Drum feed switching

Coke drum cycles (combination lower), 68, 70, 86-89 Coke drum feed switching, 51-53 Coke drum life (coking cycle), 68, 70... 

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. 

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 refinery operators were in the process of switching feed from the D drum to the C drum when a 45-degree elbow in the feed line ruptured. Investigators later determined that the 6-inch-diameter elbow was made of carbon steel instead of the 5-percent chrome alloy steel required by specifications. It seems that this section of piping was fabricated and installed in 1963. The mistake in piping fabrication was discovered 20 years later. The extent of the fire damage was such that the central unit was down for a period approaching a year and the adjacent units were each down for a few weeks. [21]... 

In the delayed coker the feed enters the bottom of the fractionator, where it mixes with recycled liquid condensed from the coke drum effluent. It is then pumped through the coking heater to one of two coke drums through a switch valve. It is at 480-500°C. Cracking and polymerization take place in the coke drum in a nominal 24-h period. Coking is a batch operation carried out in two coke drums. Coking takes place in one drum in... 

Delayed coking is the only main process in a modern petroleum refinery that is a batch-continuous process. The flow through the tube oven is continuous. The feed stream is switched between two drums. One drum is filling with coke while the other drum is being steam-stripped, cooled, decoked and warmed up (see chapter 6). The overhead vapors from the coke drums flow to a rectification unit. The rectification tower has a reservoir in the bottom where the fresh feed is combined with condensed product vapors (recycle) to make up the feed to the coker heater. 

Delayed coking is a well developed commercial process (6), and operates on a semi-continuous basis. Feed, usually vacuum residue, mixed with steam, is continuously pumped through tubular heaters in which it is heated to its incipient coking temperature. At this temperature the feed is injected into an insulated drum where coking takes place. The vapors produced in the drum during coking are continuously removed and fractionated. The fractions usually include coker naphtha and light and heavy coker gas oils. As a drum fills up, feed is switched to another drum. Meanwhile, the full drum is steam stripped, cooled and the coke drilled out. Whereas feed is continuously supplied to the drum, the coke is recovered intermittently. 

The antifoam can be added either on a continuous basis or when the foam front reaches a specified level. A series of level detectors is normally present on each drum, as shown in Figure 5. When the foam level is observed at the first level detector, the antifoam injection is started. An immediate drop in the level will be observed. At this time, the operators should be preparing the next drum to begin receiving feed. The antifoam injection will continue for some time after the drum switch occurs as the coke is being quenched with steam and water. 

The coke drums are typically installed in pairs for each fired heater. When the on-line coke drum is full of a high-density hydrocarbon residue known as petroleum coke, the heated feed is switched to an empty drum. After being filled, the full coke drum is isolated from the process flow and goes through a sequence of steps to remove the coke and prepare the drum for the next cycle, which can be from as little as 10 hours to more than 24 hours. Therefore, DCU is a semi-batch operation, in which the batch stage is represented by the filling and decoking steps sequence of the drums. Table 1 presents the sequence of typical steps and the associated duration of a 20 hours cycle DCU. 

Switch feed from full drum into empty drum. 3. Little steam—Use about 2,000 Ib/hr steam to strip lighter hydrocarbons out of the full drum to the combination 0.5... 

When switching feed out of a full coke drum into an empty drum,. the pressure on the full drum will drop. Experienced operators carefully observe the pressure on the full drum during a switch. Allowing the pressure to drop too rapidly will start a foamover. A drop of 1 or 2 psi every five minutes is about right. The top gamma-ray level indicator will show when a foamover starts during the feed switchover. To stop this foamover, the vapor valve on the full drum should be pinched closed until the drum pressure rises a few psi. 

Figure 2-5 shows the valves used during the coking cycle. Prior to switching out of a coke drum, the process operators must be sure that the empty coke drum is truly empty. Trying to speed up the cycle by cutting hot feed (900°F) into a coke drum that is incompletely drained will cause a foamover. 

Coking cycle time, 49-70 human factors, 50 time-saving techniques, 50-51 switching feed between drums, 51-53 vapor valve operation, 53 water feed drain, 53 water filling, 54-56 decoking crew, 56-57 unheading, 57-59 resid line to bottom head, 59 bot-... 

The objective in switching the feed between coke drums is to avoid foamovers. The author has witnessed several coke drum foamovers initiated by improper switching technique. Here is what transpired ... 

The proper method to control drum foamovers during a rapid switchover is to control coke drum pressure by pinching back on the outlet vapor valve to maintain the pressure in the full coke drum. While the operator at the switch valve is diverting the feed to the empty drum, he observes the full coke drum pressure. As this pressure falls during the switch, he closes off the vapor outlet valve from the full drum to hold the pressure within 5 psig of normal operating pressure. Using this technique, the switchover can be accomplished in about 20 minutes. 

One of the frequent causes of extended coke drum cycles is the difficulty in filling the drum with cooling water. Once feed is switched out of a coke drum, the coke tends to settle back and partially plug the feed nozzle in the bottom head. This effect is especially pronounced when shot (i.e., 8B-size pellet) coke is produced because of charging a low-gravity feed. Not only is the time required to fill the drum with water extended, but it becomes difficult to drain the drum. 

To keep the feed nozzle open during a switch, a small amount of steam must be cracked into the drum that is being taken off-line. By the time the feed has been diverted from a full drum, the flow of steam should be several thousand pounds per hour. In practice, the steam flow is adjusted to maintain the pressure in the feed line several psi above the pressure in the bottom of the coke drum. 

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