Delayed coking unit

Products obtained as side streams are vacuum gas oil (VGO), lube oil base stocks, and asphalt. Asphalt may be used for paving roads or may be charged to a delayed coking unit. 

In delayed coking, the reactor system consists of a short contact-time heater coupled to a large drum in which the preheated feed soaks on a hatch basis. Coke gradually forms in the drum. A delayed coking unit has at least a pair of drums. When the coke reaches a predetermined level in one drum, flow is diverted to the other so that the process is continuous. 

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

Products from catalytic cracking units are also more stable due to a lower olefin content in the liquid products. This reflects a higher hydrogen transfer activity, which leads to more saturated hydrocarbons than in thermally cracked products from delayed coking units, for example. 

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. 

Unsaturated hydrocarbons can add to the deposit-forming cyclic structures with five or six C-atoms through Diels-Alder reactions. The successive dehydrogenations produce polycyclic aromatic structures. In the third reaction class, aromatic species add to macro-radicals of the surface. Note that these radicals are greatly stabilized by the aromatic resonance. Thus, the third reaction class is more important at lower temperatures, typically, in TLE, visbreaking and delayed coking units. 

Delayed Coking. By far, the most extensive use of antifoams is in the delayed coking unit. Coking is a high-temperature thermal operation... 

A more complex example than using a microprocessor is the use of a microcomputer for a process system which requires a 16-bit memory. The control of a refinery delayed coking unit, shown in Figure 10-8, is the system that will be described. 

Hazards of Delayed Coking Unit (DCU) batch operations... 

Anyone who has ever operated a delayed coking unit can testify that large amounts of a very heavy liquid can be foamed over by depressuring a coke drum too rapidly (see Chapter 2). It follows that a constant operating pressure can stop a carry-over from a KO drum if the drum is foaming over due to fluctuations in pressure. 

How long should it take to turn a coke drum around The elapsed time from when one switches out of a full drum until it is empty and ready to receive resid again is called the cycle time. Most delayed coking units are designed for a 24-hour cycle time. Shorter cycles can be used in refineries where coke drum volume limits capacity. Each of the steps detailed in the Troubleshooting Checklist following this chapter can be contracted to save time. 

The delayed coking unit is where heavy oil residues are converted into coke, which can then be used, say, as fuel for electricity generation. The heavy oil residues are passed through a furnace - a thermal conversion unit where the long chain molecules are cracked - and into large coke drums where the coke formation actually takes place. The term delayed is used to indicate the coke formation does not take place in the furnace (which would lead to a plant shutdown) but, instead, the coke crystallizes in the large coke drums after the furnace. 

A powerful storm hit western Washington State on November 23,1998. It caused widespread damage and also interrupted the electricity supplies to the Equilon Ana-cortes refinery in Puget Sound for about 2 hours. This meant, in particular, that the delayed coking unit had to be re-started. Drum A was about one hour into a routine charging cycle when the power interruption occurred. 

Reduced particulate emissions. To reduce emission of particulates, better cyclones and high-efficiency electrostatic precipitators are being added to FCC units. At delayed coking units, enclosed coke conveyors and storage systems have been installed. 

Most refinery PM eomes from two sources - delayed coking units and the regenerators of fluid catalytic cracking (FCC) units. FCC regenerators also emit ammonia, which combines with SOx and NOx in the air to form ammonium sulfates and nitrates. According to the South Coast Air Quality Management District (AQMD) in Southern California, 1 ton of ammonia can generate 6 tons of PM 10 - airborne particulates with partiele diameters less than 10 microns. PM2.5 stands for airborne particulates with diameters less than 2.5 microns. 

FIGURE 5.1 Typical flow diagram of a delayed coking unit. 

---