Environmental Engineering Reference
In-Depth Information
Table 16.2.1 Comparison of the properties of six liquid desiccants (absorbents) at 25 C, to allow a fair
comparison, a concentration giving an equilibrium relative humidity of ERH (Equilibrium
Relative Humidity) = 50% has been chosen in each case, with the exception of sodium
chloride where ERH = 75%, this being the minimum achievable (Davies and Knowles,
2006).
Aqueous solution
Property
Unit
CaCl 2
LiBr
LiCl
MgCl
ZnCl
NaCl
Cocentration
0.36
0.39
0.26
0.31
0.52
0.26
(mass solute/
mass solution)
Hygroscopicity
%
50
50
50
50
50
75
(equilibrium RH)
Cost
US$/m 3
560
7300
4600
450
1400
180
Abundance in
m 3 /m 3
2.3 × 10 3
4.0 × 10 6
3.0 × 10 6
1.3 × 10 2
1.0 × 10 9
9.0 × 10 2
seawater a
Density
kg/m 3
1.35
1.38
1.4
1.29
1.58
1.2
Viscosity
mPa-s
4.6
1.8
2.5
6
4.7
1.8
Specific heat
kJ/kg- C 2.6
2.6
3
2.1
2.3
3.4
capacity
Thermal
W/m- C 0.56
0.48
0.56
0.52
0.46
0.58
conductivity
Diffusivity of
10 9 m 2 /s 0.54
1.17
0.9
0.91
0.8
1.86
water in the
solution
Differential heat
kJ/kg
80
no data
65
65 d
no data
no data
of dilution b
Water absorption kg/m 3
85
84
91
76
120
n.a.
capacity
Human toxicity c
L
0.14
0.23
0.10
0.49
0.03
0.66
Ecotoxicity
ml/L
4.9(2)
no data
0.06 (2)
4.3 (1)
0.001 (6)
20 (5)
(Daphnia magna)
a Volume of absorbent that could theoretically be extracted from unit volume of seawater, assuming 100% recovery
speed.
b Mass of water that, on absorption in the absorbent, will cause a 10% relative increase in equilibrium relative
humidity.
c Estimated lethal dose in humans scaled from LD50 values for rats.
d At 50 C.
Ameel et al. (1995), the absorber utilizing LiCl solution at 35 C requires about five
times the area of an absorber utilizing lithium bromide to achieve the same sorption
performance. Also, lithium chloride solutions are not practical at an absorber tem-
perature of 45 C or higher due to solubility limitations (Ameel et al., 1995). At the
same absorbent mass flow rate, the dehumidification performance of lithium chloride
is better. Where absorbent volumetric flow rate is the same, the dehumidification per-
formance of lithium chloride is almost the same as lithium bromide. Using structured
packing for dehumidification and regeneration studies of three absorbents (calcium
chloride, lithium chloride and a mixture of 50% calcium chloride and 50% lithium
chloride). Al-Farayedhi et al. (2002) show that lithium chloride has a higher rate
of liquid-phase mass transfer coefficient than the other two absorbents, owing to its
molecular weight.
 
Search WWH ::




Custom Search