IN HYDROGEN SERVICE

FIELD OF THE DISCLOSURE

[00001 ] Pressure vessels useful in hydrogen service are disclosed herein. More specifically, pressure vessels, and materials of construction therefore, suitable for use in pressure swing adsorption systems, where the pressure vessels may be subjected to repeated fluctuations in pressure (stress) during its operating lifetime, and exposure to relatively high purity hydrogen, are disclosed.

BACKGROUND

[00002] Hydrogen may change the attributes of metallic materials generally used for pressure vessels and piping systems. Industry has thus developed techniques to mitigate the deleterious effects of hydrogen, such as loss of ductility, change in transition temperatures between phases, and a tendency towards catastrophic cracking. Although these methods have proven largely successful in the prevention of unplanned failures in most structures, their application to structures for service with repeated fluctuation in stress has not proven successful. In fact, pressure vessels for use in cyclic stress

environments, fatigue service, and hydrogen charging atmospheres, such as those in pressure swing adsorption ("PSA") units, have demonstrated a vexing propensity towards failure by premature cracking. The tendency towards premature fatigue failure in pressure swing adsorption vessels has persisted despite the application of many of the techniques called for in design standards oriented towards sour gas service for carbon steels. The most commonly used carbon steel for PSA adsorber vessels is SA-516-70. It is widely known in the literature that PSA adsorber vessels made from this material have a tendency to form fatigue cracks, that these cracks are enhanced by the presence of hydrogen, and that the welds and nearby heat affected zones are more susceptible to the cracking than the base plate is. [00003] Stainless steels have also been used to form pressure-containing structures. Stainless steels are generally defined as iron-based alloys containing at least 1 1 % by weight of chromium and which possess a stable surface oxide layer which inhibits general corrosion. This class of alloys encompasses an amazingly-broad range of compositions, which generally do not resist hydrogen embrittlement in static service. Unfortunately, alloys which have been held to be suitable for hydrogen service in the literature have contained a relatively large amount of nickel to stabilize the austenitic crystal structure desired for resistance to hydrogen embrittlement, which undesirably increases the cost of the structural material and to result in low strength, as described below. The lower limit for nickel content for these alloys has generally been held to be 10% by weight.

[00004] One serious deficiency of most stainless steels is that their strength is low compared to typical carbon steels used in pressure-retaining applications. The allowable stress for stainless steels used in the ASME code is 66% of the 0.2% offset yield strength of the alloy. Typical stainless steels having at least 10% nickel possess an ASME allowable stress of only 16,000 to 20,000 psi, with compositions suitable for welding by the control of carbon content having strengths towards the low end of this range. This is less than the allowable stress of the most commonly used carbon steel for PSA adsorber vessels, SA-516-70, which has an ASME allowable stress of 20,000 psi. Thus, the stainless steels known to resist the effects of hydrogen are generally weaker than carbon steel, thus discouraging their use. A complicating secondary factor caused by the low strength is that a great thickness is required, and this great thickness increases bending restraint, and can cause heightened local stresses, which may

themselves result in premature cyclic failure, despite the heightened resistance to hydrogen fatigue cracking that such alloys possess.

[00005] One method used for strengthening stainless steels is the addition of nitrogen to the steel, which results in marked strengthening. The results of such nitrogen strengthened steels in hydrogen-charging experiments has been decidedly mixed, and only nitrogen-strengthened alloys having nickel contents of greater than 10% have previously been accepted for hydrogen service. No previous test results exist in the literature to establish the fatigue strength of nitrogen strengthened stainless steels when tested in a hydrogen environment or when pre-charged with hydrogen.

[00006] Standard welding techniques for stainless steels invariably result in a crystal structure which is a combination of austenite and ferrite crystallographic structures. The intentional compositional control of the weld filler metal to result in the formation of the ferrite phase is known in the art as a preferred method of fabrication because it reduces the incidence of cracking in the weld zone upon cooling and solidification of the weld metal. The ferrite phase has been shown to decrease the static toughness (shock resistance) of stainless steel weldments in the tritium charged condition, but no test results have been published which demonstrate the hydrogen fatigue strength of welds as a function of ferrite content. Standard weld materials for common stainless steels result in a percentage of ferrite phase which is usually 8 to15% by volume. Special welding techniques to produce lower ferrite are known, but substantially increase the cost and difficulty of welding. Prior to the date of the current invention there was no teaching or suggestion to alter the ferrite concentration to improve hydrogen fatigue strength.

[00007] The ASME Boiler and Pressure Vessel Code is a widely-accepted authority on the construction of pressure vessels. When a pressure vessel undergoes over 1 ,000 pressure cycles in its operating lifetime, the vessel is analyzed according to the rules of Section VIII, Division 2, of the ASME code. The code provides design stress levels suitable for carbon steels and 300-series austenitic stainless steels. The ASME Code does not provide guidance on austenitic stainless steels with lower concentration of nickel, or for hydrogen service. Thus, construction of pressure-retaining structures for fatigue service from such alloys has not been possible.

[00008] Another serious deficiency of stainless steels is their susceptibility to adverse forms of localized corrosion, such as pitting, crevice corrosion, and stress corrosion cracking. These forms of corrosion are combated by the use of increased concentrations of nickel and molybdenum, sometimes along with nitrogen. The addition of nickel and molybdenum undesirably increase cost. In the field of pressure and temperature swing adsorption, the gas mixture being contained by the pressure vessel and/or piping often presents the risk of localized corrosion due to concentrations of water, halogens, and/or sour gas constituents, the combination of these compounds being particularly undesirable.

SUMMARY OF THE DISCLOSURE

[00009] In contrast to the common perception that high nickel content stainless steels are required for use in cyclic hydrogen charging environments, it has now been surprisingly found that various stainless steels containing less than 10 wt. % nickel perform extremely well in hydrogen charging environments, including use as materials of construction for PSA adsorber vessels.

[000010] In one aspect, embodiments disclosed herein relate to a pressure vessel for use in cyclic hydrogen charging environments, comprising: a bottom head, a top head, and a cylindrical portion there between, at least one of which is made from a steel comprising: up to 14.5 wt. % Mn; from greater than 0 to less than 10 wt. % Ni; and greater than 0.1 wt. % N.

[00001 1 ] In another aspect, embodiments disclosed herein relate to a pressure swing adsorption system comprising: a plurality of the pressure vessels as defined above and having one or more layers of adsorbent material therein; a feed gas conveying means connected to the plurality of vessels; and a product recovery means connected to the plurality of vessels.

[000012] Other aspects and advantages will be apparent from the following description and appended claims. BRIEF DESCRIPTION OF DRAWINGS

[000013] Figure 1 is a simplified flow diagram of a pressure swing adsorption system incorporating adsorber vessels according to embodiments disclosed herein.

[000014] Figure 2 illustrates a cross-section of one example of an adsorber vessel according to embodiments disclosed herein.

[000015] Figure 3 shows the fully-reversed fatigue stress versus cyclic life curve observed for an alloy of the present invention when tested with and without charging with hydrogen.

[000016] Figure 4 shows the fully-reversed fatigue stress versus cyclic life curve observed for a weldment of the present invention having 6.6% ferrite when tested with and without charging with hydrogen.

[000017] Figure 5 shows the fully-reversed fatigue stress versus cyclic life curve observed for a weldment of the present invention having essentially no ferrite when tested with and without charging with hydrogen.

[000018] Figure 6 illustrates the crack propagation properties of the inventive materials and weldments of the current invention when tested in high purity hydrogen.

DETAILED DESCRIPTION

[000019] Figure 1 illustrates a simplified process flow diagram of a PSA system incorporating adsorber vessels according to embodiments disclosed herein. The PSA system 5 includes a plurality of adsorber vessels 1 10, 1 12, 1 14, and 1 16. Each of the adsorber vessels 1 10, 1 12, 1 14, 1 16 typically includes one or more beds of adsorbent material. The vessels 1 10, 1 12, 1 14, 1 16 are connected in parallel flow relation between a source manifold 1 18, which supplies a feed gas mixture, and a product manifold 120, which provide an outlet for unabsorbed product effluent gas. The vessels 1 10, 1 12, 1 14, 1 16 are also connected to a waste manifold 122, which provides an outlet for adsorbed components. Additionally, the vessels 1 10, 1 12, 1 14, 1 16 are connected to an equalization manifold 124, providing for equalization of pressure between two or more vessels to conserve pressure energy during operation of the system. These are four manifolds typically discussed in the art, such as in U.S. Patent No.

6,858,065. U.S. Patent No. 7,674,319 also discloses connecting vessels 10, 12, 14, 16 to a purge gas manifold 126.

[000020] The flow scheme of Figure 1 is shown for illustrative purposes only, with a limited number of valves and vessels. As one skilled in the art would appreciate, PSA adsorber vessels according to embodiments disclosed herein may be useful in any PSA system or other systems with a cyclic hydrogen charging environment or other environments that may charge the crystal lattice of the metal with atomic hydrogen, such as sour environments (including hydrogen sulfide or other reduced sulfur species), acidifying environments (such as carbon dioxide service), seawater environments, and environments with gaseous hydrogen or mixtures including gaseous hydrogen.

[000021 ] During operation, a gas mixture containing hydrogen is cyclically charged to and discharged from the vessels 1 10, 1 12, 1 14, and 1 16 to produce a high purity hydrogen product. As the adsorber vessels 1 10, 1 12, 1 14, 1 16 undergo cyclic pressurization and depressurization with a hydrogen containing gas. As the gas mixture progresses from the inlet end towards the discharge end, the concentration of hydrogen is increased, and is typically purified to at least 99% purity, and as high as 99.99999% purity. The purified hydrogen causes accelerated fatigue failure in vessels constructed from prior art materials, such as carbon steels.

[000022] It has now been surprisingly discovered that the adsorber vessels, or selected portions thereof, may be constructed with stainless steels containing less than 10 wt. % nickel while suffering no reduction in fatigue life due to hydrogen absorbed in the metal. Stainless steels useful as materials of construction of adsorber vessels or at least one portion thereof according to embodiments disclosed herein include stainless steels comprising:

from 0 up to 14.5 wt. % Mn; from greater than 0 to less than 10 wt. % Ni; and

greater than 0.1 wt. % N;

[000023] In some embodiments, the stainless steel may include from about 1 .25 wt. % to about 9.75 wt. % Ni; from about 1 .5 to about 9.5 wt. % in other embodiments, and from about 1 .5 to about 7.5 wt. % in yet other embodiments. In some embodiments, the stainless steel may include from about 14 wt. % to about 22 wt. % Cr; from about 15 to about 21 .5 wt. % in other embodiments. In some embodiments, the stainless steel may include less than 2 wt. % Mn; from about 2 to about 6 wt. % Mn in other embodiments; and from about 6 to about 15 wt. % Mn in yet other embodiments. In some embodiments, the stainless steel may have from 0 to less than 1 wt. % Mo. In some embodiments, the stainless steel may include from 0.1 to less than 0.5 wt. % N; from about 0.1 to about 0.4 wt. % N in other embodiments; and from a lower limit of about 0.1 , 0.15 or 0.2 to an upper limit of about 0.25, 0.3, 0.35, or 0.4 wt. % N in yet other embodiments, where any lower limit may be combined with any upper limit. In a preferred embodiment the amount of nitrogen by weight is about 0.2%.

[000024] In some embodiments, the stainless steel may have a 0.2% offset yield strength of at least 30 ksi; at least 35 ksi in other embodiments; at least 40 ksi in other embodiments; at least 45 ksi in other embodiments; at least 50 ksi in other embodiments; and at least 50 ksi in yet other embodiments.

[000025] The stainless steels of the present invention do not show a statistically-significant reduction in fatigue life versus fully-reversed fatigue stress amplitude (the so-called S-N curve, which is widely used in the art) between tests conducted with and without hydrogen present in the metal. A fully reversed stress amplitude means that the stress amplitude is experienced in alternating modes between tension and compression, with the scalar magnitude being the same in each direction. Such fatigue testing may be performed using an R. R. Moore rotating beam fatigue testing machine, for example.

[000026] In various embodiments, the stainless steel may be selected from the group consisting of Unified Numbering System (UNS) designations S20153 (Alloy 201 LN), S20400,S30453 (Alloy 304LN), S21904, and S24000, or mixtures thereof, representative compositions and properties of which are shown in Table 1 below.

Table 1

[000027] Referring now to Figure 2, a cross-sectional view of one example of an adsorber vessel according to embodiments disclosed herein is illustrated. The adsorber vessel 10 includes a cylindrical shell 10, an upper head 12, and a lower head 14. Heads 12, 14 may be joined to the shell 10 by welding. The heads 12, 14 may be provided with one or more nozzles 16, 17, also joined by welding, in order to facilitate the selective admission to and withdrawal of gas mixtures from the adsorber vessel 10.

[000028] It has also surprisingly been discovered that welds having a volumetric composition of ferrite lower than 8% do not show a

statistically-significant reduction in fatigue life versus fully-reversed fatigue stress amplitude between tests conducted with and without hydrogen present in the metal. Such welds may be employed both in the steels of the present invention, and in stainless steels having higher concentrations of nickel, to produce a weldment having the hydrogen-resistant properties which are the object of the present invention. The composition of the material used to form the weldments may be the same or similar in composition to that of the stainless steel used to form the portions of the vessel which the weldment connects (e.g., connecting the bottom head to the cylindrical portion, the top head to the cylindrical portion, one or more sections of the cylindrical portion, at least one nozzle to any one of the bottom head, the top head, and the cylindrical portion etc.). In other

embodiments, the weld has a greater concentration of Ni than at least one of the adjoining stainless steel portions of the adsorber vessel. In some embodiments, the weldment may have less than 8% ferrite phase by volume; less than 7% in other embodiments; less than 6% in other embodiments; less than 4% in other embodiments; less than 2% in other embodiments; and substantially free (<1 %) of ferrite phase in yet other embodiments. Examples of materials suitable for use in forming the welds may include one or more of ERNiCrMo-4 (HASTELLOY ^® C-276, generally available), ERNiCr-3 (INCONEL ^® FM-82, generally available), ER316L (S31603), and ER308L (S30803), for example.

[000029] In some embodiments, the deposited weld metal has a mean fatigue life of at least 10 million cycles at a fully reversed stress amplitude of at least 35 ksi. A fully reversed stress amplitude means that the stress amplitude is experienced in alternating modes between tension and compression, with the scalar magnitude being the same in each direction. Such fatigue testing may be performed using an R. R. Moore rotating beam fatigue testing machine (available from INSTRON, Norwood, MA), for example.

[000030] In operation, the vessel 10 may also be provided with a bed support 20 and an adsorbent system 21 . The adsorbent system 21 typically contains at least one porous adsorbent material which as an affinity for the adsorption of impurities from the gas being admitted to and withdrawn from the vessel 10. By way of non-limiting examples, adsorbents are typically selected from at least one of activated alumina, silica gel, activated carbon, molecular sieve zeolites, and molecular sieve titanates, although other types of adsorbents may be employed as suitable for the specific combination of impurities to be removed. The bed support 20 may be active or passive, and may be porous or dense, where the primary role of the bed support 20 is to mechanically support the adsorbent system 21 .

[000031 ] During typical operations, an impure gas mixture containing hydrogen is admitted to the inlet nozzle 17. This impure gas mixture may contain a variety of impurities, such as water, halogens, sour gas constituents, etc.

These constituents pose the risk of localized corrosion should they form aqueous phase deposits inside the vessel 10 or the inlet nozzles 17.

[000032] In embodiments of the adsorber vessels disclosed herein, at least one of nozzle 17 and the lower head 14 may be constructed of alloys that have a greater concentration of nickel and/or molybdenum as compared to the material(s) of construction used for the shell 10 and/or the upper head 12. For example, alloys such as UNS S21900, UNS S31653, UNS S30453, or UNS S31753 may be used when the shell 10 and upper head 12 are made from an alloy having a lower nickel and molybdenum content, such as UNS S20153.

[000033] An inlet portion of the adsorbent system 21 A may be

advantageously provided below the junction between the higher-alloyed components 16 and 14 and the lower-alloyed shell 1 1 . This inlet adsorbent 21 A is advantageously selected to remove at least water vapor from the gas mixture, thus reducing the water vapor pressure below the saturation pressure at the prevailing conditions within the adsorber vessel 10. Additionally, the bed support 20 may be provided with a limited adsorption functionality to remove at least water from the gas mixture. An example of such a partially active bed support is activated alumina. The gas mixture resulting after contact with the inlet adsorbent and/or adsorbent bed support will have a low partial pressure of water and presents a minimized risk of localized corrosion for the low-alloyed components of the vessel 10.

[000034] The adsorber vessel 10 may also be provided with a support means 35. As illustrated in Figure 2, support means 35 is a continuous skirt design. Other designs are possible, such as legs, poured ceramic bases, and many others as known to those skilled in the art.

[000035] The adsorber vessel 10 may have the same or different geometries for the top and bottom heads. In one embodiment, as illustrated in Figure 2, the shape of the bottom head 14 is chosen to possess a greater internal volume than the top head 12. In one embodiment the bottom head 14 is substantially hemispherical. The geometry of the top head 12 may then be chosen to have a lower internal volume, such as an elliptical head. This design advantageously maximizes the volume of the inlet adsorbent zone 21 A. The bottom head 14 can also be provided with an extended cylindrical section 17A that extends beyond the arcuate section of the head. This section 17A may be constructed of the same material as the rest of bottom head 14. In one embodiment, the extension 17A is integral to the head and may be formed without circumferential welding.

EXAMPLES

[000036] Example 1 : A stainless steel of composition S20153 having 4.61 % Ni, 6.73% Mn and 0.147% N was subjected to cyclic fatigue testing using fully-reversed loads in both an R. R. Moore rotating beam fatigue testing machine and a conventional axial testing machine. Samples were tested without any intentionally-added hydrogen, and after intentionally adding hydrogen by subjecting the samples to a flowing stream of heated, purified hydrogen at 300 psig at approximately 200 C for two weeks. Figure 3 shows that the fatigue life of the samples is essentially unaffected by the addition of hydrogen. None of the samples tested at fully-reversing stress amplitudes less than approximately 50 ksi ruptured, i.e. the tests were stopped without failure.

[000037] Example 2: Two pieces of UNS S20153 plate from Example 1 , nominally 3/8 inch thick, were welded using an AWS E316L filler metal having more than 10% nickel using the submerged arc process. The resulting volume percent ferrite was approximately 6.6%. Fatigue testing both with and without intentional addition of hydrogen was conducted using the same test parameters as Example 1 . Figure 4 shows that the fatigue life of the samples is essentially unaffected by the addition of hydrogen. None of the samples tested at fully-reversing stress amplitudes less than approximately 45 ksi ruptured, i.e. the tests were stopped without failure.

[000038] Example 3: Two pieces of UNS S20153 plate from Example 1 , nominally 3/8 inch thick, were welded using an AWS ENiCrMo-4 weld filler metal containing at least 45 wt. % Ni. filler metal using both Gas Tungsten Arc and Submerged Arc processes. The resulting weld was essentially free of ferrite. Fatigue testing both with and without intentional addition of hydrogen was conducted using the same test parameters as Example 1 . Figure 5 shows that the fatigue life of the samples is essentially unaffected by the addition of hydrogen. None of the samples tested at fully-reversing stress amplitudes less than approximately 45 ksi ruptured, i.e. the tests were stopped without failure.

[000039] Example 4: Test coupons were manufactured from a different melt of S20153 material, approximately ¾" thick. The composition of this melt was approximately the same as the melt used for Examples 1 through 3. The welded sample possessed approximately the same chemical characteristics of the weld of Example 3. These samples were subjected to a fatigue crack propagation test under a purified hydrogen pressure of 300 psig. Figure 7 shows that the rate of crack propagation for the welded sample was lower at every stress intensity level than the propagation rate in the plate itself.

[000040] We have discovered that stainless steels having reduced nickel content and the inventive ranges of manganese and/or nitrogen addition exhibit no loss in fatigue life when subjected to conditions which charge the metal with purified hydrogen.

[000041 ] As shown in Figs. 3-5, the inventors have surprisingly discovered that the inventive materials disclosed herein can be effectively used in vessels subject to hydrogen induced fatigue. More specifically, as shown in Figs. 3-5, each of the materials described herein were caused to absorb a large amount of hydrogen. Despite current teachings, it was surprisingly found that hydrogen had no effect on the fatigue properties of the materials.

[000042] As a result of these findings the present materials have several novel advantageous properties over conventionally used materials. For example, a lesser thickness of material may be used as compared to conventional carbon steel and still maintain an equivalent strength. The thinner materials are less stiff than the thicker conventional materials. As a result, the materials have less localized bending stress and a correspondingly lower propensity to form cracks due to fatigue.

[000043] Furthermore, as shown in Fig. 6the inventors have discovered that stainless steel weld deposits having less than 8% volume ferrite exhibit no loss in fatigue life when subjected to conditions which charge the metal with purified hydrogen. As is well known in the art, cracks normally propagate at the weld junction between materials. The lack of crack propagation at the weldment site represents a significant advantage over that of the prior art. More specifically, the welds employed in carbon steel vessels almost exclusively fail in the welds or their adjacent heat affected zones.

[000044] In addition, typical welds are generally structurally heterogeneous, especially in the carbon steel type of vessels. As a result, the weldment appears as a banded material (having a structure analogous to that of wood), and as a result it is extremely difficult to identify small cracks or defects even when using the state of the art high resolution destructive techniques. In contrast, the welds of the present invention are structurally homogeneous, making them both easier to inspect for small defects which might later grow to a large and dangerous size and more resistant the crack growth mechanism itself as a result of their surprising resistance to hydrogen assisted crack propagation.

[000045] As described above, adsorber vessels for use in cyclic hydrogen charging environments may be constructed of stainless steels having less than 10% nickel and the inventive ranges of manganese and/or nitrogen addition, resulting in adsorber vessels that are resistant to hydrogen assisted fatigue cracking, which presents a significant advantage in safety in efficacy compared to the carbon steel vessels of the prior art

[000046] Additionally, welds made according to the volumetric ferrite limitations of the present invention in stainless steel adsorber vessels made either of the inventive stainless steels, or of stainless steels having conventional nickel contents above 10%are resistant to hydrogen assisted fatigue cracking, which presents a significant advantage in safety in efficacy compared to the carbon steel vessels of the prior art.

[000047] Additionally, embodiments disclosed herein may use different materials of construction for portions of the adsorber vessels and/or weldments connecting different portions of the adsorber vessels. The use of different materials of construction may advantageously minimize effects of corrosive gases and/or liquids at the inlet of the vessel while simultaneously increasing the overall hydrogen resistance of the adsorber vessel.

[000048] While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.