Liquid purification or separation – Processes – Utilizing electrical or wave energy directly applied to...
Patent
1990-07-03
1993-07-06
Dawson, Robert A.
Liquid purification or separation
Processes
Utilizing electrical or wave energy directly applied to...
210702, 210738, 210188, 55 15, 55277, 366127, 406198, B01D 1706
Patent
active
052250890
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention refers to a method for separating particles which are dispersed in a dispersion medium, whereby an ultrasonic standing wave is generated by means of a composite resonator within a vessel carrying the dispersion. The frequency of the wave is preferably within the neighborhood of the characteristic frequency f.sub.o. The term dispersion is used here as a general term for all kinds of disperse systems. This concerns a system consisting of two (or several) phases in which the disperse phase is finely distributed in the other one, hereinafter referred to as dispersion medium or dispersion.
Whereas the invention is applicable to all types of particles (solid, liquid or gaseous disperse phases), there is a limitation in the dispersion medium to liquids or gases. The application hereby comprises both solid (suspensions), liquid (emulsions), and gaseous particles in liquids and solid (smoke) and liquid particles (fog) in gases. Naturally, the liquid can also be a molten mass. The widest field of application are hydrosoles (particles in water) and aerosoles (particles in air).
STATE OF THE ART
The precipitation of dispersed particles on a commercial scale has been achieved up to the present day by so-called flotation or by addition of so called flocculators. Flotation is limited to relatively heavy particles and flocculators chemically contaminate the dispersion, and thus cause problems in the treatment of drinking water.
Whereas the disperse and colloidal effects of ultrasonics and cleaning by means of ultrasonics are widely applied in industry, the coagulating effect of ultrasonic sound is prominently known from literature. From E. Skudrzyk, "Die Grundlagen der Akustik", Springer verlag, Wien, 1954, S. 202-205, S. 807-825; L. Bergmann "Der Ultraschall und seine Anwendung in Wissenschaft und Technik", Verlag Hirzel, Zurich, 1954; as well as K. Asai and N. Sasaki, Proceedings of the 3rd International Congress on Coal Preparation, Institut National de L'Industrie Charbonniere, Brussels-Liege, 1958, "Treatment of slick by means of ultrasonics" it follows that the frequency to be used in the applied sound is best chosen within the magnitude of the so-called characteristic frequency f.sub.o, which can be calculated from ##EQU1## constitutes the kinematic viscosity, .eta. the dynamic viscosity, .rho. the density of the dispersion medium, and r the radius of the particle. The inner friction is practically without importance for frequencies above f.sub.o, for frequencies below, however, it becomes decisive.
The characterstic frequency also defines the so-called particle velocity accordance coefficient which is the ratio between the particle velocity amplitude of the particle and the dispersion medium. In the neighborhood of the characteristic frequency the particle begins to display a measurably lower displacement than the molecules of the dispersion medium; when increasingly exceeding the characteristic frequency, the particles are less and less carried along by the sound waves. As the so-called hydrodynamic or Bernoulli forces, which contribute considerably to the coagulation of particles within the areas of the particle velocity nodes and antinodes, directly depend on the occurrence of a relative movement between particles and dispersion medium, it is normally preferable to choose the frequency of excitation higher than the characteristic frequency. In this case an increase of the frequency of more than ten times the characteristic frequency practically no longer leads to an increase of the relative movement, as the displacement of particles, related to the sound amplitude in the dispersion medium, moves towards a limit. The limit depends on the density ratio .rho.'/.rho. between particle and dispersion medium, and is achieved for a small value (e g. of gas bubbles in water) of .rho. '/.rho. at a later time (at approx. 10 f.sub.o) and for a very high value (e.g. mineral matters or metal dust in water) at an earlier time (at approx. 0.3 f.sub.o). From specialized literature i
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Benes Ewald
Hager Ferdinand
Dawson Robert A.
Reifsnyder David
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