Not being in your field, just as a person who has done CFD in water and turbomachinary fields:
1- I don't know about separators but CFD is done when it is difficult to analyze the behavior of fluid by conventional methods or calculations. Or if the most possible realistic solution to a is necessary. Or an improved design / production is needed in terms of efficiency, cost, time or output achievement. I have not seen any code or standard that asks for a CFD as a part of their requirements.
2- The engineers who are trained to use CFD. Considering the equipments, normally it's the Manufacturers that do CFD in order to assure / improve the performance of their equipments and to seek the solutions to a better designed good. Engineers designers or Vendors are normally out of CFD unless it comes to illustrate / control / improve an engineering solution that needs to be done / verified by CFD. Since CFD softwares and more importantly the knowledge of doing CFD cost money, they can hire a some small companies that are specialized in doing CFD. Unless Engineers designers or Vendors are big companies with frequent need of doing CFD simulations so they can effort the investment in man resources AND computer hardwares AND softwares.
3- CFD is applicable to anything dealing with flow. Pumps, fans, nozzles, turbines, tanks, intakes, dams, stacks, valves, etc. The need for doing a CFD is another issue that should be decided upon the need.
In your case, why you want to do a CFD for the separator and what you want to "see" with CFD analysis? is it a new design? Please note the CFD has its own limitations and one should look to the computational domain, fluid medium, and expected outcome of CFD to make the proper decision.
