1 1. Gas/LPG (approximately 13 wt%).
2 2. Naphtha (approximately 11 wt%).
3 3. Middle distillates (approximately 45 wt%) – typical light and heavy coker gas oils.
4 4. “Green petroleum coke” (approximately 31 wt%).
Figure 6.1.2.1 Area petroleum coke production 2013 and production increase plans up to 2015.
Delayed coking is carried out in order to free refinery from the “heavy fuel oil” product that is not easily marketable. In terms of investment delayed coking is a much cheaper technology compared with flexicoking or hydrocracking (factor 3–5). A further argument in favor of delayed coking can be the revalorization potential for the carbon products “regular calcinate” or “needle coke.” Moreover, delayed coking shows good product results, which like all thermal conversions have “middle distillates” as main product.
Figure 6.1.2.2 Flow sheet of delayed coking. (a) Fractionator. (b) Furnace. (c) Coke drum. (d) Crusher. (e) Coke dewatering bin. (f) Water tank. (g) Coke–water pit.
Figure 6.1.2.3 Photograph of a refining delayed coker complex: furnace, coke drums, fractionator, and dewatering bins.
Figure 6.1.2.4 shows the product yields for the North Sea crude “Brent.” All thermal conversion processes reduce the coke make and increase yield of middle distillates, especially the delayed coker unit. The goal of nearly all delayed coker units is to minimize coke yield and maximize distillate fractions as possible. However, when the objective is to produce green coke that shall serve as raw material for higher‐quality coke products (regular calcinate and needle coke), a specific feed optimum must found that, which may not result in a minimum coke yield.
Table 6.1.2.2 Green coke qualities in relation to use.
| Heating medium | Regular calcinate feed | Needle coke feed | |
|---|---|---|---|
| Sulfur content (wt%) | >2 | 1.0–3.0 | 0.1–0.8 |
| V/Ni content (mg/kg) | 200–500 | 50–200 | <50 |
| Ca/Na content (mg/kg) | 50–300 | 20–100 | <50 |
| VCM (wt%) | 9–14 | 7–9 | 5–7 |
Figure 6.1.2.4 Product yields with thermal conversion processes.
The distillate yields and thus the capacities of delayed coking can be increased by [12–14]:
1 1. Pre‐conversion of feed components [11].
2 2. Addition of light middle distillate to the feed stream.
3 3. High coke drum outlet temperature /high coker furnace outlet temperature (COT).
4 4. Low operating pressure.
5 5. Maximizing fresh feed rate by minimum recycle ratio.
6 6. Addition of coil steam.
Some quantitative effects are shown in Figure 6.1.2.5.
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.