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400 K.C. Ng et al.
with the Langmuir isotherms, the right-hand side of Eq. (10) gives the full predictions of Q st.
A good agreement between experiments and prediction is observed for vapor uptake up to
10.5% of the adsorbent mass. The second set of experimental data relates to the heat of
adsorption of CO2 on zeolites [H-ZMS-5] at 22.5 C(24), as shown in Fig. 9.3b. Good
agreement is observed for an initial uptake up to 3% of the adsorbent mass. At higher
vapor loading, however, the experimental values are observed to be higher. The isotherm
model employed here is the DA equation that has been applied to obtain predictions of
Eq. (10). Lastly, the heat of adsorption data of sulfur hexafluoride [SF6] on zeolites [NaX] at
31.6 C(24) show again a good agreement for an adsorbate uptake up to 27% of the mass of
adsorbent, as shown in Fig. 9.3c, and the Langmuir isotherm model has been assumed.
1.5. Classification of Thermally Driven Sorption Systems
Heat-driven sorption systems can be simply classified into absorption (liquid–gas) and
adsorption (solid–gas) systems, as shown in Fig. 9.4 (25). Generally speaking, the main
difference between adsorption and absorption is the nature of the sorbent pair as well as the
cycle duration time (26). It is reported that absorption cycle performance is higher than that of
adsorption cycle. However, the main drawback of absorption cycles is that they need a high
temperature heat source to operate. All absorption cycles are closed cycles. On the contrary,
adsorption cycles can be classified into open and closed cycles. Open cycle is applied in
desiccant cooling and dehumidification applications. Calcium chloride, lithium bromide, and
silica gel are commonly used as desiccants. Closed adsorption systems can be classified into
high-pressure (above atmospheric pressure) and low-pressure (sub-atmospheric pressure)
systems depending on the type of adsorbent-refrigerant employed in the system as well as
the system application. As shown in Fig. 9.4, silica gel is commonly used as an adsorbent.
1.6. Basic Closed Adsorption Cycles
The simplest heat-powered adsorption cooling or heat pump cycle composes of three heat
exchangers, namely, evaporator, condenser, and sorption element (adsorber/desorber reactor),
as shown in Fig. 9.5a. The cycle consists of four batch-operated thermodynamic
processes which are shown in Fig.9.5b, the pressure–temperature–concentration diagram
(Clapeyron diagram). The processes can be explained briefly as follows, where the subscripts
a, b, c, and d indicate the states of processes a–b, b–c, c–d, and d–a, respectively, of Fig. 9.5a.
Adsorption and evaporation process (a–b): the sorption element is connected to the
evaporator, which makes it possible to adsorb the refrigerant vapor from the evaporator.
During the adsorption process, the adsorber is cooled from Ta to Tb. Adsorption heat is
removed by a coolant that flows inside the sorption element. Refrigerant concentration in the
bed increases until reaches its maximum value, Wb.
Preheating process (b–c): the sorption element is isolated from both the evaporator and the
condenser. Heat is added by an external heat source to the sorption element at constant
refrigerant concentration, which results in increase in temperature of refrigerant from T b to
Tc. Sorption element pressure increases from that of the evaporator to the condenser pressure.