Development of novel micro swirl mixer for producing fine metal oxide nanoparticles by continuous supercritical hydrothermal method.

Novel micro swirl mixers were developed to synthesize nanoparticles, and the effect of their mixing performance on the characteristics of the synthesized nanoparticles was determined. The results were compared with those obtained using simple T-shaped mixers under the same reaction conditions. The synthesis of NiO, whose characteristics depend on the mixing performance of the mixer, was chosen as a model reaction. Initial investigations highlighted that the average particle size decreased from 32 to 23 to 20 nm as the inner diameter of the swirl mixers was decreased from 3.2 mm (Swirl mixer, SM-3.2) to 0.8 mm (Micro swirl mixer, MSM-0.8) to 0.5 mm (Micro swirl mixer, MSM-0.5), respectively. On the other hand, a similar decrease in the average particle size from 34 to 20 nm was observed with a decrease in the inner diameter of the T-shaped mixers from 1.3 mm (Tee union, T-1.3) to 0.3 mm (Micro tee union, T-0.3), respectively. Further, narrow particle size distributions were observed with a decrease in the inner diameter of each mixer. Furthermore, a computational fluid dynamics (CFD) simulation indicated an excellent mixing mechanism, which contributed to the improvement in the heating rate and the formation of nanoparticles of smaller size with a narrow particle size distribution. The result presented here indicates that the micro swirl mixers produce high-quality metal oxide nanoparticles. The size of the obtained particles with improved size distributions was comparable to that of the particles obtained using the T-shaped mixers, although the inner diameter of the swirl mixers was larger. Therefore, preliminary evidence suggests that the swirl flow mixers have the ability to produce rapid and homogeneous fluid mixing, thus controlling the particle size.