Doctoral Dissertations

Date of Award

12-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Robert M. Counce

Abstract

The adsorption of methyl iodide onto hydrogen-reduced silver-exchanged mordenite (Ag°Z) was studied. The removal of organic iodides from off-gas streams is an important step in controlling the release of radioactive iodine to the environment during the treatment of radioactive wastes or the processing of some irradiated materials. Nine well accepted mass transfer models were evaluated for their ability to adequately explain the observed CH3I uptake behavior onto the Ag°Z. Linear and multidimensional regression techniques were utilized in the estimation of the diffusion constants and other model parameters which then permitted the selection of an appropriate mass transfer model. While a number of studies have been conducted to evaluate the loading of both elemental and methyl iodide on silver-exchanged mordenite, these previous studies focused primarily on the macro scale (deep-bed) while evaluating the material under a broad range of process conditions and contaminants for total bed loading at the time of breakthrough. A few studies evaluated equilibrium or maximum loading. Thus, to date, only bulk loading data exist for the adsorption of CH3I onto Ag°Z. Hence this is believed to be the first study to quantify the controlling mass transfer mechanisms of this process. It can be concluded from the analysis of the experimental data obtained by the 'tingle-pellet" type experiments and for the process conditions used in this study that the overall mass transfer rate associated with the adsorption of CH3I onto Ag°Z is affected by both micropore and macropore diffusion. The macropore diffusion rate was significantly faster than the micropore diffusion, resulting in a two-step adsorption behavior which was adequately modeled by a bimodal pore distribution model. The micropore diffusivity was determined to be on the order of 2 x 10-14 cm2/s. The system was also shown to be isothermal under all conditions of this study. Two other conclusions were also obtained. First, the gas film resistance to mass transfer for the 1/16- and 1/8-in.-diam Ag°Z pellets can be ignored under the conditions used in this study. Finally, it was shown that by decreasing the water vapor content of the feed gas, the chemical reaction rate appeared to become the initial rate-limiting factor for the mass transfer.

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