Abstract
Effect of hydrodynamic heterogeneity on micromixing intensification in a
Taylor-Couette flow reactor (TC) with variable configurations of inner
cylinder has been investigated by adoption of a parallel competing
iodide-iodate reaction system. Two types of inner cylinder, circular
inner cylinder and lobed inner cylinder (CTC and LTC), were used to
generate hydrodynamic hydrodynamic heterogeneity for comparison of the
micromixing intensification, focusing on the effects of the Reynolds
number of the TC reactor, the acid concentration, and the feeding time.
The Segregation index (Xs ) was employed to evaluate the
micromixing efficiency. It was revealed that Xs decreases with
the increase of Reynolds number and feeding time but increases with the
increase of acid concentration for both the CTC and LTC. However, the
LTC does present a better micromixing performance at various operating
conditions than that of the CTC as affirmed by both the experimental and
computational fluid dynamics (CFD) simulation results.
Keywords: Geometry modification; Micromixing efficiency;
Segregation index; Micromixing time.
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*Corresponding author: Tel: +86-574-88182419 E-mail:
Xiaogang.Yang@nottingham.edu.cn
(XY)
Introduction
During the synthesis of various micro/nano particles, the hydrodynamics
of the mixing in the reactors has been recognised as playing a
determinant role in determining the synthesised particle properties. In
particular, micromixing, which takes place at the molecular scale, has
been considered as the rate determining step during the chemical
reaction of particle crystal growth in the reactors, especially for the
fast chemical reactions involved as pointed out by Baldyga and Bourne
(1999). A perfect micromixing condition can effectively increase the
yield of the desired particle and reduce by-products in the synthesis
process at the same time. It should be mentioned that mixing is deemed
to be effective at the macro scale, but the mixing may not have
dispersed homogeneously at the micro scale yet. Thus, the improvement of
micromixing will be beneficial to the synthesis of the particles.
Various kinds of reactors have been developed and investigated in
previous studies, aiming to improve the mixing performance. For example,
the use of a spinning disk reactor (Jacobsen and Hinrichsen, 2012),
impinging jet reactor (Liu et al. , 2014), microchannel reactor
(Shi et al. , 2012), T-shaped reactor (Gao et al. , 2015),
and high shear mixer (Qin et al. , 2017) have all been studied
before. The Taylor-Couette (TC) reactor with the features of
controllable mean residence time and narrow shear rate distribution has
also received a lot of attention. This type of reactor has a simple
configuration, where the inner cylinder rotates relative to the outer
one. Jung et al. (2000) prepared calcium carbonate particles by a
gas-liquid reaction system using a
TC reactor, and obtained three particle morphologies (cube-like,
transition and spindle). They attempted to introduce an enhancement
factor in order to characterise this type of behaviour, which is related
to the mass transfer rate and suggested that the particle shape change
is dependent on this enhancement factor. Tang et al. (2019) have
investigated the morphology change of copper sulfide nanoparticles by
using the TC reactor and they have also found that an intensified mass
transfer rate can be achieved using the TC reactor. By tracing the other
applications, Haut et al. (2003) employed a TC device to culture
animal cells and found that the device is more appropriate than a
conventional stirred tank in terms of the control of oxygen content and
cell suspension. They believed that the adoption of TC reactor can
effectively enhance mass transfer rate and create a relatively mild
environment for cell growth. Kim and his co-researchers have employed
the TC reactor to synthesise many
types of fine particles, such as cathode precursors for lithium ion
batteries (Mayra and Kim, 2015; Thai et al. , 2015; Kim and Kim
2017), barium sulfate (Aljishi et al. , 2013), L-histidine (Park
and Kim, 2018), and Guanosine 5-monophosphate (Nguyen et al. ,
2011). Their work mainly focused on the applications of the TC reactor
during various particle production processes. However, these studies
lacked the fundamental investigation of the mechanisms involved behind
the mixing, mass transfer and heat transfer processes.
During particle synthesis processes, many previous studies have
indicated that even a minor change of reactor configuration will lead to
a significant effect on the micromixing performance. Jacobsen and
Hinrichsen (2012) investigated the micromixing characteristics in a
spinning disk reactor with different feeding locations and surface
structures. In addition, by validation, they synthesised barium sulfate
particles with different reactor configurations and obtained various
particle morphologies and sizes. Bertrand et al. (2016) applied
computational fluid dynamics to simulate the micromixing in three types
of mixers from the same family but with some geometrical differences:
T-shaped tube, Y-shaped tube, and Hartridge-Roughton mixing device. Both
experimental and numerical results indicated that the Hartridge-Roughton
tube is the most efficient one. Zhu et al. (2019) synthesised
cathode precursor
Ni0.6Co0.2Mn0.2(OH)2for lithium ion batteries in a stirred tank with four different types of
impeller. They finally obtained particles with different shapes and tap
densities and consequently, different electrochemical performances. They
attributed these differences to the different flow fields generated by
four impellers. All these previous studies have revealed that
geometrical optimisation is an effective and economical approach to
improve the performance of these existing reactors. As mentioned
earlier, TC reactor has many features that are beneficial to the
particle synthesis due to the fast mass transfer and easy shear control.
However, the shear regions in the TC reactor are not locally uniformly
distributed. In order to better utilise the advantages of the overall
shear control, the modification of the classical TC reactor may improve
the features of local turbulent shear. Soos et al. (2007)
proposed a lobed profile for the inner cylinder in order to reduce the
low velocity gradient region. They found that this configuration can
successfully enhance the local shear rate in the vicinity of the inner
cylinder. Li et al. (2015) have compared the mixing performance
in the TC reactor with various cross-sectional profiles of inner
cylinder including a lobed one using CFD simulation in terms of flow
patterns, shear strain rate distribution and micromixing time. They
ascertained that the mixing performance improves when using the inner
cylinder with a lobed profile. Liu et al. (2020) have reported
the synthesis of the barium sulfate particles by using the TC reactor
with the classical circular cross-sectional profile inner cylinder and a
lobed profile inner cylinder. The experimental results clearly indicate
that the properties of the secondary particles are different in terms of
particle morphology, particle size and its distribution. The
aforementioned investigations have implications where the mixing in the
TC reactor is significantly affected by the hydrodynamics in the reactor
and also by the inner cylinder configuration adopted.
So far, the micromixing process in the TC reactor has not been clearly
understood though a number of experimental work and theoretical studies
conducted (Drozdov, 2002; Racina and Kind, 2006; and Richter et al.,
2008). Also, these previous studies have mainly focused on the TC
reactors with the circular cross-sectional profile inner cylinder and
the modifications on the configuration were subjected to the changes of
gap size and aspect ratio (DiPrima, 1984; Xiao et al. , 2002). The
impact of configuration variation on the micromixing performance,
especially the inner cylinder alteration, is still rarely studied in the
literature.
The aim of the present work is to investigate the micromixing
performance in the TC reactor with two different types of inner
cylinders and to obtain the guidelines for further enhancing the
micromixing performance through the modification of the TC reactor
configuration. A parallel
competing
system based on thee iodide-iodate
reaction proposed by Villermaux and co-workers (Villermaux et
al. , 1994; Fournier et al. , 1996) has been employed, attempting
to reveal the effects of key operating parameters on the segregation
index, which can reasonably serve as an indicator for assessing the
micromixing performance. The key parameters include the Reynolds number
based on the gap size, the reactant feeding time and the acid
concentration. In addition, the micromixing time based on the
experimental data by employing the incorporation model are also
evaluated and compared. This paper will be organised as follows. Section
2 will present the theoretical background and modelling details for
evaluating the micromixing using the iodide-iodate reaction as the
system, while Section 3 will present the experimental details for such
chemical probe in the TC reactors with micromixing performance
evaluation. Section 4 will present the results and discussion and
finally, Section 5 will summarise the conclusions derived from the
study.
Micromixing characterization modelling
Various chemical reaction schemes serving as molecular probes have been
proposed and commonly accepted by researchers to characterise the
micromixing performance for the reaction system involved. Typical
systems are single reaction systems (\(A+B\rightarrow R\)),
consecutive reaction systems
(\(A+B\rightarrow R\),\(B+R\rightarrow S\)) and parallel competing reaction systems
(\(A+B\rightarrow R\), \(A+C\rightarrow S\)). Due to the rigorous
conditions imposed for the on-line analysis of the single reaction, the
last two schemes are favoured and usually employed when measuring the
final product quality. Bourne and his co-workers proposed several
reaction systems based on the consecutive competing scheme, such as the
bromination of 1,3,5-trimethoxybenzene (Bourne and Kozicki, 1977), the
azo-coupling of 1-naphthol with
diazotised sulphanilic acid (Bourne et al. , 1981) and the
selective iodination
of
l-tyrosine (Bourne and Rohani, 1983). However, these proposed systems
and the experimental methods still have some disadvantages, especially
with their toxic, volatile and unstable nature. With a better
understanding of the mixing and chemical reactions, Villermaux et
al. (1994) and Fournier et al. (1996) proposed the use of a
parallel competing scheme based on the iodide-iodate reaction system.
Generally, the products produced in such a system are easily analysed by
using the spectrophotometric method. This method have been successfully
applied for the assessment of the micromixing efficiency in stirred
vessel reactors (Unadkat et al. , 2013; and Lemenand et
al. , 2017) and in high shear mixer (Qin et al. , 2017). As the
use of the parallel competing reaction system has the advantages of
simple control, sensitive measurement through detection of the product
concentration and low toxicity of agents,
the Villermaux
iodide-iodate
reaction system is adopted in the present study.