It is important to understand and control the effects of iron. Depending on the type of raw materials used (kao- lin clays), fresh FCC catalyst can contain 0.25-1.0 wt% iron. However, unlike sodium, the iron present in the fresh catalyst is inactive and has no impact on catalyst perfor- mance. So-called ‘added iron’ can impact FCC catalyst per- formance, especially if it is the organometallic type. Added iron is calculated as the difference between iron in the fresh catalyst and iron in the equilibrium catalyst, according to the following equation:
Low surface porosity High surface porosity
Alumina mapping
Cross - sectional analysis
Fe(Added) = Fe(Ecat) –Fe(Fresh)
Standard conditions
This added iron primarily deposits on the catalyst surface, blocking the pores. As the added iron on Ecat increases beyond a certain limit, an ‘iron cliff’ can be observed for some catalysts. This is characterised by a sudden drop in surface area and activity, leading to a drastic drop in con- version, with a corresponding increase in low-value slurry product, reducing the refining margin. As the added iron increases further, it can also lead to nodule formation on the catalyst surface. When nodules form, the apparent bulk density (ABD) of the catalyst also decreases, which can cause fluidisation and circulation issues in the unit. Since iron primarily deposits on the catalyst surface, the surface pore structure of the catalyst has the biggest impact on iron tolerance in FCC catalyst. The surface pore structure of the catalyst is determined by the type of cata- lyst manufacturing process, with the in situ process scoring over the conventional incorporated process. In the incorpo- rated manufacturing process, binders are used, which form a thick shell on the catalyst surface during calcination of the catalyst. Therefore, when the added iron deposits on these catalyst particles, it results in an additional layer of deposits, causing a double barrier to diffusion of hydrocarbons in and out of the catalyst pores, as shown in Figure 5 . This double layer leads to a sharp decline in Ecat activity when high iron feeds are processed. This phenomenon is also known as the iron cliff, and in some units using conventional FCC cat- alysts, it is observed at added iron content on Ecat as low as 0.30 wt% (3,000 ppm-wt). In contrast, in the in situ manufacturing process, binders are not used. The zeolites crystallised in the microsphere pores act as a binder, and hence there, is no binder shell present on the catalyst surface. For this reason, the in situ catalysts offer intrinsically higher surface porosity and better zeolite expo- sure for cracking hydrocarbon molecules.5 Further, when iron is deposited on the in situ catalyst, it acts as a single layer of diffusion barrier, which is why in situ catalysts offer superior pore accessibility even under high iron content environments and, therefore, significantly higher iron contamination toler - ance as compared to incorporated catalysts. As a result, the characteristic iron cliff phenomenon is not observed in units employing in situ FCC catalysts. Figure 6a compares data from different units using in situ cata- lysts versus those using incorporated catalysts. The results indicate that in situ catalysts maintain performance even at 0.80 wt% (8,000 ppm-wt) of added iron on Ecat, with- out significant loss of activity or evidence of an iron cliff.
High Fe conditions
Binder shell
Fe nodulation
Unlike nickel, vanadium is mobile under typical FCC con- ditions and can migrate to active sites on catalyst zeolite to neutralise acid sites and decrease catalyst activity. The adverse impact of vanadium is more severe in the pres- ence of sodium and steam at high temperatures, as it forms sodium vanadate, which destroys the zeolite framework on the catalyst. Vanadium is also a mild dehydrogenation agent (~25% dehydrogenation activity compared to Ni) and can contribute to increased dry gas and coke yields. To minimise the impact of vanadium, it is imperative to minimise sodium, which can enter the unit from the feed or be present in the fresh catalyst itself. Efficient desalting of crude can help minimise sodium in the feed. Sodium com- ing from the fresh catalyst depends on the type of catalyst used in the unit. Depending on the catalyst manufacturing technique, sodium in fresh catalyst can range from 0.15 wt% to 0.35 wt%. Hence, the use of low-sodium fresh cat- alysts is recommended when processing feeds with high vanadium content. In addition, vanadium passivation technologies can be employed to make vanadium immobile and inactive through various chemical reactions. Sulphur has an adverse impact on the efficacy of some vanadium traps. Hence, sulphur-tolerant vanadium passivation technology, such as BASF’s Valor, provides the best protection against contam- inant vanadium, even when processing feedstock with high sulphur content.4 The advantages of using a vanadium trap like Valor are proven in commercial FCC units. As demonstrated by Figure 4 , a European refiner was able to maintain Ecat activity despite an increase of ~1,000 ppm-wt equivalent vanadium by processing lower-cost feeds. Despite higher metals, the liquefied petroleum gas (LPG) olefins yield was maintained at similar/slightly higher levels, maximising FCC margins. Figure 5 Fundamentals of in situ FCC catalyst’s higher iron tolerance
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Catalysis 2026
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