By Norm Lieberman
There are two types of coke drum foam-overs – bad and very bad:
- Drums that carry-over during filling – Bad
- Drums that carry-over after being filled –Very Bad
Results of a drum carry-over are quite variable, depending on how much coke has been carried into the fractionator, and now the fractionator is designed to deal with carry-overs. On most newer cokers, a circulating fractionator bottoms pump, and an external filter permit modest amounts of coke (a few tons) to be extracted from the fractionator in a day or two. A typical 12′ diameter fractionator can tolerate a single carry-over of 20 or 30 tons of coke. Carry-over amounts greater than these amounts will:
- Cause cavitation of the heater charge pump
- Increase heater tube skin temperatures
- Knock the fractionator off-line
Carry-Over After Switching
I’ve had a wonderful opportunity to study this problem in great detail last month. My client has four coke drum density level indicators, drum top pressure indicators and the combined drum outlet temperature located on the switch deck. This allowed me to observe the response of the coke drums, as the operators manipulated the drain, steam, switch and vapor valves. This coker has a tendency to carry-over on almost every switch. My observations: Made after watching half a dozen switches indicated.
- Foam-overs occur from the full drum, due to a loss in drum pressure.
- Pressures must be measured at the top of the coke drum, not 100 feet away at the pressure gauge located upstream of the vapor valves. For a few minutes after the switch, these pressures may be moving in opposite directions.
- A drum pressure drop of one psi in two minutes, is sufficient to initiate a foam-over.
- Once started, foam-overs are harder to suppress, then if never allowed to start in the first place.
- Rapid coke drum switching, does not necessarily promote an increase in foam front height, if the operators closely controls the full coke drum pressure.
- A 10% – 15% reduction in coke drum density, at the lower drum level indicators proceeds, by a few minutes, a drum carry-over. However, this cannot be used as a warning for action. It’s already too late.
- The drain valve from the warming drum must be closed before switching drums. However, closing the drain valve 3 or 5 minutes before switching is looking for trouble. The warm-up condensate can fill the feed line to the empty coke drum with lighter hydrocarbons. Then, when the 920°F resid hits the 300°F to 500°F condensate in the feed line, the resulting surge in vapor pressures-up both coke drum. If the pressure in the full coke drum then is permitted to drop back to it’s pre-switch pressure, a foam-over from the full drum is likely. Naturally, an incompletely drained drum may cause a massive pressure swing. At the now defunct Western Slope Refinery, I observed a 15 psi pressure swing due to a poorly drained drum. The resulting coke drum carry-over knocked the heater charge pumps off-line.
- Maintaining an ascending pressure at the top of the full coke seemed to be the best way to suppress foam-overs. For a period of 15 minutes, starting with the time the drain valve is closed, the full coke drum pressure ought to be increased by “x” psig. But what does “x” depend on? A typical valve for “x” is one psi per five minutes. However “x’, the rate of the increase in the pressure drop profile needed to suppress foam-overs, is a function of:
- Coke drum outage. A small outage being 18′ to 22′
- Low coke drum top temperature (i.e. less than 815°F) promotes unstable foam fronts, which requires a more positive ascending pressure profile to suppress.
- Lighter coker feeds, slop in feed, low coke drum pressure, high recycle rates, excess steam in the heater passes, and other factors that increase drum vapor velocities, require a larger “x” valve.
- Many of my clients use a tremendous amount of steam in the coke drum structure, most of which goes into the coke drums, which increases velocity.