K. M. Nikolaev

Water vapor adsorption and microporous structure of carbon adsorbents. 17. Analysis of experimental isotherms for water vapor adsorption

Water vapor adsorption for various activated carbons with narrow and wide micropore volume distributions and mesopore surface areas between 40 and 300 m2/g have been investigated. For all the isotherms the point of inflection was determined, which can be taken as the point characterizing the formation of a water adsorption layer on the pore wall surface of carbon adsorbents. To do this the adsorption and desorption branches of the isotherms were approximated according to Weibulls distribution. A good correlation was obtained between values for the water monolayer capacity, calculated from the porous structure parameters of the carbons, and the adsorption values corresponding to the isotherm inflection pointsainf. For the group of carbons studied the values of relative pressure at the inflection point of the isotherms fell within the range 0.5–0.72.

Water vapor adsorption and microporous structure of carbon adsorbents. 18. Effect of surface chemistry of activated carbons on water vapor adsorption

This investigation has been devoted to a study of the chemical composition of the surfaces of activated carbons. A study has been made of the way in which changes in the surface chemistry of a series of carbons, as a result of heat treatment, affects the nature of their adsorption of water vapor. A differentiation has been made between oxygen-containing groups found on the surface of activated carbons before and after their heat treatment. It has been established that the original adsorption centers, which play a determining role in water vapor adsorption by activated carbons, comprise functional groups like strongly acidic free hydrogen ions, carboxylic and phenolic groups, situated on on the pore surface of the activated carbons. The number of these functional groups on the pore surface of the activated carbons has been correlated with the parametera0 (the number of original adsorption centers) in the isotherm equation for water vapor adsorption. The relative pressure corresponding to the formation of an adsorption layer on the surface of the activated carbons has been shown to depend on the number of original adsorption centers, the acidic functional groups.

Heats of immersion of active carbon and carbon black in water and benzene as a function of the moisture content of the adsorbents

Conclusions1.Rehydration of the active sites of carbon and carbon black formed after vacuum heat treatment of the samples at high temperatures is reflected in the heats of their immersion in benzene and water: for carbon, the heat of immersion in benzene does not change, but the heat of immersion in water increases markedly, while the heat of immersion of carbon black in benzene decreases markedly and the heat of immersion in water almost does not change.2.All four basic stages of adsorption of water vapors on active carbon: 1) the formation of water clusters, 2) formation of a random monolayer, 3) volume filling of micropores, and 4) saturation with capillary condensation phenomena in the mesopores, are reflected in the curve of the dependence of the heats of immersion of carbon in benzene on the relative vapor pressure of the preadsorbed water.3.Based on the study of the heats of immersion of AC-47 active carbon in benzene as a function of the amount of preadsorbed water, the reality of the existence of a statistical monolayer or similarity of the monolayer of water in the micropores of nongraphitized active carbon not made chemically hydrophobic, formed in the narrow region of relative water vapor pressures from O.45 to O.6, was demonstrated.

Dynamics of adsorption over a wide range of breakthrough concentrations. Communication 7. Dynamics of adsorption on a biporous adsorbent

Conclusions1.An analytical solution has been found for the model of mixed-diffusion adsorption dynamics on a biporous adsorbent in the case of a rectangular isotherm with account of three kinds of resistance to mass transfer: external, in transport pores, and in microporous zones.2.In the region of small relative breakthrough concentrations the final curves in the coordinates In y-t are linear. The loss of time of protective action in Shilovs equation is a linear function of the relaxation times which are characteristic for three different kinds of mass transfer.

Adsorption of vapors and gases by carbon sorbents with homogeneous microporous structures in the initial region of the adsorption isotherm

Examination of the adsorption isotherm equation derived from the theory of volume filling of micropores (TVFM) showed that the region of low pressures of experimental isotherms is not always described by this equation. A method for describing the initial region of adsorption isotherms using the parameters of TVFM was proposed.

The adsorptivity of organic substances with various physico-chemical properties

The adsorption isotherms of benzene,n-hexane, cyclohexane, and 1,2-dibromo-1,1,2,2-tetrafluoroethane on a nonporous carbon adsorbent, carbon black, were calculated from the results of a gas-chromatographic experiment at 373 K. A general equation of adsorption isotherm for vapors of organic substances on nonporous sorbents that was proposed earlier is shown to be valid in the range of relative pressures,p/ps, of 10−7–1 and temperatures of 293–373 K.

Adsorption behavior of Vulkan-7H graphitized carbon black

The adsorption behavior of Vulkan-7H carbon black graphitized at 3073 K was studied. Benzene adsorption isotherms at 293 K were measured by the static method and by gas chromatography. It was shown that Vulkan-7H carbon black may be considered as a supermicroporous adsorbent with a pore size of 1.05 nm.

Heats of immersion of active carbon in organic compounds. 1. Heat effects upon immersion of dry and moist active carbon in organic compounds

Heats of immersion (wetting) in organic liquids are determined experimentally for active carbon samples, dry and wet to various degrees. It is shown that the heat of immersion of dry active carbon is related to the degree of accessibility of the depth of the micropores of the sorbent to the molecules of an organic compound. It is suggested that during immersion of moist active carbon the degree of displacement of water from the micropores by molecules of the wetting liquid is related to a steric factor, the ratio between the critical diameter of the adsorbate molecule and the size of the micropore. A method is proposed for calculating heat effects during wetting of active carbon with liquids to various degrees of wetness. The calculated values were compared with experimental data.

Study of adsorption of vapors on a nonporous carbon adsorbent

Conclusions1.The isotherms of adsorption of ethyl chloride, benzene, and carbon tetrachloride vapors by a nonporous carbon black were investigated. No more than one to two adsorption layers are formed on the surface of the carbon black in adsorption of the vapors of these substances.2.A general adsorption equation which takes into consideration the effect of the molar volume and parachor of the adsorbed substance on the adsorbability was obtained for the selected vapors.

Determination of the surface area of mesopores in carbon adsorbents

Conclusions1.The use of the existing methods of calculation of the specific mesopore surface area was analyzed on the example of carbon adsorbents with a different pore structure.2.The calculations with the γ method based on the improved theory of Broekhoff and de Boer with consideration of the effect of the curvature of the liquid meniscus on the surface tension and mercury porosimetry give the closest values of the mesopore surface area for all of the adsorbents studied.