A NOTE ON GRAVIMETRIC MEASUREMENTS OF GAS-ADSORPTION EQUILIBRIA

Adsorption equilibria of He, Ar, N2, CH4 on several types of activated carbon (AC) and molecular sieves (MS) have been measured at ambient temperature in the pressure range 0 MPa-15 MPa using a microbalance (Sartorius 4104 S). The resulting experimental data on principle only allow to calculate the difference (m-rho(f)V(as)) between the net mass adsorbed m, and the buoyancy related volume V(as) of the adsorbent/adsorbate system multiplied by the density of the (fluid) adsorptive rho(f). To determine simultaneously m and V(as), the data are fitted to an analytic adsorption isotherm m = m (rho(f)T) of generalized Langmuir-Freundlich type and/or Dubinin-Radushkeviz type, and to one of four different models for the volume V(as), including the so-called helium volume, this quantity considered to be constant. - The two times four adsorption isotherms (AI) resulting in this way do not differ much for low pressures (p < 1 MPa), however tend to differ considerably at higher pressures, the helium volume related Al leading to thermodynamic inconsistencies! As a result it can be stated that the buoyancy related apparent volume V(as) of a porous adsorbent/adsorbate system strongly depends on the adsorptive used, i.e. on the size of its molecules, and also, to lesser extend, on the amount of mass adsorbed. Consequently, V(as) should not simply be identified with the helium volume but instead specified for different adsorptives and pressure ranges used in industrial adsorption processes.