Patent Application
20110283991
The present invention relates to a flameless thermal oxidizer configured for flameless thermal oxidation at optimized equivalence ratios, and a method of flameless thermal oxidation at optimized equivalence ratios.
Many industries, such as the chemical, pharmaceutical, oil refinery, power utility, and electronic industry react chemicals at high temperatures, for example about 1000 degrees Fahrenheit. The terms “react” or “reaction” refer to any endothermic or exothermic chemical reaction, such as the vaporization, synthesis, oxidation, or reduction of a chemical. For example, and though a wide variety of applications exist for vaporization, it is often desirable to vaporize liquid natural gas (LNG) so that it can be handled and distributed as a fuel source. Alternatively, chemicals and fume streams may be safely destroyed through oxidation by converting the chemical to reaction products, such as carbon dioxide and water. Thermal oxidation is a process whereby solvents and hydrocarbons combine with oxygen to form water and carbon dioxide. The products of reaction from the original mixture of solvents can thereafter be safely discharged to the atmosphere.
One type of apparatus that can be used to facilitate thermal oxidation is a flameless thermal oxidizer (FTO). The reaction is referred to as ‘flameless’ because the FTO permits the reaction of the process gas stream to occur in the absence of a flame. The flameless thermal oxidizer may be utilized, for example, to treat organic vent gases released from organic synthesis reactors and similar hydrocarbon off-gas control applications, to vaporize liquid natural gas, or to synthesize waste hydrocarbon gas or natural gas feedstock through a non-catalytic, partial oxidation process. Flameless thermal oxidizers are described, for example, in U.S. Pat. No. 6,015,540 to McAdams et al., which is incorporated herein by reference in its entirety.
In the interests of efficiency and/or safety, there exists a need to further develop and improve FTO's to facilitate a substantially complete oxidization reaction of a fume stream with reduced risk of flashback, while reducing operational costs and/or improving the available capacity of the vessel.
According to one aspect of the invention, a diptube assembly for a flameless thermal oxidizer having a matrix bed of media is provided. The diptube assembly comprises a fuel conduit at least partially positioned within the matrix bed for delivering fuel. An oxidizing agent conduit is at least partially positioned within the matrix bed for delivering oxidizing agents. The oxidizing agent conduit is separate from the fuel conduit. At least one mixing conduit is positioned within the matrix bed and configured to receive fuel from the fuel conduit and the oxidizing agent from the oxidizing agent conduit. The mixing conduit is positioned to deliver the combination of fuel and oxidizing agents into the matrix bed of media.
According to another aspect of the invention, a diptube assembly comprises a fuel conduit for carrying fuel at least partially positioned within the matrix bed. A fuel plenum is positioned within the matrix bed and configured to receive fuel from the fuel conduit. The diptube assembly further comprises an oxidizing agent conduit for delivering oxidizing agents into at least one mixing conduit. At least a portion of the mixing conduit is positioned within the fuel plenum. The portion of the mixing conduit includes at least one aperture formed in a surface thereof, wherein fuel from the fuel plenum is delivered into the mixing conduit through the aperture. A combination of fuel and oxidizing agents are delivered through the mixing conduit into the matrix bed of media of the flameless thermal oxidizer.
According to still another aspect of the invention, a flameless thermal oxidizer (FTO) is provided. The FTO comprises a vessel, a matrix bed of media contained within an interior of the vessel, and a diptube assembly at least partially positioned within the matrix bed. The diptube assembly includes a fuel conduit for carrying fuel that is at least partially positioned within the matrix bed, an oxidizing agent conduit for carrying oxidizing agents that is at least partially positioned within the matrix bed, and at least one mixing conduit that is at least partially positioned within the matrix bed that is configured to receive and combine fuel from the fuel conduit and oxidizing agents from the oxidizing agent conduit and to deliver the combination of fuel and oxidizing agents into the matrix bed of media.
According to still another aspect of the invention, a method of delivering fuel and oxidizing agents into the vessel includes the steps of distributing fuel through the fuel conduit; distributing oxidizing agents through the mixing conduit; and combining the fuel and the oxidizing agent in a mixing conduit.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing may not be to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
The invention will next be illustrated with reference to the figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate explanation of the present invention. In the figures, like items numbers refer to like elements throughout.
A reactable process stream 44 is delivered into the heated matrix bed to facilitate an oxidation reaction. The reactable process stream 44 is formed by combining a fume stream 51 containing an oxidizable material, an oxidizing agent stream 52 (such as air or oxygen), and a supplementary fuel gas stream 53 in a mixing device 50. The oxidizing agent 52 and supplementary fuel 51 promote oxidation of the fume stream 51. After the reactable process stream 44 is formed, it is fed into a feed inlet 54 of the diptube 43. The diptube 43 is an open pipe of substantially constant inner and outer diameter, which extends at least partially within the PIM matrix bed 42, as shown. The diptube 43 has an inlet 54 configured to receive the reactable process stream 44 and an outlet 55 configured to deliver the air-combustible vapor mixture into the matrix bed 42.
The reactable process stream 44 is directed into a region of the PIM matrix bed 42 where the matrix bed temperature is sufficient to react the process stream to form at least one reaction wave 56. The chemicals in the process stream typically remain substantially unreacted until reaching the reaction wave 56, where a substantial portion of the chemicals are reacted over the wave length. Preferably, the non-planar reaction wave 56 is established entirely within the boundaries of the matrix bed. The reactable process stream 44 is reacted in the reaction wave 56 to produce the reacted process stream 46. The reacted process stream 46 is then directed through the matrix bed 42, through the void space 47, and out of the vessel through the exhaust outlet 45.
The location, stability, and size of the reaction wave 56 may be controlled through a programmable control system 59.
The control system 59 is also configured to adjust the flow rates of the fume stream 51, oxidizing agent stream 52, and supplementary fuel gas stream 53 in order to limit or prevent flashback. Flashback is a phenomena which occurs when the flame speed of a flammable mixture of fuel and air is greater than the local velocity profile of the flammable mixture in the presence of an ignition source.
The FTO 40 is not generally intended for use with a flammable gas mixture because it does not include provisions to contain flashback. As described above, the FTO includes provisions, i.e., the programmable control system 59, for preventing flashback. In order to prevent flashback, the mixture of the process gas stream 51, oxidizing agent stream 52 and supplementary fuel gas stream 53 is typically maintained at least about five percentage points below the lower flammability limit and delivered into the matrix bed at a velocity that is slightly greater than the flame speed of the mixture.
According to an exemplary embodiment, the diptube assembly 10 generally comprises an outer conduit 12, a fuel conduit 14 positioned within the outer conduit 12, a fuel plenum 16 positioned at the base of the fuel conduit 14, and a plurality of mixing conduits 18 (one shown) extending through the fuel plenum 16. Only one mixing conduit 18 is shown throughout the figures for the purpose of clarity.
According to one exemplary use of the FTO 140, a mixture 19 comprising a fume stream and an oxidizing agent stream (e.g., oxygen or air) is introduced through an inlet 20 provided near the top end of outer conduit 12. The fume stream and the oxidizing agent stream may either be combined at the inlet 20 of the outer conduit 12 or any other location. The mixture 19 travels within the annular space defined between the outer conduit 12 and the fuel conduit 14. The mixture 19 is ultimately distributed into the individual mixing conduits 18.
A supplementary fuel stream 15 (e.g., natural gas, propane, kerosene, or #2 fuel oil) is introduced through an inlet 13 of the fuel conduit 14. The fuel stream 15 travels along the fuel conduit 14 and is distributed into a fuel plenum 16. The fuel plenum 16 is essentially a hollow cylinder defining a closed cylindrical region, through which the plurality of mixing conduits 18 are positioned. The fuel stream 15 and the mixture 19 are isolated until they are combined together within the interior of the mixing conduits 18 (one shown in
The lower barrier 24 is a cylindrical disc having a series of holes formed therein for accommodating the mixing conduits 18, according to an exemplary embodiment of the invention. The mixing conduits 18 may be welded to the lower barrier 24 to limit escapement of the fuel stream 15 through the lower barrier 24.
The upper barrier 23 is a cylindrical disc having a series of holes formed therein for accommodating the outlet end 26 of the fuel conduit 14 and the inlet end of each mixing conduit 18, as best shown in
The apertures 30 are formed along the segment of each mixing conduit 18 that is disposed between the upper and lower barriers 23 and 24, such that the fuel stream 15 within the fuel plenum 16 circulates into the mixing conduits 18. As best shown in
The mixing conduits 18 are tailored to deliver the reactable process stream 32 into the PIM matrix bed with sufficient local velocity to limit or prevent a flashback. Flashbacks occur when the flame speed of a flammable mixture of fuel and air is greater than the local velocity profile of the fuel and air mixture in the presence of an ignition source. Accordingly, the potential for flashback is substantially reduced by increasing the local velocity profile of the fuel and air mixture, and quickly distributing that mixture into the matrix bed.
Substantially increasing the velocity of the reactable process stream reduces the residence time for a fully integrated fuel-air mixture within the mixing conduits 18, which diminishes the potential for uncontrolled deflagration due to inventory minimization of potentially combustible gases. By delivering the reactable process stream 32 into the matrix bed at a velocity sufficiently greater than its flame speed, the fuel-air mixture may be maintained at or near the lower flammability limit of the mixture. Maintaining the fuel-air mixture at or near the lower flammability limit of the mixture minimizes FTO operational costs and maximizes the available capacity of the FTO vessel.
To meet those objectives, the number, position, cross-sectional area and length of the mixing conduits 18, individually or in combination, are tailored to deliver the reactable process stream 32 into the PIM matrix bed with sufficient local velocity to limit or prevent a flashback.
According to one exemplary embodiment, the collective cross-sectional area of the mixing conduits 18 is less than both the cross-sectional area of the fuel conduit 14 and the effective cross-sectional area of the outer conduit 12. Constricting the flow path of the mixture 19 through the mixing conduits 18 increases its local velocity, such that the local velocity of the mixture 19 (as it forms reactable process stream 32) is greater than its flame speed.
According to the exemplary embodiment, the collective cross-sectional area of the mixing conduits 18 is the sum of the interior cross-sectional areas of all of the mixing conduits 18. The effective cross-sectional area of the outer conduit 12 is the difference between the interior cross-sectional area of the outer conduit 12 and the outer cross-sectional area of the fuel conduit 14, because the mixture 19 flows in the annular area defined between the outer conduit 12 and the fuel conduit 14.
According to the exemplary embodiment, the inner diameter of each mixing conduit 18 may be from about 0.5 to about 3 inches, for example. The mixing conduit 10 may optionally include forty-two (42) mixing conduits 18. The inner diameter of the outer conduit 12 may be from about 6 to about 60 inches, for example. The inner diameter of the fuel conduit 14 that is positioned within the outer conduit may be from about 1 inch to about 6 inches, for example. A ratio of the effective cross-sectional area of the outer conduit 12 to the collective cross-sectional of the mixing conduits 18 is between about 2.5:1 and 6:1.
The velocity of the reactable process stream 32 increases as it travels through each mixing conduit 18. The velocity of the reactable process stream 32 may be between about 50 feet/second to about 250 ft/second as it travels through each mixing conduit 18. Accordingly, the length of each mixing conduit 18 is maintained at a minimum such that the local velocity of the reactable process stream 32 is greater than its flame speed. According to one exemplary embodiment, for example, the length of each mixing conduit 18 may be about 36 inches. Minimizing the flow path of the reactable process stream 32 reduces the potential for substantial deceleration of the mixture 32 within each mixing conduit 18.
According to one exemplary embodiment, the distance separating the outlet of each mixing conduit 18 and the PIM of the matrix bed 42 is also maintained at a minimum to limit or prevent substantial deceleration of the reactable process stream 32 upon exiting the mixing conduit 18, but prior to reaching the PIM matrix bed 42. The outlet of each mixing conduit 18 may be positioned within the matrix bed 42, if so desired.
Because the reactable process stream 32 is delivered directly into the matrix bed 42 through the mixing conduits 18 at a local velocity greater than its flame speed, the equivalence ratio of the reactable process stream 32 may be maintained between a range of about 0.5:1 to about 0.8:1, which is within the flammability limits for natural gas mixtures. Specifically, each mixing conduit 18 is configured to premix the fuel stream 15 and the mixture 19 (containing air) at an equivalence ratio of between a range of about 0.5:1 to about 0.8:1.
According to another aspect of the invention, the equivalence ratio of the reactable process stream 32 may be maintained above the lower flammability limit for natural gas mixtures, i.e., above 0.4:1. Broadly stated, the equivalence ratio of the reactable process stream 32 may be maintained between a range of about 0.09:1 to about 4:1. The equivalence ratio of the reactable process stream 32 may be maintained at or above the lower flammability limit for natural gas mixtures, i.e., above 0.4:1, by virtue of the high local velocity of the process stream 32 (due to the constricted-flow through the mixing conduits 18) and the close proximity of the outlet of the mixing conduits 18 to the matrix bed 42.
For the purpose of comparison, the equivalence ratio in a FTO, such as the FTO shown in
Maintaining the fuel-air mixture at least about five percentage points below its lower flammability limit may facilitate excessive fuel consumption and overly conservative vessel sizing criteria. More particularly, less supplemental fuel is required to oxidize a process stream 32 having an equivalence ratio greater than the lower flammability limit (i.e., 0.4:1) because less dilution air is required to maintain a low equivalence ratio. Thus, the cost of fuel consumed by the FTO 140 is significantly lower than that of a FTO, which is directly attributable to the diptube assembly 10 of the FTO 140.
Referring now to
In another exemplary embodiment of the mixing tube shown in
Referring back to
While exemplary embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. For example, the fuel stream 15 may be distributed through the inlet of each mixing conduit 18 and the mixture 19 may be distributed into the apertures 30 of each mixing conduit 18 to achieve a similar effect. Moreover, in lieu of the fuel plenum 16, the supplementary fuel stream 15 may be directly distributed into the mixing conduits 18 through a series of injectors, tubes or conduits (not shown) to achieve a similar effect. Also, the fuel stream 15 may be distributed directly into the fuel plenum 16 and the fuel conduit 14 may be omitted. It should be also understood that the conduits 12, 14, and 18 are not limited to a circular cross-sectional shape, as other cross-sectional shapes are envisioned, such as square, rectangular, and so forth.
Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 61/035589 | Mar 2008 | US | national |
This application is the U.S. national phase application of PCT International Application No. PCT/US2009/036724, filed Mar. 11, 2009, which claims priority to U.S. Provisional Patent Application No. 61/035,589, filed on Mar. 11, 2008, the contents of such applications being incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US09/36724 | 3/11/2009 | WO | 00 | 4/28/2011 |
