Title

High pressure adsorption of pure gases on activated carbon: Analysis of adsorption isotherms by application of potential theory and determination of heats and entropies of adsorption

Date of Award

1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Jim Schwarz

Keywords

Potential Theory of Polanyi, Dubinin-Astakhov (DA) Equation

Subject Categories

Chemical Engineering

Abstract

Analysis of high pressure adsorption isotherms on activated carbon at a number of temperatures for methane, ethane, ethylene, propane, carbon dioxide and nitrogen was carried out. Isotherms were obtained both above and below the critical temperature (Tc) for ethane, ethylene and carbon dioxide, below Tc for propane and above Tc for methane and nitrogen and up to 60 atmospheres of pressure.

The adsorption data were analyzed by the Potential Theory of Polanyi. The adsorption data were reduced to temperature independent characteristic curves for each adsorbate. The adsorbed phase molar volumes, necessary to obtain the characteristic curves, were evaluated by Dubinin's method. In this method, an exponential increase with temperature of the adsorbed phase molar volumes is postulated. It was observed that adsorption potential defined as RTln(fs/f) where fs was calculated from Ps = (T/Tc)$\sp2$Pc resulted in temperature invariant characteristic curves for each adsorbate.

The equations for adsorption isotherms for each adsorbate were developed using the Dubinin-Astakhov (DA) Equation. Values of the three parameters, $\rm W\sb0$, $\rm E\sb0$ and n were found to be temperature independent for each of the adsorbates. These parameters for each adsorbate were found to represent the experimental isotherms within 5-8%. A single equation was obtained by least square regression of the adsorption data for all the gases. From this equation, the adsorption data could be well predicted at high pressures. At low pressures, however, the errors between the experimental and predicted values were large.

Isosteric heats and differential entropies of adsorption were obtained from the experimental data. Affinity coefficients obtained by considering the ratio of polarizabilities for the adsorbate referenced to that of a standard one reduced the characteristic curves of all the adsorbates onto a generalized characteristic curve. The variation in isosteric heats of adsorption with coverage for the adsorbates could not be reduced onto a single generalized curve by these affinity coefficients. A new approximation is proposed; affinity coefficients, adapted from a method suggested by Wakasugi reduced both the adsorption data and variation of isosteric heats onto single curves. Account was taken of the electrostatic interactions between the adsorbate and the adsorbent in these affinity coefficients. From entropy considerations, it was found that adsorption was localized. (Abstract shortened with permission of author.)

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