Coking is applied to produce cracking conditions so that the yields of gas oil and gasoline are increased. Reduced crudes, cracked tars, heavy catalytic cycle oils and asphalts are used as a charge. The products are gas oil, gasoline, gas and coke.
Residual oil from the vacuum distillation unit is pumped into the bottom of the distillation column called the main fractionator. From there it is pumped, along with some injected steam, into the fuel-fired furnace and is heated to its cracking temperature of about 480 °C.
Thermal cracking begins in the pipe between the furnace and the coke drums, and finishes in the coke drum that is on-stream. The injected steam helps to minimize the deposition of coke within the furnace tubes.
The vapours from the coking drum return to the fractionator. Here the gas, gasoline and gas oil are separated and leave the unit. The heavier materials settle in the bottom and are recycled to the coking operation .
When one of the coke drum is full of the solidified coke, flow is diverted to another drum so that the furnace operation is continuous. Thus, drums are operated in pairs with one-stream while the other is being dumped. A full coke drum is removed from the process flow, steamed to strip light hydrocarbons from the coke, and cooled by water injection. More recent designs use high pressure water jets to cut the coke from the drum .