The most common form of the coking process in today's refineries is Delayed Coking where vacuum resid is thermally cracked into smaller molecules that boil at lower temperatures. Products include naphtha, gas oils and coke. Light product yield varies by feedstock but is generally around 75% conversion. Coke is sold as a fuel or specialty product into the steel and aluminum industry after calcining to remove impurities.

Vacuum resid is fed to the coker fractionator to remove as much light material as possible. Bottoms from the fractionator are heated in a direct fired furnace to more than 900˚F (480˚C) and discharged into a coke drum where thermal cracking is completed. High velocity and stream injection are used to minimize coke formation in furnace tubes. Coke deposits in the drum and cracked products are sent to the fractionator for recovery. Coke drums typically operate in the 25-50 psi (2-4 bar) range while the fractionator operates at a pressure slightly above atmospheric in the overhead accumulator. Fractionator bottoms are recycled through the furnace to extinction.

Multiple coke drums are used. As one drum is being filled with coke, others are offline for coke removal. Coke removal involves steaming, quenching, hydraulic cutting to remove solid coke from the drum and vessel preparation for return to service.

Coker light products are highly unsaturated. Coker light ends are recovered as an olefin feed source for alkylation. Coker naphtha requires desulfurization before upgrade in the Catalytic Reforming Unit. Coker gas oils are generally sent to the Hydrocracker for upgrade.

Visbreaking is a milder form of thermal cracking often used to reduce the viscosity and pour point of vacuum resid in order to meet specification for heavy fuel oil. Visbreaking helps avoid the use of expensive cutter stock required for dilution. The process is carefully controlled to predominantly crack long paraffin chains off aromatic compounds while avoiding coking reactions.

There is a tradeoff between furnace temperature and residence time for visbreaking operations. Longer residence time leads to lower furnace outlet temperatures. In general, operations are conducted between 800-930˚F (425-500˚C). Material is quenched with cold gas oil to stop the cracking process. Pressure is important to unit design and ranges between 300-750 psi (20-50 bar).