OA19834A - Duplex stainless steels and uses thereof. - Google Patents
Duplex stainless steels and uses thereof. Download PDFInfo
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- OA19834A OA19834A OA1202000211 OA19834A OA 19834 A OA19834 A OA 19834A OA 1202000211 OA1202000211 OA 1202000211 OA 19834 A OA19834 A OA 19834A
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- Prior art keywords
- duplex stainless
- stainless steel
- use according
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- urea
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- 229910001220 stainless steel Inorganic materials 0.000 title description 49
- XSQUKJJJFZCRTK-UHFFFAOYSA-N urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 claims abstract description 162
- 239000004202 carbamide Substances 0.000 claims abstract description 81
- 229910001039 duplex stainless steel Inorganic materials 0.000 claims abstract description 53
- 238000004519 manufacturing process Methods 0.000 claims abstract description 44
- 229910052761 rare earth metal Inorganic materials 0.000 claims abstract description 18
- 239000012535 impurity Substances 0.000 claims abstract description 17
- 238000000034 method Methods 0.000 claims description 41
- 239000000203 mixture Substances 0.000 claims description 38
- 229910052759 nickel Inorganic materials 0.000 claims description 37
- 229910052803 cobalt Inorganic materials 0.000 claims description 30
- 229910052750 molybdenum Inorganic materials 0.000 claims description 29
- BVCZEBOGSOYJJT-UHFFFAOYSA-N Ammonium carbamate Chemical compound [NH4+].NC([O-])=O BVCZEBOGSOYJJT-UHFFFAOYSA-N 0.000 claims description 23
- 239000011575 calcium Substances 0.000 claims description 15
- 150000002910 rare earth metals Chemical class 0.000 claims description 15
- QGZKDVFQNNGYKY-UHFFFAOYSA-N ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 claims description 10
- 229910052777 Praseodymium Inorganic materials 0.000 claims description 7
- CURLTUGMZLYLDI-UHFFFAOYSA-N carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims description 7
- 229910002092 carbon dioxide Inorganic materials 0.000 claims description 7
- 229910052746 lanthanum Inorganic materials 0.000 claims description 7
- 229910052721 tungsten Inorganic materials 0.000 claims description 6
- PWHULOQIROXLJO-UHFFFAOYSA-N manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 5
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 claims description 5
- 239000010937 tungsten Substances 0.000 claims description 5
- QCWXUUIWCKQGHC-UHFFFAOYSA-N zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 claims description 5
- 229910052726 zirconium Inorganic materials 0.000 claims description 5
- 239000005864 Sulphur Substances 0.000 claims description 3
- 229910001093 Zr alloy Inorganic materials 0.000 claims description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims description 3
- NINIDFKCEFEMDL-UHFFFAOYSA-N sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 3
- OYPRJOBELJOOCE-UHFFFAOYSA-N calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 claims description 2
- 229910052791 calcium Inorganic materials 0.000 claims description 2
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 claims description 2
- OAICVXFJPJFONN-UHFFFAOYSA-N phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 2
- 239000011574 phosphorus Substances 0.000 claims description 2
- PUDIUYLPXJFUGB-UHFFFAOYSA-N praseodymium Chemical compound [Pr] PUDIUYLPXJFUGB-UHFFFAOYSA-N 0.000 claims description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 69
- 238000005260 corrosion Methods 0.000 description 48
- 239000011651 chromium Substances 0.000 description 20
- 239000010949 copper Substances 0.000 description 16
- IJGRMHOSHXDMSA-UHFFFAOYSA-N nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 16
- 229910000831 Steel Inorganic materials 0.000 description 13
- 239000010959 steel Substances 0.000 description 13
- 239000012071 phase Substances 0.000 description 12
- 239000000243 solution Substances 0.000 description 11
- 229910000529 magnetic ferrite Inorganic materials 0.000 description 10
- 239000000463 material Substances 0.000 description 10
- 229910000859 α-Fe Inorganic materials 0.000 description 10
- 229910052757 nitrogen Inorganic materials 0.000 description 9
- 229910052804 chromium Inorganic materials 0.000 description 8
- VYZAMTAEIAYCRO-UHFFFAOYSA-N chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 7
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 description 7
- 239000010941 cobalt Substances 0.000 description 7
- 229910052760 oxygen Inorganic materials 0.000 description 7
- 239000001301 oxygen Substances 0.000 description 7
- MYMOFIZGZYHOMD-UHFFFAOYSA-N oxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 7
- 229910001566 austenite Inorganic materials 0.000 description 6
- 231100000078 corrosive Toxicity 0.000 description 6
- 231100001010 corrosive Toxicity 0.000 description 6
- 230000000694 effects Effects 0.000 description 6
- ZOKXTWBITQBERF-UHFFFAOYSA-N molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 6
- 239000011733 molybdenum Substances 0.000 description 6
- 229910052684 Cerium Inorganic materials 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 5
- KXDHJXZQYSOELW-UHFFFAOYSA-M carbamate Chemical compound NC([O-])=O KXDHJXZQYSOELW-UHFFFAOYSA-M 0.000 description 5
- 238000003786 synthesis reaction Methods 0.000 description 5
- 230000002194 synthesizing Effects 0.000 description 5
- 229910052799 carbon Inorganic materials 0.000 description 4
- 239000001569 carbon dioxide Substances 0.000 description 4
- 239000012530 fluid Substances 0.000 description 4
- 239000007789 gas Substances 0.000 description 4
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 4
- 229910052710 silicon Inorganic materials 0.000 description 4
- 239000010703 silicon Substances 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 3
- 239000003795 chemical substances by application Substances 0.000 description 3
- RYGMFSIKBFXOCR-UHFFFAOYSA-N copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 3
- 229910052802 copper Inorganic materials 0.000 description 3
- 230000001627 detrimental Effects 0.000 description 3
- 230000001965 increased Effects 0.000 description 3
- 229940064770 Zmax Drugs 0.000 description 2
- REDXJYDRNCIFBQ-UHFFFAOYSA-N aluminium(3+) Chemical class [Al+3] REDXJYDRNCIFBQ-UHFFFAOYSA-N 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 230000002708 enhancing Effects 0.000 description 2
- 238000002474 experimental method Methods 0.000 description 2
- 230000002349 favourable Effects 0.000 description 2
- 238000007654 immersion Methods 0.000 description 2
- 239000007791 liquid phase Substances 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 238000002161 passivation Methods 0.000 description 2
- 239000010935 stainless steel Substances 0.000 description 2
- 239000012085 test solution Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 101710028400 Alyref2 Proteins 0.000 description 1
- 230000035839 C max Effects 0.000 description 1
- 206010037660 Pyrexia Diseases 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- -1 compound ammonium carbamate Chemical class 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000001808 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 230000000881 depressing Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000001747 exhibiting Effects 0.000 description 1
- 239000002360 explosive Substances 0.000 description 1
- 238000009472 formulation Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009776 industrial production Methods 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000006011 modification reaction Methods 0.000 description 1
- 239000003921 oil Substances 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
Abstract
It is provided a duplex stainless steel for use in a urea production plant and/or in a urea production process, containing in weight percentage (%w): C 0,03 or less; Si 0,5 or less; Mn 2,5 or less; Cr from more than 30,0 to 35,0; Ni from 5,5 to 8,0; Co from 0,01 to 0,8; Mo from 2,0 to 2,5; W 2,5 or less; N from 0,3 to 0,6; Cu 1,0 or less; and having one or more of: Ca 0,0040 or less; Mg 0,0040 or less; one or more rare-earth elements in a total amount of 0,1 or less; the balance being Fe and impurities; and satisfying the relationship: Z = 1,062 (Ni+Co) + 4,185 Mo is between 14,95 and 19,80.
Description
DUPLEX STAINLESS STEELS AND USES THEREOF
CROSS-REFERENCE TO RELATED APPLICATIONS
This Patent application claims priority from European Patent Application No. 17210463.0 filed on December 22, 2017, the disclosure of which is incorporated by reference.
FIELD OF THE INVENTION
The présent invention relates to the use of a duplex stainless steel in highly corrosive urea environments containing ammonia carbamate at high températures and pressures.
The invention thus also relates to the use of a duplex stainless steel in a urea plant (i.e. plant for production of urea), and specifically in an apparatus, equipment or device (or a part thereof) which is exposed to concentrated ammonium carbamate at high température.
The invention also relates to an apparatus, equipment or device of a urea production plant or used in a urea production process, comprising at least a part made of a corrosion résistant duplex stainless steel.
The invention also relates to a plant and a process for the production of urea comprising at least one apparatus, equipment or device having at least a part made of a duplex stainless steel, and to a method of revamping an existing urea production plant by replacing at least a part of an apparatus, equipment or device of the plant with a part made of a duplex stainless steel. DESCRIPTION OF PRIOR ART
Duplex stainless steels are a family of stainless steels characterized by a two-phase microstructure consisting of grains of austenite and ferrite in roughly equal proportions.
The austenite-ferrite structure gives this family of stainless steels a combination of favorable properties, in particular good mechanical strength and excellent résistance to corrosion.
However, commonly available grades of duplex stainless steels, even if generally exhibiting a good corrosion résistance, are not suitable for use under very severe conditions, such as in a urea production plant and specifically in a high pressure section of a urea plant.
As it is known, urea production is based on a hightemperature, high-pressure reaction of carbon dioxide and ammonia to form ammonium carbamate, and a subséquent déhydration reaction of the ammonium carbamate to form urea and water.
In a typical urea production plant (urea plant), these processes are generally carried out in a urea synthesis reactor operating at high pressure and high température; the aqueous urea solution produced in the synthesis reactor is then progressively concentrated, with recover of unconverted reagents, in one or more recovery sections, for example in a high-pressure section, a medium-pressure section and a low-pressure section; finally, the urea is solidified in a finishing section, which normally includes a granulator or a prilling tower^ ।
Industrial level processes and plants for the production of urea are today largely based on stripping processes: the synthesis solution exiting from the reactor is subjected to heating at high pressure (substantially the same pressure of the reactor) and the ammonium carbamate décomposés into ammonia and carbon dioxide in the liquid phase; part of the ammonia, together with carbon dioxide, passes from the liquid phase to the gas phase. The gas phase collected from the stripper is condensed and recycled to the reactor.
In some industrial processes, ammonia is used as a stripping agent (ammonia-stripping process), or the stripping is performed only by supplying heat, without any stripping agent (self-stripping process, or thermal stripping process).
In other industrial processes, such as the so-called CO2-stripping process, the stripping agent is gaseous carbon dioxide.
In a urea synthesis plant operating according to the ammonia-stripping process or the self-stripping process, corrosion résistance is an essential feature.
In particular, the ammonia-stripping process and the self-stripping process hâve a high pressure section, basically comprising the urea synthesis reactor and the urea stripper (as well as auxiliary equipment and devices) , where the corrosion résistance is most important, due to the presence of the intermediate compound ammonium carbamate solution.
The ammonia-stripping process and the self-stripping process are in fact preferably performed at a maximum température of 185°C or higher (more preferably at 190°C or higher, in particular at 205°C or higher and preferably in the range 205-215°C) ; at a maximum pressure of 150 bar or higher (preferably of 156 bar or higher and more preferably of about 160 bar or higher); and with a NH3/CO2 molar ratio (so-called N/C ratio) in the range 3.2-3.6.
For example, stripping processes of the type described just above, operating at such conditions, are used in the so-called Snamprogetti Urea Technology, which is well known to the skilled person being widely used worldwide and often cited in technical texts and papers.
Thus, at least some apparatus, equipment or device of the urea plant, in particular of the high pressure section thereof, such as (but not only) the urea stripper, operate under processing conditions which are highly corrosive, particularly due to the presence of a hot and concentrated carbamate solution at high températures (185°-205°C and over) and pressures (150 bar or higher).
Similar problems are however also présent in other kinds of urea production plants also having a high pressure section.
Therefore, the high pressure section of a urea plant (in particular, but not only, in a urea plant operating according to the ammonia-stripping process or the selfstripping process) usually requires addition of a certain amount of oxygen (typically in form of a stream of inerts also including oxygen) for passivating the métal surfaces (especially, but not only, if made of austenitic stainless steels) . Use of oxygen in the high pressure section can however increase the risk to originate potentially explosive mixture and therefore there is a concern in terms of safety.
In order to reduce the use of passivation gas streams and/or to improve corrosion résistance, duplex stainless steels hâve been proposed for use in urea production plants.
For example, WO95/00674 discloses the use of a particular duplex stainless steel, the so called super duplex 30 stainless steel sold under the trademark Safurex®, for making some equipment of urea plants.
However, the super duplex stainless steels of WO95/00674, when used in a carbamate environment, may be .not fully effective at very high températures (higher than 180200°C), such as common operation températures of ammonia5 stripping or self-stripping processes. Use of known duplex stainless steels is thus confined to C02—stripping processes.
W02014/180761 discloses a shell-and-tube urea stripper, to be specifically used in an ammonia-stripping or self10 stripping process, having a bundle of tubes made of certain duplex stainless steels, namely of the Safurex® steel 29Cr— 6.5Ni-2Mo-N (ASME Code 2295-3 and UNS S32906), or the DP28W™ steel 27Cr-7.6Ni-lMo-2.3W-N (ASME Code 2496-1 and UNS S32808).
Also WO2017013180A1, WO2017013181A1 and WO2017014632A1 disclose duplex stainless steels generally suggested for use in urea plants under high température and high pressure conditions.
It can be appreciated that ail the prior art documents 20 cited above disclose duplex stainless steels which do not contain cobalt.
WO2006/049572 discloses a duplex stainless steel alloy which contains also cobalt and shows high strength, good corrosion résistance, good workability and which is 25 weldable. The proposed alloys are intended for use in the onshore and offshore sectors of the oil and gas industry, while uses under more severe corrosive conditions (such as in a urea plant/process) are not mentioned.
Therefore, even if duplex stainless steels are known 30 which hâve good corrosion résistance and are allegedly suitable for use also in a urea production plant, there is still a need for other, possibly more corrosion résistant duplex stainless steels which are suitable for use in any urea environment s, i.e. in any kind of urea production plants/processes, and specifically in an apparatus operated at high températures in contact with very corrosive fluids (containing ammonium carbamate) and also under oxygen-free conditions, such as for instance (but not only) the high pressure strippers (operated at pressure of 150 bar and more) used in an ammonia-stripping process or a self-stripping process.
DISCLOSURE OF THE INVENTION
Accordingly, it is an object of the présent invention to provide a duplex stainless steel suitable for overcoming the problem foregoing described of the prior art.
In particular, it is an object of the présent invention to provide duplex stainless steels which are specifically and fully suitable to be used in a urea environment, i.e. in contact with a fluid comprising ammonium carbamate, such as a concentrated ammonium carbamate solution, and also at températures of at least 185°C, preferably of at least 190°C and more preferably of 205°C and more, even under oxygenfree conditions.
It is also a spécifie object of the invention to provide corrosion résistant duplex stainless steels which are suitable for use in any urea environments, i.e. in any kind of urea production plants/processes, and specifically in an apparatus (such as a high pressure stripper) used in an ammonia-stripping process or a self-stripping process, and thus operating at a maximum température of 185°C or higher (preferably at 190°C or higher, in particular at 205°C or higher and preferably in the range 205-215°C); and/or at a maximum pressure of 150 bar or higher (preferably of 156 bar or higher and more preferably of about 160 bar or higher); and/or with a NH3/CO2 molar ratio (so-called N/C ratio) in the range 3.2-3.6.
The présent invention accordingly relates to a duplex stainless Steel for use in a urea production plant and/or in a urea production process, as defined in Claim 1.
The invention also relates to an apparatus, equipment or device, in particular of a urea production plant or used 10 in a urea production process, comprising at least a part made of a corrosion résistant duplex stainless steel, as defined in Claim 28.
The invention also relates to a plant and a process for the production of urea comprising at least one such 15 apparatus, equipment or device having at least a part made of a duplex stainless steel as defined in Claims 29 and 30 respectively; and to a method of revamping an existing urea production plant by replacing at least a part of an apparatus, equipment or device of the plant with a part made 20 of a duplex stainless steel as defined in Claim 31.
Advantageous preferred features of the invention are the subject matter of the dépendent claims.
The duplex stainless steels of the invention are specifically characterized by a combination of Ni, Co and 25 Mo: in fact, it has been recognized that such three éléments, used together according to spécifie composition rules, hâve an unexpected combined effect on the corrosion résistance as well as on other favorable material properties.
In fact, it has been found that these three éléments 30 (Ni, Co, Mo) effectively increase corrosion résistance of a duplex stainless steel (having the particular composition of the invention) if each element is used in a spécifie content range and the contents of the three éléments are linked to one another by a composition parameter Z which ranges between a minimum value Zmin and a maximum value Zmax.
In particular, the duplex stainless steels of the invention hâve a composition parameter Z ranging between 14,95 and 19,80, preferably between 14,95 and 19,00, more preferably between 14,95 and 18,00, even more preferably between 14,95 and 17,50.
Composition parameter Z is a parameter représentative of the combined contents of Ni, Co, Mo and defined by formula (I) :
Z = 1,062 (Ni+Co) + 4,185 Mo (I) where Ni, Co, Mo indicate the weight percentage of Ni, Co, 15 Mo respectively.
According to the invention:
14,95 Z 19,80
In other words, the inventors hâve found that duplex stainless steels having the particular compositions of the 20 invention also exhibit an excellent corrosion résistance (in particular, in urea environments) if parameter Z is maintained in the ranges defined above, i.e. if components Ni, Co and Mo are used in amounts which satisfy formula (II) :
Zmin [1,062 (Ni+Co) + 4,185 Mo] £ Zraax (II) where: ।
Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively
Zmin = 14,95
Zmax = 19,80
Experimental test confirm that duplex stainless steels according to the invention, i.e. having a combined content of Ni, Co and Mo as previously defined, satisfying formula (II) , hâve a corrosion rate in urea environment s (containing ammonium carbamate) significantly lower than prior art materials, even at high temperature/pressure and in oxygenfree conditions.
Such a resuit cannot be expected in view of the prior art teachings.
It is in fact commonly recognized in the art (as reported by several scientific papers) that the content of nickel (Ni) in austenitic steels is detrimental under low oxygen condition.
Therefore, it is commonly understood that corrosion résistance of duplex stainless steels take advantage from a low content of nickel.
On the contrary, the inventors of the présent invention hâve recognized that a certain amount of nickel, lower than in usual austenitic steels but higher than a minimum threshold, has indeed a good impact on corrosion résistance of a duplex stainless Steel, if nickel is associated with cobalt (Co) and molybdenum (Mo) according to spécifie rules.
Specifically, the duplex stainless steels of the invention hâve a content of nickel ranging between 5,5% and 8%, preferably from 6,0% to 7,5% (here and below, ail percentages are intended, if not otherwise specified, as weight percentages with respect to the total weight of the Steel).
Nickel is in fact an austenite forming element and a certain amount of nickel is needed to maintain an equilibrium between ferrite and austenite phases. From the other hand, nickel has a négative impact on intermetallic précipitation.
According to the invention, cobalt is used in combination with nickel (and replacing part of the nickel) to obtain the required balance between ferrite and austenite phases and to improve corrosion résistance.
The inventors of the présent invention hâve in fact 5 realized that the content of nickel can be reduced by replacing nickel with cobalt, that works as a partial substitute and surprisingly also has the additional advantage of improving the corrosion résistance of the duplex stainless steels having the particular compositions of the 10 invention.
Cobalt, in fact (unlike nickel), reduces the précipitations of intermetallic phases, strengthens the ferrite matrix and has a positive effect as austenite forming element.
Specifically, the duplex stainless steels of the invention hâve a content of Co in the range between 0.01% and 0,8%, preferably from 0,01% to 0,6%, more preferably from 0,02 to 0,6%, in particular from 0,04% to 0,6%.
According to the invention, the contents of nickel and 20 cobalt is also linked to the content of molybdenum.
Molybdenum is a ferrite forming element which accelerates the précipitation of intermetallic phases especially in the presence of high levels of chromium (such as in the duplex stainless steels of the invention); 25 therefore, the content of molybdenum should not exceed a maximum threshold.
On the other hand, a certain amount of molybdenum is bénéficiai for ammonium carbamate corrosion résistance and localised corrosion résistance, especially in the presence 30 of ammonium carbamate and under oxygen-free conditions.
Specifically, molybdenum is in the range between 2% and
2,5%. Preferably, the content of Mo is maintained between 2,0% and 2,4%, in particular between 2,0% and 2,3%.
The features of the invention as previously defined also provides a method to design a duplex stainless Steel for use in very corrosive environments, in particular in a urea plant/process.
In particular, the invention provides the rules for selecting an effective content of Ni, Co, Mo.
Once selected the content/amount of two out of the three components (Ni, Co, Mo), for example by taking into account the above technical considérations about expected effects of each individual éléments, the content/amount of the third component is calculated by applying the relationships of the invention.
In addition to Ni, Co and Mo, the duplex stainless steels of the invention hâve a relatively high content of chromium (Cr), which increases corrosion résistance in ammonium carbamate solution environments, and at the same time allows a good microstructure without précipitation of third phases and a good hot workability.
Chromium has in fact a bénéficiai effect on corrosion résistance and allows higher process températures in urea production applications. Chromium is also bénéficiai for other types of corrosion such as pitting or crevice. On the other hand, high amounts of chromium increase the possibility of précipitation of intermetallic phases and are detrimental to hot workability. Therefore, the amount of chromium is higher than 30% but lower than 35%, preferably ranging between 30,5 and 35%, more preferably between 30,5 and 33%, even more preferably between 30,5 and 32%, in particular between 30,5 and 31,6%.
The duplex stainless steel of the invention may also contain the following éléments:
Carbon (C) - Carbon generally improves mechanical strength; however, according to the invention high contents 5 of carbon are avoided in order to prevent précipitation of carbides. Therefore, the amount of carbon is not higher than 0,03%, preferably from 0,001% to 0,03%, more preferably from 0,001% to 0,02%.
Silicon (Si). Silicon is used as a ferrite forming 10 element and for deoxidization in the steel mill, i.e. in the manufacturing process of the duplex stainless steels. High amounts of Silicon are avoided in order to reduce the possibility of précipitation of intermetallic phases. Thus the amount of Silicon is not higher than 0,5%, preferably 15 from 0,001% to 0,5%.
Manganèse (Mn). Manganèse increases the solubility of nitrogen (N), but has also a négative impact on corrosion résistance. Therefore, the amount of manganèse is not higher than 2,5%, preferably from 0,001% to 2,5%, more preferably 20 from 0,5% to 2,2%, in particular from 1,0% to 2,2%.
Tungsten (W) . Tungsten is a ferrite forming element. Tungsten also enhances general corrosion résistance. In particular, in the same way as Cr, Mo and N, also W increases pitting and crevice résistance. However, W accelerates the 25 précipitation of intermetallic phases। so its content is maintained below 2,5%, preferably from 0,001% to 2,5%, more preferably from 0,02% to 1%.
Nitrogen (N). Nitrogen is an austenite forming element. Nitrogen also enhances the microstructure stability delaying 30 the précipitation of intermetallic phases and increases the strength of the métal matrix. Nitrogen is added also to increase the pitting and crevice corrosion résistance. For these reasons, at least 0,3% of nitrogen is used. On the other hand, higher contents of nitrogen would lead to poor hot workability, therefore the maximum value of N content is 0,6%. Thus, the content of N ranges from 0,3 to 0,6%, preferably from 0,35% to 0,6%, in particular from 0,4% to 0,6%.
Copper (Cu). Copper has in general a positive effect depressing the intermetallic précipitation kinetics, especially when relatively high amounts of Mo and W are présent. However, for urea production applications copper is a harmful element because it forms complex ions with ammonia and deteriorates corrosion résistance. Therefore, Cu content is limited to a maximum of 1%, preferably from 0,001% to 1%, preferably from 0,001% to 0,9%, more preferably from 0,001% to 0,5%, even more preferably from 0,10 to 0,45% and in particular from 0,10 to 0,40%.
Since the duplex stainless steels of the invention hâve a relatively high content of chromium (as well as nitrogen), hot workability could be negatively affected. In order to facilitate processing (in particular, hot forming) of the duplex stainless steels of the invention, one or more of the following éléments are optionally added:
Calcium (Ca): 0,004% or less, preferably from 0,001% to 0,004%f ।
Magnésium (Mg): 0,004% or less, preferably from 0,001% to 0,004%;
One or more rare-earth éléments: 0,1% or less, preferably 0,05% or less (total amount).
Preferably, the rare-earth éléments are selected in the group consisting of Lanthanum (La), Cérium (Ce),
Praseodymium (Pr) and mixtures thereof- .
Rare-earth éléments (metals) bave very high deoxidation and desulphurization capacities and also decrease the average size of inclusions- They hâve a bénéficiai effect on hot workability based on the ability to combine with impurities that can segregate at grain boundaries (such as sulphur) and modify the shape and composition of the inclusions.
The steel compositions of the invention may also include unavoidable impurities such as Phosphorus (P) and Sulphur (S). The content of P and S should however be maintained as low as possible. In particular, high amounts of S are detrimental to hot workability- Thus, the S content should be less than 0,005% and the P content should be less than 0,025%. Typical amounts are less than 0,0005% for S and less than 0,020% for P.
The ferrite content of the duplex Steel (austenoferritic alloy) according to the présent invention is also of some importance for the corrosion résistance. According to some embodiments, therefore, the ferrite content ranges from 30% to 70% by volume, preferably from 35 to 60%vol., more preferably from 40 to 60%vol. The duplex stainless steels of the invention are suitably résistant to corrosion even when exposed to ammonium carbamate at high pressure (in particular, at a maximum pressure of 150 bar and higher, preferably of 156 bar and higher, more preferably of 160 bar and higher) and high température (in particular 185°C and higher, preferably 190°C and higher, more preferably 205°C and higher), and even in oxygen-free condition.
The invention thus provides improved formulations of duplex stainless steels, fully suitable for use in very corrosive conditions such as in a urea environment, i.e. in contact with a fluid comprising ammonium carbamate, also at températures of 185°C and more (and even at 205°C and more) and even under oxygen-free conditions In particular, the duplex stainless steels of the invention are intended for use in contact with ammonium carbamate solutions having a concentration of ammonium carbamate ranging from 15%w to 95%w, in particular from 50%w to 95%w; and/or at a température of 185°C or more, in particular of 190 °C or more, in particular of 205°C or more).
The highly corrosion résistant duplex stainless steels of the invention are suitable for use in any urea environments, i.e. in any kind of urea production plants/processes, and specifically in apparatuses operated at high températures (185°C, 190°C but also 205°C and higher) in contact with fluids containing ammonium carbamate and also under oxygen-free conditions, such as for instance (but not only) the high pressure strippers used in the ammoniastripping process or the self-stripping process.
Thus, the duplex stainless steels of the invention are especially useful for manufacturing equipment and devices (or parts thereof) which are exposed to concentrated ammonium carbamate at high température, such as parts of the heat exchanger tubes and/or, or for example, tubes of strippers.
The duplex stainless steels of the invention exhibit an excellent corrosion résistance in carbamate solutions (even in oxygen-free condition) also at température of 205°C and higher.
The materials of the invention are therefore suitable to be used in a urea production plant of any kinds, including in particular the most demanding conditions of an ammonia stripping or self-stripping process.
The invention thus relates to the use of the duplex stainless steel as disclosed herein in a urea production plant, and specifically in an apparatus, equipment or device (or a part thereof) which is exposed to concentrated ammonium carbamate at high température.
The invention also relates to an apparatus, equipment or device, in particular of a urea production plant or used in a urea production process, comprising at least a part made of a corrosion résistant duplex stainless steel as disclosed herein.
The invention also relates to a plant and a process for the production of urea comprising at least one apparatus, equipment or device having at least a part made of a duplex stainless steel as disclosed herein; and to a method of revamping an existing urea production plant by replacing at least a part of an apparatus, equipment or device of the plant with a part made of a duplex stainless steel as disclosed herein.
As a resuit of the spécifie compositions of the duplex stainless steels of the invention, the following additional advantages are also achieved over the prior art, in particular in case of use in a high pressure apparatus of the urea plant:
- the corrosion rate in an piece of equipment (apparatus/device or part thereof) made of the duplex stainless steels of the invention drastically decreases with respect to a piece of equipment made of prior art materials;
- the need of passivation air is drastically reduced or even eliminated;
- the thickness of the apparatus/device, in particular of the high pressure piping loop, can be reduced, thus resulting in a significant réduction of the total weight and cost of the high pressure section, since the duplex stainless steels of the invention also hâve high mechanical characteristics;
- the température at the bottom of the stripper can be increased without increasing the corrosion rate;
- it is possible to avoid using, for the high pressure equipment, different materials with different features and prescription in terms of material spécifications.
Under particularly severe operation conditions, such as in high pressure strippers used in the ammonia-stripping process or the self-stripping process, corrosion résistance of the duplex stainless steels according to the invention can be further increased by coupling the steel with a covering layer made of Zirconium or a Zirconium alloy.
Suitable zirconium materials for such a kind of lining are disclosed, for example, in GB2157687A, EP2310792A1, EP2427711A2. Therefore, in some embodiments, the duplex stainless steel of the invention is provided with a covering layer made of Zirconium or a Zirconium alloy which covers at least a surface portion of the duplex stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
- figure 1 contains a table (Table 1) reporting the composition of exemplary samples of duplex stainless steels according to the invention, as well as of some reference samples;
- figure 2 contains a table (Table 2) reporting the results of corrosion résistance tests performed on the samples of Table 1. .
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Duplex stainless steels according to the invention contain in weight % (%w):
C max 0,03 ·
Si max 0,5
Mn max 2,5
Cr from more than 30,0 to 35,0
Ni 5,5 to 8,0
Co 0,01 to 0,8
Mo 2,0 to 2,5
W max 2,5
N 0,3 to 0,6
Cu max 1,0 and has one or more of:
Ca max 0,0040
Mg max 0,0040 one or more rare-earth éléments max 0,1 the balance being Fe and impurities (as commonly understood, impurities are ail those éléments and compounds which are not purposively added to the Steel formulation, but are however présent in small amounts being contained in | the raw materials used for manufactuririig the duplex stainless Steel).
The duplex stainless steels of the invention are further characterized in that the content of Ni, Co, Mo is such that:
Zmin £ [1,062 (Ni+Co) + 4,185 Mo] £ Zmax (II) where:
Ni, Co, Mo indicate the weight per cent âge of Ni, Co, Mo respectively;
Zmin = 14, 95;
ΖΜχ = 19,80.
In other words, the duplex stainless steels of the invention hâve a composition parameter Z, représentative of the combined contents of Ni, Co, Mo and defined by formula (I) :
Z = 1,062 (Ni+Co) + 4,185 Mo (I) where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
and wherein
14,95 Z 19,80.
In preferred embodiments, the duplex stainless steels according to the invention contain in weight % (%w):
C 0,001 to 0,03
Si 0,001 to 0,5
Mn 0,001 to 2,5
Cr from more than 30,0 to 35,0
Ni 5,5 to 8,0
Co 0,01 to 0,8
Mo 2,0 to 2,5
W 0,001 to 2,5
N 0,3 to 0,6
Cu 0,001 to 1,0 and has one or more of:
Ca max 0,0040
Mg max 0,0040 one or more rare-earth éléments, in particular selected in the group consisting of La, Ce, Pr and mixture thereof, in a total amount of max 0,1 the balance being Fe and impurities;
and wherein the content of Ni, Co, Mo is such that:
Zmin [1,062 (Ni+Co) + 4,185 Mo] £ Ζ^χ (II) where:
Zmin = 14,95;
ZMx = 19,80.
According to the invention, the composition parameter Z as above defined ranges between 14,95 and 19,80, preferably between 14,95 and 19,00, more preferably between 14,95 and 18,00, more preferably between 14,95 and 17,50.
Examples
Exemplary steel compositions according to the invention comprise, in percentages by weight:
| C: | 0,03% or | less; | |
| Si: | 0,5% | or | less; |
| Mn: | 2,5% | or | less; |
| Cr: | 30,5 | % to | , 35%; |
| Ni: | 5,5% | to | 8%; |
| Mo: | 2% to 2, | 5%; |
W: 0,02% to 1,0%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or mjore of the following: |
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth éléments in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Other embodiments of the Steel of the invention comprise, in percentages by weight:
C: 0,001% to 0,03%;
Si: 0,001% to 0,5%;
Mn: 0,001% to 2,5%;
Cr: more than 30% to 35%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,4% to 0,8%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 0,001% to 1%;
one or more of the following:
Ca: 0,001% to 0,004%;
Mg: 0,001% to 0,004%;
one or more rare earth éléments in a total amount of 0,001% to 0,1%;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Other compositions according to the invention comprise, in percentages by weight:
C: 0,001% to 0,03%; ।
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 34%; .
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth éléments in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Yet other compositions according to the invention comprises, in percentages by weight:
C: 0,02% or less;
Si: 0,001% to 0,5%;
Mn: 2,5% or less;
Cr: 30,5% to 32%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,1% to 1%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 0,15% to 0,25%;
having one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
La, Ce, Pr or other rare earth éléments: 0,05% or less the remainder being Fe and unavoidable impurities;
satisfying the relationship CRC = 1,062* (Ni + Co) + 4,185*Mo is between 14,95 to 19,80.
Other compositions according to the invention comprises, in percentages by weight:
C: 0,03¾ or less;
Si: 0,5% or less;
Mn: 0,001% to 2,2%;
Cr: 31% to 35%;
Ni: 6% to 7,5%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,01% to 0,8%;
N: 0,4% to 0,6%;
Cu: 0,9% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth éléments in a total amount of 0,05% or less; _ the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Other examples according to this invention comprise, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 35%;
Ni: 5,5% to 6,5%; .
Mo: 2% to 2,5%;
W: 0,001% to 2,5%;
Co: 0,01% to 0,6%;
N: 0,35% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one rare earth element selected from La, Ce, Pr or a combination thereof: 0,05% or less the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
For example, the présent invention relates to elementary steel compositions that comprise, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 2,2% or less;
Cr: 31% to 32%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,02% to 0,4%;
N: 0,3% to 0,6%;
Cu: 0,001% to 1%;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one rare earth element selected from La, Ce, Pr or a combination thereof: 0,05% or less the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Other exemplifying composition according to the invention comprise, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 2% or less;
Cr: 30,5% to 33%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,2% to 1%;
Co: 0,02% to 0,4%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,001% to 0,004%;
Mg: 0,001% to 0,004%;
La, Ce, Pr or other rare earth éléments: 0,001% to 0,05% the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062* (Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
Further example compositions according to the invention comprise, in percentages by weight:
C: 0,02% or less;
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 34%;
Ni: 5,5 to 8%;
Mo: 2 to 2,5%; ।
W: 0,02 to 1%;
Co: 0,02 to 0,6%;
N: 0,3 to 0,6%;
Cu: 0,20% to 0,9%;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth éléments in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062*(Ni + Co) + 4,185*Mo is between 14,95 and 19,80.
In particular, duplex stainless steels having the compositions in Table 1 were prepared and tested (in Table 1, some components are not indicated, being however in the amounts as previously disclosed).
The samples were prepared as common in the field and tested according to standard testing procedure. Samples Al to A5 were prepared by using laboratory produced materials, while sample B1 was prepared by using material from an industrial production.
In particular, corrosion tests were performed in a high pressure autoclave in ammonium carbamate solution at high pressure and high température (conditions représentative of typical operation conditions in urea plants, in particular in the tubes of a urea stripper).
In particular, the corrosion résistance of the duplex stainless steels of the invention was tested in an oxygenfree carbamate solution, having a composition simulating the worst conditions normally occurring in the tubes of a high pressure section urea stripper of a urea plant, and at a température of 208°C.
In more detail: the corrosion behavior of the laboratory heats was checked via immersion tests that were conducted in a 5-liter Zirconium autoclave. The autoclave was equipped with adéquate feed and discharge lines and a stirrer. The test solution contained a mixture of urea, ammonia and water, at concentrations similar to those of the urea synthèses process- Température and pressure for the experiments were set in the upper level of the typical ranges measured in a urea stripper, 180-210°C and 140-200 bar, respectively- The test solution was degassed before starting the tests to eliminate oxygen from the system. These experiments were designed to simulate the most severe conditions in a stripper of a urea plant without oxygen injection; note that under current working conditions in a urea plant, the stainless steel would perform even better, due to the presence of low amounts of oxygen and less aggressive conditions.
Test duration was 13 and 30 days. ASTM G31 (Standard Practice for Laboratory Immersion Corrosion Testing of Metals) standard indications were followed for test specimen préparation and the corrosion rate was measured by the gravimétrie method.
After exposures of 13 days and 30 days respectively in the oxygen-free carbamate solution, the corrosion résistance was evaluated by calculating the corrosion rate (expressed in mm/year) .
The results are shown in Table 2.
The results confirm that the samples (A1-A5; Bl) made of a duplex stainless steel according to the invention, i.e. satisfying the composition requirements of the invention (in particular with respect to the combined content of Ni, Co, Mo) , hâve a corrosion rate significantly lower than comparative samples Refl, Ref2, Ref3 and thus a better corrosion résistance.
In fact, the experimental tests confirm that when Z satisfies the requirement: 14.95 Z 19.80, corrosion values are significantly lower than those exhibited by reference materials.
Corrosion values would be even significantly lower in working conditions in a urea plant, since the experimental set-up conditions are much more aggressive.
Finally, although the invention has been disclosed in relation to the above-mentioned preferred embodiments, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the appended claims.
Claims (10)
1. Use of a duplex stainless steel in a urea production plant and/or in a urea production process, wherein the duplex stainless steel contains in weight percentage (%w) :
C 0,03 or less
Si 0,5 or less
Mn 2,5 or less
Cr from more than 30,0 to 35,0
Ni from 5,5 to 8,0
Co from 0,01 to 0,8
Mo from 2,0 to 2,5
W 2,5 or less
N from 0,3 to 0,6
Cu 1,0 or less having one or more of the following:
Ca 0,0040 or less
Mg 0,0040 or less one or more rare-earth éléments in a total amount of 0,1 or less;
the balance being Fe and impurities;
wherein the duplex stainless steel has a composition parameter (Z), représentative of the combined contents of Ni, Co, Mo and defined by formula (I):
Z = 1,062 (Ni+Co) + 4,185 Mo (I) where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
said composition parameter (Z) ranging between 14,95 and 19,80.
5
2. The use according to claim 1, wherein the duplex stainless steel contains 0,001—0,02%w C.
3. The use according to claim 1 or 2, wherein the duplex stainless steel contains 30,5—35%w Cr.
4. The use according to one of the preceding claims, 10 wherein the duplex stainless steel contains 30,5-32%w Cr.
5. The use according to one of the preceding claims, wherein the duplex stainless steel contains 30,5-31,6%w Cr.
6. The use according to one of the preceding claims, wherein the duplex stainless steel contains 0,001-0,9%w Cu.
15
7. The use according to one of the preceding claims, wherein the duplex stainless steel contains 0,10-0,90%w Cu or 0,10-0,40%w Cu.
8. The use according to one of the preceding claims, wherein the duplex stainless steel contains 0,02-0,6%w Co.
20
9. The use according to one of the preceding claims, wherein the duplex stainless steel contains 6,0-7,5%w Ni.
10. The use according to one of the preceding claims, wherein the duplex stainless steel contains Manganèse from 0,5 to 2,5%w.
25
11. The 'use according to one of the preceding âlaims, wherein the duplex stainless steel contains Manganèse from 0,5 to 2,2%w.
12. The use according to one of the preceding claims, wherein the duplex stainless steel contains Tungsten from 30 0,001 to 2,5%w.
13. The use according to one of the preceding claims, wherein the duplex stainless steel contains Tungsten from 0,02 to l,0%w.
14. The use according to one of the preceding claims, wherein the duplex stainless steel contains Calcium from 0,001 to 0,004%w.
15. The use according to one of the preceding claims, wherein the duplex stainless steel contains Magnésium (Mg) from 0,001 to 0,004%w.
16. The use according to one of the preceding claims, / wherein the duplex stainless steel contains one or more rareearth éléments in a total amount of 0,05 or less.
17. The use according to one of the preceding claims, wherein the duplex stainless steel contains one or more rareearth éléments selected in the group consisting of Lanthanum (La), Cérium (Ce), Praseodymium (Pr) and mixtures thereof.
18. The use according to one of the preceding claims, wherein the duplex stainless Steel contains, as impurities, no more than 0,025%w Phosphorus (P) and/or no more than 0,005%w Sulphur (S).
19. The use according to one of the preceding claims, wherein the duplex stainless steel is in contact with an ammonium carbamate solution having a concentration of ammonium carbamate ranging from 15%w to 95%w, in particular from 50%w to 95%w.
20. The use according to one of the preceding claims, wherein the duplex stainless steel is in contact with an ammonium carbamate solution at a température of 185°C or higher, preferably of 190°C or higher, more preferably of 205°C or higher.
21. The use according to one of the preceding claims,
wherein the duplex stainless steel is used at a maximum température of 185°C or higher, preferably of 190°C or higher, more preferably of 205°C or higher, more preferably in the range 205-215°C; and/or at a maximum pressure of 150 5 bar or higher, preferably of 156 bar or higher and more preferably of 160 bar or higher; and/or in an environment having a NH3/CO2 molar ratio in the range 3,2-3,6.
22. The use according to one of the preceding claims, wherein the urea environment is under oxygen-free 10 conditions.
23. The use according to one of the preceding claims, wherein the duplex stainless steel is used in a high pressure section of a urea plant.
24. The use according to one of the preceding claims, 15 wherein the duplex stainless steel is used in an apparatus, equipment or device performing an ammonia-stripping process or a self-stripping process in a urea production process or plant.
25. The use according to one of the preceding claims, 20 wherein the duplex stainless steel is used in a high pressure stripper configured for ammonia-stripping or self-stripping in a urea production process or plant.
26. The use according to claim 25, wherein the duplex stainless steel is coupled with a covering layer made of 25 Zirconium or a Zirconium alloy which cover at least a surface portion of the duplex stainless steel.
27. The use according to one of the preceding claims, wherein the composition parameter (Z) ranges between 14,95 and 19,00, preferably between 14,95 and 18,00, more 30 preferably between 14,95 and 17,50.
28. An apparatus, equipment or device of a urea production plant or used in a urea production process, comprising at least a part made by the use according to one of the claims from 1 to 27.
29. A plant for the production of urea, comprising at 5 least one apparatus, equipment or device according to claim 28.
30. A process for the production of urea comprising at least one step performed in an apparatus, equipment or device according to claim 28.
10 31. A method of revamping an existing urea production plant by replacing at least a part of an apparatus, equipment or device of the plant with a part made by the use according to one of the claims from 1 to 27.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP17210463.0 | 2017-12-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| OA19834A true OA19834A (en) | 2021-05-26 |
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