Membrane and Desalination Technologies - TCE Moodle Website

Adsorption Desalination: A Novel Method 415
Tests are conducted under steady state conditions of the coolant (hot water source and
cooling water source temperatures) which are accurately conditioned by a rating facility
and thus it is weather independent. The cooling water that returns from the adsorber beds and
the condenser is first mixed in a cooling water storage tank to minimize its temperature
fluctuations. It is then pumped serially through the cooling tower and an evaporator coil of a
mechanical chiller, dropping the coolant temperature to a level below that of set-point
temperature of the heater controller. Within the heater tank, the cooling water temperature
is further fine-tuned by an electrical heat input using a cascaded two-loop PID controller.
Excellent control of the outlet water temperatures is thus achieved by this control arrangement,
capitalizing on the faster time constant of heaters vs. those of the processes of cooling
tower and mechanical chiller. The control strategy used in the hot water loop is similar to that
of the cold water loop. A hot water storage tank is used to damp temperature fluctuations of
hot water returning from the desorption processes. Hot water from the mixed tank is fed into
the hot water heater tank where its temperature can be fine-tuned electrically with a similar
PID controller. Similar to the hot water circuit, the evaporator in the adsorption plant is
maintained by a constant chilled water return, which is also electrically controlled with
heaters, that simulates the cooling load. All the above-mentioned circuits have been tested
to have an accuracy of 0.3 C.
4.2. Definitions and Modeling
The energy balance equations, the isotherm equation, and the kinetics that are used for the
evaluation of the performance of the adsorption desalination plant are presented below. Tóth
isotherm equation (Eq. 13) is used to estimate the amount of water vapor that the silica gel can
adsorb at the equilibrium conditions of temperature and pressure. The adsorption kinetics is
estimated by using the well-known LDF correlation (10) as follows:
dq
dt ¼ 15Dso expð Ea=RTÞ
ðq qÞ: (14Þ
R 2 p
Here, Ds0 is the pre-exponential constant, Ea is the activation energy, Rp is the particle
radius, and q denotes the instantaneous uptake.
Figure 9.14 shows the energy balance of adsorption desalination system. As can be seen
from the control volume CV1 in Fig. 9.14, the sensible cooling heat ðQadsÞ is necessary to
reject the adsorption latent heat Qlatent ads that is developed due to adsorption of water vapor by
silica gel during adsorption process, while extracted heat by chilled water ðQchillÞ drives the
heat of evaporation Qlatent des inside the evaporator. The control volume CV2 in the same
figure shows that the heating source ðQdesÞ is needed to reject the latent heat Qlatent des that is
generated due to desorption. Heat of condensation Qlatent ads is rejected through the condenser
by a sensible cooling ðQcondÞ. The sensible and latent heats of the AD system can be defined
as follows:
1. For the adsorbent bed in adsorption mode
Qads ¼ _mcp ads Tcw;o Tcw;in ; (15Þ