SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 31
Fundamentals
Voltage driving force
There is an optimum voltage for each operating condition. One may apply too much or
too little voltage for a specific condition. A typical voltage range is given for each
module
—the optimum should be in this range.
If the voltage is too low, then the driving force is too low and not enough ions can be
moved from the feed stream into the concentrate stream. In addition, water is not being
split effectively and the ion exchange resins in the polishing bed section may not be free
of impurity ions and fully capable of trapping and moving silica for example.
When the voltage is initially lowered, the ion-exchange bed within the module will
begin to fill with ions until it reaches a steady state. During this time, more ions
will be entering than leaving the module. The symptom is that the concentrate
will not have as many ions as normal. Steady state may take 24 hours to
achieve
—during this time, product resistance will decline slowly.
If the voltage is too high, then too much water is split and the voltage driving force
becomes ineffective. A symptom of this is excess gas production in the electrode
stream, and excess current draw. Excess voltage also causes a phenomenon called
“concentration back-diffusion”. In this state, ions are forced to diffuse from the
concentrate streams into adjacent purifying chambers to achieve electrical neutrality.
When the voltage is initially raised, the ion-exchange bed within the module will
begin to discharge ions until it reaches steady state. During this time, more ions
will be exiting than entering the module. The symptom is high conductivity in the
concentrate stream. Steady state may take 24 hours to achieve. During this
time, product resistance will improve slowly.
Current densities
The current density in the
working
section of the bed (lower portion of the EDI module)
is high, caused by movement of the primary ions in the feed. There is relatively high
electrical resistance in the concentrate stream since the water there is RO-quality water
with 1-
20 μS/cm.
In the
polishing
section of the bed (upper portion of the EDI module), the concentrate
stream is full of the ions it picked up in the working bed. At 90% recovery, the
conductivity will be about 11 times the feed concentration. It can therefore be in the
range 11-
220 μS/cm. The voltage drop is now higher across the resins in the purifying
chambers (where there are very few feed ions remaining). The net result is a higher rate
of splitting water and higher concentration and transfer of protons (H
+
) and hydroxyls
(OH
-
) in this region. This results in better polishing, better removal of carbon dioxide,
better silica removal, and higher product resistivity.
Chapter 5: Water Quality Optimization