May 23, 2016 at 5:08 pm #21622
Paul R OrlowskiKeymaster
Process Engineer, MBA – Crude Oil Refining | Energy / Asset Optimization | Margin Improvement
Submitted this discussion:
Coker recycle rate
I have a few questions related to Coker recycle:
1. What in your opinion would be “best in class”? I’ve heard 3.5%, but not sure if that is attainable.
2. How can you sustainable lower your recycle rate? I’ve heard of people removing the insulation from the drum overhead lines to reduce the quench needed, but will that increase the rate the lines “coke up”?
3. Does the orientation of the quench nozzles have a significant impact on quench rate required? Is there a mixing efficiency difference in injecting orientation? Quench nozzle type perhaps?
I realize lowering the recycle rate will improve yields on a Coker, I just thought someone would have insight on how to lower it sustainably. I appreciate your feedback.
Lead Process Engineer at PDVSA INTEVEP, Venezuela
Hi Errol…nice questions, here are my comments:
1) 3.5% (1.035 TPR) recycle is certainly attainable…even zero recycle cokers can be commonly found…but I wouldn’t say that a lower recycle makes a coker “better” than other, because it largely depends on the HCGO specifications. Of course, lower is better, but as long as your HCGO with higher end point, metals and CCR can be processed downstream.
Still need a number? I’d target 5% to make everyone happy.
2) I think the current trends in optimizing recycle are oriented to the use of control loops to:
– Adjust the overhead quench as a function of the DT between the coke drum overhead temperature before and after quench.
– Adjust the HCGO wash oil flow as a function of the wash zone temperature. (Please add a DCS stop to prevent going below minimum flow)
An easier way, if you have flexible HCGO specs, is setting a minimum for both cases (usually related to the nozzles, to guarantee a good flow pattern)
3) YES. Old FW designs came with two overhead quench injection nozzles in the first elbow of the overhead line: an horizontal one (in flow direction) and a vertical one (pointing the drum). New FW designs have eliminated the vertical nozzle and kept only the horizontal, since studies have shown that it’s actual influence in the quenching process is low, and it has been reported as having high plugging tendency.
In summary, recycle rate will be dictated by the HCGO specs…but don’t be afraid to push your unit to the limits and beyond.
To control HCGO quality, do people draw fresh zone gas oil? If so, where would you route this material to?
Assuming you meant Flash Zone instead of Fresh Zone, yes, in Bechtel Thruplus DC technology (bought from Conoco) there is a Flash Zone Gas Oil cut…generally it is recycled back to the fractionator together with the fresh feed, although in some cases as in the extra heavy oil upgraders it can be sent to the syncrude pool.
FW’s SYDEC designs, on the other hand, do not have this type of side cut (recycle is internal)
Hope this helps
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