for designing gas processing equipment such as separators and flash drums etc. a process simulator would be ok
Win sim is low cost and affordable
hysys is costly and complicated and not necessarily more accurate

FOR DESIGNING FUEL CONDITIONING SKIDS ETC YOU NEED A SOFTWARE THAT CAN MODEL CONTROL VALVES, PREDICT AT WHAT FLOW HOW MUCH OF THE CONTROL VALVE WILL OPEN AND ALSO BE ABLE TO PREDICT CAVITATION AND RELIABILITY OF THE CHOSEN CONTROL VALVE. ANOTHER VERY IMPORTANT FACTOR IN DESIGNING FUEL CONDITIONING SKIDS IS THE ABILITY TO PREDICT THE TOTAL PRESSURE DROP IN THE VARIOUS PIPE SEGMENTS AND FITTINGS SUCH AS REDUCERS, EXPANDERS,NOZZLES, BENDS, ELBOWS, BALL VALVES, PLUG VALVES ETC. WITHOUT THIS CAPABILITY THE SOFTWARE WILL NOT BE OF ANY HELP IN DESIGNING FUEL CONDITIONING SKIDS. BOTH WINSIM AND HYSYS ARE LACKING IN THIS RESPECT