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Anytime there appears to be coke drum foamover, should write up the incident in the designated log. This is important information to have in determining if the antifoam is functioning properly and/or if the warmup, switching and blowdown procedures need to be further evaluated to take precautionary preventive measures.
Also, during the removal of the top head of the coke drum, the Process Technician should inspect the top drum flange neck internally and the top head for the appearance of soft coke fines. This is an indication that a possible foamover has occurred.
The following is a summary of what may be seen operationally during a foamover for Coke Drum .
NOTE: Most foamovers typically occur during the switch, however they may also occur during warm up.
(a) Spike in coke drum top Kay-Rays or similar
(b) Most noticeable is a rapid increase in the Coker Fractionator level from the extra volume of material coming over. 15 to 20% may increase immediately.
(c) A rapid increase in coker fractionator bottoms temperature, due to the hot foam.
(d) A larger than normal increase in the differential pressure between the Coker Fractionator and the Coke Drums Normal delta pressure is around 7psi. During foamover, this may increase to 11-13psi depend which is your design.
(e) Rapid loss of LCGO, HCGO and excess Naphtha production.
NOTE: The loss of production usually occurs during a switch, but during a switch with a foamover it is much worse.
(f) Rapid loss of hat temperature The hat temperature is the temperature of the Coke Drum vapors leaving the chimney tray (Tray P-1) of the Coker Fractionator
NOTE: Refer to hat temperature definition below. This loss is higher than the loss during a normal switch.
(g) Very rapid loss of overhead pressure and compressor that may set off antisurge alarms. The surge alarms on compressor would especially occur during a foamover during warm up, switch or blowdown.
1 to 2F increments.
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, at 15 psig coke drum pressure, the corresponding coke drum temperature should be at 840F +- 2F. 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 840F, which is expected to be approximately 10F 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.
Remenber How Coke Forms In A Coke Drum
Heater effluent entering a coke drum at 940F is partially vaporized. That is, part of the charge enters the drum as a vapor and the rest as a liquid. This is known as two-phase flow. The coking reactions, thermal cracking and polymerization, which began in the heater are completed in the coke drum over a two hour period. As the charge enters the coke drum the thermal cracking reactions accelerate even more due to the drop in pressure that occurs from the heater outlet to the drum. At the start of the filling cycle the coke drum will contain liquid in the bottom, then a layer of frothing liquid or foam and finally vapors. The rapid drop in pressure and increase in cracking accelerates foaming, or vapor disengagement, from the liquid. The foam layer on top of the liquid will contain reacting molecules undergoing both coking reactions. This violently reacting foam layer can be 8 to 10 feet in height. The use of antifoam is required during the last half of the filling cycle to suppress this foam layer. The antifoam is a high molecular weight silicone polymer, which acts to break the surface tension of the foam bubbles in the drum. Failure to inject antifoam could result in a foam over, whereby the foam is carried over into the Coker Fractionator or Coker Blowdown Drum . Carried over foam will continue to undergo the coking reactions, resulting in coke fines in the vapor lines and Tower bottoms. As the drum continues to fill, the liquid layer begins to concentrate coke. Thus, after several hours solid coke forms in the bottom of the coke drum forming the coke bed, on top of which is the liquid layer, and finally vapors. The vapors flow upward and out of the drum to the Coker Fractionator , however, some vapors contact the drum walls and condense. This condensed hydrocarbon that falls back into the reaction is to be recycled or further coked.
As the charge enters the bottom of the drum, it forms a main path or channel through the forming coke and liquid layers. The main charge channel branches out in various directions inside the coke bed as the drum is being filled. These channels are very important to the process since they will later be used for steam stripping and water cooling of the drum. The channels will become plugged if there is an interruption of charge or flow into the drum. Should an interruption of flow occur, then it will be very difficult if not impossible to steam strip or water cool the drum from the bottom inlet. It is therefore very important to maintain a flow at all times to keep the channels open. Thus, immediately following a drum switch, the charge line is swept clean into the coke drum with steam, and a continuous flow of steam is maintained thereafter during coke drum steam stripping. Additionally, it is equally important to follow the steam flow with water to keep the channels open for proper water quench. Some refiners have even added what is known as a proof quench to their procedure, which is a rather large flow of water for a short period of time immediately following the steam, to ensure that the coke bed stays open. Many have eliminated this step due to coke drum stress concerns. Regardless if a proof quench is done or not, the main idea is to keep the coke bed channels open. there are another point to keep in mind in order to avoid hot spot