Reaction engineering: The supercritical water hydrothermal synthesis of nano-particles

Supercritical water hydrothermal synthesis (scWHS) is a relatively simple and environmentally friendly process for the production of potentially valuable metal oxide nanoparticles. However, it has never found industrial application to date due to poor process reliability, reproducibility and control. This paper presents the conclusions of collaborative work between chemical engineers and chemists that attempts to optimise the reaction engineering of this process, with the goal of reducing or even eliminating these fundamental process flaws. Initial investigations on the mixing in a T-piece highlighted that the environment within the scWHS reactor is highly unusual in terms of conventional reaction engineering because the fluid properties were significantly different in terms of temperature, viscosity and density. This led to the development of an optimised reactor, termed the Nozzle Reactor, which was designed on the basis of Light Adsorption Imaging (LAI) and Computational Fluid Dynamics (CFD) modeling, both of which show excellent mixing mechanics. Light Adsorption Imaging is an image analysis visualization method using fluids of different densities at ambient conditions. Experimental results using the Nozzle Reactor are presented showing how different metal oxide particles can be produced including titania, ceria, zirconia, copper, zinc and silver. The reactor shows a dramatic improvement in process reproducibility (±5 m2/g for BET surface area) and in reliability such that, given further investigation, will lead to process optimisation. Preliminary evidence suggests that the reactor could eventually lead to the ability to control particle properties, such as size, composition and shape, through the manipulation of process variables.