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 DescriptionThe 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 thesoftener 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.