Boosting hydrocracker heavy feed conversion
New pore structure in nano-engineered zeolites helps to improve conversion efficiency of heavy molecules, improving product yields and qualities.
SAL TORRISI and JOHAN DEN BREEJEN
Shell Catalysts & Technologies
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To remain competitive, refiners must continuously adapt to emerging trends or risk an undesirable performance drop. This continuous quest for improvement has driven them to process ever-more difficult feeds and advantaged crudes, to produce more and differentiated fuels and to generate valuable lubricant base oils or petrochemical products. The hydrocracker has emerged as an important unit for providing the refinery with the flexibility to adapt quickly to market changes. However, a key constraint for the unit has been in the all-important cracking reaction: specifically the ability to get larger molecules into the zeolite catalyst’s narrow micropores and quickly remove them after cracking.
As refiners increase processing severity in response to emerging requirements, the limitations of the catalytic material have diminished molecular conversion efficiency and reduced product yields. This article reveals how a new nano-engineered technology, Shell’s Molecular Access Catalysts for Hydrocracking (MACH), has unlocked this constraint and describes the opportunities this creates for refiners.
Strategic objectives for refiners
As the pressure on margins continues to intensify, the reality is that refiners must continue to find ways to adapt to today’s increasingly dynamic business landscape.
Many refiners are processing lower priced opportunity crudes and very heavy residue type feeds that can be extremely difficult to deal with. Others are striving to find ways to elevate the value of the product slate by moving away from bulk commodity products to more differentiated products such as lubricant base oils and petrochemicals.
And, as a consequence of the International Maritime Organi-zation’s 2020 fuel sulphur cap and because it has a clear link with competitiveness, some refiners are striving to minimise the amount of bottoms they send to the bunker fuel pool.
For all these objectives, the hydrocracker and its catalysts are key.
The need for an enhanced zeolite
Almost all hydrocracking catalysts contain Y-zeolites: 3D, crystalline solids with well-defined structures and micropores that provide a high number of acidic catalytic sites for inducing cracking reactions.
In a conventional hydrocracker processing lighter feeds, the relatively small molecules can get in and out of the zeolite adequately and the cracking reaction is unrestricted. However, getting the heavier molecules in challenging feeds, or even typical vacuum gasoil (VGO) molecules, into the zeolite’s narrow micropores and quickly removing them as the processing severity increases has long been a key limitation. Any extra time that the molecules spend inside the ‘sizeable’ zeolite structure leads to overcracking and, consequently, lower yields of liquid products and catalyst deactivation.
To address this, Shell Catalysts & Technologies sought improved materials that would offer better accessibility for larger molecules. Following a five-year research and development journey, the result is MACH, a nano-engineered technology that enables feed molecules to be converted more efficiently into high quality fuels, lubricants, and petrochemical products, which thereby helps refiners to meet the strategic objectives described above.
Figure 1, which compares a conventional zeolite with a MACH zeolite, illustrates how the technology works. The micropores of the conventional Y-zeolite shown on the left are not optimal as the molecules cannot access all of them easily, particularly on the interior of the zeolite crystal. However, the MACH technology introduces mesopores (medium size pores) that enable a shorter pathway for the molecules to enter, react and exit the zeolite more efficiently.
The transmission electron microscopy images in Figure 2 contrast a conventional microporous Y-zeolite having a few macroporous (large port) feeder ports in the microporous Y-zeolite crystal with a MACH Y-zeolite. The MACH Y-zeolite’s structure is transformed to have a high volume of well-defined mesopores with a uniform distribution while maintaining the key features of high acidity, microporous structure, and hydrothermal stability.
In a MACH zeolite, the actual cracking reactions still take place predominantly in the microporous structure. However, the number of entry points is very much higher because of mesopore introduction. Thus, the flux of reactants and products in and out the micropores is also higher.
The cracking reaction itself is a three-step reaction in which the first and third steps happen over the metal function in the catalyst’s amorphous phase outside the zeolitic structure. The mesoporous MACH zeolite increases molecular speed to and from the metal sites that initiate and terminate the cracking reactions. Ultimately, this fosters the desired cracking reactions and reduces the likelihood of overcracking that generates undesirable light products.
Figure 3 shows the difference in the pore distributions of a conventional Y-zeolite compared with a MACH zeolite containing mesopores (the macropores are not shown). The whole pore structure is engineered to incorporate a mesoporous arrangement that has a finely tuned pore size distribution that accommodates the largest VGO molecules better. This contrasts with other mesopore incorporation techniques that have a more random approach and yield a broader distribution.
In addition, the introduction of mesopores decreases the distance molecules need to travel between acidic and metal sites. This also helps to improve hydrogenation and, therefore, to enhance the middle distillate yield and aromatics saturation.
The MACH zeolite provides step-out performance in the conversion of VGO and heavier molecules because the molecules:
• Travel one-third the distance to reach active sites, providing improved diffusion rates via the extra entrances and exits to the cracking sites; and
• Diffuse nine times better for the largest VGO molecules (see Figure 4).
Hydrocracking catalyst portfolio
Shell Catalysts & Technologies collaborates with PQ Corporation through its Zeolyst International (Zeolyst) joint venture to create hydrocracking and specialty chemical catalysts. The two organisations worked together on the MACH research and development programme.
This relationship began in 1990. Since then, Shell Catalysts & Technologies has produced three generations of hydrocracking catalysts recognised for their stability and good balance of activity and selectivity, which result in long run lengths and slow yield declines. Key to this is the organisation’s ability to develop, synthesise and manufacture new zeolites of high quality and specific molecular design.
Figure 5 shows the full portfolio of Shell Catalysts & Technologies and Zeolyst’s hydrocracking catalysts. These have been used in more than 100 hydrocrackers, mild hydrocrackers, and distillate upgrading units around the world.
The new fourth generation products based on MACH technology have been commercialised and are available in the high selectivity portion of the portfolio as Z-HD27, Z-MD07, and Z-MD17. Extension of the portfolio to the higher activity space is under way and anticipated soon.
In the original premise for developing the technology, the primary economic driver was to increase valuable liquid yields such as diesel (see Figure 5). However, Shell Catalysts & Technologies discovered many additional benefits that translate into significant economic gains for each application of a MACH product in place of a conventional product. The following section describes three diverse, real-world applications of these MACH products and the multiple economic benefits they provide that can help refiners to realise the full potential of a hydrocracker and enhance overall refinery profitability.
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