Shape Of Adsorption Isotherms Research Articles

Limiting performance mechanisms in desiccant wheel dehumidification

Desiccant cooling is an environmentally attractive alternative to conventional mechanical air-conditioning. The heart of the process is the rotary desiccant wheel which is used to dehumidify air. Recent experimental measurements of wheels with proposed alternative materials at low regeneration temperatures (<80C) have shown fewer benefits than anticipated based on the material adsorption characteristics.Here a numerical model of a desiccant wheel was used to investigate the specific influence of the desiccant equilibrium adsorption isotherm on the overall wheel performance. The heat of adsorption, moisture diffusion rate, desiccant specific heat capacity and density were varied to provide further insight into the limiting heat and mass transfer mechanisms for low temperature regeneration. In addition, an optimization analysis of the desiccant adsorption isotherm shape was performed for a range of process conditions.The results show that the extent of dehumidification is limited primarily by a combination of thermal affects caused by both the exothermic adsorption process and the carryover of heat from the regeneration stream. Braunuer Type 1 isotherms increase supply air dehumidification over a linear shape, though this is mostly due to the air inlet conditions which are more typically in the lower relative humidity range. The tendency toward Type 1 behaviour is greater when the heat of adsorption is a stronger function of the adsorbed moisture content. At moderate to high face velocities desiccant layer moisture diffusion kinetics also become important. Critically, the absolute moisture capacity has very limited influence on the performance. These findings have important implications for the design of desiccant wheels.

Comparison of textural information from argon(87 K) and nitrogen(77 K) physisorption

Argon(87 K) and nitrogen(77 K) adsorption of three types of porous samples (microporous, microporous-mesoporous and mesoporous) was evaluated. The shapes of isotherms, specific surface areas, mesopore-, micropore- and net pore volumes, pore-size distributions (PSD) and positions of micropore and mesopore PSD’s maxima were compared to ascertain the credibility of individual adsorbates for the texture information estimation. The shapes of adsorption isotherms for Ar and N2 of all samples are similar and the adsorbed amounts at relative pressure x = 0.975 differ slightly. For mesoporous samples some differences are observed between specific mesopore surface areas derived from nitrogen and argon isotherms. Radii of pore size maxima from Ar(87 K) PSD’s are on average systematically lower than from N2(77 K) for all samples. The Saito-Foley approach for ZSM-5 samples gives consistently lower mean pore radius from N2(77 K) adsorbate than from Ar(87 K). This difference probably arises from the multiplication factor, Ω, in the Saito-Foley equation which includes physical properties (magnetic susceptibility, polarizability etc. at 77 and 87 K) and is not easy to obtain with sufficient precision. The use of both adsorbates Ar(87 K) and N2(77 K) possesses some advantages as well some disadvantages and the comparison of textural properties of individual samples must be evaluated with respect to adsorbate.

Influence of Cd2+ and Cd(OH)+ Species on Adsorption to Activated Carbons in Aqueous Solutions

Cadmium(II) ions uptake by some chemically different surface activated carbons (ACs) were investigated to find out parameters to optimize the adsorption operation in aqueous solutions. The activated carbons were coal based ACs de-ashed by sequential treatments of HCl and H2F2, the de-ashed AC treated with nitric acid to introduce acidic functional groups on the surface, and contrarily out-gassed at 1000°C in helium flow to eliminate these groups. The pH alternation before and after adsorption revealed that the ion exchange between the Cd2+ species and the surface carboxylic acid was taken place for the oxidized AC, whereas the electrostatic interaction between cadmium(II) ions and Cπ electrons on the graphene layer was valid for AC possessing a little or no C-O complexes. Based on the changes in the shapes of adsorption isotherms for some pH ranges, Cd(OH)+ rather than Cd2+ is considered to be preferable adsorption species onto the acidic oxygen free out-gassed AC surfaces.

Gibbs ensemble Monte Carlo simulation of adsorption for model surfactant solution in confined slit pores

An isobaric-isothermal Gibbs ensemble Monte Carlo simulation has been carried out to study the adsorption of a model surfactant/solvent mixture in slit nanopores. The adsorption isotherms, the density distributions, and the configuration snapshots were simulated to illustrate the adsorption and self-assembly behaviors of the surfactant in the confined pores. The adsorption isotherms are stepwise: a two-step curve for the smaller (30 A) pore and a three-step one for the larger (50 A) pore. The adsorption isotherms and the interfacial aggregate structure of the surfactants in the pores with various sizes show a qualitatively consistent performance with the previous experimental observation. The micelle size distributions of the adsorbed surfactant aggregates have been analyzed in order to understand the adsorption mechanism, which suggests that the step rise in the surfactant adsorption is associated with the considerable formation of the micelle aggregates in the confined pores. The effect of the interaction between the pore surface and the surfactant on the adsorption behavior has also been investigated. The simulation results indicate that a change in the interaction can modify the shape of adsorption isotherms. A nonlinear mathematical model was used to represent the multistep adsorption isotherms. A good agreement between the model fitting and the simulation data was obtained for both the amount of adsorption and the jump point concentration.

Adsorption of carbon dioxide of 1-site and 3-site models in pillared clays: A Gibbs ensemble Monte Carlo simulation

Adsorption behavior of carbon dioxide confined in pillared clays is analyzed by using constant pressure Gibbs ensemble Monte Carlo (GEMC) method. In our simulation, 1-site and 3-site models are used to represent carbon dioxide. At the 1-site model, carbon dioxide is described as a Lennard–Jones (LJ) sphere, while at the 3-site model, carbon dioxide is modeled as a three-sites linear chain represented by EPM2 potential considering the quadrapolar effect. The potential model from Yi et al. for pillared clays is used to emphasize its quasi two-dimensional structure. Comparing the calculated results from the 1-site and the 3-site models at T = 228.15 and 258.15 K, we observe that the adsorption amount from the two models is the same basically. However, the local density presents a significant difference, because the shoulder in the main peak near the wall from 3-site model can reflect the orientation of carbon dioxide. Accordingly, in the systematical investigation to explore the effect of porosity and pore width on the adsorption of carbon dioxide in pillared clays, the 3-site model was only used. We observe that for a narrow pore of H = 1.02 nm , each isotherm shape displays type I curve, suggesting that it is not inflected by the porosity. However, for the larger pores of H = 1.70 and H = 2.38 nm , the increase of the porosity alters the shape of adsorption isotherms from a simple linear relation to the first order jump, indicating that the porosity is of very important factor to affect adsorption and phase behavior of fluids confined in pillared clays. The excess adsorptions of carbon dioxide at supercritical temperatures of T = 323.15 and 348.15 K are also investigated. We find that the maximum exists for each excess isotherm, and the optimal pressure corresponding to the maximum increases with the pore width. However, the porosity has no significant effect on the optimal pressure.

Thermodynamics of the CMMS Approach and Carbon Surface Chemistry in SO2 Adsorption

In this study, we develop the equation describing the enthalpy of adsorption corresponding to the cooperative multimolecular sorption approach proposed by Malakhov and Volkov. For different shapes of adsorption isotherms plotted from this model (and analyzed by Rutherford and Coons), we generated the corresponding enthalpy of adsorption curves. We also discuss other enthalpy plot shapes predicted by the CMMS. The new relations are verified for simultaneous description of SO2 adsorption data, and enthalpy of sorption, measured on graphitized carbon black and on activated carbon. Finally, we apply the CMMS model to description of adsorption data of SO2 measured on the series of modified activated carbons. The porosity of adsorbents was characterized via description of low-temperature N2 isotherms applying the method of Nguyen and Do. Oxygen content in carbons was measured applying the elemental analysis. From the results of the correlations between the parameters of the CMMS and elemental analysis data, it is concluded that the interaction between SO2 molecules and carbon surface oxygen groups is crucial during adsorption of this adsorbate. Our results explain and summarize up the conclusions concerning the effect of carbon surface nitrogen functionalities on SO2 sorption mechanism presented by other authors. Finally, we conclude that the CMMS approach can be successfully applied to the description of this process.

Performance evaluation of wash-coated MCM-48 monolith for adsorption of volatile organic compounds and water vapors

The adsorption equilibrium and column dynamics of five organic molecules (acetone, benzene, n-hexane, toluene, trichloroethene) and water vapors on the wash-coated MCM-48 were studied to investigate the applicability of a monolithic adsorber in the adsorption based areas. The wash-coated samples were characterized by using X-ray diffraction (XRD), scanning electron microscopic (SEM) images and nitrogen adsorption–desorption isotherms. To obtain the pure component adsorption equilibrium data at different temperatures (300.15–322.15 K), a gravimetric method was used. The wash-coated MCM-48 showed the unique feature of mesoporous media without any defects although the adsorption capacities for selected adsorbates decreased. The hybrid adsorption isotherms, presented in our recent works [J.W. Lee, J.W. Lee, W.G. Shim, S.H. Suh, H. Moon, J. Chem. Eng. Data 48 (2003) 381–386; J.S. Oh, W.G. Shim, J.W. Lee, J.H. Kim, H. Moon, G. Seo, J. Chem. Eng. Data 48 (2003) 1458–1462; J.W. Lee, W.G. Shim, H. Moon, Micropor. Mesopor. Mater. 73 (2004) 109–119; W.G. Shim, J.W. Lee, H. Moon, Micropor. Mesopor. Mater. 88 (2006) 112–125] satisfactorily correlated the adsorption equilibrium data obtained. The adsorption equilibrium amounts for water vapor were about 5–6 times higher than those of the corresponding organic compounds. To examine the behavior of adsorption breakthrough curves, we employed axi-symmetric model with hybrid isotherms for single component and a simple extended hybrid Langmuir–Sips (EHLS) equation for multicomponent. The current work demonstrates that the adsorption breakthrough patterns are significantly dependent on the equilibrium adsorption isotherm shape. In the case of multi-component mixture systems, we found that there are cooperative and competitive relationships between soluble chemicals and water vapor. Moreover, considering the prediction works, the hybrid isotherm model agrees well with the single component experimental data while the EHLS model fails to explain the multi-component systems.

Evaluation of adsorption energy distribution of microporous materials by a multivariant identification

The applicability of new BET-like adsorption models (uLBET and LBET formulas) for the examination of microporous materials of irregular structure is discussed. The models are intended to handle the main effects of random pore geometry on the adsorption isotherm shape. They are based on a generalized BET theory and employ simple (but realistic) adsorption energy–pore geometry relationships. To overcome numerical problems inherent in fitting any more advanced adsorption model, a multivariant identification procedure is proposed with a subset of parameters being fixed in each variant. The reliability assessment of the system parameters evaluation is stressed. The paper outlines formal and numerical bounds for adsorption measurements-based identifiability of irregular porous structures. The LBET-type formulas are recommended for the examination of such materials, as a relatively simple alternative versus DFT-based techniques. Numerous simulation results and exemplary examinations of an empirical isotherm are discussed.

Modeling of pore wall amorphous structures: influence of wall heterogeneity on the mechanism of adsorption: Krypton and Argon adsorption in MCM-41 pore model

Influence of the pore wall heterogeneity on the adsorption mechanism has been studied. The heterogeneity has been modeled by adsorption site distributions. Two aspects of these distributions have been analyzed: energetic and structural. It has been shown that the energy site distribution determines the low-pressure adsorption whereas the structural distribution affects the multi-layer adsorption. In the case of the adsorption of Ar atoms, one can find a distribution of adsorption sites that allows one to modify the shape of adsorption isotherms and reproduce the experimental data. In the case of Kr adsorption, we observe that the intermediate structures, between one and two layers, are not stable.

Modeling of pore wall amorphous structures: influence of wall heterogeneity on the mechanism of adsorptionKrypton and Argon adsorption in MCM-41 pore model

Influence of the pore wall heterogeneity on the adsorption mechanism has been studied. The heterogeneity has been modeled by adsorption site distributions. Two aspects of these distributions have been analyzed: energetic and structural. It has been shown that the energy site distribution determines the low-pressure adsorption whereas the structural distribution affects the multi-layer adsorption. In the case of the adsorption of Ar atoms, one can find a distribution of adsorption sites that allows one to modify the shape of adsorption isotherms and reproduce the experimental data. In the case of Kr adsorption, we observe that the intermediate structures, between one and two layers, are not stable.

Preparation of alginate–chitosan hybrid gel beads and adsorption of divalent metal ions

Naturally occurring polysaccharides such as alginic acid and chitosan have been recognized as one of the most effective adsorbents to eliminating low levels of heavy metal ions from waste water stream. The present study intended to use alginic acid and chitosan simultaneously, which are expected to form a rigid matrix structure of beads due to electrostatic interaction between carboxyl groups on alginic acid and amino groups on chitosan, and to prepare alginate–chitosan hybrid gel beads. This could be achieved for the first time by using water-soluble chitosan, which was obtained by deacetylating chitin to 36–39% degree. The water-soluble chitosan dissolved in water could remain in solution in the presence of sodium alginate, and the homogeneous solution of chitosan and alginate was dispensed into a CuCl 2 solution to give gel bead particles. The resultant beads were then reinforced by a cross-linking reaction of aldehyde groups on glutaraldehyde with amine groups on the chitosan. The cross-linking reaction made the beads durable under acidic conditions. The adsorption of Cu(II), Co(II), and Cd(II) on the beads was significantly rapid and reached at equilibrium within 10 min at 25 °C. Adsorption isotherms of the metal ions on the beads exhibited Freundlich and/or Langmuir behavior, contrary to gel beads either of alginate or chitosan showing a step-wise shape of adsorption isotherm.

Adsorption Properties and Porous Structure of η-Al2O3: Effect of Hydroxybenzenes Used as Templates

Isotherms of tetrachloromethane vapor adsorption/desorption on porous η-Al2O3 samples, which are prepared by the thermal decomposition of bayerite precipitated in the presence of several hydroxybenzenes, are measured. Using Nechaev's hypothesis that only hydroxybenzene molecules with strictly specified ionization potential are adsorbed from aqueous solutions on the oxide surface, it is demonstrated that the templated precipitation of η-Al2O3 leads to changes in the shapes of adsorption isotherms and hysteresis loops resulted from the fundamental changes in the pore shape of the forming adsorbents.

A non-BET adsorption and its LgBET model

Abstract The paper proposes new mathematical tools for interpretation of adsorption isotherms at relative pressures ranging to 0.95. Adsorption of small (nearly spherical) molecules in microporous adsorbents containing hole-like micropores is considered. In such materials formation of adsorbate clusters more compact than BET stacks is possible that leads to non-BET configurational effects. Additionally, geometrical and energetic constraints for the cluster size may be of significance. The paper shows that such effects can be handled with a rigorous mathematical model based on the BET approach. The derived general formulas may be useful in theoretical studies of effects of pores geometry on the adsorption isotherm shape. The neglecting of the geometrical constraints leads to a BET-like analytical formula (gBET equation). Moreover, an approximating formula is proposed (LgBET equation), taking into account the cluster size constraints represented by an exponential distribution function. The gBET and LgBET models may be used to draw semi-quantitative information on the shape of pores by the examination of adsorption isotherms at high relative pressures (ranging to 0.9). They provide also more reliable information on the adsorbent surface area and adsorption energy than the BET equation. Exemplary results of the new model application to analysis of N2 adsorption on activated carbons are discussed.

The fundamentals of adsorption theory for a mixture of bulky molecules in slit-shaped pores with heterogeneous wall surfaces

The fundamentals of the adsorption theory for a mixture of bulky molecules blocking more than one adsorption site on the surface in slit-shaped pores with heterogeneous wall surfaces are outlined. The adsorbate—adsorbate lateral interactions are taken into account in the quasi-chemical approximation and in the mean-field approximation. The expressions for the partial adsorption isotherms and for the binary coefficients of mixture separation and the way of isolation of the partial contributions of molecules on heterogeneous adsorption sites on pore walls are discussed. A simplified variant of adsorption theory for a binary mixture of molecules of different sizes in two-layer pores with the assumption of complete coverage of the pores is considered. The influence of the energy of binding of molecules to pore walls, lateral interactions, and the ratio of the component sizes on the shape of adsorption isotherms is analyzed. The results of calculations are compared with the experimental data for the benzene—CCl4—microporous AC carbon adsorbent system.

Selection of low swelling materials for protein adsorption from white wines

Summary The protein adsorption capacity of different materials was evaluated to assess their potential efficacy as alternative adsorbents for the removal of proteins from wines, with the purpose of finding suitable low‐swelling materials that could be used in a percolated bed. The adsorbents tested were thermally treated sodium bentonite, low‐swelling adsorbing clays, ion exchange resins and other protein adsorbents (silica gel, hydroxyapatite and alumina). The materials were evaluated by analysing both the capacity to stabilize untreated white wines, according to a heat test, and by characterizing the adsorption isotherms of bovine serum albumin (BSA) in a model wine (water, ethanol and K‐bitartrate). Breakthrough curves in a packed bed were determined experimentally for some materials, showing the influence of the adsorption isotherm shape. Some ion‐exchange resins showed a favourable behaviour and have good potential as alternative adsorbents.

Gravimetric transient response study of adsorption and reaction of propylamines on alumina

The rate and equilibrium of adsorption of n-propyl- and of di- n-propylamine on γ-alumina were studied using recording microbalances Cahn D-200. Measurements were performed both at lower temperatures where only adsorption occurs and higher temperatures where adsorption is accompanied by the elimination reaction. The shape of adsorption isotherms indicated a presence of two types of adsorption centers at lower temperatures of measurements. By a comparison of values of adsorption constants derived from equilibrium adsorption data with those calculated from kinetic results it was found that the equilibrium adsorption constants may be obtained from kinetic data in the case of slow adsorption as that of n-propylamine. This approach was used for calculation of equilibrium adsorption constants for di- n-propylamine, where the attainment of true equilibrium in adsorption experiments was impossible due to excessive time required. By modelling of di- n-propylamine adsorption–desorption experiments, it was shown that the slow increase of adsorption observed after the initial quick rise of the adsorbed amount might be explained by a rearrangement of species adsorbed. The amine molecule bound initially to individual center rearranges gradually to configuration where amine molecule is bound to two different types of sites. The models and approaches were extended to the temperature range where the deamination reaction occurs. Simultaneous recording of weight of the alumina sample and the propylene production rate indicated that for agreement of calculated and observed system behavior, the poisoning effect of ammonia and propylene has to be included into the model of the adsorption-reaction process.

Interactions in Calcium Oxalate Hydrate/Surfactant Systems

Phase transformation of calcium oxalate dihydrate (COD) into the thermodynamically stable monohydrate (COM) in anionic (sodium dodecyl sulfate (SDS)) and cationic (dodecylammonium chloride) surfactant solutions has been studied. Both surfactants inhibit, but do not stop transformation from COD to COM due to their preferential adsorption at different crystal faces. SDS acts as a stronger transformation inhibitor. The general shape of adsorption isotherms of both surfactants at the solid/liquid interface is of two-plateau-type, but differences in the adsorption behavior exist. They originate from different ionic and molecular structures of crystal surfaces and interactions between surfactant headgroups and solid surface.

Influence of surface pretreatment of bismuth and cadmium electrodes to the electric double layer and adsorption characteristics of organic compounds

Influence of the surface structure of electrodes on the electric double layer properties, as well as on the adsorption characteristics of various organic compounds has been investigated by cyclic voltammetry, impedance and electron microscopic methods at variously prepared Bi, Sb and Cd electrodes. The systematic trends of the influence of surface roughness and energetic inhomogeneity on the electric double layer characteristics (value of differential capacity, potential of diffuse minimum, Parsons-Zobel factor, inner layer capacity, roughness function) have been established. The dependencies of the shape of adsorption-desorption maxima of organic compounds and other adsorption parameters (values of attraction interaction, adsorption equilibrium constant, shape of adsorption isotherm) on the electrode surface structure have been investigated. The Debye-length dependent roughness function has been calculated. The approximate values of the linear parameter of homogeneous regions, which prevail at the surface of polycrystalline electrodes have been calculated using various theoretical models. The established values of linear parameters of homogeneous regions have been compared with the characteristics obtained from the electron microscopic studies.

95/01641 Solar generator 3 - ANU's 400m 2 paraboloidal concentrator. A status report

A montmorillonite was pillared with oxide pillars such as Al2O3, ZrO2 and TiO2 to generate permanent microporosity in the interlayers. The X-ray powder diffraction analysis and nitrogen and water adsorption measurements indicated that the pillars were stable at least up to 600°C. The slit widths (pore size) are on the order of 7–9 Å for Al2O3, 8–10 Å for ZrO2 and 14–17 Å for TiO2 pillared clay. The TiO2 pillared clay exhibited Brunauer type IV isotherm that fits BET linear equation for both nitrogen and water. Although Al2O3 and ZrO2 pillared clays followed Brunauer type I isotherm with nitrogen, their water adsorption isotherms exhibited unusual shapes and did not fit either Langmuir or BET equation. The unique pore sizes of these pillared clays and low affinity of their surfaces for water, which determine the amount and rate of water adsorption at particular relative pressure (P/PO), were demonstrated to be responsible for the unusual shapes of the isotherms. The water capacities of these pillared clays were found to be 200–300 mg/g of degassed sample, which are comparable to commercially available desiccants such as zeolites and silica gels. The application of dessicant materials in gas-fired cooling and dehumidification equipments (air conditioning systems) demands an ideal isotherm shape (moderate type I), a quality lacking both in zeolites and silica gels. Pillaring led to improved water adsorption isotherm shape as compared to the original montmorillonite. Further improvement in the shape of isotherm may be achieved by introducing higher degree of hydrophilic or polarity components in these pillared clays.

95/01643 Solar pool heating atandarda

A montmorillonite was pillared with oxide pillars such as Al2O3, ZrO2 and TiO2 to generate permanent microporosity in the interlayers. The X-ray powder diffraction analysis and nitrogen and water adsorption measurements indicated that the pillars were stable at least up to 600°C. The slit widths (pore size) are on the order of 7–9 Å for Al2O3, 8–10 Å for ZrO2 and 14–17 Å for TiO2 pillared clay. The TiO2 pillared clay exhibited Brunauer type IV isotherm that fits BET linear equation for both nitrogen and water. Although Al2O3 and ZrO2 pillared clays followed Brunauer type I isotherm with nitrogen, their water adsorption isotherms exhibited unusual shapes and did not fit either Langmuir or BET equation. The unique pore sizes of these pillared clays and low affinity of their surfaces for water, which determine the amount and rate of water adsorption at particular relative pressure (P/PO), were demonstrated to be responsible for the unusual shapes of the isotherms. The water capacities of these pillared clays were found to be 200–300 mg/g of degassed sample, which are comparable to commercially available desiccants such as zeolites and silica gels. The application of dessicant materials in gas-fired cooling and dehumidification equipments (air conditioning systems) demands an ideal isotherm shape (moderate type I), a quality lacking both in zeolites and silica gels. Pillaring led to improved water adsorption isotherm shape as compared to the original montmorillonite. Further improvement in the shape of isotherm may be achieved by introducing higher degree of hydrophilic or polarity components in these pillared clays.