The appetite for residue upgrading through the application of ebullated-bed hydrocracking is growing around the world, as it is a well-established, commercially proven technology offering very high conversion levels and can help, therefore, to minimise the amount of bottoms sent to the bunker fuel pool. Moreover, the attractiveness of ebullated-bed hydrocracking as a technology solution for refiners has been enhanced by the recent trend for greater vacuum residue (VR) feed processing flexibility, which leads to improved refinery crude diversification economics.
Ebullated-bed hydrocracking is a long-established technology that can provide conversion of up to 80% (524°C+) of very refractive VR feeds with high contaminant metals and Conradson carbon residue (CCR) contents. Since ebullated-bed hydrocracking was commercialised in the 1960s, 16 units have been built. In the last four years, its popularity has increased significantly; another six units are planned to be online by 2020.
This shift is due to both proven design upgrades and the improved crude flexibility now achievable. The first-generation units operated on crude slates very much in line with design and focused mainly on hydrodesulphurisation. In the 1990s, unit operation shifted towards higher bottoms conversion and with it came the advent of the sediment (foulant) control era.
Higher levels of sediment are formed at higher conversion levels owing to greater destabilisation of the VR feed matrix known as SARA (saturates, aromatics, resins and asphaltenes). Unit cycle lengths were thus exclusively determined by the degree of sediment-induced fouling of key downstream hardware such as separators, heaters, fractionators and exchangers. This cycle-limiting phenomenon thus constrained the feed processing flexibility of the units when high bottoms conversion was the key operational objective.
Now, however, the state of the art is such that many units are successfully operating with a variable crude diet while maintaining a high on-stream factor. Furthermore, the current lower price crude market environment is providing the opportunity for more ebullated-bed unit operators to process VR feeds derived from a crude blend featuring the design crude as the major component and one or two other compatible crudes as minor components. In some cases, this has provided higher conversion performance through the net lower sediment formation tendency of the blended crude.
Sediment continues to affect unit reliability adversely, but, by drawing on the wealth of operational and process experience that has accumulated over the years and the latest-generation, sediment control catalyst technology, it is possible to mitigate an ebullated-bed hydrocracker’s propensity for fouling sufficiently and thus meet or exceed target cycle lengths when diversifying the crude mix.
This requires proper management of crude changes, effective unit monitoring and application of best practices, but it can be a key profit-generating opportunity. This is exemplified by an ebullated-bed hydrocracker operator that utilises its unit to drive the refinery’s crude diversification economic model. This operator recently successfully increased the unit cycle length from one to two years in an operation where the crude slate changes every two to three days.
The catalyst employed plays a key role in maximising unit on-stream factor, as it is the first line of defence in controlling sediment. Criterion Catalysts & Technologies (Criterion) has made several significant advances in this area using a customised catalyst development approach linked to crude type.
For instance, it has commercialised the TEX series of catalysts that exhibits better sediment control combined with higher conversion activity for processing VR feeds derived from Middle Eastern crudes. As fresh catalyst is added on a daily basis, it can also be viewed as a process variable whereby higher amounts of catalysts are added when processing higher-sediment-forming feeds or when operating at higher bottoms conversion levels.
It is advisable for operators considering adjusting their operating window to work closely in the early stages with the unit licensor and the catalyst provider. Changing the feed changes the SARA matrix, which can lead to cycle-limiting fouling. Both the licensors and the catalyst providers have commercial and pilot plant data that enable them to rank the fouling behaviour of feeds derived from different crudes accurately.
Criterion draws on its large database in order to advise on which crudes refiners could blend in and which they should avoid. These data are also key inputs for its SEDEX model, which predicts the change in sediment level as a function of crude type or when blending crudes.
Changing other parameters, such as conversion, throughput or temperature, can also lead to enhanced rates of fouling. But, again, by working with the licensor and the catalyst supplier, substantial manoeuvrability can often be unlocked to optimise unit performance in a balanced fashion.
Ebullated-bed hydrocrackers are highly customised according to a refiner’s specific objectives and there has been a great deal of innovation in the latest grassroots designs. Earlier-generation designs were more closely integrated with downstream fluidised catalytic cracking units and the key products required further upgrading to meet the required specifications. In contrast, the design of newer-generation units features an integrated hydrocracker–high-pressure hydrotreater in the same high-pressure loop. This enables direct production of high-value transportation fuels such as ultra-low-sulphur diesel from VR.
Shell currently operates both types of ebullated-bed unit licensed technologies: Axens’ H-OilRC and Chevron Lummus Global’s LC-FINING technology. Additionally, Criterion has more than 40 years’ experience in ebullated-bed catalysts and currently supplies customised catalysts to both H-OilRC and LC-FINING units.