GB2074302A

GB2074302A - Method and Apparatus for Countercurrent Fluidised Bed Heat Exchange Between Fluids

Info

Publication number: GB2074302A
Application number: GB8012844A
Authority: GB
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1980-04-18
Filing date: 1980-04-18
Publication date: 1981-10-28
Also published as: GB2074302B

Abstract

An initially hot first fluid (24) flows in succession through a first heat exchange zone (11) wherein it fluidizes and attains thermal equilibrium with a first bed (11a) of fluidizable solids, and thereafter through at least a second heat exchange zone (13) wherein it fluidizes and attains thermal equilibrium with a second bed (13a) of fluidizable solids. An initially cold second fluid flows in succession through at least the heat exchange zone (13) wherein it fluidizes and attains thermal equilibrium with a third bed (13b) of fluidizable solids and thereafter through the first heat exchange zone (11) wherein it fluidizes and attains thermal equilibrium with a fourth bed (11b) of fluidizable solids. Fluidizable solids are circulated between the first and fourth beds (11a-11b) and between the second and third beds (13a-13b) thereby transferring heat between said first and second fluids via the circulated solids. The preferred apparatus has a further fluidized bed heat exchange zone (12). Heat exchange may be between two gases, two liquids or between a liquid and a gas or vapour. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Countercurrent Fluidised Bed Heat Exhange The present invention relates to a method and an apparatus for countercurrent fluidised bed heat exchange.

The transfer of heat from one fluid to another is often rendered difficult by the fact that one or both fluids is corrosive and/or erosive and/or at a temperature which causes problems to the materials of an indirect heat exchanger.

In one aspect, the present invention provides a method of transferring heat from a first fluid to a second fluid comprising passing the first fluid at least through a first bed of fluidisabie particles and thereby fluidising said particles and therafter through a second bed of fluidisable particles and thereby fluidising said particles, passing the second fluid at least through a third bed of fluidisable particles and thereby fluidising the particles and thereafter through a fourth bed of fluidisable particles and thereby fluidising said particles, circulating substantially fluid-free particles from the first bed to the fourth bed and from the fourth bed to the first bed and separately circulating substantially fluid-free particles from the second bed to the third bed and from the third bed to the second bed.

Preferably, particles circulate from a first region of the first bed to a first region of the fourth bed and from a second region of the fourth bed to a second region of the first bed, and preferably particles circulate from a first region of the second bed to a first region of the third bed and from a second region of the third bed to a second region of the second bed. The locations of the first and second regions is so chosen that during operation, the mean residence time of each particle is at least sufficient for thermal equilibrium with the fluid passing through the respective bed space.

In another aspect, the invention provides apparatus for use in transferring heat from a first fluid to a second fluid comprising first and second heat exchange stages comprising first, second, third and fourth spaces for containing respective beds of fluidisable particles up to respective bed depths, the first and fourth bed spaces being in the first stage and the second and third bed spaces being in the second stage, means for causing the first fluid to pass in succession through the first bed space and thereafter through the second bed space, means for causing the second fluid to pass through the third bed space and thereafter through the fourth bed space, the construction and/or arrangement being such that fluidisable particles in the beds will be fluidised by the passage of the fluids through the bed spaces, means for circulating substantially fluid-free particles from the first bed space to the fourth bed space and for circulating substantially fluid-free particles from the fourth bed space to the first bed space, and means for circulating substantially fluid-free particles from the second bed space to the third bed space and for circulating substantially fluid-free particles from the third bed space to the second bed space.

Preferably, a transfer port or duct provides communication between the bed spaces of each stage below the top level of the respective bed spaces for the circulation of particles from one bed space to anotehr.

Each transfer port or duct may be defined at its base by a bottom surface which slopes downwardly in the desired direction of circulation of particles, and preferably the slope of the bottom surface of each transfer port or duct exceeds the angle of repose of the fluidisable particles.

There may be means for passing a fluid into at least one transfer port or duct of each stage at or near to the lower end of the bottom surface thereof, and the said means may be arranged to direct the fluid into the transfer port or duct with at least a component of velocity in the desired direction of circulation of particles through the port or duct.

Preferably, the base of each fluidised bed space comprises a lower portion and an upper portion which are both constructed and/or arranged for the upward distribution into the respective bed space of either the first or second fluid, the lower portion defining the base of an entrance region of the bed space for receiving particles entering the respective bed space from the exit of a transfer port or duct, and the entrance of a transfer port or duct being disposed for receiving particles from an exit region of a respective bed space above the corresponding upper portion of the base of the bed space.

The second heat exchange stage may be above the first heat exchange stage.

The invention also comprises apparatus as described above provided with fluidisable particles up to the respective bed depths in the bed spaces.

The invention is now further described with reference to some non-limitative embodiments thereof, and with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a semi-schematic side elevation of a vertical cross section through a three-stage countercurrent fluidised bed heat exchanger; Figure 2 is a vertical sectional elevation on A-A of Figure 1 in the absence of fluidisable particles; Figure 3 is a plan sectional view on B-B of Figure 1 in the absence of fluidisable particles; and Figure 4 is a plan view like that of Figure 3 but of another embodiment.

Reference is first made to Figure 1 wherein the heat exchanger, generally designated 10 comprises a hollow parallelopiped containing three fluidised bed stages 11, 12, 13 which are verticaily separated from each other and each of which is divided into two horizontally adjacent bed spaces 11a, 11b; 12a, 12b; 13a, 13b;bya respective vertical baffle 14, 15, 1 6.

Each fluidised bed space is defined at its bottom by a respective gas distributor 17, 18, 1 9 and suitable fluidisable particles are supported on the distributors 1 7, 18, 1 9 up to a selected bed level. The baffles 14, 15, 16 are formed with ports 20, 21, 22 below the selected bed level for the circulation of particles between the two horizontal adjacent bed spaces (e.g. 1 a, 11 b) of each stage (e.g. 11), as will be more fully explained below.

As depicted in Figure 1, the part of the heat exchanger 10 to the left of the baffles 14, 1 5, 16 is for the upward passage of a gas which is initially at a high temperature, e.g. 1000 to 1 000C, and the part to the right of the baffles is for the passage of a gas which is initially cold and which is to be heated in a quasi-countercurrent manner by passage through the heat exchanger.

The hot gas enters the left side (as depicted) of the apparatus 10 from a conduit 24 into a plenum 25 at the bottom. The base of the planum is defined by an entrainment hopper 26 which receives solids which have fallen through nozzles or distributing orifices of the gas distributor 1 7 above the plenum 25. The hot gas is preferably conducted into the plenum 25 in such a manner that it entrains at least some of the solids therein so that they pass upwardly with the gas through the distributor 1 7 into the bed of fluidisable solids in the bed space 11 a. The fluidisable solids are selected to resist deterioration in the presence of the hot gas and to be relatively attrition resistant.

The solids attain a mean temperature approximately equal to that of the hot gas leaving the top of the bed space 1 1 a by heat transfer from the gas to the solids. The hot gas leaving the top of the bed space 11 a is of reduced temperature relative to the hot gas entering the apparatus 10 via conduit 25, and the gas passes upwards through the distributor 18 of the second stage 12 into a bed of fluidisable solids in the bed space 1 2a, the solids being fluidised by the gas and also heated by heat transfer from the gas.

Gas leaves the top of the bed space 12a in approximately thermal equilibrium with the particles in the bed space 1 2a and finally passes through the distributor 1 9 of the third stage into a bed of fluidisable solids in the bed space 1 3a, the solids being fluidised and heated by the gas to approximately the temperature of the gas leaving the top of the bed space 1 3a. The gas is discharged from above bed space 1 3a via a conduit 28, and may be de-dusted before being discharged to atmosphere and/or employed for some other use.

The solids in the bed spaces 1 a, 1 2a, 1 3a are pneumatically circulated (as described below) continuously or intermittently to the bed spaces 11 b,12b,13b. Each of the latter bed spaces has a distributor 29, 30, 31 which may be of the same type as the distributors 17, 1 8 and 19.Respective baffles 32, 33, 34 spaced below the distributors 29, 30, 31 extend across the right-hand side of the apparatus so as to define respective plena 35, 36, 37. Preferably, as shown, the baffles 32, 33, 34 slope downwardly towards the right-hand vertical wall of the apparatus so as to form hoppers to receive solids which have fallen through the distributors 29, 30, 31.

Cold gas which is to be heated in the apparatus 10 is passed into the top plenum 37 by means of a fan 38 and preferably enters in such a manner as to entrain any solids in the plenum and carry them up through the distributor 31 into the top bed space 1 3b. The bed space 1 3b contains particles circulated from bed space 1 3a and which have been heated in the third bed stage 1 3.

The cold gas fluidizes the solids in the bed space 1 3b and heat is transferred from the fluidized solids to the gas such that the gas passing out of the top of the bed space 1 3b is approximately at the average temperature of the particles. Particles are circulated continuously or intermittently from the bed space 1 3b to the bed space 1 3a, preferably by a pneumatic effect.

Heated gas from the bed space 1 3b is conducted to de-dusting equipment 40 (e.g. one or more cyclone separators) and separated solids are returned to the bed space 1 3b via line 41. The de-dusted heated gas is circulated via conduit 42 into plenum 36, and the circulation may be promoted by a fan 43. The gas entering plenum 36 preferably entrains solids from the top surface of baffle 33 and returns it to the bed space 1 2b which contains fluidizable particles previously heated in bed space 1 2a. The particles in space 1 2b are fluidized by the upwardly-passing heated gas which is further heated so as to leave the top of bed space 1 2b at approximately the mean temperature of particles in bed space 1 2b.

Particles are circulated continuously or intermittently from bed space 1 2b to bed space 1 2a where they are reheated.

The further heated gas leaving the top of bed space 1 2b is recovered in conduit 44, passed through solids separation equipment 45 (e.g. at least one cyclone separator) and circulated via conduit 46 into plenum 35, the circulation being assisted, if necessary, by a fan 47. Solids separated in equipment 45 are returned to the bed space 1 2b via line 48.

The gas entering plenum 35 preferably is directed so as to entrain solids from the upper surface of the baffle 32 and carry it into the bed space 1 b via the distributor 29. The bed space 1 b contains particles which previously have been heated in the hottest bed space 11 a, and the passage of the gas upwards through the bed space 1 b fluidizes particles therein and heat is transferred from the particles to the gas so that the temperature of the gas leaving the top of the bed space is approximately equal to the average temperature of particles in the bed space 1 b.

The gas leaving the top of the bed space 1 b is recovered via conduit 50, and solids are circulated, e.g. pneumatically, from bed space 1 b to bed space 1 a to be reheated therein.

For equal mass gas flow rates in the left and right side of the apparatus, there will be approximately 70% heat transfer from the hot gas to the cold gas employing the three stages 11, 12, 1 3. A greater percentage of heat transfer would be obtained with more stages, but the incremental increase decreases with each additional stage, as a rule, and for most purposes, three stages will probably be a cost-effective optimum.

The circulation of solids from one bed space (e.g. 11 a) to the other bed space (e.g. 11 b) of the same stage may be effected in any convenient manner, and when the fluids passing through the heat exchanger are both gases, it may be preferred to effect the circulation using a gas for pneumatic solids transfer.

Referring to Figures 1 and 3 in relation to the first stage 1 the vertical baffle 14 is provided with a port 21 below the top of the bed spaces 1 a and 1 b. The base of the port 21 is defined by a ramp 60 which slopes at an angle exceeding the angle of repose of the bed solids with its upper end defining part of the edge of the distributor 1 7 of space 1 a and its lower end defining part of the edge of a sump-part 129 of distributor 29 of bed space 11 b. The sump-part 129 is connected with the main part of distributor 29 by a step 130.

Towards the lower end of the ramp 60, there may be provided holes or nozzles 61 (Figure 3) for the passage of a solids-circulation gas through the ramp 60 and which nozzles 61 preferably are at least partly directed towards the bed space above the sump-part 129 to prevent blockage by bed solids. When solids accumulate on the ramp 60, the passage of gas through the nozzles 61 causes particles in the vicinity oF, and above, the nozzles 61 to be fluidized so that any forces tending to prevent the particles sliding down the ramp 60 into the sump are substantially eliminated. The gas which is passed through the nozzles 61 (when provided) in the ramp 60 may be one of the gases passing through the apparatus during use thereof or it may be some other gas.Preferably, the gas passed through the nozzles in the ramp is a portion of the gas which is passing through a heat exchange stage. Thus, referring to figure 1, it will be seen that the ramp 60 has an orifice (or nozzle) 61' to the right of baffle 14 enabling some gas to pass therethrough from the plenum and thereby cause fluidization of solids on the ramp 60 to promote solids flow into the bed space 1 b. This expedient is advantageous in avoiding the requirement for an additional supply of gas (which dilutes the gas being heated) and also for promoting the desired heat transfer from the solids entering bed space 1 1b to the gas passing therethrough. A similar provision of holes or nozzles in all the other ramps may be made.

The sump-part 129 and main part 29 of the distributor are provided with gas distribution holes or nozzles (not shown) which, during operation, fluidize solids in the bed space 11 b.

The nozzles or holes of the distributor 129 and 29 are such that bed solids are substantially uniformly fluidized in the sump and in the main part of bed space 1 b. As will be seen from Figures 1 and 3, there is a mirror-image arrangement for the solids circulation from bed space 1 b to bed space 1 1a comprising a ramp 1 60 sloping downwardly from the main distributor 29 to a sump-part 117 of distributor 17 at the base of bed space 11 a. The ramp 160 may be furnished with holes or nozzles 161 (which may be similar to and/or in addition to, the arrangement of holes or nozzles 61' of Figure 1) which are at least partly directed towards the sump above part 11 7 so that they are less likely to be blocked by bed solids.The ramp 60 is arranged to cause fluidized solids from one side of bed space 11 a to enter bed space 11 b on one side, and the ramp 1 60 is arranged to cause fluidized solids from the opposite side of bed space 11 b to enter bed space 11 a on the opposite side so that the circulation of solids is anticlockwise as shown in Figure 3, and in general, each solid particle has at least sufficient residence time in each bed to attain thermal equilibrium therein.

The pressure drop across the fluidized beds in the bed spaces on each side of the vertical baffle (14, 14, 16) in each stage should be equal. The depth of the fluidized bed in each stage for efficient heat transfer may be in the range of from 5 to 1 5 cms. over the main part of each distributor (e.g. 17, 29 in Figure 3) and somewhat greater in the sump portion of each bed space, e.g. from 12 to 30 cms. bed depth.

With good design, each stage will allow almost 50% heat recovery for equal hot and cold gas flows so that for three stages, as shown in Figure 1, the heat recovery will be about 70%.

Figure 2 shows the main features of a typical baffle 16, and it will be seen that the ports 23, 123 defined thereby are rectangular and at the bottom corners thereof. However, parts of other shapes may be employed.

Figure 4 shows an embodiment of the distributors for the "hot" and "cold" gases (or other fluids-e.g. liquids) which may be employed in place of the arrangement of distributors shown in Figure 3. In Figure 4, the distributors are depicted without their fluid-distribution nozzles or orifices for simplicity.

The distributors for the two gases are separated by a vertical baffle 200 having ports 201, 202 at its bottom corners and a port 203 at the centre of its bottom. On one side of the baffle 200 is a first gas distributor comprising a main (central) distributor part 204 and sump distributor parts 205, 206 on each side thereof and substantially aligned with the ports 201,202 respectively.

On the other side of the baffle 200, there is a central sump distributor part 207 substantially aligned with the port 203 and side distributor parts 208, 209 on each side of the central sump part 207. Preferably the distributor part 204 is substantially at the same level as the distributor parts 208, 209, and the sump distributor parts 205,206 and 207 are substantially at a common level somewhat below the level of distributor parts 204, 208 and 209.

A central ramp 210 slopes downwardly through the central port 203 from distributor part 204 to the central sump part 207, and side ramps 211 and 212 slope downwardly through the side ports 201 and 202 respectively. Side ramp 211 connects at its top end with side distributor 208 and at its bottom end with sump distributor 205, and side ramp 212 connects at its top end with side distributor 209 and at its bottom end with sump distributor 206. The ramps 210, 211,212 may be furnished with nozzles or orifices as described in relation to the previous embodiments.

When the distributors of the Figure 4 embodiment support respective beds of solids which are fluidized by respective upwardly passing gases, solids will circulate from distributor part 204 down the central ramp 210 via port 203 into sump 207, and from side distributors 208, 209 down the corresponding side ramps 211,212 via the side ports 201,202 into the sumps 205, 206 as indicated by the arrows.

It will be appreciated that other distributors employing ramps and sump portions (not described) may be devized in accordance with the principles of construction and operation described herein, without departing from the invention, and that the distributor in one stage may be different from the distributor in another stage.

The apparatus of the invention has hereinbefore been described in relation to heat transfer from one gas to another gas. However, the apparatus can also be used for the transfer of heat from one liquid to another liquid, and may also be useful is some instances for heat transfer between a liquid phase substance and a vapour or gas phase substance.

Claims

1. A method of transferring heat from a first fluid to a second fluid comprising passing the first fluid at least through a first bed of fluidizable particles and thereby fluidizing said particles and thereafter through a second bed of fluidizable particles and thereby fluidizing said particles, passing the second fluid at least through a third bed of fluidizable particles and thereby fluidizing the particles and thereafter through a fourth bed of fluidizable particles and thereby fluidizing said particles, circulating substantially fluid-free particles from the first bed to the fourth bed and from the fourth bed to the first bed and separately circulating substantially fluid-free particles from the second bed to the third bed and from the third bed to the second bed.

2. A method as in claim 1 in which particles circulate from a first region of the first bed to a first region of the fourth bed and from a second region of the fourth bed to a second region of the first bed.

3. A method as in claim 1 or claim 2 in which particles circulate from a first region of the second bed to a first region of the third bed and from a second region of the third bed to a second region of the second bed.

4. Apparatus for use in transferring heat from a first fluid to a second fluid comprising first and second heat exchange stages comprising first, second, third and fourth spaces for containing respective beds of fluidizable particles up to respective bed depths, the first and fourth bed spaces being in the first stage and the second and third bed spaces being in the second stage, means for causing the first fluid to pass in succesion through the first bed space and thereafter through the second bed space, means for causing the second fluid to pass through the third bed space and thereafter through the fourth bed space, the construction and/or anangement being such that fluidizable particles in the beds will be fluidized by the passage of the fluids through the bed spaces, means for circulating substantially fluid-free particles from the first bed space to the fourth bed space and for circulating substantially fluid-free particles from the fourth bed space to the first bed space, and means for circulating substantially fluid-free particles from the second bed space to the third bed space and for circulating substantially fluid-free particles from the third bed space to the second bed space.

5. Apparatus as in claim 4 so constructed and/or arranged that particles enter each bed space at one region and leave from another region.

6. Apparatus as in claim 4 or claim 5 in which a transfer port or duct provides communication between the bed spaces of each stage below the top level of the respective bed spaces for the circulation of particles from one bed space to another.

7. Apparatus as in claim 6 in which each transfer port or duct is defined at its base by a bottom surface which slopes downwardly in the desired direction of circulation of particles.

8. Apparatus as in claim 7 in which the slope of the bottom surface of each transfer port or duct exceeds the angle of repose of the fluidizable particles.

9. Apparatus as in claim 7 or claim 8 comprising means for passing a fluid into at least one transfer port or duct of each stage at or near to the lower end of the bottom surface thereof.

10. Apparatus as in claim 9 in which said means is arranged to direct the fluid into the transfer port or duct with at least a component of velocity in the desired direction of circulation of particles through the port or duct.

11. Apparatus as in any one of claims 6 to 10 in which the base of each fluidized bed space comprises a lower portion and an upper portion which are both constructed and/or arranged for the upward distribution into the respective bed space of either the first or second fluid, the lower portion defining the base of an entrance region of the bed space for receiving particles entering the respective bed space from the exit of a transfer port or duct, and the entrance of a transfer port or duct being disposed for receiving particles from an exit region of 9 respective bed space above the corresponding upper portion of the base of the bed space.

12. Apparatus as in any one of claims 4 to 11 in which the second heat exchange stage is above the first heat exchange stage.

13. Apparatus as in any one of claims 4 to 12 comprising fluidizable particles up to the respective bed depths in the bed spaces.

14. Apparatus for use in transferring heat from a first fluid to a second fluid substantially as hereinbefore described.

1 5. A method of transferring heat from a first fluid to a second fluid substantially as hereinbefore described.