Description of the adsorption equilibrium of organic substances absorbed from a flow of moist air by a fixed bed of active carbon moistened to equilibrium

Isotherms of the adsorption of benzene vapor from moist air on active carbon (AC) moistened to equilibrium are obtained from dynamic experiments. The experimental data on the adsorption equilibrium of some organic substances from a flow of moist air by a fixed bed of AC moistened to equilibrium are obtained. The data on the equilibrium adsorption of benzene vapor is analyzed using the Dubinin-Radushkevich equation (the theory of volume filling of micropores). It is revealed that the characteristic adsorption energy of benzene vapor decreases as the filling of the microporous volume with water molecules increases. The characteristic adsorption energy depends on the following factors: polarizability of a substance in the adsorption field created by micropores, the number of carbon atoms in the adsorbate molecules, and parameters of the porous structure. The equation for the calculation of the parameters of equilibrium adsorption of organic substances from moist air on AC moistened to equilibrium are obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 434–438, March, 1995.     

Calculation of the breakthrough time of an organic substance through a fixed layer of active carbon

The results of investigations of the adsorption dynamics of organic substances on active carbons (AC) moistened to equilibrium over a wide range of breakthrough concentrations are discussed. The features of adsorption dynamics typical for the moisture-free AC in the region of low breakthrough concentrations are also present when they are moistened to equilibrium. The equations for calculating the breakthrough time of low concentrations through a fixed layer of equilibrium moistened AC corresponding to the maximum permissible concentrations are derived.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 62–66, January, 1995.     

Adsorption of vapor mixtures of water and organic substance on activated carbons

The isotherms of water adsorption in the presence of an organic substance vapor with a specified concentration are calculated from experimental data on the joint frontal dynamics of adsorption of water vapor and several organic substances (benzene, hexane, cyclohexane, tetrachloromethane, and perfluorotripropylamine) on two samples of activated carbons. The influence of the organic substances on the equilibrium water adsorption decreases with an increase in the molecule size.     

Activated carbon oxygen content influence on water and surfactant adsorption

This research investigates the adsorption properties of three activated carbons (AC) derived from coconut, coal, and wood origin. Each carbon demonstrates different levels of resistance to 2 M NaOH treatment. The coconut AC offers the greatest and wood AC the least resistance. The influence of base treatment is mapped in terms of its effects on specific surface area, micropore volume, water adsorption, and dodecanoic acid adsorption from both water and 2 M NaOH solution. A linear relationship exists between the number of water molecules adsorbed at the B-point of the water adsorption isotherm and the oxygen content determined from elemental analysis. Surfactant adsorption isotherms from water and 2 M NaOH indicate that the AC oxygen content effects a greater dependence on affinity for surfactant than specific surface area and micropore volume. We show a linear relationship between the plateau amount of surfactant adsorbed and the AC oxygen content in both water and NaOH phases. The higher the AC oxygen content, the lower the amount of surfactant adsorbed. In contrast, no obvious relationship could be drawn between the surfactant amount adsorbed and the surface area.     

沥青基球状活性炭气相吸脱附行为研究

本文利用热重法研究了一系列沥青基球状活性炭对蒸汽及正已烷蒸汽的动态吸脱附曲线。结果表明,PSAC对苯蒸汽的吸附及再生性能优良。随吸附温度的降低、比表面积的增大、总孔容及微孔容的增大,PSAC对苯蒸汽的吸附容量增大。PSAC对正已烷的吸附速度大于对苯蒸汽的吸附速度,但其对正已烷的平衡吸附容量小于对苯蒸汽的平衡吸附容量。     

Formation of the layering boundary in the water–benzene–perfluorobenzene system

The dynamics of the interface between liquid phases in the water–benzene–perfluorobenzene system was studied in a natural experiment. The interfacial tension was found to depend on the density of the organic layer. The range of interfacial tensions in which inversion of the organic and aqueous phases takes place was determined, and the working range of a separating flask as an element of the separation scheme for the mixture was revealed.     

Adsorption studies of acetic acid dimers on activated charcoal from organic solvents

Acetic acid exists as dimers in organic solvents like benzene, toluene and xylene. Adsorption of dimeric acetic acid on activated charcoal (AC) at various temperatures from benzene, toluene and xylene solutions have been studied. The system obeys Langmuir isotherm, thus signifying a monolayer adsorption of dimers. Corrections on AC-solvent pore volume fillings, molecular cross sectional surface area of acetic acid dimers, the adsorption equilibrium constants, the free energy change and the enthalpy change values are computed at different temperatures for the three solvents. The adsorption process has been found to be physisorption type. The FTIR measurements show that the adsorbed acetic acid dimer seems to retain the cyclic structure against the open chain non-cyclic structure.     

Adsorption of organics on MSC5A in supercritical CO2, chromatographic measurements & stop & go simulation

Chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on molecular sieving carbon (MSC) in supercritical fluid CO₂ mixed with organics. Supercritical chromatograph packed with MSC was used to detect pulse responses of organics. Adsorption equilibria and adsorption dynamics parameters for organics were obtained by moment analysis of the response peaks. Dependences of adsorption equilibrium constants, K ^?, and micropore diffusivity,?D, on the amount adsorbed were examined. The dependencies of adsorption equilibrium constants, K ^?, and micropore diffusivity, D, of benzene, toluene and m-xylene, on molarity of benzene with each parameters of temperature or pressure were obtained. It was found that the values of K ^? and D for an organic substance depended on the amount adsorbed of other organics strongly. And stop & go method was used as simulation method of perturbation chromatography for investigating adsorption equilibrium and rate. Numerical solution for multicomponent chromatogram in time domain could be obtained by appropriate model equations with experimental conditions. This simulated chromatogram can be compared with experimental chromatogram to determine the adsorption equilibrium and rate parameters. In addition, molecular simulation of multicomponent adsorption equilibria was performed, and potential parameters were determined by comparing the simulation with experimental results. Simulation soft ware is Cerius2 (Version?4.2) made by MSI. The purpose of performing simulation is to elucidate an adsorption mechanism on the molecule level.     

Energies of Intermolecular Interaction and Hydration of Benzene Derivatives Molecules Adsorbed on Active Carbon from an Aqueous Solution

Energy heterogeneity of an adsorbent surface does not affect the isotherm of adsorption from aqueous solutions. Therefore, the balance of the changes in the Gibbs energy resulted from the adsorption of sparingly water-soluble substances shows that, in the whole range of filling of the adsorption phase, the pattern of the adsorption isotherm depends on the difference between the energy of the interaction of adsorbed molecules with each other and the energy of their hydration needed to displace water molecules from the adsorption phase. The standard adsorption energy of molecules of benzene and its derivatives from aqueous solutions and the difference between the energy of their interaction in the adsorption phase and the energy of their hydration are determined by the extrapolation of adsorption isotherms of these substances to the standard conditions ( 0, C 0) and to the conditions of maximal approach of the adsorbed molecules to each other ( 1). The hydration energy of the molecules of benzene and its derivatives is calculated based on the proportionality of this energy to the sum of the concentration potentials of the components in a saturated solution, where the proportionality coefficient is equal to the number of water molecules interacting with one organic molecule. Calculated energies of phenol and aniline hydration are equal to the energies of H-bonds of phenol (OH···OH₂) and amino (NH···OH₂) groups. Hydration energies of phenol and aniline derivatives vary according to the effect of a substituent in the benzene ring on the H-bond energy. Negative values of hydration energy of polar hydrophobic molecules result from their hydrophobic effect on water structure. The interaction energy of hydrated and dehydrated adsorbed molecules, whose benzene ring planes are oriented in parallel to the carbon surface, is found from the calculated hydration energies. For H-bond-forming molecules, this energy is equal to the energy of one H-bond formed upon the surface dimerization. The energy of repulsion between polar hydrophobic molecules of benzene derivatives depends on the vertical component of the dipole moment, and is the higher the larger the polar group volume.     

Dynamics of joint adsorption of mutually soluble substances by activated carbons

A model for the dynamics of isothermal absorption of a binary mixture of an organic substance, soluble in water, and water vapor in a fixed bed of activated carbon was proposed. It includes the equations of material balance and the Myers—Prausnitz model for equilibrium adsorption. The possibility of formation of the condensed phase during the adsorption of an organic substance on moist activated carbon was shown. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1491–1495, August, 1998.     

Effects of active carbon pore size distributions on adsorption of toxic organic compounds

The use of active carbons for the removal of toxic organic compounds, for example from air or smoke, is of significant interest. In this paper, the equilibrium and dynamic adsorption characteristics of two active carbons are explored; one microporous coconut based and the other micro-mesoporous derived from a synthetic resin. Benzene, acetaldehyde and acrylonitrile were chosen as the probe toxicant vapours and adsorption was measured at a temperature of 298 K. The nitrogen equilibrium data (at 77 K), analysed using the BET, Dubinin-Radushkevich equations and DFT models, showed a higher overall adsorption capacity, more supermicroporosity and a higher proportion of pores wider than 2 nm for the synthetic resin based material. A micropore distribution biased toward the ultramicropore width-range was observed for the nutshell material. As a consequence, the characteristic adsorption energies in micropores are higher for the nutshell material than the resin based carbon. The effect of these different pore size characteristics on the adsorption kinetics, obtained by fitting the data to the linear driving force (LDF) model, is that the resulting adsorption rate constants are higher across much of the relative pressure range (p/p ^s ) studied for the resin based carbon compared to the nutshell material. Significantly, the wider pores of the resin-based carbon result in higher rates of adsorption in the micropore filling domain. When evaluated under dynamic conditions in cigarette smoke, improved toxicant removal was observed using the resin based carbon.     

Adsorption phenomena at high pressures and temperatures.

Equilibrium isotherms of adsorption of methane on crystalline Rho zeolite were measured with the use of a precision volumetric gravimetric setup that was developed for determining the equilibrium and Kinetic parameters of adsorption in the pressure range of 0.1–160 MPa and temperature range of 300–600 K. The method of determining the accessible volume of an adsorption system (free volume + micropore volume) and the micropore volume of a sorbent was used. Two independent methods for calculation of micropore volume on the basis of the isotherms of excess adsorption were used. The discrepancy in the results of the calculation of the micropore volume by three independent methods is within the limits of the calculation accuracy. An evaluation of a change in the isosteric heats of the excess and absolute adsorption of methane on Rho zeolite was carried out in relation to filling and temperature. An evaluaton of the adsorption volume above the outer surface of the zeolite crystals was performed. The results of experimental investigations of methane adsorption on Rho zeolite can be used to solve the problem of encapsulation of gases by solid sorbents. kg]Key words kw]adsorption kw]micropore volume kw]surface kw]isostere kw]heat kw]zeoliteFor Part 1 see Ref. I.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 568–573, March, 1996.     

Adsorption of water vapour from humid air by selected carbon adsorbents

The water uptake by carbon molecular sieves (CMS) and graphitized carbons, all of which are used to determine volatile organic compounds in air, was investigated using a direct experimental approach. CMS, e.g. Carboxen 1002, Carboxen 1003 and Anasorb CMS adsorb substantial amounts of water, in the range 400 to 450 mg per gram of adsorbent. Graphitized carbons, e.g. Carbrogaph 5TD and Carbopack X show low water trapping, less than 30 mg g(-1) and Carbopack Y as little as 5 mg g(-1) or less. The water sorption capacity for graphitized carbons is strongly dependent on the relative humidity (RH). The change of RH from 95 to 90% decreases the amount of adsorbed water by more than a factor of 2. Two different water adsorption mechanisms are operative: adsorption on polar centers and micropore volume filling. For graphitized carbons and CMS at low RH, adsorption on polar centers is involved. For CMS, once the threshold value of relative humidity (RHth) is surpassed, micropore volume filling becomes predominant. RHth is 44 +/- 3 and 42 +/- 3% for Carboxen 1002 and 1003, respectively, and 32 +/- 3% for Anasorb CMS. The CMS mass in the trap was found not to affect the mass of retained water under condition of incomplete saturation of adsorbent bed with water. Thus, the restrictions commonly imposed on the CMS mass are not necessary. The dry purging technique is suggested to remove adsorbed water. Carbograph 5TD and Carbopack X require only a few hundred ml of dry air to remove adsorbed water entirely. Water can also be purged out from CMS; however, much larger volumes of dry air are needed.     

On the mechanisms of phenol adsorption by carbons

The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.     

Elution and frontal dynamics of adsorption of organic substances on activated carbon

The elution dynamics of adsorption on activated carbon was studied at various carrier-gas flow rates for a series of organic substances. With the help of the model of the equilibrium adsorption layer that uses the adsorption isotherm described by the theory of volume filling of micropores the outlet curves for the elution and frontal dynamics of adsorption can be adequately predicted. The effective kinetic coefficient and the parameters of the adsorption isotherm were found to be constant for the elution curves calculated both in the elution and frontal regimes of the adsorption dynamics over the whole range of concentrations studied. The effective kinetic coefficient in the mathematical model employed for the systems with microporous adsorbents is independent in fact of the nature of an adsorptive and is mainly determined by the parameters of porous structure of activated carbon and the experimental conditions of a dynamic run.     

Spontaneous liquid-gas imbibition for characterization of carbon molecular sieves

Spontaneous liquid-gas imbibition at 293.2K and 0.1 MPa was conducted to assess the micropore size and size-exclusion property of carbon molecular sieves (CMS). The CMS were firstly saturated with N(2) and then immersed into water. The volume of gas recovered by the water imbibition was measured and applied to evaluate the density of the N(2) adsorbed in the CMS. The micropore size of the CMS was determined by comparing the N(2) density from the water-N(2) imbibition with that calculated by grand canonical simulation. The micropore size evaluated by the liquid-gas imbibition coincides with that obtained by N(2) adsorption at ambient temperature. The size-exclusion property of the CMS was estimated through comparing the N(2) recovery by imbibition of liquids with increasing molecular dimensions, that is, water, benzene, and cyclohexane. The amount of N(2) recovered from benzene imbibition is dramatically less than that from the water imbibition, showing that the dominated micropore size of the CMS is smaller than 0.37 nm. Furthermore, the effect of chemical vapor deposition treatment on the porous texture of the CMS was revealed by the liquid-gas imbibition.     

Correlation between crystal texture and microporous structure of carbon adsorbents

A correlation of adsorption and X-ray data for a series of both initial and modified active carbons (AC) was performed. The AC were modified by thermal treatment and progressive activation. A linear correlation between the sizes of crystallites and micropore widths of AC was established. It was shown that the adsorption of water vapor results in an increase in the parameterL _aof crystallites of active carbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2159–2162, November, 1995.The work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-09550).     

Adsorption on nonporous and supermicroporous adsorbents

The mean values of the characteristic energy of C₆H₆ adsorption in large micropores were calculated from the adsorption isotherms of benzene vapor on carbon blacks. The supermicropores are characterized by the significant dispersion of the adsorption potential resulted from the pore-size distribution, which imparts the polymolecular character to adsorption. The effect of enhancement of the characteristic energy of adsorption was analyzed, which was caused by the overlap of the force fields of the opposite pore walls and the reduction of the adsorption film surface with micropore volume filling. The both factors are comparable by magnitude and depend on the micropore sizes.