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• I am Engineer in a residue hydrotreater unit. As this is severe service (360C+), we often see upsets in the feed filter dP and the first reactor dP increases after a few months. Troubleshooting already done on feed composition, unit conditions.

  The question concerns the feed tanks upstream the unit. For as there is water and sediments in the feed (from ADU), that should settle into the tank sump and be drained before the tank is lined up to feed the hydrotreater. I did rough estimation via Stokes law and involving the tank height, water and feed SpGr, viscosity @ T, etc...

  It shows that water would need roughly 24h to settle to the tank bottom.
  1) Is this approach correct?
  2) How to do this concerning sediments?
  3) The tanks are old, have been in VGO service before. Last inspections did not reveil any internal corrosion. Now with residue service, SpGr became closer to water, so settling of water would take more time. I scare the tanks are not adequate for this new service.

  We also think that even having an automatic BW filter, some of the tank sump comes to plug the reactor catalyst leading at least partly (the rest is coke) to the dP.

  Thank you in advance for any feedback on this.

  Dec-2022

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Answers

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

  I would like to suggest a two ways approach:

  1 - Regarding the Tank and Feedstock filter - The tank was cleaned before the change of service? If not, it's possible to carried out some chemical incompatability between the residue and the VGO in the ballast, leading to precipitation of asphaltenic compounds which can be plugging the downstream filters, another question is to ensure that the feedstock tanks have backup aiming to allow a frequent (maybe once a year) cleaning stop. Another question is if the feedstock filters have automatic backwash system, considering the hard service of this unity this is fundamental do ensure an adequate operation lifecycle.

  2 - Regarding the Hydrotreating Reactors - A special attention needs to be considered to the catalyst grading of the reactors, the feedstock was adequately characterized? If yes, it's possible to determine the best catalyst grading to ensure the desired operational lifecycle including the use of contaminants trap which tends to minimize the pressure drop in the catalyst bed during the operational campaign, mainly in residue hydrotreaters which tends to operates under high severity and feedstock's with high contaminants content (metals, etc.).

  Dec-2022