The separation of a binary system by Molecular Sieve as conceived consists of the following steps.
1. Separation of the mixture by adsorption at high pressure.
2. Desorption of adsorbed gas mixture rich in component a by lowering the system pressure （blowdown）.
3. Furher desorption by purging the residual adsorbed gas mixture with product mixture which is lean in component a.
4. Repressurization of the bed either with the product mixture or the air after which the cycle repeats.
This work discusses a method of predicting the performance of the third step when pure component isotherms and the knowledge of relative adsorptivity are available. A numerical example is given to illustrate the method and compared with the experimental findings.