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Highlights • The GSE is the only true experimental variable to quantify the extent of adsorption from a fluid (gas or liquid) on a solid adsorbent. • The concept of GSE is applicable to all types of adsorbents – porous or nonporous, homogeneous or heterogeneous. • All practically useful aspects of adsorption thermodynamics (e.g. consistency tests, heat of adsorption, heat capacity of adsorption system, etc.) and mass transfer (e.g. linear driving force model) can be described using the GSE framework. • An ideal adsorbed solution concept to estimate multicomponent gas GSE isotherms from the corresponding pure gas GSE isotherms can be postulated. • Numerical adsorptive separation process models can be formulated using the GSE framework. There is no need to calculate the actual amount adsorbed which can be misleading. Abstract The GSE is the only experimental variable to unambiguously quantify the extent of multicomponent fluid (gas or liquid)-solid adsorption. It can be directly used to describe all practically important thermodynamic and dynamic characteristics of an adsorption system as well as to formulate the relevant mass and heat conservation equations needed for design of adsorptive processes for separation and purification of fluid mixtures. [ABSTRACT FROM AUTHOR]

A practical perspective of the complex and extensive subject of adsorbate mass transfer into porous adsorbents, which is important for design of adsorptive processes for separation of fluid (gas or liquid) mixtures, is given. The state of the art for the subject may not allow reliable a priori estimation of the adsorbate mass transfer coefficients for practical adsorbents and experimental measurement of these variables may generally be needed for each specific system of interest. The Linear Driving Force (LDF) model of adsorbate mass transport may be adequate for most practical systems of interest. The LDF model is compatible with the Gibbsian surface excess frame - work of adsorption and that may have been inadvertently used in the past works. [ABSTRACT FROM AUTHOR]

A novel design of a compact rapid pressure swing adsorption system consisting of a single adsorber enclosed inside a product storage tank is proposed for application as a medical oxygen concentrator (MOC). A self-contained test unit for the process is constructed which is capable of directly and continuously producing 1-3 sl/m of 90% O2 from compressed air. Pelletized LiLSX zeolite is used as the air separation adsorbent. Steady state process performance data [bed size factor (BSF) and O2 recovery (R) as functions of total cycle time (tc)], as well as transient, cyclic, adsorber pressure, and temperature profiles are presented. A four-step Skarstrom-like pressure swing adsorption cycle was used. Two options for column pressurization, (a) using compressed feed air cocurrently or (b) using a part of the oxygen-enriched product gas counter-currently were evaluated. Option (b) exhibited superior performance. The optimum total cycle time for option (b) was 5-6 s where the BSF was lowest (∼45 kgs/TPD O2) and the corresponding R was ∼29.3%. These numbers indicate that the adsorbent inventory of a MOC can be potentially reduced by a factor of three while offering a ∼10-20% higher O2 recovery compared to a typical commercial unit. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3330-3335, 2014 [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]

Abstract: The isosteric heat of adsorption is a critical design variable in estimating the performance of an adsorptive gas separation process. The heats can be strong and complex functions of adsorbate loadings when the adsorbent is energetically heterogeneous. Ignoring these characterisitics in process design can lead to serious errors. Only calorimetric heat measurements (pure and multi-component gas) can reveal the complex nature of the adsorbent heterogeneity. Examples of calorimetrically measured heats for adsorption of pure SF6 and CO2 on a silicalite sample bonded with alumina, and those for binary CO2–C2H6 mixtures on NaX zeolite are cited to demonstrate the complexity of the subject. The loading dependence of the binary heats is found to be counter-intuitive. A simple analytic thermodynamic model of patchwise heterogeneity is proposed to describe the isosteric heat of adsorption of a single gas and those for the components of a binary gas mixture. [Copyright &y& Elsevier]

• The pure and multicomponent fluid (gas and liquid) – solid adsorption equilibria is a core variable for establishing the performance of an adsorptive separation process as well as for selection of an optimum adsorbent for a separation duty. Factors like the nature of the adsorbate-adsorbent system, the system temperature, the adsorbent porosity and the energetic homogeneity or heterogeneity of the adsorbent govern the shape of an adsorption isotherm. • The equilibrium adsorption isotherms are used to determine (a) the selectivity of adsorption between the components of a mixture, (b) the heat effects associated with the ad(de)sorption process, and (c) the driving force for adsorbate transport from the bulk fluid phase int the adsorbent particle. Analytical adsorption isotherm models are required for calculation of these properties and to facilitate numerical solution of a process performance model. • The isotherm models must satisfy several physical and thermodynamic constraints to be practically useful. • A list of analytical isotherm models covering porous and non-porous and homogeneous and heterogeneous adsorbents is presented. • Estimation of multicomponent adsorption equilibria from the corresponding pure component isotherms employing various engineering models and use of characteristic curves for coalescing isotherms of different gases at different temperatures on the same adsorbent for predictive purposes should be handled with care. These procedures must be validated using experimental data for the system of interest before their use. The fluid (gas or liquid) – solid adsorption equilibrium is a core thermodynamic property of an adsorbate-adsorbent system for establishing the efficiency of separation of a fluid mixture by an adsorptive process and for selection of an optimum adsorbent for a specified separation duty. The shape of an equilibrium adsorption isotherm of a pure gas or those for the components of a gas mixture are determined by the nature of the adsorbate-adsorbent pair, the system temperature, the porosity of the adsorbent, and the energetic heterogeneity of the adsorbent. The same factors also determine the shape of an equilibrium isotherm of a binary liquid mixture. The published literature on this subject is vast. The purpose of this article is not to review the literature but to provide a systematic practical perspective of the subject. Analytical models for fluid-solid adsorption isotherms facilitate numerical simulation of separation process performance using mathematical process models. However, the practically viable isotherm models must obey several physical and thermodynamic constraints. A selection of such isotherm models for pure and multi-component gas and binary liquid mixtures is presented. It may not, however, be possible to a priori chose a reliable isotherm model for a system of interest without extensively testing the model using experimental isotherm data for that system. The same conclusion applies to theoretical concepts for (a) predicting multi-component gas-solid adsorption equilibria from the corresponding pure gas adsorption equilibria, (b) coalescing adsorption isotherms of different pure gases at different temperatures and (c) coalescing isotherms for adsorption of trace components from a mixture (gas or a liquid) into a single characteristic curve on a given adsorbent for predictive purposes. [ABSTRACT FROM AUTHOR]

New experimental data are reportedfor equilibrium adsorption isothermand mass transfer of pure argon on a sample of pelletized lithium-exchangedlow silica X (LiLSX) zeolite. Model analysis of the data indicatethat the zeolite behaved like a nearly homogeneous adsorbent for Aradsorption while it exhibited substantial heterogeneity for adsorptionof N2and milder heterogeneity for adsorption of O2. The overall mass transfer coefficient for Ar adsorptionwas comparable in magnitude with those of N2and O2. The coefficient increased with increasing pressure and decreasedwith increasing temperature like those for N2and O2. A large skin resistance at the adsorbent particle surfacewas observed for Ar mass transfer like that for adsorption of O2and N2. [ABSTRACT FROM AUTHOR]

Effective mass transfer coefficients(κe,s–1) for adsorptionof pure N2and O2into small particles of apelletized sample oflithium exchanged low silica X (LiLSX) zeolite were estimated as functionsof gas pressure (P) and temperature (T) by analysis of experimental column dynamic data using a specificprotocol based on assumptions of a constant pattern mass transferzone (MTZ) formation and a linear driving force model for mass transport.New column breakthrough data at different Pand Tfor displacements of pure inert helium by pure N2or O2in an isobaric–isothermal adsorber wereused. The mass transfer coefficients decreased linearly with increasing Pand increased with increasing T. Datainterpretation using a “resistances in series” modelfor isothermal–isobaric mass transport of an adsorbate havinga linear adsorption isotherm into a pelletized zeolite particle suggestedthe presence of a skin resistance at the pellet surface, which incombination with resistances created by axial dispersion in the adsorbergas phase and external film contributed to most of the overall masstransfer resistance under the test conditions. The Pand Tdependence of kewere consistent with that observation. [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]

Reduction of the adsorber size of a medical oxygen concentrator (MOC) employing a generic pressure swing adsorption (PSA) technology is an ongoing research and development activity. The MOC typically produces a 90–93% O2-enriched product gas from ambient air at a rate of ≤10 L/minute (LPM) for individual use. A common practice is to reduce the total cycle time (tc, seconds) of the PSA process in order to decrease the bed size factor [BSF, pounds of adsorbent in the PSA unit per ton of contained O2per day production rate (TPDc)]. Adsorbent columns packed with very small adsorbent particles are used to enhance the adsorption kinetics for rapid PSA cycle operation. An experimental mini-PSA set up was used to measure the performance of a simulated Skarstrom-like PSA cycle using a commercial sample of LiX zeolite as the air separation sorbent. Different adsorbent particle sizes, adsorption pressures, and cycle times were tested. It was experimentally demonstrated that BSF cannot be indefinitely reduced by lowering tcbecause of finite adsorbate mass transfer and gas–solid heat transfer resistances, as well as column pressure drop during the desorption step. A BSF of ∼25–50 lbs/TPDcwith an O2recovery of ∼25–35% for production of ∼90% O2could be achieved by the PSA process using a dry, CO2-free air feed at a pressure of 3–4 atm, an adsorbent particle size of ∼0.35 mm, and a total cycle time of 3–5 s. A novel ‘snap on’ concept of a truly compact and portable MOC was proposed. [ABSTRACT FROM AUTHOR]