Coke prevention during steam cracking for ethylene production.
Steam cracking of gaseous and liquid hydrocarbons is the leading technology for the production of ethylene. Naphtha, gas oil, unconverted oils or hydrocracker residues are typical liquid feed materials. Common gaseous feedstocks are ethane, propane and butane. In the presence of dilution steam, the feedstocks are routed to the steam cracker furnaces. The cracking furnace is the heart and starting point of the ethylene production. The gas-phase reaction is called steam cracking or pyrolysis. Steam cracking is a very complex process followed by cooling, compression and separation steps. Coking is an unwanted side reaction from steam cracking. It is a major operational problem in the radiant section of steam cracker furnaces and transfer line exchangers. Steam dilution lowers the hydrocarbon partial pressure of the cracked compounds. It favours the formation of primary reaction products. The steam addition reduces the tendency of coke deposition on the furnace tubes.
Coke is an undesired but inevitable side product of the pyrolysis. Surface catalysed reactions lead to the formation of filamentous coke. In many cases, the coke formation is caused by nickel and iron on the alloy surface. Amorphous coke is formed in the gas phase. Increased pressure drop, impaired heat transfer and higher fuel consumption cause high production losses. The external tube skin temperature continuously rises. This influences the process selectivity and leads to even more rapid coke formation. The formed coke has to be removed by controlled combustion with steam and air. It is a non-productive downtime of the steam cracker furnace. Decoking cycles lead to shorter coil life of the steam cracker furnaces.
A continuous injection of a sulfiding agent is the historical method for coke reduction. DMS and DMDS are well-established additives. These sulfiding agents are believed to decompose to form sulfidic surfaces. This prevents coking and undesired chemical reactions. DMS and DMDS are very effective but have some drawbacks. Both sulfiding additives have a very bad smell and DMDS is commonly masked with odorants. It has a low flash point and requires special handling. DMDS is mainly used for steam cracking units. Storage under nitrogen pressure in closed containers is needed to avoid fire hazards.
Kurita has many years of experience with the supply and injection of polysulfides. Our polysulfides reduce the formation of undesired carbon monoxide (CO). It significantly extends the run time of the cracking furnaces. We supply DMDS but promote the use of another sulfiding agent, called CUT-COKE Technology. Kurita´s CUT-COKE is classified as non-hazardous and requires no special handling and storage. The high flash point of about 100°C reduces the risk of potential flash fires. The low sulfide odour is similar to gas oil. It does not have to be masked with odorants to cover up bad odours. Reduced material stress and low decoking times of the furnaces are further advantages of our chemical treatment.
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