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)
6.1 Reverse Osmosis (R.O)
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
rivers and lakes (surface water)
wells and springs (ground 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
“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.
“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.
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 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.
Corrosion in boilers is due to the presence of
Dissolved carbon dioxide
Dissolved salts like magnesium chloride.
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.
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
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 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.
Priming can be controlled by
Controlling the velocity of steam.
Keeping the water level lower.
Good boiler design.
Using treated water.
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.
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
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
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.
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+, SO42– , 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.
Sulphonated polystyrene R–SO3H; R–COOH ≡ RH2
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.
· Cross-linked quaternary ammonium salts. Urea-formaldehyde resin.
R–NR3OH; R–OH; R–NH2 ≡ R (OH)2
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.
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
The water is obtained by this process will have very low hardness.
Highly acidic or alkaline water can be treated by this process.
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 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.
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 CO32-.CaCO3 formed can be removed easily.
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.
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.
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.
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
Harness is the property or characteristics of water, which does not produce lather with soap solution.
Water which produces lather readily with soap solution is called soft water. Soft water is free of calcium & magnesium salts.
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 is due to the presence of soluble chlorides and sulphates of calcium and magnesium. These salts can be removed by chemical treatments only.
Alkalinity of water is due to the presence of soluble hydroxide ( OH- ), carbonate ( CO32-) 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.
The loose and slimy precipitate is called sludge.
The hard and adherent precipitate on the inner walls of the boiler is called scale.
Some droplets of liquid water are carried along with steam during the production of steam in the boiler is called priming.
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.
The water containing high concentration of dissolved salts with salty or brackish taste is called brackish water.
The solvent flows from higher concentration to lower concentration.
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