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Agglomeration and collection of fine secondary phases in flowing suspensions utilizing resonant ultrasonic fields
| Title: | Agglomeration and collection of fine secondary phases in flowing suspensions utilizing resonant ultrasonic fields |
| Author: | Tolt, Thomas Lester |
| Description: | A novel separation process based on a forced coincident excitation at ultrasonic frequencies in fluid filled tubes has been studied and developed for the agglomeration and collection of fine secondary phases in flowing suspensions. Under appropriate conditions, acoustic radiation forces can act to drive second phase inclusions to the nodes (or antinodes) of a stationary sound field and trap them there with a strength which can exceed the hydrodynamic drag exerted on the particles by a flowing fluid matrix. A review of the theoretical development of the acoustic forces acting on and between inclusions in a sound field is presented. The fields generated in a water bath using a lead zirconate-lead titanate piezoelectric disk acting a source of ultrasonic waves a short distance in front of a parallel reflector and in later experiments utilizing simple cylindrical chambers are compared to calculated stationary fields in Fabry-Perot laser resonators with parallel, plane finite mirrors. It is suggested that diffraction at the transducer and reflector surfaces is responsible for the creation of guided standing waves which have a nonuniform transverse distribution of acoustic velocity and pressure along nodal planes of minimum pressure spaced at intervals equal to one half of the acous tic wavelength in the fluid. This leads to the existence of transverse components in the acoustic radiation force in addition to an axially directed force which act to collect fine secondary phases into thin lens-shaped zones. Broadband piezoelectric transducers bonded to the ends of fluid filled glass and aluminum tubes (up to 4 cm in diameter and 25 cm in length, and ranging from 0.9 to 2 mm in wall thickness) were driven with continuous a.c. voltages at unique frequencies in the range of 0.35 to 1.4 Mhz to obtain a forced coincidence response in which the flexural vibrations of the tube wall are matched in phase and geometry with higher-order acoustic duct modes in the liquid. This excitation produced stationary fields with peak acoustic pressures on the order of 1 MPa in water which were almost uniformly strong throughout the enclosed volume of the cell. Particles ranging in size from 0.1 to 100 μm in aqueous suspensions have been collected into half wavelength spaced nodal zones in times of less than one second in cells driven with an applied electrical power on the order of 10 W. It was found that the zones could be swept to either end of the cell and removed through small ports without liquid flow by repeatedly sweeping the driving frequency over a known range and period without destroying the strong fields associated with the coincidence response. The operation of these cells is described in a variety of separation experiments including the removal of 9 μm inorganic solid particles from an aqueous electrolyte with efficiencies ranging from 80 to 98% at a continuous flow rate of the mixture on the order of 5 ml per minute. |
| Permanent Link: |
http://rave.ohiolink.edu/etdc/view?acc_num=case1055181898
http://hdl.handle.net/2374.OX/16941 |
| Date: | 2003 |
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