How can process reconfigurations and reactor enhancements improve hydroprocessing catalyst performance?
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28/07/2007
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Sal Torrisi, Criterion Catalyst & Technologies, sal.torrisi@criterioncatalysts.com
There are numerous examples of where the combination of reactor hardware and catalyst systems has significantly improved both unit reliability and catalyst performance. There are three common reasons why a catalyst system may fail to reach full cycle potential: fouling, poisoning and poor flow distribution. Fouling is a fairly common phenomenon, particularly in services where cracked stocks are processed or where solids are present in the feed (system). A combination of reactor internals, which provide particulate filtering or scale catching along with appropriate top-bed catalyst grading can have a significant impact on overall cycle length. This is particularly true for units with cycle lengths of 6-12 months that routinely shut down due to pressure drop build-up. A properly prescribed fouling abatement scheme can easily double the cycle length. Catalyst poisons, such as arsenic, silicon, nickel and vanadium, deactivate catalysts to varying degrees.
Identification of different poisons present and removal mechanisms is necessary to properly design the "protective" layers in front of the active catalyst to preserve performance throughout the cycle. Recent refinery activity in processing opportunity crudes has introduced new contaminants to some hydrotreating units (ie, arsenic) or multiple poisons (three to five) with different removal mechanisms, all of which have substantially reduced cycle length, as catalyst has deactivated more quickly. Selection of proper protective demetallisation (or “demet”) catalyst layers in combination with more robust (not necessarily more active) hydrotreating catalyst can act to produce the best overall cycle life and performance.
Finally, state-of-the-art reactor internals, including liquid distribution trays and quench assemblies, can dramatically improve overall reactor performance. This allows full catalyst utilisation and thermal uniformity so that the lowest possible SOR temperature is achieve in conjunction with minimisation of the catalyst deactivation rate. For example, in a US DHT application, a refiner experienced a series of short cycles averaging nine months in duration. While catalyst advancements helped marginally improve performance, the replacement of the liquid distributors with Shell GS HD-trays enabled this unit to nearly triple its cycle life, attributed to a 30ºF lower SOR WABT and a 2.5-fold reduction in deactivation rate. Thus, significant improvements are possible to existing reactor systems that are limited by the three problems described, and these reactor best practices have now been incorporated into new unit designs to ensure reliable operation in all hydrotreating services.
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28/07/2007
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Woody Shiflett, Chevron, wosh@chevron.com
We have worked with a process technology licensor for several applications with a number of clients that involved both new designs and reconfigurations in order to meet requirements for ULSD or FCC pretreatment. All cases are, at this time, confidential.
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28/07/2007
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Charles Cameron, Axens, charles.cameron@axens.net
Pertaining to internals, it is well known that without high-performance internals it is impossible to attain ultra-low impurity levels. This is the case for many applications, such as sulphur in gasoline and diesel fuel, diolefins in olefin monomers, C8-aromatics in paraxylene. For resid hydrotreating applications, permutable guard-bed systems (PRS) have proven to be a major success in extending the on-stream factor by eliminating the pressure drop from resid demetallisation and desulphurisation catalysts. In other words, the details count when you need to eliminate trace impurities. In hydrotreating, the slightest amount of maldistribution can lead to off-spec production, which concerns inlet distributors, catalyst grading materials, uniform catalyst bed density, quench boxes, thermocouple placement and more.
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