Patent Application
20070234974
Hydrogen reformer units are commonly used in the petrochemical industry to produce hydrogen gas from methane and steam. One common hydrogen reformer process employs a prior art reformer heater 2 of the type depicted in
In the prior art top-fired heater 2, the flue gas tunnels 14 are located entirely within the radiant firebox 4 and typically extend upwardly from the firebox floor 16 to a height of as much as seven feet or more. The prior art flue gas tunnels 14 must be constructed within the firebox 4 and have a rectangular cross-sectional shape. The side walls 20 of the prior art tunnels 14 are constructed of firebrick and the flat tops 28 of the tunnels 14 are formed of lintels or tiles which bridge between the upper ends of the side walls 20. In order to encourage uniform flow of the flue gas within the firebox 4 and to assist in obtaining a more uniform collection of flue gas in the tunnels 14 along their entire length, the flue gas openings 18 provided in the vertical side walls 20 of the tunnels 14 are smallest adjacent the discharge ends 26 of the tunnels 14 and progressively increase in size (i.e., cross-sectional area) toward the opposite longitudinal ends 30 of the tunnels 14.
Unfortunately, the flue gas tunnel systems heretofore used in the art have significant shortcomings and are costly and time-consuming to install, maintain, and repair. When constructing the prior art heater 2, several experienced bricklayers are required to erect the vertical tunnel side walls 20 within the firebox 4. In addition, the construction process is very time consuming and difficult due to the extremely limited space and maneuverability within the firebox 4 between the tube rows 6. In addition, the flat lintel or tile covers 28 of the prior art tunnels 14 undergo significant fatigue when exposed to the extreme temperature conditions within the firebox over time and tend to break. This results in the formation of cracks and holes in the covers 28 which further degrade the flue gas flow distribution and uniformity within the firebox 4 and within the tunnels 14. Also, the presence of the tunnels 14 within the already limited space between the tube rows 6 makes it difficult to construct scaffolding within the firebox 4 and to move materials into and out of the firebox 4 for inspecting and repairing the burners 12, tubes 8, and other internal components of the heater 2.
The present invention satisfies the needs and alleviates the problems discussed above. The present invention is well suited for use in both new and many existing top-fired reformer heaters and in any other type of fired heater, whether top-fired, side-fired, bottom-fired, or otherwise, which employs a flue gas tunnel system.
In one aspect, the present invention provides an improvement for a fired heater of the type comprising: a radiant firebox having an interior floor; a plurality of rows of process tubes in the firebox; and at least one flue gas tunnel which extends adjacent to at least one of the rows of tubes and has a plurality of inlet openings for receiving a flue gas from the radiant firebox. The improvement comprises the flue gas tunnel being installed such that at least most of the lateral cross section of the flue gas tunnel is below the interior floor.
In a further aspect of the present invention, the improvement preferably also comprises (a) the flue gas tunnel having a longitudinal top cover and (b) the inlet openings of the flue gas tunnel being provided through the longitudinal top cover. In addition, the longitudinal top cover preferably comprises a series of lateral brick arches in the floor of the firebox with the inlet openings preferably being laterally extending gaps in the longitudinal top cover between adjacent pairs of the lateral brick arches. The lateral slots also preferably extend through the longitudinal top cover at an angle toward a flue gas outlet end of the flue gas tunnel.
In yet another aspect, the improvement preferably further comprises: the flue gas tunnel having a first longitudinal end and a second longitudinal end opposite the first longitudinal end; the second longitudinal end being the discharge end of the flue gas tunnel; and the lateral cross section of the flue gas tunnel increasing in size from the first end to the second end.
The present invention provides several benefits and advantages over the flue gas tunnels heretofore used in the art. Examples of the benefits and advantages provided by the present invention include: eliminating the presence of and the need to construct the vertical brick side walls of the prior art tunnels within the radiant firebox; the ability to construct the flue gas tunnels offsite and install refractory linings therein prior to shipment; significantly reducing the weight of the flue gas tunnels; allowing the addition of convenient man way doors to the firebox for workers and materials; reducing the time required to erect scaffolding within the firebox for inspection and repairs; greatly increasing the available work space and maneuverability within the firebox; significantly reducing the structural support requirements and costs of the heater; and significantly improving the distribution and uniformity of the flue gas flow within the firebox and the flue gas tunnels.
Further aspects, features, and advantages of the present invention will be apparent to those in the art upon examining the accompanying drawings and upon reading the following detailed description of the preferred embodiments.
An improved heater 50 employing a first embodiment of the flue gas tunnel system provided by the present invention is depicted in
In contrast to the prior art flue gas tunnels 14, each of the inventive flue gas tunnels 52 preferably comprises: an outer metal casing 62 which is attached to the bottom of the firebox 56 and includes downwardly extending side walls 64 and a bottom 66; a layer of refractory insulation 68 applied on the interior surfaces of the side walls and bottom of the metal casing 62; an interior covering of refractory material (e.g., refractory blocks) 70 installed over the insulation layer 68; a longitudinal top cover 72 installed in the firebox floor 54; and a plurality if inlet openings 74 for receiving flue gas from the firebox 56. Consequently, at least most of the lateral cross section 55 of the inventive tunnel 52 is below the interior floor 54 of the firebox 56.
The flue gas inlet openings 74 can be of any desired type or shape and can be provided through any desired portion of the inventive tunnel which is located in the firebox 56. The openings 74 will preferably be provided through the longitudinal top cover 72 and will most preferably be slots provided in the top cover 72 as shown in
The longitudinal top cover 72 of the inventive tunnel 52 can be a flat cover formed from lintels, tiles, or other materials but will preferably be a laterally arched cover as depicted in
The slots or other openings employed in the longitudinal top 72 of the inventive tunnel 52 will preferably be of sufficient size and number that from about 5% to about 50%, and more preferably from about 10% to about 35%, of the total area of the top cover 72 is open for flue gas flow. The flue gas inlet openings 74 in the cover 72 are preferably laterally extending slots. The laterally extending slots 74 are preferably formed by providing gaps between adjacent pairs of the lateral brick arches 75 at desired locations along the length of the longitudinal top cover 72. The widths of slots 74 will preferably be in the range of from about 1 to about 8 inches. The widths of the slots 74 will more preferably be in the range of from about 1.5 inches to about 6 inches and will most preferably be in the range of from about 2 to about 4 inches. In addition, the lateral flue gas inlet slots 74 can extend vertically through the longitudinal top cover 72 or can extend through the longitudinal top cover 72 at an angle, as described below, toward the discharge end 76 of the tunnel 52.
Another improved heater 80 employing a second embodiment of the inventive flue gas tunnel system is depicted in
The increase in the cross-sectional flow area of the inventive tunnel 84 will preferably be such that the cross-sectional flow area of the tunnel 84 at any given point will at least equal, and will more preferably exceed, the total area of all of the top inlet openings 85 and/or other openings in the tunnel 84 leading up to that point. The increase in cross-sectional flow area will also preferably be substantially constant from the first end 88 of the tunnel 84 to the tunnel discharge end 86. Further, although any desired shape can be used, the inventive flue gas tunnel 84 will preferably be of constant width but will have a bottom 90 which slopes downwardly toward the discharge end 86. Typically, the bottom 90 of the inventive flue gas tunnel 84 will slope downwardly toward the tunnel discharge end 86 at a constant angle in the range of from about 2° to about 10°.
The lateral flue gas inlet slots or other openings 85 of the inventive tunnel 84 can extend vertically (i.e., directly downward) through the longitudinal top cover 92 but will more preferably be angled toward the tunnel discharge end 86. The angle of the flue gas inlet slots 85 will preferably be in the range of from about 20° to about 70° from horizontal. The angle of the flue gas inlet slots 85 will more preferably be in the range of from about 30° to about 60° from horizontal and will most preferably be about 45° from horizontal. As will be understood by those in the art, the ends of the fire bricks used to form the lateral brick arches 94 of the tunnel covers 92 can be cut as needed to provide any desired inlet slot angle.
For either of the inventive flue gas tunnel systems described above, it will also be understood that (a) the cross-sectional shape of the tunnel could alternatively be partially circular or any other shape desired, (b) the flue gas inlet openings can alternatively be holes or any other type of aperture, (c) the upper portion of the inventive tunnel can alternatively extend partially above the interior floor of the firebox, (d) if flue gas inlet openings other than lateral slots are used, such openings can also extend vertically or at an angle through the top cover, and (e) although the inventive tunnels will preferably be used throughout the heater, the inventive tunnels can alternatively be used in combination with other types of tunnels (e.g., in combination with one or more prior art tunnels 14).
Simulations were run using computer models of (a) a top-fired reformer heater utilizing the prior art flue gas tunnel system depicted in
The models simulate flue gas flow, convection heat transfer from the flue gas to the heater coils, and radiative heat transfer to the coils from the flue gas and refractory surfaces. The flue gas flow is specified as six feet per second with a temperature of 2350° F. at 25 feet above the floor of the radiant box. The tube metal temperature is specified as 1500° F. and the emissivity of the tubes is specified as 0.85, which is typical for oxidized steels. The refractory surfaces are specified as adiabatic and as having an emissivity of 0.65. The ideal gas law is used for determining flue gas density.
In the model for the prior art heater, the flue gas tunnels within the firebox have five openings on each side which are modeled as rectangular cuts through the side walls. The openings increase in size from the outlet end of the tunnel toward the opposite end of the tunnel. However, although the openings adjacent to the tunnel outlet are significantly smaller than the openings toward the opposite end of the tunnel,
The inventive flue gas tunnels employed in this simulation have two inch inlet slots in the tops thereof which are angled at 45° from horizontal and are spaced apart 20 inches from center to center. The inventive tunnels have sloped bottoms which provide a substantially constant flue gas velocity along the length of each tunnel toward the discharge end thereof. Each tunnel includes a total of 31 angled top openings and provides a significantly more uniform, improved flue gas inlet flow distribution along the length of the tunnel as illustrated in
The results of the simulation further show that 60.2 MMBtu/hr are absorbed in each row of the tubes of the improved heater using the inventive tunnels as opposed to 58.9 MMBtu/hr for the heater employing the prior art tunnels. Further, the flue gas discharge temperature calculated for the inventive system is 1727° F. versus 1740° F. for the prior art system.
Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.
