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We have observed that when we processed some crude oil that produced the feed to DCU the skins temperature in the heater increases 15 – 20 degree Celsius in a day. In order to solve this to some known crude oil we need to add Decanted oil from FCCU (3 -5 %) to keep under control this problem. We have do some analyses in the lab (transmittance number, precipitation, etc) to check and know a new crude oil before processing its VR in the DCU. Anybody do the same ? How can I know before what kind of problem could it will have in the heaters ?
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If adding 5% DCO to the Coker feed reduces the fouling rate, then the problem is likely an incompatible resid blend. DCO is very aromatic and a good solvent for asphaltenes, so blending DCO into the Coker feed will help keep the asphaltenes in solution and limit the coke formation in the heater.
When I have seen resid incompatibility the heater fouling occurs in the upper radiant section and the shock tubes of the convection section. Normal Coker heater fouling is typically in the lower radiant section of the heater. Where in the heater are you seeing this rapid fouling?
Asphaltene precipitation occurs when the solvent power of the resid is insufficient to keep all the asphaltenes in solution. Blending aromatic, high asphaltene resids with paraffinic, low asphaltene resids can result in significant asphaltene precipitation, depending on the proportions and the blending order. If resid blending is done by blending crudes together, then the order does not matter. The overall solvent power is the key to preventing the asphaltenes from coming out of solution early.
Just because the crudes are compatible at desalter temperatures does not mean that the resids will be compatible at the elevated temperatures in the Coker heater. Managing the ratios of aromatic to paraffinic resids is probably the most practical method of managing this fouling. I am not aware of a definitive test that predicts this fouling.
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We observed coking in the radiant section of the heaters. As I told we observed a increase in the skin temperature of 15-20 degrees celsius in a day. It is to much, the regular number is 0.5-0.7 degrees celsius a day
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The excessive fouling you experienced in the radiant section, was it in the upper part or in the lower part? There are a few reasons for fouling in the upper part. One reason is resid incompatibility. Another reason for fouling in that area that has been reported is inorganic contamination of the feed. One of the contaminants has been determined to be an antifoam that is used in the production of the base crude oil.
Fouling in the lower portion is typically the conventional coke formation mechanism.
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