Jul-2011

Improving residue hydrocracking performance

Trials with a catalyst system for enhanced transfer of hydrogen to asphaltenes show reduced sediment formation and fouling in ebullated-bed residue hydrocrackers

Joni Kunnas, Neste Oil
Lee Smith, HTI

Article Summary

Many ebullated-bed residue hydrocracking units are conversion limited as a result of fouling in downstream equipment. Increasingly difficult feedstocks have forced operators of ebullated-bed upgrading units to impose limitations on reactor temperatures and throughputs. Heavy oil and bitumen upgrading is often limited by poor conversion of asphaltenes, which in turn leads to the formation of sediment, coke and downstream foulants. The most likely cause is hydrogen-transfer limitations at a higher residue conversion, which cause aromatic structures in the asphaltenes to grow, aggregate and then fall out 
of solution as the oil becomes unstable.

HCat technology was developed by Headwaters’ HTI group as a molecular catalyst to limit sediment formation and fouling in ebullated-bed residue hydrocracking units. The function of the catalyst in a residue hydrocracking unit is to facilitate hydrogen transfer into the asphaltenes. This function gives higher asphaltene conversion and prevents oil instability when the converted oil is cooled in downstream equipment. The ability to hydrogenate and convert asphaltenes provides the mechanism to increase residue conversion without the associated oil instability and subsequent formation of sediment.

Neste Oil and HTI have been working together to implement the technology at Neste Oil’s Porvoo refinery and have successfully completed a 40-day commercial trial. This demonstrated a significant improvement in the ability of Neste Oil’s ebullated-bed LC-Fining unit to operate with a difficult feedstock at an elevated conversion for long periods of time. Throughout the trial, the combined feed to the unit was mostly Russian Export Blend or Urals-based vacuum residue.

The primary objective of the trial was to demonstrate the ability to control sediment in the vacuum tower bottoms product stream, while significantly increasing 560°C+ residue conversion. A second objective was to evaluate the ability of HCat to reduce product exchanger fouling rates, even at elevated residue conversions levels.

The purpose of this article is to provide an overview of the technology and the Porvoo refinery, and to review the results of the trial.

Technology overview
HCat residue hydrocracking technology is based on the in situ formation of a molecularly dispersed catalyst intimately mixed throughout the heavy oil feedstock. Molecular dispersion of the catalyst in heavy oil is achieved by blending an oil-soluble catalyst precursor with a residue feedstock using HTI’s proprietary mixing system before the feed preheat system. Figure 1 shows its addition in a simplified ebullated-bed residue hydrocracker.

As the feed is heated up to reactor conditions, the precursor breaks down and a molecularly dispersed catalyst is formed upstream of the ebullated-bed reactors. In addition, the highly polar catalyst preferentially associates with the asphaltene molecules that typically constitute the most polar fraction in heavy oil.

One of the primary functions of a hydroprocessing catalyst is to dissociate molecular hydrogen, which can readily hydrogenate unsaturated and cracked oil. Therefore, once in the ebullated-bed reactors, the catalyst facilitates hydrogen transfer, particularly into large asphaltenes that cannot readily access the solid heterogeneous catalyst (see Figure 2). This is because the catalyst remains dispersed in the heavy oil feedstock outside of the supported heterogeneous ebullated-bed catalyst.

A standard ebullated-bed residue hydrocracker without HCat is shown in Figure 3. The supported catalyst is ebullated in the reactor that defines the catalytically active zone. Large asphaltenic oil molecules responsible for sediment and fouling behaviour have limited access to the catalytically active sites in the supported catalyst.

An ebullated-bed residue hydrocracker with HCat is shown in Figure 4. The entire reactor volume now has catalytic activity, as the technology facilitates hydrogen transfer into the large asphaltenic oil molecules. This hydrogenation function limits the formation of sediment and the resulting fouling behaviour of the oil, even as the residue conversion is increased.

As a result, HCat is available to catalyse beneficial hydrogenation reactions involving asphaltenes and other large molecular components of heavy oil feedstocks that are too large to diffuse into the pores of typical supported ebullated-bed catalyst. Therefore, the catalyst complements the function of the solid supported catalysts by facilitating hydrogen addition into heavy components of the cracked residual oil. It should be made clear that the technology does not replace the function of the supported catalysts. It is, in fact, synergistic with the supported catalysts and allows the ebullated-bed reactor system to operate more efficiently.

The conversion of residue feedstocks is typically limited by the poor conversion of asphaltenes, which lead to coke and sediment formation at reactor conditions subsequent to downstream fouling. The most likely cause is hydrogen-transfer limitations at a high reactor severity, which cause aromatic structures in the asphaltenes to grow, aggregate and become unstable. This leads to mesophase and sediment formation, and fouling in the downstream separators, distillation towers and product heat exchangers. The function of HCat is to facilitate hydrogen transfer into these asphaltenes and increase asphaltene conversion. This function inhibits asphaltene instability and associated fouling.

The technology does not affect the kinetics of residue conversion. Residue conversion in an ebullated-bed hydrocracker is dependent on temperature, space velocity and other conditions. HCat does not increase or decrease this conversion. However, its ability to hydrogenate and convert asphaltenes gives a refiner the ability to raise reactor temperature and increase residue conversion without the associated oil instability and the subsequent formation of sediment and fouling that typically limits an ebullated-bed residue hydrocracker.