Mixing performance in T-shape microchannel at high flow rate for Villermaux-Dushman reaction

Microchemical Journal 155 (2020) 104662

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Mixing performance in T-shape microchannel at high flow rate for Villermaux-Dushman reaction

T

Peng lva,b,c, Liangjing Zhanga,b,c, C. Srinivasakannand, Shiwei Lia,b,c, Yuan Hea,b,c, ⁎ Kaihua Chena,b,c, Shaohua Yina,b,c, a

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China c Kunming Key Laboratory of Special Metallurgy, Kunming, Yunnan 650093, China d Chemical Engineering Department, The Petroleum Institute, Khalifa University of Science and Technology, Abu Dhabi, P.O. Box 253, United Arab Emirates b

ARTICLE INFO

ABSTRACT

Keywords: Microchannel Villermaux/Dushman Mixing performance Segregation index Reynolds number

An empirical analysis is conducted to evaluate the effect of the geometric parameters of microchannel on the mixing performance at high flow rates. The mixing performance is assessed using I−/IO3−parallel competition reaction covering the whole flow regime, and measured using segregation index (Xs) covering different lengths of outlet tube (Lout), volume of mixing chamber (Vmc) covering a wide range of Reynolds number. Results show that the channel geometric parameters have significant influence on the mixing process. The segregation index is found to increase with increase in H+concentration and volumetric flow ratio (R), while decrease with increase in the outlet (dout) and inlet tube diameter (din). The highest mixing efficiency corresponds to highest Reynolds number, c(H+) = 0.05 mol/L, R = 1, din = dout = 5 mm, Lout = 20 cm and Vmc = 0.9 cm3. Regression analysis is performed to relate the micromixing time with the influencing parameters. An optimal mixing time of 0.5 ms can be achieved in a T-shape microchannel. As compared to other forms of reactors, the optimized T-shape microchannel is identified to possess excellent mixing characteristics, which could find promising applications for high flux reaction process.

1. Introduction

width, as the path for diffusion is minimized additionally increasing the shear rate [11]. Considering this, mixing is a considerable issue in microchannels design [12]. In view of the importance of effective mixing various channel geometries such as channel convergence, bending and twisting have been attempted [13,14], however, such complex geometries have limited applications and are difficult to use on a large scale, especially in the process of preparing the precipitation reaction, which can easily block the conduit. Since early nineties, microreactors have been the subject of hundreds of scientific publications and patents [15–20]. Due to lack of knowledge and data the field uncovers a number of scientific challenges at the beginning. The mixing performance continues to be the main challenge in microchannel, particularly in cases having more than one fluid. The channel geometries are known to play an important role in micromixing [21]. Ansari et al. [22] have designed a T-joint micromixer relying on the unbalanced splits and cross collisions of fluid streams, which illustrates that the interface in the curved sub-channels can enhance mixing performance of the micromixer. Rahimi et al. [23]

Microchannels are being successfully utilized to carry out chemical reactions in tiny channels in continuous-flow mode. They facilitate sample preparation for assays, drug delivery [1], organic synthesis [2], separation and purification [3], etc., all of which in the same chip. The technique is regarded to be environmental benign as well as safe as compared to bench-top process mode, due to low volume of reagents and high rate of reaction [4–7]. Over the years, microchannels have established as completely automated and portable tools. In general, microreactors have characteristic dimension in micrometers and reaction volumes in nanoliter-to-microliter range [8–10], with the flow usually being laminar. As the reaction rates are completely controlled by diffusional resistance, it demands long channel length as result of which high-pressure drop, rendering the process uneconomical. To overcome these issues, researchers have attempted to improve mixing using different techniques even at low Reynolds numbers. In this connection molecular diffusion is improved by the reducing the channel

⁎ Corresponding authors at: State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China. E-mail address: [email protected] (S. Yin).

https://doi.org/10.1016/j.microc.2020.104662 Received 19 October 2019; Received in revised form 19 January 2020; Accepted 19 January 2020 Available online 22 January 2020 0026-265X/ © 2020 Elsevier B.V. All rights reserved.

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have reported the effect of geometrical design of double jet impingement microchannels on mixing efficiency and concludes that the microchannel with confluence angle of 135 °C works more efficiently in liquid anti-solvent precipitation and Villermaux-Dushman protocols. Additionally, numerical simulations also indicate the effect of geometric parameters of micromixers on mixing and fluid flow, for a wide range of Reynolds number, the characteristic reaction time (tr) is an important parameter to measure the mixing performance [24–27]. Although the mixing performances are studied by some literatures, the effects of channel geometry at high flow rate, have not been reported. In the present study, the efficient mixing for a wide range of Reynolds numbers (Re = 3000–42,000) in microchannel reactors with diverse geometries is investigated, using iodide-iodate chemical reaction (Villermaux-Dushman) to characterize the mixing performance. Also, the micromixing time is modeled using regression analysis. Moreover, the mixing performance of the optimized microchannel is compared to the performance of other micromixers with two-phase flow having a simple curved channel.

by mixing sodium hydroxide solution (NaOH) with boric acid solution (H3BO3) in deionized (DI) water to obtain an equimolar buffer solution of H2BO3−/H3BO3 (pH = 9.14). The powders KI and KIO3 are dissolved in DI water in separate beakers and are added to the buffer solution, respectively. The above solution should strictly follow mixing sequence so that the iodide and iodate ions coexist in a basic solution, preventing iodine formation [30–33]. The [I3−] in basic solution is detected with a UV- spectrophotometer, but it is not detected in the equipment recognition range (The optimal measurement range of the UV- spectrophotometer used in this experiment is 0.01–0.1. Thus, the experimental value of the corresponding segregation index is between 0.00013 and 0.0023).

2. Experimental

H2 BO3 + H+

2.3. Micromixing characterization The micromixing performances of microchannels with different geometries are characterized using the Villermaux-Dushman method [34–37]:

H3 BO3 (quasi

5I + IO3 + 6H+

2.1. Experimental setup

I2 + I

The schematic diagram of micromixing test system is illustrated in Fig. 1, wherein Fig. 1(a) presents the micromixing test device and Fig. 1(b) demonstrates the magnified graph of microchannel reactor. The micromixing performance of solution is determined in the microchannel reactor by Villermaux-Dushman reaction. As shown in Fig. 1(a), the micromixing test device is composed of a microchannel reactor, two advection pumps, three beakers, a spectrocell, UV-Spectrophotometer and computer. Additionally, as shown in Fig. 1(b), the microchannel reactor in micromixing test device is mainly consisted of nut, leakproof pad, booster funnel with micropore, screw thread, joint channel and flange. Together them in series, they form a uniform and straight main channel. The detailed parameters of the microchannel reactor used in the experiment are listed in Table 1. The reaction process and operation steps are as follow: the collision flow reaction is carried out in a T-shape microchannel reactor with different volumes, which further mixed reaction in a linear plastic tube to determine the mixing efficiency in continuous condition. Two kinds of fluids named solution A (dilute sulfuric acid) and solution B (composed of H3BO3, NaOH, KI and KIO3) are pumped at fixed flow rate (covering a range of 100–1000 ml/min at a flow ratio of 1:1) into the microchannel using advection pumps made by SZweico. The experimental facility also includes a linear steady flow tube with a variable inner diameter ranging from 2 to 5 mm and length in the range covering 20 to 80 cm respectively. An online UV spectrophotometer (UV-2600) is used to evaluate I3− concentration by means of the spectrophotometry at wavelength of 353 nm. The spectrocell should be immediately placed in the spectrophotometer to avoid any error due to delay in measurement. All tests are repeated several times until the experiment got the allowable error range, and the experimental data values of that are recorded. According to the approximate formula deduced by Chen et al. [28] using the double ultrasonic interference offset method, the error range of the measured flow velocity of the fluid in the steady flow tube is ± 6.3%. The experiments are conducted at atmospheric condition with the flow rate in turbulent range.

instantaneous)

3I2 + 3H2 O (veryfast)

(1) (2) (3)

I3

The neutralization reaction (1) is a quasi-instantaneous while the redox reaction (2) is also fast but much slower than reaction (1). The two reactions compete for H+ in the system. Under perfect micromixing conditions the H+ instantaneously gets distributed homogeneously which gets consumed by borate ions (H2BO3−) to form boric acid (H3BO3) according to Eq. (1). On the contrary, H+ involved in both Eqs. (1) and (2), and the I2 formed, further react with I− to yield triiodide complex (I3) according to reaction (3). The iodide concentration is determined using a linear function of the Beer–Lambert and can be easily measured by a spectrophotometer (UV-2600) at 353 nm. The segregation index (Xs) is calculated based on the measured concentrations, which helps to quantify the micromixing efficiency [38–41]:

XS = Y=

Y YST

(4)

2(VA + VB )([I2] + [I3 ]) VB [H+]0

(5)

6[IO3 ]0 6[IO3 ] + [H2 BO3 ]0

(6)

YST =

where Y is the ratio moles of acid mole consumed by reaction (2) to the total moles of acid added and YST corresponds to the total segregation where the micromixing process is extremely slow. The average of three experiments is performed, and the standard deviation is calculated to estimate the error range so as to improve the credibility of the experimental data. The range of Xs between 0 and 1 indicates partial segregation, while 0 and 1 corresponds to total segregation, respectively [36,42–45]. Furthermore, the experiment quantitatively assesses the micromixing performance at high flow rate in a T-shape microchannel. The relationship between light absorption (Abs) and concentration of I3− is assessed, and the mixed solutions of I2/I− having different concentrations are prepared to form the I3− solutions according to reaction (3). The linear relationship between Abs and the concentration of I3−, expressed as following according to Lamber-Beer law:

2.2. Solution preparation The chemicals employed for the Villermaux-Dushman reaction and corresponding concentrations are listed in Table 2. The solution is prepared as per the standard procedure detailed by Guichardon and Falk [29]. All the chemicals utilized are of analytical grade, supplied by Tianjin ZhiYuan Chemicals Company. The alkaline solution is prepared

[I3 ] =

A d

(7) −

−

where [I3 ] is the concentration of I3 , A is the absorbance of the solution, and d is the thickness of the spectrocell (0.01 m). Before the experiment, the scale of ε is determined as 2927 mol−1 m2, and the correlation coefficient being 0.999. 2

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Fig. 1. Schematic diagram of micromixing test system, (a) micromixing test device; (b) magnified graph of microchannel reactor in micromixing test device.

Table 1 Parameters of the microchannel reactors used in this experiment. Volume of mixed chamber/ml

Radius of inlet channel 1/mm

Radius of inlet channel 2/mm

Radius of outlet channel/mm

Length of inlet channel 1/mm

Length of inlet channel 2/mm

Length of outlet channel/mm

0.3 0.5 0.7 0.9

1 1.5 2 2.5

1 1.5 2 2.5

1 1.5 2 2.5

32 24 19 15

32 24 19 15

32 24 19 15

3

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effect becomes insignificant. The Xs less than 0.0001 for all the four different outlet lengths, indicates almost ideal micromixing at Re higher than 33,000. This can be explained that stripe layers are produced when fluid flows through channels and the thickness of the fluid striation laminae is dependent on the Reynolds number and the outlet length of microchannel. The fluid is further sheared, stretched, and folded with an increase of outlet length, resulting in an increase in the effective interfacial area, which contributes to an intensified mixing performance.

Table 2 The concentration of the reactants employed in the experiments. Materials of iodide-iodate reactant

Concentration (mol/L)

KI KIO3 H3BO3 NaOH H2SO4

0.0116 0.00233 0.1818 0.0909 0.02, 0.03, 0.04, 0.05, 0.06

3. Results and discussion

3.3.3. Effect of the inlet tube diameter on segregation index The inlet tube diameter is known to be an important parameter in the microreactors, as it affects the diffusion path length of the reaction. S6 shows the effect of inlet flow rate on the segregation index covering different tube diameters (2–5 mm). The flow velocity is calculated from the volumetric flow rate by dividing with the internal area of the tube. The experiments are operated by adjusting the diameter of the connecting tee inlet tube, holding the volume of the mixing chamber in the reaction zone constant. The results show that segregation index decreases with increase in the inlet tube diameter at any given flow velocity. And the lower flow velocity is, the more significant the effect becomes. A smaller inlet tube diameter contributes to an increase in the contact surface between two phase. The narrower inlet tube diameter at high flow would increase the flow overlap, generating a higher H+ zone which would decrease the contact surface and facilitate reaction (2) to form more I2. If the inlet tube diameter is very large, the contact surface would decrease, thereby to increase the H+concentration. The optimum between those two factors determines the existence of the smallest inlet size.

3.1. Effect of initial acid concentration and flow rate on segregation index S1 shows the effect of acidity concentration (H+, mol/L) on Xs covering a flow rate of 100 to 600 ml/L. The flow rate of iodide-iodate solution is varied when keeping the acid concentration constant. It is found that Xs increases marginally firstly with an increase in acid concentration until more than 0.05 mol/L, it begins to grow exponentially, which could be attributed to the very fast side reaction (Eq. (2)) in presence of more H+. With increase in acid concentration more iodine is produced which contributes to increasing the segregation index [33]. More detailed information about the figures concerning the effect of variables is presented in Supplementary material. S2 shows a decrease in segregation index with increase in Reynolds number (Re), and changes more and more slowly. At very high Re the trend flattens with the rate of decrease being insignificant. This can be due to the reaction of the large amount of H+with H2BO3− (Eq. (1)). At very high Reynolds number more efficient micromixing is promoted due to the increase in the local turbulence in the reaction ambience, namely, the first reaction proceeds more effectively reducing the undesirable products.

3.3.4. Effect of volume of mixed chamber volume on segregation index S7 shows the Reynolds number on the Xs, covering a range of mixing chamber volumes (0.3–0.9 cm3). With increase in the mixed chamber volume, the Xs decreases obviously until a Reynolds number of 35,000, beyond that, the effect becomes insignificant. A higher mixing chamber volume means a larger collision reaction zone having high interfacial area, which contributes to a reduction in the Xs. With increase in Re, the collision flow reaction comes to a near completion, and hence the volume of the mixing chamber does not show any effect on the micromixing, that is, having no significant change in the segregation index.

3.2. Effect of volume flow ratio on segregation index S3 shows the effect of flow ratio (R) (R = Va/Vb ) on Xs with the volume flow rate Va (mixed solution A), varied within a range of 100–1000 ml/min, covering Re from 3000 to 42,000. The volume flow ratio is increased by increasing the concentration of the acid solution B, keeping the stoichiometric ratio constant (the volume flow rate Vb is decreased) (na/nb = Va (Ca)/Vb (Cb)) . The results show a remarkable increase in the mixing performance with decrease in R, which could be attributed to the lower H+ concentration, contributing to a reduction in the mixing time. Moreover, the flow velocity of solution B also increases with decrease in R, further enhances the turbulence intensity at the impinging zone. The impact point of two phase gets closer to the center of the channel as R approaches to 1, reducing the molecular diffusion path length.

3.4. Micromixing time Reaction time (tr) and the characteristic micromixing time (tm) as two criteria are utilized for measuring time in the reactor, and depend on the intrinsic kinetics and the hydrodynamic condition, respectively. In order to achieve high yield and selectivity in the fast reaction systems, the characteristic micromixing time (tm) is required to be smaller than the characteristic reaction time (tr). Therefore, the process of reactor design must focus on strengthening micromixing. The characteristic reaction time (tr) of reaction (2) is expressed as following:

3.3. Effects of geometry parameters on segregation index 3.3.1. Effect of outlet tube diameter on segregation index A silicone tube having an inner diameter of 2–5 mm is connected to the outlet of the microchannel, and the results are depicted in S4. The larger the outlet tube diameter (Dout) is, the less the Xs is. On one hand, an increase in the outlet silicone tube diameter is beneficial for reducing the pressure of the mixing chamber and changing in the flow pattern of the fluid in the impingement zone, and then to enhance the turbulence at the impact point; on the other hand, it may result in an increase in the residence time due to widening of the mixing passage, generating a complete reaction.

3

tr =

1

Min ( 5 [I ]0 , 3[IO3 ]0 , 2 [H+]0 )

(8)

(r2 )t = 0

where r2 is the reaction rate of reaction (2), r2=k2CI can be calculated by the following:

Logk2 = 9.28105

Logk2 = 8.38

3.664 I

I < 0.166mol·L

1.511 I + 0.23689I

I

k2

(9)

1

0.166mol·L

CIO3−C(H+)2,

−

1

(10)

n

ci z i 2

I=

3.3.2. Effect of outlet tube length on segregation index The effects of the outlet length of microchannel on Xs are shown in S5. It is observed that the Xs decreases significantly with decrease in the outlet length, until a Reynolds number of 33,000, beyond that, the

i=1

(11)

where c is the ion concentration in the solution, and z is the valence state of the ion. As shown in Fig. 2, the intersection of the tangent at the 4

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Fig. 2. Determination of micromixing time.

initial concentration of H+with the time axis is the micromixing time. Based on the Xs, tm is found to be in the range of 0.5–1.0 s. Fig. 3 depicts the relationship between segregation index and fluid velocity at different characteristic reaction time (tr). The segregation index decreases significantly with increase in flow rate at low characteristics reaction time as compared to the high characteristic reaction time. With increase in the characteristic reaction time, it does not make any significant reduction in the reaction time characterized as critical point micromixing time (tm). When tr < tm, the reaction is controlled by mixing: [H+] participates in both quasi-instantaneous reaction (1) and fast reaction (2), segregation index still decreases with increase of flow rate. On the contrary, it is a chemical reaction controlled regime: [H+] only participates in the reaction (1), the segregation index does not change with increase in flow rate. A zero value for tm indicates the ideal micromixing conditions. The extremely low micromixing time (tm) provides a new approach for studying the ultra-fast reaction kinetics. For the kinetic measurements of ultra-fast reactions, the mixing is required to complete instantaneously before the reactions occur.

depended on the energy dissipation. Hence, tm seems to relate with the characteristic size of the reactor. As a result, the design of the reactors in the laminar flow is always labeled with high energy consumption and low energy efficiency. In this work, shortcomings of the microchannel reactor can be overcome effectively by adjusting the geometric parameters of the microchannel, flow rate and acidity at the turbulent level. The comparison exhibits the superior performance of the T-shape microreactor at high flow rates. 4. Conclusion Mixing performance in a T-shape intensified microchannel reactor is characterized by adopting iodide-iodate test reaction, known as Villermaux-Dushman reaction. In the first series of experiments, the micromixing efficiency of the microchannel is studies in terms of segregation index (XS), under initial concentration of acid solution, various total flow rates, volume flow ratio and geometry of channel (din, dout, Lout and Vmc). The results show that segregation index increases with increase in the H+ concentration and volumetric flow ratio, while decreases with increase in the outlet tube and inlet tube diameter. In particular, segregation indexes are found in the range of 0.00013–0.00047, indicating that the geometry of microchannel can establish higher micromixing efficiency. In the second series of experiments, the micromixing time is estimated with based on the optimal geometry parameters of the microchannel, and the optimal value could reach 0.5 ms, which is better than many other micromixers. Furthermore, an ideal design concept of microreactor is deduced to help suppliers and users design and select the

3.5. Comparison with other reactors Table 3 displays a comparison of the experimental condition, flow rate and tm of different reactors for two-phase flow. In general, the magnitude of tm is large as the [H+] concentration is higher. According to the turbulence theory, the acid input is engulfed immediately by the energetic vortices generated near the Kolmogorov scale. Namely, the smaller the hydrodynamic diameter is, the lower the tm is. In addition, some literatures show that the tm in laminar flow regime is strongly 5

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Fig. 3. Segregation index (Xs) vs. fluid velocity at different characteristic reaction time (tr).

microreactor more efficiently based on the comparison between this work and others.

acquisition, Project administration, Writing - original draft. Yuan He: Data curation, Formal analysis, Writing - original draft. Kaihua Chen: Data curation, Formal analysis, Writing - original draft. Shaohua Yin: Funding acquisition, Project administration, Writing - review & editing, Writing - original draft.

CRediT authorship contribution statement Peng lv: Data curation, Formal analysis, Methodology, Writing original draft. Liangjing Zhang: Data curation, Formal analysis, Methodology, Writing - original draft. C. Srinivasakannan: Writing review & editing, Writing - original draft. Shiwei Li: Funding

Declaration of competing interest The authors declare no competing financial interest.

Table 3 Comparison of the micromixing time (tm) and with other reactors in two-phase flow. Reactor

Experimental conditions

Flow rate (ml/min)

tm (ms)

Authors

T-shape High shear mixers (HSM) AccoMiX Caterpillar-0.15 Sigle-zigzag Multi-zigzag Multi-channel M1 Multi-channel M2 Flow-focousing T-mixer Helical tube reactor (HBM-HTR) Microporous tube-in-tube microchannel reactor (MTMCR) Pore-array intensified tube-in-tube microchannel reactor (PA-TMCR) SY ZY ZDT

R = 1 [H+] = 0.05 R = 1 [H+] = 0.1 R = 1 [H+] = 0.03 R = 1 [H+] = 0.0224 R = 1 [H+] = 0.02

0.5–1.0 0.09–0.6 9–44 2–12 6–93 5–75 6–15 17–41 3–60 0.78–8.9 0.14–1.5

This work Qin et al. [46] Panić et al. [47] Madhvanand et al. [48] Su et al. [49]

R = 10 [H+] = 0.16 R = 10 [H+] = 0.5 R = 10 [H+] = 0.25

100–1000 3667–7000 1–8 1–18 3–40 24–320 300–900 400–2000 10–50 4.8–40 200–1200

R = 5 [H+] = 0.2 R = 8 [H+] = 0.2 R = 1 [H+] = 0.0026

3600–8400 400–4500 64–160

2–5 0.27–1.1 4.9–12.3 8.8–22.1 6.6–16.6

Ouyang et al. [24] Li et al. [54] Woldemariam et al. [55]

R = 1 [H+] = 0.036

6

Guo et al. [50] Rahimi et al. [51] Masoud et al. [52] Luo et al. [53]

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Acknowledgements [25]

Financial aid from the following programs is gratefully acknowledged: National Natural Science Foundational of China(grant number 51604135 and 51504116), and Yunnan Ten Thousand Talents Plan Young & Elite Talents Project(grant number YNWR-QNBJ-2018-323)

[26] [27]

Supplementary materials

[28]

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2020.104662.

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