ENGINEERING CHEMISTRY Two marks question and answers

Unit 1 – Water Technology

1. How temporary and permanent hardness is caused and removed?Temporary hardness is caused by bicarbonates of calcium and magnesium salts.

Permanent hardness is due to chloride and sulphates of calcium and magnesium.Temporary hardness is removed by boiling the water, and filtration. Lime soda process or zeolite ion exchange or demineralization removes permanent hardness.

2. Reason out why CaCO3 is used as unit of hardness. 1. Alkalinity and hardness give Calcium carbonate scale. So same unit for knowing

the amount of scale. 2. The molecular mass of CaCO3 is 100, it is easy for calculations.

3. What are zeolite, classify them and give its advantages and disadvantageous.Zeolites are hydrated sodium aluminum silicate Na2Al2O. xSiO2. yH2O, where x = 2

to 10, y = 2 to 6. Zeolites are classified into a. natural zeolite – has porous structure b. synthetic zeolite – has non porous structure.

Advantages: 1. residual hardness , 10 ppm 2. regeneration is simple 3. no sludge is formed 4. equipment is compact 5. easy operation.Disadvantages: 1. turbidity is to be removed 2. acidity water cannot be used 3. softened water contains more sodium salts 4. water containing Fe, Mn make the resin irreversible.

4. Why hard water is not useful for domestic purpose?Hard water does not give lather to soap, Soap give insoluble precipitate with

hard water ions calcium and magnesium, dirt is not removed. Soap is lost, soap precipitate stick on body and clothes. Cereals are not cooked well, food taste is not good.

5. Mention the requirements of drinking water and boiler feed water.Drinking water: 1. pH – should be 6.5 to 8.52. Hardness and dissolved solids is allowed max. of 500 ppm. 3. 3. No turbidity, colour, odour, taste 4. 4. Should not have harmful bacteria and chemicals.

Boiler feed water 1. Hardness must be removed fully; total hardness value zero is good.2. Dissolved gases CO2 and O2 to be removed.3. Dissolved solids to be removed.4. No turbidity and other particles

6. What are the problems faced in boilers while using drinking/hard water?The problems are a. sludge and scale formation. b. priming and foaming.c. boiler corrosion.d. caustic embrittlement.

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7. What is caustic embrittlement? How is prevented?Caustic embrittlement is the corrosion of boiler due to the presence of soda

ash in boiler feed water.At higher temperature sodium carbonate react with water and give sodium

hydroxideNa2CO3 + 2 H2O → 2 NaOH + CO2

At steam temperature NaOH react with iron and form compound. This weakens the metal strength. Metal become brittle.

8. What is priming and foaming? Mention its effects.Priming: the liquid water carryover along with the steamFoaming: formation of permanent foam on the surface of the boiler water during steam preparation. Carryover of wet water in the steam.

9. How caustic embrttlement is controlled?Caustic embrittlement is controlled by adding sodium sulphate in boiler feed

water. Sodium sulphate fills the hair line cracks found in the metal structure and avoids sodium hydroxide entry into the cracks.

10. What are boiler water treatment and its purpose? Steam production and the life of the boiler are affected by use of hard water.

The feed water is to be treated by a. internal treatment - sodium phosphate, calgon, sodium sulphite or hydrazine are added.b. external treatment - impurities are removed by zeolite, ion exchange resins. Softening.

11. Justify every soft water is not a demineralized water but every demineralized water is soft water.In soft water hardness giving ions are removed, all other ions are not removed. In demineralized water all the ions are removed. DM water is softer water than the softened water. It is the best water.

12. Why in DM water unit first cation exchanger is used and then anion resin is used. Cation exchange resin is attacked by alkalis. The out let of anion exchanger is alkaline, so it will affect the resin. But it is not affected by acids. The acid water of cation exchanger is not affecting the anion exchanger resin.

13. List out the hardness causing salts.Ca(HCO3), Mg(HCO3), CaCl2, MgCl2, MgSO4, CaSO4

14. Mention some cation and anion exchange resinsCation exchangers – sulphonated polystyrene, phenol formaldehyde, Amberlite IR-120, Dowex-120Anion exchangers: Urea formaldehyde, melamine formaldehyde, quaternary ammonium salts.

15. Mention the causes and control of priming.Priming causes:

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1. high steam velocity 2. high water level 3. sudden boiling 4. poor boiler design

priming control: 1. control water velocity 2. keeping water level is low 3. good boiler design 4. treating the water.

16. What is desalination? Mention the methods of desalination.The process of removal of sodium chloride from the sea water or brackish water is called desalination. Desalination is carried out by 1. distillation 2. freezing 3. electro dialysis 4. reverse osmosis.

17. What is reverse osmosis? How it is used in desalination of sea water?Reverse osmosis (RO) is a separation salt from water using pressure to

force a solution through a membrane. The membrane retains the solute on one side and allows the pure solvent to pass to the other side. Solvent is forced from high solute concentration through a membrane to a region of low concentration by applying a pressure.In sea water is added on the higher concentration side and the drinking water is collected at the dilute solution side.

18. What material is used as membrane in reverse osmosis method? Give the advantages of it.Cellulose acetate, cellulose butyrate.Advantages:

1. ionic, nonionic and colloidal impurities are removed2. life time of membrane is more, replacement is simple3. good quality water is produced.

19. Write the significance of ion exchange method.a. Raw water of acidic or alkaline is also used for DM water production.b. The output water has very low or nil hardness.c. Suitable for high pressure boiler water.

20. Differentiate between soft and hard water.Hard water does not produce lather with soap solution readily due to salts of calcium and magnesium salts. It give scale on boilingSince soft water does not contain Ca, Mg salts give readily lather with soap. It is not giving scale on boiling.

21. What are salts responsible to carbonate (temporary) and non carbonate (permanent) hardness.Carbonate hardness: - Ca(HCO3)2, Mg(HCO3)2

Non carbonate hardness – CaCl2, CaSO4, MgCl2, MgSO4

22. Why is demineralized water is preferred than zeolite method water for boiler?DM water does not contain any dissolved salts, zeolite treated water contain sodium salts which cause caustic embrittlement.

23. What is the main advantage of reverse osmosis over ion exchange process?

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By applying hydrostatic pressure in excess of osmotic pressure on the concentrated side, the solvent is forced to move to the dilute side across the membrane.Reverse osmosis removes all ionic, non ionic and colloidal impurities is the main advantage.

24. How many gram of FeSO4 dissolved per litre give 210 ppm of hardness? (Fe = 56, S = 32, O = 16, Ca = 40, C = 12)100 ppm od CaCO3 = 152 ppm of FeSO4

210 ppm of hardness = (152/100)210 = 319.2 ppm,amount of ferrous sulphate to be dissolved = 319.2 mg.

25. Calculate the temporary hardness and total hardness of water sample containing Mg(HCO3)2 = 73 mg/L, Ca(HCO3)2 = 162 mg/L, MgCl2 = 95 mg/L, CaSO4 = 136 mg/L.Temporary hardness = (100/146) 73 + (100/162) 162 =150 mg/LPermanent hardness = (100/95)95 + (100/136)136 = 200 mg/L

26. How are scales and sludge formed in boilers?Sludge: During the continuous boiler operation residual salt form precipitate in the boiler. If the precipitate formed is loose and slimy it is called sludge. Scale: Fine particles are formed in the boiler water during evaporation due to the presence of salts. These particles attach strongly on the metal surface of the boiler. This hard firmly sticking layer of salt is called scale. This layer is hard to remove.

Ca(HCO3)2 → CaCO3 + H2O + CO2

27. What is the conditioning of water? Mention type of conditioning. Addition of chemicals to prevent the scale formation on the surface of the boiler is called conditioning of water. The chemicals combine with the scale forming ions and eliminate them. Two types of treatment 1. Internal 2. External conditioning methods are used.

28. What is called calgon? What is its use in water technology? The chemical Sodium hexa meta phosphate is called calgon. Sodium hexa meta phosphate combines with calcium and give soluble complex. The complex is removed by blow down. Calgon prevents the scale formation when used in boiler water.

29. What are the chemicals used in phosphate and carbonate conditioning? How they work?Sodium phosphates, tri sodium, di sodium or mono sodium phosphate salts are used for p[phosphate conditioning. Sodium carbonate is added for carbonate conditioning. Phosphate ions form insoluble calcium phosphate, carbonate form insoluble calcium carbonate. Both are removed by blow down.

30. Define alkalinity.The presence of some salts rises the pH more than 7.0. The amount of acid required to neutralize the water and bring back the water to the neutral pH

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value is called as alkalinity of the water. It is expressed as calcium carbonate equivalent in mg/L.

31. What are the sources of alkalinity?1. Dissolution of bicarbonates, carbonates, silicates etc. from the minerals.

Hydrolysis of this salt with water produce alkalinity. 2. Industrial waste water from fertilizers, paper, textiles etc discharge alkaline

water. 3. Domestic detergent use and waste water contain alkalinity4. Water treatment plant effluents discharge.

32. Mention the types of alkalinity.The alkalinity is classified into hydroxide alkalinity, carbonate alkalinity and bicarbonate alkalinity. There are five combinations are possible. 1. hydroxide alkalinity 2. carbonate alkalinity 3. bicarbonate alkalinity

4. hydroxide and carbonate alkalinity 5. carbonate and bicarbonate alkalinity

33. What are the effects of alkalinity? 1. Carbonate ions combine with calcium ions and becomes insoluble calcium

carbonate scales.2. Calcium and magnesium hydroxide precipitate forms and make the water

turbid.3. In boiler caustic embrittlement takes place.4. Itching and skin irritation takes place.5. Some biological species are sensitive to high pH.

34. Define coagulation and flocculation.Coagulation is the neutralization of the charged colloidal particles in water.

After the neutralization the particles easily combine and form flocs that can settle quickly. Flocculation is the formation of large sized particles to make it easily settled. It is the agglomeration of fine particles to bigger sized particle.

35. Briefly write about electro dialysis.It is a process of preparing drinking water from salt water using electricity

and semipermiable membrane. The ions are attracted by the opposite poles through semipermiable membranes. When both positive and negative ions are sent out the water become good drinking eater.

36.How is UV light is useful for achieving disinfection of water?

UV treatment does not kill the bacteria, instead the radiation alters bacteria cell DNA so that the organism is sterilized. The pathogens are inactivated and it will not do any harmful actions. So it cannot grow further and cause disease.

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Big questions & Answers

1. Explain the reverse osmosis process with neat sketch.Reverse osmosis (RO) is a separation process. Pressure is applied to force a solution to its

solvent to pass through a membrane. Solvent is forced from high solute concentration through a membrane to a region of low concentration by applying a pressure in excess of the osmotic pressure . This is the reverse of the normal osmosis process,

The semipermeable, allows the passage of solvent but not of solute.The reverse osmosis membrane have a dense barrier layer in the polymer matrix. to allow only water to pass through. Thin membrane of cellulose acetate is used as the semi permeable membrane. It is fixed on both sides of the perforated frame of poly methacrylate and polyamide polymer.

This process requires high pressure applied on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater.

The pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.

Draw the Diagram given in the books

2. Describe EDTA method of hardness determination and calculation. Principle

Ethelene diamine tetra acetic acid is a powerful complexing agent.

Calcium and magnesium ions reaction is

[Ca2+/Mg2+] + EBT → [Ca2+/Mg2+EBT]wine red

[Ca2+/Mg2+EBT] + H2Y2- → [Ca2+/Mg2+EDTA] + EBT (blue)

Eriochrome black T- (EBT) indicator combines with Ca2+/Mg2+ ions and give wine red colour. EDTA removes calcium ions from EBT complex, wine red colour is lost. This is the end point. ProcedureStandardization of EDTA

20 ml of the standard hard water is pipetted out into a clean conical. 10 ml of ammonia buffer and about 4 or 5 drops of Eriochrome black T indicator is added-Then titrated with EDTA. End point is wine red to blue colour. Let the EDTA volume be V1 ml.Determination of Total hardness

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50 ml of the given water sample is pipetted out into a clean conical flask. 10 ml of ammonia buffer and 4 or 5 drops of EBT indicator are added- titrated against EDTA solution. Let the value be V2 ml.

Temporary hardness100 ml of the water sample in a beaker, Heated and filtered - made up to 100

ml using distilled water. 50 ml is titrated using EDTA solution. Let the volume be V3.

Calculation of total hardness

Volume of water sample taken for total hardness = 20 mlVolume of EDTA consumed = V2 ml50 ml of water sample consumed = V2 ml of EDTATotal hardness of the water sample (m) = (20 x V1) x V2 x 1000/50 mg/L

Temporary hardness

Volume of boiled water pipetted = 50 mlVolume of EDTA consumed for temporary hardness = V3 mlTemporary hardness ‘n’ = (V3 x 20/V1 x (1000/50)

Permanent hardness = m – n mg/L as CaCO3

3. With a neat sketch explain the electro dialysis method of water treatment.In this method direct current is applied through two membranes. The negative

and positive poles are kept outside the membranes. Sodium ions move to negative pole Chloride ions move towards positive pole by passing through the membranes.

Due to this movement of ions, the ion concentration in the central compartment decreases. Salt concentration increases in the outer two compartments. On continuing this process the central compartment water becomes drinking water.

The concentrated salt solutions are drained out and fresh brackish water is admitted into the central compartment continuously for further salt removal.

When the membranes are ion selective membranes, cations with fixed functional groups RSO3

─, RCOO─ and anions groups R4N+Cl─ the electro dialysis process is speeded up.

The electro dialysis cell consists of a large number of paired set of rigid plastic membrane. Saline water is passed under a pressure of 5 to 6 kg.in -2 between membrane pairs. The electric field is applied perpendicular to the direction of water flow. In the electro dialysis alternate water outlets are of pure water and concentrated water

AdvantageIt is a compact unit. But installation cost is more. The operative cost is less. If

the electricity is available easily it is the best available method.

Draw the diagram given in the book.

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4. What are the physical quality parameters of water? Explain their significance.Physical parameters affecting water quality are colour, turbidity, taste and

odour, total dissolved solids Colour: colour is due to organic substance like tannin, inorganic salts like iron, and dyes. Carcinogenic chemicals are organic dyes give colour. For use in industries like textile, paper, food processing are severely affected by the presence of colour.

Turbidity: The turbidity is the suspension of impurities like clay, finely divided insoluble impurities. The disease producing organisms are attached to the particles and carry the disease to the consumers. Turbid particles may interfere with the water softening process by coating on the resins and reduce the ion exchange capacity. Taste and odour: iron salts, chlorine, algae and decaying vegetative parts give unpleasant taste, sulphides and mercaptons cause odour to water.

Total dissolved solids: Soluble contribute to the total dissolved solids. It is Chloride, sulphate, of Ca, Mg, Fe. It is determined by evaporation method. It should be less than 500 ppm.

5. The hardness of 106 litres of water was completely removed by passing it through a zeolite softener. The softener then require 4000 litres of sodium chloride solution containing 100 g per litre for regeneration. Calculate the hardness of the water sample (Na = 23, Cl = 35.5)

4000 litre of NaCl solution contain = 4000 x 100 g NaCl= (4000 x 50)/58.5 = 341880 g of CaCO3

Hardness of 106 litre of water = 341880 g of CaCO3

Hardness in one litre = 341880/106 = 342 ppm.

6. Describe the demineralization process of softening of hard water. What are its advantages over zeolite process. (Explain the reaction involved).

Ion exchange resins are cross linked polymers. The acid resins (RH+) contain –COOH and –SO3H groups. It exchanges its hydrogen ions with all cations of water. Anion exchange resins (R+OH-) contain quaternary ammonium hydroxide groups. It exchange its hydroxide ions with the anions of water.

Draw the diagram given in the book.

ProcessThe cation resins and anion resins are filled in separate columns.

Cation exchanger: The raw water is first sent to cation exchanger resins. The exchange reactions are

H2R + (Ca2+, Mg2+, Na+)Cl-, SO42- → R(Ca2+, Mg2+, Na+) + HCl, H2SO4

The outlet water is acidic. The outlet is sent to anion exchanging resins.Anion exchanger: Here all anions are exchanged with hydroxide ions. Hydrogen ions combine with hydroxide ions forming water. So neutral water is collected.

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R(OH)2 + HCl/H2SO4 → R(Cl-, SO42- ) + H2O

RegenerationCation resins regeneration

When the exchanger exhausted, the conductivity increases. At this stage the hydrochloric acid is admitted into the cation exchange resins in the reverse direction. The cations are replaced by H+ ions, resin is washed to remove the excess acid.

R(Ca2+, Mg2+, Na+) → H2R + (Ca2+, Mg2+, Na+)

Anion resins regenerationSodium hydroxide solution is pumped in the reverse direction of the anion exchange resins, then washed with water, and lined up for DM water production.

R(Cl-, SO42- ) + NaOH → R(OH)2 + NaCl, Na2SO4

Draw the diagram

Advantages1. Removes both cations and anions2. Raw water of highly acidic or alkaline are also used for DM water production

3. the output water has very low or nil hardness4. Zeolite removes only hardness causing ions, the total solids remain the same.

But demineralization removes all ions.

7.100 ml of water sample on titration with N/50 H2SO4 required 12.4 ml of the acid to the phenolphalein end point and 15,2 ml of the methyl orange end point. Determine the type of alkalinity present in the water.

Phenolphalein alkalinity = (12.4 x 0.02 x1000x50)/100 = 124 ppmMethyl orange alkalinity = (15.2 x 0.02 x1000x50)/100 = 152 ppmHere P > ½ M, alkalinity is due to OH- and CO3

2-

Alkalinity due to (CO32- ) carbonate = 2(M-P) = 2(152-124) = 56 ppm

Alkalinity due to hydroxide (OH- ) = (2P-M) = 2 X 124 – 152 = 96 ppm.

8. 0.5 g of CaCO3 was dissolved in HCl and the solution is made upto 500 ml with distilled water. 50 ml of the solution required 48 ml of EDTA solution for titration. 50 ml of hard water sample required 15 ml of EDTA and after boiling and filtering required 10 ml of EDTA solution. Calculate the hardness.

500 ml of standard hard water = 500 mg of CaCO3

1 ml of standard hard water = 1 mg of CaCO3

50 ml of standard hard water = 50 mg of CaCO3 = 48 ml of EDTA1 ml of EDTA = 50/48 mg of CaCO3

Total hardness 50 ml of water = 15 ml EDTA = (15 x 50/48) x 1000/50 = 312.5 ppm

Permanent hardness 50 ml of permanent water = 10 ml EDTA

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Permanent hardness = (10 x 50/48) x 1000/50 = 208.3 ppmTemporary hardness = (312.5 – 208.3) = 104.2 ppm

Extra questions

9. What is called alkalinity? How it is determined?Definition The presence of some salts rises the pH more than 7.0. The amount of acid required to neutralize the water and bring back the water to the neutral pH value is called as alkalinity of the water. It is expressed as calcium carbonate equivalent in mg/L. Sources 1. Dissolution of bicarbonates, carbonates, silicates etc. from the minerals.

Hydrolysis of this salt with water produce alkalinity. 2. Industrial waste water from fertilizers, paper, textiles etc discharge alkaline

water. 3. Domestic detergent use and waste water contain alkalinity4. Water treatment plant effluents discharge.

Types of alkalinity1. hydroxide alkalinity 2. carbonate alkalinity 3. bicarbonate alkalinity

4. hydroxide and carbonate alkalinity 5. carbonate and bicarbonate alkalinity

Determination

The types of alkalinity and the quantity of alkalinity can be determined using acid base titrations using phenolphalein and methyl orange indicators. The reactions are

OH─ + H+ → H2O ------ 1

CO32─ + H+ → HCO3─ ------ 2

HCO3─ + H+ → H2CO3 ------- 3

The volume of acid consumed upto the phenolphthalein end point of pink colour disappearance corresponds to pH 8.3 (represented as P ml) reactions 1 & 2.

After the phenolphalein end point, methyl orange indicator is added into the same water and the titration is continued till the end point of yellow to orange colour appears. This point corresponds to reaction No. 3. (pH 4.5).

The total acid used to reach the methyl orange end point starting from zero ml (including the acid added for the phenolphthalein end point) is called the total alkalinity ‘M’ ml (P ml is included).

The conditions of calculating various form of alkalinity is given in the table

Result of

Hydroxide

Carbonate

Bicarbonate

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titration

alkalinity alkalinity alkalinity

P = 0

P = T

P = ½T

P < ½T

P > ½T

0

P

0

0

2P - T

0

0

2P

2P

2(T-P)

T

0

0

T-2P

0

Calculation

The volume of acid consumed for phenolphthalein = P mlNormality of the acid taken = NThe volume of water sample taken = 50 mlThe volume of acid consumed for totalupto methyl orange indicator = T ml1. Hydroxide alkalinity (when P = T) = P x N x 50,000/50 mg/L as CaCO3

2. Carbonate alkalinity (when P= ½T) = 2P x N x 50,000/50 mg/L as CaCO3

3. Bicarbonate alkalinity (when P=0) = T x N x 50,000/50 mg/L as CaCO3

4. Hydroxide alkalinity and carbonate alkalinity (when P >½T) a. Hydroxide alkalinity = (2P-T) x N x 50,000/50 mg/L as CaCO3

b. Carbonate alkalinity = 2(T-P) x N x 50,000/50 mg/L as CaCO3

5. Carbonate and bicarbonate alkalinity (when P < ½T) a. Carbonate alkalinity = 2P x N x 50,000/50 mg/L as CaCO3

b. Bicarbonate alkalinity = (T-2P) x N x 50,000/50mg/L as CaCO3

Effects 1. Carbonate ions combine with calcium ions and becomes insoluble calcium

carbonate scales.

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2. Calcium and magnesium hydroxide precipitate forms and make the water turbid.

3. In boiler caustic embrittlement takes place.4. Itching and skin irritation takes place.5. Some biological species are sensitive to high pH.

10. Write the boiler troubles faced when untreated water is used as boiler feed water. The trouble caused by the boiler feed water are

1. carry over or priming and foaming.2. scale and sludge forming.3. corrosion.4. caustic embrittlement.

1. Priming and foamingThe carrying of liquid water along with the steam is called priming. This

steam is called ‘wet steam’, steam carry suspended particles, dissolved impurities.Reasons

1. the presence of large amount of dissolved salts greater than 300 ppm2. high steam velocity 3. sudden boiling & sudden increase in steam production4. the improper design

Remedy1. good boiler design, keeping low water level 2. avoiding rapid change of steaming rate3. minimizing foaming.

FoamingDuring the steam production formation of small permanent bubbles at the

water surface is called foaming. This foam is carried along the steam- priming. Difference in salt concentration on the surface water and in the bulk water cause foam formation. Presence of oil, finely divided solids accumulation of soluble salt in water influence foaming.Removal of foaming1. The removal of foaming agents and foam stabilizing agents, 2. Clay, oil, suspended solids 3 giving blow down.4. Providing cyclone baffle. 5. Addition of anti foaming agent

2. SludgeThe loose and slimy precipitate formed is called sludge, it is in suspension

and attach as a loosly at the boiler surface. Magnesium carbonate, magnesium chloride, magnesium sulphate, calcium chloride are sludge forming salts. Sludge prevention

1. Good soft water is to be used2. Blow down of boiler drum water is to be regularly carried.3. By adding good quality fresh water.

Scales

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Fine particles are formed in the boiler water during evaporation. This firmly sticking layer of salt is called scale. This layer is hard to remove.

Ca(HCO3)2 → CaCO3 + H2O + CO2

Sulphate ions forms hard calcium sulphate scale. Iron and aluminum oxide are trapped into the scales. Magnesium salts form magnesium hydroxide, silica deposit on the inner sides of the boiler surface.Disadvantages: 1. The scales are poor conductors, 2. Acts as an insulator of heat, consume more fuel3. Boiler efficiency decreases.Scale removal

1. The loose adherent scale is removed by wire brushing2. Thermal shock is given to loosen the scales, and then removed by brushing.3. Acids with complexing agents and anti corrosion agents are used.4. Blow down is given to reduce salt saturation.

Corrosion The reasons for corrosion are

1. presence of free acid in feed water2. hydrolysis of salt, organic fatty acids, lube oils produce acid during

hydrolysis.3. presence of dissolved gases like oxygen, carbondioxide, hydrogen sulphide 4. oxidation of manganese sulphide and hydrolysis to sulphric acid5. formation of galvanic cell

Caustic embrittlementCaustic embrittlement is a type of corrosion. When sodium carbonate present

sodium hydroxide forms.Na2CO3 +H2O → NaOH + CO2

Sodium hydroxide enters in the minute hair line cracks, due to capillary action, attack iron, form sodium ferrate.

Fe + 4OH─ → Fe3O4 + 4H+

11. Discuss the water softening by lime soda process. Mention its type.Principle of softening

Calcium, magnesium, iron & aluminum ions are made as insoluble salts, settled and filtered to give hardness free water. Calculated amount of milk of lime and soda ash is added. The reactions are

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O Mg(HCO3)2 + Ca(OH)2 → Mg(OH)2 + CaCO3 + 2H2O

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2 MgSO4 + Ca(OH)2 → Mg(OH)2 + CaSO4

Dissolved iron and aluminum salts also removed. Calcium chloride and calcium sulphate coming due to lime reaction are removed by the soda ash reaction

CaCl2 + Na2CO3 → CaCO3 + 2NaCl

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CaSO4 + Na2CO3 → CaCO3 + Na2SO4

Types of lime soda process

Two types of lime soda process is in use. They are a. cold lime soda processb. hot lime soda process

Cold lime soda processThis process is carried out in normal temperature. This process may a batch

process or a continuous processBatch process: There are two tanks with inlet, outlet. One tank will be in the chemical process, the other one will be under settling. Continuous process

It is a vertical tank. A central inner chamber is provided. The required chemicals are admitted along with water in the central column. A stirrer mixes the chemicals for uniform reaction. The precipitate comes upward through the fibre filter fitted at middle of the tank. The treated water is collected at the top and sludge is drained at the bottom. The residual hardness present in the softened water is of range 50 to 60 ppm.

Hot lime soda processThe precipitate of the cold lime soda reaction takes time to settle and get

clarified. Hot lime soda process takes place at 94 – 1000C. The efficiency of the hot lime softening is increased. The residual hardness left out in the softened water ranges 17 to 34 ppm.

Draw the diagram

12. Describe the domestic water treatment method.Domestic water treatment process involves sedimentation, coagulation,

filtration and disinfection. Sedimentation

Sedimentation is the process of allowing the water to stand or slowly to move. The heavier particles settles down. The floated particles are scrapped by screen.

The sedimentation tank may be of rectangular type, circular shape. About seventy percent of the particles are removed in settling process.

CoagulationThe fine particles (silica, organic matter, clay) are in colloidal in size which

remain as suspended particles. Coagulating agents are added to settle the particles. Aluminum salts are used as coagulating agents. For example aluminum

sulphate gives aluminum hydroxide & sulphuric acid. The sulphuric acid is removed by the alkalinity present in water.

Al2(SO4)3 + 6 H2O → 2 Al(OH)3 + 3H2SO4

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The aluminum hydroxide is the coagulating agent attracts the colloidal particles and starts the settling. If the water is acidic soda ash is added to neutralize the sulphuric acid.Filtration

The coagulated settled water is passed through a filter bed. The particles are fully removed and clear water is collected. The filter beds are cleaned when choked by the particles and used again. Activated carbon filters are used to remove taste and odour. Zeolite is used for removing if the water contains iron and manganese.Disinfection

The disinfection or sterilization removes the harmful bacteria from the filtered water. Chlorination is one method of disinfection. Bleaching powder for small scale disinfection, chlorine gas for large scale disinfection are applied.

CaOCl2 + H2O → Ca(OH)2 + Cl2 Cl2 + H2O → HOCl + HCl

HOCl → [O] + HCl The hypochlorous acid liberates nascent oxygen. This nascent oxygen oxidize the cell wall of the bacteria. The hypochlorite anion OCl- rupture the cell membrane.

The odour and bad taste producing organic chemicals are destroyed during chlorination. So chlorinating the water till the break point chlorination is reached ensures the removal of taste and odour producing chemicals. Disinfection by ozonization

Ozone is a good disinfectant. Ozone release the nascent oxygen which is a powerful oxidsing agent, react with the bacteria cells and kill them & organic impurities are oxidized and eliminated.

O3 → O2 + [O]

13. How water is sterilized?Chlorination is the commonly used for disinfection using bleaching powder or

chlorine gas for large quantity water disinfection are applied. The important points in the treatment are correct dose of disinfecting agent, good distribution to all parts of the water, sufficient contact time. The reaction of chlorination is

CaOCl2 + H2O → Ca(OH)2 + Cl2 Cl2 + H2O → HOCl + HCl

HOCl → [O] + HCl The hypochlorous acid liberates nascent oxygen. This nascent oxygen oxidize the cell wall of the bacteria. The hypochlorite anion OCl- rupture the cell membrane.

The impurities like organic molecules, oxidizable substances also react with chlorine. The chlorine required for disinfection is determined by the break point chlorination experiment. The excess chlorine is removed by adding suitable quantity of ammonia or sodium sulphite.

Na2SO3 + Cl2 → Na2SO4 + 2HCl

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Disinfection by ozonizationOzone is a good disinfectant. Ozone release the nascent oxygen which is a

powerful oxidsing agent, react with the bacteria cells and kill them & organic impurities are oxidized and eliminated.

O3 → O2 + [O]

But ozone treatment removes bacteria, colour, odour, and taste without leaving any residual effects to the water.

Disinfection by UV radiationsUltraviolet radiation kill or deactivate many pathogens. UV treatment alters

cell DNA so that the organism is sterilized. This serves to inactivate the pathogen so that it cannot multiply and cause disease. Low-pressure mercury arc lamp produce UV radiation. The water is passed through the radiation tube or radiations are passed to the flowing water.

14. Write about internal conditioning of water for boiler use.In internal treatment, the chemicals are added directly to the boiler feed water.

The chemicals keeps the harmful ions in complex form and not to form scale. The internal treatment is carried out in three ways.

1. Phosphate conditioning method 2. Calogon conditioning method 3. Carbonate conditioning method

Phosphate conditioning methodThe phosphate removes the calcium ions present in the water. The calcium

phosphate formed is a non adherent precipitate. So it will not form scales. Sodium ortho phosphates, sodium pyrophosphates are used as internal conditioning agent.

2Na3PO4 + 3CaCO3 → Ca3(PO4)2 + 3Na2CO3

The phosphate salts are selected based on the nature of the feed water. If the water is acidic trisodium phosphate is used, if water is acidic sufficient sodium hydroxide is also added.Calgon conditioning Sodium hexa meta phosphate (NaPO3)6 is called as calgon This calgon combine with calcium ions in the boiler water and form highly soluble complex. This prevents the calcium salt formation and deposition on the surface of the boiler.

Na3[Na4P6O18] → 2Na+ + [Na4P6O18]2-

2Ca2+ + [Na4P6O18]2- → 4Na+ + [Ca2P6O18]2-

The soluble calcium salt of calgon is removed by the blow down water.

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Carbonate conditioningAddition of carbonate ions make calcium as calcium carbonate and the

calcium carbonate become precipitate. The sulphate ions form soluble calcium sulphate. This carbonate conditioning makes the dangerous calcium sulphate as soluble salt and is removed by blow down water. Na2CO3 + CaSO4 → CaCO3 + Na2SO4

The excess carbonate is converted into sodium hydroxide which cause caustic embrittlement. Na2CO3 + H2O → 2NaOH + H2CO3

15. Write about external treatment of boiler water.

The water is treated before the use, Demineralization, reverse osmosis and lime soda process are external treatment methods. Write the answers for reverse osmosis process from Question 1, for demineralization process from Question 6.Water softening

Soluble ions like calcium, magnesium, iron & aluminum ions are made as insoluble salts, settled and filtered to give hardness free water. Calculated amount of milk of lime and soda ash is added. The reactions are

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O

Mg(HCO3)2 + Ca(OH)2 → Mg(OH)2 + CaCO3 + 2H2O

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2

MgSO4 + Ca(OH)2 → Mg(OH)2 + CaSO4

Dissolved iron and aluminum salts also removed.

FeSO4 Ca(OH)2 → Fe(OH)2 + CaSO4 + 2H2O + O

Calcium chloride and calcium sulphate formed during lime reaction are removed by the soda ash reaction

CaCl2 + Na2CO3 → CaCO3 + 2NaCl

CaSO4 + N a2CO3 → CaCO3 + Na2SO4

Types of lime soda process Two types of lime soda process is in use. They are a. cold lime soda process b. hot lime soda process

Cold lime soda processThis process is carried out in ambient temperature. This process may be a

batch process or a continuous process

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Batch process: Two tanks with inlet, outlet and stirrer provisions are used. In one tank chemicals are added to remove hardness, the other one will be under settling. After the settling the supernatant soft water is drained out. The sludge is collected at the bottom. Continuous process:

The raw water and the required chemicals are admitted in the central column. The stirrer mixes the chemicals for uniform reaction. The reacted water with the precipitate come upward from the bottom of the tank. The softened water raises up through the fiber filter fitted at middle of the tank. The treated water is collected at the top. Sludge is drained at the bottom.

16. Describe the purification of water by electro dialysis technique.

The brackish water (salt water) is made as a drinking water using the electro dialysis method. In this method direct current is applied through a membrane pairs. Sodium ions move to negative pole. Chloride ions moves towards positive pole through membrane.

So in the central compartment salt concentration decreases and in the other two compartments the salt concentration increases.

The concentrated salt solutions are drained out and fresh brackish water is admitted on continuous basis. When the membranes are ion selective membrane permeable to cations with fixed functional groups RSO3

─, RCOO─ and anions groups R4N+Cl─ the electro dialysis process is speeded up.

The electro dialysis cell consists of a large number of paired set of rigid plastic membrane. Saline water is passed under a pressure of 5 to 6 kgin-2 between membrane pairs. The electric field is applied perpendicular to the direction of water flow. In the electro dialysis alternate streams are of pure water and salty water.

Draw the diagram form the book.

Advantage

It is a compact unit. But installation cost is more. The operative cost is less. If the electricity is available easily it is the best method of water purification.

Unit -1

What is break point chlorination? Addition of sufficient chlorine to oxidize organic matter, ammonia, to remove odour and leaving behind free chlorine to disinfecting the disease producing bacteria.

Distinguish between hard and soft water.

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Hard water is one which does not produce lather with soap solution readily due to dissolved salts of calcium and magnesium. Soft water does not salts of calcium and magnesium, gives readily lather with soap solution.

Why is demineralized water is preferred over zeolite method for use in boiler?Demineralized water does not contain any salts, where as zeolite water contains sodium salts which cause caustic embrittlement.

Why is it necessary to chlorinate the water supply beyond break point?Draw the diagram – graph. Formation of chloramines upto the dip point and further addition of chlorine leading to increase in chlorine residuals.

What are zeolites? Mention the advantages and disadvantages of zeolite process.It is a hydrated sodium aluminosilicate. General formula Na2O.Al2O3. x SiO2. Y H2O x=2-10, y-2-6. It is porous used in water softening.Advantages: 1. Residual hardness lea than 10 ppm 2. Regeration is simple 3. No sludge formation compace and easy operation.Disadvantagee: 1. Turbidity is to be remopved 2 acidic water canne=ot be treated 3. Softened water contain more sodium salts. Fe and Mn content water regeneration is difficult.

Write the principle involved in the desalination of water by reverse osmosis.

What is reverse osmosis? By applying hydrostatic pressure in excess of osmatic pressure on the concentrated side, solvent is forced to move to the dilute side across the membrane. Reverse osmosis removes all ionic, nonionic and colloidal impurities.

How exhausted ion-exchange resins regenerated?

How scales and sludges formed in boilers?

Calculate temporary hardness and total hardness of a sample of water containing Mg(HCO3)2 = 73 mg/L, Ca(HCO3)2 = 162, MgCl2 = 95 mg/L, CaSO4 = 136 mg/L

Mg(HCO3)2 (mol.mass-146) = 73x100/146 = 50 mg/L, Ca(HCO3)2 (mol.mass-162) = 162 x 100/162 = 100 mg/L, Temporary hardness = 50 + 100 = 150 mg/L

MgCl2 (mol.mass = 95) = 95 x 100/95 = 100 mg/L, CaSO4 (mol.mass = 136) = 136 x 100/136 = 100 mg/LPermanent hardness = 100 + 100 = 200 mg/L

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