Helium volume

The helium volume is also a measure of the density of the solid material in the catalyst that is,... 

The Feldman and Sereda hypothesis [35, 43, 48, 57, 62, 64] is based on the assumption that the exchange of interlayer water in the C-S-H phase occurs. The volume of this water is equivalent to the volume of adsorbed water reduced by the volume of adsorbed nitrogen The measurements of helium volume... 

Consequently, the helium volume of a sorbent material can be considered as one of its characteristic quantities describing that volume of the material impenetrable for helium molecules on a short time scale of minutes, may be hours. It seems to be kind of lower boundary to the sorbent volumes impenetrable to other, bigger molecules. 

Helium volume measurements are also very sensitive to the state of activation of the sorbent material and to preadsorption of - for example -other gases or water vapor. Hence measurements of this type may be used on an industrial level for quality control. 

The approximation of the void volume (V ) of a sorbent material most often used in practice today is the so-called helium volume (V J, i. e. the proposition... 

In conclusion we want to emphasize that in principle it is possible to measure the absolute amount of a sorptive gas adsorbed on porous solids without using the so-called helium volume approximation by combined... 

V volume of a sorbent material seen by helium gas molecules (helium volume of (m ))... 

In calculating the adsorbed masses of nitrogen, the thermal expansion of the storage and of the adsorption vessel at higher temperatures must be taken into account. Also slight changes in the so-called helium-volume of the zeolite depending also on temperature should be considered. 

Figure 2.5. Adsorption isotherms of N2 on zeolite KE-H 555 (UOP) at different temperatures (293 K - 673 K) helium volume = 0.45 cm /g.

From eq. (3.5) the mass adsorbed (m ) can be calculated if a model for the sorbent/sorbate volume (V ) is introduced and all the necessary measurements have been performed, i. e. (fio) is known according to eq. (3.10). As outlined in Chap. 1, V is often approximated by the so-called helium volume. 

To determine the helium volume of NORIT Rl, sorption experiments with helium at 298.15 K and 323.15 K were performed using a two beam microbalance (Sartorius). Results of measurement are shown in Figure 3.9. The slope of the straight lines correlating data of the apparent weight of the sample as function of the density of the helium gas (pHe), is the negative value of sorbent s helium volume (V ), cp. eq. (3.13). 

The Gibbs excess amounts of N2 and O2 adsorbed on the zeolite SX6 have been calculated from the microbalance recordings following eq. (3.34). For data referring to the dry zeolite, the helium volume V = = 0.432cm /g... 

The Gibbs excess masses (m g ) have been calculated from experimental i2o data using numerical values of the sorbent s helium volume as given below... 

Here V is the net volume of the adsorption chamber, i. e. its geometric volume minus the volume of all fixed parts of the magnetic suspension and of the other elements which permanently are included in the chamber as for example wires of thermocouples, the basket for sorbent sample etc. Also (V ) indicates the volume of the sorbent / sorbate sample which here again is approximated by the helium volume, cp. Chaps. 2 and 3 ... 

Helium volume Macropore volume Micropore volume Bulk volume of pellets Bulk volume of powder... 

For rigid sorbent materials, combined oscillometric-gravimetric or oscillometric-manometric measurements in a saturation state of an adsorption system also allow one to determine both the total mass (m m ) and the Volume (V ) of the system without using the so-called helium volume hypothesis, cp. Fig. 5.8 and Chap. 1. 

Nevertheless, the trustworthiness of experimentally measmed isotherms should never vacillate for the effect of hehum adsorption. It was pointed out that the hehum volume can be correctly measured at 400°C [3]. In fact, not only hehmn but also the strongly adsorbed substances like water cannot be physically adsorbed at sueh a high temperature. However, the thermal expansion of adsorbent from room temperature to 400°C may result in a larger effect on dead space than hehum adsorption at room temperature. Initial luneasonable adsorption measurements may not be caused by hehum adsorption, but rather by the improper pretreatment of adsorbent sample or adsorptive gases. Monte Carlo simulation [3] also proves that for the heterogeneous interface, the values of the excess surface adsorption for hehum are identieal to zero in the limit of the statistical uncertainties. The effect of imcertainty in helium volume on... 

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