operational versus catalyst integrity factor, and the oper- ation was being optimised to account for the current unit condition. For the treat gas reduction strategy, the hydrogen avail- ability for the base operation was already near the low end of the typical range and below the recommended guide- line. The risk of a further 10% reduction was deemed to be minimal since it was still within the typical operating range. This assessment is based on the unit’s relatively high operating pressure and low sulphur feed, which indicates a low severity of operation. For units with low operating pressure or difficult feeds, a more conservative approach may be warranted. Note also that this strategy applies to diesel hydrotreating units with hydrodesulphur- isation (HDS) and hydrodenitrogenation (HDN) catalyst only, which generally do not have a ΔT limit across the reactor beds. Different constraints apply to hydrocracking and dewax- ing units due to the high cracking activity of the catalyst and stricter limits on reactor bed temperature rise. In this case, the new operating conditions resulted in an accept- able risk profile while achieving the operational objective. Takeaways Distillate production remains a key driver for refinery eco - nomics. Forced reductions in product make, either through reduced unit charge or more frequent shutdowns, can sig- nificantly compromise plant profitability. Refiners strive to optimise the operational availability of their distillate hydrotreaters by maximising unit run length. While distillate hydrotreaters are normally controlled to minimise catalyst deactivation rate, the governing con- straint can change. For hydrotreaters that are confronted with an alternative processing limitation before reaching the maximum operat- ing WABT, the rigorous DHTR-SIM model can be employed to determine the optimal strategy for extending run length. Unit-based models are calibrated to the specific opera - tion, and the analysis is informed by key unit constraints and process expertise to deliver the greatest value to the refinery. The broader lesson extends beyond this case study. Whenever an alternative operating constraint steals the spotlight, yesterday’s operating strategy should be recon- sidered. The greatest improvements in refinery performance often come from recognising that the limiting constraint has changed. Petro-SIM is a mark of KBC. The author acknowledges the following KBC colleagues for their technical contributions: Eric Streit, Senior Principal Consultant; Matt Lehr, Senior Staff Consultant; Andrew Layton, Principal Consultant; Mohan Pathak, Senior Staff Consultant. Jeff Maday is a Principal Consultant at KBC (A Yokogawa Company) with 30 years of experience in the oil refining and petrochemical industry. Since 2012, he has provided support for Profit Improvement Programs, delivering value to clients through refinery-wide optimisa - tion of key business functions. Maday holds a BS in chemical engineer- ing from the University of Akron.
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