We would like to switch our hydrocracking from maximum distillate to maximum naphtha for higher LPG and aromatics output. Any thoughts on achieving this? (PTQ Q&A)

Response to a question in the Q3 2020 issue of PTQ

Adrienne Van Kooperen
Shell Catalysts & Technologies

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Article Summary

Adrienne Van Kooperen, PhD, Senior Technical Services Engineer & Hydrocracking Specialist, Shell Catalysts & Technologies - Adrienne.Lukaski@shell.com

Many refiners are moving towards integrated crude-oil-to-chemicals plants with the projected decline in gasoline consumption by 2050 and the global growth in the demand for chemicals. This creates a substantial economic incentive to generate ‘in-house’ feedstocks for petrochemicals production either from revamping existing hardware or modifying the operation of current assets. The inherent flexibility of hydrocracking, with the ability to tune yield slates from distillate to naphtha production or anywhere in between by appropriate design of the catalytic system, provides the integrated refiner with an avenue to generate feeds for chemicals manufacturing without limitations on future return to transportation fuel production. As an active participant in the petrochemicals field since 1929, Shell possesses a long history of product and technology development in the chemicals value chain that we, as Shell Catalysts & Technologies, leverage to support our customers in repurposing molecules into the petrochemical pool.

Moderate adjustments to yield selectivity via catalyst changes alone or by swinging once-through operations into recycle mode provide refiners with handles to modify hydrocracker yield slates and product qualities for chemical applications without substantial capital investment. We have worked with a number of customers in Asia to convert conventional distillate hydrotreaters into naphtha hydrocrackers to redirect these molecules into aromatics plant feed in response to the increased demand for chemicals, while several of our domestic partners have leveraged our tailored hydrocracking systems for maximum hydrogenation and ring opening to generate high quality hydrocracked unconverted oil that swings between refinery FCC and chemical steam cracker feedstock. The latter effectively strives towards the best of both worlds by selectively cracking heavy molecules into more valuable refining products and saturating the remaining unconverted oil which improves the feed quality to the steam cracker. This saturated unconverted oil, or hydrowax, becomes an economically attractive steam cracker feedstock when the material possesses sufficient hydrogen content and the requisite end point to ensure adequate vaporisation in the steam cracker furnace.

The migration from a less active, flexible or distillate selective catalyst to a high activity and highly naphtha selective platform, or vice versa, however requires broader assessment of the end-to-end process to ensure smooth operations in all equipment sections. We have seen operations where sites changed from a naphtha selective catalyst to a more mid-distillate selective, flexible system for higher diesel production and then encountered operational issues in the work-up (fractionation) section due to the significant reduction in light ends make and the resultant shift in vapour/liquid traffic. Shell Catalysts & Technologies executed studies that identified several mitigation options ranging from adjusting operating conditions to equipment upgrades which optimised the overall hydrocracker performance and reliability.

The situation posed in this question – to switch hydrocracking from maximum distillate to maximum naphtha production – most likely requires some modifications in the work-up section to address higher vapour/liquid traffic in the fractionator and the need to debottleneck the naphtha run-down system. The degree of modification depends on the level of catalytic changes involved and the capabilities of the specific equipment on the existing unit.

Hydrocracking is an incredibly adaptable process that enables refiners to convert a wide range of feedstocks to higher quality products with excellent properties. The heart of the hydrocracking process is the catalyst system that can be designed to produce yield slates ranging from maximum distillate to maximum naphtha or to operate with the flexibility to ‘swing’ between low and high conversion modes. Depending on the product disposition, refiners can optimise the hydrocracking product slate to generate highly naphthenic/alkyl-naphthenic C6 to C9 feedstock for aromatics, LPG, and highly paraffinic light naphtha for steam cracking, or a combination of high quality transportation fuels products with highly hydrogenated hydrowax for steam cracker feedstock. Selection of the appropriate catalytic system however depends on the hydrocracker feed type, process operating window, conversion target, and product quality specifications. Similarly, the effective transition of an existing hydrocracking unit from maximum distillate operation to a maximum naphtha mode targeting chemical applications depends on the specific feedstock, operating severity and, most importantly, the need to utilise existing catalyst or the option to load a catalyst system tuned specifically to the target chemical objectives.

Feedstock to a large extent dictates the aromatics, naphthene, and paraffin content of the naphtha product. Aromatic feed sources generally yield naphtha qualities highly suited to supplying hydrocracker products for reforming operations or aromatics chemical complexes, while steam crackers benefit more from naphtha produced from highly paraffinic feeds, such as materials derived from light tight oil. When contemplating the conversion of an existing hydrocracker operating in maximum distillate service into chemical feedstock production, the refiner should work with their catalyst supplier to determine the impact of their specific feed diet and crude slate on the end naphtha product.

Operating severity also influences naphtha qualities in the aromaticity and iso- to normal paraffin ratio. Increasing conversion across a given catalyst directionally generates more naphtha with a greater degree of isomerisation and aromatics saturation. This means that there will be an optimal operating window for each catalyst system that generates the maximum amount of naphtha with the desired product qualities. The final product disposition, heavy naphtha for aromatics feedstock or lighter, more paraffinic naphtha, and LPG for steam cracking further defines where these optima lie.

Hydrocracking provides the flexibility to shift operations between production of heavy naphtha suitable as aromatics feedstock and lighter naphtha material for steam cracking with the appropriate catalyst system and operating strategy. Operating at higher conversion per pass directionally generates more light naphtha, while reduced severity produces greater yields of high quality heavy naphtha. Naphtha qualities however change more fundamentally as a function of catalyst type. Utilising a high zeolite content catalyst with low hydrogenation capacity at low to moderate severity (at lower conversion per pass) maximises the aromatic content of the naphtha product. Increasing conversion per pass by operating at higher severity drives the production of light naphtha and LPG which benefits from a lower zeolitic content catalyst with more hydrogenation capability to enhance paraffin production for steam cracker feed. Properly tuning the overall catalyst system and rationally designing the operating strategy therefore allows the refiner to deliver the highest quality chemical feedstock.

Expert evaluation of the entire hydrocracker unit capabilities from the feed diet to the catalyst workhorse to the back-end process equipment is the key to ensuring the successful, safe, and profitable conversion of a maximum distillate hydrocracker to maximum naphtha production or vice versa.

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