Mar-2018

Producing ultra low sulphur gasoline with octane retention

A drop-in catalyst enabled refiners to meet ultra low sulphur gasoline requirements with more severe feedstocks while retaining octane barrels

JIGNESH FIFADARA
Albemarle

Article Summary

Over the past decade, the refining industry has taken major steps toward accommodating the increasingly stricter regulatory requirements that aim for a low sulphur world. Europe has generally been the front runner on regulations for low sulphur, ‘clean’ transportation fuels, but the rest of the world is also gearing up to accommodate changes in maximum sulphur limits in gasoline over the next few years. While reducing emissions from existing vehicles, the lower gasoline sulphur limits will also enable more stringent vehicle emission standards and make emission control systems more effective.

There are several operational and performance unknowns associated with reducing the average sulphur content of gasoline to ≤10 ppmw. To address this challenge and meet regulatory requirements, refiners are carrying out extensive technical and economic studies of the various catalyst drop-in solutions available in the market to assess which would provide maximum profitability and performance for their refineries.

The processing of FCC naphtha takes on an even more important role in meeting the new regulatory changes. Table 1 shows typical gasoline pool blending components before treating.

Although the gasoline pool comes from various sources in the refinery, full range FCC naphtha typically accounts for 30-50% of the overall gasoline blend and is the biggest contributor to sulphur. Operating selective hydrodesulphurisation (HDS) gasoline units to process FCC naphtha effectively generally entails lower investment and operating costs compared with the alternatives available for meeting the regulatory changes.

Although process configurations and operating constraints vary between refineries, the choice of catalyst has a tremendous impact on the level of HDS activity and selectivity for octane retention. Refiners judge that they can achieve maximum refinery economic benefits by increasing operating severity and the yields of higher octane products. Robust catalyst stability, improved performance with ageing and the ability to cope with significant variations in feed components are all important catalyst parameters for reliably producing ≤10 ppmw ultra low sulphur gasoline while maximising octane barrels and profitability.

Owing to strong gasoline margins, some refiners are focusing on developing a strategy to maximise gasoline production while preparing for the upcoming regulatory changes.

Octane demand and refiner profitability

Demand for octane remains high

Although changes in gasoline regulations generally focus on product sulphur, they also concern octane quality. Gasoline with a higher octane number enables greater engine efficiency and performance. The US gasoline octane market has varied over the years and has typically been affected by changes in gasoline grade demand, crude oil quality, crude and gasoline prices, gasoline specifications and octane enhancers.

To better track pricing and historical demand for octane, Albemarle has developed a correlation for octane barrel value that combines market data from the US Energy Information Administration with commercial customer data over the last two decades. Figure 1 summa-rises the trend in the value of the road octane number, which is the average of the research octane number (RON) and motor octane number (MON).

The data show that octane values rose between 2004 and 2006 when the market was also seeing an expansion in refinery gasoline production. Crude and product prices were rising and methyl tertiary butyl ether was being phased out of the gasoline pool owing to concerns over its solubility in water and consequent contamination of water resources. From 2007 to 2011, there was a slowdown in octane demand because of the increased use of ethanol as an octane booster and lower refinery octane tar-gets. The timeframe also coincides with severe economic recessions in the global market.

However, the octane barrel value, which fluctuated periodically from 2012 to 2015 and increased to historic highs, has now plateaued. Some non-traditional drivers leading to the increased octane value include high compression engines, premium share increase, ethanol growth decline and processing more light tight oils.

The high cost of octane is expected to continue, as regulations for lower sulphur levels are already in effect. Added costs will come in the form of new refining equipment or higher severity hydrotreating of gasoline. Additionally, the lower sulphur requirements will cause refiners to blend less light naphtha into the gasoline pool owing to its low sulphur and octane values. As refiners look for new ways to improve octane quality while meeting sulphur regulations, catalyst drop-in solutions for existing selective HDS gasoline units are effective ways for them to capitalise on the market changes without large capital investments.

Profitability: octane is still the name of the game

With demand for octane remaining high, refiners have an opportunity to capitalise on profit margins by maximising octane barrels while producing ultra low sulphur gasoline. Figure 2 captures the impact of better octane retention from an existing selective HDS gasoline unit. Using data from the gasoline octane market and Albemarle’s correlation for octane barrel, the plot uses an estimated value of $1 per octane barrel for the valuation. However, the cost of octane will vary for each refiner based on the octane target, among other factors relating to refinery operations.

For example, if a refiner has a single stage unit that is processing 30 000 b/d from a selective HDS gasoline unit, an improvement of 0.5 in RON for the product could lead to approximately $5 million/y in additional profits. However, a two stage unit processing the same feed rate could expect to achieve a RON improvement greater than 1, which would lead to correspondingly larger profits for the refinery (upwards of $10 million/y).

Catalytic solutions

Conventional hydrotreating to reduce sulphur in gasoline has the unfortunate side effect of saturating olefins and consequently reducing octane. In preparation for more stringent regulation and performance optimisation, some refiners have carried out an extensive study of the various catalyst drop-in solutions available in the market for their selective HDS gasoline units. Outside the general catalyst performance characteristics a refiner typically looks for when selecting a catalyst system, the refiners wanted to challenge the status quo and strive for a solution that offered:

• Improved selectivity to save additional octane
• Increased HDS activity to handle more severe feedstocks with higher sulphur levels
• Improved carbon monoxide tolerance to prevent octane loss.

To help the refiners reach their vision of optimum performance and profitability, Albemarle proposed its next generation catalyst for selective gasoline hydrotreating process: RT-235. This industry leading catalyst was a joint development by ExxonMobil and Albemarle and the result of screening about 500 catalyst formulations by testing to ensure an optimised support structure and metals distribution.¹

RT-235 catalyst has excellent selectivity to desulphurisation reactions while significantly boosting overall desulphurisation activity. The extra desulphurisation activity can provide significant economic benefits, especially for units requiring a relatively high level of desulphurisation. The performance improvement enabled the refiners to enhance the operation of their selective HDS gasoline units by capitalising on greater octane retention and processing more severe feed-stock without additional octane loss. Both modes of operation led to significant improvements in profitability while enabling the units to produce ultra low sulphur gasoline.

The performance of RT-235 catalyst has been a major mile-stone in the selective FCC gasoline hydrotreating process. This catalyst has a proven track record. It has been applied in more than 20 commercial applications and resulted in exceptional overall profitability for its users. These operations confirm the benefit of RT-235 catalyst with octane retention from 0.1 to more than 1 RON, depending on unit configuration and feed severity. Catalyst stability and resistance to upsets have been found to be excellent and enabled users to run reliable operations with long cycle lengths.

Commercial case studies

Processing more severe feedstock while retaining octane

Refinery A has a single stage gasoline reactor and a FCC pre-treatment unit upstream of the gasoline unit. This unit meant that the average feed sulphur to the gasoline reactor was on the low side, 350-400 ppmw. The feed to the unit was a mixture of light catalytic cracker naphtha (LCN), heavy catalytic cracker naphtha (HCN), light, straight run naphtha, delayed coker gasoline and platformer pentanes. The previous cycle was not loaded with Albemarle catalyst and the current cycle was short-loaded with RT-235 catalyst because of its exceptional activity.

The main objective for the refiner was to improve profit-ability across the unit by processing more severe feed while retaining octane. Owing to the new regulations being implemented and the need to pro-duce gasoline with less than 10 ppmw sulphur, the amount of HDS across the gasoline unit had to increase.

 

Figure 3 shows the feed rate for the current cycle. For most of the cycle, the unit has consistently been processing at about 20-25% above the design feed rate. The refiner was able to operate at this rate by specifically processing more HCN feedstock. On the previous cycle, the unit was processing 5% HCN of the total feed. Now, however, the unit is processing 20-25% HCN.

Figure 3 clearly shows that this refiner was able to push the boundaries of the current cycle by increasing the feed rate and by processing more severe feedstock in the form of HCN. One way the refiner was able to increase the HCN feed to the unit so drastically was by taking a deeper cut of the HCN feedstock (see Figure 4).

Figure 4 shows the difference between the HCN T90 boiling point on the previous cycle and the current cycle. The current cycle is processing a 40-50°F deeper cut into the HCN T90 boiling point to increase the HCN feed. One of the key objectives of this refiner was to maximise gasoline production because of the favourable margins of gasoline over die-sel. However, a main drawback of taking a deeper cut into the HCN feed is that it introduces more complex sulphur species that need treating, which requires increased operating severity from the unit to reduce the product sulphur to less than 10 ppmw.

A typical HCN feed consists of mercaptans, thiophenes and benzothiophenes. To better understand the breakdown of the HCN feed components, Albemarle completed sulphur speciation testing of the HCN feed with the lower cut point from the previous cycle and compared it with the HCN feed with the deeper cut point on the current cycle (see Table 2).

  Table 2 shows that the HCN feed from the previous cycle consisted mostly of thiophenes and about 40% benzothiophenes. However, the sulphur distribution of the HCN feed from the current cycle shows a shift to mostly benzothiophenes. This shift causes the unit and catalyst system to work harder to treat the more complex sulphur species to drive product sulphur below 10 ppmw.

As the main objective of this refiner was to improve profit-ability across the unit by processing more severe feed while retaining octane, Figure 5 shows the average octane loss across this unit as the operating severity increased. This refiner was able to limit the octane loss across the unit to between 2 and 3 numbers and to process more severe feedstock while producing 5-10 ppmw sulphur product. Utilising RT-235 catalyst enabled the refiner to achieve additional profits by pushing the boundaries of this gasoline unit while retaining octane and meeting the regulatory changes.

Pushing the boundaries without sacrificing cycle length

Refinery B has a two stage gasoline reactor and the feed sulphur is about seven times higher, averaging 2700 ppmw on the current cycle, than Refinery A. This refinery has a long history of operational excellence with Albemarle catalysts, as two generations of catalyst has been utilised in the unit with a combined experience of over 10 years.

Before the change to 10 ppmw sulphur requirements, Refinery B’s preferred solution was to produce low sulphur gasoline using RT-225, a premium, low density catalyst. For the current cycle, this refiner wanted to increase the feed rate by 15% and the feed sulphur content by 600 ppmw while improving profitability from maximum octane retention. The unit primarily processes a mixture of LCN and HCN. To meet the objective, RT-235 catalyst was short-loaded for the current cycle. The performance benefits are show in Figures 6 and 7.

Figure 6 shows the normalised HDS activity of the previous cycle loaded with RT-225 catalyst over the first four years of operation. Using the same normalisation factors, the RT-235 cycle shows a clear advantage in activity, up to twice as much as in the previous cycle. Even with the current cycle processing at increased throughput and feed sulphur levels, and at higher operating severity to reduce product sulphur, the stability or deactivation rate across the cycle remains consistent with the previous cycle. This is another example of how a refiner was able to push the boundaries of a gasoline unit with RT-235 catalyst without sacrificing target cycle length for improved profitability.

Figure 7 shows a slightly different way to highlight the average octane loss across the unit as operating severity is increased. To make comparing the two cycles easier, the data were normalised to a tar-get operating severity. The data from the previous cycle show that operating below the target severity leads to octane retention, whereas operating above the target severity leads to additional octane loss. So, normalising the current cycle data to the same conditions clearly shows octane retention from the RT-235 catalyst sys-tem versus the previous cycle. Even with increased operating severity, RT-235 catalyst maintained its greater than 1 RON advantage. Based on the refinery’s economics and valuation of octane, this refiner was able to achieve an additional $12 million/y in profits from greater octane retention by utilising RT-235 catalyst.

Conclusion

With more stringent gasoline regulations being implemented worldwide, refiners are looking for solutions with minimum investment to reduce the aver-age sulphur content of gasoline. To address this, some refiners have carried out an extensive technical and economic study of various catalyst drop-in solutions available in the market for selective HDS gasoline hydrotreaters.

With the value of octane gasoline barrel at an all time high, these refiners wanted to challenge the status quo and strive for a solution that enabled them to reach optimum performance and profitability. For the units at Refineries A and B, the refiners selected Albemarle’s latest generation catalyst for selective gasoline hydrotreating processes: RT-235. This catalyst has excellent selectivity to des-ulphurisation reactions while significantly boosting over-all desulphurisation activity. RT-235 catalyst has a proven track record in numerous applications and provided exceptional overall profitability for users.

At Refinery A, RT-235 catalyst was utilised to optimise performance when making ultra low sulphur gasoline while introducing a more severe feedstock and con-trolling the octane loss across the unit to between 2 and 3 RON. At Refinery B, the transition to RT-235 catalyst resulted in twice as much HDS activity. Improved selectivity with RT-235 catalyst for octane retention enabled Refinery B to save more than 1 RON in comparison with the previous cycle, which resulted in extremely favourable refinery economics and profitability.

Reference

1. Mayo S, Greeley J, Wellons M, RT-235: Commercial performance of next generation SCANfining catalyst, paper AM-11-58 presented at the 2011 NPRA Annual Meeting, San Antonio, TX, USA, 20 Mar 2011.