Impact of reactor geometry on continuous hydrothermal synthesis mixing

Continuous hydrothermal synthesis of metal oxide nanoparticles as a continuous process has shown considerable potential for application within the speciality material industry. Many research groups have successfully produced a large variety of nanoparticles using this relatively simple and 'green' process, but currently, it has struggled to achieve any application at industrial level. The process exhibits several drawbacks, the root problem being particle accumulation within the process equipment resulting in an inability to operate at a steady state. Poor control over the mixing process results in poor process reliability and therefore poor product reproducibility. This paper shows how different reactor geometries create different mixing regimes, which could potentially create a sustainable system for continuous hydrothermal synthesis. From over 74 reactor configurations, three specific phenomena were observed, which would lead to problems during nanoparticle formation, namely, 'fluid partitioning', where the superheated water does not mix with the aqueous metal salt flow, 'back mixing', where one of the fluids moves countercurrent down the inlet of the other fluid, and 'stagnation', where steady state conditions produce an interface with minimal mixing occurring.