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Methane (CH4) and carbon dioxide (CO2) are primary components of coal seam gas (CSG). Understanding their adsorption-desorption hysteresis characteristics, along with the fundamental mechanism, is crucial for CSG exploitation and related hazards mitigation. This research focused on the representative Bulli coal seam in the Sydney Basin, Australia. Through the purpose-built indirect gravimetric high-pressure isothermal adsorption-desorption hysteresis experiment, a novel Langmuir-based desorption model, incorporating hysteresis effect and residual gas, was proposed. Quantitative characterization of the adsorption-desorption hysteresis degrees of CO2 and CH4 in coal particles of various sizes and in Φ50 mm × 100 mm intact coal samples were achieved using the improved hysteresis index (IHI). The experimental findings validated that the proposed desorption model accurately describes the desorption behavior of CO2 and CH4 in coal (R2>0.99). Based on the adsorption-desorption properties of ink-bottle-shaped micropores and pore deformation caused by gas adsorption-induced coal expansion, the occurrence mechanism of adsorption–desorption hysteresis and the fundamental reasons for the presence of residual gas were elucidated. Furthermore, the study explored the impact of CO2 and CH4 adsorption-desorption hysteresis effects on coal and gas outbursts, suggesting that coal seams rich in CO2 do not have a higher propensity for outbursts than those rich in CH4.

Metal–organic frameworks (MOFs), which possess a high degree of crystallinity and a large surface area with tunable inorganic nodes and organic linkers, exhibit high stimuli-responsiveness and molecular adsorption selectivity that enable various applications. The adsorption in MOFs changes the crystalline structure and elastic moduli. Thus, the coexistence of adsorbed/desorbed sites makes the host matrices elastically heterogeneous. However, the role of elastic heterogeneity in the adsorption– desorption transition has been overlooked. Here, we show the asymmetric role of elastic heterogeneity in the adsorption–desorption transition. We construct a minimal model incorporating adsorption-induced lattice expansion/contraction and an increase/decrease in the elastic moduli. We find that the transition is hindered by the entropic and energetic effects which become asymmetric in the adsorption process and desorption process, leading to the strong hysteretic nature of the transition. Furthermore, the adsorbed/desorbed sites exhibit spatially heterogeneous domain formation, implying that the domain morphology and interfacial area between adsorbed/desorbed sites can be controlled by elastic heterogeneity. Our results provide a theoretical guideline for designing soft porous crystals with tunable adsorption hysteresis and the dispersion and domain morphology of adsorbates using elastic heterogeneity. [ABSTRACT FROM AUTHOR]

The phenomena of methane adsorption-desorption hysteresis in coal has been observed by many scholars. The results indicate that the adsorption and desorption are not fully reversible. This paper analyses the previous studies on this phenomena and proposes an improved hysteresis index (IHI). The influences of maximum pressure and coal particle size on hysteresis degree are studied by sorption test. The mechanism of sorption hysteresis and its impact on deep coalbed methane (CBM) recovery are discussed. Results show that IHI can reflect the degree of sorption hysteresis from a fully irreversible status to a fully reversible status. The hysteresis degree increases with growing maximum pres-sure and coal particle size. The result of sorption test is an integration of sorption and diffusion, and these two processes cannot be differentiated. Therefore, a 'restricted diffusion hypothesis' is proposed to explain the present phenomena. It is due to the diffusion restriction of narrow pore throat, which is induced by the imbedded gas molecules during high pressure adsorption. The gas content of deep CBM may be very high, however, due to the sorption hysteresis, the minable gas content and the production perspective should be evaluated using desorption isotherm rather than adsorption isotherm. Besides enhancing coal permeability, some novel methods to increase desorption and diffusion rate should be considered for deep CBM recovery. [ABSTRACT FROM AUTHOR]

• A calculation method for inversing adsorption parameters is proposed by using the modified hysteresis index DHI. • The new gas flow coupling model considering the adsorption–desorption hysteresis effect is constructed in this paper. • The reliability and superiority of the new model is proven by laboratory experiments. • The effect of different degrees of hysteresis on gas extraction is quantitatively evaluated using the new model. Coal mining methane (CMM) is a major hazard source and a typical greenhouse gas in the coal industry. Gas extraction is an effective solution to eliminate coal seam gas disasters and reduce greenhouse gas emissions. Multi-field coupling model of gas flow in coal (MCMGF) is an important theoretical basis for guiding gas extraction, and the adsorption model is an important component of MCMGF. Numerous studies have shown that there is a hysteresis effect in the desorption of gas in coal compared to the adsorption process. Since gas extraction is a depressurized desorption process, HE should be taken into account when building the adsorption model. However, most of the current MCMGFs ignore the hysteresis effect, resulting in an overestimation of the amount of gas involved in the desorption process. Therefore, in this paper, the calculation method of the degree of hysteresis index (DHI) is revised and the adsorption model is modified based on systematically sorting out the adsorption–desorption hysteresis phenomenon. On this basis, a coupled gas flow model considering the hysteresis effect of adsorption and desorption was constructed, and the reliability of the model was verified by isothermal adsorption and desorption experiments on coal pillars. Eventually, the influence of different hysteresis degrees (DHI = 0, 0.25 and 0.59) on gas extraction was quantitatively evaluated by using the new model. The results show that hysteresis effect has a significant influence on gas extraction, and the larger DHI value is, the smaller the daily extraction is. Ignoring the influence of adsorption–desorption hysteresis will make the gas extraction capacity of the borehole overestimated. After 160 d of extraction, the difference between the residual gas content when adsorption–desorption hysteresis is considered and when it is ignored is up to 2.05 m3/t. This is because the adsorption–desorption hysteresis limits the ultimate amount of gas that can participate in the flow. In our previous studies, it has been found that the additional resistance to gas desorption is intrinsic to the hysteresis. With the increase of the mining depth, the in-situ stress increases rapidly, therefore the influence of coal adsorption–desorption hysteresis on gas extraction under high stress is one of the problems that need to be considered emphatically. [ABSTRACT FROM AUTHOR]

Single-, double-, and multi-walled carbon nanotubes (SWCNTs, DWCNTs, and MWCNTs), and two oxidized MWCNTs with different oxygen contents (2.51 wt % and 3.5 wt %) were used to study the effect of the wall number and surface functionalization of CNTs on their adsorption capacity and adsorption-desorption hysteresis for heavy metal ions (NiII, CdII, and PbII). Metal ions adsorbed on CNTs could be desorbed by lowering the solution pH. Adsoprtion of heavy metal ions was not completely reversible when the supernatant was replaced with metal ion-free electrolyte solution. With increasing wall number and amount of surface functional groups, CNTs had more surface defects and exhibited higher adsorption capacity and higher adsorption-desorption hysteresis index (HI) values. The coverage of heavy metal ions on the surface of CNTs, solution pH, and temperature affect the metal ion adsorption-desorption hysteresis. A possible shift in the adsorption mechanism from mainly irreversible to largely reversible processes may take place, as the amount of metal ions adsorbed on CNTs increases. Heavy metal ions may be irreversibly adsorbed on defect sites. [ABSTRACT FROM AUTHOR]

Nitrogen desorption scanning hysteresis loops (DSHLs) for large-pore MCM-41 silicas (pore diameter from 4.0 to 6.5 nm) are reported for the first time. DSHLs for MCM-41 were compared with those of conventional mesoporous silicas and no appreciable differences were found, although hysteresis loops and DSHLs for the latter were usually broader. Since desorption behavior of conventional porous silicas is appreciably influenced by pore connectivity, the observed similarity in hysteresis behavior suggests single-pore “blocking effects” for MCM-41 due to variation of pore diameter along its nonintersecting channels. It was also shown that the steepness of nitrogen desorption branches at relative pressures close to 0.4 often results from proximity of the lower pressure limit of adsorption-desorption irreversibility and consequently it is not justified to consider it as an indication of narrow pore size distribution. Thus, application of desorption data in calculations of pore size distributions may be grossly misleading. [ABSTRACT FROM AUTHOR]

A two-stage mechanism of adsorption, including nucleation and condensation, was proposed to describe the formation of the water liquid phase in carbon adsorbents. Adsorption is assumed to occur in cylindrical pores. Nucleation is described by a modified BET model, and condensation is treated by the model of a stretched liquid film on a bent surface. The onset of formation of the liquid phase (OFLP) is determined from the intersection of the adsorption isotherms for these stages. The theoretical value of the relative pressure of OFLP varies over a wide range, decreasing with a decrease in the pore radius and reactiing the relative vapor pressure of 0.178 for the spinodal state of water. The comparison method using isotherms of graphitized carbon black as the reference isotherms was applied for the determination of OFLP of water in real active carbons. This resulted in good agreement between theory and experiment. [ABSTRACT FROM AUTHOR]