In stabilizing FCC unit performance when upgrading resid feeds contaminated with variable metals content (including Ni, V, Fe, Ca), a catalyst system capable of absorbing the shock from metals contamination deserves consideration. The problem was discussed with Alexis Shackelford, technical marketing specialist at BASF, as summarized in the following:
Routine monitoring of FCCU feed quality to respond proactively [to variable metals content] will help mitigate the impact of the shock [from contaminant metals present in the feedstock]. This also helps monitor other important feed quality changes, such as gravity and concarbon residue (CCR), which will immediately impact FCCU performance, in addition to the changes in the aforementioned contaminant metals.
Active catalyst management is important for mitigating changes in feed metals, particularly for maintaining catalyst activity near the optimum level. Vanadium and sodium [Na] are by far the most important metals affecting equilibrium activity. At the simplest level, adjusting the catalyst addition rate to target a constant level of Na + V on equilibrium catalyst (Ecat) will provide a reasonably stable activity.
Catalyst deactivation models have been developed that assist in more tightly controlling catalyst activity, permitting cat crackers to operate at higher rates and higher volume expansion within constraints such as wet gas compressor, air blower, coke burn and LPG yield limits.
The effect of Ni contaminants is best mitigated with antimony [Sb] passivator. It is important to continuously adjust the Sb injection rate in response to changes in feed Ni to maintain a constant ratio, typically 0.25-0.3 Sb:Ni.
Iron and Ca affect some catalysts much more than others. Selection of a catalyst with proven Fe poisoning tolerance is therefore the best solution if Fe and Ca are causing a loss of conversion.
Many units also utilize purchased Ecat or “flush” catalyst to flush metals and increase activity. Increasing the amount of flush catalyst will help mitigate the impact of each of these metals. Monitoring Ecat performance by sending samples to the supplier is useful and should be done to supplement unit data. Long-term options include ensuring the unit is using the optimal catalyst with metals passivation technology.