Coke drums

It was standard practice to protect refinery delayed coker, coke drums, from overpressure by locating a relief valve on the overhead vapor line. However, the main reason the coke drums would overpressure was coke accumulation in the inlet of the vapor line, immediately downstream of the coke drum itself and typically just upstream of the relief valve. Thus, it was a common occurrence (for example, at the 

Suncor Coker at Ft. McMurray and the Citgo Coker in Lake Charles) for the coke drums to overpressure without the relief valves opening. In one instance, the drum pressure reached over 100 psig, even though the relief valve was set at 60 psig. 

it is standard practice to site all coke drum relief valves on the coke drum itself, and never on the vapor outlet line. 

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The delayed coking feed stream of residual oils from various upstream processes is first introduced to a fractionating tower where residual lighter materials are drawn off and the heavy ends are condensed. The heavy ends are removed and heated in a furnace to about 900 to 1,000 F and then fed to an insulated vessel called a coke drum where the coke is formed. When the coke drum is filled with product, the feed is switched to an empty parallel drum. Hot vapors from the coke drums, containing cracked lighter hydrocarbon products, hydrogen sulfide, and ammonia, are fed back to the fractionator where they can be treated in the sour gas treatment system or drawn off as intermediate products. 

Steam is then injected into the full coke drum to remove hydrocarbon vapors, water is injected to cool the coke, and the coke is removed. Typically, high pressure water jets are used to cut the coke from the drum. 

Air emissions from coking operations include the process heater flue gas emissions, fugitive emissions, and emissions that may arise from the removal of the coke from the coke drum. The injected steam is condensed and the remaining vapors are typically flared. Wastewater is generated from the coke removal and cooling operations and from the steam injection. In addition, the removal of coke from the drum can release particulate emissions and any remaining hydrocarbons to the atmosphere. 

Decoking is a routine daily occurrence accomplished by a high-pressure water jet. First the top and bottom heads of the coke drum are removed then a hole is drilled in the coke from top to hottom.. A rotating stem is lowered through the hole, spraying a jet high-pressure water sidewavs... 

Heater outlet temperature, Coke drum pressure, psig °F 900-950 15-90... 

Figure 3-3. Flow diagram of a delayed coking unit (1) coker fractionator, (2) coker heater, (3) coke drum, (4) vapor recovery column.

Decoking removal of petrolenm coke from equipment snch as coking drums hydraulic decoking nses high-velocity water streams. 

Characteristics of feedstock quality, recycle ratio, and drum pressure affect the coke yield. Highly aromatic feedstock contains more carbon per feed volume and typically produces a high coke yield. Heavy coker gas oil can be recycled back into the coker feedstock to help improve the coke yield. Also, increasing the coking drum pressure tends to increase the coke yield. Typically, a higher coke yield results in a reduced liquid product yield. 

During the past 25 years United States petroleum coke production has increased from less than 1,000,-000 tons per year to 3,400,000. Most of the 1,000,-000 tons were produced in externally fired shell stills and manually removed from the stills. Almost all of the 3,400,000 tons were produced in large "delayed coker" type coke drums and removed mechanically. As was true 25 years ago, the major use for petroleum coke is as a fuel, although the proportion so used is declining. Its use in aluminum production is growing rapidly and currently takes about 18% of the production. 

The feed stock, usually topped or reduced crude oil, is heated in pipe coils (Figure 1) from about 900° to 950° F. The oil is then fed to one of two or more vertical, insulated coke drums. The coke drums are connected by valves so that they can be switched onstream for filling, then switched off-stream for coke removal. The temperature in the drum will ordinarily be 775° to 850° F. and the pressure 4 to 60 pounds per square inch gage. Hot, coke-still vapors from the coke drum pass to a fractionator where gas and gasoline, intermediate gas oil, and heavy gas oil are separated. More or less of the heavy gas oil is recycled. The ratio of recycled heavy gas oil to fresh feed is usually less than 1 but may go up to about 1.6 (5,15,28, 40). 

The transfer line from the heating coil to the coke drum may enter near the bottom of the coke drum (15) or near the top of the coke drum (40). 

The early units used coke drums 10 feet in diameter by 40 feet high (40). More recent units use drums as lafge as 17.5 feet in diameter by 80 feet high (4). The capacity has increased correspondingly. An early unit with three drums, 16 feet in diameter by 35 feet high, had a daily capacity of 2860 barrels (15). A more recent unit using six drums, 17 feet in diameter by 80 feet high, has a capacity of over 15,000 barrels per day (28). 

Cable Decoking. With the advent of vertical coke drums, cable decoking came in as a vast improvement over manual decoking. A coiled steel cable, placed inside the coke drum, is supported on hooks. At the end of the coking cycle the bottom of the... 

Decoking by Drilling. For this method of decoking, a hydraulically controlled, rotating drill stem is mounted in a well below the coke drum. At the end of the cycle, the bottom from the coke drum is removed and a hole is drilled upward through the coke bed. The drill is then removed from the stem and knocker-bars are mounted in its place. The knocker-bars swing out from the drill stem when it is rotated, hit the coke, and break it loose from the drum. The coke falls out and is recovered (5,40). 

The nature of coke varies somewhat throughout the coke drum. In the lower section, the coke is dense and the volatile matter content is low. At the higher levels the coke becomes softer, more brittle, and the volatile matter content increases. The volatile matter in the coke at the top of the drum may be 2 to 3% higher than in the coke from the bottom of the drum (15). 

The feedstock is introduced into a furnace whose outlet temperature varies from 480 to 515°C (895-960°F). The heated feedstock enters one of a pair of coking drums where the cracking reactions continue. The cracked products leave as overheads, and coke deposits form on the inner surface of the dmm. To give continuous operation, two drums are used while one is on steam, the other is being cleaned. The temperature in the coke drum ranges from 415 to 450°C (780-840°F) at pressures from 15 to 90 psi. Overhead products go to the fractionator,... 

The coke drum is usually on-stream for about 24 h before becoming filled with porous coke. The following procedure is used to remove the coke ... 

Normally, 24 h is required to complete the cleaning operation and to prepare the coke drum for subsequent use on-stream. 

The solvent contained in the asphalt and deasphalted oil is condensed in the fractionator overhead condensers, where it can be recovered and used as lean oil for a propane/butane recovery in the absorber, eliminating the need for lean oil recirculation from the naphtha stabilizer. The solvent introduced in the coker heater and coke drums results in a significant reduction in the partial pressure of asphalt feed, compared with a regular delayed coking unit. The low asphalt partial pressure results in low coke and high liquid yields in the coking reaction. 

Coke drum a vessel in which coke is formed and which can be cut oil from the process for cleaning. 

The pre-op crew steamed out the B Unit first and vented it through the 8-inch steam vent piping. The failure occurred about four days later. An eyewitness in the operators shelter heard what sounded like a muffled explosion and ran out the back door to see if he could locate fire or smoke. Fortunately, no fire could be seen, but the collapsed coke drum was very visible. 

A major fire that resulted in 65 million in property damages occurred early in the morning (4 21 A.M.) on August 2, 1993. The central unit of three delayed coker units was the origin of the fire. Each of the three units consisted of four 100-ft. metal coke drums mounted about 40 ft. above grade. The sustained, intense fire caused other pipes in the area to rupture, releasing more hydrocarbons. Over 95 fire brigade members battled the fierce fire for nearly three hours. [21] The fire killed three people. 

On the basis of this discussion, the mechanisms of mesophase carbon fiber formation are closely related to those of needle coke, the principal differences being the extent to which the deformation and relaxation mechanisms are able to act. Because delayed coking involves relatively gentle but random deformation processes by bubble percolation and the long dwell times in the coke drum afford opportunity for extensive disclination annihilation and micro-structural relaxation, the structure of needle coke can be well defined by polarized-light microscopy (2,36). 

In delayed coking, a residual feedstock is charged to a furnace where it is rapidly heated and thermally decomposed. The heater effluent then enters a coke drum where the reaction is... 

Fractionation Section. A typical fractionation section includes the coker fractionator and attendant heat exchange equipment, the light gas oil side stream stripper and the overhead system. The coke drum overhead vapors enter the fractionator under shed trays which are located below conventional wash trays. Hot induced gas oil reflux is pumped to the wash trays to condense recycle and to wash the product vapors. The light and heavy gas oil products are condensed as sidestream products. The light gas oil product is usually steam stripped in a sidestream stripper. The overhead vapors from the fractionator are partially condensed and the gas and gasoline products are directed to the vapor recovery unit. 

Coke Dewatering and Handling System. When a coke drum is being emptied, coke and water must be collected and separated. To accomplish this, the facilities commonly used today include pit dewatering, pad dewatering, dewatering bin, and direct railcar loading. A short description of each follows. 

Direct railcar loading allows the coke to drop directly from the coke drum into a railcar. Coke and majority of fines remain in the railcar. Water drains from the railcar to a sump and is... 

A dewatering bin system is one in which the coke and water fall from the coke drum into a crusher and then, either by gravity or by slurry pump, are directed to a dewatering bin. In a gravity flow system, an innovation developed by Foster Wheeler, the coke drum and crusher are mounted on top of the dewatering bin. In a slurry system the crushed coke is pumped from a sump located directly below the coke drum and crusher to the dewatering bin. 

Three operating control variables in a delayed coker dictate the product quality and yields for a given feedstock. These variables are the heater outlet temperature, coke drum pressure and the ratio of recycle to fresh feed. 

A decrease in pressure has the effect of vaporizing more heavy hydrocarbons. As the production of desirable liquid hydrocarbons is increased at low pressures, the coke yield is correspondingly decreased. Thus, most modern delayed cokers have been designed to operate at a low coke drum pressure. 

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