Retarded coker is a refining process that breaks down residual petroleum byproducts into useful fuels. It was patented in 1891 and improved in 1913. The process produces petroleum coke, which can be used as a fuel source but is highly polluting. The process operates at high temperatures and can use various feedstocks. Visbreaking is a newer, cleaner process that has not been widely adopted.
A retarded coker is the terminology used for an oil refining process that involves the further refining of residual petroleum byproducts after the distillation of crude oil has occurred into useful fuels such as naphtha and gasoline. The process was first patented in 1891 by a Russian engineer named Vladimir Shukhov, and the Shukhov cracking process is still used as of 2011 to produce diesel fuel. In 1913, however, William Burton and Robert Humphrey patented improvements to the delayed coker methodology. This became known as the “Burton process,” which doubled the amount of gasoline that could be generated in a retarded coker. The coker unit performs a thermal cracking process on the residual oil in which it breaks or cleaves the hydrocarbon chains of molecules present into smaller, more useful hydrocarbons.
Before the retarded coker process became a common petroleum waste, known as “bottom” in the industry, it was considered a waste product of refining that was expensive to dispose of due to the environmental risks it posed to water supplies and so on . As of 2011, the retarded coker has become the most common type of petroleum refinery, with several other designs in existence, such as the fluid coker and flexicoker, focusing on the production of liquefied petroleum gas (LPG) and jet fuel . The end products of a delayed coker process include both liquid and gaseous useful fuels such as gasoline and naphtha, and a solid residue composed primarily of carbon known as petroleum coke.
Petroleum coke bears a chemical resemblance to coal and can be used in coal-fired furnaces, although it is a highly polluting fuel source that can release hazardous compounds such as lead and mercury into the atmosphere. This often means that petroleum coke must undergo an additional refining process before it can be burned as a fuel source. During the separation of petroleum coke from the useful compounds of crude oil, most of the heavy metals and sulfur-related chemicals are concentrated in the petroleum coke. These compounds would otherwise make beneficial fuel products unsuitable for everyday use.
A retard coker operates at a temperature between 779° and 842° Fahrenheit (415° and 450° Celsius), where complex molecules break down into simpler ones and some compounds act as free radicals to combine with others into olefins in the chemical alkene group. For example, the compound CnH2n+2 breaks down into Cn-2H2n-3 + C2H5. The two simpler free radicals then react to form other compounds, where C2H5 + CnH2n+2 produces CnH2n+1 + C2H6. The end products of a retarded coker can be adjusted by changing the process temperature and pressure level. A higher temperature will reduce the amount of petroleum coke that is generated, but the variation in production is relatively small. Another advantage of the retarded coker is that it can use other types of feedstocks besides the vacuum distillation residue from petroleum refining, including waste refinery sludge and feedstocks with high concentrations of metals and sulfur.
Visbreaking is another thermal cracking process that breaks down long-chain hydrocarbons through the application of heat. Unlike a retarded coker, it is a non-catalytic process that does not involve chemical reaction of the building blocks themselves, and is considered a cleaner process that produces fewer waste by-products. The visbreaking oil refining methodology is considered quite new as it has not been widely adopted by the industry as of 2011, although there are pilot plants for the method in both the United States and the Netherlands.
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