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Are there any chemicals being used for increasing the Coker furnace run length. If yes, what are the pros and cons on usage of these chemicals. How much was the furnace run length increased?

Nov-2024

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Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Cycle length of delayed coking fired heaters is always a concern for refiners. Unfortunately, I don't have notice about chemicals added to the feed which are able to significantly raise the cycle length between shutdowns for mechanical cleaning of the fired heaters tubes to remove coke, but you can try some verifications and adjustments in the process to ensure larger cycle length: 1 - Verify if the mass velocity in the fired heater tubes are adequate, normally higher mass velocity leads to larger cycle length but there is a limit which needs to be respected. If the mass velocity is excessively high, the cycle length will be limited by the pressure drop in the fired heater coil in detriment of tube metal temperature; 2 - Verify if the flow rate of steam injection in the tubes is adequate. The steam injection is applied to raise the linear velocity in the tubes, reducing the residence time which consequently reduces the coke deposition in the fired heater tubes. Lower steam injection flow rates can raise the coke deposition rate and reduce the cycle length. 3 - Check the sodium chloride in the crude oil processed by the refinery and the performance of the crude oil desalting system. As described in the literature, sodium and calcium tends to catalyze the coke precipitation reactions in the fired heaters tubes, to avoid this effect, the sodium content in the feed of delayed coking units should be controlled below 15 ppm. This point demands special control actions considering that increasingly more refiners are using sodium hydroxide in the crude oil distillation units as strategy to control the chlorides salt content in the processed crude and an eventual excessive dosage will quickly reflect in the feed quality of delayed coking unit. 4 - Study the variation of feed composition to the delayed coking unit. Some refiners can blend highly paraffinic feeds with high asphaltenic feeds, this will lead to quick asphaltenes laydown in the fired heaters tubes. It's necessary to keep an operational routine to analyze the feed composition to prevent this kind of sudden variation. 5 - Verify the recycle ratio practiced in the delayed coking unit. Lower recycle rate will lead to less coke deposition and higher distillates yield, on the other side, the recycle stream tends to be highly aromatic which will help to solubilise the asphaltenes. Furthermore, the major part of the recycle stream will vaporize the fired heater tubes, raising the velocity and reducing the residence time which will reduce the coke deposition. To ensure an adequate balance between pros and cons, it keeps the recycle ratio close to the recommended by the design of the processing unit. 6 - If there is availability in the refinery, it's possible to add decanted oil from FCC (the bottom stream of the main fractionator of FCC units) to the delayed coking unit. Decanted oil is highly aromatic and tends to help to maintain the asphaltenes solubility in the feed stream, minimizing coke laydown occurrence. According to the literature, adding to 3,0 to 5,0 % of decanted oil in the delayed coking feed it's possible to be an improvement in the fired heaters cycle length. Another strategy which can be applied to improve the cycle length of delayed coking units is the use of ceramic coatings in the internal wall of the fired heaters tubes. This coating significantly reduces the roughness of the tubes, minimizing the asphaltenes adhesion to the tube wall which will lead to a drastic reduction in the tube fouling rate.

Nov-2024