DM Plant Operation

RAW WATER & IMPURITIES Raw water is basic requirement of any

industry Its physical & chemical properties

depend on source & strata on which it flows

It picks up mineral salt from soil It further gets polluted with

multifarious organic and inorganic substances

Besides dissolved salts it contains suspended solids also

Silicate matter is also present

VARIOUS IMPURITIES Suspended particle

Alkaline salt such as Bi-Carbonates & carbonates of Ca, Mg & Na

Neutral salts such as Sulphates , Chlorides & Nitrate of Ca, Mg & Na

Dissolved metal like Fe & Manganese Organic compounds

Silica

Dissolved CO2

RAW WATER QUALITY

Parameter Unit Raw water DM water

pH 7.5 to 8.1 8.5

Total Alkalinity 440 NIL

Ca Hardness 137.5 NIL

Total hardness 330 NIL

Silica 55 0.05

Chlorides 255.6 NIL

Sulphates

ppm asCaCO3

57.5 NIL

Turbidity NTU 15 NIL

Organic ppm 5 NIL

TDS ppm > 400 1

Conductivity US/cm >600 1

Common water impurities

TOTAL DISSOLVED SOLIDS -

• The primary inorganic ions that make up TDS is Calcium Ca++, Magnesium Mg++, Sodium Na+, Iron Fe++, Manganese Mn++, Bicarbonate HCO3-, Chloride Cl-, Sulfate SO4--, Nitrate NO3-, Carbonate CO3--.

The TDS can be estimated by

measuring the specific conductance of the water

Source TDS - mg/l Distilled Water 0 Two-column Deionizer Water 8 Rain and Snow 10 Lake 170 Rivers 210 Oceans

35,000 Brine Well 125,000 Dead Sea 250,000

HARDNESS -

Temporary Hardness Salts is due to presence of HCO3 , CO3 of Ca++ & Mg++

Permanent Hardness is due to presence of SO4, NO3 , Cl of Ca++

& Mg++

Equivalent Mineral Acidity :

Sum of all ions of SO4,Cl & NO3

ALKALINITY -

are of three types :

Bicarbonate alkalinity

Carbonate alkalinity

Hydroxide & Caustic alkalinity

Total Alkalinity = HCO3 + CO3 + OH

CARBON DIOXIDE -

Rain water as it is falling through the sky absorbs Carbon Dioxide to make Carbonic Acid

The pH value will drop as the concentration of carbon dioxide increases,

and conversely will increase as the bicarbonate alkalinity content increases. H2O + CO2 <====> H2CO3 <====>

H+ + HCO3-

SILICA -

• Silica (SiO2) is an oxide of silicon, and is present in almost all minerals.

• Silica evaporates in a boiler at high temperatures and then redeposits on the turbine blades. These deposits must be periodically removed or damage to the turbine will occur.

IRON-

Iron can exist in water in one of two forms or both. Treatment depends on the form of iron present.

Waters containing "ferrous iron" are clear and colorless when drawn.

Exposure to air converts ferrous iron into the the insoluble, reddish brown "ferric iron".

Importance of Treatment

To prevent scaling on steam generating surfaces

To give adequate protection to various metallic surface from corrosion attack

To prevent turbine & its auxiliaries from failure

To achieve maximum heat utilisation for increasing efficiency

Treatment stages :

To make raw water suitable for boiler

the treatment is carried out in three

stages

Pretreatment

Demineralisation

Chemical Conditioning

Pre Treatment : The purpose of pre-treatment is to

make raw water suitable for DM plant

Pre-treatment make water free from suspended, colloidal and organic impurities

Process involves : settling and coagulation

Filtration

Settling and CoagulationRemove turbidity and suspended mattersHeavy particles of suspended matter easily removed by settling water in a tankTurbidity, micro-organism and colour are present in colloidal form, so they do not settle easilyCoagulation is a process of breaking-up of colloidal solutions into suspended solids, which settles down easilyAlum is most commonly used as coagulant because of lower cost and least corrosive to handleAl2 (SO4)3 + 3Ca (HCO3)2 = 2Al (OH)2 + 3CaSO4 + 6CO2

Filtration :

is defined as passing of fluid through porous medium to remove matters held in suspension.

It removes : Suspended silt Clay Colloidals Micro-organisms (algae, bacteria &

virus )

Backwashing is done periodically to remove dirt accumulated in Filter

DM water supply tounit

Pressure filter

StrongAcid

cation

WeakAcid

cation

Raw water inlet

Degassertank

Degassertower

StrongBaseanion

WeakBaseAnion

MixedBed

OVERVIEW OF DM PLANT

DM Water storage tank

Demineralisation System

The process of Demineralisation based on the

conversion of salts to their corresponding acid by Cation exchangers

removal of acids by Anion exchangers resin

Two exchangers are normally in series (Cation precedes anion unit )

ION EXCHANGE RESINS

Ion exchange resins are synthetic resin made by polymerisation of various organic compounds

organic compounds are styrene & Divinyl Benzene

Long chained Co polymer formed from theses compounds (80%-92%) & Minor portion Di-venenyl Benzene (8% to 20%)

Dievenyl Benzene acts as cross link to hold polymer chain

To make SAC resin polymer is treated with Concentrated H2SO4

To make anion resin the matrix is chloromethylated or aminated

Cation resin are Strongly or weekly acidic

Strongly acidic resin contain sulfonic acidic group SO3H

Weekly acidic contain Carboxylic acidic group COOH

Anion resin are Strongly or weekly basic

Strong base resin have quaternary ammonia function group

week base resin have polyamine functional group containing amine NH2 , secondary amine NHR , tertiary amine NR2

Operation of DM plant

Cation Exchanger :

WEAK ACID CATION :

Cations of Alkaline salts are removed

Exchanger Process :

2HR + CaCO3 CaR2+ H2CO3

2HR+Mg (HCO3)2 Mg R2 +2H2 CO3

Strong Acid cation :

Cation of neutral salts are removed

Exchanger Process :2HR + Na2 SO4 2NaR + H2SO4

HR + NaHCO3 NaR+H2CO3

2HR + CaSO4 CaR2 + H2SO4

HR + NaCl NaR + HCl

Exhaustion of resins can be monitored by sodium leakage from the bed as Na is monovalent and starts slipping first

At the time of Exhaustion FMA drops

.

REGENERATION of CATION UNIT :

Either H2SO4 or HCL is used as regenerant

Regeneration of Strong acid cation & Week acid cation are being done in thorough fare

Effluent of SAC is used as regenerant of WAC

Counter-current regeneration of SAC is preferred for better water quality and chemical optimisation

Effluent during cation regeneration is acidic

2NAR+ H2SO4 2HR + NA2 SO4

CaR2+2HCL 2HR + CaCL2

MgR2+2HCL 2HR + MgCL2

Anion Exchanger :

WEAK BASE ANION RESIN : removes only highly dissociated acids

(H2SO4, HCl & HNO3) from Effluent of Cation Exchanger

Cant remove either weekly dissociated Carbonic acid or Silicic acid

Exchanger Process :

ROH + HCl RCl + H2O

2ROH + H2SO4 R2SO4 +2H2O

ROH + HNO3 RNO3 + H2O

Strong Base Anion : Remove both weekly dissociated & strongly

dissociated acids Strongly dissociated acids come as slip from WBA

Exchanger Process :

ROH + H2SiO3 R2SiO3 + 2H2O

2ROH + H2CO3 R2CO3 +2H2O

ROH + HCl RCl + H2O

Exhaustion of resins can be monitored by Increase in conductivity Decrease in pH Silica Leakage Silica is the main criteria of exhaustion of anion

resin .During Silica leakage conductivity remains steady

WATER INLET

WATEROUTLET

Causticinjection

WEAK BASE ANION

STRONG BASEANION

Air vent

DraIn

ANION EXCHANGER

Air vent

REGENERATION OF ANION UNIT : NaOH is used for regeneration of Anion

unit Effluent of SBA is used regenerant of

WBA

Regeneration Process : RCl +NaOH ROH + NaCl R2SO4 + 2NaOH 2ROH + Na2SO4 R2SiO3 + 2NaOH 2ROH + Na2SiO3

Effluent during anion regeneration process is alkaline

Function of Degasser :

Carbonic acid is very week acid , it decomposes into H2O & CO2 when fall on pressurized Air

H2CO3 H2O + CO2

Non functioning of Degasser will increase load on SBA

Mix Bed Unit :

Mix Bed unit is known as polisher it produces DM water of conductivity

around 0.2 µS/cm Silica Leakage is <0.05 It consists of Mixture of Strong acid

Cation and strong base anion resin It acts as infinite series of

Demineralising pair Exhaustion point is very sharp

Mixed Bed

WATER OUTLET

Air In

Acid inlet

Caustic inlet

Drain pit

WATER INLET

AIR VENT

MIXED BED

z

OPTIMIZATION IN DM PLANT OPERATION

DM Plant operation can be optimized by :

Proper selection of regeneration system

Using atmospheric degasser

Selection of layout and resin type

Output based on water quality

Minimizing deposit

Regeneration system :

Co - current regeneration

Counter current regeneration

Counter current regeneration has

following advantages

Reduced chemical consumption

Improved water quality

Less effluent discharge

STRONG ACID CATION RESIN

Operating capacity verses Regeneration level : (Na = 40 %, Alkalinity = 50 %)

Exchange Capacity( Kg CaCO3 / M

3 of Resin )

Regenerationlevel

(Kg of HCl / M3 of Resin)Co-Current Counter current

50 46 55.2

60 51 59.5

70 55 63.36

80 58.5 66.72

90 61.5 69.12

100 64 71.52

110 66.5 73.44

120 68.5 75.3668.575.36

Increase in output of 10 to 20% incountercurrent with same regeneration level

STRONG BASE ANION RESINOperating capacity verses Regeneration level : (SO4 = 25 %, CO2 = 20 % , Silica =

25 %)

Exchange Capacity( Kg CaCO3 / M

3 of Resin )

Regenerationlevel

(Kg of NaOH / M3 ofResin) Co-Current Counter current

40 26.2 30.0

50 27.6 32.2

60 29.4 34.0

70 31.3 35.4

80 33.6 36.8

100 36.3 38.2

120 38.2 39.168.575.36

STRONG ACID CATION RESIN

Sodium leakage verses Regeneration level :

(Na / TC = 50 %); TC = Total cation

Sodium leakageRegenerationlevel

(Kg of HCl / M3 of Resin)Co-Current Counter current

50 3.9 %

60 3.0 %

70 2.5 %

80 2.0 %

100 1.5 %

120 1.2 %

150 0.9 %

Averagesodium

leakage isless than 100 ppb

(i.e. 0.1 ppm)

68.575.36

STRONG BASE ANION RESINRegeneration level verses Silica leakage :

( Silica / TA = 25 %); TA = Total Anion

Average silica leakage(PPM)

Regenerationlevel

(Kg of NaOH / M3 ofResin) Co-Current Counter current

40 0.23 0.030

50 0.19 0.025

60 0.15 0.020

80 0.085 0.012

100 0.065 0.008

120 0.048 0.00568.575.36

Selection of vessel configuration

The plant configuration will depend on

feed water composition,

water quality required

economics of operation.

A) [SAC] - [WBA]:

Is used to obtain a partially deionized

water without removal of CO2 and SiO2.

B) [SAC] - [SBA]:

Preferred for treating low mineralized

water or for small size plants.

C) [SAC] - [WBA] - [SBA]:

Well suited to treat waters with low

alkalinity, when the FMA (Cl + NO3

+ SO4) is typically > 60% of the total

anions.

D) [WAC] - [SAC] - [SBA]:

Preferred with feed waters containing

a high proportion of temporary

hardness (>60%) and low FMA.

DEGASSER :

Removing of CO2 before it reaches

to anion resin will reduce load

on anion , thus chemical

consumption will reduce

For larger plant if CO2 is greater

than 80 - 100 mg / l the use of

degasser is very ecomomical

Output based on water quality :

Raw water analysis is required on

regular basis.

Output cutoff point based on

– Conductivity comparator of SAC

– Conductivity of SBA and MB

– Silica leak from SBA and MB

High reading of conductivity comparator :

Sodium leakage Improper regeneration

Calcium sulphate precipitation in case of

H2SO4 Regeneration

To avoid CaSO4 precipitation , strength of

H2SO4 during initial stage of regeneration

should be 0.5 to 1 %

Anion output may be effected by

Organic fouling

Silica deposit

For removal of organic fouling brine

treatment is required

Silica deposit can be encountered by

double regeneration

High conductivity of Anion

•Na leakage from cation

•Chlorine leakage from Anion

•Improper regeneration.

•Organic fouling

Minimizing deposits:

Mainly three types of deposit affect resin performance:

• Caso4 precipitation on cation resin.

•Organic fouling on Anion resin

•Silica deposit on SBA resin

Effect of deposits:

•Conductivity remains on higher side.

•Silica slippage

•Low output

Remedial measures for deposits:

•Caso4

•Use low strength of H2So4(0.5 to 1%)

during regeneration

- HCL wash

•Organic fouling

•Brine wash

Silica deposit

- Double regeneration of Anion