While the continuous aspect of delayed coking is straightforward, problems arise in the batchwise filling and emptying of coke drums. The table below summarizes a typical cycle. While one drum of the pair is filling, the other drum is either steaming, quenching with water, hydraulically decoking, or warming with hot vapors.
The most commonly encountered difficulties in this batchwise operation are:
Foamovers. Partially coked resid is carried over the top of the coke drum.
Soft coke. The volatile combustible matter (VCM) of the coke is too high.
Shot coke. Coke is made in small balls instead of the usual sponge structure. This coke takes its name from its buckshot appearance.
Plugged steam-water inlet. Coke plugs the water inlet nozzle and slows down the coking cycle.
Excessive cycle length. The capacity of a coke drum varies inversely with the cycle length.
Warm-up condensate. This often mishandled stream can represent one-half volume percent on crude.
From an economic viewpoint, a foamover is the most serious of these problems.
Cooking cycle table.
1. Fill empty coke drum.
2. Switch feed from full drum into empty drum. Little steam—Use about 2,000 lb/hr steam to strip lighter hydrocarbons out of the full drum to the combination tower.
3. Big steam—Line up full drum overhead to blowdown system and use about 20,000 lb/hr steam to cool the coke partially in the bottom head.
4. Little water—Slowly start a small amount of water into the bottom of the drum while reducing steam flow. Water flow is maximized consistent with not overpressuring the drum, usually about 50 gpm.
5. Big water—Open up the main water line and fill the coke drum with water, typically at 1,000 gpm.
7. Drain water—After the water in the drum stops boiling, coke is cooled and water is drained out the bottom.
8. Remove heads—The large heads on the top and bottom of the drum are unbolted and removed by the decoking crew.
9. Drill pilot hole—Using high-pressure water, a 4-ft diameter hole is cut from top to bottom through the coke.
10. Cut coke also using high-pressure (3,000 psig) water. Starting from the top, the coke is cut and falls through the pilot hole.
11. Replace heads.
12. Steam test and purge the drum to ensure their heads do not leak and that air is removed.
13. Warm up the empty drum by backing hot vapors from the active drum through the top of the inactive drum. Condensed liquid is pumped to the FCCU; feed and vapors are vented back to the combination tower.
14. Open the vapor valve of the empty drum all the way and prepare to switch feed.
15. Idle time allowed for slippage of individual steps.
Categories: Process Troubleshooting | Tags: Coking | Leave a comment