Sequential-modular programs: in which the equations describing each process unit (module) are solved module-by-module in a stepwise manner. Iterative techniques are then used to solve the problems arising from the recycle of information.

Simultaneous (also known as equation-oriented) programs: in which the entire process is described by a set of equations, and the equations are solved simultaneously, not stepwise as in the sequential approach. Simultaneous programs can simulate the unsteady-state operation of processes and equipment, and can give faster convergence when multiple recycles are present.

For a good simulation:

  1. Good vapor-liquid equilibrium (VLE) predictions
  2. Good match between the simulation and plant data
  3. Troubleshoot the simulation