GTL unit, Pearl Shell Qatar April 2014

This was the case at S-Oil’s Onsan refinery in Ulsan, South Korea. This leading refiner was dissatisfied with the cycle length of its hydrocracker, and installing Shell reactor internals and next-generation catalysts from Criterion Catalysts & Technologies (Criterion) proved to be a robust, high-value solution. S-Oil calculates the value to its operations as in excess of $20 million a year. 

S-Oil’s Onsan facility is the fifth largest refinery in the world and has an extremely complex configuration that includes a hydrocracker, a residue fluidised catalytic cracking unit, an aromatics complex and a state-of-the-art lubricants base oils plant. This enables it to produce high-value light oil products such as gasoline, diesel and kerosene, as well as petrochemicals and group 2 and 3 lubricants base oils.

With a corporate goal of becoming the most competitive oil refiner in Asia Pacific, S-Oil is continually striving to maximise its performance and profitability. So, when the cycle length of its hydrocracker was curtailed, management took steps to rectify the situation by working with Criterion and Shell Global Solutions.

The single-stage, two-reactor, 170-bar, 75,000-bbl/d hydrocracker was originally licensed from another vendor. The unit’s most valuable output is unconverted oil, which is routed to the wax hydroisomerisation unit and then the hydrofinishing unit to make Group 3 base oils. It also produces large amounts of middle distillates and smaller quantities of less-valuable liquefied petroleum gas and naphtha.

S-Oil had desired a cycle length of 36 months but the unit had only achieved 32 months in the previous cycle and other cycles had been even shorter. Pretreatment limitations meant that, at the end of run, the unconverted oil quality tended to decline. Consequently, S-Oil had to derate the base oil unit to maintain product quality and there was little opportunity for deeper processing of vacuum gas oil (VGO). 

Meng Loong Chua, Senior Technical Service Engineer, Criterion Catalysts & Technologies, was part of the team that investigated the unit. He explains that, on the pretreatment portion, the existing catalyst was suboptimal in stability and losing activity too quickly over the cycle. There was also suboptimal catalyst utilisation that resulted in the short cycle length. “We found that there was a high radial temperature spread over the catalyst beds,” he says.

“This indicates low uniformity of vapour–liquid distribution and suggests that the catalyst bed is not being used efficiently, which can shorten cycle life.” In the cracking reactor, there was extra activity left in the last cycle and this provided the opportunity to optimise the cracking catalyst towards more middle distillate production and better hydrogenation for the unconverted oil production.

To address these issues, the team replaced the internals in both the hydrocracker’s reactors. The new hardware that it fitted included Shell high dispersion (HD) trays, Shell ultra-flat quench (UFQ) interbed internals, Shell catalyst support grids and Shell bottom baskets. 

In addition, Criterion tailored a stacked-bed catalyst system that was designed to deliver balanced activity and selectivity, an optimal product slate and significantly better-quality unconverted oil. This was informed by a series of pilot plant tests that used S-Oil’s actual feeds (see below)

LPU area Shell Qatar April 2014

Value delivered

Since the change to Criterion catalysts and Shell reactor internals, conversion at the unit can be lowered to increase the yield of unconverted oil, which is more valuable, while meeting the required product quality.

The unit has demonstrated exceptional selectivity towards the products that are most important for S-Oil. As shown in table 1, compared with the previous cycle, the yield of the less valuable products, liquefied petroleum gas and naphtha, decreased by 0.5 and 6.8% respectively. Unconverted oil from the hydrocracker (which is fed to the base oil plant) increased, which increased the yield of base oil products by 7.1%. The distillate yield remained similar. In addition, hydrogen consumption fell and catalyst stability improved.

Key to these results is the unit’s improved temperature performance and the catalyst package. As shown in Figure 1, the weighted average bed temperature (WABT) is substantially lower in the post-revamp cycle than in the pre-revamp cycle.

Mr Tae Sung Yang, Process Engineering Manager, S-Oil, is responsible for the unit’s operations. He says, “The combined team of technologists from S-Oil, Shell Global Solutions and Criterion unlocked major benefits for the economics of our refinery. We have calculated it to be more than $20 million a year.”

“After installing the new catalyst system and reactor internals, the unit achieved the 36-month cycle length that we had targeted – the first time that it had ever done so,” he continues. “In addition, we have been able to increase base oil production and to treat heavier feeds. Based on the success of this project and on the excellent working relationship that we have established, we are currently evaluating launching another revamp project with Shell Global Solutions and Criterion.” 

Figure 1

S-Oil Graph

The revamped unit has displayed enhanced temperature performance in its first cycle compared with the last cycle before the revamp.

Table 1

Vol% Yields Post-Cycle Revamp

Liquefied petroleum gas

Base –0.5


Base –6.8
Kerosene Base –2.4
Diesel Base +1.9

Base oil – light grade #1

Base oil – light grade #2 Base +0.6
Base oil – medium grade Base +3.9
Base oil – heavy grade Base +2.6
Base oil product yields Base +7.1

After the new reactor internals and catalysts were installed, naphtha yield decreased by 6.8% while the yield of base oil products increased by 7.1%.

"After installing the new catalyst system and reactor internals, the unit achieved the 36-month cycle length that we had targeted – the first time that it had ever done so"

Stacked bed catalyst systemfor S-Oil onsan refinery
Figure 2: The pre-treatment reactor and Figure 3: The cracking reactor

Stacked-bed catalyst system for S-Oil’s Onsan refinery

Criterion developed a tailored, stacked-bed catalyst system for the unit. At the top of the pretreatment reactor (Figure 2), MaxTrap[Ni, V] is used to trap the nickel and vanadium in the VGO. Then a combination of CENTINEL Gold DN-3300 and Z-503, specially selected for their ability to remove nitrogen and to provide a viscosity index boost, are used for pretreatment. In the second reactor (Figure 3), Z-673 was selected for its excellent hydrogenation properties and Z-623 for its selectivity towards distillates.

Reactor internals for S-Oil onsan refinery
Figure 4: Shell HD trays incorporate proprietary nozzles that can help to maximise catalyst utilisation.

Reactor internals for S-Oil’s Onsan refinery

 S-Oil installed a set of state-of-the-art reactor internals from Shell Global Solutions that includes:

  • Shell HD trays, which help to optimise catalyst utilisation by achieving enhanced vapour–liquid and thermal distribution, and utilise nearly 100% of the catalyst inventory see figure 4;
  • Shell UFQ interbed internals, which provide uniform gas–liquid redistribution and quench mixing between catalyst beds;
  • Shell catalyst support grids, which feature support beams and a grid screen that utilises a wedge-wire construction that is less prone to foulant build-up and clogging; and
  • Shell bottom baskets, which are designed to be extremely flat in order to increase the loading of catalyst in the bottom dome. 

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