dmplant operation& optimisation
TRANSCRIPT
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