The following technical improvements have been achieved for the production of petroleum coke in modern delayed coking plants. They result in increased plant reliability, improved work safety, and better coke quality as well as increased production capacity [16–19]:
1 1. On‐stream spalling and/or pigging (allows spalling/pigging of one or two of the four furnace coils while the other are in operation).
2 2. Homogeneous petroleum coke quality (requires temperature and recycle ratio ramps over the cycle time to adjust continuous coking conditions).
3 3. Optimum coke chamber filling (requires mass balance of feed and products and measurement of the coke chamber level) and addition of antifoam medium (silicon oil).
4 4. Batch computerization (allows automatic checking and starting of operation steps without losing time).
5 5. Automatic coke cutting with vibration alarms at coke drum wall (coke cutting with no personnel).
6 6. Automatic hydraulic coke drum bottom head.
7 7. Combination tool for drilling and cutting.
8 8. Hydraulic feed line moving.
9 9. Hydraulic coke drum bottom head closing.
10 10. Use of slide valves for bottom and top drum unheading.
Figure 6.1.2.5 Relationship between plant operating conditions and plant yield. Parameters: (a) coke drum pressure = 2.5 bar; (b) feed/recycle ratio = 1 : 1.3; (c) coke drum outlet temperature = 440 °C [15].
With these technical advancements the cycle time for delayed coking can be reduced from about 24 to 12 hours [20–23]. The time required for different steps in typical 12 hours and 24 hours coking cycles is listed in Table 6.1.2.3.
6.1.2.3.1.2 Fluid Coking
The continuous fluid coking process utilizes fluidized solids technique developed for fluid catalytic cracking, except that no catalyst is used. A flow scheme is shown in Figure 6.1.2.6.
Residue is feed to the reactor (a). The cracking reactions occur at 500–550 °C in a fluidized bed of coke particles. The coke formed in the reactor flows continuously to the heater, where it is heated up to 600–650 °C by partial combustion in a second fluidized bed. Some parts of the heated coke particles are returned to the reactor in order to supply the energy for the endothermic crack reactions and to maintain the reactor temperature. The remainder of the coke after cooling is removed as a stream of fine particle “petroleum coke.” Fluid coke is only occasionally used for production of anodes [24].
Table 6.1.2.3 Time allotted for different steps in typical 12 and 24 hours coking cycles.
| Operation step | Time (h) | |
|---|---|---|
| Coking cycle | 24‐hour cycle | 12‐hour cycle |
| Switch drum | 0.5 | a) |
| Steam out to fractionator | 0.5 | a) |
| Steam out to blowdown | 1.0 | |
| Slow water cooling | 1.0 | b) |
| Full water rate | 5.0 | b |
| Drain water and remove top head | 3.5 | 1.5 |
| Drop bottom head and feed line | 0.5 | 0.5 |
| Hydraulic decoking | 4.0 | 2.0 |
| Replace heads and feed line | 1.0 | 0.5 |
| Steam purge and pressure test | 1.0 | 0.75 |
| Drum heat up | 4.0 | 2.25 |
| Slippage allowance | 2.0 | 1.0 |
| Total | 24 hours | 12 hours |
a) Total time for both operation steps: 0.5.
b) Total time for both operation steps: 3.0.
Figure 6.1.2.6 Flow sheet of fluid coking. (a) Reactor. (b) Scrubber. (c) Burner. (d) Air blower.
6.1.2.3.1.3 Flexicoking
The flexicoking process combines fluid coking with gasification. A flow scheme of flexicoking is shown in Figure 6.1.2.7. Flexicoking is a residue conversion process that combines coke gasification with conventional fluid coking. Three fluidized beds are in operation at the reactor, heater, and gasifier. Shrinking markets for the coke obtained in substantial amounts as by‐product of delayed coking and fluid coking due to restrictions of sulfur emission encouraged the development of the flexicoking process. In flexicoking most of the coke produced is converted to low‐Btu fuel gas in the gasification reactor. This fuel gas can be desulfurized to approximately the equivalent of a 0.25 wt% sulfur fuel oil. Thus, particularly for feed stocks with a high metal and sulfur content, flexicoking can be an advantage, because it produces marketable liquid products plus fuel gas.
Thermal conversion of the feed takes place in the reactor (a) where the feed is atomized through a series of nozzles into a bed of fluidized coke particles. Coke occurs in a thin film on the fluidized particles within the bed. The coke bed is fluidized by the product vapors and by stripping steam injected
