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7/27/2019 De Mineral i Sing

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DM & PT PLANT


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DEMINERALISING PLANT

1 INTRODUCTION

1.1 PLANT CAPACITY & SALIENT FEATURES

The plant was designed to supply 55 m3/hr polished water. The potable

water is supplied to the power station and colony whereas the polished water

is consumed in boilers to produce steam (for power generation).

The plant comprises of two chains in demineralization and 3 pressure vessels

for potable water production section. There is a clarified water tank common

to both sections. From the clarified water tank, water is pumped by (2+1)

Pumps to potable water filters. After filtration, the water is supplied for use

as potable water. From the same tank, water is fed to DM water pressure

filters, active carbon filters, cation exchangers and degasser by means of

(2+1) clarified water pumps. After degasification, the water is pumped to

weak base anion, strong base anion; and mixed bed exchangers unitsworking in series. After achieving the designed quality, the polished water is

stored in DM water tanks, from where it is drawn for use in waste heat

recovery boilers. Small amount of polished water is used for internal

consumption in DM water plant i.e., regeneration in DM plant.

1.2 WATER QUALITY

(a) Feed Water Quality :

The plant is designed on the following water compositions:


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Turbidity : 30 NTU

pH : 7.3 to 8.0

Sodium & Potassium : 120 ppm as CaCO3

Calcium : 118 ppm as CaCO3

Magnesium : 55 ppm as CaCO3

Total Cations : 293 ppm as CaCO3

Sulphate : 92 ppm as CaCO3

Bicarbonates : 166 ppm as CaCO3

Chlorides : 35 ppm as CaCO3

Total Anions : 293 ppm as CaCO3

Iron : 1.0 ppm as CaCO3

Silica : 25 ppm as CaCO3

Residual Chlorine : 0.05 ppm as CaCO3

Oraganic Matter : Nil

It is assumed that the above quality of water will remain fairly constant

throughout the year. Any major variations in the same will be reflect on the

output of various ion exchangers, quality of treated water and also on

chemical consumption.

(b) Water Quality at Exit of Anion Exchangers:

Hardness : Not Detectable

Silica (Total) : 0.2 ppm as Sio2 (max)

Conductivity : 10 micro siemens (max)

Total Electrolyte : 2 ppm (max)

PH : 7.5 to 8.5

(C) D.M. Water Quality ( Exit Mixed Bed Exchangers) :

Hardness (Total) : Nil

Silica as Sio2 : 0.02 ppm (max)


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Iron as Fe : Nil

Free CO2 as CO2 : Nil

pH : 6.8 to 7.3

Conductivity : 0.1 micro siemens (max)

(d) Water Quality at Exit of Pressure Filters:

Outlet Turbidity : 2 NTU (max)

(e) Water Quality at Exit of Active Carbon Filters :

Free Chlorine Content : Nil

Iron : Nil

Turbidity : 2 NTU (max)

(f) Water Quality at Exit of Cation Exchangers :

Sodium Content : 2 ppm as CaCO3 (max)

(g) Water Quality at Exit of Degasser Tower :

CO2 : 5 ppm as CO2 (max)

2. PROCESS OF TREATMENT

The process of demineralization consists of the conversion of salts like

NaCl, CaSO4, Ca(HCO3)2 etc. to their corresponding acids like HCI,H2SO4,

H2CO3 by cation exchange resin (in hydrogen form) and removal of these

acids by anion Exchange resin (in hydroxide form), thus removing all

dissolved ionic impurities from water & converting water in pure form.

A) Ion Exchange Resins :

Ion Exchange resins used in DM water plants are synthetic organic

compounds made by copolymerisation of various organic compounds. Most

commonly used are Styrene & divinyl benzine (for the basic resin beds).

These beds are further subjected to the process of sulfonation to make cation


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resins; chloromethylation and amination to make the anion resins. These

processes attach the main functional groups to the resin matrix giving it the

cationic or anionic characteristics.

B) Cation Exchange Process :

Two types of cation exchange resins are mainly used in DM water plants,

weak acidic and strong acidic. Selection of any of these resins depends upon

the feed water quality and final water quality desired from the plant. In

softening plants, cation exchange resin is used in sodium form whereas in

DM water plants, it is used in hydrogen form.

The main cations in water are Ca++

, Mg++

& K+, These are exchanged with

mobile hydrogen ion of cation resin and is represented by the following

equations:

1. Weakly Acidic Cation Resin :

a. RH + NaHCO3 RNa + H2CO3

b. 2RH + CaCO3 R2Ca + H2CO3

c. 2RH + Mg (HCO3)2 R2Mg + 2 H2CO3

2. Strongly Acidic Cation Resin :

a. 2RH + Na2SO4 2RNa + H2SO4

b. RH + NaHCO3 RNa + H2CO3

c. 2RH + CaSO4 R2Ca + H2SO4

d. 2RH + Mg (HCO3)2 R2Mg +2 H2CO3

e. 2RH + CaSiO3 R2Ca +2H2SiO3

Water from cation exchangers is sent to degasser where carbonic acid

breakes into water & CO2.

H2CO3 H2O + CO2

In this way, Carbonic acid load is reduced.


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When the resins (weak or strong acid) are exhausted as indicated by leakage

of cations in the outlet water. These are regenerated by Hydrochloric acid or

Sulphuric acid to bring back the resin in hydrogen form & is represented by

the following equations (shown in 3).

3. Regeneration :

a. 2RNa + H2SO4 2RH + Na2SO4

b. R2Ca + 2HCI 2RH + CaCI2

c. R2Mg + H2SO4 2RH + MgSO4

C. Anion Exchange Process :

Two types of anion exchange resins are mainly used in DM water plants,

weakly basic & strongly basic.

The selection of any of these resins depends upon the feed water quality and

final water quality desired from the plant. In DM water plant, these resins

are used in hydroxide form. The main anions in water are Cl-, SO4

--, NO3

-,

CO3--

& HCO3-. These are exchanged with mobile hydroxyl ion of anion

resin and is represented by the following equations:

1. Weakly Basic Anion Resin :

a. ROH + HCL RCL + H2O

b. 2ROH+ H2SO4 R2SO4 + 2H2O

c. ROH + HNO3 RNO3 + H2O

2. Strongly Basic Anion Resin :

a. ROH + HCL RCL + H2O

b. 2ROH + H2SO4 R2SO4+ 2H2O

c. ROH + HNO3 RNO3 + H2O

d. 2ROH + H2SiO3 R2SiO3 + 2H2O

e. 2ROH + H2CO3 R2CO3 + 2H2O


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3. Regeneration :

a. RCl + NaOH ROH + NaCI

b. R2SO4 + 2NaOH 2ROH+ Na2SO4

c. RNO3 + NaOH ROH + NaNO3

d. R2SiO3 + 2NaOH 2ROH+ Na2SiO3

e. R CO3 + 2NaOH 2ROH+ Na2CO3

D. Inlet water Quality :

The feed water to ion exchange resin must be cold, clean and colourless for

efficient functioning of ion exchange resins. The water should be free from

suspended matter, organic matter, oil, heavy metals like iron and aluminum,

algae etc. These impurities would collect on or inside the resin particles and

shall reduce their ion exchange capacity. Hence such water is pre-treated by

coagulation, filtration etc. before allowing it to pass through ion exchange

resins. Ion exchange resins can act as filter also, but it causes reduction in its

ion exchange capacity and resin bed may require occasional cleaning or

replacement.

E. Mixed Bed Unit

When the water is passed through cation exchange resin and then to strongly

basic anion exchange resin, it removes most of the dissolved ions present in

the water. In order to produce still pure treated water having conductivity

less then or equal to 0.02 ppm, a mixed bed unit is used. A mixed bed

consists of mixture of strongly acidic cation exchange resin and strongly

basic anion exchange resin. Sometimes the mixed bed unit is known as

polisher because it is used for getting water having conductivity around 0.2

micromhos/cm. It acts effectively due to infinite series of demineralising

pairs. Good treated water quality can be obtained from a mixed bed unit

using relatively lower quantities of regenerants.


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PRE -TREATEMENT PLANT

This plant has the capacity to produce 3000 m3/hr of clarified water to meet

the total requirement of DM Plant and softening Plant. The variousprocesses involved are as below:

AERATION

The raw water from reservoir is being pumped through 750 mm dia inlet

pipe to the aerator. Cascade aerator is used to remove volatile impurities

from raw water. The raw water is discharged at the top of the aerator through

a concentric pipe and flows down wards in steps. By aeration, the water

absorbs O2 from atmosphere which helps in oxidation of organic matters.

The iron dissolved in the water is precipitated as Fe2O3.

COAGULATION

From the aerator, water flows directly to the flash mixer through open

cannel; where chlorine, lime and alum (Aluminum Sulphate) are dozed bythe pumps and then flows through a RCC channel by gravity. The added

chemicals are thoroughly mixed with the raw water in flash mixer agitator

(with stainless steel mixing paddle), it is operated through a reduction gear

with an electric motor. Alum as coagulant acts very efficiently in alkaline

medium. Lime furnishes residual alkalinity and thus promotes coagulation

efficiency. Chemical reaction takes place as under:

Al2(SO4)3 + 18H2O + 3Ca(OH)2 3CaSO4 + 2Al(OH)3 + 18H2O


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FLOCCULATION & CLARIFICATION

The chemically treated water is fed to the two Clariflocculators each have

capacity of 1500 M3/hr. The clariflocculators are circular tanks of 40.8 me.

in dia. The clariflocculation consists of two zones for removal of impurities.

First, the flocculation zone (dia=2 me) where the microflocs with the help of

slow speed-agitator and the second clarification zone for solid liqid

separation. Water enters the clarifier through central shaft and flows into the

flocculation zone through ports located at the top of the central shaft. With

the help of 4 nos. of slow speed agitator or floculators, microflocs is being

accumulated. The flocculated water flows towards the bottom where the

solid and liquid separation takes place. The flocs settle to the bottom and is

collected towards the centre of the clarifier by means of scrapers attached to

a rotating bridge. The sludge collected at the centre is discharged through

250 NB CI pipe. To control the discharge of sludge, a constant bleed

arrangement and blow off valve have been provided.

Clarifier and water from clarification zone over flows into launders.

Clarified water from launders flows into the discharge by using a telescopic

valve which can be varied (in position) with resect to top water level, to

achieve different discharge rates. Over and above this arrangement 250 mm

dia CI sludge valve is also provided for occasional blow off. These two

valves are located in the sludge feed and can be operated as and when

required. A rotating bridge in M. S. structure is provided for supporting the

flocculators, scraping the sludge towards discharge points. The bridge rests

on the central pillars and on the clarifier wall by end carriage. The carriage

moves on M.S. Rail laid on the top of clarifier wall. The rotation is achieved


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by operating motor and carriage drives. Power supply to various electrical

drive in the bridge is given through a slippering assembly.

CHLORINATION PLANT

The clarified water is mainly used for service water requirement (cooling of

various equipments) and to feed water to D.M. Plant and softening plant,

which is free from all suspended solids, colloidal solids and other impurities.

The analysis of raw and clarified water is given below :

SNo Property Raw Clarified

Water Water

1. pH 8.4 7.8

2. Turbidity 10.8

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