Heavy polynuclear aromatics (HPNA) formation in hydrocracking reactors is caused by condensation of aromatics and dehydrogenation of polynaphthenes. Once this happens, HPNAs are virtually impossible to convert.

Instead, they accumulate in the recycle oil of high conversion hydrocrackers. This causes HPNAs to build up to critical levels, increasing the risk of fouling in heat exchangers and catalyst deactivation.

HPNA formation limits the total conversion achievable in a “full conversion” hydrocracker, since a constant bleed of unconverted oil (UCO) from the hydrocracker, typically in the range of 2-5%, is normally applied to prevent excessive HPNA build-up. This is critical, because it can lead to a continuous loss of product upgrade value.

Managing HPNAs
TOPSOE™ has developed HPNA Trim™ (patent pending) to cost-effectively manage HPNAs in full conversion hydrocrackers. This uniquely simple process takes advantage of the heavy HPNA compounds’ extremely high boiling points.

HPNA Trim™ separates hydrocracker UCO into a light and heavy fraction by distilling the HPNAs concentrated in the heavy liquid product. It effectively reduces the required UCO bleed rate to an absolute minimum, enables the unit to operate close to true full conversion (>99%), and maximizes the yield of high-value products.

The HPNA Trim™ process has few pieces of equipment. This allows for a low-cost modular approach, making it suitable for easy retrofit to existing hydrocracking units.

The economic benefit of the HPNA Trim™ process is the added value from the difference in product values between the hydrocracker products (especially the middle distillates) and the UCO. Economic analyses show a very high return on investment with a typical simple payback time of half a year.

HPNA Trim™ can also help lower the steady-state HPNA level in the recycle oil to reduce catalyst deactivation in the hydrocracker and extend cycle length. Alternatively, it can facilitate the processing of heavier, higher endpoint vacuum gas oil or other heavy feeds while maintaining the same hydrocracker conversion and catalyst cycle length. The technology is also being used to design grassroot hydrocrackers at lower total pressure – with a lower investment requirement.