A Combined Experimental and Numerical Study on Ultrasound Effects on Particle Synthesis in Microreactors

Microreactors and ultrasound are important process intensification tools with great potential especially for multiphase processes. As an example, the synergistic effect of coupling low frequency ultrasound with flow reactors to achieve cavitation have been proven successful in clogging prevention in solid-liquid flows.

In addition to their susceptibility for clogging in the presence of solid particles, microreactors are also characterized by their poor convective mixing, and the laminar velocity profile results in a wide residence time distribution which will also affect the size distribution of particles synthesized in them. To address these drawbacks, we focus on the design, modeling and experimental validation of a microreactor coupled with high frequency ultrasound. While high frequency ultrasound will not induce cavitation in the bulk liquid, the radiation force will affect particles present in the flow and deflect their trajectories. Furthermore, it can also excite secondary flow structures (known as acoustic streaming) which will improve the mixing in the bulk fluid.

In this work, we have designed an ultrasonic microreactor using a high frequency of 1 MHz, which ensures that the ultrasound is established as a standing wave in each reactor layer. Furthermore, the standing wave in the fluidic channel forms a nodal plane in its center, and the resultant acoustic radiation force will deflect all particle trajectories towards this nodal plane and thus focus the particles. This focusing effect prevents the wall deposition of synthesized particles, and in addition each particle will experience similar residence times leading to a reduced particle size distribution. The focusing efficiency of this ultrasonic microreactor is characterized using inert particles seeded with the flow. In a second step a solid forming reaction (precipitation of barium sulfate) was performed. The effect of process parameters (e.g. sound intensity, channel size, flow rate, particle concentration) on the focus extent and the particle size distribution (measured by laser diffraction) was investigated.

To further optimize the reactor, we have also developed a modeling tool based on the coupled computational fluid dynamics (CFD) / discrete element method (DEM) approach. The DEM solver (LIGGGHTS) handles the equation of motion for each particle, while the CFD solver (OpenFOAM) solves the Navier-Stokes equations describing the fluid flow. The coupling software (CFDEM coupling) computes the momentum exchange between particles and fluid. In the DEM solver, we implemented a contact model based on the JKR model to accurately resolve particle-particle and particle-wall collisions on the micro-scale. The ultrasound effects are modeled via a continuum equation for the acoustic pressure in the suspension, which is updated as the particles move in the microfluidic domain and is used to calculate the radiation force on each particle.We will present the details of the model implementation and report the simulation results of the experiments with inert particles to identify optimum process parameters.

This work showcases how a combined experimental and numerical approach can lead to improved particle synthesis by coupling high frequency ultrasound with microreactors.