WO2006065766A2

WO2006065766A2 - Burnerless autothermal reformer mixer

Info

Publication number: WO2006065766A2
Application number: PCT/US2005/044968
Authority: WO
Inventors: Kym B. Arcuri; Kurt Schimelpfenig; James F. Leahy
Original assignee: Syntroleum Corporation
Priority date: 2004-12-14
Filing date: 2005-12-12
Publication date: 2006-06-22
Also published as: EP1835988A2; CN101111304A; AU2005316638A1; WO2006065766A3; PE20060805A1; MY139265A

Abstract

A mixing device for mixing a light hydrocarbon feed, a steam feed, and an oxygen-containing gas stream to form a feed mixture is disclosed. The autoignition of the feed mixture is prevented prior to conversion of the feed mixture to synthesis gas by subsequent contact with an active partial oxidation / reforming catalyst.

Description

BURNERLESS AUTOTHERMAL REFORMER MIXER

CROSS-REFERENCE TO RELATED APPLICATION

[001] This application claims the benefit of U.S. Provisional Application Serial No. 60/635,792 filed on December 14, 2004. The disclosure of the aforementioned provisional application is incorporated by reference in its entirety herein.

BACKGROUND OF THE INVENTION

[002] Light hydrocarbons are converted to synthesis gas by reaction with oxygen and/or oxygen-containing compounds, such as water. For example, a natural gas feed may be converted to synthesis gas by reaction with an oxygen-containing gas. [003] When water, usually present as steam, is used to oxidize, otherwise known as "reforming," the light hydrocarbon feed, it contributes both oxygen and hydrogen to the product mix. The contribution of hydrogen and the subsequent shift conversion of product CO by residual water produces a synthesis gas having relatively high ratios of hydrogen to CO. Thus steam reforming of light hydrocarbons is favored for the production of hydrogen. Reforming of light hydrocarbons with water is endothermic. Heat must be added to sustain reaction temperature. Reactor designs feature heat transfer tubing containing reforming catalyst and operating at high temperatures.

[004] Reforming of light hydrocarbons with carbon dioxide is generally done only in conjunction with recycle of byproduct CO₂. Carbon dioxide contributes both carbon and oxygen, but not hydrogen, to the product mix. Consequently, CO₂ reforming is useful in recovering carbon and oxygen values from what would otherwise be byproduct losses. The synthesis gas product is rich in carbon monoxide. When performed in conjunction with steam reforming, CO₂ reforming has the effect of reducing the H₂ to CO ratio of the product synthesis gas.

[005] Partial oxidation of light hydrocarbons by molecular oxygen contributes oxygen, but not hydrogen or carbon, to the product mix. It yields a synthesis gas having a H₂ to CO ratio lower than that of steam reforming and higher than that of CO₂ reforming. Thus, partial oxidation of light hydrocarbons by molecular oxygen is well suited to the production of synthesis gas for use in Fischer Tropsch and methanol syntheses.

[006] The partial oxidation reaction is exothermic. The exothermic nature of the reaction leads to the concept of "auto-thermal" reforming. In auto-thermal reforming, partial oxidation of the feedstock provides much of the heat needed to raise the temperature of the products. Oxidation products that would be lost in flue gas if formed external to the reaction environment become part of the product stream. The synergy of feed and fuel is further enhanced by the ability to achieve high reaction temperatures without the need for high heat transfer equipment. High temperatures favor the conversion of light hydrocarbons to product CO.

[007] The desire to achieve high carbon and thermal efficiencies has led to combinations of the reforming and partial oxidation (also referred to as "POX") processes. [008] In one combination, light hydrocarbon is first partially reformed with steam followed by partial oxidation with an oxidizing gas. This combination is widely employed in the ammonia and methanol production industries. However, this combination of steam reforming and partial oxidation requires use of costly heat transfer in the primary (steam) reformer.

[009] In a second combination, light hydrocarbon feed is preheated and partially combusted in the presence of steam. The partial combustion products are then passed over a catalyst on which steam reforming and shift conversion reactions occur. In this combination, however, the non-catalytic partial combustion of light hydrocarbons requires use of a relatively large amount of steam to avoid carbon deposition on the reforming catalyst. The steam, in turn, leads to excessive shift conversion of CO when the partial combustion products are passed over the reforming catalyst. The resulting synthesis gas product has a hydrogen to CO ratio that is in excess of the optimum for Fischer Tropsch synthesis. To adjust the ratio and improve carbon efficiency, it is necessary to recycle the excess CO₂ produced to the reactor inlet. Carbon dioxide recycle, however, is generally costly.

[010] In a third combination, the steam reforming and partial oxidation reactions occur simultaneously over a single catalyst achieving the same product mix as when the reactions occur sequentially. Combustion of the feed mixture is avoided by mixing and delivery of the feeds to the catalyst surface at rates exceeding the rate of combustion. [Oil] A device achieving ultra rapid mixing and delivery of feeds to catalyst sites is described in U.S. Patent 6,447,745. Such a device has yet to be commercialized at the Fischer Tropsch scale. The need for such ultra-rapid mixing can be greatly alleviated by the use of air in place of relatively pure oxygen as the oxidizing gas. For example, when air is used as the oxidizing gas as described in patent number 6,344,491, it is possible to extend auto-ignition lag times to several hundred milliseconds. BRIEF SUMMARY OF THE INVENTION

[012] The invention relates to a device which mixes light hydrocarbons, steam, and air and delivers the mixture to an active catalyst. The invention further relates to a process for optimizing the size and orientation of air and/or oxygen injection nozzles of the mixing device. The device and processes of the invention are well suited for use with the devices and process described in U.S. patent application Serial No. 10/924,174, Publication No. 2005/0063899 and Serial No. 10/923,931, Publication No. 2005/0066577, which are incorporated herein by reference in their entirety.

[013] In preferred embodiments, the process is conducted on a once-through basis, without recycle, at a high temperature, and at an elevated pressure. Such conditions generally utilize a feed mixture that has a tendency to self heat and autoignite. Preferably, the mixing of the natural gas and steam mixture with the oxygen-containing gas is complete before the three component gas mixture contacts an inert solid material.

[014] In some embodiments of the invention, uniform mixing is achieved by conducting the mixing at high gas velocity, by uniform distribution of the openings, and by providing adequate axial length to the mixing zone.

[015] In preferred embodiments of the invention, the mixture of natural gas, steam and oxygen-containing gas contacts the active catalyst at a sufficiently low velocity to prevent high pressure loss and catalyst erosion. In some embodiments of the invention, the three component gas mixture is decelerated in an expansion zone. Preferably, the expanding mixture does not form macroturbulent eddies nor are large void volumes and associated long duration deceleration times incurred.

BRIEF DESCRIPTION OF THE DRAWINGS

[016] Figure 1 is a schematic diagram illustrating an axial cross section of a first symmetric embodiment of the mixer.

[017] Figures IA and IB are schematics illustrating a transverse cross section of a first symmetric embodiment of the mixer.

[018] Figure 2 is a schematic illustrating an axial cross section of a second symmetric embodiment of the mixer.

[019] Figure 2A is a schematic illustrating a transverse cross section of a second symmetric embodiment of the mixer.

[020] Figure 3 is a schematic illustrating an axial cross section first asymmetric embodiment of the mixer. [021] Figure 4 is a schematic illustrating a first symmetric embodiment of the mixer further illustrating an inert solids zone and an inlet portion of an active catalyst zone.

[022] Figure 5 illustrates the results of a computational fluid dynamics analysis of an asymmetric mixer embodiment.

[023] Figure 6 illustrates the results of a computational fluid dynamics analysis of a symmetric mixer embodiment.

[024] Fig. 7 is a graph of temperature vs. time for a specific reactor configuration and feed gas composition.

DETAILED DESCRIPTION

[025] As used herein, the term "homogeneous reaction" means ignition and/or decomposition of C₂+ hydrocarbons.

[026] Referring first to Fig. 1 , a first symmetric embodiment of the mixer of the invention is illustrated in axial cross section. Fig. 1 more specifically illustrates the oxygen-containing gas inlet portion of the mixer. An inner tube 100 has a plurality of openings 110 and a gas inlet 120. A shell tube 130 has a plurality of openings 180. The space between inner tube 100 and shell tube 130 forms annular space 140. A gas inlet 170 is located at or near a top portion of the annular space 140. A jacket 150 having a gas inlet 160 surrounds at least a portion of shell tube 130. In some embodiments of the invention, an oxygen-containing gas feed is passed through gas inlet 120 into inner tube 100 and through openings 110 into annular space 140. In some embodiments of the invention, an oxygen-containing gas is also passed into gas inlet 160 and passes through openings 180 into annular space 140. In some embodiments of the invention, a mixture natural gas and steam enters annular space 140 through gas inlet 170. As used herein the term "symmetric" refers to mixers in which the oxygen-containing gas injection is symmetric relative to the centerline of natural gas and steam flow. That is, the embodiment shown in Fig. 1 is considered symmetric because as natural gas and steam flows axially through the annular space 140, an oxygen-containing gas is injected from into the annular space 140 from both sides perpendicular to the direction of flow.

[027] Figs. IA and IB illustrate transverse cross sectional views of the inner tube 100 and shell tube 130. In Fig. IA, the openings 110 and 180 are aligned. In Fig. IB, the openings 110 and 130 are offset from each other. [028] Fig 2 illustrates a second symmetric embodiment of the mixer of the invention. As seen in Fig. 2, an inner tube 200 having a plurality of openings 210 and a gas inlet 220 is shown in axial cross section, hi some embodiments of the invention, a natural gas and steam mixture passes through gas inlet 220 into inner tube 200. A shell tube 230 surrounds inner tube 200 forming an annular space 240. Annular space 240 has a gas inlet 250 at or near the top of the annular space 240. hi some embodiments of the invention, an oxygen-containing gas is passed through gas inlet 250 into annular space 240, through openings 210 and into inner tube 200.

[029] Referring to Fig. 2A, a transverse cross section showing the inner tube 200, including openings 210, and shell tube 230 is shown.

[030] Referring now to Fig. 3, a first asymmetric embodiment of the mixer is shown. An inner tube 300 having a plurality of openings 310 and a gas inlet 320 is shown in axial cross section. A shell tube 330 surrounds inner tube 300 thereby forming an annular space 340. A gas inlet 350 is located at or near a top portion of annular space 340. In some embodiments of the invention, a natural gas and steam mixture passes through gas inlet 350 into annular space 340. In some embodiments of the invention, an oxygen-containing gas passes through gas inlet 320, into inner tube 300, and through openings 310 into annular space 340. [031] Referring now to Fig. 4, a symmetric embodiment of the mixer of the invention is illustrated upstream of and fluidly connected to a burnerless autothermal reactor with which the mixer is well suited for use. An inner tube 100 having a plurality of openings 110 and a gas inlet 120 is surrounded by a shell tube 130 having a plurality of openings 180 thereby forming an annular space 140. As gas inlet 170 is located at or near a top portion of annular space 140. A jacket 150 having two gas inlets 160 surrounds a portion of shell tube 130. Inner tube 100 terminates in a tapered cone-shaped end portion 190.

[032] At or about the height of the end of inner tube 100, shown as point D, shell tube 130 flares slightly outwardly. The area between point D and E is called an expansion zone in which the volume available for the gas increases, thereby allowing the gas velocity to slow. In alternative embodiments, the diameter of the imier tube may be decreased, also resulting in an expanded volume for the gas, as is shown between points C and D of Fig. 4. It will be understood that a variety of configurations may be used with more or less rapid flaring or tapering of the shell tube or inner tube respectively, to achieve the desired expanded volume and decrease in gas velocity. As shown in the embodiment in fig. 4, inner tube 100 has no gas outlet and the air injected into inner tube 100 must pass through openings 110 and into annular space 140. [033] Illustrated in Fig. 4, between points E and F is an inert solid material zone. The inert solid material prevents the transfer of radiant heat between the expansion zone and the active partial oxidation/reforming catalyst. A number of known inert solids may be used in the inert solid material zone. In preferred embodiments, the inert solid material is randomly packed and is a catalytically inert ceramic capable of exposure to temperatures in excess of 2200⁰F without substantial chemical or physical degradation.

[034] Still referring to Fig. 4, an active catalyst zone is shown beginning at point F. It is noted that only an inlet portion of the active catalyst zone of an autothermal reactor is shown in Fig 4.

[035] In some embodiments of the invention, a natural gas and steam mixture is premixed and injected into gas inlet 170. In preferred embodiments of the invention, the natural gas and steam mixture has substantially fully developed axial flow, that is, with only insubstantial backflow or eddying, prior to reaching point A. An oxygen-containing gas is injected into inner tube 100 through gas inlet 120. The oxygen-containing gas passes through openings 110 and into annular space 140 wherein the natural gas and steam mixture contacts and begins mixing with the oxygen-containing gas. Oxygen-containing gasses useful in the invention include air, oxygen-enriched air and oxygen. Oxygen-containing gas also enters through one or both of gas inlets 160 into jacket 150, through openings 180 and into annular space 140 wherein such oxygen-containing gas also contacts the natural gas and steam mixture. Mixing of the natural gas and steam with the oxygen-containing gas begins as the oxygen-containing gas passes through openings 110 and 180 and continues as the as mixture flows through annular space 140 and through the expansion zone. In preferred embodiments of the invention, the natural gas, steam and oxygen-containing gasses are substantially uniformly mixed prior to contact with the inert solid material at point E. [036] In preferred embodiments, the diameters of the inner tube 100 and shell tube 130 are selected to yield a velocity at the inlet of the expansion zone, i.e., point D, of at least about 100 feet per second and more preferably at least about 300 feet per second. In preferred embodiments, the length of the inner tube 100 is selected to yield a mixing time of at least about 10 milliseconds, more preferably at least about 30 milliseconds, but generally not greater than about 200 milliseconds.

[037] In alternative embodiments of the invention, the flow volume of the natural gas, steam and oxygen-containing gas may be increased over the length of the oxygen-containing gas injection so as to partially or wholly offset the velocity increase with the incremental oxygen- containing gas injection. [038] Direct impingement or opposition of openings 110 and 180 may be used in some embodiments. In preferred embodiments, however, openings 110 and 180 are offset from each other.

[039] In a preferred embodiment, the quantity of steam mixed with the light hydrocarbon feed is between about 2% and about 160% by volume of the hydrocarbon portion of the light hydrocarbon feed. More preferably, the quantity of steam is 22-36%. The pressure maintained in the mixing device is 0 to 300 psig, more preferably 100 to 200 psig. The duration of time from final mixing of the oxygen-containing gas with the light hydrocarbon, e.g., natural gas, to contacting the catalyst is less than about 1000 milliseconds, more preferably less than about 300 milliseconds.

[040] The inert solid material prevents radiation of heat from the active catalyst zone to the natural gas, steam and oxygen-containing gas mixture. The inert solid material may further provide, in some embodiments, torturous passages, thereby preventing convective heat transfer.

[041] In the preferred embodiment, the velocity of the natural gas, steam and oxygen- containing gas mixture does not exceed 100 ft/sec as it contacts the surface of the active catalyst at the top of the partial oxidation and reforming section, shown in fig. 4 as point F. More preferably, this velocity does not exceed 45 ft /sec.

[042] In yet another aspect of the invention, a process for optimizing the size and location of openings connecting the oxygen-containing gas and the natural gas and steam mixture and other process conditions is provided. Figs. 5-6 illustrate the flow patterns obtained through computational fluid dynamic analysis. In Figs. 5-6, the upper portion of the colored pattern corresponds to the pre-openings, pre-oxygen-containing gas injection, flow. The openings through which an oxygen-containing gas is simulated as entering an annular space are simulated along with the mixing zone of the annular space of the mixer. The expansion zone is also illustrated at the lower portion of the colored diagrams. In each of Figs. 5 and 6, reference should be made to the scale on the left hand side which indicates gas velocity. Negative velocities indicate back-flow.

[043] Fig. 5 illustrates the results of one computational fluid dynamics process in which an asymmetric mixer configuration was used. As can be seen from Fig. 5, significant areas of negative flow, indicated by the two darkest blue regions, developed leading to poor and inadequate mixing.

[044] In contrast, Fig. 6 illustrates the computational fluid dynamics results for a symmetric mixer configuration. As can be seen from Fig. 6, no negative flow developed. [045] While the invention has been described with a limited number of embodiments, these specific embodiments are not intended to limit the scope of the invention as otherwise described and claimed herein. Modification and variations from the described embodiments exist. For example, an oxygen-containing gas could be injected into a natural gas and steam mixture along the entire length of the mixer, that is, through the entire pre-partial oxidation volume, and not just in an upper portion thereof as illustrated in the Figures. Therefore, materials which do not fulfill the selection criteria under one set of process conditions may nevertheless be used in embodiments of the invention under another set of process conditions. The incorporation of additional elements may result in beneficial properties which are not otherwise available. Also, while the processes are described as comprising one or more steps, it should be understood that these steps may be practiced in any order or sequence unless otherwise indicated. These steps may be combined or separated. Finally, any number disclosed herein should be construed to mean approximate, regardless of whether the word "about" or "approximate" is used in describing the number. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.

Claims

What Is Claimed Is:

1. A mixing device for mixing a light hydrocarbon feed, a steam feed, and an oxygen-containing gas stream to form a feed mixture wherein autoignition of the feed mixture is prevented prior to conversion of the feed mixture to synthesis gas by subsequent contact with an active partial oxidation / reforming catalyst. 5

2. A device according to claim 1 comprising: an inner tube having a plurality of openings through the inner tube walls; said inner tube surrounded by a shell tube thereby creating an annular space between the inner tube and the shell tube, said shell tube having a plurality of openings in the shell tube walls; and I₀ a jacket surrounding all or a part of the shell tube, wherein an oxygen-containing gas is fed into the inner tube and the jacket and the light hydrocarbon feed and steam feed are fed into the shell tube.

3. A device according to claim 1 comprising: an inner tube having a plurality of openings through the inner tube walls; and 15 a shell tube surrounding the inner tube thereby creating an annular space between the inner tube and the shell tube; wherein an oxygen-containing gas is fed into the inner tube and the light hydrocarbon feed and steam feed are fed into the shell tube.

4. A device according to claim 1 comprising: 0 an inner tube having a plurality of openings through the inner tube walls; and a shell tube surrounding the inner tube; wherein an oxygen-containing gas is fed into the shell tube and the light hydrocarbon feed and steam feed are fed into the inner tube.

5. A device according to claim 2 in which the inner tube and shell tube 5 are sized to effect a velocity of gas in the annular space of between about 30 and about 900 feet per second.

6. A device according to claim 3 in which the inner tube and shell tube are sized to effect a velocity of gas in the annular space of between about 30 and about 900 feet per second. 0

7. A device according to claim 4 in which the inner tube and shell tube are sized to effect a velocity of gas in the annular space of between about 30 and about 900 feet per second.

8. A device according to claim 2 in which the velocity of gas in the annular space is between about 40 and about 400 feet per second.

9. A device according to claim 3 in which the velocity of gas in the annular space is between about 40 and about 400 feet per second.

10. A device according to claim 4 in which the velocity of gas in the annular space is between about 40 and about 400 feet per second.

11. A device according to claim 2 further comprising an expansion zone downstream of the inner tube and shell tube.

12. A device according to claim 3 further comprising an expansion zone downstream of the inner tube and shell tube.

13. A device according to claim 4 further comprising an expansion zone downstream of the inner tube and shell tube.

14. A device according to claim 11 further comprising an inert solid material to prevent the transfer of radiant heat between the expansion zone and the active partial oxidation/reforming catalyst.

15. A device according to claim 12 further comprising an inert solid material to prevent the transfer of radiant heat between the expansion zone and the active partial oxidation/reforming catalyst.

16. A device according to claim 13 further comprising an inert solid material to prevent the transfer of radiant heat between the expansion zone and the active partial oxidation/reforming catalyst.

17. A device according to claim 2 wherein a previously-mixed steam and light hydrocarbon mixture pass axially through the annular space between the inner tube and the shell tube and further wherein the previously-mixed steam and light hydrocarbons have fully developed axial flow prior to contacting an uppermost first openings of either the inner tube or the shell tube.

18. A device according to claim 3 in which a previously-mixed steam and light hydrocarbon mixture pass axially through the annular space between the inner tube and the shell tube and further wherein the previously-mixed steam and light hydrocarbons have fully developed axial flow prior to contacting an uppermost first opening of the inner rube.

19. A device according to claim 4 in which a previously-mixed steam and light hydrocarbon mixture pass axially through the inner tube and further wherein the previously-mixed steam and light hydrocarbons have fully developed axial flow prior to contacting an uppermost first opening the inner tube.

20. A device according to claim 2 wherein the diameters of the inner tube and the shell tube are configured to provide an axial gas velocity within the annular space of between about 100 and about 400 feet per second.

21. A device according to claim 3 wherein the diameters of the inner tube and the shell tube are configured to provide an axial gas velocity within the annular space of between about 100 and about 400 feet per second.

22. A device according to claim 4 wherein the diameter of the inner tube is configured to provide an axial gas velocity within the inner tube of between about 100 and about 400 feet per second.

23. A device according to any of claims 2, 3 or 4 in which the natural gas, steam, and oxygen-containing gas co-exist in the mixed vapor state with no homogeneous reactions occurring prior to exposure of the gas mixture to the catalyst of the catalytic partial oxidation and reforming step.

24. A device according to any of claims 2, 3 or 4 which operates at a pressure of between about 50 and about 600 psig.

25. A device according to any of claims 2, 3 or 4 which operates at a pressure between about 50 and about 200 psig.

26. A device according to any of claims 2, 3 or 4 which operates at a temperature of between about 600 and about 1200 degrees F.

27. A device according to any of claims 2, 3, or 4 which operates at a temperature between about 850 and about 1050 degrees F.

28. A device according to any of claims 14, 15, or 16 wherein the inert solids material comprises randomly packed inert solids.

29. A device according to any of claims 14, 15, or 16 wherein the inert solids material is a catalytically inert ceramic capable of being exposed to temperatures in excess of 2200⁰F without physical or chemical degradation.

30. A process for optimizing the size and orientation in the openings of the inner tube and the shell tube of the device according to claim 2 comprising the step of utilizing computational flow dynamics for evaluating the gas velocity components for the oxygen-containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

31. A process for optimizing the size and orientation in the openings of the inner tube and the shell tube of the device according to claim 3 comprising the step of utilizing computational flow dynamics for evaluating the gas velocity components for the oxygen-containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

32. A process for optimizing the size and orientation in the openings of the inner tube of the device according to claim 4 comprising the step of utilizing computational flow dynamics for evaluating the gas velocity components for the oxygen-containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

33. A process for optimizing the size and orientation in the openings of the inner tube and the shell tube of the device according to claim 2 comprising the step of utilizing known engineering correlations for evaluating the gas velocity components for the oxygen-containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

34. A process for optimizing the size and orientation in the openings of the inner tube and the shell tube of the device according to claim 3 comprising the step of utilizing known engineering correlations for evaluating the gas velocity components for the oxygen-containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

35. A process for optimizing the size and orientation in the openings of the inner tube of the device according to claim 4 comprising the step of utilizing known engineering correlations for evaluating the gas velocity components for the oxygen- containing gas and natural gas and steam gas mixture within the mixture device for a plurality of opening sizes and orientations.

36. A device according to claim 1 wherein the oxygen-containing gas is air or oxygen-enriched air.

37. A device according to claim 1 wherein the feed mixture and device configuration are such that no homogeneous reactions occur prior to the feed mixture contacting the active catalyst.