A coker unit is a thermal cracking process in the oil refinery industry used to recover valuable elements, the most important of which being petroleum coke from main refinery process residues. The coking process involves feeding heated residual oil or pitch into the bottom of a large, upright vessel known as a coke drum, where the thermal cracking takes place. The cracking process causes the oil to separate into gas and vapor that exits at the top of the coke drum, and solidified coke that collects inside the drum. Once the coke has accumulated to a pre-determined level, the process is stopped and the coke mass is cut out of the coker unit using high-pressure water jets. The coke collected from the unit is then sent for further treatment to produce fuel or anode-grade end products.
Residual oil and coal tar pitch from the atmospheric and vacuum distillation columns in an oil refinery still contains several valuable elements, including naphtha, gas oils, and hydrocarbon gases. One of the most important of these elements is a porous, carbonaceous solid known as petroleum coke, or pet coke. Commercial-grade coke has a very high carbon content and may be used as an efficient, low-emission combustion fuel or, in the case of grades such as needle coke, for the manufacture of anodes used in steel, titanium, and aluminum smelters. These products are extracted from the refinery residues in a section of the plant known as a coker unit, typically one of the final steps in the refinery process.
The coker unit harnesses a phenomenon pioneered by William Burton and Vladimir Shukhov known as thermal cracking during which long chain hydrocarbons are broken down, or cracked, into short chain variants. This cracking process takes place in a large, upright vessel known as a coke drum. The residual oil or pitch is first heated to around 930° Fahrenheit (500° Celsius) and fed into the bottom of the coke drum where the cracking process begins. During cracking, gases and vaporized oils exit at the top of the coker unit for collection, while solid, porous coke builds up to form a mass inside the drum.
Once a sufficient accumulation of coke is present in the drum, the feed is cut off and the process ceases. When the coke in the drum has cooled sufficiently, high-pressure water jets are lowered into the top of the drum from high structures above known as de-coker derricks. These jets cut up the coke mass into smaller pieces that drop out of the bottom of the drum for collection and treatment for the production of fuel or anode-grade products. Coker units generally feature two coke drums that allow the process to continue in one drum while the other is being de-coked. Many larger refineries feature several dual-drum coker units to keep up with production demands.