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410 K.C. Ng et al.
chamber is then vacuum-sealed and suitably insulated to prevent any possibility of condensation.
The dry mass of silica gel is first measured by using a calibrated moisture balance
(MA40), same as that used in the CVVP system. Thereafter, it is further confirmed within the
reaction chamber of TGA, and the temperature of silica gel is held at 413 K for about 8 h. The
dry mass is read using the TGA balance, which is accurate to within 0.2 mg. At the end of
the regeneration process, the test system is purged by helium gas and evacuated. Thereafter,
the water vapor is continuously charged into the reaction cell and the vacuum pump keeps
pumping the vapor–gas mixture to maintain the desired vacuum and temperature conditions.
Prior to the experiment, the Platinum standard mass was used to verify the TGA system under
the moisture environment.
To avoid condensation, the temperature of the reaction cell and piping system is always
kept at least 5–10 K higher than the saturated temperature of the vapor, which is achieved by
wrapping heater tapes around the piping system and they are also well insulated to minimize
heat loss. The system is deemed to have reached thermodynamic equilibrium at desired
pressure and the temperature when the mass of the adsorbed adsorbate remains unchanged
for a period of half hour. The temperature is measured by two RTD sensors, one is used for
temperature control and other is connected to the TGA data recorder for data collection. The
pressure and the adsorbed adsorbate are measured by the capacitance monometer and TGA
balance, respectively. By repeating the above procedures, assorted range of tests could be
carried out under different settings of temperature and pressure. For every new test, system
checks should be performed diligently, namely, checking whether (a) the system is evacuated,
(b) the silica gel is regenerated, and (c) the chamber is purged with the purified inert gas.
Uncertainty of the measurements is reduced by repeating the tests (at least three times) and a
fresh batch of the silica gel is always recommended.
3.3. Adsorption Isotherms of Silica Gel–Water Pair
Isosteric heat of adsorption is a key variable for the design of adsorption chiller, which
represents a characteristic of the adsorbent and adsorbate system. At a given specific surface
loading, it could be calculated from the isotherm data at different temperatures using the
Von’t Hoff equation:
2 dðln PÞ
Qst ¼ RT : (12Þ
dðTÞ q
The isosteric heat is related to the thermal equilibrium constant in the isotherm equation,
such as Langmuir equation and Tóth equation. Therefore, it is very sensitive to the selection
of the isotherm equation (33, 34). In this study, Tóth equation (35) is used to fit the adsorption
experimental and to find the isotherm parameters and isosteric heats of adsorption. The form
of the Tóth equation used in this study is given as
q ¼
K0 expðDHads=RuTÞp
½1 þðK0=qm expðDHads=RuTÞpÞ t ; (13Þ
1=t
Š