Membrane and Desalination Technologies - TCE Moodle Website

402 K.C. Ng et al.
a b
Condenser
Evaporator
Heat sink
Low temperature heat
source
Sorption
element
Driving heat source
Heat sink
Precooling process (d–a): the sorption element is disconnected from both the evaporator
and the condenser. It is cooled down at constant refrigerant concentration from Td to Ta by
using a coolant. The adsorber pressure decreases from condenser pressure to the evaporator
pressure.
Generally, the specific cooling load is given by the product of the mass of refrigerant
circulated through the system and its latent heat. The mass of evaporated refrigerant is equal
to the concentration change during adsorption process, which can be calculated by using
Clapeyron diagram if ideal cycle is considered. The amount of heat addition depends on the
heat of adsorption. However, both the concentration change and heat of adsorption can be
extracted from the adsorption isotherms.
2. ADSORPTION CHARACTERISTICS OF SILICA GEL–WATER PAIR
2.1. Physical Adsorption of Silica Gel
ln P
P cond
P evap
Heat added
c
Heat rejection
Tevap Tcon Tads Td –1/T
Fig. 9.5. (a) Schematic diagram of the basic closed adsorption cycle. (b). Clapeyron diagram (ln P vs.
1/T) of basic closed adsorption cycle.
Silica gel is a partially dehydrated form of the polymeric colloidal silicic acid and its
chemical composition is expressed as SiO2·nH2O. Commonly, the adsorption/desorption
equation for silica gel is given by
SiO2 nH2OðsÞ ,SiO2ðn 1Þ H2OðsÞþH2OðgÞ; (11Þ
where s and g denote, respectively, solid phase and vapor phase. Commercially available
silica gels are produced from silicate (20 wt% of SiO2), the basic material in a sol–gel process
that employs aluminum ions as a growth inhibitor for the primary silica gel. The production
processes of silica gel, as described by Ito et al. (27), comprise the following:
(i) Reaction: Sodium silicate (SiO2 – 20 wt%) þ H2SO4, (ii) monomer, (iii) dimmer, (iv)
particle, (v) silica solution (aqueous aluminum sulfate), (vi) three-dimensional gel network,
(vii) pH2 washing of aluminum sulfate, and (viii) drying.
Silica gel is also described as a dehydrated colloidal gel produced under controlled
conditions from sulfuric acid and sodium silicate solution (28). Being chemically oxidized
Q cond
Q eva
b
a
d