10 Electroactive Media used in CEDI Devices

Eq. 1
This shows that the probability of a direct conductive path decreases as the intermembrane
spacing increases. The effect of intermembrane spacing on salt removal in a EDI­MB device
has also been demonstrated experimentally, as shown in Table 1.
Cell Thickness,
mm
Salt
Removal, %
Feed,
μS/cm
Product,
μS/cm
Velocity,
cm/sec
1.0 99.8 600 1.2 0.86
2.3 99.9 600 0.6 0.86
4.7 94.3 600 34 0.86
7.2 71.7 600 170 0.86
Table 1
Relationship between cell thickness and performance for a EDI­MB device
Mixed bed Resin Filler (EDI­MB) ­ Resin Packing
It has also been shown that the performance of a EDI­MB device can be improved significantly
by the use of uniform particle size ion exchange resins instead of conventional resins, which
have a Gaussian distribution of bead sizes. The uniform beads allow a higher packing density,
approaching a hexagonal close­packed structure. The effect of packing density on salt removal
is illustrated by the data in Table 2.
Feed uS/cm
Product,
MegOhm­cm
non­uniform
beads
Product,
MegOhm­cm
uniform beads
145 0.4 0.7
87 0.8 1.5
65 1.5 4.2
41 3.4 10.5
Table 2
Resin particle size distribution and performance for a EDI­MB deviceLayered Bed Resin Filler (EDI­
LB)
In the late 1980s and early 1990s there was considerable activity in the development of
layered bed (EDI­LB) devices. In this configuration the media comprise separate, sometimes
alternating layers (or in one variation, clusters) of ion­exchange resin, each layer containing
mainly one type of resin: e.g., either anion or cation resin. Liquid to be deionized flows
sequentially through the layers of resins.
For EDI­LB devices there is essentially no "enhanced transfer" regime and less limitation on
the intermembrane spacing. This is because transfer of only one type (polarity) ion is enhanced
at any given time. In order to maintain electroneutrality, the ion that is transferred out is
replaced by a co­ion resulting from splitting of water. This is illustrated in Figure 5. One of the
main design constraints is the choice of ion exchange resin, which must catalyze the water
splitting reaction at the resin/membrane interface. Resin selection must also ensure that the
electrical resistance of the layers is similar, so that the DC current is fairly evenly distributed
through the cell instead of preferentially passing through a single type of layer. It is likely that
the use of uniform particle size resins will offer some benefit to the performance of thick­cell
layered­bed devices, but that the difference will not be as dramatic as it is for a thin­cell mixedbed.