In establishing the mechanistic dynamic model of flash tank

a commonly used industrial chemical unit

the liquid-phase characteristic equation is differential

while the gas-phase characteristic equation is algebraic

treated as "quasi-steady-state". Therefore

the realistic dynamic response of pressure can not be obtained

but is completely attached to the dynamic response of the liquid-phase. In this paper

the algebraic equation of pressure was replaced with dynamic differential equation and the model to two-time scale was established. The singular perturbation method was used to precisely calculate the full-order model

and compared with the quasi-steady-state model. The control system for the established two-time scale model was designed. Compared with the performance of quasi-steady-state model

the full-order model provided better control performance

which also illustrated the rationality of modeling and its practical significance.