[ Petrochemicals ]
[ Petrochemicals ]
Background Steam cracking is an industrial-scale chemical process used to‘ break’ hydrocarbons( such as found in crude oil) to create alternative hydrocarbons which then form feedstocks for the production of useful products. For example, ethane( molecular formula C2H6, indicating that each molecule contains two carbon atoms plus six hydrogen atoms) can be cracked to create ethene( C2H4), which is a precursor in the production of plastics such as polyethene( universally known as polythene). In terms of feedstocks, steam crackers traditionally used naphtha or liquefied petroleum gas( LPG) to create products such as ethene. However, alternative feedstocks such as ethane, propane, and butane are equally viable.
Steam cracker principles In steam cracking, the feedstock is mixed with high temperature steam and in a continuous process briefly heated in a furnace in the absence of oxygen. The reaction takes place within a very short timescale, with temperatures typically from 800 ° C to 1100 ° C. The reaction breaks the hydrocarbons into smaller constituents. The cracked gas then has to be quenched to stop the cracking process. Without quenching, the cracking process could continue, ultimately breaking all the hydrocarbons down completely to their basic elements – carbon and hydrogen. Additional process stages include cleaning and separating the various constituents in the product stream. The actual products produced in the reaction depend on factors such as the type of feedstock, the hydrocarbon-tosteam ratio, the cracking temperature and furnace residence time. The use of catalysts can also influence the reaction outcome. Finding the optimum balance between parameters can help to raise conversion rates during the reaction. During the cracking process, many steam crackers are affected by the slow build-up of a hard layer of coke( a form of carbon) on surfaces inside the reactor. This impairs reactor efficiency. Furnaces are therefore typically de-coked every few months, by flushing them with steam. This converts the coke to carbon monoxide and carbon dioxide gases.
Types of furnace tubes Furnace tubes are key components of an ethene steam cracking furnace.
Wang Hao, steam cracker operator, carries out a routine inspection of the deaerator V-900. BASF-YPC is a good example of BASF’ s Verbund concept. Production plants are intelligently linked together via a network of pipelines. This saves logistics costs for transporting chemicals, raw materials and energy. Image credit: BASF SE.
Typically, furnace tubes are classified as either radiant( or radiation) tubes or convection tubes, depending on the areas where they are used. The radiant section refers to the furnace where the feedstock cracking reaction takes place. The high-temperature gas then enters the convection section for heat recovery. When selecting furnace tubes, materials engineers have to take the following potential mechanisms, amongst others, into consideration: creep, carburization, overheating, thermal shock, hydrogen embrittlement, liquid metal embrittlement, sigma-phase embrittlement, graphitization, thermal fatigue, stress corrosion cracking, oxidation, chromium depletion, metal dusting, and corrosion perforation. However, coking is seen as the most critical issue to address.
Materials for ethene crackers Given the extreme conditions – high temperatures, corrosive gases, thermal stresses, and coking – tube selection is seen as critical for steam cracking furnaces. Hence Alleima offer its 353 MA for ethene furnaces, which thanks to its high creep strength, carburization resistance, and oxidation properties is understood to help extend the time interval between decoking. Alleima’ s tubes are available with a smooth or finned design. Benefits of the finned tube are said to be the enhanced heat transfer, better temperature control, improved mechanical strength and reduced fouling, as the design promotes shedding of deposits. 353 MA is an austenitic, chromium-nickel stainless steel containing 25 % chromium and 35 % nickel, as well as nitrogen and rare earth metals. Ease of fabrication is also a consideration as regards material selection. Hence Alleima note that‘ The weldability of 353 MA is good. Suitable welding methods are manual metalarc welding with covered electrodes and gas shielded arc welding with the TIG and MIG methods as first choice. Preheating and post-weld heat treatments are normally not necessary.’ Schmidt + Clemens also have a range of solutions for steam crackers. They too indicate that proper tube metallurgy can reduce the problems caused by coking. Their website states that‘... Centralloy ® ET 45 Micro or Centralloy ® HT E alloys.... 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’. According to company material data sheets, Centralloy ® ET 45 Micro is an air melted nickel-base alloy consisting essentially of a Ni-Cr-Fe-Si matrix. Similarly, Centralloy ® HT E contains 45 % nickel, 30 % chromium, 4 % aluminium,
46 Stainless Steel World June 2026 www. stainless-steel-world. net