Q & A > Question Details
Why is heater outlet temp of VR feed ie COT temp maintained at 498C not 487 or 467 or 477 or 475 or 482 or above 500? And what is effect of temp and pressure on coke yield inside drum?
 
Answers
14/03/2017 A: Dave Collings, Jacobs, dave.collings@jacobs.com
I agree with the comment regarding rules of thumb and the impact of increasing temperature. In general, it is a good idea to maximize temperature against operating constraints.
Not many Cokers are running higher than 498degC coil outlet temperature. In order to operate at higher temperatures, you must consider the following potential consequences:
1. Higher Percentage of Shot Coke - Most Cokers today have automatic de-heading valves to address the fact that shot coke will pour out of the drum (whereas spongy coke tends to stay in the drum).
2. Hardness - Higher temperatures result in harder coke which takes longer to cut. Evaluate your cycle time to determine if you can tolerate longer cutting times. Lower Hargrove index and lower VCM values are indicators of harder coke.
3. Drum Eruptions - Harder coke and more shot coke results in more issues with drum eruptions. Automatic de-heading valves on the overhead are less common but help to mitigate the safety issues. Otherwise, administrative controls are needed to keep operators safe. If time permits, longer sit-and-soak times can help mitigate drum eruptions.
4. Coke Value - Lower Hargrove index and lower VCM values result from higher operating temperatures. For some customers, this can reduce the value of the coke. The additional recovered liquid product nearly always significantly outweighs this potential coke devaluation. This is more of an issue for those making anode grade or needle coke.
5. Water Quench - The water quench / fill period may last longer as a result of increasing heater outlet temperatures. Evaluate your cycle time to determine if you can tolerate longer quench times. Make sure that your water quench is optimized with proper process control / water-ramping.
6. Bent and Stuck Drill Stems - Harder coke and a higher percentage of shot coke can lead to a higher frequency of bent or stuck drill stem issues. Well-trained operators with good procedures can avoid these problems for the most part, but be aware of this potential issue.
7. Poor Cutting - Operators sometimes leave coke behind in the drums (typically near the bottom) since harder coke is more difficult to remove. Good training and procedures can prevent this from happening.
8. Decoking or Spalling Frequency - Increasing temperature will have a significant impact. A 3degC change is a significant step change and may result in a decrease in spalling frequency or decoking interval by 0.5 to 1.0 weeks. Depending on the time duration of the spall or decoke, this can be a significant debit to yield benefits of increasing heater outlet temperature.
9. Energy Debit - Consider the energy debit of increasing temperature which is typically very small.
I would consider increasing your temperature by 1degC each week. Evaluate the impact that it is having on the above parameters and decide if you can increase further.
17/06/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
For a given feed composition, the selected temperature is that which simultaneously meet the coke hardness and volatility specifications.
15/06/2016 A: Prabhakar Reddy, Suncor Energy Inc, preddy@suncor.com
Furnace COT is determined based on the coke drum OVHD temperature (i.e pre-quench ) not based on type of feed. For optimum yield the recommended coke drum OVHD temperature prior to quench addition is 820-835 F. So if you don't have a TI upstream of the quench, I would recommend you to shut in the vapor line quench for 1-2 hr and get the pre-quench reading if it is lower or higher then the recommended temperatures, I would make the necessary correction on the heater COT's. That's how we have set our heater COT's in the plant for both AR and VR feed
As per Lieberman rules of thumb for delayed cokers.
* 8psi pressure reduction will increase liquid yields by 1.3 vol% on feed
* 8 psi pressure reduction will decrease the coke make by 1 wt % on feed
* 10F increase in coke drum vapor line temperature increases gas & distillate yield by 1.1 vol %
* 10F increase in coke drum vapor line temperature will decrease the coke make by 0.8 wt% on feed.