Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

Entropy generation minimization (finite time thermodynamics, or thermodynamic optimization) is the method that combines into simple models the most basic concepts of heat transfer, fluid mechanics, and thermodynamics. These simple models are used in the optimization of real (irreversible) devices and processes, subject to finite‐size and finite‐time constraints. The review traces the development and adoption of the method in several sectors of mainstream thermal engineering and science: cryogenics, heat transfer, education, storage systems, solar power plants, nuclear and fossil power plants, and refrigerators. Emphasis is placed on the fundamental and technological importance of the optimization method and its results, the pedagogical merits of the method, and the chronological development of the field.

Entropy generation minimization: The new thermodynamics of finite-size devices and finite-time processes

Preface. 1. Introductory Remarks. 2. Fundamental Factors for Designing Adsorbent. 3. Sorbent Selection: Equilibrium Isotherms, Diffusion, Cyclic Processes, and Sorbent Selection Criteria. 4. Pore Size Distribution. 5. Activated Carbon. 6. Silica Gel, MCM, and Activated Alumina. 7. Zeolites and Molecular Sieves. 8. &pi -Complexation Sorbents and Applications. 9. Carbon Nanotubes, Pillared Clays, and Polymeric Resins. 10. Sorbents for Applications. Author Index. Subject Index.

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Adsorbents: Fundamentals and Applications

A pressure swing adsorption cycle comprised of blowdown, purge, pressurization, feed, pressure equalization and rinse steps provided recovery from an atmospheric air feed, essentially dry and free of carbon dioxide, of a high yield of high purity nitrogen gas and a high yield of a product gas rich in oxygen as well as recovery of a residual feed byproduct gas for recycle with the air feed.

Pressure swing adsorption

Two flexible metal-organic frameworks are presented as solid adsorbents for methane that undergo reversible phase transitions at specific methane pressures, enabling greater storage capacities of usable methane than have been achieved previously, while also providing internal heat management of the system. Natural gas — methane — is a clean and cheap fuel but its usefulness in transport applications is limited by storage problems, given its low energy density per unit volume under ambient conditions compared with petrol or diesel. One way of increasing methane storage capacity is to use tanks containing porous materials, such as metal–organic frameworks, as a storage medium. However, for every methane molecule adsorbed and desorbed there is an associated thermal fluctuation that could cause overheating or reduce storage efficiency if left unchecked. Here Jeffrey Long and colleagues describe two flexible metal–organic frameworks that undergo reversible phase transitions at specific methane pressures, enabling greater storage capacities of usable methane than have been achieved previously, while also providing internal heat management of the system. As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector1,2. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage3. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures4. Although activated carbons, zeolites, and metal–organic frameworks have been investigated extensively for CH4 storage5,6,7,8, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal–organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp2− = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp ‘step’. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents9, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.

Methane storage in flexible metal–organic frameworks with intrinsic thermal management

Non-linear unsteady processes, as different as car traffic, adsorption, sedimentation, packed-bed heat transfer and hydraulic waves, can be analysed in a unified fashion by using the concept of the travelling wave. The fundamental aspects of the ‘solitary’ waves are discussed, as well as their distinctive features, such as overall and local propagation velocities, shape modifications, appearance of shocks, and ‘coherent’ asymptotic shape. These features are governed by the velocity distribution along the wave, itself determined by the relationship between the flux and the concentration variables of the process. Qualitative rules result from the analysis of ‘operating lines’ and ‘equilibrium lines’ in the flux versus concentration diagram, in a way somewhat similar to the McCabe-Thiele analysis for steady counter-current operations.

Unifying concepts in non-linear unsteady processes Part I: Solitary travelling waves

This paper deals with pressure-swing-adsorption (PSA) experiments for removing CO2 and CH4 from a mixture with an ‘inert’ such as hydrogen or helium. Specific features are quasi cyclic steady-state and no breakthrough of the adsorbed impurity during the adsorption-production step, implying that information has to be taken inside the column, here by a set of temperature measurements. The purpose is to propose an approach for evaluating the effect on the behaviour and performance of different working conditions (feed composition, temperature, conditions of regeneration, adsorbent) under conditions of optimal bed length. We define a new criterion to represent the effective sorption capacity required in adsorption, which we call critical bed fraction (CBF), defined as the fraction of bed length occupied by the temperature and concentration fronts at incipient breakthrough of the less adsorbed component, at cyclic steady-state. The effects of the different parameters on performance (recovery and productivity) may then be expressed and analysed in terms of CBF. Some noteworthy observations are the following: for a given amount of purge, the flow velocity in the purge step may have an optimal value; a relatively high concentration of CH4 may have positive effects on both recovery and productivity when used with a high purge velocity, because it contributes to a better desorption of CO2; the advantages of an adsorbent having a high capacity per unit mass and fast internal kinetics may be offset if the density is low, implying a low capacity per unit volume.

Effects of Composition, Temperature and Purge on the Performance of the Cyclic Adsorption of CO2 and CH4 on Activated Carbon

The design of cyclic fixed-bed ion-exchange operations-I A predictive method applied to a simple softening cycle

Some consistency problems arising in the description of equilibrium adsortpion of mixtures

On presente une etude theorique du transfert couple de deux constituants independants dans un lit fixe, avec une cinetique infiniment rapide (equilibre instantane). Les bilans differentiels et les relations d’equilibre, ecrits sous une forme tres generale, sont reduits a un systeme quasi-lineaire hyperbolique, dont on etudie les courbes caracteristiques.Des exemples numeriques sont presentes pour l’adsorption, et l’echange d’ions avec et sans reactions chimiques, illustrant le fait que le reseau de courbes caracteristiques peut avoir des configurations topologiquement differentes.Ces differences impliquent une reponse differente du lit a des signaux chromatographiques (par exemple a un signal echelon) et par consequent l’etude du reseau de caracteristiques permet d’obtenir des informations tres generales sur le comportement dynamique du lit fixe.

Dynamique des transferts en lit fixe - Propriétés des équations différentielles régissant le transfert couplé de deux constituants indépendants dans une colonne chromatographique

Daniel Tondeur

Papers

Unifying concepts in non-linear unsteady processes Part I: Solitary travelling waves

Effects of Composition, Temperature and Purge on the Performance of the Cyclic Adsorption of CO2 and CH4 on Activated Carbon

The design of cyclic fixed-bed ion-exchange operations-I A predictive method applied to a simple softening cycle

Some consistency problems arising in the description of equilibrium adsortpion of mixtures

Dynamique des transferts en lit fixe - Propriétés des équations différentielles régissant le transfert couplé de deux constituants indépendants dans une colonne chromatographique