PROCESS FOR REDUCING THE FORMATION OF CARBON DEPOSITS

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 DEG 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 mu m is achieved.

Description: Process for Reducing the Formation of Carbon Deposits Description The invention relates to a process for reducing the catalytically induced formation of carbon deposits (catalytical coking) on the surfaces of components designed as heat exchangers, containers or conduits and made of a heat-resistant material that consists of an alloy containing 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, especially plants for converting hydrocarbons, for 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 carried out by means of diffusion annealing in an atmosphere containing Al. In plants for processing hot process gases having components that contain C, the deposit of carbon on the surfaces exposed to the process gas regularly occurs unter certain process conditions (depending on feedstock, pressure and temperature). The following chemical reactions, among other factors, are responsible for this: C x H y => xC + 1/2 yH2 CH 4 + H 2 O o CO + 3 H 2 2 CO => CO2 + C CO + H 2 H 2 O + C It is known that coking is significantly promoted by the catalytic influence of certain metals, such as Fe and Ni. This not only leads to the formation of thermally-insulating layers on the surfaces in question, which detracts significantly from functional capacity, particularly in heat exchanger tubes, but also causes a considerable deterioration of operating life of these components. As a result of the unavoidable diffusion of carbon into the matrix of the base material, metal carbides form; these carbides 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 accelerated. 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 components are subjected to a heat treatment at high temperatures in an atmosphere containing 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 components accordingly. Nonetheless, there remains a need for alternative solutions, in order to reduce both the formation of catalytically induced carbon deposits and the associated negative effects as effectively and as permanently as possible. The object of the invention is, firstly, to suggest an altemative 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 consists 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, especially 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 containing Al, by virtue of the fact that the diffusion annealing takes place in the temperature range of 900 to 1200 °C at least for a part of the annealing time in an atmosphere containing Cr and sufficiently long as to achieve a Cr- enrichment with a penetration depth of at least 20 μm. A metal component according to the invention, especially being designed as heat exchanger or container or conduit for a process plant for producing chemical substances and made of a heat-resistant base material 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 chemical substances are exposed to a hot process medium containing 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-enrichment with a penetration depth of at least 20 μm. Preferably the diffusion annealing (which is known per se) for enriching 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 approximately 900 to 1200 °C. In case of the two-step execution of the invention, the annealing is carried out in a first step in an atmosphere containing Cr, so Cr diffuses into the base material from the outside. The duration of this diffusion annealing is calculated as to achieve a penetration depth of at least 20 μm 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 μm, and especially of 50 μm, is achieved for the Al-enrichment. Penetration depths of at least 30 μm for Cr and at least 100 - 150 μm for Al have proved to be especially advantageous. Especially good results can be achieved at penetration depth up to 200 μm. 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 were reversed, the fact that the diffusion speed of the Cr atoms in the matrix of the base material is significantly lower than that of the Al atoms would interfere considerably with this goal. It is also advisable to carry out the diffusion annealing in the Al atmosphere at a lower temperature (preferably 100 - 200 °C lower) than the first step of the diffusion annealing. It can also be advantageous to carry out the diffusion annealing in a simultaneous atmosphere containing both Cr and Al. In doing this, it is possible 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 reforming of hydrocarbons or through other conversion of feedstock material containing C is characterized by the fact that the production ist undertaken in a process plant that contains at least one part designed as a heat exchanger (e.g. cracking tube), container or conduit, which was treated by diffusion annealing in the above-described manner in those regions of its surface which are exposed to the hot C containing process gases, wheras such diffusion annealing has effected a Cr and Al-enrichment in the surface region by means of diffusion of Cr and Al into the base material. Such a process, in particular, may be a thermal or catalytic cracking process for hydrocarbons or other carbon-containing substances (e.g. conversion of ethylene dichloride into vinyl chloride or for conversion of naphtha into light hydrocarbons), a process for producing a reduction 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 annealing 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 atmosphere alone, does indeed produce good inhibition on the surface shortly after such treatment; however, after several operation cycles, this effect is reduced drastically, and then even poorer results are obtained than with an untreated surface. A significant advantage of the invention is the long- lastingness of the protective effect, even when the components treated according to the invention are exposed to high temperatures. In a component diffusion annealed with Cr alone, for example, the inhibiting effect declines drastically by decoking at 1100 °C after a total decoking time of only 100 hours; however, this ist not the case with the invention. The surface layer enriched with Al and Cr according 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 examples. Example 1 A sample sheet (measuring 30 x 7.5 x 2 mm) of an alloy having the following composition (% by weight) Ni 34 % Cr 26 % Fe 38 % Si 2 % was subjected 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 at approximately 1100 °C to an atmosphere containing Cr. The Cr containing atmosphere was prepared by introducing Cr compounds into the furnace, decomposition of the compounds at the given annealing temperature and releasing elementary Cr. In a second annealing step following directly after the 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 testings, it was determined that an enrichment of the Cr content, up to c. 55 % to a depth of c. 35 μm and an enrichment of the Al content to c. 30 % up to a depth of c. 150 μm had occured, whereby the Ni content of the Cr-enriched diffusion layer dropped below 3 %. For checking the effectiveness ot the treatment according 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 untreated sample sheets were first subjected to a surface activation treatment for achieving in the subsequent test a „time-lapse effect," i.e., a shortening of the test periods for clearly determinable coking. In the activation treatment, both sample sheets were annealed for 5 hours at 970 β C in an N 2 atmosphere. After this, the heat treatment was continued for 1 hour at 850 °C in an H 2 atmosphere (H 2 supply 6 Nl/h). To conclude the activation treatment, 10 coking/decoking cycles of 15 minutes each were carried out on each of the two sample sheets at 830 °C in n-heptane. For quantitatively determining the coking tendency of the activated sample sheet treated according to the invention, the sheet was exposed for varying durations at 850 °C to a process gas atmosphere consisting of isobutane and N 2 (weight ratio of 2:1). This revealed a clear reduction in the catalytically induced carbon deposits on the surface exposed to the process gas, in comparison to the sample sheet of the same material that was similarly activated but had not been treated according to the invention. To determine the coking rate, the carbon deposits were measured by means of a thermal scale. The results are given in Table 1. Time in minutes Coking rate in μg/cm 2 min untreated sheet treated sheet 5 95 2.1 30 41 1.5 60 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 oxidation with air or a steam-air mixture. In order to determine 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 isobutane/N 2 process gas atmosphere (weight ratio of 2:1) again had a 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 shown in Table 2. In contrast 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 in comparison to an untreated sample sheet of the same material. In order to activate their surfaces, both the sample sheet treated according to the invention and the untreated sample sheet were first exposed for 90 minutes at 820°C to an atmosphere of 22.5 % by volume ethane, 27.5 % by volume ethylene and 50 % by volume H 2 and then decoked at 800°C in air for 30 minutes. After this, the coking rate was measured during a 3-hour exposure, again at 820°C, in the aforementioned ethane/ethylene/H 2 atmosphere. For the untreated sample sheet the coking rate was 16.0 μg/cm 2 min, while the sample sheet treated according to the invention hat a substantially lower coking rate of only 0.6 μg/cm 2 min. Comparative Test A sample sheet with the same composition as in Example 1 and with the dimensions 20 x 15 x 5 mm was exposed, under conditions corresponding to those in Example 1 , to a diffusion annealing; however, in an atmosphere containing Ai alone. In addition, an untreated comparison sheet of the same composition and form was provided. In order to activate the surface, both sheets were exposed for 90 minutes at 820°C to an atmosphere which hat the same composition as the ethane/ethylene/H 2 atmosphere in Example 3, and then decoked in air for 60 minutes at 800°C. After this, the coking rates of the sample sheets prepared in this manner were measured during a coking treatment by a 2-hour exposure in the aforementioned ethane/ethylene/H 2 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: 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 said components 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 Cr and Al. 8. A metallic component being designed as heat exchanger or container or conduit for 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 two elements 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.

Inventors: GOMMANS ROBERTUS JOANNES NICOL, JONES JOHN J, STEURBAUT CLAUDIN, VAN BUREN FRITZ R, WOERDE HENRICUS MATTHIAS, ZIMMERMANN GERHARD, GOMMANS, ROBERTUS, JOANNES, NICOLAAS, JONES, JOHN, J., STEURBAUT, CLAUDIN, VAN BUREN, FRITZ, R., WOERDE, HENRICUS, MATTHIAS, ZIMMERMANN, GERHARD
Applicants: BUREN FRITZ R VAN, DOW BENELUX, DSM NV, GOMMANS ROBERTUS JOANNES NICOL, JONES JOHN J, KTI GROUP BV, PARALLOY LTD, STEURBAUT CLAUDIN, WOERDE HENRICUS MATTHIAS, ZIMMERMANN GERHARD, DOW BENELUX N.V., DSM N.V., GOMMANS, ROBERTUS, JOANNES, NICOLAAS, JONES, JOHN, J., K.T.I. GROUP B.V., PARALLOY LTD., STEURBAUT, CLAUDIN, VAN BUREN, FRITZ, R., WOERDE, HENRICUS, MATTHIAS, ZIMMERMANN, GERHARD
Application year: 1996
Main classification: C10G9
IPC codes: B01J19, C10G9, C10G11