Mar-2018

Advances in residue hydrocracking

Recent developments in ebullated bed hydrocracking technology target high residue conversion and high quality products.

UJJAL MUKHERJEE and DAN GILLIS
Chevron Lummus Global

Article Summary

With the recent downturn in crude oil prices, the incentive to upgrade residues has also shifted as upgrading margins have been compressed. To maximise upgrading margins, technology solutions that maximise high quality, high hydrogen content transportation fuels (gasoline, kerosene, and diesel) and minimise unconverted residues are required.

Several technologies are available for residue upgrading, broadly characterised as carbon rejection technologies or hydrogen addition technologies. The principal residue conversion based on carbon rejection technology is delayed coking. Delayed coking is simple, robust, and can handle very high levels of feed contaminants. Roughly 25-30% of the residue is rejected as petroleum coke, or petcoke. Amongst hydrogen addition technologies, the principal ones are residue desulphurisation technologies (ARDS, VRDS, UFR, and OCR) and residue hydrocracking technologies such as LC-Fining. These technologies are well established and have proven to be efficient and reliable processes. Each technology has its own merits.

For conventional residues such as light sour residues, they have similar yields (see Figure 1). Residue hydrocracking’s main advantage is that its unconverted residue product is typically much higher in value than coke.

Historically, residue hydrocracking has had a slight capital cost premium and required more hydrogen because all products, including the bottoms, are hydrogenated. In recent years, the cost for high pressure equipment has gone down very significantly as has the cost of natural gas required to produce hydrogen. This reduces the capital cost gap between high pressure residue hydrocracking and low pressure processes such as delayed coking. It should be mentioned that the cost of a delayed coking project geared towards maximum diesel production has to factor in the cost of upgrading the coker gasoils and the hydrogen consumption in the hydrocracker. The global market for high sulphur residue is expected to decline in the long term with environmental regulations becoming increasingly stringent, driven by International Maritime Organization restrictions.

All of these factors support high conversion residue hydrocracking solutions, which will have the best opportunity to meet future residue upgrading projects’ requirements. What is also required to obtain the necessary financing for these projects are technologies that are based on commercially proven and reliable technologies. This requirement cannot be emphasised enough and is critical for any large project to proceed.

Vacuum residue by its very nature is difficult to process due to its high viscosity and high levels of contaminants, such as sulphur, metals, asphaltenes, and carbon residue. Any conversion leads to a destabilising effect as surrounding resins, aromatics and saturates that keep the asphaltenic cores in solution disappear at a faster rate than the asphaltenes. The propensity for rapid catalyst deactivation in the reactor, as well as fouling caused by the precipitation of heavy asphaltenic material from the surrounding aromatics, especially at high conversions, has to be very carefully managed in order to achieve on-stream factors in line with the rest of the refinery. Unfortunately, while yields, properties, and chemical hydrogen consumption can all be quite accurately measured in state-of-the-art pilot facilities such as the ones Chevron Lummus Global (CLG) has in Richmond, California, data on long-term catalyst performance, reactor stability, and fouling in the reactor effluent and fractionation circuits are only available from commercial units.

CLG is the leading licensor of residue upgrading technologies, with the most barrels in commercial operation for residue desulphurisation and residue hydrocracking, and is the only licensor to have commissioned multiple large residue hydrocracking units in the last 13 years. The data from the last five units, all commissioned in the last 10 years, have added greatly to the knowledge database required for enhancing the reliability of operating residue hydrocracking units at high severity utilising LC-Fining ebullated bed technology (see Table 1).

This article focuses on CLG’s high conversion residue hydrocracking based solutions, centred on its LC-Fining technology, a proven high conversion process. These solutions include:
• Processing of residue hydrocracked vacuum gasoil (VGO) requires special attention to the stream’s characteristics and the environment in which the VGO stream is hydrocracked. Appropriate flow schemes are explained.
• Integration with downstream delayed cokers can effectively achieve 90 wt% conversion. This has been commercially demonstrated with two of the LC-Fining licensees. It is an excellent high conversion solution for refiners with existing delayed coking as the investment cost, and hydrogen requirements are minimised.
• Conversion within the LC-Fining process can be significantly increased with the selective rejection of partially converted residue components. The LC-Max process is an integration of solvent deasphalting with LC-Fining which can increase conversion to 90 wt%.
• Replacement of the traditional ebullated bed catalyst with a slurry catalyst can achieve conversions over 95 wt%. The LC-Slurry process accomplishes this with a next generation active slurry catalyst, resulting in all products being used to make high quality finished products or suitable for downstream processing. LC-Slurry eliminates all feed quality restrictions and even SDA pitch can be hydrocracked utilising this process.

The process
The LC-Fining residue hydrocracking process has inherent flexibility to meet variations in feed quality/throughput, product quality, and reaction operating severities (temperature, space velocity, conversion, and so on, see Table 2).

This flexibility is a direct result of the ebullated catalyst bed reactor system. In an ebullated bed unit, if the metals or sulphur content of the feed increases, the product quality is maintained by increasing catalyst consumption. Conversely, catalyst consumption is reduced if the feed quality improves.

The reactor
Core to the performance of the LC-Fining process is the reactor. Fresh feed and hydrogen enter the reactor at the bottom and pass up through a catalyst bed where hydrodesulphurisation and other cracking and hydrogenation reactions occur. A portion of the product at the top of the reactor is recycled by means of an internally mounted recycle pump. This provides the flow necessary to keep the catalyst bed in a state of motion somewhat expanded over its settled level (ebullated). This ebullation is the key to the process. The reactor environment caused by the ebullation is similar to that of a continuous stirred tank reactor and consequently the reactor operates under near isothermal conditions. The ebullation also prevents any pressure drop as in the case of a fixed bed. Figure 2 is a schematic of an ebullated bed LC-Fining reactor.

The catalyst level is monitored and controlled by radioactive density detectors, where the source is contained inside the reactor and the detectors are mounted outside. Temperature is monitored by internal couples and skin couples. The performance of the ebullated bed is continuously monitored and controlled with the density detectors and temperature measurements that verify proper distribution of gas and liquid throughout the catalyst bed. Temperature deviations outside the normal expected ranges that might suggest maldistribution will cause the distributed control system (DCS) and safety instrumented system (SIS) to activate alarms and/or initiate automatic cutback actions, including reducing heater firing, increasing quench oil introduction, reducing hydrogen purity, and reducing system pressure.