Chlorides originating either in the feed or in the catalyst can introduce various complications, including intensifying corrosion concerns (NH 4 Cl), forming unwanted deposits in the fractionator and enhancing the dehydrogenation activity of nickel deposited on the catalyst. Mitigation approaches would include sound catalyst selection (avoiding high chlo- ride-containing catalysts if there is an issue) and upstream solutions, such as (again) improving desalter efficiency. Silicon (silica) contamination does not get much discus- sion or attention as it is essentially undetectable against the background of silica in the catalyst itself, and the impacts have not been thoroughly documented and quantified. It is reasonable to assume that silicon introduced in the feed (for example, from such sources as defoaming agents employed in the delayed coker) might interact with iron in a similar way as silica originating with the catalyst. While the total amount of silica contaminant is going to be very low relative to the catalyst baseline, mobile silica is the real issue. That ratio is going to be significantly higher. So, while it is tempting to draw the conclusion that silica in the feed is simply not present in large enough quantities to have an impact, this has not rigorously been shown to be true. In fact, Ketjen has lab data indicating the opposite. A catalyst specifically designed to minimise iron and silica interactions (SaFeGuard) can alleviate this impact. A Berthold Otzisk, Senior Product Manager, Process Chemicals, Kurita Europe, Berthold.otzisk@kurita-water. com In recent years, FCC catalysts have been developed that are much more tolerant of catalyst poisons (contaminants). Nevertheless, contamination of the FCC catalyst still leads to reduced product quantities or shorter cycle lengths. Contaminants act as competitive catalysts to dehydroge- nate the hydrocarbons, leading to excess hydrogen produc- tion and coke. They reach the FCC catalyst with the feed material and irreversibly destroy the zeolite crystallinity and/or the acidity. Classical impurities are metals such as nickel (Ni), vanadium (V), iron (Fe), copper (Cu), sodium (Na), calcium (Ca), or magnesium (Mg). Nickel (also Cu, V, and Fe) enters the system in the form of large porphyrin molecules, which crack onto the FCC catalyst, leaving the nickel behind. Nitrogen (N) or carbon (C) are catalyst poisons that deac- tivate or cover cracking sites on FCC catalysts. However, this is only temporary, and the catalyst activity is recovered. Catalyst destruction by metals is more pronounced and per- manent, where catalyst bed activity can only be recovered by adding fresh catalyst. Nickel is the primary competitive catalyst in the FCC, act- ing as a dehydrogenation catalyst. Dehydrogenation of hydrocarbons leads to loss of gasoline selectivity and a slight reduction in catalyst activity. By plugging catalyst pores, the conversion is reduced with the negative effects of increased delta coke on FCC heat balance. Nickel should always be con- sidered if the process unit is running against a limit. If nickel on Ecat exceeds around 500 ppm, a chemical treatment pro- gramme should be started. A nickel passivation programme reduces the negative effect of nickel by 50-70%. Alongside
nickel, vanadium is another metal that causes problems and production losses. Vanadium acts as a competitive catalyst and a true catalyst poison. Besides dehydrogenation reac- tions, it may oxidise, becoming mobile and migrate to the zeolite catalyst, permanently destroying it. There are various passivation programmes with which a reduction of nickel or vanadium dehydrogenation can be achieved. The negative influence of these metals is reduced, and the conversion and yield are increased in addition to the improved gasoline and C 3 /C 4 selectivity and longer cycle length. Best known in the industry is the use of antimony or bis- muth (Bi) to mitigate the effects of nickel. Aqueous antimony pentoxide solution (Sb 2 O 5 ) is preferred as it works much faster compared to bismuth and is easier to control. Care should be taken to ensure that the particle size of Sb 2 O 5 is preferably <5 nm in order to obtain a stable colloidal disper- sion. The more stable dispersion avoids settling problems in storage. Sodium is a catalyst poison, and residual sodium or byproducts such as Sb 2 O 3 (suspected to be carcinogenic) should not be present. When dosing Sb 2 O 5 , an average ratio of 0.35 Sb:Ni should be set. The typical base load to saturate active nickel is reached after five to seven days. An overdose of Sb2 O 5 must be avoided because Sb in LCO can poison down- stream Ni-Mo hydrotreater catalysts. Q Can you discuss your experience with using CFD for hydroprocessing reactor troubleshooting? A Zumao Chen, Engineering Fellow, Becht, zchen@ becht.com The application of computational fluid dynamics (CFD) for troubleshooting hydroprocessing reactors has proven invaluable in diagnosing complex operational challenges, optimising designs, and enhancing reactor performance. CFD, often coupled with kinetic modelling, is particularly effective in addressing flow maldistribution in hydrotreat - ing and hydrocracking reactors. For example, modelling the inlet distributor through the catalyst beds of a downflow reactor allows for improved distribution and mixing in both radial and vertical directions (see Figure 1 ). CFD analysis also enables the modelling of complex reac- tor configurations, such as ebullated bed reactors, where
Before
After
Ideal mixing would be a single colour
Figure 1 CFD analysis can improve distribution and mixing in radial and vertical directions in reactors
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Catalysis 2025
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