(33c) Design and Scale-up of Chemical Reactors for Nanoparticle Precipitation

The main purpose of this work is to develop a methodology based on both experimental data and computational fluid dynamics (CFD) simulations for design, optimization, and scale-up of reactors suitable for producing solid particles with specific characteristics. In particular, we focus on the role of reactor size and fluid dynamics on particle size distribution (PSD), morphology, composition, and crystallinity during precipitation processes.

Precipitation is the result of several mechanisms, namely, nucleation, molecular growth, and secondary processes, such as aggregation (or agglomeration) and breakage, and the driving force is super-saturation. Nucleation is the formation of the solid phase and occurs when a critical number of molecules join together to form an embryo. Stable embryos form nuclei that grow into bigger particles through molecular growth. Nucleation and growth are competing phenomena since both consume solute molecules and, therefore, particle size is the result of this competition. Very small particles are produced by high nucleation rates whereas, on the contrary, big particles are produced by low nucleation rates.

Aggregation also affects the PSD and, in fact, only if it is avoided can very small particles be produced. Usually, nanoparticle aggregation is driven by Brownian motions; in fact, nanoparticles are less sensitive to turbulent fluctuations because of their very small size. Moreover, particles interact through a combination of van der Waals attraction and electrostatic and steric repulsion forces; and it is, therefore, clear that in order to prevent aggregation one must influence this balance.

Precipitation typically consists of (turbulent) mixing of two liquid streams and, therefore, mixing affects generation of super-saturation, influences its redistribution, and causes transport of previously formed particles in regions of high supersaturation. As already reported, small particles are produced by high nucleation rates, which can be obtained if high mixing rates are realized.

The aim of this work is to investigate nanoparticle precipitation in confined impinging jets reactors (CIJR) which consist of two high velocity linear jets of fluid that collide to rapidly reduce their scale of segregation within a small volume.

Several test reactions (i.e., precipitation of inorganic substances, hydrolysis and condensation for sol-gel processes, solvent displacement for polymer precipitation) have been considered in this work and several CIJRs with similar geometrical parameters but different sizes have been studied. Reactor performances are tested under different operating conditions (i.e., jet velocities and reactant concentrations). Standard scale-up criteria based on the evaluation of the Damkhöler number are used to interpret experimental data. The Damkhöler number is calculated here as the ratio between the mixing and the precipitation times calculated in turn with the CFD model and with a simplified precipitation model.

For example, the mean particle size at the reactor outlet obtained for barium sulphate precipitation working with reactors of different size and under different jet velocities and reactant concentrations, is reported versus the Damköhler number in Figure 1. As can be seen, experiments referring to different runs lay on the same curve, proving that this approach is capable of correlating experimental data from very different operating conditions.

From Fig. 1 it is also possible to observe that when mixing is much slower than the chemical reaction (Da>>1) particles are rather big whereas only when mixing is faster than the chemical reaction (Da