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Etude de l'adsorption du benzene sur du charbon actif microporeux par determination des isothermes d'adsorption

Derived here is an energy distribution function that is associated with the Gaussian distribution of micropore sizes, which was introduced by Dubinin for describing gas adsorption on heterogeneous solids with micropores only. A global energy distribution is defined for solids with both micropores and mesopores, and a method is proposed for extracting contributions to this global function arising from the non-uniform microporous structure of a solid and from the surface heterogeneity of mesopores.

Adsorption methods for characterizing the microporosity of activated carbons are discussed critically. Three methods—the αs-method and those based on the Dubinin-Radushkevich and Jaroniec-Choma isotherm equations—are compared with respect to the parameters that characterize the microporous structure of a solid. It is shown that the isotherm equations that account for the structural heterogeneity of activated carbons give values of the micropore volume similar to that obtained by the αs-method.

A new equation for gas adsorption on microporous solids is proposed on the basis of the exponential adsorption isotherm, which is generated by the virial expansion of the spreading pressure. Expressions associated with this equation are derived also for the adsorption potential distribution, the differential molar enthalpy, and the differential molar entropy of adsorption. It is shown that this equation gives a good description of benzene adsorption on activated carbons with different degrees of microporosity and structural heterogeneity.

A single curve of the adsorbate volume W in the adsorbed phase versus the reduced adsorption potential φ correlates adsorption isotherms of saturated hydrocarbons of low molecular weight (viz. ethane, propane, and n-butane) at pressures from 0.034 to 10 mm Hg on Columbia 4LXC 1228 activated carbon at 25 and 30°C. This correlation is based on the Polanyi-Dubinin adsorption potential theory and the use of the Peng. Robinson equation of state to calculate thermodynamic properties. This correlation can be represented analytically by expanding the logarithm of W in terms of a quadratic polynomial in φ.

By measuring the time-dependent transmission of a trace contaminant gas through an adsorber bed, we obtained the volume adsorption capacity of (Columbia type 4 LXC 12/28) activated carbon at 25.0°C for four trace contaminants (acetaldehyde, ethanol, acetone and freon-113) each selected to represent an important class of organic compounds (i.e. aldehydes, primary alcohols, ketones and halocarbons). The transmission is the ratio of the concentration at the outlet of the adsorber bed to that at the inlet. Each measurement was carried out at a steady inlet concentration of about 100 ppm in helium at a total pressure of one atmosphere. Adsorption capacities at this low concentration can be determined more readily by this dynamic transmission method than by the conventional static method.