Membrane and Desalination Technologies - TCE Moodle Website

418 K.C. Ng et al.
4.3. Specific Daily Production Vs. Cycle Time
Presently, there is no available standard rating for the testing of an adsorption desalination
plant. The work of Wang and Ng (40) is the first report where they have tested the plant at the
following rating conditions:
1. The inlet temperatures of hot, cooling, and chilled water supplied to the beds and heat exchangers
of the plant are set at 85, 29.4, and 12.2 C, respectively.
2. The coolant flow rates for the evaporator is 0.0455 l/(s kW), where the kW is based on the
maximum cooling capacity across the evaporator.
3. The total cooling water flow rate is set to about 2.5 times of the evaporator flow rate, i.e., 0.114 l/
(s kW), where the ratio of water supplied to the designated adsorption beds and the condenser is
1–1.5, respectively.
4. The hot water flow rate supplied to the beds (in series flow configuration) is set to have the same
flow rate as that of the evaporator, i.e., 0.0455 l/(s kW).
Based on the above-mentioned rating conditions, the switching (preheating or precooling
cycle) time is held constant at 40 s. Too long or too short a switching time may deprive the
performance of the chiller. The performance of the experimental desalination plant is
evaluated at different half-cycle times, typically from 120 to 600 s, while the switching
time is held constant at 40 s. As can be seen from Fig. 9.15, the maximum specified water
production for a standard AD cycle is 4.7 kg/kg silica gel and this occurs at an operating cycle
time of 180 s, while increasing the half-cycle time of the adsorption cycle leads to its
reduction. It is noted that the COP (defined here as the ratio of cooling to heat input) is
generally low at 0.24–0.38 due mainly to the temperature of chilled water being produced,
typically less than 7 C. A practical half-cycle time for the desalination cycle is about 250
s without sacrificing much of the SDWP yield, i.e., 4.6 kg/kg of silica gel. It should be pointed
Fig. 9.15. Performance of the four-bed adsorption desalination system under standard operation
condition (40).