Effect of the process parameters on the length of the mass transfer zone during product withdrawal in pressure swing adsorption cycles

The effects of several process parameters on the length of the mass transfer zone (MTZ) during product withdrawal in equilibrium controlled pressure swing adsorption (PSA) operations are investigated in this study. The process parameters under consideration are the adsorbent selectivity, the initial MTZ length, the product purity, and the adsorption pressure. For locally equilibrated PSA processes the analytical solution for binary mixtures is used to develop analytical expressions for the dependence of the MTZ length on the above mentioned parameters. For equilibrium controlled PSA processes with modest mass transfer limitations the evolution of the MTZ length during product withdrawal is examined through numerical simulations using a linear driving force (LDF) model. It is observed that the trends exhibited by the locally equilibrated PSA processes pertain to PSA processes with modest mass transfer limitations as well. It is found that the length of the MTZ during product withdrawal is strongly affected by the adsorbent selectivity when the selectivity is below four, and by the initial MTZ length. The length of the MTZ during product withdrawal is modestly affected by the adsorbent selectivity when the selectivity is above eight, and by the product purity. The length of the MTZ during product withdrawal is not affected at all by the adsorption pressure for locally equilibrated PSA processes. For PSA processes with modest mass transfer limitations the length of the MTZ during product withdrawal is only mildly affected by the adsorption pressure. These results enhance the theoretical understanding of the PSA processes. They can be used in PSA design to optimize the bed size and reduce the adsorbent inventory and the pressure drop.