CA2220233A1

CA2220233A1 - Cyclone with spray electrode

Info

Publication number: CA2220233A1
Application number: CA 2220233
Authority: CA
Inventors: Christoph Wadenpohl
Original assignee: Individual
Current assignee: Universitaet Karlsruhe
Priority date: 1995-05-08
Filing date: 1996-04-30
Publication date: 1996-11-14
Also published as: DE59602285D1; EP0824376A1; JPH10506845A; AU5759196A; GR3030457T3; ATE181518T1; DK0824376T3; ES2133963T3; EP0824376B1; DE19516817C1; AU696749B2; WO1996035512A1

Abstract

The invention relates to a cyclone for separating fine particles from a stream of gas, especially soot from the exhaust gases of internal combustion engines.
According to the invention, a spray electrode (10) electrically insulated from the pure gas outlet (9) is fitted in the cyclone and the latter is electrically connected to the casing (2) of the cyclone. It is also proposed that an insulator (11) bearing the spray electrode (10) be secured inside the casing (2) at a distance from the pure gas outlet (9). The invention also relates to cyclones having an immersion pipe (8) and is applicable with advantage to both return and axial flow cyclones.

Description

CA 02220233 1997-11-0~

Cyclone With Spray Electrode The invention concerns a cyclone for the continuous separation of fine particles from a stream of gas, in particular soot from the exhaust gases of internal combustion engines in accordance with the pre-charac-terizing parts of claims 1 and 2. A cyclone is a con-ventional device in which the particles are enriched in a particle collection region via flow effects within a rotating stream formed in the cyclone. The cyclone includes a casing having an entrance region into which an inlet feeds, a separation region disposed in the flow path of the gas downstream of the entrance region, the particle collection region disposed in the flow path of the par-ticles downstream of the separation region, and a purified gas outlet connecting into the inside of the casing.

The entrance region is that region inside the cyclone immediately adjacent to the inlet. A stream of soiled gas having particles which are to be separated-out is trans-formed in the inlet region, and in particular in the separation region of the cyclone, into a rotating stream within which the particles are incident on the wall of the cyclone under the influence of centrifugal force. The particles slow down at this location and settle into the particle collection region or are discharged out of the particle collection region by a boundary layer flow. The purified gas leaves the cyclone through the purified gas outlet. The regions in the casing of a cyclone are not always clearly separable into an entrance region, sepa-ration region, and particle collection region, since they often join continuously into another. The purified gas outlet designates that area section of the inner wall CA 02220233 1997-11-0~

inside the casing which surrounds the cross sectional area through which the purified gas leaves the inside of the casing. In a cyclone not having a vortex finder, the purified gas outlet is the border of the hole in the casing through which the purified gas exits. In cyclones having a vortex finder, the purified gas outlet is the end of the vortex finder located inside the casing.

A cyclone has practical technical advantages, since it is inexpensive to produce, is robust and operates reliably when properly designed. Moreover, it can be used at high temperatures. The separation capability of a simple cyclone is, however, normally insufficient for particle sizes below a particular separation limit. The limiting grain size is between 1 ~m and 10 ~m, depending on the size of the cyclone. A cyclone having a separation limit near 1 ~m is designated a high efficiency cyclone. The centrifugal forces caused by the rotating stream are only sufficient to separate larger particles at the inner wall of the cyclone.
For very fine particles, these forces are, however, insuf-ficient for effecting a significant degree of separation.

For many applications, a larger fraction of the particles to be separated is located precisely in the region of smaller particle sizes. In e.g. diesel motor exhaust gas, approximately 80 ~ of the particle mass is soot particles of sizes below 1 ~m. The average particle size is about 0.1 ~m. Further steps must therefore be taken in order to use a cyclone in this size region.

Additional electrical forces are conventionally used to improve particle separation. A conventional realization of this idea combines a cyclone with an upstream electric agglomerator. The upstream agglomerator conditions the particles which are to be separated-out by forming agglo-merates in the agglomerator to increase the average size of the particles into a region in which the downstream cyclone CA 02220233 1997-11-0~

exhibits a better degree of separation. Electrical forces are used for agglomerate formation. The agglomerators, also designated as electrostatic precipitators, consist essentially of a flow-through pipe in which an electrode, connected to a high voltage source, is disposed. The par-ticles agglomerate in a conventional manner under the influence of the electrical field so that the agglomerates having large grain sizes can be separated in the downstream cyclone.

In order to use this conventional two-step process having an agglomerator and downstream cyclone, the particles must have a small primary size and must tend to agglomerate, i.e. be electrically conducting. Non-conducting particles form a solid layer in the agglomerator which cannot be discharged with the gas stream, rather must be removed by means of a mechanical removal of solids.

A combination of agglomerator and downstream cyclone of this kind is known in the art from the publication DE 37 32 552 A1. This document describes the manner in which an exhaust gas stream having particles which are to be separated-out, in particular diesel internal combustion engine exhaust gas having soot particles, is initially fed through an agglomerator designated as an electrofilter. The particles coagulate or agglomerate into larger agglomerates in the agglomerator. The gas leaving the agglomerator, having particle agglomerates, is fed into a downstream cyclone which is designed in a particularly compact, ring-shaped manner.

The patent publication DE 32 38 793 C2 also concerns a combination of an agglomerator and downstream cyclone of this kind. This publication describes a special feature of the upstream agglomerator. Namely, instead of using the otherwise conventional spray wire to produce ions moving towards a grounded chamber wall which ions, along their CA 02220233 1997-11-0~

path, accumulate on and charge the particles, an electric field strength less than the ionization field strength is used for agglomerate formation.

The patent publication DE 35 00 373 C2 describes a device for the removal of particles, in particular soot particles from the exhaust gas of internal combustion engines, having an electroseparator comprising spray electrodes with spray elements and a downstream centrifugal separator. This device is characterized by the fact that the centrifugal separator is fashioned from a plurality of mutually coaxial cyclones adjacent to the electroseparator and disposed in a ring-shaped manner at the exit side of the electro-separator.

The conventional combined connection of agglomerator and downstream cyclone has, in particular for fine particles, a separation capability which is still not acceptable. The constructional volume is relatively large and the pressure loss in the gas stream is substantial. In addition, greater constructive effort is necessary for manufacture.

Additional electrical forces, also within the cyclone, have also been proposed for improvement of the degree of sepa-ration of a cyclone.

For example, publication DE 37 23 153 A1 proposes sub-jecting soot particles exiting from an electrostatic precipitator, in which they are charged with the assistance of spray electrodes, to an electric field in the cyclone having substantially radial field lines. In this case, the cyclone funnel should be made from a non-conducting material and comprise a metallic layer on the inside or outside circuited as a coagulation or separation electrode.
The applied electric field is intended to improve the agglomeration or the separation of the particles, so that the separation of the particles in the cyclone is not only CA 02220233 1997-11-0~

based on centrifugal force separation, rather is also supported by the electric field.

The device described in this document requires compre-hensive protection measures to guarantee safe operation, in particular when the voltage-carrying layer is arranged on the outside of the cyclone. The manufacture of such a device is also very difficult, since it comprises differing materials. In addition, the stability with regard to temperature is questionable in view of the combina~ion of differing materials among one another as is the stability of the electrode layer. Finally, the assembly is not compact, since an upstream agglomerator is still necessary.

Other publications propose not only using a flat shape for the electrodes inside the cyclone, but also allowing them to project into the inside.

An arrangement of this kind has been proposed in US patent publication 4,010,011 for an axial flow cyclone. In an axial flow cyclone, the gas inlet and gas outlet lie across from each other so that flow reversal does not occur. In the device shown therein, the rotating stream is produced by an axial inlet with the assistance of guide blades. The spray electrode is a wire strung between the rotation-producing inlet and the vortex finder. The cyclone is provided for cleaning the intake air of internal combustion engines, i.e. it serves primarily for the separation of mineral dusts. These tend, due to their electrical proper-ties, to form bonding layers on the precipitation electrode whose removal is difficult. In order to improve removal, the patent publication proposes coating the precipitation electrode with an insulating layer of dielectric material.
Both the inlet as well as the vortex finder must be made from non-conducting material to guarantee the electrical insulation of the spray electrode. The arrangement is therefore difficult to construct.

CA 02220233 1997-11-0~

Enrichment assistance using electric forces emanating from a spray electrode disposed inside the cyclone has been studied for return flow cyclones in a theoretical work by P. Dietz, Powder Technology, 31 (1982) 221. A conventional return flow cyclone comprises a cyclone funnel and a vortex finder passing through a cover and forming the purified gas outlet at its inside end. This publication assumes that it is necessary to charge the particles via external spray devices prior to entrance into the cyclone. The configuration is therefore two-stepped, since an upstream ionization stage is required. The described cyclone is furthermore difficult to construct, since the spray electrode in the cyclone is disposed on the vortex finder.
This necessitates a high voltage isolation or attachment of the vortex finder.

Experimental investigations of this type of cyclone having an internal spray electrode have been published in a plurality of publications.

The publication by J. Petroll et al. in Freiberger For-schungshefte A 220 (1962) 175, 189 describes an electro-cyclone comprising a vortex finder made from insulating material on which a spray basket is disposed. Construction of the vortex finder as an insulator is technically difficult and the publication assumes that the slight improvement in the separation capability achievable with the spray electrode does not justify the technical difficulties.

The publication of B. Rabel et al. in Luft- und Kalte-technik (1981) 107 likewise shows an electrocyclone separator having the spray electrodes disposed on the vortex finder. This publication states that the inner components of the spray electrode lead to a decrease in the CA 02220233 1997-11-0~

separation capability and that electrocyclone separators represent a non-advantageous solution.

Another publication by J. Petroll et al., Luft- und Kalte-technik (1987) 198 once more presents the opinion of experts in this field that the corona discharge fails to improve separation behavior and that the electrodes disadvantageously influence flow inside the cyclone.

In summary, prior art considers the incorporation of spray electrodes in cyclones, in particular in reverse cyclones, to require technically difficult insulators to insulate the vortex finder and is generally considered to negatively influence the flow behavior in the cyclone. The charging zone formed by the spray electrode in the region of the purified gas outlet has also been considered to be disad-vantageous. For these reasons, cyclones having spray elec-trodes projecting into the inside of the casing have not found practical application to date.

Departing from this prior art, it is the underlying purpose of the invention to further improve a cyclone of the above mentioned kind for the separation of fine particles from a stream of gas, in particular soot from the exhaust gas of internal combustion engines, having a spray electrode pro-jecting into the inside of the casing to which an electri-cal high voltage can be applied, in such a manner that it has a higher degree of separation and is of compac~ con-struction without increased technical difficulty and ex-pense. This purpose is achieved in accordance with the features given in claims 1 and 2. Advantageous embodiments of the invention are given in the dependent claims.

The solution in accordance with the invention, based on a cyclone of the above mentioned type, consists, in a first principal aspect, in the fact that the spray electrode is electrically insulated from the purified gas outlet and the CA 02220233 1997-11-0~

purified gas outlet is connected to the casing in an elec-trically conducting manner. This cyclone configuration has the advantage that technically difficult measures for elec-trical insulation of the purified gas outlet are no longer necessary. The spray electrode is electrically insulated from the casing via an insulator, but the purified gas outlet is not. The purified gas outlet or the vortex finder can be made from metal, in particular from the same metal as the rest of the casing, and formed on the cyclone using simple production techniques such as welding.

An additional advantage is that the spray electrode can be disposed at a separation from the purified gas outlet or the vortex finder. Another principal aspect of the inven-tion, which can be realized separately or particularly advantageously in combination with the first independent claim, therefore proposes that an insulator bearing the spray electrode be mounted inside of the casing at a separation from the purified gas outlet, i.e. without direct mechanical contact with same. If the insulator bearing the spray electrode is not mounted to the purified gas outlet or to the vortex finder, rather to another location in the casing, unconventional principles of arrangement and configurations are facilitated having surprising and advantageous properties.

In a preferred embodiment, the purified gas outlet is formed on a vortex finder projecting into the inside of the casing. Also herein, the spray electrode can advantageously be electrically insulated from the vortex finder and the vortex finder can be connected to the casing in an electrically conducting manner. In this case, it is also advantageous when an insulator bearing the spray electrode is mounted inside the casing at a separation from the vortex finder.

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The invention is based on the realization that a partial destruction of the weakly bound agglomerates occurs in the turbulent rotating stream of the cyclone. The fine material which thereby results can only be separated in the cyclone to a limited extent. The use of electric field forces to assist particle separation leads to the avoidance of this break-up effect or to its compensation through reagglome-ration or at least to its reduction. It has been discovered within the framework of the invention that arrangement of a spray electrode in the inside of the cyclone can efficient-ly affect agglomerate formation or reagglomeration to such an extent that, through the improved direct use of electric forces to assist particle separation, it is even possible to do without the upstream agglomerator and thereby to achieve a very compact design. It has been discovered that, with a cyclone in accordance with the invention, even those particles can be separated which, due to their physical properties, do not tend to agglomerate under the pertaining conditions.

It has turned out that the spray electrode can be disposed and configured in such a manner that its negative influence on the flow field and the obstruction of the rotating stream are sufficiently small and sufficiently capable of compensation by the electric field that the overall degree of separation of the cyclone is increased. Up to now, one had attempted to keep the inside of a cyclone as free as possible from additional elements which could obstruct the free formation of the rotating stream. It is therefore surprising that a spray electrode projecting into the inside of the casing increases rather than decreases the separation capability.

The experimental work carried out to date led to a less positive evaluation of spray electrodes disposed inside the cyclone since, as has been discovered in accordance with the invention, it is not reasonable to use electrocyclones for the classical region of cyclone application, namely for separation of relatively coarse particles. This is true since, in this case, the positive effect of the electric forces is not compensated by the negative influence of the spray electrodes on the flow field. It has, however, been discovered within the framework of the invention that cyclones having spray electrodes can be advantageously used for the separation of fine dust, i.e. of particles smaller than 1 to 5 ~m. One has discovered that, in this size region, separation via the electric forces dominates, so that the disadvantages to the flow field are insignificant.
In conventional cyclones, the disadvantages to the guiding of the flow are, in contrast, not acceptable if high degrees of separation are required.

One reason for the skepticism of experts in the field with regard to the separation of sub-micron particles with electrocyclones using electrical field forces is that the literature imparted the point view that, in this size range, charging occurs via the relatively slow mechanism of diffusion charging. To achieve a high degree of separation, this mechanism requires long residence times not normally present in a cyclone. Although one can speed up the kinetics of charging within certain limits via high current densities, i.e. through the use of spray electrodes having a plurality of tips, there are however associated problems with deposition on the precipitation electrode in depen-dence on the particle material. Within the framework of the invention, one has discovered that particles and dusts of sufficiently high electrical conductivity are non-critical.
In particular, one has discovered that diesel soot fulfills this important criterion so that no deposition problems normally occur, even with very high electrical current den-sities desirable for achieving a high degree of separation.

One has also discovered within the framework of the invention that the charging of sub-micron particles occurs CA 02220233 1997-11-0~

more rapidly than expected based on conventional models.
For this reason, a charging and separation of particles, in particular soot particles, is also possible despite the short residence times in the cyclone.

A cyclone in accordance with the invention can be confi-gured as a return flow cyclone with which the entrance region and the purified gas outlet are disposed at the entrance side, i.e. in the region of the inlet. A return flow cyclone effects a flow reversal, since the incident gas stream must be directed back into the entrance region after establishment of the rotating stream. Conventional high efficiency cyclones are manufactured in this fashion, wherein the purified gas outlet is usually formed on a vortex finder passing through the cover of the cyclone.

Another advantageous embodiment of a cyclone in accordance with the invention is as an axial flow cyclone, wherein the entrance region and the purified gas outlet are disposed in the casing at end regions axially across from another.
There is no flow reversal in axial flow cyclones. Most con-ventional axial flow cyclones exhibit a flatter separation curve than return flow cyclones, i.e. generally have worse separation for comparable size. This is probably due to the fact that the rotating stream fundamental to particle sepa-ration is less advantageously established in such cyclones.
Configuration of prior art high efficiency cyclones as return flow cyclones having a vortex finder disposed at the cover side was therefore preferred. It has been discovered within the framework of the invention that precisely axial flow cyclones are also suitable for separation of fine particles if they are configured in accordance with the invention. An axial flow cyclone has the advantage that the pressure loss is less.

The measures in accordance with ~he invention, of elec-trically separating the spray electrode from the purified CA 02220233 1997-11-0~

gas outlet and electrically connecting the purified gas outlet to the casing or of mounting the insulator at a separation from the purified gas outlet, can be advanta-geously applied to both return flow as well as to axial flow cyclones if at least one insulator bearing the spray electrode is mounted in that region of the casing opposite the purified gas outlet in the axial direction of the casing. If the return or axial flow cyclone comprises a vortex finder, the spray electrode and the vortex finder can advantageously project into the inside of the casing from axially opposite ends of the casing. In this manner, the purified gas outlet or the vortex finder can be arranged at a separation from the spray electrode. This is technically advantageous with regard to flow and can be easily manufactured. A cyclone of this kind has an improved separation performance compared to conventional cyclones.

This arrangement has proved especially advantageous within the framework of the invention, in particular, with axial flow cyclones. This is completely surprising in view of prior art in which only return flow high efficiency cyclones were used to achieve optimum separation performance. The reason for the surprising advantages of this preferred arrangement is that the separation at small flow velocities is dominated by electric forces, since the residence time is sufficiently long. In contrast, the influence of the centrifugal force dominates the separation process at larger flow velocities. In general, the separation performance is therefore better, the smaller the flow velocity in the cyclone. This behavior is atypical for conventional cyclones, since the separation therein is improved with increasing flow velocity.

The simultaneous influence of electrical forces and centrifugal forces with relative strengths which change with flow velocity and particle size therefore leads to a stabilization of the operational performance of a cyclone CA 02220233 1997-11-0~

in accordance with the invention. The cyclone becomes less sensitive to fluctuations in the distribution of particle sizes or in the flow velocity. The corresponding optimi-zation and adjustment measures for achieving an optimal operational performance, i.e. the design, choice, and dimensioning of spray electrodes and high voltage as well as the geometric size of the cyclone, can be adjusted by one of average skill in the art to the requirements in each case using standard calculations or experimental investi-gations.

It is generally true that the separation performance is better the longer the path along which the spray electrode projects into the inside of the casing. This is also a surprising and atypical characteristic of the cyclone in accordance with the invention in view of the constructional guideline believed to date, that the inside of the cyclone be kept as free as possible from internal components.

The separation region and/or the particle collection region can advantageously be cylindrical in shape as is usually the case for axial flow cyclones A hollow truncated cone shaped configuration, as found in the cyclone funnels of conventional high efficiency cyclones, is however preferred.

A first advantageous embodiment of the arrangement of the spray electrode has at least one section of the spray electrode disposed in the entrance region. In this manner, one guarantees that the particles already enter into the region of influence of the electric field of the spray electrodes when they enter into the cyclone. This permits an efficient, early start of agglomerate formation.

A second advantageous embodiment provides that at least one section of the spray electrode be disposed in the separa-tion region. The spray electrode can thereby extend along CA 02220233 l997-ll-0 the entire separation region or only along a portion there-of. Current understanding of the inventlon considers this to be an advantageous embodiment of the invention, since the very strong and turbulent flow conditions reigning in the separation region are largely responsible for agglo-merate destruction. The arrangement of the spray electrode in this region allows for most efficient reagglomeration before the fine particles escape through the cross section of the purified gas outlet.

A third advantageous embodiment can provide that at least one section of the spray electrode be disposed in the particle collection region. A particularly advantageous embodiment further provides that the spray electrode be disposed in the entrance region and in the separation region. In this manner, the spray electrode is surrounded by the gas stream in a manner which particularly assists agglomerate formation.

A preferred feature proposes that the spray electrode be axially disposed inside the casing. An axial configuration minimizes possible negative effects on the flow field, in particular due to the rotational symmetry of the arran-gement.

The design of the spray electrode can be effected in any conventional manner, e.g. as is done in the art for the agglomerators. Spray electrodes are characterized by ~heir spray elements. Spray elements are parts of the electrode with surface portions having very small radii of curvature.
Conventional arrangements comprise spray wires or, if a larger active region is required, spray baskets or a dis-tribution of spray tips. A high charge ion concentration can be achieved using a plurality of spray tips. The spray tips are advantageously disposed in such a manner that their separatlons from the wall of the casing are largely equal to effect an even flashover voltage. The spray tips CA 02220233 1997-11-0~

are often disposed about the periphery of spray disks, which are preferred within the framework of the invention.

An advantageous improvement provides for a plurality of spray electrodes. The plurality of spray electrode can have the same or different shapes and advantageously have dif-fering applied high voltages. In this manner, spray elec-trodes can be used which are locally adapted to differing flow conditions in the cyclone to further optimize the separation behavior.

The advantages of the cyclone of this invention compared to prior art are that the degree of separation is improved and, in particular for inventive applications without com-bined upstream agglomerator, the size is reduced and the pressure losses decreased. In addition, the amount of effort required for manufacturing and safe operation of the cyclone is reduced. The invention expands the range of application of cyclones to particles well below 0.1 ~m and to particles which cannot be agglomerated or which agglo-merate poorly.

The cyclone in accordance with the invention is generally suitable for the separation of the finest of liquid and/or solid particles from gas streams. In combination with an upstream condensation stage, separation of condensable gaseous substances is also possible. For the separation of solid particles, it can e.g. be used for diesel soot sepa-ration for exhaust gas purification of stationary and non-stationary diesel engines in, for example, power plants, ships, locomotives, and motor vehicles, or for production of cleaner inert gases from combustion processes. The separation of solid particles with the assistance of the cyclone in accordance with the invention is also possible in connection with soot separation for product recycling in soot production or in soot injection in small boiler installations as well as in salt separation in the ferti-CA 02220233 l997-ll-0 lizer industry and for separation of sublimates in melting or casting processes. The separation of fluid particles is relevant e.g. for separation of so-called "blue haze" from shredders or in particle board production as well as for exhaust gas purification in painting and coating instal-lations. Gaseous substances which can be separated in combination with an upstream condensation stage include, for example, solvents in the production of liquid paints, dyes, and inks or in the form of odorous materials from smoke-house exhaust. Simultaneous separation of liquid and solid particles is possible. Examples are the separation of dust and bituminous materials in asphalt plants and in roofing paper production or for exhaust gas purification following smaller anode furnaces in the aluminum industry.

Additional advantageous features and distinguishing charac-teristics can be recognized through the representation of the drawing in the following more closely described and explained embodiment of the invention.

Figure 1 shows a schematic representation of the principle of operation of a cyclone of prior art, Figure 2 shows a cut through a cyclone of prior art, Figure 3 shows a schematic representation of a cyclone having an upstream agglomerator according to prior art, Figure 4 shows a schematic representation of a cyclone of prior art comprising a spray electrode disposed therein, Figure 5 shows a cut through a cyclone in accordance with the invention having a spray electrode disposed therein, and CA 02220233 1997-11-0~

Figure 6 shows a spray disk having spray tips.

Figure 1 is a schematic representation of the manner of operation of a high power return flow cyclone 1 of prior art. The casing 2 comprises an entrance region 4 having an inlet 3 feeding therein, a separation region 5, and a particle collection region 6 designated as cyclone funnel 27 and having the shape of a hollow truncated cone. The gas 18 which is to be purified with the particles or agglome-rates contained therein enters through the inlet 3 tangen-tially into the inside of the casing 7 into the entrance region 4. The particles describe a helix which extends into the separation region 5 and the cyclone funnel 27. Due to the centrifugal force resulting thereby, the particles are incident on the outer edge of the cyclone 1, are enriched in the particle collection region 25, and sink down through the dust exhaust opening 17 or are rinsed-out with a soiled gas stream 20. The purified gas 19 leaving the cyclone 1 travels through the cross section of the purified gas out-let 9 into a central vortex finder 8 which feeds through cover 28 into the inside of the casing 7. The purified gas outlet 9 is disposed at the entrance side so that the flow path of the gas 18 in the casing 7 must be reversed in order to exit through the purified gas outlet 9.

Figure 2 shows a schema~ic cut through a high power re~urn flow cyclone 1 of prior art comprising an apex cone 16 for stabilizing the flow spout. In order to form a stable flow spout and well-centered rotating stream to achieve a high degree of separation, prior art has proposed additional measures for influencing the inward flow of the gas through the inlet 3 into the entrance region 4 to improve formation of the rotating stream and thereby the degree of separa-tion. Towards this end, the installation of e.g. baffles or a spiral inlet have been proposed. The casing inside 7 of CA 02220233 l997-ll-0 prior art is, to the extent possible, kept free of elements obstructing the stream.

Figure 3 shows a schematic representation of a cyclone 1 having an upstream agglomerator 22 according to prior art.
The gas 18 which is to be purified and the particles con-tained therein are initially introduced to a pipe-shaped agglomerator 22 comprising a spray electrode 10. The spray electrode 10 is connected to the agglomerator casing 21 via an insulator 11 and can be connected to a high voltage source (not shown) via a high voltage feed-through 23. The spray electrode 10 has an axially directed electrode finger 15 along which spray disks 12 are disposed. Agglomerates formed in the agglomerator 22 from the particles introduced with the gas 18 enter through the inlet 3 into the cyclone 1 and are separated therein in the manner previously des-cribed. This figure clearly shows the large amount of space required by the combination assembly of prior art. One also sees that this design causes relatively large pressure losses.

Figure 4 schematically shows a high power return flow cyclone 1 of prior art comprising spray electrodes 10 projecting into the casing inside 7. Its structure corre-sponds to that of the cyclones shown in figures 1 and 3. It is distinguished by a so-called spray basket mounted to the central vortex finder 8 feeding through the cover 28. A
high voltage can be applied to this spray electrode 10 which is introduced via the vortex finder 8. For this reason, it is necessary to fashion the cover 28 as an insulator to electrically insulate the spray electrode 10 and the vortex finder 8 from the grounded casing 2. Fur-thermore, an additional insulator (not shown) is necessary to insulate the outlet-sided end of the vortex finder 8 from the connected downstream apparatus. Figure 4 clearly illustrates the complicated nature of the conventional configuration. The space charge zone emanating from the spray electrode 10 and surrounding the purified gas outlet 9 is also considered disadvantageous.

An embodiment modified relative to figure 4 has been proposed in prior art, wherein the vortex finder 8 is not made from metal rather from an insulating material. In this arrangement, the non-conducting vortex finder 8 itself constitutes the insulator bearing the spray electrode 10 and the cover 28 can be made from metal. This has, however, the disadvantage that the vortex finder 8 itself as well as its feed through the cover 28 are technically difficult and an additional high voltage feed-through is necessary for passing the electrical current to the spray basket.

Figure 5 shows a cut through a cyclone 1 in accordance with the invention. The casing 2 has an inlet 3 leading into an entrance region 4. The inlet 3 can be an axial inlet or a tangential inlet slot. Baffles or similar flow-guiding elements can be provided for in a conventional manner to optimize the inward flow. The shown preferred configuration of the inlet 3 as a helical or spiral inlet effects an improved centering of the rotating stream compared to the simple slot inlet. Adjacent to the hollow cylindrically shaped entrance region 4 along the flow path of the gas 18 which is to be purified is a hollow cylindrically shaped separation region 5 which passes into a cylindrical and hollow truncated cone shaped partlcle collection section 6.

A spray electrode 10, disposed in the casing inside 7, pro-jects into the inner volume and is electrically insulated from the casing 2 by means of an insulator 11. The spray electrode 10 can be connected to a high voltage source (not shown) via a high voltage feed-through 23. The insulator 11 is disposed in a casing extension 24 in which a zone having low flow is formed. In this manner, soiling and the asso-ciated leakage current caused by deposited particles is counteracted. A collimator 26 is additlonally provided to CA 02220233 l997-ll-0 also protect the insulator 11 from particles. One can also provide that the insulator 11 be rinsed with inlet air to keep away particlesO

The spray electrode 10 is axially disposed in the casing inside 7 and has an axially oriented rod-shaped finger 15 bearing a plurality of spray disks 12. The spray disks 12 are mounted in a parallel, separated manner on the elec-trode finger 15 and are perpendicularly penetrated by same.
The axial and rotationally symmetric configuration ob-structs formation of the rotating stream to as small a degree as possible. The diameter of the spray disks 12 or of the spray electrode 10 is advantageously 40 to 60 ~ of the inner diameter D1 of the separation region 5. In the example shown, the inner diameter D1 is 120 mm, the length D3 of the entrance region 4, separation region 5 and par-ticle collection region 6 iS 390 mm, and the inlet 3 has a width D4 of 25 mm and a height D5 of 65 mm.

The spray electrode 10 is disposed in the entrance region 4 and in a portion of the separation region 5 adjacent there-to. In this manner, an efficient agglomerate formation or reagglomeration of broken agglomerates is achieved and a greater degree of separation effected. In order for the spray electrode 10 in the entrance region 4 to be surroun-ded by the entering gas 18, the insulator 11 is accommo-dated in an upwardly disposed casing extension 24 with which the casing inside 7 iS extended in the upward direc-tion. The vortex finder 8 having the purified gas outlet 9 for the purified gas 19 projects into the casing inside 7 at the lower end of the cyclone 1 axially opposite the in-let 3 from the smaller diameter end of the hollow truncated cone shaped end of the cyclone funnel 27. It is electri-cally conducting and connected to the casing 2 in an elec-trically conducting manner. If necessary, the spray elec-trode 10 can be borne, at its end facing the vortex finder CA 02220233 1997-11-0~

8, by an additional insulator which e.g. is mounted to the vortex finder 8.

The entire assembly can, as is conventional for high efficiency cyclones, also have a vortex finder 8 projecting into the casing inside 7 from above. The insulator 11 bearing the spray electrode 10 would then be mounted at the end of the casing at the cyclone funnel side. With such a configuration, it is howe~er usually more difficult to have the spray electrode 10 project up to the entrance region 4.
In both cases, an apex cone 16 can also be provided for which, in the first case, can e.g. be configured as part of the vortex finder 8 and, in the second case, as part of the insulator 11.

A dust exhaust opening 17 is provided in the vicinity of the smaller diameter end of the cyclone funnel 27 to carry out the separated particles via a partial volume flow of the gas stream 18. The opening is configured for tangential suctioning-off of the particles. The partial volume stream of soiled gas 20 is about 1 ~ to 5 ~.

The gas 18, having fine particles, passes through the inlet 3 into the entrance region 4 and is set into rotation about the longitudinal axis of the cyclone 1. This is positively influenced when the inlet 3 is configured as a spiral in-let. In the entrance region 4, the gas flows around part of the spray electrode 10 to which a high voltage is applied.
Agglomerates of particles can be formed under the influence of the electrical field forces. In any event, the electri-cal field forces assist in separation of the particles. The rotating stream extends into the separation region 5 adja-cent to the entrance region 4. Due to the centrifugal force which is increased relative to that of finer particle due to the increased mass of the agglomerates, the agglomerates are driven primarily radially in an outward direction. In ~his manner, they are separated out of the gas stream via CA 02220233 1997-11-0~

the downwardly directed border-layer flow in the vicinity of the wall. The purified gas 19 leaves the casing inside 7 through the cross section of the purified gas outlet 9 of the central vortex finder 8. The separated particles or agglomerates are rinsed-out, as soiled gas 20, through the dust exhaust opening 17 with a partial volume stream.

The strong, turbulent rotating stream destroys agglomerates inside the casing 7. This negatively influences the degree of separation, particularly for fine particles, in conven-tional cyclones without spray electrode 10 due to the re-duced centrifugal forces. By arranging the spray electrode 10 inside the casing 7, in particular in the entrance region 4 and in the separation region 5, these disadvan-tages are compensated for so that the electric forces can act at an efficient location. In addition, the electric forces counteract size reduction of the agglomerates by effecting reagglomeration of the pieces. Furthermore, an electric field is formed between the spray electrode 10 and the casing 2 so that the charged particles are subjected to the outwardly directed Coulomb forces acting in the direc-tion of the centrifugal forces and assisting particle sepa-ration. The overall degree of separation is thereby impro-ved and the range of application extended to non-agglome-rating fine dust. The design is simultaneously technically slmple O

The degree of separation ban be optimized through appro-priate design of the geometry and flow behavior of the cyclone 1 as well as of the shape, number, arrangement and individual voltages of the one or plurality of electrodes 10. The installation or operational position of the cyclone is, of course, arbitrary since gravitational forces can be neglected.

Figure 5 shows an embodiment of a spray disk 12 having spray tips 13. It has a diameter of about 66 mm and a CA 02220233 1997-11-0~

thickness of 0.05 mm. Spray tips 13 are disposed at its periphery which are curved in the direction of the peri-phery 14. The voltage on the electrodes lies in the range between 10 kV and 20 kV. Charge carriers are set free at the spray tips 13 through application of a voltage above the so-called corona threshold voltage. This leads to charging of the particles and thereby to the formation of agglomerates.

CA 02220233 1997-11-0~

List of Reference Symbols 1 cyclone 2 casing 3 inlet 4 entrance region 5 separation region 6 particle collection region 7 casing inside 8 vortex finder 9 purified gas outlet 10 spray electrode 11 insulator 12 spray disk 13 spray tip 14 periphery 15 electrode finger 16 apex cone 17 dust exhaust opening 18 gas 19 purified gas 20 soiled gas 21 agglomerator casing 22 agglomerator 23 high voltage feed-through 24 casing extension 26 collimator 27 cyclone funnel 28 cover D1 inside diameter of 5 D2 inside diameter of 8 D3 length D4 width of 3 D5 height of 3

Claims

claims

1. Cyclone for continuous separation of fine particles from a gas stream, in particular of soot from the exhaust gas of internal combustion engines, using flow effects in a rotating stream formed in the cyclone with which the particles are enriched in a particle collection region (6), comprising a casing (2) having an entrance region (4) into which an inlet (3) feeds, a separation region (5) disposed in the flow path of the gas downstream of the entrance region (4), and a particle collection region (6) disposed in the flow path of the particles downstream of the separation region (5), the cyclone comprising a purified gas outlet (9) connecting into the casing (2) inside (7) and a spray electrode (10), projecting into the inside (7) of the casing (2), to which an electrical high voltage can be applied, the cyclone having the spray electrode (10) electrically insulated from the purified gas outlet (9), the purified gas outlet (9) connected to the casing (2) in an electrically conducting manner and with which an insulator (11) bearing the spray electrode (10) is mounted inside the casing (2) at a separation from the purified gas outlet (9), characterized in that the cyclone is adapted for the separation of fine, electrically conducting particles, in the size region below 5 µm, in particular smaller than 1 µm, wherein the spray electrode (10) is configured in such a manner that the particles form agglomerates under the influence of the electric field forces of the spray electrode (10) and an insulator (11), bearing the spray electrode (10), is disposed in a casing extension (24) in which a zone of weak flow is formed.

2. The cyclone of claim 1, characterized in that an insulator (11) bearing the spray electrode (10) can be surrounded by introduced rinsing air.

3. The cyclone of claim 1 or 2, characterized in that a collimator (26) is provided for protecting the insulator (11) bearing the spray electrode (10) from particles.

4. The cyclone of any one of the claims 1 through 3, characterized in that at least one section of the spray electrode (10) is disposed in the entrance region (4).

5. The cyclone of any one of the claims 1 through 4, characterized in that at least one section of the spray electrode (10) is disposed in the separation region (5).

6. The cyclone of any one of the claims 1 through 5, characterized in that at least one section of the spray electrode (10) is disposed in the particle collection region (6).

7. The cyclone of any one of the claims 1 through 6, characterized in that the spray electrode (10) is disposed in the entrance region (4) and in the separation region (5).

8. The cyclone of any one of the claims 1 through 7, characterized in that the spray electrode (10) is axially disposed inside the casing (7).

9. The cyclone of any one of the claims 1 through 8, characterized in that the spray electrode (10) comprises at least one spray disk (12).

10. The cyclone of any one of the claims 1 through 9, characterized in that the spray electrode (10) comprises a plurality of spray tips (13).

11. The cyclone of claims 9 and 10, characterized in that the spray tips (13) are disposed about the periphery of the spray disk (12).

12. The cyclone of claim 11, characterized in that the spray tips (13) are curved in the direction of the periphery (14) of the spray disk (12).

13. The cyclone of claims 8 and 9, characterized in that the spray electrode (10) comprises a rod-shaped electrode finger (15) axially disposed inside the casing (7) on which the at least one spray disk (12) is disposed, wherein the electrode finger (15) is oriented perpendicular to the at least one spray disk (12).

14. The cyclone of claim 13, characterized in that the spray electrode (10) comprises a plurality of spray disks (12) disposed parallel and separated on the electrode finger (15).

15. The cyclone of any one of the claims 1 through 14, characterized in that the inlet (3) is configured as an axial inlet or as a tangential slot or pipe inlet.

16. The cyclone of any one of the claims 1 through 15, characterized in that the inlet (3) is configured as a helical or spiral inlet.

17. The cyclone of any one of the claims 1 through 16, characterized in that the cyclone comprises an apex cone (16) inside (7) the casing (2).

18. The cyclone of any one of the claims 1 through 17, characterized in that the particle collection region (6) comprises a dust exhaust opening (17) for carrying out the separated particles via a partial volume of the gas stream.

19. The cyclone of claim 18, characterized in that the dust exhaust opening (17) is adapted for tangential suctioning-off of the particles.

20. The cyclone of any one of the claims 1 through 19, characterized in that a plurality of spray electrodes (10) are provided for.

21. The cyclone of claim 20, characterized in that differing high voltages can be applied to the spray electrodes (10).

22. The cyclone any one of the claims 1 through 21, characterized in that the purified gas outlet (9) is formed on a vortex finder (8) projecting into the inside (7) of the casing (2).

23. The cyclone of any one of the claims 1 through 22, characterized in that the cyclone is configured as a return flow cyclone having the entrance region (4) and the purified gas outlet (9) disposed at the entrance side.

24. The cyclone of any one of the claims 1 through 23, characterized in that the cyclone is configured as an axial flow cyclone having the entrance region (4) and the purified gas outlet (9) disposed in the casing (2) at end regions lying across from each other.

25. The cyclone of claim 23 or 24, characterized in that at least one insulator (11) bearing the spray electrode (10) is mounted at that region of the casing (2) which lies, in an axial direction of the casing (2), across from the purified gas outlet (9).

26. The cyclone of claims 22 and 25, characterized in that the spray electrode (10) and the vortex finder (8) project into the casing inside (7) from axially opposing ends of the casing.

27. The cyclone of any one of the claims 1 through 26, characterized in that the separation region (5) and/or the particle collection region (6) are cylindrically shaped.

28. The cyclone of any one of the claims 1 through 27, characterized in that the separation region (5) and/or the particle collection region (6) have a truncated cone shape.

29. Method for the continuous separation of fine particles from a stream of gas, in particular soot from the exhaust gas of internal combustion engines, via flow effects in a rotating stream formed in a cyclone by means of which the particles are enriched in a particle collection region (6), wherein the cyclone has a casing (2) comprising an entrance region (4), into which an inlet (3) feeds, a separation region (5) disposed in the flow path of the gas downstream of the entrance region (4), and a particle collection region (6) disposed in the flow path of the particles downstream of the separation region (5), the cyclone having a purified gas outlet (9) connecting into the inside (7) of the casing (2), and a spray electrode (10), to which an electrical high voltage is applied, projecting into the inside (7) of the casing (2), the cyclone having the spray electrode (10) electrically insulated from the purified gas outlet (9), the purified gas outlet (9) connected to the casing (2) in an electrically conducting manner, and with which an insulator (11) bearing the spray electrode (10) is mounted inside the casing (2) at a separation from the purified gas outlet (9), characterized in that the method is adapted for separation of fine, electrically conducting particles in the size range smaller than 5 µm, in particular less than 1 µm, wherein the spray electrode (10) is configured in such a manner that the particles form agglomerates under the influence of the electric field forces of the spray electrode (10) and the insulator (11) bearing the spray electrode (11) is disposed in a casing extension (24) in which a zone of weak flow is formed.

30. The method of claim 29, characterized in that a cyclone according to any one of the claims 1 through 28 is utilized.