• How much sodium is the limit expressed in mg/kg that can be loaded into a Visbreaker system without causing excessive scaling or coking in the oven?



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

    Unfortunately, I don't have much experience with the visbreaking process but considering the similarities with delayed coking process I believe that we can use the same considerations. Normally, it's considered a safe range of sodium concentration in the feed of delayed cokers between 10 to 15 ppm (mg/kg) in order to control the deleterious effects regarding coke laydown in the fired heaters tubes.

    The sodium concentration should be controlled at lower level as possible, but this is a trade-off nowadays considering the necessity to inject caustic soda in the crude upstream and downstream of the desalting system aiming to control the salt deposition in the crude oil distillation units as well as downstream units like hydrotreaters and delayed cokers.

    There is a growing concern related to chloride concentration in the feed of processing units which can lead to severe corrosive process due to salt deposition, this is a special issue for refiners processing crudes with high concentration of nitrogen which can lead to formation of ammonium chloride (NH4Cl) in higher temperatures than it's normally observed.

    It's necessary to carry out a deep study to determine the adequate level of caustic soda injection in the crude to control the chlorides without prejudice to run length of thermal cracking processes like visbreaking and delayed coking units once excessive dosages will accelerate the fouling in the fired heaters.



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