Membrane and Desalination Technologies - TCE Moodle Website

424 K.C. Ng et al.
Ln (P)
T B
B
B'
Adsorption
T B'
C'
A
C D
Desorption
A'
technique (45). The principles of pressure equalization are best understood by using the
P–T–q diagram shown in Fig. 9.22. At the end of adsorption/desorption processes, the
pressure of the desorber reactor is nearly equal to that of the condenser, and the refrigerant
(water) concentration reaches its minimum value (qD), which is the equilibrium uptake
corresponding to the desorption temperature and condenser pressure. On the contrary, the
pressure of adsorber is controlled by that of the evaporator and the concentration reaches it
the equilibrium uptake (qB), corresponding to the adsorber reactor pressure and the adsorption
temperature. By connecting both the adsorber and desorber reactors for a short time at the end
of adsorption/desorption processes, refrigerant vapor in the desorber is forced to move to the
adsorber by pressure swing action, making the silica gel dryer and the refrigerant uptake
decreases to qD0. The simulations pressurization in the adsorber causes silica gel state to
adsorb more refrigerant, and the uptake reaches to qB0. Mass of refrigerant circulated in the
system is then increased to (qB0 qD0), thus improving the SDWP. While maintaining the
optimal cycle time, Fig. 9.23 shows the performance of adsorption desalination plant at
different pressure equalization time intervals, namely, 10, 20, and 30 s. It can be seen that the
SDWP increases by as much as 10 kg of water/kg of adsorbent per day at a PE of 10 s.
However, an increase in the PE time beyond the optimal value reduces the SDWP due to the
decrease in the mass transfer rates.
6. DESIGN FOR LARGE COMMERCIAL ADSORPTION DESALINATION
PLANT
Most of the commercial scale desalination plants are designed to the order of million
gallons of water per day (MGD). Ng et al. (3) proposed a modular design for reactor towers of
Dq
T D'
Dq'
D'
T D
Desorber
pressure
Equalized
pressure
Adsorber
pressure
Fig. 9.22. D€uhring diagram of the basic and improved adsorption desalination plant (44).
T