Process Water Treatment

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1 CHAPTER 1 INTRODUCTION

Transcript of Process Water Treatment

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CHAPTER 1

INTRODUCTION

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1.0 INTRODUCTION[1]

Industry uses water to generate steam, to cool and heat, and to fill the varied voluminous

needs of process and general plant services. Estimating the true economic impact of this

water involves determining not only the required flow and available supply but also quality

requirements and treatment costs.

In many industrial plants, the water once used in a particular application is still of higher

quality than raw water. Therefore it can be cascaded or reused in other application requiring a

lesser quality, here it is also important to define water quality criteria for any given usage.

In order to improve economy of industrial water use, it becomes imperative to reduce waste

water volumes, recover and reuse wastewater, which can be done effectively by

FEEDWATER TREATMENT.

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1.1 FEEDWATER DEFINITION

Feedwater can be defined as the water of certain standard that is recommended to be used in

process equipment and general plant services. So that equipment performs at its best.

1.2 NEED FOR FEEDWATER

Industry uses water to generate steam, to cool to heat, and to fill the varied and voluminous

needs of process and general plant services. During these process impurities present in water

causes problems like corrosion, scale formation, embrittlement etc. To minimize such

problems it is required that raw is treated first and then allowed to enter the industrial

systems.

1.3 BASIC JOBS IN TREATMENT [2]

Feedwater treatment is basically done to minimize following problems:-

1. SCALE. Calcium and magnesium salts, chiefly dissolved in water, deposite on tube

surfaces as scale when water evoporates.This reduces heat transfer, causes overheating of

tube metal. Scale formation is avoided by (1) removing these hardness salt before water

enters boiler or (2) adding chemicals to the water to cause these solids to form a soft sludge

that can be removed by blowdown.

2. CORROSION Boilers, economizer, feedheaters, piping etc. become corroded when (1)

water is acidic or (2) dissolved gases are present. Prescribed remedies are (1) neutralizing

water acidity with an alkali (2) removing gases by dearation,followed by chemical

“scavenger” that absorbs remaining traces and (3) feeding a corrosion inhibitor together with

organic substances to protect surface of boiler and return lines.

3. CARRYOVER Slugs of water passing over with steam cause erratic superheat,

mechanical troubles with turbines. Solids passing over deposit in super heaters on turbine

blades may spoil materials in process. Alkalinity too high, oil in water, suspended solids are

main cause of foaming,Bowdown and antifoam agents combat carry over.

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4. EMBRITTLEMENT.Metal failure from embrittlement is along drum seams, under rivets

and at tube ends where water flashing to steam through any small leaks in these stressed areas

allows any sodium hydroxide in water to concentrate.Embrittlement can be checked by

maintaining low hydroxide alkalinity, avoiding leaks at stressed metal, using special

inhibiting agents.

1.4 APPLICATIONS OF FEEDWATER

Water has wide range of application in industries, considering chemical industrial

application, feedwater is used in:-

BOILER FEEDWATER

COOLING WATER

PROCESS WATER OR DEMINERALIZED WATER

In next chapters sections preparation of these waters from raw water is discussed.

But before proceeding to treatment process of feedwater, it is required that raw water

characteristics are understood.

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CHAPTER 2

RAW WATER

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2.1 RAW WATER [5]

Pure water (H2O) is colourless, tasteless, and odourless. Because water becomes

contaminated by the substances with which it comes into contact, it is not available for use in

its pure state. To some degree, water can dissolve every naturally occurring substance on the

earth. Because of this property, water has been termed a "universal solvent." Although

beneficial to mankind, the solvency power of water can pose a major threat to industrial

equipment. Corrosion reactions cause the slow dissolution of metals by water. Deposition

reactions, which produce scale on heat transfer surfaces, represent a change in the solvency

power of water as its temperature is varied. The control of corrosion and scale is a major

focus of water treatment technology.

Constituent Chemical

Formula/description

Difficulties Caused Means of Treatment

Turbidity NTU(nephlometric

turbidity unit)

imparts unsightly

appearance to water;

deposits in water

lines, process

equipment, etc.;

interferes with most

process uses

coagulation, settling, and

filtration

Hardness calcium and magnesium

salts Expressed in terms

of CaCO3 conc.or in

ppm

chief source of scale

in heat exchange

equipment, boilers,

pipe lines, etc.; forms

curds with soap,

interferes with dyeing,

etc.

softening;

demineralization; internal

boiler water treatment;

surface active agents

Free Mineral

Acid H2SO4 ,

HCI. etc.

Expressed in terms of

CaCO3 conc.or in ppm

Foam and carryover

of solids with steam;

embrittlement of

boiler steel;

bicarbonate and

carbonate produce

CO2 in steam, a

source of corrosion in

condensate lines.

lime and lime-soda

softening; acid treatment;

hydrogen zeolite

softening;

demineralization by

anion exchange

Free Mineral

Acid

H2SO4 , HCI. etc., ppm corrosion neutralization with

alkalis

pH pH=-log[H+] pH varies according to

acidic or alkaline

solids in water; most

pH can be increased by

alkalis and decreased by

acids

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natural waters have a

pH of 6.0-8.0

Sulphate SO42-

, ppm adds to solids content

of water, but in itself

is not usually

significant, combines

with calcium to form

calcium sulphate scale

demineralization, reverse

osmosis, electrodialysis,

evaporation

Chloride Cl -,ppm adds to solids content

and increases

corrosive character of

water

demineralization, reverse

osmosis, electrodialysis,

evaporation

Nitrate NO3- ,ppm adds to solids content,

useful for control of

boiler metal

embrittlement

demineralization, reverse

osmosis, electrodialysis,

evaporation

Fluoride F- ,ppm cause of mottled

enamel in teeth; also

used for control of

dental decay: not

much significant

industrially

adsorption with

magnesium hydroxide,

calcium phosphate, or

bone black; alum

coagulation

Sodium Na+ ,ppm Adds to solids content

of water: when

combined with OH-,

causes corrosion in

boilers under certain

conditions.

demineralization, reverse

osmosis, electrodialysis,

evaporation

Silica SiO2 ,ppm scale in boilers and

cooling water

systems; insoluble

turbine blade deposits

due to silica

vaporization

hot and warm process

removal by magnesium

salts; adsorption by

highly basic anion

exchange resins, in

conjunction with

demineralization, reverse

osmosis, evaporation

Iron Fe2+

(ferrous)

Fe3+

(ferric),ppm

Discolours water on

precipitation; source

of deposits in water

lines, boilers. etc.;

interferes with dyeing,

tanning, papermaking.

aeration; coagulation and

filtration; lime softening;

cation exchange; contact

filtration;

Manganese Mn2+

,ppm Discolours water on

precipitation; source

aeration; coagulation and

filtration; lime softening;

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of deposits in water

lines, boilers. etc.;

interferes with dyeing,

tanning

cation exchange; contact

filtration;

Aluminium AI3+

,ppm usually present as a

result of floc

carryover from

clarifier; can cause

deposits in cooling

systems and

contribute to complex

boiler scales

improved clarifier and

filter operation

Oxygen O2,ppm corrosion of water

lines, heat exchange

equipment, boilers,

return lines, etc.

deaeration; sodium

sulphite

Hydrogen

Sulphide

H2S,ppm cause of "rotten egg"

odour; corrosion

aeration; chlorination;

highly basic anion

exchange

Ammonia NH3,ppm corrosion of copper

and zinc alloys by

formation of complex

soluble ion

cation exchange with

hydrogen zeolite;

chlorination; deaeration

Dissolved

Solids

May contain salts in

ppm

high concentrations

are objectionable

because of process

interference and as a

cause of foaming in

boiler

lime softening and cation

exchange by hydrogen

zeolite; demineralization,

reverse osmosis,

electrodialysis,

evaporation

Suspended

Solids

Undissolved matter in

ppm

deposits in heat

exchange equipment,

boilers, water lines,

etc.

subsidence; filtration,

usually preceded by

coagulation and settling

So, these are the contents in the raw water that are required to be removed to make it usable

in industry.

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CHAPTER 3

BOILER FEEDWATER TREATMENT

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3.1 Boiler feedwater [4]

Feedwater can be defined as the water of certain standard that is recommended to be used in

process equipment and general plant services. Treatment of boiler feedwater aims at

preventing scale and sludge depositions, corrosion and carryover.

All natural waters contain varying amounts of suspended and dissolved matter as well as

dissolved gases Impurities in water become an important consideration when water is to be

used for steam generation. With the trend toward higher pressure boilers, pretreatment has

become the key to successful operation of industrial power plants. Poor quality water gives

poor quality steam.

3.2 Problem’s caused in boiler systems [3]

3.2.1 Boiler Deposits

Impurities that form deposits Bicarbonates of calcium and magnesium dissolved in water

break down under heat and give off car bon dioxide forming insoluble carbonates. These

carbonates precipitate directly on the boiler metal and or form sludge in the water that

deposits on boiler surfaces.

Sulfate and silica generally precipitate directly on the boiler metal and are much harder to

condition. Silica (sand) if present in water can form exceedingly hard scale.

Suspended or dissolved iron coming in the feedwater will also deposit on the boiler metal. Oil

and other process contaminants can form deposits as well and promote deposition of other

impurities.

Sodium compounds usually do not deposit unless the water is almost completely evaporated

to dryness, as may occur in a starved tube.

Deposits are seldom composed of one constituent alone, but are generally a mixture of

various types of solid sediments, dissolved minerals, corrosion products like rust, and other

water contaminants.

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The major problem that deposits cause is tube failure from overheating. This is due to the act

that the deposits act as an insulator and excessive deposits prevent efficient heat transfer

through the tubes to the water. This causes the metal to become overheated and over time the

metal fails.

These deposits can also cause plugging or partial obstruction of boiler tubes, leading to

starvation and subsequent overheating of the tubes. Underneath the deposit layer corrosion

may also occur. Deposits cause unscheduled outages, increased cleaning time and expenses.

Boiler deposits reduce overall operating efficiency resulting in higher fuel consumption.

3.2.2 Boiler scale formation

At prevailing temperatures in a steam boiler, both carbonate and non-carbonate hardness

forms a scale. Breakdown of hardness is shown by following equations.

Ca(HCO3)2 = CaCO3 + H2O+CO2

Mg(HCO3)2=MgCO3+ H2O+CO2

MgCO3+H2O=Mg(OH)2+CO2

Another source of boiler scale is Silica. Its thermal conductivity is very low and causes tube

failures often even with very thin silica scales.

Scale formation occurs when the salts present crystallize from a solution which is

locally supersaturated.Feedwater forms a thin film along the boiler surface and tends

to concentrate, causing hard layer formation on heating surface itself.

Scale creates problem in boiler operation because it has low degree of heat

conductivity, it causes rise in temperature causes tube and equipment failures.

Scale causes 2-3% loss in boiler efficiency.

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3.2.3 Corrosion in Boiler systems [4]

:-

Though basic corrosion is usually due to reaction of the metal with oxygen, other factors

including stresses produce different forms of attack.

Corrosion may occur in the feedwater system as a result of low pH water and the presence

of dissolved oxygen and carbon dioxide. Corrosion in the boiler itself normally occurs when

boiler water alkalinity is too low or too high or when the metal is exposed to oxygen-

bearing water during either operation or idle periods.

High temperatures and stresses tend to accelerate the corrosion. In the steam & condensate

system and pipelines corrosion is generally the result of contamination with carbon dioxide

and oxygen.

Corrosion Fatigue

Cracking in boiler metal may occur due to cyclic stresses created by rapid heating and

cooling. These stresses are concentrated at points where corrosion has roughened or pitted

the metal surface. This is usually because of improper corrosion prevention.

Sometimes even with properly treated water corrosion fatigue cracking occurs. These crack

often where a dense protective oxide film covers the metal surfaces, and cracking occurs

from the action of applied cyclic stresses.

Corrosion fatigue cracks are often thick, blunt, and across the metal grains. They start at

internal tube surfaces and are most often circumferential on the tube.

Caustic embrittlement

Caustic embrittlement or cracking is a more serious type of boiler metal failure showing up

as continuous intergranular cracks. This type of cracking occurs when the metal is stressed,

water contains caustic with a trace of silica, and some mechanism, such as a slight leak, is

present allowing the boiler water to concentrate on the stressed metal.

Caustic embrittlement is more of a problem in older boilers with riveted drums as they

cause stresses and crevices in the areas of rivets and seams. In the newer welded drum

boilers this type of cracking is less.

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3.2.4 Boiler water carryover

Boiler water carryover is the contamination of steam with boiler water solids. Common

causes of boiler water carryover are:

1. Bubbles or foam form on the surface of the boiler water and leave with the steam. This is

due to high concentrations of insoluble or soluble solids in the boiler water. Substances like

alkalis, greases, oils, fats, organic matter and suspended solids are known to cause

foaming.

2. Fine droplets of water in the form of spray or mist are thrown up into the steam space by

the bursting of rapidly rising bubbles at the steam- release surface.

3. Priming is a sudden surge of boiler water caused by a rapid change in load. It may be

caused by damaged steam-separating equipment, operation above the boiler rating, sudden

fluctuations in steam demand, or carrying too high of a water level in the steam- release

area.

4. Steam contamination may also occur from leakage of water through improperly designed

or installed steam-separating equipment in the boiler drum.

3.3 Boiler Water Treatment:-

Feedwater is filtered to remove suspended matter and if the suspended solids are very fine, a

flocculation step may be needed to enable effective filtration. The water is then subjected to

other treatments to make it suitable for the boiler. Depending on the quality of water, it may

be subjected to one or more treatments like chemical precipitation, lime-soda softening, ion-

exchange, deaeration, and reverse osmosis.

3.3.1 Filtration

Filtration is the essential first step before the chemical treatment and conditioning of the

boiler feedwater. Filtration removes or minimizes all types of suspended solid impurities. If

rust, sand (silica) etc. are not filtered out, they lead to severe scale formation, which is

difficult to clean and reduce boiler efficiency. Even the condensate feedwater must be

filtered before returning to the boiler. The boiler itself and the steam piping produce rust

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particles etc. due to corrosion and other reactions. Filtration is also necessary for any water

treatment.

3.3.2 Coagulation and flocculation

Sometimes the suspended particles in water are so fine that even cartridge filter s are unable

to remove them. In such a situation, before cartridge filtration, the water is first treated with

coagulants. Coagulation is charge neutralization of finely divided and colloidal impurities in

water into masses that can be filtered. In addition, particles have negative electrical charges,

which cause them to repel each other and resist adhering together. Coagulation, therefore,

involves neutralizing the negative charges and providing a nucleus for the suspended

particles to adhere to. Flocculation is the bridging together of coagulated particles.

Types of coagulants

Iron and aluminum salts such as ferric sulfate, ferric chloride, aluminum sulfate (alum), and

sodium aluminate are the most common coagulants.

Ferric and alumina ions each have three positive charges and therefore their effectiveness is

related to their ability to react with the negatively charged colloidal particles. These

coagulants form a floc in the water that serves like a net for collecting suspended matter.

Polyelectrolytes, which are synthetic materials, have been developed for coagulation

purposes. These consist of long chain-like molecules with positive charges. In some cases

organic polymers and special types of clay are used in the coagulation process to serve as

coagulant aids. These assist in coagulation by making the floc heavier.

3.3.3 Chemical precipitation

Chemical precipitation is a process in which chemical added reacts with dissolved minerals

in the water to produce a relatively insoluble reaction product.

Precipitation methods are used in reducing dissolved hardness, alkalinity, and silica. The

most common example is lime-soda treatment.

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Lime - soda in softening process

Calcium hydroxide (hydrated lime) reacts with soluble calcium and magnesium carbonates

to form insoluble precipitates. They form a sludge that can be removed by settling and

filtration.

Lime, therefore, can be used to reduce hardness present in the bicarbonate form (temporary

hardness) as well as decrease the amount of bicarbonate alkalinity in water. Lime reacts

with magnesium sulfate and chloride and precipitates magnesium hydroxide, but in this

process soluble calcium sulfate and chlorides are formed. Lime is not effective in removing

calcium sulfates and chlorides (permanent hardness).

Soda ash is used primarily to reduce non-bicarbonate hardness (permanent hardness). The

calcium carbonate formed by the reaction precipitates as sludge and can be filtered out. The

resulting sodium sulfate and chloride are highly soluble and non-scale forming.

Methods of lime-soda softening.

The older method of intermittent softening consists of mixing the chemicals with the water

in a tank, allowing time for reaction and forming of sludge, and filtering and drawing off the

clear water.

The modern method of continuous lime-soda softening involves use of compartmented

tanks with provision for

(a) Proportioning chemicals continuously to the incoming water

(b) Retention time for chemical reactions and sludge formation, and continuous draw-off of

softened water. Lime-soda softening is classified as hot or cold, depending on the

temperature of the water. Hot process softeners increase the rate of chemical reactions,

increase silica reduction, and produce over-all better quality water.

Coagulants used in lime-soda process

In the initial clarification process, coagulants are used to agglomerate fine suspended

particles, which can then be filtered out. Likewise, in the softening process, coagulants

speed up settling of sludge by 25 -50%. Sodium aluminate used as a coagulant in lime-soda

softening being alkaline, also contributes to the softening reactions, particularly in reducing

magnesium. Proper uses of coagulants help remove silica in the softening process. Silica

tends to be adsorbed on the floc produced by coagulation of sludge.

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Advantages of lime-soda softening

The main advantage is that in reducing hardness, alkalinity, total dissolved solids, and silica

are also reduced.

Prior clarification of the water is not usually necessary with the lime-soda

process.

Another advantage is that with continuous hot process softening some removal of

oxygen and carbondioxide can be achieved.

Fuel savings can be realized with hot process softening because of solids reduction. This

reduction decreases the conductivity of the feedwater, thereby decreasing blowdown and

conserving heat.

Disadvantages of lime-soda softening

The main disadvantage is that while hardness is reduced it is not completely removed.

Variations in raw water composition and flow rate also make control of this method difficult

since it involves adjusting the amounts of lime and soda ash being fed.

3.3.4 Deaeration of water

Dissolved oxygen in water is a major cause of boiler system corrosion. It should be

removed before the water is put in the boiler. Feedwater deaeration removes oxygen by

heating the water with steam in a deaerating heater. Part of the steam is vented, carrying

with it the bulk of the dissolved oxygen.

3.3.5 Internal treatment

Internal treatment of water inside the boiler is essential whether or not the feedwater has

been pretreated. Internal treatment compliments external treatment and is required

regardless of whether the impurities entering the boiler with the feedwater are large or small

in quantity. Internal treatment can constitute the sole treatment when boilers operate at low

pressure, large amounts of condensed steam are used for feedwater, or the raw water

available is of good quality. However, in moderate or high-pressure boilers, External

treatment of the make-up water is mandatory for good results.

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The purpose of an internal water treatment program is:

1. To react with incoming feedwater hardness and prevent it from precipitating on the

boiler metal as scale

2. To condition any suspended matter such as hardness sludge in the boiler and make it

non-adherent to the boiler metal

3. To provide antifoam protection to permit a reasonable concentration of dissolved and

suspended solids in the boiler water without foaming

4. To eliminate oxygen from the feedwater

5. To provide enough alkalinity to prevent boiler corrosion

6. To prevent scaling and protect against corrosion in the steam-condensate systems.

Chemicals used in internal treatment

Phosphates used to be the main conditioning chemical, but nowadays chelate and polymer

type chemicals are mostly used. These new chemicals have the advantage over phosphates

of maintaining scale-free metal surfaces. All internal treatment chemicals, whether

phosphate, chelate, or polymer, condition the calcium and magnesium in the feedwater.

Chelates and polymers form soluble complexes with the hardness, whereas phosphates

precipitate the hardness. Sludge conditioners are also used to aid in the conditioning of

precipitated hardness. These conditioners are selected so that they are both effective and

stable at boiler operating pressures. Synthetic organic materials are used as antifoam agents.

For feedwater oxygen scavenging, chemicals used are sodium sulfite and hydrazine.

Internal treatment for hardness

At boiler operating temperatures, calcium carbonate in the feedwater breaks down to form

calcium carbonate. Since it is relatively insoluble, it precipitates.

Sodium carbonate in the water partially breaks down to sodium hydroxide and car bon

dioxide.

Internal treatment with phosphates transforms calcium bicarbonate to calcium phosphate

and sodium carbonate. In the presence of hydroxide alkalinity, magnesium bicarbonate

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precipitates as magnesium hydroxide or reacts with silica to form magnesium silicate. These

minerals are precipitated from solution in form of sludge, which must be conditioned to

prevent its sticking to the boiler metal. The conditioned sludge is then removed from the

boiler by blowdown.

When chelate is used for internal treatment, it reacts with calcium and magnesium salts to

form soluble complexes. These complexes are in the form of dissolved solids and are

removed by blowdown.

Dispersant polymers used in conjunction with chelate produces reaction products,

which are better conditioned. They do not precipitate and are more easily removed by

blowdown.

Internal treatment for sulfates

The boiler temperature makes the calcium and magnesium sulfates in the feedwater

insoluble. With phosphates used as internal treatment, calcium reacts with the phosphate

producing hydroxyapatite, which is much easier to condition than calcium sulfate. With

chelates or polymer used as internal treatment, calcium and magnesium react with these

materials producing soluble complexes that are easily removed by blowdown.

Internal treatment for silica If silica is present in the feedwater, it tends to precipitate

directly as scale at hot spots on the boiler metal and or combines with calcium forming a

hard calcium silicate scale. In the internal treatment for silica, the boiler water alkalinity has

to be kept high enough to hold the silica in solution. Magnesium, present in most waters,

precipitates some of the silica as sludge.

Internal treatment for sludge conditioning

Internal treatment for hardness results in insoluble precipitates in the boiler that form

sludge. In addition, insoluble corrosion particulate (metal oxides) is transported to the boiler

by condensate returns and from preboiler feedwater corrosion resulting in suspended solids.

Suspended solids, carried to the boiler by feedwater or subsequently formed within the

boiler, adversely affect both boiler cleanliness and steam purity. These solids have varying

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tendency to deposit on the boiler metal. Conditioners prevent these solids from depositing

and forming corrosive or insulating boiler scale.

Some of the principal types of sludge conditioners are:

1. Starches effective on high silica feedwater and where oil contamination is a problem

2. Lignin’s effective on phosphate type sludge

3. Tannins fairly effective on high hardness feedwater

Internal treatment advantages

Internal treatment is basically simple and with the help of a qualified consultant an effective

program is easily established. Scales or deposits, corrosion and carryover are minimized

thereby improving efficiency and reducing energy consumption, preventing tube failures

and unscheduled costly repairs, and reducing deposits, corrosion and contamination in

equipment or processes.

3.3.6 Blowdown [8]

Blowdown is the discharge of boiler water containing concentrated suspended and dissolved

feedwater solids. As the blowdown water is replaced with lower solids feedwater, the boiler

water is diluted. With proper regulation of blowdown, the amount of solids in the boiler

water can be controlled. The amount of blowdown needed depends on how much feedwater

impurities a given boiler can tolerate. For example if a particular boiler can tolerate 500

ppm maximum dissolved solids, and the feedwater contains 50 ppm, it can be concentrated

only about 10 times. This means that for every 100 pounds of water fed to the boiler about

10 pounds of boiler water must be blown down to keep the dissolved solids

from exceeding 500 ppm. Total dissolved solids is not the only limiting factor in deter

mining blowdown, other considerations include suspended solids, alkalinity, silica and iron.

Continuous and intermittent blowdown

Boilers incorporate blowdown valves at low points where sludge is likely to collect.

Opening these blowdown valves for short intervals provides intermittent removal of sludge

and concentrated solids. In addition, some boilers also have a blowdown off take located

slightly below the water level in the steam release area. A small amount of water is

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continuously removed through these connections. This procedure is called intermittent

blowdown.

The use of continuous blowdown in addition to manual (bottom) blow down maintains the

residuals at more consistent levels in the boiler water.

Continuous blowdown also minimizes the amount of bottom blowdown required, with

resultant savings in fuel and chemicals. Continuous blowdown helps minimize upsets in

boiler water circulation and operation.

3.3.7 Corrosion in steam condensate system

Corrosion in steam condensate system is caused by carbon dioxide and oxygen carried into

the system by steam. Dissolved carbon dioxide in condensed steam forms corrosive

carbonic acid. If oxygen is present with carbondioxide, the corrosion rate is much higher,

and is likely to produce localized pitting. Ammonia, in combination with oxygen, attacks

copper alloys.

Prevention of steam condensate corrosion

Generally corrosion prevention is by removing oxygen from the feedwater by mechanical

(deaerator) means, by use of suitable chemicals, and pretreatment of the make-up water to

minimize potential carbon dioxide formation in the boiler. Further boiler water treatment is

done by use of volatile amines to neutralize carbon dioxide or volatile filming inhibitors to

form a barrier between the metal and the corrosive condensate. Mechanical conditions need

to checked and corrected, like poor trapping and draining of lines. Deaerator can reduce

oxygen to as low as 0.007 ppm. Since very small amounts of oxygen can cause boiler and

steam condensate system corrosion, chemical treatment is needed to assure complete

oxygen removal. Sodium sulfite and hydrazine chemicals are commonly used for this

purpose.

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3.4 Boiler feedwater specifications [3]

Table 1

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CHAPTER 4

COOLING WATER TREATMENT

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4.1 Cooling water [3]

Cooling water systems remove heat generated from a variety of industrial processes. General

schematic showing operation of cooling water

Fig 1:-

4.2 Objectives of Cooling Water Treatment

The primary objectives of cooling water treatment are to maintain the operating efficiency of

the cooling water system and to protect the equipment that contacts the cooling water. These

objectives are accomplished by controlling or minimizing deposition, corrosion, and

microbiological growth on the cooling water equipment. Treatment programs must also

address requirements for environmental compliance, safety, water conservation, and

limitation of chemical costs.

4.3 Problems occurring in cooling water systems

Water-related problems can cause system downtime, loss of equipment efficiency, the need

for capital replacement of equipment, and can increase the risk of disease from pathogenic

microorganisms. An open recirculating cooling tower system has a greater potential for these

problems than does a once-through cooling water system, due to the air- and water-mixing

design of the open recirculating system. These problems are associated with water-caused

deposits, corrosion, or microbiological organisms, and occur for various reasons:

• The cooling tower is essentially a huge air scrubber that can introduce materials such as

microorganisms, gases, dust, and dirt into the circulating water, which provides an excellent

growth environment for pathogenic microorganisms. These materials can contribute to the

formation of deposits and cause corrosion.

• If the water is not properly treated and its quality maintained, corrosion and scale and solids

deposition can occur. The potential for these problems results from the nature of the cooling

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system design and the operating conditions, including water evaporation, mineral

concentration, and water temperatures of up to 54 °C (130 °F).

• The constant addition of makeup water results in increased quantities of mineral

constituents that can form scale, deposits, and corrosion. Blowdown control and proper water

treatment can minimize these problems.

• The film fill contains small water and air passages that can become plugged, thereby

causing a reduction in cooling tower operational efficiency due to reduced water evaporation

4.3.1 Deposit Formation

Deposits that occur in cooling water systems are usually divided into two categories: scale

and fouling. The presence of either type of deposit in the heat exchangers or in the film fill

can interfere with heat transfer, thereby reducing the efficiency of operation. Deposits can

also promote under-deposit corrosion.

4.3.2 Scale formation.

Scale is formed from minerals, formerly dissolved in water, that were deposited from the

water onto heat transfer surfaces or in-flow water lines. As water is evaporated in a cooling

tower, the concentration of dissolved solids becomes greater until the solubility of a particular

scale-causing mineral salt is exceeded. When this situation occurs in an untreated cooling

water system, the scale will form on any surface in contact with the water, especially on heat

transfer surfaces.

The most common scaling minerals are calcium carbonate, calcium phosphate, calcium

sulphate, and silica, usually in that order. Formation of magnesium silicate scale is also

possible under certain conditions. Most other salts, including silica, are more soluble in hot

water than in cold water; however, most calcium and magnesium salts, including calcium

phosphate and calcium carbonate, are more soluble in cold water than in hot water. This is

called “reverse solubility.” The water temperature will increase as recirculating water passes

through the cooling system. As a result, calcium and magnesium scales may form anywhere

in the system, but most likely on heated surfaces such as heat exchangers or surface

condensers. Silica will form in areas having the lowest water temperature, such as in the

cooling tower fill.

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4.3.3 Fouling

The term “fouling” refers to the deposition of materials that are normally held in suspension

in the cooling water: mud, silt, and other SS brought into the system with the makeup water;

dust, dirt, and debris scrubbed out of the air passing through the tower; product leakage such

as oils; corrosion products from the system; and biological organisms, both living and dead.

Combinations of any or all of these materials can be present in the cooling water.

4.3.4 Cooling water corrosion

The term “corrosion” (in a cooling water system) is defined as the electrochemical

deterioration of a metal that is in contact with cooling water. Corrosion occurs when an

electric current flows from one part of the metal (anode) through the water (electrolyte) to

another part of the metal (cathode). Corrosion takes place at the anode only. The cathode is

the driving force of the corrosion action.

Specific type of corrosion called “White Rust” occurs in cooling water systems. Some

cooling towers are constructed with galvanized steel components and must not be exposed to

conditions of high pH (high alkalinity). The galvanizing process deposits a protective zinc

coating on a mild steel metal surface, resulting in increased resistance to corrosion. Failure to

avoid such exposure can result in production of “white rust” due to the corrosion of the

galvanizing coating. Eventually, this corrosion process exposes the mild steel underneath,

which then starts to corrode. White rust failures have been a common occurrence throughout

the country, mainly with newer cooling towers. Proper protection of the galvanizing material

is necessary both during start-up of a new cooling tower and during normal operations.

Specific water treatment chemicals are needed to provide this protection. Examples include

pretreatment with a high level of orthophosphate.

4.4 Treatment methods

4.4.1 Scale Control Methods.

Three basic methods are used to prevent the formation of scale in cooling water systems:

a) Remove the water scaling ingredients from the water before use. This includes softening,

RO etc.

b) Keep the scale-forming ingredients in solution. This is the most common scale-control

method used for cooling water, and it can be achieved by use of either or both of the

following two methods: adding acid, which lowers the pH of the recirculating water, or

adding a scale inhibitor (phosphonate or specific polymer), which allows higher COC (Cycles

of Concentration. COC in a steam boiler is a term that refers to the number of times the

minerals in the feedwater have been concentrated in the boiler by its operation.) to be

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maintained without scaling. Acid neutralizes (destroys) mineral alkalinity, one of the

constituents forming calcium carbonate scale; however, because of the hazards associated

with handling strong acids and the potential damage from an acid spill, the use of acid in

cooling towers is not recommended.

c) Allow the water-scaling ingredient to precipitate as sludge. Modern chemical treatment can

distort or modify scale crystals such that they cannot adhere to each other to form a hard

deposit; instead, they become a sludge that can be removed through filtration or blowdown.

4.4.1.1 Calcium Carbonate Scale Control Using Chemicals.

Acids and phosphonates are chemicals that keep scale from forming. The use of acid in

cooling towers may not be appropriate for use at sea side installations due to the associated

risk of corrosion.

4.4.1.2 Acids.

The acid most commonly used is sulphuric acid used as a diluted solution (e.g., 40%

sulphuric acid in water). The use of acids requires adequate pH control.

4.4.1.3 Phosphonates.

The phosphonates used most frequently for calcium carbonate scale control in recirculating

cooling water systems are AMP (amino-tri [methylene] phosphonic acid); HEDP (1-

hydroxyethylidene 1,1-diphosphonic acid); and PBTC (2-phosphonobutane-1,2,4-

tricarboxylic acid).

The chemical reaction of all phosphonates is similar; however, their stability varies greatly.

The presence of chlorine or other oxidants in treated cooling water favours the use of PBTC,

which is very resistant to decomposition, followed by HEDP, and finally AMP. An active

dosage of 3 to 5 ppm of either AMP or HEDP, or 1.5 to 2.5 ppm PBTC, will increase the

solubility of calcium carbonate by a factor of 3 or more relative to using no chemical

treatment. Rather than operating at a PSI of 6.0 (stable water, no scale) in an untreated

system, the cooling tower water can be used at a PSI of 4.0 without the occurrence of scale

however, in the absence of calcium scaling conditions, phosphonates can increase the

corrosion of both mild steel and copper.

4.4.1.4 Calcium Carbonate Scale Control Using Polymers.

Many different polymers are used in water treatment. For the most part, they have multi-

faceted performance capability; they can inhibit various types of scale formation. Often water

treatment products will include more than one type of polymer in the product formulation.

For control of calcium carbonate, homopolymers such as polyacrylate, polymethacrylate, and

polymaleate are used to keep calcium carbonate in solution. Dosages of 3 to 5 ppm of active

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polymer in the cooling tower water can control calcium carbonate scale formation very

effectively.

4.4.1.5 Calcium Phosphate Scale Control

Often calcium phosphate scale is formed in cooling water systems treated with a phosphate

based corrosion inhibitor program or when phosphate is present in the makeup water (i.e.,

potable or recycled water). Calcium phosphate is much less soluble in water than is calcium

carbonate. If the calcium hardness is 500 ppm and the pH is above 7.0, without any polymer

treatment calcium phosphate scale will likely form, even at the low level of 10 ppm

phosphate (as PO4) in the cooling water. Calcium phosphate solubility can be increased by a

factor of a little less than 3 by the addition of 4-ppm phosphonate or by the use of 6 to 8 ppm

of a copolymer or terpolymer specific for calcium phosphate inhibition.

4.4.1.6 Calcium Sulphate Scale Control

Calcium sulphate formation can result from high concentrations of calcium ions and sulphate

ions in the recirculating water; however, calcium sulphate is the most soluble of the scale-

forming calcium salts found in cooling tower waters having pH levels of greater than 8.0.

This means that calcium sulphate scale will not form unless some calcium ions (hardness)

remain in solution after the calcium reacts with all the carbonate and phosphate in the water.

Calcium sulphate scale may occur when the recirculating water contains calcium hardness in

the range of 500 to 700 ppm as CaCO3 and sulphate in the range of 500 to 700 ppm SO4 The

addition of 3 to 5 ppm of a copolymer of acrylate and acrylamide will allow calcium sulphate

to remain in solution at a level almost 3 times the level allowed when using no treatment.

Calcium sulphate scale rarely forms at pH levels above 8.0 in the cooling water.

4.4.1.7 Magnesium Silicate Scale Control.

Formation of magnesium silicate is possible in cooling systems, but only under certain rare

conditions. Magnesium ions (hardness) first react with hydroxyl ions (OH-) to form

magnesium hydroxide, which then react, with (absorbs) dissolved or colloidal silica. A

deposit analysis often reports this material as magnesium silicate. Since magnesium

hydroxide solubility decreases at pH levels above 9.0, this scale will usually occur only at a

pH level above 9.0 and when the magnesium hardness concentration is greater than 100 ppm.

4.4.1.8 Silica Scale Control.

Silica solubility is dependent upon temperature and pH. At pH levels greater than 8.5, silica

remains soluble (no scale) at a concentration of 250 ppm as SiO2. At pH levels of 7.5 or

below, maximum silica solubility is 150 ppm as SiO2. At maximum silica levels, silica will

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first deposit on the cooling tower slats rather than in the heat exchanger because silica is more

soluble in hot water than in cold water.

If the concentration of silica in the makeup water is above 30 ppm, it will usually be the

parameter that controls the adjustment of cooling water system COC. If the silica

concentration is high, external treatment can reduce the level of silica in the makeup water.

Summary of scale control methods Table 2

4.4.2 Fouling Control

Fouling occurs mainly due:-

Inorganic matter (mud, dirt, and corrosion products)

Organic matter (algae, bacteria, and fungus.)

4.4.2.1 Inorganic fouling control

Fouling from mud, dirt, and corrosion products can be controlled by the addition of a water-

soluble polymer dispersant, such as a polyacrylate. The addition of about 4 to 5 ppm of active

polymer, together with sufficient water velocity (e.g., 1 meter per second), can keep foulants

in suspension and prevent them from being deposited on heat transfer surfaces.

Higher dosages (5 to 20 ppm) of active polymer can be required for heavily fouled systems.

It is best to reduce the loading of SS (suspended solids) by mechanically removing them from

the system through blowdown, filtration, and physical sump cleaning.

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4.4.2.2 Organic Fouling control

Chlorine and Chlorine Release Agents.

Chlorine (Cl2) compounds are the most effective industrial oxidizing biocides and the most

widely used. Chlorine is available as a chlorine gas, dry calcium hypochlorite (HTH), liquid

sodium hypochlorite (bleach), plus several other dry products that release chlorine. When

chlorine is introduced into water, it hydrolyses to form hypochlorite ion (OCl-) and

hypochlorous acid (HOCl); it is the latter chemical that is the stronger oxidizing biocide.

The presence of hypochlorous acid is greater, proportionate to hypochlorite ion, at low pH

levels. At a pH of 5.0, hypochlorous acid exists almost exclusively. At a pH of 7.5, there are

approximately equal amounts of hypochlorous acid and hypochlorite ion. Chlorine is

effective, but to a lesser degree, as a biocide at a pH of 7.5 or greater because the

hypochlorite ion has about one-tenth the biocidal efficacy of hypochlorous acid.

A pH range of 6.5 to 7.5 is considered optimal for chlorine or chlorine-based microbiological

control programs. Above pH 7.5, relatively higher levels of chlorine are required to be

effective.

Bromine Release Agents.

Bromine (Br2) compounds are very similar to chlorine compounds. Although more expensive

than chlorine compounds, their main advantage is that bromine is more effective at higher pH

ranges (7.5 or greater) than chlorine. Bromine has a lower vapour pressure than chlorine and

is 6 times as soluble in water, making it less subject to vaporization loss from a cooling

tower. When bromine is introduced to water, it hydrolyses to form hypobromite ion (OBr-)

and hypobromous acid (HOBr). A pH range of 7.5 to 10.0 is considered optimal for the use of

bromine. Bromine release agents include dry chemicals called hydantoins and bromine salts,

such as sodium bromide. When a salt solution is mixed with an oxidizing agent, such as

bleach, and a reaction occurs, bromine is produced. In water, bromine degrades more rapidly

than chlorine. Recent developments in bromine chemistry have resulted in the production of a

bromine solution (liquid).

Ozone.

Ozone (O3) is a gas produced by passing dry air either through a strong electric field or near

an ultraviolet light. If ozone is dissolved in water, the resulting solution can be added to

cooling water. Ozone is a very strong oxidizing biocide that, if properly applied, can provide

effective control of microorganisms in cooling tower systems; however, because of safety

and operational problems associated with its manufacture and use, and the resulting high

capital and operating costs, it is neither the most economical method nor the preferred

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method for microbiological control in cooling towers under normal operations. Ozone can

increase metal corrosion and does not prevent scale.

Chlorine Dioxide.

Chlorine dioxide (ClO2) is a gas generated by mixing several chemicals. The chlorine dioxide

gas produced in this manner is subsequently dissolved in water, with the water containing the

chlorine dioxide then added to the cooling water. Chlorine dioxide must be produced in close

proximity to the point of use. It is not recommended for use on normal installations due to the

complexity of its production and safety concerns associated with its production and handling.

Hydrogen Peroxide.

Hydrogen peroxide (H2O2) is a liquid that is usually used at a concentration of 30% in water.

Hydrogen peroxide is considered one of the most environmentally friendly oxidizing biocides

because it degrades to water; however, concentrated hydrogen peroxide will react in a violent

manner when it comes into contact with organic chemicals and materials.

Non-Oxidizing Biocides.

Non-oxidizing biocides are microbiocides that act as “poisons;” they disrupt the metabolic or

reproductive processes of micro- and macro-organisms and are therefore toxic. Non-

oxidizing biocides are organic compounds that are very toxic to organisms, including human

beings and animals. They are usually liquids, but some are available as dry products (e.g.,

pellets, solids). A major consideration for their use is their persistence with respect to the

discharge limitations for water (effluent) containing these toxic substances. Also, when

choosing and applying a non-oxidizing biocide, you must consider the cooling tower

system’s operating parameters, such as pH and retention time. The applied dosages of

microbiocides should never exceed EPA maximum limits, which are always printed on the

container labels.

E.g. Quaternary ammonium salts, Dibromo-nitrilo-propionamide, Methylene bisthiocynate

4.4.3 Effective Corrosion Control.

Effective corrosion control requires maintaining appropriate pH levels, plus adding

maintenance dosages of chemical corrosion inhibitors. Chemical corrosion inhibitors form a

protective film or barrier on the cooling system metal surfaces that have been cleaned prior to

adding an initial high dosage of inhibitor. The initial high dosage of inhibitor passivates

(protects) the metal. The appropriate dosage of corrosion inhibitor must be maintained

continuously in the cooling water to ensure continuing protection. Corrosion inhibitors are

divided into three classes: cathodic, anodic, and general filmers. Corrosion inhibitors form a

protective film on either the anode, the cathode, or over the entire metal surface.

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Anodic Inhibitors.

Anodic inhibitors form a protective film coating on the anodic metal (where the metal is lost)

and thus directly control corrosion by preventing the reaction that results in corrosion. Any

unprotected areas will corrode at a much faster rate than the protected areas, a factor that

could result in pitting or localized attack of the unprotected areas. Examples of anodic

inhibitors include orthophosphate, nitrite, and orthosilicates. Under certain conditions,

molybdate can function as an anodic inhibitor.

Cathodic Inhibitors

Cathodic inhibitors form a protective film coating of the cathodic metal (where metal is not

lost) and thus indirectly prevent corrosion by interfering with the current flow required for the

electrochemical reaction to proceed.

General Inhibitors.

General inhibitors produce a protective film on the surfaces of all metals. These corrosion

inhibitors are organic compounds, such as certain phosphonates, amines, and other nitrogen

chemicals. They may be used in cooling water systems.

Corrosion Inhibitors for Specific Metals.

Some corrosion inhibitors provide corrosion control for specific metals. The protection of

copper and copper alloys requires the use of azoles, such as tolyltriazole (TTA),

benzotriazole (BZT), and butylbenzo-thiazole, which can be added to the system separately

from, or as part of a blend of, other treatment chemicals.

Galvanic or Crevice Corrosion Control

Control of galvanic or crevice corrosion is achieved primarily by engineering and mechanical

design. These types of corrosion can also be partially controlled by physical and chemical

considerations. A dielectric coupling (insulator), used to separate two different metallurgies,

can help to prevent galvanic corrosion. From a chemical perspective, adjusting pH and using

proper corrosion inhibitors will reduce corrosion.

MIC (Methyl Isocynate)

MIC is best prevented by stopping conditions that foster biological growth and by using an

effective microbiological control program. A MIC prevention program includes adequate

control (prevention) of deposits and fouling.

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Deposit Corrosion Control

Control of deposit corrosion requires maintaining deposit-free metal surfaces. This applies to

any form of deposit, such as scale, biomass, corrosion products, or foulants. The occurrence

of most types of deposits is prevented by dispersants, scale inhibitors, or biocides, along with

the maintenance of adequate flow velocities. Routine, adequate cleaning of filters and tower

sumps will help reduce deposits.

4.5 Cooling water specifications Table 3

Suspended solids None

Conductivity 50-600 uS/cm

Hardness 0.44 ppm

pH 7.8

CO2 content None

Iron <0.3 mg/L

Manganese <0.05 mg/L

Sulphate <250 mg/L

Chloride < 250 mg/L

COD < 40 mg/L

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CHAPTER 5

PROCESS WATER TREATMENT

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5.1 Process water [5]

All natural waters contain, in various concentrations, dissolved salts which dissociate in water

to form charged ions. Positively charged ions are called cations; negatively charged ions are

called anions. Ionic impurities can seriously affect the reliability and operating efficiency of a

boiler or process system. Overheating caused by the build-up of scale or deposits formed by

these impurities can lead to catastrophic tube failures, costly production losses, and

unscheduled downtime. Hardness ions, such as calcium and magnesium, must be removed

from the water supply before it can be used as boiler feedwater.

This type of water is virtually free of all contaminants both dissolved and suspended, organic

and inorganic as well free from ions, which gives it the name Demineralized (DM) water.

This high purity water is used in pharmaceuticals, electronic chip processing, high-pressure

boiler feedwater systems, power plant turbines etc.

5.2 Methods of Preparation of DM water:-

Distillation process (unable to remove volatile impurity such as alcohol)

Water softening (chemical treatment, ion exchange softening)

Reverse Osmosis (RO) (expensive method )

Ion-exchange process (most widely used and preferable)

Industrially, Ion-exchange process is preferred for producing DM water so Ion-exchange

technique is discussed in brief.

5.3 Ion-exchange process:-

Demineralization of water is the removal of essentially all inorganic salts by ion exchange. In

this process, strong acid cation resin in the hydrogen form converts dissolved salts into their

corresponding acids, and strong base anion resin in the hydroxide form removes these acids.

Demineralization produces water similar in quality to distillation at a lower cost for most

fresh waters.

5.3.1 Principle of ion-exchange process:-

Ion exchange is a process, which removes unwanted ions i.e. exchanges the mineral ions

present in the water with exchangeable counter ions. In the process of exchange, the ion

exchanger (RESIN) exchanges an equivalent number of ions. The ion exchanger has a limited

capacity for storage of ions in its skeleton (called its exchange capacity). When the ion

exchanger is exhausted the process known as regeneration revives the capacity. In short we

can say ion exchange is a stoichiometric process, every ion, which is removed from the

solution/ water, is replaced by an equivalent amount of another ionic species of the same

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charge. Resins are high molecular weight cross-linked polymer having exchangeable counter

ions. The polymer part is inert and inactive.

Exchange takes place on the counter ions as follows:-

Cation resin: R-H + M+ <====> RM + H

+

Anion resin: R-OH + X- <====> RX + OH

-

R - is the inert polymer part and H and OH are exchangeable parts.

5.3.2 Theory of Ion-Exchange Process [7]

:-

A) Cation Resin:-

The raw material used for the preparation of the cation resin is styrene and divinyl benzene.

They are first polymerized and then sulphonated to get the exchangeable sites. After

polymerization it is sulphonated with sulphuric acid. Degree of cross linkage affects pore size

and stability of the resin. More the cross linkage more the stability and less the pore size.

For hydrogen based resin, Ion-exchange takes place in following ways:-

It will be seen from the equations that the alkaline salt Calcium Bicarbonate, has been splitted

into Carbonic acid which is being weakly ionized can also be represented as free carbon

dioxide and water. This Carbondioxide formed has to be removed as it forms carbonic acid

with water and reduces capacity of ion-exchange resins.

b) Anion Resin:-

It is prepared by the co-polymerization of styrene and Divinyl benzene followed by

chloromethylation and their interaction with amine. The polymerization step is same as in

cation resin. The basic group may be derived from ammonia or an amine, and in order to

facilitate the introduction of the basic group into the polymer.

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The anion resin is much prone to get damaged than cation resin. At elevated temperature

oxidation can cause serious degradation thereby contaminating the water flowing through the

resin bed and the strong resin may be converted into a weak resin. In the dematerializing

process, the anion exchange resin that is used is in the form of R-OH.

The exchange process can be shown as:-

HCl + R-OH <====> R-Cl + H2O

H2SO4 + 2 R-OH <====> R2SO4 + 2H2O

c) Mixed Resin

The mixed bed exchanger containing both cation exchange resins and anion exchange resins

intimately mixed together. These exchangers act as polishing units, removing the ions leaking

through cation and anion exchanger. When water is passed through it, it comes into contact

alternatively with grains of cation and anion resins so that the water is subjected to an almost

infinite number of demineralization.

During operation both Cation and Anion Resins are present in equal amount in the bed. The

proper mixing of bed is very important for proper working of bed.

5.3.3 Working of a DM plant

Equipment’s used in DM plant

i. Alum dosing tank

ii. Alum dosing pumps

iii. Contact tanks

iv. Pressurized sand filters

v. Activated charcoal filters

vi. Strong acid cation bed

Vii. Degasser System

viii. Strong base anion exchangers

ix. Mixed bed exchangers

x. Neutralizing pit

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Schematic of DM plant fig:-2

Various steps involved in treatment of raw water are briefed as under-

1. Water is passed through activated carbon filter for removal of free chlorine present in

water, which otherwise will be an extra load for anion bed. The activated carbon filter can

also remove traces of organic impurities, if present in water.

2. Water is then treated in strong acid cation unit where calcium, sodium, magnesium and

other cations are replaced by hydrogen ions from the resin or in other words the neutral salts

are converted into their respective mineral acids. Hence the pH of the cation bed outlet

decreases drastically and the carbonates and bicarbonates are converted into weak carbonic

acid.

Sodium being monovalent is less efficiently removed by the cation exchanger. There is

therefore a very slight leakage of sodium (known as sodium slip) into the treated water from

the cation bed. The actual leakage of sodium is dependent on the sodium content of the raw

water and regeneration level and is not uniform throughout a run. It begins with high,

gradually reduces to a minimum then increases steadily and sharply.

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3. The water from cation bed is then passed through the degasser tower where decomposition

of carbonic acid and removal of carbon dioxide occurs. Hence the free carbon dioxide present

in the water as well as that produced by the splitting up of alkaline salts is almost all removed

by it.

4. The degassed water temporarily stored in degassed water tank, which is then pumped to

strong base anion exchanger by, degassed water pumps.

5. In anion exchanger all anions such as sulphates chlorides, dissolved silica, traces of

carbonic acid left after the degasser and other anions in the water are removed.

6. Then the water is passed through mixed bed exchanger, where traces of cations and anions

slipped through cation bed exchanger and anion bed exchangers are removed. Thus the

demineralised water is produced at the outlet of Mixed Bed unit.

7. DM water from MB outlet is stored in DM storage tanks from which it is distributed to

different consumption points of both the units.

REGENERATION:-

When the supplies of the exchangeable ions within the resins are exhausted, the treated water

quality deteriorates and the resin requires the reconversion of the resin into operating form.

This process is called as the regeneration.

For CATION exchange resins a mineral acid such as Hydrochloric acid is used for

regeneration:-

NaR + HCl <====> HR + NaCl

Sodium Hydrochloric Hydrogen Sodium

Resin acid resin Chloride

For ANION exchange resins sodium hydroxide is used for regeneration:-

RCl + NaOH <====> ROH + NaCl

Sodium Sodium Hydrogen Sodium

Resin acid resin Chloride

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5.4 DM / Process water specification: - Table 4

PARAMETER AMOUNT

Total Dissolved Solids (TDS) < 0.1mg/l

Conductivity

< 0.5μS/cm.

Silica

< 0.02mg/l

Hardness

Nil

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6.0 CONCLUSION

In this report, various types of Industrial feedwater’s and their preparation techniques are

discussed in detail.

Feedwater considered in this for topic for discussion were:-

1. BOILER FEEDWATER

2. COOLING WATER

3. PROCESS WATER/DM WATER

Comparison between Various Industrial Feedwater’s Table 5

Note:-Here Some parameters of boiler feedwater & cooling water are higher than raw

water because data here considered for both boiler feedwater & cooling water is

considered after recycle, thus average specifications are mentioned. Data obtained from

actual working equipment from Power plant (TAPP-3&4, Tarapur,Maharashtra,India).

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7.0 LIST OF FIGURES

FIGURE

NO.

DESCRIPTION

PAGE

NO.

1 Schematic for showing cooling water operation 23

2 Schematic of DM plant 36

8.0 LIST OF TABLES

TABLE

NO.

DESCRIPTION

PAGE

NO.

1 Boiler feedwater specifications 21

2 Scale control method for cooling 28

3 Cooling water specifications 32

4 DM water specifications 39

5 Comparison between various industrial feedwater’s 40

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9.0 BIBLIOGRAPHY:-

BOOKS

1. Robert Parlante, “Water In Industry”, “Handbook of Water Resource and Pollution

Control”, 1971, Pg-385-426

2. Philip Swan & William Arrot, “power Handbook Part-II”, 1951.pg-34-36

3. Unified Facilities Criteria (UFC)”, Industrial Water Treatment Operation and

Maintenance Report”, UFC 3-240-13FN, pg-33-138, 25 May 2005

4. Parsons, A.J.: “Some Aspects of Locomotive Boiler Feed Water Treatment. Of the

Institution of Locomotive Engineers”, London.pg-572-575 27th February 1957

5. Degrmonte “Handbook of water treatment”, Haslted press, pg-350-462, 1979

WEB REFERENCES

6. Chemicals used for internal treatment

http://www.gewater.com/products/chemical_water/index.jsp

7. General methods of water treatments

http://www.gewater.com/handbook/index.jsp

8. Boiler water treatment

http://www.lenntech.com/applications/process/boiler/boiler-feed-water.htm

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