Starting up hydrotreating units with cracked feeds
A catalyst sulphiding process enables the introduction of cracked or heavy feeds to HDS units without the recommended three-day delay following changeover
RandyAlexander, Reinder Van Bruggen and Pierre Dufresne
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Refiners can realise significant operating margins by processing cracked feedstocks such as coker naphtha and light cycle oil (LCO). Unfortunately, after catalyst change-outs on hydrotreating (HDS) units, catalyst manufacturers recommend that the refiner only process straight-run feeds for at least the first three days of the catalyst cycle. This delay in processing cracked feeds has several potential costs including:
• Lost profits from processing cracked and heavy feeds
• Storage costs for a three-day supply of straight-run feed
• Storage costs for three days of cracked feed that must be held aside for processing later
• The purchase of additional straight-run feed if a three-day supply is not available.
Problem with cracked feeds
Cracked feeds such as coker naphtha contain significant quantities of highly reactive olefins and diolefins, while cracked gas oils also contain polynuclear aromatics. If these components are exposed to the hyperactive catalytic sites that exist on freshly sulphided catalysts, coke and gums will form that readily deposit on the catalyst surface. Such deposits tend to obstruct the catalyst’s pores and inhibit access to the active sites, leading to a permanent loss of catalyst activity and a reduction in cycle length.
To avoid the adverse effects of cracked feeds at startup, catalyst manufacturers typically recommend a break-in period of at least three days, feeding only straight-run product to the unit. During this three-day break-in period, a small amount of soft coke will form on the catalyst surface, gradually tempering the hyperactivity of the catalyst. Once catalyst activity has stabilised, cracked feeds can be introduced to the unit without the risk of premature coke and gum formation.
Eurecat has developed an ex-situ treatment that allows the introduction of cracked or heavy feeds without the recommended three-day delay. This process, called Totsucat CFP, combines the Totsucat activation process with an additional step to protect the catalyst from the harmful effects of cracked feed, ensuring long-term reactor performance irrespective of when cracked feeds are introduced to the unit.
This is a carefully controlled procedure that gently moderates the hyperactivity that occurs immediately after catalyst sulphiding.
Catalysts treated with this are fully sulphided and activated prior to loading. The activity will mimic a catalyst that has already been conditioned with straight-run feed for three days. Cracked and heavy feedstocks can be gradually introduced during the first hours of a startup without causing permanent damage to the catalyst. Coke and gum deposits will be minimised, since the catalyst activity has been carefully stabilised following activation.
Recent unit data also show that this treatment can significantly reduce the deactivation rate of a hydrotreating unit, allowing the refiner to extend their cycle by as much as 50%.
Totsucat catalyst activation process
Unlike with other pre-sulphurisation processes, Totsucat-treated catalysts are delivered to the HDS unit totally activated and ready to work immediately. The uncertainty of incomplete activation is eliminated and startup time is greatly reduced.
The advantages of the process include:
• Load-and-go reactor startups. The catalyst is fully sulphided and activated and does not require any additional sulphiding agents or complex activation procedures
• No sulphiding exotherms since the sulphiding step is completed in Eurecat’s equipment
• No need for additional hydrogen at startup
• Minimal sour water formation during startup
• The hazards and odours associated with sulphiding chemicals are avoided since spiking agents are no longer needed at startup
• No need to measure H2S levels in the recycle gas
• Negligible amounts of H2S are released during reactor heat-up, protecting sulphur-sensitive units downstream and avoiding the need to flare undesirable byproducts
• Circulating high concentrations of wet H2S during in-situ sulphiding can cause severe corrosion problems. Totsucat eliminates this issue since only low concentrations of dry H2S will be recirculating through the system.
Starting a unit with Totsucat-treated catalysts is similar to a restart after an emergency shutdown. Once the reactor is loaded, it only requires heating up to the start-of-run temperature. Time-consuming drying steps, holding steps and tests to determine H2S concentrations are not required. Startup time is reduced to a few hours, while startup risks are minimised. The catalyst bed will achieve peak performance since sulphiding and activation are controlled at very precise conditions.
The treatment is particularly useful for:
• Critical path units where lost production time is costly
• Units with temperature limits, where effective sulphiding is difficult, if not impossible, to achieve
• Refineries with limited hydrogen supplies
• HDS units that run in parallel or series, where only one unit is taken down while the other is still operating
• Units that cannot tolerate H2S breakthrough to downstream units
• Units that utilise gas-phase startup, increasing the risk of catastrophic exotherms during in-situ sulphiding
• Units where the use of sulphiding chemicals is undesirable or uneconomical.
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