WO1991002195A1

WO1991002195A1 - Natural draft air preheater

Info

Publication number: WO1991002195A1
Application number: PCT/US1990/004274
Authority: WO
Inventors: Herbert Douglas +Di Michelson
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1989-07-31
Filing date: 1990-07-31
Publication date: 1991-02-21

Abstract

An apparatus including a chamber having a casing partly or wholly comprising a heat exchanger (1, 2, 3) for preheating air before the air is supplied to at least one burner (7). The heat exchanger (1, 2, 3) is composed of at least one heat exchange tube (2) for containing heated fluid medium which is preferably steam; preferably the heat exchanger includes a plurality of heat exchange tubes (2) which may be arranged in various configurations having opposite ends connected to manifolds (1, 3). The burner chamber surrounds a burner (7) for preheating air which is supplied to a furnace, in connection with the burner chamber, for increasing the temperature of a process fluid. The arrangement of the burner chamber and furnace permits air to be drawn past the heat exchanger and into the burner chamber by a draft created by operation of the burner (7) and the furnace. The heat exchanger (1, 2, 3) is preferably positioned to permit unobstructed air flow communication between the surroundings and the burner.

Description

NATURAL DRAFT AIR PREHEATER

1- Field Of The Invention

This invention relates to an improvement in furnace air preheaters. More specifically, the present invention is directed to an improvement in the structure of the furnace assembly through which ambient air is brought into the furnace for combustion. More particularly, the present invention is directed to the integration of a heat exchanger for preheating such air with the burner chamber casing structure of the furnace assembly. 2. Discussion Of Background And Material Information

Many commercially important processes are performed using industrial process furnaces. Such a process furnaces may have the function of increasing the temperature of a process fluid without effecting a change in state or causing chemical reactions to occur. However, the process fluid treated in such a process furnace may be heated to the extent of causing physical and/or chemical changes. High temperatures are often required in such operations, as where a hydrocarbonaceous feedstock, such as ethane, propane, naphtha, or oil, may be thermally cracked to produce less saturated products, such as acetylene, ethylene, propylene, butadiene and other products, depending on the feedstock used.

A number of commercially important conversions, such as the above-mentioned conversions, are endothermic and occur at desired rates only when elevated temperatures are reached. For example, the steam pyrolysis of various hydrocarbons to produce olefins is normally conducted at temperatures in excess of 1100°F. Catalytic reforming in the presence of hydrogen to improve octane normally proceeds at temperatures of about 900°F. In the case of catalytic steam reforming of hydrocarbons to produce hydrogen and carbon monoxide, temperatures in excess of 1000°F are normally required. Each of these processes, as well as many others, are endothermic and occur at appreciable rates only with elevated temperatures.

The furnaces suitable for such processes may generally include a refractory lined enclosure containing heat resistant metal alloy tubes through which process fluids are passed. The tubes are heated by burners which combust a suitable fuel, often mixed with air, within the refractory enclosure which provides the heat.

The thermal efficiency of the furnace processes can be 0 an important commercial and/or operational concern. Most furnace designs utilize'direct radiant heating of a portion of the process tubing for either generating reactions or preheating the material prior to entering the reaction zone of equipment downstream. The radiant heat may be from the 5 burner radiation or heat withdrawn and recycled from other reactions occurring in the furnace away from the burners. Additionally, heating the process material within the process tubing may occur in the a convective section of the furnace. The concept of preheating the process materials 0 includes the preheating combustion air from the ambient environment, usable in either the radiant or convective designs, as discussed above.

The use of tubular furnaces to carry out high- temperature conversions as practiced heretofore has 5 occasioned some difficulties as can be illustrated by consideration of typical conditions employed in a furnace in which catalytic steam reforming of methane to produce hydrogen and carbon monoxide is conducted. Typically, a light hydrocarbon is preheated to a temperature of about 30 1000°F and is introduced together with steam into catalyst- packed, vertical tubes disposed in the furnace chamber to be heated by burning a fuel. In order to supply the heat of reaction and to attain an outlet temperature in excess of 1500°F, tube wall temperatures above 1600°F are maintained 35 and the hot combustion products leave the furnace chamber at about 2100°F. Under these conditions the efficiency of the furnace is only about 40% from the standpoint of utilization in the reaction of the heat released by the combustion of the fuel. To obtain reasonable efficiency, the hot combustion products from the furnace chamber are flowed through a convection section of the furnace to preheat the gas feed and to generate steam. Thereafter the combustion products are vented.via a stack. To make use of the steam thus generated, steam must generally be the principal source of power in the associated plant thereby precluding in some instances the use of electric motors for pumps, compressors, and the like. Where large supplies of steam are not needed, the operation of reformer furnaces for hydrogen production has frequently been deemed commercially impractical.

Another approach in attempting to improve thermal efficiency was to recycle combustion gases in order to preheat combustion air. This approach was frustrated by cumbersome and expensive ductwork. Even when a regenerative-type air preheater was considered, the ductwork still was elaborate. U.S. Patent No. 3,469,946, uses a heat-transfer fluid in a combustion-air-preheating system to step up furnace efficiency.

In U.S. Patent 3,469,946, the heat-transfer fluid receives heat in a convection section of the furnace. The heat-transfer fluid is then piped to the vicinity of the burners_^ where the fluid gives up its heat to incoming combustion air. This caused a reduction in furnace chamber size by ten to thirty percent and reduction in fuel requirements; although ductwork was reduced, however, it was still required.

Another approach to achieve the preheating of combustion air is disclosed in U.S. Patent No. 3,583,691, which used a jacket around the refractory wall of a gas- fired kiln; the jacket defines an air chamber within which 5 air is heated by heat from the wall prior to being delivere to a gas-air mixer and used as combustion air.

The primary obstacles in preheating combustion air ar the additional capital and maintenance costs associated with the preheating process. It is generally recognized in the above-mentioned patents that these additional costs result from the motors, movable parts, seals and ductworks.

Related .to t is, U.S. Patent No. 3,426,733 discloses that another drawback to conventional air preheating systems has been the geometric, difficulties encountered in moving substantial quantities of gases through large ducts by means of fans.

U.S. Patent No. 3,426,733 is concerned with the relative efficiencies of different heating stream configurations and not the preheating structure itself.

U.S. Patent No. 3,469,946 teaches a structural configuration designed to minimize cumbersome and expensive ductwork. Although the piping of the heat-transferring f fluid too near the burners is seen as an expedient by which ductwork is- practically avoided, the concept and use of ductwork are still maintained. Airflow is shown as being fan-induced and passing through ductwork in which a heat exchanger is located.

U.S. Patent No. 3,583,691, also shows ductwork which is relatively extensive in nature, uses a jacket about the entire structure, and effects forced airflow using a fan. Other patents of interest are U.S. Patent No. 3,968,030 and DE 31 25 629 A 1 (German) . Thus, the prior art has taught the use of ducts to convey the combustion air to an enclosed heat exchanging means for preheating purposes with subsequent ducting to the burner chamber. The patents discussed herein are incorporated in their entirety by reference thereto. »

SUMMARY Q£ ______ INVENTION

The present invention is directed to an apparatus which includes a chamber for at least one burner wherein the chamber is composed of at least one partition including means for preheating air as the air is supplied to the burner, preferably wherein the means for preheating air is a means for exchanging heat or a heat exchanger selected from the group consisting of radiant heaters, conductive heaters, and combinations of radiant and conductive heaters, but more preferably wherein the heat exchanger is a radiant heater.

In accordance with the present invention, the means for exchanging heat includes a heated fluid median for transferring heat to the air and is composed of at least one heat exchange tube for containing the heated fluid medium, which is preferably steam; preferably the means for exchanging heat in a plurality of heat exchange tubes.

The burner chamber in accordance with the present invention also includes a manifold interconnecting a source of the heated fluid medium and the heat exchange tubes as well as a manifold interconnecting the heat exchange tubes with a pipe for discharging fluid medium from which heat has been transferred.

In a preferred embodiment, a plurality of heat exchange tubes are provided which are interconnected at their upper ends to one of the manifolds and at their lower ends to another of the manifolds, and are preferably disposed substantially perpendicular with respect to a support surface for the apparatus, and most preferably are substantially vertically disposed with respect to a horizontal support surface for the apparatus.

The heat exchange tubes of the burner chamber in accordance with the present invention may be arranged to have various configurations. In any event, the plurality of heat exchange tubes forms at least one grid section of heat exchange tubes in at least one of the partitions of the burner chamber. '

In one embodiment, the grid, section is substantially planar; another embodiment provides for at least two grid sections of heat exchange tubes which are substantially 5 planar and spaced apart from each other. Related to this, the burner chamber may be constructed to have three grid sections arranged to describe a substantially triangular open area surrounding the burner wherein each of the manifolds connected to the opposite ends of the heat exchange tubes are substantially triangular shaped. In ^' yet another embodiment, a burner chamber is constructed to have two grid sections which are diametrically opposed. In another embodiment, the burner chamber may be constructed to have a curved grid section which is substantially U-shaped as are the manifolds connected to the opposite ends of the heat exchange tubes arranged in such U-shaped configuration.

In another related embodiment, the burner chamber is composed of a curved grid section wherein the heat exchange tubes have upper and lower ends arranged in a substantially circular configuration and preferably wherein the upper ends of the heat exchange tubes are connected to a substantially circular-shaped manifold and the lower ends of the heat exchange tubes are connected to another substantially disk- shaped manifold, preferably wherein the upper ends of the heat exchange tubes are arranged in a configuration describing an upper circle and the lower heat exchange tubes are arranged in a configuration describing a lower circle wherein the diameter of the lower circle is smaller than the diameter of the upper circle so that the heat exchange tubes converge towards each other downwardly to form a generally concave burner chamber which is substantially bowl-shaped.

The present invention is also directed to an apparatus including a furnace assembly which is composed of a furnace for increasing the temperature of a process fluid, and means for preheating air operably connected to supply preheated air to the furnace, wherein the means for preheating includes a burner, and the burner chamber surrounding the burner is composed of a heat exchanger for increasing the temperature of air drawn passed the heat exchanger and into the burner chamber.

The apparatus in accordance with the present invention, as described above, permits air to be drawn past the heat exchanger and into the burner chamber by a draft created by operation of the burner and the furnace.

The apparatus in accordance with the present invention, as described has also been designed to position the heat exchanger so as to permit unobstructed air flow communication from the ambient to the burner.

In accordance with the present invention, as described above, the heat exchanger is preferably provided as an integral part of the burner chamber.

An object of the present invention is to preheat the combustion air used in furnace burners using a heat exchanger which forms a part of the casing or housing structure for the burners. To this end, the present invention features the use of ambient air supplied to the burner for combustion by a natural draft or induced air flow through a heat exchanger which is integrated into the structure of the burner casing of the furnace. In accordance with the present invention, this results in a "line of sight" or unobstructed air flow between source of the air, for example from the ambient environment, and the burner.

The structure of the furnace assembly of the present invention is advantageous in that preheating of air for combustion requires less equipment and less maintenance expenses due, at least in part, to the use of heat exchangers without the attending ductwork which would otherwise be required. Thus, the apparatus of the present invention permits a reduction or complete elimination of ductwork or double walls within which air flows to the means for preheating the air before being passed to the burner which is normally required by conventional systems. Moreover, the apparatus in accordance with the present invention permits an unobstructed flow of air for combustion from the ambient environment to the burner.

Among the advantages offered by this invention are lower initial capital costs, lower maintenance costs, greater dependability, and flexible design configurations. Other objectives, features and advantages can be realized in the context of this specification and this specifying hereinabove is not intended to be limiting accordingly. BRIEF DESCRIPTION 0£ SS. DRAWINGS The present invention is illustrated in the drawings which are only meant to be non-limiting examples and are not to be construed to restrict the invention disclosed or claimed herein.

Figure 1 is a perspective view of a burner chamber casing which shows one heat exchanger element per burner.

Figure 2 is a perspective view of a burner chamber casing which shows that multiple burners can be serviced by one heat exchanger element.

Figure 3 is perspective view of a burner chamber casing which shows the use of three planar heat exchangers as the casing walls.

Figure 4 is a perspective view of a burner chamber casing which shows a hemispherical configuration.

Figure 5 shows a cross-section of a furnace wall with an integrated heat^'exchanger.

DETAILED DESCRIPTION QE TϋE INVENTION Although title burner chamber casing described is suitable for many purposes, it is particularly suitable for use with industrial furnaces in which hydrocracking or reforming of hydrocarbons is the process.

Figure 1 is a non-limiting illustration of one embodiment In accordance with the present invention wherein external burners are used with a furnace box. Steam manifold or header 1 receives steam, which flows through steam supply conduit 10, and permits the steam to pass into a bank of tubes 2, which may be ^*referred to herein as fin tubes, attached at one end to steam header 1. Although a bank of tubes is illustrated, one could use any practical number, combination and arrangement of tubing suitable for the burner Involved. In operation, ambient air flows across or past these tubes receiving heat from the tubes and causing steam condensation within the tubes. The condensate flows down into condensate manifold or header 3, attached at the other end of the bank of fin tubes 2.

The ends of steam header 1 and condensate header 3 curve around in a generally U-shape or horseshoe fashion and are attached at their ends to front plate 4. Bottom plate 8 is attached to the bottom of condensate header 3. Fuel pipe 6 as means for providing fuel to the burner passes through an orifice provided in front plate 4 and terminates in burner 7. The interior rim of steam header 1 and the top rim of front plate 4 define an open area at the top of the burner chamber 20 which functions as an exhaust 9 for exhausting products produced by the operation of burner 7.

The burner chamber 20 is composed of steam header 1, the bank of fin tubes 2, the condensate header 3, the front plate 4 and the bottom plate 8; these elements in their entirety may also be referred to herein as the burner chamber casing 12.

Access to service the burner 7 or the interior of burner chamber casing 12 is provided through access doorway 5. During operation, doorway 5 is covered with a suitable door or other cover. Steam is provided for heat exchange through steam supply line 10, attached to steam header 1, and condensate is withdrawn from condensate header 3 through condensate drain line 11. The combustion fuel is supplied through fuel pipe 6. The exhaust opening 9 communicates with, and is preferably attached to the bottom of, a furnace so that a natural draft is created which pulls ambient air through the bank of fin tubes 2, preheating the ambient air for combustion with the fuel at burner 7. The product of combustion is then drawn through the exhaust opening 9 into the furnace. Alternatively, air can be drawn by use of an induction fan attached to the furnace. Another configuration for external burners is illustrated in Figure 2. In this embodiment steam header 31, bank of fin tubes 32 and condensate header 33 are arranged in a planar configuration. Unnumbered end plates, one being shown in an exploded view to permit interior viewing, are provided to define the burner chamber 42. Front plate 3,4 has at least one access door 35 and at least one fuel pipe 36 which pass through an orifice provided in front plate 34 and attaches to burner 37. As shown, there can be multiple burners 37 inside the burner chamber casing 42, which casing 42 is defined by steam header 31, bank of fin tubes 32, condensate header 33, bottom plate 38, front plate 34 and top plate 43 together with unnumbered end plates, which attach at both ends of the burner chamber casing. Exhaust opening 39 differs from the invention depicted in Figure 1 in being comprised of a collar 44 lining port 45. Although one exhaust opening per burner is illustrated, more could be used. In yet another configuration, three planar heat exchangers, e.g. car radiator-type heaters having their respective steam headers joined and condensate headers joined to form two separate triangular steam and condensate header structures, respectively, are provided as generally shown in Fig. 3. Although not shown, the condensate header structure of this embodiment would also have attached to it a bottom plate or floor, and an appropriate exhaust opening would be defined as an open area at the top of the burner chamber described by the interior rim of the three-sided or triangular-shaped steam header. The burner chamber of this embodiment would, therefore, be composed of the steam header, condensate heater, interconnecting heat exchange tubes and a bottom plate or floor.^* In this embodiment, the fuel supply line would pass through an opening provided in the bottom plate and connect to the burner within the burner chamber, although the fuel supply line could also pass through an opening between heat exchange tubes and connect with the burner.

In the embodiment shown in Fig. 4, a circular steam header is provided with an attached bank of fin tubes converging underneath into a single, preferably disk-shaped, condensation compartment so as to form a concave or bowl- shaped burner chamber casing, i.e., "Southern Hemisphere." In this case, condensate is drained from the bottom surface side of the condensation compartment through a pipe and exhaust escapes through the open area defined by the interior rim of the circular header structure at the top of the burner chamber. In this embodiment, however, the fuel supply line, pipe or conduit would pass through an opening between heat exchange tubes and connect with the burner. As can be seen, numerous geometric configuration may be used in accordance with the present invention, which is indicative of the great utility and flexibility of the invention. Lower costs are achieved by using the heat exchanging elements as a part of the burner chamber casing. In the case where burners internal to a furnace box are used, there could be integration of the heat exchanger with an air inlet port of a furnace wall. In this instance, the furnace wall acts as the burner chamber casing.

Figure 5 is illustrative of a cross-section of a furnace wall contemplated for use in accordance with the present invention. As shown, burner 51 is located inside the furnace box comprising a furnace box wall 55. Furnace box wall 55 is provided with port 67 within which is located heat exchanger 57. Heat exchanger 57 receives steam through steam line 59 attached to steam header 61 of heat exchanger 57. Steam condenses and flows down into steam c'ondenser header 63. This condensate flows* through condensate drain line 65. Ambient air is preheated as it is drawn past heat exchanger 57 for the combustion process at the burner 51. It can be seen that heat exchanger 57 and headers 61 and 63 form part of the integral wall boundary of wall 55 for the furnace. 12

With respect to preheating combustion air, the structure of the burner in its relation to the furnace may be conventional. Moreover, the burner may be floor-mounted, wall-mounted or roof-mounted and may be located internally or externally in relation to the furnace box or shell.

In accordance with the present invention an induced draft system could be used with the above illustrated configurations, although the economic savings would possibly not be as great.

The above configurations are provided for illustrative purposes and should not be viewed as limiting the invention herein. Those _^skilled in the art will be able to take the teachings herein and apply them to additional configurations by performing the integration of the heat exchanger into the casing for the burner chamber as has been illustrated herein. Therefore, it should be understood that although the invention has been specifically described with respect to particular means and embodiments, the foregoing description is that of preferred embodiments of the invention. The invention, however, is not limited to particulars disclosed but extends to all equivalents, various changes and modifications maybe made to the invention without departing from the spirit and scope thereof.

Claims

CLAIMS :

1. Apparatus comprising a chamber for at least one burner, said chamber comprising at least one partition including means for preheating air as said air is supplied to said at least one burner.

2. Apparatus according to claim 1, wherein said means for preheating air is a means for exchanging heat such as a radiant heater or a conductive heater or a combination thereof.

3. Apparatus according to claim 2, wherein said means for exchanging heat comprises at least one heat exchange tube for containing heated and fluid heat exchange medium, preferably a plurality of tubes with a manifold provided for interconnecting a source of said medium and the plurality of heat exchange tube and/or preferably with another manifold provided for interconnecting said plurality of heat exchange tubes with a eans for discharging the medium from which heat has been transferred.

4. Apparatus according to claim 3, wherein said plurality of tubes are disposed transversally, preferably substantially perpendicular with respect to a support surface for said apparatus, said surface being preferably substantially horizontal.

5. Apparatus according to claim 3 or claim 4, wherein said plurality of heat exchange tubes forms at least one grid section of heat exchange tubes in said partition, wherein' said grid section is a substantially planar array or a curved array, preferably with at least two grid sections of heat exchange tubes spaced apart from each other with a combustion space therebetween.

6. Apparatus according to claim 5, wherein three grid sections are arranged to describe a substantially triangular open area surrounding said burner, preferably with substantially triangular manifolds or wherein at least two grid sections are arranged substantially parallel or diametrically opposed, preferably with a curved grid section joining each of the two diametrically opposed grid sections of one end to form a generally U-shaped configuration around said burner and/or with substantially solid wall connected to ends of said two diametrically opposed grid sections at the other end and/or with a plate attached to the bottom of said another manifold.

7. Apparatus according to claim 5, wherein said grid section is substantially U-shaped or substantially circular and the manifolds or manifolds conform(s) to the cross-sectional layout of the grid section.

8. Apparatus according to claim 7, wherein said circular grid section is substantially conical with tubes conversing downwardly, preferably with a burner chamber which is substantially bowel-shaped.

9. Apparatus according to any of the preceding claims, wherein said chamber comprises a floor supporting said partition, a front wall supported on said floor opposite said partition, a roof interconnecting said front wall with said partition to form a hollow structure, and a first end plate and a second end plate attached at opposite ends of said structure to enclose an interior space containing said at least one burner, and at least one exhaust from aid interior space.

10. Apparatus according to claim 9, wherein said exhaust comprises an orifice in said roof for releasing preheated air from said chamber, such as a collar passing through said orifice, preferably with a plurality of exhausts and/or a plurality of burners, wherein said plurality of exhausts may outnumber the plurality of burners.

11. Apparatus according to any of the preceding claims further comprising a furnace operably connected to said chamber, preferably with said furnace comprising at least one furnace wall having an opening and said at least one partition being located in said opening optionally with said at least one furnace wall forming a furnace box surrounding said burner having an upper portion comprising an exhaust, conveniently formed by an open area at the top of said chamber at least partially defined by said manifold.

12. Apparatus according to any of the preceding claims further comprising means for processing hydrocarbons e.g., olefins operably connected to said furnace, optionally with means for maintaining said hydrocarbons at a required temperature for processing, such as a fractional distillation column or a reactor, and preferably with said furnace is operably connected upstream from said means for maintaining to preheat said hydrocarbons before introducing said hydrocarbons into said means for maintaining.

13. Apparatus according to claim 12, wherein said reactor is a reforming reactor, including catalytic reforming reactors sand steam reforming, cracking reactors, or pyrolysis reactors.

14. Apparatus according to claim 13, wherein said catalyst reforming reactors are selected from the group consisting of dehydrogenation reactors, isomerization reactors, dehydrocyclization reactors, and hydrocracking reactors and wherein said cracking reactors are selected from the group consisting of catalytic cracking reactors and thermal cracking reactors such as steam cracking reactors.

15. Apparatus comprising a burner arrangement comprising a burner and means for preheating ambient air in open fluid flow communication with said burner.

16. Apparatus according to claim 15, wherein said means for preheating is a heat exchanger such as a fin tube heat exchanger, preferably with a furnace operably connected to said burner arrangement.

17. Apparatus comprising: a burner chamber casing having at least one air inlet opening comprising means for preheating air as air passes through said air inlet opening wherein said means for preheating air is a heat exchanger which is an integral part of said burner chamber casing, and at least one exhaust structure.

18. Apparatus according to claim 17, further comprising a furnace operably positioned with respect to said burner chamber casing to receive preheated air, preferably arranged so that ambient air flows through said iar inlet opening by natural draft action of said furnace and preferably with said furnace is positioned above said burner chamber casing.

19. An apparatus comprising a furnace assembly, said furnace assembly comprising: a) a furnace for increasing the temperature of a process fluid; and b) a means for preheating air operably connected to supply preheated air to said furnace, said means for preheating comprising: i) a burner; and ii) a burner chamber surrounding said burner comprising a hear exchanger for increasing the temperature'of air drawn past said heat exchanger and into said burner chamber by a draft created by operation of said burner and said furnace.

20. An apparatus comprising a furnace assembly, said furnace assembly comprising: a) a furnace for increasing the temperature of a process fluid; and b) a means for preheating air operably connected to supply preheated air to said furnace, said means for preheating comprising: i) a burner; and ii) a burner chamber surrounding said burner comprising a heat exchanger positioned to permit unobstructed air flow from the ambient to said burner for increasing the temperature of air drawn past said heat exchanger and into said burner chamber.

21. An apparatus comprising a furnace assembly, said furnace assembly comprising: a) a furnace for increasing the temperature of a process fluid; and b) a means for preheating air operably connected to supply preheated air to said furnace, said means for preheating comprising: i) a burner; and ii) a burner chamber surrounding said burner comprising a heat exchanger as an integral part of said burner chamber for increasing the temperature of air drawn past said heat exchanger and into said burner chamber.

22. A process using an apparatus according to any of the preceding claims using a heated fluid medium, preferably steam, for transferring heat to air moving into the chamber.