Adsorbed Phase Density Research Articles

Characterization of methane adsorption on overmature Lower Silurian–Upper Ordovician shales in Sichuan Basin, southwest China: Experimental results and geological implications

A series of methane adsorption isotherms were measured at 35.4°C, 50.6°C, and 65.4°C at pressures up to 15.0MPa for eight dried, overmature Lower Silurian–Upper Ordovician shale samples collected from the Sichuan Basin with TOC values in the range of 1.87–5.74%. The measured maxima of excess adsorption capacity of methane range from 1.25 to 2.50cm3/g rock at 65.4°C; the maxima are slightly enhanced at 35.4°C, but all are positively correlated with total organic carbon (TOC). Both the supercritical Dubinin–Radushkevich (SDR)- and Langmuir-based excess adsorption models were found to represent the experimental excess adsorption isotherms equally well within the experimental range. The temperature-dependent densities of adsorbed methane resulting from the parameter fit of the SDR-based excess adsorption model are in the range of 297–415mg/cm3; for the Langmuir-based excess adsorption model, the adjusted densities range from 386mg/cm3 to 1027mg/cm3 and most of them are much larger than the liquid density of methane at its boiling point (424mg/cm3). Nevertheless, the maxima of absolute methane adsorption capacity fitted by both models are not significantly different and are linearly correlated. One of the contributors to the uncertainty of the gas-in-place estimation in geological conditions is the inconsistent utilization of experimental excess sorption data as “absolute sorption” values, particularly at high pressures. However, the choice of adsorption model itself (Langmuir- or SDR-based) and the fitting procedure, assuming either constant or temperature-dependent adsorbed phase density and maximum sorption capacity, do not significantly affect the estimated GIPs for the geological system studied here with depths of less than 4000m.

Impact of preparation and storage of activated carbon on the high pressure sorption of CO2

Abstract Adsorption of carbon dioxide on activated carbons has become extremely interesting in the field of energy and environment. Activated carbon is either used as a model to understand sorption processes on coals as a part of research on CO2 geological storage/Enhanced Coalbed Methane or as an adsorbent for processes such as natural gas treatment or CO2 separation from flue gas. The paper presents results of high-pressure CO2 sorption at the temperature of 318K on two similar activated carbons (Filtrasorb 400 and WAZ 0.6-2.4 mm) where one sample of WAZ was not subjected to any pretreatment procedure. Experimental results were fitted with three parameter Langmuir and therefore it was possible to calculate CO2 adsorbed phase densities. The WAZ activated carbon has a slightly lower sorption even though it was pretreated with the same procedure. The untreated sample of WAZ exhibited sorption which was lower over 15%. Calculated adsorbed phase densities differ between the activated carbons and the lowest value was obtained for the untreated WAZ sample (21.0 mol/dm3).

Combined Monte Carlo and molecular dynamics simulation of methane adsorption on dry and moist coal

A quantitative understanding of methane (CH4) adsorption on dry and moist coal and the mechanism of coal swelling is vital for successful coal bed methane (CBM) projects. CH4 adsorption isotherms of coal with moisture contents ranging from 0 to 3wt% water, the temperature effect on maximum adsorption capacity, coal swelling, and adsorbed phase density have been modeled by performing combined Monte Carlo (MC) and molecular dynamics (MD) simulations at temperatures of 308 and 370K (35 and 97°C) and at pressures up to 10MPa. Simulation results demonstrate that absolute adsorption (expressed as a mass basis) divided by bulk density is independent of temperature for CH4 on dry coal when pressure is over 8MPa. Both the adsorption capacity and adsorption rate of CH4 decrease, while coal swells as moisture content increases. These results show that the presence of water in the coal matrix reduces the interaction between the coal and methane. Our results indicate that coal–water interaction dominates and is the main contributing factor to the coal swelling. The interaction of CH4–H2O and CH4–CH4 is negligible and the absolute adsorption of CH4 on both dry and moist coal follows the Langmuir isotherm for the pressure range simulated. This study provides a quantitative understanding of the effects of moisture and temperature on CH4 adsorption, coal swelling, and the adsorbed CH4 density from a microscopic perspective. Molecular modeling proves to be a valuable and cost-effective tool for studying gas adsorption behavior in complex and complicated systems.

Understanding and application of CO2 adsorption capacity estimation models for coal types

Geosequestration is an attractive mechanism to store CO2 for geologically significant time periods in order to minimise climate changing impacts on the environment. Reservoirs such as coal seams are used as storage reservoirs. However, it is very important to estimate the quantity of CO2 that can be stored in a coal reservoir. In this study, existing storage models such as correlation models and Equations of State (EOSs) in the context of different coal types are compared, together with the experimentally-observed isotherms. Under highly dense phase non-ideal conditions, CO2 storage potential will be maximised. Therefore, the use of fugacity in place of pressure is suggested to estimate CO2 storage capacity under highly dense phase conditions. Further, four new modified correlation type models are suggested and found to be of good agreement with existing correlation-type adsorption models such as the DR and Toth models. By using various adsorbed phase density values, it was found that use of value as 1.028g/cm3 gave the most consistent results across all the proposed isotherm equations. Fugacity did not improve the results for the correlation models, nor did it worsen them. The correlation models were clearly less accurate than the EOS adsorption models, especially under highly dense phase conditions. For less dense phase conditions, all the isotherm models gave comparable results to the experimental adsorption isotherms, while for highly dense phase conditions there were varying results.

Shale Gas-in-Place Calculations Part I: New Pore-Scale Considerations

SummaryUsing focused-ion-beam (FIB)/scanning-electron-microscope (SEM) imaging technology, a series of 2D and 3D submicroscale investigations revealed a finely dispersed porous organic (kerogen) material embedded within an inorganic matrix. The organic material has pores and capillaries having characteristic lengths typically less than 100 nm. A significant portion of total gas in place appears to be associated with interconnected large nanopores within the organic material.Thermodynamics (phase behavior) of fluids in these pores is quite different; gas residing in a small pore or capillary is rarefied under the influence of organic pore walls and shows a different density profile. This raises serious questions related to gas-in-place calculations: Under reservoir conditions, what fraction of the pore volume of the organic material can be considered available as free gas, and what fraction is taken up by the adsorbed phase? How accurately is the shale-gas storage capacity estimated using the conventional volumetric methods? And finally, do average densities exist for the free and the adsorbed phases?We combine the Langmuir adsorption isotherm with the volumetrics for free gas and formulate a new gas-in-place equation accounting for the pore space taken up by the sorbed phase. The method yields a total-gas-in-place prediction. Molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature predict density profiles across the pores and show that (a) the adsorbed methane forms a 0.38-nm monolayer phase and (b) the adsorbed-phase density is 1.8–2.5 times larger than that of bulk methane. These findings could be a more important consideration with larger hydrocarbons and suggest that a significant adjustment is necessary in volume calculations, especially for gas shales high in total organic content. Finally, using typical values for the parameters, calculations show a 10–25% decrease in total gas-storage capacity compared with that using the conventional approach. The role of sorbed gas is more important than previously thought. The new methodology is recommended for estimating shale gas in place.

Microscopic analysis of adsorption in slit-like pores: layer fluctuations of particle number, layer isosteric heat and histogram of particle number

A number of measures are proposed as a microscopic means to analyse adsorption of gas on a surface and in slit pores under subcritical and supercritical conditions. Layer fluctuation of particle number provides us with information on where most of the mass interchange occurs, which can then be used as an indicator of the position of the interface separating the adsorbed phase and gas phase. The layer compressibility can be used to compare the adsorbed phase density with that of the bulk liquid. The layer isosteric heat provides an indication of the relative contribution of each layer to the overall isosteric heat. Finally, a histogram of particle number as a function of fluid–fluid particle energy is utilised to yield valuable information about the energetic structure of the adsorbed phase, for example (1) the number of neighbouring particles and (2) the evolution of the arrangement of particles.

Evaluation of an industrial pilot scale densified MOF-177 adsorbent as an on-board hydrogen storage medium

Exploring and evaluating on-board solid state hydrogen storage systems performance are of great interest for fuel cell electric vehicles development. In this report, we present gravimetric and volumetric capacities of a hydrogen storage system based on a densified MOF-177 adsorbent. This is, to our knowledge, the first thorough study of an engineered industrial scale MOFs for hydrogen storage application. The measurements were performed over the 50–120 K and 0–40 bar ranges, and modeled using micropore filling approaches. The performances of a potential 100 L vessel filled with the densified MOF-177 are inferred from the modeling parameters. A comparison of this technology with the 70 MPa compressed gas hydrogen system shows under which conditions the adsorbent offer advantages in terms of volumetric and gravimetric capacities. Further comparison with AX-21 activated carbon pellets reveals that densified MOF-177 stores about 40% more at 77 K and 35 bar. In order to get a physically sound modeling analysis, we introduced an approach to establish effective saturation pressures for supercritical adsorption. This approach insures a consistency between key model parameters and the observed liquid properties of the adsorbed phase at the lowest temperatures. We show that modeling using temperature-dependent saturation pressures and adsorbed phase densities leads to important differences in the projected usable storage capacities. Such differences can be as much as 25% at 50 K in the high pressure limit, revealing the importance of physical insights in the modeling approach.

On the Nature of the Adsorbed Hydrogen Phase in Microporous Metal−Organic Frameworks at Supercritical Temperatures

Hydrogen adsorption measurements on different metal-organic frameworks (MOFs) over the 0-60 bar range at 50 and 77 K are presented. The results are discussed with respect to the materials' surface area and thermodynamic properties of the adsorbed phase. A nearly linear correlation between the maximum hydrogen excess amount adsorbed and the Brunauer-Emmett-Teller (BET) surface area was evidenced at both temperatures. Such a trend suggests that the adsorbed phase on the different materials is similar in nature. This interpretation is supported by measurements of the adsorbed hydrogen phase properties near saturation at 50 K. In particular it was found that the adsorbed hydrogen consistently exhibits liquid state properties despite significant structural and chemical differences between the tested adsorbents. This behavior is viewed as a consequence of molecular confinement in nanoscale pores. The variability in the trend relating the surface area and the amount of hydrogen adsorbed could be explained by differences in the adsorbed phase densities. Importantly, the latter were found to lie in the known range of bulk liquid hydrogen densities. The chemical composition and structure (e.g., pore size) were found to influence mainly how adsorption isotherms increase as a function of pressure. Finally, the absolute isotherms were calculated on the basis of measured adsorbed phase volumes, allowing for an estimation of the total amounts of hydrogen that can be stored in the microporous volumes at 50 K. These amounts were found to reach values up to 25% higher than their excess counterparts, and to correlate with the BET surface areas. The measurements and analysis in this study provide new insights on supercritical adsorption, as well as on possible limitations and optimization paths for MOFs as hydrogen storage materials.

Quantum Effect-Mediated Hydrogen Isotope Mixture Separation in Slit Pore Nanoporous Materials

We use path integral simulations to investigate the separation of H2 and HD, as well as H2 and D2, in carbon slit pores of various sizes using a new improved set of carbon−hydrogen interaction parameters determined in our laboratory. As expected, the selectivity of HD over H2 is lower than that of D2 over H2 at the same conditions due to the smaller mass of HD molecules and hence larger quantum effects. In the pressure range of 0.1−10.0 bar, the selectivity is not sensitive to the pressure at the temperature of 77 K. At 40 K the selectivity shows a positive relation to the adsorbed phase density. We also report an unusual crossover effect in which the selectivity in a pore of width 0.85 nm exceeds that in a smaller pore (0.69 nm) at high densities due to enhanced quantum confinement effects when a second layer forms in the larger pore. The optimal pore widths for HD/H2 separations were identified to be 0.56−0.57 nm, with operating pressures of 10.0 and 0.1 bar for the two pore sizes, respectively. We also simulate equilibrium separation in the commercial Takeda 3 Å carbon molecular sieve, based on a slit-like pore model with a distribution of pore sizes, but find only modest equilibrium selectivity for HD over H2. It is suggested that while quantum effects are small within the pore bodies, narrow pore entrances must lead to significant quantum effects on the dynamics in order to explain literature data of faster uptake of D2 compared to H2 at 77 K in this material. Thus, kinetic molecular sieving at narrow necks, for which this material is well established, maybe a more attractive option than equilibrium separation. Alternatively, materials with controlled smaller pore sizes are needed for more efficient equilibrium HD/H2 separation. The ideal adsorption solution theory (IAST) is also examined for prediction of the binary hydrogen isotope mixture isotherms in the presence of quantum effects and is found to match simulations at all operating conditions investigated.

Anomalous Henry's law behavior of nitrogen and carbon dioxide adsorption on alkali-exchanged chabazite zeolites

This work presents adsorption equilibria of N 2 and CO 2 on a fully exchanged potassium chabazite (KCHA), sodium chabazite (NaCHA) and lithium chabazite (LiCHA) zeolites. Isotherms were measured at 273, 303 and 333 K over pressure ranges from 0.1 to 103 kPa for N 2 and from 0.001 to 103 kPa for CO 2, using a volumetric apparatus. In all cases, CO 2 adsorption capacities were remarkably higher than those for N 2 with unusual non-linear isotherm trends at pressures lower than 1.0 kPa. Accordingly, extrapolation to zero pressure for the determination of Henry's constants is problematic and could be erroneous. The maximum loadings of N 2 and CO 2 were obtained on LiCHA, and very low loadings of N 2 on KCHA (attributed to pore blockage) suggest KCHA is a promising adsorbent for N 2/CO 2 mixture separation. The adsorbed phase density of CO 2 on LiCHA was found to be fairly consistent with the theoretical value derived from van der Waals equation, while densities on NaCHA and KCHA deviated significantly from van der Waals’ values. This discrepancy was attributed to the underestimation of pore volumes in NaCHA and KCHA obtained from N 2 isotherms at 77 K. Henry's law constants were extracted from virial plots for the measured isotherms. The virial plots for CO 2 isotherms on KCHA and NaCHA exhibit “negative-slope” profiles at 273 K and pressures lower than about 1.0 kPa. The temperature dependence of the Henry's law constants obtained from the virial plots indicates that at these conditions we do not have a true equilibrium condition. Rather, we suggest that kinetic hindrance dominated CO 2 adsorption at the low coverage region partially due to pore blockage.

Thermodynamic study of the adsorbed hydrogen phase in Cu-based metal-organic frameworks at cryogenic temperatures

The hydrogen adsorption properties of two isostructural copper-based microporous metal-organic frameworks (MOFs) materials which differ in the length of the bridging ligands were investigated over the 50–100 K and 0–40 bar ranges. The measured excess adsorption isotherms were analyzed in terms of adsorption enthalpies, adsorbed phase densities and volumes. The characteristic excess maximum was found to be displaced to lower pressures and to vary less as a function of temperature on the material with the shorter ligand. This behaviour could be explained, on the basis of enthalpy calculations, by an enhanced hydrogen affinity in smaller pores. On the other hand, it was also found that the material with the shorter ligand has a reduced storage capacity. This observation could be explained, from measurements near saturation, by a reduction of both adsorbed phase density and volume. Despite their structural difference, both MOFs adsorbed hydrogen near saturation under an incompressible phase reaching bulk liquid densities. These properties suggest that repulsive forces may ultimately limit the packing of supercritical molecules in small pores despite apparently stronger solid–gas interactions.

Application of a Modified Dubinin−Radushkevich Equation to Adsorption of Gases by Coals under Supercritical Conditions

The classic Dubinin−Radushkevich (DR) equation, which is used to describe the adsorption of gases, requires a “saturation pressure” in its calculation, which is not defined for supercritical conditions. By using gas density instead of gas pressure (and adsorbed phase density rather than saturation pressure) to describe the sorption of the gas onto the surface, the DR equation can be applied in supercritical conditions. A proportionality term (k times the gas density) was added to this modified DR equation to account for possibilities such as Henry's law dissolution by the coal, errors in cell volume and helium density, and differences in the accessibility to coal between helium and the other gases. The sorption characteristics of three dry Australian bituminous coals when exposed to carbon dioxide, methane, and nitrogen were measured over the range 0−20 MPa at 53 °C. The data were fitted by the modified DR equation to within 1% of the sorption capacity across the isotherm in nearly all cases, substantiall...

High-Pressure Adsorption of Methane and Carbon Dioxide on Coal

Adsorption isotherms of methane and carbon dioxide on two kinds of Australian coals have been measured at three temperatures up to pressures of 20 MPa. The adsorption behavior is described by three isotherm equations: extended three-parameter, Langmuir, and Toth. Among these, the Toth equation is found to be the most suitable, yielding the most realistic values of pore volume of the coals and the adsorbed phase density. Also, the surface area of coals obtained from CO2 adsorption at 273 K is found to be the meaningful parameter which captures the CO2 adsorption capacity. A maximum in the excess amount adsorbed of each gas appears at a lower pressure with a decrease in temperature. For carbon dioxide, after the appearance of the maximum, an inflection point in the excess amount adsorbed is observed close to the critical density at each temperature, indicating that the decrease in the gas-phase density change with pressure influences the behavior of the excess amount adsorbed. In the context of CO2 sequest...

Ethane adsorption in slit-shaped micropores: influence of molecule orientation on adsorption capacity

Adsorption of ethane in a slit shaped micropore system has been studied by Monte Carlo molecular simulation by considering this hydrocarbon as a two interacting sites molecule. Ethane adsorption in pore sizes from 0.41 to 1.66 nm was simulated at 303 K. Microscopic characteristics of the adsorbed phase have been studied for pores of different size, comparing two density profiles: the molecule centre of mass profile and the molecular interaction site profile. Averaged angle distribution of molecule positions with respect to the slit plane across the pore width has been also obtained by simulation. These results were related to ethane molecule packing efficiency, which is also related to the adsorption capacity in terms of the adsorbed phase density. Packing efficiency presents an oscillation shape as the result of the adsorbate disorder inside the pore. Pressure influence on the adsorption has been studied by following pore filling by simulation. When pore condensation takes place and for pressures above condensation, fluid-fluid interactions are determinant in molecule disorder observed between the two adsorbed layers.

Methane Adsorption Storage Using Microporous Carbons Obtained from Coconut Shells

This paper presents an experimental and theoretical study of the adsorption equilibria of methane on activated carbon, focusing on the accurate determination of the absolute adsorbed mass from raw gravimetric measurements performed at high pressures (above 0.1 MPa). Two carbon samples were selected for these studies: a commercial sample (SRD-21) and a sample prepared in laboratory from coconut shells (CAQF-30). A gravimetric set up was used to measure the adsorption properties. Methane isotherms were obtained in the pressure range of 1 to 70 bar and under temperatures from 10 to 80∘C. Equilibrium experimental data were evaluated with the aid of three approaches labeled as D, DD and DDA, the latter being proposed for the first time in this paper. The DDA approach provided consistent and physically meaningful results for the adsorbed phase density. The adsorption isotherm that was obtained following the DDA approach matched those obtained following an approach published in the literature. The results indicate that the CAQF-30 sample, despite being prepared from carbonaceous wastes, presents competitive values for the methane adsorption parameters when compared to the commercial sample.

Multicomponent adsorption on activated carbons under supercritical conditions

Adsorption of binary mixtures onto activated carbon Norit R1 for the system nitrogen–methane–carbon dioxide was investigated over the pressure range up to 15 MPa. A new model is proposed to describe the experimental data. It is based on the assumption that an activated carbon can be characterized by the distribution function of elements of adsorption volume (EAV) over the solid–fluid potential. This function may be evaluated from pure component isotherms using the equality of the chemical potentials in the adsorbed phase and in the bulk phase for each EAV. In the case of mixture adsorption a simple combining rule is proposed, which allows determining the adsorbed phase density and its composition in the EAV at given pressure and compositions of the bulk phase. The adsorbed concentration of each adsorbate is the integral of its density over the set of EAV. The comparison with experimental data on binary mixtures has shown that the approach works reasonably well. In the case of high-pressure binary mixture adsorption, when only total amount adsorbed was measured, the proposed model allows reliably determining partial amounts of the adsorbed components.

Modeling the adsorption of pure gases on coals with the SLD model

The simplified local density/Peng–Robinson model (SLD-PR) was modified to improve its predictive capability when dealing with near-critical and supercritical adsorption systems of the type encountered in coalbed methane recovery and CO 2 sequestration. The goal was to develop efficient equation-of-state (EOS) computational frameworks for representing adsorption behavior, as well as to improve our understanding of the phenomenon. The ability of the modified SLD-PR to correlate accurately data for supercritical adsorption systems is demonstrated using adsorption measurements on activated carbon, Illinois #6 coal, Fruitland coal, and Lower Basin Fruitland coal. The results indicate that the modified SLD-PR model, which incorporates a modified repulsive parameter “ b” for the PR EOS, is capable of modeling the adsorption of pure methane, nitrogen, and CO 2 at coalbed conditions. Inclusion of a slit geometry in the adsorbent matrix yields results superior to our previous two-dimensional EOS models for the adsorbates considered. The results also indicate that accounting for the adsorption surface structure within the SLD-EOS framework is effective in improving modeling capability for high-pressure adsorption phenomena. An explanation is offered as to why the adsorbed-phase densities are close to the EOS reciprocal co-volumes. Further, the model (a) generates direct estimates for the adsorbed-phase densities (which facilitate reliable prediction of absolute gas adsorption) and (b) readily describes the observed maximum in Gibbs-adsorption isotherms of CO 2 at the temperatures and pressures encountered in coalbeds.

Adsorption of supercritical fluids in non-porous and porous carbons: analysis of adsorbed phase volume and density

In this paper, we revisit the surface mass excess in adsorption studies and investigate the role of the volume of the adsorbed phase and its density in the analysis of supercritical gas adsorption in non-porous as well as microporous solids. For many supercritical fluids tested (krypton, argon, nitrogen, methane) on many different carbonaceous solids, it is found that the volume of the adsorbed phase is confined mostly to a geometrical volume having a thickness of up to a few molecular diameters. At high pressure the adsorbed phase density is also found to be very close to but never equal or greater than the liquid phase density.

Isosteric Heats of Adsorption from Gas Mixtures

Isosteric heats of gas-mixture adsorption required in the energy balances of adsorption process design are calculated using a thermodynamically consistent multicomponent isotherm. This isotherm and the resulting heats of adsorption possess proper limiting behavior and account for adsorbates of different sizes. Binary and ternary mixtures are used as examples. Consideration of the temperature dependence of the adsorbed phase density or alternately of the saturation capacity leads to coupling of contributions from each component to isosteric heats of adsorption of other components in the mixture. For the binary example used, this reduces the difference between isosteric heats in the mixture and corresponding pure gas heats at the same fractional loading, in comparison to predictions where the saturation capacity is assumed to be independent of temperature.

On laser monitoring of transport processes in capillary porous bodies

Some trends in the diffusion of gases in capillaries exposed to resonance laser radiation are considered. The case of a free-molecular gas flow regime is studied in which the interactions of molecules with the walls of capillaries exert a substantial influence on the transport processes, the resonance radiation effect manifesting itself in the variation of such parameters as the energy and time of adsorption and the sticking probability. In fairly fine capillaries, when the surface transport of particles is significant, a change in the above parameters leads to the redistribution of the adsorbed phase density along the capillary length, as a result of which there appears a resultant flux of molecules even in the system previously in an equilibrium state. The problem of the radiation-induced drift of molecules in wide enough capillaries, when the transfer of particles over the surface may be neglected, is also considered. It is shown that radiation may cause a change in the course of the unsteady-state diffusion of gases in porous bodies.