**3.2 Model solution**

Effective diffusion coefficient of each component directly affects the time to reach the balance for the whole system during the calculation procedure. There is no absolutely accurate general calculation equation to calculate the diffusion coefficient of i-component in oil phase and gas phase, except using the empirical equation which is a relatively accurate method. The diffusion factor of i-component in oil phase usually is usually calculated by Will— Chang(1955) and that in gas phase by Chapman-Enskog empirical formula (1972). The initial K value of each component is calculated by Wilson function, and corrected by fugacity coefficient in every time step, while fugacity coefficient is calculated by PR-EOS. Compared with the computation model proposed for single component, the model is much closer to the actual simulation, since it has taken interaction among the components into consideration.
