Unit 2 Water Treatment

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UNITS IN THIS COURSE

UNIT 1 DESALINATION

UNIT 2 WATER TREATMENT

UNIT 3 STEAM GENERATION BOILERS

UNIT 4 NITROGEN GENERATION

UNIT 5 HYDROGEN GENERATION

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TABLE OF CONTENTS

Para Page

2.0. OBJECTIVES 3

2.1 WHAT IS WATER TREATMENT 4

2.3 WHY IS WATER TREATMENT NEEDED 4

2.3.1 SUSPENDED AND DISSOLVED SOLIDS WILL DAMAGE

THE BOILER INTERNALS 4

2.3.2 DISSOLVED GASES DAMAGE THE INSIDE OF BOILER TUBES 7

2.4 BOILER FEED WATER TREATMENT 7

2.4.1 FILTRATION 8

2.4.2 DEMINERALIZATION 10

2.4.3 DEAERATORS 14

2.5 CHEMICAL FEED SYSTEMS 17

2.5.1 Oxygen Scavengers 17

2.5.2 The Control of the Feedwater pH. 17

2.5.3 Steam Condensate Treatment 18

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2.0 OBJECTIVES

As a plant operator, you should have a general knowledge and understanding ofyour Companies operate the water treatment equipment.

This unit will explain the following things.

What water treatment is

Why water treatment is needed

Different types of water treatment

Mechanical treatment

Dernineralization treatment

Chlorination treatment

In Unit 1, we have already dealt with the pre-teatment of the seawater feed todesalination units.

In this unit we will look at the treatment of water that feeds and exits steamgenerators (boilers).

Steam is very important in the operation of process plants. The plants cannotoperate without steam.

The steam is generated from water. Water is the starting point.

The correct preparation and treatment of that water is therefore very important.

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2.1 WHAT IS WATER TREATMENT

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Water treatment is the removal of impurities from feed water to produce a highpurity water suitable for feeding to a boiler.

Impurities are of three types:

Suspended solids

Dissolved solids

Dissolved gases

2.3. WHY IS WATER TREATMENT NEEDED

2.3.1 SUSPENDED AND DISSOLVED SOLIDS WILL DAMAGE THE BOILERINTERNALS

Solids build up on the inside of the boiler tubes. These deposits are called

"scale".

The scale stops heat energy transferring across the tube walls to the feed

water.

"Hot spots" are created in the tube walls.

Hot steel is weaker than cold steel.

The boiler tube splits at the weak point because of the internal pressure. (SeeFigure 2.1 and 2.2).

Figure -2.1 Scale Build Up in a Pipe

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Figure 2.2 Split Boiler Tube caused by Hot Spot

Look at Figure 2.3.

See how many tubes there are in a large boiler.

One split tube could mean that we have to shut down the boiler.

That is why it is so important that we protect the boiler tubes from damage.

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Figure 2.3 Boiler Tubes

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2.3.2 DISSOLVED GASES DAMAGE THE INSIDE OF BOILER TUBES

Oxygen and carbon dioxide are the two main dissolved gases.

Oxygen causes corrosion of the steel boiler tubes by oxidation (rusting). This

corrosion produces pitting.

Carbon dioxide causes corrosion that makes grooves in the boiler tubes. [Weak

carbonic acid eats away the metal at hi temperature].

Carbon dioxide also attacks threaded connections

Figure 2.4 Pitting caused by Oxygen in Feedwater

The loss of metal due to oxygen and carbon dioxide corrosion reduces the

thickness of the boiler tube wall.

This weakens the pipe.

This reduces the working pressure of the tubes.

2.4. BOILER FEED WATER TREATMENT

When the boiler is in operation it changes the feed water to steam.

The steam is used for work in the plant.

The steam condenses. back to water.

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The steam condensate is returned back to the boiler as condensate return.

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This completes the steam cycle.

During the steam cycle some water is lost due to

boiler blow down

steam venting

leakage

being used in the process units

This lost water is replaced by make up water from the desalination plant.

The make up water is added to the condensate return system.

Before passing through the boiler again this water has to be treated toremove any impurities it may have picked up in the system.

The first step is filtration.

2.4.1 FILTRATION

Figure 2.5 , Pressure Filter

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The filter consists of a bed of porous material (media) through which the water

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is passed.

* Suspended solids in the water are trapped by the porous material.

The water at the outlet is free from suspended solids.

The filter media is in layers.

The larger holes (more porous) are at the top. The smaller ho (less porous) are

at the bottom.

After the filter has been in use for a time the filter bed becomes blocked with thesolids removed from the water.

When this happens the filter is "back washed" to clean it.

Back washing means reversing the direction of flow through the filter.

The back flow frees the trapped solids and washes them away to waste.

It is normal to have two filters in parallel.

One filter is in service. The other filter is back washed or on stand-by. (See Figure2.6).

Figure 2.6 Two Filters in Parallel

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Filter #1 is in service

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Filter #2 is out of service and is being back washed.

When the flow through a filter decreases, and when the pressure differential acrossthe filter increases, it is time to back wash the filter

Instruments on the filters indicate when the filter is blocked. Norma there is a PD1(Pressure Differential Indicator) instrument.

While it's being back washed a rotary surface washer is used to loosen materialfrom the surface of the filter bed.

Water leaves the nozzles of the rotary washer at high velocity. This makes thewasher arm rotate.

2.4.2 DEMINERALIZATION

This is the removal of dissolved solids, like CaC03, MgS04, and Na2S04

This is done by a chemical reaction called "ion exchange".

It is carried out in equipment called "mixed bed polishers".

The chemicals that do the work are called "resins".

These resins are known as "zeolites"

The zeolites are complex chemicals with very complicated chemical structures.

They are manufactured as tiny beads.

These zeolite beads have very fine pores and cracks.

The chemical action takes place in the cracks.

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Figure 2.7 Schematic of a Zeolite Bead

It can be seen that the beads are very complex.

All we need to know is that "ion exchange" takes place between the dissolved solidsin the water and the different parts of the zeolite beads.

There are two main ion exchanges.

Cation exchange - [positive ion exchange]

Anion exchange - [negative ion exchange]

Cation exchange

Hydrogen + ve ions leave the resin and replace the metals in the dissolvedsolids (Ca, Mg, Na). This forms acids in the water.

Anion exchange

Hydroxide -ve ions leave the resin and replace the acid formed.(S04,CO3,CI2,HC03) This forms water in the feed water.

All the solids are now attached to the resins.

These solids can be removed from the resins by a process called

"regeneration".

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In regeneration we wash the resins with acid and alkali to remove the metals

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and the acids.

This replaces them with hydrogen ions and hydroxide ions.

The regeneration of mixed bed exchangers has to be carefully controlled andwatched closely by the operators. (See Figure 2.8)

Figure 2.8 Main Steps in Regeneration of a Mixed Bed Exchanger.

1. This shows the mixed bed exchanger in normal operation.

2. Here the two resins are separated by loosening the bed by reverseflow backwashing.The anion resin, which is lighter, rises to the top.The cation resin, which is heavier, sinks to the bottom.

3. When the two resins are separated into two layers, each layer isregenerated at the same time.Hydrochloric acid is used for the cation. resin.

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Caustic soda is used for the anion resin. The hydrochloric acid and the

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caustic soda are followed through with pure water to rinse the layers.

4. Water is removed by blowing compressed air into the top of the exchanger.

5. The two resins are remixed by blowing compressed air through the resinsfrom the bottom of the exchanger.

6. The unit is put back on stream after a final rinse.

The waste acid/caustic soda used in regeneration plus the rinsing water areneutralised in a pit before being dumped into a sewer.

Figure 2.9 Regeneration Flow

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2.4.3 DEAERATORS

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The prime function of these pieces of equipment is to remove dissolved gases

from the boiler feed water.

The presence of dissolved gases, especially oxygen and carbon dioxide, will

cause the boiler system to corrode quickly. This is due to the high operating

temperature in the boilers.

Oxygen causes corrosion to the boiler tubes.

Carbon dioxide causes corrosion to the condensate return lines. However, it

has no bad effect on the boiler tubes.

A secondary function of the deaerators is to heat the boiler feed water so it is

nearly as hot as the water already in the boiler steam drum.

This is necessary to prevent thermal shock to the metal parts of the boiler.

It also increases the thermal energy efficiency of the boilers.

NOTE: THERMAL SHOCKIf someone throws icy cold water over you, youreceive a thermal shock because your body suddenlychanges temperature.Cold water going into a very hot piece of equipmenthas the same effect. The sudden temperature changecan damage the equipment.

The main type of deaerator used by Companies is the "spray and tray" type

These deaerators sit on top of cylindrical storage tanks.

The deaerators are different shapes depending on the manufacturer.

They all do the job in basically the same way.

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Figure 2.10 Spray and Tray Deaerator

Steam is used to remove gases from the boiler feed water for these reasons:

It is readily available in the plant

It heats the water and reduces the solubility of the gases

It does not contaminate the feed water

Only a small amount of steam has to be vented, as the steam condensesand becomes part of the feedwater.

The spray and tray deaerator works in a simple way. (See Figure 2.10).

Boiler feed water enters the top part of the deaerator vessel. It is sprayed

upward and forms small droplets.

The feed water is in a chamber full of hot steam.

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The hot steam is condensed by the feed water and gives heat energy to the

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feedwater.

The pre-heated feed water then trickles down into the tray section.

Steam is rising up through the trays and is in close contact with the down

flowing water film.

The feed water is heated to its boiling point and releases almost all of the

dissolved gases.

The released gases rise to the top of the vessel with the steam and are vented

to atmosphere.

The steam condenses at the top of the vessel in the vent condenser.

Only a small amount of steam vents from the deaerator.

Deaerated water exits at the bottom of the deaerator and goes to an insulatedstorage tank.

Feed water flow into the deaerator is controlled by a level controller on the

storage tank.

Figure 2.11 Deaerator and Deaerated Water Storage Vessel

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2.5. CHEMICAL FEED SYSTEMS

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Successful operation of boilers depends on close control of the quality of the

boiler feed water.

The feed water must be free from dissolved gases

The feed water pH must be correctly controlled

2.5.1 Oxygen Scavengers

Some oxygen still remains dissolved in the feed water after deaeration.

A chemical called hydrazine is continually injected into the deaerated water

storage vessel.

Hydrazine is called an oxygen scavenger.

Hydrazine chemically reacts with the oxygen to produce water and harmless

nitrogen gas.

Hydrazine + Oxygen --> Water + Nitrogen

N2H4 + O2 --> 2H2O + N2

Hydrazine is a strong alkali and is carcinogenic. It must be handled with great care.

2.5.2 The Control of the Feedwater pH.

acidic pH rapid corrosion of boiler internals

high alkaline pH foaming in the boiler caustic embrittlement of tubes

The control of the feed water pH is an "internal" treatment.

It is called internal as the chemical is injected. into the feedwater inside the

boiler.

pH control is normally done with a sodium phosphate program.

Tri-sodium phosphate raises the pH (Na3P04)

Di-sodium phosphate lowers the pH (Na2HP04)

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The pH of the feed water is normally kept to about 10.5.

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Figure 2.12 A Typical Chemical Feed System

2.5.3 Steam Condensate Treatment

Recall that carbon dioxide dissolved in the feed water can cause corrosion in

the steam condensate lines.

This carbon dioxide has to be removed or made harmless.

It is removed by treating the steam leaving the boiler.

Chemicals called amines are injected into the main steam header leaving the

boiler.

There are two main types of amine used:

- neutralising amines

- filming amines

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Neutralising amines combine with the CO2 and make the CO2 harmless.

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Filming amines form a film on the inside of the pipework.

This film or coating prevents the CO2 and any O2 from touching the pipework.

If the CO2 cannot touch the metal of the pipework it cannot corrode the

pipework.

The amine film is also non-wettable (water will not stick to it).

No water, CO2 or O2 in contact with the pipework = no possibility of corrosion.

Documents

Unit 2 Water Treatment