A simplitied dynamic model for a PSA air separation process is developed based on linearized mass transfer rate expressions and binary Langmuir equilibrium. Constant pressure is assumed during adsorption and desorption steps but the variation in flow rate through the column due to adsorption is aeeounted for. The model predictions, using independently measured kinetic and equilibrium data are compared with experimental results obtained in a simple two-bed air separation PSA system packed with a carbon molecular sieve adsorbent. The model is shown to provide a good representation of the experimentally observed behaviour over a wide range of conditions. Pressure swing adsorption (PSA) processes, first developed by Skarstrom (1959,1960,1972), in the form of the 'heatless drier' have found widespread application in hydrogen purification and air separation, as well as in air drying. The rational design and optimization of such processes requires a detailed mathematical model since the effects of the process variables (flow rates of purge and feed, cycle time, bed length and pressure ratio) are strongly coupled, making it difficult to arrive at an optimal combination purely by empiricism and intuition. The earlier mathematical models (Shendalman and Mitchell, 1972; Chan et al. 1981; Knaebel and Hill, 1982) were based on equilibrium theory and the effects of mass transfer resistance were ignored. The validity of such an approximation has been confirmed experimentally for air separation on a 5A zeolite adsorbent by Flores-Femandez and Kenney, 1983. However, for adsorption of CO2 on silica gel (Mitchell and Shendahnan, 1973) and C2HL on both 4A and 5A zeolites (Hassan et al., 1985)
experimental PSA studies reveal rather large deviations from the equilibrium theory predictions, suggesting that, at least for these systems, kinetic effects are probably important. Although equilibrium theory can provide valuable guidance as to the range of acceptable operating conditions and the limits of performance, the quantitative value of such models is therefore limited. Furthermore, the equilibrium theory approach is obviously restricted to systems in which the adsorptive selectivity depends on a difference in equilibrium and it is clearly not applicable to systems, such as air separation on a carbon sieve. The adsorption equilibrium isotherms for oxygen and nitrogen on carbon molecular sieves are almost identical and the preferential sorption of oxygen in these adsorbents is of kinetic origin, resulting from the higher micropore diffusivity of that species.