Water Technology Ppt 1n4q5

Water Technology

Introduction: Composed of hydrogen and oxygen  Occupies a unique position in industries.  Most important use is in the steam generation.  Water is also used as coolant in power and chemical plants.  In addition it is widely used in other fields such as production of steel, rayon, paper, atomic energy, textiles etc.

CHARACTERISTICS IMPARTED BY IMPURITIES IN WATER:

The natural water is usually contaminated by different types of impurities. Physical impurities • Color • Turbidity and sediment • Taste and odour Bacterial impurities • Microorganisms (algae, pathogenic bacteria, fungi, viruses, pathogens, parasites worms etc.) Suspended impurities • Dust particles, clay and sand • Organic suspended impurities • Silica aluminium hydroxide, ferric hydroxide colouring matter • Radioactive Substances

Chemical impurities in water: • Inorganic and organic chemicals released from dyes, paints, drugs, pesticides, textiles, tanneries etc . • Acidity from industrial wastage like acid, mine, drainage, etc. Usually acidity is caused by the presence of free CO2, mineral acid and weakly dissociated acids. • Dissolved gases (O2, CO2, NH3)

• Mineral matters have origin from rocks and industrial effluents. These include mineral acids, Ca2+, Mg2+, Na+, K+, Fe2+, Cl-, NO3-, F-, SiO2 etc.

Contaminant

Potential Health Effects from Ingestion of Water

Sources of Contaminant in Drinking Water

Antimony

Increase in blood cholesterol; decrease in blood sugar

Discharge from petroleum refineries; fire retardants; ceramics; electronics; solder

Arsenic

Skin damage or problems with circulatory systems, and may have increased risk of getting cancer

Erosion of natural deposits; runoff from orchards, runoff from glass & electronics production wastes

Asbestos (fiber >10 micrometers)

Increased risk of developing benign intestinal polyps

Decay of asbestos cement in water mains; erosion of natural deposits

Barium

Increase in blood pressure

Discharge of drilling wastes; discharge from metal refineries; erosion of natural deposits

Beryllium

Intestinal lesions

Discharge from metal refineries and coal-burning factories; discharge from electrical, aerospace, and defense industries

Cium

Kidney damage

Chromium (total)

Allergic dermatitis

Copper

Short term exposure: Gastrointestinal distress Long term exposure: Liver or kidney damage People with Wilson's Disease should consult their personal doctor if the amount of copper in their water exceeds the action level

Cyanide (as free cyanide)

Nerve damage or thyroid problems

Corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints Discharge from steel and pulp mills; erosion of natural deposits Corrosion of household plumbing systems; erosion of natural deposits

Discharge from steel/metal factories; discharge from plastic and fertilizer factories

Drinking water standards comparative table Parameters

Units

WHO standard

BIS Standard

6.5-9.2

6.5-8.5

mg/l

500

500

Sulphate

mg/l

200

200

Chloride

mg/l

250

200

Cyanides

mg/l

0.05

0.05

Fluride

mg/l

1.5

0.6-1.2

Aluminium

mg/l

0.2

Arsenic

mg/l

0.01

pH TDS (Total dissolved salts)

0.01

Drinking water standards comparative table Parameters

Units

WHO standard

BIS Standard

Cium

mg/l

0.003

0.01

Lead

mg/l

0.01

0.05

Mercury

mg/l

0.001

0.001

Sodium

mg/l

200

Zinc

mg/l

3

WHO- World Health Organization BIS - Bureau of Indian standards

5

Hard water  Has high mineral content (in contrast with soft water).  Hardness in water is defined as the presence of multivalent cations. It can cause water to form scales and a resistance to soap.  It can also be defined as water that does not produce lather with soap solutions, but produces white precipitate (scum).  Hard water minerals primarily consist of calcium (Ca2+), and magnesium (Mg2+) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates.  Types of hardness: i) Temporary hardness

ii) Permanent hardness

Temporary hardness: • Combination of calcium ions and bicarbonate ions in the water. It can be removed by boiling the water or by the addition of lime (calcium hydroxide). • Boiling promotes the formation of carbonate from the bicarbonate.

• The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water: CaCO3(s) + CO2(aq) + H2O ⇋ Ca2+(aq) + 2HCO3-(aq)

Permanent hardness: Usually caused by the presence in the water of calcium and magnesium sulfates and/or chlorides which become more soluble as the temperature rises.  Permanent hardness can be removed using a water softener or

ion exchange column, where the calcium and magnesium ions are exchanged with the sodium ions in the column.

In industrial settings, water hardness must be constantly monitored to avoid costly breakdowns in boilers, cooling towers and other equipment that comes in with water. Hardness is controlled by the addition of chemicals and by large- scale softening with zeolite (Na2Al2Si2O8.xH2O) and ion exchange resins. Unit of hardness:

Hardness is expressed in of equivalent of calcium carbonate. [It is the most insoluble salt that can be precipitated in water treatment.]

Hardness in of Calcium Carbonate Equivalents: It is customary to express hardness in of equivalents of CaCO3. The reason for choosing CaCO3 as the standard for calculating hardness of water is due to: 1. Its molecular weight is exactly 100, which makes mathematical calculations easier. 2. It is the most insoluble salt, thus can be easily precipitated in water treatment processes. The CaCO3 equivalents for various salts are as follows: 100g of CaCO3 ≡ 111g of CaCl2 ≡ 136 g of CaSO4 ≡ 95 g of MgCl2 ≡ 120 g of MgSO4 ≡ 162g of Ca(HCO3)2 ≡ 146 g of Mg(HCO3)2 ≡ 164 g of Ca(NO3)2 ≡ 44 g of CO2 ≡ 148 g of Mg(NO3)2 If x g of CaCl2 s present in a water sample, then amount of CaCl2 present on of its equivalent will be: 1 g mole of CaCl2 ≡1 g mole of CaCO3 111 g of CaCl2 ≡ 100 g of CaCO3 55.5 g of CaCl2 ≡ 50 g of CaCO3 x g of CaCl2=50/55.5*x g of CaCO3

In general calcium carbonate equivalent of hardness is given by Wt. of the substance producing hardness X Eq.wt. of CaCO3 = ---------------------------------------------------------------------------Eq.wt.of substance

Various units used to express hardness of water are as under •Parts per million (ppm): calcium carbonate equivalent per 106 parts of water •Milligrams per liter (mg/L): number of calcium carbonate equivalent present per liter of water

hardness

• Degree French (oFr): 1 part of calcium carbonate equivalent hardness per 70,000 parts • Clarke’s Degree(oCl):1 part of calcium carbonate equivalent hardness per 105 parts of water 1ppm=1mg/L=0.1 oFr=0.07oCl

Very Soft water

0-70

ppm of CaCO3

Soft water

70-140

ppm of CaCO3

Slightly hard water

140-210 ppm of CaCO3

Moderately hard

210-320 ppm of CaCO3

Hard water

320-530 ppm of CaCO3

Very hard water

› 530

ppm of CaCO3

Determination of total hardness of water: EDTA method EDTA is a weak acid and has a structure as shown below. CH 2COOH

HOOCH 2C N

CH2

CH2

N

1.a. The structure of EDTA CH 2COOH

HOOCH 2C

1.a -

CH 2COO

O OCH2C N

CH2

CH2

N CH 2COO

-

O OCH2C

1.b

-

-

1.b. The structure of tetracarboxylate [EDTA]4- ion formed by the dissociation of EDTA

Tetracarboxylate ion(1.b) is electron rich having six bonding sites.

The four carboxylate groups and the two nitrogen atoms. Each site has an electron pair available for bonding.

The [EDTA]4- anion wraps itself around a Ca2+ or Mg2+ ion so that all six electrons pairs are shared with the metal ion as shown in the figure. In this manner [EDTA]4- forms strong 1:1 complexes known as chelates with metal ions like Ca2+ and Mg 2+.

Structure of [Mg-EDTA]2- chelate

Structure of [Ca-EDTA]2- chelate

In an aqueous solution buffered at pH 10,

Erio T also dissociate forming [H-Erio T ]2- ion a blue ion that bonds with either Mg2+ or Ca

2+

ion to form a wine red complex. The

reaction of [H-Erio T]2- ion with Ca2+ and Mg2+ ions are reversible.

Mg 2+(aq)+ [ H-Erio T]2-(aq,blue)+ H2O Ca 2+(aq) + [ H-Erio T]2-(aq,blue) + H2O

[Mg-Erio T]-(aq,wine red)+ H3O (aq) [Ca-Erio T]-(aq,wine red) + H3O (aq)

When the end point is reached [EDTA]4- anion breaks up the wine red [Mg-Erio T]- and [Ca-Erio T]complexes releasing the [ H-Erio T]2- ion and hence the solution changes from wine red to permanent blue color. [Ca-Erio T]-(aq,wine red) +H3O(aq)+[EDTA]4T]2(aq,blue)+ H2O

[Ca-EDTA] aq +[ H-Erio

[Mg-Erio T]-(aq,wine red)+H3O(aq) +[EDTA]4(aq,blue) + H2O

[Mg-EDTA] aq+[ H-Erio T]2-

Procedure: Total hardness: Pipette out 50 ml of the sample of water into a clean titration flask, add 1 ml of NH3-NH4Cl buffer solution and 3-4 drops of indicator. Titrate against standard EDTA till the color changes from wine red to clear blue without any reddish tinge. Let the volume of EDTA required be v1 ml. Permanent hardness: Transfer 50 ml of the sample of water into a clean 500 ml beaker and boil gently for 20-30minutes. Cool and filter it directly into a 250 ml conical flask. Add 1 ml of buffer solution followed by 3-4 drops of indicator. Titrate against standard EDTA as described above. Let the volume of EDTA required be v2 ml.

Calculation : 1000ml of 1 M EDTA = 100g CaCO3 1ml of Z M EDTA = 100/ 1000 g of CaCO3

V1 ml of 0.01 M EDTA =

V1 X Z X 100 -------------------------- g of CaCO3 1000

50 ml of water sample contains =

V1 X Z X 100 -------------------------- g of CaCO3 1000

106 ( 1 million ) ml of water sample contains =

Similarly Permanent hardness =

( Mol.weight of CaCO3 =100g)

V2 X Z X 100 X 106 ----------------------------1000 X 50

V1 X Z X 100 X 106 ----------------------- g of CaCO3 1000 X 50

g of CaCO3

Softening of water: Ion exchange or deionization or demineralization process Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification.  An ion-exchange resin or ion-exchange polymer is an insoluble matrix (or structure) normally in the form of small (1–2 mm diameter) beads.  Insoluble cross linked long chain organic polymer and the functional groups attached to the chains are responsible for the ion exchange properties.

 Material has highly developed structure of pores on the surface of which is sites with easily trapped and released ions.  The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion-exchange  Resins containing acidic functional groups are capable of exchanging their H+ ions with other cations  Resins containing basic functional groups are capable of exchanging their anions with other anions. The ion exchange resin may be classified as • Acidic or cationic exchange resin • Basic or anion exchange resin

Cation exchange resins(RH+) They are mainly styrene-divinyl benzne copolymers which on sulphonation or carboxylation become capable to exchange their hydrogen ions with the cations in the water. Acidic or cationic exchange resin (Sulphonate form)

Anion exchange resins: They are styrene divinyl benzene or amine-formaldehyde copolymerization which contains quaternary ammonium or quaternary phosphonium or tertiary tertiary sulphonium groups as an integral part of the resin matrix. These after treated with dil NaOH becomes capable of exchanging their OH- ions with anions of water. Basic or anion exchange resin (hydroxide form)

Ion exchange or deionization or demineralization process

The hard water is ed first through cation exchange column: 2RH+ + Ca2+ 2RH+ + Mg2+

R2Ca2+ + 2H+ R2Mg2+ + 2H+

After cation exchange column the hard water is ed through anion exchange resin column : ROH- + ClRCl+ OH2ROH- + SO42R2SO42- + 2OH-

H + + OHH2O Thus water coming out from the exchange is free from cations as well as anions. Ion free water is known as deionized or demineralised water. Regeneration: Cation exchange column is regenerated by ing a solution of dil HCl or dil H2SO4. The regeneration can be represented as R2Ca2+ + 2H+ 2RH + Ca2+ Exhausted anion exchange column is regenerated by ing a solution of dil. NaOH. The regeneration can be represented as R2SO42- + 2OH-

2ROH + SO42-

Advantages: • Can be used to soften highly acidic or alkaline waters. • It produces water of very low hardness. Disadvantages: • The equipment is costly • Expensive chemicals are needed. • Output of the process is reduced if water contains turbidity.(turbidity must be below10ppm)

Mixed bed deionizer: Consists of a single cylinder containing an intimate mixture of hydrogen exchanger and strongly basic anion exchanger. Water ed through this bed comes in with two kind of the resin alternatively. The net effect of this exchanger is equivalent to ing water through a series of several cation and anion exchangers. The outgoing water from this mixed bed exchanger contains less than 1ppm of dissolved salts.

Regeneration : The mixed bed is back washed Lighter anion exchanger gets displaced Forms an upper layer above the heavier cation exchanger. Regeneration of Anion exchanger - ing caustic soda from the top and then rinsed. Regeneration of Cation exchanger – ing H2SO4 solution. PP

• Boiler feed Water (Scales and Sludges) • • •

In boilers, water evaporates continuously concentrations of the dissolved salts increases when concentrations of dissolved salts reach saturation point, they form precipitates on the inner walls of the boiler. Sludge

Scale

Loose slimy precipitate

A hard, adhering crust/coating on the inner walls of the boiler,

Scale and sludge in Boilers

Disadvantages of sludge formation • • • •

Sludge’s are poor conductor of heat. Tend to waste a portion of heat generated. Sludge’s get entrapped in the scale and both get deposited as scales. Disturbs the working of the boiler.

• •

Prevention of sludge formation: By using well softened water By frequently ‘blow-down operation’, i.e., drawing off a portion of the concentrated water.

Formation of scales may be due to

(1) Decomposition of calcium bicarbonate: Ca (HCO3)2 → CaCO3 + H2O + CO2 (In low-pressure boilers)

CaCO3 + H2O → Ca (OH) 2 (soluble) + CO2 (In high-pressure boilers)

(2) Deposition of Calcium Sulphate: Solubility of calcium sulphate in water decrease with rise of temperature. Hence CaSO4 gets precipitated as hard scale on the heated portions of the boiler.

(3) Hydrolysis of magnesium salts: Dissolved Mg salts undergo hydrolysis forming magnesium precipitate, which forms a soft type of scale

hydroxide

MgCl2 +2H2O → Mg(OH)2 + 2HCl (Scale)

(4) Presence of silica: SiO2,present deposits as calcium silicate (CaSiO3) and / or magnesium silicate (MgSiO3). These deposits stick on the inner side of the boiler surface. Difficult to remove.

Disadvantages of scale formation • Wastage of fuel Scales have a low thermal conductivity Rate of heat transfer from boiler to inside water is greatly decreased. • Lowering of boiler safety Due to scale formation, over heating of boiler is done. Causes distortion of boiler tube. High pressure boiler is unsafe to bear the pressure of the steam.

• Decrease in efficiency Scales may deposit in the valve and condensers of the boiler and choke them partially. • Danger of explosion When thick scales crack, the water comes suddenly in with over-heated iron plates.

• Removal of scales • With the help of scraper or piece of wood or wire brush. • By giving thermal shocks, if they are brittle. • By dissolving them by adding chemicals, (5-10% HCl, EDTA) if they are adherent and hard. • By frequent blow -down operation, if the scales are loosely adhering.

•

Prevention of scales formation (1) External Treatment: Includes efficient ‘softening of water’ (i.e., removing hardness-producing constituents of water)

(2) Internal Treatment: Accomplished by adding a proper chemical to the boiler water either: (a) to precipitate the scale forming impurities in the form of sludges,which can be removed by blow-down operation, or (b) to convert them into compounds, which will stay in dissolved form in water and thus do not cause any harm.

Priming and Foaming: Priming: When a boiler is steaming some particles of the liquid water are

carried along-with steam. Priming is caused by: •

the presence of large amount of dissolved solids

•

high steam velocities

•

sudden boiling

•

improper boiler design and

•

sudden increase in steam- production rate.

Foaming Production of persistent foam or bubbles in boilers. Is due to presence of substances (oils) which reduce the surface tension of water.  Priming and foaming usually occur together. They are objectionable because (i) Efficiency reduces as dissolved salts in boiler water get deposited on super-heater and turbine blades, as water evaporates. (ii) Life of the machinery may decrease as dissolved salts may enter the parts of other machinery. (iii) Maintenance of the boiler pressure becomes difficult, as actual height of the water column cannot be judged properly.

Priming can be avoided by    

Fitting mechanical steam purifiers. Avoiding rapid change in steaming rate. Maintaining low water levels in boilers. Efficient softening and filtration of the boiler-feed water.

Foaming can be avoided by  Adding anti-foaming chemicals like castor oil.  Removing oil from boiler water by adding compounds like sodium aluminate.