Sorption Isotherm Hysteresis Research Articles

Temperature dependence of adsorption hysteresis in flexible metal organic frameworks

“Breathing” and “gating” are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli. These effects are often associated with eminent steps and hysteresis in sorption isotherms. Despite significant mechanistic studies, the accurate description of stepped isotherms and hysteresis remains a barrier to the promised applications of flexible MOFs in molecular sieving, storage and sensing. Here, we investigate the temperature dependence of structural transformations in three flexible MOFs and present a new isotherm model to consistently analyse the transition pressures and step widths. The transition pressure reduces exponentially with decreasing temperature as does the degree of hysteresis (c.f. capillary condensation). The MOF structural transition enthalpies range from +6 to +31 kJ·mol−1 revealing that the adsorption-triggered transition is entropically driven. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance.

Competitive sorption and diffusion of methane and carbon dioxide mixture in Carboniferous-Permian anthracite of south Qinshui Basin, China

A comprehensive understanding of the migration and competitive adsorption of methane and carbon dioxide in coal is crucial for the injection optimization design and injection displacement evaluation of the CO2 enhanced coalbed methane recovery (ECBM) project. In this paper, adsorption-desorption tests of three gaseous mixtures (75% CH4 + 25% CO2, 50% CH4 + 50% CO2, and 25% CH4 + 75% CO2) and pure CH4 and CO2 were performed on the anthracite from the southern Qinshui Basin. The results show that (1) there was an apparent hysteresis in sorption isotherms, and the size of hysteresis loops was related to the proportion of CO2 in the mixture. There was also a non-negligible error in using the extended Langmuir model to predict mixture sorption. (2) Separation factor (SCO2-CH4) for the sorption of CH4 and CO2 mixture varied with the equilibrium pressure and the equilibrium concentration of gas components. Its value was usually between 4 and 20. During the adsorption process, the SCO2-CH4 at low pressure was generally greater than that at high pressure. Moreover, SCO2-CH4 in the desorption process was generally higher than the adsorption process. (3) Bidisperse diffusion model could better describe the diffusion process of the mixture. In general, the macropore effective diffusivity varied as a power function with pressure. Under the same pressure, the macropore effective diffusivity increased with the increasing CO2 content. Further, the macropore effective diffusivity in the desorption process was larger than that in the adsorption process. The micropore effective diffusivity and the ratio of macropore uptake to micropore uptake had no apparent relationship with the gas component concentration and pressure. The results will help to comprehensively understand the sorption processes of the CH4 and CO2 mixture and provide basic numerical simulation data for the CO2-ECBM project in the southern Qinshui Basin.

Kinetics and Equilibrium Isotherms of Water Vapor Adsorption/Desorption in Cement-Based Porous Materials

The kinetics and equilibrium of water vapor adsorption/desorption isotherm (WVSI) are fundamental information for solid–moisture interaction in the microstructure of cement-based porous materials (CBPM). This paper presents firstly the experimental data for WVSI of CBPM with a ternary binder. Mass changes in specimen are recorded in sorption tests for materials with different aggregate contents and water-to-binder (w/b) ratios under controlled ambient relative humidity. Both the sorption equilibrium and sorption kinetics are investigated through some established physical models, and the corresponding intrinsic parameters of moisture sorption are determined. For sorption kinetics, the mass change during sorption is interpreted through Elovich model, power model and pseudo-first-order and pseudo-second-order models. The mass change in sorption occurs mainly in the first 40 days, and the pseudo-second-order model shows the best adaption to all adsorption/desorption processes. For sorption equilibrium, Guggenheim–Anderson–de Boer (GAB), modified Halsey and Oswin models are used to interpret the moisture sorption capacity. The hysteresis between adsorption and desorption processes is attributed, respectively, to the mechanisms of snap-through, energy instability and pore constrictivity for low, middle and high relative humidity ranges. From the analysis, it is found that (1) through the interpretation of pseudo-second-order model the moisture sorption kinetics can be controlled by the sorption between the water molecules and the pore wall in addition to vapor diffusion; (2) the sorption capacity is sensitive to aggregate content and w/b ratio; incorporating aggregates, decreasing w/b ratio and increasing humidity gradient tend to decrease the sorption capacity of CBPMs; (3) the sorption isotherm hysteresis is well described by GAB-H model, both the extent of hysteresis and the energy constant related to pore surface tension showing clear correlation with the aggregate content and w/b ratio of CBPMs.

In-situ analysis of gas-loaded porous materials

In order to understand gas sorption processes at the molecular level it is important to correlate physico-chemical data (e.g. sorption isotherms and calorimetric analysis) with structural data. It is therefore desirable to carry out structural elucidation and calorimetric analysis under conditions that closely mimic those of the sorption/desorption experiments. However, the crystallographic analysis of samples under controlled gas environments poses significant technical challenges, particularly given the limited space associated with the sample compartment of standard commercial diffractometer. In this regard, an environmental gas cell has been developed in parallel with a pressure-programmed differential scanning calorimeter. Use of these complementary techniques has provided new insight into features such as pressure-induced phase transformations that give rise to inflections and hysteresis in sorption isotherms. The influence of guest molecules on aspects such as structural flexibility, linear thermal expansion and changes in network interpenetration will be discussed.

Evidence of irreversible CO2 intercalation in montmorillonite

Mitigation of the global climate change via sequestration of anthropogenic carbon dioxide (CO2) in geologic formations requires assessment of the reservoir storage capacity and cap rock seal integrity. The typical cap rock is shale or mudstone rich in clay minerals that may significantly affect the effectiveness of the CO2 trapping. Specific objectives of this study were to conduct experimental investigation into the processes associated with CO2 and H2O trapped in swelling clay, namely, Wyoming and Texas montmorillonite powder. Combined (same-sample) multi-technique data – manometric sorption isotherm hysteresis, diffuse reflectance infrared spectroscopy ‘trapped CO2’ fingerprints, irreversible X-ray diffraction patterns for the clay interlayer in intermediate hydration state, and HF acid digestion resulting in formation of non-extractable F:CO2 adducts – corroborate a hypothesis that carbon dioxide molecules can be irreversibly trapped via anomalous extreme confinement in the galleries associated with montmorillonite interlayer, which may result in formation of carbonates in the longer term. Validation on Arizona montmorillonite lumps substantiated the evidence that such processes may occur in natural clay deposits but possibly on a different scale and at a different rate.

δ17O Determination of Meteoric Waters

Mitigation of the global climate change via sequestration of anthropogenic carbon dioxide (CO2) in geologic formations requires assessment of the reservoir storage capacity and cap rock seal integrity. The typical cap rock is shale or mudstone rich in clay minerals that may significantly affect the effectiveness of the CO2 trapping. Specific objectives of this study were to conduct experimental investigation into the processes associated with CO2 and H2O trapped in swelling clay, namely, Wyoming and Texas montmorillonite powder. Combined (same-sample) multi-technique data – manometric sorption isotherm hysteresis, diffuse reflectance infrared spectroscopy ‘trapped CO2’ fingerprints, irreversible X-ray diffraction patterns for the clay interlayer in intermediate hydration state, and HF acid digestion resulting in formation of non-extractable F:CO2 adducts – corroborate a hypothesis that carbon dioxide molecules can be irreversibly trapped via anomalous extreme confinement in the galleries associated with montmorillonite interlayer, which may result in formation of carbonates in the longer term. Validation on Arizona montmorillonite lumps substantiated the evidence that such processes may occur in natural clay deposits but possibly on a different scale and at a different rate.

Influence of sorption isotherm hysteresis effect on indoor climate and energy demand for heating

Abstract In order to investigate the effects of sorption isotherm hysteresis on indoor climate and energy demand, a new module for precise representation of mass transfer in materials in contact with indoor air, called Humi-mur, was developed and implemented in the energy performance simulation tool, TRNSYS. This extended whole-building HAM (Heat–Air–Moisture) simulation tool was then used to investigate the impact of the moisture-buffering effect on indoor conditions in a room. Two modelling approaches were compared: including/excluding moisture-buffering effect and including/excluding hysteresis in sorption isotherm. In addition, two ventilation strategies were used: constant airflow ventilation (CAV) and relative humidity sensitive ventilation (RHS). The simulation showed that for the CAV system, the use of moisture-buffering materials did not change energy demand but improved the indoor conditions. For the RHS ventilation system, the use of the buffering materials led to a higher energy demand, but the indoor relative humidity comfort was significantly improved. The results also showed that hysteresis of the sorption isotherm impacts on indoor conditions, even if the effect is smaller for the studied material than the average moisture-buffering effect.

INFLUENCE OF HYSTERESIS IN SORPTION ISOTHERM AND MOISTURE CONDUCTIVITY ON CONDENSATION AND EVAPORATION PROCESSES

It is well known that some porous materials used as building walls show hysteresis in ad/desorption sorption isotherms. Some materials similarly show hysteresis in moisture conductivity. Sorption isotherm and moisture conductivity have an effect on heat and moisture transfer in materials, But there is little information available on the degree of the influence of hysteresis, especially that of moisture conductivity. In this study, we clarified the effect of hysteresis of sorption isotherm and moisture conductivity on heat and moisture behavior in ALC using the simultaneous heat and moisture transport equations and the independent domain theory for the experiment of repeat both condensation and evaporation process.

Infiltration pressure of a nanoporous liquid spring modified by an electrolyte

In a hydrophobic zeolite, the infiltration and defiltration of water can be controlled by adjusting external pressure, and therefore the system behaves as a “liquid spring.” Since the hysteresis of sorption isotherm is negligible and the working pressure is thermally controllable, volume memory devices can be developed based on this phenomenon. With the addition of sodium chloride, both infiltration and defiltration pressures increase, which should be attributed to the cation exchange. The temperature sensitivity of the system increases with the electrolyte concentration, beneficial to improving the output energy density.

Experimental investigation on pressurized liquid in confining nanoenvironment

In this letter, the authors report the experimental observations of pressure induced liquid motions in a confining nanoporous gel. As the pressure is sufficiently high, water molecules can move out of the initially energetically favorable gel matrix and enter the hydrophobic nanopores. As the pressure is lowered, this process is irreversible, leading to the pronounced hysteresis of sorption isotherm. This phenomenon is essential to developing advanced energy absorption systems. A first-order thermodynamics analysis is presented.

Pressure induced liquid infiltration in nanopores

In this article, we report the results of an infiltration∕defiltration experiment of a hydrophobic nanoporous silica gel. As the pressure is increased to the critical value, water can be forced into the nanopores. When the pressure is reduced, however, the “outflow” is difficult, leading to a significant hysteresis of sorption isotherm. A first-order thermodynamics analysis is performed to capture the confinement effect of nanopore walls and the mass and energy exchanges between liquid and gas phases in nanopores.

Specific surface of two hydrated cements differing in strength

Specific surface, S, of CSH-gel particles of disordered layered structure, was studied by water sorption/retention in two cement pastes differing in strength, i.e. C-33 (weaker) and C-43 (stronger), w/c=0.4. Hydration time in liquid phase was t h =1 and 6 months, followed by hydration in water vapour either on increasing stepwise the relative humidity, RH=0.5→0.95→1.0 (WS) or on its lowering in an inverse order (WR). Specific surface was estimated from evaporable (sorbed) water content, EV(110°C), assuming a bi- and three-molecular sorbed water layer at RH=0.5 or 0.95, respectively (WS). On WR it was three- and three- to four-molecular (50 to 75%), respectively, causing a hysteresis of sorption isotherm. At RH=0.5 the S increased with cement strength from 146 m 2 g -1 (C-33, 1 m) to 166 m 2 g -1 (C-43, 1 m) and with hydration time to 163 (C-33, 6 m) and to 204 m 2 g -1 (C-43, 6 m). At RH=1.0 (and 0.95), higher S-value were measured but these differences were smaller: S amounted to 190-200 m 2 g -1 in C-33 (1 and 6 m) and 198-210 m 2 g -1 in C-43 (1 and 6 m). Thus no collapse occurred on air drying of paste C-43 (6 m).

ANN approach to sorption hysteresis within a coupled hygro-thermo-mechanical FE analysis

Non-linear deformable porous media with sorption (capillary condensation) hysteresis are considered. An artificial neural network with two hidden layers is trained to interpolate the sorption hysteresis using a set of experimental data. The performance of the ANN, which is applied as a procedure in the FE code, is investigated, both from numerical, as well as from physical viewpoint. The ANN-FE code has been developed and tested for 1-D and 2-D problems concerning cyclic wetting–drying of concrete elements. In general, the application of the ANN procedure inside the classical FE program does not have any negative effect on the numerical performance of the code. The results obtained indicate that the sorption isotherm hysteresis is of importance during analysis of hygrothermal and mechanical behaviour of capillary-porous materials. The most distinct differences are observed for the saturation and displacement solutions. The ANN-FE approach seems to be an efficient way to take into account the influence of hysteresis during analysis of hygro-thermal behaviour of capillary-porous materials. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of Matrix Properties on the Performance of a Counterflow Rotary Dehumidifier

A finite difference model of a counterflow rotary dehumidifier is used to determine the effect of six matrix properties of dehumidifier performance. The matrix properties considered are the sorption isotherm shape, the maximum sorbent water content, the heat of sorption, the matrix thermal capacitance, matrix moisture diffusivity, and sorption isotherm hysteresis. The isotherm shapes and the sorption parameter values are selected to be representative of available insoluble desiccants. The results of the finite difference calculations show that the shape of the isotherm has a greater effect on dehumidification than the maximum water content. Large adsorption heat effects or matrix thermal capacitance has a detrimental impact on dehumidification. Hysteresis in the adsorption isotherm significantly impairs dehumidifier performance. The anlaysis shows that commercially available solid desiccants can result in nearly optimal dehumidifier performance.

Sorption isotherm hysteresis and turbidity phenomena in mesoporous media

Based on equilibrium sorption isotherm studies of porous Vycor discs whose circular faces were coated with a clear epoxy resin, it was possible to study the variable sorbed vapor disposition coincident with sorption isotherm hysteresis. Along the adsorption isotherm, condensate lenses were found to be more or less randomly distributed uniformly throughout the Vycor, but upon subsequent desorption, the absence of cavitation allowed the porous Vycor disc to remain condensate saturated. The desorption isotherm was, therefore, nearly horizontal until, at a sufficiently low vapor pressure, the outermost liquid-vapor interfaces became unstable. The resulting instability caused rheon jump desorption, which proceeded radially inward over an unusually small relative pressure range. This desorption process left behind residual isolated condensate ganglia in the outer portion of the porous medium in metastable equilibrium with a condensate saturated central core. After descending approximately two-thirds of the near vertical desorption isotherm, the condensate saturated core had evaporated and the remainder of the desorption hysteresis isotherm was primarily ganglia evaporation. Nonuniform desorption behavior was sensibly absent along descending scanning curves because each curve began at a partially saturated adsorption state containing randomly distributed vapor regimes from which internal rheon jump desorption could begin. This nonuniform desorption, observed with porous Vycor, may apply to a variety of porous media, thereby negating any simple application of the Kelvin equation in computing pore size distributions for these media. Such a calculation, which assumes that all pores communicate with the vapor, would lead to a considerable error in that the number of larger pores is greatly underestimated.