|M.Sc Student||Abir Hadas|
|Subject||Atomistic Calculation of Activated Carbon Adsorption|
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Moshe Sheintuch|
|Full Thesis text|
We use molecular mechanics with universal force field parameters to calculate single and multi component adsorption of phenol, Para-Bromo-Phenol and m-cresol from gas or liquid phase on activated carbon. The carbon pores are modeled by carbon nanotube of various pore diameters and different lengths. This calculation yields the Gibbs free energy change (∆G) upon adsorption which is used to predict the adsorption isotherm following reaction rate theory.
We provide atomistic calculation explanation for two phenomena: Freundlich isotherm and adsorption hysteresis. The former results from pore size distribution and specifically from ∆G dependence on pore diameter coupled with the distribution of pore sizes.
We show that evaluating the pore size distribution is a vital part in the prediction of the adsorption isotherm; we suggest a methodology for the prediction of the isotherm when the pore size distribution of the specific active carbon is known as well as for the case of unknown pore size distribution. We expand this methodology for the case of bi-solute adsorption system. Reasonable prediction of measured adsorption isotherms is demonstrated for single solute adsorption when both pore size distribution and adsorbate-adsorbate interaction are accounted for. The suggested methodology is highly sensitive to the numerous parameters it requires: the evaluation of the adsorption parameters, the pressure range taken and to accuracy in the provided values of the pore size distribution and Henry constant.
We report here the possibility of adsorption hysteresis that results from adsorbate - adsorbate attraction. The suggested explanation enables the existence of hysteresis in narrow carbon nanotubes (of 1-1.5 nm in diameter) and is different from the common explanation in literature that requires the use a continuum approach. Hysteresis may emerge due to the periodic dependence of ∆G on pore occupancy, owing to adsorbate arrangement within the pore in a single pore diameter. This behavior is translated to an isotherm with many branches. But hysteresis may also emerge due to adsorbate - adsorbate interaction that leads to a monotonic decline of the adsorption energy with pore occupancy that will lead to a single hysteresis loop.