CA2236091A1 - Process for reducing the formation of carbon deposits - Google Patents
Process for reducing the formation of carbon deposits Download PDFInfo
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- CA2236091A1 CA2236091A1 CA002236091A CA2236091A CA2236091A1 CA 2236091 A1 CA2236091 A1 CA 2236091A1 CA 002236091 A CA002236091 A CA 002236091A CA 2236091 A CA2236091 A CA 2236091A CA 2236091 A1 CA2236091 A1 CA 2236091A1
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- diffusion annealing
- carried out
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Classifications
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G9/00—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
- C10G9/14—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
- C10G9/16—Preventing or removing incrustation
-
- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G9/00—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
- C10G9/14—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
- C10G9/18—Apparatus
- C10G9/20—Tube furnaces
-
- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G9/00—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
- C10G9/14—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
- C10G9/18—Apparatus
- C10G9/20—Tube furnaces
- C10G9/203—Tube furnaces chemical composition of the tubes
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- Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Catalysts (AREA)
- Production Of Liquid Hydrocarbon Mixture For Refining Petroleum (AREA)
- Carbon And Carbon Compounds (AREA)
- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
Abstract
The invention relates to a process for reducing the formation of catalytically induced carbon deposits (coking) on the surfaces of components designed as heat exchangers, conduits or containers and made of a heat-resistant material that consists of a heat-resistant alloy containing Cr and at least one of the two elements Fe and Ni, whereas the components are exposed to hot carbonaceous process gases in process plants for producing chemical substances, especially plants for converting hydrocarbons or other substances containing C through thermal or catalytic cracking or through steam reforming or plants for producing a CO-rich reduction gas, wherein an Al-enrichment is carried out in the surface region by means of diffusion annealing in an atmosphere containing Al. Diffusion annealing is carried out in the temperature range of 900 to 1200 ~C at least for a part of the treatment time in an atmosphere containing Cr, until a Cr-enrichment having a penetration depth of at least 20 µm is achieved.
Description
4~ 5 Process for Reducing the FGr.. ~litsn of Carbon De,~,osil:.
Description The invention relates to a process for reducing the catalytically induced formation of 10 carbon deposits (catalytical coking) on the surfaces of components designed as heat exchangers, containers or conduits and made of a heat-resistant material that consisl~
of an alloy con'- ,i"g Cr and at least one of the two elements Fe and Ni, whereas the components are designated to be exposed to hot process gases in process plants for producing chemical substances, especi~lly plants for converting hydrocarbons, lor example, or other substances containing C, by means of thermal or catalytic cracking (e.g., for converting ethylene dichloride into vinyl chloride) or plants for producing a CO-rich reduction gas, wherein an Al-enrichment in the surface reg on is carriedl out by means of diffusion annealing in an atmosphere containing Al.
In plants for processing hot process gases having components that contain C, tlhe deposit of carbon on the surfaces exposed to the process gas regularly occurs unter certain process condilions (depending on feedstock, pressure and temperature'l. The following chemical reactions, among other factors, are responsiLlc for this:
CxHy =~ XC + 1/2 YH2 CH4 + H20 ~ CO + 3 H2 2 CO ~ C~2 + C
CO + H2 ~ H20 + C
It is known that coking is significantly promoted by the catalytic influence of cer~ain metals, such as Fe and Ni. This not only leads to the formation of thermally-insulating ,, 35 layers on the surfaces in question, which detracts significantly from functional c:apacity, particularly in heat exchanger tubes, but also causes a considerable deterioration of operating life of these components. As a result of the unavci~ 'Q diffusion of carbon into the matrix of the base material, metal carbides form; these car, i~os are unstable and their decomposition, due to the associated change in volume, destroys the material cohesiveness in the surface region. Small pittings form on the surface (surface roughening). In the areas of such pittings the tendency toward coking is even further enhanced, and thus the destruction of the component in question is accelerdled. From the article "Aluminized ethylene furnace tubes extend operating life"
in Oil & Gas Journal, August 31, 1987, TECHNOLOGY, it is known that this effect can be reduced by an enrichment of aluminum in the surface region of the components in question. For this purpose, these co",ponents are s' ~hjected to a heat treatment at 10 high temperatures in an atmosphere conl~i"i"g Al. Aluminum thereby diffuses from the outside into the base material (diffusion annealing).
By means of these known measures, it is possible to significantly reduce the tendency toward catalytic coking in many cases and to lengthen the operation life of co",ponents accordingly. Nonetheless, there ,t:mc.i.,s a need for alternative solutions, in orderto reduce both the formation of catalytically induced carbon deposif~ and the associated negative effects as effectively and as permanently as possible.
The object of the invention is, firstly, to suggest an alternative process for reducing the tendency to catalytic coking in components of a process plant for producing chemical substances (raw materials for further processing and end products) and, secondly, to suggest components with a reduced catalytic coking tendency. In addition, an alternative process for producing chemical substances is to be suggested in which the tendency to catalytic coking of the components is reduced.
This object is attained in a process for reducing the catalytically induced formation of carbon deposits (catalytic coking) on the surfaces of components designed as heat exchangers, containers or conduits and made of a heat-resistant material that consisl~, of a heat-resistant alloy containing Cr and at least one of the two elements Fe and Ni, whereby the components during operation time are exposed to hot C containing process gases in plants for producing chemical substances, espe~ "y in plants for converting, e.g. hydrocarbons or other C containing substances, by thermal or catalytic cracking or in plants for producing a CO-rich reduction gas, wherein an Al-enrichment is carried out on the surface region of the components by means of diffusion annealing in an atmosphere contai"il ,g Al, by virtue of the fact that the diffusion annealing takes - W O 97/16507 PCT~EP96/04728-place in the temperature range of 900 to 1200 ~C at least for a part of the anne!aling time in an atmosphere containing Cr and sufficiently long as to achieve a Cr-enrichment with a penetration depth of at least 20 I~m.
A metal component according to the invention, especially being designed as heat exchanger or container or conduit for a process plant for producing cl-e",ical substances and made of a heat-resisldl ll base ~al~ :~ ial that contains Cr and at least one of the two elements Fe and Ni, is achievable by the aforementioned process, whereas an Al-enrichment in those regions of its surface, which during production of the chell,ical substances are exposed to a hot process medium conlai"i"g C, is effected by means of diffusion annealing in an atmosphere containing Al. Such metal component is characterized by the fact that the diffusion annealing was carried out in the temperature range of 900 to 1200 ~C and at least for a part of the annealing time in an atmosphere containing Cr and sufficiently long as to achieve a Cr-enrich",enl with a penetration depth of at least 20 ,um.
Preferably the diffusion annealing (which is known per se) for enrichi, ,9 the surface regions in question with inhibiting substances should be undertaken in two steps. The diffusion annealing is carried out at a temperature within the range of approxirnately 900 to 1200 ~C. In case of the two-step execution of the invention, the annec.li.,g is carried out in a first step in an atmosphere containing Cr, so Cr diffuses into th,s base material from the outside. The duration of this diffusion annealing is calculated as to achieve a penetration depth of at least 20 ,um for the Cr-enrichment. Afterwards in a second annealing step, a further diffusion annealing is carried out in an atmosphere containing Al. Preferably, this lasts at least until a penetration depth of 20 ,um, ,and especially of 50 ,um, is achieved for the Al-enrichment. Penetration depths of at least 30 ~m for Cr and at least 100 - 150 ,um for Al have proved to be especially advantageous. Especially good results can be acl~-eved at penetration depth ulp to 200 ,um. Although greater values are technically possible, they are not advantageous, especially for reasons of cost, because they provide no improved effect. It is advisable to carry out the two diffusion annealing steps in the two-step manner in the described order, so no unwanted unevenness in respect to the distribution of the diffused Cr and Al atoms in the surface regions occurs. If the order of the diffusion steps werereversed, the fact that the diffusion speed of the Cr atoms in the matrix of the base material is signiricantly lower than that of the Al atoms would interfere considerably with this goal. It is also advis~ ble to carry out the diffusion annealing in the Al atmosphere at a lower tei"per~lure (preferably 100 - 200 ~C lower) than the first step of the diffusion anneali"g.
It can also be advant~geous to carry out the diffusion annealing in a simultaneous J
atmosphere conl-.i.l l9 both Cr and Al. In doing this, it is p~s- ' 'e to change, within certain limits, the diffusion rate of Cr and Al by setting the partial pressure in the atmosphere appropriately.
A process according to the invention for producing chemical substances through thermal or catalytic cracking or steam rt:fc n"i"9 of hydroca, I,ons or through other conversion of feedstock material cor,l~i"i.,g C is cha~cl~ri~ed by the fact that the prorluction ist undertaken in a process plant that conlai"s at least one part designed as a heat exchanger (e.g. cracking tube), container or conduit, which was treated by diffusion anneal;ng in the above-desc~iL.ed manner in those regions of its surface which are .oxrosed to the hot C cor,l.~i. ,i.,g process gases, wheras such diffusion annealing has effected a Cr and Al-enrich",ent in the surface region by means ofdiffusion of Cr and Al into the base r"alerial. Such a process, in particular, may be a thermal or catalytic cracking process for hydrocarL ons or other carbon-containing substances (e.g. conversion of ethylene dich'~ride into vinyl chloride or for conversion of naphtha into light hydrocarbons), a process for producing a redllction gas rich in CO
or a process for steam reforming of hydrocarbons.
The enrichment of Cr and Al in the surface layer of heat exchangers, containers or conduits of process plants achieved by means of diffusion anneal;. ,g according to the invention provides, compared to a corresponding treatment of the surface alone with Cr or alone with Al, a better result in respect to prevention of catalytic coking. A
diffusion annealing, e.g. in a Cr atmosphert: alone, does indeed produce good i"hibilion on the surface shortly after such treatment; however, after several operation cycles, this effect is reduced d,-dslically, and then even poorer results are obtained than with an untreated surface. A siyniricanl advantage of the invention is the long-lasli"y- ,ess of the protective effect, even when the cor"ponenls treated according to the invention are exposed to high temperatures. In a component diffusion annealed with Cr alone, for example, the inhibiting effect declines d~lically by decoking at 1100 ~C after a total decoking time of only 100 hours; however, this ist not the case with the W O 97/16507 PCT~EP96/0472;B-invention. The surface layer enriched with Al and Cr accord;.,g to the invention have proved to be extraordinarily long-lasting under normal operating conditions.
The invention will be described in greater detail with reference to the following ~xc~r,lples.
Example 1 A sample sheet (measuring 30 x 7.5 x 2 mm) of an alloy having the following 10 composition (% by weight) Ni 34%
Cr 26 %
Fe 38%
Si 2%
was s~ cted to a two-step diffusion annealing in an annealing furnace. In the first step, which lasted for approximately 6 hours, the sample sheet was exposed alt approximately 1100 ~C to an atmosphere containing Cr. The Cr corlla~ l9 atmosphere was prepared by introducing Cr compounds into the furnace, decoml~osilion of thecompounds at the given annealing temperature and r~leasi"g elementary Cr. In a second annealing step following directly afterthe first step and carried out at a lower temperature, c. 950 ~C, the sample sheet was exposed for 6 hours to an atmosphere containing Al, prepared in a corresponding manner. By material tesli,)gs, it wasdetermined that an enricl-r,~ent of the Cr content, up to c. 55 % to a depth of c. 35 I-m and an enrichment of the Al co"lt:nl to c. 30 % up to a depth of c. 150 ,um had occured, whereby the Ni content of the Cr-enriched diffusion layer dropped below 3 %.
For checking the effectiveness ot the treatment accordi"g to the invention, the treated sample sheet, along with an untreated sample sheet of the same alloy as comparison, was subjected to a coking test under standardized conditions. For this purpose', the treated and the u"l,ealed sample sheets were first subjected to a surface activation treatment for achieving in the sl Ihse~uent test a ntime-lapse effect," i.e., a sho~tening f( of the test periods for clearly cJeter",i,~ lo coking. In the activation treatment, 'both sample sheets were annealed for 5 hours at 970 ~C in an N2 atmosphere. After this, the heat treatment was continued for 1 hour at 850 ~C in an H2 atmosphere (H2 supply - W O 97/16507 PCT~EP96/04728 -6 Nl/h). To conclude the activation treatment 10 cokingldecoking cycles of 15 minutes each were carried out on each of the two sarnple sheets at 830 ~C in n-heptane.
For quantitatively determining the coking tendency of the activated sample sheettreated according to the invention the sheet was ~oxrosed for varying durations at 850 ~C to a process gas al",osphere consi~li"g of isobuPrle and N2 (weight ratio of 2:1).
This revealed a clear red~ ~-~tion in the catalytically induced carbon deposits on the surface exposed to the process gas in cori"oa,ison to the sample sheet of the same material that was similarly activated but had not been treated according to the 10 invention. To de~er"~ e the coking rate the carbon cleposil~s were measured by means of a thermal scale. The results are given in Table 1.
Time in minutes Coking rate i 1 ug/cm2 min untreated sheet treated sheet 2.1 41 1.5 34 1.1 Table 1 Example 2 It is known that the tendency toward coking increases when a coke layer is removed in advance through oxid~tion with air or a steam-air mixture. In order to deter" ,;l ,e to what extent this effect occurs in sheets treated according to the invention the coking rate of treated and untreated sample sheets was measured after respective 60-minute exposures for several such operating cycles. The isobut~ne/N2 process gas atmosphere (weight ratio of 2:1) again had a W O 97/16507 PCTrEP96/047;'8 Cycle No. Coking rate in ,ug/cm2 . min ..
untreated sheet treated sheet 8.8 1.8 2 16.9 1.4 3 20.8 1.4 4 25.0 1.4 26.9 1.9 6 32.7 1.6 7 36.5 1.6 8 37.7 1.6 9 41.5 1.6 44.6 1.6 Table 2 temperature of 850 ~C. After each coking cycle, a 15-minute decoking in air at 850 ~C
took place. The material used for the treated and untreated sample sheets had the same composition as in Example 1. Prior to the tests the same activation treatment as in Example 1 was carried out, so standard conditions existed. The results are shawn in 10 Table 2. In conl,asl to the untreated sample sheet, which displayed an increasing coking tendency (known, for example, from Oil & Gas Journal, August 15,1988, page 70) at later cycles, the coking rate of the treated sample sheet remained essentially constant, and at a very low rate.
Example 3 A sample sheet treated according to the invention as in Example 1, of the same material and with the dimensions 20 x 15 x 5 mm, was tested in a tube furnace incomparison to an untreated sample sheet of the same material. In order to activa-te their surfaces, both the sample sheet treated according to the invention and the ~ nlledled sa,l r!e sheet were first exrosed for 90 minutes at 820~C to an atmospl1ere of 22.5 % by volume ethane, 27.5 % by volume ethylene and 50 % by volume H2 and then decok-cl at 800~C in air for 30 minutes. After this, the coking rate was measured during a 3-hour F~xrosllre' again at 820~C, in the aforementioned ethane/ethylene/H2 atmosphere. For the untreated sample sheet the coking rate was 16.0 I-g/cm2 min, .i while the sample sheet treated accold;.lg to the invention hat a substantially lower coking rate of only 0.6 ,ug/cm2 min.
Comparative Test A sample sheet with the same composition as in Example 1 and with the di.nensions 20 x 15 x 5 mm was exposed, under condilions cont:sponding to those in Example 1, to a diffusion annealing; however, in an atmosphere containing Al alone. In addilion, an untreated co"~pariaon sheet of the same con.r~osilion and form was provided. In order to activate the surface, both sheetâ were exrosed for 90 minutes at 820~C to an atmosphere which hat the same composition as the ethane/ethylene/H2 atmosphere in Example 3, and then decoked in air for 60 minutes at 800~C. After this, the coking rates of the sample sheets prepa~d in this manner were measured during a coking treatment by a 2-hour exposure in the .-~rurtnlerllioned ethane/ethylene/H2 atmosphere, again at 820~C. The measurement was carried out through a comparison of sample weights before and after this coking treatment. For the Al-diffusion-annealed sample sheet, a coking rate resulted which was only 23 % of the coking rate of the untreated sample sheet. However, in Example 3 according to the invention, the coking rate of the treated sample sheet was in fact less than 4 % of the coking rate of the untreated sample sheet. This shows clearly the surprisingly high effectiveness of the invention.
Description The invention relates to a process for reducing the catalytically induced formation of 10 carbon deposits (catalytical coking) on the surfaces of components designed as heat exchangers, containers or conduits and made of a heat-resistant material that consisl~
of an alloy con'- ,i"g Cr and at least one of the two elements Fe and Ni, whereas the components are designated to be exposed to hot process gases in process plants for producing chemical substances, especi~lly plants for converting hydrocarbons, lor example, or other substances containing C, by means of thermal or catalytic cracking (e.g., for converting ethylene dichloride into vinyl chloride) or plants for producing a CO-rich reduction gas, wherein an Al-enrichment in the surface reg on is carriedl out by means of diffusion annealing in an atmosphere containing Al.
In plants for processing hot process gases having components that contain C, tlhe deposit of carbon on the surfaces exposed to the process gas regularly occurs unter certain process condilions (depending on feedstock, pressure and temperature'l. The following chemical reactions, among other factors, are responsiLlc for this:
CxHy =~ XC + 1/2 YH2 CH4 + H20 ~ CO + 3 H2 2 CO ~ C~2 + C
CO + H2 ~ H20 + C
It is known that coking is significantly promoted by the catalytic influence of cer~ain metals, such as Fe and Ni. This not only leads to the formation of thermally-insulating ,, 35 layers on the surfaces in question, which detracts significantly from functional c:apacity, particularly in heat exchanger tubes, but also causes a considerable deterioration of operating life of these components. As a result of the unavci~ 'Q diffusion of carbon into the matrix of the base material, metal carbides form; these car, i~os are unstable and their decomposition, due to the associated change in volume, destroys the material cohesiveness in the surface region. Small pittings form on the surface (surface roughening). In the areas of such pittings the tendency toward coking is even further enhanced, and thus the destruction of the component in question is accelerdled. From the article "Aluminized ethylene furnace tubes extend operating life"
in Oil & Gas Journal, August 31, 1987, TECHNOLOGY, it is known that this effect can be reduced by an enrichment of aluminum in the surface region of the components in question. For this purpose, these co",ponents are s' ~hjected to a heat treatment at 10 high temperatures in an atmosphere conl~i"i"g Al. Aluminum thereby diffuses from the outside into the base material (diffusion annealing).
By means of these known measures, it is possible to significantly reduce the tendency toward catalytic coking in many cases and to lengthen the operation life of co",ponents accordingly. Nonetheless, there ,t:mc.i.,s a need for alternative solutions, in orderto reduce both the formation of catalytically induced carbon deposif~ and the associated negative effects as effectively and as permanently as possible.
The object of the invention is, firstly, to suggest an alternative process for reducing the tendency to catalytic coking in components of a process plant for producing chemical substances (raw materials for further processing and end products) and, secondly, to suggest components with a reduced catalytic coking tendency. In addition, an alternative process for producing chemical substances is to be suggested in which the tendency to catalytic coking of the components is reduced.
This object is attained in a process for reducing the catalytically induced formation of carbon deposits (catalytic coking) on the surfaces of components designed as heat exchangers, containers or conduits and made of a heat-resistant material that consisl~, of a heat-resistant alloy containing Cr and at least one of the two elements Fe and Ni, whereby the components during operation time are exposed to hot C containing process gases in plants for producing chemical substances, espe~ "y in plants for converting, e.g. hydrocarbons or other C containing substances, by thermal or catalytic cracking or in plants for producing a CO-rich reduction gas, wherein an Al-enrichment is carried out on the surface region of the components by means of diffusion annealing in an atmosphere contai"il ,g Al, by virtue of the fact that the diffusion annealing takes - W O 97/16507 PCT~EP96/04728-place in the temperature range of 900 to 1200 ~C at least for a part of the anne!aling time in an atmosphere containing Cr and sufficiently long as to achieve a Cr-enrichment with a penetration depth of at least 20 I~m.
A metal component according to the invention, especially being designed as heat exchanger or container or conduit for a process plant for producing cl-e",ical substances and made of a heat-resisldl ll base ~al~ :~ ial that contains Cr and at least one of the two elements Fe and Ni, is achievable by the aforementioned process, whereas an Al-enrichment in those regions of its surface, which during production of the chell,ical substances are exposed to a hot process medium conlai"i"g C, is effected by means of diffusion annealing in an atmosphere containing Al. Such metal component is characterized by the fact that the diffusion annealing was carried out in the temperature range of 900 to 1200 ~C and at least for a part of the annealing time in an atmosphere containing Cr and sufficiently long as to achieve a Cr-enrich",enl with a penetration depth of at least 20 ,um.
Preferably the diffusion annealing (which is known per se) for enrichi, ,9 the surface regions in question with inhibiting substances should be undertaken in two steps. The diffusion annealing is carried out at a temperature within the range of approxirnately 900 to 1200 ~C. In case of the two-step execution of the invention, the annec.li.,g is carried out in a first step in an atmosphere containing Cr, so Cr diffuses into th,s base material from the outside. The duration of this diffusion annealing is calculated as to achieve a penetration depth of at least 20 ,um for the Cr-enrichment. Afterwards in a second annealing step, a further diffusion annealing is carried out in an atmosphere containing Al. Preferably, this lasts at least until a penetration depth of 20 ,um, ,and especially of 50 ,um, is achieved for the Al-enrichment. Penetration depths of at least 30 ~m for Cr and at least 100 - 150 ,um for Al have proved to be especially advantageous. Especially good results can be acl~-eved at penetration depth ulp to 200 ,um. Although greater values are technically possible, they are not advantageous, especially for reasons of cost, because they provide no improved effect. It is advisable to carry out the two diffusion annealing steps in the two-step manner in the described order, so no unwanted unevenness in respect to the distribution of the diffused Cr and Al atoms in the surface regions occurs. If the order of the diffusion steps werereversed, the fact that the diffusion speed of the Cr atoms in the matrix of the base material is signiricantly lower than that of the Al atoms would interfere considerably with this goal. It is also advis~ ble to carry out the diffusion annealing in the Al atmosphere at a lower tei"per~lure (preferably 100 - 200 ~C lower) than the first step of the diffusion anneali"g.
It can also be advant~geous to carry out the diffusion annealing in a simultaneous J
atmosphere conl-.i.l l9 both Cr and Al. In doing this, it is p~s- ' 'e to change, within certain limits, the diffusion rate of Cr and Al by setting the partial pressure in the atmosphere appropriately.
A process according to the invention for producing chemical substances through thermal or catalytic cracking or steam rt:fc n"i"9 of hydroca, I,ons or through other conversion of feedstock material cor,l~i"i.,g C is cha~cl~ri~ed by the fact that the prorluction ist undertaken in a process plant that conlai"s at least one part designed as a heat exchanger (e.g. cracking tube), container or conduit, which was treated by diffusion anneal;ng in the above-desc~iL.ed manner in those regions of its surface which are .oxrosed to the hot C cor,l.~i. ,i.,g process gases, wheras such diffusion annealing has effected a Cr and Al-enrich",ent in the surface region by means ofdiffusion of Cr and Al into the base r"alerial. Such a process, in particular, may be a thermal or catalytic cracking process for hydrocarL ons or other carbon-containing substances (e.g. conversion of ethylene dich'~ride into vinyl chloride or for conversion of naphtha into light hydrocarbons), a process for producing a redllction gas rich in CO
or a process for steam reforming of hydrocarbons.
The enrichment of Cr and Al in the surface layer of heat exchangers, containers or conduits of process plants achieved by means of diffusion anneal;. ,g according to the invention provides, compared to a corresponding treatment of the surface alone with Cr or alone with Al, a better result in respect to prevention of catalytic coking. A
diffusion annealing, e.g. in a Cr atmosphert: alone, does indeed produce good i"hibilion on the surface shortly after such treatment; however, after several operation cycles, this effect is reduced d,-dslically, and then even poorer results are obtained than with an untreated surface. A siyniricanl advantage of the invention is the long-lasli"y- ,ess of the protective effect, even when the cor"ponenls treated according to the invention are exposed to high temperatures. In a component diffusion annealed with Cr alone, for example, the inhibiting effect declines d~lically by decoking at 1100 ~C after a total decoking time of only 100 hours; however, this ist not the case with the W O 97/16507 PCT~EP96/0472;B-invention. The surface layer enriched with Al and Cr accord;.,g to the invention have proved to be extraordinarily long-lasting under normal operating conditions.
The invention will be described in greater detail with reference to the following ~xc~r,lples.
Example 1 A sample sheet (measuring 30 x 7.5 x 2 mm) of an alloy having the following 10 composition (% by weight) Ni 34%
Cr 26 %
Fe 38%
Si 2%
was s~ cted to a two-step diffusion annealing in an annealing furnace. In the first step, which lasted for approximately 6 hours, the sample sheet was exposed alt approximately 1100 ~C to an atmosphere containing Cr. The Cr corlla~ l9 atmosphere was prepared by introducing Cr compounds into the furnace, decoml~osilion of thecompounds at the given annealing temperature and r~leasi"g elementary Cr. In a second annealing step following directly afterthe first step and carried out at a lower temperature, c. 950 ~C, the sample sheet was exposed for 6 hours to an atmosphere containing Al, prepared in a corresponding manner. By material tesli,)gs, it wasdetermined that an enricl-r,~ent of the Cr content, up to c. 55 % to a depth of c. 35 I-m and an enrichment of the Al co"lt:nl to c. 30 % up to a depth of c. 150 ,um had occured, whereby the Ni content of the Cr-enriched diffusion layer dropped below 3 %.
For checking the effectiveness ot the treatment accordi"g to the invention, the treated sample sheet, along with an untreated sample sheet of the same alloy as comparison, was subjected to a coking test under standardized conditions. For this purpose', the treated and the u"l,ealed sample sheets were first subjected to a surface activation treatment for achieving in the sl Ihse~uent test a ntime-lapse effect," i.e., a sho~tening f( of the test periods for clearly cJeter",i,~ lo coking. In the activation treatment, 'both sample sheets were annealed for 5 hours at 970 ~C in an N2 atmosphere. After this, the heat treatment was continued for 1 hour at 850 ~C in an H2 atmosphere (H2 supply - W O 97/16507 PCT~EP96/04728 -6 Nl/h). To conclude the activation treatment 10 cokingldecoking cycles of 15 minutes each were carried out on each of the two sarnple sheets at 830 ~C in n-heptane.
For quantitatively determining the coking tendency of the activated sample sheettreated according to the invention the sheet was ~oxrosed for varying durations at 850 ~C to a process gas al",osphere consi~li"g of isobuPrle and N2 (weight ratio of 2:1).
This revealed a clear red~ ~-~tion in the catalytically induced carbon deposits on the surface exposed to the process gas in cori"oa,ison to the sample sheet of the same material that was similarly activated but had not been treated according to the 10 invention. To de~er"~ e the coking rate the carbon cleposil~s were measured by means of a thermal scale. The results are given in Table 1.
Time in minutes Coking rate i 1 ug/cm2 min untreated sheet treated sheet 2.1 41 1.5 34 1.1 Table 1 Example 2 It is known that the tendency toward coking increases when a coke layer is removed in advance through oxid~tion with air or a steam-air mixture. In order to deter" ,;l ,e to what extent this effect occurs in sheets treated according to the invention the coking rate of treated and untreated sample sheets was measured after respective 60-minute exposures for several such operating cycles. The isobut~ne/N2 process gas atmosphere (weight ratio of 2:1) again had a W O 97/16507 PCTrEP96/047;'8 Cycle No. Coking rate in ,ug/cm2 . min ..
untreated sheet treated sheet 8.8 1.8 2 16.9 1.4 3 20.8 1.4 4 25.0 1.4 26.9 1.9 6 32.7 1.6 7 36.5 1.6 8 37.7 1.6 9 41.5 1.6 44.6 1.6 Table 2 temperature of 850 ~C. After each coking cycle, a 15-minute decoking in air at 850 ~C
took place. The material used for the treated and untreated sample sheets had the same composition as in Example 1. Prior to the tests the same activation treatment as in Example 1 was carried out, so standard conditions existed. The results are shawn in 10 Table 2. In conl,asl to the untreated sample sheet, which displayed an increasing coking tendency (known, for example, from Oil & Gas Journal, August 15,1988, page 70) at later cycles, the coking rate of the treated sample sheet remained essentially constant, and at a very low rate.
Example 3 A sample sheet treated according to the invention as in Example 1, of the same material and with the dimensions 20 x 15 x 5 mm, was tested in a tube furnace incomparison to an untreated sample sheet of the same material. In order to activa-te their surfaces, both the sample sheet treated according to the invention and the ~ nlledled sa,l r!e sheet were first exrosed for 90 minutes at 820~C to an atmospl1ere of 22.5 % by volume ethane, 27.5 % by volume ethylene and 50 % by volume H2 and then decok-cl at 800~C in air for 30 minutes. After this, the coking rate was measured during a 3-hour F~xrosllre' again at 820~C, in the aforementioned ethane/ethylene/H2 atmosphere. For the untreated sample sheet the coking rate was 16.0 I-g/cm2 min, .i while the sample sheet treated accold;.lg to the invention hat a substantially lower coking rate of only 0.6 ,ug/cm2 min.
Comparative Test A sample sheet with the same composition as in Example 1 and with the di.nensions 20 x 15 x 5 mm was exposed, under condilions cont:sponding to those in Example 1, to a diffusion annealing; however, in an atmosphere containing Al alone. In addilion, an untreated co"~pariaon sheet of the same con.r~osilion and form was provided. In order to activate the surface, both sheetâ were exrosed for 90 minutes at 820~C to an atmosphere which hat the same composition as the ethane/ethylene/H2 atmosphere in Example 3, and then decoked in air for 60 minutes at 800~C. After this, the coking rates of the sample sheets prepa~d in this manner were measured during a coking treatment by a 2-hour exposure in the .-~rurtnlerllioned ethane/ethylene/H2 atmosphere, again at 820~C. The measurement was carried out through a comparison of sample weights before and after this coking treatment. For the Al-diffusion-annealed sample sheet, a coking rate resulted which was only 23 % of the coking rate of the untreated sample sheet. However, in Example 3 according to the invention, the coking rate of the treated sample sheet was in fact less than 4 % of the coking rate of the untreated sample sheet. This shows clearly the surprisingly high effectiveness of the invention.
Claims (9)
1. A process for reducing catalytically induced formation of carbon deposits (catalytic coking) on the surfaces of components designed as heat exchangers, conduits or containers and made of heat-resistant material that consists of an alloy containing Cr and at least one of the two elements Fe and Ni, whereas saidcomponents are designated to be exposed to hot carbon containing process gases in process plants for producing chemical substances, especially in plants for converting hydrocarbons or other carbon containing substances by means of thermal or catalytic cracking or steam reforming or in plants for producing a CO-rich reduction gas, whereas an Al-enrichment in the surface region of said components is effected by means of diffusion annealing in an Al containing atmosphere, characterized in, that said diffusion annealing is carried out in a temperature range of 900 to 1200 °C and at least for a part of the annealing time in an atmosphere containing Cr until a Cr-enrichment with a penetration depth of at least 20 µm has been achieved.
2. A process according to claim 1, characterized in, that said diffusion annealing is carried out in two steps, whereas in the first step the diffusion annealing is maintained in a substantially Cr containing atmosphere until achieving a given minimum penetration depth of Cr-enrichment, and whereas in the second step the diffusion annealing is carried out in a substantially Al containing atmosphere.
3. A process according to any preceeding claim, characterized in, that said Al-enrichment is maintained until the Al penetration in the matrix of the base material has reached a depth of at least 20 µm, preferably at least 50 µm.
4. A process according to claim 2, characterized in, that the durations of said two steps of said diffusion annealing is calculated as to achieve a Cr penetration depth of at least 30 µm and a Al penetration depth of at least 100 µm.
5. A process according to any preceeding claim, characterized in, that said penetration depth of Cr and Al are limited to max. 200 µm.
6. A process according to any of claims 2 - 5, characterized in, that said second step of diffusion annealing is carried out at a lower temperature, preferably at a temperature being 100 - 200 °C lower than the temperature of said first step.
7. A process according to claim 1, characterized in, that said diffusion annealing is carried out in an atmosphere containing both Crand Al.
8. A metallic component being designed as heat exchanger or container or conduitfor a process plant for producing chemical substances, said component being made of a heat resistant base material containing Cr and at least one of the twoelements Fe and Ni and being achievable by a process according to any of claims 1 - 7, whereas treating those parts of its surface, which are to be exposed to hot C-containing process mediums during the production of said chemical substances.
9. A process for producing chemical substances by thermal or catalytical cracking or steam reforming of hydrocarbons or by other methods for converting C-containing feedstocks, characterized in, that said producing chemical substances is carried out in a process plant containing at least one component according to claim 8.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19542219.8 | 1995-10-31 | ||
| DE19542219A DE19542219C2 (en) | 1995-10-31 | 1995-10-31 | Process for reducing the formation of carbon deposits |
| US1140196P | 1996-02-09 | 1996-02-09 | |
| US60/011,401 | 1996-02-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2236091A1 true CA2236091A1 (en) | 1997-05-09 |
Family
ID=26020325
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002236091A Abandoned CA2236091A1 (en) | 1995-10-31 | 1996-10-31 | Process for reducing the formation of carbon deposits |
Country Status (11)
| Country | Link |
|---|---|
| EP (1) | EP0858494B1 (en) |
| JP (1) | JPH11514681A (en) |
| KR (1) | KR19990067141A (en) |
| CN (1) | CN1201480A (en) |
| AT (1) | ATE189693T1 (en) |
| AU (1) | AU719778B2 (en) |
| CA (1) | CA2236091A1 (en) |
| HU (1) | HUP9900820A3 (en) |
| NO (1) | NO981943L (en) |
| PL (1) | PL326370A1 (en) |
| WO (1) | WO1997016507A1 (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5873951A (en) * | 1996-08-23 | 1999-02-23 | Alon, Inc. | Diffusion coated ethylene furnace tubes |
| US6537388B1 (en) | 1996-08-23 | 2003-03-25 | Alon, Inc. | Surface alloy system conversion for high temperature applications |
| FR2765594B1 (en) * | 1997-07-04 | 1999-08-27 | Inst Francais Du Petrole | CHROMED REFRACTORY STEEL, PROCESS FOR OBTAINING SAME AND USES THEREOF IN ANTI-COKAGE APPLICATIONS |
| SE529444C2 (en) * | 2005-12-02 | 2007-08-14 | Sandvik Intellectual Property | Pipes and use of the pipe |
| US20100199678A1 (en) * | 2007-09-13 | 2010-08-12 | Claus Krusch | Corrosion-Resistant Pressure Vessel Steel Product, a Process for Producing It and a Gas Turbine Component |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5575902A (en) * | 1994-01-04 | 1996-11-19 | Chevron Chemical Company | Cracking processes |
-
1996
- 1996-10-31 CA CA002236091A patent/CA2236091A1/en not_active Abandoned
- 1996-10-31 CN CN96198011A patent/CN1201480A/en active Pending
- 1996-10-31 AT AT96937304T patent/ATE189693T1/en not_active IP Right Cessation
- 1996-10-31 JP JP9517072A patent/JPH11514681A/en active Pending
- 1996-10-31 PL PL96326370A patent/PL326370A1/en unknown
- 1996-10-31 WO PCT/EP1996/004728 patent/WO1997016507A1/en not_active Application Discontinuation
- 1996-10-31 HU HU9900820A patent/HUP9900820A3/en unknown
- 1996-10-31 AU AU74962/96A patent/AU719778B2/en not_active Ceased
- 1996-10-31 KR KR1019980703087A patent/KR19990067141A/en not_active Application Discontinuation
- 1996-10-31 EP EP96937304A patent/EP0858494B1/en not_active Expired - Lifetime
-
1998
- 1998-04-29 NO NO981943A patent/NO981943L/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| NO981943D0 (en) | 1998-04-29 |
| EP0858494A1 (en) | 1998-08-19 |
| PL326370A1 (en) | 1998-09-14 |
| KR19990067141A (en) | 1999-08-16 |
| EP0858494B1 (en) | 2000-02-09 |
| JPH11514681A (en) | 1999-12-14 |
| HUP9900820A1 (en) | 1999-06-28 |
| WO1997016507A1 (en) | 1997-05-09 |
| AU719778B2 (en) | 2000-05-18 |
| ATE189693T1 (en) | 2000-02-15 |
| AU7496296A (en) | 1997-05-22 |
| HUP9900820A3 (en) | 2001-02-28 |
| NO981943L (en) | 1998-04-29 |
| CN1201480A (en) | 1998-12-09 |
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