PTQ Q4 2026 Issue

When an alternative constraint steals the spotlight

Managing furnace constraints to extend diesel hydrotreater run length

Jeff Maday KBC (A Yokogawa Company)

E very refinery has a limiting constraint. The challenge is recognising when it changes. Diesel hydrotreater (DHT) units are typically operated to mitigate catalyst deactivation because catalyst activity normally determines run length. However, constraints can shift. When they do, moving forward with the same operating strategy may no longer maximise refinery value. Higher DHT availability directly improves refinery profit - ability by maintaining high-value distillate production while reducing turnaround and catalyst replacement costs. DHT reactors are typically operated at minimum required tem - peratures to achieve product specifications. This extends the time between catalyst changeouts by preserving cata - lyst activity as long as practicable. As catalyst ages, reactor temperatures must be increased to account for decreasing catalyst activity. Run length is constrained by the maximum allowable temperature at the hydrotreating reactor outlet, which is limited by mechanical design or by undesirable reaction kinetics that can occur at elevated temperatures. Weighted average bed temper - ature (WABT) is used as a proxy for catalyst deactivation throughout the operating cycle. Operators monitor WABT to estimate end-of-run (EOR) timing and optimise unit throughput and turnaround frequency. A different operat - ing constraint, however, may emerge before the target EOR WABT is reached, leaving valuable catalyst activity unused.

This article examines one such case in which the reactor feed furnace duty emerged as the primary constraint. As a result, continuing with the previous operating strategy was no longer the optimal approach. A new strategy was developed, therefore, to use the remaining catalyst activity to extend the run length while maintaining the unit charge rate. The DHTR-SIM module in KBC’s Petro-SIM modelling suite was used to quantify the impact of this operating strategy and project the achievable run length extension. Recognising the new constraint Figure 1 depicts the DHT reactor configuration, which consisted of two parallel trains, each comprising a reac - tor feed furnace and three reactor beds with hydrogen quench injection to the inlet of beds 2 and 3. Under typical operation, hydrogen quench rates were set to maintain an equivalent outlet temperature from each of the three beds by quenching the reaction exotherms. This standard prac - tice maximises catalyst utilisation by minimising the reactor outlet temperature of the final bed, which would otherwise be the first to hit its design temperature limit, and slightly reducing catalyst deactivation by keeping beds 2 and 3 cooler. At this facility, the DHT unit was the primary refinery bottleneck. Increased DHT utilisation directly increased ultra low-sulphur diesel (ULSD) production, a key metric

DHT reactor conguration Two parallel trains with three reactor beds each

Standard operation

Hydrogen quench rates are set to maintain equivalent outlet temperatures from each bed by quenching the reaction exotherms. This maximises catalyst utilisation by reaching EOR outlet temperature on all 3 beds simultaneously.

Hydrogen recycle gas

H quench to bed 2

H quench to bed 3

Hydrogen makeup gas

Train 1

Bed 1

Bed 2

Bed 3

Treat gas

Changed constraints

Since the unit would have been limited by the furnace before reaching EOR WABT, the optimisation objective shifted from catalyst bed temperature control to managing the furnace constraint. KBC conceptualised a mitigation plan and quantied the impacts through use of the DHTR-SIM model. Analytical approach

Furnace

T, out

High sulphur diesel feed

ULSD product

Hydrogen quench practice

From Train 2

Process ow Hydrogen quench injection Hydrogen recycle gas

Hydrogen to Beds 2 and 3 quenches reaction exotherms Maintains equivalent outlet temperature from each bed (T out = T out =T out)

Figure 1 DHT reactor configuration

PTQ Q4 2026