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The existence of a skin resistance for adsorbate mass transport at the surface of a pelletized adsorbent particle or at the surface of adsorbent crystals within a particle has been proven by many macroscopic and microscopic experiments. An isothermal and isobaric column dynamic test method may be used to approximately estimate the relative magnitude of the skin resistance. [ABSTRACT FROM AUTHOR]

It is demonstrated that various combinations of models describing the adsorbed phase volume and the actual amount adsorbed (AAA) can be used to describe the experimental Gibbsian surface excess isotherm data for adsorption of a pure gas. Thus, the resulting AAA isotherm and the other adsorptive properties estimated by this approach can be ambiguous and artificial. [ABSTRACT FROM AUTHOR]

The effect of adsorbent heterogeneity on the performance of an adiabatic pressure swing adsorption (PSA) process for separation of bulk C2H4 from an inert gas (helium) using the BPL carbon as the adsorbent was numerically estimated employing a detailed mathematical model for the process. The heterogeneous Toth model was used to describe the equilibrium adsorption isotherms for C2H4 and a linear driving force model was used to describe the adsorbate mass transfer rate in the simulation. The variations in the isosteric heat of adsorption of C2H4 with adsorbate loading was included in the simulation. The study indicates that adsorbent heterogeneity negatively affects the key process performance variables by increasing the bed size factor and reducing the helium recovery. Actual performance data for a PSA process using a bench or pilot scale unit is required to reliably estimate the complexity introduced by adsorbent heterogeneity. [ABSTRACT FROM AUTHOR]

An alternative to the conventional 'pore filling model' for describing Gibbsian surface excess (GSE) isotherm from a liquid mixture on a porous adsorbent is proposed where the adsorbed phase volume is treated as a variable. The new model is tested using GSE isotherm data on various micro and mesoporous adsorbents. The adsorbed phase volume is found to be system specific but always less than the adsorbent pore volume, irrespective of the pore size. [ABSTRACT FROM AUTHOR]

The physical and thermodynamic characteristics of the Langmuir adsorption isotherm model for pure gas and gas mixtures as well as the constraints on the saturation adsorption capacity of the gases by this model are discussed. Arbitrary violation of these constraints, viz. temperature dependence of the saturation capacity and unequal saturation capacities for the components of a mixed gas, may lead to unrealistic heat of adsorption and unreliable predictions of mixed gas equilibria. Several physically and thermodynamically consistent analytical models for pure and mixed gas adsorption isotherms, which were developed using the Langmuir model as backbone, and which account for unequal -sized adsorbates, lateral interactions between adsorbed molecules, multi-layer adsorption, and adsorbent heterogeneity, are briefly reviewed. These models can be practically very useful to describe experimental isotherm data and for adsorptive process modeling. [ABSTRACT FROM AUTHOR]

The required durations of pressurization and depressurization steps of a rapid pressure swing adsorption process are primarily governed by adsorbent particle size, adsorption kinetics, column pressure drop, column length to diameter ratio, and the valve constant of the gas inlet and outlet control valve attached to the adsorbent column. A numerical model study of the influence of these variables for an adiabatic LiX zeolite column is presented using pure N as an adsorbate gas. An adsorbent particle size range of 200-350 μm was found to minimize (<1 s) the times required for the pressurization and depressurization steps. [ABSTRACT FROM AUTHOR]

Efficient desorption of selectively adsorbed N from air in a packed column of LiX zeolite by rapidly purging the adsorbent with an O enriched gas is an important element of a rapid cyclic pressure swing adsorption (RPSA) process used in the design of many medical oxygen concentrators (MOC). The amount of O purge gas used in the desorption process is a sensitive variable in determining the overall separation performance of a MOC unit. Various resistances like (a) adsorption kinetics, (b) column pressure drop, (c) non-isothermal column operation, (d) gas phase mass and thermal axial dispersions, and (e) gas-solid heat transfer kinetics determine the amount of purge gas required for efficient desorption of N. The impacts of these variables on the purge efficiency were numerically simulated using a detailed mathematical model of non-isothermal, non-isobaric, and non-equilibrium desorption process in an adiabatic column. The purge gas quantity required for a specific desorption duty (fraction of total N removed from a column) is minimum when the process is carried out under ideal, hypothetical conditions (isothermal, isobaric, and governed by local thermodynamic equilibrium). All above-listed non-idealities (a-e) can increase the purge gas quantity, thereby, lowering the efficiency of the desorption process compared to the ideal case. Items (a-c) are primarily responsible for inefficient desorption by purge, while gas phase mass and thermal axial dispersions do not affect the purge efficiency under the conditions of operation used in this study. Smaller adsorbent particles can be used to reduce the negative effects of adsorption kinetics, especially for a fast desorption process, but increased column pressure drop adds to purge inefficiency. A particle size range of ∼300-500 μm is found to require a minimum purge gas amount for a given desorption duty. The purge gas requirement can be further reduced by employing a pancake column design (length to diameter ratio, L/ D<0.2) which lowers the column pressure drop, but hydrodynamic inefficiencies (gas mal-distribution, particle agglomeration) may be introduced. Lower L/ D also leads to a smaller fraction of the column volume that is free of N at the purge inlet end, which is required for maintaining product gas purity. The simulated gas and solid temperature profiles inside the column at the end of the rapid desorption process show that a finite gas-solid heat transfer coefficient affects these profiles only in the purge gas entrance region of the column. The profiles in the balance of the column are nearly invariant to the values of that coefficient. Consequently, the gas-solid heat transfer resistance has a minimum influence on the overall integrated N desorption efficiency by O purge for the present application. [ABSTRACT FROM AUTHOR]

The effects of the sorption and the regeneration temperatures on the performance of a novel rapid thermal swing chemisorption (RTSC) process (Lee and Sircar in AIChE J. 54:2293–2302, ) for removal and recovery of CO2 from an industrial flue gas without pre-compression, pre-drying, or pre-cooling of the gas were mathematically simulated. The process directly produced a nearly pure, compressed CO2 by-product stream which will facilitate its subsequent sequestration. Na2O promoted alumina was used as the CO2 selective chemisorbent, and the preferred temperatures were found to be, respectively, 150 and 450 °C for the sorption and regeneration steps of the process. The specific cyclic CO2 production capacity of the process and the pressure of the by-product CO2 gas were substantially increased over those previously achieved by using the sorption and regeneration temperature of, respectively, 200 and 500 °C (Lee and Sircar in AIChE J. 54:2293–2302, ). The net compressed CO2 recovery from the flue gas (∼92%) did not change. However, substantially different amounts of high and low pressure steam purges were necessary for comparable degree of desorption of CO2. A first pass estimation of the capital and the operating costs of the RTSC process was carried out for a relatively moderate size application (flue gas clean up and CO2 recovery from a ∼80 MW coal fired power plant). Both costs were substantially lower than those for a conventional absorption process using MEA as the CO2 solvent (Desideri and Paolucci in Energy Convers. Manag. 40:1899–1915, ). [ABSTRACT FROM AUTHOR]