Page 195 - Industrial Ventilation Design Guidebook

4,4 WATER PROPERTIES AND TREATMENT I 5 ?

• Base exchange
» Dealkalization
* Demineralization
The base exchange process removes both the temporary and permanent
hardness salts from the water by allowing the water to flow through resin
beads containing sodium zeolite, Na 2Z.
When the permanent hardness salt CaSO 4 passes through the bed, calcium
zeolite {CaZ) and sodium sulfate (Na 2SO 4 ) are formed, which are then
flushed away.
A temporary hardness salt such as calcium carbonate (CaCO 3 ) pass-
ing through a sodium zeolite bed will produce calcium zeolite (CaZ) and
sodium carbonate (Na 2 CO 3 ). This solution is flushed away. But the tem-
porary hardness salt calcium hydrogen carbonate, Ca(HCO 3 ) 2 , passing
through a sodium zeolite bed, will produce calcium zeolite and sodium
hydrogen carbonate, NaHCO 3. The latter increases the alkalinity of wa-
ter, causing foaming of the boiler water due to the formation of sodium
hydroxide, NaOH. Similar reactions are possible involving magnesium
chloride and sodium zeolite.
After a time, depending on the concentration of salts and the flow rate,
the remaining sodium zeolite is converted to either calcium or magnesium zeo-
lite. When the zeolite becomes saturated, the resin bed must be regenerated.
The regeneration process is achieved by backwashing (flushing) the bed with
fresh water to remove some of the remaining solids, followed by passing a so-
lution of salt through the resin bed. This flushing removes the calcium chlo-
ride (CaCl) and the sodium zeolite; a final rinse removes any salt remaining,
allowing the process to continue.
The dealkalization process removes the temporary hardness in water.
This uses an acid resin bed for regeneration—in this case sulfuric acid
(H 2S0 4).
To remove the majority of the salts from water, a mixture of resins is used;
the process in this case is called demineralization.
Basically, the hardness salts of calcium and magnesium ions are ex-
changed for sodium ions in the dealkization process; the carbonate and
bicarbonate salts, which cause high levels of alkalinity, are replaced with
chloride ions. Reverse osmosis can also be used to produce demineralized
water.

Precipitation Softening. This process depends on sufficient holdup time
within a vessel to allow sedimentation and clarification to occur. A coagula-
tion chemical such as alum or iron salts added to the solution will improve the
process efficiency.

Evaporation. The process of evaporation or distillation in the past was
carried out in submerged-tube evaporators. These have been superseded by
flash-type evaporators, which are more economical to run and reduce scale
problems. The process is suitable for brackish water, where the cost of chemi-
cal methods is excessive. The resulting distilled water is not palatable and re-
quires aeration to make it potable.