Rising to the challenge

Refiners seeking opportunities to improve margins often turn to more difficult feedstocks from which to produce ultra-low-sulphur diesel (ULSD). Difficult crudes offer the potential to add value because they are cheaper. However, it is a delicate balancing act because such feedstocks are also more expensive to treat.

Catalyst performance, hydrogen consumption and regeneration yield all have an impact on the economics of processing more difficult feedstocks. The challenge in choosing a catalyst is to achieve high performance without high cost. “When the molecules in the feed are harder to treat, catalysts need to provide additional activity to deal with these difficult molecules more effectively, but they also need to deliver the expected cycle lengths and the product specifications that the refinery requires,” says Dave Casey, Business Development Manager at Criterion.

Crudes that are more difficult to process typically consume more hydrogen, which is a valuable commodity in the refinery. Therefore a catalyst that offers good hydrodesulphurisation selectivity will reduce hydrogen consumption and reduce costs. Alternatively, this may free hydrogen that can then be put to good use in another refinery operating unit.

Criterion has extended its ASCENT range of catalysts to offer high performance and maximum cycle length while reducing hydrogen consumption. DC-2534 is a cobalt–molybdenum catalyst that provides a lower-cost alternative for refiners with low-to-medium-pressure hydrotreaters. It has been designed with an improved structure that optimises promoter metal utilisation.

Casey explains: "The key to this ASCENT catalyst is that it contains a mix of type I and II active sites, which enables direct and indirect desulphurisation. This approach increases the number of active sites when compared with previous generations of ASCENT catalyst. Because a Type I catalyst removes sulphur from the molecule directly, it consumes less hydrogen."

Hydrogen consumption can be high during the start of the run. Many high-activity and Type II catalysts have a super-active period at the start of the run before stabilising over the next 30 days. During that period, the process may consume substantial amounts of hydrogen, which can cause constraints in other units on the refinery. In comparison, DC‑2534 provides a very stable start-of-run activity from day one to enable smooth refinery operations. It also facilitates desulphurisation in the poor hydrogenation environments found at end-of-run conditions where indirect desulphurisation is difficult.

Another benefit of using Type I catalysts is that they are more easily regenerated using a conventional single-stage regeneration processes, and thus give good regenerated catalyst yields at low cost. Regeneration of Type II catalysts uses a two-stage process, which is typically two to three times more costly. DC-2534’s high structural strength also means it can withstand the handling involved in removing and replacing it during regeneration. Finally, DC-2534 has a relatively low density, which means that its fill cost is lower than other alternative high-activity catalysts.

Motiva Enterprises' Norco refinery in New Orleans, USA, is currently using DC-2534 for ULSD production. The initial results suggest that the catalyst is working well. "Although it is still quite early in the cycle, the weighted average bed temperature is 4°C lower than anticipated and the actual hydrogen consumption is 5% lower than predicted performance. These are both good indications that the expected benefits are being realised," says Casey.

For more information contact David Casey.