Impact of Mixing for the Production of CuO Nanoparticles in Supercritical Hydrothermal Synthesis

Abstract

The mixing process of metal salt solutions and supercritical water is essential to supercritical water hydrothermal synthesis (SWHS) to produce nanoparticles. A computational fluid dynamics (CFD) model was developed for predicting mixing efficiency and crystallization kinetics in SWHS mixers. The mixing efficiency was calculated from the micromixing model, and the kinetics model of crystallization was built from the population balance equation. The effects of the fluid dynamics of mixing (Reynolds number and mixer diameter) on the mixing efficiency and crystallization kinetics (nucleation rate and growth rate) were investigated. The results showed that faster mixing can lead to a higher crystallization rate and the production of smaller particles with a narrower particle size distribution. INTRODUCTIONThe supercritical water hydrothermal synthesis (SWHS) method is widely used to produce metal oxide nanoparticles from metal salt aqueous solutions using high-temperature and -pressure water as the reaction medium. The extremely low dielectric constant of supercritical water can lead to a high hydrothermal synthesis reaction rate and a low solubility of metal oxides, which help to produce fine particles. 1 Compared with single atoms and bulk materials, nanomaterials have high application value because of their special size-related properties. For example, they can be used in transducers, batteries, and ceramics and as efficient photocatalysts. The SWHS method has the advantages of fast reaction, high product purity, and good environmental compatibility. Furthermore, the morphological structure and particle size of the synthesized particles can be controlled by the processing parameters such as reaction temperature, pressure, precursor concentration, and flow rate. 2−5 In the SWHS reaction, proper design of the mixer is a key factor in obtaining products with decreased particle sizes and narrow particle size distributions (PSDs). 2,6 The usual process of continuous SWHS is that metal salt solution mixes directly with supercritical water and is rapidly heated to the supercritical state. The process has advantages including high heating rate, short residence time, and easy industrial scaleup. 7,8 However, in the continuous SWHS reaction system, particle plugging and hard-to-control particle size are frequently encountered problems. The source of the problems is unreasonable design of the mixer, resulting in slow and nonuniform mixing of the metal salt solution and supercritical water. To date, various types of mixers have been developed in the study of the SWHS method for the preparation of nanoparticles. In addition to the traditional tee mixer, some new mixers have also been reported, such as Y-type, 9 nozzle-type, 10 central collisiontype, 11 and swirl-type 12 mixers. From an engineering perspective, to optimize the design of the mixer, it is necessary to acknowledge the fluid dynamics inside the mixer and the crystallization kinetics on the mixing time scale of tens of milliseconds. 13

Simulation, design and proof-of-concept of a two-stage continuous hydrothermal flow synthesis reactor for synthesis of functionalized nano-sized inorganic composite materials

“…The flow field, energy transfer and species mixing were calculated using the FLUENT module of ANSYS 14.5. The methodology was adapted from previous reports in the literature [20][21][22][23][24]. The standard k-ε-model for turbulent flow with the standard model constants implemented into the FLUENT module was used employing standard wall functions and the standard SIMPLE pressure-velocity coupling for a steady-state solution.…”

Underpinning the Interaction between NO2 and CuO Nanoplatelets at Room Temperature by Tailoring Synthesis Reaction Base and Time

“…6 More recently, the focus has shifted to p-type semiconducting and MOX nanocomposite gas sensors. 7 Compared to n-type materials, some p-type gas-sensitive materials have been reported to have significant surface reactivity to oxidizing and reducing gases at lower operating temperatures, which might lead to future advances in low energy consumption gas-sensing devices. 7a,7b,8 Cupric oxide (CuO) has attracted a great deal of attention over the last decade because of its wide range of potential applications, 9 including gas sensors.…”

Towards scalable and controlled synthesis of metal–organic framework materials using continuous flow reactors

“…One stream (generally pure solvent) is passed through a pre-heating zone where it is heated to the desired reaction temperature and this superheated stream is then brought into contact with the precursor stream(s) at a mixing point inducing very rapid reactions. The geometry of this mixing point has been shown to have important implications for product quality and operational logistics, 26,27 and a number of systems may also employ a post-mixing isothermal reaction zone to promote crystallisation and growth. After reaction the product stream passes through a heat exchanger before exiting the system via the back pressure regulator which maintains the system under the required pressures.…”