We face the same situation some years ago. We turn from a non hydrotreated VGO to a hydrotreated feed.
For sure your conversion is going to increase due to the higher crackability of the hydrotreated feed. In order to maintain the same conversion and not lose distillates production, you can apply one or several of the following alternatives:
– Decrease C/O (increase feed preheat temperature to the maximum limit)
– Decrease reaction temperature
– Decrease catalyst activity (MAT), decreasing frash catalyst addition rate
Other potential limitation you could face is problems in heat balance closure. If you are currently runing for FCCU at low severity (maximum distillates) and your regenerator temperature is close to minimum, with the hydrotreated feedstock, feed concarbon will be lower and your regenerator temperature could be even lower (below minimum).
If you have this problem and you have the possibility to recycle slurry, it could help you to compensate coke deficit (close heat balance) and increase LCO production.
Finally, if after these changes your conversion is still higher that desired, you could reformulate or change your current FCC catalyst. Catalyst manufacturers normally have catalyst specially designed to run in these scenarios. You have an excellent catayst to run in middle distillates mode in a hydrotreated scenario (Mild Hydrocracker residue)