De-mineralization of Water

DEMINERALIZATION OF WATERFOR HIGH PRESSURE BOILERS

Dilip KumarNTPC Ltd.

DEMINERALIZATION TECHNIQUES

DISTILATION

ELECTRODIALYSIS

REVERSE OSMOSIS

ION EXCHANGE

DISTILLATION

Distillation is one of the oldest

methods of water treatment and is

still in use today though not

commonly as a home treatment

method. It can effectively remove

many contaminants from drinking

water, including bacteria,

inorganic and many organic

compounds.

ELECTRODIALYSIS

REVERSE OSMOSIS

Osmosis occurs when two solutions of different

concentrations are separated from one another by a

membrane which is permeable to solvent but

impermeable to solute. Solvents flows from dilute to the

concentrated solution, until, at equilibrium, the chemical

potential of the solvent is equal on both sides of the

membrane.

REVERSE OSMOSIS CONTI...

A pressure at which just prevent the solvent flow is

called Osmotic pressure. If the pressure greater than the

osmotic pressure is applied to the concentrated solution,

the solvent can be forced through the membrane leaving

the dissolved substances behind. This method of

purifying water is

termed reverse osmosis.


REVERSE OSMOSIS CONTI...A typical reverse osmosis plant consists of the following items:

Pre-treatment including acid dosing for pH control and dosing of scale control additives.

High pressure pumps which may be high speed centrifugal, multi-stage centrifugal or reciprocating type.

The reverse osmosis membranes. The membranes or permeators are usually connected in series/ parallel stages is used as the feed to the latter stages. This increases the plant conversion.

A pressure regulating valve, this is used to maintain the necessary reject flow and control the inlet membrane pressure.

The post treatment system, this is usually includes a degasser to remove carbon dioxide formed when acid is used for pH control.

A TYPICAL REVERSE OSMOSIS PLANT

RAW WATER

LOW PRESSURE PUMPS

PARTICULATE FILTERS

HIGH PRESSURE PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


RAW WATER

LOW PRESSURE PUMPS

PARTICULATE FILTERS

HIGH PRESSURE PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


RAW WATER

LOW PRESSURE PUMPS

PARTICULATE FILTERS

HIGH PRESSURE PUMPS

RO plant

Stage-1 7 modules

Stage-24 modules

Stage-3 2 modules

CONCETRATE TO WASTE

PRODUCT WATER

DEGASSING TOWER

PRODUCT WATER PUMPS

STORAGE


Water analyses from the reverse osmosis plant at Hartlepool power station

Analyses Pre-treated water

Product water

Reject water

Conductivity µS/cm

1560 145 6050

Total hardnessmg/kg CaCO3

560 30 2700

Sodium mg/kg Na

100 15 600

Sulphate mg/kg SO4

455 15 2300

Chloride mg/kg Cl

180 23 800

DEMINERALIZATION BY ION- EXCHANGE PROCESS

Ion exchange is the reversible interchange of ions between a solid(ion exchange material) and a liquid in which there is nopermanent change in the structure of the solid. Ion exchange isused in water treatment and also provides a method ofseparation for many processes involving other liquids. It hasspecial utility in chemical synthesis, medical research, foodprocessing, mining, agriculture, and a variety of other areas. Theutility of ion exchange rests with the ability to use and reuse theion exchange material.

DEMINERALISATION SREAM

ACTIVATED CARBON FILTER (ACF)

Sl.No Characteristics Unit NTPC Specification

3.2.1

3.2.2

3.2.3

3.2.4

3.2.5

3.2.6

3.2.7

3.2.8

Total surface, Min

Particle density, wetted in water

Mean particle diameter

(i) In case of needle / cylindrical

type

(ii) In case of granular type

Adsorption capacity in terms of iodine

number, Min

Abrasion Number (by ASTM method),

Min.

Ash content, Max

Mean particle length

(i) In case of needle / cylindrical type

(ii) In case of granular type

Bulk Density, min

m2/g

g/cc

mm

mm

mg/g

% by mass

mm

mesh

Kg/m3

850

1.3 – 1.4

0.6 – 0.8

1.5 – 2.0

1000

95

7.0

2. – 2.4

4 – 16

400

ACTIVATED CARBON

ACTIVATED CARBON FILTER (ACF)

ACTIVATED CARBON

Acts on principle of adsorption which is a surface active phenomenon .

It removes residual turbidity (<2 NTU) of water to its 1/10 level.

It removes organic molecules to control color and odor.

It removes free residual chlorine present in filtered water(0.5 ppm Nil)

PREPARATION OF RESINS

TYPES OF RESIN

(R)

SAC: Strong Acid CationWAC: Weak Acid CationSBA: Strong Base Anion WBA: Weak Base Anion

R-SO3H

Sulphonic Acid

(SAC)

R-CH2CHCH3

|

COOH

Carboxylic Acid

(WAC)

CH3

|

R-CH2-NH+ OH

|

CH3

Tertiary Ammonium

(WBA) CH3

|

R-CH2-N-CH3 OH

|

CH3

Quarternary Ammonium

(SBA)

VESSEL DESIGN

WEAK ACID CATION (WAC)

Weak acid cation exchange resins derivetheir exchange activity from a carboxylicgroup (-COOH). When operated in thehydrogen form, WAC resins remove cationsthat are associated with alkalinity, producingcarbonic acid as shown:

WEAK ACID CATION (WAC) CONT….

These reactions are also reversible and permit the return of theexhausted WAC resin to the regenerated form. WAC resins are not able toremove all of the cations in most water supplies. Their primary asset istheir high regeneration efficiency in comparison with SAC resins. Thishigh efficiency reduces the amount of acid required to regenerate theresin, thereby reducing the waste acid and minimizing disposal problems.

WEAK ACID CATION (WAC) CONT….

Weak acid cation resins are used primarily for softening anddealkalization of high-hardness, high-alkalinity waters, frequently inconjunction with SAC sodium cycle polishing systems. In fulldemineralization systems, the use of WAC and SAC resins in combinationprovides the economy of the more efficient WAC resin along with the fullexchange capabilities of the SAC resin.

CATION EXCHANGE MECHANISM

START OF RUN DURING THE RUN END OF RUN

CaMgNa Ca

Na

MgCa

Mg

Na

Un-exchanged ResinNa leakage

STRONG ACID CATION (SAC)

SAC resins can neutralize strong bases and convert neutral salts into their corresponding acids.SAC resins derive their functionality from sulfonic acid groups (HSO3¯). When used in demineralization, SAC resins remove nearly all raw water cations, replacing them with hydrogen ions, as shown below:

Chemical structural formula of sulfonic strong acid cation resin (Amberlite IR-120)

(XL): cross link(PC): polymer chain(ES): exchange site(EI): exchangeable ion

STRONG ACID CATION (SAC) CONTI...

Strong acid cation exchangers function well at all pHranges. These resins have found a wide range ofapplications. For example, they are used in the sodiumcycle (sodium as the mobile ion) for softening and in thehydrogen cycle for decationization.


A measure of the total concentration of the strong acids in the cationeffluent is the free mineral acidity (FMA). In a typical service run, the FMAcontent is stable most of the time. If cation exchange were 100% efficient,the FMA from the exchanger would be equal to the theoretical mineralacidity (TMA) of the water. The FMA is usually slightly lower than the TMAbecause a small amount of sodium leaks through the cation exchanger. Theamount of sodium leakage depends on the regenerant level, the flow rate,and the proportion of sodium to the other cations in the raw water. Ingeneral, sodium leakage increases as the ratio of sodium to total cationsincreases.

Typical effluent profile for strong acid cation exchanger.


The exchange reaction is reversible. When its capacity is exhausted, theresin can be regenerated with an excess of mineral acid.

Thoroughfare Counter-flow Regeneration

EXHAUSTED CATION RESIN REGENERATION


The regeneration efficiency of WAC is very high compared to the strong acid resin. Therefore it is possible to utilize the regenerant acid strength from the strong acid unit to regenerate the weak acid unit.

DEGASIFIER DESIGN

In water demineralization, a degasifier, or degasser, is often used to remove dissolved carbon dioxide after cation exchange. The most common degassers are of the so-called forced draft or atmospheric type.

THEORY OF DEGASIFICATION

The solubility of CO2 in pure water is high: about 1.5 g/L ormore than 30 meq/L at 25°C and atmospheric pressure. When youstir the water and divide it into small droplets in an atmosphericdegasifier and blow air through the "rain", the gas tends to moveinto the air because the partial pressure of CO2 in air is much belowthe equilibrium pressure. The residual CO2 after an atmosphericdegasifier is 0.20 to 0.25 meq/L (typically 10 mg/L as CO2. Thereforesuch degassers are used when the bicarbonate concentration plusfree carbon dioxide in the feed water to separate columndemineralization systems is at least 0.6 to 0.8 meq/L.

DEGASIFIER DESIGN

After cation exchange, thebicarbonate and carbonate (if any)ions are converted to carbonic acid, orcarbon dioxide. CO2 is soluble in water,but it tends to escape into the air,much as it does in a glass of Cold drinkwhen you stir it. Using a degasser toremove CO2 reduces the ionic load onthe strong base anion resin, and theconsumption of caustic soda is thuslower.

DEGASIFIER

To be effective, the degasifier must be placed afterthe cation exchange column. Before cation exchange,the water is containing bicarbonate. After it, the cationsin water (Ca++, Mg++ and Na+ principally) are convertedto H+ ions, which combine with the HCO3

— bicarbonateanions to produce carbonic acid.

WEAK BASE ANION EXCHANGER

Weak base resin functionality originates in primary (R-NH2),secondary (R-NHR'), or tertiary (R-NR'2) amine groups. WBA resinsreadily re-move sulfuric, nitric, and hydrochloric acids, asrepresented by the following reaction:

STRONG BASE ANION EXCHANGER

SBA resins derive their functionality from quaternary ammoniumfunctional groups. When in the hydroxide form, SBA resinsremove all commonly encountered anions as shown below:

As with the cation resins, these reactions are reversible, allowing for the regeneration of the resin with a strong alkali, such as caustic soda, to return the resin to the hydroxide form.


Demineralization using strong anion resins removes silica as well asother dissolved solids. Effluent silica and conductivity are importantparameters to monitor during a demineralizer service run.

Conductivity/silica profile for strong base anion exchanger


When silica breakthrough occurs at the end of a service run, the treated watersilica level increases sharply. Often, the conductivity of the water decreasesmomentarily, then rises rapidly. This temporary drop in conductivity is easily explained.During the normal service run, most of the effluent conductivity is attributed to thesmall level of sodium hydroxide produced in the anion exchanger. When silicabreakthrough occurs, the hydroxide is no longer available, and the sodium from thecation exchanger is converted to sodium silicate, which is much less conductive thansodium hydroxide. As anion resin exhaustion progresses, the more conductive mineralions break through, causing a subsequent increase in conductivity.

EXHAUSTED ANION RESIN REGENERATION

Strong base anion exchangers are regenerated with a 5%sodium hydroxide solution. As with cation regeneration, therelatively high concentration of hydroxide drives the regenerationreaction. To improve the removal of silica from the resin bed, theregenerant caustic is usually heated to 120°F or to the temperaturespecified by the resin manufacturer. Silica removal is also enhancedby a resin bed preheat step before the introduction of warmcaustic.



The regeneration efficiency of WBA is very high compared to the strong base resin. Therefore it is possible to utilize the regenerant alkali strength from the strong base unit to regenerate the weak base unit.


Demineralizers with weak and strong base anion units canexperience silica fouling because of the use of waste causticfrom the strong base anion vessel to regenerate the weak baseanion resin during thoroughfare regeneration. To avoid this,most of the impurities from the strong base anion resin aredumped to the drain before the thoroughfare begins (generally,the first third of the regenerant). To be confident that the rightamount is dumped, an elution study can be performed.

RESIN STABILITY AND FACTORS

Oxidation

Exposing an ion exchange resin to a highly oxidative environment canshorten resin life by attacking the polymer crosslinks, which weakens thebead structure, or by chemically attacking the functional groups. One of themost common oxidants encountered in water treatment is free chlorine(Cl2). Hydrogen peroxide (H2O2), nitric acid (HNO3), chromic acid (H2CrO4),and HCl can also cause resin deterioration.Dissolved oxygen by itself does not usually cause any significant decline inperformance, unless heavy metals and/or elevated temperatures are alsopresent to accelerate degradation, particularly with anion exchange resins.


OxidationWhen a strong base anion resin experiences chemical attack, thepolymer chain usually remains intact, but the quaternaryammonium strong functional group (trimethylamine for type 1anion resins) splits off. Alternately, the strong base functionalgroups are converted to weak base tertiary amine groups, andthe resin becomes bifunctional, meaning it has both strong baseand weak base capacity. The decline in strong base (salt splitting)capacity may not be noted until more than 25% of the capacityhas been converted.


Irreversible sorption or the precipitationof a foulant within resin particles can causedeterioration of resin performance. Thefouling of anion exchange resins due to theirreversible sorption of high molecular weightorganic acids is a well-known problem.

Although fouling rarely occurs with cationexchange resins, difficulties due to thepresence of cationic polyelectrolytes in aninfluent have been known to occur.Precipitation of inorganic materials, e.g.CaSO4, can sometimes cause operatingdifficulties with cation exchange resins.

FAULING


Silica fouling:

Silica (SiO2) exists in water as a weak acid. In the ionic form,silica can be removed by strong base anion exchange resinsoperated in the hydroxide cycle. Silica can exist as a single unit,(reactive silica) and as a polymer (colloidal silica). Colloidal silicaexhibits virtually no charged ionic character and cannot beremoved by the ionic process of ion exchange. Ion exchangeresins do provide some colloidal silica reduction through thefiltration mechanism, but they are not very efficient at thisprocess.

Silica is a problem for high-pressure boilers, causing precipitation on the blades, which reduces efficiency. Both types of silica, colloidal and reactive, can cause this problem.

MIXED BED EXCHANGERS

A mixed bed exchanger has bothcation and anion resin mixedtogether in a single vessel. As waterflows through the resin bed, the ionexchange process is repeated manytimes, "polishing" the water to a veryhigh purity.Due to increasing boiler operatingpressures and the manufacture ofproducts requiring contaminant-freewater, there is a growing need forhigher water quality than cation-anion demineralizer can produce.

MIXED BED EXCHANGER REGENERATION

During regeneration, the resin is separated into distinctcation and anion fractions as shown in Figures

1. SERVICE

2. BACKWASH

3. SIMULTANEOUS REGENERATION

4. DRAIN DOWN

5. MIXING WITH AIR

6. FINAL RINSE

MIXED BED EXCHANGER REGENERATION

The resin is separated by backwashing,with the lighter anion resin settling on top ofthe cation resin. Regenerant acid isintroduced through the bottom distributor,and caustic is introduced through distributorsabove the resin bed. The regenerant streamsmeet at the boundary between the cationand anion resin and discharge through acollector located at the resin interface.Following regenerant introduction anddisplacement rinse, air and water are used tomix the resins. Then the resins are rinsed,and the unit is ready for service.

Education

De-mineralization of Water