Question 73: What is your best practice for coke drum velocity to minimize coke carryover to the fractionator?
Coke drum velocity limit depends on the capability of the coker main fractionator and downstream equipment to handle entrained coke.
Question 72: When replacing coke drums with larger diameter drums, what process and operational changes do you expect?
In 2009, we completed a group of projects at El Dorado involving larger coke drums and a deep cut vacuum tower. The coke drum project limited the scope to drum replacement with minimal changes to the balance of the unit.
Question 71: We have coker/cracked naphtha reboilers which foul on the shell side. How can we minimize the fouling? Are you practicing on-line cleaning in this service?
Coker debutanizer fouling is mainly caused by conversion of diolefins to form polymerization material. The polymerization material over time can turns into hard coke.
Question 70: How do you monitor coke drum cutting water for fines content (test methods, frequency, time in cycle) and what is your trigger level for action to reduce fines content?
There are different methods to separate the fines in cutting water. In our latest Cokers, we have hydrocyclones, provided at the discharge of the reuse water pumps. The older cokers deploy clarifier.
Question 69: What are the effects of a poorly insulated coke drum? What is a good way to assess coke drum insulation effectiveness? How can the benefits of new insulation be estimated?
Poor coke drum insulation impacts both the mechanical integrity and unit operation
Question 68: What is a typical duration of online spalling of the coker furnace? What are the guidelines on maximum steam velocity during online spall?
Prima facie, the spalling operation is dovetailed with the coke drum cycle. We try to avoid continuing with the online spall during the changeover operation. This is to eliminate any chances of foam-over or coke carryover to fractionators.
Question 67: With double-fired heaters, how long do you run between decokes (mechanical or steam-air)? What is your steam spalling interval? How much velocity steam do you maintain?
We have 8 operating Delayed Cokers across our Refineries. Of these, 2 have double fired heaters and others have conventional single fired heaters. With similar feed and Coker operating parameters, the double fired heaters are expected to have longer run length.
Question 66: How does recycle affect time between coker furnace decokes/spalling?
Natural recycle helps to keep asphaltenes in solution; therefore, higher recycle tends to decrease furnace fouling assuming a constant furnace feedrate. Higher natural recycle reduces liquid volume yield and increases furnace firing which can impact unit economics.
Question 65: What are the impacts on coker operation (yields, capacity, energy, coke quality) of FCC slurry oil in the feed?
When one of MPC’s refineries starts processing heavy Canadian resid, they add 5 to 10 volume percent of slurry oil in the feed to mitigate making shot coke. The slurry also helps meet anode grade specifications on metals and sulfur. Processing slurry backs out resid processing which can impact unit economics.
Question 64: What are the impacts on coker operation (yields, capacity, energy, coke quality) of excess VGO (1000F-) in the feed?
Our El Dorado facility has transitioned from a 950o F HVGO/VTB cut point coker feed to a +1075 F while maintaining a fairly constant feed rate to the delayed coker unit. Our experience has seen coke and off-gas yield increase while HCGO yield decreases.