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RE: Drum overheads



The pressure at the top of the Coke Drums affects the type of coke produced and the yield distribution of the gas oil and lighter products.  Increasing the drum pressure increases coke yield and gives a better coke product, in terms of pore structure, density and VCM content, while decreasing the end point of heavy gas oil product and increasing the amount of C4 and lighter vapors produced.  Pentane and heavier liquid product yields are decreased at higher pressure.  Since coke is only by-product of the process, the Coke Drum  pressure should be operated at the lowest pressure possible within equipment limitations in order to maximize the overall yield of liquid products from the coker.
Every effort must be made to avoid rapid decrease in drum operating pressure to avoid expansion of the frothy liquid foam front, in the top portion of the drum and coke carryover to the Coker Fractionator
There is a definite relationship between coke drum pressure and temperature and the tendency for the coke drum to prime (foamover).  At a given pressure, lower temperatures will make the coke drum more likely to prime by expanding the foam front.  Increasing temperatures decreases the priming tendency by reducing the foam front and makes harder coke.  For the DCU, for example: at 15 psig coke drum pressure, the corresponding coke drum temperature should be at 840°F ± 2°F. A Pressure –temperature relationship used to estimate coke drum priming characteristics may be developed with plant performance data to determine the safe coking region.. The Coke Drum overhead temperature should be held at 840°F,  which is expected to be approximately 10°F above the “safe” temperature. Operating below the coke drum “safe” temperature will make the coke drum more likely to prime and may result in a foam-over.
Best regards

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