Watertechnology-2

Applied Chemistry CODE NO: 07A1BS07/07A1BS04 I B.TECH CIVIL ENGINEERING Unit No: II Nos. of slides: 117

Applied Chemistry Water Technology-II Term: 2008-09 Unit-2 Power Point Presentations Text Books:  A text book of Engineering Chemistry by Jain & Jain,  Chemistry of Engineering Materials by C.P. Murthy, C.V. Agarwal and A. Naidu

INDEX UNIT-I PPTS Srl. No.

PPT

Module as per

Lecture

Session Planner No. Slide No. -----------------------------------------------------------------------------------------------1. Boiler Troubles L-1,2 L1,2-1 to L1,2-28 2. Water Internal Treatment L-3 L3-1 to L3-18 3. Water External Treatment Lime-Soda Process L-4,5 L4,51 to L4,5-18 4. Zeolite Process L-6 L6-1 to L6-19 5. Ion- Exchange Process L-7 L7-1 to L7-33 6. Numerical Problems L-8,9 L8,91 to L8,9-33

BOILER TROUBLES L1,2-1

In treatment of water complete elimination of all the impurities is not possible. The impurity that gives rise to certain troubles will be removed to certain extent. In modern pressure boilers and laboratories, water purer than the distilled water is required. Some of the boiler troubles caused by the use of unsuitable water are..

L1,2-2

Boiler troubles mainly 4 types they are.. 1. Carryover

2. Scale formation 3. Boiler corrosion and 4. Caustic embrittlement L1,2-3

Carryover is mainly 2 types

1. 2. L1,2-4

Priming Foaming

Priming 





Priming may defined as the violent or rapid boiling of water occurring in the boiler which results in carrying out of water with steam in in the form of a spray. When a boiler is producing steam rapidly, some particles of liquid water are carried along with the steam. This process of wet steam formation is called priming.

L1,2-5



Priming mainly caused due to very high water level. The presence of large amount of dissolved solids, high steam velocities, sudden increase in steam production rate. Sudden steam demand which leads to sudden boiling, presence of excessive foam filling the foam spare, and due to faulty boiler design.

L1,2-6



priming can be controlled by proper boiler design, fitting mechanical steam purifier, avoiding rapid change in steamrate, proper evaporation and adequate heating surfaces, uniform distribution of fuel and providing anti priming pipes, keeping the water level low and avoid sudden steam demands. Efficient softening and filtration of the boiler feed water.

L1,2-7

Foaming 





Foaming is the formation of small but stable bubbles above the surface. The main reason for foaming is being presence of fatty acids and other impurities. Foaming can be controlled by using anti-foaming chemicals, removal of concentrated boiler water and replacing it by fresh feed water. Removing oil from boiler water by adding compounds like sodium

L1,2-8

Scale formation: Some hard, sticky, adherent deposits formed on the inner surface of the boilers are known as Scales.  Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the wall surface and are difficult to remove with chisel and hammer.  Generally scales are formed due to decomposition of calcium bicarbonates, decomposition of calcium sulphates, presence of silica and hydrolysis of magnesium salts.  L1,2-9 Scales formation can be prevented by 

Boiler Corrosion: 





The chemical or electro chemical eating away of metal by its environment in a boiler is known as boiler corrosion. The main reason for this problem is the presence of excess of oxygen in water. It can be prevented by mechanical deaerator, pre-heating and chemical treatment.

L1,2-

Caustic Embrittlement









The formation of brittle and incrystalline cracks in the boiler shell is called caustic embrittlement. It is a type of boiler corrosion and the main reason for this, is the presence of alkalimetal carbonates and bicarbonates in feed water and also the presence of sodium sulphate. In lime-soda process, it is likely that, some residual Na2CO3 is still present in the softened water. This Na2CO3 decomposes to give NaOH and CO2, due to which the boiler water becomes “Caustic”.

L1,2-







This caustic water flows inside the boiler and causes some minutes haircracks, by capillary action. On evaporation of water, the dissolved caustic soda increases its concentration which attacks the surrounding area, thereby dissolving Iron of boiler as Sodium ferroate. This causes embrittlement of boiler parts such as bends, joints, reverts etc, due to which the boiler gets fail.

L1,2-

Caustic cracking can be explained by considering the following concentration cell structure..

|



Iron at bends,+ Iron at

Concentrate

joints, reverts plane surfaces

L1,2-

|

Dilute NaOH solution

NaOH solution

|-

caustic embrittlement can be prevented by 





By maintaining the pH value of water and neutralization of alkali. By using Sodium Phosphate as softening reagents, in the external treatment of boilers. Caustic embrittlement can also be prevented by adding Tannin or Lignin or Sodium sulphate which prevents the infiltration of caustic-soda solution blocking

L1,2-

FORMATION OF SLUDGE IN BOILERS

 In

boilers, because of continuous evaporation of water, the concentration of salts increase progressively and after the saturation point is reached, precipitate form on the inner walls of boiler.

SLUDGE 





Sludge is a soft, loose and slimy precipitate formed within the boiler. It is formed at comparatively colder portions of the boiler and collects in the area where flow rate is slow. These are formed by substances which have greater solubilities in hot water than in cold-water. Eg. MgCO3, MgCl2, CaCl2, MgSO4.

DIS-ADVANTAGES 



As the sludge’s are poor conductor of heat they cause loss of heat. The working of the boiler is disturbed because of chocking of pipes by the sludge.

PREVENTION  

By using well softened water. By drawing off a portion of concentrated water frequently.

ASSIGNMENT



Write a short notes on sludge formation in boilers.

SCALES Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the inner wall surface and are difficult to remove with chisel and hammer.

Scale formed inner side of the water pipe

causes of scale formation Decomposition of calcium bicarbonate:  Ca(HCO3)2  CaCO3 + H2O + CO2  In low pressure boilers, CaCO3 causes scale formation.  In High pressure boilers, CaCO3 becomes soluble.  CaCO3 + H2O  Ca(OH)2 + CO2

Decomposition of calcium sulphate: The

solubility of CaSO4 in water decreases with rise of Temperature. In super heated water CaSO4 is insoluble. This is the main cause in highpressure boilers.

Hydrolysis of Magnesium salts  Dissolved

Magnesium salts undergo hydrolysis forming Mg(OH)2 precipitate.

 MgCl2

+ 2H2O - Mg(OH)2 + 2 HCl

 Mg(OH)2

so found by hydrolysis of Magnesium salts is a soft type of scale.

Presence of Silica Silica

present in small quantities deposits as silicates like CaSiO3 and MgSiO3. These are very difficult to remove.

Disadvantages  Wastage

of fuel: The scale formation causes decreases of heat transfer. As a result over heating is required this causes consumption of fuel.  Danger of Explosion: The hot scale cracks because of expansion and water suddenly comes in contact with overheated Iron plates. This causes in formation of large amount o steam suddenly. This results high pressure causing boiler to burst.

PREVENTION  External

treatment: Efficient softening of water is to be carried out.

 Internal

treatment: Suitable chemicals are added to the boiler water either to precipitate or to convert the scale into compound

INTERNAL TREATEMENT OF HARD WATER

 Internal

treatment of boiler water is carried out by adding proper chemicals to precipitate the scale forming impurities in the form of sludge and to convert the scale forming chemicals into compounds which will stay in dissolved form in water.  This process is mainly used as a corrective treatment to remove the slight residual hardness and also sometimes to remove the corrosive

Some of the internal treatment methods used for the removed of scale formation in boilers are.  COLLODIAL

CONDITIONING  PHOSPHATE CONDITIONING  CARBONATE CONDITIONING  CALGON CONDITIONING  SODIUM ALUMINATE CONDITIONING

COLLODIAL CONDITIONING  The

addition of organic substances such as Kerosene, tannin, Gel etc., to the surface in low pressure boilers may prevent the scale formation.  These substances gets coated over the scale forming precipitates and gives a loose and non-sticky precipitates which can be removed by using blow-down operation.

PHOSPHATE CONDITIONING  The

addition of sodium phosphate in hard water reacts with the hardness causing agents and gives calcium and magnesium phosphates which are soft and non-adhere and can be removed easily by blow-down operation.  In this way, scale formation is removed in high-pressure boilers.

PHOSPHATE CONDITIONING chemical equation is

3CaCl2 + 2 Na3PO4

Ca3(PO4)2 + 6NaCl

CARBONATE CONDITIONING 

 

In low-pressure boilers, scale-formation can be avoided by adding sodium carbonate to boiler water, when CaSO4 is converted into calcium carbonate in equilibrium. CaSO4 + Na2SO4  CaCO3 + Na2SO4 Consequently, deposition of CaSO4 as scale doesn’t take place and calcium is precipitated as loose sludge of CaCO3 which can be removed by blow-down operation.

CALGON CONDITIONING  Involves

in adding calgon to boiler water. it prevents the scale and sludge formation by forming soluble complex compound with CaSO4.

 calgon

= Sodium hexa meta phosphate

Calgon formulae is 

Calgon=

(NaPO3)6

Na2[Na4(PO3)6] or 2Na+ +[Na4P6O18] -2

equation

2CaSO4 [ Na4P6O18]

[Ca2P6O18]

-2

-2

+ 2Na2SO4

SODIUM ALUMINATE CONDITIONING  Sodium

aluminate gets hydrolyzed yielding NaOH and a gelatinous precipitate of aluminum hydroxide. Thus NaAlO2 + 2H2O  NaOH + Al(OH)3 The sodium hydroxide, so-formed, precipitates some of the magnesium as Mg(OH)2, i.e.,  MgCl2 + 2NaOH  Mg(OH)2 + 2NaCl

The flocculent precipitate of Mg(OH)2 plus aluminum hydroxide, produced inside the boiler, entraps finely suspended and colloidal impurities, including oil drops and silica. The loose precipitate can be removed by predetermined blow-down operation

EXTERNAL TREATMENT OF WATER

EXTERNAL TREATMENT OF WATER  Water

used for industrial purposes such as for steam generation, should be sufficiently pure.  It should, therefore be freed from hardnessproducing salts before put to use.  The process of removing hardness-producing salts from water is known as softening of water.

In industry main three external methods employed for softening of water, they are.

1. 2. 3.

Lime-Soda process Zeolite process Ion-Exchange process

LIME-SODA PROCESS

In this method,  The

soluble calcium and magnesium salts in water are chemically converted in to insoluble compounds, by adding calculated amount of Lime and Soda.  CaCO3 and Mg(OH)2 so precipitated, these precipitates are filtered off.

Lime soda process mainly two types, they are  Cold

Lime-soda process  Hot Lime-soda process.

COLD LIME SODA PROCESS In this method, calculated quantity of chemical like lime and soda are mixed with water at room temperature.  At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily.  Consequently, it is essential to add small amounts of coagulants like alum, aluminum sulphate, sodium aluminate, etc.

 Which

hydrolyze to flocculent, gelatinous precipitate of aluminum hydroxide, and entraps fine precipitates. Use of sodium aluminate as coagulant also helps the removal of silica as well as oil, if present in water. Cold L-S process provides water, containing a residual hardness of 50 to 60 ppm.  NaAlO2 + 2H2O ---- NaOH + Al(OH)3  Al(SO4)3 + 3Ca(HCO3)2  2Al(OH)3 + 3CaSO4 + 6CO2

METHOD OF TREATMENT: Raw water and calculated quantities of chemicals ( Lime + soda + Coagulants) are fed from the top into the inner vertical circular chamber, fitted with a vertical rotating shat carrying a number of paddles.  As the raw water and chemicals flow down, there is a vigorous stirring and continuous mixing, whereby softening of water reaches up.  The softened water comes into the outer co-axial chamber, it rises upwards. 

METHOD OF TREATMENT: The heavy sludge or precipitated floc settles down the outer chamber by the time the softened water reaches up.  The softened water then passes through a filtering media this is usually made of wood fibres to ensure complete removal of sludge.  Filtered soft water finally flows out continuously through the outlet at the top.  Sludge settling at the bottom of the outer chamber is drawn of occasionally. 

HOT LIME-SODA PROCESS:     



The reaction proceeds faster The softening capacity f hot process is increased to many fold The precipitate and sludge formed settle down rapidly and hence, no coagulants are needed Much of the dissolved gases such as CO2 and air driven out of the water Viscosity of softened water is lower, so filtration of water becomes much easier. this in-turn increases the filtering capacity of filters, and Hot lime-soda process produces water of comparatively lower residual hardness of 15 to 30 ppm.

 Hot

lime-soda plant consists essentially of three parts  A ‘Reaction tank’ in which raw water, chemicals and steam are thoroughly mixed.  A ‘Conical sedimentation vessel’ in which sludge settles down, and  A ‘sand filter’ which ensures completes removal of sludge from the softened water.

ADVANTAGES OF LIME-SODA PROCESS:  



 

It is very economical If this process is combined with sedimentation with coagulation, lesser amounts o coagulants shall be needed. The process increases the pH value of the treatedwater; thereby corrosion of the distribution pipes is reduced. Besides the removal of harness, the quantity of minerals in the water is reduced. To certain extent, iron and manganese are also removed from

DIS-ADVANTAGES OF LIME-SODA PROCESS: 





For efficient and economical softening, careful operation and skilled supervision is required. Disposal of large amounts of sludge or insoluble precipitates poses a problem. However, the sludge may be disposed off in raising low-lying areas of the city. This can remove hardness only upto 15 ppm, which is not good for boilers.

ZEOLITE PROCESS

Zeolites are also known as Permutits;

Zeolites are mainly 2 types Natural

zeolites Synthetic Zeolites

Natural zeolites

Natural zeolites are nonporous, eg. NatroliteNa2O.Al2O3.4SiO2.2H2O.

Synthetic Zeolites 





Synthetic Zeolites are porous and possess gel structure. Sodium zeolites are generally used for softening of water and are simply represented as Na2Ze, where ‘Ze’ stands for insoluble zeolite. In the process, when hard water is passed through a bed of zeolite placed in an closed cylinder, the hardness causing ions like Ca+2 and Mg+2 ions are taken up by the zeolite. Sodium salts are released during the

Process:  For

softening of water by zeolite process, hard water is percolated at a specified rate through a bed retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts.

The various reactions taking place may be.. 



  

Na2Ze + Ca(HCO3)2  CaZe + 2NaHCO3 Na2Ze + Mg(HCO3)2  MgZe + 2NaHCO3 Na2Ze + MgCl2  MgZe + 2NaCl Na2Ze + CaCl2  CaZe + 2NaCl Na2Ze + MgSO4 MgZe + 2Na2SO4

Hence zeolite process removes the hardness of water effectively

Regeneration 

 

After some time, the Zeolite is completely converted into calcium and magnesium zeolites and it 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 Brine solution (10% NaCl). CaZe + 2 NaCl  Na2Ze + CaCl2 MgZe + 2 NaCl  Na2Ze + MgCl2

Advantages 

 

   

It removes the hardness completely and water of about 10 ppm hardness is produced. The equipment used is compact, occupying a small space. No impurities are precipitated, so there is no danger of sludge formation in the treated water at a later stage. The process automatically adjusts itself for variation in hardness of incoming water. It is quite clean It requires less time for softening. It requires less skill for maintenance as well as operation.

Disadvantages The treated water contains more sodium salts than in lime-soda process.  This method causes caustic embrittlement.  High turbidity water cannot be treated efficiently by this method. 

ION- EXCHANGE PROCESS

Ion exchange process also known as demineralization or de-ionization process.

 In

De-ionization process all the ions present in water are eliminated by using ion-exchange resins.  Basically resins with acidic functional group are capable of exchanging H+ ions with other cations.  Resins with functional groups are capable of exchanging OH- ions with other anions.

Resins are classified as

 Cation

Exchange Resins  Anion Exchange Resins

 Cation

Exchange Resins: These are mainly styrene divinyl benzene copolymers, which on sulphonation or carboxylation.  These are capable of exchanging their hydrogen ions with cations in water.

Anion

Exchange Resins: these are capable of exchanging their OHions with anions in water.

 In

ion-exchange process, hard water is allowed to pass through cation exchange resins, which remove Ca+2 and Mg+2 ions and exchange equivalent amount of H+ ions.  Anions exchange resins remove bicarbonates, chlorides and sulphates from water exchange equivalent amount of Oh ions.  Thus by passing hard water through cation hardness is observed by the following reactions.

Cation Exchange Resins  2RH+

+ Ca+2 - R2Ca+2 + 2H+  2RH+ + Mg+2  R2Mg+2 + 2H+ (RH+ = cation exchange resin)

Anion Exchange Resin

R’OH + Ca+2  R’Cl- + OH2R’OH- + S-2  R2S-2 + 2OH2R’OH + CO-2  R’2CO-2 + 2OH(R’OH = anion exchange resin)

 H-

And OH- ions, thus released in water from respective cation and anion exchange columns, get combined to produce water molecules.  H+ + OH-  H2O  The water coming out from the exchanger is ion free i.e., free from anions and cations. Thus water of zero hardness is obtained.

REGENERATION: 

 



When cation exchanger losses capacity of producing H+ ions and exchanger losses capacity of producing OH- ions, they are said to be exhausted. The exhausted cation exchanger is regenerated by passing it through dilute sulphruric acid. R2Ca+2 + 2H+  2RH+ + Ca+2 The exhausted anion exchanger is regenerated by passing a dilute solution of NaOH. R2SO-2 + 2OH  2R’OH- + SO-2

Merits of Ion-exchange process: 

The process can be used to soften highly acidic or alkaline water. It produces water of very low hardness (2 ppm So it is very good for treating water for use in high-pressure boilers.

Demerits of Ion-exchange process:  The

equipment is costly and more expensive chemicals are needed.  If water contains turbidity, the output of the process is reduced. The turbidity must be below 10 ppm; else it has to be removed by coagulation and filtration.

Ion exchange apparatus

Ion exchange

Ion exchange chamber internal view

Numerical problems

Water technology-2 Problems  Calculate

the lime and soda needed for softening 50,000 litres of water containing the following salts: CaSO4 = 136 mg/lit; MgCl2 = 95 mg/lit; Mg(HCO3)2 = 73 mg/lit; Ca(HCO3)2= 162 mg/lit. Given that the molar mass of Ca(HCO3)2 is 162 and that of MgCl2 is 95.

Water technology-2 Problems S.No.

Constituent

Amount (mg/lit)

MF

CaCO3 equivalent

1.

CaSO4

136

100/136

100

2.

MgCl2

95

100/95

100

3.

Mg(HCO3)2

73

100/146

50

4.

Ca(HCO3)2

162

100/162

100

Water technology-2 Problems Lime required =

74 ---- [ Ca(HCO3)2 + 2 Mg (HCO3)2 + MgCl2] 100 74 = --- [100 + 2 x 50 + 100 + 100] 100 74 400 = --- x ---= 296 mg/l 100 1 For 50000 lit of water: 50000 x 296 = 14.8 Kg of lime required.

Water technology-2 Problems 106 Soda required = ---- [CaSO4 + MgCl2] 100 106 200 = ---- x ---- = 212 mg/l 100 1 For 50000 lit of water: 50000 x 212 = 10.6 kg of soda required.

Water technology-2 Problems Calculate the quantities of lime and soda required in kgs for softening 10,000 lit of water using 82 ppm of NaAlO2 as coagulant. Analysis of water was as follows: Cacl2 = 111 ppm; Mg(HCO3)2 = 146ppm; NaCl = 58.5ppm; KCl = 74.5ppm; Dissolved CO2 = 44ppm. (At. Wt of Na = 23, Mg = 24, K = 39, Ca = 40 and Al = 27)

Water technology-2 Problems S.No.

Constituent Amount(ppm)

MF *

CaCO3 equivalent

1.

CaCl2

111

100/111

100

2.

Mg(HCO3)2

146

100/146

100

3.

NaCl

58.5

4.

KCl

74.5

5.

CO2

44

100/44

100

6.

NaAlO2

82

100/164

50

Do not contribute

Water technology-2 Problems 74 Lime required = ---- [Mg2+ + CO2 + NaAlO2] 100

mg/lit.

=

74 ---- [100 + 100 + 50] = 185

100 So for 10000 lit = 10000 X 185 = 1850000 mg = 1.85 kg

Water technology-2 Problems 106 Soda required = ---- [Ca2+ + Mg2+] 100 106 = ---- [100 + 100] = 212 mg/lit 100 For 10000 lit = 10000 X 212 = 2120000 mg = 2.12 kg.