Page 104 - Adsorption by Powders and Porous Solids
P. 104
CHAPTER 3. METHODOLOGY OF ADS0Rpirlo~
Table 3.1. Thermal transpiration: coefticients of the Takaishi and
Sensui equation, for various gases.
Gas A B C Temperature range (K)
Hydrogen
Neon
Argon
Krypton
Methane
Xenon
Helium
Nitrogen
oxygen
3.4.8. Correction for non-ideality
At the temperatures normally used for physical adsorption, the correction for the non-
ideality of real gases generally amounts to several percent. It can be reasonably taken
into account by using the first two terms of the virial equation:
where u", and vyd are the real and ideal molar volumes of the gas at a given temper-
me T and reference pressure (normally 1 bar), respectively, V is the total volume
occupied by the given amount of gas, and B, is the second molar virial coefficient,
which is usually negative under the conditions of physisorption experiments. Its
value for a large number of gases can be found in a critical compilation by Dymond
and Smith (1969) and in the thermodynamic tables published by Marsh (1985).
To determine the real amount of gas present in a given volume V, at temperature T
and pressure p, it is convenient to use the relation:
where V is expressed in cm3 and p in bar and where a=-100B,/~~~=
-(IoB,,,/RT) xpO.
Values of a for a number of gases and temperatures used in adsorption studies are
given in Table 3.2. They were calculated with help of the data provided by Dymond
and Smith (1969). A detailed study of the incidence of these corrections on the
adsorption isotherms and the BET surface areas was made by Jelinek et al. (1990).
An elegant way to avoid this non-ideality correction, especially when operating at
high pressures (up to 16.5 MPa), where it can become predominant, was proposed by
Bose et al. (1987). In their method, for each equilibrium point the density of the
adsorptive is determined experimentally from its dielectric constant, which is mea-
sured in a gas capacitance cell at the same temperature and pressure as the adsorption
system studied. The rest of their adsorption procedure is comparable to the
