Steam Cracking

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Steam cracking is the predominant method for production of olefins. Aromatics can also be produced during the steam cracking process, but olefins are typically the more desirable product. Steam cracking is a thermal process in which hydrocarbons are mixed with steam and heated rapidly (without oxygen) in order to break (“crack”) larger saturated hydrocarbons into smaller, unsaturated ones (olefins, aromatics, and other by-products).

The key features include:

• Endothermic reaction: The cracking reactions require a lot of heat input. Heat is supplied by burning fuel in the furnace/radiant section.
• Very high temperatures: Typically around 750-900 °C in the radiant section of the furnace.
• Short residence times: The time that the hydrocarbon + steam mixture remains at cracking temperature is usually on the order of milliseconds to a second or so; short enough to maximize desired cracking but minimize over-cracking, coking, and formation of unwanted byproducts.
• Steam dilution: Steam is mixed with the hydrocarbon feed. It performs a couple of functions: lowering the partial pressure of the hydrocarbon, thus helping reduce side reactions (especially coking), and helping with heat transfer.
• Quenching / rapid cooling after cracking: Once the vapor-phase cracking occurs, the hot cracked gas needs to be cooled rapidly (“quenched”) to stop further undesired reactions (secondary cracking, polymerization, coke).
• Separation & purification: After cracking, the mixture is quite complex. There are unreacted hydrocarbons, olefins, dienes, hydrogen, methane, etc. These are separated (often via compression, distillation, hydrogen removal, fractionation) into product streams.
• Coke formation and decoking: One of the operational challenges is that some of the hydrocarbon feed and reaction byproducts deposit as coke on the inner walls of the reactor tubes. Periodic decoking (e.g. by burning off coke) is needed.

Source: Steam cracking process diagram, Vakili et al., 2021.

Feedstocks

The steam cracking process can utilize a variety of hydrocarbon feedstocks, often dependent on regional availability and desired product yield:

• Light gaseous hydrocarbons: ethane, propane, butane (often from natural gas / natural gas liquids)
  • High yield of lighter olefins (ethylene, propylene), low yields of heavier by-products; lower coking tendency; may require relatively lower cracking severity.
• Naphtha (light or heavy naphtha): a liquid hydrocarbon fraction from crude oil distillation. Widely used especially in areas where natural gas liquids are less abundant.
  • Produces a wider mix: not only C2/C3 olefins but also C4s, aromatic compounds, pyrolysis gasoline, etc. Heavier feeds increase tendency to form by-products, coking, etc.
• Gas oils / heavier liquid fractions: Heavier and more complex hydrocarbon chains.
  • Generally yields more byproducts and more coke with lower selectivity to light olefins. Often pre-treated.
• Alternative / emerging feedstocks: Waste oils, pyrolysis oils from plastic/waste/biomass, tallow, vegetable oils, etc.
  • These require upgrading, purification, removing contaminants (sulfur, metals, etc.) to be usable. Using these can help in “circular” approaches to plastics, but feed quality, availability, and pre-treatment cost, are important considerations.

Products

Steam cracking can produce a variety of products. The composition of the product stream can be fine-tuned via the choice of feedstock and the reaction conditions:

• Ethylene: Used in a wide array of downstream products, including polyethylene and its many derivatives.
• Propylene: Used to make polypropylene, acrylonitrile, propylene oxide, and many more products.
• Butadiene: Primarily used to produce synthetic rubber.
• Aromatics, pyrolysis gasoline: When heavy feeds (e.g. naphtha, gas oil) are cracked, aromatics (benzene, toluene, xylene), pyrolysis gasoline (pygas), and naphthenes are commonly produced.
• Hydrogen: Typically used on-site at the chemical plant or as feed for other processes.
• Unconverted methane and ethane (light paraffins): These may be part of the by-product or recycled/unreacted feed streams.
• Coke (solid): Not a “product” in the useful sense; rather a deposit/byproduct that must be managed or removed.