UNIT – I WATER TECHNOLOGY
Introduction to boiler feed water – requirements – formation of deposits in steam boilers and heat exchangers – disadvantages (wastage of fuels, decrease in efficiency, boiler explosion) prevention of scale formation - softening of hard water – external treatment – zeolite and demineralization – internal treatment – boiler compounds (phosphate, calgon, carbonate and colloidal) – caustic embrittlement – boiler corrosion – priming and foaming – desalination of brackish water – reverse osmosis.
UNIT – I WATER TECHNOLOGY
CLASSIFICATION OF WATER:
Based on the reaction with soap, the water is classified into softand hard water. The water which does not produce lather with soapsolution readily, but forms a white precipitate is called hard water,
Thewater which produce lather easily on shaking with soap solution, iscalled soft water. Such water does not contain dissolved Ca and Mgsalts in it.
HARDNESS OF WATER:
Hardness means, which preventsthe lathering of soap. This is due to the presence of bicarbonates,sulphates, chlorides of Ca and Mg and other heavy metals dissolved init.
A sample of hard water, when treated with soap (Na or K salt ofhigher fatty acid) does not produce lather, but produces a white scumor precipitate. This precipitate is formed, due to the formation ofinsoluble soaps of Ca and Mg.
2 C17H35COONa + CaCl2 (C17H35COO)2Ca + 2 NaCl
Sodium stearate Hardness Calcium stearate
2C17H35COONa + MgSO4(C17H35COO)2Mg +2 Na2SO4
Sodium stearate Hardness Magnesium stearate
Differentiation between Hard and soft water:
Sl. No. / Hard Water / Soft Water1. / It does not give lather with soap, but gives white scum or precipitate / It gives lather with soap readily
2. / It contains chlorides, sulphates and bicarbonates of Ca & Mg / It does not contain chlorides, sulphates and bicarbonates of Ca & Mg
3. / It gives wine red colour with Eriochrome Black – T / It does not give wine red colour with Eriochrome Black – T
Hardness of water can be classified into two types:
1. Temporary or Carbonate hardness
2. Permanent or non-carbonate hardness
1. TEMPORARY HARDNESS:
It is due to the presence of dissolved carbonates and bicarbonates of Ca and Mg, heavy metals and carbonates of Fe iscalled temporary hardness.
It can be removed by either boiling thewater (or) adding lime to the water. On boiling; the bicarbonates areconverted into insoluble carbonates and hydroxides.
Ca (HCO3)2CaCO3 + H2O + CO2
Mg (HCO3)2Mg (OH)2 + CO2
2. PERMANENT HARDNESS:
It is due to the presence of chlorides andsulphates of Ca and Mg, Fe and other heavy metals is called permanenthardness.
It cannot be removed by boiling, but can be removed byLime-soda process, Zeolite process and De-Ionization process, etc.
DIFFERENCES BETWEEEN TEMPORARY & PERMANENT HARDNESS:
Sl. No. / Temporary Hardness / Permanent Hardness1. / Due to the presence of dissolved carbonates and bicarbonates of Ca and Mg , heavy metals and carbonates of Fe / Due to the presence of chlorides and sulphates of Ca and Mg, Fe and other heavy
Metals
2. / Removed by either boiling the water (or) addinglime to the water / Removed by Lime-soda process, Zeoliteprocess and De-Ionization process, etc .
DISADVANTAGES OF HARD WATER:
1. Hard water is unfit for various domestic purposes like washing,bathing and so on.
2. When hard water is used for drinking, it affects the digestive systemand it forms stones in the kidney (as Calcium oxalate)
3. When hard water is used for cooking, it consumes more fuel and time because of the presence of salts of Ca andMg, this increases the boiling point of water.
4. Hard water is not useful for industries like textile, sugar and paper because of the dissolved salts like Ca, Mg, Fe and Mn affect the followingproperties.
(a) Smooth and glossy finish to paper in paper industry
(b) Producing good lather in laundry
(c) Producing good shades and colour to fabrics in textileindustry.
5. When hard water is used for steam production, the boiler affected
bythe problems like scale-sludge formation, priming and foaming,corrosion and caustic embrittlement.
6. Hard water should not be used for laboratory analysis, because thehardness producing Ca2+ and Mg2+ ions interfere in reactions.
7. When hard water is used for concrete making, the hydration ofcement and the strength of the concrete are affected.
BOILER FEED WATER:
In industry, one of the main uses of water is generation of steamby boilers. The water fed into the boiler for the production of steam iscalled boiler feed water and it should be free from dissolved salts,gases, suspended impurities, silica and oil.
If the water contains these impurities, theproblems like – Scale and sludge formation, Boilercorrosion, Caustic embrittlement and Priming and foaming arises.
ESSENTIAL REQUIREMENTS OF BOILER FEED WATER:
Boiler feed water should be free from the following,
1. Hardness producing Ca2+ and Mg2+ ions to avoid, scale andsludge formation
2. Dissolved oxygen and CO2 to prevent boiler corrosion
3. Turbidity, oil and non-scaling salts to reduce priming andfoaming
4. Caustic alkali (NaOH) to remove caustic embrittlement
BOILER PROBLEMS:
I. SCALE AND SLUDGE FORMATION:
As water evaporates continuously in boilers for steamproduction, the concentration of dissolved salts increases. When theirsaturation points are reached, the salts are precipitated on the innerwalls of the boilers. If the precipitate formed is soft, loose, slimy andnon—adherent; it is called sludge. Sludge’s are formed by thesubstances like MgCl2, MgCO3, MgSO4 and CaCl2. These havegreater solubility in hot water than in cold water.
Sludge’s canbe easily removed by scraping off with a wire brush. These are poorconductors of heat and also cause choking of pipes.Sludge formation can be prevented by using soft water and can beremoved by blow down operation which consists of withdrawal ofportion of concentrated water from the boiler and replacing it withfresh water.
If the precipitate is hard and adherent on the inner walls, it iscalled scale. These are formed by the substances like CaSO4 and Mg(OH)2 . Scales are so hard and adherent that they are difficult toremove them even with the help of hammer and chisel.
Scales may be formed due to the following reasons:
1. Decomposition of CaCO3
2. Formation of CaSO4 precipitate
3. Hydrolysis of Mg salts
4. Presence of small quantity of SiO2
Sl. No. / Sludge (Loose deposit) / Scale (Hard deposit)1. / Sludge’s are soft and non—adherent deposits / Scales are hard and adherent deposits on the inner wall of the boiler
2. / It can be removed easily scraping off with a wirebrush / It is difficult to remove even with the help of hammer and chisel , but can be removed by chemicalreaction
3. / Sludge’s can transfer heat to some extent and areless dangerous / Scales are poor conductor of heat and are more dangerous
4. / Sludge’s are formed by substances like MgCl2,MgCO3, MgSO4 and CaCl2. / Scales are formed by the substances like CaSO4 and Mg(OH)2
Disadvantages of Scale formation
1. Wastage of fuels: Scales have low thermal conductivity, so the heat transfer from boiler to inside water is not efficient. In order to provide steady supply of heat to water, overheating is done and this causes wastage of fuel. The wastage of fuel depends on the thickness and nature of the scale.
Thickness of scale (mm) / 0.325 / 0.625 / 1.25 / 2.5 / 12Wastage of fuel / 10 % / 15 % / 50 % / 80 % / 150 %
2. Decrease in efficiency: Scales sometimes deposit in the valves and condensers of the boiler and choke. This results in decrease efficiency of the boiler.
3. Boiler explosion: Due to overheating the thick scales and sudden feed water intake may cause crack. This causes formation of a large amount of steam and high pressure is developed which may lead to explosion.
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.
- By using suitable chemicals like dil.acid (for CaCO3 scale), EDTA (for CaSO4 scale) with which they form suitable complexes.
- If the scales are loosely adhering, they can be removed by frequent blow down operations.
II. BOILER CORROSION
Boiler corrosion is the decay of boiler material by a chemical orelectrochemical attack by its environment. Boiler corrosion is mainlydue to the presence ofDissolved oxygen, Dissolved CO2and Dissolved salts like MgCl2.
Removal of oxygen by Chemical Method
Dissolved oxygen can be removed from water by the addition ofchemicals like sodium sulphite , hydrazine, etc
2Na2SO3 + O2 2Na2SO4
N2H4 + O2N2 + 2H2O
Hydrazine is the best chemical for removing dissolved oxygensince the products are water and inert nitrogen gas. Also it removesoxygen without increasing the concentration of dissolved salts.
Removal of oxygen by Mechanical De-aeration Method
Oxygen along with CO2 can be removed by this method. In thismethod water is allowed to fall slowly on the perforated plates fittedinside the tower. To reduce the pressure inside the tower, thedeaerator is connected to a vacuum pump. The sides of the tower areheated by means of steam jacket. The water flows down through anumber of perforated plates and this arrangement exposes a largesurface of water for deaeration. High temperature, low pressure andlarge exposed surface, reduces the dissolved gases (O2 and CO2) inwater.
Removal of dissolved CO2
Dissolved CO2 in water produces Carbonic acid which has a slowcorrosive effect on the boiler material. CO2 is released inside theboiler, if water used for steam generation contains bicarbonates.
Mg (HCO3)2MgCO3 + H2O + CO2
Dissolved CO2 can be removed by the addition of NH4OH
2NH4OH + CO2(NH4)2CO3 + H2O
III. CAUSTIC EMBRITTLEMENT
Caustic embrittlement means intercrystalline cracking of boilermetal. It is a type of boiler corrosion, caused by using high alkalinewater in the boiler. Boiler water usually contains small amounts ofNa2CO3. In high pressure boilers, Na2CO3 undergoes hydrolysis toproduce NaOH.
Na2CO3 + H2O 2NaOH + CO2
This NaOH flows into the minute cracks and crevices usuallypresent on the boiler material by capillary action and dissolve thesurrounding area of Iron as Sodium ferroate, Na2FeO2. This type ofelectrochemical corrosion occurs when the concentration of NaOH isabove 100 ppm. This causes brittlement of boiler parts, particularlystressed parts like bends, joints, rivets etc.
Prevention of caustic embrittlement:
1. Using Na2SO4 as softening agent instead of Na2CO3
2. Adding chemicals like tannin, lignin to the boiler water
3. Adjusting the pH of the boiler feed water around 8 – 9
IV. PRIMING AND FOAMING:
Priming
When a boiler is producing steam rapidly, some particles of theliquid water are carried along with the steam. This process of ‘Wetsteam’ formation is called priming.
Priming is caused by:
1. The presence of large amount of dissolved salts
2. High steam velocity
3. Sudden boiling
4. Improper boiler design
5. Sudden increase in steam production rate
Priming can be prevented by:
1. Fitting mechanical steam purifiers
2. Controlling the velocity of steam
3. Maintaining low water levels in boilers
4. Using treated water
5. Good boiler design
Foaming
The production of stable bubbles above the surface of water iscalled foaming. Priming and foaming, usually, occur together.
Foaming is caused by:
1. Presence of air
2. Presence of grease
3. Presence of finely divided sludge particles
Foaming can be prevented by:
1. Adding anti—foaming agents like Castor oil
2. Adding coagulants like Sodium aluminate (NaAlO2), Ferroussulphate (FeSO4)etc. to remove the finely divided sludge particlesand oil.
SOFTENING OF WATER - EXTERNAL TREATMENT
Water used for industrial purposes (steam generation) should besufficiently pure and free from hardness producing salts to avoid theproblems like boiler corrosion, scale and sludge formation, and causticembrittlement etc. The process of removing hardness producing saltsfrom water is called softening of water. In industry, the following methods are mainly used for softening ofwater.
1. Ion-Exchange or De-Mineralization or De-Ionization process
2. Desalination process (i) Electro-dialysis (ii) Reverse Osmosis
DE—MINERALIZATION METHOD:
The boiler feed water should be free from all types of hardness producing salts. Water of such quality canbe obtained through demineralization process. It is also called Ion—exchange method and De—ionization method. The process, in which the hardness producing cationslike Ca2+ and Mg2+, and anions like Cl- and SO42-are removed from thehard water using cation and anion exchange resins respectively, iscalled Ion-exchange process.
Ion –exchange resins are insoluble, cross linked, longchain organic polymers with a micro porous structure. The functionalgroups attached to the chains are responsible for ion—exchangingproperties. Resins containing acidic groups (--COOH, --SO3H) arecapable of exchanging their H+ ions with cations of hard water. Resinscontaining basic groups (--NH2, --OH) are capable of exchangingtheir anions with the anions of hard water.
PROCESS
In this process hard water is allowed to pass through twokinds of columns.
1. The first column is packed with a cation exchange resin whichexchanges H+ ions with Ca2+, Mg2+ and all other cations present in thewater.
2 RH + Ca2+R2Ca + 2 H+
2 RH + Mg2+R2Mg + 2 H+
RH + Na+ RNa + H+
2. The cation free water is now passed through the secondcolumn packed with an anion exchange resin which exchanges OHionswith Cl-, SO42- and all other anions present in the water.
ROH + Cl-RCl + OH-
2 ROH + SO42-R2SO4 + 2 OH
ROH+ HCO3-RHCO3 + OH
TheH+ and OH- ions released from cation and anion exchangercombine to give water molecules which are free from all the cationsand anions present in the hard water. The ion—free water is calledde—ionized water or de—mineralized water.
REGENERATION
Cation exchange resins are regenerated by passing dil.HCl
R2Ca + 2 HCl 2 RH + CaCl2
R2Mg + 2 HCl 2 RH + MgCl2
RNa + HCl RH + NaCl
Similarly, the anion exchange resins are regenerated by passingdil.NaOH.
RCl + NaOH ROH + NaCl
R2SO4 + 2 NaOH2 ROH + Na2SO4
Advantages:
1. This process produces water of hardness nearly 2 ppm
2. Highly acidic or alkaline water can be softened
Disadvantages:
1. The equipment is costly and more expensive chemicals areneeded.
2. Highly turbid water (more than 10 ppm) cannot be softened.
ZEOLITE (OR) PERMUTIT PROCESS:
Zeolites are naturally occurring hydrated sodium aluminosilicate. Its general formula is Na2O.Al2O3.xSiO2.yH2O. (x=2 to 10, y=2 to 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, it exchanges its sodium ions with Ca2+ and Mg2+ ions present in the hard water to form calcium and magnesium zolites. The various reactions are
Ca(HCO3)2 + Na2Ze CaZe + 2 NaHCO3
Mg(HCO3)2 + Na2Ze MgZe + 2 NaHCO3
CaSO4 + Na2Ze CaZe + Na2SO4
MgSO4 + Na2Ze MgZe + Na2SO4
CaCl2 + Na2Ze CaZe + 2 NaCl
MgCl2 + Na2Ze MgZe + 2 NaCl
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
(i)Water obtained is having hardness of 1-2 ppm.
(ii)Cheap method, 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
(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 presence of NaHCO3 and Na2CO3 causes boiler corrosion and caustic embrittlement.
(iv)Water containing Fe, Mn cannot be treated because regeneration is very difficult.
(v)Brackish water cannot be treated because the presence of Na+ ion.
DESALINATION:
Depending upon the quantity of dissolved salts, the water isgraded as follows:
1. Fresh water: It contains, <1000 ppm of dissolved salts.
2. Brackish water: It contains 1000 – 35,000 ppm of dissolved salts.
3. Sea water: It contains, >35,000 ppm of dissolved salts
The water containing dissolved salts (high percentage of NaCl)with a peculiar salty taste or brackish taste is called brackish water. It istotally unfit for drinking purposes. Sea water and brackish water canbe made available as drinking water through desalination process. Theprocess of removing common salts (NaCl) from water is known asdesalination. The need for such a method arises due to the non—availability of fresh water. Desalination is carried out by electro—dialysis and desalination methods.
REVERSE OSMOSIS:
When a semi permeable membrane separates two solutions ofdifferent concentrations, flow of solvent takes place from dilute toconcentrated solution due to Osmosis. If pressure in excess of osmoticpressure is applied, the water flows in the reverse direction on the concentrated solution side. This process is called reverse osmosis. This process is also known as hyper—filtration or super—filtration.
Reverse osmosis is used in conjunction with one or moreactivated carbon filters. This combination removes 99.9 % ofundesirable water contaminants. Using reverse osmosis method purewater is separated from sea water. The membranes used are celluloseacetate, cellulose butyrate, polysulphone, polyamides, etc.
Advantages of electro—dialysis:
1. It is most compact unit
2. The cost of installation of the plant and its operation iseconomical
3. If electricity is easily available, it is best suited.
INTERNAL TREATMENT (OR) INTERNAL CONDITIONING IN BOILERS:
Scale and sludge formation is prevented by addition of chemicals to water after taking in boilers is called as internal treatment or sequestration.
(i)Carbonate conditioning:
Scale formation can be removed by addition of Na2CO3 to boiler water. It is used only in low pressure boilers. CaSO4 is converted into CaCO3 which can be removed easily.
Na2CO3 + CaSO4 CaCO3 + Na2SO4
(ii)Phosphate conditioning:
In high pressure boilers, the scale formation can be removed by addition of Na3PO4. The CaSO4 and MgSO4 are converted into calcium and magnesium phosphates.
2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4
Three types of phosphates are mainly employed for this purpose.
- NaH2PO4 – Sodium dihydrogen phosphate (acidic) used for alkaline water.
- Na2HPO4 – Disodium hydrogen phosphate (weakly alkaline) used for weakly acidic water.
- Na3PO4 – Trisodium phosphate (alkaline) used for too acidic water.
(iii)Calgon conditioning:
Calgon is nothing but sodium hexa meta phosphate Na2[Na4(PO3)6]. It combines with calcium salts to form a soluble complex which can be removed easily.
Na2[Na4(PO3)6] + 2CaSO4 Na2[Ca2(PO3)6] + 2Na2SO4
(iv)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.
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