Shape Of Isotherms Research Articles

Evolution of wood cell wall nanopore size distribution in the hygroscopic range

Abstract Owing to technical difficulties, experimental assessment of wood cell wall pore size distribution (PSD) in the hygroscopic range still remains challenging. Here, a “trial-and-error” approach was proposed to calculate such distribution through bridging experimental and simulated sorption isotherms presented by the authors in the past. Two main assumptions were made in the calculations, namely, the generation of new and the swelling of existing cell wall pores during water sorption. The nanopore size distribution of dried cell wall derived from the experimental CO2 gas sorption isotherms was used as the initial boundary condition. Predicted pore size distributions were assessed to be fairly reasonable by comparing them at 95% relative humidity with the PSD of fully saturated cell walls derived from the solute exclusion method. The predicted distribution was relatively wide with several major peaks evolving in the hygroscopic range. The present work also showed that confounded by a wide PSD that includes mostly micropores, the shape of the experimental sorption isotherms was not reliable in assessing the sorption mechanism. The simulations suggested an alternative water sorption mechanism for wood, i.e. micropore filling of cell wall nanopores.

110th Anniversary: Comments on Heterogeneity of Practical Adsorbents

Most micro–mesoporous adsorbents used in practical adsorptive fluid (gas and liquid) separation processes are energetically heterogeneous. The adsorbent heterogeneity plays a significant role in establishing the shapes of the equilibrium adsorption isotherms (pure component or mixture), the corresponding heats of adsorption, and the adsorbate mass-transfer rates, which in turn determine the overall separation performance by a process. The sources of the adsorbent heterogeneity are discussed, and the pros and cons of several analytical heterogeneous models describing pure and multicomponent adsorption isotherms, heats, and adsorbate mass-transfer coefficients are analyzed. Several critical effects of the adsorbent heterogeneity on the equilibrium isotherms are presented with examples to describe the complexity of the subject. The overall effect of adsorbent heterogeneity on an adsorptive process performance will, of course, depend on the specific adsorbate–adsorbent–process design combination. The results ...

Synthesis of micro-mesoporous glass-analcime composite structure with soda–lime–silica glass as raw material

The micro-mesoporous composite phase structure of the analcime and the glass phase was successfully synthesized at 200∘C by using hydrothermal method, in which the soda–lime–silica glass was used as the silica source and the sodium hydroxide and aluminum oxide were used as additive. The TEM and SEM results showed that the obtained rod-shaped analcime was growing on the surface of glass particles. Thermogravimetric analysis indicated that they are stable and the adsorption isotherm shape indicated the coexistence of micropores and mesopores.

Deciphering the Relations between Pore Structure and Adsorption Behavior in Metal-Organic Frameworks: Unexpected Lessons from Argon Adsorption on Copper-Benzene-1,3,5-tricarboxylate.

Consistent adsorption characterization of metal-organic frameworks (MOFs) is imperative for their wider adoption in industry and practical applications. Current approaches are based on the conventional intuitive representation of MOF pore space as a regular network of pore compartments (cages and channels), adsorption in which occurs independently according to their geometric dimensions. Here, we demonstrate that this conventional approach is unable to describe even qualitatively the shape of Ar adsorption isotherms on hydrated and dehydrated Cu-BTC structures, one of the most well-known MOF materials. A combination of geometric characterization of MOF crystallographic structure, molecular simulation, and virtual visualization of the adsorption process reveals that the filling of the adjacent pore compartments proceeds in parallel in a complex cooperative fashion. The proposed synergistic approach helps us to understand the relations between pore structure geometric and chemical features and adsorption behavior, laying down a foundation for improved methods for MOF characterization.

Equilibrium and heat of water vapor adsorption on the surface of natural lignocellulose materials

Water vapor is to be separated from industrial gaseous streams at reduced costs. Natural lignocellulose materials have demonstrated a high water adsorption capacity and great potential in industrial gas dehydration, specifically natural gas dehydration. In this work, flax shives, a representative of abundant lignocellulose materials generated as by-products of agriculture industry, was chosen as a model to investigate their water adsorption. The focus of this work is on analyzing the surface morphology and the chemistry of the material, and investigating the effects of key parameters on the equilibrium, isotherms, surface affinity, and heat of water adsorption. The results showed that total operating pressure had the most significant effect on the equilibrium, followed by the temperature. The shape of water adsorption isotherm was changed from type III to type I as the total operating pressure was decreased and temperature was increased. The Anderson, Toth, and multi-site Langmuir models successfully described the isotherms obtained at various operating conditions. The modeling results were further used to determine the standard heat of adsorption, isosteric heat, heat of individual sites, and excess heat of liquefaction that the second and subsequent adsorbed layers released. In addition, the material surface was characterized by FE-SEM and FTIR, and the water transport through the material’s structure was visualized using optical microscopy images and video. These findings contributed to the fundamental science of water adsorption and natural lignocellulose materials and provided information for the design of drying processes based on adsorption and related interface sciences.

Decoding gas-solid interaction effects on adsorption isotherm shape: II. Polar adsorptives

A unique set of 6 polar adsorptives of relatively large dipole moment and of increasing kinetic diameter were used to probe pore volumes available and their mechanism of adsorption on a well-characterised microporous carbon. Multiple adsorption isotherm measurements were made and repeatable results with relatively small standard deviations in amount adsorbed at low relative pressures were obtained. Inconsistencies were observed between calculated Gurvitsch volumes. Sources of these were analysed and identified as contributions from one or more of: (a) molecular sieve effects; (b) molecular packing effects, and; (c) 2D molecular structure formation due to hydrogen bonding. These inconsistencies were further studied by comparison with pore volumes derived via the Dubinin-Radushkevich (DR) equation. Qualitative analyses of the micropore filling processes were proposed, and substantiated by complementary DR analyses. Although most of the isotherms showed Type I character, recasting the relative pressure axis in logarithmic format highlighted clear differences as contributions from fluid-fluid and fluid-solid interactions during pore filling. Overall, the adsorptives were classified into three groups: (a) polar adsorptives with primarily specific interactions adsorbing as a condensation process over a relatively narrow relative pressure range in a medium and late pressure range (iso-PrOH, MeOH, 2-methyl, 2-butanol, H2O); (b) polar adsorptives with potential for non-specific interactions adsorbing as a condensation process over a relatively narrow pressure range in a medium pressure range (pyridine, iso-PrOH, 2-methyl, 2-butanol); and, (c) halogenated adsorptives adsorbing with an S-shaped uptake extending over a broad relative pressure (dichloromethane).

Application of Inner Radiation Baffles in the Bridgman Process for Flattening the Temperature Profile and Controlling the Columnar Grain Structure of Directionally Solidified Ni-Based Superalloys.

The technique of flattening the temperature profile and controlling the formation of both the dendritic microstructure and grain structure in the directional solidification of nickel-based superalloy casting, using the novel inner radiation baffles (IRBs) in the Bridgman process, is presented in this paper. These baffles matched to the shape of mold and were placed horizontally along its height at various distances from the casting base. The plate castings of CMSX-4 superalloy were fabricated without and with the use of IRBs, withdrawing the mold at the rate of 6 mm/min from the heating to the cooling area of the industrial Bridgman furnace. Thermal analysis of the directional solidification of castings was carried out using the ProCAST software for a process where the various designs of the radiation baffle were applied. The results of the solidification conditions, the shape of liquidus and solidus isotherms, and grain structure obtained for the IRBs were compared with those reached for the standard ring-shaped (AERB) or perfectly adjusted (PARB) radiation baffles. The use of IRB resulted in flattening of the temperature distribution and decrease of the curvature of liquidus and solidus isotherms, as well as an increase of temperature gradient and cooling rate, compared with the process where AERB was only used. Consequently, primary dendrite arm spacing (PDAS) reached similar values across the width of casting and equaled to approximately 370 μm, reducing its average value by 26%, compared with the standard process. The change in predicted axial temperature gradient in casting was not found when thermophysical properties of molybdenum IRBs were used. The increase in graphite IRBs number in mold from seven to 14 caused the reduction of inhomogeneity of axial temperature gradient along the casting height.

On Condensation and Evaporation Mechanisms in Disordered Porous Materials.

Sorption isotherm measurement is a standard method for characterizing porous materials. However, such isotherms are generally hysteretic, differing between condensation and evaporation. Quantitative measurement of pore diameter distributions requires proper identification of the mechanisms at play, a topic which has been and remains the subject of intensive studies. In this paper, we compare high-precision measurements of condensation and evaporation of helium in Vycor, a prototypical disordered porous glass, to a model incorporating mechanisms on the single pore level through a semimacroscopic description and collective effects through lattice simulations. Our experiment determines both the average of the fluid density through volumetric measurements and its spatial fluctuations through light scattering. We show that the model consistently accounts for the temperature dependence of the isotherm shape and of the optical signal over a wide temperature range as well as for the existence of thermally activated relaxation effects. This demonstrates that the evaporation mechanism evolves from pure invasion percolation from the sample's surfaces at the lowest temperature to percolation from bulk cavitated sites at larger temperatures. The model also shows that the experimental lack of optical signals during condensation does not imply that condensation is unaffected by network effects. In fact, these effects are strong enough to make most pores to fill at their equilibrium pressure, a situation deeply contrasting the behavior for isolated pores. This implies that, for disordered porous materials, the classical Barrett-Joyner-Halenda approach, when applied to the condensation branch using an extended version of the Kelvin equation, should properly measure the true pore diameter distribution. Our experimental results support this conclusion.

Molecular Simulation of the Adsorption and Diffusion in Cylindrical Nanopores: Effect of Shape and Fluid⁻Solid Interactions.

We report on molecular simulations of model fluids composed of three tangentially bonded Lennard-Jones interaction sites with three distinct morphologies: a flexible “pearl-necklace” chain, a rigid “stiff” linear configuration, and an equilateral rigid triangular ring. The adsorption of these three models in cylindrical pores of diameters 1, 2, and 3 nm and with varying solid–fluid strength was determined by direct molecular dynamics simulations, where a sample pore was placed in contact with a bulk fluid. Adsorption isotherms of Type I, V, and H1 were obtained depending on the choice of pore size and solid–fluid strength. Additionally, the bulk-phase equilibria, the nematic order parameter of the adsorbed phase, and the self-diffusion coefficient in the direction of the pore axis were examined. It was found that both the molecular shape and the surface attractions play a decisive role in the shape of the adsorption isotherm. In general, the ring molecules showed a larger adsorption, while the fully flexible model showed the smallest adsorption. Morphology and surface strength were found to have a lesser effect on the diffusion of the molecules. An exceptional high adsorption and diffusion, suggesting an enhanced permeability, was observed for the linear stiff molecules in ultraconfinement, which was ascribed to a phase transition of the adsorbed fluid into a nematic liquid crystal.

Membrane-Coated UiO-66 MOF Adsorbents

Zirconium-based metal organic frameworks (MOFs) are promising adsorbents for various applications because the materials are typically hydrothermally stable. Most MOFs are prepared as a powder, but adsorption systems commonly require mechanically stable structures, such as pellets, spheres, or tablets. Therefore, this work presents a method of encapsulating a MOF pellet with a porous membrane to produce a mechanically stable core–shell type adsorbent structure. In particular, UiO-66 pellets were coated with a 4 wt % Matrimid solution that was subsequently phase inverted in water to generate membrane porosity. The coating is approximately 25–60 μm and comprises approximately 7 wt % of the composite material. Because the porous membrane only covers the surface of the pellet, the parent MOF isotherm shape and pore size distribution are maintained. The results show that direct pressing of a MOF powder into pellets produces a mechanically weak pellet but that a Matrimid membrane coating on the pellet increases ...

Determination of Moisture Sorption Isotherm of Crosslinked Millet Flour and Oxirane Using GAB and BET

Epoxy resin was prepared by crosslinking epoxidized oil and millet flour. The reaction was carried out at three different temperatures (25, 40, and 55°C) and zinc chloride levels (1, 2, and 3%). Moisture sorption isotherms were determined at 0.1 to 0.9 water activity (aw) using a gravimetric sorption analyzer (Q 2000). The sigmoidal shape (type II) of the resin isotherms exhibited lower equilibrium moisture content (EMC) at higher temperature. The experimental data were modeled using GAB (Guggenheim–Anderson–de Boer) and BET (Brunauer–Emmett–Teller). The EMC of the resin was significantly lower than that of the flour, which could be attributed to the decrease in the number of water-binding sites due to the creation of dense areas during crosslinking. The low root-mean-square error (RMSE) indicates that GAB and BET were suitable for predicting the water sorption isotherm for millet flour resin. The heat of sorption of the resin was large at low moisture content and increased at higher relative humidity. It is recommended that millet flour resin be used at relative humidity below 60%.

Cooperative model of the histidine and alanine adsorption on single-walled carbon nanotubes

The isotherm of histidine adsorption from an aqueous solution by carbon nanotubes MKN-SWCNT-S1 is constructed. The isotherm has the shape of a sigmoid. The alanine adsorption isotherm by these carbon nanotubes was constructed in previous work and has the shape of two-plateau isotherm. The model is considered describing the adsorption of molecules from the solution with the formation of clusters of i molecules (i = 1,2, … n) on the sorbent surface for which analytical expressions of the isotherm are obtained. This model makes it possible to clarify the sorbate structure on the sorbent surface using the experimental isotherm of adsorption.It is shown that the obtained experimental isotherm of histidine adsorption reflects the cooperative amino acid sorption, consisting in the formation on the sorbent's surface the monomers and clusters of 4 and 8 layers of histidine zwitterion ions. Experimental isotherm of alanine adsorption reflects the cooperative amino acid sorption, consisting in the formation of monomers and seven-dimensional clusters on the sorbent surface. The experimental adsorption isotherms and the proposed model were used to determine the monolayer capacities of the CNT to the investigated amino acids and to estimate specific surface area of nanotubes.

Modelling of water vapour sorption hysteresis of cement-based materials based on pore size distribution

The purpose of this paper is to develop a model for predicting water vapour sorption hysteresis of cement-based materials. The sorption hysteresis was modelled based on the pore size distribution (PSD) and the pore network. The PSD of gel and capillary pores was predicted from the adsorption isotherm. The results indicated that the distribution had two peaks at 3.8 nm and 11 nm of the pore diameters, referring to gel and small capillary pores, respectively. The predicted ratio of gel water to evaporable interlayer water was not a constant, but decreased with an increasing hydration degree. When RH was below 0.25, the predicted sorption hysteresis was solely influenced by the hysteresis of water in interlayer spaces and minor hydrates. The hysteresis in gel pores, small and large capillary pores influenced the shape of desorption isotherms within the RH ranges of 0.25–0.85, 0.35–0.98 and 0.4–1, respectively.

Evaluation of diamine-appended metal-organic frameworks for post-combustion CO2 capture by vacuum swing adsorption

Five different diamine-appended metal-organic frameworks (MOFs) that show a S-shaped CO2 isotherm are evaluated for post-combustion CO2 capture from dry flue gas using a vacuum swing adsorption process. A comprehensive simulation and optimization study using a multi-objective genetic algorithm is performed. The optimization to maximize CO2 purity and recovery showed a key link between the feed temperature, evacuation pressure and process performance. The MOFs that achieve a target CO2 purity ⩾95% and recovery ⩾90%, namely, mmen-Mn2(dobpdc) and mmen-Mg2(dobpdc) were optimized to reduce parasitic energy and increase productivity. The adsorbents, mmen-Mn2(dobpdc), mmen-Mg2(dobpdc) and Zeolite 13X showed minimum parasitic energies of 142, 152 and 167 kWhe/tonneCO2cap, respectively and maximum productivities of 0.4, 0.45 and 0.65 molCO2m-3adss-1, respectively while achieving CO2 purity ⩾95% and recovery ⩾90%. The possible low N2 affinity and the unique shape of the CO2 isotherm were found to be key reasons for a lower energy consumption.

Integrated Adsorbent Process Optimization for Minimum Cost of Electricity Including Carbon Captureby a VSA Process

Assessing vacuum swing adsorption (VSA) technology for postcombustion CO2capture and concentration (CCC) using energy and productivity indicators are useful, but its ultimate test must be the cost of electricity from a power plant including CCC. Here, our integrated optimization platform (Khurana and Farooq,AlChE J. 2017;63:2987–2995) developed earlier to simultaneously obtain the optimum adsorbent and process conditions is extended to include a comprehensive costing framework. The framework is complete with scale‐up design and column scheduling, and compliant with National Energy Technology Laboratory costing guidelines for carbon capture. This is the ultimate tool that enables integrated optimization to minimize the cost of electricity. The Shell Cansolv CO2capture system is used as the benchmark for evaluating the best performance of two VSA cycles for two adsorbents. The operating conditions and isotherm shapes necessary to achieve the lowest possible cost of electricity for the two VSA cycles are also presented to facilitate designing or searching the best adsorbent for CCC. © 2018 American Institute of Chemical EngineersAIChE J, 65: 184–195, 2019

Comparison of ab- and adsorbents for low temperature heat driven sorption cooling machines

Thermally driven cooling machines allow for reduction of CO2 emissions when using waste heat from industrial processes or cogeneration, or when using solar heat. The lower the necessary driving temperature level is the more heat sources are available to be used, or the more thermal power can be drawn from a heat source, or the smaller and cheaper the heat exchanger for a given power can be.There have been debates for decades whether adsorption chillers can be driven by heat sources at lower temperatures than absorption chillers. The paper at hand intends to continue this debate on a rather fundamental level. The inorganic salt lithium bromide which is the most common absorbent and the common adsorbent RD-Silica gel are compared to organic salts (ionic liquids) as designable absorbents on the one hand, and adsorbents designed for use at low driving temperatures, on the other hand. For that purpose, functions for the vapor pressure depression of the discussed sorbents are required. Simple correlations for the absorbents are derived in this paper, whereas the correlations for the adsorbents are taken from literature. The crystallization lines of lithium bromide derived from experimental data from two different sources are also discussed shortly.From these correlations, the minimum desorption temperature at various boundary conditions is derived for five sorbents and the influence of the desorption temperature on the change of composition in the sorbents is shown. The latter is traced back to the shape of the sorption isotherms. At small temperature lifts the zeotypes and the ionic liquid [mmim][DMP] can be operated with lowest driving temperatures, but at higher temperature lifts the zeotypes are inoperable. Finally, it is concluded that the possibility to tailoring sorbents according to the customer's needs prevails for both liquid and solid sorption as a pre-requisite to operate with very low driving temperatures.

Interactions between kaolinite clay and AOT

Interactions between kaolinite (KGa-2) and a typical anionic surfactant bis(2-ethylhexyl) sulfosuccinate sodium (AOT) have been study to obtain a better knowledge of this system. Firstly, adsorption isotherms of AOT on kaolinite are realized as a function of pH. Stability experiments have been performed between the surfactant and the clay mineral and the bed sediments structure have been studied with Small Angle X-ray Scattering (SAXS).In acidic media, the shape of the adsorption isotherm leads to the following suggestions: firstly, for surfactant concentration below the CMC a surfactant monolayer is adsorbed on the positively charged edge surfaces, and then, for surfactant concentrations above the CMC, a bilayer is adsorbed on the edge surfaces. Consequently, in the first case, the surface becomes hydrophobic and in the second case, the surface becomes hydrophilic and this assumption is verified with hydrophobicity tests. Under alkaline conditions no surfactant is adsorbed on surfaces confirming the adsorption on the edge surfaces in acidic conditions. Significant stabilization of kaolinite suspensions can be observed in the presence of high surfactant concentration, in both pH conditions. Finally, structural studies by SAXS provide data about the organization of particles in the bed sedimentation.

Effect of Processing Parameters and Shape of Blade on the Solidification of Single-Crystal CMSX-4 Ni-Based Superalloy

The effect of mold withdrawal velocity, heater and pouring temperatures and the thickness of blade root on the solidification parameters and shape of the liquidus isotherm as well as microstructure of single-crystal CMSX-4 nickel-based superalloy, manufactured by the Bridgman method, has been analyzed in the article. The temperature gradient G, solidification rate v and location of the liquidus isotherm were determined in relation to the radiation baffle on the basis of temperature measurements conducted in nine test points along the height of castings (140 mm) with different thicknesses of root (27, 15 and 7 mm) and withdrawal velocities of 3 and 5 mm/min. The increase of root thickness, relative to the airfoil, resulted in the rise of inhomogeneity of solidification parameters as well as the primary dendrite arm spacing and unsteady state solidification along the whole blade height. However, in the case of blade with constant thickness, the steady-state solidification was created at the distance of 65 to 130 mm from its base. The increase of mold withdrawal velocity caused the decrease of temperature gradient. However, the solidification rate was different from the mold withdrawal velocity, especially in the root and middle part of the airfoil. The increase of heater temperature resulted in a rise of temperature gradient and reduction of the curvature of the liquidus isotherm, whereas the pouring temperature did not affect the solidification parameters in the blade. Particularly unfavorable conditions of the solidification process could occur in the middle part of the airfoil and root because of the lowest value of G/v and the largest primary dendrite arm spacing.

Heat-source transformation thermotics: from boundary-independent conduction to all-directional replication

All objects in nature are essentially heat sources due to their non-zero temperatures, but the control of them are seldom explored in the theory of transformation thermotics, which largely results from the complicated and uncontrollable parameters given by this theory. Here we put forward an equivalence operation to overcome this challenge, and then show analytical, simulation and experimental evidence for boundary-independent conduction and all-directional replication. The former represents the counterintuitive boundary-independent behavior of thermal conduction surrounded by arbitrary boundaries; the latter denotes a thermal copy of arbitrary shaped objects, which has the accurate shape of isotherms of the objects along all directions. The equivalence operation could be an applicable method for achieving uniform heating, boundary-independent cloaking, all-directional camouflaging, etc. Our results could also give hints for both freely controlling heat transfer and delicately designing similar conduction and replication in other disciplines like electrostatics and particle diffusion.

Solubility of Copper(II) Chloride in Mixed Organic Oxygen-Containing Solvents

Solubility of copper(II) chloride at 25°C in binary organic solvents consisting of dimethyl sulfoxide, N,N-dimethylacetamide, and 1,4-dioxane was determined by the isothermal saturation method. The shape of solubility isotherms for all ternary systems under study is connected with the structural type of formed copper chloride solvates, the composition of which is defined by the solvent donor number. Solvates crystallize predominantly, where the metal ion coordinates molecules of the solvent with a greater donor power. The temperature increase in the system CuCl2–dimethylacetamide–dioxane up to 50°C leads to the formation of the cluster compound [Cu4Cl6O(DMA)4]; its structure was determined by the X-ray structural analysis method.