(418k) Multiscale Design of Aerosol Coating Reactors

Multiscale Design of Aerosol Coating Reactors

B. Buesser and S.E. Pratsinis

Particle Technology Laboratory, Institute of Process Engineering,

Department of Mechanical and Process Engineering, ETH
Zürich, 8092 Zürich, Switzerland

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
sensors or superparamagnetic² Fe₂O₃.
Typically core-shell particles are made in the liquid phase³ 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
hermetic² 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 processes³.

Here⁴, 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 model⁵ 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^6,7 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.

1.         Subramanian
NS, Diemer RB, Gai PL; E. I. du Pont de Nemours and Company (Wilmington, DE,
US); Process for making durable rutile titanium dioxide pigment by vapor phase
deposition of surface treatment. US patent 200627303(A1). 2006.

2.         Teleki A, Suter M, Kidambi PR,
Ergeneman O, Krumeich F, Nelson BJ, Pratsinis SE. Hermetically coated
superparamagnetic Fe₂O₃ particles with SiO₂
nanofilms. Chem. Mater. 2009; 21, (10), 2094-2100.

3.         Egerton TA. The modification of
fine powders by inorganic coatings. KONA. 1998; 16, 46-59.

4.         Buesser B, Pratsinis SE. Design of
gas-phase synthesis of core-shell particles by computational fluid ? aerosol
cynamics. AIChE J. 2011; DOI: 10.1002/aic.12512,

5.         Buesser B, Pratsinis SE. Design of
Aerosol Particle Coating: Thickness, Texture and Efficiency. Chem. Eng. Sci.
2010; in Press, doi: 10.1016/j.ces.2010.07.011,

6.         Teleki A, Heine MC, Krumeich F,
Akhtar MK, Pratsinis SE. In-situ coating of flame-made TiO₂
particles by nanothin SiO₂ films. Langmuir. 2008; 24, (21),
12553-12558.

7.         Teleki A, Buesser B, Heine MC,
Krumeich F, Akhtar MK, Pratsinis SE. Role of gas-aerosol mixing during in situ
coating of flame-made titania particles. Ind. Eng. Chem. Res. 2009; 48,
(1), 85-92.