Excess Adsorption Isotherms Research Articles

Comparison of commercial and new adsorbent materials for pre-combustion CO2 capture by pressure swing adsorption

The IGCC technology (Integrated Gasification Combined Cycle) with pre-combustion CO2 capture is a promising approach for near-zero CO2 emission power plants to be realized in the near future. A key challenge within this technology is the separation of the CO2/H2 gas mixture resulting from the water gas shift reaction that follows the gasification of coal. For the CO2 stream a purity of about 95% is required; additionally a CO2 capture rate of 90% is desired, which implies that both streams, H2 and CO2, are required at rather high purity (∼95%). In contrast to post- combustion capture from power plants, where a large gas stream at low pressure and low CO2 content has to be treated, in pre-combustion capture a gas mixture at up to 40bar has to be separated; therefore an adsorption based process, such as pressure swing adsorption (PSA), constitutes a promising method for CO2 removal from H2.In this work, new materials, namely USO-2-Ni MOF, UiO-67/MCM-41 Hybrid and MCM-41, are characterized in<remove-image> terms of equilibrium adsorption isotherms. Excess adsorption isotherms of CO2 and H2 on these materials are measured at different temperatures (25°C 140°C) and in a wide pressure range (up to 150bar). The experimental data are then described with a suitable isotherm model, in our case Langmuir, Sips and Quadratic. In addition, the cyclic working capacity of CO2 on each material is computed as a further assessment of the suitability of these materials for pre-combustion capture.

Adsorption from Oligomer–Monomer Solutions on the Surfaces Modified with End-Grafted Chains

A density functional theory has been applied to study adsorption from oligomer-monomer solutions on solid surfaces modified with end-grafted polymers. Chains are modeled as freely jointed tangent spheres. All spherical species interact via Lennard-Jones (12-6) potential. The grafted chains are not attracted by the surface. Fluid molecules interact with the substrate via Lennard-Jones (9-3) potential. It is shown how a shape of the relative excess adsorption isotherms depends on such parameters as the oligomer length, the grafting density, oligomer-substrate interactions, oligomer-grafted chains interactions, and oligomer-monomer interactions in the bulk solution. As attractive interactions between free and grafted chains strengthen, the relative excess adsorption isotherm can change from negative, through S-shaped with an adsorption azeotropic point, to positive in the whole concentration region. A change of oligomer-monomer interactions causes the opposite effects. For low grafting densities of attractive bonds, the relative excess of oligomers increases, reaches a maximum, and decreases. The structure of surface layers is also analyzed.

Thermodynamics of sodium maleate and chloride adsorption at an aqueous solution-gas interface

Surface tension of solutions of sodium chloride in water and sodium maleate in aqueous alkaline solutions in a wide concentration range at 298 and 303 K are measured. It is found that the dependences of surface tension of electrolyte solutions on concentration have extreme character. The obtained dependences are discussed from the standpoint of the effect of the nature of the solute and the processes of desolvation on the composition of surface layers and the surface activity of a solution. Total electrolyte adsorption isotherms are calculated using experimental surface tension and excess adsorption isotherms.

Measurements and Modeling of Excess Adsorption of Pure and Mixed Gases on Wet Coals

Pure-gas excess adsorption isotherms were measured for methane, nitrogen, and CO2 on two water-moistened coals at 319.3 K and pressures up to 12.4 MPa. The coals were lower basin Fruitland (LBF) coal, from the San Juan basin, and Illinois #6 coal, from the Illinois basin. Furthermore, gas-mixture adsorption measurements were made on the Illinois #6 coal for the three binary mixtures formed by methane, nitrogen, and CO2. The rationale for this study was (a) to provide data useful for development and testing of models for the competitive adsorption of mixed gases on coals and (b) to assess the ability of the simplified local-density model to describe the data. The pure gas measurements revealed that CO2 excess adsorption on both coals exhibited distinct maxima near the CO2 critical pressure. At a pressure of 7 MPa, the ratio of excess adsorptions for N2:CH4:CO2 was 1:2.8:4.4 for LBF coal and 1:2.7:6.3 for Illinois #6 coal. The LBF coal showed low levels of adsorption, which can be attributed to the high ash...

Excess adsorption of binary aqueous organic mixtures on various reversed-phase packing materials

Excess adsorption isotherms of acetonitrile and methanol from water were measured on eight commercial columns. Columns used in this study represent latest examples in column development and include three different poroshell columns (Kinetex-C18, Acsentis-C18 and Halo-C18) as well as conventional columns with significantly different adsorbent geometry (Allure-C18, YMC-C18) and various hybrid-silica columns (Gemini-C18, Xterra-C18 and XBridge-C18). Comparison of the excess adsorption isotherms measured on all these columns and expressed in surface specific form demonstrated significant similarity of the adsorption properties of all columns, which allows us to introduce the “standard adsorption isotherm” for reversed-phase C18-type columns. The methodology of the evaluation of the total amount of adsorbent in the column and effective surface area of the C18 modified adsorbent is also discussed. These terms are critical for successful evaluation of surface specific parameters.

Direct Measurements of Pore Fluid Density by Vibrating Tube Densimetry

The densities of pore-confined fluids were measured for the first time by means of vibrating tube densimetry (VTD). A custom-built high-pressure, high-temperature vibrating tube densimeter was used to measure the densities of propane at subcritical and supercritical temperatures (between 35 and 97 °C) and carbon dioxide at supercritical temperatures (between 32 and 50 °C) saturating hydrophobic silica aerogel (0.2 g/cm(3), 90% porosity) synthesized inside Hastelloy U-tubes. Additionally, supercritical isotherms of excess adsorption for CO(2) and the same porous solid were measured gravimetrically using a precise magnetically coupled microbalance. Pore fluid densities and total adsorption isotherms increased monotonically with increasing density of the bulk fluid, in contrast to excess adsorption isotherms, which reached a maximum and then decreased toward zero or negative values above the critical density of the bulk fluid. The isotherms of confined fluid density and excess adsorption obtained by VTD contain additional information. For instance, the maxima of excess adsorption occur below the critical density of the bulk fluid at the beginning of the plateau region in the total adsorption, marking the end of the transition of pore fluid to a denser, liquidlike pore phase. Compression of the confined fluid significantly beyond the density of the bulk fluid at the same temperature was observed even at subcritical temperatures. The effect of pore confinement on the liquid-vapor critical temperature of propane was less than ~1.7 K. The results for propane and carbon dioxide showed similarity in the sense of the principle of corresponding states. Good quantitative agreement was obtained between excess adsorption isotherms determined from VTD total adsorption results and those measured gravimetrically at the same temperature, confirming the validity of the vibrating tube measurements. Thus, it is demonstrated that vibrating tube densimetry is a novel experimental approach capable of providing directly the average density of pore-confined fluids, and hence complementary to the conventional gravimetric or volumetric/piezometric adsorption techniques, which yield the excess adsorption (the Gibbsian surface excess).

Quantifying injection solvent effects in reversed-phase liquid chromatography

Peak distortion due to the injection was measured as a function of injection solvent strength, volume, mass, retention factor, and column selectivity. The concept of a method's sensitivity (s) to injection solvent strength was mathematically defined as a vector of theoretical plate counts compared to an ideal vector that does not change with injection solvent strength. Near ideal sensitivity (s>0.90) was measured on all columns with all analytes in low volume injections of 1.25μL. Increasing the injection volume reduces the measured sensitivity from ideality to a greater extent than increasing the injection mass, with differing values for each column. Using column parameters measured from the hydrophobic-subtraction model and fitting parameters from the acetonitrile excess adsorption isotherm, differences among the columns studied are explained. Decreased ligand density and increased silanol activity provide a consistent peak shape with changes in injection volume or solvent strength. For method development, a quick test is suggested with the ratio of hydrophobic-subtraction column parameters, H/A, to predict the injection solvent sensitivity of a column. As H/A decreases, the sensitivity to injection solvent worsens. Sensitivity to organic modifiers other than acetonitrile are predicted with cited sorbed layer thickness, such that MeOH>EtOH>IPA≈THF≈MeCN, i.e., a strong MeOH diluent is more ideal (better) than a strong MeCN diluent.

Adsorption from Binary Solutions on the Polymer-Tethered Surfaces

A density functional study of adsorption from binary solutions on surfaces modified with tethered chains is presented. The tethered chains are modeled as freely jointed tangent spheres. The fluid molecules are spherical. All species interact via the Lennard-Jones (12-6) potential. The substrate is neutral with respect to all chain segments but the surface-binding segment, and it interacts with the fluid molecules via a Lennard-Jones (9-3) potential. Depending on the parameters of the model, different shapes of the relative excess adsorption isotherms are found. The density profiles of all components are analyzed. It is shown that the surface region is highly inhomogeneous. An influence of the grafting density, the length of grafted chains, the nature of the solution, and its composition on the adsorption mechanism and the structure of the bonded-phase is investigated. The theoretical predictions are consistent with the results of computer simulations and experimental data.

Absolute adsorption isotherms of gases and vapors within the lattice model

It is shown that the system of equations describing excess adsorption isotherms of the components of binary solutions previously obtained on the basis of the Ono-Kondo lattice model can be transformed to describe the absolute adsorption isotherms on microporous adsorbents of individual adsorbates at temperatures above and below critical.

Saturation properties of a supercritical gas sorbed in nanoporous materials

The description of experimental gas adsorption data in terms of an accurate model is key to understand the adsorption mechanism and its limits. As a basic feature such a model should predict correctly the conditions under which saturation occurs. However, in the absence of bulk condensation properties for a supercritical adsorbate this matter remains open to discussions. In this study, the decreasing region of excess hydrogen adsorption isotherms measured down to 50 K is used to determine the adsorbed phase volume, density and pressure corresponding to saturation. The experimental method developed for these key measurements addresses the challenges of very low temperature adsorption measurements at high pressure. Therefore, the modifications specially made to a cryostat used in conjunction with a Sievert apparatus to reach high temperature stability (±10 mK) down to 40 K are presented. The approach is implemented on the novel nanoporous materials UMCM-1 and NOTT-112 over 50-77 K and 0-40 bar. The derived hydrogen saturation properties are found to be consistent with a Dubinin-Astakhov model. Importantly, the measured adsorbed hydrogen phase volume also compares well with the pore volume obtained from Ar porosimetry. The found saturation properties provide a physical basis to calculate consistent absolute adsorption isotherms and enthalpies, and to project the ultimate adsorption capacity of a conceptual material with a maximized specific surface area. The present findings provide additional evidence that the common view on supercritical adsorption, in which it is assumed that no liquid is formed and that the only possible mechanism involves monolayer coverage, does not hold in many nanoporous materials.

Methane storage in homogeneous armchair open-ended single-walled boron nitride nanotube triangular arrays: a grand canonical Monte Carlo simulation study

The physisorption of methane in homogeneous armchair open-ended SWBNNT triangular arrays was evaluated using grand canonical ensemble Monte Carlo simulation for tubes 11.08, 13.85, 16.62, and 19.41 Å [(8,8), (10,10), (12,12), and (14,14), respectively] in diameter, at temperatures of 273, 298, 323, and 373 K, and at fugacities of 0.5-9.0 Mpa. The intermolecular forces were modeled using the Lennard-Jones potential model. The absolute, excess, and delivery adsorption isotherms of methane were calculated for the various boron nitride nanotube arrays. The specific surface areas and the isosteric heats of adsorption, Q(st), were also studied, different isotherm models were fitted to the simulated adsorption data, and the model parameters were correlated. According to the results, it is possible to reach 108% and 140% of the US Department of Energy's target for CH(4) storage (180 v/v at 298 K and 35 bar) using the SWBNNT array with nanotubes 16.62 and 19.41 Å in diameter, respectively, as adsorbent. The results show that for a van der Waals gap of 3.4 Å, there is no interstitial adsorption except for arrays containing nanotubes with diameters of >15.8 Å. Multilayer adsorption starts to occur in arrays containing nanotubes with diameters of >16.62 Å, and the minimum pressure required for multilayer adsorption is 1.0 MPa. A brief comparison of the methane adsorption capacities of single-walled carbon and boron nitride nanotube arrays was also performed.

Adsorption of styrene at a binary solution-gas interface

The surface tension isotherms of styrene solutions in 2-propanol-water solvents are obtained experimentally. Isotherms of excess and absolute adsorption at the liquid-gas interfaces are also obtained. A thermodynamic model for the adsorption of styrene from 2-propanol-water binary solvent at the solutiongas interfaces is proposed on the basis of the stoichiometric approach. The isosteric adsorption heats are calculated using the experimental isotherms of surface tension and excess adsorption of styrene from 2-propanol-water binary solvent, and it is shown that the formation of the surface layer occurs according to a mechanism combining the processes of displacement adsorption between molecules of the components of the solvent, styrene, and 2-propanol.

High Pressure Excess Isotherms for Adsorption of Oxygen and Nitrogen in Zeolites

High-pressure oxygen is an integral part of fuel cell systems, many NASA in situ resource utilization concepts, and life support systems for extravehicular activity. Due to the limited information available for system designs over wide ranges of temperature and pressure, volumetric methods are applied to measure adsorption isotherms of O(2) and N(2) on NaX and NaY zeolites covering temperatures from 105 to 448 K and pressures up to 150 bar. Experimental data measured using two apparatuses with distinctly different designs show good agreement for overlapping temperatures. Excess adsorption isotherms are modeled using a traditional isotherm model for absolute adsorption with a correction for the gas capacity of the adsorption space. Comparing two models with temperature-dependent coefficients, a virial isotherm model provides a better description than a Toth isotherm model, even with the same number of parameters. With more virial coefficients, such as a cubic form in loading and quadratic form in reciprocal temperature, the virial model can describe all data accurately over wide ranges of temperature and pressure.

Adsorption Equilibria of Light Organics on Single-Walled Carbon Nanotube Heterogeneous Bundles: Thermodynamical Aspects

Single-component adsorption equilibria of five different hydrocarbons (CH4, C2H6, C2H4, C3H8, and C3H6) on a high-purity sample of single-walled carbon nanotube heterogeneous bundles were measured using a static gravimetric method. The physicochemical properties and morphology of the bundles were thoroughly characterized to obtain information about the tube diameter distribution; the latter is shown to be well approximated by a Gaussian function with a mean diameter of 14.1 ± 1.4 A. The adsorption measurements were performed at room temperature (T ≈ 300 K) and pressures from 2 × 10−3 up to 80 bar corresponding to reduced temperatures, T/Tc, from 0.81 to 1.59 and relative pressures, p/p0, in the range 1.4 × 10−5 to 0.89, where Tc and p0 are, respectively, the critical temperature and saturation vapor pressure of the adsorptive fluid. The excess adsorption isotherms exhibit an overall type-II shape, except that of supercritical CH4 which is type-I; none of the isotherms yield a hysteresis loop at the studie...

Computer simulations for the adsorption and separation of CO2/CH4/H2/N2 gases by UMCM-1 and UMCM-2 metal organic frameworks

The adsorption of pure N2/H2/CH4/CO2 along with the adsorption and separation of mixtures thereof in two metal organic frameworks (MOFs) of UMCM-1 and UMCM-2 have been extensively studied using a hybrid method of computer simulation and adsorption theory. It is found that the excess adsorption isotherms from grand canonical Monte Carlo (GCMC) simulations basically agree with the available experimental data of pure gases, except for H2 adsorption in UMCM-1 at 298 K. Moreover, the GCMC results show that both MOF materials exhibit an excellent storage capacity for pure CH4 and CO2 at room temperature. The excess uptakes of CH4 by UMCM-1 and UMCM-2 for at 5000 kPa are 12.53 and 15.06 mmol g−1, while those of CO2 at 4500 kPa are 30.13 and 36.04 mmol g−1, respectively, which approaches and even exceeds the 30.82 mmol g−1 of MOF-177. In addition, dual-site Langmuir–Freundlich (DSLF)-based ideal adsorption solution theory (IAST) is also used to correlate the simulated adsorption isotherms of pure gases and further predict the separation of equimolar mixtures. IAST shows a good agreement with the GCMC results in most cases studied here. The selectivities of both MOF materials in CH4/H2 and CH4/N2 are insensitive to the pressure. The selectivities of both MOF materials for CH4/H2 are almost the same having a value of 4, while they are 2 for CH4/N2. By contrast, the selectivities for CO2/H2, CO2/N2 and CO2/CH4 apparently rely on the pressure, showing 16.4 and 26.9, 5.4 and 7.8, and 2.9 and 4.7 at 4000 kPa for UMCM-1 and UMCM-2, respectively. Compared with other MOFs materials, their separation ability is not prominent, but they are suitable for gas storage.

Analysis of Hydrogen Adsorption in Microporous Adsorbents at Room Temperature and High Pressures

ABSTRACTAn experimental study of hydrogen adsorption in a variety of high-surface area adsorbent materials has been conducted at room temperature and pressures up to 500 bar on high surface area activated carbons, zeolite templated carbons (ZTC), and metal organic frameworks (MOFs). For all materials, excess hydrogen adsorption isotherms were measured up to 500 bar and have been analyzed in terms of the BET surface area and pore size distribution. The materials were also evaluated for their increase in hydrogen storage density over compressed gas. It was determined that, due to the lower excess adsorption and skeletal densities for the microstructured materials, MOF-177 and ZTC have worse storage densities than compressed gas at most pressures, even when assuming a bed compaction factor of two, while the activated carbons offer marginal increases in storage density over the pressure range investigated.

Features of the Excess Adsorption Isotherms of High‐Pressure Methane Adsorption on Coal and Simulation Model

Abstract:Four coal samples of different ranks are selected to perform the adsorption measurement of high‐pressure methane (CH4). The highest equilibrium pressure of the measurement exceeds 20 MPa. Combined with the measuring results and theoretical analyses, the reasons for the peak or the maximum adsorption capacity appearing in the excess adsorption isotherms are explained. The rules of the peak occurrence are summarized. And then, based on the features of coal pore structure, the adsorption features of high‐pressure gas, the microcosmic interaction relationship of coal surface and CH4 molecule, and the coalbed methane reservoir conditions, three theoretical assumptions on the coal adsorption high‐pressure CH4 are suggested. Thereafter, on the basis of these theoretical assumptions, the Ono‐Kondo lattice model is processed for simplification and deformation. Subsequently, the equations modeling the excess adsorption isotherm of high‐pressure CH4 adsorption on coal are obtained. Through the verification on the measurement data, the fitting results indicate that it is feasible to use the Ono‐Kondo lattice mode to model the excess adsorption isotherm of high‐pressure CH4 adsorption on coal.

Influence of temperature, pressure, nanotube’s diameter and intertube distance on methane adsorption in homogeneous armchair open-ended SWCNT triangular arrays

The physisorption of methane in homogeneous armchair open-ended SWCNT triangular arrays for the tubes of diameter of 10.85, 13.57, 16.28 and 19.00 A [(8,8), (10,10), (12,12) and (14,14), respectively] at temperature of 273, 298, 323 and 373 K and at fugacity of 0.5–9.0 Mpa is evaluated by means of Grand Canonical ensemble Monte Carlo simulation. The applied intermolecular forces are modeled using Lennard-Jones potential model. The absolute, excess and delivery adsorption isotherms of methane in various carbon nanotube arrays are calculated. Besides, specific surface areas and the isosteric heats of adsorption, Q st, are studied, also different isotherm models were fitted on the simulation adsorption data, and the model parameters are correlated. A novel geometrical relationship is introduced to calculate accessible interstitial and intratubular volumes. According to our simulation results, one can reaches to 96% of the US Department of Energy target for CH4 storage of 180 v/v at 298 K and 35 bar by using the SWCNT array with nanotube’s diameter of 19 A as adsorbent. For intertube distance equal 3.4 A, no gas adsorption is observed in interstitial channel except for the arrays bigger than 15.4 A in nanotube’s diameter, and multi-layer adsorption starts to form in nanotube’s diameter of 16.28 A at the pressure of 2.0 MPa.

Further advancements in predicting adsorption equilibria using excess formalism: Calculation of adsorption excesses at the liquid/solid interface

Prediction of adsorption equilibria for ternary liquid mixtures on solid surfaces by means of adsorption data for the corresponding three binary liquid mixtures can be improved by combining the thermodynamic excess formalism with geometrical models. This new strategy for the prediction of excess adsorption isotherms is examined for four ternary adsorption systems ranging from ideal to highly non-ideal ternary mixtures. The predicted isotherms are discussed and compared with experimental ones as well as with those obtained for a model based on the absolute quantities. The results confirm: (i) superiority of predicting adsorption in terms of excess quantities, and (ii) utility of geometrical models for constructing ternary molar compositions on the basis of binary ones to predict equilibria not only for liquid mixtures alone but also for adsorption of liquid mixtures on solid surfaces.

Modeling solvation on the chemically modified silica surfaces

A detailed molecular-level description of the retention mechanism in RP-HPLC is a point of great interest to analytical chemists. For this purpose, the solvent adsorption on the octadecyl stationary-bonded phase was investigated. Preferential adsorption of solvents from the acetonitrile-water mobile phase was modeled on the silica surface with one, two, three and four organic ligands, which represents a series of non-end-capped-bonded phases with different coverage density of bonded ligands. As a result of the computer simulation, the increase of adsorbed acetonitrile around bonded ligands is observed. The number of water molecules near the modeled surface is observed as well. The results are in agreement with experimental measurement of acetonitrile excess adsorption isotherms on the series of in-house made stationary-bonded phase.