a S = Sulfur; HGI = Hardgrove.
While one coking chamber is being filled in the cycle time, the other chamber is emptied. Typical volume of a modern coke drum is about 1000 m3 with size range in diameter from 5 to 9 m and in height from 20 to 45 m. After steaming and cooling of the coke chambers, the coke is removed by drilling and cutting with high‐pressure water (up to 340 bar pressure). The green coke obtained is conveyed to its application after the draining step: either as fuel or feed for calcining. Table 6.1.2.2 shows the different green coke qualities in relation to use.
The following components can be used as feed to the delayed coking units [12]:
1 1. Crude residues (atmospheric or vacuum).
2 2. Crack components (visbroken tar, cycle oil, decant oil, or thermal crack tar).
3 3. Deasphalted residues (pitch).
4 4. Coal oils.
5 5. Used plastic materials (recycling).
The products of delayed coking processes are (basis: vacuum residue or crack components):
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
