Ui Ch 1 Hardness 6g2m16

Ch. 1 Water As Basic Utility -

Prepared By:Dhaval N.Yadav Lecturer (CL-II), Chemical Engineering Department, Shri K. J. Polytechnic, Bharuch.

Topics to be Covered 

Types of Water



Method of Removal of Hardness



Various methods of Purification of Raw Water

:

Screening,

Coagulation, Sterilization

Sedimentation,

Types of Water Water

Hard Water

Soft Water

Deminer alised

Water

Boiler Feed Water

Soft Water The water which does not contain any of the Ca or Mg salt dissolved in it is called Soft Water.  It may contain certain salts like Sodium or Potassium dissolved in it.  A sample of soft water when used with soap forms lather. 

Hard Water Hardness in water is that “Characteristics which Prevents The Lathering of Soap.”  This is because of presence of certain salts of Calcium, Magnesium and other Heavy Metals dissolved in water. 

Continued… A sample of hard water when used with soap does not produce lather, but on the other end produces white scum or precipitate.  This is formed because of formation of Insoluble Soaps of Calcium and Magnesium. 

Sludge Slimy loose precipitate called sludge suspended in water

Water Boiler wall

Sludge is a soft, loose and slimy precipitate formed within the boiler. It can be easily scrapped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. It is formed by substances which have greater solubility's in hot water than in cold water, e.g. MgCO3, MgCl2, CaCl2, MgSO4 etc., Remedy: Sludges can be removed using wire brush or mild acid

7

Scale

Hard adherent coating on inner walls of boiler

water Boiler wall

Scales are hard substances which sticks very firmly to the inner surfaces of the boiler wall. Scales are difficult to remove even with the help of a hammer and chisel. Examples: CaSO4, CaCO3, Mg(OH)2 8

Why to worry? 

Hard water does cause Soap Scum, Clogs Pipes and Clogs Boilers as Lime Scale.

Types of Hardness Hardness Temporary or Carbonate

Permanent Or Non-Carbonate

Temporary Hardness Dissolved bicarbonate (HCO3-) of Ca, Mg.  It is mostly destroyed by mere boiling of water, when bicarbonates are decomposed yielding insoluble carbonates. Ca(HCO3)2 Heat CaCO3 + H2O + CO2 

Calcium bicarbonate

Mg(HCO3)2 Magnesium Bicarbonate



Calcium Carbonate

Heat

Mg(OH)2 + 2CO2 Magnesium hydroxide

Calcium/Magnesium Carbonates thus formed being almost insoluble, are deposited as a scale at the bottom of vessel, while carbon dioxide escapes out.

Permanent Hardness 

Presence of Chlorides, Sulfates of Calcium, Magnesium, Iron and other heavy Non Carbonate Hardness is due to 2C17H35COONa + CaCl2 Sodium stearate

Hardness

(sodium soap)

(sodium soap)



Calcium stearate (Insoluble)

2C17H35COONa + MgSO4 Sodium stearate

(C17H35COO)2Ca + 2NaCl

Hardness

(C17H35COO)2Mg + 2Na2SO4 Magnesium stearate (Insoluble)

Cannot be removed by simple boiling it requires the addition of certain chemicals and process.

Permanent Hardness 

Presence of Chlorides, Sulfates of Calcium, Magnesium, Iron and other heavy Non Carbonate Hardness is due to 2C17H35COONa + CaCl2 Sodium stearate

Hardness

(sodium soap)

(sodium soap)



Calcium stearate (Insoluble)

2C17H35COONa + MgSO4 Sodium stearate

(C17H35COO)2Ca + 2NaCl

Hardness

(C17H35COO)2Mg + 2Na2SO4 Magnesium stearate (Insoluble)

Cannot be removed by simple boiling it requires the addition of certain chemicals and process.

Draw backs (or) Disadvantages of Hard Water Domestic Use

Industrial Use

1. Washing

1. Steam generation in Boilers

2. Bathing The sticky precipitate adheres on the fabric/cloth and gives spots and streaks. Fe salts stain the cloths.

3. Drinking 4. Cooking

Produces sticky scum on the bath tub and the body

Bad to the digestive system and calcium oxalate formation is possible in urinary tracts

Requires more fuel and time. Certains food don’t cook soft and also gives unpleasant taste 14

External Water Treatment Methods E x t e r n a l



The widely used methods are: Lime Soda Process Zeolite (or Permutit Process) Ion Exchange Process Phosphate Process (Conditioning) Internal Treatment

Lime Soda Process Principle: It is a process in which Lime (Ca(OH)2) and Soda (Na2CO3) are added to the hard water to convert the Soluble Ca and Mg Salts to insoluble compounds by a chemical reaction. The CaCO3 and Mg(OH)2 so precipitated are filtered off and removed easily.

LS Process Continued…. It is one of the most oldest process used for water softening.  Because of technology advancement it is now no more used.  It is further classified as Cold LS Process Hot LS Process 

LS Process Working Step-1: 

In this process a calculated quantity of Ca(OH)2 (lime) and Na2CO3 (soda) are mixed with water at room temperature and added to the hard water. The following reactions takes place depending on the nature of hardness If it is permanent hardness and due to calcium salt Ca2+ + Na2CO3

CaCO3 + 2Na+ (soda) Slimy Suspended Precipitate

If it is due to Magnesium salt Mg2+ + Ca(OH)2

Mg(OH)2 + Ca2+ (lime) Slimy Suspended Precipitate

LS Process Working If it is Temporary hardness and due to calcium salt

Ca(HCO3)2 + Ca(OH)2

2CaCO3 + 2H2O Slimy suspended precipitate

If it is due to Magnesium salt Mg(HCO3)2 + 2Ca(OH)2

2CaCO3 + Mg(OH)2 + 2H2O Slimy suspended precipitates

LS Process Working Step-2: 

The precipitates CaCO3 and Mg(OH)2 are very fine and forms sludge like precipitates in the boiler water and are difficult to remove because it does not settle easily making it difficult to filter. Finally reduces the efficiency of the boiler.



Therefore, it is essential to add small amount of Coagulant (such as Alum, Aluminium Sulfate, Sodium Aluminate etc) which hydrolyses to flocculent precipitate of Al(OH)3 which entraps the fine precipitates.

Step-2: 

When coagulants are added flocculation takes place followed by the formation of flocculants. NaAlO2 + 2H2O Coagulant

Al2(SO4)3 Aluminium sulfate

+ 3 Ca(HCO3)2 Hard water sample

NaOH + Al(OH)3 Flocculent- Gelatinous precipitate which entraps the fine precipitates of CaCO3 and Mg(OH)2

2Al(OH)3

+ CaSO4 + CO2

Flocculent- Gelatinous precipitate which entraps the fine precipitates of CaCO3 and Mg(OH)2

The Al(OH)3 formed by the addition of coagulants initiates the process of flocculation and entraps the fine precipitates and becomes heavy. The heavier flocs thus settles at the bottom and filtered off easily.

Continuous cold lime soda softener

Chemicals (soda+lime+ coagulant) feed

Hard water feed Softened water

Wood fiber filter

Stirrer paddles

Sedimented sludge (CaCO3, Mg(OH)2 22

Hot LS Process 

In this process a calculated quantity of Ca(OH)2 (lime) and Na2CO3 (soda) are mixed with hot water at a temperature range of 80 to 150oC and added to the hard water.



It consists of : Reaction tank: water, chemicals and steam are mixed Conical sedimentation tank : sludge settles down Sand filter : complete removal of sludge from the softwater is ensured

Continuous Hot Lime soda Process

Hard water feed

Super heated steam

Chemicals feed (lime and soda)

Reaction tank Soft water

Conical sedimentation tank

Precipitated sludge (CaCO3, Mg(OH)2

Sludge outlet

Filtered soft water

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Advantages & Disadvantages Advantages 1. It is very economical compared to other methods 2. Iron and manganese salts are also removed by this process 3. It increases the pH of the softened water hence corrosion is minimized also pathogenic bacteria Disadvantages 1. Disposal of large amount of sludge (insoluble precipitates) poses a problem 2. This can remove hardness to the extent of 15ppm which is not good for boilers

Zeolite (or Permutit Process) 

Zeolites are micro porous, alumino silicate minerals commonly used as commercial Adsorbents and catalyst.



The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material STILBITE, it produced large amounts of steam from water that had been adsorbed by the material. Based on this, he called the material zeolite, from the Greek ζέω (zéō), meaning "to boil" and λίθος (líthos), meaning "stone"

Zeolite (or Permutit Process) • Zeolite is a Hydrated Sodium Alumino Silicate, capable of exchanging reversibly its sodium ions for hardness producing ions in water. •The general chemical structure of zeolite is given below Na2O.Al2O3.xSiO2.yH2O (x = 2-10 and y = 2-6) •Why synthetic zeolite is better than natural zeolite for the softening of water? Ans: Natural zeolites are non-porous Micro pores of Zeolite

Porous Structure of zeolite

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Zeolite softener Hard water in

Hard water spray

Zeolite bed Gravel

Injector

Softened water NaCl storage

To sink

28

Process of softening by Zeolite method For the purification of water by the zeolite softener, hard water is ed through the zeolite bed at a specified rate. The hardness causing ions such as Ca2+, Mg2+ are retained by the zeolite bed as CaZe and MgZe respectively; while the outgoing water contains sodium salts. The following reactions takes place during softening process To remove temporary hardness

Na2Ze + Ca(HCO3)2

Scale formation

CaZe + 2NaHCO3

Hardness

Na2Ze + Mg(HCO3)2

MgZe

+ 2NaHCO3

To remove permanent hardness

water

Na2Ze + CaCl2

CaZe + 2NaCl

Na2Ze + MgSO4

MgZe

+ Na2SO4

Regeneration of Zeolite Bed CaZe (or) MgZe + 2NaCl Used Zeolite

10% brine solution

Na2Ze + CaCl2 (MgSO4) Regenerated Zeolite

Washings drained

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Limitations of Zeolite process 1.

If the water is turbid ---- then the turbidity causing particles clogs the pores of the making it inactive

Zeolite and

2.

The ions such as Mn2+ and Fe2+ forms stable complex Zeolite which can not be regenerated that easily as both metal ions bind strongly and irreversibly to the zeolite structure.

3.

Any acid present in water (acidic water) should be neutralized with soda before itting the water to the plant, since acid will hydrolyze SiO2 forming silicic acid

Advantages of Zeolite process 1.

Soft water of 10-15 ppm can be produced by this method

2.

The equipment occupies less space

3.

No impurities are precipitated, hence no danger of sludge formation in the treated water

4.

It does not require more time and more skill

Disadvantages of Zeolite process

1.

Soft water contains more sodium salts than in lime soda process

2.

It replaces only Ca2+ and Mg2+ with Na+ but leaves all the other ions like HCO3- and CO32- in the softened water (then it may form NaHCO3 and Na2CO3 which releases CO2 when the water is boiled and causes corrosion)

3.

It also causes caustic embitterment when sodium carbonate hydrolyses to give NaOH 30

Ion-Exchange/ Deionization/ Demineralization Process

ION EXCHANGE RESINS are insoluble, cross linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chain is responsible for the “ionexchange” properties. Cation exchange Resin

Resin after treatment 31





The resins containing acidic functional groups (COOH, -SO3H etc) are capable of exchanging their H+ ions with other cations, which comes in their ; whereas those containing basic functional groups ( -NH2, =NH as hydrochlorides) are capable of exchanging their anions with other ions, which comes in their . Based on the above fact the resins are classified into two types Cation Exchange Resin (Rh+) – Strongly acidic (SO3H+) and weakly acidic (COO-H+) cation exchange resins Anion Exchange resin (ROH-) – Strongly basic (R4N+OH-) and weakly basic (RNH2+OH-) anion exchange resins

Structure 



The Cation Exchange Resins are mainly Styrene Divinyl Benzene Copolymers which on Sulponation or Carboxylation become capable to exchange their hydrogen ions with the cations in water. The Anion Exchange Resins are mainly Styrene Divinyl Benzene or Amine Formaldehyde Copolymers which conatin amino or quaternary ammonium or quaternary phosponium or tertiary sulphonium group as integral part of resin matrix. These after treatment with dilute NaOH solution, becomes capable to exchange their OH- anions with anions in water.

Structure of Resin Cation exchange resin

Anion exchange resin

R = CH3 34

Ion exchange purifier or softener

Hard water

Cation exchange Resin

Gravel bed

Anion exchange Resin

Injector

Injector

Acid solution for regeneration of resin

Wastages to sink Wastages to sink

Alkaline solution for regeneration of resin

pump

Soft water

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Process Description Step-1 (ing through Cation Exchange Resin)  The hard water is ed through cation exchange column which removes all the cations like Ca+2, Mg+2 etc from it and equivalent amount of H+ ions are released from this column into water. 2RH+ + Ca2+ (hard water)

R2Ca2+ + 2 H+

2RH+ + Mg2+ (hard water)

R2Mg2+ + 2 H+

Continued… Step-2 (ing through Anion Exchange Resin)  The hard water is ed through anion exchange column, which removes all the anions like SO42-, Cl- etc. present in the water and equivalent amount of OH- ions are released from this column to water.



2ROH- + SO42- (hard water)

R2SO42+ + 2 OH-

2ROH- + Cl- (hard water)

R2Cl- + 2 OH-

H+ and OH- ions released combine to produce water molecule. H+ + OH-

H 2O

Regeneration Regeneration of Cation Exchange Resin R2Ca2+ + 2H+ (dil. HCl (or) H2SO4)

2 RH+ + Ca2+ (CaCl2, washings)

The column is washed with deionised water and washing (Ca+2 , Mg+2 and SO4-2 or Cl- ions) is ed to sink or drain.

Regeneration of Cation Exchange Resin R2SO42- + 2OH- (dil. NaOH)

2 ROH- + SO42- (Na2SO4, washings)

The column is washed with deionised water and washing (Na+ and SO4-2 or Cl- ions) is ed to sink or drain.

Advantages & Disadvantages Advantages  It is used to soften highly acidic or highly alkaline waters.  It provides water of very low hardness (nearly 2ppm). Disadvantages  Equipment is costly.  Expensive chemicals are required.  It water is turbid then the output (soft water) is reduced.Turbidity of raw water must be below 10 ppm.

Phosphate Treatment 



The zeolite and ion-exchange process removes most of the Ca and Mg from water. The traces of these impurities in water may cause scale formation in boiler. In high pressure boiler, scale formation can be avoided by addition of phosphate to boiling water. The soluble phosphates are: Disodium hydrogen phosphate : Na2HPO4 Sodium dihydrogen phosphate : Na2H2PO4 Trisodium phosphate : Na3PO4 Sodium ortho phosphate : Na2P2O7

Phosphate Treatment 



These sodium phosphate reacts with calcium and magnesium impurities to convert them to their insoluble phosphate as Ca(PO4)2 which are deposited and removed as sludge. This avoids scale formation. 3 CaSO4 + 2Na3PO4 → Ca3(PO4)2 + 3Na2SO4 3 CaCO3 + 2Na2PO4 → Ca3(PO4)2 + Na2CO3