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

Modern approaches to modeling adsorption and chromatographic operations rely, implicitly or explicitly, on a restricted number of paradigms, such as the equilibrium-stage approach and the theoretical plate, the kinematic wave, the constant pattern, the Langmuir isotherm, and a few others. Their role in organizing knowledge and experience in adsorption research is considerable, and the set of currently used paradigms expresses the state of mind, the dominant ideology in the field at a given instant. We propose a nonexhaustive inventory of these paradigms, analyze their origin and their scientific supports, and discusse their role in the progress of the field, their conservative function, and their contribution to strengthening the concerned community or to opening it to others. Illustrations are given that paradigms are sometimes paradoxes, and finally, a discussion of the missing paradigms is proposed, i.e., paradigms which have not yet been established or recognized, but which may be the keys to future progress

Paradigms and paradoxes in modeling adsorption and chromatographic separations

Flow distribution and pressure drop in 2D meshed channel circuits

A process for continuously separating the constituents of a mixture characterized in that it consists in introducing the gaseous mixture in an enclosure containing a selective adsorption agent or adsorbent, and subjecting the said mixture to a cyclical series of hot and cold steps induced by sudden variations of temperature between a temperature Tc called the "hot temperature" and a temperature TF called the "cold temperature" in order to create in the cold steps three distinct zones moving in the flow-direction at different speeds: a zone of low-speed propagation comprising a sorption front in which the concentration of gas having the most adsorbable constituents decreases; a zone called the "thermal wave" induced at times determined by the sudden cooling of the load mixture; a desorption zone where the mixture is enriched in the most adsorbable constituents.

Process for the separation or purification of mixtures by the use of a solid adsorption

In this paper, an optimisation-based approach is developed for the determination of the cyclic steady-sate (CSS) of a pressure swing adsorption process (PSA). It consists in treating the simulation problem as a single dynamic optimisation problem where the performance index is the CSS condition, the decision variables are the state variables at the start of the cycle and the constraints are given by the process model equations with associated initial conditions. The resulting optimisation problem is solved using a gradient-based non linear programming (NLP) method, e.g. SQP method, where the gradients are computed by means of four different methods: finite differences, numerical and analytical sensitivities and adjoint system methods.

Daniel Tondeur

Papers

Paradigms and paradoxes in modeling adsorption and chromatographic separations

Flow distribution and pressure drop in 2D meshed channel circuits

Process for the separation or purification of mixtures by the use of a solid adsorption

Optimisation-based simulation of a pressure swing adsorption process

Channel interlacing: A geometric concept for intensification and design of the internal structure of fluid contactors