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.