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Why it is recommended to shutdown a hydrocraker / hydroprocessing unit if the system pressure goes below 80% of operating pressure? (Other than the issue of potential coking)

Dec-2024

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

The shutdown or idling of hydroprocessing units is always a significant concern to refiners, especially when the recycle flow is lost in power outage events, for example. Some processing units are designed to immediately shut down the feed pump and fired heater aiming to cut the liquid feed to the reactors and the reaction section is depressurised. The main objective here is to avoid the contact of the feed with the catalyst under low hydrogen partial pressure which can cause coke lay-down over the catalyst. Another reason to depressurise the processing unit and promote the total shutdown is to avoid uncontrolled reactions in the catalyst beds which can cause temperature runaway leading to catalyst deactivation due to sintering and potential damages to the reactors once the quenching systems are unavailable. Another relevant question is the temperature of the reactors which should be reduced to values below 230 oC in order to avoid that the reactors contain sufficient energy to promote reactions capable of coking the catalyst beds which will cause high pressure drop in the start up. Another important issue which normally receives less attention from refiners is the potential thermal shock in critical systems of the processing units. When the recycle flow is lost, it's possible to reach high temperature variations (higher than 200 oC) in some systems like pre-heating battery exchangers leading to leaks and metallurgical damage, this is a concern also to the reactors which can be exposed to extreme temperature variations due to the temperature runway in the catalyst beds causing the risks of hydrogen embrittlement or temper embrittlement where can occur the reduction in the toughness caused by metallurgical changing in low alloys steels exposed for long periods under high temperature (between 343 oC and 577 oC), this condition can led the reactors to catastrophic failure during start up and shut down procedure once there is a changing in the ductile-to-brittle temperature. For this reason it's fundamental to ensure that the emergency depressing systems are totally available and reliable and tested periodically.

A very good reference about this topic is the article by Mr. Eric Lin and Mr. Richard Todd published in the Q2 2021 issue of PTQ Magazine, which you can VIEW HERE

Dec-2024