Coke Drum Foam-Overs: Causes and Cures

Many operators will double the amount of anti-foam injection a few minutes before switch time, then reduce it back to normal, when the drum pressures have stabilized (10 or 15 minutes later). I believe this to be a good practice. Total silicon contamination of coker naphtha will only increase by about 3%. On the other hand, this will not compensate for a sudden drop in coke drum pressure of 2 or 3 psi. Also, once the coke bed in the full drum starts to really fluff-up, a coke drum carry-over can’t be positively stopped with anti-foam injection.

To achieve an ascending pressure profile, the operators have a number of choices. My favorite one is to just pinch-back on the vapor valve from the full drum. This requires a local pressure indication from the top of the coke drum, transmitted down to the vapor valve. A more conventional method is to hold back pressure with the combined vapor line warm-up valve, which is often an HIC control valve. This method however, has the disadvantage of increasing the pressure of both the full and empty drums. That is, five times as much coke drum volume has to be pressured-up, as compared to my favorite method.

Increasing the fractionator pressure, by slowing down the wet gas compressor, and/or reducing the HCGO pump around rate also works. But this involves increasing the pressure of the fractionator and both coke drums. This is even less responsive than the previous suggestions.

Regardless of the method chosen to obtain an ascending drum pressure after the switch, the hotter the coke drum vapor outlet (upstream of the overhead vapor line quench), the more stable the coke bed will be. Also, as liquid yields increase (about one volume percent for every increase of 8°F in drum top temperature), the higher temperature off-sets the loss in liquid yield due to the 40 minutes period, when the coke drum pressure is higher than optimum. (Drum pressure should not be decreased until the steamed drum is lined-up to the blow down system).

Steaming out the coke drum

When the “little” steam (3,000 – 4,000 lbs./hr) is introduced into the coke drum, the foam front, at first, is actually suppressed. That’s because the steam will, for 10 or 15 minutes, cause an increase in the pressure in the drum being steamed. However this effect only lasts a short time. Then, the pressure in the drums will start to fall, unless the operator intervenes, to maintain an ascending pressure profile.

If the empty coke drum being switched into is cold (300°F condensate outlet), then the tendency to lose pressure, in the full drum being switched out of, is greatly enhanced. That’s because the combined vapor flow from both drums will be low, until the empty drum heats to full coking temperatures (790°F).

The combined effect of steaming a full drum, when the pressure in the drum is falling, can get pretty ugly. But, simply shutting off the “little” steam, once the coke bod begins to expand will – well, start polishing up your resume, because that will accelerate the rate of pressure decline, and pull the drum over.

Coke Drum Carry-Over While Filling

This occurs less frequently, then a foam-over after the switch. Often, the anti-foam is not added properly; diluent is added, but no silicon; a wrong anti-foam flow is displayed; the anti-foam injection point is plugged.

Or “your cup runneth over.” The gravity of the coker feed (°API) drops, and the operators neglects to cut coker feed. The bearings on the drill steam hoist motor of “A” drum are gone, and the “B” drum cycle needs to be extended just a little bit too long.

Properly calibrated K-ray level detectors give adequate warning to the alert operator. Some refiners have vibration probes (like those used to detect pump shaft vibration) on the overhead vapor line. At least for shot coke, this will identify when a coke carry-over begins. Reducing the heater outlet temperature, by 5°F to 10°F will temporarily stop the carry-over, by reducing the vapor generation in the coke drum.

Summary

Maintaining a gradually increasing pressure in the drum to be quenched, of several psi, for about 15 minutes after the switch, will off-set to some extent, marginally low drum temperatures; shortened coke drum cycles; low drum outages; higher feeds; low coke drum pressure and rapid coke drum switches.