Design of Aerosol Coating Reactors by CFD

Core-shell particles facilitate incorporation of functional particles into host (e.g. liquid or polymer) matrices like silica-coated TiO₂ pigments¹, carbon coated Cu for sensors² or superparamagnetic³ Fe₂O₃. Typically core-shell particles are made in the liquid phase⁴ but there is keen interest to develop gas-phase or aerosol coating processes that do not generate liquid by-products, offer fewer process steps, easier particle collection and hermetic³ shells. Coating of particles in the gas phase, however, is challenging, as particle motion and growth are much faster than in liquids. As a result, it is difficult to control and develop a scalable gas phase coating process. So, even commercially produced particles made by aerosol routes (e.g. pigmentary TiO₂ made by the “chloride” process) are coated by wet processes⁴.

Here⁵, gas-phase (aerosol) coating is elucidated in considerable detail, for the first time to our knowledge, by computational fluid and particle dynamics for core particles (TiO₂) and coating shells (SiO₂). Emphasis is placed on understanding the influence of process variables (coating weight fraction and mixing intensity (Figure 1) and geometry of core aerosol & shell precursor vapor) on core-shell product characteristics by a trimodal aerosol particle dynamics model⁶ accounting for SiO₂ monomer generation, coagulation and sintering. The predicted extent of complete (or hermetic) coating shells is compared to the measured photocatalytic oxidation of isopropanol by such particles^7,8 and release of acetone. As hermetic SiO₂ shells prevent the photocatalytic activity of TiO₂, the performance of coated particles is explained by the spatial distribution of shell thickness on core particles with detailed reactor flow field analysis.

Financial support from the Swiss National Science Foundation (SNF) grant # 200021-119946/1 and European Research Council is gratefully acknowledged.

Figure 1 Influence of nitrogen flow rate (mixing intensity) a) 5.8 l/min, b) 15.8 l/min and c) 30.8 l/min on the coating precursor and coating shell thickness distribution inside the aerosol coating reactor.

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