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Configuring a value-adding hydrocracker
There is a multitude of hydrocracking configuration options, and the design should always be optimised for the operator’s objectives.
With global diesel demand continuing to rise in most regions while gasoline demand stagnates or even declines, the economic imperative for many refineries to maximise their diesel yield has never been greater.
At the same time, however, many operators are capital constrained, and all have different strategic objectives, so various technology solutions are being deployed. “In my experience, every customer’s situation is unique,” says John Baric, Licensing Technology Manager, Shell Global Solutions International BV. “There can never be a silver bullet solution; a solution must always be customised, and that requires experience and an innovative mindset.”
Baric explains that Shell Global Solutions has developed two fundamental hydrocracking processes: single-stage and two-stage. But he is quick to add that these are almost always customised to the application. Each design is always tailored to the type of feedstock, the capacity of the unit and the specific processing objectives to help optimise both capital and operating costs.
Maximising yield with full conversion
The two-stage hydrocracker process configuration is best suited for large units and for processing difficult, high-nitrogen feedstocks, says Baric. "Almost all of the unconverted bottoms are recycled and conversion levels of 95-99% can be achieved," he explains. "Doing things in two stages involves more capital expenditure and consumes more energy, but it is all paid back by the extra distillate yield. The two-stage hydrocracker is typically installed as a standalone unit and does not involve integration with any other units."
Cost-effective partial conversion
Single-stage designs are a lower cost option and tend to be better suited for existing refineries where opportunities are likely to exist for profitable integration with other units. Here, the bottoms are fed to the FCC unit, to the base oil plant for production of Group 2 or Group 3 base oils, or to an ethylene cracker.
"Depending on the customer's requirements, several process configurations are available," says Baric. "The single-stage hydrocracker can be designed for flexible operation; high yield of diesel, kerosene or naphtha in recycle mode; or partial conversion in once-through mode when integrated with other downstream units. The single-stage hydrocracker can easily process a variety of feedstocks, including VGO, HCGO and DAO, producing a range of high-quality middle distillates together with unconverted bottoms."
For each application, Shell Global Solutions matches the optimum configuration with the given feedstock and the required processing objectives. Additional product slate flexibility can be achieved through the choice of catalyst system or by adjusting the process conditions.
Revamps can also be extremely cost-effective solutions: advanced reactor internals technology and leading-edge catalysts can facilitate higher reactor volume utilisation, longer catalyst run length, uniform production quality and less hot spot formation.
The reactor internals fulfil several functions, including increasing the catalyst utilisation (see Reactor internals technology, below). They also protect the catalysts from particulates and foulants, and this, according to Baric, is a key differentiator of Shell Global Solutions hardware.
"In today’s refining environment, the contribution of the hydrocracker to the refinery margin and its profitability is significant. Refiners expect – and rightly so – that these units are going to operate at high levels of utilisation and availability (or on-stream factor) for the entire planned catalyst cycle. This has become more challenging with the trend towards processing VGO originating from heavier, higher sulphur crudes and feedstocks from cokers and thermal conversion units."
In any multi-bed hydrocracking reactor, particulates can accumulate and affect the top-bed catalyst performance, even with sophisticated automatic backwash feed filters. So, Shell Global Solutions installs special filters in the top domes of all VGO hydrocrackers.
Says Baric, “This has been shown to be effective at removing the particulates that can cause pressure drop and maldistribution in the catalyst bed, which can often lead to costly unplanned shutdown of the hydrocracker. A liquid–vapour distribution tray beneath the filters enables the vapour and the liquid to have full radial dispersion across the catalyst bed, which can result in nearly 100% utilisation of the catalyst."
Proof Point: Pulau Bukom Revamp
A revamp project at Shell Singapore’s Pulau Bukom refinery is a prime example of the value of tailored process configurations and catalyst systems.
When Shell Singapore was evaluating the economic potential of building a petrochemical site alongside Pulau Bukom refinery and integrating the two complexes, deploying off-the-shelf solutions would have missed a valuable opportunity to capture additional margin.
The existing refinery featured a two-stage hydrocracker operating at high conversion. The unit focused on high conversion of VGO feeds into large quantities of distillates, such as kerosene and diesel.
However, the economics of the refinery and the petrochemical hub would be greatly enhanced, the Shell Global Solutions strategic planners calculated, if the hydrocracker were to be used to produce large amounts of unconverted oil (hydrowax) instead of the more commonly used naphtha as the feedstock for the 800,000-t/y ethylene cracker.
And so, working with Bukom management, Shell Global Solutions revamped the hydrocracker into two parallel single-stage reactors, thereby almost doubling the feed rate. This was designed to deliver 70% of its output as hydrowax, with the remaining 30% being converted into high-quality distillate products.
Srinivas Iyer, Senior Refining Technologist, Shell India Markets Pvt. Ltd, comments, “The technical modifications that were required in the hydrocracker unit were not particularly complex. What was key here was devising the initial concept and developing a tailored catalyst solution to produce a deeply hydrogenated hydrowax: it demonstrates Shell’s flexibility.”
Customers that have licensed Shell Global Solutions’ hydrocracking technology include:
- Valero, North America’s largest refiner, which is constructing a 50,000-bbl/d, unified-design, two-stage hydrocracker at each of its refineries in St Charles, Louisiana, and Port Arthur, Texas, USA;
- CNOOC, which chose the dual-service hydrocracking process for its new refinery in Guangzhou Province, China. At 80,000 bbl/d, this is the highest capacity hydrocracker operating in the country. It has the flexibility to operate in integration mode with the ethylene cracker at Nanhai Petrochemicals Complex and the refinery’s FCC unit.
- A major North American refiner, which brought a 70,000-bbl/d unit online in early 2010. It leverages a catalyst package that enables it to operate in a highly flexible regime and swing between gasoline and diesel modes, while operating in integration mode with the FCC unit.
- Grupa LOTOS, Poland’s second largest refiner, which has started up a new-generation 45,000-bbl/d DAO hydrocracker at its Gdańsk refinery; and
- North Atlantic Refining Company, which adopted Shell Global Solutions reactor internals and Criterion catalysts when it originally revamped the single-stage, 37,000-bbl/d hydrocracker at its refinery in Newfoundland, Canada.
Reactor Internals Technology
Figure 2: HD2 tray technology applies customised nozzles that use the gas flow momentum to disperse the liquid as a mist. This differentiates Shell Global Solutions technology from conventional downcomers or bubble caps because the nozzles fully and uniformly wet the entire catalyst surface and make efficient use of the top part of the catalyst bed.
The reactor internals that are used in a hydrocracker can play an important role in determining its capital cost, as they can improve the volume available for catalyst and therefore can help to reduce the reactor size. They also have a major influence on its performance as they can drive its on-stream factor, utilisation, cycle length, product yields and even product quality.
A key focus of Shell Global Solutions research and development has been on ensuring that its internals technology distributes gas and liquid uniformly, minimises thermal instabilities and maximises reactor catalyst inventory and catalyst utilisation.
The result is a reactor internals system that includes:
- HD2 (high-dispersion) trays for highly uniform vapour–liquid distribution and excellent thermal distribution (Figure 2);
- filter trays to prevent foulants from entering the catalyst beds;
- UFQ (ultra flat quench) interbed internals for uniform process and quench mixing at the interbeds;
- catalyst support grids; and
- compact bottom baskets to help maximise the catalyst volume in the bottom domes.
- Shell ConSep* trays can be applied in the distillation section of a hydrocracker. These can increase capacity by 30–100% compared with most other trays.
- The Schoepentoeter can be applied in the separators of the reactor section and in the hydrocracker main fractionator, where separation efficiency is vital. It offers excellent distribution and phase separation, even at high capacity, and operates with negligible pressure drop.
- Shell provides a special option hydrocracker product work-up section. Featuring a single main fractionator, this innovative configuration provides a 35–40% reduction in fractionation furnace energy consumption when compared with a conventional stripper–fractionator configuration. Shell’s fractionator-led configuration has been proven at Shell-designed hydrocrackers. It has operated for over 25 years at several facilities, including Shell Scotford refinery in Canada (since 1984) and the Saudi Aramco Shell Refinery Company (SASREF) refinery at Al Jubail (since 1985) Shell’s first hydrocracker design with a single main fractionator has been operating for over 15 years.
*Shell ConSep is a Shell trademark.