 135, S166-S172. (58) Teir, S.; Eloneva, S.; Zevenhoven, R. Production of precipitated calcium carbonate from calcium silicates and carbon dioxide. Energy Convers. Manage. 2005, 46, 2954–2979. (59) Söhnel, O.; Mullin, J. W. Precipitation of calcium carbonate. J. Cryst. Growth 1982, 60, 239–250. (60) Harris, D. L. Production of purified calcium carbonate, Patent number: US 3,920,800. 47 ACS Paragon Plus Environment


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(61) Lide, D. R. A ready reference book of chemical and physical data, 85th ed.; CRC: Boca Raton, FL., London, 2004. (62) Jones, A. G. Crystallization process systems; Butterworth-Heinemann: Oxford, Boston, 2002. (63) Tai, C. Y.; Chen, P.-C. Nucleation, agglomeration and crystal morphology of calcium carbonate. AIChE J. 1995, 41, 68–77.


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Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert

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