Membrane and Desalination Technologies - TCE Moodle Website

Adsorption Desalination: A Novel Method 413
Table 9.7
Correlation coefficients for the two grades of Fuji Davison silica gel + water systems (32)
Type K0 (kg/kg kPa) Qst (kJ/kg) qm (kg/kg) t Remarks
“A” (4.65 0.9) 10 10 (2.71 0.1) 103 0.4 10 By Tóth
equation
“RD” (7.30 2) 10 10 (2.693 0.1) 103 0.45 12 By Tóth
equation
“RD” (manufacturer) 2.0 10 9 2.51 103 – – Henry
Law
isosteric heat. In the literature, most of them use the Henry equation or the Langmuir equation
to fit the isothermal data and calculate isosteric heat. As shown in the figures, both Henry
equation and Langmuir equation could not fit the experiment data well, especially in the
saturation region. In addition, if only the chiller operation range was considered, both Henry
equation and Langmuir equation can fit the experimental data. Therefore, isosteric heat can
only be calculated based on the experimental data within this region.
4. THERMALLY DRIVEN ADSORPTION DESALINATION
4.1. Laboratory Scale Prototype Testing
A recent patented adsorption desalination (AD) plant, by Ng et al. (3), employs a low
temperature waste-heat source that are available in abundance from industrial processes. A
pilot plant has been constructed and operated in the Air Conditioning Laboratory at National
University of Singapore (NUS). Figure 9.13a–c show the schematic diagram and pictorial
views of the four-bed adsorption desalination plant, respectively. The AD plant would
comprise six heat exchangers, namely, four units of adsorption/desorption reactors or beds,
the condenser, and evaporator. The desalt operation of the AD cycle is similar to that of the
four-bed adsorption chiller (41), the detailed operation of which was recently reported by
Wang and Ng (40). The main difference between the operations of the AD and the chiller
cycles is that in AD, saline solution is pretreated before supplying to the evaporator and the
condensate water that emanates from the condenser is removed as pure water, using either a
vacuum pump or a 10-m high liquid-filled U-tube. The overall system operation can be
summarized as follows:
1. The saline or brackish water is first pretreated (e.g., coagulation, filtering, and de-aeration) and fed
to the evaporator. Purging of solution from the evaporator is conducted periodically for salt
concentration control.
2. The seawater is dosed into the stainless steel evaporator, where an external water circuit provides
the heating load for maintaining the evaporator’s temperature and pressure. Evaporation inside the
evaporator is enhanced by using the spray water method instead of pool boiling technique.
3. The water level inside the evaporator is regulated for both the water level and concentration and
this prevents the corrosion of the evaporation unit.