GB2173124A

GB2173124A - Removing particles from gas

Info

Publication number: GB2173124A
Application number: GB08607707A
Authority: GB
Inventors: Henry Kirsch
Original assignee: SCIENT DESIGN PLANT AND PROCES
Current assignee: SCIENT DESIGN PLANT AND PROCES
Priority date: 1985-03-29
Filing date: 1986-03-27
Publication date: 1986-10-08
Also published as: GB8607707D0; DE3610376A1

Abstract

Solid particles are removed from a hot gas by passing the gas through a bed of solid particles (13) to fluidise the bed. Water is added to the bed (14) to maintain the temperature of the bed below the boiling point of water and to maintain the moisture content of the bed at between 5% and 25% by mass. The water may be sprayed (14) on to the bed or may be provided in a paste made of solid particles withdrawn from the bed. <IMAGE>

Description

SPECIFICATION Removing particles from gas Background of the invention This invention relates to the removal of solid, finely divided particles from gases.

The invention is particularly concerned with the removal of particles from hot gases emitted by furnaces, fluidised bed boilers, dryers and the like.

In United States patent No. 3,719,029 it has been proposed to remove carbon and tar contained in a cracked gas product by spouting the product from the bottom of a vessel in which inorganic solid particles are contained, thus forming a spouted bed of solid particles in the vessel. Water of a liquid hydocarbon is sprayed in the spouted bed of solid particles to quench the product and simultaneously to allow the carbon and tar in the cracked gas product to stick and adhere onto the surface of the solid particles. The bed temperature is kept above 200 C so that water cannot exist as moisture on the solid particles, but, if it is sprayed on, merely acts as a quenching medium.

Summary of the invention According to the invention a gas containing particles is passed through a bed of fluidisable solids to fluidise the bed without entraining the fluidisable solids, and the moisture content of the bed is maintained at between 5 and 25% by mass by the addition of water, so that particles in the gas agglomerate on the particles in the bed.

Water may be added to the bed in one of two ways: (a) By spraying water on to or into the bed in an amount sufficient to maintain the temperature of the bed at below the boiling point of water and at the requisite moisture content. The water may be part of a lime or magnesite slurry and may contain binders.

(b) By continuously withdrawing some solids from the bed, wetting those solids and feeding the wetted solids back to the bed at a rate to maintain the temperature of the bed and the moisture content.

Temperature probes in the bed and moisture detecting sensors may provide control signals to adjust the rate of feed of water.

Description of the drawings Figure 1 is a diagramatic view of one separation device, Figure 2 is a similar view of a second separation device, and Figure 3 is a similar view of yet another device.

Description of embodiments In Figure 1 there is a separation vessel which has an inlet 10 and an outlet 11. Between the inlet and the outlet there is a perforated support 12 on which there is a bed 13 of fluidisable solids. Above the bed there is a water spray 14. Material overflows from the bed 13 along an overflow pipe 15.

In use dust laden gas is passed into the inlet 10 at a rate sufficient to fluidise the bed 13. At the same time water is sprayed on to the bed 13 through the spray 14 to keep the temperature of the bed 13 below 100 C. Clean gas passes from the outlet 10.

Note that if agglomerated particles from the bed 13 are light enough to escape, their large size and mass make them much easier to remove by conventional means than the original dust particles.

In Figure 2 the water spray is replaced by a system in which part of the particles overflowing into the pipe 15 is diverted into a mixer-conveyor 20 to which water is fed through a valve 21. The solids are passed to a screw feeder 22 which delivers them to a feed pipe 23. Thus some of the bed material is recirculated in a wetted condition to control the moisture content and the temperature of the bed 13.

Of the two embodiments just described the one of Figure 1 would be preferred in most cases due to its simplicity of construction and operation.

Should the particles in the incoming gas stream be too large to pass through the performations in the plate 12, the embodiment of Figure 3 is used.

In this case the plate 12 is above a swirl chamber 30 which has a tangential inlet pipe 31 along which the gas to be treated enter the chamber. Coarse particles move down to the narrow end of the chamber 30 into a blowing device 33 from which they are blown along the conduit 32 into the bed 13. The coarse particles thus also leave the bed along the pipe 15 in a wetted condition. The dust hazzard is thus decreased.

The material of the bed may be composed of the solids from the gas stream or of other materials.

Due to the low temperature of the bed, the fluidised charge can be either organic or inorganic.

In many cases a binder has to be added to the bed to ensure that the dust particle aggolerate with the bed particles. The binder could be organic or inorganic such as calcium lignosulphonate or sodium silicate.

In the process of the invention water in the form of moisture is an essential constituent of the fluidised bed. With trapped particles or such particles and a binder water forms a coating around the individual particles in the bed. This is in contrast to the prior art discussed above where tar is an essential constituent of the coating formed around particles in the spouted bed.

1. A method of removing solid particles from a gas comprising passing the gas through a bed of fluidisable solids to fluidise the bed without entraining the fluidisable solid and maintaining the moisture content of the bed at between 5 and 25% by mass by the addition of water, so that particles in the gas agglomerate on the particles in the bed.

2. The method claimed in claim 1 in which water is added to the bed by spraying in an amount sufficient to maintain the temperature of the bed at below the boiling of water and at the requisite moisture content.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Removing particles from gas Background of the invention This invention relates to the removal of solid, finely divided particles from gases. The invention is particularly concerned with the removal of particles from hot gases emitted by furnaces, fluidised bed boilers, dryers and the like. In United States patent No. 3,719,029 it has been proposed to remove carbon and tar contained in a cracked gas product by spouting the product from the bottom of a vessel in which inorganic solid particles are contained, thus forming a spouted bed of solid particles in the vessel. Water of a liquid hydocarbon is sprayed in the spouted bed of solid particles to quench the product and simultaneously to allow the carbon and tar in the cracked gas product to stick and adhere onto the surface of the solid particles. The bed temperature is kept above 200 C so that water cannot exist as moisture on the solid particles, but, if it is sprayed on, merely acts as a quenching medium. Summary of the invention According to the invention a gas containing particles is passed through a bed of fluidisable solids to fluidise the bed without entraining the fluidisable solids, and the moisture content of the bed is maintained at between 5 and 25% by mass by the addition of water, so that particles in the gas agglomerate on the particles in the bed. Water may be added to the bed in one of two ways: (a) By spraying water on to or into the bed in an amount sufficient to maintain the temperature of the bed at below the boiling point of water and at the requisite moisture content. The water may be part of a lime or magnesite slurry and may contain binders. (b) By continuously withdrawing some solids from the bed, wetting those solids and feeding the wetted solids back to the bed at a rate to maintain the temperature of the bed and the moisture content. Temperature probes in the bed and moisture detecting sensors may provide control signals to adjust the rate of feed of water. Description of the drawings Figure 1 is a diagramatic view of one separation device, Figure 2 is a similar view of a second separation device, and Figure 3 is a similar view of yet another device. Description of embodiments In Figure 1 there is a separation vessel which has an inlet 10 and an outlet 11. Between the inlet and the outlet there is a perforated support 12 on which there is a bed 13 of fluidisable solids. Above the bed there is a water spray 14. Material overflows from the bed 13 along an overflow pipe 15. In use dust laden gas is passed into the inlet 10 at a rate sufficient to fluidise the bed 13. At the same time water is sprayed on to the bed 13 through the spray 14 to keep the temperature of the bed 13 below 100 C. Clean gas passes from the outlet 10. Note that if agglomerated particles from the bed 13 are light enough to escape, their large size and mass make them much easier to remove by conventional means than the original dust particles. In Figure 2 the water spray is replaced by a system in which part of the particles overflowing into the pipe 15 is diverted into a mixer-conveyor 20 to which water is fed through a valve 21. The solids are passed to a screw feeder 22 which delivers them to a feed pipe 23. Thus some of the bed material is recirculated in a wetted condition to control the moisture content and the temperature of the bed 13. Of the two embodiments just described the one of Figure 1 would be preferred in most cases due to its simplicity of construction and operation. Should the particles in the incoming gas stream be too large to pass through the performations in the plate 12, the embodiment of Figure 3 is used. In this case the plate 12 is above a swirl chamber 30 which has a tangential inlet pipe 31 along which the gas to be treated enter the chamber. Coarse particles move down to the narrow end of the chamber 30 into a blowing device 33 from which they are blown along the conduit 32 into the bed 13. The coarse particles thus also leave the bed along the pipe 15 in a wetted condition. The dust hazzard is thus decreased. The material of the bed may be composed of the solids from the gas stream or of other materials. Due to the low temperature of the bed, the fluidised charge can be either organic or inorganic. In many cases a binder has to be added to the bed to ensure that the dust particle aggolerate with the bed particles. The binder could be organic or inorganic such as calcium lignosulphonate or sodium silicate. In the process of the invention water in the form of moisture is an essential constituent of the fluidised bed. With trapped particles or such particles and a binder water forms a coating around the individual particles in the bed. This is in contrast to the prior art discussed above where tar is an essential constituent of the coating formed around particles in the spouted bed. CLAIMS

3.The method claimed in claim 1 in which solids are continuously withdrawn from the bed and those solids are formed into a paste with water and the paste is fed back to the bed at a rate to maintain the moisture content of the bed and the temperature of the bed at below the boiling point of water.

4. The method claimed in any one of the above claims in which the water added to the bed contains a binder.

5. The method claimed in any one of the above claims in which the bed is positioned above a whirl chamber from which the gas passes into the bed, in which coarse particles are separated from the gas, and the coarse particles are collected and conveyed into the bed along a conduit.

6. A method of removing solid particles from a gas substantially as herein described with reference to Figures 1, 2 or 3 of the accompanying drawings.