GB1576937A

GB1576937A - Apparatus for removing from a gas particles suspended therein

Info

Publication number: GB1576937A
Application number: GB886277A
Authority: GB
Original assignee: ROCHEFOUCAULD M A G DE
Current assignee: ROCHEFOUCAULD M A G DE
Priority date: 1977-03-02
Filing date: 1977-03-02
Publication date: 1980-10-15

Description

(54) APPARATUS FOR REMOVING FROM A GAS PARTICLES SUSPENDED THEREIN (71) I, MATHIEU ALBERT GEORGES DE LA ROCHEFOUCAULD, a French Citizen, of 22 Boulevard Flandrin, 75116, Paris, France, do hereby declare the invention for which I pray that a patent may be granted to me and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to an apparatus for removing from a gas particles suspended therein and finds particular application in the purification of industrial fumes such as fumes from steelworks, oil-fired thermal power stations or from the calcination of household refuse.

A great number of apparatus for removing dust from gas exist; those nearest the subject matter of the invention are scrubbers and electro static filters.

In scrubbers, drops of water are passed through a gas to be de-dusted. Any particle touched by a droplet agglomerates thereon and is thereby taken out of the flow of gas. The probability of impact of the droplets on the particles is a function of the number of droplets, thus of the flow of water, but also of the relative speed of the droplets and the particles; this probability of impact is seriously affected by a phenomen of circumvention whereby the dust suspended in the gas tends to follow the movements of this gas and therefore to move aside to make way for the droplets. This effect of circumvention accentuates very rapidly when the size of the particles diminishes, but on the other hand it is reduced when the relative speed of the droplets and the particles increases.It is therefore a high relative speed between droplets and particles which characterizes the functioning of good scrubbers. However, despite. the improvements which have been made, known scrubbers tend to be bulky, and expensive to use.

In electrostatic filters, the gas suspended particles are electrically charged and are electrostatically deposited on an electrode in the form of a plate whose charge is of opposite sign to that of the charged particles. The deposited particles are then recovered from the electrode by mechanical means or by washing. The speed at which the particles approach the collector electrode diminishes very rapidly with their diameter.Therefore, if it is desired to pick up the finest particles, they must be left to dwell a long time in the electric field, implying a low speed of circulation of the gas Also, in order to achieve satisfactory operation of an electrostatic filter, it is desirable to achieve laminar gas flows, uniform gas flow rates, to maintain the electrodes and the insulators clean, to control the temperature of the gas, and accurate geometry of the electrodes. Thus, although electrostatic filters are effective they are expensive, fragile and very bulky; moreover, they are only likely to operate correctly for about three years.

A known improvement for an electrostatic filter consists in separating the zone where the particles are electrically charged from the zone where they are picked up. The distance between plates may then be considerably reduced and a more laminar flow for satisfactory speeds of passage of the gas may be obtained. This type of filter, which is very efficient, has not found industrial application, on the one hand because it is very expensive to construct and on the other hand because the cleaning of the plates requires operation of the filter to be shut down periodically.

Various attempts have been made to combine scrubbers and electrostatic filters.

In a first known apparatus water droplets are injected, at low pressure, into the shaft of a scrubber, the droplets being charged to 50 KV.

The droplets form a summarily conical sheet joining the scrubber wall. The wall is maintained at earth potential. The gas passes through this sheet at high speed and leaves its dust therewith. The improvement of the performance of the scrubber is considerable, but the method is still not very effective for ultra-fine dust. A variant of this apparatus consists in causing the gas particle mixture to first pass at high speed through Venturi in which the particles are charged with the sign contrary to that of the droplets of the sheet of water. In these arrangements the major particle removing effect is kinetic as in a conventional scrubber and the electrostatic attraction produced by the applied potential(s) is in complement of this effect to increase the probability of collision of the drop lets and the particles.

In another known apparatus a liquid is atomised and the droplets obtained are mixed with a gas particle mixture, the mixed flow then being electrostatically purified in conventional manner. The aim of this arrangement is that the droplets collected by the electrodes, continuously wash them, considerably reducing the risk of deposited particles building up on the electrodes, this making it possible to reduce the distance between ionisation electrodes and pick-up electrodes, the surface area of the electrodes and hence the volume of the filter.

In contrast with these prior arrangements the present invention provides an apparatus for removing particles suspended in a gas, comprising means for charging to a first polarity particles suspended in a gas, un upwardly extending chamber, means to cause the gas and the charged particle suspension to flow upwardly through the chamber, a supply of material for producing generally globular coalescing centres for the particles in the gas, injector means arranged to inject said material into the gaseous flow in the chamber in such a manner as to provide therein said globular coalescing centres electrically charged to an opposite polarity to the gas suspended particles, the apparatus being so arranged that in use the globular coalescing centres are practically suspended (as defined herein) in said upward gaseous flow, and means for separating the globular coalescing centres from the gas.

By "practically suspended" it is meant that the tendency of the globular coalescing centres to fall downwardly through the chamber is counteracted by the upwardly flowing stream of gas such that the globular centres tend to drift through the chamber at an overall flow rate much less than that of the upward flow rate of the gas or the rate at which they would fall naturally in the absence of the gas flow.

The object of having the globular coalescing centres practically suspended in this way is to maximize the time that they spend in contact with the gas to collect the gas suspended particles.

In one embodiment of the invention, the practically suspended globular coalescing centres drift upwardly through the chamber at a rate less than the upward flow rate of the gas.

In order that the invention may be more fully understood an embodiment thereof will now be described by way of illustrative example with reference to the single Figure of the accompanying drawing.

The apparatus includes a pipe 1 for the admission of the gas particle mixture to be puri fied, an electrode arrangement 2 interposed in the pipe 1, in which the particles suspended in the gas are electrically charged to a given polarity by means of a barbed electrode 2a held by the regulated electrical source 7 to a potential of KV with respect to earth. The electrode 2a is disposed along the axis of the pipe 1 whose walls may be taken to an electrical potential opposite to that of the axial electrode 2a. The gas passes the electrodes 2 too quickly for its suspended particles to stick to the walls. The gas with its charged particle suspension then passes from the pipe 1 into an upwardly extending chamber in the form of a conduit 3.

A series of injectors 4 is disposed around the conduit 3, the injectors being arranged to introduce into the ascending flow of gas, solid or liquid globules which act as coalescing centres for the gas suspended particles. The diameter of these globules is calibrated so that they are practically in suspension in said gas, with, on average, a very low ascending speed, which is much less than the upward flow rate of the gas particle mixture. All these globules are therefore of substantially identical weight.

The globules are also electrically charged to a polarity opposite to that of the charged particles of the gas to be purified; they are mixed with the latter and attract the particles.

One means, among others, of obtaining these globules consists of disposing on a platform 8, itself mounted on electrical insulators 9, a water tank 10 and a pump 11; the pump may be supplied by a transformer 12 with separate windings; The platform 8 is held to a potential of polarity opposite that of the charge of the gas suspended particles, by means of a regulated supply 13 the pump supplies the injectors 4 which are placed around the conduit 3 above the point of admission of the gas in this conduit. The injectors 4 thus form electrically charged droplets of substantially the same size.To avoid on the one hand the potential of the platform 8 being returned to earth by a contact of the nozzles 4 with the wall of the conduit 3 and on the other hand the periphery of the nozzles 4 from being covered with the particulate dust contained in the gas to be purified, each nozzle 4 is disposed in a short tube 14 without touching the walls thereof so that the jet of droplets of water emitted by the nozzle 4 takes along a certain amount of outside air to the inside of the conduit 3. Each tube 14 passes through the wall of the conduit 3.

In the zone of the conduit 3 located above the nozzles injecting the globules, the globules are practically suspended by the gas flow and thus have a very low upward speed; consc:- quently, the conditions for their picking up even the finest particles suspended in the gas are good.

When the gas to be purified arrives at the top of the conduit 3, it passes through a conduit 5 of smaller section towards a separator 6 which recovers the globules. This separator may be a simple decantation chamber for calibrated solid globules or a cyclonic separator for liquid globules.

The globules can then be regenerated by washing, if they are solid or for example by de cantation if it is water.

It should be noted that a secondary effect of selective dissolution of certain harmful vapours may be obtained by using a suitable liquid for the globules.

Below the point of admission of the gas to be purified in the conduit 3, there is provided an inclined plane 15 directed towards an evacuation conduit 16 opening into conduit 3.

The globules which are not associated with particles from the gas to be purified, are removed through this conduit 16.

In an alternative to the use of water droplets, the globules can be in the form of moistened solid (e.g. plastics) granules, in which case the quantity of water used is very small.

This water is advantageously recycled as follows: the water is separated from the globules leaving the coalescence conduit 3, then it is purified and serves to moisten clean globules to be reinjected into the conduit 3 through nozzles 4.

The nozzles 4 could of course be placed at the upper end of the conduit 3. Also the globules can be placed in an additional flow of air or gas before being introduced into the conduit 3.

WHAT I CLAIM IS: 1. An apparatus for removing particles suspended in a gas, comprising means for charging to a first polarity particles suspended in a gas, an upwardly extending chamber, means to cause the gas and the charged particle suspension to flow upwardly through the chamber, a supply of material for producing generally globular coalescing centres for the particles in the gas, injector means arranged to inject said material into the gaseous flow in the chamber in such a manner as to provide therein said globular coalescing centres electrically charged to an opposite polarity to the gas suspended particles, the apparatus being so arranged that in use the globular coalescing centres are practically suspended (as defined herein) in said upward gaseous flow, and means for separating the globular coalescing centres from the gas.

2. An apparatus according to claim 1 wherein said generally globular coalescing centres are liquid droplets.

3. An apparatus according to claim 1 wherein said generally globular coalescing centres are of a solid material.

4. An apparatus according to claim 3 wherein said globular coalescing centres comprise moistened granules of plastics material.

5. An apparatus according to any preceding claim wherein in use said globular coalescing centres move upwardly in said chamber at a rate less than the upward flow rate of the gas.

6. An apparatus according to claim 2 including means arranged to place the droplets in suspension in an additional flow of air or gas before being introduced into said chamber.

7. An apparatus according to claim 5 including a pipe connected to an inlet at the bottom of said chamber, to supply the gas particle mixture to the chamber, said pipe having associated therewith an electrode arrange ment for charging said particles, an outlet adjacent the top of said chamber, and a separator connected to said outlet and arranged to separate said globular coalescing centres from the gas whereby to purify the gas.

8. An apparatus according to claim 7 wherein said electrode arrangement includes a spiked wire arranged axially of the pipe, and including an electrical power source connected to said wire.

9. An apparatus according to claim 7 or 8 wherein said separator comprises a decantation chamber.

10. An apparatus according to claim 7 or 8 wherein said separator comprises a cyclonic separator.

11. An apparatus according to claim 7 or 8 including nozzles disposed between said inlet and outlet, to inject said globular coalescing centres into the chamber.

12. An apparatus according to claim 11 wherein each said nozzle is disposed in a tube without touching the wall thereof, the tube passing through the wall of said chamber.

13. An apparatus for removing particles suspended in a gas, substantially as hereinbefore described with reference to the accompanying drawing.

14. A method of removing particles suspended in a gas, comprising electrically charging the particles to a first polarity, establishing an ascending flow of said gas with the charged particles therein, introducing globules of material into said ascending gas flow, said globules being electrically charged to an opposite polarity to said particles, so as to act as coalescing centres for said particles, said globules being practically suspended (as herein defined) in said ascending gas flow, and removing said globules from the gas thereby to purify the gas.

15. A method according to claim 14 wherein said globules include a liquid selected to dissolve particular vapours from said ascending gas flow.

16. A method of removing particles suspended in a gas, substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. cantation if it is water. It should be noted that a secondary effect of selective dissolution of certain harmful vapours may be obtained by using a suitable liquid for the globules. Below the point of admission of the gas to be purified in the conduit 3, there is provided an inclined plane 15 directed towards an evacuation conduit 16 opening into conduit 3. The globules which are not associated with particles from the gas to be purified, are removed through this conduit 16. In an alternative to the use of water droplets, the globules can be in the form of moistened solid (e.g. plastics) granules, in which case the quantity of water used is very small. This water is advantageously recycled as follows: the water is separated from the globules leaving the coalescence conduit 3, then it is purified and serves to moisten clean globules to be reinjected into the conduit 3 through nozzles 4. The nozzles 4 could of course be placed at the upper end of the conduit 3. Also the globules can be placed in an additional flow of air or gas before being introduced into the conduit 3. WHAT I CLAIM IS:

1. An apparatus for removing particles suspended in a gas, comprising means for charging to a first polarity particles suspended in a gas, an upwardly extending chamber, means to cause the gas and the charged particle suspension to flow upwardly through the chamber, a supply of material for producing generally globular coalescing centres for the particles in the gas, injector means arranged to inject said material into the gaseous flow in the chamber in such a manner as to provide therein said globular coalescing centres electrically charged to an opposite polarity to the gas suspended particles, the apparatus being so arranged that in use the globular coalescing centres are practically suspended (as defined herein) in said upward gaseous flow, and means for separating the globular coalescing centres from the gas.