Ethylene furnaces, cracking liquid or gas hydrocarbon molecules in the presence of
steam, operate at high temperatures. While the cracking
operation induces coke formation that deposit alongside the
radiant coils tube walls, the tube-skin temperatures increase
up to what is the material operating limit, or until the
pressure drop due to the constriction of surface is too small.
Then the furnace is shut down and a mix of steam and air is
sent through the coils for decoking purposes.
Coil suppliers have researched on finding construction materials for ethylene
furnaces that can withstand higher operating conditions.
Several years ago, a family of alloys was developed that can
successfully operate under the extreme environment of an ethylene furnace.
This article describes new achievements in the metallurgy of
ethylene furnace applications.
As mentioned before, high temperatures and pressures are
used to crack liquid hydrocarbons and natural gas
into olefins. Several processing conditions challenge furnace
design and construction materials for the
Operating temperatures. Ethylene furnaces
usually consist of a multi-pass configuration-type coils.
Because of the cracking reaction, and coke deposit that takes
place as the feedstock is processed through the
coils, the outlet tubes of the radiant coils operate under a
higher temperature. Coke acts as a thermal barrier. It imposes
on the furnace operator and increases the tube skin
temperature. This allows the cracking temperature inside the
tube to remain the same despite the coke thickness.
Carburization resistance. Because the
carbon diffusion is thermally activated, carbon from the coke, with high
temperatures, diffuses into the metal of the tubes. The
mechanical properties, mostly the creep and thermal shock
resistance properties, are altered to the point that the tube
material becomes very brittle. The tube can fail at the first
Because of the mentioned issues, radiant coil outlet
materials have evolved from what was originally a 25Cr/35Ni
material to a higher Cr-content alloy, typically 35Cr/45Ni
It is always difficult to balance material properties. When
some developments are achieved on one hand, some drawbacks
unfortunately occur on the other hand. While a 25/35 material
has superior creep properties, its carburization resistance is
affected due to its lower Cr content as compared to 35/45.
To enhance carburization resistance of tube materials, new
alloys now contain aluminum (Al), in a limited enough
quantities to prevent the formation of low-melting point
compounds. However, in a sufficient amount, it also decreases
creep resistance properties. To restore those creep properties
back to where they were originally, tantalum (Ta) is added as
carbide former, as shown in Table 2.
In Europe, a specific pressure vessels
directive called PED is required. For a specific material such
as a heat-resisting alloy to be qualified and
recognized for its use in a pressure vessel, the manufacturer
of the alloy must obtain a particular material appraisal (PMA).
The PMA is provided by a notified body upon review of the raw
data regarding mechanical properties supplied by the
For the new Al-based alloy, the Dutch Stoomwezen was
selected as the notified body to provide the PMA. Stoomwezen
rule requires creep data reaching 1/3 of the design life, in
this case 33,000 hours of creep tests or more. Users in Europe
who have therefore positively decided to select this alloy had
their case backed up with approximately four years of creep
New alloy in service.
The Al-based alloy has now been installed in service for
over six years. Besides the material related features such as
carburization resistance and creep properties, application of
this material in service showed a lower coking rate compared to other alloys
when natural gas is used as the feedstock.
Indeed, Al plays an inerting role of the surface and delays
formation of the catalytic coke inherent to gas cracking. The
furnace run lengths are longer.
The new generation of alloy can help both maintenance (carburization
resistance, extended tube life) and process (longer run
lengths). The section criteria for alloy materials for ethylene
tubes highly depends on the problem to be solved at the furnace
Case 1. Sabic NL is a major user of the
Al-based alloy; six complete furnaces have been converted to
the new MzAl tubes, with the oldest furnace operating six
years. This furnace has not shown signs of carburization or
creep elongation. Decision criteria for the new installation
were driven by maintenance.
Case 2. This European-based ethylene operator has
three complete furnaces using the Al-based alloy tube material.
The oldest installation has been operating for four years. Only
two tubes were recently removed from the furnace of
investigation and in-depth study for the Al-diffusion pattern.
Tubing adjacent to the Al-based material, after four years,
were easily weldable, thus proving the new alloys
resistance to carburization. Additional process benefits occur
with tube metal temperatures having been modified to take full
advantage of the alloy in comparison with 35Cr-45Ni-type
Case 3. An ethylene operator in Asia opted
to change out 35/45 material to the Al-based alloy in two full
furnaces at two different gas crackers. After several decoking
cycles, the run lengths of the two furnaces increased by 20%.
The alloy manufacturer is monitoring furnace operations on a
permanent basis for this ethylene producer.
Case 4. Another European ethylene producer
is in the process of converting one complete ethane cracker.
Both process and maintenance benefits are anticipated
Case 5. Two complete naphtha cracking
furnaces were purchased by an European ethylene producer. The
alloy selection was to overcome excessive creeping of 35/45
material and address creep elongation and tube life.
Case 6. This North American producer
operates gas crackers in furnaces using very small diameter
tubes, which are sourced as the problems for run length issues.
A full Ta-based alloy furnace was delivered and is in the
process of being installed to assess process benefits and lower
Options in tube materials.
In six years, new alloy materials for ethylene tubes have
been developed and are being installed by ethylene producers
globally. While maintenance related issues such as
extensive creeping, or heavy carburization leading to tube
change are overcome to extend tube life by approximately two
years, the process benefits alone with gas cracking users make
this alloy the optimum selection. HP
1. Historical summary with
Ni Nickel Si Silicon
1 Manaurite XM
2 Manaurite XTM
3 Manaurite XO
4 Manaurite 40XO
Gilles Verdier is the director of
Metallurgy, and Frederic Carpentier is a senior
Metallurgist. Together they have 55 years of experience
in metallurgy in general, and half of this in
heat-resisting alloys. Both hold PhDs in their
discipline. Manoir Industries is one of the leaders in
supplying high-temperature alloys for the petrochemical industry.
Manoir Industries operates four plants in its petrochemical division
serving the global industry.