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When South Korea’s S-Oil Corporation wanted to boost the profitability of operations at its Ulsan refinery, its plans included overhauling the hydrocracking unit to increase the throughput, the operating cycle length and the yield of more valuable products. The main objective of this hydrocracking unit is maximising middle distillate and unconverted oil yields. The unconverted oil is then sent for lubricant dewaxing and hydrofinishing to make Group 3 base oils. Our results were the most favourable during the 2009 tests, and S-Oil selected our system for the 2010 catalyst changeout.

The key challenge for this project was to overcome the operational difficulties encountered in the previous cycle, as Mr H. I. Lee, General Manager, Cracking Process Engineering Department, S-Oil, explains. “We wanted to address the issues limiting the cycle length to 32 months rather than 36 months, and to avoid the deterioration in product quality at the end of run due to pretreatment limitations. This situation had forced us to derate the base oil unit to maintain product quality and left us with little opportunity to process deeper vacuum gas oil (VGO).” 

Worldwide, operators are looking for opportunities to upgrade feeds such as VGO from conventional and heavy crudes, deasphalted oil, coker distillates and residue fuel oil through hydrocracking. Hydrocracking also makes it possible to adjust the balance of the product slate between middle distillates and gasoline. S-Oil was considering a tailored catalyst system to help increase pretreatment activity and state-of-the-art reactor internals to go with it. 

S-Oil was aware of steps that might lead to a solution. Reducing the liquid hourly space velocity would help to improve product quality and maintain catalyst activity, but it would be necessary to maintain the pressure differential that would result from the increased reactor volume. Modifications for achieving a better balance between pretreatment and cracking activity, and a more active pretreatment for meeting unconverted oil quality constraints were possible. The company could also choose a more selective cracking catalyst to increase the yields.   

The challenge for Criterion Catalysts & Technologies (Criterion) and Shell Global Solutions was finding proven, cost-effective technologies to apply at the Ulsan refinery to address all these issues.

Joint reviews of pilot plant tests and a series of technical discussions with Shell Global Solutions and Criterion led S-Oil to decide that Criterion/Zeolyst International offered the best catalyst system for the Ulsan refinery’s hydrocracker.

S-Oil had already established a relationship with Criterion, as Li Hui Ng, Technical Service Engineer, Criterion, explains, “In 2000, S-Oil tested our amorphous silica–alumina Z-503 catalyst in its pilot plant facility. They were happy with the performance, so this catalyst was used with a competitor’s pretreatment and cracking products. S-Oil purchased this Criterion catalyst for three cycles.”      

Through its close communication and understanding of the process at Ulsan, Criterion was able to recommend an improved catalyst system for pilot plant testing: DN-3300/Z-503 for pretreatment and Z-673/Z-623 for hydrocracking. Ng says, “Our results were the most favourable during the 2009 tests, and S-Oil selected our system for the 2010 catalyst changeout.”

S-Oil had used a competitor’s catalyst since the unit’s commissioning in the late 1990s and this was to be its first experience of another vendor for the entire reactor fill. However, the catalyst Criterion proposed requires a higher operating temperature, so S-Oil questioned whether the cycle length target of 36 months could be met.   

Making this kind of change to a refinery hydrocracker is a major decision for an operator. Acknowledging this, Criterion offered S-Oil an opportunity to assess the catalyst system’s performance in a similar plant. Ng states, “We helped S-Oil reach a decision by providing reference data from customers with similar catalyst systems and we made reference visits to the Shell Godorf refinery in Germany and our manufacturing plant in Ghent, Belgium.”

S-Oil already had Shell reactor internals in two other units, the lubricants hydrotreater and the residue treater, where their performance was meeting expectations, so the decision to switch internals was based on proven technology and established performance. The new ultra-flat quench interbed internals and the high-dispersion trays supplied and installed by Shell Global Solutions help to homogenise the vapours and liquids in the reactor and eliminate thermal maldistribution in the lower catalyst beds. 

The planning and implementation phases of the project required close collaboration between S-Oil, Criterion and Shell Global Solutions. The catalyst changeout and the replacement of the reactor internals were undertaken in April 2010. The catalyst and internals solution has delivered a range of process benefits with a value S-Oil estimates at more than $20 million a year.

Introducing the Shell reactor internals meant that the catalyst volume of the hydrocracker unit increased by 10%. To mitigate any potential for increased pressure drop through this change, Criterion recommended a trilobe extreme, TX, version of its cracking catalyst. The net effects of this shape option are lower catalyst density and pressure drop, and a shorter diffusion path length. These enable S-Oil to maintain the equivalent pressure drop while increasing the catalyst volume by 10%.

The tailored catalyst system was designed to deliver balanced activity and selectivity, an optimal product slate and significantly better unconverted oil quality. 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 yield of naphtha is correspondingly lower and the middle distillate yield is similar. At the same time, hydrogen consumption has fallen and catalyst stability has improved. 

S-Oil has been able to increase base oil production and treat heavier feed compared with previous cycles. All the indications are that, for the first time in its operational history, this hydrocracker should reach a cycle length of 36 months early in 2013. S-Oil and Criterion are working closely to test and evaluate these catalyst systems and to monitor their performance.

For more information, contact Li Hui Ng.

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Criterion is the world’s largest supplier of hydroprocessing catalysts, which includes catalysts for hydrotreating, hydrocracking, hydrogenation and isomerization.
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