A NEW METHODOLOGY IN THE USE OF SUPER-CRITICAL ADSORPTION DATA TO DETERMINE THE MICROPORE SIZE DISTRIBUTION

Abstract

Adsorption of methane on the surface of graphitized thermal carbon black and in slit pores is studied using the method of Grand Canonical Monte Carlo simulation. Under the supercritical conditions and very high pressure the mass excess decreases towards zero value for a graphite surface, while for slit pores negative excess density is possible at extremely high pressures. Adsorption data obtained under supercritical conditions are increasingly used to determine the pore size distribution in the micropore range. This is largely motivated by the advances in the use of supercritical adsorption in high energy applications, such as hydrogen and methane storage in porous media. Experimental data reported as mass excess versus pressure and when these data are matched against the theoretical mass excess, significant errors can occur if the void volume used in the calculation of the mass excess is incorrectly determined. The incorrect value for the void volume leads to wrong description of the maximum in the plot of mass excess versus pressure and the part of the isotherm over the pressure region where the mass excess decreases with pressure. Because of this uncertainty in the maximum, we propose a new method in which the problems associated with this maximum of the surface excess are completely avoided. Our method involves only the relationship between the amount that is introduced into the adsorption cell and the equilibrium pressure. This information of “direct” experimental data has two distinct advantages. The first is that the data is the direct data without any manipulation (i.e. involving further calculations), and the second one is that this relationship is always monotonically increasing with pressure. We will illustrate this new method with the adsorption data of methane in a commercial Ajax activated carbon.