A Note on Gravimetric Measurements of Gas‐Adsorption Equilibria

Adsorption equilibria of He, Ar, N 2 , CH 4 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 –ρ 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 ρ f . To determine simultaneously m and V as , the data are fitted to an analytic adsorption isotherm m = m (ρ 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 AI 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.