Ethylene is the lightest olefin that largely produced in industry with current worldwide production of about 95 MM tons/ years. It is colorless, flammable gas, which produced mainly from petroleum-based feed stocks ranging from light Ethane/Propane mix to heavy naphtha and vacuum gas oils through thermal cracking in the presence of steam. Ethylene has almost no direct end uses but acts almost exclusively as an intermediate in the manufacturing of other chemicals, especially plastics. Majority of ethylene produced is used in the production of polymers and ethylene derivatives such as ethylene oxide and ethylene glycol.

 

FEEDSTOCKS

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Although there are various feedstocks (liquid and gaseous) are used for the production of ethylene, the principal feedstocks of it are naphthas, a mixture of hydrocarbons in the boiling range of 30oC to 200oC. Naphtha composition can be vary over a wide range depending on the origin, which requires quality control of the fees mixtures. In the US and the Middle East, production of ethylene from light feedstocks such as natural gas, ethane, propane and butane are preferably to be used. Production from gas oils (crude oil fractions) are also gaining importance as feedstocks in some areas of the world. Chemical analysis of the feedstock is important to ensure the required product specification and even more when the production is based on varying feedstocks.

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PROCESS PRODUCTION

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The bulk of the worldwide production is based on thermal cracking with steam. The process is called pyrolysis or steam cracking. Production can be split into four sections: 1. Feed and Furnace section, Quench and Fractionator section, Compressor and Condensate section and Hydrocarbon Separation section. The first three sections are more or less identical for all commercial processes, with the exception that primary fractionation is required only in case of a liquid feedstock. However, large variety of process routes exist for the hydrocarbon fractionation section.

 

A hydrocarbon feed stream is preheated, mixed with steam and further heated to 500oC to 700oC. The stream enters a fired tubular reactor (known as cracker or cracking heater), where under controlled conditions the feedstock is cracked at 800oC to 850oC into smaller molecules within a residence time of 0.1 to 0.5 s. after leaving the radiant coils of furnace, the product mixtures are cooled down instantaneously in the transfer line exchange (TLE) to preserve the gas composition. This quenching time is a crucial measure for severity control of the final products.

 

Other processes for ethylene production besides conventional thermal cracking include:

• Recovery from Fluid Catalytic Cracking (FCC) off gas

• Fluidized-bed cracking

• Catalytic pyrolysis

• Membrane reactor