September 16, 2017 at 3:35 am #27428
Can Crude compatibility possibly create Hotspots in Coke drum?
September 24, 2017 at 11:32 pm #27462
Hot spots are caused by inadequately cooling the coke bed in the coke drum. Sponge coke is the most forgiving coke morphology in terms of the off-line coke drum phases. Bonded or agglomerated shot coke are probably the least forgiving type of morphology. Loose b-b shot coke can be difficult to cool if the initial water rate is too high. The presence of unconverted oil in the drum from operating the drum outlet temperature too low will cause tar to remain in the coke drum which can blind sections of the coke bed from being contacted by liquid water. All of these things can result in hot spots in the coke drum.
If the resids blended together as Coker feed result in premature asphaltene precipitation, which is what is normally meant by incompatibility, the heater will show accelerated fouling and the coke morphology will likely be impacted that will make the coke bed less forgiving to the off-line steps. A short answer to the question is, yes resid incompatibility can contribute to hot spots in the coke drum. There are a number of other factors that contribute to hot spots as well. I have listed some of them below:
Resid as it enters the coke drum is roughly 20% converted and is pushed up to the bubbling liquid layer on the top of the coke bed. The average conversion of the liquid layer is roughly 60%. Coking reactions take approximately 60 to 75 minutes to complete at a reaction temperature of 825 deg F (440 deg C). Once the resid is switched out of the drum and into the other drum of the pair, the resid that just entered the drum will take 24 to 30 minutes to fully convert. If the little steam step is at too high of a rate, the steam will cool the resid and the reaction rate will slow down and full conversion of the oil that just entered the drum prior to the switch will take longer. Ensuring that all the resid in the coke drum is converted is key to minimizing hot spots in the coke drum.
Introducing liquid water at a low rate is important to prevent channeling of the water through the coke bed. For loose b-b shot coke or bonded or agglomerated shot coke I would recommend an initial water rate of no more than 75 gpm (17 m3/h) of quench water. I am aware of sites that start at an even lower initial water rate. Maintaining this low water rate for the first 60 minutes is recommended, but you need to get enough water into the coke drum to cover the coke bed with liquid water in the time allowed for quenching. This means that the initial water rate may need to be increased at 45 minutes rather than 60 minutes, depending on the maximum water rate the quench pumps can add.
Pressurized quenching at pressures higher than the coke drum operates during the coking phase will assist in getting liquid water to the coke bed and minimize the hot spots, should they be present. Once water channels through the coke bed there is not enough pressure drop through the bed to redistribute the water and steam blankets the coke and prevents liquid water from contacting the coke even if there is a large head of water above the hot spot. Higher pressure quenching reduces the volume of the steam blanket and allows more of the coke bed to be contacted by liquid water.
If you are injecting sludge during the initial water quench phase, the solids loading and solids size distribution is very important. Solids that are too large or a slurry that has too high of a solids loading can cause water to channel up through the coke bed allowing hot spots to be present.
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