Nov-2013

Catalyst raises kero-treating performance

The activity and stability of a replacement catalyst in a kerosene treater enabled a refiner to process difficult-to-treat condensate in greater quantities.

WOO KYUNG KIM, SK Innovation
JESSY TRUCKO and ERIC BAKER, UOP, A Honeywell Company

Article Summary

The recent boom in gas field production, by utilising new technologies such as hydraulic fracturing, has resulted in abundant production of natural gas. As natural gas exits the wellhead, it decompresses, causing heavier hydrocarbons to condense out of the vapour phase as the temperature and pressure decrease. These condensed heavy hydrocarbons are recovered as liquids, commonly referred to as natural gas condensates. These condensates can cover a wide boiling point range, from pentanes, through the light and heavy naphtha ranges, and all 
the way through the jet fuel and diesel ranges.

Although natural gas condensates fractionate into boiling point ranges that refiners are familiar with, they exhibit very different concentrations and species of mercaptan molecules. The mercaptans present in these condensates are typically linear mercaptans. This, combined with high concentration levels, make these feeds challenging to treat. Two effective options exist for the refiner who would like to refine these lower-cost condensates into kerosene or jet fuel: hydrotreating and mercaptan oxidation via sweetening (conversion of mercaptan to disulphide) technology. Although hydrotreating removes these mercaptans, it requires increased hydrogen supply and capacity in an existing hydrotreater or a large capital cost investment to realise. Alternatively, the UOP Merox Fixed Bed Sweetening Unit with Merox No. 10 catalyst technology requires a low capital investment and minimum operating cost, while producing on-specification kerosene or jet fuel.

Treating unit comparison and challenges
SK Innovation in Incheon, South Korea, produces jet fuel from two kerosene-sweetening units receiving the same feed blend for the production of jet fuel. The smaller kerosene treater (No. 1 KMX) was designed by UOP and loaded with Merox No. 10 catalyst. This unit has a history of operating well, but was brought down when a new and larger kerosene treater (No. 2 KMX) was installed. No. 2 KMX was not designed by UOP and was initially loaded with a competitor’s catalyst. Although SK historically processed a blend of various streams to make jet fuel, it began to produce off-specification jet fuel in No. 2 KMX when the mercaptan content increased, due to blending RasGas condensate from the South Pars field (Qatar) in the feed. Unfortunately, the company could not make its desired jet fuel specification consistently with the treating technology and catalyst in No. 2 KMX. It reduced the RasGas condensate portion of its feed and brought No. 1 KMX back on line to handle the turned-down capacity of No. 2 KMX. It needed to process jet fuel through its hydrotreater No. 1 KGHT, so its product could blend with the jet fuel pool and be on specification by offsetting the high mercaptan product from No. 2 KMX. Figure 1 illustrates this setup.

Operating data showed that No. 1 KMX with Merox No. 10 catalyst ran at over twice the space velocity of No. 2 KMX, while performing better. SK reloaded a high-activity version of the non-UOP catalyst into No. 2 KMX in late 2009, in an attempt to achieve conversion equivalent to No. 1 KMX. UOP worked with SK Incheon to optimise the operation of No. 1 KMX, and showed that it performed to specification and outperformed the new No. 2 KMX catalyst load, even though the current load of Merox No. 10 was three years old in 2011.

Once again, SK reduced No. 2 KMX’s throughput to meet specification. Further addition of the RasGas condensate to the feed caused the catalyst performance in No. 2 KMX to deteriorate quickly and exceed the maximum internal mercaptan sulphur specification of 20 wppm sulphur and the sales maximum of 30 wppm sulphur (see Table 1). Ultimately, SK switched to an easier-to-treat feed with less feed mercaptan until a better alternative could be found (see Figure 2).

SK continued to compare the performance of No. 1 KMX with Merox No. 10 catalyst against the catalyst in No. 2 KMX with the same feed. Over a two-month period, SK observed higher levels of conversion across the Merox No. 10 catalyst despite the age of the catalyst in No. 1 KMX. The formulation of the Merox No. 10 catalyst increases activity over other treating catalysts, while maintaining activity in difficult-to-treat environments by providing increased resistance to fouling by organic acids such as sodium phenolate and sodium naphthenate. Figure 3 shows the increased stability of Merox No. 10 standard fixed-bed catalyst. 

Beneficial replacement
After further evaluation of the data, UOP and SK agreed that, despite the differences in reactor vessel design, Merox No. 10 catalyst would be a beneficial replacement for the catalyst in No. 2 KMX. UOP proposed two options:
• The first option replaced the existing catalyst with UOP Merox No. 10 and kept the current unit design flow rate of 20 000 BPSD. This approach minimised the unit’s down time and allowed SK to test the performance of Merox No. 10 catalyst in a non-UOP reactor
• The revamp option replaced the existing reactor internals of No. 2 KMX and increased active catalyst volume by about 50%, increasing feed throughput to 30 000 BPSD.

SK selected option 1 for its low capital cost and quick turnaround schedule. It reloaded with Merox No. 10 catalyst without revamping the unit.

The company loaded No. 2 KMX with Merox No. 10 catalyst and evaluated the performance of the new catalyst. During testing, it processed a higher portion of RasGas condensate to No. 1 KMX and No. 2 KMX units, increasing the mercaptan load by over 250%. In addition, it realised product mercaptan concentrations of less than 8 wppm sulphur with decreased air injection rates (see Table 2).

Merox No. 10 catalyst has been in service in the No. 2 KMX reactor for almost two years now with excellent performance. The catalyst change has allowed No. 2 KMX to process 72% more mercaptan sulphur, while reducing off-spec product by more than 50% (see Table 3).