GB2236963A - Gas distribution apparatus - Google Patents

Abstract

The fluidizing flow to a fluid bed (B) is distributed to the bed by a series of fluidic control devices such as venturi devices (4). Between a gas supply channel and the bed there is a closed plenum chamber through which the devices extend. Control porting (28) connects the plenum chamber with the throats of the venturi devices, so that a drop of throat pressure in any one device relative to the others induces a flow into the venturi throat from the plenum chamber. That control flow causes separation in the diffuser region of the device, increasing the resistance to flow through the device. In this way, local pressure variations in the bed can be limited and smoothed. <IMAGE>

Description

GAS DISTRIBUTION APPARATUS This invention relates to apparatus for distributing a fluidizing gas flow in a fluidized bed installation.

In the operation of fluidized beds it has been found that undesired instabilities can arise as a result of local changes of pressure in the bed arising, perhaps, due to some random thinning of the bed material. It has been suggested that the mechanism underlying these instabilities is that localised reduction of bed density leads to an increase of gas flow in that region, which then reduces the bed density further until slugging and surges occur.

The solution adopted for this problem in the past has been to arrange that the pressure loss across a distribution plate through which the gas flow reaches the fluidized bed is of the same order of magnitude as the average pressure loss of the bed itself. Such an arrangement damps the instabilities sufficiently but it has the disadvantage that it requires a high power consumption to compensate for the pressure loss across the distribution plate.

According to the present invention, there is provided apparatus for distributing a gas flow to a fluidised bed, comprising a common gas entry space from which the gas flow is admitted to the inlets of a plurality of fluidic control devices that supply outlets occupying spaced positions in a common space for the fluidised bed said fluidic control devices comprising control porting the changes of flow through which have an amplifying effect on the rate of gas flow through the devices, the control porting of the respective devices communicating with a common plenum chamber whereby differences in pressure within the devices at said control porting causes flow through said ports such as to moderate variations in flow rate through said devices associated with said relative differences of pressure whereby to limit differences in the fluidising gas flows through the respective outlets to said common space.

The invention also comprises a fluidized bed installation having apparatus for distributing a gas flow as set out in the preceding paragraph to distribute the fluidizing flow to the fluidized bed.

The fluidic control devices can take a variety of forms. In one preferred arrangement they are venturi devices, but vortex amplifiers can also be used.

By way of example, an embodiment of the invention will be described in more detail with reference to the accompanying drawings, in which: Fig. 1 is a schematic section of a portion of an apparatus according to the invention, and Fig. 2 is a graph of comparative changes of pressure loss with mass flow in distributing the gas flow to a fluidised bed apparatus.

In Fig. 1, a fluidized bed B of particles, the bottom wall of the bed being defined by a base plate 2, is supplied with fluidizing air through a series of venturi devices 4 arranged on vertical axes to project through the base plate, their outlets opening into the common space of the bed B. The devices have screw-threaded bodies 6 that engage with the base plate 2 in a sealed manner. The lower, entry ends of the devices are engaged by a dividing plate 8 that forms the upper boundary of an air channel 10 through which air under pressure is supplied to the venturi devices. Conveniently, the dividing plate 8 is secured to the base plate by bolts 12. Whatever the method of attachment, however, it is sealed against the venturi devices 4, and with a closing wall (not shown) at their peripheries the two plates 2,8 define between them a sealed plenum chamber 14.

In known manner, the air passage through each venturi device 4 from the air channel to the bed begins with an entry section 20 converging rapidly to a minimum cross-section at a throat 22, from which a divergent diffuser section 24 extends to the outlet in the bed B. In this case, the outlet is formed by a number of radial bores 26 above the level of the base plate 2. Further radial bores 28 are provided in each venturi device extending between the throat 22 and the plenum chamber 14 to serve as control passages.

In the steady flow state, with the same conditions at the entries and outlets of all the venturi devices, the throat pressure will also be identical in each device. There will be no flow through the control passages since the plenum chamber is a sealed space. In this state, the devices behave in a conventional manner, the speed of the flow increasing as it enters the throat and then expanding again with a relatively small pressure loss in the diffuser section, typically with an essentially linear characteristic for the pressure loss in relation to the flow over the operating range of the device.

If the flow resistance at the outlet of one device is reduced from whatever cause, the flow through that device will increase, reducing the throat pressure.

As a result air is drawn into the device through the control passages 28 because the plenum chamber pressure is the average of all the throat pressures and will then be higher than the reduced throat pressure. The control flow induced into the throat of the venturi air passage of that one device causes a progressive detachment of the boundary layer of the main flow from the walls of the diffuser section in the device. The change of conditions is illustrated schematically in the two venturi devices in Fig. 1, that on the left show a normal flow pattern while on the right the boundary layer flow has become detached.

As a result of the changed flow pattern the pressure loss coefficient in the diffuser section rises in a non-linear manner and the increased rate of flow through the venturi device is strongly moderated. There will thus be less tendency towards instability due to a local change of conditions, such as thinning of the bed material.

In comparison with a conventional fluidized bed installation in which a distribution plate for the incoming gas is arranged to create a substantial pressure loss for stabilizing the bed, the arrangement described above is able to operate with much smaller pressure losses because of the non-linear stabilization that is obtained should a local deviation cause an increased gas flow in one region.

Where the bed conditions are substantially uniform throughout, each venturi device operates in its linear low loss mode. The resistance to flow through any device increases only if and when the local conditions vary measurably from the average of all the devices.

In Fig. 2, flow resistance (AP) is shown against mass flow (Q) to indicate in graphical form characteristics of pressure loss against mass flow for the distribution of an air flow to a fluidised bed with examples of the characteristics for the prior art and for the present invention. The flow resistance of a uniform depth bed is indicated by the full line curve OAB, rising steeply to a limiting value APE at about 40% of the full flow condition of of the bed, and then remaining substantially constant to the full flow condition.

The flow resistance of a conventional distribution plate with a flow resistance substantially equal to that of the fluidized bed at the full flow condition is indicated by the curve OC. The combined resistance of the bed and plate is indicated by the curve ODE. This combined characteristic shows a rapid increase of overall flow resistance within the normal operating range of the installation. Furthermore, the effectiveness of the distribution plate in smoothing local flow variations is considerably reduced at lower flow rates because it has a low rate of change of resistance relative to flow rate. A 25% drop of flow resistance in the fluidized bed at full flow rate will give an increase of flow of only some 12% because of the effect of the distribution plate. At the 40% flow rate, a flow resistance drop of 25% permits an increase of flow of 60%.

The flow resistance of a distribution arrangement of the form shown in Fig. 1, when the venturi throat pressures are equal, is indicated by the curve OF. The curve OGH thus gives the combined characteristic of bed and distribution devices. At a higher rate of flow through a venturi device there will be a greater pressure loss, and the characteristics are shown fragmentarily at I1,I2,I3 for a series of increasing rates of flow. As is known, the faster the flow through a venturi passage the lower is the static pressure at the throat; where the increased rate of flow is localised, the lowered throat pressure leads to detachment of the flow in the diffuser section, as already explained, with relative little change in the total mass flow rate to the bed.The flow resistance of venturi device so affected thus follows the much steeper characteristic HK intersecting the curves I1-I3. That is to say, the increase of flow rate that would follow a reduction of outlet pressure is very strongly clamped.

The slope of the curve HK can be influenced by the detailed design of the venturi devices. As one example, a device might be given a flow resistance characteristic: * * * AP = 0.2V2 + 2.5(V2 - V2a * in which V2 is the velocity head at the throat * and V2a is the average velocity head at the throats of all the devices of the apparatus.

Such a nozzle would have an initial pressure drop characteristic that is only some 7% that of the bed.

Nevertheless, under full flow conditions a reduction of bed resistance by 25% locally gives a flow increase through the affected nozzle of 12%, as with the distribution plate having a high flow resistance. At 40% full flow, a 25% drop in local bed resistance gives a 60% flow increase.

As another example, with a device with a datum loss 0.4 of the velocity head and a gain of one, ie. having the characteristic * P = 0.4V2* 1 ( V2 ~V2a reducing the local bed resistance by 25% under full flow conditions increases the local flow by 12%. The initial pressure drop characteristic would give an increased resistance of 29% bed resistance at full load, which is still substantially less than a conventional distribution plate, while the flow increase in response to a pressure drop at part load conditions would reduce as compared with the first example.

Instead of the bores 28, an annular control inlet passage can be formed by dividing the body of the device at this level and securing the two parts together with a small spacing between them, eg. using tie bolts with spacing washers between the parts. A spigot or other suitable location means between the parts ensures the alignment of the two successive parts of the venturi that they form.

The annular control flow can increase the sensitivity of the device by producing detachment of the flow around the complete perimeter of the venturi throat. By constricting the gap in the manner described, its size can be adjusted.

A substantially annular flow can also be achieved, of course, by a series of radial drillings, possibly opening into an annular exit slot. By varying the number of such radial passages, and/or by sleeving such passages to change their cross-section, the annular control flow can be adjusted.

Other forms of fluidic control devices can be used in place of venturi devices. For example, each outlet may receive its gas flow from a vortex amplifier. Such an amplifier typically comprises a vortex chamber having a tangential port for a control flow, this being the equivalent of the throat control passages of the venturi devices described above. The main flow through the chambers of a series of such devices between their inlets from the entry air channel and their outlets into the fluidised bed is thus controlled by the communication of their tangential control ports with the plenum chamber.

The significant feature of any type of fluidic control device employed with the present invention is that the changes of flow through the control porting has an amplifying effect on the rate of flow through the device.

Many other modifications are possible within the scope of the invention. Regarding the discharge of the flow into the fluid bed, it may be noted that the use of radial bores for the outlet flow reduces the risk of fluidized bed particles finding their way into the passages of the flow control devices where they could disrupt the required flow conditions. To this end the bores 26 may slope downwards towards their exit ends and/or be protected by overhanging lips or hoods (not shown).

Claims (12)

1. Apparatus or distributing a gas flow to a fluidised bed, comprising a common gas entry space from which the gas flow is admitted to the inlets of a plurality of fluidic control devices that supply outlets occupying spaced positions in a common space for the fluidised bed said fluidic control devices comprising control porting the changes of flow through which have an amplifying effect on the rate of gas flow through the devices, the control porting of the respective devices communicating with a common plenum chamber whereby differences in pressure within the devices at said control porting causes flow through said ports such as to moderate variations in flow rate through said devices associated with said relative differences of pressure whereby to limit differences in the fluidising gas flows through the respective outlets to said common space.

2. Apparatus according to claim 1 wherein the fluidic control devices extend between a pair of spaced plates or cells that form opposite boundaries of the plenum chamber, the devices having said inlets and outlets on the outer sides of said plates.

3. Apparatus according to claim 1 wherein the fluidic control devices provide upwardly extending gas passages with their outlet ends projecting from a supporting wall or plate above the plenum chamber.

4. Apparatus according to claim 2 together with claim 3 wherein said wall or plate is the upper of said pair of plates.

5. Apparatus according to claim 3 or claim 4 wherein the fluidic control devices have outlet passages directed transversely to said upwardly extending passages.

6. Apparatus according to claim 5 wherein said passages are directed downwardly towards their outer ends.

7. Apparatus according to claim 5 or claim 6 wherein protective hoods or lips are provided over the outer ends of said passages.

8. Apparatus according to any one of the preceding claims wherein the fluidic control devices are formed by venturi devices, the throat regions' of the venturi devices having said control porting connected to the common plenum chamber.

9. Apparatus according to claim 8 wherein said porting provides a substantially annular exit area into the throat region of at least one venturi device.

10. Apparatus according to any one of claims 1 to 7 wherein the fluidic control devices are formed by vortex amplifying devices having tangential inlets that provide said control ports connected to the common plenum chamber.

11. Apparatus for distributing gas flow constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.

12. A fluidized bed having apparatus for distributing a gas flow to the bed according to any one of the preceding claims.