Two refinery workers at desk

Excellence in refining is dynamic. When external conditions change, as they inevitably will, refiners need to adapt to ensure that their mode of operation continues to be relevant. And when they do, a revamp project can often be key to ensuring a cost-effective response, as an initiative at a Shell refinery demonstrates. 

The hydrocracker was originally designed by Shell in the 1980s as a full-conversion unit with a two-stage configuration geared towards middle distillate production. In response to growing demand for middle distillates, the unit’s capacity was later increased to about 167% of the design capacity, though it still operated as a two-stage unit. 

However, increasing demand for petrochemicals and an enhanced petrochemical margin provided the triggers that led to Shell changing the hydrocracker’s mode of operation. 

To increase its petrochemicals capacity, the company intended to build an ethylene cracker in the adjacent petrochemicals plant, so it commissioned Shell Global Solutions to help plan the investment and to understand and evaluate its technical and operational options. 

It was during this review that Shell Global Solutions’ strategic planners identified that the economics of the refinery and the petrochemicals site could be greatly enhanced by adapting the hydrocracker to produce large amounts of unconverted oil (hydrowax), which could be used as ethylene cracker feedstock.

So, working with the refinery’s management, Shell Global Solutions devised a plan to revamp the hydrocracker into two parallel, single-stage reactors (see figures 1 and 2) operating at reduced conversion, but with an increased fresh feed capacity of some 300% of the original design capacity and a catalyst cycle length of two years. The revamped hydrocracker has retained the common recycle gas compressor, the high- and low-pressureseparators, and the common distillation section.

 As with any revamp, it was critical to explore the impact on the existing major equipment. Consequently, Shell Global Solutions undertook a technical feasibility study to understand the impact of these changes on the unit’s hydraulics and heat and material balances. The study concluded that the project was viable, although there were concerns regarding hydrogen partial pressure, recycle gas purity, recycle gas compressor performance and sour water corrosiveness.

Further investigations confirmed the requirement for a recycle gas hydrogen sulphide scrubber and increased wash-water make-up flow. The engineering study concluded that several generally low-cost modifications were necessary. 

For example, in the hydrocracking reactor the project team:

  • fitted an additional feed filter to accommodate the higher feed rate;
  • installed Shell reactor internals to increase the catalyst volume and utilisation;
  • increased the fresh gas compressor capacity to cope with the increase in hydrogen consumption;
  • modified the water booster pumps and increased the wash-water make-up to minimise the ammonium bisulphide concentration in the sour water phase of the cooled reactor effluent;
  • adapted the recycle gas compressor and turbine to increase the recycle gas flow, which was required to meet minimum gas–oil ratios; and
  • installed a new recycle gas amine scrubber to improve recycle gas hydrogen purity and ease hydraulic constraints. 

This hydrocracker revamp project provides an excellent example of how making the most of existing assets can support a change in a refiner’s business drivers

Overcoming challenges

Robert Karlin, Hydrocracking Team Lead, Shell Global Solutions, has extensive experience of hydroprocessing design. He and his team have executed several hydrocracker revamps. He explains that the second-stage feed system and high-pressure reactor circuit can present significant challenges when converting to a new service. “These had originally been designed to process ‘clean’ fractionator bottoms that contained almost no sulphur or nitrogen contaminants,” he says. “The second-stage reactor had originally been designed with a cracking catalyst system for conversion, so it was going to see a major change in feedstock: fresh vacuum gas oil containing sulphur, nitrogen, Conradson carbon and metals.” 

“It was a major challenge to design a functional demetallisation, pretreatment and cracking catalyst system within the existing second-stage reactor,” Karlin continues. “And, because of the significantly higher level of sulphur and nitrogen entering the unit, we carefully evaluated the entire wash-water system to ensure that the cold section of the unit (from reactor effluent air cooler through to the fractionation system) is adequately protected against corrosion.” 

In support of the process revamp study, a pilot-plant testing programme was carried out at Shell Technology Centre Amsterdam to compare the performance of the original design basis catalyst system against three alternatives that Criterion Catalyst & Technologies had proposed. From these results, the refinery’s management was able to select the option that provided the best fit for its objectives. 

Since the unit started up in the new single-stage mode in 2010, it has delivered both the quantity and quality of hydrowax required for the ethylene cracker. 

“This hydrocracker revamp project provides an excellent example of how making the most of existing assets can support a change in a refiner’s business drivers,” says John Baric, Licensing Technology Manager, Shell Global Solutions. “In this case, we adapted the mode of operation of a hydrocracker that had been in operation for nearly 30 years in order to support the economics of a major new project. Consequently, this cost-effective revamp has had a major impact on the economics of the wider enterprise.”

Figure 1

Revamp Graphic

Pre-revamp two-stage configuration

Figure 1: Before the revamp, the hydrocracker had a two-stage configuration. It was operating at 167% of the original design capacity and focused on middle distillate production.

Figure 2

Revamp Graphic

Post-revamp single-stage configuration

Figure 2: The revamped hydrocracker operates with the two original reactors in parallel. It has a feed rate of 300% of the design capacity and about 70% of its output is hydrowax, which is fed to the ethylene cracker.

More in Industry Focus