Residuum hydrocracking: chemistry and catalysis

The hydrocracking of residuum oil fractions has been practised for nearly six decades, evolving from simpler, single catalyst ebullated bed reactor systems capable of some 65-70% vacuum bottoms conversion to the latest slurry hydrocracking systems that effectively push vacuum bottoms conversion well beyond 90% and up to 97-98%. Deep conversion of residuum enables the production of ultra low sulphur (ULS) fuels as well as feed preparation for petrochemicals production.

Throughout this development history, a key challenge has been to maintain robust and reliable operability by mitigating and ultimately controlling what is generically called sediment or sludge formation and deposition. Sediment formation has limited the performance of ebullated bed reactor systems from achieving the potential conversion levels possible with the LC-Fining reactor platform. This is also highly dependent on the feed characteristics, as shown in Figure 1.

Key to fully utilising the full reactor potential to maximising conversion are the types of catalyst utilised in conjunction with appropriate operating conditions and flow scheme. This article will review the understandings in analytical petroleum chemistry and catalysis that have enabled this technology to arrive at this level of success. Success in deep residuum hydrocracking is only possible by understanding the fundamental hydrocarbon chemistry involved and marrying it with the properly designed catalysts and optimised catalyst systems to meet high conversion objectives.

Understanding the chemistry
It is clear that fundamental concepts of the hydrocracking chemistry of residuum fractions have progressed immensely since the 1960s when the heavier components of residuum fractions were characterised by carbon residue measurements relevant to thermal visbreaking and coking operations and fluid catalytic crackers (FCC) and somewhat ill-defined insolubles in one or more alkanes.

The term ‘asphaltenes’ was associated with alkane (typically heptane) insolubles but poorly envisioned, much less understood, on a molecular level. They were highly aromatic in nature and deficient in hydrogen, but little more was fundamentally known. Empirically, it was established that sediment formation, for the most part, arose because the catalytic hydrogenation associated with the residuum hydrocracking was overly selective in hydrogenating the hydrocarbon fractions acting as the solvent for the ill-defined asphaltene components. At the same time, cracking of the side chains and naphthenic rings occurs in the asphaltene molecules, leaving the aromatic cores mostly unaffected and incompatible with the oil. These changes make the oil product more paraffinic and the unconverted asphaltene cores more aromatic and condensed than those in the feed. In consequence, a disturbance in the asphaltene-resin interactions occurs, leading to the precipitation of asphaltene as sediments.¹ Additionally, as temperature increases, the rates of thermal cracking reactions increase more rapidly than the hydrogen addition counterparts. Thus, hydrogen transfer limitations occur, which can lead to the growth of aromatic structures in the asphaltenes, making them more prone to precipitate once these compounds leave the reactor zone.

Various filtration methods and gradient of solvent experiments led from saturates, aromatics, resins and asphaltene (SARA) characterisation to asphaltene solubility profiles. However, empirical characterisation progress needed to be matched by a coherent theoretical understanding of the molecular nature of residuum oil components. Prior to the 1980s, theory on the nature and structure of asphaltenes and associated heavier ends focused on them being polymeric in nature and perhaps the result of geological transformations or conversely micellular entities consistent with a colloidal view of petroleum.

However, in the 1980s and 1990s, the extensive effort first by Boduszynski, et al and extended by Ovalles, Moir et al at Chevron Energy Technology Company² established that the molecular composition of petroleum from light ends through the heaviest residuum components was a continuum in molecular weight (the ‘Boduszynski Continuum’) and developed more and more advanced means of characterising the molecular spectrum that framed the species that lead to sediment formation under residuum hydrocracking conditions.

Enhanced asphaltene separation techniques using liquid chromatography apparatus and tailored solvent gradient sequences, coupled with in-line filtration developed in the last few years by Rogel, Ovalles, Moir and co-workers, now allow asphaltene types to be separated and characterised by their stability and tendencies to precipitate as sediment.

Today, advanced analytical techniques such as ultra high resolution Fourier transform ion cyclotron resonance mass spectrometry enable the mapping of the heavy oil molecular continuum by carbon atom count and aromaticity as indicated by double bond equivalent (DBE). Indeed, it is now possible to track reaction dynamics molecularly by DBE and carbon atom count (see Figure 2).³ Atomic force microscopy (AFM) enables us to ‘see’ various asphaltene structures.⁴ Such advances better enable the design and optimisation of catalyst chemistry, porous structure and multi-catalyst reaction systems in residuum hydrocracking.
 
The role of catalysis
Most of the vacuum residuum conversion in hydrocracking processes is thermal in nature. The catalyst or catalyst system provides a variety of concurrent benefits through various catalytic reactions: removal of sulphur (hydrodesulphurisation or HDS) for ULS fuels, nickel and vanadium (hydrodemetallisation or HDM) reactions to mitigate fouling deactivation of downstream catalyst systems, nitrogen removal (hydrogenitrogenation or HDN) and removal of micro carbon residue (HDMCR) to mitigate inhibition in subsequent processes such as vacuum gasoil (VGO) hydrocracking, and selective hydrogenation for mitigation of sediment formation plus polynuclear aromatics (PNA) reduction in lighter fractions. Other reactions such as dealkylation of asphaltenes and other higher molecular weight multi-ring aromatics must also be considered in mitigating sediment formation. Because catalysts are concurrently added and removed from both ebullated bed reactors (EBRs) and slurry hydrocracking reactors in such processes as LC-Fining and LC-Slurry, they also serve the role of transporting deposited nickel and vanadium metals and heavy, carbonaceous coke from the reaction zones.

The upshot of these catalytic reactions is that significant effort must be devoted to developing the proper catalyst chemistry as well as the distinct pore size distribution to address operational needs and targets, and similarly, the utilisation of multi-catalyst systems must be understood in order to be optimised. We will review the progress in this area in both ebullated bed (EB) catalysis (‘millimetre scale’) and slurry hydrocracking catalysis (‘micron scale’). We will also review our findings for purported nano-scale catalysts or co-catalysts in residuum hydrocracking and their subsequent implications for sediment mitigation. Processing considerations have been reviewed in some detail earlier.⁵

Catalyst development efforts on a millimetre scale
To ensure that state-of-the-art catalyst technologies are commercially available for its ebullated bed and slurry hydrocracking processes, Chevron Lummus Global (CLG), a joint venture between McDermott and Chevron, established a cooperative agreement with Advanced Refining Technologies LLC (ART), a joint venture between Grace and Chevron.

ART is a leading supplier of residuum hydroprocessing catalysts worldwide. Resources, know-how and ideas are shared between the parties to assist in the development of new catalyst technologies to meet various commercial needs. State-of-the-art research units located in Richmond, California, are utilised for ART catalyst screening studies as well as CLG/ART joint catalyst development and process optimisation studies.

The extensive database generated from these units as well as CLG’s extensive commercial experience ensure the best catalysts are selected to achieve processing objectives. This testing has led to the development of several new and enhanced catalyst technology platforms in the last 10 years.