GB2264442A - "fluidised bed apparatus and method of operation" - Google Patents

Abstract

A fluidised bed heat treatment apparatus is provided with nozzles (11) for a fluidising medium in only a central region so that the bed material is fluidised only in the central region (14) and has lateral regions (15) of unfluidised bed material over its full depth to serve as thermal insulation so that the walls (12) retaining the bed are not subject to the full temperature of the working (fluidised) portion of the bed. <IMAGE>

Description

Title: "Fluidised bed apparatus and method of operation".

Description of Invention This invention relates to fluidised bed apparatus of the kind in which a particulate material is fluidised and heated for the treatment of workpieces, and a method of operation of such a fluidised bed.

In the construction and operation of fluidised bed apparatus it is conventional for the bed to be maintained in a container, such as a tank, having a base and side walls, and for the bed to be fluidised and heated across the full area of the base so that the side walls are in contact with the hot, fluidised material which constitutes the bed.

As a result, the side walls of the container require to have sufficient structural strength to confine the mobile bed material and must be made of a material which will maintain such structural strength at the operating temperature of the bed over an extended period of time. Accordingly, the container housing the bed material is normally an expensive fabrication which requires to be constructed to high standards from specialised materials.

In many cases, the container will be of elongated form, resembling for example a trough. This is particularly the case where the apparatus is intended for the heat-treatment of rods or wires which are drawn continuously through the bed and require a relatively long-time treatment. To achieve high production rates it is accordingly necessary to utilise a long heat-treatment bed to give the required treatment time. However, thermal expansion of the fluid bed container becomes a significant problem with such long beds, thereby either limiting the effective length available, or requiring the adoption of specialised and expensive techniques to overcome the expansion problem.

The present invention, in a surprisingly simple and effective manner, significantly simplifies the design and construction of such fluidised bed apparatus and at the same time makes it possible to construct longer beds without the problems associated with thermal expansion.

According to a first aspect of the invention we provide a method of operating a fluidised bed apparatus in which the bed material is heated and fluidised only in a central region such that the heated and fluidised bed material is confined between regions of unfluidised bed material.

As a result, the side walls of a container in which the fluidised bed is maintained are largely insulated from the heated and fluidised region of the bed by the unfluidised regions of the bed material adjacent thereto. This in turn makes it possible to construct the side walls of the container in a simpler and cheaper fashion, using lighter and lower grade materials, and more relaxed engineering standards. Thus, whilst the container will need to be rather larger than conventionally required, it is simpler and cheaper to construct and maintain.

Additionally, because the side walls of the container do not achieve the operating temperature of the fluidised bed itself, the problems of thermal expansion in long trough-like containers do not arise, making it possible, if required, the utilise longer containers and higher production rates than can be achieved with conventional designs at present.

The invention further resides in fluidised bed apparatus for operation in accordance with this method, comprising a container for the bed material having a base and side walls, wherein means for heating and for fluidising bed material within the container are confined to a central region of the container such that, in use, a heated and fluidised central region of bed material is created between lateral regions of unfluidised bed material, which lateral regions extend fully from the base of the container up to the top of the bed.

In a typical arrangement, nozzles for the introduction of a fluidising medium are provided in only a central region of the base of the container. The proportion of the width of the base occupied by such fluidising nozzles will depend on the depth of the bed relative to the width of the base, since the fluidised zone will widen upwardly away from the base and consequently the greater the depth of the bed, the smaller the proportion of the width of the base that can be provided with fluidising nozzles.

The fluidising medium may be any medium conventionally used, most conveniently air, and the bed may be heated by combustion of any suitable gaseous or liquid fuel also introduced into the central region of the bed by suitable nozzles, with or without additional combustion air. Alternatively, in some cases, it may be possible to employ internal electric heating means confined to the central region of the bed.

It will be appreciated that the temperature of the bed material in the unfluidised regions will increase, when the apparatus is in use, but without direct exposure to combustion gases or electric heating elements as in the central region the temperature of the material in such unfluidised regions will steadily decrease in the direction away from the fluidised central area towards the side walls.

Theoretically, the overall width of the bed could be such that at its outside edges the bed material remains at ambient temperature. Indeed, in such a case the bed could take the form of a free-standing pile of material on a base plate without side walls. However, in practice, to avoid excessive increases in the space required, it is sufficient for the overall width of the bed to be restricted by the side walls of the container and for the thickness of the unfluidised lateral regions of the bed to be sufficient, relative to the desired operating temperature of the fluidised region, to ensure that the temperature of the bed material at the side walls is such that it is not necessary to utilise heat resistant materials for the construction of the side walls.

Thermal insulation may be provided outside the container to improve the thermal efficiency and reduce heat loss without causing the temperature of the side walls to become unacceptably high.

The invention will now be described by way of example with reference to the accompanying drawing which illustrates one embodiment of the fluidised bed apparatus in accordance with the invention, in a transverse cross-sectional view.

As illustrated, by way of example only, fluidised bed apparatus in accordance with the invention may comprise an elongated generally flat base plate 10 for supporting a bed of fluidisable particulate material such as sand. In the longitudinally central region of the bed a plurality of nozzles 11 of conventional design extend upwardly through the base plate. In the illustrated arrangement these nozzles are provided with a combustible gas and combustion air so as simultaneously to provide both fluidisation and heating of the bed material.

However, where appropriate, additional, separate fluidisation nozzles may be provided, and/or the bed may be heated by the combustion of a liquid fuel therein.

Because the nozzles are confined to a central region of the base plate 10, the bed material is heated and fluidised in only a central region 14 which extends upwardly away from the base plate and widens towards the top of the bed. Typically, the included angle between the diverging lateral margins of the fluidised region is of the order to dO" .

As can be seen, outside the fluidised central region 14, there are two lateral regions 15 of unfluidised bed material. These are confined between side walls 12 with external insulation 13.

The depth of the bed material is so chosen that at the top of the bed the heated and fluidised region does not extend up to the lateral edges of the bed, so that throughout the whole height of the bed there remains an unfluidised region of material which serves to insulate the side walls 12.

As a result, the side walls 12 can be made from materials which do not require to be heat resistant, and can be in the form of a relatively thin membrane of steel or the like.

Even with the provision of the insulating material 13, under operating conditions the temperature of the side walls 12 will remain relatively low and as a result the length of the fluidised bed may be substantially increased without encountering thermal expansion problems.

Claims (9)

CLAIMS:

1. A method of operating a fluidised bed apparatus in which the bed material is heated and fluidised only in a central region such that the heated and fluidised bed material is confined between regions of unfluidised bed material.

2. Fluidised bed apparatus for operation in accordance with the method of Claim 1 and comprising a container for the bed material having a base and side walls, wherein means for heating and for fluidising bed material within the container are confined to a central region of the container such that, in use, a heated and fluidised central region of bed material is created between lateral regions of unfluidised bed material, which lateral regions extend fully from the base of the container up to the top of the bed.

3. Apparatus according to Claim 2 wherein nozzles for the introduction of a fluidising medium are provided in only a central region of the base of the container.

4. Apparatus according to Claim 3 wherein the fluidising medium is air.

5. Apparatus according to Claim 2 or Claim 3 wherein the bed is heated by combustion of any suitable gaseous or liquid fuel also introduced into the central region of the bed by suitable nozzles.

6. Apparatus according to Claim 2 or Claim 3 wherein the bed is heated by internal electric heating means confined to the central region of the bed.

7. Apparatus according to any one of Claims 2 to 6 wherein the container is in the form of an elongated trough.

8. A method of operating fluidised bed apparatus as claimed in Claim 1 and substantially as hereinbefore described.

9. Fluidised bed apparatus constructed and operating substantially as hereinbefore described with reference to and as shown in the accompany drawing.