Adsorption In Slit-like Pores Research Articles

Studies into confined methane adsorption in shale nanopores using a simplified local density model

The large number of nanopores in shale reservoirs provide an effective storage space for gases, which can assist in accurately characterizing the confined adsorption characteristics, including the adsorption amount of fluid in nanopores that is important for a detailed understanding of shale gas storage mechanisms and production prediction. Although the simplified local density (SLD) model has been widely used to describe the behavior of confined adsorption in slit-like pores, its application in relation to cylindrical pores has not been studied in detail. In this study, the importance of fluid–fluid interaction and fluid–solid interaction has been analyzed, with the expressions for the distribution of confined attractive parameter in cylindrical pores are derived by combining with the (3/8)σff assumption to establish a new SLD model. The reliability of the new model was verified by the publicly available experimental methane excess adsorption data for shale at different temperatures and the simulation results from the Grand Canonical Monte Carlo (GCMC). Moreover, by comparing with the SLD model with the slit pore structure, it was found that the adsorption isotherms and the confined fluid density distribution pattern characterized by the SLD model with the cylindrical pores were closer to the actual reservoir conditions of shale. In addition, the densities characterized by SLD models with a (3/8)σff assumption and with a (1/2)σff assumption at different Λb have been discussed and analyzed, and it is found that the selection of a suitable Λb would help to improve the accuracy of SLD model. The results might provide new theoretical insights into confined methane adsorption, with the new SLD model providing better engineering applications in shale gas development, gas storage, and pore structure characterization.

Adsorption of organic molecules on the highly ordered MCM-41 sorbent modified by different amounts of melamine

Thermodynamic characteristics of adsorption of organic molecules on the ordered organosilylated composite MCM-41 with grafted trimethylsilyl groups (MMet) and modified by melamine deposited on the surface were studied by inverse gas chromatography. Mole changes in the internal energy and entropy of adsorption, the dispersion and specific components of the value proportional to the Helmholtz absorption energy, and the contribution of the methyl group to the heat of adsorption were calculated from the specific retention volumes. The change in the retention volumes was established to be determined by the dispersion component of the Helmholtz energy of adsorption, whereas the energy of specific interactions remains almost unchanged. After the adsorbent surface was coated by the second and third melamine layers, the changes in −ΔU and −ΔS for adsorption of the members of the homologous series of n-alkanes are so small that lie within admissible measurement inaccuracies. This phenomenon can be explained by the regularities of alkane adsorption in slit-like pores of the supramolecular structure of melamine.

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.

Density functional description of adsorption in slitlike pores modified with chain molecules: A simple model for pillaredlike materials

We propose a density functional theory to describe adsorption of Lennard-Jones fluid in slitlike pores modified by chain molecules. Specifically, the chains are bonded by their ends to the opposite pore walls, so they can form pillaredlike structure. Two models are studied. In the first model, the nonterminating segments of chains can change their configuration inside the pore upon adsorption of spherical species. In the second model, the chains configuration remains fixed, so that the system is similar to a nonuniform quenched-annealed mixture. We study capillary condensation of fluid species inside such modified pores and compare the results obtained for two models.

Description of benzene adsorption in slit-like pores. Theoretical foundations of the improved Horvath–Kawazoe method

The improved Horvath–Kawzoe method (IHKM) proposed by Ustinov and Do was studied by theoretical, numerical, and experimental approaches. The IHKM analysis was improved by incorporation of new numerical algorithms (REG and ASA). The benzene adsorption isotherms on carbon blacks and activated carbons at 293 K were measured for proving the new theoretical treatment. This theory gives a more better description of these observed adsorption isotherms. We can conclude that, presented theory can be applied to analyze others adsorption isotherms, proving reasonable pore structures for activated carbons.

Phase behaviour of symmetric binary mixtures with partially miscible componentsin slit-like pores. Application of the fundamental measure density functionalapproach

We investigate adsorption in slit-like pores of model symmetric binary mixturesexhibiting demixing in bulk phase, by using a density functional approach. Ourfocus is on the evaluation of the first-order phase transitions in adsorbed fluidsand the lines separating mixed and demixed phases. The scenario for phasetransitions is sensitive to the pore width and to the energy of adsorption.Both these parameters can change the phase diagrams of the confinedfluid. In particular, for relatively wide pores and for strong wall–fluidinteractions, the demixing line can precede the first-order transition. Moreover, acompetition between layering transitions and demixing within particularlayers also leads to further enrichment of the phase diagram.

Application of the fundamental measure density functional theory to the adsorption in cylindrical pores

In this work we have implemented the fundamental-measure density functional theory due to Kierlik and Rosinberg to describe the adsorption of Lennard-Jones molecules in cylindrical pores. The accuracy of the theory in predicting adsorption isotherms and particle density profiles is checked by comparison with grand canonical Monte Carlo simulations for a wide range of pore sizes, showing very good agreement in all cases. In addition, the theory has been applied to the adsorption in slitlike pores to study the influence of the pore geometry on this property. The results indicate that the confinement of the cylindrical geometry introduces significant differences in the shape of the adsorption isotherms and density profiles. These differences are relevant for the characterization of porous materials.

Adsorption in Slit-Like Pores of Activated Carbons: Improvement of the Horvath and Kawazoe Method

Anew thermodynamic approach has been developed in this paper to analyze adsorption in slitlike pores. The equilibrium is described by two thermodynamic conditions: the Helmholtz free energy must be minimal, and the grand potential functional at that minimum must be negative. This approach has led to local isotherms that describe adsorption in the form of a single layer or two layers near the pore walls. In narrow pores local isotherms have one step that could be either very sharp but continuous or discontinuous benchlike for a definite range of pore width. The latter reflects a so-called 0 --> 1 monolayer transition. In relatively wide pores, local isotherms have two steps, of which the first step corresponds to the appearance of two layers near the pore walls, while the second step corresponds to the filling of the space between these layers. All features of local isotherms are in agreement with the results obtained from the density functional theory and Monte Carlo simulations. The approach is used for determining pore size distributions of carbon materials. We illustrate this with the benzene adsorption data on activated carbon at 20, 50, and 80 degreesC, argon adsorption on activated carbon Norit ROX at 87.3 K, and nitrogen adsorption on activated carbon Norit R1 at 77.3 K.

Density functional approach to adsorption of a polydisperse fluid in slit-like pores

An extension of the Kierlik-Rosinberg version of density functional theory to the case of a polydisperse fluid of hard spheres in contact with a single wall and also adsorbed in slit-like pores is presented. A comparison with Monte Carlo results show that the theory is capable of precisely reproducing density profiles of a polydisperse fluid near a hard wall. Several model calculations of adsorption in slit like pores are presented. The influence of the confinement on the polydispersity, as compared with bulk fluid, is discussed.

Vapor adsorption on microporous adsorbents

Abstract A new equation describing the adsorption of vapors on microporous adsorbents is presented. This equation is based on the Ono–Kondo model for adsorption in slit-like pores. Results are compared with experimental data for nitrogen on activated carbon over the range of pressures from 0.3 Pa to 0.133 MPa. It is shown that this new equation has several advantages over the widely used Dubinin–Radushkevich (DR) isotherm. In particular, this new equation gives the correct limit of Henry’s law at low concentrations and is more general than the DR equation because it is applicable to gases at supercritical conditions.

Monte Carlo study of adsorption in energetically and geometrically nonuniform slit-like pores

The Monte Carlo simulation method is used to study the effects of nonuniformity on adsorption in slit-like pores. Three types of nonuniformity are considered: polydispersion of pore size diameters in the system, and geometrical as well as energetical heterogeneities of the pore walls. The differences and similarities in the pore filling in different systems are discussed. It is demonstrated that from measurements of the adsorption–desorption isotherms it is very difficult, if at all possible, to say anything about the origin of nonuniformity effects. On the other hand, the shape of the adsorption isotherms is, to some extent, sensitive to the distribution of nonuniformities in the system. © 1997 Elsevier Science S.A.

Grand canonical ensemble Monte Carlo simulations of adsorption in energetically heterogeneous slit-like pores

We report the results of grand ensemble Monte Carlo simulations of argon adsorption in slit-like pores with energetically non-uniform walls. Assuming random topography of adsorbing sites we have investigated how energetic heterogeneity influences the capillary condensation. We have found that energetic heterogeneity changes the first-order capillary condensation into a continuous pore filling.