22. Basic Chemistry

8/12/2019 22. Basic Chemistry

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Chemistry for Operators

Water is the only common substance that exists in three forms (ice, water, steam) at

normal temperatures. It absorbs more heat for a given temperature rise than any other

common inorganic substance. Water expands 16 times as it evaporates to form steam at

atmospheric pressure. !he steam is capable of carrying large "uantities of heat. !hese

uni"ue properties of water ma#e it an ideal raw material for power and desalination

processes. !his water is never pure and its impurities are the factors of concern in

industrial water treatment.

!he $ulti %tage &lash 'vaporators ($%&') esalination plant has units. %eawaterwhen heated and introduced to a vacuum starts to flash. !hat is, the water present in the

seawater starts to evaporate at a lower temperature than boiling due to the vacuum. !he

water that has evaporated in the &lash chambers ( istillate) is condensed and collected

from the individual esalination units.

!he distillate water obtained from the esalination units is lac#ing in essential minerals

and therefore unfit for human consumption. * "uantity of the distillate is used to generate

more process steam for the %team !urbines, however the ma+ority of the distillate is sent

to the reminerali ation plant, where, by passing through limestone, it gains the necessary

minerals to be classed as potable e.g. fit for human consumption.

!here are five ma+or problems directly associated with water "uality-

Scale Formation: %cale is a very hard substance that adheres directly to heating surfaces

forming a layer of insulation. * one sixteenth inch thic#ness of scale in a boiler can

result in a 1/.0 increase in fuel consumption. %ince we are using deioni ed water

with .1 2%3cm conductivity as the feedwater, the chance of scale formation is

negligible in the entire steam cycle.

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In sea water desalination, scaling of heat transfer tubes in multi stage flash evaporators is

a severe problem that can result in lowering the overall efficiency of desalination plant

hence increasing its capital and operating costs. %cales are normally deposits of low

solubility salts presence in sea water li#e calcium carbonate, 4a45 , magnesium

hydroxide, $g(57) / , (the so called al#aline scales) and calcium sulphate, 4a%5 . 4a%5

starts to form if the !8! is 9 11/ o4. 4a%5 salt formed can strongly adhere to the heat

transfer surfaces and is very difficult to remove.

4a45 is often reported to precipitate in favour of $g(57) / at low temperatures (i.e.

to : ;4) while at high temperatures (e.g. : to 11 ;4) $g(57) / is often the

predominant scale. 8ut some instances have been reported where $g(57) / precipitate at

low temperatures. 4a%5 will normally show up at relatively high temperatures e.g. 11;4 and above.

We are using 8elgard '< / and 8elgard '< / 0 and 8all 4leaning %ystem (84%)

for the control of scaling of 4a45 , $g(57) / = 4a%5 and particulate fouling in our

plant.

Corrosion: 4orrosion is defined as the destruction of a metal by chemical or

electromechanical reaction with its environment. It will occur when levels of oxygen

or carbon dioxide are high, where p7 values are low, where contact occurs between

dissimilar metals and in damp environment or corrosive atmospheres.

5xygen that enters the condensate system will oxidi e the protective layers on iron and

copper > oxygen converts this protective layer of magnetite to ferric oxide (?ust) and

cupric oxide which are corrosive products. 4orrosion products will enter the solution and

be transported downstream to the boilers, these deposits inhibit heat transfer across the

tube boundary, which reduces the boiler efficiency and can even shorten tube life due to

overheating. 4ontinuous monitoring and analysis of iron and copper in the steam cycle

can reduce the extent of corrosion on external and internal surfaces of boilers.

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Forms of Corrosion

(i) Galvanic Corrosion

When two different meals are in contact with each other in a conducting medium,

galvanic corrosion occurs.

5f the two metals, the one with a less reduction potential becomes anodic and gets

corroded. &or example, when inc and copper are electrically connected and partially

immersed in an electrolyte, the more active metal, i.e., inc forms the anode and gets

dissolved, whereas copper acts as cathode and gets protected.

(ii) Concentration Cell Corrosion

!his is also a #ind of galvanic corrosion that occurs when different parts of the metal are

exposed to different air concentration.

If a metal is partially immersed in a dilute solution of @a4l, the parts above and closely

ad+acent to the water line are more exposed to oxygen and hence becomes cathodic. 5n

the other hand, parts immersed to greater depth have exposed to less oxygen

concentration becomes anodic.

Factors affecting Corrosion

(i) Effect of Temperature

Increase of temperature increases the diffusion of ions in the corrosive medium, thereby

increasing the corrosion. Increase of temperature decreases the solubility of gases in the

a"ueous medium.

(ii) Effect of Humidity

4orrosion is faster in humid atmosphere. $oisture provides the electrolyte needed for

setting up a galvanic cell. !hus the rate of corrosion enhances.


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(iii) Effect of pH

Aenerally acidic media i.e., p7 B C. are more corrosive than al#aline and neutral media.

7owever amphoteric metals such as *l, Dn, %n, Eb and &e are affected by both dilute

acids and al#alies.

(iv) Effect of impurities and suspended particles

Eresence of impurities li#e 45 / , 7 / %, %5/ , fumes of 74l, 7 / %5 , and suspended particles

such as @a4l, (@7 )/ %5 and silica will get dissolved in moisture and provides the

electrolyte for setting up the galvanic cell.

(v) Silica: %ilica is a very hard substance that adheres directly to heating surfaces

forming a layer of insulation. !his layer of insulation will decrease heat transfer

efficiency. !hat is why we monitor silica in boiler water and steam cycle.

(vi) Chloride: 4hloride is one of the common contributors to stress corrosion. 7igh

levels in combination with oxygen can cause pitting of boiler tubes and carbon steel

components. 4hloride can also result in generali ed corrosion of carbon steel support

plates which can cause boiler tube F entingG. 4hloride stress corrosion results in brittlefractures which can rapidly cause failure of the turbine blade. Its presence results from

carryover of boiler water, or direct spray desuperheating with contaminated water. !his

contaminant is transported to the wet end of the turbine, at which point it dissolves in the

first moisture formed and contacts the points of operating and3 or inherent stress.

Corrosion Control

! "roper Selection of metal alloys

In our plant as recommended by the suppliers of A!s, 7?%As, %team !urbines, $%&

'vaporators and other plant e"uipment are designed to use homogenous metal alloys to

increase the corrosion resistance.


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Foaming: &oaming is a condition in which concentrations of soluble salts, aggravated by

grease, suspended solids or organic matter, create frothy bubbles or foam in the

steam space of a boiler. &oaming degrades steam "uality.

%ince we are using deioni ed water with .1 2%3cm conductivity as the feedwater, the

chance of foaming formation is negligible in the entire steam cycle.

In desalination plant, we are using 8elite $:, an effective antifoaming agent to control

foaming and priming in seawater.

Caustic Em%rittlement: 4austic embrittlement will occur when there is a high

concentration of al#aline salts. @ormally this happens when excessive trisodium phosphate or sodium hydroxide dosing. %ince we are using deioni ed water with .1

2%3cm conductivity as the feedwater coupled with good feedwater treatment with

7elamin H 7 and 7elamin H 67, the chance of caustic embrittlement is negligible in

the entire steam cycle. $oreover we do not dose trisodium phosphate or sodium

hydroxide in the boilers.

eminerali,ation %y &on E change: 4omplete removal of inorganic salts3 minerals

from the water is possible by this process and hence the name deminerali ation came up.

!he process involves usage of some synthetic chemicals called F?esinsG which exchange

their 7ydrogen and 7ydroxyl ions with the mineral ions present in water. !hus by

passing the water through a mixture of 4ation and *nion resin ($ixed bed exchanger3

tower), the salts present in the water get retained by the resin to obtain water "uality for

the 7eat ?ecovery %team Aenerators.

!his process of exchanging the ions with the salts present in the water goes on as long as

the resins posses the 7ydrogen ions and 7ydroxyl ions. 5nce all these ions are adsorbed

by the 4ations and *nions of the salts, no further purification ta#es place. *t that point

we say that the resin is F exhaustedG. !he F'xhausted resinG can be regenerated by

treating the 4ation resin with 0 7ydrochloric acid and *nion resin with to 0

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%odium hydroxide solutions. !he waste effluent arising out of the regeneration of mixed

bed and the wash water effluent collected at the sump are neutrali ed in the neutrali ation

tan# depending on the p7 of the effluent and pumped out into discharge culvert only

when the p7 is between 6.0 to C.0.

Electrochlorination "lant: 'lectrochlorinators produce %odium 7ypochlorite solution.

!his solution is dosed continuously to the seawater inta#e system at the rate of ./ to .

ppm. It is also dosed intermittently at the rate of to 0 ppm once a shift, this is an

automatic dosing triggered by the solution level present in the system. !his dosing helps

to prevent the marine growth in the system from the seawater inta#e to discharge point.

The -ole of Chemists: If the 8oiler 4hemistry3 esalination 4hemistry is notmaintained properly, we may have to experience the availability of plant losses due

to corrosion and salt deposits.

If proper chemistry is not maintained, it will lead to erratic and often elevated corrosion

rates, deposition of contaminants in both the boiler and after boiler sections, decreased

heat transfer efficiencies, causing the usage of more fuel to produce steam, increased

environmental discharge due to boiler blowdown = esalination blowdown and effluent

from deminerali ation plant regeneration etc are all typical limitations experienced in the

power and desalination industry.

!hus best possible protection of boilers, superheater, turbine = condensate sections, 7eat

recovery = 7eat re+ect sections of desalination plant re"uire that good chemistry is

maintained. ife of these e"uipment will be extended if good chemical regime is

maintained.

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.oiler Feed /ater Treatment:

!he importance of correct feed water treatment for economic operation and for extending

life of boiler and e"uipment cannot be over emphasi ed. &eed water treatment is essential

in boilers, feed systems, etc., *s all untreated waters carry natural salts, they have to be

treated to prevent scale forming .

!he three main reasons for water treatment are -

Erevention of 4orrosion in feed boiler, steam and condensate systems.

'limination of %cale. 'conomic boiler operation without carry over.

&0"1-&T2 EFFECT O3 $ .O&4E-

! issolved O ygen 4orrosion

#! Calcium salts and magnesium salts!hese salts are the JhardnessJin the boiler.%ome salts can also cause corrosion

'! Silica 4an form a very hard scale.

*! Suspended solids and dissolved solids 4ontribute to, or cause, carry over (K)

(K) 4arry over is a collective term to describe the entrainment of a relatively small "uantity of boiler water solids with

the steam. 4arry over occurs as a result of either foaming or priming, or by a combination of both. &oaming is the

formation of bubbles on the surface of the boiler resulting in the throwing over of slugs of boiler water with the steam.

!his is similar to the JbumpingJ experienced when water is boiled in an open vessel.

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Chemistry Control

Standard Conditions

System Control "arameters esired-ange Controlled %y

Condensate andFeed/ater

1. p7

/. %pecific 4onductivity

. 4ationic 4onductivity

. issolved 5xygen

0. !otal Iron

6. !otal 4opper

H. L H./

. L . M%3cm

B .

B / ppb

B / ppb

B . ppb

*mmonia = 7elamin

*mmonia = 7elamin

*mmonia = 7elamin

eaerator

$aintaining desired p7, low5 and 8lowdown for bothIron = 4opper and bycontrolled ammonia dosing

.oiler 5ater

1. 4ationic 4onductivity

/. p7

B /.0 M%3cm

:.: L H./

Nsing high purity $ waterO8lowdown

*mmonia = 7elamin

Super HeatedSteam

1. 4ationic 4onductivity

/. p7

B ./0 M%3cm

H. L H.0

*mmonia = 7elamin

*mmonia = 7elamin

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"ota%le 5ater 6uality: (-ef: "5"$7 -S. and "lant esign Conditions)

"arameters 1nit 0a imumesira%le-ange 0inimum

pH H.0 C.: L :. C.0

Conductivity 2%3cm 0 1/ L / 1

Total Hardness ppm as 4a45 0 L C0 0

m8$l+alinity ppm as 4a45 0 L C0 0

Chloride ppm / 1 L 0

-esidual Chlorine ppm . 0 . L . 0 ./

4angelier &nde . P . 0 L .

6uestions for Chemistry for Operators

Give %rief ans/er for the follo/ing:

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1. What is the principle involved in $%&Q

/. Why our distillate produced in desalination plant is not fit for humanconsumptionQ

. 7ow are we converting the distillate into potable water suitable for humanconsumptionQ

. What is scaleQ

0. What are all the ma+or salts in the seawater that normally deposits in ourdesalination plantQ

6. *t what temperature 4a45 is depositedQ

C. *t what temperature $g(57) / is depositedQ:. *t what temperature 4a%5 is depositedQ

H. What is the effect of 4a%5 Q

1 . What is 4orrosionQ

11. What is magnetiteQ

1/. 7ow boiler efficiency will be reduced by corrosionQ

1 . What is the effect of temperature on corrosionQ

1 . What is the effect of p7 on corrosionQ

10. @ame few impurities that increases the rate of corrosion.

16. 7ow chloride can cause pitting of boiler tubesQ

1C. What is sacrificial anodic method of corrosion controlQ

1:. What is impressed current method of corrosion controlQ

1H. What is the effect of oxygen on corrosionQ

/ . 7ow are we maintaining al#aline medium in our steam cycleQ

/1. 7ow are we reducing residual oxygen in our desalination plantQ

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//. What is the action of sodium hypochlorite in our desalination plantQ

/ . What is foamingQ

/ . What chemicals are we dosing to reduce foaming in our desalination plantQ

/0. Why caustic embrittlement is almost negligible in our steam cycleQ

/6. What is deminerali ationQ

/C. What is mixed bed exchangerQ

/:. 7ow are we regenerating exhausted resinsQ

/H. *t what p7 the wastewater is discharged and where does it goQ

. What is the chemical produced in the electrochlorination plantQ1. Why should we maintain proper boiler3desalination chemistryQ

/. %tate three main reasons for the water treatment.

. What is the meaning of carryoverQ

. What is the desired range of dissolved oxygen in the feed water of the7?%AsQ

0. What is the desired range of residual chlorine in the potable waterQ

Fill in the %lan+s!

1. 5ne of the responsibilities of 4hemist is the prevention of in all steam 3water sections of power = desalination plant.

/. We are using deioni ed water with 2%3cm conductivity as the feedwater, thechance of scale formation is negligible in the entire steam cycle.

. 5ur plant e"uipment are designed to use homogenous metal alloys to increasethe .

. *part from mechanical deaeration in the desalination plant, is being used to ensure complete elimination of dissolved oxygen .

0. 7elamins form a on the wall of the e"uipment and even ifthe dissolved oxygen present in the system will not come and contact with themetal .

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6. 5ur of our 7?%As ensures the desired value of less than/ ppb of dissolved oxygen.

C. In desalination plant, we are using , an effective antifoaming

agent to control foaming and priming in seawater .

:. can form a very hard scale in the boilers.

H. !he desired range of total iron in the condensates of our 7?%As is ppb.

1 . !he desired range of total copper in the feedwater of our 7?%As is ppb.

11. !he desired range of p7 in the boiler water is .

1/. !he desired range of cationic conductivity in the superheated steam is 2%3cm at /0 o4.

1 . !he desired range of p7 in the potable water is .

1 . !he maximum permissible concentration of chloride in the potable water is ppm.

10. !he desired range of residual chlorine in the potable water is ppm.

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Documents

22. Basic Chemistry