Introduction
The Alkalinity in the water is due to the ions as Carbonate (CO3) , Bicarbonte ( HCO3)
and Hydroxide ( OH) These ions are associated with Calcium, Magnecium
and Sodium. Alkalinity removal is necessary to avoid the scale formation
in the boiler and other equipments.
Alkalinity removal is achieved by using following processes.
Process Description
The dealkalization
process reduces the alkalinity in water. Along with alkalinity there is a
reduction in hardness as well as sodium associated with alkalinity.
Finally these reductions are reflected by a drop in the content of
dissolved minerals in water.
The dealkalization processes are - Split Stream Dealkalization
- Dealkalization with a weak acid cation (WAC) resin.
- Combination of WAC and Softener
In this process the
incoming water is split into two streams. One stream is softened while
the other stream is totally decationised. The softened stream and
decationised streams are blended next to yield product water free of
hardness and reduced alkalinity.
An atmospheric degasser is placed down stream of the
blending to remove the free carbon dioxide in the decationised water and
also that formed by reaction of the free mineral acidity of
decationised water with alkalinity of the softened water. This results
in a reduction of the salts in water by the extent of the alkalinity
reduced.
The softener uses strong acid cation (SAC) resin in the sodium form while the other unit utilises the SAC resin in the hydrogen form. This unit is generally referred to as the HI (Hydrogen Ion) unit.
There is a leakage of neutral salts mostly sodium due
to the presence of free mineral acids. The reactions of neutral salts
are equilibrium reactions but the reactions of alkaline salts are
unidirectional further promoted by formation of CO2 taking
the service reaction to completion. On blending the two streams, the
alkalinity of the soft water reacts with the free mineral acidity of the
HI treated water to produce carbonic acid.
Carbonic acid is actually a solution of carbon
dioxide in water which is stripped in an atmospheric degasser and
released to the atmosphere.
The split stream process thus produces soft water
with reduced alkalinity and consequently with a lower dissolved mineral
content.
The service run continues as long as
- Soft water is obtained from the softener
- Cation-free water is obtained from the HI Unit, or
- Designed throughputs are achieved through the softener or HI Unit.
When sulphuric acid is used as a regenerant for the
HI unit, the waste effluent consists of the sulphate salts of calcium,
magnesium and sodium. As calcium sulphate has very limited solubility,
care has to be taken to maintain the designed concentrations of
sulphuric acid to avoid the precipitation of calcium sulphate in the ion
exchange bed.
With hydrochloric acid (HCl) as a regenerant the HCl solution is injected normally at uniform concentration of 5% w/v.
When sulphuric acid (H2SO4) is used as a regenerant, the solution of H2SO4
is injected at a concentration ranging from 1.5% w/v to 5% w/v. The
lower concentration is used when the influent water has a high calcium
content. To conserve water during the injection step the concentration
of H2SO4 solution is progressively increased in steps as calcium is depleted from the bed.
The HI unit can operate as a downflow or an upflow unit with coflow or counterflow technique of regenerant injection .Combination of WAC and Softener
The WAC resin reacts with alkaline salts only and not with neutral salts in water. The combination of WAC and Softener is used therefore when permanent hardness is present in water and one wishes to still employ the WAC.
In this case the WAC efficiently removes the temporary hardness and the downstream softener exchanges the permanent hardness. The carbonic acid generated in the WAC dissociates to carbon dioxide which is removed in the atmospheric degasser.
This process results in removal of all hardness, reduction in alkalinity and consequent reduction in mineral content of the water.
Dealkalization with a Weak Acid Cation (WAC) Resin.
The WAC resin exchanges
cations associated with alkalinity. Consequently it is possible to
reduce alkalinity using a WAC resin. Neutral salts pass unaffected
through this resin. The resin is noted for its high capacity for
divalent cations. Hence it is ideal for the removal of temporary
hardness in water.
It has a low capacity for monovalent cations. Hence it is not
recommended for treating waters with a high proportion of sodium
alkalinity. The resin is characterised by a high efficiency of
regenerant utilization and needs only a slight excess over the
stoichiometric requirements.The WAC resin is used in the hydrogen form.
The alkaline cations in the incoming water exchange
with hydrogen ions on the resin and yield carbonic acid which is removed
in a downstream atmospheric degasser, resulting in a reduction of
alkaline cations, alkalinity and a corresponding reduction in mineral
content of water. When the resin gets depleted of hydrogen ions, the
dealkalization process cannot take place. The resin needs regeneration.
Mineral acids such as hydrochloric acid (HCl) or sulphuric acid (H2SO4) are used for its regeneration.
Advantages- This method of Alkalinity removal , removes Alkalinity as well as Calcium & Magnesium ions associated with alkalinity which results in reduction of dissolved salts.
- Alkalinity in the treated water remains consistent through out the cycle.
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