Hassan ElBanhawi

PROCESS DISCUSSION

The delayed coker is an important residue conversion process, or so-called “bottom-ofthe- barrel” process where residues from heavy, high-sulfur crudes are converted to transportation fuels.Statistically, in terms of number of units installed and total current operating capacity, it is quite easy to show that delayed coking is the residue-conversion process which is most often used today. In addition, because of its wide commercial acceptance, delayed coking has been referred to as the yardstick against which other, less commercially proven, processes must be measured.
Despite its wide commercial use, only relatively few contractors and refiners are truly knowledgeable in delayed-coking design, so that this process carries with it a “black art” connotation.

PROCESS FLOW

Coking Section: Reduced-crude or vacuum-residue fresh feed is preheated by exchange against gas oil products before entering the coker-fractionator bottom surge zone. The fresh feed is mixed with recycle condensed in the bottom section of the fractionator and is pumped by the heater charge pump through the coker heater, where the charge is rapidly heated to the desired temperature level for coke formation in the coke drums. Steam is often injected into each of the heater coils to maintain the required minimum velocity and residence time and to suppress the formation of coke in the heater tubes. The vapor-liquid mixture leaving the furnace enters the coke drum, where the trapped liquid is converted to coke and light-hydrocarbon vapors. The total vapors rise upward through the drum and leave overhead. A minimum of two drums is required for operation. One drum receives the furnace effluent, which it converts to coke and gas while the other drum is being decoked.

Fractionation Section: The coke-drum overhead vapors flow to the coker fractionator and enter below the shed section. The coke-drum effluent vapors are often “quenched” and “washed” with hot gas oil pumped back to the trayed wash section above the sheds. These operations clean and cool the effluent-product vapors and condense a recycle stream at the same time. This recycle stream, together with the fresh feed, is pumped from the coker fractionator to the coking furnace. The washed vapors pass to the rectifying section of the tower. A circulating heavy gas oil pumparound stream, withdrawn from the pumparound pan, is used to remove heat from the tower, condensing the major portion of heavy gas oil and cooling the ascending vapors. The hot pumparound stream of heavy gas oil withdrawn from the fractionator can be used to reboil the towers in the vapor-recovery plant, to preheat the charge to the unit, or to generate steam. The heavy gas oil product is partially cooled via exchange with the charge and air-cooled to storage temperature. Light gas oil product is steam-stripped to remove light ends, partially cooled via heat exchange with the charge, and air-cooled to storage temperature.
If a vapor-recovery unit is included in the design, then a sponge-oil system may be required. Lean sponge oil is withdrawn from the fractionator, cooled by heat exchange
with the rich sponge oil, and then air-cooled before flowing to the top of the sponge absorber. Rich sponge oil is returned to the top heat-transfer tray above the lean-spongeoil draw-off tray after preheat by exchange with the lean sponge oil.
The overhead vapors are partially condensed in the fractionator overhead condenser before flowing to the fractionator overhead drum. The vapor is separated from liquid in this vessel. The vapor flows under pressure control to the suction of the gas compressor in the vapor-recovery unit. The top of the fractionator is refluxed with part of the condensed hydrocarbon liquid collected in the overhead drum. The balance of this liquid is sent with the compressed vapors to the vapor-recovery unit. Sour water is withdrawn from the overhead drum and typically pumped to off-site treating facilities.

Coker Blowdown: The coke-drum blowdown system serves the purpose of recovering hydrocarbon and steam vapors generated during the quenching and steaming operation. It is designed to minimize air pollution during normal operation. The system7 includes a coker blowdown drum, blowdown condenser, blowdown settling drum, blowdown circulating oil cooler, vent-gas compressor system, and attendant pumps.
During the coke-drum cooling cycle, steam and wax tailings flow to the coker blowdown drum, where they are condensed by contact with a cooled circulating oil stream.
This circulating oil stream also dilutes the wax tailings. The diluted wax tailings are withdrawn from the bottom of the drum and recirculated after cooling in the blowdown circulating-oil cooler. Excess oil is returned to the fractionator. Light gas oil makeup is charged to the coker blowdown drum as required for dilution of the mixture.
Steam and light hydrocarbons from the top of the coker blowdown drum are condensed in the blowdown condenser before flowing to the blowdown settling drum. In the settling drum, oil is separated from condensate. The oil is pumped to refinery slop, while the water is pumped either to off-site treating facilities or to the decoking-water storage tank for reuse.
Light hydrocarbon vapors from the blowdown settling drum are compressed in the vent-gas compressor after being cooled in the vent-gas cooler and separated from the
resultant liquid in the vent-gas knockout drum. The recovered vent gas flows to the inlet of the fractionator overhead condenser. Alternatively, it may be sent directly to the fuelgas recovery system.