DYNAMIC BEHAVIOR OF LAYERED COLUMNS IN PRESSURE SWING ADSORPTION

In certain adsorption processes, several different adsorbents may be u sed, each more or less specialized for a particular task of selectivel y taking up certain components. The different adsorbents may be packed in different columns, or, alternatively, in the same column as superp osed layers. In the latter case, they necessarily experience the same conditions in terms of throughput, pressure and temperature. The dynam ic behaviour, and therefore the optimal design and operation of such b eds, Is different from that of single adsorbent beds. The passage of c oncentration waves from one layer to the other and the flow reversal i nduce particular refraction, dispersion and interference phenomena. We propose a method of analysis and representation of these phenomena, b ased on pseudo-characteristics defined as lines propagating constant v alues of particular concentrations. The pseudo-characteristics do not in general coincide with the classical lines generated by the method o f characteristics within the framework of equilibrium models, except i n some ideal situations (quasi-equilibrium without dispersion and with out pressure and velocity changes). These lines can be generated by co mputer simulations using non-ideal complex models, and offer a conveni ent and compact way to analyse the start-up and convergence towards th e cyclic regime, to represent the essential features of the concentrat ion profiles at all times, to adjust the operating times and to optimi ze the layer thickness. This analysis is illustrated using the example of hydrogen purification from a four-component mixture, using a two-b ed two-adsorbent six-step PSA process.