Ethylene cracking furnace is the main unit to produce the fundamental raw materials such as ethylene and propylene in petrochemical industry. For an ethylene plant, once it is set up, the number of ethylene cracking furnaces and their geometry configurations are fixed. Operation of the ethylene cracking furnaces system is the main engineering direction to improve the economic performance, and to meet technical and environmental requirements. Increasing demand for the petrochemical products such as ethylene and propylene is quite strong in the last several decades. Many ethylene plants nowadays have multiple cracking furnaces to process several different types of feeds. The scheduling and planning activities have been involved on the ethylene cracking furnaces system.
Ethylene production is a cornerstone of the chemical process industry. Thermal cracking is one of the most important processes in the petrochemical industry converting a hydrocarbon feedstock into more valuable products such as ethylene and propylene, and it is an endothermic process that takes place in the cracking furnaces. A cracking furnace typically consists of a long, relatively narrow furnace (radiant firebox) containing a large number of coils (process tubes) suspended in the firebox and a large number of gas burners. The furnace coils are formed from the tubes by welding. The tubes used in thermal cracking furnaces in an ethylene manufacturing process are usually made of HP and HK grades of heat resistant stainless steel. These alloys exhibit excellent properties in terms of oxidation resistance, carburization resistance, high temperature creep and thermal expansion. These tubes are exposed to high temperature (approximately 1100 °C and internal pressure of about 1 bar). There are five major licensors of ethylene plants: KBR; Technip; Linde; Shaw, Stone & Webster; and Lummus. While ethylene production differs slightly by licensor, the overall process is fairly similar. New radiant coils have been developed to enhance heat transfer and increase furnace run length, selectivity or operating capacity.
RADIANT COILS is a series
of tubes horizontal/ vertical hairpin type connected at ends (with 180° bends) or helical in construction. The radiant coil absorbs heat through radiation. They can be single pass or multi pass depending upon the process-side pressure drop allowed. The radiant coils and bends are housed in the radiant box. Radiant coil materials vary from carbon steel for low temperature services to high alloy steels for high temperature services. These are supported from the radiant side walls or hanging from the radiant roof. Material of these supports is generally high alloy steel. While designing the radiant coil, care is taken so that provision for expansion (in hot conditions) is kept.
The formation of coke in a typical Steam Cracker environment is an inevitable and complex phenomenon. Because of the pyrolysis reaction, thermally insulating hydrocarbon is deposited on the tube walls, so reducing the effective cross-sectional area. To ensure a constant mass flow through the plant, the furnace temperature must be raised continuously. Regular decoking of the furnaces is therefore necessary. However, this problem can be reduced by selecting the proper tube metallurgy, such as Centralloy® ET 45 Micro or Centralloy® HT E. These alloys are the universally preferred choice especially for “hot” outlet passes and with their carefully controlled specific additions can produce a dense and ”self-healing” oxide layer. This is especially effective in limiting coke formation and subsequent carburisation thereby increasing the overall Furnace performance and extending the Radiant Coil life.
Radiant Coil Furnaces are ideal for the process heating of liquids and gases to high temperatures over a wide range of operating pressures, where temperature and pressure combinations make traditional heating techniques impractical.
The furnace coils, at the heart of the radiant section of the furnace, encounter coking issues – carbon deposition on the coil surface – that dramatically draw back the yields. Better answers to this phenomenon could lead to improved production and/or lower environmental impact.
Super Radiant Coil (SRC) Technology Advantages:
• Increased heat transfer area / reduce volume
• Shortened residence time
• Enhanced heat transfer
• Reduced tube metal temperature
• Reduced coking rate / increased Run Length
• Reduced selectivity loss during the run length • Avoidance of tube plugging
The Super Radiant Coil (SRC) cracking tube technology utilizes a larger diameter cracking tube equipped with an internal device. Heat transfer is enhanced with radiation from the inner surface of the outer tube to the internal device. This heat is then transferred to the cracking gas by convection. This effectively reduces the heat transferred to the cracking gas through the outer tube ID film. Combined with increased area of a larger diameter outer tube the flux rate through the film is drastically reduced, and consequently, the temperature drop across the film layer is also dramatically reduced. This reduces the degree of over-cracking and coke deposition on the inside of the tube. The metal temperature of the internal device is 175°F (80°C) lower than the temperature of the outer tube, and the coke deposition over its surface is negligible.
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