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• We are an Indian refinery and recently commissioned our full conversion VGO hydrocracker unit. Within 2 months after start up, we observed higher COT's in one of the heater pass. Our heater is 4 pass heater. What could be reason for this?

  We carried out flushing with high gas/liquid flow rate with jerks as well. Still dp across the pass is very high. Finally we wanted to carry out pigging as issue still persists.

  What could be reasons for this and how to avoid such scenarios in future. Request to share your ideas and similar experiences.

  Dec-2023

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Answers

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

  This phenomena can be related to the start up procedure of the hydrocracking unit. It's important to check if the licensor recommendations and procedures were totally accomplished specially related to the heating rate of the feed and the feed quality, especially related to the hydrogen quantity and presence of heavier compounds that could accelerate the coking process of the heater pass including the presence of a coking agent like sodium. Another reason can be mechanical damage in the heating coil which can produce preferential flow in some passes and accelerate the coking process in the low velocity pass.

  Another action can be checked if there is no flame impingement over the furnace tubes which can be caused by low air to the flame or burner tip fouling.

  If these points was checked and everything is considered normal, considering the experience of the refining industry related with similar scenarios (Irving St. John Refinery in 1998), it's recommended to shutdown the hydrocracking unit to investigate the causes and avoid a potential process safety accident due to the tube rupture during furnace operation.

   

  Dec-2023