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Adsorption Desalination: A Novel Method 411
where q is the quantity of adsorbate adsorbed by the adsorbent under equilibrium conditions,
qm denotes the monolayer capacity, P; T is the equilibrium pressure and temperature of
the adsorbate in gas phase, respectively, Ru is the universal constant, DHads is the isosteric
enthalpies of adsorption, K0 the pre-exponential constant, and t is the dimensionless Tóth’s
constant. Tóth observed in his experiments that the empirical constant t was between zero and
one, but Valenzuela and Myers (33) reported that t could actually be greater than one.
Adsorption equilibrium data of water vapor in the above-mentioned silica gels were
obtained from these two independent methods. Figure 9.10 shows the comparison between
the isothermal data from the two independent experimental methods and the manufacturer’s
data (36). It could be found that the equilibrium data from the two methods were well
consistent with each other and the trends are quite compatible with the manufacturer’s data.
The isothermal maps covering entire chiller operation range for the two types of silica gel
are depicted in Figs. 9.11 and 9.12, respectively. One could observe that the equilibrium data
from the two methods under different temperature and pressure were in very good agreement
with each other. This implies that both methods are accurate of isothermal measurements
and they can be used to verify the experimental results for each test procedure. In Figs. 9.11
and 9.12, the solid lines are experimental curve fits obtained by using Tóth equation to fit the
experimental data from CVVP method. At the temperature of 348 and 358 K, the exploited
data from Tóth equation was well consistent with the experimental data from TGA.
The isotherm parameters and isosteric heats of adsorption are shown in Table 9.7. It can be
seen that DHads and qm are significantly different from those reported by Chihara and Suzuki
(37). Cremer and Davis (38) first reported a value of 2.89 10 3 kJ/kg for DHads, and Sakoda
and Suzuki (39) later reported a value of 2.8 10 3 kJ/kg for DHads. It could be easily
explained according to the effects of the selected isothermal equation on the calculation of
Fig. 9.10. Comparison of isotherm data for type RD silica gel + water system. CVVP experimental
data points: open square T = 323 K, plus T = 338 K; variable pressure TGA experimental data points:
filled circle T = 323 K, asterisk T = 338 K; solid lines are experimental curve fits using the Tóth
equation; a chiller manufacturer’s data: dotted line T = 323 K, dashed line T = 338 K (32).