Question
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Our Diesel Hydrotreater feed is containing CCR 0.01wtppm and Total Aromatics of 31.5%, if CCR of feed is in such low values, indicating coke forming tendencies of feed is very low, then what causes coking on diesel hydrotreating catalysts?
Our Vaccum gas oil hydrotreater feed contains CCR 0.85% and Asphaltenes <300ppm and product CCR is <500ppm. The route for reduction of CCR in VGO hydrotreater is by formation of coke on catalysts or is there any other mechanism of reduction of CCR without coke formation?
Dec-2022
Answers
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Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br
Despite the relatively low CCR of the feed, the coking deposition process in hydrotreating catalytic beds depends on a several factors. Considering your information, the aromatics concentration in the feed of diesel hydrotreating unit is considerable and these compounds tends to present high coke lay down rates over the catalyst. It's important to ensure that the hydrogen partial pressure of the reactor is adequate as well as if interbed quench strategy is adequate for your processing unit, if not it's possible to create favourable conditions to hot points in the catalytic bed which favours cracking and dehydrogenation reactions which are responsible for the coke deposition over the catalysts.
Regarding the VGO hydrotreater it's possible to try to reduce the asphaltenes concentration through blending the feed stream with lighter streams, but it's necessary to make a deep study about the chemical compatibility of the streams aiming to avoid the asphaltenes precipitation in the pre-heating system or even in the catalytic bed which can reduce the operating cycle of the unit due to high pressure drop in the hydrotreating reactors. Again, it's fundamental to carry out an study to analyse if the VGO hydrotreating unit is operating under adequate operational conditions like hydrogen partial pressure, temperature, interbed quench strategy, etc. in order to ensure that the processing unit is adequately optimized and prepared to deal with the available feedstock.
Dec-2022
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