Date of Award

Spring 1990

Document Type

Dissertation

Degree Name

Doctor of Engineering Science in Chemical Engineering

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Joseph W. Bozzelli

Second Advisor

Dana E. Knox

Third Advisor

Piero M. Armenante

Fourth Advisor

Richard Stephen Magee

Fifth Advisor

Jay N. Meegoda

Abstract

Soils contaminated with hazardous organic compounds have been known to threaten human health both directly through various contact mechanisms and indirectly through leaching or transfer to the food chain. Thermal desorption of contaminated soil matrices with secondary treatment of the effluent gases (collection, incineration, etc) is one of most feasible and developing technologies for cleaning of contaminated soils. There is, however, little known about the mass transfer principles of organic contaminants through the heated soil matrices. The objectives of this study is, therefore, to learn and understand details of the mass transfer processes of organic compounds in these soil matrices. We have performed several varied types of experiments to determine specific mass transfer parameters and developed a model which accurately describes the process and can be utilized to obtain optimal operation conditions.

Chromatographic response analysis and transient adsorption desorption equations of flow through soil columns have been utilized in developing the initial model. We incorporate intra (pore) and interparticle diffusion, equilibria, in addition to normal mass transfer parameters of axial dispersion and film mass transfer around particle surface.Experiments primarily consist of plug flow deposition of the contaminants on a well characterized soil column and saturation desorption of a soil bed, in addition to equilibrium tests. The plug flow deposition experiments connected with chromatographic analysis successfully yielded equilibrium constants, heats of adsorption and mass transfer parameters. In addition we identified a minimum allowable temperature (MAT), below which the organic compounds are not completely desorbed from the soil within a 1 hour operating time. Resulting equilibrium constants were strongly dependent on temperature and were revealed to follow the van't Hoff equation above the MAT's. Analysis of heats of adsorption showed that the organic - soil system can be considered as a moderately weak physical adsorption system.

Analysis results utilizing an experimental equilibrium test apparatus demonstrated that adsorption isotherms show good linearity at lower concentration and that linear zone tends to extend to higher concentrations with increasing temperature. The slopes of linear adsorption isotherms tend to decrease with increasing temperature, indicating less adsorption. As a result of desorption experiments using the equilibrium apparatus, data showed hysteresis phenomena at lower temperatures probably due to irreversibility of adsorption processes. The observed hysteresis tended to become weaker i.e. the data for desorption closely followed the adsorption isotherm as temperature increases.

An analytical solution and a numerical approach using orthogonal collocation have been utilized for the purpose of predicting the transient mass transfer behavior of organics in a soil column. The two methods result in satisfactory coincidence.

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