A computer simulation model has been developed integrating the fluid-dynamic design with the classical physical-chemical description of distillation. The software package is solid enough to work with highly non ideal compounds. The fluid-dynamic analysis of the liquid and vapor on each tray allows to develop expressions for the holdup of liquid in trays and downcomers. The model used for the steady state Simulation of the column applies material balances, energy balances, equilibrium relationships. and pressure drop relationships, having into account tray hydraulics. Such a model, considered as a previous unavoidable step to any dynamic modelling, has been applied successfully to determine the influence Of fluid-dynamic parameters in columns working with sieve, valve, and bubble-cap trays.