Decay Curve Analysis

[TusharParticipant]

Any one has idea how to find Decay Curve for the Delayed Coker if one not available from design books?
It is utilize to calculate what % of the normal flow from the Coke Drums continues at time interval after the feed stops due to thermal cracking.

[Mike KimbrellParticipant]

As I discussed in my reply for hot spots in coke drums (https://refiningcommunity.com/forums/topic/hotspots-in-coke-drums/) the coking reactions take between 60 and 75 minutes at 825 F (440 C). The reaction mass in the bubbly liquid layer on top of the coke pile is approximately 60% converted, so over approximately 30 minutes after switching feed out, all the hydrocarbons reactions have been completed and no further products are exiting the drum.

My guess is the decay is a first order exponential decay as coking reactions are first order kinetics. The mass flow rate will go from full design to zero in roughly 30 minutes once feed is lost, unless the temperatures drop. As you know, at lower temperatures the time required for complete reaction increases.

Is your question in relation to trying to size relief valves for the fractionator or some other piece of equipment? If so, you should work with the licensor to understand the assumptions and basis for the relief valve calculations. The amount of vapor and the enthalpy balance needed to properly size relief valves is tricky and a thorough understanding of your feeds and unit are necessary to ensure the relief valves are adequately sized.

[TusharParticipant]

Thanks Mike for your feedback and to provide some light about this factor.
Yes, we are doing the RV sizing for the fractionator during power failure scenario when the feed will stop at a sudden. As our unit is very old, difficult to follow up with licensor. Do you know any other method to roughly estimate this factor?

[Mike KimbrellParticipant]

Coke yield on incoming feed is roughly 30% by weight, depending on the feed properties and operating conditions of your unit. This means that the vapor rate out of the drum is 70% by weight of the drum feed. The feed at the drum inlet is roughly 20% vaporized, so 50% by weight of the feed to the drum is generated within the coke drum due to the thermal decomposition reactions.

If you used 50% by weight of the normal drum feed rate as the vapor rate from the drum during a power failure case that has to be off-set by vaporizing liquid in the fractionator on an enthalpy balance basis, I think you would be in a defensible position for sizing relief valves. You could adjust for your feed slate and operating conditions and use the delta vapor generation on a normal basis as the vapor generated by the stored energy in the coke drum during a power failure.

[calvin gaoParticipant]

One way you can field test your decay curve is that installing two digital pressure gauges at the top of the coke drum and at the far end of the overhead vapour line: by measuring the dP overtime, you can calculate the amount of vapour flow comparing with the normal vapour flow.

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