US2896922A - Ultrasonic means for changing the homogeneity of mixtures - Google Patents

Description

July 28, 1959 R. POHIL'MAN ULTRASONIC MEANS FoR l iA l lG ffiG THEHOMOGENEITY OF MIXTURES Filed June 23, 1955 IN VENTOR RE MAR PD-HLMAN ULTRASONIC MEANS FOR CHANGING THE HOMOGENEITY F MIXTURES Reirnar Pohlman, Darmstarlt, Germany, assignor to Dr. Lehfeldt & Company, G.m.b.H., a corporation of Germany Application June 23, 1955, Serial No. 517,600

Claims priority, application Germany November 15, 1954 7 Claims. (Cl. 259-4) This invention relates to a means for changing the homogeneity of mixtures of substances and particularly relates to a means for separating or mixing substances of different densities. The invention is particularly useful for mixing or separating liquids and gases, two liquid substances of different densities, or mixing or separating solid particles from a liquid or gaseous diluent.

It is known to employ ultrasonic waves for separation of substances of different densities, as for example, for degassing of liquids, precipitation of smoke particles, silicates, or other solids suspended in gasses, for precipitation of mists and the like. In such applications, sonic and prefereably ultrasonic waves are used. Since a medium changes its internal properties, such as its acoustic impedance, under compression, a steady component of pressure is produced by ultrasonic pressure waves, commonly referred to as radiation pressure, which. adds to the static pressure in the medium. Experiments have shown that, in a mixture of particles of higher and lower densities, in which a standing ultrasonic wave is produced, particles of higher density drift towards the antinodes of the wave where maximum particle velocity exists, whereas particles of lesser density tend to accumulate at the nodes of the wave Where minimum particle velocity exists. Radiation pressure appears to be in opposite directions on the left and right of a velocity antinode. When such drift caused by radiation pressure occurs, suspended particles generally coagulate to form larger agglomerates which either fall in the surrounding medium because of the force of gravity, such as in the case of smoke and mist precipitation, 01' rise, in the case of the degassing of liquids.

If the difference in the densities of substances to be separated is small, and if the viscosity of the surrounding medium is relatively high, then the separating process is a rather slow one. Closer investigation of the forces producing the aforementioned drift seems to indicate that adhesion forces exist having a directional component, directed upon the axis of the beam of ultrasonic energy. In the case of a horizontally directed ultrasonic beam, and for small differences in the densities of suspended particles, it is difficult for these particles to leave the cross-sectional area of the beam and thus effect a complete separation.

The use of ultrasonic waves for mixing or emulsifying substances of different densities, such as liquids, is well known. Whereas, in the case of separation, it is important to generate standing waves of ultrasonic energy, in the case of mixing or emulsifying by methods hitherto employed, the generation of such standing waves must be avoided, otherwise the above mentioned separating effect would prevent emulsification. Such emulsifications or mixtures can be produced by ultrasonic energy with the aid of magnetostn'ctive or piezo electric generators, or with so-called ultrasonic jets.

The efficiency of separation by these means is relatively low for the reasons mentioned above, namely, viscosity effects and the effects of adhesion forces. Moreover,

atent the three types of generators just referred to are unsuitable for gas-phase operation, that is, for gassifying liquids.

It is an objective, therefore, of the present invention to provide an improved system for changing the homogeneity of a mixture and to effect a rapid and complete separation or emulsification of substances even with only slightly different densities.

In accordance with the present invention, there is provided an improved means for changing the homogeneity of a mixture of substances of different densities, which means performs the steps of producing standing pressure waves of ultrasonic frequency in the mixture, with nodes of minimum particle velocity and antinodes of maximum particle velocity, effecting movement of the mixture in one direction parallel to the direction of the wave front in the nodes, and effecting movement of the mixture in the opposite direction parallel to the direction of the wave front in the antinodes.

Further in accordance with the invention, there is provided apparatus for changing the homogeneity of a mixture of substances of different densities comprising means for producing standing waves of ultrasonic energy in the mixture, that is, waves having nodes of minimum particle velocity and antinodes of maximum particle velocity. Further provided are means located at the nodes and antinodes for imparting movement to the mixture in directions parallel to the wave front of the ultrasonic waves.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in' the appended claims.

In the acompanying drawing, Figure 1 schematically shows an arrangement for carrying out the invention for separation of two substances of different densities in accordance with the present invention, while Figure 2 schematically shows a portion of preferred means for imparting movement to the mixture at the nodes and antinodes.

Referring now more particularly to Figure l, the apparatus illustrated comprises a vessel or tank 1 containing a liquid mixture 2, comprising a diluent and a second liquid more or less mixed therewith, or containing solid particles or gas, which is to be separated from the mixture. For producing standing waves of ultrasonic energy, there is provided an ultrasonic soundhead 3, which might be of any conventional design, either magnetostrictive or piezo electric, depending upon the particular application. Located at the opposite end of tank 1, there is an air-filled chamber 4, bounded by a foil or diaphragm 5. The distance between soundhead 3 and diaphragm 5, or the frequency of oscillations of soundhead 3 is so chosen that this distance is an integer number of half wavelengths. With such spacing between soundhead 3 and membrane 5, it is possible to produce a strong field of standing waves between soundhead 3 and diaphragm 5, at which total reflection of incident waves occurs, since the back of this membrane bounds the air-filled chamber 4. Letters A and N indicate antinodes and nodes of maximum or minimum particle velocity, respectively, produced in the liquid by the ultrasonic field.

In operation, separation of mixed substances of different densities is effected in the following manner. Soundhead 3 imparts ultrasonic energy to the mixture 2, generally beamed at diaphragm 5. This diaphragm is spaced an integer number of half wavelengths of the is produced at the diaphragm. The reflected energyis in" the proper phase to produce standing waves in the mixture between soundhead 3 and diaphragm 5, having nodes of minimum particle velocity at point N, and antinodes of maximum particle velocity at point A, as indicated in Fig. 1.

As mentioned above, steady components of pressure, so-called radiation pressure, are produced and are directed from both sides toward the antinodes of maximum particle velocity. Particles of higher density drift toward the antinodes, where they tend to accumulate. Further, as mentioned above, certain forces seem to hold these particles within the confines of the ultrasonic beam, thus tending to prevent them from falling to the bottom of tank or vessel 1 by gravity. To overcome these forces, a downward motion is imparted to the portions of the mixture in the vicinity of the antinodes A, by means of moving bands 6. This motion, substantially parallel to the wavefronts of the standing wave, removes such portions of the mixtures from within the confines of the ultrasonic beam and from the effect of forces that appear to prevent separation by gravity.

Bands 6 are guided and driven by rolls 7, as will be described below.

At the nodes, where fewer particles of higher density are present, an upward motion is imparted to the mixture so as to enhance separation of more dense and less dense particles. Denser particles collect in a zone 8 at the bottom of the tank and can be drained off through a valve 9, whereas less dense particles collect at a zone 8' close to the surface of the mixture 2, and can be drained off through a valve 9. Untreated liquid is fed to the tank 1 through feed pipes indicated at 10, which are preferably located between nodes and antinodes.

The driving arrangement for rolls 7, not shown, is preferably located on the outside of tank 1 and can be of any suitable conventional construction. Preferably, bands 6 are made of a material which causes very little reflection of ultrasonic energy, in order not to disturb the formation of a well-defined and intense standing wave pattern between soundhead 3 and diaphragm 5. For this purpose bands 6 are made of a material having an acoustic impedance approaching that of the surrounding medium as closely as possible, Where the acoustic impedance is defined as the product of sound velocity in a medium multiplied by the density of the medium. For liquids, very good matching of impedance of bands 6 and mixture 2 is achieved by making band 6 of conventional suitable synthetic plastic materials. Preferably the surface or the structure of the bands is so designed that maximum friction between the moving band and the surrounding medium is achieved so that a conveyor effect is obtained. This can be accomplished by making the surface of the bands quite rough or, particularly in the case of low viscosity liquids, by partially stamping holes in the bands and bending the material previously occupying the holes alternately to the right and left of the band as shown in Figure 2. For high viscosity liquids, a band woven of synthetic polymeric material can be used. The use of woven bands or bands with holes also has the advantage that the effective specific impedance of the band is still better matched to the surrounding medium and that the permeability of the bands to ultrasonic energy is enhanced.

The described method and apparatus is suitable for separation of minute solid particles from liquids, suchv as for example encountered in purifying of oil, as well as for separation of emulsified liquids of different densities, such as for example, water and oil in the manufacture of petroleum products. The method described is also useful for de-gassing of liquids since it prevents dwelling or" gas bubbles in the ultrasonic stream, which reduces the speed with which thorough de-gassing can be accomplished. In the case of de-gassing, it is preferable to provide tank 1 with a lid and to remove the separated gas by applying a partial vacuum by way of valve 9.

For emulsifying substances of different density, the method and apparatus operates in the reverse sense. In this case, care must be taken that particles of the substances to be mixed, either solids, liquids, or gasses, are transported to those zones of the ultrasonic field at which forces are active in a direction toward the cross section of the ultrasonic beam. For example, if it is desired to emulsify oil and water, the interface between oil and water should be lowered, by closing of valve 9', tosuch a degree that the upper portions of bands 6 are covered with oil, and to increase the velocity of motion of the bands so that as much oil as possible is conveyed into the antinodes A. The forces exerted by the ultrasonic wave cause droplets of oil to drift from the antinodes A to nodes N, thereby causing emulsification of the oil with the surrounding water. The emulsion thus formed can be drained off through openings 10 in the tank walls. In a similar manner gas can be mixed with a liquid in tank 2', by permitting the upper portion of bands 6 to he surrounded by the gas, and to increase the velocity of movement of the bands to such a degree that intense mixing of the gas or the liquid occurs. In this case, it is also preferable to provide tank 1 with a cover and to maintain the gas at a pressure above atmospheric.

Whereas I have described my invention in conjunction with vibrations of ultrasonic or super-audible frequencies, the invention is not limited to any specific frequency, since the same useful effects are produced over a very wide range of frequencies. The use of superaudible frequencies is generally desirable from the standpoint of reduction of disturbance of human beings by loud noise, and the physical dimensions of the vessel that must be employed, which might become excessively large for lower frequencies.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications. as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for changing the homogeneity of a mixture of substances of different densities, comprising means for producing standing waves of ultrasonic energy in said mixture, said waves having nodes of minimum particle velocity and antinodes of maximum particle velocity and conveyor means for imparting movement to portions of said mixture at said nodes in a direction parallel to the wavefronts of said waves, and for imparting movement to portions of said mixture at said antinodes in the direction opposite to said first-named direction.

2. Apparatus for changing the homogeneity of a mixture of substances of different densities, comprising means for producing standing waves of ultrasonic energy in said mixture, said waves having nodes of minimum particle velocity and antinodes of maximum particle velocity and conveyor means located at said nodes and antinodes for imparting movement to said mixture in directions parallel to the wavefronts of said waves.

3. Apparatus for changing the homogeneity of a mixture of substances of different densities, comprising means for producing standing waves of ultrasonic energy in said mixture, said waves having nodes of minimum particle velocity and antinodes of maximum particle velocity and conveyor means located at said nodes and antinodes for imparting movement to said mixture in directions parallel to the wavefronts of said Waves, said conveyor means being of a material having a wave resistance of a value approximately equal to the'value of thewave resistance of said mixture.

4. Apparatus for changing the homogeneity of a mixture of substances of different densities, comprising means for producing standing waves of ultrasonic energy in said mixture, said waves having nodes of minimum particle velocity and antincdes of maximum particle velocity and ribbon-type conveyor means located at said nodes and antincdes for imparting movement to said mixture in directions parallel to the wavefronts of said waves, said conveyor means being of a material having a wave resistance of a value approximately equal to the value of the wave resistance of said mixture.

5. Apparatus for separating substances of difierent 10 waves received from said soundhead, thereby to produce densities in a mixture, comprising a container for said mixture, transducer means arranged on said container for imparting ultrasonic waves to said mixture in a substantially horizontal direction, reflecting means spaced from said transducer means and adapted to reflect incident waves received from said transducer means, thereby to produce standing waves in said mixture having nodes of minimum particle velocity and antincdes of maximum particle velocity, and conveyor means for selectively imparting a downward motion to portions of said mixture solely in the vicinity of said antincdes.

6. Apparatus for separating substances of dififerent densities in a mixture, comprising a vessel for containing said mixture, an ultrasonic soundhead for imparting, along a preselected path, ultrasonic Waves in a substantially horizontal direction to said mixture, an air chamber, arranged along said preselected path so that said mixture is between said ultrasonic soundhead and said air chamber, to receive and reflect said waves, thereby to produce standing waves in said mixture having nodes of particle velocity, and antincdes of maximum particle velocity and conveyor means for selectively inrparting a downward motion to portions of said mixture standing Waves in said substances having nodes of minimum particle velocity and antincdes of maximum particle velocity, and conveyor means for moving substances of lesser densities from the upper region of the combined substances in a downward direction in the vicinity of said antincdes into the zone between said soundhead and said reflecting means.

References Cited in the file of this patent Frequency Sound-Waves of Great Intensity, Philosophical Magazine, Series 7, vol. 4, No. 22, September 1927, pg. 420. Y

Images