International audienceMicromixing is the limiting step in the progress of fast reactions and fast crystallization processes. Micromixing performance of a Nitech© glass COBR has been characterized using the competing parallel iodide-iodate test reactions. Macromixing length was also qualitatively determined by visualization of a tracer. The results show that an increase in the amplitude to baffle space ratio and a decrease of oscillation frequency improves mixing performance

Avila, M.

Poux, M

Xuereb, C

Fletcher, D.,

Aubin, Joelle

English

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Characterization of micromixing in a Continuous
Oscillatory Baffled Reactor
M. Avila, M Poux, C Xuereb, D. Fletcher, Joelle Aubin
To cite this version:
M. Avila, M Poux, C Xuereb, D. Fletcher, Joelle Aubin. Characterization of micromixing in a Contin-
uous Oscillatory Baffled Reactor. 16th European Conference on Mixing, Sep 2018, Toulouse, France.
￿hal-02373265￿
Variation of the segregation index with the oscillatory Reynolds number
cs
Operational conditions and solution concentrationsNitech© glass reactor
Characterization of Micromixing in a 
Continuous Oscillatory Baffled Reactor
M. AVILA1, M. POUX1, C. XUEREB1, D.F. FLETCHER2, J. AUBIN1
1 Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France
2 School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia
Micromixing performance
• Micromixing performance is determined by the Villermaux–Dushman reaction system [4],
based on a set of competing parallel reactions:
𝐻2𝐵𝑂3
− +𝐻+ ↔ 𝐻3𝐵𝑂3 Eq. (1) Quasi-instantaneous.
5𝐼− + 𝐼𝑂3
− + 6𝐻+ ↔ 3𝐼2 + 3𝐻2𝑂 Eq. (2) Very fast
𝐼2 + 𝐼
− ↔ 𝐼3
− Eq. (3) Quasi-instantaneous equilibrium
Parameter Value
Reactor diameter (𝐷) 15 mm
Orifice diameter (𝑑0) 7.5 mm
Tee pipe diameter (𝑑) 0.4 mm
Baffle spacing (𝑙𝑏) 16.9 mm
Oscillation frequency (𝑓) 0.5 – 1.5 Hz
Oscillation amplitude (𝑥𝑜) 2.8 – 12.68 mm
Flow rate ratio (𝑅)
3.63
7.26
Main flow rate (𝑄𝑛𝑒𝑡) 4.68 kg/h
Influence of acid concentration and oscillatory conditions Influence of flow rate ratio
Continuous oscillatory baffled reactors (COBRs) offer interesting advantages over traditional mixing processes in stirred tanks, such as effective mixing, good mass and heat transfer in laminar and transitional
flow [1], as well as plug flow with long residence times. These characteristics have been shown to improve liquid-liquid reactions [2] and crystallization processes [3].
Whilst numerous works on the characterization of COBRs have focused on the study of flow patterns, velocity profiles and RTD [4], or the conversion rate of some reactions [2, 5], there is little quantified
knowledge on the macro- and micromixing performance of these reactor types. Indeed, COBRs are most often used in applications where the kinetics are slow. However, micromixing performance in the COBR is
of interest for applications with initial fast reactions or precipitations that also require long residence time (for consecutive reactions or crystal/precipitate growth). Such applications are still challenging because
the COBR typically does not provide fast micromixing conditions, thereby leading to local segregation and a decreases in selectivity and/or product properties.
The objective of this work is to study the effect of the oscillatory conditions in the Nitech® COBR in the laminar flow regime (𝑅𝑒𝑜 < 2000) on micromixing performance using the iodide-iodate or Villermaux-
Dushman test reactions [6].
Reactant Concentration [M]
H3BO3 0.1818
NaOH 0.0909
KIO3 0.00233
KI 0.0116
H2SO4
0.03
0.015
0.0075
• The greater the yield of I2, the poorer the micromixing quality.
• Determination of product concentration through spectrophotometry.
• Mixing efficiency measured by the segregation index, 𝑋𝑆:
𝑋𝑠 =
𝑌
𝑌𝑇S
=
𝐻+𝑚𝑜𝑙𝑒𝑠 𝑐𝑜𝑛𝑠u𝑚𝑒𝑑 𝑏𝑦 𝑒𝑞. (2)
𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑖𝑜𝑑𝑖𝑑𝑒 𝑤ℎ𝑒𝑛 𝑡ℎ𝑒𝑟𝑒 𝑖𝑠 𝑡𝑜𝑡𝑎𝑙 𝑠𝑒𝑔𝑟𝑒𝑔𝑎𝑡𝑖𝑜𝑛
• Same reactant molar flow rates in both cases [7-8].
• Same trend of 𝑋𝑠 with 𝑅𝑒𝑜 for both flow rate ratios R
• 𝑋𝑠 decreases with an increase in acid flow rate.
• Influence of injection flow: Increase of instabilities in the injection zone from 𝑅𝑒inj 60 to
120.
• Macro-mixing length was not able to be determined visually due to low coloration of the
bulk stream.
Case 1 2 3 4 5 6 7
𝑓 (Hz) 0 1.5 1 1 0.5 1 1.5
𝑥𝑜 (mm) 0 2.8 4.24 6.36 12.68 12.68 12.68
𝑅𝑒𝑜 0 395 398 598 596 1191 1787
𝑋𝑠 0.38 0.36 0.35 0.30 0.05 0.10 0.22
Macro-mixing length
(# baffle cell)
Not reached Not reached 21 20 4 6 14
References
[1] Stephens G.G., Mackley M.R., (2002), Exp. Therm. Fluid Sci., 25: 583 - 594.
[2] Mazubert A., Crockatt M., Poux M., Aubin J., Roelands M., (2015), Chem. Eng. Technol., 38: 2161 - 2169.
[3] Lawton S., Steele G., Shering P., Zhao L., Laird I., Ni X.W., (2009), Org. Process Res. Dev., 13: 1357 - 1363.
[4] Hewgill M.R., Mackley M.R., Pandit A.B., Pannu S.S., (1993), Chem. Eng. Sci., 48: 799 - 809.
[5] Lobry E., Lasuye T., Gourdon C., Xuereb C., (2014), Chem. Eng. J., 259: 505 - 518.
[6] Fournier M.C., Falk L., Villermaux J., (1996), Chem. Eng. Sci., 51: 5053 - 5064.
[7] Guichardon P., Falk L., (2000), Chem. Eng. Sci., 55: 4233 - 4243.
[8] Panié S., Loebbecke S., Tuercke T., Antes J., Bošković D., (2004), Chem. Eng. J., 101: 409 - 419
Conclusions
• Mixing length and 𝑋𝑠 is directly proportional to 𝑥𝑂 and inversely
proportional to 𝑓, and not only dependent of the mixing intensity 𝑅𝑒𝑜 .
• The influence of 𝑥𝑂 and 𝑓 on the micromixing efficiency in non-negligible
when a reactive side stream is added by a tee injection before the first
baffled cell.
Future work
• Identify the relationship between 𝑋𝑠 and oscillatory and geometrical
parameters:
– Experiments with higher Τ𝑥𝑂 𝑙𝑏 values.
– Study the influence of higher 𝑅𝑒net by increasing the main flow rate.
• The influence of geometry and new configurations will be studied to
improve the micromixing performance in the COBR.
• Sensitivity problem.
– Acid concentration of 0.03 𝑀: 𝑋𝑠 only varied slightly or not at all with the oscillatory
conditions.
– Acid concentration of 0.015 𝑀 was used in further experiments.
• Presence of oscillations leads to a decrease in the 𝑋𝑠 values.
• Segregation index is not solely correlated with 𝑅𝑒𝑜.
• Constant 𝑓: an increase in 𝑥𝑂 decreases mixing length and 𝑋𝑠.
• Constant 𝑥𝑂: an increase in 𝑓, decreases mixing performance, and leads to longer mixing
lengths.
• High 𝑥𝑂 and low 𝑓 detaches the acid stream from the reactor wall, allowing it to be
engulfed by the bulk stream at shorter lengths.
