Never operated below 60% of nominal. The problems that we encounter are the ones you have described.
1) Low flow rate to feed injectors
2) Low flow rate to the main fractionator
3) Low flow air blower
4) Slide Flue gas valve closed too (poor control)
5) Low flow rate of the slurry and difficult to manage the temperature of the column bottom
6) Possible loss of catalyst
Obviously we always try to handle the problems, but have never managed to reduce the technical minimum below 60% of nominal (UOP FCCU side by side 36,000 b / d)