GB2062834A

GB2062834A - Method and apparatus for heating a fluid employing a heating gas containing sulphur oxides and water

Info

Publication number: GB2062834A
Application number: GB7937865A
Authority: GB
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1979-11-01
Filing date: 1979-11-01
Publication date: 1981-05-28
Also published as: EP0028503A3; EP0028503A2

Abstract

In the heating of a process fluid stream by a gas containing sulphur oxides and water vapour (e.g. from the combustion of coal or oil) the cold process fluid stream is placed in heat exchange with another fluid stream itself indirectly heated by said gas (e.g. via a heat transfer wall) and which is at a temperature below that of the gas but exceeding the dew point temperature of the gas, such that the outer surface temperature of said wall is always above the acid dew point, thereby avoiding acid corrosion. Heat exchangers may be utilized comprising one or more inner tubes enclosed by an outer tube, with the gas flowing over said outer tube. In Fig. 2 the said other fluid stream flows in the space between tubes 25, 26 and comprises the cold process fluid which has entered via tube 25 and is thereby preheated before entering said space. <IMAGE>

Description

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GB 2 062 834 A 1

SPECIFICATION

Method and apparatus for heating a substance employing a gas containing sulfur oxides and water

5 The present invention relates to a method and apparatus for heating a substance employing a gas containing sulfur oxides and water.

Hot gases containing sulfur oxides and water are commonly employed for heating in industry 10 and are derived by burning fuels containing sulfur. Among such fuels are solid fuels such as coals and lignites and fluid fuels such as fuel oils and hydrocarbon gases. A well-known problem arising from the use of such hot gases is that of acid 15 corrosion which occurs when the temperature of the hot gases falls to the dew point and an acidic liquid condenses on metal parts such as the low temperature heat recovery tubes of a furnace or boiler.

20 The usual method of avoiding acid corrosion is to ensure that the hot gases never contact surfaces which are at such temperatures as to cause condensation of acidic liquid. In the case of boilers or process stream heaters, the water or 25 process stream feed is preheated to a temperature exceeding the dew point of the hot gases from which heat is to be extracted, and this practice has obvious drawbacks in that an additional heat exchanger and an additional source of heat 30 therefor are required, and moreover, the heat which can be recovered from the hot gases is not as fully exploited as it might be.

An alternative approach which has been suggested is to form the surfaces of the heat 35 recovery tubes from materials which are capable of resisting acid corrosion. This alternative approach has practical limitations since it is not usually economic to form the surfaces of heat recovery tubes wholly from acid-resistant 40 material, and in practice, only those sections of the tubes which are vulnerable to acid corrosion are formed of acid-resistant materials. However, even this approach is not free from difficulties because the strength of the acid formed on the 45 exposed surface of a heat recovery tube depends on the temperature of the exposed surface, and the temperature unavoidably varies along the length of the tube. The vulnerable sections of the tubes must therefore be formed from different 50 materials each capable of resisting corrosion by the acid deposited locally thereon. Besides adding to the cost ot the heat recovery tubes, the approach is not reliable because a change in the operating conditions of the furnace can change 55 the strength of the acid deposited at each location so that materials which previously would not have been corroded become exposed to an acid strength which they may not be able to resist.

The present invention, in one aspect, provides a 60 method of heating a substance comprising passing gases containing sulfur oxides and water at a temperature above the dew point in indirect heat exchange relationship with a fluid which is at a temperature exceeding the dew point but not

65 exceeding the temperature of the gases, and passing the fluid in indirect heat exchange with the substance at a temperature below the dew point of the gases.

Preferably the fluid is passed through the 70 outermost passage of a multi-pipe heat exchanger (as herein defined), the gases passing in contact with at least part of the outer wall defining the outermost passage, and the substance is passed through an inner passage of the heat exchanger. 75 A multi-pipe heat exchanger is defined as a heat exchanger comprising an outer conduit surrounding at least one inner conduit and arranged to provide a flow channel for fluid between the outer wall(s) of the inner conduit(s) 80 . and the inner wall of the outer conduit, and wherein there may be a plurality of inner conduits arranged side-by-side within the outer conduit or one-within-the-other or disposed so that some inner conduits are side-by-side and some are one-85 within-the-other.

The substance may be passed from the said inner passage directly or indirectly into the outermost passage wherein it constitutes the said fluid.

90 The said inner passage is preferably substantially concentric with the outer passage and immediately inwards of the latter.

In another aspect, the present invention provides apparatus for heating a substance 95 employing gases containing sulfur oxides and water, comprising a first heat transfer surface disposed at a location where the gases have a temperature exceeding the dew point, means for circulating a fluid over said heat transfer surface in 100 indirect heat exchange relationship with the gases, a second heat transfer surface arranged for the passage of the fluid thereover, and means for passing said substance in indirect heat exchange relationship with the fluid.

105 The first heat transfer surface may be the outermost surface of a multi-pipe heat exchanger (as herein defined) and the second heat transfer surface is an inner surface of the multi-pipe heat exchanger.

110 The second heat transfer surface may define on its inner side the outer boundary of a first space for the substance and on its outer side the inner boundary of a second space for the fluid.

The first space bounded by the second heat 115 transfer surface may communicate with the second space at a location which is not readily accessible to said gases at the dew point whereby said substance can pass from the first space to the second space.

120 The apparatus preferably comprises walls defining a conduit for the passage of said gases, said first heat transfer surface extending across said conduit and being fixedly attached to said walls at one end only.

125 The first heat transfer surface may be arranged in at least two passes whereby during operation the said gases contact the first heat transfer surface at least twice and in which the means for passing the substance through said second space

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is arranged for passing the substance to the first of the passes which is contacted by the gases during operation. Preferably, in this type of arrangement, the means for circulating the fluid in the second 5 space is arranged for passing the fluid to the last pass which is contacted by the gases during operation.

The apparatus may, in addition, comprise means for burning a sulfur-containing fuel to 10 produce said hot gases, and means for conducting said hot gases into contact with said first heat transfer surface. The apparatus may comprise means for recovering heat from the hot gases before the hot gases contact the first heat transfer 15 surface.

The invention is now further described with reference to some illustrative diagrammatic drawings in the accompanying Figures,

wherein:—

20 Figure 1 is a diagrammatic vertical cross-sectional elevation of a known furnace employed for heating a process fluid;

Figure 2 shows diagrammatically a part of the furnace of Figure 1 incorporating apparatus in 25 accordance with the invention; and

Figure 3 shows an alternative modification to that depicted in Figure 2.

Referring first to Figure 1, the furnace 10 comprises vertical walls 11 lined with refractory 30 which define a number of sections of reduced horizontal cross-sectional area at the higher levels and which sections are connected by, e.g., sloping sections. The top section 12 is connected to a stack (not shown) for the discharge of combustion 35 gases from the top of the furnace 10.

Near the base of the furnace are provided a suitable number of burners (not shown) supported by furnace floor 13. The or each burner is supplied with fuel which is burned in a flame 14 above the 40 furnace floor 13. In the vicinity of flame 14, there is intense radiation and at more remote locations above the flame, most of the heating effect of the flame is by convection through the medium of the combustion gases and hot excess air.

45 Most fuels contain sulfur and in consequence the combustion gases contain sulfur oxides in addition to the water vapour produced by the oxidation of the hydrogen-containing components of the fuel.

50 Generally speaking, the process fluid which is to be heated is passed more or less countercurrently relative to the combustion gases so that cool fluid is employed to recover heat from the combustion gases near the top of the furnace 55 mainly by convective heat transfer, and heated fluid is finally heated mainly by radiant heat transfer in the vicinity of the flame 14. Thus, as will be seen from Figure 1, the process fluid enters the furnace 10 near the top via tube 15 and 60 passes through one (or more) sets or banks of tubes 16 disposed in a convection section 17 of the furnace for recovery of heat from the hot combustion gases passing upwardly towards the top section 12 and stack from a lower section 18 65 comprising a firebox. The fluid passes through tubes 16 in a generally countercurrent path to the combustion gases and relatively hot fluid circulates from the tubes 16 to one or more banks of tubes 19 in the lower section 18 surrounding the flame wherein a major proportion of high temperature heat is recovered from the radiation in the lower section 18. The fluid leaves the tube bank(s) 19 via outlet(s) 20 at a relatively high temperature.

Reference is now made to Figure 2 which shows, in simplified form, an arrangement of heat recovery tubes for use in the convection section 17 in accordance with the invention. In this arrangement, the fluid is heated employing a double pipe heat exchanger 21 extending across the convection section 17 and being supported at its end regions by the walls 22,23 of the furnace around the section 17. Double pipe heat exchangers are known perse, and in the illustrated arrangement, the cool fluid is passed into one end

24 of the central tube 25 of the heat exchanger 21 and circulates from the open opposite end of the central tube 25 into the surrounding annulus defined between the central tube 25 and an outer tube 26. Matters are so arranged that heat is recovered from the combustion gases at temperatures exceeding the acid dew point on the outer surface of the outer tube 26, thereby avoiding acid corrosion problems. Some of the heat thus recovered in the fluid passing between the tubes 25 and 26 is transferred to the fluid in the central tube 25 by heat transfer through the walls of tube 25, and the proportions of heat retained in the outer annulus of fluid and transferred to the innermost fluid can be varied or predetermined by appropriate dimensioning of the cross-sectional flow areas of the annulus and of the central tube, by employing appropriate materials of construction to provide the desired amount of heat transfer through tube 25, by the use of baffles (not shown), fins, studs or other extended surfaces (not shown) on appropriate parts of the fluid-contacting regions of the tubes

25 and 26 and/or gas-contacting regions of tube 26, by varying the flow rate of the fluid through the heat exchanger 21, inter alia. It is within the ordinary competence of the skilled technologist to determine which one or combination of the foregoing techniques should be employed, and to what extent.

The heated fluid is recovered from outlet 27 and may be passed to further convective heat recovery units and/or to a radiant heat recovery tube bank as described in relation to Figure 1.

It will be seen that the arrangement of Figure 2 enables cold fluid to be heated to a temperature exceeding the acid dew point without causing acid dew point corrosion of the heat exchanger 21. In one mode of construction, the heat exchanger 21 is fixed at one end only, preferably the end at which cold fluid enters and from where heated fluid is recovered. The other end is supported in such a manner that it is free to move to accommodate thermal expansion and contraction of the heat exchanger 21 and the furnace walls.

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GB 2 062 834 A 3

With reference now to Figure 3, there is depicted a double pipe heat exchanger 29 of the so-called "hairpin" type mounted in the convection section 17 of a heating furnace.

5 In this arrangement, the cold fluid feed is completely separated from the fluid which recovers heat from the upwardly passing hot combustion gases giving a greater range of variability of throughputs of the cold fluid feed 10 than in the embodiment of Figure 2 since the flow rate and initial temperature of the feed have less influence on the temperature of the outer surface of the heat exchanger 29.

The cold fluid feed is circulated into the 15 entrance 30 of the lowest section of the central tube 31 and is recovered via an outlet 32 from the highest section of the central tube 31.

The fluid feed is circulated through tube 31 generally countercurrently to a fluid in the annular 20 space 34 defined between the central tube 31 and an outer tube 33. The flow rate and temperature of the fluid in the annular space 34 are so arranged that the lowest temperature on the outside of the outer tube 33 exceeds the acid dew 25 point.

Preferably the fluid in the central tube 31 circulates countercurrently to the fluid in the annular space 34, and the latter enters the heat exchanger 29 via inlet 36 on or communicating 30 with the upper, cooler pass of the heat exchanger 29 and is recovered at a higher temperature from an outlet 37 or communicating with the lower, hotter pass. The temperature of the fluid in the cooler pass must be such that the temperature 35 of the outer surface of the wall thereof exceeds the acid dew point.

In this embodiment, it is possible to raise the temperature of the cold fluid feed by a greater amount, generally speaking, than in the 40 embodiment of Figure 2.

In a variant (not shown) of the Figure 3 arrangement, at least some of the heated fluid recovered from the outlet 32 of the central tube

32 is employed as the fluid in annular space 34. 45 In Figure 3, it will be seen that the outer tube

33 has flanged ends 38, 39 to permit access for cleaning, maintenance and repair. The central tube 31 may also be furnished with flanges (not shown) or other means of attachment in the vicinity of the

50 return bend 40 (e.g. where each end of the bend 40 is attached to the straight sections of the tube 33) for servicing and removal of the central tube 31.

As in the previous embodiment, the heat 55 transfer surfaces contacted by one or both streams of fluid may be provided with fins or other extended heat transfer surfaces and/or furnished with baffles.

It is contemplated that a plurality of double pipe 60 heat exchangers may be employed in series and/or parallel connection, and that they may be of the same type or different types. Moreover, in a further variant of the invention, it is contemplated that in place of double pipe heat exchangers, there 65 may be employed heat exchangers having at least two pipes enclosed by the outermost pipe and that the thus said enclosed pipes may be arranged side-by-side within the enclosing pipe and/or one within another inside the enclosing pipe. 70 Any feasible combination of the foregoing arrangements may also be employed without departing from the invention.

Claims

1. A method of heating a substance comprising 75 passing gases containing sulfur oxides and water at a temperature above the dew point in indirect heat exchange relationship with a fluid which is at a temperature exceeding the dew point but not exceeding the temperature of the gases, and 80 passing the fluid in indirect heat exchange with the substance at a temperature below the dew point of the gases.

2. A method as in claim 1 in which the fluid is passed through the outermost passage of a multi-

85 pipe heat exchanger (as herein defined), the gases passing in contact with at least part of the outer wall defining the outermost passage, and the substance is passed through an inner passage of the heat exchanger.

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3. A method as in claim 2 in which the substance is passed from the said inner passage directly or indirectly into the outermost passage wherein it constitutes the said fluid.

4. A method as in claim 3 in which the said 95 inner passage is substantially concentric with the outer passage and immediately inwards of the latter.

5. A method of heating a substance substantially as hereinbefore described or as

100 described with reference to the accompanying diagrammatic drawings.

6. Apparatus for heating a substance employing gases containing sulfur oxides and water, comprising a first heat transfer surface disposed at

105 a location where the gases have a temperature exceeding the dew point, means for circulating a fluid over said heat transfer surface in indirect heat exchange relationship with the gases, a second heat transfer surface arranged for the passage of

110 the fluid thereover, and means for passing said substance in indirect heat exchange relationship with the fluid.

7. Apparatus as in claim 6 in which the first heat transfer surface is the outermost surface of a

115 multi-pipe heat exchanger (as herein defined) and the second heat transfer surface is an inner surface of the multi-pipe heat exchanger.

8. Apparatus as in claim 7 in which the second heat transfer surface defines on its inner side the

120 outer boundary of a first space for the substance and on its outer side the inner boundary of a second space for the fluid.

9. Apparatus as in claim 8 in which the first space bounded by the second heat transfer surface

125 communicates with the second space at a location which is not readily accessible to said gases at the dew point whereby said substance can pass from the first space to the second space.

10. Apparatus as in claim 9 comprising walls

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GB 2 062 834 A 4

defining a conduit for the passage of said gases; said first heat transfer surface extending across said conduit and being fixedly attached to said wails at one end only.

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11. Apparatus as in any one of claims 8 to 10 in which the first heat transfer surface is arranged in at least two passes whereby during operation, the said gases contact the first heat transfer surface at least twice and in which the means for passing the 10 substance through said second space is arranged for passing the substance to the first of the passes which is contacted by the gases during operation.

12. Apparatus as in claim 11 in which the means for circulating the fluid in the second space 15 is arranged for passing the fluid to the last pass which is contacted by the gases during operation.

13. Apparatus as in any one of claims 6—12 comprising means for burning a sulfur-containing fuel to produce said hot gases, and means for

20 conducting said hot gases into contact with said first heat transfer surface.

14. Apparatus as in claim 13 comprising means for recovering heat from the hot gases before the hot gases contact the first heat transfer surface.

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15. Apparatus for heating a substance with gases containing sulfur oxides and water substantially as hereinbefore described or substantially as described with reference to the accompanying diagrammatic drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.