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quenched with water 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. | quenched with water 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. | ||
==== Typical 24 hour coke drum cycle ==== | ==== Typical 24-hour coke drum cycle ==== | ||
<center> | <center> | ||
{| class="wikitable" | {| class="wikitable" | ||
|+ ''' | |+ '''24-Hour Delayed Coking Cycle<ref>{{cite book|author=Norman P. Lieberman|title=Troubleshooting Process Operations|edition=Third Edition|publisher=Pennwell Books|year=1991|id=ISBN 0-8714-348-3}} (page 49)</ref> | ||
! | ! Step!! Time, hours | ||
|- | |- | ||
| | |align=left|On-stream drum fills with coke||align=center|24.0 | ||
|- | |||
|align=left|Switch feed from full drum to empty drum ||align=center|0.5 | |||
|- | |||
|align=left|Steam-out of the full drum||align=center|1.0 | |||
|} | |} | ||
== Petroleum coke specifications == | == Petroleum coke specifications == | ||
Revision as of 13:25, 14 February 2008
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 vacuum 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, larger units have tandem pairs of drums, some with as many as 6 drums, each of which may have diameters of up to 10 meters and overall heights of up to 43 meters.[5]
Typical schematic flow diagram
Process description
Residual oil from the vacuum distillation unit (sometimes including high-boiling oils from other sources within the refinery) is pumped into the bottom of the distillation column called the main fractionator. From there it is pumped, along with some injected steam, into the fuel-fired furnace and heated to its thermal cracking temperature of about 480 °C. Thermal cracking begins in the pipe between the furnace and the coke drums, and finishes in the coke drum that is on-stream. The injected steam helps to minimize the deposition of coke within the furnace tubes.
Pumping the incoming residual oil into the bottom of the main fractionator, rather than directly into the furnace, preheats the residual oil by having it contact the hot vapors in the bottom of the fractionator. At the same time, some of the hot vapors condense into a high-boiling liquid which recycles back into the furnace along with the hot residual oil.
As cracking takes place 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 except for any liquid or solids entrainment, and is directed to main fractionator where it is separated into the desired boiling point fractions.
The solid coke is deposited and remains in the coke drum in a porous structure that allows flow through the pores. Depending upon the overall coke drum cycle being used, a coke drum may fill in 16 to 24 hours.
After the drum is full of the solidified coke, the hot mixture from the furnace is switched to the second drum. While the second drum is filling, the full drum is steamed out to further reduce the hydrocarbon content of the petroleum coke, and then quenched with water 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.
Typical 24-hour coke drum cycle
| Step | Time, hours |
|---|---|
| On-stream drum fills with coke | 24.0 |
| Switch feed from full drum to empty drum | 0.5 |
| Steam-out of the full drum | 1.0 |
Petroleum coke specifications
The table below lists the specifications for raw petroleum coke produced in a delayed coker and the corresponding specification after the raw coke has been calcined in a rotary kiln at 1200 to 1400 °C. The raw coke is classified as sponge coke and often referred to as green coke.[7]
| Component | Green coke as produced |
Coke calcined at 1300 °C |
|---|---|---|
| Fixed carbon, wt % | 86 − 92 | 99.5 |
| Sulfur, wt % | <2.5 | <2.5 |
| Volatile matter, wt % | 8− 14 | 0.5 |
| Moisture, wt % | 6-14 | 0.1 |
| Ash, wt % | 0.25 | 0.4 |
| Iron | 0.01 | 0.02 |
| Nickel | 0.02 | 0.03 |
| Silicon | 0.02 | 0.02 |
| Vanadium | 0.02 | 0.03 |
Uses of petroleum coke
History
Petroleum coke was first made in the 1860's in the early oil refineries in Pennsylvania which boiled oil in small, iron distillation stills to recover kerosene, a much needed lamp oil. The stills were heated by wood or coal fires built underneath them, which over-heated and coked the oil near the bottom. After the distillation was completed, the still was allowed to cool and workmen could then dig out the coke and tar.[9]
In 1913, William Merriam Burton, working as a chemist for the Standard Oil of Indiana refinery at Whiting, Indiana, was granted a patent[10] for the Burton thermal cracking process that he had developed. He was later to become the president of Standard Oil of Indiana before he retired.
In 1929, based on the Burton thermal cracking process, Standard Oil of Indiana built the first delayed coker. It required very arduous manual decoking.[9]
In the late 1930's, Shell oil developed hydraulic decoking using high-pressure water at their refinery in Wood River, Illinois. That made it it possible, by having two coke drums, for delayed decoking to become a semi-continous process.[9]
From 1955 onwards, the growth in the use of delayed coking increased. As of 2002, there were 130 petroleum refineries worldwide producing 172,000 tons per day of petroleum coke and 90% of that production is from delayed cokers. Included in those worldwide data, about 59 coking units were operating in the United States and producing 114,000 tons per day of coke.[11]
Other processes for producing petroleum coke
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
- ↑ Norman P. Lieberman (1991). Troubleshooting Process Operations, Third Edition. Pennwell Books. ISBN 0-8714-348-3. (page 49)
- ↑ Petroleum coke (from the IUPAC Compendium of Chemical Terminology
- ↑ David S.J. Jones and Peter P.Pujado (Editors) (2006). Handbook of Petroleum Processing, First Edition. Springer. ISBN 1-4020-2819-9. (page 61)
- ↑ 9.0 9.1 9.2 Tutorial: Delayed Coking Fundamentals (written by Paul Ellis and Christopher Paul of the Great Lakes Carbon Corporation)
- ↑ United States Patent Number 0149667
- ↑ Staff (December 31, 2002). "2002 Worldwide Refining Survey". Oil and Gas Journal: pp. 68-111. ISSN 0030-1388.