We present here a mechanistic view of adsorption processes for separation of gas mixtures. The moving-bed adsorber (abandoned due to practical difficulties) is shown to offer orders of magnitude higher productivity than the conventional pressure swing adsorption (PSA) and yields 'perfect separation'. To realize its potential in fixed beds, simulated moving bed (SMB) with pressure or thermal swing and 3-bed and 4-bed PSAs that emulate moving-bed adsorbers have been proposed. Based on their characteristics of phase equilibria and separating agents employed, a classification of adsorption processes has been proposed as 'absorption like', 'distillation like' and 'chromatographic' processes, which provides insight into these processes and a rational basis for their design. The mechanism of separation in moving bed, SMB and the conventional PSA are shown to be similar to that of either absorption or distillation. On the other hand, the duplex, molecular-gate and rapid PSAs are shown to share the same separation mechanism that is akin to heat-transfer mechanism in heat regenerators. Subtle but significant differences in the role of 'reflux' in absorption-like and distillation-like processes and their implications in process design have been highlighted. Though process intensification in molecular-gate PSA and duplex PSA is modest compared to the SMB, they appear to hold promise since they are based on a proven technology. Their adaptability for absorption and extraction using immobilized solvent has been presented. (C) 2012 Elsevier B.V. All rights reserved.