Reactive Precipitation in Microchannels: Impact of Convective Mixing on Particle Formation

Reactive precipitation in micro- and minichannels currently draws attention of both, chemists and engineers in the field of micro process engineering. Due to intensified mixing and improved heat and mass transfer, fast chemical and thermodynamical processes involved in precipitation can be controlled readily in micro or mini structures. Particularly microchannels are a promising technology for particulate processes allowing continuous operation along with little or no backmixing. However, the sensitivity of microscale channels to blocking and fouling requires careful design and appropriate peripheral equipment. This study presents experimental results of barium sulfate precipitation from barium chloride solution and sulfuric acid in both, T-shaped and injection micromixers. The measured particle size distributions (PSD) are characterized by their first and second moment, reflecting the correlation between fluidic mixing and precipitation: faster mixing results in smaller mean particle sizes (1^st moment of the PSD). The homogeneity of the mixing process on the other hand should have impact onto the width of the distribution (2^nd moment of the PSD) due to superposition of locally formed particles to the global size distribution. The experimental particle size distributions are compared with simulations based on reduced-order modeling of the diffusive mixing process, coupled with the population balance for particle nucleation and growth. While the simulated size distributions have mean diameters between 40 nm and 68 nm, experimental data are between 90 nm and 130 nm.