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In our DCU main fractionator we have refractory laid on first tray (chimney tray), and this is done to prevent coke formation on this tray (coke drum ovhd vapours @420 deg cel comes in immediate contact with HCGO in this tray), i would like to know whether there is any way to identify if coke starts forming on this tray even after laying this refractory. our operating pressure is bottom- 0.77 kg/cm2 (g) and 310 deg cel. top - 0.53 kg/cm2(g) and 105 deg cel.
we have pressure transmitters and temperature transmitters across this chimney tray.
(our DCU plant will get commissioned by jan 2014 end, I am relatively inexperienced in this unit)
 
Answers
27/01/2014 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You might be able to see something with a gamma scan around the tray. Coke will be denser than hydrocarbon liquid on the tray, so as coke builds up on the tray you would see a layer build up with a density between that of the liquid and the refractory just above the refractory. If you want to use this approach I would recommend a base line scan shortly after start up.
Hopefully you also have underpan sprays because I don't believe that the refractory alone will prevent coke build up on the underside of the chimney tray. In fact it may increase coke build up there without sprays to quench the vapor and knock down entrained coke particles.
You should be aware that you can also get coke on the tray from coke particles that make it into the tower during normal operation or especially if there is a drum foam over.
On vacuum units some companies slope the chimney tray in the dirty wash oil and HVGO draws so that liquid level is maintained in the outlet pipe instead of on the tray to help minimize coke formation. If you have coke formation problems this might be something to consider.