Low-pressure Hysteresis Research Articles

On the Low-Pressure Hysteresis (LPH) in Gas Sorption Isotherms of Porous Carbons.

This study investigates the origin of low-pressure hysteresis (LPH) in the adsorption and desorption of three different probe molecules: carbon dioxide, nitrogen, and argon, across various adsorption temperatures (from cryogenic to room temperature), and within five different carbon materials: synthetic carbons (pristine and one post-synthetically oxidized) and natural coal. Significant attention is dedicated to elucidating LPH in oxidized samples outgassed at various temperatures (120-350°C). Experimental results show that insufficient outgassing temperature can lead to unreliable data due to artificial LPH and significantly underestimated textural properties, primarily caused by porosity blockage from substances like moisture. Conversely, in samples where heteroatoms have a stabilizing effect on texture, such as natural coal, careful consideration of outgassing temperature is crucial due to the risk of thermal degradation. Other factors contributing to LPH are adsorption temperature, and especially, kinetic limitations at cryogenic temperatures for cellulose-based carbons. Minor factors responsible for LPH are the physical state of the sample (monolith vs powder) and the flexibility of the porous system, both studied by carbon dioxide sorption. This study constitutes an important piece in the evaluation of LPH, providing practical recommendations and underlining the importance of experimental design, with implications for further research in this complex field.

Use of the Wise method as a pretreatment for the selective removal of lignin when preparing bamboo-activated carbon

The Wise method, which is known in wood science as a technique for the quantitative analysis of holocellulose, was applied as a pre-treatment to prepare a porous precursor for activated carbon. The precursor has a specific pore size structure derived from the organizational architecture of the natural plant. The lignin in the plant tissue of the bamboo was successfully removed in a stepwise fashion by the Wise method treatment. All the N2 adsorption isotherms at 77 K for the carbonized samples, including the samples treated by the Wise method, were similar in shape. Specifically, they were type I as classified by IUPAC with low-pressure hysteresis, and there was no specific effect of the selective removal of lignin from the carbonized sample. However, after CO2 activation for more than 2 h, samples treated with the Wise method had slit pores, which is type H4 as classified by IUPAC. The slit width calculated by the INNES method, which is the theory for slit-type mesopores, was ca. 2.5 nm. When samples delignified by more than 60% were activated by CO2 for 3 h, the percentage of the micropores in the total pores was ca. 40%. Samples treated by the Wise method for the stepwise selective removal of lignin, should enable pore structure derived from plant tissue to be controlled in the activated carbon.

Strategic Fast Induction Heating to Combat Hysteresis Barriers in a Flexible MOF for Rapid CO2 Desorption in Biogas Upgrading.

A major challenge in Cyclic Swing Separation using flexible adsorbents that have high equilibrium CO2 adsorption capacity is their very low-pressure hysteresis that hinders efficient desorption. Mg-Gallate MOF is such a flexible adsorbent that only begins to release CO2 at its pore closing pressure at 0.08bar and 30°C, showing very slow and inefficient desorption in pressure or temperature swing. Therefore, a novel strategy is presented that combines state of art technique Magnetic Induction Heating with a vacuum swing for fast and efficient CO2 desorption from flexible adsorbents at a moderately elevated temperature (70°C).

Pore Characteristics and Gas Preservation of the Lower Cambrian Shale in a Strongly Deformed Zone, Northern Chongqing, China

The Lower Paleozoic marine shale in southern China has undergone several strong tectonic transformations in an extensive region outside the Sichuan Basin. Although some shale strata underwent strong deformation, they still contain a significant amount of shale gas. The gas preservation mechanism in the strongly deformed shale has become the focus of attention. In this paper, the Lower Cambrian gas-bearing shale samples with a strong deformation taken from an exploration well in northern Chongqing, China, were investigated on their pore types and structure, with the aim to reveal the reason for the gas preservation. The pore types of the Lower Cambrian shale are dominated by microfractures and interparticle (interP) pores occurring mainly between clay minerals and between organic matter (OM) and clay minerals, while pores within OM that can be observed by FE-SEM (field emission-scanning electron microscopy) are rare. The shale has a low porosity, with an average of 1.56%, which is mainly controlled by the clay mineral content. The adsorption experiments of low pressure N2 (LPNA) and CO2 (LPCA) indicate that the shale is rich in micropores and small mesopores (<2–3 nm) provided mainly by OM, but mesopores with a size range of 3–50 nm are underdeveloped. The shale, as revealed by LPNA data, has dominant slit-like or plate-like pores and an obvious low-pressure hysteresis (LPH), indicating a low gas diffusion. The deformed shale samples with a removal of OM by oxidation and their isolated kerogen further indicate that the LPH is completely related to OM, without any relationship with minerals, while an undeformed shale sample, taken from another well for a comparison, has no obvious LPH for both of its OM-removed sample and kerogen. Based on a comprehensive analysis of the relative data, it is suggested that the nanopores related to OM and clay minerals in the shale were significantly altered owing to the deformation, with a result of the pores being squeezed into the slit-like shape and converted into micropores. This extraordinary pore structure of the shale formed during the deformation process should be the main preservation mechanism of shale gas.

Water adsorption characteristic and its impact on pore structure and methane adsorption of various rank coals.

Coalbed methane not only is a new clean energy source, but also has potential damage to ecological environment. Water and methane coexist in coal reservoir; understanding the adsorption of water on coal and its impact on pore structure and methane adsorption of coal is vital to evaluate the reserves and productivity of coalbed methane. In the paper, water adsorption characteristics of various rank coals are firstly investigated by ten mathematical models. The modified Dent model provides a best fit, followed by GAB and Dent models. For GAB model, the primary site adsorption is more difficult to reach saturation, and the contribution rate of the secondary site adsorption is surprisingly high at P/P0 approaching 0, which can be attributed to the possible overestimation of GAB monolayer adsorption capacity and secondary site adsorption. Besides, the low-rank coal sample YZG2 exhibits more prominent hysteresis than middle- to high-rank coals. The low-pressure hysteresis can be attributed to the water-water interactions over the primary site and the strengthened binding forces of water molecules in the water desorption process. In contrast, the high-pressure hysteresis largely depends on pore structure of coal such as ink-bottle pores, especially for the studied sample YZG2. Besides, pore analyses by low-temperature nitrogen adsorption method show that the pre-adsorbed water has remarkable influence on micropores smaller than 10 nm, and the micropores smaller than 4 nm almost disappear for water-equilibrated coals, which is closely related to the formed water clusters and capillary water in pore throats. This finding reveals that more methane gas can only be adsorbed in the larger pores of moist coal, and provides an explanation for water weakening methane adsorption capacity.

A model for elucidating the contributions of meso- and macropores to water sorption and strain in cementitious materials

Hardened cement paste and concrete have a wide pore size distribution. Micro-, meso- and macropores can contribute to water sorption isotherms. Modifying a model for Vycor glass, we describe the impact of meso- and macropores in cementitious materials on sorption, swelling and shrinkage by varying relative humidity. Swelling and shrinkage isotherms can be compared with experimental data for autoclaved aerated concrete (AAC). Apart from delayed capillary condensation due to metastable adsorbed films, trapped water in the pore network promotes the appearance of enlarged hysteresis loops in plots of linear strain versus relative humidity. The model is potentially capable of predicting the contributions of meso- and macropores to the humidity-dependent strain in ordinary Portland cement paste (OPC). But in this case a comparison with experimental results turns out to be more difficult, because sorption in slit-like micropores associated with low pressure hysteresis is the main source of the observed strain in OPC.

Adsorption Hysteresis in Open Slit-like Micropores.

Adsorption hysteresis in the low-pressure range is only rarely described in the literature. To optimise, for example, heat storage technologies, a deeper understanding of the low-pressure hysteresis (LPH) process is necessary. Here, two thermodynamically based approaches are further developed for analysing the LPH within the framework of thermodynamically irreversible processes and fractal geometry. With both methods developed, it is possible to obtain the description of the adsorption and desorption branches with high accuracy. Within the framework of the two thermodynamic models of the hysteresis loop, generalised equations are obtained with the control parameter in the form of the degree of irreversibility. This is done by taking the adsorption of water on alumina as an example. It is shown that the fractal dimension of the adsorption process is larger than the fractal dimension of the desorption branch, meaning that the phase state of the adsorbate is more symmetric during the adsorption step than in the desorption process.

Nanopore structure analysis of single wall carbon nanotube xerogels and cryogels

We fabricated two types of single wall carbon nanotube (SWCNT) gels prepared by freeze-drying (cryogels) and air-drying (xerogels) from the dispersed SWCNT hydrogels. The comparative pore structure analysis using N2 adsorption at 77 K and H2O adsorption at 298 K was applied to these SWCNT gels. The N2 adsorption isotherms of both SWCNT gels have a steep initial uptake, low-pressure adsorption hysteresis and adsorption hysteresis above P/P0 = 0.9, indicating the presence of pore mouth structures in addition to micropores and mesopores. The mesoporosity of the SWCNT cryogels was about twice higher than that of the SWCNT xerogels, while their microporosities were similar. The H2O adsorption isotherms of both SWCNT gels differ from each other along the whole P/P0 range. The H2O adsorption isotherm of the SWCNT xerogels shows an explicit adsorption below P/P0 = 0.1 and marked low-pressure adsorption hysteresis; the pore mouth volume of the SWCNT xerogels from the initial uptake of H2O was nearly three times larger than that of the SWCNT cryogels. The H2O adsorption isotherms of our SWCNTs gels give similar micropore volumes, being by ≈ 40% larger than that of N2 adsorption isotherms. The excess micropore volume evaluated by H2O adsorption comes from the pore mouth structures in which N2 cannot access. This work demonstrates that H2O adsorption and N2 adsorption analysis are crucial for elucidation of the pore mouth structure.

Synthesis of ZSM-5 zeolites from biomass power plant ash for removal of ionic dyes from aqueous solution: equilibrium isotherm, kinetic and thermodynamic analysis.

In this work, industrial biomass power plant ash was used to synthesize the ZSM-5 zeolites for the first time with the original intention to turn value-added material into wealth, and then committed to adsorption performance testing. Typical chemical structure and morphology of ZSM-5 zeolite were identified by comprehensive technologies. Uniquely, it was found that there was a low pressure hysteresis loop which was caused by crossed 10-membered rings in the N2 adsorption–desorption isotherm. To investigate the adsorption performance for dyes, the zeolite samples were used to remove cationic (MB) and anionic (CR) dyes from aqueous. The results demonstrated that when it came to adsorbing MB, the isotherm was in line with the Redlich–Peterson model (R2 > 0.99), whereas it was matched up with the Sips model (R2 > 0.99) for adsorbing CR. Kinetic models were assigned to the pseudo-second order equation (R2 ≥ 0.99) along with remarkable intraparticle diffusion. In the end, thermodynamic parameters were ΔG0 < 0, and Ea > 0. Especially, adsorbing MB was ΔS0 > 0, and ΔH0 > 0, whilst adsorbing CR was ΔS0 < 0, and ΔH0 < 0, which indicates that electrostatic interaction plays a significant role in the whole process. All in all, this work might encourage novel attempts to dispose of industrial biomass ash.

Fabrication and testing of PMOS current mirror-integrated MEMS pressure transducer

PurposeThis paper aims to describe the fabrication, packaging and testing of a resistive loadedp-channel metal-oxide-semiconductor field-effect transistor-based (MOSFET-based) current mirror-integrated pressure transducer.Design/methodology/approachUsing the concept of piezoresistive effect in a MOSFET, three identicalp-channel MOSFETs connected in current mirror configuration have been designed and fabricated using the standard polysilicon gate process and microelectromechanical system (MEMS) techniques for pressure sensing application. The channel length and width of thep-channel MOSFETs are 100 µm and 500 µm, respectively. The MOSFET M1 of the current mirror is the reference transistor that acts as the constant current source. MOSFETs M2 and M3 are the pressure-sensing transistors embedded on the diaphragm near the mid of fixed edge and at the center of the square diaphragm, respectively, to experience both the tensile and compressive stress developed due to externally applied input pressure. A flexible square diaphragm having a length of approximately 1,000 µm and thickness of 50 µm has been realized using deep-reactive ion etching of silicon on the backside of the wafer. Then, the fabricated sensor chip has been diced and mounted on a TO8 header for the testing with pressure.FindingsThe experimental result of the pressure sensor chip shows a sensitivity of approximately 0.2162 mV/psi (31.35 mV/MPa) for an input pressure of 0-100 psi. The output response shows a good linearity and very low-pressure hysteresis. In addition, the pressure-sensing structure has been simulated using the parameters of the fabricated pressure sensor and from the simulation result a pressure sensitivity of approximately 0.2283 mV/psi (33.11 mV/MPa) has been observed for input pressure ranging from 0 to 100 psi with a step size of 10 psi. The simulated and experimentally tested pressure sensitivities of the pressure sensor are in close agreement with each other.Originality/valueThis current mirror readout circuit-based MEMS pressure sensor is new and fully compatible to standard CMOS processes and has a promising application in the development CMOS-MEMS-integrated smart sensors.

Mesoscopic cage-like structured single-wall carbon nanotube cryogels

Abstract We fabricated mesoscopic cage-like structured Single-Wall Carbon Nanotube (SWCNT) cryogels having ultramicroporous necks using highly concentrated SWCNT-water inks with the aid of Zn–Al sol-gel dispersant. High-resolution transmission electron microscopy (TEM) image shows the presence of three-dimensional cage-like structures in the SWCNT cryogels, which are quite different from the bundle structures of pristine SWCNTs. Average bundle size of the SWCNT cryogels is 30% smaller than that of pristine SWCNTs according to the TEM observation. Adsorption isotherms of N2 at 77 K and Ar at 87 K for SWCNT cryogels and pristine SWCNTs have explicit adsorption hysteresis in low and high relative pressure regions, indicating the presence of necked ultramicropores and mesopores. Ar adsorption isotherms show that the SWCNT cryogels have predominant low and high relative pressure hysteresis, suggesting the presence of mesopores with necked entrances of ultramicropores. Surface areas of SWCNT cryogels by Ar and N2 adsorption are about two times larger than those of pristine SWCNTs which is in agreement with the geometrically evaluated surface area from SWCNT bundles. Mesopore volumes evaluated by adsorption of N2 and Ar are 3 and 2.3 times larger than those of pristine SWCNTs in the mesopore range above 15 nm. The marked increase in the mesoporosity of the SWCNT assemblies by freeze-drying supports the mesoscopic three-dimensional cage-like structures observed by TEM.

A model for sorption hysteresis in hardened cement paste

In hardened cement paste (C-S-H gel), water intercalated between the C-S-H layers can be adsorbed and desorbed by varying relative humidity. Sorption in the interlayer space between the C-S-H sheets is supposed to be responsible for low pressure hysteresis of water sorption isotherms. Additionally, large hysteresis loops also appear in diagrams that depict the humidity-dependent volume change of the paste. In contrast to most other porous solids, the volume of the C-S-H gel is larger along the desorption branch (drying) than along the adsorption branch (rewetting). Based on a macroscopic approach, we use a simple model to describe hysteresis in sorption and volume change diagrams. Computed hysteresis loops, which depend on the lateral extension of the interlayer spaces, are similar to loops observed experimentally.

Low Pressure Hysteresis in Materials with Narrow Slit Pores

Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy jumps by a finite value when touching slit walls are separated from each other. We consider a more realistic adhesion model by introducing a smoothly-varying adhesion force, which depends on the distance between the slit walls. The range of the attraction between the slit walls is found to have a pronounced influence on the shape of hysteresis loops at low vapor pressure. A large interaction range avoids an extraordinarily small relative vapor pressure necessary for pore closing, which is a precondition for low pressure hysteresis. Our extended model allows us to describe a discontinuity, which can appear in the desorption branch of swelling/shrinkage diagrams for hardened cement paste.

Restricted adsorption of carbon dioxide gas in fluorinated single-walled carbon nanotubes

Single-walled carbon nanotubes were fluorinated between 298 and 473 K using elemental F2, and their adsorption properties were investigated through N2 and CO2 adsorption measurements. Interestingly, a low-pressure adsorption/desorption hysteresis phenomenon was observed for the CO2 adsorption/desorption isotherms of the fluorinated nanotubes. This remarkable adsorption/desorption hysteresis may be attributed to restricted access to the inner pores of the nanotubes due to the presence of fluorinated pore gates. Diffusional limitations caused by such pore blocking effects are therefore discussed as the origin of the hysteresis. This low-pressure hysteresis may provide an insight into adsorption in the fluorinated pores, which is of particular interest for gas adsorption technologies.

Gas adsorption on commercial magnesium stearate: The origin of atypical isotherms and BET transform data

The causes of atypical nitrogen adsorption isotherm features of commercial magnesium stearate grades have been investigated through a combination of alternative adsorbates and variation of analytical conditions. Atypical features have previously been identified as having poor linearity of adsorption isotherms at relative pressures applied to the calculation of BET surface area and low pressure hysteresis between adsorption and desorption branches. These features are most pronounced for di-hydrated MgSt and are absent for the anhydrous form: the presence of hydrated water causing atypical features. The possible interaction between hydrated water molecules and nitrogen via the nitrogen quadrupole is investigated through application of non-polar adsorbates; argon and krypton. Low pressure hysteresis is investigated through alteration of equilibration conditions and through truncating adsorption isotherms to successively lower maximum relative pressures before commencing desorption. It is found that atypical gas sorption isotherms are not limited to nitrogen. The effects of these conditions on adsorption and desorption volumes are discussed and a model for the origin of atypical isotherm features is presented. The difference in gas sorption behaviour of anhydrous, mono-hydrated and di-hydrated magnesium stearate is discussed and ultimately the consequences on the calculation of BET surface area is considered.

Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths.

Fabrication of nanographene shows a promising route for production of designed porous carbons, which is indispensable for highly efficient molecular separation and energy storage applications. This process requires a better understanding of the mechanical properties of nanographene in their aggregated structure. We studied the structural and mechanical properties of nanographene monoliths compressed at 43 MPa over different times from 3 to 25 h. While in monoliths compressed over shorter time adsorption isotherms of Ar at 87 K or N2 at 77 K exhibited a prominent hysteresis due to presence of predominant mesopores, compression for long time induces a low pressure hysteresis. On the other hand, compression for 25 h increases the microporosity evaluated by Ar adsorption, not by N2 adsorption, indicating that 25 h compression rearranges the nanographene stacking structure to produce ultramicropores that can be accessible only for Ar. TEM, X-ray diffraction, and Raman spectroscopic studies indicated that the compression for 25 h unfolds double-bent-like structures, relaxing the unstable nanographene stacked structure formed on the initial compression without nanographene sheets collapse. This behavior stems from the highly elastic nature of the nanographenes.

Water vapour sorption on mudrocks

Abstract High-resolution water sorption isotherms were measured on 13 representative mudrock samples in order to assess the mechanisms of water vapour sorption and their relationship to the pore structure of mudrocks. The isotherm measurements were performed at 303 K on a gravimetric, dynamic vapour sorption device. Experimental data were interpreted by traditional physisorption models for which the validity was evaluated by relating model parameters to those obtained from nitrogen physisorption measurements. No direct relationships with the pore structure were observed, except for the Gurvich total pore volumes and the corresponding porosity data. Specific surface areas from Brunauer–Emmett–Teller theory are ambiguous and do not relate to nitrogen data, suggesting that water molecules do not adsorb as (multi-) layers covering pore walls. The volume filling theory (Dubinin–Astakhov equation) fits the water sorption data well but no relationship to the nitrogen data was observed in the studied sample set. A lower affinity of water for micropores was evident from the higher filling pressures of N 2 -based micropore volumes. The Barret–Joyner–Hallenda theory combined with N 2 physisorption measurements on moist mudrocks revealed that capillary condensation prevails close to saturation but not below about 0.94 relative pressure ( p / p 0 ). A distinct low-pressure hysteresis was observed from hysteresis scanning that was attributed to surface chemistry since capillary condensation occurs only at very high relative pressures. Analysis of mineralogical composition, total organic content (TOC) and organic maturity in relation to water sorption revealed only a weak correlation with the total clay content. In contrast, cation-exchange capacity (CEC) strongly correlates with water uptake, which evidences a surface-chemistry-controlled sorption mechanism. Tests of the influence of the exchangeable cation were inconclusive because pore system alteration due to cation-exchange probably superimposed the effect. To further assess the sorption mechanisms of water, nitrogen physisorption isotherms were measured on moisture-equilibrated mudrocks (11, 52, 75, 94% relative humidity at 298 K). Micropore analysis and cumulative pore-size distributions denote that water blocks pore throats rather than fills pore volumes at lower relative humidities. Over the entire humidity range, no direct relationship between water sorption and pore size was observed. These findings imply that water adsorption does not sequentially fill pores with increasing radii in mudrocks as relative humidity increases, as would be expected from water sorption by capillary condensation. This conclusion has important implications for the interpretation and measurement of geomechanical and petrophysical properties of mudrocks. Capillary pressures, particularly at low water saturations, are often calculated from water saturation using a concept based on the Kelvin equation for capillary condensation. Since water sorption in mudrocks seems to be controlled by surface chemistry rather than pore size, this approach is questionable. The observations reported here suggest that the water distribution in mudrock pore systems resulting from vapour equilibration differs from that obtained by fluid displacement (i.e. capillary drainage or imbibitions). A further consequence is that water vapour equilibration is a convenient, but not necessarily representative, method to obtain partially water-saturated mudrock samples for laboratory measurement of saturation-dependent geomechanical or petrophysical properties.

Dislocation model for sorption hysteresis in deformable solids

Adsorption and desorption branches of sorption isotherms for soft materials often differ from each other even at low relative vapor pressures of the adsorbate. This low vapor pressure hysteresis may result from elastic deformations produced by the capillary tension that condensates exert on pore walls. A dislocation model for materials with very narrow slit pores is proposed. The shape of adsorption/desorption isotherms of soft solids with slit pores depends on a dimensionless parameter, which is proportional to Young's modulus, the square of pore width, and inversely proportional to the adhesion energy of the solid. The model designed for soft solids can be modified to describe low vapor pressure hysteresis in more rigid materials such as hardened cement pastes.

Optimal Surface Amino-Functionalization Following Thermo-Alkaline Treatment of Nanostructured Silica Adsorbents for Enhanced CO₂ Adsorption.

Special preparation of Santa Barbara Amorphous (SBA)-15, mesoporous silica with highly hexagonal ordered, these materials have been carried out for creating adsorbents exhibiting an enhanced and partially selective adsorption toward CO2. This creation starts from an adequate conditioning of the silica surface, via a thermo-alkaline treatment to increase the population of silanol species on it. CO2 adsorption is only reasonably achieved when the SiO2 surface becomes aminated after put in contact with a solution of an amino alkoxide compound in the right solvent. Unfunctionalized and amine-functionalized substrates were characterized through X-ray diffraction, N2 sorption, Raman spectroscopy, electron microscopy, 29Si solid-state Nuclear Magnetic Resonance (NMR), and NH3 thermal programmed desorption. These analyses proved that the thermo-alkaline procedure desilicates the substrate and eliminates the micropores (without affecting the SBA-15 capillaries), present in the original solid. NMR analysis confirms that the hydroxylated solid anchors more amino functionalizing molecules than the unhydroxylated material. The SBA-15 sample subjected to hydroxylation and amino-functionalization displays a high enthalpy of interaction, a reason why this solid is suitable for a strong deposition of CO2 but with the possibility of observing a low-pressure hysteresis phenomenon. Contrastingly, CH4 adsorption on amino-functionalized, hydroxylated SBA-15 substrates becomes almost five times lower than the CO2 one, thus giving proof of their selectivity toward CO2. Although the amount of retained CO2 is not yet similar to or higher than those determined in other investigations, the methodology herein described is still susceptible to optimization.

Adsorption and desorption of nitrogen at 77 K on micro- and meso- porous materials: Study of transport kinetics

The paper presents and discusses the kinetic results of nitrogen adsorption and desorption experiments performed at 77 K on one aluminium-based and three carbon-based materials differing in their microporous and mesoporous nature. The paper focuses on an examination of the transport phenomena during the adsorption/desorption experiments in different-sized pores (micropores and mesopores) identified by both Horvath–Kawazoe and modified Kelvin equations. Of the eight kinetic relations applied to sorption kinetics data, Crank's intraparticle diffusion model is found to be the best-describing.It was observed that rates of transport phenomena significantly change with gradual pore-filling/emptying. Transport proceeds at a minimal rate for a fine micropore (diameter ≤ 1 nm) filling region and increases steeply upon filling of wider micropores (diameter 1–2 nm). The maximum transport rate is found to be during the filling of pores between 2 and 4 nm in size. Once irreversible capillary condensation in wider mesopores appears, transport of the gaseous phase is highly limited by a condensate. The desorption process takes place faster only for pure microporous activated carbon. In the case of mixed microporous/mesoporous materials, the transport kinetics is faster only during evaporation and desorption from larger mesopores. For the desorption a sharp drop in rate upon outward transport is identified and ascribed to a cavitation phenomenon. A low-pressure hysteresis observed (for subbituminous coal) between adsorption/desorption isotherms is not related to changes in transport kinetics.