Julio,
The pressure drop is selected/calculated by the process engineer, is not part of mechanical engineering! Also, the sizing is pert of process engineering followed by the mechanical design of the separator as a pressure vessel. You are asking the wrong forum...
Cheers,
gr2vessels
Gr2vessel you mean its the wrong forum because its not code specified rather process (engineer) specified. API is not inherently Mechanical Engineering.
As I recall, API 12J is just a tank with minimal internals so pressure drop should be small <1 psi if the nozzles aren't undersized. Add a demister and the drop goes up slighty. Add a filter coalescer and you start to add real pressure drop.