SnowPure Electropure EXL Series, Руководство по проектированию OEM

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
SNOWPURE, LLC, 2005-2018 VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 25
The following ionic sizes are the effective sizes
3
in aqueous solution at 25°C. These
sizes include full hydration. The larger the effective size the slower the diffusion rate
—
making removal by EDI more difficult. The larger the effective size, the more distributed
the charge thus making adsorption by the resin more difficult.
Ionic Radius, Å Cations Anions

3.0
K
+
, NH
4
+
Cl
-
, NO
3
-
3.5 OH
-
, F
-
4.0-4.5 Na
+
SO
4
-2
, CO
3
-2
6.0 Li
+
, Ca
+2
, Fe
+2
8.0-9.0 H
+
, Mg
+2
, Fe
+3
Ionic Charge
The higher the ionic charge the stronger the applied voltage will pull the ion through the
membrane. This is counterbalanced by higher degrees of hydration and larger, heavier
molecules which slow diffusion.
Selectivity Coefficients of Ions for Resin
The table below shows the selectivity of different ions for resin
4
. This is a measure of
their adsorption strength to the resin. Strong adsorption means low leakage through a
resin bed or an EDI module.
Cation Selectivity
Coefficient
Anion Selectivity
Coefficient
Li
+
0.8 HSiO
3
-
H
+
1.0 F
-
0.1
Mg
+2
1.2 HCO
3
-
0.5
Na
+
1.6 OH
-
0.6
Ca
+2
1.8 Cl
-
1.0
NH
4
+
2.0 NO
3
-
3.3
K
+
2.3 I
-
7.3
Easy Ions (Na
+
, Cl
-
, Ca
+2
, H
+
, and OH
-
)
Sodium (Na
+
), Chloride (Cl
-
), Calcium (Ca
+2
), Hydrogen ion (H
+
), and Hydroxyl ion (OH
-
)
are considered easy ions for EDI. All of these ions are adsorbed well by the resin and
have a charge that is definite and difficult to polarize. These ions are fairly easy to
remove in the “working” section of the EDI module.
Large, Weakly-charged Ions (Carbon Dioxide, Silica, Boron)
Carbon Dioxide (CO
2
), Silica (SiO
2
), and Boron (H
3
BO
3
) all have weak anionic charge
under normal operation and pH. Because of this, they are weakly adsorbed to the resins
and the applied voltage has little driving force.
To effectively remove these ions, other system strategies are used.
1. Minimize the ionic content of the feed
2. Minimize the CO
2
and HCO
3
-
in the feed
3 Lange’s Handbook of Chemistry, 12 th
Edition, Table 5.2
4 Lange’s Handbook of Chemistry, 12 th
Edition, Table 5-34, 5-35