Unit_I-L6.pptxchemistry engineeringmechanical

Chemicals

1. Prevention of Scale/Sludge Formation By “Water Softening”

2. Water Softening

3. Principles of Internal Treatment In the process, and ion is prohibited to exhibit in its original character, by complexing or converting it into other more soluble salts by adding appropriate reagents.

4. Internal Treatment

5. Consequently, deposition of CaSO 4 as scale does not take place and calcium is precipitated As loose sludge of CaSO 4 , which can be removed by blow-down operation

6. 6 7

7. External Treatment Zeolite (Permutit) Process Lime-Soda Process Ion-Exchange Process or De-ionization Process or Demineralization Process The external treatment of water is carried out before it is fed into the boiler. This treatment prevent boiler troubles .

8. Zeolite (Permutit) Process

9. Zeolite Softener A zeolite softener consist of a steel tank packed with a thick layers of Permutit. The water enters the top and passes through the bed of zeolites.

10. It operates on the principle involving alternate cycles of - softening run and regeneration run . The water is softened by passing it through the zeolite bed where Ca and Mg ions are removed from the water by zeolite and simultaneously releasing equivalent amount of Na ions in exchange. The regeneration step involves back washing, salting and rinsing before use. Principle:

11. For softening of water by zeolite process, hard water is percolated at specified rate through a bed of zeolite , housed in a cylindrical unit. Zeolite holds sodium ion loosely and can be simply represented as Na 2 Ze where Ze represents insoluble radical framework. The hardness causing ions (Ca 2+ , Mg 2+ etc.) are retained by zeolite as CaZe and MgZe respectively, while the outgoing water contains sodium salts. In the process the water becomes free from calcium and magnesium ions , the main hardness causing cations. Process: ( i ) softening run (ii) Regeneration Run ( i ) softening run

12. Process: General Diagram Sand Fine Gravels Coarse Gravels ( i ) softening run

13. Softened Water

14. Chemical Reactions: Reactions taking place during the softening process are : Small quantity of Fe and Mn present as the divalent bicarbonates may also get removed simultaneously: Fe(HCO 3 ) 2 + Na 2 Ze FeZe + 2NaHCO 3 Mn (HCO 3 ) 2 + Na 2 Ze MnZe + 2NaHCO 3

15. After sometime the zeolite is completely converted into calcium and magnesium zeolites. At this time the bed of zeolite ceases to soften water i.e. It gets exhausted. At this stage the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated sodium chloride(brine) solution when the following reactions takes place CaZe/ MgZe + 2NaCl → Na 2 Ze + CaCl 2 / MgCl 2 exhausted zeolite reclaimed zeolite washings (ii) Regeneration Run

16. Over All Representation softening of water by zeolite process (ii) Regeneration Run ( i ) softening run

17. Example 1: An exhausted Zeolite softener was regenerated by passing 150 litres of NaCl solution, having a strength of 150 g/l of NaCl. How many litres of hard water sample, having hardness of 600 ppm can be softened, using this softener? Solution : Hardness is expressed in terms of equivalents of CaCO 3 as ppm . Step I :Find the total amount of NaCl used for regeneration of zeolite Amount of NaCl used for regeneration of zeolite = 150 g NaCl in 1 litre solution 1 litre of NaCl solution contains 150 g of NaCl Therefore, 150 litres solution contains = 150 x 150g = 22500 g of NaCl = 22,500 x 1000 = 22500000 m g Na Cl Step II : Covert the amount of NaCl into CaCO 3 form Quantity of NaCl in terms of CaCO 3 equivalent hardness = 1 mole CaCO 3 = 2 Moles of NaCl (2Na+ = Ca 2+ ) 100mg of CaCO 3 = 2x 58.5 = 117 mg of NaCl ? = 22500000 mg of NaCl = 22500000 x 100 (CaCO 3 equivalent hardness) 117 = 19230750 mg CaCO3 eq. hardness. Step III : Calculate the total quantity of water softened Given that 1 litre of hard water contains 600 ppm hardness ≡ 600 mg of CaCO 3 = 0.6 g of CaCO 3 ? Litre of water = 19230750 mg of CaCO 3 = 19230750 / 600 = 32051.25 litres

18. Example 2 : A zeolite bed is exhausted by softening 3500 litres of water sample requires10 litres of 15% NaCl solution for regeneration. Calculate the hardness of water sample. Solution : Hardness is expressed in terms of equivalents of CaCO3 as parts per million. Step 1 : Find the total amount of NaCl used for regeneration of zeolite Amount of NaCl used for regeneration = 15%=15 g NaCl in 100g solution. OR 150g NaCl in 1000ml solution= 15% 1 Litre NaCl solution contains 150 g NaCl Therefore, 10 litres solution contains = 10 x 150g =1500 g of NaCl= 1500000mg of NaCl Step II : Convert the amount of NaCl into CaCO 3 form Molecular weight of CaCO 3 = 100 and that of NaCl = 58.5 Quantity of NaCl in terms of CaCO3 equivalent hardness = 1 mole of CaCO3 = 2 Moles of NaCl (2Na+ = Ca 2+ ) 100mg CaCO3 = 2x 58.5 = 117mg of NaCl ? = 1500000 mg of NaCl = 1500000 x 100 (CaCO3 equivalent hardness) 117 (twice NaCl Molecular weight) = 1500000 x 0.8547 = 1282050 mg of CaCO3 equivalent hardness. Step III : Calculate the hardness of water softened Quantity of NaCl used = Hardness of water Hardness of 3500 litres of water = 1282050 mg CaCO3 eq. hardness Therefore hardness of 1 litre water = 1282050/ 3500 = 366.3 mg/lit.