July 25, 2017 at 3:13 am #26864
“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coking cycle: high level alert that is not consistent with coke yield and drum filling (it seems as coke “floats” or is withdrawn with coking vapours) and more severe problems during decoking cycle: problems during cooling (high level measurement, it seems as coke “floats” in water),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) Excesive 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?
July 25, 2017 at 2:35 pm #26880
The symptoms you describe, sandy coke, difficulty in cooling the coke bed, plugged drain lines and bed collapse, are an indication that the temperature in the drum has not been high enough for the cycle time your unit is operating. The target drum outlet temperature should be 825 F (440 C). You may need to raise heater outlet temperatures 5 deg F or more to fully convert this new feed to the Coker. Feed stocks that have high MCR and high asphaltenes react quickly to form coke and do not require as much energy per unit mass to become coke.
The lighter more paraffinic feeds need to form aromatic rings and then react to coke which takes more time and more energy than the heavier feeds.
Time and temperature make coke, so if you cannot increase temperature then you need to give the material more time by increasing cycle time. Neither of these is desirable; however, the properties of the feed stock are the biggest lever in Coker performance.
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