Catalysts to play a large part in ultra-low sulfur fuel

Oct. 9, 2000
To meet the challenge of making diesel and gasoline with lower sulfur levels, refiners will have to depend on high hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity catalysts.

To meet the challenge of making diesel and gasoline with lower sulfur levels, refiners will have to depend on high hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity catalysts. - Yoshimasa Inoue is manager of the research and development division at Nippon Ketjen Co. Ltd., Tokyo. He has 30 years of research and development experience in hydroprocessing catalysts for petroleum and synthetic fuels. Prior to joining Nippon Ketjen, Inoue spent 16 years with the central research laboratory of Sumitomo Metal Mining Co. Ltd., where he was responsible for catalyst development and coal liquefaction. Inoue holds a degree from Tokyo Gakugei University.

Catalyst is one of the least expensive options to achieve lower sulfur levels.

Akzo Nobel Catalysts BV, Amersfoort, The Netherlands, and Nippon Ketjen Co. Ltd., Tokyo, introduced the STARS (Super Type II Active Reaction Sites) catalyst technology in 1998.

Since then, they have commercialized two STARS catalysts, Ketjenfine 757 and Ketjenfine 848 (KF 757 and KF 848). KF 848 is a NiMo catalyst geared for higher pressure applications, while KF 757 is a CoMo catalyst for low-to-moderate pressure units.

Performance results from several units worldwide show that KF 757 is meeting or exceeding pilot-plant test projections. KF 848 is also operating above expectations in two high-pressure diesel units. Operating data for these applications are being compiled.

STARS catalysts offer up to 50-60% higher HDS and HDN activity and higher stability than previous-generation catalysts.

Refiners' choices

While gasoline was subject to severe reformulation between 1980 and 1995, diesel oil is changing more between 1990 and 2010. In 2005, Western Europe will probably face its third decrease in sulfur level for diesel oil within 10 years.

In Europe, the sulfur specification for 2000 is a maximum of 350 ppm and may be as low as 50 ppm in 2005. Recent discussion has focused on further reducing the 50-ppm limit to 10 ppm.

As well as sulfur, European and US refineries may be reformulating their diesel to improve burning characteristics and decrease tailpipe emissions in response to new specifications for density, boiling range, cetane number, and polyaromatics.

In the US, recent US Environmental Protection Agency (EPA) initiatives propose reducing the diesel-sulfur specification to less than 15 ppm by 2006.

Oil companies will select from several strategies to address these changes: applying improved catalysts, operating the refinery in a different way, applying new technologies, or undertaking a combination of these options. No matter which strategy refineries choose, super high-activity catalysts will be an integral part of making ultra low-sulfur diesel.

Refiners can upgrade the performance of existing diesel HDS units by application of high-activity catalysts related to the STARS technology. This same technology can help hydrocrackers produce more clean diesel.

If capital investment is required, the MAKFining alliance offers several clean fuel technology options, including the MAKFining Premium Distillate Technology (MPDT).

STARS catalyst technology

The active phase in a hydrotreating catalyst consists of well-dispersed molybdenum sulfide (MoS2) slabs, with edges and corners decorated with nickel (Ni) or cobalt (Co).1-4 The atomically dispersed Ni or Co on the MoS2 slabs (NiMo-S or CoMo-S) is responsible for most of the catalyst activity.

The different morphologies of the MoS2 phase result in different active sites.

Type I active sites are characterized by a high dispersion of the MoS2, resulting in a relatively strong interaction between the CoMo-S (or NiMo-S) active sites and the support. This affects the electronic state of the Co (Ni) sites on the edges and corners of the MoS2 slabs, resulting in a lower intrinsic activity per active site.

Type II active sites are more fully sulfided and the MoS2 structure is less disperse, often consisting of stacks of larger slabs. The Type II active phase has a higher intrinsic activity per active site.

The higher intrinsic activity is especially beneficial in high-severity applications such as deep HDS and HDN.

Increasing the number of active sites and increasing the intrinsic activity of the active sites can significantly improve catalyst activity. Higher metals content or optimization of the dispersion of the active metals can increase the number of active sites.

Although traditional catalysts aim to increase the number of active sites, that approach has limitations as a result of catalyst-support morphology.

STARS technology follows a different approach. Although STARS maintains a high number of well-dispersed active sites, it ensures that all active sites are Type II, maximizing the activity benefits of the expensive active metals.

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For an equivalent number of active sites, the relative volume activity (RVA) of STARS catalysts is much higher than for catalysts with Type I sites (Fig. 1).

STARS CoMo catalyst

The first catalyst commercialized using STARS technology is KF 757, a CoMo catalyst for ultra deep desulfurization of mid-distillate to as low as 50 ppm sulfur.

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Fig. 2 shows that the RVA improves as product-sulfur level decreases.

The results in Fig. 2 were obtained from a large number of feedstocks, ranging from straight-run gas oil to 100% light cycle oil (LCO) with 0.8-2.0 wt % sulfur levels. Hydrogen partial pressure varied from 15 to 65 bar.

The RVA for HDS of KF 757 compared to KF 756 is 125 at 500-ppm product sulfur. Under ultra-deep desulfurization conditions, KF 757 achieves an RVA as high as 150-160, a big step forward.

The catalyst is effective in the removal of sterically hindered dibenzothiophenes, which are very difficult to remove because of the presence of alkyl groups located next to the sulfur atom.

A 2-month stability test in Refinery 1 and commercial performance in Refineries 2 and 3 demonstrate that KF 757 has equal or better stability compared to KF 752 or KF 756.

In addition to the much higher HDS activity, the catalyst has higher hydrogenation activity, resulting in lower product density than previous-generation catalysts. Product density with KF 757 is 0.5-0.9 kg/cu m lower than that from KF 756. This is of great value to refiners, who typically sell products on the basis of volume.

The new catalyst also makes a product with lower aromatics, higher cetane, higher HDN, and improved product color.

Commercial experience

Since its introduction in the spring of 1998, KF 757 has been loaded in more than 50 units worldwide. Three commercial experiences demonstrate some of the advantages of this new catalyst.

  • Refinery 1 installed KF 757 in its gas oil hydrotreater in May 1998. The refinery treats straight-run light gas oil (LGO) and heavy gas oil (HGO) in blocked operation at 30 bar hydrogen partial pressure. The catalyst was presulfided with feed only.
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Fig. 3a shows the weighted average bed temperature (WABT) in the hydrotreating unit. KF 757 experienced an 8° C. better HDS activity than the previous catalyst, KF 756.

This commercial performance is in line with the results previously obtained from pilot-plant testing and model predictions. The refiner expects to double the unit's catalyst cycle from 1 year with KF 756 to 2 years with KF 757.

  • Refinery 2 loaded KF 757 into its hydrotreater in March 1999, replacing KF 756 catalyst. The unit has an unusually high space velocity of 4.5 hr-1 and handles high amounts of cracked stocks (up to 40% heavy fluid-catalytic cracker naphtha and up to 25 % LCO).

The refinery chose the new catalyst for three reasons:

  1. To achieve flexibility to run the unit with more cracked stocks and feed with a 25% higher sulfur content, while making lower product sulfur.
  2. To enable sock loading, saving on catalyst and dense loading costs.
  3. To enable catalyst presulfiding with feed only, saving costs for ex-situ or spiked feed presulfiding.

The normalized activity of (sock loaded) KF 757 was 15-20° C. higher than (dense loaded) KF 756, higher than predicted (Fig. 3b).

Despite considerably higher operating severity during the KF 757 cycle, the activity advantage of KF 757 was about 15° C., which meant that the catalyst was stable throughout the cycle.

The unit was running at close to 400° C. shortly after start-up as a result of a combination of high intake of cracked stock, high liquid hourly space velocity (LHSV), and product sulfur as low as 100 ppm.

  • Refinery 3 is operating with up to 30% LCO, targeting 250-500 ppm diesel, with intermittent heating oil production at 0.2 wt % sulfur. The unit operates at a relatively low hydrogen partial pressure, about 25 bar at the reactor inlet.

KF 757 replaced a competitive CoMo catalyst. The refiner chose the new catalyst to achieve higher activity, better product color, and longer cycles. Previously, the product color at the end-of-run limited the cycle length to as short as 6 months.

The refinery loaded KF 757 in April 1999 and activated it with feed only.

As expected, catalyst activity has been as high as 10° C. better than the previous catalyst, and the stability has exceeded expectations.

Fig. 3c shows that the weighted average bed temperature has remained stable. Despite a high intake of LCO and low hydrogen partial pressure, the KF 757 did not show any noticeable catalyst deactivation during the first 3-4 months of operation.

Along with the excellent stability, product density reduction has been at a high level of about 20 kg/cu m. This refiner ordered a new batch of KF 757 after this successful run.

Further sulfur-reduction strategies

Without compromising cycle length, KF 757 can reduce the sulfur content to less than 350 ppm in a unit currently producing 500-ppm sulfur using previous-generation catalysts.

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While a switch of catalyst may drop the diesel-sulfur level to less than 350 ppm, it will still not allow diesel to meet the 2005 specifications of 50 ppm. Without a unit revamp, the temperature required to achieve 2005 sulfur targets will increase, resulting in higher catalyst deactivation and shorter cycles.

  • Reduction of boiling point. Reducing the diesel 95% boiling point from 370° C. to 360° C. will allow KF 757 to achieve a 13-month cycle length for 50-ppm sulfur (Fig. 4). Previous-generation catalysts would have achieved a cycle of less than 3 months under these conditions.

A commercial operation in West Europe reduced its endpoint and uses KF 757 to produce a special diesel grade with a sulfur level of 0-5 ppm. Its regular diesel grade requires a product sulfur of 20-30 ppm.

  • Unit revamp. For new units and unit revamps, the MAKFining alliance offers an ultra deep hydrodesulfurization (UD-HDS) technology (OGJ, May 3, 1999, p. 97), which combines the use of high-activity catalysts and reactor design. Combined with STARS KF 757 or KF 848, the units can produce diesel with sulfur levels less than 10 ppm.

For treating distillate feedstocks, such as LGO with or without cracked components down to 30-50 ppm sulfur, KF 757 will be the design catalyst. KF 848 is preferable for high-severity HDS, down to less than 10 ppm sulfur. Heavier, high-nitrogen feedstocks, which require higher levels of HDN as pretreatment for downstream noble metal catalysts, also use KF 848.

In many cases, the use of KF 848 with a somewhat higher design pressure will result in a more cost-effective route to aromatics reduction, cetane improvement, and ultra low sulfur than with a lower pressure, two-stage approach.

STARS NiMo catalyst

The second catalyst commercialized on basis of the STARS concept is KF 848. This catalyst has high HDA (hydrodearomatization or aromatics reduction), HDS, and HDN activity compared to its predecessors, catalysts like KF 852.

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Table 1 compares the RVA for KF 848 in diesel-type service with KF 852 for a variety of feedstock blends and medium pressure operating conditions.

Refiners can take advantage of the higher activity of the KF 848 in several ways:

  • To increase the cycle length of units that are activity limited.
  • To increase throughput while maintaining the same cycle length.
  • To produce lower aromatics diesel product.
  • To operate on heavier, more difficult feeds like light coker gas oil (LCGO) and LCO.
  • To take advantage of less-expensive feeds which are often more difficult to dearomatize and denitrogenate.
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Table 2 shows the above average catalyst activity of KF 848 in higher operating pressure applications, like hydrocracker pretreat operations. At equal operating conditions, KF 848 reduces product nitrogen levels 2 to 3 times lower than KF 843. This translates to an RVA of 140-200 compared to KF 843's RVA of 100.

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Fig. 5 shows the superior stability of KF 848 over KF 843 in a long term pilot plant test.

KF 848 has been loaded into five hydrocracker-pretreatment units and shown to greatly reduce the required operating temperature. Total installed volume in more than 10 units exceeds 1,200 tons.

Currently, KF848 is in operation in two high-pressure diesel units to produce ultra low sulfur products.

References

  1. Eijsbouts, S., Urugami, Y., Inoue, Y., and Takahashi, Y., "Improved Hydrocracker Pretreat Catalysts," Akzo Nobel Catalysts Symposium, 1998.
  2. Eijsbouts, S., Applied Catalysis, General, On the flexibility of the active phase in hydrotreating catalysts, Applied Catalysis, Vol. 158, No. 53, 1997.
  3. Inoue, Y., Urugami, Y., Takahashi, Y., and Eijsbouts, S., "High Activity HDN Catalysts" 3rd Tokyo Conference on Advanced Catalytic Science and Technology, July 1998.
  4. Gerritsen, L.A., Sonnemans, J.W.M., Lee, S.L., and Kimbara, M., ERTC Berlin, November 1998.
  5. Sarli, M.S., McGovern, S.J., Lewis, D.W., Snyder, P.W., "Improved Hydrocracker Temperature Control: Mobil Quench Zone Technology," NPRA 1993 Annual Meeting, M-93-73.

The authors-

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Leen A. Gerritsen is marketing manager of fluid-catalytic cracking (FCC) catalysts for Akzo Nobel Catalysts BV, Amersfoort, The Netherlands. He joined Akzo Nobel Catalyst in 1982 as a technical service representative for Cyanamid-Ketjen. In 1986, Gerritsen switched to FCC catalyst technical services, and in 1989, he became application research and development manager of FCC in Akzo Nobel's Amsterdam laboratory. Before his current position, he was manager of the technical services group for hydroprocessing catalysts. Gerritsen holds a PhD in chemical engineering and catalysis from the Technical University in Delft, The Netherlands.

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Frans L. Plantenga is worldwide development manager of hydroprocessing catalyst for Akzo Nobel Catalysts BV in Amersfoort. He is responsible for hydroprocessing and hydrocracking product developments. In his 20 years at Akzo, he has been technical services manager for Akzo's FCC and hydroprocessing groups as well as marketing manager for the FCC business. Plantenga holds a PhD in chemistry and physics from the University of Leiden, The Netherlands.

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Pankaj H. Desai is the commercial manager for refining catalysts at Akzo Nobel Catalysts LLC, Houston. Since he joined Akzo in 1980, he has served as a supervisor and research group manager in hydroprocessing and FCC catalyst applications research, senior technical services and development representative for FCC catalysts, technical development manager, and development manager for hydroprocessing and FCC catalysts. Desai holds a BTech degree in chemical engineering from the Indian Institute of Technology, Kanpur, India, and a PhD in chemical engineering from the University of Houston.

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Steven W. Mayo is technical service and development manager for Akzo Nobel Catalysts in Houston. He is responsible for hydroprocessing catalyst technical services and catalyst-development activities for North and South America. In his 16 years with Akzo Nobel, he has held various technical and managerial positions in the areas of hydrotreating-catalyst development, process and project engineering, catalyst manufacturing, and hydroprocessing technical service. Mayo holds a BS in chemical engineering from Texas A&M University, College Station.