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Delayed coking is one of the chemical engineering unit processes used in many petroleum refineries. In brief, the process heats the residual oil from the crude oil distillation unit in a petroleum refinery to its thermal cracking temperature in the heat transfer tubes of a furnace. This cracks the long chain hydrocarbon molecules of the residual oil into coker gas oil and petroleum coke.[1][2][3].
Some delayed coking units produce as much as 5,000 tons of coke per day.[4]
Flow diagram and process description
The diagram and description in this section are based on typical delayed coking unit with two coke drums. However, some petroleum refineries have delayed coking units with as many as 6 drums, each of which may have diameters of up to 10 meters and overall heights of up to 45 meters.[5]
Typical schematic flow diagram
Process description
The above flow diagram depicts a delayed coking unit with two coke drums. However, many coking units have more than two drums and with one or more drums on stream while
Thermal cracking begins in the furnace, continues in the transfer line between the furnace and the coke drums, and finishes in the coke drum.
As cracking continues in the drum, gas oil and lighter components are generated in vapor phase and separate from the liquid and solids. The drum effluent is vapor only except for any liquid or solids entrainment, and is directed to a fractionation column where it is separated into the desirable boiling point fractions. Solid coke is deposited in the drum in a porous structure that allows flow through the pores. All solids and uncracked residual liquid produced from the vapor and liquid feed are intended to remain in the drum.
After the drum is full of the solidified coke, the hot mixture from the furnace is switched to a second drum. While the second drum is filling, the full drum is steamed to further reduce hydrocarbon content of the petroleum coke, and then water quenched to cool it. The top and bottom heads of the full coke drum are removed, and the solid petroleum coke is then cut from the coke drum with a high pressure water nozzle, where it falls into a pit, pad, or sluiceway for reclamation to storage.
Larger cokers have several pairs of tandem drums.
Composition of coke
The table below illustrates the wide range of compositions for raw petroleum coke (referred to as green coke[6]) produced in a delayed coker and the corresponding compositions after the green coke has been calcined at 2375 °F (1302 °C):
| Component | Green coke as produced |
Coke calcined at 2375 °F |
|---|---|---|
| Fixed carbon, wt % | 80 − 95 | 98.0 − 99.5 |
| Hydrogen, wt % | 3.0 − 4.5 | 0.1 |
| Nitrogen, wt % | 0.1 − 0.5 | |
| Sulfur, wt % | 0.2 − 6.0 | |
| Volatile matter, wt % | 5 − 15 | 0.2 − 0.8 |
| Moisture, wt % | 0.5 − 10 | 0.1 |
| Ash, wt % | 0.1 − 1.0 | 0.02 − 0.7 |
| Density, g/cc | 1.2 − 1.6 | 1.9 − 2.1 |
| Metals, ppm weight: | ||
| Aluminum | 15 − 100 | 15 − 100 |
| Boron | 0.1 − 15 | 0.1 − 15 |
| Calcium | 25 − 500 | 25 − 500 |
| Chromium | 5 − 50 | 5 − 50 |
| Cobalt | 10 − 60 | 10 − 60 |
| Iron | 50 − 5000 | 50 − 5000 |
| Manganese | 2 − 100 | 2 − 100 |
| Magnesium | 10 − 250 | 10 − 250 |
| Molybdenum | 10 − 20 | 10 − 20 |
| Nickel | 10 − 500 | 10 − 500 |
| Potassium | 20 − 50 | 20 − 50 |
| Silicon | 50 − 600 | 50 − 600 |
| Sodium | 40 − 70 | 40 − 70 |
| Titanium | 2 − 60 | 2 − 60 |
| Vanadium | 5 − 500 | 5 − 500 |
References
- ↑ Gary, J.H. and Handwerk, G.E. (1984). Petroleum Refining Technology and Economics, 2nd Edition. Marcel Dekker, Inc. ISBN 0-8247-7150-8.
- ↑ Leffler, W.L. (1985). Petroleum refining for the nontechnical person, 2nd Edition. PennWell Books. ISBN 0-87814-280-0.
- ↑ Petroleum Coke Glossary
- ↑ Staff (November 2002). "2002 Refining Processes". Hydrocarbon Processing: pp. 85-147. ISSN 0887-0284.
- ↑ Delayed coking innovations and new design trends
- ↑ Petroleum coke on the website of the IUPAC Compendium of Chemical Terminology