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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

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Answers

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• Dipankar Mitra, KBC (A Yokogawa Company), Dipankar.Mitra@kbc.global

  In the current market that rewards efficient operations, many refiners are looking closely at furnace performance. In many cases, a detailed economic analysis based on yield gain and energy cost within emission limits is the appropriate, cost-effective approach to pursue. Typically, modern hydrocracker units have proper heat integration with high furnace efficiencies and effective recovery of high-level heat.

  Therefore, increased conversion will likely result in less duty and emissions than upgrading lower-value heavy materials to distillates. While the increase in furnace outlet temperature directly affects fuel consumption and emissions, its effects extend much further. As cracking levels increase, hydrogen requirements and fractionation duties also increase. Even so, increasing conversion can be beneficial unless carbon costs are exceptionally high or upgrade margins are exceptionally low. To determine an appropriate conversion level, a comprehensive analysis should be completed.

  A key aspect of the hydrocracking process involves converting crude oil into valuable components, such as fuels, lubricants, and chemicals. In order to increase the hydrocracker conversion rate, the cracking bed temperature of the unit can be increased, the nitrogen slip from the pretreat reactor can be reduced, or both can be done. Furthermore, reducing the nitrogen slip from the pretreat reactor will lead to fewer chances of forming incremental heavy polynuclear aromatics (HPNAs), which are known to foul process equipment and shorten catalyst life during conversion. While a variety of methods can be used to achieve high conversion, where each method offers a different yield/economic impact, the rate of HPNA formation increases with higher conversion and heavier feedstocks.

  By recycling unconverted oils (UCO), HPNA formation and the per pass conversion are reduced for constant overall conversion. Following are some of the ways industry can manage HPNA issues:
  - Limit feed FBP <600ºC, especially for HCGO (crack feed)
  - Monitor the UCO colour as it changes from white to yellow to orange to red as HPNA concentrations rise and increase bleed as much as possible
  - Use bulk metal pretreat catalysts to saturate/remove HPNA precursors
  - Saturate HPNAs by using superior hydrogenation function catalysts in the second-stage cracking reactor
  - Employ hot, high-pressure separator design to avoid HPNA deposits on the reactor effluent air cooler (REAC), the main location where HPNAs cause problems
  - Adjust the unit design, such as installing liquid recycle filters, designing the separator with trays, and exporting unconverted oil with high HPNA content from the bottom while recycling material from the upper tray.

   

  Dec-2022