Catalysis 2024 Issue

Passivating vanadium in FCC operations

Three in-unit trials demonstrated ability to control various oxidation states of vanadium with strong passivation technology

Corbett Senter, David M Stockwell, Bingliang Liu and Benjamin O’Berry BASF

S ixty per cent of fluid catalytic cracking (FCC) units are faced with the challenge of processing feedstocks containing at least some resid as a fraction of the feed. Vanadium passivation and resid catalyst technology are crit - ical to minimising the negative impacts of vanadium in these resid feeds since, compared to processing vacuum gas oil (VGO), resid feeds pose an additional challenge of dealing with the harmful effects of contaminants. Vanadium enters the FCC unit in porphyrin complexes present in the feed and then deposits on FCC catalyst as feed is cracked. As catalyst circulates through the FCC, the inventory moves between the riser, a reducing environment, and the regenerator, an oxidising environment containing steam and SOx. This means that, at any given time, vana - dium in the FCC exists in a variety of oxidation states, giving rise to vanadium’s notorious and detrimental mobility within the catalyst circulating inventory. The most pronounced impact of vanadium is illustrated in Figure 1. This figure shows BASF benchmarking data of FCC equilibrium catalyst (Ecat) activity vs the vanadium and sodium level on ECat collected from FCC units across the world. While there is a large amount of scatter due to the number of variables impacting Ecat activity, these data help visualise a trend well understood in the FCC industry: higher vanadium and sodium result in lower catalyst activity (all else being equal). Numerous studies by various researchers around the world describe the destructive impact of vanadium in FCC, as illustrated in Figure 1. One study demonstrated that the mechanism for the destruction of Y-zeolite in FCC catalyst is the result of a reaction involving vanadic acids, sodium, and steam. Thus, the relationship between activity and the vanadium and sodium level seen in Figure 1 is expected. Vanadium presents an additional challenge as a dehy - drogenation catalyst when in the oxidised form. While this dehydrogenation impact is not as pronounced as with nickel, it can still lead to increased hydrogen and delta coke, poten - tially introducing unit limitations due to wet gas constraints, elevated regenerator temperatures, and decreased produc - tion of valuable liquid products. While one solution to maintaining Ecat activity with higher vanadium levels is to increase fresh catalyst additions, this may not always be ideal. Increased catalyst additions result

Vanadium + sodium (ppm)

Figure 1 Benchmarking data of Ecat activity vs vanadium and sodium

in increased operating cost for FCCs; furthermore, catalyst additions are ultimately limited by the capabilities of an FCC unit’s fresh catalyst loader. Fortunately, there are several catalyst design options to counteract the negative impacts of vanadium and help mitigate extra fresh catalyst additions needed to maintain activity in the face of elevated vanadium levels. To a point, the zeolite amount or rare earth to zeolite of the FCC cat - alyst can be increased to improve activity maintenance of the catalyst. Additionally, nickel (Ni) passivation could be used to reduce delta coke resulting from any nickel con - taminant present. However, there are limitations to each of these techniques, and vanadium passivation technologies have proven to be a preferred way to deal with moderate to high levels of vanadium. Vanadium passivation and sulphur tolerance Vanadium passivators, often referred to as vanadium ‘traps’, react with vanadium in the FCC to immobilise and passivate the metal. This prevents vanadium from participating in reactions that generate hydrogen and coke or destroy zeolite and catalyst activity. Early versions of vanadium traps used alkaline earths, such as CaO or MgO, as active sites to react with vanadium. However, a competing reaction exists, which can hinder vanadium passivation. SOx present in the FCC regenerator can react with alkaline

Catalysis 2024