Depending on what you mean by 'simulation', there could be a huge number of calculations. Each of those calculations might feed back into an updated simulation, for example:
* Column internal sizing, tray efficiency calculations
* Heat exchanger detailed thermal design, vibration analysis
* Pressure drop calculations on all your equipment once the basic design design has been done and maybe again with detailed design
* Dynamic blowdown simulations
Then once the plant is built, reconciling the simulation against a test run both for any process guarantees and to improve it for the next one. Then the proud new plant owner wants to debottleneck or change feedstock...
Process simulation is not an end in itself, to be done at one stage. It's a tool that gets used throughout the design/build/operate cycle.
Regards,
Matt