Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR RENDERING METALS CORROSION RESISTANT
The invention relates to a process for rendering metals corrosion
resistant by treating them with an anti-corrosion agent.
In plants in which corrosive streams occur, such as urea plants,
an oxidizer is often added to the plant as anti-corrosion agent in order to
protect
metallic materials of construction against corrosion. In this process an oxide
scale
develops on the metal parts, which protects against corrosion. This process is
also known as passivation of the metal. Customary passivating agents are
oxygen
or an oxygen-releasing compound as described in for example US-A-2.727.069,
where preferably oxygen in the form of air is employed in a urea process. The
passivating agent is added to for example one of the raw materials but may be
introduced into the plant in various locations. The amount of oxygen used
according to US-A-2.727.069 is 0.1-3 percent by volume relative to the amount
of
COZ when used in a, urea plant fabricated from chromium nickel steel
preferably
containing 16-20% chrome, 10-14% nickel and 1.75-4% of a metal belonging to
the group of molybdenum and zirconium.
Although such addition of oxygen/air protects the metallic
materials of construction against corrosion, it has a number of drawbacks:
- the amounts of oxygen/air need to be removed from the process without
non-converted raw materials and other volatile components leaving the
process. This requires costly and energy-consuming scrubbing facilities for
these gas streams;
- the raw materials for example urea production (ammonia and carbon
dioxide) as delivered from a modern ammonia plant invariably contain
traces of hydrogen. These, in combination with the passivating air added,
may lead to the formation of flammable hydrogen/air mixtures in particular
plant areas.
The potential formation of flammable gas mixtures is a problem
especially in those locations in urea plants where non-condensable streams are
cleared of ammonia and carbon dioxide. Costly provisions are needed to prevent
or to guard against this.
It is also known that, by applying duplex steel, the addition of
oxygen/air can be reduced considerably so that the existing drawbacks present
themselves to a lesser extent. WO-95/00674 describes the application of a
duplex
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steel grade in urea plants and refers to omission of the passivating gas.
It has been found, however, that omission of the passivating gas
in the case of the duplex steel grade according to WO-95/00674 does not always
produce the desired result. With this steel grade it is still possible for
corrosion to
occur in particular plant areas such as the high-pressure section of a urea
plant. It
has been found that where the duplex steel grade according to WO-95/00674 is
employed in the carbamate-bearing liquid streams, at least 5 ppm of oxygen
needs to be present in these streams in order to prevent corrosion.
Duplex steel is a stainless steel with a ferritic-austenitic structure
with the two phases having different compositions. The duplex structure means
that chromium and molybdenum are predominantly present in the ferrite phase
and nickel and nitrogen in the austenite phase.
Duplex steel may be used especially in urea plants, where it
comes into contact with the corrosive ammonium carbamate solutions and it may
successfully be used particularly in the high-pressure section of urea plants.
Here,
the most critical items such as the cladding of high-pressure vessels, heat
exchanger tubes, seals around manholes, piping, flanges and valves are
manufactured from duplex steel.
It has been found that the corrosion resistance of duplex steel
may be improved by utilizing a ferritic-austenitic duplex steel having a
chromium
content of befinreen 28 and 35 wt% and a nickel content of between 3 and 10
wt%
with an oxidizer being brought into contact with the metal parts and with the
passivating air being completely or partly omitted. In particular, the
passivating air
is completely omitted.
As oxidizers use is preferably made of peroxides, perborates,
percarbonates, nitrites, nitrates, oxides of nitrogen or trivalent metal ions
or a
mixture of these oxidizers. In particular, 0.001-1.5 wt% peroxide,
percarbonate,
perborate, nitrite, nitrate, oxide of nitrogen or a trivalent metal ion or a
mixtures of
these oxidizers is added relative to the amount of fresh raw materials used.
If,
besides an oxidizer, passivating air is added, the amount of passivating air
will be
such as to produce an oxygen concentration in the carbamate-containing liquid
streams of less than 2 ppm, preferably less than 1 ppm.
The oxidizer is particularly added to the high-pressure section of
a urea plant, more particularly to a point between the high-pressure reactor
and
the high-pressure stripper of a urea plant. If the oxidizers are gaseous or
may be
rendered gaseous, they are preferably introduced into the urea process via the
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feedstocks ammonia and carbon dioxide. If the oxidizers are added to the high-
pressure section of a urea plant as a liquid or solid, this is preferably
effected via
the recirculated carbamate stream coming from the further recovery of the urea
solution that has formed.
As peroxides use is preferably made of hydrogen peroxide or a
peroxide of an earth alkali metal, for example barium peroxide. Organic
peroxides
for example urea peroxide may also be used. As perborate use is made of for
example sodium perborate or potassium perborate. Sodium percarbonate is used
as percarbonate. As nitrate and/or nitrite use is made of for example the
sodium
salts or potassium salts or nitric acid and/or nitrous acid. As trivalent
metal ion
use is made of for example ferrisalts.
Preferably an austenitic-ferritic duplex steel with the following
composition is used:
C: maximum 0.05 wt.%
Si maximum 0.8
: wt.%
M n 0. 3 - 4. 0
: wt. % k ,
Cr: 28-35wt.%
N i 3 - 10 wt.
:
Mo: 1.0-4.Owt.%
N 0.2-0.6wt.%
:
Cu : maximum 1.0
wt.%
W : maximum 2.0
wt.%
S : maximum 0.01
wt.%
Ce : maximum 0.2
wt.%
the balance consisting of Fe and common impurities and additives and the
ferrite
content ranging from 30 to 70 vol%.
More preferably the C content is maximum 0.03 wt.% and in
particular maximum 0.02 wt.%, the Si content is maximum 0.5 wt.%, the Cr
content
29-33 wt.%, the Ni content 3-7 wt.%, the Mo content 1-3 wt.%, in particular
1-2 wt.%, the N content 0.36-0.55 wt.% and the Mn content 0.3-1 wt%.
The ferrite content is more preferably 30-55 vol%. The Cr content
of the austenite phase is more preferably at least 25 wt.% and in particular
at least
27 wt%.
It was found that the process of the invention reduced corrosion
to a minimum and that the risk of explosible hydrogen/air mixtures ceased to
exist
or was dramatically reduced.
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In a urea plant, the reduced susceptibility to corrosion of the
aforementioned duplex steel in combination with the passivating technique
applied
allows more use to be made of pumps in place of flow by gravity. In a urea
plant,
process items such as the high-pressure carbamate condenser and the reactor no
longer need to be positioned at different elevations. All items may be placed
on
the ground, resulting in substantial investment savings.
Urea may be prepared by introducing (excess) ammonia and
carbon dioxide into a synthesis zone at a suitable pressure (for example 12-40
MPa) and a suitable temperature (for example 160-250°C), which first
results in
the formation of ammonium carbamate according to the reaction:
2NH3 + COZ ~ HZN-CO-ONH4
Dehydration of the ammonium carbamate formed then results in
the formation of urea according to the equilibrium reaction:
HZN-CO-ONH4~~ HzN-CO-NHZ + H20
The theoretically attainable conversion of ammonia and carbon
dioxide into urea is determined by the thermodynamic position of the
equilibrium
and depends on for example the NH~/COZ ratio (N/C ratio), the H20/COZ ratio
and
temperature, and can be calculated with the aid of the models described in for
example Bull. of the Chem. Soc. of Japan 1972, Vol. 45, pages 1339-1345 and
J. Applied Chem of the USSR (1981), Vol. 54, pages 1898-1901.
In the conversion of ammonia and carbon dioxide to urea there
evolves as a reaction product a urea synthesis solution which consists
essentially
of urea, water, ammonium carbamate and unbound ammonia.
Besides the urea synthesis solution, there may evolve in the
synthesis zone a gas mixture of unconverted ammonia and carbon dioxide along
with inert gases. Ammonia and carbon dioxide are removed from this gas mixture
and are preferably returned to the synthesis zone.
In practice, various processes are used for the preparation of
urea. Initially, urea was prepared in so-called conventional high-pressure
urea
plants, which at the end of the 1960s were succeeded by processes carried out
in
so-called urea stripping plants.
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A conventional high-pressure urea plant is understood to be a
urea plant in which the ammonium carbamate that has not been converted into
urea is decomposed, and the customary excess ammonia is expelled, at a
substantially lower pressure than the pressure in the synthesis reactor
itself. In a
conventional high-pressure urea plant the synthesis reactor is usually
operated at
a temperature of 180-250°C and a pressure of 15-40 MPa. In a
conventional high-
pressure urea plant, following expansion, dissociation and condensation at a
pressure of between 1.5 and 10 MPa, the reactants that are not converted into
urea are returned to the urea synthesis as a carbamate stream. In addition, in
a
conventional high-pressure urea plant, ammonia and carbon dioxide are fed
directly to the urea reactor. The N/C ratio in the urea synthesis in a
conventional
high-pressure urea process is between 3 and 5 and COZ conversion between 64
and 68%.
Initially, such conventional urea plants were designed as so-
called 'Once-Through' processes. Here, non-converted ammonia was neutralized
with acid (for example ri~tric acid) and converted into ammonia salts (for
example
ammonium nitrate). It did riot take long until these conventional Once-Through
'.
urea processes were replaced with Conventional Recycle Processes, in which all
non-converted ammonia and carbon dioxide are recycled to the urea reactor as
carbamate streams. In the recovery section, non-converted ammonia and carbon
dioxide are removed from the urea synthesis solution obtained in the synthesis
reactor, in which process a urea in water solution evolves. Next, this urea in
water
solution is converted into urea in the evaporation section by evaporating
water at
reduced pressure. Urea/water may also be separated by crystallizing urea from
urea/water mixtures.
A urea stripping plant is understood to be a urea plant in which
the ammonium carbamate that has not been converted into urea is largely
decomposed, and the customary excess ammonia is largely expelled, at a
pressure that is essentially almost equal to the pressure in the synthesis
reactor.
This decomposition/expulsion takes place in a stripper with or without
addition of a
stripping agent. In a stripping process, carbon dioxide and/or ammonia may be
used as stripping gas before these components are added to the reactor. Such
stripping is effected in a stripper installed downstream of the synthesis
reactor; in
it, the urea synthesis solution coming from the urea reactor, which contains
urea,
ammonium carbamate and water as well as ammonia, is stripped with the
stripping gas with addition of heat. It is also possible to use thermal
stripping here.
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Thermal stripping means that ammonium carbamate is decomposed and the
ammonia and carbon dioxide present are removed from the urea solution
exclusively by means of the supply of heat. Stripping may also be effected in
two
or more steps. In a known process a first, purely thermal stripping step is
followed
by a C02 stripping step with addition of heat. The gas stream containing
ammonia
and carbon dioxide exiting from the stripper is returned to the reactor
whether or
not via a high-pressure carbamate condenser.
In a urea. stripping plant the synthesis reactor is operated at a
temperature of 160-240°C and preferably at a temperature of 170-
220°C. The
pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-19.5 MPa. The
N/C ratio in the synthesis in a stripping plant is between 2.5 and 4 and COz
conversion between 58 and 65%. The synthesis may be carried out in one or two
reactors. When use is made of two reactors, the first reactor, for example,
may be
operated using virtually fresh raw materials and the second using raw
materials
entirely or partly recycled, for example from the urea recovery section.
A frequently used embodiment for the preparation of urea by a
stripping process is the Stamicarbon COZ stripping process as described in
r:
European Chemical News, Urea Supplement, of 17 January 1969, pages 17-20.
The greater part of the gas mixture obtained in the stripping operation is
condensed and adsorbed in a high-pressure carbamate condenser, after which
the ammonium carbamate stream formed is returned to the synthesis zone for the
formation of urea.
The high-pressure carbamate condenser may de designed as,
for example, a so-called submerged condenser as described in NL-A-8400839.
The submerged condenser can be placed in horizontal or vertical position. It
is,
however, particularly advantageous to carry out the condensation in a
horizontal
submerged condenser (a so-called pool condenser; see for example Nitrogen No
222, July-August 1996, pp. 29-31).
After the stripping operation, the pressure of the stripped urea
synthesis solution is reduced to a low level in the urea recovery and the
solution is
evaporated, after which urea is released and a low-pressure carbamate stream
is
circulated to the synthesis section.
Furthermore, the present process is highly suitable for improving
and optimizing existing urea plants by replacing piping and equipment items in
areas where corrosion occurs with duplex steel piping and equipment items that
are passivated in accordance with the present invention. The process is also
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particularly suitable for revamping existing urea plants by applying the steel
grade
together with the passivating technique according to the present invention in
areas
where hairline cracks may develop in duplex steel overlay welds.
The invention may be applied in all current urea processes, both
conventional urea processes and urea stripping processes. Examples of
conventional urea processes in which the invention may be applied are the so-
called 'Once-Through', Conventional 'Recycling' and Heat Recycling Processes.
Examples of urea stripping processes in which the invention may be applied are
the COz stripping process, the NH3 stripping process, the self-stripping
process,
the ACES (Advanced Process for Cost and Energy Saving) process, the IDR
(Isobaric-Double-Recycle) process and the HEC process.
The invention is illustrated by the following examples.
Example I
A quartz tube in an inertized autoclave was filled with 36 g of
k
urea, 17.5 g of a 3% hydrogen peroxide solution, 20 g of carbon dioxide and 34
g
of ammonia. An austenite-ferrite duplex steel specimen with a surface area of
36 cmz was introduced into the tube. The system was heated to 184°C,
resulting in
a pressure of 148 bar. After five days the system was cooled and
depressurized.
Upon removing the specimen, less than 3 mg of iron, chromium and nickel was
found in the remaining urea slurry.
Comparative Example A
The experiment in Example I was conducted except that 17 g of
water was used in place of 17.5 g of a 3% hydrogen peroxide solution. The
other
components were the same. Upon removing the specimen, 12 mg of iron, chrome
and nickel was found in the remaining urea slurry.
Example II
A quartz tube in an inertized autoclave was filled with 36 g of
urea, 18 g of a 5% sodium nitrite solution, 20 g of carbon dioxide and 34 g of
ammonia. An austenite-ferrite duplex steel specimen with a surface area of 36
cm2
was introduced into the tube. The system was heated to 184°C, resulting
in a
pressure of 148 bar. After five days the system was cooled and depressurized.
Upon removing the specimen, less than 3 mg of iron, chromium and nickel was
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found in the remaining urea slurry.
