chemical engineering

1. PCSIR Internship
Pilot department
WATER SOFTENING
Submitted by: Huda shah
Chemical engineering
Intern, U.E.T
Submitted to: Sir Fayaz
Junior Engineer

2. WATER SOFTENING
The process of removing hardness producing salts from water is
known as softening of water and can be accomplished by one of the
following methods.
 Lime-soda Process
 Zeolite or Permutit Process.
 Reverse Osmosis Process
Lime-soda Process
Lime-soda Process is the most widely used method of softening water.
It is based on converting the dissolved calcium and magnesium salts
into insoluble salts, which are then allowed to settle and filtered off.
The added ingredient lime Ca(OH)2 precipitates temporary hardness,
permanent magnesium hardness, iron and aluminium salts and free
acids (like CO2, H2S- etc).
Ca(HCO3)2+ Ca(OH)2 --------> 2CaCO3 + 2H2O
Mg(HCO3)2 + 2Ca(OH)2 --------> 2CaCO3 +Mg(OH)2 +2H2O
MgSO4 + Ca(OH)2 --------> Mg(OH)2 +CaSO4
FeSO4+ Ca(OH)2 --------> Fe(OH)2 +CaSO4
Types of Lime-soda Process
Lime-soda process may be carried out in the following ways
 Cold lime-soda process
 Hot lime-soda process
Cold lime-soda process
This method is usually applied for municipal water treatment. In this method calculated
quantity of chemicals (lime and soda) are mixed with water at atmospheric temperature.
At room temperature the precipitates formed are finely divided, so, they do not settle
down easily, nor can they be filtered easily. Consequently, it is essential to add small
amounts of coagulants. These coagulants forms flocs of of aluminium hydroxide, which
entraps the fine precipitates formed by the reactions of lime and soda. This process
provides water containing a residual hardness of 50 to 60 ppm.

3. Hot lime-soda process
• This method is applied for boiler feed-water treatment. This involves treating
water with chemicals at a temperature of 80 to 150 °C.
Comparison of HLSP & CLSP
• Since hot process is operated at a temperature close to the boiling point of the
solution, so, the reaction proceeds faster, while in hot softening the reactions are
completed in 15 minutes; whereas in the cold process several hours are needed.
• the softening capacity of hot process is increased to many fold,
• the precipitate and sludge formed settle rapidly and hence no coagulants are
needed,
• much of the dissolved gases (such as CO2 and air) are driven out of the solution,
• viscosity of softened water is lower, so, filtration of the water becomes, much
easier. This in turn increases the filtering capacity of filters, and hot lime-soda
produces water of comparatively lower residual hardness of 10 to 20 ppm.

4. Zeolite or Permutit Process
Zeolites (or Permutits) are Complex silicates consisting of macromolecules of several
metallic and non-metallic oxides, which form a rather crystalline structure of approximate
chemical formula Na2O. Al2O3. 2SiO2. 6H2O and they are simply represented by
Na2Ze. They hold sodium ions in loose fashion and consequently when they are treated
with a solution, an equilibrium is formed between the sodium ions held by the zeolite and
positive ions present in the solution. Thus, if the zeolite bed is kept in contact with a
solution containing heavy metal ions such as Ca2+, Mg2+, etc., there will be a tendency
for these ions to be exchanged by the sodium ions contained in zeolite.
Natural and Synthetic Zeolites
Zeolite used for water-softening purposes can be natural or synthetic. The former are
mined as such and are more durable. The later are manufactured by heating china clay
and soda ash together and cooling and crushing the resulting glass. Artificial zeolites
have a greater exchange capacity per unit weight but are less durable and more easily
effected by acids than natural zeolites.
Process details
• For softening of water by zeolites process, hard water is percolated at a
specified rate through a bed of zeolites kept in a cylinder. The hardness
causing ions (Ca2+, Mg2+, etc.) are retained by the zeolite, while the
outgoing water contains sodium salts. Reactions taking place in
the softening process are :
Regeneration
After some time, when the zeolite is completely converted into calcium and magnesium
zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage the supply of
hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a
dilute (10%) brine (NaCl) solution,
CaZe(or MgZe)+ 2NaCl -------->Na2Ze + CaCl2(orMgCl2)
The washings (containing CaCl2 and MgCl2) are led to drain and the reclaimed zeolite
bed is used again for softening.

5. Advantages of zeolite process
• It removes the hardness completely, ie., nearly zero hardness water is produced.
• The equipment used is compact occupying a small space.
• No impurities are precipitated so there is no danger of sludge formation in the
treated water at a later stage.
• The process automatically adjusts itself for different hardness of incoming water.
Disadvantages of zeolite process.
• The treated water contains more sodium salts than in lime-soda process.
• Higher capital cost as compared to LSP but lower operational cost.
• Zeolite treatment only replaces the cations (like Ca2+, Mg2+, etc.) with Na+, but
leaves all the acidic ions (like HCO3- and CO32-) in the softened water. When
such softened water is used in steam boilers, free CO2 is released. Free CO2 thus set
free is weakly acidic and is corrosive to boiler materials.

6. Osmosis
Osmosis is the spontaneous net movement of solvent molecules through a partially
permeable membrane into a region of higher solute concentration, in the direction that
tends to equalize the solute concentrations on the two sides.
Reverse Osmosis
Reverse osmosis is a separation process that uses pressure to force a solvent through a
semi-permeable membrane that retains the solute on one side and allows the pure solvent
to pass to the other side, forcing it from a region of high solute concentration through a
membrane to a region of low solute concentration by applying a pressure in excess of the
osmotic pressure.