Removal of Impurities

8/11/2019 Removal of Impurities

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REMOVAL OF IMPURITIESFROM WATER

engineering-resource.com


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Removal of Impurities from water

Removal of suspended impurities (solids)

Removal of dissolved solids (salts)



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REMOVAL OF SUSPENDEDIMPURITIES

ScreeningSedimentation

Filteration



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SCREENING

Process of arresting large and

small floating matter by passing

raw water through a screen with

large number of perforations

For removal of large things from

water

http://www.gardeninginaustralia.com/images/rainwater_tank11.JPGhttp://www.gardeninginaustralia.com/images/rainwater_tank11.JPG


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FILTERATION

process of removing colloidal matterand most of bacterial impurities bypassing water through a bed of propersized material

suspended matter, Colloidal matter,Bacterias, Colours, and odour of water

are removed.Two types of filters are commonly used



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SEDIMENTATION

Process of removing suspended matter fromwater, by keeping it quiescent (motionless) intanks so that suspended solids may settle

down at the bottom due to force of gravity. Generally carried in continuous flow type

tanks

Three types of continuous flow type tanks Horizontal flow rectangular tank

Radial flow circular tank

Vertical flow hopper bottom tank



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Sedimentation with Co-agulation

Process of removing fine sizesuspended particles by addition ofrequisite amount of suspendedchemicals called co-agulants to waterbefore sedimentation

Common co-agulants in use are

Alum K2SO4.Al2(SO4)3.24H2O

Ferrous Sulphate FeSO47H2O



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Flocculation

Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 +3CaSO4 + 6CO2co-agulant calcium bicarbonate gelatinous floc

FeSO4 + Mg(HCO3)2 Fe(OH)2 +MgCO3 + CO2 + H2O

4Fe(OH)2 +2H2O + O2 4Fe(OH)3Dissolved oxygen Floc

o Coagulants are generally added in soln for precipitation(flocculation) and for their proper mixing mixers areemployed



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REMOVAL OF DISSOLVEDSALTS/SOFTENING

Water used for steam generationshould be pure particularly w.r.t.calcium and magnesium salts

which cause scale formation inboilers

The process of removing

hardness producing salts fromwater is known as watersoftening



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WATER SOFTENING PROCESSES

1) Lime-Soda Process

Cold Lime-Soda Process Hot Lime-Soda Process

2) Zeolite or Permutit Process

3) Demineralization / DeionizationProcess



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Lime-Soda Process

Used for softening of boiler feed Converts dissolved calcium and magnesium

salts in to insoluble salts , which settle downand are filtered.

Lime Ca(OH)2 precipitates temporaryhardness, permanent magnesium hardness,iron and aluminium salts and free acids likeCO2, H2S

Added ingredient soda ash Na2CO3 reactswith soluble permanent calcium hardness. Bicarbonate as NaHCO3 .KHCO3 also requires

lime.



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Lime-Soda Process Ca(HCO3)2 + Ca(OH)2 2CaCO3 +2H2O

Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 +Mg(OH)2 +2H2O MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2 MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4 FeSO4 + Ca(OH)2 Fe(OH)2 + CaSO4

2Fe(OH)2 + H2O + 0.5O2 2Fe(OH)3 Co2 + Ca(OH)2 CaCO3 +H2O 2HCl + Ca(OH)2 CaCl2 + H2O H2SO4 + Ca(OH)2 CaSO4 + H2O

CaCl2 + Na2CO3 CaCO3 + 2NaCl CaSO4 + Na2CO3 CaCO3 + Na2SO4 2HCO3- + Ca(OH)2 CaCO3 +H2O +CO32-



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Cold Lime-Soda Process

Lime and soda are added to raw water at roomtemperature

Finely divided precipitates are formed atroom temperature.

Co-agulants are added like alum, aluminiumsulphate, sodium aluminate.

Co-agulants hydrolyse to flocculantgelatinuous precipitate aluminium

hydrooxide.entraps fine precipitates formedby the reactions of lime and soda. Residual hardness 50 to 60 ppm.



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Hot Lime-Soda Process

Process carried out at 80 to 150CAdvantages: Reaction proceeds faster Softening capacity is increased

Precipitate and sludge formed settled rapidly.Much of the dissolved gases are driven out Viscosity lower, so easy filteration. Residual hardness 15 to 30 ppm.

Plant consists of 3 partsReaction tankConical sedimentation vesselSand filter.



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Basic ion exchange softening

Whats an ion?

An ion is an atom or molecule that has a positive or

negative electrical charge

Remember: Opposites attract



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Basic ion exchange

softening

How does ion exchange work?

Hard water

Calcium ionsResin

bed

Soft water

Sodium ions

Resin exchange site

(receptor)

exchanges salt

for hardness

Hardness washed away

during backwash

Sodium added during

regeneration from salt brineengineering-resource.com


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Zeolite or Permutit Process

Zeolite or Permutits are Complex silicatesconsisting of macro molecules of severalmetallic and non metallic oxides

Crystalline structure of chemical formula

Na2O.Al2O3.2SiO2.6H2O Simply represented as Na2ZeWhen treated with a solution, equilibrium is

formed between sodium ions held by Zeolite

and positive ions present in the solution. sothere's a tendency for the positive ions to beexchanged with sodium ions of solutiondecreases.



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Zeolite or Permutit Process

Na2Ze + Ca2+

2Na+

+ CaZe (Exhausted zeolite) Na2Ze + Mg2+ 2Na+ + MgZe

Exhausted zeolite can be reclaimed by immersing it inconc. brine soln

CaZe + 2NaCl Na2Ze +CaCl2

MgZe + 2NaCl Na2Ze +MgCl2 Reclaimed zeolite

Zeolite may be

Natural: mined, more durable Synthetic: manufactured by heating felspar,china clay

and soda ash together and cooling and crushing theresulting glass, greater exchange capacity per unit

weight, less durable engineering-resource.com


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Reactions in Zeolite or PermutitProcess

Na2Ze + Ca(HCO3)2 CaZe + 2NaHCO3 Na2Ze + Mg(HCO3)2 MgZe + 2NaHCO3 Na2Ze + CaCl2 CaZe + 2NaCl Na2Ze + MgCl2 MgZe + 2NaCl Na2Ze + CaSO4 CaZe + Na2SO4 Na2Ze + MgSO4 MgZe + Na2SO4 Exhausted zeolite bed can be regenerated by

treating it with conc Brine soln

CaZe + 2NaCl Na2Ze + CaCl2 MgZe + 2NaCl Na2Ze + MgCl2 Reclaimed zeolite



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Limitations

If supply water is turbid, remove suspendedmatter otherwise pores of zeolite bed willclog and restrict flow.

Pre-treatment required if water containscolored ions like Fe2+ or Mn3+ because theseions produce Iron or manganese zeolite whichcant be generated easily.

Mineral Acids must be neutralized with sodabefore adding water to zeolite bed as theydestroy it.



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Advantages

Zero hardness

Compact equipment

No danger of sludge formation

Automatically adjustable processfor different hardness

Clean process



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Comparison between permutit and LimeSoda Process

Permutit Method Lime-Soda Method

1. Zero hardness

2. Treated water has larger

amount of sodium salts3. Capital cost is higher

4. Operation expenses are lower

5. Cant treat acidic water

6. PlantLess space

7. Raw water must be free ofsuspended matter

8. Can operate under pressure

9. No problem of settling andsludge handling

1. 15-50ppm hardness

2. Treated water has lesser

amount of sodium salts3. Capital cost is lower

4. Operation expenses are higher

5. No such limitation

6. Plant. More space

7. No such limitation

8. Cannot operate under pressure

9. Problem of difficulty in settling

and sludge handlingengineering-resource.com


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Demineralization/ DeionizationProcess

An ion exchange resin is an insolubleacid or base which can also forminsoluble salts.

An ion exchange resin consists of crosslinked polymer network to which ionizedgroups are attached.

Ion Exchangers are of two types:Hydrogen or Cation ExchangerAnion Exchangers



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Cation Exchange Resins: Main functional groups in them are -SO3H,-COOH,-OH Most stable is -SO3H , exchanges H+ rapidly 2RSO3H + Ca2+ (RSO3)2Ca + 2H+

2ROH + Mg2+ (RO)2Mg + 2H+

Hydrogen exchangers are generally represented as RH2 RH2 + Ca2+ RCa + 2H+

RH2+ Mg2+ RMg + 2H+

Regeneration carries out by passing through an excess ofstrong acid solution.

RCa + 2HCl RH2 + CaCl2 RMg + 2HCl RH2 + MgCl2



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Anion Exchange Resins: Capable of exchanging anions Main functional groups are N(CH3)2+, -OH-, -NH2,

NHCH3

N(CH3)2+

, -OH-

, are most stable and can operate inacidic alkaline solution. Represented as R(OH)2 R(OH)2 + SO42- R SO4+ 2 OH-

R(OH)2 + 2Cl - RCl2 + 2 OH-

Regenrated by passing alkaline soln RCl2 + 2 NaOH R(OH)2 + 2NaCl R SO4 + 2 NaOH R(OH)2 + Na2SO4



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Consists in passing hard water firstthrough cation exchanger bed

Then anion exchanger bed

And then through a degasifier


Removal of Impurities

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