2.5. Porosity Determinations

Zeolite 13X was subjected to porosity determinations. In particular, the porosity existing inside the zeolite grains (intragranular porosity) was determined. Moreover, grains of zeolite 13X were accommodated into a vessel so as to mimic the loading of this material in a device to be used in thermal energy storage, and the void existing between the various grains (intergranular porosity) was determined. The procedure for the determination of intergranular porosity (P_inter) was carried out in ref. [59], according to the CNR B.U. 22, ASTM D1188, and BS 1377 T15/E standards, and it can be summarized as follows: the material was placed in a cylindrical container of known volume (Vt) and weight equipped at the ends with perforated caps, ensuring its compaction by vibration on an electromechanical device (belonging to a vibrating sieve device). The system was then saturated by immersion in demineralized water for 72 h. After careful external drying of the granules for the elimination of intergranular capillary water, the sample was repositioned in the cylindrical containers by compaction on the same electromechanical device. Subsequently, the weight of the saturated granules and cylindrical containers and the total volume of the grains (Vs) were measured by hydrostatic weighing. In all experiments, the water temperature was maintained at 24.5 °C, with a density of 0.993 kg/L. Finally, the calculation of the intergranular porosity by means of the elementary relation was done using the following equation:

The procedure for determining the total porosity (Pt) and subsequently the intragranular porosity (P_intra) was carried out in ref. [59], according to the CNR—UNI 100008, ASTM D 2216, CNR B.U. 64, and ASTM D 854 standards and is as follows: the test material was dried and the dry weight and weight of the dry volume unit (γ_dry) determined. After the sample was reduced to a powder by grinding, appropriate pycnometer weighing operations were followed to determine the specific weight of the solid particles (Gs), measured in g/cm³. The calculation of the total porosity (Pt) was finally performed by means of the relation:

From the values obtained with the previous relationship, we proceeded to calculate the intragranular porosity (Pintra):