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The Gibbsian surface excess (GSE) can unequivocally measure the extent of adsorption of a gas on a solid adsorbent. A differential iso-excess heat of adsorption based on the GSE thermodynamic framework provides a physically and thermodynamically consistent description of the adsorbent heterogeneity or homogeneity. The actual amount adsorbed (AAA) cannot be experimentally measured. It must be estimated from the experimental GSE by making extraordinary assumptions regarding the nature of the adsorbed phase. Thus, the AAA is a model dependent variable. A differential iso-steric heat of adsorption based on the AAA thermodynamic framework can be misleading and artificial. The NET amount adsorbed (NAA) is an experimental variable. It is also directly related to GSE. Thus, it provides no further insight into the adsorption system vis a vis the GSE. A differential iso-NET heat of adsorption based on the NAA thermodynamic framework can be misleading and fictitious.

Separation and purification of multicomponent gas mixtures by adsorption is an established process technology. Adsorptive process design requires accurate data on multicomponent gas adsorption equilibria, kinetics, and heats as input variables. These data often cannot be predicted by using today's models, particularly for complex practical systems where the gas mixtures have adsorbates of different sizes and polarities, and the adsorbent is energetically heterogeneous. There is a large volume of pure gas and some binary gas adsorption equilibrium and kinetic data in the published literature, but multicomponent adsorption data are rare. The data for heats of adsorption are only emerging. There is a desperate need to generate a multicomponent adsorption database for better understanding of the complex phenomenon, for seriously testing existing models, and for development of new models. Two recently developed methods called “isotope exchange technique” and “microcalorimetry for adsorption heats” are recommended for measurement of the multicomponent data. Needs to satisfy thermodynamic consistencies by binary gas equilibrium data and the equilibrium models are emphasized.

Abstract: An analytical mathematical model is used to investigate the influence of minute adsorbent temperature changes on the kinetics of sorption of a gaseous adsorbate from mixture with a carrier gas using a differential closed loop recycle method. Isothermal operation may not be achieved even when a very high gas circulation rate is used. Very small changes in the adsorbent temperature during the process can cause substantial departure from isothermal uptake behavior. It is shown that the kinetic process can be assumed to be isothermal only for trace adsorbate concentrations. A criterion for validity of isothermal data analysis is proposed.

Abstract??An analytical mathematical model is used to investigate the effectiveness of forced convection for removal of the heat of adsorption from an adsorbent mass undergoing a differential adsorption process in a flow system. An example of such a process is measurement of gas adsorption kinetics using a differential adsorption bed. Isothermal operation may not be achieved even when a high gas flow rate is used, particularly if the sorption kinetics is relatively fast. Very small changes in the adsorbent temperature can cause significant departure from the isothermal uptake behavior when the heat of adsorption is moderately large. A criterion for validity of isothermal data analysis is given.

Abstract A simple analytical model of a Differential Pressure Swing Adsorption (DPSA) process on a single adsorbent particle was used to evaluate the effects of gas-solid heat transfer resistance on the cyclic working capacity of the particle. The commonly used assumption of instantaneous thermal equilibrium between the gas and the adsorbent inside an adsorber may not be valid when the gas flow rates are low, the adsorption kinetics is relatively fast, and the PSA cycle times are small.

Two pressure swing adsorption processes for separation of methane and carbon dioxide gas mixtures are described. One process simultaneously produces a high purity CH4 and a high purity CO2 product with high recoveries of both components from the feed gas. The other process only produces a high purity CH4 product with high recovery. Test data for these processes are reported and their relative advantages are discussed.

Pressure swing adsorption (PSA) processes for air separation differ by the modes and conditions of operation of the adsorption, the desorption, and the complementary steps, as well as by the types of adsorbents used. Three commercial PSA processes for air separation are reviewed and compared. The first process uses a zeolitic adsorbent and produces only an oxygen-enriched product gas. The second process uses a carbon molecular sieve and produces only a nitrogen-enriched product gas. The third process uses a zeolite and simultaneously produces both oxygen-and nitrogen-enriched product gases. The performance and separation efficiency of the last process, called the 'vacuum swing adsorption (VSA) process', are reported to be superior to the others.

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.

The effects of the gas-solid mass (k) and heat (h) transfer coefficients on the column dynamics for Type II adsorption system were analyzed by numerically solving the conservation equations for a single sorbate. Langmuir equilibrium and the linear driving force model for mass transfer were assumedThe transfer zone is larger in an adiabatic column than the corresponding zone in an isothermal column even for a small rise in column temperature. Initially proportionate pattern transfer zones are formed in both the adiabatic and the isothermal columns. As the column length increases these profiles approach to the constant pattern limit. Column adiabaticity slows down the rate of approach to constant pattern limit. On the other hand, higher adsorbate concentration and high k enhance this rate. The symmetry of the breakthrough curve may be distorted in an adiabatic column. The effects of k and h on the size of the mass transfer zone are interchangeable. Consequently, estimation of k from experimental breakthrough data by assuming thermal equilibrium between gas and solid phases may be erroneous.

Two new pressure swing adsorption processes for the simultaneous production of hydrogen or ammonia synthesis gas and carbon dioxide from a reformer off-gas feed are described. Both products are produced at high purity and recovery. Performance data for these processes are reported.