AEROSOL NANOPARTICLE PLUG FLOW REACTORS

Synthesis of Particles in an APFR

      There is considerable interest in the synthesis and use of nanosized particles for a variety of applications including superalloys and thick film conductors for the electronics industry (Kear, 1986). Furthermore, other areas of interest include measurements of magnetic susceptibility, far-infrared transmission and nuclear magnetic resonance (Granqvist and Buhrman, 1976). For these systems, it is necessary to produce fine particles of controlled size. Particle sizes can typically be in the range from 10 to 500nm.

      Owing to their size, shape, and high specific surface area these particles can also be used in pigments in cosmetics, membranes, photo catalytic reactors, catalysts and ceramic and catalytic reactors. Examples of uses of nanoparticles include SnO2 for carbon monoxide gas sensors, TiO2 for fiber optics, SiO2 for fumed silica and optical fibers, C for carbon black fillers in tires Fe for recording materials, Ni for batteries and to a lesser extent Pd, Mg, Bi and others; all these materials have been synthesized in aerosol reactors. In the bioarea, nanoparticles are used to prevent and treat wound infections in artificial bone implants, and for use in imaging the brain.

       We will use the production of aluminum particles as an example of an APFR operation, however, the analysis to other systems is analogous. A stream of argon gas saturated with Al vapor is cooled in a Aerosol Plug Flow Reactor (APFR) (Panda and Pratsinis, 1995), with a diameter of 18 mm and a length of 0.5 m, from 1600°C at a rate of 1000°C/sec. As the gas stream flows through the reactor, the nucleation and growth of Al particles take place. Flow rate of the carrier gas is 2 dm3(STP)/min and the pressure inside the PFR is 1 atm (1.013 Pa). Moving with the gas velocity U, the cooling rate inside the reactor is 1000 K/s and hence the temperature profile down the reactor is given by:

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Figure 1. Aerosol reactor and temperature profile

As we move down the reactor the gas is cooled and becomes supersaturated. Thus supersaturation leads to the nucleation of particles. This nucleation is a result of molecules colliding, escaping (evaporating) and agglomerating until a critical nucleus size is reached and a particle is formed. As these particles move down the supersaturated gas molecules condense on the particles causing them to grow in size.

  200 nm [scale for all frames]

Figure 2. Nucleation and evolution of particle growth in an APFR

Photos courtesy of Prof. Sotiris E. Pratsinis, Swiss Federal Institute of Technology (ETH Zuerich), Institute of Process Engineering (IVUK), CH-8092 Zurich, Switzerland. (Courtesy of O. I. Arabi-Katbi, S. E. Pratsinis, P. W. Morrison, Jr., and C. M. Megaridis, Combustion and Flame, 124:560-572 (2001).)

The growth of titania particles generated by oxidation of titanium isopropoxide in a premixed flame aerosol reactor collected at different positions (3-200 mm) down the reactor.

Frame (3mm) in Figure 2 shows the smaller particles near the reactor entrance and subsequent frames show the increase in particle size as they move down the reactor. This growth is also shown schematically in Figure 1. Particles obtained in the exit stream in a reactor at the University of Michigan are shown in Figure 3.

Figure 3. Nano particles formed in an aerosol reactor

Photos courtesy of Professor Richard M. Laine, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 and J. Am. Cerm. Soc. 84 p591 (2001).

In the development that follows we will model the formation and growth of aluminum nanoparticles in an AFPR. However, the algorithm applies to other reacting systems such as flame spray pyrolosis, (FPS) where the monomers are formed, then nucleate to form particles which simultaneously grow and flocculate.