Treatment of Boiler Feed Water (Softening or Conditioning Methods)

TREATMENT OF BOILER FEED WATER (SOFTENING OR CONDITIONING METHODS)

Water used for industrial purposes should be free from hardness producing substances, suspended impurities and dissolved gases etc. The process of removing hardness producing salts from water is known as softening (or) conditioning of water.

Softening of water can be done in two methods

1. Internal treatment.

2. External treatment.

1. Internal Conditioning (or) Internal Treatment (or) Boiler Compounds

It involves the removal of scale forming substance, which were not completely removed in the external treatment, by adding chemicals directly into the boiler. This chemicals are also called boiler compounds.

1. Phosphate conditioning

Scale formation can be avoided by adding sodium phosphate. It is used in high pressure boilers. The phosphate reacts with Ca²⁺ and Mg²⁺  salts to give soft sludges of calcium and magnesium phosphates.

3 CaSO₄ + 2 Na₃PO₄ → Ca₃(PO₄)₂ + 3Na₂SO₄

Generally 3 types of phosphates are employed.

(a) Trisodium phosphate - Na₃PO₄ (Too alkaline) - used for too acidic water.

(b) Disodium hydrogen phosphate - Na₂HPO₄ (weakly alkaline) - used for weakly acidic water.

(c) Sodium dihydrogen phosphate - NaH₂PO₄ (acidic) - used for alkaline water.

2. Colloidal conditioning

Scale formation can be avoided by adding colloidal conditioning agents like kerosene, agar-agar, gelatin, etc., It is used in low pressure boilers. These colloidal substances get coated over the scale forming particles and convert them into non-adherent, loose precipitate called sludge, which can be removed by blow down operation.

3. Sodium aluminate conditioning

Sodium aluminate (NaA1O₂) under goes hydrolysis in boiler water to give gelatinous white precipitate of aluminium hydroxide and sodium hydroxide.

NaA1O₂ + 2H₂O → Al(OH)₃ + NaOH

The sodium hydroxide, thus formed, precipitates magnesium as magnesium hydroxide. The gelatinous precipitates of aluminium hydroxide and magnesium hydroxide entrap the colloidal silica and finely divided solids and settled easily. This can be removed easily by blow down operations.

4. Calgon conditioning

Calgon is sodium hexa meta phosphate Na₂ [Na₄(PO₃)₆]. This substance interacts with calcium ions forming a highly soluble complex and thus prevents the precipitation of scale forming salt.

2 CaSO₄ + Na₂[Na₄(PO₃)6] → Na₂[Ca₂(PO₃)6] + 2 Na₂SO₄.

The complex Na₂ [Ca₂(PO₃)6] is soluble in water and there is no problem of sludge disposal. So calgon conditioning is better than phosphate conditioning.

 

2. External conditioning (or) External Treatment

It involves the removal of hardness producing salts from the water before feeding into the boiler. The external treatment can be done by two process

1. Demineralisation (or) Ion-exchange process

2. Zeolite process.

1. lon Exchange (or) Demineralisation process

This process removes almost all the ions (both anions and cations) present in hard water.

The soft water, produced by zeolite processes, does not contain hardness producing Ca²⁺ and Mg²⁺ ions, but it contains other ions like Na⁺, K⁺, SO^2-₄, Cl^-  etc., On the other hand D.M. (Demineralised) water does not contain both anions and cations.

Thus a soft water is not demineralised water whereas a demineralised water is soft water.

Demineralisation process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a microporous structure. The functional groups attached to the chains are responsible for the ion exchanging properties. The following two types of ion exchange resins are used.

1. Cation exchange resin (or) cation exchanger

2. Anion exchange resin (or) Anion exchanger

1. Cation exchanger

Resins containing acidic functional groups (-COOH -SO₃H) are capable of exchanging their H⁺ ions with other cations of hard water. Cation exchange resin is represented as RH₂.

Examples

(i) Sulphonated coals.

(ii) Sulphonated polystyrene.

R - SO₃H ; R-COOH ≡ RH₂

2. Anion Exchanger

Resins containing basic functional groups (-NH₂, -OH) are capable of exchanging their anions with other anions of hard water. Anion exchange resin is represented as R (OH)₂.

Examples

(i) Cross-linked quaternary ammonium salts.

(ii) Urea-formaldehyde resin.

R - NR₃OH ; R-OH; R-NH₂ ≡ R(OH)₂

Process

The hard water first passed through a cation exchange column, (Fig. 1.6) which absorbs all the cations like Ca²⁺, Mg²⁺, Na⁺, K⁺, etc., present in the hard water.

RH₂ + CaCl₂ → RCa + 2HCl

RH₂ + MgSO₄ → RMg + H₂SO₄

RH + NaCl → RNa + HCl

The cation free water is then passed through a anion exchange column, which absorbs all the anions like Cl^-, SO^2-₄, HCO^-₃, etc., present in the water.

R'(OH)₂ + 2HC1 → R'Cl₂ + 2H2O

R'(OH)₂ + H2SO₄ → R'SO₄ + 2H₂O

The water coming out of the anion exchanger is completely free from cations and anions. This water is known as demineralised water (or) deionised water.

Regeneration

When the cation exchange resin is exhausted, it can be regenerated by passing a solution of dil HCl (or) dil H₂SO₄.

RCa + 2HCl → RH₂ +CaCl₂

RNa + HCI → RH + NaCl

Similarly, when the anion exchange resin is exhausted, it can be regenerated by passing a solution of dil NaOH.

R'Cl₂ + 2NaOH → R'(OH)₂ + 2NaCl.

Advantages of ion-exchange process

(i) Highly acidic (or) alkaline water can be treated by this process.

 (ii) The water obtained by this process will have very low hardness (nearly 2. ppm).

Disadvantages of ion-exchange process

(i) Water containing turbidity, Fe and Mn cannot be treated, because turbidity reduces the output and Fe, Mn form stable compound with the resin.

(ii) The equipment is costly and more expensive chemicals are needed.

2. Zeolite (or) Permutit process

Zeolites are naturally occurring hydrated sodium aluminosilicate. Its general formula is Na₂O. Al₂O₃ . xSiO₂ .yH₂O. (x = 2 – 10, y = 2 – 6) . Natural zeolites are green sand and non-porous. The synthetic form of zeolite is known as permutit, which is porous and possess gel like structure, hence it is generally used for water softening.

Synthetic zeolite is represented by Na₂Ze. The sodium ions which are loosely held in Na₂Ze are replaced by Ca²⁺ and Mg²⁺ ions present in the water.

Process

When hard water is passed through a bed of sodium zeolite (Na₂Ze), kept in a cylinder (Fig. 1.7), it exchanges its sodium ions with Ca²⁺ and Mg²⁺ ions present in the hard water to form calcium and magnesium zeolites. The various reactions taking place during softening process are

Ca(HCO₃)₂ + Na₂Ze → CaZe + 2NaHCO₃

Mg(HCO₃) ₂ + Na₂Ze → MgZe + 2NaHCO₃

CaSO₄ + Na₂Ze → CaZe + Na₂SO₄

MgSO₄ + Na₂Ze → MgZe + Na₂SO₄

 

CaCl₂ + Na₂Ze → CaZe + 2NaCl

MgCl₂ + Na₂Ze → MgZe + 2NaCl

The softened water is enriched with large amount of sodium salts, which do not cause any hardness, but cannot be used in boilers.

Regeneration

After some time zeolite gets exhausted. The exhausted zeolite is again regenerated by treating with 10% solution of NaCl.

CaZe + 2NaCl → Na₂Ze  + CaCl₂

MgZe + 2NaCl → Na₂Ze + MgCl₂

Advantages of Zeolite process

(i) Water obtained by this process will have only hardness of 1-2 ppm.

(ii) This method is cheap, because the regenerated zeolite can be used again.

(iii) No sludge is formed during this process

(iv) The equipment used is compact and occupies a small space.

(v) Its operation is easy.

Disadvantages of Zeolite process

(i) Turbid water cannot be treated, because it blocks the pores of the zeolite bed.

(ii) Acidic water cannot be treated, because it decomposes the structure of zeolite.

(iii) The softened water contains more dissolved sodium salts

like NaHCO₃, Na₂CO₃, etc. When such water is boiled in boilers, CO₂ and NaOH is produced resulting in boiler corrosion and caustic embrittlement.

(iv) Water containing Fe, Mn cannot be treated because regeneration is very difficult.

(v) This process cannot be used for softening brackish water, because brackish water contains Na⁺ ions. So the ion exchange reaction will not occur.

 

Table 1.2 Differences between Zeolite and Demineralisation process

Zeolite Process

1. It exchanges only cations.

2. Acidic water cannot be treated because acid decomposes the zeolite.

3. The treated water contains relatively large amount of dissolved salts, which leads to priming, foaming and caustic embrittlement in boilers.

4. Water containing turbidity, Fe, Mn cannot be treated.

Demineralisation Process

1. It exchanges cations as well as anions.

2. Acidic water can be treated.

3. The treated does not contain any dissolved salts. Hence there is no priming and foaming.

4. Here also water containing turbidity, Fe, Mn cannot be treated.