Chromized refractory steel, a process for its production and...

Details Chromized refractory steel, a process for its production and... Chromized refractory steel, a process for its production and...

1998-07-06

2002-02-19

Jones, Deborah (Department: 1775)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

With prevention or removal of deleterious carbon...

C208S047000, C208S050000, C148S278000, C148S316000, C427S228000, C427S252000

Reexamination Certificate

active

06348145

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to steel articles exhibiting anti-coking properties, and to a process for producing such articles by depositing an anti-coking coating on a matrix constituted by a steel, generally a refractory steel. The process is used to manufacture articles which must be resistant to coking in a variety of refining or petrochemical processes.
The coated articles of the invention can be used in a variety of refining and petrochemical processes which involve temperatures of over 350° C.: steam reforming, dehydrogenation and visbraking, inter alia. More particularly, the invention is applicable to the manufacture of pipe stills for steam cracking intended to have a long service life at temperatures of the order of 800° C. to 1100° C.
BACKGROUND OF THE INVENTION
The carbonaceous deposit which develops in the stills during hydrocarbon conversion is generally known as coke. This coke deposit is a problem in industrial units. The formation of coke on the walls of the pipes and reactors causes a reduction in heat exchange and major blockages and thus pressure drops increase. To keep the reaction temperature constant, it may be necessary to increase the wall temperature, which risks damaging the steel constituting the walls. There is also a reduction in the selectivity of the units and, as a result, a reduction in the yield.
It is thus necessary to stop the units periodically to carry out decoking. It is thus of economic importance to develop materials or coatings which can reduce coke formation.
Many documents have described the coke formation reaction in a variety of reactions which bring hydrocarbons into contact with walls at a high temperature. The coking phenomenon has been widely described and studied for thermal cracking of hydrocarbons in particular. The leading article can be considered to be that by professor FROMENT published in 1990 in the “Review of Chemical Engineering”, volume 6, number 4, pages 292 to 328, entitled “Coke formation in the thermal cracking of hydrocarbons”. A more recent title which can be cited is that by BILLAUD, BROUTIN, BUSSON, GUERET and WEILL, the first part of which was published in the “Revue de I'Institut Francais du Pétrole”, 1992, volume 47, number 4, pages 537 to 549, entitled “Coke Formation during Hydrocarbon Pyrolysis”, and the second part of which was published in the same review in 1993, volume 48, number 2, pages 115 to 125.
To summarise the observations described in the prior art, it can be said that coke formation during thermal cracking of hydrocarbons is a complex phenomenon which involves different mechanisms, at least one of which involves reactions catalysed by the presence of oxides of metallic elements such as nickel, iron or cobalt at the walls of the apparatus used to carry out those processes. The metallic elements are generally contained in large quantities in the refractory superalloys used mainly because of the high temperatures encountered at the walls of those apparatus. That catalytic mechanism is highly preponderant: observations have shown that if that mechanism is inhibited, in the case of steam cracking it is possible to increase the length of the cycle between two still decoking stoppages necessary for implementing such a process by a factor of at least 3.
A certain number of documents are based on methods which can inhibit the catalytic formation of coke.
Japanese application P-03-104843 describes a refractory anti-coking steel for a still tube for cracking ethylene.
Further, U.S. Pat. No. 5 208 069 describes a method for passivation of the metal surface of reactor tubes coining into contact with hydrocarbons by in situ decomposition (i.e., in the assembled apparatus) of a non oxygen-containing organometallic silicon derivative under conditions in which a fine layer of ceramic material is formed on the tube surface. That method, in which deposition is carried out at atmospheric pressure or at a slight underpressure, generally does not result in the production of a deposit which is relatively uniform over the entire length of the tubes as the rate of growth of the deposit is not uniform along the entire length of the tube and thus the thickness, and the quality of the deposit, varies along the tube. Such variations entail a risk of producing very thick zones which thus have low adhesion and/or zones in which the silicon carbide deposit is of poor quality and thus of low adhesion. The pressure at which—according to the examples in that patent—vapour phase deposition of an organometallic silicon derivative is carried out is far too high and does not permit homogeneous deposition as gas diffusion distances are much smaller than in a vacuum i.e., at a pressure of less than 10
+4
Pa, for example. Further, the silicon carbide deposited is a compound with a low coefficient of expansion, while the substrate used normally has a far higher coefficient of expansion, which over time with cycles of heating and cooling, entails a non negligible risk of a loss of integrity of the silicon carbide layer at least at some points and as a result, the hydrocarbons come into contact with the superalloys which leads to an increase in the coking rate of the apparatus.
International patent application WO-A-95/18 849 describes articles, used as elements in cracking reactors, coated with a chromium-containing layer which protects against coking. The layer is applied using different plating or by painting. The chromium coatings obtained do not have sufficient resistance to coking, in particular in alternate coking and decoking cycles.
We have now discovered novel anti-coking steels, particularly for use in the applications mentioned above, which do not have the disadvantages mentioned above.
SUMMARY OF THE INVENTION
Thus the invention provides a steel article exhibiting anti-coking properties, characterized in that it comprises:
a refractory steel substrate comprising at least 0.2% by weight of carbon;
a carbon-rich diffusion barrier; and
an outer layer containing 90% to 99% by weight of chromium, characterized in that it is coated using a cementation method.
The substrate (or matrix) is generally constituted by a refractory steel, preferably containing 0.20% to 0.80% (percent) by weight of carbon.
More particularly, the steel has an austenitic grain structure at ambient temperature. A specific steel type for use in the invention is Manaurite® (registered trade mark of MANOIR INDUSTRIES).
Typical examples of steels which can be used in the invention have the main characteristics shown in the following table (the compositions are in weight %):
TABLE 1
C
Mn
Si
Ni
Cr
Fe
additions
ASTM A297 HK
0.30-
1-
1-
18-
23-28
compl.
Ti, Nb, W
Manaurite 20 ®
0.65
2
2.5
24
ASTM A297 HL
0.20-
1-
1-
18-
28-32
compl.
0.60
2
2
22
ASTM A297 HN
0.20-
1-
1-
23-
19-23
compl.
0.50
2
2
27
ASTM A297 HP
0.35-
1-
1-
33-
24-28
compl.
0.75
2
2.5
37
Manaurite
0.35-
1-
1-
33-
23-28
compl.
Nb, W
36XS ®
0.60
1.5
2
38
Manaurite XM ®
0.35-
1-
1-
33-
23-28
compl.
Nb, Ti, Zr
0.60
1.5
2
38
Manaurite XT ®
0.35-
1-
1-
42-
32-37
compl.
Nb
0.45
1.5
2
46
Manaurite
0.40-
1-
1-
43-
34-37
compl.
Nb, Ti
XTM ®
0.45
2
2
48
As indicated above, the invention relates more particularly to refractory steel articles coated by thermal chromization methods, such as chromization by cementation, in particular pack cementation or gas phase cementation.
The pack cementation process is well known to the skilled person. It has been described in a number of documents. As an example, the authors of U.S. Pat. No. 5 589 220 state that pack cementation is a process derived from CVD (Chemical Vapour Deposition) which consists of heating a pack at a high temperature in a closed or open vessel containing the metallic article to be coated for a predetermined time during which a diffusion coating is produced on the metallic article. The cementation pack in the closed or open vessel is protected from oxidation by an inert or reducing atmosphere. The cementation pack consists of a metal or alloy article or substrate for coating, surrounded

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