GB1558969A - Fluidised beds - Google Patents

Description

(54) FLUIDISED BEDS (71) We, APOLLO HEAT LIMITED, formerly SHACKLEBEST LIMITED, a British Company, of Leewood House, Clows Top, Nr. Kidderminster, Worcestershire, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a fluidised bed and in particular to a fluidised bed which is intended for use in carrying out metallurgical heat treatment processes on a component or components immersed in the fluidised bed.

To this end, it is known to provide a bed which is formed of refractory particles and a gas, or gas/air mixture, which not only provides a sufficient velocity of gas or gas/air flow for fluidising the bed but which also provides within the bed in use the desired atmosphere for providing the required heat treatment process, it being understood that heat will also be applied to the bed in any convenient manner. Such heat treatment processes may comprise incidentally carbonitriding, oxy-carbo-nitriding, carburizing neutral annealing, neutral stress relieving and neutral hardening, the gas or gas/air mixture admitted to the bed having an appropriate composition to provide the correct atmosphere for the desired treatment.

The object of the present invention is to provide an improved fluidised bed.

In accordance with the invention there is provided a fluidised bed comprising a container which contains a mass of refractory particles and which is provided with a bottom which is permeable to a gas or gas/air mixture that in use will be admitted to the interior of the container from the underside thereof for the purpose of fluidising said refractory particles and for the purpose of providing a desired treatment atmosphere within the bed when fluidised, the permeability of the permeable bottom of said container in relation to the permeability of the fluidised bed itself being so arranged that, at a velocity of gas or gas/air flow through the fluidised bed of 2.0-3.0 times the minimum velocity required for fluidisation, the pressure drop across said permeable bottom is not less than 15% of the total pressure drop across the permeable bottom and the fluidised bed.

Conveniently, the aforesaid pressure drop across said permeable bottom lies within the range of 15%-30% of the total pressure drop across the permeable bottom and the fluidised bed.

The permeable bottom of the container conveniently comprises a porous tile made of a ceramic or high temperature metallic material together with one or more filter sheets which are conveniently made of glass fibre.

The invention will now be more particularly described with reference to the accompanying drawing which is a fragmentary sectional elevation of one example of a fluidised bed constructed in accordance with the invention.

Referring to the drawing, there is shown therein a fluidised bed which comprises a container 10 having a wall 11 which is formed of any convenient material and of any convenient shape together with a permeable bottom 12, the container containing a mass of refractory particles 13 such as aluminium oxide which in use will form a fluidised bed. A gas or gas/air mixture intended both for the purpose of fluidising said particles 13 and for tht purpose of providing a treatment atmosphere for heat treating a component or components lowered into the bed when fluidised from the upper end thereof will be admitted to the container 10 in use from the underside of said permeable bottom by any convenient conduit means 14.

Also there is provided in use any suitable means for heating the fluidised bed and such means may comprise for example a further fluidised bed of annular configuration (not shown) which surrounds the abovementioned treatment bed. Alternatively, the initial heating may be effected by passing a fuel rich mixture through said permeable bottom 12 and by admitting for a limited time secondary air for combustion within the bed via apertures formed in a pipe 15 which projects into the container and which is connected to a compressed air conduit 16.

Furthermore there may be provided a hood or other closure (not shown) for the upper end of the container and through which the component or components (also not shown) to be treated will be lowered into the bed.

It is known that a certain minimum velocity of gas or gas/air flow must take place through the bed of refractory particles in order to produce fluidisation thereof. Above this minimum fluidisation velocity however the velocity of flow not only affects the heat transfer characteristics of the bed but also the rate of thermo-chemical reaction between the atmosphere within the fluidised bed and the surfaces of the component or components which are being treated in the bed. It has now been discovered that the optimum fluidisation velocity for optimum reaction rates to occur is substantially the same as for optimum heat transfer, such optimum fluidisation velocity being between 2.0 to 3.0 times the minimum fluidisation velocity at treatment temperatures.

It has also been discovered that the characteristics of the fluidised bed are most influenced by the pressure drop across the permeable bottom 12 of the container 10 and to enable uniform distribution of gas or gas/air mixture throughout the fluidised bed. and therefore fluidisation across the whole area of the treatment zone, the ratio of the pressure drop across the permeable bottom and the fluidised bed itself is important.

To this end. in accordance with the present invention, the pressure drop across said permeable bottom 12 should be not less than 15% of the total pressure drop across the permeable bottom and the fluidised bed together and desirably such pressure drop across the permeable bottom should lie within the range of 15C7o30% of the aforesaid total pressure drop.

Conveniently the aforesaid permeable bottom may be constituted by a porous tile 17 formed of a ceramic material or a higher temperature metallic material to which is added on the underside thereof one or more sheets of filter material 18 which may be formed from glass fibre, the number of sheets added to the porous tile being chosen to provide the relative pressure drop within the above stated range. It is of course to be understood that the pressure drop across the permeable bottom and also the pressure drop across the fluidised bed itself can very readily be measured by known insturments so that the performance of the fluidised bed in use can very readily be assessed and the permeability of the permeable bottom adjusted to provide the above-mentioned characteristics.

Thus in one particular example (which is not to be taken in limiting sense) there may be provided a bed which is formed of aluminium oxide particles of 250 micron size to a 20 inch depth which, at a fluidisation velocity approximately equal to 2.5 times the minimum fluidisation velocity, will give a total pressure drop of 33.4 inches water gauge. A porous ceramic tile which can be used to form part of the permeable bottom to the container typically produces a pressure drop of 4 inches water gauge and must therefore be augmented by filter sheets formed of glass fibre which may each provide a 2 inch water gauge pressure drop. Two, three or four of such filter sheets 18 would therefore be required to provide a pressure drop through the sheets when combined with the porous tile which lies within the range 15%a 30% of the pressure drop through the whole system of permeable bottom and fluidised bed. The ceramic tile 17 plus the filter sheets 18 will of course provide for even distribution of gas or gas/air mixture through the fluidised bed and at the same time will provide the desired ratio of pressure drops in accordance with the present invention.

WHAT WE CLAIM IS: 1. A fluidised bed comprising a container which contains a mass of refractory particles and which is provided with a bottom which is permeable to a gas or gas/air mixture that in use will be admitted to the interior of the container from the underside thereof for the purpose of fluidising said refractory particles and for the purpose of providing a desired treatment atmosphere within the bed when fluidised, the permeability of the permeable bottom of said container in relation to the permeability of the fluidised bed itself being so arranged that, at a velocity of gas or gas/air flow through the fluidised bed of 2.0-3.0 times the minimum velocity required for fluidisation, the pressure drop across said permeable bottom is not less than 15% of the total pressure drop across the permeable bottom and the fluidised bed.

2. A fluidised bed as claimed in Claim 1 wherein said pressure drop across said permeable bottom lies within the range of 15-30% of the total pressure drop across the permeable bottom and the fluidised bed.

3. A fluidised bed as claimed in either of the preceding Claims, wherein the permeable bottom of the container comprises a porous tile made of a ceramic material or a high temperature metallic material together with one or more filter sheets made of glass fibre.

4. A fluidised bed substantially as

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. shown) which surrounds the abovementioned treatment bed. Alternatively, the initial heating may be effected by passing a fuel rich mixture through said permeable bottom 12 and by admitting for a limited time secondary air for combustion within the bed via apertures formed in a pipe 15 which projects into the container and which is connected to a compressed air conduit 16. Furthermore there may be provided a hood or other closure (not shown) for the upper end of the container and through which the component or components (also not shown) to be treated will be lowered into the bed. It is known that a certain minimum velocity of gas or gas/air flow must take place through the bed of refractory particles in order to produce fluidisation thereof. Above this minimum fluidisation velocity however the velocity of flow not only affects the heat transfer characteristics of the bed but also the rate of thermo-chemical reaction between the atmosphere within the fluidised bed and the surfaces of the component or components which are being treated in the bed. It has now been discovered that the optimum fluidisation velocity for optimum reaction rates to occur is substantially the same as for optimum heat transfer, such optimum fluidisation velocity being between 2.0 to 3.0 times the minimum fluidisation velocity at treatment temperatures. It has also been discovered that the characteristics of the fluidised bed are most influenced by the pressure drop across the permeable bottom 12 of the container 10 and to enable uniform distribution of gas or gas/air mixture throughout the fluidised bed. and therefore fluidisation across the whole area of the treatment zone, the ratio of the pressure drop across the permeable bottom and the fluidised bed itself is important. To this end. in accordance with the present invention, the pressure drop across said permeable bottom 12 should be not less than 15% of the total pressure drop across the permeable bottom and the fluidised bed together and desirably such pressure drop across the permeable bottom should lie within the range of 15C7o30% of the aforesaid total pressure drop. Conveniently the aforesaid permeable bottom may be constituted by a porous tile 17 formed of a ceramic material or a higher temperature metallic material to which is added on the underside thereof one or more sheets of filter material 18 which may be formed from glass fibre, the number of sheets added to the porous tile being chosen to provide the relative pressure drop within the above stated range. It is of course to be understood that the pressure drop across the permeable bottom and also the pressure drop across the fluidised bed itself can very readily be measured by known insturments so that the performance of the fluidised bed in use can very readily be assessed and the permeability of the permeable bottom adjusted to provide the above-mentioned characteristics. Thus in one particular example (which is not to be taken in limiting sense) there may be provided a bed which is formed of aluminium oxide particles of 250 micron size to a 20 inch depth which, at a fluidisation velocity approximately equal to 2.5 times the minimum fluidisation velocity, will give a total pressure drop of 33.4 inches water gauge. A porous ceramic tile which can be used to form part of the permeable bottom to the container typically produces a pressure drop of 4 inches water gauge and must therefore be augmented by filter sheets formed of glass fibre which may each provide a 2 inch water gauge pressure drop. Two, three or four of such filter sheets 18 would therefore be required to provide a pressure drop through the sheets when combined with the porous tile which lies within the range 15%a 30% of the pressure drop through the whole system of permeable bottom and fluidised bed. The ceramic tile 17 plus the filter sheets 18 will of course provide for even distribution of gas or gas/air mixture through the fluidised bed and at the same time will provide the desired ratio of pressure drops in accordance with the present invention. WHAT WE CLAIM IS:

1. A fluidised bed comprising a container which contains a mass of refractory particles and which is provided with a bottom which is permeable to a gas or gas/air mixture that in use will be admitted to the interior of the container from the underside thereof for the purpose of fluidising said refractory particles and for the purpose of providing a desired treatment atmosphere within the bed when fluidised, the permeability of the permeable bottom of said container in relation to the permeability of the fluidised bed itself being so arranged that, at a velocity of gas or gas/air flow through the fluidised bed of 2.0-3.0 times the minimum velocity required for fluidisation, the pressure drop across said permeable bottom is not less than 15% of the total pressure drop across the permeable bottom and the fluidised bed.

2. A fluidised bed as claimed in Claim 1 wherein said pressure drop across said permeable bottom lies within the range of 15-30% of the total pressure drop across the permeable bottom and the fluidised bed.

3. A fluidised bed as claimed in either of the preceding Claims, wherein the permeable bottom of the container comprises a porous tile made of a ceramic material or a high temperature metallic material together with one or more filter sheets made of glass fibre.

4. A fluidised bed substantially as

hereinbefore described with reference to and as shown in the accompanying drawing.