Liu, Lu
(2021)
Fundamental study of shear controllable synthesis of fine particles using taylor-couette flow reactor.
PhD thesis, University of Nottingham.
Abstract
Fine particles generally have special physicochemical properties, such as good solubility, dispersibility, and adsorption due to their extremely large specific surface area. With such advantages, they have been attracting wide attention in the fields of pharmaceuticals, catalyst, food, cosmetics, chemicals and electronics. Among various methods to prepare fine particle materials, reactive precipitation presents its advantages in terms of simple configuration, flexible operation, and low cost.
Controllable synthesis of fine particles is still a challenge. On one hand, fine particles have a tendency to aggregate or agglomerate due to the relatively reduced effect of gravity force and the enhanced effect of other surface forces, such as van de Waals force. On the other hand, operating conditions, especially the heterogeneity of fluid flow in chemical reactors have complex interaction with particles. Fluid mixing determines the environment for chemical reaction, and fluid shear, especially turbulence induced shear, affects particle growth. Since final material performance depends on particle properties, such as particle size, morphology, porosity, and tap density, it is necessary to investigate the key steps and processes on the course of particle formation in order to realize controllable synthesis and obtain desired products consequently. Thus, this PhD work aims to build the quantitative relationships between the key hydrodynamic parameters involved in the synthesis process using the Taylor-Couette flow (TC) reactor and the synthesised particle properties by employing such reactor. The main work conducted and outcomes derived from the project are summarised as follows:
(1) In chapter 2, barium sulfate was selected as a model substance to study the interfacial phenomenon during the process of particle formation. This chapter is mainly focused on the effect of hydrodynamic process in the TC reactor on particle morphology. Three different kinds of morphology of barium sulfate particles are observed by changing Reynolds number Re=(ωdr_i)/ν, feeding rate and supersaturation in the Taylor-Couette flow reactor with a lobed inner cylinder (LTC). Such morphology transition, indicating an interfacial interaction between feed solutions and aggregated particles, is found to be dependent on fluid flow pattern. The mechanism of particle formation under the effect of fluid dynamics is proposed for the LTC.
(2) Following the work of Chapter 2, Chapter 3 is focused on the change of barium sulfate particle size and particle size distribution. The comparison of the particles synthesised using the classical Taylor-Couette flow reactor (CTC) and the LTC reactor was conducted both experimentally and numerically by applying computational fluid dynamics (CFD) modelling. Particles synthesized in the LTC show the overall smaller size with narrower size distribution than those in the CTC, which is also consistent with the CFD modelling results on the effect of shear rate distribution on the particle size distribution. It is suggested that the local turbulence intensification due to geometry modification to the LTC inner cylinder is beneficial to the synthesis of particles with smaller size. Shear induced by small turbulent eddies can have a significant impact on the synthesised particle size.
(3) Chapters 2 and 3 have confirmed that hydrodynamics plays an important role in determining the synthesised particle properties. As the mixing occurring in the TC reactor creates supersaturation, which subsequently induces chemical reaction, a fundamental study on the mixing was carried out in Chapter 4. Based on the Villermaux iodide-iodate reaction system, the segregation index (Xs) was employed as an indicator to evaluate the micromixing efficiency. It is found that the hydrodynamic heterogeneity created by the LTC can significantly enhance the micromixing efficiency. Also, it has been reaffirmed that the micromixing time achieved in the TC reactor is about three orders of magnitude lower than that of the conventional stirred tank reactor. The alteration of the configuration of the inner cylinder can be seen as an effective method to intensify the process of particle preparation.
(4) As the features of Taylor vortices in the TC reactor have been used for particle preparation in this work, the interactions between the vortices embedded with turbulent eddies and the particles were investigated based on tracking the barium sulfate particle trajectories, as discussed in Chapter 5. The simulation reveals that particle motion exhibits a helical movement, entrapped by Taylor vortices. The effective particle diffusion coefficient was introduced in this chapter, which is enhanced by increasing the inner cylinder rotational speed, especially for the LTC, implying that the deformation of Taylor vortices in the gap region of the LTC may significantly affect the entrainment of the particles by such vortices and embedded turbulent eddies. Moreover, particle radial distribution may provide a guidance for particle classification due to the axial velocity gradient, while axial dispersion can be seen as an indicator to characterise the global mixing, which is found to distribute similar to the shape of particle size distribution, indicating the existence of a strong correlation between particle property and particle dispersion due to the turbulence eddies induced shear in the TC reactor. The results from particle tracking simulation are consistent with the previous studies on the hydrodynamics of the LTC that the use of LTC can intensify the process for particle preparation.
(5) The fundamental study of barium sulfate particle preparation in this PhD project indicates that turbulence eddy induced shear and micromixing occurring in the TC reactor will be beneficial to the realisation of controllable synthesis of fine particle materials. Thus, as an extension of the application, the LTC reactor was also employed for the other reactive system, the co-preparation of Ni0.6Co0.2Mn0.2(OH)2 (NCM622) particles. Both CFD simulation and experimental results clearly show that the synthesized NCM622 particle properties have been improved in the LTC even with a reduced production time of 8 hours, compared to the conventional production method. Reactant mixing was assessed and characterized by two variables using CFD modelling with user-defined scalar (UDS). CFD modelling results show that the effective mixing at both the macro-scale and micro-scale can be quickly achieved in the LTC.
To sum up, this PhD work provides physical insight into the principle of controllable synthesis of fine particle materials. The implication is that the mixing in the TC reactor can create effective supersaturation, thereby inducing reactive precipitation under the effect of hydrodynamic heterogeneity. Such well-established mixing/micromixing environment can be quickly obtained in the TC reactors, especially in the LTC. Meanwhile, the shear induced by various length scale turbulent eddies embedded in the Taylor vortices has a strong correlation with the synthesized particle characteristics. Focuses should be placed on determination of appropriate operation parameters in the actual particle synthesis process when using the TC reactor.
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