Corrosion-protected reformer tube with internal heat exchange

Abstract

A reformer tube for producing synthesis gas by steam reforming of hydrocarbon-containing input gases is proposed where an outer shell tube is divided by means of a separating tray into the reaction chamber and an exit chamber, a dumped bed of a steam-reforming-active, solid catalyst is arranged in the reaction chamber, at least one heat exchanger tube is arranged inside the reaction chamber and inside the dumped catalyst bed whose entry end is in fluid connection with the catalyst bed and whose exit end is in fluid connection with the exit chamber, wherein gas-contacted parts of the reformer tube, in particular the at least one heat exchanger tube, are fabricated from a nickel-based alloy and coated on their inside with an aluminum diffusion layer.

Claims

1. A reformer tube for converting a hydrocarbon-containing feed into a synthesis gas product comprising carbon oxides and hydrogen under steam reforming conditions, the reformer tube comprising: (a) an outer, pressurized shell tube, wherein the outer, pressurized shell tube is divided into a reaction chamber and an exit chamber by means of a separating tray and wherein the reaction chamber is externally heatable; (b) a dumped bed of a steam-reforming-active solid catalyst arranged in the reaction chamber; (c) an entry for an input gas stream comprising an input material arranged in the region of the reaction chamber, wherein the entry for the input gas stream is in fluid connection with the dumped catalyst bed; (d) at least one heat exchanger tube arranged inside the reaction chamber and inside the dumped catalyst bed whose entry end is in fluid connection with the catalyst bed and whose exit end is in fluid connection with the exit chamber, wherein the input gas stream after entry into the reaction chamber initially flows through the catalyst bed and subsequently flows through the heat exchanger tube in countercurrent and is thus continually cooled and wherein the heat exchanger tube is in a heat exchange relationship with the dumped catalyst bed and the input gas stream flowing therethrough; and (e) a collection conduit for the synthesis gas product which is in fluid connection with the exit chamber, wherein gas-contacted metallic components of the reformer tube are made of a nickel-based alloy, wherein each gas-contacted metallic component has a gas-contacted surface, wherein the gas-contacted surfaces having a temperature during operation under defined steam reforming conditions of between 650° C. and 800° C. are equipped with an aluminum diffusion layer that is configured to retard corrosion of the gas-contacted surfaces.

2. The reformer tube according to claim 1, wherein at least one heat exchanger tube is made of a nickel-based alloy and is equipped on the inside and on the outside with an aluminum diffusion layer.

3. The reformer tube according to claim 1, wherein a sufficient amount of aluminum is applied to the inner wall of the at least one heat exchanger tube to ensure that the aluminum concentration in the diffusion layer is at least 20 wt %.

4. The reformer tube according to claim 1, wherein the at least one heat exchanger tube is helically coiled along at least a portion of a length of the at least one heat exchanger tube.

5. The reformer tube according to claim 1, wherein at least two exchanger tubes are arranged inside the dumped catalyst bed.

6. The reformer tube according to claim 1, wherein the separating tray is configured to provide gaseous separation between the reaction chamber and the exit chamber except for the input gas stream flowing through the at least one heat exchanger tube from the reaction chamber to the exit chamber.

7. The reformer tube according to claim 1, wherein the separating tray is secured in place by the separating tray.

8. The reformer tube according to claim 1, wherein the separating tray comprises an absence of perforations that allow gaseous flow directly from the reaction chamber into the exit chamber.

9. The reformer tube according to claim 1, wherein the separating tray is configured to prevent gaseous flow directly from the reaction chamber into the exit chamber.

10. A reformer furnace comprising refractorily lined or refractorily faced walls, a ceiling and a floor, an interior formed thereby, at least a one reformer tube according to claim 1, and at least one burner for heating the reformer tube is arranged in the interior or in a secondary space in fluid connection with the interior in respect of the burner flue gases.

11. The reformer furnace according to claim 10, wherein the at least one reformer tube is arranged in the interior in free-hanging or free-standing fashion, wherein the portion of the outer, pressurized shell tube comprising the reaction chamber is arranged in the interior and the portion of the outer, pressurized shell tube comprising the exit chamber is at least partially fed through the ceiling or the floor.

12. The reformer furnace according to claim 10, wherein a multiplicity of reformer tubes and burners are arranged in the interior such that the longitudinal axes of the flames generated by the burners are oriented parallel to the longitudinal axes of the reformer tubes.

13. A process for producing synthesis gas by catalytic steam reforming of hydrocarbon-containing input materials under steam reforming conditions in the presence of a steam-reforming-active, solid catalyst comprising the steps of: a. provision of an input gas stream comprising the input material and addition of reforming steam, wherein a steam-carbon ratio S/C arises from the molar ratio of the supplied reforming steam amount and the carbon present in the input material, b. catalytic conversion of the input material under steam reforming conditions into a synthesis gas products comprising carbon oxides and hydrogen, wherein the steam reforming conditions comprise operating temperatures between 650° C. and 800° C. for the gas-contacted surfaces, c. discharging and optional workup of the synthesis gas product, wherein the catalytic conversion in step (b) is effected in a reformer tube according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Developments, advantages and possible applications of the invention are apparent from the following description of exemplary embodiments and the drawing. All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims and the way in which said claims refer back to one another.

(2) The FIGURE shows a reformer tube according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) The reformer tube according to the invention depicted in the FIGURE is divided into the sections A (reaction chamber), B (exit chamber) and C (collection conduit).

(4) Via entry conduit 2 desulfurized natural gas together with reforming steam enters the reaction chamber A arranged in the upper portion of the shallow tube 3. The shell tube consists of a nickel-chromium steel for example of the type G-X45NiCrNbTi3525. The entry temperature of the input gas is 600° C., the space velocity based on the catalyst volume is typically 4000 to 5000 mN.sup.3/(m.sup.3 h).

(5) In the present exemplary embodiment the reformer tube is arranged vertically with the open tube end of the shell tube 3 in the upper position and is externally heated by means of burners (not shown in the FIGURE). During operation of the reformer tube the open tube end of the shell tube is sealed with a sealing apparatus 4, for example a flanged lid, which may be opened for overhauls and for charging and discharging of the catalyst.

(6) After entry into the shell tube the natural gas and the reforming steam enter the catalyst bed 5 which is formed from particles of a solid, nickel-based reforming catalyst. The input materials then flow upwards through the catalyst bed as indicated by flow arrows. The catalyst bed is secured in the shell tube by means of separating tray 6. Located between the separating tray and the dumped catalyst bed is a dumped bed of inert bodies 7 as a support for the catalyst.

(7) The endothermic steam reforming reaction takes place over the reforming catalyst. After leaving the catalyst bed the partially converted natural gas which comprises not only carbon oxides and hydrogen but also unconverted methane enters an open space 8 arranged at the sealed tube end 4 of the shell tube. The partially converted input gas stream subsequently enters the entry end of the coiled heat exchanger tube 9 arranged inside the dumped catalyst bed. The gas stream flowing through the heat exchanger tubes 9 gives up in countercurrent a portion of its sensible heat to the dumped catalyst bed and the input gas stream flowing through said bed. The heat exchanger tubes are made of nickel-based alloys having good resistance against metal dusting corrosion, for example Alloy 601, 602 CA, 617, 690, 692, 693, HR 160, HR 214 or so-called multilayer materials where the tubes are coated with tin-nickel or aluminium-nickel alloys. In addition, the heat exchanger tubes are provided on their insides and preferably also on the outsides with an aluminium diffusion layer as a corrosion protection layer.

(8) After flowing through the heat exchanger tubes the synthesis gas product stream enters the exit chamber B. To this end the exit ends of both heat exchanger tubes 9 are fed through the separating tray 6 and thus secured. They then open with their exit ends into the inner tube 10 which provides the connection between the heat exchanger tubes 9 and the collection conduit 11. The inner tube is likewise fabricated from one of the abovementioned metallic materials of construction and its inner wall and preferably also its outer wall are provided with an aluminium diffusion layer as a corrosion protection layer. A gas-permeable insulating material 12 is attached between the outer wall of the inner tube and the inner wall of the shell tube.

(9) The inner tube 10 is connected with the collection conduit 11 (section C) which is provided on its inside with insulating material 13 and/or a corrosion-resistant, for example ceramic, coating 14. The synthesis gas product stream is discharged from the reformer tube 1 via the collection conduit and is sent for further processing. Depending on the intended use of the synthesis gas product this may comprise a carbon monoxide conversion, a gas scrubbing operation for removal of carbon dioxide, a pressure swing adsorption for hydrogen removal and further processing stages.

Numerical Example (Invention)

(10) A reformer tube according to the invention is operated under steam reforming conditions over an operating time of 8000 operating hours. The reforming temperature was 820° C., the S/C ratio was 3.6, the entry pressure into the reforming tube was 33 bar absolute. The reforming tube was provided with two helically coiled heat exchanger tubes made of a nickel-based alloy and provided on their inner walls with an aluminium diffusion layer.

(11) After termination of reformer operation one of the heat exchanger tubes was deinstalled and material samples of its inner wall were withdrawn at various longitudinal sections. Due to the temperature profile determined the respective longitudinal coordinates correspond to different steady-state temperatures.

(12) The samples withdrawn were subjected to metallographic analysis in respect of their surface morphology and by means of SEM/EDS measurements (energy dispersive x-ray analysis) to determine the thickness and composition of the aluminium diffusion layer. None of the samples withdrawn showed any sign of metal dusting corrosion. In particular neither pitting nor any appearance of cracks the in protective layer was observed.

(13) Table 1 summarizes the thus measured layer thicknesses and average aluminium contents in the protective layer. It is apparent from the table that above 673° C. and in particular above 818° C. under the specified operating conditions a marked increase in thickness of the aluminium diffusion layer coupled with simultaneous reduction in the average aluminium contents in this layer is observed.

(14) TABLE-US-00001 TABLE 1 Layer thicknesses and average aluminium contents in the protective layer for samples from various sections of the inside of a heat exchanger tube after 8000 operating hours under steam reforming conditions Temperature ° C. Layer thickness μm Average Al content wt % 627 143 34 650 149 673 149 696 174 725 170 740 177 818 254 21

(15) The two samples obtained at 627° C. and 818° C. analyzed in more detail in respect of their horizontal layer structure. Table 2 summarizes the thus obtained local aluminium contents as a function of distance from the surface (depth).

(16) It is clearly apparent that the higher surface temperature results in a broadening/increase in thickness of the aluminium diffusion layer and the aluminium content decreases over the first 100 μm. One exception is the Al content measured directly at the surface.

(17) TABLE-US-00002 TABLE 2 Local aluminium contents as a function of distance from the surface into the inside of the workpiece (depth) for samples obtained at 627° C. and 818° C. Sample at T = 627° C. Sample at T = 818° C. Depth μm Local Al content wt % Local Al content wt % 0 25 37 25 35 24 50 37 23 75 35 22 100 34 20 125 20 20 150 4 19 175 0 17 200 0 4 225 0 6 250 0 3 Average value 34 21

(18) Table 3 summarizes the calculated Boudouard temperatures when operating the steam reforming plant at various S/C ratios and reforming temperatures. The Boudouard temperature defined as the temperature at which the activity as per equation (2a) is one.

(19) It is apparent from table 3 that the Boudouard temperature decreases with increasing S/C ratio and decreasing reforming temperature. Above the respective Boudouard temperature i.e. at an activity as per equation (2a) of less than one, metal dusting corrosion no the takes place to any appreciable extent since a thermodynamic potential therefor is no longer present.

(20) TABLE-US-00003 TABLE 3 Boudouard temperature when operating the steam reforming plant at various S/C ratios and reforming temperatures Reforming Boudouard temperature temperature S/C ° C. ° C. Case 1 3.1 900 783 Case 2 3.3 870 763 Case 3 3.5 840 742 Case 4 3.8 810 718 Case 5 4.1 780 693

Comparative Example

(21) A reformer tube was operated under the same steam reforming conditions over an operating time of 8000 operating hours as in the numerical example according to the invention. The reformer tube was provided with two helical coiled heat exchanger tubes without an aluminium diffusion layer.

(22) After termination of reformer operation one of the heat exchanger tubes was in turn deinstalled and material samples of its inner wall were withdrawn at various longitudinal sections which due to the determined temperature profile correspond to different steady-state temperatures. The withdrawn samples thus corresponded to the temperatures reported hereinbelow, the respective accompanying Boudouard activity as per equation (2b) being reported in brackets: 623° C. (9.8), 644° C. (5.8), 663° C. (3.7), 685° C. (2.2), 696° C. (1.7), 706° C. (1.3)

(23) These samples were also analyzed in respect of their surface morphology. Signs of corrosion were clearly apparent in all samples and propensity for corrosion decreased with increasing temperature in line with the Boudouard activity which fell in the same direction. The sample responding to 623° C. showed extreme corrosion while by contrast the sample corresponding to 706° C. exhibited only minor corrosion.

(24) At even higher temperatures where Boudouard activity falls below 1 significant metal dusting corrosion is no longer to be expected.

(25) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(26) The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

(27) “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

(28) “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(29) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(30) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(31) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

INDUSTRIAL APPLICABILITY

(32) The invention proposes a reformer tube enabling internal heat exchange between the input gas and the product gas partially converted into synthesis gas products, thus giving rise to advantages in terms of energy consumption during use of the reformer tube. The inventive provision of the reformer tube with an aluminium diffusion layer as a corrosion protection layer makes it possible to effectively counteract metal dusting corrosion when in particular the metallic components and the regions of the reformer tube having a surface temperature in the recited critical ranges are appropriately equipped. This results in a possible longer operating time of the reformer tube and thus in economic advantages. Corrosion protection for regions of the reformer tube having a surface temperature above the recited critical ranges is also obviated, thus permitting cost savings for costly and complex material treatment.

LIST OF REFERENCE NUMERALS

(33) [1] Reformer tube [2] Entry conduit [3] Shell tube [4] Sealing apparatus [5] Dumped catalyst bed [6] Separating tray [7] Dumped bed of inert bodies [8] Open space [9] Heat exchanger tubes [10] Inner tube [11] Collection conduit [12] Insulating layer [13] Insulating layer [14] Coating [A] Reaction chamber [B] Exit chamber [C] Collection conduit