Acoustic agglomerator

In addition to the deposition mechanisms themselves, methods for preliminary conditioning of aerosols may be used to increase the effectiveness of the deposition mechanisms subsequently apphed. One such conditioning method consists of imposing on the gas nigh-intensity acoustic vibrations to cause collisions and flocculation of the aerosol particles, producing large particles that can be separated by simple inertial devices such as cyclones. This process, termed sonic (or acoustic) agglomeration, has attained only hmited commercial acceptance. 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

Acoustic Agglomeration of Power Plant Fly Ash for Environmental and Hot Gas Cleanup... 

The Fundamentals of Acoustic Agglomeration of Small Particulates. Let us consider a polydisperse aerosol consisting of submicrometer and micron sized particles. The mean separation distance between particles would typically be about 100 micrometers. Brownian movement of the particles is caused by the collision of the thermally agitated air molecules with the particles. Also any convection currents or turbulence in the carrier gas will of course cause the particles to be partially entrained and moved in the air. If we next impose an acoustic field of acoustic pressure p, the acoustic velocity u will be given by... 

Figure 8. Final Setup of Moderate Temperature Acoustic Agglomeration Facility.

Conclusions. Our theoretical and experimental work has shown conclusively that acoustic agglomeration does result in shifting the particle size distribution from submicrometer sizes into the 10 micrometers and above size range. In order to achieve the desired 150 to 160 dB, specific acoustic powers of from 0.1 to 1 watt/cm2 are required for plane wave propagation and less than that for standing wave chambers. 

We are continuing our research with U.S. Department of Energy support to further improve our understanding of several important aspects of acoustic agglomeration such as acoustic energy absorption by hot gases and large particle concentrations,... 

Tiwary, R., and Reethof, G., "Acoustically Generated Turbulence and Its Effect on Acoustic Agglomeration , J. 

Reethof, G., "Acoustic Agglomeration of Power Plant Fly Ash , Final Report to Pittsburgh Energy Technology Center, U.S. Department of Energy, Contract DEHIC22 83 PC 602070, December 1985. 

Tiwary, R., "Acoustic Agglomeration of Micron and Submicron Particles , Ph.D. Thesis, The Pennsylvania State University, May 1985. 

George, W. and Reethof, G., "On the Fragility of Acoustically Agglomerated Submicron Fly Ash Particles , accepted for publication in the ASME Journal for Vibration, Acoustics, Stress and Reliability in Design, May 1985. 

Acoustic agglomeration—Continued results for fly ash aerosol, 250f small particulates,... 

Sonic agglomeration. Particle agglomeration can occur in ordered flow such as that which can be established in a sonic field. Here agglomeration takes place by the different velocities imparted to particles of differing inertia, by aerodynamic attractive forces between the particles, and by radiation pressure which moves the particles toward the vibration antinodes. No complete and adequate theory for acoustic agglomeration exists yet. 

In gas suspensions, where very fine particles have to be removed, US action involves agglomeration of particles in order to increase their size and, consequently, to improve the collection efficiency of conventional filters (e.g. electrostatic precipitators, cyclone separators). These filters, while effective for large particle separation, are inefficient for retaining particles smaller than 2.5 pm. Therefore, acoustic agglomeration provides a means for separating fine particles released from industrial, domestic or vehicle sources, which, analytically, constitutes an excellent method for sampling in environmental analysis. 

Acoustic agglomeration is a process in which acoustic forces cause particles to interact and, eventually, to collide. The complex mechanisms behind this process involve orthoki-netic and hydrodynamic interactions. The orthokinetic interaction is founded on the hypothesis that collisions are produced due to the different acoustic entrainments experienced by particles of different size and weight. In order to describe this mechanism, an agglomeration volume is defined around each particle as a volume where another particle can be captured [49], However, this mechanism, which constitutes the basis for most existing interaction models, can explain neither the agglomeration of monodispersed aerosols nor the way in which the agglomeration volume is refilled once the initial particles are captured. 

Acoustic agglomeration is seemingly much more efficient for large particles than for small ones, espeoially in solid-gas systems. In fact it is difficult to aohieve a high agglomeration... 

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