Electrostatic precipitators are commonly employed particulate control devices for collecting aerosols from utility boilers, incinerators and many other industrial processes [1] and also for collecting nanoparticles from aerosol or plasma reactors [2, 3]. The greatest advantage provided by an electrostatic precipitator is that the electrostatic force of highly charged particles under the influence of an external electrostatic field is usually very large, as compared to gravitational, thermal and inertial forces. It has been shown experimentally that the collection efficiency for coarse particles (larger than 100 nm) is usually larger than 99% but relatively small for sub-micrometer particles. A fundamental understanding, with the aid of numerical simulation, will shed light onto the behavior of this process and help to improve its design and operation.

In this work, the collection efficiency of particles in the nanosize range (5 - 100 nm) in a two stage parallel plate electrostatic precipitator is studied by numerical simulation based on a fundamental model of the process [4]. Specifically, the particle charging process is based on Fuchs' theory. For the collecting stage, the model employs Eulerian approach for the solid-gas flow and explicitly accounts for Brownian and eddy diffusion, turbulent flow and electrostatic migration. Calculation results indicate that particles in the nanosize range are not uniformly charged. Ultrafine particles with diameter less than 20 nm seldom acquire more than one unit of elementary charge. Larger particles (20 - 100 nm) may carry several units of charge, depending on the product of ion concentration and charging time. The simulation results also indicate that there is a local maximum in the collection efficiency in the nanosize range; a finding which is consistent with experimental observations reported in the literature. The simulation also points out that the most efficient way to increase the collection efficiency of particles in the ultrafine size range is to enhance the charging process. For particles with larger size, both the parameters in the charging stage (the product of ion concentration and charging time) and those in the collecting stage (electrostatic intensity, and length and width of the collecting cell) have an important effect on the overall collection efficiency.

References:

[1] Friedlander, S. K. Smoke, Dust and Haze: Fundamentals of Aerosol Dynamics; Oxford University Press: New York, USA, 2 ed.; 2000.

[2] Kruis, F. E.; Goossens, A.; Fissan, H. Synthesis of semiconducting nanoparticles. J. Aeros. Sci. 1996, 27, S165–S166.

[3] Karthikeyan, J.; Berndt, C. C.; Tikkanen, J.; Reddy, S.; Herman, H. Plasma spray synthesis of nanomaterial powders and deposits. Mater. Sci. Eng. A 1997, 238, 275–286.

[4] Li, M.; Christofides, P. D. Collection efficiency of nanosize particles in a two-stage electrostatic precipitator Ind. Eng. Chem. Res. in press.