ENHANCED EFFICIENCY ENDOTHERMIC REACTOR FOR SYNGAS PRODUCTION WITH FLEXIBLE HEAT RECOVERY TO MEET LOW EXPORT STEAM GENERATION

Abstract

An apparatus for carrying out endothermic reactions including a plurality of catalytic vessels, immersed in a combustion chamber having a contiguous overlaid convection chamber enclosing a top portion of the catalytic vessels wherein heat is recovered at a lower temperature level from the flue gases from the combustion chamber. The catalytic vessels may contain internal and coaxial heat recovery tubes creating an annular space filled in with a catalytic device. Both the external heat recovery through the catalyst tube outer surface and the internal heat recovery through the inner tube surface can be maximized by an enhanced catalytic device acting also as a heat transfer promoter in the process gas region. The apparatus provides enhanced and flexible heat recovery that permits to meet the request of minimum or none export steam production in one single apparatus, avoiding the need of a pre-reforming section and/or of a convective reformer downstream.

Claims

1-14. (canceled)

15. An apparatus for conducting endothermic reactions able to meet the heat balance with a null or limited amount of export steam, through a flexible heat recovery, comprising: a combustion chamber having a closed bottom end, an open top end, two opposite end walls, and two opposite side walls; an upper convection chamber, in communication with said combustion chamber through said open top end, allowing the flow of flue gas through a plurality of convection channels in flow communication with a top plenum chamber connected to an outlet flue gas channel; a plurality of catalytic vessels arranged at a distance and disposed in line or staggered along the centerline of the combustion and convection chamber in such a way that the bottom portion is immersed in the combustion chamber and the remaining top portion is immersed in the convection chamber wherein the portion of the catalytic vessel immersed in the combustion chamber is such that the flue gas temperature at its exit and inlet of the convection chamber is below the allowable limit temperature of the extended surface material; a plurality of burners disposed in parallel lines along the side walls of the combustion chamber on two or more firing levels, each provided with an adequate firing capacity or firing control, realizing an assigned firing pattern able to distribute the heat required by the endothermic process avoiding the use of heat distributor metal devices and in the same time without exceeding the allowable tube skin temperatures.

16. The apparatus of claim 15 in which the top portion of the catalytic vessels immersed in the convection chamber is provided with an extended surface made of longitudinal fins or studs having a variable extension ratio to achieve the selected level of heat recovery from the flue gas.

17. The apparatus described in claim 15, wherein each catalytic vessel contains an enhanced catalytic device realized starting from elements of a mechanical support having a random open cell structure having high void fraction (>75%), a good thermal conductivity (at least 5 W/m? K @RT) and enabling the gas flow both in axial and radial direction; the starting material for the preparation of the open cells structure being of ceramic nature belonging to the silicon carbide family, or of metallic nature belonging to the heat resistant alloys; the open cell structure being realized also starting from metal wire or strip randomly wrapped to form an irregular metal skein; the catalytic active species being added via a catalytic layer deposited on top; each catalyst element being formed in a suitable shape, so as to be stacked one over the other to fill in the inner reactor volume of each catalytic vessel; the height of the single catalyst element depending on the height obtained from the manufacturing procedure of the starting open cell structure.

18. The apparatus described in claim 15, wherein, when it is necessary to perform additional heat recovery from process syngas to meet the heat balance, each catalytic vessel has a closed bottom end and include an inner heat transfer tube having both ends open, mechanically joined on the top end and free to move axially inside the catalytic vessel.

19. The apparatus according to claim 17 wherein the annular space delimited by the catalytic vessel and the inner heat transfer tubes is filled with said enhanced catalytic device acting as a heat transfer promoter and avoiding the transmission of a mechanical stress between the catalytic vessels and the inner heat recovery tubes.

20. The apparatus according to claim 18 wherein the annular space delimited by the catalytic vessel and the inner heat transfer tubes is filled with a metal foil structured type catalyst acting as a heat transfer promoter and avoiding the transmission of a mechanical stress between the catalytic vessels and the inner heat recovery tubes.

21. The apparatus described in claim 15, wherein, when it is not necessary to perform additional heat recovery from process syngas to meet the heat balance, each catalytic vessel have an open top and bottom end, allowing the process gas flow once through, wherein the process syngas flow only downward from the top to the bottom end in countercurrent with flue gas flowing upward.

22. The apparatus according to claim 17 wherein the cylindrical space delimited by the catalytic vessels once through is filled with said enhanced catalytic device acting also as a heat transfer promoter.

23. The apparatus according to claim 21 wherein the cylindrical space delimited by the catalytic vessels once through is filled with a metal foil structured type catalyst acting as a heat transfer promoter.

24. The apparatus according to claim 17 in which the catalyst elements are shaped as annular or cylindrical elements to be stacked one over the other in the catalytic vessels.

25. The apparatus according to claim 19 in which a series of couples of flow deviators made from high alloys materials are suitably shaped and interposed between two or more catalyst elements so as to obtain a radial component of the gas flow and to increase the average gas velocity.

26. The apparatus according to claim 15 in which the catalyst active species are selected from transition metals and can be a combination of two or more metals.

27. The apparatus according to claim 15 in which the catalyst active species are chemically supported on oxidic compounds.

28. The apparatus according to claim 15 in which the catalyst active species and the chemical support are deposited on the mechanical device in the form of a thin layer.

29. The apparatus described in claim 16, wherein each catalytic vessel contains an enhanced catalytic device realized starting from elements of a mechanical support having a random open cell structure having high void fraction (>75%), a good thermal conductivity (at least 5 W/m? K @RT) and enabling the gas flow both in axial and radial direction; the starting material for the preparation of the open cells structure being of ceramic nature belonging to the silicon carbide family, or of metallic nature belonging to the heat resistant alloys; the open cell structure being realized also starting from metal wire or strip randomly wrapped to form an irregular metal skein; the catalytic active species being added via a catalytic layer deposited on top; each catalyst element being formed in a suitable shape, so as to be stacked one over the other to fill in the inner reactor volume of each catalytic vessel; the height of the single catalyst element depending on the height obtained from the manufacturing procedure of the starting open cell structure.

30. The apparatus described in claim 16, wherein, when it is necessary to perform additional heat recovery from process syngas to meet the heat balance, each catalytic vessel has a closed bottom end and include an inner heat transfer tube having both ends open, mechanically joined on the top end and free to move axially inside the catalytic vessel.

31. The apparatus according to claim 18 wherein the annular space delimited by the catalytic vessel and the inner heat transfer tubes is filled with said enhanced catalytic device acting as a heat transfer promoter and avoiding the transmission of a mechanical stress between the catalytic vessels and the inner heat recovery tubes.

32. The apparatus described in claim 16 wherein, when it is not necessary to perform additional heat recovery from process syngas to meet the heat balance, each catalytic vessel have an open top and bottom end, allowing the process gas flow once through, wherein the process syngas flow only downward from the top to the bottom end in countercurrent with flue gas flowing upward.

33. The apparatus according to claim 21 wherein the cylindrical space delimited by the catalytic vessels once through is filled with said enhanced catalytic device acting also as a heat transfer promoter.

34. The apparatus according to claim 16 in which the catalyst active species are selected from transition metals and can be a combination of two or more metals.

35. The apparatus according to claim 15 in which the catalyst active species are chemically supported on oxidic compounds.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In the drawings:

[0037] FIG. 1 represents a vertical section of a endothermic reactor according to first embodiment of the present invention where the catalytic vessels are disposed along the common center plane of the combustion and convective chambers; each catalytic vessel contains one inner concentric tube originating an annular space filled with an enhanced catalytic device;

[0038] FIG. 2 represents a vertical section of an endothermic reactor according to second embodiment of the invention wherein the catalytic vessels are open on both top and bottom ends and do not contain an inner heat transfer tube;

[0039] FIGS. 3a and 3b represents the horizontal sections of the two embodiments of FIGS. 1 and 2;

[0040] FIGS. 4a and 4b represent in vertical and horizontal sections two different configurations of the enhanced catalytic device installed in the catalytic vessels according to first and second embodiments of the invention;

[0041] FIGS. 5a and 5b represent the details of two consecutive flow deviators of the first embodiment according to FIGS. 1 and 4a;

[0042] FIGS. 6a and 6b represent the details of two consecutive flow deviators of the second embodiment according to FIGS. 2 and 4b;

[0043] FIGS. 7a and 7b represent one possible configuration of stacking catalyst elements and flow deviators respectively in the first embodiment of FIG. 4a and in the second embodiment of FIG. 4b.

DETAILED DESCRIPTION OF THE INVENTION

[0044] With reference to the drawings above illustrated, the apparatus object of the present invention consists of a plurality of catalytic vessels 10 immersed in a combustion chamber 100 equipped with burners 101. A top portion of the catalytic vessels may be immersed in a contiguous and overlaid convective chamber 200 in flow communication with the combustion chamber 100.

[0045] The catalytic vessels 10 are disposed along the common center plane of the combustion and convective chambers, at an equal distance from each other and may be disposed either in line (preferred) or staggered on a triangular pitch (first embodiment in FIG. 1 and second embodiment in FIG. 2). The combustion chamber 100, incorporating the bottom portion of the catalytic vessels, is dedicated to the production of hot flue gases by air combustion through burners 101 located on the side walls in view of the tubes and at a suitable distance to avoid any flame impingement. It should be noted that in both embodiments the design excludes the use of any other burner configuration than the side fired. The presence of an upper convection chamber of substantial length requires in fact a larger flue gas pressure drop than what would be necessary with a conventional reformer having only a radiant section. As a result, a top fired burner, which is another conventional burner design, would lead to a significant distortion of the flame leading to flame impingement on the tubes and potentially entry of the flame into the convection section which is undesirable. On the other end, the use of bottom fired burners would not allow to distribute the heat input to the catalytic vessels as necessary, while the use (here selected) of two or more levels of side fired burners, each provided with different firing capacities, or having firing capacity control per level, can ensure to follow any desired heat release pattern, realizing a combustion controlled by zone of any type of available fuel gas (purge gas from downstream syngas purification process, natural gas, refinery fuel gas, bio gas, etc. . . . ). The heat generated by combustion (LHV) is thus released at the different firing levels according to a selected firing pattern, in such a way that the heat absorption by the endothermic reaction in each firing zone follows a desired heat flux profile and that an acceptable maximum tube skin temperature is not exceeded.

TABLE-US-00001 From bottom % heat from Hflux avg. Tmax skin upward Firing LHV flue as Kcal/hr/m2 (tube OD) Zone 1 RAD ?40% ?22% ?55000 ?950? C. Zone 2 RAD ?60% ?42% ?95000 ?910? C. Zone 3 CONV None ?36% ?65000 ?890? C.

[0046] In both embodiments the upper side of the refractory housing 200 is conceived as a convection chamber, wherein the hot flue gases 3a and 3b generated in the combustion chamber 100 are allowed to flow upwards, in countercurrent with the process gas flow inside the catalytic vessels, through tight refractory ducts 201 realized by means of pre-shaped refractory materials surrounding the catalytic vessels 10; the top portion of the catalytic vessels 10 is provided with an extended surface 12 (either longitudinal fins or studs) that allows to match the desired convective heat recovery from flue gas. In the top end of the convection chamber 200 the flue gas ducts 201 surrounding each catalytic vessel are connected with a common plenum chamber 202 wherein the flue gas streams flowing upward in each refractory duct 201 are collected in the common chamber 202 and flow orthogonally to the tubes axis toward a common end 3.

[0047] The hot flue gases generated, 3a and 3b, in the combustion chamber 100 and flowing through the convective channels 201 to the top exit 3 exchange their heat content through the outer surface of the catalytic tubes 20, flowing in Countercurrent with the reacting gas inside the tubes. In the tube portion immersed in the combustion chamber 100 the prevailing heat transfer mechanism by flue gas is radiation, while in the portion immersed in the convection chamber 200 the prevailing heat transfer mechanism is convection. The exposed tube length of the bottom portion is in the range of 50% to 75% of the overall exposed tube length, and it is selected in order to limit the flue gas temperature at inlet of the convection channels 201 at a level compatible with the extended surface material (like 25Cr-20Ni SS).

[0048] In the first embodiment (FIG. 1) each catalytic vessel is closed at one end (bottom) and contains one inner concentric tube 20 originating an annular space, wherein this annular space is filled with an enhanced catalytic device 11a performing the catalytic function and contemporarily acting as a heat transfer promoter. The process gas feed 1 is distributed to the top end of the catalytic vessels 10 through an inner manifold 13 and flows downward through the annular volume filled with the enhanced catalytic device 11a where the endothermic reactions occur; at the bottom end of the catalyst vessel the reacted gas 2 inverts its direction and flows upwardly inside the heat transfer tube 20 while exchanging heat with the reacting process gas in the annular volume; in correspondence of the top outlets 2 the syngas leaves the catalyst vessels and it is collected through the outlet manifold 21 toward the downstream process. The inner tube is mechanically joined to the outer tube by means of a welded plate 15 inserted between the top flanges and thus it is fully supported at the top. The inner tube is shorter than the outer tube in order to provide a gap between the open bottom end of the inner tube and the closed bottom end of tube 10 so as to allow the gas flow from the outer to the inner tube section. The weight of each catalytic vessel is in part supported through bottom supports 14 and in part sustained through external counterweights on top, allowing the tubes thermal expansion on top.

[0049] In the second embodiment the catalytic vessels are open on both top and bottom end, and do not contain the inner heat transfer tube 20. In this simplified embodiment the process syngas 1 flow only downward (once through) inside the catalytic vessels through the enhanced catalytic device 11b from the top to the bottom end, in countercurrent with flue gas flowing upward. The reacting process gas flowing through the catalytic device 11b leaves the catalyst vessels from the bottom end and it is collected through the outlet manifold 21. The weight of the catalytic vessels is in part supported through bottom supports 14 and in part sustained through external counterweights located externally on top, allowing the tubes thermal expansion upward.

[0050] The enhanced catalytic device installed in the catalytic vessels may have two possible configurations (11a and 11b in FIGS. 4a and 4b) suitably developed for the catalytic vessels of the first embodiment and of the second embodiment.

[0051] The new and enhanced catalytic device is realized starting from a mechanical support characterized by a random open cell structure having high void fraction (>75%), a good thermal conductivity (at least 5 W/m? K RT) and enabling the gas flow both in axial and radial direction;like for example the ceramic open cells foams prepared from high conductivity materials (ex.: silicon carbide or a combination of alumina and SiC), or else the metal foams or metal skein prepared starting from heat resistant metal alloys suitably selected for the high temperatures and reducing syngas atmospheres (like for ex. NiCrFe, NiCrFeMo, NiCrAl, NiFeCrAl alloys, etc.). Open cells ceramic foams, metal foams or wire meshes are suitable mechanical supports for the deposition of the catalytic material for the endothermic process; the catalyst active species are selected from transition metals (single component or combination of more metals). In order to obtain the best heat transfer vs pressure drops and catalytic performance, the mechanical support must be characterized by: high void fraction, high thermal conductivity, high adhesion capacity, high ratio surface area/volume.

[0052] According to the present invention the catalyst mechanical support is shaped in annular elements (for the first embodiment, catalytic device 11a) or in cylindrical elements (for the second embodiment, catalytic device 11b), to be stacked one over the other inside the catalytic vessels, after deposition of the catalyst active species; flow deviators (110a and 111a for the first embodiment in FIGS. 6a and 6b, 110b and 111b for the second embodiment in FIGS. 7a and 7b) are interposed between the catalyst elements, performing the function to direct the process gas flow according to a desired flow pattern. In both embodiments the flow deviators are realized from metal sheets having suitable thickness to sustain the weight of the catalytic device stacked on top and realized from high grades alloy materials resistant to high temperatures and syngas atmospheres (like for ex. Inconel or Incoloy alloys). In the first embodiment the flow deviators are shaped as repeated couples of rings having configurations 110a and 111a (see FIGS. 5a and 5b); the first is provided with radial centering beams 112a, the second with radial connecting beams 113a.

[0053] The outer diameter of the second flow deviator 111b, which is in correspondence to the catalytic tube 10 ID, and the inner diameter in correspondence to the heat transfer tube 20 OD, have suitable tolerances to permit their insertion and removal for catalyst loading and changeover.

[0054] The first deviator encountered 110a splits the gas into two streams, one flowing toward the external surface of the inner tube 10 and the other flowing toward the inner surface of the catalyst vessel 20. The inner and outer free section areas are selected in such a way that the heat amount to be transferred from the flue gas and from the process gas (ex.: 80% flue gas and 20% process gas) is transmitted from the inner surface of the catalyst vessel 10 and from the outer surface of the inner tube 10 (CFD modelling is necessary to perform heat transfer and pressure drops calculations according to the flow regime established by the described geometry). The second flow deviator 111a is made of two rings, connected by radial beams 113a, leaving an annular free central section area allowing the axial gas flow downward.

[0055] The outer and inner section area of the first flow deviator 110a and the central free section area of the flow deviator 111a, together with the overall number of flow deviators inside one catalyst vessel, are design parameters to be optimized by means of fluid dynamic modelling so as to obtain the maximum heat transfer rate versus selected pressure drops allowable, once the tube diameters 10 and 20 have been selected. Suitable range of the free flow section areas are 20%?66% of the overall annular flow area, while the height between two consecutive flow deviators can vary from 100 to 500 mm. Depending on the height of the single catalyst element, optimized in consideration of the manufacturing procedure, the space between two consecutive flow deviators may be filled in with one or more catalyst elements, as shown in FIGS. 7a and 7b.

[0056] The overall configuration of the enhanced catalytic device as here above described, has the advantage to deviate the process gas flow in radial direction toward the inner surface of the catalyst tubes 10 and toward the outer surface of the inner tube 20, with consequent impingement of both tube walls and increase of the heat transfer coefficient up to 150% of the average value when the section area is reduced by 30%. In the same time, due to the very high porosity of the support elements (void fraction >75%), the pressure drops do not increase and instead there is an allowance for an increase of the mass flowrate per tube (?30%) that can equal the allowable pressure drops of a conventional catalyst pellets bed.

[0057] The same flow deviators pattern is repeated up to the bottom outlet grid, where the process syngas inverts its flow upward within inner tube.

[0058] In a similar way for the second embodiment two consecutive flow deviators 110b and 111b (see FIGS. 6a and 6b) are shaped as alternative couples of concentric discs and doughnuts that can deviate the process gas flow once in radial direction toward the inner surface of the tube wall 10, and then backwards to the reactor axis.

[0059] The same flow deviators pattern is repeated up to the bottom outlet grid, where the process syngas leaves the catalytic vessel.

[0060] The here presented geometries of the catalytic device, comprising the catalyst elements and of the flow deviators, are only indicative and any other combination that can ensure a radial component of the process gas flow (as for example a first ring near the outer tube 10 and a second ring near the inner tube 20) may be considered as a valid alternative.

[0061] The use of the above described enhanced catalytic device can be eventually substituted with alternative catalytic devices having similar characteristics and performance, like for example the structured catalysts realized by means of metal foils arranged in such a way to realize wall jet impingements (ex.: U.S. Pat. No. 4,985,230).

[0062] The presence of an enhanced catalytic device as above described, or else of a metal foil structured type catalyst both acting as heat transfer promoter has the advantage that the outer tube and inner tube in the first embodiment can thermally expand independently without exerting mechanical stress and that no dust and small catalyst particles are produced by catalyst pellets rupture.