Membrane and Desalination Technologies - TCE Moodle Website

Adsorption Desalination: A Novel Method 409
3.2. TGA System: Experimental Set-Up and Procedure
The TGA, as shown in Fig. 9.9, comprises the Cahn TGA unit, an evaporator, a vacuum
pumping subsystem, and the helium gas supply subsystem. Water vapor is generated separately
in a heater bath fitted with a PID controller. The pressure of the vapor is measured with
a capacitance monometer, the Edwards barocel type-622 with 10 kPa span. The steam
generator generates a flow of 50–300 ml/min and the flow rate is within the free delivery of
the vacuum pump (about 315 10 5 m 3 /s). The silica gel sample was put in the reaction
chamber of TGA that has a radiant heater for maintaining constant temperature of the sample.
Sufficient thermal insulation is incorporated to reduce heat loss. The changes in the sample
mass were measured using a microbalance. A barocel pressure sensor mounted at the bottom
of the reaction chamber is used to read partial vacuum in the system. A RTD is inserted from
the bottom of a “tee-joint” and the whole glass tube is insulated. A vacuum pump is used for
vacuuming and purging the system before the test and for maintaining the vapor testing
condition in terms of pressure and temperature during the experiment. The argon gas, for
protecting weighing bridge, is introduced from the top (from the inside) of a double-walled
tubing, while the pure adsorbate is introduced upward to the silica gel sample that is
contained within a quartz holder, and the whole assembly is subjected to a preset pressure
(P) and temperature (T). Isotherm environment of silica gel is maintained by direct radiant
heating where the heaters are placed outside (non-vacuum) the reaction chamber. Cooling to
the outside chamber (non-vacuum) is effected by passing cool air from a vortex chiller and
external fan. By balancing the heating and the cooling effects (external to reaction chamber),
a steady temperature environment for the silica gel can then be achieved readily.
In this experiment, efficiency of the Fuji Davison type A and type RD silica gel has been
tested. The silica gel, about 100–500 mg is placed inside the quartz holder and the reaction
Vacuum
System
Control panel
Evaporator
TGA
Water bath for the evaporator
Helium gas supply
Computer and data
logging
Fig. 9.9. The Cahn 2121 TGA unit with the LCD readout screen (32).