Delayed coking is a semi-continuous thermal cracking process in which a heavy hydrocarbon feedstock is converted to lighter and more valuable products and coke. The mechanism of coking can be broken down to three distinct stages.
- The feed undergoes partial vaporization and mild cracking as it passes through a specially designed coking furnace.
- The vapours undergo cracking as they pass through the coke drum to fractionation facilities downstream where products of gas, naphtha, jet fuel and gas oil are separated. The petroleum coke remains in the drum.
- The heavy hydrocarbon liquid trapped in the coke drum is subjected to successive cracking and polymerization until it is converted to vapours and coke.
The residuum (fresh feed) from the Hydrocracker Fractionation Unit enters the bottom section of the fractionator where material lighter than the desired cut point of the coker gas oil is flashed off and the remaining material combines with the recycle material condensed in the bottom of the fractionator to form the combined feed.
This combined feed is then routed to the charge furnace where the liquid is heated to its incipient coking temperature to produce vapourization and mild cracking. Steam is injected into the furnace feed line to prevent coke deposition in the furnace coils, increase tube velocity and reduce hydrocarbon partial pressure..
The vapour/liquid mixture then enters the bottom of the coke drum where the vapour experiences further cracking and the liquid experiences successive cracking and polymerization until it is completely converted to vapour and coke. The coke drum effluent vapour enters the fractionator where the hot vapours are quenched with wash oil. The condensed portion then forms the recycle stream and is recycled to the furnace for another pass through the coke drum. The condensed vapour is fractionated into gas, naphtha, jet fuel and gas oil. Gas oil and jet fuel are removed as side cuts and routed to the Gas Oil Hydrotreater and the Naphtha/Jet Hydrotreater.
The coker naphtha and gas exit the top of the fractionator and enter an overhead system where naphtha is condensed and the gas separated and routed to the Gas Recovery Unit. The naphtha is split into two streams: one is returned to the fractionator as reflux, the other is sent to the Gas Recovery Unit.
The Delayed Coking Unit has two large vertical coke drums. During operation, one will be in coking service while the other is off stream and being decoked with high pressure water jets. In the decoking operation, the coke drum is isolated, steamed to remove residual hydrocarbons, cooled by filling with water, opened then decoked in a two step process. In the first step, a pilot hole is drilled from top to bottom of the coke bed using high pressure water jets. Then the main body of coke left in the coke drum is cut into fragments which fall onto a concrete pad. The coke is then dewatered, crushed and sent to coke storage or loading facilities. The decoking water is recycled after the coke fines in the water have settled.
Used with permission (© SAIT Polytechnic, MacPhail School of Energy, 2009)