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High AT in transfer line beetween heater and drum.
After a maintenance shut down of the unit where we have some modifications in this pipe, the AT between heater and drum has increased from 18ºC until 27ºC (average values). We process heavy crudes with a 25%w concarbon and 20% C7 asphaltene.
For this crude, which is the minimum inlet drum T in order to avoid problems with coke formation? (Our current minimal T is around 468ºC)
Which is the maximum coil outlet T to avoid aceletated fouling in the heater? (now we would need to increase up to 500ºC)
We use fierglass insulation in this pipe. Have you tested any better insulation (e.g. aerogel)?
We are thinking to try an electric heat tracing, do you have any reference of this option being used in delayed cokers? Would it cause fouling in the pipe?
 
Answers
09/05/2018 A: niranjan puttaraju, reliance industries ltd, niranjanmurthyp@gmail.com
General rule of thumb is delta T across the transfer line is 10-12degC. however in your case shows too much of coking in trasfer-line. Have you checked the thermocouple? What is the before/after quench of Coke drum overhead vapor? this will indicate the problem with your unitH
12/04/2018 A: NS Murthy, GE, murthy.ns@ge.com
it appears to me that velocity in transfer line to drum is too low for such high DT. Check the turbulizing steam flow and the recycle flow in Coker feed. As regards insulation adequacy, check the surface temp over insulation. Any unusual temp excursion would imply poor insulation. Electric tracing may not be a solution in such high temp environment. By the way what is the heater run length you get between two pigging or steam air decoking. I guess it will be low too.
11/04/2018 A: William Collings, Jacobs Consultancy, dave.collings@jacobs.com
At temperatures below 800 degF coke drum overhead (pre-quench), you can expect to potentially begin having problems with more aggressive foam rises. It is crude dependent so this can vary from location to location.