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2013
Doctoral Thesis
Titel
On the rapid characterization of porous materials by optical calorimetry
Abstract
The thermal response method, measuring the temperature change of an adsorbent during adsorption of a test gas, was demonstrated to be a versatile tool that can be used for a screening of porous materials. The use of optical calorimetry removes the disadvantages of conventional calorimeters: There is no delay in sensing and the temperature effects associated with adsorption are measured in real time. Thus the possibilities of calorimetry for the characterization of porous materials can be used but with a significantly reduced time effort. Further, the use of flow conditions enable a more efficient heat and mass transfer to and from the sample. The rate-limiting step is shifted to sample internal heat and mass transfer limitations, which is a basic requirement to reduce the overall rate constant and therewith the required test time. In spite of the early development of optical calorimetry, it seems perfectly suited for a rapid screening of porous materials, especially where a significantly reduced test time and a simple operation are essential: It can be used to rapidly estimate the adsorption capacity and specific surface area of a porous material. The thermal response during desorption provides information on the reversibility of the adsorption process. This is an important criterion for several industrial applications. By varying the pressure during the test, different regions of the adsorption isotherm are addressed. Using n-butane at ambient and elevated pressure proofs the presence of meso- and macropores within a sample. Moreover, the adsorption isotherm can be approximated using a static dosing mode similar to the common volumetric gas adsorption. For the so far investigated samples, a good qualitative agreement between the adsorption isotherm and the "infratherm" is observed. Test gases of different molecular size and relative pressure can be used to evaluate the pore structure of porous materials. This was demonstrated using an activation series of activated carbon samples. An analytical description of the thermal response signal was found to fit the experimental response curves very well. The latter allows comparing adsorbents in terms of adsorption kinetics in a quantitative way using a rate constant assuming pseudo first order adsorption kinetics.
ThesisNote
Dresden, TU, Diss., 2013
Verlagsort
Dresden
Language
English
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