GB2065493A

GB2065493A - Reducing particle loss from fluidised beds

Info

Publication number: GB2065493A
Application number: GB8033820A
Authority: GB
Original assignee: Stone Platt Fluidfire Ltd
Current assignee: Stone Platt Fluidfire Ltd
Priority date: 1979-10-20
Filing date: 1980-10-20
Publication date: 1981-07-01
Also published as: GB2065493B

Abstract

The loss of particles from a fluidised bed is reduced by providing near to an upper surface of the bed a plurality of elements defining between them gaps which are sufficiently large for the bed particles to pass therethrough and are distributed substantially evenly across the bed. A heat exchanger comprises a layer of horizontal tubes (18) bearing vertical fins (19) which, in use, are submerged in a fluidised bed. A corrugated sheet (22) of expanded metal rests on the fins and is largely submerged in the bed, when fluidised. Particles tend to pass upwardly through the gaps defined by the expanded metal above the fins (19) and to pass downwardly through the expanded metal above the gaps (24) between adjacent rows of fins. <IMAGE>

Description

SPECIFICATION Apparatus comprising fluidised bed and method of reducing loss of particles from bed This invention relates to apparatus comprising a fluidised bed.

There are practical limits to the rate at which gas can be passed through a fluidised bed. If the gas velocity through the bed is too small, the bed will not be properly fluidised; If the gas velocity through the bed is too large, particles are carried away from the bed by the gas stream.

Generally, a high gas flow rate is desirable because this enables the size and/or cost of the apparatus to be reduced. In many cases, the gas flow rate is varied during use. The maximum gas flow rate is selected in view of a number of factors, including the expected proportion of operating time for which the gas flow rate will have the maximum value and the rate of loss of particles from the bed which can be tolerated. Some loss of particles is to be expected owing to degradation of particles in the bed and entrainment of the resulting fines in the gas stream.

It has been proposed to install a sieve in apparatus containing a fluidised bed so that gases leaving the apparatus must pass through the sieve. Published British Patent Application No.

2,008,732 describes a fluidised bed for burning coal particles wherein a sieve is spaced from the fluidised bed to lie between the bed and an outlet for the combustion gases and to remove from the combustion gases leaving the bed particles which are entrained in the gases.

A sieve can reduce the loss of particles from the bed but has the serious disadvantage that a substantial pressure drop occurs across the sieve, particularly as the sieve tends to become blocked by matter deposited from the gas stream.

The present invention concerns an alternative way of reducing the loss of particles from a fluidised bed at high gas flow rates. The distribution of the gas flow through a fluidised bed tends to be uneven; there are relatively high velocity regions and relatively low velocity regions.

These regions may be transitory but the presence of a stationary structure, for example a heat exchange tube, in the bed tends to impose a more permanent uneven gas flow distribution on the bed. In the relatively high velocity regions the upwards velocity of the gas is sufficiently high to cause localised upward movement of particles and where, as is generally the case, a relatively high velocity region extends to an upper surface of the bed, a mass of particles erupts from the general level of the bed. Some of these particles can become entrained in the gas stream leaving the bed. It is usual to confine the gas stream at a position somewhat downstream of the bed to a path having a cross-sectional area considerably less than the superficial area of the fluidised bed. This results in a corresponding increase in gas velocity and ability of the gas stream to entrain solid particles.It will be understood that the purpose of restriction of the crosssectional area of the gas flow path is to minimise the size and cost of the apparatus.

We have discovered a way of modifying the eruption of particles from a fluidised bed to reduce the risk of particles being carried away from the bed by the gas stream.

According to a first aspect of the invention, there is provided apparatus comprising a fluidised bed of particles and having near to an upper surface of the bed elements defining between them gaps which are sufficiently large for the particles of the bed to pass through them and which are at least approximately evenly distributed across the bed.

The elements provided in accordance with the invention are not capable of functioning as a sieve, since the gaps between the elements are larger than the particles. Surprisingly, elements defining gaps which are many times larger than the particles reduce markedly upward movement of the particle beyond the general level of the bed.

A fiuidised bed in which the gas flow rate exceeds the rate necessary just to fluidise the bed does not have a well defined surface. However, there can be discerned a general level at which eruptions of particles occur. The part of the bed which lies at this level is referred to herein as the surface of the fluidised bed.

By describing the elements as being near to the upper surface of the bed, we mean that the elements are not so far below the surface that the elements have no significant effect on the eruption of particles from the surface and are not so far away from the bed that the gas flow path is restricted to cause gas approaching the elements to have a velocity exceeding the superficial gas velocity in the fluidised bed.

Preferably, said elements and the gaps defined thereby collectively extend across an area substantially equal to the area of the surface of the bed. The open proportion of this area, that is the proportion of the area across which the elements and the gaps collectively extend which is occupied by the gaps, is preferably at least 50% and is more preferably within the range 60% to 95%.

At least some of said elements may be situated in respective positions such that, when the bed is slumped, they lie above the surface of the bed. It is also preferred that at least some of the elements should be submerged in the fluidised bed.

The preferred form of apparatus comprises a plurality of horizontal heat exchanger tubes arranged side by side and each having vertical fins. In this preferred form of apparatus, the elements are arranged to form a corrugated layer with the crest of each corrugation lying directly above a corresponding heat exchange tube and the valley of each corrugation lying directly above a gap between adjacent tubes. The elements may rest on the fins.

According to a second aspect of the invention, there is provided a method of reducing the loss of particles from a fluidised bed wherein there is provided near to an upper surface of the bed a plurality of elements defining between them gaps which are sufficiently large for particles of the bed to pass through them and which are approximately evenly distributed across the bed.

An example of apparatus embodying the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 shows a cross-section of a gas to liquid heat exchanger; Figure 2 is a plan view on an enlarged scale of certain elements of the heat exchanger; and Figure 3 shows certain parts of Fig. 1 on a scale intermediate those of Figs. 1 and 2.

The heat exchanger comprises a chamber 10 of rectangular shape, as viwed in plan, having an upper wall 11 and a lower wall 1 2. In the lower wall 1 2 adjacent to one end of the chamber, there is provided a gas inlet 1 3 and a gas outlet opening 14 is provided in an opposite end wall 1 5 adjacent to the upper wall 11. The gas outlet leads to an exhaust duct which may be a chimney in a case where gases passing through the heat exchanger are to be exhausted to the atmosphere.

The chamber 10 is divided into upper and lower parts by a horizontal wall 16 which lies somewhat nearer to the lower wall 1 2 than it does to the upper wall 11. The wall 1 6 is pervious to gases and, in the particular example illustrated, is formed by a porous ceramic plate.

Alternatively. the wall 1 6 could be in the form of a pierced metal plate. Above the wall 1 6, there lies a bed 1 7 of refractory particles, for example alumina. During operation of the heat exchanger, the bed 1 7 is fluidised by gas entering through the inlet 1 3 and leaving the chamber 10 through the outlet 1 4. In both the slumped condition of the bed illustrated and in the fluidised condition, the upper surface of the bed lies well below the outlet opening 14.

There is also provided in the upper part of the chamber 10 an array of heat exchange tubes 1 8 through which liquid is circulated. In the particular example illustrated, the tubes 1 8 are arranged side by side in a single layer, the tubes being horizontal and parallel to each other. On each tube there is provided a number of vertical fins 19, each fin being of annular form and having a diameter two to three times the diameter of the tubes 1 8. On each tube, the fins are typically spaced apart one quarter inch along the length of the tube. When the bed 1 7 is slumped. the tubes 1 8 are submerged in the bed but a part of each fin 1 9 protrudes from the bed. When the bed is fluidised, the fins are substantially submerged.

Near to the surface of the bed 1 7, there is provided a layer of elements 20 which modify the eruption of particles from the bed. Between the elements 20 are defined a large number of gaps 21 through which gases and particles of the bed can pass. In the particular example illustrated, the elements 20 are comprised by a sheet 26 of expanded metal which has been bent to a corrugated form.The pitch of the corrugation is equal to the pitch of the tubes 18 and the expanded metal rests on the fins 1 9 with the crest of each corrugation directly above a corresponding tube 1 8 and the valley of each corrugation lying directly above a gap between adjacent tubes 1 8. The radius of curvature of the corrugations may be approximately equal to the radius of the fins 1 9, so that the corrugations fit upper parts of the fins. Alternatively, as shown, the corrugations may be more severe so that the expanded metal engages each fin at two positions only and the crest of each corrugation is spaced substantially above the fins of the corresponding tube.

The sheet of corrugated expanded metal has a size and shape, as viewed in plan, corresponding to the size and shape of the chamber 10 so that there are no significant gaps between edges of the layer and the walls of the chamber. Thus, the superficial area of the layer is equal to the superficial area of the bed 1 7. As shown in Fig. 2, the gaps 21 occupy somewhat more than 50% of the superficial area of the corrugated layer. The smallest dimension of the gaps 21 between the elements 20 is a plurality of times greater than the largest dimension of the particles of the bed 1 7. For example, with particles up to 1 mm across the shortest dimension of the gaps 21 may be 4mm. It will be understood that the character of expanded metal sheet is such that the gaps 21 are distributed evenly across the entire corrugated layer.

As can be seen from Fig. 3, when the bed 1 7 is slumped, more than half of the corrugated layer comprising the elements 20 is exposed above the bed. When the bed is fluidised, masses of particles are ejected from the bed through the inclined parts 22 of the layer of expanded metal. These masses of particles fall into the adjacent valley defined by the layer of expanded metal so that there is established a circulation of particles downwardly in the gap 24 between adjacent tubes and upwardly through the gaps between the fins 1 8 on each tube. The approximate path along which particles circulate is indicated by the arrows 25.This circulation promotes efficient transfer of heat from the gases passing through the bed to liquid contained in the tubes 18, since cooler particles descend in the gaps 24 and hotter particles rise in the gaps between adjacent fins 1 9 and establish a higher temperature differential between the particles and the fins which conduct heat to the tubes.

If the apparatus is operated in the absence of the layer of corrugated expanded metal comprising the elements 20 a somewhat similar circulation of particles occurs in that there are violent eruptions of particles above the tubes 1 8 and masses of particles are thrown approximately vertically upwards from the bed. Whilst most of these particles fall back to the bed, some become entrained in the gas stream leaving the bed. As can be seen from Fig. 1, the path along which the gas flows is restricted at the outlet opening 14 and as the gas approaches this opening its velocity increases. Particles which are thown upwardly from the bed a significant proportion of the distance from the bed to the opening 1 4 in a region near to the end wall 1 5 are likely to be entrained in the gas stream.The risk of particles becoming entrained in the gas stream could be reduced by increasing the distance between the bed and the outlet opening 14 but since some particles are thrown up, in the absence of the element 20, approximately six inches from the bed, this would result in the upper wall 11 being spaced unduly far above the wall 1 6 and the apparatus would be unduly bulky.

The provision of the elements 20 so modifies the eruption of particles from the bed 1 7 that few particles rise above the apex of each corrugation of the layer comprising the elements 20 and the maximum height to which particles are thrown is reduced, as compared with operation in the absence of the elements 20. The reasons for this effect are not fully understood but it has been observed that masses of particles tend to be thrown vertically upwards in the absence of the elements 20 and, in the presence of the elements 20, masses of particles are thrown in nonvertical directions approximately perpendicular to the inclined parts 22. Few particles appear to emerge from the apex of each corrugation.

When the bed is fluidised without the elements 20 being present, and also when the bed is fluidised with the elements 20 positioned as shown in the drawing, bubbles appear to rise through the fluidised bed to the upper surface thereof. In the absence of the elements 20, each bubble bursts at the surface of the bed and throws a substantial mass of particles approximately vertically upwards. The elements 20 appear to disrupt the bubbles and lead to a consequent reduction in the eruption of particles from the surface of the bed. It is particularly noticeable that particles are not thrown so far above the bed when the elements 20 are used, as the particles are thrown in the absence of the elements 20.

We have compared the rates at which particles are carried from a fluidised bed by the fluidising gas in the absence of elements near to the upper surface of the bed with the rates achieved under substantially identical conditions except for the provision of elements near to the upper surface of the bed and defining between them gaps which are substantially larger than the particles. These elements were in the form of a net of wire filaments having a mesh size of 2.5mm. The net was corrugated, the pitch of the corrugations being approximately 50 mm and the vertical distance from the apex of one corrugation to the bottom of an adhacent corrugation being approximately 25mm. The bed was formed of particles having an average diameter of 800 micron.

Air was passed through the bed at a superficial velocity of 1 8m per second in the absence of the wire net and particles were separated from the fluidising gas downstream of the bed by means of a cyclone having a freeboard of 500mm. The weight of particles separated by the cyclone in a known time was measured. The procedure was repeated with the same fluidising gas velocity but with the wire net present near to the surface of the bed. The fluidising gas velocity was then adjusted to 2.37m per second and measurements carried out in the absence of and in the presence of with wire net. The results of these tests were as follows: Fluidising Gas Loss from bed Loss from bed Velocity without net with net 1.8 m/s. 26 g/hr. 2.3 g/hr.

2.37 m/s 460 g/hr. 26 g/hr.

Although the tests hereinbefore described were carried out with a net which was at least partly submerged in the fluidised bed, it is believed that a similar reduction in the rate of loss of particles from the bed can be achieved by positioning a net or the like above the level of the upper surface of the fluidised bed in the region to which particles are thrown by eruption of bubbles at the surface of the bed. In a case where the bed is shallow, by which we mean a bed having a slumped depth not exceeding 50mm, the net may be spaced above the surface of the slumped bed by a distance of up to five times the depth of the slumped bed.

Although the invention is useful generally in apparatus comprising fluidised beds, it is especially advantageous in the case of heat exchangers where heat is to be transferred between the fluidising gas and a further medium. The invention enables the apparatus to be operated with a higher fluidising gas flow rate than could otherwise be tolerated. A higher gas flow rate enables more heat to be transferred within unit volume of the heat exhanger and so enables the size of a heat exchanger designed for a particular duty to be reduced.

Claims

CLAIMS 1. Apparatus comprising a fluidised bed of particles and having near to an upper surface of the bed elements defining between them gaps which are sufficiently large for the particles of the bed to pass through them and which are at least approximately evenly distributed across the bed. 2. Apparatus according to Claim 1 wherein said elements and the gaps defined thereby collectively extend across an area substantially equal to the area of the surface of the bed. 3. Apparatus according to Claim 1 or Claim 2 wherein at least 50% of the area across which the elements and gaps collectively extend is open. 4. Apparatus according to any preceding claim wherein at least some of said elements are situated in respective positions such that, when the bed is slumped, they lie above the surface of the bed. 5. Apparatus according to any preceding claim wherein at least some of said elements are situated in respective positions such that, when the bed is fluidised, they are submerged in the bed. 6. Apparatus according to any preceding claim comprising a plurality of horizontal heat exchanger tubes arranged side by side and each having vertical fins, said elements being arranged to form a corrugated layer with the crest of each corrugation lying directly above a corresponding heat exchange tube and the valley of each corrugation lying directly above a gap between adjacent tubes. 7. Apparatus according to Claim 6 wherein at least some of said elements rest on the fins of the tubes. 8. A method of reducing the loss of particles from a fluidised bed wherein there is provided near to an upper surface of the bed a plurality of elements defining between them gaps which are sufficiently large for the particles of the bed to pass through them and which are approximately evenly distributed across the bed. 9. A method according to Claim 8 wherein, when the bed is fluidised, at least some of said elements are at least partly submerged in the bed and particles move in both directions between a part of the bed lying beneath the elements and a part of the bed lying above the elements. 10. Apparatus substantially as herein described with reference to and as illustrated in the accompanying drawing. 11. A method substantially as herein described with reference to the accompanying drawing.

1. Apparatus comprising a fluidised bed of particles and having near to an upper surface of the bed elements defining between them gaps which are sufficiently large for the particles of the bed to pass through them and which are at least approximately evenly distributed across the bed, at least some of said elements being substantially entirely submerged in the fluidised bed and substantially entirely exposed above the slumped bed.

Claims 4 and 5 deleted; later claims renumbered.

1

2. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.

CLAIM (2 Mar 1981)