Delayed Coker - Schematic Flow Diagram and Description

Schematic Flow Diagram and Description

The flow diagram and description in this section are based on a delayed coking unit with a single pair of coke drums and one feedstock furnace. However, as mentioned above, larger units may have as many as 4 pairs of drums (8 drums in total) as well as a furnace for each pair of coke drums.

Residual oil from the vacuum distillation unit (sometimes including high-boiling oils from other sources within the refinery) 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 heated to its thermal 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.

Pumping the incoming residual oil into the bottom of the main fractionator, rather than directly into the furnace, preheats the residual oil by having it contact the hot vapors in the bottom of the fractionator. At the same time, some of the hot vapors condense into a high-boiling liquid which recycles back into the furnace along with the hot residual oil.

As cracking takes place in the drum, gas oil and lighter components are generated in vapor phase and separate from the liquid and solids. The drum effluent is vapor except for any liquid or solids entrainment, and is directed to main fractionator where it is separated into the desired boiling point fractions.

The solid coke is deposited and remains in the coke drum in a porous structure that allows flow through the pores. Depending upon the overall coke drum cycle being used, a coke drum may fill in 16 to 24 hours.

After the drum is full of the solidified coke, the hot mixture from the furnace is switched to the second drum. While the second drum is filling, the full drum is steamed out to reduce the hydrocarbon content of the petroleum coke, and then quenched with water to cool it. The top and bottom heads of the full coke drum are removed, and the solid petroleum coke is then cut from the coke drum with a high pressure water nozzle, where it falls into a pit, pad, or sluiceway for reclamation to storage.