Membrane and Desalination Technologies - TCE Moodle Website

406 K.C. Ng et al.
Table 9.5
Surface area, micropore volume, and pore size data of various parent silica gels (29)
Sample
name
SiO2 (3A) 0.69 10 6
SiO2 (RD)
0.74 10 6
SiO2
(A þþ )
0.75 10 6
S BET (m 2 /kg) V 0.99 (m 3 /kg) Daver
(nm)
MPD
(A ˚ )
4.22 10 4 2.44 24.48 0.90 10 6
4.48 10 4 2.42 24.27 0.93 10 6
4.53 10 4 2.42 24.08 0.96 10 6
capacity of RD type silica gel is remarkably higher than that of type A ++ silica gel when P/P0
exceeds 0.4. So RD silica gel is recommended for adsorption desalination application.
3. ISOTHERMAL ADSORPTION OF WATER VAPOR ONTO TWO TYPES
OF SILICA GELS
The common types of silica gel found in the commercial adsorption chillers are the Fuji
Davison type “A,” and “RD.” Thermo-physical properties of both types of silica gels are
reported by Ng et al. (31); the data are provided by Fiji Silysia Chemical Ltd., Japan, and
summarized in Table 9.6. One can observe that the thermo-physical properties of these two
types of silica gels are similar. Although the type “RD” silica gel has a slightly higher thermal
conductivity than type “A” silica gel, the most significant difference between these two types
of silica gels lies in their water vapor uptake characteristics.
Adsorption isotherms of silica gel þ water have were experimentally investigated by Ng
et al. (31) and later by Wang et al. (32). They used two independent experimental methods, viz.
the constant volume–variable pressure (CVVP) system and the variable pressure TGA. The
experimental isotherm data of silica gel and water system were compared by these two
methods and the isosteric heat of adsorption was derived from the equilibrium information.
Two types of silica gels (Fuji Davison type A and type RD) were investigated under an
assortment of chillers’ operation conditions (temperatures from 303 to 358 K and pressures
from 500 to 7,000 Pa). The data collected from the two independent experiments were compared
with each other and their trends are consistent with those of the chiller manufacturer.
3.1. Constant Volume–Variable Pressure Test Facility
Dubinin–Radushkevich Horvath–Kawazoe
S mic (m 2 /kg) V mic (m 3 /kg) V mic (m 3 /kg) MPW
(A ˚ )
3.20 10 4 3.00 10 4 12.23
3.29 10 4 3.17 10 4 12.52
3.41 10 4 3.17 10 4 12.27
S BET: specific surface area measured by BET method; V 0.99: the total pore volume estimated at relative pressure
0.99; Daver = (4.0 V0.99)/SBET, according to the cylindrical pore model; MPD: mean pore diameter;
S mic: micropore surface area; V mic: volume of micropore; MPW: mean pore width.
As shown in Fig. 9.8, the CVVP system mainly comprises a charging tank (material
SS304) with volume of 572.64 27 cm 3 inclusive of rated piping and valves, a dosing
tank (material SS304) with volume of 698.47 32 cm 3 inclusive of rated piping and valves,