X-ray Microtomography: a Useful Method to Study the Dynamics of Organic and Water Vapours Adsorption on Carbon

The suitability of the X-ray microtomographic technique to study the dynamics of vapor adsorption on carbon beds was assessed in this work. This technique was tested using three different systems: CH3I, water vapor and a mixture of waterand organic vapors. Image analysis of the carbon bed cross-sections obtained by X-ray microtomography allows determining the adsorption front progress in the case of organic vapor and mixture of water and organic vapor whereas the existence of this front was not so obvious in the case of water vapor. The tomographic data show flat profile, i.e. a continuous water uptake with time. Experimental results obtained for organic vapors were interpreted on the basis of the Wheeler-Jonas equation: a good agreement was found between experimental and theoretical breakthrough times. Introduction Activated carbons are largely used to adsorb organic compounds in industrial processes as well as for civil and military protection. Water vapor is usually present in gas streams, ranging from low concentrations to the saturated state. So, when modeling organic vapor adsorption on activated carbon filters, the effects of water-vapor coadsorption has to be taken into account [1-3]. Both the preadsorbed water and the water present in the contaminated air stream can have a deleterious effect on the adsorption capacity of the activated carbon. As organic vapors can go from badly smelling, over noxious and ecotoxic to highly toxic, a correct estimation of filter breakthrough times is essential. This explains the interest in the coadsorption behavior of water and organic vapors on activated carbon. However, this attention has been, almost exclusively, focused on the static adsorption capacity, i.e. the usual adsorption isotherm [4]. Even though this static capacity is very important, adsorption and coadsorption are essentially dynamic phenomena. The first purpose of this work was to assess the suitability of x-ray microtomography to follow an adsorption process taking place in an activated carbon filter. The very first use of X-ray imaging, in radiography mode, to study adsorption in porous materials was performed by Dubinin et al. [5]. In this work, X-ray contrast substances were used to analyze the nature of the mass transfer limiting step, i.e. adsorption in micropores or transport in meso and macropores, in activated carbons. X-ray radiographies served as visual supports to explain adsorption kinetics data but no quantification was performed. Fifteen years later Wittwer and Lavanchy [6] used a medical tomograph to visualize the adsorption of organic vapors in activated carbon. To our knowledge, this non-destructive 3D-technique has not been exploited much since the early 90's. During the last decade, microtomographs with improved resolution were developed and commercialized. The objective of the present study was to