GB2162935A

GB2162935A - Fluid beds

Info

Publication number: GB2162935A
Application number: GB08420290A
Authority: GB
Inventors: William Bradshaw; Douglas Handley
Original assignee: PETTINGALE Ltd
Current assignee: PETTINGALE Ltd
Priority date: 1984-08-09
Filing date: 1984-08-09
Publication date: 1986-02-12
Also published as: GB8420290D0; GB2162935B

Abstract

A fluid bed is divided into eight zones by a series of radiator plates (1 to 9) of a heat exchanger. Fluidising gas enters each of the zones at a temperature T, and the line 14 shows how the temperature of the material within each zone decreases. Water enters the radiator plate at 18 and leaves at 19, successively flowing through the plates, with the water temperature in each of the plates being shown by line 16. The mean temperature (20) of the material in the fluid bed is typically 65 DEG C, but the water flowing through the heat exchanger may leave at 19 at a temperature which is at or above 100 DEG C. <IMAGE>

Description

SPECIFICATION Fluid beds The present invention relates to fluid beds and to a method of exchanging heat using fluid beds. The invention is particularly, although not exclusively, applicable to the recovery of heat from the gas discharges of boilers and dryers, and to the generation of hot water or steam.

Heat generation and drying devices commonly discharge or exhaust the combustion products or the gases used and or generated during drying to the atmosphere thus wasting heat energy.

A series of devices exist which attempt to recover at least some of the energy contained in this exhaust.

One such prior proposal has been to locate a heat exchanger in the exhaust duct and pass water through the heat exchanger. However, the heat transfer coefficient of this system is extremely low and is typically 3 b.t.u.'s/ft2/bF/h. Furthermore, a large surface area of heat exchanger is required to handle common throughputs and is, as a result, very expensive. A further major problem with this type of device is that for discharge gases which contain "contaminants", e.g. sulphur as found in most oil and coal fired units, any major drop in the temperature of these exhaust gases would cause condensation of liquids as the temperature fell below that of the condensation level.The resultant condensate can be of a highly corrosive nature and, alternatively or additionally, may be in a solid form such as a tar To operate with a corrosive condensate it is necessary to provide the large heat exchanger surfaces with expensive corrosion resistant material. Even then the surfaces become coated with an insulating solid tar like residue which reduces the heat transfer efficiency or coefficient to an unacceptable level.

Further devices exist which attempt to convert contaminated exhaust gases into clean infeed gases by passing a gas through a heat exchanger to give a gas/gas heat exchange, but these devices are all subject to the problems described above in relation to the heat exchanger through which water is passed.

Another method used to recover energy from the exhaust gases is that of a "spray condense" device through which discharge gases are passed so cooling the gases and raising the temperature of the incoming spray of water. Again, such devices are extremely inefficient.

In applications where the exhaust gases are of a clean nature-U.k. North Sea Gas may be considered to be such-for some applications the exhaust will commonly contain other secondary gases-such as air in the case of gas fired boilers. The effect of these secondary gases is to reduce the effective dew point of the finally cooled gases to the point where the temperature of the water being heated by the exhaust gases is reduced. Typical outlet figures would be 50-60"c.

A further method used to recover the exhaust heat is a fluid bed exchanger comprising a nest of tubes through which water to be heated is passed located within a bed of material suitable for fluidisation, such as sand.

The fluid bed has a greatly improved heat transfer co-efficient over the other previously referred to heat exchangers, and may be typically 70 b.t.u's/ft2/ F/h. However, for all practical considerations the temperature in a fluid bed is uniform, such evidence being well documented, for example in Heat Transmission by McAdams. Figure 1 is a graphical representation showing the exhaust gas temperature across a fluid bed and the temperature of the fluid within the tubes as it flows through the bed. It can be seen that the temperature of the fluid leaving the tubes, shown at 10, can never be higher than the uniform temperature of the gas in, and leaving the bed shown at 1 2. The difference between the maximum fluid temperature and the temperature of the gas leaving the bed is shown es "A" on the figure.

When the fluid flowing through the- heat exchanger is water at atmospheric pressure, the most useful form that this can be used in is when it is at 1 00 C or more, i.e. when the water is boiling or is steam. Accordingly the temperature of the gas in and leaving the bed, as shown at 12, must be greater than 1 00 C + A if the water temperature is to rise to, or above 1 00 C. Thus the gas leaving the fluid bed takes with it a lot of heat which is wasted, and it is necessary for the exhaust gas entering the fluid bed to have a temperature which is considerably higher than 1 00,C + A.

According to one aspect of the present invention in a fluid bed the material comprising the fluid bed is arranged to be restricted to be fluidised within zones and a heat exchanger is arranged to exchange heat with two or more of the zones. With such a fluid bed it may be possible to cause the temperature of fluid within the heat exchanger to be brought cioser to the temperature- of the gas entering the fluid bed and causing the fluidisation of the bed than can be the case in a conventional fluid bed in which no such zones exist. The fluid bed and the heat exchanger maHe used in fluidised bed based systems for efficient heat recovery from combustion waste gases or dryer exhaust gases.

One or more of the zones may be of a size which is arranged to restrict the lateral or axial movement of the particles in the fluid bed of that zone to create a more gradual-temperature gradient from the inlet to the outlet of the fluid bed than is achieved in a conventional fluid bed in which no such restriction is provided and substantially the whole of the fluid bed is at the same temperature. Providing the temperature gradient increases the difference between the temperature of fluid within the heat exchanger and the material within the bed and therefore increases the thermal driv ing force allowing or causing the heat ex change.

The heat exchanger may be arranged to pass fluid in such a way that it successively exchanges heat with the zones. By succes sively exchanging heat with the zones the fluid in the heat exchanger may be brought closer to the temperature of the gas entering and fluidising each zone as the fluid ex changes heat with the successive zones.

The heat exchanger may be arranged to define at least some of the zones thus provid ing a simplified structure in which the heat exchanger serves the additional function of providing the zones and allowing heat to be exchanged with zones located on either side of the heat exchanger when the heat ex changer is located between adjacent zones.

The heat exchanger may include a plurality of plates defining the zones which provide a large surface area with which to exchange heat with the fluid bed.

The fluid bed may include- a gas inlet ar ranged to supply the gas-to the fluid bed material in order to fluidise the same, the gas inlet being common to all of the zones. Alter natively or additionally the gas outlet to the zones may be common to each of the zones.

Two or more fluid beds, each including a plurality of zones, may be arranged in series with the gas leaving the fluidised bed of a preceding fluid bed being arranged to be supplied to fluidise the bed of a following fluid bed.

According to another aspect of the present invention a method of exchanging heat using a fluidised bed comprises causing the material in the bed to move in zones and passing fluid to exchange heat with the fluid bed through a heat exchanger which exchanges heat with two or more of the zones. The fluid in the heat exchanger may be arranged to succes sively exchange heat with the zones.

The method may further comprise passing fluid through the heat exchanger which at least partly defines the zones.

The method may also comprise causing the fluid leaving the heat exchanger to be at a greater temperature than the temperature of the gas leaving the heat exchanger.

The method may include passing the gas to fluidise the material in the zones through a common inlet and, alternatively or addition ally, passing the gas leaving the material-in the zones through a common outlet.

The method may involve fluidising the ma terial in-the zones with a gas which includes condensible vapour and allowing the gas leaving the zones to be at a temperature-which is either above the point at which condensation of the vapour occurs or above that at which the condensates boil.

The method may include supplying the gas to fluidise the material at a temperature which is above the boiling point of the fluid passing through the heat exchanger, the temperature of the gas leaving the fluid bed being below the temperature at which the fluid in the heat exchanger boils, and removing fluid from the heat exchanger which is at or above the temperature at which that fluid boils.

The present invention also includes the method of exchanging heat using the fluid bed as well as the fluid bed when used in the method of exchanging heat.

The invention may be carried into practice in various ways but several embodiments will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 2 shows a zonal fluid bed comprising a series of separate zones which progressively increase the temperature of fluid within the heat exchanger as the fluid flows across the zones; Figure 3 shows an exhaust gas system including a zonal fluid bed arranged to receive exhaust gas containing condensable vapours; Figure 4 shows an exhaust gas system including a zonal fluid bed, the water entering the heart exchanger being arranged to be preheated by the gas leaving the fluid bed, and Figure 5 shows a boiler system including two zonal fluid beds in series.

Referring now to Figure 2, a graphical represenation is shown of the temperature 14 of the material within the zones of the fluid bed or of the fluidising gas within the bed (these are generally -equivalent) and the water temperature 1 6 across the bed. The representation relates to a zonal fluid bed in which the water flows through a series of radiator plates (shown 1 to 9 in-the drawing), which plates serve to divide the fluid bed into eight zones or compartments and give thin film flow on their surface facing the bed. The reduced size of the compartments limits the axial and lateral thermal mixing of the particles compris ing the fluidised bed, and gives a more.linear fall of the temperature across each zone to its exit temperature than is achieved in the conventional fluid bed shown in Figure 1 where virtually the whole- bed is at a common tem perature.

Each of the zones or compartments are fed from a common gas source, which may be an exhaust gas, at temperature T. The gas source may-be a plenum chamber or, in a preferred form, inverted sparge pipes. The partitions numbered 9 to 1 respectively are at progres sively increasing temperatures from the water inlet side 1 8 to the water outlet 19, the mean zonal-bed.temperatures being marked h t9 a.

It is apparent from Figure 2 that not. dnly are the temperature differences producing the heat transfer larger than in a conventional fluid bed but also the exit fluid temperature from the radiator plates can be larger than the mean exit gas temperature shown as 20 thus increasing the efficiency of heat recovery.

The radiator plates may be sealed and may include turbulence promotors: that is the fluid flowing within the plates, such as water, gas or other, may be caused to flow with turbulence thereby ensuring greater heat transfer within the fluid being heated.

Although it is thought that the water flowing through the heat exchanger obtains the same heat content as can be achieved by a comparable conventional fluid bed, the temperature of the water leaving the heat exchanger can be raised significantly above -the mean temperature 20 (typically 65"C) of the fluid bed. Thus where the mean temperature of the fluid bed is below 1 00, it is possible to obtain water from the heat exchanger which is either boiling or in steam form and is consequently immediately usuable without necessarily requiring further heat input to obtain a useful form or state.As the heat content removed from the fluid bed is thought to be the same as is able to be removed from a conventional fluid bed it follows that the flow rate of water in the embodiment will have to be- less than the flow rate in a comparable conventional fluid bed if the increase in temperature is to be achieved.

The discharge pipe from the heat exchanger may be extended by other means such as serpentine form pipes, gilled tubes etc., to afford a further increase in temperature as a function of increased surface area in the higher temperature zone and/or passed back through the incoming high temperature gases, if desired.

In instances where certain contaminents exist in the incoming gas stream that could foul the fluid bed or damage the heat exchanger by condensing therein, the system may be designed to give a discharge temperature at the surface of the bed which is above the condensation temperature of the gases. This allows the use of low cost heat exchangers since corrosion resistant materials need not be used in the fluid bed. Lime or magnesia may be dosed to the bed to remove acid gas and prevent corrosion. A second conventional heat exchanger may be fitted, as will be referred to later in Figure 3, which can heat water prior to that water passing through the zonal fluid bed.

Although the present invention is described throughout this specification as being applicable to extracting heat from gas it will be appreciated that what the present invention provides is a heat exchanger which provides a closer temperature relationship between two fluids and thus the principle of the invention may be used when it is desired to effect a heat transfer in the opposite way to that described either where direction of heat transfer is the same and it is desired to cool the gas flowing through the fluid bed, rather than the purpose being to use its heat, or where it is desired to heat the gas flowing through the bed.

In Figure 3 there is shown an exhaust gas heat exchange system in which the gas passes first through a zonal fluid bed 22, which may be the same as that shown in Figure 2, and then through a primary heat exchanger 24, whose construction will not be detailed herein, before passing out to the atmosphere through an outlet 26.

The gas entering the zonal fluid bed in this embodiment may be 600"C, and the gas leaving the bed 22 may be at 250"C. Thus, although the gas may contain condensable vapours which may be corrosive or damaging when condensed, these vapours do not condense in the bed 22 which allows the fluid bed material to remain in a fluidisable condition and permits any partitions. defining the zones in the bed to be made of relatively low cost mild steel.

The gas containing the condensable vapours then passes through the primary heat exchanger 24 where sensible heat, and possibly also latent heat is recovered. The condensable vapours are either condensed in the primary heat exchanger or in a separation zone (not shown) before the condensates are removed through a line 27. Where the exhaust gas contains water vapour and the vapour is condensed in the primary heat exchanger 24 the latent heat of the vapour is given up to the secondary heat exchanger. If the exhaust gas contains some air which itself contains water vapour then the latent heat of that water vapour condensing provides an additional free source of energy for the system.

The water which is desired to be heated by the exhaust gas is fed in counter-current to the gas by first passing it into the secondary heat exchanger 24 through a line 28, then into the partitions defining the zones of the fluid bed through a line 30, and then across and back over the inlet of the gases to the fluid bed prior to passing out of the system through a line 32. The water may leave the system either as hot or boiling water or, as is more likely in this embodiment, as steam, by example at over 250"C.

The system shown in Figure 3 is suitable for uses where "contaminated" gases are passing through the heat exchanger as the water which is being heated by the exchanges need never come into direct contact with the gas. Where the gases are uncontaminated or it is of no consequence if the heated water.

becomes contaminated, a spray condensor 34 may provide the source for the water to be heated, as shown in Figure 4.

The system shown in Figure 4 is suitable for use in the recovery of heat from gases which have been derived from the drying of cornflakes or baking bread, for example, where water vapour is coming off in the presence of air.

The hot gas enters a zonal fluid bed 36 at an inlet 38 and then passes up through a cascade 40, past the spray condensor 34 and out through a duct 42. The gas may enter the zonal fluid bed at 200 C-and leave at 60"C without any of the water vapour having condensed in the bed as the outlet temperature is still above the dew point. In a conventional fluid bed heat exchanger, if water entering the heat exchanger were at 70 the effect of the fluid bed, whose outlet temperature is at 60'C, would-be to cool that water whereas, with thefluid bed of this embodiment, it may be possible to heat that water to boiling point or above. The water vapour in the gas is condensed as it flows up through the cascade and past the spray condensor.The spray condensor or spray water feed 34 is located close to the outlet duct 42 to give the minimum gas discharge temperature, but other sprays may be located lower in the column, preferably using water feed in series from collection points to ensure a temperature gradient falling towards the final gas outlet duct 42. This is done to minimise the water vapour content of the discharged gases and accords with Dalton's Law.

The water supply for the heat exchanger is provided by the spray condensor and the water vapour condensing from the exhaust gas through the cascade. The exhaust gas heats up the water as it flows -down the cascade, and adds condensed water vapour to the water supply. The water eventually falls into a sump 44 adjacent to the fluid bed.

A pump 46 removes water from the sump and passes it, through a line 48, into the heat exchanger of the fluid bed. The zonal fluid bed may be of the same construction as that shown in Figure 2. The water or steam, having passed through the fluid bed is then fed through a line 50, across the path of the hot gases coming into the fluid bed, and away through an outlet line 52.

The system shown in Figure 5 is particularly suitable as a hot water or steam generator or a generator of both hot water and steam.

Three heat exchangers are shown, the first two being zonal fluid beds 54 and 56 located adjacent to each other, and a primary heat exchanger 58.

The hot gases from a burner 60, which may be gas, passes through the zonal fluid bed 54, then between a partition between the two beds before passing through the zonal fluid bed 56, through the primary heat exchanger and out through an outlet line 61.

The passage of the gas through the fluid beds may be assisted fans and,tor blowers if required, and the zonal fluid beds may be of the same construction as that described in relation to Figure 2.

Gas burners generally include water vapour and it is the-7htention that this vapour does not change phase and yield up its latent heat until it reaches the primary heat exchanger 50. A line 62 may be provided beneath the primary heat exchanger to draw off the condensate, if required. Thus the system is able to take advantage of both the sensible heat and the latent heat, The water to be heated in the boiler flows in counter current to the burner gases first via a line 64, and is pressurised or driven by a pump 65, through the primary heat exchanger, through a line 66 to the fluid bed 56, then through a line 68 to the fluid bed 54 and then back and across the gas inlet to the fluid bed 56 before being drawn off as steam or hot water through a line-70. An overload 72 and pressure regulation valve74 may be provided in the line 70.

Although the embodiment shown in Figure 5 shows three heat exchangers being used it will be appreciated that any desired number of combination could be provided.

Claims

1. A fluid bed in which the material com prising the bed is arranged tò be fluidised within zones, the fluid bed including a heat exchanger arranged to exchange heat with two or more of the zones.

2. A fluid bed as claimed in Claim 1 in which the size of one or more of the zones is arranged to restrict lateral or axial movement -- of particles of the material comprising the bed.

3. A fluid bed as claimed in Claim 2 including a temperature gradient from the inlet to the outlet of the material in the fluid bed.

4. A fluid bed as claimed in any preceding claim in which fluid in the heat exchanger is arranged to exchange heat with successive zones.

5. A fluid bed as claimed in any preceding claim in which the heat exchanger at least partly defines at least some of the zones.

6. A fluid bed as claimed in Claim 5 in which the heat exchanger includes a plurality of plates which define the zones.

7. A fluid bed including a gas inlet ar ranged to supply the gas to fluidise the material in the bed, the gas inlet being common to all of the zones.

8. A fluid bed as claimed in any preceding claim including a gas outlet for the gas which has fluidised the material in the bed, the gas outlet being common to all of the zones.

9. A fluid bed as claimed in any preceding claim arranged to transfer heat from the ma terial comprising the bed to a fluid within' the heat exchanger.

10. A fluid bed substantially as herein described with reference to, and as shown in any of Figures 2 to 5.

11. Two fluid beds, each as claimed in any preceding claim in which the gas which leaves one fluid bed having fluidised the material in that bed is arranged to be passed through the material in the other fluid bed to fluidise the material in that other fluid bed.

1 2. A method of exchanging heat using a fluidised bed comprising causing the material in the bed to move in zones, and passing fluid to exchange heat with the material in the bed through a heat exchanger to exchange heat with two or more zones.

1 3. A method as claimed in Claim 1 2 comprising the fluid in the heat exchanger exchanging heat with successive zones.

14. A method as claimed in Claim 1 2 or 1 3 comprising passing heat through a heat exchanger which at least partly defines at least some of the zones.

1 5. A method as claimed in any of Claims 1 2 to 14 comprising causing the temperature of the fluid leaving the heat exchanger to be at a greater temperature than the temperature at which the gas which has been used to fluidise the material in the bed leaves the fluid bed.

1 6. A method as claimed in any of Claims 1 2 to 1 5 comprising passing the gas to fluidise the material in the zones through a common inlet.

1 7. A method as claimed in any of Claims 1 2 to 1 6 comprising passing the gas which has been used to fluidise the material in the zones through a common outlet.

1 8. A method as claimed in any of Claims 1 2 to 1 7 comprising fluidising the material in the zones with a gas which includes condensable vapour, the gas leaving the zones being at a temperature which is above the point at which condensation of the vapour occurs.

1 9. A method as claimed in any of Claims 1 2 to 1 7 comprising fluidising the material in the zones with a gas which includes condensable vapour, the gas leaving the zones being at a temperature which is above that at which the condensates in the vapour boil.

20. A method as claimed in any of Claims 1 2 to 1 9 comprising supplying the gas to fluidise the material in the bed at a temperature which is above the boiling point of the fluid passing through the heat exchanger, the temperature of the gas leaving the fluid bed being below the temperature at which the fluid in the heat exchanger boils, the fluid in the heat exchanger being heated by the fluid bed to a temperature which is at or above the temperature at which that fluid boils.

21. A method of exchanging heat using a fluidised bed substantially as herein described with reference to, and as shown in any of Figures 2 to 5.

22. A method of exchanging heat as claimed in any of Claims 12 to 21 including a fluid bed as claimed in any of Claims 1 to 10 or two fluid beds as claimed in Claim 11.