Method of transferring particles from one pressure zone to another pressure zone

One exemplary embodiment can include a method of controlling a catalyst transfer vessel cycle of loading and unloading based on an expected level of a catalyst in a downstream vessel for a hydrocarbon conversion unit. The method can include calculating an expected catalyst level in the downstream vessel based on the unloading of the catalyst transfer vessel to adjust a setpoint to change the catalyst transfer vessel cycle of loading and unloading.