Supercritical water is simply the scientific term for hot, pressurised water and it has different properties to those you would normally expect of water, one of which is to allow the synthesis of nanoparticles. In hydrothermal synthesis the hot, pressurised water is mixed with a metal salt solution, such as iron nitrate solution, and a reaction occurs (the salt is dehydrated) and nanoparticles form.
Supercritical water hydrothermal synthesis (scWHS) is a relatively simple and environmentally friendly process for the production of potentially valuable nanoparticles. Prior to the developments at Promethean Particles, it had never found industrial application to date due to poor process reliability, reproducibility and control. This synthetic approach has been known for many years but only as a one-pot, batch process, which is low yielding, inherently difficult to scale-up and which has product reproducibility issues; making the process continuous would solve all these problems.
The step from batch to continuous hydrothermal synthesis had been held back by engineering issues around mixing the heated fluid and the aqueous metal salt flow. T piece reactors were originally used but these are now generally accepted as problematic because the reactors frequently block, discouraging the commercial use of this technique to produce nanoparticles, until now. Promethean’s patented reactor design shows a dramatic improvement in process reproducibility and reliability as well as providing the ability to control particle properties, such as size, composition and shape, through the manipulation of process variables.
Continuous hydrothermal synthesis produces nanoparticulate materials by mixing superheated/supercritical water with a solution of a metal salt. i.e. rather than slowly heating the entire contents of a batch vessel, two fluids are continuously mixed together.
When water is heated towards its critical point (374°C, 218 atm.), the ionic product [H+][OH-] increases and the superheated fluid is technically supercritical, rather than near-critical.
The enhanced levels of OH- were first exploited for continuous nanoparticle production by Adschiri and Arai, in 1992, who showed that under these conditions hydrolysis of metal salts (MLx) is immediately followed by a dehydration step.
Hydrolysis: M Lx + xOH– → M(OH)x + xL–
Dehydration: M(OH)x → MOx/2 + x/2 H2O
Our Reactor Design.
The Nozzle Reactor is a customised design that uses the bouyancy induced eddies to produce an ‘ideal’ mixing scenario. The figure above illustrates our patented reactor; it is a pipe-in-pipe concentric configuration in which the internal pipe has an open-ended nozzle. The supercritical water is fed downwards through the internal pipe and out the end of the ‘Nozzle’; the aqueous metal salt stream is fed counter-currently upwards through the outer pipe.
Simulations have been employed to model the flow regime, mixing environment and conditions of the scWHS reactor, as shown in the figure below. The steady state concentration map (figure below) illustrates that as the two reactant fluids are introduced, the mixing is instantaneous and strong.
As the particles are produced as an aqueous dispersion we avoid the problems of particle agglomeration and can formulate the particles easily.
As the particles flow out of the reactor they are in dispersion and are never handled as dry powders, thus avoiding agglomeration. This is a major advantage over dry powder techniques where the particles clump together, defeating the object of using nanoparticles, and requiring further processing such as milling.
Formulation, for example adding a surfactant or transfer to another solvent system, is widely needed for nanoparticle applications. As we operate a continuous flow process formulation agents can be added online, with customer-ready material exiting the reactor in a one-step process.
Promethean Particles have already demonstrated the scale-up potential of the technology with the development of a pilot scale reactor system. Furthermore, Promethean have recently demonstrated the capability of their pilot scale reactor to manufacture nanomaterials continuously for 60 hours to highlight the commercialisation and potential for industrial scale up of their technology.
SHYMAN is a €10M EU FP7 project which is based on the core Promethean’s proprietary technology and aims to scale up the system used by Promethean Particles by building a plant that will increase Promethean’s manufacturing capability from its current scale of 1-10 tons per year to over 1000 tonnes per year. For more information on the project, please visit the consortium website: www.shyman.eu
Read more about the process system.
Reaction engineering: The supercritical water hydrothermal synthesis of nano-particles The Journal of Supercritical Fluids, Volume 37, Issue 2, April 2006, Pages 209-214 Edward Lester, Paul Blood, Joanne Denyer, Donald Giddings, Barry Azzopardi, Martyn Poliakoff
A versatile flow visualisation technique for quantifying mixing in a binary system: application to continuous supercritical water hydrothermal synthesis (SWHS) Chemical Engineering Science, Volume 59, Issue 14, July 2004, Pages 2853-2861
Paul J Blood, Joanne P Denyer, Barry J Azzopardi, Martyn Poliakoff, Edward Lester
Abbott, S. and Holmes, N. (2013). Nanocoatings: Principles and Practice – From Research to Production. Pennsylvania, USA: DEStech Publications. p62-63.