Question
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Increasing hydrocracker reactor heater temperature is a typical strategy when upgrading heavy feedstocks but should be balanced against higher energy costs and emissions considerations. Further complicating matters is fouling and corrosion of hydrocracker unit heat exchangers by unconverted oils (UCOs). What trends do you see in resolving increased fouling from UCOs?
Dec-2022
Answers
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Ole Frej Alkilde, Topsoe, ofc@topsoe.com:
When the severity of a hydrocracking unit is increased by increasing the reactor temperatures and/or processing heavier feedstocks, the fouling tendency of the UCO increases, and ultimately the UCO can become unstable, and a solid phase will precipitate out. The reason for this is an increased content of HPNA. The conventional way to control the HPNAs is to limit overall conversion by drawing a UCO bleed stream from the unit, typically 2-5% of the feed rate to the unit.
Other strategies are to reduce the reactor temperatures by using a more active hydrocracking catalyst, control the endpoint of the VGO feed to the hydrocracking unit, or revamp the unit by installing a hot separator, which removes the heavy product from the reactor effluent air cooler and thereby reduces the fouling. It is also possible to selectively remove HPNA by various industrially proven technologies like carbon bed absorption or advanced separation like Topsoe’s proprietary HPNA-Trim. This can reduce the UCO bleed rate from the unit by 60-80% and thereby increase overall conversion without reducing unit cycle length.
Dec-2022
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