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 furnace.
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 thermal shock.
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 material.
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 manufacturer.
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 tests.
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 level.
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 material.
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 to occur.
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 coking rates.
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
| Fig. 1. Historical summary with features. |
Ni Nickel Si Silicon
1 Manaurite XM
2 Manaurite XTM
3 Manaurite XO
4 Manaurite 40XO
|The author |
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.