US2992700A - Electrostatic air cleaning device and method - Google Patents

Description

ooooo 'Tkooooooooo L. SILVERMAN ETA].

ELECTROSTATIC AIR CLEANING DEVICE AND METHOD Filed March 11, 1959 INVENTORS LESLIE SILVERMAN DAVID M. ANDERSON United States ?atent ELECTROSTATIC AIR CLEANING DEVICE AND METHOD The present invention relates to a method and apparatus for aerosol filtration, and more particularly, to a method and apparatus utilizing a triboelectrified fluidized bed for aerosol filtration.

An aerosol is a suspension of particles in a gaseous medium, usually air, the particulate constituents being dusts, fumes, smokes, or mists, or combinations thereof. By aerosol filtration is meant the elimination of the particles just enumerated from the gaseous medium, thereby purifying the air or other medium. The use of an electric charge or electrostatic effects in aerosol filtration has been known for some time. For example, there are available for commercial use in the metallurgical and heavy chemical in dustries a variety of electric precipitators to collect metallic oxide fumes and acid mists. In such devices the aerosol is usually first subjected to an ionizing field and then is passed to a separate collecting field wherein the particulate constituents are collected. In one particular type of apparatus, as just described, voltages in excess of 25,000 volts are not unusual and, while accidents from electric shock are rare, the risk in the use of such apparatus is very real. Also, such devices are inefficient in the sense that large amounts of electric power are absorbed during the operation thereof.

In order to avoid the use of devices of the type just described, there has been investigated the possibility of using mechanically induced electrostatic charges on fibrous media and/or fabrics. However, a serious problem encountered in the use of fibrous and fabric media is the difliculty of continuously replacing the charge which is constantly leaking off to ground from the surfaces of the fibers. While some success has been found in continually charging the surface of fabric media, it has been discovered that for optimum utilization of electrostatic forces a longer path of aerosol travel is necessary than can be presently attained with fabrics, due to the requirement that the surface charge producing the electrostatic forces be distributed uniformly throughout the media and not merely on the surface layers. As a result, several methods have been considered for continually charging a deep fibrous bed, but the necessity for obtaining intimate contact between the fibrous surfaces and the charging surfaces creates problems which heretofore have not been readily solved.

The present invention has to do with a method and apparatus for establishing and maintaining an electrostatically charged medium to efiect the efficient filtration of an aerosol passing therethrough. The medium referred to consists of a fluidized bed composed of material having a certain inherent chargeability and ability to hold a charge characteristics as will be hereinafter further described. By fluidized is meant a condition of the bed in which there is undergoing a gas-solid contacting process in which finely divided solid particles making up the bed are lifted and agitated by a rising stream of process gas, which is the aerosolin this instance. The degree of agitation produced depends upon the velocity of the aerosol therethrough. At some low velocity rate, for example, the amount of lifting would be slight, the bed behaving like a boiling liquid, and so is known by the term boiling bed. As the velocity of the aerosol through the boiling bed is increased, a point is reached where the pressure drop be comes equal to the sum of the weight of the bed per unit cross-sectional area plus the friction of the bed against the wall containing it. At this point, for free-flowing materials making up the bed, the latter expands and assumes a more open arrangement so that the gas can flow without the pressure drop exceeding the unit bed weight. Another increment of velocity of the aerosol results in overall circulation of the bed, often with transient upward-flowing gas streams in channels that contain relatively few par ticles and downflow of clumps of particles. This is called aggregative fluidization which is the preferred state of the medium or bed to carry out the aerosol filtration according to this invention.

The electrostatic charging of the medium by this process is accomplished by triboelectrification which may be described as the building up of an electrostatic charge as a result of the intimate contact between dissimilar materials, and is otherwise known as friction-charging. In order to acomplish triboelectrification of the medium or bed hereinbefore described, there is used as the substance of the medium a packed bed of spheres of a chargeable material. Since beds of spherical materials are easily fluidized, they provide the contact necessary for triboelectrification, caused by the random motion and turnover of the fluidized spheres. To meet the requirement of contact between dissimilar charging materials, the walls of the container in contact with the spheres may provide sufiicient contact provided enough mixing of the bed is attained. In the alternative, a bed of mixed materials may be relied upon, or certain other particular means as hereinafter more particularly described may be resorted to in order to accomplish the building up of the electrostatic forces and maintaining them.

While the theory of triboelectrification has not been fully and satisfactorily explained, it has been determined that the charge generated on two dissimilar materials is independent of the relative motion, contact time, and pressure between the two and depends only on the intimate contact and subsequent separation of the two materials. In this connection, it is the inherent chargeability and ability to hold an electrostatic charge which are the most important properties for this purpose. This ability to hold a charge, as is understood in the art, is related to the surface resistivity of the material when dry and its water absorption characteristics. In general, certain plastic materials possess high surface resistivity and low moisture absorption properties and therefore are capable of holding a charge for considerable periods of time. Hence, it has been discovered that certain types of plastic materials are suitable for making up the fluidized bed of this invention, as will be later described.

It is thus a principal object of this invention to provide a method and apparatus for establishing a medium for the eflicient filtration of an aerosol.

It is a further object of this invention to provide a method and apparatus for utilizing electrostatic effects for filtering airborne particulate constituents.

Still another object of this invention is to provide a method and apparatus for removing suspensions of particles from a gaseous medium by a continually maintained electrostatic field.

It is still another object of this invention to provide a method of preparing by triboelectrification a fluidized bed in which the processed gas is subject to electrostatic filtration.

Other objects and features of this invention will become more readily apparent to those skilled in the art as the disclosure is made in the following detailed description of a preferred embodiment of the invention as: illustrated in the accompanying sheet of drawings in which:

FIGURE 1 is a typical arrangement of apparatus for accomplishing aerosol filtration according to the principles of this invention;

FIGURE 2 is an expanded, detailed view of the fluidized bed shown schematically in FIGURE 1; and

FIGURE 3 is a section along 33 of FIGURE 2.

Referring to FIGURE 1, there is shown a typical filtration apparatus embodying the principles of this invention. Apparatus 10 consists of a vertically disposed cylindrical tubular section or container 12' which is constructed of material of high electrical resistance such as some suitable plastic as, in this particular instance, polystyrene. Container 12 is provided with a bottom inlet tube 14 and top exit tube 16. Within container 12 there is mounted a screen or mesh 18 for supporting a bed 20, as later described, and a pair of pressure taps 22 and 24 across bed to permit measurement of the pressure drop thereacross. A fiow straightener 26 consisting, for example, of a plurality of strawlike elements made of paper or plastic material may be provided upstream of bed 20 to eliminate turbulence of the air to the extent possible. Within inlet tube 14 there may be provided a rotameter (not shown) to measure the flow rate of the air therethrough, and a standard-type coarse filter (not shown) utilizing fiber glass to screen out the coarser particles in the air. A vibrator unit 28 of conventional design may be attached in convenient fashion to the outside of container 12 adjacent bed 20 for a purpose to be later described.

Referring to FIGURES 2 and 3 for the details of fluidized bed 20 made according to the principles of this invention, there is shown a cylindrical sleeve 30 fitted into container 12 for supporting fluidized bed 20. Adjacent the bottom of sleeve 30 there is provided the supporting screen 18 of conductive material of sufliciently fine mesh to support the spherical pellets of bed 20. Screen 18 may be 120-mesh copper screening mounted by an annular ring 34 to the interior surface of sleeve 30. In the space above screen 1-8 occupied by bed 20 there are provided lengths 34 of copper wire 38' at various levels as illustrated. For this purpose the outer circumference of sleeve 30 is provided with a plurality of annular grooves 36 which may also be a single, helical-type groove. Wire 38 is wound in grooves 36 and is threaded through suitably placed holes or openings 40 to produce the arrangement as illustrated in FIGURES 2 and 3. The spacing of the Wire lengths is great enough to permit free movement of the pellets (to be later described) therebetween. Copper wire 38 is, if desired, grounded at one end by a lead 39 as illustrated. The other end would be embedded or terminated in sleeve 30. Fluidized bed 20 is completed by filling in the volume above screen 18 and covering the top wire length 34 by particles or pellets of material having the ability to hold an electrostatic charge and capable of being so charged. In general, plastic materials possess high surface resistivities and low moisture absorption properties thereby making them very suitable for this purpose. One material found especially suitable is polystyrene in the formof unlubricated spherical beads such as that made by Koppers Company, Pittsburgh, Pennsylvania, and sold under the trade name -Dylene. It has been found that the spherical shape of the particles is most suitable for this purpose because materials of this shape are easily fluidized and thereby provide the contact necessary for triboelectrification to take place. The spacing of wire lengths 34 as compared to the size of the pellets is such as to permit the relative free movement of the pellets between wire lengths 34.

Once bed 20 is completed as described above, the aerosol (such as dirty air) is pumped through by means, not shown, up through inlet 14, through bed 20 and out through outlet pipe 16. The velocity of the fluid is increased until the pressure drop across the bed as measured by appropriate instruments utilizing taps 22 and 24 becomes equal to the sum of the weight of the bed per unit cross-sectional area, plus the friction of the bed against the walls. It has been found, as already noted, that at this point for free-flowing materials such as that utilized herein, the bed expands in volume and assumes a more open arrangement so that the aerosol can flow through without the pressure drop exceeding the unit bed Weight. The pore spaces eventually become so large that no stable arrangement can exist and another increment in velocity results in overall circulation of the bed. This is calledaggregate fluidization which, as noted above, is the desired state to produce the necessary triboelectrification of bed 20. Once this state of bed 20 is accomplished, the particles or pellets in bed 20 will build up a substantial electrostatic charge due to contact with the wall of sleeve 30 and their contact with wire 38 of the copper screening interspersed throughout bed 20. Due to the fact that as the charge builds up there is a tendency for the particles to stick to and coat the walls of sleeve 30 and thesurface of the copper wire 38 making up the mesh, it may be desirable for improving efliciency to provide some vibrating unit such as 28 schematically illustrated which may be any commercially available unit capable of imparting a slight degree of vibration. This will prevent the building up of the particles of material along the wall and the wires as noted and will assist in a uniform building up of electrostatic charge throughout the fluidized bed 20. It should be understood that for certain types of aerosols where high velocities are possible there may be sufiicient agitation of the particles in bed 20 to prevent the building up of the coatings and hence vibrating unit 28 would not be necessary.

It should be noted that under some particular arrangement it may not be necessary to include the wire lengths 34 throughout bed 20 if conditions are such that with the aerosol being filtered and with the type of material being used for bed 20 it is found that the circulation produced is sufiicient to build up a uniform electrostatic charge as a result of contact with the walls of cylinder 30 of suitable dissimilar material. In addition, it may be desired to obtain the dissimilar contact by utilizing a mixture of pellets of different materials. Furthermore, instead of utilizing wire lengths 34, a suitable cylinder of screen arranged concentrically in sleeve 30 or suitable perforated sections arranged concentrically in sleeve 30 may be used. Other materials which have been found to be suitable for the charging spheres are styrenedivinylbenzene microspheres made by Dow Chemical Company and ureaformaldehyde microballoons manufactured by Colton Chemical Company.

By the method and apparatus just described, it is seen that the aerosol or other medium to be filtered provides all the energy required to accomplish the filtering action without the necessity for resorting to extremely high and dangerous voltages. It has been discovered in connection with these arrangements that only a slight loss of pressure of the order of .5 inch of water is required to accomplish this filtering action and that the filtering efliciency is of the order of 95% to 99% depending upon the particular type of aerosol and the type of particles to be removed and the type of materials being used to make up the fluidized bed. This efliciency is equal generally to that obtained by the aforementioned equipment designed utilizing extremely highly dangerous voltages.

For example, efliciency determinations made on oneinch deep beds of 200M (Mg polystyrene spheres with atmospheric dust show that unhindered beds (without wire 38) operating at 20% expansion (30 f.p.m.) and a charged level of ()0.086 e.s.o/em. surface result in efficiencies of 97% to 98%. A mechanically generated gentian violet aerosol (mg=0.49M, Qp=e/part.) has been collected at 94.5% to 98.5% efliciency at 50 f.p.m. using a 50-50 mesh fraction of polystyrene spheres charged to levels of from l.0-35.0 e.s.u./gm. (neg) by fluidization in a hindered bed (utilizing wire 38). 7

It is thus seen that there has been provided a method and apparatus for accomplishing aerosol filtration utilizing electrostatic effects capable of continually recharging the fluidized medium so that the filtration action can be made to continue indefinitely. To permit occasional cleaning of the bed, two of the filters may be used in parallel so that one bed can be functioning while the other is being dumped for cleaning or replaced. In addition, it is seen that the method and apparatus is capable of very high energy efiiciency in that it does not require the introduction of high energy or electrical levels to accomplish the results since the aerosol medium accomplishes or brings about the electrostatic charging of the medium while at the same time the filtration is made to occur. Another advantage of the method and apparatus herein described is that it is inherently simple, and equipment built according to the principles of this invention should have a much smaller initial cost than other types of electrostatic equipment described hereinabove.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of this invention and that numerous modifications or alterations thereof may be made without departing from the spirit and the scope of the invention as set forth in the appended claims.

We claim:

1. In an electrostatic filter for the elimination of particulate constituents from an aerosol, a bed of divided elements formed of material having ability to be charged with and maintain a charge of static electricity, at least one element of material dissimilar to said divided elements material in contact with said bed, and means for maintaining said bed in a boiling, fluidized state by flowing upwardly therethrough said aerosol under a pressure drop across said bed substantially equal to the sum of the Weight of said bed per unit cross-sectional area plus the pressure drop equal to the friction of said bed against the wall of said filter causing circulation of said divided elements and continuous, intimate contact between said divided elements and said one element to cause the triboelectrification of said boiling bed and thereby the electrostatic removal of said particulate constituents from said aerosol passing therethrough.

2. In an electrostatic filter for the elimination of particulate constituents from an aerosol, a bed of spherical elements formed of plastic material having ability to be charged with and maintain a charge of static electricity, at least one element of electrically conductive, metallic material in contact with said bed, and means for maintaining said bed in a boiling, fluidized state by flowing upwardly therethrough said aerosol under pressure causing circulation of said spherical elements and continuous, intimate contact between said spherical elements and said one element to cause the triboelectrification of said boiling bed and thereby the electrostatic removal of said particulate constituents from said aerosol passing therethrough.

3. In an electrostatic filter for the elimination of particulate constituents from an aerosol, a bed of spherical elements formed of plastic material having ability to be charged with and maintain a charge of static electricity selected from the group consisting of polystyrene, styrenedivinylbenzene, and urea-formaldehyde, a plurality of metallic, wire-like elements extending through said bed spaced to permit relatively free movement of said spherical elements therebetween, and means for maintaining said bed in a boiling, fluidized state by flowing upwardly therethrough said aerosol under pressure causing circulation of said spherical elements and continuous,

intimate contact between said spherical elements and said wire-like elements to cause the triboelectrification of said boiling bed and thereby the electrostatic removal of said particulate constituents from said aerosol passing therethrough.

4. The electrostatic filter of claim 3 in which a vibratory unit is mounted adjacent said boiling bed to prevent sticking of said spherical elements to said wire-like elements.

5. A method for eleotrostatically filtering the particulate constituents from an aerosol passing through a bed of divided elements formed of material having ability to be charged with and maintain a charge of static electricity, said bed having at least one other element of material dissimilar to said divided elements material in contact therewith, comprising the step of forcing said aerosol through said bed under sufficient pressure to maintain said bed in an agitated, boiling state of overall circulation of the divided elements to cause the triboelectrification of said bed due to continuous intimate contact between said elements thereby simultaneously bringing about the electrostatic removal of said particulate constituents from said aerosol.

6. A method for electrostatically filtering the particulate constituents from an aerosol passing through a bed of divided elements formed of material having ability to be charged with and maintain a charge of static electricity, said bed having at least one other stationary element of material dissimilar to said divided elements mate rial in contact therewith, comprising the steps of forcing said aerosol through said bed under sufiicient pressure to maintain said bed in an agitated, boiling state of overall circulation of the divided elements to cause the triboelectrification of said bed due to continuous, intimate contact between said divided and stationary elements thereby simultaneously bringing about the electrostatic removal of said particulate constituents from said aerosol, and vibrating said bed by external means to prevent sticking of said divided elements to said stationary element.

References Cited in the file of this patent UNITED STATES PATENTS 793,745 Shields July 4, 1905 1,405,613 Milliken Feb. 7, 1922 2,114,682 Gumaer Apr. 19, 1938 2,564,316 Veron Aug. 14, 1951 2,597,927 Endres May 27, 1952 2,612,966 Nicol Oct. 7, 1952. 2,825,299 Herman Mar. 4, 1958 2,924,294 Johnstone Feb. 9, 1960 FOREIGN PATENTS 292,479 Great Britain a- June 13, 1929

Images