Continuous production for nanoparticles

Cat Pumps model 5CP pump skid installed at Promethean Particles’ pilot-scale reactor.
Cat Pumps model 5CP pump skid installed at Promethean Particles’ pilot-scale reactor.

Promethean Particles, a spin-out company from the University of Nottingham (UK), produces nanoparticles using continuous hydrothermal technology and is in the process of developing a large reactor that incorporates four Cat Pumps Model 3801 Stainless Steel pumps in order to scale-up production to commercial levels.

In hydrothermal synthesis the hot, pressurised water is mixed with a metal salt solution, such as iron nitrate, and a reaction occurs (the salt is dehydrated) and nanoparticles are formed. 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. It is the high pressure positive displacement triplex plunger pumps that create the continuous pressure and fluid flow that is a key element in continuous production.

Promethean Particles is participating in the EU FP7 funded SHYMAN project (Sustainable Hydrothermal Manufacturing of Nanomaterials). This project is based on Promethean’s proprietary technology and is worth €10 million. The aim of the project is to scale up nanomaterial production whilst maintaining a sustainable, low cost process for producing high quality materials.

Supercritical water The University of Nottingham started research into optimising supercritical water hydrothermal synthesis (scWHS) for nanoparticles about 14 years ago. The result of the University’s work was the development of Promethean Particles’ Nozzle Reactor, a customised design that uses the buoyancy induced eddies to produce an 'ideal' mixing scenario in a pipe-in-pip 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. The reactor has shown 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.

Research and development commenced with bench scale reactors and later a pilot scale reactor approximately 30 times larger was developed. The reactor now being built is 80 times larger than the pilot plant reactor. The four Cat Pumps Model 3801 high pressure triplex plunger pumps will be used for pumping deionised water through a heater that raises the temperature to the super-critical condition of 374 °C at 218 atmospheres.

First consideration

Triplex PD plunger pumps are typically the chemical engineer’s first consideration when high pressure and high operating efficiency is required. Triplex PD pumps actually increase in efficiency even further at higher pressures and are most widely used in applications where the pumped liquid is of low to medium viscosity and has little inherent lubricity.

There are two significant advantages of reciprocating positive displacement pumps to be considered. Firstly, as they are almost 100% positive they will always deliver a predictable volumetric flow irrespective of pressure. This is because on the inlet stroke the pump takes in a fixed volume of liquid and physically moves it through and out of the pump with virtually no losses or inefficiencies. Secondly, the pump performance is independent of specific gravity so the input power and output flow are unaffected by the liquid density.

Brian Hubbard, general manager of Cat Pumps UK explains: “We worked with Promethean from the early stages of the scale-up project, helping the scientists to develop a pump specification and then offering the most elegant solution both technically and commercially. We recommended our Model 3801 as it offers the required combination of flow, pressure, materials compatibility and features, including the facility to flush the pump seals which future-proofs the installation. By pairing up two pumps for the highest capacity duty, and two more solo pumps for the mid-flows, we’ve also given Promethean a high level of commonality and standardisation.”

Operating efficiency With the pilot reactor clearly showing that scaling to a much larger commercial reactor was viable, some major decisions had to be made in terms of the pumping capability for the de-ionised water feed. “From our experience with the bench and pilot reactors, we knew that positive displacement pumps were the only viable solution for scaling up the process to commercial viability,” said Lewis Neve, lead chemical engineer on the SHYMAN project.

“Originally the pilot plant used two single head piston-activated diaphragm pumps and these displayed marked signs of pulsation. A further issue was that they were at the edge of their capability range in terms of pressure and flow, so they would not be suitable for the much larger reactor.”

Following a thorough review of pump technologies available it was accepted that the high pressure triplex plunger pump would be the most efficient and cost-effective for reactor up-scaling. The high-pressure triplex plunger pump provided that all-important additional pressure and required flow rate, so Promethean Particles had all the margins that were needed for the planned production output goal.

 “We chose Cat Pumps because they are a well-established international manufacturer and could provide details of similar projects where their pumps had run for many years and this gave us confidence,” continues Lewis.

“From the start they were hands-on and came to us on a number of occasions to discuss the technical details and what they could bring to the process and the design of the plant. In terms of design and production output, the pumps were a huge item for us, so we were very impressed that they could do everything that we wanted and even more.”