Environmental Engineering Reference
In-Depth Information
These processes may occur simultaneously. The flame reactor is the most widely
used reactor in the fabrication of inorganic particles by the aerosol method. In the
reaction chamber, the aerosol precursor and oxygen are mixed with each other and then
burned; inert gases and fuels such as hydrogen or methane may also be present. The
reaction stoichiometry for a liquid phase reaction can be described by the following
equations.
2H
2
+ O
2
2H
2
O
(Eq. 2.8)
SiCl
4
+ 2H
2
O SiO
2
+ 4HCl
(Eq. 2.9)
Since the reaction occurs with water vapor, the process is referred to as flame
hydrolysis. The gas coming out of the furnace contains silica particles, gaseous
hydrochloric acid, hydrogen, and a small amount of chlorine. The agglomerates are
collected in cyclone separators, which may be followed by a bag filter. This flame
process can be used to fabricate different products such as alumina and titania,
commercial products produced from SiCl
4
and TiCl
4
. For the gas phase reaction, the
overall reactions are as follows (Pratsinis, 1998).
TiCl
4
+ O
2
TiO
2
+2Cl
2
(Eq. 2.10)
SiCl
4
+ O
2
SiO
2
+2Cl
2
(Eq. 2.11)
The size of the primary particles depends on the temperature, resident time, and
material properties. At high temperatures, the individual particles grow by particle
coalescence, which typically occurs by contact. Therefore, the synthesis of large
individual particles commonly results in agglomerates. However, at low temperatures,
particle coalescence takes place slowly due to the decrease on the number of collisions,
producing fractal-like agglomerates with a high specific surface area. The other key
material property controlling particle size is the solid state diffusion coefficient, D (as D
increases the particle size tends also to increase), residence time, temperature history and
volumetric loading of solids. Since solid state diffusion is an activated process, there is a
strong temperature effect on particle formation in aerosol reactors.
2.2.6 Sonochemical Method
The sonochemical method has been used extensively to generate novel materials
such as metals, transition metals, and semiconductors (Ramesh et al., 1997; Gedanken,
2004; Okitsu et al., 2005). Sonochemical methods apply sonic and ultrasonic waves to
chemical processing; through the sonochemical method, molecules undergo a chemical
reaction due to the powerful ultrasound radiation (20 kHz-10 MHz) (Suslick et al.,
1991). Sonochemistry enhances or promotes chemical reactions and mass transfer,
thereby enabling shorter reaction cycles, cheaper reagents, and the use of less extreme
Search WWH ::
Custom Search