Dmplant Operation& Optimisation

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

330

NIL

55

0.05

Chlorides

255.6

NIL

Sulphates

57.5

NIL

Total hardness Silica

ppm as CaCO3

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 0 ✔ Distilled Water ✔ Two-column Deionizer Water 8 10 ✔ Rain and Snow 170 ✔ Lake ✔ Rivers 210 ✔ Oceans 35,000 125,000 ✔ Brine Well 250,000 ✔ Dead Sea

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 Coagulation Remove turbidity and suspended matters Heavy particles of suspended matter easily removed by settling water in a tank Turbidity, micro-organism and colour are present in colloidal form, so they do not settle easily Coagulation is a process of breaking-up of colloidal solutions into suspended solids, which settles down easily Alum is most commonly used as coagulant because of lower cost and least corrosive to handle Al2 (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

OVERVIEW OF DM PLANT Weak Acid cation

Raw water inlet

Pressure filter

Strong Acid cation

Degasser tower

Degasser tank

DM Water storage tank Weak Base Anion

Strong Base anion

Mixed Bed DM water supply to unit

Demineralisation System The process of Demineralisation based on the ✔ conversion of salts to their corresponding acid by Cation exchangers ✔ removal

of acids exchangers resin

by Anion

✔ 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 Divenenyl 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 2ROH + H2SO4 ROH + HNO3

RCl + H2O R2SO4 +2H2O 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

ANION EXCHANGER Air vent

WATER INLET

Caustic injection WEAK BASE ANION Air vent

STRONG BASE ANION WATER OUTLET

D r a I n

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

unit ✔ Effluent of SBA is used regenerant of WBA Regeneration Process : RCl +NaOH R2SO4 + 2NaOH R2SiO3 + 2NaOH Na2SiO3

ROH + NaCl 2ROH + Na2SO4 2ROH +

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 AIR VENT Caustic inlet WATER INLET

Mixed Bed

Acid inlet

WATER OUTLET

z

Air In Drain pit

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 %) Regeneration level (Kg of HCl / M

3

of Resin)

Exchange Capacity ( Kg CaCO

Co-Current

3

/ M 3 of Resin )

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

75.36

Increase in output of 10 to 20% in countercurrent with same regeneration level 68.575.36

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

(Kg of NaOH / M3 of Resin)

6 8.57 5.36

Exchange Capacity ( Kg CaCO3 / M3 of Resin )

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

39.1

STRONG ACID CATION RESIN Sodium leakage verses Regeneration level : (Na / TC = 50 %); TC = Total cation

Regeneration level

(Kg of HCl / M3 of Resin)

6 8.57 5.36

Sodium leakage Co-Current

50

3.9 %

60

3.0 %

70

2.5 %

80

2.0 %

100

1.5 %

120

1.2 %

150

0.9 %

Counter current

Average sodium leakage is less than 100 ppb (i.e. 0.1 ppm)

STRONG BASE ANION RESIN Regeneration level verses Silica leakage : ( Silica / TA = 25 %); TA = Total Anion Regeneration level

(Kg of NaOH / M3 of Resin)

6 8.57 5.36

Average silica leakage (PPM)

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

0.005

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