not sure i would suggest a radiator for a water separator for a few reasons
1- contanimation from oil used in cutting and threading pipe used to connect radiator.
2- rust scale and debris can work lose ruining tools or paintjob
3- not sure if a radiator is designed to hold that kind of pressure. most steam or hot water systems only operate at a few psi not 100 psi or more. not really sure if this is safe