Water Technology

WATER TECHNOLOGY

1 Introduction

2 Hard water and soft water

3 Boiler Feed water

3.1 Formation of deposits in steam boilers and Heat exchangers

3.2 Caustic embrittlement

3.3 Boiler corrosion

3.4 Priming and Foaming

4 Softening or conditioning Methods

4.1 Zeolite or Permutit process

4.2 Ion-Exchange (or) De- ionization (or) Demineralisation Process

5 Internal Treatment (or) Internal Conditioning (or) Boiler Compounds

6 Desalination of Brackish water

6.1 Reverse Osmosis (R.O)

1 INTRODUCTION

Water is essential for the existence of human beings, animals and plants. Though 80% of the earth‟s surface is occupied by water, less than 1% of the water is available for ready use.

The main sources of water are

Rain

rivers and lakes (surface water)

wells and springs (ground water)

sea water

Among the above sources of water, rain water is the purest form of water but it is very difficult to collect whereas sea water is the most impure form.

Thus, surface and ground water are normally used for industrial and domestic purposes. Such water must be free from undesirable impurities.

“The process of removing all types of impurities from water and making it fit for domestic or industrial purposes are called water treatment.” Before treating water one must know the nature as well as the amount of impurities.

2 HARD WATER AND SOFT WATER

Hard Water

“Water which does not produce lather with soap solution, but produces white precipitate is called hard water”.

This is due to the presence of dissolved Ca and Mg salts.

Soft Water

“Water which produces lather readily with soap solution is called soft water.”

This is due to the absence of Ca and Mg salts.

3 BOILER FEED WATER

In Industry, one of the chief uses of water is generation of steam by boilers. The water fed into the boiler for the production of steam is called boiler feed water.

Requirements of boiler feed water

It should have zero hardness.

It must be free from dissolved gases like O₂, CO₂, etc.

It should be free from suspended impurities.

It should be free from dissolved salts and alkalinity

It should be free from turbidity and oil.

It should be free from hardness causing and scale forming constituents like Ca and Mg salts.

Formation of deposits (Scales and Sludges) in boilers and heat exchangers

In a boiler, water is continuously converted into steam. Due to this continuous evaporation of water, the concentration of soluble matters increases progressively. Then the salts separating out from the solution in the order of their solubility, the lease soluble ones separating out first.

(i) Sludge

If the precipitate is loose and slimy it is called sludges.

Sludges are formed by substances like MgCl₂, MgCO₃, MgSO₄ and CaCl₂.

They have greater solubilities in hot water than cold water.

( i i ) Scale

 If the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale.

Scales are formed by substances like Ca (HCO₃)₂, CaSO₄ and Mg(OH) ₂.

Disadvantages of Scale Formation

Wastage of fuels

Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a supply of heat to water, excessive or over-heating is done. This causes increase in fuel consumption. The wastage of fuel depends upon the thickness and the nature of scale.

(ii) Decrease in efficiency

Scales sometimes deposit in the valves and condensers of the boiler and choke them partially. This results in decrease in efficiency of the boiler.

(iii) Boiler explosion

When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler.

Prevention of scale formation

At the initial stage, scales can be removed using scraper, wire brush etc.

If scales are brittle, they can be removed by thermal shocks.

If the scales are loosely adhering, they can be removed by frequent blow down operation.

Caustic Embrittlement

Caustic embrittlement is a form of corrosion caused by a high concentration of sodium

Hydroxide in the boiler feed water.

It is characterized by the formation of irregular intercrystalline cracks on the boiler metal, particularly at places of high local stress such as bends and joints.

Causes of caustic embrittlement

Boiler water usually contains a small amount of Na₂CO₃. In high pressure boilers, Na₂CO₃ undergoes hydrolysis to produce NaOH.

Na ₂ CO ₃ + H ₂ O → 2NaOH +CO₂

This NaOH flows into the minute hairline cracks present on the boiler material by capillary action and dissolves the surrounding area of iron as sodium ferroate, Na₂FeO₂.

Fe + 2NaOH  → Na ₂ FeO ₂+H₂

This  type of  electrochemical  corrosion  occurs  when  the  concentration  of NaOH is above 100 ppm. This causes embrittlement of boiler parts, particularly the stressed parts like bends, joints, rivets, etc.

Caustic embrittlement can be prevented by

Using sodium phosphate as the softening agent instead of sodium carbonate.

Adding chemicals such as tannin, lignin to the boiler water. They block the hairline cracks.

Adjusting the pH of the feed water carefully between 8 and 9.

Boiler Corrosion

Corrosion in boilers is due to the presence of

Dissolved oxygen

Dissolved carbon dioxide

Dissolved salts like magnesium chloride.

Dissolved oxygen

The presence of dissolved oxygen is responsible for corrosion in boilers. Water containing dissolved oxygen when heated in a boiler, free oxygen is evolved, which corrodes the boiler material.

 4Fe + 6H₂O + 3O₂        → 4 Fe(OH)₃

Dissolved carbon dioxide

When water containing bicarbonates is heated, carbon dioxide is evolved which makes the water acidic. Carbon dioxide dissolved in water forms carbonic acid. This leads to intense local corrosion called pitting corrosion.

Ca(HCO₃)₂  → CaCO₃ + H₂O + CO₂

CO₂ + H₂O  → H₂CO₃

Dissolved magnesium chloride

When water containing dissolved magnesium chloride is used in a boiler, hydrochloric acid is produced. HCl attacks the boiler in a chain-like reaction producing hydrochloric acid again and again which corrodes boiler severely.

MgCl₂ + 2H₂O → 2HCl + Mg (OH)₂

Fe + 2 HCl     → FeCl₂ + H₂

FeCl₂ + 2H₂O → Fe (OH)₂ + 2 HCl

Corrosion by HCl can be avoided by the addition of alkali to the boiler water.

Prevention of boiler corrosion

Removal of dissolved oxygen and carbon dioxide can be done either chemically or mechanically.

Chemical method

For the removal of dissolved oxygen, sodium sulphite, hydrazines are used.

2Na₂SO₃ + O₂        → 2Na₂SO₄

N₂H₄  + O₂       → N₂ + 2H₂O

Hydrazine is the ideal compound for the removal of dissolved O₂ as it forms only water and inert nitrogen gas during the reaction.

Dissolved CO₂ is removed by the addition of ammonium hydroxide. 2NH₄OH + CO₂ → (NH₄)₂CO₃ + H₂O

Mechanical method

Oxygen along with carbon dioxide can be removed mechanically by the de-aeration method

In this method, water is allowed to fill in slowly on the perforated plates fitted inside the tower.

To reduce the pressure inside the tower, the de-aerator is connected to a vacuum pump. The sides of the tower are heated by means of a steam jacket. This is based on the principle that the solubility of a gas in water is directly proportional to pressure and inversely proportional to temperature.

High temperature, low pressure and a large exposed surface, reduces the dissolved gases (O₂ and CO ₂) in water.

The water flows down through a number of perforated plates and this arrangement exposes a large surface of water for de-aeration.

Priming and Foaming

During the production of steam in the boiler, due to rapid boiling, some droplets of liquid water are carried along with steam. Steam containing droplets of liquid water is called wet steam.

These droplets of liquid water carry with them some dissolved salts and suspended impurities. This phenomenon is called carry over. It occurs due to priming and foaming.

Priming

Priming is the process of production of wet steam. Priming is caused by

High steam velocity.

Very high water level in the boiler.

Sudden boiling of water.

Very poor boiler design.

Prevention

Priming can be controlled by

Controlling the velocity of steam.

Keeping the water level lower.

Good boiler design.

Using treated water.

Foaming

The formation of stable bubbles above the surface of water is called foaming. These bubbles are carried over by steam leading to excessive priming.

Foaming is caused by the

Presence of oil and grease.

Presence of finely divided particles.

Prevention

Foaming can be prevented by

Adding coagulants like sodium aluminate, aluminium hydroxide.

Adding anti-foaming agents like synthetic polyamides.

4 PREVENTION OF SCALE FORMATION (OR) SOFTENING OF HARD WATER

The process of removing hardness – producing salts from water is known as softening or conditioning of water. Since water is a source for industrial purpose. It is mandatory to soften water to make it free from hardness producing substances, suspended impurities and dissolved gases, etc.

Softening of water can be done by two methods.

i) External treatment ii) Internal treatment.

External Treatment or Conditioning

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

Zeolite (or) Permutit process

1 Zeolite (or) Permutit process

Zeolites are naturally occuring hydrated sodium aluminosilicate minerals. The chemical formula is Na₂O.Al₂O₃.XSiO₂.YH₂O. The synthetic form of zeolite is called permutit and is represented by Na₂Ze.

In this process the hard water is allowed to perlocate through sodium zeolite. The sodium ions which are loosely held in this compound are replaced by Ca²⁺ and Mg²⁺ ions.

When zeolite comes in contact with hard water, it exchanges its sodium ions with calcium and magnesium ions of hard water to form calcium and magnesium zeolites.

As sodium ions do not give any hardness to water, the effluent will be soft. The exhausted zeolite is again regenerated by treated with 5 to 10 percent of sodium chloride solution.

Na₂Ze + Ca(HCO)₂      -- -- >      CaZe + 2NaHCO₃

Na₂Ze + Mg(HCO)₂        -- -- >        MgZe  + 2NaHCO₃

Na₂Ze + MgCl₂         -- -- >             MgZe  + 2NaCl

Na₂Ze + CaCl₂           -- -- >            CaZe + 2NaCl

Na₂Ze + CaSO₄          -- -- >            CaZe + Na ₂SO₄

Na₂Ze + MgSO₄          -- -- >           MgZe  + Na ₂SO₄

Regeration

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

CaZe + 2 NaCl    →       Na₂Ze + CaCl₂

MgZe + 2 NaCl →         Na₂Ze +MgCl₂

Advantages

No sludge is formed during this process.

Water of nearly zero hardness is obtained.

This method is very cheap because the generated permutit can be used again.

The equipment used is compact and occupies a small space.

Its operation is also easy.

The process can be made automatic and continuous.

Disadvantages

This process cannot be used for turbid and acidic water as they will destroy the zeolite bed.

This treatment replaces only the cations, leaving all the anions like

(HCO₃)^–  and(CO₃)^2– in the soft water.

· When such water is boiled in boilers, CO₂ is liberated. Free CO₂ is weakly acidic in nature and extremely corrosive to boiler metal.

Na₂CO₃ + H₂O→2NaOH + CO₂

Due to the formation of sodium hydroxide, the water becomes alkaline and can cause cause caustic embrittlement.

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

This process cannot be used for softening brackish water. Because brackish water also contains Na⁺ ions. So, the ions exchange reaction will not take place.

2 Ion exchange or Demineralisation process

Ion exchange or demineralisation process removes almost all the ions (both anions and cations) present in the hard water.

The soft water, produced by lime-soda and zeolite processes, does not contain hardness producing Ca²⁺ and Mg²⁺ ions, but it will contain other ions like Na⁺, K⁺, SO₄^2– , Cl^– etc.,

On the other hand demineralised (DM) water does not contain both anions and cations. Thus a soft water is not demineralised water whereas demineralised water is soft water.

This process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a micro process structure. The functional groups attached to the chains are responsible for the ion exchanging properties.

(i) 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:

Sulphonated coals

Sulphonated polystyrene R–SO₃H; R–COOH ≡ RH₂

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:

· Cross-linked quaternary ammonium salts. Urea-formaldehyde resin.

R–NR₃OH; R–OH; R–NH₂ ≡ R (OH)₂

Process

The hard water first passed through a cation exchange 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^–, SO₄², HCO₃^–, etc., present in the water.

R' (OH) ₂ + 2HCl → R'Cl₂ + 2H₂O R'(OH) ₂ + H₂SO₄ → R'SO₄ + 2H₂O

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

Regeneration

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

RCa + 2HCl       → RH₂ + CaCl₂

RNa + HCl         → RH + NaCl

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

R'Cl₂ + 2 NaOH   → R'(OH)₂ + 2 NaCl

Advantages

The water is obtained by this process will have very low hardness.

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

Disadvantages

The equipment is costly.

More explosive chemicals are needed for this process.

Water containing turbidity, Fe and Mn cannot be treated, because turbidity reduces the output and Fe, Mn form stable compound with the resins

.

5 INTERNAL TREATMENT

Internal treatment involves adding chemicals directly to the water in the boilers for removing dangerous scale – forming salts which were not completely removed by the external treatment for water softening. This method is used to convert scale to sludge which can be removed by blow-down operation.

Calgon conditioning

Carbonate conditioning

Phosphate conditioning

Colloidal conditioning

Calgon conditioning

Calgon is sodium hexa meta phosphate with a

Composition Na₂(Na₄ (PO₃)₆). A highly soluble complex containing Ca is formed by replacing the sodium ions and thus prevents their formation of scale forming salts like CaSO₄. The reaction is as follows:

2CaSO ₄ + Na ₂ [Na ₄ (PO₃) ₆]  →        Na ₂ [Ca ₂ (PO₃) ₆] + 2Na ₂SO₄

Since the complex is highly soluble there is no problem of sludge disposal.

Carbonate conditioning

Scale formation due to CaSO₄ in low pressure boilers can be avoided by adding Na₂CO₃ to the boilers.

CaSO₄ + Na₂ CO₃ →     CaCO₃ + Na₂SO₄

The forward reaction is favored by increasing the concentration of CO₃^2-.CaCO₃ formed can be removed easily.

Phosphate conditioning

In high pressure boilers, CaSO₄ scale whose solubility decrease with increase of temperature. Such scale can be converted into soft sludge by adding excess of soluble phosphates.

3CaSO₄ + 2Na₃ PO₄ →    Ca₃ (PO₄)₂ +2Na₂SO₄

There are three types of phosphates employed for this purpose.

Tri-sodium phosphate – Na₃PO₄ (too alkaline): used for too acidic water.

Di-sodium hydrogen phosphate – Na₂HPO₄ (weakly alkaline): Used for weakly acidic water.Mono sodium di hydrogen phosphate NaH₂PO₄ (acidic) used for alkaline acidic water.

Colloidal conditioning

The colloidal conditioning agents are kerosene, agar-agar, gelatin, glue, etc. They are Used in low pressure boilers. The colloidal substances convert scale forming substance like CaCO₃, CaSO₄ into a Non-adherent, loose precipitate called sludge, which can be removed by blow-down Operation.

6 DESALINATION OF BRACKISH WATER

Depending upon the quantity of dissolved solids, water is graded as Fresh water has < 1000 ppm of dissolved solids. Brackish water has > 1000 but < 35,000 ppm of Dissolved solids.

Sea water has > 35,000 ppm of dissolved solids.

Water          containing   dissolved    salts  with   a        peculiar  salty  or  brackish taste  is called brackish water. It is totally unfit for drinking purpose. Sea water and brackish water can be made available as drinking water through desalination process.

The removal of dissolved solids (NaCl) from water is known as desalination process. The need for such a method arises due to the non-availability of fresh water. Desalination is carried out either by electro dialysis or by reverse osmosis.

Reverse Osmosis

When two solutions of different concentrations are separated by a semi-permeable membrane, flow of solvent takes place from a region of low concentration to high concentration until the concentration is equal on both the sides. This process is called osmosis.

The driving forces in this phenomenon are called osmotic pressure. If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow reverses, i.e., solvent is forced to move from higher concentration to lower concentration .This is the principle of reverse osmosis. Thus, in reverse osmosis method pure water is separated from its dissolved solids.

sing this method pure water is separated from sea water. This process is also known as super-titration. The membranes used are cellulose acetate, cellulose butyrate, etc.

Advantages

The life time of the membrance is high.

It can be replaced within few minutes.

It removes ionic as well as non-ionic, colloidal impurities

Due to simplicity  low capital cost, low operating, this process is used for converting sea water into drinking water

Glossary

Hardness

Harness is the property or characteristics of water, which does not produce lather with soap solution.

Soft water

Water which produces lather readily with soap solution is called soft water. Soft water is free of calcium & magnesium salts.

Temporary hardness

Temporary hardness is due to the presence of bicarbonates of calcium and magnesium. Since these salts can be easily removed by simple physical methods such as boiling and filtering.

Permanent hardness

Permanent hardness is due to the presence of soluble chlorides and sulphates of calcium and magnesium. These salts can be removed by chemical treatments only.

Alkalinity

Alkalinity of water is due to the presence of soluble hydroxide ( OH^- ), carbonate ( CO₃^2-) and bicarbonate ( HCO3-) ions.

Boiler feed water

The water which is free from dissolved salts, dissolved gases, hardness, oils and alkalinity is known as boiler feed water.

Sludge

The loose and slimy precipitate is called sludge.

Scale

The hard and adherent precipitate on the inner walls of the boiler is called scale.

Priming

Some droplets of liquid water are carried along with steam during the production of steam in the boiler is called priming.

Foaming

The formation of stable bubbles above the surface of water is called foaming. Caustic embrittlement

Formation of irregular, intergranular cracks at the welded joints, rivets etc. in high pressure boilers is called caustic embrittlement.

Brackish water

The water containing high concentration of dissolved salts with salty or brackish taste is called brackish water.

Reverse osmosis

The solvent flows from higher concentration to lower concentration.