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Chapter: Chemistry - Water Technology

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Important Questions and Answers: Water Technology

Chemistry - Water Technology - Important Questions and Answers: Water Technology


01. Define boiler feed water.

 

The water fed into the boiler for the production of steam is known as boiler feed water.

 

02. What are the requirements of boiler feed water?

 

It should free from

 

(i). Suspended solids. (ii). Dissolved salts like MgCl2

 

(iii). Hardness. (iv). Alkalinity. (v). Dissolved gases like O2 and CO2

 

03. What are the disadvantages using hard water in boiler?

 

Scale and sludge formation.  2. Caustic Embrittlement.

 

3. Priming and Foaming.          4. Boiler corrosion.

 

04. What are the disadvantages formation of deposits in steam boilers and heat exchangers?

 

Wastage of fuels. 2. Decrease in efficiency. 3. Boiler explosion.

 

05. Define Boiler explosion.

 

When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler.

 

06. What are the methods used to be prevention of scale formation?

 

Prevention of scales formation:          (i). External Treatment – zeolite process

 

(ii). Internal Treatment.- carbonate conditioning.

 

07. What are the methods used to be softening hard water?

 

(i). External Treatment – zeolite process, Demineralisation process (ii). Internal Treatment.- carbonate conditioning, calgon conditioning.

 

08. What is meant by internal conditioning of water? Give one example.

 

Treating the boiler water in the boiler itself by adding chemicals to remove scale forming substance is called internal conditioning.

 

Ex: carbonate conditioning, phosphate conditioning, calgon conditioning.

 

09. What is meant by external conditioning of water? Give one example.

 

Treating the boiler water before feeding it into boiler is called external conditioning. Ex: zeolite process, Demineralisation process

 

10. Distinguish between internal contioning and external conditioning methods.


Internal contioning

To remove hardness producing salts, chemicals were added to the boiler water in the boiler itself and that treatment is known as internal conditioning.

 

External conditioning

Treating the boiler water before feeding it into boiler is called external contioning.

 

11. What is meant by caustic embrittlement? How is it prevented?

 

Caustic embrittlement means intercrystalline cracking of boiler metal. It is prevented by using softening agent like sodium phosphate and by adding tannin and lignin.

 

12. What is desalination?

 

The process of removing common salt (NaCl) from the water is known as desalination. The water contains dissolved salts with brackish taste is called brackish water.

Desalination = Removal of common salt (NaCl ) from water

 

13. What are boiler compounds? Mention two different boiler compounds and their actions.

 

The chemicals directly added into the boiler for removing scale forming substances is known as boiler compounds. Ex: Sodium carbonate and Sodium phosphate

 

14. What is Calgon? How does it function in water treatment?

 

Calgon – Sodium Hexa Meta Phosphate

 

It interacts with calcium ions forming a highly soluble complex and thus prevents the precipitation of scale forming salt.

 

15. What is Reverse osmosis?

 

When two solutions of different concentrations are separated by a semipermeable membrane, when a pressure is applied on the concentrated side, the solvent flow from higher concentration to lower concentration.

 

16. What are the advantages of reverse osmosis method?

 

Low capital cost, easy operating.

 

RO method is used for converting sea water into drinking water.

 

It removes all types of impurities like non-ionic and colloidal.

 

The life time of membrane is high and it can be replaced within few minutes.


 

01. Define Boiler Feed Water. What are the requirements of Boiler Feed Water?

 

Boiler Feed Water: The water fed into boiler for the production of steam is called boiler feed water.

 

It should be free from turbidity, oil, dissolved gases, alkali and hardness producing substances.

 

Requirements of Boiler Feed Water:

 

i). It should has zero hardness.

 

If hardness present in boiler feed water, it produces scales and sludges, which prevents efficient heat transfer.

 

ii). It must free from dissolved gases like O2 and CO2.

If dissolved gases present in boiler feed water, it leads to boiler corrosion.

 

iii). It should be free from suspended impurities.

 

If it is present in boiler feed water, it produces wet steam.

 

iv). It should be free from dissolved salts and alkalinity.

 

If it is present in boiler feed water, it produces caustic embrittlement, which causes brittlement of boiler parts.

 

 02. What is Zeolite? How is water softened by zeolite? Give equations.

 

Zeolite:Sodium Aluminum Orthosilicate.

 

Na2O.Al2O3.xSiO2YH2O.


The synthetic form of zeolite is known as PERMUTIT, which is porous and possess gel structure and Ze stands for insoluble zeolite.

 

Principle: The sodium ions which are loosely held in Na2Ze are replaced by Ca2+ and Mg2+ ions present in water.

Process: When hard water is passed through a bed of zeolite placed in a closed cylinder, the hardness causing ions like Ca2+ and Mg2+ ions are taken up by zeolite. Sodium salts are released during the reaction as byproduct.

 

For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder.

 

The hardness causing ions like Ca2+ and Mg2+are retained by the zeolite as CaZe and MgZe. While the outgoing water contains sodium salts.

 

The various reactions taking place may be

 

Na2Ze + Ca(HCO)2     -- --- >     CaZe + 2NaHCO3

Na2Ze + Mg(HCO)2     -- --- >  MgZe  + 2NaHCO3

Na2Ze + MgCl2    -- --- >  MgZe  + 2NaCl

Na2Ze + CaCl2           -- --- >      CaZe + 2NaCl

Na2Ze + CaSO4          -- --- >     CaZe + Na 2SO4

Na2Ze + MgSO4        -- --- >     MgZe  + Na 2SO4

 

Regeration

 

 

After some time zeolite gets exhausted. The exhausted zeolite is again regerated by treating with 10%solution of NaCl.

 

CaZe + 2 NaCl    →       Na2Ze + CaCl2

 

MgZe + 2 NaCl          Na2Ze +MgCl2

 

 

Advantages: (i). Output water has only 1-2 ppm. (ii). Operation is easy.

 

(iii). No sludge is formed during the process.

 

 

 

03. Explain formation of deposits in steam boilers and heat exchangers.

 

Steamboilers:       Sealed vessel where water is converted to steam.

 

A steam boiler is a type of generator that is used to create steam.

 

Heat exchangers: A device for transferring heat from one medium to another.

 

Heat exchangers are designed to remove excess heat from aircraft engines, optics, x-ray tubes, lasers, power supplies, military equipment, and many other types of equipment that require cooling beyond what air-cooled heat sinks can provide.

Formation of deposits: Scale and sludge formation.

 

Scale:If the precipitate forms hard and adherent coating on the inner walls of the boiler is known as scale. It is formed by the substances like CaSO4, Mg(OH)2 and Ca(HCO3)2.

Scales are difficult to remove even with the help of hammer and chisel. Scales are the main sources of boiler troubles. Formation

 

of scales may be due to

 

(i). Decomposition of calcium bicarbonate. (ii). Deposition of calcium sulphate.

 

(iii). Hydrolysis of magnesium salts. (iv). Presence of silica.

 

Sludge: In boiler, water contains the precipitate loose and slimy is known as sludge. It is formed by the substances like CaCl2, MgCl2, MgSO4 and MgCO3.

 

Disadvantages:

 

(i). Poor conductor of heat.

 

(ii). Excessive sludge formation disturbs the working of the boiler.

 

(iii). It forms along with scales, then former gets entrapped in the latter and both get deposited scales.

 

Prevention of sludge formation: (i). By using well softened water.

 

(ii). By frequently below down opearation.

 

 

04. What are the disadvantages in scale formation? Explain in detail.(Or)

 

What are the disadvantages formation of deposits in steam boilers and heat exchangers?(or)

 

Write short notes on (i). Wastage of fuels. (ii). Decrease in efficiency. (iii). Boiler Explosion.

Disadvantages:    (i). Wastage of fuels.

 

(ii). Decrease in

 

efficiency. (iii). Boiler

 

Explosion.

 

(i). Wastage of fuels:


(ii). Decrease in efficiency:

 

Scales may sometimes deposit in the valves and condensers of the boiler and choke then partially. This results in decrease in efficiency of the boiler.

 

(iii). Boiler Explosion:

 

When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler.

 

6. Explain prevention of scales formation.

 

Prevention of scales formation:       (i). External Treatment.

 

(ii). Internal Treatment.

 

(i). External Treatment:Softening of water (i.e) removing hardness- producing constituents of water. Ex: Zeolite process and Demineralization process.

 

(ii). Internal Treatment:Sequestration process.

 

An ion is prohibited to exhibit its original character by complexing or converting it into other more soluble salt by adding suitable reagent.

 

An internal treatment – by adding proper chemical to the boiler water either

 

(a). To precipitate the scale forming impurities in the form of sludge, which can be remove 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 Blow down operation: It is partial removal of hard water through top at the bottom of boiler, when extent of hardness in the boiler becomes alarmingly high.

Internal treatments methods are, generally, followed by blow-down operation , so that accumulated sludge is removed. Internal treatment methods are,

(i). Colloidal Conditioning

(ii). Phosphate Conditioning

(iii). Carbonate Conditioning

(iv). Calgon Conditioning.

 

07. Write short notes on Caustic Embrittlement.

Caustic Embrittlement. = Intercrystalline cracking of boiler metal

 

It is a type of boiler corrosion, caused by using highly alkaline water in the boiler.

 

In boiler water, it contains a small amount of sodium Carbonate. In high pressure, it decomposes to give sodium hydroxide.

Na2CO3 + H2O ---  2NaOH + CO2

 

Then this NaOH reacts with boiler material and it forms sodium Ferrate and this leads to brittlement of boiler parts like joints, bends etc.,

Fe +2NaOH --- Na2FeO2 + H2

 

It is prevented by         or Avoided by

 

(i). By using sodium phosphate as softening reagent instead of sodium carbonate. (ii). By adding Tannin or Lignin to boiler water for blocks hair-cracks.

 

(iii). By adding sodium sulphate to boiler water also blocks hair-cracks.

 

08. Explain Internal conditioning methods of softening hard water.

 

To remove hardness producing salts, chemicals were added to the boiler water in the boiler itself and

 

that treatment is called internal conditioning.

 

(i). Colloidal Conditioning (ii). Phosphate Conditioning

 

(iii). Carbonate Conditioning (iv). Calgon Conditioning.

 

(i) Colloidal Conditioning:

 

In  low-pressure  boilers,  scale  formation  can  be  avoided  by  adding  organic  substances  like kerosene ,tannin, agar-agar (a gel), etc., which get coated over the scale forming precipitates, thereby yielding non-sticky and loose deposits, which can easily be removed by pre-determined blow-down operations.

(ii). Phosphate conditioning:

 

Three  types  of  phosphates- mono,  di  and  trisodium  phosphates  are  employed  in phosphate conditioning. The advantages of phosphate conditioning over carborate conditioning are

(i). It can be applied to high pressure boilers and

 

(ii) It can be used for softening/ conditioning acidic, neutral or alkaline water sample.

If acidic water is to be conditioned, trisodium phosphate can be used. For neutral and alkaline water samples disodium phosphate and monosodium phosphate can be used respectively.

 

(iii) Carbonate conditioning:

 

In low pressure boilers, calcium ions are converted into soft and loose sludge by adding sodium carbonate solution. It forms soft CaCO3 which can be removed by blow-down operation.

 

Calgon conditioning:

 

Calgon interacts with calcium ions forming a highly soluble complex and it prevents the

 

Calgon=Sodium HexaMeta phospha)6precipitation of scale forming salt. The complex Na2[Na4(PO3)6] is soluble in water and no problem for its sludge disposal.

 

         Write short notes on Boiler corrosion / Explain boiler corrosion in detail / Write short notes on Boiler troubles – Boiler corrosion.

 

Boiler corrosion taken place in boiler in the presenceë  of gases like dissolved oxygen,û dissolved CO2 and dissolved salts.

 

Dissolved oxygen: It attacks the boiler material at high temperature and causes the Corrosion.

4Fe +6H2O + 3O2 --- > 4Fe(OH)3

 

It is removed by chemical and mechanical method. Chemical Method: Sodium sulphite, Hydrazine used to remove dissolved oxygen. Mechanical Method: To remove by De-aeration method.

 

Water spraying in a perforated plate-fitted tower, heated from sides and connected to vaccum pump. High temperature, low pressure and large exposed surface reduces the dissolved oxygen in water.


Dissolved CO2:    It produces carbonic acid, which is acidic and corrosive in nature

CO2 +H2O ---  H2CO3 (Carbonic acid )

 

It can be removed by adding calculated amount of NH4OH into water and also removed by de-aeration method.

 

 

Dissolved salts(MgCl2): Salts like CaCl2 and MgCl2 undergoes hydrolysis at higher temperature , to give HCl , which corrodes the boiler

MgCl2 + 2H2O --- Mg(OH)2       +2HCl

Fe +2HCl   --- FeCl2 + H2

FeCl2 + 2H2O ---- Fe(OH)2 +2HCl

 

It can be avoided by adding alkali into the boiler water.

 

10. Explain priming and foaming (carry over).

 

1.Priming:  It is the process of production of wet steam.

 

Wet steam – Steam contains droplets of liquid water

 

It is caused by

 

       High steam velocity

 

         Very high level water in the boiler

 

           Sudden boiling of water

 

iv).Very poor boiler design.

 

 

Prevention:  It is controlled by

 

         Controlling the velocity of steam

 

         Keeping the water level lower

 

         Good boiler design

 

4) Using treated water

2) Foaming:

 

The formation of stable bubbles above the surface of water is called foaming

 

These bubbles are carried over by steam leading to excessive priming. It is caused by

 

       Presence of oil and grease

 

         Presence of finely divided particles. It can be prevented by

 

i). Adding coagulants like sodium aluminate

 

ii).Adding anti-foaming agents like synthetic polyamides.

 

11. What is Desalination? Describe desalination of by Reverse Osmosis method with neat diagram.(or)Explain the reverse osmosis process for desalination of brackish water in detail.

Desalination =      Removal of common salt-NaCl from water.

Brackish water = Water containing dissolved salts with a peculiar salty taste. Ex: Sea water

 

Reverse

Osmosis(R

 

O):

 

When a pressure greater than osmotic pressure applied on the concentrated side, the solvent flow takes place from higher concentration to lower concentration is known as reverse osmosis.

 

               In this RO process, Pure water is separated from salt water.

 

               This RO process is also known as Super filtration or Hyper filtration

 

               When the pressure is applied from the higher concentration side, the solvent flow takes place to lower side and these two concentrations are separated by semipermeable membrane, the salt water is converted into pure water.

 

               The membranes used as cellulose acetate, polyamide and some polymers.

 

Advantages & RO method:

1. Low capital cost, easy operating.

 2. It is used for converting sea water into drinking water

 3. It removes all types of impurities like non-ionic and colloidal

 4. The life time of membrane is high and it can be replaced within few minutes.


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