High-pressure Adsorption Isotherms Research Articles

Water Injection Into Coal Seams for Outburst Prevention: The Coupling Effect of Gas Displacement and Desorption Inhibition.

Gassy coal seams generally have low permeability and dry coal bodies, which are susceptible to coal and gas outburst hazards in the process of mining. Water injection into coal seams can significantly alter the gas release rate and flow behavior. However, water has dual effects on coal seams: gas displacement and water-locking, and the coupling mechanism of these two effects is not clear in the whole process of coal seam water injection. By measuring high-pressure gas adsorption isotherms and gas diffusion initial velocity, it was found that both the Langmuir adsorption constant a and gas diffusion initial velocity ΔP decrease with the increase of water content, which would reduce outburst risks. Through the self-developed integrated experimental device of "gas adsorption + water injection displacement + gas desorption″, the changing rules of gas displacement amount, desorption amount, and water lock amount under different water injection conditions were studied. The results show that when the water injection ratio increases from 6 to 25%, the gas displacement would increase from 0.62 to 1.16 mL/g, with an increase of 87.09%. Also, at the same time, the gas desorption capacity would decrease from 4.86 to 4.05 mL/g after pressure relief, with a decrease of 16.67%. The amount of water-locking increased from 0.11 to 0.38 mL/g. The effect of water injection to control coal and gas outburst occurs in two different water injection stages. In the process of water injection, water plays a major role in gas displacement, which is conducive to reducing the gas content of the coal seam. After the completion of water injection, the static water pressure remaining in the coal seam can reduce the gas emission rate. The combination of these two effects can effectively reduce the risk of outbursts.

Enhanced ammonia adsorption-desorption properties of synthesized zeolite-carbon composite with the effect of Si/Al ratio

Effective ammonia (NH3) adsorbent is crucial for removing undecomposed NH3 residue after splitting into N2 and H2 fuel. In this study, a zeolite-activated carbon (AC) hybrid was successfully synthesized with a trace AC amount and different Si/Al ratios via hydrothermal method. The NH3 adsorption–desorption properties and mechanisms were investigated through two major activity tests, employing high pressure adsorption isotherm analyzer and breakthrough set up. Incorporating AC at 1 wt%, effectively shortened the regeneration period of the pure zeolite by ∼50% due to the AC’s pore-widening effect on zeolite network, reducing the trapping of adsorbed NH3 in the zeolite pores. Interestingly, an optimum Si/Al ratio of 1.6 led to the highest specific surface area of 977.57 m2/g by the formation of zeolite mixture with coexisting NaX (faujasite zeolite with high Al atoms per cell) and NaY (higher crystalline faujasite zeolite with low Al atoms per cell). These improved structural characteristics consequently exhibited the highest NH3 (99.9995 %) storage capacity of 15.6 mmol/g at a partial pressure of 300 kPa in isotherm analysis at 25 °C. Besides, the initial breakthrough adsorption capacity for dilute NH3 (5 %/N2 balance) under atmospheric conditions reached 8.9 mmol/g, accurately equivalent to isotherm analysis result at 5 kPa. This reproducibility of consistent adsorption uptake within low pressure region in both tests was credited to the compositional presence of both acidic and alkaline surface functionalities in the optimum zeolite-AC hybrid as stable NH3 adsorption sites. These active sites also provided a uniform dynamic adsorption capacity of ∼4 mmol/g throughout 15 cycles. Conclusively, this work highlighted the importance of zeolite composition and AC-incorporated proportion for the H2 purification aspect.

Comparative analysis of CO2 and N2 adsorption on zeolite 4A: Thermodynamics parameters and site energy distribution investigations

Adsorption equilibrium data for CO2 and N2 on zeolite 4A were volumetrically measured at temperatures ranging from 278 K to 328 K and gas pressures up to 4 MPa. The experimental data was correlated using the high-pressure Langmuir, Sips, and DA adsorption isotherm models. The high-pressure DA model was found to be the optimal model for describing the adsorption isotherms on basis of nonlinear regression analysis. Subsequently, the thermodynamic parameters, including isosteric heat of adsorption (Hst), adsorption phase specific heat capacity (cpa), adsorption phase entropy (sa), and enthalpy (ha), considering the actual adsorption state, were estimated. The results demonstrate that the adsorption of CO2 on zeolite 4A is greater than that of N2. The higher Hst and ha values and the lower sa values of CO2 compared to N2, suggest a stronger surface interaction of CO2 with zeolite 4A, resulting in a more ordered arrangement of CO2 molecules than N2. Site energy distribution theory analysis indicates that less energy is required for CO2 adsorption and greater site energy heterogeneity of CO2 than those of N2 on zeolite 4A. The obtained results evidence that the studied zeolite 4A could be a promising candidate adsorbent for greenhouse gas capture.

Aspects of Gas Storage: Confined Geometry Effects on the High-Pressure Adsorption Behavior of Supercritical Fluids.

During the last decades, major progress was made concerning the understanding of subcritical low-pressure adsorption of fluids like nitrogen and argon at their boiling temperatures in nanoporous materials. It was possible to understand how structural properties affect the shape of the adsorption isotherms. However, within the context of gas storage applications, supercritical high-pressure gas adsorption is important. A key feature here is that the experimentally determined surface excess adsorption isotherm may exhibit a characteristic maximum at a certain pressure. For a given temperature and adsorptive/adsorbent system, the surface excess maximum (and the corresponding adsorbed amount) is related to the storage capacity of the adsorbent. However, there is still a lack of understanding of how key textural properties such as surface area and pore size affect details of the shape of supercritical high-pressure adsorption isotherms. To address these open questions, we have performed a systematic experimental study assessing the effect of pore size/structure on the supercritical adsorption isotherms of pure fluids such as C2H4, CO2, and SF6 over a wider range of temperatures and pressures on a series of model materials exhibiting well-defined pore sizes, i.e., ordered micro- and mesoporous materials (e.g., NaY zeolite, KIT-6 silica, and MCM-48 silica). A fundamental result of our experiments is a unique fluid-independent correlation between the pressure of the surface excess maximum pmax (at a given temperature) and the pore size (by taking into account the kinetic diameter of the fluid and the underlying effective attractive fluid-wall interaction). Summarizing, our results suggest important structure-property relationships, allowing one to determine, for given thermodynamic conditions, important information related to the optimal operating conditions for supercritical adsorption applications. The insights may also serve as a basis for optimizing and tailoring the properties of nanoporous adsorbent materials for gas storage applications.

Influence of competitive adsorption, diffusion, and dispersion of CH4 and CO2 gases during the CO2-ECBM process

The displacement mechanisms of adsorbed CH4 by injected CO2 at high pressures in CO2-enhanced coalbed methane (CO2-ECBM) processes involve competitive adsorption, diffusion, and dispersion. Inherent coal heterogeneity and the complexity of the CO2-CH4 displacement model make CO2-ECBM modelling and optimal process design challenging. The pure gas isotherms and Langmuir adsorptions constants for CO2 and CH4 were determined using the manometric method. Notably, CO2 shows higher adsorption capacity than methane, and Langmuir volume (VL) for CO2 is 2 to 2.3 times that of methane. High pressure adsorption isotherms and experimentally determined diffusion coefficients set the sorption kinetics. The diffusion coefficient of CH4 for all four samples was in the range between 1.35×10-12 and 7.42×10-12 m2/s, while for CO2, it varied from 10.9×10-12 to 15.48×10-12 m2/s. The new concept of caucus time was introduced in the sorption kinetics of CO2 and CH4. Each sample has distinct caucus time and was positively correlated with Langmuir volumes of CO2 and CH4 with R2 > 0.9. The critical desorption pressure above which CH4 exhibits retrograde adsorption behaviour is evaluated from co-adsorption isotherms adopting Ideal Adsorption Solution Theory (IAST). The critical desorption pressure for the four samples were 1220, 460, 575 and 740 psi respectively. IAST also predicts a critical mole fraction (∼30 %) in the gas phase above which CO2 becomes the dominant adsorbed phase on coal surfaces. The results of Advective-Dispersion modelling at a larger scale illustrate the value of the Dispersion coefficient in forecasting and screening. Since the time available for diffusion versus dispersion depends upon flowrate, high values of dispersion coefficient can negatively influence favourable competitive adsorption expectations through higher operational cost in CO2 separation. This work suggests the existence of both an optimal operational pressure and flowrate to maximize the potential of CO2-ECBM technology demonstrated in competitive adsorption modelling.

Post-synthetic ligand exchange as a route to improve the affinity of ZIF-67 towards CO2

The Zeolitic Imidazolate Framework 67 (ZIF-67) is a highly promising material owing to its exceptional thermal stability, large specific surface area, cost-effectiveness, and versatile applications. One of the potential applications of ZIF-67 is gas separation processes, among which the separation of CO2/CH4 mixtures has attracted great interest nowadays in the biogas sector. However, when it comes to CO2/CH4 separation, ZIF-67 falls short as it lacks the desired selectivity despite its high adsorption capacity. This limitation arises from its relatively low affinity towards CO2. In this study, we have addressed this issue by partially exchanging the ligand of ZIF-67, specifically replacing 2-methylimidazole with 1,2,4 (1H) triazole, which introduces an additional nitrogen atom. This modification resulted in ZIF-67 showing significantly enhanced affinity towards CO2 and, as a result, greater selectivity towards CO2 over CH4. The modified materials underwent thorough characterization using various techniques, and their adsorption capacity was evaluated through high-pressure adsorption isotherms. Furthermore, their separation performance was assessed using the Ideal Solution Adsorption Theory, which provided valuable insights into their potential for efficient gas separation.

Kinetic process assessment of H2 purification over highly porous carbon sorbents under multicomponent feed conditions

As a universal energy carrier, the need for pure H2 is ever increasing due to its ubiquitous role in petrochemical refining, metal reduction, and the up-and-coming fuel cell market. Hydrogen produced from steam methane reforming (SMR) is typically laden with impurities such as CO2, CO, and CH4 and a full efficiency screening for potential H2 purification sorbents requires evaluating the thermodynamic and kinetic behaviors associated with multicomponent pressure swing adsorption (PSA). As such, in this study we assessed three commercially available activated carbons with high surface area and pore volume for the PSA upgrading of H2 from simulated SMR off-gas stream consisting of H2/CO/CH4/CO2 (75/5/5/15 vol%). In addition to high-pressure adsorption isotherms for pure gases, H2 purity and recovery, and H2 productivity were estimated from cyclic PSA experiments, while actual (CO + CH4 + CO2)/H2 selectivity values were estimated from breakthrough experiments. For the best performing material, the results demonstrated H2 purity and recovery of 99.6 and 55.3 %, respectively with a productivity of 18.3 molH2/kg.h and multicomponent (CO + CH4 + CO2)/H2 selectivity of 59.86 %. Moreover, the affinity of the different adsorbates toward the activated carbons presented from the most adsorbed to the least adsorbed gas was in the order of CO2 > CH4 > CO ≫ H2. The H2 purification over these carbon-based adsorbents was found to be an equilibrium-controlled process.

Influence of regeneration conditions on cyclic CO2 adsorption on NaA zeolite at high pressures

This work aimed to determine the influence of different regeneration conditions on CO2 adsorption on NaA zeolite at high pressures through cyclic processes. To reach this goal, NaA characterization, high-pressure adsorption isotherms, heat of adsorption and working capacities for CO2/NaA system were performed. NaA zeolite characterization showed a high crystalline and stable microporous structure with 0.43 nm pore diameter and 247 m² g-1 specific area. CO2 adsorption isotherms are strongly favorable, showing adsorption uptakes equal to 4.73, 4.54 and 4.44 mol kg-1 (c.a. 20 wt%) at 40 bar and 20, 30 and 40 °C, respectively. Jensen-Seaton model fits experimental data and was used to determine CO2/NaA heats of adsorption from 10 to 40 kJ mol-1, which are characteristic of physisorption process. This was corroborated by constant working capacity for all regeneration processes, reaching 1.44, 3.27, and 4.27 mol kg-1 for REG-1 (1 bar, 30 °C), REG-2 (vacuum, 30 °C) and REG-3 (vacuum, 150 °C) regeneration processes, respectively. The working capacity simulation showed a strong influence of pressure and practically insensitive to adsorption temperature from 20° to 40°C. Therefore, NaA zeolite is a regenerative material showing a high potential for utilization in high pressure cyclic adsorption processes, such as fixed and fluidized beds, for CO2 separation when pressure and temperature are changed in blowdown/regeneration step.

Comparative study of methane adsorption of Middle-Upper Ordovician marine shales in the western Ordos Basin, NW China: Insights into impacts of moisture on thermodynamics and kinetics of adsorption

Impacts of water molecules on subsurface shale gas adsorption is a challenging topic in the assessment of shale gas resources and the selection of optimized exploitation plans. And scholars have recognized that water molecules occupy adsorption sites on hydrophilic pore walls. However, the impacts of water molecules on the thermodynamics and kinetics of methane adsorption in shales remains poorly understood. We document a series of high-pressure methane adsorption isotherms of dry and moisture-equilibrated samples of the Middle-Upper Ordovician Wulalike shale at 20–120 °C and up to 30 MPa. Quantitative analyses of the thermodynamic and kinetic characteristics are carried out. Results show that water molecules do not change the impacts of temperature and pressure on the trend of isotherms, thermodynamics, and kinetics of methane adsorption. The impacts of temperature, pressure, methane adsorption capacity, and water molecules on the methane adsorption are basically functioned by modifying the interactions between the adsorbate and the adsorbent. The presence of water molecules in shales leads to a decrease of ∼ 77.06% in the capacity of methane adsorption, an increase of ∼ 70.46% in the adsorption heat (Qst), a decrease of ∼ 96.62% in the standard entropy (△S0), and a decrease of ∼ 1/9 to 1/5 in the Bangham adsorption rate (kb). The impacts of water molecules on the decline of methane adsorption at high pressure is lower than that at low pressure, however, on the decrease of methane adsorption rate higher than that at low pressure. The study also reflects the methane adsorption into the three-dimensional network system of kerogen. This work can improve our understanding of subsurface supercritical adsorption under reservoir conditions.

Hybrid Pore-Scale Adsorption Model for CO2 and CH4 Storage in Shale

Making reliable estimates of gas adsorption in shale remains a challenge because the variability in their mineralogy and thermal maturity results in a broad distribution of pore-scale properties, including size, morphology and surface chemistry. Here, we demonstrate the development and application of a hybrid pore-scale model that uses surrogate surfaces to describe supercritical gas adsorption in shale. The model is based on the lattice density functional theory (DFT) and considers both slits and cylindrical pores to mimic the texture of shale. Inorganic and organic surfaces associated with these pores are accounted for by using two distinct adsorbate–adsorbent interaction energies. The model is parametrized upon calibration against experimental adsorption data acquired on adsorbents featuring either pure clay or pure carbon surfaces. Therefore, in its application to shale, the hybrid lattice DFT model only requires knowledge of the shale-specific organic and clay content. We verify the reliability of the model predictions by comparison against high-pressure CO2 and CH4 adsorption isotherms measured at 40 °C in the pressure range 0.01–30 MPa on four samples from three distinct plays, namely, the Bowland (UK), Longmaxi (China), and Marcellus shale (USA). Because it uses only the relevant pore-scale properties, the proposed model can be applied to the analysis of other shales, minimizing the heavy experimental burden associated with high-pressure experiments. Moreover, the proposed development has general applicability meaning that the hybrid lattice DFT can be used for the characterization of any adsorbent featuring morphologically and chemically heterogeneous surfaces.

Relationship between the Geological Origins of Pore-Fracture and Methane Adsorption Behaviors in High-Rank Coal.

Coal is characterized by a complex pore-fracture network and functional groups, which are derived from various geological origins and which further affect methane adsorption. To explore the relationship between the geological origins of pore-fractures and methane adsorption behaviors, we conducted pore structure tests and adsorption isotherms on six Qinshui high-rank coals. The pores and fractures were observed using an optical microscope (OM), a field emission scanning electron microscope (FESEM), and a high-resolution transmission electron microscope (HRTEM), and the pore structure parameters were determined using mercury intrusion and low-pressure N2 and CO2 adsorption. High-pressure CH4 adsorption isotherms were obtained at 30 °C using the manometric method. Results show that the Qinshui high-rank coals develop five stages of pore size distribution, consisting of the smaller micropore stage (0.3–1 nm), the larger micropore and smaller mesopore stage (1–10 nm), the mesopore and smaller macropore stage (10–110 nm), the microfracture stage (0.11–40 μm), and the larger macropore stage (>40 μm). The micropores dominate the total pore volume (PV) and specific surface area (SSA). Pores and fractures of various morphologies and sizes have different geological origins, which are related to coalification and stress field evolution. Methane adsorption on coals mainly occurs in the micropores as a form of volume filling. The maximum pore size for complete gas filling (MPSCGF) ranges from 0.60 to 0.88 nm in Qinshui high-rank coals. The coal-forming geological processes, such as coalification and stress field evolution, contribute to various pores and fractures, which show different pore sizes and functional groups. The geological origins of pores and fractures control the methane adsorption behaviors in coals by way of the pore size and functional groups. Surface coverage-related methane adsorption behavior occurs in fractures, primary pores, and large-scale secondary pores, while micropore filling is the methane adsorption behavior in macromolecular pores and small-scale secondary pores. The aim of this study is to provide a new insight into the methane adsorption on coals from the geological process of the formation and modification of pores and fractures.

Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Carbon dioxide (CO2) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH4) and CO2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH4 and CO2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH4 and CO2 on coals. Next, the adsorption mechanisms of high-pressure CH4 and CO2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH4 is less than that of CO2, while its reverse pressure is greater than that of CO2. The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH4 excess adsorption is due to the saturation near the pore wall, while the CO2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO2-enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

Exploring periodic mesoporous organosilicas for ethane–ethylene adsorption–separation

Ethylene is a core building block in the chemical industry and its separation from ethane is very challenging due to high energy requirements. Adsorption-based processes can be an alternative to current processes based on cryogenic distillation. In this work, we explore for the first time the application of periodic mesoporous organosilica (PMO) materials, whose surface properties can be tuned with different functional groups. With the aim of correlating the PMO structure with ethane and ethylene adsorption, eight different PMO materials have been prepared, viz. the phenylene-bridged PMO, PMO aminated at the organic bridges, and PMO functionalized by silylation of free silanol (inorganic) moieties. High pressure adsorption isotherms were measured, and the separation selectivity and phase diagrams of a binary mixture of ethane and ethylene were estimated. Results have shown that, overall, the PMO studied tend to be more selective towards ethylene than ethane probably due to the interactions between the quadrupole moment of ethylene and the free silanols in the samples. After silylation, the novel materials presented surfaces with higher affinity towards ethane than those of the pristine material. For the aminated samples, functionalization with primary amines originated materials displaying better selectivity towards ethylene than those functionalized with secondary or tertiary amines.

Enhanced nitrogen adsorption capacity on Ca2+ ion-exchanged hierarchical X zeolite

The aim of this paper is to modify the X zeolite adsorbent to enhance nitrogen adsorption capacity. To this end, a novel procedure for modification of X zeolite, including both NH4+ treatment and Ca2+ ion-exchange, is introduced which results in a hierarchical mesopore-micropore structure with multilayer N2 adsorption behavior. The presented adsorbents, “hierarchical X zeolite” (HX) and “Ca2+ exchanged hierarchical X zeolite” (CaHX), are compared with the available microporous 13X zeolites, modified by Na+ (NaX), and mixed Ca2+ /Na+ cations (CaNaX). The properties of these adsorbents are characterized by N2 adsorption–desorption in 77 K, XRD, and XRF analysis. The high-pressure N2 adsorption isotherms are measured at 283, 298, and 313 K up to 35 bar. It is shown by increasing the pressure, N2 adsorption capacity of HX and CaHX continuously increases (without saturation), in comparison with NaX and CaNaX that approaches to a saturation point. The N2 adsorption capacity of CaHX is 4.70 mmol/g at 35 bar and 298 K, which is 57% and 102% more than CaNaX (2.98 mmol/g) and NaX (2.33 mmol/g), respectively. Considerable potential improvements in the N2 adsorption capacity of CaHX are explained by the multilayer adsorption of N2 at high pressures in mesopores, as a dominant mechanism. Additionally, the isosteric heat of adsorption of hierarchical X zeolites (HX and CaHX), calculated by Clausius-Clapeyron equation, are lower than NaX and CaNaX that leads to the more facile regeneration. Adsorption isotherms are also modeled by Langmuir, Dual-Site Langmuir (DSL), Freundlich, and Langmuir-Freundlich (Sips) models, and related parameters determined. These adsorbents can be used effectively in the helium purification by the PSA process, which is based on nitrogen adsorption.

Measurement of gas contents in shale reservoirs – impact of gas density and implications for gas resource estimates

Gas shale reservoirs pose unique measurement challenges due to their ultra-low petrophysical properties and complicated pore structures. A small variation in an experimental parameter, under high-pressure conditions, may result in huge discrepancies in gas contents and the resource estimates derived from such data. This study illustrates the impact of the equation of state on the gas content determined for a shale sample. The gas content was determined from laboratory-measured high-pressure methane adsorption isotherms and theoretically described by a hybrid type model. The modelling involved the use of the Dubinin–Radushkevich isotherm to obtain the adsorbed phase density followed by the Langmuir isotherm to describe the resultant absolute adsorptions. Significant variations were observed in measured adsorption isotherms due to the variations in gas densities calculated from different equations of states. The model parameters and the gas in-place volumes estimated from those parameters also varied significantly.

Carbon dioxide and methane sorption characteristics of Damodar Valley and Upper Assam shale, India

Shale gas has become one of the most significant unconventional hydrocarbon resources. The gas is adsorbed in shale micropores, more precisely in the pores of clay minerals and organic matter. In this work, studies were done on the shale samples of Damodar valley and Upper Assam Basin. High pressure adsorption isotherms of methane (CH4) and carbon dioxide (CO2) were analysed to assess the gas storage capacity. The organic geochemistry, thermal maturity and mineralogy are analysed by Rock-Eval pyrolysis, petrographic study and XRD study respectively. The purpose of this study was to understand the factors controlling the variations in the nature of high pressure CH4 and CO2 sorption capacities. The factors include organic-inorganic constituents, and pore characteristics. Results show that the shale samples have a higher CO2 sorption capacity compared to that of methane. However, the CO2 sorption capacity shows a sharp decrease after 70 bar. Under similar temperature and pressure condition CO2: CH4 ratio increases with increasing pressure up to a pressure of 70 bar and then shows a sharp decrease. Petrographic study shows that vitrinite is the most dominant maceral group. Unlike clay minerals, organic matter shows a robust control over gas sorption in the studied shale samples. Sorption potential of carbon dioxide and methane increases positively with vitrinite content but, decreases with rising mineral matter and thermal maturity. Vitrinite content also controls the CO2: CH4 ratio, and the ratio bears no relation with thermal maturity. Micropores, rather than mesopores, mostly influence the gas sorption.

A Novel Data-Driven Method to Estimate Methane Adsorption Isotherm on Coals Using the Gradient Boosting Decision Tree: A Case Study in the Qinshui Basin, China

The accurate determination of methane adsorption isotherms in coals is crucial for both the evaluation of underground coalbed methane (CBM) reserves and design of development strategies for enhancing CBM recovery. However, the experimental measurement of high-pressure methane adsorption isotherms is extremely tedious and time-consuming. This paper proposed the use of an ensemble machine learning (ML) method, namely the gradient boosting decision tree (GBDT), in order to accurately estimate methane adsorption isotherms based on coal properties in the Qinshui basin, China. The GBDT method was trained to correlate the adsorption amount with coal properties (ash, fixed carbon, moisture, vitrinite, and vitrinite reflectance) and experimental conditions (pressure, equilibrium moisture, and temperature). The results show that the estimated adsorption amounts agree well with the experimental ones, which prove the accuracy and robustness of the GBDT method. A comparison of the GBDT with two commonly used ML methods, namely the artificial neural network (ANN) and support vector machine (SVM), confirms the superiority of GBDT in terms of generalization capability and robustness. Furthermore, relative importance scanning and univariate analysis based on the constructed GBDT model were conducted, which showed that the fixed carbon and ash contents are primary factors that significantly affect the adsorption isotherms for the coal samples in this study.

A reference high-pressure CH4 adsorption isotherm for zeolite Y: results of an interlaboratory study

This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess methane adsorption isotherms on NIST Reference Material RM 8850 (Zeolite Y), at 25 °C up to 7.5 MPa. Twenty laboratories participated in the study and contributed over one-hundred adsorption isotherms of methane on Zeolite Y. From these data, an empirical reference equation was determined, along with a 95% uncertainty interval (Uk=2). By requiring participants to replicate a high-pressure reference isotherm for carbon dioxide adsorption on NIST Reference Material RM 8852 (ZSM-5), this interlaboratory study also demonstrated the usefulness of reference isotherms in evaluating the performance of high-pressure adsorption experiments.

On the pressure and temperature dependence of adsorption densities and other thermodynamic properties in gas shales

Black shale source rocks have become a key natural gas resource in the US and China. Unlike in conventional hydrocarbon reservoirs, a significant fraction of the gas in shales is adsorbed onto the inordinate specific surface areas (SSA) of micropores inside organic matter with densities exceeding those of bulk gas by an order of magnitude. Yet, the distribution of adsorbed versus bulk gas remains poorly understood. Experiments measure excess adsorption isotherms, which are corrected to absolute adsorption amounts by a factor that involves the density, ρabs, of the adsorption layer(s). Constant values of ρabs are widely used in the literature but, we argue, are inconsistent with the pressure- and temperature-dependence of adsorption behavior. In this work, we propose a more reliable approach that assumes a constant volume of each adsorption layer that can be determined from the SSA of the substrate.We measure nine high-pressure methane excess adsorption isotherms up to 15 MPa for three shale samples from China and Germany at temperatures of 65, 75, and 95 °C, and low-pressure carbon dioxide and nitrogen isotherms to estimate the SSA. We also investigate another 24 isotherms at 35, 50, and 65 °C for 8 Chinese shale samples and 3 isotherms at 60, 90, and 120 °C for an Argentinian shale sample from the literature. A new variation of the multi-layer Ono-Kondo lattice model is introduced to derive the temperature and pressure dependent adsorption layer densities, which are subsequently used to obtain the absolute adsorption isotherms of interest. This practical methodology has the potential to significantly improve our estimates of shale gas-in-place resources, and adsorption modeling more broadly.

Effect of additives in the nucleation and growth of methane hydrates confined in a high-surface area activated carbon material

A high-surface area activated carbon material (PPAC) prepared from a petroleum residue has been used as a host structure to promote the nucleation and growth of confined methane hydrates after the incorporation of additives. Sodium dodecyl sulfate (SDS), Leucine and Tetrahydrofuran (THF) have been evaluated either dissolved in the pre-impregnation media (ultrapure water) or incorporated in the carbon surface using a mechanochemical approach. High-pressure methane adsorption isotherms show that the mechanochemical approach does not improve the storage performance of the original carbon material due to the steric effects by the bulkier organic functionalities. Furthermore, the incorporation of the additives shifts the nucleation in narrow pores to higher pressures.On the contrary, when the additives are dissolved in water, high-pressure methane isotherms anticipate a modification in the kinetics and/or thermodynamics of the nucleation process. Whereas gas hydrates grown in large pores and/or the external surface exhibits a significant reduction in the induction time, the nucleation process in narrow micropores becomes more uniform and occurs in a narrower pressure window, slightly shifted to higher pressures. However, no changes in the total uptake at 10 MPa could be found. The situation is different when THF is dissolved in water. In this case, the nucleation takes place at pressures below the natural process (<3–4 MPa), the total amount of methane stored at 10 MPa being slightly limited. In-situ synchrotron X-ray diffraction studies anticipate a sI structure for the pure water methane hydrates, while a combination of sI and sII is identified for the THF-based system.