Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
A TUBE FOR THERMAL CRACKING OR REFORMING OF HYDROCARBON
BACKGROUND OF THE INVENTION
The present invention relates to a reactor tube
for thermal cracking or reforming hydrocarbons, in particular
the reactor tube which prevents deposition and building up
of solid carbon thereon accompanied by chemical reaction
of hydrocarbon and further prevents carburization.
Reactor for thermal cracking and reforming of
hydrocarbon used here is in tubular form and passes hydro-
carbon therethrough in liquid or gaseous form under high
pressures and temperatures for thermal cracking or reforming,
in the presence or absence of a catalyst layer. The material
heretofore used for such reactors is Fe-Cr-Ni austenitic
heat resisting steel which contains large amount of Ni and
Cr and has been generally applied for equipments at high-
temperature use. It has been the usual practice to increase
the Ni content to enhance heat resisting property of tube
material to be used at higher temperatures.
Since thermal cracking or reforming of hydrocarbons
accompanies deposition of solid carbon, when reaction is
continued by using such reactor tube made of Fe-Cr-Ni steel
as aforesaid, solid carbon inevitably deposits and builds
up on the wall surface (inner wall surface, outer wall
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surface or both inner and outer wall surfaces depending on
the way of use of reactor tube) to be in contact with
hydrocarbons. When such deposit of solid carbon is left
unattended, it not only obstructs the passage of fluid
containing hydrocarbon through the tube but also remarkably
reduces overall heat transfer coefficient in the reaction
heat supplied or removed from outside the tube and thus
operation becomes difficult to be continued. As the result
periodical shutdown of operation is required for removal
of carbon deposits by various methods, so-called decoking,
although the reactor is to be operated continuously as a
rule. Besides, the conventional reactor tube as aforesaid
presents such problems as deterioration of tube material due
to carburizing through reaction wall surface, particularly
a remarkable reduction of ductility and danger of generation
of cracks due to embrittling of tube material under high
pressures.
In order to solve the above problems, we have
carried out intensive research and found out that the reason
for the remarkable carbon deposits in the reactor tube made
of Fe~Cr-Ni heat resisting steel is that Ni contained in the
steel material acts catalytically to accelerate deposition
of solid carbon on the tube surface through hydrocarbon and
that there exists a correlation between the amount of solid
carbon deposits and Ni content in the tube material and by
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restricting such Ni content, it is possible to inhibit and
prevent deposition of solid carbon on the tube surface.
As for carburization, when steel material of the tube
contains appropriate amount of Mn and Nb, carburization
from the tube wall surface is effectively restricted and
deterioration of tube material can be prevented.
SUMMARY OF THE INVENTION
The present invention was accomplished based on
the aforesaid analysis. This invention offers the reactor
tube whose reacting layer (inner wall layer) to be in
contact with hydrocarbon is made of Fe-Cr heat resisting
steel free from Ni or Fe-Cr-Ni heat resisting steel
containing up to 10% of Ni so that it does not substantially
exhibit the said catalytic action to accelerate deposition
of solid carbon and whose reacting layer is covered by the
outer layer made of the conventional material for use in
equipments at high temperatures such as Fe-Cr-Ni heat resist-
ing steel. By employing such double layer structure for the
reactor tube, we have succeeded in inhibiting the deposition
of solid carbon resulted by reaction as much as possible
and ensuring stable operation without carrying out decoying
for long time while maintaining the required characteristics
of the reactor tube used under high temperatures and high
pressures.
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More specifically the present invention provides
reactor tube for thermally cracking or reforming hydrocarbons
with which deposition of solid carbon accompanied by reaction
can be prevented by forming the reacting layer in the reac-
tion zone to be in contact with hydrocarbons, with heat re-
sisting steel comprising, in terms of % by weight, 0.01 -
1.5%C,up to 3% Si, up to 15% Mn, 13 to 30% Cr, up to 0.15%
N and the balance substantially Fe and by forming covering
layer which cowers the said reacting layer and is fused
thereto at the boundary surface with heat resisting steel
comprising, in terms of % by weight, 0.1 to 0.6% C, up to
2.5% Si, up to 2% Mn, 20 to 30% Cr, 18 to 40% Ni, up to 0.15%
N and the balance substantially Fe.
Another object of the present invention is to pro-
vide the reactor tube for thermally cracking or reforming
hydrocarbons wherein deposition of solid carbon on the tube
surface is prevented and carburization through the tube
surface is inhibited by forming the reacting layer of the
reactor tube in the reaction zone to be contact with hydro-
carbons, with Fe-Cr-Mn-Nb heat resisting steel comprising
in terms of % by weight 0.3 to l.S% C, up to 3% Si, 6 to 15%
Mn, 20 to 30% Cr, up to 3% Nb, up to 0.15% N and the balance
substantially Fe or Fe-Cr-Mn-Nb-Ni heat resisting steel
obtained by replacing some amount of Fe by Ni. in the amount
up to 10% and by forming the covering layer which cover the
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L97667
said reacting layer and fused thereto at the boundary sur-
face with Fe-Cr-Ni heat resisting steel comprising in terms
of % by weight 0.1 to 0.6% C, up to 2.5% Si, up to 2% Mn,
20 to 30%Cr, 18 to 40% Ni, up to 0.15% N and the balance
substantially Fe or Fe-Cr-Ni heat resisting steel obtained
by replacing some amount of Fe by one or more elements
selected from Mo, W and Nb, in a combined amount of up to
5% by weight.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a front elevation view partly broken away
and showing a reactor tube according to the invention;
Fig. 2 is a view in section taken along the line
II-II in Fig.1;
Figs. 3 and 4 are sectional views of reactor tube
according to other examples of the invention;
Fig. 5 is a graph showing the correlation between
Ni content of reactor tube material and the amount of depo-
sit of solid carbon on the reacting layer surface;
Fig. 6 is a graph showing the amount of carbon
increment by carburizing into the reacting layer;
Fig. 7 is a graph to indicate the amount of solid
carbon deposits on the reacting layer surface.
DETAILED DESCRIPTION OF THE INVENTION
When the reaction zone of the tube which is brought
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in contact with hydrocarbons is in the inner surface of the
tube, the reacting layer 1 at the inside as illustrated in
Figs. 1 and 2, is made of a heat resisting steel of Fe-Cr
ferritic type or martensitic type free from Ni, or of Fe-Cr
-Ni heat resisting steel of ferritic, ferritic-austenitic or
martensitic type containing up to about 10% of Ni.
The said Fe-Cr heat resisting steel may be for
example, the one rnade of 13 to 30% Cr (% by weight, the same
as hereinafter), 0.01 to 1.5% C, up to 2.5% Si, up to 2.0%
Mn, up to 0.15% N and the balance substantially Fe or the
one wherein some amount of Fe is replaced by one or more of
Mo, W and Nb in a combined amount of up to 5.0% to obtain a
further improved characteristics of the material.
The covering layer 2 which covers the outside of
the said reacting layer 1 and is made of Fe-Cr-Ni austenitic
heat resisting steel usually used for tubes of this type is
fused to the aforesaid reacting layer 1 at the boundary
surface~to obtain a double layer structure. Contrary to the
above, when the reaction zone of the tube which comes contact
with hydrocarbons is in the outer surface of the tube, the
reacting layer 1 of the aforementioned chemical compositions
is provided on the outside and the covering layer 2 of the
aforesaid chemical compositions is provided on the inside
as shown in Fig. 3.
When both inner surface and outer surface of reac-
tor tube become the reaction zone, reacting layers 1, 1 may
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be provided at both surfaces and covering layer 2 may be
interposed between the both reacting layers 1,1, as illust-
rated in Fig. 4.
Fig. 5 indicates the correlation between the amount
of solid carbon deposits (in mg/cm ) and the Ni content (%),
in the reactor tube made of Fe-Cr-Ni heat resisting steel
(18% Cr, 0.8% C, 1.5% Si, 1.1% Mn, 0.05% N, up to 35% Ni,
43.5 to 78.55% Fe) (Experimental conditions: Amount of
ethane supplied is 400 cc/min, S/C=1.5, duration of passage
of ethane gas is one hour, inner diameter of tube is 110 mm,
temperature is 900C; wherein, S/C is Mol H20/atomic C)
As shown in the drawing, the amount of deposits of
solid carbon increases with the increment of Ni content in
the tube material. For example, the Ni content of ~e-Cr-Ni
heat resisting steel which has been used for the material
of reactor tube of this type is about 35% and it coincides
with the fact that remarkable deposition of solid carbon
was unavoidable with the conventional reactor tube. It is
because, as mentioned above, Ni on the surface of tube wall
acts catalytically to accelerate deposition of solid carbon.
Based upon this fact supported by experiments, maximum con-
tent of Ni in the present invention is limited to about 10.0%
or preferrably up to about 5.0% in order to inhibit and
prevent deposition of solid carbon as much as possible.
Covering layer 2 which covers the reacting layer 1
may be made of Fe-Cr-Ni austenitic heat resisting steel
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having been usually used for the tube of this type. The
examples of such chemical compositions of steel may be in
terms of % by weight 20 to 30% Cr, l to 40% Ni, 0.01 to
0.6% C, up to 2.5% Si, up to 2.0% Mn, up to 0.15% N and the
balance substantially Fe or the chemical compositions of
steel wherein some amount of Fe is replaced by one or more
elements selected from Mo, W and Nb in a combined contents
of up to 5.0%.
According to the present invention, the reacting
layer 1 is made of heat resisting steel free from Ni or con-
taining Ni within the range where it does not substantially
act catalytically to deposit solid carbon and the covering
layer 2 covers the reacting layer 1, both the reacting layer
and the covering-layer being joined together by fusing at
the boundary surface.
The reactor tube is thus given a double layer con-
struction wherein covering layer 2 which covers the said
reacting layer 1 is fused to the reacting layer 1 at the
boundary surface and through these means, deposition of
solid carbon on the reacting layer wall surface of the tube
is effectively inhibited and the tube is simultaneously pro-
vided with such mechanical properties as high strength and
high creep rupture strength at high temperatures, the pro-
perties possessed by Fe-Cr-Ni austenitic heat resisting steel
and thus the reactor tube becomes a more preferred reactor
tube to be used under high pressures and high temperatures.
Another embodiment of the present invention is the
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reactor tube having reacting layer 1 made of substantially
Ni-free Fe-Cr-Mn-Nb heat resisting steel or low nickel
Fe-Cr-Mn-Nb-Ni heat resisting steel containing up to 10% of
Ni.
A preferred embodiment of the said Fe-Cr-Mn-Nb
heat resisting steel may be the one made of in terms of % by
weight 20 to 30% Cr, 0.3 to 1.5% C, up to 3% Si, 6 to 15% Mn,
up to 3% Nb, up to 0.15% N and the balance being substantial-
ly Fe.
A preferred example of low nickel Fe-Cr-Mn-Nb-Ni
heat resisting steel may be the one wherein Fe of the said
Fe-Cr-Mn-Nb heat resisting s-teel is replaced in part by up
to 10% of Ni, i.e., the heat resisting steel made of in
terms of % by weight 20 to 30% Cr, up to 10% Ni, 0.3 to 1.5%
C, up to 3% Si, 6 to 15% Mn, up to 3% Nb, up to 0.15% N and
the balance being substantially Fe.
The heat resisting steel forming the covering
layer may be compristing Fe-Cr-Ni austenitic heat resisting
steel generally used for the material of the tube of the
type described. For example, the steel comprising in terms
of % by weight 20 to 30% Cr, 18 to 40% Ni, 0.1 to 0.6% C,
up to 2.5% Si, up to 2% Mn, up to 0.15% N and the balance
being substantially Fe or the one with the above steel com-
position wherein Fe in the above steel is replaced in part
by one or more elements selected from Mo, W and Nb in a com-
bined amount of up to 5%.
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In the above examples of the present invention,
the chemical compositions of Fe~Cr or low-Ni Fe-Cr-Ni heat
resisting steel and Fe-Cr-Mn-Nb or low-Ni Fe-Cr-Mn-Nb-Ni heat
resisting steel constituting reacting layer 1 and Fe-Cr-Ni
austenitic heat resisting steel constituting covering layer
2 are only explanatory and appropriate modifications and
changes such as some increase or decrease of the proportions
of the components beyond the above ranges or small addition
or removal of components to or from those described above are
also useful.
Fig.-6 shows the results of carburizing test con-
ducted to find the influence of Mn and Nb contained in the
tube material upon carburization into reactor tube.
Carburizing conditions: Carburizing treatment through inner
wall surface of the tube by using solid carburizing agent.
Treating temperature: 1100C
Duration of treatment: 500 hrs.
The following three sample reactor tubes A, B and C were
used for test.
Reactor tube A (double layer tube structure)
Inner reacting layer:
Layer thickness 2mm
Low nickel Fe-Cr-Mn-Nb-Ni heat resisting steel (25%
Cr, 5% Ni, 0.6% C, 2.0% Si, 8.1% Mn, 0.45% Nb and
0.05% N)
Outer covering layer:
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Layer thickness lOmm
Fe-Cr-Ni heat resisting steel (25% Cr, 35% Ni, 0.48%
C, 1.5% Si, 1.0% Mn and 0.05% N)
Reactor tube B (double layer tube structure)
Inner reacting layer:
Layer thickness 2mm
Low nickel Fe-Cr-Ni Heat resisting steel (25% Cr, 5%
Ni, 1.0% C, 2.0% Si, 1.1% Mn and 0.05% N)
Outer covering layer:
Layer thickness lOmm
Same Fe-Cr-Ni heat resisting steel as used for the
reactor tube A described above.
Reactor tube C (single layer tube - equivalent to the
conventional reactor tube having been generally used)
Layer thickness 12mm
Same Fe-Cr-Ni heat resisting steel as used for the
outer covering layer of the reactor tube _ described
above.
In Fig. 6, curves A, B and C respectively show the
results obtained with reactor tubes A, B and C. As shown by
this figure, in the case of the reactor tube C having single
layer construction made of the material equivalent to those
used for the conventional reactor tubes (0.4 C - 25 Cr - 1 Mn
- 35 Ni), carbon increases over 2.0% due to carburizing at
the tube wall surface indicating remarkable carburization
towards the inside of the tube wall.
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Whereas in the case of reactor tube B having the
inner layer made of low nickel tube material (1 C - 25 Cr -
1 Mn - 5 Ni), carbon increment is subs-tantially smaller than
that of the reactor tube described above. In the case of the
reactor tube A wherein the inner layer is made of low nickel
material containing Nb and large amount of Mn (0.6 C - 25 Cr
- 8 Mn - 0.5 Nb - 5 Ni), carbon increment due to carburiza-
tion is extremely small, being less than about 0.3%. Such
effect to inhibit carburization is improved when Mn is in-
creasingly contained and Nb is added. Therefore, in orderto obtain anti-carburizing effect by Mn and Nb, the reacting
layer 1 facing the reaction zone is made of the material con-
taining Nb and large amounts of Mn. The content of Mn is
defined to be at least 6%. However, if Mn is excesively
contained, ductility decreases remarkably and cast products
are susceptible to crac}cs upon solidifying in the casting
process and therefore the maxirnum limit of Mn content is
defined to be 15%. When Nb is contained in a large amount
SIGMA phase precipitates during use at high temperatures and
ductility remarkably decreases and consequently the maximum
limit of Nb is defined to be 3%.
Fig. 7 is a graph giving comparison of the amounts
of solid carbon deposits on the tube wall surface in thermal
cracking and reforming reaction test of hydrocarbon wherein
the inside of the tube is used for reaction zone, and the
three kinds of reactor tubes D, E and F are made of the
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material containing large amount of Mn and Nb or the material
of low Mn content.
Test conditions: Ethane supply 400cc/min,
S/C = 1.5, temperature 900C, duration one hour and
inner dia. of reactor tube llOmm.
Reactor tube D and E have double layer construction
made up of reacting layer (thickness 2mm) at the inside and
covering layer (thickness lOmm) at the outside. Inner layer
in reactor tube D is made of low nickel Fe-Cr-Mn-Nb-Ni heat
resisting steel contianing Nb and a large amount of Mn,
whereas reacting layer of reactor tube E is formed by low
nickel Fe-Cr-Ni heat resisting steel containing only very
small amount of Mn. Covering layers are in both cases madeof
Fe-Cr-Ni heat resisting steel usually used for the material
of reactor tube of the type described.
Reactor tube F is the conventional reactor tube of
single layer construction which is made of the same tube
material being used for the covering layer of reactor tube
A and B described above. Chemical compositions of the mater-
ial of the respective reactor tube is as follows:
Reacting layer of reactor tube D: 24.2% Cr, 4.8% Ni,
0.56% C, 1.9% Si, 8.81% Mn, 0.51% Nb and 0.05% N.
Reacting layer of reactor tube E: 25.2% Cr, 4.3% Ni,
0.96% C, 1.76% Si, 1.3A% Mn and 0.05% N.
Reactor tube F: 25.1% Cr, 35.5% Ni. 0.~3% C, 1.3% Si,
1.2% Mn and 0.05% N.
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D, E and F in Fig. 7 show the results obtained with the
aforesaid reactor tubes D, E and F. As it is evident in the
figure, amount of solid carbon deposits on the tube wall
surface of reactor tube D whose reacting layer is made of
low-Ni Fe-Cr-Mn-Nb-Ni heat resisting steel containing Nb and
large arnount of Mn is outstandingly smaller than that of the
reactor tube F made of conventional Fe-Cr-Ni heat resisting
steel, just like the case of reactor tube E having reacting
layer made of low nickel Fe-Cr-Ni heat resisting steel con-
taining lesser Mn and free from Nb, indicating an outstandinganti-coking property. It is known from the above that even
when such large amount of Mn as described above is contained
together with Nb in low nickel Fe-Cr-Ni heat resisting steel
wherein Ni content is restricted to up to 10%, solid carbon
deposits inhibiting effect is not impeded. From these ex-
periments it is understood that the reactor tube whose react-
ing layer facing reaction zone is made of Fe-Cr-Mn-Nb or Fe-
Cr-Mn-Nb-Ni heat resisting steel with defined Ni, Mn and Nb
contents as aforesaid suffers only very small amount of solid
carbon deposits on tube wall surface and has an excellent
characteristics in reistance to carburization. The reasons
for specifying the C conten-t in heat resisting steS~ as
aforesiad are as follows:
In Fe-Cr-Mn-Nb or Fe-Cr-Mn-Nb-Ni heat resisting steel,
if the C content is too low, SIGMA phase precipitates during
use at high temperatures and ductility remarkably decreases.
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Besides, the lower the C content, the higher the solidifi-
cation temperature of molten alloy and as the result in the
centrifugal casting of the reactor tube of double layer
construction in the manner described later, which is the
purpose of this invention, molten alloy of the reacting
layer solidifies promptly after casting and thus inferior
fusing is made at the boundary of the reacting layer and
the covering layer and casting of reactor tube with sound
double layer construction becomes difficult. Such difficul-
ties may be overcome by increasing C content. However, whenC content is high, Materials constituting the covering layer
deteriorates due to diffusion tranfer of carbon content
from reacting layer to covering layer during the use of the
reactor at high temperatures. Therefore C content is
defined to be 0.3 to 1.5%. Among the metalelements con-
tained in the reacting layer, the contents of Cr, Si, and
N are determined by the following:
Cr, with the co-existence of Ni, has the effect
to austenitize cast steel structure and thus increasestrength
at high temperatures and increase oxidation resistance.
Cr content must be at least 20% in order to obtain the re-
quired strength and oxidation resistance at the temperatures
especially above 1000C. The aforesaid effect is reinforced
as the content of Cr increases but when it becomes too high,
decrease of ductility after use becomes excessive and
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therefore upper limit is 30%.
Si serves as a deoxidant during melting of cast
steel and also improves anti-carburizing property. However,
the Si content must be up to 3.0% since an excess of Si will
lead to impaired weldability.
N serves in the form of a solid solution to
stabilize and reinforce the austenitic phase, forms a
nitride and carbonitride with Nb and Cr, produces refined
grains by finely dispersed and precipitated nitride and
carbo-nitride and prevents grain growth, thus contributing
to the improvement of creep rupture strength. Preferably
the upper limit of theN content is 0.15% since the presence
of an excess of N permits excessive precipitation of nitride
and carbonitride, formation of coarse particles of nitride
and carbonitride and impairment of resistance to weldability.
Among Nb, Mo and W that are selectively contained
in covering layer, Nb contributes to the improvement of
castability and also forms Nb carbo-nitrides to disperse
finely in austenitic phase, thus reinforcing austenitic
matrix and greatly increasing creep rupture strength and
at the same time it makes the casting structure finer and
irnproves weldability. However, when its content becomes
too high, creep rupture strength decreases to the contrary
and also ductility is reduced. Therefore the Nb content is
preferred to be up to 5%.
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Nb usually contains Ta which is the element
having the same effect as Nb and therefore a combined amount
of Nb and Ta should be up to 5% when Nb contains Ta.
Mo and W also form carbo-nitrides and strengthen
austenitic structure in the same manner as Nb while its
effect is magnified in the co-presence of Nb. However,
when a combined content of Nb~Mo+W exceeds 5%, it reduces
ductility as in the case of Nb alone, and it also becomes
economically unpreferrable. Whether they are used independ-
ently or conjointly, combined content Mo and/or W is pre-
ferred to be up to 5%. The reactor tube having double layer
structure according to the present invention is pre-ferably
made by centrifugal casting. At the casting, molten metal
made of Fe-Cr-Ni heat resisting steel containing larger
amount of Ni for forming of outer covering layer is poured
into the mold for centrifugal casting to obtain the covering
layer of desired thickness. Immediately after it solidifies
up to the inner wall surface, molten metal of Fe-Cr or low
nickel Fe-Cr-Ni or Fe-Cr-Mn-Nb or low nickel Fe-Cr-Mn-Nb-Ni
heat resisting steel for forming inner reacting layer is
poured to cast the reacting layer of desired thickness.
Then rotation of mold is continued to complete casting.
By this process, the inner reacting layer and the outer
covering layer jointly form a thin fused layer at their
adjacent boundary, thus providing a tube of double layers
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metalluragically joined together. In the above casting,
in order to securely fuse the two layers at their boundary,
it is preferred that the heat resisting steel of reacting
layer has lower melting temperature than that of the heat
resisting steel of covering layer. The desirable mutual
relation of these melting temperatures is easily obtained
by relatively adjusting the chemical compositions of each
heat resisting steel, mainly their carbon content, within
the range as defined above. There are no specific restrict-
ions as to other casting conditions. Casting temperatureof molten metal may be adjusted to the temperature for
example 150 C higher than the melting temperature as convent-
ionally practised and upon necessity to protect the inner
surface of reacting layer from air oxidation, an appropriate
flux may be applied according to the usual method.
In the conventional centrifugal casting of double
layer tube, it has been the usual practise to cast the re-
acting layer before the inner surface of covering layer
solidifies, because if the molten alloy for reacting layer
is poured after the covering layer had solidified up to its
inner surface, fusing of the two layers at the boundary
becomes incomplete and strong bondage of the layers can
not be obtained. however, in such method, although a firm
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bondage of the two layers may be obtained, the molten alloys
constituting the two layers excessively mix so that not
only it becomes impossible to form each layer for the desired
thickness but also the composition of alloy constituting
respective layer deviates from what has been intended for
and consequently the objective double layer tube can not be
obtained. Whereas in the case of the double layer tube of
the present invention, reacting layer is cast after the
covering layer solidifies up to its inner surface so that
the excessive mixing of two layers does not occur and the
aforesaid troubles accompanied by it can be avoided and
yet in spite of such casting method (casting of reacting
layer after solidification of inner surface of covering
layer) the two layers are firmly joined together. The
reason for it is that the heat resisting steel used for the
reacting layer of this invention which has the composition
as defined above has a wide temperature range between the
start and the end of solidification and consequently even
when the molten metal of reacting layer comes contact with
the solidified inner surface of the covering layer, it does
not solidify immediately so t'nat an appropriate thickness of
fused layer is formed at thei.r boundary surface. Besides,
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at such moment, covering layer does not remelt in any large
amount and the said fused layer gains the minimum thickness
required for firmly bonding the two layers and thus an
ideal double layer structure is obtained.
To make double layer tube, it is also possible to
employ the method for example to combine centrifugal cast-
ing with spraying and first a cast tube of single layer is
formed by centrifugal casting and then desired alloy is
covered by spraying in the said tube surface. However,
when the centrifugal casting as aforesaid is employed, it is
not only possible to obtain a firm bonding of the two layers
but also to give desired thickness to each layer and further
to select the appropriate chemical composition for the
alloy of each layer so that it satisfies the desired
characteristics of the material.
An example of manufacture of the reactor tube of
this invention by centrifugal casting may be that molten
metal of Fe-Cr-Ni heat resiting steel with high Ni content
(25.5% Cr, 35.0% Ni, 0.45% C, 1.0% Si, 0.8% Mn, 0.06% N and
the balance substantially Fe) used for covering layer and
molten metal of Fe-Cr-Mn-Nb heat resisting steel (25.5% Cr,
0.6% O 2.0% Six 9.1% Mn, 0. 45% Nb, 0. 05% N and the balance
substantially Fe) used for reacting layer are prepared in a
high frequency induction melting furnace, 20 kg of the said
alloy for molten covering layer is poured into the mold for
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~L3L97~6~7
eentrifugal casting to form a covering layer of 134mm in
outer diameter, 15mm in thickness and 500mm in length and
immediately after solidification of its inner surface, 10 kg
of molten alloy used for reacting layer is poured to form
the reacting layer of lOmm in thickness, thus obtaining the
reactor tube G having concentric double layer construction
without mixing inner and outer layer alloys to each other
and the two layers fused metallurgically at their boundary.
Another embodiment of the manufacture according
to this invention is that in the same method of preparation
of molten metal as aforesaid, high Ni content of Fe-Cr-Ni
heat resisting molten steel alloy (26.0% Cr, 35.9% Ni, 0.44
% C, 1.2% Si, 1.0% Mn, 0.04% N and the balance substantially
Fe) is prepared for the covering layer alloy and low nickel
content of Fe-Cr-Mn-Nb-Ni heat resisting molten steel alloy
(25.3% Cr, 6.5% Ni, 0.55% C, 1.3% Si, 12.2% Mn, 0.65% Nb,
0.06% N and the balance substantially Fe) is prepared for
the reaeting layer alloy and under the same centrifugal
easting conditions as in the foregoing examples, 20 kg of
molten alloy of covering layer and 10 ~g of molten alloy
of reacting layer are easting to obtain the reactor tube H
with eoneentrie double layer eonstruetion without mixing
alloys of the inner and outer layers to eaeh other and the
two layers are metallurgieally joined together. With the
aforesaid reactor tubes G and H, the inner reacting layers
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were processed to obtain wall thickness of 2mm respectively
and inner diameter was made to be lOlmm and solid carbon
deposition test (anti-coking test) and carburizing test
were conducted on each of them. In all these tests, the
test conditions were same as those used for the tests
described before. With the reactor tube G, amount of solid
carbon deposit was 0.08 mg/cm2, carburized amount (the
amount of C increment) measured at the depths of 0.5mm, 1.5
mm, 2.5mm and 5.5mm from tube wall surface were respective-
ly 0,30%, 0.25%, 0.14% and 0.017%.
In the case of reactor tube H, the amount of solidcarbon deposit was 0.12 mg/cm and carburized amount (the
amount of C increment) measured at the depths of 0.5mm, 1,5
mm, 2.5mm and 5.5mm were respectively 0.25%, 0.21%, 0.11%
and 0.05%-
Both of the said two reactor tubes G and H had
superior anti-coking property and anti-carburizing charact-
eristics when compared to those of the single alyer reactor
tube only made of the conventional heat resisting steel used
for outer layer (see the graph C of Fig. 6 and graph F of
Fig. 7).
As aforesaid, the reactor tube of this invention
has the reacting layer made of Fe-Cr, low nickel Fe-Cr-Ni,
Fe-Cr-Mn-Nb or low nickel Fe-Cr-Mn-Nb-Ni heat resisting
steel and therefore deposition of solid carbon on the
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surface of tube wall resulted by chemical reaction of
hydrocarbons is effectively inhibited. Particularly when
the reacting layer is made of Fe-Cr-Mn-Nb or low nickel Fe-
Cr-Mn-Nb-Ni heat resisting steel, solid carbon deposits and
carburization are effectively inhibited. And besides, since
the reacting layer is covered by the high Ni content Fe-Cr
-Ni austenitic heat resisting steel layer which is joined
together with the reacting layer, the reactor tube gains the
high temperature characteristics to sufficiently withstand
the use at temperatures exceeding 500C and at pressures
exceeding atmospheric pressure. Therefore when used for
high temperature and high pressure thermal cracking of
hydrocarbons alone or its mixture with steam, oxygen con-
taining gas etc. to obtain low molecular hydrocarbons, or
for the manufacture of gaseous mixture containing hydrogen
or carbon oxide, it enables to maintain stable operation for
long time without various troubles caused by solid carbon
deposits or deterioration or damage of tube material due to
carburization.
The present invention is not limited to the
fGregoing embodiments but can be embodied into various
modifications without deviating from the spirit of this
invention by the ordinary skilled persons in the field of
technology to which the present invention belongs and is a
matter of course that those modifications belong to the
scope of claims.
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