Environmental Engineering Reference
In-Depth Information
salt rejection [103]. Although many methods have been developed for boron nitride pro-
duction, developing a high-purity manufacturing method with diameters on the order of
those investigated by molecular dynamics has proven to be challenging [104].
Zeolite membranes have also been developed after molecular dynamics simulations
suggested their desalination potential [7]. These membranes for water desalination com-
prised a microstructure of aluminosilicate minerals with pore sizes ranging from 3 to 8 Ǻ.
These membranes have been shown to reject 98.6% of salt with a 3.5 MPa applied pres-
sure and 0.1 M NaCl feed solution; however, rejection drops to ~90% as the concentration
of NaCl is increased to 0.3 M [7]. Another challenge to zeolite membrane technology is
that salt can travel through the intercrystal defects within the zeolite structure, so further
development must be done for defect-free zeolite membranes. Signiicant progress has
been made to this effect with the development of a rapid thermal process (RTP) added to
the membrane manufacture. Scanning electron microscopy (SEM) and dye testing showed
signiicant decrease in grain boundary size or lexibility while it was observed that RTP
increased separation factor to 28 over 3 for conventionally processed membranes for sepa-
ration of
p
-xylene to
o
-xylene [105]. Zeolite loading in commercial thin ilm composite and
RO membranes is also under investigation, showing increasing lux for increasing zeo-
lite loading, but not yet reaching the lux found in advanced commercial RO membranes
[7]. Molecular dynamics has also suggested another membrane type by demonstrating an
induced electro-osmosis or internal recirculation between heterogeneously charged pores,
which can be considered a simplistic model of the ubiquitous heterogeneous structures
found in nature [77] with structure similar to aquaporins. These simulations suggest a
water velocity between 0.0711 and 0.842 m/s can be achieved if this type of heterogeneous
structure is achieved [75].
27.3.4 Humidification-Dehumidification Desalination
Humidiication-dehumidiication (HDH) desalination processes rely on distillation to
achieve desalination [106]. Simple technology is required because of their ability to func-
tion at atmospheric pressure; generally, HDH units require two exchangers, an evaporator
to humidify air and a condenser to collect the freshwater generated as shown in the basic
system schematic in Figure 27.14. The process begins by allowing dry air to absorb vapor
Heater
Heating steam,
M
s
,
T
s
Inlet hot water,
M
w
,
T
wi
Outlet cold
water,
M
w
,
T
co
Humidifier
Condenser
Packing
material
Condensate,
M
d
Intake air,
M
a
,
H
i
,
W
i
Outlet
brine,
M
w
,
T
wo
Inlet seawater,
M
w
,
T
ci
FIGURE 27.14
Schematic of conventional HDH process. (From Ettouney, H.,
Desalination
, 183, 341, 2005.)
