Sodium Monitoring in Water Treatment Processes
AG/AI-024_4
4
Process Description
Basic Measurement/ Analysis Theory
All on-line sodium monitors are based on the use of a sodium ion selective
electrode and a reference electrode. The measurement is very similar to that of pH,
with the electrode pair also responding to hydrogen ions and changes in sample
temperature.
The hydrogen ion interference is controlled by raising the pH of the sample
(reducing the hydrogen ion concentration). This is achieved by adding an alkali
– usually ammonia gas or a volatile amine. Temperature effects are reduced by
monitoring temperature and carrying out automatic temperature compensation.
The monitor is calibrated by use of solutions of known sodium ion concentration,
with the electrode pair producing a millivolt output proportional to the sodium
ion concentration.
Cation Exchange Bed
Cations are exchanged for hydrogen ions in this bed until all the hydrogen
ions in the bed have been removed. At this stage (known as resin bed
exhaustion) the levels of dissolved solids in the water will increase and
the resin will need to be regenerated by passing acid (usually sulphuric
or hydrochloric) to reinstate the hydrogen ions in the resin.
Mixed Bed
The mixed bed contains a mixture of both cation and anion exchange resins
to further reduce the dissolved solids content, and thus produce the quality
of make
-
up water required.
Condensate Polishing Plant (CPP)
This type of plant is used mainly on once
-
through type boilers. It takes the
condensate from the steam turbine/condenser and purifies or polishes it to
reduce the concentrations of any impurities such as sodium chloride, sulphate
etc., which may have entered the system via condenser tube leaks. Condensate
polishing plants (CPP's – see figure on previous page) only normally remove
small levels of contaminants, therefore only have a mixed bed stage.
Measuring Points
Cation Bed Outlet
Monovalent cations such as sodium (Na
+
) are the first ions to breakthrough when
cation resin exhaustion occurs. The ability of the sodium monitor to detect very
low levels of sodium makes it a much more sensitive method of detecting resin
exhaustion than conductivity.
Mixed Bed Outlet
Here again, sodium monitors are used to check the quality of the final water
and detect exhaustion of the cation exchange resin within the mixed bed. It is
essential that the sodium concentration in the final water is kept as low as
possible to minimize thermal losses due to blowdown
.
Condensate Polishing Plant (CPP) Outlet
As with the complete demineralisation plant, the CPP has a sodium monitor on
the outlet from the mixed bed unit to detect exhaustion of the cation resin. This is
particularly important on once
-
through boilers as any sodium in the feed water
after the CPP is deposited in the boiler, causing corrosion and potentially, the premature
shutdown of the system.