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"Sand-type" coke morphology
Typically, we process high asphaltene, high-MCR vacuum residues in our Delayed Coker, that produce a shot-coke morphology.
But in one of our Delayed Coker Units we have recently changed feedstock quality, to light (low-MCR, low ashaltenes). This light VR is producing a transition coke (asphaltene / MCR ratio about 0,6) with low particle size (less than 1 mm), quite loose, that looks like "sand". This coke give us a lot of problems during coking cycle: high level alert that is not consistent with coke yield and drum filling (it seems as coke "floats" or is withdrawn with coking vapours) and more severe problems during decoking cycle: problems during cooling (high level measurement, it seems as coke "floats" in water), difficulties in cooling (more time required, it seems as coke release more heat that usually), plugging problems in drainage line and bed collapse during coke cutting.
Definitively, the coke bed formed is very loose, not compact and "mobile".
We suspect that it is due to: 1) Excessive velocity in the coke drum (due to higher gas production); 2) Light feedstock require more time in order to obtain a compact coke.
How could we improve the coke morphology to avoid this problem? Which should be the changes in operating conditions to avoid these problems?

 
Answers
25/07/2017 A: Ruben Miravalles, Repsol SA, rumiravallesgu@repsol.com
William Collings: Besides the operating conditions you mention, do you consider that an increase in coking cycle (residence time) should also help to avoid this kind of sand coke and improve the consistency of the bed?
25/07/2017 A: William Collings, Jacobs Consultancy, dave.collings@jacobs.com
Sandy Coke is often a sign of relatively low heater outlet temperature. Premature foam rises are also often a sign of relatively low heater outlet temperature also.
A coke drum overhead temperature (pre-quench) of less than 810degF is relatively low. Greater than 820degF is relatively high. Likewise, a heater outlet temperature of less than 905degF is relatively low and greater than 915degF is relatively high. Increasing temperature may help you. Since the Coke drum temperature varies throughout the cycle, you will want to compare each drum an hour before quench.
Plus 5degF would be a large increase. Great for yield too but may impact cutting time due to harder Coke. May result in more shot Coke and less sandy Coke. If no shot Coke, an increase in temperature should drive you to less sandy and more spongy Coke.
Coke drum cooling is a challenge for most Cokers. Cooling under pressure (rather than at minimum pressure) helps. Some have found success in tailoring the duration and magnitude of the initial water spike. Too much is said to cause channeling and poor cooling.
Recycle, HVGO point, and pressure are other handle to consider. Based on your symptoms, less recycle, less pressure, and higher cut point should drive you toward more shot Coke, better yields, and it may address the problems you are having. The opposite drives you to a more spongy structure and poorer yields & shot Coke avoidance.
Some say sponge Coke cools well, shot Coke cools well, but transition Coke doesn't. You may or may not find this to be the truth in your case.
Also, it is not unusual to have some Coke that floats.