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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Chad Perrott, Albemarle Corporation, chad.perrott@albemarle.com
UCO heat exchanger fouling is common in conversion hydrocracking units with an insufficient purge. This leads to a build-up of heavy polynuclear aromatics (HPNAs). The HPNAs are most often associated with coronene and ovalene, which have a red/orange colour. These HPNAs are increasingly hard to convert when recycled through the hydrocracker because their ring count will typically increase. Historically, the only way to manage them was with increased purging of HPNAs to prevent additional condensation reactions or the addition of adsorbents.
Currently, bulk metal catalysts (BMC) such as the proprietary Nebula or the proprietary Celestia are advanced formulations with up to 200% higher hydrodearomatisation (HDA) activity and hydrodesulphurisation (HDS) activity capable of saturating HPNA precursors like coronene and ovalene. Saturating these multi-ring aromatics can minimise fouling in hydrocracker UCO streams, leading to reduced UCO purge rates.
In addition, overall reactor HDS and HDN activity will also increase because of the loaded BMC. Higher HDN activity results in lower reactor WABTs for equal nitrogen (N) slip to the hydrocracking catalyst. Lower hydrocracker pre-treat WABTs increase the cycle time spent in the kinetic aromatic saturation mode, thus avoiding the thermodynamically controlled regime where dehydrogenation is present and more HPNAs are produced. Therefore, the trend towards increased use of BMCs in hydrocrackers can increase profitability through less UCO purge and reliability through fewer fouling events.
Dec-2022
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