Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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
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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
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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
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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
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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
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--8--
~ 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.
