SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY COIMBATORE First Year BE/B.TECH ( ) Engineering Chemistry- I

Transcription

1 SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY COIMBATORE Define hard water and soft water? First Year BE/B.TECH ( ) Engineering Chemistry- I UNIT-I- Water Technology Water which does not produce lather with soap solution but forms white precipitate is called hard water. Water which lather easily with soap solution is called soft water. 2. Give chemical test to detect hardness of water? Hardness can be detected by treating water with soap as hard water forms a white precipitate with soap solution. 3. Mention the common units used for expressing hardness of water? Parts per million (ppm) Milligrams per litre (mg/l) Clarke s degree ( 0 Cl) Degree French( 0 Fr) 4. Define ppm. It is the parts of calcium carbonate equivalent hardness per 10 6 parts of water. 5. Why do we express hardness of water in terms of calcium carbonate equivalent? The choice of calcium carbonate is due to the fact that Its molecular weight 100,equivalent weight 50 which makes calculation easier It is the most insoluble salt that can be precipitated in water treatment. 6. What are the types of alkalinity? Even though many substances may contribute to the alkalinity of water, depending on the anion that is present in water, alkalinity is classified into the following types. a) Hydroxide alkalinity : presence of OH - ion b) Bicarbonate alkalinity :presence of HCO 3 - ion c) Carbonate alkalinity : presence of CO 3 2- ion 7. Alkalinity of water cannot be due to the simultaneous presence of OH -, CO 3 2- HCO 3 -. Give reason. OH - ions readily react with HCO 3 - ions to form CO 3 2- ions. OH - + HCO What is known as boiler feed water? CO H 2 O The water fed into the boiler for the production of steam is called boiler feed water. 9. What are the requirements of boiler feed water? Boiler water must be free from dissolved gases and salts, suspended impurities, silica and oil. and

2 10. What are the problems encountered in boiler feed water? Scale and sludge formation. Caustic embrittlement Boiler corrosion Carry over (priming and foaming) 11. Distinguish between sludge and scale. S.No Sludge Scale 1 It is soft, loose and slimy precipitate formed in the boiler during steam generation. It is hard, adherent coating formed on the inner walls of the boiler during steam generation. 2 The main sludge forming substances are The main scale forming substances MgCO 3, MgCl 2, MgSO 4 are CaSO 4,SiO What is meant by boiler corrosion? How is it prevented? Boiler corrosion is due to the presence of dissolved gases such as O 2, CO 2 and the salts like MgCl 2 (acid producing salts). Mechanical deaeration is a method used to remove dissolved gases such as O 2, CO 2. The principles involved in this process are the solubility of a gas is directly proportional to pressure and inversely proportional to temperature. 13. What is caustic embrittlement? Caustic embrittlement is a type of electro chemical corrosion which occurs in high pressure boilers when water containing NaCO 3 is used. Na 2 CO 3 + H 2 O 2NaOH + CO 2 The NaOH so produced flows into minute irregular inter granular cracks and dissolve and corrodes the surrounding area of iron as sodium ferrate. 14. How is caustic embrittlement prevented? Caustic embrittlement can be prevented by using sodium phosphate as softening agent instead of Na 2 CO 3. adding chemicals such as tannin, lignin to the boiler water. They are used to block the hair cracks present in the boiler material. 15. What is meant by priming and foaming? What are their effects in boilers? Priming: The production of wet steam by rapid boiling of water in boilers. Foaming: The formation of stable bubbles above the surface of water and lead to excessive foaming. Disadvantages: Reduce boiler efficiency; maintenance of boiler pressure is difficult. 16. What is meant by carry over? Steam sometimes may be associated with small droplets of water. Such steam containing liquid water is called wet steam. These droplets carry with them some dissolved salts and sludge material present in water. This phenomenon is called carry over. It occurs mainly due to priming and foaming

3 17. What is aeration? Why is it required? Aeration is a process in which water is mixed with air. The main purpose of aeration is To remove CO 2, H 2 S and other volatile substances causing bad taste and odour. To remove impurities like Fe and Mn as their respective hydroxides. 18. What are boiler compounds? Give examples. The chemicals added inside the boilers to remove scale forming substances are called boiler compounds. Example: sodium phosphate, calgon. 19. What is external treatment of water? It involves the removable of scale and sludge forming impurities from the water before feeding to the boiler. 20. What is the purpose of internal treatment of boiler feed water? It involves the removal of scale forming substances which were not completely removed by external treatment by adding chemicals directly inside the boiler during evaporation itself. So, it is called corrective treatment. 21. What is blow down operation? It is a process in which portions of water containing a large amount of dissolved salts are replaced by fresh water frequently during steam generation. 22. What is calgon? What is it role in water treatment? Calgon is sodium hexa phosphate. It forms a highly soluble complex with Ca + ions of hard water and prevents the formation of scale in boilers. 23. Every soft water is not a demineralization water whereas every demineralization water is soft water. Justify. Softening involves removal of only hardness causing ions where as demineralization Involves removal of all ions present in water. Hence the above statement is justified. 24. How will you regenerate the column in demineralization process? The exhausted cation exchange column and anion exchange column are regenerated by passing diluted hydro chloric acid or diluted sulphuric acid and diluted sodium hydroxide respectively. 25. What is desalination? Name few methods of converting sea water into fresh water? The process of removal of extra common salt from sea water is described as desalination or desalting. The various desalination processes are Distillation Freezing Reverse osmosis 26. What are the requisites of water for municipal supply? Water should be clean, colourless and odourless. It should be free from pathogenic organisms. It should not be excessively hard or soft.

4 27. Name any two of the important disadvantages of using hard water for domestic purposes. Hard water when used for drinking affects our digestive system and also leads to the formation of stones in kidney. Hard water when used for washing does not lather with soap easily and thereby causing wastage of soap. 28. What is coagulation? The process of removing fine suspended and colloidal impurities by adding the requisite amount of coagulant to water. Coagulant (alum) gets hydrolyzed to form a insoluble precipitate, which will entrap finely divided particles and colloidal impurities which settle down easily. 29. What is a disinfectant? Give examples. The process by which harmful bacteria are destroyed to make the water safe for drinking is called disinfection. Chemicals used for this purpose are called disinfectant. Examples: chlorine, bleaching powder, ozone and UV rays. 30. Why is chloramine better than chlorine for sterilization of water? Chloramines is quite stable Chloramines does not import any disagreeable odour or bad taste to treated water. 31. What is meant by break point chlorination? The process of adding chlorine to kill bacteria and to oxidize the organic impurities present in water and the point at which the residual increases is called break point chlorination. 32. What are the hardness causing salts? Mg(HCO 3 ) 2, MgCl 2, MgSO 4 Ca(HCO 3 ) 2, CaCl 2, CaSO How will you remove dissolved oxygen and dissolved carbon dioxide in water? For the removal of dissolved oxygen, chemicals- sodium sulphite or hydrazine is added. Dissolved carbon dioxide is removed by the addition of ammonium hydroxide. 34. How is priming caused? Priming may be caused by Very high water level High steam velocity Sudden boiling of water Very poor boiler design 35. How is priming controlled? Priming may be controlled by Keeping the water level lower Good boiler design Using treated water Controlling the velocity of water.

5 36. Suggest few methods to prevent foaming. Foaming can be prevented by Adding coagulants like sodium aluminate, ferrous sulphate etc.. to remove the finely divided sludge particle. Adding anti foaming agents to remove foaming and stabilizing agents. 37. Name few internal conditioning methods. Colloidal conditioning EDTA conditioning Electrical conditioning Treatment with NaAlO Give example for cation exchangers. Sulphonated coal Sulphonated polystyrene Phenol formaldehyde resin 39. Give example for anion exchangers. Urea formaldehyde resin Melamine formaldehyde resin Cross linked quaternary ammonium salts 40. What are the advantages of ion-exchange method? This process can be used to treat highly acidic and alkaline water. This process produces water of very low hardness say 2ppm.Hence; it is very good for use in high pressure boilers. 41. What are the dis-advantages of ion-exchange method? The equipment is costly and more expensive chemicals and resins are required. Turbid water and iron, manganese containing water cannot be treated with this method. Because iron, manganese are irreversibly exchanged and turbidity reduces the efficiency of the process. 42. Explain reverse osmosis. The natural osmosis can be reversed by applying pressure higher than the osmotic Pressure on the concentrated solution side. 43. Explain ultraviolet treatment. UV rays are powerful disinfectant.they are produced by passing electric current through mercury vapour lamp. When water is allowed to flow in this stream, UV rays from the lamp destroys pathogens completely 44. What is the role of buffer in EDTA titrations? EDTA forms complexes with bivalent ions like calcium and magnesium ions present in water. These complexes are stable only when ph is maintained between 9 and10. Thus EDTA titration is carried out by maintaining the ph of 9-10 by using ammonia buffer.

6 45. Principle behind the EDTA titration. Hardness in water is due to the presence of dissolved carbonates, bicarbonates, chlorides and sulphates of calcium and magnesium. Eriochrome Black T is used as indicator. Initially the indicator forms an unstable complex with calcium and magnesium ions which is wine red in colour. When EDTA reacts with this complex, Ca/Mg-EDTA complex is formed and the indicator is released and hence colour changes from wine red to steel blue. This determines the end point. Ca/Mg + EBT Ca/Mg-EBT (Wine red colour) Unstable complex Ca/Mg-EBT + EDTA (Ca/Mg-EDTA) + EBT (Blue Colour) Stable complex 46. What is temporary hardness or carbonate hardness? Temporary hardness in water is due to the presence of dissolved bicarbonates of calcium and magnesium and other metal ions. Temporary hardness is removed by boiling. 47. What is permanent hardness or non carbonate hardness? Permanent hardness in water is due to the presence of dissolved chlorides and sulphates of calcium and magnesium and other heavy metal ions. Permanent hardness Cannot be removed by boiling, but requires chemical treatment. 48. A sample water contains 204 gm of calcium sulphate per liter.calculate the hardness in terms of calcium carbonate equivalent. Equivalent of calcium carbonate = = Mass of hardness producing substance * Molecular weight of calcium carbonate Molecular weight of hardness producing substance Equivalent of calcium carbonate = 204*100 ppm = 150 ppm ml of sample of water required 50ml of 0.01N EDTA for titration. Calculate the total hardness. 1 ml of 1M EDTA = 100 mg of calcium carbonate 1 ml of 0.01M EDTA = 1 mg of calcium carbonate In the estimation, 500ml of sample of water required 50ml of 0.01N EDTA = 50*1 mg of calcium carbonate equivalent. = 50 mg of calcium carbonate equivalent 1000 ml of sample of water requires = 50*1000 mg 500 = 100 mg of calcium carbonate equivalent total hardness= 100 ppm 50. How is water sterilized by ozone. Ozone is a powerful disinfectant and is readily absorbed by water. Ozone is highly unstable and breaks down to give nascent oxygen. O 3 O 2 + [O] The nascent oxygen is a powerful oxidizing agent and kills the bacterias.

7 51. What is the cause for alkalinity of natural water? Presence of soluble i) hydroxide (OH - ), ii) carbonate (CO 3 2- ) and iii) bicarbonate (HCO 3 - ). 52. Soft water is not demineralized water whereas demineralized water is soft water- Justify. The soft water, produced by lime-soda and zeolite processes, does not contain hardness producing Ca 2+ and Mg 2+ ions, but it will contain other hand demineralised water does not contain both anions and cations. PART- B 1. What is meant by carbonate and non-carbonate hardness of water? Explain with examples. 2. Describe the principle and method involved in the determination of different types and amount of alkalinity of water. 3. What is break-point chlorination? State its significance 4. Define the term desalination with neat diagram, describe desalination by reverse osmosis method. 5. What are boiler troubles? How are they caused? Suggest steps to minimize the boiler troubles. 6. Describe briefly the different steps in the purification of water for drinking purposes what is the usage of breakpoint chlorination? (or) Out line the various stages of domestic water treatment in sequence. 7. Describe de-mineralisation process of water softening. Explain the reaction involved. 8. How is the exhausted resin regenerated in an ion-exchanger? What are the merits and demerits of ion-exchange method? 9. What are the various methods by which disinfection of domestic water is carried out. Explain. 10. Explain the EDTA method of estimation of hardness of water. 11. Explain the following boiler troubles suggesting the remedical methods: (i) Sludge and scale formation (ii)caustic embrittlement. 12. Describe briefly the various methods of internal conditioning of boiler feed water.