STEAM METHANE REFORMER TUBE OUTLET ASSEMBLY

Abstract

The present invention relates a steam methane reformer tube outlet assembly and a method of assembling or retrofitting same. More specifically, it relates to an exposed flanged tube outlet of a reformer designed to mitigate metal dusting corrosion, dew point condensation-related metal fatigue and cracking, and over-temperature induced metal failures such as hydrogen attack.

Claims

1. A flanged tube outlet assembly of a steam methane reformer assembly comprising: at least one or more reformer tubes having an inlet for allowing a process gas to be introduced into said tube outlet assembly for the removal of said process gas, wherein said process gas exiting an outlet port is syngas, said tube outlet assembly is disposed outside the confines of the reformer and includes a reformer tube having an interior space accommodating an internal insulation can therein wherein said insulation can is fitted in the interior space of the reformer tube, and the exterior of said reformer tube is covered with insulation extending in close proximity to the tube-flange weld neck; the outlet port disposed upstream of the distal end of said insulating can for delivering said syngas to downstream process units, and said insulation can is connected to a blind flange and extends into the reformer tube toward the outlet port, wherein the gap between the can and the interior of said reformer tube is larger at the distal end than at the blind flange end.

2. The flanged tube outlet assembly of claim 1, wherein the internal insulation can is selected from the group consisting of a fully tapered can, a partially tapered can, a fully stepped can or a partially stepped can.

3. The flanged tube outlet assembly of claim 2, wherein the internal can has an angled or curved distal end.

4. The flanged tube outlet assembly of claim 1, wherein the insulation can disposed in the interior of said reformer tube is designed to maintain areas of the tube outlet upstream of the distal end of the insulation can above the temperature range favorable to metal dusting, while areas with temperatures favorable to high rates of metal dusting are restricted to regions of low syngas flow within the annular gap thereby having a reduced rate of metal dusting corrosion.

5. The flanged tube outlet assembly of claim 1, wherein the larger gap size at the distal end of the insulation can maintains the tube outlet above the syngas dew point temperature, and the reduced gap size toward the blind flange keeps the flanges below the threshold temperature for high temperature hydrogen attack.

6. The flanged tube outlet assembly of claim 2, wherein the gap of the tapered or stepped insulation can at the distal end ranges between about 0.15 to 1 inches, and the gap at the blind flange end ranges between about 0.1 to 0.5 inches.

7. A flanged tube outlet assembly of a steam methane reformer assembly comprising: at least one or more reformer tubes having an inlet for allowing the process gas to be introduced into a tube outlet assembly for removal of the process gas, wherein said process exiting an outlet port is syngas, said tube outlet assembly is disposed outside the confines of the reformer and includes a reformer tube having an interior space accommodating an internal insulation can therein wherein said insulation can is tapered or stepped in the interior space of the reformer tube and wherein the exterior of said reformer tube is covered with insulation extending in close proximity to the tube-flange weld neck; the outlet port is disposed upstream of the distal end of said insulation can for delivering said syngas to downstream process units, and said insulation can is connected to a blind flange and extends into the reformer tube toward the outlet port and securely connected to the blind flange, wherein the gap between the can and the interior of said reformer tube is in the range between about 0.1 to 0.5 inches at the blind flange end of said tube outlet, and 0.1 to 1 inches at the distal end, allowing a larger volume of hot syngas to be maintained at the distal end of the gap so the tube metal temperature in the vicinity of the distal end of the can is above metal dusting favorable temperatures, yet regulating the flow of hot gas towards the flange to maintain the whole length of the tube outlet above the syngas dew point temperatures to eliminate condensation/evaporation thermal cycling induced fatigue cracking while lowering the flange temperatures to minimize occurrence over-temperature induced metal failures.

8. The flanged tube outlet assembly of a steam methane reformer of claim 7, wherein the internal can is either partially or fully tapered or stepped, and optionally with the distal end angled or curved.

9. The flanged tube outlet assembly of claim 7, wherein the internal insulation can is selected from the group consisting of a fully tapered can, a partially tapered can, a fully stepped can or a partially stepped can.

10. The flanged tube outlet assembly of claim 7, wherein the internal can has an angled or curved distal end.

11. Processing a hydrocarbon feedstock in a can or bottom fired stream methane reformer, comprising: reactor tubes with a coating, an aluminum diffusion coating applied by pack cementation, wherein the coating is applied to the inner walls of the tube outlet assembly extending from the distal end down into the tube a distance of 72 inches, and having a thickness of 10-300 m, to minimize tube exposure to carbon supersaturated environments and substantially reduce metal dusting at temperature ranging from about 900-1400 F.

12. A method of preventing metal dusting corrosion of a reformer tube utilized in a steam methane reformer application, comprising: introducing a hydrocarbon feedstock at the bottom of the reactor tubes, wherein an upper portion of the inner walls of a tube outlet assembly is coated with a composition 30-40 wt % Al, which is resistant to metal dusting.

13. The method of claim 12, wherein the coating has a thickness of 10-300 m and extends from the distal end of the tube down into the tube a distance of 72 inches.

14. The method of claim 12, wherein the tube is a micro-alloyed HP-Nb-MA (micro-alloyed) steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other aspects, features, and advantages of the present invention will be more apparent from the following drawings, wherein:

[0028] FIG. 1 is a schematic representation of a related art bottom-fired cylindrical reformer with tube outlets disposed outside the confines of the reformer;

[0029] FIGS. 2a and 2b are a schematic representation of a related art tube outlet assembly;

[0030] FIGS. 3a, 3b and 3c are schematic representations of a flanged tube outlet assembly of a reformer tube in accordance with one exemplary embodiment of the invention;

[0031] FIGS. 4a and 4b are depictions of another exemplary embodiment of the tube outlet assembly in which the insulation can is tapered and a distal end that is angled or curved;

[0032] FIGS. 5a and 5b depict the computational fluid dynamics of a conventional tube outlet assembly;

[0033] FIG. 6 depicts the computational fluid dynamics of a tube outlet assembly in accordance with FIG. 3a; and

[0034] FIG. 7 depicts the computational fluid dynamics of a tube outlet assembly in accordance with FIG. 4a.

[0035] FIG. 8 depicts the computational fluid dynamics results showing the improvement in tube outlet reliability against various material degradation mechanisms for the present invention over the related art.

[0036] FIG. 9 illustrates the coating composition vs. distance from the surface of the coating to the substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention addresses the susceptibility of tube outlets to the aforementioned material degradation mechanisms that lead to premature tube failure in steam methane reformers. Specifically, this invention is utilized with a flanged tube outlet assembly of a steam methane reformer, an example of which is a bottom-fed cylindrical reformer. As utilized herein the term bottom-fed cylindrical reformer or reactor will be understood by those skilled in the art to refer to a can reformer or the like where feed gas is introduced into the bottom of the reformer tubes, and the burners are fired at the bottom of the reformer, and the process gas and flue gas flow co-currently from the bottom to the top of the reformer. In this type of reformer, the tube outlet is outside the furnace refractory wall/roof and exposed to the ambient.

[0038] Referring to the figures and commencing with FIG. 1, a bottom fired can reformer is depicted generally at 100, including reformer tubes 101 through which syngas exits the reformer at temperatures ranging from 1450-1650 F. Syngas flows upwards and exits the reformer tube through side port 102. Internal insulation (not shown) comprising of a cylindrically-shaped can and filled with insulation material such as ceramic fiber blanket, is positioned in the interior of tube outlet 101 and prevents the hot syngas from making direct contact with the flange and thereby overheating it. Generally, the flanges are made of carbon steel and it is necessary to keep its temperature below 400 F. In instances where the flanges are made of stainless steel, a higher temperature (up to 800 F.) is tolerable. External insulation 103 also limits heat losses from the tube outlet and prevents rapid cooling of the syngas. As noted above, the tube outlet is located outside the reformer 100 where, unless the insulation design prevents internal surfaces of the tube outlet and flanges from entering specific temperature ranges, it can be susceptible to material degradation mechanisms such as metal dusting, high temperature hydrogen attack and dew point condensation induced failures.

[0039] With reference to FIG. 2a, the external insulation 206a is typically one inch thick and extends a few inches above the outlet port 207a. The internal insulation can 208a is typically cylindrically-shaped. As determined through failure root cause analysis and Computational Fluid Dynamics (CFD) modeling, the effect of this insulation arrangement was found to be lacking. The modeling results in FIG. 5a depict that this insulation scheme is insufficient and will lead to rapid failure of the tube outlet because areas of the tube metal below the distal end of the insulation can and in the vicinity of the outlet port 207a are in the temperature range of 900-1400 F., which are favorable to high rates of metal dusting corrosion in syngas environments. The term metal dusting or metal dusting corrosion as utilized herein will be understood by those skilled in the art to mean a form of carburization that leads to material loss, occurring in high carbon activity environments between 570 F.-1550 F., with maximum rates happening typically between 900-1400 F. but highly dependent on the process conditions.

[0040] The very short height of the external insulation leads to increased heat losses and low flange temperatures. In this example, the maximum temperature on the weld flange was found to be 237 F. While this is beneficial to minimizing the occurrence of high temperature hydrogen attack, metal temperatures for the upper parts of the tube outlet are below the syngas dew point temperature, which is 311 F. in this case. As a result, water will condense on the inner walls of the tube. At a lower location where the tube is hotter, the water evaporates. This repeated condensation/evaporation cycle can cause thermal fatiguing and cracking of the reformer tube. In other cases too, the condensed water can become slightly acidic due to dissolved gases such as CO.sub.2, and can cause corrosion of the tube. These material degradation mechanisms are herein referred to as dew point condensation related failures. The term high temperature hydrogen attack as utilized herein will be understood by those skilled in the art to mean a form of decarburization at elevated temperatures (typically >400 F. for carbon steel) whereby hydrogen can dissociate into atomic form and diffuse into steel, reacting with unstable carbides to form methane gas. This eventually leads to cracking and equipment failure.

[0041] FIG. 2b illustrates another embodiment of the related art in which the thickness and height of the external insulation 206b have been increased. The internal insulation can 208b is cylindrically-shaped. As can be seen in CFD results of FIG. 5b, this reduces heat losses and shifts the areas of the tube outlet with temperatures favorable to metal dusting further up. While this is an improvement over the previous design in that the flanges temperatures are higher (maximum is 330 F.), there are still tube metal areas below the distal end of the insulation can that fall in the metal dusting favorable temperature band. Increasing the annular gap size to increase convective flow of hot syngas in that region to further shift up the metal dusting favorable temperature band invariably exposes the flanges to more hot syngas and can cause overheating. Therefore there is a need for an insulation design that balances these opposing temperature constraints and leads to a desired tube metal temperature profile.

[0042] Referring now to an exemplary embodiment of the invention, as shown in FIGS. 3(a), 3(b) and 3(c), the tube outlet assembly 300a-c is utilized in the steam reformer 100 shown in FIG. 1, and replaces the conventional tube assembly of FIG. 2a or 2b.

[0043] An internal insulation can of the tube outlet assembly 300a-c includes a blind flange 311a-c and a non-cylindrical can 308a-c that is positioned in the interior space of the steam reformer tube 305a-c. The can portion 308a-c fits into the inside of the reformer tube and is securely attached to the blind flange 311a-c such as through a weld. Internal insulation can 308a-c is a sheet metal formed into the non-cylindrical can and filled with insulation material and extends toward the outlet port 307a-c at its distal end.

[0044] In an assembled form of the tube assembly 300a-c as shown, the internal insulation can 308a-c is tapered or stepped as shown in FIG. 3(a)-3(c) toward the distal end extending into the tube 305a-c. The tapering or stepping can be partialup to any length of the can, such as all the way to the blind flange as shown in FIG. 3(a), or halfwayas shown in FIG. 3(b). The extent of the taper dictates the amount of hot syngas that circulates in the annular gap towards the flange, allowing a larger volume of hot syngas to be maintained at the entrance of the gap so that the tube metal temperature up to the distal end of the can is above the high rates metal dusting temperatures, yet limiting the flow of hot gas towards the flange. Preferably, the gap between the insulation can and the reformer tube inside diameter ranges between about 0.25 and 1 inches at the distal end, and between 0.1 to 0.25 inches at the blind flange end. This ensures that the section of tube outlet between the distal end of the can and tube/flange weld neck 312a-c can be maintained above the syngas dew point temperature to avoid dew point condensation induced failures, but with the flange kept at low enough temperatures (e.g., below 400 F. for carbon steel flanges) to prevent the occurrence of high temperature hydrogen attack. FIG. 3(c) shows an embodiment where the internal can is stepped. The effect of the stepped can with a larger gap at the distal end than at the blind flange end is analogous to the tapering shown in FIG. 3(a), but may be easier to fabricate. A partially stepped can analogous to FIG. 3(b) can also be employed.

[0045] As illustrated in FIGS. 4(a) and 4(b), other exemplary embodiments are shown where the tube outlet assembly has a tapered can which is angled (413a) or curved (413b) at the distal end, with the longer side being located opposite the syngas outlet port 407(a-b). This arrangement allows the non-outlet side of the tube outlet to always remain above metal dusting favorable temperatures. The angled or curved end of the insulation can at the distal end also acts to direct hot gas towards the opposite side of the tube, ensuring that that side stays above metal dusting favorable temperatures. This way, sections of tube outlet with temperatures favorable to metal dusting are shifted to low syngas flow areas downstream of the bottom of the internal insulation can where the rate of metal dusting corrosion is greatly diminished. This embodiment is suitable in situations where the temperature of the process gas entering the tube outlet is relatively low at around 1500 F.

[0046] The choice of internal can design for the tube assembly outlet will depend on the process conditions and geographic location of the reformer. For processes where the temperature of the syngas exiting the reformer is very high (>1600 F.), a shallow taper or stepping will be most appropriate as it is not desirable to have large volumes of very hot syngas contact the flanges. Conversely, if the reformer is located in a very cold climate, then a more pronounced tapering or stepping will be appropriate as more syngas can be directed into the gap to help maintain temperatures above the dew point. By considering the process conditions and climate, an appropriate internal and external insulation tube outlet assembly design can be selected that greatly improves its reliability and lifespan.

[0047] Alternatively, or in addition to the redesign of the internal insulation can 308a-c, the inner walls of the reactor tubes are coated with an aluminum diffusion coating by pack cementation process. While the reactor tube material or substrate can be austenitic stainless steels, nickel based alloys and nickel centrifugal cast alloys, it is preferred that it is a micro-alloyed nickel centrifugal cast alloy such as an HP-Nb-MA (micro-alloyed) material with addition of carbides or intermetallic compounds forming elements to improve microstructural stability in long term exposure to high temperatures, and, therefore, better resistance to high temperature stress and creep deformation.

[0048] The coating process on the inside walls of the reactor tubes is performed in a furnace or retort with a protective atmosphere. The substrate material to be aluminized is prepared so that it is free of surface flaws or defect detrimental to the coating process. Therefore, thorough cleaning and grit blasting is used for preparing the surface such that there will be minimal contamination during the coating process. The inside of the reactor tubes are then packed with pack compound which consists of an aluminum source, an activator which is normally a halogen compound, and an inert phase. At high temperature, a chemical reaction occurs that a gaseous aluminum halide forms and the aluminum is transferred by the gas to the reactor tube inside diameter surface (i.e., interior wall). The gas decomposes at the substrate surface depositing aluminum and releasing the halogen activator. The halogen activator returns to the pack and reacts with the Al source again. Thus, the transfer process continues until all of the aluminum in the pack is consumed or until the process is stopped by cooling. The aluminum diffuses into the reactor tube inside diameter surface forming metal aluminides diffusion coating at temperatures ranging from 700 to 1100 C. The coating thickness is controlled by time and temperature.

[0049] In order to have sufficient metal dusting protection and also maintain the mechanical properties of the reactor tube, the thickness of aluminizing diffusion coating can be in the range of about 10-300 m, and is preferably controlled to a range of about 60 to 100 m. The weldability of the reactor tube is preferably evaluated by ASTM A488 after coating.

[0050] The composition of the coating is dependent on the substrate chemistry. For HP-Nb microalloyed reactor tubes, the chemical composition as measured by energy dispersive spectroscopy (EDS) preferably can have an aluminum content 20-50 wt % from the surface to the substrate interface, preferably an aluminum content 30-35 wt % from the surface to a distance at least 50% of the coating thickness. A typical example of the coating composition vs. distance from surface is shown in FIG. 9.

[0051] For ease of explanation, and with reference to FIGS. 3(a)-(c), the coating is applied to the distal end of the can that is above the high rate metal dusting temperatures, with the coated portion of the tube extending from the tube-flange weld neck (312 a) of the tube (i.e., the terminal end of the reformer tube) down into the tube for a distance of 72 inches, and has a thickness of about 10-300 m, as discussed above. Thus, during the processing of the natural gas feedstock and its conversion to syngas at metal dusting temperature of approximately 900-1,400 F., metal dusting is substantially reduced if not outright eliminated.

[0052] The invention is further explained through the following examples, which compare the base case with a standard design at the outlet tube, and those based on various embodiments of the invention, which are not to be construed as limiting the present invention.

Comparative Example

[0053] FIG. 5(a) depicts the CFD modeling results for the related art flanged tube outlet assembly design shown in FIG. 2(a). In this design, the external insulation is 1 inch thick and 3.5 inches above the centerline of the outlet port. The internal insulation can is cylindrically shaped. As the syngas exits the furnace and enters the tube outlet assembly, it proceeds from being heated in the radiant section to losing heat to the ambient in the tube outlet. In the tube outlet assembly design shown, inadequate external insulation and a conventional internal can design leads to heat losses and the tube metal temperatures below the distal end of the internal can fall in the temperature range favorable to high rates of metal dusting, 900-1400 F., as shown in FIG. 5a. In this design, the maximum flange temperature shown is 237 F. This is beneficial for avoiding high flange temperatures. On the other hand, the temperatures on the top part of the tube are below the syngas dew point, which is 311 F. in this case. As a result, the tube outlet will be prone to dew point condensation related failures.

[0054] In an alternative example of the related art, and as shown in FIG. 2(b), the thickness and height of the external insulation have been increased but the internal insulation can 208b is still cylindrically-shaped. As can be seen in CFD results exhibited in FIG. 5(b), it reduces heat losses and the maximum flange temperature is 330 F. This shifts the areas of the tube outlet with temperatures favorable to metal dusting further up, but there are still tube metal areas below the distal end of the insulation can that fall in the metal dusting favorable temperature range. Increasing the annular gap size will increase convective flow of hot syngas in that region and likely lead to higher than desired flange temperatures.

Example 1

[0055] The design which is the subject of this invention involves an internal insulation can that is tapered where the annular gap is larger at the distal end than at the blind flange end (FIG. 6). In this example, the gaps at the distal end and blind flange ends are 0.25 and 0.1 inches, respectively. By this design, a larger volume of hot syngas initially enters the gap. This helps shift the areas of the tube with temperatures favorable to metal dusting to above the distal end of the insulation can where because of very little flow of syngas, metal dusting corrosion rates are greatly decreased. However, because the gap narrows towards the blind flange, decreased amounts of hot gas makes contact with the flange thereby keeping it cooler to avoid overheating it, but maintaining it above the syngas dew point temperature to avoid dew point condensation induced failures. Plots of the circumferentially averaged inner wall tube temperature for the prior art and FIG. 6 are shown in FIG. 8. As can be seen, all areas below the distal end of the can are above the upper temperature limit for high rates of metal dusting (1400 F.), whereas tube temperatures for both cases of the related art shows susceptibility to metal dusting in those areas. This susceptibility is more pronounced for the configuration of FIG. 5a. The maximum flange temperature is also higher (i.e., 341 F. for the FIG. 6 design), reducing susceptibility to dew point condensation induced failures.

Example 2

[0056] The results shown in FIG. 7 depict another embodiment of the present invention. In this case, the external insulation is the same as in FIGS. 5b and 6 (2.75 thick and extends to 2 below the weld neck) but the internal can is tapered and its distal end is angled. Since the angled end is longer, areas of the tube metal opposite the outlet side of the tube outlet always remain above metal dusting favorable temperatures. The angled or curved end of the insulation can also acts to direct hot gas towards the opposite side of the tube, ensuring that that side also stays above metal dusting favorable temperatures. This way, sections of tube outlet with temperatures favorable to metal dusting are shifted to low syngas flow areas above the bottom of the internal insulation can where the rate of metal dusting corrosion is greatly diminished. Referring to FIG. 8 again, the circumferentially averaged inner wall tube temperature for FIG. 7 is also shown. As can be seen, the internal can design of this invention leads to all areas below the bottom of the can to be well above the upper threshold (1400 F.) for high rates metal dusting corrosion. Furthermore, the maximum flange temperature for the FIG. 7 design is 391 F., allowing the entire length of the tube outlet to be maintained above the syngas dew point temperature to stop thermal cycling fatigue, but minimizing the flange temperatures to help eliminate occurrence of high temperature hydrogen attack on the flanges.

[0057] Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.