Adsorbent Helium Density Measurement and Its Effect on Adsorption Isotherms at High Pressure

Abstract

On the basis of an experimental study in a large temperature range, it is shown that “helium densities” of adsorbents measured at room temperature could be erroneous due to a non-negligible effect of helium adsorption. It is proposed that the density obtained with helium at high temperature, for instance, at the regeneration temperature of the adsorbent, be considered as the adsorbent density. Using the corrected densities of 3A, 4A, 5A, and 13X zeolites and of activated and graphitized carbons and of silica gel, we experimentally determined the adsorption of helium on the above mentioned adsorbents at room temperature and in a large pressure range up to 500 MPa. The shape of the adsorption isotherm reveals no saturation at high pressure. These experimental data are in agreement with Monte Carlo simulations of adsorption of a Lennard-Jones gas by a rigid plane as well as by a microporous rigid solid interface. We also examined implications of the new helium density of activated carbon for our previous measurements of adsorption at high pressure. The result is the disappearance of the inexplicable negative part of the isotherms and even a renewed increase in the curves at high pressure. Moreover, a comparison with Monte Carlo simulations of argon adsorption on microporous graphite is in good agreement with the shape of the adsorption curve at high pressure. Finally, the role of the microporous structure of adsorbents and of the gas−adsorbent interaction in adsorption at high pressure is discussed.

The use of small angle neutron scattering with contrast matching and variable adsorbate partial pressures in the study of porosity in activated carbons

“…It is known that helium density measurements in these materials performed at room temperature can be erroneous [25]. Indeed, GCMC calculations suggest [26] that the average density of helium confined in sub-nanometer pores may exceed the bulk density by orders of magnitude due to gas interaction with the surface.…”

Capillary condensation and adsorption of binary mixtures

“…v p can be experimentaly determined by the helium displacement method [37] and, in the numerical simulations, by the procedure described in [38].…”

Net, excess and absolute adsorption in mixed gas adsorption

“…For the excess adsorbed amount, one alternative approach to defining the accessible and non-accessible volumes is the use of very high pressure adsorption data as discussed by Malbrunot et al (1997), who carried out experiments up to 500 MPa. These authors suggested that "the ideal method according to the Gibbs surface definition would be to measure the adsorbent density for each gas with the gas itself, but this may not be practical."…”