Drum overheads

[Anonymous]

Hey Guys/GalsPlease review this info and let me know if it is acurate. Any and all input of any nature will be greatly apreciated.Thank YouAntonio Carranza  Overhead Vapor LinesThe vapor overhead line runs from the top of the coke drum to the fractionator. The temperature in the line is around 443°C (830°F) . The temperature is decreased by about 28°C (50°F) by injecting hot heavy coker gas oil into the line as quench oil. This prevents coking in the line. The heavy coker gas oil is a wash oil coating the inside of the pipe. If the liquid layer dries out, coke starts to form.Some refineries leave the insulation off the overhead lines to help drop the temperature and keep the inside wetted. Prevention of coke in the line is important since this will increase the pressure in the coke drum thus increasing reflux of gas oil in the drum. Decreasing coke drum pressure increasesliquid yield (decreases coke yield). Also, high pressure drops in overhead lines can cause foaming in the coke drum during the drum switch. Vapor line sizes are very large in order to obtain the minimum amount of pressure drop. One refinery used two 760 mm (30 inch) vapor lines in parallel.

[Anonymous]

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
España2005

[coker squirrelParticipant]

We are experimenting with running the quench oil on temperature control.  We are looking for an optimum point so that we can keep the vapor line “wet” enough to prevent coking and at the same time hold down the recycle ratio and keep the Fractionator Flash Zone more consistent through slump (warm-up and drum switches).  At what temperature do we need to keep the coke drum overhead line cooled down to prevent coking?

[Anonymous]

We injected between 2 to 3% ( weight)  of the total flow goint to the heater  and the temperature is between 805 and 800 degrees ºF ; with this  we have eliminated the formation of coke in our units in overhead  lines going to the frac. , except in the vertical line just leaving the coke drum , where it always has been formed coke between the flange of the drum and the flange of the pipe

[Anonymous]

Can anybody sgare a picture of a plugged OVHD line after a foam over event?

[Anonymous]

send me a email I will sent some picture

espaa2005

[Author]

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