Normally dynamic simulations are performed before the commissioning to find out possible upsets situations and mitigation ways.
During troubleshooting and debottlenecking, process simulation model is built for the unit operation investigate the possible cause of bottleneck. Actual plant operating data is matched in the process simulation model. The deviations from the original plant design are reviewed by comparing the actual plant operation model and original process simulation case. The affected equipment and units operations (vessels, pumps, compressors, distillation column, control valves, etc) are checked for size for actual plant operations. Possible bottleneck are identified and those are resized for current plant operations as well as future expected operations.