Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 94/16035 PCT/US94/00107
- 1 -
TREATING OXIDIZED STEELS IN
LOW-SULFUR REFORMING PROCESSES
BACKGROUND OF THE INVENTION
The present invention relates to improved techniques
for catalytic reforming, particularly, catalytic reforming
under low-sulfur conditions. More specifically, the
invention relates to the discovery and control of problems
particularly acute with low-sulfur reforming processes.
Catalytic reforming is well known in the petroleum
industry and involves the treatment of naphtha fractions to
improve octane rating by the production of aromatics. The
more important hydrocarbon reactions which occur during the
reforming operation include the dehydrogenation of
cyclohe.~anes to aromatics, dehydroisomerization of
alkylcyclopentanes to aromatics, and dehydrocyclization of
acyclic hydrocarbons to aromatics. A number of other
reactions also occur, including the dealkylation of
alkylbenzenes, isomerization of paraffins, and hydrocracking
reactions which produce light gaseous hydrocarbons, e.g.,
methane, ethane, propane and butane. It is important to
minimize hydrocracking reactions during reforming as they
decrease the yield of gasoline boiling products and
hydrogen.
Because there is a demand for high octane gasoline,
extensive research has been devoted to the development of
improved reforming catalysts and catalytic reforming
processes. Catalysts for successful reforming processes
must possess good selectivity. That is, they should be
effective for producing high yields of liquid products in
the gasoline boiling range containing large concentrations
of high octane number aromatic hydrocarbons. Likewise,
there should be a low yield of light gaseous hydrocarbons.
The catalysts should possess good activity to minimize
excessively high temperatures for producing a certain
S~JBS'fITUTE SiiEET (RULE 261
WO 94/16035 PCT/US94/OOlU:
~1~3~~~
- 2 -
quality of products. It is also necessary for the catalysts
to either possess good stability in order that the activity
and selectivity characteristics can be retained during
prolonged periods of operation; or be sufficiently
regenerable to allow frequent regeneration without loss of
performance.
Catalytic reforming is also an important process for
the chemical industry. There is an increasingly larger
demand for aromatic hydrocarbons for use in the manufacture
of various chemical products such as synthetic fibers,
insecticides, adhesives, detergents, plastics, synthetic
rubbers, pharmaceutical products, high octane gasoline,
perfumes, drying oils, ion-exchange resins, and various
other products well known to those skilled in the art.
An important technological advance in catalytic
reforming has recently emerged which involves the use of
large-pore zeolite catalysts. These catalysts are further
characterized by the presence of an alkali or alkaline earth
metal and are charged with one or more Group VIII metals.
This type of catalyst has been found to advantageously
provide higher selectivity and longer catalytic life than
those previously used.
Having discovered selective catalysts with acceptable
cycle lives, successful commercialization seemed inevitable.
Unfortunately, it was subsequently discovered that the
highly selective, large pore zeolite catalysts containing a
Group VIII metal were unusually susceptible to sulfur
poisoning. See U.S. Patent No. 4,456,527.
Generally, sulfur occurs in petroleum and syncrude
stocks as hydrogen sulfide, organic sulfides, organic
disulfides, mercaptans, also known as thiols, and aromatic
ring compounds such as thiophene, benzothiophene and related
compounds.
Conventionally, feeds with substantial amounts of
sulfur, for example, those with more than 10 ppm sulfur,
have been hydrotreated with conventional catalysts under
SUBSTITUTE SHEET (RULE 261
~
CA 02153229 2003-04-16
- 3 -
conventional conditions, thereby changing the form of
most of the sulfur in the feed to hydrogen sulfide. Then,
the hydrogen sulfide is removed by distillation,
stripping or related techniques.
One conventional method for removing residual
hydrogen sulfide and mercaptan sulfur is the use of
sulfur sorbents. See, for example, U.S. Patent Nos.
4,204,997 and 4,163,706. The concentration of sulfur in
this form can be reduced to considerably less than 1 ppm
by using the appropriate sorbents and conditions, but it
has been found to be difficult to remove sulfur to less
than 0.1 ppm or to remove residual thiophene sulfur. See,
for example, U.S. Patent No. 4,179,361, and particularly
Example 1 of that patent. Zlery low space velocities are
required to remove thiophene sulfur, requiring large
reaction vessels filled with sorbent. Even with these
precautions, traces of thiophene sulfur still can be
f ound .
Thus, improved methods for removing residual sulfur,
and in particular, thiophene sulfur, from a hydrotreated
naphtha feedstock were developed. See, for example, U.S.
Patent Nos. 4,741,819 and 4,925,549. These alternative
methods include contacting the naphtha feedstock with
molecular hydrogen under reforming conditions in the
presence of a less sulfur sensitive reforming catalyst,
thereby converting trace sulfur compounds to HZS, and
forming a first effluent. The second effluent is
contacted with a highly selective reforming catalyst
under severe reforming conditions. Accordingly, when
using the highly sulfur sensitive catalysts, those
skilled in the art go to great extremes to remove sulfur
from the hydrocarbon feed. By doing so, the catalyst life
is extended for significant periods of time.
CA 02153229 2003-04-16
- 4 -
While low-sulfur systems using highly selective
large-pore zeolite catalysts were initially effective, it
was discovered that a shut down of the reactor system may
be necessary after only a matter of weeks. The reactor
system of one test plant had regularly become plugged
after only such brief operating periods. The plugs were
found to be those associated with coking. However,
although coking within catalyst particles is a common
problem in hydrocarbon processing, the extent and rate of
coke plug formation far exceeded any expectation.
SL~ARY OF THE INVENTION
Accordingly, one object of an aspect of the
invention is to provide a method for reforming
hydrocarbons under conditions of low sulfur which avoids
the aforementioned problems found to be associated with
the use of highly sensitive reforming catalysts and of
low-sulfur reforming processes.
It has been surprisingly found that coke plugs in
low sulfur reactor systems contained particles and
droplets of metal; the droplets ranging in size of up to
a few microns. This observation led to the startling
realization that there are new, profoundly serious,
problems which were not of concern with conventional
reforming techniques where process sulfur levels are
significantly higher. More particularly, it was
discovered that problems existed which threatened the
effective and economic operability of the systems, and
the physical integrity of the equipment as well. It was
also discovered that these problems emerged due to the
low-sulfur conditions, and to some extent, the low levels
of water.
For the last forty years, catalytic reforming
reactor systems have been constructed of ordinary mild
steel (e. g., 21/ Cr 1 Mo). Over time, experience has shown
that the systems can operate successfully for about
CA 02153229 2003-04-16
- 4a -
twenty years without significant loss of physical
strength. However, the discovery of the metal particles
and droplets in the coke plugs eventually lead to an
investigation of the physical
vV0 94/16035 ~ ~ ~ PCT/US94/00107
- 5 -
characteristics of the reactor system. Quite surprisingly,
conditions were discovered which are symptomatic of a
potentially severe physical degradation of the entire
e
reactor system, including the furnace tubes, piping, reactor
walls and other environments such as catalysts that contain
iron and metal screens in the reactors. Ultimately, it was
discovered that this problem is associated with the
excessive carburization of the steel which causes an
embrittlement of the steel due to injection of process
l0 carbon into the metal. Conceivably, a catastrophic physical
failure of the reactor system could result.
With conventional reforming techniques carburization
simply is not a problem or concern; nor was it expected to
be in contemporary low-sulfur/low-water systems. And, it
was assumed that conventional process equipment could be
used. Apparently, however, the sulfur present in
conventional systems effectively inhibits carburization.
Somehow in conventional processes the process sulfur
interferes with the carburization reaction. But with
e~°:: emely low-sulfur systems, this inherent protection no
lo:. her exists .
The problems associated with carburization only begin
with carburization of the physical system. The
carburization of the steel walls leads to "metal dusting"; a
release of catalytically active particles and melt droplets
of metal due to erosion of the metal.
The active metal particulates provide additional sites
for coke formation in the system. While catalyst
deactivation from coking is generally a problem which must
be addressed in reforming, this new significant source of
coke formation leads to a new problem of coke plugs which
excessively aggravates the problem. In fact, it was found
that the mobile active metal particulates and coke particles
metastasize coking generally throughout the system. The
active metal particulates actually induce coke formation on
themselves and anywhere that the particles accumulate in the
SUBSTiTIJTE SKEET (RULE 26b
WO 94/16035 PCT/US94/OOlI~.
- 6 -
system resulting in coke plugs and hot regions of exothermic
demethanation reactions.
Additionally, new reactor systems are often heat
treated to remove stress. Such procedures, for example,
heating in air at least 1650°F for 1-2 hours, often produce '
an oxide scale up to 50 ~.m thick on 347 stainless and
thicker on mild steels. In carburizing environments, these
oxide scales reduce to finely particulate Fe, Ni metal which
is extremely reactive for coking and can infect underlying
steel with carburization and pitting. Thus, the use of
oxidized steels in such environments and of heat treatments
which result in the oxidation of such steels should be
avoided.
As a result of the above reactions, an unmanageable and
premature coke-plugging of the reactor system occurs which
can lead to a system shut-down within weeks of start-up.
Use of the process and reactor system of the present
invention, however, overcomes these problems.
Therefore, another aspect of the invention relates to a
method for reforming hydrocarbons comprising contacting the
hydrocarbons with a reforming catalyst, preferably a
large-pore zeolite catalyst including an alkali or alkaline
earth metal and charged with one or more Group VIII metals,
in reactor systems having oxidized surfaces.
Yet another aspect of the invention relates to
a reactor system including means for providing a resistance
to carburization and metal dusting which is an improvement
over conventional mild steel systems in a method for
reforming hydrocarbons using a reforming catalyst such as a
large-pore zeolite catalyst including an alkaline earth
metal and charged with one or more Group VIII metals under
conditions of low sulfur, the resistance being such that
embrittlement will be less than about 2.5 mm/year,
preferably less than 1.5 mm/year, more preferably less than
1 mm/year, and most preferably less than 0.1 mm/year.
SUBSTiME SHEfT (RULE 26~
CA 02153229 2005-02-04
- 6a -
In accordance with an aspect of the invention, a
method for reforming hydrocarbons comprises (i) treating
a reforming reactor system, at least one surface thereof
to be exposed to hydrocarbons comprising a metal oxide or
metal oxides, by coating at least a portion of the
surface of the reforming rector system comprising the
metal oxides) with a material more resistant to
carburization than the portion prior to coating, reacting
the material with the metal oxide on the surfaces and
fixating or removing at least a portion of the oxide I
the metal oxide from the reactor system, and (ii)
reforming hydrocarbons in the reactor system under
conditions of low sulfur.
In accordance with another aspect of the invention,
a method for protecting a reactor system comprises (i)
treating a reactor system, at least one surface thereof
comprising a metal oxide or metal oxides to be exposed to
hydrocarbons, by coating at least a portion of the
surface of the rector system comprising the metal
oxides) with a material more resistant under reaction
conditions to carburization than the portion prior to
coating, reacting the material with the metal oxide on
the surfaces and fixating or removing at least a portion
of the oxide in the metal oxide from the reactor system,
and (ii) reacting a hydrocarbon in the reactor system
under elevated temperatures under conditions of low
sulfur.
According to an aspect of the present invention,
there is provided a method for increasing the
carburization resistance of an oxidized portion of a low
sulfur catalytic reforming reactor system, comprising the
steps of
applying a carburization resistant material to an
oxidized portion of a low sulfur catalytic reforming
reactor system comprising a layer of metal oxides;
CA 02153229 2005-02-04
-6b-
reacting said carburization resistant material with
the metal oxides to form a carburization inhibiting
layer, thereby fixating the metal oxides;
diffusing said carburization resistant material
through said carburization inhibiting layer; and
reacting said carburization resistant material with
a steel portion underlying said carburization inhibiting
layer to form a protective layer.
According to another aspect of the present
invention, there is provided a carburization resistant
layer for protecting an oxidized portion of a low sulfur
catalytic reforming reactor system having metal oxides,
comprising:
a top carburization inhibiting layer comprising
fixated metal oxides; and
a bottom protective layer on a steel portion of a
low sulfur catalytic reforming reactor system underlying
said top carburization inhibiting layer.
According to a further aspect of the present
invention, there is provided a method of reforming
hydrocarbons, comprising the step of catalytically
reforming, under low sulfur conditions, hydrocarbons in a
catalytic reforming reactor system comprising a
carburization resistant layer formed on an oxidized
portion of said catalytic reforming reactor system,
wherein said carburization resistant layer is exposed to
said hydrocarbons and comprises a top carburization
inhibiting layer comprising fixated metal oxides and a
bottom protective layer underlying said top carburization
inhibiting layer formed by applying carburization
resistant material comprising a metal selected from the
group consisting of tin, arsenic, antimony, bismuth, and
mixtures thereof.
WO 94/16035 7 °~ ~ PCT/US94I00107
_ 7 _
BRLEF DESCRIPTION OF THE FIGURES
Figure 1 is a photomicrograph (reflected lighted
light: 200x. 1 cm = 50 ~Cm) of a surface of a 347 stainless
steel sample which was heat treated in an electric furnace
at 1650°F air for 1 hour. The photomicrograph shows that
the heat treatment in air produced a uniformly thin (5 ~cm)
and adherent oxide coating.
Figure 2 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~,m) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. At high magnification, the complex oxide
scale is shown. The inner dark band and crystals adjacent
to the steel (bright) are composed of ferrochromite
(FeCrz04); the outer, brighter band is magnetite (essentially
Fe3o4). The escalloped pattern on the steel surface results
from a tendency of oxidation to preferentially attack grain
boundaries.
Figure 3 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~cm) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. A sample treated for five days at 1000°F in
the carburizing atmosphere did not coke. Nevertheless, the
magnetite band of the oxide coating was completely altered
to finely porous iron metal, which should be extremely
reactive in a carburizing environment. The ferrochromite
(dark) was unaltered.
Figure 4 is a photomicrograph (scale bar at right) of a
surface of a 347 stainless steel sample which was heat
treated in an electric furnace at 1650°F in air for 1 hour.
The photomicrograph is an SEM electron backscatter image of
the sample run for 5 days at 1000°F.
Figure 5 is a photograph (approximately 2x
magnification) of a sample of 347 stainless steel heat
treated in an electric furnace at 1650°F in air for 1 hour.
The sample was exposed to a carburizing atmosphere for five
SUSST1T1JTE SHEET (RULE 261
WO 94/16035 PCT/US9410010,
8
hours at 1150°F. The sample coked abundantly on the
oxidized surfaces but not on the unoxidized surfaces.
Figure 6 is a photograph (approximately 2x
magnification) of a sample of 347 stainless steel which was
heat treated in an electric furnace at 1650°F in air for 1
hour. The sample was exposed to a carburizing atmosphere
for two weeks at 1150°F. A protective tin coating exhibited
protection against coking and carburization.
Figure 7 is a photograph of a sample of 347 stainless
steel which was heat treated in an electric furnace
at 1650°F in air for 1 hour. The sample coked severely on
the oxidized surfaces after only 24 hours at 1025°F in a
carburizing atmosphere. The chip at right is raw 347
stainless steel.
Figure 8 is a photograph of a surface of a 347
stainless steel sample which was heat treated in an electric
furnace at 1650°F in air for 1 hour. Abundant coking
occurred on the oxidized surfaces after only 2Z hours
at 1200°F in a carburizing atmosphere. The chip at right
is 304 stainless steel.
Figure 9 is a photomicrograph (reflected light, 200x, 1
cm = 50 ~,m) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. The sample was exposed to a carburizing
atmosphere at 1200°F for 22 hours. A metal dusting pit
(center) formed on the steel surface under the actively
coking oxide layer. No dusting occurred on the unoxidized
surfaces.
Figure 10 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~Cm) of the surface of the 347 stainless steel
sample described in Figure 9. The sample carburized
at 1200°F along most of the surface. The chromium-rich
oxide layer (gray) persisted and resisted coking.
Figure 11 is a photomicrograph (reflected light, 1250x,
1 cm = 8 Vim) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
SU8ST1T1)TE SHEET (RULE 26b
WO 94/16035 PCT/US94/00107
2~~a229
_ g
air for 1 hour. The sample was coated with tin paint. The
ferrochromite layer and crystals (dark) persisted on the
steel surface (bright, center). The magnetite layer was
completely replaced by a series of iron stannides (shades of
gray). Some unreacted tin spheres (bright, top) were
present.
Figure 12 is,a photomicrograph (reflected light, 250x,
1 cm = 8 /sm) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. On an unoxidized surface, a protective tin
coating reacted directly with the steel to form a series of
nickel iron stannides.
Figure 13 is a photomicrograph (reflected light, 1250x,
1 cm = 8 Vim) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. After two weeks of exposure to a
carburizing atmosphere at 1150°F, tin in a tin containing
protective coating consumed the iron in the ferrochromite
layer and crystals, penetrated the chromium oxide layer, and
2o reacted to form a continuous layer of nickel iron stannide
on the underlying steel. The remaining oxide is eskolaite
( Crz03 )
Figure 14 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~cm) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 1 hour. After two weeks at 1150°F, a thicker
continuous layer of nickel iron stannide (dark) on the
unoxidized surface of the steel was formed. The thin
brighter layer under the stannide is the chromium-rich,
nickel-poor steel layer.
Figure 15 is a photomicrograph (scale bar at right) of
a surface of a 347 stainless steel sample which was heat
treated in an electric furnace at 1650°F in air for 1 hour.
The freshly stannided sample exhibited a stannide (bright)
layer on top of a ferrochromite oxide layer (dark). Steel
at left.
StIBSTIT1JTF SHEET (RULE 261
WO 94/16035 PCT/US94/0010,
- 10 -
Figure 16 is a photomicrograph (scale bar at right) of
a surface of a 347 stainless steel sample which was heat
treated in an electric furnace at 1650°F .in air for 1 hour.
After two weeks at 1150°F in a carburizing atmosphere, the
stannide migrated under the eskolaite (Cr203) layer (dark).
Figure 17 is a photomicrograph (reflected light, 200x,
1 cm = 50 ~,m) of a surface of a 347 stainless steel sample
which was heat treated in a flame furnace at 1650°F in air
for 2Z hours. This treatment produced a much thicker oxide
scale than the treatment in the electric furnace. The scale
is approximately 40 ~m thick.
Figure 18 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~,m) of a surface of a 347 stainless steel sample
which was heat treated in a flame furnace at 1650°F in air
for 2Z hours. The scale is more complex than the scale
obtained by heat treatment in the electric furnace. The
outermost layer (bright) is hematite (Fe203). Under the
hematite layer is a layer of magnetite (darker). Under the
magnetite layer is a layer of ferrochromite shot with a
fine, nickel-rich metal dust. A thin layer (darkest) of
pure ferrochromite coats the steel surface (very bright).
Figure 19 is a photograph (approximately 2x) of a
sample of a 347 stainless steel which was heat treated in a
flame furnace at 1650°F in air for 2~ hours. When exposed
to a carburizing atmosphere for three hours at 1050°F, the
sample coked profusely on the oxidized surfaces. Moreover,
some coke also formed on the unoxidized surfaces.
Figure 20 is a photograph of a sample of 347 stainless
steel which was heat treated in a flame furnace at 1650°F in
air for 22 hours. The sample which had received a
protective coating of tin paint was nearly coke free after
five days at 1150°F.
Figure 21 is a photomicrograph (reflected light, 200x,
1 cm = 50 ~,m) of a sample of 347 stainless steel which was
heat treated in a flame furnace at 1650°F in air for 2z
hours. After five days at 1150°F in a carburizing
SUBSZTT1TTE SHEET (RULE 261
WO 94/16035 ~ ,~.~, PCT/US94/00107
- 11 -
atmosphere, a stannide layer (bright gray) formed atop a
layer of oxide (darker) on the steel surface that had been
painted with plain tin paint. The oxides are heavily veined
with stannide. A ball of unreacted tin rests on the
stannide surface.
Figure 22 is a photomicrograph (reflected light, 200x,
1 cm = 50 ~Cm) of a surface of a 347 stainless steel sample
which was heat treated in a flame furnace at 1650°F in air
for 2= hours. On the oxidized surface that had been painted
with Fe203-modified tin paint, the exterior stannide layer
was locally breached exposing the underlying oxide. This is
the surface shown in Figure 20.
Figure 23 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~cm) of a surface of a 347 stainless steel sample
which was heat treated in a flame furnace at 1650°F in air
for 2i hours. On the surface treated with plain tin paint,
a continuous layer of stannide (gray) was formed on the
steel under the oxide.
Figure 24 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~cm) of a surface of a 347 stainless steel sample
which was heat treated in a flame furnace at 1650°F in air
for 2Z hours. The side painted with ferruginous paint
exhibited a sparse and discontinuous stannide under the
oxide.
Figure 25 is a photomicrograph (reflected light, 200x,
1 cm = 50 ~cm) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 2i hours. The exterior of the sample was exposed to
air during the heat treatment. This produced a complex
oxide scale similar to that produced in the flame furnace
but thinner.
Figure 26 is a photomicrograph (reflected light, 200x,
1 cm = 50 Vim) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
Nz for 2 = hours. Only a trace of oxide was formed.
SIJBS'OTUfE SHEfT (RULf 261
WO 94/16035 PCT/US94/0010,
- 12 -
Figure 27 is a photomicrograph (reflected light, 1250x,
1 cm = 8 um) of a surface of a 347 stainless steel sample
heat treated in an electric furnace at 1650°F in air for 2z
hours. An exterior layer of hematite (gray), underlain by
magnetite (gray), underlain by ferrochromite (dark) shot
with fine metal particles is shown.
Figure 28 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~cm) of a surface of a 347 stainless steel sample
heat treated in an electric furnace at 1650°F in N2 for 22
hours. On the nitrogen exposed side, scattered pockets
filled with oxide and chloride (dark) and an edge zone with
abundant grain boundary carbides or possibly nitrides is
shown. The carbide enriched zone is about 15 ~.m thick.
Figure 29 is a photomicrograph (reflected light, 200x,
1 cm = 50 ~,m) of a surface of a 347 stainless steel sample
which was heat treated in an electric furnace at 1650°F in
air for 2z hours. When this sample was exposed to the
carburizing atmosphere at 900°F for five days, the oxide
surface erupted with coke.
Figure 30 is a photomicrograph (reflected light, 1250x,
1 cm = 8 ~.m) of the surface of the 347 stainless steel
sample shown in Figure 29. The oxide was reduced to finely
porous metal (bright white) and coke (dark) deposited on
this metal. The steel surface (bright white) is at the
bottom.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The metallurgical terms used herein are to be given
their common metallurgical meanings as set forth in THE
METALS HANDBOOK of the American Society of Metals. For
example, "carbon steels" are those steels having no
specified minimum quantity for any alloying element (other
than the commonly accepted amounts of manganese, silicon and
copper) and containing only an incidental amount of any
element other than carbon, silicon, manganese, copper,
sulfur and phosphorus. "Mild steels" are those carbon
SUBSTTTITfE SHEET (RULf 261
WO 94116035 PCT/US94/00107
- 13 -
steels with a maximum of about 0.25% carbon. Alloy steels
are those steels containing specified quantities of alloying
elements (other than carbon and the commonly accepted
amounts of manganese, copper, silicon, sulfur and
phosphorus) within the limits recognized for constructional
alloy steels, added to effect changes in mechanical or
physical properties. Alloy steels will contain less than
10% chromium. Stainless steels are any of several steels
containing at least l0, preferably 12 to 30%, chromium as
the principal alloying element.
One focus of the invention is to provide an improved
method for reforming hydrocarbons using a reforming
catalyst, particularly a large pore zeolite catalyst
including an alkali or alkaline earth metal and charged with
one or more Group VIII metals which is sulfur sensitive,
under conditions of low sulfur. Such a process, of course,
must demonstrate better resistance to carburization than
conventional low-sulfur reforming techniques, yet contain
little sulfur available to poison the catalyst.
One solution for the problem addressed by the present
invention is to pretreat existing oxidized reactor systems
to prevent the reduction of oxide scales to finely porous
Fe, Ni metal and improve resistance to carburization and
metal dusting during reforming using a reforming catalyst
such as the aforementioned sulfur sensitive large-pore
zeolite catalyst under conditions of low sulfur.
By "reactor system", as used herein, there is intended
at least one reforming reactor and its corresponding furnace
means and piping. This term also includes other reactors
and their corresponding furnaces and piping wherein the
carburization is a problem under low sulfur conditions or
those systems wherein the aforementioned sulfur sensitive
large-pore zeolite catalysts are utilized. Such systems
include reactor systems used in processes for
dehydrogenation and thermal dealkylation of hydrocarbons.
Thus, by "reaction conditions" as used hebein, there is
SIJBS'tlTITTE SHEET (RULE 261
WO 94/16035 ~,r'.a PCT/US94/0010'~
- 14 -
intended, those conditions required to convert the feed
hydrocarbons to a desired product.
The aforementioned problems with low-sulfur reforming
can be effectively addressed by a selection of an
appropriate reactor system material for contact with the
hydrocarbons during processing. Typically, reforming
reactor systems have been constructed of mild steels, or
alloy steels such as typical chromium steels, with
insignificant carburization and dusting. For example, under
conditions of standard reforming, 2; Cr furnace tubes can
last twenty years. However, it was found that these steels
are unsuitable under low-sulfur reforming conditions. They
rapidly become embrittled by carburization within about one
year. For example, it was found that 22 Cr 1 Mo steel
carburized and embrittled more than 1 mm/year.
Furthermore, it was found that materials considered
under standard metallurgical practice to be resistant to
coking and carburization are not necessarily resistant under
low-sulfur reforming conditions. For example, nickel-rich
alloys such as Incoloy 800 and 825; Inconel 600; Marcel and
Haynes 230, are unacceptable as they exhibit excessive
coking and dusting.
However, 300 series stainless steels, preferably 304,
316, 321 and 347, are acceptable as materials for at least
portions of the reactor system according to the present
invention which contact the hydrocarbons. They have been
found to have a resistance to carburization greater than
mild steels and nickel-rich alloys.
In some areas of the reactor systems, localized
3o temperatures can become excessively high during reforming
(e.g., 900-1250°F). This is particularly the case in
SIIBSTTfITTE SHEET (RULE 26L
dV0 94/16035 PCT/US94/00107
_ ~5 _ ~1.5~~:2g
furnace tubes, and in catalyst beds where exothermic
demethanation reactions occur within normally occurring coke
balls causing localized hot regions. While still preferred
to mild steels and nickel-rich alloys, the 300 series
stainless steels do exhibit some coking and dusting at
around~1000°F. Thus, while useful, the 300 series stainless
steels are not the most preferred material for use in the
present invention.
Chromium-rich stainless steels such as 446 and 430 are
even more resistant to carburization than 300 series
stainless steels. However, these steels are not as
desirable for heat resisting properties (they tend to become
brittle).
Resistant materials which are preferred over the 300
series stainless steels for use in the present invention
include copper, tin, arsenic, antimony, bismuth, chromium,
germanium, indium, selenium, tellurium and brass, and
intermetallic compounds and alloys thereof (e.g., Cu-Sn
alloys, Cu-Sb alloys, stannides, antimonides, bismuthides,
etc.). Steels and even nickel-rich alloys containing these
metals can also show reduced carburization.
Reactor systems previously exposed to an oxidative
atmosphere are not preferred when such systems utilize the
aforementioned sulfur sensitive large-pore zeolite catalyst
systems. For example, heating such reactor systems in air
to remove stress may promote the formation of oxide scales.
When these oxide scales reduce, they may form finely porous
SIJBS'~1T1JTE SHEET (RULE 261
WO 94/16035 PCT/US94/0010~
16 -
Fe, Ni metal, which is extremely reactive for coking and can
infect underlying steel with carburization and pitting.
These previously oxidized steels may, according to the
present invention, be treated with carburization resistant
materials to prevent the formation of the finely porous Fe,
Ni metal, which significantly reduces coking, carburization
and metal dusting under reaction conditions. Such materials
may also allow for the removal of oxygen from reactor walls
with the possible release of water and/or the fixation of
the oxides by forming a continuous protective coating over
the oxide.
In a preferred embodiment, these materials are provided
as a plating, cladding, paint (e. g., oxide paints) or other
coating to a base construction material. This is
particularly advantageous since conventional construction
materials such as mild steels can still be used with only
the surface contacting the hydrocarbons being treated. Of
these, tin is especially preferred as it reacts with the
surface to provide a coating having excellent carburization
resistance at higher temperatures, and which resists peeling
and flaking of the coating. Also, it is believed that a tin
containing layer can be as thin as 1/10 micron and still
prevent carburization.
In addition, it has been observed that with the use of
such reactor systems, tin attacks the sulfided metal
surfaces including FeS replacing sulfur and releasing HzS.
Thus, application of resistant materials such as tin to a
StJBSTTT1JTE SHEET (RUSE 261
WO 94/16035 ~ ~ PCT/US94/00107
- 17 -
reactor system to prevent coking, carburization and metal
dusting can also protect sulfur sensitive catalysts when
applied to previously sulfided reactor systems.
Where practical, it is preferred that the resistant
materials be applied in a paint-like formulation
(hereinafter "paint") to a new or existing reactor system.
Such a paint can be sprayed, brushed, pigged, etc. on
reactor system surfaces such as mild steels or stainless
steels. It is most preferred that such a paint be a
decomposable, reactive, tin-containing paint which reduces
to a reactive tin and forms metallic stannides (e. g., iron
stannides and nickel/iron stannides) upon heating in a
reducing atmosphere.
It is most preferred that the aforementioned paint
contain at least four components (or their functional
equivalents); (i) a hydrogen decomposable tin compound, (ii)
a solvent system, (iii) a finely divided tin metal and (iv)
tin oxide as a reducible sponge/dispersing/binding agent.
The paint should contain finely divided solids to minimize
settling, and should not contain non-reactive materials
which will prevent reaction of reactive tin with surfaces of
the reactor system.
As the hydrogen decomposable tin compound, tin
octanoate or neodecanoate is particularly useful.
Commercial formulations of this compound itself are
available and will partially dry to an almost
chewing-gum-like layer on a steel surface; a layer which
SUBSnTITTE SHEET (RULE 26b
WO 94/16035 PCT/US94/0010'~
- 18 -
will not crack and/or split. This property is necessary for
any coating composition used in this context because it is
conceivable that the coated material will be stored for
months prior to treatment with hydrogen. Also, if parts are
coated prior to assembly they must be resistant to chipping
during construction. As noted above, tin octanoate is
available commercially. It is reasonably priced, and will
decompose smoothly to a reactive tin layer which forms iron
stannide in hydrogen at temperatures as low as 600°F.
Tin octanoate should not be used alone in a paint,
however. It is not sufficiently viscous. Even when the
solvent is evaporated therefrom, the remaining liquid will
drip and run on the coated surface. In practice, for
example, if such were used to coat a horizontal furnace
tube, it would pool at the bottom of the tube.
Component (iv), the tin oxide sponge/dispersing/binding
agent, is a porous tin-containing compound wrich can
sponge-up an organo-metallic tin compound, yet still be
reduced to active tin in the reducing atmosphere. In
addition, tin oxide can be processed through a colloid mill
to produce very fine particles which resist rapid settling.
The addition of tin oxide will provide a paint which becomes
dry to the touch, and resists running.
Unlike typical paint thickeners, component (iv) is
selected such that it becomes a reactive part of the coating
when reduced. It is not inert like formed silica; a typical
StIBSnTUTE SHEET (RULE 26L
WO 94/16035 ~ ~ , PCT/US94/00107
- 19 -
paint thickener which would leave an unreactive surface
coating after treatment.
Finely divided tin metal, component (iii), is added to
insure that metallic tin is available to react with the
surface to be coated at as low a temperature as possible,
even in a non-reducing atmosphere. The particle size of the
tin is preferably one to five microns which allows excellent
coverage of the surface to be coated with tin metal.
Non-reducing conditions can occur during drying of the paint
and welding of pipe joints. The presence of metallic tin
ensures that even when part of the coa°ing is not completely
reduced, tin metal will be present to react and form the
desired stannide layer.
The solvent should be non-toxic, and effective for
rendering the paint sprayable and spreadable when desired.
It should also evaporate quickly and have compatible solvent
properties for the hydrogen decomposable tin compound.
Isopropyl alcohol is most preferred, while hexane and
pentane can be useful, if necessary. Acetone, however,
tends to precipitate organic tin compounds.
In one embodiment, there can be used a tin paint of 20
percent Tin Ten-Cem (stannous octanoate in octanoic acid or
neodecanoate in neodecanoic acid), stannic oxide, tin metal
powder and isopropyl alcohol.
SUBS11TLTTE SHEET (RULE 261
WO 94/16035 PCT/US94/0010'~
- 20 -
The tin paint can be applied in many ways. For
example, furnace tubes of the reactor system can be painted
individually or as modules. A reforming reactor system
according to the present invention can contain various
numbers of furnace tube modules (e. g., about 24 furnace tube
modules) of suitable width, length and height (e. g., about
feet long, about 4 feet wide, and about 40 feet in
height). Typically, each module will include two headers of
suitable diameter, preferably about 2 feet in diameter,
10 which are connected by about four to ten u-tubes of suitable
length (e. g., about 42 feet long). Therefore, the total
surface area to be painted in the modules can vary widely;
for example, in one embodiment it can be about 16,500 ft2.
Painting modules rather than the tubes individually can
be advantageous in at least four respects; (i) painting
modules rather than individual tubes should avoid heat
destruction of the tin paint as the components of the
modules are usually heat treated at extremely elevated
temperatures during production; (ii) painting modules will
likely be quicker and less expensive than painting tubes
individually; (iii) painting modules should be more
efficient during production scheduling; and (iv) painting of
the modules should enable painting of welds.
However, painting the modules may not enable the tubes
to be as completely coated with paint as if the tubes were
painted individually. If coating is insufficient, the tubes
can be coated individually.
SUBSTtnT~E SHEET (RULE 26b
T
WO 94/16035 ~ ~ PCT/US94I00107
- 21 -
It is preferable that the paint be sprayed into the
tubes and headers. Sufficient paint should be applied to
fully coat the tubes and headers. After a module is
sprayed, it should be left to dry for about 24 hours
followed by application of a slow stream of heated nitrogen
(e.g., about 150°F for about 24 hours). Thereafter, it is
preferable that a second coat of paint be applied and also
dried by the procedure described above. After the paint has
been applied, the modules should preferably be kept under a
slight nitrogen pressure and should not be exposed to
temperatures exceeding about 200°F prior to installation,
nor should they be exposed to water except during
hydrotesting.
Iron bearing reactive paints are also useful in the
present invention. Such an iron bearing reactive paint will
preferably contain various tin compounds to which iron has
been added in amounts up to one third Fe/Sn by weight.
The addition of iron can, for example, be in the form
of Fe203. The addition of iron to a tin containing paint
should afford noteworthy advantages; in particular: (i) it
should facilitate the reaction of the paint to form iron
stannides thereby acting as a flux; (ii) it should dilute
the nickel concentration in the stannide layer thereby
providing better protection against coking; and (iii) it
should result in a paint which affords the anti-coking
protection of iron stannides even if the underlying surface
does not react well.
9U1BSTTTITfE SHEET (RULE 261
WO 94/16035 PCT/US94/0010'~
- 22 -
Yet another means for preventing carburization, coking,
and metal dusting in the low-sulfur reactor system comprises
the application of a metal coating or cladding to chromium
rich steels contained in the reactor system. These metal
coatings or claddings may be comprised of tin, antimony,
bismuth, germanium, indium, selenium, tellurium or arsenic.
Tin is especially preferred. These coatings or claddings
may be applied by methods including electroplating, vapor
depositing, and soaking of the chromium rich steel in a
molten metal bath.
It has been found that in reactor systems where
carburization, coking, and metal dusting are particularly
problematic that the coating of the chromium-rich,
nickel-containing steels with a layer of tin in effect
creates a double protective layer. There results an inner
chromium rich layer which is resistant to carburization,
coking, and metal dusting and an outer tin layer which is
also resistant to carburization, coking and metal dusting.
This occurs because when the tin coated chromium rich steel
is exposed to typical reforming temperatures, such as about
1200°F, it reacts with the steel to form nickel-rich iron
nickel stannides. Thereby, the nickel is preferentially
leached from the surface of the steel leaving behind a layer
of chromium rich steel. In some instances, it may be
desirable to remove the iron nickel stannide layer from the
stainless steel to expose the chromium rich steel layer.
SUBSiTTIJTE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
23 _21~~z~~
For example, it was found that when a n cladding Haas
applied to a 304 grade stainless steel and neated at abut
1200°F there resulted a chromium rich steel layer containing
about 17% chromium and substantially no nickel, comparable
to 430 grade stainless steel.
When applying the tin metal coating or cladding to the
chromium rich steel, it may be desirable to vary the
thickness of the metal coating or cladding to achieve the
desired resistance against carburization, coking, and metal
dusting. This can be done by, e.g., adjusting the amount of
time the chromium rich steel is soaked in a molten tin bath.
This will also affect the thickness of the resulting
chromium rich steel layer. It may also be desirable to vary
the operating temperature, or to vary the composition of the
chromium rich steel which is coated which in order to
control the chromium concentration in the chromium rich
steel layer produced.
It has additionally been found that tin-coated steels
can be further protected from carburization, metal dusting,
and coking by a post-treatment process which involves
application of a thin oxide coating, preferably a chromium
oxide, such as Cr203. This coating will be thin, as thin as
a few Vim. Application of such a chromium oxide will protect
aluminum as well as tin coated steels, such as Alonized
steels, under low-sulfur reforming conditions.
The chromium oxide layer can be applied by various
methods including: application of a chromate or dichromate
SUBSTTTITr~ SHE~'~ (RUL~ 261
WO 94/16035 PCT/US94/0010'~
- 24 -
paint followed by a reduction process; vapor treatment with
an organo-chromium compound; or application of a chromium
metal plating followed by oxidation of the resulting
chromium plated steel.
Examination of tin-electroplated steels which have been
subjected to low-sulfur reforming conditions for a
substantial period of time has shown that when a chromium
oxide layer is produced on the surface of the stannide layer
or under the stannide layer, the chromium oxide layer does
not cause deterioration of the stannide layer, but appears
to render the steel further resistant to carburization,
coking and metal dusting. Accordingly, application of a
chromium oxide layer to either tin or aluminum coated steels
will result in steels which are further resistant to
carburization and coking under the low-sulfur reforming
conditions. This post-treatment process has particular
applications for treating tin or aluminum coated steels
which, after prolonged exposure to low-sulfur reforming
conditions, are in need of repair.
While not wishing to be bound by theory, it is believed
that the suitability of various materials for the present
invention can be selected and classified according to their
responses to carburizing atmospheres. For example, iron,
cobalt, and nickel form relatively unstable carbides which
will subsequently carburize, coke and dust. Elements such
as chromium, niobium, vanadium, tungsten, molybdenum,
tantalum and zirconium, will form stable carbides which are
SUBS'TiTIJTE SHFE'T (RULE 261
WO 94116035 PCT/US94100107
~~~3~zs
- 25 -
more resistant to carburization coking and dusting.
Elements such as tin, antimony and bismuth do not form
carbides or coke. And, these compounds can form stable
compounds with many metals such as iron, nickel and copper
under reforming conditions. Stannides, antimonides and
bismuthides, and compounds of lead, mercury, arsenic,
germanium, indium, tellurium, selenium, thallium, sulfur and
oxygen are also resistant. A final category of materials
include elements such as silver, copper, gold, platinum and
refractory oxides such as silica and alumina. These
materials are resistant and do not form carbides, or react
with other metals in a carburizing environment under
reforming conditions.
Because different areas of the reactor system of the
invention (e.g., different areas in a furnace) can be
exposed to a wide range of temperatures, the material
selection and thickness of coating can be staged, such that
better carburization resistances are used in those areas of
the system experiencing the highest temperatures. In any
case, the carburization resistant coating should be used in
amounts such that the metal oxides present in the reactor
system do not consume the entire protective coating. It is
preferred that any remaining oxide in the oxidated surfaces
is fixated. By "fixated", as used herein, it is meant
applying a coating of the carburization resistant coating
over the oxidized metal such that the oxide does not form
finely porous Fe, Ni metal and the like, which is extremely
SUBSTITUTE SHEET (RULE 261
WO 94!16035 ~ PCT/US94/OOlOi
- 26 -
reactive for coking and may infect underlying steel with
carburization and pitting.
With regard to materials selection, it was discovered
that oxidized Group VIII metal surfaces such as iron, nickel
and cobalt are more active in terms of coking and
carburization than their unoxidized counterparts. For
example, it was found that an air roasted sample of 347
stainless steel was significantly more active than an
unoxidized sample of the same steel. This is believed to be
l0 due to a re-reduction of oxidized steels which produces very
fine-grained iron and/or nickel metals. Such metals are
especially active for carburization and coking. Thus, it is
desirable to avoid these materials as much as possible
during oxidative regeneration processes, such as those
typically used in catalytic reforming. However, it has been
found that an air roasted 300 series stainless steel coated
with tin can provide similar resistances to coking and
carburization as unroasted samples of the same tin coated
300 series stainless steel.
Furthermore, it will be appreciated that oxidation will
be a problem in systems where sulfur sensitivity of the
catalyst is not of concern, and sulfur is used to passivate
the metal surfaces. If sulfur levels in such systems ever
become insufficient, any metal sulfides which have formed on
metal surfaces would, after oxidation and reduction, be
reduced to fine-grained metal. This metal would be highly
reactive for coking and carburization. Potentially, this
SlJBSTITiTfE SKEET (RULE 261
VVO 94/16035 PCT/US94/00107
2l~~zz~
- 27 -
can cause a catastrophic failure of the metallurgy, or a
major coking event.
As noted above, excessively high temperatures can occur
in the catalyst beds when exothermic demethanation reactions
within cokeballs cause localized hot regions. These hot
spots also pose a problem in conventional reforming reactor
systems (as well as other areas of chemical and
petrochemical processing).
For example, the center pipe screens of reformers have
been observed to locally waste away and develop holes;
ultimately resulting in catalyst migration. In conventional
reforming processes the temperatures within cokeballs during
formation and burning are apparently high enough to overcome
the ability of process sulfur to poison coking,
carburization, and dusting. The metal screens, therefore,
carburize and are more sensitive to wasting by intergranular
oxidation (a type of corrosion) during regeneration. The
screen openings enlarge and holes develop.
Thus, the teachings of the present invention are
applicable to conventional reforming, as well as other areas
of chemical and petrochemical processing. For example, the
aforementioned platings, claddings and coatings can be used
in the preparation of center pipe screens to avoid excessive
hole development and catalyst migration. In addition, the
teachings can be applied to any furnace tubes which are
subjected to carburization, coking and metal dusting, such
as furnace tubes in coker furnaces.
SU8ST1T1JTE SHEET (RULE 261
CA 02153229 2003-04-16
- 28 -
In addition, since the techniques described herein
can be used to control carburization, coking, and metal
dusting at excessively high temperatures, they can be
used in cracking furnaces operating at from about 1400°
to about 1700°F. For example, the deterioration of steel
occurring in cracking furnaces operating at those
temperatures can be controlled by application of various
metal coatings. These metal coatings can be applied by
melting, electroplating, and painting. Painting is
particularly preferred.
For example, a coating of antimony applied to iron
bearing steels protects these steels from carburization,
coking and metal dusting under the described cracking
conditions. In fact, an antimony paint applied to iron
bearing steels will provide protection against
carburization, coking, and metal dusting at 1600°F.
A coating of bismuth applied to nickel rich steel
alloys (e. g., INCONELTM 600) can protect those steels
against carburization, coking, and metal dusting under
cracking conditions. This has been demonstrated at
temperatures of up to 1600°F.
Bismuth coatings may also be applied to iron bearing
steels and provide protection against carburization,
metal dusting, and coking under cracking conditions.
Also, a metal coating comprising a combination of
bismuth, antimony, and/or tin can be used.
Looking again to low-sulfur reforming, other
techniques can also be used to address the problem
discovered according
WO 94/16035 PCT/US94/00107
- 29 -
to the present invention. They can be used in conjunction
with an appropriate material selection for the reactor
system, or they can be used alone. Preferred from among the
additional techniques is the addition of non-sulfur,
anti-carburizing and anti-coking agents) during the
reforming process. These agents can be added continuously
during processing and function to interact with those
surfaces of the reactor system which contact the
hydrocarbons, or they may be applied as a pretreatment to
the reactor system.
While not wishing to bound by theory it is believed
that these agents interact with the surfaces of the reactor
system by decomposition and surface attack to form iron
and/or nickel intermetallic compounds, such as stannides,
antimonides, bismuthides, plumbides, arsenides, etc. Such
intermetallic compounds are resistant to carburization,
coking and dusting and can protect the underlying
metallurgy.
The intermetallic compounds are also believed to be
more stable than the metal sulfides which were formed in
systems where HZS was used to passivate the metal. These
compounds are not reduced by hydrogen as are metal sulfides.
As a result, they are less likely to leave the system than
metal sulfides. Therefore, the coiltinuous addition of a
carburization inhibitor with the feed can be minimized.
Preferred non-sulfur anti-carburizing and anti-coking
agents include organo-metallic compounds such as organo-tin
SUBST1T1T~E SHEET (RULE 26b
WO 94/16035 PCT/US94IOOl0i
~,~~~~,~9
compounds, organo-antimony compounds, organo-bismuth
compounds, organo-arsenic compounds, and organo-lead
compounds. Suitable organo-lead compounds include
tetraethyl and tetramethyl lead. Organo-tin compounds such
as tetrabutyl tin and trimethyl tin hydride are especially
preferred.
Additional specific organo-metallic compounds include
bismuth neodecanoate, chromium octoate, copper naphthenate,
manganese carboxylate, palladium neodecanoate, silver
neodecanoate, tetrabutylgermanium, tributylantimony,
triphenylantimony, triphenylarsine, and zirconium octoate.
How and where these agents are added to the reactor
system is not critical, and will primarily depend on
particular process design characteristics. For example,
they can be added continuously or discontinuously with the
feed .
However, adding the agents to the feed is not preferred
as they would tend to accumulate in the initial portions of
the reactor system. This may not provide adequate
protection in the other areas of the system.
It is preferred that the agents be provided as a
coating prior to construction, prior to start-up, or in-situ
(i.e., in an existing system). If added in-situ, it should
be done right after catalyst regeneration. Very thin
coatings can be applied. For example, it is believed that
when using organo-tin compounds, iron stannide coatings as
thin as 0.1 micron can be effective.
SlIBSTTT1JTE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
215329
- 31 -
A preferred method of coating the agents on an existing
or new reactor surface, or a new or existing furnace tube is
to decompose an organometallic compound in a hydrogen
atmosphere at temperatures of about 900°F. For organo-tin
compounds, for example, this produces reactive metallic tin
on the tube surface. At these temperatures the tin will
further react with the surface metal to passivate it.
Optimum coating temperatures will depend on the
particular organometallic compound, or the mixtures of
compounds if alloys are desired. Typically, an excess of
the organometallic coating agent can be pulsed into the
tubes at a high hydrogen flow rate so as to carry the
coating agent throughout the system in a mist. The flow
rate can then be reduced to permit the coating metal mist to
coat and react with the furnace tube or reactor surface.
Alternatively, the compound can be introduced as a vapor
which decomposes and reacts with the hot walls of the tube
or reactor in a reducing atmosphere.
As discussed above, reforming reactor systems
susceptible to carburization, metal dusting and coking can
be treated by application of a decomposable coating
containing a decomposable organometallic tin compound to
those areas of the reactor system most susceptible to
carburization. Such an approach works particularly well in
a temperature controlled furnace.
However, such control is not always present. There are
"hot spots" which develop in the reactor system,
SU8ST1T1JTF SHEET (RULE 261
WO 94/16035 PCT/US94/OOlOi
- 32 -
particularly in the furnace tubes, where the organometallic
compound can decompose and form deposits. Therefore,
another aspect of the invention is a process which avoids
such deposition in reforming reactor systems where
temperatures are not closely controlled and exhibit areas of
high temperature hot spots.
Such a process involves preheating the entire reactor
system to a temperature of from 750 to 1150°F, preferably
900 to 1100°F, and most preferably about 1050°F, with a hot
stream of hydrogen gas. After preheating, a colder gas
stream at a temperature of 400 to 800°F, preferably 500
to 70o°F, and most preferably about 550°F, containing a
vaporized organometallic tin compound and hydrogen gas is
introduced into the preheated reactor system. This gas
mixture is introduced upstream and can provide a
decomposition "wave" which travels throughout the entire
reactor system. Essentially this process works because
the hot hydrogen gas produces a uniformly heated surface
which will decompose the colder organometallic gas as it
travels as a wave throughout the reactor system. The colder
gas containing the organometallic tin compound will
decompose on the hot surface and coat the surface. The
organometallic tin vapor will continue to move as a wave to
treat the hotter surfaces downstream in the reactor system.
Thereby, the entire reactor system can have a uniform
coating of the organometallic tin compound. It may also be
desirable to conduct several of these hot-cold temperature
SU$STTTiTfE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
_ 33 ~.I c~ ~~
cycles to ensure that the entire reactor system has been
uniformly coated with the organometallic tin compound.
In operation of the reforming reactor system according
to the present invention, naphtha will be reformed to form
aromatics. The naphtha feed is a light hydrocarbon,
preferably boiling in the range of about 70°F to 450°F, more
preferably about 100 to 350°F. The naphtha feed will
contain aliphatic or paraffinic hydrocarbons. These
aliphatics are converted, at least in part, to aromatics in
the reforming reaction zone.
In the "low-sulfur" system of the invention, the feed
will preferably contain less than 100 ppm sulfur and more
preferably, less than 50 ppm sulfur. When using a large
pore zeolite catalyst, the feed will preferably contain less
than 100 ppb sulfur, more preferably, less than 50 ppb
sulfur, more preferably, less than 10 ppb sulfur, and even
more preferably, less than 5 ppb sulfur. If necessary, a
sulfur sorber unit can be employed to remove small excesses
of sulfur.
Preferred reforming process conditions include a
temperature between 700 and 1050°F, more preferably between
850 and 1025°F; and a pressure between 0 and 400 psig, more
preferably between 15 and 150 psig; a recycle hydrogen rate
sufficient to yield a hydrogen to hydrocarbon mole ratio for
the feed to the reforming reaction zone between 0.1 and 20,
more preferably between 0.5 and 10; and a liquid hourly
space velocity for the hydrocarbon feed over the reforming
SUBSTTTIJTE SHEET (RULE 261
WO 94/16035 PCT/L1S94/0010'i
_ _
34
catalyst of between 0.1 and 10, more preferably between 0.5
and 5. At these temperatures, tin reacts with the oxidized
metals to replace oxygen in the metals with tin.
To achieve the suitable reformer temperatures, it is
often necessary to heat the furnace tubes to high
temperatures. These temperatures can often range from 600
to 1800°F, usually from 850 and 1250°F, and more often from
900 and 1200°F.
As noted above, the problems of carburization, coking
and metal dusting in low-sulfur systems have been found to
associated with excessively high, localized process
temperatures of the reactor system, and are particularly
acute in the furnace tubes of the system where particularly
high temperatures are characteristic. In conventional
reforming techniques where high levels of sulfur are
present, furnace tube skin temperatures of up to 1175°F at
end of run are typical. Yet, excessive carburization,
coking and metal dusting was not observed. In low-sulfur
systems, however, it has been discovered that excessive and
rapid carburization, coking and metal dusting occurred with
CrMo steels at temperatures above 950°F, and stainless
steels at temperatures above 1025°F.
Accordingly, another aspect of the invention is to
lower the temperatures of the metal surfaces inside the
furnace tubes, transfer-lines and/or reactors of the
reforming system below the aforementioned levels. For
example, temperatures can be monitored using thermocouples
S1J9ST1TUTF SHEET (RULE 261
WO 94/16035 ~ ~ ~ PCT/US94/00107
- 35 -
attached at various locations in the reactor system. In the
case of furnace tubes, thermocouples can be attached to the
outer walls thereof, preferably at the hottest point of the
furnace (usually near the furnace outlet). When necessary,
adjustments in process operation can be made to maintain the
temperatures at desired levels.
There are other techniques for reducing exposure of
system surfaces to undesirably high temperatures as well.
For example, heat transfer areas can be used with resistant
(and usually more costly) tubing in the final stage where
temperatures are usually the highest.
In addition, superheated hydrogen can be added between
reactors of the reforming system. Also, a larger catalyst
charge can be used. And, the catalyst can be regenerated
more frequently. In the case of catalyst regeneration, it
is best accomplished using a moving bed process where the
catalyst is withdrawn from the final bed, regenerated, and
charged to the first bed.
Carburization and metal dusting can also be minimized
in the low-sulfur reforming reactor system of the invention
by using certain other novel equipment configurations and
process conditions. For example, the reactor system can be
constructed with staged heaters and/or tubes. In other
words, the heaters or tubes which are subjected to the most
extreme temperature conditions in the reactor system can be
constructed of materials more resistant to carburization
than materials conventionally used in the construction of
SlI9STTT1JTE SHEET (RULE 261
WO 94/16035 ~~~ PCT/US94/OOlOi
- 36 -
reforming reactor systems; materials such as those described
above. Heaters or tubes which are not subjected to extreme
temperatures can continue to be constructed of conventional
materials.
By using such a staged design in the reactor system, it
is possible to reduce the overall cost of the system (since
carburization resistant materials are generally more
expensive than conventional materials) while still providing
a reactor system which is sufficiently resistant to
carburization and metal dusting under low-sulfur reforming
conditions. Additionally, this should facilitate the
retrofitting of existing reforming reactor systems to render
them carburization and metal dusting resistant under
low-sulfur operating conditions; since a smaller portion of
the reactor system would need replacement or modification
with a staged design.
The reactor system can also be operated using at least
two temperature zones; at least one of higher and one of
lower temperature. This approach is based on the
observation that metal dusting has a temperature maximum and
minimum, above and below which dusting is minimized.
Therefore, by "higher" temperatures, it is meant that the
temperatures are higher than those conventionally used in
reforming reactor systems and higher than the temperature
maximum for dusting. By "lower" temperatures it is meant
that the temperature is at or about the temperatures which
SUBST1TLTTF SHEET (RULE 26L
WO 94/16035 PCT/US94/00107
~~.~3~29
- 37 -
reforming processes are conventionally conducted, and falls
below that in which dusting becomes a problem.
Operation of portions of the reactor system in
different temperature zones should reduce metal dusting as
less of the reactor system is at a temperature conducive for
metal dusting. Also, other advantages of such a design
include improved heat transfer efficiencies and the ability
to reduce equipment size because of the operation of
portions of the system at higher temperatures. However,
l0 operating portions of the reactor system at levels below and
above that conducive for metal dusting would only minimize,
not completely avoid, the temperature range at which metal
dusting occurs. This is unavoidable because of temperature
fluctuations which will occur during day to day operation of
the reforming reactor system; particularly fluctuations
during shut-down and start-up of the system, temperature
fluctuations during cycling, and temperature fluctuations
which will occur as the process fluids are heated in the
reactor system.
Another approach to minimizing metal dusting relates to
providing heat to the system using superheated raw materials
(such as e.g., hydrogen), thereby minimizing the need to
heat the hydrocarbons through furnace walls.
Yet another process design approach involves providing
a pre-existing reforming reactor system with larger tube
diameters and/or higher tube velocities. Using larger tube
diameters and/or higher tube velocities will minimize the
SUBSTmT~E SHEET (RULE 261
WO 94/16035 PCT/US94/00107
~~r~3'
- 38 -
exposure of the heating surfaces in the reactor system to
the hydrocarbons.
As noted above, catalytic reforming is well known in
the petroleum industry and involves the treatment of naphtha
fractions to improve octane rating by the production of
aromatics. The more important hydrocarbon reactions which
occur during the reforming operation include the
dehydrogenation of cyclohexanes to aromatics,
dehydroisomerization of alkycyclopentanes to aromatics, and
dehydrocyclization of acyclic hydrocarbons to aromatics. In
addition, a number of other reactions also occur, including
the dealkylation of alkylbenzenes, isomerization of
paraffins, and hydrocracking reactions which produce light
gaseous hydrocarbons, e.g., methane, ethane, propane and
butane, which hydrocracking reactions should be minimized
during reforming as they decrease the yield of gasoline
boiling products and hydrogen. Thus, "reforming" as used
herein refers to the treatment of a hydrocarbon feed through
the use of one or more aromatics producing reactions in
order to provide an aromatics enriched product (i.e., a
product whose aromatics content is greater than in the
feed).
While the present invention is directed primarily to
catalytic reforming, it will be useful generally in the
production of aromatic hydrocarbons from various hydrocarbon
feedstocks under conditions of low sulfur. That is, while
catalytic reforming typically refers to the conversion of
S1lBSTfTIJTE SHEET (RULE 261
WO 94/16035 ~ ~ PCT/US94/00107
- 39 -
naphthas, other feedstocks can be treated as well to provide
an aromatics enriched product. Therefore, while the
conversion of naphthas is a preferred embodiment, the
present invention can be useful for the conversion or
aromatization of a variety of feedstocks such as paraffin
hydrocarbons, olefin hydrocarbons, acetylene hydrocarbons,
cyclic paraffin hydrocarbons, cyclic olefin hydrocarbons,
and mixtures thereof, and particularly saturated
hydrocarbons.
Examples of paraffin hydrocarbons are those having 6 to
10 carbons such as n-hexane, methylpentane, n-haptane,
methylhexane, dimethylpentane and n-octane. Examples of
acetylene hydrocarbons are those having 6 to 10 carbon atoms
such as hexyne, heptyne and octyne. Examples of acyclic
paraffin hydrocarbons are those having 6 to 10 carbon atoms
such as methylcyclopentane, cyclohexane, methylcyclohexane
and dimethylcyclohexane. Typical examples of cyclic olefin
hydrocarbons are those having 6 to 10 carbon atoms such as
methylcyclopentene, cyclohexene, methylcyclohexene, and
dimethylcyclohexene.
The present invention will also be useful for reforming
under low-sulfur conditions using a variety of different
reforming catalysts. Such catalyst include, but are not
limited to Noble Group VIII metals on refractory inorganic
oxides such as platinum on alumina, Pt/Sn on alumina and
Pt/Re on alumina; Noble Group VIII metals on a zeolite such
as Pt, Pt/Sn and Pt/Re on zeolites such as L-zeolites,
SUBST1T1JTE SHEET (RULE 261
CA 02153229 2003-04-16
- 40 -
ZSM-5, silicalite and beta; and Nobel Group VIII metals
on alkali- and alkaline-earth exchanged L-zeolites.
A preferred embodiment of the invention involves the
use of a large-pore zeolite catalyst including an alkali
or alkaline earth metal and charged with one or more
Group VIII metals. Most preferred is the embodiment where
such a catalyst is used in reforming a naphtha feed.
The term "large pore zeolite" is indicative
generally of a zeolite having an effective pore diameter
of 6 to 15 Angstroms. Preferable large pore crystalline
zeolites which are useful in the present invention
include the type L zeolite, zeolite X, zeolite Y and
faujasite. These have apparent pore sizes on the order to
7 to 9 Angstroms. Most preferably the zeolite is a type L
zeolite.
The composition of type L zeolite expressed in terms
of mole ratios of oxides, may be represented by the
following formula:
2 0 ( 0 . 9-1 . 3 ) M2 /n0 : A1 z03 ( 5 . 2-6 . 9 ) S 7.02 : yH20
In the above formula M represents a cation, n represents
the valence of M, and y may be any value from 0 to about
9. Zeolite L, its X-ray diffraction pattern, its
properties, and method for its preparation are described
in detail in, for example, U.S. Patent No. 3,216,789.
The actual formula may vary without changing the
crystalline structure. For example, the mole ratio of
silicon to aluminum (Si/Al) may vary from 1.0 to 3.5.
CA 02153229 2003-04-16
- 41 -
The chemical formula for zeolite Y expressed in
terms of mole ratios of oxides may be written as:
( 0 . 7-1 .1 ) Na20 : A1z03 : xS i02 : yH20
In the above formula, x is a value greater than 3 and up
to about 6. y may be a value up to about 9. Zeolite Y has
a characteristic X-ray powder diffraction pattern which
may be employed with the above formula for
identification. Zeolite Y is described in more detail in
U.S. Patent No. 3,130,007.
Zeolite X is a synthetic crystalline zeolitic
molecular sieve which may be represented by the formula:
(0.7-1.1)Mz/nO:A1203: (2.0-3.0)S102:yH20
In the above formula, N represents a metal, particularly
alkali and alkaline earth metals, n is the valence of M,
and y may have any value up to about 8 depending on the
identity of N and the degree of hydration of the
crystalline zeolite. Zeolite X, its X-ray diffraction
pattern, its properties, and method for its preparation
are described in detail in U.S. Patent No. 2,882,244.
An alkali or alkaline earth metal is preferably
present in the large-pore zeolite. That alkaline earth
metal may be either barium, strontium or calcium,
preferably barium. The alkaline earth metal can be
incorporated into the zeolite by synthesis, impregnation
or ion exchange. Barium is preferred to the other
alkaline earths because it results in a somewhat less
acidic catalyst. Strong acidity is
WO 94/16035 PCT/US94/0010'7
- 42 -
undesirable in the catalyst because it promotes cracking,
resulting in lower selectivity.
In another embodiment, at least part of the alkali
metal can be exchanged with barium using known techniques
for ion exchange of zeolites. This involves contacting the
zeolite with a solution containing excess Ba++ions. In this
embodiment the barium should preferably constitute from 0.1%
to 35% by weight of the zeolite.
The large-pore zeolitic catalysts used in the invention
are charged with one or more Group VIII metals, e.g.,
nickel, ruthenium, rhodium, palladium, iridium or platinum.
The preferred Group VIII metals are iridium and particularly
platinum. These are more selective with regard to
dehydrocyclization and are also more stable under the
dehydrocyclization reaction conditions than other Group VIII
metals. If used, the preferred weight percentage of
platinum in the catalyst is between 0.1% and 5%.
Group VIII metals are introduced into large-pore
zeolites by synthesis, impregnation or exchange in an
aqueous solution of appropriate salt. When it is desired to
introduce two Group VIII metals into the zeolite, the
operation may be carried out simultaneously or sequentially.
To obtain a more complete understanding of the present
invention, the following examples illustrating certain
aspects of the invention are set forth. It should be
SIJBS'I'ITiJTE SHEET (RULE 261
~ T
PCT/US94/00107
WO 94/16035
- 43 -
understood, however, that the invention is not limited in
any way to the specific details set forth therein.
EXAMPLES
High-temperature stress relief procedures applied to
steel~in air during construction of refinery reactor and
furnace systems can produce oxide scales. Such oxide scales
are reactive in carburizing environments. The following
paragraphs examine the scales produced in typical oxidative
treatments (an hour or two at 1650°F in air); how these
scales behave in carburizing environments; and how the
scales respond to direct application of protective tin
paint.
The materials investigated were samples of type 347
stainless steel prepared by heat treating in air in an
electric furnace for one hour at 1650°F. Samples were
treated for up to 2? hours in air in both electric and
flame-fired furnaces. These materials were examined as
prepared in the oxidized state. Additional samples were
exposed to carburizing conditions in a bench carburization
apparatus for various durations of a few hours to one week
at temperatures from 850°F to 1200°F. Other samples were
painted with protective tin paint, cured (reduced) and
examined. Other protected samples were exposed to
carburizing conditions at 1150°F for up to two weeks.
Petrographic microscopy analysis revealed that the
oxide scales that form at high temperature in air can be
S1JBS11ME SHEET (RULE 261
WO 94/16035 PCT/US94/001f7
- 44 -
thick and complex. They typically consist of three layers.
The outermost is hematite - Fe203; a middle layer is
magnetite - Fe304; the innermost is ferrochromite - FeCr204.
The magnetite may contain some chromium. The chromite layer
may contain fine, nickel-rich metal inclusions. In a
carburizing environment, the oxides largely become reduced
to a fine-grained porous iron metal deposit which is
extremely reactive and coke at temperatures as low as 850°F.
For this reason, oxidized surfaces can result in serious
coking problems.
A protective coating of, e.g., tin paint may be
directly applied upon oxidized steel surfaces. The tin
reacts with the iron in the oxide scale to form a
coke-inhibiting layer of, e.g., iron stannide on chromium
oxide. If the scale is not so thick that it completely
consumes the tin, the remaining tin penetrates the chromium
oxide layer to react with the steel surface.
The following paragraphs provide a description of
various observations made when examining samples of 347
stainless steel which has been exposed to oxidizing and/or
carburizing atmospheres. These observations are further
illustrated in Figures 1-30. The above and other
observations are illustrated below.
A 347 stainless steel furnace tube heat-treated
at 1650°F in air for 1 hour was cut into smaller pieces
suitable for petrographic examination. A fresh sample was
SUBSTiTITTE SHEET (RULE 26b
WO 94/16035 PCT/US94/00107
- 45 -
mounted in epoxy resin, ground, and polished for examination
with petrographic and scanning electron microscopes.
Additional samples were tested in a bench carburization
apparatus for various durations at 1000°F, 1025°F,
1050°F,
1150°F, and 1200°F in an atmosphere of approximately 1%
toluene in 7% propane in hydrogen. These samples were then
prepared for petrographic microscopy analysis.
Another sample was tin-passivated using tin paint. The
paint was allowed to dry overnight. The sample was then
heated in 4% hydrogen in nitrogen at 200°F per hour to 950°F
and held for 20 hours. This sample was then exposed to the
carburizing atmosphere for two weeks at 1150°F. After one
week, the carburization test was momentarily interrupted to
examine the sample.
Petrographic microscopy analysis of the fresh,
heat-treated steel revealed uniformly thin (5 ~cm) and
adherent oxide scales on the air-exposed surfaces
(Figure 1). At high magnification, a duplex scale
consisting of a thin inner layer and scattered crystals of
ferrochromite and a thicker outer layer of magnetite is
shown (Figure 2). The oxide scale shallowly penetrated the
steel surface along grain boundaries producing an escalloped
pattern.
SEM-EDX analyses showed that the steel surface was
depleted in chromium and relatively enriched in nickel.
Thus, it is believed that the composition would be more
SUBS?1T1JTE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
46 -
reactive in the carburizing environment an unoxidized normal
type 347 stainless steel composition.
The alteration of the composition of the steel surface
illustrates that the oxide soale is enriched in chromium and
depleted in nickel. The weight % of Cr, Fe and Ni for each
layer is set forth below.
Inner Oxide Outer Oxide
wt . % ( FeCr~04 ) ( Fe~04 )
% Cr 65.0 6.8
Fe 28.6 91.2
N1 6.8 1.0
A sample of this oxidized steel was exposed for 5 days
at 1000°F in the carburizing atmosphere. The sample did not
coke in this test. The magnetite layer of the oxide scale
was completely altered to porous, fine-grained iron metal
(Figures 3 and 4).
The tendency of oxide scales to reduce to fine reactive
metal in the carburizing atmosphere is illustrated in
Figure 5. After only five hours at 1150°F, the oxidized
surfaces had coked abundantly, whereas the raw steel was
merely slightly tarnished.
A coating of tin paint provided protection against
coking and carburization for two weeks at 1150°F on all
surfaces including the oxidized surfaces as shown in
Figure 6.
Similar vigorous coking on oxidized surfaces was
observed on samples exposed to the carburizing atmosphere
SUBSTTT1JTE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
- 47 -
for 25 hours at 1025°F and for 2Z hours at 1200°F (Figures 7
and 8 ) .
A photomicrograph of a sample exposed to the
carburizing atmosphere at 1200°F (Figure 9) shows not only
that the iron oxide had completely decomposed but that
pitting and carburization attack had locally occurred on the
underlying steel itself. This shows that an oxide scale or
accumulation of oxide particulates can infect an otherwise
resistant steel with a coking problem. The raw surfaces of
this sample were merely tarnished. At high magnification,
it is shown that the chromium-rich oxide persists and
continues to offer some protection to the steel (Figure 10).
Figures 11 and 12 show at high magnification how the
tin paint had reacted with the oxidized and raw steel
surfaces, respectively, through the reduction step.
Figure 11 shows that the iron oxide layer had thoroughly
reacted with the tin to produce a series of iron stannide
compounds. The layer and crystals of ferrochromite
persisted under the stannide. Some excess tin remained on
top of the stannide. SEM-EDX analysis of the ferrochromite
gave:
Cr 58.7
Fe 30.4
N1 2.3
Nb 4.1
No 0.2
Sn 6.9
SUBStITIJtE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
- 48 -
On the raw steel surface (Figure 12), a smooth,
continuous layer of stannide (two phases) about 4 um
thick formed.
Because the stannide had not directly coated
the steel on the oxidized surface, it was thought
that direct application of the protective tin paint
to an oxidized surface might not be practical.
Surprisingly, it worked very well. Even more
surprisingly, after two weeks at 1150°F in the
carburizing atmosphere, the stannide layer had
migrated through the ferrochromite layer to attack
the steel surface beneath, resulting in a continuous
coating of stannide directly on the steel
(Figures 13, 15 and 16). Most of the ferrochromite
had been reduced to eskolaite (Cr203). This
experiment also showed that both the eskolaite and
the stannide inhibit coking. SEM-EDX analysis of
the eskolaite gave:
% Cr 72.7
Fe 7.1
Ni 1.3
Nb 15.5
Mo 0.0
Sn 9.2
Figure 14 shows that direct application of tin
paint to a raw steel surface produces a smooth and
continuous stannide coating on the steel. This
coating was about 15 ~,m thick after two weeks in the
SUBST'(TUTE SIiFET (RULE 26L
WO 94/16035 PCT/US94100107
-'~4~~~'.2 ~ r9
carburizing atmosphere at 1150°F. Some differences
in the thickness of stannide layers on different
samples was expected. However, most of the
variation in thickness on this sample is believed to
be a result of various degrees of reaction of
available tin with the underlying steel (compare
Figure 12).
Samples of 347 stainless steel furnace tube
that had been heat-treated in air in a gas-fired
furnace for 2Z hours were examined after exposure to
a carburizing atmosphere of approximately 1% toluene
in 7% propane in hydrogen for three hours at 1050°F.
Another sample had one oxidized surface and four raw
surfaces painted with a ferruginous tin paint with
Fe203 - and the other oxidized surface painted with
plain tin paint - PM 30o A2. The ferruginous paint
had been reduced in 50/50 HZNZ at 950°F. The plain
tin paint was not reduced prior to exposure to the
carburizing atmosphere. The sample was then treated
in the carburizing atmosphere at 1150°F for five
days.
Figures 17 and 18 show that the heat treatment
produced a thicker and more complicated oxide scale
than the previous treatment. The scale averaged
about 40 ~m in thickness, It consisted of an outer
layer of hematite - Fe203; a middle layer of
magnetite - Fe304; an inner layer of ferrochromite
SUBST1T1T~ SHEET (RULE 261
WO 94/16035 PCT/US94/00107
~~ ~~'~~~'~ -
50 -
shot with fine (< 1 ~m grains) nickel-rich metal;
and a thin, innermost layer of pure ferrochromite.
The clouds of metal grains in the ferrochromite
layer are believed to be the result of
low-temperature decomposition of a wiistite-type
phase. A chromium depleted, nickel enriched zone at
the surface of the steel in this sample was not
detected as with the oxidized sample. SEM-EDX
analyses of the oxides gave:
wt.% Ferrochromite Magnetite Hematite
Cr 48.2 1.2 0.5
Fe 44.7 98.6 99,g
Ni 5.9 0.2 0.0
Nb 0.8 0.0 0.0
Mo 0.9 0.5 0.0
The oxidized surfaces coked profusely after
only three hours at 1050°F (Figure 19). A little
coking also occurred on adjacent raw surfaces.
Surprisingly, however, the tin paint provided nearly
complete protection against coking and carburization
after five days at 1150°F (Figure 20).
The underside of the sample in Figure 20 had
been painted with plain tin paint. A
photomicrograph (Figure 21) shows that the resulting
stannide effectively coated and veined the
ferrochromite layer, sealing off the reactive
SUBSITtITTE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
- 51 -
nickel-rich metal particles and protecting the
underlying steel.
The upper side of the sample in Figure 20 had
been painted with a ferruginous variety of the tin
paint, which contained 5% fine powdered FeZ03.
Unexpectedly, we have observed that the presence of
some fine-grained iron oxide helps the tin in the
paint react with stainless steel, producing thicker
coatings of stannide.
l0 Figure 22 shows that stannide had effectively
coated most of the ferrochromite. However, locally
the stannide coating was breached exposing the
underlying ferrochromite and steel to the
carburizing atmosphere. Active coating at these
localities peeled back some of the protective
stannide, exposing more of the steel to attack.
At high magnification in Figure 2a, it is shown
that the plain tin paint was able to penetrate the
ferrochromite layer to produce a continuous stannide
coating directly on the steel. In Figure 24, it is
shown that some stannide formed directly on the
steel surface but it is mostly spotty and
discontinuous.
The above results show that it is possible to
directly apply the protective tin paint directly to
a thick layer of oxide on steel, but it is preferred
that the oxide not be so thick that it overwhelms
SUBSTmJTE SHEET (RULE 261
WO 94/16035 PCT/US94/0010~
52 -
the tin in the paint. Thus, it is preferred to
apply enough tin to react with the underlying steel
itself. These results also demonstrate that with
abundant iron in an oxide scale, it is not necessary
to add iron, oxide to the paint formula.
A 347 stainless steel furnace tube sample was
heat treated in air and Nz for 2i hours in an
electric furnace and exposed to a carburizing
atmosphere of approximately 1% toluene in 7% propane
in hydrogen at 900°F for 5 days.
A fresh, heat-treated steel sample revealed a
thick, complex oxide scale on the air-exposed
surface (Figures 25 and 27). A nitrogen-exposed
surface had a trace of oxide and chloride attack
(Figures 26 and 28) and, more curiously, a ten ~Cm
edge zone enriched in scattered chromium-rich
carbide grains. Some carbides or possibly nitrides
also occur scattered along the surface of the steel.
Similar carbides had appeared in a sample of
stainless steel heated in air at 1300°F for 24
hours. These carbides were attributed to reaction
with CO2.
A sample of heat-treated steel was exposed to
the carburizing atmosphere at 900°F for five days.
Some coking and dusting was observed on the oxidized
surface (Figures 29 and 30). Figure 30 shows that
the oxide was initially reduced to finely porous
SUBSTTTUTE SHEET (RULE 261
WO 94/16035 ~ ~' ~, ~ PCT/US94/00107
- 53 -
iron metal which then reacted with the carburizing
atmosphere.
Conclusions
The oxide scales that form on steels at high
.temperature in air can be thick and complex. They
typically consist of three layers. The outermost is
hematite - Fe203; a middle layer is magnetite - Fe304;
the innermost is ferrochromite - FeCr204. The
magnetite may contain some chromium. The chromite
layer may contain fine, nickel-rich metal
inclusions. In a carburizing environment, the
oxides largely become reduced to fine-grained iron
metal which is extremely reactive and cokes at
temperatures as low as 850°F. For this reason,
oxidized surfaces can result in serious coking
problems resulting from carburization.
By applying a protective coating of, e.g., tin
paint directly upon an oxidized steel surface, the
coating reacts with the iron in the oxide scale to
form a carburization inhibiting layer of iron
stannide on chromium oxide. If the scale is not so
thick that it completely consumes the tin, the
remaining tin penetrates the chromium oxide layer to
react with the steel surface. A continuous layer of
protective stannide can form on the steel this way.
SUBSTtTLTrE SHEET (RULE 261
WO 94/16035 PCT/US94/00107
54 -
2~1~~22°~ -
While the invention has been described above in
terms of preferred embodiments, it is to be
understood that variations and modifications may be
used. Such variations and modifications to the
above preferred embodiments which will be readily
evident to those skilled in the art, and which are
to be considered within the scope of the invention
as defined by the following claims.
SUBSTTTLTfE SHEET (RULE 261
